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Andreas M. Antonopoulos 2017-01-12 13:07:50 +07:00
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@ -1,9 +1,9 @@
[[bitcoin_network_ch06]] [[bitcoin_network_ch08]]
== The Bitcoin Network == The Bitcoin Network
=== Peer-to-Peer Network Architecture === Peer-to-Peer Network Architecture
((("bitcoin network", id="ix_ch06-asciidoc0", range="startofrange")))((("bitcoin network","architecture of")))((("peer-to-peer networks")))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, that they are all equal, that there are no "special" nodes, and that 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, decentralized, and open. The preeminent 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 network", id="ix_ch08-asciidoc0", range="startofrange")))((("bitcoin network","architecture of")))((("peer-to-peer networks")))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, that they are all equal, that there are no "special" nodes, and that 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, decentralized, and open. The preeminent 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'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. Decentralization of control is a core design principle and that can only be achieved and maintained by a flat, decentralized 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. Decentralization of control is a core design principle and that can only be achieved and maintained by a flat, decentralized P2P consensus network.
@ -47,7 +47,7 @@ image::images/msbt_0603.png["BitcoinNetwork"]
=== Network Discovery === Network Discovery
((("bitcoin network","discovery", id="ix_ch06-asciidoc1", range="startofrange")))((("network discovery", id="ix_ch06-asciidoc2", range="startofrange")))((("nodes","network discovery and", id="ix_ch06-asciidoc3", range="startofrange")))((("peer-to-peer networks","discovery by new nodes", id="ix_ch06-asciidoc4", range="startofrange")))When a new node boots up, it must discover other bitcoin nodes on the network in order to participate. To start this process, a new node must discover at least one existing node on the network and connect to it. The geographic location of other nodes is irrelevant; the bitcoin network topology is not geographically defined. Therefore, any existing bitcoin nodes can be selected at random. ((("bitcoin network","discovery", id="ix_ch08-asciidoc1", range="startofrange")))((("network discovery", id="ix_ch08-asciidoc2", range="startofrange")))((("nodes","network discovery and", id="ix_ch08-asciidoc3", range="startofrange")))((("peer-to-peer networks","discovery by new nodes", id="ix_ch08-asciidoc4", range="startofrange")))When a new node boots up, it must discover other bitcoin nodes on the network in order to participate. To start this process, a new node must discover at least one existing node on the network and connect to it. The geographic location of other nodes is irrelevant; the bitcoin network topology is not geographically defined. Therefore, any existing bitcoin nodes can be selected at random.
((("peer-to-peer networks","connections")))To connect to a known peer, nodes establish a TCP connection, usually to port 8333 (the port generally known as the one used by bitcoin), or an alternative port if one is provided. Upon establishing a connection, the node will start a "handshake" (see <<network_handshake>>) by transmitting a((("version message"))) +version+ message, which contains basic identifying information, including: ((("peer-to-peer networks","connections")))To connect to a known peer, nodes establish a TCP connection, usually to port 8333 (the port generally known as the one used by bitcoin), or an alternative port if one is provided. Upon establishing a connection, the node will start a "handshake" (see <<network_handshake>>) by transmitting a((("version message"))) +version+ message, which contains basic identifying information, including:
@ -126,7 +126,7 @@ $ bitcoin-cli getpeerinfo
((("peer-to-peer networks","automatic management, overriding")))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. ((("peer-to-peer networks","automatic management, overriding")))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 and network problems, and can organically grow and shrink as needed without any central control.(((range="endofrange", startref="ix_ch06-asciidoc4")))(((range="endofrange", startref="ix_ch06-asciidoc3")))(((range="endofrange", startref="ix_ch06-asciidoc2")))(((range="endofrange", startref="ix_ch06-asciidoc1"))) 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 and network problems, and can organically grow and shrink as needed without any central control.(((range="endofrange", startref="ix_ch08-asciidoc4")))(((range="endofrange", startref="ix_ch08-asciidoc3")))(((range="endofrange", startref="ix_ch08-asciidoc2")))(((range="endofrange", startref="ix_ch08-asciidoc1")))
=== Full Nodes === Full Nodes
@ -154,7 +154,7 @@ This process of comparing the local blockchain with the peers and retrieving any
[[spv_nodes]] [[spv_nodes]]
=== Simplified Payment Verification (SPV) Nodes === Simplified Payment Verification (SPV) Nodes
((("nodes","SPV", id="ix_ch06-asciidoc5", range="startofrange")))((("nodes","lightweight", id="ix_ch06-asciidoc5a", range="startofrange")))((("simplified payment verification (SPV) nodes", id="ix_ch06-asciidoc6", range="startofrange")))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 _simplified 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. ((("nodes","SPV", id="ix_ch08-asciidoc5", range="startofrange")))((("nodes","lightweight", id="ix_ch08-asciidoc5a", range="startofrange")))((("simplified payment verification (SPV) nodes", id="ix_ch08-asciidoc6", range="startofrange")))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 _simplified 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.
((("blockchains","on SPV nodes")))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 because 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. ((("blockchains","on SPV nodes")))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 because 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.
@ -185,11 +185,11 @@ image::images/msbt_0607.png["SPVSynchronization"]
Because SPV nodes need to retrieve specific transactions in order to selectively verify them, they also create a privacy risk. Unlike full blockchain nodes, which collect all transactions within each block, the SPV node's requests for specific data can inadvertently reveal the addresses in their wallet. For example, a third party monitoring a network could keep track of all the transactions requested by a wallet on an SPV node and use those to associate bitcoin addresses with the user of that wallet, destroying the user's privacy. Because SPV nodes need to retrieve specific transactions in order to selectively verify them, they also create a privacy risk. Unlike full blockchain nodes, which collect all transactions within each block, the SPV node's requests for specific data can inadvertently reveal the addresses in their wallet. For example, a third party monitoring a network could keep track of all the transactions requested by a wallet on an SPV node and use those to associate bitcoin addresses with the user of that wallet, destroying the user's privacy.
Shortly after the introduction of SPV/lightweight nodes, the bitcoin developers added a feature called _bloom filters_ to address the privacy risks of SPV nodes. Bloom filters allow SPV nodes to receive a subset of the transactions without revealing precisely which addresses they are interested in, through a filtering mechanism that uses probabilities rather than fixed patterns.(((range="endofrange", startref="ix_ch06-asciidoc6")))(((range="endofrange", startref="ix_ch06-asciidoc5a")))(((range="endofrange", startref="ix_ch06-asciidoc5"))) Shortly after the introduction of SPV/lightweight nodes, the bitcoin developers added a feature called _bloom filters_ to address the privacy risks of SPV nodes. Bloom filters allow SPV nodes to receive a subset of the transactions without revealing precisely which addresses they are interested in, through a filtering mechanism that uses probabilities rather than fixed patterns.(((range="endofrange", startref="ix_ch08-asciidoc6")))(((range="endofrange", startref="ix_ch08-asciidoc5a")))(((range="endofrange", startref="ix_ch08-asciidoc5")))
=== Bloom Filters === Bloom Filters
((("bitcoin network","bloom filters and", id="ix_ch06-asciidoc7", range="startofrange")))((("bloom filters", id="ix_ch06-asciidoc8", range="startofrange")))((("Simplified Payment Verification (SPV) nodes","bloom filters and", id="ix_ch06-asciidoc9", range="startofrange")))A bloom filter is a probabilistic search filter, a way to describe a desired pattern without specifying it exactly. Bloom filters offer an efficient way to express a search pattern while protecting privacy. They are used by SPV nodes to ask their peers for transactions matching a specific pattern, without revealing exactly which addresses they are searching for. ((("bitcoin network","bloom filters and", id="ix_ch08-asciidoc7", range="startofrange")))((("bloom filters", id="ix_ch08-asciidoc8", range="startofrange")))((("Simplified Payment Verification (SPV) nodes","bloom filters and", id="ix_ch08-asciidoc9", range="startofrange")))A bloom filter is a probabilistic search filter, a way to describe a desired pattern without specifying it exactly. Bloom filters offer an efficient way to express a search pattern while protecting privacy. They are used by SPV nodes to ask their peers for transactions matching a specific pattern, without revealing exactly which addresses they are searching for.
In our previous analogy, a tourist without a map is asking for directions to a specific address, "23 Church St." If she asks strangers for directions to this street, she inadvertently reveals her destination. A bloom filter is like asking, "Are there any streets in this neighborhood whose name ends in R-C-H?" A question like that reveals slightly less about the desired destination than asking for "23 Church St." Using this technique, a tourist could specify the desired address in more detail as "ending in U-R-C-H" or less detail as "ending in H." By varying the precision of the search, the tourist reveals more or less information, at the expense of getting more or less specific results. If she asks a less specific pattern, she gets a lot more possible addresses and better privacy, but many of the results are irrelevant. If she asks for a very specific pattern, she gets fewer results but loses privacy. In our previous analogy, a tourist without a map is asking for directions to a specific address, "23 Church St." If she asks strangers for directions to this street, she inadvertently reveals her destination. A bloom filter is like asking, "Are there any streets in this neighborhood whose name ends in R-C-H?" A question like that reveals slightly less about the desired destination than asking for "23 Church St." Using this technique, a tourist could specify the desired address in more detail as "ending in U-R-C-H" or less detail as "ending in H." By varying the precision of the search, the tourist reveals more or less information, at the expense of getting more or less specific results. If she asks a less specific pattern, she gets a lot more possible addresses and better privacy, but many of the results are irrelevant. If she asks for a very specific pattern, she gets fewer results but loses privacy.
@ -246,7 +246,7 @@ Bitcoin's implementation of bloom filters is described in Bitcoin Improvement Pr
In response to a +getdata+ message from the node, peers will send a +merkleblock+ message that contains only block headers for blocks matching the filter and a merkle path (see <<merkle_trees>>) for each matching transaction. The peer will then also send +tx+ messages containing the transactions matched by the filter. In response to a +getdata+ message from the node, peers will send a +merkleblock+ message that contains only block headers for blocks matching the filter and a merkle path (see <<merkle_trees>>) for each matching transaction. The peer will then also send +tx+ messages containing the transactions matched by the filter.
The node setting the bloom filter can interactively add patterns to the filter by sending a((("filteradd message"))) +filteradd+ message. To clear the bloom filter, the node can send a((("filterclear message"))) +filterclear+ message. Because it is not possible to remove a pattern from a bloom filter, a node has to clear and resend a new bloom filter if a pattern is no longer desired.(((range="endofrange", startref="ix_ch06-asciidoc9")))(((range="endofrange", startref="ix_ch06-asciidoc8")))(((range="endofrange", startref="ix_ch06-asciidoc7"))) The node setting the bloom filter can interactively add patterns to the filter by sending a((("filteradd message"))) +filteradd+ message. To clear the bloom filter, the node can send a((("filterclear message"))) +filterclear+ message. Because it is not possible to remove a pattern from a bloom filter, a node has to clear and resend a new bloom filter if a pattern is no longer desired.(((range="endofrange", startref="ix_ch08-asciidoc9")))(((range="endofrange", startref="ix_ch08-asciidoc8")))(((range="endofrange", startref="ix_ch08-asciidoc7")))
[[transaction_pools]] [[transaction_pools]]
=== Transaction Pools === Transaction Pools
@ -295,7 +295,4 @@ Each node receiving this alert message will verify it, check for expiration, and
((("Bitcoin Core client","alerts, configuring")))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. ((("Bitcoin Core client","alerts, configuring")))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.
Other implementations of the bitcoin protocol might handle the alert in different ways. ((("mining","hardware, alerts and")))Many hardware-embedded bitcoin mining systems do not implement the alert message function because they have no user interface. It is strongly recommended that miners running such mining systems subscribe to alerts via a mining pool operator or by running a lightweight node just for alert purposes.(((range="endofrange", startref="ix_ch06-asciidoc0"))) Other implementations of the bitcoin protocol might handle the alert in different ways. ((("mining","hardware, alerts and")))Many hardware-embedded bitcoin mining systems do not implement the alert message function because they have no user interface. It is strongly recommended that miners running such mining systems subscribe to alerts via a mining pool operator or by running a lightweight node just for alert purposes.(((range="endofrange", startref="ix_ch08-asciidoc0")))