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1、2: Application Layer,1,DNS: Domain Name System,People: many identifiers: SSN, name, Passport # Internet hosts, routers: IP address (32 bit) - used for addressing datagrams “name”, e.g., gaia.cs.umass.edu - used by humans Q: map between IP addresses and name ?,Domain Name System: distributed database
2、 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 networks “edge”,2: Application Layer,2,DNS name server
3、s,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 hosts IP address, name can perform name/address translation for that hosts name,Why
4、not centralize DNS? single point of failure traffic volume distant centralized database maintenancedoesnt scale!,2: Application Layer,3,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
5、mapping returns mapping to local name server dozen root name servers worldwide,2: Application Layer,4,Simple DNS example,host surf.eurecom.fr wants IP address of gaia.cs.umass.edu 1. Contacts its local DNS server, dns.eurecom.fr 2. dns.eurecom.fr contacts root name server, if necessary 3. root name
6、server contacts authoritative name server, dns.umass.edu, if necessary,requesting host surf.eurecom.fr,gaia.cs.umass.edu,root name server,authorititive name server dns.umass.edu,1,2,3,4,5,6,2: Application Layer,5,DNS example,Root name server: may not know authoratiative name server may know intermed
7、iate name server: who to contact to find authoritative name server,requesting host surf.eurecom.fr,gaia.cs.umass.edu,root name server,1,2,3,4,5,6,authoritative name server dns.cs.umass.edu,7,8,2: Application Layer,6,DNS: iterated queries,recursive query: puts burden of name resolution on contacted n
8、ame server heavy load? iterated query: contacted server replies with name of server to contact “I dont know this name, but ask this server”,requesting host surf.eurecom.fr,gaia.cs.umass.edu,root name server,1,2,3,4,5,6,authoritative name server dns.cs.umass.edu,7,8,iterated query,2: Application Laye
9、r,7,DNS: caching and updating records,once (any) name server learns mapping, it caches mapping cache entries timeout (disappear) after some time update/notify mechanisms under design by IETF RFC 2136 http:/www.ietf.org/html.charters/dnsind-charter.html,2: Application Layer,8,DNS records,DNS: distrib
10、uted db storing resource records (RR),Type=NS name is domain (e.g. ) value is IP address of authoritative name server for this domain,Type=A name is hostname value is IP address,Type=CNAME name is an alias name for some “cannonical” (the real) name value is cannonical name,Type=MX value is hostname
11、of mailserver associated with name,2: Application Layer,9,DNS protocol, messages,DNS protocol : query and repy messages, both with same message format,msg header identification: 16 bit # for query, repy to query uses same # flags: query or reply recursion desired recursion available reply is authori
12、tative,2: Application Layer,10,DNS protocol, messages,Name, type fieldsfor a query,RRs in reponse to query,records for authoritative servers,additional “helpful” info that may be used,2: Application Layer,11,Socket programming,Socket API introduced in BSD4.1 UNIX, 1981 Now Industry standard Availabl
13、e on many operating systems explicitly created, used, released by apps client/server paradigm two types of transport service via socket API: unreliable datagram reliable, byte stream-oriented,Goal: learn how to build client/server application that communicate using sockets,2: Application Layer,12,Wh
14、at is “socket”?,A socket is a virtual connection between two applications Using a socket, two processes can communicate with each other The socket is the major communication tool for Internet applications A socket is bi-directional (full-duplex) transmission A socket can be created dynamically,2: Ap
15、plication Layer,13,Network,Socket as a virtual connection between two processes,(physical connection),Host B,Process 2,Host A,Process 1,Information Hiding,2: Application Layer,14,Socket as a client/server model,2: Application Layer,15,Socket,OS abstraction (not hardware) Created dynamically Persists
16、 only while application runs Referenced by a descriptor,2: Application Layer,16,Descriptor,Small integer One per active socket Used in all operations on socket Generated by OS when socket created Only meaningful to application that owns socket In UNIX, integrated with file descriptors,2: Application Layer,17,Creating a Socket,Application calls socket functionOS returns descriptor for socket Descriptor valid until application closes socket or exits Common: protofamily = PF_INET, type = SOCK_STREAM,SOCK_DGRAM or SOCK_RAW,
