Network node

1. A network node having a normal mode and a coordinator mode, comprising:

a communication unit arranged to send and receive electronic messages within a group of network nodes,

a counter and an incrementing unit, the incrementing unit being arranged to increment the counter at regular time intervals,

a counter manager, wherein the counter manager is arranged to

in normal mode:

upon receiving a counter synchronization message, adjust the counter to a received counter value in the received counter synchronization message, and start a synchronization timeout period,

upon expiry of the synchronization timeout period without receiving a counter synchronization message during the synchronization timeout period, switch the network node to coordinator mode,

in coordinator mode:

periodically send a counter synchronization message, comprising a current value of the counter, to the network nodes in the group.

2. A network node as claimed in claim 1 , comprising:

a key storage arranged for storing a cryptographic key, wherein in normal mode,

the communication unit is arranged to reject a received message after failing to verify an authentication of the received message with the cryptographic key, and to reject after failing to verify that a received counter value obtained from the received message differs less than a delay threshold from a counter value stored in the counter of the network node, and

the communication unit is arranged to include a counter value stored in the counter of the network node in a message and to authenticate said message with the cryptographic key before sending said message.

3. A network node as claimed in claim 1 having an initialization mode, the network node being in initialization mode after start-up of the network node, wherein in initialization mode

the communication unit is arranged to accept a received counter synchronization message after verifying an authentication of a received message with a cryptographic key,

the counter manager is arranged to

start the synchronization timeout period, and

upon expiry of the synchronization timeout period without receiving a counter synchronization message during the synchronization timeout period, switch to coordinator mode,

upon receiving a counter synchronization message within the synchronization timeout period, adjusting the counter to a counter value in the counter synchronization message received within the synchronization timeout period, starting a synchronization timeout period, and switch to normal mode.

4. A network node as claimed in claim 1 having an initialization mode, wherein

in initialization mode,

the counter manager is arranged to send a counter synchronization request message to the network nodes in the group, and

in coordinator mode,

the counter manager is arranged to send a counter synchronization message, comprising a current value of the counter, to the network nodes in the group upon receiving a counter synchronization request message.

5. A network node as claimed in claim 4 , wherein

in initialization mode the counter manager is arranged to

start a request delay timeout period,

upon expiry of the request delay timeout period without receiving a counter synchronization request message and without receiving a counter synchronization message, sending the counter synchronization request message to the network nodes in the group,

upon receiving any one of a counter synchronization message and a counter synchronization request message, terminating the request delay timeout period.

6. A network node as claimed in claim 1 , wherein the counter is randomized after start-up of the network node.

7. A network node as claimed in claimed 1 , wherein

in coordinator mode, the counter manager is arranged to

upon receiving a counter synchronization message, send a resign message comprising a counter value stored in the counter,

upon receiving a resign message from another network node, comparing the counter value stored in the counter with a received counter value obtained from the received resign message from the other network node,

if the received counter value received from the other network node is larger than the counter value stored in the counter, send a resigned message comprising the counter value stored in the counter and the received counter value from the other network node, adjust the counter to the received counter value from the other network node and switch to normal mode,

in normal mode, the counter manager is arranged to, upon receiving the resigned message from the other network node, adjusting the counter to the counter value received from the other network node in the resigned message.

8. A network node as claimed in claim 7 , wherein

in coordinator mode, the counter manager is arranged to

if the received counter value from the other network node is less than the counter value stored in the counter, send a member request message, if no membership message is received in response within a member request period then, send a resigned message comprising the counter value stored in the counter and the received counter value from the other network node, adjust the counter to the received counter value from the other network node and switch to normal mode,

in normal mode, the counter manager is arranged to, upon receiving a member request message to respond with a membership message.

9. A network node as claimed in claim 1 arranged to receive a stateless command in a message.

10. A network method having a normal mode and a coordinator mode, the method comprising:

sending and receiving electronic messages within a group of network nodes,

incrementing a counter at regular time intervals,

by a network node in normal mode:

upon receiving a counter synchronization message, adjusting the counter to a received counter value in the received counter synchronization message, and start a synchronization timeout period,

upon expiry of the synchronization timeout period without receiving a counter synchronization message during the synchronization timeout period, switching the method to coordinator mode, and

by the network node in coordinator mode:

periodically send a counter synchronization message to the network nodes in the group, said periodically sent counter synchronization message comprising a current value of the counter.

11. A computer readable non-transitory medium having stored therein instructions for causing a processor to execute a method having a normal mode and a coordinator mode, the medium comprising code for:

sending and receiving electronic messages within a group of network nodes,

incrementing a counter at regular time intervals, by a network node in normal mode:

upon receiving a counter synchronization message, adjusting the counter to a received counter value in the received counter synchronization message, and start a synchronization timeout period,

upon expiry of the synchronization timeout period without receiving a counter synchronization message during the synchronization timeout period, switching the method to coordinator mode, and

by the network node in coordinator mode:

periodically send a counter synchronization message to the network nodes in the group, said periodically sent counter synchronization message comprising a current value of the counter.