/127 between routers?

[Sam Wilson]

On 5 Jan 2010, at 21:50, Mark Smith wrote:

> On Tue, 5 Jan 2010 20:26:55 +0100
> Gert Doering <gert at space.net> wrote:
>
>> Hi,
>>
>> On Tue, Jan 05, 2010 at 08:59:58AM -0800, Michael K. Smith -  
>> Adhost wrote:
>>> We use /64's for interfaces/interface sets/ and route /48's  
>>> through and
>>> to those interfaces.  [..]
>>> Granted, I do use /128's for loopback addresses.
>>
>> Same here.  For similar reasons.  Eventually, CGAs and SEND might  
>> show
>> up, and then I might want /64s - and since a single /48 will be  
>> sufficient
>> to number all routers we'll ever have, I stopped doing my IPv4  
>> worries
>> ("oh, this is all so wasteful") here.
>>
>> I wouldn't want to enter a religious war here, though.  /64s might be
>> useful one day, or might be not.  /120 and /112 work as well.  So  
>> pick
>> something you're comfortable with.
>>
>
> Why doesn't anybody put a price on managing multiple prefix lengths?
> Maybe because they're so used to paying it with IPv4 that they don't
> recognise it as a cost?
>
> I'm for 'less is more'. /64s everywhere is less complexity and with
> a /48 as an individual/end-site, you get 65536 of them. As an ISP with
> a /32 you get 65K /48s, and if you use four of those for your own
> internal addressing you'll have 256K /64s - how many ISPs have that
> many internal point-to-point or otherwise links and loopbacks?
>
> Why be conservative when the corresponding cost is to to record,
> maintain and constantly have to work with and get right, differing
> prefix lengths? If it's a /64 or nothing, you can never get it wrong.

Let me delurk and offer the following:

- IPv6 allows arbitrary prefix lengths (RFC 4291 2.5 para 1)
- IPv6 allows maximum 64-bit subnet prefix lengths in the currently  
allocated global unicast address space (RFC 4291 2.5.1 para 3)
- current IPv6 implementations clearly do not enforce this restriction

The second bullet point above seems to reflect the 8+8 architecture  
proposed by Mike O'Dell [1] or the network+host architecture of  
Novell IPX and the like.  Part of O'Dell's argument was that routing  
kit should be optimised for making routing decisions only on the  
upper 64 bits of an address.  What worries me in Mark's proposal (if  
it's that not inflating the strength of what he's saying) is that we  
might end up with 8+8 by stealth - CPE which only recognises 64-bit  
prefixes, big-iron routers that route 64-bit prefixes in the fast  
path and relegate anything longer to slower processing.  It feels as  
though someone should be deciding either that subnet prefixes should  
never be longer than 64 bits or championing the right for prefixes to  
be any old length you want.

> Personally I'm considering a slight exception to the above by
> using /128s on router etc. loopbacks, however that is for the
> convenience of being able to see device IPv6 addresses/IDs by  
> filtering
> route table output e.g. "show ipv6 route | inc /128". That's a  
> doubling
> of the addressing management cost over just using /64s everywhere,
> which is why I'm still debating it a bit

The same argument applies to management systems - what happens when  
the only systems available assume that the longest subnet prefixes  
is /64 because that's what's currently in use (RFC 4291 2.5 vs  
2.5.1)?  What about staff training?

> I think the key realisation I've come to with IPv6 is that you get
> enough address space that you don't just have to worrying about
> allocating it out to make things work, you can allocate it out to also
> make addressing *convenient* to work with, with as simple as possible
> usually also being most convenient. Addressing convenience isn't a
> property we've had with IPv4 for a long time (e.g. slicing up a  
> Class B
> at the 3/4th octets, using the 3rd octet as subnet numbers). With IPv6
> we get it back, and we get more of it than we ever had with IPv4.

I (FWIW) think the key realisation with IPv6 is that we have too many  
bits available and don't really know what to do with them.  :-)


Sam Wilson
Network Team, IT Infrastructure
Information Services, The University of Edinburgh
Edinburgh, Scotland, UK




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