1 HTTP Messages Herng-Yow Chen. 2 Outline The Flow of Messages The Parts of a Message The various methods that request message support The various status

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HTTP Messages

Herng-Yow Chen

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Outline The Flow of Messages The Parts of a Message The various methods that request message

support The various status codes that are returned

with response message What the various headers do

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Messages Commute Inbound to the Origin Server

proxy1 proxy2 proxy3

GET /index.html http/1.0

HTTP/1.0 200 ok

Content-type: text/html

…

Inbound(to server)

Outbound(to user agent)

clientserver

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All messages flow downstream

proxy1

proxy2

proxy3

proxy1

proxy2

proxy3

No messages ever go upstream

Request(flowing downstream)

Response(flowing downstream)

client

client

server

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The Parts of a Message

HTTP/1.0 200 ok

Content-type: text/plain

Content-length: 19

Hi I’m a message

Start line

Headers

Bodyclient server

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Request Message Syntax

<method> <request-URL> <version>

<headers>

<entity-body>

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Response Message Syntax

<version> <status> <reason-phrase><headers>

<entity-body>

• Note that version numbers are not treated as fractional numbers. That is, HTTP/1.22 is higher version than HTTP/1.3.

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Message Syntax

client

GET /pics/hychen.jpg HTTP/1.0Host: www.csie.ncnu.edu.tw

HTTP/1.0 200 OKContent-type: image/jpegContent-length: 12345

www.csie.ncnu.edu.tw

HTTP request message containsThe command and the URI

HTTP response message containsThe result of the transaction

Internet

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Message Syntax

(a)Request message

(b)Response message

GET /test/hi-here.txt HTTP/1.1Accept: text/*

Host: www.csie.ncnu.edu.tw

HTTP/1.0 200 ok


Content-length: 19

Hi! I’m a message!

Start line

Headers

Body

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Common HTTP method Method Message

body? GET No HEAD No POST Yes PUT Yes TRACE No OPTIONS No DELETE NO

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Status code Overall range Define range Category

100-199 100-101 informational

200-299 200-206 successful

300-399 300-305 redirection

400-499 400-415 client error

500-599 500-505 server error

Common status codes 200 OK 401 Unauthorized 404 Not found

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Headers HTTP head field add additional information

to request and response messages. They are basically just lists of name/value pairs.

HTTP specification defines several header fields. Applications also are free to invent their own home-brewed headers.

Each HTTP has a simple syntax:a name, followed by a colon (:), followed by optional whitespace, followed by field value, followed by CRLF.

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Header classification General headers

Can appear in both request and response message Request headers

Provide more information about the request Response headers

Provide more information about the response Entity headers

Describe body size and contents, or the resource itself Extension headers

New headers that are not defined in the specification

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Common header examples Header example

Date: Tue, 3 Oct 1997 02:16:03 GMT Content-length: 15040 Content-type: image/gif Accept: image/gif, image/jpeg, text/html

Header continuation lines Long header lines can be made more readable by

breaking them into multiple lines, preceding each extra line with at least one space or tab character. For example,

Server: Test ServerVersion 1.0

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Entity bodies The third part of an HTTP message is the

optional entity body. Entity bodies are the payload of the HTTP messages.

HTTP message can carry many kinds of digital data: images, video, HTML documents, software applications, credit card transactions, electronic mail, and so on.

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Version 0.9 Messages

client

client

GET /pics/hychen.jpg

No version number www.csie.ncnu.edu.tw


* The request contained merely the method and the request URL, and * the response contained only the entity.* No version number, no status code, no headers were included.

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Methods Note that not all methods are implemented by

every server. To be compliant with HTTP/1.1, a server need

implement only the GET and HEAD methods for its resources.

Even when servers do implement all of these common methods (listed in the slide 10), some methods such as DELETE and PUT most likely have restricted use. The restrictions generally are set up in the server’s configuration, so they are vary from site to site and from server to server.

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GET


GET /~hychen/index.html HTTP/1.1


Accept: *

client

Request message

HTTP/1.1 200 ok

Content-Type: text/html

Content-Length: 617

<html>

<HEAD><TITLE>Home Page</TITLE>

…

Response message

* The most common method. It usually is used to ask a server to send a resource.

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HEAD


HEAD /~hychen/index.html HTTP/1.1


Accept: *

client

Request message

HTTP/1.1 200 ok

Content-Type: text/html

Content-Length: 617

Response message

No entity body

* It behaves exactly like the GET method, but the server returns only the headers in the response.

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HEAD No entity body is ever returned. This allows

a client to inspect the headers for a resource without having to actually get the resource. Using the header, you can Find out about a resource (e.g., determine its

type) without getting it. See if an object exists, by looking at the status

code of the response. Test if the resource has been modified, by

looking at the headers.

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PUT


PUT /product-list.txt HTTP/1.1



Updated product list coming soon!

client

Request message

HTTP/1.1 201 Created

Location: Http://www.csie.ncnu.edu.tw/product-list.txt

Content-Type: text/plain

Content-Length: 47

Http://www.csie.ncnu.edu.tw/product-list.txt

Response message

Server updates/creates resource ”/product-list.txt” and writes it to its disk.

The PUT method writes documents to a server, in the inverse of the way that GET reads documents from the server.

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PUT Some publishing systems (e.g. frontpage, dream

waver) let you create web pages in a local computer and install them directly on a web server via PUT method.

The semantics of the PUT method are for the server to take the body of the request and either use it to create a new document named by the requested URL, or, if that URL already exists, use the body to replace it.

Because PUT allows you to change content, many web servers require you to log in with a password fore you can perform a PUT.

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POST PUT is used to deposit data into a resource

on the server (e.g., a file). In comparison, the POST method was designed to send input data to the server.

In practice, it is used to support HTML forms. The data from a filled-in from typically is sent to a server gateway program to process it.

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POST

POST /check.cgi HTTP/1.1



Content-Length: 18

item=bandsaw 2647

HTTP/1.1 200 OK


Context-lenght: 37

The bandsaw model 2647 is in stock!

client


Request message

Response message

YES!

item=bandsaw 2647

Inventory list

Inventory check

Browser sticks data in entity body of message

CGI program

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TRACE When a client makes a request, that request may

have to travel through firewalls, proxies, gateways, or other applications. Each of these has the opportunity to modify the original HTTP request.

The TRACE method allows clients to see how its request looks when it finally makes it to the server.

The TRACE method is used primarily for diagnostics; i.e., verifying that requests are going through the request/response chain as intended.

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TRACE

client

Proxy


TRACE /product-list.txt HTTP/1.1Accept:*Host : www.csie.ncnu.edu.tw

Request messageTRACE /product-list.txt HTTP/1.1Host : www.csie.ncnu.edu.twAccept:*Via:1.1 proxy.ncnu.edu.tw

HTTP/1.1 200 OKContent-type: text/plainContent-length: 96

TRACE /product-list.txt HTTP/1.1Host: www.csie.ncnu.edu.twAccept: *Via: 1.1 proxy.ncnu.edu.tw

Response messageHTTP/1.1 200 OKContent-type: text/plainContent-length: 96Via:1.1 proxy.ncnu.edu.tw

TRACE /product-list.txt HTTP/1.1Host: www.csie.ncnu.edu.twAccept: *Via: 1.1 proxy.ncnu.edu.tw

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OPTIONS The OPTIONS method asks the server tell

us about the various supported capabilities of the web server.

Some servers may support particular operations on on particular kinds of objects.

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OPTIONS

client www.csie.ncnu.edu.tw

Request messageOPTIONS * HTTP/1.1Host : www.csie.ncnu.edu.twAccept : *

Response messageHTTP/1.1 200 OK

Allow: GET, POST, PUT, OPTIONS

Context-length: 0

Since the request is for options on all resources, the server just returns the methods it supports for its resources.

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DELETE The DELETE method asks the server to

delete the resource specified by the requested URL.

However, the client application is not guaranteed that the delete is carried out. This is because the HTTP specification allows the server to override the request without telling the client.

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DELETE

client www.csie.ncnu.edu.tw

Request messageDELETE /product-list.txt HTTP/1.1Host : www.csie.ncnu.edu.tw



Content-Length: 54

I have your delete request,

will take time to process.

File ”product-list.txt”

removed from server’s disk

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Extension Methods HTTP was designed to be field-extensible,

so new features wouldn’t cause older software to fail.

Extension Method are method that are not defined in HTTP/1.1 specification. They provide developers with a means of extending the capabilities of the HTTP services their servers implement on the resources that the servers manage.

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WebDAV HTTP extension Method DescriptionLOCK Allows a user to “lock” a resource–

for example, you could lock a resourcewhile you are editing it

MKCOL Allows a user to create a resource

COPY Facilitates copying resources on a server

MOVE Moves a resource on a server

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Status Codes 100-199: Informational Status Code

Status Code Reason Phrase

100 Continue

101 Switching Protocols

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200-299: success status code

Status Code Reason Phrase200 OK

201 Created

202 Accepted

203 Not-Authoritative Information

204 Not Content

205 Reset Content

206 Partial Content

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300-399: Redirection Status Code

Status code Reason Phrase300 Multiple Choices

301 Moved Permanently

302 Found

303 See other

304 Not Modified

305 Use Proxy

306 (Unused)

307 Temporary Redirect

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300-399:Redirection Status Codes

clientwww.csie.ncnu.edu.t

w

Request messageGet /pet-products.txt HTTP/1.1Host: www.csie.ncnu.edu.twAccept: *

Response messageHTTP/1.1 301 OKLocation: http://english.csie.ncnu.edu.twContent-length: 56Content-type: text/plainPlease go to our partner site,English.csie.ncnu.edu.tw

clientenglish.csie.ncnu.edu.t

w

Request messageGet / HTTP/1.1Host: english.csie.ncnu.edu.twAccept: *


Content-type: text/html

Content-length: 3307 …

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300-399:Redirection Status Codes (cont.)

client

Request messageGet /m_faculty.html HTTP/1.1Host: www.csie.ncnu.edu.twAccept: *If-Modified-Since: Wed, Sept. 3 2003 02:16:00 GMT


Has not changed

Client has previously requested copy of:

http://www.csie.ncnu.edu.tw/m_faculty.html

client

Response messageHTTP/1.1 304 Not Modified

…

Browswe displays local copy, since the original has not changed since we last requested it

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400-499: Client Errors Status Codes

Status Code Reason Phrase400 Bad Request401 Unauthorized402 Payment Requested403 Forbidden404 Not Found405 Method Not Allowed406 Not Acceptable407 Proxy Authentication Required 408 Request Timeout409 Conflict

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400-499: Client Errors Status Codes

Status Code Reason Phrase410 Gone

411 Length Required

412 Precondition Failed

413 Request Entity Too Large

414 Request URI Too Long

415 Unsupported Media Type

416 Requested Range Not Satisfiable

417 Expectation Failed

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500-599: Server Error Status Code Reason Phrase

500 Internal Server Error

501 Not Implemented

502 Bad Gateway

503 Service Unavailable

504 Gateway Timeout

505 HTTP Version Not Supported

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Headers General headers

E.g., Date: Tue, 3 Oct 2003 02:16:00 GMT Request headers

E.g., Accept: */* Response headers

E.g., Server: Apache/1.3 Entity headers

E.g., Content-Type: text/html; charset=iso-latin-1 Extension headers

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General HeadersHeader Description Connection Allow clients and servers to specify

options about the request/response connection

Date MIME-Version Trailer List the set of headers that are in the

trailer of a message encoded with thechunked transfer encoding

Transfer-Encoding Upgrade Via

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General caching headers

Header Description Cache-Control Used to pass caching directions alon

gwith the message

Progma Another way to pass direction along the

message, though not specific caching

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General caching headers

Header Description Cache-Control Used to pass caching directions alon

gwith the message

Progma Another way to pass direction along the

message, though not specific caching

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Request HeadersHeader Description Client-IP From Host Referer UA-Color UA-CPU UA-Disp UA-OS UA-Pixels User-Agent

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Accept Headers

Header Description Accept what media types Accept-Charset Accept-Encoding Accept-Language TE what extension transfer

codings are okay to use

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Request HeadersHeader Description Client-IP From Host Referer UA-Color UA-CPU UA-Disp UA-OS UA-Pixels User-Agent

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Accept Headers

Header Description Accept what media types Accept-Charset Accept-Encoding Accept-Language TE what extension transfer

codings are okay to use

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Conditional requester headers

Header Description Expect If-Match If-Modified-Since If-None-Match If-Range If-Unmodified-Since Range

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Request security headers

Header Description Authorization Cookie Cookie2

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Proxy request headers

Header Description Max-Forwards Proxy-Authorization Proxy-Connection

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Response headers

Header Description Age Public Retry-After Server Title Warning

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Negotiation headers

Header Description Accept-Ranges Vary

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Response security headers

Header Description Proxy-Authenticate Set-Cookie Set-Cookie2 WWW-Authenticate

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Entity Headers

Header Description Content-Base Content-Encoding Content-Language Content-Length Content-Location Content-MD5 Content-Range Content-Type

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Entity caching headers

Header Description ETag Expires Last-Modified

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HTTP protocol information http://www.w3.org/Protocols/

The W3C architecture page for HTTP. http://www.ietf.org/rfc/rfc2621.txt

RFC 2616, “Hypertext Transfer Protocol- HTTP/1.1”, is the official specification for HTTP/1.1, the current version of the HTTP protocol.

Documents

1 HTTP Messages Herng-Yow Chen. 2 Outline The Flow of Messages The Parts of a Message The various methods that request message support The various status