Application Layer Web HTTP Instructor Anirban Mahanti Office

Application Layer: Web & HTTP Instructor: Anirban Mahanti Office: ICT 745 Email: mahanti@cpsc. ucalgary. ca Class Location: ICT 121 Lectures: MWF 12: 00 – 12: 50 Notes derived from “Computer Networking: A Top Down Approach Featuring the Internet”, 2005, 3 rd edition, Jim Kurose, Keith Ross, Addison-Wesley. Slides are adapted from the companion web site of the book, as modified by Anirban Mahanti (and Carey Williamson). CPSC 441: WWW/HTTP 1

Outline r Introduction to App Layer Protocols r Brief History of WWW r Architecture r HTTP Connections r HTTP Format r Web Performance r Cookies CPSC 441: WWW/HTTP 2

Network applications: some jargon Process: program running user agent: interfaces within a host. with user “above” and network “below”. r within same host, two processes communicate r implements user using interprocess interface & application communication (defined -level protocol by OS). m Web: browser m E-mail: mail reader r processes running in m streaming audio/video: different hosts media player communicate with an application-layer protocol CPSC 441: WWW/HTTP 3

Applications and application-layer protocols Application: communicating, distributed processes m m m e. g. , e-mail, Web, P 2 P file sharing, instant messaging running in end systems (hosts) exchange messages to implement application transport network data link physical Application-layer protocols m m m one “piece” of an app define messages exchanged by apps and actions taken use communication services provided by lower layer protocols (TCP, UDP) application transport network data link physical CPSC 441: WWW/HTTP 4

App-layer protocol defines r Types of messages exchanged, eg, request & response messages r Syntax of message types: what fields in messages & how fields are delineated r Semantics of the fields, ie, meaning of information in fields r Rules for when and how processes send & respond to messages Public-domain protocols: r defined in RFCs r allows for interoperability r eg, HTTP, SMTP Proprietary protocols: r eg, Ka. Za. A CPSC 441: WWW/HTTP 5

Client-server paradigm Typical network app has two pieces: client and server Client: application transport network data link physical r initiates contact with server (“speaks first”) r typically requests service from server, r Web: client implemented in browser; e-mail: in mail reader Server: r provides requested service to client request reply application transport network data link physical r e. g. , Web server sends requested Web page, mail server delivers e-mail CPSC 441: WWW/HTTP 6

Processes communicating across network r process sends/receives messages to/from its socket r socket analogous to door m m sending process shoves message out door sending process asssumes transport infrastructure on other side of door which brings message to socket at receiving process host or server process controlled by app developer process socket TCP with buffers, variables Internet TCP with buffers, variables controlled by OS r API: (1) choice of transport protocol; (2) ability to fix a few parameters (lots more on this later) CPSC 441: WWW/HTTP 7

Addressing processes: r For a process to receive messages, it must have an identifier r Every 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: m m HTTP server: 80 Mail server: 25 r More on this later CPSC 441: WWW/HTTP 8

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 CPSC 441: WWW/HTTP 9

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 CPSC 441: WWW/HTTP 10

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 providing: 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? CPSC 441: WWW/HTTP 11

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. , Dialpad) TCP TCP TCP or UDP typically UDP CPSC 441: WWW/HTTP 12

Outline r Introduction to App Layer Protocols r Brief History of WWW r Architecture r HTTP Connections r HTTP Format r Web Performance r Cookies CPSC 441: WWW/HTTP 13

History of the Web r World Wide Web, “Web”, “WWW” r Tim Berners-Lee at CERN in 1991 m Demonstrated prototype at a conf. in ’ 91 m Text-based r Marc Andreessen developed the first graphical Web browser in 1993: Mosaic r Andreessen founds Netscape Communications r Browser war starts around 1995 -96 r America Online buys Netscape in 1998 CPSC 441: WWW/HTTP 14

Some “Web” Terminology r Web page may contain links to other pages (sometimes also called Web Objects) r Object can be HTML file, JPEG image, Java applet, audio file, … r Web pages are “Hypertexts” m One page points to another m Proposed by Prof. Vannevar Bush in 1945! r Each object is addressable by a URL: http: //www. someschool. edu/some. Dept/pic. gif protocol host name path name CPSC 441: WWW/HTTP 15

Outline r Introduction to App Layer Protocols r Brief History of WWW r Architecture r HTTP Connections r HTTP Format r Web Performance r Cookies CPSC 441: WWW/HTTP 16

HTTP overview HTTP: hypertext transfer protocol r Web’s application layer protocol r client/server model m client: browser that requests, receives, “displays” Web objects m server: Web server sends objects in response to requests r HTTP 1. 0: RFC 1945 r HTTP 1. 1: RFC 2616 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 CPSC 441: WWW/HTTP 17

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 CPSC 441: WWW/HTTP 18

Outline r Introduction to App Layer Protocols r Brief History of WWW r Architecture r HTTP Connections r HTTP Format r Web Performance r Cookies CPSC 441: WWW/HTTP 19

HTTP connections Non-persistent 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 m m Pipelined Non-pipelined CPSC 441: WWW/HTTP 20

Response time modeling Definition of RTT: 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 time to transmit file RTT file received time CPSC 441: WWW/HTTP 21

Classical HTTP/1. 0 initiate TCP connection http: //www. somewhere. com/index. html RTT GET index. html references: page 1. jpg, page 2. jpg, page 3. jpg. time to transmit index. hml RTT file received GET page 1. jpg time to transmit page 1. jpg CPSC 441: WWW/HTTP 22

Persistent HTTP Nonpersistent HTTP issues: r requires 2 RTTs per object r OS must work and allocate host resources for each TCP connection r but browsers often open parallel TCP connections to fetch referenced objects Persistent HTTP r server leaves connection open after sending response r subsequent HTTP messages between same client/server are sent over 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 CPSC 441: WWW/HTTP 23

Outline r Introduction to App Layer Protocols r Brief History of WWW r Architecture r HTTP Connections r HTTP Format r Web Performance r Cookies CPSC 441: WWW/HTTP 24

HTTP request message r HTTP request message: m 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 header Connection: close lines Accept-language: fr Carriage return, line feed indicates end of message (extra carriage return, line feed) CPSC 441: WWW/HTTP 25

HTTP request message: general format CPSC 441: WWW/HTTP 26

HTTP Methods r GET: retrieve a file (95% of requests) r HEAD: just get meta-data (e. g. , mod time) r POST: submitting a form to a server r PUT: store enclosed document as URI r DELETE: removed named resource r LINK/UNLINK: in 1. 0, gone in 1. 1 r TRACE: http “echo” for debugging (added in 1. 1) r CONNECT: used by proxies for tunneling (1. 1) r OPTIONS: request for server/proxy options (1. 1) CPSC 441: WWW/HTTP 27

HTTP response message status line (protocol status code status phrase) header lines data, e. g. , requested HTML file 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. . . CPSC 441: WWW/HTTP 28

HTTP Response Status Codes r 1 XX: Informational (def’d in 1. 0, used in 1. 1) 100 Continue, 101 Switching Protocols r 2 XX: Success 200 OK, 206 Partial Content r 3 XX: Redirection 301 Moved Permanently, 304 Not Modified r 4 XX: Client error 400 Bad Request, 403 Forbidden, 404 Not Found r 5 XX: Server error 500 Internal Server Error, 503 Service Unavailable, 505 HTTP Version Not Supported CPSC 441: WWW/HTTP 29

Trying out HTTP (client side) for yourself 1. Telnet to your favorite Web server: telnet www. eurecom. fr 80 Opens TCP connection to port 80 (default HTTP server port) at www. eurecom. fr. Anything typed in sent to port 80 at www. eurecom. fr 2. Type in a GET HTTP request: GET /~ross/index. html HTTP/1. 0 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! CPSC 441: WWW/HTTP 30

Outline r Introduction to App Layer Protocols r Brief History of WWW r Architecture r HTTP Connections r HTTP Format r Web Performance r Cookies CPSC 441: WWW/HTTP 31

Web Proxy Caching Objective: satisfy client request without involving origin server resulting in reduced server & network load, low latency to response r user sets browser: Web accesses via cache r browser sends all HTTP requests to cache m m object in cache => cache hit: returns object else cache requests object from origin server, then returns object to client 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 Cache acts as both client and server origin server est u q e Pr T nse o p HT res P T HT origin server CPSC 441: WWW/HTTP 32

Web Caching Hierarchy national/international proxy cache regional proxy cache local proxy cache (e. g. , local ISP, University) client CPSC 441: WWW/HTTP 33

Why Cache? 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 CPSC 441: WWW/HTTP 34

Some Issues r All objects can’t be cached m E. g. , dynamic objects r Cache consistency m strong m weak r Cache Replacement Policies m Variable size objects m Varying cost of not finding an object (a “miss”) in the cache r Prefetch? m A large fraction of the requests are one-times CPSC 441: WWW/HTTP 35

Weak Consistency r Each cached copy has a TTL beyond which it must be validated with the origin server r TTL = freshness life time – age m freshness life time: often heuristically calculated; sometimes based on MAX_AGE or EXPIRES headers m age = current time (at client) – timestamp on object (time at which server generated response) r Age Penalty? CPSC 441: WWW/HTTP 36

Conditional GET: client-side caching r Goal: don’t send object if client has up-to-date cached version r client: specify date of cached copy in HTTP request If-modified-since: <date> r server: response contains no object if cached copy is up-to -date: HTTP/1. 0 304 Not Modified server client HTTP request msg If-modified-since: <date> HTTP response object not modified HTTP/1. 0 304 Not Modified HTTP request msg If-modified-since: <date> HTTP response object modified HTTP/1. 0 200 OK <data> CPSC 441: WWW/HTTP 37

Content distribution networks (CDNs) r The content providers are the CDN customers. Content replication r CDN company installs hundreds of CDN servers throughout Internet m in lower-tier ISPs, close to users r CDN replicates its customers’ content in CDN servers. When provider updates content, CDN updates servers origin server in North America CDN distribution node CDN server in S. America CDN server in Europe CDN server in Asia CPSC 441: WWW/HTTP 38

Cookies: keeping “state” 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 Example: m m m 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 CPSC 441: WWW/HTTP 39

Cookies: keeping “state” (cont. ) client ebay: 8734 Cookie file amazon: 1678 ebay: 8734 usual http request msg usual http response + Set-cookie: 1678 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 n server da try i tab n 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 CPSC 441: WWW/HTTP 40

Cookies (continued) What cookies can bring: r authorization r shopping carts r recommendations r user session state (Web e-mail) 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 CPSC 441: WWW/HTTP 41

Web & HTTP r The major application on the Internet m A large fraction of traffic is HTTP r Client/server model: m Clients make requests, servers respond to them m Done mostly in ASCII text (helps debugging!) r Various headers and commands r Web Caching & Performance r Content Distribution Networks CPSC 441: WWW/HTTP 42