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RE: VoIP over IPsec

  • From: Kuhtz, Christian
  • Date: Tue Feb 18 14:09:16 2003

On Tue, 18 Feb 2003, Petri Helenius wrote:

> > Maybe a stupid question... why would you need GRE tunneling while IPsec
> > has a tunnel mode of its own?

> Probably because a major router vendor, despite of repeated customer
> declined to implement routing across such tunnel mode.

> So if the router uses tunnel mode (as per the RFC) despite the GRE
> tunnel the packet has three IP headers... So that's 160 bits ethernet
> layer 1 + 18 bytes ethernet layer 2 overhead, 24 bytes for the GRE
> tunnel, 20 bytes for the IPsec tunnel mode IP header, 10 - 12 bytes for
> the ESP header, 16 bytes for the initialization vector, 20 bytes for the
> original IP header and finally 20 bytes for the RTP header. With a 40
> byte payload that adds up to 188 bytes on the wire of which 78% is
> overhead...


On Crisco, if memory serves, default payload is 160 for G.711, not 40.  The
sizing goes in multiples of 80s.


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