Re: Re: URI comparison rules - IPv6 addresses

Dale.Worleycomcast.net wrote:
>    From: "Vijay K. Gurbani" <vkgalcatel-lucent.com>
> 
>    Jeroen van Bemmel wrote:
>    > Vijay,
>    > It's not only IPv6: what about 127.0.0.1 versus
127.000.000.1?
> 
>    Jeroen: Pedantically speaking, you are probably
right.  But
>    in practice we do not generally see leading zeros in
an IPv4
>    octet.
> 
> Even worse, in some places, including some early RFCs,
the leading
> zero is used to indicate that the octet is represented
in octal!
> 
> But I think Jeroen's point is actually well-taken, when
comparing
> representations of IP addresses (not DNS names), the
comparison is
> implicitly of the address represented, not the textual
> representation.  And this applies in IPv4 as well as
IPv6.
> 
> In regard to loop detection, there are two approaches: 
(1) Whatever
> attempts to detect loops can canonicalize the addresses
before
> comparing them or whatever. 

I agree with all the above.

> (2) Since there are a limited number of
> likely representations of any address, having different
entities use
> different representations will only delay loop
detection, not prevent
> it.  And loops will be detected even if address
comparisons have
> occasional false negatives.

While the number is *limited*, the limit is pretty large.

Consider an IPv4 address where each of the components
requires only two 
digits. Then each can be represented two ways - with two
digits or 
three. That means there are 16 different valid
representations of the 
address. Of course it would be much worse for Ipv6
addresses.

Port numbers are worse. The syntax for the port number is
1*DIGIT. So a 
port number can be represented with *any* number of leading
zeros.

Bottom line - I think you can't rely on exhausting all the
permutations 
of hostport before detecting a loop.

	Thanks,
	Paul


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   From: Paul Kyzivat <pkyzivatcisco.com>

   > (2) Since there are a limited number of
   > likely representations of any address, having
different entities use
   > different representations will only delay loop
detection, not prevent
   > it.  And loops will be detected even if address
comparisons have
   > occasional false negatives.

   While the number is *limited*, the limit is pretty
large.

True, but I expect in practice there will be no cases with
more than 3
representations of the same address used by any one SIP
agent unless
it's deliberately trying to defeat loop detection.  That
case is
ameliorated by (1) a malicious agent would have to handle
the message
"every time around the loop", which would place a
high burden on it,
and (2) loops will eventually be detected by Max-Forwards in
any case.

Dale


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