Re: [Softwires] Proposed Unified Address Mapping for encapsulation and double-translation

[Jacni Qin <[e-mail]>]

hi,

One comment on the example you proposed, please see below,

On 10/9/2011 7:30 PM, GangChen wrote:
> ...
> I'm not sure what formula you have to deduce this *requirement*.
> Different sharing ratios should implicitly be expressed by different
> IPv6 prefix lengths when an operator would deploy PD. At this point,
> you can't help even you encode whole IPv4 address in suffix. As for
> the question you raised, I think that are operational issues more than
> technical issues. I take your example as explanation for your better
> understanding.
>
> A small ISP has its Rule IPv4 prefix such as 10/8, while its total
> IPv6 block has fairly long prefix like /48 or a /56.
>
> Then an operator could strategically pre-config a relatively small
> IPv4 subnet to 4v6 domain as rule IPv4 prefix, like 10/24. Then you
> could get more space in IPv6 prefix to put PSID information. See that
> is quite flexible if you do have a good operational planning.
>
I get it, but let me try in another way.
The IPv6 addressing must not be affected, that's one of the basic 
objectives of MAP we'll agree on, right?

So, firstly the IPv6 address planning should be done natively according 
to the service requirements, the expected scale of subscribers, and the 
address space assigned respectively.
For example, if you plan to assign a /60 to each subscriber in a given 
domain with 1M subscribers totally, an IPv6 prefix of "/40" is required 
(probably shorter given the efficiency loss of hierarchical aggregation).
Then, this becomes the assumption (or restriction) on which we've got 20 
bits space to play with our art.

The next step is to decide the sharing ratio, say, 2k ports per 
subscriber, the PSID will be of 5 bits. Here the tradeoff is to use how 
many bits borrowed from ports or from the first 32 bits.
Then in this case, an IPv4 prefix of "/17" is required to accommodate 
the 1M subscribers (through "15+5").
If we're lucky to get a block of "/17", one piece of MAP rule is enough. 
Otherwise, we'll have to face another tradeoff, smaller number of rules 
or smaller number of fragmented blocks used.

ISPs' cases are quite different, while the philosophy of address 
planning is not, and should be kept in mind when we design the MAP.


Cheers,
Jacni

>