[bitcoin-dev] [BIP Proposal] New "feefilter" p2p message

Alex Morcos morcos at gmail.com
Tue Feb 16 20:20:26 UTC 2016


Hi,

I'm proposing the addition of a new optional p2p message to help reduce
unnecessary network traffic.  The draft BIP is available here and pasted
below:
https://gist.github.com/morcos/9aab223c443c9258c979

The goal of this message is to take advantage of the fact that when a node
has reached its mempool limit, there is a minimum fee below which no
transactions are accepted to the mempool.  Informing peers of this minimum
would save them inv'ing your node for those transaction id's and save your
node requesting them if they are not in your recentRejects filter.

This message is optional and may be ignored as a protocol rule.  There is
also an option to turn off sending the messages in the implementation.

Thanks to Suhas Daftuar, Greg Maxwell, and others for helping develop the
idea.

-Alex

Draft BIP text:

<pre>
  BIP: <unassigned>
  Title: feefilter message
  Author: Alex Morcos <morcos at chaincode.com>
  Status: Draft
  Type: Standards Track
  Created: 2016-02-13
</pre>

==Abstract==

Add a new message, "feefilter", which serves to instruct peers not to send
"inv"'s to the node for transactions with fees below the specified fee rate.

==Motivation==

The concept of a limited mempool was introduced in Bitcoin Core 0.12 to
provide protection against attacks or spam transactions of low fees that
are not being mined. A reject filter was also introduced to help prevent
repeated requests for the same transaction that might have been recently
rejected for insufficient fee. These methods help keep resource utilization
on a node from getting out of control.

However, there are limitations to the effectiveness of these approaches.
The reject filter is reset after every block which means transactions that
are inv'ed over a longer time period will be rerequested and there is no
method to prevent requesting the transaction the first time.  Furthermore,
inv data is sent at least once either to or from each peer for every
transaction accepted to the mempool and there is no mechanism by which to
know that an inv sent to a given peer would not result in a getdata request
because it represents a transaction with too little fee.

After receiving a feefilter message, a node can know before sending an inv
that a given transaction's fee rate is below the minimum currently required
by a given peer, and therefore the node can skip relaying an inv for that
transaction to that peer.

==Specification==

# The feefilter message is defined as a message containing an int64_t where
pchCommand == "feefilter"
# Upon receipt of a "feefilter" message, the node will be permitted, but
not required, to filter transaction invs for transactions that fall below
the feerate provided in the feefilter message interpreted as satoshis per
kilobyte.
# The fee filter is additive with a bloom filter for transactions so if an
SPV client were to load a bloom filter and send a feefilter message,
transactions would only be relayed if they passed both filters.
# Inv's generated from a mempool message are also subject to a fee filter
if it exists.
# Feature discovery is enabled by checking protocol version >= 70013

==Considerations==
The propagation efficiency of transactions across the network should not be
adversely affected by this change. In general, transactions which are not
accepted to your mempool are not relayed and the funcionality implemented
with this message is meant only to filter those transactions.  There could
be a small number of edge cases where a node's mempool min fee is actually
less than the filter value a peer is aware of and transactions with fee
rates between these values will now be newly inhibited.

Feefilter messages are not sent to whitelisted peers if the
"-whitelistforcerelay" option is set. In that case, transactions are
intended to be relayed even if they are not accepted to the mempool.

There are privacy concerns with deanonymizing a node by the fact that it is
broadcasting identifying information about its mempool min fee. To help
ameliorate this concern, the implementaion quantizes the filter value
broadcast with a small amount of randomness, in addition, the messages are
broadcast to different peers at individually randomly distributed times.

If a node is using prioritisetransaction to accept transactions whose
actual fee rates might fall below the node's mempool min fee, it may want
to consider setting "-nofeefilter" to make sure it is exposed to all
possible txid's.

==Backward compatibility==

Older clients remain fully compatible and interoperable after this change.
The sending of feefilter messages can be disabled by unsetting the
"-feefilter" option.

==Implementation==

https://github.com/bitcoin/bitcoin/pull/7542
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