TSVWG                                                           J. Touch
Internet Draft                                    Independent Consultant
Intended status: Standards Track                          March 27, 2023
Intended updates: 768
Expires: September 2023

                         Transport Options for UDP


   Transport protocols are extended through the use of transport header
   options. This document extends UDP by indicating the location,
   syntax, and semantics for UDP transport layer options.

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
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   This Internet-Draft will expire on September 27, 2023.

Copyright Notice

   Copyright (c) 2023 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
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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document. Please review these documents

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   carefully, as they describe your rights and restrictions with
   respect to this document. Code Components extracted from this
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   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. The UDP Option Area........................................... 5
   6. The UDP Surplus Area Structure................................ 8
   7. The Option Checksum (OCS)..................................... 8
   8. UDP Options.................................................. 10
   9. Safe UDP Options............................................. 13
      9.1. End of Options List (EOL)............................... 13
      9.2. No Operation (NOP)...................................... 14
      9.3. Alternate Payload Checksum (APC)........................ 14
      9.4. Fragmentation (FRAG).................................... 16
      9.5. Maximum Datagram Size (MDS)............................. 21
      9.6. Maximum Reassembled Datagram Size (MRDS)................ 21
      9.7. Echo request (REQ) and echo response (RES).............. 22
      9.8. Timestamps (TIME)....................................... 22
      9.9. Authentication (AUTH)................................... 23
      9.10. Experimental (EXP)..................................... 25
   10. UNSAFE Options.............................................. 26
      10.1. UNSAFE Encryption (UENC)............................... 26
      10.2. UNSAFE Experimental (UEXP)............................. 26
   11. Rules for designing new options............................. 27
   12. Option inclusion and processing............................. 28
   13. UDP API Extensions.......................................... 29
   14. UDP Options are for Transport, Not Transit.................. 30
   15. UDP options vs. UDP-Lite.................................... 31
   16. Interactions with Legacy Devices............................ 31
   17. Options in a Stateless, Unreliable Transport Protocol....... 32
   18. UDP Option State Caching.................................... 33
   19. Updates to RFC 768.......................................... 33
   20. Interactions with other RFCs (and drafts)................... 33
   21. Multicast Considerations.................................... 34
   22. Security Considerations..................................... 35
   23. IANA Considerations......................................... 36
   24. References.................................................. 37
      24.1. Normative References................................... 37
      24.2. Informative References................................. 37
   25. Acknowledgments............................................. 40

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   Appendix A. Implementation Information.......................... 41

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 defines 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.

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

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

   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 as IP payloads larger than
      permitted by IP datagram maximum sizes; note that each UDP
      fragment is itself transmitted as a UDP packet with its own

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   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.

   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

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   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. Thus Teredo
   extensions are compatible with UDP options.

5. 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 16). The UDP
   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

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      |Version|  IHL  |Type of Service|          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 16).
   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".

6. 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), 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 7.

   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 8. These options continue
   until the end of the surplus area or can end earlier using the EOL
   (end of list) option, followed by zeroes.

7. The Option Checksum (OCS)

   The Option Checksum (OCS) option is 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 for alignment

   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

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   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 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 22.

   The design enables traversal of errant middleboxes that incorrectly
   compute the UDP checksum over the entire IP payload [Fa18], 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

   >> 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.

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

   >> If 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 12).

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8. 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.

                   |  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.

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   >> UDP options MUST be interpreted in the order in which they occur
   in the surplus area.

   The following UDP options are currently defined:

             Kind    Length    Meaning
             0*      -         End of Options List (EOL)
             1*      -         No operation (NOP)
             2*      6         Alternate 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)  Authentication (AUTH)
             10-126  (varies)  UNASSIGNED (assignable by IANA)
             127     (varies)  RFC 3692-style experiments (EXP)
             128-191           RESERVED

             192     (varies)  Encryption (UENC)
             193-253           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 alter the UDP data payload and thus
   do not interfere with use of that data by legacy endpoints. Options
   indicated by Kind values in the range 192..255 are known as UNSAFE
   options because they do alter the UDP data payload and thus would
   interfere with legacy endpoints. UNSAFE option nicknames are
   expected to begin with "U", which should be avoided for safe option
   nicknames (see Section 23). RESERVED and RESERVED-UNSAFE are not
   assignable by IANA and not otherwise defined at this time.

   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 UDP payloads, thus is
   SAFE in actual use, as discussed further in Section 9.4.

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

   >> An endpoint supporting UDP options MUST support those marked with
   a "*" above: EOL, NOP, APC, FRAG, MDS, MRDS, REQ, and RES. This

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   includes both recognizing and being able to generate these options
   if configured to do so. These are called "must-support" options.

   >> An endpoint supporting UDP options MUST treat unsupported options
   in the UNSAFE range as terminating all option processing.

   >> 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). 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 are used only in with the FRAG option, in a manner
   that prevents them from being silently ignored but passing the UDP
   payload to the user when not supported. This ensures their safe use
   in environments that might include legacy receivers (See Section

   >> Receivers supporting UDP options MUST silently drop all UDP
   options in a datagram containing an UNSAFE option when any UNSAFE
   option it contains is unknown. See Section 10 for further discussion
   of UNSAFE options.

   >> Except for NOP, EXP, and UEXP, each option SHOULD NOT occur more
   than once in a single UDP datagram. 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.

   >> EXP and UEXP MAY occur more than once, but SHOULD NOT occur more
   than once using the same ExID (see Sections 9.10 and 10.2).

   >> Only the OCS and the AUTH and UENC options 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 a value
   dependent on the contents of the surplus area. Otherwise,
   interactions between those values, the OCS, and the AUTH and UENC
   options could be unpredictable.

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   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). The exception is only for lengths that imply a
   physical impossibility, e.g., smaller than two for conventional
   options and four for extended length options. Impossible lengths
   should indicate a malformed surplus area and all options silently
   discarded. Lengths other than those expected should result in safe
   options being ignored and skipped over, as with any other unknown
   safe option.

   >> Option lengths MUST NOT exceed the IP length of the overall IP
   datagram. If this occurs, the options MUST be treated as malformed
   and all options dropped, and the event MAY be logged for diagnostics
   (logging SHOULD be rate limited).

   >> "Must-support" options other than NOP and EOL MUST come before
   other options.

   The requirement that must-support options come before others is
   intended to allow for endpoints to implement DOS protection, as
   discussed further in Section 22.

9. 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 10). The following subsections
   describe safe options defined in this document.

9.1. 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.

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   >> NOPs SHOULD NOT be used as padding before the EOL option. As a
   one byte option, it 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 be treated as zero regardless of their content
   and are not 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 [Fa22]).

9.2. 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

   >> 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.

   This issue is discussed further in Section 22.

9.3. Alternate Payload Checksum (APC)

   The Alternate Payload Checksum (APC, Kind=2) option provides a
   stronger alternative 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 an
   "alternate" to the UDP checksum that covers the user data - not to
   the OCS (the latter covers the surplus area only). Unlike the UDP
   checksum, APC does not include the IP pseudoheader or UDP header,
   thus it does not need to be updated by NATs when IP addresses or UDP

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   ports are rewritten. Its purpose is to detect user data errors that
   the UDP checksum, when used, 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
   that protection because it would need to be updated whenever the UDP
   pseudoheader changed, e.g., during NAT address and port translation;
   because this is not the case, APC does not cover the pseudoheader.

   >> UDP packets with incorrect APC checksums MUST be passed to the
   application by default, e.g., with a flag indicating APC failure.

   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.

   >> UDP packets with unrecognized APC lengths MUST be 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.

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9.4. Fragmentation (FRAG)

   The Fragmentation (FRAG, Kind=3) option supports UDP fragmentation
   and reassembly, which can be used to transfer UDP messages larger
   than limited 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. FRAG
   is designed similar to 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. 10.1). Like
   fragmentation, such options would not be safely used on UDP payloads
   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 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 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.

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

              Figure 10   UDP non-terminal FRAG option format

   In the non-terminal FRAG option format, 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
   are applied to this UDP fragment. Non-terminal fragments never have
   options after the fragment.

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

   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

   >> 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 9.6).

   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.

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

                Figure 11   UDP terminal FRAG option format

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   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 after the original UDP datagram. UDP
   options that apply to the reassembled datagram are contained in the
   partially reassembled payload, 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 measurements.

   If RDOS is larger than Frag. Start of the first fragment (offset=8),
   then all the per-packet options occur before the user data as it is
   reassembled. If RDOS points to the end of the original IP packet,
   then there are no per-packet options. If RDOS is smaller than Frag.
   Start of the first fragment, then it indicates the end of the per-
   fragment options (of the first fragment) and the start of per-packet
   options (of the reassembled user data). The RDOS field enables the
   FRAG option to precede other options in every fragment and to enable
   all packet options to precede user data, enabling easier support for
   reassembly offload via DMA and to support limited router option
   processing hardware.

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

   The Fragment Offset is 16 bits and indicates the location of the UDP
   payload fragment in bytes from the beginning of the original UDP
   header. The option Len field indicates whether there are more
   fragments (Len=10) or no more fragments (Len=12).

   >> 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.

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   It should be noted that fragments may 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 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

   Any per-datagram UDP options, if used, follow the FRAG option in the
   final fragment and would be included in the reassembled UDP packet.
   Processing of those options would commence after reassembly. This is
   especially important for UNSAFE options, which are interpreted only
   after FRAG.

   In general, UDP packets are fragmented as follows:

   1. Create a UDP packet with data and UDP options, which we will call
      "D". Note that the UDP options treat the data area as UDP user
      data and thus must follow that data.

      Process these UDP options before the rest of the fragmentation
      steps below. Note that 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 the 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.

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   3. Fragment "D" into chunks of size no larger than "S"-10 each, with
      one final chunk no larger than "S"-12. Note that all the non-FRAG
      options in step #1 need not be limited to the terminal fragment,
      i.e., the Dgram Opt. Start pointer can indicate the start of the
      original surplus area anywhere in the reassembled data.

      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 segment, 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 zero-data UDP packet
      followed by the word-aligned OCS, the FRAG option, and any
      additional UDP options, followed by the FRAG data chunk.

      The last chunk includes the non-FRAG options noted in step #1
      after the end of the FRAG data. These UDP options apply to the
      reassembled user data as a whole when received.

   5. Process the pre-reassembly UDP options of 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,
   the entire UDP packet is processed with any trailing UDP options
   applying to the reassembled user data.

   The OCS value for the reassembled datagram SHOULD be zero, because
   either the original UDP CS=0 or OCS!=0 in each of the fragments.

   Reassembly failures at the receiver result in silent discard of any
   per-fragment options and fragment contents. To emulate the behavior
   of a legacy host, any initial fragments received but not
   successfully reassembled SHOULD each generate a zero-length UDP
   application message.

   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 support limited header

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9.5. 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 need 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; it does not indicate a known path MTU and
   thus MUST NOT be used to limit transmissions.

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

                     Figure 12   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
   retransmission. It 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 9.6),
   e.g., when EMTU_R is larger than the required minimums (576 for IPv4
   [RFC791] and 1500 for IPv6 [RFC8200]). It can also be used with
   DPLPMTUD [RFC8899] to provide a hint to maximum DPLPMTU, though it
   MUST NOT prohibit transmission of larger UDP packets (or fragments)
   used as DPLPMTU probes.

9.6. Maximum Reassembled Datagram Size (MRDS)

   The Maximum Reassembled Segment Size (MRDS, Kind=5) option is a 16-
   bit indicator of the largest reassembled UDP segment 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 9.4), 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 13   UDP MRDS option format

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

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

   The echo request (REQ, Kind=6) and echo response (RES, Kind=7)
   options provide a means for UDP options to be used to provide UDP
   packet-level acknowledgements. One such use is described as part of
   the UDP options variant of packetization layer path MTU discovery
   (PLPMTUD) [Fa22]. The options both have the format indicated in
   Figure 14, in which the token has no internal structure or meaning.

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

                 Figure 14   UDP REQ and RES options format

   Each of these option kinds appears at most once in each UDP packet,
   as with other options. Note also that the FRAG option is not used
   when sending DPLPMTUD probes to determine a PLPMTU [Fa22].

9.8. 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
   [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 15   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.

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   >> 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) 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.

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

9.9. Authentication (AUTH)

   The Authentication (AUTH, Kind=9) option is intended to allow UDP to
   provide a similar type of authentication as the TCP Authentication
   Option (TCP-AO) [RFC5925]. AUTH covers the UDP user data. AUTH
   supports NAT traversal in a similar manner as TCP-AO [RFC6978].

   Figure 16 shows the UDP AUTH format, whose contents are identical to
   that of the TCP-AO option, with the addition of a 32-bit unsigned
   sequence number. The sequence number is used to differentiate
   otherwise identical datagrams for cryptographic purposes; it is
   intended to not repeat during the lifetime of a security
   association, but are otherwise meaningless (e.g., they can be
   monotonically increased except during rollover). Because AUTH
   sequence numbers are not coordinated and not reliably transmitted,
   in contrast to TCP, they cannot be used to derive session traffic
   keys. During an association, the one-byte KeyID and ReceiveNextKeyID

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   (RNKID) fields serve the same purpose as for TCP-AO, allowing the
   active keys used in either direction to change in a coordinated

                   | Kind=9 |  Len   | KeyID  | RNKID  |
                   |         Sequence Number           |
                   | MAC...                            |
                   | ...MAC                            |

                     Figure 16   UDP AUTH option format

   Like TCP-AO, AUTH is not negotiated in-band. Its use assumes both
   endpoints have populated Master Key Tuples (MKTs), used to exclude
   non-protected traffic.

   TCP-AO generates unique traffic keys from a hash of TCP connection
   parameters. UDP lacks a three-way handshake to coordinate
   connection-specific values, such as TCP's Initial Sequence Numbers
   (ISNs) [RFC9293], thus AUTH's Key Derivation Function (KDF) uses
   zeroes as the value for both ISNs. This means that the AUTH reuses
   keys when socket pairs are reused, unlike TCP-AO.

   >> UDP packets with incorrect AUTH HMACs MUST be passed to the
   application by default, e.g., with a flag indicating AUTH failure.

   >> UDP fragments with individual incorrect AUTH HMACs MUST be
   accumulated and passed to the application by default as part of the
   reassembled packet.

   >> If used with UDP fragments, AUTH MUST be configured to cover the
   UDP option area (because fragments have an empty UDP data area).

   Like all non-UNSAFE UDP options, AUTH needs to be silently ignored
   when failing. This silently-ignored behavior ensures that option-
   aware receivers operate the same as legacy receivers unless

   In addition to the UDP user data (which is always included), AUTH
   can be configured to either include or exclude the surplus area
   (again, the latter is not allowed for UDP fragments), in a similar

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   way as can TCP-AO can optionally exclude TCP options. When UDP
   options are covered, the OCS value and AUTH (and later, UENC) hash
   areas are zeroed before computing the AUTH hash. It is important to
   consider that options not yet defined might yield unpredictable
   results if not confirmed as supported, e.g., if they were to contain
   other hashes or checksums that depend on the surplus area contents.
   This is why such dependencies are not permitted except as defined
   for the OCS and the AUTH (and later, UENC) option.

   Similar to TCP-AO-NAT, AUTH (and later, UENC) can be configured to
   support NAT traversal, excluding (by zeroing out) one or both of the
   UDP ports and corresponding IP addresses [RFC6978].

9.10. Experimental (EXP)

   The Experimental option (EXP, Kind=127) is reserved for experiments
   [RFC3692]. Only one such 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].

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

                     Figure 17   UDP EXP option format

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

   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 23). Assigned ExIDs can be

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   used in either the EXP or UEXP options (see Section 10.2 for the

10. 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.
   Note that this still results in the receipt of a zero-length UDP

10.1. UNSAFE Encryption (UENC)

   UNSAFE encryption (UENC, Kind=192) has the same format as AUTH
   (Section 9.9), except that it encrypts (modifies) the user data. It
   provides a similar encryption capability as TCP-AO-ENC, in a similar
   manner [To18]. Its fields, coverage, and processing are the same as
   for AUTH, except that UENC encrypts the user data and (when
   configured to) the portion of the surplus area that occurs after
   UENC, although it can (optionally) depend on options that precede it
   (with certain fields zeroed, as per AUTH, e.g., providing
   authentication over the surplus area). Like AUTH, UENC can be
   configured to be compatible with NAT traversal.

   Because UDP lacks TCP's Initial Sequence Numbers (ISNs), those
   values are zero for the purposes of computing traffic keys based on
   the TCP-AO approach.

10.2. UNSAFE Experimental (UEXP)

   The UNSAFE Experimental option (UEXP, Kind=254) is reserved for
   experiments [RFC3692]. As with EXP, only one such UEXP value is

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   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.

11. 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 other option content.

   >> New options MUST NOT depend on the content of other options.

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

   >> New options MUST NOT declare their order relative to other
   options, whether new or old.

   >> 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 intend optionally for
   modification of any UDP options, including their new areas, in

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

   o  >> Only FRAG and UNSAFE options are permitted to modify the UDP

   The following recommendation helps enable efficient zero-copy

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

12. Option inclusion and processing

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

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

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

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

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

   >> 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 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)
               or ((OCS != 0 and passes) 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 when 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

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   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.

   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.

   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).

13. 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

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        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 as indicated
      by the application. Datagrams not containing these required
      options MUST be silently dropped and MAY be logged.

   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.

   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 [Fa22].

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

14. 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

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   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 intended for use only by the transport endpoints.
   They are no more (or less) appropriate to be modified in-transit
   than any other portion of the transport datagram.

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

15. UDP options vs. UDP-Lite

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

   UDP (protocol 17) already defines the UDP Length field as the limit
   of the UDP checksum, but by default also limits the data provided to
   the application as that which precedes the UDP Length. A goal of
   UDP-Lite is to deliver data beyond UDP Length as a default, which is
   why a separate transport protocol number was required.

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

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

16. 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.

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   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.

17. 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 MUST allow for
   message reordering and loss.

   The above requirements prevent using any option that cannot be
   safely ignored unless it is hidden inside the FRAG area (i.e.,

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   UNSAFE options). Legacy systems also always need to be able to
   interpret the transport fragments as individual UDP packets.

18. 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
   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.

19. 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

20. 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

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   indicates - allowing UDP options in addition to the trailer options
   of the IPv6 payload. The result, if UDP options were used, is shown
   in Figure 19.

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

         Figure 19   TE trailers and UDP options used concurrently

   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].

21. 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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22. 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 UDP 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. Except when enabled, UDP
   options cause no additional work at the receiver. At most, the
   required options (if enabled) result in a responding option in the
   next transmitted packet, but no options (including ECHO) ever
   initiate UDP responses 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). Despite the name, 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]) or in UDP by the
   Authentication (AUTH) option (Section 9.9) and UNSAFE Encryption
   (UENC) option (Section 10). Transport headers are also protected as
   payload when using IP security (IPsec) [RFC4301].

   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 8.

   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 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).

   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.

23. IANA Considerations

   Upon publication, IANA is hereby requested to create a new registry
   for UDP Option Kind numbers, similar to that for TCP Option Kinds;
   this assumes the creation of a new UDP registry group in which UDP
   Option Kinds would be the only entry.

   Initial values of the UDP Option Kind registry are as listed in
   Section 8. Additional values in this registry are to be assigned
   from the UNASSIGNED values in Section 8 by IESG Approval or
   Standards Action [RFC8126]. Those assignments are subject to the

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   conditions set forth in this document, particularly (but not limited
   to) those in Section 11.

   Although option nicknames are not used in-band, new UNSAFE safe
   option values SHOULD commence with the letter "U" and avoid that
   letter as commencing safe options.

   Upon publication, IANA is hereby requested to create a new registry
   for UDP Experimental Option Experiment Identifiers (UDP ExIDs) for
   use in a similar manner as TCP ExIDs [RFC6994]. UDP ExIDs can be
   used in either (or both) the EXP or UEXP options. This registry is
   initially empty. Values in this registry are to be assigned by IANA
   using first-come, first-served (FCFS) rules [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 11.

24. References

24.1. Normative References

   [Fa22]    Fairhurst, G., T. Jones, "Datagram PLPMTUD for UDP
             Options," draft-ietf-tsvwg-udp-options-dplpmtud, Sep.

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

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

   [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.

24.2. Informative References

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

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

   [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.

   [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.

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   [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.

   [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.

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   [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.

25. Acknowledgments

   This work benefitted from feedback from Erik Auerswald, Bob Briscoe,
   Ken Calvert, Ted Faber, Gorry Fairhurst (including OCS for
   misbehaving middlebox traversal), C. M. Heard (including combining
   previous FRAG and LITE options into the new FRAG), Tom Herbert, Mark
   Smith, 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 2-Word-v2.0.template.dot.

Authors' Addresses

   Joe Touch
   Manhattan Beach, CA 90266 USA

   Phone: +1 (310) 560-0334
   Email: touch@strayalpha.com

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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         Default use OCS
           net.ipv4.udp_opt_apc   0         Default include APC
           net.ipv4.udp_opt_frag  0         Default fragment
           net.ipv4.udp_opt_mds   0         Default include MDS
           net.ipv4.udp_opt_mrds  0         Default include MRDS
           net.ipv4.udp_opt_req   0         Default include REQ
           net.ipv4.udp_opt_resp  0         Default include RES
           net.ipv4.udp_opt_time  0         Default include TIME
           net.ipv4.udp_opt_auth  0         Default include AUTH
           net.ipv4.udp_opt_exp   0         Default include EXP
           net.ipv4.udp_opt_uenc  0         Default include UENC
           net.ipv4.udp_opt_uexp  0         Default 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_UENC   Enable UDP UENC option
           UDP OPT UEXP   Enable UDP UEXP option

   Send/sendto parameters:

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

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

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

   System variables

   net.ipv4.udp_opt_junk   0

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   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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