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Versions: 00 01 02 rfc3828                               Standards Track
    Network Working Group                                        L-A. Larzon
    INTERNET-DRAFT                            Lulea University of Technology
    Expires: January 2003                                       M. Degermark
                                                                     S. Pink
                                                   The University of Arizona
                                                       L-E. Jonsson (Editor)
                                                       G. Fairhurst (Editor)
                                                      University of Aberdeen
                                                                August, 2003
                              The UDP-Lite Protocol
    Status of this memo
       This document is an Internet-Draft and is in full conformance with
       all provisions of Section 10 of RFC2026.
       Internet-Drafts are working documents of the Internet Engineering
       Task Force (IETF), its areas, and its working groups. Note that other
       groups may also distribute working documents as Internet-Drafts.
       Internet-Drafts are draft documents valid for a maximum of six months
       and may be updated, replaced, or obsoleted by other documents at any
       time. It is inappropriate to use Internet-Drafts as reference
       material or cite them other than as "work in progress".
       The list of current Internet-Drafts can be accessed at
       The list of Internet-Draft Shadow Directories can be accessed at
       Please direct comments to the TSV WG mailing list: tsvwg@ietf.org
       This document describes the UDP-Lite protocol, which is similar to
       UDP [RFC-768], but can also serve applications that in error-prone
       network environments prefer to have partially damaged payloads
       delivered rather than discarded. If this feature is not used, UDP-
       Lite is semantically identical to UDP.
    Table of Contents
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       1.  Introduction...................................................2
       2.  Terminology....................................................3
       3.  Protocol Description...........................................3
          3.1.  Fields....................................................3
          3.2.  Pseudo Header.............................................4
          3.3.  Application Interface.....................................4
          3.4.  IP Interface..............................................5
          3.5.  Jumbograms................................................5
       4.  Lower Layer Considerations.....................................6
       5.  Compatibility with UDP.........................................6
       6.  Security Considerations........................................7
       7.  IANA Considerations............................................8
       8.  References.....................................................8
          8.1.  Normative References......................................8
          8.2.  Informative References....................................9
       9.  Acknowledgements...............................................10
       10.  Authors' Addresses............................................11
       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
       Second, all links that support IP transmission should use a strong
       link layer integrity check (e.g. CRC-32 [LINK]), 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 forwaded when requested [LINK]. Several radio technologies (e.g.
       [3GPP-QoS])  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).
       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
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       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
       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
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                        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 as those 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 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.
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    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 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-1889]) 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.
    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.
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    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 [LINK] 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 [LINK]. 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.
    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, XXXX
       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
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       "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.
       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, or
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       Authentication Header, AH) 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.  Note that omitting an integrity check can, under
       certain circumstances, compromise confidentiality [Bell98].
       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, stream ciphers,
       which do not cause error propagation.  Proper use of stream ciphers
       can be quite difficult, especially when authentication-checking is
       omitted [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, XXXX [INSERT NUMBER BEFORE PUBLICATION],
       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.
       IANA assignment instruction:
           The IANA must reserve an IP protocol number for UDP-Lite.
           IANA - Please NOTE the name of the registry entry MUST be
           "UDPLite", as detailed above.
       [END OF NOTE]
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    8.  References
    8.1.  Normative References
       [RFC-768]   Postel, J., "User Datagram Protocol", RFC 768 (STD6),
                   August 1980.
       [RFC-791]   Postel, J., "Internet Protocol", RFC 791 (STD5),
                   September 1981.
       [RFC-793]   Postel, J., "Transmission Control Protocol", RFC 793
                   (STD7), 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", RFC 2119 (BCP15), March 1997.
       [RFC-2460]  Deering, S., and R. Hinden, "Internet Protocol, Version 6
                   (IPv6) Specification", RFC 2460, December 1998.
    8.2.  Informative References
       [Bell98]    Bellovin, S.M., "Cryptography and the Internet",
                   Proceedings of CRYPTO Ì98, August, 1988.
       [BB01]      Bellovin, S.M., and M. Blaze, "Cryptographic Modes of
                   Operation for the Internet", 2nd  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., T. Stockhammer, M. Westerlund. 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",
                   draft-ietf-avt-ilbc-codec-01.txt, Internet Draft, Work in
                   Progress, March 2003.
       [ISO-14496] ISO/IEC International Standard 1446 (MPEG-4),
                   "Information Technology € Coding of Audio-Visual
                   Objects", January 2000.
       [ITU-H.263] "Video Coding for Low Bit Rate Communication," ITU-T
                   Recommendation H.263, January 1998.
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       [ITU-H.264] "Draft ITU-T Recommendation and Final Draft International
                   Standard of Joint Video Specification",
                   ITU-T Recommendation H.264, May 2003.
       [LINK]      Phil Karn, Editor, "Advice for Internet Subnetwork
                   Designers", Work in Progress, IETF.
       [RFC-1889]  Schulzrinne, H., Casner, S., Frederick, R., and
                   V. Jacobson, "RTP: A Transport Protocol for Real-Time
                   Applications", RFC 1889, January 1996.
       [RFC-2026]  Bradner, S., "The Internet Standards Process", RFC 2026,
                   October 1996.
       [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 206, November 1998.
       [RFC-3267]  Sjoberg, J., M. Westerlund, A. Lakeaniemi, and Q. Xie,
                   "Real-Time Transport Protocol (RTP) Payload Format and
                   File Storage Format for the Adaptiove 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),
    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 and thereby improving the
       clarity of this document.
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    10.  Authors' Addresses
       Lars-Ake Larzon
       Department of CS & EE
       Lulea University of Technology
       S-971 87 Lulea, Sweden
       Email: lln@cdt.luth.se
       Mikael Degermark
       Department of Computer Science
       The University of Arizona
       P.O. Box 210077
       Tucson, AZ 85721-0077, USA
       Email: micke@cs.arizona.edu
       Stephen Pink
       The University of Arizona
       P.O. Box 210077
       Tucson, AZ 85721-0077, USA
       Email: steve@cs.arizona.edu
       Lars-Erik Jonsson
       Ericsson AB
       Box 920
       S-971 28 Lulea, Sweden
       Email: lars-erik.jonsson@ericsson.com
       Godred Fairhurst
       Department of Engineering
       University of Aberdeen
       Aberdeen, AB24 3UE, UK
       Email: gorry@erg.abdn.ac.uk
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    Full Copyright Statement
       Copyright (C) The Internet Society (2002). All Rights Reserved.
       This document and translations of it may be copied and furnished to
       others, and derivative works that comment on or otherwise explain it
       or assist in its implementation may be prepared, copied, published
       and distributed, in whole or in part, without restriction of any
       kind, provided that the above copyright notice and this paragraph are
       included on all such copies and derivative works.  However, this
       document itself may not be modified in any way, such as by removing
       the copyright notice or references to the Internet Society or other
       Internet organizations, except as needed for the purpose of
       developing Internet standards in which case the procedures for
       copyrights defined in the Internet Standards process must be
       followed, or as required to translate it into languages other than
       The limited permissions granted above are perpetual and will not be
       revoked by the Internet Society or its successors or assigns.
       This document and the information contained herein is provided on an
    This Internet-Draft expires December, 2003.
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       Document History 02j - This section is intended to assist the AD in
       review of the document. It must be deleted by the RFC Editor.
       (1)  IANA Assignemnet Name chnage UDP-Lite renamed UDPLite to
            increase the portability of the code to operating systems that
            use the "-" character as a part of the mapping function (i.e.
            not allowed in the protocol ID).
            Having done this, I now worry a little that this may now divorce
            the RFC from the previous published work --- should we also
            refer people to UDP-Lite?
       (2) Text added to 2nd para, section 3.1 to say pseudo header always
       (3) Text added to 2nd para, section 3.1 to say initial checksum value
       is zero.
       (4) Section 5, added IPv6 text: 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).
       (5) BSD Code behaviour? This is a protocol problem with a BSD
       implementation, not a spec fault.
       (6) Examples added of applications
       (7) Examples of systems that would use it
       (8) Security issues (text requested by IESG).
       (9) Minor NiTs with written English corrected.
       (10) Introduction starts rather strangely - can we fix this?
       (11) Security AD Text Revised, and now OK.
       (12) Revised security note:
       When IPSEC is used with ESP payload encryption, there is no
       visibility of the transport header, and therefore a link can not
       determine which transport layer protocol is used, and would not be
       able to determine the value of the Checksum Coverage field. UDP-Lite
       provides no benefit in this case, and the link MUST provide a
       standard integrity check.
       [END OF NOTE]
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