Joe Touch skrev: > > > Iljitsch van Beijnum wrote: >> On 19 mei 2009, at 22:26, Joe Touch wrote: > >>> The problem is that when a 1480 packet arrives and you do ROHC, it's >>> 1450 only most of the time; sometimes it's 1481 because it needs the >>> full original header and the ROHC stream ID. Those "big" packets are >>> required by ROHC to resync, and would be dropped after IPsec. >> In those cases the packet would be truncated so there's enough for ROHC >> to resync but obviously the receiver doesn't get the original packet so >> the sender will have to retransmit. This is not a problem if the >> retransmitted packet does compress so it fits. > > Ah - OK, that works. Not efficiently, but it works. >
For TCP yes, not for UDP or other protocols. >> So as long as we can accurately predict which packets will compress >> enough to fit upon retransmission it's safe to drop them if they do and >> we send a too big if they don't. > >>> Even if TCP does fast retransmit, at some point the stream needs to get >>> through a payload of 1481, which will never get through. The result is >>> that ROHC will never resync, and *everything* will get thrown on the >>> floor... >> Didn't you just suggest to just send enough of the original packet to >> make ROHC do its thing? That piece of the packet should be small enough >> to always fit. > > Yes, except that I also suggested sending the packet without any ROHC on > it too. That works only if ROHC is NOT used in the computation to > increase the MTU. > > I.e., IMO ROHC ought NOT be used to increase the MTU. It ought to be > used primarily to reduce header overhead. It's a bit silly to do all > this work for 2%. I am starting to think the same, simply declare the MTU that you know will always work. With an IPsec tunnel, you will anyway have a smaller MTU than the regular link one, so descreasing it some bytes more should not be an issue. Cheers Magnus Westerlund IETF Transport Area Director & TSVWG Chair ---------------------------------------------------------------------- Multimedia Technologies, Ericsson Research EAB/TVM ---------------------------------------------------------------------- Ericsson AB | Phone +46 10 7148287 Färögatan 6 | Mobile +46 73 0949079 SE-164 80 Stockholm, Sweden| mailto: [email protected] ----------------------------------------------------------------------
