Transcript Document

HTTP
Hypertext Transport Protocol
Hypertext Transfer Protocol
(HTTP)
 A communications protocol
 Used to transfer or convey information on the World
Wide Web
 Original purpose was to provide a way to publish and
retrieve HTML hypertext pages
 Development of HTTP was coordinated by
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W3C (World Wide Web Consortium)
IETF (Internet Engineering Task Force)
 Culminating in the publication of a series of RFCs
 Most notably RFC 2616 (June 1999)
 Defines HTTP/1.1, the version of HTTP in common
use today
Hypertext Transfer Protocol
(HTTP)
 HTTP is a request/response protocol between
clients and servers

Client makes an HTTP request
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Referred to as the user agent
 A web browser, spider, or other end-user tool
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Server responds
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Called the origin server
 Stores or creates resources such as HTML files and
images
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In between the user agent and origin server may
be several intermediaries
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proxies, gateways, tunnels, etc.
Hypertext Transfer Protocol
(HTTP)
 HTTP does not need to use TCP/IP or its
supporting layers
 HTTP:
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Can be implemented on top of any other protocol
on the Internet, or on other networks
Only presumes a reliable transport
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Any protocol that provides such guarantees can be
used
Hypertext Transfer Protocol
(HTTP)
 An HTTP client initiates a request by establishing a
Transmission Control Protocol (TCP) connection to a
particular port on a host
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Port 80 by default
An HTTP server listening on that port waits for the client
to send a request message
 Upon receiving the request, the server sends back
 A status line
 E.g. "HTTP/1.1 200 OK“
 A message of its own
 Body of which is perhaps the requested file, an error
message, or some other information
Hypertext Transfer Protocol
(HTTP)
 Resources to be accessed by HTTP are
identified using
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Uniform Resource Identifiers (URIs)
Or, more specifically, URLs
 Using the http: or https URI schemes
Request message
 The request message consists of the
following:
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Request line
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E.g. GET /images/logo.gif HTTP/1.1
 Requests the file logo.gif from the /images directory
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Headers
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E.g. Accept-Language: en
An empty line
An optional message body
Request message
 The request line and headers must all end
with CRLF
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A carriage return followed by a line feed
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ASCII Code 13 followed by an ASCII Code 10
An empty line must consist of only CRLF and
no other whitespace
In the HTTP/1.1 protocol, all headers except
Host are optional
HTTP Methods
 HTTP defines eight methods
 Indicates
the desired action to be
performed on the identified resource
 Sometimes referred to as "verbs"
Request Methods
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HEAD
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Asks for the response identical to the one that would
correspond to a GET request, but without the response body
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Useful for retrieving meta-information written in response
headers, without having to transport the entire content
GET
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Requests a representation of the specified resource
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By far the most common method used on the Web today
Should not be used for operations that cause side-effects
 Using it for actions in web applications is a common misuse
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See 'safe methods' below
POST
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Submits data to be processed (e.g. from an HTML form) to the
identified resource
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The data is included in the body of the request
May result in the creation of a new resource or the updates of
existing resources or both
Request methods
 PUT
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Uploads a representation of the specified resource
 DELETE
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Deletes the specified resource
 TRACE
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Echoes back the received request
 so a client can see what intermediate servers are adding
or changing in the request
 OPTIONS
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Returns the HTTP methods that the server supports
 Can be used to check the functionality of a web server
 CONNECT
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Converts the request connection to a transparent TCP/IP
tunnel
 Usually to facilitate SSL-encrypted communication
(HTTPS) through an unencrypted HTTP proxy
Request methods
 HTTP servers are supposed to
implement at least:
GET and HEAD methods
 OPTIONS method
 Whenever possible
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Request methods
 Safe methods
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Some methods (e.g. HEAD or GET) are defined as safe, which means
they are intended only for information retrieval and should not change the
state of the server
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In other words, they should not have side effects
Unsafe methods (such as POST, PUT and DELETE) should be displayed
to the user in a special way
 Typically as buttons rather than links
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Make the user aware of possible obligations
 Such as a button that causes a financial transaction
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Despite the required safety of GET requests they can cause changes on
the server
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For example, a Web server may use the retrieval through a simple
hyperlink to initiate deletion of a domain database record, thus causing a
change of the server's state as a side-effect of a GET request
 This is discouraged, because it can cause problems for Web caching, search
engines and other automated agents, which can make unintended changes on
the server
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Another case is that a GET request may cause the server to create a
cache space
Request methods
 Idempotent methods and Web Applications
 Methods GET, HEAD, PUT and DELETE are defined to be idempotent
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The RFC allows a user-agent, such as a browser to assume that any
idempotent request can be retried without informing the user
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This is done to improve the user experience when connecting to
unresponsive or heavily-loaded web servers
However, note that the idempotence is not assured by the protocol or
web server
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Multiple identical requests should have the same effect as a single request
Methods OPTIONS and TRACE, being safe, are inherently idempotent
It is perfectly possible to write a web application in which (eg) a database
insert or update is triggered by a GET request - this would be a very normal
example of what the spec refers to as "a change in server state"
This misuse of GET can combine with the retry behavior above to
produce erroneous transactions and used, as intended, for document
retrieval only
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For this reason GET should be avoided for anything transactional
HTTP versions
 HTTP has evolved into multiple, mostly
backwards-compatible protocol versions.
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RFC 2145 describes the use of HTTP version
numbers
The client tells in the beginning of the request the
version it uses, and the server uses the same or
earlier version in the response
HTTP versions
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0.9
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HTTP/1.0 (May 1996)
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Deprecated
Supports only one command, GET — which does not specify the HTTP version
Does not support headers
Since this version does not support POST, the client can't pass much information to the
server
This is the first protocol revision to specify its version in communications
Still in wide use, especially by proxy servers
HTTP/1.1 (June 1999)
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Current version; persistent connections enabled by default and works well with proxies.
Supports request pipelining
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Allows multiple requests to be sent at the same time
Allows the server to prepare for the workload and potentially transfer the requested
resources more quickly to the client
HTTP/1.2
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The initial 1995 working drafts were prepared by the W3C and submitted to the IETF
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an Extension Mechanism for HTTP
proposed the Protocol Extension Protocol, abbreviated PEP
PEP was originally intended to become a distinguishing feature of HTTP/1.2
In later PEP working drafts, however, the reference to HTTP/1.2 was removed
The experimental RFC 2774, HTTP Extension Framework, largely subsumed PEP.
It was published in February 2000
Status codes
 In HTTP/1.0 and since, the first line of the HTTP
response is called the status line
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Includes a
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The way the user agent handles the response
primarily depends on
1.
2.
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Numeric status code (such as "404")
Textual reason phrase (such as "Not Found").
the code
the response headers
Custom status codes can be used
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If the user agent encounters a code it does not
recognize, it can use the first digit of the code to
determine the general class of the response.
Status codes
 Standard reason phrases are only recommendations
 Can be replaced with "local equivalents" at the web
developer's discretion
 If the status code indicated a problem
 The user agent might display the reason phrase to the user
to provide further information about the nature of the problem
 The standard also allows the user agent to attempt to
interpret the reason phrase
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This might be unwise since the standard explicitly specifies
that
 Status codes are machine-readable
 Reason phrases are human-readable.
Status Codes
 1xx Informational
 2xx Success
 3xx Redirection
 4xx Client Error
 5xx Server Error
1xx Informational
 Request received, continuing process.
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This class of status code indicates a
provisional response
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Consists only of the Status-Line and optional
headers
Terminated by an empty line
Since HTTP/1.0 did not define any 1xx status
codes, servers MUST NOT send a 1xx
response to an HTTP/1.0 client except under
experimental conditions
1xx Informational
 100 Continue
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The server has received the request headers
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The client should proceed to send the request body
 in the case of a request for which a body needs to be sent
 for example, a POST request
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If the request body is large, sending it to a server when a
request has already been rejected based upon inappropriate
headers is inefficient
To have a server check if the request could be accepted based
on the request's headers alone, a client must send
 Expect: 100-continue as a header in its initial request
 see RFC 2616 §14.20: Expect header)
 Check if a 100 Continue status code is received in response before
 Continuing
or
 receive 417 Expectation Failed and not continue
 101 Switching Protocols
 102 Processing (WebDAV)
2xx Success
 The action was successfully received,
understood, and accepted
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This class of status code indicates that the
client's request was successfully received,
understood, and accepted
2xx Success
 200 OK
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Standard response for successful HTTP requests.
 201 Created
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Request has been fulfilled and resulted in a new resource being create
 202 Accepted
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Request has been accepted for processing
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Request might or might not eventually be acted upon
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The processing has not been completed
It might be disallowed when processing actually takes place
203 Non-Authoritative Information (since HTTP/1.1)
204 No Content
205 Reset Content
206 Partial Content
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Notice that a file has been partially downloaded.
Used by tools like wget to enable resuming of interrupted downloads, or split a
download into multiple simultaneous streams.
 207 Multi-Status (WebDAV)
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The message body that follows is an XML message and can contain a number of
separate response codes, depending on how many sub-requests were made.
3xx Redirection
 The client must take additional action to complete the
request
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This class of status code indicates that further action
needs to be taken by the user agent in order to fulfill
the request
The action required MAY be carried out by the user
agent without interaction with the user if and only if the
method used in the second request is GET or HEAD
A user agent SHOULD NOT automatically redirect a
request more than 5 times, since such redirections
usually indicate an infinite loop
3xx Redirection
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300 Multiple Choices
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Indicates multiple options for the URI that the client may follow.
Canbe used to present different format options for video, list files with different extensions, or word sense disambiguation.
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301 Moved Permanently
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302 Found
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The response to the request can be found under another URI using a GET method.
304 Not Modified
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Most popular redirect code, but also an example of industrial practice contradicting the standard.
HTTP/1.0 specification (RFC 1945) required the client to perform a temporary redirect (the original describing phrase was
"Moved Temporarily"), but popular browsers implemented it as a 303 See Other.
Therefore, HTTP/1.1 added status codes 303 and 307 to disambiguate between the two behaviors.
However, the majority of Web applications and frameworks still use the 302 status code as if it were the 303.
303 See Other (since HTTP/1.1)
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This and all future requests should be directed to the given URI.
Indicates the request URL has not been modified since last requested.
Typically, the HTTP client provides a header like the If-Modified-Since header to provide a time with which to compare
Utilizing this saves bandwidth and reprocessing on both the server and client.
305 Use Proxy (since HTTP/1.1)
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Many HTTP clients (such as Mozilla [1] and Internet Explorer) don't correctly handle responses with this status code,
primarily for security reasons
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306 Switch Prox
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307 Temporary Redirect (since HTTP/1.1)
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No longer used.
In this occasion, the request should be repeated with another URI, but future requests can still be directed to the original
URI.
In contrast to 303, the request method should not be changed when reissuing the original request.
For instance, a POST request must be repeated using another POST request
4xx Client Error
 The request contains bad syntax or cannot be fulfilled
 The 4xx class of status code is intended for cases in which
the client seems to have erred
 Except when responding to a HEAD request, the server
SHOULD include an entity containing an explanation of
the error situation, and whether it is a temporary or
permanent condition
 These status codes are applicable to any request method
 User agents SHOULD display any included entity to the
user
4xx Client Error
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400 Bad Request
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401 Unauthorized
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Original intention was that this code might be used as part of some form of digital cash or
micropayment scheme
Has not happened, and this code has never been used
403 Forbidden
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Similar to 403 Forbidden, specifically for use when authentication is possible but has
failed or not yet been provided
402 Payment Required
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The request contains bad syntax or cannot be fulfilled.
Request was a legal request, but the server is refusing to respond to it
Unlike a 401 Unauthorized response, authenticating will make no difference
404 Not Found
405 Method Not Allowed
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Request made to a URL using a request method not supported by that URL
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Using GET on a form which requires data to be presented via POST
Using PUT on a read-only resource
406 Not Acceptable
407 Proxy Authentication Required
408 Request Timeout
409 Conflict
4xx Client Error
 410 Gone
 Indicates that the resource requested is no longer
available and will not be available again
 Should be used when a resource has been
intentionally removed
 In practice, a 404 Not Found is often issued instead
 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
4xx Client Error
 422 Unprocessable Entity (WebDAV)
Request was well-formed but was unable to be followed due to
semantic errors
423 Locked (WebDAV)
 The resource that is being accessed is locked
424 Failed Dependency (WebDAV)
 The request failed due to failure of a previous request (e.g. a
PROPPATCH).
425 Unordered Collection
 Defined in drafts of WebDav Advanced Collections
 Not present in "Web Distributed Authoring and Versioning (WebDAV)
Ordered Collections Protocol"
426 Upgrade Required
 The client should switch to TLS/1.0.
449 Retry With
 A Microsoft extension: The request should be retried after doing the
appropriate action.
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5xx Server Error
 The server failed to fulfill an apparently valid request
 Response status codes beginning with the digit "5" indicate
cases in which the server is aware that it has erred or is
incapable of performing the request
 Except when responding to a HEAD request, the server
SHOULD include an entity containing an explanation of
the error situation, and whether it is a temporary or
permanent condition
 User agents SHOULD display any included entity to the
user
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These response codes are applicable to any request method
5xx Server Error
 500 Internal Server Error
 501 Not Implemented
 502 Bad Gateway
 503 Service Temporarily Unavailable
 504 Gateway Timeout
 505 HTTP Version Not Supported
 506 Variant Also Negotiates
 507 Insufficient Storage (WebDAV)
 509 Bandwidth Limit Exceeded
 Not an official HTTP status code
 Still used by many servers
 510 Not Extended (RFC 2774)
Persistent connections
 In HTTP/0.9 and 1.0, the connection is closed after a single
request/response pair.
 In HTTP/1.1 a keep-alive-mechanism was introduced, where a
connection could be reused for more than one request.
 Such persistent connections reduce lag perceptibly, because
the client does not need to re-negotiate the TCP connection
after the first request has been sent.
 Version 1.1 of the protocol also introduced:
 Chunked transfer encoding to allow content on persistent
connections to be streamed, rather than buffered
 HTTP pipelining, which allows clients to send some types of
requests before the previous response has been received,
further reducing lag
Main
article: HTTP persistent connections
HTTP session state
 HTTP can occasionally pose problems for Web developers and
applications since HTTP is stateless
 The advantage of a stateless protocol is that hosts do not
need to retain information about users between requests
 This forces the use of alternative methods for maintaining
users' state
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E.g. when a host would like to customize content for a user who
has visited before
One common method for solving this problem involves the
use of sending and requesting cookies
Other methods include
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Server side sessions
Hidden variables
 When current page is a form
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URL encoded parameters
 Such as /index.php?userid=3
Secure HTTP
 There are currently two methods of
establishing a secure HTTP connection:
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The https URI scheme
The HTTP 1.1 Upgrade header
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Introduced by RFC 2817
 Browser support for the Upgrade header is
nearly non-existent
 The https URI scheme is still the dominant
method of establishing a secure HTTP
connection
Secure HTTP
 https URI scheme
 A URI scheme syntactically identical to the http:
scheme used for normal HTTP connections
 Signals the browser to use an added encryption
layer of SSL/TLS to protect the traffic
 SSL – Secure Sockets Layer
 TLS – Transport Layer Security
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Main
SSL is especially suited for HTTP since it can
provide some protection even if only one side of
the communication is authenticated
In the case of HTTP transactions over the
Internet, typically, only the server side is
authenticated
article: https
Secure HTTP
 HTTP 1.1 Upgrade header
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HTTP 1.1 introduced support for the Upgrade header.
In the exchange
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The client begins by making a clear-text request, which is later
upgraded to TLS
Either the client or the server may request (or demand) that the
connection be upgraded
The most common usage is a clear-text request by the client followed
by a server demand to upgrade the connection, which looks like this:
 Client:
 GET /encrypted-area HTTP/1.1
 Host: www.example.com
 Server:
 HTTP/1.1 426 Upgrade Required
 Upgrade: TLS/1.0, HTTP/1.1
 Connection: Upgrade
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The server returns a 426 status-code because 400 level codes
indicate a client failure
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Correctly alerts legacy clients that the failure was client-related
Secure HTTP
 Benefits of using this method for establishing a secure
connection are:
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Removes messy and problematic redirection and URL
rewriting on the server side
Allows virtual hosting (single IP, multiple domain-names)
of secured websites
Reduces user confusion by providing a single way to
access a particular resource
 A weakness with this method is:
 Requirement for secure HTTP cannot be specified in the
URI
 In practice, the (untrusted) server will thus be
responsible for enabling secure HTTP, not the (trusted)
client
Sample
 Following is a sample conversation between
an HTTP client and an HTTP server running
on www.example.com, port 80
Sample Client Request
GET /index.html HTTP/1.1
Host: www.example.com
 Client request
 Followed by a blank line
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Request ends with a double newline
In the form of a carriage return followed by a line
feed
The "Host" header
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Distinguishes between various DNS names
sharing a single IP address
 Allows name-based virtual hosting.
 Optional in HTTP/1.0, mandatory in HTTP/1.1
Sample Server Response
HTTP/1.1 200 OK
Date: Mon, 23 May 2005 22:38:34 GMT
Server: Apache/1.3.27 (Unix) (Red-Hat/Linux)
Last-Modified: Wed, 08 Jan 2003 23:11:55 GMT
Etag: "3f80f-1b6-3e1cb03b"
Accept-Ranges: bytes
Content-Length: 438
Connection: close
Content-Type: text/html; charset=UTF-8
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Server response
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ETag (entity tag) header is used to determine if the URL cached is identical to the requested URL on
the server.
Content-Type specifies the Internet media type of the data conveyed by the http message
Content-Length indicates its length in bytes.
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The webserver publishes its ability to respond to requests for certain byte ranges of the document by
setting the header
Accept-Ranges: bytes
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Followed by a blank line and text of the requested page
This is useful if the connection was interrupted before the data was completely transferred to the client
Connection: close
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It is stated, that the webserver will close the TCP connection immediately after the transfer of this
package.