4个值唯一地定义一条TCP连接。
HTTP The Definitive Guide
A computer might have several TCP connections open at any one time. TCP keeps all these
connections straight through port numbers.
Port numbers are like employees' phone extensions. Just as a company's main phone number gets you
to the front desk and the extension gets you to the right employee, the IP address gets you to the right
computer and the port number gets you to the right application. A TCP connection is distinguished by
four values:
<source-IP-address, source-port, destination-IP-address,
destination-port>
Together, these four values uniquely define a connection.