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From: "Steven M. Bellovin" <smb () research att com>
Date: Thu, 13 Sep 2001 21:43:48 -0400

In message <912A91BC69F4D3119D1B009027D0D40C01BB45A7 () exchange1 secure insweb co
m>, Vandy Hamidi writes:
It is working now.  I've done it with Linksys and Netopia DSL routers.
Software client on the laptop that DOES tunnel mode ESP.  No AH and running
through a PAT and it works flawlessly.  I just want to know how it works,
I've already determined that it does.
The point where my logic fails is where PAT relies on modifying the TCP/UDP
port numbers, an ESP packet has a standard IP header with an additional
protocol 50 ESP header.  Since there is no ports to change to create a table
to keep track of which packet came from which internal client, what is used
to keep track.
Someone said something about the UDP encapsulation, but what about the
NETOPIA which doesn't do that?

I repeat -- it doesn't do PAT.  Some "routers" -- they're really no 
such thing, of course; they're NAT boxes and/or bridges -- allow one 
host behind them to speak IPsec.  If a host emits a packet using ESP, 
it's tagged as *the* IPsec user; return IPsec packets are routed to 
that host.  (Some of these boxes may use manual configuration instead 
or in addition.)  You can't have two IPsec hosts, because there's no 
way to know which should receive incoming packets -- there's no 
relationship between inbound and outbound SPIs.

As for the UDP encapsulation -- yes, the IETF's IPsec working group is 
moving in that direction.  But it's not standardized yet, and there may 
be patent issues to sort through.

                --Steve Bellovin, http://www.research.att.com/~smb

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