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Technical Bulletin
SoundPoint
®
IP, SIP 2.1
5
Behavior When the Primary Server Connection Fails
For Outgoing Calls (INVITE Fallback)
When the user initiates a call, the phone will go through the following steps to
connect the call:
1. Try to make the call using the working server.
2. If the working server does not respond correctly to the INVITE, then try
and make a call using the next server in the list (even if there is no current
registration with these servers). This could be the case if the Internet
connection has gone down, but the registration to the working server has
not yet expired.
3. If the second server is also unavailable, the phone will try all possible
servers (even those not currently registered) until it either succeeds in
making a call or exhausts the list at which point the call will fail.
At the start of a call, server availability is determined by SIP signaling failure.
SIP signaling failure depends on the SIP protocol being used as described
below.
• If TCP is used, then the signaling fails if the connection fails or the Send
fails.
• If UDP is used, then the signaling fails if ICMP is detected or if the signal
times out. If the signaling has been attempted with all servers in the list
and this is the last server then the signaling fails after the complete UDP
timeout defined in RFC 3261. If it is not the last server in the list, the
maximum number of retries using the configurable retry timeout is used.
For more information, refer to Server <server/> on page 95 and
Registration <reg/> on page 149 of the SIP 2.0 Administrator's Guide.
For Incoming Calls (Incoming Call Fallback)
The primary call server can use mechanisms for detecting that the Internet
connection is down and route incoming calls through the PSTN link to the
back-up gateway/router on-site. Since the phone is simultaneously registered
to both servers, it will receive calls through the gateway even if the primary
registration has not expired. This is a key advantage of the new behavior
introduced in SIP 2.1.
Warning
If DNS is used to resolve the address for Servers, the DNS server is unavailable,
and the TTL for the DNS records has expired, the phone will attempt to contact the
DNS server to resolve the address of all servers in its list before initiating a call.
These attempts will timeout, but the timeout mechanism can cause long delays (for
example, two minutes) before the phone call proceeds “using the working server”.
To mitigate this issue, long TTLs should be used. It is strongly recommended that
an on-site DNS server is deployed as part of the redundancy solution.