Re: [homenet] on prefix comparison

[Alexandru Petrescu <[e-mail]>]

Pierre, just a small doubt, but I agree with you in general.

Le 08/10/2014 13:58, Alexandru Petrescu a écrit :
[...]
>> Equality is never considered alone. Actually, most of the time, you
>> will find considerations such as: The prefix is not included or does
>> not include any other Assigned Prefix with a higher precedence.

It is hard to say whether a prefix is included into another or not.  We 
do not have a published algorithm to say what it means for a prefix to 
include another.

In general, we have a common understanding (and not published algorithm) 
about what it means 'longest prefix match'.  But that compares an 
address to a prefix, not a prefix to a prefix.

Sure, one could assume that an address is just a /128 prefix and execute 
longest prefix match with it as if it were an address.

But then again which prefix has the role of the address and which is the 
role of the prefix?  In other words, when hearing two prefixes on a link 
and want to compare them, which of them should be compared against the 
other by using the longest-prefix match?  There are two possibilities 
and two different outputs for a particular tuple of prefixes, depending 
on the order of this longest prefix match.

Of course, I do not mention the easy case which compares two prefixes of 
precisely same length.

Just because the length is different may make think that the prefixes 
are different.  Or otherwise one could be aggregated into another.  But 
there are several types of aggregation: matching up to the shortest 
length, matching up to middle, up to longest length, beyond the longest 
length.

These cases are not documented and people may implement them in many 
different ways with different outputs when trying to tell whether this 
or that prefix are equal or included into one another.

Alex