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Can a browser communicate with another browser on the same network directly?

I'm playing around trying to find a way to communicate between two browsers on the same network to establish WebRTC without a server roundtrip (no STUN/ICE/TURN). Basically an alternative to the approach found here, where the "handshake" is done via copy/mail/pasting.
After sifting through all the cross-browser-communication examples I could find (like via cookies or WebTCP) plus a bunch of questions on SO (like here), I'm back to wondering a simple thing:
Question:
If Alice and Bob visit the same page foo.html while on the same network and they know each others' internal assigned IP addresses, are there any ways they can communicate purely with what is available on the browser?
This excludes non-standard APIs like Mozilla TCP_Socket_API, but other than that all "tricks" are allowed (img tags, iframes, cookies, etc.).
I'm just curious if I can listen to someone on the same network "broadcasting" something via the browser at all.
Edit:
foo.html will be on static server, no logic, no ICE, no shortcut.
Edit:
Still not a solution but a websocket server as Chrome extension comes closer. Example here: almost pure browser serverless WebRTC

Yes, you can establish a direct connection between two browsers over the local network using WebRTC. It requires the use of ICE, but that does not mean that an outside STUN or TURN server is needed. If the browsers are on the same network, ICE will succeed with only the local candidates of each browser.
STUN/TURN is needed only in order to guarantee that two endpoints can establish a connection even when they are in different networks and behind NATs.
In fact, if you use most of the WebRTC example applications (such as apprtc) with two browsers connected in a local network, ICE is most likely to select and use the pair of local addresses. In this case a channel allocation on a TURN server will be made, but it will not get used.
In your WebRTC application, you can disable the use of STUN/TURN by passing empty iceServers when you create the PeerConnection.

While the MDN documentation lists WebSocketServer as a client API, I don't think this is accurate (maybe they wanted to document there how to write a server).
At the moment, I know no standard way to create a server socket on a web browser. I know a couple of attacks to scan the local network but most of them rely on an active server outside the network, that is you connect to a server and get JavaScript back which opens a WebSocket connection. Via that connection, I can take full control over the client and have it open more WebSockets with local IP addresses to scan the internal network.
If internal web sites don't implement CORS correctly (see here), I can access all internal web sites where the current user is currently logged in. That is a devious attack vector which allows external attackers to browser internal documents without cracking anything. This page has a demo of the attack.
Even Flash won't let you create a server socket.
If you allow a Java applet and the Java version on the client is very old or the user blindly clicked "OK", then you can create server sockets.
Related:
Socket Server in Javascript (in browsers)?

This could be explained easily. The answer is it's not possible. In order for alice and bob to communicate at all without a third-party, at least one of them needs to be listening for incoming connections. Not possible using a standard web browser alone.

You can take a look at this
https://github.com/jed/browserver-client
I think that you can easily create an http server with javascript and send messages from one browser to another
With Nodejs you can achieve the same.

Call ajax when value is inserted on database [duplicate]

Of course I am aware of Ajax, but the problem with Ajax is that the browser should poll the server frequently to find whether there is new data. This increases server load.
Is there any better method (even using Ajax) other than polling the server frequently?

Yes, what you're looking for is COMET http://en.wikipedia.org/wiki/Comet_(programming). Other good Google terms to search for are AJAX-push and reverse-ajax.

Yes, it's called Reverse Ajax or Comet. Comet is basically an umbrella term for different ways of opening long-lived HTTP requests in order to push data in real-time to a web browser. I'd recommend StreamHub Push Server, they have some cool demos and it's much easier to get started with than any of the other servers. Check out the Getting Started with Comet and StreamHub Tutorial for a quick intro. You can use the Community Edition which is available to download for free but is limited to 20 concurrent users. The commercial version is well worth it for the support alone plus you get SSL and Desktop .NET & Java client adapters. Help is available via the Google Group, there's a good bunch of tutorials on the net and there's a GWT Comet adapter too.

Nowadays you should use WebSockets.
This is 2011 standard that allows to initiate connections with HTTP and then upgrade them to two-directional client-server message-based communication.
You can easily initiate the connection from javascript:
var ws = new WebSocket("ws://your.domain.com/somePathIfYouNeed?args=any");
ws.onmessage = function (evt)
{
var message = evt.data;
//decode message (with JSON or something) and do the needed
};
The sever-side handling depend on your tenchnology stack.

Look into Comet (a spoof on the fact that Ajax is a cleaning agent and so is Comet) which is basically "reverse Ajax." Be aware that this requires a long-lived server connection for each user to receive notifications so be aware of the performance implications when writing your app.
http://en.wikipedia.org/wiki/Comet_(programming)

Comet is definitely what you want. Depending on your language/framework requirements, there are different server libraries available. For example, WebSync is an IIS-integrated comet server for ASP.NET/C#/IIS developers, and there are a bunch of other standalone servers as well if you need tighter integration with other languages.

I would strongly suggest to invest some time on Comet, but I dont know an actual implementation or library you could use.
For an sort of "callcenter control panel" of a web app that involved updating agent and call-queue status for a live Callcenter we developed an in-house solution that works, but is far away from a library you could use.
What we did was to implement a small service on the server that talks to the phone-system, waits for new events and maintains a photograph of the situation. This service provides a small webserver.
Our web-clients connects over HTTP to this webserver and ask for the last photo (coded in XML), displays it and then goes again, asking for the new photo. The webserver at this point can:
Return the new photo, if there is one
Block the client for some seconds (30 in our setup) waiting for some event to ocurr and change the photograph. If no event was generated at that point, it returns the same photo, only to allow the connection to stay alive and not timeout the client.
This way, when clients polls, it get a response in 0 to 30 seconds max. If a new event was already generated it gets it immediately), otherwise it blocks until new event is generated.
It's basically polling, but it somewhat smart polling to not overheat the webserver. If Comet is not your answer, I'm sure this could be implemented using the same idea but using more extensively AJAX or coding in JSON for better results. This was designed pre-AJAX era, so there are lots of room for improvement.
If someone can provide a actual lightweight implementation of this, great!

An interesting alternative to Comet is to use sockets in Flash.

Yet another, standard, way is SSE (Server-Sent Events, also known as EventSource, after the JavaScript object).

Comet was actually coined by Alex Russell from Dojo Toolkit ( http://www.dojotoolkit.org ). Here is a link to more infomration http://cometdproject.dojotoolkit.org/

There are other methods. Not sure if they are "better" in your situation. You could have a Java applet that connects to the server on page load and waits for stuff to be sent by the server. It would be a quite a bit slower on start-up, but would allow the browser to receive data from the server on an infrequent basis, without polling.

You can use a Flash/Flex application on the client with BlazeDS or LiveCycle on the server side. Data can be pushed to the client using an RTMP connection. Be aware that RTMP uses a non standard port. But you can easily fall back to polling if the port is blocked.

It's possible to achive what you're aiming at through the use of persistent http connections.
Check out the Comet article over at wikipedia, that's a good place to start.
You're not providing much info but if you're looking at building some kind of event-driven site (a'la digg spy) or something along the lines of that you'll probably be looking at implementing a hidden IFRAME that connects to a url where the connection never closes and then you'll push script-tags from the server to the client in order to perform the updates.

Might be worth checking out Meteor Server which is a web server designed for COMET. Nice demo and it also is used by twitterfall.

Once a connection is opened to the server it can be kept open and the server can Push content a long while ago I did with using multipart/x-mixed-replace but this didn't work in IE.
I think you can do clever stuff with polling that makes it work more like push by not sending content unchanged headers but leaving the connection open but I've never done this.

You could try out our Comet Component - though it's extremely experimental...!

please check this library https://github.com/SignalR/SignalR to know how to push data to clients dynamically as it becomes available

You can also look into Java Pushlets if you are using jsp pages.

Might want to look at ReverseHTTP also.

Script on server to interact with clients Javascript

I currently have a javascript library that is using a JSON file to print them on the screen in an interactive way. (::We are using D3JS Library)
When we are on a client, we can easily delete, edit and create some nodes, that are updated in the JSON every 5-10 seconds.
The problem comes from two main facts :
First the automatic function that call itself every x seconds could make data corruption if we are doing some stuff on the datas already represented on the screen.
Then the project has been made in order to permit 5 people to interact together. When they are present onto the same session we cannot decently make them refresh every 5 seconds, that cause many overhead and doesn't avoid data corruption.
We have mainly thought about a solution only made with javascript and some AJAX but we realize that it should be reconsidered with a trigger that inform the client that the datas are no longer OK.
We are thinking currently about opening a script onto a server in order to attribute on each client an ID.
The goal would be to detect the modification done on the JSON file (on the server). But the point where we are stuck is :
1) Is there a best scripting language to interact server/web?
2) Which type of things should we use to make the clients update their datas? (socket right?)
About the second point the easiest way would be to call a JS function be we aren't aware of the possibilities given by the shell codes...
Sorry about the fact that we are happy developpers but maybe not enough skilled to solve this problem.
Thanks for your helps !

You can achieve that using pure javascript with the new WebSocket feature.
http://www.html5rocks.com/en/tutorials/websockets/basics/
Edit:
WebSocket is a web technology providing full-duplex communications channels over a single TCP connection. The WebSocket API is being standardized by the W3C, and the WebSocket protocol has been standardized by the IETF as RFC 6455.
WebSocket is designed to be implemented in web browsers and web servers, but it can be used by any client or server application. The WebSocket Protocol is an independent TCP-based protocol. Its only relationship to HTTP is that its handshake is interpreted by HTTP servers as an Upgrade request.[1] The WebSocket protocol makes possible more interaction between a browser and a web site, facilitating live content and the creation of real-time games. This is made possible by providing a standardized way for the server to send content to the browser without being solicited by the client, and allowing for messages to be passed back and forth while keeping the connection open.

direct client-to-client message

I have been working with node.js for a while, now when I'm looking deeper into it, for a chat aplication instead of sending message as client - server - client, there must be some possible ways for direct client to client message sending?

Browsers tend to communicate with servers via HTTP. Some implement other protocols, like websockets & SPDY, but again, these are mostly client-server protocols.
Some plug-ins (like Flash & Java) can open ports and communicate client-client. (AFAIK, haven't actually used them.)
Chrome is the only browser I'm aware of that can (soon) open TCP and UDP sockets from Javascript and do direct client-client communication. At the moment normal web apps can't do this, your app needs to be run as a "Chrome Packaged App", with a special manifest file.
Here are the docs, a blog post describing the feature and a browserify module that can behave like the net node.js module in the browser.
EDIT: This should probably not be tagged as [node.js] since you're trying to run in browsers (not in your node vm), this is a Javascript / Browser question.

This is maybe out of date question, but take a look on PeerJS.
It requires server only as a connection manager (broker). But all communication is done between clients directly.

This does not have anything with server. If you need something like that and if clients are flash you can use RTMFP . For JS i google this library which is js bridge for RTMFP, I dont know how it works. At the end you can write you own library to chat beetween clients but this is much harder(IP addresses are behind NAT, etc...)

I think answer for your question is here
PS Also exist open-source in-browser server which written using JS, but I didn't google it quickly. If you find it, please notify me.

If you just don't want to write your own server you can use:
https://httprelay.io
Use AJAX calls to communicate between peers.

Is there a Telnet library for JavaScript?

We have a network camera. It has an HTTP server to provides the current image. There is also a Telnet interface for controlling the camera (i.e. trigger, focus, etc.). I would like to add an HTML page to the camera that would provide a simple interface (we already have client software we write). I can "GET" the image and display that, but I would also like to have controls that use the Telnet interface to control the camera. So a button might have JavaScript code behind it that connects to the camera via Telnet (logs in) and issues the command to trigger the camera.
I know that JavaScript/browsers support connecting to the same host via XMLHttpRequest. In this case I would be looking to open a socket on port 23 and send text. I also know that I can do this through Flash, Java, or some other technology, but I would prefer to use JavaScript only. If that is possible.

Thomaschaaf is correct, while HTML5 introduces websockets you'll find they still require special server support as they post HTTP style information upon opening the socket:
JS/HTML5 WebSocket: Connect without HTTP call
The best way, currently, to have true sockets is to either
use a flash or Java component on the webpage that does the actual socket work.
use a proxy server with websockets that can handle the additional protocol overhead of websockets and connect to the real tcp/ip port with plain sockets.
The jsterm example Matt linked does the latter, and if your webcans are behind a firewall it will not work in your situation without also implementing another server.
There are libraries that implement the first method, two are linked here for convenience, many others can be found using a search engine:
http://stephengware.com/proj/javasocketbridge/ (Java)
http://matthaynes.net/blog/2008/07/17/socketbridge-flash-javascript-socket-bridge/ (Flash)

jsTerm is an HTML5 implementation of a Telnet client.
You'll need a browser that supports HTML5 WebSockets. WebSockets is the only method of doing non-HTTP requests with pure JavaScript.

Currently there is no way to do socket connections with JavaScript only.
But what you are searching for is a socket connection ;)
https://developer.mozilla.org/en/XML_Extras

If I interpret the question liberally as "is there a remote connectivity library for Javascript", then the answer is yes (quoting from https://xtermjs.org/):
A web based SSH2 client using xterm.js, socket.io, and ssh2: https://github.com/billchurch/WebSSH2
HTML5 Based SSHv2 Web Client with E2E encryption utilising xterm.js, SJCL & websockets: https://github.com/stuicey/SSHy
I've tried WebSSH2 with node.js briefly, it worked for me - I managed to connect to a Linux-based server with it.
(I know this probably doesn't help the OP but this is a 7-year old question anyway. Maybe it helps others who are needing an answer to a similar problem.)