6.2.3. Requestor's Payload Size

6.2.3. Requestor's Payload Size

The requestor's UDP payload size (encoded in the RR CLASS field) is the number of octets of the largest UDP payload that can be reassembled and delivered in the requestor's network stack. Note that path MTU, with or without fragmentation, could be smaller than this.

Values lower than 512 MUST be treated as equal to 512.

The requestor SHOULD place a value in this field that it can actually receive. For example, if a requestor sits behind a firewall that will block fragmented IP packets, a requestor SHOULD NOT choose a value that will cause fragmentation. Doing so will prevent large responses from being received and can cause fallback to occur. This knowledge may be auto-detected by the implementation or provided by a human administrator.

Note that a 512-octet UDP payload requires a 576-octet IP reassembly buffer. Choosing 1280 on an IPv6 requestor node might be beneficial, as it is guaranteed to not require IP-layer fragmentation [RFC2460]. Choosing 1410 bytes on an IPv4 requestor has a reasonable chance to fit in a single Ethernet frame [RFC2132]. Larger values are more likely to require fragmentation or be dropped by a device along the path (e.g., a firewall or middlebox). Consideration for the most common deployment scenarios is strongly encouraged.