Re: 6MAN WG Last Call: draft-ietf-6man-rfc3484-revise-05.txt

[Arifumi Matsumoto <[e-mail]>]

Dave,

another point below.

On 2012/02/14, at 8:55, Dave Thaler wrote:

>> -----Original Message-----
>> From: Dave Thaler
>> Sent: Monday, February 13, 2012 2:01 PM
>> To: Dave Thaler; 'Chris Grundemann'; 'Brian E Carpenter'
>> Cc: '[e-mail]'; 'Brian Haberman'; 'Bob Hinden'
>> Subject: RE: 6MAN WG Last Call: draft-ietf-6man-rfc3484-revise-05.txt
>> 
>> Yet another problem in draft-ietf-6man-rfc3484-revise...
>> 
>> Section 2.4 (Private IPv4 address scope):
>> [...]
>>>  The algorithm currently specified in RFC 3484 is based on the
>>>  assumption that a source address with a small scope cannot reach a
>>>  destination address with a larger scope.
>> [...]
>> 
>> The above sentence is simply not true, it was NOT based on such an assumption
>> at all.  It was based on the assumption that it was
>> less likely to work.   There's two reasons why it's less likely to work.
>> First, it might or might not be able to reach it (the text overstates by saying it
>> cannot... it was acknowledged that it may or may not).
>> Second, if it goes through a NAT, it might not work for protocols that embed IP
>> addresses in payloads.
>> [...]
>> 
>>>  Due to this assumption, in the presence of both a NATed private IPv4
>>>  address and a transitional address (like 6to4 or Teredo), the host
>>>  will choose the transitional IPv6 address to access dual-stack peers
>>>  [I-D.denis-v6ops-nat-addrsel].  Choosing transitional IPv6
>>>  connectivity over native IPv4 connectivity, particularly where the
>>>  transitional connectivity is unmanaged, is not considered to be
>>>  generally desirable.
>>> 
>>>  This issue can be fixed by changing the address scope of private IPv4
>>>  addresses to global.
>> 
>> Section 10 of RFC 3484 contained many examples.   -revise contains
>> no such example of what it's talking about, so I have to guess.  Let's look at 3
>> cases.
>> 
>> Case 1:
>> D set = { global IPv6, global IPv4 }
>> S set = { Teredo IPv6, RFC1918 IPv4 }
>> 
>> Under RFC 3484 rules, Destination Address Selection would prefer the Teredo
>> connectivity under rule 2 (Prefer matching scope).
>> 
>> Under -revise rules, Destination Address Selection would still prefer the Teredo
>> connectivity under rule 6 (Prefer higher precedence), since the precedence of
>> the (non-Teredo) destination address
>> beats the precedence of the IPv4 address.   Hence -revise
>> does not change the behavior in this case.
> 
> Dmitry Anipko pointed out that rule 5 (Prefer matching label) would cause
> the -revise rules to prefer IPv4.  Still, I'd prefer a solution that doesn't solve
> this problem by creating another one (case 3).   That is, we should fix a problem
> rather than just move it around.
> 
> I'll think about this and  see if I can come back with a proposal.

>> Case 3:
>> D set = { global IPv4 = 1.2.3.4 }
>> S set = { NAT-ed IPv4 = 10.2.3.4, global IPv4 = 128.66.3.4 }
>> 
>> Under RFC 3484 rules, Source Address Selection would prefer the global IPv4
>> address under Rule 2(Prefer appropriate scope).
>> Under -revise rules, Source Address Selection would instead prefer the NAT'ed
>> IPv4 under Rule 8 (Longest matching prefix).
>> 
>> This is broken.   I don't see a real case the proposed change
>> fixes, I only see real cases it breaks.


AFAIK, neither RFC 3484 nor -revise specifies source address selection algorithm
for an IPv4 destination address. Simply, it is out of scope of these documents.

Do you want to cover these issues in the revision ?

Best regards, 

> 
> -Dave
> 
>> 
>> Case 2:
>> D set = { Teredo IPv6, global IPv4 }
>> 
>> Not an interesting case because Teredo addressing should be disabled when a
>> host has a global IPv4 address.
>> 
>> Case 3:
>> D set = { global IPv4 = 1.2.3.4 }
>> S set = { NAT-ed IPv4 = 10.2.3.4, global IPv4 = 128.66.3.4 }
>> 
>> Under RFC 3484 rules, Source Address Selection would prefer the global IPv4
>> address under Rule 2(Prefer appropriate scope).
>> Under -revise rules, Source Address Selection would instead prefer the NAT'ed
>> IPv4 under Rule 8 (Longest matching prefix).
>> 
>> This is broken.   I don't see a real case the proposed change
>> fixes, I only see real cases it breaks.
>> 
>> -Dave
> 
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