TCP Vegas implementation in |ns3|

TCP Vegas implementation in |ns3|
------------------------------------------------

This chapter describes TCP Vegas implementation in |ns3|. This implementation is contained in the following files:

.. sourcecode:: text

    src/internet/model/tcp-vegas.{cc,h}

Model Description
*****************

TCP Vegas is a pure delay-based congestion control algorithm implementing a proactive
scheme that tries to prevent packet drops by maintaining a small backlog at the bottleneck queue.

Vegas continuously samples the RTT and computes the actual throughput a connection achieves using
Equation (1) and compares it with the expected throughput calculated in Equation (2).
The difference between these 2 sending rates in Equation (3) reflects the amount of
extra packets being queued at the bottleneck.
 
               actual = cwnd / RTT        (1)
               expected = cwnd / BaseRTT  (2)
               diff = expected - actual   (3)
 
To avoid congestion, Vegas linearly increases/decreases its congestion window to ensure
the diff value fall between the 2 predefined thresholds, alpha and beta.
diff and another threshold, gamma, are used to determine when Vegas should change from
its slow-start mode to linear increase/decrease mode.

Following the implementation of Vegas in Linux, we use 2, 4, and 1 as the default values
of alpha, beta, and gamma, respectively.

References
==========

.. [Brakmo1995] L. S. Brakmo and L. L. Peterson, "TCP Vegas: end to end congestion avoidance on a global Internet," 
in IEEE Journal on Selected Areas in Communications, vol. 13, no. 8, pp. 1465-1480, Oct 1995. 
DOI=http://dx.doi.org/10.1109/49.464716 

Validation
==========
The TCP Vegas model is tested using :cpp:class:`TcpVegasTestSuite` class defined in `src/internet/test/tcp-vegas-test.{cc,h}`.  
This test suit can be run using the following commands:

::

  $ ./waf configure --enable-examples --enable-tests
  $ ./waf build
  $ ./test.py -s tcp-vegas-test
  
Example
=======
TCP Vegas can be simulated using the example `tcp-variants-comparison.cc` located in ``examples/tcp``.

[n.p, 2016-04-06 12:56:50]

Looks ok, some minor comments (see also previous review)

https://codereview.appspot.com/290510043/diff/1/src/internet/model/tcp-conges...
File src/internet/model/tcp-congestion-ops.h (right):

https://codereview.appspot.com/290510043/diff/1/src/internet/model/tcp-conges...
src/internet/model/tcp-congestion-ops.h:122: }
This is duplicated from Veno (we need to remember it)

https://codereview.appspot.com/290510043/diff/1/src/internet/model/tcp-socket...
File src/internet/model/tcp-socket-base.cc (right):

https://codereview.appspot.com/290510043/diff/1/src/internet/model/tcp-socket...
src/internet/model/tcp-socket-base.cc:1456: 
This is already in the ns-3-dev repo

https://codereview.appspot.com/290510043/diff/1/src/internet/model/tcp-socket...
src/internet/model/tcp-socket-base.cc:1470: m_congestionControl->CongestionState
(m_tcb, TcpSocketState::CA_DISORDER);
Duplicated from Veno

https://codereview.appspot.com/290510043/diff/1/src/internet/test/tcp-vegas-t...
File src/internet/test/tcp-vegas-test.h (right):

https://codereview.appspot.com/290510043/diff/1/src/internet/test/tcp-vegas-t...
src/internet/test/tcp-vegas-test.h:5: class TcpVegasTest
To remove, useless