TRDP is used to exchange PD (process data) and MD (message data) on the ETB/ECN. TRDP is IP-based and operates above the TCP/UDP transport layer. The protocol stack is as follows:

TRDP uses UDP or TCP to transmit data:

  • Process Data uses UDP

  • Message Data can use either UDP or TCP, which is selected by the user. When the length of Message Data (Max: 64K bytes) exceeds the maximum MTU (approximately 1500 bytes), 

    the message needs to be transmitted in segments, and TCP can be used

The port numbers used by TRDP

ComId (Communication Identifier)

The ComId is carried in the PDU header, and the ComId is the unique identifier of the user data in the TRDP PDU.

The ComId + the source IP is the unique identifier of the TRDP message in the train.

The ComId is set to 0 in the following scenarios:

  • PDUs with unspecified user data

  • Acknowledgment messages for Message Data

  • Error messages for Message Data

The ComId is of the UINT32 type, among which 1-999 are reserved for special purposes. When users define their own ComId, they should not use the reserved ComId.