[ns] Adding a minimum RTO to TCP

[Sally Floyd <floyd@aciri.org>]

NS has long had a maxrto_ variable to set a maximum value for the
TCP RTO (retransmit timeout), but it has not had a minrto_ variable.
So I have just added minrto_ to TCP, and for the moment I have set
the default to 0.

In conformance with RFCC 2988 on "Computing TCP's Retransmission
Timer", I would like to set the default value of minrto_ in NS to
one second.  If anyone has strong objects to this, please let me
know.

The validation tests are as follows:
% ns test-suite-tcp-init-win.tcl slowLink
% ns test-suite-tcp-init-win.tcl slowLink1
% ns test-suite-tcp-init-win.tcl slowLink2

All of these tests use a minrto_ of 1 second, and use a scenario
with a 9.6Kbps link.

- Sally
http://www.aciri.org/floyd/