比特币结点间使用TCP相互链接。数据包格式如下：
// Message format
// (4) message start
// (12) command
// (4) size
// (4) checksum
// (x) datamessage start是用来区分不同网络，主网(MAIN)和测试网(TESTNET)，见代码chainparams.cpp中关于pchMessageStart的赋值。command命令，该数据包的作用，command的类型见protocol.h中命名空间NetMsgType的变量。
namespace NetMsgType {
/**
* 刚连上的双方会相互发送version，包含结点版本号，结点服务类型，时间戳等
* @see https://bitcoin.org/en/developer-reference#version
*/
extern const char *VERSION;
/**
* 通知对方version已收到，可以发送其它消息了。
* @see https://bitcoin.org/en/developer-reference#verack
*/
extern const char *VERACK;
/**
* 收到关于其它结点的IP信息
* @see https://bitcoin.org/en/developer-reference#addr
*/
extern const char *ADDR;
/**
* 混合的hash数组，包含交易的hash、区块的hash
* @see https://bitcoin.org/en/developer-reference#inv
*/
extern const char *INV;
/**
* 对方请求数据
* @see https://bitcoin.org/en/developer-reference#getdata
*/
extern const char *GETDATA;
/**
* The merkleblock message is a reply to a getdata message which requested a
* block using the inventory type MSG_MERKLEBLOCK.
* @since protocol version 70001 as described by BIP37.
* @see https://bitcoin.org/en/developer-reference#merkleblock
*/
extern const char *MERKLEBLOCK;
/**
* The getblocks message requests an inv message that provides block header
* hashes starting from a particular point in the block chain.
* @see https://bitcoin.org/en/developer-reference#getblocks
*/
extern const char *GETBLOCKS;
/**
* The getheaders message requests a headers message that provides block
* headers starting from a particular point in the block chain.
* @since protocol version 31800.
* @see https://bitcoin.org/en/developer-reference#getheaders
*/
extern const char *GETHEADERS;
/**
* The tx message transmits a single transaction.
* @see https://bitcoin.org/en/developer-reference#tx
*/
extern const char *TX;
/**
* The headers message sends one or more block headers to a node which
* previously requested certain headers with a getheaders message.
* @since protocol version 31800.
* @see https://bitcoin.org/en/developer-reference#headers
*/
extern const char *HEADERS;
/**
* The block message transmits a single serialized block.
* @see https://bitcoin.org/en/developer-reference#block
*/
extern const char *BLOCK;
/**
* The getaddr message requests an addr message from the receiving node,
* preferably one with lots of IP addresses of other receiving nodes.
* @see https://bitcoin.org/en/developer-reference#getaddr
*/
extern const char *GETADDR;
/**
* 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.
* @since protocol version 60002.
* @see https://bitcoin.org/en/developer-reference#mempool
*/
extern const char *MEMPOOL;
/**
* The ping message is sent periodically to help confirm that the receiving
* peer is still connected.
* @see https://bitcoin.org/en/developer-reference#ping
*/
extern const char *PING;
/**
* The pong message replies to a ping message, proving to the pinging node that
* the ponging node is still alive.
* @since protocol version 60001 as described by BIP31.
* @see https://bitcoin.org/en/developer-reference#pong
*/
extern const char *PONG;
/**
* The notfound message is a reply to a getdata message which requested an
* object the receiving node does not have available for relay.
* @since protocol version 70001.
* @see https://bitcoin.org/en/developer-reference#notfound
*/
extern const char *NOTFOUND;
/**
* The filterload message tells the receiving peer to filter all relayed
* transactions and requested merkle blocks through the provided filter.
* @since protocol version 70001 as described by BIP37.
* Only available with service bit NODE_BLOOM since protocol version
* 70011 as described by BIP111.
* @see https://bitcoin.org/en/developer-reference#filterload
*/
extern const char *FILTERLOAD;
/**
* The filteradd message tells the receiving peer to add a single element to a
* previously-set bloom filter, such as a new public key.
* @since protocol version 70001 as described by BIP37.
* Only available with service bit NODE_BLOOM since protocol version
* 70011 as described by BIP111.
* @see https://bitcoin.org/en/developer-reference#filteradd
*/
extern const char *FILTERADD;
/**
* The filterclear message tells the receiving peer to remove a previously-set
* bloom filter.
* @since protocol version 70001 as described by BIP37.
* Only available with service bit NODE_BLOOM since protocol version
* 70011 as described by BIP111.
* @see https://bitcoin.org/en/developer-reference#filterclear
*/
extern const char *FILTERCLEAR;
/**
* The reject message informs the receiving node that one of its previous
* messages has been rejected.
* @since protocol version 70002 as described by BIP61.
* @see https://bitcoin.org/en/developer-reference#reject
*/
extern const char *REJECT;
/**
* Indicates that a node prefers to receive new block announcements via a
* "headers" message rather than an "inv".
* @since protocol version 70012 as described by BIP130.
* @see https://bitcoin.org/en/developer-reference#sendheaders
*/
extern const char *SENDHEADERS;
/**
* The feefilter message tells the receiving peer not to inv us any txs
* which do not meet the specified min fee rate.
* @since protocol version 70013 as described by BIP133
*/
extern const char *FEEFILTER;
/**
* Contains a 1-byte bool and 8-byte LE version number.
* Indicates that a node is willing to provide blocks via "cmpctblock" messages.
* May indicate that a node prefers to receive new block announcements via a
* "cmpctblock" message rather than an "inv", depending on message contents.
* @since protocol version 70014 as described by BIP 152
*/
extern const char *SENDCMPCT;
/**
* Contains a CBlockHeaderAndShortTxIDs object - providing a header and
* list of "short txids".
* @since protocol version 70014 as described by BIP 152
*/
extern const char *CMPCTBLOCK;
/**
* Contains a BlockTransactionsRequest
* Peer should respond with "blocktxn" message.
* @since protocol version 70014 as described by BIP 152
*/
extern const char *GETBLOCKTXN;
/**
* Contains a BlockTransactions.
* Sent in response to a "getblocktxn" message.
* @since protocol version 70014 as described by BIP 152
*/
extern const char *BLOCKTXN;
};size数据包大小，checksum核验和，data不同命令数据格式不一样。
多线程接收数据，见net.cpp中MCConnman::ThreadMessageHandler()函数，先拷贝结点数组，对每个结点调用net_processing.cpp中的函数PeerLogicValidation::ProcessMessages(CNode* pfrom, std::atomic<bool>& interruptMsgProc)，从数据流中得到数据包，然后将数据交给ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStream& vRecv, int64_t nTimeReceived, const CChainParams& chainparams, CConnman* connman, const std::atomic<bool>& interruptMsgProc, bool enable_bip61)函数处理。
ProcessMessage中对不同command进行处理。command交互，既有请求应答式，也有推送式。
