Pioneering communications with Session Initiation Protocol


Pioneering communications with Session Initiation Protocol

SIP is a signalling protocol used to create, manage and terminate sessions in an IP-based network. Sessions can range from simple two-way telephone calls to collaborative multi-media conferences.

SIP is the first protocol to enable multi-user sessions regardless of media content. It defines how one or more participant’s end devices can create, modify and terminate a connection whether the content is voice, video, data or Web-based.

SIP is the session control mechanism for the next-generation cellular network.


SIP is becoming one of the most significant protocols since HTTP and SMTP (resembling both in some ways). It serves four major purposes:

  • establishment of user location
  • feature negotiation
  • call management (for example: adding, dropping, or transferring participants)
  • changing the features of a session in progress


  • SIP messages are text-based.
  • SIP re-uses several existing and mature internet services and protocols – DNS, RTP, RSVP, etc.
  • Service providers can define SIP extensions for new applications.
  • SIP is transport layer independent, flexibly connecting users independent of the underlying infrastructure.
  • SIP supports multi-device feature levelling and negotiation. For example, if a service or session initiates video and voice, voice can still be transmitted to devices that aren’t video-enabled.


To describe the payload of message content and characteristics, SIP uses the Internet’s Session Description Protocol (SDP) to describe the characteristics of the end devices. It also works with a number of other protocols, including LDAP for location, RADIUS for authentication and RTP for real-time transmissions, among many others.

Components of a SIP Session

A SIP user agent (UA) is a logical network end-point that is used to create or receive SIP messages and thereby manage a SIP session. A SIP UA can perform the role of a User Agent Client (UAC), which sends SIP requests, and the User Agent Server (UAS), which receives the requests and returns a SIP response. Infrastructure elements include Proxy servers, Registrars and Redirect servers.

SIP Sessions

The following presents a typical exchange of requests and responses:

This shows only a few of the basic SIP commands:

  • INVITE: Invites a user to a call
  • ACK: Acknowledgement is used to facilitate reliable message exchange for INVITEs
  • BYE: Terminates a connection between users

The first digit of a Status Code defines the category of response. Two types are found in the example:

  • 1xx: Provisional – request received, continuing to process the request
  • 2xx: Success – the action was successfully received, understood and accepted

The other types are: Redirection (3xx), Client Error (4xx), Server Error (5xx) and Global Failure (6xx).


  • SIP is flexible, extensible and open.
  • Programmers can add new bits of information to messages without compromising connections.
  • The similarity of SIP to HTTP philosophies allows developers to quickly understand the principles and get involved in development.
  • Using SIP, service providers can freely choose among standards-based components and quickly harness new technologies.
  • Users can locate and contact one another regardless of media content or number of participants.

SIP-based services

  • Local and long-distance telephony
  • Presence & Instant Messaging
  • IP Centrex/Hosted PBX
  • Voice messaging
  • Click-to-Call
  • Rich media conferencing
  • Unified Communications
  • CallCenter-CRM integration


  • IP phones
  • Personal computers
  • Network proxy servers
  • VOIP gateways
  • Media servers
  • Application servers

Used by

  • Vonage
  • Deltathree
  • Microsoft
  • MCI
  • Nokia
  • Research in Motion
  • 3G Community