Network Working Group Lars-Erik Jonsson, Ericsson
INTERNET-DRAFT Ghyslain Pelletier, Ericsson
Expires: December 2003
June 27, 2003
RObust Header Compression (ROHC):
A Compression Profile for IP
<draft-ietf-rohc-ip-only-02.txt>
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
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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...............................3
3.2. Handling multiple levels of IP headers.................3
3.3. Constant IP-ID.........................................4
3.4. Additional mode transition logic.......................6
3.5. Initialization.........................................7
3.6. Packet types...........................................8
4. Security Considerations.......................................9
5. IANA Considerations...........................................9
6. Acknowledgements..............................................9
7. References....................................................9
8. Authors' Addresses...........................................10
Appendix A. Detailed procedures for canceling mode transitions..11
A.1. Transition from Optimistic to Reliable mode................11
A.2. Transition from Unidirectional to Reliable mode............12
A.3. Transition from Reliable to Optimistic mode................12
A.4. Transition back to Unidirectional mode.....................13
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
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way of terminating the static header chain, and the capability to
compress IP header chains of arbitrary length.
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
One difference for IP-only compression, compared to IP/UDP
compression, is related to the termination of the static chain in IR
headers. For the UDP profile, the chain always ends with a UDP header
part, which per definition provides the boundaries for the chain. The
UDP header is also the last header in the uncompressed packet (except
for potential application header). For the IP-only profile, 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).
3.2. Handling multiple levels of IP headers
The ROHC IR and IR-DYN packets defined in [RFC-3095] are used to
communicate static and/or dynamic parts of a context. For each of the
compression profiles defined in [RFC-3095], there is a single last
header in the header chain that clearly marks the termination of the
static chain. The length of the dynamic chain is then inferred from
the static chain in the IR header itself, or from the static chain in
the context for the IR-DYN header. The length of both static and
dynamic chains may thus be of arbitrary length and may, in theory,
initialize a context with an arbitrary number of IP levels.
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However, the general compressed header formats defined in [RFC-3095,
section 5.7.] specifies that a most two levels of IP headers (the
'Inner' and the 'Outer' level of IP headers) may be included in a
compressed header. Specifically, the format defined for Extension 3
[RFC-3095, section 5.7.5.] can only carry the 'Outer' IP header. In
addition, while list compression may be used to compress other types
of headers, it cannot be used to compress additional IP headers as IP
headers may not be part of an extension header chain in compressed
headers [ROHC, section 5.8.].
For the compression profiles defined in [RFC-3095], the consequence
is that at most two levels of IP headers can be compressed. In other
words, the presence of additional IP headers at best partially
disables header compression and compression will not go beyond the IR
state (as only IR and IR-DYNs can be used in such case).
For the compression of IP headers only, the additional IP headers
would however not have to cause header compression to be disabled
because there is no single packet type that ends the compressed
chain. The excess IP headers could simply be left uncompressed by
implicitly terminating the static and dynamic chains after at most
two levels of IP headers.
The IP-only profile defined in this document takes one step further
and supports compression of an arbitrary number of IP levels. This is
achieved by adding a dynamic chain to the general format of
compressed headers, to include the header part of each IP level in
excess of the first two.
As explained above, the static chain within IR packets can be of
arbitrary length, and the chain is terminated by the presence of a
non-IP header (not IPinIP nor IPv6). The dynamic chain is structured
analogously.
For compressed headers, the information related to the initial two IP
headers is carried as for the IP/UDP profile, and a chain of dynamic
header information is added to the end of the compressed header for
each and every additional IP header. This additional data structure
is thus exactly the same as the one used in IR and IR-DYN packets.
The length of the chain is inferred from the chain of static
parameters in the context. While a dynamic chain carries dynamically
changing parameters using an uncompressed representation, this
ensures that flows with arbitrary levels of IP headers will not
impair compression efficiency.
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3.3. Constant IP-ID
Most IPv4 stacks assigns IP-ID according to the value of a counter
increasing by one for each outgoing packet. ROHC UDP compresses the
IP-ID field using offset IP-ID encoding based on the UDP SN [RFC-
3095]. For stacks generating IP-ID values using a pseudo-random
number generator, the field is not compressed and is sent as-is in
its entirety as additional octets after the compressed header.
Cases have also been found where an IPv4 stack uses a constant value
for the IP Identifier. When the IP-ID field is constant, it cannot be
compressed using offset IP-ID encoding and the field must be sent in
its entirety. This overhead can be avoided with the addition of a
flag within the dynamic part of the chain used to initialize the IPv4
header, as follow:
Dynamic part:
+---+---+---+---+---+---+---+---+
| Type of Service |
+---+---+---+---+---+---+---+---+
| Time to Live |
+---+---+---+---+---+---+---+---+
/ Identification / 2 octets
+---+---+---+---+---+---+---+---+
| DF|RND|NBO|SID| 0 |
+---+---+---+---+---+---+---+---+
/ Generic extension header list / variable length
+---+---+---+---+---+---+---+---+
SID: Static IP Identifier.
For IR and IR-DYN packets, the logic is the same as for ROHC UDP
with the addition that field(SID) must be kept in the context.
For compressed headers other than IR and IR-DYN:
If value(RND) = 0 and context(SID) = 0, hdr(IP-ID) is
compressed using Offset IP-ID encoding (see [RFC-3095 section
4.5.5]) using p = 0 and default-slope(IP-ID offset) = 0.
If value(RND) = 0 and context(SID) = 1, hdr(IP-ID) is constant
and compressed away; hdr(IP-ID) is the value of context(IP-ID).
If value(RND) = 1, IP-ID is the uncompressed hdr(IP-ID). IP-ID
is then passed as additional octets at the end of the
compressed header, after any extensions.
Note: Only IR and IR-DYN packets can update context(SID).
Note: All other fields are the same as for ROHC UDP [RFC-3095].
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3.4. Additional mode transition logic
The profiles defined in [ROHC] operate using different modes of
compression. A mode transition can be requested once a packet has
reached the decompressor by sending feedback indicating the desired
mode. As per the specifications found in [ROHC], the compressor is
compelled to honor such request.
The Mode parameter for the value mode = 0, for packet types UOR-2, IR
and IR-DYN, is redefined to allow the compressor to decline a mode
transition requested by the decompressor:
Mode: Compression mode. 0 = (C)ancel Mode Transition
Upon receiving the Mode parameter set to '0', the decompressor MUST
stay in its current mode of operation and SHOULD refrain from sending
further mode transition requests for a certain amount of time.
More specifically, with reference to the parameters C_TRANS, C_MODE,
D_TRANS and D_MODE defined in [ROHC, section 5.6.1.], the following
modifications apply when the compressor cancels a mode transition:
Parameters for the compressor side:
- C_MODE:
This value must not be changed when sending mode information
within packets when the mode parameter set to '0' (as a
response to a mode transition request from the decompressor).
- C_TRANS:
C_TRANS is (P)ending when receiving a mode transition request
from the decompressor. C_TRANS is set to (D)one when the
compressor receives an ACK for a UOR-2, IR-DYN, or IR packet
sent with the mode parameter set to the mode in use at the time
when the mode transition request was initiated.
Parameters for the decompressor side:
- D_MODE:
D_MODE MUST remain unchanged when receiving a UOR-2, an IR-DYN,
or an IR packet sent with the mode parameter set to '0'.
- D_TRANS:
D_TRANS is (P)ending when a UOR-2, IR-DYN, or IR packet sent
with the mode parameter set to '0' is received. It is set to
(D)one when a packet of type 1 or 0 corresponding to the
unchanged mode is received.
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The resulting mode transition procedure is described below:
Compressor Decompressor
----------------------------------------------
C_MODE = X | | D_MODE = X
| Mode Request(Y) +-<-<-<-| D_TRANS = I
| +-<-<-<-<-<-<-<-+ |
C_TRANS = P |-<-<-<-+ |
C_MODE = X | |
|->->->-+ IR/IR-DYN/UOR-2(SN,C) |
| +->->->->->->->-+ |
|->-.. +->->->-| D_TRANS = P
|->-.. | D_MODE = X
| ACK(SN,X) +-<-<-<-|
| +-<-<-<-<-<-<-<-+ |
C_TRANS = D |-<-<-<-+ |
| |
|->->->-+ X-0, X-1* |
| +->->->->->->->-+ |
| +->->->-| D_TRANS = D
| |
where X: mode in use before the mode transition was initiated
Y: mode requested by the decompressor
C: (C)ancel mode transition
3.5. 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.
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3.6. 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 of [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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Note that the list of dynamic chains for the additional IP headers in
compressed packets do not have a sequence number at the end of the
chain, as SN is present within compressed base headers.
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 authors would like to thank Carsten Bormann and Fredrik Lindstr÷m
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.
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[PROTOCOL] "Assigned Internet Protocol Numbers", IANA registry at:
http://www.iana.org/assignments/protocol-numbers
8. Authors' Addresses
Lars-Erik Jonsson
Ericsson AB
Box 920
SE-971 28 Lulea, Sweden
Phone: +46 920 20 21 07
Fax: +46 920 20 20 99
Email: lars-erik.jonsson@ericsson.com
Ghyslain Pelletier
Box 920
Ericsson AB
SE-971 28 Lulea, Sweden
Phone: +46 920 20 24 32
Fax: +46 920 20 20 99
Email: ghyslain.pelletier@epl.ericsson.se
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Appendix A. Detailed procedures for canceling mode transitions
[Author's note: This appendix may be removed before publication]
The profiles defined in [ROHC] operate using different modes of
compression: Unidirectional (U-Mode), Bi-directional Optimistic (O-
Mode) and Bi-directional Reliable (R-Mode). Compression always starts
in the U-Mode, and mode transitions can only be initiated by the
decompressor [ROHC, section 5.6.]. A mode transition can be requested
once a packet has reached the decompressor by sending feedback
indicating the desired mode.
With reference to the parameters C_TRANS, C_MODE, D_TRANS and D_MODE
defined in [ROHC, section 5.6.1.], the following sub-sections
describe the resulting procedures when a compressor declines a mode
transition request from the decompressor as described in section 3.4.
A.1. Transition from Optimistic to Reliable mode
When the decompressor initiates a mode transition from Optimistic to
Reliable mode, the cancellation of the transition procedure is
described as follows:
Compressor Decompressor
----------------------------------------------
| |
| ACK(R)/NACK(R) +-<-<-<-| D_TRANS = I
| +-<-<-<-<-<-<-<-+ |
C_TRANS = P |-<-<-<-+ |
C_MODE = O | |
|->->->-+ IR/IR-DYN/UOR-2(SN,C) |
| +->->->->->->->-+ |
|->-.. +->->->-| D_TRANS = P
|->-.. | D_MODE = O
| ACK(SN,O) +-<-<-<-|
| +-<-<-<-<-<-<-<-+ |
C_TRANS = D |-<-<-<-+ |
| |
|->->->-+ UO-0, UO-1* |
| +->->->->->->->-+ |
| +->->->-| D_TRANS = D
The compressor must not send packet types 1 or 0 when C_TRANS is P,
i.e. not until it has received an ACK for a UOR-2, IR-DYN, or IR
packet sent with the mode transition parameter set to C. When the
decompressor receives a UOR-2, IR-DYN, or IR packet sent with the
mode transition parameter set to C, it must keep the value D_MODE to
O and it sets D_TRANS to P. When the decompressor receives packet
types 0 or 1, after having ACKed a UOR-2, IR-DYN, or IR packet, it
sets D_TRANS to D.
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A.2. Transition from Unidirectional to Reliable mode
The cancellation of a transition from Unidirectional to Reliable mode
follows the same procedure as defined in section 4.2 above.
A.3. Transition from Reliable to Optimistic mode
When the decompressor initiates a mode transition from Reliable to
Optimistic mode, the cancellation of the transition procedure is
described as follows:
Compressor Decompressor
----------------------------------------------
| |
| ACK(O)/NACK(O) +-<-<-<-| D_TRANS = I
| +-<-<-<-<-<-<-<-+ |
C_TRANS = P |-<-<-<-+ |
C_MODE = R | |
|->->->-+ IR/IR-DYN/UOR-2(SN,C) |
| +->->->->->->->-+ |
|->-.. +->->->-| D_MODE = R
|->-.. |
| ACK(SN,R) +-<-<-<-|
| +-<-<-<-<-<-<-<-+ |
C_TRANS = D |-<-<-<-+ |
| |
|->->->-+ R-0, R-1* |
| +->->->->->->->-+ |
| +->->->-| D_TRANS = D
| |
The compressor must not send packet types 1 or 0 when C_TRANS is P,
i.e. not until it has received an ACK for a UOR-2, IR-DYN, or IR
packet sent with the mode transition parameter set to C. When the
decompressor receives a UOR-2, IR-DYN, or IR packet sent with the
mode transition parameter set to C, it must keep the value D_MODE to
R. When the decompressor receives packet types 0 or 1, after having
ACKed a UOR-2, IR-DYN, or IR packet, it sets D_TRANS to D.
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A.4. Transition back to Unidirectional mode
When the decompressor initiates a mode transition from Reliable or
Optimistic mode back to Unidirectional mode, the cancellation of the
transition procedure is described as follows:
Compressor Decompressor
----------------------------------------------
| |
| ACK(U)/NACK(U) +-<-<-<-| D_TRANS = I
| +-<-<-<-<-<-<-<-+ |
C_TRANS = P |-<-<-<-+ |
C_MODE = O/R| |
|->->->-+ IR/IR-DYN/UOR-2(SN,C) |
| +->->->->->->->-+ |
|->-.. +->->->-|
|->-.. |
| ACK(SN,O/R) +-<-<-<-|
| +-<-<-<-<-<-<-<-+ |
C_TRANS = D |-<-<-<-+ |
| R-0, R-1* or |
|->->->-+ UO-0, UO-1* |
| +->->->->->->->-+ |
| +->->->-| D_TRANS = D
D_MODE = O/R
When the decompressor receives a UOR-2, IR-DYN, or IR packet sent
with the mode transition parameter set to C, it must keep the value
D_MODE to the bi-directional mode already in use (either O- or R-
mode). After ACKing the first UOR-2(C), IR-DYN(C), or IR(C), the
decompressor MUST continue to send feedback with the Mode parameter
set to the bi-directional mode in use (either O- or R-mode) until it
receives packet types 0 or 1. When the decompressor receives packet
types 0 or 1, after having ACKed a UOR-2, IR-DYN, or IR packet, it
sets D_TRANS to D.
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