Unable to SIP Signal Using TCP on SPA3102

Answered Question
Jun 24th, 2009

I changed my SIP transport to TCP but when I telnet to SPA IP on specified TCP port, nothing is listening? What could I be missing?

Thanks.

I have this problem too.
0 votes
Correct Answer by Alberto Montilla about 7 years 5 months ago

Dear Sir;

Your assumption is right. The ATA will not listen on the port. It just initiates the TCP connection towards the server.

Regards
Alberto

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William Childs Thu, 06/25/2009 - 07:46

Telnet is not supported on Linksys ATAs. The SIP transport field dictates how the SIP messaging will be sent to the Proxy server.

rais Thu, 06/25/2009 - 08:12

Thanks for the reply.

Please note that I didn't mean 'telnet' for MGMT purposes, but for checking out if there is a TCP port listening on a device or not...e.g. you may check your cisco.com connectivity by using: 'telnet cisco.com 80' and you will receive some information or at least get a connection.

When I enable TCP as a transport for SIP on ATA say on port 5000, I don't get listening port on the device. What else should be done to enable TCP transport. If nothing is listening is it an ok behavior? Is the ATA not suppose to listen on a TCP port the way it does on UDP 5060 port?

Thanks.

Correct Answer
Alberto Montilla Mon, 06/29/2009 - 06:33

Dear Sir;

Your assumption is right. The ATA will not listen on the port. It just initiates the TCP connection towards the server.

Regards
Alberto

rais Mon, 06/29/2009 - 08:10

Thanks for your reply Alberto,

If ATA wouldn't listen on TCP port, it would render the ATA to act only as a UAC [not as a UAS] when TCP is used...is that correct? When a call comes in for this ATA, the SIP server would know the location of this ATA [but it registered using TCP] but how would it initiate a connection to it or forward the call to this ATA? Since it is not listening on any ports.

Regards.

William Childs Mon, 06/29/2009 - 09:17

The ATA should still use the SIP port that is defined. Meaning it will only use TCP on the transport layer of the TCP/OSI model to communicate over IP. It would still choose the destination port (as defined one the ATA) but it would be sent via TCP instead of UDP. TCP meaning it has retransmission of lost packets, and support for security. When you say it registered via TCP that is a bit of a misnomer in that it actually regstered using SIP but it ran on top of the TCP protocol and carried by IP. The completion of the call would be handled by source and destination IP/port and the type of transmission (at the Application layer) would be a SIP transmission. Hope this helps.

Bill

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