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Bitcoin is structured as a peer-to-peer network architecture on top of the Internet. The term peer-to-peer or P2P means that the computers that participate in the network are peers to each other, they are all equal there are no "special" nodes and all nodes share the burden of providing network services. The network nodes interconnect in a mesh network with a "flat" topology. There is no "server", no centralized service and no hierarchy within the network. Nodes in a peer-to-peer network both provide and consume services at the same time, with reciprocity acting as the incentive for participation. Peer-to-peer networks are inherently resilient, de-centralized and open. The pre-eminent example of a P2P network architecture was the early Internet itself, where nodes on the IP network were equal. Today's Internet architecture is more hierarchical, but the Internet Protocol still retains its flat-topology essence. Beyond bitcoin, the largest and most successful application of P2P technologies is file sharing, with Napster as the pioneer and bittorrent as the most recent evolution of the architecture. Bitcoin is structured as a peer-to-peer network architecture on top of the Internet. The term peer-to-peer or P2P means that the computers that participate in the network are peers to each other, they are all equal there are no "special" nodes and all nodes share the burden of providing network services. The network nodes interconnect in a mesh network with a "flat" topology. There is no "server", no centralized service and no hierarchy within the network. Nodes in a peer-to-peer network both provide and consume services at the same time, with reciprocity acting as the incentive for participation. Peer-to-peer networks are inherently resilient, de-centralized and open. The pre-eminent example of a P2P network architecture was the early Internet itself, where nodes on the IP network were equal. Today's Internet architecture is more hierarchical, but the Internet Protocol still retains its flat-topology essence. Beyond bitcoin, the largest and most successful application of P2P technologies is file sharing, with Napster as the pioneer and bittorrent as the most recent evolution of the architecture.
Bitcoins P2P network architecture is much more than a topology choice. Bitcoin is a peer-to-peer digital cash system by design, and the network architecture is both a reflection and a foundation of that core characteristic. De-centralization of control is a core design principle and that can only be achieved and maintained by a flat, de-centralized P2P consensus network. Bitcoin's P2P network architecture is much more than a topology choice. Bitcoin is a peer-to-peer digital cash system by design, and the network architecture is both a reflection and a foundation of that core characteristic. De-centralization of control is a core design principle and that can only be achieved and maintained by a flat, de-centralized P2P consensus network.
The term "bitcoin network" refers to the collection of nodes running the bitcoin P2P protocol. In addition to the bitcoin P2P protocol, there are other protocols such as Stratum, that are used for mining and lightweight or mobile wallets. These additional protocols are provided by gateway routing servers that access the bitcoin network using the bitcoin P2P protocol and then extend that network to nodes running other protocols. For example, Stratum servers connect Stratum mining nodes via the Stratum protocol to the main bitcoin network and bridge the Stratum protocol to the bitcoin P2P protocol. We use the term "extended bitcoin network" to refer to the overall network that includes the bitcoin P2P protocol, pool mining protocols, the Stratum protocol and any other related protocols connecting the components of the bitcoin system. The term "bitcoin network" refers to the collection of nodes running the bitcoin P2P protocol. In addition to the bitcoin P2P protocol, there are other protocols such as Stratum, that are used for mining and lightweight or mobile wallets. These additional protocols are provided by gateway routing servers that access the bitcoin network using the bitcoin P2P protocol and then extend that network to nodes running other protocols. For example, Stratum servers connect Stratum mining nodes via the Stratum protocol to the main bitcoin network and bridge the Stratum protocol to the bitcoin P2P protocol. We use the term "extended bitcoin network" to refer to the overall network that includes the bitcoin P2P protocol, pool mining protocols, the Stratum protocol and any other related protocols connecting the components of the bitcoin system.
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Mining nodes compete to create new blocks by running specialized hardware to solve the proof-of-work algorithm. Some mining nodes are also full nodes, maintaining a full copy of the blockchain while others are lightweight nodes participating in pool mining and depending on a pool server to maintain a full node. The mining function is shown in the full node above as a black circle named "Mining". Mining nodes compete to create new blocks by running specialized hardware to solve the proof-of-work algorithm. Some mining nodes are also full nodes, maintaining a full copy of the blockchain while others are lightweight nodes participating in pool mining and depending on a pool server to maintain a full node. The mining function is shown in the full node above as a black circle named "Mining".
User wallets may be part of a full node, as is usually the case with desktop bitcoin clients. Increasingly many user wallets, especially those running on resource constrained devices such as smart phones, are SPV nodes. The wallet function is shown above as a green circle named "Wallet". User wallets may be part of a full node, as is usually the case with desktop bitcoin clients. Increasingly many user wallets, especially those running on resource- constrained devices such as smart phones, are SPV nodes. The wallet function is shown above as a green circle named "Wallet".
In addition to the main node types on the bitcoin P2P protocol, there are servers and nodes running other protocols, such as specialized mining pool protocols and lightweight client access protocols. In addition to the main node types on the bitcoin P2P protocol, there are servers and nodes running other protocols, such as specialized mining pool protocols and lightweight client access protocols.
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=== The Extended Bitcoin Network === The Extended Bitcoin Network
The main bitcoin network, running the bitcoin P2P protocol consists of between 7,000 to 10,000 nodes running various versions of the bitcoin reference client (Bitcoin Core) and a few hundred nodes running various other implementations of the bitcoin P2P protocol, such as BitcoinJ, Libbitcoin and btcd. A small percentage of the nodes on the bitcoin P2P network are also mining nodes, competing in the mining process, validating transactions and creating new blocks. Various large companies interface with the bitcoin network by running full-node clients based on the Bitcoin Core client, with full copies of the blockchain and a network node, but without mining or wallet functions. These nodes act as network edge routers, allowing various other services (exchanges, wallets, block explorers, merchant payment processing) to be built on top. The main bitcoin network, running the bitcoin P2P protocol, consists of between 7,000 to 10,000 nodes running various versions of the bitcoin reference client (Bitcoin Core) and a few hundred nodes running various other implementations of the bitcoin P2P protocol, such as BitcoinJ, Libbitcoin and btcd. A small percentage of the nodes on the bitcoin P2P network are also mining nodes, competing in the mining process, validating transactions and creating new blocks. Various large companies interface with the bitcoin network by running full-node clients based on the Bitcoin Core client, with full copies of the blockchain and a network node, but without mining or wallet functions. These nodes act as network edge routers, allowing various other services (exchanges, wallets, block explorers, merchant payment processing) to be built on top.
The extended bitcoin network includes the network running the bitcoin P2P protocol, described above, as well as nodes running specialized protocols. Attached to the main bitcoin P2P network are a number of pool servers and protocol gateways that connect nodes running other protocols, mostly pool mining nodes (see <<mining>>) and lightweight wallet clients, which do not carry a full copy of the blockchain. The extended bitcoin network includes the network running the bitcoin P2P protocol, described above, as well as nodes running specialized protocols. Attached to the main bitcoin P2P network are a number of pool servers and protocol gateways that connect nodes running other protocols. These other protocol nodes are mostly pool mining nodes (see <<mining>>) and lightweight wallet clients, which do not carry a full copy of the blockchain.
The diagram below shows the extended bitcoin network with the various types of nodes, gateway servers, edge routers and wallet clients and the various protocols they use to connect to each other. The diagram below shows the extended bitcoin network with the various types of nodes, gateway servers, edge routers and wallet clients and the various protocols they use to connect to each other.
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How does a new node find peers? While there are no special nodes in bitcoin, there are some long running stable nodes that are listed in the client as _seed nodes_. While a new node does not have to connect with the seed nodes, it can use them to quickly discover other nodes in the network. In the Bitcoin Core client, the option to use the seed nodes is controlled by the option switch +-dnsseed+, which is set to 1, to use the seed nodes, by default. Alternatively, a bootstrapping node that knows nothing of the network must be given the IP address of at least one bitcoin node after which it can establish connections through further introductions. The command line argument +-seednode+ can be used to connect to one node just for introductions, using it as a DNS seed. After the initial seed node is used to form introductions, the client will disconnect from it and use the newly discovered peers. How does a new node find peers? While there are no special nodes in bitcoin, there are some long running stable nodes that are listed in the client as _seed nodes_. While a new node does not have to connect with the seed nodes, it can use them to quickly discover other nodes in the network. In the Bitcoin Core client, the option to use the seed nodes is controlled by the option switch +-dnsseed+, which is set to 1, to use the seed nodes, by default. Alternatively, a bootstrapping node that knows nothing of the network must be given the IP address of at least one bitcoin node after which it can establish connections through further introductions. The command line argument +-seednode+ can be used to connect to one node just for introductions, using it as a DNS seed. After the initial seed node is used to form introductions, the client will disconnect from it and use the newly discovered peers.
Once one or more connections is established, the new node will send an +addr+ message containing its own IP address, to its neighbors. The neighbors will in turn forward the +addr+ message to their neighbors, ensuring that the newly connected node becomes well known and better connected. Additionally, the newly connected node can send +getaddr+ to the neighbors asking them to return a list of IP addresses of other peers. That way, a node can find peers to connect to and advertise its existence on the network for other nodes to find it. Once one or more connections is established, the new node will send an +addr+ message containing its own IP address, to its neighbors. The neighbors will in turn forward the +addr+ message to their neighbors, ensuring that the newly connected node becomes well known and better connected. Additionally, the newly connected node can send +getaddr+ to the neighbors, asking them to return a list of IP addresses of other peers. That way, a node can find peers to connect to and advertise its existence on the network for other nodes to find it.
[[address_propagation]] [[address_propagation]]
.Address Propagation and Discovery .Address Propagation and Discovery
image::images/AddressPropagation.png["AddressPropagation"] image::images/AddressPropagation.png["AddressPropagation"]
A node must connect to a few different peers in order to establish diverse paths into the bitcoin network. These paths are not reliable, nodes come and go, and so the node must continue to discover new nodes as it loses old connections as well as assist other nodes when they bootstrap. Only one connection is needed to bootstrap, as the first node can offer introductions to its peer nodes and those peers can offer further introductions. Its also unnecessary and wasteful of network resources to connect to more than a handful of nodes. After bootstrapping a node will remember its most recent successful peer connections, so that if it is rebooted it can quickly reestablish connections with its former peer network. If none of the former peers respond to its connection request, the node can use the seed nodes to bootstrap again. A node must connect to a few different peers in order to establish diverse paths into the bitcoin network. Paths are not reliable, nodes come and go, and so the node must continue to discover new nodes as it loses old connections as well as assist other nodes when they bootstrap. Only one connection is needed to bootstrap, as the first node can offer introductions to its peer nodes and those peers can offer further introductions. It's also unnecessary and wasteful of network resources to connect to more than a handful of nodes. After bootstrapping a node will remember its most recent successful peer connections, so that if it is rebooted it can quickly reestablish connections with its former peer network. If none of the former peers respond to its connection request, the node can use the seed nodes to bootstrap again.
On a node running the Bitcoin Core client, you can list the peer connections with the command +getpeerinfo+: On a node running the Bitcoin Core client, you can list the peer connections with the command +getpeerinfo+:
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