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Exchange 5.5 Message Routing 
 


Added: 01/22/2002, Hits: 4,152, Rating: 0, Comments: 0, Votes: 0
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By Brian Talbert

With the exception of small businesses, the typical Exchange deployment often involves more than a single server, often multiple sites, and often multiple connectors such as X.400 or SMTP (IMS). With such a complexity of delivery options, how does Exchange determine where to deliver messages? It uses a message routing concept of Address Spaces.

Exchange Server Addressing
Before looking at the specifics of the Address Space, however, let's take a look at a few types of Addresses that appear in an Exchange system. Exchange can automatically create the following addresses:
  • Native Exchange (X.500 Style)

  • X.400

  • SMTP

  • Microsoft Mail


Exchange Addresses (EX): Native Exchange Addresses are based on X.500 addresses. X.500 is the DIRECTORY standard. So an Exchange Address is an object path within the
hierarchical Exchange directory. This full path is also referred to as a "Distinguished Name". Distinguished Names are used for communication within an Exchange Organization.

The format of an EX address is as follows:

/o=Organization/ou=Site/cn=RecipientContainer/cn=MailBox

ex. /o=Acme/ou=NorthAmerica/cn=Recipients/cn=JohnDoe

X.400 Address (X400): X.400 Addresses are used when a Distinguished Name is not known or available. X.400 Addresses are also referred to as "O/R", or Originator/Recipient addresses. They are based on a global X.400 Address Space. X.400 addresses are comprised of a variety of fields. Not all are required, but you must include enough such that each address within the X.400 name space is unique.

The following are some of the more command fields:
  • c =Country

  • a =Administrative management domain or ADMD

  • p =Private management domain or PRMD

  • o =Organization

  • ou1 =Organizational units (ou1, ou2, ou3, and ou4)

  • cn =Common name

  • q =Generation qualifier

  • i =Initials

  • s =Surname

  • g =Given Name


Addresses are constructed by appending these fields to one another with a semicolon. By default, these values map to the following Exchange values:
  • Surname and Given Name = Recipients Name

  • Organization = Site Name

  • PRMD = Exchange Organization

  • ADMD = (‘space’ by default)

  • Country code


For Example:
c=us;a= ;p=Acme;o=NorthAmerica;s=Doe;g=John

Note the space for the ADMD. This is an important default requirement!

SMTP (SMTP): SMPT addresses are internet addresses. They are also hierarchical in nature, using a concept of Domains. There are several "Top Level" Domains, that include "COM", "EDU", "GOV", and "MIL". Each Top Level Domain has many sub-domains and these sub-domains can be further subdivided. The address typically takes the format of:

mailbox@sub-domain.domain

By default, Exchange generates SMTP address of the format:

mailbox@site.organization.com

ex. JohnDoe@NorthAmerica.Acme.com

MSMAIL (MS): Microsoft mail uses the concept of "Networks" and "Post Offices". A typical MSMAIL recipient would have an address such as:

Acme/NorthAmerica/JohnDoe

The Address Space
An Address Space can be considered the LEAST amount of information NECESSARY to determine the connection to use to reach a recipient.

Consider a scenario where you have just designed a railway system. You have created tracks that provide the following "Service Table":

Line 1 = Washington, DC to Durham, NC
Line 2 = Washington, DC to Denver, CO
Line 3 = Washington, DC to Cumberland, MD
Line 4 = Washington, DC to Pittsburgh, PA to Cumberland, MD
Line 5 = Washington, DC to Wilmington, NC to Atlanta, GA

A passenger arrives in Washington DC and informs you that she will be visiting some friends at 123 Main St. in Durham, NC.

What is the LEAST amount of information you need to know to make a ticket? You clearly need to know that she is going to NC. But you have a train that goes to Wilmington as well as a train that goes to Atlanta. So, you need to also know where in NC she is going. In this case Durham. But do you need to know that she is going to house number 123 or even that she is going to Main street? No.

So, your "Address Space", or the LEAST amount of information you need to know, is "Durham, NC". This is because you know that all passengers going to Durham, NC, regardless of street address or house number, can get there via Line 1. With this information you can refer back to your service table and offer her a ticket on Line 1.

Exchange creates an "Address Space" in much the same way. Consider an Exchange organization of ACME, with a Site in Chicago which is connected via an SMTP connector to a site in Dallas and another SMTP connector to a site in Denver. From Chicago you need to send an email to JaneDoe@Denver.Acme.com. What is the least amount of information you need to know? It is "Denver.Acme.com". What if the address was JaneDoe@Engineer.Denver.Acme.Com? You still only need "Denver.Acme.Com" because ALL messages for Denver go through this connector, regardless of department.

The GWART
Exchange keeps track of these "Address Spaces" much like our Train scenario. Instead of a "Service Table" though, Exchange uses the Gateway Address Routing Table (GWART). The GWART is simply a text file that lists all known messaging routes between sites. The file, "gwart0.mta" contains the current routing configuration and is saved in the EXCHSRVRMTADATA directory. When changes are made to the GWART is is first copied to "gwart1.mta" in the same directory. Therefore after changes have been made the PREVIOUS routing table is always available in this file.

The GWART includes the TYPE of route (EX, X400, SMTP, etc.), the Address Space included, the COST of the route, and connector to use for this Address Space. The following is an example of a few entries that might be found in a GWART:

EX,/o=Acme/ou=NorthAmerica;,1,Site Connector (NorthAmerica),

EX,/o=Acme/ou=Europe;2,Site Connector (NorthAmerica), Site Connector (Europe),

SMTP, *, 1, Internet Mail Service (MexicoCity),

Notice the leading TYPE (EX, SMTP), followed by an address space. Remember that the address space is simply the least amount of information necessary to make the routing decision. Note the address space for SMTP. Being a single asterisk, this idicates that ALL SMTP traffic should go through this connector.





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