On 25. aug. 2014, at 10:19, Sebastian Moeller wrote: > Hi Michael, > > On Aug 25, 2014, at 10:01 , Michael Welzl <mich...@ifi.uio.no> wrote: > [...] >> >>> This is a case where a local proxy server can actually make a big >>> difference to you. The connections between your mobile devices and the >>> local proxy server have a short RTT and so all timeouts can be nice and >>> short, and then the proxy deals with the long RTT connections out to the >>> Internet. >> >> Adding a proxy to these considerations only complicates them: it's a hard >> enough trade-off when we just ask ourselves: how large should a buffer for >> the sake of link layer retransmissions be? (which is closely related to the >> question: how often should a link layer try to retransmit before giving up?) >> That's what my emails were about. I suspect that we don't have a good >> answer to even these questions, and I suspect that we'd better off having >> something dynamic than fixed default values. > > What about framing the retransmissions not in number but rather in > time? For example the maximum of either time to transmit a few (say 3?) > packet at the current data rate (or maybe one rate lower than current to > allow setoriating signal quality) or 20ms (pulled out of thin air, would need > some research). The first should make sure we actually retransmit to overcome > glitches, and the second should make sure that RTT does not increase to > dramatically. This basically assumes that for reasonable interactive traffic > we only have a given RTT budget and should make sure not to overspend ;)
That would be VERY good I think!!!! As for the actual recommendations, I think we'll also need a lower minimum to avoid that a single random collision that has nothing to do with any real overload causes a TCP reaction. THAT number could be 3, for example. Then, we typically have a default value of retransmissions in today's equipment - I think that number is usually something in the order of 10. So yes, that limit could be replaced with something like "min(10 retransmissions, 20ms)". I actually wonder what magnitude we can really reach in a practical system - e.g., is 20ms way beyond realistic in a super crowded network? Worth analyzing I guess. Cheers, Michael _______________________________________________ Bloat mailing list Bloat@lists.bufferbloat.net https://lists.bufferbloat.net/listinfo/bloat
