Network Working Group Qian Zhang, (ed.), Microsoft Research Asia
Internet Draft Robert Hancock, Siemens/Roke Manor
Expires: September 2002 HongBin Liao, Microsoft Research Asia
Stephen McCann, Siemens/Roke Manor
Paul Ollis, Siemens/Roke Manor
Richard Price, Siemens/Roke Manor
Abigail Surtees, Siemens/Roke Manor
Mark A West, Siemens/Roke Manor
Ya-Qin Zhang, Microsoft Research Asia
Wenwu Zhu, Microsoft Research Asia
March, 2002
TCP/IP Header Compression for ROHC (ROHC-TCP)
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [RFC2026].
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
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Internet-Drafts are draft documents valid for a maximum of six
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at any time. It is inappropriate to use Internet- Drafts as
reference material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
Existing TCP/IP header compression schemes do not work well on noisy
links, especially the one with high bit error rate and long
roundtrip time. For many bandwidth limited links where header
compression is essential, such characteristics are common. In
addition, existing schemes [VJHC, IPHC] have not addressed how to
compress TCP options such as SACK [RFC2018, RFC2883] and Timestamps
[RFC1323].
A robust and efficient header compression scheme for TCP/IP, called
ROHC-TCP, is proposed within the basic RObust Header Compression
[ROHC] framework.
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Table of Contents
Status of this Memo................................................1
Abstract...........................................................1
1. Introduction....................................................5
2. Terminology.....................................................5
3. Background......................................................7
3.1 Existing TCP/IP header compression schemes..................7
3.2 Classification of header fields.............................8
3.3 Characteristics of the short-lived TCP transfers............9
3.4 Classification of shareable header fields..................10
4. TCP/IP header compression framework............................12
4.1 Compression and decompression states.......................12
4.1.1 Compressor states.....................................12
4.1.1.1. Initialization and Refresh (IR) state...........12
4.1.1.2. First Order (FO) State..........................13
4.1.1.3. Second Order (SO) State.........................13
4.1.2. Decompressor states..................................13
4.2 Modes of operation.........................................13
4.2.1. Unidirectional mode -- U-mode........................14
4.2.2. Bi-directional mode -- B-mode........................14
4.3. Impairment considerations.................................14
5. The Protocol...................................................15
5.1. Data structures...........................................15
5.2. ROHC-TCP packet formats from compressor to decompressor...15
5.3. Parameters needed for mode transition in ROHC-TCP.........16
5.4 Robustness and efficiency maintenance......................16
5.5 Operation in unidirectional mode...........................17
5.5.1. Compressor state and logic...........................17
5.5.1.1. State transition logic (U-mode).................18
5.5.2. Decompressor logic...................................21
5.5.2.1. No Context State................................21
5.5.2.2. Full Context State..............................22
5.6. Operations in bi-directional mode.........................22
5.6.1. Compressor states and logic..........................22
5.6.1.1. Master Sequence Number (MSN) for packet
acknowledging............................................23
5.6.1.2. State transition logic..........................23
5.6.1.3. Compression logic and packets used..............23
5.6.2. Decompressor states and logic........................24
5.7. Mode transitions..........................................24
5.7.1. Compression and decompression during mode transitions25
5.7.2. Transition from Unidirectional to Bidirectional mode.26
5.7.3. Transition to Unidirectional mode....................26
5.8. Implementation optimizations..............................27
5.8.1. Determine the value N................................27
5.8.2. Determine the frequency of updating context..........27
5.8.3. Tracking-based TCP congestion window estimation......27
5.8.3.1. General principle of congestion window tracking.27
5.8.3.2. Tracking based on Sequence Number...............28
5.8.3.3. Tracking based on Acknowledgment Number.........29
5.8.3.4. Further discussion on congestion window tracking31
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5.8.3.5. Determine the value K in congestion window
estimation...............................................31
5.8.4. Possible optimization in U-mode......................32
6. Coding Scheme and compressed packet header format..............33
6.1. Fixed-payload encoding....................................33
6.2. ROHC Profile for compression of TCP/IP....................34
7. Security considerations.......................................46
8. Acknowledgements...............................................46
9. References.....................................................47
10. Authors' addresses............................................49
Appendix A - Detailed classification of header fields.............51
A.1. General classification....................................51
A.1.1. IP header fields.....................................52
A.1.1.1. IPv6 header fields..............................52
A.1.1.2. IPv4 header fields..............................53
A.1.2. TCP header fields....................................56
A.1.3. Summary for IP/TCP...................................56
A.2. Analysis of change patterns of header fields..............57
A.2.1. IP header............................................59
A.2.1.1. IP Traffic-Class / Type-Of-Service (TOS)........59
A.2.1.2. ECN Flags.......................................60
A.2.1.3. IP Identification...............................60
A.2.1.4. Don't Fragment (DF) flag........................62
A.2.1.5. IP Hop-Limit / Time-To-Live (TTL)...............62
A.2.2. TCP header...........................................62
A.2.2.1. Sequence number.................................63
A.2.2.2. Acknowledgement number..........................64
A.2.2.3. Reserved........................................64
A.2.2.4. Flags...........................................64
A.2.2.5. Checksum........................................65
A.2.2.6. Window..........................................66
A.2.2.7. Urgent pointer..................................66
A.2.3. Options..............................................66
A.2.3.1. Options overview................................66
A.2.3.2. Option field behavior...........................67
A.3. Some observations.........................................72
A.3.1. Implicit acknowledgements............................72
A.3.2. Field dependence and packet behavior.................72
A.3.3. Correlation and size constraint for TCP options......72
A.3.4. Short-lived flows....................................73
A.3.5. Shared data..........................................76
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Document History
00 January 31, 2002 First release.
01 March 16, 2002 Revise structure of the draft;
Refine operation mode and mode transition of
the state machine;
Add solution for context replication;
Add profile for context replication;
Refine the TCP/IP behavior analysis.
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1. Introduction
The necessity and importance of doing TCP/IP header compression on
low- or medium-speed links have been discussed in [IPHC].
However, some links, such as cellular links, have characteristics
that make header compression as defined in [IPHC, VJHC] perform less
than well. The most important characteristic is the lossy behavior
of cellular links, where a bit error rate (BER) as high as 1e-3 must
be accepted to keep the radio resources efficiently utilized. In
severe operating situations, the BER can be as high as 1e-2.
Although TCP traffic has usually tended to be sent over reliable
links, there are still many scenarios where TCP header compression
will be implemented over less reliable links [RFC-3150, PILC-ARQ],
making robustness an important objective also for the new TCP
compression scheme. The other problematic characteristic is the
long round-trip time (RTT) of the cellular link, which is due to FEC
combined with interleaving, processing delay and delays from the
transport in the radio network. A typical RTT varies between a few
hundred milliseconds to one second. The link layer ARQ will make
this RTT even larger.
Other shortcomings of existing TCP/IP header compression schemes
([IPHC, VJHC]) are that headers of handshaking packets (SYNs and
FINs), and TCP option fields (e.g., SACK or timestamps) are not
compressed. In fact, the presence of timestamps effectively
disables header compression. Especially, if many short-lived TCP
connections run across the link, the compression of the handshaking
packets may greatly improve the overall header compression ratio.
Thus a viable header compression scheme for cellular links must be
able to handle loss on the link between the compression and
decompression point as well as loss before the compression point.
Meanwhile, the new header compression scheme needs to consider how
to efficiently compress the short-lived TCP transfers and the TCP
options, such as SACK and Timestamp.
Bandwidth is the most costly resource in cellular links. Processing
power is very cheap in comparison. Implementation or computational
simplicity of a header compression scheme is therefore of less
importance than its compression ratio and robustness.
This document describes a new header compression scheme for TCP/IP
header compression (ROHC-TCP), which is robust against packet loss
and hence achieves good performance over lossy links.
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 [RFC2119].
Compressed header format
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A compressed header format describes how to compress each field in
the chosen protocol stack. It consists of two parts: a bit
pattern to indicate to the decompressor which format is being used,
followed by a list of the compressed versions of each field.
Context
The context of the compressor is the state it uses to compress a
header. The context of the decompressor is the state it uses to
decompress a header. Either of these or the two in combination
are usually referred to as "context", when it is clear which is
intended. The context contains relevant information from previous
headers in the packet stream, such as static fields and possible
reference values for compression and decompression. Moreover,
additional information describing the packet stream is also part
of the context, for example information about how the IP
Identifier field changes and the initial window size.
Context identifiers
On some channels, the ability to transport multiple packet streams
is required. It can also be feasible to have channels dedicated
to individual packet streams. Therefore, ROHC uses a distinct
context identifier space per channel and can eliminate context
identifiers completely for one of the streams when few streams
share a channel.
Context sharing
Essentially sharing a field between a number of contexts.
Multiple flows all share the same context entry.
Context replication
Initialize a new context from an existing one and overwrite some
of the values to create the new context.
Loss propagation
Loss of headers, due to errors in (i.e., loss of or damage to)
previous header(s) or feedback.
Profile
A [ROHC] profile is a description of how to compress a certain
protocol stack over a certain type of link. Each profile includes
one or more sets of compressed header formats and a state machine
to control the compressor and the decompressor.
Robustness
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The performance of a header compression scheme can be described
with three parameters: compression efficiency, robustness, and
compression transparency. A robust scheme tolerates loss and
residual errors on the link over which header compression takes
place without losing additional packets or introducing additional
errors in decompressed headers.
3. Background
This session provides a background to the subject of TCP/IP header
compression. The fundamentals of general header compression had
been introduced in [ROHC]. Here two existing TCP/IP header
compression schemes are described. Their drawbacks are then
discussed, following by the classification of TCP/IP header fields.
After that, the characteristics of the short-lived TCP transfers are
discussed, following by the behavior analysis of TCP/IP header
fields among multiple short-lived connections.
3.1 Existing TCP/IP header compression schemes
There are two typical existing TCP/IP header compression schemes,
which are VJHC [VJHC] and IPHC [IPHC], respectively. Both of them
rely on transmitting only the difference from the previous header in
order to reduce the large overhead of TCP/IP header.
Although VJHC works well over reliable links, it is vulnerable on
lossy links, because even a single bit error results in loss of
synchronization between the compressor and decompressor.
Considering the high bit error rate in cellular links, when used
over unreliable links, it induces many additional errors due to
inconsistent contexts between the compressor and the decompressor.
Then the decompressor must send the request for resynchronization
and in the meanwhile discard all compressed header. A fatal result
of this effect is that it prevents TCP Fast Retransmit algorithm
[E2E] from being fired and always causes TCP retransmission timeout.
This effect is shown in detail in [MOBI96].
IPHC proposes two simple mechanisms, the TWICE algorithm and the
full header request mechanism, to reduce the errors due to the
inconsistent contexts between the compressor and the decompressor.
The TWICE algorithm assumes that only the Sequence Number field of
TCP segments are changing during the connection and the deltas among
consecutive packets are constant in most cases. However, these
assumptions are not always true, especially when TCP Timestamp and
SACK options are used. The full header request mechanism uses
explicit request for retransmission of an uncompressed packet to
allow resynchronization without waiting for a TCP timeout. It needs
a feedback channel, which is unavailable in some circumstances. Even
when the feedback channel is available, this mechanism still cannot
perform well enough if the roundtrip time of this link is very long.
Once a packet is corrupted on the noisy link, there are still
several consecutive packets dropped due to the inconsistency between
the compressor and the decompressor.
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Meanwhile, both IPHC and VJHC had not compressed the headers of
handshaking packets (SYNs and FINs), and TCP option fields (e.g.,
SACK or timestamps).
3.2 Classification of header fields
Header compression is possible due to the fact that there is much
redundancy between header field values within packets, especially
between consecutive packets. To utilize these properties for header
compression, it is important to understand the change patterns of
the various header fields.
All header fields have been classified in detail in appendix A. The
fields are first classified at a high level and then some of them
are studied more in detail. Finally, the appendix concludes with
recommendations on how the various fields should be handled by
header compression algorithms. The main conclusion that can be
drawn is that most of the header fields can easily be compressed
away since they never or seldom change. Only several fields need
more sophisticated mechanisms. These fields are:
- IPv4 Identification (16 bits) - IP-ID
- TCP Sequence Number (32 bits) - SN
- TCP Acknowledgement Number (32 bits) - ACKN
- TCP Reserved (4 bits)
- TCP ECN flags (2 bits) - ECN
- TCP Window (16 bits) - WINDOW
- TCP Options
- Maximum Segment Size (4 octets) - MSS
- Window Scale (3 octets) - WSopt
- SACK Permitted (2 octets)
- TCP SACK - SACK
- TCP Timestamp (32 bits) - TS
It is known that the change patterns of several TCP fields (for
example, Sequence Number, Acknowledgement Number, Window, etc.) are
completely different from the ones of RTP, which had already
discussed in detail in [ROHC], and are very hard to predict. The
analysis in Appendix A reveals that an understanding of the sequence
and acknowledgement number behavior are essential for a TCP
compression scheme.
Note that 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. Missing packets or retransmissions can
cause the TCP sequence number to fluctuate within the limits of this
window. The jumps in acknowledgement number are also bounded by
this TCP window.
The dependency between sequence number and acknowledgement number
had been mentioned in Appendix. It's well-known that most TCP
connections only have one-way traffic (for example, web browsing and
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FTP downloading). That is, on the forward path (from server to
client), only Sequence Number changes and Acknowledgement Number
remains constant at most time; on the backward path (from client to
server), only Acknowledgement Number changes and Sequence Number
remain constant at most time.
The analysis in Appendix A also reveals that in the TCP case, there
is no obvious candidate for a field to offer the Master sequence
number to compress IP-ID field than in the RTP case. The assignment
of IP-ID values can be done in various ways, which are Sequential
jump, Random, and Sequential, respectively. However, designers of
IPv4 stacks for cellular terminals SHOULD use an assignment policy
close to sequential.
An interesting thing for TCP options is that, most TCP options, such
as MSS, WSopt, SACK-Permitted, etc., may appear only on a SYN
segment. Every implementation should (and we expect that most will)
ignore unknown options on SYN segments.
Headers specific to Mobile IP (for IPv4 or IPv6) do not receive any
special treatment in this document. They are compressible, however,
and it is expected that the compression efficiency for Mobile IP
headers will be good enough due to the handling of extension header
lists and tunneling headers. The handling of this issue is conforms
with [ROHC].
3.3 Characteristics of the short-lived TCP transfers
There are commonly cases where there will be multiple short-live TCP
connections between the same cellular links. Two types of scenarios
are given as examples, which can be illustrated in Figure 3.3.1. and
Figure 3.3.2.
Mobile Base
Terminals Station Server
| \ /
+---+ ~ ~ ~ |
| | . . . | +---------+
|MT | | ======================= | Server |
+---+ ~ ~ ~ | +---------+
Cellular Link Wired Network
Figure 3.3.1. : Scenario for multiple connections between the same
mobile terminal and server over cellular links
In Figure 3.3.1., mobile terminal is connected to base station over
cellular link, and the base station is connected to a server through
wired (or possibly wireless) networks. There are multiple
connections between the mobile terminal and the server
simultaneously or near simultaneously (open within a relatively
short space of time).
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In Figure 3.3.2., one mobile terminal is connected to base station
over cellular link, and the base station is connected to multiple
web servers through wired (or possibly wireless) networks. User in
the mobile terminal may send multiple requests to different web
servers simultaneously or near simultaneously.
Mobile Base Web
Terminal Station Servers
+----------+
| ~ ~ ~ \ / ||============= | Server 1 |
| | || +----------+
+---+ | ||
|MT | | || . . . . . .
| | | ||
+---+ | ||
| || +----------+
|============||============= | Server n |
+----------+
Cellular Link Wired Network
Figure 3.3.2. : Scenario for mobile terminal send requests to
multiple web servers over cellular links
For multiple connections with same source-IP, which occur
simultaneously or near simultaneously, the IP header part of the
contexts will probably be quite similar. Certain aspects of the TCP
context may also be similar.
For example, it may be that one of the port numbers will stay the
same (the service port) and the other will change by a small amount
relative to the just-closed connection (the ephemeral port).
Short-lived TCP transfers will degrade the performances of header
compression schemes which establish a new context by sending full
headers initially. The way ROHC TCP compression operates, then, is
to replicate the context among the short-live TCP flows so as to
improve the compression efficiency.
The new connection initializes a new context, (partially) copied the
context from an existing one, and send out partial uncompressed
packet to indicate the fields that need to be updated in the new
context.
3.4 Classification of shareable header fields
Context replication is possible due to the fact that there is much
similarity in header field values and context values among multiple
simultaneously or near simultaneously short-lived connections. To
utilize these properties for header compression, it is important to
understand the shareable characteristics for the various header
fields and context values.
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All header fields and related context values have been classified in
detail in appendix A. The main conclusion that can be drawn is that
most part of the IP sub-context, some TCP fields, and some context
values can easily be shared since they seldom change or change with
only a small jump. These fields are:
IPv4 Header (inner and/or outer)
Field Class Shareable
------------------------------------------------
Header Length STATIC-KNOWN Yes
ToS CHANGING Yes
Packet Length INFERRED N/A
Identification CHANGING Yes
Time To Live CHANGING Yes
Protocol STATIC N/A
Header Checksum INFERRED N/A
Source Address STATIC-DEF N/A
Destination Address STATIC-DEF N/A
IPv6 Header (inner and/or outer)
Field Class Shareable
------------------------------------------------
Version STATIC N/A
Traffic Class CHANGING Yes
Flow Label STATIC-DEF N/A
Payload Length INFERRED N/A
Next Header STATIC N/A
Hop Limit CHANGING Yes
Source Address STATIC-DEF N/A
Destination Address STATIC-DEF N/A
TCP Header
Field Class Shareable
------------------------------------------------
Source Port STATIC-DEF Yes
Destination Port STATIC-DEF Yes
Data Offset INFERRED N/A
Window CHANGING Yes
Reserved Bits CHANGING Yes
Init-Window (Context) CHANGING Yes
TCP Options
Option SYN-only Shareable
-----------------------------------------------------
Maximum Segment Size Option Yes Yes
Window Scale Option Yes Yes
SACK-Permitted Option Yes Yes
Timestamps Option No Yes
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4. TCP/IP header compression framework
Similar to the ROHC framework, the ROHC-TCP protocol achieves its
compression gain by establishing state information at both ends of
the link, i.e., at the compressor and at the decompressor.
4.1 Compression and decompression states
Header compression with ROHC can be characterized as an interaction
between two state machines, one compressor machine and one
decompressor machine, each instantiated once per context. The
compressor and the decompressor have three and two states,
respectively. Both machines start in the lowest compression state
and transit gradually to higher states. Transitions need not be
synchronized between the two machines. The compressor transits back
to lower states when it does not have sufficient confidence to stay
at the high state. The decompressor will transit back only when
context damage is detected.
Subsequent sections present an overview of the state machines and
their corresponding states, respectively, starting with the
compressor.
4.1.1 Compressor states
There are three compressor states in ROHC-TCP: Initialization and
Refresh (IR) state, First Order (FO), and Second Order (SO) states.
The compressor starts in the lowest compression state (IR) and
transits gradually to the higher compression state. The compressor
will always operate in the highest possible compression state, under
the constraint that the compressor is sufficiently confident that
the decompressor has the information necessary to decompress a
header, which is compressed according to the state.
+----------+
+----------+ <--------> | FO State | <--------> +----------+
| IR State | +----------+ | SO State |
+----------+ <----------------------------------> +----------+
Decisions about transitions between the various compression states
are taken by the compressor on the basis of:
- variations in packet headers
- positive feedback from decompressor (Acknowledgments -- ACKs)
- negative feedback from decompressor (Negative ACKs -- NACKs)
- robustness confidence level decision
How transitions are performed is explained in detail in session 5.7.
for each mode of operation.
4.1.1.1. Initialization and Refresh (IR) state
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The purpose of IR state is to initialize or refresh the static parts
of the context at the decompressor. In this state, the compressor
sends full header (or partial full header) periodically. The
compressor leaves the IR state only when it is confident that the
decompressor has correctly received the static information.
4.1.1.2. First Order (FO) State
The purpose of FO state is to efficiently transmit the difference
between the two consecutive packets in the TCP stream. When
operating in this state, the compressor and the decompressor should
have the same context. Only compressed packet is transmitted from
the compressor to the decompressor in this state. The compressor
transits back to IR state only when it finds that the context of
decompressor may be inconsistent, or there are remarkable changes in
the TCP/IP header.
4.1.1.3. Second Order (SO) State
The purpose of SO state is to efficiently transmit the fixed-payload
data. The compressor enters this state when it is sufficiently
confident that the decompressor has got the constant payload size of
the data transferring.
The compressor leaves this state and transits to the FO state when
the current payload size no longer conforms to the constant payload.
The compressor transits back to IR state only when it finds that the
context of decompressor may be inconsistent, or there are remarkable
changes in the TCP/IP header.
4.1.2. Decompressor states
The decompressor starts in its lowest compression state, "No
Context" and gradually transits to higher state, "Full Context". The
decompressor state machine normally never leaves the "Full Context"
state once it has entered this state.
+--------------+ +--------------+
| No Context | <---> | Full Context |
+--------------+ +--------------+
Initially, while working in the "No Context" state, the decompressor
has not yet successfully decompressed a packet. Once a packet has
been decompressed correctly, the decompressor can transit to the
"Full Context" state, and only upon repeated failures will it
transit back to lower state.
When state transitions are performed is explained in detail in
session 5.7.
4.2 Modes of operation
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There are two modes in ROHC-TCP, called Unidirectional and Bi-
directional mode, respectively. The mode transitions are conformed
to ROHC framework. However, the operations of each mode are
different.
4.2.1. Unidirectional mode -- U-mode
When in U-mode, packets are sent in one direction only: from
compressor to decompressor. Therefore, feedbacks from decompressor
to the compressor are unavailable under this mode.
In U-mode, transitions between compressor states are performed only
on account of the robustness confidence level and irregularities in
the header field change patterns in the compressed packet stream.
The error detection and error recovery is reply on the TCP protocol
itself. Due to the lack of feedback for correctness acknowledgement
and initiation of error recovery, compression in the Unidirectional
mode will be less efficient.
Compression with ROHC-TCP MUST starts in the Unidirectional mode.
Transition to the Bidirectional mode can be performed as soon as a
packet has reached the decompressor and it has replied with a
feedback packet indicating that a mode transition is desired (see
session 5.7).
4.2.2. Bi-directional mode -- B-mode
The Bidirectional mode is similar to the Unidirectional mode. The
difference is that a feedback channel is used to send error recovery
requests and (optionally) acknowledgments of significant context
updates from decompressor to compressor (not, however, for pure
sequence number updates). In this mode, the number of context
values can be reduced more efficiently.
B-mode aims to maximize compression efficiency and sparse usage of
the feedback channel. The frequency of context invalidation may be
lower than for U-mode.
4.3. Impairment considerations
Impairments to headers can be classified into the following types:
(1) the lower layer was not able to decode the packet and did not
deliver it to ROHC,
(2) the lower layer was able to decode the packet, but discarded it
because of a detected error,
(3) ROHC detected an error in the generated header and discarded the
packet, or
(4) ROHC did not detect that the regenerated header was damaged and
delivered it to upper layers.
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Impairments cause loss of individual headers. Some impairment
scenarios also cause context invalidation, which in turn results in
loss propagation. Loss propagation and impairments resulting in
loss or discarding of single packets both contribute to the packet
loss seen by upper layers.
Examples of context invalidating scenarios are:
(a) Impairment of type (4) on the forward channel, causing the
decompressor to update its context with incorrect information;
(b) Loss/error burst of headers: Impairments of types (1), (2) and
(3) on a number of consecutive headers that is large enough to
cause the decompressor to lose the context synchronization;
Scenario (a) is mitigated by the CRC carried in all headers. The
larger the CRC, the lower the chance of context invalidation caused
by (a). The CRC of headers is usually 3 bits and sometimes 7 or 8
bits.
Scenario (b) is mitigated by the CRC check.
ROHC detects damaged headers using CRCs over the original headers.
The small headers (how to define the small header is tend to be
further discussed) in this document include a 3-bit CRC. For the
smallest headers, damage is thus detected with a probability of
roughly 7/8.
The above analysis suggests that U-mode may be more prone than O-
mode to context invalidation. On the other hand, the CRC present in
all U/O-mode headers permits quick detection of context invalidation.
5. The Protocol
5.1. Data structures
As stated in [ROHC], the ROHC protocol is based on a number of
parameters that form part of the negotiated channel state and the
per-context state. For ROHC-TCP case, some parameters in Per-
channel and Per-context need to be modified.
PROFILES in Per-channel parameter:
Define a unique nonnegative integer to indicate the TCP/IP
profile supported by the decompressor. The compressor MUST NOT
compress using a profile not in PROFILES.
Profiles in Per-context parameter:
Profile 0x0005 is for TCP/IP compression.
5.2. ROHC-TCP packet formats from compressor to decompressor
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ROHC-TCP uses four packet types to identify compressed headers, and
three for initialization/refresh. The format of a compressed packet
does not depend on the mode.
Packet type: IR
This packet type communicates the static part of the context. It
can optionally also communicate the dynamic part of the context.
Packet type: IR-DYN
This packet type communicates the dynamic part of the context.
Packet type: IR-UPDATE
This packet type communicates the static and dynamic parts of the
replicated context.
5.3. Parameters needed for mode transition in ROHC-TCP
The packet types IR (with dynamic information), IR-DYN, and IR-
UPDATE are common for all modes.
Feedback of types ACK, NACK carry master sequence number, and
feedback packets can also carry a mode parameter indicating the
desired compression mode: U, or B.
As a shorthand, the notation PACKET(mode) is used to indicate which
mode value a packet carries. For example, an ACK with mode
parameter B is written ACK(B).
5.4 Robustness and efficiency maintenance
Considering the changing pattern of several TCP fields, Window-based
LSB encoding [ROHC], which does not assume the linear changing
pattern of the target header fields, is more suitable to encode
those TCP fields both efficiently and robustly.
Using ROHC-TCP, the compressor and decompressor maintain a context
value. To provide robustness, the compressor can maintain more than
one context value for each field. These values represent the r most
likely candidatesÆ values for the context at the decompressor.
ROHC-TCP ensures that the compressed header contains enough
information so that the uncompressed header can be extracted no
matter which one of the compressor context values is actually stored
at the decompressor. The only problem arises if the decompressor
has a context value that does not belong to the set of values stored
at the compressor; this situation is detected by a checksum over the
uncompressed header and the packet is discarded at the decompressor.
If more than one value for a field is stored in the compressor
context, W-LSB encoding will only succeed if sufficient LSBs are
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sent to infer correct value of the field regardless of the precise
value stored in the decompressor context.
Storing more context values at the compressor reduces the chance
that the decompressor will have a context value different from any
of the values stored at the compressor (which could cause the packet
to be decompressed incorrectly).
As an example, an implementation may choose to store the last r
values of each field in the compressor context. In this case
robustness is guaranteed against up to r - 1 consecutive dropped
packets between the compressor and the decompressor. Thus, by
keeping the value r large enough, the compressor rarely gets out of
synchronization with the decompressor.
However, the trade-off is that the larger the number of context
values is, the compressed header will be larger because it must
contain enough information to decompress relative to any of the
candidate context values.
To reduce the number of context value r, the compressor needs some
form of feedback to get sufficient confidence that a certain context
value will not be used as a reference by the decompressor. Then the
compressor can remove that value and all other values older than it
from its context. Obviously, when a feedback channel is available,
confidence can be achieved by proactive feedback in the form of ACKs
from the decompressor. A feedback channel, however, is unavailable
or expensive in some environments. For TCP/IP header compression,
an implicit feedback can be obtained from the nature feedback-loop
of TCP protocol itself.
Since TCP is a window-based protocol, a new segment cannot be
transmitted without getting the acknowledgment of segment in the
previous window. Upon receiving the new segment, the compressor can
get enough confidence that the decompressor has received the segment
in the previous window and then shrink the context window by
removing all the values older than that segment.
This is to say, the context window of ROHC-TCP, the number of
context value r, can be controlled by the TCP congestion window.
How to estimate TCP congestion window is an implementation issue
that can be determined by the compressor. A tracking based TCP
congestion window estimation algorithm is discussed in the Section
5.8.4 as an optional enhancement for the compressor.
5.5 Operation in unidirectional mode
5.5.1. Compressor state and logic
Below is the state machine for the compressor in Unidirectional mode.
Details of the transitions between states and compression logic are
given subsequent to the figure.
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Optimistic approach
+------>------>------>------>------>------>------>------>------+
| |
| Optimistic approach F-Optimistic approach |
| +------>------>------+ +------>------>------+ |
| | | | | |
| | v | v v
+----------+ +----------+ +----------+
| IR State | | FO State | | SO State |
+----------+ +----------+ +----------+
^ ^ | ^ | |
| | Update | | F-Update | |
| +------<------<------+ +------<------<------+ |
| |
| Update |
+------<------<------<------<------<------<------<------<------+
5.5.1.1. State transition logic (U-mode)
The transition logic for compression states in Unidirectional mode
is based on four principles: the optimistic approach principle, the
F-optimistic approach principle, the need for update, and the need
for F-update.
In ROHC-TCP, the compressor will start in the IR state and perform
different logics in different states. The following sub-sections
will describe the logic for each compressor sate in detail.
5.5.1.1.1. Optimistic approach, upwards transition
Transition to the FO or SO state from IR state in Unidirectional
mode is carried out according to the optimistic approach principle.
This means that the compressor transits to FO/SO state when it is
fairly confident that the decompressor has received enough
information to correctly decompress packets sent according to the
FO/SO state.
When the compressor is in the IR state, it will stay there until it
assumes that the decompressor has correctly received the context
information.
The compressor normally obtains its confidence about decompressor
status by sending uncompressed or partial full headers periodically
according to the IR/SO state. If the decompressor receives any of
these packets, it will be in sync with the compressor. For
robustness, the number of consecutive uncompressed or partial full
headers to send should larger than the context window. The interval
for sending the uncompressed or partial full header is not defined
in this document.
5.5.1.1.1.1. Optional transition for short-live TCP transfers
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This approach is introduced in ROHC-TCP to compress short-lived TCP
transfers more efficiently.
The key message of this approach is that the compressor should try
to speed up the initialization process. This approach can be
applied is the compressor is able to see the SYN packet. The
compressor enters the IR state when it receives the packet with SYN
bit set and sends IR packet. When it receives the first data packet
of the transfer, it should transit to FO state because that means
the decompressor has received the packet with SYN bit set and
established the context successfully at its side. Using this
mechanism can significantly reduce the number of context initiation
headers.
5.5.1.1.1.2. Optional operation in IR state
In IR state, the compressor can send full header (or partial full
header) periodically with an exponentially increasing period, which
is so-called compression slow-start [IPHC].
The main idea of this optional operation is controlling the size of
context window by dynamically TCP congestion window estimation.
After a packet has been sent out, the compressor invokes the
Algorithm SEQ and Algorithm ACK introduced in Session 5.8. to track
the congestion windows of the two one-way traffics with different
directions in a TCP connection. Suppose that the estimated
congestion windows are cwnd_seq and cwnd_ack, while the estimated
slow start thresholds are ssthresh_seq and ssthresh_ack,
respectively. Let
W(cwnd_seq, ssthresh_seq, cwnd_ack, ssthresh_ack) =
K*MAX(MAX(cwnd_seq, 2*ssthresh_seq),
MAX(cwnd_ack, 2*ssthresh_ack)).
If the value of context window, r, is larger than W(cwnd_seq,
ssthresh_seq, cwnd_ack, ssthresh_ack), r can be reduced. K is an
implementation parameter that will be further discussed in Section
5.8.
If the number of the compress packets been send gets larger than
W(cwnd_seq, ssthresh_seq, cwnd_ack, ssthresh_ack), the compressor
transits to FO/SO state.
If the compressor finds that the payload size of consecutive packets
is a constant value and one of such packets is removed from the
context window, which means the decompressor has known the exact
value of the constant size, it may transit to SO state. Otherwise it
will transit to the FO state.
If the compressor transits to the IR state from the higher states,
the compressor will re-initialize the algorithm for tracking TCP
congestion window.
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5.5.1.1.2. F-Optimistic approach, upwards transition
Transition to the SO state from FO state in Unidirectional mode is
carried out according to the F-optimistic approach principle. This
means that the compressor transits to SO state when it is fairly
confident that the decompressor has received enough information to
correctly decompress packets sent according to the SO state.
For transition from the FO to the SO state, the compressor should be
confident that the decompressor has all parameters needed to
decompress according to a fixed pattern.
5.5.1.1.2.1. Optional operation in FO state
After a packet has been sent out, the compressor invokes the
Algorithm SEQ and Algorithm ACK introduced in Session 5.8., to track
the congestion windows of the two one-way traffics with different
directions in a TCP connection. Suppose that the estimated
congestion windows are cwnd_seq and cwnd_ack, while the estimated
slow start thresholds are ssthresh_seq and ssthresh_ack,
respectively. Let
W(cwnd_seq, ssthresh_seq, cwnd_ack, ssthresh_ack) =
K*MAX(MAX(cwnd_seq, 2*ssthresh_seq),
MAX(cwnd_ack, 2*ssthresh_ack)).
If the value of context window, r, is larger than W(cwnd_seq,
ssthresh_seq, cwnd_ack, ssthresh_ack), r can be reduced. K is an
implementation parameter, which can be set to the same value as in
the IR state.
If the compressor finds that the payload size of consecutive packets
is a constant value and one of such packets has been removed from
the context window, which means the decompressor has known the exact
value of the constant size, it may transit to the SO state.
5.5.1.1.3. Need for update, downwards transition
If the static context of transfers changed or the compressor will not
confident whether the decompressor has the synchronized context, the
compressor will transit to the IR state.
5.5.1.1.3.1 Optional operation for downwards transition
The compressor must immediately transit back to the IR state when
the header to be compressed, falls behind the context window, i.e.
it is older than all the packets in context.
If the context window contains only one packet, which means there is
a long jump in the packet sequence number or acknowledge number, the
compressor will transit to the IR state. Here, a segment causes a
long jump when the distance between its sequence number (or
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acknowledgment number) and CMAXSN (or CMAXACK) is larger than the
estimated congestion window size, i.e.,
|sequence number (acknowledgement number) - CMAXSN (CMAXACK)| >
estimated congestion window size.
5.5.1.1.4. Need for F-update, downwards transition
The compressor must also immediately transit back to the FO state
when the header to be compressed does not conform to the established
pattern.
5.5.1.1.4.1. Optional operation in SO state
After a packet has been sent out, the compressor invokes the
Algorithm SEQ and Algorithm ACK to track the congestion windows of
the two one-way traffics with different directions in a TCP
connection. Suppose that the estimated congestion windows are
cwnd_seq and cwnd_ack, while the estimated slow start thresholds are
ssthresh_seq and ssthresh_ack, respectively. Let
W(cwnd_seq, ssthresh_seq, cwnd_ack, ssthresh_ack) =
K*MAX(MAX(cwnd_seq, 2*ssthresh_seq),
MAX(cwnd_ack, 2*ssthresh_ack)).
If the value of context window, r, is larger than W(cwnd_seq,
ssthresh_seq, cwnd_ack, ssthresh_ack), r can be reduced. K is an
implementation parameter, which can be set to the same value as in
the IR state.
If the payload size of the packets in context window doesn't keep
constant, the compressor transits to the FO state.
5.5.2. Decompressor logic
The logic of the decompressor is simpler compared to the compressor.
5.5.2.1. No Context State
The decompressor starts in this state. Upon receiving an IR or IR-
DYN packet, the decompressor should first verify the correctness of
this packet. Then it will determine that whether this is a full
header packet or PARTIAL-FULL one. For a PARTIAL-FULL header, the
decompressor will re-build a new context from the existing one and
make the necessary update. Otherwise, the decompressor will just
update the context. Finally, the decompressor will use this packet
as the reference packet.
After that, the decompressor will pass it to the system's network
layer and transit to Full Context State. Conformed to ROHC
framework [ROHC], only IR or IR-DYN packets may be decompressed in
No Context state.
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5.5.2.2. Full Context State
The operations of decompressor in Full Context state can be
summarized as follows:
a) Upon receiving an IR or IR-DYN packet, the decompressor should
verify the correctness of its header by CRC check. If the
verification succeeds, the decompressor will update the context and
use this packet as the reference packet. Consequently, the
decompressor will convert the packet into the original packet and
pass it to the network layer of the system.
b) Upon receiving the other type of packet, the decompressor will
decompress it. After that, the decompressor MUST verify the
correctness of the decompressed packet. If the verification
succeeds, the decompressor passes it to the system's network layer.
Then the decompressor will use it as the reference value if this
packet is not older than the current reference packet.
c) If consequent N packets fail to be decompressed, the decompressor
should transit downwards to No Context State. N is an
implementation parameter that will be further discussed in Section
5.8.
5.6. Operations in bi-directional mode
5.6.1. Compressor states and logic
Optimistic approach / ACK
+------>------>------>------>------>------>------>------>------+
| |
| Optimistic appr. / ACK F-Optimistic appr. /ACK ACK |
| +------>------>------+ +------>--- -->-----+ +->--+
| | | | | | |
| | v | v | v
+----------+ +----------+ +----------+
| IR State | | FO State | | SO State |
+----------+ +----------+ +----------+
^ ^ | ^ | |
| | NACK / Update | | F-Update | |
| +------<------<------+ +------<------<------+ |
| |
| NACK / Update |
+------<------<------<------<------<------<------<------<------+
Below is the state machine for the compressor in bi-directional mode.
The details of each state, state transitions and compression logic
are given subsequent to the figure.
The bi-directional mode makes use of feedback from decompressor to
compressor for OPTIONAL improving the transitions among different
states. Feedback packet types ACK and NACK are conform to the ones
defined in [ROHC].
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5.6.1.1. Master Sequence Number (MSN) for packet acknowledging
Feedback of types ACK and NACK carry the information about sequence
number/ acknowledgement number from decompressor to the compressor.
Unfortunately, sequence number/acknowledgement number field is not
guaranteed to be present in every IP protocol stack. Meanwhile, the
size of the sequence number/acknowledgement number field is rather
large, which is not so efficient if it is carried in the feedback
packet directly. To overcome this problem ROHC-TCP introduces a
control field called the Master Sequence Number (MSN) field. This
field is present in every m compressed header and can be used to
infer the acknowledged sequence number.
The value of m is chosen for the best trade-off between compression
efficiency and the acknowledging efficiency.
5.6.1.2. State transition logic
The transition logic for compression states in bi-directional mode
has much in common with the logic of the Unidirectional mode. The
optimistic / F-optimistic approach principles and transitions
occasioned by the need for update/F-update work in the same way as
described in session 5.5.1. However, the bi-directional mode makes
use of feedback from decompressor to compressor for transitions in
the backward direction and for OPTIONAL improved forward transition.
5.6.1.2.1. Negative acknowledgments (NACKs), downward transition
Negative acknowledgments (NACKs) is also called context requests.
Upon reception of a NACK the compressor transits back to the IR
state and sends updates (IR-DYN, or possibly IR) to the decompressor.
NACKs carry the MSN of the latest packet successfully decompressed.
5.6.1.2.2. Optional acknowledgments, upwards transition
In addition to NACKs, positive feedback (ACKs) MAY also be used for
packets in the Bi-directional mode. Upon reception of an ACK for an
updating packet, the compressor knows that the decompressor has
received the acknowledged packet and the transition to a higher
compression state can be carried out immediately. This
functionality is optional, so a compressor MUST NOT expect to get
such ACKs initially.
5.6.1.3. Compression logic and packets used
The compression logic is the same for the bi-directional mode as for
the Unidirectional mode (see section 5.5.1.).
In the IR state, the compressor can transit to the FO or SO state
once it receives a valid ACK(B) for an IR packet sent (an ACK(B) can
only be valid if it refers to a packet sent earlier). If the packet
referred by the feedback is in the context window, the compressor
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will remove the packets older than the referred packet from the
context window. Because ACK(B) means that the packet referred by
feedback has been the reference of the decompressor, the compressor
doesn't need to keep older packets.
If the compressor is in the FO or SO state, it will remove the
packets older than the referred packet by the feedback from the
context window.
Upon receiving an NACK(B), the compressor transits back to IR state.
5.6.2. Decompressor states and logic
The decompression states and the state transition logic are the same
as in the Unidirectional case (see section 5.2.2.). What differs is
the feedback logic.
When an IR packet passes the verification, send an ACK(B). When an
IR-DYN packet or other packet is correctly decompressed, optionally
send an ACK(B). In both cases, the feedback packet will carry the
master sequence number information about the nearest data packet
that had MSN.
In the Full Context state, when the verification check of x out of
the last y decompressed packets have failed, context damage SHOULD
be assumed and a NACK(B) SHOULD be sent. The decompressor moves to
the No Context state and discards all packets until an update which
passes the verification check is received.
Note that appropriate values for x and y, are related to the
residual error rate of the link. When the residual error rate is
close to zero, x = y = 1 may be appropriate.
In the No Context state, when any packet fails the verification,
send an NACK(B). The decompressor discards all packets until a
static update (IR) which passes the verification check is received.
5.7. Mode transitions
The decision to move from one compression mode to another is taken
by the decompressor and the possible mode transitions are shown in
the figure below. Subsequent sessions describe how the transitions
are performed together with exceptions for the compression and
decompression functionality during transitions.
+-------------------------+
| Unidirectional (U) mode |
+-------------------------+
| ^
| | Feedback(U)
| |
Feedback(B) | |
v |
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+-------------------------+
| Bidirectional (B) mode |
+-------------------------+
5.7.1. Compression and decompression during mode transitions
The following sections assume that, for each context, the compressor
and decompressor maintain a variable whose value is the current
compression mode for that context. The value of the variable
controls, for the context in question, which packet types to use,
which actions to be taken, etc.
As a safeguard against residual errors, all feedback sent during a
mode transition MUST be protected by a CRC, i.e., the CRC option
MUST be used. A mode transition MUST NOT be initiated by feedback
which is not protected by a CRC.
The subsequent subsections define exactly when to change the value
of the MODE variable. When ROHC transits between compression modes,
there are several cases where the behavior of compressor or
decompressor must be restricted during the transition phase. These
restrictions are defined by exception parameters that specify which
restrictions to apply. The transition descriptions in subsequent
sessions refer to these exception parameters and defines when they
are set and to what values. All mode-related parameters are listed
below together with their possible values, with explanations and
restrictions:
Parameters for the compressor side:
- C_MODE:
Possible values for the C_MODE parameter are (U)NIDIRECTIONAL,
(B)IDIRECTIONAL. C_MODE MUST be initialized to U.
- C_TRANS:
Possible values for the C_TRANS parameter are (P)ENDING and
(D)ONE. C_TRANS MUST be initialized to D. When C_TRANS is P,
it is REQUIRED
1) that the compressor only use packet formats common to all
modes,
2) that mode information is included in packets sent, at least
periodically,
4) that new mode transition requests be ignored.
Parameters for the decompressor side:
- D_MODE:
Possible values for the D_MODE parameter are (U)NIDIRECTIONAL
and (B)IDIRECTIONAL. D_MODE MUST be initialized to U.
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- D_TRANS:
Possible values for the D_TRANS parameter are (I)NITIATED,
(P)ENDING and (D)ONE. D_TRANS MUST be initialized to D. A
mode transition can be initiated only when D_TRANS is D. While
D_TRANS is I, the decompressor sends a NACK or ACK carrying a
CRC option for each received packet.
5.7.2. Transition from Unidirectional to Bidirectional mode
When there is a feedback channel available, the decompressor may at
any moment decide to initiate transition from Unidirectional to
Bidirectional mode. Any feedback packet carrying a CRC can be used
with the mode parameter set to O. The decompressor can then
directly start working in Optimistic mode. The compressor transits
from Unidirectional to Bidirectional mode as soon as it receives any
feedback packet that has the mode parameter set to B and that passes
the CRC check. The transition procedure is described below:
Compressor Decompressor
----------------------------------------------
| |
| ACK(B)/NACK(B) +-<-<-<-| D_MODE = B
| +-<-<-<-<-<-<-<-+ |
C_MODE = B |-<-<-<-+ |
| |
If the feedback packet is lost, the compressor will continue to work
in Unidirectional mode, but as soon as any feedback packet reaches
the compressor it will transit to Bidirectional mode.
5.7.3. Transition to Unidirectional mode
The decompressor can force a transition back to Unidirectional mode
if it desires to do so. A three-way handshake MUST be carried out
to ensure correct transition on the compressor side. The transition
procedure is described below:
After ACKing the IR-DYN(U), or IR(U), the decompressor MUST continue
to send feedback with the Mode parameter set to U until it receives
a U-mode packet.
Compressor Decompressor
----------------------------------------------
| |
| ACK(U)/NACK(U) +-<-<-<-| D_TRANS = I
| +-<-<-<-<-<-<-<-+ |
C_TRANS = P |-<-<-<-+ |
C_MODE = U | |
|->->->-+ IR/IR-DYN |
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| +->->->->->->->-+ |
|->-.. +->->->-|
|->-.. |
| ACK(MSN,U) +-<-<-<-|
| +-<-<-<-<-<-<-<-+ |
C_TRANS = D |-<-<-<-+ |
| |
|->->->-+ U-mode packet |
| +->->->->->->->-+ |
| +->->->-| D_TRANS = D, D_MODE= U
5.8. Implementation optimizations
5.8.1. Determine the value N
We should distinguish out of synchronization from the packet errors
cause by the link. So considering the error condition of the link,
N should be higher than the packet burst error length, a practical
range of N is around 8~10.
5.8.2. Determine the frequency of updating context
The choice of the frequency of updating context, ACK(B), is a
balance between the efficiency and robustness, i.e. sending ACK(B)
more frequently improves the compression robustness but adds more
system overhead, and the vice versa. From a practical view, the
ACK(B) SHOULD be sent for every 4~8 successfully decompressed
packets.
5.8.3. Tracking-based TCP congestion window estimation
As originally outlined in [CAC] and specified in [RFC2581], TCP is
incorporated with four congestion control algorithms: slow-start,
congestion-avoidance, fast retransmit, and fast recovery. The
effective window of TCP is mainly controlled by the congestion
window and may change during the entire connection life. ROHC-TCP
designs a mechanism to track the dynamics of TCP congestion window,
and control the number of context value r of windowed LSB encoding
by the estimated congestion window. By combining the windowed LSB
encoding and TCP congestion window tracking, ROHC-TCP can achieve
better performance over high bit-error-rate links.
Note that in one-way TCP traffic, only the information about
sequence number or acknowledgment number is available for tracking
TCP congestion window. ROHC-TCP does not require that all one-way
TCP traffics must cross the same compressor. The detail will be
described in the following sections.
5.8.3.1. General principle of congestion window tracking
The general principle of congestion window tracking is as follows.
The compressor imitates the congestion control behavior of TCP upon
receiving each segment, in the meantime, estimates the congestion
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window (cwnd) and the slow start threshold (ssthresh). Besides the
requirement of accuracy, there are also some other requirements for
the congestion window tracking algorithms:
- Simplex link. The tracking algorithm SHOULD always only take
Sequence Number or Acknowledgment Number of a one-way TCP
traffic into consideration. It SHOULD NOT use Sequence Number
and Acknowledgment Number of that traffic simultaneously.
- Misordering resilience. The tracking algorithm SHOULD work
well while receiving misordered segments.
- Multiple-links. The tracking algorithm SHOULD work well when
not all the one-way TCP traffics are crossing the same link.
- Slightly overestimation. If the tracking algorithm cannot
guarantee the accuracy of the estimated cwnd and ssthresh, it is
RECOMMANDED that it produces a slightly overestimated one.
The following sections will describe two congestion window tracking
algorithms, which use Sequence Number and Acknowledgment Number of a
one-way TCP traffic, respectively.
5.8.3.2. Tracking based on Sequence Number
This algorithm (Algorithm SEQ) contains 3 states: SLOW-START,
CONGESTION-AVOIDANCE, and FAST-RECOVERY, which are equivalent to the
states in TCP congestion control algorithms. It maintains 2
variables: cwnd and ssthresh.
+-------------+
| |
---------------->| CONGESTION- |
| | AVOIDANCE |
| ----| |<---
+------------+ | +-------------+ |
| | | |
| SLOW-START | | |
| | | +-------------+ |
+------------+ | | | |
| |-->| FAST- |----
| | RECOVERY |
---------------->| |
+-------------+
Initially, this algorithm starts in state SLOW-START with ssthresh
set to ISSTHRESH and cwnd set to IW.
Upon receiving a segment, if it is the first segment, which is not
necessary to be the SYN segment, the algorithm sets the current
maximum Sequence Number (CMAXSN) and the current minimum Sequence
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Number (CMINSN) to this segment's sequence number; otherwise, the
algorithm takes a procedure according to the current state.
- SLOW-START
* If the new Sequence Number (NSN) is larger than CMAXSN,
increase cwnd by the distance between NSN and CMAXSN, and
update CMAXSN and CMINSN based on the following rules:
CMAXSN = NSN
if (CMAXSN - CMINSN) > cwnd)
CMINSN = cwnd - CMAXSN;
If the cwnd is larger than ssthresh, the algorithm transits to
CONGESTION-AVOIDANCE state;
* If the distance between NSN and CMAXSN is less than or equal
to 3*MSS, ignore it;
* If the distance is larger than 3*MSS, halve the cwnd and set
ssthresh to MAX(cwnd, 2*MSS). After that, the algorithm
transits into FAST-RECOVERY state.
- CONGESTION-AVOIDANCE
* If NSN is larger than CMAXSN, increase cwnd by ((NSN-
CMAXSN)*MSS)/cwnd and then update CMAXSN and CMINSN based on
the following rules:
CMAXSN = NSN
if (CMAXSN - CMINSN) > cwnd)
CMINSN = cwnd - CMAXSN;
* If the distance between NSN and CMAXSN is less than or equal
to 3*MSS, ignore it;
* If the distance is larger than 3*MSS, halve the cwnd and set
ssthresh to MAX(cwnd, 2*MSS). After that, the algorithm
transits into FAST-RECOVERY state.
- FAST-RECOVERY
* If NSN is larger than or equal to CMAXSN + cwnd, the algorithm
transits into CONGESTION-AVOIDANCE state;
* Otherwise, ignore it.
In this algorithm, MSS is denoted as the estimated maximum segment
size. The implementation can use the MTU of the link as an
approximation of this value. ISSHRESH and IW are the initial values
of ssthresh and cwnd, respectively. ISSTHRESH MAY be arbitrarily
high. IW SHOULD be set to 4*MSS.
5.8.3.3. Tracking based on Acknowledgment Number
Zhang (ed.), et al. [Page 29]
draft-ietf-rohc-tcp-01.txt
+-------------+
| |
---------------->| CONGESTION- |
| | AVOIDANCE |
| ----| |<---
+------------+ | +-------------+ |
| | | |
| SLOW-START | | |
| | | +-------------+ |
+------------+ | | | |
| |-->| FAST- |----
| | RECOVERY |
---------------->| |
+-------------+
This algorithm (Algorithm ACK) maintains 3 states: SLOW-START,
CONGESTION-AVOIDANCE and FAST-RECOVERY, which are equivalent to the
states in TCP congestion control algorithms. It also maintains 2
variables: cwnd and ssthresh.
Initially, this algorithm starts in state SLOW-START with ssthresh
set to ISSTHRESH and cwnd set to IW.
Upon receiving a segment, if it is the first segment, which is not
necessary to be the SYN segment, the algorithm sets the current
maximum Acknowledgment Number (CMAXACK) to this segment's
acknowledgment number; otherwise, the algorithm takes a procedure
according to the current state.
- SLOW-START
* If the new Acknowledgment Number (NEWACK) is larger than
CMAXACK, increase cwnd by the distance between NEWACK and
CMAXACK, set duplicate ack counter (NDUPACKS) to 0, and update
CMAXACK accordingly; If the cwnd is larger than ssthresh, the
algorithm transits to CONGESTION-AVOIDANCE state;
* If NEWACK is equal to CMAXACK, increase the NDUPACKS by 1. If
NDUPACKS is greater than 3, halve the cwnd and set ssthresh to
MAX(cwnd, 2*MSS). Consequently, the algorithm transits into
FAST-RECOVERY state;
* Otherwise, set NDUPACKS to 0.
- CONGESTION-AVOIDANCE
* If NEWACK is larger than CMAXACK, increase cwnd by ((NEWACK-
CMAXACK)*MSS)/cwnd, set NDUPACKS to 0 and update CMAXACK
accordingly;
* If NEWACK is equal to CMAXACK, increase NDUPACKS by 1. If
NDUPACKS is greater than 3, halve the cwnd and set ssthresh to
Zhang (ed.), et al. [Page 30]
draft-ietf-rohc-tcp-01.txt
MAX(cwnd, 2*MSS). After that, the algorithm transits into
FAST-RECOVERY state;
* Otherwise, set NDUPACKS to 0.
- FAST-RECOVERY
* If NEWACK is larger than CMAXACK, set NDUPACKS to 0.
Consequently, the algorithm transits into CONGESTION-AVOID
state;
* Otherwise, ignore it.
In this algorithm, MSS is denoted as the estimated maximum segment
size. The implementation can use the MTU of the link as an
approximation of this value. ISSHRESH and IW are the initial values
of ssthresh and cwnd, respectively. ISSTHRESH MAY be arbitrarily
high. IW SHOULD be set to 4*MSS.
5.8.3.4. Further discussion on congestion window tracking
In some cases, it is inevitable for the tracking algorithms to
overestimate the TCP congestion window. Also, it SHOULD be avoided
that the estimated congestion window gets significantly smaller that
the actual one. For all of these cases, ROHC-TCP simply applies two
boundaries on the estimated congestion window size. One of the two
boundaries is the MIN boundary, which is the minimum congestion
window size and whose value is determined according to the [INITWIN];
the other boundary is the MAX boundary, which is the maximum
congestion window size. There are two possible approaches to
setting this MAX boundary. One is to select a commonly used maximum
TCP socket buffer size. The other one is to use the simple equation
W=sqrt(8/3*l), where W is the maximum window size and l is the
typical packet loss rate.
If ECN mechanism is deployed, according to [RFC2481] and [ECN], the
TCP sender will set the CWR (Congestion Window Reduced) flag in the
TCP header of the first new data packet sent after the window
reduction, and the TCP receiver will reset the ECN-Echo flag back to
0 after receiving a packet with CWR flag set. Thus, the CWR flag
and the ECN-Echo flag's transition from 1 to 0 can be used as
another indication of congestion combined with other mechanisms
mentioned in the tracking algorithm.
5.8.3.5. Determine the value K in congestion window estimation
As mentioned above, the context window SHOULD be shrunk when its
size gets larger than K*MAX(MAX(cwnd_seq, 2*ssthresh_seq),
MAX(cwnd_ack, 2*ssthresh_ack)). Since the Fast Recovery algorithm
was introduced in TCP, several TCP variants had been proposed, which
are different only in the behaviors of Fast Recovery. Some of them
need several RTTs to be recovered from multiple losses in a window.
Zhang (ed.), et al. [Page 31]
draft-ietf-rohc-tcp-01.txt
Ideally, they may send L*W/2 packets in this stage, where L is the
number of lost packets and W is the size of the congestion window
where error occurs. Some recent work [REQTCP] on improving TCP
performance allows transmitting packets even when receiving
duplicate acknowledgments. Due to the above concerns, it'd better
keep K large enough so as to prevent shrinking context window
without enough confidence that corresponding packets had been
successfully received.
Considering the bandwidth-limited environments and the limited
receiver buffer, a practical range of K is around 1~2. From the
simulation results, K=1 is good enough for most cases.
5.8.4. Possible optimization in U-mode
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 it may be the situation that a compressor and
decompressor could be co-located as illustrated in Figure 5.8.4. It
may then be possible for the compressor and decompressor at each end
of the link to exchange information. In such a scenario, ROHC-TCP
can make further optimization on context size for windowed LSB
encoding.
In Figure 5.8.4., C-SN represents the compressor for the sequence
number traffic that deployed in Host A, D-SN represents the
decompressor for the sequence number traffic that deployed in Host B.
Similarly, C-ACK represents the compressor for the acknowledgement
number traffic that deployed in Host B, D-ACK represents the
decompressor for the acknowledgement number traffic that deployed in
Host A.
Host A Host B
+------------------+ +------------------+
| | | |
| +---------+ | SN \ | +---------+ |
| | C-SN | | ~ ~ ~ ~ | | D-SN | |
| +---------+ | / | +---------+ |
| /|\ | | |
| | | | |
| +---------+ | / | +---------+ |
| | D-ACK | | ~ ~ ~ ~ | | C-ACK | |
| +---------+ | \ ACK | +---------+ |
| | | |
+------------------+ +------------------+
Figure 5.8.4. Illustration of Possible optimization in U-mode.
It is known that acknowledgement numbers (from C-ACK to D-ACK) are
generally taken from the sequence numbers (from C-SN to D-SN) in the
opposite direction. Since an acknowledgement cannot be generated
Zhang (ed.), et al. [Page 32]
draft-ietf-rohc-tcp-01.txt
for a packet that has not passed across the link, this offers an
efficient way of estimating the TCP congestion window.
Denote the current sequence number that is sending out from C-SN as
SN-New, denote the sequence number that been acknowledged by the D-
ACK as SN-Old, then the TCP congestion window (cwnd-bidir) can be
represented as
cwnd-bidir = SN-New - SN-Old.
Having this new estimated congestion window, the control parameter W
that is calculated in Section 5.2.1. will be re-calculated as
W(cwnd_seq, ssthresh_seq, cwnd_ack, ssthresh_ack)
= min ( W(cwnd_seq, ssthresh_seq, cwnd_ack, ssthresh_ack),
cwnd-bidir)
6. Coding Scheme and compressed packet header format
Following the requirement of TCP/IP header compression [REQTCP],
ROHC-TCP should fit into the ROHC framework. Thus, ROHC-TCP will
conform to the general format and the reserved packet types defined
in [ROHC].
In this document, ROHC-TCP adopts EPIC-LITE as the coding framework.
The detail of EPIC-LITE scheme had been discussed in [EPIC]. EPIC-
LITE is used to generate new ROHC profiles. This scheme takes as
its input a list of fields in the protocol stack to be compressed,
and for each field a choice of one or more compression techniques.
Using this input EPIC-LITE derives a set of compressed header
formats that can be used to quickly and efficiently compress and
decompress headers.
A TCP/IP profile is proposed to describe the behaviors of each field
in TCP/IP header.
6.1. Fixed-payload encoding
For some applications, such as bulk data transferring, etc., the
payload size of each packet is usually a constant value, e.g. 1460
bytes. In such a case, the sequence number and acknowledgment number
can be represented as the following equation:
SEQ (or ACK) = m * PAYLOAD + n.
If all the packets in context window have the same 'n', only 'm'
need be transmitted to the decompressor. The decompressor can
assign the value of PAYLOAD by the packet size of the reference
packet. Then, the decompressor can obtain the sequence number or
acknowledgment number after correctly decoding 'm', and use them as
the reference values. This encoding method is called fixed-payload
encoding.
Zhang (ed.), et al. [Page 33]
draft-ietf-rohc-tcp-01.txt
6.2. ROHC Profile for compression of TCP/IP
This session describes a ROHC profile for the compression of TCP/IP.
< need to be further refine>
Note that the probabilities listed for each encoding method are
initial estimates only. These need to be refined with more accurate
values from genuine TCP/IP streams.
The profile for TCP/IP compression is given below:
only uses the following toolbox methods:
- STATIC-KNOWN
- STATIC-UNKNOWN
- STATIC
- IRREGULAR
- LSB
- VALUE
- C
profile_identifier 0xFFFF
max_formats 60
; to be refined
max_sets 48
bit_alignment 8
npatterns 224
CO_packet TCP-IP-CO
IR_packet TCP-IP-IR
IR-DYN_packet TCP-IP-DYN
TCP-IP-CO = INFERRED-IP-CHECKSUM(IPv4-co-header)
TCP-co-header
skip-msn
CRC(3,80%) | CRC(7,20%)
;
; since we want to have a MSN in some packets and not in others,
; there needs to be some way to indicate that there is a
; context value present.
;
; we need to agree on the degree of CRC protection that is
; appropriate for the various packets
;
TCP-IP-IR = INFERRED-IP-CHECKSUM(IPv4-ir-header)
TCP-ir-header
msn-ir
CRC(8,100%)
TCP-IP-DYN = INFERRED-IP-CHECKSUM(IPv4-dyn-header)
Zhang (ed.), et al. [Page 34]
draft-ietf-rohc-tcp-01.txt
TCP-dyn-header
msn-dyn
CRC(8,100%)
IPv4-co-header = version
header_len
tos-co
ecn
length
ip-id-co
rf_flag
df_flag-co
mf_flag
offset
ttl-co
protocol
ip_chksum
src_address-co
dst_address-co
IPv4-ir-header = version
header_len
tos-dyn
ecn
length
ip-id-dyn
rf_flag
df_flag-dyn
mf_flag
offset
ttl-dyn
protocol
ip_chksum
src_address-ir
dst_address-ir
IPv4-dyn-header = version
header_len
tos-dyn
ecn
length
ip-id-dyn
rf_flag
df_flag-dyn
mf_flag
offset
ttl-dyn
protocol
ip_chksum
src_address-co
dst_address-co
version = VALUE(4,4,100%)
Zhang (ed.), et al. [Page 35]
draft-ietf-rohc-tcp-01.txt
header_len = VALUE(4,5,100%)
tos-co = STATIC(99%) | IRREGULAR(6,1%)
tos-dyn = IRREGULAR(6,100%)
ecn = STACK-TO-CONTROL(2)
length = INFERRED-SIZE(16,128)
ip-id-co = NBO(16) ; check byte-order
FORMAT(ip-id-seq-co, ip-id-rnd)
STATIC(100%) ; nbo flag
ip-id-dyn = NBO(16) ; check byte-order
FORMAT(ip-id-seq-dyn, ip-id-rnd)
IRREGULAR(16,100%)
IRREGULAR(1,100%) ; nbo flag
ip-id-seq-co = LSB(4,3,70%) | LSB(6,8,15%) |
LSB(8,8,15%) | IRREGULAR(16,30%)
ip-id-rnd = IRREGULAR(16,100%)
ip-id-seq-dyn = IRREGULAR(16,100%)
;
; ip-id-seq-co should be refined as follows:
;
; ip-id-seq-co = LSB(4,3,70%) | LSB(6,8,15%) |
; LSB(8,8,10%) | IRREGULAR(16,5%)
;
rf_flag = STACK-TO-CONTROL(1)
df_flag-co = STATIC(80%) | IRREGULAR(20%)
df-flag-dyn = IRREGULAR(1,100%)
mf_flag = VALUE(1,0,100%)
offset = VALUE(13,0,100%)
ttl-co = STATIC(99%) | IRREGULAR(8,1%)
ttl-dyn = IRREGULAR(8,100%)
protocol = VALUE(8,6,100%)
Zhang (ed.), et al. [Page 36]
draft-ietf-rohc-tcp-01.txt
; need to take account of extension
; headers at some point
;
ip_chksum = VALUE(16,0,100%)
src_address-co = STATIC(100%)
src_address-ir = IRREGULAR(32,100%)
dst_address-co = STATIC(100%)
dst_address-ir = IRREGULAR(32,100%)
TCP-header-co = source_port-co
dest_port-co
seqno
ackno
data_offset
flags-co
window-co
tcp_chksum
urg_ptr-co
TCP-header-dyn = source_port-co
dest_port-co
seqno
ackno
data_offset
flags-dyn
window-dyn
tcp_chksum
urg_ptr-dyn
TCP-header-ir = source_port-ir
dest_port-ir
seqno
ackno
data_offset
flags-dyn
window-dyn
tcp_chksum
urg_ptr-dyn
source_port-co = STATIC(100%)
source_port-ir = IRREGULAR(16,100%)
dest_port-co = STATIC(100%)
dest_port-ir = IRREGULAR(16,100%)
seqno = STACK-TO-CONTROL(32)
Zhang (ed.), et al. [Page 37]
draft-ietf-rohc-tcp-01.txt
ackno = STACK-TO-CONTROL(32)
;
; the following encode-method, 'seqno-co' and 'ackno-co' should be
; refined or removed if 'seqno-and-ackno-coÆ is used.
;
seqno-co = LSB(8,63,5%) | LSB(14,4096,80%) |
LSB(20,16384,10%) | IRREGULAR(32,5%)
seqno-dyn = IRREGULAR(32,100%)
ackno-co = LSB(8,0,10%) | LSB(14,0,80%) |
LSB(20,0,5%) | IRREGULAR(32,5%)
ackno-dyn = IRREGULAR(32,100%)
data_offset = STACK-TO-CONTTROL(4)
window-co = STATIC(80%) | LSB(12,63,10%) |
IRREGULAR(16,10%)
window-dyn = IRREGULAR(16,100%)
tcp_chksum = IRREGULAR(16,100%)
urg-ptr = STACK-TO-CONTROL(16)
urg_ptr-co = STATIC(99%) | IRREGULAR(16,1%)
urg_ptr-dyn = IRREGULAR(16,100%)
flags-co = reserved-co
cwr
ece
urg
ack
psh
rst-syn-fin-co
flags-dyn = reserved-dyn
IRREGULAR(2,100%)
urg
IRREGULAR(1,100%)
psh-dyn
rst-syn-fin-dyn
get-rf = ROTATE(4,1)
STACK-FROM-CONTROL(1)
reserved-co = get-rf
Zhang (ed.), et al. [Page 38]
draft-ietf-rohc-tcp-01.txt
FORMAT(reserved-unused, reserved-used)
STATIC(100%) ; format selector
reserved-dyn = get-rf
FORMAT(reserved-unused, reserved-used)
IRREGULAR(1,100%) ; format selector
reserved-unused = VALUE(5,0,100%)
reserved-used = reserved-flag
reserved-flag
reserved-flag
reserved-flag
reserved-flag
reserved-flag = IRREGULAR(1,100%)
cwr-ece = STACK-TO-CONTROL(2)
cwr = IRREGULAR(1,100%)
ece = IRREGULAR(1,100%)
urg = STACK-TO-CONTROL(1)
ack-co = VALUE(1,1,99.9%) | VALUE(1,0,0.1%)
ack-dyn = IRREGULAR(1,100%)
psh-co = FORMAT(reg-psh-co, irreg-psh)
STATIC(100%) ; format selector
psh-dyn = FORMAT(reg-psh-dyn, irreg-psh)
IRREGULAR(1,100%) ; format selector
reg-psh-co = STATIC(90%) | N(IRREGULAR(1,9%) |
IRREGULAR(1,9%)
reg-psh-dyn = IRREGULAR(1,100%)
irreg-psh = IRREGULAR(1,100%)
rst-syn-fin-co = no-flag(98%) | rst-only(1%) | fin-only(1%)
no-flag = VALUE(3,0x0,100%)
rst-only = VALUE(3,0x4,100%)
syn-only = VALUE(3,0x2,100%)
fin-only = VALUE(3,0x1,100%)
rst-syn-fin-dyn = IRREGULAR(3,100%)
Zhang (ed.), et al. [Page 39]
draft-ietf-rohc-tcp-01.txt
;
; Since some TCP options MUST NOT occur in non-SYN packets and the
; compressed packet is only used for non-SYN packets, several
; encode-method, such as 'sack-permitted', should be removed from
; encode-method 'tcp-options-co'. Also, TCP No-Operation options is
; omitted here. In summary, encode-method 'tcp-options-co' should be
; refined.
;
tcp-options-co = ROTATE(3,1) ; get TCP data offset
LIST(4,1,32,-160,8,
U(OPTIONAL(tcp-sack-co)),
U(OPTIONAL(tcp-timestamp-co)),
U(OPTIONAL(tcp-mss)),
U(OPTIONAL(tcp-end)),
U(OPTIONAL(tcp-wscale)),
U(OPTIONAL(sack-permitted)),
U(OPTIONAL(tcp-generic-co)),
U(OPTIONAL(tcp-generic-co)),
5,8,2,0,3,4)
STATIC(100%) ; option order
STATIC(95%) | IRREGULAR(8,5%) ; option presence
;
; Encode-method 'tcp-options-dyn' should be refined as describe
; above.
;
tcp-options-dyn = ROTATE(3,1) ; get TCP data offset
LIST(4,1,32,-160,8,
U(OPTIONAL(tcp-sack-dyn)),
U(OPTIONAL(tcp-timestamp-dyn)),
U(OPTIONAL(tcp-mss)),
U(OPTIONAL(tcp-end)),
U(OPTIONAL(tcp-wscale)),
U(OPTIONAL(sack-permitted)),
U(OPTIONAL(tcp-generic-dyn)),
U(OPTIONAL(tcp-generic-dyn)),
5,8,2,0,3,4)
IRREGULAR(24,100%) ; option order
IRREGULAR(8,100%) ; option presence
;
; need further discussion
;
tcp-sack-co = VALUE(8,5,100%) ; type
Zhang (ed.), et al. [Page 40]
draft-ietf-rohc-tcp-01.txt
STACK-TO-CONTROL(8) ; length
LIST(8,1,8,-16,0,
OPTIONAL(sack-block-co),
OPTIONAL(sack-block-co),
OPTIONAL(sack-block-co),
OPTIONAL(sack-block-co))
STATIC(90%) | IRREGULAR(8,10%) ; order
sack-block-presence
sack-block-presence = VALUE(1,1,100%)
VALUE(1,1,50%) | VALUE(1,0,50%)
VALUE(1,1,20%) | VALUE(1,0,80%)
VALUE(1,1,5%) | VALUE(1,0,95%)
tcp-sack-dyn = VALUE(8,5,100%) ; type
STACK-TO-CONTROL(8) ; length
LIST(8,1,8,-16,0,
OPTIONAL(sack-block-dyn),
OPTIONAL(sack-block-dyn),
OPTIONAL(sack-block-dyn),
OPTIONAL(sack-block-dyn))
IRREGULAR(8,10%) ; order
sack-block-presence-dyn
sack-block-presence-dyn = IRREGULAR(1,100%)
IRREGULAR(1,100%)
IRREGULAR(1,100%)
IRREGULAR(1,100%)
sack-block-dyn = sack-element-dyn
sack-element-dyn
sack-block-co = sack-element-co
sack-element-co
sack-element-dyn = INFERRED-OFFSET-LIST(32) ; offset from base
STACK-TO-CONTROL(32) ; preserve new base
Zhang (ed.), et al. [Page 41]
draft-ietf-rohc-tcp-01.txt
IRREGULAR(32,100%)
sack-element = INFERRED-OFFSET-LIST(32) ; offset from base
STACK-TO-CONTROL(32) ; preserve new base
STATIC(50%) | LSB(16,32768,30%) |
LSB(24,1048576,20%)
tcp-timestamp-co = VALUE(8,8,100%) ; type
VALUE(8,10,100%) ; length
ts-entry
ts-entry
ts-entry-co = LSB(12,0,60%) | LSB(16,0,30%) |
IRREGULAR(32,10%)
ts-entry-dyn = IRREGULAR(32,100%)
tcp-mss = VALUE(8,2,100%) ; type
VALUE(8,4,100%) ; length
IRREGULAR-PADDED(16,11,99%) | IRREGULAR(16,1%)
tcp-end = VALUE(8,0,100%) ; type
tcp-wscale = VALUE(8,3,100%) ; type
VALUE(8,3,100%) ; length
IRREGULAR-PADDED(8,4,100%) ; scale
tcp-sack-permitted = VALUE(8,4,100%) ; type
VALUE(8,2,100%) ; length
tcp-generic-co = STATIC(100%) ; type
STACK-TO-CONTROL(8) ; length
UNCOMPRESSED(8,1,8,-16) ; value
tcp-generic-dyn = IRREGULAR(8,100%) ; type
STACK-TO-CONTROL(8) ; length
UNCOMPRESSED(8,1,8,-16) ; value
get-seq-ack = ROTATE(2,1)
STACK-FROM-CONTROL(2) ; cwr/ece
ROTATE(4,1)
STACK-FROM-CONTROL(2) ; ecn
STACK-FROM-CONTROL(32) ; ack
STACK-FROM-CONTROL(32) ; seq
Zhang (ed.), et al. [Page 42]
draft-ietf-rohc-tcp-01.txt
seqno-and-ackno-co = FORMAT(bulk-data-co, bulk-ack-co, interactive-
co)
STATIC(100%) ; format selector
seqno-and-ackno-dyn = FORMAT(bulk-data-dyn, bulk-ack-dyn,
interactive-dyn)
IRREGULAR(2,100%) ; format-selector
bulk-data-co = data-seqno-co
data-ackno-co
ecn-co
bulk-ack-co = ack-seqno-co
ack-ackno-co
ecn-co
interactive-co = interact-seqno-co
interact-ackno-co
ecn-co
bulk-data-dyn = seqno-dyn
ackno-dyn
ecn-dyn
bulk-ack-dyn = seqno-dyn
ackno-dyn
ecn-dyn
interactive-dyn = seqno-dyn
ackno-dyn
ecn-dyn
;
; To be refined.
;
data-seqno-co = LSB(14,0,50%) | LSB(16,32768,30%) |
LSB(20,65536,20%)
data-ackno-co = STATIC(50%) | LSB(10,0,30%) | LSB(16,0,20%)
ecn-co = FORMAT(no-ecn-co, use-ecn-co)
no-ecn-co = VALUE(2,0,100%)
VALUE(2,0,100%)
use-ecn-co = IRREGULAR(4,100%)
Zhang (ed.), et al. [Page 43]
draft-ietf-rohc-tcp-01.txt
seqno-dyn = IRREGULAR(32,100%)
ackno-dyn = IRREGULAR(32,100%)
ecn-dyn = FORMAT(no-ecn-dyn, use-ecn-dyn)
IRREGULAR(1,100%) ; format selector
no-ecn-dyn = VALUE(2,0,100%)
VALUE(2,0,100%)
use-ecn-dyn = IRREGULAR(2,100%)
IRREGULAR(2,100%)
ack-seqno-co = STATIC(50%) | LSB(10,0,30%) | LSB(16,0,20%)
ack-ackno-co = LSB(14,0,50%) | LSB(16,0,30%) |
LSB(20,0,20%)
interact-seqno-co = LSB(10,256,35%) | LSB(14,2048,50%) |
LSB(18,4096,15%)
interact-ackno-co = LSB(10,0,35%) | LSB(14,0,50%) | LSB(18,0,15%)
msn-ir = MSN-IRREGULAR(16,10%)
msn-dyn = MSN-LSB(3,0,90%) | MSN-LSB(7,0,9%) | MSN-
IRREGULAR(16,1%)
;
; context-updating support
;
; The following is the profile for ROHC context-updating. To be
; refined.
;
TCP-IP-UPDATE = base-context-id
INFERRED-IP-CHECKSUM(IPv4-update-header)
TCP-update-header
msn-dyn
CRC(8,100%)
Base-context-id = VALUE(0,0,60%) ; this context is updated from
; a context, base context, with
; the same CID, i.e. this
; context is updated from
; itself.
| IRREGULAR(N,40%) ; the context is updated from
; the base context with a
; different CID. N is the
Zhang (ed.), et al. [Page 44]
draft-ietf-rohc-tcp-01.txt
; number of bits for a CID.
; The compressor choose a CID as the base of the
; new CID. The decompressor decompress the
; base-context-id from TCP-IP-UPDATE and copy the
; content of base context to the new context.
IPv4-update-header = version
Header_len
tos-update
ecn
length
ip-id-update
rf-flag
df-flag-dyn
mf-flag
offset
ttl-update
protocol
ip-chhsum
src-address-update
dst-address-update
tos-update = STATIC(99%) | IRREGULAR(6,1%)
ip-id-update = NBO(16)
FORMAT(ip-id-seq-update, ip-id-rnd-update)
STATIC(99%) | IRREGULAR(1,100%) ; nbo flag
ip-id-seq-update = LSB(4,3,70%) | LSB(6,8,15%)
| LSB(8,8,10%) | IRREGULAR(16,5%)
ip-ip-rnd-update = IRREGULAR(16,100%)
ttl-update = STATIC(90%) | IRREGULAR(8,10%)
src-address-update = STATIC(90%) | IRREGULAR(32,10%)
dst-address-update = STATIC(90%) | IRREGULAR(32,10%)
TCP-header-update = source-port-update
dest-port-update
seqno
ackno
data_offset
flags-dyn
window-update
tcp_chksum
urg_ptr-dyn
source-port-update = LSB(4,3,70%) | LSB(6,8,15%)
| LSB(8,8,10%) | IRREGULAR(16,5%)
Zhang (ed.), et al. [Page 45]
draft-ietf-rohc-tcp-01.txt
dest-port-update = LSB(4,3,70%) | LSB(6,8,15%)
| LSB(8,8,10%) | IRREGULAR(16,5%)
; DELTA encoding may be more efficient here.
; However, the delta should be the difference
; between the new port number and the port
; number in the base context, instead of the
; difference between packets.
window-update = STATIC(80%) | LSB(12,63,10%)
| IRREGULAR(16,100%)
tcp-options-update = ROTATE(3,1) ; get the data_offset
LIST(4,1,32,-160,8,
U(OPTIONAL(tcp-sack-dyn)),
U(OPTIONAL(tcp-timestamp-update)),
U(OPTIONAL(tcp-mss-update)),
U(OPTIONAL(tcp-end)),
U(OPTIONAL(tcp-wscale-update)),
U(OPTIONAL(sack-permitted-update)),
U(OPTIONAL(tcp-generic-dyn)),
U(OPTIONAL(tcp-generic-dyn)),
5,8,2,0,3,4)
STATIC(%90) | IRREGULAR(24,10%) ; order
STATIC(%90) | IRREGULAR(8,10%) ; presence
tcp-timestamp-update = VALUE(8,8,100%)
VALUE(8,10,100%)
ts-entry-update
ts-entry-update
ts-entry-update = LSB(12,0,60%) | LSB(16,0,30%)
| IRREGULAR(32,10%)
tcp-mss-update = VALUE(8,2,100%)
VALUE(8,4,100%)
STATIC(90%) | IRREGULAR-PADDED(16,11,9%)
| IRREGULAR(16,1%)
tcp-wscale = VALUE(8,3,100%)
VALUE(8,3,100%)
STATIC(90%) | IRREGULAR-PADDED(8,4,10%)
sack-permitted-update = VALUE(8,4,100%)
VALUE(8,2,100%)
7. Security considerations
ROHC-TCP is conformed to ROHC framework. Consequently the security
considerations for ROHC-TCP match those of [ROHC].
8. Acknowledgements
Zhang (ed.), et al. [Page 46]
draft-ietf-rohc-tcp-01.txt
Header compression schemes from [VJHC, IPHC, ROHC] have been
important sources of ideas and knowledge. This document also
benefited from discussion on the rohc mailing list about some key
issues.
9. References
[RFC2026] S. Bradner, "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996.
[VJHC] V. Jacobson, "Compressing TCP/IP headers for low-speed
serial links", RFC 1144, February 1990.
[IPHC] M. Degermark, B. Nordgren, and S. Pink, "IP Header
Compression", RFC 2507, February 1999.
[RFC2018] M. Mathis, J. Mahdavi, S. Floyd, and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
[RFC2883] S. Floyd, J. Mahdavi, M. Mathis, and M. Podolsky, "An
Extension to the Selective Acknowledgement (SACK) Option for TCP",
RFC 2883, July 2000.
[RFC1323] V. Jacobson, R. Braden, and D. Borman, "TCP Extensions
for High Performance", RFC 1323, May 1992.
[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[E2E] V. Jacobson, "Fast Retransmit", Message to the end2end-
interest mailing list, April 1990.
[Mobi96] M. Degermark, M. Engan, B. Nordgren, and Stephen Pink,
"Low-loss TCP/IP header compression for wireless networks", In
the Proceedings of MobiCom, 1996.
[RFC3095] Bormann (ed.), et al., "RObust Header Compression (ROHC)",
RFC 3095, July 2001.
[CAC] V. Jacobson, "Congestion avoidance and control", In ACM
SIGCOMM '88, 1988.
[RFC2581] M. Allman, V. Paxson, and W. R. Stevens, "TCP Congestion
Control", RFC 2581, April 1999.
[RFC2481] K. Ramakrishnan, S. Floyd, "A Proposal to add Explicit
Congestion Notification (ECN) to IP", RFC 2481, January 1999.
[ECN] K. K. RamaKrishnan, Sally Floyd, D. Black, "The Addition of
Explicit Congestion Notification (ECN) to IP", Internet Draft
(work in progress), June, 2001. <draft-ietf-tsvwg-ecn-04.txt>
Zhang (ed.), et al. [Page 47]
draft-ietf-rohc-tcp-01.txt
[REQTCP] L-E. Jonsson, "Requirements for ROHC IP/TCP header
compression", Internet Draft (work in progress), June 20, 2001.
[LTX] M. Allman, H. Balakrishnan, and S. Floyd, "Enhancing TCP's
Loss Recovery Using Limited Transmit", Internet Draft (work in
progress), August 2000. <draft-ietf-tsvwg-limited-xmit-00.txt>
[RFC3096] M. Degermark, "Requirements for robust IP/UDP/RTP header
compression", RFC 3096, July 2001.
[INITWIN] M. Allman, S. Floyd, and C. Partridge, "Increasing TCP's
Initial Window", Internet Draft (work in progress), May 2001.
<draft-ietf-tsvwg-initwin-00.txt>
[EPIC] Richard Price et al, "Framework for EPIC-LITE", <draft-ietf-
rohc-epic-lite-00.txt>, Internet Draft (work in progress), Oct.
2001.
[RFC791] Postel, J., "Internet Protocol", STD 5, September 1981.
[RFC793] Postel, J., "Transmission Control Protocol", STD 7,
DARPA, September 1981.
[RFC1072] Jacobson, V., and R. Braden, "TCP Extensions for Long-
Delay Paths", LBL, ISI, October 1988.
[RFC1146] Zweig, J., and C. Partridge, "TCP Alternate Checksum
Options", UIUC, BBN, March 1990.
[RFC1323] Jacobson, V., Braden, R., and D. Borman, "TCP
Extensions for High Performance", LBL, ISI, Cray Research, May 1992.
[RFC1644] Braden, R. "T/TCP -- TCP Extensions for Transactions
Functional Specification", ISI, July 1994.
[RFC1693] Connolly, T., et al, "An Extension to TCP : Partial
Order Service", University of Delaware, November 1994.
[RFC2001] Stevens, W., TCP Slow Start, Congestion Avoidance,
Fast Retransmit, and Fast Recovery Algorithms, NOAO, January 1997
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and Romanow, A.,
TCP Selective Acknowledgement Options, April 1996.
[RFC2026] Bradner, S., "The Internet Standards Process - Revision
3", October 1996
[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP
MD5 Signature Option", Cisco Systems, August 1998.
[RFC2474] Nichols, K., et al Definition of the Differentiated
Services Field (DS Field) in the IPv4 and IPv6 Headers, December
Zhang (ed.), et al. [Page 48]
draft-ietf-rohc-tcp-01.txt
1998
[RFC 2508] Casner, S., Jacobson, V., Compressing IP/UDP/RTP
Headers for Low-Speed Serial Links, Cisco Systems, February 1999
[RFC2509] Engan, M., et al, IP Header Compression over PPP,
February 1999
[RFC2883] Floyd, S., et al, An Extension to the Selective
Acknowledgement (SACK) Option for TCP, July 2000
[ECN-X] Spring, N., Wetherall, D., Ely, D., Robust ECN
Signaling with Nonces, draft-ietf-tsvwg-tcp-nonce-02.txt,
University of Washington, October 2001.
[IANA] http://www.iana.org/assignments/tcp-parameters
[RFC3150] Dawkins, S., Montenegro, G., Kojo, M. and V. Magret, "End-
to-end Performance Implications of Slow Links", BCP 48, RFC 3150,
July 2001.
[TCP-WIRELESS] Inamura, H., Montenegro, G., Ludwig, R., Gurtov, A.
and F. Khafizov, "TCP over 2.5G and 3G Wireless Networks", draft-
ietf-pilc-2.5g3g-07.txt (work in progress), February 2002.
[EIFEL] Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm for
TCP", draft-ietf-tsvwg-tcp-eifel-alg-03.txt (work in progress),
February 2002.
[PILC-ASYM] Balakrishnan, , Padmanabhan, V., Fairhurst, G. and M.
Sooriyabandara, "TCP Performance Implications of Network Path
Asymmetry", draft-ietf-pilc-asym-07.txt (work in progress), November
2001.
[PILC-ARQ] G. Fairhurst, L. Wood, ôAdvice to link designers on link
Automatic Repeat reQuest (ARQ)ö, draft-ietf-pilc-link-arq-issues-
03.txt (work in progress), Feb. 2002.
10. Authors' addresses
Qian Zhang Tel: +86 10 62617711-3135
Email: qianz@microsoft.com
HongBin Liao Tel: +86 10 62617711-3156
Email: i-hbliao@microsoft.com
Ya-Qin Zhang Tel: +86 10 62617711
Email: yzhang@microsoft.com
Wenwu Zhu Tel: +86 10 62617711-5405
Email: wwzhu@microsoft.com
Microsoft Research Asia
Zhang (ed.), et al. [Page 49]
draft-ietf-rohc-tcp-01.txt
Beijing Sigma Center
No.49, Zhichun Road, Haidian District
Beijing 100080, P.R.C.
Robert Hancock Tel: +44 1794 833601
Email: robert.hancock@roke.co.uk
Stephen McCann Tel: +44 1794 833341
Email: stephen.mccann@roke.co.uk
Paul Ollis Tel: +44 1794 833168
Email: paul.ollis@roke.co.uk
Richard Price Tel: +44 1794 833681
Email: richard.price@roke.co.uk
Abigail Surtees Tel: +44 1794 833131
Email: abigail.surtees@roke.co.uk
Mark A West Tel: +44 1794 833311
Email: mark.a.west@roke.co.uk
Roke Manor Research Ltd
Romsey, Hants, SO51 0ZN
United Kingdom
Zhang (ed.), et al. [Page 50]
draft-ietf-rohc-tcp-01.txt
Appendix A - Detailed classification of header fields
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.
Since the IP header does exhibit some slightly different behavior
from that previously presented in [ROHC] for the RTP case, it is
also included in this document.
It is intentional that much of the classification text from [ROHC]
has been borrowed. This is for easier reading rather than inserting
many references to that document.
Again based on the format presented in [ROHC] TCP/IP header fields
are classified and analyzed in two steps. First, we have a general
classification in Section A.1, 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. A less stable but
more detailed analysis of the change characteristics is then done in
Section A.4. Finally, Section A.5 summarizes this appendix with
conclusions about how the various header fields should be handled by
the header compression scheme to optimize compression and
functionality.
A.1. General classification
Definitions (and some text) copied from [ROHC] Appendix A.
Differences between IP field behavior between [ROHC] (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:
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.
STATIC These fields are expected to be constant throughout the
lifetime of the packet stream. Static information must
in some way be communicated once.
STATIC-DEF STATIC fields whose values define a packet stream. They
are in general handled as STATIC.
STATIC-KNOWN These STATIC fields are expected to have well-known
Zhang (ed.), et al. [Page 51]
draft-ietf-rohc-tcp-01.txt
values and therefore do not need to be communicated at
all.
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 A.2, CHANGING fields are further examined and classified on
the basis of their expected change behavior.
A.1.1. IP header fields
A.1.1.1. 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 [ROHC]
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.
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.
Payload Length
Zhang (ed.), et al. [Page 52]
draft-ietf-rohc-tcp-01.txt
Information about packet length (and, consequently, payload
length) is expected to be provided by the link layer. The field
is therefore classified as INFERRED.
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 [ROHC]. 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.
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 |
+--------------+--------------+
A.1.1.2. IPv4 header fields
+---------------------+-------------+----------------+
| Field | Size (bits) | Class |
+---------------------+-------------+----------------+
| Version | 4 | STATIC |
| Header Length | 4 | STATIC-KNOWN |
| DSCP* | 6 | CHANGING |
| ECT flag* | 1 | CHANGING |
| CE flag* | 1 | CHANGING |
Zhang (ed.), et al. [Page 53]
draft-ietf-rohc-tcp-01.txt
| 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 [ROHC]
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.
Header Length
As long 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.
Packet Length
Information about packet length is expected to be provided by
the link layer. The field is therefore classified as INFERRED.
Flags
The Reserved flag must be set to zero, as defined in [RFC791].
In [ROHC] the field is therefore classified as STATIC-KNOWN.
However, 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. It is unclear exactly how reserved
bits should be handled, given that the possible future uses
cannot be predicted. 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
Zhang (ed.), et al. [Page 54]
draft-ietf-rohc-tcp-01.txt
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.
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 would still be zero.
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.
Header Checksum
The header checksum protects individual hops from processing a
corrupted header. When almost all IP header information is
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], 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, an
additional check is necessary. Therefore, it is expected than
some additional checksum (such as a CRC) will be used to
validate correct decompression.
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:
Zhang (ed.), et al. [Page 55]
draft-ietf-rohc-tcp-01.txt
+--------------+----------------+
| Class | Size (octets) |
+--------------+----------------+
| INFERRED | 4 |
| STATIC* | 1.5 |
| STATIC-DEF | 8 |
| STATIC-KNOWN*| 2.25 |
| CHANGING* | 4.25 |
+--------------+----------------+
* differs from [ROHC]
A.1.2. TCP header fields
+---------------------+-------------+----------------+
| 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 |
+---------------------+-------------+----------------+
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.
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 is not
necessary to carry this explicitly. The field is therefore
classified as INFERRED.
A.1.3. Summary for IP/TCP
Zhang (ed.), et al. [Page 56]
draft-ietf-rohc-tcp-01.txt
Summarizing this for IP/TCP one obtains
+----------------+----------------+----------------+
| 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)
A.2. 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 Section A.1, considering the fields which were
labeled CHANGING in that classification.
The CHANGING fields are separated into five different subclasses:
STATIC These are fields that were classified as
CHANGING on a general basis, but are
classified as STATIC here due to certain
additional assumptions.
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.
RARELY-CHANGING (RC) These are fields that change their values
occasionally and then keep their new values.
ALTERNATING These fields alternate between a small number
of different values.
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.
Zhang (ed.), et al. [Page 57]
draft-ietf-rohc-tcp-01.txt
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, 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 ECN flags | Value | IRREGULAR | UNKNOWN |
+------------------------+-------------+-------------+------------+
| TCP CWR flag | Value | IRREGULAR | UNKNOWN |
+------------------------+-------------+-------------+------------+
| TCP ECE flag | Value | IRREGULAR | UNKNOWN |
+------------------------+-------------+-------------+------------+
| TCP URG flag | Value | IRREGULAR | UNKNOWN |
+------------------------+-------------+-------------+------------+
| TCP ACK flag | Value | SEMISTATIC | KNOWN |
+------------------------+-------------+-------------+------------+
| TCP PSH flag | Value | IRREGULAR | UNKNOWN |
+------------------------+-------------+-------------+------------+
| TCP RST flag | Value | IRREGULAR | UNKNOWN |
Zhang (ed.), et al. [Page 58]
draft-ietf-rohc-tcp-01.txt
+------------------------+-------------+-------------+------------+
| TCP SYN flag | Value | SEMISTATIC | KNOWN |
+------------------------+-------------+-------------+------------+
| TCP FIN flag | Value | SEMISTATIC | KNOWN |
+------------------------+-------------+-------------+------------+
| TCP Window | Value | ALTERNATING | KNOWN |
+------------------------+-------------+-------------+------------+
| TCP Checksum | Value | IRREGULAR | UNKNOWN |
+------------------------+-------------+-------------+------------+
| 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.
A.2.1. IP header
A.2.1.1. IP Traffic-Class / Type-Of-Service (TOS)
The Traffic-Class (IPv6) or Type-Of-Service (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 [RFC791].
The TOS byte was initially described in [RFC791] as 3 bits of
precedence followed by 3 bits of TOS and 2 reserved bits (defined to
be 0). [RFC1122] extended this to specify 5 bits of TOS, although
the meanings of the additional 2 bits were not defined. [RFC1349]
defined the 4th bit of TOS to be 'minimize monetary cost'.
The next significant change was in [RFC2474] which reworked the TOS
octet as 6 bits of DSCP (DiffServ Code Point) plus 2 unused bits.
Most recently [RFC2780] 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.
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The DSCP (as for TOS in ROHC RTP) is not expected to change
frequently.
Initially we describe the ECN flags as specified in [RFC2481].
Subsequently, a suggested update is described which would alter the
behavior of the flags.
In [RFC2481] 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' and indicates
congestion in the network. The CE flag is expected to be randomly
(and comparatively rarely, although this is dependent upon the
network congestion state) set to '1'.
A recent draft [nonce] 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 depends upon the level of congestion in the
network and this depends upon many factors. However, it is likely
that the probability is non-trivial and may, in many cases, be
significant.
A.2.1.3. IP Identification
The Identification field (IP ID) of the IPv4 header is there to
identify which fragments constitute a datagram when reassembling
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)
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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:
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 fewer 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.
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 should not use this
IP ID assignment policy.
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 [IPv4], 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 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
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requiring fewer 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, it is noted that
the IP ID was 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/replicate the IP
compression context between a number of flows (between the same end-
points, or at least from the same server) would offer potential
benefits. However, this would only have any real impact where there
were a large number of flows that can share their context. The
detail about the context sharing/replication can be found in Section
A.3.
A.2.1.4. Don't Fragment (DF) flag
Due to the use of path-MTU discovery [RFC1191], the value is more
likely to be '1', than found in UDP/RTP streams since DF should be
periodically set to check for fragmentation in the end-to-end path.
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' (periodically being set to '1') or '1'.
A.2.1.5. 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.
A.2.2. TCP header
Any discussion of compressability of TCP fields borrows heavily from
[VJHC]. However, the premise of how the compression is performed is
slightly different and the protocol has evolved slightly in the
intervening time.
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A.2.2.1. 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). Meanwhile, 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
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
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.
It is also noted that the SYN and FIN flags (which have to be
acknowledged) consume 1 byte of sequence space.
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A.2.2.2. 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. 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.
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
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.
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.
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)
CWR (Congestion Window Reduced)
'1' to signal congestion window reduced on ECN. Will generally
be set in individual packets. Here, the probability of this
flag being set is essentially equivalent to the probability of
CE being signaled in the data-flow direction. (This refers to
CE being signaled somewhere in the end-to-end path; not
necessarily prior to compression).
ECE (Echo Congestion Experience)
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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
(The difference in bit-patterns for the negotiation is so that
it will work with broken stacks that reflect the value of
reserved bits)
URG (Urgent Flag)
'1' to indicate urgent data [unlikely with any flag other than
ACK]
ACK (Acknowledgement)
'1' for all except the initial 'SYN' packet
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)
RST (Reset Connection)
'1' to reset a connection [unlikely with any flag other than ACK]
SYN (Synchronize Sequence Number)
'1' for the SYN/SYN-ACK only at the start of a connection
FIN (End of Data : FINished)
'1' to indicate 'no more data' [unlikely with any flag other
than ACK]
Carried as the end-to-end check for the TCP data. See [RFC 1144]
for a discussion of why this should be carried. There may be more
complex interactions with error detection and robustness that would
have to be addressed in a TCP header compression scheme. The TCP
checksum has to be considered as randomly changing.
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May oscillate randomly between 0 and the receiverÆs window limit
(for the connection).
In practice, the window will either not change, or may alternate
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 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.
A.2.2.7. 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.
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.
A.2.3.1. Options overview
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Table 2 classifies the IANA known options together with their
associated RFCs, if applicable [IANA].
+------+--------+------------------------------------+-----------+
| Kind | Length | Meaning | RFC |
| |(octets)| | |
+------+--------+------------------------------------+-----------+
| 0 | - | End of Option List | [RFC793] |
| 1 | - | No-Operation | [RFC793] |
| 2 | 4 | Maximum Segment Size | [RFC793] |
| 3 | 3 | WSopt - Window Scale | [RFC1323] |
| 4 | 2 | SACK Permitted | [RFC2018] |
| 5 | N | SACK | [RFC2018] |
| 6 | 6 | Echo (obsoleted by option 8) | [RFC1072] |
| 7 | 6 | Echo Reply (obsoleted by option 8) | [RFC1072] |
| 8 | 10 | TSopt - Time Stamp Option | [RFC1323] |
| 9 | 2 | Partial Order Connection Permitted | [RFC1693] |
| 10 | 3 | Partial Order Service Profile | [RFC1693] |
| 11 | 6 | CC | [RFC1644] |
| 12 | 6 | CC.NEW | [RFC1644] |
| 13 | 6 | CC.ECHO | [RFC1644] |
| 14 | 3 | Alternate Checksum Request | [RFC1146] |
| 15 | N | Alternate Checksum Data | [RFC1146] |
| 16 | | Skeeter | |
| 17 | | Bubba | |
| 18 | 3 | Trailer Checksum Option | |
| 19 | 18 | MD5 Signature Option | [RFC2385] |
| 20 | | SCPS Capabilities | |
| 21 | | Selective Negative Acknowledgements| |
| 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
A.2.3.2. 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 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
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would not otherwise coincide with the end of the TCP header.
[RFC793]
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 [RFC793].
There is no data associated with this option, a compression
scheme must simply be able to encode its presence.
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 [RFC793].
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 are 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.
The scale factor is encoded logarithmically, as a power of 2
(presumably to be implemented by binary shifts).
This option is sent only in a SYN segment (a segment with the SYN
bit on), hence the window scale is fixed in each direction when a
connection is opened. A Window Scale option in a segment without a
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SYN bit should be ignored. The Window field in a SYN segment
itself is never scaled [RFC1323]
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 [RFC2018]. It MUST NOT be sent on non-SYN
segments.
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 [RFC2018].
If SACK has been negotiated (through an exchange of SACK-
Permitted options), then this option may occur when dropped
segments are noticed by the receiver. Because this identifies
ranges of blocks within the receiverÆs window, this can be viewed
as a base value with a number of offsets. 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.
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Alternative proposals such as DSACK [RFC2883] do not
fundamentally change the behavior of the SACK block, from the
point of view of the information contained within it.
6. - 7.
These options are generally not used in practice. It is obsoleted
by the Timestamp option. For transparency it is desirable that a
compression scheme be able to encode it. However, there is no
benefit in attempting any more sophisticated treatment than
viewing it as a generic 'option').
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 [RFC1323].
Timestamps are quite commonly used. If timestamp options are
exchanged in the connection set-up phase, then they will 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 [RFC1323]). 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, [RFC3150]. However, the proposed Eifel
algorithm [EIFEL] makes use of timestamps and so, currently, it
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is recommended that timestamps are used for cellular-type links
[TCP-WIRELESS].
The following layouts are recommended for sending options on non-
SYN segments, to achieve maximum feasible alignment of 32-bit and
64-bit machines.
+--------+--------+--------+--------+
| NOP | NOP | TSopt | 10 |
+--------+--------+--------+--------+
| TSval timestamp |
+--------+--------+--------+--------+
| TSecr timestamp |
+--------+--------+--------+--------+
With regard to compression, it is further noted that the range of
resolutions for the timestamp suggested in [RFC1323] is quite
wide (1ms to 1s per 'tick'). This (along with the perhaps wide
variation in RTT) makes it hard to select a set of encodings that
will be optimal in all cases.
9. - 15.
Those options are 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
The primary motivation for this option is to allow BGP to protect
itself against the introduction of spoofed TCP segments into the
connection stream. 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.
Unlike other TCP extensions (e.g., the Window Scale option
[RFC1323]), 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
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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.
A.3. Some observations
A.3.1. 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.
It may be that there are other such cues that may be used in certain
circumstances to improve compression efficiency.
A.3.2. Field dependence 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, however, predictable manner. However, TCP fields, such
as sequence number tend to change more unpredictably, because of the
influence of external factors (application behaviors, size of TCP
windows, etc.).
It's well-known that most TCP connections only have one-way traffic
(for example, web browsing and FTP downloading). That is, on the
forward path (from server to client), only Sequence Number changes
and Acknowledgement Number remains constant at most time; on the
backward path (from client to server), only Acknowledgement Number
changes and Sequence Number remain constant at most time. Better
knowledge of TCP packet behaviors will help the design of the packet
format for compressed TCP header and achieve good tradeoff between
complexity and efficiency.
A.3.3. Correlation and 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 as
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indicated that both sides understand the extension. Other TCP
options, such as MD5 Digest, Timestamp also tend to be send when the
connection initiated (with 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-byte words. This means
that the total size of the header plus option must be less than or
equal to 60 bytes -- this leaves 40 bytes for options.
A.3.4. 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 or at least send from the same source host.
As a particular example, consider a mobile user browsing several web
pages (this is more the case with HTTP/1.0 than HTTP/1.1) from the
same web server. Meanwhile, this mechanism can also be when all the
connections have the same Source-IP. In this case, multiple users
in the same cell may browse different web pages from the same server.
In those cases, multiple short-lived web flows are occurred
simultaneously (or near simultaneously within a relatively short
space of time). Then, the IP header part of those contexts will
probably be almost identical. There are only small jump among the
IP_IDs for those contexts. Meanwhile, certain aspects of the TCP
context may also be similar. For example, it may be that one of the
port numbers will stay the same (the service port) and the other
will change by a small amount relative to the just-closed connection
(the ephemeral port).
Thus, support for sub-context sharing, or context replication
(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 to
tell at a middle-box 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. In general, there are much more
fields that are similar to the ones in other context than the
exactly fixed/static ones. Thus it may be more useful to think
about using "context replication" to initialize a new context from
an existing one, rather than re-using an existing one.
A brief study on the TCP/IP field behavior among 'shareable' packet
stream is given in the following.
TERMS
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'shareable' - Two packet streams are defined as shareable if they
belong to the same profile (ROHC/TCP, etc.) AND have
at least the identical Source IP address.
- The shareable property of a field specifies how
similar the value in a new context is to the existing
one. It has the following values:
'N/A' - The field is unnecessary to be shared since
it can be inferred or used to define a packet
stream
'No' - The field is impossible to be shared since
its change pattern between two packet streams
are irregular
'Yes' - The field is possible to be shared. Specific
encoding method can be used to improve the
compression efficiency.
IPv4 Header (inner and/or outer)
Field Class Shareable
------------------------------------------------
Version STATIC N/A
Header Length STATIC-KNOWN Yes
ToS CHANGING Yes (1)
Packet Length INFERRED N/A
Identification CHANGING Yes (2)
Reserved flag STATIC-KNOWN No (3)
Don't Fragment flag STATIC No
More Fragments flag STATIC-KNOWN No
Fragment Offset STATIC-KNOWN No
Time To Live CHANGING Yes
Protocol STATIC N/A
Header Checksum INFERRED N/A
Source Address STATIC-DEF N/A
Destination Address STATIC-DEF N/A
(1) ToS is marked based on the applicationÆs requirement.
Considering that the shareable connections usually belong to same
type of traffic, it can be regarded as shareable.
(2) The shareable context for this field includes IP-ID, NBO, and
RND flags.
(3) Since the possible future behavior of the 'Reserved Flag' cannot
be predicted, it is considered as not shareable.
IPv6 Header (inner and/or outer)
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Field Class Shareable
------------------------------------------------
Version STATIC N/A
Traffic Class CHANGING No
Flow Label STATIC-DEF N/A
Payload Length INFERRED N/A
Next Header STATIC N/A
Hop Limit CHANGING Yes
Source Address STATIC-DEF N/A
Destination Address STATIC-DEF N/A
TCP Header
Field Class Shareable
------------------------------------------------
Source Port STATIC-DEF Yes (4)
Destination Port STATIC-DEF Yes (4)
Sequence Number CHANGING No (5)
Acknowledgement Number CHANGING No
Data Offset INFERRED N/A
Reserved Bits CHANGING Yes (6)
Control Bits
URG CHANGING No
ACK CHANGING No
PSH CHANGING No
RST CHANGING No
SYN CHANGING No
FIN CHANGING No
Window CHANGING Yes (7)
CHECKSUM CHANGING No
Urgent Pointer CHANGING No
(4) On the server side, the port number should be well-known value.
On the client side, the port number is selected by OS automatically.
Whether the port number is shareable depends on how the OS chooses
port number. However, most implementation uses a simple scheme
which just search next available port number.
(5) With the deployment of TCP Initial Sequence Number Randomization,
the Sequence Number will be impossible to share at all. Thus, this
field will not be regarded as shareable.
(6) Don't include ECN flags if ECT is enabled
(7) The Window, here, should be referred as the initial value (or
maximum value) of RWND. Since sharable packet streams are likely to
have the same initial RWND, it would optimize the SYN packet size
for short-lived TCP traffics.
ECN Flags
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Field Class Shareable
------------------------------------------------
ECT CHANGING No (8)
CE CHANGING No
ECN CHANGING No
CWR CHANGING No
(8) Considering that the IP ECN bits will also make use of the ECN
nonce scheme. None of the ECN flags could be regarded as shareable.
TCP Options
Option SYN-only (9) Shareable
-----------------------------------------------------
End of option list Option No No
No-Operation Option No No
Maximum Segment Size Option Yes Yes
Window Scale Option Yes Yes
SACK-Permitted Option Yes Yes
SACK Option No No
Timestamps Option No Yes
(9) SYN-only indicates whether the options only appear in SYN packet
or not. For 'Yes', the option only appears in SYN packet; otherwise,
the option may appear in any packets.
Most TCP options are used only in SYN packet. Some options, such as
MSS, Window Scale, SACK-Permitted and etc., tend to have the same
value among shareable packet streams. Since TCP options may not be
included in the context if the header compression scheme doesn't
support context sharing. Thus, to support context sharing, the
compressor should maintain such TCP options in the context.
The criterion to determine whether two contexts can be shareable is
an implementation issue. For simplicity, contexts with the same
Source-IP should be considered as shareable contexts, and only the
most recent one should be used as the candidate to be shared.
To provide a simple and effective way to handle context
sharing/replication issue, it is not necessary to handle the inter-
profile sharing.
A.3.5. Shared data
At the risk of drifting into solution space, 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 it may be the case that a compressor and
decompressor could be co-located. It may then be possible for the
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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.
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