SIP (Session Initiation Protocol)

From NENA Knowledge Base

SIP (Session Initiation Protocol) is a protocol specified by the IETF (RFC 3261) that defines a method for establishing multimedia sessions over the Internet. Used as the call signaling protocol in VoIP, NENA i2 and NENA i3.

SIP Term Definition
BYE A SIP transaction used to terminate a session.
CANCEL A SIP transaction which is used to cancel an INVITE transaction which has not yet completed.
Call-Info A SIP header field which contains a URI referring to some kind of data relevant to a call, and a “purpose” parameter describing what the URI refers to.  Used to carry URIs to such entities as Additional Data for the Call and Caller, and call/Incident Tracking Identifiers.
From A SIP header that describes the caller’s notion of its own identity (Address of Record).  From is generally not treated as reliable unless it is protected by an Identity header.
INFO A SIP transaction used to pass information from the caller to the called party.
INVITE A SIP transaction used to initiate a session.
re-INVITE A SIP INVITE transaction within an established session used to change the parameters of a call or refresh a session. See INVITE.
Method The primary function that an SIP request is meant to call on a server. This method is carried in the request message itself.
Transaction A SIP transaction occurs between a UAC and a UAS. The SIP transaction comprises all messages from the first request sent from the UAC to the UAS up to a final response (non-1xx) sent from the UAS to the UAC. If the request is INVITE, and the final response is a non-2xx, the SIP transaction also includes an ACK to the response. The ACK for a 2xx response to an INVITE request is a separate SIP transaction.
MESSAGE A SIP method which passes information, often an Instant Message, between endpoints in the body of the SIP message.
NOTIFY A SIP method used to send a notification to a subscriber of the occurrence of an asynchronous event.
OPTIONS A SIP method used to request the SIP protocol options supported by an endpoint.
P-A-I (P-Asserted-Identity) A header field in a SIP message containing a URI that the originating network asserts is the correct identity of the caller.

Also known as: PAI

REFER A SIP method that is used as part of a transfer operation to refer a call to another endpoint.
RequestURI That part of a SIP message that indicates where the call is being routed towards. SIP Proxy servers commonly change the Request ID (“retargeting”) to route a call towards the intended recipient.
Resource Priority A header field used on SIP calls to indicate priority that proxy servers give to specific calls. The Resource Priority header field does not indicate that a call is an emergency call (see Request-URI).
REGISTER A SIP method that is used to communicate the availability and address of an endpoint to the proxy server that directs incoming calls.
UPDATE A SIP method used to update parameters in a call not yet established.
REFER/Replaces Use of the SIP REFER method together with a Replaces header as part of a transfer operation to indicate that a new leg is to be created that replaces an existing call leg.
SUBSCRIBE / NOTIFY The two actions in an asynchronous event notification system.  The subscription is the request to receive notifications of the events.  The Notify is the notification of the event itself. Also refers to the SIP methods used for this purpose.
PRACK A SIP message used to reliably acknowledge receipt of an otherwise unreliable message transmission.


Relevant NENA Documents


NENA-STA-010, NENA i3 Standard for Next Generation 9-1-1


External References