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Hypertext Transfer Protocol Hypertext Transfer Protocol Ing. Pierluigi Gallo HTTP Hypertext Transfer Protocol used in the WWW protocol used for communication between web browsers and web servers client-server paradigm TCP port 80 RFC 1945 Ing. Pierluigi Gallo Introduction to HTTP 80% of Internet flows are HTTP connections Early protocol is HTTP 0.9 read only Today we use HTTP 1.0 read, input, delete, ... New version: HTTP 1.1 performance optimizations Ing. Pierluigi Gallo HTTP Overview Client (browser) sends HTTP request to server Request specifies affected URL Request specifies desired operation Server performs operation on URL Server sends response Request and reply headers are in pure text Ing. Pierluigi Gallo Static Content and HTML Most static web content is written in HTML HTML allows Text formatting commands Embedded objects Links to other objects Server need not understand or interpret HTML Ing. Pierluigi Gallo URI,URN,URL RFC 3305 Uniform Resource Identifier Identifies a resource Uniform Resource Name The name of the resource with in a namespace like a person’s name Uniform Resource Locator How to find the resource, a URI that says how to find the resource like a person street address URI concept is more general than its use in web pages (XML, …) Ing. Pierluigi Gallo HTTP - URLs URL Uniform Resource Locator protocol (http, ftp, news) host name (name.domain name) port (80, 8080, …) directory path to the resource resource name absolute relative http://www.tti.unipa.it/~pg/pg/Teaching.html http://xxx.myplace.com:80/cgi-bin/t.exe Ing. Pierluigi Gallo URI examples http://example.org/absolute/URI/with/absolute/ path/to/resource.txt ftp://example.org/resource.txt urn:issn:1535-3613 /relative/URI/with/absolute/path/to/resource.txt relative/path/to/resource.txt ../../../resource.txt ./resource.txt#frag01 Ing. Pierluigi Gallo HTTP - methods Methods GET retrieve a URL from the server simple page request depending on the requested page: run a CGI program run a CGI with arguments attached to the URL POST preferred method for forms processing run a CGI program parameterized data in sysin more secure and private Ing. Pierluigi Gallo HTTP - methods Methods (cont.) PUT Used to transfer a file from the client to the server HEAD requests URLs status header only used for conditional URL handling for performance enhancement schemes retrieve URL only if not in local cache or date is more recent than cached copy DELETE deletes page from server Ing. Pierluigi Gallo req-resp approach client-server network protocol in use by the World-Wide Web since 1990 request-response HTTP request messages for HTML pages, images, scripts and styles sheets. Web servers handle these requests by returning response messages that contain the requested resource. Ing. Pierluigi Gallo Example of an HTTP Exchange Client Server GET www.cs.virginia.edu Retrieve Data From Disk Ing. Pierluigi Gallo Fetching Multiple Objects Most web-pages contain embedded objects (e.g., images, backgrounds, etc) Browser requests HTML page Server sends HTML file Browser parses file and requests embedded objects Server sends requested objects Ing. Pierluigi Gallo Fetching Embedded Objects Client Server GET www.cs.virginia.edu Retrieve Data From Disk GET image.gif Retrieve Image From Disk Ing. Pierluigi Gallo HTTP Request Packets Sent from client to server Consists of HTTP header header is hidden in browser environment contains: content type / mime type content length user agent - browser issuing request content types user agent can handle HTTP 1.1 is the and a URL latest version GET /simtec/httpgallery/introduction/ HTTP/1.1 Accept:*/* Accept-Language: en-gb Accept-Encoding: gzip, deflate User-Agent: Mozilla/4.0 (compatible; MSIE 6.0) Host: www.httpwatch.com Ing. PierluigiConnection: Gallo Keep-Alive HTTP Request Headers Precede HTTP Method requests headers are terminated by a blank line Header Fields: From Accept Accept-Encoding Accept Language Ing. Pierluigi Gallo HTTP 1.1 vs 1.0 Additional Methods (PUT, DELETE, TRACE, CONNECT + GET, HEAD, POST) Additional Headers Transfer Coding (chunk encoding) Persistent Connections (content-length matters) Request Pipelining Ing. Pierluigi Gallo HTTP 1.0 Client opens a separate TCP connection for each requested object Object is served and connection is closed Advantages maximum concurrency Limitations TCP connection setup/tear-down overhead TCP slow start overhead Ing. Pierluigi Gallo HTTP 1.0 connect() Client SYN Server SYN, ACK write() ACK, GET www.cs.virginia.edu Retrieve Data From Disk close() connect() SYN SYN, ACK write() ACK, GET image.gif Retrieve Image From Disk Ing. Pierluigiclose() Gallo HTTP 1.1 To avoid a connection per object model, HTTP 1.1 supports persistent connections Client opens TCP connection to server All requests use same connection Problems Less concurrency Server does not know when to close idle connections Ing. Pierluigi Gallo HTTP 1.1 connect() Client SYN Server SYN, ACK write() ACK, GET www.cs.virginia.edu Retrieve Data From Disk write() GET image.gif Retrieve Image From Disk close() Ing. Pierluigi Gallo Server Side Close() Client Server connect() SYN Set timeout SYN, ACK Reset write() ACK, GET www.cs.virginia.edu timeout Retrieve Data From Disk GET image.gif write() Retrieve Image From Disk Timeout! Ing. Pierluigi Gallo close() CGI Scripts Common Gateway Interface web server software can delegate the generation of web pages to a stand-alone application, an executable file. CGI scripts are URLs with a .cgi extension The script is a program (e.g., C, JAVA, …) When the URL is requested, server invokes the named script, passing to it client info Script outputs HTML page to standard output (redirected to server) Server sends page to client Ing. Pierluigi Gallo CGI Execution fork() CGI Server Script Send page Request Response Ing. Pierluigi Gallo Modified-Since: Used with GET to make a conditional GET if requested document has not been modified since specified date a Modified 304 header is sent back to client instead of document client can then display cached version Ing. Pierluigi Gallo Status Header “HTTP/1.0 sp code” Codes: 1xx - reserved for future use 2xx - successful, understood and accepted 3xx - further action needed to complete 4xx - bad syntax in client request 5xx - server can’t fulfill good request Ing. Pierluigi Gallo HTTP Response Headers Sent by server to client browser Status Header Entities Content-Encoding: Content-Length: Content-Type: Expires: Last-Modified: extension-header Body – content (usually html) Ing. Pierluigi Gallo Status Codes 200 OK 401 unauthorized 201 created 403 forbidden 202 accepted 404 not found 204 no content 500 int. server error 301 moved perm. 501 not impl. 302 moved temp 502 bad gateway 304 not modified 503 svc not avail 400 bad request Ing. Pierluigi Gallo Statelessness Because of the Connect, Request, Response, Disconnect nature of HTTP it is said to be a stateless protocol i.e. from one web page to the next there is nothing in the protocol that allows a web program to maintain program “state” (like a desktop program). “state” can be maintained by “witchery” or “trickery” if it is needed Ing. Pierluigi Gallo Maintaining program “state” Hidden variables (<input type=hidden> Sessions Special header tags interpreted by the server Used by ASP, PHP, JSP Implemented at the language api level Ing. Pierluigi Gallo HTTPS Secure Hypertext Transport Protocol Ing. Pierluigi Gallo SSL vs S-HTTP Secure Sockets Layer (SSL) protocol has become the Internet’s key “secure protocol” supports security across a variety of Internet transfer protocols (FTP, HTTP, IRC, etc…) Support public keys and private key. Ing. Pierluigi Gallo Introduction S-HTTP Secure Hypertext Transport Protocols (S-HTTP) - Is a modified version of the Hypertext Transport Protocols (HTTP). - Encryption for Web documents. - Provides the client (browser) the ability to verify message by using a Message Authentication Code (MAC). - Primary purpose is to enable commercial transactions within a wide range of applications. Ing. Pierluigi Gallo S-HTTP Methods Signature Encryption Message sender Authenticity Ing. Pierluigi Gallo S-HTTP vs HTTP HTTP message S-HTTP message message body encrypted message message header body message header Ing. Pierluigi Gallo 3 steps how server create S-HTTP message Server Encrypt Methods List KPCS-7 RSA Diffie- Hellman Encrypt Method KPCS-7 Client Encrypt Methods List KPCS-7 RSA Diffie- Hellman Server compares encryption lists and selection. Ing. Pierluigi Gallo from the plain text to the encrypted message Public-Key Cryptography Standards (PKCS) S-http header Client’s PKCS-7 <html> !@##$$%%* PKCS-7 This is This is !@##$$%%* @***&^&^%$ my PKCS-7 @***&^&^%$ @#$$@@!&^ my Session message @#$$@@!&^ Key message </html> RFC 2315. Used to sign and/or encrypt messages under a PKI. Used also for certificate dissemination (for instance as a response to a PKCS#10 message). Formed the basis for S/MIME, which is as of 2010 based on RFC 5652, an updated Cryptographic Message Syntax Standard (CMS). Often used for single sign-on. Ing.PKCS Pierluigi Gallo Cryptographic Algorithm and digital signature modes for S-HTTP S-HTTP provides message protection in 3 ways: Digital signature Message authentication Message encryption Ing. Pierluigi Gallo Further readings Hypertext Transfer Protocol -- HTTP/1.1 RFC 2616 Hypertext Transfer Protocol -- HTTP/1.0 RFC 1945 Upgrading
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