IC consensus layer

The job consensus layer of the IC is to order transaction requests so that all replicas in a subnet will process transaction requests in the same order. There are many protocols in the literature for this problem. The IC uses a new consensus protocol, which is described here at a high level. For more details, see the paper https://eprint.iacr.org/2021/1330 (in particular, Protocol ICC1 in that paper).

Any secure consensus protocol should guarantee two properties, which (roughly stated) are:

• safety: all replicas in fact agree on the same ordering of transaction requests, and
• liveness: all replicas should make steady progress.

The IC consensus protocol is design to be

• extremely simple, and
• robust (performance degrades gracefully when some replicas are malicious).

As discussed in the introduction, we assume

• a subnet of [math]\displaystyle{ n }[/math] replicas, and
• at most [math]\displaystyle{ f \lt n/3 }[/math] of the replicas are faulty.

Faulty replicas may behave in an arbitrary, malicious way.

We assume that communication is asynchronous, with no a priori bound on the delay of messages sent between replicas. In fact, the scheduling of message delivery may be completely under adversarial control. The IC consensus protocol guarantees safety under this very weak communication assumption.

However, to achieve liveness, we need to assume a form of partial synchrony

,

which (roughly stated) says that there exists a bound [math]\displaystyle{ \Delta }[/math] such that from time to time, messages will be delivered within time [math]\displaystyle{ \Delta }[/math]. The bound [math]\displaystyle{ \Delta }[/math] does not have to be known in advance (the protocol can adapt itself to an unknown [math]\displaystyle{ \Delta }[/math] value).