![]() ![]() The number is reduced by one each time the request is forwarded. ![]() The Max-Forwards header limits the number of times a request can be forwarded on its way to the recipient. The number increases by one for each additional request of the same type.įormat CSeq: (number) (request type) Max-Forwards ![]() For example, CSeq: 15 INVITE means that is the 15th invite request. The CSeq header specifies the number of requests of each type that have been sent. A Content-Length of 0 indicates that there is no message body.įormat Content-Length: (number of bytes in message body) CSeq The Content-Length header specifies the size of the message content in bytes. The From header specifies who the call is coming from. The Contact header specifies a caller domain name or IP address and a transport type. The Contact header identifies the most direct route for sending future requests to the requesting device. This guarantees that the Call-ID number will be globally unique, since no two devices will have the same IP address. To ensure that each Call-ID identifier is globally unique, a random number is generated (which often looks like this: f_169eac17a017b0a4e0adfa8_I), and the sender’s IP address is appended to this number. The Call-ID SIP Header creates a globally unique identifier for the call. If a SIP request is routed through multiple devices, each UAC adds its own VIA header to the request before sending it on.įormat Via: SIP/(protocol version)/(transport type) (UAC):(protocol port) branch=(branch number) Call-ID The Via header routes SIP responses to the correct device, similar to a return address on a package. The Via header identifies a call’s path with the protocol name, protocol version, transport type, user agent client (UAC), the protocol port for the request and a branch parameter which serves as a unique identifier for each SIP transaction. The To header specifies the recipient of the call. A BYE request is used to terminate calls. While there are many other SIP headers, the nine outlined below supply the minimum required information to initiate a call over a SIP trunking network. Usually, an INVITE message initiates a session-essentially a phone call-on the SIP protocol. We'll try and run through some of the most common SIP Headers in this blogpost. Including the correct SIP Header and correctly formatting these SIP Headers is critical to ensure that requests are successfully routed to the right recipients. What is a SIP Header?Ī SIP Header is a component of a SIP message that is used to convey information about the SIP message. Why not try out our SIP Trunking product and see for yourself how SIP headers work? You can set up a Telnyx Portal account in minutes. SIP message requests use SIP headers to supply information about the requested action or information. User agents and network servers use message requests to locate, invite, and manage calls. SIP message requests are critical to successfully utilizing SIP trunking technology. ![]()
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