The ability of the VCS to receive incoming calls made via URI
dialing relies on the presence of DNS records for each domain
the VCS is hosting.
These records can be of various types including:

A records

•

, which provide the IPv4 address of the VCS

AAAA records

•

, which provide the IPv6 address of the VCS

Service (SRV) records

•

, which specify the FQDN of the VCS

and the port on it to be queried for a particular protocol and
transport type.

NAPTR records

•

, which specify SRV record and transport

preferences for a SIP domain.

You should provide an SRV or NAPTR record for each
combination of domain hosted and protocol and transport type
enabled on the VCS.

URI Dialing for Incoming Calls

Configuring H.323 SRV Records

Annex O of H.323 [15]

defines the procedures for using DNS to locate

gatekeepers and endpoints and for resolving H.323 URL aliases. It also defines
parameters for use with the H.323 URL.
The VCS supports two types of SRV record as defined by this Annex. These are
Location and Call, with

_ Service

set to

_ h323ls

and

_ h323cs

respectively.

If you wish the VCS to be contactable via H.323 URI dialing, you should provide
at least a Location SRV record, as it provides the most flexibility and the simplest
configuration.

Location SRV Records

For each domain hosted by the VCS, you should configure a Location SRV record
as follows:

_ Service

•

_ h323ls

_ Proto

•

_ udp

Port

•

is the port number that has been configured via

VCS Configuration >

Protocols > H.323

as the

Registration UDP port

Call SRV Records

Call SRV records (and A/AAAA records) are intended primarily for use by endpoints
which cannot participate in a location transaction, exchanging LRQ and LCF. The
configuration of a Call SRV record should be as follows:

_ Service

•

_ h323cs

_ Proto

•

_ tcp

Port

•

is the port number that has been configured via

VCS Configuration >

Protocols > H.323

as the

Call signaling TCP port

SRV Record Format

The format of SRV records is defined by

RFC 2782 [3]

as:

_ Service. _ Proto.Name TTL Class SRV Priority Weight Port Target

For the VCS, these will be as follows:

_ Service

•

and

_ Proto

will be different for H.323 and SIP, and will depend on the protocol and transport type being used.

Name

•

is the domain in the URI that the VCS is hosting (e.g. example.com)

Port

•

is the port on the VCS that has been configured to listen for that particular service and protocol combination

Target

•

is the FQDN of the VCS.

Configuring SIP SRV Records

RFC 3263 [16]

describes the DNS procedures

used to resolve a SIP URI into the IP address,
port, and transport protocol of the next hop to
contact.
If you wish the VCS to be contactable via SIP
URI dialing, you should configure an SRV record
for each SIP transport protocol enabled on the
VCS (i.e. UDP, TCP or TLS) as follows:

Valid combinations of

•

_ Service

and

_ Proto

are:

_ sips. _ tcp

•

_ sip. _ tcp

•

_ sip. _ udp

•

Port

•

is the port number that has been

configured via

VCS Configuration > Protocols

> SIP

as the

port

for that particular

transport protocol.

Process

When an incoming call has been placed using URI dialing, the
VCS will have been located by the calling system via one of the
DNS record lookups described above. The VCS will receive the
request containing the dialled URI in the form

user@example.

com

. The VCS will then check its local registrations and FindMe

names and if any are an exact match, the call will be routed to
the appropriate device(s).