<br/>
.. _howto:
-
+
Modeling Hints
##############
Of course, this is only one possible way to model these things. YMMV ;)
+.. _understanding_lv08
+Understanding the default TCP model
+*****************************
+When simulating a data transfer between two hosts, you may be surprised
+by the obtained simulation time. Lets consider the following platform:
+
+.. code-block:: xml
+
+ <host id="A" speed="1Gf"/>
+ <host id="B" speed="1Gf"/>
+
+ <link id="link1" latency="10ms" bandwidth="1Mbps"/>
+
+ <route src="A" dst="B>
+ <link_ctn id="link1/>
+ </route>
+
+If host `A` sends `100kB` (a hundred kilobytes) to host `B`, one could expect
+that this communication would take `0.81` seconds to complete according to a
+simple latency-plus-size-divided-by-bandwidth model (0.01 + 8e5/1e6 = 0.81).
+However, the default TCP model of SimGrid is a bit more complex than that. It
+accounts for three phenomena that directly impact the simulation time even
+on such a simple example:
+ - The size of a message at the application level (i.e., 100kB in this
+ example) is not the size that will actually be transferred over the
+ network. To mimic the fact that TCP and IP headers are added to each packet of
+ the original payload, the TCP model of SimGrid empirically considers that
+ `only 97% of the nominal bandwidth` are available. In other words, the
+ size of your message is increased by a few percents, whatever this size be.
+
+ - In the real world, the TCP protocol is not able to fully exploit the
+ bandwidth of a link from the emission of the first packet. To reflect this
+ `slow start` phenomenon, the latency declared in the platform file is
+ multiplied by `a factor of 13.01`. Here again, this is an empirically
+ determined value that may not correspond to every TCP implementations on
+ every networks. It can be tuned when more realistic simulated times for
+ short messages are needed though.
+
+ - When data is transferred from A to B, some TCP ACK messages travel in the
+ opposite direction. To reflect the impact of this `cross-traffic`, SimGrid
+ simulates a flow from B to A that represents an additional bandwidth
+ consumption of `0.05`. The route from B to A is implicity declared in the
+ platfrom file and uses the same link `link1` as if the two hosts were
+ connected through a communication bus. The bandwidth share allocated to the
+ flow from A to B is then the available bandwidth of `link1` (i.e., 97% of
+ the nominal bandwidth of 1Mb/s) divided by 1.05 (i.e., the total consumption).
+ This feature, activated by default, can be disabled by adding the
+ `--cfg=network/crosstraffic:0` flag to command line.
+
+As a consequence, the time to transfer 100kB from A to B as simulated by the
+default TCP model of SimGrid is not 0.81 seconds but
+
+.. code-block:: python
+
+ 0.01 * 13.01 + 800000 / ((0.97 * 1e6) / 1.05) = 0.996079 seconds.
