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

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

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.

As of FORK_BLOCK_NUMBER, rlp([nonce, gasPrice, gasLimit, to, value, data, senderV, senderR, senderS]) will continue to be a valid transaction as defined in EIP-155. As of FORK_BLOCK_NUMBER, rlp([0, [nonce, gasPrice, gasLimit, to, value, data, senderV, senderR, senderS]]) will be functionally equivalent where the second item of the outer array will be handled exactly the same as an EIP-155 transaction is handled.

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.

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.

Clients can differentiate between the transaction types by noting that the RLP decoded transaction has 2 elements rather than 9 or by noting that the second element is a list rather than a value.

TBD

TBD

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), or read the second item as a value when it is encoded as an array.

Copyright and related rights waived via CC0.