TZI Digitale Medien und Netze © 2000 Carsten Bormann / Jörg Ott rohc Robust Header Compression 49. IETF December 2000 San Diego Chairs: Carsten Bormann Mikael Degermark Mailing List:

TZI Digitale Medien und Netze © 2000 Carsten Bormann / Jörg Ott ROHC RObust Header Compression uHeader compression is prerequisite for all-IP wireless uWireless = lossy, long latency (multiple packets in flight) uProblem: RFC2508 (CRTP) causes loss propagation on packet losses with long RTT links Basic idea: uNo delta encoding! uExpose (LSBs of) the RTP sequence number in the compressed packet; key everything off that –R-mode (reliable): Use ACKs to synchronize state –O-mode (optimistic): Use CRCs to verify synchronization –U-mode (unidirectional): Send info often enough

TZI Digitale Medien und Netze © 2000 Carsten Bormann / Jörg Ott ROHC WG uChairs: Carsten Bormann (TZI), Mikael Degermark (U Arizona) uhttp:// Work Items: uRobust Header Compression for IP/UDP/RTP –Needed for e2e VoIP/video conferencing as well as streaming –Transparent solution nearing completion (Dec 2000 timeframe) –Non-transparent extensions may follow uRobust Header Compression for TCP –Starting now (robustness can easily aided by L2 retransmission)

TZI Digitale Medien und Netze © 2000 Carsten Bormann / Jörg Ott RTP ROHC document status: WG last-call uEnd-date: about 1400Z udraft-ietf-rohc-rtp-lower-layer-guidelines-00.txt (Oct 12) –No last-call comments yet udraft-ietf-rohc-rtp-requirements-03.txt (Nov 20) –Few last-call comments udraft-ietf-rohc-rtp-06.txt (Nov 29): RTP ROHC –Main deliverable –156 pages –~ 15 WG last-call comments so far

TZI Digitale Medien und Netze © 2000 Carsten Bormann / Jörg Ott ROHC: Charter (4) Goals and Milestones uMar: I-D on Requirements for IP/UDP/RTP HC. uMay: I-D of layer-2 design guidelines. uMay: I-D(s) proposing IP/UDP/RTP HC schemes. uMay: I-D of Requirements for IP/TCP HC. uJun: Requirements for IP/UDP/RTP HC submitted to IESG (Inf.) uJul: Requirements for IP/TCP HC submitted to IESG (Inf.) uJul: Resolve possibly multiple IP/UDP/RTP HC schemes into a single scheme. uAug: I-D on IP/TCP header compression scheme. uSep: Layer-2 design guidelines submitted to IESG (Inf.)  TCP g/l uSep: IP/UDP/RTP HC scheme submitted to IESG (PS) uDec: IP/TCP HC scheme submitted to IESG (PS) uJan: Possible recharter of WG to develop additional HC schemes. Done in last-call Start now To do

TZI Digitale Medien und Netze © 2000 Carsten Bormann / Jörg Ott ROHC TCP – why develop separately? uThe requirements for robustness may be less stringent –Can do retransmission at link layer (see PILC) uLess stringent time constraints on development uDifferent protocol than RTP (obviously) u2507 is not enough: Options like SACK, timestamps –Need to compress ECN bits well! uSolicit wider input wrt next generation TCP compression –But is this maybe still a researchy topic? uTwo drafts right now: –draft-ietf-rohc-tcp-taroc-00.txt –TCP over EPIC (distributed on mailing list)

TZI Digitale Medien und Netze © 2000 Carsten Bormann / Jörg Ott ROHC TCP Requirements uDifferent link properties –No residual errors, but may have packet loss uRobustness: –Should not disable [might even help] TCP mechanisms t fast retransmit, fast repair, etc –MUST NOT generate damaged headers (that can pass TCP chksum!) –Must deal with current and future TCPs t SACK, timestamp, ECN, Diffserv, Initial TCP negotiation, etc –TCP sequence numbers and IP ID less predictable uMight want it to work well for short-lived TCP transfers? uSolve known problems with TCP Checksum –Window scale option – satellite links (loss of 64K undetectable) –window field decrement + seq no increment (rfc1144)

TZI Digitale Medien und Netze © 2000 Carsten Bormann / Jörg Ott Call for help uROHC TCP design will be influenced by link layers: –Loss rate, loss patterns, residual bit error rate, latency, latency distribution, queueing behavior, channel variants, … uROHC TCP needs documented information on link layers –What is out there that will be used below ROHC TCP –What can we expect in the next ~ 5 years t In particular, what would be reasonable to build uLink layer designers need information about ROHC TCP –Document our assumptions so they know what to select uROHC TCP Lower Layer Guidelines Document u(Help with the ROHC TCP scheme is appreciated, too) uwww.dmn.tzi.org/ietf/rohc