HTTP Hypertext Transfer Protocol Jan 14 2019 HTTP messages HTTP is the language that web clients and web servers use to talk to each other HTTP is largely under the hood but a basic understanding can be helpful Each message whether a request or a response has three parts 1 2 3 The request or the response line A header section The body of the message 2 What the client does part I The client sends a message to the server at a particular port 80 is the default The first part of the message is the request line containing A method HTTP command such as GET or POST A document address and An HTTP version number Example GET index html HTTP 1 0 3 Other methods Other methods beside GET and POST are HEAD Like GET but ask that only a header be returned PUT Request to store the entity body at the URI DELETE Request removal of data at the URI LINK Request header information be associated with a document on the server UNLINK Request to undo a LINK request OPTIONS Request information about communications options on the server TRACE Request that the entity body be returned as received used for debugging 4 What the client does part II The second part of a request is optional header information such as What the client software is What formats it can accept All information is in the form Name Value Example User Agent Mozilla 2 02Gold WinNT I Accept image gif image jpeg A blank line ends the header 5 Client request headers Accept type subtype type subtype Accept Language en fr de The browser or other client program sending the request From dave acm org Preferred language For example English French German User Agent string Specifies media types that the client prefers to accept Email address of user of client program Cookie name value Information about a cookie for that URL Multiple cookies can be separated by commas 6 What the client does part III The third part of a request after the blank line is the entity body which contains optional data The entity body part is used mostly by POST requests The entity body part is always empty for a GET request 7 What the server does part I The server response is also in three parts The first part is the status line which tells The HTTP version A status code A short description of what the status code means Example HTTP 1 1 404 Not Found Status codes are in groups 100 199 200 299 300 399 400 499 500 599 Informational The request was successful The request was redirected The request failed A server error occurred 8 Common status codes 200 OK 301 Moved Permanently You can t do this and we won t tell you why 404 Not Found There is a syntax error in your request 403 Forbidden URL temporarily out of service keep the old one but use this one for now 400 Bad Request URI was moved but here s the new address for your records 302 Moved temporarily Everything worked here s the data No such document 408 Request Time out 504 Gateway Time out Request took too long to fulfill for some reason 9 What the server does part II The second part of the response is header information ended by a blank line Example Content Length 2532 Connection Close Server GWS 2 0 Date Sun 01 Dec 2002 21 24 50 GMT Content Type text html Cache control private Set Cookie All on PREF ID 05302a93093ec661 TM 1038777890 LM 10 one line 38777890 S yNWNjraftUz299RH expires Sun 17 Jan2038 19 14 07 GMT path domain google com 10 Viewing the response There is a header viewer at http www delorie com web headers html with nasty jittery advertisements Example 2 3 GetResponses in the Gittleman book does the same thing Here s an example from GetResponses java GetResponses http www cis upenn edu matuszek cit5972003 index html Status line HTTP 1 1 200 OK Response headers Date Wed 10 Sep 2003 00 26 53 GMT Server Apache 1 3 26 Unix PHP 4 2 2 mod perl 1 27 mod ssl 2 8 10 OpenSSL 0 9 6e Last Modified Tue 09 Sep 2003 19 24 50 GMT ETag 1c1ad5 1654 3f5e2902 Accept Ranges bytes Content Length 5716 Keep Alive timeout 15 max 100 Connection Keep Alive Content Type text html 11 The GetResponses program I Here s just the skeleton of the program that provided the output on the last slide import java net import java io public class GetResponses public static void main String args try interesting code goes here catch Exception e e printStackTrace 12 The GetResponses program II Here s the interesting part of the code URL url new URL args 0 URLConnection c url openConnection System out println Status line System out println t c getHeaderField 0 System out println Response headers String value int n 1 while true value c getHeaderField n if value null break System out println t c getHeaderFieldKey n value 13 Server response headers Server NCSA 1 3 Content Type type subtype Name and version of the server Should be of a type and subtype specified by the client s Accept header Set Cookie name value options Requests the client to store a cookie with the given name and value 14 What the server does part III The third part of a server response is the entity body This is often an HTML page But it can also be a jpeg a gif plain text etc anything the browser or other client is prepared to accept 15 The meta http equiv tag The meta http equiv string content string tag may occur in the head of an HTML document http equiv and content typically have the same kinds of values as in the HTTP header This tag asks the client to pretend that the information actually occurred in the header The information is not really in the header This tag is available because you have little direct control over what is in the header unless you write your own server As usual not all browsers handle this information the same way Example meta http equiv Set Cookie content value n expires date path url 16 Summary HTTP is a fairly straightforward protocol with a lot of possible kinds of predefined header information More kinds can be added so long as client and server agree A request from the client consists of three parts 1 2 3 A header line A block of header information ending with a blank line The optional entity body containing data A response from the server consists of the same three parts HTTP headers are under the hood information not normally displayed to the user As with most of the things covered in CIT597 We have covered only the fundamentals Much more detail can be found on the Web 17 The End 18
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