o Reflect changed default such that keepalives are always enabled by   default now.  Discuss why that's good.  Note that there are still   some situations where turning it off may be advantageous, including   situations where there are network outages and it's desirable to   have TCP sessions last beyond the outage.
o Reflect changed default such that keepalives are always enabled by
  default now.  Discuss why that's good.  Note that there are still
  some situations where turning it off may be advantageous, including
  situations where there are network outages and it's desirable to
  have TCP sessions last beyond the outage.

Reviewed by:	fenner
Suggested by:	silby

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