[lwip-devel] [bug #27847] TCP fast retransmit error

[hanhui]

URL:
  <http://savannah.nongnu.org/bugs/?27847>

                 Summary: TCP fast retransmit error
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: hanhui03
            Submitted on: Thu Oct 29 14:28:15 2009
                Category: TCP
                Severity: 3 - Normal
              Item Group: Faulty Behaviour
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: 
            lwIP version: CVS Head

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Details:

In recent days, I use a TCP echo server test lwip stack.
When I use a mass of data test and check the packet. I found many TCP
"Out-Of-Order" and "Retransmit" packet(lwip send).
After detailed investigation, then I found the problem is in lwip's Fast
Retransmit arithmetic.
lwip has error in decide duplicate ACK!
windows send large amounts of data, usually send consecutive packets and has
same ACK(> 3 packets). then lwip decide that need fast retransmit.

These are the rules BSD stacks use to determine if an ACK is a
duplicate:

An ACK is a duplicate if:

(1) it has the same sequence number as the largest number we've seen,
(2) it has the same window as the last ACK,
(3) we have outstanding data that has not been ACKed
(4) The packet was not carrying any data.






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