Network Working Group                                        L-A. Larzon
INTERNET-DRAFT                            Lulea University of Technology
Expires: June 2003                                          M. Degermark
                                                                 S. Pink
                                               The University of Arizona
                                                   L-E. Jonsson (editor)
                                                   G. Fairhurst (editor)
                                                  University of Aberdeen
                                                        December 5, 2002

                          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
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   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.

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Table of Contents

   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.....................................5
   5.  Compatibility with UDP.........................................6
   6.  Security Considerations........................................7
   7.  IANA Considerations............................................7
   8.  References.....................................................8
      8.1.  Normative References......................................8
      8.2.  Informative References....................................8
   9.  Acknowledgements...............................................8
   10.  Authors' Addresses............................................9

1.  Introduction

   Why another transport protocol?

   First, there is a class of applications that prefer to have damaged
   data delivered rather than discarded by the network. A number of
   codecs for voice and video fall into this class. These codecs are
   designed to cope better with errors in the payload than with loss of
   entire packets.

   Second, there are a number of link technologies where data can be
   partially damaged. Several radio technologies exhibit this behavior
   when operating at a point where cost and delay are sufficiently low.

   Third, intermediate layers should not prevent error-tolerant
   applications to run well in the presence of such links. The
   intermediate layers are IP and the transport layer. 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 [UDP-LITE]. The error-
   detection mechanism of the transport layer must be able to protect
   vital information such as headers, but also to optionally ignore

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   errors best dealt with by the application. What should 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",
   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

                    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.

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   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. UDP-Lite packets with a Checksum Coverage greater than the
   IP length MUST also be discarded.

   Checksum is the 16-bit one's complement of the one's complement sum
   of a pseudo-header of information 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].
   If the computed checksum is 0, it is transmitted as all ones (the
   equivalent in one's complement arithmetic).

   The transmitted checksum MUST NOT be all zeroes. If an application
   using UDP-Lite wishes to have no protection of the packet payload, it
   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

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

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   UDP-Lite packet, and verify the entire packet. Applications that want
   to define the payload as partially insensitive to bit errors (e.g.
   error tolerant codecs using RTP [RFC-1889]) should do that 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 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 indicated checksum coverage, 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 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 of 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 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 wants them, frames carrying UDP-Lite packets need
   not be discarded by lower layers 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 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. At least the sensitive part would thus be treated in
   exactly the same way as UDP packets.

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   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 discard damaged packets. 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 the
   application if partial error detection, based on the partial checksum
   feature of UDP-Lite, is implemented also by link layers, as discussed
   above. Obviously, 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
   [INSERT IANA NUMBER BEFORE PUBLICATION], that allows a receiver to
   identify whether UDP or UDP-Lite is used. A system unaware of UDP-
   Lite will in general return an ICMP Protocol Unreachable error
   message to the sender. This simple method of detecting UDP-Lite
   unaware systems is the primary benefit of having separate protocol

   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 be 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

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

   Anyway, since UDP-Lite has now been assigned its own protocol
   identifier, there is no need to consider the 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 on UDP-Lite packets where the
   insensitive part has been damaged.

   The IPSec integrity check (Encapsulation Security Protocol, ESP, or
   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 discarding of the UDP-Lite packet by the IP

   Encryption is also an issue when using UDP-Lite. 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 strong encryption transforms today exhibit this behavior, for
   reasons obvious from a security point of view. There exist encryption
   transforms, stream ciphers, which do not spread errors in this way
   when the damage occurs in the insensitive part of the packet.

7.  IANA Considerations

   has been assigned for UDP-Lite.


       IANA assignment instruction:
       - The IANA must reserve an IP protocol number for UDP-Lite.


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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

   [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.

   [UDP-LITE]  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 Elisabetta Carrara and Mats Naslund for
   reviewing the security considerations chapter, and to Peter Eriksson
   for doing a language review and thereby improving the clarity of this

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10.  Authors' Addresses

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

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

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

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

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

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This Internet-Draft expires June 5, 2003.

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