Point 2 point IPs between ASes

Mark Andrews marka at isc.org
Thu Jun 29 23:08:09 CST 2017


In message <20170629150630.glfvte2ures27p2n at Vurt.local>, Job Snijders writes:
> On Wed, Jun 28, 2017 at 11:09:25PM +0200, Thomas Bellman wrote:
> > On 2017-06-28 17:03, William Herrin wrote:
> > > The common recommendations for IPv6 point to point interface numbering are:
> > > 
> > > /64
> > > /124
> > > /126
> > > /127
> > 
> > I thought the only allowed subnet prefix lengths for IPv6 were /64 and
> > /127.  RFC 4291 states:
> > 
> >    For all unicast addresses, except those that start with the binary
> >    value 000, Interface IDs are required to be 64 bits long and to be
> >    constructed in Modified EUI-64 format.
> > 
> > (and addresses starting with 000 are only used for special things,
> > like the localhost address ::1).  And then RFC 6164 adds /127 to the
> > allowed prefix lengths.
> > 
> > I know that many devices allow you to configure any subnet size, but
> > is there any RFC allowing you to use e.g. /124 or /126?
> 
> Breaking the law! Some IETFers will come hunt you do, be aware! ;-)
> 
> Here is some historical perspective looking at the IETF standarsd and
> current Internet-Drafts:
> 
> RFC 3513 "only /64 is valid"
> RFC 3627 "don't use /127, use /126 if you must"
> RFC 4291 "reaffirming: only /64 is valid"
> RFC 6164 "a /127 is OK to use too"
> RFC 6583 "there are problems with /64"
> RFC 7421 "/64 is the best!"
> RFC 7608 "every prefix length must be forward-able"
> RFC 4291bis-07 "fine, /64 and /127 are valid, but nothing else!"
> draft-bourbaki-6man-classless-ipv6-00 "IPv6 is classless FFS"
> RFC 4291bis-08 "fine, /64 and /127 are valid, and anything defined in future standards, and anything configured manually"
> 
> Quoting from 4291bis-08: 
> 
> """
>     Interface Identifiers are 64 bit long except if the first three bits
>     of the address are 000, or when the addresses are manually
>     configured, or by exceptions defined in standards track documents.
>     The rationale for using 64 bit Interface Identifiers can be found in
>     [RFC7421]. An example of a standards track exception is [RFC6164]
>     that standardises 127 bit prefixes on inter-router point-to-point
>     links.
> 
> 	Note: In the case of manual configuration, the Prefix and    
> 	Interface Identifier can be any length as long as they add up to    
> 	128.
> """
>     source: https://tools.ietf.org/rfcdiff?url2=draft-ietf-6man-rfc4291bis-08.txt
>     full file: https://tools.ietf.org/html/draft-ietf-6man-rfc4291bis-08
> 
> So, what it boils down to: if you want to use SLAAC, you should use a
> /64, if you don't need SLAAC, do whatever makes sense for you. And never
> be greedy: give your end-users a /48, there is plenty of space to go
> around.

And that should apply to cell phones as well.  A single /64 from a
ISP to a customer is a stop gap assignment.  3GPP supports DHCP-PD
it should be enabled in the back ends.

> Kind regards,
> 
> Job
-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at isc.org


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