This chapter contains sections titled:

• SIP Functionality

• SIP Entities

• Message Format

• The Start Line in SIP Responses: the Status Line

• The Start Line in SIP Requests: the Request Line

• Header Fields

• Message Body

• SIP Transactions

• Message Flow for Session Establishment

• SIP Dialogs

• Extending SIP

• Caller Preferences and User Agent Capabilities

• Reliability of Provisional Responses

• Preconditions

• Event Notification

• Signaling Compression

• Content Indirection

• The REFER Method

• Globally Routable User Agent URIs (GRUU)

• NAT Traversal