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Network Working Group                                         A. ShachamRequest for Comments: 2393                                         CiscoCategory: Standards Track                                     R. Monsour                                                                   Hi/fn                                                              R. Pereira                                                                TimeStep                                                               M. Thomas                                                      AltaVista Internet                                                           December 1998IP Payload Compression Protocol (IPComp)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 Internet Society (1998).  All Rights Reserved.Abstract   This document describes a protocol intended to provide lossless   compression for Internet Protocol datagrams in an Internet   environment.1. Introduction   IP payload compression is a protocol to reduce the size of IP   datagrams.  This protocol will increase the overall communication   performance between a pair of communicating hosts/gateways ("nodes")   by compressing the datagrams, provided the nodes have sufficient   computation power, through either CPU capacity or a compression   coprocessor, and the communication is over slow or congested links.   IP payload compression is especially useful when encryption is   applied to IP datagrams.  Encrypting the IP datagram causes the data   to be random in nature, rendering compression at lower protocol   layers (e.g., PPP Compression Control Protocol [RFC-1962])   ineffective.  If both compression and encryption are required,   compression MUST be applied before encryption.Shacham, et. al.            Standards Track                     [Page 1]

RFC 2393                         IPComp                    December 1998   This document defines the IP payload compression protocol (IPComp),   the IPComp packet structure, the IPComp Association (IPCA), and   several methods to negotiate the IPCA.   Other documents shall specify how a specific compression algorithm   can be used with the IP payload compression protocol.  Such   algorithms are beyond the scope of this document.1.1. Specification of Requirements   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 inRFC 2119 [RFC-2119].2. Compression Process   The compression processing of IP datagrams has two phases:   compressing of outbound IP datagrams ("compression") and   decompressing of inbound datagrams ("decompression").  The   compression processing MUST be lossless, ensuring that the IP   datagram, after being compressed and decompressed, is identical to   the original IP datagram.   Each IP datagram is compressed and decompressed by itself without any   relation to other datagrams ("stateless compression"), as IP   datagrams may arrive out of order or not arrive at all.  Each   compressed IP datagram encapsulates a single IP payload.   Processing of inbound IP datagrams MUST support both compressed and   non-compressed IP datagrams, in order to meet the non-expansion   policy requirements, as defined insection 2.2.   The compression of outbound IP datagrams MUST be done before any IP   security processing, such as encryption and authentication, and   before any fragmentation of the IP datagram.  In addition, in IP   version 6 [RFC-2460], the compression of outbound IP datagrams MUST   be done before the addition of either a Hop-by-Hop Options header or   a Routing Header, since both carry information that must be examined   and processed by possibly every node along a packet's delivery path,   and therefore MUST be sent in the original form.   Similarly, the decompression of inbound IP datagrams MUST be done   after the reassembly of the IP datagrams, and after the completion of   all IP security processing, such as authentication and decryption.Shacham, et. al.            Standards Track                     [Page 2]

RFC 2393                         IPComp                    December 19982.1. Compressed Payload   The compression is applied to a single array of octets, which are   contiguous in the IP datagram.  This array of octets always ends at   the last octet of the IP packet payload.  Note: a contiguous array of   octets in the IP datagram may be not contiguous in physical memory.   In IP version 4 [RFC-0791], the compression is applied to the upper   layer protocol (ULP) payload of the IP datagram.  No portion of the   IP header or the IP header options is compressed.   In the IPv6 context, IPComp is viewed as an end-to-end payload, and   MUST not apply to hop-by-hop, routing, and fragmentation extension   headers.  The compression is applied starting at the first IP Header   Option field that does not carry information that must be examined   and processed by nodes along a packet's delivery path, if such IP   Header Option field exists, and continues to the ULP payload of the   IP datagram.   The size of a compressed payload, generated by the compression   algorithm, MUST be in whole octet units.   As defined insection 3, an IPComp header is inserted immediately   preceding the compressed payload.  The original IP header is modified   to indicate the usage of the IPComp protocol and the reduced size of   the IP datagram.  The original content of the Next Header (IPv6) or   protocol (IPv4) field is stored in the IPComp header.   The decompression is applied to a single contiguous array of octets   in the IP datagram.  The start of the array of octets immediately   follows the IPComp header and ends at the last octet of the IP   payload.  If the decompression process is successfully completed, the   IP header is modified to indicate the size of the decompressed IP   datagram, and the original next header as stored in the IPComp   header.  The IPComp header is removed from the IP datagram and the   decompressed payload immediately follows the IP header.2.2. Non-Expansion Policy   If the total size of a compressed ULP payload and the IPComp header,   as defined insection 3, is not smaller than the size of the original   ULP payload, the IP datagram MUST be sent in the original non-   compressed form.  To clarify:  If an IP datagram is sent non-   compressed, no IPComp header is added to the datagram.  This policy   ensures saving the decompression processing cycles and avoiding   incurring IP datagram fragmentation when the expanded datagram is   larger than MTU.Shacham, et. al.            Standards Track                     [Page 3]

RFC 2393                         IPComp                    December 1998   Small IP datagrams are likely to expand as a result of compression.   Therefore, a numeric threshold should be applied before compression,   where IP datagrams of size smaller than the threshold are sent in the   original form without attempting compression.  The numeric threshold   is implementation dependent.   An IP datagram with payload that has been previously compressed tends   not to compress any further.  The previously compressed payload may   be the result of external processes, such as compression applied by   an upper layer in the communication stack, or by an off-line   compression utility.  An adaptive algorithm should be implemented to   avoid the performance hit.  For example, if the compression of i   consecutive IP datagrams of an IPCA fails, the next k IP datagrams   are sent without attempting compression.  If the next j datagrams are   also failing to compress, the next k+n datagrams are sent without   attempting compression.  Once a datagram is compressed successfully,   the normal process of IPComp restarts.  Such an adaptive algorithm,   including all the related thresholds, is implementation dependent.   During the processing of the payload, the compression algorithm MAY   periodically apply a test to determine the compressibility of the   processed data, similar to the requirements of [V42BIS].  The nature   of the test is algorithm dependent.  Once the compression algorithm   detects that the data is non-compressible, the algorithm SHOULD stop   processing the data, and the payload is sent in the original non-   compressed form.3. Compressed IP Datagram Header Structure   A compressed IP datagram is encapsulated by modifying the IP header   and inserting an IPComp header immediately preceding the compressed   payload.  This section defines the IP header modifications both in   IPv4 and IPv6, and the structure of the IPComp header.3.1. IPv4 Header Modifications   The following IPv4 header fields are set before transmitting the   compressed IP datagram:      Total Length         The length of the entire encapsulated IP datagram, including         the IP header, the IPComp header and the compressed payload.      Protocol         The Protocol field is set to 108, IPComp Datagram, [RFC-1700].Shacham, et. al.            Standards Track                     [Page 4]

RFC 2393                         IPComp                    December 1998      Header Checksum         The Internet Header checksum [RFC-0791] of the IP header.   All other IPv4 header fields are kept unchanged, including any header   options.3.2. IPv6 Header Modifications   The following IPv6 header fields are set before transmitting the   compressed IP datagram:      Payload Length         The length of the compressed IP payload.      Next Header         The Next Header field is set to 108, IPComp Datagram, [RFC-         1700].   All other IPv6 header fields are kept unchanged, including any non-   compressed header options.   The IPComp header is placed in an IPv6 packet using the same rules as   the IPv6 Fragment Header.  However if an IPv6 packet contains both an   IPv6 Fragment Header and an IPComp header, the IPv6 Fragment Header   MUST precede the IPComp header in the packet.3.3.  IPComp Header Structure   The four-octet header has the following structure:   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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |  Next Header  |     Flags     | Compression Parameter Index |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Next Header        8-bit selector.  Stores the IPv4 Protocol field or the IPv6 Next        Header field of the original IP header.   Flags        8-bit field.  Reserved for future use.  MUST be set to zero.        MUST be ignored by the receiving node.Shacham, et. al.            Standards Track                     [Page 5]

RFC 2393                         IPComp                    December 1998   Compression Parameter Index (CPI)        16-bit index.  The CPI is stored in network order.  The values        0-63 define well-known compression algorithms, which require no        additional information, and are used for manual setup.  The        values themselves are identical to IPCOMP Transform identifiers        as defined in [SECDOI].  Consult [SECDOI] for an initial set of        defined values and for instructions on how to assign new values.        The values 64-255 are reserved for future use.  The values        256-61439 are negotiated between the two nodes in definition of        an IPComp Association, as defined insection 4.  Note: When        negotiating one of the well-known algorithms, the nodes MAY        select a CPI in the pre-defined range 0-63.  The values        61440-65535 are for private use among mutually consenting        parties.  Both nodes participating can select a CPI value        independently of each other and there is no relationships        between the two separately chosen CPIs.  The outbound IPComp        header MUST use the CPI value chosen by the decompressing node.        The CPI in combination with the destination IP address uniquely        identifies the compression algorithm characteristics for the        datagram.4. IPComp Association (IPCA) Negotiation   To utilize the IPComp protocol, two nodes MUST first establish an   IPComp Association (IPCA) between them.  The IPCA includes all   required information for the operation of IPComp, including the   Compression Parameter Index (CPI), the mode of operation, the   compression algorithm to be used, and any required parameter for the   selected compression algorithm.  The IPComp mode of operation is   either a node-to-node policy where IPComp is applied to every IP   packet between the nodes, or an ULP session based policy where only   selected ULP sessions between the nodes are using IPComp.  For each   IPCA, a different compression algorithm may be negotiated in each   direction, or only one direction may be compressed.  The default is   "no IPComp compression".   The IPCA is established by dynamic negotiations or by manual   configuration.  The dynamic negotiations SHOULD use the Internet   Security Association and Key Management Protocol [ISAKMP], where   IPSec is present.  The dynamic negotiations MAY be implemented   through a different protocol.4.1. Use of ISAKMP   For IPComp in the context of IP Security, ISAKMP provides the   necessary mechanisms to establish IPCA.  IPComp Association is   negotiated by the initiator using a Proposal Payload, which wouldShacham, et. al.            Standards Track                     [Page 6]

RFC 2393                         IPComp                    December 1998   include one or more Transform Payloads.  The Proposal Payload would   specify a compression protocol in the protocol id field and each   Transform Payload would contain the specific compression method(s)   being offered to the responder.   In the Internet IP Security Domain of Interpretation (DOI), IPComp is   negotiated as the Protocol ID PROTO_IPCOMP.  The compression   algorithm is negotiated as one of the defined IPCOMP Transform   Identifiers.4.2. Use of Non-ISAKMP Protocol   The dynamic negotiations MAY be implemented through a protocol other   than ISAKMP.  Such protocol is beyond the scope of this document.4.3. Manual Configuration   Nodes may establish IPComp Associations using manual configuration.   For this method, a limited number of Compression Parameters Indexes   (CPIs) is designated to represent a list of specific compression   methods.5. Security Considerations   When IPComp is used in the context of IPSec, it is believed not to   have an effect on the underlying security functionality provided by   the IPSec protocol; i.e., the use of compression is not known to   degrade or alter the nature of the underlying security architecture   or the encryption technologies used to implement it.   When IPComp is used without IPSec, IP payload compression potentially   reduces the security of the Internet, similar to the effects of IP   encapsulation [RFC-2003].  For example, IPComp may make it difficult   for border routers to filter datagrams based on header fields.  In   particular, the original value of the Protocol field in the IP header   is not located in its normal positions within the datagram, and any   transport layer header fields within the datagram, such as port   numbers, are neither located in their normal positions within the   datagram nor presented in their original values after compression.  A   filtering border router can filter the datagram only if it shares the   IPComp Association used for the compression.  To allow this sort of   compression in environments in which all packets need to be filtered   (or at least accounted for), a mechanism must be in place for the   receiving node to securely communicate the IPComp Association to the   border router.  This might, more rarely, also apply to the IPComp   Association used for outgoing datagrams.Shacham, et. al.            Standards Track                     [Page 7]

RFC 2393                         IPComp                    December 19986. References   [RFC-0791] Postel, J., Editor, "Internet Protocol", STD 5,RFC 791,              September 1981.   [RFC-1700] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2,RFC 1700, October 1994.  Or see:http://www.iana.org/numbers.html   [RFC-2460] Deering, S., and R. Hinden, "Internet Protocol, Version 6              (IPv6) Specification",RFC 2460, December 1998.   [RFC-1962] Rand, D., "The PPP Compression Control Protocol (CCP)",RFC 1962, June 1996.   [RFC-2003] Perkins, C., "IP Encapsulation within IP",RFC 2003,              October 1996.   [RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [ISAKMP]   Maughan, D., Schertler, M., Schneider, M., and J. Turner,              "Internet Security Association and Key Management Protocol              (ISAKMP)",RFC 2408, November 1998.   [SECDOI]   Piper, D., "The Internet IP Security Domain of              Interpretation for ISAKMP",RFC 2407, November 1998.   [V42BIS]   CCITT, "Data Compression Procedures for Data Circuit              Terminating Equipment (DCE) Using Error Correction              Procedures", Recommendation V.42 bis, January 1990.Shacham, et. al.            Standards Track                     [Page 8]

RFC 2393                         IPComp                    December 1998Authors' Addresses   Abraham Shacham   Cisco Systems   170 West Tasman Drive   San Jose, California 95134   United States of America   EMail: shacham@cisco.com   Robert Monsour   Hi/fn Inc.   2105 Hamilton Avenue, Suite 230   San Jose, California 95125   United States of America   EMail: rmonsour@hifn.com   Roy Pereira   TimeStep Corporation   362 Terry Fox Drive   Kanata, Ontario K2K 2P5   Canada   EMail: rpereira@timestep.com   Matt Thomas   AltaVista Internet Software   30 Porter Road   Littleton, Massachusetts 01460   United States of America   EMail: matt.thomas@altavista-software.comWorking Group   The IP Payload Compression Protocol (IPPCP) working group can be   contacted through its chair:   Naganand Dorswamy   Bay Networks   EMail: naganand@baynetworks.comShacham, et. al.            Standards Track                     [Page 9]

RFC 2393                         IPComp                    December 1998Full Copyright Statement   Copyright (C) The Internet Society (1998).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Shacham, et. al.            Standards Track                    [Page 10]