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

[Rémi Després <[e-mail]>]

Le 23 oct. 2011 à 18:25, Maoke a écrit :

> hi Remi,
>  
> 2011/10/21 Rémi Després <[e-mail]>
> Maoke,
> Please see below.
> 
> Le 20 oct. 2011 à 16:48, Maoke a écrit :
> 
>> 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? ;-) 
> 
> Yes, thanks.
> 
>>  
>>  
>>  - 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).
> 
> Oops, another bug :-(.
> The right formula for ri is:  ri = m - (32 - hi) 
> 
>> 
>>   (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)
>>  
> 
>  
>  is this another bug? i think:

>  
> |<------------        x       ---------------->|
> |            |<---------   n   ----------->|<k>|
> +------------+---------+-------------+-----+---+
> |Common Pref.|Rule idx | IPv4 suffix |PSID |   |
> +------------+---------+-------------+-----+---+
> |            |<---------   m  ------>|
> |<--- c ---->|<-- ri ->|<--- hi ---->| 

Correct (with the correction made in your next mail: hi replaced by 32 - hi)

> and thus c = x - (n + k), right? ;-)

Indeed.
Thanks fot the careful review.

Cheers,
RD