Hello,
Below is the "netstat -p tcp" output on a FreeBSD 4.3 Machine.
(with a couple of arrows to indicate lines of interest)
---------------------------------------------------
tcp:
181061 packets sent
181039 data packets (262144000 bytes)
-----> 0 data packets (0 bytes) retransmitted
0 resends initiated by MTU discovery
21 ack-only packets (0 delayed)
0 URG only packets
0 window probe packets
0 window update packets
1 control packet
254774 packets received
135892 acks (for 262144001 bytes)
------> 38908 duplicate acks
0 acks for unsent data
0 packets (0 bytes) received in-sequence
------> 19 completely duplicate packets (0 bytes)
------> 19 old duplicate packets
0 packets with some dup. data (0 bytes duped)
1 out-of-order packet (0 bytes)
0 packets (0 bytes) of data after window
0 window probes
76732 window update packets
0 packets received after close
0 discarded for bad checksums
0 discarded for bad header offset fields
0 discarded because packet too short
1 connection request
0 connection accepts
0 bad connection attempts
0 listen queue overflows
1 connection established (including accepts)
6 connections closed (including 0 drops)
1 connection updated cached RTT on close
1 connection updated cached RTT variance on close
0 connections updated cached ssthresh on close
0 embryonic connections dropped
135892 segments updated rtt (of 91461 attempts)
0 retransmit timeouts
0 connections dropped by rexmit timeout
0 persist timeouts
0 connections dropped by persist timeout
0 keepalive timeouts
0 keepalive probes sent
0 connections dropped by keepalive
12355 correct ACK header predictions
0 correct data packet header predictions
--------------------------------------------------------------------
In order to understand the effect of reordering better,
a)the value of "Fast Retransmit Threshold" in tcp_input.c is modified
to 1
b)packets of the connection are distributed over 2 links.
1. Is it possible to have so many duplicate acks but not
still have a Fast Retransmit ? (TCP source clearly shows that
there is a possibility of this but is it not surprising
that probability the probability is so high?)
2. What does the field "Old duplicate packets" (in this case, ACKs) mean ?
(This was the first TCP session after reboot and the
earlier TCP sessions apparently closed without any problem).
TIA,
Murthy
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