One document matched: draft-ietf-rohc-formal-notation-05.txt
Differences from draft-ietf-rohc-formal-notation-04.txt
Robust Header Compression R. Finking
Internet-Draft Siemens/Roke Manor
Expires: August 25, 2005 C. Bormann
Universitaet Bremen TZI
G. Pelletier
Ericsson AB
February 21, 2005
Formal Notation for Robust Header Compression (ROHC-FN)
draft-ietf-rohc-formal-notation-05.txt
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document defines ROHC-FN: a formal notation to unambiguously
specify header compression field encodings, when defining new
profiles within the ROHC (RFC3095 [4]) framework. ROHC-FN offers a
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library of encoding methods that are often used in ROHC profiles, and
can thereby help simplifying future profile development work.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview of ROHC-FN . . . . . . . . . . . . . . . . . . . . . 5
3.1 Scope of ROHC-FN . . . . . . . . . . . . . . . . . . . . . 5
3.2 Fundamentals of ROHC-FN . . . . . . . . . . . . . . . . . 6
3.2.1 Fields and Encodings . . . . . . . . . . . . . . . . . 6
3.2.2 Structures . . . . . . . . . . . . . . . . . . . . . . 7
3.3 Example using IPv4 . . . . . . . . . . . . . . . . . . . . 9
4. Normative Definition of ROHC-FN . . . . . . . . . . . . . . . 12
4.1 Overall Structure of a Specification . . . . . . . . . . . 12
4.2 Constant Definitions . . . . . . . . . . . . . . . . . . . 13
4.3 Attributes . . . . . . . . . . . . . . . . . . . . . . . . 13
4.3.1 Attribute References . . . . . . . . . . . . . . . . . 14
4.4 Expressions . . . . . . . . . . . . . . . . . . . . . . . 14
4.4.1 Integer Literals . . . . . . . . . . . . . . . . . . . 15
4.4.2 Boolean Literals . . . . . . . . . . . . . . . . . . . 15
4.4.3 Boolean Operators . . . . . . . . . . . . . . . . . . 15
4.4.4 Integer Operators . . . . . . . . . . . . . . . . . . 16
4.4.5 Comparison Operators . . . . . . . . . . . . . . . . . 16
4.5 let Statements . . . . . . . . . . . . . . . . . . . . . . 16
4.6 Comments . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.6.1 End of line comments . . . . . . . . . . . . . . . . . 17
4.6.2 Block comments . . . . . . . . . . . . . . . . . . . . 18
4.7 Library Encoding Methods . . . . . . . . . . . . . . . . . 18
4.7.1 uncompressed_value . . . . . . . . . . . . . . . . . . 18
4.7.2 compressed_value . . . . . . . . . . . . . . . . . . . 19
4.7.3 irregular . . . . . . . . . . . . . . . . . . . . . . 20
4.7.4 static . . . . . . . . . . . . . . . . . . . . . . . . 20
4.7.5 lsb . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.7.6 crc . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.8 Profile-specific Encoding Methods . . . . . . . . . . . . 23
4.9 Structures . . . . . . . . . . . . . . . . . . . . . . . . 23
4.9.1 "this" . . . . . . . . . . . . . . . . . . . . . . . . 23
4.9.2 Simple Structures . . . . . . . . . . . . . . . . . . 23
4.9.3 Arguments and Structures . . . . . . . . . . . . . . . 26
4.9.4 Multiple Formats . . . . . . . . . . . . . . . . . . . 27
4.9.5 Control Fields . . . . . . . . . . . . . . . . . . . . 30
5. Security considerations . . . . . . . . . . . . . . . . . . . 32
6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 32
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 32
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 33
A. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
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A.1 Reserved Keywords . . . . . . . . . . . . . . . . . . . . 33
A.2 Characters . . . . . . . . . . . . . . . . . . . . . . . . 34
A.3 Literals . . . . . . . . . . . . . . . . . . . . . . . . . 36
A.4 Identifiers . . . . . . . . . . . . . . . . . . . . . . . 36
A.5 Operators . . . . . . . . . . . . . . . . . . . . . . . . 36
A.6 Expressions . . . . . . . . . . . . . . . . . . . . . . . 37
A.7 Constants . . . . . . . . . . . . . . . . . . . . . . . . 37
A.8 Field Names . . . . . . . . . . . . . . . . . . . . . . . 37
A.9 Attributes . . . . . . . . . . . . . . . . . . . . . . . . 37
A.10 Encoding Methods . . . . . . . . . . . . . . . . . . . . . 38
A.11 Structures . . . . . . . . . . . . . . . . . . . . . . . . 38
B. Bit-level Worked Example . . . . . . . . . . . . . . . . . . . 39
B.1 Example Packet Format . . . . . . . . . . . . . . . . . . 39
B.2 Initial Encoding . . . . . . . . . . . . . . . . . . . . . 39
B.3 Basic Compression . . . . . . . . . . . . . . . . . . . . 40
B.4 Inter-packet compression . . . . . . . . . . . . . . . . . 42
B.5 Variable Length Discriminators . . . . . . . . . . . . . . 45
B.6 Default encoding . . . . . . . . . . . . . . . . . . . . . 48
B.7 Control Fields . . . . . . . . . . . . . . . . . . . . . . 49
Intellectual Property and Copyright Statements . . . . . . . . 52
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1. Introduction
ROHC-FN is a formal notation designed to help with the definition of
ROHC (RFC3095 [4]) header compression profiles. ROHC-FN offers a
library of encoding methods that are often used in ROHC profiles, so
new profiles can be specified without the need to redefine this
library from scratch.
Informally, an encoding method is a function that maps between
uncompressed data and compressed data. The simplest encoding methods
only have one input and one output: the input is an uncompressed
field and the output is the compressed version of the field. More
complex encoding methods can compress multiple fields at the same
time, e.g. "list" encoding from RFC3095 [4], which is designed to
compress an ordered list of fields.
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 RFC2119 [2].
o Profile
A ROHC (RFC 3095 [4]) profile is a description of how to compress
a certain protocol stack over a certain type of link. Each
profile includes packet formats which describe how to compress the
headers and a state machine to control the actions of each
endpoint.
o Field
ROHC-FN divides the protocol header to be compressed into a set of
contiguous bit patterns known as fields.
o Control field
Control fields are transmitted from a ROHC compressor to a ROHC
decompressor, but are not part of the uncompressed header itself.
o Encoding method
Encoding methods are functions that can be applied to compress
fields in a protocol header.
o Library of encoding methods
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The library of encoding methods contains a number of commonly used
encoding methods for compressing header fields.
3. Overview of ROHC-FN
This section gives an overview of ROHC-FN. It also explains how
ROHC-FN can be used to specify the compression of header fields as
part of a ROHC profile.
3.1 Scope of ROHC-FN
This section describes the scope of the ROHC-FN. It explains how the
formal notation relates to the ROHC framework and to specific ROHC
profiles.
The ROHC framework provides the general principles for performing
ROHC compression. It defines the concept of a profile, which makes
ROHC a general platform for different compression schemes. It sets
link layer requirements, and in particular negotiation requirements
for all ROHC profiles. It defines a set of common functions such as
Context Identifiers (CIDs), padding and segmentation. It also
defines common packet formats (IR, IR-DYN, Feedback, Short-CID
expander, etc.), and finally it defines a generic, profile
independent, feedback mechanism.
A ROHC profile is a description of how to compress a certain protocol
stack over a certain type of link. For example, ROHC profiles are
available for RTP/UDP/IP and many other protocol stacks.
Each ROHC profile contains the following two components:
1. Packet formats, for compressing and decompressing headers; and
2. State machine, for maintaining synchronisation of the context.
The purpose of the packet formats is to define how to compress and
decompress headers. The packet formats define one or more compressed
versions of each uncompressed header; inversely, the packet formats
define how to relate a compressed header back to the original
uncompressed header.
The packet formats will typically compress headers relative to a
context of field values from previous headers in a flow. This
improves the overall compression ratio, because this takes into
account redundancies between headers of successive packets.
The purpose of the state machine is to ensure that the profile is
robust against bit errors and dropped packets. The state machine
manages the context, providing feedback and other mechanisms to
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ensure that the compressor and decompressor contexts are kept
synchronised.
The ROHC-FN is designed to help in the specification of the packet
formats used in ROHC profiles. It offers a library of encoding
methods for compressing fields, and a mechanism for combining these
encoding methods to create packet formats tailored to a specific
protocol stack. The state machine for the profiles is beyond the
scope of ROHC-FN, and it must be provided separately as part of a
complete profile specification.
3.2 Fundamentals of ROHC-FN
There are two fundamental elements to the formal notation:
1. Fields and their encodings, which define the mapping between a
field's uncompressed and compressed values.
2. Structures, which define lists of uncompressed fields and the
lists of compressed fields they map onto.
These two fundamental elements are at the core of the notation and
are outlined below.
3.2.1 Fields and Encodings
The creation of bindings between fields and encoding methods is
indicated as follows:
field ::= encoding_method
When writing the above statement, the symbol "::=" means "is encoded
as". This statement does not represent an assignment operation from
the right hand side to the left side. Instead, it is a two-way
mapping in that it both represents the compression and the
decompression operation in a single statement, where variables take
on values through a process of two-way matching. Two-way matching is
a binary operation that attempts to make the operands the same
(similar to the unification process in logic). The operands
represent one unspecified data object, and values can be matched from
either operand.
Fields have attributes. Attributes describe various things about the
field, including the length of the field and whereabouts the field
appears in the header. For example:
field:has_context
indicates whether or not a context entry exists for this field.
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See Section 4.3 for more details on field attributes.
An encoding method (including the parameters specified with the
method) creates a reversible binding between the attributes of a
field . At the compressor, a packet format can be used if a set of
bindings that is successful for all fields can be found. At the
decompressor, the operation is reversed using the same bindings and
the fields are filled according to the specified bindings.
For example, the 'static' encoding method creates a binding between
the attribute corresponding to the uncompressed value of the field
and the attribute corresponding to the value of the field in the
context.
o For the compressor, the 'static' binding is successful when both
the context value and the uncompressed value are the same. If the
two values differ then the binding fails.
o For the decompressor, the 'static' binding succeeds for a packet
type only if a valid context entry containing the value of the
uncompressed field exists. Otherwise, the binding will fail and
an alternative encoding method must be used.
3.2.2 Structures
Structures provide a mechanism for combining fields and their
encoding methods into larger units. Structures are defined using the
"===" symbol. These can then be used as encoding methods in other
structures:
structure ===
{
uc_format = field_1,
field_2,
:
:
field_n;
control_fields = ctrl_field_1,
ctrl_field_2,
:
:
ctrl_field_n;
default_methods =
{
field_a ::= encoding_method_9;
field_e ::= encoding_method_8;
: :
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: :
ctrl_field_3 ::= encoding_method_2;
};
co_format_0 = field_a,
:
:
field_b
{
field_a ::= encoding_method_1;
: :
: :
field_b ::= encoding_method_2;
ctrl_field_1 ::= encoding_method_3
};
co_format_1 = field_c,
:
:
field_d
{
field_c ::= encoding_method_4;
: :
: :
field_d ::= encoding_method_5;
};
:
:
co_format_n = field_y,
:
:
field_z
{
field_y ::= encoding_method_foo;
: :
: :
field_z ::= encoding_method_bar;
};
};
In the example above, the comma separated list "uc_format" indicates
the order of fields in the uncompressed header. After this is
another comma separated list, "control_fields", which defines one or
more control fields. Finally, a number of packet formats for the
compressed data follow, each beginning with the reserved prefix
"co_format". These also have a field order list, which consists of:
o fields that occur in the uncompressed header; or
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o "control fields", that are additional information added to the
compressed packet during compression.
In the example packet formats defined by "co_format" also indicate an
list of field encodings, which is typical usage. A "uc_format" may
also include a field order list, though the one in the example
doesn't. The field encodings list contains the encoding methods for
each field. These are defined inside braces for the fields in the
preceding field order list. Fields that have no encoding methods
defined in this field order list are encoded using the default
encodings specified in "default_methods" (see Section 4.9.4.3).
Fields from the uncompressed header have the same name as they do in
the compressed header. If there are any fields which are present
exclusively in the compressed header but which do have an
uncompressed value, they must be declared in the "control_fields"
section of the structure (see Section 4.9.5 for more details on
defining control fields). In the example above, all fields appearing
in the compressed header are also found in the uncompressed field
order list, or the control field list. However it is possible to
have fields which appear in neither an uncompressed field order list
nor the control field list. Fields which have no "uncompressed"
value, such as a checksum on the compressed header, fall into this
category.
Following the compressed field order list,
3.3 Example using IPv4
This section gives an overview of how the notation is used by means
of an example. The example will develop the formal notation for an
encoding method capable of compressing a single, well-known header:
the IPv4 header.
The first step is to specify the overall structure of the IPv4
header. To do this, we use a structure which we will call
"ipv4_header". Structures are defined in Section 4.9. This is
notated as follows:
ipv4_header ===
{
The statement above defines the encoding method "ipv4_header" as a
structure, the definition of which follows the opening brace.
Definitions within the pair of braces are local to "ipv4_header".
This scoping mechanism helps to clarify which fields belong to which
headers: it is also useful when compressing complex protocol stacks
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with several headers and fields, often sharing the same names.
The next step is to specify the fields contained in the uncompressed
IPv4 header. This is accomplished using ROHC-FN as follows:
uc_format = version, % [ 4 ]
header_length, % [ 4 ]
tos, % [ 6 ]
ecn, % [ 2 ]
length, % [ 16 ]
id, % [ 16 ]
reserved, % [ 1 ]
dont_frag, % [ 1 ]
more_fragments, % [ 1 ]
offset, % [ 13 ]
ttl, % [ 8 ]
protocol, % [ 8 ]
checksum, % [ 16 ]
src_addr, % [ 32 ]
dest_addr; % [ 32 ]
The numbers in square brackets give the field width in bits. Note
that these are mere comments that do not have any formal meaning.
The fields contained in the compressed header can then be specified.
Exactly what appears in this list of fields depends on the encoding
methods used to encode the uncompressed fields -- it may be possible
to compress certain fields down to 0 bits, in which case they do not
need to be sent in the compressed header at all.
co_format = src_addr, % [ 32 ]
dest_addr, % [ 32 ]
length, % [ 16 ]
id, % [ 16 ]
ttl, % [ 8 ]
protocol, % [ 8 ]
tos, % [ 6 ]
ecn, % [ 2 ]
dont_frag % [ 1 ]
{
Note that the order of the fields in the compressed header is
independent of the order of the fields in the uncompressed header.
The next step is to specify the encoding methods for each field in
the IPv4 header. These are taken from encoding methods in the
ROHC-FN library, as well as from additional encoding methods defined
in the profile specification itself. Since the intention here is to
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illustrate the use of the notation, rather than to describe the
optimum method of compressing IPv4 headers, this example uses only
three predefined encoding methods.
The "uncompressed_value" encoding method (defined in Section 4.7.1)
can compress any field whose uncompressed length and value are fixed.
No compressed bits need to be sent because the uncompressed field can
be reconstructed using its known size and value. The
"uncompressed_value" encoding method is used to compress five fields
in the IPv4 header, as described below:
version ::= uncompressed_value (4, 4);
header_length ::= uncompressed_value (4, 5);
reserved ::= uncompressed_value (1, 0);
more_fragments ::= uncompressed_value (1, 0);
offset ::= uncompressed_value (13, 0);
The first parameter indicates the length of the uncompressed field in
bits, and the second parameter gives its integer value.
The "irregular" encoding method (defined in Section 4.7.3) can be
used to encode any field whose length is fixed, or can be calculated
using an expression. It is a general encoding method that can be
used for fields to which no other encoding method applies. All of
the bits in the uncompressed field are present in the compressed
format as well; hence this encoding does not achieve any compression.
tos ::= irregular (6);
ecn ::= irregular (2);
length ::= irregular (16);
id ::= irregular (16);
dont_frag ::= irregular (1);
ttl ::= irregular (8);
protocol ::= irregular (8);
src_addr ::= irregular (32);
dest_addr ::= irregular (32);
Finally, the third encoding method is specific only to IPv4 headers,
"inferred_ip_v4_header_checksum":
checksum ::= inferred_ip_v4_header_checksum;
};
This is a specific encoding method for calculating the IP checksum
from the rest of the header values. Like the "uncompressed_value"
encoding method, no compressed bits need to be sent, since the field
value can be reconstructed at the decompressor.
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However, unlike "uncompressed_value", the meaning of
"inferred_ip_v4_header_checksum" is not defined in the ROHC-FN
library of encoding methods, nor is it defined by another structure
elsewhere in the formal notation given in the example above. Its
definition can be given either using plain English text or using the
formal notation as part of the profile definition itself.
Finally the definition of the structure is closed with a closing
brace. At this point, the above example has defined the format of
the compressed IPv4 header, and provided enough information to allow
an implementation to construct the compressed header from an
uncompressed header and vice versa.
4. Normative Definition of ROHC-FN
This section gives the normative definition of ROHC-FN. ROHC-FN is a
referentially transparent, declarative language with no side effects.
4.1 Overall Structure of a Specification
A ROHC-FN specification consists of a sequence of zero or more
constant definitions (Section 4.2), an optional global control field
list (Section 4.9.5) and one or more encoding method definitions,
given in the form of structures (Section 4.9).
Structures define an encoding method by giving one or more formats
for uncompressed packets and one or more formats for compressed
packets. These formats are linked by so-called fields, each of which
describes a certain part of an uncompressed and/or a compressed
format.
The properties of a field are defined by defining an encoding method
for it and/or by use of "let" statements. Encoding methods can be
defined in FN using a structure or can be predefined encoding
methods. Predefined encoding methods can be defined in the text
accompanying a formal specification or they can be those defined in
the present document.
Each encoding method and each constant has an identifier. All of
these identifiers have global scope. It is illegal to have multiple
instances of the same identifier. It is also illegal to use any of
the following as identifiers for encoding methods:
o "let", "this"
o "control_fields", "default_methods"
o "uncomp_hdr_start", "uncomp_length", "uncomp_value"
o "comp_hdr_start", "comp_length", "comp_value"
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o identifiers starting either with "uc_format" or "co_format"
4.2 Constant Definitions
Constant values can be defined using the "=" operator. Identifiers
for constants must be all upper case. For example:
SOME_CONSTANT = 3;
Constants are defined by an expression on the right hand side of the
"=" operator . The expression must yield a constant value. That is,
the expression must be one whose terms are all either constants or
literals and not structure parameters or field attributes (see
Section 4.4).
Constants have global scope. Constants must be defined at the top
level, outside of any structure definition (noting that "=" has a
different meaning inside a structure see Section 4.9). Because the
FN is referentially transparent constants are entirely equivalent to
the value they refer to and are completely interchangeable with that
value. Similarly, since the language has no side effects a constant
may never change its value.
4.3 Attributes
In ROHC-FN, the properties of a field are defined by an encoding
method. The encoding method's formal semantics are specified using a
set of attributes. This set of attributes entirely characterises the
relationship between the uncompressed and compressed representation
of a field. Both of these representations are bit strings.
The notation defines six attributes, three for the uncompressed field
and a corresponding three for the compressed field. The attributes
available for each field are as follows:
uncompressed attributes of a field:
o "uncomp_value", "uncomp_length" and "uncomp_hdr_start",
compressed attributes of a field:
o "comp_value", "comp_length" and "comp_hdr_start".
The two value attributes contain the respective numerical values of
the field, i.e. "uncomp_value" gives the numerical value of the
uncompressed aspect of the field, and the attribute "comp_value"
gives the numerical value of the compressed aspect of the field. The
numerical values are derived by interpreting the bit string in the
field as an unsigned binary number, most-significant bit first.
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The two length attributes indicate the length in bits of the
associated bit string; "uncomp_length" for the uncompressed
representation, and "comp_length" for the compressed representation.
Finally, the two "hdr_start" attributes indicate the offset in bits
of the start of the field from the start of the header;
"uncomp_hdr_start" for the position in the uncompressed header, and
"comp_hdr_start" for the position of the field in the compressed
header.
Attributes are undefined unless they are bound to a value in which
case they become defined. The defined value of an attribute can not
be changed, bindings are permanent in the FN. Defined values are
required for all compressed attributes of fields which appear in the
compressed header and for all uncompressed attributes of fields which
appear in it the uncompressed header. If two conflicting bindings
are given for a field attribute then the binding fails along with the
packet format in which the binding was defined.
Note that uncompressed attributes do not always reflect an aspect of
the uncompressed header. Some fields do not originate from the
uncompressed header, but are control fields. In particular note that
the "uncomp_hdr_start" attribute has no useful meaning if the field
is a control field (see Section 4.9.5).
4.3.1 Attribute References
Attributes of a particular field are referred to formally by using
the field's name followed by a ":" and the attribute's identifier.
For example:
ip_id_behavior:uncomp_value
gives the uncompressed value of the ip_id_behaviour field.
4.4 Expressions
ROHC-FN includes the usual infix style of expressions, with
parentheses "(" and ")" used for grouping. Expressions can be made
up of any of the components described in the following subsections.
In summary, the semantics of expressions are generally as in the C
programming language, with the following additions and exceptions:
o There is no limit on the range of integers.
o For modulo, the expression "mod(k,v)" is used instead of C
language "k % v". Note that the '%' is a comment character in
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ROHC-FN.
o "x ^ y" evaluates to x raised to the power of y.
o "log2(w)" evaluates to the smallest integer k where w <= 2^k, i.e.
it returns the smallest number of bits in which value v can be
stored.
Expressions may refer to any of the attributes of each field (as
described in Section 4.3), and also to any defined constant (see
Section 4.2).
If any of the attributes or constants used in the expression are
undefined, the value of the expression is undefined. Undefined
expressions cause the environment (e.g. the packet format) in which
they are used to fail if a defined value is required. Defined values
are required for all compressed attributes of fields which appear in
the compressed header and for all uncompressed attributes of fields
which appear in it the uncompressed header.
Note that expressions cannot be used as encoding methods directly
because they do not completely characterise an field. Expressions
only specify a single value whereas a field is made up of several
values: its attributes. If for example the expression was used to
define the uncompressed value of a field, the length of the
uncompressed field would be undefined at the decompressor. For
example, the following is illegal:
tcp_list_length ::= (data_offset + 20) / 4;
4.4.1 Integer Literals
Integers can be expressed as decimal values, binary values (prefixed
by 0b), or hexadecimal values (prefixed by 0x). Negative integers
are prefixed by a "-" sign (note that there is no unary minus
operator).
4.4.2 Boolean Literals
The boolean literals are "false", which has a value of 0, and "true",
which has a value of 1.
4.4.3 Boolean Operators
The following "boolean" operators are available, which take boolean
arguments and return a boolean result:
o &&, for logical "and". Returns true if both boolean1 and boolean2
are true. Returns false otherwise.
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o ||, for logical "or". Returns true if at least one of boolean1 or
boolean2 is true. Returns false otherwise.
o !, for logical not. Returns true if boolean is false. Returns
false otherwise.
4.4.4 Integer Operators
The following "integer" operators are available, which take integer
arguments and return an integer result:
o ^, for exponentiation. "x ^ y" returns the value of "x" to the
power of "y".
o *, / for multiplication and division. "x * y" returns the product
of "x" and "y". "x / y" returns the quotient, rounded down to the
next lowest integer.
o +, - for addition and subtraction. "x + y" returns the sum of "x"
and "y". "x - y" returns the difference.
o mod(k, v) for modulo. "mod(x,y)" returns "x" modulo "y"; x - y *
(x / y).
o log2(w) for logarithm to base 2. Log2(x) returns the smallest
integer k where x <= 2^k, i.e. it returns the smallest number of
bits in which value x can be stored.
4.4.5 Comparison Operators
The following "comparison" operators are available, which take
integer arguments and return a boolean result:
o ==, !=, for equality and its negative. "x == y" returns true if x
is equal to y. Returns false otherwise. "x != y" returns true if
x is not equal to y. Returns false otherwise.
o <, >, for less than and greater than. "x < y" returns true if x
is less than y. Returns false otherwise. "x > y" returns true if
x is greater than y. Returns false otherwise.
o >=, <=, for less than or equal and greater than or equal, the
inverse functions of <, >. "x >= y" returns false if x is less
than y. Returns true otherwise. "x <= y" returns false if x is
greater than y. Returns true otherwise.
4.5 let Statements
A "let" statement takes a boolean expression as a parameter. It can
be used to assert that the expression has a specific value, in order
to choose a particular packet format from a list of possible formats
specified in a structure (see Section 4.9)
let (<boolean expression>)
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A "let" statement must only be used inside a field encodings list
(see Section 4.9).
There are three possible results when an expression is asserted in a
let statement:
o The boolean expression evaluates to false, in which case the
assertion fails,
o All terms in the boolean expression are defined and it evaluates
to true, in which case the assertion succeeds,
o Some or all of the terms in the boolean expression are undefined.
If the undefined terms had the correct values the expression would
evaluate to true. In this case the undefined terms become bound
by the expression and the assertion succeeds.
If asserting the boolean expression fails, the packet format
containing the expression fails, i.e. the packet format it belongs
to cannot be selected by the compressor.
"let" is a reserved word.
4.6 Comments
Comments do not affect the formal meaning of what is notated, but can
be used to improve readability. Their use is optional.
Free English text can be inserted into a profile definition to
explain why something has been done a particular way, to clarify the
intended meaning of the notation, or to elaborate on some point. To
this end, the two commenting styles described in the subsections
below can be used.
Comments may help provide clarifications to the reader, and serve
different purposes to implementers. Comments should thus not be
considered of lesser importance when inserting then into the formal
definition of a profile; these should be consistent with the
normative part of the profile.
4.6.1 End of line comments
The end of line comment style makes use of the "%" comment character.
Any text between the "%" character and the end of the line has no
formal meaning. For example:
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%-----------------------------------------------------------------
% IR-REPLICATE packet formats
%-----------------------------------------------------------------
% The following fields are included in all of the IR-REPLICATE
% packet formats:
%
uc_format = discriminator, % [ 8 ] bits
tcp.seq_number, % [ 32 ] bits
tcp.flags.ecn, % [ 2 ] bits
4.6.2 Block comments
The block comment style makes use of the "/*" and "*/" delimiters.
Any text between the "/*" and the "*/" has no formal meaning. For
example:
/******************************************************************
* IR-REPLICATE packet formats
*****************************************************************/
/* The following fields are included in all of the IR-REPLICATE
* packet formats:
*/
uc_format = discriminator, /* 8 bits */
tcp.seq_number, /* 32 bits */
tcp.flags.ecn, /* 2 bits */
The block comment style allows comments to be nested, unlike some
programming languages such as C, C++ or Java.
4.7 Library Encoding Methods
ROHC (RFC 3095 [4]) contains a number of different techniques for
compressing header fields (LSB encoding, value encoding, etc.). Most
of these techniques are part of the ROHC-FN library so that they can
be reused when creating new ROHC profiles. The notation for these is
described below. Encoding methods can be defined using structures
(see section Section 4.9). It is also possible for a profile to
define its own set of encoding methods using the formal notation or
using a textual definition.
4.7.1 uncompressed_value
The "uncompressed_value" encoding method is used to encode header
fields for which the uncompressed value can be defined using a
mathematical expression (including constant values):
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field ::= uncompressed_value(<uncomp_length_expression>,
<uncomp_value_expression>);
where the value of the "uncomp_length_expression" binds with the
field's "uncomp_length" attribute, and the value of the
"uncomp_value_expression" binds with the field's "uncomp_value"
attribute. The "comp_length" attribute is bound to zero since the
field does not appear in the compressed header. Note however that it
is still legal to refer to it in a compressed format field order
list, but it has a length of zero. The "comp_value" attribute is not
bound by this encoding method.
As an example of the usage of "uncompressed_value" encoding, the IPv6
header version number is a four bit field that always has the value
6:
version ::= uncompressed_value (4, 6);
Another example of value encoding, using an expression to calculate
the length:
padding ::= uncompressed_value(nbits - 8, 0);
4.7.2 compressed_value
The "compressed_value" encoding method is used to define fields in
the compressed header for which there is no counter-part in the
uncompressed header. It can be used to set compressed fields whose
value can be defined using a mathematical expression (including
constant values):
field ::= compressed_value(<comp_length_expression>,
<comp_value_expression>);
where the value of the "comp_length_expression" binds with the
field's "comp_length" attribute, and the value of the
"comp_value_expression" binds with the field's "comp_value"
attribute. The "uncomp_length" attribute is bound to zero since the
field does not appear in the uncompressed header. Note however that
it is still legal to refer to it in an uncompressed format field
order list, but it has a length of zero. The "uncomp_value"
attribute is not bound by this encoding method.
One possible use of this encoding method is to define padding in the
compressed header:
pad_to_octet_boundary ::= compressed_value (3, 0);
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A more common use is to define a discriminator field to make it
possible to differentiate between different packet formats within a
structure. For convenience, the notation provides syntax for
specifying value encoding in the form of a binary string. The binary
string to be encoded is simply given in single quotes. For example:
discriminator ::= '01101';
This has exactly the same meaning as:
discriminator ::= compressed_value(5, 13);
4.7.3 irregular
The "irregular" encoding method is used to encode a field in the
compressed packet with a bit pattern identical to the original field
in the uncompressed packet. e.g.
field ::= irregular (<expression>);
where the value of "expression" binds with the "uncomp_length"
attribute of the field.
For example, the checksum field of the TCP header is a sixteen bits
field that does not follow any pattern:
tcp_checksum ::= irregular (16);
The expression can be used to derive the length of the field from the
value of another field, and the length does not have to be constant.
4.7.4 static
The "static" encoding method compresses a field whose length and
value are the same as for a previous header in the flow, i.e. where
the field completely matches an existing entry in the context:
field ::= static;
The field's "uncomp_value" and "uncomp_length" attributes bind with
their respective values in the context.
Since the field value is the same as a previous field value, the
entire field can be reconstructed from the context, so it is
compressed to zero bits and does not appear in the compressed header.
For example, the source port of the TCP header is a field whose value
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does not change from one packet to the next for a given flow:
src_port ::= static;
4.7.5 lsb
The Least Significant Bit encoding method, "lsb", compresses a field
whose value differs by a small amount from the value stored in the
context.
field ::= lsb (num_lsbs_param, offset_param);
Here, "num_lsbs_param" is the number of least significant bits to
use, and "offset_param" is the interpretation interval offset. The
parameter "num_lsbs_param" binds with the "comp_length" attribute,
and the "uncomp_value" attribute binds with (context_value -
offset_param + comp_value).
The "lsb" encoding method can compress a field whose value lies
between (context_value - offset_param) and (context _value -
offset_param + 2^num_lsbs_param - 1) inclusively. In particular, if
offset_param = 0 then the field value can only stay the same or
increase relative to the previous header in the flow. If
offset_param = -1 then it can only increase, whereas if offset_param
= 2^num_lsbs_param then it can only decrease.
The compressor may not be capable to determine the exact context
value that will be used by the decompressor, since some packets that
would have updated the context may have been lost or damaged.
However, from feedback received or by making assumptions, the
compressor can limit the candidate set of values. The compressor
then chooses an encoding such that no matter which context value in
the candidate set the decompressor uses, the resulting decompression
is correct. If that is not possible, the lsb encoding method fails
(which typically results in a less efficient packet format being
chosen by the compressor). As "reasonable" assumptions may not
always be correct, lsb encoding is intended to be used in conjunction
with methods that validate the output of the decompression process,
such as the crc method described in Section 4.7.6.
The compressed field takes up the specified number of bits in the
compressed header (i.e. num_lsbs_param).
For example, the tcp sequence number:
tcp_sequence_number ::= lsb (14, 8192);
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See the ROHC specification (RFC 3095 [4]) for additional details on
LSB encoding, where the parameter "k" corresponds to the parameter
"num_lsbs_param" and where interpretation interval offset "p"
corresponds to the parameter "offset_param".
4.7.6 crc
The "crc" encoding method provides a CRC calculated over a block of
data. The block of data is represented using either the
"uncomp_value" or "comp_value" attribute of a field. The "crc"
method takes a number of parameters:
o the number of bits for the CRC (crc_bits),
o the bit-pattern for the polynomial (bit_pattern),
o the initial value for the CRC register (initial_value),
o the value of the block of data (block_data_value); and
o the size in octets of the block of data (block_data_length).
I.e.:
field ::= crc (num_bits, bit_pattern, initial_value,
block_data_value, block_data_length);
The CRC is calculated in least significant bit (LSB) order.
The following CRC polynomials are defined in RFC 3095 [4], in
Sections 5.9.1 and 5.9.2:
8-bit
C(x) = x^0 + x^1 + x^2 + x^8
bit_pattern = 0xe0
7-bit
C(x) = x^0 + x^1 + x^2 + x^3 + x^6 + x^7
bit_pattern = 0x79
3-bit
C(x) = x^0 + x^1 + x^3
bit_pattern = 0x06
For example:
% 3 bit CRC, C(x) = x^0 + x^1 + x^3
crc_field ::= crc(3, 0x6, 0xF, this:comp_value, this:comp_length);
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4.8 Profile-specific Encoding Methods
The library of encoding methods provides a basic and a generic set of
field encoding methods. Some additional encodings specific to a
particular protocol may however be needed, such as for methods that
infer the value of a field from other values. These methods are
defined based on the properties of the protocol being compressed.
Profiles may define additional encoding methods; the scope of these
methods is then local to the profile definition itself, and they can
be used as part of the formal definition of the profile as any other
methods from the library (see section Section 4.7).
Profile-specific encoding methods must be rigorously defined using
either the ROHC-FN syntax or in plain text, as long as its definition
provides enough information to unambiguously implement the encoding
method in the compressor and the decompressor. These methods should
be no less complete than the methods provided herein.
4.9 Structures
Structures are used for defining new encoding methods in a formal
specification. They compose groups of individual fields into
contiguous blocks. Structures can be thought of as compound encoding
methods; they have names and may have parameters and can be used in
the same way as any other encoding method. Since structures can
contain references to other structures, complicated headers can be
broken down into manageable pieces.
This section describes the various features of structures, starting
out with the simplest.
4.9.1 "this"
Within a structure it is possible to refer to the field it is
encoding, using the keyword "this". This is useful for gaining
access to the attributes of the field being encoded. For example it
is often useful to know the total uncompressed length of the header
which is being encoded.
4.9.2 Simple Structures
A structure can be used to specify a single fixed encoding. This is
its simplest form. For example:
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compound_encoding_method ===
{
uc_format = field_1, % [ 4 ]
field_2; % [ 12 ]
co_format = field_2, % [ 0 ]
field_1 % [ 4 ]
{
field_1 ::= irregular (4);
field_2 ::= uncompressed_value (12, 9);
};
};
The above begins with the structure's identifier,
"compound_encoding_method". The identifier is followed by "===",
which indicates that this is a structure definition. The definition
of the structure then follows inside curly braces, "{" and "}". The
first item in the definition is the "uc_format" field order list,
which gives the order of the fields in the uncompressed header. This
is followed by the compressed header field order list. This list is
in turn followed by the field encodings list for the compressed
header, which gives the encoding method for each field. The
different components of this example are described in more detail
below.
4.9.2.1 Uncompressed Format
The uncompressed field order list is defined by "uc_format", which
specifies the fields of the uncompressed header in the order that
appear in the uncompressed header. In the example, this is "field_1"
followed by "field_2". This means that a field being encoded by this
structure is divided into two subfields, "field_1" and "field_2".
The total uncompressed lengths of these two fields therefore equals
the length of the field being encoded. Formally:
field_1:uncomp_length + field_2:uncomp_length == this:uncomp_length
In the example we have just two fields but any number of subfields
may be used. This relationship applies to however many fields are
actually used.Note that the arrangement of fields specified in the
uncompressed field order list is up to the notator. Any arrangement
of fields that correctly describes the content of the uncompressed
header may be chosen -- this need not be the same as the one
described in the specifications for the protocol header being
compressed. However, the bits of the uncompressed format must remain
in the same order.
For example, there may be a protocol whose header contains a 16 bits
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sequence number, but whose sessions tend to be short lived. This
would mean that the high bits of the sequence number are almost
always constant. The "uc_format" could reflect this by splitting the
original uncompressed field into two fields, one field to represent
the almost-always-zero part of the sequence number, and a second
field to represent the significant part.
An uncompressed format may contain a field encodings list. Encoding
methods specified therein are used whenever a packet with that
uncompressed format is being encoded. The encoding of a packet with
a given uncompressed format can only succeed if all of its encoding
methods and let statements succeed (see Section 4.5).
The total length of an uncompressed header must be defined. The
length of each of the fields in an uncompressed header must also be
defined. This means that the bindings in the "uc_format",
"co_format" and "default_methods" (see below) field encodings lists
must between them define the "uncomp_length" attribute of evey field
in an uncompressed header so that there is an unambiguous mapping
from the bits in the uncompressed header to the fields listed in each
"uc_format" field order list.
4.9.2.2 Compressed Format
Similar to the uncompressed field order list, the compressed data
will appear in the order specified by the compressed field order list
given for a compressed format. Each individual field is encoded in
the manner given for that field in the field encodings list, which is
in braces and follows immediately after the compressed field order
list. The total length of the compressed data will be the total of
the compressed lengths of all the individual fields. The annotation
for these fields indicates that they are zero and 4 bits long, making
a total of 4 bits.
Note that the order of the fields specified in a compressed format
field order list, does not have to match the order they appear in the
"uc_format" field order list. It may be desirable to reorder the
fields in the compressed header for alignment the compressed header
to the octet boundary, or for other reasons. In the above example,
the order is in fact the opposite of that in the uncompressed header.
The field encodings list specifies that the encoding for "field_1",
is "irregular", which takes up four bits in both the compressed
header and uncompressed header. The encoding for "field_2" is
"uncompressed_value", which means that the field has a fixed value,
so it can be compressed to zero bits. The value it takes is 9, and
it is 12 bits wide in the uncompressed header.
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Fields like "field_2", which compress to zero bits in length, may be
omitted from the compressed field order list. This is because their
position in the list is not significant. So, without changing the
meaning, the above example could be notated as follows:
compound_encoding_method ===
{
uc_format = field_1, % [ 4 ]
field_2; % [ 12 ]
co_format = field_1 % [ 4 ]
{
field_1 ::= irregular (4);
field_2 ::= uncompressed_value (12, 9);
};
};
The total length of a compressed header must be defined. The length
of each of the fields in a compressed header must also be defined.
This means that the bindings in the "uc_format", "co_format" and
"default_methods" (see below) field encodings lists must between them
define the "comp_length" attribute of evey field in a compressed
header so that there is an unambiguous mapping from the bits in the
compressed header to the fields listed in each "co_format" field
order list.
4.9.3 Arguments and Structures
Structures may take arguments, which have some control over the
mapping between compressed and uncompressed fields. These are
specified immediately after the structure name, in parentheses, as a
comma separated list. For example:
poor_mans_lsb(variable_length) ===
{
uc_format = constant_bits,
variable_bits;
co_format = variable_bits
{
constant_bits ::= static;
variable_bits ::= irregular(variable_length);
};
};
As with any encoding method, all arguments are values, rather than
fields. Although entire fields cannot be passed as arguments, it is
possible to pass their attributes instead.
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4.9.4 Multiple Formats
Structures can also define multiple formats for a given header. This
allows different compression methods to be used depending on what is
the most efficient way of compressing a particular header.
For example, a field may have a fixed value most of the time, but the
fixed value may occasionally change. Using a single format for the
structure, this field would have to be encoded using "irregular" (see
Section 4.7.3), even though the value only changes rarely. However,
by using the structure to define multiple formats, we can provide two
alternative encodings; one for when the value remains fixed and
another for when the value changes.
This is the topic of the following sub-sections.
4.9.4.1 Naming Convention
When compressed formats are defined, they must be defined using names
beginning with the reserved prefix "co_format". Similarly
uncompressed formats must be defined using names beginning with
"uc_format".
Format names must be unique within the structure to which they
belong.
4.9.4.2 Format Discrimination
Each of the compressed formats has its own field order list and field
encodings list. A compressor may pick any of these alternative
formats to compress a header, as long the field encodings it employs
can be used with the uncompressed header. For example, the
compressor could not choose to use a compressed format that had a
"static" encoding for a field whose value had just changed.
More formally, the compressor can choose any combination of an
uncompressed format and a compressed format for which all fields
"succeed", i.e. the encoding methods and let-statements succeed (see
Section 4.5). If there are multiple successful combinations, the
compressor can choose any one. Otherwise if there is no successful
combination, the encoding method defined by the structure "fails".
Because the compressor has a choice, it must be possible for the
decompressor to discriminate between the different packet formats. A
simple approach to this problem is for each compressed format to
include a "discriminator" that uniquely identifies that particular
"co_format". A discriminator is a control field; it is not derived
from any of the uncompressed field values (see Section 4.7.2).
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4.9.4.3 Default Encoding Methods - default_methods
When using multiple packet formats, default bindings may be specified
for each field or attribute. The default encoding methods specify
the encoding method to use for a field if no encoding method for that
field elsewhere. This is helpful to keep the definition of the
packet formats concise, as the same encoding method need not be
repeated for every format.
The syntax for specifying default bindings is similar to that used to
specify a compressed or uncompressed format. However there is no
field order list for the default encoding methods, only the field
encodings list is given. The field order is specified individually
for each "co_format" and "uc_format". For example:
default_methods =
{
field_1 ::= uncompressed_value (4,1);
field_2 ::= uncompressed_value (4,2);
field_3 ::= lsb(3,-1);
let(field_4:uncomp_length == 4);
};
Fields for which there is a default encoding method do not need to be
specified in the field encodings list of any format that uses the
default encoding method for that field. The default encoding method
for a field may be overridden by specifying explicitly an encoding
method for that field. If a default encoding method is not
overridden, and that encoding method always compresses the field down
to zero bits, then the field can also be omitted from the compressed
format field order list, since, like any other zero bit field, its
position in the field order list is not significant.
The field encodings list of default_methods may also contain default
bindings for individual attributes by using "let" statements. If a
default binding is given for an individual attribute, that binding
may be overridden by another binding for that attribute or the field
to which it belongs. The overriding binding may either be another
let statement, or an encoding method.
It is allowed to override one default binding but still use another.
Overriding one default binding does not imply that other default
bindings are also being overridden. It is also allowed to supply
default bindings for some but not all fields.
Note that any and all default methods can be overridden. Therefore
to notate that a "let" statement or encoding method must be applied
to every compressed format of a structure, the "uc_format" field
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encodings list(s) should be used. "uc_format" field encodings lists
can not be overridden.
4.9.4.4 Example of Multiple Formats
Putting this altogether, here is a complete example of a structure
with multiple compressed formats:
test_multiple_formats ===
{
uc_format = field_1, % [ 4 ]
field_2, % [ 4 ]
field_3; % [ 24 ]
default_methods =
{
field_1 ::= static;
field_2 ::= uncompressed_value(4, 2);
field_3 ::= lsb(4, 0);
};
co_format_0 = discriminator, % [ 1 ]
field_3 % [ 4 ]
{
discriminator ::= '0';
};
co_format_1 = discriminator, % [ 1 ]
field_1, % [ 4 ]
field_3 % [ 24 ]
{
discriminator ::= '1';
field_1 ::= irregular(4);
field_3 ::= irregular(24);
};
};
Note the following:
o "field_1" and "field_3" both have default encoding methods
specified for them, which are used in "format_0", but is
overridden in "format_1"; "field_2" however is not overridden.
o "field_1" and "field_2" have default encoding methods which
compress to zero bits. When these are used in "co_format_0", the
field names do not appear in either the field order list or in the
field encodings list.
o "field_3" has an encoding method which does not compress to zero
bits, so whilst "field_3" is absent from the field encoding list
of "format_0"', it still needs to appear in the field order list
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to specify whereabouts it goes in the compressed packet.
o in the example, all the uncompressed header fields have default
encoding methods specified for them, but this is not a
requirement. It is perfectly allowable to only specify default
encodings for some or even none of the uncompressed header fields.
o in the example all the default encoding methods are on fields from
the uncompressed header, but this is not a requirement. It is
perfectly allowable to specify default encoding methods for
control fields.
4.9.5 Control Fields
Control fields are defined using the "control_fields" list, which
specifies any fields that do not appear in the uncompressed header
but which have an uncompressed value (specifically those with a
non-zero uncomp_length). Such fields may be used to help compress
fields from the uncompressed header more efficiently. A control
field could be used to improve efficiency by representing some
commonality between a number of the uncompressed fields, or by
representing some information about the flow that is not explicitly
contained in the protocol headers.
For example in IP, the behaviour of the IP ID field in a flow varies
depending on how the endpoints handle IP IDs. Sometimes the
behaviour is effectively random, sometimes the IP ID follows a
predictable sequence, and at other times it stays fixed at zero.
This information is never communicated explicitly in the uncompressed
header, but to compress the field efficiently, its behaviour must be
communicated somehow. A control field is used:
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ipv4 ===
{
uc_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 ]
:
:
};
The control_fields list is equivalent to the "uc_format" field order
list for fields that do not appear in the uncompressed header, that
is it defines a field that has the same properties (the same
attributes etc) as fields appearing in the uncompressed header.
Control fields are initialised by using the appropriate encoding
methods and/or by using let statements. For example this control
field is used to scale down a field in the uncompressed header by a
factor of 8 before encoding it with LSB encoding. Scaling it down
makes the LSB encoding more efficient.
example_struct() ===
{
uc_format = field_1;
control_fields = ctrl_field;
format = ctrl_field
{
let(ctrl_field:uncomp_value == field_1:uncomp_value / 8);
let(ctrl_field:uncomp_length == field_1:length - 3);
ctrl_field ::= lsb(4, 0);
};
};
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Control fields may also be used with global scope. In this case
their declaration must be outside of any structure. They are then
visible within any structure.
5. Security considerations
This draft describes a formal notation similar to ABNF RFC 2234 [3],
and hence is not believed to raise any security issues.
6. Contributors
Although no longer listed as an author, Richard Price did almost all
of the foundational work on the formal notation and also produced the
original formal notation internet draft on which this document is
based. Many thanks to him for doing that groundwork on which this
document stands.
7. Acknowledgements
A number of important concepts and ideas have been borrowed from ROHC
RFC 3095 [4].
Thanks to Mark West, Eilert Brinkmann and particularly Kristofer
Sandlund for their cooperation and feedback from notating the TCP
profile, and also for their review comments.
Thanks to Rob Hancock and Stephen McCann for early work on the formal
notation. The authors would also like to thank Christian Schmidt,
Qian Zhang, Hongbin Liao, Max Riegel and Lars-Erik Jonsson for their
comments and valuable input.
Finally thanks to Caroline Daniels and Alan Finney for doing some
excellent last minute review work.
8. References
[1] Bradner, S., "The Internet Standards Process -- Revision 3",
BCP 9, RFC 2026, October 1996.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Crocker, D. and P. Overall, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[4] 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.,
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Yoshimura, T. and H. Zheng, "RObust Header Compression (ROHC):
Framework and four profiles: RTP, UDP, ESP, and uncompressed",
RFC 3095, July 2001.
Authors' Addresses
Robert Finking
Siemens/Roke Manor
Roke Manor Research Ltd.
Romsey, Hampshire SO51 0ZN
UK
Phone: +44 (0)1794 833189
Email: robert.finking@roke.co.uk
URI: http://www.roke.co.uk
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28334
Germany
Phone: +49 421 218 7024
Fax: +49 421 218 7000
Email: cabo@tzi.org
Ghyslain Pelletier
Ericsson AB
Box 920
LuleÈÑ SE-971 28
Sweden
Phone: +46 (0) 8 404 29 43
Email: ghyslain.pelletier@ericsson.com
Appendix A. Syntax
This section gives a formal definition of the ROHC-FN syntax in ABNF
(see RFC 2234 [3]).
A.1 Reserved Keywords
Some keywords are defined and reserved in ROHC-FN. These keywords
cannot be reused as identifiers by the notator.
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o co_format - struct syntax
o comp_hdr_start - attribute
o comp_length - attribute
o comp_value - attribute
o compressed_value - primitive encoding method
o default_methods - struct syntax
o irregular - primitive encoding method
o let - primitive encoding method
o lsb - primitive encoding method
o static - primitive encoding method
o uc_format - struct syntax
o uncomp_hdr_start - attribute
o uncomp_length - attribute
o uncomp_value - attribute
o uncompressed_value - primitive encoding method
reserved_word ::= primitive_encoding_method_name |
attribute_identifier | struct_reserved_words
A.2 Characters
Because ABNF [3] symbols are case insensitive, it is necessary to
define explicit symbols for each of the lower case characters which
we use in the reserved words of our grammar. Fortunately there are
no fundamental components of the FN syntax which are in upper case,
otherwise we would have to define each capital letter separately
also.
a = %x61
b = %x62
c = %x63
d = %x64
e = %x65
f = %x66
g = %x67
h = %x68
i = %x69
j = %x6a
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k = %x6b
l = %x6c
m = %x6d
n = %x6e
o = %x6f
p = %x70
q = %x71
r = %x72
s = %x73
t = %x74
u = %x75
v = %x76
w = %x77
x = %x78
y = %x79
z = %x7a
lower-case-letter = %x61-7a ; a-z
upper-case-letter = %x41-5a ; A-Z
binary-digit = "0" / "1"
octal-digit = binary-digit / "2" / "3" / "4" / "5" / "6" / "7"
decimal-digit = octal-digit / "8" / "9"
hexadecimal-digit = decimal-digit / %x61-66
open-bracket = "("
close-bracket = ")"
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open-brace = "{"
close-brace = "}"
equals-sign = "="
underscore = "_"
comma = ","
semi-colon = ";"
single-quote = "'"
A.3 Literals
decimal-literal = 1*decimal-digit
binary-literal = "0".b 1*binary-digit
octal-literal = "0".o 1*octal-digit
hexadecimal-literal = "0".x 1*hexadecimal-digit
numeric-literal = decimal-literal / binary-literal / octal-literal /
hexadecimal-literal
boolean-literal = t.r.u.e / f.a.l.s.e
A.4 Identifiers
lower-case-identifier = (lower-case-letter *(lower-case-letter /
decimal-digit / underscore)) ; The original EBNF had "-
reserved-word" here, meaning "except reserved words", but ABNF has no
equivalent construct. Notwithstanding this fact, any automated tool
should enforce the reservation of reserved words in this fashion.
upper-case-identifier = upper-case-letter *(upper-case-letter /
decimal-digit / underscore)
A.5 Operators
exponential-operator = "^"
multiplicative-operator = "*" / "/"
additive-operator = "+" / "-"
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unary-minus = "-"
A.6 Expressions
parenthesised-expression = open-bracket arithmetic-expression
close-bracket
primitive-expression = numeric-literal / constant-name /
field-attribute / parenthesised-expression / (unary-minus
primitive-expression)
exponential-expression = primitive-expression *(exponential-operator
primitive-expression)
multiplicative-expression = exponential-expression
*(multiplicative-operator exponential-expression)
additive-expression = multiplicative-expression *(additive-operator
multiplicative-expression)
arithmetic-expression = additive-expression
A.7 Constants
constant-name = upper-case-identifier
constant-value = constant-name / expression
constant-definition = constant-name equals-sign constant-value
A.8 Field Names
field-name = lower-case-identifier
annotated-field-name = field-name [ "[" constant "]" ]
A.9 Attributes
attribute-category = (c.o.m.p) / (u.n.c.o.m.p)
attribute-name = (l.e.n.g.t.h) / (v.a.l.u.e) /
(h.d.r.underscore.s.t.a.r.t)
attribute-identifier = attribute-category underscore attribute-name
field-attribute = field-name ":" attribute-identifier
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A.10 Encoding Methods
primitive-encoding-method-name =
(c.o.m.p.r.e.s.s.e.d.underscore.v.a.l.u.e) / (i.r.r.e.g.u.l.a.r) /
(l.s.b) / (s.t.a.t.i.c) /
(u.n.c.o.m.p.r.e.s.s.e.d.underscore.v.a.l.u.e)
uncompressed-value-shorthand = single-quote *binary-digit
single-quote
external-encoding-method-name = underscore lower-case-identifier
non-primitive-encoding-method-name = structure-name /
external-encoding-method-name
encoding-method-parameter-list = open-bracket arithmetic-expression
*(comma arithmetic-expression) close-bracket
encoding-method = uncompressed-value-shorthand /
(encoding-method-name [encoding-method-parameter-list])
field-encoding = field-name "::=" encoding-method
A.11 Structures
structure-name = lower-case-identifier
field-order-list = [ annotated-field-name *(comma
annotated-field-name) ]
field-encodings-list = open-brace *(field-encoding semi-colon)
close-brace
uncompressed-format-prefix =
(u.n.c.o.m.p.r.e.s.s.e.d.underscore.f.o.r.m.a.t)
uncompressed-format = uncompressed-format-prefix [underscore
lower-case-identifier] equals-sign field-order-list; semi-colon
compressed-format-prefix =
(c.o.m.p.r.e.s.s.e.d.underscore.f.o.r.m.a.t)
compressed-format = compressed-format-prefix [underscore
lower-case-identifier] equals-sign field-order-list
field-encodings-list semi-colon
default-methods-id ::= (d.e.f.a.u.l.t.underscore.m.e.t.h.o.d.s)
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default-methods = default-methods-id equals-sign field-encodings-list
semi-colon
uncompressed-format-list = *uncompressed-format
compressed-format-list = 1*compressed-format
structure-body = open-brace uncompressed-format-list
[default-methods] compressed-format-list close-brace
structure-definition = structure-name "===" structure-body semi-colon
struct-reserved-words = uncompressed-format-prefix /
compressed-format-prefix / default-methods-id;
Appendix B. Bit-level Worked Example
This section gives a worked example at the bit level, showing how a
simple profile describes the compression of real data from an
imaginary protocol header. The example used has been kept fairly
simple, whilst still aiming to illustrate some of the intricacies
that arise in use of the notation. In particular, fields have been
kept short to make it possible to read the binary representation of
the headers by eye, without too much difficulty.
B.1 Example Packet Format
Our imaginary header is just 16 bits long, and consists of the
following fields:
1. version number - 2 bits
2. type - 2 bits
3. flow id - 4 bits
4. sequence number - 4 bits
5. flag bits - 4 bits
So for example 0101000100010000 indicates a packet with a version
number of one, a type of one, a flow id of one, a sequence number of
one, and all flag bits set to zero.
B.2 Initial Encoding
An initial definition based solely on the above information is:
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eg_header ===
{
uc_format = version_no, % [ 2 ]
type, % [ 2 ]
flow_id, % [ 4 ]
sequence_no, % [ 4 ]
flag_bits; % [ 4 ]
co_format_initial = version_no, % [ 2 ]
type, % [ 2 ]
flow_id, % [ 4 ]
sequence_no, % [ 4 ]
flag_bits % [ 4 ]
{
version_no ::= irregular(2);
type ::= irregular(2);
flow_id ::= irregular(4);
sequence_no ::= irregular(4);
flag_bits ::= irregular(4);
};
};
This defines the packet format nicely, but doesn't actually offer any
compression. If we use it to encode the above header, we get:
Uncompressed header: 0101000100010000
Compressed header: 0101000100010000
This is because we have stated that all fields are irregular - i.e.
we haven't specified anything about their behaviour.
B.3 Basic Compression
In order to achieve any compression we need to notate more knowledge
about the header and it's behaviour in a flow. For example, we may
know the following facts about the header:
1. version number - indicates which version of the protocol this is,
always one for this version of the protocol
2. type - may take any value.
3. flow id - may take any value.
4. sequence number - make take any value
5. flag bits - contains three flags, a, b and c, each of which may
be set or clear, and a reserved flag bit, which is always clear
(i.e. zero).
We could notate this knowledge as follows:
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eg_header ===
{
uc_format = version_no, % [ 2 ]
type, % [ 2 ]
flow_id, % [ 4 ]
sequence_no, % [ 4 ]
abc_flag_bits, % [ 3 ]
reserved_flag; % [ 1 ]
co_format_basic = version_no, % [ 0 ]
type, % [ 2 ]
flow_id, % [ 4 ]
sequence_no, % [ 4 ]
abc_flag_bits, % [ 3 ]
reserved_flag % [ 0 ]
{
version_no ::= uncompressed_value(2,1);
type ::= irregular(2);
flow_id ::= irregular(4);
sequence_no ::= irregular(4);
abc_flag_bits ::= irregular(3);
reserved_flag ::= uncompressed_value(1,0);
};
};
Using this simple scheme, we have successfully encoded the fact that
one of the fields has a permanently fixed value of one, and therefore
contains no useful information. We have also encoded the fact that
the final flag bit is always zero, which again contains no useful
information. Both of these facts have been notated using the
uncompressed_value encoding method (see Section 4.7.1)
Note that we could have omitted the "0 bits" fields from the
definition of the compressed_data if we wished, since the only
purpose of that list is to indicate the order in the compressed
header; zero bit fields don't actually appear and so can be omitted
from the field order list.
Using this new encoding on the above header, we get:
Uncompressed header: 0101000100010000
Compressed header: 0100010001000
Which reduces the amount of data we need to transmit by roughly 20%.
However, this encoding fails to take advantage of relationships
between values of a field in one packet and its value in subsequent
packets. For example, every header in the following sequence is
compressed by the same amount despite the similarities between them:
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Uncompressed header: 0101000100010000
Compressed header: 0100010001000
Uncompressed header: 0101000101000000
Compressed header: 0100010100000
Uncompressed header: 0111000101110000
Compressed header: 1100010111000
B.4 Inter-packet compression
The profile we have defined so far has not compressed the sequence
number or flow ID fields at all, since they can take any value.
However the value of these fields in one header has a very simple
relationship to their value in previous headers:
o the sequence number is unusual, it increases by three each time,
o the flow_id stays the same, it always has the same value that it
did in the previous header in the flow,
o the abc_flag_bits stay the same most of the time, they usually
have the same value that they did in the previous header in the
flow,
An obvious way of notating this is as follows:
% This obvious encoding will not work (correct encoding below)
eg_header ===
{
uc_format = version_no, % [ 2 ]
type, % [ 2 ]
flow_id, % [ 4 ]
sequence_no, % [ 4 ]
abc_flag_bits, % [ 3 ]
reserved_flag; % [ 1 ]
co_format_obvious = type, % [ 2 ]
abc_flag_bits % [ 3 ]
{
version_no ::= uncompressed_value(2,1);
type ::= irregular(2);
flow_id ::= static;
sequence_no ::= lsb(0,-3);
abc_flag_bits ::= irregular(3);
reserved_flag ::= uncompressed_value(1,0);
};
};
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This dependency on previous packets is notated using the static and
LSB encoding methods (see Section 4.7.4 and Section 4.7.5
respectively).
However there are a few problems with the above notation. Firstly,
and most importantly, the flow_id field is notated as "static" which
means that it doesn't change from packet to packet. However, the
notation does not indicate how to communicate the value of the field
initially. It's all very well saying "it's the same value as last
time", but there must have been a first time where we define what
that value is, so that it can be referred back to. The above
notation provides no way of communicating that. Similarly with the
sequence number - there needs to be a way of communicating its
initial value.
Secondly, the sequence number field is communicated very efficiently
in zero bits, but it is not at all robust against packet loss. If a
packet is lost then there is no way to handle the missing sequence
number.
Finally, although the flag bits are usually the same as in the
previous header in the flow, the profile doesn't make any use of this
fact; since they are sometimes not the same as those in the previous
header, it is not safe to say that they are always the same, so
static encoding can't be used solely. We solve all three of these
problems below, robustness first, since it is simplest.
When communicating sequence numbers, or any other field encoding with
LSB encoding, a very important consideration for the notator is how
robust against packet loss the compressed protocol should be. This
will vary a lot from protocol stack to protocol stack. For example
RTP has a high setup cost, so the compressed stream needs to be
robust against fairly high packet loss. Things are different with
TCP, where robustness to loss of just a few packets is sufficient.
For the example protocol we'll assume short, low overhead flows and
say we need to be robust to the loss of just one packet, which we can
achieve with two bits of LSB encoding (one bit isn't enough since the
sequence number increases by three each time - see Section 4.7.5 ).
To communicate initial values for the sequence number and flow ID
fields, and to take advantage of the fact that the flag bits are
usually the same as in the previous header, we need to depart from
the single packet format encoding we are currently using and instead
use multiple packet formats:
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eg_header ===
{
uc_format = version_no, % [ 2 ]
type, % [ 2 ]
flow_id, % [ 4 ]
sequence_no, % [ 4 ]
abc_flag_bits, % [ 3 ]
reserved_flag; % [ 1 ]
co_format_irregular = discriminator, % [ 1 ]
type, % [ 2 ]
flow_id, % [ 4 ]
sequence_no, % [ 4 ]
abc_flag_bits % [ 3 ]
{
discriminator ::= '0';
version_no ::= uncompressed_value(2,1);
type ::= irregular(2);
flow_id ::= irregular(4);
sequence_no ::= irregular(4);
abc_flag_bits ::= irregular(3);
reserved_flag ::= uncompressed_value(1,0);
};
co_format_compressed = discriminator, % [ 1 ]
type, % [ 2 ]
sequence_no % [ 2 ]
{
discriminator ::= '1';
version_no ::= uncompressed_value(2,1);
type ::= irregular(2);
flow_id ::= static;
sequence_no ::= lsb(2,-3);
abc_flag_bits ::= static;
reserved_flag ::= uncompressed_value(1,0);
};
};
Note that we have had to add a discriminator field, so that the
decompressor can tell which packet format has been used by the
compressor. The format with a static flow ID and LSB encoded
sequence number, is now 5 bits long, a saving of over 60% on the size
of the single packet format, almost a 70% saving on the size of the
uncompressed header. Note that despite having to add the
discriminator field, this format is still the same size as the
original incorrect naive notation, because this notation takes
advantage of the fact that the abc flag bits rarely change.
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However, the original packet format (with an irregular flow ID and
sequence number) has also grown by one bit due to the addition of the
discriminator. An important consideration when creating multiple
packet formats is whether the extra format occurs frequently enough
that the average compressed header length is shorter as a result.
For example, if in fact the flag bits always changed between packet
headers, the static encoding could never be used; all we would have
achieved is to lengthen the irregular packet format by one bit.
Using the above notation, we now get:
Uncompressed header: 0101000100010000
Compressed header: 00100010001000
Uncompressed header: 0101000101000000
Compressed header: 10100 ; 00100010100000
Uncompressed header: 0111000101110000
Compressed header: 11100 ; 01100010111000
The first header in the stream is compressed the same way as before,
except that it now has the extra 1 bit discriminator at the start
(0). When a second header arrives, with the same flow ID as the
first and its sequence number three higher, it can now be compressed
in two possible ways, either using co_format_compressed or in the
same way as previously, using co_format_irregular.
Note that we show all possible encodings of a packet as defined by a
given profile, separated by semi-colons. Either of the above
encodings for the packet could be produced by a valid implementation,
although a good implementation would always aim to make the
compressed size as small as possible and an optimum implementation
would pick the encoding which led to the best compression of the
packet stream (which is not necessarily the smallest encoding for a
particular packet).
B.5 Variable Length Discriminators
Suppose we do some analysis on flows of our example protocol and
discover that whilst it is usual for successive packets to have the
same flags, on the occasions when they don't, the packet is almost
always a "flags set" packet, in which all three of the abc flags are
set. To encode the flow more efficiently a packet format needs to be
written to reflect this.
This now gives a total of three packet formats, which means we need
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three discriminators to differentiate between them. The obvious
solution here is to increase the number of bits in the discriminator
from one to two and for example use discriminators 00, 01, and 10.
However we can do slightly better than this.
Any uniquely identifiable discriminator will suffice, so we can use
00, 01 and 1. If the discriminator starts with 1, that's the whole
thing. If it starts with 0 the decompressor knows it has to check
one more bit to determine the packet kind.
Note that care must be taken when using variable length
discriminators. For example it would be erroneous to use 0, 01 and
10 as discriminators since after reading an initial 0, the
decompressor would have no way of knowing if the next bit was a
second bit of discriminator, or the first bit of the next field in
the packet stream. 0, 10 and 11 however would be OK as the first bit
again indicates whether or not there are further discriminator bits
to follow.
This gives us the following:
eg_header ===
{
uc_format = version_no, % [ 2 ]
type, % [ 2 ]
flow_id, % [ 4 ]
sequence_no, % [ 4 ]
abc_flag_bits, % [ 3 ]
reserved_flag; % [ 1 ]
co_format_irregular = discriminator, % [ 2 ]
type, % [ 2 ]
flow_id, % [ 4 ]
sequence_no, % [ 4 ]
abc_flag_bits % [ 3 ]
{
discriminator ::= '00';
version_no ::= uncompressed_value(2,1);
type ::= irregular(2);
flow_id ::= irregular(4);
sequence_no ::= irregular(4);
abc_flag_bits ::= irregular(3);
reserved_flag ::= uncompressed_value(1,0);
};
co_format_flags_set = discriminator, % [ 2 ]
type, % [ 2 ]
sequence_no % [ 2 ]
{
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discriminator ::= '01';
version_no ::= uncompressed_value(2,1);
type ::= irregular(2);
flow_id ::= static;
sequence_no ::= lsb(2,-3);
abc_flag_bits ::= uncompressed_value(3,7);
reserved_flag ::= uncompressed_value(1,0);
};
co_format_flags_static = discriminator, % [ 1 ]
type, % [ 2 ]
sequence_no % [ 2 ]
{
discriminator ::= '1';
version_no ::= uncompressed_value(2,1);
type ::= irregular(2);
flow_id ::= static;
sequence_no ::= lsb(2,-3);
abc_flag_bits ::= static;
reserved_flag ::= uncompressed_value(1,0);
};
Here is some example output:
Uncompressed header: 0101000100010000
Compressed header: 000100010001000
Uncompressed header: 0101000101000000
Compressed header: 10100 ; 000100010100000
Uncompressed header: 0111000101110000
Compressed header: 11100 ; 001100010111000
Uncompressed header: 0111000110101110
Compressed header: 011100 ; 001100011010111
Here we have a very similar sequence to last time, except that there
is now an extra message on the end which has the flag bits set. The
encoding for the first message in the stream is now one bit larger,
the encoding for the next two messages is the same as before, since
that packet format has not grown, thanks to the use of variable
length discriminators. Finally the packet that comes through with
all the flag bits set can be encoded in just six bits, only one bit
more than the most common packet format. Without the extra packet
format, this last packet would have to be encoded using the longest
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packet format and would have taken up 14 bits. This represents a
saving of almost 60% for this kind of packet.
B.6 Default encoding
There is some redundancy in the notation used to define the profile
in that the same encoding method is used for the same fields several
times in different formats, but the field is redefined explicitly
each time. If the encoding for any of these fields changed in the
future (e.g. if the reserved flag became permanently set to 1
instead of 0), then every packet format would have to be modified to
reflect this change.
This problem can be avoided by specifying a default encoding for
these fields, which also leads to a more concisely notated profile:
eg_header ===
{
uc_format = version_no, % [ 2 ]
type, % [ 2 ]
flow_id, % [ 4 ]
sequence_no, % [ 4 ]
abc_flag_bits, % [ 3 ]
reserved_flag; % [ 1 ]
default_methods =
{
version_no ::= uncompressed_value(2,1);
type ::= irregular(2);
flow_id ::= static;
sequence_no ::= lsb(2,-3);
reserved_flag ::= uncompressed_value(1,0);
};
co_format_irregular = discriminator, % [ 2 ]
type, % [ 2 ]
flow_id, % [ 4 ]
sequence_no, % [ 4 ]
abc_flag_bits % [ 3 ]
{
discriminator ::= '00';
flow_id ::= irregular(4); % overrides default
sequence_no ::= irregular(4); % overrides default
abc_flag_bits ::= irregular(3);
};
co_format_flags_set = discriminator, % [ 2 ]
type, % [ 2 ]
sequence_no % [ 2 ]
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{
discriminator ::= '01';
abc_flag_bits ::= uncompressed_value(3,7);
};
co_format_flags_static = discriminator, % [ 1 ]
type, % [ 2 ]
sequence_no % [ 2 ]
{
discriminator ::= '1';
abc_flag_bits ::= static;
};
};
The above profile behaves in exactly the same way as the one notated
previously, since it has the same meaning. Note that the purposes
behind the different formats become clearer with the default encoding
methods factored out; all that remains are the encodings which are
relevant to each specific format. Note also that default encoding
methods which compress down to zero bits have become completely
implicit. For example none of the compressed formats mentions
"version_no" explicitly, neither the field order list (no need, it's
zero bits long) nor the field encodings list (no need it's specified
in the default encoding methods).
B.7 Control Fields
One inefficiency in the compression scheme we have produced thus far
is that it uses two bits to provide the LSB encoded sequence number
with robustness for the loss of just one packet. In theory only one
bit should be needed. The root of the problem is the unusual
sequence number that the protocol uses - it counts up in increments
of three. In order to encode it at maximum efficiency we need to
translate this into a field that increments by one each time. We do
this using a control field.
Control fields are extra data that are communicated in the compressed
packet, which are not direct encodings of fields in the uncompressed
header. They can be used to communicate extra information in the
compressed packet, which allows other fields to be compressed more
efficiently.
The control field which we introduce scales the sequence number down
by a factor of three. Instead of encoding the original sequence
number in the compressed packet, we encode the scaled sequence
number, allowing us to have robustness to the loss of one packet by
using just one bit of LSB encoding:
eg_header ===
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{
uc_format = version_no, % [ 2 ]
type, % [ 2 ]
flow_id, % [ 4 ]
sequence_no, % [ 4 ]
abc_flag_bits, % [ 3 ]
reserved_flag; % [ 1 ]
control_fields = scaled_seq_no;
default_methods =
{
version_no ::= uncompressed_value(2,1);
type ::= irregular(2);
flow_id ::= static;
reserved_flag ::= uncompressed_value(1,0);
% need modulo maths to calculate scaling correctly,
% due to 4 bit wrap around
let(scaled_seq_no:uncomp_value
== ((mod(15 - sequence_no:uncomp_value, 3) * 16
+ sequence_no:uncomp_value) / 3));
scaled_seq_no ::= lsb(1,-1);
};
co_format_irregular = discriminator, % [ 2 ]
type, % [ 2 ]
flow_id, % [ 4 ]
scaled_seq_no, % [ 4 ]
abc_flag_bits % [ 3 ]
{
discriminator ::= '00';
flow_id ::= irregular(4); % overrides default
scaled_seq_no ::= irregular(4); % overrides default
abc_flag_bits ::= irregular(3);
};
co_format_flags_set = discriminator, % [ 2 ]
type, % [ 2 ]
scaled_seq_no % [ 1 ]
{
discriminator ::= '01';
abc_flag_bits ::= uncompressed_value(3,7);
};
co_format_flags_static = discriminator, % [ 1 ]
type, % [ 2 ]
scaled_seq_no % [ 1 ]
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{
discriminator ::= '1';
abc_flag_bits ::= static;
};
};
Here is some example output:
Uncompressed header: 0101000100010000
Compressed header: 000100010001000
Uncompressed header: 0101000101000000
Compressed header: 1010 ; 000100010100000
Uncompressed header: 0111000101110000
Compressed header: 1110 ; 001100010111000
Uncompressed header: 0111000110101110
Compressed header: 01110 ; 001100011010111
In it's final form, we see that this gives us a saving of a further
bit in most packets, reducing the average size of the flow by around
20%.
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