The VCS supports SIP over UDP, TCP and TLS transport protocols. You can configure whether or
not incoming calls using each protocol are supported, and if so, the ports on which the VCS will
listen for such calls.

Using the VCS as a SIP Registrar

In order for a SIP endpoint to be contactable via its registered alias, it must register its location
with a SIP Registrar. The VCS can act as a SIP Registrar for up to 20 domains.
SIP aliases always take the form

username@domain

. To enable the VCS to act as a SIP Registrar,

you must

configure it with the SIP Domain

(s) for which it will be authoritative. It will then accept

registration requests for any endpoints attempting to register with an alias that includes that
domain.
If no Domains are configured, then the VCS will not act as a SIP Registrar.

Proxying Registration Requests

If the VCS has no domains configured, or it receives a registration request for a domain for which
it is not acting as a Registrar, then the VCS may proxy the registration request. This depends on
the

SIP Registration Proxy Mode

setting, as follows;

Off

: the VCS will not proxy any registration requests. The request will be rejected with a “

403

Forbidden” message.

Proxy to Known Only

: the VCS will proxy the registration request but only to its neighbors.

Proxy to any

: the VCS will proxy the registration requests in accordance with its call policy (e.g.

administrator policy and transforms). See

Call Processing

for more information.

•

•
•

About SIP on the VCS

The VCS supports the SIP protocol: it is both a SIP Proxy and SIP Registrar, and will provide
interworking between SIP and H.323 calls. In order to support SIP,

SIP mode

must be enabled

and at least one of the SIP transport protocols must be active.

Using the VCS as a SIP Proxy Server

When in

SIP mode

, the VCS may act as a SIP Proxy Server. The role of a Proxy Server is to forward

requests (such as REGISTER and INVITE) from endpoints or other Proxy Servers. These requests
are forwarded on to other Proxy Servers or to the destination endpoint.
Whether or not the VCS acts as a SIP Proxy Server, and its exact behavior when proxying requests,
is determined by the

SIP Registration Proxy Mode

setting. This in turn depends on the presence

of Route Set information in the request header and whether or not the Proxy Server from which the
request was received is a Neighbor of the VCS.
A Route Set can specify the path that must be taken when requests are being proxied between
an endpoint and its Registrar. For example, when a REGISTER request is proxied by a VCS, the
VCS adds a Path header component to the request which signals that the VCS must be included
on any call to that endpoint. The information is usually required in situations where firewalls exist
and the media must follow a specified path in order to successfully traverse the firewall. For more
information about the path header field, see RFC 3327 [

When the VCS proxies a request that contains existing Route Set information, it will forward it
directly to the URI specified in the path. Any call policy configured on the VCS will therefore be
bypassed. This may present a security risk if the information in the Route Set cannot be trusted.
For this reason, you can configure the VCS with three different behaviors when proxying requests,
as follows:

If the

SIP Registration Proxy Mode

setting is

Off

, the VCS will not proxy any requests that have

an existing Route Set. Requests that do not have an existing Route Set will still be proxied in
accordance with existing call policy (e.g. zone searches and transforms). This setting provides
the highest level of security.
If the setting is

Proxy to Known Only

, the VCS will proxy requests with an existing Route Set

only if the request was received from a Neighbor zone (including Traversal Client and Traversal
Server zones). Requests that do not have an existing Route Set will be proxied in accordance
with existing call policy.
If the setting is

Proxy to any

, the VCS will proxy all requests. Those with existing Route Sets

will be proxied to the specified URI; those without will be proxied in accordance with existing
call policy.

•

SIP Registration Expiry

SIP endpoints must periodically re-register with the SIP Registrar in order to prevent their
registration expiring. You can determine the interval with which SIP endpoints must register with
the VCS.

SIP Overview

Working with SIP

This setting also impacts the VCS’s behavior when acting as a

SIP Proxy Server

This setting applies only when the VCS is acting as a SIP Registrar, and to endpoints
registered with the VCS. It does not apply to endpoints whose registrations are being
proxied through the VCS.

At least one of these protocols must be set to a

Mode

in order for SIP functionality to

be supported.

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