Use the acknowledged subset, not the whole wish list
02OACK means Option Acknowledgment. It contains opcode 6 followed by accepted option/value pairs; it does not repeat the filename or mode.
Separate invalid-server comparison: OACK adds tsize even though the request did not contain tsize. The client follows the RFC’s should by sending ERROR code 8; this transfer terminates.
Source for this drawing: RFC 2347 · Read RFC source: Negotiation Protocol · p.3 ↗
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.
Accepted values are option-specific. Blksize can decrease; timeout must echo the request. Negotiation is not a universal numeric-minimum rule.
Read the updated RFC section ↗Only the client initiates option negotiation. The server must not include an option in OACK that the client did not request.
- Only the client initiates option negotiation. The server must not include an option in OACK that the client did not request.
- The server may acknowledge supported options whose values it accepts. Unsupported options should be omitted, not cause an ERROR just because they are unsupported. Unacknowledged options must be ignored by both peers as though never requested.
- Alternative or invalid values follow the individual option specification. In this example RFC 2348 permits blksize 512 in response to 1024, while RFC 2349 timeout must match the requested value. Do not apply one option’s rule to every option.
- A client receiving an unrequested option should send ERROR code 8 and terminate. This is distinct from the server’s must-not-add rule. A client that otherwise rejects OACK sends ERROR code 8 and terminates too.
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 ↗