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Network Working Group                               M. Degermark, EditorRequest for Comments: 3096                         University of ArizonaCategory: Informational                                        July 2001Requirements for robust IP/UDP/RTP header compressionStatus of this Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2001).  All Rights Reserved.Abstract   This document contains requirements for robust IP/UDP/RTP (Internet   Protocol/User Datagram Protocol/Real-Time Transport Protocol) header   compression to be developed by the ROHC (Robust Header Compression)   WG.  It is based on the ROHC charter, discussions in the WG, the 3GPP   document "3GPP TR 23.922", version 1.0.0 of October 1999, as well as   contributions from 3G.IP.1.  Introduction   The goal of the ROHC WG is to develop header compression schemes that   perform well over links with high error rates and long link round   trip times.  The schemes must perform well for cellular links built   using technologies such as WCDMA, EDGE, and CDMA-2000.  However, the   schemes should also be applicable to other future link technologies   with high loss and long round trip times.   The following requirements have, more or less arbitrarily, been   divided into three groups.  The first group deals with requirements   concerning the impact of an header compression scheme on the rest of   the Internet infrastructure.  The second group concerns what kind of   headers that must be compressed efficiently.  The final group   concerns efficiency requirements and requirements which stem from the   properties of the anticipated link technologies.2. Header compression requirements   Several current standardization efforts in the cellular arena aim at   supporting voice over IP and other real-time services over IP, e.g.,   GERAN (specified by the ETSI SMG2 standards group), and UTRANDegermark                    Informational                      [Page 1]

RFC 3096            Requirements for IP/UDP/RTP ROHC           July 2001   (specified by the 3GPP standards organization).  It is critical for   these standardization efforts that a suitable header compression   scheme is developed before completion of the Release 2000 standards.   Therefore, it is imperative that the ROHC WG keeps its schedule.2.1 Impact on Internet infrastructure   1. Transparency: When a header is compressed and then decompressed,   the resulting header must be semantically identical to the original   header.  If this cannot be achieved, the packet containing the   erroneous header must be discarded.   Justification: The header compression process must not produce   headers that might cause problems for any current or future part of   the Internet infrastructure.   2. Ubiquity: Must not require modifications to existing IP (v4 or   v6), UDP, or RTP implementations.   Justification: Ease of deployment.   Note: The ROHC WG may recommend changes that would increase the   compression efficiency for the RTP streams emitted by   implementations.  However, ROHC cannot rely on such recommendations   being followed.2.2 Supported headers and kinds of RTP streams   1. IPv4 and IPv6: Must support both IPv4 and IPv6.   Justification: IPv4 and IPv6 will both be around during the   foreseeable future.   2. Mobile IP: The kinds of headers used by Mobile IP{v4,v6} should be   compressed efficiently.  For IPv4 these include headers of tunneled   packets.  For IPv6 these include headers containing the Routing   Header, the Binding Update Destination Option, and the Home Address   option.   Justification: It is very likely that Mobile IP will be used by   cellular devices.   3. Genericity: Must support compression of headers of arbitrary RTP   streams.Degermark                    Informational                      [Page 2]

RFC 3096            Requirements for IP/UDP/RTP ROHC           July 2001   Justification: There must be a generic scheme which can compress   reasonably well for any payload type and traffic pattern.  This does   not preclude optimizations for certain media types where the traffic   pattern is known, e.g., for low-bandwidth voice and low-bandwidth   video.   Note: This applies to the RTP stream before as well as after it has   passed through an internet.   4. IPSEC: ROHC should be able to compress headers containing IPSEC   subheaders.   Note: It is of course not possible to compress the encrypted part of   an ESP header, nor the cryptographic data in an AH header.2.3 Efficiency   1. Performance/Spectral Efficiency: Must provide low relative   overhead under expected operating conditions; compression efficiency   should be better than forRFC 2508 under equivalent operating   conditions.  The error rate should only marginally increase the   overhead under expected operating conditions.   Justification: Spectrum efficiency is a primary goal.RFC 2508 does   not perform well enough.   Note: The relative overhead is the average header overhead relative   to the payload.  Any auxiliary (e.g., control or feedback) channels   used by the scheme should be taken into account when calculating the   header overhead.   2. Error propagation: Error propagation due to header compression   should be kept at an absolute minimum.  Error propagation is defined   as the loss or damage of headers subsequent to headers lost or   damaged by the link, even if those subsequent headers are not lost or   damaged.   Justification: Error propagation reduces spectral efficiency and   reduces quality.  CRTP suffers severely from error propagation.   Note: There are at least two kinds of error propagation; loss   propagation, where an error causes subsequent headers to be lost, and   damage propagation, where an error causes subsequent headers to be   damaged.Degermark                    Informational                      [Page 3]

RFC 3096            Requirements for IP/UDP/RTP ROHC           July 2001   3a. Handover loss events: There must be a way to run ROHC where loss   events of length 150 milliseconds are handled efficiently and where   loss or damage propagation is not introduced by ROHC during such   events.   Justification: Such loss events can be introduced by handover   operations in a (radio) network between compressor and decompressor.   Handover operations can be frequent in cellular systems.  Failure to   handle handover well can adversely impact spectrum efficiency and   quality.   3b. Handover context recreation: There must be a way to run ROHC that   deals well with events where the route of an RTP conversation changes   such that either the compressor or the decompressor of the   conversation is relocated to another node, where the context needs to   be recreated.  ROHC must not introduce erroneous headers during such   events, and should not introduce packet loss during such events.   Justification: Such events can be frequent in cellular systems where   the compressor/decompressor on the network side is close to the radio   base stations.   Note: In order to aid developers of context recreation schemes where   context is transfered to the new node, the specification shall   outline what parts of the context is to be transfered, as well as   conditions for its use.  Procedures for context recreation (and   discard) without such context transfer will also be provided.   4. Link delay: Must operate under all expected link delay conditions.   5. Processing delay: The scheme must not contribute significantly to   system delay budget.   6. Multiple links: The scheme must perform well when there are two or   more cellular links in the end-to-end path.   Justification: Such paths will occur.   Note: loss on previous links will cause irregularities in the RTP   stream reaching the compressor.  Such irregularities should only   marginally affect performance.   7a. Packet Misordering: The scheme should be able to compress when   there are misordered packets in the RTP stream reaching the   compressor.  No misordering is expected on the link between   compressor and decompressor.Degermark                    Informational                      [Page 4]

RFC 3096            Requirements for IP/UDP/RTP ROHC           July 2001   Justification: Misordering happens regularly in the Internet.   However, since the Internet is engineered to run TCP reasonably well,   excessive misordering will not be common and need not be handled with   optimum efficiently.   7b. Moderate Packet Misordering: The scheme should efficiently handle   moderate misordering (2-3 packets) in the packet stream reaching the   compressor.   Note: This kind of reordering is common.   8. Unidirectional links/multicast: Must operate (possibly with less   efficiency) over links where there is no feedback channel or where   there are several receivers.   9. Configurable frame size fluctuation: It should be possible to   restrict the number of different frame sizes used by the scheme.   Justification: Some radio technologies support only a limited number   of frame sizes efficiently.   Note: Somewhat degraded performance is to be expected when such   restrictions are applied.   Note: This implies that a list of "good" frame sizes must be made   available and that ROHC may pick a suitable header format to utilize   available space as well as possible.   10. Delay: ROHC should not noticeably add to the end-to-end delay.   Justification: RTP packets carrying data for interactive voice or   video have a limited end-to-end delay budget.   Note: This requirement is intended to prevent schemes that achieve   robustness at the expense of delay, for example schemes that require   that header i+x, x>0, is received before header i can be   decompressed.   Note: Together with 2.3.5, this requirement implies that ROHC will   not noticeably add to the jitter of the RTP stream, other than what   is caused by variations in header size.   Note: This requirement does not prevent a queue from forming upstream   a bottleneck link.  Nor does it prevent a compressor from utilizing   information from more than one header in such a queue.   11. Residual errors: For a residual bit-error rate of at most 1e-5,   the ROHC scheme must not increase the error rate.Degermark                    Informational                      [Page 5]

RFC 3096            Requirements for IP/UDP/RTP ROHC           July 2001   Justification: Some target links have a residual error rate, i.e,   rate of undetected errors, of this magnitude.   Note: In the presence of residual bit-errors, ROHC will need error   detection mechanisms to prevent damage propagation.  These mechanisms   will catch some residual errors, but those not caught might cause   damage propagation.  This requirement states that the reduction of   the damage rate due to the error detection mechanisms must not be   less than the increase in error rate due to damage propagation.3.  IANA Considerations   A protocol which meets these requirements, e.g., [ROHC], will require   the IANA to assign various numbers.  This document by itself,   however, does not require any IANA involvement.4.  Security Considerations   A protocol specified to meet these requirements, e.g., [ROHC], must   be able to compress packets containing IPSEC headers according to the   IPSEC requirement, 2.2.4.  There may be other security aspects to   consider in such protocols.  This document by itself, however, does   not add any security risks.5.  Editor's Address   Mikael Degermark   Dept. of Computer Science   University of Arizona   P.O. Box 210077   Tucson, AZ 85721-0077   USA   Phone: (May-July): +46 70 833-8933   Phone: +1 520 621-3489   Fax:   +1 520 621-4246   EMail: micke@cs.arizona.eduDegermark                    Informational                      [Page 6]

RFC 3096            Requirements for IP/UDP/RTP ROHC           July 20016.  References   [TR]      3GPP TR 23.922 version 1.0.0, 3rd Generation partnership             Project; Technical Specification Group Services and Systems             Aspects; Architecture for an All IP network.   [TS]      TS 22.101 version 3.6.0: Service Principles   [RFC768]  Postel, J., "User Datagram Protocol", STD 6,RFC 768,             August 1980.   [RFC791]  Postel, J., "Internet Protocol", STD 5,RFC 791, September             1981.   [RFC1144] Jacobson, V., "Compressing TCP/IP Headers for Low-Speed             Serial Links",RFC 1144, February 1990.   [CRTP]    Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP Headers             for Low-Speed Serial Links",RFC 2508, February 1999.   [OHC]    Bormann, C., Editor, "Robust Header Compression (ROHC)",RFC3095, June 2001.Degermark                    Informational                      [Page 7]

RFC 3096            Requirements for IP/UDP/RTP ROHC           July 20017.  Full 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.Degermark                    Informational                      [Page 8]