possible bug when sending FAX

Julius <justa@xxxxxxxxxxx> · Mon, 28 Sep 2009 17:13:17 +0300

Hello,

I've recently tried to send fax using t.38 via 
CiscoGW->gnugkProxy->PattonGW. Success rate was only ~50%. Before 
addressing mailing list I've tried my best to troubleshoot the problem 
myself. So:

Connectivity scheme is pretty simple:

CiscoGw(ip:10.242.0.101)-----(ip:10.242.0.1)gnugkProxy(ip:10.242.1.10)-----(ip:10.242.1.11)PattonGW

gnugk config:

[Gatekeeper::Main]
Fourtytwo=42
Name=gk1
Home=10.242.1.10
UseBroadcastListener=0
TimeToLive=300
TraceLevel=0

[GkStatus::Auth]
rule=password
admin=YZ943Tg7NsY=

[RoutedMode]
GKRouted=1
H245Routed=1
CallSignalPort=1720

[CallTable]
DefaultCallDurationLimit=3600

[Proxy]
Enable=1
InternalNetwork=10.242.0.0/24

[RasSrv::RRQFeatures]
AcceptEndpointIdentifier=1
AcceptGatewayPrefixes=1

I've taken packet traces of failed call on both interfaces (10.242.0.1 
and 10.242.1.10) (please see them attached).
I've tried to understand some of this, but of course I could be wrong:
1. Call gets signaled (ARQ, ACF, all kinds of signaling and negotiations).
2. Two unidirectional g729 RTP streams opened (2 x openLogicalChannel 
(g729), 2 x openLogicalChannelAck)
3. Gateways detect fax and negotiate to tear down g729 RTP streams and 
open new t.38 RTP streams.
4. Two unidirectional t.38 RTP streams opened (2 x openLogicalChannel 
(t.38), 2 x openLogicalChannelAck)

By inspecting Cisco side packets from step 4 I've noticed weird thing 
(cisco.pcap packet 221):
In openLogicalChannel message GNUGK correctly rewrites Pattons ip 
address 10.242.1.11 with his own 10.242.1.10. But in 
openLogicalChannelAck message GNUGK signals 10.242.1.11 and UDP ports 
originally reported by Patton. As the result Cisco starts sending RTP 
packets directly to Pattons ip 10.242.1.11 instead of sending them to 
GNUGK's ip 10.242.1.10.
In cases when fax transmission is successful GNUGK rewrites ip address 
and UDP ports correctly (I can send packet traces of good session if 
needed).
Not sure if this only coincidence but in failed fax transmissions both 
openLogicalChannel and openLogicalChannelAck gets packed in single IP 
packet, in successful fax transmissions each message gets sent in 
separate packet.

Sadly I'm unable to locate the cause of a bug in source code, so I'm 
addressing this forum in hope that someone more skilled than me could 
look into this.

Thanks.

Julius
Attachment:
cisco.pcap

Description: Binary data
Attachment:
patton.pcap

Description: Binary data
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