Agree on a size or stop negotiation
02A willing server returns an Option Acknowledgment (OACK). Its block size cannot be greater than the requested value; the client either accepts the offered size or sends ERROR code 8 to terminate.
Separate read fallback: DATA1 arrives without OACK, so no larger blksize was agreed. Use ordinary 512-byte data blocks; do not interpret the requested 1428 as active.
Source for this drawing: RFC 2348 · Read RFC source: Blocksize agreement · p.2 ↗
New to this? Start hereA quick picture + helpful background
A message needs an address.
Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.
Helpful first, if these ideas are unfamiliar
These pictures are analogies to help you learn. The linked RFC explains the actual rules.
Terms in this drawing
Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
A server’s accepted block size is less than or equal to the client request, within the option’s valid range. An acceptable smaller value becomes the value used, not merely a hint.
- A server’s accepted block size is less than or equal to the client request, within the option’s valid range. An acceptable smaller value becomes the value used, not merely a hint.
- The client must use the OACK value or send ERROR code 8 to terminate. An ERROR packet is not acknowledged or retransmitted; the diagram does not promise its delivery (RFC 1350 section7).
- Only the client initiates option negotiation. RFC 2347 says the server must not add an unrequested option and the client should respond to such an OACK with ERROR8 and terminate. The drawing chooses that recommended rejection branch; it does not upgrade should to a new uppercase MUST.
- If blksize is not acknowledged, ignore it. A legacy read may begin with DATA1 at the ordinary 512 size; a legacy write begins with ACK0. An ERROR response is a rejection, not permission to silently use the requested larger size.
A selected octet-mode example, not a complete TFTP implementation. One octet is an eight-bit byte. Follow the actual RFC source and its companion option-negotiation rules.
Read the RFC section ↗