Date: Thu, 03 Sep 2015 21:13:29 +1000 From: Lawrence Stewart <lstewart@freebsd.org> To: hiren panchasara <hiren@strugglingcoder.info>, freebsd-net@freebsd.org Subject: Re: Value of congestion window (cwnd) when loss is detected Message-ID: <55E82B59.6000202@freebsd.org> In-Reply-To: <20150903005405.GN68814@strugglingcoder.info> References: <20150903005405.GN68814@strugglingcoder.info>
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On 09/03/15 10:54, hiren panchasara wrote: > 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. You want to read up about window inflation during fast recovery in RFC 5681 followed by 3782, and then consult Stevens vol 2 to understand how variables are used for different purposes depending on connection state and which code path was taken (something I greatly dislike and would love to change one day). Cheers, Lawrence
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