Re: encoding Subnet ID in link-local addrs (Re: about violation of standards)

[Alexandre Petrescu <[e-mail]>]

Le 26/04/2019 à 20:48, Manfredi (US), Albert E a écrit :
> -----Original Message----- From: ipv6 <[e-mail]> On
> Behalf Of Alexandre Petrescu
> 
>> In  my setting, the Subnet ID in the LL address is unique per
>> subnet,
> not per host.  There is a subnet formed by the front bumper interface
> in the follower car and the rear bumper interface of the lead car.
>> 
>> The Subnet ID in an LL address would be unique per a set of hosts
>> in
> that subnet.
> 
> Instead of messing with LL standards, why not use ULAs? For example,
> the front car determines the 54 random bits in the ULA prefix. Same
> logic as you are using now, without requiring any DNS, and without
> violating anything.

I need the cars to send RAs to each other.  I think RAs have src a LL 
address.  Hence the need to use LLs.

Second, a ping to LL responding ok means the channels (frequency, GHz) 
are set correctly.  A ping to LL not responding means the channels are 
not set correctly.  Were that LL a GUA/ULA the conclusion is not the 
same, because IP routing.

HEre are the details:

Outside a car there are multiple subnets.  Each subnet is on a different 
5.9GHz channel.  The only addresses sure to be present are the LL addresses.

If I ping a paarticular address (fe80:1::1) and it responds it means 
that the channel is set correctly.    If it does not respond, it means 
the channel is set incorrectly.

If on the other hand I used GUAs or ULAs instead of LLs, the fact that a 
ping responds may mean the router between the subnets forwards, even 
though the channels might be set incorrectly.

Thus, when I ping fe80:1::1 and it responds I am sure I am in that 
fe80:1::/32 subnet and the channels are set correctly.  A negative 
answer means the channels are set incorrectly.

I hope I explained clearly enough, otherwise I can explain further.

>> More details about the problem: RFC4291 54 0 bits would refuse to
>> change
> to accomodate this solution's Subnet ID in an LL address; another
> detail is that one particular OS would not allow the manual
> configuration of an LL address setting some of these 54 0 bits.
> 
> I would always look for an equally simple solution that doesn’t need
> to violate standards. It is the path of least resistance.

I agree with you to look for simplest solution not violating standards.

In addition, I would also look for what immediately available 
implementation can do.

I the deployments where I work, people are much harsher about technology 
than here.

Note that the ULA recommendation I received at IETF I did take and 
deploy in some cars - but inside the cars, not outside.

> Alternatively, you can do what MANET does, which is essentially to
> use IP addresses as flat address space.

YEs.  Many people suggest that.

Flat addressing space on OCB may work but has some indeterminism.  There 
is no bridging in OCB, there is much dynamicity, etc.  SOme things 
happen and you dont know why.  I am afraid a convoy of more than 
800meter long will not scale (workr eliably) if MANET IP flat address 
space is used.

But only trying it in practice can prove me right or wrong.  And trying 
it in practice I cant do in the immediate months. (a convoy of more than 
800m).

On another hand, claiming that a flat IP space on OCB on a convoy of 
more than 800m long works ok should also be backed by some experimental 
evidence.

Alex

> 
> Bert
>