Subject: Re: Unique SSRC IDs?

From: S_S_Dinesh/India/[email-masked]
Date: Mon, 8 May 2000 14:10:13 +0530

Martin Mauve wrote:
>
> It therefore seems that the problem of unique ID distribution has to
> be solved anyway (addressing the concerns you mentioned).

I'm not so sure. Its possible (and is often done), to generate unique
IDs without a central server to guarantee that uniqueness. The problem
is that these IDs are long. Its not an issue for many protocols that use
them, but RTP is different since its packets are so short. Having a 20
byte UUID in the header for a G.723.1 voice packet is simply too much
overhead. Thats why RTP uses this SSRC, which is short, and then binds
it to a unique ID (note that this ID is chosen locally and is still
unique), the CNAME, sent in an RTCP packet.

So, a global unique ID distribution service is only needed for protocols
where the cost of transferring a 20 or so byte identifier is too high. I
somehow doubt this is a very large set of protocols.

-Jonathan R.

--
Jonathan D. Rosenberg                       200 Executive Drive
Chief Scientist                             Suite 120
dynamicsoft                                 West Orange, NJ 07052
[email-masked]                     FAX:   (732) 741-4778
http://www.cs.columbia.edu/~jdrosen         PHONE: (732) 741-7244
http://www.dynamicsoft.com

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From: S_S_Dinesh/India/[email-masked]
Date: Mon, 8 May 2000 14:10:23 +0530

There are several problems with a centralized unique ID allocator:

- yet another point of failure (calls to mind Tanenbaum's dictum about
distributed systems which fail because a system fails that you didn't
even know existed)

- yet another target for a high-payoff denial-of-service attack (neat -
I can shut down the world's telephone, TV and radio by shutting down the
number server...)

- how do you keep me (and various copies of my little virus) from
sucking up all identifiers for some high-profile event?

- firewalls, NATs

- making sure that identifiers get returned - kind of a global DHCP
problem, just much worse

- fast joins for channel surfing add another step (and with channel
surfing, the number of requests to this server could be quite large)

Thus, I'd think that some test code that exercises the SSRC allocation
would be a far more useful contribution. If we ever want to get to
5-nines reliability, we can't keep adding centralized servers whose
failure brings down the whole phone system, radio and TV.
--
Henning Schulzrinne   http://www.cs.columbia.edu/~hgs