On Aug 9, 2010, at 11:35 AM, Jari Arkko wrote:

A DS-Lite home gateway SHOULD NOT operate a NAT function on a B4
interface, as the NAT function will be performed by the AFTR in the
service provider's network.

I am not sure what it means to operate a NAT function on an interface. 
Presumably it would be operated between two interfaces. Perhaps you wanted to 
say that "the GW SHOULD NOT operate a NAT function".

If you look at natd in a traditional unix/linux kernel, the NAT deamon is 
associated to an outgoing interface. The semantic is, whatever packet leaving 
through that interface (according to the routing table) will be translated 
according to the specified NAT rules. In general, it means, the SRC address 
will be rewritten to the IP address configured on that interface.
I initially wanted to say the home gateway SHOULD NOT operate NAT, but then we 
thought it would be too restrictive: a CPE may have 2 outgoing interfaces, one 
through a "regular" IPv4 path and one through a B4/DS-Lite path.
It is OK to NAT on the former path, but not on the later.

   - Alain.

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