Biggest mistake for IPv6: It's not backwards compatible, developers admit

Sam Wilson Sam.Wilson at
Wed Apr 1 19:25:06 CEST 2009

Pardon me while I delurk.

On 27 Mar 2009, at 14:01, Benny Amorsen wrote:

> You could have added extra, optional fields to the IP header,  
> containing
> extra source or destination IP addresses. NAT devices would then move
> the original IP address into the new src ip address header and put the
> NAT-address into the src ip address. If the host at the remote end
> understood the extra header, it would copy it into the extra dst ip
> address header of the return packet, and the NAT would know where to
> send the packet without connection tracking. This mechanism would also
> make it possible to directly address hosts behind NAT. If a particular
> host doesn't understand the new header fields, it should simply ignore
> it, and the NAT then has to handle the packet using connection  
> tracking.

It's called loose source routing and it's been in IPv4 since time  
began.  It's a security nightmare which is why it's not used (and why  
the PIP proposal for what became IPv6 wasn't developed).  It was a  
great debugging tool in more innocent times.


Sam Wilson
Network Team, IT Infrastructure
Information Services, The University of Edinburgh
Edinburgh, Scotland, UK

The University of Edinburgh is a charitable body, registered in
Scotland, with registration number SC005336.

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