RFC3261: SIP:13.3.1.4接受INVITE

13.3.1.4 The INVITE is Accepted
13.3.1.4接受INVITE

   The UAS core generates a 2xx response.  This response establishes a dialog, and therefore follows the procedures of Section 12.1.1 in addition to those of Section 8.2.6.

​UAS核心生成2xx响应。该响应建立了一个对话，因此除了遵循第8.2.6节的程序外，还遵循第12.1.1节的程序。

   A 2xx response to an INVITE SHOULD contain the Allow header field and the Supported header field, and MAY contain the Accept header field. Including these header fields allows the UAC to determine the features and extensions supported by the UAS for the duration of the call, without probing.

对INVITE的2xx响应应包含Allow报头字段和Supported报头字段，并且可能包含Accept报头字段。包括这些报头字段允许UAC在呼叫期间确定UAS支持的功能和扩展，而无需进行探测。

   If the INVITE request contained an offer, and the UAS had not yet sent an answer, the 2xx MUST contain an answer.  If the INVITE did not contain an offer, the 2xx MUST contain an offer if the UAS had not yet sent an offer.

如果INVITE请求包含offer，而UAS尚未发送answer，则2xx必须包含answer。如果INVITE不包含offer，如果UAS尚未发送offer，则2xx必须包含offer。

   Once the response has been constructed, it is passed to the INVITE server transaction.  Note, however, that the INVITE server transaction will be destroyed as soon as it receives this final response and passes it to the transport.  Therefore, it is necessary to periodically pass the response directly to the transport until the ACK arrives.  The 2xx response is passed to the transport with an interval that starts at T1 seconds and doubles for each retransmission until it reaches T2 seconds (T1 and T2 are defined in Section 17).  Response retransmissions cease when an ACK request for the response is received.  This is independent of whatever transport protocols are used to send the response.

​一旦构建了响应，就将其传递给INVITE服务器事务。但是，请注意，一旦INVITE服务器事务接收到该最终响应并将其传递给传输，它就会被销毁。因此，有必要周期性地将响应直接传递到传输，直到ACK到达。2xx响应以从T1秒开始的间隔传递到传输，每次重传加倍，直到达到T2秒（T1和T2在第17节中定义）。当接收到针对响应的ACK请求时，响应重传停止。这与用于发送响应的任何传输协议无关。

      Since 2xx is retransmitted end-to-end, there may be hops between UAS and UAC that are UDP.  To ensure reliable delivery across these hops, the response is retransmitted periodically even if the transport at the UAS is reliable.

由于2xx是端到端重传的，因此UAS和UAC之间可能存在UDP跳。为了确保跨这些跳的可靠传递，即使UAS的传输是可靠的，也会定期重新传输响应。

   If the server retransmits the 2xx response for 64*T1 seconds without receiving an ACK, the dialog is confirmed, but the session SHOULD be terminated.  This is accomplished with a BYE, as described in Section 15.

​如果服务器在没有收到ACK的情况下重新发送2xx响应64*T1秒，则对话得到确认，但会话应该终止。这是通过BYE完成的，如第15节所述。