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Transport Options for UDP

Document Type Active Internet-Draft (tsvwg WG)
Author Dr. Joseph D. Touch
Last updated 2024-06-11 (Latest revision 2024-03-21)
Replaces draft-touch-tsvwg-udp-options
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Revised I-D Needed - Issue raised by WGLC
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Dec 2023
Submit " Transport Options for UDP" as a Proposed Standard RFC
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TSVWG                                                           J. Touch
Internet Draft                                    Independent Consultant
Intended status: Standards Track                          March 21, 2024
Intended updates: 768
Expires: September 2024

                         Transport Options for UDP


   Transport protocols are extended through the use of transport header
   options. This document updates RFC 768 (UDP) by indicating the
   location, syntax, and semantics for UDP transport layer options
   within the surplus area after the end of the UDP user data but
   before the end of the IP length.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Internet-Drafts are draft documents valid for a maximum of six
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   at any time.  It is inappropriate to use Internet-Drafts as
   reference material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 21, 2024.

Copyright Notice

   Copyright (c) 2024 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

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   ( 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 ..................................................3
   2. Conventions used in this document .............................3
   3. Terminology ...................................................3
   4. Background ....................................................4
   5. UDP Option Intended Uses ......................................5
   6. UDP Option Design Principles ..................................6
   7. The UDP Option Area ...........................................7
   8. The UDP Surplus Area Structure ...............................10
   9. The Option Checksum (OCS) ....................................10
   10. UDP Options .................................................12
   11. SAFE UDP Options ............................................16
      11.1. End of Options List (EOL) ..............................17
      11.2. No Operation (NOP) .....................................17
      11.3. Additional Payload Checksum (APC) ......................18
      11.4. Fragmentation (FRAG) ...................................19
      11.5. Maximum Datagram Size (MDS) ............................26
      11.6. Maximum Reassembled Datagram Size (MRDS) ...............27
      11.7. Echo request (REQ) and echo response (RES) .............27
      11.8. Timestamps (TIME) ......................................28
      11.9. Authentication (AUTH), RESERVED Only ...................30
      11.10. Experimental (EXP) ....................................30
   12. UNSAFE Options ..............................................31
      12.1. UNSAFE Compression (UCMP) ..............................32
      12.2. UNSAFE Encryption (UENC) ...............................32
      12.3. UNSAFE Experimental (UEXP) .............................32
   13. Rules for designing new options .............................32
   14. Option inclusion and processing .............................33
   15. UDP API Extensions ..........................................35
   16. UDP Options are for Transport, Not Transit ..................37
   17. UDP options vs. UDP-Lite ....................................37
   18. Interactions with Legacy Devices ............................38
   19. Options in a Stateless, Unreliable Transport Protocol .......39
   20. UDP Option State Caching ....................................39
   21. Updates to RFC 768 ..........................................40
   22. Interactions with other RFCs (and drafts) ...................40
   23. Multicast Considerations ....................................41
   24. Security Considerations .....................................42

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   25. IANA Considerations .........................................44
   26. References ..................................................45
      26.1. Normative References ...................................45
      26.2. Informative References .................................46
   27. Acknowledgments .............................................49
   Appendix A. Implementation Information ..........................50

1. Introduction

   Transport protocols use options as a way to extend their
   capabilities. TCP [RFC9293], SCTP [RFC9260], and DCCP [RFC4340]
   include space for these options but UDP [RFC768] 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",
   "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)
   indicates 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:

   o  IP datagram [RFC791][RFC8200] - an IP packet, composed of the IP
      header and an IP payload area

   o  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.

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   o  UDP packet - the more contemporary term used herein to refer to a
      user datagram [RFC768]

   o  User - the upper layer application, protocol, or service that
      produces and consumes content that UDP transfers

   o  Surplus area - the area of an IP payload that follows a UDP
      packet; this area is used for UDP options in this document

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

   o  (UDP) User data - the user data field of a UDP packet [RFC768]

   o  UDP Length - the length field of a UDP header [RFC768]

   o  Must-support options - UDP options that all implementations are
      required to support. Their use in individual UDP packets is

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, Window Scale,
   Timestamp, and Authentication Options [RFC9293][RFC5925][RFC7323].

   Header options are used both in stateful (connection-oriented, e.g.,
   TCP [RFC9293], SCTP [RFC9260], DCCP [RFC4340]) and stateless
   (connectionless, e.g., IPv4 [RFC791], 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 [RFC768]. The UDP header was intended to be a minimal
   addition to IP, providing only ports and a checksum for error
   detection. This document extends UDP to provide a trailer area for
   such options, located after the UDP user data.

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   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,
   SCTP). There are a number of reasons why Internet historians suggest
   that 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 concurrent design of
   multi-segment multiplexing protocols, however, so 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

   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 [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 have no relation
   to the surplus area discussed in this document. As a consequence,
   Teredo extensions are compatible with UDP options.

5. UDP Option Intended Uses

   UDP options 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., NATs.
   It adds features that may, in the future, protect transport
   integrity and validate source identity (authentication), as well as
   those that may also encrypt the user payload, while still protecting
   the UDP transport header - unlike DTLS. They also enable
   packetization-layer path MTU discovery (PLPMTUD) over UDP, known as
   Datagram Packetization Layer Path Maximum Transmission Unit
   Discovery DPLPMTUD [Fa24], providing a means for probe packet
   validation without affecting the user data plane, as well as
   providing explicit indication of the receiver transport reassembly

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   UDP was originally intended to assume such capabilities could be
   provided by the user or by a layer above UDP. 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.

6. UDP Option Design Principles

   UDP options have been designed based on the following core
   principles. Each is an observation about (preexisting) UDP [RFC768]
   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.

      State required or maintained by the endpoints must 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

   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 bidirectionally needs to be defined in a separate

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

      Past experience confirms that static length limits will always
      need to be exceeded. Each implementation can limit how long/many
      options there are, but the specification should not introduce
      such a limit.

   4. UDP options are not intended replace or replicate other

      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 to instrument UDP to
      replace the need for network testing devices.

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   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 may 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 [Fa24]) or
      left undefined (TIME).

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

      By default, even when option checksums (OCS, APC),
      authentication, or encryption fail, every received packet is
      passed (possibly with an empty data payload) to the user
      application. Options that do not modify user data should (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 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 options. 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, the IPv4 ID field is deprecated for such use

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 detail in Section 18). The UDP

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   Option area is created when the UDP Length indicates a smaller
   transport payload than implied by the IP header.

   For IPv4, IP Total Length field indicates the total IP datagram
   length (including 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), as shown in Figure 1 [RFC791]. As a result, the
   typical (and largest valid) value for UDP Length is:

      UDP_Length = IPv4_Total_Length - IPv4_IHL * 4

      |Version|  IHL  |   Diff Svcs   |          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) ...
      |         UDP Source Port       |     UDP Destination Port      |
      |          UDP Length           |         UDP Checksum          |

                   Figure 1 IPv4 datagram with UDP header

   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 HDR 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 typical (and largest valid) value for UDP Length

       UDP_Length = IPv6_Payload_Length - sum(extension header lengths)

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      |Version| Traffic Class |             Flow Label                |
      |         Payload Length        |   Next Hdr    |   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          |

                   Figure 2 IPv6 datagram with UDP header

   In both cases, the space available for the UDP packet is indicated
   by IP, either directly in the base header (for IPv4) or by adding
   information in the extensions (for IPv6). In either case, this
   document will refer to this available space as the "IP transport

   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

                Figure 3 IP transport payload vs. 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 [RFC768] (discussed further in Section 18).
   This document uses the surplus area for UDP options.

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   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, using zeroes 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.

   The OCS contains an optional ones-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 defined using
   a TLV (type, length, and optional value) syntax similar to that of
   TCP [RFC9293], as detailed in Section 10. These options continue
   until the end of the surplus area or can end earlier using the EOL
   (end of list) option, followed by zeroes (discussed further in
   Section 10).

9. The Option Checksum (OCS)

   The Option Checksum (OCS) option is a conventional Internet checksum
   [RFC791] 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... |

      Figure 4 UDP OCS format, here using one zero byte for alignment

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   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 non-standard
   (i.e., non-option) uses of that area and accidental errors. It is
   not intended to detect attacks, as discussed further in Section 24.
   OCS is not intended to prevent future non-standard 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, 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 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 [RFC791] and for IPv6 [RFC8200] when UDP payload
   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

   The benefits are similar to allowing UDP checksums to be zero, but
   the risks differ. 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 OCS to
   ensure such traversal. In cases where such traversal risks can
   safely be ignored, such as controlled environments, over paths where

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   traversal is validated, or where upper layer protocols
   (applications, libraries, etc.) can adapt (by enabling OCS when
   packet exchange fails), and when bit errors at the UDP layer would
   be detected by other layers (as with the UDP checksum) OCS can be
   disabled, e.g., to conserve energy or processing resources or when
   it can improve performance. This is why zeroing OCS is only safe
   when UDP checksum is also zero, but 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 be
   delivered to the application layer, even if the OCS fails, unless
   the endpoints have negotiated otherwise for this UDP packet's socket

   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 typically a minimum of two bytes in length as shown
   in Figure 5, excepting only the one-byte options "No Operation"
   (NOP) and "End of Options List" (EOL) described below.

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

                     Figure 5 UDP option default format

   The Kind field is always one byte. The Length field 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.

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

                    Figure 6 UDP option extended format

   >> 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 where rate-limiting permits.

   >> 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

   >> Any UDP option other than EOL and NOP MAY use either the default
   or extended option formats.

   >> 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.

   The following UDP options are currently defined:

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             Kind    Length    Meaning
             0*      -         End of Options List (EOL)
             1*      -         No operation (NOP)
             2*      6         Additional payload checksum (APC)
             3*      10/12     Fragmentation (FRAG)
             4*      4         Maximum datagram size (MDS)
             5*      4         Maximum reassembled datagram size (MRDS)
             6*      6         Request (REQ)
             7*      6         Response (RES)
             8       10        Timestamps (TIME)
             9       (varies)  RESERVED for Authentication (AUTH)
             10-126  (varies)  UNASSIGNED (assignable by IANA)
             127     (varies)  RFC 3692-style experiments (EXP)
             128-191           RESERVED

             192     (varies)  RESERVED for Compression (UCMP)
             193     (varies)  RESERVED for Encryption (UENC)
             194-253           UNASSIGNED-UNSAFE (assignable by IANA)
             254     (varies)  RFC 3692-style experiments (UEXP)
             255               RESERVED-UNSAFE

   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 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 should be avoided for SAFE option nicknames (see Section 25).
   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.

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   >> An endpoint supporting UDP options MUST support those marked with
   a "*" 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.

   >> All other SAFE options (without a "*") 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
   SAFE options (i.e., in the same way a legacy receiver would ignore
   all UDP options). That includes options whose length does not
   indicate the specified value(s), as long as the length is not
   inherently invalid (i.e., smaller than 2 for the default and 4 for
   the extended formats).

   >> UNSAFE options MUST be used only with the FRAG option, in a
   manner that prevents them from being silently ignored while still
   passing up potentially modified UDP payload. This ensures their safe
   use in environments that might include legacy receivers (See Section
   12), because the UDP 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.

   >> Each option SHOULD NOT occur more than once in a single UDP
   datagram, the only exceptions being NOP, EXP, and UEXP. If an option
   other than these occurs more than once, a receiver MUST interpret
   only the first instance of that option and MUST ignore all others.
   Section 24 provides additional advice for DOS issues that involve
   large numbers of options, whether valid, unknown, or repeating.

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

   >> Only the OCS, AUTH, and UENC options include fields within the
   options that depend on the contents of the surplus area. AUTH and
   UENC are never used together, as UENC would serve both purposes.
   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
   remaining contents of the surplus area, i.e., the area after the

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   last option (presumably EOL). Otherwise, interactions between those
   values, the OCS, and the AUTH and UENC options could be
   unpredictable. This does not prohibit future uses of the entire
   surplus area; space that would have occurred after the EOL can be
   used as a UDP option instead, i.e., rather than using the EOL option
   and trying to defining meaning beyond it, define a new option that
   uses the remaining surplus area as an option itself, in conjunction
   with an assigned UDP option codepoint and length to unambiguously
   indicate the meaning of that area. This also 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).

   >> Impossible lengths SHOULD be treated as an indication of a
   malformed surplus area and all options SHOULD silently be discarded.
   This includes lengths that imply a physical impossibility, e.g.,
   smaller than two for conventional options and four for extended
   length options. Lengths other than those expected result in safe
   options being ignored and skipped over, as with any other unknown
   safe option.

   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).

   >> Option lengths MUST NOT exceed the IP length of the overall IP
   datagram. A receiver MUST drop all options in such a malformed
   packet and the event MAY be logged for diagnostics (logging SHOULD
   be rate limited).

   >> "Must-support" options other than NOP and EOL MUST be placed by
   the transmitter before other UDP options and a receiver MUST drop
   all UDP options in such malformed packet (i.e., in which this
   ordering is not honored) and that event MAY be logged for
   diagnostics (logging SHOULD be rate limited).

   The requirement that must-support options come before others is
   intended to allow for endpoints to implement DOS protection, as
   discussed further in Section 24.

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.

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11.1. End of Options List (EOL)

   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
   options without needing to use NOP options (see the following
   section) as padding to fill all available option space.

                                 | Kind=0 |

                       Figure 7 UDP EOL option format

   >> When the UDP options do not consume the entire surplus area, the
   last non-NOP option MUST be EOL.

   >> NOPs SHOULD NOT be used as padding before the EOL option. As a
   one-byte option, EOL need not be otherwise aligned.

   >> All bytes in the surplus area after EOL MUST be set to zero on

   >> Bytes after EOL in the surplus area MAY be checked as being zero
   on receipt, but MUST NOT be otherwise processed (except for OCS
   calculation, which zeros would not affect) and MUST NOT be passed to
   the user (e.g., as part of the surplus area).

   Requiring the post-option surplus area to be zero prevents side-
   channel uses of this area, requiring instead that all use of the
   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 UDP datagram length without affecting the UDP user data length,
   e.g., for UDP DPLPMTUD (Section 4.1 of [Fa24]).

11.2. No Operation (NOP)

   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
   along 16-bit, 32-bit, or 64-bit boundaries.

                                 | Kind=1 |

                       Figure 8 UDP NOP option format

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   >> UDP packets SHOULD NOT use more than seven consecutive NOPs,
   i.e., to support alignment up to 8-byte boundaries. UDP packets
   SHOULD NOT use NOPs at the end of the options area as a substitute
   for EOL followed by zero-fill. NOPs are intended to assist with
   alignment, not as other padding or fill.

   >> Receivers persistently experiencing packets with more than seven
   consecutive NOPs SHOULD log such events, at least occasionally, as a
   potential DOS indicator.

   NOPs are not reported to the user, whether used per-datagram or per-
   fragment (as defined in Section 11.4).

   This issue is discussed further in Section 24.

11.3. Additional Payload Checksum (APC)

   The Additional Payload Checksum (APC, Kind=2) option provides a
   stronger supplement to the checksum in the UDP header, using a 32-
   bit CRC of the conventional UDP user data payload only (excluding
   the IP pseudoheader, UDP header, and surplus 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 supplement to the OCS
   because the latter covers the surplus area only. Its purpose is to
   detect user data errors that the UDP checksum might not detect.

   A CRC32c has been chosen because of its ubiquity and use in other
   Internet protocols, including iSCSI and SCTP. The option contains
   the CRC32c in network standard byte order, as described in

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

                       Figure 9 UDP APC option format

   When present, the APC always contains a valid CRC checksum. There
   are no reserved values, including the value of zero. If the CRC is
   zero, this must indicate a valid checksum (i.e., it does not
   indicate that the APC is not used; instead, the option would simply
   not be included if that were the desired effect).

   APC does not protect the UDP pseudoheader; only the current UDP
   checksum provides that protection (when used). APC cannot provide

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   that protection because it would need to be updated whenever the UDP
   pseudoheader changed, e.g., during NAT address and port translation.

   >> UDP packets with incorrect APC checksums SHOULD be passed to the
   application, e.g., with a flag indicating APC failure. This is the
   default behavior for APC.

   Like all SAFE UDP options, APC needs to be silently ignored when
   failing by default, unless the receiver has been configured to do
   otherwise. Although all UDP option-aware endpoints support APC
   (being in the required set), this silently-ignored behavior ensures
   that option-aware receivers operate the same as legacy receivers
   unless overridden. Another reason is because APC may fail even where
   the user data has not been corrupted, such as when its contents have
   been intentionally overwritten e.g. by a middlebox to update
   embedded ports numbers or IP addresses. Such overwrites could be
   intentional and not widely known; defaulting to silent ignore
   ensures that option-aware endpoints do not change how users or
   applications operate unless explicitly directed to do otherwise.

   >> UDP packets with unrecognized APC lengths MUST receive the same
   treatment as UDP packets with incorrect APC checksums.

   Ensuring that unrecognized APC lengths are treated as incorrect
   checksums enables future variants of APC to be treated as APC-like.

   APC is reported to the user and useful only per-datagram, because
   fragments have no UDP user data.

11.4. Fragmentation (FRAG)

   The Fragmentation (FRAG, Kind=3) option supports UDP fragmentation
   and reassembly, which can be used to transfer UDP messages larger
   than allowed by the IP receive MTU (EMTU_R [RFC1122]). FRAG includes
   a copy of the same UDP transport ports in each fragment, enabling
   them to traverse Network Address (and port) Translation (NAT)
   devices, in contrast to the behavior of IP fragments. FRAG is
   typically used with the UDP MDS and MRDS options to enable more
   efficient use of large messages, both at the UDP and IP layers. The
   design of FRAG is similar to that of the IPv6 Fragmentation Header
   [RFC8200], except that the UDP variant uses a 16-bit Offset measured
   in bytes, rather than IPv6's 13-bit Fragment Offset measured in 8-
   byte units. This UDP variant avoids creating reserved fields.

   The FRAG header also enables use of options that modify the contents
   of the UDP payload, such as encryption (UENC, see Sec. 12.2). Like
   fragmentation, such options would not be safely used on UDP payloads

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   because they would be misinterpreted by legacy receivers. FRAG
   allows use of these options, either on fragments or on a whole,
   unfragmented message (i.e., an "atomic" fragment at the UDP layer,
   similar to atomic IP datagrams [RFC6864]). This is safe because FRAG
   hides the payload from legacy receivers by placing it within the
   surplus area.

   >> When FRAG is present, it SHOULD come as early as possible in the
   UDP options list.

   When present, placing FRAG first can simplify some implementations,
   notably using hardware acceleration that assumes a fixed location
   for the FRAG option. However, there are cases where FRAG cannot
   occur first, such as when combined with per-fragment UENC or UCMP.
   In those cases, encryption or compression (or both) would precede
   FRAG when they also encrypt or compress the fragment option itself.
   Additional cases could include recoding, such as may be used to
   support forward error correction (FEC) over a group of fragments.
   FRAG not being first might result in software (so-called "slow
   path") option processing, or might also be accommodated via a small
   set of known cases.

   >> 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
   the user data received sent to the user.

   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
   header), which would not affect the receiver unless the presence of
   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 can be used more safely in the presence of legacy receivers.

   The FRAG option has two formats; non-terminal fragments use the
   shorter variant (Figure 10) and terminal fragments use the longer
   (Figure 11). The latter includes stand-alone fragments, i.e., when
   data is contained in the FRAG option but reassembly is not required.

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

              Figure 10   UDP non-terminal FRAG option format

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   Most fields are common to both FRAG option formats. The option Len
   field indicates whether there are more fragments (Len=10) or no more
   fragments (Len=12).

   Frag. Start indicates the location of the beginning 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 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
   FRAG option) are applied to this UDP fragment.

   The Frag. Offset field indicates the location of this fragment
   relative to the original UDP datagram (prior to fragmentation or
   after reassembly), measured from the start of the original UDP
   datagram's header.

   The Identification field is a 32-bit value that, when used in
   combination with the IP source address, UDP source port, IP
   destination address, and UDP destination port, uniquely identifies
   the original UDP datagram.

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

                Figure 11   UDP terminal FRAG option format

   The terminal FRAG option format adds a Reassembled Datagram Option
   Start (RDOS) pointer, measured from the start of the original UDP
   datagram header, indicating the end of the reassembled data and the
   start of the surplus area within the original UDP datagram. UDP
   options that apply to the reassembled datagram are contained in the
   partially reassembled surplus area, as indicated by RDOS. UDP
   options that occur within the fragment are processed on the fragment
   itself. This allows either pre-reassembly or post-reassembly UDP
   option effects, such as using UENC on each fragment while also using
   TIME on the reassembled datagram for round-trip latency

   An example showing the relationship between UDP fragments and the
   original UDP datagram is provided in Figure 12. In this example, the
   trailer containing per-datagram options resides entirely within the
   terminal fragment, but this need not always be the case.

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          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       ~    |
        +--|----------+------------+    |
           |                            |

            Figure 12   UDP fragments and Original UDP datagram

   The FRAG option does not need a "more fragments" bit because it
   provides the same indication by using the longer, 12-byte variant,
   as shown in Figure 11.

   >> The FRAG option MAY be used on a single fragment, in which case
   the Frag. Offset would be zero and the option would have the 12-byte

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   >> Endpoints supporting UDP options MUST be capable of fragmenting
   and reassembling at least 2 fragments, for a total of at least 3,000
   bytes (see MRDS in Section 11.6).

   Use of the single fragment variant can be helpful in supporting use
   of UNSAFE options without undesirable impact to receivers that do
   not support either UDP options or the specific UNSAFE options.

   During fragmentation, the UDP header checksum of each fragment
   remains constant. It does not depend on the fragment data (which
   appears in the surplus area) because all fragments have a zero-
   length user data field.

   >> The Identification field is a 32-bit value that MUST be unique
   over the expected fragment reassembly timeout.

   >> The Identification field SHOULD be generated in a manner similar
   to that of the IPv6 Fragment ID [RFC8200].

   >> UDP fragments MUST NOT overlap.

   >> Similar to IPv6 reassembly [RFC8200], if any of the fragments
   being reassembled overlap with any other fragments being reassembled
   for the same UDP packet, reassembly of that UDP packet MUST be
   abandoned and all the fragments that have been received for that UDP
   packet must be discarded, and no ICMP error messages should be sent
   in this case (to avoid a potential DOS attack turning into an ICMP
   storm in the reverse direction).

   >> Note that fragments might be duplicated in the network. Instead
   of treating these exact duplicate fragments as overlapping
   fragments, an implementation MAY choose to detect this case and drop
   exact duplicate fragments while keeping the other fragments
   belonging to the same UDP packet.

   UDP fragmentation relies on a fragment expiration timer, which can
   be preset or could use a value computed using the UDP Timestamp

   >> The default UDP reassembly expiration timeout SHOULD be no more
   than 2 minutes.

   >> UDP reassembly expiration MUST NOT generate an ICMP error. Such
   events are not an IP error and can be addressed by the
   user/application layer if desired.

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   >> UDP reassembly space SHOULD be limited to reduce the impact of
   DOS attacks on resource use.

   >> UDP reassembly space limits SHOULD NOT be computed as a shared
   resource across multiple sockets, to avoid cross-socketpair DOS

   >> Individual UDP fragments MUST NOT be forwarded to the user. The
   reassembled datagram is received only after complete reassembly,
   checksum validation, and continued processing of the remaining UDP

   Per-fragment UDP options, if used in addition to FRAG, occur before
   the fragment data. They typically occur after the FRAG option,
   except where they modify the FRAG option itself (e.g., UENC or
   UCMP). Per-fragment options are processed before the fragment is
   included in the reassembled datagram. Such options can be useful to
   protect the reassembly process itself, e.g., to prevent the
   reassembly cache from being polluted (using AUTH or UENC).

   >> Fragments of a single datagram MAY use different sets of options.
   It is expected to be computationally expensive to validate
   uniformity across all fragments and there may be legitimate reasons
   for including options in a fragment but not all fragments (e.g.,
   MDS, MRDS).

   If an option is used per-fragment but defined as not usable per-
   fragment, it is treated the same as any other unknown option.

   Per-datagram UDP options, if used, reside in the surplus area of the
   original UDP datagram. Processing of those options occurs after
   reassembly is complete. This enables the safe use of UNSAFE options,
   which are required to result in discarding the entire UDP datagram
   if they are unknown to the receiver or otherwise fail (see Section

   In general, UDP packets are fragmented as follows:

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   1. Create a UDP packet with data and UDP options. This is the
      original UDP datagram, which we will call "D". The UDP options
      follow the UDP user data and occur in the surplus area, just as
      in an unfragmented UDP datagram with UDP options.

      >> UDP options for the original packet MUST be fully prepared
      before the rest of the fragmentation steps that follow here.

      >> The UDP checksum of the original packet SHOULD be set to zero
      because it is never transmitted. Equivalent protection is
      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 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
      each fragment's UDP checksum is non-zero.

   2. Identify the desired fragment size, which we will call "S". This
      value should take into account the path MTU (if known) and allow
      space for per-fragment options.

   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
      chunk no larger 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 need not be limited to the terminal fragment, i.e., the
      RDOS pointer can indicate the start of the original surplus area
      anywhere in the reassembled datagram.

      Note: per packet options can occur either at the end of the
      original user data or be placed after the FRAG option of the
      first fragment, with the Reassembled Datagram Option Start (RDOS)
      in the terminal FRAG option set accordingly. This includes its
      use in atomic fragments, where the terminal option is the initial
      and only fragment.

   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
      FRAG option, and any additional per-fragment UDP options,
      followed by the FRAG data chunk.

   5. Complete the processing associated with creating these additional
      per-fragment UDP options for each fragment.

   Receivers reverse the above sequence. They process all received
   options in each fragment. When the FRAG option is encountered, the
   FRAG data is used in reassembly. After all fragments are received,

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   the entire UDP packet is processed with any trailing UDP options
   applying to the reassembled user data.

   >> Reassembly failures at the receiver result in silent discard of
   any per-fragment options and fragment contents and such failures
   SHOULD NOT generate zero-length frames to the user.

   >> Finally, because fragmentation processing can be expensive, the
   FRAG option SHOULD be avoided unless the original datagram requires
   fragmentation or it is needed for "safe" use of UNSAFE options.

   >> Users MAY also select the FRAG option to provide limited support
   for UDP options in systems that have access to only the initial
   portion of the data in incoming or outgoing packets, with the caveat
   that such packets would be silently ignored by legacy receivers
   (that do not support UDP options).

   FRAG is not reported to the user, whether used per-datagram or per-
   fragment (as defined in Section 11.4).

11.5. Maximum Datagram Size (MDS)

   The Maximum Datagram Size (MDS, Kind=4) option is a 16-bit hint of
   the largest unfragmented UDP packet that an endpoint believes can be
   received. 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 headers only [RFC9293]. The space needed for IP and
   UDP options needs to be adjusted by the sender when using the value
   indicated. The value transmitted is based on EMTU_R, the largest IP
   datagram 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 believes.

   >> MDS does not indicate a known path MTU and thus MUST NOT be used
   to limit transmissions.

                   | Kind=4 | Len=4  |    MDS size     |

                     Figure 13   UDP MDS option format

   >> The UDP MDS option MAY be used as a hint for path MTU discovery
   [RFC1191][RFC8201], but this may be difficult because of known
   issues with ICMP blocking [RFC2923] as well as UDP lacking automatic

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   MDS is more likely to be useful when coupled with IP source
   fragmentation or UDP fragmentation to limit the largest reassembled
   UDP message as indicated by MRDS (see Section 11.6), e.g., when
   EMTU_R is larger than the required minimums (576 for IPv4 [RFC791]
   and 1500 for IPv6 [RFC8200]).

   >> MDS can be used with DPLPMTUD [RFC8899] to provide a hint to the
   packetization layer path MTU (PLPMTU) value, though it MUST NOT
   prohibit transmission of larger UDP packets used as DPLPMTUD probes.

   MDS is reported to the user, whether used per-datagram or per-
   fragment (as defined in Section 11.4). When used per-fragment, the
   report should be the minimum of the MDS values received per-

11.6. Maximum Reassembled Datagram Size (MRDS)

   The Maximum Reassembled Datagram Size (MRDS, Kind=5) option is a 16-
   bit indicator of the largest reassembled UDP datagram that can be
   received. MRDS is the UDP equivalent of IP's EMTU_R but the two are
   not related [RFC1122]. Using the FRAG option (Section 11.4), UDP
   packets can be transmitted as transport fragments, each in their own
   (presumably not fragmented) IP datagram and be reassembled at the
   UDP layer.

                   | Kind=5 | Len=4  |    MRDS size    |

                     Figure 14   UDP MRDS option format

   >> Endpoints supporting UDP options MUST support a local MRDS of at
   least 3,000 bytes.

   NOPs are not reported to the user, whether used per-datagram or per-
   fragment (as defined in Section 11.4).

   MRDS is reported to the user, whether used per-datagram or per-
   fragment (as defined in Section 11.4). When used per-fragment, the
   report should be the minimum of the MRDS values received per-

11.7. Echo request (REQ) and echo response (RES)

   The echo request (REQ, Kind=6) and echo response (RES, Kind=7)
   options provides UDP packet-level acknowledgements as a capability
   for use by upper layer protocols, e.g., user applications,

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   libraries, operating systems, etc. Both the REQ and RES are under
   the control of these upper layers, i.e., UDP itself never
   automatically responds to a REQ with a RES. Instead, the REQ is
   delivered to the upper layer, which decides whether and when to
   issue a RES.

   One such use is described as part of DPLPMTUD [Fa24]. The options
   both have the format indicated in Figure 15, in which the token has
   no internal structure or meaning.

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

                 Figure 15   UDP REQ and RES options format

   >> As advice to upper layer protocol/library designers, when
   supporting REQ/RES and responding with a RES, the upper layer SHOULD
   respond with the most recently received REQ token.

   >> REQ/RES MUST be disabled by default, i.e., arriving REQs are
   silently ignored and RES cannot be issued unless REQ/RES is actively
   enabled, e.g., for DPLPMTUD or other known use by an upper layer

   For example, an application needs to explicitly enable the
   generation of a RES response by DPLPMTUD when using UDP Options

   >> 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
   to a received request.

   REQ and RES option kinds appear at most once each in each UDP
   packet, as with most other options. Note also that the FRAG option
   is not used when sending DPLPMTUD probes to determine a PLPMTU

   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,
   the report should indicate the most recently received token.

11.8. Timestamps (TIME)

   The Timestamp (TIME, Kind=8) option exchanges two four-byte unsigned
   timestamp fields. It serves a similar purpose to TCP's TS option

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   [RFC7323], enabling UDP to estimate the round-trip time (RTT)
   between hosts. For UDP, this RTT can be useful for establishing UDP
   fragment reassembly timeouts or transport-layer rate-limiting

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

                     Figure 16   UDP TIME option format

   TS Value (TSval) and TS Echo Reply (TSecr) are used in a similar
   manner to the TCP TS option [RFC7323]. On transmitted UDP packets
   using the option, TS Value is always set based on the local "time"
   value. Received TSval and TSecr values are provided to the
   application, which can pass the TSval value to be used as TSecr on
   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 transmitter, i.e., from a previously received UDP packet.

   >> TIME MAY use an RTT estimate based on nonzero Timestamp values as
   a hint for fragmentation reassembly, rate limiting, or other
   mechanisms that benefit from such an estimate.

   >> an application MAY use TIME to compute this RTT estimate for
   further use by the user.

   UDP timestamps are modeled after TCP timestamps and have similar
   expectations. In particular, they are expected to be:

   o  Values are monotonic and non-decreasing except for anticipated
      number-space rollover events

   o  Values should "increase" (allowing for rollover, i.e., modulo the
      field size excepting zero) according to a typical 'tick' time

   o  A request is defined as TSval being non-zero and a reply is
      defined as TSecr being non-zero.

   o  A receiver should always respond to a request with the highest
      TSval received (allowing for rollover), which is not necessarily
      the most recently received.

   Rollover can be handled as a special case or more completely using
   sequence number extension [RFC9187], however zero values need to be
   avoided explicitly.

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   >> TIME values MUST NOT use zeros as valid time values, because they
   are used as indicators of requests and responses.

   TIME is reported to the user, whether used per-datagram or per-
   fragment (as defined in Section 11.4). When used per-fragment, the
   report should be the minimum and maximum of each of the timestamp
   values received per-fragment.

   >> Use of TIME per-fragment is NOT RECOMMENDED. Exceptions include
   supporting diagnostics on the reassembly process itself, which may
   be more appropriate to handle within the UDP option processing

11.9. Authentication (AUTH), RESERVED Only

   The Authentication (AUTH, Kind=9) option is reserved for all UDP
   authentication mechanisms [To24]. AUTH is expected to cover the UDP
   user data and UDP options, with possible additional coverage of
   portions of the IP and UDP headers and potentially also support for
   NAT traversal, in a similar manner as TCP-AO [RFC6978].

   Like APC, AUTH is a SAFE option because it would not modify the UDP
   user data. AUTH may fail even where the user data has not been
   corrupted, such as when its contents have been overwritten. Such
   overwrites could be intentional and not widely known; defaulting to
   silent ignore ensures that option-aware endpoints do not change how
   users or applications operate unless explicitly directed to do

11.10. Experimental (EXP)

   The Experimental option (EXP, Kind=127) is allocated for experiments
   [RFC3692]. Only one such value is allocated because experiments are
   expected to use an Experimental ID (ExIDs) to differentiate
   concurrent use for different purposes, using UDP ExIDs registered
   with IANA according to the approach developed for TCP experimental
   options [RFC6994].

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

                     Figure 17   UDP EXP option format

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   >> The length of the experimental option MUST be at least 4 to
   account for the Kind, Length, and the 16-bit UDP ExID identifier
   (similar to TCP ExIDs [RFC6994]).

   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,
   where present, iss used to decrease the chance of the entire ExID
   occurring in legacy use of the TCP EXP option. This extended variant
   provides no similar use for UDP EXP because ExIDs are required.

   The UDP EXP option also includes an extended length format, where
   the option LEN is 255 followed by two bytes of extended length.

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

                 Figure 18   UDP EXP extended option format

   Assigned UDP experimental IDs (ExIDs) assigned from a single
   registry managed by IANA (see Section 25). Assigned ExIDs can be
   used in either the EXP or UEXP options (see Section 12.3 for the

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

   >> 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

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   if an UNSAFE option appears outside the context of a fragment or
   reassembled fragments.

12.1. UNSAFE Compression (UCMP)

   The UNSAFE Compression (UCMP, Kind=192) option is reserved for all
   UDP compression mechanisms. UCMP is expected to cover the UDP user
   data and some (e.g., later, in sequence) UDP options.

12.2. UNSAFE Encryption (UENC)

   The UNSAFE Encryption (UENC, Kind=193) option is reserved for all
   UDP encryption mechanisms. UENC is expected to cover the UDP user
   data and some (e.g., later, in sequence) UDP options, with possible
   additional protection of portions of the IP and UDP headers and
   potentially also support for NAT traversal, in a similar manner as
   TCP-AO [RFC6978].

12.3. UNSAFE Experimental (UEXP)

   The UNSAFE Experimental option (UEXP, Kind=254) is reserved for
   experiments [RFC3692]. As with EXP, only one such UEXP value is
   reserved because experiments are expected to use an Experimental ID
   (ExIDs) to differentiate concurrent use for different purposes,
   using UDP ExIDs registered with IANA according to the approach
   developed for TCP experimental options [RFC6994].

   Assigned ExIDs can be used with either the UEXP or EXP options.

13. Rules for designing new options

   The UDP option Kind space allows for the definition of new options,
   however the currently defined options do not allow for arbitrary new
   options. The following is a summary of rules for new options and
   their rationales:

   >> 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

   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 UDP option capable receivers.

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   >> 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, excepting only those
   contained within the UNSAFE option; areas that need to remain
   unmodified include the IP header, IP options, the UDP user data, and
   the 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

   >> 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:

   o  >> 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.

   The following recommendation helps enable efficient zero-copy

   o  >> FRAG SHOULD be the first option, when present.

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).

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   >> 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.

   >> All options except UNSAFE options MUST result in the UDP user
   data being passed to the application layer, regardless of whether
   all options are processed, 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 RFC1122)
           if the UDP checksum passes or is zero then
               if ((OCS != 0 and 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 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 as used only inside the FRAG area
   ensures that the resulting UDP data will be silently dropped in both
   legacy and options-aware receivers. 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.

   I.e., 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) should the
   receiver require it being present and correct.

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   I.e., for all options:

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

   o  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.

   o  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 programmer interface (API),
   summarized as follows (with Unix-style command as an example)

   o  Method to create new receive ports

        o E.g., bind(handle, recvaddr(optional), recvport)

   o  Receive, which returns data octets, source port, and source

        o E.g., recvfrom(handle, srcaddr, srcport, data)

   o  Send, which specifies data, source and destination addresses, and
      source and destination ports

        o E.g., sendto(handle, destaddr, destport, data)

   This API is extended to support options as follows:

   o  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 MAY be logged.

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   o  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, regardless of
      whether all options are processed, supported, or succeed.

   o  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).

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

   o  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 [Fa24].

   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 24). 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

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   a per-packet basis. Such control over fragmentation is critical to

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 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

   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 here are
   specified 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,
   e.g., IPsec), UDP options remain visible to devices on the network

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.

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   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 [RFC768]. 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
   [RFC768] [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., CheckPoint, use a
   default "relaxed UDP length verification" to avoid falsely
   interpreting this inconsistency as an attack.

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   There are known uses of UDP exchanges of zero-length UDP user data
   packets, notably in the TIME protocol [RFC868]. 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 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 allow for 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, 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 may have
   similar benefit 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

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   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 RFC 768 as follows:

   o  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.

   o  This document extends the UDP API to support the use of UDP

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 [RFC768] [RFC1122].

   Teredo extensions (TE) define use of a similar difference between
   these lengths for trailers [RFC4380][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.

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                            Outer IP Length
      | IP Hdr | UDP Hdr | IPv6 packet/len | TE trailer | surplus  |
                          Inner IPv6 Length
                              UDP Length

         Figure 19   TE trailers and UDP options used concurrently

   UDP options cannot be supported when a UDP packet has no independent
   UDP Length. The only known 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 [RFC768]. The technique has been proposed for
   deprecation [Jo19].

   This document is consistent 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) would cause the UDP header to remain
   uncompressed. Upon receipt of a compressed UDP header, Section A.1.3
   of that document 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., Sec. 10.10 of Static
   Context Header Compression [RFC8724].

23. Multicast Considerations

   UDP options are primarily intended for unicast use. Using these
   options over multicast 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 group confirm support
   for the options in use.

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24. 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
   in this section further.

   Note that any user application that considers UDP options to affect
   security need not enable them. However, their use does not impact
   security in a way substantially different than TCP options; both
   enable the use of a control channel that has the potential for
   abuse. Similar to TCP, there are many options that, if unprotected,
   could be used by an attacker to interfere with communication.

   UDP options create new potential opportunities for DDOS attacks,
   notably through the use of fragmentation. When enabled, UDP options
   cause additional work at the receiver, however, of the "must-
   support" options, only REQ (e.g., when used with DPLPMTUD [Fa24])
   will cause the upper layer to initiate a UDP response in the absence
   of user transmission.

   The use of UDP packets with inconsistent IP and UDP Length fields
   has the potential to trigger a buffer overflow error if not properly
   handled, e.g., if space is allocated based on the smaller field and
   copying is based on the larger. However, there have been no reports
   of such vulnerability and it would rely on inconsistent use of the
   two fields for memory allocation and copying.

   UDP options are not covered by DTLS (datagram transport-layer
   security). Neither TLS [RFC8446] (transport layer
   security, for TCP) nor DTLS [RFC9147] (TLS for UDP) protect the
   transport layer; both operate as a shim layer solely on the user
   data of transport packets, protecting only their contents.

   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 document. Transport security is provided in TCP by the TCP
   Authentication Option (TCP-AO [RFC5925]) and (when defined) in UDP
   by the Authentication (AUTH) option (Section 11.9) and (when
   defined) the UNSAFE Encryption (UENC) option (Section 12). Transport
   headers are also protected as payload when using IP security (IPsec)

   UDP options use the TLV syntax similar to that of TCP. This syntax
   is known to require serial processing and may pose a DOS risk, e.g.,
   if an attacker adds large numbers of unknown options that must be
   parsed in their entirety, as is the case for IPv6 [RFC8504].

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   >> Implementations concerned with the potential for UDP options
   introducing a vulnerability MAY implement only the required UDP
   options and SHOULD also limit processing of TLVs, either in number
   of non-padding options or total length, or both. The number of non-
   zero TLVs allowed in such cases MUST be at least as many as the
   number of concurrent options supported with an additional few to
   account for unexpected unknown options, but should also consider
   being adaptive and based on the implementation, to avoid locking in
   that limit globally.

   E.g., if a system supports 10 different option types that could
   concurrently be used, it is expected to allow up to around 13-14
   different options in the same packet. This document avoids
   specifying a fixed minimum, but recognizes that a given system
   should not expect to receive more than a few unknown option types
   per packet.

   Because required options come first and at most once each (with the
   exception of NOPs, which should never need to come in sequences of
   more than seven in a row), this limits their DOS impact. Note that
   TLV formats for options does require serial processing, but any
   format that allows future options, whether ignored or not, could
   introduce a similar DOS vulnerability.

   >> Implementations concerned with the potential for DOS attacks
   involving large numbers of UDP options, either implemented or
   unknown, or excessive sequences of valid repeating options (e.g.,
   NOPs) SHOULD detect excessive numbers of such occurrences and limit
   resources they use, either through silent packet drops. Such
   responses MUST be logged. Specific thresholds for such limits will
   vary based on implementation and are thus not included here.

   >> Implementations concerned with the potential for UDP
   fragmentation introducing a vulnerability SHOULD implement limits on
   the number of pending fragments.

   UDP security should never rely solely on transport layer 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 surplus
   area until after those options are processed. All other options
   default to being silently ignored at the transport layer but may be
   dropped either if that default is overridden (e.g., by
   configuration) or discarded at the application layer (e.g., using
   information about the options processed that are passed along with
   the UDP packet).

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   UDP fragmentation introduces its own set of security concerns, which
   can be handled in a manner similar to IP reassembly or TCP segment
   reordering [CERT18]. In particular, the number of UDP packets
   pending reassembly and effort used for reassembly is typically
   limited. In addition, it may be useful to assume a reasonable
   minimum fragment size, e.g., that non-terminal fragments should
   never be smaller than 500 bytes.

   UDP options, like any options, have the potential to expose option
   information to on-path attackers, unless the options themselves are
   encrypted (as might be the case with some configurations of UENC,
   when defined). Application protocol designers should ensure that
   information in UDP options is not used with the assumption of
   privacy unless UENC provides that capability. Application protocol
   designers using secure payload contents (e.g., via DTLS) should be
   aware that UDP options add information that is not inside the UDP
   payload and thus not protected by the same mechanism, and that
   alternate mechanisms (again, as might be the case with some
   configurations of UENC) may be additionally required to protect
   against information disclosure.

   Some UDP options are never passed to the receiving application,
   notably FRAG, NOP, and EOL. They are not intended to convey
   information, either by their presence (FRAG, EOL) or number (NOP).
   It may also be useful to provide the options received in a reference
   order (i.e., sorted by option number), to avoid the order of options
   being used as a covert channel.

   >> Implementations concerned with the potential use of UDP options
   as a covert channel MAY consider limiting use of some or all
   options. Such implementations SHOULD ensure FRAG, NOP, and EOL are
   not passed to the receiving user and SHOULD return options in an
   order not related to their sequence in the received packet.

25. IANA Considerations

   Upon publication, IANA is hereby requested to create a new registry
   group for UDP Options, consisting of UDP Option Kind numbers and UDP
   Option Experimental IDs (ExIDs).

   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.

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   >> Although option nicknames are not used in-band, new UNSAFE option
   names SHOULD commence with the capital letter "U" and avoid either
   uppercase or lowercase "U" as commencing safe options.

   UDP Experimental Option Experiment Identifiers (UDP ExIDs) are
   intended for use in a similar manner as TCP ExIDs [RFC6994]. UDP
   ExIDs can be used in either (or both) the UDP EXP (Section 11.10) or
   UEXP (Section 12.3) options. UDP ExID entries consist of a 16-bit
   ExID (in network-standard order), and (as with the TCP ExID) will
   preferentially also include a short description and acronym for use
   in documentation. UDP ExIDs are always 16 bits because their use in
   EXP and UEXP options is required and thus do not need a larger
   number space to decrease the probability of accidental occurrence
   with non-ExID uses of the experimental options, as is the case with
   TCP ExIDs.

   Values in this registry are to be assigned by IANA using first-come,
   first-served (FCFS) rules applied to both the ExID value and the
   acronym [RFC8126]. Options using these ExIDs are subject to the same
   conditions as new options, i.e., they too are subject to the
   conditions set forth in this document, particularly (but not limited
   to) those in Section 13.

   The UDP ExID registry is intended to align with the TCP ExID
   registry, where all UDP ExIDs are also TCP ExIDs, and the first 16
   bits of all TCP ExIDs are also UDP ExIDs. Whether these are
   represented as separate registries or as a single registry is at the
   discretion of IANA. If as separate registries, this document
   requests that each ExID request in either registry generates a
   corresponding entry in the other registry. If as a single registry,
   please include the direction that "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".

26. References

26.1. Normative References

   [Fa24]    Fairhurst, G., T. Jones, "Datagram PLPMTUD for UDP
             Options," draft-ietf-tsvwg-udp-options-dplpmtud, Jan.

   [RFC768]  Postel, J., "User Datagram Protocol," RFC 768, August

   [RFC791]  Postel, J., "Internet Protocol," RFC 791, Sept. 1981.

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   [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts --
             Communication Layers," RFC 1122, Oct. 1989.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels," BCP 14, RFC 2119, March 1997.

   [RFC5925] Touch, J., A. Mankin, R. Bonica, "The TCP Authentication
             Option," RFC 5925, June 2010.

   [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
             2119 Key Words," RFC 2119, May 2017.

26.2. Informative References

   [Fa18]    Fairhurst, G., T. Jones, R. Zullo, "Checksum Compensation
             Options for UDP Options", draft-fairhurst-udp-options-cco,
             Oct. 2018.

   [Hi15]    Hildebrand, J., B. Trammel, "Substrate Protocol for User
             Datagrams (SPUD) Prototype," draft-hildebrand-spud-
             prototype-03, Mar. 2015.

   [Jo19]    Jones, T., G. Fairhurst, "Change Status of RFC 2675 to
             Historic," draft-jones-6man-historic-rfc2675, May 2019.

   [RFC868]  Postel, J., K. Harrenstien, "Time Protocol," RFC 868, May

   [RFC1071] Braden, R., D. Borman, C. Partridge, "Computing the
             Internet Checksum," RFC 1071, Sept. 1988.

   [RFC1191] Mogul, J., S. Deering, "Path MTU discovery," RFC 1191,
             November 1990.

   [RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery," RFC
             2923, September 2000.

   [RFC3095] Bormann, C. (Ed), et al., "RObust Header Compression
             (ROHC): Framework and four profiles: RTP, UDP, ESP, and
             uncompressed," RFC 3095, July 2001.

   [RFC3385] Sheinwald, D., J. Satran, P. Thaler, V. Cavanna, "Internet
             Protocol Small Computer System Interface (iSCSI) Cyclic
             Redundancy Check (CRC)/Checksum Considerations," RFC 3385,
             Sep. 2002.

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   [RFC3692] Narten, T., "Assigning Experimental and Testing Numbers
             Considered Useful," RFC 3692, Jan. 2004.

   [RFC3828] Larzon, L-A., M. Degermark, S. Pink, L-E. Jonsson (Ed.),
             G. Fairhurst (Ed.), "The Lightweight User Datagram
             Protocol (UDP-Lite)," RFC 3828, July 2004.

   [RFC4301] Kent, S. and K. Seo, "Security Architecture for the
             Internet Protocol", RFC 4301, Dec. 2005.

   [RFC4340] Kohler, E., M. Handley, and S. Floyd, "Datagram Congestion
             Control Protocol (DCCP)", RFC 4340, March 2006.

   [RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
             Network Address Translations (NATs)," RFC 4380, Feb. 2006.

   [RFC6081] Thaler, D., "Teredo Extensions," RFC 6081, Jan 2011.

   [RFC6864] Touch, J., "Updated Specification of the IPv4 ID Field,"
             RFC 6864, Feb. 2013.

   [RFC6935] Eubanks, M., P. Chimento, M. Westerlund, "IPv6 and UDP
             Checksums for Tunneled Packets," RFC 6935, April 2013.

   [RFC6978] Touch, J., "A TCP Authentication Option Extension for NAT
             Traversal", RFC 6978, July 2013.

   [RFC6994] Touch, J., "Shared Use of Experimental TCP Options," RFC
             6994, Aug. 2013.

   [RFC7323] Borman, D., R. Braden, V. Jacobson, R. Scheffenegger
             (Ed.), "TCP Extensions for High Performance," RFC 7323,
             Sep. 2014.

   [RFC8085] Eggert, L., G. Fairhurst, G. Shepherd, "UDP Usage
             Guidelines," RFC 8085, Feb. 2017.

   [RFC8126] Cotton, M., B. Leiba, T. Narten, "Guidelines for Writing
             an IANA Considerations Section in RFCs," RFC 8126, June

   [RFC8200] Deering, S., R. Hinden, "Internet Protocol Version 6
             (IPv6) Specification," RFC 8200, Jul. 2017.

   [RFC8201] McCann, J., S. Deering, J. Mogul, R. Hinden (Ed.), "Path
             MTU Discovery for IP version 6," RFC 8201, Jul. 2017.

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   [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
             Version 1.3," RFC 8446, Aug. 2018.

   [RFC8504] Chown, T., J. Loughney, T. Winters, "IPv6 Node
             Requirements," RFC 8504, Jan. 2019.

   [RFC8724] Minaburo, A., L. Toutain, C. Gomez, D. Barthel, JC.,
             "SCHC: Generic Framework for Static Context Header
             Compression and Fragmentation," RFC 8724, Apr. 2020.

   [RFC8899] Fairhurst, G., T. Jones, M. Tuxen, I. Rungeler, T. Volker,
             "Packetization Layer Path MTU Discovery for Datagram
             Transports," RFC 8899, Sep. 2020.

   [RFC9040] Touch, J., M. Welzl, S. Islam, "TCP Control Block
             Interdependence," RFC 9040, Jul. 2021.

   [RFC9147] Rescorla, E., H. Tschofenig, N. Modadugu, "Datagram
             Transport Layer Security Version 1.3," RFC 9147, Apr.

   [RFC9187] Touch, J., "Sequence Number Extension for Windowed
             Protocols," RFC 9187, Jan. 2022.

   [RFC9260] Stewart, R., M. Tuxen, K. Nielsen, "Stream Control
             Transmission Protocol", RFC 9260, June 2022.

   [RFC9293] Eddy, W. (Ed.), "Transmission Control Protocol," STD 7,
             RFC 9293, Aug. 2022.

   [CERT18]  CERT Coordination Center, "TCP implementations vulnerable
             to Denial of Service,", Vulnerability Note VU 962459,
             Software Engineering Institute, CMU, 2018,

   [To18]    Touch, J., "A TCP Authentication Option Extension for
             Payload Encryption," draft-touch-tcp-ao-encrypt, Jul.

   [To24]    Touch, J., "The UDP Authentication Option," draft-touch-
             tsvwg-udp-auth-opt, Mar. 2024.

   [Zu20]    Zullo, R., T. Jones, and G. Fairhurst, "Overcoming the
             Sorrows of the Young UDP Options," 2020 Network Traffic
             Measurement and Analysis Conference (TMA), IEEE, 2020.

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27. 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 (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.

   This document was prepared using

Authors' Addresses

   Joe Touch
   Manhattan Beach, CA 90266 USA

   Phone: +1 (310) 560-0334

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Appendix A.Implementation Information

   The following information is provided to encourage interoperable API

   System-level variables (sysctl):

           Name                   default   meaning
           net.ipv4.udp_opt       0         UDP options available
           net.ipv4.udp_opt_ocs   1         Use OCS
           net.ipv4.udp_opt_apc   0         Include APC
           net.ipv4.udp_opt_frag  0         Fragment
           net.ipv4.udp_opt_mds   0         Include MDS
           net.ipv4.udp_opt_mrds  0         Include MRDS
           net.ipv4.udp_opt_req   0         Include REQ
           net.ipv4.udp_opt_resp  0         Include RES
           net.ipv4.udp_opt_time  0         Include TIME
           net.ipv4.udp_opt_auth  0         Include AUTH
           net.ipv4.udp_opt_exp   0         Include EXP
           net.ipv4.udp_opt_ucmp  0         Include UCMP
           net.ipv4.udp_opt_uenc  0         Include UENC
           net.ipv4.udp_opt_uexp  0         Include UEXP

   Socket options (sockopt), cached for outgoing datagrams:

           Name           meaning
           UDP_OPT        Enable UDP options (at all)
           UDP_OPT_OCS    Use UDP OCS
           UDP_OPT_APC    Enable UDP APC option
           UDP_OPT_FRAG   Enable UDP fragmentation
           UDP OPT MDS    Enable UDP MDS option
           UDP OPT MRDS   Enable UDP MRDS option
           UDP OPT REQ    Enable UDP REQ option
           UDP OPT RES    Enable UDP RES option
           UDP_OPT_TIME   Enable UDP TIME option
           UDP OPT AUTH   Enable UDP AUTH option
           UDP OPT EXP    Enable UDP EXP option
           UDP_OPT_UCMP   Enable UDP UCMP option
           UDP_OPT_UENC   Enable UDP UENC option
           UDP OPT UEXP   Enable UDP UEXP option

   Send/sendto parameters:

         (Same as sysctl, with different prefixes)

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   Connection parameters (per-socketpair cached state, part UCB):

           Name          Initial value
           opts_enabled  net.ipv4.udp_opt
           ocs_enabled   net.ipv4.udp_opt_ocs

   >> The JUNK option is included for debugging purposes, and MUST NOT
   be enabled otherwise.

   System variables

   net.ipv4.udp_opt_junk   0

   System-level variables (sysctl):

           Name                   default   meaning
           net.ipv4.udp_opt_junk  0         Default use of junk

   Socket options (sockopt):

           Name          params   meaning
           UDP_JUNK      -        Enable UDP junk option
           UDP_JUNK_VAL  fillval  Value to use as junk fill
           UDP_JUNK_LEN  length   Length of junk payload in bytes

   Connection parameters (per-socketpair cached state, part UCB):

           Name          Initial value
           junk_enabled  net.ipv4.udp_opt_junk
           junk_value    0xABCD
           junk_len      4

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