Robust Header Compression R. Finking Internet-Draft Siemens/Roke Manor Expires: April 28, 2005 G. Pelletier Ericsson AB R. Price Cogent Defence and Security Networks October 28, 2004 Formal Notation for Robust Header Compression (ROHC-FN) draft-ietf-rohc-formal-notation-04.txt Status of this Memo This document is an Internet-Draft and is subject to all provisions of section 3 of RFC 3667. By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she become aware will be disclosed, in accordance with RFC 3668. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on April 28, 2005. Copyright Notice Copyright (C) The Internet Society (2004). Abstract This document defines ROHC-FN: a formal notation for specifying how to compress and decompress fields from an arbitrary protocol stack. ROHC-FN is intended to simplify the creation of new compression Finking, et al. Expires April 28, 2005 [Page 1] Internet-Draft ROHC-FN October 2004 profiles to fit within the ROHC (RFC3095 [4]) framework. 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 . . . . . . . . . . . . . . 11 4.1 Overall Structure of a Specification . . . . . . . . . . . 11 4.2 Constant Definitions . . . . . . . . . . . . . . . . . . . 12 4.3 Field Attributes . . . . . . . . . . . . . . . . . . . . . 12 4.4 Expressions . . . . . . . . . . . . . . . . . . . . . . . 13 4.5 Expressions: NOTE:Merge+Remove . . . . . . . . . . . . . . 15 4.6 Field References . . . . . . . . . . . . . . . . . . . . . 16 4.7 Reserved Keywords . . . . . . . . . . . . . . . . . . . . 16 4.7.1 "let" . . . . . . . . . . . . . . . . . . . . . . . . 16 4.7.2 "this" . . . . . . . . . . . . . . . . . . . . . . . . 17 4.8 Comments . . . . . . . . . . . . . . . . . . . . . . . . . 17 4.8.1 End of line comments . . . . . . . . . . . . . . . . . 17 4.8.2 Block comments . . . . . . . . . . . . . . . . . . . . 18 4.9 Encoding Methods . . . . . . . . . . . . . . . . . . . . . 18 4.9.1 uncompressed_value . . . . . . . . . . . . . . . . . . 18 4.9.2 compressed_value . . . . . . . . . . . . . . . . . . . 19 4.9.3 irregular . . . . . . . . . . . . . . . . . . . . . . 20 4.9.4 static . . . . . . . . . . . . . . . . . . . . . . . . 20 4.9.5 lsb . . . . . . . . . . . . . . . . . . . . . . . . . 20 4.9.6 crc . . . . . . . . . . . . . . . . . . . . . . . . . 21 4.10 Profile-specific Encoding Methods . . . . . . . . . . . 22 4.11 Structures . . . . . . . . . . . . . . . . . . . . . . . 23 4.11.1 Simple Structures . . . . . . . . . . . . . . . . . 23 4.11.2 Arguments and Structures . . . . . . . . . . . . . . 25 4.11.3 Multiple Formats . . . . . . . . . . . . . . . . . . 26 4.11.4 Recursive Structures . . . . . . . . . . . . . . . . 29 4.12 Lists . . . . . . . . . . . . . . . . . . . . . . . . . 30 4.12.1 Notation . . . . . . . . . . . . . . . . . . . . . . 31 4.12.2 List Encoding . . . . . . . . . . . . . . . . . . . 34 5. Security considerations . . . . . . . . . . . . . . . . . . 38 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 39 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 39 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 39 A. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 A.1 Reserved Keywords . . . . . . . . . . . . . . . . . . . . 40 A.2 Characters . . . . . . . . . . . . . . . . . . . . . . . . 41 Finking, et al. Expires April 28, 2005 [Page 2] Internet-Draft ROHC-FN October 2004 A.3 Literals . . . . . . . . . . . . . . . . . . . . . . . . . 43 A.4 Identifiers . . . . . . . . . . . . . . . . . . . . . . . 43 A.5 Opertators . . . . . . . . . . . . . . . . . . . . . . . . 43 A.6 Expressions . . . . . . . . . . . . . . . . . . . . . . . 43 A.7 Constants . . . . . . . . . . . . . . . . . . . . . . . . 44 A.8 Field Names . . . . . . . . . . . . . . . . . . . . . . . 44 A.9 Attributes . . . . . . . . . . . . . . . . . . . . . . . . 44 A.10 Encoding Methods . . . . . . . . . . . . . . . . . . . . 44 A.11 Structures . . . . . . . . . . . . . . . . . . . . . . . 45 B. Bit-level Worked Example . . . . . . . . . . . . . . . . . . 46 B.1 Example Packet Format . . . . . . . . . . . . . . . . . . 46 B.2 Initial Encoding . . . . . . . . . . . . . . . . . . . . . 46 B.3 Basic Compression . . . . . . . . . . . . . . . . . . . . 47 B.4 Inter-packet compression . . . . . . . . . . . . . . . . . 49 B.5 Variable Length Discriminators . . . . . . . . . . . . . . 52 B.6 Default encoding . . . . . . . . . . . . . . . . . . . . . 55 Intellectual Property and Copyright Statements . . . . . . . 57 Finking, et al. Expires April 28, 2005 [Page 3] Internet-Draft ROHC-FN October 2004 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 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 Field ROHC-FN divides the protocol header to be compressed into a set of contiguous bit patterns known as fields. o Library of encoding methods The library of encoding methods contains a number of commonly used encoding methods for compressing header fields. 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 to compress the headers and a state machine to control the actions of each endpoint. Finking, et al. Expires April 28, 2005 [Page 4] Internet-Draft ROHC-FN October 2004 3. Overview of ROHC-FN This section gives an overview of ROHC-FN and explains how it can be used to specify how to compress 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 is common to all profiles: it defines 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 can be further subdivided into 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 must 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 successive headers. 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 ensure that the compressor and decompressor contexts are kept synchronised. Finking, et al. Expires April 28, 2005 [Page 5] Internet-Draft ROHC-FN October 2004 The ROHC-FN is designed to help in the specification of the packet formats for use 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". It 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 the 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. More specifically, this statement creates a reversible binding between the attributes of a field and the encoding method (including the parameters specified with the method). 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 Finking, et al. Expires April 28, 2005 [Page 6] Internet-Draft ROHC-FN October 2004 and the attribute corresponding to the value of the field in the context. o For the compressor, this binding is successful when both values are the same for a packet format that sends no bits for that field. Otherwise, a packet format using another encoding method that is successful when the parameters are not equal is used (such as a method that would send the field uncompressed). o For the decompressor, the same binding succeeds for a packet type that sends no bits for that field if a valid context entry containing the value of the uncompressed field exists. Otherwise, the binding will fail decompression for that packet type. Fields have attributes. Attributes describe various things about the field, including the length and whereabouts they appear in the header. For example: field:has_context indicates whether or not a context entry exists for this field. 3.2.2 Structures Structures provide a mechanism for combining fields and their encoding methods into larger units. Structures are defined using the "===" operator. These can then be used as encoding methods in other structures. structure === { uncompressed_format = field_1, field_2, : : field_n; compressed_format_0 = field_a, : : field_b { field_a ::= encoding_method_1; : : : : field_b ::= encoding_method_2; }; Finking, et al. Expires April 28, 2005 [Page 7] Internet-Draft ROHC-FN October 2004 compressed_format_1 = field_c, : : field_d { field_c ::= encoding_method_3; : : : : field_d ::= encoding_method_4; }; : : compressed_format_n = field_y, : : field_z { field_y ::= encoding_method_foo; : : : : field_z ::= encoding_method_bar; }; }; In the example above, the comma separated list "uncompressed_format" indicates the order of fields in the uncompressed header. This list is followed by several packet formats for the compressed data, each beginning with the keyword "compressed_format". Packet formats defined by "compressed_format" also indicate an ordered list of fields. Items in this list consist either of: o a compressed representation of fields that occur in the uncompressed header; or o "control fields", that are additional information added to the compressed packet during compression. Fields from the uncompressed header will have the same name as they do in the compressed header. So in the example above, "field_a" would be a control field if it didn't appear in the uncompressed field order list . Following the compressed field order list, encoding methods are defined inside braces for all the compressed and uncompressed fields. Fields that have no encoding methods will be handled using "default_methods" (see TBAref below). Finking, et al. Expires April 28, 2005 [Page 8] Internet-Draft ROHC-FN October 2004 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 (defined in Section 4.11), which we will call "ipv4_header". This is notated as follows: ipv4_header === { This 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 with several headers and fields, often sharing the same names. The next step is to specify the fields contained in the uncompressed IPv4 header, which is accomplished using ROHC-FN as follows: uncompressed_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 Finking, et al. Expires April 28, 2005 [Page 9] Internet-Draft ROHC-FN October 2004 need to be sent in the compressed header at all. compressed_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 additional encoding methods defined in the profile specification itself. Since the intention here is to 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 4.9.1) 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 4.9.3) 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. Finking, et al. Expires April 28, 2005 [Page 10] Internet-Draft ROHC-FN October 2004 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. 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 as an example above. Its definition can be given either in the English language 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. 4.1 Overall Structure of a Specification A ROHC-FN specification consists of a sequence of zero or more constant definitions (Section 4.2) and one or more encoding method definitions, given in the form of structures (Section 4.11). 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. Finking, et al. Expires April 28, 2005 [Page 11] Internet-Draft ROHC-FN October 2004 The properties of a field are defined by defining an encoding method for it, typically in the compressed format. This encoding method can be one defined in a structure or it can be a predefined encoding method. Predefined encoding methods can be defined in the text accompanying a formal specification, or they can be defined in the present document. 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 can be defined by any expression on the right hand side of the "=" operator (see Section 4.4). 4.3 Field Attributes In ROHC-FN, the properties of a field are defined by an encoding method. The encoding method‚ÇÖs 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 seven attributes, three for the uncompressed field, three for the compressed field and one to assert the existence of a context entry for the field. The attributes available for each field are: o "uncomp_value", "uncomp_length" and "uncomp_hdr_start" -- uncompressed attributes of the field o "comp_value", "comp_length" and "comp_hdr_start" -- compressed attributes of the field o "has_context" -- context information Attributes of a particular field are referred formally by using the field's reference (see Section 4.6, followed by a ":" and the attribute's identifier. For example: tcp_ip.options.list_length:uncomp_value gives the numerical uncompressed value of the field referenced. The attributes are explained in more detail below. The two value attributes contain the respective numeric values of the field as a non-negative integer by encoding the bit string most-significant bit first, i.e. "uncomp_value" gives the numerical Finking, et al. Expires April 28, 2005 [Page 12] Internet-Draft ROHC-FN October 2004 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 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 "has_context" attribute indicates whether there is any "context" available for the field. The context keep for a particular field contains information about previous value(s) of the field. This information is needed for encoding methods, such as "static" and "lsb" (see section Section 4.9). These methods refer back to the previous value of the field. This attribute is particularly useful for list encoding, as it can be necessary for the notator to find out if context information is available or not (see section Section 4.12.2). 4.4 Expressions Expressions can be made up of any of the following components: Integers Integers can be expressed as decimal values, binary values (prefixed by 0b), or hex values (prefixed by 0x). Negative integers are prefixed by a "-" sign. Integer operations The operators +, -, *, / and ^ are available, along with ( and ) for grouping. Note that k / v is undefined if k is not an integer multiple of v (i.e. if it does not evaluate to an integer). However, k // v is always defined. The precedence for each of the operators, along with parentheses is given below (higher precedence first): (, ) ^ *, / +, - x ^ y Evaluates to x raised to the power of y. x / y Evaluates to the integer division of x by y, i.e. x divided by y, rounded down to the nearest integer. It is undefined when y Finking, et al. Expires April 28, 2005 [Page 13] Internet-Draft ROHC-FN October 2004 is zero. mod (k, v) Evaluates to k - v * (k / v). log2 (w) Evaluates to the smallest integer k where v <= 2^k, i.e. it returns the smallest number of bits in which value v can be stored. Boolean operations The following boolean operators are available: &&, for logical and ||, for logical or !, for logical not The boolean values are 0 (false) and 1 (true). boolean1 && boolean2 Returns true if both boolean1 and boolean2 are true. Returns false otherwise. boolean1 || boolean2 Returns true if at least one of boolean1 or boolean2 is true. Returns false otherwise. !boolean Returns true if boolean is false. Returns false otherwise. Comparison operations The following comparison operators are available: ==, != for equality ("is equal" and "is not equal", respectively) <, >, <=, >= for comparison ("is smaller than", "is larger than", "is smaller than or equal to" and "is larger than or equal to" respectively) 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. Finking, et al. Expires April 28, 2005 [Page 14] Internet-Draft ROHC-FN October 2004 x < y Returns true if x is less than y. Returns false otherwise. x <= y Returns true if x is less than or equal to y. Returns false otherwise. x > y Returns true if x is greater than y. Returns false otherwise. x >= y Returns true if x is greater than or equal to y. Returns false otherwise. 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 are illegal. Expressions cannot be used as encoding methods. This is because they cannot completely characterise an uncompressed field; in particular, the length of the uncompressed field would be undefined for the decompressor. 4.5 Expressions: NOTE:Merge+Remove ROHC-FN includes the usual infix style of expressions, with parentheses "(" and ")" used for grouping. Expressions can be made up of any of the following components: Integers 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). Integer, comparison and boolean operations The following operators are defined on integers. Their precedence and semantics generally is as in the C programming language, with the following exceptions: Finking, et al. Expires April 28, 2005 [Page 15] Internet-Draft ROHC-FN October 2004 There is no limit on the range of integers. The expression div(k,v) is only defined if k is an integer multiple of v (i.e. it always evaluates to an integer, with no residue). The expression k/v is always defined (for v != 0) and is evaluated as in C. The expression mod(k,v) is used instead of C language k % v, as the "%" character is the comment character. x ^ y evaluates to x raised to the power of y. log2(w) Evaluates to the smallest integer k where v <= 2^k, i.e. it returns the smallest number of bits in which value v can be stored. field/attribute reference syntax ("." and ":") ! (unary), function application f(x) ^ * / + - < <= > >= == != & | && || 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, i.e., not succeed. It is possible to test if an expression has an undefined value by comparing it to the keyword "null". For example: field == null 4.6 Field References A field reference followed by a dot and a field name refers to the named field that is an immediate child within the referenced field. [needs fixing] 4.7 Reserved Keywords 4.7.1 "let" The reserved keyword "let" takes a boolean expression as a parameter. Finking, et al. Expires April 28, 2005 [Page 16] Internet-Draft ROHC-FN October 2004 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: let () When the boolean expression evaluates to false, the packet format containing the expression fails, i.e. this packet format cannot be selected by the compressor. A "let" statement is always part of a field encoding list. 4.7.2 "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 length of the uncompressed header which is being encoded. 4.8 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. 4.8.1 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: Finking, et al. Expires April 28, 2005 [Page 17] Internet-Draft ROHC-FN October 2004 %----------------------------------------------------------------- % IR-REPLICATE packet formats %----------------------------------------------------------------- % The following fields are included in all of the IR-REPLICATE % packet formats: % uncompressed_format = discriminator, % [ 8 ] bits tcp.seq_number, % [ 32 ] bits tcp.flags.ecn, % [ 2 ] bits 4.8.2 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: */ uncompressed_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. 4.9 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 4.11). It is also possible for a profile to define its own set of encoding methods using the formal notation or using a textual definition. 4.9.1 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): Finking, et al. Expires April 28, 2005 [Page 18] Internet-Draft ROHC-FN October 2004 field ::= uncompressed_value (uncomp_length_param, ); where the "uncomp_length_param" binds with the field's "uncomp_length" attribute, and where is a mathematical expression. The value of binds with the field's "uncomp_value" attribute. For example, the IPv6 header version number is a four bits field that always has the value 6: version ::= uncompressed_value (4, 6); Another example of value encoding, using an expression: data_offset ::= expression(4, (uncomp_value(tcp_ip.options.list_length) + 160) / 32); In both examples above, since the value is either fixed or described entirely in terms of a known expression, it is omitted from the compressed header. 4.9.2 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_param, ); where the "comp_length_param" binds with the field's "comp_length" attribute, and where is a mathematical expression. The value of binds with the field's "comp_value" attribute. One possible use of this encoding method is to define padding in the compressed header: pad_to_octet_boundary ::= compressed_value (3, 0); Another 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'; Finking, et al. Expires April 28, 2005 [Page 19] Internet-Draft ROHC-FN October 2004 This has exactly the same meaning as: discriminator ::= compressed_value(5, 13); 4.9.3 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 (); where "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. 4.9.4 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 does not change from one packet to the other for a given flow: src_port ::= static; 4.9.5 lsb The Least Significant Bit encoding method, "lsb", compresses a field Finking, et al. Expires April 28, 2005 [Page 20] Internet-Draft ROHC-FN October 2004 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 compressed field takes up the specified number of bits in the compressed header (i.e. num_lsbs_param). For example, a sequence number used as a control field that can only increase: msn ::= lsb (2, -1); 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". 4.9.6 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 Finking, et al. Expires April 28, 2005 [Page 21] Internet-Draft ROHC-FN October 2004 o the size inoctets 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: crc_field ::= crc (3, 0x6, 0xF, 0x3, 40) % 3 bits % C(x) = x^0 + x^1 + x^3 4.10 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 4.9). 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. Finking, et al. Expires April 28, 2005 [Page 22] Internet-Draft ROHC-FN October 2004 4.11 Structures Structures are used for defining new encoding methods in a formal specification. They can 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. 4.11.1 Simple Structures A structure can be used to specify a single fixed encoding. This is its simplest form. For example: compound_encoding_method === { uncompressed_format = field_1, % [ 4 ] field_2; % [ 12 ] compressed_format = field_2, % [ 0 ], field_1 % [ 4 ] { field_1 ::= irregular (4); field_2 ::= uncompressed_value (12, 9); }; }; The above begins with the structure name, "compound_encoding_method". This name 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 "uncompressed_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. The encoding methods defined for the fields must define the "uncomp_length" attribute so there is an unambiguous mapping from the bits in the uncompressed header to the fields listed in the field order list. Finking, et al. Expires April 28, 2005 [Page 23] Internet-Draft ROHC-FN October 2004 4.11.1.1 control_fields Control fields are defined using the "control_fields" list, which specifies control fields that do not appear in the uncompressed header but that are used for compression. [Editor - write more here + include in example] 4.11.1.2 uncompressed_format The uncompressed field order list is defined by "uncompressed_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". 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 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 "uncompressed_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 this uncompressed format is being encoded, regardless of the selected compressed format. If an uncompressed format contains let-statements, the encoding of a packet with this uncompressed format can only succeed if the specified expressions evaluate to true (see Section [TBA]). 4.11.1.3 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. Finking, et al. Expires April 28, 2005 [Page 24] Internet-Draft ROHC-FN October 2004 Note that the order of the fields specified in "compressed_format" field order list, does not have to match the order they appear in the "uncompressed_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. 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 === { uncompressed_format = field_1 % [ 4 ], field_2 % [ 12 ]; compressed_format = field_1 % [ 4 ] { field_1 ::= irregular (4); field_2 ::= uncompressed_value (12, 9); }; }; 4.11.2 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: Finking, et al. Expires April 28, 2005 [Page 25] Internet-Draft ROHC-FN October 2004 poor_mans_lsb(variable_length) === { uncompressed_format = constant_bits, variable_bits; compressed_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. 4.11.3 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 4.9.3), 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. 4.11.3.1 Naming Convention When multiple compressed formats are defined, they must be defined using names beginning with "compressed_format", and each name must be unique. In fact this is true even if only one format is given, but in that case, simply "compressed_format" will do, since there are no alternatives to differentiate between. Similarly, if multiple uncompressed formats are defined, they must be defined using names beginning with "uncompressed_format". 4.11.3.2 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 Finking, et al. Expires April 28, 2005 [Page 26] Internet-Draft ROHC-FN October 2004 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 succeed and there are solutions for all the let-statements (see Section 4.7.1). If there is no such combination, the encoding method defined by the structure "fails". If there are multiple such combinations, the compressor can choose one. On the other hand, it must be possible for the decompressor to discriminate between the different packet formats that the compressor may choose from. A simple approach to this problem is for each compressed format to include a "discriminator" that uniquely identifies that particular format. A discriminator is a control field; it is not derived from any of the uncompressed field values (see Section 4.9.2). 4.11.3.3 Default Encoding Methods - default_methods When using multiple compressed packet formats, default encoding methods can be specified for each field. The default encoding methods specify the encoding method to use for a field if a given compressed format does not specify the encoding method for that field. This is helpful to keep the definition of the packet formats concise, as the same encoding method need not be repeated for every compressed format. The syntax for specifying default encoding methods is similar to that used to specify a compressed format, except that there is no need to specify a field order list for the default encoding methods, since the field order is specified individually for each format; only the field encodings list is given. For example: default_methods = { field_1 ::= uncompressed_value (4,1); field_2 ::= uncompressed_value (4,2); field_3 ::= lsb(3,-1); } Fields for which there is a default encoding method do not need to be specified in the field encodings list of any compressed format which wishes to use the default encoding method for that field. The default encoding method may however be overridden by specifying an explicit encoding method for that field. If a default encoding Finking, et al. Expires April 28, 2005 [Page 27] Internet-Draft ROHC-FN October 2004 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 let-statements. In this case every compressed format of the structure can only succeed if the specified expressions evaluate to true. Note that let-statements can not be overridden in compressed formats. 4.11.3.4 Example of Multiple Formats Putting this altogether, here is a complete example of a structure with multiple compressed formats: test_multiple_formats === { uncompressed_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); }; compressed_format_0 = discriminator, % [ 1 ] field_3 % [ 4 ] { discriminator ::= '0'; }; compressed_format_1 = discriminator, % [ 1 ] field_1, % [ 4 ] field_3 % [ 24 ] { discriminator ::= '1'; field_1 ::= irregular(4); field_3 ::= irregular(24); }; }; Note the following: Finking, et al. Expires April 28, 2005 [Page 28] Internet-Draft ROHC-FN October 2004 o "field_1" and "field_3" both have default encoding methods specified for them, which are used in "compressed_format_0", but is overridden in "compressed_format_1"; "field_2" however is not overridden. Overriding one of the default encoding methods does not imply that all default encoding methods must be overridden. o "field_1" and "field_2" have default encoding methods which compress to zero bits, when these are used in "compressed_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 "compressed_format_0"'s field encodings list, it still needs to appear in the field order list 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. 4.11.4 Recursive Structures It is possible to define structures recursively, by having one or more of the compressed formats of a structure encode a field using the structure itself. For example: static_32_list(num_bytes) === { uncompressed_format_end = field_1; % [ 32 ] bits uncompressed_format_mid = field_1, % [ 32 ] bits tail; % [ num_bytes - 32 ] bits compressed_format_end_of_list = field_1 % [ 32 ] bits { field_1 ::= static_or_irregular(32); }; compressed_format_mid_list = field_1, % [ 32 ] bits tail % [ num_bytes - 32 ] bits { field_1 ::= static_or_irregular(32); tail ::= static_32_list(num_bytes - 32); }; }; Finking, et al. Expires April 28, 2005 [Page 29] Internet-Draft ROHC-FN October 2004 static_or_irregular(length) === { uncompressed_format = field; compressed_format_irregular = discriminator, % [ 1 ] bits field % [ length ] bits { discriminator ::= '0'; field ::= irregular(length); }; compressed_format_static = discriminator, % [ 1 ] bits field % [ 0 ] bits { discriminator ::= '1'; field ::= static; }; }; The "static_or_irregular" structure will encode a field as either irregular, or static if there is a context value to refer back too. The "static_32_list" uses the "static_or_irregular" structure and one other encoding method, itself. It encodes an arbitrary length sequence of 32 bit fields as "irregular (32)", or static where possible. We could use this for example to encode a CSRC list: csrc_list ::= static_32_list(96); % 32 bits per item, 96 bits = 3 items This is exactly equivalent to notating the following: csrc_list_1 ::= static_or_irregular(32); csrc_list_2 ::= static_or_irregular(32); csrc_list_3 ::= static_or_irregular(32); In this case the recursive notation simply provides a mechanism to choose the number of list items at run time; the literal "96" in the above example could easily have been an expression. It is possible to notate extremely complicated list structures using the above technique. However, special syntax is provided which simplifies the notating of lists considerably. This is discussed in the next section. 4.12 Lists The above section has described how structures can be used to build individual fields into larger units, but largely for a fixed order of Finking, et al. Expires April 28, 2005 [Page 30] Internet-Draft ROHC-FN October 2004 uncompressed fields. Through the use of multiple uncompressed formats it is possible to cater for variable field order/presence using the above notation, but it quickly becomes cumbersome. This section presents notation aimed specifically at encoding lists of fields, where the number and even the type of fields may vary from header to header in the flow. 4.12.1 Notation List notation is similar to that for structures above, the sections below describe it. 4.12.1.1 List name The notation for naming a list structure is the same as for ordinary structures, except that the structure name must begin with "list", and the first argument of the structure is always "list_length_in_bytes" and is the list length in bytes. This parameter must always be present, even if not used explicitly, since it is used implicitly when encoding the list. 4.12.1.2 List body The notation for the body of the list structure definition is different from that described previously for ordinary structures. The body only contains definitions for the formats of all the possible list entries, and so the whole structure has an appearance similar to that of the compressed format field encodings, in an ordinary structure. There must be at least one such entry. For example: list_csrc(list_length_in_bytes) === { list_item ::= irregular(32); }; This defines a list of 32 bit irregular values. The number of items in the list is determined by the length of the list in bytes. Since this list structure defines no padding mechanism, the list length must be a multiple of 32, otherwise list_csrc encoding will fail, for example: csrc_list ::= list_csrc(32); % OK csrc_list ::= list_csrc(48); % Not OK End of list padding is discussed in the next section. Finking, et al. Expires April 28, 2005 [Page 31] Internet-Draft ROHC-FN October 2004 4.12.1.3 List Termination With the wide variety of protocols in use today, there are a number of different mechanisms used which indicate the end of a list. Most commonly the list length is specified, and when that length is reached then the end of the list is reached. However, other lists are terminated by an end-of-list "sentinel". In order to indicate the use of a sentinel in the uncompressed list, list structures have a reserved field, "end_of_list_sentinel", which defines the list item which defines the end-of-list marker. In addition to this, the notator may define an "end_of_list_pad", which specifies how to encode the pad bytes which occur after the end of the list is reached, but before the total list length is reached. The pad is only needed if the end of the list may be reached with bytes to spare. This is the case with the TCP options list for example: list_tcp_options(list_length_in_bytes) === { end_of_list_sentinel ::= value(8, 0); end_of_list_pad ::= value(8, 1); : : etc. }; Note that if a list is always terminated by an end of list sentinel, with no padding afterwards, the "list_length_in_bytes" parameter can be passed to the list structure unbound, since it will be bound to the length of the list (inclusive of the end-of-list sentinel). It is an error for the "list_length_in_bytes" to be passed unbound to a list structure which specifies an end_of_list_pad; the use of the pad requires "list_length_in_bytes" to have a bound value. Conversely, if no "end_of_list_pad" is specified and "list_length_in_bytes" is bound, then it must match the list size (regardless of whether the list uses an end-of-list sentinel), or else the encoding will fail. Finally, if no "end_of_list_sentinel" is specified, the list_length_in_bytes must be passed to the list structure bound, otherwise there is no way to tell when the end of the list has been reached. 4.12.1.4 Use of the has_context attribute For each list item, a boolean flag is defined, called "has_context", which is available as an attribute of the field (see sectionSection 4.3 for more information on attributes). When the list item is encoded, this flag is set to true or false depending on whether the Finking, et al. Expires April 28, 2005 [Page 32] Internet-Draft ROHC-FN October 2004 item is new or not. New list items have no context, and so can not use encoding methods such as "lsb" or "static", which rely on context being present. The purpose of this flag is to enable a list item to be compressed in different ways depending on the availability of context information for that list item. Typical behaviour for a list item is to be encoded as "irregular" when there is no context available, and "static" once the context becomes available. However this does not suit all list items. For example in TCP options, the no operation (or NOP) item has a fixed value of 1, so there is no need to specify two alternative encodings for it. The timestamp field on the other hand is constantly changing, so static encoding would always fail, meaning it would have to be resent in full every time - better instead to use "lsb" encoding, or even a struct with several alternative "lsb" encodings. For example: list_tcp_options(list_length_in_bytes) === { end_of_list_sentinel ::= value(8, 0); end_of_list_pad ::= value(8, 1); : : timestamp ::= tcp_timestamp_list_item(); : : : : etc. }; tcp_timestamp_list_item() === { uncompressed_format = type, length, timestamp_value, timestamp_echo_reply; default_methods = { type ::= value(8, 8); length ::= value(8, 10); }; compressed_format_first = timestamp_value, timestamp_echo_reply { let (this:has_context == false); timestamp_value ::= irregular(32); timestamp_echo_reply ::= irregular(32); }; Finking, et al. Expires April 28, 2005 [Page 33] Internet-Draft ROHC-FN October 2004 compressed_format_subsequent = timestamp_value, timestamp_echo_reply { let (this:has_context == true); timestamp_value ::= lsb(16, 0); timestamp_echo_reply ::= lsb(16, 0); }; }; Note that no discriminator has been used to differentiate between the two compressed formats, since the "has_context" flag fulfils that role. It would be redundant to add a discriminator here since the "has_context" flag is automatically included in the encoding of the list, see next section for details on exactly how lists are encoded. This is NOT the case for fields which are not list items; non-list fields must encode a discriminator explicitly. 4.12.2 List Encoding The way a list structure is encoded is referred to as "type 0 list encoding" and is defined in RFC-3095 [4], section 5.8. "List Compression", as a generic list compression scheme. Type 0 list encoding includes features which make it highly efficient at encoding the sorts of behaviour that occur in real protocols, such as items disappearing from the middle of list and perhaps reappearing later in the flow. Notating this sort of behaviour directly would require a large amount of notation and would be hard to test and therefore error prone. The strength of type 0 list encoding is that it separates out the items that occur in the list from the order in which they occur. The list items are stored in a table at both the compressor and the decompressor. Updates to the table are transmitted if new list items are seen, otherwise, only references into the table (known as index items) need to be transmitted, rather than the list items themselves. Moreover, even the index items only need to be transmitted when there is a change to the list. 4.12.2.1 Formal Notation For List Encoding This section specifies the encoding of lists, using the formal notation. Note that the notation given here is given only for the purposes of defining how lists are encoded - it is not necessary for a notator to reproduce this notation every time he/she wishes to encode a list, it all happens automatically. The type 0 list encoding starts with a header, which specifies the Finking, et al. Expires April 28, 2005 [Page 34] Internet-Draft ROHC-FN October 2004 format of the index item list. In particular it specifies whether short or long index items are to be used, and how many of them there are: list_header(num_list_items_param, highest_index_param) === { uncompressed_format = ; compressed_format = encoding_type [ 2 ], % ET generation_id_present [ 1 ], % GP xi_field_size [ 1 ], % PS list_item_count [ 4 ] % CC { encoding_type ::= '00'; generation_id_present ::= '0'; xi_size ::= xi_size_encoding(highest_index_param); list_item_count ::= compressed_value(4, num_list_items_param); }; }; % calculates the index item (xi) size flag xi_size_encoding(highest_index_param) === { uncompressed_format = ; compressed_format_4_bit_field = xi_field_size { let(highest_index_param < 2^3); xi_field_size = '0'; }; compressed_format_8_bit_field = xi_field_size { let(highest_index_param < 2^7); xi_field_size = '1'; }; }; Note that "num_list_items_param" must be derived from the header being compressed, and that "highest_index_param" comes from the compressor's knowledge of the items in the "xi list" (see below). "highest_index_param" is set to the maximum table index in the list. This means that even if the table currently contains greater than 8 items, the "xi_field_size" flag could still be zero, as long as the highest index referred to in the list is below 8 (note items are indexed from zero upwards). Finking, et al. Expires April 28, 2005 [Page 35] Internet-Draft ROHC-FN October 2004 Immediately following the header is the index item list (or "xi list"). This is a contiguous list of index items, which specify what table indices to look up to find out what is in the list. Each index item (or "xi") starts with a single bit flag, which indicates whether context is available for this item or not, followed by either three of seven bits to indicate the index into the table where the item is stored, depending on the table size: xi_list(xi_count_param, xi_size_param) === { uncompressed_format = ; default_methods = { xi_1 ::= xi(xi_size_param); xi_2 ::= xi(xi_size_param); }; compressed_format_mid = xi_1 [ xi_size_param ], xi_2 [ xi_size_param ], tail [ (xi_count_param - 2) * xi_size_param ]; { let(xi_count_param > 2); tail ::= xi_list(xi_count_param - 2, xi_size_param); }; compressed_format_even_end = xi_1 [ xi_size_param ], xi_2 [ xi_size_param ]; { let(xi_count_param == 2); }; % need a four bit pad at the end of the xi list if there are an % odd number in the list, and xi size is four bits compressed_format_odd_end = xi_1 [ xi_size_param ], pad [ 8 - xi_size_param ]; { let(xi_count_param == 1); pad ::= compressed_value(8 - xi_size_param, 0); }; }; xi(xi_size_param) === { uncompressed_format = ; Finking, et al. Expires April 28, 2005 [Page 36] Internet-Draft ROHC-FN October 2004 compressed_format_new = new, table_index { let(this:has_context == false); new ::= '1'; table_index ::= irregular(xi_size_param); }; compressed_format_old = new, table_index { let(this:has_context == true); new ::= '0'; table_index ::= irregular(xi_size_param); }; }; Note that "xi_size_param" must be calculated by the compressor to be either 4 or 8 (it can be derived by multiplying the "xi_field_size" flag by 4 and adding 4), and that the "has_context" attribute of each xi must be bound by the compressor to the value of the "has_context" attribute of the corresponding list item. Immediately following the xi list, is the encodings list. This is the actual list of encodings for all the items referred to by the xi list. The items occur in the same order as they do in the xi list, each encoded in the manner specified by the notator. Assuming the following generic notation for lists: list (list_length_in_bytes) === { end_of_list_sentinel ::= sentinel_encoding; end_of_list_pad ::= pad_encoding; item_type_1 ::= item_type_1_encoding; item_type_2 ::= item_type_2_encoding; : : : : item_type_n ::= item_type_n_encoding; }; The item list is encoded as follows: item_list_encoding(list_length_in_bytes, list_end_reached) === { uncompressed_format ::= item, tail; default_methods = Finking, et al. Expires April 28, 2005 [Page 37] Internet-Draft ROHC-FN October 2004 { let (bytes_left == list_length_in_bytes - item:uncomp_length); tail ::= item_list_encoding(bytes_left, bytes_left == 0); }; compressed_format_list_end = { let(list_length_in_bytes == 0); let(list_end_reached == true); }; compressed_format_sentinel = item, tail { item ::= sentinel_encoding; tail ::= item_list_encoding(bytes_left, true); }; compressed_format_pad = item, tail { let (list_end_reached == true); item ::= pad_encoding; tail ::= item_list_encoding(bytes_left, true); }; compressed_fomat_type_1 = item, tail { let (list_end_reached == false); item ::= item_type_1_encoding; }; compressed_fomat_type_2 = item, tail { let (list_end_reached == false); item ::= item_type_2_encoding; }; : : : : compressed_fomat_type_n = item_, tail { let (list_end_reached == false); item ::= item_type_n_encoding; }; }; 5. Security considerations This draft describes a formal notation similar to ABNF RFC 2234 [3], Finking, et al. Expires April 28, 2005 [Page 38] Internet-Draft ROHC-FN October 2004 and hence is not believed to raise any security issues. 6. Acknowledgements A number of important concepts and ideas have been borrowed from ROHC RFC 3095 [4]. Thanks to Mark West, Eilert Brinkmann and Kristofer Sandlund for their cooperation and feedback from notating the TCP profile. Thanks to Rob Hancock and Stephen McCann for putting up with the authors' arguments and making helpful suggestions, frequently against the tide! The authors would also like to thank Carsten Bormann, Christian Schmidt, Qian Zhang, Hongbin Liao, Max Riegel and Lars-Erik Jonsson for their comments and encouragement. We haven't always agreed, but the arguments have been fun! 7 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., Yoshimura, T. and H. Zheng, "RObust Header Compression (ROHC): Framework and four profiles: RTP, UDP, ESP, and uncompressed", RFC 3095, July 2001. Finking, et al. Expires April 28, 2005 [Page 39] Internet-Draft ROHC-FN October 2004 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 Ghyslain Pelletier Ericsson AB Box 920 LuleÈÑ SE-971 28 Sweden Phone: +46 (0) 8 404 29 43 EMail: ghyslain.pelletier@ericsson.com Richard Price Cogent Defence and Security Networks Queensway Meadows Industrial Estate Meadows Road Newport, Gwent NP19 4SS Phone: +44 (0)1794 833681 EMail: richard.price@cogent-dsn.com URI: http://www.cogent-dsn.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. o comp_hdr_start - attribute o comp_length - attribute o comp_value - attribute o compressed_format - struct syntax Finking, et al. Expires April 28, 2005 [Page 40] Internet-Draft ROHC-FN October 2004 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 uncomp_hdr_start - attribute o uncomp_length - attribute o uncomp_value - attribute o uncompressed_format - struct syntax 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 k = %x6b l = %x6c Finking, et al. Expires April 28, 2005 [Page 41] Internet-Draft ROHC-FN October 2004 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 = ")" open-brace = "{" close-brace = "}" Finking, et al. Expires April 28, 2005 [Page 42] Internet-Draft ROHC-FN October 2004 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 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 Opertators exponential-operator = "^" multiplicative-operator = "*" / "/" additive-operator = "+" / "-" unary-minus = "-" A.6 Expressions parenthesised-expression = open-bracket arithmetic-expression close-bracket Finking, et al. Expires April 28, 2005 [Page 43] Internet-Draft ROHC-FN October 2004 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 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 Finking, et al. Expires April 28, 2005 [Page 44] Internet-Draft ROHC-FN October 2004 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) 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 Finking, et al. Expires April 28, 2005 [Page 45] Internet-Draft ROHC-FN October 2004 [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 packet format. 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: Finking, et al. Expires April 28, 2005 [Page 46] Internet-Draft ROHC-FN October 2004 eg_header === { uncompressed_format = version_no [ 2 ], type [ 2 ], flow_id [ 4 ], sequence_no [ 4 ], flag_bits [ 4 ]; compressed_format = 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 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 don't know anything about their behaviour. B.3 Basic Compression In order to achieve any compression we need to notate our 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: Finking, et al. Expires April 28, 2005 [Page 47] Internet-Draft ROHC-FN October 2004 eg_header === { uncompressed_format = version_no [ 2 ], type [ 2 ], flow_id [ 4 ], sequence_no [ 4 ], abc_flag_bits [ 3 ], reserved_flag [ 1 ]; compressed_format = 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 4.9.1) Note that we could just as well have omitted the "0 bits" fields from the definition of the compressed_data if we so 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. 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 any advantage of relationships between values of a field in one packet and its value in subsequent packets. For example, every packet in the following sequence is compressed the same amount despite the similarities between them: Finking, et al. Expires April 28, 2005 [Page 48] Internet-Draft ROHC-FN October 2004 Uncompressed header: 0101000100010000 Compressed header: 0100010001000 Uncompressed header: 0101000100100000 Compressed header: 0100010010000 Uncompressed header: 0111000100110000 Compressed header: 1100010011000 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: the sequence number increases by one each time, the flow_id stays the same, it always has the same value that it did in the previous header in the flow, 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 === { uncompressed_format = version_no [ 2 ], type [ 2 ], flow_id [ 4 ], sequence_no [ 4 ], abc_flag_bits [ 3 ], reserved_flag [ 1 ]; compressed_format = type [ 2 ], abc_flag_bits [ 3 ] { version_no ::= uncompressed_value(2,1); type ::= irregular(2); flow_id ::= static; sequence_no ::= lsb(0,-1); abc_flag_bits ::= irregular(3); reserved_flag ::= uncompressed_value(1,0); }; }; Finking, et al. Expires April 28, 2005 [Page 49] Internet-Draft ROHC-FN October 2004 This dependency on previous packets is notated using the static and LSB encoding methods (see Section 4.9.4 and Section 4.9.5 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 fill in 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 all the time. We solve all three of these problems below, robustness first, since it is simplest. When communicating sequence numbers a very important consideration for the notator is how robust the compressed protocol needs to be against packet loss. This will vary a lot from protocol to protocol. 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 a single bit of LSB encoding (see Section 4.9.5 ). 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: Finking, et al. Expires April 28, 2005 [Page 50] Internet-Draft ROHC-FN October 2004 eg_header === { uncompressed_data = version_no [ 2 ], type [ 2 ], flow_id [ 4 ], sequence_no [ 4 ], abc_flag_bits [ 3 ], reserved_flag [ 1 ]; compressed_format_0 = 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); }; compressed_format_1 = discriminator [ 1 ], type [ 2 ], sequence_no [ 1 ] { discriminator ::= '1'; version_no ::= uncompressed_value(2,1); type ::= irregular(2); flow_id ::= static; sequence_no ::= lsb(1,-1); 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 4 bits long, less than a third of the size of the single packet format, and a quarter of the size of the uncompressed header. Note that despite having to add the discriminator field, this format is even smaller than the original incorrect naȯve notation, which was 5 bits long, because this notation takes advantage of the fact that the abc flag bits rarely change. Finking, et al. Expires April 28, 2005 [Page 51] Internet-Draft ROHC-FN October 2004 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 sequence number in the example protocol counted up in steps of three, not one, then the LSB encoding could never be used; all we would have just 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: 0101000100100000 Compressed header: 1010 ; 00100010010000 Uncompressed header: 0111000100110000 Compressed header: 1110 ; 01100010011000 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 one higher, it can now be compressed in two possible ways, either using format_1 or in the same way as previously, using format_0. 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 of course 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. Finking, et al. Expires April 28, 2005 [Page 52] Internet-Draft ROHC-FN October 2004 This now gives a total of three packet formats, which means we need three discriminators to differentiate between them. The obvious solution here is to increase the number of bits in the discriminator from 1 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 it would be erroneous to use e.g. 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 === { uncompressed_data = version_no [ 2 ], type [ 2 ], flow_id [ 4 ], sequence_no [ 4 ], abc_flag_bits [ 3 ], reserved_flag [ 1 ]; compressed_format_0 = 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); }; compressed_format_1 = discriminator [ 2 ], type [ 2 ], sequence_no [ 1 ] { Finking, et al. Expires April 28, 2005 [Page 53] Internet-Draft ROHC-FN October 2004 discriminator ::= '01'; version_no ::= uncompressed_value(2,1); type ::= irregular(2); flow_id ::= static; sequence_no ::= lsb(1,-1); abc_flag_bits ::= uncompressed_value(3,7); reserved_flag ::= uncompressed_value(1,0); }; compressed_format_2 = discriminator [ 1 ], type [ 2 ], sequence_no [ 1 ] { discriminator ::= '1'; version_no ::= uncompressed_value(2,1); type ::= irregular(2); flow_id ::= static; sequence_no ::= lsb(1,-1); abc_flag_bits ::= static; reserved_flag ::= uncompressed_value(1,0); }; Here is some example output: Uncompressed header: 0101000100010000 Compressed header: 000100010001000 Uncompressed header: 0101000100100000 Compressed header: 1010 ; 000100010010000 Uncompressed header: 0111000100110000 Compressed header: 1110 ; 001100010011000 Uncompressed header: 0111000101001110 Compressed header: 01110 ; 001100010100111 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 five bits, only one bit more than the most common packet format. Finking, et al. Expires April 28, 2005 [Page 54] Internet-Draft ROHC-FN October 2004 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 === { uncompressed_data = 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(1,-1); reserved_flag ::= uncompressed_value(1,0); }; compressed_format_0 = discriminator [ 2 ], type [ 2 ], flow_id [ 4 ], sequence_no [ 4 ], abc_flag_bits [ 3 ] { discriminator ::= '00'; flow_id ::= irregular(4); sequence_no ::= irregular(4); % overrides default abc_flag_bits ::= irregular(3); }; compressed_format_1 = discriminator [ 2 ], type [ 2 ], sequence_no [ 1 ] { discriminator ::= '01'; abc_flag_bits ::= uncompressed_value(3,7); Finking, et al. Expires April 28, 2005 [Page 55] Internet-Draft ROHC-FN October 2004 }; compressed_format_2 = discriminator [ 1 ], type [ 2 ], sequence_no [ 1 ] { 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, either the field order list (no need, it's zero bits long) or in the field encodings list (no need it's specified in the default encoding methods). Finking, et al. 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Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Finking, et al. Expires April 28, 2005 [Page 57]