[Rfc3261] Sip-via Header

In many cases, the SIP does not go directly to the target host, but goes through many intermediate node servers. In the request message, the Via header field indicates the nodes that have passed through (each node passes through, add a via header). In the response message, the via header field indicates the node that the message will go through next (each time the request is returned from the original path, a via header is deleted from each node ).

The basic format of via is: via header domain ID (that is, "via"): Header Domain value

The value of the Via header field added to the current node should contain the SIP version and network transmission protocol of the current node, the domain name or IP address of the current node, the port number, and several optional attribute items. Property items are separated from previous projects and property items. The following is an example of a typical via header field:

Via: SIP/2.0/udp 192.0.2.1: 506

The optional attribute items of via generally include: "maddr", "TTL", "received", and "branc". Their meanings and usage are described below.

 

The via header maddr, TTL, and sent-by components will be set when
The request is processed by the transport layer (section 18 ).