1 Introduction

This document describes Microsoft extensions to the Session Initiation Protocol (SIP). SIP is used by terminals to establish, modify, and terminate multimedia sessions or calls. SIP is specified in [RFC3261], [RFC3262], [RFC3263], and [RFC3863].

Microsoft has added extensions for NTLM/Kerberos Authentication, for presence, for optimization of subscriptions, and for notifications and contact management. These extensions are used by Windows Messenger and the Real-Time Communications (RTC) Client API.

Sections 1.5, 1.8, 1.9, 2, and 3 of this specification are normative. All other sections and examples in this specification are informative.

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