more than one DNS server has been configured on the VCS, the query will be sent to all servers
at the same time, and all responses will be prioritized by the VCS with only the most relevant
SRV record being used.) If available, this SRV record will return information (e.g. the FQDN
and listening port) about either the device itself or the authoritative H.323 gatekeeper for that
domain.

•

If the domain part of the URI address was resolved successfully using an H.323 Location SRV
record (i.e. for

_ h323ls) then the VCS will send an A/AAAA record query for each name record

returned. These will resolve to one or more IP addresses, and the VCS then sends, in priority
order, an LRQ for the full URI to those IP addresses.

•

If the domain part of the URI address was resolved using an H.323 Call Signaling SRV record
(i.e. for

_ h323cs) then the VCS will send an A/AAAA record query for each name record

returned. These will resolve to one or more IP addresses, and the then routes the call, in
priority order to the IP addresses returned in those records. (An exception to this is where
the original dial string has a port specified - e.g. user@example.com:1719 - in which case the
address returned is queried via an LRQ for the full URI address.).

2. If a relevant SRV record cannot be located:

•

If the Include address record setting for the DNS zone being queried is set to On, the system
will fall back to looking for an A or AAAA record for the domain in the URI. If such a record is
found, the call will be routed to that IP address and the search will terminate.
Note: if the A and AAAA records that are found at this domain are for systems other than
those that support SIP or H.323, the VCS will still forward the call to this zone, and the call will
therefore fail. For this reason, we recommend that this setting is left as the default Off.

•

If the Include address record setting for the DNS zone being queried is set to Off, the VCS
will not query for A and AAAA records and instead will continue with the search, querying the
remaining lower priority zones.

SIP

The VCS supports the SIP resolution process as outlined in

RFC 3263 [16]

. An example of how the

VCS implements this process is as follows:
1. The VCS will send a NAPTR query for the domain in the URI. If available, the result set of this

query will describe a prioritized list of SRV records and transport protocols that should be used to
contact that domain.
If no NAPTR records are present in DNS for this domain name then the VCS will use a default list
of _sips._tcp.<domain>, _sip._tcp.<domain> and _sip._udp.<domain> for that domain as if they
had been returned from DNS.

•

The VCS will send SRV queries for each result returned from the NAPTR record lookup. A
prioritized list of A/AAAA records returned is built.

•

The VCS will send an A/AAAA record query for each name record returned by the SRV record
lookup.

The above steps will result in a tree of IP addresses, port and transport protocols to be used
to contact the target domain. The tree is sub-divided by NAPTR record priority and then by SRV
record priority. When the tree of locations is used, the searching process will stop on the first
location to return a response that indicates that the target destination has been contacted.

2. If the search process does not return a relevant SRV record:

•

URI resolution process using DNS