Subject: [Tsvwg] Fast retransmit and RTO re-start

From: Gil Jose-josegil1 <[email-masked]>
Date: Mon, 16 Jun 2003 12:57:28 +0100

I would like to know (confirmation) if the RTO timer is re-started when a
fast retransmit happens, that is, the RTO re-started as soon as the detected
lost segment is retransmitted.  I couldn't get a clear answer on this
specific question from reading RFC 2581 and RFC 2988. I would appreciate if
somebody can let me know the answer or point me to any other appropriate
RFC.

Would any of the three duplicate ACKs alter the current calculated RTO
value?

Thank you

Jose
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From: Gil Jose-josegil1 <[email-masked]>
Date: Mon, 16 Jun 2003 14:08:51 +0100

Thanks for the answer, Reiner. I see, the timer is switched off after the
third dupack. Now the paragraph below makes sense and is consistent with
what I read in some computer networking books (it was actually this
paragraph the one that confused me because the way I was reading it meant to
me that the RTO should not be re-started after fast retransmit since the RTO
would be already running for the lost packet).

>Then RFC2988 continues:
>    (5.1) Every time a packet containing data is sent (including a
>          retransmission), if the timer is not running, start it running
>          so that it will expire after RTO seconds (for the current value
>          of RTO).

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From: Reiner Ludwig <[email-masked]>
Date: Mon, 16 Jun 2003 14:38:49 +0200

>somebody can let me know the answer or point me to any other appropriate

The retransmission timer is turned off when the 3rd DUPACK arrives. That's 
the part that's not explicitly mentioned in any RFC as far as I know. It's 
in the original BSD code, though.

Then RFC2988 continues:

>Would any of the three duplicate ACKs alter the current calculated RTO

Usually not. However, if those DUPACKs would carry timestamps, one could 
actually retrofit the RTO. The same is true for any other DUPACK beyond the 
3rd one ...

///Reiner