![]() An odd and an even port were reserved for each application layer application or protocol. An ongoing transfer of file data over the data connection can be aborted using an interrupt message sent over the control connection.įTP needs two ports (one for sending and one for receiving) because it was originally designed to operate on top of Network Control Protocol (NCP), which was a simplex protocol that utilized two port addresses, establishing two connections, for two-way communications. The numbers represent the code for the response and the optional text represents a human-readable explanation or request (e.g. For example, "200" (or "200 OK") means that the last command was successful. The server responds over the control connection with three-digit status codes in ASCII with an optional text message. Further changes were introduced to the passive mode at that time, updating it to extended passive mode. īoth modes were updated in September 1998 to support IPv6. In this mode, the client uses the control connection to send a PASV command to the server and then receives a server IP address and server port number from the server, which the client then uses to open a data connection from an arbitrary client port to the server IP address and server port number received.
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