[RTW] Criteria for what one can do in Javascript vs what one has to do inside the browser

[Harald Alvestrand]

Trying to sort out thoughts in my own head.... I've run a few 
experiments, and thought a few thoughts.

There are a couple of things that should NOT be permitted from 
Javascript in an RTC-Web enabled application, and there are a couple of 
things that just can't be done at the present stage of our codebase.

Things that should not be permitted:

- Sending an UDP packet with script-specified content to a 
script-specified destination. This has far too much potential for havoc 
(imagine them being fired at business critical systems, DNS servers or 
SNMP monitoring ports).

- Setting up a TCP connection to a script-specified destination and 
sending script-specified data down it. Same issues as above.

The traditional defense against the second type is the "same origin 
policy", policed by browsers in their implementation of XmlHttpGet, 
WebSockets and similar interfaces, which also limits requests to 
more-or-less valid HTTP (but see 
http://www.alvestrand.no/ietf/http-traps.html for some fun abuses that 
used to work...)

So far, we have assumed that the STUN handshake is our defense against 
the first one, and that it's OK to send out a moderate amount of 
STUN-formatted UDP packets to ports and IP addresses chosen by the 
script, believing that the STUN format prevents them from being parsed 
as valid packets by other protocols.

(Query: What other operations need to be protected against?)

Things that can't be done:

- Anything that requires timing of events within 20-100 ms of each other
- Anything that depends on multithreading behaviour in the browser

In both cases, Javascript just doesn't work that way.

I think the TCP constraint + the UDP constraint means that we can't 
implement SIP or XMPP in Javascript without a gateway that talks 
SIP-over-HTTP - you just can't get around the security features.

I think the timing constraint means that if you implement ICE in 
Javascript, you're going to need to have seriously relaxed timing 
constraints - the standard specifies that you should try candidates at 
<complicated expression that usually turns out to be 20 ms>.

What else are serious limitations on what we can or cannot do?

What are the consequences?

                          Harald