许多表示法和运行时不允许向后传输连接。
Many notations and runtimes do not permit backward flowing connections.
原因是在响应可用之前，传输连接可能会超时。
This is because the transport connection might timeout before the response is available.
它使用一个传输连接同时发送和接收响应，如下面的清单4所示。
It USES a single transport connection for sending and receiving the response, as shown in Listing 4 below.
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