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

H323

The system 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).
The system will then send out another query for an A/AAAA record for the FQDN
returned in the SRV record. If available, this will return the actual IP address of
the gatekeeper. 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

Without DNS, calls made using URI dialing will still be placed if the
destination endpoint is locally registered, or registered to a neighbor
system as locating these this does not require the use of DNS.

Enabling URI Dialing

URI Resolution Process via DNS