draft-ietf-rohc-tcp-field-behavior-02.txt   draft-ietf-rohc-tcp-field-behavior-03.txt 
Network Working Group M. West Network Working Group M. West
Internet-Draft S. McCann Internet-Draft S. McCann
Expires: September 1, 2003 Siemens/Roke Manor Expires: February 16, 2005 Siemens/Roke Manor Research
March 3, 2003 August 18, 2004
TCP/IP Field Behavior TCP/IP Field Behavior
draft-ietf-rohc-tcp-field-behavior-02.txt draft-ietf-rohc-tcp-field-behavior-03.txt
Status of this Memo Status of this Memo
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and any of which I become aware will be disclosed, in accordance with
RFC 3668.
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved. Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract Abstract
This memo 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. An example of this analysis can be seen in RFC 3095 [31]. detail. An example of this analysis can be seen in RFC 3095 [34].
This memo performs a similar role for the compression of TCP/IP. This memo performs a similar role for the compression of TCP/IP.
Change History
-00 : Initial version
-01 : Corrections and clarifications from review comments plus
analysis of shareable fields
-02 : Re-write shareable field section + incorporate Gorry's
comments
-03 : Correct references
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 . . . . . . . . . . . . . . . . . . . . . 5
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 replicable header fields . . . . . . . . . 12 4. Classification of replicable header fields . . . . . . . . . . 11
4.1 IPv4 Header (inner and/or outer) . . . . . . . . . . . . . . 13 4.1 IPv4 Header (inner and/or outer) . . . . . . . . . . . . . 12
4.2 IPv6 Header (inner and/or outer) . . . . . . . . . . . . . . 14 4.2 IPv6 Header (inner and/or outer) . . . . . . . . . . . . . 13
4.3 TCP Header . . . . . . . . . . . . . . . . . . . . . . . . . 15 4.3 TCP Header . . . . . . . . . . . . . . . . . . . . . . . . 14
4.4 TCP Options . . . . . . . . . . . . . . . . . . . . . . . . 16 4.4 TCP Options . . . . . . . . . . . . . . . . . . . . . . . 15
4.5 Summary of replication . . . . . . . . . . . . . . . . . . . 16 4.5 Summary of replication . . . . . . . . . . . . . . . . . . 15
5. Analysis of change patterns of header fields . . . . . . . . 17 5. Analysis of change patterns of header fields . . . . . . . . . 15
5.1 IP header . . . . . . . . . . . . . . . . . . . . . . . . . 19 5.1 IP header . . . . . . . . . . . . . . . . . . . . . . . . 18
5.1.1 IP Traffic-Class / Type-Of-Service (TOS) . . . . . . . . . . 19 5.1.1 IP Traffic-Class / Type-Of-Service (TOS) . . . . . . . 18
5.1.2 ECN Flags . . . . . . . . . . . . . . . . . . . . . . . . . 20 5.1.2 ECN Flags . . . . . . . . . . . . . . . . . . . . . . 18
5.1.3 IP Identification . . . . . . . . . . . . . . . . . . . . . 20 5.1.3 IP Identification . . . . . . . . . . . . . . . . . . 19
5.1.4 Don't Fragment (DF) flag . . . . . . . . . . . . . . . . . . 22 5.1.4 Don't Fragment (DF) flag . . . . . . . . . . . . . . . 21
5.1.5 IP Hop-Limit / Time-To-Live (TTL) . . . . . . . . . . . . . 23 5.1.5 IP Hop-Limit / Time-To-Live (TTL) . . . . . . . . . . 21
5.2 TCP header . . . . . . . . . . . . . . . . . . . . . . . . . 23 5.2 TCP header . . . . . . . . . . . . . . . . . . . . . . . . 21
5.2.1 Sequence number . . . . . . . . . . . . . . . . . . . . . . 23 5.2.1 Sequence number . . . . . . . . . . . . . . . . . . . 22
5.2.2 Acknowledgement number . . . . . . . . . . . . . . . . . . . 24 5.2.2 Acknowledgement number . . . . . . . . . . . . . . . . 23
5.2.3 Reserved . . . . . . . . . . . . . . . . . . . . . . . . . . 25 5.2.3 Reserved . . . . . . . . . . . . . . . . . . . . . . . 23
5.2.4 Flags . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 5.2.4 Flags . . . . . . . . . . . . . . . . . . . . . . . . 24
5.2.5 Checksum . . . . . . . . . . . . . . . . . . . . . . . . . . 26 5.2.5 Checksum . . . . . . . . . . . . . . . . . . . . . . . 25
5.2.6 Window . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 5.2.6 Window . . . . . . . . . . . . . . . . . . . . . . . . 25
5.2.7 Urgent pointer . . . . . . . . . . . . . . . . . . . . . . . 27 5.2.7 Urgent pointer . . . . . . . . . . . . . . . . . . . . 25
5.3 Options . . . . . . . . . . . . . . . . . . . . . . . . . . 27 5.3 Options . . . . . . . . . . . . . . . . . . . . . . . . . 25
5.3.1 Options overview . . . . . . . . . . . . . . . . . . . . . . 27 5.3.1 Options overview . . . . . . . . . . . . . . . . . . . 26
5.3.2 Option field behavior . . . . . . . . . . . . . . . . . . . 28 5.3.2 Option field behavior . . . . . . . . . . . . . . . . 27
6. Other observations . . . . . . . . . . . . . . . . . . . . . 36 6. Other observations . . . . . . . . . . . . . . . . . . . . . . 33
6.1 Implicit acknowledgements . . . . . . . . . . . . . . . . . 36 6.1 Implicit acknowledgements . . . . . . . . . . . . . . . . 33
6.2 Shared data . . . . . . . . . . . . . . . . . . . . . . . . 36 6.2 Shared data . . . . . . . . . . . . . . . . . . . . . . . 33
6.3 TCP header overhead . . . . . . . . . . . . . . . . . . . . 36 6.3 TCP header overhead . . . . . . . . . . . . . . . . . . . 33
6.4 Field independence and packet behavior . . . . . . . . . . . 37 6.4 Field independence and packet behavior . . . . . . . . . . 34
6.5 Short-lived flows . . . . . . . . . . . . . . . . . . . . . 37 6.5 Short-lived flows . . . . . . . . . . . . . . . . . . . . 34
6.6 Master Sequence Number . . . . . . . . . . . . . . . . . . . 38 6.6 Master Sequence Number . . . . . . . . . . . . . . . . . . 35
6.7 Size constraint for TCP options . . . . . . . . . . . . . . 38 6.7 Size constraint for TCP options . . . . . . . . . . . . . 35
7. Security considerations . . . . . . . . . . . . . . . . . . 39 7. Security considerations . . . . . . . . . . . . . . . . . . . 36
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 39 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 36
References . . . . . . . . . . . . . . . . . . . . . . . . . 39 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 42 9.1 Normative References . . . . . . . . . . . . . . . . . . . . 36
Full Copyright Statement . . . . . . . . . . . . . . . . . . 43 9.2 Informative References . . . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 39
Intellectual Property and Copyright Statements . . . . . . . . 41
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 [31] for the RTP case, it from that previously presented in RFC 3095 [34] 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
[31] has been borrowed. This is for easier reading rather than [34] 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 [31] TCP/IP header Again based on the format presented in RFC 3095 [34] 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
the header compression scheme to optimize compression and the header compression scheme to optimize compression and
functionality. functionality.
A general question raised by this analysis is that of what 'baseline' A general question raised by this analysis is that of what 'baseline'
definition of all possible TCP/IP implementations is to be definition of all possible TCP/IP implementations is to be
considered? For the purposes of this document, a relatively up-to- considered? For the purposes of this document, a relatively
date (as of the time of writing) implementation is considered, with a up-to-date (as of the time of writing) implementation is considered,
view to ensuring compatibility with legacy implementations. with a view to ensuring compatibility with legacy implementations.
The general requirement for transparency is also seen to be more The general requirement for transparency is also seen to be more
interesting. A number of recent proposals for extensions to TCP make interesting. A number of recent proposals for extensions to TCP make
use of some of the previously 'reserved' bits. It is therefore clear use of some of the previously 'reserved' bits. It is therefore clear
that a 'reserved' bit cannot be taken to have a guaranteed zero that a 'reserved' bit cannot be taken to have a guaranteed zero
value, but may change. Ideally, this should be accommodated by the value, but may change. Ideally, this should be accommodated by the
compression profile. compression profile.
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 [21]. document are to be interpreted as described in RFC 2119 [23].
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
[31] Appendix A. Differences between IP field behavior between RFC [34] Appendix A. Differences between IP field behavior between RFC
3095 [31] (i.e. IP/UDP/RTP behavior for audio and video 3095 [34] (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.
o STATIC o STATIC
These fields are expected to be constant throughout the These fields are expected to be constant throughout the
lifetime of the packet stream. Static information must in some lifetime of the packet stream. Static information must in some
way be communicated once. way be communicated once.
o STATIC-DEF o STATIC-DEF
STATIC fields whose values define a packet stream. They are in STATIC fields whose values define a packet stream. They are in
general handled as STATIC. general handled as STATIC.
o STATIC-KNOWN o STATIC-KNOWN
These STATIC fields are expected to have well-known values and These STATIC fields are expected to have well-known values and
therefore do not need to be communicated at all. therefore do not need to be communicated at all.
o CHANGING o CHANGING
These fields are expected to vary in some way: randomly, within These fields are expected to vary in some way: randomly, within
a limited value set or range, or in some other manner. a limited value set or range, or in some other manner.
In this section, each of the IP and TCP header fields is assigned to In this section, each of the IP and TCP header fields is assigned to
one of these classes. For all fields except those classified as one of these classes. For all fields except those classified as
CHANGING, the motives for the classification are also stated. In CHANGING, the motives for the classification are also stated. In
section 4, CHANGING fields are further examined and classified on the section 4, CHANGING fields are further examined and classified on the
basis of their expected change behavior. basis of their expected change behavior.
3.1 IP header fields 3.1 IP header fields
skipping to change at page 5, line 24 skipping to change at page 6, line 22
| 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 [31] Figure 1
* differs from RFC 3095 [34]
[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
classified as STATIC. classified as STATIC.
o Flow Label o Flow Label
This field may be used to identify packets belonging to a This field may be used to identify packets belonging to a
specific packet stream. If not used, the value should be set specific packet stream. If not used, the value should be set
to zero. Otherwise, all packets belonging to the same stream to zero. Otherwise, all packets belonging to the same stream
must have the same value in this field, it being one of the must have the same value in this field, it being one of the
fields that define the stream. The field is therefore fields that define the stream. The field is therefore
classified as STATIC-DEF. classified as STATIC-DEF.
o Payload Length o Payload Length
Information about packet length (and, consequently, payload Information about packet length (and, consequently, payload
length) is expected to be provided by the link layer. The length) is expected to be provided by the link layer. The
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 [34].
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
therefore classified as STATIC-DEF. therefore classified as STATIC-DEF.
This might be considered as a slightly simplistic view. This might be considered as a slightly simplistic view.
However for now the IP addresses are associated with the However for now the IP addresses are associated with the
transport layer connection. More complex flow-separation transport layer connection. More complex flow-separation
could, of course, be considered. could, of course, be considered.
Total size of the fields in each class: Total size of the fields in each class:
+--------------+--------------+ +--------------+--------------+
| Class | Size (octets)| | Class | Size (octets)|
+--------------+--------------+ +--------------+--------------+
skipping to change at page 6, line 43 skipping to change at page 7, line 36
+--------------+--------------+ +--------------+--------------+
| Class | Size (octets)| | Class | Size (octets)|
+--------------+--------------+ +--------------+--------------+
| INFERRED | 2 | | INFERRED | 2 |
| STATIC | 1.5 | | STATIC | 1.5 |
| STATIC-DEF | 34.5 | | STATIC-DEF | 34.5 |
| STATIC-KNOWN | 0 | | STATIC-KNOWN | 0 |
| CHANGING | 2 | | CHANGING | 2 |
+--------------+--------------+ +--------------+--------------+
Figure 2
3.1.2 IPv4 header fields 3.1.2 IPv4 header fields
+---------------------+-------------+----------------+ +---------------------+-------------+----------------+
| Field | Size (bits) | Class | | Field | Size (bits) | Class |
+---------------------+-------------+----------------+ +---------------------+-------------+----------------+
| Version | 4 | STATIC | | Version | 4 | STATIC |
| Header Length | 4 | STATIC-KNOWN | | Header Length | 4 | STATIC-KNOWN |
| DSCP* | 6 | CHANGING | | DSCP* | 6 | CHANGING |
| ECT flag* | 1 | CHANGING | | ECT flag* | 1 | CHANGING |
| CE flag* | 1 | CHANGING | | CE flag* | 1 | CHANGING |
| Packet Length | 16 | INFERRED | | Packet Length | 16 | INFERRED |
skipping to change at page 7, line 25 skipping to change at page 8, line 25
| 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 [31] Figure 3
* differs from RFC 3095 [34]
[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 behavior 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.
o Header Length o Header Length
As long as no options are present in the IP header, the header As long as no options are present in the IP header, the header
length is constant and well known. If there are options, the length is constant and well known. If there are options, the
fields would be STATIC, but it is assumed here that there are fields would be STATIC, but it is assumed here that there are
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 [31] the field is therefore classified as [1]. In RFC 3095 [34] 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
fragmentation is generally not expected. As discussed in the fragmentation is generally not expected. As discussed in the
RTP case, only the first fragment will contain the transport RTP case, only the first fragment will contain the transport
layer protocol header; subsequent fragments would have to be layer protocol header; subsequent fragments would have to be
compressed with a different profile. In terms of the effect of compressed with a different profile. In terms of the effect of
header overhead, if fragmentation does occur then the first header overhead, if fragmentation does occur then the first
fragment, by definition, should be relatively large, minimizing fragment, by definition, should be relatively large, minimizing
the header overhead. In the case of TCP, fragmentation should the header overhead. In the case of TCP, fragmentation should
not be common due to a combination of initial MSS negotiation not be common due to a combination of initial MSS negotiation
and subsequent use of path-MTU discovery. The More Fragments and subsequent use of path-MTU discovery. The More Fragments
skipping to change at page 8, line 31 skipping to change at page 9, line 23
layer protocol header; subsequent fragments would have to be layer protocol header; subsequent fragments would have to be
compressed with a different profile. In terms of the effect of compressed with a different profile. In terms of the effect of
header overhead, if fragmentation does occur then the first header overhead, if fragmentation does occur then the first
fragment, by definition, should be relatively large, minimizing fragment, by definition, should be relatively large, minimizing
the header overhead. In the case of TCP, fragmentation should the header overhead. In the case of TCP, fragmentation should
not be common due to a combination of initial MSS negotiation not be common due to a combination of initial MSS negotiation
and subsequent use of path-MTU discovery. The More Fragments and subsequent use of path-MTU discovery. The More Fragments
flag is therefore classified as STATIC-KNOWN. However, a flag is therefore classified as STATIC-KNOWN. However, a
profile could accept that this flag may be set in order to cope profile could accept that this flag may be set in order to cope
with fragmentation. with fragmentation.
o Fragment Offset o Fragment Offset
Under the assumption that no fragmentation occurs, the fragment Under the assumption that no fragmentation occurs, the fragment
offset is always zero. The field is therefore classified as offset is always zero. The field is therefore classified as
STATIC-KNOWN. Even if fragmentation were to be further STATIC-KNOWN. Even if fragmentation were to be further
considered, then only the first fragment would contain the TCP considered, then only the first fragment would contain the TCP
header and the fragment offset of this packet would still be header and the fragment offset of this packet would still be
zero. zero.
o Protocol o Protocol
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 a not, will the field change its value during the lifetime of a
stream. The field is therefore classified as STATIC. stream. The field is therefore classified as STATIC.
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 [31], which uses a CRC to verify the Compare this with ROHC [34], 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 [37], entire payload; and known weaknesses in the TCP checksum [40],
an additional check is necessary. Therefore, it is expected an additional check is necessary. Therefore, it is expected
than 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 |
skipping to change at page 9, line 36 skipping to change at page 10, line 22
+--------------+--------------+ +--------------+--------------+
| 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 [31] Figure 4
* differs from RFC 3095 [34]
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 |
| Reserved | 4 | CHANGING | | Reserved | 4 | CHANGING |
| CWR flag | 1 | CHANGING | | CWR flag | 1 | CHANGING |
skipping to change at page 10, line 26 skipping to change at page 11, line 4
| ACK flag | 1 | CHANGING | | ACK flag | 1 | CHANGING |
| PSH flag | 1 | CHANGING | | PSH flag | 1 | CHANGING |
| RST flag | 1 | CHANGING | | RST flag | 1 | CHANGING |
| SYN flag | 1 | CHANGING | | SYN flag | 1 | CHANGING |
| FIN flag | 1 | CHANGING | | FIN flag | 1 | CHANGING |
| Window | 16 | CHANGING | | Window | 16 | CHANGING |
| Checksum | 16 | CHANGING | | Checksum | 16 | CHANGING |
| Urgent Pointer | 16 | CHANGING | | Urgent Pointer | 16 | CHANGING |
| Options | 0(-352) | CHANGING | | Options | 0(-352) | CHANGING |
+---------------------+-------------+----------------+ +---------------------+-------------+----------------+
Figure 5
o Source and Destination ports o Source and Destination ports
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.
o Data Offset o Data Offset
The number of 4 octet words in the TCP header, thus indicating The number of 4 octet words in the TCP header, thus indicating
The start of the data. It is always a multiple of 4 octets. The start of the data. It is always a multiple of 4 octets.
It can be re-constructed from the length of any options and It can be re-constructed from the length of any options and
thus it is not necessary to carry this explicitly. The field thus it is not necessary to carry this explicitly. The field
is therefore classified as INFERRED. is therefore classified as INFERRED.
3.3 Summary for IP/TCP 3.3 Summary for IP/TCP
Summarizing this for IP/TCP one obtains Summarizing this for IP/TCP one obtains
+----------------+----------------+----------------+ +----------------+----------------+----------------+
| Class \ IP ver | IPv6 (octets) | IPv4 (octets) | | Class \ IP ver | IPv6 (octets) | IPv4 (octets) |
+----------------+----------------+----------------+ +----------------+----------------+----------------+
| INFERRED | 2 + 4 bits | 4 + 4 bits | | INFERRED | 2 + 4 bits | 4 + 4 bits |
| 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 |
skipping to change at page 11, line 16 skipping to change at page 11, line 33
+----------------+----------------+----------------+ +----------------+----------------+----------------+
| INFERRED | 2 + 4 bits | 4 + 4 bits | | INFERRED | 2 + 4 bits | 4 + 4 bits |
| 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 |
+----------------+----------------+----------------+ +----------------+----------------+----------------+
Figure 6
(excludes options, which are all classified as CHANGING) (excludes options, which are all classified as CHANGING)
4. Classification of replicable header fields 4. Classification of replicable header fields
Where multiple flows either overlap in time or occur sequentially Where multiple flows either overlap in time or occur sequentially
within a short space of time there can be a great deal of similarity within a short space of time there can be a great deal of similarity
in header field values. Such commonality of field values is in header field values. Such commonality of field values is
reflected in the compression context. Thus, it should be possible to reflected in the compression context. Thus, it should be possible to
utilise links between fields across different flows to improve the utilise links between fields across different flows to improve the
compression ratio. In order to do this, it is important to compression ratio. In order to do this, it is important to
skipping to change at page 12, line 34 skipping to change at page 13, line 4
| Reserved flag | CHANGING | No (4) | | Reserved flag | CHANGING | No (4) |
| Don't Fragment flag | CHANGING | No | | Don't Fragment flag | CHANGING | No |
| More Fragments flag | STATIC-KNOWN | N/A | | More Fragments flag | STATIC-KNOWN | N/A |
| Fragment Offset | STATIC-KNOWN | N/A | | Fragment Offset | STATIC-KNOWN | N/A |
| Time To Live | CHANGING | Yes | | Time To Live | CHANGING | Yes |
| Protocol | STATIC | N/A | | Protocol | STATIC | N/A |
| Header Checksum | INFERRED | N/A | | Header Checksum | INFERRED | N/A |
| Source Address | STATIC-DEF | Yes | | Source Address | STATIC-DEF | Yes |
| Destination Address | STATIC-DEF | Yes | | Destination Address | STATIC-DEF | Yes |
+-----------------------+---------------+------------+ +-----------------------+---------------+------------+
(1) The DSCP is marked based on the application's requirements. If (1) The DSCP is marked based on the application's requirements. If
it can be assumed that replicable connections often carry the same it can be assumed that replicable connections often carry the same
type of traffic, the DSCP may be regarded as replicable. However, type of traffic, the DSCP may be regarded as replicable. However,
issues such as re-marking will need to be taken into account. issues such as re-marking will need to be taken into account.
(2) It is not possible for the ECN bits to be replicated (note that (2) It is not possible for the ECN bits to be replicated (note that
use of the ECN nonce scheme [35] is anticipated). However, it use of the ECN nonce scheme [38] is anticipated). However, it
seems likely that all TCP flows between ECN-capable hosts will use 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 ECN, the use (or not) of ECN for flows between the same end-points
might be considered replicable. See also note (4). might be considered replicable. See also note (4).
(3) The replicable context for this field includes the IP-ID, NBO, (3) The replicable context for this field includes the IP-ID, NBO,
and RND flags (as described in ROHC RTP). This highlights that and RND flags (as described in ROHC RTP). This highlights that
the replication is of the context, rather than just the header the replication is of the context, rather than just the header
field values and, as such, needs to be considered based on the field values and, as such, needs to be considered based on the
exact nature of compression applied to each field. exact nature of compression applied to each field.
(4) Since the possible future behavior of the 'Reserved Flag' cannot (4) Since the possible future behavior of the 'Reserved Flag' cannot
be predicted, it is not considered as replicable. However, it be predicted, it is not considered as replicable. However, it
might be expected that the behavior of the reserved flag between 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 behavior might carry of packet formats (for example) based on this behavior might carry
across to the new flow. In the case of packet formats, this can across to the new flow. In the case of packet formats, this can
probably be considered as a compressor-local decision. probably be considered as a compressor-local decision.
4.2 IPv6 Header (inner and/or outer) 4.2 IPv6 Header (inner and/or outer)
skipping to change at page 18, line 12 skipping to change at page 17, line 52
+------------------------+-------------+-------------+-------------+ +------------------------+-------------+-------------+-------------+
| TCP Checksum | Value | IRREGULAR | UNKNOWN | | TCP Checksum | Value | IRREGULAR | UNKNOWN |
+------------------------+-------------+-------------+-------------+ +------------------------+-------------+-------------+-------------+
| TCP Urgent Pointer | Value | IRREGULAR | KNOWN | | TCP Urgent Pointer | Value | IRREGULAR | KNOWN |
+------------------------+-------------+-------------+-------------+ +------------------------+-------------+-------------+-------------+
| TCP Options | Value | IRREGULAR | UNKNOWN | | TCP Options | Value | IRREGULAR | UNKNOWN |
+------------------------+-------------+-------------+-------------+ +------------------------+-------------+-------------+-------------+
Table 1 : Classification of CHANGING header fields Table 1 : Classification of CHANGING header fields
Figure 11
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/DSCP (IPv4) field might The Traffic-Class (IPv6) or Type-Of-Service/DSCP (IPv4) field might
be 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 [5] 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 the meanings of the additional 2 bits were not defined. RFC 1349
[10] defined the 4th bit of TOS to be 'minimize monetary cost'. The [10] defined the 4th bit of TOS to be 'minimize monetary cost'. The
next significant change was in RFC 2474 [23] which reworked the TOS next significant change was in RFC 2474 [25] which reworked the TOS
octet as 6 bits of DSCP (DiffServ Code Point) plus 2 unused bits. octet as 6 bits of DSCP (DiffServ Code Point) plus 2 unused bits.
Most recently RFC 2780 [29] identified the 2 reserved bits in the TOS Most recently RFC 2780 [32] identified the 2 reserved bits in the TOS
or 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 [24]. Initially we describe the ECN flags as specified in RFC 2481 [26] and
Subsequently, a suggested update is described which would alter the RFC 3168 [27]. Subsequently, a suggested update is described which
behavior of the flags. would alter the behavior of the flags.
In RFC 2481 [24] there are 2 separate flags, the ECT (ECN Capable In RFC 2481 [26] 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'. It is set to '0' by The CE flag is only used if ECT is set to '1'. It is set to '0' by
the sender and can be set to '1' by an ECN capable router in the the sender and can be set to '1' by an ECN capable router in the
network to indicate congestion. Thus the CE flag is expected to be network to indicate congestion. Thus the CE flag is expected to be
randomly set to '1' with a probability dependent upon the congestion randomly set to '1' with a probability dependent upon the congestion
state of the network and the position of the compressor in the path. 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 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 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 sender will rarely, if ever, see the CE bit set to '1'.
A recent draft [35] suggests an alternative view of these 2 bits. A recent, experimental proposal [38] suggests an alternative view of
This considers the two bits together as having 4 possible codepoints. these 2 bits. This considers the two bits together as having 4
Meanings are then assigned to the codepoints: possible 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
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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, in RFC 3095 [31] it considered to be essentially the same. However, in RFC 3095 [34] it
is noted that the IP ID is generally inferred from the RTP Sequence is noted that the IP ID is generally inferred from the RTP Sequence
Number. There is no obvious candidate in the TCP case for a field to Number. There is no obvious candidate in the TCP case for a field to
offer this '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,
skipping to change at page 23, line 8 skipping to change at page 22, line 44
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.
It would also be desirable to be able to predict the sequence number It would also be desirable to be able to predict the sequence number
from some regularity. However, this also appears to be difficult to from some regularity. However, this also appears to be difficult to
do. For example, during bulk data transfer the sequence number will do. For example, during bulk data transfer the sequence number will
tend to go up by 1 MSS per packet (assuming no packet loss). Higher tend to go up by 1 MSS per packet (assuming no packet loss). Higher
level values have been seen to have an impact as well, where sequence level values have been seen to have an impact as well, where sequence
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
[3] commonly applies, coalescing small packets where possible to [3] commonly applies, coalescing small packets where possible to
reduce the basic header overhead. This may also mean that is less reduce the basic header overhead. This may also mean that is less
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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. The algorithm is specified in RFC 2581 [28]. every 2 data segments. The algorithm is specified in RFC 2581 [31].
An ACK need not always be send immediately on receipt of a data An ACK need not always be send immediately on receipt of a data
segment, but must be sent within 500ms and should be generated for at segment, but must be sent within 500ms and should be generated for at
least every second full sized segment (MSS) of received data. It may least every second full sized segment (MSS) of received data. It may
be seen from this that the delta for the acknowledgement number is be seen from this that the delta for the acknowledgement number is
roughly twice that of the sequence number. This is not always the roughly twice that of the sequence number. This is not always the
case and the discussion about sequence number irregularity should be case and the discussion about sequence number irregularity should be
applied. 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
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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.
5.2.3 Reserved 5.2.3 Reserved
This field is reserved and as such might be expected to be zero. This field is reserved and as such might be expected to be zero.
This can no longer be assumed due to future proofing as it is only a This can no longer be assumed due to future proofing as it is only a
matter of time before a suggestion for using the flag is made. matter of time before a suggestion for using the flag is made.
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). The nonce- either be unused [always 0] or randomly changing). The
sum is the 1-bit sum of the ECT codepoints, as described in nonce-sum is the 1-bit sum of the ECT codepoints, as described
[35]. in [38].
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. The flag will be set once per be set in individual packets. The flag will be set once per
loss event. Thus, the probability of it being set is loss event. Thus, the probability of it being set is
proportional to the degree of congestion in the network, but proportional to the degree of congestion in the network, but
less likely to be set than the CE flag. less likely to be set than the CE flag.
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
special meaning: special meaning:
CWR + ECN-E are both set with SYN to indicate desire to use ECN CWR + ECN-E are both set with SYN to indicate desire to use ECN
CWR only is set in SYN-ACK to agree ECN CWR only is set in SYN-ACK to agree ECN
(The difference in bit-patterns for the negotiation is so that it (The difference in bit-patterns for the negotiation is so that it
will work with broken stacks that reflect the value of reserved will work with broken stacks that reflect the value of reserved
bits) bits)
o URG (Urgent Flag) o URG (Urgent Flag)
'1' to indicate urgent data [unlikely with any flag other than '1' to indicate urgent data [unlikely with any flag other than
ACK] ACK]
o ACK (Acknowledgement) o ACK (Acknowledgement)
'1' for all except the initial 'SYN' packet '1' for all except the initial 'SYN' packet
o PSH (Push Function Field) o PSH (Push Function Field)
generally accepted to be randomly '0' or '1'. However, may be generally accepted to be randomly '0' or '1'. However, may be
biased more to one value than the other (this is largely down biased more to one value than the other (this is largely down
to the implementation of the stack) to the implementation of the stack)
o RST (Reset Connection) o RST (Reset Connection)
'1' to reset a connection [unlikely with any flag other than '1' to reset a connection [unlikely with any flag other than
ACK] ACK]
o SYN (Synchronize Sequence Number) o SYN (Synchronize Sequence Number)
'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 [6] Carried as the end-to-end check for the TCP data. See RFC 1144 [6]
for a discussion of why this should be carried. A header compression for a discussion of why this should be carried. A header compression
scheme should not rely upon the TCP checksum for robustness, though, scheme should not rely upon the TCP checksum for robustness, though,
and should apply appropriate error-detection mechanisms of its own. and should apply appropriate error-detection mechanisms of its own.
The TCP checksum has to be considered as randomly changing. The TCP checksum has to be considered as randomly changing.
skipping to change at page 26, line 48 skipping to change at page 26, line 14
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 [38] associated RFCs, if applicable, from
<http://www.iana.org/assignments/tcp-parameters>
+------+--------+------------------------------------+----------+-----+ +------+--------+------------------------------------+----------+-----+
| Kind | Length | Meaning | RFC | Use | | Kind | Length | Meaning | RFC | Use |
| |(octets)| | | | | |(octets)| | | |
+------+--------+------------------------------------+----------+-----+ +------+--------+------------------------------------+----------+-----+
| 0 | - | End of Option List | RFC 793 | * | | 0 | - | End of Option List | RFC 793 | * |
| 1 | - | No-Operation | RFC 793 | * | | 1 | - | No-Operation | RFC 793 | * |
| 2 | 4 | Maximum Segment Size | RFC 793 | * | | 2 | 4 | Maximum Segment Size | RFC 793 | * |
| 3 | 3 | WSopt - Window Scale | RFC 1323 | * | | 3 | 3 | WSopt - Window Scale | RFC 1323 | * |
| 4 | 2 | SACK Permitted | RFC 2018 | * | | 4 | 2 | SACK Permitted | RFC 2018 | * |
| 5 | N | SACK | RFC 2018 | * | | 5 | N | SACK | RFC 2018 | * |
skipping to change at page 27, line 38 skipping to change at page 27, line 4
| 20 | | SCPS Capabilities | | | | 20 | | SCPS Capabilities | | |
| 21 | | Selective Negative Acks | | | | 21 | | Selective Negative Acks | | |
| 22 | | Record Boundaries | | | | 22 | | Record Boundaries | | |
| 23 | | Corruption experienced | | | | 23 | | Corruption experienced | | |
| 24 | | SNAP | | | | 24 | | SNAP | | |
| 25 | | Unassigned (released 12/18/00) | | | | 25 | | Unassigned (released 12/18/00) | | |
| 26 | | TCP Compression Filter | | | | 26 | | TCP Compression Filter | | |
+------+--------+------------------------------------+----------+-----+ +------+--------+------------------------------------+----------+-----+
Table 2 Description of common TCP options Table 2 Description of common TCP options
Figure 12
The 'use' column is marked with '*' to indicate those options that The 'use' column is marked with '*' to indicate those options that
are most likely to be seen in TCP flows. 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
the Data Offset field. This is used at the end of all options, the Data Offset field. This is used at the end of all options,
not the end of each option, and need only be used if the end of not the end of each option, and need only be used if the end of
the options would not otherwise coincide with the end of the the options would not otherwise coincide with the end of the
TCP header. Defined in RFC 793 [2]. TCP header. Defined in RFC 793 [2].
There is no data associated with this option, a compression There is no data associated with this option, a compression
scheme must simply be able to encode its presence. scheme must simply be able to encode its presence. However,
note that since this options marks the end of the list and the
TCP options are 4-octet aligned, there may be octets of padding
(defined to be '0' in [2]) after this option.
1. No-Operation 1. No-Operation
This option code may be used between options, for example, to This option code may be used between options, for example, to
align the beginning of a subsequent option on a word boundary. align the beginning of a subsequent option on a word boundary.
There is no guarantee that senders will use this option, so There is no guarantee that senders will use this option, so
receivers must be prepared to process options even if they do receivers must be prepared to process options even if they do
not begin on a word boundary RFC 793 [2]. not begin on a word boundary RFC 793 [2].
There is no data associated with this option, a compression There is no data associated with this option, a compression
scheme must simply be able to encode its presence. scheme must simply be able to encode its presence.
This may be done by noting that the option simply maintains a This may be done by noting that the option simply maintains a
certain alignment and that compression need only convey this certain alignment and that compression need only convey this
alignment. In this way, padding can just be removed. alignment. In this way, padding can just be removed.
2. Maximum Segment Size 2. Maximum Segment Size
If this option is present, then it communicates the maximum If this option is present, then it communicates the maximum
receive segment size at the TCP that sends this segment. This receive segment size at the TCP that sends this segment. This
field must only be sent in the initial connection request field must only be sent in the initial connection request
(i.e., in segments with the SYN control bit set). If this (i.e., in segments with the SYN control bit set). If this
option is not used, any segment size is allowed RFC 793 [2]. option is not used, any segment size is allowed RFC 793 [2].
This option is very common. The segment size is a 16-bit This option is very common. The segment size is a 16-bit
quantity. Theoretically this could take any value, however quantity. Theoretically this could take any value, however
there are a number of values that are more common. For there are a number of values that are more common. For
example, 1460 bytes is very common for TCP/IPv4 over Ethernet example, 1460 bytes is very common for TCP/IPv4 over Ethernet
(though with the increased prevalence of tunnels, for example, (though with the increased prevalence of tunnels, for example,
smaller values such as 1400 have become more popular). 536 smaller values such as 1400 have become more popular). 536
bytes is the default MSS value. This may allow for common bytes is the default MSS value. This may allow for common
values to be encoded more efficiently. values to be encoded more efficiently.
3. Window Scale Option (WSopt) 3. Window Scale Option (WSopt)
skipping to change at page 30, line 13 skipping to change at page 29, line 9
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 [33] 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 [4]. 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 [4]). 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 31, line 21 skipping to change at page 30, line 7
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 [9]. 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 are exchanged in the connection set-up phase, then they are
expected to appear on all subsequent segments. If this expected to appear on all subsequent segments. If this
exchange does not happen, then they will not appear for the exchange does not happen, then they will not appear for the
remainder of the flow. 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 [9]). 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 [32]. However, the discussed in, for example, RFC 3150 [35]. However, the
proposed Eifel algorithm [36] makes use of timestamps and so, proposed Eifel algorithm [39] 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 [34]. cellular-type links [37].
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 [9] 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 [12] 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.
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.
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 [11]. 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 [11]. 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 [11]. 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
skipping to change at page 33, line 29 skipping to change at page 31, line 42
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 [7] 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 [7]. 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.
19. MD5 Digest 19. MD5 Digest
Every segment sent on a TCP connection to be protected against Every segment sent on a TCP connection to be protected against
spoofing will contain the 16-byte MD5 digest produced by spoofing will contain the 16-byte MD5 digest produced by
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
[9]), 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
SYN segments. This is not a problem for this option, since the non-SYN segments. This is not a problem for this option, since
SYN, ACK sent during connection negotiation will not be signed the SYN, ACK sent during connection negotiation will not be
and will thus be ignored. The connection will never be made, signed and will thus be ignored. The connection will never be
and non-SYN segments with options will never be sent. More made, and non-SYN segments with options will never be sent.
importantly, the sending of signatures must be under the More 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 This only means that their behavior is not described in detail
as a compression scheme is not expected to be optimised for as a compression scheme is not expected to be optimised for
these options. However any unrecognised option must be these options. However any unrecognised option must be
transparently carried by a TCP compression scheme in order to transparently carried by a TCP compression scheme in order to
work efficiently in the presence of new or rare options. 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
skipping to change at page 36, line 11 skipping to change at page 34, line 11
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 [36]
and [32]. Most of these schemes are entirely compatible with header and [35]. 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 36, line 39 skipping to change at page 34, line 39
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 [15] than HTTP/1.1 browsing (this is more the case with HTTP/1.0 [15] than HTTP/1.1
[20]). [22]).
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 12 skipping to change at page 35, line 12
It is noted that although TCP is connection oriented, it is hard for It is noted that although TCP is connection oriented, it is hard for
a compressor to tell whether or not a TCP flow has finished. For a compressor to tell whether or not a TCP flow has finished. For
example, even in the 'bi-directional' link case, seeing a FIN and the example, even in the 'bi-directional' link case, seeing a FIN and the
ACK of the FIN at the compressor/decompressor does not mean that the ACK of the FIN at the compressor/decompressor does not mean that the
FIN cannot be retransmitted. Thus it may be more useful to think FIN cannot be retransmitted. Thus it may be more useful to think
about initializing a new context from an existing one, rather than about initializing a new context from an existing one, rather than
re-using an existing one. re-using an existing one.
As mentioned previously, in Section 5.1.3, the IP header can clearly As mentioned previously, in Section 5.1.3, the IP header can clearly
be shared between any transport-layer flows between the same two end- be shared between any transport-layer flows between the same two
points. There may be limited scope for initialisation of a new TCP end-points. There may be limited scope for initialisation of a new
header from an existing one. The port numbers are the most obvious TCP header from an existing one. The port numbers are the most
starting point. obvious starting point.
6.6 Master Sequence Number 6.6 Master Sequence Number
As pointed out earlier in Section 5.1.3 there is no obvious candidate As pointed out earlier in Section 5.1.3 there is no obvious candidate
for a 'master sequence number' in TCP. Moreover, it is noted that for a 'master sequence number' in TCP. Moreover, it is noted that
such a master sequence number is only required to allow a such a master sequence number is only required to allow a
decompressor to acknowledge packets in bi-directional mode. It can decompressor to acknowledge packets in bi-directional mode. It can
also be seen that such a sequence number would not be required for also be seen that such a sequence number would not be required for
every packet. every packet.
skipping to change at page 38, line 33 skipping to change at page 36, line 33
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
Fairhurst. Fairhurst.
Qian Zhang and Hongbin Liao contributed the extensive analysis of Qian Zhang and Hongbin Liao contributed the extensive analysis of
shareable header fields. shareable header fields.
Any remaining misrepresentation or misinterpretation of information Any remaining misrepresentation or misinterpretation of information
is entirely the fault of the authors of this draft. is entirely the fault of the authors.
References 9. References
9.1 Normative References
9.2 Informative 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] Nagle, J., "Congestion control in IP/TCP internetworks", RFC [3] Nagle, J., "Congestion control in IP/TCP internetworks", RFC
896, January 1984. 896, January 1984.
skipping to change at page 39, line 26 skipping to change at page 37, line 29
[10] 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.
[11] 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.
[12] 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.
[13] Atkinson, R., "IP Authentication Header", RFC 1826, August [13] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
1995. November 1998.
[14] Atkinson, R., "IP Encapsulating Security Payload (ESP)", RFC [14] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
1827, August 1995. (ESP)", RFC 2406, November 1998.
[15] Berners-Lee, T., Fielding, R. and H. Nielsen, "Hypertext [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.
[16] Bellovin, S., "Defending Against Sequence Number Attacks", RFC [16] Bellovin, S., "Defending Against Sequence Number Attacks", RFC
1948, May 1996. 1948, May 1996.
[17] 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.
[18] 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.
[19] 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.
[20] Fielding, R., Gettys, J., Mogul, J., Nielsen, H. and T. [20] Bradner, S., "IETF Rights in Contributions", BCP 78, RFC 3667,
February 2004.
[21] Bradner, S., "Intellectual Property Rights in IETF Technology",
BCP 79, RFC 3668, February 2004.
[22] 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.
[21] Bradner, S., "Key words for use in RFCs to Indicate Requirement [23] 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.
[22] Heffernan, A., "Protection of BGP Sessions via the TCP MD5 [24] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998. Signature Option", RFC 2385, August 1998.
[23] Nichols, K., Blake, S., Baker, F. and D. Black, "Definition of [25] 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.
[24] Ramakrishnan, K. and S. Floyd, "A Proposal to add Explicit [26] 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.
[25] Degermark, M., Nordgren, B. and S. Pink, "IP Header [27] Ramakrishnan, K., Floyd, S. and D. Black, "The Addition of
Explicit Congestion Notification (ECN) to IP", RFC 3168,
September 2001.
[28] Degermark, M., Nordgren, B. and S. Pink, "IP Header
Compression", RFC 2507, February 1999. Compression", RFC 2507, February 1999.
[26] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP Headers for [29] 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.
[27] Engan, M., Casner, S. and C. Bormann, "IP Header Compression [30] Engan, M., Casner, S. and C. Bormann, "IP Header Compression
over PPP", RFC 2509, February 1999. over PPP", RFC 2509, February 1999.
[28] Allman, M., Paxson, V. and W. Stevens, "TCP Congestion [31] Allman, M., Paxson, V. and W. Stevens, "TCP Congestion
Control", RFC 2581, April 1999. Control", RFC 2581, April 1999.
[29] Bradner, S. and V. Paxson, "IANA Allocation Guidelines For [32] 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.
[30] Floyd, S., Mahdavi, J., Mathis, M. and M. Podolsky, "An [33] 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.
[31] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H., [34] 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.
[32] Dawkins, S., Montenegro, G., Kojo, M. and V. Magret, "End-to- [35] Dawkins, S., Montenegro, G., Kojo, M. and V. Magret,
end Performance Implications of Slow Links", BCP 48, RFC 3150, "End-to-end Performance Implications of Slow Links", BCP 48,
July 2001. RFC 3150, July 2001.
[33] Balakrishnan, , Padmanabhan, V., Fairhurst, G. and M. [36] Balakrishnan, Padmanabhan, V., Fairhurst, G. and M.
Sooriyabandara, "TCP Performance Implications of Network Path Sooriyabandara, "TCP Performance Implications of Network Path
Asymmetry", RFC 3449, December 2002. Asymmetry", RFC 3449, December 2002.
[34] Inamura, H., Montenegro, G., Ludwig, R., Gurtov, A. and F. [37] Inamura, H., Montenegro, G., Ludwig, R., Gurtov, A. and F.
Khafizov, "TCP over Second (2.5G) and Third (3G) Generation Khafizov, "TCP over Second (2.5G) and Third (3G) Generation
Wireless Networks", RFC 3481, February 2003. Wireless Networks", RFC 3481, February 2003.
[35] Spring, N., Wetherall, D. and D. Ely, "Robust ECN Signaling [38] Spring, N., Wetherall, D. and D. Ely, "Robust Explicit
with Nonces", draft-ietf-tsvwg-tcp-nonce-04.txt (work in Congestion Notification (ECN) Signaling with Nonces", RFC
progress), October 2002. 3540, June 2003.
[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- [39] Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm for
pilc-link-design-13.txt (work in progress), February 2003. TCP", RFC 3522.txt, April 2003.
[38] <http://www.iana.org/assignments/tcp-parameters> [40] Karn, Bormann, C., Fairhurst, G., Grossman, D., Ludwig, R.,
Mahdavi, J., Montenegro, G., Touch, J. and L. Wood, "Advice for
Internet Subnetwork Designers", BCP 89, RFC 3819, July 2004.
Authors' Addresses Authors' Addresses
Mark A. West Mark A. West
Siemens/Roke Manor Siemens/Roke Manor Research
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
EMail: mark.a.west@roke.co.uk EMail: mark.a.west@roke.co.uk
URI: http://www.roke.co.uk URI: http://www.roke.co.uk
Stephen McCann Stephen McCann
Siemens/Roke Manor Siemens/Roke Manor Research
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 Intellectual Property Statement
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