CPSC 3600 - DAY 7
FEBRUARY 1, 2017

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INTERNET APPLICATIONS (PRESENTATION)
------------------------------------

	.Application-Layer Protocols
		.There are two broad types of application-layer protocols
		when specifying details of communication.
			.Private communication - two applications communicate
			over the internet so that only those two communciate
			.Standardized service: a service that has multiple
			clients access the service; this means the programmer
			must be precise to make sure everyone can interoperate
			correctly.

	.Representation and Transfer
		.Data Representation - Syntax of data items that are exchanged
		specific during transfer, translation of integers, characters
		and files between computers
		.Data Transfer - Interaction between client and server, message
		syntax and semantics, valid and invalid exchange error handling,
		termination of interaction.

WEB PROTOCOLS
-------------

	.HyperText Markup Language (HTML) - A representation standard used to
	specify the contents and layout of a web page.
	.Uniform Resource Locator (URL) - A representation standard that 
	specifies the format and meaning of web page identifiers
	.HyperText Transfer Protocol (HTTP) - A transfer protocol that specifies
	how a browser interacts with a web server to transfer data.

	.HTML - General Characteristics:
		.Uses textual representation
		.Describes pages containing multimedia
		.Declarative paradigm, not procedural
		.Provides markup specifications instead of formatting
		.Permits a hyperlink to be embedded in an object.
		.Allows a document to include metadata.
	.Uniform Resource Locator
		.Specifies the web page itself
		
	http://www.scriptingmaster.com/xhtml/using-colors.asp
	^----^ ^----------------------^^---^^----------------^
	protocol      host name        folder name     file name

	.HTTP
		.the primary transfer protocol that a browser uses to interact
		with a web server.
			.Uses textual control messages
			.Transfers binary data files
			.Download and upload data
			.Caching

+-----------------------+---------------------------------------------+
| Request		|	Description			      |
+-----------------------+---------------------------------------------+
| Get			| Requests a document; server responds by     |
|			| sending status information followed by copy |
+-----------------------+---------------------------------------------+
| Head			| Requests status information; server sends   |
|			| status, but does not send copy of doc.      |
+-----------------------+---------------------------------------------+
| Post			| Sends data to server; server appends data to|
|			| specified item.			      |
+-----------------------+---------------------------------------------+
| Put			| Sends data to a server; the server uses data|
|			| to completely replace specify item 	      |	
+-----------------------+---------------------------------------------+


HOW HTTP WORKS
--------------

	.STATUS CODES
		.200: OK
		.400: BAD REQUEST
		.404: NOT FOUND

	.Caching in Browsers
		.A browser can reduce downtimes by saving a copy of each image
		in a cache on a user's disk and using than cached copy
		.A browser makes a HEAD request and compares the last-modified
		date of the server's copy to the last modified date on the 
		cached copy.
		.Improves browser speed.

FILE TRANSFER PROTOCOL
----------------------

	.File: fundamental storage abstraction which holds an arbitrary object.
	.File transfer between computers is complicated by computer's hetero-
	geneity.
	.The first FTP applications were written in early 1970s
	.Arbitrary file contents
	.Bidirectional transfer
	.Support for authentication and ownership
	.Ability to browse folders
	.Textual control messages.
	.Accommodates heterogeneity
	.Client creates a control connection, which is reserved for commands
	.To respond, the server creates a data connection over which directory
	listing and files are sent.
		.inverted relationship between client and server.

ELECTRONIC MAIL
---------------

	.Email has, since the 1960s, allowed a user on one computer to send 
	message to other user, without users been online at same time.
		.Email interface application for composing and reading emails
		.Mail transfer application, which invokes SMTP to send messages