Re: [rtcweb] Sebastien Cubaud's non-ICE mechanism (Re: Summary of ICE discussion)

Randell Jesup <randell-ietf@jesup.org> Tue, 11 October 2011 00:16 UTC

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Date: Mon, 10 Oct 2011 20:11:44 -0400
From: Randell Jesup <randell-ietf@jesup.org>
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Subject: Re: [rtcweb] Sebastien Cubaud's non-ICE mechanism (Re: Summary of ICE discussion)
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On 10/10/2011 7:04 PM, Eric Rescorla wrote:
> On Mon, Oct 10, 2011 at 1:50 PM, Harald Alvestrand<harald@alvestrand.no>;  wrote:
>> Changing the subject to keep threads separate....
>>
>> On 10/09/2011 12:00 PM, sebastien.cubaud@orange.com wrote:
>>> Here are the steps I foresee before allowing the establishment of a media
>>> session:
>>>
>>> - Let's consider A (a RTC-Web compliant browser) connected to server S and
>>> wishing to share real-time media with destination B (potentially a SIP
>>> endpoint or a browser)
>>> - A&    B learn via the signalling channel the triple @IP, transport proto
>>> and associated listening port of the remote media
>>> - A sends a few RTP packets to B (3 as in RFC 2833/4733 or more?).- This
>>> would allow the mechanism to resist against packet loss -. The format of such
>>> packets are to be discussed
>>> - Assuming B receives these packets, it then sends via the signalling
>>> channel an information from the media path unknown from S (i.e. not accessible via
>>> JS).
>>> I propose to use the min of the sequence number of the RTP packets
>>> received (which is random per RFC 3550)
>>
>> The sequence number is a 16-bit number, so there are 16 bits of randomness
>> to play with here.
>> An attack based on just returning a random number will succeed 1 out of
>> 65.536 times; if any of the 3 packets' sequence numbers are acceptable, it
>> will succeed 1 out of 21.845 times.

If you use the sequence number and the timestamp, you have 48 bits of 
entropy...


-- 
Randell Jesup
randell-ietf@jesup.org