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tcp: stop spurious rescue retransmissions

Authored by rscheff on Apr 25 2021, 11:03 AM.



As stale data from a prior SACK loss recovery episode persists,
a subsequent reordering event may trigger a spurious rescue
retransmission, which in turn misbehaves due to other stale

To prevent sending an erraneous rescue retransmission, clear
the sackhint.delivered_data value, once finished with a SACK
loss recovery.

Furthermore, initialize snd_fack to th_ack on a partial ack,
to pull it forward from whereever the last proper SACK may
have been.

Test Plan

Perform long-duration testing with a low background of packet
reordering and very few losses (or sufficient reordering for
dupthresh to be exceeded).

Reported by: pho@
MFC after: 3 days
Sponsored by: NetApp, Inc.

Diff Detail

rG FreeBSD src repository
Automatic diff as part of commit; lint not applicable.
Automatic diff as part of commit; unit tests not applicable.