The web and HTTP

three fields of the request line

method field, URL, and HTTP version field

headers

predefined informational fields (ex: resource location, browser information, cookies)

Status Codes - 300s

redirection

primary goals for HTTP/2

reduce perceived latency by enabling request and response multiplexing over a single TCP connection, provide request prioritization and server push, and provide efficient compression of HTTP header fields.

two types of HTTP messages

request messages and response messages

200 OK

request succeeded and the information is returned in the response

404 Not Found

requested document not found on this server

301 Moved Permanently

requested object has been permanently moved

Head-of-the-Line (HOL) blocking

sending all the objects in a web page over a single TCP connection

HTTP responses:

sent by servers to respond to a request

Status Codes - 500s

server errors

three sections of a HTTP response message

status line, six header lines, and entity body

status codes: 200s

success

304 Not Modified

tells the cache that it can go ahead and forward its cached copy of the object to the requesting browser

two components of a URL

the hostname of the server that houses the object and the object's path name.

entity body

the meat of the message

three fields of the status line

the protocol version field, a status code, and a corresponding status message

POST message

the user is still requesting the a web page from the server but the specific contents of the web page depend on what the user entered into the form fields.

PUT method

uplaodes new file (object) to server - completely replaces file that exists at specified URL with content in entity body of POST HTTP request message

GET method

used when the browser requests and object, with the requested object identifies in the URL field

POST method

web page often include form input - user input sent from client to server in entity body of HTTP POST request message

HEAD method

- asks for a response that's identical to the get request but without the respons body - could be used to determine the size of and object that would be retrieved without actually retrieving that object. - requests header (only) that would be returned if specified URL were requested with an HTTP GET Method

HTTP Request message

GET /somedir/page.html HTTP/1.1 Host: www.someschool.edu Connection: close User-agent: Mozilla/5.0 Accept-language: fr

method field values

GET, POST, HEAD, PUT, and DELETE.

HTTP Request response behavior

HTTP request messages for the objects in the page to the server. The server receives the requests and responds with HTTP response messages that contain the objects. HTTP uses TCP as its underlying transport protocol (rather than running on top of UDP). The HTTP client first initiates a TCP connection with the server. Once the connection is established, the browser and the server processes access TCP through their socket interfaces.

HTTP response message

HTTP/1.1 200 OK Connection: close Date: Tue, 18 Aug 2015 15:44:04 GMT Server: Apache/2.2.3 (CentOS) Last-Modified: Tue, 18 Aug 2015 15:11:03 GMT Content-Length: 6821 Content-Type: text/html (data data data data data ...)

web servers

Implement the server side of HTTP

header lines

Subsequent lines of an HTTP request message

objects

a file that is addressable by a single URL

request line

The first line of an HTTP request message

400 Bad Request

The request could not be understood by the server

505 HTTP Version Not Supported

The requested HTTP protocol version is not supported by the server

stateless protocol

a server maintains no information about the clients (HTTP)

solution for HOL blocking

break each message into small frames, and interleave the request and response messages on the same TCP connection.

Status Codes - 400s

client error

two programs of Hypertext Transfer Protocol (HTTP)

client program and server program

web page

consists of object

HTTP

defines the structure of these messages and how the client and server exchange the messages

conditional GET

don't send object if cache has up-to-data cached version - no object transmission delay (or use of network resources)

web browsers

implement the client side of HTTP

status line

indicate that the server is using HTTP/1.1 and everything is OK

status codes: 100s

informational ("here is an update to you amazon order-seller has shipped your item")