hosted PBX/VOIP thru VPN?
jtodd at loligo.com
Tue Nov 11 22:03:44 CST 2008
On Nov 11, 2008, at 6:17 PM, Lorell Hathcock wrote:
> My customer wants to try to improve performance to his ATAs by
> creating a
> VPN from his network to the VOIP provider's network through the
> I have to admit, the idea caught me flat footed. At the outset, it
> like we would want to do it just to improve security for end users.
> my customer wants it because he thinks it will improve performance
> voice quality). We are suffering from poor VOIP quality due to the
> Sprint /
> Cogent depeering and subsequent squirming by our vendors.
> The only reason I can think that VOIP thru a VPN would help is that
> *perhaps* routers in the middle on ASNs I have no control over *may*
> prioritize VPN traffic higher than regular traffic. They opposite
> also be true.
> Specifically the ASNs in the middle are Level 3, Sprint and Time
> Thoughts? Should I try to dissuade him from this if performance is
> his main
The implementation of a VPN indeed would probably not result in an
improvement of your customer' RTP packet delivery, either for jitter
or packet loss. If you wish to see if RTP is being meddled with, try
changing the RTP port numbers on the ATA or on the remote side to
something less typical of the RTP port range - try something <10000.
While some deep-packet inspections will dig through each packet to
categorize and stomp on RTP voice audio, it is probably not the case
that anyone in the path you describe is applying anything other than
port number and protocol (UDP/TCP) inspections, if they are doing any
such punitive QOS at all.
I would be very interested to learn if you or anyone does know of a
transit carrier who is de-pref'ing RTP packets as a peered transit
provider (or non-paid peering partner.) This doesn't mean static "end
customers" - I'm really talking about traffic that is ingress/egress
from some other ASN, even if that ASN is paying for transit. This
would be a fairly major departure from any type of QOS or de-
preferencing that I've seen before, and I'm sure the list would be
interested in any results as well.
The root cause of the problem your customer describes also needs to be
identified - that will tell you a lot. Wireshark a few calls and see
what you can see on the RTP packet path. Without more specifics on
the "bad performance", it will be difficult to determine if this is
even a network issue at all - maybe it's just a sub-standard ITSP,
gateway, or even PSTN path on the far side of the equation.
A very slight chance exists that due to round-robin routing you are
getting out-of-order packets in one or both directions of the media
stream. RTP does not recover well from OOO packets. Try some
traceroutes in the RTP port range to see what happens. You can see
one direction for the traceroute UDP outbound and watch for multi-
pathing, and you can see the other direction with wireshark on inbound
OOO RTP streams to your customer. If the problem is out-of-order RTP
packets, then there are some things that a GRE tunnel plus some
creative route announcements/static routes might be able to solve, and
those are left as an exercise for the reader. But a "VPN" is almost
always going to be the wrong answer for VoIP performance increases -
GRE is better suited for encapsulating UDP, and I run VoIP connections
over GRE all the time due to the perverse and unfortunate routing
situation for my home network, which resides at the end of a consumer-
grade cable IP connection. I would not recommend even GRE as a matter
of course for VoIP RTP transport; I merely say that it is possible,
and in some fringe cases the only solution available.
FWIW: Snom (a SIP-based desk phone) now includes a built-in IPSEC
tunnel protocol stack so the phone can securely establish connections
back to the PBX or other endpoints. The reasons for doing this are
not clearly not performance-oriented - they are security-oriented. It
even will encapsulate traffic from any hosts attached to the one-port
switch on the back. Desk phones are getting pretty scary. I'm
waiting for "sh ip bgp" for my Cisco 7960...
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