Chapter_2_V6.3 copy.pptx

Chapter 2 Application Layer Computer Networking: A Top Down Approach 6th edition Jim Kurose, Keith Ross Addison-Wesley March 2012 Application Layer 2-1 Chapter 2: outline 2.1 principles of network applications 2.2 Web and HTTP 2.3 FTP 2.4 electronic mail 2.6 P2P applications  SMTP, POP3, IMAP 2.5 D...

Chapter 2 Application Layer Computer Networking: A Top Down Approach 6th edition Jim Kurose, Keith Ross Addison-Wesley March 2012 Application Layer 2-1 Chapter 2: outline 2.1 principles of network applications 2.2 Web and HTTP 2.3 FTP 2.4 electronic mail 2.6 P2P applications  SMTP, POP3, IMAP 2.5 DNS Application Layer 2-2 Chapter 2: application layer our goals:  conceptual, implementation aspects of network application protocols  transport-layer service models  client-server paradigm  peer-to-peer paradigm  learn about protocols by examining popular application-level protocols      HTTP FTP SMTP / POP3 / IMAP DNS creating network applications  socket API Application Layer 2-3 Some network apps        e-mail web text messaging remote login P2P file sharing multi-user network games streaming stored video (YouTube, Hulu, Netflix)       voice over IP (e.g., Skype) real-time video conferencing social networking search … … Application Layer 2-4 Creating a network app write programs that:  run on (different) end systems  communicate over network  e.g., web server software communicates with browser software no need to write software for network-core devices  network-core devices do not run user applications  applications on end systems allows for application transport network data link physical application transport network data link physical application transport network data link physical Application Layer 2-5 Application architectures possible structure of applications:  client-server  peer-to-peer (P2P) Application Layer 2-6 Client-server architecture server:    always-on host permanent IP address data centers for scaling clients:  client/server    communicate with server may be intermittently connected may have dynamic IP addresses do not communicate directly with each other Application Layer 2-7 P2P architecture     no always-on server arbitrary end systems directly communicate peers request service from other peers, provide service in return to other peers  self scalability – new peers bring new service capacity, as well as new service demands peers are intermittently connected and change IP addresses peer-peer Application Layer 2-8 Processes communicating process: program running within a host   within same host, two processes communicate using inter-process communication (defined by OS) processes in different hosts communicate by exchanging messages clients, servers client process: process that initiates communication server process: process that waits to be contacted Application Layer 2-9 Sockets   process sends/receives messages to/from its socket socket analogous to door  sending process shoves message out door  sending process relies on transport infrastructure on other side of door to deliver message to socket at receiving process application process socket application process transport transport network network link physical Internet link controlled by app developer controlled by OS physical Application Layer 2-10 Addressing processes    to receive messages, process must have identifier host device has unique 32-bit IP address Q: does IP address of host on which process runs  A:suffice for no, many identifying the processes can be process? running on same host   identifier includes both IP address and port numbers associated with process on host. example port numbers:  HTTP server: 80  mail server: 25  to send HTTP message to gaia.cs.umass.edu web server:  IP address: 128.119.245.12  port number: 80  more shortly… Application Layer 2-11 App-layer protocol defines     types of messages exchanged,  e.g., request, response message syntax:  what fields in messages & how fields are delineated message semantics  meaning of information in fields rules for when and how processes send & respond to messages open protocols:  defined in RFCs  allows for interoperability  e.g., HTTP, SMTP proprietary protocols:  e.g., Skype Application Layer 2-12 What transport service does an app need? data integrity  some apps (e.g., file transfer, web transactions) require 100% reliable data transfer  other apps (e.g., audio) can tolerate some loss timing  some apps (e.g., Internet telephony, interactive games) require low delay to be “effective” throughput  some apps (e.g., multimedia) require minimum amount of throughput to be “effective”  other apps (“elastic apps”) make use of whatever throughput they get security  encryption, data integrity, … Application Layer 2-13 Transport service requirements: common apps application data loss throughput file transfer e-mail Web documents real-time audio/video no loss no loss no loss loss-tolerant stored audio/video interactive games text messaging loss-tolerant loss-tolerant no loss elastic no elastic no elastic no audio: 5kbps-1Mbps yes, 100’s msec video:10kbps-5Mbps same as above yes, few secs few kbps up yes, 100’s msec elastic yes and no time sensitive Application Layer 2-14 Internet transport protocols services TCP service: UDP service:       reliable transport between sending and receiving process flow control: sender won’t overwhelm receiver congestion control: throttle sender when network overloaded does not provide: timing, minimum throughput guarantee, security connection-oriented: setup required between client and server processes  unreliable data transfer between sending and receiving process does not provide: reliability, flow control, congestion control, throughput guarantee, security, or connection setup, Application Layer 2-15 Internet apps: application, transport protocols application e-mail remote terminal access Web file transfer streaming multimedia Internet telephony application layer protocol underlying transport protocol SMTP Telnet HTTP FTP HTTP (e.g., YouTube), RTP SIP, RTP, proprietary (e.g., Skype) TCP TCP TCP TCP TCP or UDP TCP or UDP Application Layer 2-16 Chapter 2: outline 2.1 principles of network applications 2.6 P2P applications  app architectures  app requirements 2.2 Web and HTTP 2.3 FTP 2.4 electronic mail  SMTP, POP3, IMAP 2.5 DNS Application Layer 2-17 Web and HTTP First, a review…     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, www.someschool.edu/someDept/pic.gif e.g., host name path name Application Layer 2-18 HTTP overview HTTP: hypertext transfer protocol   Web’s application layer protocol client/server model  client: browser that requests, receives, (using HTTP protocol) and “displays” Web objects  server: Web server sends (using HTTP protocol) objects in response to requests HT TP req ues t HT TP res pon se PC running Firefox browser t es u req server se n running po s e r Apache Web P T server HT TP T H iphone running Safari browser Application Layer 2-19 HTTP overview (continued) 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 Application Layer 2-20 HTTP connections non-persistent HTTP persistent HTTP  at most one object sent  multiple over TCP objects connection can be sent over single TCP  connection then closed connection  downloading multiple objects required multiple connections between client, server Application Layer 2-21 Non-persistent HTTP 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 2. HTTP client sends HTTP request message (containing URL) into TCP connection socket. Message indicates that client wants object someDepartment/home.i time ndex (contains text, references to 10 jpeg images) 1b. HTTP server at host www.someSchool.edu waiting for TCP connection at port 80. “accepts” connection, notifying client 3. HTTP server receives request message, forms response message containing requested object, and sends message into its socket Application Layer 2-22 Non-persistent HTTP (cont.) 5. HTTP client receives time 4. HTTP server closes TCP connection. response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects 6. Steps 1-5 repeated for each of 10 jpeg objects Application Layer 2-23 Non-persistent HTTP: response time RTT (definition): time for a small packet to travel from client to server and back HTTP 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  non-persistent HTTP response time = 2RTT+ file transmission time initiate TCP connection RTT request file time to transmit file RTT file received time time Application Layer 2-24 Persistent HTTP non-persistent HTTP issues:    requires 2 RTTs per object OS overhead for each TCP connection 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 sent over open connection client sends requests as soon as it encounters a referenced object as little as one RTT Application Layer 2-25 for all the referenced HTTP request message   two types of HTTP messages: request, response HTTP request message:  ASCII (human-readable format) carriage return character line-feed character request line (GET, POST, GET /index.html HTTP/1.1\r\n HEAD commands) Host: www-net.cs.umass.edu\r\n User-Agent: Firefox/3.6.10\r\n Accept: text/html,application/xhtml+xml\r\n headerAccept-Language: en-us,en;q=0.5\r\n linesAccept-Encoding: gzip,deflate\r\n Accept-Charset: ISO-8859-1,utf-8;q=0.7\r\n Keep-Alive: 115\r\n carriage return, Connection: keep-alive\r\n line feed at start \r\n of line indicates end of header lines Application Layer 2-26 HTTP request message: general format method sp URL header field name sp value version cr cr value cr request line header lines ~ ~ header field name lf lf ~ ~ ~ ~ cr lf lf entity body ~ ~ body Application Layer 2-27 Uploading form input POST method:   web page often includes form input input is uploaded to server in entity body URL method:   uses GET method input is uploaded in URL field of request line: www.somesite.com/animalsearch?monkeys&banana Application Layer 2-28 Method types HTTP/1.0:    GET POST HEAD  asks server to leave requested object out of response HTTP/1.1:    GET, POST, HEAD PUT  uploads file in entity body to path specified in URL field DELETE  deletes file specified in the URL field Application Layer 2-29 HTTP response status codes status code appears in 1st line in server-toclient response message.  some sample codes:  200 OK  request succeeded, requested object later in this msg 301 Moved Permanently  requested object moved, new location specified later in this msg (Location:) 400 Bad Request  request msg not understood by server 404 Not Found  requested document not found on this server 505 HTTP Version Not Supported Application Layer 2-30 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 Host: cis.poly.edu by typing this in (hit carriage return twice), you send this minimal (but complete) GET request to HTTP server 3. look at response message sent by HTTP server! use Wireshark to look at captured HTTP request/response) Application Layer 2-31 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 HT TP proxy server req ues t res pon se est u req e P ns T o T p H es r TP T H H client TTP client st e u req P T se n o HT p origin res P T server HT origin server Application Layer 2-32 More about Web caching  cache acts as both client and server  server for original requesting client  client to origin server  typically cache is installed by ISP (university, company, residential ISP) 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 (so too Application Layer 2-33 Chapter 2: outline 2.1 principles of network applications 2.6 P2P applications  app architectures  app requirements 2.2 Web and HTTP 2.3 FTP 2.4 electronic mail  SMTP, POP3, IMAP 2.5 DNS Application Layer 2-34 Electronic mail outgoing message queue user mailbox 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., Outlook, Thunderbird, iPhone mail client user agent mail server user agent SMTP mail server user agent SMTP SMTP mail server user agent user agent user agent Application Layer 2-35 Electronic mail: mail servers mail servers:    mailbox contains incoming messages for user message queue of outgoing (to be sent) mail messages SMTP protocol between mail servers to send email messages  client: sending mail server  “server”: receiving mail server user agent mail server user agent SMTP mail server user agent SMTP SMTP mail server user agent user agent user agent Application Layer 2-36 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 interaction (like HTTP, FTP)  commands: ASCII text Application Layer 2-37 Scenario: Alice sends message to Bob 4) SMTP client sends Alice’s message over the TCP connection 5) Bob’s mail server places the message in Bob’s mailbox 6) Bob invokes his user agent to read message 1) Alice uses UA to compose message “to” [email protected] 2) Alice’s UA sends message to her mail server; message placed in message queue 3) client side of SMTP opens TCP connection with Bob’s mail server 1 user agent 2 mail server 3 Alice’s mail server user agent mail server 4 6 5 Bob’s mail server Application Layer 2-38 Mail message format SMTP: protocol for exchanging email msgs  header lines, e.g.,  To:  From:  Subject:  header blank line body Body: the “message”  ASCII characters only Application Layer 2-39 Mail access protocols user agent SMTP SMTP mail access protocol user agent (e.g., POP, IMAP) sender’s mail server   receiver’s mail server SMTP: delivery/storage to receiver’s server mail access protocol: retrieval from server  POP: Post Office Protocol  IMAP: Internet Mail Access Protocol  HTTP: gmail, Hotmail, Yahoo! Mail, etc. Application Layer 2-40 Chapter 2: outline 2.1 principles of network applications 2.6 P2P applications  app architectures  app requirements 2.2 Web and HTTP 2.3 FTP 2.4 electronic mail  SMTP, POP3, IMAP 2.5 DNS Application Layer 2-41 DNS: domain name system people: many identifiers:  SSN, name, passport # Internet hosts, routers:  IP address (32 bit) - used for addressing datagrams  “name”, e.g., www.yahoo.com used by humans Q: how to map between IP address and name, and vice versa ? Domain Name System:   distributed database implemented in hierarchy of many name servers application-layer protocol: hosts, name servers communicate to resolve names (address/name translation) Application Layer 2-42 EXAMPLE: Request URL www.someschool.edu        1. The same user machine runs the client side of the DNS application. 2. The browser extracts the hostname, www.someschool.edu, from the URL and passes the hostname to the client side of the DNS application. 3. The DNS client sends a query containing the hostname to a DNS server. 4. The DNS client eventually receives a reply, which includes the IP address for the hostname. 5. Once the browser receives the IP address from DNS, it can initiate a TCP connection to the HTTP server process located at port 80 at that IP address. Application Layer 2-43 DNS: a distributed, hierarchical database Root DNS Servers … com DNS servers yahoo.com amazon.com DNS servers DNS servers … org DNS servers pbs.org DNS servers edu DNS servers poly.edu umass.edu DNS serversDNS servers client wants IP for www.amazon.com; 1st approx:    client queries 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 www.amazon.com Application Layer 2-44