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

[Rémi Després <[e-mail]>]

Hi all,

1.
Here is a unified Address Mapping, for both encapsulation and double-translation approaches, that is PROPOSED for IPv4 residual deployment across IPv6-only routing domains.

It results from an informal meeting among participants having different preferences between double-translation and encapsulation solutions (Xing Li, Congxiao Bao, Wojciech Dec, myself, at the end of the Beijing meeting).

The idea is to have a common format that is good for both types of solutions.
In particular:
- It enriches Double translation by introducing a deterministic way to distinguish a translated IPv4 packet from a native IPv6 packet.
- It enriches Encapsulation by expressing, in clear format within an IPv6 address, an IPv4 address, an IPv4 prefix, or an IPv4 address + port-set ID. 


2.
(a)
The IPv6 Source address of an IPv4 packet from a CE is:

a1- If the CE has an exclusive or shared IPv4 address:

<--------- 64 ------------><8 ><------ L >= 32 -------><48-L><8 >
+-------------+--------+---+---+----------------+------+-----+---+
| IPv6 prefix |CE index| 0 | V |  IPv4 address  | PSID |  0  | L |
+-------------+--------+---+---+----------------+------+-----+---+

a2- If the CE has an IPv4 prefix:

<--------- 64 ------------><8 ><-- L < 32 --><--- 48-L -----><8 >
+-------------+--------+---+---+-------------+---------------+---+
| IPv6 prefix |CE index| 0 | V | IPv4 prefix |         0     | L |
+-------------+--------+---+---+-------------+---------------+---+

(b)
V is the mark that characterizes IPv6 packets that are in reality IPv4 packets.
Its value differs from any permitted value of this octet in IPv6 IIDs  (ref RFC 4291).

It is understood that, if double Translation coexists with single translation, concerned ISPs may notify their CEs to use the U octet of RFC 6052 instead of V.

An unambiguous mark is fortunately possible because currently permitted IIDs have in their first octet either bit6 = 0 (the "u" bit"), or bit6 = 1 and bit7= 0 (the "g" bit).
With V having "u" = 1 (signifying Universal scope) AND "g" = 1, distinction is therefore deterministic.

The proposed V is = 00000011. 
(With other values of this octet, other IID formats can be defined in case some would be useful in the future.)

Note that, if and when a consensus is reached in Softwire, an extension of RFC 4291 will have to be submitted to 6MAN. 

(c)                                   
A Destination address from a CE to the outside IPv4 Internet is:
<--------- 64 ------------>< 8 ><----   32 ----><--- 16 ----><8 >
+--------------------------+----+---------------+------------+---+
|     BR subnet prefix     | V  |  IPv4 address |      0     |32 |
+--------------------------+----+---------------+------------+---+

Note that if double-translation CEs are notified to use U instead of V, the last octet becomes 0 per RFC  6052.


3.
All comments are most welcome.
  
Kind regards to all,
RD