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					   Computer Networks
                                           Lecture 3: application layer


                            Prof. Younghee Lee




* Some part of this teaching materials are prepared referencing the
  lecture note made by F. Kurose, Keith W. Ross(U. of Massachusetts)



                                                                   1   1
                               Prof. Younghee Lee
Some network apps
   E-mail                          Internet telephone
   Web                             Real-time video
   Instant messaging                conference
   Remote login                    Massive parallel
   P2P file sharing                 computing
   Multi-user network               IPTV
    games                            VoIP
   Streaming stored video       
    clips



                                                          2   2
                        Prof. Younghee Lee
Application architectures
 Client-server
 Peer-to-peer (P2P)
 Hybrid of client-server and P2P




                                         3   3
                    Prof. Younghee Lee
Hybrid of client-server and P2P
Napster
  – File transfer P2P
  – File search centralized:
     » Peers register content at central server
     » Peers query same central server to locate content

Instant messaging
  – Chatting between two users is P2P
  – Presence detection/location centralized:
     » User registers its IP address with central server when it comes
       online
     » User contacts central server to find IP addresses of buddies


                                                                    4    4
                            Prof. Younghee Lee
 Processes communicating

Process: program running         Client process: process
  within a host.                    that initiates
 within same host, two
                                    communication
  processes communicate          Server process: process
  using inter-process               that waits to be
  communication (defined            contacted
  by OS).
 processes in different         Note: applications with
  hosts communicate by            P2P architectures have
  exchanging messages             client processes & server
                                  processes

                                                       5      5
                      Prof. Younghee Lee
Addressing processes
   For a process to receive         Identifier includes both
    messages, it must have            the IP address and port
    an identifier                     numbers associated
   A host has a unique32-            with the process on the
    bit IP address                    host.
   Q: does the IP address
                                     Example port numbers:
    of the host on which the
    process runs suffice for           – HTTP server: 80
    identifying the process?           – Mail server: 25
   Answer: No, many                 More on this later
    processes can be
    running on same host


                                                           6     6
                         Prof. Younghee Lee
App-layer protocol defines
   Types of messages
    exchanged, e.g., request       Public-domain protocols:
    & response messages             defined in RFCs
   Syntax of message types:  allows for
    what fields in messages &
    how fields are delineated        interoperability
   Semantics of the fields, i.e.,  e.g., HTTP, SMTP
    meaning of information in Proprietary protocols:
    fields                          e.g., KaZaA
   Rules for when and how
    processes send & respond
    to messages


                                                         7    7
                          Prof. Younghee Lee
    Transport service requirements of common apps



         Application      Data loss       Bandwidth            Time Sensitive

          file transfer   no loss         elastic              no
                 e-mail   no loss         elastic              no
     Web documents        no loss         elastic              no
real-time audio/video     loss-tolerant   audio: 5kbps-1Mbps   yes, 100’s msec
                                          video:10kbps-5Mbps
  stored audio/video      loss-tolerant   same as above        yes, few secs
   interactive games      loss-tolerant   few kbps up          yes, 100’s msec
  instant messaging       no loss         elastic              yes and no

            TCP?     UDP?



                                                                       8     8
                                    Prof. Younghee Lee
Internet transport protocols services

TCP service:
   connection-oriented: setup     UDP service:
    required between client and     unreliable data transfer
    server processes                 between sending and
   reliable transport between       receiving process
    sending and receiving process  does not provide:
   flow control: sender won’t       connection setup, reliability,
    overwhelm receiver               flow control, congestion
                                     control, timing, or
   congestion control: throttle
                                     bandwidth guarantee
    sender when network overloaded
   does not provide: timing,
    minimum bandwidth guarantees Q: why bother? Why is there a
                                     UDP?


                                                                9     9
                             Prof. Younghee Lee
    Internet apps: application, transport protocols


                            Application               Underlying
           Application      layer protocol            transport protocol

                   e-mail   SMTP [RFC 2821]           TCP
remote terminal access      Telnet [RFC 854]          TCP
                    Web     HTTP [RFC 2616]           TCP
            file transfer   FTP [RFC 959]             TCP
  streaming multimedia      proprietary               TCP or UDP
                            (e.g. RealNetworks)
     Internet telephony     proprietary
                            (e.g., Vonage,Dialpad)    typically UDP




                                                                      10   10
                                 Prof. Younghee Lee
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:


    www.someschool.edu/someDept/pic.gif

          host name                    path name
                                                   11   11
                      Prof. Younghee Lee
HTTP overview
HTTP: hypertext transfer
  protocol
   Web’s application layer
    protocol
                                     PC running
   client/server model               Explorer
     – client: browser that
        requests, receives,
        “displays” Web objects
     – server: Web server sends                        Server
        objects in response to                         running
        requests                                     Apache Web
                                                       server
   HTTP 1.0: RFC 1945
   HTTP 1.1: RFC 2068
                                       Mac running
                                        Navigator


                                                        12   12
                            Prof. Younghee Lee
HTTP overview (continued)
Uses TCP:                                    HTTP is “stateless”
   client initiates TCP connection               server maintains no
    (creates socket) to server, port               information about past
                                                   client requests
    80
   server accepts TCP connection
                                       Protocols that maintain “state”
    from client
                                          are complex!           aside
   HTTP messages (application-
                                        past history (state) must be
    layer protocol messages)
                                          maintained
    exchanged between browser
    (HTTP client) and Web server        if server/client crashes, their

    (HTTP server)                         views of “state” may be
                                          inconsistent, must be
   TCP connection closed
                                          reconciled
                                            Soft State


                                                                      13    13
                              Prof. Younghee Lee
HTTP connections
Nonpersistent HTTP                 Persistent HTTP
 At most one object is             Multiple objects can be
  sent over a TCP                    sent over single TCP
  connection.                        connection between
 HTTP/1.0 uses                      client and server.
  nonpersistent HTTP                HTTP/1.1 uses persistent
                                     connections in default
                                     mode




                                                        14      14
                          Prof. Younghee Lee
    Nonpersistent HTTP
Suppose user enters URL
  www.someSchool.edu/someDepartment/home.index
                                                (contains text,
     1a. HTTP client initiates TCP             references to 10
        connection to HTTP server                                 jpeg images)
        (process) at www.someSchool.edu
        on port 80                      1b. HTTP server at host
                                            www.someSchool.edu waiting
                                            for TCP connection at port 80.
                                            “accepts” connection, notifying
                                            client
    2. HTTP client sends HTTP
       request message (containing
       URL) into TCP connection         3. HTTP server receives request
       socket. Message indicates that      message, forms response
       client wants object                 message containing requested
       someDepartment/home.index           object, and sends message into
                                           its socket

 time
                                                                      15    15
                                 Prof. Younghee Lee
    Nonpersistent HTTP (cont.)

                                                4. HTTP server closes TCP
         5. HTTP client receives response          connection.
            message containing html file,
            displays html. Parsing html file,
            finds 10 referenced jpeg objects


time 6. Steps 1-5 repeated for each of
           10 jpeg objects




                                                                        16   16
                                   Prof. Younghee Lee
Response time modeling
Definition of RTT: time to send a
   small packet to travel from
   client to server and back.
Response time:                      initiate TCP
 one RTT to initiate TCP           connection
   connection                                RTT
 one RTT for HTTP request and           request
   first few bytes of HTTP               file
                                                             time to
   response to return                        RTT
                                                             transmit
 file transmission time                                     file
                                           file
total = 2RTT+transmit time                 received


                                                   time   time



                                                             17     17
                            Prof. Younghee Lee
   HTTP request message
     two types of HTTP messages: request, response
     HTTP request message:
      – ASCII (human-readable format)


  request line
 (GET, POST,         GET /somedir/page.html HTTP/1.1
HEAD commands)       Host: www.someschool.edu
                     User-agent: Mozilla/4.0
             header Connection: close
               lines Accept-language:fr

 Carriage return,
     line feed       (extra carriage return, line feed)
  indicates end
    of message                                            18   18
                              Prof. Younghee Lee
HTTP request message: general format




                                       19   19
                 Prof. Younghee Lee
  Trying out HTTP (client side) for yourself


1. Telnet to your favorite Web server:
  telnet cis.poly.edu 80       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


2. Type in a GET HTTP request:
       GET /~ross/ HTTP/1.1             By typing this in (hit carriage
       Host: cis.poly.edu               return twice), you send
                                        this minimal (but complete)
                                        GET request to HTTP server

3. Look at response message sent by HTTP server!

                                                                      20     20
                              Prof. Younghee Lee
Let’s look at HTTP in action
 telnet example
 Ethereal example




                                          21   21
                     Prof. Younghee Lee
User-server state: cookies
Many major Web sites use            Example:
  cookies                                – Susan access Internet
Four components:                           always from same PC
                                         – She visits a specific e-
   1) cookie header line of
                                           commerce site for first time
      HTTP response message
                                         – When initial HTTP
   2) cookie header line in
                                           requests arrives at site,
      HTTP request message
                                           site creates a unique ID
   3) cookie file kept on user’s           and creates an entry in
      host, managed by user’s              backend database for ID
      browser
   4) back-end database at Web
      site



                                                                22        22
                           Prof. Younghee Lee
    Cookies: keeping “state” (cont.)

                   client                             server
     Cookie file        usual http request msg           server
                        usual http response +          creates ID
  ebay: 8734            Set-cookie: 1678              1678 for user

     Cookie file
                        usual http request msg
 amazon: 1678               cookie: 1678                cookie-
 ebay: 8734                                             specific
                       usual http response msg           action
one week later:
                        usual http request msg
     Cookie file                                         cookie-
                            cookie: 1678
  amazon: 1678                                          spectific
  ebay: 8734           usual http response msg            action

                                                                      23   23
                                 Prof. Younghee Lee
 Cookies (continued)
                                                   aside
What cookies can bring:           Cookies and privacy:
 authorization                    cookies permit sites to

 shopping carts
                                    learn a lot about you
                                   you may supply name
 recommendations
                                    and e-mail to sites
 user session state (Web
                                   search engines use
  e-mail)
                                    redirection & cookies to
                                    learn yet more
                                   advertising companies
                                    obtain info across sites


                                                       24      24
                       Prof. Younghee Lee
 Web caches (proxy server)
Goal: satisfy client request without involving origin server
   user sets browser: Web
    accesses via cache                                         origin
                                                               server
   browser sends all HTTP
    requests to cache                               Proxy
    – object in cache: cache                        server
      returns object             client

    – else cache requests object
      from origin server, then
      returns object to client



                                   client
                                                               origin
                                                               server

                                                               25       25
                               Prof. Younghee Lee
More about Web caching
   Cache acts as both client and       Why Web caching?
    server                                 Reduce response time for
   Typically cache is installed by         client request.
    ISP (university, company,              Reduce traffic on an
    residential ISP)                        institution’s access link.
                                           Internet dense with caches
                                            enables “poor” content
                                            providers to effectively deliver
                                            content (but so does P2P file
                                            sharing)




                                                                      26       26
                               Prof. Younghee Lee
FTP: the file transfer protocol

                    FTP                   file transfer
                            FTP                            FTP
                    user   client                         server
                 interface
      user
     at host                 local file                            remote file
                             system                                system


    transfer file to/from remote host
    client/server model
       – client: side that initiates transfer (either to/from remote)
       – server: remote host
    ftp: RFC 959
    ftp server: port 21

                                                                          27     27
                             Prof. Younghee Lee
FTP: separate control, data connections
   FTP client contacts FTP server             TCP control connection
    at port 21, specifying TCP as                       port 21
    transport protocol
   Client obtains authorization over
    control connection                          TCP data connection
                                         FTP          port 20         FTP
   Client browses remote directory client                           server
    by sending commands over
    control connection.
                                         Server opens a second TCP
   When server receives a                data connection to transfer
    command for a file transfer, the
                                          another file.
    server opens a TCP data
    connection to client                 Control connection: “out of

   After transferring one file, server   band”
    closes connection.                   FTP server maintains “state”:
                                          current directory, earlier
                                          authentication
                                                                    28    28
                               Prof. Younghee Lee
FTP commands, responses
Sample commands:
   sent as ASCII text over control
                                           Sample return codes
    channel                                   status code and phrase (as in
                                               HTTP)
   USER username
                                              331 Username OK,
   PASS password                              password required
   LIST return list of file in               125 data connection
    current directory                          already open; transfer
                                               starting
   RETR filename retrieves                   425 Can’t open data
    (gets) file                                connection
   STOR filename stores (puts)               452 Error writing file
    file onto remote host



                                                                      29       29
                                  Prof. Younghee Lee
     Electronic Mail                                                           outgoing
                                                                          message queue

Three major components:                                                     user mailbox
   user agents                                                 user
   mail servers                                               agent
   simple mail transfer protocol: SMTP              mail
                                                                                  user
                                                    server
Why user agent and mail                                                          agent
 server? ; instead of just mail                                 SMTP     mail
    application over end host                                           server      user
User Agent                                  SMTP                                   agent
 a.k.a. “mail reader”
 composing, editing, reading mail messages                     SMTP
 e.g., Eudora, Outlook, elm, Netscape         mail                               user
   Messenger                                  server                             agent
 outgoing, incoming messages stored on
   server
                                                                 user
Directory?                                                      agent
 DNS: name to ip address, ip address to name          user
 LDAP: white page, yellow page                       agent
                                                                                   30      30
                                          Prof. Younghee Lee
    Electronic Mail: mail servers
Mail Servers
                                                     user
   mailbox contains incoming                       agent
    messages for user                       mail
                                                                       user
   message queue of outgoing (to          server
                                                                      agent
    be sent) mail messages
                                                     SMTP     mail
   SMTP protocol between mail
                                                             server         user
    servers to send email
    messages                          SMTP                                 agent

     – client: sending mail server                   SMTP
                                                                       user
     – “server”: receiving mail             mail
                                                                      agent
       server                              server

                                                      user
                                                     agent
                                             user
                                            agent
                                                                      31      31
                             Prof. Younghee Lee
Electronic Mail: SMTP [RFC 2821]
   uses TCP to reliably transfer email message from
    client to server, port 25
   direct transfer: sending server to receiving server
   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

                                                          32   32
                          Prof. Younghee Lee
Scenario: Alice sends message to Bob
1) Alice uses UA to compose          4) SMTP client sends Alice’s
   message and “to”                     message over the TCP
   bob@someschool.edu                   connection
2) Alice’s UA sends message to       5) Bob’s mail server places the
   her mail server; message             message in Bob’s mailbox
   placed in message queue           6) Bob invokes his user agent to
3) Client side of SMTP opens            read message
   TCP connection with Bob’s
   mail server



       1                                   mail
                    mail
                                          server          user
       user        server
              2                                          agent
      agent          3                             6
                                 4           5

                                                                  33    33
                              Prof. Younghee Lee
Sample SMTP interaction
S:   220 hamburger.edu
C:   HELO crepes.fr
S:   250 Hello crepes.fr, pleased to meet you
C:   MAIL FROM: <alice@crepes.fr>
S:   250 alice@crepes.fr... Sender ok
C:   RCPT TO: <bob@hamburger.edu>
S:   250 bob@hamburger.edu ... 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 hamburger.edu closing connection

                                                34      34
                      Prof. Younghee Lee
Try SMTP interaction for yourself:
   telnet servername 25
 see 220 reply from server
 enter HELO, MAIL FROM, RCPT TO, DATA, QUIT
  commands
above lets you send email without using email client
  (reader)




                                                       35   35
                       Prof. Younghee Lee
SMTP: final words
   SMTP uses persistent               Comparison with HTTP:
    connections
                                          HTTP: pull
   SMTP requires message
    (header & body) to be in 7-bit        SMTP: push
    ASCII                                 both have ASCII
   SMTP server uses                       command/response interaction,
    CRLF.CRLF to determine end             status codes
    of message
                                          HTTP: each object
                                           encapsulated in its own
                                           response msg
                                          SMTP: multiple objects sent in
                                           multipart msg



                                                                   36       36
                              Prof. Younghee Lee
Mail message format
SMTP: protocol for exchanging
  email msgs
RFC 822: standard for text                        header
                                                                blank
  message format:
                                                                 line
 header lines, e.g.,
    – To:
    – From:                                       body
    – Subject:
    different from SMTP commands!
   body
    – the “message”, ASCII
      characters only




                                                           37      37
                             Prof. Younghee Lee
       Message format: multimedia extensions

      MIME: multimedia mail extension, RFC 2045, 2056
      additional lines in msg header declare MIME content type



                                  From: alice@crepes.fr
        MIME version              To: bob@hamburger.edu
                                  Subject: Picture of yummy crepe.
         method used              MIME-Version: 1.0
       to encode data             Content-Transfer-Encoding: base64
                                  Content-Type: image/jpeg
     multimedia data
      type, subtype,              base64 encoded data .....
parameter declaration             .........................
                                  ......base64 encoded data
         encoded data

                                                                  38   38
                                 Prof. Younghee Lee
Mail access protocols
                 SMTP          SMTP                    access     user
          user
         agent                                        protocol   agent


                sender’s mail     receiver’s mail
                    server             server
 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.




                                                                         39   39
                                 Prof. Younghee Lee
    POP3 protocol                        S:   +OK POP3 server ready
                                         C:   user bob
authorization phase                      S:   +OK
                                         C:   pass hungry
   client commands:                     S:   +OK user successfully logged   on
     – user: declare username
     – pass: password                    C:   list
                                         S:   1 498
   server responses                     S:   2 912
     – +OK                               S:   .
     – -ERR                              C:   retr 1
                                         S:   <message 1 contents>
transaction phase, client:               S:   .
   list: list message numbers           C:   dele 1
   retr: retrieve message by            C:   retr 2
    number                               S:   <message 1 contents>
   dele: delete                         S:   .
   quit                                 C:   dele 2
                                         C:   quit
                                         S:   +OK POP3 server signing off
                                                                    40       40
                             Prof. Younghee Lee

				
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