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Review: Which protocol is used to move email messages around in the Internet?

Review: Which protocol is used to move email messages around in the Internet? Describe how a message is moved from the sender’s UA to the receiver’s mailbox? What does Pop3 do? When do we use it? What is the major problem with the current Internet email system?.

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Review: Which protocol is used to move email messages around in the Internet?

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  1. Review: • Which protocol is used to move email messages around in the Internet? • Describe how a message is moved from the sender’s UA to the receiver’s mailbox? • What does Pop3 do? When do we use it? • What is the major problem with the current Internet email system?

  2. www.someschool.edu/someDept/pic.gif path name host name Web and HTTP First some jargon • Web page consists of objects • Object can be HTML file, JPEG image, Java applet, audio file,… • Web page consists of base HTML-file which includes several referenced objects • Each object is addressable by a URL • Example URL:

  3. HTTP: hypertext transfer protocol Web’s application layer protocol client/server model client: browser that requests, receives, “displays” Web objects server: Web server sends objects in response to requests HTTP 1.0: RFC 1945 HTTP 1.1: RFC 2068 HTTP overview HTTP request PC running Explorer HTTP response HTTP request Server running Apache Web server HTTP response Mac running Navigator

  4. Over TCP: client initiates TCP connection (creates socket) to server, port 80 server accepts TCP connection from client HTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server) TCP connection closed HTTP overview (continued)

  5. HTTP request message • two types of HTTP messages: request, response • HTTP request message: • ASCII (human-readable format) request line (GET, POST, HEAD commands) GET /somedir/page.html HTTP/1.1 Host: www.someschool.edu User-agent: Mozilla/4.0 Connection: close Accept-language:fr (extra carriage return, line feed) header lines Carriage return, line feed indicates end of message

  6. HTTP request message: general format

  7. HTTP response message status line (protocol status code status phrase) HTTP/1.1 200 OK Connection close Date: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 …... Content-Length: 6821 Content-Type: text/html data data data data data ... header lines data, e.g., requested HTML file

  8. 1. Telnet to your favorite Web server: Trying out HTTP (client side) for yourself Opens TCP connection to port 80 (default HTTP server port) at cis.poly.edu. Anything typed in sent to port 80 at cis.poly.edu telnet cis.poly.edu 80 2. Type in a GET HTTP request: By typing this in (hit carriage return twice), you send this minimal (but complete) GET request to HTTP server GET /~ross/ HTTP/1.1 Host: cis.poly.edu 3. Look at response message sent by HTTP server!

  9. HTTP is stateless. two requests are treated independently. Why stateless? What is the problem with a stateless http? E-commence: People buy things by making many requests. Need the ability to bind the requests from the same customer together. Solution: cookies User-server state: cookies

  10. client server usual http request msg usual http response + Set-cookie: 1678 Cookie file Cookie file Cookie file amazon: 1678 ebay: 8734 ebay: 8734 amazon: 1678 ebay: 8734 cookie- specific action usual http request msg cookie: 1678 usual http request msg cookie: 1678 usual http response msg usual http response msg cookie- spectific action Cookies: keeping “state” (cont.) server creates ID 1678 for user entry in backend database access access one week later:

  11. What cookies can bring: authorization shopping carts recommendations user session state (Web e-mail) Cookies (continued) aside Cookies and privacy: • cookies permit sites to learn a lot about you • you may supply name and e-mail to sites • search engines use redirection & cookies to learn yet more • advertising companies obtain info across sites

  12. Some issues in HTTP: • Mainly due to its popularity • Cache support. • Insufficient in http/1.0, improved in http/1.1 • Intermediate nodes, encoding, etc • Dynamically generated date • Not reliable in http/1.0 • Performance • Persistent or non-persistent TCP connection • Download the whole file or part of a file • User preference • Security

  13. DNS services Hostname to IP address translation Host aliasing Canonical and alias names Mail server aliasing Load distribution Replicated Web servers: set of IP addresses for one canonical name DNS: Domain Name System

  14. Root DNS Servers org DNS servers edu DNS servers com DNS servers poly.edu DNS servers umass.edu DNS servers pbs.org DNS servers yahoo.com DNS servers amazon.com DNS servers Distributed, Hierarchical Database

  15. 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 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) k RIPE London (also Amsterdam, Frankfurt) i Autonomica, Stockholm (plus 3 other locations) m WIDE Tokyo e NASA Mt View, CA f Internet Software C. Palo Alto, CA (and 17 other locations) b USC-ISI Marina del Rey, CA l ICANN Los Angeles, CA DNS: Root name servers 13 root name servers worldwide

  16. 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 and mail). • Can be maintained by organization or service provider

  17. Local Name Server • Does not strictly belong to hierarchy • Each ISP (residential ISP, company, 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.

  18. 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 root DNS server Iterative Queries 2 3 TLD DNS server 4 5 6 7 1 8 authoritative DNS server dns.cs.umass.edu requesting host cis.poly.edu gaia.cs.umass.edu

  19. root DNS server 2 3 6 7 TLD DNS server 4 local DNS server dns.poly.edu 5 1 8 authoritative DNS server dns.cs.umass.edu requesting host cis.poly.edu gaia.cs.umass.edu Recursive queries recursive query: • puts burden of name resolution on contacted name server • heavy load?

  20. 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 update/notify mechanisms under design by IETF RFC 2136 http://www.ietf.org/html.charters/dnsind-charter.html DNS: caching and updating records

  21. DNS: distributed db storing resource records (RR) Type=NS name is domain (e.g. foo.com) value is IP address of authoritative name server for this domain RR format: (name, value, type, ttl) DNS records • Type=A • name is hostname • value is IP address • Type=CNAME • name is alias name for some “cannonical” (the real) name www.ibm.com is really servereast.backup2.ibm.com • value is cannonical name • Type=MX • value is name of mail server associated with name

  22. Example: Aix IN A 192.168.42.2 IN AAAA 3ffe:b80:1f8d:2:204:acff:fe17:bf38 IN MX 5 aix.unpbook.com. IN MX 10 mailhost.unpbook.com. Aix-4 IN A 192.168.42.2 ftp IN CNAME linux.unpbook.com www IN CNAME linux.unpbook.com • DNS uses UDP to exchange information • Query is initiated from a system call: gethostbyname, gethostbyaddr.

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