Freeswitch中文用户手册(第四章 SIP)----2
2015-03-18 15:43
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转载：http://www.cnblogs.com/cnsanshao/p/3309640.html
通过 B2BUA 呼叫
在真实世界中，bob 和 alice 肯定要经常改变位置，那么它们的 SIP 地址也会相应改变，并且，如果他们之中有一个或两个处于 NAT 的网络中时，直接通信就更困难了。所以，他们通常会借助于一个服务器来相互通信。通过注册到服务器上，他们都可以获得一个服务器上的 SIP 地址。注册服务器的地址一般是不变的，因此他们的 SIP 地址就不会发生变化，因而，他们总是能够进行通信。
我们让他们两个都注册到 FreeSWITCH 上。上面已经说过，FreeSWITCH 监听的端口是 SIP 默认的端口 5060。bob 和 alice 注册后，他们分别获得了一个服务器的地址（SIP URI）：sip:bob@192.168.4.4 和 sip:alice@192.168.4.4（默认的端口号 5060 可以省略）。
下面是 bob 呼叫 alice 的流程。需要指出，如果 bob 只是发起呼叫而不接收呼叫，他并不需要向 FreeSWITCH 注册（有些软交换服务器需要先注册才能发起呼叫，但 SIP 是不强制这么做的）。
------------------------------------------------------------------------
recv 1118 bytes from udp/[192.168.4.4]:26000 at 13:31:39.938891:
------------------------------------------------------------------------
INVITE sip:alice@192.168.4.4 SIP/2.0
Via: SIP/2.0/UDP 192.168.4.4:26000;branch=z9hG4bK-d8754z-56adad736231f024-1---d8754z-;rport
Max-Forwards: 70
Contact: <sip:bob@192.168.4.4:26000>
To: "alice"<sip:alice@192.168.4.4>
From: "Bob"<sip:bob@192.168.4.4>;tag=15c8325a
Call-ID: YWEwYjNlZTZjOWZjNDg3ZjU3MjQ3MTA1ZmQ1MDM5YmQ.
CSeq: 1 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY, MESSAGE, SUBSCRIBE, INFO
Content-Type: application/sdp
User-Agent: X-Lite release 1014k stamp 47051
Content-Length: 594
上面的消息中省略了 SDP 的内容，我们将留到以后再探讨。bob 的 UAC 通过 INVITE 消息向 FreeSWITCH 发起请求。bob 的 UAC 用的是 X-Lite（User-Agent），它运行在端口 26000 上（实际上，它默认在端口也是 5060，但由于在我的实验环境下它也是跟 FreeSWITCH 运行在一台机器上，已被占用，因此它需要选择另一个端口）。其中，From 为主叫用户的地址，To 为被叫用户的地址。此时 FreeSWITCH 作为一个 UAS 接受请求并进行响应。它得知 bob 要呼叫 alice，需要在自己的数据库中查找 alice 是否已在服务器上注册，好知道应该怎么找到 alice。但在此之前，它先通知 bob 它已经收到了他的请求。
------------------------------------------------------------------------
send 345 bytes to udp/[192.168.4.4]:26000 at 13:31:39.940278:
------------------------------------------------------------------------
SIP/2.0 100 Trying
Via: SIP/2.0/UDP 192.168.4.4:26000;branch=z9hG4bK-d8754z-56adad736231f024-1---d8754z-;rport=26000
From: "Bob"<sip:bob@192.168.4.4>;tag=15c8325a
To: "alice"<sip:alice@192.168.4.4>
Call-ID: YWEwYjNlZTZjOWZjNDg3ZjU3MjQ3MTA1ZmQ1MDM5YmQ.
CSeq: 1 INVITE
User-Agent: FreeSWITCH-mod_sofia/1.0.trunk-16981M
Content-Length: 0
FreeSWITCH 通过 100 Trying 消息告诉 bob “我已经收到你的消息了，别着急，我正在联系 alice 呢...” 该消息称为呼叫进展消息。
------------------------------------------------------------------------
send 826 bytes to udp/[192.168.4.4]:26000 at 13:31:39.943392:
------------------------------------------------------------------------
SIP/2.0 407 Proxy Authentication Required
Via: SIP/2.0/UDP 192.168.4.4:26000;branch=z9hG4bK-d8754z-56adad736231f024-1---d8754z-;rport=26000
From: "Bob"<sip:bob@192.168.4.4>;tag=15c8325a
To: "alice" <sip:alice@192.168.4.4>;tag=B4pem31jHgtHS
Call-ID: YWEwYjNlZTZjOWZjNDg3ZjU3MjQ3MTA1ZmQ1MDM5YmQ.
CSeq: 1 INVITE
User-Agent: FreeSWITCH-mod_sofia/1.0.trunk-16981M
Accept: application/sdp
Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, UPDATE, INFO, REGISTER, REFER,
NOTIFY, PUBLISH, SUBSCRIBE
Supported: timer, precondition, path, replaces
Allow-Events: talk, presence, dialog, line-seize, call-info, sla,
include-session-description, presence.winfo, message-summary, refer
Proxy-Authenticate: Digest realm="192.168.4.4",
nonce="31c5c3e0-cc6e-46c8-a661-599b0c7f87d8", algorithm=MD5, qop="auth"
Content-Length: 0
但就在此时，FreeSWITCH 发现 bob 并不是授权用户，因而它需要确认 bob 的身份。它通过发送带有 Digest 验证信息的 407 消息来通知 bob（注意，这里与注册流程中的 401 不同）。
------------------------------------------------------------------------
recv 319 bytes from udp/[192.168.4.4]:26000 at 13:31:39.945314:
------------------------------------------------------------------------
ACK sip:alice@192.168.4.4 SIP/2.0
Via: SIP/2.0/UDP 192.168.4.4:26000;branch=z9hG4bK-d8754z-56adad736231f024-1---d8754z-;rport
To: "alice" <sip:alice@192.168.4.4>;tag=B4pem31jHgtHS
From: "Bob"<sip:bob@192.168.4.4>;tag=15c8325a
Call-ID: YWEwYjNlZTZjOWZjNDg3ZjU3MjQ3MTA1ZmQ1MDM5YmQ.
CSeq: 1 ACK
Content-Length: 0
bob 回送 ACK 证实消息向 FreeSWITCH 证实已收到认证要求。并重新发送 INVITE，这次，附带了验证信息。
------------------------------------------------------------------------
recv 1376 bytes from udp/[192.168.4.4]:26000 at 13:31:39.945526:
------------------------------------------------------------------------
INVITE sip:alice@192.168.4.4 SIP/2.0
Via: SIP/2.0/UDP 192.168.4.4:26000;branch=z9hG4bK-d8754z-87d60b47b6627c3a-1---d8754z-;rport
Max-Forwards: 70
Contact: <sip:bob@192.168.4.4:26000>
To: "alice"<sip:alice@192.168.4.4>
From: "Bob"<sip:bob@192.168.4.4>;tag=15c8325a
Call-ID: YWEwYjNlZTZjOWZjNDg3ZjU3MjQ3MTA1ZmQ1MDM5YmQ.
CSeq: 2 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY, MESSAGE, SUBSCRIBE, INFO
Content-Type: application/sdp
Proxy-Authorization: Digest username="bob",realm="192.168.4.4",
nonce="31c5c3e0-cc6e-46c8-a661-599b0c7f87d8",
uri="sip:alice@192.168.4.4",response="327887635344405bcd545da06763c466",
cnonce="c164b74f625ff2161bd8d47dba3a0ee2",nc=00000001,qop=auth,
algorithm=MD5
User-Agent: X-Lite release 1014k stamp 47051
Content-Length: 594
这里也省略了 SDP 消息体。
------------------------------------------------------------------------
send 345 bytes to udp/[192.168.4.4]:26000 at 13:31:39.946349:
------------------------------------------------------------------------
SIP/2.0 100 Trying
Via: SIP/2.0/UDP 192.168.4.4:26000;branch=z9hG4bK-d8754z-87d60b47b6627c3a-1---d8754z-;rport=26000
From: "Bob"<sip:bob@192.168.4.4>;tag=15c8325a
To: "alice"<sip:alice@192.168.4.4>
Call-ID: YWEwYjNlZTZjOWZjNDg3ZjU3MjQ3MTA1ZmQ1MDM5YmQ.
CSeq: 2 INVITE
User-Agent: FreeSWITCH-mod_sofia/1.0.trunk-16981M
Content-Length: 0
FreeSWITCH 重新回 100 Trying，告诉 bob 呼叫进展情况。
至此，bob 与 FreeSWITCH 之间的通信已经初步建立，这种通信的通道称作一个信道（channel）。该信道是由 bob 的 UA 和 FreeSWITCH 的一个 UA 构成的，我们称它为 FreeSWITCH 的一条腿，叫做 a-leg。
接下来 FreeSWITCH 要建立另一条腿，称为 b-leg。它通过查打本地数据库，得到了 alice 的位置，接着启动一个 UA（用作 UAC），向 alice 发送 INVITE 消息。如下：
------------------------------------------------------------------------
send 1340 bytes to udp/[192.168.4.4]:5090 at 13:31:40.028988:
------------------------------------------------------------------------
INVITE sip:alice@192.168.4.4:5090;rinstance=e7d5364c81f2b879;transport=UDP SIP/2.0
Via: SIP/2.0/UDP 192.168.4.4;rport;branch=z9hG4bKey90QUyHZQXNN
Route: <sip:alice@192.168.4.4:5090>;rinstance=e7d5364c81f2b879;transport=UDP
Max-Forwards: 69
From: "Bob" <sip:bob@192.168.4.4>;tag=Dp9ZQS3SB26pg
To: <sip:alice@192.168.4.4:5090;rinstance=e7d5364c81f2b879;transport=UDP>
Call-ID: 0d74ac35-cca4-122d-81a2-2990e5b2bd3e
CSeq: 130069214 INVITE
Contact: <sip:mod_sofia@192.168.4.4:5060>
User-Agent: FreeSWITCH-mod_sofia/1.0.trunk-16981M
Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, UPDATE, INFO, REGISTER, REFER,
NOTIFY, PUBLISH, SUBSCRIBE
Supported: timer, precondition, path, replaces
Allow-Events: talk, presence, dialog, line-seize, call-info, sla,
include-session-description, presence.winfo, message-summary, refer
Content-Type: application/sdp
Content-Disposition: session
Content-Length: 313
X-FS-Support: update_display
Remote-Party-ID: "Bob" <sip:bob@192.168.4.4>;party=calling;screen=yes;privacy=off
你可以看到，该INVITE 的 Call-ID 与前面的不同，说明这是另一个 SIP 会话（Session）。另外，它还多了一个 Remote-Party-ID，它主要是用来支持来电显示。因为，在 alice 的话机上，希望显示 bob 的号码，显示呼叫它的那个 UA（负责 b-leg的那个 UA）没什么意义。与普通的 POTS 电话不同，在 SIP 电话中，不仅能显示电话号码（这里是 bob），还能显示一个可选的名字（“Bob”）。这也说明了 FreeSWITCH 这个 B2BUA 本身是一个整体，它虽然是以一个单独的 UA 呼叫 alice，但还是跟负责 bob 的那个 UA 有联系--就是这种背靠背的串联。
------------------------------------------------------------------------
recv 309 bytes from udp/[192.168.4.4]:5090 at 13:31:40.193634:
------------------------------------------------------------------------
SIP/2.0 100 Trying
Via: SIP/2.0/UDP 192.168.4.4;rport=5060;branch=z9hG4bKey90QUyHZQXNN
To: <sip:alice@192.168.4.4:5090;rinstance=e7d5364c81f2b879;transport=UDP>
From: "Bob" <sip:bob@192.168.4.4>;tag=Dp9ZQS3SB26pg
Call-ID: 0d74ac35-cca4-122d-81a2-2990e5b2bd3e
CSeq: 130069214 INVITE
Content-Length: 0
跟上面的流程差不多，alice回的呼叫进展。此时，alice 的 UA 开始振铃。
------------------------------------------------------------------------
recv 431 bytes from udp/[192.168.4.4]:5090 at 13:31:40.193816:
------------------------------------------------------------------------
SIP/2.0 180 Ringing
Via: SIP/2.0/UDP 192.168.4.4;rport=5060;branch=z9hG4bKey90QUyHZQXNN
Contact: <sip:alice@192.168.4.4:5090;rinstance=e7d5364c81f2b879;transport=UDP>
To: <sip:alice@192.168.4.4:5090;rinstance=e7d5364c81f2b879;transport=UDP>;tag=3813e926
From: "Bob"<sip:bob@192.168.4.4>;tag=Dp9ZQS3SB26pg
Call-ID: 0d74ac35-cca4-122d-81a2-2990e5b2bd3e
CSeq: 130069214 INVITE
User-Agent: Zoiper rev.5415
Content-Length: 0
180也是呼叫进展消息，它说明，我这边已经准备好了，alice 的电话已经响了，她听到了一会就会接听。
send 1125 bytes to udp/[192.168.4.4]:26000 at 13:31:40.270533:
------------------------------------------------------------------------
SIP/2.0 183 Session Progress
Via: SIP/2.0/UDP 192.168.4.4:26000;branch=z9hG4bK-d8754z-87d60b47b6627c3a-1---d8754z-;rport=26000
From: "Bob"<sip:bob@192.168.4.4>;tag=15c8325a
To: "alice" <sip:alice@192.168.4.4>;tag=cDg7NyjpeSg4m
Call-ID: YWEwYjNlZTZjOWZjNDg3ZjU3MjQ3MTA1ZmQ1MDM5YmQ.
CSeq: 2 INVITE
Contact: <sip:alice@192.168.4.4:5060;transport=udp>
User-Agent: FreeSWITCH-mod_sofia/1.0.trunk-16981M
Accept: application/sdp
Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, UPDATE, INFO, REGISTER, REFER,
NOTIFY, PUBLISH, SUBSCRIBE
Supported: timer, precondition, path, replaces
Allow-Events: talk, presence, dialog, line-seize, call-info, sla,
include-session-description, presence.winfo, message-summary, refer
Content-Type: application/sdp
Content-Disposition: session
Content-Length: 267
Remote-Party-ID: "alice" <sip:alice@192.168.4.4>
FreeSWITCH 在收到 alice 的 180 Ringing 消息后，便告诉 bob 呼叫进展情况，183 与 180 不同的是，它包含 SDP，即接下来它会向 bob 发送 RTP 的媒体流，就是回铃音。
------------------------------------------------------------------------
recv 768 bytes from udp/[192.168.4.4]:5090 at 13:31:43.251980:
------------------------------------------------------------------------
SIP/2.0 200 OK
Via: SIP/2.0/UDP 192.168.4.4;rport=5060;branch=z9hG4bKey90QUyHZQXNN
Contact: <sip:alice@192.168.4.4:5090;rinstance=e7d5364c81f2b879;transport=UDP>
To: <sip:alice@192.168.4.4:5090;rinstance=e7d5364c81f2b879;transport=UDP>;tag=3813e926
From: "Bob"<sip:bob@192.168.4.4>;tag=Dp9ZQS3SB26pg
Call-ID: 0d74ac35-cca4-122d-81a2-2990e5b2bd3e
CSeq: 130069214 INVITE
Allow: INVITE, ACK, CANCEL, BYE, NOTIFY, REFER, MESSAGE, OPTIONS, INFO, SUBSCRIBE
Content-Type: application/sdp
User-Agent: Zoiper rev.5415
Content-Length: 226
alice 接听电话以后，其 UA 向 FreeSWITCH 送 200 OK，即应答消息。
------------------------------------------------------------------------
send 436 bytes to udp/[192.168.4.4]:5090 at 13:31:43.256692:
------------------------------------------------------------------------
ACK sip:alice@192.168.4.4:5090;rinstance=e7d5364c81f2b879;transport=UDP SIP/2.0
Via: SIP/2.0/UDP 192.168.4.4;rport;branch=z9hG4bKF72SSpFNv0K8g
Max-Forwards: 70
From: "Bob" <sip:bob@192.168.4.4>;tag=Dp9ZQS3SB26pg
To: <sip:alice@192.168.4.4:5090;rinstance=e7d5364c81f2b879;transport=UDP>;tag=3813e926
Call-ID: 0d74ac35-cca4-122d-81a2-2990e5b2bd3e
CSeq: 130069214 ACK
Contact: <sip:mod_sofia@192.168.4.4:5060>
Content-Length: 0
FreeSWITCH 向 alice 回送证实消息，证实已经知道了。至此，b-leg已经完全建立完毕，多半这时 alice 已经开始说话了：“Hi, bob，你好……”
------------------------------------------------------------------------
send 1135 bytes to udp/[192.168.4.4]:26000 at 13:31:43.293311:
------------------------------------------------------------------------
SIP/2.0 200 OK
Via: SIP/2.0/UDP 192.168.4.4:26000;branch=z9hG4bK-d8754z-87d60b47b6627c3a-1---d8754z-;rport=26000
From: "Bob"<sip:bob@192.168.4.4>;tag=15c8325a
To: "alice" <sip:alice@192.168.4.4>;tag=cDg7NyjpeSg4m
Call-ID: YWEwYjNlZTZjOWZjNDg3ZjU3MjQ3MTA1ZmQ1MDM5YmQ.
CSeq: 2 INVITE
Contact: <sip:alice@192.168.4.4:5060;transport=udp>
User-Agent: FreeSWITCH-mod_sofia/1.0.trunk-16981M
Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, UPDATE, INFO, REGISTER, REFER,
NOTIFY, PUBLISH, SUBSCRIBE
Supported: timer, precondition, path, replaces
Allow-Events: talk, presence, dialog, line-seize, call-info, sla,
include-session-description, presence.winfo, message-summary, refer
Session-Expires: 120;refresher=uas
Min-SE: 120
Content-Type: application/sdp
Content-Disposition: session
Content-Length: 267
Remote-Party-ID: "alice" <sip:alice@192.168.4.4>
与此同时，它也给 bob 送应答消息，告诉他电话已经接通了，可以跟 alice 说话了。在需要计费的情况下，应该从此时开始对 bob 的电话计费。bob 的 UA 收到该消息后启动麦克风让 bob 讲话。
------------------------------------------------------------------------
recv 697 bytes from udp/[192.168.4.4]:26000 at 13:31:43.413025:
------------------------------------------------------------------------
ACK sip:alice@192.168.4.4:5060;transport=udp SIP/2.0
Via: SIP/2.0/UDP 192.168.4.4:26000;branch=z9hG4bK-d8754z-ef53864320037c04-1---d8754z-;rport
Max-Forwards: 70
Contact: <sip:bob@192.168.4.4:26000>
To: "alice"<sip:alice@192.168.4.4>;tag=cDg7NyjpeSg4m
From: "Bob"<sip:bob@192.168.4.4>;tag=15c8325a
Call-ID: YWEwYjNlZTZjOWZjNDg3ZjU3MjQ3MTA1ZmQ1MDM5YmQ.
CSeq: 2 ACK
Proxy-Authorization: Digest username="bob",realm="192.168.4.4",
nonce="31c5c3e0-cc6e-46c8-a661-599b0c7f87d8",
uri="sip:alice@192.168.4.4",response="327887635344405bcd545da06763c466",
cnonce="c164b74f625ff2161bd8d47dba3a0ee2",nc=00000001,qop=auth,
algorithm=MD5
User-Agent: X-Lite release 1014k stamp 47051
Content-Length: 0
bob 在收到应答消息后也需要回送证实消息。至此 a-leg 也建立完毕。双方正常通话。
------------------------------------------------------------------------
recv 484 bytes from udp/[192.168.4.4]:5090 at 13:31:49.949240:
------------------------------------------------------------------------
BYE sip:mod_sofia@192.168.4.4:5060 SIP/2.0
Via: SIP/2.0/UDP 192.168.4.4:5090;branch=z9hG4bK-d8754z-2146ae0ddd113efe-1---d8754z-;rport
Max-Forwards: 70
Contact: <sip:alice@192.168.4.4:5090;rinstance=e7d5364c81f2b879;transport=UDP>
To: "Bob"<sip:bob@192.168.4.4>;tag=Dp9ZQS3SB26pg
From: <sip:alice@192.168.4.4:5090;rinstance=e7d5364c81f2b879;transport=UDP>;tag=3813e926
Call-ID: 0d74ac35-cca4-122d-81a2-2990e5b2bd3e
CSeq: 2 BYE
User-Agent: Zoiper rev.5415
Content-Length: 0
终于聊完了，alice 挂断电话，发送 BYE 消息。
------------------------------------------------------------------------
send 543 bytes to udp/[192.168.4.4]:5090 at 13:31:49.950425:
------------------------------------------------------------------------
SIP/2.0 200 OK
Via: SIP/2.0/UDP 192.168.4.4:5090;branch=z9hG4bK-d8754z-2146ae0ddd113efe-1---d8754z-;rport=5090
From: <sip:alice@192.168.4.4:5090;rinstance=e7d5364c81f2b879;transport=UDP>;tag=3813e926
To: "Bob"<sip:bob@192.168.4.4>;tag=Dp9ZQS3SB26pg
Call-ID: 0d74ac35-cca4-122d-81a2-2990e5b2bd3e
CSeq: 2 BYE
User-Agent: FreeSWITCH-mod_sofia/1.0.trunk-16981M
Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, UPDATE, INFO, REGISTER, REFER,
NOTIFY, PUBLISH, SUBSCRIBE
Supported: timer, precondition, path, replaces
Content-Length: 0
FreeSWITCH 返回 OK，b-leg 释放完毕。
------------------------------------------------------------------------
send 630 bytes to udp/[192.168.4.4]:26000 at 13:31:50.003165:
------------------------------------------------------------------------
BYE sip:bob@192.168.4.4:26000 SIP/2.0
Via: SIP/2.0/UDP 192.168.4.4;rport;branch=z9hG4bKggvjUH0rS99tc
Max-Forwards: 70
From: "alice" <sip:alice@192.168.4.4>;tag=cDg7NyjpeSg4m
To: "Bob" <sip:bob@192.168.4.4>;tag=15c8325a
Call-ID: YWEwYjNlZTZjOWZjNDg3ZjU3MjQ3MTA1ZmQ1MDM5YmQ.
CSeq: 130069219 BYE
Contact: <sip:alice@192.168.4.4:5060;transport=udp>
User-Agent: FreeSWITCH-mod_sofia/1.0.trunk-16981M
Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, UPDATE, INFO, REGISTER, REFER,
NOTIFY, PUBLISH, SUBSCRIBE
Supported: timer, precondition, path, replaces
Reason: Q.850;cause=16;text="NORMAL_CLEARING"
Content-Length: 0
接下来 FreeSWITCH 给 bob 发送 BYE，通知要拆线了。出于对 bob 负责，它包含了挂机原因（Hangup Cause），此处 NOMAL_CLEARING 表示正常释放。
------------------------------------------------------------------------
recv 367 bytes from udp/[192.168.4.4]:26000 at 13:31:50.111765:
------------------------------------------------------------------------
SIP/2.0 200 OK
Via: SIP/2.0/UDP 192.168.4.4;rport=5060;branch=z9hG4bKggvjUH0rS99tc
Contact: <sip:bob@192.168.4.4:26000>
To: "Bob"<sip:bob@192.168.4.4>;tag=15c8325a
From: "alice"<sip:alice@192.168.4.4>;tag=cDg7NyjpeSg4m
Call-ID: YWEwYjNlZTZjOWZjNDg3ZjU3MjQ3MTA1ZmQ1MDM5YmQ.
CSeq: 130069219 BYE
User-Agent: X-Lite release 1014k stamp 47051
Content-Length: 0
bob 回送 OK，a-leg 释放完毕，通话结束。从下图可以很形象地看出 FreeSWITCH 的两条“腿”-- a-leg 和 b-leg。
B2BUA
整个呼叫流程图示如下：
bob (UAC) [ UAS-UAC ] (UAS) alice
| | | |
| INVITE | | |
|-------------------->| | |
| 100 Trying | | |
|<--------------------| | |
| 407 Authentication Required |
|<--------------------| | |
| ACK | | |
|-------------------->| | |
| INVITE | | |
|-------------------->| | |
| 100 Trying | | INVITE |
|<--------------------< >-------------------->|
| | | 100 Trying |
| | |<--------------------|
| 183 Progress | | 180 Ringing |
|<--------------------< |<--------------------|
| | | 200 OK |
| | |<--------------------|
| 200 OK | | ACK |
|<--------------------< >-------------------->|
| ACK | | |
|-------------------->| | |
| |
| Call Connected |
| |
| | | BYE |
| | |<--------------------|
| BYE | | 200 OK |
|<--------------------< >-------------------->|
| 200 OK | | |
|-------------------->| | |
| | | |
从流程图可以看出，右半部分跟上一节“UA间直接呼叫”一样，而左半部分也类似。这就更好的说明了实际上有 4 个 UA （两对）参与到了通信中，并且，有两个 Session。
再论 SIP URI
上面我们介绍了一些 FreeSWITCH 的基本概念，并通过一个真正的呼叫流程讲解了一下 SIP。由于实验中所有 UA 都 运行在一台机器上，这可能会引起迷惑，如果我们有三台服务器，那么情况可能是：
/---------------\
| FreeSWITCH |
| 192.168.0.1 |
\ --------------/
sip:bob@192.168.0.1 / \ sip:alice@192.168.0.1
/ \
/ \
/-----------------\ /-----------------\
| bob | | alice |
| 192.168.0.100 | | 192.168.0.200 |
\-----------------/ \-----------------/
sip:bob@192.168.0.100 sip:alice@192.168.0.200
alice 注册到 FreeSWITCH，bob呼叫她时，使用她的服务器地址，即 sip:alice@192.168.0.1，FreeSWITCH 接到请求后，查找本地数据库，发现 alice 的实际地址是 sip:alice@192.168.0.200，便可以建立呼叫。
SIP URI 除使用 IP 地址外，也可以使用域名，如 sip:alice@freeswitch.org.cn。更高级也更复杂的配置则需要 DNS 的 SRV 记录，在此就不做讨论了。
通过 B2BUA 呼叫
在真实世界中，bob 和 alice 肯定要经常改变位置，那么它们的 SIP 地址也会相应改变，并且，如果他们之中有一个或两个处于 NAT 的网络中时，直接通信就更困难了。所以，他们通常会借助于一个服务器来相互通信。通过注册到服务器上，他们都可以获得一个服务器上的 SIP 地址。注册服务器的地址一般是不变的，因此他们的 SIP 地址就不会发生变化，因而，他们总是能够进行通信。
我们让他们两个都注册到 FreeSWITCH 上。上面已经说过，FreeSWITCH 监听的端口是 SIP 默认的端口 5060。bob 和 alice 注册后，他们分别获得了一个服务器的地址（SIP URI）：sip:bob@192.168.4.4 和 sip:alice@192.168.4.4（默认的端口号 5060 可以省略）。
下面是 bob 呼叫 alice 的流程。需要指出，如果 bob 只是发起呼叫而不接收呼叫，他并不需要向 FreeSWITCH 注册（有些软交换服务器需要先注册才能发起呼叫，但 SIP 是不强制这么做的）。
------------------------------------------------------------------------
recv 1118 bytes from udp/[192.168.4.4]:26000 at 13:31:39.938891:
------------------------------------------------------------------------
INVITE sip:alice@192.168.4.4 SIP/2.0
Via: SIP/2.0/UDP 192.168.4.4:26000;branch=z9hG4bK-d8754z-56adad736231f024-1---d8754z-;rport
Max-Forwards: 70
Contact: <sip:bob@192.168.4.4:26000>
To: "alice"<sip:alice@192.168.4.4>
From: "Bob"<sip:bob@192.168.4.4>;tag=15c8325a
Call-ID: YWEwYjNlZTZjOWZjNDg3ZjU3MjQ3MTA1ZmQ1MDM5YmQ.
CSeq: 1 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY, MESSAGE, SUBSCRIBE, INFO
Content-Type: application/sdp
User-Agent: X-Lite release 1014k stamp 47051
Content-Length: 594
上面的消息中省略了 SDP 的内容，我们将留到以后再探讨。bob 的 UAC 通过 INVITE 消息向 FreeSWITCH 发起请求。bob 的 UAC 用的是 X-Lite（User-Agent），它运行在端口 26000 上（实际上，它默认在端口也是 5060，但由于在我的实验环境下它也是跟 FreeSWITCH 运行在一台机器上，已被占用，因此它需要选择另一个端口）。其中，From 为主叫用户的地址，To 为被叫用户的地址。此时 FreeSWITCH 作为一个 UAS 接受请求并进行响应。它得知 bob 要呼叫 alice，需要在自己的数据库中查找 alice 是否已在服务器上注册，好知道应该怎么找到 alice。但在此之前，它先通知 bob 它已经收到了他的请求。
------------------------------------------------------------------------
send 345 bytes to udp/[192.168.4.4]:26000 at 13:31:39.940278:
------------------------------------------------------------------------
SIP/2.0 100 Trying
Via: SIP/2.0/UDP 192.168.4.4:26000;branch=z9hG4bK-d8754z-56adad736231f024-1---d8754z-;rport=26000
From: "Bob"<sip:bob@192.168.4.4>;tag=15c8325a
To: "alice"<sip:alice@192.168.4.4>
Call-ID: YWEwYjNlZTZjOWZjNDg3ZjU3MjQ3MTA1ZmQ1MDM5YmQ.
CSeq: 1 INVITE
User-Agent: FreeSWITCH-mod_sofia/1.0.trunk-16981M
Content-Length: 0
FreeSWITCH 通过 100 Trying 消息告诉 bob “我已经收到你的消息了，别着急，我正在联系 alice 呢...” 该消息称为呼叫进展消息。
------------------------------------------------------------------------
send 826 bytes to udp/[192.168.4.4]:26000 at 13:31:39.943392:
------------------------------------------------------------------------
SIP/2.0 407 Proxy Authentication Required
Via: SIP/2.0/UDP 192.168.4.4:26000;branch=z9hG4bK-d8754z-56adad736231f024-1---d8754z-;rport=26000
From: "Bob"<sip:bob@192.168.4.4>;tag=15c8325a
To: "alice" <sip:alice@192.168.4.4>;tag=B4pem31jHgtHS
Call-ID: YWEwYjNlZTZjOWZjNDg3ZjU3MjQ3MTA1ZmQ1MDM5YmQ.
CSeq: 1 INVITE
User-Agent: FreeSWITCH-mod_sofia/1.0.trunk-16981M
Accept: application/sdp
Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, UPDATE, INFO, REGISTER, REFER,
NOTIFY, PUBLISH, SUBSCRIBE
Supported: timer, precondition, path, replaces
Allow-Events: talk, presence, dialog, line-seize, call-info, sla,
include-session-description, presence.winfo, message-summary, refer
Proxy-Authenticate: Digest realm="192.168.4.4",
nonce="31c5c3e0-cc6e-46c8-a661-599b0c7f87d8", algorithm=MD5, qop="auth"
Content-Length: 0
但就在此时，FreeSWITCH 发现 bob 并不是授权用户，因而它需要确认 bob 的身份。它通过发送带有 Digest 验证信息的 407 消息来通知 bob（注意，这里与注册流程中的 401 不同）。
------------------------------------------------------------------------
recv 319 bytes from udp/[192.168.4.4]:26000 at 13:31:39.945314:
------------------------------------------------------------------------
ACK sip:alice@192.168.4.4 SIP/2.0
Via: SIP/2.0/UDP 192.168.4.4:26000;branch=z9hG4bK-d8754z-56adad736231f024-1---d8754z-;rport
To: "alice" <sip:alice@192.168.4.4>;tag=B4pem31jHgtHS
From: "Bob"<sip:bob@192.168.4.4>;tag=15c8325a
Call-ID: YWEwYjNlZTZjOWZjNDg3ZjU3MjQ3MTA1ZmQ1MDM5YmQ.
CSeq: 1 ACK
Content-Length: 0
bob 回送 ACK 证实消息向 FreeSWITCH 证实已收到认证要求。并重新发送 INVITE，这次，附带了验证信息。
------------------------------------------------------------------------
recv 1376 bytes from udp/[192.168.4.4]:26000 at 13:31:39.945526:
------------------------------------------------------------------------
INVITE sip:alice@192.168.4.4 SIP/2.0
Via: SIP/2.0/UDP 192.168.4.4:26000;branch=z9hG4bK-d8754z-87d60b47b6627c3a-1---d8754z-;rport
Max-Forwards: 70
Contact: <sip:bob@192.168.4.4:26000>
To: "alice"<sip:alice@192.168.4.4>
From: "Bob"<sip:bob@192.168.4.4>;tag=15c8325a
Call-ID: YWEwYjNlZTZjOWZjNDg3ZjU3MjQ3MTA1ZmQ1MDM5YmQ.
CSeq: 2 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY, MESSAGE, SUBSCRIBE, INFO
Content-Type: application/sdp
Proxy-Authorization: Digest username="bob",realm="192.168.4.4",
nonce="31c5c3e0-cc6e-46c8-a661-599b0c7f87d8",
uri="sip:alice@192.168.4.4",response="327887635344405bcd545da06763c466",
cnonce="c164b74f625ff2161bd8d47dba3a0ee2",nc=00000001,qop=auth,
algorithm=MD5
User-Agent: X-Lite release 1014k stamp 47051
Content-Length: 594
这里也省略了 SDP 消息体。
------------------------------------------------------------------------
send 345 bytes to udp/[192.168.4.4]:26000 at 13:31:39.946349:
------------------------------------------------------------------------
SIP/2.0 100 Trying
Via: SIP/2.0/UDP 192.168.4.4:26000;branch=z9hG4bK-d8754z-87d60b47b6627c3a-1---d8754z-;rport=26000
From: "Bob"<sip:bob@192.168.4.4>;tag=15c8325a
To: "alice"<sip:alice@192.168.4.4>
Call-ID: YWEwYjNlZTZjOWZjNDg3ZjU3MjQ3MTA1ZmQ1MDM5YmQ.
CSeq: 2 INVITE
User-Agent: FreeSWITCH-mod_sofia/1.0.trunk-16981M
Content-Length: 0
FreeSWITCH 重新回 100 Trying，告诉 bob 呼叫进展情况。
至此，bob 与 FreeSWITCH 之间的通信已经初步建立，这种通信的通道称作一个信道（channel）。该信道是由 bob 的 UA 和 FreeSWITCH 的一个 UA 构成的，我们称它为 FreeSWITCH 的一条腿，叫做 a-leg。
接下来 FreeSWITCH 要建立另一条腿，称为 b-leg。它通过查打本地数据库，得到了 alice 的位置，接着启动一个 UA（用作 UAC），向 alice 发送 INVITE 消息。如下：
------------------------------------------------------------------------
send 1340 bytes to udp/[192.168.4.4]:5090 at 13:31:40.028988:
------------------------------------------------------------------------
INVITE sip:alice@192.168.4.4:5090;rinstance=e7d5364c81f2b879;transport=UDP SIP/2.0
Via: SIP/2.0/UDP 192.168.4.4;rport;branch=z9hG4bKey90QUyHZQXNN
Route: <sip:alice@192.168.4.4:5090>;rinstance=e7d5364c81f2b879;transport=UDP
Max-Forwards: 69
From: "Bob" <sip:bob@192.168.4.4>;tag=Dp9ZQS3SB26pg
To: <sip:alice@192.168.4.4:5090;rinstance=e7d5364c81f2b879;transport=UDP>
Call-ID: 0d74ac35-cca4-122d-81a2-2990e5b2bd3e
CSeq: 130069214 INVITE
Contact: <sip:mod_sofia@192.168.4.4:5060>
User-Agent: FreeSWITCH-mod_sofia/1.0.trunk-16981M
Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, UPDATE, INFO, REGISTER, REFER,
NOTIFY, PUBLISH, SUBSCRIBE
Supported: timer, precondition, path, replaces
Allow-Events: talk, presence, dialog, line-seize, call-info, sla,
include-session-description, presence.winfo, message-summary, refer
Content-Type: application/sdp
Content-Disposition: session
Content-Length: 313
X-FS-Support: update_display
Remote-Party-ID: "Bob" <sip:bob@192.168.4.4>;party=calling;screen=yes;privacy=off
你可以看到，该INVITE 的 Call-ID 与前面的不同，说明这是另一个 SIP 会话（Session）。另外，它还多了一个 Remote-Party-ID，它主要是用来支持来电显示。因为，在 alice 的话机上，希望显示 bob 的号码，显示呼叫它的那个 UA（负责 b-leg的那个 UA）没什么意义。与普通的 POTS 电话不同，在 SIP 电话中，不仅能显示电话号码（这里是 bob），还能显示一个可选的名字（“Bob”）。这也说明了 FreeSWITCH 这个 B2BUA 本身是一个整体，它虽然是以一个单独的 UA 呼叫 alice，但还是跟负责 bob 的那个 UA 有联系--就是这种背靠背的串联。
------------------------------------------------------------------------
recv 309 bytes from udp/[192.168.4.4]:5090 at 13:31:40.193634:
------------------------------------------------------------------------
SIP/2.0 100 Trying
Via: SIP/2.0/UDP 192.168.4.4;rport=5060;branch=z9hG4bKey90QUyHZQXNN
To: <sip:alice@192.168.4.4:5090;rinstance=e7d5364c81f2b879;transport=UDP>
From: "Bob" <sip:bob@192.168.4.4>;tag=Dp9ZQS3SB26pg
Call-ID: 0d74ac35-cca4-122d-81a2-2990e5b2bd3e
CSeq: 130069214 INVITE
Content-Length: 0
跟上面的流程差不多，alice回的呼叫进展。此时，alice 的 UA 开始振铃。
------------------------------------------------------------------------
recv 431 bytes from udp/[192.168.4.4]:5090 at 13:31:40.193816:
------------------------------------------------------------------------
SIP/2.0 180 Ringing
Via: SIP/2.0/UDP 192.168.4.4;rport=5060;branch=z9hG4bKey90QUyHZQXNN
Contact: <sip:alice@192.168.4.4:5090;rinstance=e7d5364c81f2b879;transport=UDP>
To: <sip:alice@192.168.4.4:5090;rinstance=e7d5364c81f2b879;transport=UDP>;tag=3813e926
From: "Bob"<sip:bob@192.168.4.4>;tag=Dp9ZQS3SB26pg
Call-ID: 0d74ac35-cca4-122d-81a2-2990e5b2bd3e
CSeq: 130069214 INVITE
User-Agent: Zoiper rev.5415
Content-Length: 0
180也是呼叫进展消息，它说明，我这边已经准备好了，alice 的电话已经响了，她听到了一会就会接听。
send 1125 bytes to udp/[192.168.4.4]:26000 at 13:31:40.270533:
------------------------------------------------------------------------
SIP/2.0 183 Session Progress
Via: SIP/2.0/UDP 192.168.4.4:26000;branch=z9hG4bK-d8754z-87d60b47b6627c3a-1---d8754z-;rport=26000
From: "Bob"<sip:bob@192.168.4.4>;tag=15c8325a
To: "alice" <sip:alice@192.168.4.4>;tag=cDg7NyjpeSg4m
Call-ID: YWEwYjNlZTZjOWZjNDg3ZjU3MjQ3MTA1ZmQ1MDM5YmQ.
CSeq: 2 INVITE
Contact: <sip:alice@192.168.4.4:5060;transport=udp>
User-Agent: FreeSWITCH-mod_sofia/1.0.trunk-16981M
Accept: application/sdp
Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, UPDATE, INFO, REGISTER, REFER,
NOTIFY, PUBLISH, SUBSCRIBE
Supported: timer, precondition, path, replaces
Allow-Events: talk, presence, dialog, line-seize, call-info, sla,
include-session-description, presence.winfo, message-summary, refer
Content-Type: application/sdp
Content-Disposition: session
Content-Length: 267
Remote-Party-ID: "alice" <sip:alice@192.168.4.4>
FreeSWITCH 在收到 alice 的 180 Ringing 消息后，便告诉 bob 呼叫进展情况，183 与 180 不同的是，它包含 SDP，即接下来它会向 bob 发送 RTP 的媒体流，就是回铃音。
------------------------------------------------------------------------
recv 768 bytes from udp/[192.168.4.4]:5090 at 13:31:43.251980:
------------------------------------------------------------------------
SIP/2.0 200 OK
Via: SIP/2.0/UDP 192.168.4.4;rport=5060;branch=z9hG4bKey90QUyHZQXNN
Contact: <sip:alice@192.168.4.4:5090;rinstance=e7d5364c81f2b879;transport=UDP>
To: <sip:alice@192.168.4.4:5090;rinstance=e7d5364c81f2b879;transport=UDP>;tag=3813e926
From: "Bob"<sip:bob@192.168.4.4>;tag=Dp9ZQS3SB26pg
Call-ID: 0d74ac35-cca4-122d-81a2-2990e5b2bd3e
CSeq: 130069214 INVITE
Allow: INVITE, ACK, CANCEL, BYE, NOTIFY, REFER, MESSAGE, OPTIONS, INFO, SUBSCRIBE
Content-Type: application/sdp
User-Agent: Zoiper rev.5415
Content-Length: 226
alice 接听电话以后，其 UA 向 FreeSWITCH 送 200 OK，即应答消息。
------------------------------------------------------------------------
send 436 bytes to udp/[192.168.4.4]:5090 at 13:31:43.256692:
------------------------------------------------------------------------
ACK sip:alice@192.168.4.4:5090;rinstance=e7d5364c81f2b879;transport=UDP SIP/2.0
Via: SIP/2.0/UDP 192.168.4.4;rport;branch=z9hG4bKF72SSpFNv0K8g
Max-Forwards: 70
From: "Bob" <sip:bob@192.168.4.4>;tag=Dp9ZQS3SB26pg
To: <sip:alice@192.168.4.4:5090;rinstance=e7d5364c81f2b879;transport=UDP>;tag=3813e926
Call-ID: 0d74ac35-cca4-122d-81a2-2990e5b2bd3e
CSeq: 130069214 ACK
Contact: <sip:mod_sofia@192.168.4.4:5060>
Content-Length: 0
FreeSWITCH 向 alice 回送证实消息，证实已经知道了。至此，b-leg已经完全建立完毕，多半这时 alice 已经开始说话了：“Hi, bob，你好……”
------------------------------------------------------------------------
send 1135 bytes to udp/[192.168.4.4]:26000 at 13:31:43.293311:
------------------------------------------------------------------------
SIP/2.0 200 OK
Via: SIP/2.0/UDP 192.168.4.4:26000;branch=z9hG4bK-d8754z-87d60b47b6627c3a-1---d8754z-;rport=26000
From: "Bob"<sip:bob@192.168.4.4>;tag=15c8325a
To: "alice" <sip:alice@192.168.4.4>;tag=cDg7NyjpeSg4m
Call-ID: YWEwYjNlZTZjOWZjNDg3ZjU3MjQ3MTA1ZmQ1MDM5YmQ.
CSeq: 2 INVITE
Contact: <sip:alice@192.168.4.4:5060;transport=udp>
User-Agent: FreeSWITCH-mod_sofia/1.0.trunk-16981M
Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, UPDATE, INFO, REGISTER, REFER,
NOTIFY, PUBLISH, SUBSCRIBE
Supported: timer, precondition, path, replaces
Allow-Events: talk, presence, dialog, line-seize, call-info, sla,
include-session-description, presence.winfo, message-summary, refer
Session-Expires: 120;refresher=uas
Min-SE: 120
Content-Type: application/sdp
Content-Disposition: session
Content-Length: 267
Remote-Party-ID: "alice" <sip:alice@192.168.4.4>
与此同时，它也给 bob 送应答消息，告诉他电话已经接通了，可以跟 alice 说话了。在需要计费的情况下，应该从此时开始对 bob 的电话计费。bob 的 UA 收到该消息后启动麦克风让 bob 讲话。
------------------------------------------------------------------------
recv 697 bytes from udp/[192.168.4.4]:26000 at 13:31:43.413025:
------------------------------------------------------------------------
ACK sip:alice@192.168.4.4:5060;transport=udp SIP/2.0
Via: SIP/2.0/UDP 192.168.4.4:26000;branch=z9hG4bK-d8754z-ef53864320037c04-1---d8754z-;rport
Max-Forwards: 70
Contact: <sip:bob@192.168.4.4:26000>
To: "alice"<sip:alice@192.168.4.4>;tag=cDg7NyjpeSg4m
From: "Bob"<sip:bob@192.168.4.4>;tag=15c8325a
Call-ID: YWEwYjNlZTZjOWZjNDg3ZjU3MjQ3MTA1ZmQ1MDM5YmQ.
CSeq: 2 ACK
Proxy-Authorization: Digest username="bob",realm="192.168.4.4",
nonce="31c5c3e0-cc6e-46c8-a661-599b0c7f87d8",
uri="sip:alice@192.168.4.4",response="327887635344405bcd545da06763c466",
cnonce="c164b74f625ff2161bd8d47dba3a0ee2",nc=00000001,qop=auth,
algorithm=MD5
User-Agent: X-Lite release 1014k stamp 47051
Content-Length: 0
bob 在收到应答消息后也需要回送证实消息。至此 a-leg 也建立完毕。双方正常通话。
------------------------------------------------------------------------
recv 484 bytes from udp/[192.168.4.4]:5090 at 13:31:49.949240:
------------------------------------------------------------------------
BYE sip:mod_sofia@192.168.4.4:5060 SIP/2.0
Via: SIP/2.0/UDP 192.168.4.4:5090;branch=z9hG4bK-d8754z-2146ae0ddd113efe-1---d8754z-;rport
Max-Forwards: 70
Contact: <sip:alice@192.168.4.4:5090;rinstance=e7d5364c81f2b879;transport=UDP>
To: "Bob"<sip:bob@192.168.4.4>;tag=Dp9ZQS3SB26pg
From: <sip:alice@192.168.4.4:5090;rinstance=e7d5364c81f2b879;transport=UDP>;tag=3813e926
Call-ID: 0d74ac35-cca4-122d-81a2-2990e5b2bd3e
CSeq: 2 BYE
User-Agent: Zoiper rev.5415
Content-Length: 0
终于聊完了，alice 挂断电话，发送 BYE 消息。
------------------------------------------------------------------------
send 543 bytes to udp/[192.168.4.4]:5090 at 13:31:49.950425:
------------------------------------------------------------------------
SIP/2.0 200 OK
Via: SIP/2.0/UDP 192.168.4.4:5090;branch=z9hG4bK-d8754z-2146ae0ddd113efe-1---d8754z-;rport=5090
From: <sip:alice@192.168.4.4:5090;rinstance=e7d5364c81f2b879;transport=UDP>;tag=3813e926
To: "Bob"<sip:bob@192.168.4.4>;tag=Dp9ZQS3SB26pg
Call-ID: 0d74ac35-cca4-122d-81a2-2990e5b2bd3e
CSeq: 2 BYE
User-Agent: FreeSWITCH-mod_sofia/1.0.trunk-16981M
Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, UPDATE, INFO, REGISTER, REFER,
NOTIFY, PUBLISH, SUBSCRIBE
Supported: timer, precondition, path, replaces
Content-Length: 0
FreeSWITCH 返回 OK，b-leg 释放完毕。
------------------------------------------------------------------------
send 630 bytes to udp/[192.168.4.4]:26000 at 13:31:50.003165:
------------------------------------------------------------------------
BYE sip:bob@192.168.4.4:26000 SIP/2.0
Via: SIP/2.0/UDP 192.168.4.4;rport;branch=z9hG4bKggvjUH0rS99tc
Max-Forwards: 70
From: "alice" <sip:alice@192.168.4.4>;tag=cDg7NyjpeSg4m
To: "Bob" <sip:bob@192.168.4.4>;tag=15c8325a
Call-ID: YWEwYjNlZTZjOWZjNDg3ZjU3MjQ3MTA1ZmQ1MDM5YmQ.
CSeq: 130069219 BYE
Contact: <sip:alice@192.168.4.4:5060;transport=udp>
User-Agent: FreeSWITCH-mod_sofia/1.0.trunk-16981M
Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, UPDATE, INFO, REGISTER, REFER,
NOTIFY, PUBLISH, SUBSCRIBE
Supported: timer, precondition, path, replaces
Reason: Q.850;cause=16;text="NORMAL_CLEARING"
Content-Length: 0
接下来 FreeSWITCH 给 bob 发送 BYE，通知要拆线了。出于对 bob 负责，它包含了挂机原因（Hangup Cause），此处 NOMAL_CLEARING 表示正常释放。
------------------------------------------------------------------------
recv 367 bytes from udp/[192.168.4.4]:26000 at 13:31:50.111765:
------------------------------------------------------------------------
SIP/2.0 200 OK
Via: SIP/2.0/UDP 192.168.4.4;rport=5060;branch=z9hG4bKggvjUH0rS99tc
Contact: <sip:bob@192.168.4.4:26000>
To: "Bob"<sip:bob@192.168.4.4>;tag=15c8325a
From: "alice"<sip:alice@192.168.4.4>;tag=cDg7NyjpeSg4m
Call-ID: YWEwYjNlZTZjOWZjNDg3ZjU3MjQ3MTA1ZmQ1MDM5YmQ.
CSeq: 130069219 BYE
User-Agent: X-Lite release 1014k stamp 47051
Content-Length: 0
bob 回送 OK，a-leg 释放完毕，通话结束。从下图可以很形象地看出 FreeSWITCH 的两条“腿”-- a-leg 和 b-leg。
B2BUA
整个呼叫流程图示如下：
bob (UAC) [ UAS-UAC ] (UAS) alice
| | | |
| INVITE | | |
|-------------------->| | |
| 100 Trying | | |
|<--------------------| | |
| 407 Authentication Required |
|<--------------------| | |
| ACK | | |
|-------------------->| | |
| INVITE | | |
|-------------------->| | |
| 100 Trying | | INVITE |
|<--------------------< >-------------------->|
| | | 100 Trying |
| | |<--------------------|
| 183 Progress | | 180 Ringing |
|<--------------------< |<--------------------|
| | | 200 OK |
| | |<--------------------|
| 200 OK | | ACK |
|<--------------------< >-------------------->|
| ACK | | |
|-------------------->| | |
| |
| Call Connected |
| |
| | | BYE |
| | |<--------------------|
| BYE | | 200 OK |
|<--------------------< >-------------------->|
| 200 OK | | |
|-------------------->| | |
| | | |
从流程图可以看出，右半部分跟上一节“UA间直接呼叫”一样，而左半部分也类似。这就更好的说明了实际上有 4 个 UA （两对）参与到了通信中，并且，有两个 Session。
再论 SIP URI
上面我们介绍了一些 FreeSWITCH 的基本概念，并通过一个真正的呼叫流程讲解了一下 SIP。由于实验中所有 UA 都 运行在一台机器上，这可能会引起迷惑，如果我们有三台服务器，那么情况可能是：
/---------------\
| FreeSWITCH |
| 192.168.0.1 |
\ --------------/
sip:bob@192.168.0.1 / \ sip:alice@192.168.0.1
/ \
/ \
/-----------------\ /-----------------\
| bob | | alice |
| 192.168.0.100 | | 192.168.0.200 |
\-----------------/ \-----------------/
sip:bob@192.168.0.100 sip:alice@192.168.0.200
alice 注册到 FreeSWITCH，bob呼叫她时，使用她的服务器地址，即 sip:alice@192.168.0.1，FreeSWITCH 接到请求后，查找本地数据库，发现 alice 的实际地址是 sip:alice@192.168.0.200，便可以建立呼叫。
SIP URI 除使用 IP 地址外，也可以使用域名，如 sip:alice@freeswitch.org.cn。更高级也更复杂的配置则需要 DNS 的 SRV 记录，在此就不做讨论了。
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