some questions of tcp congestion window

some questions of tcp congestion window

[Dong Fang]

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?

Re: some questions of tcp congestion window

[Dong Fang]

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? :)

Re: some questions of tcp congestion window

[Eric Dumazet]

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 ;)