Status of This Memo

This is an Internet Standards Track document.¶

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.¶

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9868.¶

Copyright Notice

Copyright (c) 2025 IETF Trust and the persons identified as the document authors. All rights reserved.¶

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶

Table of Contents

1. Introduction

Transport protocols use options as a way to extend their capabilities. TCP [RFC9293], the Stream Control Transmission Protocol (SCTP) [RFC9260], and the Datagram Congestion Control Protocol (DCCP) [RFC4340] include space for these options, but UDP [RFC0768] currently does not. This document updates RFC 768 with an extension to UDP that provides space for transport options including their generic syntax and semantics for their use in UDP's stateless, unreliable message protocol. The details of the impact on RFC 768 are provided in Section 21. This extension does not apply to UDP-Lite, as discussed further in Section 17.¶

2. Conventions Used in This Document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

In this document, the characters ">>" preceding an indented line(s) indicate a statement using the key words listed above. This convention aids reviewers in quickly identifying or finding the portions of this RFC covered by these key words.¶

3. Terminology

The following terminology is used in this document:¶

IP datagram [RFC0791] [RFC8200]:

An IP packet, composed of the IP header (including any IPv4 options) and an IP payload area (including any IPv6 extension headers or other shim headers).¶

Must-support options:

UDP options that all implementations are required to support. Their use in individual UDP packets is optional.¶

SAFE options:

UDP options that are designed to be safe to ignore for a receiver that does not understand them. Such options do not alter the UDP user data or signal a change in what its contents represent.¶

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 and UDP port number (most widely known from TCP [RFC0793]).¶

Surplus area:

The area of an IP payload that follows a UDP packet; this area is used for UDP options in this document.¶

UDP packet:

The more contemporary term used herein to refer to a user datagram [RFC0768].¶

UDP fragment:

One or more components of a UDP packet and its UDP options that enable transmission over multiple IP payloads, larger than permitted by the maximum size of a single IP packet; note that each UDP fragment is itself transmitted as a UDP packet with its own options.¶

(UDP) User data:

The user data field of a UDP packet [RFC0768].¶

UDP Length:

The length field of a UDP header [RFC0768].¶

UNSAFE options:

UDP options that are not designed to be safe for a receiver that does not understand them to ignore. Such options could alter the UDP user data or signal a change in what its contents represent, but there are restrictions on how they can be transmitted; these restrictions are noted in Sections 10 and 12.¶

User:

The upper layer application, protocol, or service that produces and consumes content that UDP transfers.¶

User datagram:

A UDP packet, composed of a UDP header and UDP payload; as discussed herein, that payload need not extend to 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 datagram is redefined, where a UDP datagram can span more than one IP datagram through UDP fragmentation.¶

4. Background

Many protocols include a default, invariant header and an area for header options that varies from packet to packet. These options enable the protocol to be extended for use in particular environments or in ways unforeseen by the original designers. Examples include TCP's Maximum Segment Size (MSS), Window Scale, Timestamp, and Authentication Options [RFC9293] [RFC5925] [RFC7323].¶

Header options are used both in stateful (connection-oriented, e.g., TCP [RFC9293], SCTP [RFC9260], and DCCP [RFC4340]) and stateless (connectionless, e.g., IPv4 [RFC0791] and IPv6 [RFC8200]) protocols. In stateful protocols, they can help extend the way in which state is managed. In stateless protocols, their effect is often limited to individual packets, but they can have an aggregate effect on a sequence of packets as well.¶

UDP is one of the most popular protocols that lacks space for header options [RFC0768]. The UDP header was intended to be a minimal addition to IP, providing only port numbers and a checksum for error detection. This document extends UDP to provide a trailer area for such options, located after the UDP user data.¶

UDP options are possible because UDP includes its own length field, separate from that of the IP header. Other transport protocols infer transport payload length from the IP datagram length (TCP, DCCP, and SCTP). Internet historians have suggested a number of possible 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 length of the UDP user data as distinct from zero padding required for systems that require writes that are not byte-aligned. These suggestions are not consistent with earlier versions of UDP or with the concurrent design of multi-segment, multiplexing protocols; however, the real reason remains unknown. Regardless, this field presents an opportunity to differentiate the UDP user data from the implied transport payload length, which this document leverages to support a trailer options field.¶

There are other ways to include additional header fields or options in protocols that otherwise are not extensible. In particular, in- band encoding can be used to differentiate transport payload from additional fields, such as was proposed in [Hi15]. This approach can cause complications for interactions with legacy devices and is thus not considered further in this document.¶

IPv6 Teredo extensions (TEs) [RFC4380] [RFC6081] use a similar inconsistency between UDP and IPv6 packet lengths to support 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. 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 are compatible with UDP options.¶

5. UDP Option Intended Uses

UDP options can be used to provide a soft control plane to UDP. They enable capabilities available in other transport protocols, such as fragmentation and reassembly, that enable UDP frames larger than the IP MTU to traverse devices that rely on transport ports, e.g., Network Address Translations (NATs), without additional mechanisms or state. They add features that could, in the future, protect transport integrity and validate source identity (authentication), as well as those that could encrypt the user payload while still protecting the UDP transport header -- unlike Datagram Transport Layer Security (DTLS) [RFC9147]. They also enable Packetization Layer Path MTU Discovery (PLPMTUD) over UDP, known as Datagram Packetization Layer Path Maximum Transmission Unit Discovery (DPLPMTUD) [RFC9869], providing a means for probe packet validation without affecting the user data plane, as well as providing explicit indication of the receiver transport reassembly size.¶

UDP originally assumed that such capabilities would be provided by the user or by a layer above UDP [RFC0768]. However, enough protocols have evolved to use UDP directly, so such an intermediate layer would be difficult to deploy for legacy applications. UDP options leverage the opportunity presented by the surplus area to enable these extensions within the UDP transport layer itself. Among the use cases where this approach could be of benefit are request- response protocols such as DNS over UDP [He24].¶

6. UDP Option Design Principles

UDP options have been designed based on the following core principles. Each is an observation about (preexisting) UDP [RFC0768] in the absence of these extensions that this document does not intend to change or a lesson learned from other protocol designs.¶

1. UDP is stateless; UDP options do not change that fact.¶

   The state required or maintained by the endpoints is intended to be managed either by the application or a layer/library on behalf of the application. Reassembly of fragments is the only limited exception where this document introduces a notion of state to UDP.¶

2. UDP is unidirectional; UDP options do not change that fact.¶

   Responses to options are initiated by the application or a layer/library on behalf of the application. A mechanism that requires bidirectionality needs to be defined in a separate document.¶

3. UDP options have no length limit separate from that of the UDP packet itself.¶

   Past experience with other protocols confirms that static length limits will always need to be exceeded, e.g., as has been an issue with TCP options and IPv4 addresses. Each implementation can limit how long/many options there are, but a specification is more robust when it does not introduce such a limit.¶

4. UDP options are not intended to replace or replicate other protocols.¶

   This includes NTP, ICMP (notably echo), etc. UDP options are intended to introduce features useful for applications, not to either replace these other protocols nor instrument UDP to replace the need for network testing devices.¶

5. UDP options are a framework, not a protocol.¶

   Options can be defined in this initial document even when the details are not sufficient to specify a complete protocol. Uses of such options could then be described or supplemented in other documents. Examples herein include REQ/RES and TIME; in both cases, the option format is defined, but the protocol that uses these is specified elsewhere (REQ/RES for DPLPMTUD [RFC9869]) or left undefined (TIME).¶

6. The UDP option mechanism and UDP options themselves are intended to default to the same behavior experienced by a legacy receiver.¶

   By default, even when option checksums (OCS, APC), authentication, or decryption fail, all received packets (with the exception of UDP fragments) are passed (possibly with an empty data payload) to the user application. Options that do not modify user data are intended to (by default) result in the user data also being passed, even if, e.g., option checksums or authentication fails. It is always the user's or application's obligation to override this default behavior explicitly.¶

These principles are intended to enable the design and use of UDP options with minimal impact to legacy UDP endpoints, preferably none. UDP is -- and remains -- a minimal transport protocol. Additional capability is explicitly activated by user applications or libraries acting on their behalf.¶

Finally, UDP options do not attempt to match the number of zero- length UDP datagrams received by legacy and option-aware receivers from a source using UDP fragmentation (see Section 11.4). Legacy receivers interpret every UDP fragment as a zero-length packet (because they do not perform reassembly), but option-aware receivers would reassemble the packet as a non-zero-length packet. Zero-length UDP packets have been used as "liveness" indicators (see Section 5 of [RFC8085]), but such use is dangerous because they lack unique identifiers (the IPv6 base header has none, and the IPv4 ID field is deprecated for such use [RFC6994]).¶

7. The UDP Option Area

The UDP transport header includes demultiplexing and service identification (port numbers), an error detection checksum, and a field that indicates the UDP datagram length (including UDP header). The UDP Length field is typically redundant with the size of the maximum space available as a transport protocol payload, as determined by the IP header (see details in Section 18). The UDP option area is created when the UDP Length indicates a smaller transport payload than implied by the IP header.¶

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 indicated in the IP header (in 4-byte words) as the "Internet Header 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., when intervening shim headers are present such as IP Security (IPsec) [RFC4301] or for IP Payload Compression (IPComp) [RFC3173].¶

The upper bound for UDP Length when Protocol = 17 is given by:¶

   UDP_Length <= IPv4_Total_Length - IPv4_IHL * 4

If shim headers are present, this upper bound must be reduced by the sum of the lengths of shim headers that precede the UDP header.¶

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version|  IHL  |   DSCP    |ECN|          Total Length         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Identification        |Flags|      Fragment Offset    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Time to Live | Proto=17 (UDP)|        Header Checksum        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       Source Address                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Destination Address                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... zero or more IP Options (using space as indicated by IHL) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...     zero or more shim headers (each indicating size)      ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         UDP Source Port       |     UDP Destination Port      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          UDP Length           |         UDP Checksum          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

For IPv6, the IP Payload Length field indicates the transport payload after the base IPv6 header, which includes the IPv6 extension headers and space available for the transport protocol, as shown in Figure 2 [RFC8200]. Note that the Next Header field in IPv6 might not indicate UDP (i.e., 17), e.g., when intervening IP extension headers are present. For IPv6, the lengths of any additional IP extensions are indicated within each extension [RFC8200], so the upper bound for UDP Length is given by:¶

  UDP_Length <= IPv6_Payload_Length - sum(extension header lengths)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| Traffic Class |             Flow Label                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Payload Length        |  Next Header  |   Hop Limit   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
|                       Source Address (128 bits)               |
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
|                    Destination Address (128 bits)             |
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... zero or more IP Extension headers (each indicating size)  ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         UDP Source Port       |     UDP Destination Port      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          UDP Length           |         UDP Checksum          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

In both cases, the space available for the UDP packet is indicated by IP, either directly in the base header or by adding information in the shim headers or extensions. In either case, this document will refer to this available space as the "IP transport payload".¶

As a result of this redundancy, there is an opportunity to use the UDP Length field as a way to break up the IP transport payload into two areas -- that intended as UDP user data and an additional "surplus area" (as shown in Figure 3).¶

                         IP transport payload
          <------------------------------------------------->
+--------+---------+----------------------+------------------+
| IP Hdr | UDP Hdr |     UDP user data    |   surplus area   |
+--------+---------+----------------------+------------------+
          <------------------------------>
                     UDP Length

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 a requirement of UDP [RFC0768] (discussed further in Section 18). This document uses the surplus area for UDP options.¶

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 the UDP Length field as a "trailer options offset".¶

8. The UDP Surplus Area Structure

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 2-byte Option Checksum (OCS) field aligned to the first 2- byte boundary (relative to the start of the IP datagram) of that area, 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 UDP Length of 8), as long as there remains enough space for the aligned OCS and the options used.¶

>> UDP options MAY begin at any UDP length offset.¶

>> 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 area silently discarded.¶

These alignment bytes, coupled with OCS as computed over the 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 avoid the need for byte-swapping in its implementation.¶

The OCS contains an optional one's complement sum that detects errors in the surplus area, which is not otherwise covered by the UDP checksum, as detailed in Section 9.¶

The remainder of the surplus area consists of options, all except two of which are defined using a TLV (type, length, and optional value) syntax similar to that of TCP [RFC9293], as detailed in Section 10 (types No Operation (NOP) and End of Options List (EOL) have an implicit length of one byte). These options continue until the end of the surplus area or can end earlier using the EOL option, followed by zeroes (discussed further in Section 10).¶

9. The Option Checksum (OCS)

The Option Checksum (OCS) option is a conventional Internet checksum [RFC0791] that detects errors in the surplus area. The OCS option contains a 16-bit checksum that is aligned to the first 2-byte boundary, preceded by zeroes for padding (if needed), as shown in Figure 4.¶

                +--------+--------+--------+--------+
                |         UDP data         |    0   |
                +--------+--------+--------+--------+
                |       OCS       |  UDP options... |
                +--------+--------+--------+--------+

The OCS consists of a 16-bit Internet checksum [RFC1071], computed 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 errors in a similar way that the UDP checksum protects the UDP user data (when not zero).¶

The primary purpose of the OCS is to detect existing nonstandard (i.e., non-option) uses of that area and accidental errors. It is not intended to detect attacks, as discussed further in Section 25. 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 with UDP options.¶

The design enables traversal of errant middleboxes that incorrectly compute the UDP checksum over the entire IP payload [Fa18] [Zu20], rather than only the UDP header and UDP payload (as indicated by the UDP header length). Because the OCS is computed over the surplus area and its length and then inverted, the OCS effectively negates 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 same in either case.¶

>> 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 indicated by a zero OCS value.¶

>> 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 detecting nonstandard uses of the option area (of which there are none currently known).¶

Like the UDP checksum, the OCS is optional under certain circumstances and contains zero when not used. UDP checksums can be zero for IPv4 [RFC0791] and for IPv6 [RFC8200] when the UDP payload is already covered by another checksum, as might occur for tunnels [RFC6935]. The same exceptions apply to the OCS when used to detect bit errors; an additional exception occurs for its use in the UDP datagram prior to fragmentation or after reassembly (see Section 11.4).¶

The benefits are similar to allowing UDP checksums to be zero, but the risks differ. The OCS is additionally important to ensure packets with UDP options can traverse errant middleboxes [Zu20]. 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 risks can safely be ignored, such as controlled environments, over paths where traversal is validated, or where upper layer protocols (applications, libraries, etc.) can adapt (by enabling the OCS when 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 disabled, e.g., to conserve energy or processing resources or when 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 in that case.¶

The OCS covers the surplus area as formatted for transmission and is processed immediately upon reception.¶

>> If the receiver validation of the OCS fails, all options MUST be ignored and the surplus area silently discarded.¶

>> UDP user data that is validated by a correct UDP checksum MUST by default be delivered to the application layer, even if the OCS fails, unless the endpoints have negotiated otherwise for this UDP packet's socket pair.¶

When not used (i.e., containing zero), the OCS is assumed to be "correct" for the purpose of accepting UDP datagrams at a receiver (see Section 14).¶

10. UDP Options

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 of Options List (EOL) described below.¶

                +--------+--------+-------
                |  Kind  | Length | (remainder of option...)
                +--------+--------+-------

The Kind field is always one byte and is named after the corresponding TCP field (though other protocols refer to this as "Type"). The Length field, which indicates the length in bytes of the entire option, including Kind and Length, is one byte for all lengths below 255 (including the Kind and Length bytes). A Length of 255 indicates use of the UDP option extended format shown in 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 field or Extended Length field, whichever is applicable.¶

                +--------+--------+--------+--------+
                |  Kind  |  255   | Extended Length |
                +--------+--------+--------+--------+
                | (remainder of option...)          |
                +--------+--------+--------+--------+

>> 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 values outside this range MUST be silently dropped as invalid and logged.¶

>> All logging SHOULD be rate limited. Excess logging events can be coalesced and reported as a count or can be silently dropped if needed to avoid resource overloading.¶

>> Option Lengths (or Extended Lengths, where applicable) smaller than the minimum for the corresponding Kind MUST be treated as an error. Such errors call into question the remainder of the surplus area and thus MUST result in all UDP options being silently discarded.¶

>> 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 EOL never use either length format.¶

>> Any UDP option whose length is larger than 254 MUST use the UDP option extended format shown in Figure 6.¶

>> For compactness, UDP options SHOULD use the smallest option format possible.¶

>> 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 which they appear in the UDP fragment option area (see Section 11.4).¶

The following UDP options are currently defined:¶

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 legacy endpoints or when the option is unsupported. Options indicated by Kind values in the range 192..255 are known as UNSAFE options because they might interfere with use by legacy receiving endpoints (e.g., an option that alters the UDP data payload).¶

UNSAFE option nicknames are expected to begin with capital "U", which needs to be avoided for SAFE option nicknames (see Section 26). RESERVED and RESERVED-UNSAFE are not assignable by IANA and not otherwise defined at this time. The AUTH, UCMP, and UENC reservations are intended for all future options supporting authentication, compression, and encryption, respectively, and remain reserved until assigned for those uses.¶

Although the FRAG option modifies the original UDP payload contents (i.e., is UNSAFE with respect to the original UDP payload), it is used only in subsequent fragments with zero-length UDP user data payloads, thus is SAFE in actual use, as discussed further in Section 11.4.¶

These options are defined in the following subsections. Options 0 and 1 use the same values as for TCP.¶

>> An endpoint supporting UDP options MUST support those marked with an "*" above: EOL, NOP, APC, FRAG, MDS, MRDS, REQ, and RES. This includes both recognizing and being able to generate these options if configured to do so. These are called "must-support" options.¶

The set of must-support options is defined herein. New options are not eligible for this designation.¶

>> All other SAFE options (without an "*") MAY be implemented, and their use SHOULD be determined either out-of-band or negotiated, notably if needed to detect when options are silently ignored by legacy receivers.¶

>> Receivers supporting UDP options MUST silently ignore unknown or 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 not indicate (one of) the value(s) stated in the option's specification. A malformed FRAG option is an exception to this rule; it SHALL be treated as an unsupported UNSAFE option.¶

>> Options with inherently invalid Length field values, i.e., those that indicate underruns of the option itself or overruns of the surplus area (pointing past the end of the IP payload), MUST be treated as an indication of a malformed surplus area, and all options MUST silently be discarded.¶

Receivers cannot generally treat unexpected option lengths as invalid, as this would unnecessarily limit future revision of options (e.g., defining a new APC that is defined by having a different length).¶

>> When UNSAFE options are present, the UDP user data MUST be empty, and any transport payload MUST be contained in a FRAG option (see Section 11.4). Recall that such options may alter the transport payload or signal a change in what its contents represent. This restriction ensures their safe use in environments that might include legacy receivers (see Section 12), because the transport payload occurs inside the FRAG option area and is silently discarded by legacy receivers.¶

>> Receivers supporting UDP options that receive unsupported options in the UNSAFE range MUST terminate all option processing and MUST silently drop all UDP options in that datagram. See Section 12 for further discussion of UNSAFE options.¶

>> 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 these four occurs more than once, a receiver MUST interpret only the first instance of that option and MUST ignore later instances. Section 25 provides additional advice for Denial of Service (DoS) issues that involve large numbers of options, whether valid, unknown, or repeating.¶

>> NOP MAY occur multiple times, either in succession or between other options, for option alignment. Additional repetition constraints are indicated in Section 11.2.¶

>> If FRAG occurs more than once, the options area MUST be considered malformed and MUST NOT be processed.¶

>> EXP and UEXP MAY occur more than once but SHOULD NOT occur more than once using the same Experimental ID (ExID) (see Sections 11.10 and 12.3).¶

>> Options other than OCS, AUTH, and UENC MUST NOT include fields whose values depend on the contents of the surplus area.¶

AUTH and UENC are always computed as if their hash and the OCS are zero; the OCS is always computed as if its contents are zero and after the AUTH or UENC hash has been computed.¶

>> Future options MUST NOT be defined as having an option field value 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 option (presumably EOL).¶

If future options were to depend on the contents or presence of other options, interactions between those values, the OCS, and the AUTH and UENC options could be unpredictable. This does not prohibit options that modify later options (in order of appearance within a packet), such as would typically be the case for compression (UCMP).¶

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 new UDP option over that area instead. Using an option for this purpose is safer than treating the region as an exception, because its use can be verified based on option Kind and Length.¶

>> AUTH and UENC MUST NOT be used concurrently.¶

AUTH and UENC are never used together because UENC would serve both purposes.¶

>> "Must-support" options other than NOP and EOL MUST be placed by the transmitter before other SAFE UDP options. A receiver MAY drop all UDP options if this ordering is not honored. Such events MAY be logged for diagnostic purposes.¶

The requirement that must-support options come before others is intended to allow for endpoints to implement DoS protection, as discussed further in Section 25.¶

11. SAFE UDP Options

SAFE UDP options can be silently ignored by legacy receivers without 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 unusable by legacy receivers (Section 12). The following subsections describe SAFE options defined in this document.¶

                               +--------+
                               | Kind=0 |
                               +--------+

                            +--------+
                            | Kind=1 |
                            +--------+

                +--------+--------+--------+--------+
                | Kind=2 | Len=6  |    CRC32c...    |
                +--------+--------+--------+--------+
                |  CRC32c (cont.) |
                +--------+--------+

                +--------+--------+--------+--------+
                | Kind=3 | Len=10 |   Frag. Start   |
                +--------+--------+--------+--------+
                |           Identification          |
                +--------+--------+--------+--------+
                |  Frag. Offset   |
                +--------+--------+

               +--------+--------+--------+--------+
               | Kind=3 | Len=12 |   Frag. Start   |
               +--------+--------+--------+--------+
               |           Identification          |
               +--------+--------+--------+--------+
               |  Frag. Offset   |Reass DgOpt Start|
               +--------+--------+--------+--------+

       Constituent UDP Fragments         Original UDP Datagram

     +-------------+------------+
     | Src Port    | Dst Port   |
     +-------------+------------+
     | UDP Len (8) | UDP Chksum |
     +-------------+------------+
     |     OCS     | K=3   L=10 |      +-------------+------------+
     +-------------+------------+      | Src Port    | Dst Port   |
  ,--| Frag. Start | Identifi-  ~      +-------------+------------+
  |  +-------------+------------+      | UDP L.(RDOS)| UDP Chksum |
  |  ~ cation      | Frag. Off. |----->+-------------+------------+
  |  +-------------+------------+      | Frag Data from 1st Frag. |
  |  ~ Per-Fragment Options     ~      |             .            |
  '->+-------------+------------+      ~             .            ~
     ~      Fragment Data       ~      |             .            |
     +-------------+------------+  ,-->+-------------+------------+
                                   |   | Frag Data from 2nd Frag. |
     +-------------+------------+  |   |             .            |
     | Src Port    | Dst Port   |  |   ~             .            ~
     +-------------+------------+  |   |             .            |
     | UDP Len (8) | UDP Chksum |  | ,>+-------------+------------+
     +-------------+------------+  | | |     OCS     | UDP Options|
     |     OCS     | K=3   L=12 |  | | +-------------+            +
     +-------------+------------+  | | ~             .            ~
  ,--| Frag. Start | Identifi-  ~  | | +-------------+------------+
  |  +-------------+------------+  | |
  |  ~ cation      | Frag. Off. |--' |
  |  +-------------+------------+    |
  |  |  RDOS       | Frag.Opts. |    |
  '->+--|----------+------------+    |
     ~  |   Fragment Data       ~    |
     +--|----------+------------+    |
        |                            |
        '----------------------------'

                +--------+--------+--------+--------+
                | Kind=4 | Len=4  |    MDS size     |
                +--------+--------+--------+--------+

            +--------+--------+--------+--------+---------+
            | Kind=5 | Len=5  |    MRDS size    |MRDS segs|
            +--------+--------+--------+--------+---------+

                +--------+--------+-----------------+
                |  Kind  | Len=6  |      token      |
                +--------+--------+-----------------+
                  1 byte   1 byte       4 bytes

     +--------+--------+------------------+------------------+
     | Kind=8 | Len=10 |      TSval       |      TSecr       |
     +--------+--------+------------------+------------------+
       1 byte   1 byte       4 bytes            4 bytes

            +----------+----------+----------+----------+
            | Kind=127 |   Len    |      UDP ExID       |
            +----------+----------+----------+----------+
            |  (option contents, as defined)...         |
            +----------+----------+----------+----------+

            +----------+----------+----------+----------+
            | Kind=127 |   255    |   Extended Length   |
            +----------+----------+----------+----------+
            |      UDP ExID       |(option contents...) |
            +----------+----------+----------+----------+

12. UNSAFE Options

UNSAFE options are not safe to ignore and can be used unidirectionally or without soft-state confirmation of UDP option capability. They are always used only when the user data occurs inside a reassembled set of one or more UDP fragments, such that if UDP fragmentation is not supported, the enclosed UDP user data would be silently dropped anyway.¶

>> Applications using UNSAFE options SHOULD NOT also use zero-length UDP packets as signals, because they will arrive when UNSAFE options fail. Those that choose to allow such packets MUST account for such events.¶

>> UNSAFE options MUST be used only as part of UDP fragments, used either per-fragment or after reassembly.¶

>> Receivers supporting UDP options MUST silently drop the UDP user data of the reassembled datagram if any fragment or the entire datagram includes an UNSAFE option whose UKind is not supported or if an UNSAFE option appears outside the context of a fragment or reassembled fragments.¶

13. Rules for Designing New Options

The UDP option Kind space allows for the definition of new options; however, the currently defined options (including AUTH, UENC, and UCMP) do not allow for arbitrary new options. The following is a summary of rules for new options and their rationales:¶

>> New options MUST NOT be defined as "must-implement", i.e., they are not eligible for the asterisk ("*") designation used in Section 10.¶

This document defines the minimum set of "must-implement" UDP options. All new options are included at the discretion of a given implementation.¶

>> New options MUST NOT modify the content of options that precede them (in order of appearance and thus processing).¶

>> The fields of new options MUST NOT depend on the content of other options.¶

UNSAFE options can both depend on and vary user data content because they are contained only inside UDP fragments and thus are processed only by receivers capable of handling UDP options.¶

>> New options MUST NOT declare their order relative to other options, whether new or old, even as a preference.¶

>> At the sender, new options MUST NOT modify UDP packet content anywhere except within their option field, except only those contained within the UNSAFE option; areas that need to remain unmodified include the IP header, IP options, UDP user data, and surplus area (i.e., other options).¶

>> Options MUST NOT be modified in transit. This includes those already defined as well as new options.¶

>> New options MUST NOT require or allow that any UDP options (including themselves) or the remaining surplus area be modified in transit.¶

>> All options MUST indicate whether they can be used per-fragment and, if so, MUST also indicate how their success or failure is reported to the user. This document RECOMMENDS that options be useful per-fragment and also RECOMMENDS that options used per- fragment be reported to the user as a finite aggregate (e.g., a sum, a flag, etc.) rather than individually.¶

Note that only certain of the initially defined options violate these rules:¶

• >> The FRAG option modifies UDP user data, splitting it across multiple IP packets. UNSAFE options MAY modify the UDP user data, e.g., by encryption, compression, or other transformations. All other (SAFE) options MUST NOT modify the UDP user data.¶

14. Option Inclusion and Processing

The following rules apply to option inclusion by senders and processing by receivers.¶

>> Senders MAY add any option, as configured by the API.¶

>> All "must-support" options MUST be processed by receivers, if present (presuming UDP options are supported at that receiver).¶

>> Non-"must-support" options MAY be ignored by receivers, if present, e.g., based on API settings.¶

>> All options MUST be processed by receivers in the order encountered in the options area.¶

>> Unless configuration settings direct otherwise, all options except UNSAFE options MUST result in the UDP user data being passed to the upper layer protocol or application, regardless of whether all options are processed, are supported, or succeed.¶

The basic premise is that, for options-aware endpoints, the sender decides what options to add and the receiver decides what options to handle. Simply adding an option does not force work upon a receiver, with the exception of the "must-support" options.¶

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 whether to accept or ignore some of its options as follows (in order):¶

        if the UDP checksum fails then
            silently drop the entire UDP packet (per RFC 1122)
        if the UDP checksum passes or is zero then
            if (OCS != 0 and OCS fails) or
               (OCS == 0 and UDP CS != 0) then
                deliver the UDP user data but ignore other options
                (this is required to emulate legacy behavior)
            if (OCS != 0 and OCS passes) or
               (OCS == 0 and UDP CS == 0) then
                deliver the UDP user data after parsing
                and processing the rest of the options,
                regardless of whether each is supported or succeeds
                (again, this is required to emulate legacy behavior)

The design of the UNSAFE options ensures that the resulting UDP data will be silently dropped in both legacy receivers and options-aware receivers that do not recognize those options. Again, note that this still results in the delivery of a zero-length UDP packet.¶

Options-aware receivers can drop UDP packets with option processing errors via either an override of the default UDP processing or at the application layer.¶

That is, all options are treated the same, in that the transmitter can add it as desired and the receiver has the option to require it or not. Only if it is required (e.g., by API configuration) would the receiver require it being present and correct.¶

That is, for all options:¶

• if the option is not required by the receiver, then UDP packets missing the option are accepted.¶

• if the option is required (e.g., by override of the default behavior at the receiver) and missing or incorrectly formed, silently drop the UDP packet.¶

• if the UDP packet is accepted (either because the option is not required or because it was required and correct), then pass the option with the UDP packet via the API. Note that FRAG, NOP, and EOL are not passed to the user (see Section 15).¶

>> Any options whose length exceeds that of the UDP packet (i.e., intending to use data that would have been beyond the surplus area) SHOULD be silently ignored (again to model legacy behavior).¶

15. UDP API Extensions

UDP currently specifies an Application Programming Interface (API), summarized as follows (with Unix-style command as an example) [RFC0768]:¶

• Method to create new receive ports¶

  • e.g., bind(handle, recvaddr(optional), recvport)¶

• Receive, which returns data octets, source port, and source address¶

  • e.g., recvfrom(handle, srcaddr, srcport, data)¶

• Send, which specifies data, source and destination addresses, and source and destination ports¶

  • e.g., sendto(handle, destaddr, destport, data)¶

This API is extended to support options as follows:¶

• Extend the method to create receive ports to include per-packet and per-fragment receive options that are required or omitted as indicated by the application.¶

  >> Datagrams not containing these required options MUST be silently dropped and SHOULD be logged.¶

• Extend the method to create receive ports to have a means to indicate that all packets containing UDP options that are received on a particular socket pair are to be discarded.¶

  >> The default value for the setting to drop all packets containing UDP options MUST be to process packets containing UDP options normally (i.e., not to discard them).¶

• Extend the receive function to indicate the per-packet options and their parameters as received with the corresponding received datagram. Note that per-fragment options are handled within the processing of each fragment.¶

  >> Options and their processing status (success/fail) MUST be available to the user (i.e., application layer or upper layer protocol/service), both for the packet and for the fragment set, except for FRAG, NOP, and EOL; those three options are handled within UDP option processing only. As a reminder (from Section 14), all options except UNSAFE options MUST result in the UDP user data being passed to the application layer (unless overridden in the API), regardless of whether all options are processed, supported, or succeed.¶

• For fragments, success for an option is reported only when all fragments succeed for that option.¶

  >> Per-fragment option status reporting SHOULD default as needed (e.g., not computed and/or not passed up to the upper layers) to minimize overhead unless actively requested (e.g., by the user/application layer).¶

  >> SAFE options associated with fragments are accumulated when associated with the reassembled packet; values MAY be coalesced, e.g., to indicate that only an AUTH failure of a fragment occurred, rather than not indicating the AUTH status of each fragment.¶

• Extend the send function to indicate the options to be added to the corresponding sent datagram. This includes indicating which options apply to individual fragments vs. which apply to the UDP packet prior to fragmentation, if fragmentation is enabled. This includes a minimum datagram length, such that the options list ends in EOL and additional space is zero-filled as needed. It also includes a maximum fragment size, e.g., as discovered by DPLPMTUD, whether implemented at the application layer per [RFC8899] or in conjunction with other UDP options [RFC9869].¶

Examples of API instances for Linux and FreeBSD are provided in Appendix A to encourage uniform cross-platform implementations.¶

APIs are not intended to provide user control over option order, especially on a per-packet basis, as this could create a covert channel (see Section 25). Similarly, APIs are not intended to provide user/application control over UDP fragment boundaries on a per-packet basis; although, they are expected to allow control over which options, including fragmentation, are enabled (or disabled) on a per-packet basis. Such control over fragmentation is critical to DPLPMTUD.¶

16. UDP Options Are for Transport, Not Transit

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 the UDP payload, and as such, it might be tempting to consider whether this is a generally useful approach to extending IP.¶

Unfortunately, the surplus area exists only for transports that include their own transport layer payload length indicator. TCP and SCTP include header length fields that already provide space for transport options by indicating the total length of the header area, such that the entire remaining area indicated in the network layer (IP) is the transport payload. UDP-Lite already uses the UDP Length field to indicate the boundary between data covered by the transport 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 [RFC3828].¶

UDP options are transport options. They are no more (or less) appropriate to be modified in-transit than any other portion of the transport datagram.¶

>> Generally, transport headers, options, and data are not intended to be modified in-transit. UDP options are no exception and are specified here as "MUST NOT be altered in transit".¶

However, note that the UDP option mechanism provides no specific protection against in-transit modification of the UDP header, UDP payload, or surplus area, except as provided by the OCS or the options selected (e.g., AUTH or UENC).¶

Unless protected by encryption (e.g., UENC or via other layers, like IPsec), UDP options remain visible to devices on the network path. The decision to not require mandatory encryption for UDP options to prevent such visibility was made because the key distribution and management infrastructure necessary to support such encryption does not exist in many of the deployment scenarios of interest, notably those that use UDP directly as a stateless and connectionless transport protocol (e.g., see [He24]).¶

17. UDP Options vs. UDP-Lite

UDP-Lite provides partial checksum coverage so that UDP packets with errors in some locations can be delivered to the user [RFC3828]. It uses a different transport protocol number (136) than UDP (17) to interpret the UDP Length field as the prefix covered by the UDP checksum.¶

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 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 why a separate transport protocol number was required.¶

UDP options do not use or need a separate transport protocol number because the data beyond the UDP Length offset (surplus data) is not provided to the application by default. That data is interpreted exclusively within the UDP transport layer.¶

UDP-Lite cannot support UDP options, either as proposed here or in any other form, because the entire payload of the UDP packet is already defined as user data and there is no additional field in which to indicate a surplus area for options. The UDP Length field in UDP-Lite is already used to indicate the boundary between user data covered by the checksum and user data not covered.¶

18. Interactions with Legacy Devices

It has always been permissible for the UDP Length to be inconsistent with the IP transport payload length [RFC0768]. Such inconsistency has been utilized in UDP-Lite using a different transport number. There are no known systems that use this inconsistency for UDP [RFC3828]. It is possible that such use might interact with UDP options, i.e., where legacy systems might generate UDP datagrams that appear to have UDP options. The OCS provides protection against such events and is stronger than a static "magic number".¶

UDP options have been tested as interoperable with Linux, macOS, and Windows Cygwin and worked through NAT devices. These systems successfully delivered only the user data indicated by the UDP Length field and silently discarded the surplus area.¶

One reported embedded device passes the entire IP datagram to the UDP application layer. Although this feature could enable application-layer UDP option processing, it would require that conventional UDP user applications examine only the UDP user data.¶

This feature is also inconsistent with the UDP application interface [RFC0768] [RFC1122].¶

It has been reported that Alcatel-Lucent's "Brick" Intrusion Detection System has a default configuration that interprets inconsistencies between UDP Length and IP Length as an attack to be reported. Note that other firewall systems, e.g., Check Point, use a default "relaxed UDP length verification" to avoid falsely interpreting this inconsistency as an attack.¶

There are known uses of UDP exchanges of zero-length UDP user data packets, notably in the TIME protocol [RFC0868]. The need to support such packets is also noted in the UDP usage guidelines [RFC8085]. Some of the mechanisms in this document can generate more zero- length UDP packets for a UDP option-aware endpoint than for a legacy (non-aware) endpoint (e.g., based on some error conditions), and some can generate fewer (e.g., fragment reassembly). Because such packets inherently carry no unique transport header or transport content, endpoints are already expected to be tolerant of their (inadvertent) replication or loss by the network, so such variations are not expected to be problematic.¶

19. Options in a Stateless, Unreliable Transport Protocol

There are two ways to interpret options for a stateless, unreliable protocol -- an option is either local to the message or intended to affect a stream of messages in a soft-state manner. Either interpretation is valid for defined UDP options.¶

It is impossible to know in advance whether an endpoint supports a UDP option.¶

>> All UDP options other than UNSAFE ones MUST be ignored if not supported or upon failure (e.g., APC).¶

>> All UDP options that fail MUST result in the UDP data still being sent to the application layer by default to ensure equivalence with legacy devices.¶

UDP options that rely on soft-state exchange need to allow message reordering and loss, in the same way as UDP applications [RFC8085].¶

The above requirements prevent using any option that cannot be safely ignored unless it is hidden inside the FRAG area (i.e., UNSAFE options). Legacy systems also always need to be able to interpret the transport fragments as individual UDP packets.¶

20. UDP Option State Caching

Some TCP connection parameters, stored in the TCP Control Block (TCB), can be usefully shared either among concurrent connections or between connections in sequence, known as TCP Sharing [RFC9040]. Although UDP is stateless, some of the options proposed herein could have similar benefits in being shared or cached. We call this UCB sharing, or UDP Control Block sharing, by analogy. Just as TCB sharing is not a standard because it is consistent with existing TCP specifications, UCB sharing would be consistent with existing UDP specifications, including this one. Both are implementation issues that are outside the scope of their respective specifications, and so UCB sharing is outside the scope of this document.¶

21. Updates to RFC 768

This document updates [RFC0768] as follows:¶

• This document defines the meaning of the IP payload area beyond the UDP length but within the IP Length as the surplus area used herein for UDP options.¶

• This document extends the UDP API to support the use of UDP options.¶

22. Interactions with Other RFCs (and drafts)

This document clarifies the interaction between UDP Length and IP Length that is not explicitly constrained in either UDP or the host requirements [RFC0768] [RFC1122].¶

Teredo extensions (TEs) define use of a similar difference between these lengths for trailers [RFC4380] [RFC6081]. In [RFC6081], TE defines the length of an IPv6 payload inside UDP as pointing to less than the end of the UDP payload, enabling trailing options for that IPv6 packet:¶

...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 the UDP payload length (i.e., the Length value in the UDP header minus the UDP header size)¶

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, which could end before the enclosing IPv4 or IPv6 header indicates -- allowing UDP options in addition to the trailer options of the IPv6 payload. The result, if UDP options were used, is shown in Figure 19.¶

                         Outer IP Length
    <---------------------------------------------------------->
   +--------+---------+------------------------------+----------+
   | IP Hdr | UDP Hdr | IPv6 packet/len | TE trailer | surplus  |
   +--------+---------+------------------------------+----------+
                       <--------------->
                       Inner IPv6 Length
             <-------------------------------------->
                           UDP Length

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 for (but not limited to) IPv6 Jumbograms [RFC2675]. Note that although this technique is "Standard", the specification did not "update" UDP [RFC0768]. Another such case arises when UDP is proxied via HTTP [RFC9298], as the UDP header is omitted and only the UDP user data is transported.¶

This document is consistent with the UDP profile for RObust Header Compression (ROHC) [RFC3095], noted here:¶

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 after the UDP payload that is covered by the IP Length.¶

ROHC compresses UDP headers only when this match succeeds. It does not prohibit UDP headers where the match fails; in those cases, ROHC default rules (Section 5.10 of [RFC3095]) would cause the UDP header to remain uncompressed. Upon receipt of a compressed UDP header, Appendix A.1.3 of [RFC3095] indicates that the UDP length is "INFERRED"; in uncompressed packets, it would simply be explicitly provided.¶

This issue of handling UDP header compression is more explicitly described in more recent specifications, e.g., Section 10.10 of [RFC8724].¶

23. Multicast and Broadcast Considerations

UDP options are primarily intended for unicast use. Using these options over multicast or broadcast IP requires careful consideration, e.g., to ensure that the options used are safe for different endpoints to interpret differently (e.g., either to support or silently ignore) or to ensure that all receivers of a multicast or broadcast group confirm support for the options in use.¶

24. Network Management Considerations

UDP options use and configuration may be useful to track and manage remotely. IP Flow Information Export (IPFIX) [RFC7011] Information Elements for UDP options have been defined in [Bo24]. Similar to what has been done for TCP [RFC9648], a YANG model [RFC7950] for use by network management protocols (e.g., NETCONF [RFC6241] or RESTCONF [RFC8040]) may be developed. Development of these models is outside the scope of this document.¶

25. Security Considerations

There are a number of security issues raised by the introduction of options to UDP. Some are specific to this variant, but others are associated with any packet processing mechanism; all are discussed further in this section.¶

26. IANA Considerations

IANA has created the "User Datagram Protocol (UDP)" registry group, which consists of the "UDP Option Kind Numbers" registry and a pointer to the unified "TCP/UDP Experimental Option Experiment Identifiers (TCP/UDP ExIDs)" registry. Note that the "TCP experimental IDs (ExIDs)" registry has been renamed as the "TCP/UDP Experimental Option Experiment Identifiers (TCP/UDP ExIDs)" registry, and is a unified registry for both TCP and UDP ExIDs. IANA has added 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 can be used with TCP or their first 16 bits can be used with UDP. Use with each transport (TCP, UDP) is indicated in the protocol column, as defined in RFC 9868.¶

Initial values of the UDP Option Kind registry are as listed in Section 10, including those both assigned and reserved. Additional values in this registry are to be assigned from the Unassigned values in Section 10 by IESG Approval or Standards Action [RFC8126]. Those assignments are subject to the conditions set forth in this document, particularly (but not limited to) those in Section 13.¶

>> Although option nicknames are not used in-band, new UNSAFE option names MUST commence with the capital letter "U" and new SAFE options MUST NOT commence with either uppercase or lowercase "U".¶

IANA has added the following note to the "UDP Option Kind Numbers" indicating entries are mandatory to implement when UDP options are supported. No new options may be created that are mandatory to implement in all UDP options implementations.¶

Codepoints 0-7 MUST be supported on any implementation supporting UDP options. All others are supported at the discretion of each implementation.¶

UDP Experimental Option Experiment Identifiers (UDP ExIDs) are intended for use in a similar manner as TCP ExIDs [RFC6994]. Both TCP and UDP ExIDs are managed as a single, unified registry 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 maintain them separately. This new TCP/UDP ExIDs registry has entries for both transports, although each codepoint needs to be 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 in UDP. IANA has added a "Protocol" field to the registry and updated the current TCP ExIDs to be indicated as defined for TCP. New assignments are to indicate the transport for which it is defined.¶

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 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 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 UEXP options is required and thus do not need a larger codepoint value to decrease the probability of accidental occurrence with non-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 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 described for TCP [RFC6994].¶

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 and the acronym [RFC8126]. UDP options using these ExIDs are subject to the same conditions as new UDP options, i.e., they too are subject to the conditions set forth in this document, particularly (but not limited to) those in Section 13.¶

27. References

Appendix A. Implementation Information

The following information is provided to encourage consistent naming for API implementations.¶

System-level variables (sysctl):¶

Socket options (sockopt), cached for outgoing datagrams:¶

Send/sendto parameters:¶

• (Same as sysctl, with different prefixes)¶

Connection parameters (per-socket pair cached state, part UCB):¶

NB: The JUNK option is included for debugging purposes and is not intended to be enabled otherwise.¶

System variables:¶

net.ipv4.udp_opt_junk 0¶

System-level variables (sysctl):¶

Socket options (sockopt):¶

Connection parameters (per-socket pair cached state, part UCB):¶

Acknowledgments

This work benefitted from feedback from Erik Auerswald, Bob Briscoe, Ken Calvert, Ted Faber, Gorry Fairhurst (including OCS for errant middlebox traversal), C. M. Heard (editor of this document, including combining previous FRAG and LITE options into the new FRAG, as well as Figure 12), Tom Herbert, Tom Jones, Mark Smith, Carl Williams, and Raffaele Zullo, as well as discussions on the IETF TSVWG and SPUD email lists.¶

This work was partly supported by USC/ISI's Postel Center.¶

Authors' Addresses