Request for Comments: 3843 G. Pelletier
Category: Standards Track Ericsson
June 2004
RObust Header Compression (ROHC): A Compression Profile for IP
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004).
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 (Encapsulating Security Payload),
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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology. . . . . . . . . . . . . . . . . . . . . . . . . . 2
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 . . . . . . . . . . . . . . . . . . . . . 8
3.6. Packet Types . . . . . . . . . . . . . . . . . . . . . . 8
3.7. The CONTEXT_MEMORY Feedback Option . . . . . . . . . . . 10
4. Security Considerations. . . . . . . . . . . . . . . . . . . . 10
5. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 10
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
7. Normative References . . . . . . . . . . . . . . . . . . . . . 11
Appendix A. Detailed Procedures for Canceling Mode Transitions. . 12
A.1. Transition from Optimistic to Reliable Mode. . . . . . . 12
A.2. Transition from Unidirectional to Reliable Mode. . . . . 13
A.3. Transition from Reliable to Optimistic Mode. . . . . . . 13
A.4. Transition Back to Unidirectional Mode . . . . . . . . . 14
Authors’ Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 16
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 (Encapsulating Security Payload),
IP/UDP, and also a profile for uncompressed packet streams. The
profile for uncompressed data was defined to provide a 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 not yet 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 represents 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 of
compressing 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].
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 differences 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 in
principle. 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 the handling of multiple IP headers. Unless stated
explicitly below, mechanisms and formats are the same 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 a 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). Alternatively, the compressor can
choose to end the static chain at any IP header, and indicate this by
setting the MSB of the IP version field to 1 (0xC for IPv4 or 0xE for
IPv6). The decompressor must store this indication in the context
for correct decompression of subsequent headers. Note that the IP
version field in decompressed headers must be restored to its
original value.
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.
However, the general compressed header formats defined in [RFC-3095,
section 5.7.] specifies that at 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 one single ’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 [RFC-3095, 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, as the compressor will only be allowed
to send IR and IR-DYN packets in such cases.
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 goes 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). Alternatively, the chain may be
explicitly terminated with a special code value in the IP version
field, as described in section 3.1. 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. Thus, this additional data
structure is 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.
3.3. Constant IP-ID
Most IPv4 stacks assign an 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].
3.4. Additional Mode Transition Logic
The profiles defined in [RFC-3095] 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 [RFC-3095], the compressor
is compelled to honor such requests.
For the IP profile defined in this document, the Mode parameter for
the value mode = 0 (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 the declined mode for a certain
amount of time.
More specifically, with reference to the parameters C_TRANS, C_MODE,
D_TRANS, and D_MODE defined in [RFC-3095, 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 if the mode parameter is 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
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.
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], or where the IP version field indicates
termination (see section 3.1). 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], or where the IP
version field indicates termination (see section 3.1). 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.
3.6. Packet Types
Except for one new feedback option (see section 3.7), 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 :