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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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