[ns] Adding RIO code to NS

[Sally Floyd <floyd@aciri.org>]

I have added a slightly-modified version of the RIO code from the
contributed code web page to the NS distribution.  The validation
test is "./test-all-rio" in tcl/test.  The first few tests in the
RIO validation test suite use the flow ID to identify some flows
as IN and other flows as OUT, with OUT packets four times more
likely to be dropped than IN packets.  The last test in the RIO
validation test suite uses token bucket policing and tagging to
mark IN and OUT packets from each flow, with OUT packets dropped
before IN packets.

The RIO contributed code is from Yun Wang, based on the code used by
Wenjia Fang, and was ported to ns-2.1b6 by J. Scott.  The incorporation
into the NS distribution was done with debugging from Ratul Mahajan.

I restructured the RIO code somewhat to use code from red.cc, and
added some subroutines to red.cc in the process, but if there was
time, a lot more could still be done...

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