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Versions: 00 01 02 rfc2509                               Standards Track
Internet Engineering Task Force       IPng, ISSLL and AVT Working Groups
INTERNET-DRAFT                                             Mathias Engan
draft-engan-ip-compress-02.txt        CDT/Lulea University of Technology
                                                               S. Casner
                                                        Precept Software
                                                              C. Bormann
                                                 Universitaet Bremen TZI
                                                       December 17, 1997
                                                           Expires: 6/98


                     IP Header Compression over PPP




Status of this Memo

   This document is  an  Internet-Draft.   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.

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   and  may be updated, replaced, or obsoleted by other documents at any
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   Distribution of this document is unlimited.


                                Abstract
      This document describes an  option  for  negotiating  the  use  of
      header  compression on IP datagrams transmitted over the Point-to-
      Point Protocol [RFC1661]. It defines extensions to the PPP Control
      Protocols for IPv4 and IPv6 [RFC1332, RFC2023]. Header compression
      may be applied to IPv4 and IPv6 datagrams in combination with TCP,
      UDP and RTP transport protocols as specified in [IPHC] and [CRTP].

   [Note to IANA, to be removed before publication:  The PPP protocol
   type assignments suggested in this document were selected from those
   unassigned in ftp://ftp.isi.edu/in-notes/iana/assignments/ppp-numbers
   on December 7, 1997.  These selections were presented to the PPPEXT
   working group at IETF in Washington, DC on December 9, 1997 and were
   approved as being in the appropriate ranges for this protocol.]


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1. Introduction

   The IP Header Compression (IPHC) defined in [IPHC] may  be  used  for
   compression  of  both IPv4 and IPv6 datagrams or packets encapsulated
   with multiple IP headers. IPHC is also capable  of  compressing  both
   TCP  and  UDP  transport  protocol  headers.  The  IP/UDP/RTP  header
   compression defined in [CRTP] fits within the  framework  defined  by
   IPHC so that it may also be applied to both IPv4 and IPv6 packets.

   In order to establish compression of IP datagrams  sent  over  a  PPP
   link each end of the link must agree on a set of configuration param-
   eters for the compression. The process of negotiating link parameters
   for  network layer protocols is handled in PPP by a family of network
   control protocols (NCPs).  Since there are separate NCPs for IPv4 and
   IPv6,  this document defines configuration options to be used in both
   NCPs to negotiate parameters for the compression scheme.

   IPHC relies on the link layer's ability  to  indicate  the  types  of
   datagrams carried in the link layer frames. In this document nine new
   types for the PPP Data Link Layer Protocol Field  are  defined  along
   with their meaning.

   In general, header compression schemes that  use  delta  encoding  of
   compressed  packets  require  that  the  lower layer does not reorder
   packets between compressor and decompressor. IPHC uses delta encoding
   of compressed packets for TCP and RTP.  The IPHC specification [IPHC]
   includes methods that allow link layers that may reorder  packets  to
   be  used with IPHC.  Since PPP does not reorder packets these mechan-
   isms are disabled by default.  When using reordering mechanisms  such
   as  multiclass multilink PPP [MCML], care must be taken so that pack-
   ets that share the same compression context are not reordered.


2. Configuration Option

   This document specifies a new compression protocol value for the IPCP
   IP-Compression-Protocol  option  as  specified in [RFC1332].  The new
   value and the associated option format are described in section 2.1.

   The option format is structured to allow  future  extensions  to  the
   IPHC scheme.

        NOTE:  The specification of link and network layer  parame-
        ter  negotiation  for  PPP  [RFC1661], [RFC1331], [RFC1332]
        does not prohibit multiple instances of  one  configuration
        option but states that the specification of a configuration
        option must explicitly allow multiple instances.  From  the
        current  specification  of the IPCP IP-Compression-Protocol



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        configuration option [RFC1332, p 6] it follows that it  can
        only be used to select a single compression protocol at any
        given time.

        NOTE:  [RFC1332] is not explicit about whether  the  option
        negotiates  the  capabilities  of  the  receiver  or of the
        sender.  In keeping with current practice, we  assume  that
        the  option  describes the capabilities of the decompressor
        (receiving side) of the peer that sends the Config-Req.


2.1. Configuration Option Format

Both the network control protocol for IPv4, IPCP [RFC1332] and the  IPv6
NCP,  IPV6CP  [RFC2023]  may  be used to negotiate IP Header Compression
parameters for their respective protocols.  The format of the configura-
tion option is the same for both IPCP and IPV6CP.


   Description

      This NCP configuration option is used to negotiate parameters  for
      IP  Header  Compression.   The  option format is summarized 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |    Length     |    IP-Compression-Protocol    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           TCP_SPACE           |         NON_TCP_SPACE         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         F_MAX_PERIOD          |          F_MAX_TIME           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           MAX_HEADER          |          suboptions...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
      2

   Length
      >= 14

      The length may be increased if the presence of additional  parame-
      ters is indicated by additional suboptions.

   IP-Compression-Protocol
      0061 (hex)



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   TCP_SPACE
      The TCP_SPACE field is  two  octets  and  indicates  the   maximum
      value  of a context identifier in the space of context identifiers
      allocated for TCP.

         Suggested value: 15

      TCP_SPACE must be at least 0 and at most 255 (The value 0  implies
      having one context).

   NON_TCP_SPACE
      The NON_TCP_SPACE field is two octets and  indicates  the  maximum
      value  of a context identifier in the space of context identifiers
      allocated for non-TCP. These context identifiers  are  carried  in
      COMPRESSED_NON_TCP,   COMPRESSED_UDP   and  COMPRESSED_RTP  packet
      headers.

         Suggested value: 15

      NON_TCP_SPACE must be at least 0 and at most 65535  (The  value  0
      implies having one context).

   F_MAX_PERIOD
      Maximum interval between full headers.  No more than  F_MAX_PERIOD
      COMPRESSED_NON_TCP   headers   may  be  sent  between  FULL_HEADER
      headers.

         Suggested value: 256

      A value of zero implies infinity, i.e. there is no  limit  to  the
      number of consecutive COMPRESSED_NON_TCP headers.

   F_MAX_TIME
      Maximum time interval between  full  headers.   COMPRESSED_NON_TCP
      headers may not be sent more than F_MAX_TIME seconds after sending
      the last FULL_HEADER header.

         Suggested value: 5 seconds

      A value of zero implies infinity.

   MAX_HEADER
      The largest header size in octets that may be compressed.

         Suggested value: 168 octets

      The value of MAX_HEADER should be large enough so  that  at  least
      the  outer  network  layer  header can be compressed.  To increase



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      compression efficiency MAX_HEADER should be set to a  value  large
      enough to cover common combinations of network and transport layer
      headers.

   suboptions
      The suboptions field consists of zero or  more  suboptions.   Each
      suboption  consists  of  a  type field, a length field and zero or
      more parameter octets, as defined  by  the  suboption  type.   The
      value of the length field indicates the length of the suboption in
      its entirety, including the lengths of the type and length fields.

       0                   1                   2
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |    Length     |  Parameters...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



2.2 RTP-Compression Suboption

   The RTP-Compression suboption is included in the NCP  IP-Compression-
   Protocol option for IPHC if IP/UDP/RTP compression is to be enabled.

   After successful negotiation of parameters for IP Header  Compression
   the   use   of   Protocol  Identifiers  FULL_HEADER,  COMPRESSED_TCP,
   COMPRESSED_TCP_NODELTA and COMPRESSED_NON_TCP is enabled,  regardless
   of the prescence of an RTP-Compression suboption.


   Description

      Enable use of Protocol Identifiers COMPRESSED_RTP,  COMPRESSED_UDP
      and CONTEXT_STATE as specified in [CRTP].

       0                   1
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |    Length     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
      1

   Length
      2





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3. Multiple Network Control Protocols

   The IPHC protocol is able to compress both IPv6 and  IPv4  datagrams.
   Both  IPCP  and  IPV6CP are able to negotiate option parameter values
   for IPHC.  These values apply to the compression of packets where the
   outer header is an IPv4 header and an IPv6 header, respectively.


3.1. Sharing Context Identifier Space

   For the compression and  decompression  of  IPv4  and  IPv6  datagram
   headers  the context identifier space is shared.  While the parameter
   values are independently negotiated, sharing the  context  identifier
   spaces  becomes more complex when the parameter values differ.  Since
   the compressed packets share context identifier space,  the  compres-
   sion  engine  must allocate context identifiers out of a common pool;
   for compressed packets, the decompressor has to examine  the  context
   state to determine what parameters to use for decompression.

   Context identifier  spaces  are  not  shared  between  TCP  and  non-
   TCP/UDP/RTP.   Doing so would require additional mechanisms to ensure
   that no error can occur when switching from using a context  identif-
   ier for TCP to non-TCP.


4. Demultiplexing of Datagrams

   The IPHC specification [IPHC] defines four header  formats  for  dif-
   ferent   types  of  compressed  headers.  They  are  compressed  TCP,
   compressed TCP with no delta encoding, compressed non-TCP with 8  bit
   CID  and  compressed non-TCP with 16 bit CID. The two non-TCP formats
   may be distinguished by their contents  so  both  may  use  the  same
   link-level  identifier.   A  fifth  header format, the full header is
   distinct from a regular header in that it carries additional informa-
   tion   to   establish  shared  context  between  the  compressor  and
   decompressor.

   The specification of IP/UDP/RTP  Header  Compression  [CRTP]  defines
   four   additional  formats  of  compressed  headers.   They  are  for
   compressed UDP and compressed RTP (on top of UDP), both  with  either
   8-  or  16-bit CIDs.  In addition, there is an explicit error message
   from the decompressor to the compressor.

   The link layer must be able to indicate  these  header  formats  with
   distinct  values.  Nine PPP Data Link Layer Protocol Field values are
   specified below.





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   FULL_HEADER
      The frame contains a full header as specified  in  [CRTP]  Section
      3.3.1.  This  is  the  same as the FULL_HEADER specified in [IPHC]
      Section 5.3.
         Value: 0061 (hex)

   COMPRESSED_TCP
      The frame contains a datagram with a compressed  header  with  the
      format as specified in [IPHC] Section 6a.
         Value: 0063 (hex)

   COMPRESSED_TCP_NODELTA
      The frame contains a datagram with a compressed  header  with  the
      format as specified in [IPHC] Section 6b.
         Value: 2063 (hex)

   COMPRESSED_NON_TCP
      The frame contains a datagram with a compressed  header  with  the
      format as specified in either Section 6c or Section 6d of [IPHC].
         Value: 0065 (hex)

   COMPRESSED_RTP_8
      The frame contains a datagram with a compressed  header  with  the
      format as specified in [CRTP] Section 3.3.2, using 8-bit CIDs.
         Value: 0069 (hex)

   COMPRESSED_RTP_16
      The frame contains a datagram with a compressed  header  with  the
      format as specified in [CRTP] Section 3.3.2, using 16-bit CIDs.
         Value: 2069 (hex)

   COMPRESSED_UDP_8
      The frame contains a datagram with a compressed  header  with  the
      format as specified in [CRTP] Section 3.3.3, using 8-bit CIDs.
         Value: 0067 (hex)

   COMPRESSED_UDP_16
      The frame contains a datagram with a compressed  header  with  the
      format as specified in [CRTP] Section 3.3.3, using 16-bit CIDs.
         Value: 2067 (hex)

   CONTEXT_STATE
      The frame is a link-level message sent from  the  decompressor  to
      the compressor as specified in [CRTP] Section 3.3.5.
         Value: 2065 (hex)






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5. References

   [CRTP]     Casner, S., Jacobson, V., "Compressing IP/UDP/RTP  Headers
              for  Low-Speed Serial Links", Internet-Draft
              draft-ietf-avt-crtp (work in progress), November 21, 1997,
              expires May 1998.

   [IPHC]     Degermark, M., Nordgren, B., Pink, S., "Header Compression
              for IP", Internet-Draft draft-degermark-ipv6-hc (work in
              progress), November 18, 1997, expires May 1998.

   [RFC2023]  Haskin, E., Allan, E., "IP Version 6 over PPP", RFC  2023,
              October 1996.

   [RFC1144]  Jacobson, V., "Compressing   TCP/IP   Headers   for   Low-
              Speed Serial Links", RFC 1144, February 1990.

   [RFC1332]  McGregor, G., "The PPP Internet Protocol Control  Protocol
              (IPCP)", RFC 1332, May 1992.

   [RFC1889]  Schulzrinne, H., Casner, S., Frederick, R., and  Jacobson,
              V.,  "RTP:  A  Transport  Protocol  for real-time applica-
              tions", RFC 1889, January 1996.

   [RFC1661]  Simpson, W., ed.,  "The  Point-To-Point  Protocol  (PPP)",
              RFC 1661, July 1994.

   [MCML]     Bormann, C.,  "The  Multi-Class  Extension  to  Multi-Link
              PPP", Internet-Draft draft-ietf-issll-isslow-mcml (work in
              progress), May 1997, expires November 1997.

6. Security Considerations

   Negotiation of the option defined here imposes no additional security
   considerations beyond those that otherwise apply to PPP [RFC1661].

   The use of header compression can, in  rare  cases,  cause  the  mis-
   delivery of packets. If necessary, confidentiality of packet contents
   should be assured by encryption. Encryption applied at the  IP  layer
   (e.g.,  using  IPSEC  mechanisms) precludes header compression of the
   encrypted headers, though compression of the   outer  IP  header  and
   authentication/security  headers  is  still  possible as described in
   [IPHC].  For RTP packets, full header compression is possible if  the
   RTP  payload is encrypted by itself without encrypting the UDP or RTP
   headers, as described in [RFC1889].  This method is appropriate  when
   the UDP and RTP header information need not be kept confidential.





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7. Authors' Addresses

   Mathias Engan
   CDT/Dept of Computer Communication
   Lulea University of Technology
   S-971 87 Lulea, Sweden
   Phone: +46 920 72288
   Mobile: +46 70 522 8109
   Fax: +46 920 72801
   EMail: engan@cdt.luth.se

   Stephen L. Casner
   Precept Software, Inc.
   1072 Arastradero Road
   Palo Alto, CA 94304
   United States
   EMail: casner@precept.com

   Carsten Bormann
   Universitaet Bremen FB3 TZI
   Postfach 330440
   D-28334 Bremen, GERMANY
   EMail: cabo@tzi.uni-bremen.de
   Phone: +49.421.218-7024
   Fax: +49.421.218-7000


























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