Changing the CIDR Assignment Strategy

[Daniel Karrenberg <]

We will circulate a revision of ripe-72 shortly. We suggest to change
the recommendation for address space reservations. Below we give the
reasoning behind this. Discussion Welocme

For the NCC team

Daniel

Previously the RIPE NCC has recommended IRs to reserve some address
space contiguous to assigned address space for future expansion.  The
reasoning was that this would further aggregation and keep the routing
table sizes down in the long term, while being slightly inefficient on
address space usage in the short term. 

However recently we had to start delegating blocks from 194.x.y, when
193.x.y had slightly more than 25% of address space assigned.  This
caused us to review assignements and we have found a lot of reserved
address space causing fragmentation.  This experience suggests that the
address space usage problems created by those reservations outweigh the
possible aggregation benefits. 

If a block of equal size to the one actually assigned is reserved,
address space utilisation is halved in return for a possible 50%
reduction in routing table size. 

Relatively speaking this looks like a good tradeoff.  In absolute terms
however substantial amounts of address space are traded in to save 1 (in
words: one) additional routing table entry. 

If some address space is reserved and exactly that amount is needed
later the reserved space can be assigned and aggregated into the same
CIDR route, not expanding the routing tables at all, saving one
additional routing table entry.  Note that this small absolute gain can
only be realised if the reserved space indeed fits the need exactly! 

If no reservation is made at all and more address space is needed at a
later stage, another block of appropriate size can always be assigned. 
Since there are now two blocks these can be aggregated into two routes
instead of the one which would have resulted if a suitable reservation
had been made. 

The effect of the reserved address space is not only decreased
utilisation but also creates fragmentation of the address space which
makes it difficult to find block of appropriate size for new requests. 

Considering this it does not make sense to reserve anything but very
small amounts of address space or unused parts of CIDR blocks.  Thus the
current recommendation is to reserve only address space that is needed
to "round" the requested space to the next suitable block boundary.