Larger Transaction Size

Summary

Historically, messages transmitted over the network must not exceed the IPv6 MTU size to ensure a fast and reliable network delivery. Solana has used a conservative MTU size of 1280 bytes, which after accounting for some of the overhead, leaves a maximum transaction size of 1232 bytes for data in the payload.

With QUIC, we can now use a larger transaction size than the original 1280 bytes. This proposal presents a new transaction size of 4096 bytes.

Motivation

The current transaction size limit of 1232 bytes is too small for many developer use cases. While there have been some attempts to artificially increase the size capacity with address lookup tables, the current cap still is artificially limiting the number of signatures and size of other data.

Use cases that are currently limited by the transaction size include:

• ZK proofs such as what is used within Confidential Balances
• Untruncated Winternitz one-time signatures
• Nested Multisig used by corporations under Squads
• Onchain cryptography signature schemes without precompiles such as BLS

and many more. A larger transaction size would also help remove the need for address lookup tables, which have been a source of complexity on the validator client as well as a frictional developer experience for many developers.

A number of developers have also resorted to using Jito bundles to artificially increase the size of their transactions. This is a current workaround that developers are using, but it does not have the same guarantees as a transaction does on the protocol level, namely that bundles are not guaranteed to be atomic like singular transactions.

For these reasons, increasing the current transaction size limit would enable developers to create new applications and use cases that were previously unfeasible.

New Terminology

Not at this time.

Detailed Design

Increasing the transaction size limit above the current MTU max size is possible with the use of QUIC. QUIC's RFC 9000 specification does not have an explicit maximum stream size, allowing for larger transactions to be sent. The larger transaction size would only be supported by the v1 transaction format as described in SIMD-0385. Clients will need to support accepting the larger transaction sizes on their network ports and not reject the larger size during replay.

Alternatives Considered

Alternative designs considered:

• Having a transaction loading feature that would allow developers to load the transaction parts in a buffer and then be able to execute them at the end. This method is no longer considered as it requires a much higher level of latency on the application level and a decent amount of complexity within the validator
• Bundles at the protocol level. This would not solve all problems that are solved by larger transaction sizes. Bundles would still limit the ability for developers to use cryptographic signature schemes that have large proof sizes.

Impact

Developers would have to update with applications to use the new transaction format designed in SIMD-0385 to take advantage of the larger transaction size. Those developers that had previously been using address lookup tables would be required to update the new transactions with the full address list instead of the address lookup table and its indices.

In consideration of what size the new transaction size limit should increase to, Jito's bundle sizes that were used to increase the overall transaction size should be considered. The following is the rough distribution of the transaction sizes:

• 2048 bytes or lower - 50% of bundles
• 6144 bytes or lower = 65% of all bundles
• 9216 bytes or lower = 100% of all bundles

In consideration of the above, the use cases mentioned within the motivation section, and the current page size for managing memory within a validator, a new transaction size limit of 4096 bytes is proposed. This new limit should cover a majority of use cases mentioned within the motivation section, as well as enable most of the developers using Jito bundles for larger transactions to migrate to the new transaction format. Similarly, since the new limit proposed can accommodate the max accounts used in ALTs directly in the transaction, developers are able to migrate from v0 transactions to v1 transactions.

A number of changes are required to be made to the validator to support the new transaction size limit. Consensus would need to be updated to support the larger transaction size, as larger transactions included in a block would be marked as invalid by the cluster today. The scheduler would also need to be modified to support the larger transaction sizes. While this proposal does not introduce a new fee structure around bytes in a transaction, the scheduler should prioritize larger transactions differently, requiring developers to pay a higher priority fee to land their transaction.

Testing for this new transaction size limit should be done extensively to ensure that performance on the cluster is not adversely affected. This testing should include the different use cases mentioned within the motivation section, as well as the different sizes of transactions that are currently being used by developers on the network.

Security Considerations

Larger transaction sizes could results in a number of bandwidth issues on the cluster than need to be tested thoroughly to ensure that performance is not adversely affected.

Drawbacks

There is an argument to be made that an increase in the transaction size limit above the max MTU packet size would incur some network overhead due to fragmentation retransmits. For a validator, this could mean maintaining in-memory buffers for larger transactions compared to just receiving and processing a single UDP packet. These issues will need to be addressed in the scheduler and how priority fees for larger transactions are handled.

Backwards Compatibility

As all of these changes are implemented with the v1 transaction format, the increase on the transaction size limit does not affect or break v0 or legacy transactions.