Robust Header Compression G. Pelletier
Internet-Draft L. Jonsson
Expires: January 2, 2006 K. Sandlund
Ericsson
M. West
Siemens/Roke Manor
July 1, 2005
RObust Header Compression (ROHC):A Profile for TCP/IP (ROHC-TCP)
draft-ietf-rohc-tcp-10.txt
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Copyright (C) The Internet Society (2005).
Abstract
Existing TCP/IP header compression schemes do not work well when used
over links with significant error rates and long round-trip times.
For many bandwidth-limited links where header compression is
essential, such characteristics are common. In addition, existing
schemes have not addressed how to compress TCP options such as SACK
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(Selective Acknowledgements) and Timestamps.
This document specifies a ROHC (Robust Header Compression) profile
for compression of TCP/IP packets. The profile, called ROHC-TCP, is
a robust header compression scheme for TCP/IP that provides improved
compression efficiency and enhanced capabilities for compression of
various header fields including TCP options.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1 Existing TCP/IP Header Compression Schemes . . . . . . . 6
3.2 Classification of TCP/IP Header Fields . . . . . . . . . 7
3.3 Characteristics of Short-lived TCP Transfers . . . . . . 8
4. Overview of the TCP/IP Profile . . . . . . . . . . . . . . . 9
4.1 General Concepts . . . . . . . . . . . . . . . . . . . . 9
4.2 Profile Operation . . . . . . . . . . . . . . . . . . . 9
4.3 Packet Formats and Encoding Methods . . . . . . . . . . 10
4.4 Context Replication . . . . . . . . . . . . . . . . . . 10
4.5 Irregular Chain . . . . . . . . . . . . . . . . . . . . 10
4.6 TCP Options . . . . . . . . . . . . . . . . . . . . . . 10
4.7 Compressing Extension Headers . . . . . . . . . . . . . 10
5. Compressor and Decompressor Logic . . . . . . . . . . . . . 11
5.1 Considerations for the Feedback Channel . . . . . . . . 11
5.2 Compressor Operation . . . . . . . . . . . . . . . . . . 11
5.2.1 Initialization . . . . . . . . . . . . . . . . . . . 11
5.2.2 Compression Logic . . . . . . . . . . . . . . . . . 11
5.2.3 Optimistic Approach . . . . . . . . . . . . . . . . 12
5.2.4 Periodic Context Refreshes . . . . . . . . . . . . . 12
5.2.5 Feedback Logic . . . . . . . . . . . . . . . . . . . 13
5.2.6 Context Replication . . . . . . . . . . . . . . . . 13
5.3 Decompressor Operation . . . . . . . . . . . . . . . . . 14
5.3.1 Decompressor States and Logic . . . . . . . . . . . 14
5.3.2 Reconstruction and Verification . . . . . . . . . . 16
5.3.3 Actions upon CRC Failure . . . . . . . . . . . . . . 16
5.3.4 Feedback Logic . . . . . . . . . . . . . . . . . . . 17
5.3.5 Context Replication . . . . . . . . . . . . . . . . 17
6. Encodings in ROHC-TCP . . . . . . . . . . . . . . . . . . . 18
6.1 Control Fields in ROHC-TCP . . . . . . . . . . . . . . . 18
6.1.1 Master Sequence Number (MSN) . . . . . . . . . . . . 18
6.1.2 IP-ID Behavior . . . . . . . . . . . . . . . . . . . 19
6.1.3 Explicit Congestion Notification (ECN) . . . . . . . 19
6.2 Compressed Header Chains . . . . . . . . . . . . . . . . 20
6.3 Compressing TCP Options with List Compression . . . . . 21
6.3.1 List Compression . . . . . . . . . . . . . . . . . . 21
6.3.2 Table-based Item Compression . . . . . . . . . . . . 22
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6.3.3 Encoding of Compressed Lists . . . . . . . . . . . . 23
6.3.4 Item Table Mappings . . . . . . . . . . . . . . . . 24
6.3.5 Compressed Lists in Dynamic Chain . . . . . . . . . 26
6.3.6 Irregular Chain Items for TCP Options . . . . . . . 26
6.3.7 Replication of TCP Options . . . . . . . . . . . . . 26
6.4 Profile-specific Encoding Methods . . . . . . . . . . . 26
6.4.1 inferred_mine_header_checksum() . . . . . . . . . . 26
6.4.2 inferred_ip_v4_header_checksum() . . . . . . . . . . 27
6.4.3 inferred_ip_v4_length() . . . . . . . . . . . . . . 27
6.4.4 inferred_ip_v6_length() . . . . . . . . . . . . . . 28
6.4.5 inferred_offset() . . . . . . . . . . . . . . . . . 28
6.4.6 Scaled TCP Sequence Number Encoding . . . . . . . . 29
6.4.7 Scaled Acknowledgement Number Encoding . . . . . . . 30
6.5 CRC Calculations . . . . . . . . . . . . . . . . . . . . 30
7. Packet Types . . . . . . . . . . . . . . . . . . . . . . . . 31
7.1 Initialization and Refresh Packets (IR) . . . . . . . . 31
7.2 Context Replication Packets (IR-CR) . . . . . . . . . . 33
7.3 Compressed Packets (CO) . . . . . . . . . . . . . . . . 35
8. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . 35
8.1 Design rationale for compressed base headers . . . . . . 36
8.2 Formal Definition in ROHC-FN . . . . . . . . . . . . . . 39
8.3 Feedback Formats and Options . . . . . . . . . . . . . . 100
8.3.1 Feedback Formats . . . . . . . . . . . . . . . . . . 100
8.3.2 Feedback Options . . . . . . . . . . . . . . . . . . 101
9. Security Consideration . . . . . . . . . . . . . . . . . . . 104
10. IANA Considerations . . . . . . . . . . . . . . . . . . . 104
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 105
12. References . . . . . . . . . . . . . . . . . . . . . . . . 105
12.1 Normative References . . . . . . . . . . . . . . . . . . 105
12.2 Informative References . . . . . . . . . . . . . . . . . 106
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 107
Intellectual Property and Copyright Statements . . . . . . . 108
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1. Introduction
There are several reasons to perform header compression on low- or
medium-speed links for TCP/IP traffic, and these have already been
discussed in RFC 2507 [RFC2507]. Additional considerations that make
robustness an important objective for a TCP [RFC793] compression
scheme are introduced in [REQS]. Finally, existing TCP/IP header
compression schemes (RFC 1144 [RFC1144], RFC 2507 [RFC2507]) are
limited in their handling of the TCP options field and cannot
compress the headers of handshaking packets (SYNs and FINs).
It is thus desirable for a header compression scheme to be able to
handle loss on the link between the compression and decompression
point as well as loss before the compression point. The header
compression scheme also needs to consider how to efficiently compress
short-lived TCP transfers and TCP options, such as SACK (RFC 2018
[RFC2018], RFC 2883 [RFC2883]) and Timestamps (RFC 1323 [RFC1323]).
The ROHC WG has developed a header compression framework on top of
which various profiles can be defined for different protocol sets, or
for different compression strategies. This document defines a TCP/IP
compression profile for the ROHC framework [RFC3095], compliant with
the requirements on ROHC TCP/IP header compression [REQS].
Specifically, it describes a header compression scheme for TCP/IP
header compression (ROHC-TCP) that is robust against packet loss and
that offers enhanced capabilities, in particular for the compression
of header fields including TCP options. The profile identifier for
TCP/IP compression is 0x0006.
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].
This document reuses some of the terminology found in RFC 3095
[RFC3095]. In addition, this document uses or defines the following
terms:
Base context
The base context is a context that has been validated by both the
compressor and the decompressor. A base context can be used as
the reference when building a new context using replication.
Base CID
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The Base Context Identifier is the CID used to identify the Base
Context, where information needed for context replication can be
extracted from.
Context replication
Context replication is the mechanism that establishes and
initializes a new context based on another existing valid context
(a base context). This mechanism is introduced to reduce the
overhead of the context establishment procedure, and is especially
useful for compression of multiple short-lived TCP connections
that may be occurring simultaneously or near-simultaneously.
ROHC Context Replication (ROHC-CR)
"ROHC-CR" in this document normatively refers to the context
replication mechanism for ROHC profiles defined in [ROHC-CR].
ROHC Formal Notation (ROHC-FN)
"ROHC-FN" in this document normatively refers to the formal
notation for ROHC profiles defined in [ROHC-FN], including the
library of encoding methods it specifies.
ROHC-TCP Packet Types
ROHC-TCP uses two different packet types: the Initialization and
Refresh (IR) packet type, and the Compressed packet type (CO).
Short-lived TCP Transfer
Short-lived TCP transfers refer to TCP connections transmitting
only small amounts of data for each single connection. Short TCP
flows seldom need to operate beyond the slow-start phase of TCP to
complete their transfer, which also means that the transmission
ends before any significant increase of the TCP congestion window
may occur.
3. Background
This chapter provides some background information on TCP/IP header
compression. The fundamentals of general header compression can be
found in [RFC3095]. In the following sections, two existing TCP/IP
header compression schemes are first described along with a
discussion of their limitations, followed by the classification of
TCP/IP header fields. Finally, some of the characteristics of short-
lived TCP transfers are summarized.
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The behavior analysis of TCP/IP header fields among multiple short-
lived connections may be found in [TCP-BEH].
3.1 Existing TCP/IP Header Compression Schemes
Compressed TCP (CTCP) and IP Header Compression (IPHC) are two
different schemes that may be used to compress TCP/IP headers. Both
schemes transmit only the differences from the previous header in
order to reduce the large overhead of the TCP/IP header.
The CTCP (RFC 1144 [RFC1144]) compressor detects transport-level
retransmissions and sends a header that updates the context
completely when they occur. While CTCP works well over reliable
links, it is vulnerable when used over less reliable links as even a
single packet loss results in loss of synchronization between the
compressor and the decompressor. This in turn leads to the TCP
receiver discarding all remaining packets in the current window
because of a checksum error. This effectively prevents the TCP Fast
Retransmit algorithm (RFC 2001) from being triggered. In such case,
the compressor must wait until the TCP timeout to resynchronize.
To reduce the errors due to the inconsistent contexts between
compressor and decompressor when compressing TCP, IPHC (RFC 2507
[RFC2507]) improves somewhat on CTCP by augmenting the repair
mechanism of CTCP with a local repair mechanism called TWICE and with
a link-level nacking mechanism to request a header that updates the
context.
The TWICE algorithm assumes that only the Sequence Number field of
TCP segments are changing with the deltas between consecutive packets
being constant in most cases. This assumption is however not always
true, especially when TCP Timestamps and SACK options are used.
The full header request mechanism requires a feedback channel that
may be unavailable in some circumstances. This channel is used to
explicitly request that the next packet be sent with an uncompressed
header to allow resynchronization without waiting for a TCP timeout.
In addition, this mechanism does not perform well on links with long
round-trip time.
Both CTCP and IPHC are also limited in their handling of the TCP
options field. For IPHC, any change in the options field (caused by
timestamps or SACK, for example) renders the entire field
uncompressible, while for CTCP such a change in the options field
effectively disables TCP/IP header compression altogether.
Finally, existing TCP/IP compression schemes do not compress the
headers of handshaking packets (SYNs and FINs). Compressing these
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packets may greatly improve the overall header compression ratio for
the cases where many short-lived TCP connections share the same link.
3.2 Classification of TCP/IP 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 TCP/IP
header compression, it is important to understand the change patterns
of the various header fields.
All fields of the TCP/IP packet header have been classified in detail
in [TCP-BEH]. The main conclusion is that most of the header fields
can easily be compressed away since they seldom or never change. The
following fields do however require more sophisticated mechanisms:
* IPv4 Identification (16 bits) - IP-ID
* TCP Sequence Number (32 bits) - SN
* TCP Acknowledgment Number (32 bits) - ACKN
* TCP Reserved (4 bits)
* TCP ECN flags (2 bits) - ECN
* TCP Window (16 bits) - WINDOW
* TCP Options
+ Maximum Segment Size (32 bits) - MSS
+ Window Scale (24 bits) - WSopt
+ SACK Permitted (16 bits)
+ TCP SACK (80, 144, 208 or 272 bits) - SACK
+ TCP Timestamp (80 bits) - TS
The assignment of IP-ID values can be done in various ways, which are
Sequential, Sequential jump, Random or constant to a value of zero.
However, designers of IPv4 stacks for cellular terminals should use
an assignment policy close to Sequential. Some IPv4 stacks do use a
sequential assignment when generating IP-ID values but do not
transmit the contents of this field in network byte order; instead it
is sent with the two octets reversed. In this case, the compressor
can compress the IP-ID field after swapping the bytes. Consequently,
the decompressor also swaps the bytes of the IP-ID after
decompression to regenerate the original IP-ID. In RFC 3095
[RFC3095], the IP-ID is generally inferred from the RTP Sequence
Number. However, with respect to TCP compression, the analysis in
[TCP-BEH] reveals that there is no obvious candidate to this purpose
among the TCP fields.
The change pattern of several TCP fields (Sequence Number,
Acknowledgment Number, Window, etc.) is very hard to predict and
differs entirely from the behavior of RTP fields discussed in
[RFC3095]. Of particular importance to a TCP/IP header compression
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scheme is the understanding of the sequence and acknowledgment number
[TCP-BEH].
Specifically, the sequence number can be anywhere within a range
defined by the TCP window at any point on the path (i.e. wherever a
compressor might be deployed). Missing packets or retransmissions
can cause the TCP sequence number to fluctuate within the limits of
this window. The TCP window also bound the jumps in acknowledgment
number.
Another important behavior of the TCP/IP header is the dependency
between the sequence number and the acknowledgment number. It is
well known that most TCP connections only have one-way traffic (web
browsing and FTP downloading, for example). This means that on the
forward path (from server to client), only the sequence number is
changing while the acknowledgment number remains constant for most
packets; on the backward path (from client to server), only the
acknowledgment number is changing and the sequence number remains
constant for most packets.
With respect to TCP options, it is noted that most options (such as
MSS, WSopt, SACK-permitted, etc.) may appear only on a SYN segment.
Every implementation should (and we expect 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, for reasons similar as those
described in [RFC3095].
3.3 Characteristics of Short-lived TCP Transfers
Recent studies shows that the majority of TCP flows are short-lived
transfers with an average and a median size no larger than 10KB.
Short-lived TCP transfers will degrade the performance of header
compression schemes that establish a new context by initially sending
full headers.
It is hard to improve the performance for a single, unpredictable,
short-lived connection. However, there are common cases where there
will be multiple TCP connections between the same pair of hosts. A
mobile user browsing several web pages from the same web server is
one example(this is more the case with HTTP/1.0 than HTTP/1.1).
In such case, multiple short-lived TCP/IP flows occur simultaneously
or near simultaneously within a relatively short time interval. It
may be expected that most (if not all) of the IP header of the these
connections will be almost identical to each other, with only small
relative jumps for the IP-ID field.
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Furthermore, a subset of the TCP fields may also be very similar from
one connection to another. For example, one of the port numbers may
be reused (the service port) while the other (the ephemeral port) may
be changed only by a small amount relative to the just-closed
connection.
With regard to header compression, this means that parts of a
compression context used for a TCP connection may be reusable for
another TCP connection. A mechanism supporting context replication,
where a new context is initialized from an existing one, provide
useful optimizations for a sequence of short-lived TCP connections.
Context replication is possible due to the fact that there is much
similarity in header field values and context values among multiple
simultaneous or near simultaneous connections. All header fields and
related context values have been classified in detail in [TCP-BEH].
The main conclusion is that most part of the IP sub-context, some TCP
fields, and some context values can easily be replicated since they
seldom change or change with only a small jump.
4. Overview of the TCP/IP Profile
4.1 General Concepts
Many of the concepts behind the ROHC-TCP profile are similar to those
described in RFC 3095 [RFC3095]. Like for other ROHC profiles, ROHC-
TCP makes use of the ROHC protocol as described in [RFC3095], in
sections 5.1 to 5.2.6. This includes data structures, reserved
packet types, general packet formats, segmentation and initial
decompressor processing.
4.2 Profile Operation
Header compression with ROHC can be conceptually characterized as the
interaction of a compressor with a decompressor state machine. The
compressor's task is to minimally send the information needed to
successfully decompress a packet, based on a certain confidence
regarding the state of the decompressor context.
For ROHC-TCP compression, the compressor normally starts compression
with the initial assumption that the decompressor has no useful
information to process the new flow, and sends Initialization and
Refresh (IR) packets. Alternatively, the compressor may also support
Context Replication (CR) and use IR-CR packets [ROHC-CR] which
attempts to reuse context information related to another flow.
The compressor can then adjust the compression level based on its
confidence that the decompressor has the necessary information to
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successfully process the compressed packets (CO) that it selects. In
other words, the task of the compressor is to ensure that the
decompressor operates in the state that allows decompression of the
most efficient CO packet, and to allow the decompressor to move to
that state as soon as possible otherwise. The decompressor state
machine has three states: the No Context (NC), the Static Context
(SC) and the Full Context (FC).
4.3 Packet Formats and Encoding Methods
The packet formats used for ROHC-TCP are defined using the formal
notation, ROHC-FN. The formal notation is used to provide an
unambiguous representation of the packet formats and a clear
definition of the encoding methods. The encoding methods used in the
packet formats for ROHC-TCP are defined in [ROHC-FN].
4.4 Context Replication
For ROHC-TCP, context replication may be particularly useful for
short-lived TCP flows [REQS]. ROHC-TCP therefore supports context
replication as defined in ROHC-CR [ROHC-CR]; the compressor MAY
support context replication, while a decompressor implementation is
REQUIRED to support decompression of the IR-CR packet type.
4.5 Irregular Chain
The ROHC-TCP profile defines an irregular chain for each header type,
in addition to the static and dynamic chains as used in RFC 3095
[RFC3095].
The irregular chain handles fields for which no predictable change
pattern could be identified, i.e. fields from the TCP, IP and
extension headers that have an irregular behavior and therefore have
to be included in each compressed packet. This chain is attached to
compressed packet in order to make it possible to carry arbitrary
combinations of headers.
4.6 TCP Options
The TCP options in ROHC-TCP are compressed using a downscaled version
of the list compression in [RFC3095], allowing option content to be
established so that TCP options can be added or removed from the
packet without having to send the entire option uncompressed.
4.7 Compressing Extension Headers
In RFC 3095 [RFC3095], list compression is used to compress extension
headers. ROHC-TCP compresses the same type of extension headers as
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in [RFC3095]. However, these headers are treated exactly as other
headers and thus have a static chain, a dynamic chain, an irregular
chain and a chain for context replication.
The consequence is that headers appearing in or disappearing from the
flow being compressed will lead to changes to the static chain.
However, the change pattern of extension headers is not deemed to
impair compression efficiency with respect to this design strategy.
5. Compressor and Decompressor Logic
The header compression logic as described in this chapter is a
simplified version of the one found in [RFC3095].
5.1 Considerations for the Feedback Channel
The ROHC-TCP profile may be used in environments with or without
feedback capabilities from decompressor to compressor. ROHC-TCP
however assumes that if a ROHC feedback channel is available and if
this channel is used at least once by the decompressor for a specific
ROHC-TCP context, this channel will be used during the entire
compression operation for that context. If the connection is broken
and the feedback channel disappears, compression should be restarted.
To parallel RFC 3095 [RFC3095], this is similar to allowing only one
mode transition per compressor: from the initial unidirectional mode
to the bi-directional mode of operation, with the transition being
triggered by the reception of the first packet containing feedback
from the decompressor. This effectively means that ROHC-TCP does not
explicitly define any operational modes.
5.2 Compressor Operation
5.2.1 Initialization
The static context of ROHC TCP streams can be initialized in either
of two ways:
1. By using an IR packet as in Section 7.1, where the profile is six
(6) and the static chain ends with the static part of a TCP
header.
2. By replicating an existing context using the mechanism defined by
ROHC-CR. This is done with the IR-CR packet defined in
Section 7.2, where the profile number is six (6).
5.2.2 Compression Logic
The task of the compressor is to determine what data must be sent
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when compressing a TCP/IP packet, so that the decompressor can
successfully reconstruct the original packet based on its current
state. The selection of the type of compressed header to send thus
depends on a number of factors, including:
* The change behavior of header fields in the stream, e.g.
conveying the necessary information within the restrictions of
the set of available packet formats;
* The compressor's level of confidence regarding decompressor
state, e.g. by using an optimistic approach through repetition
of context updates or from the reception of decompressor
feedback (ACKs and/or NACKs);
* Additional robustness required for the flow, e.g. periodic
repetition of static and dynamic information using IR and IR-
DYN packets when decompressor feedback is not expected.
The impact of these factors on the compressor's packet type selection
is described more in detail in the following subsections.
In this section, a "higher compression state" means that less data
will be sent in compressed packets, i.e. smaller compressed headers
are used, while a lower compression state means that a larger amount
of data will be sent using larger compressed headers.
5.2.3 Optimistic Approach
When ROHC-TCP is used over lossy links, all information needs to be
repeated by the compressor until it is fairly confident that the
decompressor has received the information contained in the packet.
Therefore, if field X in the uncompressed packet changes value, the
compressor must send packets containing an encoding of field X until
it has gained confidence that the decompressor has received at least
one packet containing the new value for X. The compressor SHOULD
choose a compressed format with the smallest header that can convey
the changes needed to fulfil the optimistic approach condition used.
5.2.4 Periodic Context Refreshes
When the optimistic approach is used, there will always be a
possibility of decompression failures since the decompressor may not
have received sufficient information for correct decompression.
Therefore, until the decompressor has established a feedback channel,
the compressor SHOULD periodically move to a lower compression state
and send IR and/or IR-DYN packets. These refreshes can be based on
timeouts, on the number of compressed packets sent for the flow or
any other strategy the implementer chooses.
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5.2.5 Feedback Logic
The compressor makes use of feedback from the decompressor to move to
a lower compression state (NACKs), and optionally to move to a higher
compression state (ACKs).
The reception of either positive feedback (ACKs) or negative feedback
(NACKs) establishes the feedback channel from the decompressor for
the context for which the feedback was received. Once there is an
established feedback channel for a specific context, the compressor
can make use of this feedback to estimate the current state of the
decompressor. This helps increasing the compression efficiency by
providing the information needed for the compressor to achieve the
necessary confidence level. When the feedback channel is
established, it becomes superfluous for the compressor to send
periodic refreshes.
5.2.5.1 Optional Acknowledgements (ACKs)
The compressor can optionally use acknowledgment feedback (ACKs) to
move to a higher compression state. If a feedback channel is
available, the decompressor MAY use positive feedback (ACKs) to
acknowledge successful decompression of packets. Upon reception of
an ACK for a context-updating packet, the compressor knows that the
decompressor has received the acknowledged packet and the changes in
the packet flow up to the acknowledged packet does not need to be
transmit in the next compressed packet(s).
This functionality is optional, so a compressor MUST NOT expect to
get such ACKs initially or during normal operation, even if a
feedback channel is available or established.
5.2.5.2 Negative Acknowledgements (NACKs)
Negative acknowledgments (NACKs or STATIC-NACKs) are also called
context requests and indicate that the decompressor context has been
invalidated. Upon reception of a negative acknowledgment, the
compressor transits to a lower compression state. On reception of a
NACK packet, the compressor should re-send all dynamic information
(via an IR or IR-DYN packet) next time it compresses a packet for the
indicated flow. On reception of a STATIC-NACK packet, the compressor
should re-send all static and all dynamic information (via an IR
packet) next time it compresses a packet for the indicated flow.
5.2.6 Context Replication
For a compressor to support context replication, the additional
compressor and feedback logic defined in ROHC-CR [ROHC-CR] must be
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added to the compressor operation.
5.3 Decompressor Operation
The three states of the decompressor are No Context (NC), Static
Context (SC) and Full Context (FC). The decompressor starts in its
lowest compression state, the NC state. Successful decompression
will always move the decompressor to the FC state. The decompressor
state machine normally never leaves the FC state once it has entered
this state; only repeated decompression failures will force the
decompressor to transit downwards to a lower state.
Below is the state machine for the decompressor. Details of the
transitions between states and decompression logic are given in the
sub-sections following the figure.
Success
+-->------>------>------>------>------>--+
| |
No Static | No Dynamic Success | Success
+-->--+ | +-->--+ +--->----->---+ +-->--+
| | | | | | | | |
| v | | v | v | v
+-----------------+ +---------------------+ +-------------------+
| No Context (NC) | | Static Context (SC) | | Full Context (FC) |
+-----------------+ +---------------------+ +-------------------+
^ | ^ |
| Static Context Damage | | Context Damage |
+-----<------<------<-----+ +-----<------<------<-----+
5.3.1 Decompressor States and Logic
5.3.1.1 No Context (NC) State
Initially, while working in the NC state, the decompressor has not
yet successfully decompressed a packet.
Upon receiving an IR packet, the decompressor MUST verify the
correctness of this packet by validating its header using the CRC
verification. If the decompressed packet is successfully verified,
the decompressor will update the context and use this packet as the
reference packet. Once a packet has been decompressed correctly, the
decompressor can transit to the FC state, and only upon repeated
failures will it transit back to a lower state.
Allowing decompression:
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In the No Context state, only packets carrying sufficient
information on the static fields (i.e. IR packets) can be
decompressed. If decompression may not be performed, the packet
MUST be discarded.
Feedback logic:
In the No Context state, the decompressor SHOULD send a STATIC-
NACK if a packet of a type other than IR is received, or if an IR
packet has failed the CRC verification, subject to the feedback
rate limitation as describedin Section 5.3.4.
5.3.1.2 Static Context (SC) State
From the SC state, the decompressor moves to the NC state if static
context damage is detected. How the decompressor detects static
context damage should be based on the residual error rate, where a
low error rate should make the decompressor assume damage more often
than on a high rate link. The decompressor may send feedback, as
described below, when assuming static context damage.
Allowing decompression:
In the Static Context state, only packets carrying sufficient
information on the dynamic fields covered by an 8-bit CRC may be
decompressed (e.g. IR and IR-DYN). If decompression may not be
performed, the packet is discarded.
Feedback logic:
In the Static Context state, the decompressor SHOULD send a
STATIC-NACK when decompression of an IR, an IR-DYN or a CO packet
carrying a 7-bit CRC fails and if static context damage is
assumed, subject to the feedback rate limitation as describedin
Section 5.3.4. If any other packet type is received, the
decompressor SHOULD treat it as a CRC mismatch when deciding if a
NACK is to be sent.
5.3.1.3 Full Context (FC) State
In the FC state, the decompressor moves to the SC state and discards
all packets until a packet carrying a 7- or 8-bit CRC that
successfully verifies is received if context damage is detected. How
the decompressor detects static context damage should be based on the
residual error rate, where a low error rate should make the
decompressor assume damage more often than on a high rate link. The
decompressor may send feedback, as described below, when assuming
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context damage.
Upon receiving an IR or an IR-DYN packet, the decompressor MUST
validate the correctness of its header using CRC verification. 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.
Upon receiving other types of packet, the decompressor will
decompress it. The decompressor MUST verify the correctness of the
decompressed packet by CRC check. If this verification succeeds, the
decompressor passes the decompressed packet to the system's network
layer. The decompressor will then use this packet as the reference
packet.
If the received packet is older than the current reference packet
(based on the Master Sequence Number (MSN) in the compressed packet),
the decompressor MAY refrain from using this packet as the new
reference packet, even if the correctness of its header was
successfully verified.
Allowing decompression:
In the Full Context state, decompression may be attempted
regardless of the type of packet received.
Feedback logic:
In the Full Context state, the decompressor SHOULD send a NACK
when decompression of any packet type fails and if context damage
is assumed, subject to the feedback rate limitation as describedin
Section 5.3.4.
5.3.2 Reconstruction and Verification
The CRC carried within compressed headers MUST be used to verify
decompression. When the decompression is verified and successful,
the decompressor updates the context with the information received in
the current header; otherwise if the reconstructed header fails the
CRC verification, these updates MUST NOT be performed.
5.3.3 Actions upon CRC Failure
When a CRC verification fails, the decompressor MUST discard the
packet. The actions to be taken when CRC verification fails
following the decompression of an IR-CR packet are specified in
[ROHC-CR]. For other packet types carrying a CRC, if feedback is
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used the logic specified in Section 5.3.4 and in Section 5.3.1 must
be followed when CRC verification fails.
Note: Decompressor implementations may attempt corrective or repair
measures prior to performing the above actions, and the result of any
attempt MUST be validated using the CRC verification.
5.3.4 Feedback Logic
The decompressor may send positive feedback (ACKs) to initially
establish the feedback channel for a particular flow. Either
positive feedback (ACKs) or negative feedback (NACKs) will establish
this channel. The decompressor will then use the feedback channel to
send error recovery requests and (optionally) acknowledgments of
significant context updates.
Once the decompressor establishes a feedback channel, the compressor
will operate using an optimistic logic. In particular, this means
that the compressor will rely on specific decompressor feedback
logic:
* the decompressor will send negative acknowledgments in the case
where context damage is assumed or in other failure situations;
* the decompressor is not strictly expected to send feedback upon
successful decompression, other than for the purpose of
improving compression efficiency.
Once the feedback channel is established, the decompressor is
REQUIRED to continue sending feedback (subject to the feedback rate
limiting considerations later in this section) for the lifetime of
the packet stream, as per the logic defined for each state.
When decompression fails, the feedback rate SHOULD be limited. For
example, feedback could be sent only when decompression of several
consecutive packets have failed. In addition, the decompressor
should also limit the rate at which feedback is sent on successful
decompression, if sent at all. The decompressor may limit the
feedback rate by sending feedback for one out of a number of packets
providing the same type of feedback.
The decompressor MAY optionally send ACKs upon successful
decompression of any packet type. In particular, when an IR, an IR-
DYN or any CO packet carrying a 7- or 8-bit CRC is correctly
decompressed, the decompressor may optionally send an ACK.
5.3.5 Context Replication
ROHC-TCP supports context replication, therefore the decompressor
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MUST implement the additional decompressor and feedback logic defined
in ROHC-CR [ROHC-CR].
6. Encodings in ROHC-TCP
This section describes a ROHC profile for TCP/IP compression. The
profile identifier for ROHC-TCP is 0x0006.
6.1 Control Fields in ROHC-TCP
A control field is a field that is transmitted from the compressor to
the decompressor, but is not part of the uncompressed header. Values
for control fields can be set up in the context of both the
compressor and the decompressor.
In ROHC-TCP, a number of control fields are used by the decompressor
in its interpretation of the packet formats for packets received from
the compressor. These control fields are not a part of the
uncompressed header, but are explicitly transmitted inside ROHC-TCP
packets. Once established at the decompressor, the values of these
fields should be kept until updated by another packet.
6.1.1 Master Sequence Number (MSN)
Feedback packets of types ACK and NACK carry information about
sequence number or acknowledgment number from decompressor to
compressor to uniquely identify a reference packet in the flow.
Unfortunately, there is no guarantee that sequence number and
acknowledgment number fields will take on unique values for each
packet in a TCP/IP flow. In addition, the combined size of the
sequence number field and the acknowledgment number field is rather
large, and they can therefore not be carried efficiently within the
feedback packet.
To overcome this problem, ROHC-TCP introduces a control field called
the Master Sequence Number (MSN) field. The MSN field is created at
the compressor, rather than using one of the fields already present
in the uncompressed header. The compressor increments the value of
the MSN by one for each packet that it sends.
The MSN field has the following two functions:
1. Differentiating between packets when sending feedback data.
2. Inferring the value of incrementing fields such as the IP-ID.
The MSN field is present in every packet sent by the compressor. The
MSN is LSB encoded within the CO packets, and the 16-bit MSN is sent
in full in IR/IR-DYN packets. The decompressor always sends the MSN
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as part of the feedback information. The compressor can later use
the MSN to infer which packet the decompressor is acknowledging.
When the MSN is initialized, it is initialized to a random value.
The compressor should only initialize a new MSN for the initial IR or
IR-CR packet sent for a CID that corresponds to a context that is not
already associated with this profile. In other words, if the
compressor reuses the same CID to compress many TCP flows one after
the other, the MSN is not reinitialized but rather continues to
increment monotonously.
For context replication, the compressor does not use the MSN of the
base context when sending the IR-CR packet, unless the replication
process overwrites the base context (i.e. BCID == CID). Instead,
the compressor uses the value of the MSN if it already exists in the
context being associated with the new flow (CID); otherwise, the MSN
is initialized to a new value.
6.1.2 IP-ID Behavior
The IP-ID field of the IPv4 header can have different change
patterns. RFC 3095 [RFC3095] describes three behaviors: sequential
(NBO), sequential byte-swapped, and random (RND). In addition, this
profile uses a fourth behavior, the constant zero IP-ID behavior as
defined in RFC 3843 [RFC3843] (SID).
The compressor monitors changes in the value of the IP-ID field for a
number of packets, to identify which one of the above listed behavior
is the closest match to the observed change pattern. The compressor
can then select packet formats based on the identified field
behavior.
If more than one level of IP headers is present, ROHC-TCP can assign
a sequential behavior (NBO or byte-swapped) only to the IP-ID of
innermost IP header. This is because only this IP-ID is likely to
have a close correlation with the MSN (see also Section 6.1.1).
Therefore, a compressor MUST assign either the constant zero IP-ID or
the random IP-ID behavior to tunneling headers.
The control field for the IP-ID behavior determines which set of
packet formats will be used. Note that these control fields are also
used to determine the contents of the irregular chain item for each
IP header.
6.1.3 Explicit Congestion Notification (ECN)
When ECN [RFC3168] is used once on a stream, it can be expected that
the ECN bits will change quite often. ROHC-TCP maintains a control
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field in the context to indicate if ECN is used or not. This control
field is transmitted in the dynamic chain of the TCP header, and its
value can be updated using specific compressed headers carrying a
7-bit CRC.
When this control field indicates that ECN is being used, items of IP
and TCP headers in the irregular chain will include bits used for
ECN. To preserve octet-alignment, all of the TCP reserved bits are
transmitted and, for outer IP headers, the entire TOS/TC field is
included in the irregular chain.
The design rationale behind this is the possible use of the "full-
functionality option" of section 9.1 of RFC 3168 [RFC3168].
6.2 Compressed Header Chains
Some packet types use one or more chains containing sub-header
information. The function of a chain is to group items based on
similar characteristics, i.e. grouping fields that either are static,
dynamic or irregular in behavior. Chaining is done by appending each
item to the chain in their order of appearance in the original
header, starting from the fields in the outermost header.
Static chain:
The static chain consists of one item for each header of the chain
of protocol headers to be compressed, starting from the outermost
IP header and ending with a TCP header. In the formal description
of the packet formats, this static chain item for each header type
is labelled format_<protocol name>_static.
Dynamic chain:
The dynamic chain consists of one item for each header of the
chain of protocol headers to be compressed, starting from the
outermost IP header and ending with a TCP header. It should be
noted that the dynamic chain item for the TCP header also contains
a compressed list of TCP options (see Section 6.3). In the formal
description of the packet formats, the dynamic chain item for each
header type is labelled format_<protocol name>_dynamic.
Replicate chain:
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The replicate chain consists of one item for each header in the
chain of protocol headers to be compressed, starting from the
outermost IP header and ending with a TCP header. It should be
noted that the replicate chain item for the TCP header also
contains a compressed list of TCP options (see Section 6.3). In
the formal description of the packet formats, this replicate chain
item for each header type is labelled format_<protocol
name>_replicate. Header fields that are not present in the
replicate chain are replicated from the base context.
Irregular chain:
The structure of the irregular chain is analogous to the structure
of the static chain. For each compressed packet, the irregular
chain is appended at the specified location in the general format
of the compressed packets as defined in Section 7.3. This chain
also includes the irregular chain items for TCP options as defined
in Section 6.3.6.
Note that the format of the irregular chain for the innermost IP
header differs from the format of outer IP headers, since this
header is a part of the compressed base header. The name of the
chain item for the innermost header is postfixed with
"_innermost_irregular", while the irregular chain item for outer
IP headers is postfixed by "_outer_irregular". The format of the
irregular chain item for the outer IP headers also determined
using a flag for TTL/Hoplimit; this flag is defined in the format
of some of the compressed base headers.
6.3 Compressing TCP Options with List Compression
This section describes in details how list compression is applied to
the TCP options. In the definition of the packet formats for ROHC-
TCP, the most frequent type of TCP options are described. Each of
these options has an uncompressed format, a
format_<option_type>_list_item format and a
format_<option_type>_irregular format, where <option_type> is the
name of the actual field item in the option list.
6.3.1 List Compression
The TCP options in the uncompressed packet can be represented as an
ordered list, whose order and presence are most of the time constant
between packets. The generic structure of such a list is as follows:
+--------+--------+--...--+--------+
list: | item 1 | item 2 | | item n |
+--------+--------+--...--+--------+
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The basic principles of list-based compression are the following:
1) When a context is being initialized, a complete representation
of the compressed list of options is transmitted. All options
that have any content are present in the compressed list of items
sent to the decompressor.
Then, once the context has been initialized:
2) When the structure AND the content of the list are not
changing, no information about the list is sent in compressed
headers.
3) When the structure of the list is constant, and when only the
content of one or more options that are defined within the
irregular format is changing, no information about the list needs
to be sent in compressed headers; the irregular content is sent as
part of the irregular chain (as described in Section 6.3.6 in the
general compressed packet format (Section 7.3).
4) When the structure of the list changes, a compressed list is
sent in the compressed header, including a representation of its
structure and order.
6.3.2 Table-based Item Compression
The Table-based item compression compresses individual items sent in
compressed lists. The compressor assigns a unique identifier,
"Index", to each item "Item" of a list.
Compressor Logic
The compressor conceptually maintains an Item Table containing all
items, indexed using "Index". The (Index, Item) pair is sent
together in compressed lists until the compressor gains enough
confidence that the decompressor has observed the mapping between
items and their respective index. Confidence is obtained from the
reception of an acknowledgment from the decompressor, or by
sending (Index, Item) pairs using the optimistic approach. Once
confidence is obtained, the index alone is sent in compressed
lists to indicate the presence of the item corresponding to this
index.
The compressor may reassign an existing index to a new item, by
re-establishing the mapping using the procedure described above.
Decompressor Logic
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The decompressor conceptually maintains an Item Table that
contains all (Index, Item) pairs received. The Item Table is
updated whenever an (Index, Item) pair is received and
decompression is successfully verified using the CRC. The
decompressor retrieves the item from the table whenever an Index
without an accompanying Item is received.
If an index without an accompanying item is received for which the
value of the "Known" flag is zero, the header MUST be discarded
and a NACK SHOULD be sent.
6.3.3 Encoding of Compressed Lists
Each item present in a compressed list is represented by:
o an index into the table of items, and
o a bit indicating if a compressed representation of the item is
present in the list.
o an item (if the presence bit is set)
If the presence bit is not set, the item must already be known by the
decompressor.
A compressed list of TCP options uses the following encoding:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Reserved |PS | m |
+---+---+---+---+---+---+---+---+
| XI_1, ..., XI_m | m octets, or m * 4 bits
/ --- --- --- ---/
| : Padding : if PS = 0 and m is odd
+---+---+---+---+---+---+---+---+
| |
/ item_1, ..., item_n / variable
| |
+---+---+---+---+---+---+---+---+
Reserved: Must be set to zero.
PS: Indicates size of XI fields:
PS = 0 indicates 4-bit XI fields;
PS = 1 indicates 8-bit XI fields.
m: Number of XI item(s) in the compressed list.
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XI_1, ..., XI_m: m XI items. Each XI represents one TCP option in
the uncompressed packet, in the same order as they appear in the
uncompressed packet.
The format of an XI item is as follows:
+---+---+---+---+
PS = 0: | X | Index |
+---+---+---+---+
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
PS = 1: | X | Reserved | Index |
+---+---+---+---+---+---+---+---+
X: Indicates whether the item present in the list:
X = 1 indicates that the item corresponding to the Index is
sent in the item_1, ..., item_n list;
X = 0 indicates that the item corresponding to the Index is not
sent.
Reserved: Set to zero when sending, ignored when received.
Index: An index into the item table. See Section 6.3.4
When 4-bit XI items are used and, the XI items are placed in octets
in the following manner:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| XI_k | XI_k + 1 |
+---+---+---+---+---+---+---+---+
Padding: A 4-bit padding field is present when PS = 0 and the
number of XIs is odd. The Padding field is set to zero when
sending and ignored when receiving.
Item 1, ..., item n:
Each item corresponds to an XI with X = 1 in XI 1, ..., XI m.
Each entry in the item list is formatted as expressed by
format_<option_type>_list_item in Section 8 .
6.3.4 Item Table Mappings
The item table for TCP options list compression is limited to 16
different items, since it is unlikely that any packet stream will
contain a larger number of unique options.
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The mapping between TCP option type and table indexes are listed in
the table below:
+-----------------+---------------+
| Option name | Table index |
+-----------------+---------------+
| NOP | 0 |
| EOL | 1 |
| MSS | 2 |
| WINDOW SCALE | 3 |
| TIMESTAMP | 4 |
| SACK-PERMITTED | 5 |
| SACK | 6 |
| Generic options | 7-15 |
+-----------------+---------------+
Some TCP options are used more frequently than others. To simplify
their compression, a part of the item table is reserved for these
option types, as shown on the table above. The decompressor MUST use
these mappings between item and indexes to decompress TCP options
compressed using list compression.
It is expected that the option types for which an index is reserved
in the item table will only appear once in a list. However, if an
option type is detected twice in the same options list and if both
options have a different content, the compressor should compress the
second occurrence of the option type by mapping it to a generic
compressed option. Otherwise, if the options have the exact same
content, the compressor can still use the same table index for both.
The NOP option
The NOP option can appear more than once in the list. However,
since its value is always the same, no context information needs
to be transmitted. Multiple NOP options can thus be mapped to the
same index. Since the NOP option does not have any content when
compressed as a list_item, it will never be present in the item
list. For consistency, the compressor should still establish an
entry in the list by setting the presence bit, as done for the
other type of options.
The EOL option
The size of the compressed format for the EOL option can be larger
than one octet, and it is defined so that it includes the option
padding. This is because the EOL should terminate the parsing of
the options, but it can also be followed by padding octets that
all have the value zero.
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The Generic option
The generic option can be used to compress any type of TCP option
that do not have a reserved index in the item table.
6.3.5 Compressed Lists in Dynamic Chain
A compressed list for TCP options that is part of the dynamic chain
(e.g. in IR or IR-DYN packets) MUST have all its list items present,
i.e. all x-bits in the XI list must be set.
6.3.6 Irregular Chain Items for TCP Options
The list_item represents the option inside the compressed item list,
and the irregular format is used for the option fields that are
expected to change with each packet. When an item of the specified
type is present in the current context, these irregular fields are
present in each compressed packet, as part of the irregular chain.
Since many of the TCP option types are expected to stay static for
the duration of a flow, many of the irregular_formats are empty.
The irregular chain for TCP options is structured analogously to the
structure of the current TCP options in the uncompressed packet. If
a compressed list is present in the compressed packet, then the
irregular chain for TCP options MUST NOT contain irregular items for
the list items that are transmitted inside the compressed list (i.e.
items in the list that have the x-bit set in its xi). The items that
are not present in the compressed list, but are present in the
current list, MUST have their respective irregular items present in
the irregular chain.
6.3.7 Replication of TCP Options
The entire table of TCP options items is always replicated when using
the IR-CR packet. In the IR-CR packet, the current list of options
for the new flow is also transmitted as a compressed list in the
IR-CR packet.
6.4 Profile-specific Encoding Methods
This section defines encoding methods that are specific to this
profile. These methods are used in the formal definition of the
packet formats in Section 8.
6.4.1 inferred_mine_header_checksum()
This encoding method compresses the minimal encapsulation header
checksum. This checksum is defined in RFC 2004 [RFC2004] as follows:
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Header Checksum
The 16-bit one's complement of the one's complement sum of all
16-bit words in the minimal forwarding header. For purposes of
computing the checksum, the value of the checksum field is 0.
The IP header and IP payload (after the minimal forwarding
header) are not included in this checksum computation.
The "inferred_mine_header_checksum()" encoding method compresses the
minimal encapsulation header checksum down to a size of zero bit,
i.e. no bits are transmitted in compressed headers for this field.
Using this encoding method, the decompressor infers the value of this
field using the above computation.
6.4.2 inferred_ip_v4_header_checksum()
This encoding method compresses the header checksum field of the IPv4
header. This checksum is defined in RFC 791 [RFC791] as follows:
Header Checksum: 16 bits
A checksum on the header only. Since some header fields change
(e.g., time to live), this is recomputed and verified at each
point that the internet header is processed.
The checksum algorithm is:
The checksum field is the 16 bit one's complement of the one's
complement sum of all 16 bit words in the header. For purposes
of computing the checksum, the value of the checksum field is
zero.
The "inferred_ip_v4_header_checksum()" encoding method compresses the
IPv4 header checksum down to a size of zero bit, i.e. no bits are
transmitted in compressed headers for this field. Using this
encoding method, the decompressor infers the value of this field
using the above computation.
6.4.3 inferred_ip_v4_length()
This encoding method compresses the total length field of the IPv4
header. The total length field of the IPv4 header is defined in RFC
791 [RFC791] as follows:
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Total Length: 16 bits
Total Length is the length of the datagram, measured in octets,
including internet header and data. This field allows the
length of a datagram to be up to 65,535 octets.
The "inferred_ip_v4_length()" encoding method compresses the IPv4
header checksum down to a size of zero bit, i.e. no bits are
transmitted in compressed headers for this field. Using this
encoding method, the decompressor infers the value of this field by
counting in octets the length of the entire packet after
decompression.
6.4.4 inferred_ip_v6_length()
This encoding method compresses the payload length field in the IPv6
header. This length field is defined in RFC 2460 [RFC2460] as
follows:
Payload Length: 16-bit unsigned integer
Length of the IPv6 payload, i.e., the rest of the packet
following this IPv6 header, in octets. (Note that any
extension headers present are considered part of the payload,
i.e., included in the length count.)
The "inferred_ip_v6_length()" encoding method compresses the payload
length field of the IPv6 header down to a size of zero bit, i.e. no
bits are transmitted in compressed headers for this field. Using
this encoding method, the decompressor infers the value of this field
by counting in octets the length of the entire packet after
decompression.
6.4.5 inferred_offset()
This encoding method compresses the data offset field of the TCP
header.
The inferred_offset encoding method is used on the data offset field
of the TCP header. This field is defined in RFC 793 as:
Data Offset: 4 bits
The number of 32 bit words in the TCP Header. This indicates
where the data begins. The TCP header (even one including
options) is an integral number of 32 bits long.
The "inferred_offset()" encoding method compresses the data offset
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field of the TCP header down to a size of zero bit, i.e. no bits are
transmitted in compressed headers for this field. Using this
encoding method, the decompressor infers the value of this field by
first decompressing the TCP options list, and by then setting data
offset = (options length / 4) + 5.
6.4.6 Scaled TCP Sequence Number Encoding
On some TCP streams, such as data transfers, the payload size will be
constants over periods of time. For such streams, the TCP sequence
number is bound to increase by multiples of the payload size between
packets. ROHC-TCP provides a method to use scaled compression of the
TCP sequence number to improve compression efficiency in such case.
When scaling the TCP sequence number, the residue is the sequence
number offset from a multiple of the payload size. The precondition
for the compressor to start using this type of encoding is that the
compressor must be confident that the decompressor has received a
number of packets sufficient to establish the value of the residue of
the scaling function.
This confidence can be established by sending a number of packets
that are compressed using an unscaled representation of the sequence
numbers, when the payload size is constant. The compressor can then
start using the scaled sequence number encoding, where the sequence
number is first downscaled by the value of the payload size and then
LSB encoded.
Packets incoming to the compressor for which the value of the residue
is different than the one that has previously been established MUST
be sent in a compressed packet that carries the sequence number
compressed using its unscaled representation, until a stable residue
value can once again be established at the decompressor.
Note that when the sequence number wraps around, the value of the
residue of the scaling function is likely to change, even when the
payload size remains constant. When this occurs, the compressor MUST
reestablish the new residue value using the unscaled representation
of the sequence number as described above.
Note also that the scaling function applied to the TCP sequence
number does not use an explicit scaling factor, such as the TS_STRIDE
used in RFC 3095 [RFC3095]. Instead, the payload size is used as the
scaling factor; as this value can be inferred from the length of the
packet, there is no need to transmit this field explicitly.
The expressions for compressing and decompressing the scaled sequence
number are specified in the definitions of the packet format
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Section 8.2.
6.4.7 Scaled Acknowledgement Number Encoding
Similar to the pattern exhibited by sequence numbers, the expected
increase in the TCP Acknowledgment number will often be a multiple of
the packet size. For the Sequence Number, the compression scheme can
use the payload size of the packets as a scaling factor (see section
6.1.6 above).
For the Acknowledgment Number, the scaling factor depends on the size
of packets flowing in the opposite direction; this information might
not be available to the compressor/decompressor pair. For this
reason, ROHC-TCP uses an explicit scaling factor to compress the TCP
Acknowledgment Number.
For the compressor to use the scaled acknowledgment number encoding,
it MUST first explicitly transmit the value of the scaling factor
(ack_stride) to the decompressor, using one of the packet types that
can carry this information. Once the value of the scaling factor is
established, before using this scaled encoding the compressor must
have enough confidence that the decompressor has successfully
calculated the residue of the scaling function for the acknowledgment
number. This is done the same way as for the scaled sequence number
encoding (see Section 6.4.6 above).
Once the compressor has gained enough confidence that both the value
of the scaling factor and the value of the residue have been
established in the decompressor, the compressor can start compressing
packets using the scaled representation of the Acknowledgment Number.
The compressor MUST NOT use the scaled acknowledgment number encoding
with the value of the scaling factor (ack_stride) set to zero.
The compressor MAY use the scaled acknowledgment number encoding;
what value it will use as the scaling factor is up to the compressor
implementation. In the case where there is a co-located decompressor
processing packets of the same TCP flow in the opposite direction,
the scaling factor for the acknowledgment numbers can be set to the
same value as the scaling factor of the sequence numbers used for
that flow.
6.5 CRC Calculations
The 3-bit and 7-bit CRCs both cover the entire uncompressed header
chain. Note that there is no separation between CRC-STATIC or CRC-
DYNAMIC fields in ROHC-TCP, as opposed to profiles defined in
[RFC3095].
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7. Packet Types
ROHC-TCP uses two different packet types: the Initialization and
Refresh (IR) packet type, and the Compressed packet type (CO).
Each packet type defines a number of packet formats: three packet
formats are defined for the IR type, and two sets of ten base header
formats are defined for the CO type with one additional format that
is common to both sets.
7.1 Initialization and Refresh Packets (IR)
ROHC-TCP uses the basic structure of the ROHC IR and IR-DYN packets
as defined in [RFC3095] (section 5.2.3. and 5.2.4. respectively).
The 8-bit CRC is computed according to section 5.9.1 of [RFC3095].
Packet type: IR
This packet type communicates the static part and the dynamic part
of the context.
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For the ROHC-TCP IR packet, the value of the x bit must be set to
one. It has the following format:
0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if for small CIDs and (CID != 0)
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 1 0 1 | IR type octet
+---+---+---+---+---+---+---+---+
: :
/ 0-2 octets of CID / 1-2 octets if for large CIDs
: :
+---+---+---+---+---+---+---+---+
| Profile | 1 octet
+---+---+---+---+---+---+---+---+
| CRC | 1 octet
+---+---+---+---+---+---+---+---+
| |
/ Static chain / variable length
| |
- - - - - - - - - - - - - - - -
| |
/ Dynamic chain / variable length
| |
- - - - - - - - - - - - - - - -
| |
/ Payload / variable length
| |
- - - - - - - - - - - - - - - -
CRC: 8-bit CRC, computed according to section 5.9.1 of [RFC3095].
Static chain: See Section 6.2.
Dynamic chain: See Section 6.2.
Payload: The payload of the corresponding original packet, if any.
The presence of a payload is inferred from the packet length.
Packet type: IR-DYN
This packet type communicates the dynamic part of the context.
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The ROHC-TCP IR-DYN packet has the following format:
0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if for small CIDs and (CID != 0)
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 0 0 0 | IR-DYN type octet
+---+---+---+---+---+---+---+---+
: :
/ 0-2 octets of CID / 1-2 octets if for large CIDs
: :
+---+---+---+---+---+---+---+---+
| Profile | 1 octet
+---+---+---+---+---+---+---+---+
| CRC | 1 octet
+---+---+---+---+---+---+---+---+
| |
/ Dynamic chain / variable length
| |
- - - - - - - - - - - - - - - -
| |
/ Payload / variable length
| |
- - - - - - - - - - - - - - - -
CRC: 8-bit CRC, computed according to section 5.9.1 of [RFC3095].
Dynamic chain: See Section 6.2.
Payload: The payload of the corresponding original packet, if any.
The presence of a payload is inferred from the packet length.
7.2 Context Replication Packets (IR-CR)
Context replication requires a dedicated IR packet format that
uniquely identifies the IR-CR packet for the ROHC-TCP profile. This
section defines the profile-specific part of the IR-CR packet
[ROHC-CR].
Packet type: IR-CR
This packet type communicates a reference to a base context along
with the static and dynamic parts of the replicated context that
differs from the base context.
The ROHC-TCP IR-CR packet follows the general format of the ROHC CR
packet, as defined in ROHC-CR [ROHC-CR], section 3.4.2. With
consideration to the extensibility of the IR packet type defined in
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RFC 3095 [RFC3095], the ROHC-TCP profile supports context replication
through the profile specific part of the IR packet. This is achieved
using the bit (x) left in the IR packet header for "Profile specific
information". For ROHC-TCP, this bit is defined as a flag indicating
whether this packet is an IR packet or an IR-CR packet. For the
ROHC-TCP IR-CR packet, the value of the x bit must be set to zero.
The ROHC-TCP IR-CR has the following format:
0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if for small CIDs and (CID != 0)
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 1 0 0 | IR-CR type octet
+---+---+---+---+---+---+---+---+
: :
/ 0-2 octets of CID / 1-2 octets if for large CIDs
: :
+---+---+---+---+---+---+---+---+
| Profile | 1 octet
+---+---+---+---+---+---+---+---+
| CRC | 1 octet
+---+---+---+---+---+---+---+---+
| B | CRC7 | 1 octet
+---+---+---+---+---+---+---+---+
: Reserved | Base CID : 1 octet, for small CID, if B=1
+---+---+---+---+---+---+---+---+
: :
/ Base CID / 1-2 octets, for large CIDs,
: : if B=1
+---+---+---+---+---+---+---+---+
| |
/ Replicate chain / variable length
| |
- - - - - - - - - - - - - - - -
| |
/ Payload / variable length
| |
- - - - - - - - - - - - - - - -
B: B = 1 indicates that the Base CID field is present.
CRC7: The CRC over the original, uncompressed, header. This 7-bit
CRC is computed according to section 3.4.1.1 of [ROHC-CR].
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Replicate chain: See Section 6.2.
Payload: The payload of the corresponding original packet, if any.
The presence of a payload is inferred from the packet length.
7.3 Compressed Packets (CO)
The ROHC-TCP CO packets communicate irregularities in the packet
header. All CO packets carry a CRC and can update the context.
The general format for a compressed TCP header is as follows:
0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if for small CIDs and CID 1-15
+---+---+---+---+---+---+---+---+
| first octet of base header | (with type indication)
+---+---+---+---+---+---+---+---+
: :
/ 0, 1, or 2 octets of CID / 1-2 octets if large CIDs
: :
+---+---+---+---+---+---+---+---+
/ remainder of base header / variable number of octets
+---+---+---+---+---+---+---+---+
: :
/ Irregular Chain / variable
: :
--- --- --- --- --- --- --- ---
: :
/ TCP Options Irregular Part / variable
: :
--- --- --- --- --- --- --- ---
The base header in the figure above is the compressed representation
of the innermost IP header and the TCP header in the uncompressed
packet. The full set of base headers are described in Section 8.
Irregular chain: See Section 6.2.
TCP options irregular part: See Section 6.3.6.
8. Packet Formats
This section describes the set of compressed TCP/IP packet formats.
The normative description of the packet formats is given using a
formal notation, the ROHC-FN [ROHC-FN]. The formal description of
the packet formats specifies all of the information needed to
compress and decompress a header relative to the context.
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In particular, the notation provides a list of all the fields present
in the uncompressed and compressed TCP/IP headers, and defines how to
map from each uncompressed packet to its compressed equivalent and
vice versa. See the ROHC-FN [ROHC-FN] for an explanation of the
formal notation itself, and for a description of the encoding methods
used to compress each of the fields in the TCP/IP header.
Note that the formal definition of the packet formats for ROHC-TCP
includes comments that follow a specific syntax. These comments,
called annotations, make use of square brackets as delimiters;
numbers in between the "[" and the "]" are used to provide additional
information about the expected number of bits for the field(s) that
appears as a right-hand operand. These are not normative in any way.
8.1 Design rationale for compressed base headers
The compressed packet formats are defined as two separate sets: one
set for the packets where the innermost IP header contains a
sequential IP-ID (either network byte order or byte swapped), and one
set for the packets without sequential IP-ID (either random, zero, or
no IP-ID).
These two sets of packet formats are referred to as the "sequential"
and the "random" set of packet format.
In addition, there is a common compressed packet that can be used
regardless of the type of IP-ID behavior. This common packet can
transmit rarely changing fields and also send the frequently changing
field coded in variable lengths. The common packet format can also
change the value of control fields such as IP-ID and ECN behavior.
All compressed base headers contain a 3-bit CRC, unless they update
control fields such as "ip_id_behavior" or "ecn_used" that affect the
interpretation of subsequent packets. Packets that can modify these
control fields will carry a 7-bit CRC instead.
The encoding methods used in the compressed base headers are based on
the following design criteria:
o MSN
Since the MSN is a number generated by the compressor, it only
needs to be large enough to ensure robust operation and to
accommodate a small amount of reordering. Therefore, each
compressed base header contains 4 bits of MSN. To handle
reordering, the LSB offset value is set to p=4.
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o Sequence number
ROHC-TCP has the capability to handle bulk data transfers
efficiently, for which the sequence number is expected to
increase by about 1460 bytes (which can be represented by 11
bits). For the compressed base headers to handle
retransmissions (i.e. negative delta to the sequence number),
the LSB interpretation interval must handle negative offsets
about as large as positive offset, which means that one more
bit is needed.
Also, for ROHC-TCP to be robust to losses, two additional bits
are added to the LSB encoding of the sequence number. This
means that the base headers should contain at least 14 bits of
LSB-encoded sequence number when present. According to the
logic above, the LSB offset value p is set to be as large as
the positive offset, i.e. p = 2^(k-1)-1, where k is the number
of LSB-encoded bits that are transmitted in the base header.
o Acknowledgment number
The design criterion for the acknowledgment number is similar
to that of the sequence number. However, often only every
other data packet is acknowledged, which means that the
expected delta value is twice as large as for sequence numbers.
Therefore, at least 15 bits of acknowledgment number should be
used in compressed base headers. Since the acknowledgment
number is expected to constantly increase, and the only
exception to this is packet reordering (either on link or pre-
link), the negative offset for LSB encoding is set to be 25% of
the total interval, i.e. p = 2^(k-2)-1. The offset value p has
been set the same way as for the sequence number, i.e. p =
2^(k-1)-1.
o Window
The TCP window field is expected to increase in increments of
similar size as the sequence number, and therefore the design
criterion for the TCP window has been to send at least 14 bits
when used.
o IP-ID
For the "sequential" set of packet formats, all the compressed
base headers contains LSB encoded IP-ID offset bits. The
requirement is that at least 3 bits of IP-ID should always be
present, but it is preferable to use 4 to 7 bits. When k=3,
p=1 and if k>3, then p=3 since the offset is expected to
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increase most of the time.
Each set of packet formats contains ten different compressed base
headers. The reason for having this large number of packets is that
the TCP sequence number, TCP acknowledgment number, TCP window and
MSN are frequently changing in a non-linear pattern. All of the
compressed base headers transmit LSB-encoded MSN bits, the push flag
and a CRC, and in addition to this, all the base headers in the
sequential packet format set contains LSB encoded IP-ID bits.
The following packet formats exist in both the sequential and random
packet format sets:
o Format 1: This packet format transmits changes to the TCP sequence
number and its principal use should be on the downstream of a data
transfer.
o Format 2: This packet format transmits the TCP sequence number in
scaled form, and will normally be used on the downstream of a data
transfer where the payload size is constant for multiple packets.
o Format 3: This packet format transmits changes in the TCP
acknowledgment number, and will be used in the acknowledgment
direction of data transfer.
o Format 4: This packet format is similar to format 3, but sends a
scaled TCP acknowledgment number.
o Format 5: This packet format transmits both the TCP sequence
number and the acknowledgment number, and should be particularly
useful for streams that send data in both directions.
o Format 6: This packet format is similar to format 5, but sends the
TCP sequence number in scaled form, when the payload size is
static for certain intervals in a data stream.
o Format 7: This packet format transmits changes to both the TCP
sequence number and the TCP window, and is expected to be useful
for any type of data transfer.
o Format 8: This packet format transmits changes to both the TCP
acknowledgment number and the TCP window, and is expected to be
useful for the acknowledgment streams of data connections.
o Format 9: This packet format is similar to format 7, but sends the
TCP sequence number in scaled form to allow higher compression
rates on streams with a constant payload size,
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o Format 10: This packet format is used to transmit changes to some
of the more seldom changing fields in the streams, such as ECN
behavior, RST/SYN/FIN flags, the TTL/Hop Limit and the TCP options
list. This format carries a 7-bit CRC, since it can change the
contents of the irregular chain in later packets. Note that this
can be seen as a reduced form of the common packet format.
o Common packet format: The common packet format can be used for all
kinds of IP-ID behavior, and should be used when some of the more
rarely changing fields in the IP or TCP header changes. Since
this packet format can be used to change what set of packet
formats is to be used for future packets, it carries a 7-bit CRC
to reduce the probability of context corruption. This packet can
basically change all the dynamic fields in the IP and TCP header,
and it uses a large set of flags to control which fields that are
present in the packet.
8.2 Formal Definition in ROHC-FN
% List of encoding methods that are expected to be predefined
% by the FN:
%
irregular (length) === "predefined by FN";
static === "predefined by FN";
compressed_value (length, value) === "predefined by FN";
lsb (lsbs, offset) === "predefined by FN";
crc (b, p, i, d, l) === "predefined by FN";
uncompressed_value (length, value) === "predefined by FN";
% Encoding methods not specified in FN syntax:
%
inferred_mine_header_checksum === "insert reference here";
inferred_ip_v6_length === "insert reference here";
inferred_ip_v4_header_checksum === "insert reference here";
inferred_ip_v4_length === "insert reference here";
inferred_offset === "insert reference here";
list_tcp_options(nbits, ack_value) === "insert reference here";
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Constants
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
IP_ID_BEHAVIOR_SEQUENTIAL = 0;
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED = 1;
IP_ID_BEHAVIOR_RANDOM = 2;
IP_ID_BEHAVIOR_ZERO = 3;
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Global control fields
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% The control fields here reside in the context and are persistent
% between packets. Some of these (such as the sequence number
% scaling/residue are never transmitted explicitly, but are inferred
% from other values.
%
control_fields = ecn_used, %[ 1 ]
msn, %[ 16 ]
ip_inner_ecn, %[ 2 ]
seq_number_scaled, %[ 32 ]
seq_number_residue, %[ 32 ]
ack_stride, %[ 16 ]
ack_number_scaled, %[ 16 ]
ack_number_residue; %[ 16 ]
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% General structures
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
static_or_irreg32(flag) ===
{
uc_format = field; %[ 32 ]
co_format_irreg_enc = field, %[ 32 ]
{
let (flag == 1);
field ::= irregular(32);
};
co_format_static_enc = field, %[ 0 ]
{
let (flag == 0);
field ::= static;
};
};
static_or_irreg16(flag) ===
{
uc_format = field; %[ 16 ]
co_format_irreg_enc = field, %[ 16 ]
{
let (flag == 1);
field ::= irregular(16);
};
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co_format_static_enc = field, %[ 0 ]
{
let (flag == 0);
field ::= static;
};
};
static_or_irreg8(flag) ===
{
uc_format = field; %[ 8 ]
co_format_irreg_enc = field, %[ 8 ]
{
let (flag == 1);
field ::= irregular(8);
};
co_format_static_enc = field, %[ 0 ]
{
let (flag == 0);
field ::= static;
};
};
variable_length_32_enc(flag) ===
{
uc_format = field; %[ 32 ]
co_format_not_present = field, %[ 0 ]
{
let(flag == 0);
field ::= static;
};
co_format_8_bit = field, %[ 8 ]
{
let(flag == 1);
field ::= lsb(8, 63);
};
co_format_16_bit = field, %[ 16 ]
{
let(flag == 2);
field ::= lsb(16, 16383);
};
co_format_32_bit = field, %[ 32 ]
{
let(flag == 3);
field ::= irregular(32);
};
};
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variable_length_16_enc(flag) ===
{
uc_format = field; %[ 16 ]
co_format_not_present = field, %[ 0 ]
{
let(flag == 0);
field ::= static;
};
co_format_8_bit = field, %[ 8 ]
{
let(flag == 1);
field ::= lsb(8, 63);
};
co_format_16_bit = field, %[ 16 ]
{
let(flag == 2);
field ::= irregular(16);
};
};
optional32 (flag) ===
{
uc_format = item; % 0 or 32 bits
co_format_present = item, %[ 32 ]
{
let (flag == 1);
item ::= irregular (32);
};
co_format_not_present = item, %[ 0 ]
{
let (flag == 0);
item ::= compressed_value (0, 0);
};
};
lsb_7_or_31 ===
{
uc_format = item; % 7 or 31 bits
co_format_lsb_7 = discriminator, %[ 1 ]
item, %[ 7 ]
{
discriminator ::= '0';
item ::= lsb (7, 8);
};
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co_format_lsb_31 = discriminator, %[ 1 ]
item, %[ 31 ]
{
discriminator ::= '1';
item ::= lsb (31, 256);
};
};
opt_lsb_7_or_31 (flag) ===
{
uc_format = item; % 32 bits
co_format_present = item, % 8 or 32 bits
{
let (flag == 1);
item ::= lsb_7_or_31;
};
co_format_not_present = item, %[ 0 ]
{
let (flag == 0);
item ::= compressed_value (0, 0);
};
};
crc3 (data_value, data_length) ===
{
uc_format = ;
co_format = crc_value, %[ 3 ]
{
crc_value ::= crc(3, 0x06, 0x07, data_value, data_length);
};
};
crc7 (data_value, data_length) ===
{
uc_format = ;
co_format = crc_value, %[ 7 ]
{
crc_value ::= crc(7, 0x79, 0x7f, data_value, data_length);
};
};
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% IPv6 Destination options header
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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ip_dest_opt ===
{
uc_format = next_header, %[ 8 ]
length, %[ 8 ]
value; % n bits
default_methods =
{
next_header ::= static;
length ::= static;
value ::= static;
};
co_format_dest_opt_static = next_header, %[ 8 ]
length, %[ 8 ]
{
next_header ::= irregular(8);
length ::= irregular(8);
};
co_format_dest_opt_dynamic = value, % n bits
{
value ::= irregular(length:uncomp_value * 64 + 48);
};
co_format_dest_opt_replicate_0 = discriminator, %[ 8 ]
{
discriminator ::= '00000000';
};
co_format_dest_opt_replicate_1 = discriminator, %[ 8 ]
length, %[ 8 ]
value, % n bits
{
discriminator ::= '10000000';
length ::= irregular(8);
value ::= irregular(
length:uncomp_value * 64 + 48);
};
};
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% IPv6 Hop-by-Hop options header
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
ip_hop_opt ===
{
uc_format = next_header, %[ 8 ]
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length, %[ 8 ]
value; % n bits
default_methods =
{
next_header ::= static;
length ::= static;
value ::= static;
};
co_format_hop_opt_static = next_header, %[ 8 ]
length, %[ 8 ]
{
next_header ::= irregular(8);
length ::= irregular(8);
};
co_format_hop_opt_dynamic = value, % n bits
{
value ::= irregular(length:uncomp_value * 64 + 48);
};
co_format_hop_opt_replicate_0 = discriminator, %[ 8 ]
{
discriminator ::= '00000000';
};
co_format_hop_opt_replicate_1 = discriminator, %[ 8 ]
length, %[ 8 ]
value, % n bits
{
discriminator ::= '10000000';
length ::= irregular(8);
value ::= irregular(
length:uncomp_value * 64 + 48);
};
};
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% IPv6 Routing header
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
ip_rout_opt ===
{
uc_format = next_header, %[ 8 ]
length, %[ 8 ]
value; % n bits
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default_methods =
{
next_header ::= static;
length ::= static;
value ::= static;
};
co_format_rout_opt_static = next_header, %[ 8 ]
length, %[ 8 ]
value, % n bits
{
next_header ::= irregular(8);
length ::= irregular(8);
value ::= irregular(length:uncomp_value * 64 + 48);
};
co_format_rout_opt_dynamic =
{
};
co_format_rout_opt_replicate_0 = discriminator, %[ 8 ]
{
discriminator ::= '00000000';
};
co_format_rout_opt_replicate_1 = discriminator, %[ 8 ]
length, %[ 8 ]
value, % n bits
{
discriminator ::= '10000000';
length ::= irregular(8);
value ::= irregular(length:uncomp_value * 64 + 48);
};
};
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% GRE Header
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
optional_checksum (flag_value) ===
{
uc_format = value, % 0 or 16 bits
reserved1; % 0 or 16 bits
co_format_cs_present = value, %[ 16 ]
reserved1, %[ 0 ]
{
let (flag_value == 1);
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value ::= irregular (16);
reserved1 ::= uncompressed_value (16, 0);
};
co_format_not_present = value, %[ 0 ]
reserved1, %[ 0 ]
{
let (flag_value == 0);
value ::= compressed_value (0, 0);
reserved1 ::= compressed_value (0, 0);
};
};
gre_proto ===
{
uc_format = protocol; %[ 16 ]
default_methods =
{
};
co_format_ether_v4 = discriminator, %[ 1 ]
{
discriminator ::= compressed_value (1, 0);
protocol ::= uncompressed_value (16, 0x0800);
};
co_format_ether_v6 = discriminator, %[ 1 ]
{
discriminator ::= compressed_value (1, 1);
protocol ::= uncompressed_value (16, 0x86DD);
};
};
gre ===
{
uc_format = c_flag, %[ 1 ]
r_flag, %[ 1 ]
k_flag, %[ 1 ]
s_flag, %[ 1 ]
reserved0, %[ 9 ]
version, %[ 3 ]
protocol, %[ 16 ]
checksum_and_res, % 0 or 32 bits
key, % 0 or 32 bits
sequence_number; % 0 or 32 bits
default_methods =
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{
c_flag ::= static;
r_flag ::= static;
k_flag ::= static;
s_flag ::= static;
reserved0 ::= uncompressed_value (9, 0);
version ::= static;
protocol ::= static;
key ::= static;
checksum_and_res ::= optional_checksum (c_flag:uncomp_value);
};
co_format_gre_static = protocol, %[ 1 ]
c_flag, %[ 1 ]
r_flag, %[ 1 ]
k_flag, %[ 1 ]
s_flag, %[ 1 ]
version, %[ 3 ]
key, % 0 or 32 bits
{
protocol ::= gre_proto;
c_flag ::= irregular (1);
r_flag ::= irregular (1);
k_flag ::= irregular (1);
s_flag ::= irregular (1);
version ::= irregular (3);
key ::= optional32 (k_flag:uncomp_value);
sequence_number ::= static;
};
co_format_gre_dynamic = checksum_and_res, % 0 or 16 bits
sequence_number, % 0 or 32 bits
{
sequence_number ::= optional32 (s_flag:uncomp_value);
};
co_format_gre_replicate_0 = discriminator, %[ 8 ]
checksum_and_res, % 0 or 16 bits
sequence_number, % 0, 8 or 32 bits
{
discriminator ::= '00000000';
sequence_number ::= opt_lsb_7_or_31 (s_flag:uncomp_value);
};
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co_format_gre_replicate_1 = discriminator, %[ 8 ]
c_flag, %[ 1 ]
r_flag, %[ 1 ]
k_flag, %[ 1 ]
s_flag, %[ 1 ]
reserved, %[ 1 ]
version, %[ 3 ]
checksum_and_res, % 0 or 16 bits
key, % 0 or 32 bits
sequence_number, % 0 or 32 bits
{
discriminator ::= '10000000';
c_flag ::= irregular (1);
r_flag ::= irregular (1);
k_flag ::= irregular (1);
s_flag ::= irregular (1);
reserved ::= '0';
version ::= irregular (3);
key ::= optional32 (k_flag:uncomp_value);
sequence_number ::= optional32 (s_flag:uncomp_value);
};
co_format_gre_irregular = checksum_and_res, % 0 or 16 bits
sequence_number, % 0, 8 or 32 bits
{
sequence_number ::= opt_lsb_7_or_31 (s_flag:uncomp_value);
};
};
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% MINE header
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
mine ===
{
uc_format = next_header, %[ 8 ]
s_bit, %[ 1 ]
res_bits, %[ 7 ]
checksum, %[ 16 ]
orig_dest, %[ 32 ]
orig_src; % 0 or 32 bits
default_methods =
{
next_header ::= static;
s_bit ::= static;
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res_bits ::= static;
checksum ::= inferred_mine_header_checksum;
orig_dest ::= static;
orig_src ::= static;
};
co_format_mine_static = next_header, %[ 8 ]
s_bit, %[ 1 ]
res_bits, %[ 7 ]
orig_dest, %[ 32 ]
orig_src, % 0 or 32 bits
{
next_header ::= irregular (8);
s_bit ::= irregular (1);
res_bits ::= irregular (7);
% include reserved - no benefit in removing them
orig_dest ::= irregular (32);
orig_src ::= optional32 (s_bit:uncomp_value);
};
co_format_mine_dynamic =
{
};
co_format_mine_replicate_0 = discriminator, %[ 8 ]
checksum, %[ 0 ]
{
discriminator ::= '00000000';
};
co_format_mine_replicate_1 = discriminator, %[ 8 ]
s_bit, %[ 1 ]
res_bits, %[ 7 ]
orig_dest, %[ 32 ]
orig_src, % 0 or 32 bits
{
discriminator ::= '10000000';
s_bit ::= irregular (1);
res_bits ::= irregular (7);
orig_dest ::= irregular (32);
orig_src ::= optional32 (s_bit:uncomp_value);
};
};
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Authentication Header (AH)
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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ah ===
{
uc_format = next_header, %[ 8 ]
length, %[ 8 ]
res_bits, %[ 16 ]
spi, %[ 32 ]
sequence_number, %[ 32 ]
auth_data; % n bits
default_methods =
{
next_header ::= static;
length ::= static;
res_bits ::= static;
spi ::= static;
sequence_number ::= static;
auth_data ::= irregular (length:uncomp_value * 32 - 32);
};
co_format_ah_static = next_header, %[ 8 ]
length, %[ 8 ]
spi, %[ 32 ]
{
next_header ::= irregular(8);
length ::= irregular (8);
spi ::= irregular (32);
};
co_format_ah_dynamic = res_bits, %[ 16 ]
sequence_number, %[ 32 ]
auth_data, % n bits
{
res_bits ::= irregular (16);
sequence_number ::= irregular (32);
};
co_format_ah_replicate_0 = discriminator, %[ 8 ]
sequence_number, % 8 or 32 bits
auth_data, % n bits
{
discriminator ::= '00000000';
sequence_number ::= lsb_7_or_31;
};
co_format_ah_replicate_1 = discriminator, %[ 8 ]
length, %[ 8 ]
res_bits, %[ 16 ]
spi, %[ 32 ]
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sequence_number, %[ 32 ]
auth_data, % n bits
{
discriminator ::= '10000000';
length ::= irregular (8);
res_bits ::= irregular (16);
spi ::= irregular (32);
sequence_number ::= irregular (32);
};
co_format_ah_irregular = sequence_number, % 8 or 32 bits
auth_data, % n bits
{
sequence_number ::= lsb_7_or_31;
};
};
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% ESP header (NULL encrypted)
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
esp_null ===
{
uc_format = spi, %[ 32 ]
sequence_number, %[ 32 ]
next_header; %[ 8 ]
default_methods =
{
spi ::= static;
%
% Next header will always be present in the trailer part,
% but sometimes it will ALSO be present in the header
% (static chain only).
%
nh_field ::= static; % Control field
next_header ::= static;
sequence_number ::= static;
};
co_format_esp_static = nh_field, %[ 8 ]
spi, %[ 32 ]
{
% identify next header assume next 96 bits skipped
% to get to end of packet (i.e. this is anchored from
% the end of the packet, not the start)
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%
nh_field ::= compressed_value(8, next_header:uncomp_value);
next_header ::= irregular (8); % At packet end!
spi ::= irregular (32);
};
co_format_esp_dynamic = sequence_number, %[ 32 ]
{
sequence_number ::= irregular (32);
};
co_format_esp_replicate_0 = discriminator, %[ 8 ]
sequence_number, % 8 or 32 bits
{
discriminator ::= '00000000';
sequence_number ::= lsb_7_or_31;
};
co_format_esp_replicate_1 = discriminator, %[ 8 ]
spi, %[ 32 ]
sequence_number, %[ 32 ]
{
discriminator ::= '10000000';
spi ::= irregular (32);
sequence_number ::= irregular (32);
};
co_format_esp_irregular = sequence_number, % 8 or 32 bits
{
sequence_number ::= lsb_7_or_31;
};
};
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Structures common for IPv4 and IPv6
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
irreg_tos_tc ===
{
uc_format = tos_tc; %[ 6 ]
co_format_tos_tc_present = tos_tc, %[ 6 ]
{
let(ecn_used:uncomp_value == 1);
tos_tc ::= irregular (6);
};
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co_format_tos_tc_not_present = tos_tc, %[ 0 ]
{
let(ecn_used:uncomp_value == 0);
tos_tc ::= static;
};
};
ip_irreg_ecn ===
{
uc_format = ip_ecn_flags; %[ 2 ]
co_format_tc_present = ip_ecn_flags, %[ 2 ]
{
let(ecn_used:uncomp_value == 1);
ip_ecn_flags ::= irregular (2);
};
co_format_tc_not_present = ip_ecn_flags, %[ 0 ]
{
let(ecn_used:uncomp_value == 0);
ip_ecn_flags ::= static;
};
};
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% IPv6 Header
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
fl_enc ===
{
uc_format = flow_label;
co_format_fl_zero = discriminator, %[ 1 ]
flow_label, %[ 0 ]
reserved, %[ 4 ]
{
discriminator ::= '0';
flow_label ::= uncompressed_value (20, 0);
reserved ::= '0000';
};
co_format_fl_non_zero = discriminator, %[ 1 ]
flow_label, %[ 20 ]
{
discriminator ::= '1';
flow_label ::= irregular (20);
};
};
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% The argument flag should only be used if this flag was set when
% processing a compressed base header, if not, the flag should be
% zero.
ipv6 (ttl_irregular_chain_flag) ===
{
uc_format = version, %[ 4 ]
tos_tc, %[ 6 ]
ip_ecn_flags, %[ 2 ]
flow_label, %[ 20 ]
payload_length, %[ 16 ]
next_header, %[ 8 ]
ttl_hopl, %[ 8 ]
src_addr, %[ 128 ]
dst_addr; %[ 128 ]
default_methods =
{
version ::= uncompressed_value (4, 6);
tos_tc ::= static;
ip_ecn_flags ::= static;
flow_label ::= static;
payload_length ::= inferred_ip_v6_length;
next_header ::= static;
ttl_hopl ::= static;
src_addr ::= static;
dst_addr ::= static;
};
co_format_ipv6_static = version_flag, %[ 1 ]
reserved, %[ 2 ]
flow_label, % 5 or 21 bits
next_header, %[ 8 ]
src_addr, %[ 128 ]
dst_addr, %[ 128 ]
{
version_flag ::= '1';
reserved ::= '00';
flow_label ::= fl_enc;
next_header ::= irregular (8);
src_addr ::= irregular(128);
dst_addr ::= irregular(128);
};
co_format_ipv6_dynamic = tos_tc, %[ 6 ]
ip_ecn_flags, %[ 2 ]
ttl_hopl, %[ 8 ]
{
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tos_tc ::= irregular (6);
ip_ecn_flags ::= irregular (2);
ttl_hopl ::= irregular (8);
};
co_format_ipv6_replicate = tos_tc, %[ 6 ]
ip_ecn_flags, %[ 2 ]
{
tos_tc ::= irregular (6);
ip_ecn_flags ::= irregular (2);
};
co_format_ipv6_outer_irregular_without_ttl
= tos_tc, % 0 or 6 bits
ip_ecn_flags, % 0 or 2 bits
{
% for 'outer' headers only, irregular chain is required
%
tos_tc ::= irreg_tos_tc;
ip_ecn_flags ::= ip_irreg_ecn;
let(ttl_irregular_chain_flag == 0);
};
co_format_ipv6_outer_irregular_with_ttl
= tos_tc, % 0 or 6 bits
ip_ecn_flags, % 0 or 2 bits
ttl_hopl, %[ 8 ]
{
% for 'outer' headers only, irregular chain is required
%
tos_tc ::= irreg_tos_tc;
ip_ecn_flags ::= ip_irreg_ecn;
let(ttl_irregular_chain_flag == 1);
ttl_hopl ::= irregular(8);
};
% Note that the ECN bits are stored in the global control field
% so that they can be output in TCP irregular chain.
co_format_ipv6_innermost_irregular =
{
let(ip_inner_ecn:uncomp_value ==
ip_ecn_flags:uncomp_value);
};
};
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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% IPv4 Header
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
ip_id_enc_dyn (behavior) ===
{
uc_format = ip_id; %[ 16 ]
co_format_ip_id_seq = ip_id, %[ 16 ]
{
let ((behavior == 0) || (behavior == 1) || (behavior == 2));
%
% In dynamic chain, but random, seq, and seq-swapped are 16
% bits
%
ip_id ::= irregular(16);
};
co_format_ip_id_zero = ip_id, %[ 0 ]
{
let (behavior == 3);
%
% Zero IPID
%
ip_id ::= uncompressed_value (16, 0);
};
};
ip_id_enc_irreg (behavior) ===
{
uc_format = ip_id; %[ 16 ]
co_format_ip_id_seq = ip_id, %[ 0 ]
{
let (behavior == 0); % sequential
ip_id ::= static; % Nothing to send in irregular chain
};
co_format_ip_id_seq_swapped = ip_id, %[ 0 ]
{
let (behavior == 1); % sequential-swapped
ip_id ::= static; % Nothing to send in irregular chain
};
co_format_ip_id_rand = ip_id, %[ 16 ]
{
let (behavior == 2); % random
ip_id ::= irregular (16);
};
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co_format_ip_id_zero = ip_id, %[ 0 ]
{
let (behavior == 3); % zero
ip_id ::= uncompressed_value (16, 0);
};
};
ip_id_behavior_enc ===
{
uc_format = ip_id_behavior; %[ 2 ]
default_methods =
{
ip_id_behavior ::= irregular(2);
};
co_format_sequential = ip_id_behavior, %[ 2 ]
{
let (ip_id_behavior:uncomp_value ==
IP_ID_BEHAVIOR_SEQUENTIAL);
};
co_format_sequential_swapped = ip_id_behavior, %[ 2 ]
{
let (ip_id_behavior:uncomp_value ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED);
};
co_format_random = ip_id_behavior, %[ 2 ]
{
let (ip_id_behavior:uncomp_value == IP_ID_BEHAVIOR_RANDOM);
};
co_format_zero = ip_id_behavior, %[ 2 ]
{
let (ip_id_behavior:uncomp_value == IP_ID_BEHAVIOR_ZERO);
};
};
% The argument flag should only be used if this flag was set when
% processing a compressed base header, if not, the flag should be
% zero.
%
ipv4 (ttl_irregular_chain_flag) ===
{
uc_format = version, %[ 4 ]
hdr_length, %[ 4 ]
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tos_tc, %[ 6 ]
ip_ecn_flags, %[ 2 ]
length, %[ 16 ]
ip_id, %[ 16 ]
rf, %[ 1 ]
df, %[ 1 ]
mf, %[ 1 ]
frag_offset, %[ 13 ]
ttl_hopl, %[ 8 ]
protocol, %[ 8 ]
checksum, %[ 16 ]
src_addr, %[ 32 ]
dst_addr; %[ 32 ]
control_fields = ip_id_behavior; %[ 2 ]
default_methods =
{
version ::= static;
hdr_length ::= uncompressed_value (4, 5);
protocol ::= static;
tos_tc ::= static;
ip_ecn_flags ::= static;
ttl_hopl ::= static;
df ::= static;
mf ::= uncompressed_value (1, 0);
rf ::= static;
frag_offset ::= uncompressed_value (13, 0);
ip_id ::= uncompressed_value (16, 0);
ip_id_behavior ::= static;
src_addr ::= static;
dst_addr ::= static;
checksum ::= inferred_ip_v4_header_checksum;
length ::= inferred_ip_v4_length;
};
co_format_ipv4_static = version_flag, %[ 1 ]
reserved, %[ 7 ]
protocol, %[ 8 ]
src_addr, %[ 32 ]
dst_addr, %[ 32 ]
{
version_flag ::= '0';
reserved ::= '0000000';
protocol ::= irregular (8);
src_addr ::= irregular(32);
dst_addr ::= irregular(32);
};
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co_format_ipv4_dynamic = reserved, %[ 5 ]
df, %[ 1 ]
ip_id_behavior, %[ 2 ]
tos_tc, %[ 6 ]
ip_ecn_flags, %[ 2 ]
ttl_hopl, %[ 8 ]
ip_id, % 0/16 bits
{
reserved ::= '00000';
%
% compressor chooses behavior of IP-ID
%
ip_id_behavior ::= ip_id_behavior_enc;
df ::= irregular (1);
tos_tc ::= irregular (6);
ip_ecn_flags ::= irregular (2);
ttl_hopl ::= irregular (8);
ip_id ::= ip_id_enc_dyn (
ip_id_behavior:uncomp_value);
};
co_format_ipv4_replicate_0 = discriminator, %[ 8 ]
ip_id, % 0 or 16 bits
tos_tc, %[ 6 ]
ip_ecn_flags, %[ 2 ]
{
discriminator ::= '00000000';
ip_id_behavior ::= static;
ip_id ::= ip_id_enc_irreg (
ip_id_behavior:uncomp_value);
tos_tc ::= irregular (6);
ip_ecn_flags ::= irregular (2);
};
co_format_ipv4_replicate_1 = discriminator, %[ 5 ]
df, %[ 1 ]
ip_id_behavior, %[ 2 ]
tos_tc, %[ 6 ]
ip_ecn_flags, %[ 2 ]
ttl_hopl, %[ 8 ]
ip_id, % 0/16 bits
{
discriminator ::= '10000';
df ::= irregular (1);
tos_tc ::= irregular (6);
ip_ecn_flags ::= irregular (2);
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ttl_hopl ::= irregular (8);
%
% compressor chooses behavior of IP-ID
% 00 = sequential
% 01 = sequential byteswapped
% 10 = random
% 11 = zero
%
ip_id_behavior ::= ip_id_behavior_enc;
ip_id ::= ip_id_enc_dyn (ip_id_behavior:uncomp_value);
};
co_format_ipv4_outer_irregular_without_ttl =
ip_id, % 0 or 16 bits
tos_tc, % 0 or 6 bits
ip_ecn_flags, % 0 or 2 bits
{
ip_id_behavior ::= static;
ip_id ::= ip_id_enc_irreg (
ip_id_behavior:uncomp_value);
tos_tc ::= irreg_tos_tc;
ip_ecn_flags ::= ip_irreg_ecn;
let(ttl_irregular_chain_flag == 0);
};
co_format_ipv4_outer_irregular_with_ttl =
ip_id, % 0 or 16 bits
tos_tc, % 0 or 6 bits
ip_ecn_flags, % 0 or 2 bits
ttl_hopl, %[ 8 ]
{
ip_id_behavior ::= static;
ip_id ::= ip_id_enc_irreg (
ip_id_behavior:uncomp_value);
tos_tc ::= irreg_tos_tc;
ip_ecn_flags ::= ip_irreg_ecn;
let(ttl_irregular_chain_flag == 1);
ttl_hopl ::= irregular(8);
};
% Note that the ECN bits are stored in the global control field
% so that they can be output in TCP irregular chain.
co_format_ipv4_innermost_irregular = ip_id, % 0 or 16 bits
{
ip_id_behavior ::= static;
ip_id ::= ip_id_enc_irreg (
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ip_id_behavior:uncomp_value);
let(ip_inner_ecn:uncomp_value ==
ip_ecn_flags:uncomp_value);
};
};
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% TCP Options
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
% EOL marks the end of the option list and, based on
% the description in RFC 793 and the BSB TCP code,
% nothing after this should be processed...
% So, ignore everything after the EOL option
% (according to 793 it must be 0)
%
% The length of the padding needs to be transmitted with the
% compressed list since the length of the list can be unknown to
% the decompressor.
%
tcp_opt_eol(nbits) === {
uc_format = type, %[ 8 ]
padding; % (nbits - 8) bits
default_methods =
{
type ::= uncompressed_value (8, 0);
pad_len ::= static;
padding ::= uncompressed_value (nbits - 8, 0);
};
co_format_eol_list_item = pad_len, % 8 bits
padding, %[ 0 ]
{
pad_len ::= compressed_value(8, nbits - 8);
};
co_format_eol_irregular =
{
let(nbits - 8 == pad_len:uncomp_value);
};
};
tcp_opt_nop ===
{
uc_format = type; %[ 8 ]
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default_methods =
{
type ::= uncompressed_value (8, 1);
};
co_format_nop_list_item =
{
};
co_format_nop_irregular =
{
};
};
tcp_opt_mss ===
{
uc_format = type, %[ 8 ]
length, %[ 8 ]
mss; %[ 16 ]
default_methods =
{
type ::= uncompressed_value (8, 2);
length ::= uncompressed_value (8, 4);
mss ::= static;
};
co_format_mss_list_item = mss, %[ 16 ]
{
mss ::= irregular (16);
};
co_format_mss_irregular =
{
};
};
tcp_opt_wscale ===
{
uc_format = type, %[ 8 ]
length, %[ 8 ]
wscale; %[ 8 ]
default_methods =
{
type ::= uncompressed_value (8, 3);
length ::= uncompressed_value (8, 3);
wscale ::= static;
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};
co_format_wscale_list_item = wscale, %[ 8 ]
{
wscale ::= irregular (8);
};
co_format_wscale_irregular =
{
};
};
ts_lsb ===
{
uc_format = tsval;
%
% Few bits (7 and 14) bits can only increase, while the larger
% formats allow decreasing timestamp to allow prelink reordering.
%
co_format_tsval_7 = discriminator, %[ 1 ]
tsval, %[ 7 ]
{
discriminator ::= '0';
tsval ::= lsb (7, -1);
};
co_format_tsval_14 = discriminator, %[ 2 ]
tsval, %[ 14 ]
{
discriminator ::= '10';
tsval ::= lsb (14, -1);
};
co_format_tsval_21 = discriminator, %[ 3 ]
tsval, %[ 21 ]
{
discriminator ::= '110';
tsval ::= lsb (21, 0x00040000);
};
co_format_tsval_29 = discriminator, %[ 3 ]
tsval, %[ 29 ]
{
discriminator ::= '111';
tsval ::= lsb (29, 0x04000000);
};
};
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tcp_opt_tsopt ===
{
uc_format = type, %[ 8 ]
length, %[ 8 ]
tsval, %[ 32 ]
tsecho; %[ 32 ]
default_methods =
{
type ::= uncompressed_value (8, 8);
length ::= uncompressed_value (8, 10);
};
co_format_tsopt_list_item = tsval, %[ 32 ]
tsecho, %[ 32 ]
{
tsval ::= irregular (32);
tsecho ::= irregular (32);
};
co_format_tsopt_irregular = tsval, % 16, 24 or 32 bits
tsecho, % 16, 24 or 32 bits
{
tsval ::= ts_lsb;
tsecho ::= ts_lsb;
};
};
sack_var_length_enc (base) ===
{
uc_format = sack_field; %[ 32 ]
control_fields = sack_offset, %[ 32 ]
{
let (sack_offset:uncomp_value ==
sack_field:uncomp_value - base);
let (sack_offset:uncomp_length == 32);
let (sack_field:uncomp_length == 32);
};
co_format_lsb_15 = discriminator, %[ 1 ]
sack_offset, %[ 15 ]
{
discriminator ::= '0';
sack_offset ::= lsb (15, -1);
};
co_format_lsb_22 = discriminator, %[ 2 ]
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sack_offset, %[ 22 ]
{
discriminator ::= '10';
sack_offset ::= lsb (22, -1);
};
co_format_lsb_30 = discriminator, %[ 2 ]
sack_offset, %[ 30 ]
{
discriminator ::= '11';
sack_offset ::= lsb (30, -1);
};
};
tcp_opt_sack_block (prev_block_end) ===
{
uc_format = block_start, %[ 32 ]
block_end; %[ 32 ]
co_format_0 = block_start, % 16, 24 or 32 bits
block_end, % 16, 24 or 32 bits
{
block_start ::= sack_var_length_enc (prev_block_end);
block_end ::= sack_var_length_enc (block_start);
};
};
tcp_opt_sack(ack_value) ===
{
%
% The ACK value from the TCP header is needed as input parameter.
%
uc_format = type, %[ 8 ]
length, %[ 8 ]
block_1, %[ 64 ]
block_2, % 0 or 64 bits
block_3, % 0 or 64 bits
block_4; % 0 or 64 bits
default_methods =
{
length ::= static;
type ::= uncompressed_value (8, 5);
block_2 ::= uncompressed_value (0, 0);
block_3 ::= uncompressed_value (0, 0);
block_4 ::= uncompressed_value (0, 0);
};
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co_format_sack1_list_item = discriminator,
block_1,
{
let(length:uncomp_value == 10);
discriminator ::= '00000001';
block_1 ::= tcp_opt_sack_block (ack_value);
};
co_format_sack2_list_item = discriminator,
block_1,
block_2,
{
let(length:uncomp_value == 18);
discriminator ::= '00000010';
block_1 ::= tcp_opt_sack_block (ack_value);
block_2 ::= tcp_opt_sack_block (block_1_end:uncomp_value);
};
co_format_sack3_list_item = discriminator,
block_1,
block_2,
block_3,
{
let(length:uncomp_value == 26);
discriminator ::= '00000011';
block_1 ::= tcp_opt_sack_block (ack_value);
block_2 ::= tcp_opt_sack_block (block_1_end:uncomp_value);
block_3 ::= tcp_opt_sack_block (block_2_end:uncomp_value);
};
co_format_sack4_list_item = discriminator,
block_1,
block_2,
block_3,
block_4,
{
let(length:uncomp_value == 34);
discriminator ::= '00000100';
block_1 ::= tcp_opt_sack_block (ack_value);
block_2 ::= tcp_opt_sack_block (block_1_end:uncomp_value);
block_3 ::= tcp_opt_sack_block (block_2_end:uncomp_value);
block_4 ::= tcp_opt_sack_block (block_3_end:uncomp_value);
};
co_format_sack_unchanged_irregular = discriminator,
block_1,
block_2,
block_3,
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block_4,
{
discriminator ::= '00000000';
block_1 ::= static;
block_2 ::= static;
block_3 ::= static;
block_4 ::= static;
};
co_format_sack1_irregular = discriminator,
block_1,
{
let(length:uncomp_value == 10);
discriminator ::= '00000001';
block_1 ::= tcp_opt_sack_block (ack_value);
};
co_format_sack2_irregular = discriminator,
block_1,
block_2,
{
let(length:uncomp_value == 18);
discriminator ::= '00000010';
block_1 ::= tcp_opt_sack_block (ack_value);
block_2 ::= tcp_opt_sack_block (block_1_end:uncomp_value);
};
co_format_sack3_irregular = discriminator,
block_1,
block_2,
block_3,
{
let(length:uncomp_value == 26);
discriminator ::= '00000011';
block_1 ::= tcp_opt_sack_block (ack_value);
block_2 ::= tcp_opt_sack_block (block_1_end:uncomp_value);
block_3 ::= tcp_opt_sack_block (block_2_end:uncomp_value);
};
co_format_sack4_irregular = discriminator,
block_1,
block_2,
block_3,
block_4,
{
let(length:uncomp_value == 34);
discriminator ::= '00000100';
block_1 ::= tcp_opt_sack_block (ack_value);
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block_2 ::= tcp_opt_sack_block (block_1_end:uncomp_value);
block_3 ::= tcp_opt_sack_block (block_2_end:uncomp_value);
block_4 ::= tcp_opt_sack_block (block_3_end:uncomp_value);
};
};
tcp_opt_sack_permitted ===
{
uc_format = type, %[ 8 ]
length; %[ 8 ]
default_methods =
{
type ::= uncompressed_value (8, 4);
length ::= uncompressed_value (8, 2);
};
co_format_sack_permitted_list_item =
{
};
co_format_sack_permitted_irregular =
{
};
};
tcp_opt_generic ===
{
uc_format = type, %[ 8 ]
length_msb, %[ 1 ]
length_lsb, %[ 7 ]
contents; % n bits
control_fields = option_static, %[ 1 ]
{
let (option_static:uncomp_length == 1);
};
default_methods =
{
type ::= static;
%
% lengths are always < 128
% (i.e. the msb is always 0)
%
length_msb ::= uncompressed_value (1, 0);
length_lsb ::= static;
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contents ::= static;
};
co_format_generic_list_item = type, %[ 8 ]
option_static, %[ 1 ]
length_lsb, %[ 7 ]
contents, % n bits
{
type ::= irregular (8);
option_static ::= irregular (1);
length_lsb ::= irregular (7);
contents ::= irregular (length_len:uncomp_value * 8 - 16);
};
% Used when context of option has option_static set to one
%
co_format_generic_irregular_static =
{
let(option_static:uncomp_value == 1);
};
% An item that can change, but currently is unchanged
%
co_format_generic_irregular_stable = discriminator, %[ 8 ]
{
let(option_static:uncomp_value == 0);
discriminator ::= '11111111';
};
% An item that can change, and has changed compared to context.
% Length is not allowed to change here, since a length change is
% most likely to cause new NOPs or an EOL length change.
%
co_format_generic_irregular_full = discriminator, %[ 8 ]
contents, % n bits
{
let(option_static:uncomp_value == 0);
discriminator ::= '00000000';
contents ::= irregular (
length_lsb:uncomp_value * 8 - 16);
};
};
tcp_list_presence_enc(list_length, presence, ack_value) ===
{
uc_format = tcp_options;
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co_format_list_not_present = tcp_options, %[ 0 ]
{
let (presence == 0);
tcp_options ::= static;
};
co_format_list_present = tcp_options, % 8 + n*8 bits
{
let (presence == 1);
tcp_options ::= list_tcp_options(list_length, ack_value);
};
};
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% TCP Header
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
port_replicate(flags) ===
{
uc_format = port; %[ 16 ]
co_format_port_static_enc = port, %[ 0 ]
{
let(flags == 0b00);
port ::= static;
};
co_format_port_lsb8 = port, %[ 8 ]
{
let(flags == 0b01);
port ::= lsb (8, 64);
};
co_format_port_irr_enc = port, %[ 16 ]
{
let(flags == 0b10);
port ::= irregular (16);
};
};
zero_or_irr16_enc(flag) ===
{
uc_format = field; %[ 16 ]
co_format_non_zero = field, %[ 16 ]
{
let(flag == 0);
field ::= irregular (16);
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};
co_format_zero = field, %[ 0 ]
{
let(flag == 1);
field ::= uncompressed_value (16, 0);
};
};
ack_enc_dyn(flag) ===
{
uc_format = ack_number; %[ 32 ]
co_format_ack_non_zero = ack_number, %[ 32 ]
{
let(flag == 0);
ack_number ::= irregular (32);
};
co_format_ack_zero = ack_number, %[ 0 ]
{
let(flag == 1);
ack_number ::= uncompressed_value (32, 0);
};
};
tcp_ecn_flags_enc ===
{
uc_format = tcp_ecn_flags; %[ 2 ]
co_format_irreg = tcp_ecn_flags, %[ 2 ]
{
let(ecn_used:uncomp_value == 1);
tcp_ecn_flags ::= irregular(2);
};
co_format_unused =
{
let(ecn_used:uncomp_value == 0);
tcp_ecn_flags ::= static;
};
};
tcp_res_flags_enc ===
{
uc_format = tcp_res_flags; %[ 4 ]
co_format_irreg = tcp_res_flags, %[ 4 ]
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{
let(ecn_used:uncomp_value == 1);
tcp_res_flags ::= irregular(4);
};
co_format_unused =
{
let(ecn_used:uncomp_value == 0);
tcp_res_flags ::= uncompressed_value(4, 0);
};
};
tcp_irreg_ip_ecn ===
{
uc_format = ip_ecn_flags; %[ 2 ]
co_format_tc_present = ip_ecn_flags, %[ 2 ]
{
let(ecn_used:uncomp_value == 1);
ip_ecn_flags ::= compressed_value(2,
ip_inner_ecn:uncomp_value);
};
co_format_tc_not_present = ip_ecn_flags, %[ 0 ]
{
let(ecn_used:uncomp_value == 0);
ip_inner_ecn ::= static; % Global control field
ip_ecn_flags ::= compressed_value(0,0); % Nothing transmit
};
};
rsf_index_enc ===
{
uc_format = rsf_flag; %[ 3 ]
co_format_none = rsf_idx, %[ 2 ]
{
rsf_idx ::= '00';
rsf_flag ::= uncompressed_value (3, 0x00);
};
co_format_rst_only = rsf_idx, %[ 2 ]
{
rsf_idx ::= '01';
rsf_flag ::= uncompressed_value (3, 0x04);
};
co_format_syn_only = rsf_idx, %[ 2 ]
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{
rsf_idx ::= '10';
rsf_flag ::= uncompressed_value (3, 0x02);
};
co_format_fin_only = rsf_idx, %[ 2 ]
{
rsf_idx ::= '11';
rsf_flag ::= uncompressed_value (3, 0x01);
};
};
optional_2bit_padding(used_flag) ===
{
uc_format = ;
co_format_used = padding, %[ 2 ]
{
let(used_flag == 1);
padding ::= compressed_value (2, 0x0);
};
co_format_unused = padding,
{
let(used_flag == 0);
padding ::= compressed_value (0, 0x0);
};
};
tcp ===
{
uc_format = src_port, %[ 16 ]
dst_port, %[ 16 ]
seq_number, %[ 32 ]
ack_number, %[ 32 ]
data_offset, %[ 4 ]
tcp_res_flags, %[ 4 ]
tcp_ecn_flags, %[ 2 ]
urg_flag, %[ 1 ]
ack_flag, %[ 1 ]
psh_flag, %[ 1 ]
rsf_flags, %[ 3 ]
window, %[ 16 ]
checksum, %[ 16 ]
urg_ptr, %[ 16 ]
options; % n bits
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default_methods =
{
src_port ::= static;
dst_port ::= static;
seq_number ::= static;
ack_number ::= static;
rsf_flags ::= static;
psh_flag ::= irregular (1);
urg_flag ::= static;
ack_flag ::= uncompressed_value (1, 1);
urg_ptr ::= static;
window ::= static;
checksum ::= irregular (16);
tcp_ecn_flags ::= static;
tcp_res_flags ::= static;
};
co_format_tcp_static = src_port, %[ 16 ]
dst_port, %[ 16 ]
{
src_port ::= irregular(16);
dst_port ::= irregular(16);
};
co_format_tcp_dynamic = ecn_used, %[ 1 ]
ack_stride_zero, %[ 1 ]
ack_zero, %[ 1 ]
urp_zero, %[ 1 ]
tcp_res_flags, %[ 4 ]
tcp_ecn_flags, %[ 2 ]
urg_flag, %[ 1 ]
ack_flag, %[ 1 ]
psh_flag, %[ 1 ]
rsf_flags, %[ 3 ]
msn, %[ 16 ]
seq_number, %[ 32 ]
ack_number, % 0 or 32 bits
window, %[ 16 ]
checksum, %[ 16 ]
urg_ptr, % 0 or 16 bits
ack_stride, % 0 or 16 bits
options, % n bits
{
ecn_used ::= irregular (1);
ack_stride_zero ::= irregular (1);
ack_zero ::= irregular (1);
urp_zero ::= irregular (1);
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ack_flag ::= irregular (1);
urg_flag ::= irregular (1);
psh_flag ::= irregular (1);
tcp_ecn_flags ::= irregular (2);
rsf_flags ::= irregular (3);
tcp_res_flags ::= irregular (4);
msn ::= irregular (16);
seq_number ::= irregular (32);
window ::= irregular (16);
checksum ::= irregular (16);
urg_ptr ::= zero_or_irr16_enc(urp_zero:comp_value);
ack_number ::= ack_enc_dyn(ack_zero:comp_value);
ack_stride ::= zero_or_irr16_enc(
ack_stride_zero:comp_value);
data_offset ::= uncompressed_value(4, data_offset_value);
options ::= list_tcp_options(
(data_offset_value - 5) * 32,
ack_number:uncomp_value);
};
co_format_tcp_replicate = reserved, %[ 2 ]
window_presence, %[ 1 ]
list_present, %[ 1 ]
src_port_presence, %[ 2 ]
dst_port_presence, %[ 2 ]
ack_presence, %[ 1 ]
urp_presence, %[ 1 ]
urg_flag, %[ 1 ]
ack_flag, %[ 1 ]
psh_flag, %[ 1 ]
rsf_flags, %[ 2 ]
ecn_used, %[ 1 ]
msn, %[ 16 ]
seq_number, %[ 32 ]
src_port, % 0, 8 or 16 bits
dst_port, % 0, 8 or 16 bits
window, % 0 or 16 bits
urg_point, % 0 or 16 bits
ack_number, % 0 or 32 bits
ecn_padding, % 0 or 2 bits
tcp_res_flags, % 0 or 4 bits
tcp_ecn_flags, % 0 or 2 bits
options, % n bits
{
reserved ::= '00';
list_present ::= irregular (1);
msn ::= irregular (16);
urg_flag ::= irregular (1);
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ack_flag ::= irregular (1);
psh_flag ::= irregular (1);
rsf_flags ::= rsf_index_enc;
ecn_used ::= irregular (1);
src_port_presence ::= compressed_value(2,
src_port_presence_value);
dst_port_presence ::= compressed_value(2,
dst_port_presence_value);
src_port ::= port_replicate(src_port_presence_value);
dst_port ::= port_replicate(dst_port_presence_value);
seq_number ::= irregular(32);
ack_presence ::= compressed_value(1, ack_presence_value);
window_presence ::= compressed_value(1,
window_presence_value);
urp_presence ::= compressed_value(1, urg_presence_value);
ack_number ::= static_or_irreg32(ack_presence_value);
window ::= static_or_irreg16(
window_presence_value);
urg_point ::= static_or_irreg16(urp_presence_value);
ecn_padding ::= optional_2bit_padding(
ecn_used:comp_value);
tcp_res_flags ::= tcp_res_flags_enc;
tcp_ecn_flags ::= tcp_ecn_flags_enc;
data_offset ::= uncompressed_value(4,
data_offset_value);
options ::= tcp_list_presence_enc
((data_offset_value - 5) * 32,
list_present:comp_value,
ack_number:uncomp_value);
};
% ECN from innermost IP header is taken from global control field
%
co_format_tcp_irregular = ip_ecn_flags, % 0 or 2 bits
tcp_res_flags, % 0 or 4 bits
tcp_ecn_flags, % 0 or 2 bits
checksum, %[ 16 ]
{
ip_ecn_flags ::= tcp_irreg_ip_ecn;
tcp_ecn_flags ::= tcp_ecn_flags_enc;
tcp_res_flags ::= tcp_res_flags_enc;
checksum ::= irregular (16);
};
};
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Structures used in compressed base headers
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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tos_tc_enc(flag) ===
{
uc_format = tos_tc; %[ 6 ]
co_format_static = tos_tc, %[ 0 ]
{
let (flag == 0);
tos_tc ::= static;
};
co_format_irreg = tos_tc, %[ 6 ]
padding, %[ 2 ]
{
let (flag == 1);
tos_tc ::= irregular(6);
padding ::= compressed_value (2, 0);
};
};
ip_id_lsb (behavior, k, p) ===
{
uc_format = ip_id, %[ 16 ]
{
let (ip_id:uncomp_length == 16);
};
co_format_nbo = ip_id_offset, % k bits
{
let (behavior == 0);
let (ip_id_offset:uncomp_value ==
ip_id:uncomp_value - msn:uncomp_value);
let (ip_id_offset:uncomp_length == 16);
ip_id_offset ::= lsb (k, p);
};
co_format_non_nbo = ip_id_offset, % k bits
{
let (behavior == 1);
let (ip_id_nbo:uncomp_value ==
(ip_id:uncomp_value / 256) +
(ip_id:uncomp_value & 255) * 256);
let (ip_id_nbo:uncomp_length == 16);
let (ip_id_offset:uncomp_value ==
ip_id_nbo:uncomp_value - msn:uncomp_value);
let (ip_id_offset:uncomp_length == 16);
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ip_id_offset ::= lsb (k, p);
};
};
dont_fragment(version) ===
{
uc_format = df; %[ 1 ]
co_format_v4 = df, %[ 1 ]
{
let (version == 4);
df ::= irregular(1);
};
co_format_v6 = df,
{
let (version == 6);
df ::= compressed_value(1,0);
};
};
% Structures for updating the scaling control fields.
%
seq_number_scaling(payload_size) ===
{
uc_format = seq_number;
co_format_no_payload =
{
let(payload_size == 0);
let (seq_number_residue:uncomp_value ==
seq_number:uncomp_value);
let (seq_number_scaled:uncomp_value == 0);
};
co_format_with_payload =
{
let(payload_size != 0);
let(seq_number_residue:uncomp_value ==
mod(seq_number:uncomp_value, payload_size));
let(seq_number:uncomp_value ==
seq_number_scaled:uncomp_value * payload_size +
seq_number_residue:uncomp_value);
};
}
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ack_number_scaling ===
{
uc_format = ack_number;
co_format_stride_not_set =
{
let(ack_stride:uncomp_value == 0);
let (ack_number_residue:uncomp_value ==
ack_number:uncomp_value);
let (ack_number_scaled:uncomp_value == 0);
};
co_format_stride_set =
{
let(ack_stride:uncomp_value != 0);
let(ack_number_residue:uncomp_value ==
mod(ack_number:uncomp_value, payload_size));
let(ack_number:uncomp_value ==
ack_number_scaled:uncomp_value * ack_stride:uncomp_value +
ack_number_residue:uncomp_value);
};
}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Actual start of compressed packet formats
% Important note:
% The base header is the compressed representation
% of the innermost IP header AND the TCP header.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% ttl_irregular_chain_flag is an "output argument" that should be
% passed to the processing of the irregular chain for outer
% IP headers.
%
co_baseheader(payload_size, ttl_irregular_chain_flag) ===
{
uc_format_v4 = version, %[ 4 ]
header_length, %[ 4 ]
tos_tc, %[ 6 ]
ip_ecn_flags, %[ 2 ]
length, %[ 16 ]
ip_id, %[ 16 ]
rf, %[ 1 ]
df, %[ 1 ]
mf, %[ 1 ]
frag_offset, %[ 13 ]
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ttl_hopl, %[ 8 ]
next_header, %[ 8 ]
checksum, %[ 16 ]
src_addr, %[ 32 ]
dest_addr, %[ 32 ]
src_port, %[ 16 ]
dest_port, %[ 16 ]
seq_number, %[ 32 ]
ack_number, %[ 32 ]
data_offset, %[ 4 ]
tcp_res_flags, %[ 4 ]
tcp_ecn_flags, %[ 2 ]
urg_flag, %[ 1 ]
ack_flag, %[ 1 ]
psh_flag, %[ 1 ]
rsf_flags, %[ 3 ]
window, %[ 16 ]
tcp_checksum, %[ 16 ]
urg_ptr, %[ 16 ]
options_list, % n bits
{
let (version:uncomp_value == 4);
};
uc_format_v6 = version, %[ 4 ]
tos_tc, %[ 6 ]
ip_ecn_flags, %[ 2 ]
flow_label, %[ 20 ]
payload_length, %[ 16 ]
next_header, %[ 8 ]
ttl_hopl, %[ 8 ]
src_addr, %[ 128 ]
dest_addr, %[ 128 ]
src_port, %[ 16 ]
dest_port, %[ 16 ]
seq_number, %[ 32 ]
ack_number, %[ 32 ]
data_offset, %[ 4 ]
tcp_res_flags, %[ 4 ]
tcp_ecn_flags, %[ 2 ]
urg_flag, %[ 1 ]
ack_flag, %[ 1 ]
psh_flag, %[ 1 ]
rsf_flags, %[ 3 ]
window, %[ 16 ]
tcp_checksum, %[ 16 ]
urg_ptr, %[ 16 ]
options_list, % n bits
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{
let (version:uncomp_value == 6);
let (ip_id_behavior:uncomp_value == IP_ID_BEHAVIOR_RANDOM);
};
control_fields = ip_id_behavior, % 2 bits
{
let (version:uncomp_length == 4);
seq_number ::= seq_number_scaling(payload_size);
ack_number ::= ack_number_scaling;
};
default_methods =
{
version ::= static;
tos_tc ::= static;
ip_ecn_flags ::= static;
ttl_hopl ::= static;
next_header ::= static;
src_addr ::= static;
dest_addr ::= static;
flow_label ::= static;
payload_length ::= inferred_ip_v6_length;
header_length ::= uncompressed_value (4,5);
length ::= inferred_ip_v4_length;
ip_id ::= irregular(16);
ip_id_behavior ::= static;
rf ::= static;
df ::= static;
mf ::= static;
frag_offset ::= static;
checksum ::= inferred_ip_v4_header_checksum;
src_port ::= static;
dest_port ::= static;
seq_number ::= static;
ack_number ::= static;
data_offset ::= inferred_offset;
tcp_ecn_flags ::= static;
psh_flag ::= irregular (1);
urg_flag ::= uncompressed_value (1, 0);
ack_flag ::= uncompressed_value (1, 1);
window ::= static;
tcp_checksum ::= irregular(16);
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urg_ptr ::= static;
rsf_flags ::= uncompressed_value (3, 0);
tcp_res_flags ::= static;
options_list ::= static;
};
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Common compressed packet format
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
co_format_co_common = discriminator, %[ 7 ]
ttl_hopl_outer_flag, %[ 1 ]
ack_flag, %[ 1 ]
psh_flag, %[ 1 ]
rsf_flags, %[ 2 ]
msn, %[ 4 ]
seq_indicator, %[ 2 ]
ack_indicator, %[ 2 ]
ack_stride_indicator, %[ 1 ]
window_indicator, %[ 1 ]
ip_id_indicator, %[ 2 ]
urg_ptr_present, %[ 1 ]
ecn_used, %[ 1 ]
tos_tc_present, %[ 1 ]
ttl_hopl_present, %[ 1 ]
list_present, %[ 1 ]
ip_id_behavior, %[ 2 ]
urg_flag, %[ 1 ]
df, %[ 1 ]
header_crc, %[ 7 ]
seq_number, % 0, 8, 16, 32 bits
ack_number, % 0, 8, 16, 32 bits
ack_stride, % 0 or 16 bits
window, % 0 or 16 bits
ip_id, % 0, 8, 16 bits
urg_ptr, % 0 or 16 bits
tos_tc, % 0 or 8 bits
ttl_hopl, % 0 or 8 bits
options_list, % n bits
{
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discriminator ::= '1111101';
ttl_hopl_outer_flag::= irregular(1);
%
% Need to bind argument so that user can pass it on to the
% structure for IPv4/IPv6 irregular chain.
%
let(ttl_irregular_chain_flag ==
ttl_hopl_outer_flag:uncomp_value);
tos_tc_present ::= irregular(1);
ttl_hopl_present ::= irregular(1);
ack_flag ::= irregular(1);
psh_flag ::= irregular(1);
msn ::= lsb (4, 3);
df ::= dont_fragment(version:uncomp_value);
header_crc ::= crc7(this:uncomp_value,
this:uncomp_length);
urg_flag ::= irregular(1);
urg_ptr_present ::= irregular(1);
ecn_used ::= irregular(1);
list_present ::= irregular(1);
ip_id_behavior ::= ip_id_behavior_enc;
rsf_flags ::= rsf_index_enc;
window_indicator ::= irregular(1);
ip_id_indicator ::= irregular(2);
seq_indicator ::= irregular(2);
ack_indicator ::= irregular(2);
ack_stride_indicator ::= irregular(1);
seq_number ::= variable_length_32_enc(
seq_indicator:comp_value);
ack_number ::= variable_length_32_enc(
ack_indicator:comp_value);
ack_stride ::= static_or_irreg16(
ack_stride_indicator:comp_value);
window ::= static_or_irreg16(
window_indicator:comp_value);
ip_id ::= variable_length_16_enc(
ip_id_indicator:comp_value);
urg_ptr ::= static_or_irreg16(urg_ptr_present:comp_value);
ttl_hopl ::= static_or_irreg8(ttl_hopl_present:comp_value);
tos_tc ::= tos_tc_enc(tos_tc_present:comp_value);
options_list ::= tcp_list_presence_enc
((data_offset:uncomp_value - 5) * 32,
list_present:comp_value,
ack_number:uncomp_value);
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};
% 0 1 2 3 4 5 6 7
% +---+---+---+---+---+---+---+---+ tho controls
% | 1 1 1 1 1 0 1 |tho| irregular chain TTL/Hoplimit
% +===+===+===+===+===+===+===+===+
% |ACK|PSH| RSF | MSN |
% +---+---+---+---+---+---+---+---+
% | sn | a_sn |ast|win| ip_id |
% +---+---+---+---+---+---+---+---+
% |urg|ecn|tos|ttl|lst|IPIDbeh|URG|
% +---+---+---+---+---+---+---+---+
% |DF | CRC |
% +---+---+---+---+---+---+---+---+
% / SN / 0, 8, 16, 32 bits,
% --- --- --- --- --- --- --- --- indicated by sn
% / ACK_SN / 0, 8, 16, 32 bits,
% --- --- --- --- --- --- --- --- indicated by a_sn
% / ACK_STRIDE / 0 or 16 bits, indicated by ast
% --- --- --- --- --- --- --- ---
% / WINDOW / 0 or 16 bits, indicated by win
% --- --- --- --- --- --- --- ---
% / IP-ID / 0, 8, 16 bits, indicated by ip_id
% --- --- --- --- --- --- --- ---
% / URG-PTR / 16 bits, if urg=1
% --- --- --- --- --- --- --- ---
% / TOS / 8 bits, if tos=1
% --- --- --- --- --- --- --- ---
% / TTL / 8 bits, if ttl=1
% --- --- --- --- --- --- --- ---
% / options_list / n*8 bits, if lst=1
% --- --- --- --- --- --- --- ---
% Send LSBs of sequence number
%
co_format_rnd_1 = discriminator, %[ 8 ]
seq_number, %[ 16 ]
msn, %[ 4 ]
psh_flag, %[ 1 ]
header_crc, %[ 3 ]
{
let ((ip_id_behavior:uncomp_value == IP_ID_BEHAVIOR_RANDOM) ||
(ip_id_behavior:uncomp_value == IP_ID_BEHAVIOR_ZERO));
discriminator ::= '10111110';
msn ::= lsb(4, 4);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
psh_flag ::= irregular (1);
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seq_number ::= lsb(16, 32767);
};
% 0 1 2 3 4 5 6 7
% +---+---+---+---+---+---+---+---+
% | 1 0 1 1 1 1 1 0 |
% +===+===+===+===+===+===+===+===+
% | SN... |
% +---+---+---+---+---+---+---+---+
% | ...SN |
% +---+---+---+---+---+---+---+---+
% | MSN |PSH| CRC |
% +---+---+---+---+---+---+---+---+
% Send scaled sequence number LSBs
%
co_format_rnd_2 = discriminator, %[ 4 ]
seq_number_scaled, %[ 4 ]
msn, %[ 4 ]
psh_flag, %[ 1 ]
header_crc, %[ 3 ]
{
let ((ip_id_behavior:uncomp_value == IP_ID_BEHAVIOR_RANDOM) ||
(ip_id_behavior:uncomp_value == IP_ID_BEHAVIOR_ZERO));
discriminator ::= '1100';
msn ::= lsb(4, 4);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
psh_flag ::= irregular (1);
seq_number_scaled ::= lsb(4, 7);
seq_number_residue ::= static;
};
% 0 1 2 3 4 5 6 7
% +---+---+---+---+---+---+---+---+
% | 1 1 0 0 | SN_SCALED |
% +===+===+===+===+===+===+===+===+
% | MSN |PSH| CRC |
% +---+---+---+---+---+---+---+---+
% Send acknowledgement number LSBs
%
co_format_rnd_3 = discriminator, %[ 1 ]
ack_number, %[ 15 ]
msn, %[ 4 ]
psh_flag, %[ 1 ]
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header_crc, %[ 3 ]
{
let ((ip_id_behavior:uncomp_value == IP_ID_BEHAVIOR_RANDOM) ||
(ip_id_behavior:uncomp_value == IP_ID_BEHAVIOR_ZERO));
discriminator ::= '0';
msn ::= lsb(4, 4);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
psh_flag ::= irregular (1);
ack_number ::= lsb(15, 8191);
};
% 0 1 2 3 4 5 6 7
% +---+---+---+---+---+---+---+---+
% | 0 | ACK_SN... |
% +===+===+===+===+===+===+===+===+
% | ...ACK_SN |
% +---+---+---+---+---+---+---+---+
% | MSN |PSH| CRC |
% +---+---+---+---+---+---+---+---+
% Send acknowledgement number scaled
%
co_format_rnd_4 = discriminator, %[ 4 ]
ack_number_scaled, %[ 4 ]
msn, %[ 4 ]
psh_flag, %[ 1 ]
header_crc, %[ 3 ]
{
let ((ip_id_behavior:uncomp_value == IP_ID_BEHAVIOR_RANDOM) ||
(ip_id_behavior:uncomp_value == IP_ID_BEHAVIOR_ZERO));
discriminator ::= '1101';
msn ::= lsb(4, 4);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
psh_flag ::= irregular (1);
ack_number_scaled ::= lsb(4, 3);
ack_number_residue ::= static;
};
% 0 1 2 3 4 5 6 7
% +---+---+---+---+---+---+---+---+
% | 1 1 0 1 | ACK_SN_SCALED |
% +===+===+===+===+===+===+===+===+
% | MSN |PSH| CRC |
% +---+---+---+---+---+---+---+---+
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% Send ACK and sequence number
%
co_format_rnd_5 = discriminator, %[ 3 ]
psh_flag, %[ 1 ]
msn, %[ 4 ]
header_crc, %[ 3 ]
seq_number, %[ 14 ]
ack_number, %[ 15 ]
{
let ((ip_id_behavior:uncomp_value == IP_ID_BEHAVIOR_RANDOM) ||
(ip_id_behavior:uncomp_value == IP_ID_BEHAVIOR_ZERO));
discriminator ::= '100';
msn ::= lsb(4, 4);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
psh_flag ::= irregular (1);
ack_number ::= lsb(15, 8191);
seq_number ::= lsb(14, 8191);
};
% 0 1 2 3 4 5 6 7
% +---+---+---+---+---+---+---+---+
% | 1 0 0 |PSH| MSN |
% +===+===+===+===+===+===+===+===+
% | CRC | SN.. |
% +---+---+---+---+---+---+---+---+
% | ...SN... |
% +---+---+---+---+---+---+---+---+
% |..S| ACK_SN... |
% +---+---+---+---+---+---+---+---+
% | ...ACK_SN |
% +---+---+---+---+---+---+---+---+
% Send both ACK and scaled sequence number LSBs
%
co_format_rnd_6 = discriminator, %[ 5 ]
header_crc, %[ 3 ]
psh_flag, %[ 1 ]
ack_number, %[ 15 ]
msn, %[ 4 ]
seq_number_scaled, %[ 4 ],
{
let ((ip_id_behavior:uncomp_value == IP_ID_BEHAVIOR_RANDOM) ||
(ip_id_behavior:uncomp_value == IP_ID_BEHAVIOR_ZERO));
discriminator ::= '10110';
msn ::= lsb(4, 4);
header_crc ::= crc3 (this:uncomp_value,
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this:uncomp_length);
psh_flag ::= irregular (1);
ack_number ::= lsb(15, 8191);
seq_number_scaled ::= lsb(4, 7);
seq_number_residue ::= static;
};
% +---+---+---+---+---+---+---+---+
% | 1 0 1 1 0 | CRC |
% +===+===+===+===+===+===+===+===+
% |PSH| ACK_SN... |
% +---+---+---+---+---+---+---+---+
% | ...ACK_SN... |
% +---+---+---+---+---+---+---+---+
% | MSN | SN_SCALED |
% +---+---+---+---+---+---+---+---+
% Send sequence number and window
%
co_format_rnd_7 = discriminator, %[ 4 ]
seq_number, %[ 14 ]
window, %[ 14 ]
msn, %[ 4 ]
psh_flag, %[ 1 ]
header_crc, %[ 3 ]
{
let ((ip_id_behavior:uncomp_value == IP_ID_BEHAVIOR_RANDOM) ||
(ip_id_behavior:uncomp_value == IP_ID_BEHAVIOR_ZERO));
discriminator ::= '1010';
msn ::= lsb(4, 4);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
psh_flag ::= irregular (1);
seq_number ::= lsb(14, 8191);
window ::= lsb(14, 8191);
};
% +---+---+---+---+---+---+---+---+
% | 1 0 1 0 | SN... |
% +===+===+===+===+===+===+===+===+
% | SN...
% +---+---+---+---+---+---+---+---+
% | ...SN | WINDOW... |
% +---+---+---+---+---+---+---+---+
% | WINDOW |
% +---+---+---+---+---+---+---+---+
% | MSN |PSH| CRC |
% +---+---+---+---+---+---+---+---+
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% Send ACK and window
%
co_format_rnd_8 = discriminator, %[ 8 ]
ack_number, %[ 16 ]
window, %[ 16 ]
msn, %[ 4 ]
psh_flag, %[ 1 ]
header_crc, %[ 3 ]
{
let ((ip_id_behavior:uncomp_value == IP_ID_BEHAVIOR_RANDOM) ||
(ip_id_behavior:uncomp_value == IP_ID_BEHAVIOR_ZERO));
discriminator ::= '10111111';
msn ::= lsb(4, 4);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
psh_flag ::= irregular (1);
ack_number ::= lsb(16, 16383);
window ::= irregular(16);
};
% +---+---+---+---+---+---+---+---+
% | 1 0 1 1 1 1 1 1 |
% +===+===+===+===+===+===+===+===+
% | |
% + ACK_SN +
% | |
% +---+---+---+---+---+---+---+---+
% | |
% + WINDOW +
% | |
% +---+---+---+---+---+---+---+---+
% | MSN |PSH| CRC |
% +---+---+---+---+---+---+---+---+
% Send scaled sequence number and window.
%
co_format_rnd_9 = discriminator, %[ 6 ]
seq_number_scaled, %[ 4 ]
window, %[ 14 ]
msn, %[ 4 ]
psh_flag, %[ 1 ]
header_crc, %[ 3 ]
{
let ((ip_id_behavior:uncomp_value == IP_ID_BEHAVIOR_RANDOM) ||
(ip_id_behavior:uncomp_value == IP_ID_BEHAVIOR_ZERO));
discriminator ::= '101110';
msn ::= lsb(4, 4);
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header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
psh_flag ::= irregular (1);
window ::= lsb(14, 8191);
seq_number_scaled ::= lsb(4, 3);
seq_number_residue ::= static;
};
% +---+---+---+---+---+---+---+---+
% | 1 0 1 1 1 0 |SN_SC..|
% +===+===+===+===+===+===+===+===+
% |..SN_SC| WINDOW... |
% +---+---+---+---+---+---+---+---+
% | ...WINDOW |
% +---+---+---+---+---+---+---+---+
% | MSN |PSH| CRC |
% +---+---+---+---+---+---+---+---+
% A packet halfway between co_common and compressed packets
% Can send LSBs of TTL, RSF flags, change ECN behavior and
% options list
%
co_format_rnd_10 = discriminator, %[ 7 ]
ecn_used, %[ 1 ]
list_present, %[ 1 ]
header_crc, %[ 7 ]
msn, %[ 4 ]
psh_flag, %[ 1 ]
ttl_hopl, %[ 3 ]
rsf_flags, %[ 2 ]
seq_number, %[ 14 ]
ack_number, %[ 16 ]
options_list, % 0 or X bits
{
let ((ip_id_behavior:uncomp_value == IP_ID_BEHAVIOR_RANDOM) ||
(ip_id_behavior:uncomp_value == IP_ID_BEHAVIOR_ZERO));
discriminator ::= '1011110';
msn ::= lsb(4, 4);
header_crc ::= crc7 (this:uncomp_value,
this:uncomp_length);
psh_flag ::= irregular (1);
list_present ::= irregular(1);
options_list ::= tcp_list_presence_enc
((data_offset:uncomp_value - 5) * 32,
list_present:comp_value,
ack_number:uncomp_value);
rsf_flags ::= rsf_index_enc;
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ecn_used ::= irregular(1);
ttl_hopl ::= lsb(3, 3);
seq_number ::= lsb(14, 8191);
ack_number ::= lsb(16, 16383);
};
% +---+---+---+---+---+---+---+---+
% | 1 0 1 1 1 1 1 |ECN|
% +===+===+===+===+===+===+===+===+
% |lst| CRC |
% +---+---+---+---+---+---+---+---+
% | MSN |PSH| TTL |
% +---+---+---+---+---+---+---+---+
% | RSF | SN... |
% +---+---+---+---+---+---+---+---+
% | ..SN |
% +---+---+---+---+---+---+---+---+
% | |
% + ACK_SN +
% | |
% +---+---+---+---+---+---+---+---+
% / options_list / n*8 bits, if lst=1
% --- --- --- --- --- --- --- ---
% Send LSBs of sequence number
%
co_format_seq_1 = discriminator, %[ 4 ]
ip_id, %[ 4 ]
seq_number, %[ 16 ]
msn, %[ 4 ]
psh_flag, %[ 1 ]
header_crc, %[ 3 ]
{
let ((ip_id_behavior:uncomp_value ==
IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior:uncomp_value ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator ::= '1010';
msn ::= lsb(4, 4);
ip_id ::= ip_id_lsb (ip_id_behavior:uncomp_value, 4,
3);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
psh_flag ::= irregular (1);
seq_number ::= lsb(16, 32767);
};
% +---+---+---+---+---+---+---+---+
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% | 1 0 1 0 | IP-ID |
% +===+===+===+===+===+===+===+===+
% | |
% + SN +
% | |
% +---+---+---+---+---+---+---+---+
% | MSN |PSH| CRC |
% +---+---+---+---+---+---+---+---+
% Send scaled sequence number LSBs
%
co_format_seq_2 = discriminator, %[ 5 ]
ip_id, %[ 7 ]
seq_number_scaled, %[ 4 ]
msn, %[ 4 ]
psh_flag, %[ 1 ]
header_crc, %[ 3 ]
{
let ((ip_id_behavior:uncomp_value ==
IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior:uncomp_value ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator ::= '11001';
msn ::= lsb(4, 4);
ip_id ::= ip_id_lsb (ip_id_behavior:uncomp_value, 7,
3);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
psh_flag ::= irregular (1);
seq_number_scaled ::= lsb(4, 7);
seq_number_residue ::= static;
};
% +---+---+---+---+---+---+---+---+
% | 1 1 0 0 1 | IP-ID... |
% +===+===+===+===+===+===+===+===+
% | ...IP-ID | SN_SCALED |
% +---+---+---+---+---+---+---+---+
% | MSN |PSH| CRC |
% +---+---+---+---+---+---+---+---+
% Send acknowledgement number LSBs
%
co_format_seq_3 = discriminator, %[ 4 ]
ip_id, %[ 4 ]
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ack_number, %[ 16 ]
msn, %[ 4 ]
psh_flag, %[ 1 ]
header_crc, %[ 3 ]
{
let ((ip_id_behavior:uncomp_value ==
IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior:uncomp_value ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator ::= '1001';
msn ::= lsb(4, 4);
ip_id ::= ip_id_lsb (ip_id_behavior:uncomp_value, 4,
3);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
psh_flag ::= irregular (1);
ack_number ::= lsb(16, 16383);
};
% +---+---+---+---+---+---+---+---+
% | 1 0 0 1 | IP-ID |
% +===+===+===+===+===+===+===+===+
% | |
% + ACK_SN +
% | |
% +---+---+---+---+---+---+---+---+
% | MSN |PSH| CRC |
% +---+---+---+---+---+---+---+---+
% Send scaled acknowledgement number scaled
%
co_format_seq_4 = discriminator, %[ 1 ]
ack_number_scaled, %[ 4 ]
ip_id, %[ 3 ]
msn, %[ 4 ]
psh_flag, %[ 1 ]
header_crc, %[ 3 ]
{
let ((ip_id_behavior:uncomp_value ==
IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior:uncomp_value ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator ::= '0';
msn ::= lsb(4, 4);
%
% Note that due to having very few ip_id bits, no reordering
% offset
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%
ip_id ::= ip_id_lsb (ip_id_behavior:uncomp_value, 3,
1);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
psh_flag ::= irregular (1);
ack_number_scaled ::= lsb(4, 3);
ack_number_residue ::= static;
};
% +---+---+---+---+---+---+---+---+
% | 0 | ACK_SN_SCALED | IP-ID |
% +===+===+===+===+===+===+===+===+
% | MSN |PSH| CRC |
% +---+---+---+---+---+---+---+---+
% Send ACK and sequence number
%
co_format_seq_5 = discriminator, %[ 4 ]
ip_id, %[ 4 ]
ack_number, %[ 16 ]
seq_number, %[ 16 ]
msn, %[ 4 ]
psh_flag, %[ 1 ]
header_crc, %[ 3 ]
{
let ((ip_id_behavior:uncomp_value ==
IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior:uncomp_value ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator ::= '1000';
msn ::= lsb(4, 4);
ip_id ::= ip_id_lsb (ip_id_behavior:uncomp_value, 4,
3);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
psh_flag ::= irregular (1);
ack_number ::= lsb(16, 16383);
seq_number ::= lsb(16, 32767);
};
% +---+---+---+---+---+---+---+---+
% | 1 0 0 0 | IP-ID |
% +===+===+===+===+===+===+===+===+
% | |
% + ACK_SN +
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% | |
% +---+---+---+---+---+---+---+---+
% | |
% + SN +
% | |
% +---+---+---+---+---+---+---+---+
% | MSN |PSH| CRC |
% +---+---+---+---+---+---+---+---+
% Send both ACK and scaled sequence number LSBs
%
co_format_seq_6 = discriminator, %[ 6 ]
seq_number_scaled, %[ 4 ]
ip_id, %[ 6 ]
ack_number, %[ 16 ]
msn, %[ 4 ]
psh_flag, %[ 1 ]
header_crc, %[ 3 ]
{
let ((ip_id_behavior:uncomp_value ==
IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior:uncomp_value ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator ::= '110110';
seq_number_scaled ::= lsb(4, 7);
seq_number_residue ::= static;
ip_id ::= ip_id_lsb (ip_id_behavior:uncomp_value, 6,
3);
ack_number ::= lsb(16, 16383);
msn ::= lsb(4, 4);
psh_flag ::= irregular (1);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
};
% +---+---+---+---+---+---+---+---+
% | 1 1 0 1 0 0 |SN_SC..|
% +===+===+===+===+===+===+===+===+
% |..SN_SC| IP-ID |
% +---+---+---+---+---+---+---+---+
% | |
% + ACK_SN +
% | |
% +---+---+---+---+---+---+---+---+
% | MSN |PSH| CRC |
% +---+---+---+---+---+---+---+---+
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% Send sequence number and window
%
co_format_seq_7 = discriminator, %[ 5 ]
seq_number, %[ 14 ]
ip_id, %[ 5 ]
window, %[ 16 ]
msn, %[ 4 ]
psh_flag, %[ 1 ]
header_crc, %[ 3 ]
{
let ((ip_id_behavior:uncomp_value == IP_ID_BEHAVIOR_SEQUENTIAL)
||
(ip_id_behavior:uncomp_value ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator ::= '11010';
msn ::= lsb(4, 4);
ip_id ::= ip_id_lsb (ip_id_behavior:uncomp_value, 5,
3);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
psh_flag ::= irregular (1);
seq_number ::= lsb(14, 8191);
window ::= irregular(16);
};
% +---+---+---+---+---+---+---+---+
% | 1 1 0 1 0 | SN.. |
% +===+===+===+===+===+===+===+===+
% | ..SN.. |
% +---+---+---+---+---+---+---+---+
% | ..SN | IP-ID |
% +---+---+---+---+---+---+---+---+
% | |
% + WINDOW +
% | |
% +---+---+---+---+---+---+---+---+
% | MSN |PSH| CRC |
% +---+---+---+---+---+---+---+---+
% Send ACK and window
%
co_format_seq_8 = discriminator, %[ 5 ]
window, %[ 14 ]
ip_id, %[ 5 ]
ack_number, %[ 16 ]
msn, %[ 4 ]
psh_flag, %[ 1 ]
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header_crc, %[ 3 ]
{
let ((ip_id_behavior:uncomp_value ==
IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior:uncomp_value ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator ::= '11000';
msn ::= lsb(4, 4);
ip_id ::= ip_id_lsb (ip_id_behavior:uncomp_value, 5,
3);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
psh_flag ::= irregular (1);
ack_number ::= lsb(16, 32767);
window ::= lsb(14, 8191);
};
% +---+---+---+---+---+---+---+---+
% | 1 1 0 0 0 | WINDOW.. |
% +===+===+===+===+===+===+===+===+
% | ..WINDOW.. |
% +---+---+---+---+---+---+---+---+
% | ..WINDOW | IP-ID |
% +---+---+---+---+---+---+---+---+
% | |
% + ACK +
% | |
% +---+---+---+---+---+---+---+---+
% | MSN |PSH| CRC |
% +---+---+---+---+---+---+---+---+
% Send scaled sequence number and window.
%
co_format_seq_9 = discriminator, %[ 6 ]
ip_id, %[ 6 ]
seq_number_scaled, %[ 4 ]
window, %[ 16 ]
msn, %[ 4 ]
psh_flag, %[ 1 ]
header_crc, %[ 3 ]
{
let ((ip_id_behavior:uncomp_value == IP_ID_BEHAVIOR_SEQUENTIAL)
||
(ip_id_behavior:uncomp_value ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator ::= '110111';
msn ::= lsb(4, 4);
ip_id ::= ip_id_lsb (ip_id_behavior:uncomp_value, 6,
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3);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
psh_flag ::= irregular (1);
window ::= irregular(16);
seq_number_scaled ::= lsb(4, 7);
seq_number_residue ::= static;
};
% +---+---+---+---+---+---+---+---+
% | 1 1 0 1 1 1 |IP-ID..|
% +===+===+===+===+===+===+===+===+
% | ..IP-ID | SN_SCALED |
% +---+---+---+---+---+---+---+---+
% | |
% + WINDOW +
% | |
% +---+---+---+---+---+---+---+---+
% | MSN |PSH| CRC |
% +---+---+---+---+---+---+---+---+
% A packet halfway between co_common and compressed packets
% Can send LSBs of TTL, RSF flags, change ECN behavior and
% options list
%
co_format_seq_10 = discriminator, %[ 4 ]
ip_id, %[ 4 ]
list_present, %[ 1 ]
header_crc, %[ 7 ]
msn, %[ 4 ]
psh_flag, %[ 1 ]
ttl_hopl, %[ 3 ]
ecn_used, %[ 1 ]
ack_number, %[ 15 ]
rsf_flags, %[ 2 ]
seq_number, %[ 14 ]
options_list, % Nx8 bits
{
let ((ip_id_behavior:uncomp_value ==
IP_ID_BEHAVIOR_SEQUENTIAL) ||
(ip_id_behavior:uncomp_value ==
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
discriminator ::= '1011';
msn ::= lsb(4, 4);
ip_id ::= ip_id_lsb (ip_id_behavior:uncomp_value, 4,
3);
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header_crc ::= crc7 (this:uncomp_value,
this:uncomp_length);
psh_flag ::= irregular (1);
list_present ::= irregular(1);
options_list ::= tcp_list_presence_enc
((data_offset:uncomp_value - 5) * 32,
list_present:comp_value,
ack_number:uncomp_value);
rsf_flags ::= rsf_index_enc;
ecn_used ::= irregular(1);
ttl_hopl ::= lsb(3, 3);
seq_number ::= lsb(14, 8191);
ack_number ::= lsb(15, 8191);
};
% +---+---+---+---+---+---+---+---+
% | 1 0 1 1 | IP-ID |
% +===+===+===+===+===+===+===+===+
% |lst| CRC |
% +---+---+---+---+---+---+---+---+
% | MSN |PSH| TTL |
% +---+---+---+---+---+---+---+---+
% |ECN| ACK_SN... |
% +---+---+---+---+---+---+---+---+
% | ..ACK_SN |
% +---+---+---+---+---+---+---+---+
% | RSF | SN.. |
% +---+---+---+---+---+---+---+---+
% | ..SN |
% +---+---+---+---+---+---+---+---+
% / options_list / n*8 bits, if lst=1
% --- --- --- --- --- --- --- ---
};
8.3 Feedback Formats and Options
8.3.1 Feedback Formats
This section describes the feedback format for ROHC-TCP. ROHC-TCP
uses the ROHC feedback format described in section 5.2.2 of
[RFC3095].
All feedback formats carry a field labelled SN. The SN field
contains LSBs of the Master Sequence Number (MSN) described in
Section 6.1.1. The sequence number to use is the MSN corresponding
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to the header that caused the feedback information to be sent. If
that MSN cannot be determined, for example when decompression fails,
the MSN to use is that corresponding to the latest successfully
decompressed header.
FEEDBACK-1
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| MSN |
+---+---+---+---+---+---+---+---+
MSN: The lsb-encoded master sequence number.
A FEEDBACK-1 is an ACK. In order to send a NACK or a STATIC-NACK,
FEEDBACK-2 must be used.
FEEDBACK-2
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
|Acktype| MSN |
+---+---+---+---+---+---+---+---+
| MSN |
+---+---+---+---+---+---+---+---+
/ Feedback options /
+---+---+---+---+---+---+---+---+
Acktype:
0 = ACK
1 = NACK
2 = STATIC-NACK
3 is reserved (MUST NOT be used for parsability)
MSN: The lsb-encoded master sequence number.
Feedback options: A variable number of feedback options, see
Section 8.3.2. Options may appear in any order.
8.3.2 Feedback Options
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A ROHC-TCP Feedback option has variable length and the following
general format:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Opt Type | Opt Len |
+---+---+---+---+---+---+---+---+
/ option data / Opt Len octets
+---+---+---+---+---+---+---+---+
Each ROHC-TCP feedback option can appear at most once within a
FEEDBACK-2.
8.3.2.1 The CRC option
The CRC option contains an 8-bit CRC computed over the entire
feedback payload, without the packet type and code octet, but
including any CID fields, using the polynomial of section 5.9.1 of
[RFC3095]. If the CID is given with an Add-CID octet, the Add-CID
octet immediately precedes the FEEDBACK-1 or FEEDBACK-2 format. For
purposes of computing the CRC, the CRC fields of all CRC options are
zero.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Opt Type = 1 | Opt Len = 1 |
+---+---+---+---+---+---+---+---+
| CRC |
+---+---+---+---+---+---+---+---+
When receiving feedback information with a CRC option, the compressor
MUST verify the information by computing the CRC and comparing the
result with the CRC carried in the CRC option. If the two are not
identical, the feedback information MUST be ignored.
8.3.2.2 The REJECT option
The REJECT option informs the compressor that the decompressor does
not have sufficient resources to handle the flow.
+---+---+---+---+---+---+---+---+
| Opt Type = 2 | Opt Len = 0 |
+---+---+---+---+---+---+---+---+
When receiving a REJECT option, the compressor stops compressing the
packet stream, and should refrain from attempting to increase the
number of compressed packet streams for some time. Any FEEDBACK
packet carrying a REJECT option MUST also carry a CRC option.
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8.3.2.3 The MSN-NOT-VALID option
The MSN-NOT-VALID option indicates that the MSN of the feedback is
not valid. A compressor MUST NOT use the MSN of the feedback to find
the corresponding sent header when this option is present.
+---+---+---+---+---+---+---+---+
| Opt Type = 3 | Opt Len = 0 |
+---+---+---+---+---+---+---+---+
8.3.2.4 The MSN option
The MSN option provides 2 additional bits of MSN.
+---+---+---+---+---+---+---+---+
| Opt Type = 4 | Opt Len = 1 |
+---+---+---+---+---+---+---+---+
| MSN | Reserved |
+---+---+---+---+---+---+---+---+
8.3.2.5 The LOSS option
The LOSS option allows the decompressor to report the largest
observed number of packets lost in sequence.
+---+---+---+---+---+---+---+---+
| Opt Type = 7 | Opt Len = 1 |
+---+---+---+---+---+---+---+---+
| longest loss event (packets) |
+---+---+---+---+---+---+---+---+
The decompressor MAY choose to ignore the oldest loss events. Thus,
the value reported may decrease. Since setting the reference window
too small can reduce robustness, a FEEDBACK packet carrying a LOSS
option SHOULD also carry a CRC option. The compressor MAY choose to
ignore decreasing loss values.
8.3.2.6 The CONTEXT_MEMORY Feedback Option
The CONTEXT_MEMORY option informs the compressor that the
decompressor does not have sufficient memory resources to handle the
context of the packet stream, as the stream is currently compressed.
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0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Opt Type = 9 | Opt Len = 0 |
+---+---+---+---+---+---+---+---+
When receiving a CONTEXT_MEMORY option, the compressor SHOULD take
actions to compress the packet stream in a way that requires less
decompressor memory resources, or stop compressing the packet stream.
8.3.2.7 Unknown option types
If an option type unknown to the compressor is encountered, it must
continue parsing the rest of the FEEDBACK packet, which is possible
since the length of the option is explicit, but MUST otherwise ignore
the unknown option.
9. Security Consideration
Because encryption eliminates the redundancy that header compression
schemes try to exploit, there is some inducement to forego encryption
of headers in order to enable operation over low-bandwidth links.
However, for those cases where encryption of data (and not headers)
is sufficient, TCP does specify an alternative encryption method in
which only the TCP payload is encrypted and the headers are left in
the clear. That would still allow header compression to be applied.
A malfunctioning or malicious header compressor could cause the
header decompressor to reconstitute packets that do not match the
original packets but still have valid IP, and TCP headers and
possibly also valid TCP checksums. Such corruption may be detected
with end-to-end authentication and integrity mechanisms that will not
be affected by the compression. Moreover, this header compression
scheme uses an internal checksum for verification of reconstructed
headers. This reduces the probability of producing decompressed
headers not matching the original ones without this being noticed.
Denial-of-service attacks are possible if an intruder can introduce
(for example) bogus IR, CO or FEEDBACK packets onto the link and
thereby cause compression efficiency to be reduced. However, an
intruder having the ability to inject arbitrary packets at the link
layer in this manner raises additional security issues that dwarf
those related to the use of header compression.
10. IANA Considerations
The ROHC profile identifier 0x00XX <# Editor's Note: To be replaced
before publication #> has been reserved by the IANA for the profile
defined in this document.
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<# Editor's Note: To be removed before publication #>
A ROHC profile identifier must be reserved by the IANA for the
profile defined in this document. Profiles 0x0000-0x0005 have
previously been reserved, which means this profile could be 0x0006.
As for previous ROHC profiles, profile numbers 0xnnXX must also be
reserved for future updates of this profile. A suggested
registration in the "RObust Header Compression (ROHC) Profile
Identifiers" name space would then be:
Profile Usage Document
identifier
0x0006 ROHC TCP [RFCXXXX (this)]
0xnn06 Reserved
11. Acknowledgements
The authors would like to thank Qian Zhang, Hong Bin Liao, Richard
Price and Fredrik Lindstroem for their work with early versions of
this specification. Thanks also to Robert Finking and Carsten Borman
for valuable input.
12. References
12.1 Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le,
K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K.,
Wiebke, T., Yoshimura, T., and H. Zheng, "RObust Header
Compression (ROHC): Framework and four profiles: RTP, UDP,
ESP, and uncompressed", RFC 3095, July 2001.
[RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
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[ROHC-CR] Pelletier, G., "Robust Header Compression (ROHC): Context
Replication for ROHC profiles",
I-D draft-ietf-rohc-context-replication-06.txt,
October 2004.
[ROHC-FN] Finking, R. and G. Pelletier, "Formal Notation for Robust
Header Compression (ROHC-FN)",
I-D draft-ietf-rohc-formal-notation-09.txt, June 2005.
12.2 Informative References
[REQS] Jonsson, L-E., "Requirements on ROHC IP/TCP header
compression", I-D draft-ietf-rohc-tcp-requirements-08.txt,
September 2004.
[RFC1144] Jacobson, V., "Compressing TCP/IP Headers for Low-Speed
Serial Links", RFC 1144, February 1990.
[RFC1323] Jacobson, V., Braden, R., and D. Borman, "TCP Extensions
for High Performance", RFC 1323, May 1992.
[RFC2004] Perkins, C., "Minimal Encapsulation within IP", RFC 2004,
October 1996.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
[RFC2507] Degermark, M., Nordgren, B., and S. Pink, "IP Header
Compression", RFC 2507, February 1999.
[RFC2883] Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An
Extension to the Selective Acknowledgment (SACK) Option
for TCP", RFC 2883, July 2000.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
[RFC3843] Jonsson, L. and G. Pelletier, "RObust Header Compression
(ROHC): A compression profile for IP", RFC 3843,
June 2003.
[TCP-BEH] West, M. and S. McCann, "TCP/IP Field Behavior",
I-D draft-ietf-rohc-tcp-field-behavior-04.txt,
October 2004.
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Authors' Addresses
Ghyslain Pelletier
Ericsson
Box 920
Lulea SE-971 28
Sweden
Phone: +46 (0) 8 404 29 43
Email: ghyslain.pelletier@ericsson.com
Lars-Erik Jonsson
Ericsson
Box 920
Lulea SE-971 28
Sweden
Phone: +46 (0) 8 404 29 61
Email: lars-erik.jonsson@ericsson.com
Kristofer Sandlund
Ericsson
Box 920
Lulea SE-971 28
Sweden
Phone: +46 (0) 8 404 41 58
Email: kristofer.sandlund@ericsson.com
Mark A West
Siemens/Roke Manor
Roke Manor Research Ltd.
Romsey, Hampshire SO51 0ZN
UK
Phone: +44 1794 833311
Email: mark.a.west@roke.co.uk
URI: http://www.roke.co.uk
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The IETF invites any interested party to bring to its attention any
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this standard. Please address the information to the IETF at
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This document and the information contained herein are provided on an
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ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
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