    S  <#n>      Send a block
    R  <#n>      Receive a block
    T  <ERR#n-n> Timeout error(no activity within x time

    NAK<ERR#n-n> Block sent not acknowledged as OK by remote
                 computer.
    CS <ERR#n-n> Checksum error. Received total not what was sent.
    BS <ERR#n-n> Bad start. Improper data format usually.
    WB <ERR#n-n> Wrong Block number received.
    FT <ERR#n-n> Filetype specified is invalid.
    SP <ERR#n-n> Wrong data received during a special packet.

    SYNC ERROR!  Irrecoverable synchronization error.
