rfc9868v1.txt   rfc9868.txt 
Internet Engineering Task Force (IETF) J. Touch Internet Engineering Task Force (IETF) J. Touch
Request for Comments: 9868 Independent Consultant Request for Comments: 9868 Independent Consultant
Updates: 768 C. Heard, Ed. Updates: 768 C. Heard, Ed.
Category: Standards Track Unaffiliated Category: Standards Track Unaffiliated
ISSN: 2070-1721 September 2025 ISSN: 2070-1721 October 2025
Transport Options for UDP Transport Options for UDP
Abstract Abstract
Transport protocols are extended through the use of transport header Transport protocols are extended through the use of transport header
options. This document updates RFC 768 (UDP) by indicating the options. This document updates RFC 768 (UDP) by indicating the
location, syntax, and semantics for UDP transport layer options location, syntax, and semantics for UDP transport layer options
within the surplus area after the end of the UDP user data but before within the surplus area after the end of the UDP user data but before
the end of the IP datagram. the end of the IP datagram.
skipping to change at line 67 skipping to change at line 67
9. The Option Checksum (OCS) 9. The Option Checksum (OCS)
10. UDP Options 10. UDP Options
11. SAFE UDP Options 11. SAFE UDP Options
11.1. End of Options List (EOL) 11.1. End of Options List (EOL)
11.2. No Operation (NOP) 11.2. No Operation (NOP)
11.3. Additional Payload Checksum (APC) 11.3. Additional Payload Checksum (APC)
11.4. Fragmentation (FRAG) 11.4. Fragmentation (FRAG)
11.5. Maximum Datagram Size (MDS) 11.5. Maximum Datagram Size (MDS)
11.6. Maximum Reassembled Datagram Size (MRDS) 11.6. Maximum Reassembled Datagram Size (MRDS)
11.7. Echo Request (REQ) and Echo Response (RES) 11.7. Echo Request (REQ) and Echo Response (RES)
11.8. Timestamps (TIME) 11.8. Timestamp (TIME)
11.9. Authentication (AUTH), RESERVED Only 11.9. Authentication (AUTH), RESERVED Only
11.10. Experimental (EXP) 11.10. Experimental (EXP)
12. UNSAFE Options 12. UNSAFE Options
12.1. UNSAFE Compression (UCMP) 12.1. UNSAFE Compression (UCMP)
12.2. UNSAFE Encryption (UENC) 12.2. UNSAFE Encryption (UENC)
12.3. UNSAFE Experimental (UEXP) 12.3. UNSAFE Experimental (UEXP)
13. Rules for Designing New Options 13. Rules for Designing New Options
14. Option Inclusion and Processing 14. Option Inclusion and Processing
15. UDP API Extensions 15. UDP API Extensions
16. UDP Options Are for Transport, Not Transit 16. UDP Options Are for Transport, Not Transit
17. UDP Options vs. UDP-Lite 17. UDP Options vs. UDP-Lite
18. Interactions with Legacy Devices 18. Interactions with Legacy Devices
19. Options in a Stateless, Unreliable Transport Protocol 19. Options in a Stateless, Unreliable Transport Protocol
20. UDP Option State Caching 20. UDP Option State Caching
21. Updates to RFC 768 21. Updates to RFC 768
22. Interactions with Other RFCs (and drafts) 22. Interactions with Other RFCs
23. Multicast and Broadcast Considerations 23. Multicast and Broadcast Considerations
24. Network Management Considerations 24. Network Management Considerations
25. Security Considerations 25. Security Considerations
25.1. General Considerations Regarding the Use of Options 25.1. General Considerations Regarding the Use of Options
25.2. Considerations Regarding On-Path Attacks 25.2. Considerations Regarding On-Path Attacks
25.3. Considerations Regarding Option Processing 25.3. Considerations Regarding Option Processing
25.4. Considerations for Fragmentation 25.4. Considerations for Fragmentation
25.5. Considerations for Providing UDP Security 25.5. Considerations for Providing UDP Security
25.6. Considerations Regarding Middleboxes 25.6. Considerations Regarding Middleboxes
26. IANA Considerations 26. IANA Considerations
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Lite, as discussed further in Section 17. Lite, as discussed further in Section 17.
2. Conventions Used in This Document 2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in "OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
In this document, the characters ">>" preceding an indented line(s) In this document, the characters ">>" at the beginning of a paragraph
indicate a statement using the key words listed above. This indicate a statement using the key words listed above. This
convention aids reviewers in quickly identifying or finding the convention aids reviewers in quickly identifying or finding the
portions of this RFC covered by these key words. portions of this RFC covered by these key words.
3. Terminology 3. Terminology
The following terminology is used in this document: The following terminology is used in this document:
IP datagram [RFC0791] [RFC8200]: An IP packet, composed of the IP IP datagram [RFC0791] [RFC8200]: An IP packet, composed of the IP
header (including any IPv4 options) and an IP payload area header (including any IPv4 options) and an IP payload area
(including any IPv6 extension headers or other shim headers). (including any IPv6 extension headers or other shim headers).
Must-support options: UDP options that all implementations are Must-support options: UDP Options that all implementations are
required to support. Their use in individual UDP packets is required to support. Their use in individual UDP packets is
optional. optional.
SAFE options: UDP options that are designed to be safe to ignore for SAFE Options: UDP Options that are designed to be safe to ignore for
a receiver that does not understand them. Such options do not a receiver that does not understand them. Such options do not
alter the UDP user data or signal a change in what its contents alter the UDP user data or signal a change in what its contents
represent. represent.
Socket pair: A pair of sockets defining a UDP exchange, defined by a Socket pair: A pair of sockets defining a UDP exchange, defined by a
remote socket and a local socket, each composed of an IP address remote socket and a local socket, each composed of an IP address
and UDP port number (most widely known from TCP [RFC0793]). and UDP port number (most widely known from TCP [RFC0793], which
has been obsoleted by [RFC9293]).
Surplus area: The area of an IP payload that follows a UDP packet; Surplus area: The area of an IP payload that follows a UDP packet;
this area is used for UDP options in this document. this area is used for UDP Options in this document.
UDP packet: The more contemporary term used herein to refer to a UDP packet: The more contemporary term used herein to refer to a
user datagram [RFC0768]. user datagram [RFC0768].
UDP fragment: One or more components of a UDP packet and its UDP UDP fragment: One or more components of a UDP packet and its UDP
options that enable transmission over multiple IP payloads, larger Options that enable transmission over multiple IP payloads, larger
than permitted by the maximum size of a single IP packet; note than permitted by the maximum size of a single IP packet; note
that each UDP fragment is itself transmitted as a UDP packet with that each UDP fragment is itself transmitted as a UDP packet with
its own options. its own options.
(UDP) User data: The user data field of a UDP packet [RFC0768]. (UDP) User data: The user data field of a UDP packet [RFC0768].
UDP Length: The length field of a UDP header [RFC0768]. UDP Length: The length field of a UDP header [RFC0768].
UNSAFE options: UDP options that are not designed to be safe for a UNSAFE Options: UDP Options that are not designed to be safely
receiver that does not understand them to ignore. Such options ignored by a receiver that does not understand them. Such options
could alter the UDP user data or signal a change in what its could alter the UDP user data or signal a change in what its
contents represent, but there are restrictions on how they can be contents represent, but there are restrictions on how they can be
transmitted; these restrictions are noted in Sections 10 and 12. transmitted; these restrictions are noted in Sections 10 and 12.
User: The upper layer application, protocol, or service that User: The upper layer application, protocol, or service that
produces and consumes content that UDP transfers. produces and consumes content that UDP transfers.
User datagram: A UDP packet, composed of a UDP header and UDP User datagram: A UDP packet, composed of a UDP header and UDP
payload; as discussed herein, that payload need not extend to the payload; as discussed herein, the UDP payload need not extend to
end of the IP datagram. In this document, the original intent the end of the IP datagram. In this document, the original intent
that a UDP datagram corresponds to the user portion of a single IP that a UDP datagram corresponds to the user portion of a single IP
datagram is redefined, where a UDP datagram can span more than one datagram is redefined, where a UDP datagram can span more than one
IP datagram through UDP fragmentation. IP datagram through UDP fragmentation.
4. Background 4. Background
Many protocols include a default, invariant header and an area for Many protocols include a default, invariant header and an area for
header options that varies from packet to packet. These options header options that varies from packet to packet. These options
enable the protocol to be extended for use in particular environments enable the protocol to be extended for use in particular environments
or in ways unforeseen by the original designers. Examples include or in ways unforeseen by the original designers. Examples include
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managed. In stateless protocols, their effect is often limited to managed. In stateless protocols, their effect is often limited to
individual packets, but they can have an aggregate effect on a individual packets, but they can have an aggregate effect on a
sequence of packets as well. sequence of packets as well.
UDP is one of the most popular protocols that lacks space for header UDP is one of the most popular protocols that lacks space for header
options [RFC0768]. The UDP header was intended to be a minimal options [RFC0768]. The UDP header was intended to be a minimal
addition to IP, providing only port numbers and a checksum for error addition to IP, providing only port numbers and a checksum for error
detection. This document extends UDP to provide a trailer area for detection. This document extends UDP to provide a trailer area for
such options, located after the UDP user data. such options, located after the UDP user data.
UDP options are possible because UDP includes its own length field, UDP Options are possible because UDP includes its own length field,
separate from that of the IP header. Other transport protocols infer separate from that of the IP header. Other transport protocols infer
transport payload length from the IP datagram length (TCP, DCCP, and transport payload length from the IP datagram length (TCP, DCCP, and
SCTP). Internet historians have suggested a number of possible SCTP). Internet historians have suggested a number of possible
reasons why the design of UDP includes this field, e.g., to support reasons why the design of UDP includes this field, e.g., to support
multiple UDP packets within the same IP datagram or to indicate the multiple UDP packets within the same IP datagram or to indicate the
length of the UDP user data as distinct from zero padding required length of the UDP user data as distinct from zero padding required
for systems that require writes that are not byte-aligned. These for systems that cannot write an arbitrary number of bytes of data.
suggestions are not consistent with earlier versions of UDP or with These suggestions are not consistent with earlier versions of UDP or
the concurrent design of multi-segment, multiplexing protocols; with the concurrent design of multi-segment, multiplexing protocols;
however, the real reason remains unknown. Regardless, this field however, the real reason remains unknown. Regardless, this field
presents an opportunity to differentiate the UDP user data from the presents an opportunity to differentiate the UDP user data from the
implied transport payload length, which this document leverages to implied transport payload length, which this document leverages to
support a trailer options field. support a trailer options field.
There are other ways to include additional header fields or options There are other ways to include additional header fields or options
in protocols that otherwise are not extensible. In particular, in- in protocols that otherwise are not extensible. In particular, in-
band encoding can be used to differentiate transport payload from band encoding can be used to differentiate transport payload from
additional fields, such as was proposed in [Hi15]. This approach can additional fields, such as was proposed in [Hi15]. This approach can
cause complications for interactions with legacy devices and is thus cause complications for interactions with legacy devices and is thus
not considered further in this document. not considered further in this document.
IPv6 Teredo extensions (TEs) [RFC4380] [RFC6081] use a similar IPv6 Teredo extensions [RFC4380] [RFC6081] use a similar
inconsistency between UDP and IPv6 packet lengths to support inconsistency between UDP and IPv6 packet lengths to support
trailers, but in this case, the values differ between the UDP header trailers, but in this case, the values differ between the UDP header
and an IPv6 length contained as the payload of the UDP user data. and an IPv6 length contained as the payload of the UDP user data.
This allows IPv6 trailers in the UDP user data but has no relation to This allows IPv6 trailers in the UDP user data but has no relation to
the surplus area discussed in this document. As a consequence, TEs the surplus area discussed in this document. As a consequence,
are compatible with UDP options. Teredo extensions are compatible with UDP Options.
5. UDP Option Intended Uses 5. UDP Option Intended Uses
UDP options can be used to provide a soft control plane to UDP. They UDP Options can be used to provide a soft control plane to UDP. They
enable capabilities available in other transport protocols, such as enable capabilities available in other transport protocols, such as
fragmentation and reassembly, that enable UDP frames larger than the fragmentation and reassembly, that enable UDP frames larger than the
IP MTU to traverse devices that rely on transport ports, e.g., IP MTU to traverse devices that rely on transport ports, e.g.,
Network Address Translations (NATs), without additional mechanisms or Network Address Translations (NATs), without additional mechanisms or
state. They add features that could, in the future, protect state. They add features that could, in the future, protect
transport integrity and validate source identity (authentication), as transport integrity and validate source identity (authentication), as
well as those that could encrypt the user payload while still well as those that could encrypt the user payload while still
protecting the UDP transport header -- unlike Datagram Transport protecting the UDP transport header -- unlike Datagram Transport
Layer Security (DTLS) [RFC9147]. They also enable Packetization Layer Security (DTLS) [RFC9147]. They also enable Packetization
Layer Path MTU Discovery (PLPMTUD) over UDP, known as Datagram Layer Path MTU Discovery (PLPMTUD) over UDP, known as Datagram
Packetization Layer Path Maximum Transmission Unit Discovery Packetization Layer Path Maximum Transmission Unit Discovery
(DPLPMTUD) [RFC9869], providing a means for probe packet validation (DPLPMTUD) [RFC9869], providing a means for probe packet validation
without affecting the user data plane, as well as providing explicit without affecting the user data plane, as well as providing explicit
indication of the receiver transport reassembly size. indication of the receiver transport reassembly size.
UDP originally assumed that such capabilities would be provided by UDP originally assumed that such capabilities would be provided by
the user or by a layer above UDP [RFC0768]. However, enough the user or by a layer above UDP [RFC0768]. However, enough
protocols have evolved to use UDP directly, so such an intermediate protocols have evolved to use UDP directly, so such an intermediate
layer would be difficult to deploy for legacy applications. UDP layer would be difficult to deploy for legacy applications. UDP
options leverage the opportunity presented by the surplus area to Options leverage the opportunity presented by the surplus area to
enable these extensions within the UDP transport layer itself. Among enable these extensions within the UDP transport layer itself. Among
the use cases where this approach could be of benefit are request- the use cases where this approach could be of benefit are request-
response protocols such as DNS over UDP [He24]. response protocols such as DNS over UDP [He24].
6. UDP Option Design Principles 6. UDP Option Design Principles
UDP options have been designed based on the following core UDP Options have been designed based on the following core
principles. Each is an observation about (preexisting) UDP [RFC0768] principles. Each is an observation about preexisting behavior of UDP
in the absence of these extensions that this document does not intend [RFC0768] in the absence of these extensions that this document does
to change or a lesson learned from other protocol designs. not intend to change or a lesson learned from other protocol designs.
1. UDP is stateless; UDP options do not change that fact. 1. UDP is stateless; UDP Options do not change that fact.
The state required or maintained by the endpoints is intended to The state required or maintained by the endpoints is intended to
be managed either by the application or a layer/library on behalf be managed either by the application or a layer/library on behalf
of the application. Reassembly of fragments is the only limited of the application. Reassembly of fragments is the only limited
exception where this document introduces a notion of state to exception where this document introduces a notion of state to
UDP. UDP.
2. UDP is unidirectional; UDP options do not change that fact. 2. UDP is unidirectional; UDP Options do not change that fact.
Responses to options are initiated by the application or a layer/ Responses to options are initiated by the application or a layer/
library on behalf of the application. A mechanism that requires library on behalf of the application. A mechanism that requires
bidirectionality needs to be defined in a separate document. bidirectionality needs to be defined in a separate document.
3. UDP options have no length limit separate from that of the UDP 3. UDP Options have no length limit separate from that of the UDP
packet itself. packet itself.
Past experience with other protocols confirms that static length Past experience with other protocols confirms that static length
limits will always need to be exceeded, e.g., as has been an limits will always need to be exceeded, e.g., as has been an
issue with TCP options and IPv4 addresses. Each implementation issue with TCP options and IPv4 addresses. Each implementation
can limit how long/many options there are, but a specification is can limit how long/many options there are, but a specification is
more robust when it does not introduce such a limit. more robust when it does not introduce such a limit.
4. UDP options are not intended to replace or replicate other 4. UDP Options are not intended to replace or replicate other
protocols. protocols.
This includes NTP, ICMP (notably echo), etc. UDP options are This includes NTP, ICMP (notably echo), etc. UDP Options are
intended to introduce features useful for applications, not to intended to introduce features useful for applications, not to
either replace these other protocols nor instrument UDP to either replace these other protocols nor instrument UDP to
replace the need for network testing devices. replace the need for network testing devices.
5. UDP options are a framework, not a protocol. 5. UDP Options are a framework, not a protocol.
Options can be defined in this initial document even when the Options can be defined in this initial document even when the
details are not sufficient to specify a complete protocol. Uses details are not sufficient to specify a complete protocol. Uses
of such options could then be described or supplemented in other of such options could then be described or supplemented in other
documents. Examples herein include REQ/RES and TIME; in both documents. Examples herein include REQ/RES and TIME; in both
cases, the option format is defined, but the protocol that uses cases, the option format is defined, but the protocol that uses
these is specified elsewhere (REQ/RES for DPLPMTUD [RFC9869]) or these is specified elsewhere (REQ/RES for DPLPMTUD [RFC9869]) or
left undefined (TIME). left undefined (TIME).
6. The UDP option mechanism and UDP options themselves are intended 6. The UDP Option mechanism and UDP Options themselves are intended
to default to the same behavior experienced by a legacy receiver. to default to the same behavior experienced by a legacy receiver.
By default, even when option checksums (OCS, APC), By default, even when option checksums (OCS, APC),
authentication, or decryption fail, all received packets (with authentication, or decryption fail, all received packets (with
the exception of UDP fragments) are passed (possibly with an the exception of UDP fragments) are passed (possibly with an
empty data payload) to the user application. Options that do not empty data payload) to the user application. Options that do not
modify user data are intended to (by default) result in the user modify user data are intended to (by default) result in the user
data also being passed, even if, e.g., option checksums or data also being passed, even if, e.g., option checksums or
authentication fails. It is always the user's or application's authentication fails. It is always the user's or application's
obligation to override this default behavior explicitly. obligation to override this default behavior explicitly.
These principles are intended to enable the design and use of UDP These principles are intended to enable the design and use of UDP
options with minimal impact to legacy UDP endpoints, preferably none. Options with minimal impact to legacy UDP endpoints, preferably none.
UDP is -- and remains -- a minimal transport protocol. Additional UDP is -- and remains -- a minimal transport protocol. Additional
capability is explicitly activated by user applications or libraries capability is explicitly activated by user applications or libraries
acting on their behalf. acting on their behalf.
Finally, UDP options do not attempt to match the number of zero- Finally, UDP Options do not attempt to match the number of zero-
length UDP datagrams received by legacy and option-aware receivers length UDP datagrams received by legacy and option-aware receivers
from a source using UDP fragmentation (see Section 11.4). Legacy from a source using UDP fragmentation (see Section 11.4). Legacy
receivers interpret every UDP fragment as a zero-length packet receivers interpret every UDP fragment as a zero-length packet
(because they do not perform reassembly), but option-aware receivers (because they do not perform reassembly), but option-aware receivers
would reassemble the packet as a non-zero-length packet. Zero-length would reassemble the packet as a non-zero-length packet. Zero-length
UDP packets have been used as "liveness" indicators (see Section 5 of UDP packets have been used as "liveness" indicators (see Section 5 of
[RFC8085]), but such use is dangerous because they lack unique [RFC8085]), but such use is dangerous because they lack unique
identifiers (the IPv6 base header has none, and the IPv4 ID field is identifiers (the IPv6 base header has none, and the IPv4 ID field is
deprecated for such use [RFC6994]). deprecated for such use [RFC6864]).
7. The UDP Option Area 7. The UDP Option Area
The UDP transport header includes demultiplexing and service The UDP transport header includes demultiplexing and service
identification (port numbers), an error detection checksum, and a identification (port numbers), an error detection checksum, and a
field that indicates the UDP datagram length (including UDP header). field that indicates the UDP datagram length (including UDP header).
The UDP Length field is typically redundant with the size of the The UDP Length field is typically redundant with the size of the
maximum space available as a transport protocol payload, as maximum space available as a transport protocol payload, as
determined by the IP header (see details in Section 18). The UDP determined by the IP header (see details in Section 18). The UDP
option area is created when the UDP Length indicates a smaller Option area is created when the UDP Length indicates a smaller
transport payload than implied by the IP header. transport payload than implied by the IP header.
For IPv4, the IP Total Length field indicates the total IP datagram For IPv4, the IP Total Length field indicates the total IP datagram
length (including the IP header), and the size of the IP options is length (including the IP header), and the size of the IP options is
indicated in the IP header (in 4-byte words) as the "Internet Header indicated in the IP header (in 4-byte words) as the "Internet Header
Length" (IHL) [RFC0791], as shown in Figure 1. In exceptional cases, Length" (IHL) [RFC0791], as shown in Figure 1. In exceptional cases,
the Protocol field in IPv4 might not indicate UDP (i.e., 17), e.g., the Protocol field in IPv4 might not indicate UDP (i.e., 17), e.g.,
when intervening shim headers are present such as IP Security (IPsec) when intervening shim headers are present such as IP Security (IPsec)
[RFC4301] or for IP Payload Compression (IPComp) [RFC3173]. [RFC4301] or for IP Payload Compression (IPComp) [RFC3173].
skipping to change at line 374 skipping to change at line 375
|Version| IHL | DSCP |ECN| Total Length | |Version| IHL | DSCP |ECN| Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |Flags| Fragment Offset | | Identification |Flags| Fragment Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time to Live | Proto=17 (UDP)| Header Checksum | | Time to Live | Proto=17 (UDP)| Header Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address | | Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Address | | Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... zero or more IP Options (using space as indicated by IHL) ... ... zero or more IP options (using space as indicated by IHL) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... zero or more shim headers (each indicating size) ... ... zero or more shim headers (each indicating size) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP Source Port | UDP Destination Port | | UDP Source Port | UDP Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP Length | UDP Checksum | | UDP Length | UDP Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: IPv4 Datagram with UDP Header Figure 1: IPv4 Datagram with UDP Header
skipping to change at line 441 skipping to change at line 442
| IP Hdr | UDP Hdr | UDP user data | surplus area | | IP Hdr | UDP Hdr | UDP user data | surplus area |
+--------+---------+----------------------+------------------+ +--------+---------+----------------------+------------------+
<------------------------------> <------------------------------>
UDP Length UDP Length
Figure 3: IP Transport Payload vs. UDP Length Figure 3: IP Transport Payload vs. UDP Length
In most cases, the IP transport payload and UDP Length point to the In most cases, the IP transport payload and UDP Length point to the
same location, indicating that there is no surplus area. This is not same location, indicating that there is no surplus area. This is not
a requirement of UDP [RFC0768] (discussed further in Section 18). a requirement of UDP [RFC0768] (discussed further in Section 18).
This document uses the surplus area for UDP options. This document uses the surplus area for UDP Options.
The surplus area can commence at any valid byte offset, i.e., it need The surplus area can commence at any valid byte offset, i.e., it need
not be 16-bit or 32-bit aligned. In effect, this document redefines not be 16-bit or 32-bit aligned. In effect, this document redefines
the UDP Length field as a "trailer options offset". the UDP Length field as a "trailer options offset".
8. The UDP Surplus Area Structure 8. The UDP Surplus Area Structure
UDP options use the entire surplus area, i.e., the contents of the IP UDP Options use the entire surplus area, i.e., the contents of the IP
payload after the last byte of the UDP payload. They commence with a payload after the last byte of the UDP payload. They commence with a
2-byte Option Checksum (OCS) field aligned to the first 2- byte 2-byte Option Checksum (OCS) field aligned to the first 2- byte
boundary (relative to the start of the IP datagram) of that area, boundary (relative to the start of the IP datagram) of that area,
adding zeroes before OCS as needed for alignment. The UDP option adding zeroes before OCS as needed for alignment. The UDP Option
area can be used with any UDP payload length (including zero, i.e., a area can be used with any UDP payload length (including zero, i.e., a
UDP Length of 8), as long as there remains enough space for the UDP Length of 8), as long as there remains enough space for the
aligned OCS and the options used. aligned OCS and the options used.
>> UDP options MAY begin at any UDP length offset. >> UDP Options MAY begin at any UDP Length offset.
>> Option area bytes used for alignment before the OCS MUST be zero. >> Option area bytes used for alignment before the OCS MUST be zero.
If this is not the case, all options MUST be ignored and the surplus If this is not the case, all options MUST be ignored and the surplus
area silently discarded. area silently discarded.
These alignment bytes, coupled with OCS as computed over the These alignment bytes, coupled with OCS as computed over the
remainder of the surplus area, ensure that the one's complement sum remainder of the surplus area, ensure that the one's complement sum
of the surplus area is zero. OCS is half-word (2-byte) aligned to of the surplus area is zero. OCS is half-word (2-byte) aligned to
avoid the need for byte-swapping in its implementation. avoid the need for byte-swapping in its implementation.
The OCS contains an optional one's complement sum that detects errors The OCS contains an optional one's complement sum that detects errors
in the surplus area, which is not otherwise covered by the UDP in the surplus area, which is not otherwise covered by the UDP
checksum, as detailed in Section 9. checksum, as detailed in Section 9.
The remainder of the surplus area consists of options, all except two The remainder of the surplus area consists of options, all except two
of which are defined using a TLV (type, length, and optional value) of which are defined using a TLV (type, length, and optional value)
syntax similar to that of TCP [RFC9293], as detailed in Section 10 syntax similar to that of TCP [RFC9293], as detailed in Section 10
(types No Operation (NOP) and End of Options List (EOL) have an (types No Operation (NOP) and End of Options List (EOL) have an
implicit length of one byte). These options continue until the end implicit length of one byte). These options continue until the end
of the surplus area or can end earlier using the EOL option, followed of the surplus area or can end earlier using the EOL Option, followed
by zeroes (discussed further in Section 10). by zeroes (discussed further in Section 10).
9. The Option Checksum (OCS) 9. The Option Checksum (OCS)
The Option Checksum (OCS) option is a conventional Internet checksum The Option Checksum (OCS) Option is a conventional Internet checksum
[RFC0791] that detects errors in the surplus area. The OCS option [RFC0791] that detects errors in the surplus area. The OCS Option
contains a 16-bit checksum that is aligned to the first 2-byte contains a 16-bit checksum that is aligned to the first 2-byte
boundary, preceded by zeroes for padding (if needed), as shown in boundary, preceded by zeroes for padding (if needed), as shown in
Figure 4. Figure 4.
+--------+--------+--------+--------+ +--------+--------+--------+--------+
| UDP data | 0 | | UDP data | 0 |
+--------+--------+--------+--------+ +--------+--------+--------+--------+
| OCS | UDP options... | | OCS | UDP Options... |
+--------+--------+--------+--------+ +--------+--------+--------+--------+
Figure 4: UDP OCS Format, Here Using One Zero Byte for Alignment Figure 4: UDP OCS Format, Here Using One Zero Byte for Alignment
The OCS consists of a 16-bit Internet checksum [RFC1071], computed The OCS consists of a 16-bit Internet checksum [RFC1071], computed
over the surplus area and including the length of the surplus area as over the surplus area and including the length of the surplus area as
an unsigned 16-bit value. The OCS protects the surplus area from an unsigned 16-bit value. The OCS protects the surplus area from
errors in a similar way that the UDP checksum protects the UDP user errors in a similar way that the UDP checksum protects the UDP user
data (when not zero). data (when not zero).
The primary purpose of the OCS is to detect existing nonstandard The primary purpose of the OCS is to detect existing nonstandard
(i.e., non-option) uses of that area and accidental errors. It is (i.e., non-option) uses of that area and accidental errors. It is
not intended to detect attacks, as discussed further in Section 25. not intended to detect attacks, as discussed further in Section 25.
OCS is not intended to prevent future nonstandard uses of the surplus OCS is not intended to prevent future nonstandard uses of the surplus
area nor does it enable shared use with mechanisms that do not comply area nor does it enable shared use with mechanisms that do not comply
with UDP options. with UDP Options.
The design enables traversal of errant middleboxes that incorrectly The design enables traversal of errant middleboxes that incorrectly
compute the UDP checksum over the entire IP payload [Fa18] [Zu20], compute the UDP checksum over the entire IP payload [Fa18] [Zu20],
rather than only the UDP header and UDP payload (as indicated by the rather than only the UDP header and UDP payload (as indicated by the
UDP header length). Because the OCS is computed over the surplus UDP header length). Because the OCS is computed over the surplus
area and its length and then inverted, the OCS effectively negates area and its length and then inverted, the OCS effectively negates
the effect that incorrectly including the surplus has on the UDP the effect that incorrectly including the surplus has on the UDP
checksum. As a result, when OCS is non-zero, the UDP checksum is the checksum. As a result, when OCS is non-zero, the UDP checksum is the
same in either case. same in either case.
>> The OCS MUST be non-zero when the UDP checksum is non-zero. >> The OCS MUST be non-zero when the UDP checksum is non-zero.
>> When the UDP checksum is zero, the OCS MAY be unused and is then >> When the UDP checksum is zero, the OCS MAY be unused and is then
indicated by a zero OCS value. indicated by a zero OCS value.
>> UDP option implementations MUST default to using the OCS (i.e., as >> UDP Option implementations MUST default to using the OCS (i.e., as
a non-zero value); users overriding that default take the risk of not a non-zero value); users overriding that default take the risk of not
detecting nonstandard uses of the option area (of which there are detecting nonstandard uses of the option area (of which there are
none currently known). none currently known).
Like the UDP checksum, the OCS is optional under certain Like the UDP checksum, the OCS is optional under certain
circumstances and contains zero when not used. UDP checksums can be circumstances and contains zero when not used. UDP checksums can be
zero for IPv4 [RFC0791] and for IPv6 [RFC8200] when the UDP payload zero for IPv4 [RFC0791] and for IPv6 [RFC8200] when the UDP payload
is already covered by another checksum, as might occur for tunnels is already covered by another checksum, as might occur for tunnels
[RFC6935]. The same exceptions apply to the OCS when used to detect [RFC6935]. The same exceptions apply to the OCS when used to detect
bit errors; an additional exception occurs for its use in the UDP bit errors; an additional exception occurs for its use in the UDP
datagram prior to fragmentation or after reassembly (see datagram prior to fragmentation or after reassembly (see
Section 11.4). Section 11.4).
The benefits are similar to allowing UDP checksums to be zero, but The benefits are similar to allowing UDP checksums to be zero, but
the risks differ. The OCS is additionally important to ensure the risks differ. The OCS is additionally important to ensure
packets with UDP options can traverse errant middleboxes [Zu20]. packets with UDP Options can traverse errant middleboxes [Zu20].
When the cost of computing an OCS is negligible, it is better to use When the cost of computing an OCS is negligible, it is better to use
the OCS to ensure such traversal. In cases where such traversal the OCS to ensure such traversal. In cases where such traversal
risks can safely be ignored, such as controlled environments, over risks can safely be ignored, such as controlled environments, over
paths where traversal is validated, or where upper layer protocols paths where traversal is validated, or where upper layer protocols
(applications, libraries, etc.) can adapt (by enabling the OCS when (applications, libraries, etc.) can adapt (by enabling the OCS when
packet exchange fails), and when bit errors at the UDP layer would be packet exchange fails), and when bit errors at the UDP layer would be
detected by other layers (as with the UDP checksum), the OCS can be detected by other layers (as with the UDP checksum), the OCS can be
disabled, e.g., to conserve energy or processing resources or when disabled, e.g., to conserve energy or processing resources or when
performance can be improved. This is why zeroing the OCS is only performance can be improved. This is why zeroing the OCS is only
safe when UDP checksum is also zero and why OCS might still be used safe when UDP checksum is also zero and why OCS might still be used
skipping to change at line 570 skipping to change at line 571
default be delivered to the application layer, even if the OCS fails, default be delivered to the application layer, even if the OCS fails,
unless the endpoints have negotiated otherwise for this UDP packet's unless the endpoints have negotiated otherwise for this UDP packet's
socket pair. socket pair.
When not used (i.e., containing zero), the OCS is assumed to be When not used (i.e., containing zero), the OCS is assumed to be
"correct" for the purpose of accepting UDP datagrams at a receiver "correct" for the purpose of accepting UDP datagrams at a receiver
(see Section 14). (see Section 14).
10. UDP Options 10. UDP Options
UDP options are a minimum of two bytes in length as shown in UDP Options are a minimum of two bytes in length as shown in
Figure 5, except only the one-byte options No Operation (NOP) and End Figure 5, except only the one-byte options No Operation (NOP) and End
of Options List (EOL) described below. of Options List (EOL) described below.
+--------+--------+------- +--------+--------+------------~~------------+
| Kind | Length | (remainder of option...) | Kind | Length | (remainder of option...) |
+--------+--------+------- +--------+--------+------------~~------------+
Figure 5: UDP Option Default Format Figure 5: UDP Option Default Format
The Kind field is always one byte and is named after the The Kind field is always one byte and is named after the
corresponding TCP field (though other protocols refer to this as corresponding TCP field (though other protocols refer to this as
"Type"). The Length field, which indicates the length in bytes of "Type"). The Length field, which indicates the length in bytes of
the entire option, including Kind and Length, is one byte for all the entire option, including Kind and Length, is one byte for all
lengths below 255 (including the Kind and Length bytes). A Length of lengths below 255 (including the Kind and Length bytes). A Length of
255 indicates use of the UDP option extended format shown in 255 indicates use of the UDP Option extended format shown in
Figure 6. The Extended Length field is a 16-bit field in network Figure 6. The Extended Length field is a 16-bit field in network
standard byte order. The length of the option refers to its Length standard byte order. The length of the option refers to its Length
field or Extended Length field, whichever is applicable. field or Extended Length field, whichever is applicable.
+--------+--------+--------+--------+ +--------+--------+--------+--------+
| Kind | 255 | Extended Length | | Kind | 255 | Extended Length |
+--------+--------+--------+--------+ +--------+--------+--------+--------+
| (remainder of option...) | | (remainder of option...) |
+--------+--------+--------+--------+ +--------+--------+--------+--------+
Figure 6: UDP Option Extended Format Figure 6: UDP Option Extended Format
>> The UDP length MUST be at least as large as the UDP header (8) and >> The UDP Length MUST be at least as large as the UDP header (8) and
no larger than the IP transport payload. Datagrams with length no larger than the IP transport payload. Datagrams with length
values outside this range MUST be silently dropped as invalid and values outside this range MUST be silently dropped as invalid and
logged. logged.
>> All logging SHOULD be rate limited. Excess logging events can be >> All logging SHOULD be rate limited. Excess logging events can be
coalesced and reported as a count or can be silently dropped if coalesced and reported as a count or can be silently dropped if
needed to avoid resource overloading. needed to avoid resource overloading.
>> Option Lengths (or Extended Lengths, where applicable) smaller >> Option Lengths (or Extended Lengths, where applicable) smaller
than the minimum for the corresponding Kind MUST be treated as an than the minimum for the corresponding Kind MUST be treated as an
error. Such errors call into question the remainder of the surplus error. Such errors call into question the remainder of the surplus
area and thus MUST result in all UDP options being silently area and thus MUST result in all UDP Options being silently
discarded. discarded.
>> Any UDP option other than NOP or EOL whose length is 254 or less >> Any UDP Option other than NOP or EOL whose length is 254 or less
MUST use the UDP option default format shown in Figure 5. NOP and MUST use the UDP Option default format shown in Figure 5. NOP and
EOL never use either length format. EOL never use either length format.
>> Any UDP option whose length is larger than 254 MUST use the UDP >> Any UDP Option whose length is larger than 254 MUST use the UDP
option extended format shown in Figure 6. Option extended format shown in Figure 6.
>> For compactness, UDP options SHOULD use the smallest option format >> For compactness, UDP Options SHOULD use the smallest option format
possible. possible.
>> UDP options MUST be interpreted in the order in which they occur >> UDP Options MUST be interpreted in the order in which they occur
in the surplus area or, in the case of UDP fragments, in the order in in the surplus area or, in the case of UDP fragments, in the order in
which they appear in the UDP fragment option area (see Section 11.4). which they appear in the UDP fragment option area (see Section 11.4).
The following UDP options are currently defined: The following UDP Options are currently defined:
+=========+==========+==========================================+ +=========+==========+==========================================+
| Kind | Length | Meaning | | Kind | Length | Meaning |
+=========+==========+==========================================+ +=========+==========+==========================================+
| 0* | - | End of Options List (EOL) | | 0* | - | End of Options List (EOL) |
+---------+----------+------------------------------------------+ +---------+----------+------------------------------------------+
| 1* | - | No Operation (NOP) | | 1* | - | No Operation (NOP) |
+---------+----------+------------------------------------------+ +---------+----------+------------------------------------------+
| 2* | 6 | Additional Payload Checksum (APC) | | 2* | 6 | Additional Payload Checksum (APC) |
+---------+----------+------------------------------------------+ +---------+----------+------------------------------------------+
| 3* | 10/12 | Fragmentation (FRAG) | | 3* | 10/12 | Fragmentation (FRAG) |
+---------+----------+------------------------------------------+ +---------+----------+------------------------------------------+
| 4* | 4 | Maximum Datagram Size (MDS) | | 4* | 4 | Maximum Datagram Size (MDS) |
+---------+----------+------------------------------------------+ +---------+----------+------------------------------------------+
| 5* | 5 | Maximum Reassembled Datagram Size (MRDS) | | 5* | 5 | Maximum Reassembled Datagram Size (MRDS) |
+---------+----------+------------------------------------------+ +---------+----------+------------------------------------------+
| 6* | 6 | Request (REQ) | | 6* | 6 | Request (REQ) |
+---------+----------+------------------------------------------+ +---------+----------+------------------------------------------+
| 7* | 6 | Response (RES) | | 7* | 6 | Response (RES) |
+---------+----------+------------------------------------------+ +---------+----------+------------------------------------------+
| 8 | 10 | Timestamps (TIME) | | 8 | 10 | Timestamp (TIME) |
+---------+----------+------------------------------------------+ +---------+----------+------------------------------------------+
| 9 | (varies) | RESERVED for Authentication (AUTH) | | 9 | (varies) | RESERVED for Authentication (AUTH) |
+---------+----------+------------------------------------------+ +---------+----------+------------------------------------------+
| 10-126 | (varies) | Unassigned (assignable by IANA) | | 10-126 | (varies) | Unassigned (assignable by IANA) |
+---------+----------+------------------------------------------+ +---------+----------+------------------------------------------+
| 127 | (varies) | RFC3692-style experiments (EXP) | | 127 | (varies) | RFC3692-style experiments (EXP) |
+---------+----------+------------------------------------------+ +---------+----------+------------------------------------------+
| 128-191 | | Reserved | | 128-191 | | Reserved |
+---------+----------+------------------------------------------+ +---------+----------+------------------------------------------+
| 192 | (varies) | Reserved for Compression (UCMP) | | 192 | (varies) | Reserved for UNSAFE Compression (UCMP) |
+---------+----------+------------------------------------------+ +---------+----------+------------------------------------------+
| 193 | (varies) | Reserved for Encryption (UENC) | | 193 | (varies) | Reserved for UNSAFE Encryption (UENC) |
+---------+----------+------------------------------------------+ +---------+----------+------------------------------------------+
| 194-253 | | Unassigned-UNSAFE (assignable by IANA) | | 194-253 | | Unassigned-UNSAFE (assignable by IANA) |
+---------+----------+------------------------------------------+ +---------+----------+------------------------------------------+
| 254 | (varies) | RFC3692-style experiments (UEXP) | | 254 | (varies) | RFC3692-style UNSAFE experiments (UEXP) |
+---------+----------+------------------------------------------+ +---------+----------+------------------------------------------+
| 255 | | Reserved-UNSAFE | | 255 | | Reserved-UNSAFE |
+---------+----------+------------------------------------------+ +---------+----------+------------------------------------------+
Table 1 Table 1
Options indicated by Kind values in the range 0..191 are known as Options indicated by Kind values in the range 0..191 are known as
SAFE options because they do not interfere with use of that data by SAFE Options because they do not interfere with use of UDP user data
legacy endpoints or when the option is unsupported. Options by legacy endpoints or when the option is unsupported. Options
indicated by Kind values in the range 192..255 are known as UNSAFE indicated by Kind values in the range 192..255 are known as UNSAFE
options because they might interfere with use by legacy receiving Options because they might interfere with use by legacy receiving
endpoints (e.g., an option that alters the UDP data payload). endpoints (e.g., an option that alters the UDP data payload).
UNSAFE option nicknames are expected to begin with capital "U", which UNSAFE Option nicknames are expected to begin with capital "U", which
needs to be avoided for SAFE option nicknames (see Section 26). needs to be avoided for SAFE Option nicknames (see Section 26).
RESERVED and RESERVED-UNSAFE are not assignable by IANA and not RESERVED and RESERVED-UNSAFE are not assignable by IANA and not
otherwise defined at this time. The AUTH, UCMP, and UENC otherwise defined at this time. The AUTH, UCMP, and UENC
reservations are intended for all future options supporting reservations are intended for all future options supporting
authentication, compression, and encryption, respectively, and remain authentication, compression, and encryption, respectively, and remain
reserved until assigned for those uses. reserved until assigned for those uses.
Although the FRAG option modifies the original UDP payload contents Although the FRAG Option modifies the original UDP payload contents
(i.e., is UNSAFE with respect to the original UDP payload), it is (i.e., is UNSAFE with respect to the original UDP payload), it is
used only in subsequent fragments with zero-length UDP user data used only in subsequent fragments with zero-length UDP user data
payloads, thus is SAFE in actual use, as discussed further in payloads, thus is SAFE in actual use, as discussed further in
Section 11.4. Section 11.4.
These options are defined in the following subsections. Options 0 These options are defined in the following subsections. Options 0
and 1 use the same values as for TCP. and 1 use the same values as for TCP.
>> An endpoint supporting UDP options MUST support those marked with >> An endpoint supporting UDP Options MUST support those marked with
an "*" above: EOL, NOP, APC, FRAG, MDS, MRDS, REQ, and RES. This an "*" above: EOL, NOP, APC, FRAG, MDS, MRDS, REQ, and RES. This
includes both recognizing and being able to generate these options if includes both recognizing and being able to generate these options if
configured to do so. These are called "must-support" options. configured to do so. These are called "must-support" options.
The set of must-support options is defined herein. New options are The set of must-support options is defined herein. New options are
not eligible for this designation. not eligible for this designation.
>> All other SAFE options (without an "*") MAY be implemented, and >> All other SAFE Options (without an "*") MAY be implemented, and
their use SHOULD be determined either out-of-band or negotiated, their use SHOULD be determined either out-of-band or negotiated,
notably if needed to detect when options are silently ignored by notably if needed to detect when options are silently ignored by
legacy receivers. legacy receivers.
>> Receivers supporting UDP options MUST silently ignore unknown or >> Receivers supporting UDP Options MUST silently ignore unknown or
malformed SAFE options (i.e., in the same way a legacy receiver would malformed SAFE Options (i.e., in the same way a legacy receiver would
ignore all UDP options). An option is malformed when its length does ignore all UDP Options). An option is malformed when its length does
not indicate (one of) the value(s) stated in the option's not indicate (one of) the value(s) stated in the option's
specification. A malformed FRAG option is an exception to this rule; specification. A malformed FRAG Option is an exception to this rule;
it SHALL be treated as an unsupported UNSAFE option. it SHALL be treated as an unsupported UNSAFE Option.
>> Options with inherently invalid Length field values, i.e., those >> Options with inherently invalid Length field values, i.e., those
that indicate underruns of the option itself or overruns of the that indicate underruns of the option itself or overruns of the
surplus area (pointing past the end of the IP payload), MUST be surplus area (pointing past the end of the IP payload), MUST be
treated as an indication of a malformed surplus area, and all options treated as an indication of a malformed surplus area, and all options
MUST silently be discarded. MUST silently be discarded.
Receivers cannot generally treat unexpected option lengths as Receivers cannot generally treat unexpected Option Lengths as
invalid, as this would unnecessarily limit future revision of options invalid, as this would unnecessarily limit future revision of options
(e.g., defining a new APC that is defined by having a different (e.g., defining a new APC that is defined by having a different
length). length).
>> When UNSAFE options are present, the UDP user data MUST be empty, >> When UNSAFE Options are present, the UDP user data MUST be empty,
and any transport payload MUST be contained in a FRAG option (see and any transport payload MUST be contained in a FRAG Option (see
Section 11.4). Recall that such options may alter the transport Section 11.4). Recall that such options may alter the transport
payload or signal a change in what its contents represent. This payload or signal a change in what its contents represent. This
restriction ensures their safe use in environments that might include restriction ensures their safe use in environments that might include
legacy receivers (see Section 12), because the transport payload legacy receivers (see Section 12), because the transport payload
occurs inside the FRAG option area and is silently discarded by occurs inside the FRAG Option area and is silently discarded by
legacy receivers. legacy receivers.
>> Receivers supporting UDP options that receive unsupported options >> Receivers supporting UDP Options that receive unsupported options
in the UNSAFE range MUST terminate all option processing and MUST in the UNSAFE range MUST terminate all option processing and MUST
silently drop all UDP options in that datagram. See Section 12 for silently drop all UDP Options in that datagram. See Section 12 for
further discussion of UNSAFE options. further discussion of UNSAFE Options.
>> Other than FRAG, NOP, EXP, and UEXP, each option SHOULD NOT occur >> Other than FRAG, NOP, EXP, and UEXP, each option SHOULD NOT occur
more than once in a single UDP datagram. If an option other than more than once in a single UDP datagram. If an option other than
these four occurs more than once, a receiver MUST interpret only the these four occurs more than once, a receiver MUST interpret only the
first instance of that option and MUST ignore later instances. first instance of that option and MUST ignore later instances.
Section 25 provides additional advice for Denial of Service (DoS) Section 25 provides additional advice for Denial of Service (DoS)
issues that involve large numbers of options, whether valid, unknown, issues that involve large numbers of options, whether valid, unknown,
or repeating. or repeating.
>> NOP MAY occur multiple times, either in succession or between >> NOP MAY occur multiple times, either in succession or between
skipping to change at line 773 skipping to change at line 774
zero; the OCS is always computed as if its contents are zero and zero; the OCS is always computed as if its contents are zero and
after the AUTH or UENC hash has been computed. after the AUTH or UENC hash has been computed.
>> Future options MUST NOT be defined as having an option field value >> Future options MUST NOT be defined as having an option field value
dependent on the content or presence of other options or on the dependent on the content or presence of other options or on the
remaining contents of the surplus area, i.e., the area after the last remaining contents of the surplus area, i.e., the area after the last
option (presumably EOL). option (presumably EOL).
If future options were to depend on the contents or presence of other If future options were to depend on the contents or presence of other
options, interactions between those values, the OCS, and the AUTH and options, interactions between those values, the OCS, and the AUTH and
UENC options could be unpredictable. This does not prohibit options UENC Options could be unpredictable. This does not prohibit options
that modify later options (in order of appearance within a packet), that modify later options (in order of appearance within a packet),
such as would typically be the case for compression (UCMP). such as would typically be the case for compression (UCMP).
Note that there is no need to reserve area after the last UDP option Note that there is no need to reserve area after the last UDP Option
for future uses, because any such use can be supported by defining a for future uses, because any such use can be supported by defining a
new UDP option over that area instead. Using an option for this new UDP Option over that area instead. Using an option for this
purpose is safer than treating the region as an exception, because purpose is safer than treating the region as an exception, because
its use can be verified based on option Kind and Length. its use can be verified based on option Kind and Length.
>> AUTH and UENC MUST NOT be used concurrently. >> AUTH and UENC MUST NOT be used concurrently.
AUTH and UENC are never used together because UENC would serve both AUTH and UENC are never used together because UENC would serve both
purposes. purposes.
>> "Must-support" options other than NOP and EOL MUST be placed by >> "Must-support" options other than NOP and EOL MUST be placed by
the transmitter before other SAFE UDP options. A receiver MAY drop the transmitter before other SAFE UDP Options. A receiver MAY drop
all UDP options if this ordering is not honored. Such events MAY be all UDP Options if this ordering is not honored. Such events MAY be
logged for diagnostic purposes. logged for diagnostic purposes.
The requirement that must-support options come before others is The requirement that must-support options come before others is
intended to allow for endpoints to implement DoS protection, as intended to allow for endpoints to implement DoS protection, as
discussed further in Section 25. discussed further in Section 25.
11. SAFE UDP Options 11. SAFE UDP Options
SAFE UDP options can be silently ignored by legacy receivers without SAFE UDP Options can be silently ignored by legacy receivers without
affecting the meaning of the UDP user data. They stand in contrast affecting the meaning of the UDP user data. They stand in contrast
to UNSAFE options, which modify UDP user data in ways that render it to UNSAFE Options, which modify UDP user data in ways that render it
unusable by legacy receivers (Section 12). The following subsections unusable by legacy receivers (Section 12). The following subsections
describe SAFE options defined in this document. describe SAFE Options defined in this document.
11.1. End of Options List (EOL) 11.1. End of Options List (EOL)
The End of Options List (EOL, Kind=0) option indicates that there are The End of Options List (EOL, Kind=0) Option indicates that there are
no more options. It is used to indicate the end of the list of no more options. It is used to indicate the end of the list of
options without needing to use NOP options (see the following options without needing to use NOP Options (see the following
section) as padding to fill all available option space. section) as padding to fill all available option space.
+--------+ +--------+
| Kind=0 | | Kind=0 |
+--------+ +--------+
Figure 7: UDP EOL Option Format Figure 7: UDP EOL Option Format
>> When the UDP options do not consume the entire surplus area or the >> When the UDP Options do not consume the entire surplus area or the
options area of a UDP fragment, the last non-NOP option MUST be EOL. options area of a UDP fragment, the last non-NOP Option MUST be EOL.
>> NOPs SHOULD NOT be used as padding before the EOL option. As a >> NOPs SHOULD NOT be used as padding before the EOL Option. As a
one-byte option, EOL need not be otherwise aligned. one-byte option, EOL need not be otherwise aligned.
>> All bytes after EOL in the surplus area or the options area of a >> All bytes after EOL in the surplus area or the options area of a
UDP fragment MUST be set to zero on transmit. UDP fragment MUST be set to zero on transmit.
>> Bytes after EOL in the surplus area or the options area of a UDP >> Bytes after EOL in the surplus area or the options area of a UDP
fragment MAY be checked as being zero on receipt but MUST NOT be fragment MAY be checked as being zero on receipt but MUST NOT be
otherwise processed (except for OCS calculation, which zeros would otherwise processed (except for OCS calculation, which zeros would
not affect) and MUST NOT be passed to the user. not affect) and MUST NOT be passed to the user.
>> If a receiver elects to check the bytes following EOL and finds >> If a receiver elects to check the bytes following EOL and finds
that they are not all set to zero, it MUST silently discard the that they are not all set to zero, it MUST silently discard the
options area. In this case, the UDP user data MUST be delivered to options area. In this case, the UDP user data MUST be delivered to
the application layer, unless the socket has been explicitly the application layer, unless the socket has been explicitly
configured to do otherwise, as decided by the upper layer or configured to do otherwise, as decided by the upper layer or
negotiated with the other endpoint. negotiated with the other endpoint.
Requiring the post-option surplus area to be zero prevents side- Requiring the post-option surplus area to be zero prevents side-
channel uses of this area, instead requiring that all use of the channel uses of this area, instead requiring that all use of the
surplus area be UDP options supported by both endpoints. It is surplus area be UDP Options supported by both endpoints. It is
useful to allow this area to be used for zero padding to increase the useful to allow this area to be used for zero padding to increase the
UDP datagram length without affecting the UDP user data length, e.g., UDP datagram length without affecting the UDP user data length, e.g.,
for UDP DPLPMTUD (Section 4.1 of [RFC9869]). for UDP DPLPMTUD (Section 4.4 of [RFC9869]).
11.2. No Operation (NOP) 11.2. No Operation (NOP)
The No Operation (NOP, Kind=1) option is a one-byte placeholder, The No Operation (NOP, Kind=1) Option is a one-byte placeholder,
intended to be used as padding, e.g., to align multi-byte options intended to be used as padding, e.g., to align multi-byte options
along 16-bit, 32-bit, or 64-bit boundaries. along 16-bit, 32-bit, or 64-bit boundaries.
+--------+ +--------+
| Kind=1 | | Kind=1 |
+--------+ +--------+
Figure 8: UDP NOP Option Format Figure 8: UDP NOP Option Format
>> UDP packets SHOULD NOT use more than seven consecutive NOPs, i.e., >> UDP packets SHOULD NOT use more than seven consecutive NOPs, i.e.,
skipping to change at line 875 skipping to change at line 876
consecutive NOPs SHOULD log such events, at least occasionally, as a consecutive NOPs SHOULD log such events, at least occasionally, as a
potential DoS indicator. potential DoS indicator.
NOPs are not reported to the user, whether used per-datagram or per- NOPs are not reported to the user, whether used per-datagram or per-
fragment (as defined in Section 11.4). fragment (as defined in Section 11.4).
This issue is discussed further in Section 25. This issue is discussed further in Section 25.
11.3. Additional Payload Checksum (APC) 11.3. Additional Payload Checksum (APC)
The Additional Payload Checksum (APC, Kind=2) option provides a The Additional Payload Checksum (APC, Kind=2) Option provides a
stronger supplement to the checksum in the UDP header, using a 32- stronger supplement to the checksum in the UDP header, using a 32-
bit Cyclic Redundancy Check (CRC) of the conventional UDP user data bit Cyclic Redundancy Check (CRC) of the conventional UDP user data
payload only (excluding the IP pseudoheader, UDP header, and surplus payload only (excluding the IP pseudoheader, UDP header, and surplus
area). It is not an alternative to the UDP checksum because it does area). It is not an alternative to the UDP checksum because it does
not cover the IP pseudoheader or UDP header, and it is not a not cover the IP pseudoheader or UDP header, and it is not a
supplement to the OCS because the latter covers the surplus area supplement to the OCS because the latter covers the surplus area
only. Its purpose is to detect user data errors that the UDP only. Its purpose is to detect user data errors that the UDP
checksum might not detect. checksum might not detect.
A CRC32c has been chosen because of its ubiquity and use in other A CRC32c has been chosen because of its ubiquity and use in other
skipping to change at line 910 skipping to change at line 911
zero is a potentially valid checksum. As such, it does not indicate zero is a potentially valid checksum. As such, it does not indicate
that the APC is not used; instead, the option would simply not be that the APC is not used; instead, the option would simply not be
included if that were the desired effect. included if that were the desired effect.
The APC does not protect the UDP pseudoheader; only the current UDP The APC does not protect the UDP pseudoheader; only the current UDP
checksum provides that protection (when used). The APC cannot checksum provides that protection (when used). The APC cannot
provide that protection because it would need to be updated whenever provide that protection because it would need to be updated whenever
the UDP pseudoheader changed, e.g., during NAT address and port the UDP pseudoheader changed, e.g., during NAT address and port
translation (see [RFC1141]). translation (see [RFC1141]).
>> UDP packets with incorrect APC checksums SHOULD be passed to the >> UDP packets with incorrect APC Option checksum fields SHOULD be
application with an indication of APC failure. This is the default passed to the application with an indication of APC Option checksum
behavior for APC. failure. This is the default behavior for APC.
>> Like all SAFE UDP options, the APC MUST be silently ignored when >> Like all SAFE UDP Options, the APC MUST be silently ignored when
failing, unless the receiver has been explicitly configured to do failing, unless the receiver has been explicitly configured to do
otherwise. otherwise.
Although all UDP option-aware endpoints support the APC (being in the Although all UDP Option aware endpoints support the APC (being in the
required set), this silently ignored behavior ensures that option- required set), this silently ignored behavior ensures that option-
aware receivers operate the same as legacy receivers unless aware receivers operate the same as legacy receivers unless
overridden. Another reason is because the APC check could fail even overridden. Another reason is because the APC check could fail even
where the user data has not been corrupted, such as when its contents where the user data has not been corrupted, such as when its contents
have been intentionally overwritten, e.g., by a middlebox to update have been intentionally overwritten, e.g., by a middlebox to update
embedded port numbers or IP addresses. Such overwrites could be embedded port numbers or IP addresses. Such overwrites could be
intentional and not widely known; defaulting to silent ignore ensures intentional and not widely known; defaulting to silent ignore ensures
that option-aware endpoints do not change how users or applications that option-aware endpoints do not change how users or applications
operate unless explicitly directed to do otherwise. operate unless explicitly directed to do otherwise.
>> UDP packets with unrecognized APC lengths MUST receive the same >> UDP packets with unrecognized APC lengths MUST receive the same
treatment as UDP packets with incorrect APC checksums. treatment as UDP packets with incorrect APC Option checksum fields.
Ensuring that unrecognized APC lengths are treated as incorrect Ensuring that unrecognized APC lengths are treated as incorrect
checksums enables future variants of APC to be treated like APC. checksums enables future variants of APC to be treated similarly.
The APC is reported to the user and useful only per-datagram, because The APC is reported to the user and useful only per-datagram, because
fragments have no UDP user data. fragments have no UDP user data.
11.4. Fragmentation (FRAG) 11.4. Fragmentation (FRAG)
The Fragmentation (FRAG, Kind=3) option supports UDP fragmentation The Fragmentation (FRAG, Kind=3) Option supports UDP fragmentation
and reassembly, which can be used to transfer UDP messages larger and reassembly, which can be used to transfer UDP messages larger
than allowed by the IP receive MTU (Effective MTU for Receiving than allowed by the IP Effective MTU for Receiving (EMTU_R)
(EMTU_R) [RFC1122]). FRAG includes a copy of the same UDP transport [RFC1122]. FRAG includes a copy of the same UDP transport ports in
ports in each fragment, enabling them to traverse stateless Network each fragment, enabling them to traverse stateless Network Address
Address (and port) Translation (NAT) devices, in contrast to the (and port) Translation (NAT) devices, in contrast to the behavior of
behavior of IP fragments [RFC4787]. FRAG is typically used with the IP fragments [RFC4787]. FRAG is typically used with the UDP MDS and
UDP MDS and MRDS options to enable more efficient use of large MRDS Options to enable more efficient use of large messages, both at
messages, both at the UDP and IP layers. The design of FRAG is the UDP and IP layers. The design of FRAG is similar to that of the
similar to that of the IPv6 Fragmentation Header [RFC8200], except IPv6 Fragmentation Header [RFC8200], except that the UDP variant uses
that the UDP variant uses a 16-bit Offset measured in bytes, rather a 16-bit Offset measured in bytes, rather than IPv6's 13-bit Fragment
than IPv6's 13-bit Fragment Offset measured in 8-byte units. This Offset measured in 8-byte units. This UDP variant avoids creating
UDP variant avoids creating reserved fields. reserved fields.
The FRAG header also enables use of options that modify the contents The FRAG header also enables use of options that modify the contents
of the UDP payload, such as encryption (UENC, see Section 12.2). of the UDP payload, such as encryption (UENC, see Section 12.2).
Like FRAG, such options would not be safely used on UDP payloads Like FRAG, such options would not be safely used on UDP payloads
because they would be misinterpreted by legacy receivers. FRAG because they would be misinterpreted by legacy receivers. FRAG
allows use of these options, either on fragments or on a whole, allows use of these options, either on fragments or on a whole,
unfragmented message (i.e., an "atomic" fragment at the UDP layer, unfragmented message (i.e., an "atomic" fragment at the UDP layer,
similar to atomic IP datagrams [RFC6864]). This is safe because FRAG similar to atomic IP datagrams [RFC6864]). This is safe because FRAG
hides the payload from legacy receivers by placing it within the hides the payload from legacy receivers by placing it within the
surplus area. surplus area.
>> When FRAG is present, it SHOULD come as early as possible in the >> When FRAG is present, it SHOULD come as early as possible in the
UDP options list. UDP Options list.
When present, placing FRAG first can simplify some implementations, When present, placing FRAG first can simplify some implementations,
notably those using hardware acceleration that assume a fixed notably those using hardware acceleration that assume a fixed
location for the FRAG option. However, there are cases where FRAG location for the FRAG Option. However, there are cases where FRAG
cannot occur first, such as when combined with per-fragment UENC or cannot occur first, such as when combined with per-fragment UENC or
UCMP. In those cases, encryption or compression (or both) would UCMP. In those cases, encryption or compression (or both) would
precede FRAG when they also encrypt or compress the fragment option precede FRAG when they also encrypt or compress the fragment option
itself. Additional cases could include recoding, such as could be itself. Additional cases could include recoding, such as could be
used to support Forward Error Correction (FEC) over a group of used to support Forward Error Correction (FEC) over a group of
fragments. FRAG not being first might result in software (so-called fragments. FRAG not being first might result in software (so-called
"slow path") option processing or might also be accommodated via a "slow path") option processing or might also be accommodated via a
small set of known cases. small set of known cases.
>> When FRAG is present, the UDP user data MUST be empty. If the >> When FRAG is present, the UDP user data MUST be empty. If the
user data is not empty, all UDP options MUST be silently ignored and user data is not empty, all UDP Options MUST be silently ignored and
the user data received sent to the user. the user data received MUST be sent to the user.
Legacy receivers interpret FRAG messages as zero-length user data UDP Legacy receivers interpret FRAG messages as zero-length user data UDP
packets (i.e., UDP Length field is 8, the length of just the UDP packets (i.e., UDP Length field is 8, the length of just the UDP
header), which would not affect the receiver unless the presence of header), which would not affect the receiver unless the presence of
the UDP packet itself were a signal (see Section 5 of [RFC8085]). In the UDP packet itself were a signal (see Section 5 of [RFC8085]). In
this manner, the FRAG option also helps hide UNSAFE options so they this manner, the FRAG Option also helps hide UNSAFE Options so they
can be used more safely in the presence of legacy receivers. can be used more safely in the presence of legacy receivers.
The FRAG option has two formats: non-terminal fragments use the The FRAG Option has two formats: non-terminal fragments use the
shorter variant (Figure 10) and terminal fragments use the longer shorter variant (Figure 10) and terminal fragments use the longer
(Figure 11). The latter includes stand-alone fragments, i.e., when (Figure 11). The latter includes stand-alone fragments, i.e., when
data is contained in the FRAG option but reassembly is not required. data is contained in the FRAG Option but reassembly is not required.
+--------+--------+--------+--------+ +--------+--------+--------+--------+
| Kind=3 | Len=10 | Frag. Start | | Kind=3 | Len=10 | Frag. Start |
+--------+--------+--------+--------+ +--------+--------+--------+--------+
| Identification | | Identification |
+--------+--------+--------+--------+ +--------+--------+--------+--------+
| Frag. Offset | | Frag. Offset |
+--------+--------+ +--------+--------+
Figure 10: UDP Non-Terminal FRAG Option Format Figure 10: UDP Non-Terminal FRAG Option Format
Most fields are common to both FRAG option formats. The option Len Most fields are common to both FRAG Option formats. The option Len
field indicates whether there are more fragments (Len=10) or no more field indicates whether there are more fragments (Len=10) or no more
fragments (Len=12). fragments (Len=12).
The Frag. Start field indicates the location of the beginning of the The Frag. Start field indicates the location of the beginning of the
fragment data, measured from the beginning of the UDP header of the fragment data, measured from the beginning of the UDP header of the
fragment. The fragment data follows the remainder of the UDP options fragment. The fragment data follows the remainder of the UDP Options
and continues to the end of the IP datagram (i.e., the end of the and continues to the end of the IP datagram (i.e., the end of the
surplus area). Those options (i.e., any that precede or follow the surplus area). Those options (i.e., any that precede or follow the
FRAG option) are applied to this UDP fragment. FRAG Option) are applied to this UDP fragment.
The Frag. Offset field indicates the location of this fragment The Frag. Offset field indicates the location of this fragment
relative to the original UDP datagram (prior to fragmentation or relative to the original UDP datagram (prior to fragmentation or
after reassembly), measured from the start of the original UDP after reassembly), measured from the start of the original UDP
datagram's header. datagram's header.
The Identification field is a 32-bit value that, when used in The Identification field is a 32-bit value that, when used in
combination with the IP source address, UDP source port, IP combination with the IP source address, UDP source port, IP
destination address, and UDP destination port, uniquely identifies destination address, and UDP destination port, uniquely identifies
the original UDP datagram. the original UDP datagram.
skipping to change at line 1036 skipping to change at line 1037
+--------+--------+--------+--------+ +--------+--------+--------+--------+
| Kind=3 | Len=12 | Frag. Start | | Kind=3 | Len=12 | Frag. Start |
+--------+--------+--------+--------+ +--------+--------+--------+--------+
| Identification | | Identification |
+--------+--------+--------+--------+ +--------+--------+--------+--------+
| Frag. Offset |Reass DgOpt Start| | Frag. Offset |Reass DgOpt Start|
+--------+--------+--------+--------+ +--------+--------+--------+--------+
Figure 11: UDP Non-Terminal FRAG Option Format Figure 11: UDP Non-Terminal FRAG Option Format
The terminal FRAG option format adds a Reassembled Datagram Option The terminal FRAG Option format adds a Reassembled Datagram Option
Start (RDOS) pointer, measured from the start of the original UDP Start (RDOS) pointer, measured from the start of the original UDP
datagram header, indicating the end of the reassembled data and the datagram header, indicating the end of the reassembled data and the
start of the surplus area within the original UDP datagram. UDP start of the surplus area within the original UDP datagram. UDP
options that apply to the reassembled datagram are contained in the Options that apply to the reassembled datagram are contained in the
reassembled surplus area, as indicated by RDOS. UDP options that reassembled surplus area, as indicated by RDOS. UDP Options that
occur within the fragment are processed on the fragment itself. This occur within the fragment are processed on the fragment itself. This
allows either pre-reassembly or post-reassembly UDP option effects, allows either pre-reassembly or post-reassembly UDP Option effects,
such as using UENC on each fragment while also using TIME on the such as using UENC on each fragment while also using TIME on the
reassembled datagram for round-trip latency measurements. reassembled datagram for round-trip latency measurements.
An example showing the relationship between UDP fragments and the An example showing the relationship between UDP fragments and the
original UDP datagram is provided in Figure 12. In this example, the original UDP datagram is provided in Figure 12. In this example, the
trailer containing per-datagram options resides entirely within the trailer containing per-datagram options resides entirely within the
terminal fragment, but this need not always be the case. terminal fragment, but this need not always be the case.
Constituent UDP Fragments Original UDP Datagram Constituent UDP Fragments Original UDP Datagram
skipping to change at line 1090 skipping to change at line 1091
| +-------------+------------+ | | +-------------+------------+ |
| | RDOS | Frag.Opts. | | | | RDOS | Frag.Opts. | |
'->+--|----------+------------+ | '->+--|----------+------------+ |
~ | Fragment Data ~ | ~ | Fragment Data ~ |
+--|----------+------------+ | +--|----------+------------+ |
| | | |
'----------------------------' '----------------------------'
Figure 12: UDP Fragments and Original UDP Datagram Figure 12: UDP Fragments and Original UDP Datagram
The FRAG option does not need a "more fragments" bit (as used by IP The FRAG Option does not need a "more fragments" bit (as used by IP
fragmentation) because it provides the same indication by using the fragmentation) because it provides the same indication by using the
longer, 12-byte variant, as shown in Figure 11. longer, 12-byte variant, as shown in Figure 11.
>> The FRAG option MAY be used on a single fragment; in which case, >> The FRAG Option MAY be used on a single fragment; in this case,
the Frag. Offset would be zero and the option would have the 12-byte the Frag. Offset would be zero and the option would have the 12-byte
format. format.
>> Endpoints supporting UDP options MUST be capable of fragmenting >> Endpoints supporting UDP Options MUST be capable of fragmenting
and reassembling at least two fragments, each of a size that will fit and reassembling at least two fragments, each of a size that will fit
within the standard Ethernet MTU of 1,500 bytes. For further within the standard Ethernet MTU of 1,500 bytes. For further
details, please see Section 11.6. details, please see Section 11.6.
Use of the single fragment variant can be helpful in supporting use Use of the single fragment variant can be helpful in supporting use
of UNSAFE options without undesirable impact to receivers that do not of UNSAFE Options without undesirable impact to receivers that do not
support either UDP options or the specific UNSAFE options. support either UDP Options or the specific UNSAFE Options.
During fragmentation, the UDP header checksum of each fragment During fragmentation, the UDP header checksum of each fragment
remains constant. It does not depend on the fragment data (which remains constant. It does not depend on the fragment data (which
appears in the surplus area) because all fragments have a zero- appears in the surplus area) because all fragments have a zero-
length user data field. length user data field.
>> The Identification field is a 32-bit value that MUST be unique >> The Identification field is a 32-bit value that MUST be unique
over the expected fragment reassembly timeout. over the expected fragment reassembly timeout.
>> The Identification field SHOULD be generated in a manner similar >> The Identification field SHOULD be generated in a manner similar
skipping to change at line 1135 skipping to change at line 1136
in this case (to avoid a potential DoS attack turning into an ICMP in this case (to avoid a potential DoS attack turning into an ICMP
storm in the reverse direction). storm in the reverse direction).
>> Note that fragments might be duplicated in the network. Instead >> Note that fragments might be duplicated in the network. Instead
of treating these exact duplicate fragments as overlapping fragments, of treating these exact duplicate fragments as overlapping fragments,
an implementation MAY choose to detect this case and drop exact an implementation MAY choose to detect this case and drop exact
duplicate fragments while keeping the other fragments belonging to duplicate fragments while keeping the other fragments belonging to
the same UDP packet. the same UDP packet.
UDP fragmentation relies on a fragment expiration timer, which can be UDP fragmentation relies on a fragment expiration timer, which can be
preset or could use a value computed using the UDP Timestamp option. preset or could use a value computed using the UDP Timestamp Option.
>> The default UDP reassembly expiration timeout SHOULD be no more >> The default UDP reassembly expiration timeout SHOULD be no more
than 2 minutes. than 2 minutes.
>> UDP reassembly expiration MUST NOT generate an ICMP error. Such >> UDP reassembly expiration MUST NOT generate an ICMP error. Such
events are not an IP error and can be addressed by the user/ events are not an IP error and can be addressed by the user/
application layer if desired. application layer if desired.
>> UDP reassembly space SHOULD be limited to reduce the impact of DoS >> UDP reassembly space SHOULD be limited to reduce the impact of DoS
attacks on resource use. attacks on resource use.
>> UDP reassembly space limits SHOULD NOT be computed as a shared >> UDP reassembly space limits SHOULD NOT be computed as a shared
resource across multiple sockets, to avoid cross-socket pair DoS resource across multiple sockets, to avoid cross-socket pair DoS
attacks. attacks.
>> Individual UDP fragments MUST NOT be forwarded to the user. The >> Individual UDP fragments MUST NOT be forwarded to the user. The
reassembled datagram is received only after complete reassembly, reassembled datagram is received only after complete reassembly,
checksum validation, and continued processing of the remaining UDP checksum validation, and continued processing of the remaining UDP
options. Options.
Per-fragment UDP options, if used in addition to FRAG, occur before Per-fragment UDP Options, if used in addition to FRAG, occur before
the fragment data. They typically occur after the FRAG option, the fragment data. They typically occur after the FRAG Option,
except where they modify the FRAG option itself (e.g., UENC or UCMP). except where they modify the FRAG Option itself (e.g., UENC or UCMP).
Per-fragment options are processed before the fragment is included in Per-fragment options are processed before the fragment is included in
the reassembled datagram. Such options can be useful to protect the the reassembled datagram. Such options can be useful to protect the
reassembly process itself, e.g., to prevent the reassembly cache from reassembly process itself, e.g., to prevent the reassembly cache from
being polluted (using AUTH or UENC). being polluted (using AUTH or UENC).
>> Fragments of a single datagram MAY use different sets of options. >> Fragments of a single datagram MAY use different sets of options.
It is expected to be computationally expensive to validate uniformity It is expected to be computationally expensive to validate uniformity
across all fragments, and there could be legitimate reasons for across all fragments, and there could be legitimate reasons for
including options in a fragment but not all fragments (e.g., MDS and including options in a fragment but not all fragments (e.g., MDS and
MRDS). MRDS).
If an option is used per-fragment but defined as not usable per- If an option is used per-fragment but defined as not usable per-
fragment, it is treated the same as any other unknown option. fragment, it is treated the same as any other unknown option.
Per-datagram UDP options, if used, reside in the surplus area of the Per-datagram UDP Options, if used, reside in the surplus area of the
original UDP datagram. Processing of those options occurs after original UDP datagram. Processing of those options occurs after
reassembly is complete. This enables the safe use of UNSAFE options, reassembly is complete. This enables the safe use of UNSAFE Options,
which are required to result in discarding the entire UDP datagram if which are required to result in discarding the entire UDP datagram if
they are unknown to the receiver or otherwise fail (see Section 12). they are unknown to the receiver or otherwise fail (see Section 12).
In general, UDP packets are fragmented as follows: In general, UDP packets are fragmented as follows:
1. Create a UDP packet with data and UDP options. This is the 1. Create a UDP packet with data and UDP Options. This is the
original UDP datagram, which we will call "D". The UDP options original UDP datagram, which we will call "D". The UDP Options
follow the UDP user data and occur in the surplus area, just as follow the UDP user data and occur in the surplus area, just as
in an unfragmented UDP datagram with UDP options. in an unfragmented UDP datagram with UDP Options.
>> UDP options for the original packet MUST be fully prepared >> UDP Options for the original packet MUST be fully prepared
before the rest of the fragmentation steps that follow here. before the rest of the fragmentation steps that follow here.
>> The UDP checksum of the original packet SHOULD be set to zero >> The UDP checksum of the original packet SHOULD be set to zero
because it is never transmitted. Equivalent protection is because it is never transmitted. Equivalent protection is
provided if each fragment has a non-zero OCS value, as will be provided if each fragment has a non-zero OCS value, as will be
the case if each fragment's UDP checksum is non-zero. Similarly, the case if each fragment's UDP checksum is non-zero. Similarly,
the OCS value of the original packet SHOULD be zero if each the OCS value of the original packet SHOULD be zero if each
fragment will have a non-zero OCS value, as will be the case if fragment will have a non-zero OCS value, as will be the case if
each fragment's UDP checksum is non-zero. each fragment's UDP checksum is non-zero.
2. Identify the desired fragment size, which we will call "S". This 2. Identify the desired fragment size, which we will call "S". This
value is calculated to take into account the path MTU (if known) value is calculated to take into account the path MTU (if known)
and to allow space for per-fragment options. and to allow space for per-fragment options.
3. Fragment "D" into chunks of size no larger than "S"-12 each (10 3. Fragment "D" into chunks of size no larger than "S"-12 each (10
for the non-terminal FRAG option and 2 for OCS), with one final for the non-terminal FRAG Option and 2 for OCS), with one final
chunk no larger than "S"-14 (12 for the terminal FRAG option and chunk no larger than "S"-14 (12 for the terminal FRAG Option and
2 for OCS). Note that all the per-datagram options in step #1 2 for OCS). Note that all the per-datagram options in step #1
need not be limited to the terminal fragment, i.e., the RDOS need not be limited to the terminal fragment, i.e., the RDOS
pointer can indicate the start of the original surplus area pointer can indicate the start of the original surplus area
anywhere in the reassembled datagram. anywhere in the reassembled datagram.
4. For each chunk of "D" in step #3, create a UDP packet with no 4. For each chunk of "D" in step #3, create a UDP packet with no
user data (UDP Length=8) followed by the word-aligned OCS, the user data (UDP Length=8) followed by the word-aligned OCS, the
FRAG option, and any additional per-fragment UDP options, FRAG Option, and any additional per-fragment UDP Options,
followed by the FRAG data chunk. followed by the FRAG data chunk.
5. Complete the processing associated with creating these additional 5. Complete the processing associated with creating these additional
per-fragment UDP options for each fragment. per-fragment UDP Options for each fragment.
Receivers reverse the above sequence. They process all received Receivers reverse the above sequence. They process all received
options in each fragment. When the FRAG option is encountered, the options in each fragment. When the FRAG Option is encountered, the
FRAG data is used in reassembly. After all fragments are received, FRAG data is used in reassembly. After all fragments are received,
the entire UDP packet is processed with any trailing UDP options the entire UDP packet is processed with any trailing UDP Options
applying to the reassembled user data. applying to the reassembled user data.
>> Reassembly failures at the receiver result in silent discard of >> Reassembly failures at the receiver result in silent discard of
any per-fragment options and fragment contents, and such failures any per-fragment options and fragment contents, and such failures
SHOULD NOT generate zero-length frames to the user. SHOULD NOT generate zero-length frames to the user.
>> Finally, because fragmentation processing can be expensive, the >> Finally, because fragmentation processing can be expensive, the
FRAG option SHOULD be avoided unless the original datagram requires FRAG Option SHOULD be avoided unless the original datagram requires
fragmentation or it is needed for "safe" use of UNSAFE options. fragmentation or it is needed for "safe" use of UNSAFE Options.
>> The FRAG option MAY also be used to provide limited support for >> The FRAG Option MAY also be used to provide limited support for
UDP options in systems that have access to only the initial portion UDP Options in systems that have access to only the initial portion
of the data in incoming or outgoing packets, as such systems could of the data in incoming or outgoing packets, as such systems could
potentially access per-fragment options. Such packets would, of potentially access per-fragment options. Such packets would, of
course, be silently ignored by legacy receivers that do not support course, be silently ignored by legacy receivers that do not support
UDP options. UDP Options.
The presence of the FRAG option is not reported to the user. The presence of the FRAG Option is not reported to the user.
11.5. Maximum Datagram Size (MDS) 11.5. Maximum Datagram Size (MDS)
The Maximum Datagram Size (MDS, Kind=4) option is a 16-bit hint of The Maximum Datagram Size (MDS, Kind=4) Option is a 16-bit hint of
the largest UDP packet or UDP fragment that an endpoint believes can the largest UDP packet or UDP fragment that an endpoint believes can
be received without use of IP fragmentation. It helps UDP be received without use of IP fragmentation. It helps UDP
applications limit the largest UDP packet that can be sent without applications limit the largest UDP packet that can be sent without
UDP fragmentation and helps UDP fragmentation determine the largest UDP fragmentation and helps UDP fragmentation determine the largest
UDP fragment to send -- in both cases, to avoid IP fragmentation. UDP fragment to send -- in both cases, to avoid IP fragmentation.
As with the TCP Maximum Segment Size (MSS) option [RFC9293], the size As with the TCP Maximum Segment Size (MSS) Option [RFC9293], the size
indicated is the IP layer MTU decreased by the fixed IP and UDP indicated is the IP layer MTU decreased by the fixed IP and UDP
headers only [RFC9293]. The space needed for IP and UDP options headers only [RFC9293]. The space needed for IP and UDP Options
needs to be adjusted by the sender when using the value indicated. needs to be adjusted by the sender when using the value indicated.
The value transmitted is based on EMTU_R, the largest IP datagram The value transmitted is based on EMTU_R, the largest IP datagram
that can be received (i.e., reassembled at the receiver) [RFC1122]. that can be received (i.e., reassembled at the receiver) [RFC1122].
However, as with TCP, this value is only a hint at what the receiver However, as with TCP, this value is only a hint at what the receiver
believes, as when used with PLPMTUD at the UDP layer, as discussed believes, as when used with PLPMTUD at the UDP layer, as discussed
later in this section. later in this section.
>> MDS does not indicate a known path MTU and thus MUST NOT be used >> MDS does not indicate a known path MTU and thus MUST NOT be used
to limit transmissions. to limit transmissions.
+--------+--------+--------+--------+ +--------+--------+--------+--------+
| Kind=4 | Len=4 | MDS size | | Kind=4 | Len=4 | MDS size |
+--------+--------+--------+--------+ +--------+--------+--------+--------+
Figure 13: UDP MDS Option Format Figure 13: UDP MDS Option Format
>> The UDP MDS option MAY be used as a hint for path MTU discovery >> The UDP MDS Option MAY be used as a hint for path MTU discovery
[RFC1191] [RFC8201], but this could be difficult because of known [RFC1191] [RFC8201], but this could be difficult because of known
issues with ICMP blocking [RFC2923] as well as UDP lacking automatic issues with ICMP blocking [RFC2923] as well as UDP lacking automatic
retransmission. retransmission.
MDS is more likely to be useful when coupled with IP source MDS is more likely to be useful when coupled with IP source
fragmentation or UDP fragmentation to limit the largest reassembled fragmentation or UDP fragmentation to limit the largest reassembled
UDP message as indicated by MRDS (see Section 11.6), e.g., when UDP message as indicated by MRDS (see Section 11.6), e.g., when
EMTU_R is larger than the required minimums (576 for IPv4 [RFC0791] EMTU_R is larger than the required minimums (576 for IPv4 [RFC0791]
and 1500 for IPv6 [RFC8200]). and 1500 for IPv6 [RFC8200]).
skipping to change at line 1290 skipping to change at line 1291
Packetization Layer Path MTU (PLPMTU) value, though it MUST NOT Packetization Layer Path MTU (PLPMTU) value, though it MUST NOT
prohibit transmission of larger UDP packets used as DPLPMTUD probes. prohibit transmission of larger UDP packets used as DPLPMTUD probes.
MDS is reported to the user, whether used per-datagram or per- MDS is reported to the user, whether used per-datagram or per-
fragment (as defined in Section 11.4). When used per-fragment, the fragment (as defined in Section 11.4). When used per-fragment, the
reported value is the minimum of the MDS values received per- reported value is the minimum of the MDS values received per-
fragment. fragment.
11.6. Maximum Reassembled Datagram Size (MRDS) 11.6. Maximum Reassembled Datagram Size (MRDS)
The Maximum Reassembled Datagram Size (MRDS, Kind=5) option is a 16- The Maximum Reassembled Datagram Size (MRDS, Kind=5) Option is a 16-
bit indicator of the largest reassembled UDP datagram that can be bit indicator of the largest reassembled UDP datagram that can be
received, including the UDP header and any per-datagram UDP options, received, including the UDP header and any per-datagram UDP Options,
accompanied by an 8-bit indication of how many UDP fragments can be accompanied by an 8-bit indication of how many UDP fragments can be
reassembled. MRDS size is the UDP equivalent of IP's EMTU_R, but the reassembled. The MRDS size field is the UDP equivalent of IP's
two are not related [RFC1122]. Using the FRAG option (Section 11.4), EMTU_R, but the two are not related [RFC1122]. Using the FRAG Option
UDP packets can be transmitted as transport fragments, each in their (Section 11.4), UDP packets can be transmitted as transport
own (presumably not fragmented) IP datagram, and be reassembled at fragments, each in their own (presumably not fragmented) IP datagram,
the UDP layer. MRDS segs is the number of UDP fragments that can be and be reassembled at the UDP layer. MRDS segs is the number of UDP
reassembled. fragments that can be reassembled.
+--------+--------+--------+--------+---------+ +--------+--------+--------+--------+---------+
| Kind=5 | Len=5 | MRDS size |MRDS segs| | Kind=5 | Len=5 | MRDS size |MRDS segs|
+--------+--------+--------+--------+---------+ +--------+--------+--------+--------+---------+
Figure 14: UDP MRDS Option Format Figure 14: UDP MRDS Option Format
>> Endpoints supporting UDP options MUST support a local MRDS size of >> Endpoints supporting UDP Options MUST support a local MRDS size of
at least 2,926 bytes for IPv4 and 2,886 bytes for IPv6. Support for at least 2,926 bytes for IPv4 and 2,886 bytes for IPv6. Support for
larger values is encouraged. larger values is encouraged.
>> Endpoints supporting UDP options MUST support a local MRDS segs >> Endpoints supporting UDP Options MUST support a local MRDS segs
value of at least 2. Support for larger values is encouraged. value of at least 2. Support for larger values is encouraged.
These parameters plus the Path MTU (PMTU) allow a sender to compute These parameters plus the Path MTU (PMTU) allow a sender to compute
the size of the largest pre-fragmentation UDP packet that a receiver the size of the largest pre-fragmentation UDP packet that a receiver
will guarantee to accept. Suppose that MMS_S is the PMTU less the will guarantee to accept. MMS_S is defined as the PMTU less the size
size of the IP header and the UDP header, i.e., the maximum UDP of the IP header and the UDP header, i.e., the maximum UDP message
message size that can be successfully sent in a single UDP datagram size that can be successfully sent in a single UDP datagram if there
if there are no IP options or extension headers and no UDP per- are no IP options or extension headers and no UDP per-fragment
fragment options. options. Given the above size definitions, the size of the largest
pre-fragmentation UDP packet that the receiver will guarantee to
Then, the size of the largest pre-fragmentation UDP packet that the accept is the smaller of the MRDS size and the following:
receiver will guarantee to accept is the smaller of the MRDS size and
(MMS_S - 12) * (MRDS segs) - 2 - (Total Per-Frag IP/UDP Options) + 8 (MMS_S - 12) * (MRDS segs) - 2 - (Total Per-Frag IP/UDP Options) + 8
where Total Per-Frag IP/UDP Options includes the size of all IP In the above expression, the Total Per-Frag IP/UDP Options includes
options and extension headers and all per-fragment UDP options, the size of all IP options and extension headers and all per-fragment
except for OCS and FRAG, that are in the sequence of UDP fragments. UDP Options, except for OCS and FRAG, that are in the sequence of UDP
fragments.
>> If no MRDS option has been received, a sender MUST assume that >> If no MRDS Option has been received, a sender MUST assume that
MRDS size is 2,926 bytes for IPv4 and 2,886 bytes for IPv6 and that MRDS size is 2,926 bytes for IPv4 and 2,886 bytes for IPv6 and that
MRDS segs is 2, i.e., the minimum values allowed. MRDS segs is 2, i.e., the minimum values allowed.
MRDS is reported to the user, whether used per-datagram or per- MRDS is reported to the user, whether used per-datagram or per-
fragment (as defined in Section 11.4). When used per-fragment, the fragment (as defined in Section 11.4). When used per-fragment, the
reported value is the minimum of the MRDS values received per- reported value is the minimum of the MRDS values received per-
fragment. fragment.
11.7. Echo Request (REQ) and Echo Response (RES) 11.7. Echo Request (REQ) and Echo Response (RES)
The echo Request (REQ, Kind=6) and echo Response (RES, Kind=7) The echo Request (REQ, Kind=6) and echo Response (RES, Kind=7)
options provides UDP packet-level acknowledgments as a capability for Options provide UDP packet-level acknowledgments as a capability for
use by upper layer protocols, e.g., user applications, libraries, use by upper layer protocols, e.g., user applications, libraries,
operating systems, etc. Both the REQ and RES are under the control operating systems, etc. Both the REQ and RES are under the control
of these upper layers, i.e., UDP option support described in this of these upper layers, i.e., UDP Option support described in this
document never automatically responds to a REQ with a RES. Instead, document never automatically responds to a REQ with a RES. Instead,
the REQ is delivered to the upper layer, which decides whether and the REQ is delivered to the upper layer, which decides whether and
when to issue a RES. when to issue a RES.
One such use is described as part of DPLPMTUD [RFC9869]. This use One such use is described as part of DPLPMTUD [RFC9869]. This use
case is described as part of UDP options but is logically considered case is described as part of UDP Options but is logically considered
to be a capability of an upper layer that uses UDP options. The to be a capability of an upper layer that uses UDP Options. The
options both have the format indicated in Figure 15, in which the options both have the format indicated in Figure 15, in which the
token has no internal structure or meaning. token has no internal structure or meaning.
+--------+--------+-----------------+ +--------+--------+-----------------+
| Kind | Len=6 | token | | Kind | Len=6 | token |
+--------+--------+-----------------+ +--------+--------+-----------------+
1 byte 1 byte 4 bytes 1 byte 1 byte 4 bytes
Figure 15: UDP REQ and RES Options Format Figure 15: UDP REQ and RES Options Format
>> As advice to upper layer protocol/library designers, when >> As advice to upper layer protocol/library designers, when
supporting REQ/RES and responding with a RES, the upper layer SHOULD supporting REQ/RES and responding with a RES, the upper layer SHOULD
respond with the most recently received REQ token. respond with the most recently received REQ token.
>> If the implementation includes a layer/library that produces and >> If the implementation includes a layer/library that produces and
consumes REQ/RES on behalf of the user/application, then that layer consumes REQ/RES on behalf of the user/application, then that layer
MUST be disabled by default; in which case, REQ/RES are simply sent MUST be disabled by default; in this case, REQ/RES are simply sent
upon request by the user/application and passed to it when received, upon request by the user/application and passed to it when received,
as with most other UDP options. as with most other UDP Options.
For example, an application needs to explicitly enable the generation For example, an application needs to explicitly enable the generation
of a RES response by DPLPMTUD when using UDP Options [RFC9869]. of a RES Option by DPLPMTUD when using UDP Options [RFC9869].
>> The token transmitted in a RES option MUST be a token received in >> The token transmitted in a RES Option MUST be a token received in
a REQ option by the transmitter. This ensures that the response is a REQ Option by the transmitter. This ensures that the response is
to a received request. to a received request.
REQ and RES option kinds each appear at most once in each UDP packet, REQ and RES Option kinds each appear at most once in each UDP packet,
as with most other options. A single packet can include both as with most other options. A single packet can include both
options, though they would be otherwise unrelated to each other. options, though they would be otherwise unrelated to each other.
Note also that the FRAG option is not used when sending DPLPMTUD Note also that the FRAG Option is not used when sending DPLPMTUD
probes to determine a PLPMTU [RFC9869]. probes to determine a PLPMTU [RFC9869].
REQ and RES are reported to the user, whether used per-datagram or REQ and RES are reported to the user, whether used per-datagram or
per-fragment (as defined in Section 11.4). When used per-fragment, per-fragment (as defined in Section 11.4). When used per-fragment,
the reported value indicates the most recently received token. the reported value indicates the most recently received token.
11.8. Timestamps (TIME) 11.8. Timestamp (TIME)
Timestamps are provided as a capability to be used by applications Timestamps are provided as a capability to be used by applications
and other upper layer protocols. They are based on a notion of time and other upper layer protocols. They are based on a notion of time
as a monotonically non-decreasing unsigned integer, with wraparound. as a monotonically non-decreasing unsigned integer, with wraparound.
They are defined the same way as TCP Protection Against Wrapped They are defined the same way as TCP Protection Against Wrapped
Sequence (PAWS) numbers, i.e., "without any connection to [real- Sequence (PAWS) numbers, i.e., "without any connection to [real-
world, classical physics wall-clock] time" [RFC7323]. They are quite world, classical physics wall-clock] time" [RFC7323]. They are quite
similar to the behavior of relativistic time or the individual similar to the behavior of relativistic time or the individual
scalars of Lamport clocks [La78]. However, if desired, they can scalars of Lamport clocks [La78]. However, if desired, they can
correspond to real-world time, e.g., as used for round-trip time correspond to real-world time, e.g., as used for round-trip time
(RTT) estimation. This option makes no assertions as to which is the (RTT) estimation. This option makes no assertions as to which is the
case; the decision is up to the application layer using this option. case; the decision is up to the application layer using this option.
The Timestamp (TIME, Kind=8) option exchanges two four-byte unsigned The Timestamp (TIME, Kind=8) Option exchanges two four-byte unsigned
timestamp fields. It serves a similar purpose to TCP's TS option timestamp fields. It serves a similar purpose to TCP's Timestamp
[RFC7323], enabling UDP to estimate the RTT between hosts. For UDP, (TS) Option [RFC7323], enabling UDP to estimate the RTT between
this RTT can be useful for establishing UDP fragment reassembly hosts. For UDP, this RTT can be useful for establishing UDP fragment
timeouts or transport-layer rate limiting [RFC8085]. reassembly timeouts or transport-layer rate limiting [RFC8085].
+--------+--------+------------------+------------------+ +--------+--------+------------------+------------------+
| Kind=8 | Len=10 | TSval | TSecr | | Kind=8 | Len=10 | TSval | TSecr |
+--------+--------+------------------+------------------+ +--------+--------+------------------+------------------+
1 byte 1 byte 4 bytes 4 bytes 1 byte 1 byte 4 bytes 4 bytes
Figure 16: UDP TIME Option Format Figure 16: UDP TIME Option Format
TS Value (TSval) and TS Echo Reply (TSecr) are used in a similar TS Value (TSval) and TS Echo Reply (TSecr) are used in a similar
manner to the TCP TS option [RFC7323]. On transmitted UDP packets manner to the TCP TS Option [RFC7323]. On transmitted UDP packets
using the option, TSval is always set based on the local "time" using the option, TSval is always set based on the local "time"
value. Received TSval and TSecr values are provided to the value. Received TSval and TSecr field contents are provided to the
application, which can pass the TSval value to be used as TSecr on application, which can pass the received TSval to be used as TSecr in
UDP messages sent in response (i.e., to echo the received TSval). A UDP messages sent in response (i.e., to echo the received TSval). A
received TSecr of zero indicates that the TSval was not echoed by the received TSecr of zero indicates that the TSval was not echoed by the
transmitter, i.e., from a previously received UDP packet. transmitter, i.e., from a previously received UDP packet.
>> TIME MAY use an RTT estimate based on non-zero Timestamp values as >> TIME MAY use an RTT estimate based on non-zero Timestamp values as
a hint for fragmentation reassembly, rate limiting, or other a hint for fragmentation reassembly, rate limiting, or other
mechanisms that benefit from such an estimate. mechanisms that benefit from such an estimate.
>> An application MAY use TIME to compute this RTT estimate for >> An application MAY use TIME to compute this RTT estimate for
further use by the user. further use by the user.
skipping to change at line 1465 skipping to change at line 1466
>> TIME values MUST NOT use zeros as valid time values, because they >> TIME values MUST NOT use zeros as valid time values, because they
are used as indicators of requests and responses. are used as indicators of requests and responses.
TIME is reported to the user, whether used per-datagram or per- TIME is reported to the user, whether used per-datagram or per-
fragment (as defined in Section 11.4). When used per-fragment, the fragment (as defined in Section 11.4). When used per-fragment, the
reported value is the minimum and maximum of each of the timestamp reported value is the minimum and maximum of each of the timestamp
values received per-fragment. values received per-fragment.
>> Use of TIME per-fragment is NOT RECOMMENDED. Exceptions include >> Use of TIME per-fragment is NOT RECOMMENDED. Exceptions include
supporting diagnostics on the reassembly process itself, which could supporting diagnostics on the reassembly process itself, which could
be more appropriate to handle within the UDP option processing be more appropriate to handle within the UDP Option processing
implementation. implementation.
11.9. Authentication (AUTH), RESERVED Only 11.9. Authentication (AUTH), RESERVED Only
The Authentication (AUTH, Kind=9) option is reserved for all UDP The Authentication (AUTH, Kind=9) Option is reserved for all UDP
authentication mechanisms [To24]. AUTH is expected to cover the UDP authentication mechanisms [To24]. AUTH is expected to cover the UDP
user data and UDP options, with possible additional coverage of the user data and UDP Options, with possible additional coverage of the
IP pseudoheader and UDP header and potentially also support for NAT IP pseudoheader and UDP header and potentially also support for NAT
traversal (i.e., by zeroing the remote socket -- the source IP traversal (i.e., by zeroing the remote socket -- the source IP
address and UDP port -- before computing the check), the latter in a address and UDP port -- before computing the check), the latter in a
similar manner as per TCP Authentication Option (TCP-AO) NAT similar manner as per TCP Authentication Option (TCP-AO) NAT
traversal [RFC6978]. traversal [RFC6978].
Like APC, AUTH is a SAFE option because it does not modify the UDP Like APC, AUTH is a SAFE Option because it does not modify the UDP
user data. AUTH could fail even where the user data has not been user data. AUTH could fail even where the user data has not been
corrupted, such as when its contents have been overwritten. Such corrupted, such as when its contents have been overwritten. Such
overwrites could be intentional and not widely known; defaulting to overwrites could be intentional and not widely known; defaulting to
silent ignore ensures that option-aware endpoints do not change how silent ignore ensures that option-aware endpoints do not change how
users or applications operate unless explicitly directed to do users or applications operate unless explicitly directed to do
otherwise. When a socket pair relies on AUTH, e.g., upon otherwise. When a socket pair relies on AUTH, e.g., upon
configuration of a security policy, this default is expected to be configuration of a security policy, this default is expected to be
overridden, where incoming packets without AUTH or with a failed AUTH overridden, where incoming packets without AUTH or with a failed AUTH
check would be silently dropped, such that only authenticated packets check would be silently dropped, such that only authenticated packets
would be sent to the user. This approach enables security checks for would be sent to the user. This approach enables security checks for
skipping to change at line 1501 skipping to change at line 1502
library. library.
A specification for using AUTH is expected to define the coordination A specification for using AUTH is expected to define the coordination
of AUTH security parameters and configuration of the socket pair when of AUTH security parameters and configuration of the socket pair when
those parameters are installed. That specification is expected to those parameters are installed. That specification is expected to
address rules for when AUTH is required upon transmission and when address rules for when AUTH is required upon transmission and when
the presence and correct validation of AUTH is required on reception. the presence and correct validation of AUTH is required on reception.
11.10. Experimental (EXP) 11.10. Experimental (EXP)
The Experimental (EXP, Kind=127) option is allocated for experiments The Experimental (EXP, Kind=127) Option is allocated for experiments
[RFC3692]. Only one such value is allocated because experiments are [RFC3692]. Only one such value is allocated because experiments are
expected to use an Experimental ID (ExID) to differentiate concurrent expected to use an Experimental ID (ExID) to differentiate concurrent
use for different purposes, using UDP ExIDs registered with IANA use for different purposes, using UDP ExIDs registered with IANA
according to the approach developed for TCP experimental options according to the approach developed for TCP experimental options
[RFC6994]. [RFC6994].
+----------+----------+----------+----------+ +----------+----------+----------+----------+
| Kind=127 | Len | UDP ExID | | Kind=127 | Len | UDP ExID |
+----------+----------+----------+----------+ +----------+----------+----------+----------+
| (option contents, as defined)... | | (option contents, as defined)... |
+----------+----------+----------+----------+ +----------+----------+----------+----------+
Figure 17: UDP EXP Option Format Figure 17: UDP EXP Option Format
>> The length of the Experimental option MUST be at least 4 to >> The length of the Experimental Option MUST be at least 4 to
account for the Kind, Len, and 16-bit UDP ExID (similar to TCP ExIDs account for the Kind, Len, and 16-bit UDP ExID (similar to TCP ExIDs
[RFC6994]). [RFC6994]).
The UDP EXP option uses only 16-bit ExIDs, unlike TCP ExIDs. In TCP, The UDP EXP Option uses only 16-bit ExIDs, unlike TCP ExIDs. In TCP,
the first 16 bits of the ExID is unique; the additional 16 bits, the first 16 bits of the ExID is unique; the additional 16 bits,
where present, are used to decrease the chance of the entire ExID where present, are used to decrease the chance of the entire ExID
occurring in legacy use of the TCP EXP option. This extended variant occurring in legacy use of the TCP EXP Option. This extended variant
provides no similar use for UDP EXP because ExIDs are required. provides no similar use for UDP EXP because ExIDs are required.
The UDP EXP option also includes an extended length format, where the The UDP EXP Option also includes an Extended Length format, where the
option Len is 255, followed by two bytes of extended length. option Len is 255, followed by two bytes of Extended Length.
+----------+----------+----------+----------+ +----------+----------+----------+----------+
| Kind=127 | 255 | Extended Length | | Kind=127 | 255 | Extended Length |
+----------+----------+----------+----------+ +----------+----------+----------+----------+
| UDP ExID |(option contents...) | | UDP ExID |(option contents...) |
+----------+----------+----------+----------+ +----------+----------+----------+----------+
Figure 18: UDP EXP Extended Option Format Figure 18: UDP EXP Extended Option Format
Assigned UDP Experimental IDs (ExIDs) are assigned from a combined UDP Experimental IDs (ExIDs) are assigned from a combined TCP/UDP
TCP/UDP ExID registry managed by IANA (see Section 26). Assigned ExID registry managed by IANA (see Section 26). Assigned ExIDs can
ExIDs can be used in either the EXP or UEXP options (see Section 12.3 be used in either the EXP or UEXP Options (see Section 12.3 for the
for the latter). latter).
12. UNSAFE Options 12. UNSAFE Options
UNSAFE options are not safe to ignore and can be used UNSAFE Options are not safe to ignore and can be used
unidirectionally or without soft-state confirmation of UDP option unidirectionally or without soft-state confirmation of UDP Option
capability. They are always used only when the user data occurs capability. They are always used only when the user data occurs
inside a reassembled set of one or more UDP fragments, such that if inside a reassembled set of one or more UDP fragments, such that if
UDP fragmentation is not supported, the enclosed UDP user data would UDP fragmentation is not supported, the enclosed UDP user data would
be silently dropped anyway. be silently dropped anyway.
>> Applications using UNSAFE options SHOULD NOT also use zero-length >> Applications using UNSAFE Options SHOULD NOT also use zero-length
UDP packets as signals, because they will arrive when UNSAFE options UDP packets as signals, because they will arrive when UNSAFE Options
fail. Those that choose to allow such packets MUST account for such fail. Those that choose to allow such packets MUST account for such
events. events.
>> UNSAFE options MUST be used only as part of UDP fragments, used >> UNSAFE Options MUST be used only as part of UDP fragments, used
either per-fragment or after reassembly. either per-fragment or after reassembly.
>> Receivers supporting UDP options MUST silently drop the UDP user >> Receivers supporting UDP Options MUST silently drop the UDP user
data of the reassembled datagram if any fragment or the entire data of the reassembled datagram if any fragment or the entire
datagram includes an UNSAFE option whose UKind is not supported or if datagram includes an UNSAFE Option whose Kind is not supported or if
an UNSAFE option appears outside the context of a fragment or an UNSAFE Option appears outside the context of a fragment or
reassembled fragments. reassembled fragments.
12.1. UNSAFE Compression (UCMP) 12.1. UNSAFE Compression (UCMP)
The UNSAFE Compression (UCMP, Kind=192) option is reserved for all The UNSAFE Compression (UCMP, Kind=192) Option is reserved for all
UDP compression mechanisms. UCMP is expected to cover the UDP user UDP compression mechanisms. UCMP is expected to cover the UDP user
data and some (e.g., later or in sequence) UDP options. data and some (e.g., later, in sequence) UDP Options.
12.2. UNSAFE Encryption (UENC) 12.2. UNSAFE Encryption (UENC)
The UNSAFE Encryption (UENC, Kind=193) option is reserved for all UDP The UNSAFE Encryption (UENC, Kind=193) Option is reserved for all UDP
encryption mechanisms. UENC is expected to provide all of the encryption mechanisms. UENC is expected to provide all of the
services of the AUTH option (Section 11.9) and in addition to encrypt services of the AUTH Option (Section 11.9) and in addition to encrypt
the UDP user data and some (e.g., later or in sequence) UDP options, the UDP user data and some (e.g., later or in sequence) UDP Options,
in a similar manner as TCP Authentication Option Encryption (TCP-AO- in a similar manner as TCP Authentication Option Extension for
ENC) [To18]. Payload Encryption (TCP-AO-ENC) [To18].
12.3. UNSAFE Experimental (UEXP) 12.3. UNSAFE Experimental (UEXP)
The UNSAFE Experimental (UEXP, Kind=254) option is reserved for The UNSAFE Experimental (UEXP, Kind=254) Option is reserved for
experiments [RFC3692]. As with EXP, only one such UEXP value is experiments [RFC3692]. As with EXP, only one such UEXP value is
reserved because experiments are expected to use an Experimental ID reserved because experiments are expected to use an Experimental ID
(ExIDs) to differentiate concurrent use for different purposes, using (ExIDs) to differentiate concurrent use for different purposes, using
UDP ExIDs registered with IANA according to the approach developed UDP ExIDs registered with IANA according to the approach developed
for TCP experimental options [RFC6994]. for TCP experimental options [RFC6994].
Assigned ExIDs can be used with either the UEXP or EXP options. Assigned ExIDs can be used with either the UEXP or EXP Options.
13. Rules for Designing New Options 13. Rules for Designing New Options
The UDP option Kind space allows for the definition of new options; The UDP Option Kind space allows for the definition of new options;
however, the currently defined options (including AUTH, UENC, and however, the currently defined options (including AUTH, UENC, and
UCMP) do not allow for arbitrary new options. The following is a UCMP) do not allow for arbitrary new options. The following is a
summary of rules for new options and their rationales: summary of rules for new options and their rationales:
>> New options MUST NOT be defined as "must-implement", i.e., they >> New options MUST NOT be defined as "must-implement", i.e., they
are not eligible for the asterisk ("*") designation used in are not eligible for the asterisk ("*") designation used in
Section 10. Section 10.
This document defines the minimum set of "must-implement" UDP This document defines the minimum set of "must-implement" UDP
options. All new options are included at the discretion of a given Options. All new options are included at the discretion of a given
implementation. implementation.
>> New options MUST NOT modify the content of options that precede >> New options MUST NOT modify the content of options that precede
them (in order of appearance and thus processing). them (in order of appearance and thus processing).
>> The fields of new options MUST NOT depend on the content of other >> The fields of new options MUST NOT depend on the content of other
options. options.
UNSAFE options can both depend on and vary user data content because UNSAFE Options can both depend on and vary user data content because
they are contained only inside UDP fragments and thus are processed they are contained only inside UDP fragments and thus are processed
only by receivers capable of handling UDP options. only by receivers capable of handling UDP Options.
>> New options MUST NOT declare their order relative to other >> New options MUST NOT declare their order relative to other
options, whether new or old, even as a preference. options, whether new or old, even as a preference.
>> At the sender, new options MUST NOT modify UDP packet content >> At the sender, new options MUST NOT modify UDP packet content
anywhere except within their option field, except only those anywhere outside their option field, excepting only UNSAFE Options;
contained within the UNSAFE option; areas that need to remain areas that need to remain unmodified include the IP header, IP
unmodified include the IP header, IP options, UDP user data, and options, UDP user data, and surplus area (i.e., other options).
surplus area (i.e., other options).
>> Options MUST NOT be modified in transit. This includes those >> Options MUST NOT be modified in transit. This includes those
already defined as well as new options. already defined as well as new options.
>> New options MUST NOT require or allow that any UDP options >> New options MUST NOT require or allow that any UDP Options
(including themselves) or the remaining surplus area be modified in (including themselves) or the remaining surplus area be modified in
transit. transit.
>> All options MUST indicate whether they can be used per-fragment >> All options MUST indicate whether they can be used per-fragment
and, if so, MUST also indicate how their success or failure is and, if so, MUST also indicate how their success or failure is
reported to the user. This document RECOMMENDS that options be reported to the user. It is RECOMMENDED that new options be designed
useful per-fragment and also RECOMMENDS that options used per- to support per-fragment use; it is also RECOMMENDED that options used
fragment be reported to the user as a finite aggregate (e.g., a sum, per-fragment be reported to the user as a finite aggregate (e.g., a
a flag, etc.) rather than individually. sum, a flag, etc.) rather than individually.
Note that only certain of the initially defined options violate these With one exception, UNSAFE Options are used when UDP user data needs
rules: to be modified:
* >> The FRAG option modifies UDP user data, splitting it across >> The FRAG Option modifies UDP user data, splitting it across
multiple IP packets. UNSAFE options MAY modify the UDP user data, multiple IP packets. UNSAFE Options MAY modify the UDP user data,
e.g., by encryption, compression, or other transformations. All e.g., by encryption, compression, or other transformations. All
other (SAFE) options MUST NOT modify the UDP user data. other (SAFE) options MUST NOT modify the UDP user data.
14. Option Inclusion and Processing 14. Option Inclusion and Processing
The following rules apply to option inclusion by senders and The following rules apply to option inclusion by senders and
processing by receivers. processing by receivers.
>> Senders MAY add any option, as configured by the API. >> Senders MAY add any option, as configured by the API.
>> All "must-support" options MUST be processed by receivers, if >> All "must-support" options MUST be processed by receivers, if
present (presuming UDP options are supported at that receiver). present (presuming UDP Options are supported at that receiver).
>> Non-"must-support" options MAY be ignored by receivers, if >> Options that are not "must-support" options MAY, if present, be
present, e.g., based on API settings. ignored by receivers, based, e.g., on API settings.
>> All options MUST be processed by receivers in the order >> All options MUST be processed by receivers in the order
encountered in the options area. encountered in the options area.
>> Unless configuration settings direct otherwise, all options except >> Unless configuration settings direct otherwise, all options except
UNSAFE options MUST result in the UDP user data being passed to the UNSAFE Options MUST result in the UDP user data being passed to the
upper layer protocol or application, regardless of whether all upper layer protocol or application, regardless of whether all
options are processed, are supported, or succeed. options are processed, are supported, or succeed.
The basic premise is that, for options-aware endpoints, the sender The basic premise is that, for options-aware endpoints, the sender
decides what options to add and the receiver decides what options to decides what options to add and the receiver decides what options to
handle. Simply adding an option does not force work upon a receiver, handle. Simply adding an option does not force work upon a receiver,
with the exception of the "must-support" options. with the exception of the "must-support" options.
Upon receipt, the receiver checks various properties of the UDP Upon receipt, the receiver checks various properties of the UDP
packet and its options to decide whether to accept or drop the UDP packet and its options to decide whether to accept or drop the UDP
skipping to change at line 1692 skipping to change at line 1692
(OCS == 0 and UDP CS != 0) then (OCS == 0 and UDP CS != 0) then
deliver the UDP user data but ignore other options deliver the UDP user data but ignore other options
(this is required to emulate legacy behavior) (this is required to emulate legacy behavior)
if (OCS != 0 and OCS passes) or if (OCS != 0 and OCS passes) or
(OCS == 0 and UDP CS == 0) then (OCS == 0 and UDP CS == 0) then
deliver the UDP user data after parsing deliver the UDP user data after parsing
and processing the rest of the options, and processing the rest of the options,
regardless of whether each is supported or succeeds regardless of whether each is supported or succeeds
(again, this is required to emulate legacy behavior) (again, this is required to emulate legacy behavior)
The design of the UNSAFE options ensures that the resulting UDP data The design of the UNSAFE Options ensures that the resulting UDP data
will be silently dropped in both legacy receivers and options-aware will be silently dropped in both legacy receivers and options-aware
receivers that do not recognize those options. Again, note that this receivers that do not recognize those options. Again, note that this
still results in the delivery of a zero-length UDP packet. still results in the delivery of a zero-length UDP packet.
Options-aware receivers can drop UDP packets with option processing Options-aware receivers can drop UDP packets with option processing
errors via either an override of the default UDP processing or at the errors via either an override of the default UDP processing or at the
application layer. application layer.
That is, all options are treated the same, in that the transmitter Put another way, all options are treated the same, in that the
can add it as desired and the receiver has the option to require it transmitter can add each option as desired and the receiver has the
or not. Only if it is required (e.g., by API configuration) would choice to require a given option or not. Only if a particular option
the receiver require it being present and correct. is indicated as mandatory by a receiver (e.g., by API configuration)
would the receiver need to confirm it being present and correct.
That is, for all options: In summary, for all options:
* if the option is not required by the receiver, then UDP packets * if the option is not required by the receiver, then UDP packets
missing the option are accepted. missing the option are accepted.
* if the option is required (e.g., by override of the default * if the option is required (e.g., by override of the default
behavior at the receiver) and missing or incorrectly formed, behavior at the receiver) and missing or incorrectly formed,
silently drop the UDP packet. silently drop the UDP packet.
* if the UDP packet is accepted (either because the option is not * if the UDP packet is accepted (either because the option is not
required or because it was required and correct), then pass the required or because it was required and correct), then pass the
skipping to change at line 1754 skipping to change at line 1755
This API is extended to support options as follows: This API is extended to support options as follows:
* Extend the method to create receive ports to include per-packet * Extend the method to create receive ports to include per-packet
and per-fragment receive options that are required or omitted as and per-fragment receive options that are required or omitted as
indicated by the application. indicated by the application.
>> Datagrams not containing these required options MUST be >> Datagrams not containing these required options MUST be
silently dropped and SHOULD be logged. silently dropped and SHOULD be logged.
* Extend the method to create receive ports to have a means to * Extend the method to create receive ports to have a means to
indicate that all packets containing UDP options that are received indicate that all packets containing UDP Options that are received
on a particular socket pair are to be discarded. on a particular socket pair are to be discarded.
>> The default value for the setting to drop all packets >> The default value for the setting to drop all packets
containing UDP options MUST be to process packets containing UDP containing UDP Options MUST be to process packets containing UDP
options normally (i.e., not to discard them). Options normally (i.e., not to discard them).
* Extend the receive function to indicate the per-packet options and * Extend the receive function to indicate the per-packet options and
their parameters as received with the corresponding received their parameters as received with the corresponding received
datagram. Note that per-fragment options are handled within the datagram. Note that per-fragment options are handled within the
processing of each fragment. processing of each fragment.
>> Options and their processing status (success/fail) MUST be >> Options and their processing status (success/fail) MUST be
available to the user (i.e., application layer or upper layer available to the user (i.e., application layer or upper layer
protocol/service), both for the packet and for the fragment set, protocol/service), both for the packet and for the fragment set,
except for FRAG, NOP, and EOL; those three options are handled except for FRAG, NOP, and EOL; those three options are handled
within UDP option processing only. As a reminder (from within UDP Option processing only. As a reminder (from
Section 14), all options except UNSAFE options MUST result in the Section 14), all options except UNSAFE Options MUST result in the
UDP user data being passed to the application layer (unless UDP user data being passed to the application layer (unless
overridden in the API), regardless of whether all options are overridden in the API), regardless of whether all options are
processed, supported, or succeed. processed, supported, or succeed.
* For fragments, success for an option is reported only when all * For fragments, success for an option is reported only when all
fragments succeed for that option. fragments succeed for that option.
>> Per-fragment option status reporting SHOULD default as needed >> Per-fragment option status reporting SHOULD default as needed
(e.g., not computed and/or not passed up to the upper layers) to (e.g., not computed and/or not passed up to the upper layers) to
minimize overhead unless actively requested (e.g., by the user/ minimize overhead unless actively requested (e.g., by the user/
application layer). application layer).
>> SAFE options associated with fragments are accumulated when >> SAFE Options associated with fragments are accumulated when
associated with the reassembled packet; values MAY be coalesced, associated with the reassembled packet; values MAY be coalesced,
e.g., to indicate that only an AUTH failure of a fragment e.g., to indicate that only an AUTH failure of a fragment
occurred, rather than not indicating the AUTH status of each occurred, rather than not indicating the AUTH status of each
fragment. fragment.
* Extend the send function to indicate the options to be added to * Extend the send function to indicate the options to be added to
the corresponding sent datagram. This includes indicating which the corresponding sent datagram. This includes indicating which
options apply to individual fragments vs. which apply to the UDP options apply to individual fragments vs. which apply to the UDP
packet prior to fragmentation, if fragmentation is enabled. This packet prior to fragmentation, if fragmentation is enabled. This
includes a minimum datagram length, such that the options list includes a minimum datagram length, such that the options list
ends in EOL and additional space is zero-filled as needed. It ends in EOL and additional space is zero-filled as needed. It
also includes a maximum fragment size, e.g., as discovered by also includes a maximum fragment size, e.g., as discovered by
DPLPMTUD, whether implemented at the application layer per DPLPMTUD, whether implemented at the application layer per
[RFC8899] or in conjunction with other UDP options [RFC9869]. [RFC8899] or in conjunction with other UDP Options [RFC9869].
Examples of API instances for Linux and FreeBSD are provided in Examples of API instances for Linux and FreeBSD are provided in
Appendix A to encourage uniform cross-platform implementations. Appendix A to encourage uniform cross-platform implementations.
APIs are not intended to provide user control over option order, APIs are not intended to provide user control over option order,
especially on a per-packet basis, as this could create a covert especially on a per-packet basis, as this could create a covert
channel (see Section 25). Similarly, APIs are not intended to channel (see Section 25). Similarly, APIs are not intended to
provide user/application control over UDP fragment boundaries on a provide user/application control over UDP fragment boundaries on a
per-packet basis; although, they are expected to allow control over per-packet basis; they are, however, expected to allow control over
which options, including fragmentation, are enabled (or disabled) on which options, including fragmentation, are enabled (or disabled) on
a per-packet basis. Such control over fragmentation is critical to a per-packet basis. Such control over fragmentation is critical to
DPLPMTUD. DPLPMTUD.
16. UDP Options Are for Transport, Not Transit 16. UDP Options Are for Transport, Not Transit
UDP options are indicated in the surplus area of the IP payload that UDP Options are indicated in the surplus area of the IP payload that
is not used by UDP. That area is really part of the IP payload, not is not used by UDP. That area is really part of the IP payload, not
the UDP payload, and as such, it might be tempting to consider the UDP payload, and as such, it might be tempting to consider
whether this is a generally useful approach to extending IP. whether this is a generally useful approach to extending IP.
Unfortunately, the surplus area exists only for transports that Unfortunately, the surplus area exists only for transports that
include their own transport layer payload length indicator. TCP and include their own transport layer payload length indicator. TCP and
SCTP include header length fields that already provide space for SCTP include header length fields that already provide space for
transport options by indicating the total length of the header area, transport options by indicating the total length of the header area,
such that the entire remaining area indicated in the network layer such that the entire remaining area indicated in the network layer
(IP) is the transport payload. UDP-Lite already uses the UDP Length (IP) is the transport payload. UDP-Lite already uses the UDP Length
field to indicate the boundary between data covered by the transport field to indicate the boundary between data covered by the transport
checksum and data not covered, and so there is no remaining area checksum and data not covered, and so there is no remaining area
where the length of the UDP-Lite payload as a whole can be indicated where the length of the UDP-Lite payload as a whole can be indicated
[RFC3828]. [RFC3828].
UDP options are transport options. They are no more (or less) UDP Options are transport options. They are no more (or less)
appropriate to be modified in-transit than any other portion of the appropriate to be modified in-transit than any other portion of the
transport datagram. transport datagram.
>> Generally, transport headers, options, and data are not intended >> Generally, transport headers, options, and data are not intended
to be modified in-transit. UDP options are no exception and are to be modified in-transit. UDP Options are no exception and are
specified here as "MUST NOT be altered in transit". specified here as "MUST NOT be altered in transit".
However, note that the UDP option mechanism provides no specific However, note that the UDP Option mechanism provides no specific
protection against in-transit modification of the UDP header, UDP protection against in-transit modification of the UDP header, UDP
payload, or surplus area, except as provided by the OCS or the payload, or surplus area, except as provided by the OCS or the
options selected (e.g., AUTH or UENC). options selected (e.g., AUTH or UENC).
Unless protected by encryption (e.g., UENC or via other layers, like Unless protected by encryption (e.g., UENC or via other layers, like
IPsec), UDP options remain visible to devices on the network path. IPsec), UDP Options remain visible to devices on the network path.
The decision to not require mandatory encryption for UDP options to The decision to not require mandatory encryption for UDP Options to
prevent such visibility was made because the key distribution and prevent such visibility was made because the key distribution and
management infrastructure necessary to support such encryption does management infrastructure necessary to support such encryption does
not exist in many of the deployment scenarios of interest, notably not exist in many of the deployment scenarios of interest, notably
those that use UDP directly as a stateless and connectionless those that use UDP directly as a stateless and connectionless
transport protocol (e.g., see [He24]). transport protocol (e.g., see [He24]).
17. UDP Options vs. UDP-Lite 17. UDP Options vs. UDP-Lite
UDP-Lite provides partial checksum coverage so that UDP packets with UDP-Lite provides partial checksum coverage so that UDP packets with
errors in some locations can be delivered to the user [RFC3828]. It errors in some locations can be delivered to the user [RFC3828]. It
uses a different transport protocol number (136) than UDP (17) to uses a different transport protocol number (136) than UDP (17) to
interpret the UDP Length field as the prefix covered by the UDP interpret the UDP Length field as the prefix covered by the UDP
checksum. checksum.
UDP (protocol 17) already defines the UDP Length field as the limit UDP (protocol 17) already defines the UDP Length field as the limit
of the UDP checksum but by default also limits the data provided to of the UDP checksum but by default also limits the data provided to
the application as that which precedes the UDP Length. A goal of the application as that which precedes the UDP Length. A goal of
UDP-Lite is to deliver data beyond UDP Length as a default, which is UDP-Lite is to deliver data beyond UDP Length as a default, which is
why a separate transport protocol number was required. why a separate transport protocol number was required.
UDP options do not use or need a separate transport protocol number UDP Options do not use or need a separate transport protocol number
because the data beyond the UDP Length offset (surplus data) is not because the data beyond the UDP Length offset (surplus data) is not
provided to the application by default. That data is interpreted provided to the application by default. That data is interpreted
exclusively within the UDP transport layer. exclusively within the UDP transport layer.
UDP-Lite cannot support UDP options, either as proposed here or in UDP-Lite cannot support UDP Options, either as proposed here or in
any other form, because the entire payload of the UDP packet is any other form, because the entire payload of the UDP packet is
already defined as user data and there is no additional field in already defined as user data and there is no additional field in
which to indicate a surplus area for options. The UDP Length field which to indicate a surplus area for options. The UDP Length field
in UDP-Lite is already used to indicate the boundary between user in UDP-Lite is already used to indicate the boundary between user
data covered by the checksum and user data not covered. data covered by the checksum and user data not covered.
18. Interactions with Legacy Devices 18. Interactions with Legacy Devices
It has always been permissible for the UDP Length to be inconsistent It has always been permissible for the UDP Length to be inconsistent
with the IP transport payload length [RFC0768]. Such inconsistency with the IP transport payload length [RFC0768]. Such inconsistency
has been utilized in UDP-Lite using a different transport number. has been utilized in UDP-Lite using a different transport number
There are no known systems that use this inconsistency for UDP [RFC3828]. There are no known systems that use this inconsistency
[RFC3828]. It is possible that such use might interact with UDP for UDP. It is possible that such use might interact with UDP
options, i.e., where legacy systems might generate UDP datagrams that Options, i.e., where legacy systems might generate UDP datagrams that
appear to have UDP options. The OCS provides protection against such appear to have UDP Options. The OCS provides protection against such
events and is stronger than a static "magic number". events and is stronger than a static "magic number".
UDP options have been tested as interoperable with Linux, macOS, and UDP Options have been tested as interoperable with Linux, macOS, and
Windows Cygwin and worked through NAT devices. These systems Windows Cygwin and worked through NAT devices. These systems
successfully delivered only the user data indicated by the UDP Length successfully delivered only the user data indicated by the UDP Length
field and silently discarded the surplus area. field and silently discarded the surplus area.
One reported embedded device passes the entire IP datagram to the UDP One reported embedded device passes the entire IP datagram to the UDP
application layer. Although this feature could enable application- application layer. Although this feature could enable application-
layer UDP option processing, it would require that conventional UDP layer UDP Option processing, it would require that conventional UDP
user applications examine only the UDP user data. user applications examine only the UDP user data. This feature is
also inconsistent with the UDP application interface [RFC0768]
This feature is also inconsistent with the UDP application interface [RFC1122].
[RFC0768] [RFC1122].
It has been reported that Alcatel-Lucent's "Brick" Intrusion It has been noted that Alcatel-Lucent's "Brick" Intrusion Detection
Detection System has a default configuration that interprets System has a default configuration that interprets inconsistencies
inconsistencies between UDP Length and IP Length as an attack to be between UDP Length and IP Length as an attack to be reported. Note
reported. Note that other firewall systems, e.g., Check Point, use a that other firewall systems, e.g., Check Point, use a default
default "relaxed UDP length verification" to avoid falsely "relaxed UDP Length verification" to avoid falsely interpreting this
interpreting this inconsistency as an attack. inconsistency as an attack.
There are known uses of UDP exchanges of zero-length UDP user data There are known uses of UDP exchanges of zero-length UDP user data
packets, notably in the TIME protocol [RFC0868]. The need to support packets, notably in the TIME protocol [RFC0868]. The need to support
such packets is also noted in the UDP usage guidelines [RFC8085]. such packets is also noted in the UDP usage guidelines [RFC8085].
Some of the mechanisms in this document can generate more zero- Some of the mechanisms in this document can generate more zero-length
length UDP packets for a UDP option-aware endpoint than for a legacy UDP packets for a UDP Option aware endpoint than for a legacy
(non-aware) endpoint (e.g., based on some error conditions), and some endpoint (e.g., based on some error conditions), and some can
can generate fewer (e.g., fragment reassembly). Because such packets generate fewer (e.g., fragment reassembly). Because such packets
inherently carry no unique transport header or transport content, inherently carry no unique transport header or transport content,
endpoints are already expected to be tolerant of their (inadvertent) endpoints are already expected to be tolerant of their (inadvertent)
replication or loss by the network, so such variations are not replication or loss by the network, so such variations are not
expected to be problematic. expected to be problematic.
19. Options in a Stateless, Unreliable Transport Protocol 19. Options in a Stateless, Unreliable Transport Protocol
There are two ways to interpret options for a stateless, unreliable There are two ways to interpret options for a stateless, unreliable
protocol -- an option is either local to the message or intended to protocol -- an option is either local to the message or intended to
affect a stream of messages in a soft-state manner. Either affect a stream of messages in a soft-state manner. Either
interpretation is valid for defined UDP options. interpretation is valid for defined UDP Options.
It is impossible to know in advance whether an endpoint supports a It is impossible to know in advance whether an endpoint supports a
UDP option. UDP Option.
>> All UDP options other than UNSAFE ones MUST be ignored if not >> All UDP Options other than UNSAFE ones MUST be ignored if not
supported or upon failure (e.g., APC). supported or upon failure (e.g., APC).
>> All UDP options that fail MUST result in the UDP data still being >> All UDP Options that fail MUST result in the UDP data still being
sent to the application layer by default to ensure equivalence with sent to the application layer by default to ensure equivalence with
legacy devices. legacy devices.
UDP options that rely on soft-state exchange need to allow message UDP Options that rely on soft-state exchange need to allow message
reordering and loss, in the same way as UDP applications [RFC8085]. reordering and loss, in the same way as UDP applications [RFC8085].
The above requirements prevent using any option that cannot be safely The above requirements prevent using any option that cannot be safely
ignored unless it is hidden inside the FRAG area (i.e., UNSAFE ignored unless it is hidden inside the FRAG area (i.e., UNSAFE
options). Legacy systems also always need to be able to interpret Options). Legacy systems also always need to be able to interpret
the transport fragments as individual UDP packets. the transport fragments as individual UDP packets.
20. UDP Option State Caching 20. UDP Option State Caching
Some TCP connection parameters, stored in the TCP Control Block Some TCP connection parameters, stored in the TCP Control Block
(TCB), can be usefully shared either among concurrent connections or (TCB), can be usefully shared either among concurrent connections or
between connections in sequence, known as TCP Sharing [RFC9040]. between connections in sequence, known as TCB sharing [RFC9040].
Although UDP is stateless, some of the options proposed herein could Although UDP is stateless, some of the options proposed herein could
have similar benefits in being shared or cached. We call this UCB have similar benefits in being shared or cached. We call this UCB
sharing, or UDP Control Block sharing, by analogy. Just as TCB sharing, or UDP Control Block sharing, by analogy. Just as TCB
sharing is not a standard because it is consistent with existing TCP sharing is not a standard because it is consistent with existing TCP
specifications, UCB sharing would be consistent with existing UDP specifications, UCB sharing would be consistent with existing UDP
specifications, including this one. Both are implementation issues specifications, including this one. Both are implementation issues
that are outside the scope of their respective specifications, and so that are outside the scope of their respective specifications, and so
UCB sharing is outside the scope of this document. UCB sharing is outside the scope of this document.
21. Updates to RFC 768 21. Updates to RFC 768
This document updates [RFC0768] as follows: This document updates [RFC0768] as follows:
* This document defines the meaning of the IP payload area beyond * This document defines the meaning of the IP payload area beyond
the UDP length but within the IP Length as the surplus area used the UDP Length but within the IP Length as the surplus area used
herein for UDP options. herein for UDP Options.
* This document extends the UDP API to support the use of UDP * This document extends the UDP API to support the use of UDP
options. Options.
22. Interactions with Other RFCs (and drafts) 22. Interactions with Other RFCs
This document clarifies the interaction between UDP Length and IP This document clarifies the interaction between UDP Length and IP
Length that is not explicitly constrained in either UDP or the host Length that is not explicitly constrained in either UDP or the host
requirements [RFC0768] [RFC1122]. requirements [RFC0768] [RFC1122].
Teredo extensions (TEs) define use of a similar difference between Teredo extensions define use of a similar difference between these
these lengths for trailers [RFC4380] [RFC6081]. In [RFC6081], TE lengths for trailers [RFC4380] [RFC6081]. In [RFC6081], Teredo
defines the length of an IPv6 payload inside UDP as pointing to less extensions define the length of an IPv6 payload inside UDP as
than the end of the UDP payload, enabling trailing options for that pointing to less than the end of the UDP payload, enabling trailing
IPv6 packet: options for that IPv6 packet:
| ...the IPv6 packet length (i.e., the Payload Length value in the | ...the IPv6 packet length (i.e., the Payload Length value in the
| IPv6 header plus the IPv6 header size) is less than or equal to | IPv6 header plus the IPv6 header size) is less than or equal to
| the UDP payload length (i.e., the Length value in the UDP header | the UDP payload length (i.e., the Length value in the UDP header
| minus the UDP header size) | minus the UDP header size)
UDP options are not affected by the difference between the UDP user UDP Options are not affected by the difference between the UDP user
payload end and the payload IPv6 end; both would end at the UDP user payload end and the payload IPv6 end; both would end at the UDP user
payload, which could end before the enclosing IPv4 or IPv6 header payload, which could end before the enclosing IPv4 or IPv6 header
indicates -- allowing UDP options in addition to the trailer options indicates -- allowing UDP Options in addition to the trailer options
of the IPv6 payload. The result, if UDP options were used, is shown of the IPv6 payload. The result, if UDP Options were used, is shown
in Figure 19. in Figure 19.
Outer IP Length Outer IP Length
<----------------------------------------------------------> <---------------------------------------------------------->
+--------+---------+------------------------------+----------+ +--------+---------+------------------------------+----------+
| IP Hdr | UDP Hdr | IPv6 packet/len | TE trailer | surplus | | IP Hdr | UDP Hdr | IPv6 packet/len | TE trailer | surplus |
+--------+---------+------------------------------+----------+ +--------+---------+------------------------------+----------+
<---------------> <--------------->
Inner IPv6 Length Inner IPv6 Length
<--------------------------------------> <-------------------------------------->
UDP Length UDP Length
Figure 19: TE Trailers and UDP Options Used Concurrently Figure 19: TE Trailers and UDP Options Used Concurrently
UDP options cannot be supported when a UDP packet has no independent UDP Options cannot be supported when a UDP packet has no independent
UDP Length. One such case is when UDP Length==0 in IPv6, intended UDP Length. One such case is when UDP Length==0 in IPv6, intended
for (but not limited to) IPv6 Jumbograms [RFC2675]. Note that for (but not limited to) IPv6 Jumbograms [RFC2675]. Note that
although this technique is "Standard", the specification did not although this technique is "Standard", the specification did not
"update" UDP [RFC0768]. Another such case arises when UDP is proxied "update" UDP [RFC0768]. Another such case arises when UDP is proxied
via HTTP [RFC9298], as the UDP header is omitted and only the UDP via HTTP [RFC9298], as the UDP header is omitted and only the UDP
user data is transported. user data is transported.
This document is consistent with the UDP profile for RObust Header This document is consistent with the UDP profile for RObust Header
Compression (ROHC) [RFC3095], noted here: Compression (ROHC) [RFC3095], noted here:
| The Length field of the UDP header MUST match the Length field(s) | The Length field of the UDP header MUST match the Length field(s)
| of the preceding subheaders, i.e., there must not be any padding | of the preceding subheaders, i.e., there must not be any padding
| after the UDP payload that is covered by the IP Length. | after the UDP payload that is covered by the IP Length.
ROHC compresses UDP headers only when this match succeeds. It does ROHC compresses UDP headers only when this match succeeds. It does
not prohibit UDP headers where the match fails; in those cases, ROHC not prohibit UDP headers where the match fails; in those cases, ROHC
default rules (Section 5.10 of [RFC3095]) would cause the UDP header default rules (Section 5.10 of [RFC3095]) would cause the UDP header
to remain uncompressed. Upon receipt of a compressed UDP header, to remain uncompressed. Upon receipt of a compressed UDP header,
Appendix A.1.3 of [RFC3095] indicates that the UDP length is Appendix A.1.3 of [RFC3095] indicates that the UDP Length is
"INFERRED"; in uncompressed packets, it would simply be explicitly "INFERRED"; in uncompressed packets, it would simply be explicitly
provided. provided.
This issue of handling UDP header compression is more explicitly This issue of handling UDP header compression is more explicitly
described in more recent specifications, e.g., Section 10.10 of described in more recent specifications, e.g., Section 10.10 of
[RFC8724]. [RFC8724].
23. Multicast and Broadcast Considerations 23. Multicast and Broadcast Considerations
UDP options are primarily intended for unicast use. Using these UDP Options are primarily intended for unicast use. Using these
options over multicast or broadcast IP requires careful options over multicast or broadcast IP requires careful
consideration, e.g., to ensure that the options used are safe for consideration, e.g., to ensure that the options used are safe for
different endpoints to interpret differently (e.g., either to support different endpoints to interpret differently (e.g., either to support
or silently ignore) or to ensure that all receivers of a multicast or or silently ignore) or to ensure that all receivers of a multicast or
broadcast group confirm support for the options in use. broadcast group confirm support for the options in use.
24. Network Management Considerations 24. Network Management Considerations
UDP options use and configuration may be useful to track and manage UDP Options use and configuration may be useful to track and manage
remotely. IP Flow Information Export (IPFIX) [RFC7011] Information remotely. IP Flow Information Export (IPFIX) [RFC7011] Information
Elements for UDP options have been defined in [Bo24]. Similar to Elements for UDP Options have been defined in [RFC9870]. Similar to
what has been done for TCP [RFC9648], a YANG model [RFC7950] for use what has been done for TCP [RFC9648], a YANG model [RFC7950] for use
by network management protocols (e.g., NETCONF [RFC6241] or RESTCONF by network management protocols (e.g., NETCONF [RFC6241] or RESTCONF
[RFC8040]) may be developed. Development of these models is outside [RFC8040]) may be developed. Development of these models is outside
the scope of this document. the scope of this document.
25. Security Considerations 25. Security Considerations
There are a number of security issues raised by the introduction of There are a number of security issues raised by the introduction of
options to UDP. Some are specific to this variant, but others are options to UDP. Some are specific to this variant, but others are
associated with any packet processing mechanism; all are discussed associated with any packet processing mechanism; all are discussed
further in this section. further in this section.
25.1. General Considerations Regarding the Use of Options 25.1. General Considerations Regarding the Use of Options
Note that any user application that considers UDP options to Note that any user application that considers UDP Options to
adversely affect security need not enable them. However, their use adversely affect security need not enable them. However, their use
does not impact security in a substantially different way than TCP does not impact security in a substantially different way than TCP
options; both enable the use of a control channel that has the options; both enable the use of a control channel that has the
potential for abuse. Similar to TCP, there are many options that, if potential for abuse. Similar to TCP, there are many options that, if
unprotected, could be used by an attacker to interfere with unprotected, could be used by an attacker to interfere with
communication. communication.
UDP options are not covered by DTLS [RFC9147]. Neither TLS [RFC8446] UDP Options are not covered by DTLS [RFC9147]. Neither TLS [RFC8446]
(Transport Layer Security for TCP) nor DTLS (TLS for UDP) protect the (Transport Layer Security for TCP) nor DTLS (TLS for UDP) protect the
transport layer; both operate as a shim layer solely on the user data transport layer; both operate as a shim layer solely on the user data
of transport packets, protecting only their contents. of transport packets, protecting only their contents.
Just as TLS does not protect the TCP header or its options, DTLS does Just as TLS does not protect the TCP header or its options, DTLS does
not protect the UDP header or the new options introduced by this not protect the UDP header or the new options introduced by this
document. Transport security is provided in TCP by the TCP document. Transport security is provided in TCP by the TCP
Authentication Option (TCP-AO) [RFC5925] and (when defined) in UDP by Authentication Option (TCP-AO) [RFC5925] and (when defined) in UDP by
the Authentication (AUTH) option (Section 11.9) and (when defined) the Authentication (AUTH) Option (Section 11.9) and (when defined)
the UNSAFE Encryption (UENC) option (Section 12). Transport headers the UNSAFE Encryption (UENC) Option (Section 12). Transport headers
are also protected as payload when using IP security (IPsec) are also protected as payload when using IP security (IPsec)
[RFC4301]. [RFC4301].
Some UDP options are never passed to the receiving application, Some UDP Options are never passed to the receiving application,
notably FRAG, NOP, and EOL. They are not intended to convey notably FRAG, NOP, and EOL. They are not intended to convey
information, either by their presence (FRAG, EOL) or number (NOP). information, either by their presence (FRAG, EOL) or number (NOP).
It could also be useful to provide the options received in a It could also be useful to provide the options received in a
reference order (e.g., sorted by option number) to avoid the order of reference order (e.g., sorted by option number) to avoid the order of
options being used as a covert channel. options being used as a covert channel.
All logging is rate limited to avoid logging itself becoming a All logging is rate limited to avoid logging itself becoming a
resource vulnerability. resource vulnerability.
25.2. Considerations Regarding On-Path Attacks 25.2. Considerations Regarding On-Path Attacks
UDP options, like any options, have the potential to expose option UDP Options, like any options, have the potential to expose option
information to on-path attackers, unless the options themselves are information to on-path attackers, unless the options themselves are
encrypted (as might be the case with some configurations of UENC, encrypted (as might be the case with some configurations of UENC,
when defined). Application protocol designers are expected to ensure when defined). Application protocol designers are expected to ensure
that information in UDP options is not used with the assumption of that information in UDP Options is not used with the assumption of
privacy unless UENC provides that capability. Application protocol privacy unless UENC provides that capability. Application protocol
designers using secure payload contents (e.g., via DTLS) are expected designers using secure payload contents (e.g., via DTLS) are expected
to be aware that UDP options add information that is not inside the to be aware that UDP Options add information that is not inside the
UDP payload and thus not protected by the same mechanism and that UDP payload and thus not protected by the same mechanism and that
alternate mechanisms (again, as might be the case with some alternate mechanisms (again, as might be the case with some
configurations of UENC) could be additionally required to protect configurations of UENC) could be additionally required to protect
against information disclosure. against information disclosure.
>> Implementations concerned with the potential use of UDP options as >> Implementations concerned with the potential use of UDP Options as
a covert channel MAY consider limiting use of some or all options. a covert channel MAY consider limiting use of some or all options.
Such implementations SHOULD return options in an order not related to Such implementations SHOULD return options in an order not related to
their sequence in the received packet. their sequence in the received packet.
UDP options create new potential opportunities for Distributed DoS UDP Options create new potential opportunities for Distributed DoS
(DDos) attacks, notably through the use of fragmentation. When (DDos) attacks, notably through the use of fragmentation. When
enabled, UDP options cause additional work at the receiver; however, enabled, UDP Options cause additional work at the receiver; however,
of the "must-support" options, only REQ (e.g., when used with of the "must-support" options, only REQ (e.g., when used with
DPLPMTUD [RFC9869]) will cause the upper layer to initiate a UDP DPLPMTUD [RFC9869]) will cause the upper layer to initiate a UDP
response in the absence of user transmission. response in the absence of user transmission.
>> Implementations concerned with the potential for DoS attacks >> Implementations concerned with the potential for DoS attacks
involving large numbers of UDP options, either implemented or involving large numbers of UDP Options, either implemented or
unknown, or excessive sequences of valid repeating options (e.g., unknown, or excessive sequences of valid repeating options (e.g.,
NOPs) SHOULD detect excessive numbers of such occurrences and limit NOPs) SHOULD detect excessive numbers of such occurrences and limit
resources they use, e.g., through silent packet drops. Such resources they use, e.g., through silent packet drops. Such
responses SHOULD be logged. Specific thresholds for such limits will responses SHOULD be logged. Specific thresholds for such limits will
vary based on implementation and are thus not included here. vary based on implementation and are thus not included here.
25.3. Considerations Regarding Option Processing 25.3. Considerations Regarding Option Processing
UDP options use the TLV syntax similar to that of TCP. This syntax UDP Options use the TLV syntax similar to that of TCP. This syntax
is known to require serial processing and could pose a DoS risk, is known to require serial processing and could pose a DoS risk,
e.g., if an attacker adds large numbers of unknown options that need e.g., if an attacker adds large numbers of unknown options that need
to be parsed in their entirety, as is the case for IPv6 [RFC8504]. to be parsed in their entirety, as is the case for IPv6 [RFC8504].
The use of UDP packets with inconsistent IP and UDP Length fields has The use of UDP packets with inconsistent IP and UDP Length fields has
the potential to trigger a buffer overflow error if not properly the potential to trigger a buffer overflow error if not properly
handled, e.g., if space is allocated based on the smaller field and handled, e.g., if space is allocated based on the smaller field and
copying is based on the larger field. However, there have been no copying is based on the larger field. However, there have been no
reports of such vulnerability, and it would rely on inconsistent use reports of such vulnerability, and it would rely on inconsistent use
of the two fields for memory allocation and copying. of the two fields for memory allocation and copying.
Because required options come first and at most once each (with the Because required options come first and at most once each (with the
exception of NOPs, which never need to come in sequences of more than exception of NOPs, which never need to come in sequences of more than
seven in a row), their DoS impact is limited. Note that TLV formats seven in a row), their DoS impact is limited. Note that TLV formats
for options do require serial processing, but any format that allows for options do require serial processing, but any format that allows
future options, whether ignored or not, could introduce a similar DoS future options, whether ignored or not, could introduce a similar DoS
vulnerability. vulnerability.
>> Implementations concerned with the potential for UDP options >> Implementations concerned with the potential for UDP Options
introducing a vulnerability MAY implement only the required UDP introducing a vulnerability MAY implement only the required UDP
options and SHOULD also limit processing of TLVs, in number of non- Options and SHOULD also limit processing of TLVs, in number of non-
padding options, total length, or both. The number of non-zero TLVs padding options, total length, or both. The number of non-zero TLVs
allowed in such cases MUST be at least as many as the number of allowed in such cases MUST be at least as many as the number of
concurrent options supported with an additional few to account for concurrent options supported with an additional few to account for
unexpected unknown options but SHOULD also consider being adaptive unexpected unknown options but SHOULD also consider being adaptive
and based on the implementation to avoid locking in that limit and based on the implementation to avoid locking in that limit
globally. globally.
For example, if a system supports 10 different option types that For example, if a system supports 10 different option types that
could concurrently be used, it is expected to allow up to around could concurrently be used, it is expected to allow up to around
13-14 different options in the same packet. This document avoids 13-14 different options in the same packet. This document avoids
skipping to change at line 2174 skipping to change at line 2174
not expect to receive more than a few unknown option types per not expect to receive more than a few unknown option types per
packet. packet.
25.4. Considerations for Fragmentation 25.4. Considerations for Fragmentation
UDP fragmentation introduces its own set of security concerns, which UDP fragmentation introduces its own set of security concerns, which
can be handled in a manner similar to IP reassembly or TCP segment can be handled in a manner similar to IP reassembly or TCP segment
reordering [CERT18]. In particular, the number of UDP packets reordering [CERT18]. In particular, the number of UDP packets
pending reassembly and effort used for reassembly is typically pending reassembly and effort used for reassembly is typically
limited. In addition, it could be useful to assume a reasonable limited. In addition, it could be useful to assume a reasonable
minimum fragment size, e.g., that non-terminal fragments are never be minimum fragment size, e.g., that non-terminal fragments are never
smaller than 500 bytes. smaller than 500 bytes.
>> Implementations concerned with the potential for UDP fragmentation >> Implementations concerned with the potential for UDP fragmentation
introducing a vulnerability SHOULD implement limits on the number of introducing a vulnerability SHOULD implement limits on the number of
pending fragments. pending fragments.
25.5. Considerations for Providing UDP Security 25.5. Considerations for Providing UDP Security
UDP security is not intended to rely solely on transport layer UDP security is not intended to rely solely on transport layer
processing of options. UNSAFE options are the only type that share processing of options. UNSAFE Options are the only type that share
fate with the UDP data because of the way that data is hidden in the fate with the UDP data because of the way that data is hidden in the
surplus area until after those options are processed. All other surplus area until after those options are processed. All other
options default to being silently ignored at the transport layer but options default to being silently ignored at the transport layer but
could be dropped if that default is either overridden (e.g., by could be dropped if that default is either overridden (e.g., by
configuration) or discarded at the application layer (e.g., using configuration) or discarded at the application layer (e.g., using
information about the options processed that are passed along with information about the options processed that are passed along with
the UDP packet). the UDP packet).
Options providing UDP security, e.g., AUTH and UENC, require endpoint Options providing UDP security, e.g., AUTH and UENC, require endpoint
key and security parameter coordination, which UDP options (being key and security parameter coordination, which UDP Options (being
stateless) do not facilitate. These parameters include whether and stateless) do not facilitate. These parameters include whether and
when to override the defaults described herein, especially at the when to override the defaults described herein, especially at the
transmitter as to when emitted packets need to include AUTH and at transmitter as to when emitted packets need to include AUTH and at
the receiver as to whether (and when) packets with failed AUTH and/or the receiver as to whether (and when) packets with failed AUTH and/or
without AUTH (or that fail the AUTH checks) are not to be forwarded without AUTH (or that fail the AUTH checks) are not to be forwarded
to the user/application. to the user/application.
25.6. Considerations Regarding Middleboxes 25.6. Considerations Regarding Middleboxes
Some middleboxes operate as UDP relays, forwarding data between a UDP Some middleboxes operate as UDP relays, forwarding data between a UDP
socket and another transport socket by modifying the IP and/or UDP socket and another transport socket by modifying the IP and/or UDP
headers without properly acting as a protocol endpoint (i.e., an headers without properly acting as a protocol endpoint (i.e., an
application layer proxy). In such cases, a sender might add UDP application layer proxy). In such cases, a sender might add UDP
options that could be stripped by the middlebox before the packet is Options that could be stripped by the middlebox before the packet is
forwarded to the second socket. A remote application will not forwarded to the second socket. A remote application will not
receive the options (for SAFE options, the payload data will be receive the options (for SAFE Options, the payload data will be
received; for UNSAFE options, the payload data will not be received). received; for UNSAFE Options, the payload data will not be received).
In such cases, the application will function as it would if In such cases, the application will function as it would if
communicating with a remote endpoint that does not support UDP communicating with a remote endpoint that does not support UDP
options. Options.
Additionally, [Zu20] reports that packets containing UDP options do Additionally, [Zu20] reports that packets containing UDP Options do
not traverse certain Internet paths; most likely, those options were not traverse certain Internet paths; most likely, those options were
stripped (e.g., by resetting the IP Length to correspond to the UDP stripped (e.g., by resetting the IP Length to correspond to the UDP
length, truncating the surplus area) or packets with options were Length, truncating the surplus area) or packets with options were
dropped. UDP options do not function over such paths. dropped. UDP Options do not function over such paths.
26. IANA Considerations 26. IANA Considerations
IANA has created the "User Datagram Protocol (UDP)" registry group, IANA has created the "User Datagram Protocol (UDP)" registry group,
which consists of the "UDP Option Kind Numbers" registry and a which consists of the "UDP Option Kind Numbers" registry and a
pointer to the unified "TCP/UDP Experimental Option Experiment pointer to the unified "TCP/UDP Experimental Option Experiment
Identifiers (TCP/UDP ExIDs)" registry. Note that the "TCP Identifiers (TCP/UDP ExIDs)" registry. Note that the "TCP
experimental IDs (ExIDs)" registry has been renamed as the "TCP/UDP experimental IDs (ExIDs)" registry has been renamed as the "TCP/UDP
Experimental Option Experiment Identifiers (TCP/UDP ExIDs)" registry, Experimental Option Experiment Identifiers (TCP/UDP ExIDs)" registry,
and is a unified registry for both TCP and UDP ExIDs. IANA has added and is a unified registry for both TCP and UDP ExIDs. IANA has added
the following note to the unified TCP/UDP ExID registry: the following note to the unified TCP/UDP ExID registry:
| Note 16-bit ExIDs can be used with either TCP or UDP; 32-bit ExIDs | 16-bit ExIDs can be used with either TCP or UDP; 32-bit ExIDs can
| can be used with TCP or their first 16 bits can be used with UDP. | be used with TCP or their first 16 bits can be used with UDP. Use
| Use with each transport (TCP, UDP) is indicated in the protocol | with each transport (TCP, UDP) is indicated in the protocol
| column, as defined in RFC 9868. | column, as defined in RFC 9868.
Initial values of the UDP Option Kind registry are as listed in Initial values of the "UDP Option Kind Numbers" registry are as
Section 10, including those both assigned and reserved. Additional listed in Section 10, including those both assigned and reserved.
values in this registry are to be assigned from the Unassigned values Additional values in this registry are to be assigned from the
in Section 10 by IESG Approval or Standards Action [RFC8126]. Those Unassigned values in Section 10 by IESG Approval or Standards Action
assignments are subject to the conditions set forth in this document, [RFC8126]. Those assignments are subject to the conditions set forth
particularly (but not limited to) those in Section 13. in this document, particularly (but not limited to) those in
Section 13.
>> Although option nicknames are not used in-band, new UNSAFE option >> Although option nicknames are not used in-band, new UNSAFE Option
names MUST commence with the capital letter "U" and new SAFE options names MUST commence with the capital letter "U" and new SAFE Options
MUST NOT commence with either uppercase or lowercase "U". MUST NOT commence with either uppercase or lowercase "U".
IANA has added the following note to the "UDP Option Kind Numbers" IANA has added the following note to the "UDP Option Kind Numbers"
indicating entries are mandatory to implement when UDP options are indicating entries are mandatory to implement when UDP Options are
supported. No new options may be created that are mandatory to supported. No new options may be created that are mandatory to
implement in all UDP options implementations. implement in all UDP Options implementations.
| Codepoints 0-7 MUST be supported on any implementation supporting | Codepoints 0-7 MUST be supported on any implementation supporting
| UDP options. All others are supported at the discretion of each | UDP Options. All others are supported at the discretion of each
| implementation. | implementation.
UDP Experimental Option Experiment Identifiers (UDP ExIDs) are UDP Experimental Option Experiment Identifiers (UDP ExIDs) are
intended for use in a similar manner as TCP ExIDs [RFC6994]. Both intended for use in a similar manner as TCP ExIDs [RFC6994]. Both
TCP and UDP ExIDs are managed as a single, unified registry because TCP and UDP ExIDs are managed as a single, unified registry because
such options could be used for both transport protocols and because such options could be used for both transport protocols and because
the option space is large enough that there is no clear need to the option space is large enough that there is no clear need to
maintain them separately. This new TCP/UDP ExIDs registry has maintain them separately. This new TCP/UDP ExIDs registry has
entries for both transports, although each codepoint needs to be entries for both transports, although each codepoint needs to be
explicitly defined for each transport protocol in which it is used, explicitly defined for each transport protocol in which it is used,
i.e., defining a codepoint in TCP does not imply it has a similar use i.e., defining a codepoint in TCP does not imply it has a similar use
in UDP. IANA has added a "Protocol" field to the registry and in UDP. IANA has added a "Protocol" field to the registry and
updated the current TCP ExIDs to be indicated as defined for TCP. updated the current TCP ExIDs to be indicated as defined for TCP.
New assignments are to indicate the transport for which it is New assignments are to indicate the transport for which it is
defined. defined.
TCP/UDP ExIDs can be used in either (or both) the UDP EXP TCP/UDP ExIDs can be used in either (or both) the UDP EXP
(Section 11.10) or UEXP (Section 12.3) options. TCP/UDP ExID entries (Section 11.10) or UEXP (Section 12.3) Options. TCP/UDP ExID entries
for use in UDP consist of a 16-bit ExID (in network-standard order), for use in UDP consist of a 16-bit ExID (in network-standard order),
and (as with the original TCP ExIDs) will preferentially also include and (as with the original TCP ExIDs) will preferentially also include
a short description and acronym for use in documentation. TCP/UDP a short description and acronym for use in documentation. TCP/UDP
ExIDs used for UDP are always 16 bits because their use in EXP and ExIDs used for UDP are always 16 bits because their use in EXP and
UEXP options is required and thus do not need a larger codepoint UEXP Options is required and thus do not need a larger codepoint
value to decrease the probability of accidental occurrence with non- value to decrease the probability of accidental occurrence with non-
ExID uses of the experimental options, as is the case with TCP ExIDs ExID uses of the experimental options, as is the case with TCP ExIDs
(e.g., when using 32-bit ExIDs). ExIDs defined solely for TCP (e.g., when using 32-bit ExIDs). ExIDs defined solely for TCP
options could be either 16 or 32 bits and all ExIDs (including now options could be either 16 or 32 bits and all ExIDs (including now
UDP) need to be unique in their first 16 bits, as originally UDP) need to be unique in their first 16 bits, as originally
described for TCP [RFC6994]. described for TCP [RFC6994].
Values in the TCP/UDP ExID registry are to be assigned by IANA using Values in the TCP/UDP ExID registry are to be assigned by IANA using
first-come, first-served (FCFS) rules applied to both the ExID value the First Come First Served (FCFS) policy [RFC8126], which applies to
and the acronym [RFC8126]. UDP options using these ExIDs are subject both the ExID value and the acronym. UDP Options using these ExIDs
to the same conditions as new UDP options, i.e., they too are subject are subject to the same conditions as new UDP Options, i.e., they too
to the conditions set forth in this document, particularly (but not are subject to the conditions set forth in this document,
limited to) those in Section 13. particularly (but not limited to) those in Section 13.
27. References 27. References
27.1. Normative References 27.1. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980, DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>. <https://www.rfc-editor.org/info/rfc768>.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
skipping to change at line 2319 skipping to change at line 2320
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC9869] Fairhurst, G. and T. Jones, "Datagram PLPMTUD for UDP [RFC9869] Fairhurst, G. and T. Jones, "Datagram Packetization Layer
Options", RFC 9869, DOI 10.17487/RFC9869, September 2025, Path MTU Discovery (DPLPMTUD) for UDP Options", RFC 9869,
DOI 10.17487/RFC9869, October 2025,
<https://www.rfc-editor.org/info/rfc9869>. <https://www.rfc-editor.org/info/rfc9869>.
27.2. Informative References 27.2. Informative References
[Bo24] Boucadair, M. and T. Reddy.K, "Export of UDP Options
Information in IP Flow Information Export (IPFIX)", Work
in Progress, Internet-Draft, draft-ietf-opsawg-tsvwg-udp-
ipfix-14, 22 July 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-opsawg-
tsvwg-udp-ipfix-14>.
[CERT18] CERT Coordination Center, "TCP implementations vulnerable [CERT18] CERT Coordination Center, "TCP implementations vulnerable
to Denial of Service", Vulnerability Note VU#962459, to Denial of Service", Vulnerability Note VU#962459,
Software Engineering Institute, CMU, 2018, Software Engineering Institute, CMU, 2018,
<https://www.kb.cert.org/vuls/id/962459>. <https://www.kb.cert.org/vuls/id/962459>.
[Fa18] Fairhurst, G., Jones, T., and R. Zullo, "Checksum [Fa18] Fairhurst, G., Jones, T., and R. Zullo, "Checksum
Compensation Options for UDP Options", Work in Progress, Compensation Options for UDP Options", Work in Progress,
Internet-Draft, draft-fairhurst-udp-options-cco-00, 19 Internet-Draft, draft-fairhurst-udp-options-cco-00, 19
October 2018, <https://datatracker.ietf.org/doc/html/ October 2018, <https://datatracker.ietf.org/doc/html/
draft-fairhurst-udp-options-cco-00>. draft-fairhurst-udp-options-cco-00>.
skipping to change at line 2552 skipping to change at line 2547
<https://www.rfc-editor.org/info/rfc9293>. <https://www.rfc-editor.org/info/rfc9293>.
[RFC9298] Schinazi, D., "Proxying UDP in HTTP", RFC 9298, [RFC9298] Schinazi, D., "Proxying UDP in HTTP", RFC 9298,
DOI 10.17487/RFC9298, August 2022, DOI 10.17487/RFC9298, August 2022,
<https://www.rfc-editor.org/info/rfc9298>. <https://www.rfc-editor.org/info/rfc9298>.
[RFC9648] Scharf, M., Jethanandani, M., and V. Murgai, "YANG Data [RFC9648] Scharf, M., Jethanandani, M., and V. Murgai, "YANG Data
Model for TCP", RFC 9648, DOI 10.17487/RFC9648, October Model for TCP", RFC 9648, DOI 10.17487/RFC9648, October
2024, <https://www.rfc-editor.org/info/rfc9648>. 2024, <https://www.rfc-editor.org/info/rfc9648>.
[RFC9870] Boucadair, M. and T. Reddy.K, "Export of UDP Options
Information in IP Flow Information Export (IPFIX)",
RFC 9870, DOI 10.17487/RFC9870, October 2025,
<https://www.rfc-editor.org/info/rfc9870>.
[To18] Touch, J. D., "A TCP Authentication Option Extension for [To18] Touch, J. D., "A TCP Authentication Option Extension for
Payload Encryption", Work in Progress, Internet-Draft, Payload Encryption", Work in Progress, Internet-Draft,
draft-touch-tcp-ao-encrypt-09, 19 July 2018, draft-touch-tcp-ao-encrypt-09, 19 July 2018,
<https://datatracker.ietf.org/doc/html/draft-touch-tcp-ao- <https://datatracker.ietf.org/doc/html/draft-touch-tcp-ao-
encrypt-09>. encrypt-09>.
[To24] Touch, J. D., "The UDP Authentication Option", Work in [To24] Touch, J. D., "The UDP Authentication Option", Work in
Progress, Internet-Draft, draft-touch-tsvwg-udp-auth-opt- Progress, Internet-Draft, draft-touch-tsvwg-udp-auth-opt-
00, 3 March 2024, <https://datatracker.ietf.org/doc/html/ 00, 3 March 2024, <https://datatracker.ietf.org/doc/html/
draft-touch-tsvwg-udp-auth-opt-00>. draft-touch-tsvwg-udp-auth-opt-00>.
skipping to change at line 2579 skipping to change at line 2579
Appendix A. Implementation Information Appendix A. Implementation Information
The following information is provided to encourage consistent naming The following information is provided to encourage consistent naming
for API implementations. for API implementations.
System-level variables (sysctl): System-level variables (sysctl):
+=======================+=========+=======================+ +=======================+=========+=======================+
| Name | Default | Meaning | | Name | Default | Meaning |
+=======================+=========+=======================+ +=======================+=========+=======================+
| net.ipv4.udp_opt | 0 | UDP options available | | net.ipv4.udp_opt | 0 | UDP Options available |
+-----------------------+---------+-----------------------+ +-----------------------+---------+-----------------------+
| net.ipv4.udp_opt_ocs | 1 | Use OCS | | net.ipv4.udp_opt_ocs | 1 | Use OCS |
+-----------------------+---------+-----------------------+ +-----------------------+---------+-----------------------+
| net.ipv4.udp_opt_apc | 0 | Include APC | | net.ipv4.udp_opt_apc | 0 | Include APC |
+-----------------------+---------+-----------------------+ +-----------------------+---------+-----------------------+
| net.ipv4.udp_opt_frag | 0 | Fragment | | net.ipv4.udp_opt_frag | 0 | Fragment |
+-----------------------+---------+-----------------------+ +-----------------------+---------+-----------------------+
| net.ipv4.udp_opt_mds | 0 | Include MDS | | net.ipv4.udp_opt_mds | 0 | Include MDS |
+-----------------------+---------+-----------------------+ +-----------------------+---------+-----------------------+
| net.ipv4.udp_opt_mrds | 0 | Include MRDS | | net.ipv4.udp_opt_mrds | 0 | Include MRDS |
skipping to change at line 2615 skipping to change at line 2615
| net.ipv4.udp_opt_uexp | 0 | Include UEXP | | net.ipv4.udp_opt_uexp | 0 | Include UEXP |
+-----------------------+---------+-----------------------+ +-----------------------+---------+-----------------------+
Table 2 Table 2
Socket options (sockopt), cached for outgoing datagrams: Socket options (sockopt), cached for outgoing datagrams:
+==============+=============================+ +==============+=============================+
| Name | Meaning | | Name | Meaning |
+==============+=============================+ +==============+=============================+
| UDP_OPT | Enable UDP options (at all) | | UDP_OPT | Enable UDP Options (at all) |
+--------------+-----------------------------+ +--------------+-----------------------------+
| UDP_OPT_OCS | Use UDP OCS | | UDP_OPT_OCS | Use UDP OCS |
+--------------+-----------------------------+ +--------------+-----------------------------+
| UDP_OPT_APC | Enable UDP APC option | | UDP_OPT_APC | Enable UDP APC Option |
+--------------+-----------------------------+ +--------------+-----------------------------+
| UDP_OPT_FRAG | Enable UDP fragmentation | | UDP_OPT_FRAG | Enable UDP fragmentation |
+--------------+-----------------------------+ +--------------+-----------------------------+
| UDP OPT MDS | Enable UDP MDS option | | UDP OPT MDS | Enable UDP MDS Option |
+--------------+-----------------------------+ +--------------+-----------------------------+
| UDP OPT MRDS | Enable UDP MRDS option | | UDP OPT MRDS | Enable UDP MRDS Option |
+--------------+-----------------------------+ +--------------+-----------------------------+
| UDP OPT REQ | Enable UDP REQ option | | UDP OPT REQ | Enable UDP REQ Option |
+--------------+-----------------------------+ +--------------+-----------------------------+
| UDP OPT RES | Enable UDP RES option | | UDP OPT RES | Enable UDP RES Option |
+--------------+-----------------------------+ +--------------+-----------------------------+
| UDP_OPT_TIME | Enable UDP TIME option | | UDP_OPT_TIME | Enable UDP TIME Option |
+--------------+-----------------------------+ +--------------+-----------------------------+
| UDP OPT AUTH | Enable UDP AUTH option | | UDP OPT AUTH | Enable UDP AUTH Option |
+--------------+-----------------------------+ +--------------+-----------------------------+
| UDP OPT EXP | Enable UDP EXP option | | UDP OPT EXP | Enable UDP EXP Option |
+--------------+-----------------------------+ +--------------+-----------------------------+
| UDP_OPT_UCMP | Enable UDP UCMP option | | UDP_OPT_UCMP | Enable UDP UCMP Option |
+--------------+-----------------------------+ +--------------+-----------------------------+
| UDP_OPT_UENC | Enable UDP UENC option | | UDP_OPT_UENC | Enable UDP UENC Option |
+--------------+-----------------------------+ +--------------+-----------------------------+
| UDP OPT UEXP | Enable UDP UEXP option | | UDP OPT UEXP | Enable UDP UEXP Option |
+--------------+-----------------------------+ +--------------+-----------------------------+
Table 3 Table 3
Send/sendto parameters: Send/sendto parameters:
* (Same as sysctl, with different prefixes) * (Same as sysctl, with different prefixes)
Connection parameters (per-socket pair cached state, part UCB): Connection parameters (per-socket pair cached state, part UCB):
+==============+======================+ +==============+======================+
| Name | Initial Value | | Name | Initial Value |
+==============+======================+ +==============+======================+
| opts_enabled | net.ipv4.udp_opt | | opts_enabled | net.ipv4.udp_opt |
+--------------+----------------------+ +--------------+----------------------+
| ocs_enabled | net.ipv4.udp_opt_ocs | | ocs_enabled | net.ipv4.udp_opt_ocs |
+--------------+----------------------+ +--------------+----------------------+
Table 4 Table 4
NB: The JUNK option is included for debugging purposes and is not NB: The JUNK Option is included for debugging purposes and is not
intended to be enabled otherwise. intended to be enabled otherwise.
System variables: System variables:
net.ipv4.udp_opt_junk 0 net.ipv4.udp_opt_junk 0
System-level variables (sysctl): System-level variables (sysctl):
+=======================+=========+=====================+ +=======================+=========+=====================+
| Name | Default | Meaning | | Name | Default | Meaning |
skipping to change at line 2712 skipping to change at line 2712
| junk_len | 4 | | junk_len | 4 |
+--------------+-----------------------+ +--------------+-----------------------+
Table 7 Table 7
Acknowledgments Acknowledgments
This work benefitted from feedback from Erik Auerswald, Bob Briscoe, This work benefitted from feedback from Erik Auerswald, Bob Briscoe,
Ken Calvert, Ted Faber, Gorry Fairhurst (including OCS for errant Ken Calvert, Ted Faber, Gorry Fairhurst (including OCS for errant
middlebox traversal), C. M. Heard (editor of this document, including middlebox traversal), C. M. Heard (editor of this document, including
combining previous FRAG and LITE options into the new FRAG, as well combining previous FRAG and LITE Options into the new FRAG, as well
as Figure 12), Tom Herbert, Tom Jones, Mark Smith, Carl Williams, and as Figure 12), Tom Herbert, Tom Jones, Mark Smith, Carl Williams, and
Raffaele Zullo, as well as discussions on the IETF TSVWG and SPUD Raffaele Zullo, as well as discussions on the IETF TSVWG and SPUD
email lists. email lists.
This work was partly supported by USC/ISI's Postel Center. This work was partly supported by USC/ISI's Postel Center.
Authors' Addresses Authors' Addresses
Joe Touch Joe Touch
Independent Consultant Independent Consultant
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