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Michalis Kargakis 2014-07-23 00:06:19 +03:00
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=== Peer-to-Peer Network 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.
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.
=== Nodes Types and Roles
@ -37,11 +37,11 @@ image::images/BitcoinNodeTypes.png["BitcoinNodeTypes"]
=== 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 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.
[[bitcoin_network]]
.The extended bitcoin network showing various node types, gateways and protocols
@ -121,7 +121,7 @@ $ bitcoin-cli getpeerinfo
To override the automatic management of peers and to specify a list of IP addresses, users can provide the option +-connect=<IPAddress>+ and specify one or more IP addresses. If this option is used, the node will only connect to the selected IP addresses, instead of discovering and maintaining the peer connections automatically.
If there is no traffic on a connection, nodes will periodically send a message to maintain the connection. If a node has not communicated on a connection for more than 90 minutes it is assumed to be disconnected and a new peer will be sought. Thus the network dynamically adjusts to transient nodes, network problems, and can organically grow and shrink as needed without any central control.
If there is no traffic on a connection, nodes will periodically send a message to maintain the connection. If a node has not communicated on a connection for more than 90 minutes, it is assumed to be disconnected and a new peer will be sought. Thus the network dynamically adjusts to transient nodes, network problems, and can organically grow and shrink as needed without any central control.
=== Full Nodes
@ -129,7 +129,7 @@ Full nodes are nodes that maintain a full blockchain, with all transactions. Mor
Full blockchain nodes maintain a complete and up-to-date copy of the bitcoin blockchain, with all the transactions, which they independently build and verify, starting with the very first block (genesis block) and up to the latest known block in the network. A full blockchain node can independently and authoritatively verify any transaction, without recourse or reliance on any other node or source of information. The full blockchain node relies on the network to receive updates about new blocks of transactions, which it then verifies and incorporates into its local copy of the blockchain.
Running a full blockchain node gives you the pure bitcoin experience: independent verification of all transactions without the need to rely on, or trust, any other systems. It's easy to tell if you're running a full node because it requires several gigabytes of persistent storage (disk space) to store the full blockchain. If you need a lot of disk and it takes 2-3 days to "sync" to the network you are running a full node. That is the price of complete independence and freedom from central authority.
Running a full blockchain node gives you the pure bitcoin experience: independent verification of all transactions without the need to rely on, or trust, any other systems. It's easy to tell if you're running a full node because it requires several gigabytes of persistent storage (disk space) to store the full blockchain. If you need a lot of disk and it takes 2-3 days to "sync" to the network, you are running a full node. That is the price of complete independence and freedom from central authority.
There are a few alternative implementations of full-blockchain bitcoin clients, built using different programming languages and software architectures. However, the most common implementation is the reference client Bitcoin Core, also known as the Satoshi Client. More than 90% of the nodes on the bitcoin network run various versions of Bitcoin Core. It is identified as "Satoshi" in the sub-version string sent in the +version+ message and shown by the command +getpeerinfo+ as we saw above, for example +/Satoshi:0.8.6/+.
@ -139,11 +139,11 @@ The first thing a full node will do once it connects to peers is try to construc
The process of "syncing" the blockchain starts with the +version+ message, as that contains +BestHeight+, a node's current blockchain height (number of blocks). A node will see the +version+ messages from its peers, know how many blocks they each have and be able to compare to how many blocks it has in its own blockchain. Peered nodes will exchange a +getblocks+ message that contains the hash (fingerprint) of the top block on their local blockchain. One of the peers will be able to identify the received hash as belonging to a block that is not at the top, but rather belongs to an older block, thus deducing that its own local blockchain is longer than its peer's.
The peer that has the longer blockchain has more blocks that the other node and can identify which blocks the other node needs to "catch up". It will identify the first 500 blocks to share and transmit their hashes using an +inv+ (inventory) message. The node missing these blocks will then retrieve them, by issuing a series of +getdata+ messages requesting the full block data and identifying the requested blocks using the hashes from the +inv+ message.
The peer that has the longer blockchain, has more blocks than the other node and can identify which blocks the other node needs to "catch up". It will identify the first 500 blocks to share and transmit their hashes using an +inv+ (inventory) message. The node missing these blocks will then retrieve them, by issuing a series of +getdata+ messages requesting the full block data and identifying the requested blocks using the hashes from the +inv+ message.
Let's assume for example that a node only has the genesis block. It will then receive an +inv+ message from its peers containing the hashes of the next 500 blocks in the chain. It will start requesting blocks from all its connected peers, spreading the load and ensuring that it doesn't overwhelm any peer with requests. The node keeps track of how many blocks are "in transit" per peer connection, meaning blocks that it has requested but not received, checking that it does not exceed a limit (MAX_BLOCKS_IN_TRANSIT_PER_PEER). This way, if it needs a lot of blocks, it will only request new ones as previous requests are fulfilled, allowing the peers to control the pace of updates and not overwhelming the network. As each block is received, it is added to the blockchain as we will see in the next chapter <<blockchain>>. The local blockchain is gradually built up, more blocks are requested and received and the process continues until the node catches up to the rest of the network.
Let's assume for example that a node only has the genesis block. It will then receive an +inv+ message from its peers containing the hashes of the next 500 blocks in the chain. It will start requesting blocks from all of its connected peers, spreading the load and ensuring that it doesn't overwhelm any peer with requests. The node keeps track of how many blocks are "in transit" per peer connection, meaning blocks that it has requested but not received, checking that it does not exceed a limit (MAX_BLOCKS_IN_TRANSIT_PER_PEER). This way, if it needs a lot of blocks, it will only request new ones as previous requests are fulfilled, allowing the peers to control the pace of updates and not overwhelming the network. As each block is received, it is added to the blockchain as we will see in the next chapter <<blockchain>>. The local blockchain is gradually built up, more blocks are requested and received and the process continues until the node catches up to the rest of the network.
This process of comparing the local blockchain with the peers and retrieving any missing blocks happens any time a node goes offline for any period of time. Whether a node has been offline for a few minutes and is missing a few blocks, or a month and is missing a few thousand blocks, it starts by sending +getblocks+, gets an +inv+ response and starts downloading the missing blocks.
This process of comparing the local blockchain with the peers and retrieving any missing blocks happens any time a node goes offline for any period of time. Whether a node has been offline for a few minutes and is missing a few blocks, or a month and is missing a few thousand blocks, it starts by sending +getblocks+, gets an +inv+ response, and starts downloading the missing blocks.
[[inventory_synchronization]]
.Node synchronizing the blockchain by retrieving blocks from a peer
@ -153,7 +153,7 @@ image::images/InventorySynchronization.png["InventorySynchronization"]
Not all nodes have the ability to store the full blockchain. Many bitcoin clients are designed to run on space- and power-constrained devices, such as smartphones, tablets or embedded systems. For such devices, a _simple payment verification_ (SPV) method is used to allow them to operate without storing the full blockchain. These types of clients are called SPV clients or lightweight clients. As bitcoin adoption surges, the SPV node is becoming the most common form of bitcoin node, especially for bitcoin wallets.
SPV nodes download only the block headers and do not download the transactions included in each block. The resulting chain of blocks, without transactions, is 1,000 times smaller than the full blockchain. SPV nodes cannot construct a full picture of all the UTXO that is available for spending, as they do not know about all the transactions on the network. SPV nodes verify transactions using a slightly different methodology that relies on peers to provide partial views of relevant parts of the blockchain on-demand.
SPV nodes download only the block headers and do not download the transactions included in each block. The resulting chain of blocks, without transactions, is 1,000 times smaller than the full blockchain. SPV nodes cannot construct a full picture of all the UTXOs that are available for spending, as they do not know about all the transactions on the network. SPV nodes verify transactions using a slightly different methodology that relies on peers to provide partial views of relevant parts of the blockchain on-demand.
As an analogy, a full node is like a tourist in a strange city, equipped with a detailed map of every street and every address. By comparison, an SPV node is like a tourist in a strange city asking random strangers for turn-by-turn directions while knowing only one main avenue. While both tourists can verify the existence of a street by visiting it, the tourist without a map doesn't know what lies down any of the side streets and doesn't know what other streets exist. Positioned in front of 23 Church Street, the tourist without a map cannot know if there are a dozen other "23 Church Street" addresses in the city and whether this is the right one. The map-less tourist's best chance is to ask enough people and hope some of them are not trying to mug the tourist.
@ -163,11 +163,11 @@ For example, when examining a transaction in block 300,000, a full node links al
An SPV node cannot be persuaded that a transaction exists in a block, when it does not in fact exist. The SPV node establishes the existence of a transaction in a block by requesting a merkle path proof and by validating the proof-of-work in the chain of blocks. However, a transaction's existence can be "hidden" from an SPV node. An SPV node can definitely prove that a transaction exists but cannot verify that a transaction, such as a double-spend of the same UTXO, doesn't exist because it doesn't have a record of all transactions. This type of attack can be used as a Denial-of-Service attack or as a double-spending attack against SPV nodes. To defend against this, an SPV node needs to connect randomly to several nodes, to increase the probability that it is in contact with at least one honest node. SPV nodes are therefore vulnerable to network partitioning attacks or Sybil attacks, where they are connected to fake nodes or fake networks and do not have access to honest nodes or the real bitcoin network.
For most practical purposes, well-connected SPV nodes are secure enough, striking the right balance between resource needs, practicality and security. For the truly security conscious, however, nothing beats running a full blockchain node.
For most practical purposes, well-connected SPV nodes are secure enough, striking the right balance between resource needs, practicality, and security. For the truly security conscious, however, nothing beats running a full blockchain node.
[TIP]
====
A full blockchain node verifies a transaction by checking the chain of thousands of blocks below it and checks the UTXO is not spent, whereas an SPV node checks how deep the block is buried by a handful of blocks above it.
A full blockchain node verifies a transaction by checking the chain of thousands of blocks below it and checks that the UTXO is not spent, whereas an SPV node checks how deep the block is buried by a handful of blocks above it.
====
To get the block headers, SPV nodes use a +getheaders+ message instead of +getblocks+. The responding peer will send up to 2000 block headers using a single +headers+ message. The process is otherwise the same as that used by a full node to retrieve full blocks. SPV nodes also set a filter on the connection to peers, to filter the stream of future blocks and transactions sent by the peers. Any transactions of interest are retrieved using a +getdata+ request. The peer generates a +tx+ message containing the transactions, in response.
@ -244,13 +244,13 @@ The node setting the bloom filter can interactively add patterns to the filter b
[[transaction_pools]]
=== Transaction Pools
Almost every node on the bitcoin network maintains a temporary list of unconfirmed transactions called the memory pool or transaction pool. Nodes use this pool to keep track of transactions that are known to the network but are not yet included in the blockchain. For example, a node that holds a users wallet will use the transaction pool to track incoming payments to the users wallet that have been received on the network but are not yet confirmed.
Almost every node on the bitcoin network maintains a temporary list of unconfirmed transactions called the memory pool or transaction pool. Nodes use this pool to keep track of transactions that are known to the network but are not yet included in the blockchain. For example, a node that holds a user's wallet will use the transaction pool to track incoming payments to the user's wallet that have been received on the network but are not yet confirmed.
As transactions are received and verified, they are added to the transaction pool and relayed to the neighboring nodes to propagate on the network.
Some node implementations also maintain a separate pool of orphaned transactions as detailed in <<orphan_transactions>>. If a transaction's inputs refer to a transaction that is not yet known, a missing parent, then the orphan transaction will be stored temporarily in the orphan pool until the parent transaction arrives.
When a transaction is added to the transaction pool, the orphan pool is checked for any orphans that reference this transaction's outputs (its children). Any orphans found are pulled from the orphan pool and validated using the above checklist. If valid, they are also added to the transaction pool, completing the chain that started with the parent transaction. In light of the newly added transaction which is no longer an orphan, the process is repeated recursively looking for any further descendants, until no more descendants are found. Through this process, the arrival of a parent transaction triggers a cascade reconstruction of an entire chain of interdependent transactions by re-uniting the orphans with their parents all the way down the chain.
When a transaction is added to the transaction pool, the orphan pool is checked for any orphans that reference this transaction's outputs (its children). Any orphans found, are pulled from the orphan pool and validated using the above checklist. If valid, they are also added to the transaction pool, completing the chain that started with the parent transaction. In light of the newly added transaction which is no longer an orphan, the process is repeated recursively looking for any further descendants, until no more descendants are found. Through this process, the arrival of a parent transaction triggers a cascade reconstruction of an entire chain of interdependent transactions by re-uniting the orphans with their parents all the way down the chain.
Both the transaction pool and orphan pool (where implemented) are stored in local memory and are not saved on persistent storage, rather they are dynamically populated from incoming network messages. When a node starts, both pools are empty and are gradually populated with new transactions received on the network.
@ -271,9 +271,9 @@ Alert messages are propagated by the +alert+ message. The alert message contains
* subVer - The client software version that this alert applies to
* Priority - An alert priority level, currently unused
Alerts are cryptographically signed by a public key. The corresponding private key is held by a few select members of the core development team. The digital signature ensures that fake alerts will not be propagated on the network.
Alerts are cryptographically signed by a public key. The corresponding private key is held by a few selected members of the core development team. The digital signature ensures that fake alerts will not be propagated on the network.
Each node receiving this alert message will verify it, check for expiration and propagate it to all its peers, thus ensuring rapid propagation across the entire network. In addition to propagating the alert, each node may implement a user interface function to present the alert to the user.
Each node receiving this alert message will verify it, check for expiration, and propagate it to all its peers, thus ensuring rapid propagation across the entire network. In addition to propagating the alert, each node may implement a user interface function to present the alert to the user.
In the Bitcoin Core client, the alert is configured with the command line option +-alertnotify+, which specifies a command to run when an alert is received. The alert message is passed as a parameter to the alertnotify command. Most commonly, the alertnotify command is set to generate an email message to the administrator of the node, containing the alert message. The alert is also displayed as a pop-up dialog in the graphical user interface (bitcoin-Qt) if it is running.