% % x-kernel v3.3 % % Copyright (c) 1996,1993,1991,1990 Arizona Board of Regents % \subsection {The {\var net} Entry}\label{simNetAppendix} The {\var net} entries in the {\var xsim.data} file specify the kinds of networks to include in the simulated internet, the parameters for these networks (e.g., bandwidth, latency), and the IP addresses for all networks, routers, and hosts involved in the simulation. The connectivity for the entire internetwork is specified by the {\var net} entries alone. In the {\var xsim.data} file, a particular {\var net} entry must come {\it after} the entries of all hosts and routers that it connects. The syntax of a network entry is defined as follows: \begin{verbatim} net ; connections = [instance]
, [instance]
,... ; args = , , ... ; \end{verbatim} The {\var net} line specifies the type of network. Two types of networks are currently defined: Ethernet ({\var ETH}) and point-to-point ({\var PP}). The properties of each type are documented in subsequent sections. The {\var } field specifies the network IP address. The host portion of this address can be 0, although the full address may be specified, as is done in the example given in the previous section. The {\var connections} line lists the IP addresses of the hosts and routers connected to this particular network; only the host part of the IP address should be specified. The optional {\var instance} is used only when specifying a host, and indicates the name of the driver instance that is connected at that IP address (e.g., {\var ethd/1}). There are several things to keep in mind when specifying IP addresses for networks, hosts, and routers. \begin{itemize} \item The simulator only saves the last two bytes of an IP address in a trace entry. This is done to conserve space. Therefore, to get unambiguous trace results, each host and router should be uniquely identified by the last two bytes of its IP address. \item The simulator calculates the full IP address for a host or router by performing a logical OR between the network IP address (specified in the {\var net} line) and the host IP address (from the {\var connections} line). \item For a large internetwork, judicious choices of IP addresses can make the job of specifying routing tables in the {\var router} entry easier. See Section~\ref{router_entry} for information on how to pick IP addresses so that it is easier to use the routing ``shortcuts'' provided by the simulator. \end{itemize} The {\var args} line includes parameters specific to the type of the network being defined, and are used to define aspects of the network such as its bandwidth, the network MTU, and the associated framing overhead. Parameters for Ethernet are listed in Section~\ref{ETH}, paremeters for point-to-point links are given in Section~\ref{PP}. A syntactic point to be aware of is that no spaces should be included anywhere between the argument and its value: {\var rate=10Mb}. \subsubsection{Ethernet ({\var ETH})}\label{ETH} The Ethernet network type simulates a multiple access network with optional collision detection. Note that this is really an abstract network type, capable of simulating networks with a variety of characteristics. If no arguments are specified, the network's default configuration is that of a 10Mbps Ethernet, with the exception that collision detection is disabled. However, the bandwidth, latency, packet overhead, and MTU can be changed to produce networks with behaviors different from that of the standard Ethernet. The following arguments can be specified on the {\var args} line of an Ethernet entry. \begin{verbatim} collisions= on | off \end{verbatim} Enables/disables Ethernet collisions. When this option is off, two or more hosts are allowed to successfully transmit on the Ethernet simultaneously. When on, if two hosts send simultaneously, a collision is detected and the hosts perform the Ethernet exponential backoff algorithm. The default is {\var off}. \begin{verbatim} delay=[unit] \end{verbatim} Sets the latency, that is, the time it takes for a packet to propagate from one end of the network to the other. The optional unit can be {\var us} (microseconds), {\var ms} (milliseconds), or {\var s} (seconds). The default value is 0, and the default unit is microseconds. \begin{verbatim} maxPacket= \end{verbatim} Specifies, in bytes, the maximum packet size for the network. Packets larger than this maximum should be fragmented by a higher-level protocol; if they are not, they will be dropped. The default {\var maxPacket} for {\var ETH} is 1518, comprising 1500 bytes of payload, 14 bytes of Ethernet header, and a 4-byte CRC. \begin{verbatim} minPacket= \end{verbatim} Specifies, in bytes, the minimum packet size for that network. Packets smaller than the minimum will be padded. The default {\var minPacket} for {\var ETH} networks is 64 bytes, 14 of which are for the Ethernet header, and 4 for the CRC. \begin{verbatim} pktHeader= \end{verbatim} For use with {\var minPacket} and {\var maxPacket}. The simulator adds the value of {\var pktHeader} to the length of the message pushed to SIM to determine the packet's length. In {\var ETH}, since the header is attached by the {\var ethd} protocol, the network's {\var pktHeader} defaults to 0. In other words, the size of a {\it packet} is given by the size of the {\it message} pushed to SIM, plus {\var pktHeader}. If {\it packet} $<$ {\var minPacket}, it is padded; if {\it packet} $>$ {\var maxPacket}, it is dropped. When using {\var ETH} networks to model other types of networks, one may want to use negative values for {\var pktHeader} when the modeled network's header is less than 14 bytes. \begin{verbatim} pktOverhead= \end{verbatim} Conceptually, this option adds the specified number of bytes to each packet as it is transmitted on the network. The intended use is to model the overhead consumed by framing information. The added {\var pktOverhead} does not count as part of the packet length from the standpoint of {\var minPacket} or {\var maxPacket}. The default is 8, as Intel's Lance uses 8 bytes for framing. \begin{verbatim} rate=[unit] | T1 | T3 \end{verbatim} Sets the bandwidth, in units per second, for the network. The rate can be a positive real number followed immediately by an optional unit. The unit is one of: {\var MB}, {\var Mb}, {\var KB}, {\var Kb}, {\var B}, or {\var b}. A {\var B} in all units represents bytes and {\var b} denotes bits. For instance, {\var 4.5Mb} specifies a bandwidth of 4.5 mega{\it bits} per second. If no unit is specified, the default is bytes. {\var T1} and {\var T3} can also be specified as rates. {\var T1} corresponds to approximately 1.5Mbps, and {\var T3} is about 45Mbps. For Ethernet, the default bandwidth is 10Mbps. \subsubsection {Point-to-point Links (PP)}\label{PP} The point-to-point network type can be used to connect two routers or hosts. It differs from the Ethernet in that it should be thought of as two links, one in each direction. Packets flowing in one direction cannot affect flows in the other direction. The following arguments can be used to modify the behavior of a simulated point-to-point link. They have the same meanings as those specified above for the Ethernet. \begin{verbatim} delay=[unit] # default is 0 minPacket= # default is 0 maxPacket= # default is 1500 pktHeader= # default is 0 pktOverhead= # default is 0 rate=[unit] | T1 | T3 # MUST be specified \end{verbatim}