# EIP 2718: Typed Transaction Envelope Source

Author Micah Zoltu https://ethereum-magicians.org/t/eip-2718-typed-transaction-envelope/4355 Draft Standards Track Core 2020-06-13

## Simple Summary

Defines a new transaction type that is an envelope for future transaction types.

## Abstract

As of FORK_BLOCK_NUMBER, rlp([TransactionType, Payload]) will be a valid transaction where TransactionType is a number identifying the format of the transaction and payload is the transaction, whose definition is defined in future EIPs. The first new transaction type will be a wrapped legacy transaction with the format rlp([0, rlp([nonce, gasPrice, gasLimit, to, value, data, v, r, s])]).

## Motivation

In the past, when we have wanted to add new transaction types we have had to ensure they were backward compatible with all other transactions, meaning that you could differentiate them based only on the encoded payload, and it was not possible to have a transaction that matched both types. This was seen in EIP-155 where the new value was bit-packed into one of the encoded fields. There are multiple proposals in discussion that define new transaction types such as one that allows EOA accounts to execute code directly within their context, one that enables someone besides msg.sender to pay for gas, and proposals related to layer 0 multi-sig transactions. These all need to be defined in a way that is mutually compatible, which quickly becomes burdensome to EIP authors and to clients who now have to follow complex rules for differentiating transaction type.

By introducing an envolope transaction type, we only need to ensure backward compatibility with existing transactions and from then on we just need to solve the much simpler problem of ensuring there is no numbering conflict between TransactionTypes.

## Specification

As of FORK_BLOCK_NUMBER, rlp([nonce, gasPrice, gasLimit, to, value, data, v, r, s]) (legacy transaction) will no longer be a valid Ethereum transaction over the devp2p protocol or in a block.

As of FORK_BLOCK_NUMBER, all transactions sent over devp2p or included in a block MUST be of the form rlp([TransactionType, Payload]) where TransactionType is a number that represents the type of the transcation and Payload is an opaque value (byte array) whose interpretation is dependent on the TransactionType. Transactions SHOULD include the TransactionType in any signatures they include to minimize the chance of unintentional replay attacks between different transaction types. The transaction hash of all transactions MUST be keccak256(rlp([TransactionType, Payload]))

As of FORK_BLOCK_NUMBER, rlp([0, rlp([nonce, gasPrice, gasLimit, to, value, data, v, r, s])]) will be a valid transaction where the Payload will be signed/processed/handled exactly the same as legacy transactions were processed/handled.

Between FORK_BLOCK_NUMBER - 1 and FORK_BLOCK_NUMBER clients SHOULD wrap all transactions in their local pending pool as type 0 transactions. Clients MAY choose to flush their pending pool instead, if it is not possible to do the wrapping in time for the next block, though this is discouraged.

As of FORK_BLOCK_NUMBER, clients SHOULD accept incoming transactions over user/application facing APIs such as JSON-RPC in both the new wrapped format and the legacy format. If a legacy format transaction is received, the client MUST wrap it in a type 0 envelope before sending it over devp2p or including it in a block.

## Rationale

### TransactionType high bit

Setting the high bit of the TransactionType so that a decoder could identify transaction type early on in the decoding process was discussed, but decided against. The problem with this solution is that due to the way RLP encoding works, this would result in the TransactionType being 5 bytes long rather than 1 byte long. RLP decoders also do not need to know the shape of the result before decoding, and thus in most (possibly all) clients it will be just be a matter of first decoding the RLP transaction, and then checking to see if the decoded result is a 2 item array or a 9 item array, which is likely simpler than checking the high bit of the first item.

### TransactionType selection algorithm

There was discussion about defining the TransactionType identifier assignment/selection algorithm in this standard. While it would be nice to have a standardized mechanism for assignment, at the time of writing of this standard there is not a strong need for it so it was deemed out of scope. A future EIP may introduce a standard for TransactionType identifier assignment if it is deemed necessary.

### Opaque second item rather than an array

By having the second item of the array just be opaque bytes, rather than a list, we can support different encoding formats for the transaction payload in the future, such as SSZ or a fixed width format.

### n-item list instead of 2-item list

We could have chosen the format rlp([TransactionType, ...]) where ... represents whatever items the transaction wants. This format however would require us to remain vigilant in the future about using the shape of the transaction to identify its type, as it is possible that there could be an overlap between an EIP-2718 transaction type and a legacy transaction type with a nonce that matches the TransactionType. By having a strict 2-item array with the second item potentially being an encoded list, we avoid needing to worry about any conflict between TransactionTypes beyond choosing uinque numbers.

### ORIGIN and CALLER

There was discussion about having ORIGIN and CALLER opcodes be dependent on the transaction type, so that each transaction type could define what those opcodes returned. However, there is a desire to make transaction type opaque to the contracts to discourage contracts treating different different types of transactions differently and there also were concerns over backward compatibility with existing contracts which make assumptions about ORIGIN and CALLER opcodes. Going forward, we will assume that all transaction types will have an address that reasonably represents a CALLER of the first EVM frame and ORIGIN will be the same address in all cases. If a transaction type needs to supply additional information to contracts, they will need a new opcode.

### Hashing the outer transaction

If you submit a transaction prior to the fork block and it is mined after the fork block, the transaction hash of the mined transaction will not match the the transaction hash of the transaction you originally submitted. This is because the mined transaction will be a wrapped one, while the submitted transaction will be unwrapped. Applications that use the transaction hash to lookup the transaction result will fail to ever find it or see it ever mined. Clients can optionally choose to track type 0 transactions internally by both hashes for a period of time so that lookups of the old hash won’t fail for the user, but this is not required for consensus.

### Signing the inner transaction for type 0

While this EIP recommends that the envelope be included in any signatures, it also specifies that for Transaction Type 0 the envelope is not included in the signature. This EIP disobayes its own recommendation because we don’t want all signing tools to break on the fork block and require updates to start working again.

## Backwards Compatibility

Clients can differentiate between the legacy transactions and typed transactions by noting that the RLP decoded transaction has 2 elements rather than 9.

TBD

TBD

## Security Considerations

Due to the way RLP encoding works, the risk of a new transaction being mis-interpreted as a legacy transaction is deemed low. The authors of this EIP consider it unlikely that there are any transaction decoders in production that will accept a transaction with not enough elements (perhaps reading off the end of the array).

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