Microsoft Exchange 2000 Server Service Pack 1 (SP1) includes a feature that permits Exchange Instant Messaging to operate in polling mode. Support for polling mode was added to address connectivity issues that may occur when network address translation (NAT), firewalls, or both are present between a client and a server.

Because Exchange Instant Messaging was not designed to support NAT and firewalls, the polling mode feature was added to Exchange 2000. However, the addition of the polling mode feature may cause server instability in certain configurations and topologies. Because of the number of incidents associated with this feature and the scope of changes to the product that would have to be made to fully resolve these issues, Microsoft will no longer support the polling mode feature for Exchange Instant Messaging.

To address any connectivity issues, Microsoft Office Live Communications Server 2003 Standard Edition was designed to solve NAT and firewall problems for instant messaging. Live Communications Server 2003 handles bidirectional connectivity by using Session Initiation Protocol (SIP), and clients can open a single, full-duplex connection to the server. This permits the server to send and receive traffic over an existing connection to the server that was opened by the client. Since its release in October 2003, Live Communications Server has been successfully used in several large-scale deployments in a broad range of industries. These deployments all have varying firewall requirements and scenarios. For more information about Live Communications Server, visit the following Microsoft Web site:

Exchange Instant Messaging is designed as a lightweight instant messaging solution that operates on top of the Rendezvous Vector Protocol (RVP). The RVP protocol is an extension of HTTP/1.1. Clients register their IP address with servers so that they can be reached when new messages are available for them. Then the clients create RVP requests and send them over new connections to the server. Servers also create RVP requests and send them back to the client over new connections. Therefore, Exchange Instant Messaging requires full connectivity from client to server and from server to client to operate correctly.

Exchange Instant Messaging requires the server to be able to reach the client. Because there are environments that involve client-side NAT and client-side firewalls, Exchange Instant Messaging in Exchange 2000 Server SP1 introduced a polling mode feature that permitted clients and servers to operate in existing NAT and firewall environments. This feature added a lightweight, store-and-forward model for instant messaging so that clients could quickly retrieve queued messages that were destined for them when an NAT infrastructure or a firewall infrastructure exists between the client and the server.

This optional polling mode feature has been the source of most of the support issues for Exchange Instant Messaging since Exchange Instant Messaging was released in 2000. Because this store-and-forward model dramatically affects the performance and the characteristics of the system, several issues may occur in the code base. Networks and environments that must use the polling mode feature in Exchange Instant Messaging are affected by these issues and cannot use Exchange Instant Messaging. However, this does not affect most Exchange Instant Messaging customers because most enterprise networks do not require the polling mode feature.