This article is part 3 in a series that discusses namespace planning, load balancing principles, client connectivity, and certificate planning.
Over the last several months, we have routinely fielded questions on how various clients connect to the infrastructure once Exchange 2013 is deployed. Our goal with this article is to articulate the various connectivity scenarios you may encounter in your designs. To that end, this article will begin with a walk through of a deployment that consists of Exchange 2007 and Exchange 2010 in a multi-site architecture and show how the connectivity changes with the introduction of Exchange 2013.
Figure 1: Exchange 2007 & Exchange 2010 Multi-Site Architecture
As you can see from the above diagram, this environment contains three Active Directory sites:
To understand the client connectivity before we instantiate Exchange 2013 into the environment, let’s look at the four users.
The Autodiscover namespace, autodiscover.contoso.com, as well as, the internal SCP records resolve to the CAS2010 infrastructure located in Site1. Outlook clients and ActiveSync clients (on initial configuration) will submit Autodiscover requests to the CAS2010 infrastructure and retrieve configuration settings based on their mailbox’s location. The Autodiscover service on Exchange 2010 can process Autodiscover requests for both Exchange 2007 and Exchange 2010 mailboxes.
For more information on how Autodiscover requests are performed, see the whitepaper, Understanding the Exchange 2010 Autodiscover Service.
For internal Outlook clients using RPC/TCP connectivity whose mailboxes exist on Exchange 2010, they will connect to the Exchange 2010 RPC Client Access array endpoint (assuming one exists). Keep in mind the importance of configuring the RPC Client Access array endpoint correctly, as documented in Ambiguous URLs and their effect on Exchange 2010 to Exchange 2013 Migrations.
For internal Outlook clients using RPC/TCP connectivity whose mailboxes exist on Exchange 2007, they will connect directly to the Exchange 2007 Mailbox server instance hosting the mailbox.
For more information, see the article Upgrading Outlook Web App to Exchange 2010.
For more information, see the article Upgrading Exchange ActiveSync to Exchange 2010.
Exchange 2013 has now been deployed in Site1 following the guidance documented within the Exchange Deployment Assistant. As a result, Outlook Anywhere has been enabled on all Client Access servers within the infrastructure and the mail.contoso.com and autodiscover.contoso.com namespaces have been moved to resolve to Exchange 2013 Client Access server infrastructure.
Figure 2: Exchange 2013 Coexistence with Exchange 2007 & Exchange 2010 in a Multi-Site Architecture
To understand the client connectivity now that Exchange 2013 exists in the environment, let’s look at the four users.
The Autodiscover external namespace, autodiscover.contoso.com, as well as, the internal SCP records resolve to the CAS2013 infrastructure located in Site1. Outlook clients and ActiveSync clients (on initial configuration) will submit Autodiscover requests to the CAS2013 infrastructure and depending on the mailbox version, different behaviors occur:
For internal Outlook clients using RPC/TCP connectivity whose mailboxes exist on Exchange 2010, they will still connect to the Exchange 2010 RPC Client Access array endpoint.
For internal Outlook clients using RPC/TCP connectivity whose mailboxes exist on Exchange 2007, they will still connect directly to the Exchange 2007 Mailbox server instance hosting the mailbox.
When you have an Exchange 2013 mailbox you are using Outlook Anywhere, both within the corporate network and outside of the corporate network; RPC/TCP connectivity no longer exists for Exchange 2013 mailboxes.
In Exchange 2007/2010, the way Outlook Anywhere was implemented is that you had one namespace you could configure. In Exchange 2013, you have both an internal host name and an external host name. Think of it as having two sets of Outlook Anywhere settings, one for when you are connected to the corporate domain, and another for when you are not. You will see this returned to the Outlook client in the Autodiscover response via what looks like a new provider, ExHTTP. However, ExHTTP isn’t an actual provider, it is a calculated set of values from the EXCH (internal Outlook Anywhere) and EXPR (External Outlook Anywhere) settings. To correctly use these settings, the Outlook client must be patched to the appropriate levels (see the Exchange 2013 System Requirements for more information). Outlook will process the ExHTTP in order – internal first and external second.
The default Exchange 2013 internal Outlook Anywhere settings don’t require HTTPS. By not requiring SSL, the client should be able to connect and not get a certificate pop-up for the mail and directory connections. However, you will still have to deploy a certificate that is trusted by the client machine for Exchange Web Services and OAB downloads.
In order to support access for Outlook Anywhere clients whose mailboxes are on legacy versions of Exchange, you will need to make some changes to your environment which are documented in the steps within the Exchange Deployment Assistant. Specifically, you will need to enable Outlook Anywhere on your legacy Client Access servers and enable NTLM in addition to basic authentication for the IIS Authentication Method.
The Exchange 2013 Client Access server’s RPC proxy component sees the incoming connections, authenticates and chooses which server to route the request to (regardless of version), proxying the HTTP session to the endpoint (legacy CAS or Exchange 2013 Mailbox server).
For Outlook Web App, the user experience will depend on the mailbox version and where the mailbox is located.
For Exchange ActiveSync clients, the user experience will depend on the mailbox version and where the mailbox is located. In addition, Exchange 2013 no longer supports the 451 redirect response – Exchange 2013 will always proxy ActiveSync requests.
Coexistence with Exchange Web Services is rather simple.
Offline Address Book
Like with Exchange Web Services, Autodiscover will provide the Offline Address Book URL.
It’s important to understand that when CAS2013 proxies to a legacy Exchange Client Access server, it constructs a URL based on the server FQDN, not a load balanced namespace or the InternalURL value.But how does CAS2013 choose which legacy Client Access server to proxy the connection?
When a CAS2013 starts up, it connects to Active Directory and enumerates a topology map to understand all the Client Access servers that exist within the environment. Every 50 seconds, CAS2013 will send a lightweight request to each protocol end point to all the Client Access servers in the topology map; these requests have a user agent string of HttpProxy.ClientAccessServer2010Ping (yes, even Exchange 2007 servers are targeted with that user string). CAS2013 expects a response - a 200/300/400 response series indicates the target server is up for the protocol in question; a 502, 503, or 504 response indicates a failure. If a failure response occurs, CAS2013 immediately retries to determine if the error was a transient error. If this second attempt fails, CAS2013 marks the target CAS as down and excludes it from being a proxy target. At the next interval (50 seconds), CAS2013 will attempt to determine the health state of the down CAS to determine if it is available.
The IIS log on a legacy Client Access server will contain the ping events. For example:
2014-03-11 14:00:00 W3SVC1 DF-C14-02 22.214.171.124 HEAD /ecp - 443 - 192.168.1.42 HTTP/1.1 HttpProxy.ClientAccessServer2010Ping - - coe-e14-1.coe.lab 302 0 0 277 170 02014-03-11 14:00:00 W3SVC1 DF-C14-02 126.96.36.199 HEAD /PowerShell - 443 - 192.168.1.27 HTTP/1.1 HttpProxy.ClientAccessServer2010Ping - - coe-e14-1.coe.lab 401 0 0 309 177 152014-03-11 14:00:00 W3SVC1 DF-C14-02 188.8.131.52 HEAD /EWS - 443 - 192.168.1.134 HTTP/1.1 HttpProxy.ClientAccessServer2010Ping - - coe-e14-1.coe.lab 401 0 0 245 170 02014-03-11 14:00:00 W3SVC1 DF-C14-02 184.108.40.206 GET /owa - 443 - 192.168.1.220 HTTP/1.1 HttpProxy.ClientAccessServer2010Ping - - coe-e14-1.coe.lab 301 0 0 213 169 1712014-03-11 14:00:01 W3SVC1 DF-C14-02 220.127.116.11 HEAD /Microsoft-Server-ActiveSync/default.eas - 443 - 192.168.1.29 HTTP/1.1 HttpProxy.ClientAccessServer2010Ping - - coe-e14-1.coe.lab 401 2 5 293 194 312014-03-11 14:00:04 W3SVC1 DF-C14-02 18.104.22.168 HEAD /OAB - 443 - 10.166.18.213 HTTP/1.1 HttpProxy.ClientAccessServer2010Ping - - coe-e14-1.coe.lab 401 2 5 261 170 171
If for some reason, you would like to ensure a particular CAS2010 is never considered a proxy endpoint (or want to remove it for maintenance activities), you can do so by executing the following cmdlet on Exchange 2010 (note that this feature does not exist on Exchange 2007):
Set-ClientAccessServer <server> -IsOutofService $True
All this discussion about HTTP-based clients is great, but what about POP and IMAP clients? Like the HTTP-based client counterparts, IMAP and POP clients are also proxied from the Exchange 2013 Client Access server to a target server (whether that be an Exchange 2013 Mailbox server or a legacy Client Access server). However, there is one key difference, there is no health-checking on the target IMAP/POP services.
When the Exchange 2013 Client Access server receives a POP or IMAP request, it will authenticate the user and perform a service discovery.
CAS2013 will choose a server to proxy the request based on the incoming connection’s configuration. If the incoming connection is over an encrypted channel, CAS2013 will try to locate an SSL proxy target first, TLS next, plaintext lastly. If the incoming connection is over a plaintext channel, CAS2013 will try to locate a plaintext proxy target first, SSL next, TLS lastly.
Hopefully this information dispels some of the myths around proxying and redirection logic for Exchange 2013 when coexisting with either Exchange 2007 or Exchange 2010. Please let us know if you have any questions.
Ross Smith IV Principal Program Manager Office 365 Customer Experience