Re: [Softwires] 4rd-addmapping - Possible independence of the IPv6 addressing plan from IPv4 prefixes

[Maoke <[e-mail]>]

hi Remi,

thanks a lot for the reply! it is very helpful and my latest understanding
almost approach it. but still some minor clarifications.
2011/10/20 Rémi Després <[e-mail]>

> Hi Maoke,
>
> Thank you for your questions concerning relationships between IPv6 and IPv4
> addressing plans.
>
> Here are address-planning steps that illustrate how an IPv6 addressing plan
> can be kept independent from IPv4 prefixes used for IPv4 residual
> deployment, even if CEs must be able to have different sharing ratios.
>
>   (A) IPv6 considerations
> (A1) Determine the maximum number N of CEs you want to support, a power of
> 2 (N = 2 ^ n).
> (A2) Choose the length x of IPv6 prefixes you want to assign to ordinary
> customers (e.g. x = 60)
> (A2) Multiply M by a margin coefficient K, a power of two (K = 2 ^ k), to
> take into account that:
>
>

here the M should be a typo of "N", right? ;-)


>
>  - Some privileged customers may be assigned IPv6 prefixes of length x',
> shorter than x, to have larger addressing spaces than ordinary customers,
> both in IPv6 and IPv4.
>  - Due to the hierarchy of routable prefixes, many theoretically
> delegatable prefixes may not be actually delegatable (ref: host density
> ratio of RFC 3194).
>
>   (B) IPv4 considerations
> (B1) List all (non overlapping) IPv4 prefixes Hi that are available for
> IPv4 residual deployment.
> (B2) Take enough of them, among the shortest ones, to get a total space
> whose size M is a power of two (M = 2 ^ m), and includes a good proportion
> of the available IPv4 space (ref
> www.ietf.org/mail-archive/web/softwires/current/msg02261.html).
> (B3) For each IPv4 prefix Hi of length hi, choose a "Rule index" Ri of
> length ri = m - hi. All these indexes must be non overlapping prefixes (e.g.
> 0, 10, 110, 111 for one /10, one /11, and two /12).
>
>   (C) After (A) and (B)
> (C1) Derive the length c of the "Common prefix" C that will appear at the
> beginning of all delegatable prefixes (c = n + k - m)
> (C2) Take for C any prefix of length c that starts with a RIR-allocated
> IPv6 prefix
> (C3) For each IPv4 prefix Hi, make a rule in which it is the IPv4 prefix,
> and in which the IPv6 prefix is the Common prefix C followed by the Rule
> index Ri.
>
> I hope this can work on your favorite example(s).
> If not, please let me know.
>

it seems you didn't mention the PSID part. my understanding is, after the
above, when we have the, e.g., /60, the PSID is attached after the /60. then
the CE delegated prefix (including IPv4 address suffix and PSID) may have
different lengths if the sharing ratio is variant for different shared IPv4
addresses. but the c + m (or n + k, or x (right?)) , including the common
prefix and the ipv4-address-related bits, is a constant for every
CE-delegated network in the domain, right?

on the other hand, there is a limitation. if c + m > 64, the above address
planning is not deployable, but with our effort of maximum compression, we
believe the undeployable case rarely happens for normal providers, right?


> Note that this is by no means to say that all ISPs should plan addresses
> this way (i.e. with a relationship between lengths of CE IPv6 prefixes and
> sharing ratios). It is just to show that this  is workable.
>
>

sure. but i believe this will helps operators very much, considering up to
now no operators have experienced that.

thanks,
maoke


>
> Regards,
> RD
>
>