Protocol Plugins

This document describes the protocol plugins implemented in Octez.

Protocol-specific shell plugins, simply called protocol plugins, implement extra APIs needed by the shell in order to interact with the economic protocol, beyond the one provided by the protocol environment. This code is not strictly speaking part of the protocol code base, so this is not subject to on-chain governance (see voting procedure), but it is still protocol-dependent, which means that it may vary with different protocols. For instance, the plugin code for protocol Alpha is located in file src/proto_alpha/lib_plugin/plugin.ml. Thus, a specific version is included in the Octez node for each protocol version (recall that a new release of Octez is usually delivered for each new protocol proposal, see Release System)

In contrast to the Octez code, plugins are protocol-specific and as such, know how to read the content of an operation.

So what kind of features may a protocol plugin provide? For instance, protocol plugins do not define the context, or restrict the validity of operations. In turn protocol plugins may, for example:

  • perform protocol-dependent computations that require data not available in the amendable part of the protocol like accessing the current time to reason on timestamps (see Consensus filter);

  • preserve the opacity/abstraction barrier of the protocol’s internal data by performing computations on internal data without revealing it: e.g., there are some RPCs that can introspect the protocol-dependent content for certain operations;

  • implement some common operations that are customized for each protocol (e.g., Prevalidator filters).

Prevalidator filters

Prevalidator filters, which are detailed in the rest of this page, enable the node to discard some operations (that cannot be included in the next block) faster and restrict the operations it propagates to the network. Filters are implemented as a node plugin and a specific filter is delivered with each protocol version. When the chain switches to a new protocol, the node installs its corresponding filters, in lieu of the filters of the previous protocol. Notice that prevalidator filters are not mandatory, their absence does not break the Tezos blockchain protocol.

The interface of the prevalidator plugin is described at the mempool plugin API

The different kinds of prevalidator filters are described below.

Fees filter

A very affordable technique for attackers is based on flooding the network with, valid but useless, zero-fees operations. This is why the prevalidator filter currently restricts operations based on their associated fees, to reject “too cheap” or “zero-fees” operations. This can be configured via the minimal_fees, minimal_nanotez_per_gas_unit and minimal_nanotez_per_byte (see filter RPCs) parameters of the filter configuration of your node.

This filtering strategy is implemented in the prefilter (see The Prevalidator).

Consensus filter

For technical reasons, the economic protocol cannot reject consensus operations based on their timestamp. Consequently, many consensus operations are declared as valid by the economic protocol while being timestamped too early or too late. The plugin filter aims to narrow down the number of consensus operations declared as valid based on several heuristics. In particular, the filter can use the current timestamp while the protocol cannot.

This filter classifies a consensus operation according to the current block proposal as follows:

  • Outdated if it concerns a previous head at a previous level,

  • Branch_refused if it concerns a previous head at the same level but in the previous round,

  • Branch_delayed if it concerns a round in the future.

This filtering strategy is implemented in the prefilter (see The Prevalidator).

Prechecking of manager operations

The aim of the precheck filter is to avoid fully executing manager operations before deciding whether to gossip them to the network.

The detailed description of this feature is given in Prechecking of manager operations. For operations other than manager operations, the precheck filter is a no-op, which entails that these operations need to be fully executed to decide their propagation (see The Prevalidator).

One operation manager per manager per block

The mempool filters, prefilter, precheck and postfilter also ensure that, since the last head update (the last valid block which increased the chain’s fitness), only one operation per manager is propagated. All other received operations originating from the same manager will be classified as Branch_delayed and will not be propagated.

This criterion is used only by the prevalidator to decide the propagation of operations. A baker can still include several operations originating from the same manager in a single block, provided that it gets them in time (note that they can be propagated by nodes using different versions or implementations).

Alternatively, a user can inject an operation with the same manager and the same counter, but with a higher fee to replace an already existing operation in the prevalidator. Only one of the two operations will be eventually included in a block. To be able to replace the first operation, the fee and the “fee/gas limit” ratio of the second one is supposed to be higher than the first’s by a factor (currently fixed to 5%). In case of successful replacement, the old operation is re-classified as Outdated.

Filters RPCs

Filters are tunable by several parameters, whose values can be retrieved and changed by users via the following RPC calls, respectively:

  • rpc get /chains/<chain>/mempool/filter

  • rpc post /chains/<chain>/mempool/filter

The following parameters can be thus inspected and modified:

  • minimal_fees: type int, default 100

  • minimal_nanotez_per_gas_unit: type int, default 100

  • minimal_nanotez_per_byte: type int, default 1000

  • allow_script_failure: type bool, default true

  • clock_drift : type Period.t option, default None

For example, the following command modifies the minimal_fees parameter (and resets all the other parameters to their default values):

tezos-client rpc post /chains/main/mempool/filter with '{ "minimal_fees": "42" }'

Changing filters default configuration

Changing filters configuration may not have the expected outcome. Because the configuration is only changed on your node, and not on all the gossip network nodes, this will only impact how operations of your node are propagated. For example, assuming there are three nodes A, B and C such that A is connected to B and B is connected to C. Assume that A modifies its filter configuration so that minimal_fees is now 0. Then A may propagate an operation with 0 fee to B. However, because B has the default filter configuration, this operation will not be propagated to C (so C may never see it).