文档链接：《Extensible Messaging and Presence Protocol (XMPP): Core》
客户和服务器之间端口为5222，服务器之间的端口为5269.
通信协议格式：
|--------------------|
| <stream> |
|--------------------|
| <presence> |
| <show/> |
| </presence> |
|--------------------|
| <message to='foo'> |
| <body/> |
| </message> |
|--------------------|
| <iq to='bar'> |
| <query/> |
| </iq> |
|--------------------|
| 
|
|--------------------|
| </stream> |
客户端和服务器之间维持一个TCP连接，服务器之间维持两个TCP连接。
想传输数据之间必须得通过验证，否则不予处理
To:用于客户端到服务器端，
From:用于服务器到客户端
Id:用于服务器到客户端，是服务器端生成的会话id号
Xml:lang：包括在客户端的初始流种，指定XML数据的字体，服务器端应该记住此设置，若客户端没有设置，则服务器使用其默认配置，并在应答流中通知给客户端。
| initiating to receiving | receiving to initiating
---------+---------------------------+-----------------------
to | hostname of receiver | silently ignored
from | silently ignored | hostname of receiver
id | silently ignored | session key
xml:lang | default language | default language
version | signals XMPP 1.0 support | signals XMPP 1.0 support
若客户端在初始化流包中包含了版本字段，则服务器在应答中必须包含一个<features/>元素，从而让客户端知道服务器支持的特性是哪些,以便客户端和服务器就某些特性进行协商。
流级别的错误是不可恢复的，因此一旦出现，则探测到错误的实体必须发送一个流错误信息给对方，并且发送</stream>结束符，关闭底层TCP连接。
一次简单的会话示例：
C: <?xml version='1.0'?>
<stream:stream
to='example.com'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
version='1.0'>
S: <?xml version='1.0'?>
<stream:stream
from='example.com'
id='someid'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
version='1.0'>
encryption, authentication, and resource binding
C: <message from='juliet@example.com'
to='romeo@example.net'
xml:lang='en'>
C: <body>Art thou not Romeo, and a Montague?</body>
C: </message>
S: <message from='romeo@example.net'
to='juliet@example.com'
xml:lang='en'>
S: <body>Neither, fair saint, if either thee dislike.</body>
S: </message>
C: </stream:stream>
S: </stream:stream>
一次发生错误的会话：
C: <?xml version='1.0'?>
<stream:stream
to='example.com'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
version='1.0'>
S: <?xml version='1.0'?>
<stream:stream
from='example.com'
id='someid'
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
version='1.0'>
encryption, authentication, and resource binding
C: <message xml:lang='en'>
<body>Bad XML, no closing body tag!
</message>
S: <stream:error>
<xml-not-well-formed
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
S: </stream:stream>
出于安全性的考虑，在客户和服务器之间，服务器之间可能会要求先进行TLS 安全性的验证，而且必须在SASL协商之前完成。
客户到服务器示例：
Step 1: 客户端初始化到服务器端的流:
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
to='example.com'
version='1.0'>
Step 2: 服务器的应答，回送一个<stream>标签，并指明会话id:
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
id='c2s_123'
from='example.com'
version='1.0'>
Step 3: 服务器发送STARTTLS extension 给客户端，并且指明验证机制和其他支持的流特性:
<stream:features>
<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
<required/>
</starttls>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>DIGEST-MD5</mechanism>
<mechanism>PLAIN</mechanism>
</mechanisms>
</stream:features>
Step 4:客户端发送STARTTLS命令给服务器:
<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
Step 5: 服务器通知客户端，它被允许继续处理:
<proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
Step 5 (alt): 服务器通知客户，TLS协商失败，关闭流和TCP连接:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
</stream:stream>
Step 6:客户端和服务器端准备在现有的TCP连接上完成TLS验证。
Step 7: 若TLS验证成功，客户端初始化一个到服务器的新流:
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
to='example.com'
version='1.0'>
Step 7 (alt): 若TLS验证失败,服务器关闭TCP连接
Step 8:服务器回送应答信息，包含了一个流头部和其他可用的流特性：
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
from='example.com'
id='c2s_234'
version='1.0'>
<stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>DIGEST-MD5</mechanism>
<mechanism>PLAIN</mechanism>
<mechanism>EXTERNAL</mechanism>
</mechanisms>
</stream:features>
Step 9: 客户端继续进行SASL验证。
服务器到服务器示例：
Step 1: Server1 initiates stream to Server2:
<stream:stream
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'
to='example.com'
version='1.0'>
Step 2: Server2 responds by sending a stream tag to Server1:
<stream:stream
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'
from='example.com'
id='s2s_123'
version='1.0'>
Step 3: Server2 sends the STARTTLS extension to Server1 along with authentication mechanisms and any other stream features:
<stream:features>
<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
<required/>
</starttls>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>DIGEST-MD5</mechanism>
<mechanism>KERBEROS_V4</mechanism>
</mechanisms>
</stream:features>
Step 4: Server1 sends the STARTTLS command to Server2:
<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
Step 5: Server2 informs Server1 that it is allowed to proceed:
<proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
Step 5 (alt): Server2 informs Server1 that TLS negotiation has failed and closes stream:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
</stream:stream>
Step 6: Server1 and Server2 attempt to complete TLS negotiation via TCP.
Step 7: If TLS negotiation is successful, Server1 initiates a new stream to Server2:
<stream:stream
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'
to='example.com'
version='1.0'>
Step 7 (alt): If TLS negotiation is unsuccessful, Server2 closes TCP connection.
Step 8: Server2 responds by sending a stream header to Server1 along with any available stream features:
<stream:stream
xmlns='jabber:server'
xmlns:stream='http://etherx.jabber.org/streams'
from='example.com'
id='s2s_234'
version='1.0'>
<stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>DIGEST-MD5</mechanism>
<mechanism>KERBEROS_V4</mechanism>
<mechanism>EXTERNAL</mechanism>
</mechanisms>
</stream:features>
Step 9: server1继续进行SASL验证
在TLS验证后，进行SASL验证
客户到服务器示例：
Step 1: 客户端初始到服务器的流:
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
to='example.com'
version='1.0'>
Step 2:服务器发送应答:
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
id='c2s_234'
from='example.com'
version='1.0'>
Step 3:服务器通知客户可选的验证机制:
<stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>DIGEST-MD5</mechanism>
<mechanism>PLAIN</mechanism>
</mechanisms>
</stream:features>
Step 4: 客户端选择一种验证机制:
<auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
mechanism='DIGEST-MD5'/>
Step 5: 服务器发送BASE64编码过的应答数据给客户端:
<challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
cmVhbG09InNvbWVyZWFsbSIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIixxb3A9ImF1dGgi
LGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGhtPW1kNS1zZXNzCg==
</challenge>
解码后的数据为:
realm="somerealm",nonce="OA6MG9tEQGm2hh",\
qop="auth",charset=utf-8,algorithm=md5-sess
Step 5 (alt):服务器返回错误给客户端:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<incorrect-encoding/>
</failure>
</stream:stream>
Step 6: 客户端发送一个BASE64编码的应答数据给服务器:
<response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
dXNlcm5hbWU9InNvbWVub2RlIixyZWFsbT0ic29tZXJlYWxtIixub25jZT0i
T0E2TUc5dEVRR20yaGgiLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLG5jPTAw
MDAwMDAxLHFvcD1hdXRoLGRpZ2VzdC11cmk9InhtcHAvZXhhbXBsZS5jb20i
LHJlc3BvbnNlPWQzODhkYWQ5MGQ0YmJkNzYwYTE1MjMyMWYyMTQzYWY3LGNo
YXJzZXQ9dXRmLTgK
</response>
解码后的应答消息是:
username="somenode",realm="somerealm",\
nonce="OA6MG9tEQGm2hh",cnonce="OA6MHXh6VqTrRk",\
nc=00000001,qop=auth,digest-uri="xmpp/example.com",\
response=d388dad90d4bbd760a152321f2143af7,charset=utf-8
Step 7: 服务器发送第二个BASE64编码的challenge数据给客户:
<challenge xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZAo=
</challenge>
解码后的challenge是:
rspauth=ea40f60335c427b5527b84dbabcdfffd
Step 7 (alt): 服务器返回错误给客户:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<temporary-auth-failure/>
</failure>
</stream:stream>
Step 8: 客户返回应答消息给服务器:
<response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
Step 9: 服务器通知客户成功通过验证:
<success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
Step 9 (alt):服务器通知客户验证失败:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<temporary-auth-failure/>
</failure>
</stream:stream>
Step 10:客户端初始化一个到服务器的新流:
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
to='example.com'
version='1.0'>
Step 11: 服务器返回一个流头部，包含了服务器支持的流特性:
<stream:stream
xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
id='c2s_345'
from='example.com'
version='1.0'>
<stream:features>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
<session xmlns='urn:ietf:params:xml:ns:xmpp-session'/>
</stream:features>