Subject: SIP Via hiding

From: Henning Schulzrinne <[email-masked]>
Date: Fri, 20 Mar 1998 21:15:32 -0500

It may be desirable in some circumstances to hide the path of a SIP
message. This is relatively easy by having each proxy apply encryption
to its Via information. While this is sufficient to allow
loop-prevention, hidden Vias do not allow for re-tracing one's steps.

E.g. with C as client and proxies P1 and P2, we would have after P2:

Via: P2
Via: E[P1]
Via: C

if P1 encrypts. On the return trip, P2 has no way of knowing that it
needs to send on the reply to P1.

If everybody encrypts, this is easy: each proxy not just encrypts its
own address, but also the previous-hop address. In the case where only
P1 encrypts, P2 obviously can't just put P1's address in the Via, as
would defeat the whole hiding.

Note that Via hiding relies on the cooperation of the next-hop host: if
I want to reveal to the rest of the world where the packet came from,
nothing can stop me.

Given these limitations, I'd like WG opinion on whether Via hiding is
worthwhile.