CMPE 80N: Introduction to Networking and the Internet
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1 CMPE 80N: Introduction to Networking and the Internet Katia Obraczka Computer Engineering UCSC Baskin Engineering Lecture 10 CMPE 80N Fall'10 1
2 Announcements Forum assignment #2 posted. Due Nov. 5 th. Project #2 coming up soon. Stay tuned. CMPE 80N Fall'10 2
3 Last class The Web and HTTP. Cookies. Web caching. CMPE 80N Fall'10 3
4 Today The Web and HTTP (Cont d). Web caching (cont d). DNS. CMPE 80N Fall'10 4
5 User-server state: cookies Many major Web sites use cookies Four components: 1) cookie header line of HTTP response message 2) cookie header line in HTTP request message 3) cookie file kept on user s host, managed by user s browser 4) back-end database at Web site Example: Susan always access Internet always from PC visits specific e- commerce site for first time when initial HTTP requests arrives at site, site creates: unique ID entry in backend database for ID CMPE 80N Fall'10 5
6 Cookies: keeping state (cont.) client ebay 8734 cookie file ebay 8734 amazon 1678 one week later: ebay 8734 amazon 1678 usual http request msg usual http response Set-cookie: 1678 usual http request msg cookie: 1678 usual http response msg usual http request msg cookie: 1678 usual http response msg server Amazon server creates ID 1678 for user create entry cookiespecific action cookiespectific action access access backend database CMPE 80N Fall'10 6
7 Cookies (continued) What cookies can bring: authorization shopping carts recommendations user session state (Web ) How to keep state : protocol endpoints: maintain state at sender/receiver over multiple transactions cookies: http messages carry state aside Cookies and privacy: cookies permit sites to learn a lot about you CMPE 80N Fall'10 7
8 Web Caching CMPE 80N Fall'10 8
9 Web Caches (proxy server) Goal: satisfy client request without involving origin server CMPE 80N Fall'10 9
10 Web Caches (proxy server) Goal: satisfy client request without involving origin 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 client client Proxy server origin server origin server CMPE 80N Fall'10 10
11 More about Web caching Cache acts as both client and server. Why Web caching? reduce response time for client request reduce traffic on an institution s access link. Internet dense with caches: enables poor content providers to effectively deliver content. CMPE 80N Fall'10 11
12 Caching example origin servers public Internet 1.5 Mbps access link institutional network 10 Mbps LAN CMPE 80N Fall'10 12
13 Caching example public Internet origin servers public Internet origin servers institutional network 1.5 Mbps access link 10 Mbps LAN institutional network 1.5 Mbps access link 10 Mbps LAN institutional cache CMPE 80N Fall'10 13
14 DNS: Domain Name System CMPE 80N Spring'10 14
15 DNS: Domain Name System People: many identifiers: SSN, name, passport # Internet hosts, routers: IP address (32 bit) - used for addressing. name, e.g., ww.yahoo.com - used by humans. Q: map between IP addresses and name? Domain Name System: distributed database implemented in hierarchy of many name servers. CMPE 80N Spring'10 15
16 DNS services hostname to IP address translation. DNS Why not centralize DNS? single point of failure traffic volume maintenance doesn t scale! CMPE 80N Spring'10 16
17 Distributed, Hierarchical Database Root DNS Servers com DNS servers org DNS servers edu DNS servers yahoo.com DNS servers amazon.com DNS servers pbs.org DNS servers poly.edu umass.edu DNS servers DNS servers Client wants IP for 1 st approx: client queries a root server to find com DNS server client queries com DNS server to get amazon.com DNS server client queries amazon.com DNS server to get IP address for CMPE 80N Spring'10 17
18 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 CMPE 80N Spring'10 18
19 DNS: Root name servers e NASA Mt View, CA f Internet Software C. Palo Alto, CA (and 36 other locations) a Verisign, Dulles, VA c Cogent, Herndon, VA (also LA) d U Maryland College Park, MD g US DoD Vienna, VA h ARL Aberdeen, MD j Verisign, ( 21 locations) k RIPE London (also 16 other locations) i Autonomica, Stockholm (plus 28 other locations) m WIDE Tokyo (also Seoul, Paris, SF) b USC-ISI Marina del Rey, CA l ICANN Los Angeles, CA 13 root name servers worldwide (labeled a-m). CMPE 80N Spring'10 19
20 TLD and Authoritative Servers Top-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp. Network Solutions maintains servers for com TLD Educause for edu TLD Authoritative DNS servers: organization s DNS servers, providing authoritative hostname to IP mappings for organization s servers (e.g., Web, mail). can be maintained by organization or service provider CMPE 80N Spring'10 20
21 Local Name Server does not strictly belong to hierarchy each ISP (residential ISP, company, university) has one. also called default name server when host makes DNS query, query is sent to its local DNS server acts as proxy, forwards query into hierarchy CMPE 80N Spring' : Application Layer 21
22 DNS name root DNS server resolution example Host at cis.poly.edu wants IP address for gaia.cs.umass.edu iterated query: contacted server replies with name of server to contact I don t know this name, but ask this server local DNS server dns.poly.edu requesting host cis.poly.edu TLD DNS server 6 authoritative DNS server dns.cs.umass.edu gaia.cs.umass.edu CMPE 80N Spring'10 22
23 DNS name resolution example root DNS server recursive query: puts burden of name resolution on contacted name server heavy load? 2 local DNS server dns.poly.edu TLD DNS server 1 8 requesting host cis.poly.edu authoritative DNS server dns.cs.umass.edu gaia.cs.umass.edu CMPE 80N Spring'10 23
24 DNS: caching and updating records Once (any) name server learns mapping, it caches mapping cache entries timeout (disappear) after some time TLD servers typically cached in local name servers Thus root name servers not often visited CMPE 80N Spring'10 24
25 DNS records DNS: distributed db storing resource records (RR) Type=A RR format: (name, value, type, ttl) name is hostname value is IP address Type=NS name is domain (e.g. foo.com) value is hostname of authoritative name server for this domain Type=CNAME name is alias name for some canonical (the real) name is really servereast.backup2.ibm.com value is canonical name Type=MX value is name of mailserver associated with name CMPE 80N Spring'10 25
26 DNS protocol, messages DNS protocol : query and reply messages, both with same message format msg header identification: 16 bit # for query, reply to query uses same # flags: query or reply recursion desired recursion available reply is authoritative CMPE 80N Spring'10 26
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