【正文】 1st approx: ? Client queries a root server to find DNS server ? Client queries DNS server to get DNS server ? Client queries DNS server to get IP address for 2: Application Layer 65 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 13 root name servers worldwide b USCISI Marina del Rey, CA l ICANN Los Angeles, CA e NASA Mt View, CA f Inter Software C. Palo Alto, CA (and 17 other locations) i Autonomica, Stockholm (plus 3 other locations) k RIPE London (also Amsterdam, Frankfurt) m WIDE Tokyo a Verisign, Dulles, VA c Cogent, Herndon, VA (also Los Angeles) d U Maryland College Park, MD g US DoD Vienna, VA h ARL Aberdeen, MD j Verisign, ( 11 locations) 2: Application Layer 66 TLD and Authoritative Servers ? Toplevel domain (TLD) servers: responsible for , , , edu, etc, and all toplevel country domains uk, fr, ca, jp. ? Network solutions maintains servers for TLD ? Educause for edu TLD ? Authoritative DNS servers: anization’s DNS servers, providing authoritative hostname to IP mappings for anization’s servers (., Web and mail). ? Can be maintained by anization or service provider 2: Application Layer 67 Local Name Server ? Does not strictly belong to hierarchy ? Each ISP (residential ISP, pany, university) has one. ?Also called “default name server” ? When a host makes a DNS query, query is sent to its local DNS server ? Acts as a proxy, forwards query into hierarchy. 2: Application Layer 68 requesting host root DNS server local DNS server 1 2 3 4 5 6 authoritative DNS server 7 8 TLD DNS server Example ? Host at wants IP address for 2: Application Layer 69 requesting host root DNS server local DNS server 1 2 4 5 6 authoritative DNS server 7 8 TLD DNS server 3 Recursive queries recursive query: ? puts burden of name resolution on contacted name server 。 message placed in message queue 3) Client side of SMTP opens TCP connection with Bob’s mail server 4) SMTP client sends Alice’s message over the TCP connection 5) Bob’s mail server places the message in Bob’s mailbox 6) Bob invokes his user agent to read message user agent mail server mail server user agent 1 2 3 4 5 6 2: Application Layer 53 Sample SMTP interaction S: 220 C: HELO S: 250 Hello , pleased to meet you C: MAIL FROM: S: 250 ... Sender ok C: RCPT TO: S: 250 ... Recipient ok C: DATA S: 354 Enter mail, end with . on a line by itself C: Do you like ketchup? C: How about pickles? C: . S: 250 Message accepted for delivery C: QUIT S: 221 closing connection 2: Application Layer 54 Try SMTP interaction for yourself: ? tel servername 25 ? see 220 reply from server ? enter HELO, MAIL FROM, RCPT TO, DATA, QUIT mands above lets you send without using client (reader) 2: Application Layer 55 SMTP: final words ? SMTP uses persistent connections ? SMTP requires message (header amp。 cookies to learn yet more ? advertising panies obtain info across sites aside 2: Application Layer 38 Web caches (proxy server) ? user sets browser: Web accesses via cache ? browser sends all HTTP requests to cache ? object in cache: cache returns object ? else cache requests object from origin server, then returns object to client Goal: satisfy client request without involving origin server client Proxy server client origin server origin server 2: Application Layer 39 More about Web caching ? Cache acts as both client and server ? Typically cache is installed by ISP (university, pany, residential ISP) Why Web caching? ? Reduce response time for client request. ? Reduce traffic on an institution’s access link. ? Inter dense with caches enables “poor” content providers to effectively deliver content (but so does P2P file sharing) 2: Application Layer 40 Caching example Assumptions ? average object size = 100,000 bits ? avg. request rate from institution’s browsers to origin servers = 15/sec ? delay from institutional router to any origin server and back to router = 2 sec Consequences ? utilization on LAN = 15% ? utilization on access link = 100% ? total delay = Inter delay + access delay + LAN delay = 2 sec + minutes + milliseconds origin servers public Inter institutional work 10 Mbps LAN Mbps access link institutional cache 2: Application Layer 41 Caching example (cont) Possible solution ? increase bandwidth of access link to, say, 10 Mbps Consequences ? utilization on LAN = 15% ? utilization on access link = 15% ? Total delay = Inter delay + access delay + LAN delay = 2 sec + msecs + msecs ? often a costly upgrade origin servers public Inter institutional work 10 Mbps LAN 10 Mbps access link institutional cache 2: Application Layer 42 Caching example (cont) Install cache ? suppose hit rate is .4 Consequence ? 40% requests will be satisfied almost immediately ? 60% requests satisfied by origin server ? utilization of access link reduced to 60%, resulting in negligible delays (say 10 msec) ? total avg delay = Inter delay + access delay + LAN delay = .6*() secs + milliseconds secs origin servers public Inter institutional work 10 Mbps LAN Mbps access link institutional cache 2: Application Layer 43 Conditional GET ? Goal: don’t send object if cache has uptodate cached versi