In message <002b01cca054$9be42920$d3ac7b60$@nic.in>, Gaurav Kansal writes: > Thanks a lot Mark. > But I don't understand the calculation part. > Is there any source available from which I can get detail information > regarding the same??????
The DNS protocol is defined in RFC 1034 and RFC 1035. This all comes from a basic analysis of how the data is packed into a DNS packet. > Thanks and Regards, > Gaurav Kansal > 9910118448 > > > > -----Original Message----- > From: Mark Andrews [mailto:ma...@isc.org] > Sent: Friday, 11 November, 2011 12:14 PM > To: Gaurav Kansal > Cc: bind-us...@isc.org > Subject: Re: Reason for Limited number of Root DNS Servers > > > In message <004c01cca034$259d4870$70d7d950$@nic.in>, 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 > > > > 9910118448 > > 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 > message? > > 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 > -- > Mark Andrews, ISC > 1 Seymour St., Dundas Valley, NSW 2117, Australia > PHONE: +61 2 9871 4742 INTERNET: ma...@isc.org > -- Mark Andrews, ISC 1 Seymour St., Dundas Valley, NSW 2117, Australia PHONE: +61 2 9871 4742 INTERNET: ma...@isc.org _______________________________________________ Please visit https://lists.isc.org/mailman/listinfo/bind-users to unsubscribe from this list bind-users mailing list bind-users@lists.isc.org https://lists.isc.org/mailman/listinfo/bind-users
