The document discusses the format of HTTP messages, including requests and responses. An HTTP request contains a request line with the method, URL, and HTTP version. It also includes headers and an optional body. The response contains a status line with the HTTP version, status code, and reason phrase. It also includes headers and an optional body. The document provides examples of common request methods, status codes, and header types included in HTTP messages.

1. `UNIT I: MESSAGE FORMAT

2. MESSAGE FORMAT:
Earlier:
• HTTP is a request-response protocol.
It specifies a set of rules that clients and servers use to
communicate:

3. • An HTTP server process is created on a port (80), which waits
for clients to establish a TCP connection.
• An HTTP client initiates a TCP connection with the HTTP
server at the designated port.
• The HTTP server accepts this connection.
• The HTTP client then sends a request for a resource to the
server.
• Upon receiving the request, the server processes the request,
performs the desired task, and sends a response back to the
client.
• The HTTP server closes the TCP connection.
• The HTTP client receives the response containing information
and processes it.

4. HTTP STATELESS:
• HTTP client wants to get resource from the server, it has
to follow these steps.
• Web server treats every request as a new request.
• There is no way to specify that some requests are
related.

5. ADVANTAGE:
• Client/Server need not retain information between
successive requests.
• It is successive related requests is sometimes necessary.
• As HTTP protocol is inherently stateless, designer of the
web pages must use alternative methods such as
Cookies, URL rewriting, etc.

7. HTTP Message
Request
Message
Response
Message
Types:

9. REQUEST MESSAGE:
A request message is sent by a web client to the web server.
• Request Line
• Header
• Empty Line
• Optional Body

10. HTTP REQUEST MESSAGE FORMAT:
Request line
Header
Empty line
Body (available for some messages)

11. EXAMPLE:

12. REQUEST LINE:
• Request Type
• URL
• HTTP Version
Two consecutive parts are separated by a space.

13. SYNTAX:
Request
Type
URL
HTTP
Version
Space

14. HTTP REQUEST LINE EXAMPLE:

15. REQUEST TYPE (METHOD):
• It indicates the type of the request, a client wants to send.
• A method makes a message either a request or a command
to the server.
• Request messages are used to retrieve data from the server.
• A command tells the server to do a specific task.

16. Fields/
Methods
Description
GET It is used to retrieve information from the given server using a
given URI. Requests using GET should only retrieve data and
should have no other effect on the data.
HEAD Same as GET, but transfers the status line and header section
only.
POST It is used to send data to the server, for example, customer
information, file upload, etc. using HTML forms.
PUT Replaces all current representations of the target resource with
the uploaded content.
PATCH This is similar to PUT method except that if specifies a list of
differences that must be applied on the existing file.
COPY The HTTP protocol used to copy a file from one location to
another. URL specified in the request line specifies the location
of the source file and is specified in the entity header. Then
must be property to accept the COPY method.

17. Fields/
Methods
Description
MOVE It is similar to the COPY method except that it deletes the
source file. The location of the source file is specified by
the URL in the request line and the location of the target
file.
DELETE It is used to remove a document from the server and
deleted is specified by a URL in the request line.
LINK It is used to create a link or links from one document to
another. The URL in the request line specifies the location
of the source file and the entity header and location of
target document.
UNLINK It is used to remove a link or links created by the LINK
method.

18. Fields/
Methods
Description
OPTIONS It is used to retrieve the set of methods supported by the
server and used to check whether a server is functioning
properly before performing other tasks.
CONNECT It is used to convert a request connection into the
transparent TCP/IP tunnel and facilitate Secured Socket
Layer (SSL) encrypted communication through an
unencrypted HTTP proxy server.
TRACE It is used to instruct the web server to echo the request
back to the client and then see what additions or changes
are done by the immediate servers.

19. Methods
Fields /
Methods
Description Safe
GET • They are used to retrieve information from
the server.
• They can’t change the state of the server and
don’t have any harmful side effects except
certain harmless effects such as caching,
logging, etc.
Yes
HEAD Yes
OPTIONS Yes
TRACE Yes

20. Methods
Fields /
Methods
Description Unsafe
DELETE • They may change the state of the server.
• These methods have harmful side effects
and hence are vulnerable.
• Sensitive web servers are usually not
configured to accept these methods.
Yes
MOVE Yes
UNLINK Yes

21. URL:
Studied in previous section.

22. HTTP VERSION:
• HTTP/1.0
• HTTP/1.1 – it is current version.

24. RESPONSE METHOD:
In response to the request message, a response message is sent
by a server to the client.
• A status line
• A header
• An empty line
• An optional body

25. HTTP RESPONSE MESSAGE FORMAT:
Status line
Header
Empty line
Body (available for some messages)

26. STATUS LINE:
• HTTP version
• Status code
• Status phrase
Two consecutive parts are separated by a space.

27. SYNTAX:
HTTP
Version
Status
code
Status
phrase
Space

28. EXAMPLE OF HTTP RESPONSE LINE:

29. EXAMPLE:

30. HTTP VERSION:
This field specifies the version of the HTTP protocol being
used by the server.
Current Version: HTTP/1.1

31. STATUS CODE:
• It is a three-digit code that indicates the status of the
response.
• They are classified with respect to their functionality into
five groups:

32. 3-Digit
Code
Name Description
1xx series Informational • This class of status codes represents
provisional responses.
2xx series Success • This class of status codes indicates that the
client’s request are received, understood,
and accepted successfully.
3xx Redirectional • It indicates additional actions must be taken
by the client to complete the request.
• The user agent make actions in order to
fulfill the request automatically, provided it
uses either HEAD or GET method.

33. 3-Digit
Code
Name Description
4xx Client error • They are used to indicate the client request
had an error and therefore it can’t be
fulfilled.
• Except for the HEAD method, the body of
the response message contains explanation
that caused the error.
• The user agent should display the error
message to inform the server.

34. 3-Digit
Code
Name Description
5xx Server error • It indicates the server encountered some
problem and request can’t be satisfied at
this time.
• The reason of the failure is embedded in the
message body.
• It is also indicated whether failure is
temporary or permanent.
• The user agent should display a message
on the screen to make the user aware of the
server failure.

35. HTTP STATUS CODE:
1xx series – Information
Status code Status phrase Description
100 Continue The server has received the request
headers and the client can continue further
sending the request body where required.
Example:
A client can to send the request body for
POST method. In way client can avoid
sending a large message body if request is
already rejected.
101 Switching The server switches the protocol on
receiving a request from the client to do the
same.
102 Processing The server has received the request which
is currently under process and no response
is available yet.

36. 2xx series – Success
Status code Status phrase Description
200 Ok The request is OK.
201 Created The request is complete, and a new
resource is created .
202 Accepted The request is accepted for processing,
but the processing is not complete.
203 Non-Authoritative
Information
The information in the entity header is
from a local or third-party copy, not from
the original server.
204 No Content A status code and a header are given in
the response, but there is no entity-body
in the reply
205 Reset Content The browser should clear the form used
for this transaction for additional input.

37. Status code Status phrase Description
206 Partial Content The server is returning partial data of
the size requested. Used in response to
a request specifying a Range header.
The server must specify the range
included in the response with
the Content-Range head
207 Multi-Status The body of the message is an XML file
that contains a number of response
codes. The number of response codes
depends on the number of sub-requests
made.

38. 3xx series – Redirection
Status code Status phrase Description
300 Multiple Choices A link list. The user can select a link and
go to that location. Maximum five
addresses .
301 Moved Permanently The requested page has moved to a new
URL.
302 Found The requested page has moved
temporarily to a new URL.
303 See Other The requested page has moved
temporarily to a new URL.
304 Not Modified This is the response code to an If-
Modified-Since or If-None-
Match header, where the URL has not
been modified since the specified date.

39. 4xx series – Client Error
Status code Status phrase Description
400 Bad Request The server did not understand the
request.
401 Unauthorized The requested page needs a
username and a password.
403 Forbidden Access is forbidden to the requested
page.
404 Not Found The server can not find the requested
page.
405 Method Not Allowed The method specified in the request is
not allowed.
406 Not Acceptable The server can only generate a
response that is not accepted by the
client.
408 Request Timeout The request took longer than the
server was prepared to wait.

40. Status
code
Status phrase Description
409 Conflict The request could not be completed
because of a conflict.
410 Gone The requested page is no longer
available.
411 Length Required The "Content-Length" is not defined.
The server will not accept the request
without it.
412 Precondition Failed The precondition given in the request
evaluated to false by the server.
413 Request Entity Too Large The server will not accept the request,
because the request entity is too large.
414 Request-URI Too Long The URI specified was too long for this
server to process.
415 Unsupported Media Type The server doesn’t support the media
type specified.

41. Status
code
Status phrase Description
416 Requested Range Not
Satisfiable
The requested byte range is not
available and is out of bounds.
417 Expectation Failed The expectation given in an Expect
request-header field could not be met
by this server.
422 Unprocessable Entity The request contains semantic errors
and can’t be fullfilled.
423 Locked The requested resource is locked.
424 Failed Dependency The request has failed due to the
failure of the previous request.

42. 5xx series – Server Error
Status code Status phrase Description
500 Internal Server Error The request was not completed. The
server met an unexpected condition.
501 Not Implemented The request was not completed. The
server did not support the functionality
required.
502 Bad Gateway The request was not completed. The
server received an invalid response
from the upstream server.
503 Service Unavailable The request was not completed. The
server is temporarily overloading or
down.
504 Gateway Timeout The gateway has timed out.
505 HTTP Version Not
Supported
The server does not support the "http
protocol" version.

45. HEADERS:
• They are important part of both request message and response
message.
• The characteristics of the resource requested and the data that are
provided.
Example:
• A client may want to accept image files only in some specified format.
• Similarly, the server may provide additional information about the
resource being sent such as the length of message content or last
modification date of the resource, etc.
• The headers are separated by an empty line from the request and
response body.

46. HTTP REQUEST HEADER FORMAT:
Request Header contains of three parts:
General Header
Request Header
Entity Header

47. HTTP RESPONSE HEADER FORMAT:
Response Header contains of three parts:
General Header
Response Header
Entity Header

48. • A header consists of single line or multiple lines.
• Each line is a single header form:
• It is not a case-sensitive; but header value may be.
• If header line starts with a space, it is considered to be a part
of previous header line.
Header-name: Header value
Example:
• If a long header value is broken into multiple lines.
Sun, 06 Nov 1994 08:49:37 GMT ; RFC 822, updated by RFC
1123

49. SYNTAX:
Header
name:
Header
value
Space(s)

51. GENERAL HEADERS:
They are present in both request and response message, but have somewhat
different meaning in each.

52. REQUEST HEADER:
It contains information about client sending the request as well as the
data format that the client expects.

54. RESPONSE HEADER:
It contains the information about the server and the data being sent.

55. ENTITY HEADER:
It is present in both request and response message and information
about the message body.

57. REFERENCE:
https://www.slideshare.net/origamiaddict/http-40249449
https://www.httpdebugger.com/http/http_header.html