How Communicator Uses SDP and ICE To Establish a Media Channel
From the OCS Team blog:
This article describes the steps taken by Office Communicator to establish a Communicator call between an OC client sitting on a typical home network, connected to the Internet using a NAT router and another OC client placed on the company's internal network. The user initiating the call will be Alice and the data and logs are collected from Alice's computer
The main problem when establishing a media connection (audio or video) between Alice and Bob is finding a way media can travel through the intermediate network, without being blocked. This is where SDP, ICE, STUN and TURN come into the picture.
SDP
Office Communicator uses SDP (Session Description Protocol) to provide initialization parameters for the media stream in an audio or audio/video session. It is a proposed standard published by IETF in several RFCs (e.g. RFC 4566) and completely based on ASCII, which makes it easy to read.
Although SDP helps initializing media flow between two entities, every client is only describing its own view of the connection. If you ever wondered, what side of the media stream the advertised IP addresses in the SDP blob belong to, remember SDP as the "Self Description Protocol".
ICE
The Interactive Connectivity Establishment (ICE) Extensions protocol is used to establish media flow between two endpoints. In typical deployments, NATs or firewalls might exist between the two endpoints that are intended to communicate. NATs and firewalls are deployed to provide private address space and to "secure" the private networks to which the endpoints belong. This type of deployment blocks incoming traffic. If the endpoint advertises its local interface address, the remote endpoint might not be able to reach it. Advertising the address exposed by the NAT or firewall is not as straightforward, because the endpoints would first need to determine the external routable mapping address created by the NAT (NAT-mapped address) for its local interface address. Moreover, NATs and firewalls exhibit differint behavior in the way they create the NAT-mapped addresses. Section 5 of [IETFDRAFT-STUN-02] provides an overview of NAT types.
ICE provides a mechanism to assist media in traversing NATs without requiring the endpoints to be aware of their network topologies. ICE assists by identifying one or more transport addresses, which the two endpoints can potentially use to communicate and ICE determines which transport address is best for both endpoints to use for their media session.
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