A prefix can only contain another prefix if its prefix length is smaller.
Here is some C-code that provides what you are looking for.
Cheers,
- Pierre
/* Tell whether a prefix contains an address */
uint8_t prefix_contains(const struct in6_addr *p, uint8_t plen, const struct
in6_addr *addr)
{
int blen = plen >> 3;
if(blen && memcmp(p, addr, blen))
return 0;
int rem = plen & 0x07;
if(rem && ((p->s6_addr[blen] ^ addr->s6_addr[blen]) >> (8 - rem)))
return 0;
return 1;
}
/* Tell whether a prefix contains another prefix */
uint8_t prefix_include(const struct in6_addr *p1, uint8_t plen1, const struct
in6_addr *p2, uint8_t plen2)
{
if(plen1 > plen2)
return 0;
return prefix_contains(p1, plen1, p2);
}
/* Tell whether two prefixes are colliding */
uint8_t prefix_collision(const struct in6_addr *p1, uint8_t plen1, const struct
in6_addr *p2, uint8_t plen2)
{
return prefix_include(p1, plen1, p2, plen2) || prefix_include(p2,
plen2, p1, plen1);
}
Le 8 oct. 2014 à 14:13, Alexandru Petrescu <[email protected]> a
écrit :
> 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
>
>
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