Network Working Group Mikael Degermark (editor) /University of Arizona INTERNET-DRAFT Sweden Expires: November 20, 2000 May 20, 2000 Requirements for IP/UDP/RTP robust header compression Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This document is a submission of the IETF ROHC WG. Comments should be directed to its mailing list, rohc@cdt.luth.se. Abstract This document gives draft requirements for robust IP/UDP/RTP header compression to be developed by the ROHC WG. It is based on the charter, the 3GPP document "3GPP TR 23.922", version 1.0.0 of october 1999 [TR], as well as contributions from 3G.IP. Degermark (Ed) [Page 1] INTERNET-DRAFT Requirements for IP/UDP/RTP ROHC May 20, 2000 0. History and Change Log May 20, 2000 - draft-ietf-rohc-rtp-requirements-01.txt added "robust" to the title of the document. 2.1.2 Mobile-IP: Added requirement for compression of Home Address option. 2.3.3 Cellular handover: Added note to requirement stating that handover procedure will not be prescribed as the procedure will be highly system-dependent. However, it must be possible to run ROHC such that handover is an efficient operation. 2.3.7 Packet misordering: Added note to explain why only moderate misordering needs to be handled efficiently. 2.3.10 Delay. New requirement. March 29, 2000 - draft-ietf-rohc-rtp-requirements-00.txt Initial version of this document. Distributed over the ROHC WG mailing list prior to 47th IETF in Adelaide. Degermark (Ed) [Page 2] INTERNET-DRAFT Requirements for IP/UDP/RTP ROHC May 20, 2000 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 roundtrip times. The schemes must perform well for cellular links build 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 roundtrip 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 UTRAN (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. Degermark (Ed) [Page 3] INTERNET-DRAFT Requirements for IP/UDP/RTP ROHC May 20, 2000 2.1 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. 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. 2.3 Efficiency 1. Performance/Spectral Efficiency: Must provide low relative overhead under expected operating conditions; compression efficiency should be better than for RFC2508 under equivalent error conditions. The error rate should only marginally increase the overhead under expected operating conditions. Justification: Spectrum efficiency is a primary goal. RFC2508 does not perform well enough. Notes: 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 of packets subsequent to packets damaged by the link, even if those subsequent packets are not damaged. Justification: Error propagation reduces spectral efficiency and reduces voice quality. CRTP suffers severely from error propagation. Degermark (Ed) [Page 4] INTERNET-DRAFT Requirements for IP/UDP/RTP ROHC May 20, 2000 3. Cellular handover: Cellular handover must be supported. The header compression scheme should not cause packet loss after handover. Justification: Handover can be a frequent operation in cellular systems. Failure to handle it well can adversely impact spectrum efficiency and voice quality. Note: "Supported" does not imply that a handover procedure will be prescribed by the ROHC specification. Handover is a highly system- dependent operation as is handover frequency and characteristics of loss events related to handover. "Supported" does imply that there must be a way to run ROHC that deals well with handover in target systems. The specification shall outline possible handover schemes and pitfalls. The specification will outline what parts of the context is to be transfered, as well as conditions for using transfered context, to aid developers of handover implementations that transfer context between compressors or decompressors. 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. 7. Packet Misordering: The scheme must tolerate moderate misordering in the packet stream reaching the compressor. No misordering is expected on the link between compressor and decompressor. Justification: Misordering happens regularly in the Internet. Note: Since the Internet is engineered to run TCP reasonably well, excessive misordering will not be common, and need not be handled efficiently. Moderate misordering (up to 2-3 packets) should be handled efficiently. 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 header size fluctuation: It should be possible to restrict the number of different header sizes used by the scheme. Degermark (Ed) [Page 5] INTERNET-DRAFT Requirements for IP/UDP/RTP ROHC May 20, 2000 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. 10. Delay: The scheme should not delay packets. Justification: RTP packets carrying data for interactive voice or video have a limited end-to-end delay budget. ROHC should not noticeably add to the end-to-end delay. Note: This requirement is intended to prevent schemes that achieve robustness at the expense of delay, for example schemes that require that packet i+x, x>0, is received before packet 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. 3. Editor's address Mikael Degermark Tel (May-July): +46 70 833-8933 Dept. of Computer Science Tel: +1 520 621-3489 University of Arizona Fax: +1 520 621-4246 P.O. Box 210077 Tucson, AZ 85721-0077 USA EMail: micke@cs.arizona.edu 4. 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 [RFC-768] J. Postel, User Datagram Protocol, RFC 761, August 1980. [RFC-791] J. Postel, Internet Protocol, RFC 791, September 1981. [RFC-1144] V. Jacobson, Compressing TCP/IP Headers for Low-Speed Serial Links, RFC 1144, February 1990. [CRTP] S. Casner, V. Jacobson, Compressing IP/UDP/RTP Headers Degermark (Ed) [Page 6] INTERNET-DRAFT Requirements for IP/UDP/RTP ROHC May 20, 2000 for Low-Speed Serial Links, RFC 2508. This draft expires November 20, 2000. Degermark (Ed) [Page 7]