DN04 应用层2.ppt

DNS: Domain Name System People: many identifiers: SSN, name, Passport # Internet hosts, routers: IP address (32 bit) - used for addressing datagrams “name”, e.g., - used by humans Q: map between IP addresses and name ? Domain Name System: distributed database implemented in hierarchy of many name servers application-layer protocol host, routers, name servers to communicate to resolve names (address/name translation) note: core Internet function implemented as application-layer protocol complexity at network’s “edge” DNS name servers no server has all name-to-IP address mappings local name servers: each ISP, company has local (default) name server host DNS query first goes to local name server authoritative name server: for a host: stores that host’s IP address, name can perform name/address translation for that host’s name Why not centralize DNS? single point of failure traffic volume distant centralized database maintenance doesn’t scale! DNS: Root name servers contacted by local name server that can not resolve name root name server: contacts authoritative name server if name mapping not known gets mapping returns mapping to local name server ~ dozen root name servers worldwide Simple DNS example host surf.eurecom.fr wants IP address of 1. Contacts its local DNS server, dns.eurecom.fr 2. dns.eurecom.fr contacts root name server, if necessary 3. root name server contacts authoritative name server, , if necessary DNS example Root name server: may not know authoratiative name server may know intermediate name server: who to contact to find authoritative name server DNS: iterated queries recursive query（递归查询）: puts burden of name resolution on contacted name server heavy load? iterated query（迭代查询）: contacted server replies with name of server to contact “I don’t know this name, but ask this server” DNS: caching and updating records once (any) name server learns mapping, it caches mapping cache entries timeout (disappear) after some time update/notify mechanisms