TSGWG                                                          J. Touch
Internet Draft                                                  USC/ISI
Intended status: Experimental                           January 3, 2017
Expires: July 2017

                         Transport Options for UDP

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   Transport protocols are extended through the use of transport header
   options. This document experimentally extends UDP to provide a
   location, syntax, and semantics for transport layer options.

Table of Contents

   1. Introduction...................................................2
   2. Conventions used in this document..............................2
   3. Background.....................................................3
   4. The UDP Option Area............................................3
   5. UDP Options....................................................6
      5.1. End of Options List (EOL).................................7
      5.2. No Operation (NOP)........................................7
      5.3. Option Checksum (OCS).....................................8
      5.4. Alternate Checksum (ACS)..................................8
      5.5. Lite (LITE)...............................................9
      5.6. Experimental (EXP).......................................11
   6. Whose options are these?......................................11
   7. UDP options vs. UDP-Lite......................................12
   8. Interactions with Legacy Devices..............................12
   9. Options in a Stateless, Unreliable Transport Protocol.........13
   10. Security Considerations......................................14
   11. IANA Considerations..........................................14
   12. References...................................................14
      12.1. Normative References....................................14
      12.2. Informative References..................................14
   13. Acknowledgments..............................................15

1. Introduction

   Transport protocols use options as a way to extend their
   capabilities. TCP [RFC793], SCTP [RFC4960], and DCCP [RFC4340]
   include space for these options but UDP [RFC768] currently does not.
   This document defines an experimental extension to UDP that provides
   space for transport options including their generic syntax and
   semantics for their use in UDP's stateless, unreliable message

2. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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   In this document, these words will appear with that interpretation
   only when in ALL CAPS. Lowercase uses of these words are not to be
   interpreted as carrying significance described in RFC 2119.

   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. Background

   Many protocols include a default header and an area for header
   options. 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

   These options are used both in stateful (connection-oriented, e.g.,
   TCP [RFC793], SCTP [RFC4960], DCCP [RFC4340]) and stateless
   (connectionless, e.g., IPv4 [RFC791], IPv6 [RFC2460] 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 as well. One example of such uses is Substrate Protocol for
   User Datagrams (SPUD) [Tr15], and this document is intended to
   provide an out-of-band option area as an alternative to the in-band
   mechanism currently proposed [Hi15].

   UDP is one of the most popular protocols that lacks space for
   options [RFC768]. The UDP header was intended to be a minimal
   addition to IP, providing only ports and a data checksum for
   protection. This document experimentally extends UDP to provide a
   trailer area for options located after the UDP data payload.

4. The UDP Option Area

   The UDP transport header includes demultiplexing and service
   identification (port numbers), a checksum, and a field that
   indicates the UDP datagram length (including UDP header). The UDP
   Length length field is typically redundant with the size of the
   maximum space available as a transport protocol payload (see also
   discussion in Section 8).

   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

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   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

   For IPv6, the IP Payload Length field indicates the datagram after
   the base IPv6 header, which includes the IPv6 extension headers and
   space available for the transport protocol, as shown in Figure 2
   [RFC2460]. 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 [RFC2460], so the typical (and
   largest valid) value for UDP Length is:

       UDP_Length = IPv6_Payload_Length - sum(extension header lengths)

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

      |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 transport payload

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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 transport payload

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

                             IP transport payload
      | IP Hdr | UDP Hdr |     UDP user data    |   surplus area   |
                           UDP Length

               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. It is
   important to note that this is not a requirement of UDP [RFC768]
   (discussed further in Section 8). UDP-Lite used the difference in
   these pointers to indicate the partial coverage of the UDP Checksum,
   such that the UDP user data, UDP header, and UDP pseudoheader (a
   subset of the IP header) are covered by the UDP checksum but
   additional user data in the surplus area is not covered [RFC3828].
   This document uses the surplus area for UDP transport options.

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   The UDP option area is thus defined as the location between the end
   of the UDP payload and the end of the IP datagram as a trailing
   options area. This area can occur 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 offset".

   UDP options are defined using a syntax similar to that of TCP
   [RFC793]. They are typically a minimum of two bytes in length as
   shown in Figure 4, excepting only the one byte options "No
   Operation" (NOP) and "End of Options List" (EOL) described below.

                        |  Kind  | Length |

                    Figure 4 UDP option default format

   >> UDP options MAY occur at any UDP length offset.

   >> The UDP length MUST be at least as large as the UDP header (8)
   and no larger than the IP transport payload. Values outside this
   range MUST be silently discarded as invalid and logged where rate-
   limiting permits.

   Others have considered using values of the UDP Length that is larger
   than the IP transport payload as an additional type of signal. Using
   a value smaller than the IP transport payload is expected to be
   backward compatible with existing UDP implementations, i.e., to
   deliver the UDP Length of user data to the application and silently
   ignore the additional surplus area data. Using a value larger than
   the IP transport payload would either be considered malformed (and
   be silently dropped) or could cause buffer overruns, and so is not
   considered silently and safely backward compatible. Its use is thus
   out of scope for the extension described in this document.

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

5. UDP Options

   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       2         Option checksum (OCS)
             3       4         Alternate checksum (ACS)
             4       4         Lite (LITE)
             128-253           RESERVED
             254     N(>=4)    RFC 3692-style experiments (EXP)
             255               RESERVED

   These options are defined in the following subsections.

5.1. End of Options List (EOL)

   The End of Options List (EOL) option indicates that there are no
   more options. It is used to indicate the end of the list of options
   without needing to pad the options to fill all available option

                                 | Kind=0 |

                      Figure 5 UDP EOL option format

   >> When the UDP options do not consume the entire option area, the
   last non-NOP option SHOULD be EOL (vs. filling the entire option
   area with NOP values).

   >> All bytes after EOL MUST be ignored by UDP option processing.

5.2. No Operation (NOP)

   The No Operation (NOP) option is a one byte placeholder, intended to
   be used as padding, e.g., to align multi-byte options along 16-bit
   or 32-bit boundaries.

                                 | Kind=1 |

                      Figure 6 UDP NOP option format

   >> If options longer than one byte are used, NOP options SHOULD be
   used at the beginning of the UDP options area to achieve alignment
   as would be more efficient for active (i.e., non-NOP) options.

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5.3. Option Checksum (OCS)

   The Option Checksum (OCS, Kind = 2) is an 8-bit ones-complement sum
   (Ones8) that covers only the UDP options, from the first option as
   indicated by the UDP Length to the last option as indicated by EOL
   (where present) or the IP Payload Length. OCS can be calculated by
   computing the 16-bit ones-complement sum and "folding over" the
   result (using carry wraparound). Note that OCS is direct, i.e., it
   is not negated or adjusted if zero (unlike the Internet checksum as
   used in IPv4, TCP, and UDP headers). OCS protects the option area
   from errors in a similar way that the UDP checksum protects the UDP
   user data.

                            | Kind=2 | Ones8  |

                      Figure 7 UDP OCS option format

   >> When present, the option checksum SHOULD occur as early as
   possible, preferably preceded by only NOP options for alignment and
   the LITE option if present.

   >> If the option checksum fails, all options MUST be ignored and any
   trailing surplus data silently discarded.

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

5.4. Alternate Checksum (ACS)

   The Alternate Checksum (ACS) is a CRC16 of the UDP payload only. It
   does not include the IP pseudoheader or UDP header, and so need not
   be updated by NATs when IP addresses or UDP ports are rewritten. Its
   purpose is to detect errors that the UDP checksum might not detect.

                   | Kind=3 | Len=4  |     CRC16sum    |

                      Figure 8 UDP ACS option format

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5.5. Lite (LITE)

   The Lite option is intended to provide equivalent capability to the
   UDP Lite transport protocol [RFC3828]. UDP Lite allows the UDP
   checksum to cover only a prefix of the UDP data payload, to protect
   critical information (e.g., application headers) but allow
   potentially erroneous data to be passed to the user. This feature
   helps protect application headers but allows for application data
   errors. Some applications are impacted more by a lack of data than
   errors in data, e.g., voice and video.

   >> When the Lite option is active, it MUST come first in the UDP
   options list.

   The Lite option is intended to support the same API as for UDP Lite
   to allow applications to send and receive data that has a marker
   indicating the portion protected by the UDP checksum and the portion
   not protected by the UDP checksum.

   The option includes a 2-byte offset that indicates the length of the
   portion of the UDP data that is not covered by the UDP checksum.

                   | Kind=4 | Len=4  |      Offset     |

                      Figure 9 UDP LITE option format

   At the sender, the option is formed using the following steps:

   1. Create a LITE option, ordered as the first UDP option (Figure

   2. Calculate the location of the start of the options as an absolute
      offset from the start of the UDP header and place that length in
      the last two bytes of the LITE option.

   3. Swap all four bytes of the LITE option with the first 4 bytes of
      the LITE data area (Figure 11).

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      | UDP Hdr |  user data   |  Lite data   |LITE| other opts  |
              UDP Length

            Figure 10   Lite option formation - LITE goes first

      | UDP Hdr |  user data   |  Lite data   |LITE| other opts  |
                                ^^^^           ^^^^
                                  |              |

   Figure 11   Lite option before transmission - swap LITE and front of
                                 LITE data

   The resulting packet has the format shown in Figure 12. Note that
   the UDP length now points to the LITE option, and the LITE option
   points to the start of the option area.

      | UDP Hdr |  user data   |LITE| Lite data |Ldat| other opts  |
       <---------------------->    |             ^
              UDP Length           +-------------+

                  Figure 12   Lite option as transmitted

   A legacy endpoint receiving this packet will discard the LITE option
   and everything that follows, including the lite data and remainder
   of the UDP options. The UDP checksum will protect only the user
   data, not the LITE option or lite data.

   Receiving endpoints capable of processing UDP options will do the

   1. Process options as usual. This will start at the LITE option.

   2. When the LITE option is encountered, record its location as the
      start of the LITE data area and swap the four bytes there with
      the four bytes at the location indicated inside the LITE option,
      which indicates the start of all of the options, including the
      LITE one (one past the end of the lite data area).

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   3. Continue processing the remainder of the options, which are now
      in the format shown in Figure 11.

   The purpose of this swap is to support UDP Lite operation and UDP
   options without requiring the entire lite data area to be moved
   after the UDP option area.

5.6. Experimental (EXP)

   The Experimental option (EXP) 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].

   >> 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]).

6. Whose options are these?

   UDP options are indicated in an 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 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 are transport options. Generally, transport
   datagrams are not intended to be modified in-transit. However, the
   UDP option mechanism provides no specific protection against in-
   transit modification of the UDP header, UDP payload, or UDP option

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7. UDP options vs. UDP-Lite

   UDP-Lite provides partial checksum coverage, so that 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

   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 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 options support a similar service to UDP-Lite by terminating the
   UDP options with an EOL option. The additional data not covered by
   the UDP checksum follows that EOL option, and is passed to the user
   separately. The difference is that UDP-Lite provides the un-
   checksummed user data to the application by default, whereas UDP
   options can provide the same capability only for endpoints that are
   negotiated in advance (i.e., by default, UDP options would silently
   discard this non-checksummed data). Additionally, in UDP-Lite the
   checksummed and non-checksummed payload components are adjacent,
   whereas in UDP options they are separated by the option area -
   which, minimally, must consist of at least one EOL option.

   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 separate 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.

8. 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

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   that appear to have UDP options. The UDP OCS provides protection
   against such events and is stronger than a static "magic number".

   UDP options have been tested as interoperable with Linux, Max OS-X,
   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.

   There was one embedded device reported that passed the entire IP
   transport payload to the user UDP socket. This is already
   inconsistent with UDP and host requirements [RFC768] [RFC1122], as
   it presents the entire IP transport payload to the user (including
   the transport header) instead of presenting the transport payload to
   the corresponding to the transport protocol, where the transport
   header would have been removed.

   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.

   (TBD: test with UDP checksum offload and UDP fragmentation offload)

9. 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.

   >> UDP options MUST allow for silent failure on first receipt.

   >> UDP options that rely on soft-state exchange MUST allow for
   message reordering and loss.

   >> A UDP option MUST be silently optional until confirmed by
   exchange with an endpoint.

   It is useful that the above requirements prevent using UDP options
   to implement transport-layer fragmentation and reassembly unless
   that capability has been negotiated with an endpoint in advance for

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   a socket pair. Legacy systems would need to be able to interpret the
   transport payload fragments as individual transport datagrams.

10. Security Considerations

   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 a vulnerability and it would rely on inconsistent use of the
   two fields for memory allocation and copying.

11. 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.
   Initial values of this registry are as indicated herein. Additional
   values in this registry are to be assigned by IESG Approval or
   Standards Action [RFC5226].

   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]. Values in this
   registry are to be assigned by IANA using first-come, first-served
   (FCFS) rules [RFC5226].

12. References

12.1. Normative References

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

12.2. Informative References

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

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

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

   [RFC793]  Postel, J., "Transmission Control Protocol" RFC 793,
             September 1981.

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

   [RFC2460] Deering, S., R. Hinden, "Internet Protocol Version 6
             (IPv6) Specification," RFC 2460, Dec. 1998.

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

   [RFC4960] Stewart, R. (Ed.), "Stream Control Transmission Protocol",
             RFC 4960, September 2007.

   [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.

   [RFC5226] Narten, T., H. Alvestrand, "Guidelines for Writing an IANA
             Considerations Section in RFCs," RFC 5226, May 2008.

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

   [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.

   [Tr15]    Trammel, B. (Ed.), M. Kuelewind (Ed.), "Requirements for
             the design of a Substrate Protocol for User Datagrams
             (SPUD)," draft-trammell-spud-req-04, May 2016.

13. Acknowledgments

   This work benefitted from feedback from Bob Briscoe, Ken Calvert,
   Ted Faber, Gorry Fairhurst, C. M. Heard, Tom Herbert, as well as
   discussions on the IETF SPUD email list.

   This document was prepared using 2-Word-v2.0.template.dot.

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Authors' Addresses

   Joe Touch
   4676 Admiralty Way
   Marina del Rey, CA 90292 USA

   Phone: +1 (310) 448-9151
   Email: touch@isi.edu

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