The RObust Header Compression (ROHC) Framework
RFC 4995

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Network Working Group                                       L-E. Jonsson
Request for Comments: 4995                                  G. Pelletier
Category: Standards Track                                    K. Sandlund
                                                                Ericsson
                                                               July 2007

             The RObust Header Compression (ROHC) Framework

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   The Robust Header Compression (ROHC) protocol provides an efficient,
   flexible, and future-proof header compression concept.  It is
   designed to operate efficiently and robustly over various link
   technologies with different characteristics.

   The ROHC framework, along with a set of compression profiles, was
   initially defined in RFC 3095.  To improve and simplify the ROHC
   specifications, this document explicitly defines the ROHC framework
   and the profile for uncompressed separately.  More specifically, the
   definition of the framework does not modify or update the definition
   of the framework specified by RFC 3095.

Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................4
      2.1. Acronyms ...................................................4
      2.2. ROHC Terminology ...........................................4
   3. Background (Informative) ........................................7
      3.1. Header Compression Fundamentals ............................7
      3.2. A Short History of Header Compression ......................7
   4. Overview of Robust Header Compression (ROHC) (Informative) ......8
      4.1. General Principles .........................................8
      4.2. Compression Efficiency, Robustness, and Transparency ......10
      4.3. Developing the ROHC Protocol ..............................10

Jonsson, et al.             Standards Track                     [Page 1]
RFC 4995                   The ROHC Framework                  July 2007

      4.4. Operational Characteristics of the ROHC Channel ...........11
      4.5. Compression and Master Sequence Number (MSN) ..............13
      4.6. Static and Dynamic Parts of a Context .....................13
   5. The ROHC Framework (Normative) .................................14
      5.1. The ROHC Channel ..........................................14
           5.1.1. Contexts and Context Identifiers ...................14
           5.1.2. Per-Channel Parameters .............................15
           5.1.3. Persistence of Decompressor Contexts ...............16
      5.2. ROHC Packets and Packet Types .............................16
           5.2.1. General Format of ROHC Packets .....................17
                  5.2.1.1. Format of the Padding Octet ...............17
                  5.2.1.2. Format of the Add-CID Octet ...............18
                  5.2.1.3. General Format of Header ..................18
           5.2.2. Initialization and Refresh (IR) Packet Types .......19
                  5.2.2.1. ROHC IR Packet Type .......................20
                  5.2.2.2. ROHC IR-DYN Packet Type ...................20
           5.2.3. ROHC Initial Decompressor Processing ...............21
           5.2.4. ROHC Feedback ......................................22
                  5.2.4.1. ROHC Feedback Format ......................23
           5.2.5. ROHC Segmentation ..................................25
                  5.2.5.1. Segmentation Usage Considerations .........25
                  5.2.5.2. Segmentation Protocol .....................26
      5.3. General Encoding Methods ..................................27
           5.3.1. Header Compression CRCs, Coverage and Polynomials ..27
                  5.3.1.1. 8-bit CRCs in IR and IR-DYN Headers .......27
                  5.3.1.2. 3-bit CRC in Compressed Headers ...........27
                  5.3.1.3. 7-bit CRC in Compressed Headers ...........28
                  5.3.1.4. 32-bit Segmentation CRC ...................28
           5.3.2. Self-Describing Variable-Length Values .............29
      5.4. ROHC UNCOMPRESSED -- No Compression  (Profile 0x0000) .....29
           5.4.1. IR Packet ..........................................30
           5.4.2. Normal Packet ......................................31
           5.4.3. Decompressor Operation .............................31
           5.4.4. Feedback ...........................................32
   6. Overview of a ROHC Profile (Informative) .......................32