All nodes include the routing function to participate in the network and may include other functionality. All nodes validate and propagate transactions and blocks, and discover and maintain connections to peers. In the full node example above, the routing function is indicated by an orange circle named "Network Routing Node".
Some nodes, called full nodes, also maintain a complete and up-to-date copy of the blockchain. Full nodes can autonomously and authoritatively verify any transaction without external reference. Some nodes maintain only a subset of the blockchain and verify transactions using a method called _Simple Payment Verification_ or SPV. These nodes are known as SPV or Lightweight nodes. In the full node example above, the full node blockchain database function is indicated by a blue circle named "Full Blockchain". SPV nodes are drawn without the blue circle, showing that they do not have a full copy of the blockchain.
Some nodes, called full nodes, also maintain a complete and up-to-date copy of the blockchain. Full nodes can autonomously and authoritatively verify any transaction without external reference. Some nodes maintain only a subset of the blockchain and verify transactions using a method called _Simplified Payment Verification_ or SPV. These nodes are known as SPV or Lightweight nodes. In the full node example above, the full node blockchain database function is indicated by a blue circle named "Full Blockchain". SPV nodes are drawn without the blue circle, showing that they do not have a full copy of the blockchain.
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 "Miner".
@ -149,15 +149,15 @@ This process of comparing the local blockchain with the peers and retrieving any
.Node synchronizing the blockchain by retrieving blocks from a peer
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.
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.
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.
Simple 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.
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 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 6 more blocks on top of it is proof, by proxy, that the transaction was not a double-spend.
As you can see from the table above, while the block size increases rapidly, from 4KB with 16 transactions to a block size of 16 MB to fit 65,535 transactions, the merkle path required to prove the inclusion of a transaction increases much more slowly, from 128 bytes to only 512 bytes. With merkle trees, a node can download just the block headers (80 bytes per block) and still be able to identify a transaction's inclusion in a block by retrieving a small merkle path from a full node, without storing or transmitting the vast majority of the blockchain which may be several gigabytes in size. Nodes which do not maintain a full blockchain, called Simple Payment Verification or SPV nodes use merkle paths to verify transactions without downloading full blocks.
As you can see from the table above, while the block size increases rapidly, from 4KB with 16 transactions to a block size of 16 MB to fit 65,535 transactions, the merkle path required to prove the inclusion of a transaction increases much more slowly, from 128 bytes to only 512 bytes. With merkle trees, a node can download just the block headers (80 bytes per block) and still be able to identify a transaction's inclusion in a block by retrieving a small merkle path from a full node, without storing or transmitting the vast majority of the blockchain which may be several gigabytes in size. Nodes which do not maintain a full blockchain, called Simplified Payment Verification or SPV nodes use merkle paths to verify transactions without downloading full blocks.
=== Merkle Trees and Simple Payment Verification (SPV)
=== Merkle Trees and Simplified Payment Verification (SPV)
Merkle trees are used extensively by Simple Payment Verification nodes. SPV nodes don't have all transactions and do not download full blocks, just block headers. In order to verify that a transaction is included in a block, without having to download all the transactions in the block, they use an _authentication path_, or merkle path.
Merkle trees are used extensively by Simplified Payment Verification nodes. SPV nodes don't have all transactions and do not download full blocks, just block headers. In order to verify that a transaction is included in a block, without having to download all the transactions in the block, they use an _authentication path_, or merkle path.
Consider for example an SPV node that is interested in incoming payments to an address contained in its wallet. The SPV node will establish a bloom filter on its connections to peers to limit the transactions received to only those containing addresses of interest. When a peer sees a transaction that matches the bloom filter, it will send that block using a +merkleblock+ message. The +merkleblock+ message contains the block header as well as a merkle path that links the transaction of interest to the merkle root in the block. The SPV node can use this merkle path to connect the transaction to the block and verify that the transaction is included in the block. The SPV node also uses the block header to link the block to the rest of the blockchain. The combination of these two links, between the transaction and block, and between the block and blockchain, proves that the transaction is recorded in the blockchain. All in all, the SPV node will have received less than a kilobyte of data for the block header and merkle path, an amount of data that is more than a thousand times less than a full block (about 1 megabyte currently).
Minh T. Nguyen
10 years ago