Why this is broken [was Re: Extending a /64]

[Brian E Carpenter <[e-mail]>]

On 17-Nov-20 04:26, Tony Whyman wrote:
> On 16/11/2020 14:50, Joel M. Halpern wrote:
>> Tony, why are you embedding the 39 bit airplane ID into the IPv6 
>> address.  That seems to be the fundamental thing that then has you 
>> need very short prefixes, and doing other difficult operations.
>>
>> And if the answer is "ICAO said so", then we have a problem of really 
>> smart aviation engineers doing network engineering.
>>
>> Yours,
>> Joel
> 
> Perhaps the best way to answer this question is to look at the alternative.
> 
> Aircraft MNPs are non-topological 

Thank you for that clear statement. Internet routing *is* topological.
Therefore, you cannot use bits in the routing prefix for non-topological
information. Therefore, you cannot put 39 magic bits into the routing
prefix; the Internet doesn't work like that. End of story. Back to the
drawing board.

This is so badly conceived that I suggest an IETF liaison statement to
ICAO may be needed.

There's nothing to stop you putting those 39 bits into the Interface
Identifier of the aircraft's IPv6 addresses. That would need a slightly
different version of RFC7217, but would still allow 25 entropy bits.

However, you should be aware that IP addresses are intrinsically
forgeable. I'm not sure I would want to fly on an aircraft whose
identity might be trivially forged. Also, it would be trivial for
an attacker to observe that a particular address belongs to a
particular aircraft.

One more comment below...

> and hence could be densely allocated. 
> However, if you want to go the distance and specify the minimum length 
> identifier then:
> 
> 1. You need to set up a new registration database for aircraft (in order 
> to assign the new identifier) and one that is global rather than state 
> based - which is the present approach.
> 
> 2. You need to update the specification for the Aircraft Personality 
> Module and organise the fleet wide upgrade necessary to ensure that 
> every affected aircraft gets a new ICAO global Aircraft Identifier.
> 
> 3. Safety Authorities will now identify a new hazard potentially 
> resulting from a mis-match between the existing ICAO 24-bit identifier 
> (which still has to be used for radar, ADS-B, TCAS, etc.), and the new 
> airframe identifier.
> 
> 4. We need to develop procedures and systems for mitigating the new 
> hazard, assuming it is deemed serious enough.
> 
> 5. The protocol gateways that are planned to avoid having to upgrade the 
> existing ATC Systems in the same timeframe would now have to include a 
> lookup table between the "old" NSAP Addresses and the "new" IPv6 Addresses.
> 
> 6. The Safety Authority would identify a new hazard resulting from a 
> table configuration/maintenance error and procedures/mitigations would 
> be needed to counter the threat.
> 
> 7. The address mapping would have to extend to the application protocols.
> 
> In other words - a lot of negatives each with a potentially large cost 
> attached to them.
> 
> The next step is to accept that it is worth "wasting" a few more bits 
> and using the existing ICAO 24-bit aircraft identifier instead of some 
> new global aircraft identifier. This avoids issues 1 to 4 above.
> 
> In order to avoid 5, 6  and 7, we need to have a simple algorithm to 
> convert between aircraft ATN/OSI NSAP Addresses and ATN/IPS IPv6 
> Addresses. 

Are you aware that we already "solved" that in 1996 (https://tools.ietf.org/html/rfc1888), and scrapped it in 2005?

Regards
   Brian



> The existing (legacy) NSAP Address format for aircraft 
> addresses starts off with some constant prefix and is then followed by 
> an airline identifier, the ICAO 24-bit identifier, an 8 bit subnet id 
> and a 48-bit End System id (it follows a structure originally proposed 
> by NIST). The last two can be readily combined into a 64-bit host 
> identifier. Indeed, if we based the MNP on the ICAO 24-bit then the only 
> thing stopping a simple algorithm is the missing airline identifier part 
> - this cannot be readily predicted from the ICAO 24-bit identifier 
> without access to the registration database.
> 
> In order to avoid a lookup table in the protocol gateway, etc. we need 
> to have an airline identifier in the MNP. 15 bits appears to be the 
> minimum we can get away with (in order to avoid a new airline 
> identifier, the proposal is to generate a 15-bit identifier by encoding 
> the existing ICAO three character airline identifier using ITA-2 letter 
> shift). Hence 24 +15 = 39.
> 
> The industry's network providers also regard embedding an airline 
> identifier in the MNP as highly desirable. It is used in the existing  
> system for diagnostic purposes and firewalls.
> 
> This is not an "ICAO says so" at all. This is practical engineering to 
> deliver a new system as an evolution from an existing system and with 
> limited budgets. Start with a clean sheet of paper and you could use a 
> lot less that 39 bits. Unfortunately, we do not have that luxury.
> 
> Tony
> 
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