Re: [P2PSIP] HIP vs. TLS/DTLS/SRTP (was HIP pros and cons)

[Eric Rescorla <[e-mail]>]

> On Tue, 18 Dec 2007, Eric Rescorla wrote:
> 
> > At Mon, 17 Dec 2007 17:30:24 -0500,
> > Philip Matthews wrote:
> >> In all three proposals, media packets would flow directly between the
> >> X and Y, and not hop-by-hop around the overlay. So when ESP was used,
> >> there would be no need to use STRP for media, or TLS or DTLS for
> >> signaling.
> >
> > This is arguably a bug, not a feature.
> >
> > SRTP was explicitly designed to have very low overhead: just the
> > bits of the authentication tag itself, with no header, etc. The
> > rationale for this design was that RTP packets tend to be very
> > small and so the overhead for the header, IV, etc. was significant.
> > In cases where that type of constraint applies, then wrapping the
> > RTP in ESP would be bad.
> 
> I think the difference is around 18 bytes:
> 
> http://dasan.sejong.ac.kr/~wisa04/ppt/1A1.ppt
> 
> In practice, the difference is insignificant according to these results:
> 
> Bilien et at: Secure VoIP: call establishment and media protection:
> http://www.minisip.org/publications/secvoip-minisip-camera.pdf

I don't see that this paper is at all relevant to the question of whether 18
bytes of per-packet overhead is significant. In any case, if you want
to argue this point, I would advise you to take it up in AVT, since
low overhead was one of the principal design considerations for
SRTP.

-Ekr

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