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.
- A Basic SIP Call Flow (telconotes.wordpress.com)
- Introduction to SIP (telconotes.wordpress.com)
- VoIP Basics (telconotes.wordpress.com)