Firewall Traversal; About Firewall Traversal; Expressway Solution; How Does It Work - Cisco TelePresence Video Communication Server Administrator's Manual

Firewall traversal

This section describes how to configure your VCS Control and VCS Expressway in order to traverse
firewalls.
It includes the following information:
an overview of firewall traversal
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how to configure the VCS as a
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firewall traversal protocols and ports
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firewall configuration guidelines
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an overview of ICE and TURN services
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About firewall traversal

The purpose of a firewall is to control the IP traffic entering your network. Firewalls will generally block
unsolicited incoming requests, meaning that any calls originating from outside your network will be
prevented. However, firewalls can be configured to allow outgoing requests to certain trusted
destinations, and to allow responses from those destinations. This principle is used by Cisco's
Expressway technology to enable secure traversal of any firewall.

Expressway solution

The Expressway solution consists of:
a VCS Expressway or Border Controller located outside the firewall on the public network or in the
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DMZ, which acts as the firewall traversal server
a VCS Control, Gatekeeper, MXP endpoint or other traversal-enabled endpoint located in a private
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network, which acts as the firewall traversal client
The two systems work together to create an environment where all connections between the two are
outbound, i.e. established from the client to the server, and thus able to successfully traverse the
firewall.

How does it work?

The traversal client constantly maintains a connection via the firewall to a designated port on the
traversal server. This connection is kept alive by the client sending packets at regular intervals to the
server. When the traversal server receives an incoming call for the traversal client, it uses this existing
connection to send an incoming call request to the client. The client then initiates the necessary
outbound connections required for the call media and/or signaling.
This process ensures that from the firewall's point of view, all connections are initiated from the
traversal client inside the firewall out to the traversal server.
For firewall traversal to function correctly, the VCS Expressway must have one traversal server zone
configured on it for each client system that is connecting to it (this does not include traversal-enabled
endpoints which register directly with the VCS Expressway; the settings for these connections are
configured in a different way). Likewise, each VCS client must have one traversal client zone
configured on it for each server that it is connecting to.
The ports and protocols configured for each pair of client-server zones must be the same. See the
Quick guide to VCS traversal client - server configuration
on each system. Because the VCS Expressway listens for connections from the client on a specific
port, you are recommended to create the traversal server zone on the VCS Expressway before you
create the traversal client zone on the VCS Control.
Cisco VCS Administrator Guide (X6.1)
traversal client and as a traversal server
for a summary of the required configuration
Firewall traversal
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