Date: Wed, 2 Sep 2015 17:54:05 -0700 From: hiren panchasara <hiren@strugglingcoder.info> To: freebsd-net@FreeBSD.org Subject: Value of congestion window (cwnd) when loss is detected Message-ID: <20150903005405.GN68814@strugglingcoder.info>
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--/UvyDgxjlFfP/4zZ Content-Type: text/plain; charset=us-ascii Content-Disposition: inline I am failing to understand the reason behind this behavior. What should the congestion window (snd_cwnd) be set to when we hit loss? It seems that we set it to 1 segment right now. https://svnweb.freebsd.org/base/head/sys/netinet/tcp_input.c?revision=286227&view=markup#l2531 I also see that in the simulations I did. Sender side pcap can be found at: https://people.freebsd.org/~hiren/pcaps/single_packet_loss.pcap Trying to send 50kb of data from freebsd 10.2 server to freebsd client. Initial cwnd is 10 so we blast out 10 packets but 1 packet gets dropped: seq 2897:4345. We get 3 dupacks and we retransmit it. But as soon as we detect this loss, we reduce cwnd to 1 segment. In fact, we could've used data in SACK to see how much we could send on the n/w, imo. 3rd dup ack (which triggered the retransmit) looks like this: IP 192.168.11.10.41674 > 192.168.10.10.http: Flags [.], ack 2897, win 12579, options [nop,nop,TS val 4236220288 ecr 3905376863,nop,nop,sack 1 {4345:10137}], length 0 And the retransmit: IP 192.168.10.10.http > 192.168.11.10.41674: Flags [.], seq 2897:4345, ack 172, win 12579, options [nop,nop,TS val 3905376894 ecr 4236220288], length 1448 At this point in time, sender knows that it has sent 23169 bytes (last packet server sent was seq 21721:23169) and received ack for 10137 bytes minus a missing packet = 8689 bytes. i.e. 6 packets. So, there is at least that much room on n/w at that point in time. We can go conservative and halve that. i.e. 3 packets. That is still better than going down to 1 packet. Is there something basic I am missing here? Any insights would be helpful. Cheers, Hiren --/UvyDgxjlFfP/4zZ Content-Type: application/pgp-signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.22 (FreeBSD) iQF8BAEBCgBmBQJV55otXxSAAAAAAC4AKGlzc3Vlci1mcHJAbm90YXRpb25zLm9w ZW5wZ3AuZmlmdGhob3JzZW1hbi5uZXRBNEUyMEZBMUQ4Nzg4RjNGMTdFNjZGMDI4 QjkyNTBFMTU2M0VERkU1AAoJEIuSUOFWPt/l5YUH/06EVmLj18+NgLRZb8qmvbZZ RTSQm8jK37ha1CjXS4VJItXo3FImTEiJd/NMqnYUFlKrn9DymVZt03q+9H3E2hl4 ZtZ9mBkfvklwk0euaE0N5Ip7RVVS6cE5sYRCmNUe1IEomjBKpv8sOVZsHQ4+ajfX qS/3rbHvIl83VHq7jE7bZmXrjc75Ckd0Wm7gSNDAfoDOpDjMAXg6rjx6TH+KH/Eh uoesRscqm1XpQKJk8zx2xrF71as+n6TeXICneoCpdzNOWnctf8jA1F/cjNAHbScJ /mhg67GjtFAmPCV/g2F7clTL+3xSVaBscbEyY0EjFRnkbCpE1M0D5vQZ9qKh6Ko= =Trgd -----END PGP SIGNATURE----- --/UvyDgxjlFfP/4zZ--
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