Network Working Group                                            D. RandRequest for Comments: 1962                                        NovellCategory: Standards Track                                      June 1996               The PPP Compression Control Protocol (CCP)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.Abstract   The Point-to-Point Protocol (PPP) [1] provides a standard method for   transporting multi-protocol datagrams over point-to-point links.  PPP   also defines an extensible Link Control Protocol.   This document defines a method for negotiating data compression over   PPP links.Table of Contents   1.     Introduction ..........................................    1   2.     Compression Control Protocol (CCP) ....................    2      2.1       Sending Compressed Datagrams ....................    3   3.     Additional Packets ....................................    4      3.1       Reset-Request and Reset-Ack .....................    4   4.     CCP Configuration Options .............................    5      4.1       Proprietary Compression OUI .....................    7      4.2       Other Compression Types .........................    8   SECURITY CONSIDERATIONS ......................................    9   REFERENCES ...................................................    9   ACKNOWLEDGEMENTS .............................................    9   CHAIR'S ADDRESS ..............................................    9   AUTHOR'S ADDRESS .............................................    91.  Introduction   In order to establish communications over a PPP link, each end of the   link must first send LCP packets to configure and test the data link   during Link Establishment phase.  After the link has been   established, optional facilities may be negotiated as needed.Rand                        Standards Track                     [Page 1]RFC 1962                    PPP Compression                    June 1996   One such facility is data compression.  A wide variety of compression   methods may be negotiated, although typically only one method is used   in each direction of the link.   A different compression algorithm may be negotiated in each   direction, for speed, cost, memory or other considerations, or only   one direction may be compressed.2.  Compression Control Protocol (CCP)   The Compression Control Protocol (CCP) is responsible for   configuring, enabling, and disabling data compression algorithms on   both ends of the point-to-point link.  It is also used to signal a   failure of the compression/decompression mechanism in a reliable   manner.   CCP uses the same packet exchange mechanism as the Link Control   Protocol (LCP).  CCP packets may not be exchanged until PPP has   reached the Network-Layer Protocol phase.  CCP packets received   before this phase is reached should be silently discarded.   The Compression Control Protocol is exactly the same as the Link   Control Protocol [1] with the following exceptions:   Frame Modifications      The packet may utilize any modifications to the basic frame format      which have been negotiated during the Link Establishment phase.   Data Link Layer Protocol Field      Exactly one CCP packet is encapsulated in the PPP Information      field, where the PPP Protocol field indicates type hex 80FD      (Compression Control Protocol).      When individual link data compression is used in a multiple link      connection to a single destination, the PPP Protocol field      indicates type hex 80FB (Individual link Compression Control      Protocol).   Code field      In addition to Codes 1 through 7 (Configure-Request, Configure-      Ack, Configure-Nak, Configure-Reject, Terminate-Request,      Terminate-Ack and Code-Reject), two additional Codes 14 and 15      (Reset-Request and Reset-Ack) are defined for this protocol.      Other Codes should be treated as unrecognized and should result in      Code-Rejects.Rand                        Standards Track                     [Page 2]RFC 1962                    PPP Compression                    June 1996   Timeouts      CCP packets may not be exchanged until PPP has reached the      Network-Layer Protocol phase.  An implementation should be      prepared to wait for Authentication and Link Quality Determination      to finish before timing out waiting for a Configure-Ack or other      response.  It is suggested that an implementation give up only      after user intervention or a configurable amount of time.   Configuration Option Types      CCP has a distinct set of Configuration Options.2.1.  Sending Compressed Datagrams   Before any compressed packets may be communicated, PPP must reach the   Network-Layer Protocol phase, and the Compression Control Protocol   must reach the Opened state.   One or more compressed packets are encapsulated in the PPP   Information field, where the PPP Protocol field indicates type hex   00FD (Compressed datagram).  Each of the compression algorithms may   use a different mechanism to indicate the inclusion of more than one   uncompressed packet in a single Data Link Layer frame.   When using multiple PPP links to a single destination, there are two   methods of employing data compression.  The first method is to   compress the data prior to sending it out through the multiple links.   The second is to treat each link as a separate connection, that may   or may not have compression enabled.  In the second case, the PPP   Protocol field MUST be type hex 00FB (Individual link compressed   datagram).   Only one primary algorithm in each direction is in use at a time, and   that is negotiated prior to sending the first compressed frame.  The   PPP Protocol field of the compressed datagram indicates that the   frame is compressed, but not the algorithm with which it was   compressed.   The maximum length of a compressed packet transmitted over a PPP link   is the same as the maximum length of the Information field of a PPP   encapsulated packet.  Larger datagrams (presumably the result of the   compression algorithm increasing the size of the message in some   cases) may be sent uncompressed, using its standard form, or may be   sent in multiple datagrams, if the compression algorithm supports it.   Each of the compression algorithms must supply a way of determining   if they are passing data reliably, or they must require the use of aRand                        Standards Track                     [Page 3]RFC 1962                    PPP Compression                    June 1996   reliable transport such as LAPB [3].  Vendors are strongly encouraged   to employ a method of validating the compressed data, or recognizing   out-of-sync compressor/decompressor pairs.3.  Additional Packets   The Packet format and basic facilities are already defined for LCP   [1].   Up-to-date values of the CCP Code field are specified in the most   recent "Assigned Numbers" RFC [2].  This specification concerns the   following values:      14      Reset-Request      15      Reset-Ack3.1.  Reset-Request and Reset-Ack   Description      CCP includes Reset-Request and Reset-Ack Codes in order to provide      a mechanism for indicating a decompression failure in one      direction of a compressed link without affecting traffic in the      other direction.  A decompression failure may be determined by      periodically passing a hash value, performing a CRC check on the      decompressed data, or other mechanism.  It is strongly suggested      that some mechanism be available in all compression algorithms to      validate the decompressed data before passing the data on to the      rest of the system.      A CCP implementation wishing to indicate a decompression failure      SHOULD transmit a CCP packet with the Code field set to 14      (Reset-Request), and the Data field filled with any desired data.      Once a Reset-Request has been sent, any Compressed packets      received are discarded, and another Reset-Request is sent with the      same Identifier, until a valid Reset-Ack is received.      Upon reception of a Reset-Request, the transmitting compressor is      reset to an initial state.  This may include clearing a      dictionary, resetting hash codes, or other mechanisms.  A CCP      packet MUST be transmitted with the Code field set to 15 (Reset-      Ack), the Identifier field copied from the Reset-Request packet,      and the Data field filled with any desired data.      On receipt of a Reset-Ack, the receiving decompressor is reset to      an initial state.  This may include clearing a dictionary,      resetting hash codes, or other mechanisms.  Since there may be      several Reset-Acks in the pipe, the decompressor MUST be reset forRand                        Standards Track                     [Page 4]RFC 1962                    PPP Compression                    June 1996      each Reset-Ack which matches the currently expected identifier.   A summary of the Reset-Request and Reset-Ack packet formats is shown   below.  The fields are transmitted from left to right.    0                   1                   2                   3    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Code      |  Identifier   |            Length             |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |    Data ...   +-+-+-+-+   Code      14 for Reset-Request;      15 for Reset-Ack.   Identifier      On transmission, the Identifier field MUST be changed whenever the      content of the Data field changes, and whenever a valid reply has