PLANET VIP-350PT - Manual (Page 101)

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Expires
Set the default expiration time of registration; measured in seconds. We suggest a value not less
than 1200 seconds to avoid heavy network traffic due to registration. This expiration may modified
(shorten) by SIP registrar server, and IP Phone will re-register to server when 50% of the modified
expiration time has been passed. It, however, will re-register to SIP registrar after 90 seconds on
registration failure.
Default is 3600 seconds.
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Rport
Enable the support of RFC3581, an extension to the Session Initiation Protocol (SIP) for Symmetric
Response Routing.
When SIP operates on UDP, responses to requests are returned to the source address the request
came from, and to the port written into the topmost Via header field value of the request. This
behavior is not desirable in many cases, most notably, when the client is behind a Network Address
Translator (NAT). This extension defines a new parameter for the Via header field, called "rport",
that allows a client to request that the server send the response back to the source IP address and
port from which the request originated.
A client sends an INVITE to a proxy server which looks like, in part:
INVITE sip:[email protected] SIP/2.0
Via: SIP/2.0/UDP 10.1.1.1:4540;
rport ;branch=z9hG4bKkjshdyff
If the proxy does not recognize this extension, you SHOULD disable this feature such that IP Phone
will not insert this “rport” parameter into Via header while sending SIP requests.
Default is enabled to help NAT traversal if your SIP proxy/soft-switch supports it (for
backward compatibility.you may disable it because some server think the rport parameter
bears wrong format and silently drop this reques).