1. 1 IEEE 802.21 MEDIA INDEPENDENT HANDOVER DCN: 21-10-0168-00-srho Title: DNS Based SFF Discovery Date Submitted: 2010-8-11 Authors or Source(s): Dapeng Liu(China Mobile) Abstract: Discussion of DNS based SFF IP address discovery. 1

2. 22 IEEE 802.21 presentation release statements This document has been prepared to assist the IEEE 802.21 Working Group. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.21. The contributor is familiar with IEEE patent policy, as stated in Section 6 of the IEEE-SA Standards Board bylaws and in Understanding Patent Issues During IEEE Standards Development http://standards.ieee.org/board/pat/faq.pdf> Section 6 of the IEEE-SA Standards Board bylawshttp://standards.ieee.org/guides/bylaws/sect6-7.html#6http://standards.ieee.org/board/pat/faq.pdf

3. Background 1 、 DNS Resource Record A domain name identifies a node. Each node has a set of resource record which is the resource information associated with the particular name. 2 、 RR type A:a host address In DNS queries, the result of a query for a name may contain multiple IP addresses.

4. Question 21-09-0198-02-srho4 How to select an IP address from the RR which satisfies the two requirement: Provide basic load-balancing functionality among the hosts corresponding to the selected IP address. Assure that the selected IP address is the closest result.

5. Example For example,if we want to visit sina’s webserver,the return result is : You have eight type A records for the same DNS name. Each of your eight type A records has a separate address. 21-09-0198-02-srho5

6. Solution DNS Round Robin is a mechanism for choosing an IP address from the list returned by a DNS server so that all clients won't get the same IP address every time. Netmask ordering is a mechanism for further optimizing which IP address is used by attempting to determine the closest result. 21-09-0198-02-srho6

7. DNS Round Robin The order in which IP addresses from the list are returned is the basis for the term round robin. With each DNS response, the IP address sequence in the list is permuted. Usually, basic IP clients attempt connections with the first address returned from a DNS query so that on different connection attempts clients would receive service from different providers, thus distributing the overall load among servers. 21-09-0198-02-srho7

8. Netmask Ordering we should use the longest match and not just pull the first IP address off the list. Longest match rule in destination address selection described in RFC 3484: Rule 9: Use longest matching prefix. When DA and DB belong to the same address family (both are IPv6 or both are IPv4): If CommonPrefixLen(DA, Source(DA)) > CommonPrefixLen(DB, Source(DB)), then prefer DA. Similarly, if CommonPrefixLen(DA, Source(DA)) < CommonPrefixLen(DB, Source(DB)), then prefer DB 21-09-0198-02-srho8

9. Example2: A client with an IP address of 192.168.0.1 queries for Webserver.test.net and receives the following list: Webserver.test.net A 192.168.1.10 Webserver.test.net A 192.168.5.20 Webserver.test.net A 192.168.6.30 Webserver.test.net A 192.168.0.40 Webserver.test.net A 192.168.4.50 With RFC3484 in effect, the client will always use the 192.168.0.40 address as it is the longest match, negating the effects of DNS round-Robin. 21-09-0198-02-srho9

10. Reference 1 、 http://tools.ietf.org/html/rfc1034 http://tools.ietf.org/html/rfc1034 2 、 http://www.ietf.org/rfc/rfc3484.txt http://www.ietf.org/rfc/rfc3484.txt 3 、 http://blogs.technet.com/b/networking/ archive/2009/04/17/dns-round-robin-and-destination-ip-address- selection.aspx 4 、 Computer Network(Fourth Edition) Andrew S.Tanenbaum 21-09-0198-02-srho10

11. 11 Thanks!