Network Working Group L-E. Jonsson
INTERNET-DRAFT Ericsson
Expires: July 2003 January 23, 2003
RObust Header Compression (ROHC):
A Compression Profile for IP
<draft-ietf-rohc-ip-only-01.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
The original RObust Header Compression (ROHC) RFC, RFC 3095, defines
a framework for header compression, along with compression protocols
(profiles) for IP/UDP/RTP, IP/ESP, IP/UDP, and also a profile for
uncompressed packet streams. However, no profile was defined for
compression of IP only, which has been identified as a missing piece
in RFC 3095. This document defines a ROHC compression profile for IP,
similar to the IP/UDP profile defined by RFC 3095, but simplified to
exclude UDP, and enhanced to compress IP header chains of arbitrary
length.
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Table of Contents
1. Introduction..................................................2
2. Terminology...................................................3
3. ROHC IP Compression (Profile 0x0004)..........................3
3.1. Static chain termination and multiple IP headers.......3
3.2. Initialization.........................................4
3.3. Packet types...........................................5
4. Security Considerations.......................................6
5. IANA Considerations...........................................6
6. Acknowledgements..............................................6
7. References....................................................6
8. Author's Address..............................................7
1. Introduction
The original RObust Header Compression (ROHC) RFC [RFC-3095] defines
a framework for header compression, along with compression protocols
(profiles) for IP/UDP/RTP, IP/ESP, IP/UDP, and also a profile for
uncompressed packet streams. The profile for uncompressed data was
defined to provide means to encapsulate all traffic over a link
within ROHC packets. Through this profile, the lower layers do not
have to provide multiplexing for different packet types, but instead
ROHC can handle any packet stream, even if compression profiles for
all kinds of packet streams have yet not been defined or implemented
over the link.
Although the profile without compression is simple and can tunnel
arbitrary packets, it has of course a major weakness in that it does
not compress the headers at all. When considering that normally all
packets are expected to be IP [RFC-791, RFC-2460] packets, and that
the IP header often represent a major part of the total header, a
useful alternative to no compression would for most packets be
compression of the IP header only. Unfortunately, such a profile was
not defined in [RFC-3095], and this has thus been identified as an
important missing piece in the ROHC toolbox.
This document addresses this missing compression support and defines
a ROHC compression profile for IP [RFC-791, RFC-2460] only, similar
to the IP/UDP profile defined by [RFC-3095], but simplified to
exclude UDP. Due to the similarities with the IP/UDP profile, the IP
compression profile is described based on the IP/UDP profile, mainly
covering differences. The most important differences are a different
way of terminating the static header chain, and the capability to
compress IP header chains of arbitrary length.
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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 [RFC-2119].
ROHC UDP
"ROHC UDP" in this document refers to the IP/UDP profile
(Profile 0x0002) as defined in [RFC-3095].
3. ROHC IP Compression (Profile 0x0004)
In general, there are no major difference between the ROHC UDP
profile and the IP profile (ROHC IP) defined in this document, since
the removal of UDP has no impact on the compression mechanisms. As
for ROHC UDP, the compressor generates a 16-bit sequence number which
increases by one for each packet compressed in the packet stream,
simply called SN below. The most important difference between this
profile and ROHC UDP is about static chain termination and handling
of multiple IP headers. Unless stated explicitly below, mechanisms
and formats are as for ROHC UDP.
3.1. Static chain termination and multiple IP headers
The most important difference for IP-only compression, compared to
IP/UDP compression, is about how to terminate compression, i.e. how
to end the static chain in IR headers. For the UDP profile, the chain
always ends with a UDP header part, which per definition terminates
compression, and the UDP header is also the last header in the
uncompressed packet (except for potential application header). For
the case of IP-only compression, there is no single last header that
per profile definition terminates the chain. Instead, the static
chain is terminated if the "Next Header / Protocol" field of a static
IP header part indicates anything but IP (IPinIP or IPv6).
Another difference with IP-only compression is related to the
potential compression of multiple IP headers. ROHC UDP can compress
at most two IP headers, but additional IP headers would completely
disable the use of header compression, since the compressed chain
must end with the UDP header part. However, as there is no single
packet type that ends the compressed chain with the IP profile,
additional IP headers would not have to cause header compression to
be disabled. By implicitly terminating the chain after at most 2 IP
headers, additional IP headers could just be left uncompressed.
The IP profile defined in this document goes one step further, and
supports compression of an arbitrary number of IP headers. The static
chain can obviously be of an arbitrary length, and is simply
terminated through the presence of a non-IP header (not IPinIP nor
IPv6). The dynamic chain is structured analogously. In compressed
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packet headers, header information related to the initial two IP
headers are carried as in the IP/UDP profile, while a dynamic header
chain is added at the end of the compressed header for each and every
additional IP header. The data structure used for these header
portions in compressed headers is thus exactly the same as the one
used in IR and IR-DYN packets.
The following sections describe how the IP profile differs from the
IP/UDP profile, providing the details of the general principles
described in the previous paragraph.
3.2. Initialization
The static context for ROHC IP compression can be initialized in
either of two ways:
1) By using an IR packet as in ROHC UDP, where the profile is
0x0004, and the static chain ends with the static part of an
IP header, where the Next Header/Protocol field has any value but
IPinIP (4) or IPv6 (41) [PROTOCOL]. At the compressor, SN is
initialized to a random value when the first IR packet is sent.
2) By reusing an existing context. This is done with an IR-DYN
packet, identifying profile 0x0004, where the dynamic chain
corresponds to the prefix of the existing static chain, ending
with an IP header where the Next Header/Protocol field has any
value but IPinIP (4) or IPv6 (41) [PROTOCOL]. At the compressor,
SN is initialized to a random value when the first IR-DYN packet
is sent.
For ROHC IP, the dynamic part of an IR or IR-DYN packet is similar to
the one for ROHC UDP, with a two-octet field containing the SN
present at the end of the dynamic chain in IR and IR-DYN packets. It
should be noted that the static and dynamic chains have an arbitrary
length, and the SN is added only once, at the end of the dynamic
chain in IR and IR-DYN packets. Note further that additional dynamic
chains in compressed packets do not have this sequence number at the
end of the chain, as SN is present within compressed base headers.
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3.3. Packet types
The only packet format that differs from ROHC UDP is the general
format for compressed packets, which has no UDP checksum in the end.
Instead, it ends with a list of dynamic header portions, one for each
IP header above the initial two (if any, as indicated by the presence
of corresponding header portions in the static chain).
The general format for a compressed header is thus as follows:
0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
: Add-CID octet : |
+---+---+---+---+---+---+---+---+ |
| first octet of base header | |
+---+---+---+---+---+---+---+---+ |
: : |
/ 0, 1, or 2 octets of CID / |
: : |
+---+---+---+---+---+---+---+---+ |
/ remainder of base header / |
+---+---+---+---+---+---+---+---+ |
: : |
/ Extension / |
: : |
--- --- --- --- --- --- --- --- |
: : |
+ IP-ID of outer IPv4 header +
: : (see section 5.7 or [RFC-3095])
--- --- --- --- --- --- --- ---
/ AH data for outer list / |
--- --- --- --- --- --- --- --- |
: : |
+ GRE checksum + |
: : |
--- --- --- --- --- --- --- --- |
: : |
+ IP-ID of inner IPv4 header + |
: : |
--- --- --- --- --- --- --- --- |
/ AH data for inner list / |
--- --- --- --- --- --- --- --- |
: : |
+ GRE checksum + |
: : |
--- --- --- --- --- --- --- ---
: List of :
/ Dynamic chains / variable, given by static chain
: for additional IP headers : (includes no SN)
--- --- --- --- --- --- --- ---
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4. Security Considerations
The security considerations of [RFC-3095] apply equally to this
document, without exceptions or additions.
5. IANA Considerations
ROHC profile identifier 0x0004 has been reserved by the IANA for the
profile defined in this document.
{ NOTE TO IANA - TO BE REMOVED BEFORE PUBLICATION }
A ROHC profile identifier must be reserved by the IANA for the
profile defined in this document. Profile number 0x0004 has
previously been saved for this purpose, and should thus be used.
As for previous ROHC profiles, profile numbers 0xnn04 must also be
reserved for future variants of this profile. A suggested
registration in the "RObust Header Compression (ROHC) Profile
Identifiers" name space would then be:
OLD: 0xnn04 To be Assigned by IANA
NEW: 0x0004 ROHC IP [RFCXXXX (this)]
0xnn04 Reserved
{ END OF NOTE }
6. Acknowledgements
The author would like to thank Carsten Bormann and Ghyslain Pelletier
for valuable input and review.
7. References
[RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC-3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima,
H., Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T.,
Le, K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro,
K., Wiebke, T., Yoshimura, T. and H. Zheng, "Robust
Header Compression (ROHC)", RFC 3095, July 2001.
[RFC-791] Postel, J., "Internet Protocol", RFC 791, September 1981.
[RFC-2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[PROTOCOL] "Assigned Internet Protocol Numbers", IANA registry at:
http://www.iana.org/assignments/protocol-numbers
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8. 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 July 23, 2003.
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