draft-ietf-rohc-tcp-field-behavior-01.txt   draft-ietf-rohc-tcp-field-behavior-02.txt 
Network Working Group M. West Network Working Group M. West
Internet-Draft S. McCann Internet-Draft S. McCann
Expires: April 28, 2003 Siemens/Roke Manor Expires: September 1, 2003 Siemens/Roke Manor
October 28, 2002 March 3, 2003
TCP/IP Field Behavior TCP/IP Field Behavior
draft-ietf-rohc-tcp-field-behavior-01.txt draft-ietf-rohc-tcp-field-behavior-02.txt
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
skipping to change at page 1, line 32 skipping to change at page 1, line 32
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at http:// The list of current Internet-Drafts can be accessed at http://
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This Internet-Draft will expire on April 28, 2003. This Internet-Draft will expire on September 1, 2003.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved. Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract Abstract
This draft describes TCP/IP field behavior in the context of header This memo describes TCP/IP field behavior in the context of header
compression. compression.
Header compression is possible thanks to the fact that most header Header compression is possible thanks to the fact that most header
fields do not vary randomly from packet to packet. Many of the fields do not vary randomly from packet to packet. Many of the
fields exhibit static behavior or change in a more or less fields exhibit static behavior or change in a more or less
predictable way. When designing a header compression scheme, it is predictable way. When designing a header compression scheme, it is
of fundamental importance to understand the behavior of the fields in of fundamental importance to understand the behavior of the fields in
detail. detail. An example of this analysis can be seen in RFC 3095 [31].
This memo performs a similar role for the compression of TCP/IP.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 5
3. General classification . . . . . . . . . . . . . . . . . . . 5 3. General classification . . . . . . . . . . . . . . . . . . . 5
3.1 IP header fields . . . . . . . . . . . . . . . . . . . . . . 6 3.1 IP header fields . . . . . . . . . . . . . . . . . . . . . . 6
3.1.1 IPv6 header fields . . . . . . . . . . . . . . . . . . . . . 6 3.1.1 IPv6 header fields . . . . . . . . . . . . . . . . . . . . . 6
3.1.2 IPv4 header fields . . . . . . . . . . . . . . . . . . . . . 7 3.1.2 IPv4 header fields . . . . . . . . . . . . . . . . . . . . . 7
3.2 TCP header fields . . . . . . . . . . . . . . . . . . . . . 10 3.2 TCP header fields . . . . . . . . . . . . . . . . . . . . . 10
3.3 Summary for IP/TCP . . . . . . . . . . . . . . . . . . . . . 11 3.3 Summary for IP/TCP . . . . . . . . . . . . . . . . . . . . . 11
4. Classification of shareable header fields . . . . . . . . . 12 4. Classification of replicable header fields . . . . . . . . . 12
5. Analysis of change patterns of header fields . . . . . . . . 15 4.1 IPv4 Header (inner and/or outer) . . . . . . . . . . . . . . 13
5.1 IP header . . . . . . . . . . . . . . . . . . . . . . . . . 18 4.2 IPv6 Header (inner and/or outer) . . . . . . . . . . . . . . 14
5.1.1 IP Traffic-Class / Type-Of-Service (TOS) . . . . . . . . . . 18 4.3 TCP Header . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.1.2 ECN Flags . . . . . . . . . . . . . . . . . . . . . . . . . 18 4.4 TCP Options . . . . . . . . . . . . . . . . . . . . . . . . 16
5.1.3 IP Identification . . . . . . . . . . . . . . . . . . . . . 19 4.5 Summary of replication . . . . . . . . . . . . . . . . . . . 16
5.1.4 Don't Fragment (DF) flag . . . . . . . . . . . . . . . . . . 21 5. Analysis of change patterns of header fields . . . . . . . . 17
5.1.5 IP Hop-Limit / Time-To-Live (TTL) . . . . . . . . . . . . . 21 5.1 IP header . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.2 TCP header . . . . . . . . . . . . . . . . . . . . . . . . . 21 5.1.1 IP Traffic-Class / Type-Of-Service (TOS) . . . . . . . . . . 19
5.2.1 Sequence number . . . . . . . . . . . . . . . . . . . . . . 22 5.1.2 ECN Flags . . . . . . . . . . . . . . . . . . . . . . . . . 20
5.2.2 Acknowledgement number . . . . . . . . . . . . . . . . . . . 23 5.1.3 IP Identification . . . . . . . . . . . . . . . . . . . . . 20
5.2.3 Reserved . . . . . . . . . . . . . . . . . . . . . . . . . . 23 5.1.4 Don't Fragment (DF) flag . . . . . . . . . . . . . . . . . . 22
5.2.4 Flags . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 5.1.5 IP Hop-Limit / Time-To-Live (TTL) . . . . . . . . . . . . . 23
5.2.5 Checksum . . . . . . . . . . . . . . . . . . . . . . . . . . 25 5.2 TCP header . . . . . . . . . . . . . . . . . . . . . . . . . 23
5.2.6 Window . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 5.2.1 Sequence number . . . . . . . . . . . . . . . . . . . . . . 23
5.2.7 Urgent pointer . . . . . . . . . . . . . . . . . . . . . . . 25 5.2.2 Acknowledgement number . . . . . . . . . . . . . . . . . . . 24
5.3 Options . . . . . . . . . . . . . . . . . . . . . . . . . . 26 5.2.3 Reserved . . . . . . . . . . . . . . . . . . . . . . . . . . 25
5.3.1 Options overview . . . . . . . . . . . . . . . . . . . . . . 26 5.2.4 Flags . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
5.3.2 Option field behavior . . . . . . . . . . . . . . . . . . . 27 5.2.5 Checksum . . . . . . . . . . . . . . . . . . . . . . . . . . 26
6. Other observations . . . . . . . . . . . . . . . . . . . . . 35 5.2.6 Window . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
6.1 Implicit acknowledgements . . . . . . . . . . . . . . . . . 35 5.2.7 Urgent pointer . . . . . . . . . . . . . . . . . . . . . . . 27
6.2 Shared data . . . . . . . . . . . . . . . . . . . . . . . . 35 5.3 Options . . . . . . . . . . . . . . . . . . . . . . . . . . 27
6.3 TCP header overhead . . . . . . . . . . . . . . . . . . . . 35 5.3.1 Options overview . . . . . . . . . . . . . . . . . . . . . . 27
6.4 Field independence and packet behavior . . . . . . . . . . . 36 5.3.2 Option field behavior . . . . . . . . . . . . . . . . . . . 28
6.5 Short-lived flows . . . . . . . . . . . . . . . . . . . . . 36 6. Other observations . . . . . . . . . . . . . . . . . . . . . 36
6.6 Master Sequence Number . . . . . . . . . . . . . . . . . . . 37 6.1 Implicit acknowledgements . . . . . . . . . . . . . . . . . 36
6.7 Size constraint for TCP options . . . . . . . . . . . . . . 37 6.2 Shared data . . . . . . . . . . . . . . . . . . . . . . . . 36
7. Security considerations . . . . . . . . . . . . . . . . . . 37 6.3 TCP header overhead . . . . . . . . . . . . . . . . . . . . 36
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 38 6.4 Field independence and packet behavior . . . . . . . . . . . 37
References . . . . . . . . . . . . . . . . . . . . . . . . . 38 6.5 Short-lived flows . . . . . . . . . . . . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 41 6.6 Master Sequence Number . . . . . . . . . . . . . . . . . . . 38
Full Copyright Statement . . . . . . . . . . . . . . . . . . 42 6.7 Size constraint for TCP options . . . . . . . . . . . . . . 38
7. Security considerations . . . . . . . . . . . . . . . . . . 39
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 39
References . . . . . . . . . . . . . . . . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 42
Full Copyright Statement . . . . . . . . . . . . . . . . . . 43
1. Introduction 1. Introduction
This document describes TCP/IP field behavior, as it is essential to This document describes TCP/IP field behavior, as it is essential to
understand this before correct assumptions about header compression understand this before correct assumptions about header compression
can be made. can be made.
Since the IP header does exhibit some slightly different behavior Since the IP header does exhibit some slightly different behavior
from that previously presented in RFC 3095 [28] for the RTP case, it from that previously presented in RFC 3095 [31] for the RTP case, it
is also included in this document. is also included in this document.
It is intentional that much of the classification text from RFC 3095 It is intentional that much of the classification text from RFC 3095
[28] has been borrowed. This is for easier reading rather than [31] has been borrowed. This is for easier reading rather than
inserting many references to that document. inserting many references to that document.
Again based on the format presented in RFC 3095 [28] TCP/IP header Again based on the format presented in RFC 3095 [31] TCP/IP header
fields are classified and analyzed in two steps. First, we have a fields are classified and analyzed in two steps. First, we have a
general classification in Section 3 where the fields are classified general classification in Section 3 where the fields are classified
on the basis of stable knowledge and assumptions. The general on the basis of stable knowledge and assumptions. The general
classification does not take into account the change characteristics classification does not take into account the change characteristics
of changing fields because those will vary more or less depending on of changing fields because those will vary more or less depending on
the implementation and on the application used. Section 4 considers the implementation and on the application used. Section 4 considers
how field values can be used to optimize short-lived flows. A less how field values can be used to optimize short-lived flows. A less
stable but more detailed analysis of the change characteristics is stable but more detailed analysis of the change characteristics is
then done in Section 5. Finally, Section 6 summarizes with then done in Section 5. Finally, Section 6 summarizes with
conclusions about how the various header fields should be handled by conclusions about how the various header fields should be handled by
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It is unclear exactly how reserved bits should be handled, given that It is unclear exactly how reserved bits should be handled, given that
the possible future uses cannot be predicted. It is accepted that if the possible future uses cannot be predicted. It is accepted that if
these currently reserved bits were used, then efficiency may be these currently reserved bits were used, then efficiency may be
reduced. However, the compression scheme should still offer a useful reduced. However, the compression scheme should still offer a useful
solution. solution.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [18]. document are to be interpreted as described in RFC 2119 [21].
3. General classification 3. General classification
The following definitions (and some text) are copied from RFC 3095 The following definitions (and some text) are copied from RFC 3095
[28] Appendix A. Differences between IP field behavior between RFC [31] Appendix A. Differences between IP field behavior between RFC
3095 [28] (i.e. IP/UDP/RTP behavior for audio and video 3095 [31] (i.e. IP/UDP/RTP behavior for audio and video
applications) and this document have been identified. applications) and this document have been identified.
At a general level, the header fields are separated into 5 classes: At a general level, the header fields are separated into 5 classes:
o INFERRED o INFERRED
These fields contain values that can be inferred from other These fields contain values that can be inferred from other
values, for example the size of the frame carrying the packet, values, for example the size of the frame carrying the packet,
and thus do not have to be handled at all by the compression and thus do not have to be handled at all by the compression
scheme. scheme.
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| ECT flag* | 1 | CHANGING | | ECT flag* | 1 | CHANGING |
| CE flag* | 1 | CHANGING | | CE flag* | 1 | CHANGING |
| Flow Label | 20 | STATIC-DEF | | Flow Label | 20 | STATIC-DEF |
| Payload Length | 16 | INFERRED | | Payload Length | 16 | INFERRED |
| Next Header | 8 | STATIC | | Next Header | 8 | STATIC |
| Hop Limit | 8 | CHANGING | | Hop Limit | 8 | CHANGING |
| Source Address | 128 | STATIC-DEF | | Source Address | 128 | STATIC-DEF |
| Destination Address | 128 | STATIC-DEF | | Destination Address | 128 | STATIC-DEF |
+---------------------+-------------+----------------+ +---------------------+-------------+----------------+
* differs from RFC 3095 [28] * differs from RFC 3095 [31]
[The DSCP, ECT and CE flags were amalgamated into the Traffic Class [The DSCP, ECT and CE flags were amalgamated into the Traffic Class
octet in RFC 3095.] octet in RFC 3095.]
o Version o Version
The version field states which IP version is used. Packets The version field states which IP version is used. Packets
with different values in this field must be handled by with different values in this field must be handled by
different IP stacks. All packets of a packet stream must different IP stacks. All packets of a packet stream must
therefore be of the same IP version. Accordingly, the field is therefore be of the same IP version. Accordingly, the field is
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field is therefore classified as INFERRED. field is therefore classified as INFERRED.
o Next Header o Next Header
This field will usually have the same value in all packets of a This field will usually have the same value in all packets of a
packet stream. It encodes the type of the subsequent header. packet stream. It encodes the type of the subsequent header.
Only when extension headers are sometimes present and sometimes Only when extension headers are sometimes present and sometimes
not, will the field change its value during the lifetime of the not, will the field change its value during the lifetime of the
stream. The field is therefore classified as STATIC. stream. The field is therefore classified as STATIC.
The classification of STATIC is inherited from RFC 3095 [28]. The classification of STATIC is inherited from RFC 3095 [31].
However, it should be pointed out that the next header field is However, it should be pointed out that the next header field is
actually determined by the type of the following header. Thus, actually determined by the type of the following header. Thus,
it might be more appropriate to view this as an inference, it might be more appropriate to view this as an inference,
although this depends upon the specific implementation of the although this depends upon the specific implementation of the
compression scheme. compression scheme.
o Source and Destination addresses o Source and Destination addresses
These fields are part of the definition of a stream and must These fields are part of the definition of a stream and must
thus be constant for all packets in the stream. The fields are thus be constant for all packets in the stream. The fields are
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| Don't Fragment flag*| 1 | CHANGING | | Don't Fragment flag*| 1 | CHANGING |
| More Fragments flag | 1 | STATIC-KNOWN | | More Fragments flag | 1 | STATIC-KNOWN |
| Fragment Offset | 13 | STATIC-KNOWN | | Fragment Offset | 13 | STATIC-KNOWN |
| Time To Live | 8 | CHANGING | | Time To Live | 8 | CHANGING |
| Protocol | 8 | STATIC | | Protocol | 8 | STATIC |
| Header Checksum | 16 | INFERRED | | Header Checksum | 16 | INFERRED |
| Source Address | 32 | STATIC-DEF | | Source Address | 32 | STATIC-DEF |
| Destination Address | 32 | STATIC-DEF | | Destination Address | 32 | STATIC-DEF |
+---------------------+-------------+----------------+ +---------------------+-------------+----------------+
* differs from RFC 3095 [28] * differs from RFC 3095 [31]
[The DSCP, ECT and CE flags were amalgamated into the TOS octet in [The DSCP, ECT and CE flags were amalgamated into the TOS octet in
RFC 3095. RFC 3095.
The DF flag behaviour is considered later. The DF flag behavior is considered later.
The reserved field is discussed below.] The reserved field is discussed below.]
o Version o Version
The version field states which IP version is used. Packets The version field states which IP version is used. Packets
with different values in this field must be handled by with different values in this field must be handled by
different IP stacks. All packets of a packet stream must different IP stacks. All packets of a packet stream must
therefore be of the same IP version. Accordingly, the field is therefore be of the same IP version. Accordingly, the field is
classified as STATIC. classified as STATIC.
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no options. The field is therefore classified as STATIC-KNOWN. no options. The field is therefore classified as STATIC-KNOWN.
o Packet Length o Packet Length
Information about packet length is expected to be provided by Information about packet length is expected to be provided by
the link layer. The field is therefore classified as INFERRED. the link layer. The field is therefore classified as INFERRED.
o Flags o Flags
The Reserved flag must be set to zero, as defined in RFC 791 The Reserved flag must be set to zero, as defined in RFC 791
[1]. In RFC 3095 [28] the field is therefore classified as [1]. In RFC 3095 [31] the field is therefore classified as
STATIC-KNOWN. However, it is expected that reserved fields may STATIC-KNOWN. However, it is expected that reserved fields may
be used at some future point. It appears unwise to select an be used at some future point. It appears unwise to select an
encoding that would preclude the use of a compression profile encoding that would preclude the use of a compression profile
for a future change in the use of reserved fields. For this for a future change in the use of reserved fields. For this
reason the alternative encoding of CHANGING is suggested. It reason the alternative encoding of CHANGING is suggested. It
would also be possible to have more than one compression would also be possible to have more than one compression
profile, in one of which this field was considered to be profile, in one of which this field was considered to be
STATIC-KNOWN. STATIC-KNOWN.
The More Fragments (MF) flag is expected to be zero because The More Fragments (MF) flag is expected to be zero because
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o Header Checksum o Header Checksum
The header checksum protects individual hops from processing a The header checksum protects individual hops from processing a
corrupted header. When almost all IP header information is corrupted header. When almost all IP header information is
compressed away, there is no point in having this additional compressed away, there is no point in having this additional
checksum; instead it can be regenerated at the decompressor checksum; instead it can be regenerated at the decompressor
side. The field is therefore classified as INFERRED. side. The field is therefore classified as INFERRED.
We note that the TCP checksum does not protect the whole TCP/IP We note that the TCP checksum does not protect the whole TCP/IP
header, but only the TCP pseudo-header (and the payload). header, but only the TCP pseudo-header (and the payload).
Compare this with ROHC [28], which uses a CRC to verify the Compare this with ROHC [31], which uses a CRC to verify the
uncompressed header. Given the need to validate the complete uncompressed header. Given the need to validate the complete
TCP/IP header; the cost of computing the TCP checksum over the TCP/IP header; the cost of computing the TCP checksum over the
entire payload; and known weaknesses in the TCP checksum, an entire payload; and known weaknesses in the TCP checksum [37],
additional check is necessary. Therefore, it is expected than an additional check is necessary. Therefore, it is expected
some additional checksum (such as a CRC) will be used to than some additional checksum (such as a CRC) will be used to
validate correct decompression. validate correct decompression.
o Source and Destination addresses o Source and Destination addresses
These fields are part of the definition of a stream and must These fields are part of the definition of a stream and must
thus be constant for all packets in the stream. The fields are thus be constant for all packets in the stream. The fields are
therefore classified as STATIC-DEF. therefore classified as STATIC-DEF.
Total size of the fields in each class: Total size of the fields in each class:
+--------------+--------------+ +--------------+--------------+
| Class | Size (octets)| | Class | Size (octets)|
+--------------+--------------+ +--------------+--------------+
| INFERRED | 4 | | INFERRED | 4 |
| STATIC* | 1.5 | | STATIC* | 1.5 |
| STATIC-DEF | 8 | | STATIC-DEF | 8 |
| STATIC-KNOWN*| 2.25 | | STATIC-KNOWN*| 2.25 |
| CHANGING* | 4.25 | | CHANGING* | 4.25 |
+--------------+--------------+ +--------------+--------------+
* differs from RFC 3095 [28] * differs from RFC 3095 [31]
3.2 TCP header fields 3.2 TCP header fields
+---------------------+-------------+----------------+ +---------------------+-------------+----------------+
| Field | Size (bits) | Class | | Field | Size (bits) | Class |
+---------------------+-------------+----------------+ +---------------------+-------------+----------------+
| Source Port | 16 | STATIC-DEF | | Source Port | 16 | STATIC-DEF |
| Destination Port | 16 | STATIC-DEF | | Destination Port | 16 | STATIC-DEF |
| Sequence Number | 32 | CHANGING | | Sequence Number | 32 | CHANGING |
| Acknowledgement Num | 32 | CHANGING | | Acknowledgement Num | 32 | CHANGING |
| Data Offset | 4 | INFERRED | | Data Offset | 4 | INFERRED |
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| STATIC | 1 + 4 bits | 1 + 4 bits | | STATIC | 1 + 4 bits | 1 + 4 bits |
| STATIC-DEF | 38 + 4 bits | 12 | | STATIC-DEF | 38 + 4 bits | 12 |
| STATIC-KNOWN | - | 2 + 2 bits | | STATIC-KNOWN | - | 2 + 2 bits |
| CHANGING | 17 + 4 bits | 19 + 6 bits | | CHANGING | 17 + 4 bits | 19 + 6 bits |
+----------------+----------------+----------------+ +----------------+----------------+----------------+
| Totals | 60 | 40 | | Totals | 60 | 40 |
+----------------+----------------+----------------+ +----------------+----------------+----------------+
(excludes options, which are all classified as CHANGING) (excludes options, which are all classified as CHANGING)
4. Classification of shareable header fields 4. Classification of replicable header fields
For multiple flows that overlap in time (or occur sequentially within Where multiple flows either overlap in time or occur sequentially
a relatively short space of time), there can be much similarity in within a short space of time there can be a great deal of similarity
header field values (and hence context values) among those in header field values. Such commonality of field values is
connections. To utilize these properties for header compression, it reflected in the compression context. Thus, it should be possible to
is important to understand the shareable characteristics for the utilise links between fields across different flows to improve the
various header fields and context values. compression ratio. In order to do this, it is important to
understand the 'replicable' characteristics of the various header
fields.
The intent here is to use an existing context as a baseline and to The key concept is that of 'replication', where an existing context
'replicate' it to create a new context. The fields that have changed is used as a baseline and replicated to initialise a new context.
will be updated in the replicated context. Hence it is important to Those fields that are the same are then automatically initialised in
understand the degree of commonality between fields in different the new context. Those that have changed will be updated or
flows. overwritten with values from the initialisation packet that triggered
the replication. This section considers the commonality between
fields in different flows.
It should be noted, however, that replication is based on contexts
(rather than just field values) and so compressor created fields that
are part of the context may also be included. These, of course, are
dependent upon the nature of the compression protocol (ROHC profile)
being applied.
A brief analysis of the relationship of TCP/IP fields among A brief analysis of the relationship of TCP/IP fields among
'shareable' packet streams is given here. 'replicable' packet streams follows.
'N/A' The field need not be shared since it can be inferred or 'N/A' -- The field need not be considered in the replication
is used to define a packet flow. process as it is inferred or known 'a priori' (and,
therefore, does not appear in the context).
'No' The field cannot be shared since its change pattern 'No' -- The field cannot be replicated since its change pattern
between two packet flows is uncorrelated. between two packet flows is uncorrelated.
'Yes' The field can be shared. Specific encoding methods can 'Yes' -- The field may be replicated. This does not guarantee
be used to improve the compression efficiency. that the field value will be the same across two candidate
streams, only that it might be possible to exploit
replication to increase the compression ratio. Specific
encoding methods can be used to improve the compression
efficiency.
IPv4 Header (inner and/or outer) 4.1 IPv4 Header (inner and/or outer)
Field Class Shareable +-----------------------+---------------+------------+
------------------------------------------------ | Field | Class | Replicable |
Version STATIC N/A +-----------------------+---------------+------------+
Header Length STATIC-KNOWN Yes | Version | STATIC | N/A |
DSCP CHANGING Yes (1) | Header Length | STATIC-KNOWN | N/A |
Packet Length INFERRED N/A | DSCP | CHANGING | No (1) |
Identification CHANGING Yes (2) | ECT flag | CHANGING | No (2) |
Reserved flag STATIC-KNOWN No (3) | CE flag | CHANGING | No (2) |
Don't Fragment flag STATIC No | Packet Length | INFERRED | N/A |
More Fragments flag STATIC-KNOWN No | Identification | CHANGING | Yes (3) |
Fragment Offset STATIC-KNOWN No | Reserved flag | CHANGING | No (4) |
Time To Live CHANGING Yes | Don't Fragment flag | CHANGING | No |
Protocol STATIC N/A | More Fragments flag | STATIC-KNOWN | N/A |
Header Checksum INFERRED N/A | Fragment Offset | STATIC-KNOWN | N/A |
Source Address STATIC-DEF N/A | Time To Live | CHANGING | Yes |
Destination Address STATIC-DEF N/A | Protocol | STATIC | N/A |
| Header Checksum | INFERRED | N/A |
| Source Address | STATIC-DEF | Yes |
| Destination Address | STATIC-DEF | Yes |
+-----------------------+---------------+------------+
(1) DSCP is marked based on the application's requirement. (1) The DSCP is marked based on the application's requirements. If
Considering that shareable connections usually belong to same type it can be assumed that replicable connections often carry the same
of traffic, it may be regarded as shareable. type of traffic, the DSCP may be regarded as replicable. However,
issues such as re-marking will need to be taken into account.
(2) The shareable context for this field includes IP-ID, NBO, and RND (2) It is not possible for the ECN bits to be replicated (note that
flags (as described in ROHC RTP). use of the ECN nonce scheme [35] is anticipated). However, it
seems likely that all TCP flows between ECN-capable hosts will use
ECN, the use (or not) of ECN for flows between the same end-points
might be considered replicable. See also note (4).
(3) Since the possible future behavior of the 'Reserved Flag' cannot (3) The replicable context for this field includes the IP-ID, NBO,
be predicted, it is not considered as shareable. However, it and RND flags (as described in ROHC RTP). This highlights that
might be expected that the behaviour of the reserved flag between the replication is of the context, rather than just the header
field values and, as such, needs to be considered based on the
exact nature of compression applied to each field.
(4) Since the possible future behavior of the 'Reserved Flag' cannot
be predicted, it is not considered as replicable. However, it
might be expected that the behavior of the reserved flag between
the same end-points will be similar. In this case, any selection the same end-points will be similar. In this case, any selection
of packet formats (for example) based on this behaviour might of packet formats (for example) based on this behavior might carry
carry across to the new flow. In the case of packet formats, this across to the new flow. In the case of packet formats, this can
can probably be considered as a compressor-local decision. probably be considered as a compressor-local decision.
IPv6 Header (inner and/or outer) 4.2 IPv6 Header (inner and/or outer)
Field Class Shareable +-----------------------+---------------+------------+
------------------------------------------------ | Field | Class | Replicable |
Version STATIC N/A +-----------------------+---------------+------------+
Traffic Class CHANGING No | Version | STATIC | N/A |
Flow Label STATIC-DEF N/A | Traffic Class | CHANGING | Yes (1) |
Payload Length INFERRED N/A | ECT flag | CHANGING | No (2) |
Next Header STATIC N/A | CE flag | CHANGING | No (2) |
Hop Limit CHANGING Yes | Flow Label | STATIC-DEF | N/A |
Source Address STATIC-DEF N/A | Payload Length | INFERRED | N/A |
Destination Address STATIC-DEF N/A | Next Header | STATIC | N/A |
TCP Header | Hop Limit | CHANGING | Yes |
| Source Address | STATIC-DEF | Yes |
| Destination Address | STATIC-DEF | Yes |
+-----------------------+---------------+------------+
Field Class Shareable (1) See comment about DSCP field for IPv4, above.
------------------------------------------------
Source Port STATIC-DEF Yes (4)
Destination Port STATIC-DEF Yes (4)
Sequence Number CHANGING No (5)
Acknowledgement Number CHANGING No
Data Offset INFERRED N/A
Reserved Bits CHANGING No (6)
Control Bits
URG CHANGING No
ACK CHANGING No
PSH CHANGING No
RST CHANGING No
SYN CHANGING No
FIN CHANGING No
Window CHANGING Yes (7)
CHECKSUM CHANGING No
Urgent Pointer CHANGING No
(4) On the server side, the port number is likely to be a well-known (2) See comment about ECT and CE flags for IPv4, above.
4.3 TCP Header
+-----------------------+---------------+------------+
| Field | Class | Replicable |
+-----------------------+---------------+------------+
| Source Port | STATIC-DEF | Yes (1) |
| Destination Port | STATIC-DEF | Yes (1) |
| Sequence Number | CHANGING | No (2) |
| Acknowledgement Number| CHANGING | No |
| Data Offset | INFERRED | N/A |
| Reserved Bits | CHANGING | No (3) |
| Flags | | |
| CWR | CHANGING | No (4) |
| ECE | CHANGING | No (4) |
| URG | CHANGING | No |
| ACK | CHANGING | No |
| PSH | CHANGING | No |
| RST | CHANGING | No |
| SYN | CHANGING | No |
| FIN | CHANGING | No |
| Window | CHANGING | Yes |
| Checksum | CHANGING | No |
| Urgent Pointer | CHANGING | Yes (5) |
+-----------------------+---------------+------------+
(1) On the server side, the port number is likely to be a well-known
value. On the client side, the port number is generally selected value. On the client side, the port number is generally selected
by the stack automatically. Whether the port number is shareable by the stack automatically. Whether the port number is replicable
depends upon how the stack chooses the port number. However, most depends upon how the stack chooses the port number. However, most
implementations use a simple scheme which sequentially picks the implementations use a simple scheme which sequentially picks the
next available port number. This is clearly exploitable in next available port number. This is clearly exploitable in a
compression. compression scheme.
(5) With the recommendation (and expected deployment) of TCP Initial (2) With the recommendation (and expected deployment) of TCP Initial
Sequence Number randomization, defined in RFC 1948 [13], it will Sequence Number randomization, defined in RFC 1948 [16], it will
be impossible to share the sequence number. Thus, this field will be impossible to share the sequence number. Thus, this field will
not be regarded as shareable. not be regarded as replicable.
(6) Clearly the ECN flags are not considered shareable (if ECT is
enabled). Otherwise, the same caveats as with the IP reserved
flag (see point (3), above) apply.
(7) The window here should be referred to as the initial value (or
maximum value) of RWND. Since sharable packet streams are likely
to have the same initial RWND, this can help optimize the SYN
packet size for short-lived TCP flows.
ECN Flags (3) See comment (4) for the IPv4 header, above.
Field Class Shareable (4) See comment (2) on ECN flags for the IPv4 header, above.
------------------------------------------------
ECT CHANGING No (8)
CE CHANGING No
ECN CHANGING No
CWR CHANGING No
(8) We assume that the IP ECN bits will make use of the ECN nonce (5) The urgent pointer is very rarely used. This means that, in
scheme. Therefore, none of the ECN flags can be regarded as practice, the field may be considered replicable.
shareable.
TCP Options 4.4 TCP Options
Option SYN-only (9) Shareable +---------------------------+--------------+------------+
----------------------------------------------------- | Option | SYN-only (1) | Replicable |
End of option list Option No No +---------------------------+--------------+------------+
No-Operation Option No No | End of Option List | No | No (2) |
Maximum Segment Size Option Yes Yes | No-Operation | No | No (2) |
Window Scale Option Yes Yes | Maximum Segment Size | Yes | Yes |
SACK-Permitted Option Yes Yes | Window Scale | Yes | Yes |
SACK Option No No | SACK-Permitted | Yes | Yes |
Timestamps Option No Yes | SACK | No | No |
| Timestamp | No | No |
+---------------------------+--------------+------------+
(9) SYN-only indicates whether the options only appear in SYN packet (1) This indicates whether the option only appears in SYN packet or
or not. For 'yes', the option only appears in SYN packet; not. Options that are not 'SYN-only' may appear in any packet.
otherwise, the option may appear in any packet.
Many TCP options are used only in SYN packets. Some options, such Many TCP options are used only in SYN packets. Some options, such
as MSS, Window Scale, SACK-Permitted etc., will tend to have the as MSS, Window Scale, SACK-Permitted etc., will tend to have the
same value among shareable packet streams. same value among replicable packet streams.
Thus, to support context sharing, the compressor should maintain Thus, to support context sharing, the compressor should maintain
such TCP options in the context (even though they only appear in such TCP options in the context (even though they only appear in
the SYN segment). the SYN segment).
(2) Since these options have fixed values, they could be regarded as
replicable. However, the only interesting thing to convey about
these options is their presence: if it is known that such an
option exists, its value is defined.
4.5 Summary of replication
From the above analysis, it can be seen that there are reasonable
grounds for exploiting redundancy between flows, as well as between
packets within a flow. Simply consider the advantage of being able
to elide the source and destination addresses for a repeated
connection between two IPv6 endpoints. There will also be a cost (in
terms of complexity and robustness) for replicating contexts, and
this must be considered when deciding what constitutes an appropriate
solution.
The final point to note for the use of replication is that it
requires the compressor to have a suitable degree of confidence that
the source data is present and correct at the decompressor. This may
place some restrictions on which of the 'changing' fields, in
particular, can be utilised during replication.
5. Analysis of change patterns of header fields 5. Analysis of change patterns of header fields
To design suitable mechanisms for efficient compression of all header To design suitable mechanisms for efficient compression of all header
fields, their change patterns must be analyzed. For this reason, an fields, their change patterns must be analyzed. For this reason, an
extended classification is done based on the general classification extended classification is done based on the general classification
in 2, considering the fields which were labeled CHANGING in that in 2, considering the fields which were labeled CHANGING in that
classification. classification.
The CHANGING fields are separated into five different subclasses: The CHANGING fields are separated into five different subclasses:
skipping to change at page 16, line 22 skipping to change at page 17, line 31
| Sequential | Delta | STATIC | KNOWN | | Sequential | Delta | STATIC | KNOWN |
| -----------+-------------+-------------+-------------+ | -----------+-------------+-------------+-------------+
| IP Id(4) Seq. jump | Delta | RC | LIMITED | | IP Id(4) Seq. jump | Delta | RC | LIMITED |
| -----------+-------------+-------------+-------------+ | -----------+-------------+-------------+-------------+
| Random | Value | IRREGULAR | UNKNOWN | | Random | Value | IRREGULAR | UNKNOWN |
+------------------------+-------------+-------------+-------------+ +------------------------+-------------+-------------+-------------+
| IP DF flag(4) | Value | RC | UNKNOWN | | IP DF flag(4) | Value | RC | UNKNOWN |
+------------------------+-------------+-------------+-------------+ +------------------------+-------------+-------------+-------------+
| IP TTL(4) / Hop Lim(6) | Value | ALTERNATING | LIMITED | | IP TTL(4) / Hop Lim(6) | Value | ALTERNATING | LIMITED |
+------------------------+-------------+-------------+-------------+ +------------------------+-------------+-------------+-------------+
| TCP Sequence Number ª Delta ª IRREGULAR ª LIMITED ª | TCP Sequence Number | Delta | IRREGULAR | LIMITED |
+------------------------+-------------+-------------+-------------+ +------------------------+-------------+-------------+-------------+
ª TCP Acknowledgement Numª Delta ª IRREGULAR ª LIMITED ª | TCP Acknowledgement Num| Delta | IRREGULAR | LIMITED |
+------------------------+-------------+-------------+-------------+ +------------------------+-------------+-------------+-------------+
ª TCP Reserved ª Value ª RC ª UNKNOWN ª | TCP Reserved | Value | RC | UNKNOWN |
+------------------------+-------------+-------------+-------------+ +------------------------+-------------+-------------+-------------+
ª TCP flags ª | TCP flags | | | |
ª ECN flags ª Value ª IRREGULAR ª UNKNOWN ª | ECN flags | Value | IRREGULAR | UNKNOWN |
ª CWR flag ª Value ª IRREGULAR ª UNKNOWN ª | CWR flag | Value | IRREGULAR | UNKNOWN |
ª ECE flag ª Value ª IRREGULAR ª UNKNOWN ª | ECE flag | Value | IRREGULAR | UNKNOWN |
ª URG flag ª Value ª IRREGULAR ª UNKNOWN ª | URG flag | Value | IRREGULAR | UNKNOWN |
ª ACK flag ª Value ª SEMISTATIC ª KNOWN ª | ACK flag | Value | SEMISTATIC | KNOWN |
ª PSH flag ª Value ª IRREGULAR ª UNKNOWN ª | PSH flag | Value | IRREGULAR | UNKNOWN |
ª RST flag ª Value ª IRREGULAR ª UNKNOWN ª | RST flag | Value | IRREGULAR | UNKNOWN |
ª SYN flag ª Value ª SEMISTATIC ª KNOWN ª | SYN flag | Value | SEMISTATIC | KNOWN |
ª FIN flag ª Value ª SEMISTATIC ª KNOWN ª | FIN flag | Value | SEMISTATIC | KNOWN |
+------------------------+-------------+-------------+-------------+ +------------------------+-------------+-------------+-------------+
ª TCP Window ª Value ª ALTERNATING ª KNOWN ª | TCP Window | Value | ALTERNATING | KNOWN |
+------------------------+-------------+-------------+-------------+ +------------------------+-------------+-------------+-------------+
ª TCP Checksum ª Value ª IRREGULAR ª UNKNOWN ª | TCP Checksum | Value | IRREGULAR | UNKNOWN |
+------------------------+-------------+-------------+-------------+ +------------------------+-------------+-------------+-------------+
ª TCP Urgent Pointer ª Value ª IRREGULAR ª KNOWN ª | TCP Urgent Pointer | Value | IRREGULAR | KNOWN |
+------------------------+-------------+-------------+-------------+ +------------------------+-------------+-------------+-------------+
ª TCP Options ª Value ª IRREGULAR ª UNKNOWN ª | TCP Options | Value | IRREGULAR | UNKNOWN |
+------------------------+-------------+-------------+-------------+ +------------------------+-------------+-------------+-------------+
Table 1 : Classification of CHANGING header fields Table 1 : Classification of CHANGING header fields
The following subsections discuss the various header fields in The following subsections discuss the various header fields in
detail. Note that table 1 and the discussions below do not consider detail. Note that table 1 and the discussions below do not consider
changes caused by loss or reordering before the compression point. changes caused by loss or reordering before the compression point.
5.1 IP header 5.1 IP header
5.1.1 IP Traffic-Class / Type-Of-Service (TOS) 5.1.1 IP Traffic-Class / Type-Of-Service (TOS)
The Traffic-Class (IPv6) or Type-Of-Service (IPv4) field might be The Traffic-Class (IPv6) or Type-Of-Service/DSCP (IPv4) field might
expected to change during the lifetime of a packet stream. This be expected to change during the lifetime of a packet stream. This
analysis considers several RFCs that describe modifications to the analysis considers several RFCs that describe modifications to the
original RFC 791 [1]. original RFC 791 [1].
The TOS byte was initially described in RFC 791 [1] as 3 bits of The TOS byte was initially described in RFC 791 [1] as 3 bits of
precedence followed by 3 bits of TOS and 2 reserved bits (defined to precedence followed by 3 bits of TOS and 2 reserved bits (defined to
be 0). RFC 1122 [4] extended this to specify 5 bits of TOS, although be 0). RFC 1122 [5] extended this to specify 5 bits of TOS, although
the meanings of the additional 2 bits were not defined. RFC 1349 [9] the meanings of the additional 2 bits were not defined. RFC 1349
defined the 4th bit of TOS to be 'minimize monetary cost'. The next [10] defined the 4th bit of TOS to be 'minimize monetary cost'. The
significant change was in RFC 2474 [20] which reworked the TOS octet next significant change was in RFC 2474 [23] which reworked the TOS
as 6 bits of DSCP (DiffServ Code Point) plus 2 unused bits. Most octet as 6 bits of DSCP (DiffServ Code Point) plus 2 unused bits.
recently RFC 2780 [26] identified the 2 reserved bits in the TOS or Most recently RFC 2780 [29] identified the 2 reserved bits in the TOS
traffic class octet for experimental use with ECN. or traffic class octet for experimental use with ECN.
For the purposes of this classification, it is therefore proposed to For the purposes of this classification, it is therefore proposed to
classify the TOS (or traffic class) octet as 6 bits for the DSCP and classify the TOS (or traffic class) octet as 6 bits for the DSCP and
2 additional bits. This may be expected to be 0 or to contain ECN 2 additional bits. This may be expected to be 0 or to contain ECN
data. From a future proofing perspective, it is preferable to assume data. From a future proofing perspective, it is preferable to assume
the use of ECN, especially with respect to TCP. the use of ECN, especially with respect to TCP.
It is also considered important that the profile works with legacy It is also considered important that the profile works with legacy
stacks, since these will be in existence for some considerable time stacks, since these will be in existence for some considerable time
to come. For simplicity, this will be considered as 6 bits of TOS to come. For simplicity, this will be considered as 6 bits of TOS
information and 2 bits of ECN data, so the fields are always information and 2 bits of ECN data, so the fields are always
considered to be structured the same way. considered to be structured the same way.
The DSCP (as for TOS in ROHC RTP) is not expected to change The DSCP (as for TOS in ROHC RTP) is not expected to change
frequently (although it could change mid-flow, for example as a frequently (although it could change mid-flow, for example as a
result of a route change). result of a route change).
5.1.2 ECN Flags 5.1.2 ECN Flags
Initially we describe the ECN flags as specified in RFC 2481 [21]. Initially we describe the ECN flags as specified in RFC 2481 [24].
Subsequently, a suggested update is described which would alter the Subsequently, a suggested update is described which would alter the
behaviour of the flags. behavior of the flags.
In RFC 2481 [21] there are 2 separate flags, the ECT (ECN Capable In RFC 2481 [24] there are 2 separate flags, the ECT (ECN Capable
Transport) flag and the CE (Congestion Experienced) flag. The ECT Transport) flag and the CE (Congestion Experienced) flag. The ECT
flag, if negotiated by the TCP stack, will be '1' for all data flag, if negotiated by the TCP stack, will be '1' for all data
packets and '0' for all 'pure acknowledgement' packets. This means packets and '0' for all 'pure acknowledgement' packets. This means
that the behavior of the ECT flag is linked to behavior in the TCP that the behavior of the ECT flag is linked to behavior in the TCP
stack. Whether this can be exploited for compression is not clear. stack. Whether this can be exploited for compression is not clear.
The CE flag is only used if ECT is set to '1' and indicates The CE flag is only used if ECT is set to '1'. It is set to '0' by
congestion in the network. The CE flag is expected to be randomly the sender and can be set to '1' by an ECN capable router in the
(and comparatively rarely, although this is dependent upon the network to indicate congestion. Thus the CE flag is expected to be
network congestion state) set to '1'. randomly set to '1' with a probability dependent upon the congestion
state of the network and the position of the compressor in the path.
So, a compressor located close to the receiver in a congested network
will see the CE bit set frequently, but a compressor located close to
a sender will rarely, if ever, see the CE bit set to '1'.
A recent draft [30] suggests an alternative view of these 2 bits. A recent draft [35] suggests an alternative view of these 2 bits.
This considers the two bits together as having 4 possible codepoints. This considers the two bits together as having 4 possible codepoints.
Meanings are then assigned to the codepoints: Meanings are then assigned to the codepoints:
00 Not ECN capable 00 Not ECN capable
01 ECN capable, no congestion [known as ECT(0)] 01 ECN capable, no congestion [known as ECT(0)]
10 ECN capable, no congestion [known as ECT(1)] 10 ECN capable, no congestion [known as ECT(1)]
11 Congestion experienced 11 Congestion experienced
The use of 2 codepoints for signaling ECT allows the sender to detect The use of 2 codepoints for signaling ECT allows the sender to detect
when a receiver is not reliably echoing congestion information. when a receiver is not reliably echoing congestion information.
For the purposes of compression, this update means that ECT(0) and For the purposes of compression, this update means that ECT(0) and
ECT(1) are equally likely (for an ECN capable flow) and that '11' ECT(1) are equally likely (for an ECN capable flow) and that '11'
will be relatively rarely seen. The probability of seeing a will be relatively rarely seen. The probability of seeing a
congestion indication depends upon the level of congestion in the congestion indication is discussed above in the description of the CE
network and this depends upon many factors. However, it is likely flag.
that the probability is non-trivial and may, in many cases, be
significant.
It is suggested that, for the purposes of compression, ECN with It is suggested that, for the purposes of compression, ECN with
nonces is assumed as the baseline, although the compression scheme nonces is assumed as the baseline, although the compression scheme
must be able to transparently compress the original ECN scheme. must be able to transparently compress the original ECN scheme.
5.1.3 IP Identification 5.1.3 IP Identification
The Identification field (IP ID) of the IPv4 header is there to The Identification field (IP ID) of the IPv4 header is there to
identify which fragments constitute a datagram when reassembling identify which fragments constitute a datagram when reassembling
fragmented datagrams. The IPv4 specification does not specify fragmented datagrams. The IPv4 specification does not specify
skipping to change at page 20, line 16 skipping to change at page 21, line 29
between the two solutions above. Finally, even for IPv4, header between the two solutions above. Finally, even for IPv4, header
compression could be designed without any additional information for compression could be designed without any additional information for
the ID field included in compressed headers. To use such schemes, it the ID field included in compressed headers. To use such schemes, it
must be known which assignment policy for the ID field is being used must be known which assignment policy for the ID field is being used
by the sender. That might not be possible to know, which implies by the sender. That might not be possible to know, which implies
that the applicability of such solutions is very uncertain. However, that the applicability of such solutions is very uncertain. However,
designers of IPv4 stacks for cellular terminals should use an designers of IPv4 stacks for cellular terminals should use an
assignment policy close to sequential. assignment policy close to sequential.
With regard to TCP compression, the behavior of the IP ID field is With regard to TCP compression, the behavior of the IP ID field is
considered to be essentially the same. However, it is noted that the considered to be essentially the same. However, in RFC 3095 [31] it
IP ID was generally inferred from the RTP Sequence Number. There is is noted that the IP ID is generally inferred from the RTP Sequence
no obvious candidate in the TCP case for a field to offer this Number. There is no obvious candidate in the TCP case for a field to
'master sequence number' role. offer this 'master sequence number' role.
Clearly from a busy server the observed behavior may well be quite Clearly from a busy server the observed behavior may well be quite
erratic. This is a case where the ability to share the IP erratic. This is a case where the ability to share the IP
compression context between a number of flows (between the same end- compression context between a number of flows (between the same end-
points) could offer potential benefits. However, this would only points) could offer potential benefits. However, this would only
have any real impact where there were a large number of flows between have any real impact where there were a large number of flows between
one machine and the server. If context sharing is being considered, one machine and the server. If context sharing is being considered,
then it is preferable to share the IP part of the context. then it is preferable to share the IP part of the context.
5.1.4 Don't Fragment (DF) flag 5.1.4 Don't Fragment (DF) flag
Due to the use of path-MTU discovery RFC 1191 [7], the value is more Due to the use of path-MTU discovery RFC 1191 [8], the value is more
likely to be '1', than found in UDP/RTP streams since DF should be likely to be '1', than found in UDP/RTP streams since DF should be
periodically set to check for fragmentation in the end-to-end path. set to check for fragmentation in the end-to-end path. In practice
In practice it is hard to predict the behavior of this field. it is hard to predict the behavior of this field. However, it is
However, it is considered that the most likely case is that it will considered that the most likely case is that it will generally stay
generally stay at either '0' (periodically being set to '1') or '1'. at either '0' or '1'. When using PMTU discovery [8] it is expected
that DF will always be set to '1', although a host may end PMTU
discovery by clearing the DF bit to '0'.
5.1.5 IP Hop-Limit / Time-To-Live (TTL) 5.1.5 IP Hop-Limit / Time-To-Live (TTL)
The Hop-Limit (IPv6) or Time-To-Live (IPv4) field is expected to be The Hop-Limit (IPv6) or Time-To-Live (IPv4) field is expected to be
constant during the lifetime of a packet stream or to alternate constant during the lifetime of a packet stream or to alternate
between a limited number of values due to route changes. between a limited number of values due to route changes.
5.2 TCP header 5.2 TCP header
Any discussion of compressability of TCP fields borrows heavily from Any discussion of compressability of TCP fields borrows heavily from
RFC 1144 [5]. However, the premise of how the compression is RFC 1144 [6]. However, the premise of how the compression is
performed is slightly different and the protocol has evolved slightly performed is slightly different and the protocol has evolved slightly
in the intervening time. in the intervening time.
5.2.1 Sequence number 5.2.1 Sequence number
An understanding of the sequence and acknowledgement number behavior An understanding of the sequence and acknowledgement number behavior
are essential for a TCP compression scheme. are essential for a TCP compression scheme.
At the simplest level the behavior of the sequence number can be At the simplest level the behavior of the sequence number can be
described relatively easily. However, there are a number of described relatively easily. However, there are a number of
skipping to change at page 21, line 27 skipping to change at page 22, line 40
Although there are common MSS values, these can be quite variable. Although there are common MSS values, these can be quite variable.
Given this variability and the range of window sizes it is hard Given this variability and the range of window sizes it is hard
(compared with the RTP case, for example) to select a 'one size fits (compared with the RTP case, for example) to select a 'one size fits
all' encoding for the sequence number. (The same argument applies all' encoding for the sequence number. (The same argument applies
equally to the acknowledgement number). equally to the acknowledgement number).
We note that the increment of the sequence number in a packet is the We note that the increment of the sequence number in a packet is the
size of the data payload of that packet (including the SYN and FIN size of the data payload of that packet (including the SYN and FIN
flags; see later). This is, of course, exactly the relationship that flags; see later). This is, of course, exactly the relationship that
RFC 1144 [5] exploits to compress the sequence number in the most RFC 1144 [6] exploits to compress the sequence number in the most
efficient case. This technique may not be directly applicable to a efficient case. This technique may not be directly applicable to a
robust solution, but may be a useful relationship to consider. robust solution, but may be a useful relationship to consider.
However, at any point on the path (i.e. wherever a compressor might However, at any point on the path (i.e. wherever a compressor might
be deployed), the sequence number can be anywhere within a range be deployed), the sequence number can be anywhere within a range
defined by the TCP window. This is a combination of a number of defined by the TCP window. This is a combination of a number of
values (buffer space at the sender; advertised buffer size at the values (buffer space at the sender; advertised buffer size at the
receiver; and TCP congestion control algorithms). Missing packets or receiver; and TCP congestion control algorithms). Missing packets or
retransmissions can cause the TCP sequence number to fluctuate within retransmissions can cause the TCP sequence number to fluctuate within
the limits of this window. the limits of this window.
skipping to change at page 22, line 5 skipping to change at page 23, line 18
number behavior has been observed with an 8 kbyte repeating pattern - number behavior has been observed with an 8 kbyte repeating pattern -
- 5 segments of 1460 bytes followed by 1 segment of 892 bytes. It - 5 segments of 1460 bytes followed by 1 segment of 892 bytes. It
appears that implementation and how data is presented to the stack appears that implementation and how data is presented to the stack
can affect the sequence number. can affect the sequence number.
It has been suggested that the TCP window can be tracked by the It has been suggested that the TCP window can be tracked by the
compressor, allowing it to bound the size of these jumps. compressor, allowing it to bound the size of these jumps.
For interactive flows (for example telnet), the sequence number will For interactive flows (for example telnet), the sequence number will
change by small irregular amounts. In this case the Nagle algorithm change by small irregular amounts. In this case the Nagle algorithm
commonly applies, coalescing small packets where possible to reduce [3] commonly applies, coalescing small packets where possible to
the basic header overhead. This may also mean that is less likely reduce the basic header overhead. This may also mean that is less
that predictable changes in the sequence number will occur. likely that predictable changes in the sequence number will occur.
The Nagle algorithm is an optimisation and not required to be used.
Also, applications can disable the Nagle algorithm (which may be done
to mitigate the delays associated with its use).
It is also noted that the SYN and FIN flags (which have to be It is also noted that the SYN and FIN flags (which have to be
acknowledged) consume 1 byte of sequence space. acknowledged) consume 1 byte of sequence space.
5.2.2 Acknowledgement number 5.2.2 Acknowledgement number
Much of the information about the sequence number applies equally to Much of the information about the sequence number applies equally to
the acknowledgement number. However, there are some important the acknowledgement number. However, there are some important
differences. differences.
For bulk data transfers there will tend to be 1 acknowledgement for For bulk data transfers there will tend to be 1 acknowledgement for
every 2 data segments. It may be seen from this that the delta for every 2 data segments. The algorithm is specified in RFC 2581 [28].
the acknowledgement number is roughly twice that of the sequence An ACK need not always be send immediately on receipt of a data
number. This is not always the case and the discussion about segment, but must be sent within 500ms and should be generated for at
sequence number irregularity should be applied. least every second full sized segment (MSS) of received data. It may
be seen from this that the delta for the acknowledgement number is
roughly twice that of the sequence number. This is not always the
case and the discussion about sequence number irregularity should be
applied.
As an aside, a common implementation bug was 'stretch ACKs' As an aside, a common implementation bug was 'stretch ACKs'
(acknowledgements may be generated less frequently than every two (acknowledgements may be generated less frequently than every two
full-size data segments). full-size data segments). This pattern can also occur following loss
on the return path.
Since the acknowledgement number is cumulative, dropped packets in Since the acknowledgement number is cumulative, dropped packets in
the forward path will result in the acknowledgement number remaining the forward path will result in the acknowledgement number remaining
constant for a time in the reverse direction. Retransmission of a constant for a time in the reverse direction. Retransmission of a
dropped segment can then cause a substantial jump in the dropped segment can then cause a substantial jump in the
acknowledgement number. These jumps in acknowledgement number are acknowledgement number. These jumps in acknowledgement number are
bounded by the TCP window, just as for the jumps in sequence number. bounded by the TCP window, just as for the jumps in sequence number.
In the acknowledgement case, information about the advertised In the acknowledgement case, information about the advertised
received window gives a bound to the size of any ACK jump. received window gives a bound to the size of any ACK jump.
skipping to change at page 23, line 4 skipping to change at page 24, line 25
5.2.4 Flags 5.2.4 Flags
o ECN-E (Explicit Congestion Notification) o ECN-E (Explicit Congestion Notification)
'1' to echo CE bit in IP header. Will be set in several '1' to echo CE bit in IP header. Will be set in several
consecutive headers (until 'acknowledged' by CWR) consecutive headers (until 'acknowledged' by CWR)
If ECN nonces get used, then there will be a 'nonce-sum' (NS) If ECN nonces get used, then there will be a 'nonce-sum' (NS)
bit in the flags, as well. Again, transparency of the reserved bit in the flags, as well. Again, transparency of the reserved
bits is crucial for future-proofing this compression scheme... bits is crucial for future-proofing this compression scheme.
From an efficiency/compression standpoint, the NS bit will From an efficiency/compression standpoint, the NS bit will
either be unused [always 0] or randomly changing) either be unused [always 0] or randomly changing). The nonce-
sum is the 1-bit sum of the ECT codepoints, as described in
[35].
o CWR (Congestion Window Reduced) o CWR (Congestion Window Reduced)
'1' to signal congestion window reduced on ECN. Will generally '1' to signal congestion window reduced on ECN. Will generally
be set in individual packets. Here, the probability of this be set in individual packets. The flag will be set once per
flag being set is essentially equivalent to the probability of loss event. Thus, the probability of it being set is
CE being signalled in the data-flow direction. (This refers to proportional to the degree of congestion in the network, but
CE being signalled somewhere in the end-to-end path; not less likely to be set than the CE flag.
necessarily prior to compression).
o ECE (Echo Congestion Experience) o ECE (Echo Congestion Experience)
If 'congestion experienced' is signaled on a received IP If 'congestion experienced' is signaled on a received IP
header, this is echoed through the ECE bit in segments sent by header, this is echoed through the ECE bit in segments sent by
the receiver until acknowledged by seeing the CWR bit set. the receiver until acknowledged by seeing the CWR bit set.
Clearly in periods of high congestion and/or long RTT, this Clearly in periods of high congestion and/or long RTT, this
flag will be frequently set to '1'. flag will be frequently set to '1'.
During connection open (SYN and SYN/ACK packets) the ECN bits have During connection open (SYN and SYN/ACK packets) the ECN bits have
skipping to change at page 24, line 17 skipping to change at page 25, line 39
'1' for the SYN/SYN-ACK only at the start of a connection '1' for the SYN/SYN-ACK only at the start of a connection
o FIN (End of Data : FINished) o FIN (End of Data : FINished)
'1' to indicate 'no more data' [unlikely with any flag other '1' to indicate 'no more data' [unlikely with any flag other
than ACK] than ACK]
5.2.5 Checksum 5.2.5 Checksum
Carried as the end-to-end check for the TCP data. See RFC 1144 [5] Carried as the end-to-end check for the TCP data. See RFC 1144 [6]
for a discussion of why this should be carried. There may be more for a discussion of why this should be carried. A header compression
complex interactions with error detection and robustness that would scheme should not rely upon the TCP checksum for robustness, though,
have to be addressed in a TCP header compression scheme. The TCP and should apply appropriate error-detection mechanisms of its own.
checksum has to be considered as randomly changing. The TCP checksum has to be considered as randomly changing.
5.2.6 Window 5.2.6 Window
May oscillate randomly between 0 and the receiver's window limit (for May oscillate randomly between 0 and the receiver's window limit (for
the connection). the connection).
In practice, the window will either not change, or may alternate In practice, the window will either not change, or may alternate
between a relatively small number of values. Particularly when between a relatively small number of values. Particularly when
closing (the value is getting smaller), the change in window is closing (the value is getting smaller), the change in window is
likely to be related to the segment size, but it is not clear that likely to be related to the segment size, but it is not clear that
skipping to change at page 25, line 26 skipping to change at page 26, line 48
multiple of 32 bits. multiple of 32 bits.
Optional header fields are identified by an option kind field. Optional header fields are identified by an option kind field.
Options 0 and 1 are exactly one octet that is their kind field. All Options 0 and 1 are exactly one octet that is their kind field. All
other options have their one octet kind field, followed by a one other options have their one octet kind field, followed by a one
octet length field, followed by length-2 octets of option data. octet length field, followed by length-2 octets of option data.
5.3.1 Options overview 5.3.1 Options overview
Table 2 classifies the IANA known options together with their Table 2 classifies the IANA known options together with their
associated RFCs, if applicable, from IANA [34] associated RFCs, if applicable, from IANA [38]
+------+--------+------------------------------------+------------+ +------+--------+------------------------------------+----------+-----+
ª Kind ª Length ª Meaning ª RFC ª | Kind | Length | Meaning | RFC | Use |
ª ª(octets)ª ª ª | |(octets)| | | |
+------+--------+------------------------------------+------------+ +------+--------+------------------------------------+----------+-----+
ª 0 ª - ª End of Option List ª RFC 793 ª | 0 | - | End of Option List | RFC 793 | * |
ª 1 ª - ª No-Operation ª RFC 793 ª | 1 | - | No-Operation | RFC 793 | * |
ª 2 ª 4 ª Maximum Segment Size ª RFC 793 ª | 2 | 4 | Maximum Segment Size | RFC 793 | * |
ª 3 ª 3 ª WSopt - Window Scale ª RFC 1323 ª | 3 | 3 | WSopt - Window Scale | RFC 1323 | * |
ª 4 ª 2 ª SACK Permitted ª RFC 2018 ª | 4 | 2 | SACK Permitted | RFC 2018 | * |
ª 5 ª N ª SACK ª RFC 2018 ª | 5 | N | SACK | RFC 2018 | * |
ª 6 ª 6 ª Echo (obsoleted by option 8) ª RFC 1072 ª | 6 | 6 | Echo (obsoleted by option 8) | RFC 1072 | |
ª 7 ª 6 ª Echo Reply (obsoleted by option 8) ª RFC 1072 ª | 7 | 6 | Echo Reply (obsoleted by option 8) | RFC 1072 | |
ª 8 ª 10 ª TSopt - Time Stamp Option ª RFC 1323 ª | 8 | 10 | TSopt - Time Stamp Option | RFC 1323 | * |
ª 9 ª 2 ª Partial Order Connection Permitted ª RFC 1693 ª | 9 | 2 | Partial Order Connection Permitted | RFC 1693 | |
ª 10 ª 3 ª Partial Order Service Profile ª RFC 1693 ª | 10 | 3 | Partial Order Service Profile | RFC 1693 | |
ª 11 ª 6 ª CC ª RFC 1644 ª | 11 | 6 | CC | RFC 1644 | |
ª 12 ª 6 ª CC.NEW ª RFC 1644 ª | 12 | 6 | CC.NEW | RFC 1644 | |
ª 13 ª 6 ª CC.ECHO ª RFC 1644 ª | 13 | 6 | CC.ECHO | RFC 1644 | |
ª 14 ª 3 ª Alternate Checksum Request ª RFC 1146 ª | 14 | 3 | Alternate Checksum Request | RFC 1146 | |
ª 15 ª N ª Alternate Checksum Data ª RFC 1146 ª | 15 | N | Alternate Checksum Data | RFC 1146 | |
ª 16 ª ª Skeeter ª ª | 16 | | Skeeter | | |
ª 17 ª ª Bubba ª ª | 17 | | Bubba | | |
ª 18 ª 3 ª Trailer Checksum Option ª ª | 18 | 3 | Trailer Checksum Option | | |
ª 19 ª 18 ª MD5 Signature Option ª RFC 2385 ª | 19 | 18 | MD5 Signature Option | RFC 2385 | |
ª 20 ª ª SCPS Capabilities ª ª | 20 | | SCPS Capabilities | | |
ª 21 ª ª Selective Negative Acknowledgementsª ª | 21 | | Selective Negative Acks | | |
ª 22 ª ª Record Boundaries ª ª | 22 | | Record Boundaries | | |
ª 23 ª ª Corruption experienced ª ª | 23 | | Corruption experienced | | |
ª 24 ª ª SNAP ª ª | 24 | | SNAP | | |
ª 25 ª ª Unassigned (released 12/18/00) ª ª | 25 | | Unassigned (released 12/18/00) | | |
ª 26 ª ª TCP Compression Filter ª ª | 26 | | TCP Compression Filter | | |
+------+--------+------------------------------------+------------+ +------+--------+------------------------------------+----------+-----+
Table 2 Description of common TCP options Table 2 Description of common TCP options
The 'use' column is marked with '*' to indicate those options that
are most likely to be seen in TCP flows.
5.3.2 Option field behavior 5.3.2 Option field behavior
Generally speaking all option fields have been classified as Generally speaking all option fields have been classified as
changing. This section describes the behavior of each option changing. This section describes the behavior of each option
referenced within an RFC, listed by 'kind' indicator. referenced within an RFC, listed by 'kind' indicator.
0. End of option list 0. End of option list
This option code indicates the end of the option list. This This option code indicates the end of the option list. This
might not coincide with the end of the TCP header according to might not coincide with the end of the TCP header according to
skipping to change at page 28, line 5 skipping to change at page 29, line 7
This option may be sent in a SYN segment by TCP : This option may be sent in a SYN segment by TCP :
(1) to indicate that it is prepared to do both send and (1) to indicate that it is prepared to do both send and
receive window scaling, and receive window scaling, and
(2) to communicate a scale factor to be applied to its (2) to communicate a scale factor to be applied to its
receive window. receive window.
The scale factor is encoded logarithmically, as a power of 2 The scale factor is encoded logarithmically, as a power of 2
(presumably to be implemented by binary shifts). Note: the (presumably to be implemented by binary shifts). Note: the
window in the SYN segment itself is never scaled RFC 1072 [3]. window in the SYN segment itself is never scaled RFC 1072 [4].
This option may be sent in an initial segment (i.e., a segment This option may be sent in an initial segment (i.e., a segment
with the SYN bit on and the ACK bit off). It may also be sent with the SYN bit on and the ACK bit off). It may also be sent
in a segment, but only if a Window Scale option was received in in a segment, but only if a Window Scale option was received in
the initial segment. A Window Scale option in a segment the initial segment. A Window Scale option in a segment
without a SYN bit should be ignored. The Window field in a SYN without a SYN bit should be ignored. The Window field in a SYN
segment itself is never scaled RFC 1323 [8] segment itself is never scaled RFC 1323 [9]
The use of window scaling does not affect the encoding of any The use of window scaling does not affect the encoding of any
other field during the life-time of the flow. It is only the other field during the life-time of the flow. It is only the
encoding of the window scaling option itself that is important. encoding of the window scaling option itself that is important.
The window scale must be between 0 and 14 (inclusive). The window scale must be between 0 and 14 (inclusive).
Generally smaller values would be expected (a window scale of Generally smaller values would be expected (a window scale of
14 allows for a 1Gbyte window, which is extremely large). 14 allows for a 1Gbyte window, which is extremely large).
4. SACK-Permitted 4. SACK-Permitted
This option may be sent in a SYN by a TCP that has been This option may be sent in a SYN by a TCP that has been
extended to receive (and presumably process) the SACK option extended to receive (and presumably process) the SACK option
once the connection has opened RFC 2018 [15]. once the connection has opened RFC 2018 [18].
There is no data in this option, all that is required is for There is no data in this option, all that is required is for
the presence of the option to be encoded. the presence of the option to be encoded.
5. SACK 5. SACK
This option is to be used to convey extended acknowledgment This option is to be used to convey extended acknowledgment
information over an established connection. Specifically, it information over an established connection. Specifically, it
is to be sent by a data receiver to inform the data transmitter is to be sent by a data receiver to inform the data transmitter
of non- contiguous blocks of data that have been received and of non- contiguous blocks of data that have been received and
queued. The data receiver is awaiting the receipt of data in queued. The data receiver is awaiting the receipt of data in
later retransmissions to fill the gaps in sequence space later retransmissions to fill the gaps in sequence space
between these blocks. between these blocks.
At that time, the data receiver will acknowledge the data At that time, the data receiver will acknowledge the data
normally by advancing the left window edge in the normally by advancing the left window edge in the
Acknowledgment Number field of the TCP header. It is important Acknowledgment Number field of the TCP header. It is important
to understand that the SACK option will not change the meaning to understand that the SACK option will not change the meaning
of the Acknowledgment Number field, whose value will still of the Acknowledgment Number field, whose value will still
specify the left window edge, i.e., one byte beyond the last specify the left window edge, i.e., one byte beyond the last
sequence number of fully-received data RFC 2018 [15]. sequence number of fully-received data RFC 2018 [18].
If SACK has been negotiated (through an exchange of SACK- If SACK has been negotiated (through an exchange of SACK-
Permitted options), then this option may occur when dropped Permitted options), then this option may occur when dropped
segments are noticed by the receiver. Because this identifies segments are noticed by the receiver. Because this identifies
ranges of blocks within the receiver's window, this can be ranges of blocks within the receiver's window, this can be
viewed as a base value with a number of offsets. The base viewed as a base value with a number of offsets. The base
value (left edge of the first block) can be viewed as offset value (left edge of the first block) can be viewed as offset
from the TCP acknowledgement number. There can be up to 4 SACK from the TCP acknowledgement number. There can be up to 4 SACK
blocks in a single option. SACK blocks may occur in a number blocks in a single option. SACK blocks may occur in a number
of segments (if there is more out-of-order data 'on the wire') of segments (if there is more out-of-order data 'on the wire')
and this will typically extend the size of or add to the and this will typically extend the size of or add to the
existing blocks. existing blocks.
Alternative proposals such as DSACK RFC 2883 [27] do not Alternative proposals such as DSACK RFC 2883 [30] do not
fundamentally change the behavior of the SACK block, from the fundamentally change the behavior of the SACK block, from the
point of view of the information contained within it. point of view of the information contained within it.
6. Echo 6. Echo
This option carries information that the receiving TCP may send This option carries information that the receiving TCP may send
back in a subsequent TCP Echo Reply option (see below). A TCP back in a subsequent TCP Echo Reply option (see below). A TCP
may send the TCP Echo option in any segment, but only if a TCP may send the TCP Echo option in any segment, but only if a TCP
Echo option was received in a SYN segment for the connection. Echo option was received in a SYN segment for the connection.
When the TCP echo option is used for RTT measurement, it will When the TCP echo option is used for RTT measurement, it will
be included in data segments, and the four information bytes be included in data segments, and the four information bytes
will define the time at which the data segment was transmitted will define the time at which the data segment was transmitted
in any format convenient to the sender RFC 1072 [3]. in any format convenient to the sender RFC 1072 [4].
The Echo option is generally not used in practice -- it is The Echo option is generally not used in practice -- it is
obsoleted by the Timestamp option. However, for transparency obsoleted by the Timestamp option. However, for transparency
it is desirable that a compression scheme be able to transport it is desirable that a compression scheme be able to transport
it. (However, there is no benefit in attempting any more it. (However, there is no benefit in attempting any more
sophisticated treatment than viewing it as a generic 'option'). sophisticated treatment than viewing it as a generic 'option').
7. Echo Reply 7. Echo Reply
A TCP that receives a TCP Echo option containing four A TCP that receives a TCP Echo option containing four
information bytes will return these same bytes in a TCP Echo information bytes will return these same bytes in a TCP Echo
Reply option. This TCP Echo Reply option must be returned in Reply option. This TCP Echo Reply option must be returned in
the next segment (e.g., an ACK segment) that is sent. If more the next segment (e.g., an ACK segment) that is sent. If more
than one Echo option is received before a reply segment is than one Echo option is received before a reply segment is
sent, the TCP must choose only one of the options to echo, sent, the TCP must choose only one of the options to echo,
ignoring the others; specifically, it must choose the newest ignoring the others; specifically, it must choose the newest
segment with the oldest sequence number (see RFC 1072 [3]). segment with the oldest sequence number (see RFC 1072 [4]).
The Echo option is generally not used in practice -- it is The Echo option is generally not used in practice -- it is
obsoleted by the Timestamp option. However, for transparency obsoleted by the Timestamp option. However, for transparency
it is desirable that a compression scheme be able to transport it is desirable that a compression scheme be able to transport
it. (However, there is no benefit in attempting any more it. (However, there is no benefit in attempting any more
sophisticated treatment than viewing it as a generic 'option'). sophisticated treatment than viewing it as a generic 'option').
8. Timestamps 8. Timestamps
This option carries two four-byte timestamp fields. The This option carries two four-byte timestamp fields. The
Timestamp Value field (TSval) contains the current value of the Timestamp Value field (TSval) contains the current value of the
timestamp clock of the TCP sending the option. The Timestamp timestamp clock of the TCP sending the option. The Timestamp
Echo Reply field (TSecr) is only valid if the ACK bit is set in Echo Reply field (TSecr) is only valid if the ACK bit is set in
the TCP header; if it is valid, it echoes a timestamp value the TCP header; if it is valid, it echoes a timestamp value
that was sent by the remote TCP in the TSval field of a that was sent by the remote TCP in the TSval field of a
Timestamps option. When TSecr is not valid, its value must be Timestamps option. When TSecr is not valid, its value must be
zero. The TSecr value will generally be from the most recent zero. The TSecr value will generally be from the most recent
Timestamp option that was received; however, there are Timestamp option that was received; however, there are
exceptions that are explained below. A TCP may send the exceptions that are explained below. A TCP may send the
skipping to change at page 30, line 17 skipping to change at page 31, line 20
Echo Reply field (TSecr) is only valid if the ACK bit is set in Echo Reply field (TSecr) is only valid if the ACK bit is set in
the TCP header; if it is valid, it echoes a timestamp value the TCP header; if it is valid, it echoes a timestamp value
that was sent by the remote TCP in the TSval field of a that was sent by the remote TCP in the TSval field of a
Timestamps option. When TSecr is not valid, its value must be Timestamps option. When TSecr is not valid, its value must be
zero. The TSecr value will generally be from the most recent zero. The TSecr value will generally be from the most recent
Timestamp option that was received; however, there are Timestamp option that was received; however, there are
exceptions that are explained below. A TCP may send the exceptions that are explained below. A TCP may send the
Timestamps option (TSopt) in an initial segment (i.e., segment Timestamps option (TSopt) in an initial segment (i.e., segment
containing a SYN bit and no ACK bit), and may send a TSopt in containing a SYN bit and no ACK bit), and may send a TSopt in
other segments only if it received a TSopt in the initial other segments only if it received a TSopt in the initial
segment for the connection RFC 1323 [8]. segment for the connection RFC 1323 [9].
Timestamps are quite commonly used. If timestamp options are Timestamps are quite commonly used. If timestamp options are
exchanged in the connection set-up phase, then they will appear exchanged in the connection set-up phase, then they are
on all subsequent segments. If this exchange does not happen, expected to appear on all subsequent segments. If this
then they will not appear for the remainder of the flow. exchange does not happen, then they will not appear for the
remainder of the flow.
Note that currently it is assumed that the negotiation of Note that currently it is assumed that the negotiation of
options such as timestamp occurs in the SYN packets. However, options such as timestamp occurs in the SYN packets. However,
should this ever be allowed to change (allowing timestamps to should this ever be allowed to change (allowing timestamps to
be enabled during an existing connection, for example), the be enabled during an existing connection, for example), the
presence of the option should still be handled correctly. presence of the option should still be handled correctly.
Because the value being carried is a timestamp, it is logical Because the value being carried is a timestamp, it is logical
to expect that the entire value need not be carried. There is to expect that the entire value need not be carried. There is
no obvious pattern of increments that might be expected, no obvious pattern of increments that might be expected,
however. however.
An important reason for using the timestamp option is to allow An important reason for using the timestamp option is to allow
detection of sequence space wrap-around (Protection Against detection of sequence space wrap-around (Protection Against
Wrapped Sequence-number, or PAWS RFC 1323 [8]). It is not Wrapped Sequence-number, or PAWS RFC 1323 [9]). It is not
expected that this is serious concern on the links that TCP expected that this is serious concern on the links that TCP
header compression would be deployed on, but it is important header compression would be deployed on, but it is important
that the integrity of this option is maintained. This issue is that the integrity of this option is maintained. This issue is
discussed in, for example, RFC 3150 [29]. However, the discussed in, for example, RFC 3150 [32]. However, the
proposed Eifel algorithm [31] makes use of timestamps and so, proposed Eifel algorithm [36] makes use of timestamps and so,
currently, it is recommended that timestamps are used for currently, it is recommended that timestamps are used for
cellular-type links [32]. cellular-type links [34].
With regard to compression, it is further noted that the range With regard to compression, it is further noted that the range
of resolutions for the timestamp suggested in RFC 1323 [8] is of resolutions for the timestamp suggested in RFC 1323 [9] is
quite wide (1ms to 1s per 'tick'). This (along with the quite wide (1ms to 1s per 'tick'). This (along with the
perhaps wide variation in RTT) makes it hard to select a set of perhaps wide variation in RTT) makes it hard to select a set of
encodings that will be optimal in all cases. encodings that will be optimal in all cases.
9. Partial Order Connection (POC) permitted 9. Partial Order Connection (POC) permitted
This option represents a simple indicator communicated between This option represents a simple indicator communicated between
the two peer transport entities to establish the operation of the two peer transport entities to establish the operation of
the POC protocol RFC 1693 [11] the POC protocol RFC 1693 [12]
The Partial Order Connection option is in practice never seen, The Partial Order Connection option is in practice never seen,
and so the only requirement is that the header compression and so the only requirement is that the header compression
scheme should be able to encode it. scheme should be able to encode it.
10. POC service profile 10. POC service profile
This option serves to communicate the information necessary to This option serves to communicate the information necessary to
carry out the job of the protocol -- the type of information carry out the job of the protocol -- the type of information
that is typically found in the header of a TCP segment. that is typically found in the header of a TCP segment.
skipping to change at page 31, line 34 skipping to change at page 32, line 36
11. Connection Count (CC) 11. Connection Count (CC)
This option is part of the implementation of TCP Accelerated This option is part of the implementation of TCP Accelerated
Open (TAO) that effectively bypasses the TCP Three-Way Open (TAO) that effectively bypasses the TCP Three-Way
Handshake (3WHS). TAO introduces a 32-bit incarnation number, Handshake (3WHS). TAO introduces a 32-bit incarnation number,
called a "connection count" (CC) that is carried in a TCP called a "connection count" (CC) that is carried in a TCP
option in each segment. A distinct CC value is assigned to option in each segment. A distinct CC value is assigned to
each direction of an open connection. The implementation each direction of an open connection. The implementation
assigns monotonically increasing CC values to successive assigns monotonically increasing CC values to successive
connections that it opens actively or passively RFC 1644 [10]. connections that it opens actively or passively RFC 1644 [11].
This option is in practice never seen, and so the only This option is in practice never seen, and so the only
requirement is that the header compression scheme should be requirement is that the header compression scheme should be
able to encode it. able to encode it.
12. CC.NEW 12. CC.NEW
Correctness of the TAO mechanism requires that clients generate Correctness of the TAO mechanism requires that clients generate
monotonically increasing CC values for successive connection monotonically increasing CC values for successive connection
initiations. Receiving a CC.NEW causes the server to initiations. Receiving a CC.NEW causes the server to
invalidate its cache entry and do a 3WHS. RFC 1644 [10]. invalidate its cache entry and do a 3WHS. RFC 1644 [11].
This option is in practice never seen, and so the only This option is in practice never seen, and so the only
requirement is that the header compression scheme should be requirement is that the header compression scheme should be
able to encode it. able to encode it.
13. CC.ECHO 13. CC.ECHO
When a server host sends a segment, it echoes the connection When a server host sends a segment, it echoes the connection
count from the initial in a CC.ECHO option, which is used by count from the initial in a CC.ECHO option, which is used by
the client host to validate the segment RFC 1644 [10]. the client host to validate the segment RFC 1644 [11].
This option is in practice never seen, and so the only This option is in practice never seen, and so the only
requirement is that the header compression scheme should be requirement is that the header compression scheme should be
able to encode it. able to encode it.
14. Alternate Checksum Request 14. Alternate Checksum Request
This option may be sent in a SYN segment by a TCP to indicate This option may be sent in a SYN segment by a TCP to indicate
that the TCP is prepared to both generate and receive checksums that the TCP is prepared to both generate and receive checksums
based on an alternate algorithm. During communication, the based on an alternate algorithm. During communication, the
alternate checksum replaces the regular TCP checksum in the alternate checksum replaces the regular TCP checksum in the
checksum field of the TCP header. Should the alternate checksum field of the TCP header. Should the alternate
checksum require more than 2 octets to transmit, the checksum checksum require more than 2 octets to transmit, the checksum
may either be moved into a TCP Alternate Checksum Data Option may either be moved into a TCP Alternate Checksum Data Option
and the checksum field of the TCP header be sent as 0, or the and the checksum field of the TCP header be sent as 0, or the
data may be split between the header field and the option. data may be split between the header field and the option.
Alternate checksums are computed over the same data as the Alternate checksums are computed over the same data as the
regular TCP checksum RFC 1146 [6] regular TCP checksum RFC 1146 [7]
This option is in practice never seen, and so the only This option is in practice never seen, and so the only
requirement is that the header compression scheme should be requirement is that the header compression scheme should be
able to encode it. It would only occur in connection set-up able to encode it. It would only occur in connection set-up
(SYN) packets. (SYN) packets.
Even if this option were used, it would not affect the handling Even if this option were used, it would not affect the handling
of the checksum, since this should be carried transparently in of the checksum, since this should be carried transparently in
any case. any case.
15. Alternate Checksum Data 15. Alternate Checksum Data
This field is used only when the alternate checksum that is This field is used only when the alternate checksum that is
negotiated is longer than 16 bits. These checksums will not negotiated is longer than 16 bits. These checksums will not
fit in the checksum field of the TCP header and thus at least fit in the checksum field of the TCP header and thus at least
part of them must be put in an option. Whether the checksum is part of them must be put in an option. Whether the checksum is
split between the checksum field in the TCP header and the split between the checksum field in the TCP header and the
option or the entire checksum is placed in the option is option or the entire checksum is placed in the option is
determined on a checksum by checksum basis. The length of this determined on a checksum by checksum basis. The length of this
option will depend on the choice of alternate checksum option will depend on the choice of alternate checksum
algorithm for this connection RFC 1146 [6]. algorithm for this connection RFC 1146 [7].
If an alternative checksum were negotiated in the connection If an alternative checksum were negotiated in the connection
set-up, then this option may appear on all subsequent packets set-up, then this option may appear on all subsequent packets
(if needed to carry the checksum data). However, this option (if needed to carry the checksum data). However, this option
is in practice never seen, and so the only requirement is that is in practice never seen, and so the only requirement is that
the header compression scheme should be able to encode it. the header compression scheme should be able to encode it.
16. -- 18. 16. -- 18.
Are non-RFC references and are not considered in this document. Are non-RFC references and are not considered in this document.
skipping to change at page 33, line 23 skipping to change at page 34, line 27
applying the MD5 algorithm to a concatenated block of data. applying the MD5 algorithm to a concatenated block of data.
Upon receiving a signed segment, the receiver must validate it Upon receiving a signed segment, the receiver must validate it
by calculating its own digest from the same data (using its own by calculating its own digest from the same data (using its own
key) and comparing the two digest. A failing comparison must key) and comparing the two digest. A failing comparison must
result in the segment being dropped and must not produce any result in the segment being dropped and must not produce any
response back to the sender. Logging the failure is probably response back to the sender. Logging the failure is probably
advisable. advisable.
Unlike other TCP extensions (e.g., the Window Scale option Unlike other TCP extensions (e.g., the Window Scale option
[8]), the absence of the option in the SYN, ACK segment must [9]), the absence of the option in the SYN, ACK segment must
not cause the sender to disable its sending of signatures. not cause the sender to disable its sending of signatures.
This negotiation is typically done to prevent some TCP This negotiation is typically done to prevent some TCP
implementations from misbehaving upon receiving options in non- implementations from misbehaving upon receiving options in non-
SYN segments. This is not a problem for this option, since the SYN segments. This is not a problem for this option, since the
SYN, ACK sent during connection negotiation will not be signed SYN, ACK sent during connection negotiation will not be signed
and will thus be ignored. The connection will never be made, and will thus be ignored. The connection will never be made,
and non-SYN segments with options will never be sent. More and non-SYN segments with options will never be sent. More
importantly, the sending of signatures must be under the importantly, the sending of signatures must be under the
complete control of the application, not at the mercy of the complete control of the application, not at the mercy of the
remote host not understanding the option. remote host not understanding the option.
MD5 digest information should, like any cryptographically MD5 digest information should, like any cryptographically
secure data, be incompressible. Therefore the compression secure data, be incompressible. Therefore the compression
scheme must simply transparently carry this option, if it scheme must simply transparently carry this option, if it
occurs. occurs.
20. -- 26. 20. -- 26.
Are non-RFC references and are not considered in this document. Are non-RFC references and are not considered in this document.
This only means that their behavior is not described in detail
as a compression scheme is not expected to be optimised for
these options. However any unrecognised option must be
transparently carried by a TCP compression scheme in order to
work efficiently in the presence of new or rare options.
In the discussion above regarding timestamps it is pointed out that In the discussion above regarding timestamps it is pointed out that
there is the possibility (at some time in the future) of negotiations there is the possibility (at some time in the future) of negotiations
being permitted more generally than in the SYN packets at connection being permitted more generally than in the SYN packets at connection
set-up. Although there seems to be no compelling need to optimise set-up. Although there seems to be no compelling need to optimise
for this, it is worth pointing out that the compression scheme should for this, it is worth pointing out that the compression scheme should
be able to cope with arbitrary options appearing at any point within be able to cope with arbitrary options appearing at any point within
the flow. the flow. There is also no guarantee that a compression scheme will
see the SYN packets of a connection set-up.
6. Other observations 6. Other observations
6.1 Implicit acknowledgements 6.1 Implicit acknowledgements
There may be a small number of cues for 'implicit acknowledgements' There may be a small number of cues for 'implicit acknowledgements'
in a TCP flow. Even if the compressor only sees the data transfer in a TCP flow. Even if the compressor only sees the data transfer
direction, for example, then seeing a packet without the SYN flag set direction, for example, then seeing a packet without the SYN flag set
implies that the SYN packet has been received. implies that the SYN packet has been received.
It is worth pointing out the implication of the requirement for There is a clear requirement for the deployment of compression to be
compression to be topologically independent. This means that it is topologically independent. This means that it is not actually
not actually possible to be sure that seeing a data packet at the possible to be sure that seeing a data packet at the compressor
compressor guarantees that the SYN packet has been correctly received guarantees that the SYN packet has been correctly received by the
by the decompressor. decompressor (as the SYN packet may have taken an alternative path).
However, it may be that there are other such cues that may be used in However, it may be that there are other such cues that may be used in
certain circumstances to improve compression efficiency. certain circumstances to improve compression efficiency.
6.2 Shared data 6.2 Shared data
At the risk of drifting into solution space, it can be seen that It can be seen that there are two distinct deployments -- one where
there are two distinct deployments -- one where the forward and the forward and reverse paths share a link and one where they do not.
reverse paths share a link and one where they do not.
In the former case it may be the case that a compressor and In the former case a compressor and decompressor could be co-located.
decompressor could be co-located. It may then be possible for the It may then be possible for the compressor and decompressor at each
compressor and decompressor at each end of the link to exchange end of the link to exchange information. This could lead to possible
information. This could lead to possible optimizations. optimizations.
For example, acknowledgement numbers are generally taken from the For example, acknowledgement numbers are generally taken from the
sequence numbers in the opposite direction. Since an acknowledgement sequence numbers in the opposite direction. Since an acknowledgement
cannot be generated for a packet that has not passed across the link, cannot be generated for a packet that has not passed across the link,
this offers an efficient way of encoding acknowledgements. this offers an efficient way of encoding acknowledgements.
6.3 TCP header overhead 6.3 TCP header overhead
For a TCP bulk data-transfer the overhead is not that onerous, For a TCP bulk data-transfer the overhead is not that onerous,
particularly compared to the typical RTP voice case. Although particularly compared to the typical RTP voice case. Although
spectral efficiency is clearly an important goal, it does not seem spectral efficiency is clearly an important goal, it does not seem
critical to extract every last bit of compression gain. critical to extract every last bit of compression gain.
However, in the acknowledgement direction (i.e. for 'pure' However, in the acknowledgement direction (i.e. for 'pure'
acknowledgement headers) the overhead could be said to be infinite acknowledgement headers) the overhead could be said to be infinite
(since there is no data being carried). This is why optimizations (since there is no data being carried). This is why optimizations
for the acknowledgement path may be considered useful. for the acknowledgement path may be considered useful.
There are a number of schemes for manipulating TCP acknowledgements There are a number of schemes for manipulating TCP acknowledgements
to reduce the ACK bandwidth. Many of these are documented in [33] to reduce the ACK bandwidth. Many of these are documented in [33]
and [29]. Most of these schemes are entirely compatible with header and [32]. Most of these schemes are entirely compatible with header
compression, without requiring any particular support from either. compression, without requiring any particular support from either.
While it is not expected that a compression scheme will support While it is not expected that a compression scheme will support
experimental options, it is useful that these be considered when experimental options, it is useful that these be considered when
developing header compression schemes, and vice versa. developing header compression schemes, and vice versa.
6.4 Field independence and packet behavior 6.4 Field independence and packet behavior
It should be apparent that direct comparisons with the highly It should be apparent that direct comparisons with the highly
'packet' based view of RTP compression are hard. RTP header fields 'packet' based view of RTP compression are hard. RTP header fields
tend to change regularly per-packet and many fields (IPv4 IP ID, RTP tend to change regularly per-packet and many fields (IPv4 IP ID, RTP
skipping to change at page 35, line 30 skipping to change at page 36, line 38
makes compression more challenging and makes it harder to select a makes compression more challenging and makes it harder to select a
set of encodings that can successfully trade-off efficiency and set of encodings that can successfully trade-off efficiency and
robustness. robustness.
6.5 Short-lived flows 6.5 Short-lived flows
It is hard to see what can be done to improve performance for a It is hard to see what can be done to improve performance for a
single, unpredictable, short-lived connection. However, there are single, unpredictable, short-lived connection. However, there are
commonly cases where there will be multiple TCP connections between commonly cases where there will be multiple TCP connections between
the same pair of hosts. As a particular example, consider web the same pair of hosts. As a particular example, consider web
browsing (this is more the case with HTTP/1.0 [12] than HTTP/1.1 browsing (this is more the case with HTTP/1.0 [15] than HTTP/1.1
[17]). [20]).
When a connection closes, it is either the last connection between When a connection closes, it is either the last connection between
that pair of hosts or it is likely that another connection will open that pair of hosts or it is likely that another connection will open
within a relatively short space of time. In this case, the IP header within a relatively short space of time. In this case, the IP header
part of the context will probably be almost identical. Certain part of the context will probably be almost identical. Certain
aspects of the TCP context may also be similar. aspects of the TCP context may also be similar.
Support for context replication is discussed in more detail in Support for context replication is discussed in more detail in
Section 4. Overall, support for sub-context sharing, or initializing Section 4. Overall, support for sub-context sharing, or initializing
one context from another offers useful optimizations for a sequence one context from another offers useful optimizations for a sequence
skipping to change at page 37, line 5 skipping to change at page 38, line 11
where segment data starts with a 4-bit field which gives the total where segment data starts with a 4-bit field which gives the total
size of the header (including options) in 32-bit words. This means size of the header (including options) in 32-bit words. This means
that the total size of the header plus option must be less than or that the total size of the header plus option must be less than or
equal to 60 bytes -- this leaves 40 bytes for options. equal to 60 bytes -- this leaves 40 bytes for options.
7. Security considerations 7. Security considerations
Since this document only describes TCP field behavior there are no Since this document only describes TCP field behavior there are no
direct security concerns raised by it. direct security concerns raised by it.
This memo is intended to be used to aid the compression of TCP/IP
headers. Where authentication mechanisms such as IPsec AH [13] are
used, it is important that compression is transparent. Where
encryption methods such as IPsec ESP [14] are used, the TCP fields
may not be visible, preventing compression.
8. Acknowledgements 8. Acknowledgements
Many IP and TCP RFCs (hopefully all of which have been collated Many IP and TCP RFCs (hopefully all of which have been collated
below) have been sources of ideas and knowledge, together with header below) have been sources of ideas and knowledge, together with header
compression schemes from RFC 1144, RFC 2509 and RFC 3095, and of compression schemes from RFC 1144, RFC 2509 and RFC 3095, and of
course the detailed analysis of RTP/UDP/IP in RFC 3095. course the detailed analysis of RTP/UDP/IP in RFC 3095.
This draft also benefited from discussion on the rohc mailing list This draft also benefited from discussion on the rohc mailing list
and in various corridors (virtual or otherwise) about many key and in various corridors (virtual or otherwise) about many key
issues; special thanks to Qian Zhang, Carsten Bormann and Gorry issues; special thanks to Qian Zhang, Carsten Bormann and Gorry
skipping to change at page 37, line 31 skipping to change at page 38, line 43
is entirely the fault of the authors of this draft. is entirely the fault of the authors of this draft.
References References
[1] Postel, J., "Internet Protocol", STD 5, RFC 791, September [1] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981. 1981.
[2] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, [2] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981. September 1981.
[3] Jacobson, V. and R. Braden, "TCP extensions for long-delay [3] Nagle, J., "Congestion control in IP/TCP internetworks", RFC
896, January 1984.
[4] Jacobson, V. and R. Braden, "TCP extensions for long-delay
paths", RFC 1072, October 1988. paths", RFC 1072, October 1988.
[4] Braden, R., "Requirements for Internet Hosts - Communication [5] Braden, R., "Requirements for Internet Hosts - Communication
Layers", STD 3, RFC 1122, October 1989. Layers", STD 3, RFC 1122, October 1989.
[5] Jacobson, V., "Compressing TCP/IP headers for low-speed serial [6] Jacobson, V., "Compressing TCP/IP headers for low-speed serial
links", RFC 1144, February 1990. links", RFC 1144, February 1990.
[6] Zweig, J. and C. Partridge, "TCP alternate checksum options", [7] Zweig, J. and C. Partridge, "TCP alternate checksum options",
RFC 1146, March 1990. RFC 1146, March 1990.
[7] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, [8] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990. November 1990.
[8] Jacobson, V., Braden, B. and D. Borman, "TCP Extensions for [9] Jacobson, V., Braden, B. and D. Borman, "TCP Extensions for
High Performance", RFC 1323, May 1992. High Performance", RFC 1323, May 1992.
[9] Almquist, P., "Type of Service in the Internet Protocol Suite", [10] Almquist, P., "Type of Service in the Internet Protocol Suite",
RFC 1349, July 1992. RFC 1349, July 1992.
[10] Braden, B., "T/TCP -- TCP Extensions for Transactions [11] Braden, B., "T/TCP -- TCP Extensions for Transactions
Functional Specification", RFC 1644, July 1994. Functional Specification", RFC 1644, July 1994.
[11] Connolly, T., Amer, P. and P. Conrad, "An Extension to TCP : [12] Connolly, T., Amer, P. and P. Conrad, "An Extension to TCP :
Partial Order Service", RFC 1693, November 1994. Partial Order Service", RFC 1693, November 1994.
[12] Berners-Lee, T., Fielding, R. and H. Nielsen, "Hypertext [13] Atkinson, R., "IP Authentication Header", RFC 1826, August
1995.
[14] Atkinson, R., "IP Encapsulating Security Payload (ESP)", RFC
1827, August 1995.
[15] Berners-Lee, T., Fielding, R. and H. Nielsen, "Hypertext
Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996. Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.
[13] Bellovin, S., "Defending Against Sequence Number Attacks", RFC [16] Bellovin, S., "Defending Against Sequence Number Attacks", RFC
1948, May 1996. 1948, May 1996.
[14] Stevens, W., "TCP Slow Start, Congestion Avoidance, Fast [17] Stevens, W., "TCP Slow Start, Congestion Avoidance, Fast
Retransmit, and Fast Recovery Algorithms", RFC 2001, January Retransmit, and Fast Recovery Algorithms", RFC 2001, January
1997. 1997.
[15] and, M., Floyd, S. and A. Romanow, "TCP Selective [18] and, M., Floyd, S. and A. Romanow, "TCP Selective
Acknowledgment Options", RFC 2018, October 1996. Acknowledgment Options", RFC 2018, October 1996.
[16] Bradner, S., "The Internet Standards Process -- Revision 3", [19] Bradner, S., "The Internet Standards Process -- Revision 3",
BCP 9, RFC 2026, October 1996. BCP 9, RFC 2026, October 1996.
[17] Fielding, R., Gettys, J., Mogul, J., Nielsen, H. and T. [20] Fielding, R., Gettys, J., Mogul, J., Nielsen, H. and T.
Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC
2068, January 1997. 2068, January 1997.
[18] Bradner, S., "Key words for use in RFCs to Indicate Requirement [21] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997. Levels", BCP 14, RFC 2119, March 1997.
[19] Heffernan, A., "Protection of BGP Sessions via the TCP MD5 [22] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998. Signature Option", RFC 2385, August 1998.
[20] Nichols, K., Blake, S., Baker, F. and D. Black, "Definition of [23] Nichols, K., Blake, S., Baker, F. and D. Black, "Definition of
the Differentiated Services Field (DS Field) in the IPv4 and the Differentiated Services Field (DS Field) in the IPv4 and
IPv6 Headers", RFC 2474, December 1998. IPv6 Headers", RFC 2474, December 1998.
[21] Ramakrishnan, K. and S. Floyd, "A Proposal to add Explicit [24] Ramakrishnan, K. and S. Floyd, "A Proposal to add Explicit
Congestion Notification (ECN) to IP", RFC 2481, January 1999. Congestion Notification (ECN) to IP", RFC 2481, January 1999.
[22] Degermark, M., Nordgren, B. and S. Pink, "IP Header [25] Degermark, M., Nordgren, B. and S. Pink, "IP Header
Compression", RFC 2507, February 1999. Compression", RFC 2507, February 1999.
[23] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP Headers for [26] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP Headers for
Low-Speed Serial Links", RFC 2508, February 1999. Low-Speed Serial Links", RFC 2508, February 1999.
[24] Engan, M., Casner, S. and C. Bormann, "IP Header Compression [27] Engan, M., Casner, S. and C. Bormann, "IP Header Compression
over PPP", RFC 2509, February 1999. over PPP", RFC 2509, February 1999.
[25] Allman, M., Paxson, V. and W. Stevens, "TCP Congestion [28] Allman, M., Paxson, V. and W. Stevens, "TCP Congestion
Control", RFC 2581, April 1999. Control", RFC 2581, April 1999.
[26] Bradner, S. and V. Paxson, "IANA Allocation Guidelines For [29] Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
Values In the Internet Protocol and Related Headers", BCP 37, Values In the Internet Protocol and Related Headers", BCP 37,
RFC 2780, March 2000. RFC 2780, March 2000.
[27] Floyd, S., Mahdavi, J., Mathis, M. and M. Podolsky, "An [30] Floyd, S., Mahdavi, J., Mathis, M. and M. Podolsky, "An
Extension to the Selective Acknowledgement (SACK) Option for Extension to the Selective Acknowledgement (SACK) Option for
TCP", RFC 2883, July 2000. TCP", RFC 2883, July 2000.
[28] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H., [31] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le, K., Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le, K.,
Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K., Wiebke, T., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K., Wiebke, T.,
Yoshimura, T. and H. Zheng, "RObust Header Compression (ROHC): Yoshimura, T. and H. Zheng, "RObust Header Compression (ROHC):
Framework and four profiles: RTP, UDP, ESP, and uncompressed", Framework and four profiles: RTP, UDP, ESP, and uncompressed",
RFC 3095, July 2001. RFC 3095, July 2001.
[29] Dawkins, S., Montenegro, G., Kojo, M. and V. Magret, "End-to- [32] Dawkins, S., Montenegro, G., Kojo, M. and V. Magret, "End-to-
end Performance Implications of Slow Links", BCP 48, RFC 3150, end Performance Implications of Slow Links", BCP 48, RFC 3150,
July 2001. July 2001.
[30] Spring, N., Wetherall, D. and D. Ely, "Robust ECN Signaling [33] Balakrishnan, , Padmanabhan, V., Fairhurst, G. and M.
with Nonces", draft-ietf-tsvwg-tcp-nonce-02.txt (work in Sooriyabandara, "TCP Performance Implications of Network Path
progress), October 2001. Asymmetry", RFC 3449, December 2002.
[31] Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm for [34] Inamura, H., Montenegro, G., Ludwig, R., Gurtov, A. and F.
TCP", draft-ietf-tsvwg-tcp-eifel-alg-03.txt (work in progress),
February 2002.
[32] Inamura, H., Montenegro, G., Ludwig, R., Gurtov, A. and F. Khafizov, "TCP over Second (2.5G) and Third (3G) Generation
Khafizov, "TCP over 2.5G and 3G Wireless Networks", draft-ietf- Wireless Networks", RFC 3481, February 2003.
pilc-2.5g3g-06.txt (work in progress), February 2002.
[33] Balakrishnan, , Padmanabhan, V., Fairhurst, G. and M. [35] Spring, N., Wetherall, D. and D. Ely, "Robust ECN Signaling
Sooriyabandara, "TCP Performance Implications of Network Path with Nonces", draft-ietf-tsvwg-tcp-nonce-04.txt (work in
Asymmetry", draft-ietf-pilc-asym-07.txt (work in progress), progress), October 2002.
November 2001.
[34] <http://www.iana.org/assignments/tcp-parameters> [36] Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm for
TCP", draft-ietf-tsvwg-tcp-eifel-alg-07.txt (work in progress),
December 2002.
[37] Karn, , "Advice for Internet Subnetwork Designers", draft-ietf-
pilc-link-design-13.txt (work in progress), February 2003.
[38] <http://www.iana.org/assignments/tcp-parameters>
Authors' Addresses Authors' Addresses
Mark A. West Mark A. West
Siemens/Roke Manor Siemens/Roke Manor
Roke Manor Research Ltd. Roke Manor Research Ltd.
Romsey, Hants SO51 0ZN Romsey, Hants SO51 0ZN
UK UK
Phone: +44 (0)1794 833311 Phone: +44 (0)1794 833311
skipping to change at page 41, line 7 skipping to change at page 42, line 7
Roke Manor Research Ltd. Roke Manor Research Ltd.
Romsey, Hants SO51 0ZN Romsey, Hants SO51 0ZN
UK UK
Phone: +44 (0)1794 833341 Phone: +44 (0)1794 833341
EMail: stephen.mccann@roke.co.uk EMail: stephen.mccann@roke.co.uk
URI: http://www.roke.co.uk URI: http://www.roke.co.uk
Full Copyright Statement Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved. Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of Internet organizations, except as needed for the purpose of
 End of changes. 

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