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Do we have a guaranteed delivery of the message sent via Implicit messaging in Ethernet/IP?

Implicit messaging uses UDP, so my guess is no, but this seems strange given that Implicit messaging is used for real-time control, so I wonder maybe Ethernet/IP provides guaranteed delivery on top of UDP?

If no, should I implement something on top of it? We do not care much about real-time thing, but we need a guaranteed delivery and we are forced to use implicit messaging

Archeg
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    How is the question too broad? Expected answers: `yes, it guarantees `, `no, but you should not care about that `, `no, and you should implement it yourself` – Archeg Aug 16 '16 at 13:29
  • Implicit messaging is about reporting realtime data with minimal latency. Hence, a guaranteed packet of data that took 100ms too long is worthless (it might have well have been "lost"). Explicit messaging is the opposite. It does have/support delivery acknowledgement, but at the cost of latency. – franji1 Aug 18 '16 at 12:46

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This article gives a good overview of it. UDP is faster than using a connection oriented transport and can be multicast which makes it a pretty good fit. Reliablility however is it's drawback.

If data is updating frequently this isn't such a problem. In other applications I've used PGM on top of this for the reliability. But this was for PC to PC comms.

I'm assuming that CIP Safety must use something similar.

FloatingKiwi
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No, there is no guarenteed delivery. If a device along the route decides to drop all the UDP packets, they will not be delivered. The connection will then close due to a receive timeout. This timeout is specified by means of a CIP timeout multiplier which defines the maximum allowed number of frame losses in a sequence. The minimum ist four.

Regarding the term "Real Time", you have to distinguish between hard realtime systems and soft realtime systems. EtherNet/IP is definetly soft-realtime, meaning that those systems are typically correct, i.e. delivering in due time. There is no guarantee that the data will be available within a certain time at the peer as in a hard-realtime network.

Safety requirements in this domain are often covered by redundancy, plausibility checks and fail safe states.

Marc Bommert
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