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.

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.

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:

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.

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

The currently-available type identifiers are:

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).

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.

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.

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:

• Before Bitcoin Core 0.9.0, the response to the mempool message was only one inv message. An inv message is limited to 50,000 inventories, so a node with a memory pool larger than 50,000 entries would not send everything. Later versions of Bitcoin Core send as many inv messages as needed to reference its complete memory pool.

• The mempool message is not currently fully compatible with the filterload message’s BLOOM_UPDATE_ALL and BLOOM_UPDATE_P2PUBKEY_ONLY flags. Mempool transactions are not sorted like in-block transactions, so a transaction (tx2) spending an output can appear before the transaction (tx1) containing that output, which means the automatic filter update mechanism won’t operate until the second-appearing transaction (tx1) is seen—missing the first-appearing transaction (tx2). It has been proposed in Bitcoin Core issue #2381 that the transactions should be sorted before being processed by the filter.

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.

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.

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.

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):

• Fail if there are unused hashes in the hashes list.

• Fail if there are unused flag bits—except for the minimum number of bits necessary to pad up to the next full byte.

• Fail if the hash of the merkle root node is not identical to the merkle root in the block header.

• Fail if the block header is invalid. Remember to ensure that the hash of the header is less than or equal to the target threshold encoded by the nBits header field. Your program should also, of course, attempt to ensure the header belongs to the best block chain and that the user knows how many confirmations this block has.

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.

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.

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;

• Transaction Response: Bitcoin Core and BitcoinJ will send it in response to a getdata message that requests the transaction with an inventory type of MSG_TX.

• MerkleBlock Response: Bitcoin Core will send it in response to a getdata message that requests a merkle block with an inventory type of MSG_MERKLEBLOCK. (This is in addition to sending a merkleblock message.) Each tx message in this case provides a matched transaction from that block.

• Unsolicited: BitcoinJ will send a tx message unsolicited for transactions it originates.

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.

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.

Each encapsulated network IP address currently uses the following structure:

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:

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.

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.

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.

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.

• Size of the bit field in bytes (nFilterBytes), up to a maximum of 36,000: (-1 / log(2)**2 * n * log(p)) / 8

• Hash functions to use (nHashFuncs), up to a maximum of 50: nFilterBytes * 8 / n * log(2)

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:

• nHashNum is the sequence number for this hash function, starting at 0 for the first hash iteration and increasing up to the value of the nHashFuncs field (minus one) for the last hash iteration.

• 0xfba4c795 is a constant optimized to create large differences in the seed for different values of nHashNum.

• nTweak is a per-filter constant set by the client to require the use of an arbitrary set of hash functions.

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: 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).

• TXIDs: the transaction’s SHA256(SHA256()) hash.

• Outpoints: each 36-byte outpoint used this transaction’s input section is individually compared to the filter.

• Signature Script Data: each element pushed onto the stack by a data-pushing opcode in a signature script from this transaction is individually compared to the filter. This includes data elements present in P2SH redeem scripts when they are being spent.

• PubKey Script Data: each element pushed onto the the stack by a data-pushing opcode in any pubkey script from this transaction is individually compared to the filter. (If a pubkey script element matches the filter, the filter will be immediately updated if the BLOOM_UPDATE_ALL flag was set; if the pubkey script is in the P2PKH format and matches the filter, the filter will be immediately updated if the BLOOM_UPDATE_P2PUBKEY_ONLY flag was set. See the subsection below for details.)

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.

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:

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.

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.

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.

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.

The following service identifiers have been assigned.

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