+SimGrid can take network irregularities such as a slow startup or changing behavior depending on the message size into account.
+The values provided by default were computed through data fitting one the timings of either packet-level simulators or direct
+experiments on real platforms. These default values should be OK for most users, but simulation realism is really important to
+you, you probably want to recalibrate the models (i.e., devise sensible values for your specific settings). This section only
+describes how to pass new values to the models while the calibration process involved in the computation of these values is
+described :ref:`in the relevant chapter <models_calibration>`.
+
+We found out that many networking effects can be realistically modeled with three following correction factors. They were shown
+to be enough to capture slow-start effects, the different transmission modes of MPI systems (eager vs. rendez-vous mode), or the
+non linear effects of wifi sharing.
+
+**Option** ``network/latency-factor`` **Default:** 1.0, but overridden by most models
+
+This option specifies a multiplier to apply to the platform latency. The factor can either be a constant to apply to any
+communication, or it can depend on the message size. The ``CM02`` model does not use any correction factor, so the
+latency-factor remains to 1. The ``LV08`` model sets it to 13.01 to model slow-start, while the ``SMPI`` model has several
+values, each for an interval of sizes. The default SMPI setting given below specifies for example that a message smaller than
+257 bytes will get a latency multiplier of 2.01467 while a message which size is in [15424, 65472] will get a latency multiplier
+of 3.48845. The ``wifi`` model goes further and uses a callback in the program to compute the factor that must be non-linear in
+this case.
+
+This multiplier is applied to the latency computed from the platform, that is the sum of all link latencies over the :ref:`network
+path <platform_routing>` used by the considered communication.
+
+Constant factors are easy to express, but the interval-based syntax used in SMPI is somewhat complex. It expects a set of
+factors separated by semicolons, each of the form ``boundary:factor``. For example if your specification is
+``0:1;1000:2;5000:3``, it means that on [0, 1000) the factor is 1. On [1000,5000), the factor is 2 while the factor is 3 for
+5000 and beyond. If your first interval does include size=0, then the default value of 1 is used before. Changing the factor
+callback is not possible from the command line and must be done from your code, as shown in `this example
+<https://framagit.org/simgrid/simgrid/tree/master/examples/cpp/network-factors/s4u-network-factors.cpp>`_. Note that the chosen
+model only provide some default setting, not more. You can pick a ``LV08`` to get some of the settings, and override the latency
+with an interval-based value.
+
+SMPI default value: 65472:11.6436; 15424:3.48845; 9376:2.59299; 5776:2.18796; 3484:1.88101; 1426:1.61075; 732:1.9503;
+257:1.95341;0:2.01467 (interval boundaries are sorted automatically). These values were computed by data fitting on the Stampede
+Supercomputer at TACC, with optimal deployment of processes on nodes. To accurately model your settings, you should redo the
+:ref:`calibration <models_calibration>`.
+
+**Option** ``network/bandwidth-factor`` **Default:** 1.0, but overridden by most models
+
+Setting this option automatically adjusts the bandwidth used by any given communication. As with latency-factor above, the value
+can be a constant (``CM02`` uses 1 -- no correction -- while ``LV08`` uses 0.97 to discount TCP headers while computing the
+payload bandwidth), interval-based (as the default provided by the ``SMPI``), or using in-program callbacks (as with ``wifi``).
+
+SMPI default value: 65472:0.940694;15424:0.697866;9376:0.58729;5776:1.08739;3484:0.77493;1426:0.608902;732:0.341987;257:0.338112;0:0.812084
+This was also computed on the Stampede Supercomputer.
+
+**Option** ``network/weight-S`` **Default:** depends on the model
+
+Value of the bottleneck sharing, that is used to calculate RTT. Described in `Accuracy Study and Improvement of Network
+Simulation in the SimGrid Framework <http://mescal.imag.fr/membres/arnaud.legrand/articles/simutools09.pdf>`_
+
+Default values for ``CM02`` is 0. ``LV08`` sets it to 20537 while both ``SMPI`` and ``IB`` set it to 8775.
+
+.. _cfg=network/loopback:
+
+Configuring loopback link
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Several network model provide an implicit loopback link to account for local
+communication on a host. By default it has a 10GBps bandwidth and a null latency.
+This can be changed with ``network/loopback-lat`` and ``network/loopback-bw``
+items.
