Re: [v6ops] Scope of Unique Local IPv6 Unicast Addresses (Fwd: New Version Notification for draft-gont-6man-ipv6-ula-scope-00.txt)

Fernando Gont <fgont@si6networks.com> Fri, 12 February 2021 23:13 UTC

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To: Fred Baker <fredbaker.ietf@gmail.com>
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From: Fernando Gont <fgont@si6networks.com>
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Date: Fri, 12 Feb 2021 20:12:33 -0300
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Subject: Re: [v6ops] Scope of Unique Local IPv6 Unicast Addresses (Fwd: New Version Notification for draft-gont-6man-ipv6-ula-scope-00.txt)
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Hi, Fred,

On 12/2/21 19:37, Fred Baker wrote:
> 
> 
>> On Jan 5, 2021, at 5:20 PM, Fernando Gont <fgont@si6networks.com> 
>> wrote:
>> 
>> ULAs are formally part of the GUA space. However, the 
>> characteristics of ULAs do not seem to match the definition of 
>> global scope from RFC4007 (IPv6 Scope Addr Architecture). ULA seem 
>> to have a scope of scope(link-local) < scope(ULA) < scope(GUA).
> 
> I think I might describe it using the phrase "routing scope" or 
> something akin to it. An address, any address, is usable within the 
> range it is advertised to in routing,

Indeed --- which is always smaller than the scope of the address in
terms of uniqueness.

Both the uniqueness scope and the routing scope of ULAs are smaller than
that of GUAs.



 > configure BGP accordingly. Link-local addresses are confined to a
 > given LAN not because someone said so in an RFC, but because routing
 > implementations do not advertise a certain prefix off-LAN,

There's a mix of things for ULAs:
1) Their uniqueness scope is the local link. So those addresses are 
meaningless out of the link.

2) Because of #1, even if you wanted to route link-locals, if you have a 
router with multiple interfaces you wouldn't known on which of them you 
should forward the packet.

3) The RFCs do prohibit that such packets are forwarded across links



 > and ULAs
 > are confined to a given domain not because someone wrote it in an RFC,
 > but because routers are not configured to (are configured to not)
 > advertise them to external BGP peers and (hopefully) BGP peers refuse
 > them if inappropriately advertised to them.

The question is: if you wanted to route ULAs globally -- could you?

i.e., who's acceptable as a origin AS for a ULA prefix? And, what if the 
same prefix is used by multiple parties?  (the ULA spec only considers 
the case where a handful of ULA-based addresses are interconnected --- 
but that's quite different from ULAs having uniqueness global scope... 
since this would imply that *all* ULA prefixes employed by *all* 
networks are unique)



> We're trying very hard to construct a useful definition of 
> "site-local" without saying so, and it might be worthwhile to 
> actually say so.

I seem to agree with you :-)

Thanks,
-- 
Fernando Gont
SI6 Networks
e-mail: fgont@si6networks.com
PGP Fingerprint: 6666 31C6 D484 63B2 8FB1 E3C4 AE25 0D55 1D4E 7492