本文详细阐述了在Session Initiation Protocol (SIP)中如何正确使用扩展功能。当服务器需要应用特定扩展来生成响应时，必须确保客户端已通过Supported头字段表明支持该扩展。若客户端不支持所需扩展，则服务器应仅依赖基本SIP功能或其他受支持的扩展。在极少数情况下，如果缺少特定扩展支持则无法处理请求，服务器可以发送421(Extension Required)响应，并在响应的Require头字段中列出所需的扩展选项标签。
A UAS that wishes to apply some extension when generating the
response MUST NOT do so unless support for that extension is
indicated in the Supported header field in the request. If the
desired extension is not supported, the server SHOULD rely only on
baseline SIP and any other extensions supported by the client. In
rare circumstances, where the server cannot process the request
without the extension, the server MAY send a 421 (Extension Required)
response. This response indicates that the proper response cannot be
generated without support of a specific extension. The needed
extension(s) MUST be included in a Require header field in the
response. This behavior is NOT RECOMMENDED, as it will generally
break interoperability.
Any extensions applied to a non-421 response MUST be listed in a
Require header field included in the response. Of course, the server
MUST NOT apply extensions not listed in the Supported header field in
the request. As a result of this, the Require header field in a
response will only ever contain option tags defined in standards-
track RFCs.
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