[bdNOG] Plain Simple v4 routing over v6

Scott Weeks surfer at mauigateway.com
Tue Mar 10 01:56:10 BDT 2020


Hello,

I didn't see an answer to your question, so I thought I'd try.

First: "everyone might think of flame throwing but I would really 
appreciate a positive feedback on the question"

I would hope no one throws flame over something like this.  I 
believe this is one thing a NOG is for and hope anyone would feel 
comfortable asking these questions here.  We all learn better in 
a collaborative environment.

::  Recently I was asked the question in an Interview that why 
:: cannot we simply route v4 in an v6 network without the help 
:: of a Tunneling protocol or such.

Your question is not answered by this part of my response, but 
for others the tunneling protocols are a transition mechanism, 
which should be avoided at this point for many networks.  Opt 
for dual stack when designing an IPv6/IPv4 network.  The two 
tunneling transition mechanisms are 4in6 and 6in4.

https://en.wikipedia.org/wiki/6in4

https://en.wikipedia.org/wiki/4in6

The reason you can't do it without a tunnel is the protocols are
not compatible with each other.  If an IPv4 address showed up on 
an IPv6 interface the receiving router would have no idea what 
to do with it.  Likewise, if an IPv6 addressed packet showed up 
on an IPv4 interface the router has no idea what to do with it.  
In both cases the packet would be dropped as it's not understood.
Perhaps that'd show up in the interface errors as an "unknown 
protocol"?

The tunnels are a way to 'hide' the underlying protocol from the 
transporting protocol until the underlying protocol's packet gets 
to a router that understands it.

Hopefully, someone with more experience than me can speak up 
with better details that I used.

scott






--- manjan.ahmed.chowdhury at gmail.com wrote:

From: Manjan Ahmed Chowdhury <manjan.ahmed.chowdhury at gmail.com>
To: nog at bdnog.org
Subject: [bdNOG] Plain Simple v4 routing over v6
Date: Sat, 7 Mar 2020 07:58:41 +0600

Hello Everyone,
This is my first interaction with bdNOG and looking forward to some
positive feedback. Unfortunately this is pretty simple question and
everyone might think of flame throwing but I would really appreciate a
positive feedback on the question. Recently I was asked the question in an
Interview that why cannot we simply route v4 in an v6 network without the
help of a Tunneling protocol or such.
My answer was something like following but it seemed that the Board was not
happy with the question and neither did they answer it:
1. TTL field vs Hop-Limit field due to which when a v4 route is received on
a v6 host the next-hop is changed from v4 to v6 so the TTL field is gone or
reset to 0.
2. v4 Routes are associated with an Interface and v4 address whereas in v6
a route is associated with a Link and uses Link Local address only.
3. In v4 reverse route lookups are done using ARP resolution to find the
next-hop interface whereas in v6 there is no concept of ARP as it is part
of stateless autoconfiguration with predefined algorithms.

Can anyone please let me know if I was wrong in any statement?

Best Regards,
Manjan Ahmed Chowdhury


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