[bitcoin-dev] Proposal to Remove BIP35 P2P 'mempool' Message

[Will Clark <will@256k1.dev>]

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.


-- 


Cheers,
Will