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CS 640

Network Applications. Outline Simple Mail Transfer Protocol The Web, HTTP. CS 640. 1. Simple Mail Transfer Protocol. Basic protocol for email exchange over the Internet Runs on top of TCP SMTP is NOT an interactive protocol, unlike, say, FTP

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CS 640

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  1. Network Applications Outline Simple Mail Transfer Protocol The Web, HTTP CS 640 1

  2. Simple Mail Transfer Protocol • Basic protocol for email exchange over the Internet • Runs on top of TCP • SMTP is NOT an interactive protocol, unlike, say, FTP • Messages are queued and spooled by SMTP agent • Users interact with email application • Lots! • Application interfaces with Message Transfer Agent or mail daemon • Sendmail on UNIX • Setup and configured by admins. • SMTP specifies how MTA’s pass email across the Internet CS 640 2

  3. Simple Mail Transfer Protocol • Client uses email application to construct and send messages • Addresses consist of machine name and mailbox address • Mailbox is usually the same as one’s login but you can have aliases • Destination machine can also be an alias@cs.wisc.edu • Message is passed to mail spooler which is part of MTA • Application communicates with MTA via email transfer protocol • Post Office Protocol (POP3) is common, but not very secure • Our department uses IMAP • MTA’s on remote systems listen for incoming mail on well known port (25) • Messages are delivered in two parts – header and body • Header format has exact specification = RFC 822 • Body content types are specified by MIME CS 640 3

  4. SMTP Example From:akella@cs.wisc.edu To: “Srini" <seshan@cs.cmu.edu> Subject: Hi Date: Tues, 11 Aug 2001 10:46:16 -0400 MIME-Version: 1.0 X-Mailer: Internet Mail Service (5.5.1960.3) Content-Type: text/plain Content-Transfer-Encoding: 7bit Hi, Srini! Blah! -Aditya header Aditya Akella UW-madison Srini Seshan CMU body CS 640 4

  5. Sample SMTP Session tux34(21)% telnet smtp.cs.wisc.edu 25 Trying 128.105.6.11... Connected to schroeder.cs.wisc.edu (128.105.6.11). 220 schroeder.cs.wisc.edu ESMTP Sendmail 8.11.3; Tue, 11 Sep 2001 14:09:52 -0500 HELO akella.cs.wisc.edu 250 schroeder.cs.wisc.edu Hello tux34.cs.wisc.edu [128.105.111.134], pleased to meet you MAIL FROM:<akella@cs.wisc.edu> 250 2.1.0 <akella@cs.wisc.edu> ... Sender ok RCPT TO:<someone@tds.net> 250 2.1.5 <jgast@tds.net>... Recipient ok DATA 354 Enter mail, end with "." on a line by itself To: someone@tds.net Test for CS640 . 250 2.0.0. f8BJAeq14849 Message accepted for delivery QUIT 221 2.0.0 schroeder.cs.wisc.edu closing connection Connection closed by foreign host. tux34(22)% Envelope Header Body CS 640 5

  6. Web/WWW Components • Structural Components • Clients/browsers – to dominant implementations • Servers – run on sophisticated hardware • Caches – many interesting implementations • Internet – the global infrastructure which facilitates data transfer • Semantic Components • Hyper Text Transfer Protocol (HTTP) • Hyper Text Markup Language (HTML) • eXtensible Markup Language (XML) • Uniform Resource Identifiers (URIs) CS 640 6

  7. WWW Structure • Clients use browser application to send URIs via HTTP to servers requesting a Web page • Web pages constructed using HTML (or other markup language) and consist of text, graphics, sounds plus embedded files • Servers (or caches) respond with requested Web page • Or with error message • Client’s browser renders Web page returned by server • Page is written using Hyper Text Markup Language (HTML) • Displaying text, graphics and sound in browser • Writing data as well • The entire system runs over standard networking protocols (TCP/IP, DNS,…) CS 640 7

  8. Uniform Resource Identifiers • Web resources need names/identifiers – Uniform Resource Identifiers (URIs) • Resource can reside anywhere on the Internet • URIs are a somewhat abstract notion • A pointer to a resource to which request methods can be applied to generate potentially different responses • A request method is eg. fetching or changing the object • Instance: http://www.foo.com/index.html • Protocol, server, resource • Most popular form of a URI is the Uniform Resource Locator (URL) • Differences between URI and URL are beyond scope • RFC 2396 CS 640 8

  9. HTTP Basics • Protocol for client/server communication • The heart of the Web • Very simple request/response protocol • Client sends request message, server replies with response message • Stateless • Relies on URI naming mechanism • Three versions have been used • 09/1.0 • RFC 1945 (original RFC is now expired) • 1.1 – developed to enhance performance, caching, compression • RFC 2068 • 1.0 dominates today but 1.1 is catching up CS 640 9

  10. HTTP Request Messages • GET – retrieve document specified by URL • PUT – store specified document under given URL • HEAD – retrieve info. about document specified by URL • POST – give information (eg. annotation) to the server Less common ones… • OPTIONS – retrieve information about available options • DELETE – remove document specified by URL • TRACE – loopback request message • CONNECT – for use by caches CS 640 10

  11. HTTP Request Format request-line ( request request-URI HTTP-version) headers (0 or more) <blank line> body (only for POST request) • First type of HTTP message: requests • Client browsers construct and send message • Typical HTTP request: • GET http://www.cs.wisc.edu/index.html HTTP/1.0 CS 640 11

  12. HTTP Response Format status-line (HTTP-version response-code response-phrase) headers (0 or more) <blank line> body • Second type of HTTP message: response • Web servers construct and send response messages • Typical HTTP response: • HTTP/1.0 301 Moved Permanently Location: http://www.wisc.edu/cs/index.html CS 640 12

  13. HTTP Response Codes • 1xx – Informational – request received, processing • 2xx – Success – action received, understood, accepted • 3xx – Redirection – further action necessary • 4xx – Client Error – bad syntax or cannot be fulfilled • 5xx – Server Error – server failed CS 640 13

  14. HTTP Headers • Both requests and responses can contain a variable number of header fields • Consists of field name, colon, space, field value • 17 possible header types divided into three categories • Request • Response • Body • Example: Date: Friday, 27-Apr-01 13:30:01 GMT • Example: Content-length: 3001 CS 640 14

  15. HTTP/1.0 Network Interaction • Clients make requests to port 80 on servers • Uses DNS to resolve server name • Clients make separate TCP connection for each URL • Some browsers open multiple TCP connections • Netscape default = 4 • Server returns HTML page • Many types of servers with a variety of implementations • Apache is the most widely used • Freely available in source form • Client parses page • Requests embedded objects CS 640 15

  16. HTTP/1.1 Performance Enhancements • HTTP/1.0 is a “stop and wait” protocol • Separate TCP connection for each file • Connect setup and tear down is incurred for each file • Inefficient use of packets • Server must maintain many connections in TIME_WAIT • Mogul and Padmanabahn studied these issues in ’95 • Resulted in HTTP/1.1 specification focused on performance enhancements • Persistent connections • Pipelining • Enhanced caching options • Support for compression CS 640 16

  17. Persistent Connections and Pipelining • Persistent connections • Use the same TCP connection(s) for transfer of multiple files • Reduces packet traffic significantly • May or may not increase performance from client perspective • Load on server increases • Pipelining • Pack as much data into a packet as possible • Requires length field(s) within header • May or may not reduce packet traffic or increase performance • Page structure is critical CS 640 17

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