Notes from the Lab

Outlook and Protocol Sequences

June 9th, 2006

I was recently testing different troubleshooting scenarios for MAPI client connectivity issues.

MAPI clients connect to the Exchange IS service, and there are a few ways to do that. Normally, Exchange 2003 registers six interfaces (provided, the only installed network protocol is TCP/IP). There are only two that relevant to a MAPI client (Outlook):

1. “ncacn_ip_tcp” - uses TCP. Uses one of the higher (1024 and above) TCP ports randomly (that is by default) selected by the server when Exchange IS starts. This can be manually configured on a server - see Knowledge Base Article 270836 for details
2. “ncacn_np” - uses Named Pipes. Uses TCP port 445.

Outlook XP attempts to use ncacn_ip_tcp first. If this fails, it falls back to ncacn_np. From my experience it works much slower through ncacn_np. Outlook 2003 only uses ncacn_ip_tcp. In TCP ports term this means that if you mistakenly block higher ports, Outlook XP will work slow.

As you can see from the above, ncacn_ip_tcp is the most important protocol sequence from the Outlook connectivity standpoint. In order for this protocol sequence to work, you need to make sure that two TCP ports on the server can be reached by the client:

1. Port 135
2. Higher port selected by Exchange IS.

Port 135 is used by the RPC Endpoint Mapper, so the client needs to reach it in order to receive information about the TCP port that is actually used by Exchange IS.

There are two ways to determine (for troubleshooting purposes) the port used by the Exchange IS:

1. We can simply define it as described in Knowledge Base Article 270836
2. We can use rpcdump-like tool

Rpcdump is a part of Windows 2003 Resource Kit and available for download from the Microsoft web site. It simply gives you the list of RPC interfaces registered on the server. Run in in verbose mode (-v) and look for the interface with protocol sequence “ncacn_ip_tcp” and UUID “a4f1db00-ca47-1067-b31f-00dd010662da“. The TCP port number will be in brackets after the IP address. You can use a simple telnet client to test connectivity.

Entry Filed under: Exchange