SIP INVITE Request-line & R-URI

The INVITE is the first SIP message that a caller will send to the recipient in order to establish a call. This post focuses in the first line of the INVITE and describes its components and usage.

This first line is actually called “Request-line” and it consists of three components: the method name, Request-URI (R-URI), and the protocol version.

Assuming that A calls B, the Request-line of A’s INVITE will look like this:

 INVITE sip:B@bbb.com SIP/2.0

 The first component in the line specifies the Method used in this Request. In that case it is an INVITE, stating that A wishes to initiate a session with B.

 The second component is the R-URI and it identifies the recipient of this SIP request (in this case it is B). In a SIP environment every user is identified by a SIP Uniform Resource Identifier (SIP URI) which basically contains the contact information of a SIP user and it is formatted as: sip:username@hostname.

In this example, the username is B and the hostname is bbb.com. A SIP URI can also have other forms, but this will be covered in another post.

Finally the third component specifies the protocol (SIP) and version (2.0) used.

The R-URI is an important parameter for the signaling process and it will be analyzed in later posts as well.

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About TelcoNotes

IP & VoIP networking

Posted on March 1, 2013, in VoIP and tagged , . Bookmark the permalink. Leave a comment.

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