Subject: Re: Anomalous TCP behaviour?

From: Vern Paxson <[email-masked]>
Date: Tue, 25 Jan 2000 01:35:46 PST

> 	Is it normal for TCP to skip a certain number of bytes and send
> them later when it receives dup acks?

No, it isn't.  But, depending on how you recorded the traffic, it's
possible that it's sending in order but the packets are being lost
before they make it to the packet filter (e.g., because the NIC card
is dropping them).

> ## Also it is obvious from the tcpdump o/p that the Fast retransmit
> mechanism doesnt get kicked off after 3 dup acks - is this normal
> behaviour?

Again, no, but you need to check the advertised window on each of the
dups.  If it changes, then the counting for # of duplicates starts over.

>      outoforder pkts:           9           outoforder pkts:           0      

What does outoforder pkts mean here?

		Vern

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From: "Shawn Ostermann" <[email-masked]>
Date: Tue, 25 Jan 2000 08:42:58 -0500

> What does outoforder pkts mean here?

That's what you'll see with tcptrace when it sees a retransmitted
segment without having seen the original.  It could be caused by
missing packets in the trace file, but it's more commonly the case
that you're grabbing the packets near the receiver rather than near
the sender.  In either case, you can't tell for sure whether the
segments are out of order or a retransmission (although it's almost
always the latter, of course).

--sdo
-------------------------------------------------------------------------
   Dr. Shawn Ostermann  -  Associate Professor  -  Ohio University
      322 Stocker Center, Ohio University, Athens, Ohio  45701-2979
 [email-masked] -- FAX: (740)593-0007 -- Voice: (740)593-1234
    http://ace.cs.ohiou.edu/~osterman   http://jarok.cs.ohiou.edu

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From: Anupama Sundaresan <[email-masked]>
Date: Tue, 25 Jan 2000 12:39:13 -0600 (CST)

> always the latter, of course).

If Tcptrace displays the statistics for both the transmitter and the
receiver and if this is the sender side statistics then what does the out
of order packets mean? Does it mean that the packet gets lost even before
it comes to tcpdump or does it mean that tcpdump somehow missed the first
transmission and what was recorded as an outoforder packet is actually a
retransmission?