Supported – indicates that the UA can send or receive reliable provisional responses. Require in a request – indicates that the UAS must send all provisional responses reliably. Require in a reliable provisional response – indicates that the response is to be sent reliably.
Supported – indicates the UA supports automatic or manual answering of a request. Require – indicates that the UA has to understand the extension when placed in an INVITE. Accept-Contact – the extension parameter “answermode” is placed in this header field when the Require header field is used..
Supported – indicates that a UA understands the early-session content-disposition type. Require – indicates that a UA requires the early-session disposition type.
Supported in a SUBSCRIBE – indicates that the UA is willing to process a list. Require in responses to a SUBSCRIBE and in all NOTIFYs within that subscription – indicates that the UA has subscribed to an event list.
Supported – indicates that a UA understands the extension. Require in a REGISTER request – indicates that the registrar is not expected to process the registration unless it supports the GRUU extension.
Supported – indicates that the UA supports the Join header field. Require – indicates that the UA wants explicit failure notification if Join is not supported.
REFER method refers to multiple
resources in a single request
Supported – the UA can handle multiple resources in a REFER request. Require – indicates the REFER request contains a pointer to a URI list in the Refer-To header field and the body contains a resource list document describing multiple REFER targets.
Supported – indicates that a UA can accept a REFER request without establishing an implicit subscription. Require – can be present with the Refer-Sub: false header field.
Supported – UA supports the Path header field. If found found in a REGISTER request, intermediate proxies can determine whether to offer Path service for that request. Require – added if an intermediate proxy requires that the registrar support Path for a request.
Supported – indicates that the offer contains only “optional” or “none” strength-tags. Require – indicates that the offer contains one or more “mandatory” strength-tags, or only “optional” or “none” strength-tags.
Proxy-Require – indicates that proxy servers do not forward the request unless they can provide the requested privacy service. Proxies remove this option tag before forwarding the request if the desired privacy function has been performed.
Conference establishment using
request-contained lists
Supported – added to the response to an OPTIONs request when the conference server can handle INVITEs with a ‘recipient-list’ body. Require – added to an INVITE when the UAC includes the set of participants in the body of its request to create an ad-hoc conference.
Supported – added to the response to an OPTIONs request when the UA can handle MESSAGEs with a ‘recipient-list’ body. Require – added to a MESSAGE when the UAC includes the set of recipients in the body.
Supported – added to the response to an OPTIONs request when the server can handle SUBSCRIBEs with a ‘recipient-list’ body. Require – added to a SUBSCRIBE when the UAC creates a resource list that it wants to subscribe to and includes the set of recipients in the body.
Supported – indicates the UA supports the Replaces header field. Require – indicates the UA wants explicit failure notification when the Replaces header field is not supported.
Resource-Priority and Accept-Resource-Priority header fields
Supported in OPTIONS – indicates the UA supports the resource-priority mechanism for emergency communications. Require – indicates that the UA wants explicit failure notification if resource priority is not supported.
Alternative network address types of the SDP grouping framework
Supported – indicates that the UA understands the ANAT semantics as defined in RFC 4091. Require – indicates that the UA has generated an offer that uses ANAT semantics.
Supported – indicates that the UAC supports the security agreement mechanism. Require or Proxy-Require – indicates that proxy servers are required to use the security agreement mechanism. Require in 494 or 421 responses – indicates that the UAC must use the security agreement mechanism.
Supported – indicates that the UA can perform refreshes according to 4028. Require in a request – means that the UAS must understand the session timer extension to process the request. Require in a response – indicates that the UAC must look for the Session-Expires header field in the response, and process accordingly.
