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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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