TFTP (`tftp`)

Layer: Application and control
Header: variable length
Notes: Application protocols

Reached from

Lower layer	Selector	Value
`udp`	`dst_port` / `src_port`	69 (0x0045)

Field	Type	Default	Description
`opcode`	`enum(16)`	`1` (rrq)	TFTP opcode (1 = RRQ, 2 = WRQ, 3 = DATA, 4 = ACK, 5 = ERROR, 6 = OACK).
`filename`	`cstring`	`"boot.bin"`	File name (NUL-terminated).
`mode`	`cstring`	`"octet"`	Transfer mode: netascii, octet or mail (NUL-terminated).
`options`	`list<struct>` — rest of the PDU	`[]`	Option name / value string pairs (RFC 2347): blksize, timeout, tsize, windowsize, …

Each item is a group of the fields below.

Field	Type	Default	Auto	Description
`name`	`cstring`	`"blksize"`
`value`	`cstring`	`"1428"`

The next layer is selected by opcode; when the field is Auto the builder writes the value of the protocol stacked above.