TCP conection problems IBM VM -> FreeBSD

[Chuck Swiger]

On Mar 23, 2007, at 10:16 AM, Torbjorn Granlund wrote:
>>> 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 vm.se.lsoft.com.58679 > bang.swox.se.smtp: S
>>> 678305700:678305700(0) win 8192 <mss 1420,wscale
>>> 0,nop,nop,nop,timestamp 2317060084 0>
>>> 19:45:56.940154 IP bang.swox.se.smtp > vm.se.lsoft.com.58679: S
>>> 3183232720:3183232720(0) ack 678305701 win 57344 <mss
>>> 1460,nop,wscale 0,nop,nop,timestamp 24588210 2317060084>
>>
>>   Notice the ACK from vm.se.lsoft.com 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.

It's not intentional.  I might be confusing us both.  :-)

> (1) There is no ACK sent from vm.se.lsoft.com.  The only ACKs sent
>     are i  the other directions, as per the tcpdumps above.

Uh, yes.  I meant this packet:

   bang.swox.se.smtp > vm.se.lsoft.com.58679: S 3183232720:3183232720 
(0) ack 678305701

> (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.

You're right-- the SYN+ACK reply to a connection open (bare SYN)  
should reply with SEQ = ISS + 1 to acknowledge the SYN (which counts  
as a byte).

-- 
-Chuck