Centralized Sign-In

Based on the previously described sign-in process, administrators without a Director in a multiple Front End pool environment face a dilemma of determining which pool should handle the initial sign-in and authentication for all users. The Lync client applications use weighted DNS SRV record lookups to find a pool, and only one pool can be considered the most preferred. When a Director exists, these DNS records typically point at the Director, which then handles sending the user a SIP 301 Redirect message with the primary and backup registrar pool information. Without a Director, one of the Front End pools in the environment must be responsible for handling these tasks for all users.

Historically, the Director role played a much bigger part in the sign-in process every single day, and careful planning was required to ensure that there was enough processing capacity available to handle the bulk of sign-in and authentication traffic occurring during the morning hours in each region. The benefit of a dedicated Director from an internal perspective was that these initial authentication requests were offloaded from Front End servers.

However, since Office Communications Server 2007 R2, the client applications have maintained a file called endpointconfiguration.cache in the local settings folder of a user’s PC. This file contains the user’s primary pool and preferred server in a pool so that on subsequent sign-in attempts the client will actually attempt to first contact the server in the file before falling back to any DNS SRV record lookups and potentially leveraging a Director.

This means that although a Director can certainly offload authentication and sign-in traffic from a Front End pool for a user’s first sign-in, it’s of little benefit internally on later sign-in attempts. The clients are generally bypassing the Director altogether by leveraging their local cache information. Of course, if that cache is removed or fails at some point, the Director will be used again, but many organizations have begun to accept that temporary traffic increase to a Front End pool.