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gaty
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Joined: 01 Mar 2017
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 Posted: 05/09/2018 15:37 Post subject: How to configure IP multicasts for UDP Ethernet CPs / CMs |
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How do you configure and use IP multicasts for UDP connections with Industrial Ethernet CPs / CMs?
https://support.industry.siemens.com/cs/document/9822272/
IP multicast is a special connection option that with Industrial Ethernet CPs / CMs can only be supported or configured for UDP connections (UDP - User Datagram Protocol). IP multicast was introduced for sending messages from one station to numeroase partner stations.
The messages are sent with a special multicast address. The IP address range of 224.0.1.0 to 239.255.255.255 is reserved for the multicast addresses.
A broadcast message is sent to all the stations in the network. Broadcast messages are needed, for example, when searching for the MAC address of an IP address (ARP request). This is why a communication group has to generally be able to process and evaluate broadcast messages.
If there are too many broadcast messages on the network, the network performance drops. This is because the single modules have to process all broadcast messages to determine which ones are meant for them.
In the case of S7-300 / S7-400 Industrial Ethernet CPs you should pay attention to the following points with regard to broadcast messages:
| Code: | After receipt, the broadcast messages are filtered out as high priority in all S7-300 / S7-400 Industrial Ethernet CPs. All messages which are not usable (ARP request, for example), are discarded immediately. This prevents the negative influence of broadcast messages on the other connections.
The S7-300/400 Industrial Ethernet CPs can send but not receive data via configured broadcast connections.
With STEP 7 / NCM V5.1 + SP2 and higher it is possible to configure IP multicast for UDP connections via S7-300 / S7-400 Industrial Ethernet CPs in order to send a message to a specific group of communication partners.
Behavior of the Industrial CPs / CMs
Most of the protocols that work on OSI Layer 2 have distinct MAC addresses, so-called MAC multicast addresses. The communication processor is blocked for protocols with MAC multicast addresses.
If IP multicast is configured, then only the configured multicast address is enabled in the controller. The communication processor continues to be immune to broadcast messages from the network. Each configured multicast group is registered in the controller. This is why IP multicast is a better solution than broadcast when messages are to be sent to groups of partner stations. It is not the user who registers the multicast groups in the controller.
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Multicast groups are supported by S7-300 Industrial Ethernet CPs.
https://support.industry.siemens.com/cs/document/16767769/what-differences-are-there-between-the-industrial-ethernet-cps-of-simatic-s7-300
https://support.industry.siemens.com/cs/attachments/16767769/IE_CP_S7-300_e.pdf
Multicast groups are supported by S7-400 Industrial Ethernet CPs.
https://support.industry.siemens.com/cs/document/15368142/which-communication-services-are-supported-by-the-industrial-ethernet-cps-of-simatic-s7-400
https://support.industry.siemens.com/cs/attachments/15368142/15368142_IE_CP_S7400_e.pdf
The data length is limited to 2048 bytes just as with UDP.
The communication processor continues to be immune to broadcast loads.
The requirement is of course that all the partner stations also support IP multicast.
The messages are sent without any security mechanisms (acknowledgment).
IP multicast messages can be sent via routers to third-party IP subnetworks.
The messages are sent without an acknowledgment request because the UDP protocol does not provide for acknowledgments. If, for example, a message is sent to 100 partners, then 100 acknowledgments would arrive simultaneously (one acknowledgment per partner). Such floods of data could not be evaluated by the sender module.
Configuration of multicast connections in STEP 7 V5
Insert a new connection of the "UDP connection" type in NetPro. As connection partner you select "All multicast stations".
Fig. 1
In the Properties dialog of the UDP connection you open the "Addresses" tab. The multicast group is defined here. The IP addresses 224.0.1.0 to 239.255.255.255 are available specially for IP multicast. Messages from this IP address range are recognized as multicast messages by each module. The IP address range is used exclusively for IP multicast via UDP connections.
The local port and the remote port can be released between 1 and 65535. The first multicast group is created with the IP address 224.0.1.0. The user can change the IP address of the multicast group. The IP-address range of 224.0.1.0 to 239.255.255.255 is possible.
Fig. 2
In NetPro, the UDP connection is displayed in the list of configured connections. In the case of a multicast connection the entry "All multicast nodes" is displayed in the "Partner" column.
It is recommended to comply with the following rules when configuring multicast connections:
Select the same port numbers for the local port and the remote port of a multicast connection
Pay attention to the selection of multicast addresses
Configuration recommendations
It is recommended to comply with the following rules when configuring multicast connections:
Select the same port numbers for the local port and the remote port of a multicast connection
Pay attention to the selection of multicast addresses
Select the same port numbers for the local port and the remote port of a multicast connection
If the messages have passed the controller, only the port number is relevant.
Exactly one multicast connection is configured between two stations. In Station 1 and Station 2 the same port numbers have to be selected for the local port and the remote port of the multicast connection.
In this way Station 1 can receive the data sent by Station 2 to the multicast group, and Station 2 can receive the data sent by Station 1 to the multicast group.
Table 1
Configuration of multicast connections in STEP 7 V14 SP1 or higher (TIA Portal)
The following document includes instructions for configuring multicast and broadcast connections in STEP 7 V14 SP1 or higher (TIA Portal).
https://support.industry.siemens.com/cs/attachments/9822272/9822272_Multicast_Broadcast_Configuartion_en.pdf
Special points when selecting the multicast addresses
With IP multicast, the 3 lowest value bytes of the IP address are copied into the last 3 bytes of the MAC address 01:00:5E:00:00:00. This is then the MAC address that is entered in the controllers for the separate multicast groups. This ensures that the messages can pass the respective controllers. Furthermore, the top bit of the first copied byte of the address is ignored and is always 0.
The created MAC address is also visible as target MAC address in the messages on the LAN.
A multicast message is sent from Station 1 to Station 2.
Since Station 1 and Station 2 are in the same multicast group and all the selected port numbers are identical, unrestricted bidirectional data exchange is possible between Station 1 and Station 2.
However, the multicast messages sent by Station 1 and Station 2 are also received by Station 3.
This is because the multicast groups 224.0.1.0 and 225.0.1.0 have the same MAC address: 01:00:5E:00:01:00.
Note
Different multicast IP addresses in fact represent the same multicast group. Therefore, you should pay attention when assigning the multicast group in order to avoid unwanted messages on non-addressed stations. This is due to a regulation based on the valid RFC 1112 (internet standard).
The Industrial Ethernet CPs / CMs support the IGMP protocol (Internet Group Management Protocol). Since the IGMP protocol makes IP multicast possible in the internet, stations in a third-party IP subnetwork can be reached via routers. The administration of the multicast groups is done in the routers to which recipients of a multicast group are connected directly. IGMP provides functions with which the Industrial Ethernet CP / CM can inform a router that it would like to receive the multicast messages of a specific multicast group. Multicast routing protocols handle the coordination between the routers.
Modules that support multicast connections
Table 3 shows which S7-1200 / S7-1500 modules support multicast connections and broadcast connections.
Table 4 shows which S7-300 / S7-400 modules support multicast connections and broadcast connections.
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