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6.6. Consistency Check (CC)

The CC message is used to check secure message counters and issue challenge-responses. A CC message MUST be sent as a secured RPL message.

6.6.1. Format of the CC Base Object​

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RPLInstanceID |R|    Flags    |           CC Nonce            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                                                               +
|                                                               |
+                            DODAGID                            +
|                                                               |
+                                                               +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Destination Counter                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Option(s)...
+-+-+-+-+-+-+-+-+

                Figure 18: The CC Base Object

  • RPLInstanceID: 8-bit field indicating the topology instance associated with the DODAG, as learned from the DIO.

  • R: The 'R' flag indicates whether the CC message is a response. A message with the 'R' flag cleared is a request; a message with the 'R' flag set is a response.

  • Flags: The 7 bits remaining unused in the Flags field are reserved for flags. The field MUST be initialized to zero by the sender and MUST be ignored by the receiver.

  • CC Nonce: 16-bit unsigned integer set by a CC request. The corresponding CC response includes the same CC nonce value as the request.

  • DODAGID: 128-bit field, contains the identifier of the DODAG root.

  • Destination Counter: 32-bit unsigned integer value indicating the sender's estimate of the destination's current security counter value. If the sender does not have an estimate, it SHOULD set the Destination Counter field to zero.

  • Unassigned bits of the CC Base are reserved. They MUST be set to zero on transmission and MUST be ignored on reception.

The Destination Counter value allows new or recovered nodes to resynchronize through CC message exchanges. This is important to ensure that a Counter value is not repeated for a given security key even in the event of devices recovering from a failure that created a loss of Counter state. For example, where a CC request or other RPL message is received with an initialized counter within the message Security section, the provision of the Incoming Counter within the CC response message allows the requesting node to reset its Outgoing Counter to a value greater than the last value received by the responding node; the Incoming Counter will also be updated from the received CC response.

6.6.2. CC Options​

This specification allows for the CC message to carry the following options:

  • 0x00 Pad1
  • 0x01 PadN
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