Who are the Exchange Autodiscover clients? | Part 03#36

Page 1 of 16 | Who are the Exchange Autodiscover clients? | Part 03#36

Written by Eyal Doron | o365info.com | Copyright © 2012-2015

Who are the Exchange Autodiscover

clients? | Part 03#36

Throughout the current article series, the term “Autodiscover client\s” will be

mentioned dozens or even hundreds of times.

The obvious question could be – who or what are these “Autodiscover clients”?

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Autodiscover Client\server model versus server to

server model

Technically speaking, every Exchange client or other client that “know” how to

locate Exchange CAS server, how to request Autodiscover information and what to

“do” with the Autodiscover information could be considered as – Autodiscover

client.

If we want to be more specific, in the Exchange based environment, the term

“Autodiscover clients”, relates to the following type of Exchange clients:

Outlook client (RPC/HTTP/S)

Mobile mail client (ActiveSync client)

Web-based client (HTTP/HTTPS)

Other Exchange servers






One way that we can use to classify the different type of Autodiscover clients is by

using the “client to server” or, “server to a server” classification.

1. Client to server

An example of a “standard” Exchange Autodiscover client could be:

Outlook client.

Outlook clients use the Autodiscover services for:

Locating the Exchange CAS server who will enable them to access their mailbox, for

creating a new Outlook mail profile, for getting information about different

Exchange services such as – Offline address Book, Free\Busy time and more.

2. Server to Server model

Fewer know the perspective of the Autodiscover services, is a relationship that

could be described as: “Server to Server.”

In this scenario, the “client” is not a standard client such as Outlook but instead, the

client is an Exchange server who needs information from another Exchange server.

The ability of the “client” (the source Exchange server) to find or locate the

“destination Exchange server” that can provide him the required information, is

based on using the Autodiscover service.

An example for the Autodiscover “Server to Server model” could be

Mail migration in Office 365 environment | Exchange server as

Autodiscover client

In the Office 365 environment, migration such as – Cutover migration, Stage

migration or Hybrid migration, is implemented by relying on the Exchange On-

Premise Autodiscover service.

The migration batch that we create from the Office 365 admin center will use the

domain name suffix in the username credentials that we provide for locating the

Exchange on-Premises server.






Note – later we will review in more details this Autodiscover mechanism in which

the “client” use the part of the domain name suffix for locating Autodiscover

Endpoint.

Office 365 Hybrid environment | Exchange server as Autodiscover client

The term “Hybrid environment”, is a term that describes a relationship between two

different Exchange organization (Exchange Online and Exchange on-Premises

server) that can operate as one entity and share between them different types of

information and services.

The ability to share information between the cloud infrastructure (Exchange Online)

and the Exchange On-Premise infrastructure such as – Availability service

(Free/Busy time), is based on the Autodiscover infrastructure of each of the

environments (cloud and On-Premise).

When a user from the “cloud” needs to view Free/Busy time of On-Premise user, the

Exchange Online server will use the Autodiscover process for locating the Exchange

On-Premise server.

Who are the Exchange mail clients?






Another term that will be used very often throughout the current article series is

the term – “Exchange client”.

Exchange environment can serve many types of the different mail client.

The main charterer that difference one mail client from another is the protocol that

the client use.

Although all the “mail protocol” has the same purpose of – enable a user to see the

content of his mailbox, each of the mail protocols to behaves differently, speaks

other “language” and interacts differently with Exchange infrastructure.

Exchange server supports four types of mail clients.






Outlook mail client – Outlook mail client described sometimes as an MAPI

client because, this client uses a dedicated mail protocol named – MAPI

(Mail application program interface).

The MAPI protocol is not a transport protocol and, for this reason, Outlook

mail client uses a “combination” of protocols. For example – MAPI over RPC

(Remote Procedure Call) or another type of combination such as when using

the Outlook Anywhere settings that use the following combination – MAPI

over RPC over HTTPS.

Exchange 2013 will support a new “combination” in which, the RPC protocol

will be “taken out” from the equation.

Webmail client – mail web clients are clients that use the HTTP or the HTTPS

protocol for accessing the user mailbox.

Mobile mail clients – the access protocol for a mobile client such as a

smartphone, tablets, etc., is the ActiveSync protocol.

Internet mail client – the term “internal mail client” relates to the mail client

that uses the POP3 and IMAP4 protocol for retrieving mail from the user

mailbox and use the SMTP protocol for sending mail.

Besides of the different protocol this of this mail client use, there are more

categories that we can use for this mail client.

In the following diagram, we can see that the Exchange CAS server, provide his

services for a variety of mail clients, using a variety of mail protocols such as –

Outlook mail client that uses RPC protocol, or RPC/HTTPS (Outlook Anywhere)

protocol, mobile devices that use the ActiveSync protocol and “web-based mail

client” such as OWA.

The Exchange CAS server “know” how to adapt himself to each of these different

protocols, that has different charters, communication methods, etc.






The “Exchange Autodiscover” mail clients

Versus the phrase – “All men are created equal”, the Exchange clients are not

created equal and each of them has a specific charter and requirements.

Relating to our main subject – the Exchange Autodiscover infrastructure, not all the

“Exchange client” need or “consume” the Exchange Autodiscover in the same way.






For example, Autodiscover protocol enables Exchange client to automatically locate

available Exchange CAS server but some of the Exchange client such as the OWA

client specifies manually the name of the Exchange server whom they access,

instead of using an “automatic process” that will reveal at the end the required

Exchange CAS server name.

The most prominent Exchange Autodiscover client is the – Outlook. The Outlook

client is fully dependent on the Exchange Autodiscover infrastructure.

Another Exchange client such as – mobile client (ActiveSync) and OWA (web client)

are partially dependent on the Exchange Autodiscover infrastructure.

For example, a mobile client (ActiveSync) can locate the required Exchange CAS

server using Autodiscover query or instead manually type the required Exchange

CAS server name.






The Exchange Autodiscover infrastructure serves for additional purposes such as:

providing the required information needed for creating a new mail profile (relevant,

mainly for Outlook mail client), information about available Exchange web services,

and information about the supported authentication protocol from the Exchange

CAS server and so on.

For the sake of full disclosure, I must admit that I know for sure the Exchange

Autodiscover is “mandatory” for Outlook client, but for the rest of the Exchange

clients – mobile (ActiveSync) client and OWA (web client) it’s not so clear how does

this client interaction with the Exchange Autodiscover services.

For example, part of the Autodiscover information that is provided by the Exchange

server includes a specific section of authentication protocol requirements for the

Exchange web client (OWA mail client) but it’s not so clear how and if the OWA client

uses this information.

The dependency of the different Exchange client in the Exchange Autodiscover

services table






In the following table, we can see “Level of use” of the “Autodiscover services”, by

the different Exchange clients.

The different Exchange mail client and the process of

automatically locating the Exchange CAS server

By default, Outlook client doesn’t know the name of the Exchange CAS server that

will serve him. The only way for Outlook client to locate and find “their Exchange

CAS server” is, by using the Autodiscover process.

These are some exceptions for this “rule” because technically, we can provide

Outlook client the Exchange CAS server name manually or, by using a local

configuration file, but this method is not recommended and additionally, there are

many other configuration settings that we will need to provide to the Outlook client

beside of the name of the Exchange CAS server.

Mobile (ActiveSync) clients will try to locate their Exchange CAS server by using the

Autodiscover process and, this is the “recommended way”.

In a scenario in which the Autodiscover process failed or the in some old mobile

devices the Mobile client (ActiveSync) doesn’t support Autodiscover; we can






manually provide the name of the Public facing Exchange CAS server who will

accept the communication request of the Mobile (ActiveSync) client.

Regarding a webmail client such as OWA, the OWA mail client doesn’t use the

Autodiscover protocol for “locating” the “element” that will “lead him” to his mailbox

because, when using a webmail client, the basic assumption is that the user will

have to know in advance the name of the Exchange mail server that will enable him

to access his mailbox.

Get information about available Exchange web services

The only way for “informing” Outlook client about the availability Exchange web

services is via the Autodiscover process, in which the Exchange CAS server sends

the Outlook client the URL address of the existing Exchange web services.

Regarding the “other Exchange clients” – mobile (ActiveSync) client and OWA (web

client), it’s not so clear, how does this client get the information about available

Exchange web services.

As far as I know, a client such as OWA, doesn’t need to know about available

Exchange web services, the OWA client just needs to apply for information and the

Exchange server is responsible for locating the required resources (Exchange web

services) and fetching the required information for the OWA client.

And again, I’m not sure that this theory accurate in 100%.

Get the information required for the creation of a new mail profile.

Outlook client must be configured with a “mail profile” that serves as a “logical

container” for all the communication settings that will be required for the

communication channel between the Outlook client and Exchange CAS server.

The only optional way for Outlook mail client to get the required configuration

setting for the mail profile is – by using the Autodiscover services.

The Exchange CAS server will accept the Outlook client Autodiscover request for

information and based on specific characters of the Exchange client such as – the

Exchange Mailbox server who hosts the recipient mailbox, the geographic location

of the client and more, provide a custom answer.

Other






In the “other” section, we can relate to any type of configuration settings or services

that can be provided as part of the Autodiscover process to Exchange client.

For example, as mention, part of the Autodiscover information that is provided by

Exchange includes a specific section that relates to the OWA mail client.

Because I’m not sure how this information does is “used” by OWA mail client, I will

classify this information in the “other” section.

Autodiscover one-time procedure or ongoing

communication channel?

In the former section, we have to review the nature of the relationships that exists

between the Exchange clients and their Exchange CAS server and the way that the

Autodiscover infrastructure help client to find their “destination Exchange CAS

server” and help the Exchange CAS server to provide the necessary information for

his client.

The question that could appear now is – Does the Autodiscover can be considered

as a “one-time event”? Or does the Autodiscover is an “ongoing” communication

channel between the Exchange CAS server and his clients?






The answer to these questions is that the Autodiscover process is not a static

process!






The Autodiscover process is not a “one-time time process.”

We can classify the scenarios of Autodiscover relationships between the Exchange

clients and their Exchange CAS server into two major categories:

1. First-time Autodiscover process

2. Ongoing Autodiscover process

The Autodiscover process between the client and the server starts with a “first

handshake” between the Autodiscover client and the Autodiscover Endpoint.

In this “preliminary process,” the Autodiscover Endpoint provides the information to

his client, but this doesn’t mean that starting from today onwards, the Autodiscover

client will use this information forever!






The more suitable term for the Autodiscover relationships that exists between the

Autodiscover client and the Autodiscover Endpoint is a “dialog.”

For example, Exchange clients, such as Outlook, are accessing the Autodiscover

Endpoint, on an hourly basis to check for “new information” or updates relating to

the Exchange infrastructure.

Another example could be – each time that the mail client such as Outlook is

“restarted,” the client will try to access the Autodiscover Endpoint, looking for a new

information or verifying that he has the required information.









Documents

Who are the Exchange Autodiscover clients? | Part 03#36