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Network Working Group                                        L-A. LarzonRequest for Comments: 3828                Lulea University of TechnologyCategory: Standards Track                                   M. Degermark                                                                 S. Pink                                               The University of Arizona                                                       L-E. Jonsson, Ed.                                                                Ericsson                                                       G. Fairhurst, Ed.                                                  University of Aberdeen                                                               July 2004The 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) (RFC768), 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 2004Table of Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .22.  Terminology. . . . . . . . . . . . . . . . . . . . . . . . . .33.  Protocol Description . . . . . . . . . . . . . . . . . . . . .33.1.  Fields . . . . . . . . . . . . . . . . . . . . . . . . .43.2.  Pseudo Header. . . . . . . . . . . . . . . . . . . . . .53.3.  Application Interface. . . . . . . . . . . . . . . . . .53.4.  IP Interface . . . . . . . . . . . . . . . . . . . . . .63.5.  Jumbograms . . . . . . . . . . . . . . . . . . . . . . .64.  Lower Layer Considerations . . . . . . . . . . . . . . . . . .65.  Compatibility with UDP . . . . . . . . . . . . . . . . . . . .76.  Security Considerations. . . . . . . . . . . . . . . . . . . .87.  IANA Considerations. . . . . . . . . . . . . . . . . . . . . .88.  References . . . . . . . . . . . . . . . . . . . . . . . . . .98.1.  Normative References . . . . . . . . . . . . . . . . . .98.2.  Informative References . . . . . . . . . . . . . . . . .99.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .1010. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . .1111. Full Copyright Statement . . . . . . . . . . . . . . . . . . .121.  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 Format3.1.  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 differsLarzon, 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 20043.4.  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 20045.  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 20046.  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 20048.  References8.1.  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,RFC793, 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",BCP89,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 200410.  Authors' Addresses   Lars-Ake Larzon   Department of CS & EE   Lulea University of Technology   S-971 87 Lulea, Sweden   EMail: lln@csee.ltu.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.ukLarzon, et al.              Standards Track                    [Page 11]

RFC 3828                   UDP-Lite Protocol                   July 200411.  Full Copyright Statement   Copyright (C) The Internet Society (2004).  This document is subject   to the rights, licenses and restrictions contained inBCP 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 inBCP 78 andBCP 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 athttp://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-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Larzon, et al.              Standards Track                    [Page 12]