[Mip6] Issue 73: v4 mapped address in IPv6 header

["Hesham Soliman" <[e-mail]>]

Folks, 

This is the final issue listed on the tracker. This one is a bit long.

Issue Text:
-----------

the IPv4 mapped address has a special meaning by RFC
2553 API.  It is not preferable to use the mapped address in IPv6
headers (See the following the drafts)
	draft-itojun-v6ops-v4mapped-harmful
	draft-cmetz-v6ops-v4mapped-api-harmful

In our code based on KAME, the IPv6 implementation discard a IPv6
header which has the v4 mapped address for sanity at ip6_input() and
ip6_rthadr2().  We also need to add the mapped address in an address
list (the list of all addresses which the node has) to receive the
header.  This is somehow uncomfortable because the mapped address is
actually not routable.

>From Hesham:
=> No one suggested that it should/would be routable. It's simply
used to keep the packet format. There is no routing based on this 
information. 

>From Koshiro:
=> I am not sure whether it's just an implementation issues.  But putting
the mapped address in the address list in order to process the DSMIP
IPv6 header means the mapped address may be chosen as a source address
even the address is actually not routable.  To avoid this, we need
e.g. an additional flag to distinguish the mapped address from others.
I think some implementers will not accept this.

The above is not only the reason again the mapped address in the IPv6
header.  Please refer the draft-*-harmful.  So, my idea is to put HoA
in IPv6 header and kind of IPv4 CoA option to idicate it's IPv4 CoA.

BTW, if you just want to keep the packer format, I think it's better
to use compatible address, or 6to4 address, or newly-defined address
for this purpose.

Analysis:
---------

The resons listed in the issue text (and other reasons discussed in the DT)
as well as their rebuttal are listed in this section. The first reason for
using a different address format was that the use of mapped address was not
recommended. The issue text refers to two drafts above. Those two drafts
were discussed several years ago in 2002 (first v6ops meeting). The only
issue that was agreed on in those drafts was that the mapped address should
not be used as a routable address. Therefore, the issue misinterprets the
agreement in the community. Also, the mapped address is not used as a
routable address in DSMIP. The drafts referred to above did suggest the
removal of the v4 mapped address altogether from IPv6, but this suggestion
was rejected and the drafts were not adopted. Mapped addresses are supported
in all major OSs (with the exception of KAME). 

The issue text also suggests the use of a different address format
(compatible address, 6-to-4, or a new address format). The compatible
address format was deprecated from the IPv6 address architecture and the
mapped format is the recommended format for embedding IPv4 addresses in
IPv6. 6-to-4 addresses imply a specific tunnelling behaviour (tunnelling to
the v4 address), which is not useful for our purposes. A new address format
will be no different from the mapped address, which is designed for this
purpose. 

Another concern that was raised against the use of the mapped address was
that they are "implicit" in nature ad do not explicitly show the IPv4
address. However, IP stacks must check the src address in the packet to
insure that is in fact a legal address (e.g. not multicast) in ip6_input. 


Recommendation:
--------------

My recommendation is to reject this issue for several reasons:
a. There is no clear problem with the current format, i.e. what breaks?
b. We've already removed the alt-CoA option in a previous issue, so if we
accept this issue we'd have to introduce a new address format for DSMIP.
This can take a long time and will yield the same result. Although, if there
is something specific in the mapped address format that will cause problems,
and a new address format will solve this problem then I'm personally ok with
the new address format. However, we need to understand what that problem is.



Regards,
Hesham






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