Re: [mpls] Available IP version numbers.

If we are defining a protocol carried with its own ethertype (e.g. 
MPLS), there is no neeed for an IP version differentiation.

It can in fact be argued that IPv6 could have used a different header 
format, and assumed that the media woudl indicate the new protocol.
However, because we knew that it needed to interwork with, be mixed 
with, and be diagnosed with existing IPv4 packets it was far more robust 
to build the packet format in such a way that it was reliably 
distinguishable from IPv4.  Which means a different starting nibble.

We could have skipped it.  (Historically, we got there the other way. 
We started by thinking we would use the same ethertype for both.  And 
then also realized that had problems.  So we have a belt and suspenders; 
dual identification.  Which is often a good design paradigm.)

Yours,
Joel

PS: In the MPLS context the is further complicated by devices which look 
at the data after the end of the stack and try to guess what it is. 
Typically to support ECMP / LAG.

On 4/19/2021 2:53 PM, ietf@sergey.dev wrote:
> Hi Loa,
> In an adjacent thread ("mpls open dt & the first nibble discussion") I raised a question about why do we even expect to take something from IP numbering. Would appreciate your feedback on this one.
> 
> 
> -----Original Message-----
> From: mpls <mpls-bounces@ietf.org> On Behalf Of Loa Andersson
> Sent: Saturday, April 17, 2021 1:13 AM
> To: mpls <mpls@ietf.org>
> Subject: [mpls] Available IP version numbers.
> 
> DT,
> 
> We had a discussion on how many IP version numbers are available.
> 
> It should be remembered that in IANA “Reserved” really means “Reserved, do not assign”.
> 
> 0,1,5,7,8,9 and 15 are reserved.
> 2,3,10,11,12,13 and 14 are unassigned
> 4 and 6 are assigned
> 
> To make the reserved requires a standard track RFC.
> 
> So we 7 IP version numbers available, that is a sufficient low number to make me nervous, if I owned the registry.
> 
> I would nit count on having more IP version numbers assigned to “us”, especially since we already have an agreement (PWE3), accepting to get two numbers and committing to not use more.
> 
> /Loa
> 
> Sent from my iPhone
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