Fix bugs in SACK recovery.

Every call to sender.NextSeg does not need to iterate from the
front of the writeList as in a given recovery episode we can cache
the last nextSeg returned. There cannot be a lower sequenced segment
that matches the next call to NextSeg as otherwise we would have
returned that instead in the previous call.

This fixes the issue of excessive CPU usage w/ large send buffers
where we spend a lot of time iterating from the front of the list on
every NextSeg invocation.

Further the following other bugs were also fixed:
  * Iteration of segments never sent in NextSeg() when looking for segments for
    retransmission that match step1/3/4 of the NextSeg algorithm
  * Correctly setting rescueRxt only if the rescue segment was actually sent.
  * Correctly initializing rescueRxt/highRxt when entering SACK recovery.
  * Correctly re-arming the timer only on retransmissions when SACK is in use
    and not for every segment being sent as it was being done before.
  * Copy over xmitTime and xmitCount on segment clone.
  * Move writeNext along when skipping over SACKED segments. This is required
    to prevent spurious retransmissions where we end up retransmitting data
    that was never lost.

PiperOrigin-RevId: 310387671

0 files changed, 0 insertions, 0 deletions