Tim Chown wrote:
Hi Fred,
That's interesting. There has been some talk about multiple ISATAP site
routers, and scalability. Can you comment on how ISATAP performed in this
network of thousands of (mobile) hosts?
I don't have any scaling figures from actual experiments, but scaling is
bounded by the frequency of the polling interval for router advertisements.
The spec currently lists 15min as the recommended minimum, so a density
of 900 nodes per ISATAP router would produce 1 RS/RA exchange per second.
Additional overhead is incurred for nodes that are actiively using the
router to carry traffic vis-a-vis NS/NA exchanges, but that overhead
is the same as for peer-peer communications between nodes.
If a generalized path MTU discovery mechanism were deployed, there would
be still more overhead for the tunnel encapsulator and decapsulator to
police the path MTU of the tunnel. But, I strongly believe that generalized
path MTU discovery is *not a good thing* for wireless networks. Reason
being - large packets on a lossy media (e.g., IEEE 802.11) incur increased
exposure to bit error rates, probability of loss, delay variance, etc. So,
there is no good reason to incur the extra overhead for generalized path
MTU discovery on such media - this also saves state in the routers.
(I assume you meant hundreds or thousands, not hundreds of thousands)
In a single cluster - hundreds or thousands. In a large MANET with
clustering - hundreds OF thousands or more. You also need to get into
analysis of the relative density/sparseness of each cluster. For example,
having hundreds of thousands of nodes show up in the same cluster at the
same point of time might cause trouble! (But, in this case, the wireless
media itself would become congested long before the ISATAP routers.)
Fred Templin
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