Description of the Windows Messenger Reverse Connection Process Used by Remote Assistance

This article describes the reverse connection method that Windows Messenger uses to connect two users who are establishing a Remote Assistance session. To establish a successful connection when one of the two parties (novice or expert) is behind a firewall or Network Address Translation (NAT) device, you can use Windows Messenger to initiate the Remote Assistance session.

NOTE: In this article, "novice" refers to the user who is requesting help, and "expert" refers to the user who is providing help. The scenario in this article assumes that the novice is requesting Remote Assistance from the expert.

The following scenario describes the sequence of events that occurs when the novice invites the expert to a Remote Assistance session.

1. The novice sends a Remote Assistance invitation request to the Messenger server using Windows Messenger. This invitation request gets transmitted from the Messenger server to the expert.
2. The expert accepts the invitation, and then sends a reply to the novice, which gets sent to the Messenger server and gets transmitted to the novice.
3. The expert uses Windows Messenger to send the expert ticket that contains the expert's TCP/IP address and a dynamic port number to the Messenger server, which gets transmitted to the novice. The dynamic port is a random port above 1024.
   
   NOTE: The expert ticket contains essentially the same information as a novice ticket, but it provides the expert's connection information instead.
4. The novice receives the expert ticket, and then creates a ticket that is sent to the Messenger server, which gets transmitted to the expert. The novice ticket is used in cases where the connection cannot be made using the expert ticket.
5. The novice attempts a Remote Assistance Remote Desktop Protocol (RDP) connection using the expert ticket that contains the expert's IP address and dynamic port. (Windows Messenger is not used at this time.)
6. The expert receives the packet, and then validates the contents of the novice ticket before a Remote Assistance session is started.
7. If the expert does not receive a Remote Assistance connection attempt from the novice in five seconds, the expert sends a forward connect using the novice ticket that contains the IP address and port number to the novice's computer.

NOTE: RDP is the protocol used by Remote Assistance to communicate.

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