Network Working Group Lars-Erik Jonsson
INTERNET-DRAFT Ericsson
Expires: April 2003 October 23, 2002
Requirements on ROHC TCP/IP Header Compression
<draft-ietf-rohc-tcp-requirements-05.txt>
Status of this memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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This document is a submission of the IETF ROHC WG. Comments should be
directed to the ROHC WG mailing list, rohc@ietf.org.
Abstract
This document contains requirements on the TCP/IP header compression
scheme (profile) to be developed by the ROHC WG. The document
discusses the scope of TCP compression, performance considerations,
assumptions on the surrounding environment, as well as IPR concerns.
The structure of this document is inherited from the document
defining RTP/UDP/IP requirements for ROHC.
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1. Introduction
The goal of the ROHC WG is to develop header compression schemes that
perform well over links with high error rates and long link roundtrip
times. The schemes must perform well for cellular links, using
technologies such as WCDMA, EDGE, and CDMA-2000. However, the schemes
should also be applicable to other future link technologies with high
loss and long roundtrip times.
The main objective for ROHC has been robust compression of
IP/UDP/RTP, but the WG is also chartered to develop new header
compression solutions for IP/TCP [RFC-791, RFC-793]. Since TCP
traffic, in contrast to RTP, has usually been sent over reliable
links, existing schemes for TCP [RFC-1144, RFC-2507] have not
experienced the same robustness problems as RTP compression. However,
there are still many scenarios where TCP header compression will be
implemented over less reliable links [RFC-3150, PILC-ARQ], making
robustness an important objective also for the new TCP compression
scheme. Other, equally important, objectives for ROHC TCP compression
are: improved compression efficiency, enhanced capabilities for
compression of header fields including TCP options, and finally
incorporation of TCP compression into the ROHC framework [RFC-3095].
2. Header Compression Requirements
The following requirements have, more or less arbitrarily, been
divided into five groups. The first group deals with requirements
concerning the impact of a header compression scheme on the rest of
the Internet infrastructure. The second group defines what kind of
headers must be compressed efficiently, while the third and fourth
groups concern performance requirements and capability requirements
which stem from the properties of the anticipated link technologies.
Finally, the fifth section discusses Intellectual Property Rights
related to ROHC TCP compression.
2.1. Impact on Internet Infrastructure
1. Transparency: When a header is compressed and then decompressed,
the resulting header must be semantically identical to the
original header. If this cannot be achieved, the packet
containing the erroneous header must be discarded.
Justification: The header compression process must not produce
headers that might cause problems for any current or future part
of the Internet infrastructure.
Note: The ROHC WG has not found a case where "semantically
identical" is not the same as "bitwise identical".
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2. Ubiquity: Must not require modifications to existing IP (v4 or
v6) or TCP implementations.
Justification: Ease of deployment.
Note: The ROHC WG may recommend changes that would increase the
compression efficiency for the TCP streams emitted by
implementations. However, ROHC cannot rely on such
recommendations being followed.
Note: Several TCP variants are currently in use on the Internet.
This requirement implies that the header compression scheme must
work efficiently and correctly for all expected TCP variants.
2.2. Supported Headers and Kinds of TCP Streams
1. IPv4 and IPv6: Must support both IPv4 and IPv6. This means that
all possible changes in the IP header fields must be handled by
the compression scheme, and commonly changing fields should be
compressed efficiently. Compression must not be disabled if IPv4
Options or IPv6 Extensions are present. The compression scheme
must further consider as normal operation the scenario where
Explicit Congestion Notification (ECN) [RFC-3168] is applied and
support efficient compression also in the case when the ECN bits
are used.
Justification: IPv4 and IPv6 will both be around for the
foreseeable future, and Options/Extensions are expected to be
more commonly used. ECN is expected to have a breakthrough and be
widely deployed, especially in combination with TCP.
2. Mobile IP: The kinds of headers used by Mobile IP{v4,v6} must be
supported and should be compressed efficiently. For IPv4 these
include headers of tunneled packets. For IPv6 they include
headers containing the Routing Header, the Binding Update
Destination Option, and the Home Address Option.
Justification: It is very likely that Mobile IP will be used by
cellular devices.
3. Generality: Must handle all headers from arbitrary TCP streams.
Justification: There must be a generic scheme which can compress
reasonably well for any TCP traffic pattern. This does not
preclude optimizations for certain traffic patterns.
4. IPSEC: The scheme should be able to compress headers containing
IPSEC sub-headers.
Justification: IPSEC is expected to be used to provide necessary
end-to-end security.
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Note: It is not possible to compress the encrypted part of an ESP
header, nor the cryptographic data in an AH header.
5. TCP: All fields supported by [RFC-2507] should be handled with
efficient compression, and so also the cases when the SYN, FIN or
TCP ECN [RFC-3168] bits are set.
Justification: These bits are expected to be commonly used.
6. TCP options: The scheme must support compression of packets with
any TCP option present, even if the option itself is not
compressed. Further, for some commonly used options the scheme
should provide compression mechanisms also for the options.
Justification: Since various TCP options are commonly used,
applicability of the compression scheme would be significantly
reduced if packets with options could not be compressed.
Note: Options that should be compressed are:
- Selective Acknowledgement (SACK), [RFC-2018, RFC-2883]
- Timestamp, [RFC-1323]
2.3. Performance Issues
1. Performance/Spectral Efficiency: The scheme must provide low
relative overhead under expected operating conditions;
compression efficiency should be better than for RFC2507 under
equivalent operating conditions.
Justification: Spectrum efficiency is a primary goal.
Note: The relative overhead is the average header overhead
relative to the payload. Any auxiliary (e.g., control or
feedback) channels used by the scheme should be taken into
account when calculating the header overhead.
2. Losses between compressor and decompressor: The scheme should make
sure that losses between compressor and decompressor do not
result in losses of subsequent packets, or cause damage to the
context that result in incorrect decompression of subsequent
packet headers.
Justification: Even though link layer retransmission in most cases
is expected to almost eliminate losses between compressor and
decompressor, there are still many scenarios where TCP header
compression will be implemented over less reliable links [RFC-
3150, PILC-ARQ]. In such cases, loss propagation due to header
compression could affect certain TCP mechanisms that are capable
of handling some losses, and have a negative impact on the
performance of TCP loss recovery.
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3. Residual errors in compressed headers: Residual errors in
compressed headers may result in delivery of incorrectly
decompressed headers not only for the damaged packet itself, but
also for subsequent packets, since errors may be saved in the
context state. For TCP, the compression scheme is not required to
implement explicit mechanisms for residual error detection, but
the compression scheme must not affect TCP's end-to-end
mechanisms for error detection.
Justification: For links carrying TCP traffic, the residual error
rate is expected to be insignificant. However, residual errors
may still occur, especially in the end-to-end path, and therefore
it is crucial that TCP is not prevented from handling these.
Note: This requirement implies that the TCP checksum must be
carried unmodified in all compressed headers.
Note: The error detection mechanism in TCP may be able to detect
residual bit errors, but the mechanism is not designed for this
purpose, and might actually provide a rather weak protection.
Therefore, although it is not a requirement on the compression
scheme, the decompressor should discard packets which are known
to contain residual errors.
4. Short-lived TCP transfers: The scheme should provide mechanisms
for efficient compression of short-lived TCP transfers,
minimizing the size of context initiation headers.
Justification: Many TCP transfers are short-lived. This may lead
to a low gain for header compression schemes that for all new
packet streams require full headers to be sent initially and
allow small compressed headers only after the initiation phase.
Note: This requirement implies that mechanisms for "context
sharing" (concurrent packet streams share context information) or
"context re-use" (new contexts can be built on information from
previous contexts) should be considered.
5a. Moderate Packet Misordering: The scheme should efficiently handle
moderate misordering (2-3 packets) in the packet stream reaching
the compressor.
Justification: This kind of misordering is common.
5b. Packet Misordering: The scheme must be able to correctly handle
and preferably compress also when there are misordered packets in
the TCP stream reaching the compressor.
Justification: Misordering happens regularly in the Internet.
However, since the Internet is engineered to run TCP reasonably
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well, excessive misordering will not be common and need not be
handled with optimum efficiency.
6. Processing delay: The scheme should not contribute significantly
to the system delay budget.
2.4. Capability Requirements Related to Link Layer Characteristics
1. Unidirectional links: Must be possible to implement (possibly with
less efficiency) without explicit feedback messages from
decompressor to compressor.
Justification: There are links that do not provide a feedback
channel or where feedback is not desirable for other reasons.
2. Misordering between compressor and decompressor: The header
compression scheme must be able to handle misordered packets
between compressor and decompressor, but can assume that packet
misordering is indicated to the decompressor by the lower layers.
Justification: When compression is applied over tunnels,
misordering often cannot be completely avoided. A header
compression scheme that prohibits misordering between compressor
and decompressor would therefore not be applicable in many
tunneling scenarios. However, in the case of tunneling, it is
usually possible to get misordering indications. Therefore, the
compression scheme does not have to support detection of
misordering, but can assume that such information is available
from lower layers.
3. Link delay: Must operate under all expected link delay conditions.
4. Header compression coexistence: The scheme must fit into the ROHC
framework together with other ROHC profiles (e.g. [RFC-3095]).
2.5. Intellectual Property Rights (IPR)
The ROHC WG must spend effort to achieve a high degree of
confidence that there is no IPR covering a final compression
solution for TCP.
Justification: Currently there is no TCP header compression
scheme available that can efficiently compress the packet headers
of modern TCP, e.g. with SACK, ECN, etc. ROHC is expected to fill
this gap by providing a ROHC TCP scheme that is applicable in the
wide area Internet, not only over error-prone radio links. It
must thus attempt to be as future-proof as possible, and only
unencumbered solutions will be acceptable to the Internet at
large.
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3. IANA Considerations
A protocol which meets these requirements will require the IANA to
assign various numbers. This document by itself, however, does not
require any IANA involvement.
4. Security Considerations
A protocol specified to meet these requirements must be able to
compress packets containing IPSEC headers according to the IPSEC
requirement, 2.2.4. There may be other security aspects to consider
in such protocols. This document by itself, however, does not add
any security risks.
5. Acknowledgements
This document has evolved through fruitful discussions with and input
from Gorry Fairhurst, Carsten Bormann, Mikael Degermark, Mark West,
Jan Kullander, Qian Zhang, Richard Price, and Aaron Falk. The
document quality was significantly improved thanks to Peter Eriksson,
who made a thorough linguistic review.
6. References
[RFC-791] Jon Postel, Internet Protocol, RFC 791, September 1981.
[RFC-793] Jon Postel, Transport Control Protocol, RFC 793,
September 1981.
[RFC-1144] Van Jacobson, "Compressing TCP/IP Headers for Low-Speed
Serial Links", RFC 1144, February 1990.
[RFC-2507] Mikael Degermark, Bjorn Nordgren, Stephen Pink, "IP
Header Compression", RFC 2507, February 1999.
[RFC-3096] Mikael Degermark, "Requirements for IP/UDP/RTP header
compression", RFC 3096, July 2001.
[RFC-3095] Carsten Bormann, et. al., "Robust Header Compression
(ROHC)", RFC 3095, July 2001.
[RFC-1323] Van Jacobson, Bob Braden, Dave Borman, "TCP Extensions
for High Performance", RFC 1323, May 1992.
[RFC-2018] Matt Mathis, Jamshid Mahdavi, Sally Floyd, Allyn
Romanow, "TCP Selective Acknowledgement Option", RFC
2018, October 1996.
[RFC-2883] Sally Floyd, Jamshid Mahdavi, Matt Mathis, Matthew
Podolsky, "An Extension to the Selective Acknowledgement
(SACK) Option for TCP", RFC 2883, July 2000.
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[RFC-3168] K. K. Ramakrishnan, Sally Floyd, David L. Black, "The
Addition of Explicit Congestion Notification (ECN) to
IP", RFC 3168, September 2001.
[RFC-3150] Spencer Dawkins, Gabriel Montenegro, Markku Kojo,
Vincent Magret, "End-to-end Performance Implications of
Slow Links", RFC 3150, July 2001.
[PILC-ARQ] Gorry Fairhurst, Lloyd Wood, "Advice to link designers
on link Automatic Repeat reQuest (ARQ)", Internet Draft
(work in progress), March 2002.
<draft-ietf-pilc-link-arq-issues-04.txt>
7. Author's Address
Lars-Erik Jonsson Tel: +46 920 20 21 07
Ericsson AB Fax: +46 920 20 20 99
Box 920
SE-971 28 Lulea
Sweden EMail: lars-erik.jonsson@ericsson.com
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This Internet-Draft expires April 23, 2003.
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