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words about Attribute tweaking in CSMA dox

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This commit is contained in:
Craig Dowell
2010-04-16 16:06:14 -07:00
parent c51501aef8
commit 4567f96113
1 changed files with 50 additions and 4 deletions
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54
src/devices/csma/csma.h
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@@ -133,13 +133,18 @@
*
* The CsmaChannel provides following Attributes:
*
* - DataRate: The bitrate for packet transmission on connected devices;
* - Delay: The speed of light transmission delay for the channel.
* - DataRate: The bitrate for packet transmission on connected devices;
* - Delay: The speed of light transmission delay for the channel.
*
* \section CsmaNetDeviceModel CSMA Net Device Model
*
* The CSMA network device appears somewhat like an Ethernet device. The
* CsmaNetDevice provides following Attributes:
* The CSMA network device appears somewhat like an Ethernet device,, but it
* is important to understand that it does not model an Ethernet device. As
* mentioned above, it does not model collisions. The CSMA model also provides
* users with the ability to configure the device into other states that are
* not compatible with strictly Ethernet or IEEE 802.3 devices.
*
* The CsmaNetDevice provides following Attributes:
*
* - Address: The Mac48Address of the device;
* - SendEnable: Enable packet transmission if true;
@@ -151,6 +156,47 @@
* - Rx: A trace source for received packets;
* - Drop: A trace source for dropped packets.
*
* We recommend thinking carefully about changing these Attributes, since
* it can result in behavior that surprises users. We allow this because
* we believe flexibility is important. As an example of a possibly
* surprising effect of changing Attributes, consider the following:
*
* The Mtu Attribute indicates the Maximum Transmission Unit to the device.
* This is the size of the largest Protocol Data Unit (PDU) that the
* device can send. This Attribute defaults to 1500 bytes and corresponds
* to a number found in RFC 894, "A Standard for the Transmission of IP
* Datagrams over Ethernet Networks." The number is actually derived
* from the maximum packet size for 10Base5 (full-spec Ethernet) networks --
* 1518 bytes. If you subtract DIX encapsulation overhead for Ethernet
* packets (18 bytes) you will end up with a maximum possible data size (MTU)
* of 1500 bytes. One can also find that the MTU for IEEE 802.3 networks
* is 1492 bytes. This is because LLC/SNAP encapsulation adds an extra eight
* bytes of overhead to the packet. In both cases, the underlying network
* hardware is limited to 1518 bytes, but the MTU is different because the
* encapsulation is different.
*
* If one leaves the Mtu Attribute at 1500 bytes and changes the encapsulation
* mode Attribute to Llc, the result will be a network that encapsulates 1500
* byte PDUs with LLC/SNAP framing resulting in packets of 1526 bytes. This
* would be illegal in many networks, but we allow you do do this. This
* results in a simulation that quite subtly does not reflect what you might
* be expecting since a real device would balk at sending a 1526 byte packet.
*
* There also exist jumbo frames (1500 < MTU <= 9000 bytes) and super-jumbo
* (MTU > 9000 bytes) frames that are not officially sanctioned by IEEE but
* are available in some high-speed (Gigabit) networks and NICs. In the
* CSMA model, one could leave the encapsulation mode set to Dix, and set the
* Mtu to 64000 bytes -- even though an associated CsmaChannel DataRate was
* left at 10 megabits per second (certainly not Gigabit Ethernet). This
* would essentially model an Ethernet switch made out of vampire-tapped
* 1980s-style 10Base5 networks that support super-jumbo datagrams, which is
* certainly not something that was ever made, nor is likely to ever be made;
* however it is quite easy for you to configure.
*
* Be careful about assumptions regarding what CSMA is actually modelling and
* how configuration (Attributes) may allow you to swerve considerably away
* from reality.
*
* The CsmaNetDevice supports the assignment of a "receive error model."
* This is an ErrorModel object that is used to simulate data corruption
* on the link.