[VoiceOps] Odd network problem with SIP

Jay Ashworth jra at baylink.com
Sat Jun 21 11:37:19 EDT 2014

I have a client using the Spitfire dialler on Win7 to dial via SIP to a
carrier called VoxTelecom; I infer they're a virtual carrier because the
SIP goes to their Sonus SBC at a single IP, but the media sessions go all
over creation.

The client has been having completion trouble, and the dialler folks said
"crappy circuit; too much jitter and packet loss", tested from the dialler
using PingPlotter.  I don't think it was jitter, I think it was ping response
deviation; PingPlotter doesn't appear to actually test jitter.

But to tick the boxes, I had road runner out; they replaced a ubee D2 modem
with their New Hawtness Arris D3 modem; no change.  All his analog measurements
were pristine, he told me.

So next step, check the router.  Log in to the Fortigate 40D; control panel
won't paint properly.  This *may* have been an IE10 compatibility mode
red-herring, but I temporarily swapped it for a 20C, which I could talk to.

Set up the inbound DNATs for udp/5060 SIP and upd/49152-49252 RTP.  

Ran some calls.

They're not seeing my ACK after they send me an SDP.  They get the invite, but
not the ACK.  I send RTP, but they don't bother cause they think the call's
not set up yet.

Wireshark on the dialler... that ACK packet *has a bad IP header checksum*.

Not the earlier packets; just the ACK.  Huh?

So, assume it's the OS, somehow; reboot.  65 Windows updates and an hour later...

Set everything back up, and run more test calls.  This time, the Invite has a bad
IP header checksum.  Look at clock, 6pm EDT.  Give up, go home.

I'm going to go back to the original router Monday morning, and check the 
compatibility mode theory, but has anyone ever seen "just certain sent packets
show up with a bad IP header checksum, as monitored on-machine"?

(I know that the original 40D router might well *have* a problem, and that the 
checksum thing is orthogonal to the original problem -- since older pcaps show
clean setups -- but at least I can confirm I have the NATs configured right.)

-- jra

Jay R. Ashworth                  Baylink                       jra at baylink.com
Designer                     The Things I Think                       RFC 2100
Ashworth & Associates       http://www.bcp38.info          2000 Land Rover DII
St Petersburg FL USA      BCP38: Ask For It By Name!           +1 727 647 1274

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