-----------------------------------------------------------------------
  BIP: 37
  Title: Connection Bloom filtering
  Author: Mike Hearn <hearn@google.com>, Matt Corallo <bip@bluematt.me>
  Status: Accepted
  Type: Standards Track
  Created: 2012-10-24
-----------------------------------------------------------------------

[[abstract]]
Abstract
~~~~~~~~

This BIP adds new support to the peer-to-peer protocol that allows peers
to reduce the amount of transaction data they are sent. Peers have the
option of setting _filters_ on each connection they make after the
version handshake has completed. A filter is defined as a
http://en.wikipedia.org/wiki/Bloom_filter[Bloom filter] on data derived
from transactions. A Bloom filter is a probabilistic data structure
which allows for testing set membership - they can have false positives
but not false negatives.

This document will not go into the details of how Bloom filters work and
the reader is referred to Wikipedia for an introduction to the topic.

[[motivation]]
Motivation
~~~~~~~~~~

As Bitcoin grows in usage the amount of bandwidth needed to download
blocks and transaction broadcasts increases. Clients implementing
_simplified payment verification_ do not attempt to fully verify the
block chain, instead just checking that block headers connect together
correctly and trusting that the transactions in a chain of high
difficulty are in fact valid. See the Bitcoin paper for more detail on
this mode.

Today, link:Simplified_Payment_Verification[SPV] clients have to
download the entire contents of blocks and all broadcast transactions,
only to throw away the vast majority of the transactions that are not
relevant to their wallets. This slows down their synchronization
process, wastes users bandwidth (which on phones is often metered) and
increases memory usage. All three problems are triggering real user
complaints for the Android "Bitcoin Wallet" app which implements SPV
mode. In order to make chain synchronization fast, cheap and able to run
on older phones with limited memory we want to have remote peers throw
away irrelevant transactions before sending them across the network.

[[design-rationale]]
Design rationale
~~~~~~~~~~~~~~~~

The most obvious way to implement the stated goal would be for clients
to upload lists of their keys to the remote node. We take a more complex
approach for the following reasons:

* Privacy: Because Bloom filters are probabilistic, with the false
positive rate chosen by the client, nodes can trade off precision vs
bandwidth usage. A node with access to lots of bandwidth may choose to
have a high FP rate, meaning the remote peer cannot accurately know
which transactions belong to the client and which don't. A node with
very little bandwidth may choose to use a very accurate filter meaning
that they only get sent transactions actually relevant to their wallet,
but remote peers may be able to correlate transactions with IP addresses
(and each other).
* Bloom filters are compact and testing membership in them is fast. This
results in satisfying performance characteristics with minimal risk of
opening up potential for DoS attacks.

[[specification]]
Specification
~~~~~~~~~~~~~

[[new-messages]]
New messages
^^^^^^^^^^^^

We start by adding three new messages to the protocol:

* `filterload`, which sets the current Bloom filter on the connection
* `filteradd`, which adds the given data element to the connections
current filter without requiring a completely new one to be set
* `filterclear`, which deletes the current filter and goes back to
regular pre-BIP37 usage.

Note that there is no filterremove command because by their nature,
Bloom filters are append-only data structures. Once an element is added
it cannot be removed again without rebuilding the entire structure from
scratch.

The `filterload` command is defined as follows:

[cols=",,,",options="header",]
|=======================================================================
|Field Size |Description |Data type |Comments
|? |filter |uint8_t[] |The filter itself is simply a bit field of
arbitrary byte-aligned size. The maximum size is 36,000 bytes.

|4 |nHashFuncs |uint32_t |The number of hash functions to use in this
filter. The maximum value allowed in this field is 50.

|4 |nTweak |uint32_t |A random value to add to the seed value in the
hash function used by the bloom filter.

|1 |nFlags |uint8_t |A set of flags that control how matched items are
added to the filter.
|=======================================================================

See below for a description of the Bloom filter algorithm and how to
select nHashFuncs and filter size for a desired false positive rate.

Upon receiving a `filterload` command, the remote peer will immediately
restrict the broadcast transactions it announces (in inv packets) to
transactions matching the filter, where the matching algorithm is
specified below. The flags control the update behaviour of the matching
algorithm.

The `filteradd` command is defined as follows:

[cols=",,,",options="header",]
|==================================================================
|Field Size |Description |Data type |Comments
|? |data |uint8_t[] |The data element to add to the current filter.
|==================================================================

The data field must be smaller than or equal to 520 bytes in size (the
maximum size of any potentially matched object).

The given data element will be added to the Bloom filter. A filter must
have been previously provided using `filterload`. This command is useful
if a new key or script is added to a clients wallet whilst it has
connections to the network open, it avoids the need to re-calculate and
send an entirely new filter to every peer (though doing so is usually
advisable to maintain anonymity).

The `filterclear` command has no arguments at all.

After a filter has been set, nodes don't merely stop announcing
non-matching transactions, they can also serve filtered blocks. A
filtered block is defined by the `merkleblock` message and is defined
like this:

[cols=",,,",options="header",]
|=======================================================================
|Field Size |Description |Data type |Comments
|4 |version |uint32_t |Block version information, based upon the
software version creating this block

|32 |prev_block |char[32] |The hash value of the previous block this
particular block references

|32 |merkle_root |char[32] |The reference to a Merkle tree collection
which is a hash of all transactions related to this block

|4 |timestamp |uint32_t |A timestamp recording when this block was
created (Limited to 2106!)

|4 |bits |uint32_t |The calculated difficulty target being used for this
block

|4 |nonce |uint32_t |The nonce used to generate this block… to allow
variations of the header and compute different hashes

|4 |total_transactions |uint32_t |Number of transactions in the block
(including unmatched ones)

|? |hashes |uint256[] |hashes in depth-first order (including standard
varint size prefix)

|? |flags |byte[] |flag bits, packed per 8 in a byte, least significant
bit first (including standard varint size prefix)
|=======================================================================

See below for the format of the partial merkle tree hashes and flags.

Thus, a `merkleblock` message is a block header, plus a part of a merkle
tree which can be used to extract identifying information for
transactions that matched the filter and prove that the matching
transaction data really did appear in the solved block. Clients can use
this data to be sure that the remote node is not feeding them fake
transactions that never appeared in a real block, although lying through
omission is still possible.

[[extensions-to-existing-messages]]
Extensions to existing messages
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The `version` command is extended with a new field:

[cols=",,,",options="header",]
|=======================================================================
|Field Size |Description |Data type |Comments
|1 byte |fRelay |bool |If false then broadcast transactions will not be
announced until a filter\{load,add,clear} command is received. If
missing or true, no change in protocol behaviour occurs.
|=======================================================================

SPV clients that wish to use Bloom filtering would normally set fRelay
to false in the version message, then set a filter based on their wallet
(or a subset of it, if they are overlapping different peers). Being able
to opt-out of inv messages until the filter is set prevents a client
being flooded with traffic in the brief window of time between finishing
version handshaking and setting the filter.

The `getdata` command is extended to allow a new type in the `inv`
submessage. The type field can now be `MSG_FILTERED_BLOCK (== 3)` rather
than `MSG_BLOCK`. If no filter has been set on the connection, a request
for filtered blocks is ignored. If a filter has been set, a
`merkleblock` message is returned for the requested block hash. In
addition, because a `merkleblock` message contains only a list of
transaction hashes, transactions matching the filter should also be sent
in separate tx messages after the merkleblock is sent. This avoids a
slow roundtrip that would otherwise be required (receive hashes, didn't
see some of these transactions yet, ask for them). Note that because
there is currently no way to request transactions which are already in a
block from a node (aside from requesting the full block), the set of
matching transactions that the requesting node hasn't either received or
announced with an inv must be sent and any additional transactions which
match the filter may also be sent. This allows for clients (such as the
reference client) to limit the number of invs it must remember a given
node to have announced while still providing nodes with, at a minimum,
all the transactions it needs.

[[filter-matching-algorithm]]
Filter matching algorithm
^^^^^^^^^^^^^^^^^^^^^^^^^

The filter can be tested against arbitrary pieces of data, to see if
that data was inserted by the client. Therefore the question arises of
what pieces of data should be inserted/tested.

To determine if a transaction matches the filter, the following
algorithm is used. Once a match is found the algorithm aborts.

1.  Test the hash of the transaction itself.
2.  For each output, test each data element of the output script. This
means each hash and key in the output script is tested independently.
*Important:* if an output matches whilst testing a transaction, the node
might need to update the filter by inserting the serialized COutPoint
structure. See below for more details.
3.  For each input, test the serialized COutPoint structure.
4.  For each input, test each data element of the input script (note:
input scripts only ever contain data elements).
5.  Otherwise there is no match.

In this way addresses, keys and script hashes (for P2SH outputs) can all
be added to the filter. You can also match against classes of
transactions that are marked with well known data elements in either
inputs or outputs, for example, to implement various forms of
link:Smart property[Smart property].

The test for outpoints is there to ensure you can find transactions
spending outputs in your wallet, even though you don't know anything
about their form. As you can see, once set on a connection the filter is
*not static* and can change throughout the connections lifetime. This is
done to avoid the following race condition:

1.  A client sets a filter matching a key in their wallet. They then
start downloading the block chain. The part of the chain that the client
is missing is requested using getblocks.
2.  The first block is read from disk by the serving peer. It contains
TX 1 which sends money to the clients key. It matches the filter and is
thus sent to the client.
3.  The second block is read from disk by the serving peer. It contains
TX 2 which spends TX 1. However TX 2 does not contain any of the clients
keys and is thus not sent. The client does not know the money they
received was already spent.

By updating the bloom filter atomically in step 2 with the discovered
outpoint, the filter will match against TX 2 in step 3 and the client
will learn about all relevant transactions, despite that there is no
pause between the node processing the first and second blocks.

The nFlags field of the filter controls the nodes precise update
behaviour and is a bit field.

* `BLOOM_UPDATE_NONE (0)` means the filter is not adjusted when a match
is found.
* `BLOOM_UPDATE_ALL (1)` means if the filter matches any data element in
a scriptPubKey the outpoint is serialized and inserted into the filter.
* `BLOOM_UPDATE_P2PUBKEY_ONLY (2)` means the outpoint is inserted into
the filter only if a data element in the scriptPubKey is matched, and
that script is of the standard "pay to pubkey" or "pay to multisig"
forms.

These distinctions are useful to avoid too-rapid degradation of the
filter due to an increasing false positive rate. We can observe that a
wallet which expects to receive only payments of the standard
pay-to-address form doesn't need automatic filter updates because any
transaction that spends one of its own outputs has a predictable data
element in the input (the pubkey that hashes to the address). If a
wallet might receive pay-to-address outputs and also pay-to-pubkey or
pay-to-multisig outputs then BLOOM_UPDATE_P2PUBKEY_ONLY is appropriate,
as it avoids unnecessary expansions of the filter for the most common
types of output but still ensures correct behaviour with payments that
explicitly specify keys.

Obviously, nFlags == 1 or nFlags == 2 mean that the filter will get
dirtier as more of the chain is scanned. Clients should monitor the
observed false positive rate and periodically refresh the filter with a
clean one.

[[partial-merkle-branch-format]]
Partial Merkle branch format
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

A _Merkle tree_ is a way of arranging a set of items as leaf nodes of
tree in which the interior nodes are hashes of the concatenations of
their child hashes. The root node is called the _Merkle root_. Every
Bitcoin block contains a Merkle root of the tree formed from the blocks
transactions. By providing some elements of the trees interior nodes
(called a _Merkle branch_) a proof is formed that the given transaction
was indeed in the block when it was being mined, but the size of the
proof is much smaller than the size of the original block.

[[constructing-a-partial-merkle-tree-object]]
Constructing a partial merkle tree object
+++++++++++++++++++++++++++++++++++++++++

* Traverse the merkle tree from the root down, and for each encountered
node:
** Check whether this node corresponds to a leaf node (transaction) that
is to be included OR any parent thereof:
*** If so, append a '1' bit to the flag bits
*** Otherwise, append a '0' bit.
** Check whether this node is a internal node (non-leaf) AND is the
parent of an included leaf node:
*** If so:
**** Descend into its left child node, and process the subtree beneath
it entirely (depth-first).
**** If this node has a right child node too, descend into it as well.
*** Otherwise: append this node's hash to the hash list.

[[parsing-a-partial-merkle-tree-object]]
Parsing a partial merkle tree object
++++++++++++++++++++++++++++++++++++

As the partial block message contains the number of transactions in the
entire block, the shape of the merkle tree is known before hand. Again,
traverse this tree, computing traversed node's hashes along the way:

* Read a bit from the flag bit list:
** If it is '0':
*** Read a hash from the hashes list, and return it as this node's hash.
** If it is '1' and this is a leaf node:
*** Read a hash from the hashes list, store it as a matched txid, and
return it as this node's hash.
** If it is '1' and this is an internal node:
*** Descend into its left child tree, and store its computed hash as L.
*** If this node has a right child as well:
**** Descend into its right child, and store its computed hash as R.
**** If L == R, the partial merkle tree object is invalid.
**** Return Hash(L || R).
*** If this node has no right child, return Hash(L || L).

The partial merkle tree object is only valid if:

* All hashes in the hash list were consumed and no more.
* All bits in the flag bits list were consumed (except padding to make
it into a full byte), and no more.
* The hash computed for the root node matches the block header's merkle
root.
* The block header is valid, and matches its claimed proof of work.
* In two-child nodes, the hash of the left and right branches was never
equal.

[[bloom-filter-format]]
Bloom filter format
^^^^^^^^^^^^^^^^^^^

A Bloom filter is a bit-field in which bits are set based on feeding the
data element to a set of different hash functions. The number of hash
functions used is a parameter of the filter. In Bitcoin we use version 3
of the 32-bit Murmur hash function. To get N "different" hash functions
we simply initialize the Murmur algorithm with the following formula:

`nHashNum * 0xFBA4C795 + nTweak`

i.e. if the filter is initialized with 4 hash functions and a tweak of
0x00000005, when the second function (index 1) is needed h1 would be
equal to 4221880218.

When loading a filter with the `filterload` command, there are two
parameters that can be chosen. One is the size of the filter in bytes.
The other is the number of hash functions to use. To select the
parameters you can use the following formulas:

Let N be the number of elements you wish to insert into the set and P be
the probability of a false positive, where 1.0 is "match everything" and
zero is unachievable.

The size S of the filter in bytes is given by
`(-1 / pow(log(2), 2) * N * log(P)) / 8`. Of course you must ensure it
does not go over the maximum size (36,000: selected as it represents a
filter of 20,000 items with false positive rate of < 0.1% or 10,000
items and a false positive rate of < 0.0001%).

The number of hash functions required is given by `S * 8 / N * log(2)`.

[[copyright]]
Copyright
~~~~~~~~~

This document is placed in the public domain.
