HTTP

1
HTTP

• By
• Jaron Weese
• DatacomII

2
What is HTTP?

• According to RFC 1945, HTTP is an
  application-level protocol with the lightness and
  speed necessary for distributed, collaborative,
  hypermedia information systems. It is a generic,
  stateless, object-oriented protocol which can be
  used for many tasks, such as name servers and
  distributed object management systems, through
  extension of its request methods.

3
HUH?

• Yeah, that was my first response.

4
However, to truly understand HTTP, three other
subjects must first be addressed.

• URL
• URI
• HTML

5
URL

• Uniform Resource Locator
• A compact representation of the location and
  access method for a resource available via the
  Internet
• A networked extension of a file name Stallings
  p. 715
• And..

6
URL Cont.

• Since, different resources are accessible in
  different ways, the URL also indicates which
  access method must be used to retrieve the
  object. Stallings p. 715
• General form

7
URL Schemes

• ftp
• http
• mailto
• news
• telnet

8
Format of the HTTP URL

• Http/// with default port set
  at 80
• Example http//128.5.17.3280/some path or,
  thanks to DNS, we merely see http//www.schoolsuck
  s.com
• Path is any extension after the main page/files
  /pages whatever.

9
URI

• According to Stallings, URI is a generic WWW
  identifier which provides for a list of naming or
  addressing schemes.
• Or, what we would call a DNS. ?

10
HTML

• Hypertext Markup Language
• Standard language used to make documents on the
  web
• Uses tags to allow connection to other
  documents
• Not bound to any operating systems features
• Not as concerned with appearance as structure

11
So what does all of that have to do with HTTP?
12
HTTP
HTTP can be used in any client-server
application involving hypertext and is a protocol
for transmitting information with the efficiency
necessary for making hypertext jumps. The data
transferred can be plain text, graphics, sound,
etc Stallings pp. 719-720
13
HTTP is a stateless protocol

• each transaction is treated independently
• Therefore, a new TCP connection between client
  and server is created for each transaction and
  then terminated when the transaction completes.
• Stallings p. 721

14
So how does it work?

• TCP Connection is opened and an HTTP request is
  sent by the user agent to the original server.
• When the HTTP request is received, the TCP
  Connection is closed by the original server.
• Once the page is found, the TCP connection is
  reopened and a response is sent from the original
  server to the user agent.

15
When a request is made as described by w3.org.

• . The client (typically a Web Browser) makes an
  HTTP connection to the host.
• . The server accepts the connection.
• . The client sends a request for a document --
  one line of text, such as GET path_from_URL
• . The server closes the connection and fetches
  the requested document.
• . The server reopens the connection and sends the
  requested document.
• . The client accepts the document.
• . When the send is complete, the server closes
  the connection.
• . The client terminates the HTTP connection.

16
A more complex request/response path involves
intermediaries.

• Not an end-to-end TCP connection
• It consists of one or more intermediate systems
  connected by TCP connections
• Three forms identified
• Proxy
• Gateway
• Tunnel

17
Forms of Intermediate systems

• A Proxy acts on behalf of other clients and
  presents requests from other clients to a server.
  Like a firewall.
• A Gateway is a server that appears to the client
  as if it were an origin server.
• A Tunnel performs no operations on HTTP requests
  and responses, rather it acts simply as a relay
  point. Stallings pp. 722-723

18
More on Intermediaries.
19
A Cache.

• A cache is a facility that may store previous
  requests and responses for handling new
  requests. Stallings p. 723
• Not all requests and responses can be cached.

20
Simple-Request and Simple-Response messages
21
Entity Header and Body

• According to Newtons Telecom Dictionary, an
  entity is an active element within an OSI layer
  or sublayer. p. 303
• Entity Header provides information about the
  Entity Body
• Entity Body consists of an arbitrary sequence of
  octets. When an entity body is present in a
  message, the interpretation of the octets in the
  body is determined by the entity header fields.
  Stallings p. 734

22
Status Codes

• RFC 2068 gives five different status codes
• 1xx Informational - Request Received, continuing
  process
• 2xx Success - The action was successfully
  received, understood, and accepted
• 3xx Redirection - Further action must be taken
  in order to complete the request
• 4xx Client Error - The request contains bad
  syntax or cannot be fulfilled
• 5xx Server Error - The server failed to fulfill
  an apparently valid request

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Status Codes cont.

• 301 - Moved Permanently
• 400 - Bad Request
• 401 - Unauthorized
• 403 - Forbidden
• 404 - Not Found
• 405 - Method Not Allowed
• 406 - Not Acceptable
• 415 - Unsupported Media Type

24
The End...

• For further reading the Datacom book provides
  more detail. Pages 715, 719-736
• Other sources.
• RFCs 1945, 2068, 2616
• www.w3.org/Protocols/HTTP/HTTP2.html
• www.jmarshall.com/easy/http/