New Routing and Transport Features in Exchange Server 2007
Exchange Server 2007 addresses performance and scalability with major redesigns in three areas:
First, Exchange Server 2007 is a true 64-bit application with program architecture that provides vastly improved memory management and much more efficient I/O processes.
As a result of increased addressing capabilities you can configure up to 50 storage groups; a dramatic increase to the limit of 4 storage groups in earlier versions of exchange.
The 64-bit environment also provides better database handling using the 64-bit version of the Extensible Storage Engine, increased cache size which reduces hard drive request, and a larger page size 8kb which can handle larger messages in a single I/O operation.
Finally, Exchange Server 2007 completely revises the transport pipeline allowing Exchange Server 2007 to handle substantially more messaging traffic than earlier versions.
Here is a quick overview of the changes made to the transport and routing architecture in Exchange Server 2007.
Transport and Routing
The “Transport Pipeline” describes all of the components within the exchange server deployment that guide and store message data during its journey from its source to its final destination.
The pipeline components include server roles, routing, queues and connections. The diagram below shows an overview of the transport pipeline:
Image Courtesy of Microsoft TechNet
Hub Transport Server
The main hub for all message traffic is the now appropriately renamed Hub Transport Server, which was called the Bridgehead Server in previous versions of Exchange. Every message sent and received must pass through the Hub Transport Server without exception.
This topology guarantees that all messages are processed according to the policies of the organization. In today’s messaging environment, compliance and protection within the network has become just as important if not more important than compliance and protection at the perimeter of the organization’s network.
The Hub Transport subscribes to the active directory where all of the configuration settings are stored including: transport rules, journaling rules, and connector configurations. Storing all the configuration data in active directory allows for simpler deployment and configuration of multiple Hub Transport servers in larger organizations.
Another new feature of the Hub Server role is the ability to provide anti-spam and anti-virus protection services. Although these services have traditionally been deployed on the perimeter of the network (through the Edge Transport server), increased mobility and wireless networking has changed network topology increasing threats from internally connected transient clients.
Anti-spam and anti-virus features are disabled by default under the Hub Transport Server role. To enable them you must register the agents in the configuration file then run scripts using the management shell.
Edge Transport Server
A new name was also given to the Edge Transport Server, which was called the Gateway Server in previous versions of exchange.
Along with the Hub Transport server, the Edge Transport server makes up the main transport route for all message data arriving to or leaving from the network. The Edge Transport server acts as a single point of contact with the Internet and is designed for security.
Typically, the Edge Transport Server should be a standalone server and not have access to the Active Directory Service. Instead, it should use the Active Directory Application Mode (ADAM) service, which receives required data from the Active Directory via the EdgeSync services, which are run on the Hub Transport server.
EdgeSync provides a secure one-way replication service that performs scheduled updates from the Active Directory so that the information within ADAM remains current.
SMTP Transport Stack
Exchange Server 2007 has a completely rewritten Transport Stack that is provided via the Microsoft Exchange Transport service, instead of the Internet Information Services (IIS), as in previous versions of Exchange.
By eliminating dependence on the IIS and running the stack as a network service, Exchange Server 2007 has increased security and simplified deployment.
The transport stack no longer uses SMTP event sinks to filter messages, but now uses Transport Agents. As messages are processed through the stack, Transport Agents are allowed to access the message and perform actions such as content filtering, address rewriting, etc.
Active Directory Routing
Unlike previous versions of Exchange which required you to define routing groups and connectors, Exchange Server 2007 uses the Active Directory site topology to determine routing.
This new design is meant to simplify deployment and maintenance, reduce network bandwidth overhead, and create a more deterministic routing topology.
Exchange Server 2007 has eliminated the use of link state tables. If a destination is unreachable, the message is simply queued in the cache of the Hub Server closest to the destination, and is delivered once the destination is back online or an update occurs in the routing tables.
The Delayed Fan Out feature delays message bifurcation by delegating the process to a hub which is closer to the destination servers. By delaying bifurcation, less bandwidth is required to send messages with multiple destinations.
The Direct Relay resolution feature attempts to send messages directly from source to destination without the use of intermediate Hub Servers, which reduces hops and speeds up message delivery.
Direct Relay can be overridden using the Hub Site feature which forces traffic to pass through a defined Hub Server. You can also use cmdlets to override the Active Directory link cost to set Exchange specific cost value to links.
Connectors within the organization are implicitly created using the site link data from the Active Directory, but you must manually define connectors to send and receive email outside of the organization.
When you install the Edge and Hub servers, default receive connectors are created which are sufficient for most deployments. Send connectors must be configured between the Hub and Edge servers and between the Edge server and the Internet.
You can configure these connectors manually or by subscribing the Edge server to the Active Directory, in which case the required default Send connectors will be created by the EdgeSync service.