Network Working Group Ghyslain Pelletier, Editor, Ericsson AB
INTERNET-DRAFT Lars-Erik Jonsson, Ericsson AB
Expires: April 2005 Mark A West, Siemens/Roke Manor
Richard Price, Siemens/Roke Manor
Kristofer Sandlund, Effnet
October 25, 2004
RObust Header Compression (ROHC):
A Profile for TCP/IP (ROHC-TCP)
<draft-ietf-rohc-tcp-08.txt>
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Abstract
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.
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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 (RFC 1144 [14], RFC 2507 [21]) have not addressed how to
compress TCP options such as SACK (Selective Acknowledgements) (RFC
2018 [20], RFC 2883 [22]) and Timestamps (RFC 1323 [15]).
Table of Contents
1. Introduction.....................................................4
2. Terminology......................................................4
3. Background.......................................................5
3.1. Existing TCP/IP Header Compression Schemes..................5
3.2. Classification of TCP/IP Header Fields......................6
3.3. Characteristics of Short-lived TCP Transfers................8
4. Overview of the TCP/IP Profile...................................9
4.1. General Concepts............................................9
4.2. Context Replication.........................................9
4.3. State Machines and Profile Operation........................9
4.4. Packet Formats and Encoding Methods.........................9
4.5. Irregular Chain............................................10
4.6. TCP Options................................................10
4.6.1. Compressing TCP Options with List Compression.........10
4.6.1.1. List Compression.................................10
4.6.1.2. Table-based Item Compression.....................11
4.6.1.3. Item Tables......................................12
4.6.1.4. Constraints to List Compression..................13
4.6.2. Item Table Mappings...................................13
4.6.3. Replication of TCP Options............................14
4.6.4. Compressing Extension Headers.........................14
4.6.5. Explicit Congestion Notification (ECN) in TCP Headers.14
5. Compressor and decompressor State Machines......................15
5.1. Compressor States and Logic................................15
5.1.1. Initialization and Refresh (IR) State.................15
5.1.2. Compression (CO) State................................16
5.1.3. Feedback Logic........................................16
5.1.4. State Transition Logic................................16
5.1.4.1. Optimistic Approach, Upward Transition...........16
5.1.4.2. Optional Acknowledgements (ACKs), Upward Transition
..........................................................17
5.1.4.3. Timeouts, Downward Transition....................17
5.1.4.4. Negative ACKs (NACKs), Downward Transition.......17
5.1.4.5. Need for Updates, Downward Transition............17
5.1.5. State Machine Supporting Context Replication..........17
5.2. Decompressor States and Logic..............................18
5.2.1. No Context (NC) State.................................19
5.2.2. Static Context (SC) State.............................19
5.2.3. Full Context (FC) State...............................19
5.2.4. Allowing Decompression................................20
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5.2.5. Reconstruction and Verification.......................20
5.2.6. Actions upon CRC Failure..............................20
5.2.7. Feedback Logic........................................20
6. ROHC-TCP - TCP/IP Compression (Profile 0x0006)..................22
6.1. Profile-specific Encoding Methods..........................22
6.1.1. inferred_mine_header_checksum().......................22
6.1.2. inferred_ip_v4_header_checksum()......................22
6.1.3. inferred_ip_v4_length()...............................23
6.1.4. inferred_ip_v6_length()...............................23
6.1.5. inferred_offset().....................................24
6.1.6. tcpopt_eol_padding_length.............................24
6.2. Considerations for the Feedback Channel....................24
6.3. Master Sequence Number (MSN)...............................25
6.4. CRC Calculations...........................................26
6.5. Initialization.............................................26
6.6. Packet Types...............................................27
6.6.1. Initialization and Refresh Packets (IR)...............27
6.6.2. Context Replication Packets (IR-CR)...................29
6.6.3. Compressed Packets (CO)...............................30
6.7. Packet Formats.............................................30
6.7.1. General Structures....................................31
6.7.2. Extension Headers.....................................34
6.7.2.1. IPv6 DEST opt header.............................34
6.7.2.2. IPv6 HOP opt header..............................34
6.7.2.3. IPv6 Routing Header..............................35
6.7.2.4. GRE Header.......................................36
6.7.2.5. MINE header......................................39
6.7.2.6. Authentication Header (AH) header................40
6.7.2.7. Encapsulation Security Payload (ESP) header......41
6.7.3. IP Header.............................................43
6.7.3.1. Structures Common for IPv4 and IPv6..............43
6.7.3.2. IPv6 Header......................................43
6.7.3.3. IPv4 Header......................................45
6.7.4. TCP Header............................................49
6.7.5. TCP Options...........................................55
6.7.6. Structures used in Compressed Base Headers............63
6.7.7. Compressed Base Headers...............................64
6.8. Feedback Formats and Options...............................77
6.8.1. Feedback Formats......................................77
6.8.2. Feedback Options......................................78
6.8.3. The CONTEXT_MEMORY Feedback Option....................79
7. Security Consideration..........................................79
8. IANA Considerations.............................................79
9. Acknowledgments.................................................80
10. Authors' Addresses.............................................80
11. References.....................................................81
11.1. Normative references......................................81
11.2. Informative References....................................82
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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 [21]. Additional considerations that makes
robustness an important objective for a TCP compression scheme are
introduced in [10]. Finally, existing TCP/IP header compression
schemes (RFC 1144 [14], RFC 2507 [21]) 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
[20], RFC 2883 [22]) and Timestamps (RFC 1323 [15]).
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 [2], compliant with the
requirements on ROHC TCP/IP header compression [10].
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 [1].
This document reuses some of the terminology found in RFC 3095 [2].
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
The Base Context Identifier is the CID used to identify the Base
Context, where information needed for context replication can
be extracted from.
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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 [3].
ROHC Formal Notation (ROHC-FN)
"ROHC-FN" in this document normatively refers to the formal
notation for ROHC profiles defined in [4], including the library of
encoding methods it specifies.
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 may be
found in [2]. 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.
The behavior analysis of TCP/IP header fields among multiple short-
lived connections may be found in [11].
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.
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The CTCP (RFC 1144 [14]) 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
[21]) 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
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 [11]. The main conclusion is that most of the header fields can
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easily be compressed away since they never or seldom change. The
following fields do however require more sophisticated mechanisms:
- IPv4 Identification (16 bits) - IP-ID
- TCP Sequence Number (32 bits) - SN
- TCP Acknowledgement Number (32 bits) - ACKN
- TCP Reserved (4 bits)
- TCP ECN flags (2 bits) - ECN
- TCP Window (16 bits) - WINDOW
- TCP Options
- Maximum Segment Size (4 octets) - MSS
- Window Scale (3 octets) - WSopt
- SACK Permitted (2 octets)
- TCP SACK - SACK
- TCP Timestamp (32 bits) - TS
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 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 [2], the IP-ID is
generally inferred from the RTP Sequence Number. However, with
respect to TCP compression, the analysis in [11] reveals that there
is no obvious candidate to this purpose among the TCP fields.
The change pattern of several TCP fields (Sequence Number,
Acknowledgement Number, Window, etc.) is very hard to predict and
differs entirely from the behavior of RTP fields discussed in [2]. Of
particular importance to a TCP/IP header compression scheme is the
understanding of the sequence and acknowledgement number [11].
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 acknowledgement
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 acknowledgement number remains constant for most
packets; on the backward path (from client to server), only the
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sequence number is changing and the acknowledgement 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 [2].
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 (this
is more the case with HTTP/1.0 than HTTP/1.1) is one example.
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.
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 [11]. The
main conclusion is that most part of the IP sub-context, some TCP
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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 [2]. Like for other ROHC profiles, ROHC-TCP
makes use of the ROHC protocol as described in [2], 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. Context Replication
For ROHC-TCP, context replication may be particularly useful for
short-lived TCP flows [10]. ROHC-TCP therefore supports context
replication as defined in ROHC-CR [3]; the compressor MAY support
context replication, while a decompressor implementation is REQUIRED
to support decompression of the IR-CR packet type.
4.3. State Machines and Profile Operation
Header compression with ROHC can be characterized as an interaction
between two state machines, one compressor machine and one
decompressor machine, each instantiated once per context.
For ROHC-TCP compression, the compressor has two states and the
decompressor has three states. The two compressor states are the
Initialization and Refresh (IR) state, and the Compression (CO)
state. The three states of the decompressor are No Context (NC),
Static Context (SC) and Full Context (FC). The compressor may also
implement a third state, the Context Replication (CR) state, to
support context replication ROHC-CR [3]. Transitions need not be
synchronized between the two state machines.
4.4. Packet Formats and Encoding Methods
The packet formats used for ROHC-TCP and found in this document 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 [4].
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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 [2].
This irregular chain handles fields of tunneling headers and of
extension headers, for which the change pattern is classified as
IRREGULAR and that have to be sent in each compressed packet.
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 6.6.3).
Note that the TCP header and the innermost IP header are not a part
of the irregular chain. This is because the irregular fields of these
headers are included in the base header of the compressed packet.
4.6. TCP Options
4.6.1. Compressing TCP Options with List Compression
The options in the TCP header are compressed using list compression
as defined by the ROHC-FN [4]. The following subsections explain how
this encoding is applied to the TCP options in more details.
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
an format_[option_type]_irregular format, where [option_type] is the
name of the actual field item in the option list.
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. These irregular fields are
present in each compressed packet, as part of the irregular chain.
4.6.1.1. List Compression
The TCP options in the uncompressed packet can be structured 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 |
+--------+--------+--...--+--------+
The basic principles of list-based compression are the following:
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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.
2) While 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; the irregular content is sent as part of the
irregular chain in the general compressed packet format
(section 6.6.3).
4) When the structure of the list changes, a compressed list is
sent, including a representation of its structure and order.
4.6.1.2. Table-based Item Compression
The Table-based item compression compressses individual items sent in
compressed lists. The compressor assigns a unique identifier "Index"
to each "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 by
receiving an acknowledgment from the decompressor or by sending L
(Index, Item) pairs (not necessarily consecutively). The value for
L is maintained by the compressor. After such confidence is
obtained, the Index alone is sent in compressed lists to indicate
the corresponding Item.
The compressor may reassign an existing Index to a new item, and
then needs to re-establish the mapping as described above.
Decompressor Logic
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 sucessfully verified usng the CRC. The
decompressor retrieves the item from the table whenever an Index
without an accompanying Item is received.
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4.6.1.3. Item Tables
Compressor Logic
The compressor uses the following structure to represent an entry
in the Item Table:
+-------+------+---------+--------------------------+
Index i | Known | Item | Counter | MSN_1, MSN_2, ..., MSN_L |
+-------+------+---------+--------------------------+
The flag "Known" indicates whether the mapping between Index i and
Item has been established, i.e., if Index i can be sent in
compressed lists without its corresponding Item.
The "Counter" field is useful to obtain confidence that the
context at the decompressor contains the (Index, Item) pair.
The list of sequence numbers, [MSN 1, ..., MSN L], is useful in
relating an acknowledgment received from the decompressor with the
(Index, Item) pair, meaning that it is now part of the
decompressor context.
The flag "Known" is initially set to a value of zero. It is also
set to zero whenever Index i is assigned to a new Item. "Known" is
set to a non-zero value when either of the following conditions
occur:
a) The corresponding (Index, Item) pair is acknowledged;
b) Counter >= L (confidence based of the optimistic approach).
When the compressor sets the flag "Known", the sequence number
list can be discarded.
Decompressor Logic
The decompressor uses the following structure to represent an
entry in the Item Table:
+-------+------+
Index i | Known | item |
+-------+------+
The flag "Known" is initially set to a value of zero. "Known" is
set to a non-zero value when the decompressor receives an (Index,
Item) pair and inserts the Item into the table at position Index.
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.
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4.6.1.4. Constraints to List Compression
List compression, as defined in the ROHC-FN [4], allows 7-bit indexes
to be used in the Item table. For ROHC-TCP, the compressor MUST use
the low-order 4 bits of the item count (i.e. large_xi of [4], section
5.5.5) to describe an index. In other words, the compressor MUST NOT
map items with indexes larger than a value of 15. This is because no
more than 16 different options are expected to be used in a TCP flow.
4.6.2. Item Table Mappings
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.
The compressor can thus omit from the compressed packet format an
option type that corresponds to a reserved item in the item table.
This is because the type of the option can be known based on the
index number.
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 occurence 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
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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 consistence, the compressor should still established an
entry in the list by setting the presence bit, like for the other
type of options.
The EOL option
The size of the compressed format for the EOL option can be of
more than one octets, 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 of
undefined format.
The Generic option
The generic option can be used to compress any type of TCP
options, in particular those that do not have a reserved index in
the item table.
4.6.3. Replication of TCP Options
TCP options can be replicated. When parts (or all) of the options are
replicated, the entire item table in the context is replicated. The
list of options for the new flow is then transmitted as a generic
compressed list, like for other compressed packets.
4.6.4. Compressing Extension Headers
In RFC 3095 [2], list compression is used to compress extension
headers. ROHC-TCP compresses the same type of extension headers.
However, these headers are treated exactly as other headers and thus
have a static chain, a dynamic chain as well as an irregular chain
(see also section 4.5 above).
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.
4.6.5. Explicit Congestion Notification (ECN) in TCP Headers
When the ECN is used in the TCP headers, the TOS/TC fields of all IP
headers in this flow must be sent uncompressed in all packets. This
is because of the possible use of the "full-functionality option" of
section 9.1 of RFC 3168 [23].
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[EditorÆs Note: Here it should be described how this is handled]
[ in packet formats. I.e. for ecn_used û some ]
[ packet types can setup the value of the context]
[ flag ecn_used. If it is set, the TOS/TC of all ]
[ IP headers are transmitted in the irregular ]
[ chain of all compressed packets. ]
5. Compressor and decompressor State Machines
The header compression state machines and their associated logic as
specified in this section are a simplified version of the ones found
in [2].
5.1. Compressor States and Logic
The two compressor states are the Initialization and Refresh (IR)
state, and the Compression (CO) state. The compressor always starts
in the lower compression state (IR). The compressor will normally
operate in the higher compression state (CO), under the constraint
that the compressor is sufficiently confident that the decompressor
has the information necessary to reconstruct a header compressed
according to this state.
The figure below shows the state machine for the compressor. The
details of each state, state transitions, and compression logic are
given in sub-sections following the figure.
Optimistic approach / ACK ACK
+------>------>------>------+ +->-+
| | | |
| v | v
+----------+ +----------+
| IR State | | CO State |
+----------+ +----------+
^ |
| Timeout / NACK / STATIC-NACK |
+-------<-------<-------<--------+
The transition from IR state to CO state is based on the following
principles: the need for update and the optimistic approach principle
or, if a feedback channel is established, feedback received from the
decompressor.
5.1.1. Initialization and Refresh (IR) State
The purpose of the IR state is to initialize the static parts of the
context at the decompressor or to recover after failure. In this
state, the compressor sends complete header information. This
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includes static and non-static fields in uncompressed form plus some
additional information.
The compressor stays in the IR state until it is fairly confident
that the decompressor has received the static information correctly.
5.1.2. Compression (CO) State
The purpose of the CO state is to efficiently communicate
irregularities in the packet stream when needed while maintaining the
most optimal compression ratio. When operating in this state, the
compressor normally sends most or all of the information in a
compressed form.
5.1.3. Feedback Logic
The compressor state machine makes use of feedback from decompressor
to compressor for transitions in the backward direction, and
optionally to improve the forward transition.
The reception of either positive feedback (ACKs) or negative feedback
(NACKs) establishes the feedback channel from the decompressor. Once
there is an established feedback channel, the compressor makes use of
this feedback for optionally improving the transitions among
different states. 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.
5.1.4. State Transition Logic
The compressor makes its decisions about when to transit between the
IR and the CO states on the basis of:
- variations in the packet headers
- positive feedback from decompressor (Acknowledgements -- ACKs)
- negative feedback from decompressor (Negative ACKS -- NACKs)
- confidence level regarding error-free decompression of a packet
5.1.4.1. Optimistic Approach, Upward Transition
Transition to the CO state is carried out according to the optimistic
approach principle. This means that the compressor transits to the CO
state when it is fairly confident that the decompressor has received
enough information to correctly decompress packets sent according to
the higher compression state.
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In general, there are many approaches where the compressor can obtain
such information. A simple and general approach can be achieved by
sending uncompressed or partial full headers periodically.
5.1.4.2. Optional Acknowledgements (ACKs), Upward Transition
The compressor can also transit to the CO state based on feedback
received by the decompressor. 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 transition to the CO
state can be carried out immediately.
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.
5.1.4.3. Timeouts, Downward Transition
When the optimistic approach is used (i.e. until a feedback channel
is established), there will always be a possibility of failure since
the decompressor may not have received sufficient information for
correct decompression. Therefore, unless the decompressor has
established a feedback channel, the compressor MUST periodically
transit to the IR state.
5.1.4.4. Negative ACKs (NACKs), Downward Transition
Negative acknowledgments (NACKs) are also called context requests.
Upon reception of a NACK, the compressor transits back to the IR
state and sends updates (such as IR-DYN or IR) to the decompressor.
5.1.4.5. Need for Updates, Downward Transition
When the header to be compressed does not conform to the established
pattern or when the compressor is not confident whether the
decompressor has the synchronized context, the compressor will
transit to the IR state.
5.1.5. State Machine Supporting Context Replication
For a profile supporting context replication, the additional
compressor logic (including corresponding state transition and
feedback logic) defined by ROHC-CR [3] must be added to the
compressor state machine described above.
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The following figure shows the resulting state machine:
Optimistic approach / ACK
+--->------>------>------>------>------>------>---+
| |
| BCID Selection Optimistic approach / ACK | ACK
| +------>----->------+ +----->----->----->-----+ | +->-+
| | | | | | | |
| | v | v v | v
+---------+ +---------+ +---------+
| IR | | CR | | CO |
| State | | State | | State |
+---------+ +---------+ +---------+
^ ^ | |
| | NACK / STATIC-NACK | |
| +---<-----<-----<----+ |
| |
| Timeout / NACK / STATIC-NACK |
+-----<-------<-------<-------<-------<-------<----+
5.2. Decompressor States and Logic
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) |
+-----------------+ +---------------------+ +-------------------+
^ | ^ |
| k_2 out of n_2 failures | | k_1 out of n_1 failures |
+-----<------<------<-----+ +-----<------<------<-----+
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5.2.1. No Context (NC) State
Initially, while working in the NC state, the decompressor has not
yet successfully decompressed a packet.
Upon receiving an IR or an IR-DYN packet, the decompressor will
verify the correctness of this packet by validating its header using
the CRC check. 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.
5.2.2. Static Context (SC) State
In the SC state, the decompressor assumes static context damage when
the CRC check of k_2 out of the last n_2 decompressed packets have
failed. The decompressor moves to the NC state and discards all
packets until a packet (e.g. IR or IR-DYN packet) that successfully
passes the verification check is received. The decompressor may send
feedback (see section 5.2.7) when assuming static context damage.
Note that appropriate values for k and n, are related to the residual
error rate of the link. When the residual error rate is close to
zero, k = n = 1 may be appropriate.
5.2.3. Full Context (FC) State
In the FC state, the decompressor assumes context damage when the CRC
check of k_1 out of the last n_1 decompressed packets have failed,
(where k and n are related to the residual error rate of the link as
in section 5.2.2). The decompressor moves to the SC state and
discards all packets until a packet carrying a 7- or 8-bit CRC that
successfully passes the verification check is received. The
decompressor may send feedback (see section 5.2.7) when assuming
context damage.
Upon receiving an IR or an IR-DYN packet, the decompressor SHOULD
verify the correctness of its header using CRC validation. 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
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value, if it is not older than the current reference packet (based on
sequence numbers in the compressed packet or in the uncompressed
header).
5.2.4. Allowing Decompression
In the No Context state, only packets carrying sufficient information
on the static fields (i.e. IR packets) can be decompressed.
In the Static Context state, only packets carrying a 7- or 8-bit CRC
may be decompressed (i.e. IR, IR-DYN and some CO packets).
In the Full Context state, decompression may be attempted regardless
of the type of packet received.
If decompression may not be performed, the packet is discarded.
As per ROHC-CR [3], IR-CR packets may be decompressed in any state.
5.2.5. 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
check, these updates MUST NOT be performed.
5.2.6. Actions upon CRC Failure
When a CRC check 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 [3]. For other
packet types carrying a CRC, if feedback is used the logic specified
in section 5.2.7 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 verified using the CRC check.
5.2.7. 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) acknowledgements of
significant context updates.
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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 acknowledgements in case
when 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 the forward state transition.
Once the feedback channel is established, the decompressor is
REQUIRED to continue sending feedback for the lifetime of the packet
stream as follow:
In NC 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
check.
In SC 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 (see also section 5.2.2).
If any other packet type is received, the decompressor SHOULD
treat it as a CRC mismatch when deciding if feedback is to be
sent.
In FC state:
The decompressor SHOULD send a NACK when decompression of any
packet type fails and if context damage is assumed (see also
section 5.2.3).
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 compressor may optionally send an ACK.
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Finally, when the decompressor ACKs an IR packet, it MUST use the CRC
option (see [2], section 5.7.6.3) when sending this feedback. This is
necessary to ensure that a context does not erroneously become a
candidate for later use as a base context for replication [3].
6. ROHC-TCP - TCP/IP Compression (Profile 0x0006)
This section describes a ROHC profile for TCP/IP compression. The
profile identifier for ROHC-TCP is 0x0006.
6.1. 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 6.7.
6.1.1. inferred_mine_header_checksum()
This encoding method compresses the minimal encapsulation header
checksum. This checksum is defined in RFC 2004 [25] as follow:
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.
6.1.2. inferred_ip_v4_header_checksum()
This encoding method compresses the header checksum field of the IPv4
header. This checksum is defined in RFC 791 [5] 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.
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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.
6.1.3. 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 [5] as follows:
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.
6.1.4. inferred_ip_v6_length()
This encoding method compresses the payload length field in the IPv6
header. This length field is defined in RFC 2460 [9] as follow:
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.
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6.1.5. inferred_offset()
This encoding method compresses
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
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.
6.1.6. tcpopt_eol_padding_length
The tcpopt_eol_padding_length is used in compressed lists for
representing the TCP end-of-list option. Because the EOL option is
followed by padding, represented using a number of octets all set to
zero, the length of the padding must be transmitted to the
decompressor within the compressed form of the list item.
The compressor calculates the padding length from the data offset
field and the number of options octets remaining after the EOL option
is encountered.
The decompressor uses this value to reconstruct the EOL option
padding and the data offset field.
6.2. 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 is
used at least once by the decompressor, this channel will be present
during the entire compression operation. Otherwise, if the connection
is broken and the channel disappears, header compression should be
restarted.
To parallel RFC 3095 [2], 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
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from the decompressor. This effectively means that ROHC-TCP does not
explicitly define any operational modes.
6.3. Master Sequence Number (MSN)
Feedback packets of types ACK and NACK carry information about
sequence number or acknowledgement number from decompressor to
compressor. Unfortunately, there is no guarantee that sequence number
and acknowledgement number fields will be used by every IP protocol
stack. In addition, the combined size of the sequence number field
and the acknowledgement 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 packets 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
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.
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6.4. CRC Calculations
The 3-bit and 7-bit CRCs both cover the entire uncompressed header
chain. Note that there is no division between CRC-STATIC or CRC-
DYNAMIC fields in ROHC-TCP, as opposed to profiles defined in [2].
6.5. Initialization
The static context of ROHC TCP streams can be initialized in either
two ways:
1) By using an IR packet as in section 6.6.1, where the profile is
six (6) and the static chain ends with the static part of a TCP
packet.
2) By replicating an existing context using the mechanism defined by
ROHC-CR. This is done with the IR-CR packet defined in section 6.6.2,
where the profile number is six (6) and the static replication chain
ends with the static part of a TCP packet.
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6.6. Packet Types
ROHC-TCP defines two different packet types: the Initialization and
Refresh (IR) packet type, and the Compressed packet type (CO). Each
type corresponds to one of the possible states of the compressor.
Each packet type also defines a number of packet formats: 30 packet
formats are defined for compressed headers (CO), and two for
initialization and refresh (IR).
Finally, the profile-specific part of the IR-CR packet [3] is also
defined in this section.
6.6.1. Initialization and Refresh Packets (IR)
ROHC-TCP uses the basic structure of the ROHC IR and IR-DYN packets
as defined in [2] (section 5.2.3. and 5.2.4. respectively). The 8-bit
CRC is computed according to section 5.9.1 of [2].
o Packet type: IR
This packet type communicates the static part and the dynamic part
of the context.
For the ROHC-TCP IR packet, the value of the x bit must be set to
zero. 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 0 | IR type octet
+---+---+---+---+---+---+---+---+
: :
/ 0-2 octets of CID / 1-2 octets if for large CIDs
: :
+---+---+---+---+---+---+---+---+
| Profile | 1 octet
+---+---+---+---+---+---+---+---+
| CRC | 1 octet
+---+---+---+---+---+---+---+---+
| |
/ Profile Specific Part / variable length
| |
- - - - - - - - - - - - - - - -
| |
/ Payload / variable length
| |
- - - - - - - - - - - - - - - -
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CRC: 8-bit CRC, computed according to section 5.9.1 of [2].
Profile Specific_Part: The format of this field is defined using
the formal notation in section 6.7. It consists in the
static chain, the dynamic chain, the irregular chain and the
TCP options.
Payload: The payload of the corresponding original packet, if
any. The presence of a payload is inferred from the packet
length.
o Packet type: IR-DYN
This packet type communicates the dynamic part of the context.
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
+---+---+---+---+---+---+---+---+
| |
/ Profile_Specific_Part / variable length
| |
- - - - - - - - - - - - - - - -
| |
/ Payload / variable length
| |
- - - - - - - - - - - - - - - -
CRC: 8-bit CRC, computed according to section 5.9.1 of [2].
Profile_Specific_Part: The format of this field is defined using
the formal notation in section 6.7. It consists in the
dynamic chain, the irregular chain and the TCP options only.
Payload: The payload of the corresponding original packet, if
any. The presence of a payload is inferred from the packet
length.
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6.6.2. 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.
o 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 [3], section 3.4.2. With consideration
to the extensibility of the IR packet type defined in RFC 3095 [2],
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 one.
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 1 | 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
+---+---+---+---+---+---+---+---+
| | present if B = 1,
/ Base CID / 1 octet if for small CIDs, or
| | 1-2 octets if for large CIDs
+---+---+---+---+---+---+---+---+
| |
| Profile_Specific_Part / variable length
| |
- - - - - - - - - - - - - - - -
| |
/ Payload / variable length
| |
- - - - - - - - - - - - - - - -
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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 [3].
Profile Specific Part: Static and dynamic subheader information
used for replication. The format of this field is defined
using the formal notation in section 6.7.
Payload: The payload of the corresponding original packet, if
any. The presence of a payload is inferred from the packet
length.
6.6.3. 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 bits
+---+---+---+---+---+---+---+---+
: :
/ Header Chain Irregular Part / variable (see section 1.1.1)
: :
--- --- --- --- --- --- --- ---
: :
/ TCP Options Irregular Part / variable (see section 6.7.5)
: :
--- --- --- --- --- --- --- ---
6.7. 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 [4]. 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 [4] for an explanation of the formal
notation itself, and 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.
The following constants are defined to improve readability of the
packet formats in this section:
IPPROTO_TCP = 6
IPPROTO_IP = 255 % place-holder for IP header in chain
6.7.1. General Structures
static_or_irreg32(flag) ===
{
uncompressed_format = field; %[ 32 ]
format_irreg_enc = field, %[ 32 ]
{
let (flag == 1);
field ::= irregular(32);
};
format_static_enc = field, %[ 0 ]
{
let (flag == 0);
field ::= static;
};
};
static_or_irreg16(flag) ===
{
uncompressed_format = field; %[ 16 ]
format_irreg_enc = field, %[ 16 ]
{
let (flag == 1);
field ::= irregular(16);
};
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format_static_enc = field, %[ 0 ]
{
let (flag == 0);
field ::= static;
};
};
static_or_irreg8(flag) ===
{
uncompressed_format = field; %[ 8 ]
format_irreg_enc = field, %[ 8 ]
{
let (flag == 1);
field ::= irregular(8);
};
format_static_enc = field, %[ 0 ]
{
let (flag == 0);
field ::= static;
};
};
tlv_header ===
{
uncompressed_format = length, %[ 8 ]
option_value; % n bits
format_0 = length, %[ 8 ]
option_value, % n bits
{
length ::= irregular (8);
option_value ::= irregular (length:uncomp_value * 64 û
64);
};
};
optional32 (flag) ===
{
uncompressed_format = item; % 0 or 32 bits
format_present = item, %[ 32 ]
{
let (flag == 1);
item ::= irregular (32);
};
format_not_present = item, %[ 0 ]
{
let (flag == 0);
item ::= compressed_value (0, 0);
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};
};
lsb_7_or_31 ===
{
uncompressed_format = item; % 7 or 31 bits
format_lsb_7 = discriminator, %[ 1 ]
item, %[ 7 ]
{
discriminator ::= '0';
item ::= lsb (7, 8);
};
format_lsb_31 = discriminator, %[ 1 ]
item, %[ 31 ]
{
discriminator ::= '1';
item ::= lsb (31, 256);
};
};
opt_lsb_7_or_31 (flag) ===
{
uncompressed_format = item; % 32 bits
format_present = item, % 8 or 32 bits
{
let (flag == 1);
item ::= lsb_7_or_31;
};
format_not_present = item, %[ 0 ]
{
let (flag == 0);
item ::= compressed_value (0, 0);
};
};
crc3 (data_value, data_length) ===
{
uncompressed_format = ;
compressed_format = crc_value, %[ 3 ]
{
crc_value ::= crc(3, 0x06, 0x07, data_value,
data_length);
};
};
crc7 (data_value, data_length) ===
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{
uncompressed_format = ;
compressed_format = crc_value, %[ 7 ]
{
crc_value ::= crc(7, 0x79, 0x7f, data_value,
data_length);
};
};
6.7.2. Extension Headers
6.7.2.1. IPv6 DEST opt header
ip_dest_opt ===
{
uncompressed_format = next_header, %[ 8 ]
length_and_value; % 8 + n bits
default_methods =
{
next_header ::= static;
length_and_value ::= static;
};
format_dest_opt_static = next_header, %[ 8 ]
{
next_header ::= irregular(8);
};
format_dest_opt_dynamic = length_and_value, % 8 + n bits
{
length_and_value ::= tlv_header;
};
format_dest_opt_replicate_0 = discriminator, %[ 8 ]
{
discriminator ::= '00000000';
};
format_dest_opt_replicate_1 = discriminator, %[ 8 ]
length_and_value, % 8 + n bits
{
discriminator ::= '10000000';
length_and_value ::= tlv_header;
};
};
6.7.2.2. IPv6 HOP opt header
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ip_hop_opt ===
{
uncompressed_format = next_header, %[ 8 ]
length_and_value; % 8 + n bits
default_methods =
{
next_header ::= static;
length_and_value ::= static;
};
format_hop_opt_static = next_header, %[ 8 ]
{
next_header ::= irregular(8);
};
format_hop_opt_dynamic = length_and_value, % 8 + n bits
{
length_and_value ::= tlv_header;
};
format_hop_opt_replicate_0 = discriminator, %[ 8 ]
{
discriminator ::= '00000000';
};
format_hop_opt_replicate_1 = discriminator, %[ 8 ]
length_and_value, % 8 + n bits
{
discriminator ::= '10000000';
length_and_value ::= tlv_header;
};
};
6.7.2.3. IPv6 Routing Header
ip_rout_opt ===
{
uncompressed_format = next_header, %[ 8 ]
length_and_value; % 8 + n bits
default_methods =
{
next_header ::= static;
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length_and_value ::= static;
};
format_rout_opt_static = next_header, %[ 8 ]
length_and_value, % 8 + n bits
{
next_header ::= irregular(8);
length_and_value ::= tlv_header;
};
format_rout_opt_dynamic =
{
};
format_rout_opt_replicate_0 = discriminator, %[ 8 ]
{
discriminator ::= '00000000';
};
format_rout_opt_replicate_1 = discriminator, %[ 8 ]
length_and_value, % 8 + n bits
{
discriminator ::= '10000000';
length_and_value ::= tlv_header;
};
};
6.7.2.4. GRE Header
optional_checksum (flag_value) ===
{
uncompressed_format = value, % 0 or 16 bits
reserved1; % 0 or 16 bits
format_cs_present = value, %[ 16 ]
reserved1, %[ 0 ]
{
let (flag_value == 1);
value ::= irregular (16);
reserved1 ::= uncompressed_value (16, 0);
};
format_not_present = value, %[ 0 ]
reserved1, %[ 0 ]
{
let (flag_value == 0);
value ::= compressed_value (0, 0);
reserved1 ::= compressed_value (0, 0);
};
};
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gre_proto ===
{
uncompressed_format = protocol; %[ 16 ]
default_methods =
{
};
format_ether_v4 = discriminator, %[ 1 ]
{
discriminator ::= compressed_value (1, 0);
protocol ::= uncompressed_value (16, 0x0800);
};
format_ether_v6 = discriminator, %[ 1 ]
{
discriminator ::= compressed_value (1, 1);
protocol ::= uncompressed_value (16, 0x86DD);
};
};
gre ===
{
uncompressed_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 =
{
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);
};
format_gre_static = protocol, %[ 1 ]
c_flag, %[ 1 ]
r_flag, %[ 1 ]
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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);
sequence_number ::= static;
};
format_gre_dynamic = checksum_and_res,% 0 or 16 bits
sequence_number, % 0 or 32 bits
{
sequence_number ::= optional32 (s_flag);
};
format_gre_replicate_0 = discriminator, % 8 bits
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);
};
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);
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reserved ::= '0';
version ::= irregular (3);
key ::= optional32 (k_flag);
sequence_number ::= optional32 (s_flag);
};
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);
};
};
6.7.2.5. MINE header
mine ===
{
uncompressed_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;
res_bits ::= static;
checksum ::= inferred_mine_header_checksum;
orig_dest ::= static;
orig_src ::= static;
};
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);
};
format_mine_dynamic =
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{
};
format_mine_replicate_0 = discriminator, %[ 8 ]
checksum, %[ 0 ]
{
discriminator ::= '00000000';
};
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);
};
};
6.7.2.6. Authentication Header (AH) header
ah ===
{
uncompressed_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 û
32);
};
format_ah_static = next_header, %[ 8 ]
length, %[ 8 ]
spi, %[ 32 ]
{
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next_header ::= irregular(8);
length ::= irregular (8);
spi ::= irregular (32);
};
format_ah_dynamic = res_bits, %[ 16 ]
sequence_number, %[ 32 ]
auth_data, % n bits
{
res_bits ::= irregular (16);
sequence_number ::= irregular (32);
};
format_ah_replicate_0 = discriminator, %[ 8 ]
sequence_number, % 8 or 32 bits
auth_data, % n bits
{
discriminator ::= '00000000';
sequence_number ::= lsb_7_or_31;
};
format_ah_replicate_1 = discriminator, %[ 8 ]
length, %[ 8 ]
res_bits, %[ 16 ]
spi, %[ 32 ]
sequence_number, %[ 32 ]
auth_data, % n bits
{
discriminator ::= '10000000';
length ::= irregular (8);
res_bits ::= irregular (16);
spi ::= irregular (32);
sequence_number ::= irregular (32);
};
format_ah_irregular = sequence_number, % 8 or 32 bits
auth_data, % n bits
{
sequence_number ::= lsb_7_or_31;
};
};
6.7.2.7. Encapsulation Security Payload (ESP) header
esp_null ===
{
uncompressed_format = spi, %[ 32 ]
sequence_number, %[ 32 ]
next_header; %[ 8 ]
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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;
};
format_esp_static = next_header, %[ 8 ]
{
% 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)
nh_field ::= compressed_value(8, next_header);
next_header ::= irregular (8);
};
format_esp_dynamic = sequence_number, %[ 32 ]
{
sequence_number ::= irregular (32);
};
format_esp_replicate_0 = discriminator, %[ 8 ]
sequence_number, % 8 or 32 bits
{
discriminator ::= '00000000';
sequence_number ::= lsb_7_or_31;
};
format_esp_replicate_1 = discriminator, %[ 8 ]
spi, %[ 32 ]
sequence_number, %[ 32 ]
{
discriminator ::= '10000000';
spi ::= irregular (32);
sequence_number ::= irregular (32);
};
format_esp_irregular = sequence_number, % 8 or 32 bits
{
sequence_number ::= lsb_7_or_31;
};
};
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6.7.3. IP Header
6.7.3.1. Structures Common for IPv4 and IPv6
irreg_tos_tc(flag) ===
{
uncompressed_format = tos_tc; %[ 6 ]
format_tos_tc_present = tos_tc , %[ 6 ]
{
let(flag == 1);
tos_tc ::= irregular (6);
};
format_tos_tc_not_present = tos_tc , %[ 0 ]
{
let(flag == 0);
tos_tc ::= static;
};
};
ip_irreg_ecn(flag) ===
{
uncompressed_format = ip_ecn_flags; %[ 2 ]
format_tc_present = ip_ecn_flags, %[ 2 ]
{
let(flag == 1);
ip_ecn_flags ::= irregular (2);
};
format_tc_not_present = ip_ecn_flags, %[ 0 ]
{
let(flag == 0);
ip_ecn_flags ::= static;
};
};
6.7.3.2. IPv6 Header
fl_enc ===
{
uncompressed_format = flow_label;
format_fl_zero = discriminator,
flow_label,
reserved,
{
discriminator ::= '0';
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flow_label ::= uncompressed_value (20, 0);
reserved ::= '0000';
};
format_fl_non_zero = discriminator,
flow_label,
{
discriminator ::= '1';
flow_label ::= irregular (20);
};
};
ipv6 ===
{
uncompressed_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;
};
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);
};
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format_ipv6_dynamic = tos_tc, %[ 6 ]
ip_ecn_flags, %[ 2 ]
ttl_hopl, %[ 8 ]
{
tos_tc ::= irregular (6);
ip_ecn_flags ::= irregular (2);
ttl_hopl ::= irregular (8);
};
format_ipv6_replicate = tos_tc, %[ 6 ]
ip_ecn_flags, %[ 2 ]
{
tos_tc ::= irregular (6);
ip_ecn_flags ::= irregular (2);
};
format_ipv6_outer_irregular(ecn_used_flag) =
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 (ecn_used_flag);
ip_ecn_flags ::= ip_irreg_ecn (ecn_used_flag);
};
% Can be non-octet-aligned, but combined with the TCP irregular
% it will be made octet-aligned
format_ipv6_innermost_irregular(ecn_used_flag) =
ip_ecn_flags, % 0 or 2 bits
{
ip_ecn_flags ::= ip_irreg_ecn (ecn_used_flag);
};
};
6.7.3.3. IPv4 Header
ip_id_enc_dyn (behavior) ===
{
uncompressed_format = ip_id; %[ 16 ]
format_ip_id_seq = ip_id,
{
let ((behavior == 0) || (behavior == 1) || (behavior == 2));
% In dynamic chain, but random, seq, and seq-swapped are 16 bits
ip_id ::= irregular(16);
};
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format_ip_id_zero = ip_id,
{
let (behavior == 3);
% Zero IPID
ip_id ::= uncompressed_value (16, 0);
};
};
ip_id_enc_irreg (behavior) ===
{
uncompressed_format = ip_id; % 0 or 16
format_ip_id_seq = ip_id,
{
let (behavior == 0); % sequential
ip_id ::= static; % Nothing to send in irregular chain
};
format_ip_id_seq_swapped = ip_id,
{
let (behavior == 1); % sequential-swapped
ip_id ::= static; % Nothing to send in irregular chain
};
format_ip_id_rand = ip_id,
{
let (behavior == 2); % random
ip_id ::= irregular (16);
};
format_ip_id_zero = ip_id,
{
let (behavior == 3); % zero
ip_id ::= uncompressed_value (16, 0);
};
};
ip_id_behavior_enc ===
{
uncompressed_format = ip_id_behavior; %[ 2 ]
format_sequential = ip_id_behavior,
{
ip_id_behavior ::= '00';
};
format_sequential_swapped = ip_id_behavior,
{
ip_id_behavior ::= '01';
};
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format_random = ip_id_behavior,
{
ip_id_behavior ::= '10';
};
format_zero = ip_id_behavior,
{
ip_id_behavior ::= '11';
};
};
ipv4 ===
{
uncompressed_format = version, %[ 4 ]
hdr_length, %[ 4 ]
protocol, %[ 8 ]
tos_tc, %[ 6 ]
ip_ecn_flags,%[ 2 ]
ttl_hopl, %[ 8 ]
df, %[ 1 ]
mf, %[ 1 ]
rf, %[ 1 ]
frag_offset, %[ 13 ]
ip_id, %[ 16 ]
src_addr, %[ 32 ]
dst_addr, %[ 32 ]
checksum, %[ 16 ]
length; %[ 16 ]
control_fields = ip_id_behavior; % 2 bits
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;
};
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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);
};
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
% 00 = sequential
% 01 = sequential byteswapped
% 10 = random
% 11 = zero
%
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);
};
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);
tos_tc ::= irregular (6);
ip_ecn_flags ::= irregular (2);
};
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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);
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);
};
format_ipv4_outer_irregular(ecn_used_flag) = ip_id,
tos_tc,
ip_ecn_flags,
{
ip_id_behavior ::= static;
ip_id ::= ip_id_enc_irreg (ip_id_behavior);
tos_tc ::= irreg_tos_tc (ecn_used_flag);
ip_ecn_flags ::= ip_irreg_ecn (ecn_used_flag);
};
% Can be non-octet-aligned, but combined with the TCP irregular
% it will be made octet-aligned
format_ipv4_innermost_irregular(ecn_used_flag) =
ip_id, % 0 or 16 bits
ip_ecn_flags, % 0 or 2 bits
{
ip_id_behavior ::= static;
ip_id ::= ip_id_enc_irreg (ip_id_behavior);
ip_ecn_flags ::= ip_irreg_ecn (ecn_used_flag);
};
};
6.7.4. TCP Header
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port_replicate(flags) ===
{
uncompressed_format = port; %[ 16 ]
format_port_static_enc = port, %[ 0 ]
{
let(flags == 00);
port ::= static;
};
format_port_lsb8 = port, %[ 8 ]
{
let(flags == 01);
port ::= lsb (8, 64);
};
format_port_irr_enc = port, %[ 16 ]
{
let(flags == 10);
port ::= irregular (16);
};
};
urg_enc_dyn(flag) ===
{
uncompressed_format = urg_ptr;
format_urg_zero = urg_ptr,
{
let(flag == 0);
urg_ptr ::= irregular (16);
};
format_urg_non_zero = urg_ptr,
{
let(flag == 1);
urg_ptr ::= uncompressed_value (16, 0);
};
};
ack_enc_dyn(flag) ===
{
uncompressed_format = ack_number;
format_ack_zero = ack_number,
{
let(flag == 0);
ack_number ::= irregular (32);
};
format_ack_non_zero = ack_number,
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{
let(flag == 1);
ack_number ::= uncompressed_value (32, 0);
};
};
tcp_ecn_flags_enc(flag) ===
{
uncompressed_format = tcp_ecn_flags;
format_irreg = tcp_ecn_flags,
{
let(flag == 1);
tcp_ecn_flags ::= irregular(2);
};
format_unused =
{
let(flag == 0);
tcp_ecn_flags ::= static;
};
};
tcp_res_flags_enc(flag) ===
{
uncompressed_format = tcp_res_flags;
format_irreg = tcp_res_flags,
{
let(flag == 1);
tcp_res_flags ::= irregular(4);
};
format_unused =
{
let(flag == 0);
tcp_res_flags ::= uncompressed_value(4, 0);
};
};
rsf_index_enc ===
{
uncompressed_format = rsf_flag;
format_none = rsf_idx,
{
rsf_idx ::= '00';
rsf_flag ::= uncompressed_value (3, 0x00);
};
format_rst_only = rsf_idx,
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{
rsf_idx ::= '01';
rsf_flag ::= uncompressed_value (3, 0x01);
};
format_syn_only = rsf_idx,
{
rsf_idx ::= '10';
rsf_flag ::= uncompressed_value (3, 0x02);
};
format_fin_only = rsf_idx,
{
rsf_idx ::= '11';
rsf_flag ::= uncompressed_value (3, 0x04);
};
};
tcp ===
{
uncompressed_format = src_port, %[ 16 ]
dst_port, %[ 16 ]
rsf_flags, %[ 3 ]
psh_flag, %[ 1 ]
urg_flag, %[ 1 ]
ack_flag, %[ 1 ]
data_offset, %[ 4 ]
tcp_ecn_flags,%[ 2 ]
tcp_res_flags,%[ 4 ]
urg_ptr, %[ 16 ]
window, %[ 16 ]
checksum, %[ 16 ]
seq_number, %[ 32 ]
ack_number, %[ 32 ]
options; % n bits
control_fields = msn, % 16 bits
ecn_used; % 1 bit
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;
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checksum ::= irregular (16);
tcp_ecn_flags ::= static;
tcp_res_flags ::= static;
};
format_tcp_static = src_port, %[ 16 ]
dst_port, %[ 16 ]
{
next_header ::= uncompressed_value (8, 6);
src_port ::= irregular(16);
dst_port ::= irregular(16);
};
format_tcp_dynamic = ecn_used, %[ 1 ]
ack_flag, %[ 1 ]
urg_flag, %[ 1 ]
psh_flag, %[ 1 ]
ack_zero, %[ 1 ]
urp_zero, %[ 1 ]
rsf_flags, %[ 3 ]
tcp_ecn_flags, %[ 2 ]
tcp_res_flags, %[ 4 ]
padding, %[ 1 ]
msn, %[ 16 ]
seq_number, %[ 32 ]
ack_number, % 0 or 32 bits
window, %[ 16 ]
checksum, %[ 16 ]
urg_ptr, % 0 or 16 bits
options, % n bits
{
ecn_used ::= irregular (1);
ack_zero ::= irregular (1);
urp_zero ::= irregular (1);
ack_flag ::= irregular (1);
urg_flag ::= irregular (1);
psh_flag ::= irregular (1);
tcp_ecn_flags ::= irregular (2);
padding ::= '0';
rsf_flags ::= irregular (3);
tcp_res_flags ::= irregular (4);
msn ::= irregular (16);
seq_number ::= irregular (32);
window ::= irregular (16);
checksum ::= irregular (16);
urg_ptr ::= urg_enc_dyn(urp_zero);
ack_number ::= ack_enc_dyn(ack_zero);
options ::=
list_tcp_options(list_length);
};
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format_tcp_replicate = reserved, %[ 1 ]
list_present, %[ 1 ]
src_port_presence, %[ 2 ]
dst_port_presence, %[ 2 ]
ack_number, %[ 1 ]
urp_presence, %[ 1 ]
urg_flag, %[ 1 ]
ack_flag, %[ 1 ]
psh_flag, %[ 1 ]
rsf_flags, %[ 2 ]
tcp_ecn_flags, %[ 2 ]
ecn_used, %[ 1 ]
msn, %[ 16 ]
seq_number, %[ 32 ]
src_port, % 0, 8 or 16 bits
dst_port, % 0, 8 or 16 bits
urg_point, % 0 or 16 bits
ack_number, % 0 or 32 bits
tcp_ecn_flags, % 0 or 2 bits
tcp_res_flags, % 0 or 4 bits
options_list, % n bits
{
reserved ::= '0';
options_replicate ::= irregular (1);
msn ::= irregular (16);
urg_flag ::= irregular (1);
ack_flag ::= irregular (1);
psh_flag ::= irregular (1);
rsf_flags ::= rsf_index_enc;
ecn_used ::= irregular (1);
tcp_ecn_flags ::= irregular (2);
src_port ::=
port_replicate(src_port_presence);
dst_port ::=
port_replicate(dst_port_presence);
seq_number ::= irregular(32);
ack_number ::=
static_or_irreg32(ack_presence);
window ::=
static_or_irreg16(window_presence);
urg_point ::=
static_or_irreg16(urp_presence);
options_list ::= tcp_list_presence_enc
((data_offset:uncomp_value + 20) / 4,
list_present);
};
% Note that this structure can be non-octet-aligned, but it is known
% that is will always be used together with an innermost_irregular
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% structure that will make it octet-aligned
format_tcp_irregular = tcp_ecn_flags,
tcp_res_flags,
checksum, %[ 16 ]
{
tcp_ecn_flags ::= tcp_ecn_flags_enc (ecn_used_flag);
tcp_res_flags ::= tcp_res_flags_enc (ecn_used_flag);
checksum ::= irregular (16);
};
};
6.7.5. TCP Options
tcp_opt_mss ===
{
uncompressed_format = type, %[ 8 ]
length, %[ 8 ]
mss; %[ 16 ]
default_methods =
{
type ::= uncompressed_value (8, 2);
length ::= uncompressed_value (8, 4);
mss ::= static;
};
format_mss_list_item = mss, %[ 16 ]
{
mss ::= irregular (16);
};
format_mss_irregular =
{
};
};
tcp_opt_wscale ===
{
uncompressed_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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format_wscale_list_item = wscale, %[ 8 ]
{
wscale ::= irregular (8);
};
format_wscale_irregular =
{
};
};
ts_lsb ===
{
uncompressed_format = tsval;
format_tsval_15 = discriminator, %[ 1 ]
tsval, %[ 15 ]
{
discriminator ::= '0';
tsval ::= lsb (15, 128);
};
format_tsval_22 = discriminator, %[ 2 ]
tsval, %[ 22 ]
{
discriminator ::= '10';
tsval ::= lsb (22, 256);
};
format_tsval_30 = discriminator, %[ 2 ]
tsval, %[ 20 ]
{
discriminator ::= '11';
tsval ::= lsb (30, 512);
};
};
tcp_opt_tsopt ===
{
uncompressed_format = type, %[ 8 ]
length, %[ 8 ]
tsval, %[ 32 ]
tsecho; %[ 32 ]
default_methods =
{
type ::= uncompressed_value (8, 8);
length ::= uncompressed_value (8, 10);
};
format_tsopt_list_item = tsval, %[ 32 ]
tsecho, %[ 32 ]
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{
tsval ::= irregular (32);
tsecho ::= irregular (32);
};
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) ===
{
uncompressed_format = sack_field; %[ 32 ]
default_methods =
{
let (sack_offset:uncomp_value ==
sack_field:uncomp_value - base);
let (sack_offset:uncomp_length == 32);
};
format_lsb_15 = discriminator, %[ 1 ]
sack_offset, %[ 15 ]
{
discriminator ::= '0';
sack_offset ::= lsb (15, -1);
};
format_lsb_22 = discriminator, %[ 2 ]
sack_offset, %[ 22 ]
{
discriminator ::= '10';
sack_offset ::= lsb (22, -1);
};
format_lsb_30 = discriminator, %[ 2 ]
sack_offset, %[ 30 ]
{
discriminator ::= '11';
sack_offset ::= lsb (30, -1);
};
};
tcp_opt_sack_block (prev_block_end) ===
{
uncompressed_format = block_start, %[ 32 ]
block_end; %[ 32 ]
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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.
uncompressed_format = type, %[ 8 ]
length, %[ 8 ]
block_1, % n bits
block_2, % n bits
block_3, % n bits
block_4; % n bits
default_methods =
{
type ::= uncompressed_value (8, 5);
length ::= irregular (8);
block_1 ::= uncompressed_value (0, 0);
block_2 ::= uncompressed_value (0, 0);
block_3 ::= uncompressed_value (0, 0);
block_4 ::= uncompressed_value (0, 0);
};
format_sack1_list_item = length,
block_1,
{
length ::= uncompressed_value (8, 10);
block_1 ::= tcp_opt_sack_block (ack_value);
};
format_sack2_list_item = length,
block_1,
block_2,
{
length ::= uncompressed_value (8, 18);
block_1 ::= tcp_opt_sack_block (ack_value);
block_2 ::= tcp_opt_sack_block
(block_1_end:uncomp_value);
};
format_sack3_list_item = length,
block_1,
block_2,
block_3,
{
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length ::= uncompressed_value (8, 26);
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);
};
format_sack4_list_item = length,
block_1,
block_2,
block_3,
block_4,
{
length ::= uncompressed_value (8, 34);
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);
};
format_sack_irregular =
{
};
};
%
% 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 trasmitted with the
% compressed list since the length of the list can be unknown to the
% decompressor.
%
tcp_opt_eol ===
{
uncompressed_format = type, %[ 8 ]
padding; % (n * 8) bits
default_methods =
{
type ::= uncompressed_value (8, 0);
pad_len ::= static;
padding ::= static;
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};
format_eol_list_item = pad_len, % 8 bits
padding, %[ 0 ]
{
pad_len ::= tcpopt_eol_padding_length;
padding ::= uncompressed_value (pad_bits, 0);
};
format_eol_irregular =
{
};
};
tcp_opt_nop ===
{
uncompressed_format = type; %[ 8 ]
default_methods =
{
type ::= uncompressed_value (8, 1);
};
format_nop_list_item =
{
};
format_nop_irregular =
{
};
};
tcp_opt_sack_permitted ===
{
uncompressed_format = type, %[ 8 ]
length; %[ 8 ]
default_methods =
{
type ::= uncompressed_value (8, 1);
length ::= uncompressed_value (8, 2);
};
format_sack_permitted_list_item =
{
};
format_sack_permitted_irregular =
{
};
};
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tcp_opt_generic ===
{
uncompressed_format = type, %[ 8 ]
length_msb, %[ 1 ]
length_lsb, %[ 7 ]
contents; % n bits
default_methods =
{
type ::= static;
%
% lengths are always < 128
% (i.e. the msb is always 0)
%
length_msb ::= uncompressed_value (1, 0);
length_lsb ::= static;
contents ::= irregular (length_len:uncomp_value
* 8 - 16);
};
format_generic_list_item = type, %[ 8 ]
option_static, %[ 1 ]
length_lsb, %[ 7 ]
contents, % n bits
{
type ::= irregular (8);
option_static ::= '0';
length_lsb ::= irregular (7);
};
format_generic_replicate_0 = discriminator, %[ 8 ]
{
discriminator ::= '00000000';
contents ::= static;
};
format_generic_replicate_1 = discriminator, %[ 8 ]
type, %[ 8 ]
option_static, %[ 1 ]
length_lsb, %[ 7 ]
contents, % n bits
{
discriminator ::= '10000000';
type ::= irregular (8);
option_static ::= '0';
length_lsb ::= irregular (7);
};
format_generic_irregular_stable = discriminator, %[ 8 ]
contents, %[ 0 ]
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{
discriminator ::= '00000000';
contents ::= static;
};
format_generic_irregular_full = discriminator, %[ 1 ]
length_lsb, %[ 7 ]
contents, % n bits
{
discriminator ::= '1';
length_lsb ::= irregular (7);
contents ::= irregular (length_lsb:uncomp_value
* 8 - 16);
};
};
list_tcp_options(list_length_in_bytes) ===
{
% Length is not known a priori on decompressor, so we use a sentinel.
end_of_list_sentinel ::= uncompressed_value(8, 0);
end_of_list_padding ::= uncompressed_value(8, 1);
mss ::= tcp_opt_mss;
wscale ::= tcp_opt_wscale;
tsopt ::= tcp_opt_tsopt;
sack ::= tcp_opt_sack;
sack_permitted ::= tcp_opt_sack_permitted;
eol ::= tcp_opt_eol;
nop ::= tcp_opt_nop;
generic ::= tcp_opt_generic;
};
tcp_list_presence_enc(list_length, presence) ===
{
uncompressed_format = tcp_options;
format_list_not_present = tcp_options,
{
let (presence == 0);
tcp_options ::= static;
};
format_list_present = tcp_options,
{
let (presence == 1);
tcp_options ::= list_tcp_options(list_length);
};
};
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6.7.6. Structures used in Compressed Base Headers
tos_tc_enc(flag) ===
{
uncompressed_format = tos_tc; %[ 6 ]
format_static = tos_tc, %[ 0 ]
{
let (flag == 0);
tos_tc ::= static;
};
format_irreg = tos_tc, %[ 6 ]
padding,
{
let (flag == 1);
tos_tc ::= irregular(6);
padding ::= compressed_value (2, 0);
};
};
rsf_static_or_byte_enc(flag) ===
{
uncompressed_format = rsf_flags;
format_static = rsf_flags, %[ 0 ]
{
let (flag == 0);
rsf_flags ::= static;
};
format_irreg = rsf_flags, %[ 3 ]
{
let (flag == 1);
rsf_flags ::= irregular(3);
reserved ::= compressed_value (5, 0);
};
};
ip_id_lsb (behavior, msn, k, p) ===
{
uncompressed_format = ip_id;
default_methods =
{
let (ip_id:uncomp_length == 16);
};
format_nbo = ip_id_offset,
{
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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);
};
format_non_nbo = ip_id_offset,
{
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);
ip_id_offset ::= lsb (k, p);
};
};
dont_fragment(version) ===
{
uncompressed_format = df;
compressed_format_v4 = df,
{
let (version == 4);
df ::= irregular(1);
};
compressed_format_v6 = df,
{
let (version == 6);
df ::= compressed_value(1,0);
};
};
6.7.7. Compressed Base Headers
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Actual start of compressed packet formats
% Important note: The base header is the compressed representation of
% the innermost IP header AND the TCP header.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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co_baseheader ===
{
uncompressed_format_v4 = version,
header_length,
tos_tc,
ip_ecn_flags,
length,
ip_id,
df,
mf,
rf,
frag_offset,
ttl_hopl,
next_header,
checksum,
src_addr,
dest_addr,
src_port,
dest_port,
seq_number,
ack_number,
data_offset,
tcp_ecn_flags,
tcp_res_flags,
urg_flag,
ack_flag,
psh_flag,
rsf_flags,
tcp_checksum,
urg_ptr,
window,
tcp_options,
{
let (version:uncomp_value == 4);
};
uncompressed_format_v6 = version,
tos_tc,
ip_ecn_flags,
flow_label,
payload_length,
next_header,
ttl_hopl,
src_addr,
dest_addr,
src_port,
dest_port,
seq_number,
ack_number,
data_offset,
tcp_ecn_flags,
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tcp_res_flags,
urg_flag,
ack_flag,
psh_flag,
rsf_flags,
tcp_checksum,
urg_ptr,
window,
{
let (version:uncomp_value == 6);
};
control_fields = msn, % 16 bits
ecn_used, % 1 bit
ip_id_behavior; % 2 bits
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;
header_length ::= uncompressed_value (4,5);
length ::= inferred_ip_v4_length;
ip_id ::= irregular(16);
rf ::= static;
df ::= static;
mf ::= static;
frag_offset ::= static;
checksum ::= inferred_ip_checksum;
src_port ::= static;
dest_port ::= static;
seq_number ::= static;
ack_number ::= static;
data_offset ::= inferred_offset;
tcp_ecn ::= static;
psh_flag ::= irregular (1);
urg_flag ::= uncompressed_value (1, 0);
ack_flag ::= uncompressed_value (1, 1);
window ::= static;
tcp_checksum ::= irregular(16);
urg_ptr ::= static;
seq_number_scaled ::= static;
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payload_size ::= static;
rsf_flags ::= uncompressed_value (3, 0);
let (version:uncomp_length == 4);
let (seq_number_scaled:uncomp_length == 32);
let (seq_number_scaled:uncomp_value ==
seq_number:uncomp_value /
payload_size:uncomp_value);
let (seq_number_residue:uncomp_length == 32);
let (seq_number_residue:uncomp_value ==
mod(seq_number:uncomp_value,
payload_size:uncomp_value));
};
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Common compressed packet format
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
format_co_common = discriminator, %[ 4 ]
msn, %[ 4 ]
padding1, %[ 1 ]
header_crc, %[ 7 ]
urg_flag, %[ 1 ]
ack_flag, %[ 1 ]
psh_flag, %[ 1 ]
df, %[ 1 ]
ecn_used, %[ 1 ]
ip_id_present, %[ 1 ]
ip_id_behavior, %[ 2 ]
seq_present, %[ 1 ]
ack_present, %[ 1 ]
window_present, %[ 1 ]
urg_ptr_present, %[ 1 ]
tos_tc_present, %[ 1 ]
ttl_hopl_present, %[ 1 ]
rsf_flags_present, %[ 1 ]
ecn_flags_present, %[ 1 ]
seq_number, % 0 or 32 bits
ack_number, % 0 or 32 bits
ip_id, % 0 or 16 bits
window, % 0 or 16 bits
urg_ptr, % 0 or 16 bits
ip_ecn_flags, % 0 or 2 bits
tcp_ecn_flags, % 0 or 2 bits
tcp_res_flags, % 0 or 4 bits
ttl_hopl, % 0 or 8 bits
tos_tc, % 0 or 8 bits
rsf_flags, % 0 or 8 bits
ecn_flags, % 0 or 8 bits
{
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discriminator ::= '1110';
padding1 ::= compressed_value (1, 0);
msn ::= lsb (3, -1);
psh_flag ::= irregular(1);
header_crc ::= crc7(this:uncomp_value,
this:uncomp_length);
ack_flag ::= irregular(1);
ip_id_behavior ::= ip_id_behavior_enc;
df ::= dont_fragment(version:uncomp_value);
ecn_used ::= irregular(1);
urg_flag ::= irregular(1);
ip_id_present ::= irregular(1);
seq_present ::= irregular(1);
window_present ::= irregular(1);
ack_present ::= irregular(1);
urg_ptr_present ::= irregular(1);
tos_tc_present ::= irregular(1);
ttl_hopl_present ::= irregular(1);
rsf_flags_present ::= irregular(1);
ecn_flags_present ::= irregular(1);
seq_number ::= static_or_irreg32(seq_present);
window ::= static_or_irreg16(window_present);
ack_number ::= static_or_irreg32(ack_present);
ip_id ::= static_or_irreg16(ip_id_present);
urg_ptr ::= static_or_irreg16(urg_present);
ttl_hops ::= static_or_irreg8(ttl_hopl_present);
ip_ecn_flags ::= ip_irreg_ecn(ecn_used_flag);
tcp_ecn_flags ::= tcp_ecn_flags_enc(ecn_used_flag);
tcp_res_flags ::= tcp_res_flags_enc(ecn_used_flag);
tos_tc ::= tos_tc_enc(tos_tc_present);
rsf_flags ::= rsf_static_or_byte_enc(rsf_present);
};
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% IP-ID Sequential Packet CO packet format base headers
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% XXX: Note that the following discriminators are now free for use
% dumpeding sequential packet formats:
% 11000101, 11000100, 110001101, 11000110000, 1100011001, 11000110001
format_seq_0 = discriminator, %[ 1 ]
header_crc, %[ 3 ]
psh_flag, %[ 1 ]
msn, %[ 3 ]
ip_id, %[ 4 ]
seq_number, %[ 12 ]
{
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let ((ip_id_behavior:uncomp_value == 0) ||
(ip_id_behavior:uncomp_value == 1));
discriminator ::= '0';
msn ::= lsb (3, -1);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
ip_id ::= ip_id_lsb (ip_id_behavior, msn, 4, -1);
seq_number ::= lsb (12, 1023);
psh_flag ::= irregular (1);
};
format_seq_1 = discriminator, %[ 3 ]
header_crc, %[ 3 ]
rsf_flags, %[ 2 ]
ip_id, %[ 8 ]
psh_flag, %[ 1 ]
msn, %[ 3 ]
seq_number, %[ 12 ]
{
let ((ip_id_behavior:uncomp_value == 0) ||
(ip_id_behavior:uncomp_value == 1));
discriminator ::= '100';
msn ::= lsb (3, -1);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
ip_id ::= ip_id_lsb (ip_id_behavior, msn, 8, -1);
seq_number ::= lsb (12, 1023);
psh_flag ::= irregular (1);
rsf_flags ::= rsf_index_enc;
};
format_seq_2 = discriminator, %[ 3 ]
ip_id, %[ 4 ]
psh_flag, %[ 1 ]
ack_number, %[ 16 ]
msn, %[ 3 ]
header_crc, %[ 3 ]
seq_number_scaled, %[ 10 ]
{
let ((ip_id_behavior:uncomp_value == 0) ||
(ip_id_behavior:uncomp_value == 1));
discriminator ::= '101';
msn ::= lsb (3, -1);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
ip_id ::= ip_id_lsb (ip_id_behavior, msn, 4, -1);
ack_number ::= lsb (16, 0);
psh_flag ::= irregular (1);
seq_number_scaled ::= lsb (10, 511);
};
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format_seq_3 = discriminator, %[ 6 ]
seq_number_scaled, %[ 10 ]
psh_flag, %[ 1 ]
header_crc, %[ 7 ]
msn, %[ 3 ]
ip_id, %[ 5 ]
ecn_used, %[ 1 ]
list_present, %[ 1 ]
ack_number, %[ 14 ]
options_list, % n bits
{
let ((ip_id_behavior:uncomp_value == 0) ||
(ip_id_behavior:uncomp_value == 1));
discriminator ::= '110101';
msn ::= lsb (3, -1);
header_crc ::= crc7 (this:uncomp_value,
this:uncomp_length);
ip_id ::= ip_id_lsb (ip_id_behavior, msn, 5, -1);
ack_number ::= lsb (14, 0);
psh_flag ::= irregular (1);
seq_number_scaled ::= lsb (10, 511);
ecn_used ::= irregular(1);
list_present ::= irregular(1);
options_list ::= tcp_list_presence_enc(list_length,
list_present);
};
format_seq_4 = discriminator, %[ 5 ]
msn, %[ 3 ]
psh_flag, %[ 1 ]
header_crc, %[ 7 ]
ttl_hopl, %[ 8 ]
ip_id, %[ 6 ]
tos_tc, %[ 6 ]
seq_number, %[ 12 ]
{
let ((ip_id_behavior:uncomp_value == 0) ||
(ip_id_behavior:uncomp_value == 1));
discriminator ::= '11001';
msn ::= lsb (3, -1);
psh_flag ::= irregular (1);
header_crc ::= crc7 (this:uncomp_value,
this:uncomp_length);
ttl_hopl ::= irregular (8);
ip_id ::= ip_id_lsb (ip_id_behavior, msn, 6, -1);
tos_tc ::= irregular (6);
seq_number ::= lsb (12, 1023);
};
format_seq_5 = discriminator, %[ 7 ]
psh_flag, %[ 1 ]
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msn, %[ 3 ]
ip_id, %[ 13 ]
ack_flag, %[ 1 ]
header_crc, %[ 3 ]
seq_number, %[ 12 ]
{
let ((ip_id_behavior:uncomp_value == 0) ||
(ip_id_behavior:uncomp_value == 1));
discriminator ::= '1100000';
msn ::= lsb (3, -1);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
ip_id ::= ip_id_lsb (ip_id_behavior, msn, 6, -1);
seq_number ::= lsb (12, 1023);
psh_flag ::= irregular (1);
ack_flag ::= irregular (1);
};
format_seq_6 = discriminator, %[ 6 ]
list_present, %[ 1 ]
psh_flag, %[ 1 ]
ack_flag, %[ 1 ]
header_crc, %[ 7 ]
ecn_used, %[ 1 ]
msn, %[ 4 ]
seq_number, %[ 11 ]
ip_id, %[ 8 ]
ttl_hopl, %[ 8 ]
options_list, % n bits
{
let ((ip_id_behavior:uncomp_value == 0) ||
(ip_id_behavior:uncomp_value == 1));
discriminator ::= '110111';
msn ::= lsb (4, -1);
header_crc ::= crc7 (this:uncomp_value,
this:uncomp_length);
ack_flag ::= irregular(1);
ip_id ::= ip_id_lsb (ip_id_behavior, msn, 8, -1);
ttl_hopl ::= irregular (8);
seq_number ::= lsb (11, 511);
psh_flag ::= irregular (1);
ecn_used ::= irregular (1);
list_present ::= irregular(1);
options_list ::= tcp_list_presence_enc(list_length,
list_present);
};
format_seq_7 = discriminator, %[ 8 ]
psh_flag, %[ 1 ]
msn, %[ 4 ]
header_crc, %[ 3 ]
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seq_number, %[ 16 ]
ip_id, %[ 16 ]
{
let ((ip_id_behavior:uncomp_value == 0) ||
(ip_id_behavior:uncomp_value == 1));
discriminator ::= '11000111';
msn ::= lsb (4, -1);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
ip_id ::= irregular (16);
seq_number ::= lsb (16, 32767);
psh_flag ::= irregular (1);
};
format_seq_8 = discriminator, %[ 7 ]
msn, %[ 3 ]
ip_id, %[ 6 ]
psh_flag, %[ 1 ]
header_crc, %[ 3 ]
seq_number_scaled, %[ 12 ]
ack_number, %[ 16 ]
window, %[ 16 ]
{
let ((ip_id_behavior:uncomp_value == 0) ||
(ip_id_behavior:uncomp_value == 1));
discriminator ::= '1100001';
msn ::= lsb (3, -1);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
ip_id ::= ip_id_lsb (ip_id_behavior, msn, 6, -1);
seq_number_scaled ::= lsb (12, 1023);
psh_flag ::= irregular (1);
ack_number ::= lsb (16, 0);
window ::= irregular (16);
};
format_seq_9 = discriminator, %[ 6 ]
seq_number_scaled, %[ 10 ]
window, %[ 16 ]
psh_flag, %[ 1 ]
msn, %[ 3 ]
header_crc, %[ 3 ]
ip_id, %[ 5 ]
tos_tc, %[ 6 ]
ack_number, %[ 14 ]
{
let ((ip_id_behavior:uncomp_value == 0) ||
(ip_id_behavior:uncomp_value == 1));
discriminator ::= '110100';
msn ::= lsb (3, -1);
header_crc ::= crc3 (this:uncomp_value,
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this:uncomp_length);
tos_tc ::= irregular (6);
ip_id ::= ip_id_lsb (ip_id_behavior, msn, 5, -1);
ack_number ::= lsb (14, 0);
psh_flag ::= irregular (1);
window ::= irregular (16);
seq_number_scaled ::= lsb (10, 511);
};
format_seq_10 = discriminator, %[ 6 ]
list_present, %[ 1 ]
ip_id, %[ 5 ]
msn, %[ 4 ]
seq_number_scaled, %[ 32 ]
payload_size, %[ 16 ]
psh_flag, %[ 1 ]
ack_number, %[ 15 ]
header_crc, %[ 7 ]
window, %[ 13 ]
seq_number, %[ 12 ]
{
let ((ip_id_behavior:uncomp_value == 0) ||
(ip_id_behavior:uncomp_value == 1));
discriminator ::= '110110';
msn ::= lsb (4, -1);
header_crc ::= crc7 (this:uncomp_value,
this:uncomp_length);
ip_id ::= ip_id_lsb (ip_id_behavior, msn, 5, -1);
seq_number ::= lsb (12, 1023);
ack_number ::= lsb (15, 0);
psh_flag ::= irregular (1);
window ::= lsb (13, 4095);
seq_number_scaled ::= irregular (32);
payload_size ::= irregular (16);
list_present ::= irregular(1);
options_list ::= tcp_list_presence_enc(list_length,
list_present);
};
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% IP-ID Random/Zero Packet CO packet format base headers
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% XXX: Note: The discriminator '00000' was freed up by
% reducing the number of packet formats
format_rnd_0 = discriminator, %[ 2 ]
seq_number, %[ 14 ]
psh_flag, %[ 1 ]
header_crc, %[ 3 ]
msn, %[ 4 ]
{
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let ((ip_id_behavior:uncomp_value == 2) ||
(ip_id_behavior:uncomp_value == 3));
discriminator ::= '01';
msn ::= lsb(4, -1);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
seq_number ::= lsb (13, 4095);
psh_flag ::= irregular (1);
ecn_used ::= irregular (1);
};
format_rnd_1 = discriminator, %[ 3 ]
psh_flag, %[ 1 ]
ack_number, %[ 2 ]
rsf_flags, %[ 2 ]
msn, %[ 3 ]
header_crc, %[ 3 ]
seq_number_scaled, %[ 10 ]
{
let ((ip_id_behavior:uncomp_value == 2) ||
(ip_id_behavior:uncomp_value == 3));
discriminator ::= '101';
msn ::= lsb(3,-1);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
ack_number ::= lsb (2, 1);
psh_flag ::= irregular (1);
rsf_flags ::= rsf_index_enc;
seq_number_scaled ::= lsb (10, 511);
};
format_rnd_2 = discriminator, %[ 3 ]
list_present, %[ 1 ]
ecn_used, %[ 1 ]
msn, %[ 3 ]
ttl_hopl, %[ 8 ]
psh_flag, %[ 1 ]
header_crc, %[ 7 ]
seq_number, %[ 16 ]
{
let ((ip_id_behavior:uncomp_value == 2) ||
(ip_id_behavior:uncomp_value == 3));
discriminator ::= '110';
msn ::= lsb(3,-11);
header_crc ::= crc7 (this:uncomp_value,
this:uncomp_length);
ttl_hopl ::= irregular (8);
seq_number ::= lsb (16, 16383);
psh_flag ::= irregular (1);
ecn_used ::= irregular (1);
list_present ::= irregular(1);
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options_list ::= tcp_list_presence_enc(list_length,
list_present);
};
format_rnd_3 = discriminator, %[ 4 ]
psh_flag, %[ 1 ]
header_crc, %[ 3 ]
ack_number, %[ 16 ]
msn, %[ 4 ]
seq_number_scaled, %[ 12 ]
{
let ((ip_id_behavior:uncomp_value == 2) ||
(ip_id_behavior:uncomp_value == 3));
discriminator ::= '0011';
msn ::= lsb(4, -1);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
ack_number ::= lsb (15, 0);
psh_flag ::= irregular (1);
seq_number_scaled ::= lsb (12, 1023);
};
format_rnd_4 = discriminator, %[ 3 ]
seq_number, %[ 13 ]
psh_flag, %[ 1 ]
msn, %[ 4 ]
header_crc, %[ 3 ]
{
let ((ip_id_behavior:uncomp_value == 2) ||
(ip_id_behavior:uncomp_value == 3));
discriminator ::= '100';
msn ::= lsb(4,-1);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
seq_number ::= lsb (13, 2047);
psh_flag ::= irregular (1);
urg_ptr ::= irregular (16);
urg_flag ::= irregular(1);
};
format_rnd_5 = discriminator, %[ 5 ]
header_crc, %[ 3 ]
psh_flag, %[ 1 ]
ack_number, %[ 15 ]
msn, %[ 4 ]
seq_number_scaled, %[ 12 ]
{
let ((ip_id_behavior:uncomp_value == 2) ||
(ip_id_behavior:uncomp_value == 3));
discriminator ::= '00011';
msn ::= lsb(4, -1);
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header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
ack_number ::= lsb (15, 0);
psh_flag ::= irregular (1);
seq_number_scaled ::= lsb (12, 1023);
};
format_rnd_6 = discriminator, %[ 5 ]
msn, %[ 3 ]
header_crc, %[ 7 ]
psh_flag, %[ 1 ]
ack_number, %[ 16 ]
list_present, %[ 1 ]
ecn_used, %[ 1 ]
seq_number_scaled, %[ 14 ]
{
let ((ip_id_behavior:uncomp_value == 2) ||
(ip_id_behavior:uncomp_value == 3));
discriminator ::= '00101';
msn ::= lsb(3, -1);
header_crc ::= crc7 (this:uncomp_value,
this:uncomp_length);
ttl_hopl ::= irregular (8);
ack_number ::= lsb (16, 0);
psh_flag ::= irregular (1);
seq_number_scaled ::= lsb (14, 4095);
ecn_used ::= irregular (1);
list_present ::= irregular(1);
options_list ::= tcp_list_presence_enc(list_length,
list_present);
};
format_rnd_7 = discriminator, %[ 5 ]
header_crc, %[ 3 ]
window, %[ 16 ]
psh_flag, %[ 1 ]
ack_number, %[ 15 ]
msn, %[ 4 ]
seq_number_scaled, %[ 12 ]
{
let ((ip_id_behavior:uncomp_value == 2) ||
(ip_id_behavior:uncomp_value == 3));
discriminator ::= '00001';
msn ::= lsb(4, -1);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
ack_number ::= lsb (15, 0);
psh_flag ::= irregular (1);
window ::= irregular (16);
seq_number_scaled ::= lsb (12, 1023);
};
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format_rnd_8 = discriminator, %[ 5 ]
msn, %[ 3 ]
ack_number, %[ 32 ]
psh_flag, %[ 1 ]
header_crc, %[ 3 ]
seq_number, %[ 12 ]
{
let ((ip_id_behavior:uncomp_value == 2) ||
(ip_id_behavior:uncomp_value == 3));
discriminator ::= '00100';
msn ::= lsb(3, -1);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
seq_number ::= lsb (12, 1023);
ack_number ::= irregular (32);
psh_flag ::= irregular (1);
};
format_rnd_9 = discriminator, %[ 5 ]
header_crc, %[ 3 ]
window, %[ 16 ]
psh_flag, %[ 1 ]
ack_number, %[ 15 ]
msn, %[ 4 ]
seq_number_scaled, %[ 12 ]
{
let ((ip_id_behavior:uncomp_value == 2) ||
(ip_id_behavior:uncomp_value == 3));
discriminator ::= '00010';
msn ::= lsb(4, -1);
header_crc ::= crc3 (this:uncomp_value,
this:uncomp_length);
ack_number ::= lsb (15, 0);
psh_flag ::= irregular (1);
window ::= irregular (16);
seq_number_scaled ::= lsb (12, 1023);
};
};
6.8. Feedback Formats and Options
6.8.1. 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 [2].
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All feedback formats carry a field labeled SN. The SN field contains
LSBs of the Master Sequence Number (MSN) described in section 6.3.
The sequence number to use is the MSN corresponding 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 parseability)
MSN: The lsb-encoded master sequence number.
Feedback options: A variable number of feedback options, see
section 6.8.2. Options may appear in any order.
6.8.2. Feedback Options
ROHC-TCP uses the same feedback options as the options defined in
section 5.7.6 of [2], with the following exceptions:
1) The MSN replaces RTP SN in the feedback information.
2) The CLOCK option ([2], section 5.7.6.6) is not used.
3) The JITTER option ([2], section 5.7.6.7) is not used.
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6.8.3. 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.
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.
7. 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.
8. 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.
<# Editor's Note: To be removed before publication #>
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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
9. Acknowledgments
The authors would like to thank Qian Zhang and Hong Bin Liao for
their work with early versions of this specification. Thanks also to
Fredrik Lindstroem for reviewing the packet formats, as well as to
Carsten Bormann and Robert Finking for valuable input.
10. Authors' Addresses
Ghyslain Pelletier
Ericsson AB
Box 920
SE-971 28 Lulea, Sweden
Phone: +46 8 404 29 43
Fax: +46 920 996 21
EMail: ghyslain.pelletier@ericsson.com
Lars-Erik Jonsson
Ericsson AB
Box 920
SE-971 28 Lulea, Sweden
Phone: +46 8 404 29 61
Fax: +46 920 996 21
EMail: lars-erik.jonsson@ericsson.com
Mark A West
Roke Manor Research Ltd
Romsey, Hants, SO51 0ZN
United Kingdom
Pelletier, et. al [Page 80]
INTERNET-DRAFT ROHC Profile for TCP/IP October 25, 2004
Phone: +44 1794 833311
Email: mark.a.west@roke.co.uk
Richard Price
Roke Manor Research Ltd
Romsey, Hants, SO51 0ZN
United Kingdom
Phone: +44 1794 833681
Email: richard.price@roke.co.uk
Kristofer Sandlund
Effnet AB
Stationsgatan 69
S-972 34 Lulea
Sweden
Phone: +46 920 609 17
Fax: +46 920 609 27
EMail: kristofer.sandlund@effnet.com
11. References
11.1. Normative references
[1] S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[2] 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.
[3] Pelletier, G., "Robust Header Compression (ROHC): Context
Replication for ROHC profiles", Internet Draft (work in
progress), <draft-ietf-rohc-context-replication-03.txt>, July
2004.
[4] R. Price, R. Finking and G. Pelletier, "Formal Notation for
Robust Header Compression (ROHC-FN)", Internet Draft (work in
progress), <draft-ietf-rohc-formal-notation-03.txt>, July 2004.
[5] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.
[6] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
Pelletier, et. al [Page 81]
INTERNET-DRAFT ROHC Profile for TCP/IP October 25, 2004
[7] S. Bradner, "The Internet Standards Process - Revision 3", RFC
2026, October 1996.
[8] S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[9] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
11.2. Informative References
[10] Jonsson, L-E., "Requirements on ROHC IP/TCP header compression",
Internet Draft (work in progress), <draft-ietf-rohc-tcp-
requirements-07.txt>, June 2004.
[11] West, M. and S. McCann, "TCP/IP Field Behavior", Internet Draft
(work in progress), <draft-ietf-rohc-tcp-field-behavior-03.txt>,
July 2004.
[12] Jonsson, L-E. and G. Pelletier, "RObust Header Compression
(ROHC): A compression profile for IP", RFC 3843, June 2003.
[13] Jacobson, V., and R. Braden, "TCP Extensions for Long-Delay
Paths", LBL, ISI, October 1988.
[14] Jacobson, V.,"Compressing TCP/IP Headers for Low-Speed Serial
Links", RFC 1144, February 1990.
[15] Jacobson, V., Braden, R. and D. Borman, "TCP Extensions for High
Performance", RFC 1323, May 1992.
[16] Braden, R. "T/TCP -- TCP Extensions for Transactions Functional
Specification", ISI, July 1994.
[17] Connolly, T., et al, "An Extension to TCP: Partial Order
Service", University of Delaware, November 1994.
[18] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", RFC
1889, January 1996.
[19] Stevens, W., "TCP Slow Start, Congestion Avoidance, Fast
Retransmit, and Fast Recovery Algorithms", NOAO, January 1997.
[20] Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
Pelletier, et. al [Page 82]
INTERNET-DRAFT ROHC Profile for TCP/IP October 25, 2004
[21] Degermark, M., Nordgren, B. and S. Pink, "IP Header
Compression", RFC 2507, February 1999.
[22] Floyd, S., Mahdavi, J., Mathis, M. and M. Podolsky, "An
Extension to the Selective Acknowledgement (SACK) Option for
TCP", RFC 2883, July 2000.
[23] Ramakrishnan, K., Floyd and D. Black, "The Addition of Explicit
Congestion Notification (ECN) to IP", RFC 3168, September 2001.
[24] Jacobson, V., "Fast Retransmit", Message to the end2end-interest
mailing list, April 1990.
[25] Perkins, C., ôMinimal Encapsulation within IPö, RFC 2004,
October 1996.
Pelletier, et. al [Page 83]
INTERNET-DRAFT ROHC Profile for TCP/IP October 25, 2004
Copyright Statement
Copyright (C) The Internet Society (2004). 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.
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
This Internet-Draft expires April 25, 2005.
Pelletier, et. al [Page 84]
