Re: some questions of tcp congestion window

[Eric Dumazet <eric.dumazet@gmail.com>]

On Thu, 2013-08-08 at 15:34 -0400, Dong Fang wrote:
> 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：
> >     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 == 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 == 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 =
> >     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 == sshresh << 1, enter
> >     CA progress, right?
> can anybody help me? :)

I suggest you download packetdrill and build a test ;)