Appendix I

Order of Headers

                                          2 bytes
----------------------------------------------------------
|  Local Medium  |  Internet  |  Datagram  |  TFTP Opcode  |
----------------------------------------------------------

TFTP Formats

Type Op # Format without header

      2 bytes    string   1 byte     string   1 byte
      -----------------------------------------------
RRQ/  | 01/02 |  Filename  |   0  |    Mode    |   0  |
WRQ    -----------------------------------------------

      2 bytes    2 bytes       n bytes
      ---------------------------------
DATA  | 03    |   Block #  |    Data    |
      ---------------------------------

      2 bytes    2 bytes
      -------------------
ACK   | 04    |   Block #  |
      --------------------

      2 bytes  2 bytes        string    1 byte
      ----------------------------------------
ERROR | 05    |  ErrorCode |   ErrMsg   |   0  |
      ----------------------------------------

Initial Connection Protocol for reading a file

1. Host A sends a "RRQ" to host B with source= A's TID, destination= 69.
2. Host B sends a "DATA" (with block number= 1) to host A with source= B's TID, destination= A's TID.

Error Codes

Value	Meaning
0	Not defined, see error message (if any).
1	File not found.
2	Access violation.
3	Disk full or allocation exceeded.
4	Illegal TFTP operation.
5	Unknown transfer ID.
6	File already exists.
7	No such user.

Internet User Datagram Header [2]

(This has been included only for convenience. TFTP need not be implemented on top of the Internet User Datagram Protocol.)

Format

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          Source Port          |       Destination Port        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Length             |           Checksum            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Values of Fields

• Source Port: Picked by originator of packet.
• Dest. Port: Picked by destination machine (69 for RRQ or WRQ).
• Length: Number of bytes in UDP packet, including UDP header.
• Checksum: Reference 2 describes rules for computing checksum. (The implementor of this should be sure that the correct algorithm is used here.) Field contains zero if unused.

Note: TFTP passes transfer identifiers (TID's) to the Internet User Datagram protocol to be used as the source and destination ports.

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