User Guide > IPDU

IPDU

Starting with version 2.0 of TCN TimeAnalysis, the concept of an Interaction layer Protocol Data Unit (IPDU) is introduced. An IPDU is basically a data message that originates in a specific source Host and can be simultaneously multicasted via several different Ports of the source Host. Furthermore, when received by an intermediate Host (a gateway), the same IPDU can be immediately transmitted onto new networks and busses via ports belonging to the gateway Host. This is illustrated in the figure below in which blue lines indicate the fan-out of a specific IPDU; starting in Host1 one and the same IPDU is first multicasted through one LIN-port and two CAN-ports. Arriving in the Host named Gateway2, the IPDU is immediately routed to its two Ethernet Ports and transmitted again. The entity responsible for performing the routing is the PDU Router, which is available in all Hosts in an architecture.

In practice, for every Ethernet network and bus involved in forwarding an IPDU, the user has to define the different frames that carry the IPDU in their payload fields. This is no difference from earlier versions of TCN TimeAnalysis; the new thing is that, for example, a CAN frame has to explicitly indicate the IPDU that it is to carry across a specific CAN bus. Consequently, certain attributes earlier associated with the CAN frame, like bit size and period, have been transferred to the new IPDU object in version 2.0. These properties are described below.

New in version 2.0 is also that the user can define explicit Ethernet frames to carry an IPDU across a switched Ethernet network. Like before, it is also possible to define UDP datagrams to carry an IPDU. The primary difference is that only UDP datagrams can carry large IDPUs that require fragmentation. That is, if the resulting datagram does not fit into a single Ethernet frame, as governed by the Link’s MTU (Maximum Transmission Unit), it will be split up and carried in a string of two or more Ethernet frames. In the receiving link partner Host, the original UDP datagram will be reassembled again and the contained IPDU can then be extracted.

Note, however, that the PDU Router does not allow an IPDU that arrives to a Host in a CAN, LIN or Ethernet frame to be gatewayed to an outbound UDP datagram or vice versa.

Furthermore, it is currently not allowed to route an IPDU so that it is received by any Host via more than one ingress port of that Host, as this could cause a cycle to occur. Thus the fan-out of an IPDU through the different Hosts in a system model must be in the form of a tree. To make it easier for the user, the GUI will actively make sure that the user does not make routing decisions that cause cycles to occur, for example, by filtering away forbidden target Hosts during the construction of a new frame or datagram.

Finally, one or more Timers can be defined to measure the latency required to transmit a specific IPDU from its source Host to one or several of the target end Hosts of the IPDU.

Properties

Offset

This parameter specifies the point in time when the task that generates an IPDU at the end of its execution is released for the very first time.

Random Offset

If this box is checked, a random offset will be selected automatically for the IPDU task when a simulation starts. The offset will be chosen from the interval [0, P[, P denoting the period of the IPDU task.

Period

The task that produces an IPDU at the end of its execution will be released periodically as specified by this parameter.

Bit size

This parameter specifies the bit size of an IPDU.

Note that if an IPDU is gatewayed and, for example, first transmitted in a CAN frame and then in an Ethernet frame, the maximum size of the IPDU is determined by the CAN frame. Thus, this puts an upper limit of 64 bits or 8 bytes on the IPDU’s size.

The same is true when an IPDU is carried in a LIN frame.

The maximum length of the payload of a FlexRay frame is up to 254 bytes or 2,032 bits.

In the case of an Ethernet frame, the size is typically 1,500 bytes or 12,000 bits, but this is ultimately determined by the current value of the Maximum Transmission Unit (MTU) of the egress Ethernet port.

Finally, if an IPDU is placed in a UDP datagram, the upper limit is 524,056 bits or 65,507 bytes, which is determined by the Length field of the UDP datagram header. In this case, if the final packet size is larger than the MTU of the egress Ethernet port, the UDP datagram will be fragmented and sent as a string of two or more Ethernet frames.

The lower limit of the size of an IPDU is always 0 bits.

If the Random size box is checked, the data size of each new IPDU generated will be selected with equal probability in the specified interval.