Siemens optiPoint 410 entry Administrator's Manual page 42

Technical Overview
DNS SRV
The phone supports the use of DNS SRV record lookups to allow SIP serv-
ers to be located. This mechanism is described in detail in RFC 3263 - Lo-
cating SIP Servers.
If the location to which a SIP message is to be sent is defined as a name
as opposed to an IP address a DNS SRV lookup will be performed. An ex-
ample query being:
_sip._tcp.example.com.
This indicates a query for a SIP server supporting the TCP transport proto-
col. The transport used in this query is determined by the SIP transport
menu setting ! page 217. The DNS server may return an IP address for
the requested SIP server or may return a single name or list of names
which require further A or AAAA record lookups to determine an IP ad-
dress. The response to a DNS SRV query will also contain information re-
garding the Time To Live for the information returned, the port address to
which requests should be sent and weighting information relating to load
balancing of requests.
DNS SRV and failover
Lists of candidate SIP server names are often returned in response to DNS
SRV queries to allow failover mechanisms to be implemented which in-
crease overall system availability.
If the phone sends a request to the first address in the list but fails to re-
ceive a response (currently the failover time is 6 seconds, this is not con-
figurable) the address is placed in a "penalty box" which means that it will
not be tried again until a specific time interval has past (currently pre-set to
1 minute). The request is sent to the next SIP server in the list and the pro-
cess continues. The penalty box mechanism ensures that the responsive-
ness of the phone is maintained by not continually retrying SIP servers that
are failing to respond.
For example the request to the first SIP server in the list fails, a call to an-
other user hosted on the SIP server will result in the user experiencing a 6
second delay before the failover to the secondary occurs. This means the
"Calling.." status will remain on the display until the second SIP server is
contacted. All subsequent messages for this call will go to the second SIP
server until the first SIP server is removed from the penalty box and it can
be tried once again to see if it is back in service. Note this mechanism is
independent of call setup. The first SIP server will not be retried necessar-
ily when the next call is established, only when it is removed from the pen-
alty box.
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