Is FTP Communication Synchronous?

Whenever FTP Client issues USER command to FTP server,

(1) Server immediately replies with TCP Ack

(2) After that, It will send FTP 331 response (asking for password)

Both the communications (1) and (2) are synchronous开发者_运维知识库 or Async?

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The communication itself is synchronous. There is a command channel and a data channel opened for each transfer. Both are synchronous in the sense that one command must be completed until you can issue a new one.

(NOTE this has nothing to do with how the sockets are handled synchronously or asynchronously on client or server side)