Internet DRAFT - draft-ietf-tsvwg-udp-lite
draft-ietf-tsvwg-udp-lite
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)
Ericsson
G. Fairhurst (Editor)
University of Aberdeen
August, 2003
The UDP-Lite Protocol
<draft-ietf-tsvwg-udp-lite-02.txt>
Status of this memo
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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.
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
packets.
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
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].
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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
(UDPLite) [INSERT IANA NUMBER BEFORE PUBLICATION], 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
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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.
[NOTE, REMOVE BEFORE PUBLICATION]
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),
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 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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This Internet-Draft expires December, 2003.
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[NOTE, REMOVE BEFORE PUBLICATION]
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
present.
(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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