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HTTP and Electronic Mail Services

HTTP and Electronic Mail Services. Objectives: TCP, UDP, HTTP protocols HTTP clients and servers Electronic mail. TCP/IP As A Protocol Suite. Recall: TCP sender, receiver establish “connection” before exchanging data segments initialize TCP variables: seq. #s

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HTTP and Electronic Mail Services

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  1. HTTP and Electronic Mail Services Objectives: • TCP, UDP, HTTP protocols • HTTP clients and servers • Electronic mail

  2. TCP/IP As A Protocol Suite

  3. Recall:TCP sender, receiver establish “connection” before exchanging data segments initialize TCP variables: seq. #s buffers, flow control info (e.g. RcvWindow) client: connection initiator Socket clientSocket = new Socket("hostname","port number"); server: contacted by client Socket connectionSocket = welcomeSocket.accept(); Three way handshake: Step 1:client host sends TCP SYN segment to server specifies initial seq # no data Step 2:server host receives SYN, replies with SYNACK segment server allocates buffers specifies server initial seq. # Step 3: client receives SYNACK, replies with ACK segment, which may contain data TCP Connection Management

  4. TCP Connection Establishment client server CLOSED Active open; SYN Passive open SYN, seq=x SYN/SYN+ACK LISTEN SYN_SENT SYN+ACK, seq=y, ack=x+1 SYN_RCVD SYN+ACK/ACK ACK, ack=y+1 ACK Established Solid line for client Dashed line for server

  5. TCP Connection Termination client server closing FIN_WAIT1 FIN CLOSE_WAIT ACK LAST_ACK FIN_WAIT2 FIN TIME_WAIT ACK timed wait CLOSED CLOSED

  6. Transport Layer Transport Layer Network Layer Network Layer Multiplexing/Demultiplexing HTTP • Use same communication channel between hosts for several logical communication processes • How does Mux/DeMux work? • Sockets: doors between process & host • UDP socket: (dest. IP, dest. Port) • TCP socket: (src. IP, src. port, dest. IP, dest. Port) FTP Telnet

  7. Connectionless demux • UDP socket identified by two-tuple: • (dest IP address, dest port number) • When host receives UDP segment: • checks destination port number in segment • directs UDP segment to socket with that port number • IP datagrams with different source IP addresses and/or source port numbers are directed to the same socket

  8. TCP socket identified by 4-tuple: source IP address source port number dest IP address dest port number recv host uses all four values to direct segment to appropriate socket Server host may support many simultaneous TCP sockets: each socket identified by its own 4-tuple Web servers have different sockets for each connecting client non-persistent HTTP will have different socket for each request Connection-oriented demux

  9. Example Standardized Services DNS FTP (your Assignment #1) SCP Ping Finger Telnet, SSH SMTP POP IMAP HTTP (your assignment #3)

  10. 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:

  11. 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

  12. Steps For Fetching URLs • The browser makes up an absolute URL if URL is relative • The browser examines the protocol part and the host part • Browser uses DNS to resolve host name to IP address • For HTTP protocol, the browser makes a TCP connection to IP address and port 80 • The port maybe :portno following the hostname in URL • The browser sends a GET request as in: GET /path/somefile.htm HTTP/1.0 • The server sends the file and closes the connection. • Browser renders file. If it is HTML file then any image referenced by some <img> tag, then go back to step 4

  13. Uses 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 is “stateless” server maintains no information about past client requests HTTP overview (continued) Protocols that maintain “state” are complex! • past history (state) must be maintained • if server/client crashes, their views of “state” may be inconsistent, must be reconciled

  14. Nonpersistent HTTP At most one object is sent over a TCP connection. HTTP/1.0 uses nonpersistent HTTP Persistent HTTP Multiple objects can be sent over single TCP connection between client and server. HTTP/1.1 uses persistent connections in default mode HTTP connections

  15. Suppose user enters URL www.someSchool.edu/someDepartment/home.index 1a. HTTP client initiates TCP connection to HTTP server (process) at www.someSchool.edu on port 80 Nonpersistent HTTP (contains text, references to 10 jpeg images) 1b. HTTP server at host www.someSchool.edu waiting for TCP connection at port 80, “accepts” connection and notifies the client 2. HTTP client sends HTTP request message (containing URL) into TCP connection socket. Message indicates that client wants object someDepartment/home.index 3. HTTP server receives request message, forms response message containing requested object, and sends message into its socket time

  16. 5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects Nonpersistent HTTP (cont.) 4. HTTP server closes TCP connection. time 6.Steps 1-5 repeated for each of 10 jpeg objects

  17. initiate TCP connection RTT request file time to transmit file RTT file received time time Response time modeling Definition of RTT: time to send a small packet to travel from client to server and back. Response time: • one RTT to initiate TCP connection • one RTT for HTTP request and first few bytes of HTTP response to return • file transmission time total = 2RTT+transmit time

  18. Nonpersistent HTTP issues: requires 2 RTTs per object OS must work and allocate host resources for each TCP connection but browsers often open parallel TCP connections to fetch referenced objects Persistent HTTP server leaves connection open after sending response subsequent HTTP messages between same client/server are sent over the same connection Persistent without pipelining: client issues new request only when previous response has been received one RTT for each referenced object Persistent with pipelining: default in HTTP/1.1 client sends requests as soon as it encounters a referenced object as little as one RTT for all the referenced objects Persistent HTTP

  19. 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

  20. Many major Web sites use cookies Four components: 1) cookie header line in the HTTP response message 2) cookie header line in HTTP request message 3) cookie file kept on user’s host and managed by user’s browser 4) back-end database at Web site User-server state: cookies

  21. 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:

  22. What cookies can bring: authorization shopping carts recommendations one-click shopping 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

  23. Goal: don’t send object if cache has up-to-date cached version cache: specify date of cached copy in HTTP request If-modified-since: <date> server: response contains no object if cached copy is up-to-date: HTTP/1.0 304 Not Modified HTTP response HTTP/1.0 304 Not Modified Conditional GET server cache HTTP request msg If-modified-since: <date> object not modified HTTP request msg If-modified-since: <date> object modified HTTP response HTTP/1.0 200 OK <data>

  24. Three major components: user agents mail servers simple mail transfer protocol: SMTP User Agent a.k.a. “mail reader” composing, editing, reading mail messages e.g., Eudora, Outlook, elm, Netscape Messenger, Thunderbird outgoing, incoming messages stored on server user agent user agent user agent user agent user agent user agent SMTP SMTP SMTP mail server mail server mail server outgoing message queue user mailbox Electronic Mail

  25. Email using SMTP/IMAP/POP • Email consists of two components • Simple Mail Transfer Protocol (SMTP) for email clients to send out emails (e.g. smtp.nd.edu, port 25) • Internet Message Access Protocol (IMAP) or Post Office Protocol (POP) • email clients to receive your emails (e.g. imap.nd.edu, port 143) • You can use telnet to “talk” to these servers directly • E.g. type ‘telnet smtp.nd.edu 25’ and then type ‘help’

  26. Email using SMTP/IMAP/POP • For SMTP, your client (say Outlook), connects to smtp.nd.edu and then delivers an email destined for friend@aol.com • Smtp.nd.edu then locates the SMTP server responsible for AOL. These servers may delegate to other SMTP servers. Eventually it reaches friend@aol.com • Friend will use IMAP or POP to retrieve this email

  27. uses TCP to reliably transfer email message from client to server, port 25 direct transfer: sending server to receiving server no middle server in-between! three phases of transfer handshaking (greeting) transfer of messages closure command/response interaction commands: ASCII text response: status code and phrase messages must be in 7-bit ASCII (mostly) Electronic Mail: SMTP [RFC 2821]

  28. SMTP: protocol for exchanging email messages RFC 2822: standard for text message format: header lines, e.g., To: From: Subject: differentfrom SMTP commands! body the “message”, ASCII characters only (mostly) Mail message format header blank line body

  29. MIME: multimedia mail extension, RFC 2045, 2056 additional lines in msg header declare MIME content type From: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data ..... ......................... ......base64 encoded data Message format: multimedia extensions MIME version method used to encode data multimedia data type, subtype, parameter declaration encoded data

  30. SMTP: delivery/storage to receiver’s server Mail access protocol: retrieval from server POP: Post Office Protocol [RFC 1939] authorization (agent <-->server) and download IMAP: Internet Mail Access Protocol [RFC 1730] more features (more complex) manipulation of stored msgs on server HTTP: Hotmail , Yahoo! Mail, etc. user agent user agent sender’s mail server SMTP Mail access protocols SMTP access protocol receiver’s mail server

  31. authorization phase client commands: user: declare username pass: password server responses +OK -ERR transaction phase, client: list: list message numbers retr: retrieve message by number dele: delete quit POP3 protocol S: +OK POP3 server ready C: user bob S: +OK C: pass hungry S: +OK user successfully logged on C: list S: 1 498 S: 2 912 S: . C: retr 1 S: <message 1 contents> S: . C: dele 1 C: retr 2 S: <message 1 contents> S: . C: dele 2 C: quit S: +OK POP3 server signing off

  32. Email

  33. SMTP uses persistent connections SMTP requires message (header & body) to be in 7-bit ASCII SMTP server uses CRLF.CRLF to determine end of message Comparison with HTTP: HTTP: pull SMTP: push both have ASCII command/response interaction, status codes HTTP: each object encapsulated in its own response msg SMTP: multiple objects sent in multipart msg SMTP versus HTTP

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