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

[Tim Chown <[e-mail]>]

On 13 Feb 2012, at 22:01, Dave Thaler wrote:

> 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.
> [...]

I certainly agree that that wording can be improved.

Tim

> 
>>  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.
> 
> 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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