[AusNOG] TCP - Fast Retransmit / Normal Retransmit times

[PRK]

On 2015-02-10 21:34, Edwin Groothuis wrote:
> On 10/02/2015 6:13 pm, PRK wrote:

>> 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?

> 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.

The scenario I'm taking about is when a packet gets dropped, so the 
receiver sends back duplicate acks, to indicate it missed that sequence.

The sender gets those three acks, so initiates a fast retransmit of the 
missing sequence.

However, that fast transmit also suffers the same fate as the original 
packet, and never reaches the receiver.

So the sender thinks it's initiated a fast retransmit, the receiver has 
dup acked all the packets it got in the window size as they weren't the 
missing sequence.

Now, how long does the sender wait, after sending that fast retransmit 
and not getting an ack, before it initiates a normal re-transmit.

prk.