From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dong Fang Subject: Re: some questions of tcp congestion window Date: Thu, 08 Aug 2013 15:34:41 -0400 Message-ID: <5203F2D1.3030104@gmail.com> References: <5203F0DD.1090007@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE To: netdev@vger.kernel.org, Dong Fang Return-path: Received: from mail-pa0-f50.google.com ([209.85.220.50]:56271 "EHLO mail-pa0-f50.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757639Ab3HHHfH (ORCPT ); Thu, 8 Aug 2013 03:35:07 -0400 Received: by mail-pa0-f50.google.com with SMTP id fb10so3092228pad.9 for ; Thu, 08 Aug 2013 00:35:06 -0700 (PDT) In-Reply-To: <5203F0DD.1090007@gmail.com> Sender: netdev-owner@vger.kernel.org List-ID: On 08/08/2013 03:26 PM, Dong Fang wrote: > hi, all > > I'am reading the tcp/ip network source code(kernel 3.10), i was > fogged by the congestion window. so, i want to confirm something > about it: > > Note: sack is disable. and A is sender, B is receiver. > > 1. at time t, the number of packets in flight is 100. like this=EF=BC= =9A > u, u+1, u+2, ..., u+99 > and suppose u is lost and all other packets are not. so bwtween > t:t+RTT we would have retransmitted u and received 99 dupack. > all of this dupack's ack =3D=3D u, right? > > when A recieved 3 dupack, it changed to TCP_CA_Recovery state. > in this state, the congestion window won't grow any more. > then, A retransmit U packet. after that, if A receive a new ack, > acked all the packets in flight, so the sock state is changed > to TCP_CA_Open. right? > > 2. at time t, the number of packets in flight is 100, like this: > u, u+1, u+2, ..., u+99 > and suppose u and u+5 is lost and all other packets are not. > between t:t+RTT, we should have retransmitted u and received 99 > dupack, all of this dupack's ack =3D=3D u, right? > > when A recieved 3 dupack, it changed to Recovery state, then > retransmit U packet, after that, if A receive a new ack, > this ack is only acked for u+5, at this time, current sshresh =3D > cwnd/2 + 5(the first 5 packets was acked), but cwnd > sshresh. > so A won't send any packets to B, and B won't send any ack to > A too, because B have beed send 99 dupack and 1 new ack, it > have done its work, right? the only way to let B send ack to > A is, the retransmit timeout of u+5 packet. > > when A was retransmit u+5 packet to B, then B send packet ack > for u+100 to A, this time, the A's cwnd =3D=3D sshresh << 1, ente= r > CA progress, right? can anybody help me? :)