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[lwip-devel] [bug #33058] Fast retransmit transmits too much (disables n

From: Simon Goldschmidt
Subject: [lwip-devel] [bug #33058] Fast retransmit transmits too much (disables nagle)
Date: Tue, 12 Apr 2011 19:11:32 +0000
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:2.0) Gecko/20100101 Firefox/4.0


                 Summary: Fast retransmit transmits too much (disables nagle)
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: goldsimon
            Submitted on: Di 12 Apr 2011 19:11:31 GMT
                Category: TCP
                Severity: 2 - Minor
              Item Group: Faulty Behaviour
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: 1.4.1
            lwIP version: CVS Head



While reviewing bug #32417, I discovered that TCP fast retransmit transmits
too much data because it disables the nagle algorithm (see definition of
tcp_do_output_nagle(): if the TF_INFR flag is set, it always returns true).

Now while this isn't too big a problem, I think we can just leave that check
away: if the segment in question (which gets moved from unacked to unsent by
tcp_rexmit_fast()) has already been sent, this means that before, one of the
other checks in tcp_do_output_nagle() fired and as such, the next time
tcp_output() is called, at least one segment will be sent.

The problem with the current check is that tcp_rexmit_fast() setting the
TF_INFR flag leads to sending all data from the unsent queue, including the
segment held back by nagle before.


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