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RFC2018, "TCP Selective Acknowledgment Options", October 1996

Source of RFC: tcplw (tsv)

Errata ID: 1610

Status: Verified
Type: Technical

Reported By: Matt Mathis
Date Reported: 2008-11-22
Verifier Name: Lars Eggert
Date Verified: 2008-12-19

Section 5.1 says:

   The use of time-outs as a fall-back mechanism for detecting dropped
   packets is unchanged by the SACK option.  Because the data receiver
   is allowed to discard SACKed data, when a retransmit timeout occurs
   the data sender MUST ignore prior SACK information in determining
   which data to retransmit.

It should say:

   The use of time-outs as a fall-back mechanism for detecting dropped
   packets is unchanged by the SACK option.  Because the data receiver
   is allowed to discard SACKed data, when a retransmit timeout occurs
   the data sender SHOULD ignore prior SACK information in determining
   which data to retransmit.

Notes:

At least one OS (Linux) violates the MUST to good effect: Even when timeout
driven, it keeps old SACK data so it can avoid retransmitting data already at
the receiver. Thus even under severe bandwidth exhaustion, 100% of the data
delivered to the receiver causes forward progress and the system is not subject
to classical congestion collapse (that is, congestion collapse from
unnecessarily-retransmitted packets).

When this draft is reopened, this text should be further refined to address a
number of additional issues. In particular:

- It has been observed that clearing the scoreboard on timeouts sometimes
causes very inefficient network utilization, with large quantities of
duplicated data delivered to the receiver.

- There is some risk of deadlock if the timeout was caused a corrupted
scoreboard or if the receiver reneges SACK blocks. It is important that the
checks for reneging and inconsistent scoreboards are robust. Furthermore,
there probably should be a mandatory fall back mechanism, such as requiring
classical fast retransmit and new reno behavior, or ultimately under repeated
timeouts with no forward progress, clearing the scoreboard.

- Making SACK more robust in the presence of timeouts may increase the risk of
congestion collapse associated with cascaded bottlenecks, because it may
enable TCP to function under unreasonably high loss rates.


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