Subject: Re: [bitcoin-dev] Revisiting NODE_BLOOM: Proposed BIP

From: Matt Corallo <[email-masked]>
Date: Fri, 21 Aug 2015 17:55:58 +0000

Revised copy follows. re: mentioning the HTTP seeding stuff, I'm not
sure we want to encourage more people aside from bitcoinj to use
that...I thought about adding a DNS seed section to this bip, but
decided against it...still, I think we should add the option to select
service bits to DNS seeds ASAP.

Re: need to "shard" the blockchain: not sure what you're referring to
here. The bloom filter stuff requires you to download the chain
in-order, sure, but you have to do that for headers anyway, and
hopefully your total data isnt too much more than headers alone.

Anyone have the best reference for the DoS issues?

BIP: ?
Title: NODE_BLOOM service bit
Author: Matt Corallo <[email-masked]>, Peter Todd <[email-masked]>
Type: Standards Track (draft)
Created: 20-08-2015

Abstract
========

This BIP extends BIP 37, Connection Bloom filtering, by defining a
service bit to allow peers to advertise that they support bloom filters
explicitly. It also bumps the protocol version to allow peers to
identify old nodes which allow bloom filtering of the connection despite
lacking the new service bit.

Motivation
==========

BIP 37 did not specify a service bit for the bloom filter service, thus
implicitly assuming that all nodes that serve peers data support it.
However, the connection filtering algorithm proposed in BIP 37, and
implemented in several clients today, has been shown to provide little
to no privacy[1], as well as being a large DoS risk on some nodes[2].
Thus, allowing node operators to disable connection bloom filtering is a
much-needed feature.

Specification
=============

The following protocol bit is added:

    NODE_BLOOM = (1 << 2)

Nodes which support bloom filters should set that protocol bit.
Otherwise it should remain unset. In addition the protocol version is
increased from 70002 to 70011 in the reference implementation. It is
often the case that nodes which have a protocol version smaller than
70011, but larger than 70000 support bloom filtered connections without
the NODE_BLOOM bit set, however clients which require bloom filtered
connections should avoid making this assumption.

NODE_BLOOM is distinct from NODE_NETWORK, and it is legal to advertise
NODE_BLOOM but not NODE_NETWORK (eg for nodes running in pruned mode
which, nonetheless, provide filtered access to the data which they do have).

If a node does not support bloom filters but receives a "filterload",
"filteradd", or "filterclear" message from a peer the node should
disconnect that peer immediately. For backwards compatibility, in
initial implementations, nodes may choose to only disconnect nodes which
have the new protocol version set and attempt to send a filter command.

While outside the scope of this BIP it is suggested that DNS seeds and
other peer discovery mechanisms support the ability to specify the
services required; current implementations simply check only that
NODE_NETWORK is set.

Design rational
===============

A service bit was chosen as applying a bloom filter is a service.

The increase in protocol version is for backwards compatibility. In
initial implementations, old nodes which are not yet aware of NODE_BLOOM
and use a protocol version < 70011 may still send filter* messages to a
node without NODE_BLOOM. This feature may be removed after there are
sufficient NODE_BLOOM nodes available and SPV clients have upgraded,
allowing node operators to fully close the bloom-related DoS vectors.

Reference Implementation
========================

https://github.com/bitcoin/bitcoin/pull/6579

Copyright
=========

This document is placed in the public domain.

References
==========

[1] http://eprint.iacr.org/2014/763
[2] ???? is one example where the issues were found, though others
independently discovered issues as well. Sample DoS exploit code
available at https://github.com/petertodd/bloom-io-attack.

On 08/21/15 05:42, Peter Todd wrote:
> On Thu, Aug 20, 2015 at 10:38:19PM -0700, Peter Todd via bitcoin-dev wrote:
>>> Motivation
>>> ==========
>>>
>>> BIP 37 did not specify a service bit for the bloom filter service, thus
>>> implicitly assuming that all nodes that serve peers data support it.
>>> However, the connection filtering algorithm proposed in BIP 37, and
>>> implemented in several clients today, has been shown to provide little
>>> to no privacy, as well as being a large DoS risk on some nodes. Thus,
>>> allowing node operators to disable connection bloom filtering is a
>>> much-needed feature.
>>
>> I'd reference that paper on bloom filters re: the "little to no privacy"
>> issue. There's also a post in the bitcoinj mailing list somewhere IIRC
>> talking about the default settings, and how they don't provide any
>> privacy.
> 
> Oh, and we should also point out that Bloom filters have scaling issues,
> as each application of the filter has to scan the whole blockchain -
> with future blocksize increases these issues increase, in some proposals
> quite dramatically. The underlying idea also conflicts with some
> proposals to "shard" the blockchain, again suggesting that we need a bit
> to handle future upgrades to more scalable designs.
>