Protocol specification

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Common standards


Usually, when a hash is computed within bitcoin, it is computed twice. Most of the time SHA-256 hashes are used, however RIPEMD-160 is also used when a shorter hash is desirable (for example when creating a bitcoin address).

Example of double-SHA-256 encoding of string "hello":

2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824 (first round of sha-256)
9595c9df90075148eb06860365df33584b75bff782a510c6cd4883a419833d50 (second round of sha-256)

For bitcoin addresses (RIPEMD-160) this would give:

2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824 (first round is sha-256)
b6a9c8c230722b7c748331a8b450f05566dc7d0f (with ripemd-160)

Common structures

All integers are encoded in big endian. (This is different from Bitcoin).

Message structure

Field Size Description Data type Comments
4 magic uint32_t Magic value indicating message origin network, and used to seek to next message when stream state is unknown
12 command char[12] ASCII string identifying the packet content, NULL padded (non-NULL padding results in packet rejected)
4 length uint32_t Length of payload in number of bytes
4 checksum uint32_t First 4 bytes of sha256(sha256(payload))
? payload uchar[] The actual data

Known magic values:

Magic value Sent over wire as
0xE9BEB4D9 E9 BE B4 D9

Variable length integer

Integer can be encoded depending on the represented value to save space. Variable length integers always precede an array/vector of a type of data that may vary in length.

Value Storage length Format
< 0xfd 1 uint8_t
<= 0xffff 3 0xfd followed by the length as uint16_t
<= 0xffffffff 5 0xfe followed by the length as uint32_t
- 9 0xff followed by the length as uint64_t

Variable length string

Variable length string can be stored using a variable length integer followed by the string itself.

Field Size Description Data type Comments
? length var_int Length of the string
? string char[] The string itself (can be empty)

Network address

When a network address is needed somewhere, this structure is used. This protocol and structure supports IPv6, but note that the original client currently only supports IPv4 networking. Network addresses are not prefixed with a timestamp in the version message.

Field Size Description Data type Comments
4 time uint32 the Time
8 services uint64_t same service(s) listed in version
16 IPv6/4 char[16] IPv6 address. Network byte order. The original client only supports IPv4 and only reads the last 4 bytes to get the IPv4 address. However, the IPv4 address is written into the message as a 16 byte IPv4-mapped IPv6 address

(12 bytes 00 00 00 00 00 00 00 00 00 00 FF FF, followed by the 4 bytes of the IPv4 address).

2 port uint16_t port number, network byte order

Hexdump example of Network address structure

0000   00 00 00 00 00 00 00 01  00 00 00 00 00 00 00 00  ................
0010   00 00 FF FF 0A 00 00 01  20 8D                    ........ .

Network address:
 00 00 00 00 00 00 00 01                         - 1 (NODE_NETWORK: see services listed under version command)
 00 00 00 00 00 00 00 00 00 00 FF FF 0A 00 00 01 - IPv6: ::ffff: or IPv4:
 20 8D                                           - Port 8333

Inventory Vectors

Inventory vectors are used for notifying other nodes about objects they have or data which is being requested.

Inventory vectors consist of the following data format:

Field Size Description Data type Comments
1+ type var_int Identifies the object type linked to this inventory
32 hash char[32] Hash of the object

Message types


When a node creates an outgoing connection, it will immediately advertise its version. The remote node will respond with its version. No futher communication is possible until both peers have exchanged their version.


Field Size Description Data type Comments
4 version int32_t Identifies protocol version being used by the node
8 services uint64_t bitfield of features to be enabled for this connection
8 timestamp int64_t standard UNIX timestamp in seconds
26 addr_recv net_addr The network address of the node receiving this message
26 addr_from net_addr The network address of the node emitting this message
8 nonce uint64_t Node random nonce, randomly generated every time a version packet is sent. This nonce is used to detect connections to self.
? user_agent var_str User Agent (0x00 if string is 0 bytes long)
4 start_height int32_t Currently unused. (Might someday be used to specify the time this node was last online so that messages since then can be relayed.)

A "verack" packet shall be sent if the version packet was accepted.

The following services are currently assigned:

Value Name Description
1 NODE_NETWORK This is a normal network node.


The verack message is sent in reply to version. This message consists of only a message header with the command string "verack".


Provide information on known nodes of the network. Non-advertised nodes should be forgotten after typically 3 hours


Field Size Description Data type Comments
1+ count var_int Number of address entries (max: 1000)
30x? addr_list (uint32_t + net_addr)[] Address of other nodes on the network. The uint32_t is a timestamp


Allows a node to advertise its knowledge of one or more objects. It can be received unsolicited, or in reply to getmessages.

Payload (maximum payload length: 50000 bytes):

Field Size Description Data type Comments
? count var_int Number of inventory entries
(33+)x inventory inv_vect[] Inventory vectors


getdata is used in response to inv, to retrieve the content of a specific object, and is usually sent after receiving an inv packet, after filtering known elements.

Payload (maximum payload length: 50000 bytes):

Field Size Description Data type Comments
? count var_int Number of inventory entries
36x? inventory inv_vect[] Inventory vectors