TFTP ackPacket

ACK packets (opcode 4) acknowledge the receipt of a DATA packet. They are the simplest TFTP packet type - just an opcode and a block number.

Source file: tftp.c
Hammer concepts: h_int_range · h_sequence

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ACK Packet Layout

 2 bytes    2 bytes
+---------+---------+
| Opcode  | Block # |
+---------+---------+

According to the RFC:

• Opcode - 2 bytes, value 4
• Block # - 2 bytes, the block number being acknowledged (range 0–65535)

A block number of 0 is used to acknowledge a WRQ before any DATA packets are sent.

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Step 1: Opcode

H_RULE(opc, h_int_range(h_uint16(), 4, 4));

This only accepts a value of exactly 4.

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Step 2: Block Number

H_RULE(blockNum, h_int_range(h_uint16(), 0, UINT16_MAX));

Unlike DATA packets (which start at 1), ACK block numbers start at 0.

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Step 3: Assemble the ACK Parser

H_RULE(ackPacket, h_sequence(opc, blockNum, h_end_p(), NULL));

h_end_p() ensures there are no extra bytes after the block number, since an ACK packet is exactly 4 bytes.

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Summary

Concept	Hammer Function	Why It Matters in TFTP
Exact value match	h_int_range(p, 4, 4)	Opcode must be exactly 4
Full range	h_int_range(p, 0, MAX)	Block number can be 0–65535
Parse in order	h_sequence	Opcode followed by block number

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