ActiveSync and Exchange web service client protocol connectivity flow in Exchange 2013/2010 coexistence | 4/4 | 23#23

of 21 | Part 23#23 | ActiveSync and Exchange web service client protocol
connectivity flow in Exchange 2013/2010 coexistence environment | 4/4
Written by Eyal Doron | o365info.com | Copyright © 2012-2015
ACTIVESYNC AND EXCHANGE WEB SERVICE
CLIENT PROTOCOL CONNECTIVITY FLOW IN
EXCHANGE 2013/2010 COEXISTENCE
ENVIRONMENT | 4/4 | 23#23
The current article is the fourth article of four articles series, on the subject of:
“Exchange 2013/2010 coexistence environment and mail client protocol
connectivity flow”. In this article, our main focus is reviewing two types of client
protocol connectivity flow in Exchange 2013/2010 coexistence environment:
 ActiveSync client protocol connectivity flow
 Exchange web services client protocol connectivity flow

Exchange 2013/2010 coexistence | ActiveSync
client protocol connectivity flow
Exchange ActiveSync clients (Mobile clients) are always considered as “external
client” because, the network infrastructure of mobile client is based on a public
mobile network. Mobile client (ActiveSync client) will always need to address the
Public facing Exchange CAS server and for this reason, the “connection point”
(Exchange CAS server) that will accept the mobile (ActiveSync) client communication
requests, must be configured as: a “Public facing Exchange CAS server”.

Note – other Exchange clients such as Outlook and OWA, that can connect the
internal (or to the external) Exchange infrastructure.
When the ActiveSync (Mobile) client connects the Public facing Exchange CAS
server, based on the provided user credentials, the Public facing Exchange CAS
server finds out where is the user mailbox is hosted and “route” (Proxy) the
communication request to the internal Exchange infrastructure.
The “internal routing” of the ActiveSync (mobile) client communication request is
implemented by using the internal ActiveSync URL address.
Scenario 1: mobile (ActiveSync) client | User mailbox located on New York
site.
Scenario charters: mobile (ActiveSync) client, need to get access to his mailbox.
 Exchange user type: Exchange 2010 client (Exchange user whom his mailbox is
hosted on the Exchange 2010 mailbox server).
 Exchange mailbox server location: the Exchange 2010 Mailbox server who hosts
the user mailbox, is located on the New York site.
The ActiveSync client protocol connectivity flow, will be implemented as follows:
1. Mobile (ActiveSync) client, connects the “New York Public facing Exchange CAS”
by using the server name: mail.o365info.com and provides his user credentials.
2. CAS2013 uses the user credentials and performs the Active Directory lookup.
CAS2013 determines that:
o The user mailbox version is: 2010
o The Exchange 2010 mailbox server that host the user mailbox is located at the
New York site
o There is a local Exchange CAS 2010 in the site (the New York site)
3. CAS2013 will proxy the ActiveSync client request + the ActiveSync user
credentials to the CAS2010 in the local site server by using the internal Exchange
2010 CAS ActiveSync URL address (. (Number 2).
4. The CAS2010 will accept the request and “forward” (Proxy) the ActiveSync client
connection request to the Exchange 2010 Mailbox server (Number 3).
5. Exchange 2010 mailbox server “fetch” the required user mailbox content and
send back the data to the CAS2010 (Number 4).
6. CAS2010 proxy back the informationdata to CAS2013 (Number 5).

7. CAS2013 provides the required information to the external ActiveSync client
(Number 6).

Scenario 2: mobile (ActiveSync) client | User mailbox located on Los Angles
site | Destination site = Intranet site | No local Exchange 2010 CAS
Scenario charters: mobile (ActiveSync) client, need to get access to his mailbox.
the user mailbox, is located on the Los Angles site.
 The New York site, doesn’t have a “local” Exchange 2010 CAS.
Since in our scenario, the Exchange 2010 mailbox is hosted on Exchange 2010
Mailbox server on other sites (Los Angles site) and, since there is no “local Exchange
2010 CAS”, Exchange 2013 CAS will proxy by himself the external mobile client
request to the “remote Exchange 2010 CAS” that is located on Los Angles site
(Number 2).
Note – the rest of the process is identical to the steps that we have already
reviewed in – Scenario 1: ActiveSync client | Exchange user mailbox on the same
Active Directory site.

Scenario 3: mobile (ActiveSync) client | User mailbox located on Madrid site |
Destination site = Public facing Exchange site | Regional namespace
Before we start with the specific details of the “Madrid ActiveSync user” briefly
review the charters of this specific scenario.
By default, ActiveSync (Mobile) client will use the Exchange Autodiscover
infrastructure for getting the “server name” that will accept their request. In a
scenario of a “Madrid ActiveSync user”, the name of the Exchange server who

should be provided for the ActiveSync client as part of the Autodiscover process is,
the name of “Madrid Public facing Exchange CAS server”:europe.mail.o365info.com
By default, the preferred method for ActiveSync client is to use the Exchange
Autodiscover services for getting all the required ActiveSync profile settings and the
host name of the Exchange server who will serve as: “ActiveSync server” but, In
some scenarios, ActiveSync client the Autodiscover services are not used and
instead, the mobile user uses a “manual method” in which he provides the
“Exchange server name”.
For example: when a “Madrid ActiveSync user” want to access his mailbox, he can
provide the primary namespace: mail.o365info.com (option A in the diagram) as the
Exchange a1 host name or, use the host name of the “Madrid Public facing
Exchange CAS server”:europe.mail.o365info.com (option B in the diagram)
In case that the “Madrid ActiveSync user” use the primary
namespace: mail.o365info.com, the connection request will be accepted by the “New
York Public facing Exchange CAS server”.
The “New York Public facing Exchange CAS server” will need to know how to
“handles” this request because in our scenario, the ActiveSync user mailbox is
hosted on another Exchange site: the Madrid site.
The basic assumption could be that in this case, the “New York Public facing
Exchange CAS server” will “redirect the “Madrid ActiveSync user” to his Exchange
server but Exchange 2013 CAS will not use the redirection method.
In this scenario, the “New York Public facing Exchange CAS server” will not redirect
the ActiveSync request, but instead, proxy the connection request to the “Madrid
Exchange CAS server”.

Scenario charters: mobile (ActiveSync) client, need to get access to his
mailbox.
the user mailbox, is located on the Madrid site.
 The Madrid site considers as Public facing Exchange site and the “Madrid Public
facing Exchange CAS server” are published with a regional
namespace: mail.o365info.com

The special charter of this scenario is – that the user’s mailbox, is located on a
different Exchange site and additionally, the destination site is a “Public facing
Exchange site”
In former versions of Exchange server, in a scenario in which the mobile
(ActiveSync) client connects a Public facing Exchange CAS server, and the Exchange
server recognizes that the mobile (ActiveSync) client mailbox is located in a different
Exchange site + the “other Exchange site” considers as: Public facing Exchange site,
the “response” of the Public facing Exchange CAS server was a: redirection message
to the mobile (ActiveSync) client.
The mobile (ActiveSync) client was supposed to accept the “redirection message”
and create a new communication channel with the “other Public facing Exchange
CAS server (the “Madrid Public facing Exchange CAS server” in our scenario).
The method of redirecting mobile (ActiveSync) client was implemented by using a
message that described as: ”451 redirects message”.
The problem with the ”451 redirects message” was that – many mobile (ActiveSync)
clients, did not know how to “handle” the redirection message and the result were:
communication failure. In other words, the mobile (ActiveSync) client didn’t
“understand” the redirection message, and that he was “instructed” to connect
“other ActiveSync Exchange servers.

For this reason, the behavior of Exchange CAS 2013 server is different because, the
Exchange CAS 2013 server will not implement any more the redirection method
(451 redirect message) for mobile (ActiveSync) clients. Instead, when Exchange 2013
CAS recognizes that the ActiveSync mailbox is located on another Exchange site, he
will Proxy the request “on behalf” of the ActiveSync to the “destination Exchange
server”.
In our scenario, the New York Public facing Exchange CAS server “know” that the
user mailbox is located at the Madrid site and additionally, that the Madrid
considers as a Public facing Exchange site (has a Public facing Exchange CAS server).
Theoretically, the New York Public facing Exchange CAS server can send a
redirection command to the mobile (ActiveSync) client, but instead, the New York
Exchange 2013 CAS will choose to use the Proxy method.
It’s clear that this method is not efficient from the point of view of the “New York
Public facing Exchange 2013 CAS server” because theoretically, the “Madrid Public
facing Exchange CAS server” should have served the “Madrid ActiveSync (mobile)
client, but using the “Proxy method”, will ensure that the mobile (ActiveSync) client
communication will be successfully completed.

The ActiveSync client protocol connectivity flow, will be implemented as follows:
1. Madrid Mobile (ActiveSync) client, connects the “New York Public facing
Exchange CAS” by using the server name: mail.o365info.com and provides his
user credentials.
o the Exchange 2010 mailbox server that host the user mailbox is located at the
Madrid site
3. CAS2013 will not send a redirection request to the Madrid ActiveSync client, but
instead, proxy the ActiveSync client request + the ActiveSync user credentials to
the “Madrid Public facing Exchange CAS server” by using the external “Madrid
Public facing Exchange CAS server” ActiveSync URL address (Number 2).
4. The “Madrid Public facing Exchange CAS server” will accept the request and
“forward” (Proxy) the ActiveSync client connection request to the “internal
Madrid Exchange 2010 Mailbox server” (Number 3).
5. The “internal Madrid Exchange 2010 Mailbox server” “fetch” the required user
mailbox content and send back the data to the “Madrid Public facing Exchange
CAS server” (Number 4).
6. “Madrid Public facing Exchange CAS server” proxy back the informationdata to
“New York Public facing Exchange CAS server” (Number 5).
7. “New York Public facing Exchange CAS server” provides the required information
to the external ActiveSync client (Number 6).

Additional reading
 Understanding Proxying and Redirection
 Exchange ActiveSync Returned an HTTP 451 Error
 How Exchange Server 2010 CAS Proxy & Redirection works for Exchange
ActiveSync

Exchange 2013/2010 coexistence | Exchange
web service client protocol connectivity flow
The subject of Exchange web services connectivity flow In Exchange 2013/2010
coexistence environment could be a bit confusing because, it’s not clear who is the
“element” that provides the Exchange web services to the Exchange 2010 clients.
Is the element is Exchange 2013 CAS that implements the standard Proxy
mechanism of proxy, Exchange 2010 clients request to the Exchange 2010 CAS or
other scenario in which the Exchange 2010 client connects directly to the Exchange
2010 CAS and asks for a specific Exchange web service.
Just a general note: the most “important Exchange web service client” is the Outlook
client. Its truth that there are other Exchange web service clients, but the Exchange
client that is most dependent on the Exchange web service is: the Outlook mail
client.
For this reason, when we mention the subject of “Exchange web services and client
protocol connectivity flow”, most of the time, the client that we are refereeing is
Outlook.

Technically, the “element” that provides the Exchange web service to the Exchange
2010 clients can be: Exchange CAS 2010 based server or, Exchange CAS 2013 based
server.
The answer for the question: who is the element the provide Exchange web service
to Exchange 2010 clients, depend on the specific implementation of the namespace
infrastructure.
 In case that the Exchange 2010 web service’s URL address namespace is identical
to the Exchange 2013 web service’s URL address namespace, the Exchange web
service’s element that will serve Exchange 2010 clients is the Exchange 2013 CAS.
 In case that the Exchange 2010 web service’s URL address namespace is not
identical to the Exchange 2013 web service’s URL address namespace, the
Exchange web service’s element that will serve Exchange 2010 clients is the
Exchange 2010 CAS.

Note – In the current article, we will not get into a detailed explanations of this
concept, and if you want a more thorough review, please read the articles:
 Exchange web services in an Exchange 2013 coexistence environment | Part 1/2
 Exchange web services in an Exchange 2013 coexistence environment | Part 2/2
To simplify the description of the Exchange web services client protocol connectivity
flow, we will use the scenario of the “best practice” configuration in which the
“element” that will serve as a “focal point” foe Exchange 2010 client that requests
Exchange web services will be the Exchange 2013 CAS.
The charter of this scenario is that the Exchange 2013 CAS and the Exchange 2010
CAS will use an identical namespace of the Exchange web services.

For example: the URL address that was configured in Exchange 2013 CAS for the
Exchange web services is: https://mail.o365info.com/EWS/Exchange.asmx and, the
same URL address is configured in the Exchange 2010 CAS server for the URL
address for Exchange web services.
In this scenario, Exchange 2010 client will address the Exchange 2013 CAS for
Exchange web services and the Exchange 2013 CAS will Proxy their request to the
Exchange 2010 CAS.
Exchange web service and Autodiscover services
To be able to understand better the implementation of the Exchange web services
in an Exchange 2013 coexistence environment, it’s important that we will know of
the relationships that exist between the Exchange Autodiscover infrastructure and
the Exchange web services infrastructure.
The Exchange web service is “build on” the Autodiscover information that is
provided by the Exchange CAS server.
When the Exchange 2010 client address Exchange 2013 CAS and asks for
Autodiscover information, the Exchange 2013 CAS will provide the Autodiscover
information that includes the URL address of the Exchange web services. For
example: https://mail.o365info.com/EWS/Exchange.asmx
The Exchange 2010 client uses this information for locating the Exchange CAS
server who will provide him the required Exchange web services. In our scenario,
the URL address includes the host name: mail.o365info.com. This host name will
point the Exchange 2010 client to the Exchange 2013 CAS server.
In the following diagram, we can see an example for the client protocol connectivity
flow that described the “full flow” of Exchange 2010 clients.
Phase 1/2 – Autodiscover services
1. Exchange 2010 client, address the Exchange 2013 CAS as an Autodiscover
Endpoint.
2. The Autodiscover responds that the CAS2013 “provide” includes the URL address
URL address of the Exchange web
services: https://mail.o365info.com/EWS/Exchange.asmx

Phase 2/2 – Exchange web services
When the Exchange 2010 client needs a specific web service such as availability
service (Free/Busy time), he will use the URL address that he got from the
Autodiscover response: https://mail.o365info.com/EWS/Exchange.asmx
This URL includes the host name: mail.o365info.com that will be “translated” to the
IP address of Exchange 2013 CAS.
When the Exchange 2013 CAS gets the Exchange 2010 request for an Exchange web
service, he “understand” that the Exchange mail client is Exchange 2010 client and,
for this reason, he will “forward” (Proxy) the requites to the Exchange 2010 CAS. The
Exchange 2013 CAS addresses the Exchange 2010 CAS that will need to provide the
required Exchange web services to the Exchange 2010 client by using the Exchange
2010 CAS FQDN. In our scenario: Excas2010.o365info.com

Scenario 1: Internal/External Exchange web service’s client | User mailbox
located on New York site.
Scenario charters: Exchange web service client, need to get a specific Exchange web
service.
the user mailbox, is located at the New York site.
The Exchange web service client protocol connectivity flow, will be implemented as
follows:
The preliminary process for the Exchange web services is the Autodiscover process,
in which the external Exchange 2010 client gets the Autodiscover information that
includes the URL address of the Exchange web service.

1. Exchange web service clients use the following URL address:
https://mail.o365info.com/EWS/Exchange.asmx, connect the “New York Public
facing Exchange CAS” and provides his user credentials.
o The Exchange 2010 mailbox server that host the user mailbox is located at the
New York site
o There is a local Exchange CAS 2010 in the site (the New York site)
3. CAS2013 will proxy the Exchange web service client request to the CAS2010 in
the local site. The “proxy “process, includes the user credentials that were
provided by the Exchange web service user (Number 2).
4. The CAS2010 will accept the request and “forward” (Proxy) the Exchange web
service request to the Exchange 2010 Mailbox server (Number 3).
5. Exchange 2010 Mailbox server “reply back” and send the required information to
CAS2010 (Number 4).
6. CAS2010 proxy backs the information to CAS2013 (Number 5).
7. CAS2013 provides the required information to the external Exchange web
service client (Number6).

Additional reading
 Client Connectivity in an Exchange 2013 Coexistence Environment
 Understanding Client Access Protocol Connectivity Flow – Microsoft Exchange
Server 2013
 Overview of Exchange Server 2010 CAS Proxying and Redirection
 Microsoft Exchange Server 2013 Deployment and Coexistence

The Exchange 2013 coexistence article series index page

ActiveSync and Exchange web service client protocol connectivity flow in Exchange 2013/2010 coexistence | 4/4 | 23#23

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