[AusNOG] TCP - Fast Retransmit / Normal Retransmit times

[Edwin Groothuis]

On 10/02/2015 6:13 pm, PRK wrote:
> Hi all,
> 
> Anyone full bottle on TCP at a base protocol level?
> 
> In particular, what's the expected behaviour if a Fast Retransmit (of a
> packet that's been lost/dropped) also gets lost/dropped?
> 
> How long would be expected before a normal retransmit should be sent?
> 
> I've been working through
> http://repo.hackerzvoice.net/depot_madchat/ebooks/TCP-IP_Illustrated/tcp_time.htm
> and other web resources, and I can see that a Fast Retransmit is after 3
> duplicate Acks are received - makes sense.
> 
> Separately, a normal retransmit will back off exponentially at 1.5, 3,
> 6, 12, 24, 48 & 64 seconds. Again, makes sense.
> 
> But I can't quite marry the two into the same situation. If the Fast
> Retransmit is lost/dropped, would I expect the 1.5 seconds until a
> normal retransmit, or would I expect 64 seconds, or somewhere inbetween?

The initiative for a Fast Retransmit lays by the receiver, while the
initiative for a normal retransmit lays by the sender.

If a Fast Retransmit sequence (3 duplicate ACKs) doesn't get received by
the sender of the original packet, then the sender will detect the
timeout and retransmit the original packet with the normal 1.5, 3, 6 etc
seconds interval.

Of course this is not the only way packet loss gets identified. The
receiver can send ACKs with Selective ACK Options send to indicate which
packets have not been received, and the sender can know the average RTT
and detect that ACKs for some of the data have not been received in the
expected time.

Edwin