Re: Are IPv6 auto-configured addresses transient?

[Brian Haberman <[e-mail]>]

Hi Margaret,

Margaret Wasserman wrote:
> 
> Hi Vijayrajan,
> 
> On Oct 7, 2009, at 12:25 PM, Vijayrajan ranganathan wrote:
> 
>> Hi Everyone,
>> Is there a notion that auto-configured IPv6 addresses based on
>> globally unique prefixes are transient compared to manually configured
>> ones?
> 
> Auto-configured global IPv6 addresses are "leased" to an interface for a 
> specified "lifetime".  They expire at the end of that lifetime if the 
> lifetime is not renewed.
> 
>> As an IPv6 application developer, would I have to factor in this
>> "transiency" of autoconf addresses in my design all the time? How safe
>> & normal
>> is it to replace all manual IPv6 address configuration with
>> auto-configuration in a large IPv6 deployment esp in an environment
>> that is very sensitive
>> to non-availability of addresses?
> 
> Typically, IPv6 router advertisements will advertise fairly long (at 
> least several hour) lifetimes for their prefixes, and those lifetimes 
> are renewed each time a hosts receives another router advertisement, 
> which should happen quite often.
> 
> Also, when the address lifetime expires, the addresses do not go away, 
> they are merely deprecated.  They will continue to be used for ongoing 
> communication, and may be used for new communication if no other 
> addresses are available.  So, prefix lifetimes offer a way to replace an 
> existing address with a new address when the network configuration 
> changes, but they do not prevent continued local communication if the 
> only router (or every router) goes down for an extended period, for 
> example.
> 
>> Another related question, is it common for a site's global prefix(es)
>> to change? In this regard, are they any different from an IANA
>> assigned IPv4
>> network-id for example?
> 
> Most global prefixes in IPv6 are provider-assigned, so they would change 
> if you change ISPs, or perhaps if your ISP changes their network 
> configuration.  Some registries do seem to over provider-independent 
> IPv6 addresses, but I don't know all of the details about how you would 
> get one of those and/or how you would get an ISP to route it.  You'd 
> need to contact a registry and your ISP to work that out.
> 

I *think* that Vijay is asking a more general question about 
applications rather than how to get IPv6 addresses (or service).  If I 
interpreted his earlier comments correctly, he is asking what does an 
application know about an address that it is using.  For example, if 
Vijay's application is long-lived would it be affected by an address 
becoming deprecated and no longer reachable.  In this case, the address 
stays reachable for some period of time until the deprecated prefix is 
removed from the routing system.

I suppose there is a related question as to how an application that uses 
IP addresses in referrals should behave.  Would an application find it 
useful to know the lifetime associated with the auto-configured addresses?

Regards,
Brian