SIP - Response Only Header Fields

Min-Expires

The Min-Expires header field is used in a 423 Interval Too Brief response from a registrar rejecting a REGISTER request in which one or more Contacts have an expiration time that is too short.

• The header field contains an integer number of seconds that represents the minimum expiration interval that the registrar will accept.

• A client receiving this header field can update the expiration intervals of the registration request accordingly and resend the REGISTER request.

Min-SE

The Min-SE header field is a required header field in a 422 Session Timer Interval Too Small response.

The response may also be present in an INVITE or UPDATE containing a Session-Expires header field. It contains an integer number of seconds.

Proxy-Authenticate

The Proxy-Authenticate header field is used in a 407 Proxy Authentication Required authentication challenge by a proxy server to a UAC.

It contains the nature of the challenge so that the UAC may formulate credentials in a Proxy- Authorization header field in a subsequent request.

SIP-ETag

The SIP-ETag header field is part of the SIP publication mechanism. The SIP-ETag header field is returned in a 2xx response to a PUBLISH request.

• It contains an entity tag uniquely identifying the state information that has been processed.

• This entity tag can then be used to do conditional publications on this data including refreshing, modifying, and removing.

Unsupported

The Unsupported header field is used to indicate features that are not supported by the server.

The header field is used in a 420 Bad Extension response to a request containing an unsupported feature listed in a Require header field.

Example:
Unsupported: rel100

WWW-Authenticate

The WWW-Authenticate header field is used in a 401 Unauthorized authentication challenge by a UA or registrar server to a UAC.

It contains the nature of the challenge so that the UAC may formulate credentials in a Proxy-Authorization header field in a subsequent request.

RSeq

The RSeq header field is used in provisional (1xx class) responses to INVITEs to request reliable transport.

The header field may only be used if the INVITE request contained the Supported: rel100 header field.

• If present in a provisional response, the UAC should acknowledge receipt of the response with a PRACK method.

• The RSeq header field contains a reliable sequence number that is an integer randomly initialized by the UAS.

• Each subsequent provisional response sent reliably for this dialog will have a monotonically increasing RSeq number.

• The UAS will retransmit a reliably sent response until a PRACK is received with a RAck containing the reliable sequence number and CSeq.