Re: [rrg] moving towards recommendation: the current plan

[William Herrin <bill at herrin.us>]

On Mon, Nov 16, 2009 at 8:37 PM, Noel Chiappa <jnc at mercury.lcs.mit.edu> wrote:
>    > From: Michael Menth <menth at informatik.uni-wuerzburg.de>
>
>    > it touches a fundamental problem of Loc/ID split solutions based on
>    > address rewriting when hosts have only a single local address but their
>    > edge network is multhomed to the outside world.
>
> I'm not sure I understand what the problem is; can you explain in a little
> more detail?

Hi Noel,

A is a client talking to a multihomed server B.

Ip addresses:
A: 1
B: 2 (internal) 3, 4 (external)

Round trip from A to B:
A picks address 3 via DNS.
A->(1,3)->(1,2)->B->(2,1)->[(3,1),(4,1)]->A

The problem is that A received a return packet from B that might have
been from 3 and might have been from 4. But A has no idea what to do
with a packet from 4.

Per Michael, the solution is either:

1. Stateful NAT. The translator on B's network handles both network
paths and remembers that the communication from A came to destination
3 so that he can put source 3 back on the outbound packet.

2. Host modification. The inbound NAT adds an IP extension with the
original destination address. The host echos this in the return
packet, providing the outbound NAT with the info he needs to set the
correct external source address.

Regards,
Bill Herrin

-- 
William D. Herrin ................ herrin at dirtside.com  bill at herrin.us
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