Microsoft Knowledge Base Article 819836 contains the following summary:

This article describes how to troubleshoot and resolve most of the known multiplayer issues in the Microsoft games that are listed in the "Applies to" section. If the methods in this article do not resolve the issue, your network setup may be configured incorrectly. Microsoft Games Product Support does not provide support for configuring your personal network, Internet Service Provider (ISP), firewall or routers. If you suspect that your personal network, ISP, firewall or router is the cause of your issue, contact the manufacturer of your hardware or software, or the provider of your Internet services for more information.

When you try to start or play the games that are listed in the "Applies to" section, you may experience one or more of the following symptoms:

•    You cannot connect to an online game site or session.
•    You cannot join a multiplayer game.
•    You cannot host a multiplayer game.
•    You are disconnected from a multiplayer game session.
•    You receive an error message when you try to play or connect to a multiplayer session.
•    You may experience slow performance during a multiplayer game.