<link_ctn id="link2"/>
</route>
-.. _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 implicitly declared in the
- platform 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.
