Reason for Limited number of Root DNS Servers
marka at isc.org
Fri Nov 11 06:43:57 UTC 2011
In message <email@example.com>, Gaurav Kansal writes:
> Dear All,
> Somewhere I read that number of ROOT DNS servers is limited to 13 because of
> protocol limitation of DNS and UDP.
> Exact writing was "A combination of limits in the DNS and certain
> protocols, namely the practical size of unfragmented User Datagram Protocol
> (UDP) packets, resulted in a limited number of root server addresses that
> can be accommodated in DNS name query responses. This limit has determined
> the number of name server installations at (currently) 13 clusters, serving
> the needs of the entire public Internet worldwide."
> As root DNS are running in anycast so number is not an issue at all. But I
> don't understand where exactly is this limitation exists???
> Please some elaborate on this.
> Thanks and Regards,
> Gaurav Kansal
Actually despite the words above it has *nothing* to do
with unfragmented UDP and everything to with being able to
reassemble fragmented UDP.
All IPv4 hosts MUST accept fragmented packets up to 576
octets (RFC 791). DNS's 512 octet UDP limit was choosen to
ensure that the UDP datagram can always be reassembled and
for there to be room for some IP options in addition to the
IP and UDP headers.
Originally there wasn't commonality in the root server's
names. Then it was said if we make the maximum use of
compression how root servers can we fit into a DNS/UDP
The first NS record takes 31 octets (1 + 2 + 2 + 4 + 2 + 20).
Additional a NS records for . takes 15 octets (1 octets for
the name, 2 octets for the class, 2 octets for the type, 4
octets for the ttl, 2 octet for length and 4 of actual data).
A "A" record with a compressed ownername takes 16 octets
(2 octets for the name, 2 octets for the class, 2 octets for the
type, 4 octets for the ttl, 2 octet for length and 4 of actual data).
Then there is the 12 octet header and the 5 octet question.
Doing the math on priming queries you get the following:
13 names uses 436 octets
14 names uses 467 octets
15 names uses 498 octets
If you have a referral to the root with a maximum sized qname
it takes 482 octets (12 + 255 + 4 + 31 + 15 * 12).
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742 INTERNET: marka at isc.org
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