If you use the port range registry key settings to reduce the ports that can be used for media, we recommend that you do this according to the minimums described in this section.
For client endpoints, the port range should not be reduced to the point where it can compromise the ability of the media stack to negotiate audio, video, and desktop sharing communication ports during session setup or during a call. More specifically, for an Office Communicator 2007 R2 client, the minimum port range should be 40. A smaller range of ports can result in errors during call transfer, when starting desktop sharing, and conference escalation scenarios.
Configuring a minimum of 40 ports will enable the client to evaluate the candidate transport addresses that it can use to stream audio, video, and data to another client, as described in the IETF Interactive Connectivity Establishment (ICE) protocol. Candidate addresses include local addresses and an address on the A/V Access Edge server. A minimum of 40 ports in the port range will also accommodate any escalations from a peer-to-peer call to a conference. (An escalation of a peer-to-peer call to a conference triggers a temporary doubling of the ports in use.)
The registry keys for these settings are as follows:
REG_DWORD 40039 (for example)
REG_DWORD 40000 (for example)