Fall back deliberately or terminate negotiation
05An option request does not force the server to negotiate. Ordinary TFTP replies can start a transfer without those options; an ERROR is a different outcome.
Separate request-with-options that received ERROR: the client chooses the permitted new request without option pairs. This is a fresh attempt with an unknown result, not guaranteed recovery.
Source for this drawing: RFC 2347 · Read RFC source: Negotiation Protocol · p.4 ↗
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.
For a read request, DATA block 1 instead of OACK starts ordinary TFTP without acknowledged options. For a write request, ACK block 0 instead of OACK does the same. Requested options must not silently become active.
- For a read request, DATA block 1 instead of OACK starts ordinary TFTP without acknowledged options. For a write request, ACK block 0 instead of OACK does the same. Requested options must not silently become active.
- Without an acknowledged blksize option, base TFTP data blocks use up to 512 data octets. The four-octet DATA header is additional. A server ignoring extensions is not promising the requested blocksize.
- A client rejecting an OACK sends ERROR code 8 and terminates. Unsupported options should be omitted by the server; a supported option with an invalid value follows that option’s specification and can have different outcomes.
- If the server returns ERROR for a request carrying options, the client may attempt a new request without options. That retry is optional and not guaranteed to succeed; it does not bypass permissions or fix a missing file. The extension adds no security.
Fictional picnic.txt in octet mode; chosen sizes and values are examples. Comparisons reset state. This RFC uses lowercase must/should/may; those distinctions are retained. No real files or UDP transfers are used.
Read the RFC section ↗