TRDP Message Data

The TRDP protocol defines the exchange of MD (Message Data) between a Caller and one or more Repliers on a confirmed TRDP service. Since it is only used for real-time message data, the length of the message is limited to 64 kb.

Roles

In the process of Message Data interaction, there are two roles:

• Caller: The source device of MD messages in the Push communication mode; the destination device of MD messages in the Pull communication mode.

• Replier: The destination device of MD messages in the Push communication mode; the source device of MD messages in the Pull communication mode.

TRDP provides three MD transmission methods:

• The request message has no response ('notification')

• The request message has a response but no confirmation ('request without confirmation')

• The request message has a response and confirmation ('request with confirmation')

TRDP Message Data

Push communication mode

It supports various modes such as point-to-point, point-to-multipoint, the source device knows the destination device, and the source device doesn't know the destination device.

Pull communication mode

It supports various modes such as point-to-point, point-to-multipoint, the destination device knows the source device, and the destination device doesn't know the source device.

MD message format

The IEC standard defines the MD message format as follows:

TRDP Message Data

Field	Length	Value	Description
SequenceCounter	4 bytes	Counting starts from 0
ProtocolVersion	2 bytes	Fixed value	High byte：MainVersion Low byte：SubVersion For example: '0102' = Version 1.2
MsgType	2 bytes	'4D6E'H ('Mn') '4D72'H ('Mr') '4D70'H ('Mp') '4D71'H ('Mq') '4D63'H ('Mc') '4D65'H ('Me')	'Mn' = Notification (Request without reply) 'Mr' = MD Request with reply 'Mp' = MD Reply without confirmation 'Mq' = MD Reply with confirmation 'Mc' = MD Confirm 'Me' = MD error
comId	4 bytes	Set by the user	Identifier of user data, supporting Unspecified message (ComId = 0)
etbTopoCnt	4 bytes	0..2³²-1	The ETB topography counter: • Used as the train's address in IEC 61375-2-5 (parameter 'etbTopoCnt') • Set by the user • Must be set for all ETB-based communications • If a valid opTrnTopoCnt is set, this field must also be set • Set to 0 in other cases
opTrnTopoCnt	4 bytes		The operational train topography counter: • Refer to the description in Section 5.3.3 for usage • Set by the user • Set this field when information in the operational train directory is required (e.g., routing operations) • Set this field when the source device uses the operational train dictionary to resolve the destination IP (e.g., resolving the URI 'vcu.leadVeh.anyCst.anyClTrn.lTrn') • Set to 0 in other cases
DatasetLength	4 bytes	0 ..65388	Length of user data, in bytes, excluding padding bytes
ReplyStatus	4 bytes	< 0: NOK 0: OK > 0: user reply status	Set when the Replier reports the execution result of the request message, or when the caller sends a confirmation message. The Replier carries this field in the reply message. If it is an MD error message, the values may be as follows: • –1 – reserved • –2 – session abort • –3 – no replier instance (at replier side) • –4 – no memory (at replier side) • –5 – no memory (local) • –6 – no reply • –7 – not all replies • –8 – no confirm • –9 – reserved • –10 – sending failed
SessionId	16 bytes	Calculated	• Can identify a set of "request-reply" session or "request-reply-confirm" session • Can identify a set of "request-error" session
ReplyTimeOut	4 bytes	1..2³²-1 0==Infinite time	Used in request/reply Set to 0 in 'Mn', 'Mp', 'Mc' and 'Me'
SourceURI	32 bytes		• For addressing function • It is a null-terminated string • Padded with 0 at the end • The URI only contains the user part, excluding the host part and @ • It may be an all-0 empty string
DestinationURI	32 bytes		• For addressing function • It should be a null-terminated string • Padded with 0 at the end • The URI only contains the user part, excluding the host part and @ • It may be an all-0 empty string
HeaderFCS	4 bytes		• Calculated based on the PD-PDU header • Exclude the headerFcs itself during calculation
Dataset	0..65388		Must be an integer multiple of 4 bytes. Pad with zeros if insufficient.