Network Working Group Ghyslain Pelletier, Ericsson
INTERNET-DRAFT
Expires: November 2003 May 23, 2002
RObust Header Compression (ROHC):
Context Replication for ROHC Profiles
<draft-pelletier-rohc-context-replication-00.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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Abstract
This document defines context replication, an alternative to the
context initialization procedure found in ROHC (Robust Header
Compression) [RFC-3095]. Profiles defining support for context
replication may use the mechanism described herein to establish a new
context based on another already existing context.
Context replication is introduced to reduce the overhead of the
context establishment procedure, and may be especially useful for the
compression of multiple short-lived flows that may be occurring
simultaneously or near-simultaneously, such as for example short-
lived TCP flows.
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Table of contents
1. Introduction....................................................3
2. Terminology.....................................................4
3. Context replication for ROHC profiles...........................4
3.1. Robustness considerations.................................4
3.2. Compressor logic..........................................5
3.2.1. Selection of the Base Context...........................5
3.2.2. Feedback logic..........................................6
3.2.2.1. Negative Acknowledgements (NACKs).....................6
3.2.2.2. Optional Acknowledgements (ACKs)......................7
3.3. Decompressor logic........................................7
3.3.1. Replication and context initialization..................7
3.3.2. Actions upon failure....................................7
3.4. Packet Formats............................................8
3.4.1. Checksums in the IR-REPLICATE packet....................8
3.4.1.1. 7-bit CRC.............................................9
3.4.1.2. 8-bit CRC.............................................9
3.4.2. General Format of the IR-REPLICATE packet...............9
4. Security considerations........................................11
5. Acknowledgements...............................................11
6. References.....................................................11
6.1. Normative References.....................................11
6.2. Informative References...................................12
7. Author's address...............................................12
Appendix A. Replication chains....................................13
A.1. Replication of the IPv6 Header [RFC-2460]................13
A.2. Replication of the IPv4 Header [RFC-791].................14
A.3. Replication of the UDP Header [RFC-768]..................16
A.4. Replication of the UDP-Lite Header [UDP-Lite]............17
A.5. Replication of the TCP Header [RFC-793]..................18
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1. Introduction
There is often some redundancy between header fields of different
flows passing through the same compressor-decompressor pair. This
means that some of the information needed to initialize the context
for compressing the headers of a new flow may already be present at
the decompressor. It may be desirable to reuse this information and
remove some of the overhead normally required for the initialization
of a new header compression context.
Reducing the overhead of the context establishment procedure is
particularly useful when multiple short-lived connections (or flows)
occurs simultaneously, or near-simultaneously, between the same
compressor-decompressor pair. Such flows may lead to lower header
compression gains, as each new packet stream requires the entire
headers to be sent initially and smaller compressed headers may only
be sent thereafter.
Context replication allows some header fields, such as the IP source
and/or destination addresses (16 octets each for IPv6), to be omitted
within the IR packet specially defined for replication. It also
allows other fields, such as source and/or destination ports, to be
either omitted or sent in a compressed form from the very first
packet of the header compressed flow. In addition, this mechanism
allows contexts from different profiles to be used with context
replication, where for obvious reasons only header fields common to
both profiles can possibly be replicated.
Context replication is herein defined as a general ROHC mechanism;
its support may be defined for any ROHC profile. However, although
the benefits of context replication are not limited to any particular
protocol, it is best motivated for TCP compression. Specifically,
many TCP transfers are short-lived; a behavior analysis of TCP/IP
header fields among multiple short-lived connections may be found in
[TCP-BEH]. In addition, [TCP-REQ] introduces considerations and
requirements for the ROHC-TCP profile [ROHC-TCP] to efficiently
compress such short-lived TCP transfers.
For profiles supporting this mechanism, context replication is
performed by the compressor first initializing a new context for the
new flow. This context is then populated using parts of an existing
context, i.e. a base context, to create the replicated context. The
compressor then sends to the decompressor a packet that contains a
reference to the selected base context, along with some data for the
fields that need to be updated when creating the replicated context.
Finally, the decompressor creates the replicated context based on the
reference to the base context along with the uncompressed and
compressed data from the received packet.
This document specifies the context replication procedure for ROHC
profiles. It defines the general compressor and decompressor logic
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used during context replication, as well as the general format of the
special IR packet required for this procedure. Profiles defining
support for context replication must further specify the specific
format of this packet.
The fundamentals of general header compression may be found in [RFC-
3095]. These are assumed to be understood throughout this document.
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 RFC2119.
This document reuses some of the terminology found in [RFC-3095]. In
addition, this document defines the following terms:
Base context
A base context is a context that has been validated by both the
compressor and the decompressor. A base context can be used by the
compressor as the reference when building a new context using
replication.
Base CID
The Base Context Identifier is the CID used to identify the Base
Context, where information needed for context replication can
be extracted from.
Context replication
Context replication is the mechanism that initializes a new context
based on another already existing context (a base context).
3. Context Replication for ROHC profiles
For profiles defining its support, context replication may be used as
an alternative to the context initialization procedure found in [RFC-
3095]. This section describes the compressor and decompressor logic
as well as the IR packet format used with context replication.
3.1. Robustness considerations
Context replication deviates from the initialization procedure
defined in [RFC-3095] by its capacity to achieve a certain level of
compression already from the first packet used to initialize the
context for a new flow. It is therefore of particular importance that
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the context replication procedure be reliable. This requires that a
base context suitable for replication be used, that the integrity of
the initialization packet be guaranteed and finally that the outcome
of the replication process be verified.
The primary mechanisms used to achieve robustness of the context
replication procedure are the selection of the base context based on
prior feedback from the decompressor and the use of checksums.
Specifically, the compressor must obtain enough confidence that a
base context corresponding to the one selected for replication is
available at the decompressor before initiating the replication
procedure. The most reliable way to select the base context is thus
to choose a context that has previously been acknowledged by the
decompressor.
In addition, the presence of a CRC covering the information used to
initialize the context ensures the integrity of the IR header used
for replication. Finally, an additional CRC calculated over the
original uncompressed header allows the decompressor to validate the
reconstructed header and the outcome of the replication process.
3.2. Compressor logic
For profiles defining support for context replication, the compressor
may replace any IR/IR-DYN packets during the context establishment
procedure (i.e. in IR state) with the IR-REPLICATE packet, if an
already existing context can be selected as a base context for
replication.
3.2.1. Selection of a Base Context
When initiating context replication, the compressor MUST select a
context that has previously been acknowledged by the decompressor as
the base context, and this base context must be valid at replication
time. This also implies that a compressor is not allowed to use the
context replication mechanism if a feedback channel is not present.
Note however that this cannot provide the guarantee that the selected
context is still part of the state managed by the decompressor when
the IR-REPLICATE will be received.
More specifically, [RFC-3095] defines the context identifier (CID) as
a reference to the state information (i.e. the context) used for
compression and decompression. Multiple packet streams with different
contexts may thus share a channel, and the CID space along with its
representation within packet formats may be negotiated as part of the
channel state. However, because [RFC-3095] does not explicitly define
context state management between compressor and decompressor, and in
particular for connection-oriented flows (e.g. TCP), only a high
degree of confidence can be achieved when selecting a base context.
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The criteria whether an existing context is a suitable base context
for replication for a new flow are left to implementations. For
simplicity, contexts with the same Source-IP and/or Destination-IP
may be considered as replicable contexts, and the most recent one
should be selected as the candidate to be replicated.
Finally, the Base CID within the packet format of the IR-REPLICATE
may be assigned a different value than the context identifier
associated to the new flow (i.e. Base CID != CID); otherwise the base
context is overwritten with the new context by the replication
process.
3.2.2. Feedback logic
Context replication is designed to operate over links where a
feedback channel is available. This is necessary to ensure that the
information used to create a new context is synchronized between the
compressor and the decompressor. In addition, context replication may
also make use of feedback from decompressor to compressor for
transition back to the IR state and for OPTIONAL improved forward
transition.
[RFC-3095, section 5.2.2.] specifies the required format for the
feedback field within the general ROHC packet format to be used by
all profiles; the feedback information is structured using two
possible formats: FEEDBACK-1 and FEEDBACK-2. In particular, FEEDBACK-
2 can carry one of three possible types of feedback information: ACK,
NACK or STATIC-NACK.
3.2.2.1. Negative Acknowledgements (NACKs)
A STATIC-NACK sent by the decompressor may indicate that a valid
context could not be initialized by the decompressor during context
replication, and the corresponding context has been invalidated.
Upon reception of a STATIC-NACK, the compressor MUST transit back to
its initial no context state and SHOULD refrain from sending IR-
REPLICATE packets using the same base context. The compressor SHOULD
re-initialize the decompressor context using an IR packet.
A NACK sent by the decompressor may indicate that a valid context has
been successfully initialized but that the decompression of one or
more subsequent packets has failed.
Upon reception of a NACK, the compressor may assume that the static
part of the decompressor context is valid but that the dynamic part
is invalid, and take actions accordingly.
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3.2.2.2. Optional Acknowledgements (ACKs)
An ACK may be sent by the decompressor to indicate that a context has
been successfully initialized during context replication.
Upon reception of an ACK, the compressor may assume that the context
replication procedure was successful and transit from its initial
state (e.g. IR state) to a higher compression state.
3.3. Decompressor logic
3.3.1. Replication and context initialization
Upon reception of an IR-REPLICATE packet, the decompressor first
determines its content (RFC-3095, section 5.2.6). The profile
indicated in the IR-REPLICATE packet determines how it is to be
processed. If the CRC (8-bit CRC) fails to verify the packet, the
packet MUST be discarded.
If the profile as indicated in the IR-REPLICATE packet defines the
use of the Base CID and if its corresponding field is present within
the packet format, this field is used to identify the base context;
otherwise the CID is used.
The decompressor then creates a new context using the information
present in the IR-REPLICATE packet together with the identified base
context, and decompresses the original header. The decompressor
validates the resulting header using the CRC calculated over the
original uncompressed header; if the decompessor fails to validate
the header, the actions specified in section 3.3.2 must be taken.
3.3.2. Actions upon failure
For profiles supporting context replication, the feedback logic of a
decompressor is similar to the logic used for context initialization,
as described in [RFC-3095].
Specifically, when the decompressor fails to validate the context
following the decompression of one or more initial IR-REPLICATE
packets, it MUST invalidate the context and remain in its initial
state. In addition, the decompressor SHOULD send a STATIC-NACK.
If the context has been successfully validated from the decompression
of one or more initial IR-REPLICATE packets, the decompressor SHOULD
send a NACK when it fails to verify the context following the
decompression of one or more subsequent IR-REPLICATE packets.
The decompressor SHOULD send an ACK when it succeeds to validate the
context as a result of the decompression of one or more IR-REPLICATE
packets.
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3.4. Packet Formats
The format of the IR-REPLICATE packet has been designed under the
following constraints:
a) it must be possible to either overwrite a CID during context
replication, or to use a CID different than the Base CID for
the replicated context;
b) it must be possible to replicate from a base context using a
profile different than the one associated with the replicated
context, for fields specifically common to both profiles;
c) it must be possible to selectively include or exclude from the
packet format some fields that may be replicable;
d) it must be possible for some fields that may be replicable to be
represented within the packet format using either a compressed or
an uncompressed form;
e) it must be possible for the decompressor to verify the success of
the replication procedure;
f) it is anticipated that profiles other than [ROHC-TCP] will also
define support for context replication, therefore it is desirable
that the packet format be as profile independent as possible.
3.4.1. CRCs in the IR-REPLICATE packet
The IR packet, as defined in [RFC-3095], is used to communicate
static and/or dynamic parts of a context, and typically initialize
the context. The static and dynamic chains of IR packets contains an
uncompressed representation of the original header.
The IR packet format includes an 8-bit CRC, calculated over the
initial part of the IR packet. This CRC is meant to protect any
information that initialize the context. In particular, its coverage
always includes any CID information as well as the profile used to
interpret the remainder of the IR packet.
The purpose of the 8-bit CRC is to ensure the integrity of the IR
header itself. Profiles may extend the coverage of this CRC to
include the entire IR header, thus allowing the verification of the
integrity of the entire uncompressed header. However because the
format of the IR packet is common to all ROHC profiles and verified
as part of the initial processing of a ROHC decompressor (see [RFC-
3095, section 5.2.6.]), profiles may not redefine this CRC beyond the
extent of its coverage.
[RFC-3095] also defines a 3-bit CRC and a 7-bit CRC for compressed
headers, used to verify proper decompression and validate the
context. This type of CRC is calculated over the original
uncompressed header, as it is not sufficient to only protect the
compressed data being exchanged between compressor and decompressor
to ensure a robust reconstruction of the original header.
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There is thus a clear distinction in purposes between the 8-bit CRC
found in the IR packet and the 3-bit or 7-bit CRC found in compressed
headers. With context replication, where the IR-REPLICATE packet may
contain both compressed as well as uncompressed information and omit
entirely replicable fields, this distinction in no longer present.
Profiles supporting context replication MUST define a CRC over the
original uncompressed header as part of the profile specific
information in the IR-REPLICATE packet. This is necessary to allow a
decompressor to verify that the replication process has succeeded.
3.4.1.1. 7-bit CRC
The 7-bit CRC in the IR-REPLICATE packet is calculated over all
octets of the entire original header, before replication, in the same
manner as described in [RFC-3095, section 5.9.2].
The initial content of the CRC register is to be preset to all 1's.
The CRC polynomial used for the 7-bit CRC in the IR-REPLICATE is:
C(x) = 1 + x + x^2 + x^3 + x^6 + x^7
3.4.1.2. 8-bit CRC
The coverage of the 8-bit CRC in the IR-REPLICATE packet is profile-
dependent, but it MUST cover at least the initial part of the packet
ending with the Profile field and if present, the Base CID field. For
profiles that define the usage of the Base CID within the packet
format of the IR-REPLICATE as optional, the CRC MUST also cover the
information used to indicate the presence of this field within the
packet. Any other information which initializes the context of the
decompressor should also be protected by the CRC.
The initial content of the CRC register is to be preset to all 1's.
The CRC polynomial used for the 8-bit CRC in the IR-REPLICATE is:
C(x) = 1 + x + x^2 + x^8
3.4.2. General Format of the IR-REPLICATE packet
The context replication mechanism requires a dedicated IR packet
format that uniquely identifies the IR-REPLICATE packet. This packet
communicates the static and the dynamic parts of the replicated
context. It may also communicate a reference to a base context.
With consideration to the extensibility of the IR packet type defined
in [RFC-3095], support for replication can be added using the profile
specific part of the IR packet. Note that there is one bit, (x), left
in the IR header for "Profile specific information". The definition
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of this bit is profile-specific. Thus, profiles supporting context
replication may use this bit as a flag indicating whether the packet
is an IR packet or an IR-REPLICATE packet. Note also that profiles
may define an alternative method to identify the IR-REPLICATE packet
within the profile specific information, instead of using this bit.
The IR-REPLICATE header associates a CID with a profile, and
initializes the context using the context replication mechanism. It
is not recommended to use this packet to repair a damaged context.
The IR-REPLICATE has the following general format:
0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if for small CIDs and (CID != 0)
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 1 0 x | IR type octet
+---+---+---+---+---+---+---+---+
: :
/ 0-2 octets of CID / 1-2 octets if for large CIDs
: :
+---+---+---+---+---+---+---+---+
| Profile | 1 octet
+---+---+---+---+---+---+---+---+
| CRC | 1 octet
+---+---+---+---+---+---+---+---+
| |
/ profile specific information / variable length
| |
+---+---+---+---+---+---+---+---+
| |
| Static replication chain / variable length
| |
+---+---+---+---+---+---+---+---+
| |
/ Dynamic replication chain / variable length
| |
- - - - - - - - - - - - - - - -
| |
/ Payload / variable length
| |
- - - - - - - - - - - - - - - -
x: Profile specific information. Interpreted according to the
profile indicated in the Profile field.
Profile: The profile to be associated with the CID. In the IR-
REPLICATE packet, the profile identifier is abbreviated to the
8 least significant bits. It selects the highest-number
profile in the channel state parameter PROFILES that matches
the 8 LSBs given (see also [RFC-3095]).
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CRC: 8-bit CRC computed using the polynomial of section 3.4.1.2.
Profile specific information: The contents of this part of the IR-
REPLICATE packet are defined by the individual profiles. This
information is interpreted according to the profile indicated
in the Profile field. It MUST include a 7-bit CRC over the
original uncompressed header using the polynomial of section
3.4.1.1.
Static replication chain: A chain of static subheader information
used for replication.
Dynamic replication chain: A chain of dynamic subheader
information used for replication. What dynamic information is
present is inferred from the static replication chain.
Payload: The payload of the corresponding original packet, if any.
The presence of a payload is inferred from the packet length.
4. Security considerations
This document does not bring any new additional security
considerations than those already listed in [ROHC-TCP].
5. Acknowledgements
The author would like to thank Lars-Erik Jonsson, Richard Price, Mark
West and HongBin Liao for valuable input to this document.
6. References
6.1. Normative References
[RFC-768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC-791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC-793] Postel, J., "Transmission Control Protocol," RFC 793
(STD7), September 1981.
[RFC-2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[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,
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K., Wiebke, T., Yoshimura, T. and H. Zheng, "RObust
Header Compression (ROHC): Framework and four profiles:
RTP, UDP, ESP, and uncompressed", RFC 3095, July 2001.
[UDP-Lite] Larzon, L., Degermark, M., Pink, S., Jonsson, L.,
Fairhurst, G., "The UDP-Lite Protocol", Internet draft
(work in progress), December 2002, <draft-ietf-tsvwg-
udp-lite-01.txt>
6.2. Informative References
[ROHC-TCP] Pelletier, G., Zhang, Q., Jonsson, L-E., Liao, H., West,
M., "RObust Header Compression (ROHC): TCP/IP Profile
(ROHC-TCP)", Internet Draft (work in progress), <draft-
ietf-rohc-tcp-04.txt>, May 2003.
[TCP-REQ] Jonsson, L-E., "Requirements on ROHC IP/TCP header
compression", Internet Draft (work in progress),
<draft-ietf-rohc-tcp-requirements-05.txt>, October 2002.
[TCP-BEH] West, M. and S. McCann, "TCP/IP Field Behavior", Internet
Draft (work in progress), <draft-ietf-rohc-tcp-field-
behavior-02.txt>, March 2003.
[RFC-2026] Bradner, S., "The Internet Standards Process", RFC 2026,
October 1996.
7. Author's address
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. Replication chains
This section provides examples of static and dynamic replication
chains for IPv6, IPv4, UDP, UDP-Lite and TCP.
A.1. Replication of the IPv6 Header [RFC-2460]
Static part:
+---+---+---+---+---+---+---+---+
| F | N | S | D |Flow Label(msb)|
+---+---+---+---+---+---+---+---+
/ Flow Label (lsb) / 2 octets, if F = 1
+---+---+---+---+---+---+---+---+
| Next Header | 1 octet, if N = 1
+---+---+---+---+---+---+---+---+
/ Source Address / 16 octets, if S = 1
+---+---+---+---+---+---+---+---+
/ Destination Address / 16 octets, if D = 1
+---+---+---+---+---+---+---+---+
Dynamic part:
+---+---+---+---+---+---+---+---+
| Traffic Class | 1 octet
+---+---+---+---+---+---+---+---+
| Hop Limit | 1 octet
+---+---+---+---+---+---+---+---+
/ Generic extension header list / variable length
+---+---+---+---+---+---+---+---+
Eliminated:
Payload Length
Version
Replicable:
Flow Label (Flow Label must be all '0's)
Flow Label must not be used, i.e. this field must be all '0's.
Note that the Flow Label(msb) field is valid only if F = 1.
Next Header
The type of the following header in the static replication
chain must be the same type as the one found in the static
chain of the base context.
Source Address
Destination Address
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Extras:
F, N, S and D: replication flags.
Generic extension header list: see [RFC-3095, section 5.7.7.3].
CRC-DYNAMIC: Payload Length field (octets 5-6).
CRC-STATIC: All other fields (octets 1-4, 7-40).
CRC coverage for extension headers is defined in [RFC-3095, section
5.8.7].
Note: The Next Header field indicates the type of the following
header in the static chain, rather than being a copy of the Next
Header field of the original IPv6 header. See also [RFC-30905,
section 5.7.7.8].
Note: When using context replication from a base context where the
static part contains multiple IP levels for a flow with a different
number of IP levels, only the outer IP header can be replicated.
A.2. Replication of the IPv4 Header [RFC-791]
Static part:
+---+---+---+---+---+---+---+---+
| P | S | D | DF|RND|NBO| 0 |
+---+---+---+---+---+---+---+---+
| Protocol | 1 octets, if P = 1
+---+---+---+---+---+---+---+---+
/ Source Address / 4 octets, if S = 1
+---+---+---+---+---+---+---+---+
/ Destination Address / 4 octets, if D = 1
+---+---+---+---+---+---+---+---+
Dynamic part:
+---+---+---+---+---+---+---+---+
| Type of Service |
+---+---+---+---+---+---+---+---+
| Time to Live |
+---+---+---+---+---+---+---+---+
/ Identification / 2 octets
+---+---+---+---+---+---+---+---+
/ Generic extension header list / variable length
+---+---+---+---+---+---+---+---+
Eliminated:
IHL, Total Length, MF flag, Fragment Offset, Header Checksum,
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Options, Padding and Version. See also [RFC-3095, section
5.7.7.4].
Replicable:
Protocol
The type of the following header in the static replication
chain must be the same type as the one found in the static
chain of the base context.
Source Address
Destination Address
Extras:
P, S and D: replication flags.
RND, NBO. See [RFC-3095, section 5.7].
CRC-DYNAMIC: Total Length, Identification, Header Checksum
(octets 3-4, 5-6, 11-12).
CRC-STATIC: All other fields (octets 1-2, 7-10, 13-20)
CRC coverage for extension headers is defined in [ROHC, section
5.8.7].
Note: The Protocol field indicates the type of the following header
in the static chain, rather than being a copy of the Protocol field
of the original IPv4 header. See also [RFC-3095, section 5.7.7.8].
Note: When using context replication from a base context where the
static part contains multiple IP levels for a flow with a different
number of IP levels, only the outer IP header can be replicated.
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INTERNET-DRAFT Context Replication for ROHC profiles May 23 2002
A.3. Replication of the UDP Header [RFC-768]
<# Author's Is it possible to identify a useful scenario where #>
<# note : replication of the UDP part of a context is #>
<# desirable? It is not clear how much value there is #>
<# the UDP header. For UDP, the static and dynamic #>
<# chains found in [RFC-3095] could be used. #>
Static part:
+---+---+---+---+---+---+---+---+
| S | SC| D | DC| C | 0 |
+---+---+---+---+---+---+---+---+
: : Present if S = 1 and
/ Source Port / 1 octet if SC = 1
: : 2 octets if SC = 0
+---+---+---+---+---+---+---+---+
: : Present if D = 1 and
/ Destination Port / 1 octet if DC = 1
: : 2 octets if DC = 0
+---+---+---+---+---+---+---+---+
Dynamic part:
+---+---+---+---+---+---+---+---+
/ Checksum / 2 octets, present if C = 1
+---+---+---+---+---+---+---+---+
Eliminated:
Length
The Length field of the UDP header MUST match the Length field(s)
of the preceding subheaders, i.e., there must not be any padding
after the UDP payload that is covered by the IP Length.
Replicable:
Source Port
Destination Port
Checksum
Extras:
S, SC, D, DC, C: replication flags.
CRC-DYNAMIC: Length field, Checksum (octets 5-8).
CRC-STATIC: All other fields (octets 1-4).
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INTERNET-DRAFT Context Replication for ROHC profiles May 23 2002
A.4. Replication of the UDP-Lite Header [UDP-Lite]
<# Author's Is it possible to identify a useful scenario where #>
<# note : replication of the UDP-Lite part of a context is #>
<# desirable? It is not clear how much value there #>
<# is to replicate parts of the UDP-Lite header. For #>
<# UDP-Lite, the static and dynamic chains defined in #>
<# the UDP-Lite profile could be used. #>
Static part:
+---+---+---+---+---+---+---+---+
| S | SC| D | DC| C | CC| 0 |
+---+---+---+---+---+---+---+---+
: : Present if S = 1 and
/ Source Port / 1 octet if SC = 1
: : 2 octets if SC = 0
+---+---+---+---+---+---+---+---+
: : Present if D = 1 and
/ Destination Port / 1 octet if DC = 1
: : 2 octets if DC = 0
+---+---+---+---+---+---+---+---+
Dynamic part:
+---+---+---+---+---+---+---+---+
: : Present if C = 1 and
/ Checksum Coverage / 1 octet if CC = 1
: : 2 octets if CC = 0
+---+---+---+---+---+---+---+---+
/ Checksum / 2 octets
+---+---+---+---+---+---+---+---+
Replicable:
Source Port
Destination Port
Checksum Coverage
Checksum
Extras:
S, SC, D, DC, C, CC: replication flags.
CRC-DYNAMIC: Checksum Coverage field, Checksum field (octets 5-8).
CRC-STATIC: All other fields (octets 1-4).
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INTERNET-DRAFT Context Replication for ROHC profiles May 23 2002
A.5. Replication of the TCP Header [RFC-793]
Static part:
+---+---+---+---+---+---+---+---+
| S | SC| D | DC| W | U | M | 0 |
+---+---+---+---+---+---+---+---+
: : Present, if S = 1 and
/ Source Port / 1 octet, if SC = 1
: : 2 octets, if SC = 0
+---+---+---+---+---+---+---+---+
: : Present, if D = 1 and
/ Destination Port / 1 octet, if DC = 1
: : 2 octets, if DC = 0
+---+---+---+---+---+---+---+---+
Dynamic part:
+---+---+---+---+---+---+---+---+
/ Master Sequence Number / 2 octets, if M = 1
+---+---+---+---+---+---+---+---+
/ Sequence Number / 4 octets
+---+---+---+---+---+---+---+---+
/ Acknowledgement Number / 4 octets
+---+---+---+---+---+---+---+---+
| Data Offset | Reserved | 1 octet
+---+---+---+---+---+---+---+---+
|CWR|ECE|URG|ACK|PSH|RST|SYN|FIN| 1 octet
+---+---+---+---+---+---+---+---+
/ Window / 2 octets, present if W = 1
+---+---+---+---+---+---+---+---+
/ Checksum / 2 octets
+---+---+---+---+---+---+---+---+
/ Urgent Pointer / 2 octets, present if U = 1
+---+---+---+---+---+---+---+---+
/ Options / variable length
+---+---+---+---+---+---+---+---+
Eliminated:
Nothing.
Extras:
S, SC, D, DC, W, U, M: replication flags.
Replicable:
Source Port
Destination Port
Master Sequence Number
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INTERNET-DRAFT Context Replication for ROHC profiles May 23 2002
Window
Urgent Pointer
CRC-DYNAMIC: Length field, Checksum (octets 5-8).
CRC-STATIC: All other fields (octets 1-4).
Note: This chain is always the last chain in the IR-Replicate packet.
Full Copyright Statement
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This Internet-Draft expires November 23, 2003.
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