Subject: Re: question about ipv6 interface id

From: Erik Nordmark <[email-masked]>
Date: Thu, 13 Jan 2005 11:37:48 -0800

[Sorry for the delayed reply]

liqunhui wrote:
> hi Could anybody tell me why we need a 64bit fixed length interface
> id? and do we really need a network that contains so many hosts?

The primary motivation as I understand it is to allow stateless address 
autoconfiguration, not only for IEEE 802 links, but also links that use 
the new 64-bit EUI-64 link-layer addresses.

When the 64 bit IID was decided, there was also some hope that the 
Universal bit in the IID would be useful as part of some multihoming 
solution that separates the identifier and locator aspects of an IP 
address. However, I haven't seen any use of this when trying to come up 
with ideas in the multi6 WG, and I haven't seen any other proposals in 
the multi6 WG try to use this bit.
So I think in practice, the U bit in the IID is useful to reduce the 
risk of address conflicts when there are different methods for picking 
interface IDs (stateless address autoconfiguration, DHCPv6, manually 
assigned, temporary addresses, CGAs, etc), but doesn't have any useful 
semantics for the peer. So it might very well be possible to make the 
existence of the U bit a property of the actual IP-over-foo specification.

> If router advertise a prefix which is longer than 64bit,then the
> possibility of address duplication is increasing.for example ,it will
> overwrite 56 bits of the 64bit interface id when router advertise a
> 120bit length prefix.so only 8 bits is left for interface id.how can
> we deal with this?

The current specification (RFC 2462) says that the length of the 
advertised prefix plus the length of the interface ID (for the 
particular IP-over-foo) must equal 128.
So there will not be any overwriting of bits.

If you want to use 120 bit prefixes then you either have to manually 
assign the addresses (I think folks do this for links between routers) 
or use DHCPv6 (but I'm not certain DHCPv6 implementations support this).

   Erik

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