Truncation Bit

[Jim Reid]

>>>>> "Kevin" == Kevin Darcy <kcd at daimlerchrysler.com> writes:

    Kevin> DNS UDP packets are arbitrarily limited to 512 bytes.

Limited to 512 bytes, yes. Arbitrarily, no. The 512 byte limit is a
relic of the early days of the Arpanet. 576 bytes was the maximum size
of an IP datagram that could be guaranteed to cross the network
without being fragmented. This was in the days when a 56k or 64k
leased line was *fast*, after all. So a number of UDP based protocols
restricted themselves to 512 bytes of data so that when the UDP and IP
headers were added to the packet, the 576 byte limit wasn't exceeded.
Although a UDP header and regular IP header only add up to 28 bytes, a
512 byte payload on UDP packets was the obvious limit to choose.