Announcements. The World Wide Web. Goals of Today s Lecture. The World Wide Web HTML. Web Components

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1 Announcements The World Wide Web Project #1 - Milestone 1 Due 11pm Tonight No slip days! EE 122: Intro to Communication Networks Fall 2007 (WF 4-5:30 in Cory 277) Lisa Fowler / Vern Paxson TAs: Lisa Fowler, Daniel Killebrew, Jorge Ortiz Homework #2 Out now *Hard* deadline of 11PM Tues Oct 9 Materials with thanks to Vern Paxson, Jennifer Rexford, Ion Stoica, and colleagues at Princeton and UC Berkeley 1 First Midterm coming soon Fri Oct 12 2 Goals of Today s Lecture Main ingredients of the Web URIs, HTML, HTTP Key properties of HTTP Request-response, stateless, and resource meta-data Performance of HTTP Parallel connections, persistent connections, pipelining Web components Clients, proxies, and servers Caching vs. replication The World Wide Web 3 4 Web Components Content Objects Clients Send requests / Receive responses s Receive requests / Send responses Store or generate the responses Proxies Placed between clients and servers Act as a server for the client, and a client to the server Provide extra functions Caching, anonymization, logging, transcoding, filtering access HTML A Web page has several components Base HTML file Referenced objects (e.g., images) Content: How? HyperText Markup Language (HTML) Representation of hypertext documents in ASCII format Web browsers interpret HTML when rendering a page Several functions: Format text, reference images, embed hyperlinks (HREF) Straight-forward to learn Syntax easy to understand Authoring programs can auto-generate HTML Source almost always available Explicit or transparent ( interception ) 5 6 1

2 URI Uniform Resource Identifier (URI) Uniform Resource Locator (URL) Provides a means to get the resource Content: How? Uniform Resource Name (URN) Names a resource independent of how to get it urn:ietf:rfc:3986 is a standard URN for RFC URL Syntax Content: How? protocol://hostname[:port]/directorypath/resource protocol hostname port directory path resource http, ftp, https, smtp, rtsp, etc. FQDN, IP address Defaults to protocol s standard port e.g. http: 80/tcp https: 443/tcp Hierarchical, often reflecting file system Identifies the desired resource Can also extend to program executions: 0B%40Bulk&MsgId=2604_ _29699_1123_1261_0_289 17_3552_ &Search=&Nhead=f&YY=31454&order= down&sort=date&pos=0&view=a&head=b 8 HTTP Client-: How? HyperText Transfer Protocol (HTTP) Client-server protocol for transferring resources Important properties: Request-response protocol Reliance on a global URI namespace Resource metadata Stateless ASCII format % telnet 80 GET /vern/ HTTP/1.0 <blank line, i.e., CRLF> 9 Client-to- Communication HTTP Request Message Request line: method, resource, and protocol version Request headers: provide information or modify request Body: optional data (e.g., to POST data to the server) request line carriage return line feed indicates end of message GET /somedir/page.html HTTP/1.1 Host: header User-agent: Mozilla/4.0 lines Connection: close Accept-language: fr (blank line) Not optional 10 Client-to- Communication -to-client Communication HTTP Request Message Request line: method, resource, and protocol version Request headers: provide information or modify request Body: optional data (e.g., to POST data to the server) Request methods include: GET: Return current value of resource, run program, HEAD: Return the meta-data associated with a resource POST: Update resource, provide input to a program, Headers include: Useful info for the server HTTP Response Message Status line: protocol version, status code, status phrase Response headers: provide information Body: optional data status line (protocol, status code, status phrase) HTTP/ OK Connection close Date: Thu, 06 Aug :00:15 GMT : Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun Content-Length: 6821 Content-Type: text/html (blank line) data data data data data... data e.g. desired language 11 e.g., requested HTML file 12 header lines 2

3 -to-client Communication HTTP Response Message Status line: protocol version, status code, status phrase Response headers: provide information Body: optional data Response code classes Similar to other ASCII app. protocols like SMTP Code 1xx 2xx 3xx 4xx 5xx Class Informational Success Redirection Client error error Example 100 Continue 200 OK 304 Not Modified 404 Not Found 503 Service Unavailable 13 Web : Generating a Response Return a file URL matches a file (e.g., /www/index.html) returns file as the response generates appropriate response header Generate response dynamically URL triggers a program on the server runs program and sends output to client Return meta-data with no body 14 HTTP Resource Meta-Data HTTP is Stateless Client-: How? Meta-data Info about a resource A separate entity Examples: Size of a resource Last modification time Type of the content Data format classification e.g., Content-Type: text/html Enables browser to automatically launch an appropriate viewer Borrowed from s Multipurpose Internet Mail Extensions (MIME) Usage example: Conditional GET Request Client requests object If-modified-since If object hasn t changed, server returns HTTP/ Not Modified No body in the server s response, only a header 15 Stateless protocol Each request-response exchange treated independently s not required to retain state This is good Improves scalability on the server-side Don t have to retain info across requests Can handle higher rate of requests Order of requests doesn t matter This is bad Some applications need persistent state Need to uniquely identify user or store temporary info e.g., Shopping cart, user preferences and profiles, usage tracking, 16 State in a Stateless Protocol: Cookies Client-side state maintenance Client stores small (?) state on behalf of server Client sends state in future requests to the server Can provide authentication Request Response Set-Cookie: XYZ Request Cookie: XYZ 17 State in a Stateless Protocol: HTTP Authentication Tool to limit access to server documents Basic HTTP Authentication Client can add an Authorization header to GET request Base64-encoded concatenation of username, a colon, & password If client doesn t provide header, server responds with a 401 Unauthorized and a WWW-Authenticate header does not honor request until valid authorization received Stateless: Must happen on each request Is this secure? Is this security? No. Authentication is not security, but provides a piece 18 3

4 Security Sneak-Peek: HTTPS Transport Layer Security (TLS) Came after Secure Sockets Layer (SSL) Shim between App layer (e.g. HTTP) and Transport layer (e.g. TCP) Provides authentication and communication privacy Putting It All Together Client-: How? Client- Request-Response HTTP Stateless Get state with cookies Content URI/URL HTML Meta-data Web Browser Is the client Client-: In Practice Generates HTTP requests User types URL, clicks a hyperlink or bookmark, clicks reload or submit Automatically downloads embedded images Submits the requests (fetches content) Via one or more HTTP connections Presents the response Parses HTML and renders the Web page Invokes helper applications (e.g., Acrobat, RealPlayer) Or not! Control vs. Accessibility Maintains cache Stores recently-viewed objects and ensures freshness 21 Web Client-: In Practice Handle client request: 1. Accept a TCP connection 2. Read and parse the HTTP request message 3. Translate the URI to a resource 4. Determine whether the request is authorized 5. Generate and transmit the response Web site vs. Web server Web site: one or more Web pages and objects united to provide the user an experience of a coherent collection Web server: program that satisfies client requests for Web resources 22 A Brief History Tim Berners-Lee & Robert Cailliau propose and coin WorldWideWeb - HTML, HTTP, URI 1991 Tim Berners-Lee writes first web browser 1993 Mosaic 1 - Supports images Netscape 1 CSS introduced Netscape 2 & 3 Internet Explorer - Multiple connections, cookies, <CENTER> - Separates content from structure - Frames, JavaScript, mouseover 5 Minute Break Boom! now HTML 4.0 XML Dynamic content: DHTML/W3C DOM Commerce Web 2.0 (coined 2004) Accessibility, mobility, internationalization, voice, media, - Tables, scripting, style sheets, - The network is the platform - Open data, user participation, rich user experience Questions Before We Proceed? Future: Web everywhere, everyone, everything, everyhow, more useful, you!

5 HTTP Performance Most Web pages have multiple objects ( items ) e.g., HTML file and a bunch of embedded images How do you retrieve those objects? One item at a time Fetch HTTP Items: Stop & Wait Client Start fetching page Request item 1 Transfer item 1 Request item 2 Transfer item 2 Time What transport behavior does this remind you of? Finish; display page Request item 3 Transfer item 3 2 RTTs per object HTTP Performance Concurrent Requests & Responses Most Web pages have multiple objects ( items ) e.g., HTML file and a bunch of embedded images Use multiple connections in parallel How do you retrieve those objects? One item at a time What are the performance implications? Where does the analogy possibly break down? What else can we do? 27 Does not necessarily maintain order of responses Client = Why? = Why? Network = Why? R1 T1 Is this fair? N parallel connections use bandwidth N times more aggressively than just one What s a reasonable/fair limit as traffic competes with that of other users? R2 T2 R3 T3 28 Pipelined Requests & Responses Batch requests and responses Reduce connection overhead Multiple requests sent in a single batch Small items (common) can also share segments Maintains order of responses Item 1 always arrives before item 2 How is this different from concurrent requests/responses? This isn t everything. Something s missing Client Request 1 Request 2 Request 3 Transfer 1 Transfer 2 Transfer 3 29 Persistent Connections Enables multiple transfers per connection Maintain TCP connection across multiple requests Including transfers subsequent to current page Client or server can tear down connection Performance advantages: Avoid overhead of connection set-up and tear-down Allow TCP to learn more accurate RTT estimate Allow TCP congestion window to increase i.e., leverage previously discovered bandwidth Default in HTTP/1.1 Can use this to batch requests on a single connection Example: 5 objects, RTT=50ms 30 5

6 ?? Many clients transfer same information Generates redundant server and network load Clients experience unnecessary latency Clients ISP-1 Backbone ISP ISP-2 31 Caching How? Modifier to GET requests: If-modified-since returns not modified if resource not modified since specified time Response header: Expires how long it s safe to cache the resource No-cache ignore all caches; always get resource directly from server No, really, how? Step 1: Caching Step 2: Step 3: Profit! Exploit locality of reference 32 Caching on the Client Caching with Reverse Proxies Example: Conditional GET Request Return resource only if it has changed at the server Save server resources! Request from client to server: GET /~ee122/fa07/ HTTP/1.1 Host: inst.eecs.berkeley.edu User-Agent: Mozilla/4.03 If-Modified-Since: Sun, 27 Aug :25:50 GMT <CRLF> Cache documents close to server decrease server load Typically done by content providers Only works for static content Reverse proxies How? Client specifies if-modified-since time in request compares this against last modified time of desired resource returns 304 Not Modified if resource has not changed. or a 200 OK with the latest version otherwise 33 Clients ISP-1 Backbone ISP ISP-2 34 Caching with Forward Proxies Cache documents close to clients reduce network traffic and decrease latency Typically done by ISPs or corporate LANs Forward proxies Reverse proxies ISP-1 Backbone ISP ISP-2 Caching w/ Content Distribution Networks Integrate forward and reverse caching functionality One overlay network (usually) administered by one entity e.g., Akamai Provide document caching Pull: Direct result of clients requests Push: Expectation of high access rate Also do some processing Handle dynamic web pages Transcoding Clients

7 Caching with CDNs (cont.) CDN Example Akamai Forward proxies Clients ISP-1 Backbone ISP CDN ISP-2 Akamai: Creates new domain names for each client content provider e.g.,i.a.cnn.net = CNAME custom.i.cnn.net.edgesuite.net custom.i.cnn.net.edgesuite.net = CNAME a1921.g.akamai.net Customer/Content Provider: Modifies content Embedded URLs reference new domains CNN page s HREF s refer to i.a.cnn.net Pushes content out to Akamai as it changes Or: Akamai pulls it on demand w/ usual caching mech. Both CNN & Akamai have control over load distribution CDN Example Akamai DNS server for cnn.com cnn.com Akamaizes its content. lookup a1921.g.akamai.net local DNS server akamai.net DNS servers Akamaized response object has inline URLs for secondary content at (after resolving CNAMEs) a1921.g.akamai.net and other Akamai-managed DNS names. a Akamai servers store/cache secondary content for Akamaized services. b GET DNS Lookup 2 - Fetch page w/ Akamaized content 3 - DNS Lookup for Akamai URLs 4 - Fetch content c 39 Caching vs. Replication Why move content closer to users? Reduce latency for the user Reduce load on the network and the server How? Caching Replicate content on demand after a request Store the response message locally for future use Challenges: May need to verify if the response has changed and some responses are not cacheable Replication Planned replication of content in multiple locations Update of resources handled outside of HTTP Can replicate scripts that create dynamic responses 40 Hosting: Multiple Sites Per Machine Multiple Web sites on a single machine Hosting company runs the Web server on behalf of multiple sites (e.g., and Problem: GET /index.html or Solutions: Multiple server processes on the same machine Have a separate IP address (or port) for each server Include site name in HTTP request Single Web server process with a single IP address Client includes Host header (e.g., Host: Required header with HTTP/ Hosting: Multiple Machines Per Site Replicate a popular Web site across multiple machines Helps to handle the load Places content closer to clients Helps when content isn t cacheable by proxies/cdns Problem: Want to direct client to a particular replica Why? Balance load across server replicas Pair clients with nearby servers Solution #1: Manual selection by clients Each replica has its own site name A Web page lists the replicas (e.g., by name, location) and asks clients to click on a hyperlink to pick 42 7

8 Hosting: Multiple Machines Per Site Solution #2: single IP address, multiple machines Run multiple machines behind a single IP address Hosting: Multiple Machines Per Site Solution #3: multiple addresses, multiple machines Same name but different addresses for all of the replicas Configure DNS server to return different addresses Load Balancer Internet Ensure all packets from a single TCP connection go to the same replica Conclusions Key ideas underlying the Web Uniform Resource Identifier (URI), Locator (URL) HyperText Markup Language (HTML) HyperText Transfer Protocol (HTTP) Browser helper applications based on content type Performance implications Concurrent connections, pipelining, persistent conns. Main Web components Clients, servers, proxies, CDNs Next lecture: drilling down to the link layer K & R ,