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Network Working Group Alex McKenzie
RFC # 613 BBN-NET
NIC # 21525 January 21, 1974
Network connectivity: A response to RFC #603
Network topology is a complicated political and economic question with
obvious technical overtones. I shall not attempt, in this note, to
cover all the possible arguments which might be made, but merely to
respond directly to the points raised in RFC #603.
1. The important consideration in deciding whether it is good or
bad to have a node (AMES) be four connected is not how many circuits
are affected by a node failure; rather one should consider how well
the network is still connected after a node failure. For example,
if ALL nodes in the network were four-connected I doubt that anyone
would argue that this was bad for reliability. The weaknesses are
not the three-connected and four-connected nodes but rather the
ONE-connected (Hawaii, London) and two-connected nodes. I must
agree with Burchfiel's implied argument that it is better to have
two adjacent three-connected nodes than to have a four-connected
node adjacent to a two-connected node; unfortunately the realities
of installing interfaces and common carrier services cause the
Network to expand in sub-optimal ways.
2. "Loops" are not good per se, they appear good because the act of
making loops increases the connectivity and thereby reduces the
effect of multiple failures. Adding more circuits costs ARPA money,
both capital cost for IMP interfaces and recurring cost for the
circuits. The network group at BBN has suggested to ARPA several
times that "connectivity should be increased" but it was only late
in December 1973 that we made specific suggestions for the locations
of additional circuits. These recommendations were not based on
building loops (although they may have that effect) but were based
on breaking the long chains of IMPs which have occurred as the
Network has grown. ARPA and NAC are now presumably in the process
of evaluating our suggestions, and perhaps formulating other
possibilities.
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McKenzie [Page 1]