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@ -130,7 +130,7 @@ If there is no traffic on a connection, nodes will periodically send a message t
((("blockchains","full nodes and")))((("full nodes")))((("nodes","full")))Full nodes are nodes that maintain a full blockchain with all transactions. More accurately, they probably should be called "full blockchain nodes." In the early years of bitcoin, all nodes were full nodes and currently the Bitcoin Core client is a full blockchain node. In the past two years, however, new forms of bitcoin clients have been introduced that do not maintain a full blockchain but run as lightweight clients. We'll examine these in more detail in the next section.
((("blockchains","on full nodes")))Full blockchain nodes maintain a complete and up-to-date copy of the bitcoin block chain with all the transactions, which they independently build and verify, starting with the very first block (genesis block) and building up to the latest known block in the network. A full block chain node can independently and authoritatively verify any transaction without recourse or reliance on any other node or source of information. The full block chain 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 block chain.
((("blockchains","on full nodes")))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 building 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 20+ gigabytes of persistent storage (disk space) to store the full blockchain. If you need a lot of disk and it takes two to three days to sync to the network, you are running a full node. That is the price of complete independence and freedom from central authority.
@ -157,21 +157,21 @@ image::images/msbt_0606.png["InventorySynchronization"]
((("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.
((("block chains","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 block chain. 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 block chain 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.
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. Although 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 mapless tourist's best chance is to ask enough people and hope some of them are not trying to mug him.
Simplified payment verification verifies transactions by reference to their _depth_ in the block chain instead of their _height_. Whereas a full block chain node will construct a fully verified chain of thousands of blocks and transactions reaching down the block chain (back in time) all the way to the genesis block, an SPV node will verify the chain of all blocks and link that chain to the transaction of interest.
Simplified payment verification verifies transactions by reference to their _depth_ in the blockchain instead of their _height_. Whereas a full blockchain node will construct a fully verified chain of thousands of blocks and transactions reaching down the blockchain (back in time) all the way to the genesis block, an SPV node will verify the chain of all blocks and link that chain to the transaction of interest.
For example, when examining a transaction in block 300,000, a full node links all 300,000 blocks down to the genesis block and builds a full database of UTXO, establishing the validity of the transaction by confirming that the UTXO remains unspent. An SPV node cannot validate whether the UTXO is unspent. Instead, the SPV node will establish a link between the transaction and the block that contains it, using a((("merkle trees","SPV and"))) _merkle path_ (see <<merkle_trees>>). Then, the SPV node waits until it sees the six blocks 300,001 through 300,006 piled on top of the block containing the transaction and verifies it by establishing its depth under blocks 300,006 to 300,001. The fact that other nodes on the network accepted block 300,000 and then did the necessary work to produce six more blocks on top of it is proof, by proxy, that the transaction was not a double-spend.
An SPV node cannot be persuaded that a transaction exists in a block when the transaction 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 vulnerability can be used in a denial-of-service attack or for 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. This need to randomly connect means that SPV nodes also are 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 infallible security, however, nothing beats running a full block chain node.
For most practical purposes, well-connected SPV nodes are secure enough, striking the right balance between resource needs, practicality, and security. For infallible security, however, nothing beats running a full blockchain node.
[TIP]
====
((("simplified payment verification (SPV) nodes","verification")))A full block chain node verifies a transaction by checking the entire chain of thousands of blocks below it in order to guarantee that the UTXO is not spent, whereas an SPV node checks how deep the block is buried by a handful of blocks above it.
((("simplified payment verification (SPV) nodes","verification")))A full blockchain node verifies a transaction by checking the entire chain of thousands of blocks below it in order to guarantee that the UTXO is not spent, whereas an SPV node checks how deep the block is buried by a handful of blocks above it.
====
((("block headers","getting on SPV nodes")))To get the block headers, SPV nodes use a((("getheaders message"))) +getheaders+ message instead of +getblocks+. The responding peer will send up to 2,000 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 messages"))) +tx+ message containing the transactions, in response. <<spv_synchronization>> shows the synchronization of block headers.
@ -180,7 +180,7 @@ For most practical purposes, well-connected SPV nodes are secure enough, strikin
.SPV node synchronizing the block headers
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 block chain 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")))
@ -247,7 +247,7 @@ The node setting the bloom filter can interactively add patterns to the filter b
[[transaction_pools]]
=== Transaction Pools
((("bitcoin network","transaction pools")))((("transaction pools")))((("transactions","unconfirmed, pools of")))((("unconfirmed transactions")))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 block chain. 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.
((("bitcoin network","transaction pools")))((("transaction pools")))((("transactions","unconfirmed, pools of")))((("unconfirmed transactions")))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.
@ -257,13 +257,13 @@ When a transaction is added to the transaction pool, the orphan pool is checked
((("orphan transaction pool","storage")))((("transaction pools","storage")))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.
Some implementations of the bitcoin client also maintain a UTXO database or UTXO pool, which is the set of all unspent outputs on the block chain. Although the name "UTXO pool" sounds similar to the transaction pool, it represents a different set of data. Unlike the transaction and orphan pools, the UTXO pool is not initialized empty but instead contains millions of entries of unspent transaction outputs, including some dating back to 2009. The UTXO pool may be housed in local memory or as an indexed database table on persistent storage.
Some implementations of the bitcoin client also maintain a UTXO database or UTXO pool, which is the set of all unspent outputs on the blockchain. Although the name "UTXO pool" sounds similar to the transaction pool, it represents a different set of data. Unlike the transaction and orphan pools, the UTXO pool is not initialized empty but instead contains millions of entries of unspent transaction outputs, including some dating back to 2009. The UTXO pool may be housed in local memory or as an indexed database table on persistent storage.
Whereas the transaction and orphan pools represent a single node's local perspective and might vary significantly from node to node depending upon when the node was started or restarted, the UTXO pool represents the emergent consensus of the network and therefore will vary little between nodes. Furthermore, the transaction and orphan pools only contain unconfirmed transactions, while the UTXO pool only contains confirmed outputs.
=== Alert Messages
((("alert messages")))((("bitcoin network","alert messages")))Alert messages are a seldom used function, but are nevertheless implemented in most nodes. Alert messages are bitcoin's "emergency broadcast system," a means by which the core bitcoin developers can send an emergency text message to all bitcoin nodes. This feature is implemented to allow the core developer team to notify all bitcoin users of a serious problem in the bitcoin network, such as a critical bug that requires user action. The alert system has only been used a handful of times, most notably in early 2013 when a critical database bug caused a multiblock fork to occur in the bitcoin block chain.
((("alert messages")))((("bitcoin network","alert messages")))Alert messages are a seldom used function, but are nevertheless implemented in most nodes. Alert messages are bitcoin's "emergency broadcast system," a means by which the core bitcoin developers can send an emergency text message to all bitcoin nodes. This feature is implemented to allow the core developer team to notify all bitcoin users of a serious problem in the bitcoin network, such as a critical bug that requires user action. The alert system has only been used a handful of times, most notably in early 2013 when a critical database bug caused a multiblock fork to occur in the bitcoin blockchain.
Alert messages are propagated by the +alert+ message. The alert message contains several fields, including: