118
D14049.04
JULY 2008
Grey Headline
(continued)
TANDBERG
VIDEO COMMUNICATIONS SERVER
ADMINISTRATOR GUIDE
Introduction
Getting Started
Overview and
Status
System
Configuration
VCS
Configuration
Zones and
Neighbors
Call
Processing
Bandwidth
Control
Firewall
Traversal
Appendices
Applications
Maintenance
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.
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
•
e
, and
configure at least one DNS Serve
•
r
.
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
1.
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
A/
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.
user@example.com: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,
2.
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
1.
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.
The system will send SRV queries for each
2.
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
3.
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
alias@domain.com
. 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
