Compressing TCP/IP Headers for Low-Speed Serial Links
RFC 1144

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Network Working Group                                     V. Jacobson/1/
   Request for Comments: 1144                                           LBL
                                                              February 1990

                          Compressing TCP/IP Headers

                          for Low-Speed Serial Links

   Status of this Memo

   This RFC is a proposed elective protocol for the Internet community and
   requests discussion and suggestions for improvement.  It describes a
   method for compressing the headers of TCP/IP datagrams to improve
   performance over low speed serial links.  The motivation, implementation
   and performance of the method are described.  C code for a sample
   implementation is given for reference.  Distribution of this memo is
   unlimited.

   NOTE: Both ASCII and Postscript versions of this document are available.
         The ASCII version, obviously, lacks all the figures and all the
         information encoded in typographic variation (italics, boldface,
         etc.).  Since this information was, in the author's opinion, an
         essential part of the document, the ASCII version is at best
         incomplete and at worst misleading.  Anyone who plans to work
         with this protocol is strongly encouraged obtain the Postscript
         version of this RFC.

   ----------------------------
     1. This work was supported in part by the U.S. Department of Energy
   under Contract Number DE-AC03-76SF00098.



   Contents

   1  Introduction                                                        1

   2  The problem                                                         1

   3  The compression algorithm                                           4

      3.1 The basic idea . . . . . . . . . . . . . . . . . . . . . . . .  4

      3.2 The ugly details . . . . . . . . . . . . . . . . . . . . . . .  5

         3.2.1 Overview. . . . . . . . . . . . . . . . . . . . . . . . .  5

         3.2.2 Compressed packet format. . . . . . . . . . . . . . . . .  7

         3.2.3 Compressor processing . . . . . . . . . . . . . . . . . .  8

         3.2.4 Decompressor processing . . . . . . . . . . . . . . . . . 12

   4  Error handling                                                     14

      4.1 Error detection  . . . . . . . . . . . . . . . . . . . . . . . 14

      4.2 Error recovery . . . . . . . . . . . . . . . . . . . . . . . . 17

   5  Configurable parameters and tuning                                 18

      5.1 Compression configuration  . . . . . . . . . . . . . . . . . . 18

      5.2 Choosing a maximum transmission unit . . . . . . . . . . . . . 20

      5.3 Interaction with data compression  . . . . . . . . . . . . . . 21

   6  Performance measurements                                           23

   7  Acknowlegements                                                    25

   A  Sample Implementation                                              27

      A.1 Definitions and State Data . . . . . . . . . . . . . . . . . . 28

      A.2 Compression  . . . . . . . . . . . . . . . . . . . . . . . . . 31

                                      i



      A.3 Decompression  . . . . . . . . . . . . . . . . . . . . . . . . 37

      A.4 Initialization . . . . . . . . . . . . . . . . . . . . . . . . 41

      A.5 Berkeley Unix dependencies . . . . . . . . . . . . . . . . . . 41

   B  Compatibility with past mistakes                                   43

      B.1 Living without a framing `type' byte . . . . . . . . . . . . . 43

      B.2 Backwards compatible SLIP servers  . . . . . . . . . . . . . . 43

   C  More aggressive compression                                        45

   D  Security Considerations                                            46

   E  Author's address                                                   46

                                      ii


   RFC 1144               Compressing TCP/IP Headers          February 1990

   1  Introduction

   As increasingly powerful computers find their way into people's homes,
   there is growing interest in extending Internet connectivity to those
   computers.  Unfortunately, this extension exposes some complex problems
   in link-level framing, address assignment, routing, authentication and
   performance.  As of this writing there is active work in all these
   areas.  This memo describes a method that has been used to improve
   TCP/IP performance over low speed (300 to 19,200 bps) serial links.

   The compression proposed here is similar in spirit to the Thinwire-II
   protocol described in [5].  However, this protocol compresses more
   effectively (the average compressed header is 3 bytes compared to 13 in
   Thinwire-II) and is both efficient and simple to implement (the Unix
   implementation is 250 lines of C and requires, on the average, 90us (170
   instructions) for a 20MHz MC68020 to compress or decompress a packet).

   This compression is specific to TCP/IP datagrams./2/  The author