TCP/IP streams are bidirectional, but there is a limited way of sending "out of band" data, though it is not used as much.
It's not real "out of band" data: that's something wholly invented by the Unix socket API. TCP itself just has an "urgent pointer", which addresses some byte further in the data stream that the receiver doesn't have yet, with the intent that higher-level protocols could use it as a signal to flush any data up to that pointer to observe whatever the urgent message is. There's nothing in the protocol itself to actually send a message separately from the rest of the stream.
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It's not real "out of band" data: that's something wholly invented by the Unix socket API. TCP itself just has an "urgent pointer", which addresses some byte further in the data stream that the receiver doesn't have yet, with the intent that higher-level protocols could use it as a signal to flush any data up to that pointer to observe whatever the urgent message is. There's nothing in the protocol itself to actually send a message separately from the rest of the stream.