S7 routing support for multi-subnet PLC access

[gijzelaerr]

Summary

Implement S7 routing support to reach PLCs behind gateways across multiple subnets.

Background

In complex industrial networks, PLCs are often segmented into different subnets connected through S7 routing-capable devices (S7-300/400 CPs, S7-1500 with routing enabled). S7 routing allows a client to reach a PLC on a remote subnet by specifying a routing path through intermediate devices.

What needs to be done

• Implement routing TSAP construction (encoding subnet ID and rack/slot of the target)
• Implement extended connection setup with routing information in the COTP CR (Connection Request)
• Support multi-hop routing (routing through multiple gateways)
• Add routing parameters to connect() or add a connect_routed() method
• Handle routing-specific error codes and connection failures
• Add server-side routing support (optional, for testing)
• Add tests with routing scenarios
• Add documentation and examples

API Design Considerations

# Option A: Extended connect
client.connect("192.168.1.1", 0, 2, routing={"subnet": 0x0001, "dest_rack": 0, "dest_slot": 1})

# Option B: Dedicated method
client.connect_routed("192.168.1.1", router_rack=0, router_slot=2,
                      subnet=0x0001, dest_rack=0, dest_slot=1, dest_tsap=0x0102)

References

• Siemens documentation on S7 routing configuration
• COTP CR routing parameter encoding in existing snap7 C library