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

Am 08.11.2008 um 20:59 schrieb Ethan Blanton:

> 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

Hi Ethan,

you are completely right. However, in the example of section 5.1 it is  
assumed that no segment is lost.
IMHO the example is not correct (anymore) since it is assumed that a  
DUPACK can be piggyback on a data packet.
According to 2581bis a DUPACK carries no data.

Alex

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// Dipl.-Inform. Alexander Zimmermann
// Department of Computer Science, Informatik 4
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Re: [tcpm] RFC 2883 (D-SACK), Section 5.1 Replication by the network

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