Network Working Group                                        L-A. Larzon
Request for Comments: 3828                Lulea University of Technology
Category: Standards Track                                   M. Degermark
                                                                S. Pink
                                              The University of Arizona
                                                      L-E. Jonsson, Ed.
                                                               Ericsson
                                                      G. Fairhurst, Ed.
                                                 University of Aberdeen
                                                              July 2004


          The Lightweight User Datagram Protocol (UDP-Lite)

Status of this Memo

  This document specifies an Internet standards track protocol for the
  Internet community, and requests discussion and suggestions for
  improvements.  Please refer to the current edition of the "Internet
  Official Protocol Standards" (STD 1) for the standardization state
  and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

  Copyright (C) The Internet Society (2004).

Abstract

  This document describes the Lightweight User Datagram Protocol (UDP-
  Lite), which is similar to the User Datagram Protocol (UDP) (RFC
  768), but can also serve applications in error-prone network
  environments that prefer to have partially damaged payloads delivered
  rather than discarded.  If this feature is not used, UDP-Lite is
  semantically identical to UDP.

















Larzon, et al.              Standards Track                     [Page 1]

RFC 3828                   UDP-Lite Protocol                   July 2004


Table of Contents

  1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
  2.  Terminology. . . . . . . . . . . . . . . . . . . . . . . . . .  3
  3.  Protocol Description . . . . . . . . . . . . . . . . . . . . .  3
      3.1.  Fields . . . . . . . . . . . . . . . . . . . . . . . . .  4
      3.2.  Pseudo Header. . . . . . . . . . . . . . . . . . . . . .  5
      3.3.  Application Interface. . . . . . . . . . . . . . . . . .  5
      3.4.  IP Interface . . . . . . . . . . . . . . . . . . . . . .  6
      3.5.  Jumbograms . . . . . . . . . . . . . . . . . . . . . . .  6
  4.  Lower Layer Considerations . . . . . . . . . . . . . . . . . .  6
  5.  Compatibility with UDP . . . . . . . . . . . . . . . . . . . .  7
  6.  Security Considerations. . . . . . . . . . . . . . . . . . . .  8
  7.  IANA Considerations. . . . . . . . . . . . . . . . . . . . . .  8
  8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
      8.1.  Normative References . . . . . . . . . . . . . . . . . .  9
      8.2.  Informative References . . . . . . . . . . . . . . . . .  9
  9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
  10. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 11
  11. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 12

1.  Introduction

  This document describes a new transport protocol, UDP-Lite, (also
  known as UDPLite).  This new protocol is based on three observations:

  First, there is a class of applications that benefit from having
  damaged data delivered rather than discarded by the network.  A
  number of codecs for voice and video fall into this class (e.g., the
  AMR speech codec [RFC-3267], the Internet Low Bit Rate Codec [ILBRC],
  and error resilient H.263+ [ITU-H.263], H.264 [ITU-H.264; H.264], and
  MPEG-4 [ISO-14496] video codecs).  These codecs may be designed to
  cope better with errors in the payload than with loss of entire
  packets.

  Second, all links that support IP transmission should use a strong
  link layer integrity check (e.g., CRC-32 [RFC-3819]), and this MUST
  be used by default for IP traffic.  When the under-lying link
  supports it, certain types of traffic (e.g., UDP-Lite) may benefit
  from a different link behavior that permits partially damaged IP
  packets to be forwarded when requested [RFC-3819].  Several radio
  technologies (e.g., [3GPP]) support this link behavior when operating
  at a point where cost and delay are sufficiently low.  If error-prone
  links are aware of the error sensitive portion of a packet, it is
  also possible for the physical link to provide greater protection to
  reduce the probability of corruption of these error sensitive bytes
  (e.g., the use of unequal Forward Error Correction).




Larzon, et al.              Standards Track                     [Page 2]

RFC 3828                   UDP-Lite Protocol                   July 2004


  Third, intermediate layers (i.e., IP and the transport layer
  protocols) should not prevent error-tolerant applications from
  running well in the presence of such links.  IP is not a problem in
  this regard, since the IP header has no checksum that covers the IP
  payload.  The generally available transport protocol best suited for
  these applications is UDP, since it has no overhead for
  retransmission of erroneous packets, in-order delivery, or error
  correction.  In IPv4 [RFC-791], the UDP checksum covers either the
  entire packet or nothing at all.  In IPv6 [RFC-2460], the UDP
  checksum is mandatory and must not be disabled.  The IPv6 header does
  not have a header checksum and it was deemed necessary to always
  protect the IP addressing information by making the UDP checksum
  mandatory.

  A transport protocol is needed that conforms to the properties of
  link layers and applications described above [LDP99].  The error-
  detection mechanism of the transport layer must be able to protect
  vital information such as headers, but also to optionally ignore
  errors best dealt with by the application.  The set of octets to be
  verified by the checksum is best specified by the sending
  application.

  UDP-Lite provides a checksum with an optional partial coverage.  When
  using this option, a packet is divided into a sensitive part (covered
  by the checksum) and an insensitive part (not covered by the
  checksum).  Errors in the insensitive part will not cause the packet
  to be discarded by the transport layer at the receiving end host.
  When the checksum covers the entire packet, which should be the
  default, UDP-Lite is semantically identical to UDP.

  Compared to UDP, the UDP-Lite partial checksum provides extra
  flexibility for applications that want to define the payload as
  partially insensitive to bit errors.

2.  Terminology

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
  document are to be interpreted as described in [RFC-2119].

3.  Protocol Description

  The UDP-Lite header is shown in figure 1.  Its format differs from
  UDP in that the Length field has been replaced with a Checksum
  Coverage field.  This can be done since information about UDP packet
  length can be provided by the IP module in the same manner as for TCP
  [RFC-793].




Larzon, et al.              Standards Track                     [Page 3]

RFC 3828                   UDP-Lite Protocol                   July 2004


      0              15 16             31
     +--------+--------+--------+--------+
     |     Source      |   Destination   |
     |      Port       |      Port       |
     +--------+--------+--------+--------+
     |    Checksum     |                 |
     |    Coverage     |    Checksum     |
     +--------+--------+--------+--------+
     |                                   |
     :              Payload              :
     |                                   |
     +-----------------------------------+

     Figure 1: UDP-Lite Header Format

3.1.  Fields

  The fields Source Port and Destination Port are defined as in the UDP
  specification [RFC-768].  UDP-Lite uses the same set of port number
  values assigned by the IANA for use by UDP.

  Checksum Coverage is the number of octets, counting from the first
  octet of the UDP-Lite header, that are covered by the checksum.  The
  UDP-Lite header MUST always be covered by the checksum.  Despite this
  requirement, the Checksum Coverage is expressed in octets from the
  beginning of the UDP-Lite header in the same way as for UDP.  A
  Checksum Coverage of zero indicates that the entire UDP-Lite packet
  is covered by the checksum.  This means that the value of the
  Checksum Coverage field MUST be either 0 or at least 8.  A UDP-Lite
  packet with a Checksum Coverage value of 1 to 7 MUST be discarded by
  the receiver.  Irrespective of the Checksum Coverage, the computed
  Checksum field MUST include a pseudo-header, based on the IP header
  (see below).  UDP-Lite packets with a Checksum Coverage greater than
  the IP length MUST also be discarded.

  The Checksum field is the 16-bit one's complement of the one's
  complement sum of a pseudo-header of information collected from the
  IP header, the number of octets specified by the Checksum Coverage
  (starting at the first octet in the UDP-Lite header), virtually
  padded with a zero octet at the end (if necessary) to make a multiple
  of two octets [RFC-1071].  Prior to computation, the checksum field
  MUST be set to zero.  If the computed checksum is 0, it is
  transmitted as all ones (the equivalent in one's complement
  arithmetic).

  Since the transmitted checksum MUST NOT be all zeroes, an application
  using UDP-Lite that wishes to have no protection of the packet
  payload should use a Checksum Coverage value of 8.  This differs



Larzon, et al.              Standards Track                     [Page 4]

RFC 3828                   UDP-Lite Protocol                   July 2004


  from the use of UDP over IPv4 in that the minimal UDP-Lite checksum
  always covers the UDP-Lite protocol header, which includes the
  Checksum Coverage field.

3.2.  Pseudo Header

  UDP and UDP-Lite use the same conceptually prefixed pseudo header
  from the IP layer for the checksum.  This pseudo header is different
  for IPv4 and IPv6.  The pseudo header of UDP-Lite is different from
  the pseudo header of UDP in one way: The value of the Length field of
  the pseudo header is not taken from the UDP-Lite header, but rather
  from information provided by the IP module.  This computation is done
  in the same manner as for TCP [RFC-793], and implies that the Length
  field of the pseudo header includes the UDP-Lite header and all
  subsequent octets in the IP payload.

3.3.  Application Interface

  An application interface should allow the same operations as for UDP.
  In addition to this, it should provide a way for the sending
  application to pass the Checksum Coverage value to the UDP-Lite
  module.  There should also be a way to pass the Checksum Coverage
  value to the receiving application, or at least let the receiving
  application block delivery of packets with coverage values less than
  a value provided by the application.

  It is RECOMMENDED that the default behavior of UDP-Lite be set to
  mimic UDP by having the Checksum Coverage field match the length of
  the UDP-Lite packet and verify the entire packet.  Applications that
  wish to define the payload as partially insensitive to bit errors
  (e.g., error tolerant codecs using RTP [RFC-3550]) should do this by
  an explicit system call on the sender side.  Applications that wish
  to receive payloads that were only partially covered by a checksum
  should inform the receiving system by an explicit system call.

  The characteristics of the links forming an Internet path may vary
  greatly.  It is therefore difficult to make assumptions about the
  level or patterns of errors that may occur in the corruption
  insensitive part of the UDP-Lite payload.  Applications that use
  UDP-Lite should not make any assumptions regarding the correctness of
  the received data beyond the position indicated by the Checksum
  Coverage field, and should, if necessary, introduce their own
  appropriate validity checks.








Larzon, et al.              Standards Track                     [Page 5]

RFC 3828                   UDP-Lite Protocol                   July 2004


3.4.  IP Interface

  As for UDP, the IP module must provide the pseudo header to the UDP-
  Lite protocol module (known as the UDPLite module).  The UDP-Lite
  pseudo header contains the IP addresses and protocol fields of the IP
  header, and also the length of the IP payload, which is derived from
  the Length field in the IP header.

  The sender IP module MUST NOT pad the IP payload with extra octets,
  since the length of the UDP-Lite payload delivered to the receiver
  depends on the length of the IP payload.

3.5.  Jumbograms

  The Checksum Coverage field is 16 bits and can represent a Checksum
  Coverage value of up to 65535 octets.  This allows arbitrary checksum
  coverage for IP packets, unless they are Jumbograms.  For Jumbograms,
  the checksum can cover either the entire payload (when the Checksum
  Coverage field has the value zero), or else at most the initial 65535
  octets of the UDP-Lite packet.

4.  Lower Layer Considerations

  Since UDP-Lite can deliver packets with damaged payloads to an
  application that wishes to receive them, frames carrying UDP-Lite
  packets need not be discarded by lower layer protocols when there are
  errors only in the insensitive part.  For a link that supports
  partial error detection, the Checksum Coverage field in the UDP-Lite
  header MAY be used as a hint of where errors do not need to be
  detected.  Lower layers MUST use a strong error detection mechanism
  [RFC-3819] to detect at least errors that occur in the sensitive part
  of the packet, and discard damaged packets.  The sensitive part
  consists of the octets between the first octet of the IP header and
  the last octet identified by the Checksum Coverage field.  The
  sensitive part would thus be treated in exactly the same way as for a
  UDP packet.

  Link layers that do not support partial error detection suitable for
  UDP-Lite, as described above, MUST detect errors in the entire UDP-
  Lite packet, and MUST discard damaged packets [RFC-3819].  The whole
  UDP-Lite packet is thus treated in exactly the same way as a UDP
  packet.

  It should be noted that UDP-Lite would only make a difference to an
  application if partial error detection, based on the partial checksum
  feature of UDP-Lite, is implemented also by link layers, as discussed
  above.  Partial error detection at the link layer would only make a
  difference when implemented over error-prone links.



Larzon, et al.              Standards Track                     [Page 6]

RFC 3828                   UDP-Lite Protocol                   July 2004


5.  Compatibility with UDP

  UDP and UDP-Lite have similar syntax and semantics.  Applications
  designed for UDP may therefore use UDP-Lite instead, and will by
  default receive the same full packet coverage.  The similarities also
  ease implementation of UDP-Lite, since only minor modifications are
  needed to an existing UDP implementation.

  UDP-Lite has been allocated a separate IP protocol identifier, 136
  (UDPLite), that allows a receiver to identify whether UDP or UDP-Lite
  is used.  A destination end host that is unaware of UDP-Lite will, in
  general, return an ICMP "Protocol Unreachable" or an ICMPv6 "Payload
  Type Unknown" error message (depending on the IP protocol type).
  This simple method of detecting UDP-Lite unaware systems is the
  primary benefit of having separate protocol identifiers.

  The remainder of this section provides the rationale for allocating a
  separate IP protocol identifier for UDP-Lite, rather than sharing the
  IP protocol identifier with UDP.

  There are no known interoperability problems between UDP and UDP-Lite
  if they were to share the protocol identifier with UDP.
  Specifically, there is no case where a potentially problematic packet
  is delivered to an unsuspecting application; a UDP-Lite payload with
  partial checksum coverage cannot be delivered to UDP applications,
  and UDP packets that only partially fill the IP payload cannot be
  delivered to applications using UDP-Lite.

  However, if the protocol identifier were to have been shared between
  UDP and UDP-Lite, and a UDP-Lite implementation was to send a UDP-
  Lite packet using a partial checksum to a UDP implementation, the UDP
  implementation would silently discard the packet, because a
  mismatching pseudo header would cause the UDP checksum to fail.
  Neither the sending nor the receiving application would be notified.
  Potential solutions to this could have been:

  1) explicit application in-band signaling (while not using the
     partial checksum coverage option) to enable the sender to learn
     whether the receiver is UDP-Lite enabled or not, or

  2) use of out-of-band signaling such as H.323, SIP, or RTCP to convey
     whether the receiver is UDP-Lite enabled.

  Since UDP-Lite has been assigned its own IP protocol identifier,
  there is no need to consider this possibility of delivery of a UDP-
  Lite packet to an unsuspecting UDP port.





Larzon, et al.              Standards Track                     [Page 7]

RFC 3828                   UDP-Lite Protocol                   July 2004


6.  Security Considerations

  The security impact of UDP-Lite is related to its interaction with
  authentication and encryption mechanisms.  When the partial checksum
  option of UDP-Lite is enabled, the insensitive portion of a packet
  may change in transit.  This is contrary to the idea behind most
  authentication mechanisms: authentication succeeds if the packet has
  not changed in transit.  Unless authentication mechanisms that
  operate only on the sensitive part of packets are developed and used,
  authentication will always fail for UDP-Lite packets where the
  insensitive part has been damaged.

  The IPsec integrity check (Encapsulation Security Protocol, ESP
  [RFC-2406], or Authentication Header, AH [RFC-2402]) is applied (at
  least) to the entire IP packet payload. Corruption of any bit within
  the protected area will then result in the IP receiver discarding the
  UDP-Lite packet.

  When IPsec is used with ESP payload encryption, a link can not
  determine the specific transport protocol of a packet being forwarded
  by inspecting the IP packet payload.  In this case, the link MUST
  provide a standard integrity check covering the entire IP packet and
  payload.  UDP-Lite provides no benefit in this case.

  Encryption (e.g., at the transport or application levels) may be
  used.  If a few bits of an encrypted packet are damaged, the
  decryption transform will typically spread errors so that the packet
  becomes too damaged to be of use.  Many encryption transforms today
  exhibit this behavior.  There exist encryption transforms, and stream
  ciphers, which do not cause error propagation.  Note that omitting an
  integrity check can, under certain circumstances, compromise
  confidentiality [Bellovin98].  Proper use of stream ciphers poses its
  own challenges [BB01].  In particular, an attacker can cause
  predictable changes to the ultimate plaintext, even without being
  able to decrypt the ciphertext.

7.  IANA Considerations

  A new IP protocol number, 136 has been assigned for UDP-Lite.  The
  name associated with this protocol number is "UDPLite".  This ensures
  compatibility across a wide range of platforms, since on some
  platforms the "-" character may not form part of a protocol entity
  name.








Larzon, et al.              Standards Track                     [Page 8]

RFC 3828                   UDP-Lite Protocol                   July 2004


8.  References

8.1.  Normative References

  [RFC-768]    Postel, J., "User Datagram Protocol", STD 6, RFC 768,
               August 1980.

  [RFC-791]    Postel, J., "Internet Protocol", STD 5, RFC 791,
               September 1981.

  [RFC-793]    Postel, J., "Transmission Control Protocol", STD 7, RFC
               793, September 1981.

  [RFC-1071]   Braden, R., Borman, D. and C. Partridge, "Computing the
               Internet Checksum", RFC 1071, September 1988.

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

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

8.2.  Informative References

  [Bellovin98] Bellovin, S.M., "Cryptography and the Internet", in
               Proceedings of CRYPTO '98, August 1988.

  [BB01]       Bellovin, S. and M. Blaze, "Cryptographic Modes of
               Operation for the Internet", Second NIST Workshop on
               Modes of Operation, August 2001.

  [3GPP]       "Technical Specification Group Services and System
               Aspects; Quality of Service (QoS) concept and
               architecture", TS 23.107 V5.9.0, Technical Specification
               3rd  Generation Partnership Project, June 2003.

  [H.264]      Hannuksela, M.M., Stockhammer, T., Westerlund, M. and D.
               Singer, "RTP payload Format for H.264 Video", Internet
               Draft, Work in Progress, March 2003.

  [ILBRC]      S.V. Andersen, et. al., "Internet Low Bit Rate Codec",
               Work in Progress, March 2003.

  [ISO-14496]  ISO/IEC International Standard 1446 (MPEG-4),
               "Information Technology Coding of Audio-Visual Objects",
               January 2000.





Larzon, et al.              Standards Track                     [Page 9]

RFC 3828                   UDP-Lite Protocol                   July 2004


  [ITU-H.263]  "Video Coding for Low Bit Rate Communication," ITU-T
               Recommendation H.263, January 1998.

  [ITU-H.264]  "Draft ITU-T Recommendation and Final Draft
               International Standard of Joint Video Specification",
               ITU-T Recommendation H.264, May 2003.

  [RFC-3819]   Karn, Ed., P., Bormann, C., Fairhurst, G., Grossman, D.,
               Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J. and
               L. Wood, "Advice for Internet Subnetwork Designers", BCP
               89, RFC 3819, July 2004.

  [RFC-3550]   Schulzrinne, H., Casner, S., Frederick, R. and V.
               Jacobson, "RTP: A Transport Protocol for Real-Time
               Applications", RFC 3550, July 2003.

  [RFC-2402]   Kent, S. and R. Atkinson, "IP Authentication Header",
               RFC 2402, November 1998.

  [RFC-2406]   Kent, S. and R. Atkinson, "IP Encapsulating Security
               Payload (ESP)", RFC 2406, November 1998.

  [RFC-3267]   Sjoberg, J., Westerlund, M., Lakeaniemi, A. and Q. Xie,
               "Real-Time Transport Protocol (RTP) Payload Format and
               File Storage Format for the Adaptive Multi-Rate (AMR)
               and Adaptive Multi-Rate Wideband (AMR-WB) Audio Codecs",
               RFC 3267, June 2002.

  [LDP99]      Larzon, L-A., Degermark, M. and S. Pink, "UDP Lite for
               Real-Time Multimedia Applications", Proceedings of the
               IEEE International Conference of Communications (ICC),
               1999.

9.  Acknowledgements

  Thanks to Ghyslain Pelletier for significant technical and editorial
  comments.  Thanks also to Steven Bellovin, Elisabetta Carrara, and
  Mats Naslund for reviewing the security considerations chapter, and
  to Peter Eriksson for a language review, thereby improving the
  clarity of this document.











Larzon, et al.              Standards Track                    [Page 10]

RFC 3828                   UDP-Lite Protocol                   July 2004


10.  Authors' Addresses

  Lars-Ake Larzon
  Department of CS & EE
  Lulea University of Technology
  S-971 87 Lulea, Sweden

  EMail: [email protected]


  Mikael Degermark
  Department of Computer Science
  The University of Arizona
  P.O. Box 210077
  Tucson, AZ 85721-0077, USA

  EMail: [email protected]


  Stephen Pink
  The University of Arizona
  P.O. Box 210077
  Tucson, AZ 85721-0077, USA

  EMail: [email protected]


  Lars-Erik Jonsson
  Ericsson AB
  Box 920
  S-971 28 Lulea, Sweden

  EMail: [email protected]


  Godred Fairhurst
  Department of Engineering
  University of Aberdeen
  Aberdeen, AB24 3UE, UK

  EMail: [email protected]










Larzon, et al.              Standards Track                    [Page 11]

RFC 3828                   UDP-Lite Protocol                   July 2004


11.  Full Copyright Statement

  Copyright (C) The Internet Society (2004).  This document is subject
  to the rights, licenses and restrictions contained in BCP 78, and
  except as set forth therein, the authors retain all their rights.

  This document and the information contained herein are provided on an
  "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
  OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
  ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
  INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
  INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
  WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

  The IETF takes no position regarding the validity or scope of any
  Intellectual Property Rights or other rights that might be claimed to
  pertain to the implementation or use of the technology described in
  this document or the extent to which any license under such rights
  might or might not be available; nor does it represent that it has
  made any independent effort to identify any such rights.  Information
  on the procedures with respect to rights in RFC documents can be
  found in BCP 78 and BCP 79.

  Copies of IPR disclosures made to the IETF Secretariat and any
  assurances of licenses to be made available, or the result of an
  attempt made to obtain a general license or permission for the use of
  such proprietary rights by implementers or users of this
  specification can be obtained from the IETF on-line IPR repository at
  http://www.ietf.org/ipr.

  The IETF invites any interested party to bring to its attention any
  copyrights, patents or patent applications, or other proprietary
  rights that may cover technology that may be required to implement
  this standard.  Please address the information to the IETF at ietf-
  [email protected].

Acknowledgement

  Funding for the RFC Editor function is currently provided by the
  Internet Society.









Larzon, et al.              Standards Track                    [Page 12]