On Thu, Feb 08, 2007 at 10:44:41PM +0000, Michael Rogers wrote: > Matthew Toseland wrote: > > I don't understand. We can indicate how long a packet had to wait for > > coalescing, and therefore record a value based on the network delay, > > with a bit of the CPU delay thrown in, no? > > Right, we can get an accurate measure of the RTT by using timestamps, > but we still have to delay retransmissions by an extra 100ms in case the > ack is being held for coalescing (we won't know until it arrives). The > extra lag makes congestion control harder - you can't really do things > like fast retransmission if the acks for packets that were sent almost > simultaneously can be separated by 100ms.
Fast retransmission = If I get acks for 2,3,4 but not 1, then I resend 1, after X RTTs have elapsed? > > Maybe I'm getting too caught up in the TCP way of doing things, but when > I look at the amount of work that's gone into the design of TCP and the > number of subtle problems they've found, I start to doubt that we can do > better from scratch... > > > 52 = 5% * 1000. The probability of a packet being dropped is less than > > 5% on most useful links. > > Sorry, I don't see how the loss rate of the link is relevant - I'm > talking about the overhead of sending an ack straight away (52 bytes) > versus the overhead of retransmitting a packet unnecessarily (1000 > bytes). Unnecessary retransmissions happen when the RTT variance is > high, which it is at the moment because acks are held for anywhere > between 0 and 100ms. Currently we retransmit a packet if it hasn't been > acked for 4 * RTT + MAX_DELAY, but we don't know what fraction of acks > arrive after 4 * RTT + MAX_DELAY. If it's more than 5% then it would be > cheaper to send the acks straight away. Why would it be more than 4*RTT+MAX_DELAY? > > Cheers, > Michael -------------- next part -------------- A non-text attachment was scrubbed... Name: signature.asc Type: application/pgp-signature Size: 189 bytes Desc: Digital signature URL: <https://emu.freenetproject.org/pipermail/devl/attachments/20070209/d31f62e3/attachment.pgp>
