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Network Working Group                                         A. ShachamRequest for Comments: 3173                                       JuniperObsoletes:2393                                               B. MonsourCategory: Standards Track                                     Consultant                                                              R. Pereira                                                                   Cisco                                                               M. Thomas                                                              Consultant                                                          September 2001IP 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 (2001).  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 [RFC1962])   ineffective.  If both compression and encryption are required,   compression must be applied before encryption.Shacham, et al.             Standards Track                     [Page 1]

RFC 3173            IP Payload Compression Protocol       September 2001   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 [RFC2119].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 [RFC2460], 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 3173            IP Payload Compression Protocol       September 20012.1. 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 [RFC0791], the compression is applied to the payload   of the IP datagram, starting at the first octet following the IP   header, and continuing through the last octet of the datagram.  No   portion of the IP header or the IP header options is compressed.   Note: In the case of an encapsulated IP header (e.g., tunnel mode   encapsulation in IPsec), the datagram payload is defined to start   immediately after the outer IP header; accordingly, the inner IP   header is considered part of the payload and 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 an 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 payload and the IPComp header, as   defined insection 3, is not smaller than the size of the original   payload, the IP datagram MUST be sent in the original non-compressed   form.  To clarify: If an IP datagram is sent non-compressed, noShacham, et al.             Standards Track                     [Page 3]

RFC 3173            IP Payload Compression Protocol       September 2001   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 the   MTU.   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 several IP   datagrams, say k, are sent without attempting compression.  If then   the next j datagrams also fail to compress, a larger number of   datagrams, say k+n, 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:Shacham, et al.             Standards Track                     [Page 4]

RFC 3173            IP Payload Compression Protocol       September 2001      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, [RFC1700].      Header Checksum         The Internet Header checksum [RFC0791] 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,         [RFC1700].   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.  Note: Other IPv6   headers may be present between the IPv6 Fragment Header and the   IPComp header.Shacham, et al.             Standards Track                     [Page 5]

RFC 3173            IP Payload Compression Protocol       September 20013.3.  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.   Compression Parameter Index (CPI)      16-bit index.  The CPI is stored in network order.  The values      0-63 designate 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 relationship 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.Shacham, et al.             Standards Track                     [Page 6]

RFC 3173            IP Payload Compression Protocol       September 20014. 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 policy for establishing IPComp may be either a node-to-node   policy where IPComp is applied to every IP packet between the nodes,   or a session-based policy where only selected sessions between the   nodes employ IPComp.   Two nodes may choose to negotiate IPComp in either or both   directions, and they may choose to employ a different compression   algorithm in each direction.  The nodes MUST, however, negotiate a   compression algorithm in each direction for which they establish an   IPCA: there is no default compression algorithm.   No compression algorithm is mandatory for an IPComp implementation.   The IPCA is established by dynamic negotiations or by manual   configuration.  The dynamic negotiations SHOULD use the Internet Key   Exchange protocol [IKE], where IPsec is present.  The dynamic   negotiations MAY be implemented through a different protocol.4.1. Use of IKE   For IPComp in the context of IP Security, IKE provides the necessary   mechanisms and guidelines for establishing IPCA.  Using IKE, IPComp   can be negotiated as stand-alone or in conjunction with other IPsec   protocols.   An IPComp Association is negotiated by the initiator using a Proposal   Payload, which includes one or more Transform Payloads.  The Proposal   Payload specifies the IP Payload Compression Protocol in the protocol   ID field and each Transform Payload contains the specific compression   algorithm(s) being offered to the responder.   The CPI is sent in the SPI field of the proposal, with the SPI size   field set to match.  The CPI SHOULD be sent as a 16-bit number, with   the SPI size field set to 2.  Alternatively, the CPI MAY be sent as a   32-bit value, with the SPI size field set to 4.  In this case, the   16-bit CPI number MUST be placed in the two least significant octets   of the SPI field, while the two most significant octets MUST be set   to zero, and MUST be ignored by the receiving node.  The receiving   node MUST be able to process both forms of the CPI proposal.Shacham, et al.             Standards Track                     [Page 7]

RFC 3173            IP Payload Compression Protocol       September 2001   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.   The following attributes are applicable to IPComp proposals:      Encapsulation Mode         To propose a non-default Encapsulation Mode (such as Tunnel         Mode), an IPComp proposal MUST include an Encapsulation Mode         attribute.  If the Encapsulation Mode is unspecified, the         default value of Transport Mode is assumed.      Lifetime         An IPComp proposal uses the Life Duration and Life Type         attributes to suggest life duration to the IPCA.   When IPComp is negotiated as part of a Protection Suite, all the   logically related offers must be consistent.  However, an IPComp   proposal SHOULD NOT include attributes that are not applicable to   IPComp.  An IPComp proposal MUST NOT be rejected because it does not   include attributes of other protocols in the Protection Suite that   are not relevant to IPComp.  When an IPComp proposal includes such   attributes, those attributes MUST be ignored when setting the IPCA,   and therefore ignored in the operation of IPComp.   Implementation note:      A node can avoid the computation necessary for determining the      compression algorithm from the CPI if it is using one of the      well-known algorithms; this can save time in the decompression      process.  A node can do this by negotiating a CPI equal in value      to the pre-defined Transform identifier of that compression      algorithm.  Specifically: A node MAY offer a CPI in the pre-      defined range by sending a Proposal Payload that MUST contain a      single Transform Payload, which is identical to the CPI.  When      proposing two or more Transform Payloads, a node MAY offer CPIs in      the pre-defined range by using multiple IPComp proposals -- each      MUST include a single Transform Payload.  To clarify: If a      Proposal Payload contains two or more Transform Payloads, the CPI      MUST be in the negotiated range.  A receiving node MUST be able to      process each of these proposal forms.Shacham, et al.             Standards Track                     [Page 8]

RFC 3173            IP Payload Compression Protocol       September 2001   Implementation note:      IPCAs become non-unique when two or more IPComp sessions are      established between two nodes, and the same well-known CPI is used      in at least two of the sessions.  Non-unique IPCAs pose problems      in maintaining attributes specific to each IPCA, either negotiated      (e.g., lifetime) or internal (e.g., the counters of the adaptive      algorithm for handling previously compressed payload).  To ensure      the uniqueness of IPCAs between two nodes, when two or more of the      IPCAs use the same compression algorithm, the CPIs SHOULD be in      the negotiated range.  However, when the IPCAs are not required to      be unique, for example when no attribute is being utilized for      these IPCAs, a well-known CPI MAY be used.  To clarify: When only      a single session using a particular well-known CPI is established      between two nodes, this IPCA is unique.4.2. Use of Non-IKE Protocol   The dynamic negotiations MAY be implemented through a protocol other   than IKE.  Such a 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 [RFC2003].  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 filteredShacham, et al.             Standards Track                     [Page 9]

RFC 3173            IP Payload Compression Protocol       September 2001   (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.6. IANA Considerations   This document does not require any IANA actions.  The well-known   numbers used in this document are defined elsewhere; see [SECDOI].7. Changes made sinceRFC 2393   This section summarizes the changes in this document fromRFC 2393 of   which an implementer ofRFC 2393 should be aware.  All the changes   are meant to clarify the negotiation of an IPComp Association (IPCA)   using IKE [IKE] in the context of IPsec.   1) Added a clarification that IPComp can be negotiated stand-alone or      bundled with other protocols in a Protection Suite.   2) Defined the CPI in the SPI field of an IKE proposal: two-octet      field is a SHOULD, four-octet a MAY.  Defined the placement of the      16-bit CPI in a four-octet field.  Specified that a receiver MUST      process both field sizes.   3) Added wording to define the default Encapsulation Mode to be      Transport Mode.  Required that an IPComp proposal MUST include an      Encapsulation Mode attribute when it suggests a non-default      encapsulation, such as Tunnel Mode.   4) Added the Lifetime attribute to the list of supported attributes      (along with Transport Mode).   5) Specified the handling of attributes of transforms in a Protection      Suite that are not applicable to IPComp: These attributes SHOULD      NOT be included in an IPComp proposal and MUST be ignored when      setting IPCA and in the operation of IPComp.  IPComp      implementations MUST never reject an IPCOMP proposal that does not      include attributes of other transforms.   6) Added implementation notes on the negotiation and usage of CPIs in      the predefined (well-known) range.Shacham, et al.             Standards Track                    [Page 10]

RFC 3173            IP Payload Compression Protocol       September 20018. References   [RFC0791] Postel, J., Editor, "Internet Protocol", STD 5,RFC 791,             September 1981.   [RFC1700] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2,RFC1700, October 1994.  Or see:http://www.iana.org/numbers.html   [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6             (IPv6) Specification",RFC 2460, December 1998.   [RFC1962] Rand, D., "The PPP Compression Control Protocol (CCP)",RFC1962, June 1996.   [RFC2003] Perkins, C., "IP Encapsulation within IP",RFC 2003,             October 1996.   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate             Requirement Levels",BCP 14,RFC 2119, March 1997.   [IKE]     Harkins, D. and D. Carrel, "The Internet Key Exchange             (IKE)",RFC 2409, 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 11]

RFC 3173            IP Payload Compression Protocol       September 2001Authors' Addresses   Abraham Shacham   Juniper Networks, Inc.   1194 North Mathilda Avenue   Sunnyvale, California 94089   United States of America   EMail: shacham@shacham.net   Bob Monsour   18 Stout Road   Princeton, New Jersey 08540   United States of America   EMail: bob@bobmonsour.com   Roy Pereira   Cisco Systems, Inc.   55 Metcalfe Street   Ottawa, Ontario K1P 6L5   Canada   EMail: royp@cisco.com   Matt Thomas   3am Software Foundry   8053 Park Villa Circle   Cupertino, California 95014   United States of America   EMail: matt@3am-software.comComments   Comments should be addressed to the ippcp@external.cisco.com mailing   list and/or the author(s).Shacham, et al.             Standards Track                    [Page 12]

RFC 3173            IP Payload Compression Protocol       September 2001Full Copyright Statement   Copyright (C) The Internet Society (2001).  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.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Shacham, et al.             Standards Track                    [Page 13]