Chapter 2 Application layer r 2 1 Principles

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Chapter 2: Application layer r 2. 1 Principles of network applications r 2. 2

Chapter 2: Application layer r 2. 1 Principles of network applications r 2. 2 Web and HTTP v Lab assignment r 2. 3 FTP r Online gaming r 2. 4 Electronic Mail v v v SMTP (simple mail transfer protocol) POP 3, IMAP Lab assignment r 2. 6 P 2 P file sharing r 2. 7 VOIP r 2. 8 Socket programming with TCP v v Introduce c sock program Programming assignment r 2. 9 Socket programming with UDP r 2. 10 Building a Web server r 2. 5 DNS (domain name service) 2: Application Layer 1

Chapter 2: Application Layer Our goals: r conceptual, implementation aspects of network application protocols

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

Some network apps r E-mail r Internet telephone r Web r Real-time video r

Some network apps r E-mail r Internet telephone r Web r Real-time video r Instant messaging r P 2 P file sharing r Multi-user network games r Streaming stored video clips conference r Massive parallel computing v Grid computing 2: Application Layer 3

Creating a network app Write programs that v v v run on different end

Creating a network app Write programs that v v v 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 v v Network core devices do not function at app layer This design allows for rapid app development application transport network data link physical 2: Application Layer 4

Chapter 2: Application layer r 2. 1 Principles of r 2. 6 P 2

Chapter 2: Application layer r 2. 1 Principles of r 2. 6 P 2 P file sharing r r 2. 8 Socket programming r r r network applications 2. 2 Web and HTTP 2. 3 FTP Online gaming 2. 4 Electronic Mail v v SMTP, POP 3, IMAP r 2. 5 DNS r 2. 7 VOIP with TCP v v Introduce c sock program Programming assignment r 2. 9 Socket programming with UDP r 2. 10 Building a Web server 2: Application Layer 5

Application architectures r Client-server r Peer-to-peer (P 2 P) r Hybrid of client-server and

Application architectures r Client-server r Peer-to-peer (P 2 P) r Hybrid of client-server and P 2 P 2: Application Layer 6

Client-server architecture server: v v v always-on host permanent IP address server farms for

Client-server architecture server: v v v always-on host permanent IP address server farms for scaling clients: v client/server v v v communicate with server may be intermittently connected may have dynamic IP addresses do not communicate directly with each other 2: Application Layer 7

Pure P 2 P architecture r no always-on server r arbitrary end systems directly

Pure P 2 P architecture r no always-on server r arbitrary end systems directly communicate r peers are intermittently connected and change IP addresses r example: Gnutella, Bit. Torrent Highly scalable But difficult to manage 2: Application Layer 8

Hybrid of client-server and P 2 P Skype v voice-over-IP P 2 P application

Hybrid of client-server and P 2 P Skype v voice-over-IP P 2 P application centralized server: finding address of remote party: client-client connection: direct (not through server) Instant messaging (e. g. , MSN) v v Chatting between two users is P 2 P 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 2: Application Layer 9

Processes communicating Process: program running within a host. r within same host, two processes

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 P 2 P architectures have both client processes & server processes 2: Application Layer 10

Addressing processes r For a process to r Identifier includes receive messages, it both

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

App-layer protocol defines r Types of messages exchanged, e. g. , request & response

App-layer protocol defines r Types of messages exchanged, e. g. , request & response messages r Syntax of message types: what fields in messages & how fields are delineated r Semantics of the fields, i. e. , meaning of information in fields r Rules for when and how processes send & respond to messages Public-domain protocols: r defined in RFCs v Requests for Comments r allows for interoperability r e. g. , HTTP, SMTP Proprietary protocols: r e. g. , Ka. Za. A 2: Application Layer 12

What transport service does an app need? Data loss r some apps (e. g.

What transport service does an app need? Data loss r some apps (e. g. , audio) can tolerate some loss r other apps (e. g. , file transfer, telnet) require 100% reliable data transfer Timing r some apps (e. g. , Internet telephony, interactive games) require low delay to be “effective” Bandwidth r some apps (e. g. , multimedia) require minimum amount of bandwidth to be “effective” r other apps (“elastic apps”) make use of whatever bandwidth they get 2: Application Layer 13

Transport service requirements of common apps Data loss Bandwidth Time Sensitive file transfer e-mail

Transport service requirements of common apps Data loss Bandwidth Time Sensitive file transfer e-mail Web documents real-time audio/video no loss-tolerant no no no yes, 100’s msec stored audio/video interactive games instant messaging loss-tolerant no loss elastic audio: 5 kbps-1 Mbps video: 10 kbps-5 Mbps same as above few kbps up elastic Application yes, few secs yes, 100’s msec yes and no 2: Application Layer 14

Internet transport protocols services TCP service: r connection-oriented: setup r r required between client

Internet transport protocols services TCP service: r connection-oriented: setup r r required between client and server processes 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 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? 2: Application Layer 15

Internet apps: application, transport protocols Application e-mail remote terminal access Web file transfer streaming

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 [RFC 2821] Telnet [RFC 854] HTTP [RFC 2616] FTP [RFC 959] proprietary (e. g. Real. Networks) proprietary (e. g. , Vonage, Dialpad) TCP TCP TCP or UDP typically UDP 2: Application Layer 16

Chapter 2: Application layer r 2. 1 Principles of network applications v v app

Chapter 2: Application layer r 2. 1 Principles of network applications v v app architectures app requirements r 2. 2 Web and HTTP r Online gaming r 2. 4 Electronic Mail v SMTP, POP 3, IMAP r 2. 5 DNS r 2. 6 P 2 P file sharing r 2. 7 VOIP r 2. 8 Socket programming with TCP v v Introduce c sock program Programming assignment r 2. 9 Socket programming with UDP r 2. 10 Building a Web server 2: Application Layer 17

Web and HTTP First some jargon r Web page consists of objects r Object

Web and HTTP First some jargon 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/some. Dept/pic. gif path name host name What if URL: www. ucf. edu/students ? 2: Application Layer 18

HTTP overview HTTP: hypertext transfer protocol r Web’s application layer protocol r client/server model

HTTP overview HTTP: hypertext transfer protocol r Web’s application layer protocol r client/server model v client: browser that requests, receives, “displays” Web objects v server: Web server sends objects in response to requests r HTTP 1. 0: RFC 1945 r HTTP 1. 1: RFC 2068 HT TP req ues PC running HT t TP res Explorer pon se st ue q e r P nse Server T o p running HT es r P T Apache Web HT server Mac running Navigator 2: Application Layer 19

HTTP overview (continued) Uses TCP: r client initiates TCP connection (creates socket) to server,

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 (applicationlayer 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 aside 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 2: Application Layer 20

HTTP connections Nonpersistent HTTP r At most one object is sent over a TCP

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? 2: Application Layer 21

Nonpersistent HTTP (contains text, Suppose user enters URL references to 10 www. some. School.

Nonpersistent HTTP (contains text, Suppose user enters URL references to 10 www. some. School. edu/some. Department/index. html jpeg images) Client Server 1 a. HTTP client initiates TCP connection to HTTP server (process) at www. some. School. edu on port 80 2. HTTP client sends HTTP time request message (containing URL) into TCP connection socket. Message indicates that client wants object some. Department/index. html 1 b. HTTP server at host www. some. School. 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 2: Application Layer 22

Nonpersistent HTTP (cont. ) 4. HTTP server closes TCP 5. HTTP client receives response

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 2: Application Layer 23

Response time modeling RRT (round-trip time): time to send a small packet to travel

Response time modeling RRT (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 = 2 RTT+transmit time initiate TCP connection RTT request file RTT file received time to transmit file time 2: Application Layer 24

Persistent HTTP Nonpersistent HTTP issues: r requires 2 RTTs per object r OS overhead

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 v 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 2: Application Layer 25

HTTP request message r two types of HTTP messages: request, response r HTTP request

HTTP request message r two types of HTTP messages: request, response r HTTP request message: v ASCII (human-readable format) Protocol No. request line (GET, POST, HEAD commands) GET /somedir/page. html HTTP/1. 1 Host: www. someschool. edu User-agent: Mozilla/4. 0 header Connection: close lines Accept-language: fr Carriage return, line feed indicates end of message (extra carriage return, line feed) 2: Application Layer 26

HTTP request message: general format 2: Application Layer 27

HTTP request message: general format 2: Application Layer 27

Uploading form input Post method: r Uses POST method r Web page often includes

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 2: Application Layer 28

Method types HTTP/1. 0 r GET r POST r HEAD v asks server to

Method types HTTP/1. 0 r GET r POST r HEAD v asks server to leave requested object out of response HTTP/1. 1 r GET, POST, HEAD r PUT v uploads file in entity body to path specified in URL field r DELETE v deletes file specified in the URL field 2: Application Layer 29

HTTP response message status line (protocol status code status phrase) header lines data, e.

HTTP response message status line (protocol status code status phrase) header lines data, e. g. , requested HTML file, image HTTP/1. 1 200 OK Connection close Date: Thu, 06 Aug 1998 12: 00: 15 GMT Server: Apache/1. 3. 0 (Unix) Last-Modified: Mon, 22 Jun 1998 …. . . Content-Length: 6821 Content-Type: text/html data data. . . 2: Application Layer 30

HTTP response status codes In first line in server->client response message. A few sample

HTTP response status codes In first line in server->client response message. A few sample codes: 200 OK v request succeeded, requested object later in this message 301 Moved Permanently v requested object moved, new location specified later in this message (Location: ) one way of URL redirection 400 Bad Request v request message not understood by server 404 Not Found v requested document not found on this server 505 HTTP Version Not Supported 2: Application Layer 31

Trying out HTTP (client side) for yourself 1. Telnet to your favorite Web server:

Trying out HTTP (client side) for yourself 1. Telnet to your favorite Web server: telnet www. cs. ucf. edu 80 Opens TCP connection to port 80 (default HTTP server port) at cis. poly. edu. Anything typed in sent to port 80 at www. cs. ucf. edu 2. Type in a GET HTTP request: GET /~czou/CDA 4527/example. html HTTP/1. 1 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! 2: Application Layer 32

Let’s look at HTTP in action r Telnet example v “GET” must be Capital

Let’s look at HTTP in action r Telnet example v “GET” must be Capital letters! v Must have “host” header! • For web proxy reason • www. cs. ucf. edu ignores “host” command content v What if type in “HTTP/1. 0” ? r Ethereal example 2: Application Layer 33

Web Proxy Introduction r Client A r Proxy P Web B r A B:

Web Proxy Introduction r Client A r Proxy P Web B r A B: telnet B: 80 GET /~czou/CDA 4527/notes. html HTTP/1. 1 Host: B r A P B: telnet P: 80 GET /~czou/CDA 4527/notes. html HTTP/1. 1 Host: B 2: Application Layer 34

User-server state: cookies Many major Web sites use cookies: Web server to identify user

User-server state: 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 C: Documents and SettingsCliff. ZouCookies 2) Corresponding info on backend database at Web server Example: v v v Susan access Internet always from same PC She visits a specific ecommerce 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 2: Application Layer 35

Cookies: keeping “state” (cont. ) client usual http request msg usual http response +

Cookies: keeping “state” (cont. ) client usual http request msg usual http response + ebay: 8734 Set-cookie: 1678 Cookie file amazon: 1678 ebay: 8734 usual http request msg cookie: 1678 usual http response msg Cookie file amazon: 1678 ebay: 8734 cookiespecific action ss acce ac ce one week later: e Amazon. com da ntry tab in b creates ID as ac e ke nd 1678 for user ss Cookie file server usual http request msg cookie: 1678 usual http response msg cookiespectific action Ethereal Example (old amazon cookie, browser cookie option, test new google cookie) 2: Application Layer 36

Cookies (continued) What cookies can bring: r authorization r shopping carts r recommendations r

Cookies (continued) What cookies can bring: r authorization r shopping carts r recommendations r user session state (Web e-mail) Cookies for Firefox and IE: FF: tools -> options -> privacy -> cookies -> view cookies IE: Internet options -> general -> settings (temporary Internet files) -> view files aside 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 2: Application Layer 37

Web caches (proxy server) Goal: satisfy client request without involving origin server r user

Web caches (proxy server) Goal: satisfy client request without involving origin server r user sets browser: Web accesses via cache r browser sends all HTTP requests to cache v v If object in cache: cache returns object Else, cache requests object from origin server, then returns object to client origin server HT client. HTTP TP req Proxy server ues t res pon se t s ue q re P nse o T p HT es r TP T H client est u q e Pr T nse o p HT res P T HT origin server 2: Application Layer 38

More about Web caching r Cache acts as both client and server r Typically

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 P 2 P file sharing) origin server HT client. HTTP TP req Proxy server ues t res pon se t s ue q re P nse o T p HT es r TP T H client st e u req P T nse o p HT res P T HT IE proxy setup “Internet option”-> “connections” ->”LAN settings”->”proxy server” 2: Application Layer 39

Caching example Assumptions r average object size = 100 K bits r avg. request

Caching example Assumptions r average object size = 100 K bits r avg. request rate from institution’s browsers to origin servers = 15/sec r delay from institutional router to any origin server and back to router = 2 sec Consequences origin servers public Internet 1. 5 Mbps access link institutional network 10 Mbps LAN r utilization on LAN = 15% r utilization on access link = 100% r total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + milliseconds institutional cache 2: Application Layer 40

Caching example (cont) Possible solution r increase bandwidth of access link to, say, 10

Caching example (cont) Possible solution r increase bandwidth of access link to, say, 10 Mbps Consequences origin servers public Internet r utilization on LAN = 15% r utilization on access link = 15% = Internet delay + access delay + LAN delay = 2 sec + msecs r often a costly upgrade 10 Mbps access link r Total delay institutional network 10 Mbps LAN institutional cache 2: Application Layer 41

Caching example (cont) origin servers Install cache r suppose hit rate is. 4 Consequence

Caching example (cont) origin servers Install cache r suppose hit rate is. 4 Consequence public Internet r 40% requests will be r r r = satisfied almost immediately (say 1 msec) 60% requests satisfied by origin server utilization of access link reduced to 60%, resulting in negligible delays (say 10 msec) total avg delay = Internet delay + access delay + LAN delay. 6*(2. 01) secs +. 4*(0. 001) secs < 1. 4 secs 1. 5 Mbps access link institutional network 10 Mbps LAN institutional cache 2: Application Layer 42

Conditional GET (act by cache) r Let cache to update its server cached info

Conditional GET (act by cache) r Let cache to update its server cached info if necessary r cache: specify date of cached copy in HTTP request If-modified-since: <date> HTTP request msg If-modified-since: <date> HTTP response object not modified HTTP/1. 1 304 Not Modified r server: response contains no object if cached copy is up-to -date: HTTP/1. 0 304 Not Modified Ethereal example (load one page, and reload it) HTTP request msg If-modified-since: <date> HTTP response object modified HTTP/1. 1 200 OK <data> 2: Application Layer 43

Expire HTTP Header (act by sever) r Conditional GET v Cache actively keeps its

Expire HTTP Header (act by sever) r Conditional GET v Cache actively keeps its content fresh r Can a sever be responsible for cache refresh? v HTTP header option: Expire v Server tells cache when an object need update v Expires: Fri, 30 Oct 2005 14: 19: 41 GMT 2: Application Layer 44