sip中180振铃消息的解释
Explaination of 180 ringing message in sip
当爱丽丝给鲍勃打电话时，鲍勃给爱丽丝发送了一条 180 振铃消息。那为什么from字段是Alice的uri，to字段是Bob的uri呢？我指的是 RFC 3261。
F2 180 振铃鲍勃 -> 爱丽丝
SIP/2.0 180 铃声
通过：SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9
;收到=192.0.2.101
发件人：爱丽丝 ;tag=9fxced76sl
收件人：Bob ;tag=8321234356
来电号码：3848276298220188511@atlanta.example.com
CSeq: 1 个邀请
联系人：
内容长度：0
相同的 RFC 包含答案
8.2.6.2 Headers and Tags
The From field of the response MUST equal the From header field of
the request. The Call-ID header field of the response MUST equal the
Call-ID header field of the request. The CSeq header field of the
response MUST equal the CSeq field of the request. The Via header
field values in the response MUST equal the Via header field values
in the request and MUST maintain the same ordering.
If a request contained a To tag in the request, the To header field
in the response MUST equal that of the request. However, if the To
header field in the request did not contain a tag, the URI in the To
header field in the response MUST equal the URI in the To header
field; additionally, the UAS MUST add a tag to the To header field in
the response (with the exception of the 100 (Trying) response, in
which a tag MAY be present). This serves to identify the UAS that is
responding, possibly resulting in a component of a dialog ID. The
same tag MUST be used for all responses to that request, both final
and provisional (again excepting the 100 (Trying)). Procedures for
the generation of tags are defined in Section 19.3.
根据 RFC 3261，
From Header 标识 REQUEST 的发起者。
To Header 表示 REQUEST 的接收者。
因此，在收到响应时，From 和 To headers 不会有任何变化。
当爱丽丝给鲍勃打电话时，鲍勃给爱丽丝发送了一条 180 振铃消息。那为什么from字段是Alice的uri，to字段是Bob的uri呢？我指的是 RFC 3261。
F2 180 振铃鲍勃 -> 爱丽丝
SIP/2.0 180 铃声
通过：SIP/2.0/TCP client.atlanta.example.com:5060;branch=z9hG4bK74bf9 ;收到=192.0.2.101
发件人：爱丽丝 ;tag=9fxced76sl
收件人：Bob ;tag=8321234356
来电号码：3848276298220188511@atlanta.example.com
CSeq: 1 个邀请
联系人：
内容长度：0
相同的 RFC 包含答案
8.2.6.2 Headers and Tags
The From field of the response MUST equal the From header field of the request. The Call-ID header field of the response MUST equal the Call-ID header field of the request. The CSeq header field of the response MUST equal the CSeq field of the request. The Via header field values in the response MUST equal the Via header field values in the request and MUST maintain the same ordering.
If a request contained a To tag in the request, the To header field in the response MUST equal that of the request. However, if the To header field in the request did not contain a tag, the URI in the To header field in the response MUST equal the URI in the To header field; additionally, the UAS MUST add a tag to the To header field in the response (with the exception of the 100 (Trying) response, in which a tag MAY be present). This serves to identify the UAS that is responding, possibly resulting in a component of a dialog ID. The same tag MUST be used for all responses to that request, both final and provisional (again excepting the 100 (Trying)). Procedures for the generation of tags are defined in Section 19.3.
根据 RFC 3261， From Header 标识 REQUEST 的发起者。 To Header 表示 REQUEST 的接收者。 因此，在收到响应时，From 和 To headers 不会有任何变化。