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URI Dialing

TANDBERG VIDEO COMMUNICATIONS SERVER ADMINISTRATOR GUIDE

Overview URI Resolution Process via DNS Enabling URI Dialing

A URI address typically takes the form name@example.com, where name is the alias and example.com is the domain.

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.

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 alias@domain.com. A local transform is then configured to strip the @domain, and the search is made locally for alias.

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

H323

1.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.

2.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

1.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.<domain>, _sip._tcp.<domain> and _sip._udp.<domain> for that domain as if they had been returned from DNS.

2.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.

3.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.

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.

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 zone, and

•configure at least one DNS Server.

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.

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.

Introduction	Getting Started	Overview and	System	VCS	Zones and	Call	Bandwidth
		Status	Configuration	Configuration	Neighbors	Processing	Control
D14049.03					114
MAY 2008

Firewall Maintenance Appendices Traversal