General purpose execution layer requests

Abstract

This proposal defines a general purpose framework for storing contract-triggered requests. It extends the execution header and body with a single field each to store the request information. This inherently exposes the requests to the consensus layer, which can then process each one.

Motivation

The proliferation of smart contract controlled validators has caused there to be a demand for additional EL triggered behaviors. By allowing these systems to delegate administrative operations to their governing smart contracts, they can avoid intermediaries needing to step in and ensure certain operations occur. This creates a safer system for end users.

Specification

Execution Layer

Request

A request consists of a request_type prepended to an opaque byte array request_data:

request = request_type ++ request_data

Let requests be the list of all request objects in the block in ascending order by type. For example:

[0x00_request_0, 0x01_request_0, 0x01_request_1, 0x02_request_0, ...]

The ordering of requests within a type is to be defined by each request type.

Block structure

The block body is appended with a list of requests. RLP encoding of the extended block body structure is computed as follows:

block_body_rlp = rlp([
    field_0,
    ...,
    # Latest block body field before `requests`
    field_n,
    [request_0, ..., request_k],
])

Block Header

Extend the header with a new 32 byte value requests_root.

Let requests_root be the root of a Merkle-Patricia trie keyed by the index in the list of requests. This is equivalent to how the transaction trie root is computed.

The requests_root field value is therefore computed as follows:

def compute_trie_root_from_indexed_data(data):
    trie = Trie.from([(i, obj) for i, obj in enumerate(data)])
    return trie.root

block.header.requests_root = compute_trie_root_from_indexed_data(block.body.requests)

Consensus Layer

Each proposal may choose how to extend the beacon chain types to include the new EL request.

Rationale

Opaque byte array rather than an RLP array

By having the bytes of request_data array from second byte on be opaque bytes, rather than an RLP (or other encoding) list, we can support different encoding formats for the transaction payload in the future such as SSZ, LEB128, or a fixed width format.

Request source and validity

This EIP makes no strict requirement where a request may come from nor when/how a request must be validated. This is to provide future protocol designers maximum flexibility.

The authors' recommendations on source and validity of requests are:

• The source of requests should be from the execution of transactions. More specifically, transactions which make calls to designated system contracts that store the request in account. The storage would later be retrieved by a post-block system call to the contract. Alternatively, if the system call does not need to be inherently concerned with rate limiting, it could rely simply on emitting an event which is later parsed post-block by the system and converted into a request.
• A request's validity can often not be fully verified at the execution layer. This is why they are referred to merely as "requests"; they do not carry the authority on their own to unilaterally catalyze an action. We expect the system contracts to perform whatever validation is possible by the EL and then pass it on to the CL for further validation.

Ordering

The ordering across types is ascending by type. This is to simplify the process of verifying that all requests which were committed to in requests_root were found in the block.

An alternative could be to order by when the request was generated within the block. Since it's expected that many requests will be accumulated at the end of the block via system calls, this would be difficult to enforce. Therefore, ordering by type is the most straightforward ordering which ensures integrity.

Intra-type

Within the same type, order is not defined. This is because the data of the request is opaque as far as this EIP is concerned. Therefore, it is to be determined by each request type individually.

Backwards Compatibility

No backward compatibility issues found.

Test Cases

TODO

Security Considerations

Needs discussion.

Copyright

Copyright and related rights waived via CC0.