Re: Are IPv6 auto-configured addresses transient?

[Margaret Wasserman <mrw at sandstorm.net>]

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.

Margaret