RE: [SIP] symmetric RTP as a solution for NAT traversal

[Jonathan Rosenberg <[e-mail]>]

 

> -----Original Message-----
> From: Christian Huitema [mailto:[e-mail]]
> Sent: Thursday, March 15, 2001 10:22 PM
> To: Jonathan Rosenberg; [e-mail]; [e-mail];
> [e-mail]
> Subject: RE: [SIP] symmetric RTP as a solution for NAT traversal
> 
> 
> Jonathan,
> 
> Your solution is interesting, but it does not deal with the 
> case of two
> PCs, both behind a NAT. 

It does. The document discusses the solution. It uses an external RTP
translator. Not optimal; I recognize that. Other solutions may be possible.

> Another way to solve the problem is to let the
> PC who is behind a NAT learn the external mapping of the RTP 
> port, e.g.
> that 10.0.0.9:3456 maps to 123.45.67.89:7891. There are quite 
> a few ways
> to do that, some of which could well end up being standardized. That
> would certainly be a nice complement to the symmetric RTP trick.

Absolutely. There are several tractable solutions if nats conform to the
UDP requirements which you have outlined, Christian, in:

http://search.ietf.org/internet-drafts/draft-huitema-natreq4udp-00.txt



> 
> However, there is a slight problem. We cannot assume that 
> NATs are aware
> of RTP's requirement for "port pairs", an even port for RTP, the next
> port for RTCP. We may well learn that port 3456 maps to
> 123.45.67.89:7891, and port 3457 maps to 123.45.67.89:9872. Now, if we
> intend to open the SDP spec and create attributes for the "symmetric
> RTP", we should perhaps also create an attribute specifying the RTCP
> port, when that port is not equal to RTP+1.

If we do some kind of solution where the external entity tells both UAs
their public addresses, then yes, this will be needed.

> 
> A mild objection to the symmetric RTP is the risk of session 
> hijacking.
> Arguably, that is a risk you can assume if the alternative is 
> no session
> at all, but you should still consider it...

How does symmetric RTP create a risk of hijacking?

-Jonathan R.

---
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