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The “SIP/2.0 403 Forbidden” response is a status code in the Session Initiation Protocol (SIP), which is a protocol used for initiating, maintaining, modifying, and terminating real-time sessions that involve video, voice, messaging, and other communications between two or more endpoints on IP networks.

The “403 Forbidden” response indicates that the request was understood by the server, but the server refuses to authorize it. This can occur when the server has insufficient permissions to access the requested resource, or when the client lacks the necessary credentials to access the resource.

The “From:”, “To:”, “Call-ID:”, “CSeq:”, “Server:”, “Allow:”, “Supported:”, “Session-Expires:”, “Content-Length:”, “Via:”, “Contact:”, and “Record-Route:” headers are all headers that can be included in a SIP message. The “From:”, “To:”, and “Call-ID:” headers are required in all SIP requests and responses, while the other headers are optional.

SIP 403 Header Field Explanations

The “From: ” header contains the address of the user making the request.

The “To: ” header contains the address of the user being called.

The “Call-ID: ” header is a globally unique identifier for the call.

The “CSeq: ” header is a sequence number that is incremented for each new request sent by the client.

The “Server: ” header contains information about the server handling the request.

The “Allow: ” header lists the methods that the server supports.

The “Supported: ” header lists the optional features that the client supports.

The “Session-Expires: ” header indicates how long the session is allowed to be active.

The “Content-Length: ” header indicates the size of the message body in bytes.

The “Via: ” header contains the path taken by the request.

The “Contact: ” header contains the contact address of the client.

The “RecordRoute: ” header is used to reverse the route taken by the response.

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