TCP conection problems IBM VM -> FreeBSD

Torbjorn Granlund tg at
Fri Mar 23 17:17:00 UTC 2007

Chuck Swiger <cswiger at> writes:

  On Mar 22, 2007, at 12:00 PM, Torbjorn Granlund wrote:
  >>   The second line should have been SYN+ACK'ing the
  >>   ISN of 27523124.  vm is sending a RST to that because the sequence
  >>   #'s don't match.  It's also odd that the set of options being
  >> listed
  >>   don't correspond at all...if you run the tcpdump for several
  >> minutes,
  >>   can you track down other SYN requests which do correspond?
  > These are the ones the correspond.  They come in bursts like that.  If
  > I let it run a little longer, I get output like this:
  > 19:45:56.939958 IP > S
  > 678305700:678305700(0) win 8192 <mss 1420,wscale
  > 0,nop,nop,nop,timestamp 2317060084 0>
  > 19:45:56.940154 IP > S
  > 3183232720:3183232720(0) ack 678305701 win 57344 <mss
  > 1460,nop,wscale 0,nop,nop,timestamp 24588210 2317060084>
  Notice the ACK from is off by one, but the timestamp
  option corresponds.  Looks to be a bug with the vm machine, the bang
  machine is behaving properly per the TCP requirements.
Now you're confusing me.

(1) There is no ACK sent from  The only ACKs sent
    are i  the other directions, as per the tcpdumps above.
(2) The ACKs are not off by one, or if you prefer, ACKs on TCP are
    always off by one.  They are one higher than one would expect.


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