Subject: [tcpm] Re: tcpm Digest, Vol 16, Issue 5

From: Arjuna Sathiaseelan <[email-masked]>
Date: Thu, 11 Aug 2005 21:00:48 +0100

Dear Reiner,
  As I was going through "The Eifel Algorithm Making TCP Robust
Against Spurious Retransmissions" paper - a question arose - so would
like to clarify with you.

>From the paper:
"On receipt of the first ACK after the timeout, the sender must
interpret this ACK as acknowledging the retransmission, and
must assume that all other outstanding segments have also
been lost"

You say that a spurious retransmission timeout, could cause go-back-n
retransmissions..i.e the ACKs the sender receives are mistaken to be
those of the retransmitted packets rather than the original packets
and the sender which is already in slow start could end up
retransmitting those packets again. Am I right?

Now the question - if the receiver receives all the original packets -
wouldnt it send a cumulative ACK acknowledging all the packets and
thus the sender would know all the packets have been received at the
receiver. So the sender need not retransmit all those packets again.
So why the sender "must" assume that the outstanding packets have been
lost? Is it because of the assumption that the receiver may have
reneged? I am not able to comprehend this. Please let me know. Thanks.

Regards,
Arjuna

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