Date: Fri, 08 Feb 2002 20:27:50 +0530 From: "murthy kn" <knmurthy30@hotmail.com> To: silby@silby.com Cc: freebsd-net@FreeBSD.ORG Subject: Re: Duplicate Acks and Fast Retransmit Message-ID: <F113wNIRt1FFK7dcLdZ000045ab@hotmail.com>
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Hello,
Thanks for the useful suggestions. I have one small doubt which
I feel is reordering is happening.
>
> > 1. In my understanding, this should make the "sending" TCP get into Fast
> > Retransmit mode each time it receives 2 identical acks (or 1 duplicate
>ack).
> > However, I am not seeing this behaviour and would like
> > to get your advice on if I am missing something.
>
>One of the reasons this appears to be the case is because I don't see
>duplicate acks (be careful to distinguish between duplicate acks and
>window updates.) In fact, I don't really see much in the way of packet
>reordering.
>
(A small portion of earlier dump is attached below)
10:16:02.513687 10.10.10.1.1024 > 10.10.10.2.5001: . 240369:241817(1448) ack
1 win 32942 <nop,nop,timestamp 103899 99400> (DF)
10:16:02.513700 10.10.10.2.5001 > 10.10.10.1.1024: . ack 240369 win 33304
<nop,nop,timestamp 99400 103899> (DF)
10:16:02.513714 10.10.10.1.1024 > 10.10.10.2.5001: . 241817:243265(1448) ack
1 win 32942 <nop,nop,timestamp 103899 99400> (DF)
10:16:02.513757 10.10.10.2.5001 > 10.10.10.1.1024: . ack 243265 win 31856
<nop,nop,timestamp 99400 103899> (DF)
10:16:02.513791 10.10.10.2.5001 > 10.10.10.1.1024: . ack 243265 win 33304
<nop,nop,timestamp 99400 103899> (DF)
--------> From what I understand by reading Stevens (TCP/IP Illustrated Vol
1 pg 279), this is an
example of Window Update (The window advertised by the receiver
has changed and this is NOT an example of reordering)
10:16:02.513841 10.10.10.1.1024 > 10.10.10.2.5001: P 244713:246161(1448) ack
1 win 32942 <nop,nop,timestamp 103899 99400> (DF)
10:16:02.513889 10.10.10.2.5001 > 10.10.10.1.1024: . ack 243265 win 33304
<nop,nop,timestamp 99400 103899> (DF)
---------------> However, AFAIK, this is an example of Duplicate
Ack due to Out-or-order arrival (i,e reorder) and NOT a Window
update as the receiver is advertising the same window.
The reciever has gone out of his usual ACK every other segment
mode and generating an "immediate ACK for an out-of-order segment" in
compliance with the TCP standards.
Please correct me if I am wrong.
If the above is true, I am understanding why sender is
not generating a "Fast Retransmit" though he is receiving
many such "Dup Acks" and "Rex threshold" is 1.
10:16:02.513905 10.10.10.1.1024 > 10.10.10.2.5001: . 243265:244713(1448) ack
1 win 32942 <nop,nop,timestamp 103899 99400> (DF)
10:16:02.513944 10.10.10.2.5001 > 10.10.10
Thanks,
Murthy
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