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91 lines
4.0 KiB
Markdown
# P2P Network Message
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The Bitcoin Cash Peer-to-Peer (P2P) Network protocol is a binary protocol used by Full Nodes and [SPV](/protocol/simple-payment-verification) Nodes, transmitted via TCP. The P2P network is similar to a gossip network, where nodes listen for messages and then relays that message to its other peers if it believes that message's content is valid.
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P2P network messages do not necessarily have a reply and there is no way to unambiguously connect a sent message to a reply, although many communications are often request/response pairs. Nodes may handle incoming messages in parallel, so a message reply order cannot be assumed. Messages that cannot be fulfilled are sometimes dropped with no reply, and sometimes replied to via a `reject` message.
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These design decisions were made with consideration to communication with untrusted/uncooperative partners.
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*Developer Notes: A common message strategy is to wait for any message that provides the required data (with a timeout), and then separately issue the request in a retry loop to multiple peers.*
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## Message Format
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The P2P network has a variety of message types. All P2P messages follow a binary format with the following structure:
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| Field | Length | Format |
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|--|--|--|
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| net magic | 4 bytes | byte array<sup>[(BE)](/protocol/misc/endian/big)</sup> |
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| command string | 12 bytes | string<sup>[(BE)](/protocol/misc/endian/big)</sup> |
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| payload byte count | 4 bytes | integer<sup>[(LE)](/protocol/misc/endian/little)</sup> |
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| payload checksum | 4 bytes | byte array<sup>[(BE)](/protocol/misc/endian/big)</sup> |
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| payload | variable | |
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### Net Magic
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The `net magic` field is always `E3E1F3E8`. Any message received that does not begin with the `net magic` should be ignored.
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The `net magic` is designed to be unlikely to occur in normal data--the characters are rarely used upper ASCII, are not valid as UTF-8, and produce a large 32-bit integer with any alignment. `E3E1F3E8` is the ASCII string, "cash", with each byte's highest bit set.
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### Command String
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The `command string` is a fixed-length 12 byte ASCII string. The command string is used to determine the type of message being transmitted. Messages with unknown an unrecognized `command string` are ignored.
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The following messages are considered standard by all node implementations.
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| Command String | Name |
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| -- | -- |
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| version | [Handshake: Version](/protocol/network/messages/version) |
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| verack | [Handshake: Acknowledge Version](/protocol/network/messages/verack) |
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| ping | [Ping](/protocol/network/messages/ping) |
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| pong | [Pong](/protocol/network/messages/pong) |
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| addr | |
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| getblocks | |
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| inv | |
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| mempool | |
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| getheaders | |
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| headers | |
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| getdata | |
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| block | |
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| tx | |
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| merkleblock | |
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| notfound | |
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| reject | |
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| sendheaders | |
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| feefilter | |
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| getaddr | |
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| filterload | |
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| filteradd | |
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| filterclear | |
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The following messages are well known, but not implemented by all node implementations.
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| Command String | Name | Supported Implementations |
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| -- | -- | -- |
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| sendcmpct | | |
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| get_xthin | | |
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| xthinblock | | |
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| thinblock | | |
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| get_xblocktx | | |
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| xblocktx | | |
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### Payload Byte Count
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The payload byte count is the size of the payload, encoded as a [little-endian](/protocol/misc/endian/little) 4-byte integer. The total max size of any message is `268,435,456` bytes (256 MiB), and the header for a message is always 24 bytes, therefore the max value of the payload byte count is `268,435,432` bytes. The payload byte count may be zero, but must not be negative.
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### Payload Checksum
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The message checksum is the first 4 bytes of a double-sha256 hash of the payload. The checksum is transmitted as a byte array, and is encoded as [big-endian](/protocol/misc/endian/big).
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### Payload
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The message payload is defined by the message type.
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# Node Specific Behavior
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## Bitcoin ABC
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### Payload Checksum
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Bitcoin ABC does not validate the message checksum since messages are sent via TCP which has its own checksum paradigm. |