Network Working Group M. West
Internet-Draft S. McCann
Expires: September 1, 2003 Siemens/Roke Manor
March 3, 2003
TCP/IP Field Behavior
draft-ietf-rohc-tcp-field-behavior-02.txt
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This memo describes TCP/IP field behavior in the context of header
compression.
Header compression is possible thanks to the fact that most header
fields do not vary randomly from packet to packet. Many of the
fields exhibit static behavior or change in a more or less
predictable way. When designing a header compression scheme, it is
of fundamental importance to understand the behavior of the fields in
detail. An example of this analysis can be seen in RFC 3095 [31].
This memo performs a similar role for the compression of TCP/IP.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 5
3. General classification . . . . . . . . . . . . . . . . . . . 5
3.1 IP header fields . . . . . . . . . . . . . . . . . . . . . . 6
3.1.1 IPv6 header fields . . . . . . . . . . . . . . . . . . . . . 6
3.1.2 IPv4 header fields . . . . . . . . . . . . . . . . . . . . . 7
3.2 TCP header fields . . . . . . . . . . . . . . . . . . . . . 10
3.3 Summary for IP/TCP . . . . . . . . . . . . . . . . . . . . . 11
4. Classification of replicable header fields . . . . . . . . . 12
4.1 IPv4 Header (inner and/or outer) . . . . . . . . . . . . . . 13
4.2 IPv6 Header (inner and/or outer) . . . . . . . . . . . . . . 14
4.3 TCP Header . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.4 TCP Options . . . . . . . . . . . . . . . . . . . . . . . . 16
4.5 Summary of replication . . . . . . . . . . . . . . . . . . . 16
5. Analysis of change patterns of header fields . . . . . . . . 17
5.1 IP header . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.1.1 IP Traffic-Class / Type-Of-Service (TOS) . . . . . . . . . . 19
5.1.2 ECN Flags . . . . . . . . . . . . . . . . . . . . . . . . . 20
5.1.3 IP Identification . . . . . . . . . . . . . . . . . . . . . 20
5.1.4 Don't Fragment (DF) flag . . . . . . . . . . . . . . . . . . 22
5.1.5 IP Hop-Limit / Time-To-Live (TTL) . . . . . . . . . . . . . 23
5.2 TCP header . . . . . . . . . . . . . . . . . . . . . . . . . 23
5.2.1 Sequence number . . . . . . . . . . . . . . . . . . . . . . 23
5.2.2 Acknowledgement number . . . . . . . . . . . . . . . . . . . 24
5.2.3 Reserved . . . . . . . . . . . . . . . . . . . . . . . . . . 25
5.2.4 Flags . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
5.2.5 Checksum . . . . . . . . . . . . . . . . . . . . . . . . . . 26
5.2.6 Window . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
5.2.7 Urgent pointer . . . . . . . . . . . . . . . . . . . . . . . 27
5.3 Options . . . . . . . . . . . . . . . . . . . . . . . . . . 27
5.3.1 Options overview . . . . . . . . . . . . . . . . . . . . . . 27
5.3.2 Option field behavior . . . . . . . . . . . . . . . . . . . 28
6. Other observations . . . . . . . . . . . . . . . . . . . . . 36
6.1 Implicit acknowledgements . . . . . . . . . . . . . . . . . 36
6.2 Shared data . . . . . . . . . . . . . . . . . . . . . . . . 36
6.3 TCP header overhead . . . . . . . . . . . . . . . . . . . . 36
6.4 Field independence and packet behavior . . . . . . . . . . . 37
6.5 Short-lived flows . . . . . . . . . . . . . . . . . . . . . 37
6.6 Master Sequence Number . . . . . . . . . . . . . . . . . . . 38
6.7 Size constraint for TCP options . . . . . . . . . . . . . . 38
7. Security considerations . . . . . . . . . . . . . . . . . . 39
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 39
References . . . . . . . . . . . . . . . . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 42
Full Copyright Statement . . . . . . . . . . . . . . . . . . 43
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1. Introduction
This document describes TCP/IP field behavior, as it is essential to
understand this before correct assumptions about header compression
can be made.
Since the IP header does exhibit some slightly different behavior
from that previously presented in RFC 3095 [31] for the RTP case, it
is also included in this document.
It is intentional that much of the classification text from RFC 3095
[31] has been borrowed. This is for easier reading rather than
inserting many references to that document.
Again based on the format presented in RFC 3095 [31] TCP/IP header
fields are classified and analyzed in two steps. First, we have a
general classification in Section 3 where the fields are classified
on the basis of stable knowledge and assumptions. The general
classification does not take into account the change characteristics
of changing fields because those will vary more or less depending on
the implementation and on the application used. Section 4 considers
how field values can be used to optimize short-lived flows. A less
stable but more detailed analysis of the change characteristics is
then done in Section 5. Finally, Section 6 summarizes with
conclusions about how the various header fields should be handled by
the header compression scheme to optimize compression and
functionality.
A general question raised by this analysis is that of what 'baseline'
definition of all possible TCP/IP implementations is to be
considered? For the purposes of this document, a relatively up-to-
date (as of the time of writing) implementation is considered, with a
view to ensuring compatibility with legacy implementations.
The general requirement for transparency is also seen to be more
interesting. A number of recent proposals for extensions to TCP make
use of some of the previously 'reserved' bits. It is therefore clear
that a 'reserved' bit cannot be taken to have a guaranteed zero
value, but may change. Ideally, this should be accommodated by the
compression profile.
It is unclear exactly how reserved bits should be handled, given that
the possible future uses cannot be predicted. It is accepted that if
these currently reserved bits were used, then efficiency may be
reduced. However, the compression scheme should still offer a useful
solution.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [21].
3. General classification
The following definitions (and some text) are copied from RFC 3095
[31] Appendix A. Differences between IP field behavior between RFC
3095 [31] (i.e. IP/UDP/RTP behavior for audio and video
applications) and this document have been identified.
At a general level, the header fields are separated into 5 classes:
o INFERRED
These fields contain values that can be inferred from other
values, for example the size of the frame carrying the packet,
and thus do not have to be handled at all by the compression
scheme.
o STATIC
These fields are expected to be constant throughout the
lifetime of the packet stream. Static information must in some
way be communicated once.
o STATIC-DEF
STATIC fields whose values define a packet stream. They are in
general handled as STATIC.
o STATIC-KNOWN
These STATIC fields are expected to have well-known values and
therefore do not need to be communicated at all.
o CHANGING
These fields are expected to vary in some way: randomly, within
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
one of these classes. For all fields except those classified as
CHANGING, the motives for the classification are also stated. In
section 4, CHANGING fields are further examined and classified on the
basis of their expected change behavior.
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3.1 IP header fields
3.1.1 IPv6 header fields
+---------------------+-------------+----------------+
| Field | Size (bits) | Class |
+---------------------+-------------+----------------+
| Version | 4 | STATIC |
| DSCP* | 6 | CHANGING |
| ECT flag* | 1 | CHANGING |
| CE flag* | 1 | CHANGING |
| Flow Label | 20 | STATIC-DEF |
| Payload Length | 16 | INFERRED |
| Next Header | 8 | STATIC |
| Hop Limit | 8 | CHANGING |
| Source Address | 128 | STATIC-DEF |
| Destination Address | 128 | STATIC-DEF |
+---------------------+-------------+----------------+
* differs from RFC 3095 [31]
[The DSCP, ECT and CE flags were amalgamated into the Traffic Class
octet in RFC 3095.]
o Version
The version field states which IP version is used. Packets
with different values in this field must be handled by
different IP stacks. All packets of a packet stream must
therefore be of the same IP version. Accordingly, the field is
classified as STATIC.
o Flow Label
This field may be used to identify packets belonging to a
specific packet stream. If not used, the value should be set
to zero. Otherwise, all packets belonging to the same stream
must have the same value in this field, it being one of the
fields that define the stream. The field is therefore
classified as STATIC-DEF.
o Payload Length
Information about packet length (and, consequently, payload
length) is expected to be provided by the link layer. The
field is therefore classified as INFERRED.
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o Next Header
This field will usually have the same value in all packets of a
packet stream. It encodes the type of the subsequent header.
Only when extension headers are sometimes present and sometimes
not, will the field change its value during the lifetime of the
stream. The field is therefore classified as STATIC.
The classification of STATIC is inherited from RFC 3095 [31].
However, it should be pointed out that the next header field is
actually determined by the type of the following header. Thus,
it might be more appropriate to view this as an inference,
although this depends upon the specific implementation of the
compression scheme.
o Source and Destination addresses
These fields are part of the definition of a stream and must
thus be constant for all packets in the stream. The fields are
therefore classified as STATIC-DEF.
This might be considered as a slightly simplistic view.
However for now the IP addresses are associated with the
transport layer connection. More complex flow-separation
could, of course, be considered.
Total size of the fields in each class:
+--------------+--------------+
| Class | Size (octets)|
+--------------+--------------+
| INFERRED | 2 |
| STATIC | 1.5 |
| STATIC-DEF | 34.5 |
| STATIC-KNOWN | 0 |
| CHANGING | 2 |
+--------------+--------------+
3.1.2 IPv4 header fields
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+---------------------+-------------+----------------+
| Field | Size (bits) | Class |
+---------------------+-------------+----------------+
| Version | 4 | STATIC |
| Header Length | 4 | STATIC-KNOWN |
| DSCP* | 6 | CHANGING |
| ECT flag* | 1 | CHANGING |
| CE flag* | 1 | CHANGING |
| Packet Length | 16 | INFERRED |
| Identification | 16 | CHANGING |
| Reserved flag* | 1 | CHANGING |
| Don't Fragment flag*| 1 | CHANGING |
| More Fragments flag | 1 | STATIC-KNOWN |
| Fragment Offset | 13 | STATIC-KNOWN |
| Time To Live | 8 | CHANGING |
| Protocol | 8 | STATIC |
| Header Checksum | 16 | INFERRED |
| Source Address | 32 | STATIC-DEF |
| Destination Address | 32 | STATIC-DEF |
+---------------------+-------------+----------------+
* differs from RFC 3095 [31]
[The DSCP, ECT and CE flags were amalgamated into the TOS octet in
RFC 3095.
The DF flag behavior is considered later.
The reserved field is discussed below.]
o Version
The version field states which IP version is used. Packets
with different values in this field must be handled by
different IP stacks. All packets of a packet stream must
therefore be of the same IP version. Accordingly, the field is
classified as STATIC.
o Header Length
As long as no options are present in the IP header, the header
length is constant and well known. If there are options, the
fields would be STATIC, but it is assumed here that there are
no options. The field is therefore classified as STATIC-KNOWN.
o Packet Length
Information about packet length is expected to be provided by
the link layer. The field is therefore classified as INFERRED.
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o Flags
The Reserved flag must be set to zero, as defined in RFC 791
[1]. In RFC 3095 [31] the field is therefore classified as
STATIC-KNOWN. However, it is expected that reserved fields may
be used at some future point. It appears unwise to select an
encoding that would preclude the use of a compression profile
for a future change in the use of reserved fields. For this
reason the alternative encoding of CHANGING is suggested. It
would also be possible to have more than one compression
profile, in one of which this field was considered to be
STATIC-KNOWN.
The More Fragments (MF) flag is expected to be zero because
fragmentation is generally not expected. As discussed in the
RTP case, only the first fragment will contain the transport
layer protocol header; subsequent fragments would have to be
compressed with a different profile. In terms of the effect of
header overhead, if fragmentation does occur then the first
fragment, by definition, should be relatively large, minimizing
the header overhead. In the case of TCP, fragmentation should
not be common due to a combination of initial MSS negotiation
and subsequent use of path-MTU discovery. The More Fragments
flag is therefore classified as STATIC-KNOWN. However, a
profile could accept that this flag may be set in order to cope
with fragmentation.
o Fragment Offset
Under the assumption that no fragmentation occurs, the fragment
offset is always zero. The field is therefore classified as
STATIC-KNOWN. Even if fragmentation were to be further
considered, then only the first fragment would contain the TCP
header and the fragment offset of this packet would still be
zero.
o Protocol
This field will usually have the same value in all packets of a
packet stream. It encodes the type of the subsequent header.
Only when extension headers are sometimes present and sometimes
not, will the field change its value during the lifetime of a
stream. The field is therefore classified as STATIC.
o Header Checksum
The header checksum protects individual hops from processing a
corrupted header. When almost all IP header information is
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compressed away, there is no point in having this additional
checksum; instead it can be regenerated at the decompressor
side. The field is therefore classified as INFERRED.
We note that the TCP checksum does not protect the whole TCP/IP
header, but only the TCP pseudo-header (and the payload).
Compare this with ROHC [31], which uses a CRC to verify the
uncompressed header. Given the need to validate the complete
TCP/IP header; the cost of computing the TCP checksum over the
entire payload; and known weaknesses in the TCP checksum [37],
an additional check is necessary. Therefore, it is expected
than some additional checksum (such as a CRC) will be used to
validate correct decompression.
o Source and Destination addresses
These fields are part of the definition of a stream and must
thus be constant for all packets in the stream. The fields are
therefore classified as STATIC-DEF.
Total size of the fields in each class:
+--------------+--------------+
| Class | Size (octets)|
+--------------+--------------+
| INFERRED | 4 |
| STATIC* | 1.5 |
| STATIC-DEF | 8 |
| STATIC-KNOWN*| 2.25 |
| CHANGING* | 4.25 |
+--------------+--------------+
* differs from RFC 3095 [31]
3.2 TCP header fields
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+---------------------+-------------+----------------+
| Field | Size (bits) | Class |
+---------------------+-------------+----------------+
| Source Port | 16 | STATIC-DEF |
| Destination Port | 16 | STATIC-DEF |
| Sequence Number | 32 | CHANGING |
| Acknowledgement Num | 32 | CHANGING |
| Data Offset | 4 | INFERRED |
| Reserved | 4 | CHANGING |
| CWR flag | 1 | CHANGING |
| ECE flag | 1 | CHANGING |
| URG flag | 1 | CHANGING |
| ACK flag | 1 | CHANGING |
| PSH flag | 1 | CHANGING |
| RST flag | 1 | CHANGING |
| SYN flag | 1 | CHANGING |
| FIN flag | 1 | CHANGING |
| Window | 16 | CHANGING |
| Checksum | 16 | CHANGING |
| Urgent Pointer | 16 | CHANGING |
| Options | 0(-352) | CHANGING |
+---------------------+-------------+----------------+
o Source and Destination ports
These fields are part of the definition of a stream and must
thus be constant for all packets in the stream. The fields are
therefore classified as STATIC-DEF.
o Data Offset
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.
It can be re-constructed from the length of any options and
thus it is not necessary to carry this explicitly. The field
is therefore classified as INFERRED.
3.3 Summary for IP/TCP
Summarizing this for IP/TCP one obtains
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+----------------+----------------+----------------+
| Class \ IP ver | IPv6 (octets) | IPv4 (octets) |
+----------------+----------------+----------------+
| INFERRED | 2 + 4 bits | 4 + 4 bits |
| STATIC | 1 + 4 bits | 1 + 4 bits |
| STATIC-DEF | 38 + 4 bits | 12 |
| STATIC-KNOWN | - | 2 + 2 bits |
| CHANGING | 17 + 4 bits | 19 + 6 bits |
+----------------+----------------+----------------+
| Totals | 60 | 40 |
+----------------+----------------+----------------+
(excludes options, which are all classified as CHANGING)
4. Classification of replicable header fields
Where multiple flows either overlap in time or occur sequentially
within a short space of time there can be a great deal of similarity
in header field values. Such commonality of field values is
reflected in the compression context. Thus, it should be possible to
utilise links between fields across different flows to improve the
compression ratio. In order to do this, it is important to
understand the 'replicable' characteristics of the various header
fields.
The key concept is that of 'replication', where an existing context
is used as a baseline and replicated to initialise a new context.
Those fields that are the same are then automatically initialised in
the new context. Those that have changed will be updated or
overwritten with values from the initialisation packet that triggered
the replication. This section considers the commonality between
fields in different flows.
It should be noted, however, that replication is based on contexts
(rather than just field values) and so compressor created fields that
are part of the context may also be included. These, of course, are
dependent upon the nature of the compression protocol (ROHC profile)
being applied.
A brief analysis of the relationship of TCP/IP fields among
'replicable' packet streams follows.
'N/A' -- The field need not be considered in the replication
process as it is inferred or known 'a priori' (and,
therefore, does not appear in the context).
'No' -- The field cannot be replicated since its change pattern
between two packet flows is uncorrelated.
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'Yes' -- The field may be replicated. This does not guarantee
that the field value will be the same across two candidate
streams, only that it might be possible to exploit
replication to increase the compression ratio. Specific
encoding methods can be used to improve the compression
efficiency.
4.1 IPv4 Header (inner and/or outer)
+-----------------------+---------------+------------+
| Field | Class | Replicable |
+-----------------------+---------------+------------+
| Version | STATIC | N/A |
| Header Length | STATIC-KNOWN | N/A |
| DSCP | CHANGING | No (1) |
| ECT flag | CHANGING | No (2) |
| CE flag | CHANGING | No (2) |
| Packet Length | INFERRED | N/A |
| Identification | CHANGING | Yes (3) |
| Reserved flag | CHANGING | No (4) |
| Don't Fragment flag | CHANGING | No |
| More Fragments flag | STATIC-KNOWN | N/A |
| Fragment Offset | STATIC-KNOWN | N/A |
| Time To Live | CHANGING | Yes |
| Protocol | STATIC | N/A |
| Header Checksum | INFERRED | N/A |
| Source Address | STATIC-DEF | Yes |
| Destination Address | STATIC-DEF | Yes |
+-----------------------+---------------+------------+
(1) The DSCP is marked based on the application's requirements. If
it can be assumed that replicable connections often carry the same
type of traffic, the DSCP may be regarded as replicable. However,
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
use of the ECN nonce scheme [35] is anticipated). However, it
seems likely that all TCP flows between ECN-capable hosts will use
ECN, the use (or not) of ECN for flows between the same end-points
might be considered replicable. See also note (4).
(3) The replicable context for this field includes the IP-ID, NBO,
and RND flags (as described in ROHC RTP). This highlights that
the replication is of the context, rather than just the header
field values and, as such, needs to be considered based on the
exact nature of compression applied to each field.
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(4) Since the possible future behavior of the 'Reserved Flag' cannot
be predicted, it is not considered as replicable. However, it
might be expected that the behavior of the reserved flag between
the same end-points will be similar. In this case, any selection
of packet formats (for example) based on this behavior might carry
across to the new flow. In the case of packet formats, this can
probably be considered as a compressor-local decision.
4.2 IPv6 Header (inner and/or outer)
+-----------------------+---------------+------------+
| Field | Class | Replicable |
+-----------------------+---------------+------------+
| Version | STATIC | N/A |
| Traffic Class | CHANGING | Yes (1) |
| ECT flag | CHANGING | No (2) |
| CE flag | CHANGING | No (2) |
| Flow Label | STATIC-DEF | N/A |
| Payload Length | INFERRED | N/A |
| Next Header | STATIC | N/A |
| Hop Limit | CHANGING | Yes |
| Source Address | STATIC-DEF | Yes |
| Destination Address | STATIC-DEF | Yes |
+-----------------------+---------------+------------+
(1) See comment about DSCP field for IPv4, above.
(2) See comment about ECT and CE flags for IPv4, above.
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4.3 TCP Header
+-----------------------+---------------+------------+
| Field | Class | Replicable |
+-----------------------+---------------+------------+
| Source Port | STATIC-DEF | Yes (1) |
| Destination Port | STATIC-DEF | Yes (1) |
| Sequence Number | CHANGING | No (2) |
| Acknowledgement Number| CHANGING | No |
| Data Offset | INFERRED | N/A |
| Reserved Bits | CHANGING | No (3) |
| Flags | | |
| CWR | CHANGING | No (4) |
| ECE | CHANGING | No (4) |
| URG | CHANGING | No |
| ACK | CHANGING | No |
| PSH | CHANGING | No |
| RST | CHANGING | No |
| SYN | CHANGING | No |
| FIN | CHANGING | No |
| Window | CHANGING | Yes |
| Checksum | CHANGING | No |
| Urgent Pointer | CHANGING | Yes (5) |
+-----------------------+---------------+------------+
(1) On the server side, the port number is likely to be a well-known
value. On the client side, the port number is generally selected
by the stack automatically. Whether the port number is replicable
depends upon how the stack chooses the port number. However, most
implementations use a simple scheme which sequentially picks the
next available port number. This is clearly exploitable in a
compression scheme.
(2) With the recommendation (and expected deployment) of TCP Initial
Sequence Number randomization, defined in RFC 1948 [16], it will
be impossible to share the sequence number. Thus, this field will
not be regarded as replicable.
(3) See comment (4) for the IPv4 header, above.
(4) See comment (2) on ECN flags for the IPv4 header, above.
(5) The urgent pointer is very rarely used. This means that, in
practice, the field may be considered replicable.
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4.4 TCP Options
+---------------------------+--------------+------------+
| Option | SYN-only (1) | Replicable |
+---------------------------+--------------+------------+
| End of Option List | No | No (2) |
| No-Operation | No | No (2) |
| Maximum Segment Size | Yes | Yes |
| Window Scale | Yes | Yes |
| SACK-Permitted | Yes | Yes |
| SACK | No | No |
| Timestamp | No | No |
+---------------------------+--------------+------------+
(1) This indicates whether the option only appears in SYN packet or
not. Options that are not 'SYN-only' may appear in any packet.
Many TCP options are used only in SYN packets. Some options, such
as MSS, Window Scale, SACK-Permitted etc., will tend to have the
same value among replicable packet streams.
Thus, to support context sharing, the compressor should maintain
such TCP options in the context (even though they only appear in
the SYN segment).
(2) Since these options have fixed values, they could be regarded as
replicable. However, the only interesting thing to convey about
these options is their presence: if it is known that such an
option exists, its value is defined.
4.5 Summary of replication
From the above analysis, it can be seen that there are reasonable
grounds for exploiting redundancy between flows, as well as between
packets within a flow. Simply consider the advantage of being able
to elide the source and destination addresses for a repeated
connection between two IPv6 endpoints. There will also be a cost (in
terms of complexity and robustness) for replicating contexts, and
this must be considered when deciding what constitutes an appropriate
solution.
The final point to note for the use of replication is that it
requires the compressor to have a suitable degree of confidence that
the source data is present and correct at the decompressor. This may
place some restrictions on which of the 'changing' fields, in
particular, can be utilised during replication.
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5. Analysis of change patterns of header fields
To design suitable mechanisms for efficient compression of all header
fields, their change patterns must be analyzed. For this reason, an
extended classification is done based on the general classification
in 2, considering the fields which were labeled CHANGING in that
classification.
The CHANGING fields are separated into five different subclasses:
o STATIC
These are fields that were classified as CHANGING on a general
basis, but are classified as STATIC here due to certain
additional assumptions.
o SEMISTATIC
These fields are STATIC most of the time. However,
occasionally the value changes but reverts to its original
value after a known number of packets.
o RARELY-CHANGING (RC)
These are fields that change their values occasionally and then
keep their new values.
o ALTERNATING
These fields alternate between a small number of different
values.
o IRREGULAR
These, finally, are the fields for which no useful change
pattern can be identified.
To further expand the classification possibilities without increasing
complexity, the classification can be done either according to the
values of the field and/or according to the values of the deltas for
the field.
When the classification is done, other details are also stated
regarding possible additional knowledge about the field values and/or
field deltas, according to the classification. For fields classified
as STATIC or SEMISTATIC, the case could be that the value of the
field is not only STATIC but also well KNOWN a priori (two states for
SEMISTATIC fields). For fields with non-irregular change behavior,
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it could be known that changes usually are within a LIMITED range
compared to the maximal change for the field. For other fields, the
values are completely UNKNOWN.
Table 1 classifies all the CHANGING fields on the basis of their
expected change patterns. (4) refers to IPv4 fields and (6) refers
to IPv6.
+------------------------+-------------+-------------+-------------+
| Field | Value/Delta | Class | Knowledge |
+========================+=============+=============+=============+
| IP TOS(4) / Tr.Class(6)| Value | RC | UNKNOWN |
+------------------------+-------------+-------------+-------------+
| IP ECT flag(4) | Value | RC | UNKNOWN |
+------------------------+-------------+-------------+-------------+
| IP CE flag(4) | Value | RC | UNKNOWN |
+------------------------+-------------+-------------+-------------+
| Sequential | Delta | STATIC | KNOWN |
| -----------+-------------+-------------+-------------+
| IP Id(4) Seq. jump | Delta | RC | LIMITED |
| -----------+-------------+-------------+-------------+
| Random | Value | IRREGULAR | UNKNOWN |
+------------------------+-------------+-------------+-------------+
| IP DF flag(4) | Value | RC | UNKNOWN |
+------------------------+-------------+-------------+-------------+
| IP TTL(4) / Hop Lim(6) | Value | ALTERNATING | LIMITED |
+------------------------+-------------+-------------+-------------+
| TCP Sequence Number | Delta | IRREGULAR | LIMITED |
+------------------------+-------------+-------------+-------------+
| TCP Acknowledgement Num| Delta | IRREGULAR | LIMITED |
+------------------------+-------------+-------------+-------------+
| TCP Reserved | Value | RC | UNKNOWN |
+------------------------+-------------+-------------+-------------+
| TCP flags | | | |
| ECN flags | Value | IRREGULAR | UNKNOWN |
| CWR flag | Value | IRREGULAR | UNKNOWN |
| ECE flag | Value | IRREGULAR | UNKNOWN |
| URG flag | Value | IRREGULAR | UNKNOWN |
| ACK flag | Value | SEMISTATIC | KNOWN |
| PSH flag | Value | IRREGULAR | UNKNOWN |
| RST flag | Value | IRREGULAR | UNKNOWN |
| SYN flag | Value | SEMISTATIC | KNOWN |
| FIN flag | Value | SEMISTATIC | KNOWN |
+------------------------+-------------+-------------+-------------+
| TCP Window | Value | ALTERNATING | KNOWN |
+------------------------+-------------+-------------+-------------+
| TCP Checksum | Value | IRREGULAR | UNKNOWN |
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+------------------------+-------------+-------------+-------------+
| TCP Urgent Pointer | Value | IRREGULAR | KNOWN |
+------------------------+-------------+-------------+-------------+
| TCP Options | Value | IRREGULAR | UNKNOWN |
+------------------------+-------------+-------------+-------------+
Table 1 : Classification of CHANGING header fields
The following subsections discuss the various header fields in
detail. Note that table 1 and the discussions below do not consider
changes caused by loss or reordering before the compression point.
5.1 IP header
5.1.1 IP Traffic-Class / Type-Of-Service (TOS)
The Traffic-Class (IPv6) or Type-Of-Service/DSCP (IPv4) field might
be expected to change during the lifetime of a packet stream. This
analysis considers several RFCs that describe modifications to the
original RFC 791 [1].
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
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
[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
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
or traffic class octet for experimental use with ECN.
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
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
the use of ECN, especially with respect to TCP.
It is also considered important that the profile works with legacy
stacks, since these will be in existence for some considerable time
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
considered to be structured the same way.
The DSCP (as for TOS in ROHC RTP) is not expected to change
frequently (although it could change mid-flow, for example as a
result of a route change).
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5.1.2 ECN Flags
Initially we describe the ECN flags as specified in RFC 2481 [24].
Subsequently, a suggested update is described which would alter the
behavior of the flags.
In RFC 2481 [24] there are 2 separate flags, the ECT (ECN Capable
Transport) flag and the CE (Congestion Experienced) flag. The ECT
flag, if negotiated by the TCP stack, will be '1' for all data
packets and '0' for all 'pure acknowledgement' packets. This means
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.
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
network to indicate congestion. Thus the CE flag is expected to be
randomly set to '1' with a probability dependent upon the congestion
state of the network and the position of the compressor in the path.
So, a compressor located close to the receiver in a congested network
will see the CE bit set frequently, but a compressor located close to
a sender will rarely, if ever, see the CE bit set to '1'.
A recent draft [35] suggests an alternative view of these 2 bits.
This considers the two bits together as having 4 possible codepoints.
Meanings are then assigned to the codepoints:
00 Not ECN capable
01 ECN capable, no congestion [known as ECT(0)]
10 ECN capable, no congestion [known as ECT(1)]
11 Congestion experienced
The use of 2 codepoints for signaling ECT allows the sender to detect
when a receiver is not reliably echoing congestion information.
For the purposes of compression, this update means that ECT(0) and
ECT(1) are equally likely (for an ECN capable flow) and that '11'
will be relatively rarely seen. The probability of seeing a
congestion indication is discussed above in the description of the CE
flag.
It is suggested that, for the purposes of compression, ECN with
nonces is assumed as the baseline, although the compression scheme
must be able to transparently compress the original ECN scheme.
5.1.3 IP Identification
The Identification field (IP ID) of the IPv4 header is there to
identify which fragments constitute a datagram when reassembling
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fragmented datagrams. The IPv4 specification does not specify
exactly how this field is to be assigned values, only that each
packet should get an IP ID that is unique for the source-destination
pair and protocol for the time the datagram (or any of its fragments)
could be alive in the network. This means that assignment of IP ID
values can be done in various ways, which we have separated into
three classes:
o Sequential jump
This is the most common assignment policy in today's IP stacks.
A single IP ID counter is used for all packet streams. When
the sender is running more than one packet stream
simultaneously, the IP ID can increase by more than one between
packets in a stream. The IP ID values will be much more
predictable and require less bits to transfer than random
values, and the packet-to-packet increment (determined by the
number of active outgoing packet streams and sending
frequencies) will usually be limited.
o Random
Some IP stacks assign IP ID values using a pseudo-random number
generator. There is thus no correlation between the ID values
of subsequent datagrams. Therefore there is no way to predict
the IP ID value for the next datagram. For header compression
purposes, this means that the IP ID field needs to be sent
uncompressed with each datagram, resulting in two extra octets
of header. IP stacks in cellular terminals that need optimum
header compression efficiency should not use this IP ID
assignment policy.
o Sequential
This assignment policy keeps a separate counter for each
outgoing packet stream and thus the IP ID value will increment
by one for each packet in the stream, except at wrap around.
Therefore, the delta value of the field is constant and well
known a priori. This assignment policy is the most desirable
for header compression purposes. However, its usage is not as
common as it perhaps should be.
In order to avoid violating RFC 791 [1], packets sharing the
same IP address pair and IP protocol number cannot use the same
IP ID values. Therefore, implementations of sequential
policies must make the ID number spaces disjoint for packet
streams of the same IP protocol going between the same pair of
nodes. This can be done in a number of ways, all of which
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introduce occasional jumps, and thus makes the policy less than
perfectly sequential. For header compression purposes less
frequent jumps are preferred.
It should be noted that the ID is an IPv4 mechanism and is therefore
not a problem for IPv6. For IPv4 the ID could be handled in three
different ways. First, we have the inefficient but reliable solution
where the ID field is sent as-is in all packets, increasing the
compressed headers by two octets. This is the best way to handle the
ID field if the sender uses random assignment of the ID field.
Second, there can be solutions with more flexible mechanisms
requiring less bits for the ID handling as long as sequential jump
assignment is used. Such solutions will probably require even more
bits if random assignment is used by the sender. Knowledge about the
sender's assignment policy could therefore be useful when choosing
between the two solutions above. Finally, even for IPv4, header
compression could be designed without any additional information for
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
by the sender. That might not be possible to know, which implies
that the applicability of such solutions is very uncertain. However,
designers of IPv4 stacks for cellular terminals should use an
assignment policy close to sequential.
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
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
offer this 'master sequence number' role.
Clearly from a busy server the observed behavior may well be quite
erratic. This is a case where the ability to share the IP
compression context between a number of flows (between the same end-
points) could offer potential benefits. However, this would only
have any real impact where there were a large number of flows between
one machine and the server. If context sharing is being considered,
then it is preferable to share the IP part of the context.
5.1.4 Don't Fragment (DF) flag
Due to the use of path-MTU discovery RFC 1191 [8], the value is more
likely to be '1', than found in UDP/RTP streams since DF should be
set to check for fragmentation in the end-to-end path. In practice
it is hard to predict the behavior of this field. However, it is
considered that the most likely case is that it will generally stay
at either '0' or '1'. When using PMTU discovery [8] it is expected
that DF will always be set to '1', although a host may end PMTU
discovery by clearing the DF bit to '0'.
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5.1.5 IP Hop-Limit / Time-To-Live (TTL)
The Hop-Limit (IPv6) or Time-To-Live (IPv4) field is expected to be
constant during the lifetime of a packet stream or to alternate
between a limited number of values due to route changes.
5.2 TCP header
Any discussion of compressability of TCP fields borrows heavily from
RFC 1144 [6]. However, the premise of how the compression is
performed is slightly different and the protocol has evolved slightly
in the intervening time.
5.2.1 Sequence number
An understanding of the sequence and acknowledgement number behavior
are essential for a TCP compression scheme.
At the simplest level the behavior of the sequence number can be
described relatively easily. However, there are a number of
complicating factors that also need to be considered.
For transferring in-sequence data packets, the sequence number will
increment for each packet by between 0 and an upper limit defined by
the MSS (Maximum Segment Size).
Although there are common MSS values, these can be quite variable.
Given this variability and the range of window sizes it is hard
(compared with the RTP case, for example) to select a 'one size fits
all' encoding for the sequence number. (The same argument applies
equally to the acknowledgement number).
We note that the increment of the sequence number in a packet is the
size of the data payload of that packet (including the SYN and FIN
flags; see later). This is, of course, exactly the relationship that
RFC 1144 [6] exploits to compress the sequence number in the most
efficient case. This technique may not be directly applicable to a
robust solution, but may be a useful relationship to consider.
However, at any point on the path (i.e. wherever a compressor might
be deployed), the sequence number can be anywhere within a range
defined by the TCP window. This is a combination of a number of
values (buffer space at the sender; advertised buffer size at the
receiver; and TCP congestion control algorithms). Missing packets or
retransmissions can cause the TCP sequence number to fluctuate within
the limits of this window.
It would also be desirable to be able to predict the sequence number
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from some regularity. However, this also appears to be difficult to
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
level values have been seen to have an impact as well, where sequence
number behavior has been observed with an 8 kbyte repeating pattern -
- 5 segments of 1460 bytes followed by 1 segment of 892 bytes. It
appears that implementation and how data is presented to the stack
can affect the sequence number.
It has been suggested that the TCP window can be tracked by the
compressor, allowing it to bound the size of these jumps.
For interactive flows (for example telnet), the sequence number will
change by small irregular amounts. In this case the Nagle algorithm
[3] commonly applies, coalescing small packets where possible to
reduce the basic header overhead. This may also mean that is less
likely that predictable changes in the sequence number will occur.
The Nagle algorithm is an optimisation and not required to be used.
Also, applications can disable the Nagle algorithm (which may be done
to mitigate the delays associated with its use).
It is also noted that the SYN and FIN flags (which have to be
acknowledged) consume 1 byte of sequence space.
5.2.2 Acknowledgement number
Much of the information about the sequence number applies equally to
the acknowledgement number. However, there are some important
differences.
For bulk data transfers there will tend to be 1 acknowledgement for
every 2 data segments. The algorithm is specified in RFC 2581 [28].
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
least every second full sized segment (MSS) of received data. It may
be seen from this that the delta for the acknowledgement number is
roughly twice that of the sequence number. This is not always the
case and the discussion about sequence number irregularity should be
applied.
As an aside, a common implementation bug was 'stretch ACKs'
(acknowledgements may be generated less frequently than every two
full-size data segments). This pattern can also occur following loss
on the return path.
Since the acknowledgement number is cumulative, dropped packets in
the forward path will result in the acknowledgement number remaining
constant for a time in the reverse direction. Retransmission of a
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dropped segment can then cause a substantial jump in the
acknowledgement number. These jumps in acknowledgement number are
bounded by the TCP window, just as for the jumps in sequence number.
In the acknowledgement case, information about the advertised
received window gives a bound to the size of any ACK jump.
5.2.3 Reserved
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
matter of time before a suggestion for using the flag is made.
5.2.4 Flags
o ECN-E (Explicit Congestion Notification)
'1' to echo CE bit in IP header. Will be set in several
consecutive headers (until 'acknowledged' by CWR)
If ECN nonces get used, then there will be a 'nonce-sum' (NS)
bit in the flags, as well. Again, transparency of the reserved
bits is crucial for future-proofing this compression scheme.
From an efficiency/compression standpoint, the NS bit will
either be unused [always 0] or randomly changing). The nonce-
sum is the 1-bit sum of the ECT codepoints, as described in
[35].
o CWR (Congestion Window Reduced)
'1' to signal congestion window reduced on ECN. Will generally
be set in individual packets. The flag will be set once per
loss event. Thus, the probability of it being set is
proportional to the degree of congestion in the network, but
less likely to be set than the CE flag.
o ECE (Echo Congestion Experience)
If 'congestion experienced' is signaled on a received IP
header, this is echoed through the ECE bit in segments sent by
the receiver until acknowledged by seeing the CWR bit set.
Clearly in periods of high congestion and/or long RTT, this
flag will be frequently set to '1'.
During connection open (SYN and SYN/ACK packets) the ECN bits have
special meaning:
CWR + ECN-E are both set with SYN to indicate desire to use ECN
CWR only is set in SYN-ACK to agree ECN
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(The difference in bit-patterns for the negotiation is so that it
will work with broken stacks that reflect the value of reserved
bits)
o URG (Urgent Flag)
'1' to indicate urgent data [unlikely with any flag other than
ACK]
o ACK (Acknowledgement)
'1' for all except the initial 'SYN' packet
o PSH (Push Function Field)
generally accepted to be randomly '0' or '1'. However, may be
biased more to one value than the other (this is largely down
to the implementation of the stack)
o RST (Reset Connection)
'1' to reset a connection [unlikely with any flag other than
ACK]
o SYN (Synchronize Sequence Number)
'1' for the SYN/SYN-ACK only at the start of a connection
o FIN (End of Data : FINished)
'1' to indicate 'no more data' [unlikely with any flag other
than ACK]
5.2.5 Checksum
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
scheme should not rely upon the TCP checksum for robustness, though,
and should apply appropriate error-detection mechanisms of its own.
The TCP checksum has to be considered as randomly changing.
5.2.6 Window
May oscillate randomly between 0 and the receiver's window limit (for
the connection).
In practice, the window will either not change, or may alternate
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between a relatively small number of values. Particularly when
closing (the value is getting smaller), the change in window is
likely to be related to the segment size, but it is not clear that
this necessarily offers any compression advantage. When the window
is opening, the effect of 'Silly-Window Syndrome' avoidance should be
remembered. This prevents the window opening by small amounts that
would encourage the sender to clock out small segments.
When thinking about what fields might change in a sequence of TCP
segments, it should be noted that the receiver can generate 'window
update' segments in which only the window advertisement changes.
5.2.7 Urgent pointer
From a compression point of view, the Urgent Pointer is interesting
because it offers an example where 'semantically identical'
compression is not the same as 'bitwise identical'. This is because
the value of the Urgent Pointer is only valid if the URG flag is set.
However, from the discussion of the TCP Checksum above, it should be
realized that this enforces bitwise transparency of the scheme and so
this argument is not particularly important.
If the URG flag is set, then this pointer indicates the end of the
urgent data and so can be point to anywhere in the window. This may
be set (and changing) over several segments. Note that urgent data
is rarely used, since it is not a particularly clean way of managing
out-of-band data.
5.3 Options
Options occupy space at the end of the TCP header. All options are
included in the checksum. An option may begin on any byte boundary.
The TCP header must be padded with zeros to make the header length a
multiple of 32 bits.
Optional header fields are identified by an option kind field.
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
octet length field, followed by length-2 octets of option data.
5.3.1 Options overview
Table 2 classifies the IANA known options together with their
associated RFCs, if applicable, from IANA [38]
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+------+--------+------------------------------------+----------+-----+
| Kind | Length | Meaning | RFC | Use |
| |(octets)| | | |
+------+--------+------------------------------------+----------+-----+
| 0 | - | End of Option List | RFC 793 | * |
| 1 | - | No-Operation | RFC 793 | * |
| 2 | 4 | Maximum Segment Size | RFC 793 | * |
| 3 | 3 | WSopt - Window Scale | RFC 1323 | * |
| 4 | 2 | SACK Permitted | RFC 2018 | * |
| 5 | N | SACK | RFC 2018 | * |
| 6 | 6 | Echo (obsoleted by option 8) | RFC 1072 | |
| 7 | 6 | Echo Reply (obsoleted by option 8) | RFC 1072 | |
| 8 | 10 | TSopt - Time Stamp Option | RFC 1323 | * |
| 9 | 2 | Partial Order Connection Permitted | RFC 1693 | |
| 10 | 3 | Partial Order Service Profile | RFC 1693 | |
| 11 | 6 | CC | RFC 1644 | |
| 12 | 6 | CC.NEW | RFC 1644 | |
| 13 | 6 | CC.ECHO | RFC 1644 | |
| 14 | 3 | Alternate Checksum Request | RFC 1146 | |
| 15 | N | Alternate Checksum Data | RFC 1146 | |
| 16 | | Skeeter | | |
| 17 | | Bubba | | |
| 18 | 3 | Trailer Checksum Option | | |
| 19 | 18 | MD5 Signature Option | RFC 2385 | |
| 20 | | SCPS Capabilities | | |
| 21 | | Selective Negative Acks | | |
| 22 | | Record Boundaries | | |
| 23 | | Corruption experienced | | |
| 24 | | SNAP | | |
| 25 | | Unassigned (released 12/18/00) | | |
| 26 | | TCP Compression Filter | | |
+------+--------+------------------------------------+----------+-----+
Table 2 Description of common TCP options
The 'use' column is marked with '*' to indicate those options that
are most likely to be seen in TCP flows.
5.3.2 Option field behavior
Generally speaking all option fields have been classified as
changing. This section describes the behavior of each option
referenced within an RFC, listed by 'kind' indicator.
0. End of option list
This option code indicates the end of the option list. This
might not coincide with the end of the TCP header according to
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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
the options would not otherwise coincide with the end of the
TCP header. Defined in RFC 793 [2].
There is no data associated with this option, a compression
scheme must simply be able to encode its presence.
1. No-Operation
This option code may be used between options, for example, to
align the beginning of a subsequent option on a word boundary.
There is no guarantee that senders will use this option, so
receivers must be prepared to process options even if they do
not begin on a word boundary RFC 793 [2].
There is no data associated with this option, a compression
scheme must simply be able to encode its presence.
This may be done by noting that the option simply maintains a
certain alignment and that compression need only convey this
alignment. In this way, padding can just be removed.
2. Maximum Segment Size
If this option is present, then it communicates the maximum
receive segment size at the TCP that sends this segment. This
field must only be sent in the initial connection request
(i.e., in segments with the SYN control bit set). If this
option is not used, any segment size is allowed RFC 793 [2].
This option is very common. The segment size is a 16-bit
quantity. Theoretically this could take any value, however
there are a number of values that are more common. For
example, 1460 bytes is very common for TCP/IPv4 over Ethernet
(though with the increased prevalence of tunnels, for example,
smaller values such as 1400 have become more popular). 536
bytes is the default MSS value. This may allow for common
values to be encoded more efficiently.
3. Window Scale Option (WSopt)
This option may be sent in a SYN segment by TCP :
(1) to indicate that it is prepared to do both send and
receive window scaling, and
(2) to communicate a scale factor to be applied to its
receive window.
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The scale factor is encoded logarithmically, as a power of 2
(presumably to be implemented by binary shifts). Note: the
window in the SYN segment itself is never scaled RFC 1072 [4].
This option may be sent in an initial segment (i.e., a segment
with the SYN bit on and the ACK bit off). It may also be sent
in a segment, but only if a Window Scale option was received in
the initial segment. A Window Scale option in a segment
without a SYN bit should be ignored. The Window field in a SYN
segment itself is never scaled RFC 1323 [9]
The use of window scaling does not affect the encoding of any
other field during the life-time of the flow. It is only the
encoding of the window scaling option itself that is important.
The window scale must be between 0 and 14 (inclusive).
Generally smaller values would be expected (a window scale of
14 allows for a 1Gbyte window, which is extremely large).
4. SACK-Permitted
This option may be sent in a SYN by a TCP that has been
extended to receive (and presumably process) the SACK option
once the connection has opened RFC 2018 [18].
There is no data in this option, all that is required is for
the presence of the option to be encoded.
5. SACK
This option is to be used to convey extended acknowledgment
information over an established connection. Specifically, it
is to be sent by a data receiver to inform the data transmitter
of non- contiguous blocks of data that have been received and
queued. The data receiver is awaiting the receipt of data in
later retransmissions to fill the gaps in sequence space
between these blocks.
At that time, the data receiver will acknowledge the data
normally by advancing the left window edge in the
Acknowledgment Number field of the TCP header. It is important
to understand that the SACK option will not change the meaning
of the Acknowledgment Number field, whose value will still
specify the left window edge, i.e., one byte beyond the last
sequence number of fully-received data RFC 2018 [18].
If SACK has been negotiated (through an exchange of SACK-
Permitted options), then this option may occur when dropped
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segments are noticed by the receiver. Because this identifies
ranges of blocks within the receiver's window, this can be
viewed as a base value with a number of offsets. The base
value (left edge of the first block) can be viewed as offset
from the TCP acknowledgement number. There can be up to 4 SACK
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')
and this will typically extend the size of or add to the
existing blocks.
Alternative proposals such as DSACK RFC 2883 [30] do not
fundamentally change the behavior of the SACK block, from the
point of view of the information contained within it.
6. Echo
This option carries information that the receiving TCP may send
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
Echo option was received in a SYN segment for the connection.
When the TCP echo option is used for RTT measurement, it will
be included in data segments, and the four information bytes
will define the time at which the data segment was transmitted
in any format convenient to the sender RFC 1072 [4].
The Echo option is generally not used in practice -- it is
obsoleted by the Timestamp option. However, for transparency
it is desirable that a compression scheme be able to transport
it. (However, there is no benefit in attempting any more
sophisticated treatment than viewing it as a generic 'option').
7. Echo Reply
A TCP that receives a TCP Echo option containing four
information bytes will return these same bytes in a TCP Echo
Reply option. This TCP Echo Reply option must be returned in
the next segment (e.g., an ACK segment) that is sent. If more
than one Echo option is received before a reply segment is
sent, the TCP must choose only one of the options to echo,
ignoring the others; specifically, it must choose the newest
segment with the oldest sequence number (see RFC 1072 [4]).
The Echo option is generally not used in practice -- it is
obsoleted by the Timestamp option. However, for transparency
it is desirable that a compression scheme be able to transport
it. (However, there is no benefit in attempting any more
sophisticated treatment than viewing it as a generic 'option').
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8. Timestamps
This option carries two four-byte timestamp fields. The
Timestamp Value field (TSval) contains the current value of the
timestamp clock of the TCP sending the option. The Timestamp
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
that was sent by the remote TCP in the TSval field of a
Timestamps option. When TSecr is not valid, its value must be
zero. The TSecr value will generally be from the most recent
Timestamp option that was received; however, there are
exceptions that are explained below. A TCP may send the
Timestamps option (TSopt) in an initial segment (i.e., segment
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
segment for the connection RFC 1323 [9].
Timestamps are quite commonly used. If timestamp options are
exchanged in the connection set-up phase, then they are
expected to appear on all subsequent segments. If this
exchange does not happen, then they will not appear for the
remainder of the flow.
Note that currently it is assumed that the negotiation of
options such as timestamp occurs in the SYN packets. However,
should this ever be allowed to change (allowing timestamps to
be enabled during an existing connection, for example), the
presence of the option should still be handled correctly.
Because the value being carried is a timestamp, it is logical
to expect that the entire value need not be carried. There is
no obvious pattern of increments that might be expected,
however.
An important reason for using the timestamp option is to allow
detection of sequence space wrap-around (Protection Against
Wrapped Sequence-number, or PAWS RFC 1323 [9]). It is not
expected that this is serious concern on the links that TCP
header compression would be deployed on, but it is important
that the integrity of this option is maintained. This issue is
discussed in, for example, RFC 3150 [32]. However, the
proposed Eifel algorithm [36] makes use of timestamps and so,
currently, it is recommended that timestamps are used for
cellular-type links [34].
With regard to compression, it is further noted that the range
of resolutions for the timestamp suggested in RFC 1323 [9] is
quite wide (1ms to 1s per 'tick'). This (along with the
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perhaps wide variation in RTT) makes it hard to select a set of
encodings that will be optimal in all cases.
9. Partial Order Connection (POC) permitted
This option represents a simple indicator communicated between
the two peer transport entities to establish the operation of
the POC protocol RFC 1693 [12]
The Partial Order Connection option is in practice never seen,
and so the only requirement is that the header compression
scheme should be able to encode it.
10. POC service profile
This option serves to communicate the information necessary to
carry out the job of the protocol -- the type of information
that is typically found in the header of a TCP segment.
The Partial Order Connection option is in practice never seen,
and so the only requirement is that the header compression
scheme should be able to encode it.
11. Connection Count (CC)
This option is part of the implementation of TCP Accelerated
Open (TAO) that effectively bypasses the TCP Three-Way
Handshake (3WHS). TAO introduces a 32-bit incarnation number,
called a "connection count" (CC) that is carried in a TCP
option in each segment. A distinct CC value is assigned to
each direction of an open connection. The implementation
assigns monotonically increasing CC values to successive
connections that it opens actively or passively RFC 1644 [11].
This option is in practice never seen, and so the only
requirement is that the header compression scheme should be
able to encode it.
12. CC.NEW
Correctness of the TAO mechanism requires that clients generate
monotonically increasing CC values for successive connection
initiations. Receiving a CC.NEW causes the server to
invalidate its cache entry and do a 3WHS. RFC 1644 [11].
This option is in practice never seen, and so the only
requirement is that the header compression scheme should be
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able to encode it.
13. CC.ECHO
When a server host sends a segment, it echoes the connection
count from the initial in a CC.ECHO option, which is used by
the client host to validate the segment RFC 1644 [11].
This option is in practice never seen, and so the only
requirement is that the header compression scheme should be
able to encode it.
14. Alternate Checksum Request
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
based on an alternate algorithm. During communication, the
alternate checksum replaces the regular TCP checksum in the
checksum field of the TCP header. Should the alternate
checksum require more than 2 octets to transmit, the checksum
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
data may be split between the header field and the option.
Alternate checksums are computed over the same data as the
regular TCP checksum RFC 1146 [7]
This option is in practice never seen, and so the only
requirement is that the header compression scheme should be
able to encode it. It would only occur in connection set-up
(SYN) packets.
Even if this option were used, it would not affect the handling
of the checksum, since this should be carried transparently in
any case.
15. Alternate Checksum Data
This field is used only when the alternate checksum that is
negotiated is longer than 16 bits. These checksums will not
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
split between the checksum field in the TCP header and the
option or the entire checksum is placed in the option is
determined on a checksum by checksum basis. The length of this
option will depend on the choice of alternate checksum
algorithm for this connection RFC 1146 [7].
If an alternative checksum were negotiated in the connection
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set-up, then this option may appear on all subsequent packets
(if needed to carry the checksum data). However, this option
is in practice never seen, and so the only requirement is that
the header compression scheme should be able to encode it.
16. -- 18.
Are non-RFC references and are not considered in this document.
19. MD5 Digest
Every segment sent on a TCP connection to be protected against
spoofing will contain the 16-byte MD5 digest produced by
applying the MD5 algorithm to a concatenated block of data.
Upon receiving a signed segment, the receiver must validate it
by calculating its own digest from the same data (using its own
key) and comparing the two digest. A failing comparison must
result in the segment being dropped and must not produce any
response back to the sender. Logging the failure is probably
advisable.
Unlike other TCP extensions (e.g., the Window Scale option
[9]), the absence of the option in the SYN, ACK segment must
not cause the sender to disable its sending of signatures.
This negotiation is typically done to prevent some TCP
implementations from misbehaving upon receiving options in non-
SYN segments. This is not a problem for this option, since the
SYN, ACK sent during connection negotiation will not be signed
and will thus be ignored. The connection will never be made,
and non-SYN segments with options will never be sent. More
importantly, the sending of signatures must be under the
complete control of the application, not at the mercy of the
remote host not understanding the option.
MD5 digest information should, like any cryptographically
secure data, be incompressible. Therefore the compression
scheme must simply transparently carry this option, if it
occurs.
20. -- 26.
Are non-RFC references and are not considered in this document.
This only means that their behavior is not described in detail
as a compression scheme is not expected to be optimised for
these options. However any unrecognised option must be
transparently carried by a TCP compression scheme in order to
work efficiently in the presence of new or rare options.
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In the discussion above regarding timestamps it is pointed out that
there is the possibility (at some time in the future) of negotiations
being permitted more generally than in the SYN packets at connection
set-up. Although there seems to be no compelling need to optimise
for this, it is worth pointing out that the compression scheme should
be able to cope with arbitrary options appearing at any point within
the flow. There is also no guarantee that a compression scheme will
see the SYN packets of a connection set-up.
6. Other observations
6.1 Implicit acknowledgements
There may be a small number of cues for 'implicit acknowledgements'
in a TCP flow. Even if the compressor only sees the data transfer
direction, for example, then seeing a packet without the SYN flag set
implies that the SYN packet has been received.
There is a clear requirement for the deployment of compression to be
topologically independent. This means that it is not actually
possible to be sure that seeing a data packet at the compressor
guarantees that the SYN packet has been correctly received by the
decompressor (as the SYN packet may have taken an alternative path).
However, it may be that there are other such cues that may be used in
certain circumstances to improve compression efficiency.
6.2 Shared data
It can be seen that there are two distinct deployments -- one where
the forward and reverse paths share a link and one where they do not.
In the former case a compressor and decompressor could be co-located.
It may then be possible for the compressor and decompressor at each
end of the link to exchange information. This could lead to possible
optimizations.
For example, acknowledgement numbers are generally taken from the
sequence numbers in the opposite direction. Since an acknowledgement
cannot be generated for a packet that has not passed across the link,
this offers an efficient way of encoding acknowledgements.
6.3 TCP header overhead
For a TCP bulk data-transfer the overhead is not that onerous,
particularly compared to the typical RTP voice case. Although
spectral efficiency is clearly an important goal, it does not seem
critical to extract every last bit of compression gain.
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However, in the acknowledgement direction (i.e. for 'pure'
acknowledgement headers) the overhead could be said to be infinite
(since there is no data being carried). This is why optimizations
for the acknowledgement path may be considered useful.
There are a number of schemes for manipulating TCP acknowledgements
to reduce the ACK bandwidth. Many of these are documented in [33]
and [32]. Most of these schemes are entirely compatible with header
compression, without requiring any particular support from either.
While it is not expected that a compression scheme will support
experimental options, it is useful that these be considered when
developing header compression schemes, and vice versa.
6.4 Field independence and packet behavior
It should be apparent that direct comparisons with the highly
'packet' based view of RTP compression are hard. RTP header fields
tend to change regularly per-packet and many fields (IPv4 IP ID, RTP
sequence number and RTP timestamp, for example) typically change in a
dependent manner. However, TCP fields, such as sequence number tend
to change more unpredictably, partly because of the influence of
external factors (size of TCP windows, application behavior, etc.)
Also, the field values tend to change indpendently. Overall, this
makes compression more challenging and makes it harder to select a
set of encodings that can successfully trade-off efficiency and
robustness.
6.5 Short-lived flows
It is hard to see what can be done to improve performance for a
single, unpredictable, short-lived connection. However, there are
commonly cases where there will be multiple TCP connections between
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
[20]).
When a connection closes, it is either the last connection between
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
part of the context will probably be almost identical. Certain
aspects of the TCP context may also be similar.
Support for context replication is discussed in more detail in
Section 4. Overall, support for sub-context sharing, or initializing
one context from another offers useful optimizations for a sequence
of short-lived connections.
It is noted that although TCP is connection oriented, it is hard for
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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
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
about initializing a new context from an existing one, rather than
re-using an existing one.
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-
points. There may be limited scope for initialisation of a new TCP
header from an existing one. The port numbers are the most obvious
starting point.
6.6 Master Sequence Number
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
such a master sequence number is only required to allow a
decompressor to acknowledge packets in bi-directional mode. It can
also be seen that such a sequence number would not be required for
every packet.
While the sequence number only needs to be 'sparse', it is clear that
there is a requirement for an explicitly added sequence number.
There are no obvious ways of guaranteeing the unique identity of a
packet other than by adding such a sequence number (sequence and
acknowledgement numbers can both remain the same, for example).
As a further note, support for re-ordering of compressed packets
would require a sequence number external to the compressed packet.
This is so that re-ordering could be identified prior to attempting
decompression.
6.7 Size constraint for TCP options
It can be seen from the above analysis, most TCP options, such as
MSS, WSopt, SACK-Permitted, may appear only on a SYN segment. Every
implementation should (and we expect that most will) ignore unknown
options on SYN segments. TCP options will be sent on non-SYN
segments only when an exchange of options on the SYN segments has
indicated that both sides understand the extension. Other TCP
options, such as MD5 Digest, Timestamp also tend to be sent when the
connection is initiated (i.e. in the SYN packet).
The total header size is also an issue. The TCP header specifies
where segment data starts with a 4-bit field which gives the total
size of the header (including options) in 32-bit words. This means
that the total size of the header plus option must be less than or
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equal to 60 bytes -- this leaves 40 bytes for options.
7. Security considerations
Since this document only describes TCP field behavior there are no
direct security concerns raised by it.
This memo is intended to be used to aid the compression of TCP/IP
headers. Where authentication mechanisms such as IPsec AH [13] are
used, it is important that compression is transparent. Where
encryption methods such as IPsec ESP [14] are used, the TCP fields
may not be visible, preventing compression.
8. Acknowledgements
Many IP and TCP RFCs (hopefully all of which have been collated
below) have been sources of ideas and knowledge, together with header
compression schemes from RFC 1144, RFC 2509 and RFC 3095, and of
course the detailed analysis of RTP/UDP/IP in RFC 3095.
This draft also benefited from discussion on the rohc mailing list
and in various corridors (virtual or otherwise) about many key
issues; special thanks to Qian Zhang, Carsten Bormann and Gorry
Fairhurst.
Qian Zhang and Hongbin Liao contributed the extensive analysis of
shareable header fields.
Any remaining misrepresentation or misinterpretation of information
is entirely the fault of the authors of this draft.
References
[1] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[2] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
[3] Nagle, J., "Congestion control in IP/TCP internetworks", RFC
896, January 1984.
[4] Jacobson, V. and R. Braden, "TCP extensions for long-delay
paths", RFC 1072, October 1988.
[5] Braden, R., "Requirements for Internet Hosts - Communication
Layers", STD 3, RFC 1122, October 1989.
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[6] Jacobson, V., "Compressing TCP/IP headers for low-speed serial
links", RFC 1144, February 1990.
[7] Zweig, J. and C. Partridge, "TCP alternate checksum options",
RFC 1146, March 1990.
[8] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[9] Jacobson, V., Braden, B. and D. Borman, "TCP Extensions for
High Performance", RFC 1323, May 1992.
[10] Almquist, P., "Type of Service in the Internet Protocol Suite",
RFC 1349, July 1992.
[11] Braden, B., "T/TCP -- TCP Extensions for Transactions
Functional Specification", RFC 1644, July 1994.
[12] Connolly, T., Amer, P. and P. Conrad, "An Extension to TCP :
Partial Order Service", RFC 1693, November 1994.
[13] Atkinson, R., "IP Authentication Header", RFC 1826, August
1995.
[14] Atkinson, R., "IP Encapsulating Security Payload (ESP)", RFC
1827, August 1995.
[15] Berners-Lee, T., Fielding, R. and H. Nielsen, "Hypertext
Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.
[16] Bellovin, S., "Defending Against Sequence Number Attacks", RFC
1948, May 1996.
[17] Stevens, W., "TCP Slow Start, Congestion Avoidance, Fast
Retransmit, and Fast Recovery Algorithms", RFC 2001, January
1997.
[18] and, M., Floyd, S. and A. Romanow, "TCP Selective
Acknowledgment Options", RFC 2018, October 1996.
[19] Bradner, S., "The Internet Standards Process -- Revision 3",
BCP 9, RFC 2026, October 1996.
[20] Fielding, R., Gettys, J., Mogul, J., Nielsen, H. and T.
Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC
2068, January 1997.
[21] Bradner, S., "Key words for use in RFCs to Indicate Requirement
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Levels", BCP 14, RFC 2119, March 1997.
[22] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998.
[23] Nichols, K., Blake, S., Baker, F. and D. Black, "Definition of
the Differentiated Services Field (DS Field) in the IPv4 and
IPv6 Headers", RFC 2474, December 1998.
[24] Ramakrishnan, K. and S. Floyd, "A Proposal to add Explicit
Congestion Notification (ECN) to IP", RFC 2481, January 1999.
[25] Degermark, M., Nordgren, B. and S. Pink, "IP Header
Compression", RFC 2507, February 1999.
[26] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP Headers for
Low-Speed Serial Links", RFC 2508, February 1999.
[27] Engan, M., Casner, S. and C. Bormann, "IP Header Compression
over PPP", RFC 2509, February 1999.
[28] Allman, M., Paxson, V. and W. Stevens, "TCP Congestion
Control", RFC 2581, April 1999.
[29] Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
Values In the Internet Protocol and Related Headers", BCP 37,
RFC 2780, March 2000.
[30] Floyd, S., Mahdavi, J., Mathis, M. and M. Podolsky, "An
Extension to the Selective Acknowledgement (SACK) Option for
TCP", RFC 2883, July 2000.
[31] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le, K.,
Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K., Wiebke, T.,
Yoshimura, T. and H. Zheng, "RObust Header Compression (ROHC):
Framework and four profiles: RTP, UDP, ESP, and uncompressed",
RFC 3095, July 2001.
[32] Dawkins, S., Montenegro, G., Kojo, M. and V. Magret, "End-to-
end Performance Implications of Slow Links", BCP 48, RFC 3150,
July 2001.
[33] Balakrishnan, , Padmanabhan, V., Fairhurst, G. and M.
Sooriyabandara, "TCP Performance Implications of Network Path
Asymmetry", RFC 3449, December 2002.
[34] Inamura, H., Montenegro, G., Ludwig, R., Gurtov, A. and F.
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Khafizov, "TCP over Second (2.5G) and Third (3G) Generation
Wireless Networks", RFC 3481, February 2003.
[35] Spring, N., Wetherall, D. and D. Ely, "Robust ECN Signaling
with Nonces", draft-ietf-tsvwg-tcp-nonce-04.txt (work in
progress), October 2002.
[36] Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm for
TCP", draft-ietf-tsvwg-tcp-eifel-alg-07.txt (work in progress),
December 2002.
[37] Karn, , "Advice for Internet Subnetwork Designers", draft-ietf-
pilc-link-design-13.txt (work in progress), February 2003.
[38] <http://www.iana.org/assignments/tcp-parameters>
Authors' Addresses
Mark A. West
Siemens/Roke Manor
Roke Manor Research Ltd.
Romsey, Hants SO51 0ZN
UK
Phone: +44 (0)1794 833311
EMail: mark.a.west@roke.co.uk
URI: http://www.roke.co.uk
Stephen McCann
Siemens/Roke Manor
Roke Manor Research Ltd.
Romsey, Hants SO51 0ZN
UK
Phone: +44 (0)1794 833341
EMail: stephen.mccann@roke.co.uk
URI: http://www.roke.co.uk
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Full Copyright Statement
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West & McCann Expires September 1, 2003 [Page 42]
