What are the mandatory header in SIP?

The Call-ID header field is mandatory in all SIP requests and responses. It is used to uniquely identify a call between two user agents. A Call-ID must be unique across calls. All registrations for a user agent should use the same Call-ID.

What is CSeq?

CSeq. The CSeq header specifies the number of requests of each type that have been sent. For example, CSeq: 15 INVITE means that is the 15th invite request. The number increases by one for each additional request of the same type.

What is Max forward in SIP?

For SIP, the max value is 255 (0-255) but default value is 70 as per RFC 3261. When a call is made from SIP leg to SS7 leg with a Max forward value > 31, it will be treated as Hop Counter 31 on SIP side and decremented to 30.

Is 100 trying mandatory?

Purpose of 100 Trying is to stop retransmission on INVITE, which means > that once UAC receives 100 trying then it can be sure that next hop has > received the message. Since next hop may not be consumer of INVITE, > hence To-tag is not mandatory (discouraged otherwise as well). > > > >

What is the Via header?

The Via general header is added by proxies, both forward and reverse, and can appear in the request or response headers. It is used for tracking message forwards, avoiding request loops, and identifying the protocol capabilities of senders along the request/response chain.

What is prack in SIP?

PRACK is used to acknowledge the receipt of a reliable transfer of provisional response (1XX). Generally PRACK is generated by a client when it receive a provisional response containing an RSeq reliable sequence number and a supported:100rel header. PRACK contains (RSeq + CSeq) value in the rack header.

What does prack mean in SIP?

Provisional Response Acknowledgement
SIP PRACK (Provisional Response Acknowledgement)

Is 180 ringing mandatory?

Neither 100 nor 180 is mandatory.

What is the CSeq header for SIP INFO messages?

The RFC on SIP INFO messages states CSeq header values follow the mechanism in RFC3261: The Info Package mechanism does not define a delivery order mechanism. Info Packages can rely on the CSeq header field [RFC3261] to detect if an INFO request is received out of order.

What is the CSeq number in a request?

The CSeq header field is a required header field in every request. It contains a decimal number that increases for each request. Usually, it increases by 1 for each new request, with the exception of CANCEL and ACK requests, which use the CSeq number of the INVITE request to which it refers.

Do CSeq header values have to be incremental?

SIP RFC 3261does indicate that the CSeq header values MUST be incremental but it depends of the party initiating the request. Section 12.2.1.1of this RFC states:

What is a from header in SIP request?

A From header field may contain a tag used to identify a particular call. It may contain a display name, in which case the URI is enclosed in <>. It is a mandatory header. The Call-ID header field is mandatory in all SIP requests and responses.