Message Routing in Exchange 2007

Exchange 2007 simplifies the management of the physical layout of Exchange by eliminating the need for routing groups and relying on the Active Directory site architecture instead. Servers with the Hub Transport role accept messages from Mailbox servers, determine the location of the destination mailbox database, and deliver the message to a Hub Transport server in the remote Active Directory site.

I am sure you might have read about Exchange 2007 roles. Let me try and brief it again:-
The best way to think of Exchange 2007 server roles is to think of a server that has the necessary software and configuration to perform only a specific set of functions. This makes installing servers with dedicated functions much easier. Dedicated server roles are also more secure because only the necessary software is installed, thus reducing the attack surface. With Exchange 2007, the server roles are assigned with the setup.

There are five basic server roles:
-Mailbox Server Role
-Client Access Role
-Hub Transport Role
-Unified Messaging Role
-Edge Transport Role

Note:- In small and medium-size organizations, a single physical server will usually host more than one server role except in the case of the Edge Transport role. Edge Transport must run on its own server.

Mailbox Server Role:-
-The Mailbox server role is responsible for mailbox and public folder databases and for allowing direct connectivity from MAPI/RPC clients. Clients such as Outlook 2003 or Outlook 2007 using MAPI/RPC will connect directly to the MAPI interface on the Mailbox server role. The Hub Transport and Client Access server roles are required for a fully functioning e-mail environment, but they do not necessarily have to be on the same physical server.

-The Mailbox server role must exist on its own physical server if it is being installed as part of a clustered mailbox server environment. In that case, the Hub Transport, Client Access, and Unified Messaging server roles must be on separate physical hardware. A server handling a mailbox server role will typically be configured with significantly more RAM, hard disk space, and processor capacity than the other server roles.

-High-availability options for Mailbox servers include local continuous replication, single copy clusters, and clustered continuous replication. (http://technet.microsoft.com/en-us/library/bb124721(EXCHG.80).aspx)

Client Access Server Role:-
The Client Access server is considered a middle-tier server; this server role handles communications between non-MAPI clients and the Mailbox server role. In order to have a fully functioning e-mail environment, the Client Access server role must be functioning.

The following are some of the functions of the Client Access server role supports:
-Outlook Web Access clients
-ActiveSync-enabled mobile devices
-Outlook Anywhere (RPC over HTTP) clients
-POP3 and IMAP4 clients
-Offline Address Book Web distribution
-Web services such as Autodiscover and the Availability service
-Web Services that require access to user mailboxes

The Client Access server accepts connections from these clients via HTTP, POP3, or IMAP4 and then passes requests on to the Mailbox server via MAPI over RPC. Each Active Directory site that contains a Mailbox server role must also contain at least one Client Access Server.

Hub Transport Server Role:-
The Hub Transport server role is responsible for all message delivery regardless of whether the message is being delivered from one mailbox to another in the same mailbox database, a Mailbox server in the same Active Directory site, a server in a remote Active Directory site, or outside of the organization. At least one Hub Transport server role is required in each Active Directory site that contains a Mailbox server.
For smaller organizations with a single server Exchange implementation, the Hub Transport server can perform most of the message hygiene functions performed by the Edge Transport server role to connect Exchange to outside world. However, separating message hygiene functions to a separate server role located on the perimeter network is more secure.

Unified Messaging Role:-
The Unified Message server role is considered a middle-tier system and is an entirely new concept for Exchange 2007. This server role integrates voicemail and inbound faxing with Exchange mailboxes. The Unified Messaging server requires an IP-based telephone switch or a traditional PBX-to-IP gateway (PBX stands for public branch exchange).

The following functions are handled by the Unified Messaging server role:
-Provides voicemail for users of the IP-based phone system or through the PBX-to-IP gateway including voicemail greetings and options. Inbound voicemail is recorded as a WMA file and stored as a message in a user's Inbox.
-Accepts inbound faxes that are designated for specific mailboxes, converts the fax to a TIFF file, and stores that message in a user's Inbox.
-Allows a user to dial in to the Unified Messaging server to retrieve voicemail, listen to e-mail messages, review their calendar, or change appointments.
-Provides voice menus and prompting call menus acting as an auto-attendant system.

Edge Transport Role:-
The Edge Transport server role is an entirely new role. In the past, Exchange servers could be implemented as an additional tier of message hygiene protection. However, there are a number of reasons that you might not want to use Exchange servers as perimeter message hygiene systems:

-In order to process delivery reports, nondelivery reports, and address rewrites, the information store service must be running and the default mailbox database must be mounted.
-Placing an Exchange 2000/2003 server in the perimeter network requires many ports to be opened on the firewall from the perimeter network to the internal network.
-Allowing inbound e-mail directly to an Exchange server could jeopardize both Exchange and Active Directory.

Following are some of the characteristics of the Edge Transport server role:

-The Edge Transport server role should be deployed in the perimeter network.
-It can be managed with Exchange Management Shell scripts and the Exchange Management Console in much the same way a regular Exchange server is managed.
-The only components required to run the Edge Transport role are the message transport system and an instance of the Active Directory Application Mode (ADAM) database.
(http://technet.microsoft.com/en-us/library/cc738377(WS.10).aspx)
-Features such as transport rules can be implemented in the perimeter network and provide message policy enforcement for messages entering or leaving the organization that is separate from that provided on the internal network.
-Connectivity between internal Hub Transport servers and Edge Transport servers can be authenticated and the data stream encrypted
-The content filter functionality and other anti-spam and message security tools are built in, as is the ability to add third-party content filtering/message hygiene tools.
-Microsoft Forefront Security for Exchange Server can be employed on the Edge Transport server role for virus detection and quarantine.

Message Routing

Exchange 2007 simplifies the management of the physical layout of Exchange by eliminating the need for routing groups and relying on the Active Directory site architecture instead. Servers with the Hub Transport role accept messages from Mailbox servers, determine the location of the destination mailbox database, and deliver the message to a Hub Transport server in the remote Active Directory site.

The Hub Transport server role is at the center of the message transport architecture. The Hub Transport server maintains Send and Receive connectors that are responsible for receiving mail from the Internet, sending mail to the Internet, sending mail to remote Hub Transport servers, and receiving mail from remote Hub Transport servers. All messages must be processed by the Hub Transport system regardless of whether they will be delivered to a local mailbox or a remote recipient.

Messages enter the Exchange 2007 message transport system through one of three possible mechanisms:
--A message can be submitted to the Hub Transport via SMTP,
--A Mailbox server's store driver,
--The file system's Pickup folder .
The Hub Transport relies on Active Directory for configuration, topology, and recipient information and thus must have access to domain controllers and global catalog servers. Once a message is submitted to the Hub Transport system, it enters the message queuing system where the message Categorizer reviews it and determines how to deliver it.

There are five possible queues that can be found on a Hub Transport server:

-The Submission Queue
-The Poison Message Queue
-The Unreachable Domain Queue
-The Local Delivery Queue
-The Remote Delivery Queue
-Submission Queue is the queue in which messages are placed when they enter the Hub Transport server (via SMTP, store driver, or pickup folder). The categorizer processes the messages as they arrive in this queue. The submission queue is also called the categorizer queue or the submit queue.
-Poison Message Queue is the queue in which messages are placed if there is a problem that prevents the message from being categorized.
-Unreachable Domain Queue is the queue in which messages are placed if there is no route available.
-Local Delivery Queue are queues in which messages are placed if they are to be delivered to a Mailbox server in the same Active Directory site.
-Remote Delivery Queue are queues in which messages are placed to be delivered to remote Hub Transport servers or outside of the organization. The remote delivery queue is the only type of queue available on the Edge Transport server role.

Message Categorization

The Categorizer component is the Hub Transport component that watches the submission queue. As messages arrive in the submission queue, the Categorizer picks them up and processes them.

Following are some of the steps involved in message categorization :-

--Expand any distribution lists, if applicable, by querying the global catalog.
--Resolve recipient addresses to determine which recipients are local, remote, or outside of the organization.
--Apply message transport rules to the message.
--Split the message into multiple parts if the message is going to local and remote recipients; this process is called bifurcation.
--Examine the message sender, recipients, header, body, and attachments and apply message transport rules that apply to the message.
--Convert the message to the appropriate message format (Summary-TNEF, MIME, or UUENCODE) depending on its destination.
--Determine the next "hop" for the message.
--Place the message in to appropriate local or remote delivery queue.

Message Transport Components:-

To work with Exchange Server and troubleshoot message transport problems you should know the internal workings of Exchange message routing.
There are five Message Transport Components:-
--Submission Queue
--Store Driver
--Microsoft Exchange Mail Submission Service
--Pickup Directory
--Categorizer

Note:- Messages from outside your Exchange organization enter the transport pipeline through an SMTP Receive Connector. Messages inside enter the pipeline through the Hub Transport server role.

Submission QueueEach Transport server role (Hub or Edge Transport) has one submission queue that is created by the categorizer when Exchange Transport Service starts. It stores all messages on the local hard disk until they are processed by the categorizer for delivery. They are then finally removed from this queue.

Store Driver Messages sent by a mailbox user enter the transport pipeline when they reach the sender’s outbox. The store driver on the Hub Transport retrieves it from the user’s Outbox and then transfers it to the submission queue. After the message has been successfully added to the submission queue, it is moved from the sender’s Outbox to the sender’s Sent Items. Messages are stored in MAPI format and must be converted to Summary Transport Neutral Encapsulation Format (S/TNEF) before being placed in the Submission Queue. This conversion is the job of the store driver, too. If this conversion is unsuccessful, a non-delivery report (NDR) is generated.

Microsoft Exchange Mail Submission Service The Microsoft Exchange Mail Submission Service is a notification service that runs on Mailbox server roles. It notifies the Hub Transport server role to pick up the message from the sender’s Outbox. If there are multiple Hub Transport server roles on one Active Directory site, the Message Exchange Mail Submission service attempts to evenly distribute notifications between each transport role using static load balancing.

Pickup Directory Each message that is transferred to the pickup directory has been successfully submitted to the submission queue via the categorizer. Messages placed in the Pickup Directory must be in the appropriate format and have read/write permissions configured. It allows you to take a properly formatted text file and have the Hub Transport server role process and deliver it. This can be very helpful when mail flow is being validated in the organization or relaying specific messages or returning to the transport pipeline. Even 3rd party applications may place messages in the Pickup directory rather than communicating directly with the Exchange Server.

Categorizer The categorizer always picks the oldest message from the Submission queue and checks whether this message has to be routed internally in the Exchange organization or externally.

On each Hub Transport server the categorizer performs the following tasks:

--Identification and verification of recipients
--Expansion of distribution lists
--Determination of routing paths
--Conversion of content formats
--Application of message policies

Comments

  1. Very Precise and informative blog. Thanks for posting it.

    ReplyDelete

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