Hi All, According to this article :http://theessentialexchange.com/blogs/michael/archive/2008/01/16/exchange-active-directory-and-wins.aspx DSProxy is primarily used to support old versions of Outlook (versions that were out before Active Directory was released). Versions of Outlook that old presume that the Exchange Server is running all of the LDAP directory services as well, since for Exchange Server 5.5 and all prior releases of Exchange Server, that was true. DSProxy runs on the Exchange Server and forwards any directory service requests from client computers to NSPI on an appropriate GC server. Newer versions of Outlook will usually connect to DSProxy only once - in order to receive a referral to a GC that Exchange uses. This happens during the creation of a user's profile. Once that information is received, Outlook stores the information in the user's profile and that GC is used for future requests. From what I understand - i) Older MAPI clients will connect to the NSPI interface on Exchange for GAL lookups, and Exchange will proxy this connection to a GC. ii) Newer MAPI clients will connect to the NSPI interface on Exchange only one, and then after this connect to the NSPI interface on the GC for GAL lookups. I'm curious - for Exchange servers holding, say, thousands of users, is there a noticeable performance increase with using the second method? And, if the first method is called DSProxy, what is the second method called?

I've never noticed a discernable peformance hit on the GCs through NSPI. Having said that, it's always good practice to monitor performance on your DC/GCs - I believe there is a counter for NSPI. And if you think about it, how many times do your users actually need to browse/search the GAL or other address lists? Usually they will just use the local resolved names cache. Also, if they are using Outlook in cached mode they will use the OAB by default. The NSPI concurrency is going to be low generally and the impact will be spread by users hitting an in-site GC (i.e. once that is local to them). Not sure if the second method has a special name. Alexei

Thanks Alexei... The Outlook <> GC connection when using NSPI referral...is that a MAPI session, an NSPI session or named something else?

My understanding is that NSPI is part of MAPI. In other words, it will be an RPC connection to the GC. Alexei

Hi, That is NSPI session over RPC. And the DSProxy component provides Outlook (MAPI0 clients with an address book service. DSProxy contains two parts: NSPI Proxy which serves for old Outlook clients. Referral Service which serves for newer Outlook clients. The name should be proxy and referral. Thanks Allen