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NOMOS-SDP

Introduction​

This document defines a mechanism enabling validators to declare their participation in specific protocols that require a known and agreed-upon list of participants. Some examples of this are Data Availability and the Blend Network. We create a single repository of identifiers which is used to establish secure communication between validators and provide services. Before being admitted to the repository, the validator proves that it locked at least a minimum stake.

Requirements​

The requirements for the protocol are defined as follows:

  • A declaration must be backed by a confirmation that the sender of the declaration owns a certain value of the stake.
  • A declaration is valid until it is withdrawn or is not used for a service-specific amount of time.

Overview​

The SDP enables nodes to declare their eligibility to serve a specific service in the system, and withdraw their declarations.

Protocol Actions​

The protocol defines the following actions:

  • Declare: A node sends a declaration that confirms its willingness to provide a specific service, which is confirmed by locking a threshold of stake.
  • Active: A node marks that its participation in the protocol is active according to the service-specific activity logic. This action enables the protocol to monitor the node's activity. We utilize this as a non-intrusive differentiator of node activity. It is crucial to exclude inactive nodes from the set of active nodes, as it enhances the stability of services.
  • Withdraw: A node withdraws its declaration and stops providing a service.

The logic of the protocol is straightforward:

  1. A node sends a declaration message for a specific service and proves it has a minimum stake.
  2. The declaration is registered on the ledger, and the node can commence its service according to the service-specific service logic.
  3. After a service-specific service-providing time, the node confirms its activity.
  4. The node must confirm its activity with a service-specific minimum frequency; otherwise, its declaration is inactive.
  5. After the service-specific locking period, the node can send a withdrawal message, and its declaration is removed from the ledger, which means that the node will no longer provide the service.

💡 The protocol messages are subject to a finality that means messages become part of the immutable ledger after a delay. The delay at which it happens is defined by the consensus.

Construction​

In this section, we present the main constructions of the protocol. First, we start with data definitions. Second, we describe the protocol actions. Finally, we present part of the Bedrock Mantle design responsible for storing and processing SDP-related messages and data.

Data​

In this section, we discuss and define data types, messages, and their storage.

Service Types​

We define the following services which can be used for service declaration:

  • BN: for Blend Network service.
  • DA: for Data Availability service.
class ServiceType(Enum):
    BN="BN"  # Blend Network
    DA="DA"  # Data Availability

A declaration can be generated for any of the services above. Any declaration that is not one of the above must be rejected. The number of services might grow in the future.

Minimum Stake​

The minimum stake is a global value that defines the minimum stake a node must have to perform any service.

The MinStake is a structure that holds the value of the stake stake_threshold and the block number it was set at: timestamp.

class MinStake:
    stake_threshold: StakeThreshold
    timestamp: BlockNumber

The stake_thresholds is a structure aggregating all defined MinStake values.

stake_thresholds: list[MinStake]

For more information on how the minimum stake is calculated, please refer to the Nomos documentation.

Service Parameters​

The service parameters structure defines the parameters set necessary for correctly handling interaction between the protocol and services. Each of the service types defined above must be mapped to a set of the following parameters:

  • session_length defines the session length expressed as the number of blocks; the sessions are counted from block timestamp.
  • lock_period defines the minimum time (as a number of sessions) during which the declaration cannot be withdrawn, this time must include the period necessary for finalizing the declaration (which might be implicit) and provision of a service for least a single session; it can be expressed as the number of blocks by multiplying its value by the session_length.
  • inactivity_period defines the maximum time (as a number of sessions) during which an activation message must be sent; otherwise, the declaration is considered inactive; it can be expressed as the number of blocks by multiplying its value by the session_length.
  • retention_period defines the time (as a number of sessions) after which the declaration can be safely deleted by the Garbage Collection mechanism; it can be expressed as the number of blocks by multiplying its value by the session_length.
  • timestamp defines the block number at which the parameter was set.
class ServiceParameters:
    session_length: NumberOfBlocks
    lock_period: NumberOfSessions
    inactivity_period: NumberOfSessions
    retention_period: NumberOfSessions
    timestamp: BlockNumber

The parameters is a structure aggregating all defined ServiceParameters values.

parameters: list[ServiceParameters]

Identifiers​

We define the following set of identifiers which are used for service-specific cryptographic operations:

  • provider_id: used to sign the SDP messages and to establish secure links between validators; it is Ed25519PublicKey.
  • zk_id: used for zero-knowledge operations by the validator that includes rewarding (Zero Knowledge Signature Scheme (ZkSignature)).

Locators​

A Locator is the address of a validator which is used to establish secure communication between validators. It follows the multiaddr addressing scheme from libp2p, but it must contain only the location part and must not contain the node identity (peer_id).

The provider_id must be used as the node identity. Therefore, the Locator must be completed by adding the provider_id at the end of it, which makes the Locator usable in the context of libp2p.

The length of the Locator is restricted to 329 characters.

The syntax of every Locator entry must be validated.

The common formatting of every Locator must be applied to maintain its unambiguity, to make deterministic ID generation work consistently. The Locator must at least contain only lower case letters and every part of the address must be explicit (no implicit defaults).

Declaration Message​

The construction of the declaration message is as follows:

class DeclarationMessage:
    service_type: ServiceType
    locators: list[Locator]
    provider_id: Ed25519PublicKey 
    zk_id: ZkPublicKey

The locators list length must be limited to reduce the potential for abuse. Therefore, the length of the list cannot be longer than 8.

The message must be signed by the provider_id key to prove ownership of the key that is used for network-level authentication of the validator. The message is also signed by the zk_id key (by default all Mantle transactions are signed with zk_id key).

Declaration Storage​

Only valid declaration messages can be stored on the ledger. We define the DeclarationInfo as follows:

class DeclarationInfo:
    service: ServiceType
    provider_id: Ed25519PublicKey
    zk_id: ZkPublicKey
    locators: list[Locator]
    created: BlockNumber
    active: BlockNumber
    withdrawn: BlockNumber
    nonce: Nonce

Where:

  • service defines the service type of the declaration;
  • provider_id is an Ed25519PublicKey used to sign the message by the validator;
  • zk_id is used for zero-knowledge operations by the validator that includes rewarding (Zero Knowledge Signature Scheme (ZkSignature));
  • locators are a copy of the locators from the DeclarationMessage;
  • created refers to the block number of the block that contained the declaration;
  • active refers to the latest block number for which the active message was sent (it is set to created by default);
  • withdrawn refers to the block number for which the service declaration was withdrawn (it is set to 0 by default).
  • The nonce must be set to 0 for the declaration message and must increase monotonically by every message sent for the declaration_id.

We also define the declaration_id (of a DeclarationId type) that is the unique identifier of DeclarationInfo calculated as a hash of the concatenation of service, provider_id, locators and zk_id. The implementation of the hash function is blake2b using 256 bits of the output.

declaration_id = Hash(service||provider_id||zk_id||locators)

The declaration_id is not stored as part of the DeclarationInfo but it is used to index it.

All DeclarationInfo references are stored in the declarations and are indexed by declaration_id.

declarations: list[declaration_id]

Active Message​

The construction of the active message is as follows:

class ActiveMessage:
    declaration_id: DeclarationId
    nonce: Nonce
    metadata: Metadata

where metadata is a service-specific node activeness metadata.

The message must be signed by the zk_id key associated with the declaration_id.

The nonce must increase monotonically by every message sent for the declaration_id.

Withdraw Message​

The construction of the withdraw message is as follows:

class WithdrawMessage:
    declaration_id: DeclarationId
    nonce: Nonce

The message must be signed by the zk_id key from the declaration_id.

The nonce must increase monotonically by every message sent for the declaration_id.

Indexing​

Every event must be correctly indexed to enable lighter synchronization of the changes. Therefore, we index every declaration_id according to EventType, ServiceType, and Timestamp. Where EventType = { "created", "active", "withdrawn" } follows the type of the message.

events = {
    event_type: {
        service_type: {
            timestamp: {
                declarations: list[declaration_id]
            }
        }
    }
}

Protocol​

Declare​

The Declare action associates a validator with a service it wants to provide. It requires sending a valid DeclarationMessage (as defined in Declaration Message), which is then processed (as defined below) and stored (as defined in Declaration Storage).

The declaration message is considered valid when all of the following are met:

  • The sender meets the stake requirements.
  • The declaration_id is unique.
  • The sender knows the secret behind the provider_id identifier.
  • The length of the locators list must not be longer than 8.
  • The nonce is increasing monotonically.

If all of the above conditions are fulfilled, then the message is stored on the ledger; otherwise, the message is discarded.

Active​

The Active action enables marking the provider as actively providing a service. It requires sending a valid ActiveMessage (as defined in Active Message), which is relayed to the service-specific node activity logic (as indicated by the service type in Common SDP Structures).

The Active action updates the active value of the DeclarationInfo, which means that it also activates inactive (but not expired) providers.

The SDP active action logic is:

  1. A node sends a ActiveMessage transaction.
  2. The ActiveMessage is verified by the SDP logic. a. The declaration_id returns an existing DeclarationInfo. b. The transaction containing ActiveMessage is signed by the zk_id. c. The withdrawn from the DeclarationInfo is set to zero. d. The nonce is increasing monotonically.
  3. If any of these conditions fail, discard the message and stop processing.
  4. The message is processed by the service-specific activity logic alongside the active value indicating the period since the last active message was sent. The active value comes from the DeclarationInfo.
  5. If the service-specific activity logic approves the node active message, then the active field of the DeclarationInfo is set to the current block height.

Withdraw​

The withdraw action enables a withdrawal of a service declaration. It requires sending a valid WithdrawMessage (as defined in Withdraw Message). The withdrawal cannot happen before the end of the locking period, which is defined as the number of blocks counted since created. This lock period is stored as lock_period in the Service Parameters.

The logic of the withdraw action is:

  1. A node sends a WithdrawMessage transaction.
  2. The WithdrawMessage is verified by the SDP logic: a. The declaration_id returns an existing DeclarationInfo. b. The transaction containing WithdrawMessage is signed by the zk_id. c. The withdrawn from DeclarationInfo is set to zero. d. The nonce is increasing monotonically.
  3. If any of the above is not correct, then discard the message and stop.
  4. Set the withdrawn from the DeclarationInfo to the current block height.
  5. Unlock the stake.

Garbage Collection​

The protocol requires a garbage collection mechanism that periodically removes unused DeclarationInfo entries.

The logic of garbage collection is:

For every DeclarationInfo in the declarations set, remove the entry if either:

  1. The entry is past the retention period: withdrawn + retention_period < current_block_height.
  2. The entry is inactive beyond the inactivity and retention periods: active + inactivity_period + retention_period < current_block_height.

Query​

The protocol must enable querying the ledger in at least the following manner:

  • GetAllProviderId(timestamp), returns all provider_ids associated with the timestamp.
  • GetAllProviderIdSince(timestamp), returns all provider_ids since the timestamp.
  • GetAllDeclarationInfo(timestamp), returns all DeclarationInfo entries associated with the timestamp.
  • GetAllDeclarationInfoSince(timestamp), returns all DeclarationInfo entries since the timestamp.
  • GetDeclarationInfo(provider_id), returns the DeclarationInfo entry identified by the provider_id.
  • GetDeclarationInfo(declaration_id), returns the DeclarationInfo entry identified by the declaration_id.
  • GetAllServiceParameters(timestamp), returns all entries of the ServiceParameters store for the requested timestamp.
  • GetAllServiceParametersSince(timestamp), returns all entries of the ServiceParameters store since the requested timestamp.
  • GetServiceParameters(service_type, timestamp), returns the service parameter entry from the ServiceParameters store of a service_type for a specified timestamp.
  • GetMinStake(timestamp), returns the MinStake structure at the requested timestamp.
  • GetMinStakeSince(timestamp), returns a set of MinStake structures since the requested timestamp.

The query must return an error if the retention period for the delegation has passed and the requested information is not available.

The list of queries may be extended.

Every query must return information for a finalized state only.

Mantle and ZK Proof​

For more information about the Mantle and ZK proofs, please refer to Mantle Specification.

Appendix​

Future Improvements​

Refer to the Mantle Specification for a list of potential improvements to the protocol.

References​

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