Perhaps some of you have been following the thread on the NANOG mailing list
concerning the current growth rate of the core BGP routing table. This
reminded me of some questions that came up after Geoff Huston's presentation
at the last IETF regarding the possible origins of this growth. Geoff's
presentation included a heuristic `piecewise fit' of the historical BGP
routing table size data going back to the pre-CIDR period at the beginning
of 1994 (see http://www.telstra.net/ops/bgptable.html, or Tony Bates' similar
plot at http://www.employees.org/~tbates/cidr.hist.plot.html). Geoff
hypothesized that while the pre-CIDR period showed exponential growth in the
number of prefixes stored by core routers, the period after the introduction
of CIDR shows sub-exponential growth. However, both his and Tony's data (as
well as my own at http://xoanon.colorado.edu/routing/BGP_table_size.html)
clearly show that the current growth rate is once again exponential.
If Geoff's hypothesis about the qualitative difference in the growth rate
during the mid-1994 to 1997 period is correct, then something must have
changed in the way core ISPs are using BGP in recent years. This raises the
following questions:
1. What engineering practices are producing the increased growth
rate? Possibilities here include advertising aggregateable
address space as several pieces to balance traffic across
multiple links, as well as lack of BGP expertise among new
providers' engineers.
2. Can BGP be easily extended to accomodate the technical and
economic requrements that motivate these practices while
reducing their impact on core router resources? Given a
thorough understanding of the problems ISPs are trying to
solve, it seems likely that a better solution can be found
through careful modification of the routing protocol rather
than operational hacks that achieve the desired goal as a
side-effect.
3. If no scheme can reduce the amount of resources required to
accomodate these requirements, can a feedback mechanism
be designed that prevents their frivolous use? This could
be economic or technical in nature, but a simple policy
mechanism that could be enforced unilaterally would best
match the current business model of most ISPs.
-- Sean McCreary mccreary@colorado.edu
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