Re: [tcpm] RFC 2883 (D-SACK), Section 5.1 Replication by the network

Ethan Blanton <eblanton@cs.ohiou.edu> Sat, 08 November 2008 20:12 UTC

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Date: Sat, 8 Nov 2008 14:59:19 -0500
From: Ethan Blanton <eblanton@cs.ohiou.edu>
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Subject: Re: [tcpm] RFC 2883 (D-SACK), Section 5.1 Replication by the network
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Alexander Zimmermann spake unto us the following wisdom:
> RFC 2883, Section 5.1 states that without D-SACK a TCP sender would
> not know that a packet had been replicated in the network if the last  
> ACK was piggybacked on a data packet.
>
> According 2581(bis) a TCP receiver should send immediately an DUPACK
> when an out-of-order segment arrives. Furthermore, 2581bis says that a
> DUPACK carries no data.
>
> So, according to the DUPACK definition of 2581bis it seems to me that we
> have no problem to detect a packet replication in case D-SACK is not
> present. Right?

The problem here is that you have no idea whether the dupack was
generated by a packet already received (entirely below RCV.NXT), or a
packet following a lost or as yet un-received packet (entirely beyond
RCV.NXT).  Both of these conditions generate a duplicate
acknowledgment.

Ethan

-- 
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