Date: Wed, 06 Feb 2002 22:47:24 +0530 From: "murthy kn" <knmurthy30@hotmail.com> To: freebsd-net@FreeBSD.ORG Subject: Duplicate Acks and Fast Retransmit Message-ID: <F105JNwPmxd22RQY3Wh0000109b@hotmail.com>
next in thread | raw e-mail | index | archive | help
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 _________________________________________________________________ Join the world’s largest e-mail service with MSN Hotmail. http://www.hotmail.com To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?F105JNwPmxd22RQY3Wh0000109b>