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Re: PPP over IPSec (without L2TP)?

Bernard Aboba wrote:
In L2TP it is perfectly possible to apply
filters to achieve the same level of security. In fact, if
anything the argument went the other way -- because L2TP
does user authentication, when run over IPSEC its security
is stronger than that of IPSEC tunnel mode implementations
that only do machine authentication and therefore have no
idea who the user is.
Excuse me? I'm not an expert on L2TP, but I believe that's not
entirely correct. Let me quote RFC-2661.
9.1 Tunnel Endpoint Security

   The tunnel endpoints may optionally perform an authentication
   procedure of one another during tunnel establishment.  This
   authentication has the same security attributes as CHAP, and has
   reasonable protection against replay and snooping during the tunnel
   establishment process. This mechanism is not designed to provide any
   authentication beyond tunnel establishment; it is fairly simple for a
   malicious user who can snoop the tunnel stream to inject packets once
   an authenticated tunnel establishment has been completed

   For authentication to occur, the LAC and LNS MUST share a single
   secret.  Each side uses this same secret when acting as authenticatee
   as well as authenticator. Since a single secret is used, the tunnel
   authentication AVPs include differentiating values in the CHAP ID
   fields for each message digest calculation to guard against replay

I believe L2TP only authenticates the machines, while it's PPP that authenticates
the users.

There was also some talk about filtering of packets. As you see below, RFC-2661
wishes to put the filtering above its layer, to the PPP layer. Achieving this
should be easier in practice if you have less protocol layers between IPSec and PPP.

9.4 L2TP and IPsec


   IPsec also defines access control features that are  required of a
   compliant IPsec implementation. These features allow filtering of
   packets based upon network and transport layer characteristics such
   as IP address, ports, etc. In the L2TP tunneling model, analogous
   filtering is logically performed at the PPP layer or network layer
   above L2TP.  These network layer access control features may be
   handled at the LNS via vendor-specific authorization features based
   upon the authenticated PPP user, or at the network layer itself by
   using IPsec transport mode end-to-end between the communicating
   hosts. The requirements for access control mechanisms are not a part
   of the L2TP specification and as such are outside the scope of this

As to the re-ordering of packets by IPSec.. IPSec already does sequence numbers. It shouldn't
be too difficult to define a new IPSec SA attribute negotiable by IKE that says "sequenced
delivery of packets required". The recieving IPSec implementation would perhaps try to re-order
packets during a few milliseconds or whatever, and drop packets that come after that.

Ari Huttunen                   phone: +358 9 859 900
Senior Software Engineer       fax  : +358 9 8599 0452

Data Fellows Corporation       http://www.DataFellows.com

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