[AusNOG] TCP - Fast Retransmit / Normal Retransmit times

[Tony]

I think the original reply from Edwin had some good information. 

The way I read it, the Fast Retransmit is initiated by the receiver sending 3+ ACKs back for a segment that has not been received yet. If the receiver still doesn't get the segment in question, it's going to keep ACKing for it.
If you read this RFC:

https://tools.ietf.org/html/rfc5681#section-3.2

you can see that there is a backoff for retransmitting the lost segment if the ACK's keep coming back for the lost segment. There is also a method for getting back to 'normal' transmission after such an event.



________________________________
From: PRK <ausnog at digitaljunkie.net>
To: ausnog at lists.ausnog.net 
Sent: Tuesday, 10 February 2015, 22:31
Subject: Re: [AusNOG] TCP - Fast Retransmit / Normal Retransmit times


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.



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