1. Chapter 2: Application layer Adopted from textbook’s slides 2: Application Layer 1

2. 2 Chapter 2: Application layer r 2.1 Principles of network applications r 2.2 Web and HTTP  Lab assignment r 2.3 FTP r Online gaming r 2.4 Electronic Mail  SMTP (simple mail transfer protocol)  POP3, IMAP  Lab assignment r 2.5 DNS (domain name service) r 2.6 P2P file sharing r 2.7 Socket programming with TCP  Introduce c sock program  Programming assignment  Socket programming with UDP r VOIP basic principle

3. 2: Application Layer 3 Chapter 2: Application Layer Our goals: r conceptual, implementation aspects of network application protocols  transport-layer service models  client-server paradigm  peer-to-peer paradigm r learn about protocols by examining popular application-level protocols  HTTP  FTP  SMTP / POP3 / IMAP  DNS  VOIP r programming network applications  socket API

4. 2: Application Layer 4 Some network apps r E-mail r Web r Instant messaging r P2P file sharing r Multi-user network games r streaming stored video (YouTube, Hulu, Netflix) r Internet telephone (skype) r Real-time video conference r Massive parallel computing  Grid computing r social networking r search r ……

5. 2: Application Layer 5 Creating a network app Write programs that  run on different end systems and  communicate over a network.  e.g., Web: Web server software communicates with browser software No software written for devices in network core  Network core devices do not function at app layer  This design allows for rapid app development application transport network data link physical application transport network data link physical application transport network data link physical

6. 2: Application Layer 6 Chapter 2: Application layer r 2.1 Principles of network applications r 2.2 Web and HTTP r 2.3 FTP r Online gaming r 2.4 Electronic Mail  SMTP,  POP3, IMAP r 2.5 DNS r 2.6 P2P file sharing r 2.7 Socket programming with TCP  Introduce c sock program  Programming assignment  Socket programming with UDP r VOIP basic principle

7. 2: Application Layer 7 Application architectures r Client-server r Peer-to-peer (P2P) r Hybrid of client-server and P2P

8. 2: Application Layer 8 Client-server architecture server:  always-on host  permanent IP address  server farms for scaling clients:  communicate with server  may be intermittently connected  may have dynamic IP addresses  do not communicate directly with each other client/server

9. Application Layer2-9 P2P architecture r no always-on server r arbitrary end systems directly communicate r 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 r peers are intermittently connected and change IP addresses  complex management peer-peer

10. 2: Application Layer 10 Processes communicating Process: program running within a host. r within same host, two processes communicate using inter-process communication (defined by OS). r processes in different hosts communicate by exchanging messages Client process: process that initiates communication Server process: process that waits to be contacted r Note: applications with P2P architectures have both client processes & server processes

11. 2: Application Layer 11 Addressing processes r For a process to receive messages, it must have an identifier r A host has a unique 32-bit IP address r Q: does the IP address of the host on which the process runs suffice for identifying the process? r Answer: No, many processes can be running on same host r Identifier includes both the IP address and port numbers associated with the process on the host. r Example port numbers:  HTTP server: 80  Mail server: 25  SSH server: 22 r More on this later

12. Application Layer2- 12 App-layer protocol defines r types of messages exchanged,  e.g., request, response r message syntax:  what fields in messages & how fields are delineated r message semantics  meaning of information in fields r rules for when and how processes send & respond to messages open protocols: r defined in RFCs r allows for interoperability r e.g., HTTP, SMTP proprietary protocols: r e.g., Skype

13. Application Layer2- 13 What transport service does an app need? data integrity r some apps (e.g., file transfer, web transactions) require 100% reliable data transfer r other apps (e.g., audio) can tolerate some loss timing r 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, …

14. Application Layer2- 14 Transport service requirements: common apps application file transfer e-mail Web documents real-time audio/video stored audio/video interactive games text messaging data loss no loss loss-tolerant no loss throughput elastic audio: 5kbps-1Mbps video:10kbps-5Mbps same as above few kbps up elastic time sensitive no yes, 100’s msec yes, few secs yes, 100’s msec yes and no

15. 2: Application Layer 15 Internet transport protocols services TCP service: r connection-oriented: setup required between client and server processes r reliable transport between sending and receiving process r flow control: sender won’t overwhelm receiver r congestion control: throttle sender when network overloaded r does not provide: timing, minimum bandwidth guarantees UDP service: r unreliable data transfer between sending and receiving process r does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee Q: why bother? Why is there a UDP?

16. Application Layer2- 16 Internet apps: application, transport protocols application e-mail remote terminal access Web file transfer streaming multimedia Internet telephony DNS application layer protocol SMTP [RFC 2821] Telnet [RFC 854] HTTP [RFC 2616] FTP [RFC 959] HTTP (e.g., YouTube), RTP [RFC 1889] SIP, RTP, proprietary (e.g., Skype) [RFC 1035, 1123, 2181] underlying transport protocol TCP TCP or UDP UDP or TCP

17. Securing TCP TCP & UDP r no encryption r cleartext passwds sent into socket traverse Internet in cleartext SSL(secure socket layer) r provides encrypted TCP connection r data integrity r end-point authentication SSL is at app layer r Apps use SSL libraries, which “talk” to TCP SSL socket API  cleartext passwds sent into socket traverse Internet encrypted  More on SSL later Application Layer 2- 17

18. 2: Application Layer 18 Chapter 2: Application layer r 2.1 Principles of network applications r 2.2 Web and HTTP r 2.3 FTP r Online gaming r 2.4 Electronic Mail  SMTP,  POP3, IMAP r 2.5 DNS r 2.6 P2P file sharing r 2.7 Socket programming with TCP  Introduce c sock program  Programming assignment  Socket programming with UDP r VOIP basic principle

19. 2: Application Layer 19 Web and HTTP First some jargons r Web page consists of objects r Object can be HTML file, JPEG image, Java applet, audio file,… r Web page consists of base HTML-file which includes several referenced objects r Each object is addressable by a URL (Uniform Resource Locator ) r Example URL: www.someschool.edu/someDept/pic.gif host name path name What if URL: www.ucf.edu/students ?

20. Default Webpage Filename r When a URL is specified in a web browser without a specific filename at the end, the web server looks for a default page to show r Each OS defines its own default page names that you can use, such as:  index.html, index.htm, default.htm, index.php…  If the directory has no default files, browser will display a list of all the files in that directory (or deny it when configured)  Possibly cause security and privacy leakage 2: Application Layer 20

21. 2: Application Layer 21 HTTP overview HTTP: hypertext transfer protocol r Web’s application layer protocol r client/server model  client: browser that requests, receives, “displays” Web objects  server: Web server sends objects in response to requests r HTTP 1.0: RFC 1945 r HTTP 1.1: RFC 2068 PC running Firefox browser iphone running Safari browser HTTP request HTTP response HTTP request HTTP response

22. 2: Application Layer 22 HTTP overview (continued) Uses TCP: r client initiates TCP connection (creates socket) to server, port 80 r server accepts TCP connection from client r HTTP messages (application- layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server) r TCP connection closed HTTP is “stateless” r server maintains no information about past client requests Protocols that maintain “state” are complex! r past history (state) must be maintained r if server/client crashes, their views of “state” may be inconsistent, must be reconciled aside

23. 2: Application Layer 23 HTTP connections Nonpersistent HTTP r At most one object is sent over a TCP connection. r HTTP/1.0 uses nonpersistent HTTP Persistent HTTP r Multiple objects can be sent over single TCP connection between client and server. r HTTP/1.1 uses persistent connections in default mode Q. Why change to persistent HTTP?

24. 2: Application Layer 24 Nonpersistent HTTP Suppose user enters URL www.someSchool.edu/someDepartment/index.html 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/index.html 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 time (contains text, references to 10 jpeg images) Client Server

25. 2: Application Layer 25 Nonpersistent HTTP (cont.) 5. HTTP client receives 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 4. HTTP server closes TCP connection. time

26. 2: Application Layer 26 Response time modeling RTT (round-trip time): time to send a small packet to travel from client to server and back. Response time: r one RTT to initiate TCP connection r one RTT for HTTP request and first few bytes of HTTP response to return r file transmission time total = 2RTT+ file transmit time time to transmit file initiate TCP connection RTT request file RTT file received time

27. 2: Application Layer 27 Persistent HTTP Nonpersistent HTTP issues: r requires 2 RTTs per object r OS overhead for each TCP connection r browsers often open parallel TCP connections to fetch referenced objects Persistent HTTP r server leaves connection open after sending response  Time-out close after idle a while r subsequent HTTP messages between same client/server sent over open connection Persistent without pipelining: r client issues new request only when previous response has been received r one RTT for each referenced object Persistent with pipelining: r default in HTTP/1.1 r client sends requests as soon as it encounters a referenced object r as little as one RTT for all the referenced objects

28. HTTP request message r two types of HTTP messages: request, response r HTTP request message:  ASCII (human-readable format) Application Layer 2-28 request line (GET, POST, HEAD commands ) header lines carriage return, line feed at start of line indicates end of header lines GET /index.html HTTP/1.1\r\n Host: www-net.cs.umass.edu\r\n User-Agent: Firefox/3.6.10\r\n Accept: text/html,application/xhtml+xml\r\n Accept-Language: en-us,en;q=0.5\r\n Accept-Encoding: gzip,deflate\r\n Accept-Charset: ISO-8859-1,utf-8;q=0.7\r\n Keep-Alive: 115\r\n Connection: keep-alive\r\n \r\n carriage return character line-feed character

29. 2: Application Layer 29 HTTP request message: general format

30. 2: Application Layer 30 Uploading form input Post method: r Uses POST method r Web page often includes form input r Input content is uploaded to server in “entity body” in request message URL method: r Uses GET method r Input is uploaded in URL field of request line: www.somesite.com/animalsearch?monkeys&banana

31. 2: Application Layer 31 Method types HTTP/1.0 r GET r POST r HEAD  asks server to leave requested object out of response  Similar to get  For debugging purpose HTTP/1.1 r GET, POST, HEAD r PUT  uploads file in entity body to path specified in URL field r DELETE  deletes file specified in the URL field

32. Application Layer2- 32 HTTP response message status line (protocol status code status phrase) header lines data, e.g., requested HTML file HTTP/1.1 200 OK\r\n Date: Sun, 26 Sep 2010 20:09:20 GMT\r\n Server: Apache/2.0.52 (CentOS)\r\n Last-Modified: Tue, 30 Oct 2007 17:00:02 GMT\r\n ETag: "17dc6-a5c-bf716880"\r\n Accept-Ranges: bytes\r\n Content-Length: 2652\r\n Keep-Alive: timeout=10, max=100\r\n Connection: Keep-Alive\r\n Content-Type: text/html; charset=ISO-8859- 1\r\n \r\n data data data data data...

33. 2: Application Layer 33 HTTP response status codes 200 OK  request succeeded, requested object later in this message 301 Moved Permanently  requested object moved, new location specified later in this message (Location:)  one way of URL redirection 400 Bad Request  request message not understood by server 404 Not Found  requested document not found on this server 505 HTTP Version Not Supported In first line in server->client response message. A few sample codes:

34. 2: Application Layer 34 Trying out HTTP (client side) for yourself 1. Telnet to your favorite Web server: Opens TCP connection to port 80 (default HTTP server port) at cs.ucf.edu. Anything typed in sent to port 80 at www.cs.ucf.edu telnet www.cs.ucf.edu 80 2. Type in a GET HTTP request: GET /~czou/CNT4704-14/example.html HTTP/1.0 Host: www.cs.ucf.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!

35. 2: Application Layer 35 Let’s look at HTTP in action r Telnet example  “GET” must be Capital letters!  Must have “host” header! For web proxy reason –A proxy can know where to forward the GET request  What if type in “HTTP/1.0” ? r Wireshark example

36. 2: Application Layer 36 Web Proxy Introduction r Client A  Web B r A  B: (suppose B is “www.cs.ucf.edu”) telnet B:80 GET /~czou/CNT4704-14/notes.html HTTP/1.1 Host: B r A  Proxy  B: telnet Proxy:80 GET /~czou/CNT4704-14/notes.html HTTP/1.1 Host: B

37. 2: Application Layer 37 Web caches (proxy server) r user sets browser: Web accesses via cache r browser sends all HTTP requests to cache  If object in cache: cache returns object  Else, cache requests object from origin server, then returns object to client Goal: satisfy client request without involving origin server client proxy server client HTTP request HTTP response HTTP request origin server origin server HTTP response

38. 2: Application Layer 38 More about Web caching r Cache acts as both client and server r Typically cache is installed by ISP (university, company, residential ISP) Why Web caching? r Reduce response time for client request. r Reduce traffic on an institution’s access link. r Internet dense with caches enables “poor” content providers to effectively deliver content (but so does P2P file sharing) Akamai client proxy server client HTTP request HTTP response HTTP request origin server origin server HTTP response

39. 2: Application Layer 39 Cache Maintained by Browser r Each Browser also keeps caching previously obtained Web contents r If the “back” button is pressed, the local cached version of a page may be displayed instead of a new request being sent to the web server.  You need to click “refresh” or “reload” to let the browser send new requests. r Just like institutional cache, browser cache achieves the similar performance improvement r HTTP protocol helps the caching procedure

40. 2: Application Layer 40 Conditional GET (act by cache) r Let cache to update its cached info if necessary r cache: specify date of cached copy in HTTP request If-modified-since: r server: response contains no object if cached copy is up- to-date: HTTP/1.0 304 Not Modified cache server HTTP request msg If-modified-since: HTTP response HTTP/1.1 304 Not Modified object not modified HTTP request msg If-modified-since: HTTP response HTTP/1.1 200 OK object modified Wireshark example (load course page, and reload it)

41. 2: Application Layer 41 Expire HTTP Header (act by sever) r Conditional GET  Cache actively keeps its content fresh r Can a sever be responsible for cache refresh?  HTTP header option: Expire  Server tells cache when an object need update  Expires: Fri, 30 Oct 2005 14:19:41 GMT

42. Application Layer2- 42 Caching example: origin servers public Internet institutional network 1 Gbps LAN 1.54 Mbps access link assumptions:  avg object size: 100K bits  avg request rate from browsers to origin servers:15/sec  avg data rate to browsers: 1.50 Mbps  RTT from institutional router to any origin server: 2 sec  access link rate: 1.54 Mbps consequences:  LAN utilization: 0.15%  access link utilization = 99%  total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + usecs problem!

43. Application Layer2- 43 assumptions:  avg object size: 100K bits  avg request rate from browsers to origin servers:15/sec  avg data rate to browsers: 1.50 Mbps  RTT from institutional router to any origin server: 2 sec  access link rate: 1.54 Mbps consequences:  LAN utilization: 0.15%  access link utilization = 99%  total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + usecs Caching example: fatter access link origin servers 1.54 Mbps access link 154 Mbps msecs Cost: increased access link speed (not cheap!) 9.9% public Internet institutional network 1 Gbps LAN

44. institutional network 1 Gbps LAN Application Layer2- 44 Caching example: install local cache origin servers 1.54 Mbps access link local web cache assumptions:  avg object size: 100K bits  avg request rate from browsers to origin servers:15/sec  avg data rate to browsers: 1.50 Mbps  RTT from institutional router to any origin server: 2 sec  access link rate: 1.54 Mbps consequences:  LAN utilization: 0.15%  access link utilization = ?  total delay = Internet delay + access delay + LAN delay = 2 sec + ? + usecs How to compute link utilization, delay? Cost: web cache (cheap!) public Internet

45. Application Layer2- 45 Caching example: install local cache Calculating access link utilization, delay with cache: r suppose cache hit rate is 0.4  40% requests satisfied at cache, 60% requests satisfied at origin origin servers 1.54 Mbps access link  access link utilization:  60% of requests use access link  data rate to browsers over access link = 0.6*1.50 Mbps =.9 Mbps  Access link utilization = 0.9/1.54 = 58%  Total delay  = 0.6 * (delay from origin servers) +0.4 * (delay when satisfied at cache)  = 0.6 (2.01) + 0.4 (~msecs)  = ~ 1.2 secs  less than with 154 Mbps link (and cheaper too!) public Internet institutional network 1 Gbps LAN local web cache

46. 2: Application Layer 46 User-server state: cookies cookies: Many major Web sites use cookies: Web server to identify user (user’s ID, preference) 1) cookie file kept on user’s host, managed by user’s browser 2) Corresponding info on backend database at Web server Example:  Susan access Internet always from same PC  She visits a specific e- commerce site for first time  When initial HTTP requests arrives at site, site creates a unique ID and creates an entry in backend database for ID

47. 2: Application Layer 47 Cookie File Management r Cookies management for Firefox and IE: FF: tools -> options -> privacy -> remove individual cookies IE: Internet options -> general -> settings (in Browse history) -> view files r Where is the Cookie file?  It changes a lot with different browsers and different versions  IE 7, IE8: ??  Firefox: ?? FF 15: “option”->”privacy” -> “remove individual cookies”

48. 2: Application Layer 48 Cookies: keeping “state” (cont.) client server usual http request msg usual http response + Set-cookie: 1678 usual http request msg cookie: 1678 usual http response msgusual http request msg cookie: 1678 usual http response msg cookie- specific action cookie- spectific action Amazon.com creates ID 1678 for user entry in backend database access Cookie file amazon: 1678 ebay: 8734 Cookie file ebay: 8734 Cookie file amazon: 1678 ebay: 8734 one week later: Wireshark Example (old google cookie, browser cookie option, test new google cookie)

49. 2: Application Layer 49 Cookies (continued) What cookies can bring: r authorization r shopping carts r recommendations r user session state (Web e- mail) r Customized search results (e.g., google, obitz.com) Cookies and privacy: r cookies permit sites to learn a lot about you r you may supply name and e-mail to sites r search engines use redirection & cookies to learn yet more r advertising companies obtain info across sites aside Maintain “state” over stateless HTTP:  protocol endpoints: maintain state at sender/receiver over multiple transactions  cookies: http messages carry state