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RE: VoIP over IPsec

  • From: Ejay Hire
  • Date: Mon Feb 17 14:00:12 2003

There is some work on the SRTP protocol, but finding a cpe that will work with it is unlikely in the near future.  If you had a gateway server at each site then you might be able to use a back-to-back ua and srtp between the sites.  (That sounds kludgey.

-----Original Message-----
From: Charles Youse []
Sent: Monday, February 17, 2003 12:34 PM
To: Stephen Sprunk; Charlie Clemmer
Subject: RE: VoIP over IPsec

So do you suppose that in my scenario, I'd be better off leaving the VoIP out of the encrypted tunnels and use a separate [cleartext] path for them?

I'm worried about the security implications, not because I feel there is a huge security risk but because I'm sure the topic will be brought up.  (Communicating over one provider's backbone provides little opportunity for third parties to snoop packets between points, of course.)  

Has the issue of VoIP security ever been addressed?  I suppose I should really do my homework.


-----Original Message-----
From: Stephen Sprunk []
Sent: Monday, February 17, 2003 1:22 PM
To: Charlie Clemmer
Subject: Re: VoIP over IPsec

Thus spake "Charlie Clemmer" <>
> Stephen, I know this is outside of Charles' original inquiry, but I'm not
> familiar with this "qos pre-classify" feature. Since we would be
> voice traffic ... at what point would you classify it? If I classify it
> before it goes into the tunnel and gets encrypted, would that
> classification last once it's encrypted? If we try to classify after it's
> been encrypted, how can we tell it's voice traffic? It seems to me that
> jitter from both the actual encryption process as well as that associated
> with basic serialization would be the potential death of VoIP in this
> scenario, but I'm not sure mechanisms available to help resolve that risk.

In the default IOS code path, encryption happens before QOS (and after GRE).
Modern IOS versions copy the DSCP when encapsulating/ encrypting packets, so
DSCP-based QOS will still work, but IP- and port-based QOS will not.

More importantly, encryption is slow; even hardware encryption is
significantly slower than the rest of the forwarding process.  It's also
FIFO by default, meaning that large data packets can get stuck ahead of your
VoIP packets, causing jitter.

'qos pre-classify' adds a second QOS stage before encryption, which allows
you to classify packets in their unencrypted state and, more importantly,
adds PQ capability to the encryption stage.

For more information:


Stephen Sprunk         "God does not play dice."  --Albert Einstein
CCIE #3723         "God is an inveterate gambler, and He throws the
K5SSS        dice at every possible opportunity." --Stephen Hawking

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