URI dialing can make use of DNS to enable
endpoints registered with different systems to
locate and call each other. With URI dialing, it
is possible to find an endpoint by using DNS to
locate the domain in the URI address and then
query that domain for the alias.
Without URI dialing, you would need to
neighbor all the systems to each other in order
for one system to be able to locate an endpoint
registered to another system. This does not
scale well as the number of systems grows. It
is also inconvenient for making one-off calls to
endpoints registered with previously unknown
systems.
H.323 endpoints should register with the VCS
using a URI address in order to be reachable
using URI dialing. SIP endpoints always
register with an AOR in the form of a URI.

URI dialing is enabled separately for outgoing
and incoming calls.

Outgoing Calls

To enable endpoints registered to your VCS to
place calls to non-locally registered endpoints
directly using URI dialing, you must:

configure at least one DNS zon

•

, and

configure at least one DNS Serve

•

This is described in the section

Configuring URI

dialing for outgoing calls

Incoming Calls

To enable endpoints registered to your VCS to
receive calls directly from non-locally registered
endpoints using URI dialing, you must:

ensure all endpoints are registered with a

•

URI address
configure appropriate DNS records,

•

depending on the protocols and transport
types you wish to use.

This is described in the section

Configuring URI

dialing for Incoming calls

Firewall Traversal Calls

To configure your system so that you can place
and receive calls using URI dialing through a
firewall, see the section

URI Dialing and firewall

traversal

Overview

If a DNS zone and/or a DNS server have not been configured on the local VCS, calls to non-locally registered endpoints could still be placed if
the local VCS is neighbored with another VCS that has been configured for DNS. In this case, any URI dialed calls will go via the neighbor. This
configuration is useful if you want all URI dialing to be made via one particular system, e.g. a VCS Expressway.

When a VCS is attempting to locate a
destination URI address using the DNS system,
the general process is as follows:

H323

The VCS will send a query (via its DNS

server) for a SRV record for the domain in the
URI. If available, this SRV record will return
information about the authoritative H.323
gatekeeper for that domain (e.g. its FQDN
and listening port).

If the domain part of the URI address was

•

resolved successfully using an H.323
Location SRV record (i.e. for

_ h323ls)

then the FQDN returned is queried via an
LRQ for the full URI address.
If the domain part of the URI address was

•

resolved using an H.323 Call Signaling
SRV record (i.e. for

_ h323cs)

or an

AAAA

record lookup then the call is routed

directly to the IP address returned in that
record. An exception to this is where the
original dial string has a port specified
(e.g.

[email protected]:1720

) in which

case the address returned is queried via
an LRQ for the full URI address.
Once its IP address has been discovered,
the system will query that gatekeeper for
the URI.

If a relevant SRV record cannot be located,

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.

SIP

The system 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._tls.

_sip._

tcp.

and

_sip._udp.

for

that domain as if they had been returned
from DNS.
The system will send SRV queries for each

result returned from the NAPTR record
lookup. A prioritized list of A/AAAA records
returned is built. If no SRV records are found
then the domain name from the URI is added
as the only entry in list of A/AAAA records to
lookup.
The system 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.

There is an exception to the
requirement for H.323 endpoints to
register with the VCS using a URI

address in order to be reachable using URI
dialing. This is the case where endpoints
register with an

alias

, and incoming calls are

made to

[email protected]

. A local transform

is then configured to strip the

@domain

, and

the search is made locally for

alias

If the VCS does not have DNS
configured, calls made using URI dialing
will still be placed if the destination

endpoint is locally registered, or registered to a
neighbor system. This is because these
endpoints are not located via a DNS query.

Enabling URI Dialing

URI Resolution Process via DNS