P2P Network

This section describes the Bitcoin P2P network protocol (but it is not a specification). It does not describe the discontinued direct IP-to-IP payment protocol, the BIP70 payment protocol, the GetBlockTemplate mining protocol, or any network protocol never implemented in an official version of Bitcoin Core.

All peer-to-peer communication occurs entirely over TCP.

Note: unless their description says otherwise, all multi-byte integers mentioned in this section are transmitted in little-endian order.

Constants And Defaults

The following constants and defaults are taken from Bitcoin Core’s chainparams.cpp source code file.

Network Default Port Start String Max nBits
Mainnet 8333 0xf9beb4d9 0x1d00ffff
Testnet 18333 0x0b110907 0x1d00ffff
Regtest 18444 0xfabfb5da 0x207fffff

Note: the testnet start string and nBits above are for testnet3; the original testnet used a different string and higher (less difficult) nBits.

Command line parameters can change what port a node listens on (see -help). Start strings are hardcoded constants that appear at the start of all messages sent on the Bitcoin network; they may also appear in data files such as Bitcoin Core’s block database. The nBits displayed above are in big-endian order; they’re sent over the network in little-endian order.

Bitcoin Core’s chainparams.cpp also includes other constants useful to programs, such as the hash of the genesis blocks for the different networks as well as the alert keys for mainnet and testnet.

Protocol Versions

The table below lists some notable versions of the P2P network protocol, with the most recent versions listed first. (If you know of a protocol version that implemented a major change but which is not listed here, please open an issue.)

As of Bitcoin Core 0.12.0, the most recent protocol version is 70012.

Version Initial Release Major Changes
70012 Bitcoin Core 0.12.0
BIP130:
• Added sendheaders message
70002 Bitcoin Core 0.9.0
(Mar 2014)
• Send multiple inv messages in response to a mempool message if necessary

BIP61:
• Added reject message
70001 Bitcoin Core 0.8.0
(Feb 2013)
• Added notfound message.

BIP37:
• Added filterload message.
• Added filteradd message.
• Added filterclear message.
• Added merkleblock message.
• Added relay field to version message
• Added MSG_FILTERED_BLOCK inventory type to getdata message.
60002 Bitcoin Core 0.7.0
(Sep 2012)
BIP35:
• Added mempool message.
• Extended getdata message to allow download of memory pool transactions
60001 Bitcoin Core 0.6.1
(May 2012)
BIP31:
• Added nonce field to ping message
• Added pong message
60000 Bitcoin Core 0.6.0
(Mar 2012)
BIP14:
• Separated protocol version from Bitcoin Core version
31800 Bitcoin Core 0.3.18
(Dec 2010)
• Added getheaders message and headers message.
31402 Bitcoin Core 0.3.15
(Oct 2010)
• Added time field to addr message.
311 Bitcoin Core 0.3.11
(Aug 2010)
• Added alert message.
209 Bitcoin Core 0.2.9
(May 2010)
• Added checksum field to message headers.
106 Bitcoin Core 0.1.6
(Oct 2009)
• Added receive IP address fields to version message.

Message Headers

All messages in the network protocol use the same container format, which provides a required multi-field message header and an optional payload. The message header format is:

Bytes Name Data Type Description
4 start string char[4] Magic bytes indicating the originating network; used to seek to next message when stream state is unknown.
12 command name char[12] ASCII string which identifies what message type is contained in the payload. Followed by nulls (0x00) to pad out byte count; for example: version\0\0\0\0\0.
4 payload size uint32_t Number of bytes in payload. The current maximum number of bytes (MAX_SIZE) allowed in the payload by Bitcoin Core is 32 MiB—messages with a payload size larger than this will be dropped or rejected.
4 checksum char[4] Added in protocol version 209.

First 4 bytes of SHA256(SHA256(payload)) in internal byte order.

If payload is empty, as in verack and getaddr messages, the checksum is always 0x5df6e0e2 (SHA256(SHA256(<empty string>))).

The following example is an annotated hex dump of a mainnet message header from a verack message which has no payload.

f9beb4d9 ................... Start string: Mainnet
76657261636b000000000000 ... Command name: verack + null padding
00000000 ................... Byte count: 0
5df6e0e2 ................... Checksum: SHA256(SHA256(<empty>))

Data Messages

The following network messages all request or provide data related to transactions and blocks.

Overview Of P2P Protocol Data Request And Reply Messages

Many of the data messages use inventories as unique identifiers for transactions and blocks. Inventories have a simple 36-byte structure:

Bytes Name Data Type Description
4 type identifier uint32_t The type of object which was hashed. See list of type identifiers below.
32 hash char[32] SHA256(SHA256()) hash of the object in internal byte order.

The currently-available type identifiers are:

Type Identifier Name Description
1 MSG_TX The hash is a TXID.
2 MSG_BLOCK The hash is of a block header.
3 MSG_FILTERED_BLOCK The hash is of a block header; identical to MSG_BLOCK. When used in a getdata message, this indicates the response should be a merkleblock message rather than a block message (but this only works if a bloom filter was previously configured). Only for use in getdata messages.

Type identifier zero and type identifiers greater than three are reserved for future implementations. Bitcoin Core ignores all inventories with one of these unknown types.

Block

The block message transmits a single serialized block in the format described in the serialized blocks section. See that section for an example hexdump. It can be sent for two different reasons:

  1. GetData Response: Nodes will always send it in response to a getdata message that requests the block with an inventory type of MSG_BLOCK (provided the node has that block available for relay).

  2. Unsolicited: Some miners will send unsolicited block messages broadcasting their newly-mined blocks to all of their peers. Many mining pools do the same thing, although some may be misconfigured to send the block from multiple nodes, possibly sending the same block to some peers more than once.

GetBlocks

The getblocks message requests an inv message that provides block header hashes starting from a particular point in the block chain. It allows a peer which has been disconnected or started for the first time to get the data it needs to request the blocks it hasn’t seen.

Peers which have been disconnected may have stale blocks in their locally-stored block chain, so the getblocks message allows the requesting peer to provide the receiving peer with multiple header hashes at various heights on their local chain. This allows the receiving peer to find, within that list, the last header hash they had in common and reply with all subsequent header hashes.

Note: the receiving peer itself may respond with an inv message containing header hashes of stale blocks. It is up to the requesting peer to poll all of its peers to find the best block chain.

If the receiving peer does not find a common header hash within the list, it will assume the last common block was the genesis block (block zero), so it will reply with in inv message containing header hashes starting with block one (the first block after the genesis block).

Bytes Name Data Type Description
4 version uint32_t The protocol version number; the same as sent in the version message.
Varies hash count compactSize uint The number of header hashes provided not including the stop hash. There is no limit except that the byte size of the entire message must be below the MAX_SIZE limit; typically from 1 to 200 hashes are sent.
Varies block header hashes char[32] One or more block header hashes (32 bytes each) in internal byte order. Hashes should be provided in reverse order of block height, so highest-height hashes are listed first and lowest-height hashes are listed last.
32 stop hash char[32] The header hash of the last header hash being requested; set to all zeroes to request an inv message with all subsequent header hashes (a maximum of 500 will be sent as a reply to this message; if you need more than 500, you will need to send another getblocks message with a higher-height header hash as the first entry in block header hash field).

The following annotated hexdump shows a getblocks message. (The message header has been omitted.)

71110100 ........................... Protocol version: 70001
02 ................................. Hash count: 2

d39f608a7775b537729884d4e6633bb2
105e55a16a14d31b0000000000000000 ... Hash #1

5c3e6403d40837110a2e8afb602b1c01
714bda7ce23bea0a0000000000000000 ... Hash #2

00000000000000000000000000000000
00000000000000000000000000000000 ... Stop hash

GetData

The getdata message requests one or more data objects from another node. The objects are requested by an inventory, which the requesting node typically previously received by way of an inv message.

The response to a getdata message can be a tx message, block message, merkleblock message, or notfound message.

This message cannot be used to request arbitrary data, such as historic transactions no longer in the memory pool or relay set. Full nodes may not even be able to provide older blocks if they’ve pruned old transactions from their block database. For this reason, the getdata message should usually only be used to request data from a node which previously advertised it had that data by sending an inv message.

The format and maximum size limitations of the getdata message are identical to the inv message; only the message header differs.

GetHeaders

Added in protocol version 31800.

The getheaders message requests a headers message that provides block headers starting from a particular point in the block chain. It allows a peer which has been disconnected or started for the first time to get the headers it hasn’t seen yet.

The getheaders message is nearly identical to the getblocks message, with one minor difference: the inv reply to the getblocks message will include no more than 500 block header hashes; the headers reply to the getheaders message will include as many as 2,000 block headers.

Headers

Added in protocol version 31800.

The headers message sends one or more block headers to a node which previously requested certain headers with a getheaders message.

Bytes Name Data Type Description
Varies count compactSize uint Number of block headers up to a maximum of 2,000. Note: headers-first sync assumes the sending node will send the maximum number of headers whenever possible.
Varies headers block_header Block headers: each 80-byte block header is in the format described in the block headers section with an additional 0x00 suffixed. This 0x00 is called the transaction count, but because the headers message doesn’t include any transactions, the transaction count is always zero.

The following annotated hexdump shows a headers message. (The message header has been omitted.)

01 ................................. Header count: 1

02000000 ........................... Block version: 2
b6ff0b1b1680a2862a30ca44d346d9e8
910d334beb48ca0c0000000000000000 ... Hash of previous block's header
9d10aa52ee949386ca9385695f04ede2
70dda20810decd12bc9b048aaab31471 ... Merkle root
24d95a54 ........................... Unix time: 1415239972
30c31b18 ........................... Target (bits)
fe9f0864 ........................... Nonce

00 ................................. Transaction count (0x00)

Inv

The inv message (inventory message) transmits one or more inventories of objects known to the transmitting peer. It can be sent unsolicited to announce new transactions or blocks, or it can be sent in reply to a getblocks message or mempool message.

The receiving peer can compare the inventories from an inv message against the inventories it has already seen, and then use a follow-up message to request unseen objects.

Bytes Name Data Type Description
Varies count compactSize uint The number of inventory entries.
Varies inventory inventory One or more inventory entries up to a maximum of 50,000 entries.

The following annotated hexdump shows an inv message with two inventory entries. (The message header has been omitted.)

02 ................................. Count: 2

01000000 ........................... Type: MSG_TX
de55ffd709ac1f5dc509a0925d0b1fc4
42ca034f224732e429081da1b621f55a ... Hash (TXID)

01000000 ........................... Type: MSG_TX
91d36d997037e08018262978766f24b8
a055aaf1d872e94ae85e9817b2c68dc7 ... Hash (TXID)

MemPool

Added in protocol version 60002.

The mempool message requests the TXIDs of transactions that the receiving node has verified as valid but which have not yet appeared in a block. That is, transactions which are in the receiving node’s memory pool. The response to the mempool message is one or more inv messages containing the TXIDs in the usual inventory format.

Sending the mempool message is mostly useful when a program first connects to the network. Full nodes can use it to quickly gather most or all of the unconfirmed transactions available on the network; this is especially useful for miners trying to gather transactions for their transaction fees. SPV clients can set a filter before sending a mempool to only receive transactions that match that filter; this allows a recently-started client to get most or all unconfirmed transactions related to its wallet.

The inv response to the mempool message is, at best, one node’s view of the network—not a complete list of unconfirmed transactions on the network. Here are some additional reasons the list might not be complete:

There is no payload in a mempool message. See the message header section for an example of a message without a payload.

MerkleBlock

Added in protocol version 70001 as described by BIP37.

The merkleblock message is a reply to a getdata message which requested a block using the inventory type MSG_MERKLEBLOCK. It is only part of the reply: if any matching transactions are found, they will be sent separately as tx messages.

If a filter has been previously set with the filterload message, the merkleblock message will contain the TXIDs of any transactions in the requested block that matched the filter, as well as any parts of the block’s merkle tree necessary to connect those transactions to the block header’s merkle root. The message also contains a complete copy of the block header to allow the client to hash it and confirm its proof of work.

Bytes Name Data Type Description
80 block header block_header The block header in the format described in the block header section.
4 transaction count uint32_t The number of transactions in the block (including ones that don’t match the filter).
Varies hash count compactSize uint The number of hashes in the following field.
Varies hashes char[32] One or more hashes of both transactions and merkle nodes in internal byte order. Each hash is 32 bytes.
Varies flag byte count compactSize uint The number of flag bytes in the following field.
Varies flags byte[] A sequence of bits packed eight in a byte with the least significant bit first. May be padded to the nearest byte boundary but must not contain any more bits than that. Used to assign the hashes to particular nodes in the merkle tree as described below.

The annotated hexdump below shows a merkleblock message which corresponds to the examples below. (The message header has been omitted.)

01000000 ........................... Block version: 1
82bb869cf3a793432a66e826e05a6fc3
7469f8efb7421dc88067010000000000 ... Hash of previous block's header
7f16c5962e8bd963659c793ce370d95f
093bc7e367117b3c30c1f8fdd0d97287 ... Merkle root
76381b4d ........................... Time: 1293629558
4c86041b ........................... nBits: 0x04864c * 256**(0x1b-3)
554b8529 ........................... Nonce

07000000 ........................... Transaction count: 7
04 ................................. Hash count: 4

3612262624047ee87660be1a707519a4
43b1c1ce3d248cbfc6c15870f6c5daa2 ... Hash #1
019f5b01d4195ecbc9398fbf3c3b1fa9
bb3183301d7a1fb3bd174fcfa40a2b65 ... Hash #2
41ed70551dd7e841883ab8f0b16bf041
76b7d1480e4f0af9f3d4c3595768d068 ... Hash #3
20d2a7bc994987302e5b1ac80fc425fe
25f8b63169ea78e68fbaaefa59379bbf ... Hash #4

01 ................................. Flag bytes: 1
1d ................................. Flags: 1 0 1 1 1 0 0 0

Note: when fully decoded, the above merkleblock message provided the TXID for a single transaction that matched the filter. In the network traffic dump this output was taken from, the full transaction belonging to that TXID was sent immediately after the merkleblock message as a tx message.

Parsing A MerkleBlock Message

As seen in the annotated hexdump above, the merkleblock message provides three special data types: a transaction count, a list of hashes, and a list of one-bit flags.

You can use the transaction count to construct an empty merkle tree. We’ll call each entry in the tree a node; on the bottom are TXID nodes—the hashes for these nodes are TXIDs; the remaining nodes (including the merkle root) are non-TXID nodes—they may actually have the same hash as a TXID, but we treat them differently.

Example Of Parsing A MerkleBlock Message

Keep the hashes and flags in the order they appear in the merkleblock message. When we say “next flag” or “next hash”, we mean the next flag or hash on the list, even if it’s the first one we’ve used so far.

Start with the merkle root node and the first flag. The table below describes how to evaluate a flag based on whether the node being processed is a TXID node or a non-TXID node. Once you apply a flag to a node, never apply another flag to that same node or reuse that same flag again.

Flag TXID Node Non-TXID Node
0 Use the next hash as this node’s TXID, but this transaction didn’t match the filter. Use the next hash as this node’s hash. Don’t process any descendant nodes.
1 Use the next hash as this node’s TXID, and mark this transaction as matching the filter. The hash needs to be computed. Process the left child node to get its hash; process the right child node to get its hash; then concatenate the two hashes as 64 raw bytes and hash them to get this node’s hash.

Any time you begin processing a node for the first time, evaluate the next flag. Never use a flag at any other time.

When processing a child node, you may need to process its children (the grandchildren of the original node) or further-descended nodes before returning to the parent node. This is expected—keep processing depth first until you reach a TXID node or a non-TXID node with a flag of 0.

After you process a TXID node or a non-TXID node with a flag of 0, stop processing flags and begin to ascend the tree. As you ascend, compute the hash of any nodes for which you now have both child hashes or for which you now have the sole child hash. See the merkle tree section for hashing instructions. If you reach a node where only the left hash is known, descend into its right child (if present) and further descendants as necessary.

However, if you find a node whose left and right children both have the same hash, fail. This is related to CVE-2012-2459.

Continue descending and ascending until you have enough information to obtain the hash of the merkle root node. If you run out of flags or hashes before that condition is reached, fail. Then perform the following checks (order doesn’t matter):

For a detailed example of parsing a merkleblock message, please see the corresponding merkle block examples section.

Creating A MerkleBlock Message

It’s easier to understand how to create a merkleblock message after you understand how to parse an already-created message, so we recommend you read the parsing section above first.

Create a complete merkle tree with TXIDs on the bottom row and all the other hashes calculated up to the merkle root on the top row. For each transaction that matches the filter, track its TXID node and all of its ancestor nodes.

Example Of Creating A MerkleBlock Message

Start processing the tree with the merkle root node. The table below describes how to process both TXID nodes and non-TXID nodes based on whether the node is a match, a match ancestor, or neither a match nor a match ancestor.

  TXID Node Non-TXID Node
Neither Match Nor Match Ancestor Append a 0 to the flag list; append this node’s TXID to the hash list. Append a 0 to the flag list; append this node’s hash to the hash list. Do not descend into its child nodes.
Match Or Match Ancestor Append a 1 to the flag list; append this node’s TXID to the hash list. Append a 1 to the flag list; process the left child node. Then, if the node has a right child, process the right child. Do not append a hash to the hash list for this node.

Any time you begin processing a node for the first time, a flag should be appended to the flag list. Never put a flag on the list at any other time, except when processing is complete to pad out the flag list to a byte boundary.

When processing a child node, you may need to process its children (the grandchildren of the original node) or further-descended nodes before returning to the parent node. This is expected—keep processing depth first until you reach a TXID node or a node which is neither a TXID nor a match ancestor.

After you process a TXID node or a node which is neither a TXID nor a match ancestor, stop processing and begin to ascend the tree until you find a node with a right child you haven’t processed yet. Descend into that right child and process it.

After you fully process the merkle root node according to the instructions in the table above, processing is complete. Pad your flag list to a byte boundary and construct the merkleblock message using the template near the beginning of this subsection.

NotFound

Added in protocol version 70001.

The notfound message is a reply to a getdata message which requested an object the receiving node does not have available for relay. (Nodes are not expected to relay historic transactions which are no longer in the memory pool or relay set. Nodes may also have pruned spent transactions from older blocks, making them unable to send those blocks.)

The format and maximum size limitations of the notfound message are identical to the inv message; only the message header differs.

Tx

The tx message transmits a single transaction in the raw transaction format. It can be sent in a variety of situations;

For an example hexdump of the raw transaction format, see the raw transaction section.

Control Messages

The following network messages all help control the connection between two peers or allow them to advise each other about the rest of the network.

Overview Of P2P Protocol Control And Advisory Messages

Note that almost none of the control messages are authenticated in any way, meaning they can contain incorrect or intentionally harmful information. In addition, this section does not yet cover P2P protocol operation over the Tor network; if you would like to contribute information about Tor, please open an issue.

Addr

The addr (IP address) message relays connection information for peers on the network. Each peer which wants to accept incoming connections creates an addr message providing its connection information and then sends that message to its peers unsolicited. Some of its peers send that information to their peers (also unsolicited), some of which further distribute it, allowing decentralized peer discovery for any program already on the network.

An addr message may also be sent in response to a getaddr message.

Bytes Name Data Type Description
Varies IP address count compactSize uint The number of IP address entries up to a maximum of 1,000.
Varies IP addresses network IP address IP address entries. See the table below for the format of a Bitcoin network IP address.

Each encapsulated network IP address currently uses the following structure:

Bytes Name Data Type Description
4 time uint32 Added in protocol version 31402.

A time in Unix epoch time format. Nodes advertising their own IP address set this to the current time. Nodes advertising IP addresses they’ve connected to set this to the last time they connected to that node. Other nodes just relaying the IP address should not change the time. Nodes can use the time field to avoid relaying old addr messages.

Malicious nodes may change times or even set them in the future.
8 services uint64_t The services the node advertised in its version message.
16 IP address char IPv6 address in big endian byte order. IPv4 addresses can be provided as IPv4-mapped IPv6 addresses
2 port uint16_t Port number in big endian byte order. Note that Bitcoin Core will only connect to nodes with non-standard port numbers as a last resort for finding peers. This is to prevent anyone from trying to use the network to disrupt non-Bitcoin services that run on other ports.

The following annotated hexdump shows part of an addr message. (The message header has been omitted and the actual IP address has been replaced with a RFC5737 reserved IP address.)

fde803 ............................. Address count: 1000

d91f4854 ........................... Epoch time: 1414012889
0100000000000000 ................... Service bits: 01 (network node)
00000000000000000000ffffc0000233 ... IP Address: ::ffff:192.0.2.51
208d ............................... Port: 8333

[...] .............................. (999 more addresses omitted)

Alert

Added in protocol version 311.

The alert message warns nodes of problems that may affect them or the rest of the network. Each alert message is signed using a key controlled by respected community members, mostly Bitcoin Core developers.

To ensure all nodes can validate and forward alert messages, encapsulation is used. Developers create an alert using the data structure appropriate for the versions of the software they want to notify; then they serialize that data and sign it. The serialized data and its signature make up the outer alert message—allowing nodes which don’t understand the data structure to validate the signature and relay the alert to nodes which do understand it. The nodes which actually need the message can decode the serialized data to access the inner alert message.

The outer alert message has four fields:

Bytes Name Data Type Description
Variable alert bytes compactSize uint The number of bytes in following alert field.
Variable alert uchar The serialized alert. See below for a description of the current alert format.
Variable signature bytes compactSize uint The number of bytes in the following signature field.
Variable signature uchar A DER-encoded ECDSA (secp256k1) signature of the alert signed with the developer’s alert key.

Although designed to be easily upgraded, the format of the inner serialized alert has not changed since the alert message was first introduced in protocol version 311.

Bytes Name Data Type Description
4 version int32_t Alert format version. Version 1 from protocol version 311 through at least protocol version 70002.
8 relayUntil int64_t The time beyond which nodes should stop relaying this alert. Unix epoch time format.
8 expiration int64_t The time beyond which this alert is no longer in effect and should be ignored. Unix epoch time format.
4 ID int32_t A unique ID number for this alert.
4 cancel int32_t All alerts with an ID number less than or equal to this number should be canceled: deleted and not accepted in the future.
Varies setCancel count compactSize uint The number of IDs in the following setCancel field. May be zero.
Varies setCancel int32_t Alert IDs which should be canceled. Each alert ID is a separate int32_t number.
4 minVer int32_t This alert only applies to protocol versions greater than or equal to this version. Nodes running other protocol versions should still relay it.
4 maxVer int32_t This alert only applies to protocol versions less than or equal to this version. Nodes running other protocol versions should still relay it.
Varies user_agent count compactSize uint The number of user agent strings in the following setUser_agent field. May be zero.
Varies setUser_agent compactSize uint + string If this field is empty, it has no effect on the alert. If there is at least one entry is this field, this alert only applies to programs with a user agent that exactly matches one of the strings in this field. Each entry in this field is a compactSize uint followed by a string—the uint indicates how many bytes are in the following string. This field was originally called setSubVer; since BIP14, it applies to user agent strings as defined in the version message.
4 priority int32_t Relative priority compared to other alerts.
Varies comment bytes compactSize uint The number of bytes in the following comment field. May be zero.
Varies comment string A comment on the alert that is not displayed.
Varies statusBar bytes compactSize uint The number of bytes in the following statusBar field. May be zero.
Varies statusBar string The alert message that is displayed to the user.
Varies reserved bytes compactSize uint The number of bytes in the following reserved field. May be zero.
Varies reserved string Reserved for future use. Originally called RPC Error.

The annotated hexdump below shows an alert message. (The message header has been omitted.)

73 ................................. Bytes in encapsulated alert: 115
01000000 ........................... Version: 1
3766404f00000000 ................... RelayUntil: 1329620535
b305434f00000000 ................... Expiration: 1330917376

f2030000 ........................... ID: 1010
f1030000 ........................... Cancel: 1009
00 ................................. setCancel count: 0

10270000 ........................... MinVer: 10000
48ee0000 ........................... MaxVer: 61000
00 ................................. setUser_agent bytes: 0
64000000 ........................... Priority: 100

00 ................................. Bytes In Comment String: 0
46 ................................. Bytes in StatusBar String: 70
53656520626974636f696e2e6f72672f
666562323020696620796f7520686176
652074726f75626c6520636f6e6e6563
74696e67206166746572203230204665
627275617279 ....................... Status Bar String: "See [...]"
00 ................................. Bytes In Reserved String: 0

47 ................................. Bytes in signature: 71
30450221008389df45f0703f39ec8c1c
c42c13810ffcae14995bb648340219e3
53b63b53eb022009ec65e1c1aaeec1fd
334c6b684bde2b3f573060d5b70c3a46
723326e4e8a4f1 ..................... Signature

Alert key compromise: Bitcoin Core’s source code defines a particular set of alert parameters that can be used to notify users that the alert signing key has been compromised and that they should upgrade to get a new alert public key. Once a signed alert containing those parameters has been received, no other alerts can cancel or override it. See the ProcessAlert() function in the Bitcoin Core alert.cpp source code for the parameters of this message.

FilterAdd

Added in protocol version 70001 as described by BIP37.

The filteradd message tells the receiving peer to add a single element to a previously-set bloom filter, such as a new public key. The element is sent directly to the receiving peer; the peer then uses the parameters set in the filterload message to add the element to the bloom filter.

Because the element is sent directly to the receiving peer, there is no obfuscation of the element and none of the plausible-deniability privacy provided by the bloom filter. Clients that want to maintain greater privacy should recalculate the bloom filter themselves and send a new filterload message with the recalculated bloom filter.

Bytes Name Data Type Description
Varies element bytes compactSize uint The number of bytes in the following element field.
Varies element uint8_t[] The element to add to the current filter. Maximum of 520 bytes, which is the maximum size of an element which can be pushed onto the stack in a pubkey or signature script. Elements must be sent in the byte order they would use when appearing in a raw transaction; for example, hashes should be sent in internal byte order.

Note: a filteradd message will not be accepted unless a filter was previously set with the filterload message.

The annotated hexdump below shows a filteradd message adding a TXID. (The message header has been omitted.) This TXID appears in the same block used for the example hexdump in the merkleblock message; if that merkleblock message is re-sent after sending this filteradd message, six hashes are returned instead of four.

20 ................................. Element bytes: 32
fdacf9b3eb077412e7a968d2e4f11b9a
9dee312d666187ed77ee7d26af16cb0b ... Element (A TXID)

FilterClear

Added in protocol version 70001 as described by BIP37.

The filterclear message tells the receiving peer to remove a previously-set bloom filter. This also undoes the effect of setting the relay field in the version message to 0, allowing unfiltered access to inv messages announcing new transactions.

Bitcoin Core does not require a filterclear message before a replacement filter is loaded with filterload. It also doesn’t require a filterload message before a filterclear message.

There is no payload in a filterclear message. See the message header section for an example of a message without a payload.

FilterLoad

Added in protocol version 70001 as described by BIP37.

The filterload message tells the receiving peer to filter all relayed transactions and requested merkle blocks through the provided filter. This allows clients to receive transactions relevant to their wallet plus a configurable rate of false positive transactions which can provide plausible-deniability privacy.

Bytes Name Data Type Description
Varies nFilterBytes compactSize uint Number of bytes in the following filter bit field.
Varies filter uint8_t[] 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 An arbitrary 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 outpoints corresponding to a matched pubkey script are added to the filter. See the table in the Updating A Bloom Filter subsection below.

The annotated hexdump below shows a filterload message. (The message header has been omitted.) For an example of how this payload was created, see the filterload example.

02 ......... Filter bytes: 2
b50f ....... Filter: 1010 1101 1111 0000
0b000000 ... nHashFuncs: 11
00000000 ... nTweak: 0/none
00 ......... nFlags: BLOOM_UPDATE_NONE

Initializing A Bloom Filter

Filters have two core parameters: the size of the bit field and the number of hash functions to run against each data element. The following formulas from BIP37 will allow you to automatically select appropriate values based on the number of elements you plan to insert into the filter (n) and the false positive rate (p) you desire to maintain plausible deniability.

Note that the filter matches parts of transactions (transaction elements), so the false positive rate is relative to the number of elements checked—not the number of transactions checked. Each normal transaction has a minimum of four matchable elements (described in the comparison subsection below), so a filter with a false-positive rate of 1 percent will match about 4 percent of all transactions at a minimum.

According to BIP37, the formulas and limits described above provide support for bloom filters containing 20,000 items with a false positive rate of less than 0.1 percent or 10,000 items with a false positive rate of less than 0.0001 percent.

Once the size of the bit field is known, the bit field should be initialized as all zeroes.

Populating A Bloom Filter

The bloom filter is populated using between 1 and 50 unique hash functions (the number specified per filter by the nHashFuncs field). Instead of using up to 50 different hash function implementations, a single implementation is used with a unique seed value for each function.

The seed is nHashNum * 0xfba4c795 + nTweak as a uint32_t, where the values are:

If the seed resulting from the formula above is larger than four bytes, it must be truncated to its four most significant bytes (for example, 0x8967452301 & 0xffffffff → 0x67452301).

The actual hash function implementation used is the 32-bit Murmur3 hash function.

Warning icon Warning: the Murmur3 hash function has separate 32-bit and 64-bit versions that produce different results for the same input. Only the 32-bit Murmur3 version is used with Bitcoin bloom filters.

The data to be hashed can be any transaction element which the bloom filter can match. See the next subsection for the list of transaction elements checked against the filter. The largest element which can be matched is a script data push of 520 bytes, so the data should never exceed 520 bytes.

The example below from Bitcoin Core bloom.cpp combines all the steps above to create the hash function template. The seed is the first parameter; the data to be hashed is the second parameter. The result is a uint32_t modulo the size of the bit field in bits.

MurmurHash3(nHashNum * 0xFBA4C795 + nTweak, vDataToHash) % (vData.size() * 8)

Each data element to be added to the filter is hashed by nHashFuncs number of hash functions. Each time a hash function is run, the result will be the index number (nIndex) of a bit in the bit field. That bit must be set to 1. For example if the filter bit field was 00000000 and the result is 5, the revised filter bit field is 00000100 (the first bit is bit 0).

It is expected that sometimes the same index number will be returned more than once when populating the bit field; this does not affect the algorithm—after a bit is set to 1, it is never changed back to 0.

After all data elements have been added to the filter, each set of eight bits is converted into a little-endian byte. These bytes are the value of the filter field.

Comparing Transaction Elements To A Bloom Filter

To compare an arbitrary data element against the bloom filter, it is hashed using the same parameters used to create the bloom filter. Specifically, it is hashed nHashFuncs times, each time using the same nTweak provided in the filter, and the resulting output is modulo the size of the bit field provided in the filter field. After each hash is performed, the filter is checked to see if the bit at that indexed location is set. For example if the result of a hash is 5 and the filter is 01001110, the bit is considered set.

If the result of every hash points to a set bit, the filter matches. If any of the results points to an unset bit, the filter does not match.

The following transaction elements are compared against bloom filters. All elements will be hashed in the byte order used in blocks (for example, TXIDs will be in internal byte order).

The following annotated hexdump of a transaction is from the raw transaction format section; the elements which would be checked by the filter are emphasized in bold. Note that this transaction’s TXID (01000000017b1eab[...]) would also be checked, and that the outpoint TXID and index number below would be checked as a single 36-byte element.

01000000 ................................... Version

01 ......................................... Number of inputs
|
| 7b1eabe0209b1fe794124575ef807057
| c77ada2138ae4fa8d6c4de0398a14f3f ......... Outpoint TXID
| 00000000 ................................. Outpoint index number
|
| 49 ....................................... Bytes in sig. script: 73
| | 48 ..................................... Push 72 bytes as data
| | | 30450221008949f0cb400094ad2b5eb3
| | | 99d59d01c14d73d8fe6e96df1a7150de
| | | b388ab8935022079656090d7f6bac4c9
| | | a94e0aad311a4268e082a725f8aeae05
| | | 73fb12ff866a5f01 ..................... Secp256k1 signature
|
| ffffffff ................................. Sequence number: UINT32_MAX

01 ......................................... Number of outputs
| f0ca052a01000000 ......................... Satoshis (49.99990000 BTC)
|
| 19 ....................................... Bytes in pubkey script: 25
| | 76 ..................................... OP_DUP
| | a9 ..................................... OP_HASH160
| | 14 ..................................... Push 20 bytes as data
| | | cbc20a7664f2f69e5355aa427045bc15
| | | e7c6c772 ............................. PubKey hash
| | 88 ..................................... OP_EQUALVERIFY
| | ac ..................................... OP_CHECKSIG

00000000 ................................... locktime: 0 (a block height)

Updating A Bloom Filter

Clients will often want to track inputs that spend outputs (outpoints) relevant to their wallet, so the filterload field nFlags can be set to allow the filtering node to update the filter when a match is found. When the filtering node sees a pubkey script that pays a pubkey, address, or other data element matching the filter, the filtering node immediately updates the filter with the outpoint corresponding to that pubkey script.

Automatically Updating Bloom Filters

If an input later spends that outpoint, the filter will match it, allowing the filtering node to tell the client that one of its transaction outputs has been spent.

The nFlags field has three allowed values:

Value Name Description
0 BLOOM_UPDATE_NONE The filtering node should not update the filter.
1 BLOOM_UPDATE_ALL If the filter matches any data element in a pubkey script, the corresponding outpoint is added to the filter.
2 BLOOM_UPDATE_P2PUBKEY_ONLY If the filter matches any data element in a pubkey script and that script is either a P2PKH or non-P2SH pay-to-multisig script, the corresponding outpoint is added to the filter.

In addition, because the filter size stays the same even though additional elements are being added to it, the false positive rate increases. Each false positive can result in another element being added to the filter, creating a feedback loop that can (after a certain point) make the filter useless. For this reason, clients using automatic filter updates need to monitor the actual false positive rate and send a new filter when the rate gets too high.

GetAddr

The getaddr message requests an addr message from the receiving node, preferably one with lots of IP addresses of other receiving nodes. The transmitting node can use those IP addresses to quickly update its database of available nodes rather than waiting for unsolicited addr messages to arrive over time.

There is no payload in a getaddr message. See the message header section for an example of a message without a payload.

Ping

The ping message helps confirm that the receiving peer is still connected. If a TCP/IP error is encountered when sending the ping message (such as a connection timeout), the transmitting node can assume that the receiving node is disconnected. The response to a ping message is the pong message.

Before protocol version 60000, the ping message had no payload. As of protocol version 60001 and all later versions, the message includes a single field, the nonce.

Bytes Name Data Type Description
8 nonce uint64_t Added in protocol version 60000 as described by BIP31.

Random nonce assigned to this ping message. The responding pong message will include this nonce to identify the ping message to which it is replying.

The annotated hexdump below shows a ping message. (The message header has been omitted.)

0094102111e2af4d ... Nonce

Pong

Added in protocol version 60001 as described by BIP31.

The pong message replies to a ping message, proving to the pinging node that the ponging node is still alive. Bitcoin Core will, by default, disconnect from any clients which have not responded to a ping message within 20 minutes.

To allow nodes to keep track of latency, the pong message sends back the same nonce received in the ping message it is replying to.

The format of the pong message is identical to the ping message; only the message header differs.

Reject

Added in protocol version 70002 as described by BIP61.

The reject message informs the receiving node that one of its previous messages has been rejected.

Bytes Name Data Type Description
Varies message bytes compactSize uint The number of bytes in the following message field.
Varies message string The type of message rejected as ASCII text without null padding. For example: “tx”, “block”, or “version”.
1 code char The reject message code. See the table below.
Varies reason bytes compactSize uint The number of bytes in the following reason field. May be 0x00 if a text reason isn’t provided.
Varies reason string The reason for the rejection in ASCII text. This should not be displayed to the user; it is only for debugging purposes.
Varies extra data varies Optional additional data provided with the rejection. For example, most rejections of tx messages or block messages include the hash of the rejected transaction or block header. See the code table below.

The following table lists message reject codes. Codes are tied to the type of message they reply to; for example there is a 0x10 reject code for transactions and a 0x10 reject code for blocks.

Code In Reply To Extra Bytes Extra Type Description
0x01 any message 0 N/A Message could not be decoded. Be careful of reject message feedback loops where two peers each don’t understand each other’s reject messages and so keep sending them back and forth forever.
0x10 block message 32 char[32] Block is invalid for some reason (invalid proof-of-work, invalid signature, etc). Extra data is the rejected block’s header hash.
0x10 tx message 32 char[32] Transaction is invalid for some reason (invalid signature, output value greater than input, etc.). Extra data is the rejected transaction’s TXID.
0x11 block message 32 char[32] The block uses a version that is no longer supported. Extra data is the rejected block’s header hash.
0x11 version message 0 N/A Connecting node is using a protocol version that the rejecting node considers obsolete and unsupported.
0x12 tx message 32 char[32] Duplicate input spend (double spend): the rejected transaction spends the same input as a previously-received transaction. Extra data is the rejected transaction’s TXID.
0x12 version message 0 N/A More than one version message received in this connection.
0x40 tx message 32 char[32] The transaction will not be mined or relayed because the rejecting node considers it non-standard—a transaction type or version unknown by the server. Extra data is the rejected transaction’s TXID.
0x41 tx message 32 char[32] One or more output amounts are below the dust threshold. Extra data is the rejected transaction’s TXID.
0x42 tx message   char[32] The transaction did not have a large enough fee or priority to be relayed or mined. Extra data is the rejected transaction’s TXID.
0x43 block message 32 char[32] The block belongs to a block chain which is not the same block chain as provided by a compiled-in checkpoint. Extra data is the rejected block’s header hash.

The annotated hexdump below shows a reject message. (The message header has been omitted.)

02 ................................. Number of bytes in message: 2
7478 ............................... Type of message rejected: tx
12 ................................. Reject code: 0x12 (duplicate)
15 ................................. Number of bytes in reason: 21
6261642d74786e732d696e707574732d
7370656e74 ......................... Reason: bad-txns-inputs-spent
394715fcab51093be7bfca5a31005972
947baf86a31017939575fb2354222821 ... TXID

SendHeaders

The sendheaders message tells the receiving peer to send new block announcements using a headers message rather than an inv message.

There is no payload in a sendheaders message. See the message header section for an example of a message without a payload.

VerAck

The verack message acknowledges a previously-received version message, informing the connecting node that it can begin to send other messages. The verack message has no payload; for an example of a message with no payload, see the message headers section.

Version

The version message provides information about the transmitting node to the receiving node at the beginning of a connection. Until both peers have exchanged version messages, no other messages will be accepted.

If a version message is accepted, the receiving node should send a verack message—but no node should send a verack message before initializing its half of the connection by first sending a version message.

Bytes Name Data Type Description
4 version int32_t The highest protocol version understood by the transmitting node. See the protocol version section.
8 services uint64_t The services supported by the transmitting node encoded as a bitfield. See the list of service codes below.
8 timestamp int64_t The current Unix epoch time according to the transmitting node’s clock. Because nodes will reject blocks with timestamps more than two hours in the future, this field can help other nodes to determine that their clock is wrong.
8 addr_recv services uint64_t Added in protocol version 106.

The services supported by the receiving node as perceived by the transmitting node. Same format as the ‘services’ field above. Bitcoin Core will attempt to provide accurate information. BitcoinJ will, by default, always send 0.
16 addr_recv IP address char Added in protocol version 106.

The IPv6 address of the receiving node as perceived by the transmitting node in big endian byte order. IPv4 addresses can be provided as IPv4-mapped IPv6 addresses. Bitcoin Core will attempt to provide accurate information. BitcoinJ will, by default, always return ::ffff:127.0.0.1
2 addr_recv port uint16_t Added in protocol version 106.

The port number of the receiving node as perceived by the transmitting node in big endian byte order.
8 addr_trans services uint64_t The services supported by the transmitting node. Should be identical to the ‘services’ field above.
16 addr_trans IP address char The IPv6 address of the transmitting node in big endian byte order. IPv4 addresses can be provided as IPv4-mapped IPv6 addresses. Set to ::ffff:127.0.0.1 if unknown.
2 addr_trans port uint16_t The port number of the transmitting node in big endian byte order.
8 nonce uint64_t A random nonce which can help a node detect a connection to itself. If the nonce is 0, the nonce field is ignored. If the nonce is anything else, a node should terminate the connection on receipt of a version message with a nonce it previously sent.
Varies user_agent bytes compactSize uint Number of bytes in following user_agent field. If 0x00, no user agent field is sent.
Varies user_agent string Renamed in protocol version 60000.

User agent as defined by BIP14. Previously called subVer.
4 start_height int32_t The height of the transmitting node’s best block chain or, in the case of an SPV client, best block header chain.
1 relay bool Added in protocol version 70001 as described by BIP37.

Transaction relay flag. If 0x00, no inv messages or tx messages announcing new transactions should be sent to this client until it sends a filterload message or filterclear message. If 0x01, this node wants inv messages and tx messages announcing new transactions.

The following service identifiers have been assigned.

Value Name Description
0x00 Unnamed This node is not a full node. It may not be able to provide any data except for the transactions it originates.
0x01 NODE_NETWORK This is a full node and can be asked for full blocks. It should implement all protocol features available in its self-reported protocol version.

The following annotated hexdump shows a version message. (The message header has been omitted and the actual IP addresses have been replaced with RFC5737 reserved IP addresses.)

72110100 ........................... Protocol version: 70002
0100000000000000 ................... Services: NODE_NETWORK 
bc8f5e5400000000 ................... Epoch time: 1415483324

0100000000000000 ................... Receiving node's services
00000000000000000000ffffc61b6409 ... Receiving node's IPv6 address
208d ............................... Receiving node's port number

0100000000000000 ................... Transmitting node's services
00000000000000000000ffffcb0071c0 ... Transmitting node's IPv6 address
208d ............................... Transmitting node's port number

128035cbc97953f8 ................... Nonce

0f ................................. Bytes in user agent string: 15
2f5361746f7368693a302e392e332f ..... User agent: /Satoshi:0.9.2.1/

cf050500 ........................... Start height: 329167
01 ................................. Relay flag: true