Collaboration Tutorial :: Special Edition :: NAT Voice/Video Nightmare

Hi guys, when you are studying for CCNA , NAT seems just this passing by topic that you playing around.

you play the little game of changing ip address, that FUN.

but WAIT , until you get into Collaboration world, it becoms a nightmare.

Disclamer : to be able to follow you will need a basic undrestanfin about what NAT do.

a quick review of what NAT do , fo those who skipped NAT classes

you have a packet in : Packet_1 ={@IP_1 : Port_1} and when it traverse a NAT Device we have a packet out with Packet_2 = { @IP_2 : Port_2}

now lets ask this question , what is the layer (refering to the OSI ) is NAT.

well NAT plays with to information, IP and Port which makes it a L3/L4 Porotocol.

Now knowing this is the very basics of why NAT is a nightmare for collaboration.

Well for Collaboration we play around with :

• Media using RTP/RTCP protocols
• whos controls this media , well its the SIG using commonly H.323 & SIP

Now H.323 & SIP are L5 Protocol

what does H.323 & SIP do? well they instruct endpoint how to make calls voice/video between them

and for H.323 & SIP they know about this endpoint via IP Addresse and Ports.

So the nightmare here is when a H.323/SIP packets traverses a NAT device, the NAT only see the L3/L4 portion of that packet and changes the information, but it keeps the L5 information intact this leads into a packet is inconsistent addressing and voice/video will not be able to route correclty.


This issue causes generally sometime a one-way audio or one-way video.


Want to hear about other nightmares, stay tuned

Cheers,