TFTP Protocol & Lock-Step Architecture Simulation

What is Trivial File Transfer Protocol (TFTP)?

Trivial File Transfer Protocol (TFTP) is a simplified, UDP-based file transfer protocol running over UDP Port 69 (RFC 1350). Designed with a minimal footprint, TFTP lacks user authentication, directory listing capabilities, and encryption. It is extensively used in enterprise networking for PXE network booting, router/switch firmware upgrades, and automated configuration backups.

TFTP Connection Architecture & Transfer Identifiers (TID)

• Initial Request (Port 69): The client initiates communication by sending a Read Request (RRQ) or Write Request (WRQ) packet to the server's well-known UDP Port 69.
• Dynamic TID Allocation: Upon receiving the request, the TFTP server allocates a new random high UDP port (Transfer Identifier / TID, e.g., 52314) to handle the file session. All subsequent data packets and acknowledgments bypass Port 69 and flow exclusively between the client's source port and the new TID port.
• Lock-Step Flow Control: TFTP employs a strict Stop-and-Wait protocol. The server transmits a 512-byte block (`DATA`), and must receive an acknowledgment (`ACK`) from the client for that exact block number before transmitting the next block.
• Termination: A file transfer completes when a `DATA` packet containing less than 512 bytes of payload (0 to 511 bytes) is received and acknowledged.

TFTP Packet Opcodes

• Opcode 1 (RRQ): Read Request — requests permission to download a specified file.
• Opcode 2 (WRQ): Write Request — requests permission to upload a specified file.
• Opcode 3 (DATA): Data packet carrying a 2-byte Block Number and up to 512 bytes of data.
• Opcode 4 (ACK): Acknowledgment packet echoing the 2-byte Block Number received.
• Opcode 5 (ERROR): Error packet carrying error codes (e.g., File Not Found, Access Violation, Disk Full).