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Network Working Group                                   Alex McKenzieRFC # 613                                               BBN-NETNIC # 21525                                             January 21, 1974Network connectivity:  A response to RFC #603Network topology is a complicated political and economic question withobvious technical overtones.  I shall not attempt, in this note, tocover all the possible arguments which might be made, but merely torespond 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.       [ This RFC was put into machine readable form for entry ]       [ into the online RFC archives by Alex McKenzie with    ]       [ support from GTE, formerly BBN Corp.            10/99 ]McKenzie                                                        [Page 1]