how does wireshark dissect the protocol above TCP?

edit

In this packet:

No.     Time                          Source                Destination           Protocol Length Info
      5 2012-01-18 13:49:07.908220    10.46.206.14          10.46.212.81          FTP      65     Request: USER root

Frame 5: 65 bytes on wire (520 bits), 65 bytes captured (520 bits)
Ethernet II, Src: Wistron_31:77:41 (00:1f:16:31:77:41), Dst: Zte_03:da:35 (00:19:c6:03:da:35)
Internet Protocol Version 4, Src: 10.46.206.14, Dst: 10.46.212.81   
Transmission Control Protocol, Src Port: 56053, Dst Port: 21, Seq: 1, Ack: 22, Len: 11    
    Source Port: 56053
    Destination Port: 21
    [Stream index: 0]
    [TCP Segment Len: 11]
    Sequence number: 1    (relative sequence number)
    [Next sequence number: 12    (relative sequence number)]
    Acknowledgment number: 22    (relative ack number)
    0101 .... = Header Length: 20 bytes (5)
    Flags: 0x018 (PSH, ACK)
    Window size value: 16419
    [Calculated window size: 65676]
    [Window size scaling factor: 4]
    Checksum: 0xb6e1 [unverified]
    [Checksum Status: Unverified]
    Urgent pointer: 0
    [SEQ/ACK analysis]
    TCP payload (11 bytes)
File Transfer Protocol (FTP)
    USER root\r\n
        Request command: USER
        Request arg: root

-----from TCP layer,there is no info indicate upper application is ftp,how does wireshark know it ,thanks