SIP (Session Initiation Protocol)

VoIP equipment on a laptop
TwilightEye / GettyImages

SIP stands for Session Initiation Protocol. It is complementary with VoIP since it provides signaling functions to it. Apart from VoIP, it is used in other multimedia technologies as well, like online games, video, and other services. SIP was developed along with another signaling protocol, H.323, which was used as the signaling protocol for VoIP before SIP. Now, SIP is has replaced it to a large extent.

SIP deals with communication sessions, which are the periods of time during which parties communicate. These include Internet telephone calls, multimedia conferences, and distribution, etc. SIP provides the necessary signaling for creating, modifying and terminating sessions with one or more communicating participants.

SIP works in roughly the same way as other common protocols like HTTP or SMTP. It carries out the signaling by sending small messages, consisting of a header and a body.

SIP Functions

SIP is an enabler-protocol for VoIP and Telephony in general, due to the following features it has:

Name Translation and User Location: SIP translates an address to a name and thus reaches the called party at any location. It does a mapping of session description to location and ensures support for details of the nature of the call.

Feature negotiation: Not all communicating parties (which may be more than two) have the necessary features. For example, not everyone may have video support. SIP allows the group the negotiate for the features.

Call participant management: SIP allows a participant to make or cancel connections to other users during a call. Users can also be transferred or placed on hold.

Call feature changes: SIP allows a user to change a call’s characteristics during the call. For example, as a user, you may want to enable or disable video, especially while a new user joins a session.

Media negotiation: This mechanism enables negotiation of the media used in a call, like selecting the appropriate codec for call establishment between various devices.

The Structure of a SIP message

SIP works by having the communicating devices sending and receiving messages. A SIP message carries a lot of information which help identify the session, control timing, and describe the media. Below is a list of what a message briefly contains:

  • Protocol information (e.g. version)
  • Session information (creator, name, etc.)
  • Participant information (email, phone, etc.)
  • Bandwidth information
  • Encryption information
  • Time description (active time and repeat time)
  • Media description (media name, title, address, etc.)
  • Media bandwidth information
  • Media encryption key