Subject: [Tsvwg] (no subject)

From: Lili Wang <[email-masked]>
Date: Tue, 16 Jul 2002 17:38:20 -0400 (EDT)

Henderson, Thomas R wrote:
>Data that has been sent before a timeout event should be considered as
>having left the network.
 
  I agree. I think that it is good that HighData is reset after an
timeout.

>Now consider the case in which the receiver is sending SACK
>blocks.  After
>a timeout, the sender SHOULD expunge SACK information.  The sender MAY
>then use subsequently received SACK information to avoid unnecessary
>retransmissions.  However, will the receiver provide enough
>information?  It
>depends on how many SACK blocks exist between HighRxt and HighData.  If
>there are three or less, then enough SACK information should be
>replenished,
>and any subsequent dupacks are more likely to indicate the loss of a
>retransmitted segment.  If there are more than three, however, since the
>receiver is instructed by RFC 2018 to repeat the most recent SACK blocks
>and
>not the oldest SACK blocks, we could again be in a situation where false
>fast retransmissions occur (Ethan has pointed out to me this problem in a
>separate email).

  It is pointed out in RFC 2582 that  "false fast retransmission"
may occur because the sender could not infer which received packets
triggered the duplicate acknowledgment without SACK
information. However, 
it is specified in RFC 2018 that the first SACK block reflects
the contiguous  block of data containing the segment which triggered the
ACK unless that segment advanced the Acknowledgment Number field in
 the header."  So, the sender may avoid  "false fast retransmission"
problem using SACK information.  Reseting HighData  may be OK.

Lili

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