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Robust Header Compression (ROHC) Bakeoff July/August 2001 Roke Manor Research, UK

Robust Header Compression (ROHC) Bakeoff July/August 2001 Roke Manor Research, UK. Carsten Bormann, TZI. London. RFC 3095 Interop Testing (‘Bay Cough’). When? Just before IETF-51 (31st July – 3rd August) Who?

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Robust Header Compression (ROHC) Bakeoff July/August 2001 Roke Manor Research, UK

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  1. Robust Header Compression (ROHC)Bakeoff July/August 2001Roke Manor Research, UK Carsten Bormann, TZI

  2. London RFC 3095 Interop Testing (‘Bay Cough’) • When? • Just before IETF-51 (31st July – 3rd August) • Who? • Anyone with enough of an implementation…(half a dozen “expressions of interest” so far) • What? • Focus on RTP profile with IPv4 • Where? • Siemens/Roke Manor, Southampton • How? • Currently discussing technical details on the ROHC mailing list

  3. Who was there • Implementations • Effnet • Ericsson • Nokia • Siemens • Testers • Ericsson • Support • Universität Bremen • University of Arizona Would you trust these people with your packets?

  4. What did we test • ROHC over PPP • But without the negotiation protocol • Also: ROHC over Ethernet (as test harness, only) • Context setup, CID numbering, Mode changes, Packet formats, Feedback formats • Continuous stream of RTP packets • Basic voice call, including silence suppression/talkspurts • IP ID: • Irregular IP ID increments*, Byte swapped IP ID*, Random IP ID* • Change of IP TOS, Change of IP TTL, Change of RTP PT • Combination of IP / RTP field changes* • Wrap-around test (for IP ID, SN and TS) • Different TS_STRIDE (900 / 30ms) • Timer-based compression

  5. What were the problems • CRCs • LSB or MSB, and what exactly is the initialization • Solved: use PPP CRC (RFC1662, = Ethernet CRC) as guidance • http://www.dmn.tzi.org/ietf/rohc/rohc-crc.pl • Include this as example code in draft standard • Inefficiency in mode transition figures • The decompressor can stop sending CRCs in the middle of not only 5.6.3, but 5.6.5 or 5.6.6 as well (set D_TRANS to P) • Transition to timer-based compression requires care • In uncertainty period, need to keep k large enough to cover both interpretation intervals (window-based and timer-based)

  6. Other clarifications • Padding is not included in CRC (5.9.1) • In dynamic chain, there is one zero byte (or, if GP is set, two bytes) for the generic header extension list • NBO=1 really means network byte order • SN updates with CRC update TS and ID implicitly, unless there are explicit TS and ID fields in the packet • Switch type-0/-1 decoder between modes before mode transition completes • Yes, we are really sending CC twice in the RTP dynamic chain (ouch, but very little efficiency loss)

  7. Results • RFC 3095 appears to be a solid spec • Four implementations are out there that interoperate • Packet traces and example programs on ROHC web site • Another interop is needed to complete the work! • Re-test where implementation bugs were found • Profiles 0, 2, 3 • IPv6, extension headers, GRE, … • List-based compression (extension headers, CSRC) • More rigorous robustness testing • November in Tucson, AZ? • Avg day high 22 °C

  8. http://www.dmn.tzi.org/ietf/rohc

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