Proposal to Remove BIP35 P2P 'mempool' Message

I'd like to discuss potential removal of the BIP35 P2P `mempool` message.

Supporting the BIP35 `mempool` message for arbitrary peers is bad for privacy while being relatively inefficient for trusted peers. Its original intention was to be publicly callable, but it is now (in Bitcoin Core) gated behind stricter Net Permissions which make it accessible to trusted peers only.

When serving trusted clients one alternative might be to use the `savemempool` RPC, which can then be loaded directly (in whole) by the client. This does currently have some shortcomings, namely that the client will lose the contents of their mempool in the process, so if they have different policies some transactions may be lost. Currently there is no way to load and de-duplicate a mempool dumped by this RPC into an existing mempool, although a PR has been opened to Bitcoin Core enabling this functionality so it may be available in the not-too-distant future: https://github.com/bitcoin/bitcoin/pull/27460

In my view dumping and loading a dumped mempool to sync two trusted nodes (or bootstrap one node) makes more sense via RPC as more transaction data can be included in the sync (e.g. transaction expiry time), but there is an argument to be made that syncing via P2P message would be more convenient.

N.B. that two (un-patched) bitcoin nodes cannot currently sync from each other using the `mempool` P2P message as there is no functionality to _send_ these messages, only to service them.

Removing this message would also provide an (albeit small) clean-up to the P2P codebase, bringing with it the usual benefits in terms of maintainability etc.

I have a draft PR open for the removal of the mempool message here: https://github.com/bitcoin/bitcoin/pull/27426

However, before moving forward, I want to ensure that there are no active use cases or technical opposition to its removal from readers of this list. To that end, I kindly request your input on the following questions:

1. Are there any parties who still directly rely on the BIP35 P2P `mempool` message? If so, please share your use case and any potential impact that the removal might have on your operations.

2. Do you foresee any issues or negative consequences resulting from the removal of the `mempool` message? If so, please elaborate on the potential problems and their severity.

From a quick search of node implementations I can see `btcd`, `libbitcoin` and `BitcoinJ` all have BIP35 messages specified, but I have not checked more deeply to see if they are using/servicing/ignoring them, and it's difficult to gauge upstream usage by other projects without outreach like this...

I look forward to hearing your thoughts.

> When serving trusted clients one alternative might be to use the
> `savemempool` RPC, which can then be loaded directly (in whole) by the
> client.

It was common in the past for lightweight clients to load a BIP37 filter and then send a `getdata` for requesting `mempool`. In that case, the node would filter the mempool transactions and only send transactions matching the filter to the client (plus false positives, which the client could choose to keep very low).

The above approach minimized the amount of data that needed to be transferred, which can be very important for lite clients on metered or bandwidth-limited connections---especially considering that lite clients on poor connections (e.g. mobile) might get disconnected frequently and so need to re-request the filtered mempool every time they reconnect to acquire any new unconfirmed transactions that arrived while they were disconnected.

By comparison, during a period of backlog (the natural state, we hope), the mempool contents in the `savemempool` format are about 300 MB. I think that's a bit much to potentially be sending to lite clients just so they can learn about any unconfirmed transactions which arrived since they last connected.

Although I understand and support the desire to remove problematic public interfaces like BIP37 and BIP35, I think we should also be aiming to build interfaces which make it easier for people to use third-party wallets with their own trusted nodes. Right now, it's possible to use[*] `getheaders`, BIP157/8, and `getdata(block)` with your own node to learn about all confirmed transactions affecting your wallet. It's also possible now to use BIP37 and BIP35 to get unconfirmed transactions in a bandwidth-efficient manner, if your connection is allowlisted.

I would personally like to see lite clients that use a trusted node receive a replacement for BIP35/7 support before those protocols are removed. (Of course, I'd also like to see support for BIP324 and for something like countersign so that authenticated and encrypted connections from a lite client to a trusted node are easy to setup.)

Thanks,

-Dave

[*]: Requires an authenticated connection to be secure (and should
      ideally be encrypted).

Hi Will,

I shared some numbers and feedback as comment [0] on your PR wanted to answer question 1. for completeness here too.

> Its original intention was to be publicly callable, but it is now (in

Bitcoin Core) gated behind stricter Net Permissions which make it accessible to trusted peers only.

Bitcoin Core does only gate processing of mempool messages on NetPermissionFlags::Mempool when bloom filters are disabled [1]. While these are disabled by default, more than 20% (see PR comment) of nodes on the network have bloom filters enabled. These nodes all respond to mempool messages with INV messages.

> 1. Are there any parties who still directly rely on the BIP35 P2P

`mempool` message?

I've been receiving on average about 20 mempool messages per hour to a well-connected NODE_BLOOM Bitcoin Core node. I've seen multiple messages from the user agent /BitcoinKit:0.1.0/, /bitcoinj:0.*.*/Bitcoin Wallet:*/, /WalletKit:0.1.0/, and /bread:2.1/. Similarly, the node responds to the clients with INVs up to the max number of 50k entries and with smaller (bloom) filtered INVs.

0xB10C

[0]: https://github.com/bitcoin/bitcoin/pull/27426#issuecomment-1529678174 [1]: https://github.com/bitcoin/bitcoin/blob/d89aca1bdbe52406f000e3fa8dda12c46dca9bdd/src/net_processing.cpp#LL4603C52-L4603