Users cannot receive file transfers, audio, or video behind a NAT device in Windows Messenger 5.0

In Microsoft Office Live Communications Server 2003 and in Live Communications Server 2005 you may experience the following symptoms:

• A Microsoft Windows Messenger user, "User A," can successfully transfer files to or send audio or video to another Windows Messenger user, "User B." (This behavior occurs when no users are behind a network address translation (NAT) device)
• "User A" cannot receive file transfers, audio, or video from "User B" when any one of the users is behind a NAT device.

This issue occurs if one of the Windows Messenger 5.x users or one of the Live Communications Server 2005 users is behind a NAT device. In this case, the user who is behind the NAT device receives a private IP address from that device.

When "User A" starts a session, such as a file transfer session, with "User B," IP address information is sent between computers from "User A" to "User B." However, if "User A" is behind a NAT device, this IP address information is a private IP address. When "User B" tries to connect to this private IP address to send a file or to start an audio or video session, "User B" is not successful. Therefore, file transfer traffic or audio and video traffic, only travels in one direction between these two users.

If you have a UPnP-enabled NAT device that is configured between the Windows Messenger 5.x users or the Live Communications Server 2005 users, all the Windows Messenger features work as expected.

Note Transfers of audio, video, and data files is peer-to-peer traffic that occurs between computers that are running Windows Messenger 5.x or Live Communications Server 2005. This traffic does not pass through the Live Communications Server computer.

For more information about Windows Messenger, visit the following Microsoft Web site: For more information about how to deploy Live Communications Server 2003, visit the following Microsoft Web site: