Firewall Traversal

posted 6 years ago by Ben Cordero

In the world of video conferencing, one of the most annoying aspects of networks are firewalls. When setting up calls, a lot of ports are needed. RTP requires a port for media in each direction, plus their RTCP port (usually RTP port + 1). Double that number again for the audio channels and just for luck, add an extra port for BFCP/H.239 content.

For a company that relies on video conferencing, multiplying this many ports by the number of external calls they expect to have concurrently, and this poses a problem.

Ideally, firewalls shouldn't be needed and the problem goes away. Alternatively, poke some holes through your corporate firewall. This doesn't work since now you're exposing ports that are expecting a high volume of random (possibly encrypted) udp data. It also doesn't work since RTP traffic usually uses dynamic ports which is why protocols such as SDP exist in the first place.

Tandberg developed a very nifty solution for firewall traversal which exploits the useful fact that most firewalls are implemented to allow outgoing traffic, and prevent incoming traffic.

Essentially, setup one box outside the firewall (known as the traversal server) and one box inside the firewall (the traversal client). The client connects to the server and creates a path of two way communication through the firewall. When the server gets messages from the outside world, it can play the proxy role, add some routing information and send it to the inside world to the traversal client.

It's a great solution, it just involves trusting some expensive and proprietary boxes that all your calls have to go through. But that's fine, you use encryption[1].

So, you're not going to lower your firewalls, nor poke holes in them, nor use a series of standard protocols that have been designed in the open to solve this very problem. You need a more... trusting solution.

One of the solutions of the most paranoid (yes, they really do this) is to get two C90s[2]. One sits in the internal network and the other in a DMZ. Then, plug the inputs of one to the outputs of the other and setup a call. Others point the camera at each other's screen and have a physical separation; probably sitting in a vacuum box.

Ahh, the lengths some people go through. Of course, there are other ways to do this c.f. The Skype method.

[1] Only, it is encrypted between peers, and that box needs to be able to decrypt and modify some headers to do it's job.
[2] http://www.tandberg.com/telepresence-products/telepresence-engine-c90.jsp Note, the video mentions 'firewall traversal' with a nice graphic of a VCS.

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