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CHAPTER 3. HTML5

the authors of the specication. It has been debated, as to whether the

specication should be completed before implementation or vice-versa. It

has been decided that they have to go along with each other, as complet-

ing the specication rst, is quite likely to have implementation issues and

the implementation feedback is important to modify the specication. And

having the implementation done rst, restricts the specication from being

redesigned, as applications which rely on these implementations might stop

working.[33]

3.2 HTML5 and JavaScript

JavaScript has been around much before HTML5 and is one of the most

popular programming languages on the web today. It was originally de-

signed as a lightweight interpreted client-side language that would appeal

to non-professional programmers, and complement Sun Microsystem’s ‘Java’

programming language. Because it runs at the client-side (usually a web

browser), it can respond quickly to user actions making the application more

responsive. The increasing speed of JavaScript engines has also added to the

success of the language.

JavaScript is also one of the most misunderstood programming languages

and was initially not taken seriously by many professional programmers.

This misunderstanding is only exacerbated by its name. The prex Java

incorrectly suggests it is related to Java and the Script sux suggests it is

not a real programming language. Also the initial versions of the language

did not have a lot of functionality such as error handling, inheritance and

inner functions. Although these are supported now and the current version

of the language is a complete object-oriented language, the previous versions

created a low opinion about the language’s capability. JavaScript was also

known for some of its major design aws, this had given room to a lot of

bad programming, which added to the languages bad reputation and these

bad designs cannot be easily removed, as applications depending on those

aws might fail. The literature on the language is also of very poor quality.

There are in fact very few good books, which actually portray the language

the right way.

Despite its aws, at its core JavaScript is a very powerful, expressive pro-

gramming language. It has also emerged as the only language that most pop-

ular web browsers share support for. It is the language of the web browser

and now with HTML5 it is the only scripting language endorsed by the draft.

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CHAPTER 3. HTML5

One of the main reasons JavaScript is popular as a browser language is

because of its support for asynchronous execution of code. Although this

feature slightly complicates the readability of the code, it brings in quite a

few advantages which make up for this. For example from a user experience

(UX) perspective this is important to ensure that web pages are responsive

and take as little time to execute. The asynchronous features of JavaScript

ensures this with function callbacks. A ‘function callback’ is a programming

feature where methods may be passed to another method as an argument.

This allows programmers to specify methods that can to be called on any

event. This does require additional eort in understanding the code ow due

to its non-linear execution path and hencea proper understanding ofthis pro-

gramming paradigm is important for a developer to be able to write robust

code. To better understand this let us look at the following code example,

setTimeout() is an in-built JavaScript method that accepts two arguments,

setTimeout(function() { console.log(’Hello’); }, , 3000);

the rst being a function callback and the second a duration in milliseconds.

This method invokes the function specied in the rst argument, after a time

period equal to the duration specied in the second argument.

With the above code, the string ‘Hello’ is displayed after a delay of 3 sec-

onds. If we now introduce another statement to display an additional string

as shown below, we notice a dierence in the output, This now prints the

setTimeout(function() { console.log(’Hello’); }, , 3000);

console.log(’ World’);

string ‘ World’ rst and then ‘Hello’ after adelay of3 seconds. Thus thedelay

does not aect the code following it and execution proceeds. This example

highlights the asynchronous nature of execution in JavaScript. This asyn-

chronous or non-blocking feature of JavaScript is not to be confused with

concurrency. Since the browser implementation decides how non-blocking

code actually works, JavaScript is usually implemented as a single thread.

Later on in this chapter we look at how multi-threading is implemented in

JavaScript with the newly introduced HTML5 Web workers API.

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CHAPTER 3. HTML5

In order tostoreand transmit information between web server and clients, we

need to dene a data interchange format. This brings us to another impor-

tant in-built feature of JavaScript. JavaScript brings with it the ‘JavaScript

Object Notation’ (JSON)[12], which is a lightweight human-readable data

interchange format. It is as the name suggests, derived from the JavaScript

language. Despite this it is a language-independent format, with parsers

available in most of the commonly used languages. The JSON format is

used for serializing and transmitting data over a network connection, and

is primarily used between web servers and clients replacing XML. A JSON

representation is basically a collection of name-value pairs. The names and

values are separated by the semi-colon character (:). An example of a JSON

representation of an object that describes a person is shown below,

{

"firstName": "John",

"lastName" : "Doe",

"age"

: 26,

"address" :

{

"streetAddress": "Baker Street, 5 C 391",

"city"

: "Espoo",

"country"

: "Finland",

"postalCode"

: "02150"

"phoneNumber":

[

{

"type" : "home",

"number": "358 456-7890"

{

"type" : "fax",

"number": "358 123-4567"

}

]

}

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CHAPTER 3. HTML5

HTML5 consists of numerous individual features and in the remainder of

this chapter we introduce those HTML5 features which are signicant to our

P2P VOD streaming implementation. These include the following,

 The <video> element, to allow native videoplayback within the HTML

user agent.

 The File API, to access the le system.

 The IndexedDB API, to maintain and organize an index of the les

and application settings.

 The Web workers API, that enables multi-threading.

 The Web sockets API, that allows bi-directional communication be-

tween the client and servers.

 The WebRTC API, that enables browser-to-browser communication.

 JavaScript helper libraries to parse the HTML DOM and to perform

Message Integrity Checks.

One of themajor challenges ofdealingwith HTML5 at this stagewith regards

to writing this thesis, is the immaturity of the implementations and changes

in the API due to the ongoing standardization process.

3.3 The <video> tag

The video element is an important addition to HTML. It has been intro-

duced for the purpose of playing videos and movies and to replace the <ob-

ject> element, which was earlier used among other things to embed video

content intoweb pages. Theintroduction ofthis element means thatbrowsers

would need to implement native video players so that videos can be played

without the need for any plugins. Even though the video tag is a work in

progress, it has been implemented by almost all major browsers to date. To

demonstrate the media API and media events, theW3C has published a page

,which when viewed with a browser that supports the <video> element,

displays the properties, events and media types it supports.

When we talk about video players embedded in the browser, the next nat-

ural question would be which video formats are supported. This however is

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CHAPTER 3. HTML5

not clear at the moment as the current HTML5 draft does not specify which

video formats browsers should support. Video formats are comprised of a

combination of containers and codecs. Video containers are the le formats

which contain the video stream and other related information, such as the

meta-data of the video le, the title of the video, a thumbnail picture pre-

view, the video stream bit rate, audio les with markers for synchronization

and so on. Video codecs are the implementation of the algorithms used for

renderingthe video streams. Although it would be ideal if every browser sup-

ports a standard set of video formats, this is currently not the case. For this

reason the video tag accepts multiple source URLs, and the browser plays

the rst resource that it supports. The online resource ‘Dive Into HTML5’

details the browser support for the various video formats[33], and since this

is constantly changing (and will continue to do so until standardized), it is

important that developers are aware of this information.

3.4 File API

The HTML5 File API specication[37] is still being drafted by the W3C Web

Applications (WebApps) Working Group. The implementation of this API

within the browser would allow HTML5 applications to handle les via the

browser in astandardized way. It has been designed to allow the web browser

to create a sandboxed lesystem for each web domain, which means if we had

two domains say www.yahoo.com and www.google.com, the les created by

each of these domains would not be accessible to each other. The JavaScript

code could request two types of lesystem storage, namely temporary and

persistent storage. In temporary storage the les stored can be removed

at the browser’s discretion and in persistent it can only be removed by the

application. Of course, neither of these would stop the user from manually

removing these les from the hard disk if they so desired. The latter storage

requires the user to grant access to the application.

It is important to note that the File API cannot be used to access the le

system outside the browsers sandboxed lesystem. Providing such access

needs to be debated and designed carefully, as it brings up security concerns.

As depicted in the Figure 3.1, each browser that supports the HTML5 File

API will have a separate sandboxed lesystem even if they are of the same

domain. Files within each of these sandboxed lesystem cannot reference

les that are outside their respective lesystem. This would mean that each

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CHAPTER 3. HTML5

Figure 3.1: An overview of the HTML5 browser lesystem.

last node in the Figure 3.1 would not be able to reference les outside the

respective node. This would not even be possible from the temporary to the

persistent lesystem for the same domain within the same browser. Appli-

cations could be designed to allow the moving and copying of les within

the temporary and persistent lesystem of the same domain and browser. In

order to do this with dierent browsers, the domain web server would have

to be used to relay the data.

The currently published version of the File API draft[? ] has been im-

plemented by major browser vendors.

3.5 IndexedDB API

The Indexed Database (IndexedDB) API[27] is another specication by the

W3C Web Applications (WebApps) Working Group. The W3C group has re-

cently been looking into the option of having aclient side persistent database

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CHAPTER 3. HTML5

within the browser. This would give applications the ability to save in-

formation within the browser, so that they may access information even

when oine. The WebSQL API was introduced so that information could

be recorded and retrieved based on well established Structured Query Lan-

guage (SQL) statements, however towardsthe end of the2010 theW3C group

decided that introducing SQL statements within JavaScript, was not a very

compatible design and dropped it. This was then replaced by the IndexedDB

API, which has a more JavaScript related API which deals with with data in

the form ofthe JavaScriptObject Notation (JSON). The HTML5IndexedDB

API allows applications to have a persistent storage of information locally

within the browser. The availability of this stored oine information is the

same as the File API discussed previously, where the information is saved

separately for dierent domains and browsers.

The indexed database consists of object stores, these are the data structures

that contain all the data. In relational databases these are known as tables.

The indexed database consists of three types of transaction modes which

allow access and modication to the object stores, they are ’readonly’, ’read-

write’ and ’versionchange’ transactions. The ’readonly’ transactions where

data is not modied, can have many instances running in parallel, as these

instances would not have any impact on each other. The ’readwrite’ trans-

action on the other hand can only have one instance accessing a given object

store running at a time, in order to ensure that transactions are mutually

exclusive. It is also important to note that the object store schema cannot be

modied in this transaction mode. Finally the ’versionchange’ transaction

mode is the only one in which the object store schema can be modied.

The IndexedDB API brings in a unique problemas to how thebrowser should

handle the data when the object store schema has been updated. We should

remember that when a new version has been deployed, all previous versions

would be available with the user base, so if the designed application cannot

aord to lose previously stored data, it would have to write code that han-

dles the smooth transition of data from all the previous versions to the new

object store schema. This is a problem that is specic to having a database

on the client side, as server side databases would only have to be migrated

from one schema version to another and would be re ected globally. This is

also where the ’versionchange’ transaction mode becomes important.

CHAPTER 3. HTML5

3.6 Web Workers

The Web Workers API[18] is another specication by the W3C Web Ap-

plications (WebApps) Working Group. One of the obstacles to porting of

server-heavy applications to client-side JavaScript is the single-threaded en-

vironment of JavasScript. JavaScript does allowing asynchronous execution

but this does not necessarily mean concurrency. As the asynchronous non-

blocking code need not necessarily mean that threads are employed. In order

to handle computationally intensive tasks, the HTML5 Web Workers API

was introduced. This species an API to spawn background scripts in web

applications.

With this it is now possible to run separate JavaScript les concurrently.

Workers can utilize thread-like message passing ,to pass strings or JSON ob-

jects to JavaScript code that spawned the worker. Workers are meant for

resource intensive task and are not meant to be used in large numbers as

it could hog up the users system resources. It should ideally be used for

CPU intensive scripts that would otherwise keep the application from being

responsive.

It is important to free the worker by calling the close() method after it has

completed its task. If not this might keep the allocated resources from being

used. Workers are not thread safe, and for this reason they have some re-

strictions, such as accessing the DOM or the document object. Workers can

spawn other workers, this can help break down resource intensive tasks to

complete faster. Employing workers is an important design decision, which

can help make heavy client-side computation quicker.

3.7 Web Sockets

The Web Sockets API[17] is also a specication maintained by the W3C Web

Applications (WebApps) Working Group. A server push and a client pull are

means of delivering content from a server computer (web server) to a client

computer (browser). Unitl now browsers only supported client pull mecha-

nisms, and server push was not directly provided to web developers. This is

amajor drawback, as servers were not able to inform clients when they had

any updated information.

There have been several workarounds in achieving server push mechanisms

they are namely Pushlets, Long polling, Browser plug-ins and third party

CHAPTER 3. HTML5

JavaScript libraries. Some of the best practices of abusing the HTTP proto-

col such as long polling have been discussed in RFC 6202[25]. The HTML5

WebSocket API is the rst standardization to help browsers implement a

common mechanism of server push delivery.

The WebSocket protocol[13] has been designed for the the WebSocket API,

this protocol helps with bi-directional communication i.e. allowing a server

to communication with a client and vice-versa. It helps to achieve this by

using a single Transmission Control Protocol (TCP)[34] connection, rather

than by creatinga new TCP connection each time the server or client want to

communicate with each other. This protocol is an independent TCP-based

protocol, and is treated as a HTTP upgrade during the handshakeprocedure.

Just like HTTP, it can work on the default port 80, and for secure connection

it works on port 443 which is based on the Transport Layer Security (TLS)

protocol[38].

Unlike most of the previously discussed HTML5 APIs, this Web Sockets API

requires support from a server component as well. Hence it is not enough if

the browser alone implements the Web Sockets API, the server would also

have to implement the protocol to support this communication. There are a

number of server side implementations of the Web Socket protocol in most

of the well known languages like Python, C sharp, Java and even JavaScript.

It might be a little surprising to know that JavaScript is used outside the

browser, however this has been around for some time now (in fact it has

been available soon after JavaScript was released for the browser in 1994).

Node.js

is one recent notable example of a server-side implementation of

JavaScript.

3.8 Additional libraries

In addition to the above discussed API’s our HTML5 implementation of

the Browser-to-browser video streaming application uses a few JavaScript

libraries that are brie y described here.

3.8.1 jQuery

jQuery is a JavaScript library designed to simplify client-side scripting of

HTML, It is designed to make it easy to navigate and manipulate the DOM,

http://nodejs.org/

CHAPTER 3. HTML5

select DOM elements and handle events (among several others things). It

simplies web application development on the client-side and makes the

JavaScript code more readable. We make use of this library to ease the

development of our video streaming web application.

3.8.2 WebToolKit MD5

In our implementation of video streaming over a P2P network, we need to

ensure the integrity of the pieces of the video stream received by a peer from

another host on the network. One way to do this is with use of Message

Integrity Codes (MIC). A MIC algorithm ensures that a given message will

always produce the same MIC, assuming the same algorithm is used at both

ends.

The MD5 Message-Digest Algorithm is a widely used cryptographic hash

function that produces a 128-bit hash value[42]. MD5 is commonly used to

check data integrity. An MD5 hash is typically expressed as a 32-character

hexadecimal number. The WebToolKit MD5 script is a JavaScript imple-

mentation of the MD5 algorithm, and is used to process a variable length

message into axed-length output of128bits. Wemakeuse ofthis JavaScript

library to generate a MIC for each piece of the video stream received by the

web browser.

The list of HTML5 APIs discussed in this chapter, is by no means ex-

haustive. HTML5 oers many more APIs for development of rich web ap-

plications. We have conned our discussion to the HTML5 features / APIs

that are relevant to the implementation of our P2P VOD solution.

Documents you may be interested

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