6.2.5. Payload Size Selection

6.2.5. Payload Size Selection

The requestor's maximum payload size can serve as an indication of the path MTU as it is the largest size the requestor can reassemble. When sending messages over paths where some link in the path has an MTU less than the EDNS payload size, large responses can cause IP fragmentation.

There is a trade-off between choosing a large payload size and avoiding IP fragmentation. Choosing a payload size of 4096 octets as a starting point is reasonable. This size is large enough to handle most responses without incurring the overhead of TCP while still being small enough to have a reasonable chance of not causing fragmentation on many links.

A requestor MAY choose to implement a fallback to smaller advertised sizes to work around firewall or other network limitations. A requestor SHOULD choose to use a fallback mechanism that begins with a large size, such as 4096. If that fails, a fallback around the range of 1280-1410 bytes SHOULD be tried, as it has a reasonable chance to fit within a single Ethernet frame. Failing that, a requestor MAY choose a 512-byte packet, which with large answers may cause a TCP retry.

Values of less than 512 bytes MUST be treated as equal to 512 bytes.