6.2.4. Responder's Payload Size

6.2.4. Responder's Payload Size

The responder's UDP payload size can vary depending on the request, or as a result of local policy. A responder MAY place a value in this field that differs from the value specified by the requestor. A responder's payload size information (in the OPT record in the response) is advisory information for use by the requestor when formulating subsequent queries. While the responder's payload size is expected to remain relatively constant across queries sent to the same responder, the responder may vary this value based on network conditions or per-query policy restrictions. For example, a responder may wish to reply with a smaller payload size when replying to a new or unknown requestor, or when replying over a limited-capacity link, to reduce the amplification effect of denial-of-service attacks.

To maximize the benefit of EDNS, responders SHOULD choose a payload size of at least 1232 bytes for IPv4 and 1232 bytes for IPv6. This size is chosen to avoid fragmentation on most common deployment scenarios.