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) Ericsson 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 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 http://www.ietf.org/ietf/lid-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Please direct comments to the TSV WG mailing list: tsvwg@ietf.org Abstract 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. Larzon, et al. [Page 1] INTERNET-DRAFT UDP-Lite December 5, 2002 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 Larzon, et al. [Page 2] INTERNET-DRAFT UDP-Lite December 5, 2002 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", "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]. 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. Larzon, et al. [Page 3] INTERNET-DRAFT UDP-Lite December 5, 2002 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 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 to mimic UDP by having the Checksum Coverage field match the length of the Larzon, et al. [Page 4] INTERNET-DRAFT UDP-Lite December 5, 2002 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. Larzon, et al. [Page 5] INTERNET-DRAFT UDP-Lite December 5, 2002 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 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 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 Larzon, et al. [Page 6] INTERNET-DRAFT UDP-Lite December 5, 2002 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 receiver. 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 A new IP protocol number, XXXX [INSERT NUMBER BEFORE PUBLICATION], has been assigned for UDP-Lite. [NOTE, REMOVE BEFORE PUBLICATION] IANA assignment instruction: - The IANA must reserve an IP protocol number for UDP-Lite. [END OF NOTE] Larzon, et al. [Page 7] INTERNET-DRAFT UDP-Lite December 5, 2002 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), 1999. 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 document. Larzon, et al. [Page 8] INTERNET-DRAFT UDP-Lite December 5, 2002 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 Larzon, et al. [Page 9] INTERNET-DRAFT UDP-Lite December 5, 2002 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 English. 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