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SIP early Dialog state with SIP 180 Ringing response from Bob to Alice
U2:
  • creates an 'early' dialog by answering with a provisional response with a To tag;
  • copies all Record-Route headers from the request into the response;
  • adds a Contact header field to the response;
  • constructs the state of the dialog:
    • the Route set is set to the list of URIs in the Record-Route header fields from the request, taken in order and preserving all URI parameters;
    • the remote target is set to the URI from the Contact header field of the request;
    • the remote sequence number is set to the value of the sequence number in the CSeq header field of the request;
    • the local sequence number is empty;
    • the call identifier component of the dialog ID is set to the value of the Call-ID field in the request;
    • the local tag component of the dialog ID is set to the tag in the To field in the response to the request;
    • the remote tag component of the dialog ID is set to the tag from the From field in the request;
    • the remote URI is set to the URI in the From field;
    • the local URI is set to the URI in the To field.
  • sends the 180 response by using the Via header fields (not shown).
P2:
  • forwards the 180 response by using the Via header fields (not shown).
P1:
  • forwards the 180 response by using the Via header fields (not shown).
U1:
  • establishes an 'early' dialog when receiving the provisional response with a To tag;
  • constructs the state of the dialog:
    • the Route set is set to the list of URIs in the Record-Route header fields from the response, taken in reverse order and preserving all URI parameters;
    • the remote target is set to the URI from the Contact header field of the response;
    • the local sequence number is set to the value of the sequence number in the CSeq header field of the request;
    • the remote sequence number is empty;
    • the call identifier component of the dialog ID is set to the value of the Call-ID field in the request;
    • the local tag component of the dialog ID is set to the tag in the From field in the request;
    • the remote tag component of the dialog ID is set to the tag in the To field of the response;
    • the remote URI is set to the URI in the To field;
    • the local URI is set to the URI in the From field.