Computer Networks. Web and HTTP HTTP

Transcription

1 Computer Networks HTTP 1 Web and HTTP First some jargon 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 Example URL: host name path name 2 1

2 HTTP overview HTTP: hypertext transfer protocol Web s applicaoon layer protocol client/ model client: browser that requests, receives, displays Web objects : Web sends objects in response to requests Uses TCP: PC running Explorer Mac running Navigator HTTP request HTTP request Server running Apache Web client inioates TCP connecoon (creates socket) to, port 80 accepts TCP connecoon from client HTTP messages (applicaoon- layer protocol messages) exchanged between browser (HTTP client) and Web (HTTP ) TCP connecoon closed HTTP is stateless maintains no informaoon about past client requests Protocols that maintain state are complex! past history (state) must be maintained if /client crashes, their views of state may be inconsistent, must be reconciled 3 HTTP connecoons Nonpersistent HTTP At most one object is sent over a TCP connecoon. Persistent HTTP MulOple objects can be sent over single TCP connecoon between client and. 4 2

3 Nonpersistent HTTP Suppose user enters URL (contains text, references to 10 jpeg images) 1a. HTTP client inioates TCP connecoon to HTTP (process) at on port HTTP client sends HTTP request message (containing URL) into TCP connecoon socket. Message indicates that client wants object somedepartment/ home.index 5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects 1b. HTTP at host waiong for TCP connecoon at port 80. accepts connecoon, noofying client 3. HTTP receives request message, forms response message containing requested object, and sends message into its socket 4. HTTP closes TCP connecoon. Ome 6. Steps 1-5 repeated for each of 10 jpeg objects 5 Non- Persistent HTTP: Response Ome DefiniOon of RTT: Ome for a small packet to travel from client to and back. Response Ome: one RTT to inioate TCP connecoon one RTT for HTTP request and first few bytes of HTTP response to return file transmission Ome total = 2RTT+transmit Ome initiate TCP connection RTT request file RTT file received time time time to transmit file 6 3

4 Persistent HTTP Nonpersistent HTTP issues: requires 2 RTTs per object OS overhead for each TCP connecoon browsers oeen open parallel TCP connecoons to fetch referenced objects Persistent HTTP leaves connecoon open aeer sending response subsequent HTTP messages between same client/ sent over open connecoon client sends requests as soon as it encounters a referenced object as lifle as one RTT for all the referenced objects 7 HTTP request message two types of HTTP messages: request, response HTTP request message: ASCII (human- readable format) request line (GET, POST, HEAD commands) header lines GET /somedir/page.html HTTP/1.1 Host: User-agent: Mozilla/4.0 Connection: close Accept-language:fr Carriage return, line feed indicates end of message (extra carriage return, line feed) 8 4

5 HTTP request message: general format 9 Uploading form input Post method: Web page oeen includes form input Input is uploaded to in enoty body URL method: Uses GET method Input is uploaded in URL field of request line:

6 Method types HTTP/1.0 GET POST HEAD asks to leave requested object out of response HTTP/1.1 GET, POST, HEAD PUT uploads file in enoty body to path specified in URL field DELETE deletes file specified in the URL field 11 message status line (protocol status code status phrase) data, e.g., requested HTML file header lines HTTP/ OK Connection close Date: Thu, 06 Aug :00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun Content-Length: 6821 Content-Type: text/html data data data data data

7 status codes In first line in - > client response message. A few sample codes: 200 OK request succeeded, requested object later in this message 301 Moved Permanently requested object moved, new locaoon specified later in this message (LocaOon:) 304 Not modified See condioonal GET 400 Bad Request request message not understood by 404 Not Found requested document not found on this 505 HTTP Version Not Supported 13 User- state: cookies Many major Web sites use cookies Four components: 1) cookie header line of HTTP response message 2) cookie header line in HTTP request message 3) cookie file kept on user s host, managed by user s browser 4) back- end database at Web site Example: Susan always access Internet always from PC visits specific e- commerce site for first Ome when inioal HTTP requests arrives at site, site creates: unique ID entry in backend database for ID 14 7

8 Cookies: keeping state (cont.) client ebay 8734 cookie file ebay 8734 amazon 1678 one week later: ebay 8734 amazon 1678 usual http request msg usual http response Set-cookie: 1678 usual http request msg cookie: 1678 usual http response msg usual http request msg cookie: 1678 usual http response msg Amazon creates ID 1678 for user create entry cookiespecific action cookiespectific action access access backend database 15 What cookies can bring: authorizaoon shopping carts Cookies (cononued) recommendaoons user session state (Web e- mail) How to keep state : protocol endpoints: maintain state at sender/receiver over mulople transacoons cookies: hfp messages carry state aside Cookies and privacy: cookies permit sites to learn a lot about you you may supply name and e- mail to sites 16 8

9 Web caches (proxy ) Goal: satisfy client request without involving origin 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, then returns object to client client client HTTP request HTTP request Proxy HTTP request origin origin 17 More about Web caching cache acts as both client and typically cache is installed by ISP (university, company, residenoal ISP) Why Web caching? reduce response Ome for client request reduce traffic on an insotuoon s access link. Internet dense with caches: enables poor content providers to effecovely deliver content (but so does P2P file sharing) 18 9

10 Caching example AssumpOons average object size = 1,000,000 bits avg. request rate from insotuoon s browsers to origin s = 15/sec delay from insotuoonal router to any origin and back to router = 2 sec Consequences uolizaoon on LAN = 15% uolizaoon on access link = 100% total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + milliseconds network public Internet 15 Mbps access link 100 Mbps LAN origin s cache 19 Caching example (cont) possible soluoon increase bandwidth of access link to, say, 100 Mbps consequence uolizaoon on LAN = 15% uolizaoon on access link = 15% Total delay = Internet delay + access delay + LAN delay = 2 sec + msecs + msecs oeen a costly upgrade network public Internet 100 Mbps access link 100 Mbps LAN origin s cache 20 10

11 Caching example (cont) possible soluoon: install cache suppose hit rate is 0.4 consequence 40% requests will be saosfied almost immediately 60% requests saosfied by origin uolizaoon of access link reduced to 60%, resulong in negligible delays (say 10 msec) total avg delay = Internet delay + access delay + LAN delay =. 6*(2.01) secs +.4*milliseconds < 1.4 secs network public Internet 15 Mbps access link origin s 100 Mbps LAN cache 21 CondiOonal GET Goal: don t send object if cache has up- to- date cached version cache: specify date of cached copy in HTTP request If-modified-since: <date> : response contains no object if cached copy is up- to- date: HTTP/ Not Modified cache HTTP request msg If-modified-since: <date> HTTP/ Not Modified HTTP request msg If-modified-since: <date> HTTP/ OK <data> object not modified object modified 22 11