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425 lines
20 KiB
Markdown
425 lines
20 KiB
Markdown
{% comment %}
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This file is licensed under the MIT License (MIT) available on
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http://opensource.org/licenses/MIT.
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{% endcomment %}
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{% assign filename="_includes/guide_p2p_network.md" %}
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## P2P Network
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{% include helpers/subhead-links.md %}
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{% autocrossref %}
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The Bitcoin network protocol allows full nodes
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([peers][peer]{:#term-peer}{:.term}) to collaboratively maintain a
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[peer-to-peer network][network]{:#term-network}{:.term} for block and
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transaction exchange. Many SPV clients also use this protocol to connect
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to full nodes.
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Consensus rules do not cover networking, so Bitcoin programs may use
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alternative networks and protocols, such as the [high-speed block relay
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network][] used by some miners and the [dedicated transaction
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information servers][electrum server] used by some wallets that provide
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SPV-level security.
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To provide practical examples of the Bitcoin peer-to-peer network, this
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section uses Bitcoin Core as a representative full node and [BitcoinJ][]
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as a representative SPV client. Both programs are flexible, so only
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default behavior is described. Also, for privacy, actual IP addresses
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in the example output below have been replaced with [RFC5737][] reserved
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IP addresses.
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{% endautocrossref %}
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### Peer Discovery
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{% include helpers/subhead-links.md %}
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{% autocrossref %}
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When started for the first time, programs don't know the IP
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addresses of any active full nodes. In order to discover some IP
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addresses, they query one or more DNS names (called [DNS seeds][dns
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seed]{:#term-dns-seed}{:.term}) hardcoded into Bitcoin Core and
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BitcoinJ. The response to the lookup should include one or more [DNS
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A records][] with the IP addresses of full nodes that may accept new
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incoming connections. For example, using the [Unix `dig`
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command][dig command]:
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;; QUESTION SECTION:
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;seed.bitcoin.sipa.be. IN A
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;; ANSWER SECTION:
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seed.bitcoin.sipa.be. 60 IN A 192.0.2.113
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seed.bitcoin.sipa.be. 60 IN A 198.51.100.231
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seed.bitcoin.sipa.be. 60 IN A 203.0.113.183
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[...]
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The DNS seeds are maintained by Bitcoin community members: some of them
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provide dynamic DNS seed servers which automatically get IP addresses
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of active nodes by scanning the network; others provide static DNS
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seeds that are updated manually and are more likely to provide IP
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addresses for inactive nodes. In either case, nodes are added to the
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DNS seed if they run on the default Bitcoin ports of 8333 for mainnet
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or 18333 for testnet.
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<!-- paragraph below based on Greg Maxwell's email in
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http://comments.gmane.org/gmane.comp.bitcoin.devel/5378 -->
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DNS seed results are not authenticated and a malicious seed operator or
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network man-in-the-middle attacker can return only IP addresses of
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nodes controlled by the attacker, isolating a program on the attacker's
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own network and allowing the attacker to feed it bogus transactions and
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blocks. For this reason, programs should not rely on DNS seeds
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exclusively.
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Once a program has connected to the network, its peers can begin to send
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it `addr`
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(address<!--noref-->) messages with the IP addresses and port numbers of
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other peers on the network, providing a fully decentralized method of
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peer discovery. Bitcoin Core keeps a record of known peers in a
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persistent on-disk database which usually allows it to connect directly
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to those peers on subsequent startups without having to use DNS seeds.
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However, peers often leave the network or change IP addresses, so
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programs may need to make several different connection attempts at
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startup before a successful connection is made. This can add a
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significant delay to the amount of time it takes to connect to the
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network, forcing a user to wait before sending a transaction or checking
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the status of payment.
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<!-- reference for "Bitcoin Core...11 seconds" below:
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https://github.com/bitcoin/bitcoin/pull/4559 -->
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To avoid this possible delay, BitcoinJ always uses dynamic DNS seeds to
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get IP addresses for nodes believed to be currently active.
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Bitcoin Core also tries to strike a balance between minimizing delays
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and avoiding unnecessary DNS seed use: if Bitcoin Core has entries in
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its peer database, it spends up to 11 seconds attempting to connect to
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at least one of them before falling back to seeds; if a connection is
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made within that time, it does not query any seeds.
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<!-- reference for Bitcoin Core behavior described below: search for
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"FixedSeeds" in src/net.cpp; BitcoinJ has IPv4 seeds in its chainparams
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and a function to use them, but I don't see that function being used in
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any of the examples/wallet templates (but I'm not Java fluent, so
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maybe PEBKAC). -@harding -->
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Both Bitcoin Core and BitcoinJ also include a hardcoded list of IP
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addresses and port numbers to several dozen nodes which were active
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around the time that particular version of the software was first
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released. Bitcoin Core will start attempting to connect to these nodes
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if none of the DNS seed servers have responded to a query within 60
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seconds, providing an automatic fallback option.
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As a manual fallback option, Bitcoin Core also provides several
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command-line connection options, including the ability to get a list of
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peers from a specific node by IP address, or to make a persistent
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connection to a specific node by IP address. See the `-help` text for
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details. BitcoinJ can be programmed to do the same thing.
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**Resources:** [Bitcoin Seeder][], the program run by several of the
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seeds used by Bitcoin Core and BitcoinJ. The Bitcoin Core [DNS Seed
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Policy][]. The hardcoded list of IP addresses used by Bitcoin Core and
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BitcoinJ is generated using the [makeseeds script][].
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{% endautocrossref %}
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### Connecting To Peers
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{% include helpers/subhead-links.md %}
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{% autocrossref %}
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Connecting to a peer is done by sending a `version` message, which
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contains your version number, block, and current time to the remote
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node. The remote node responds with its own `version` message. Then both
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nodes send a `verack` message to the other node to indicate the
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connection has been established.
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Once connected, the client can send to the remote node `getaddr` and `addr` messages to gather additional peers.
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In order to maintain a connection with a peer, nodes by default will send a message to peers before 30 minutes of inactivity. If 90 minutes pass without a message being received by a peer, the client will assume that connection has closed.
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{% endautocrossref %}
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### Initial Block Download
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{% include helpers/subhead-links.md %}
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{% autocrossref %}
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Before a full node can validate unconfirmed transactions and
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recently-mined blocks, it must download and validate all blocks from
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block 1 (the block after the hardcoded genesis block) to the current tip
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of the best block chain. This is the Initial Block Download (IBD) or
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initial sync.
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Although the word "initial" implies this method is only used once, it
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can also be used any time a large number of blocks need to be
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downloaded, such as when a previously-caught-up node has been offline
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for a long time. In this case, a node can use the IBD method to download
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all the blocks which were produced since the last time it was online.
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Bitcoin Core uses the IBD method any time the last block on its local
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best block chain has a block header time more than 24 hours in the past.
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Bitcoin Core 0.10.0 will also perform IBD if its local best block chain is
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more than 144 blocks lower than its local best headers chain (that is,
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the local block chain is more than about 24 hours in the past).
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{% endautocrossref %}
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#### Blocks-First
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{% include helpers/subhead-links.md %}
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{% autocrossref %}
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Bitcoin Core (up until version [0.9.3][bitcoin core 0.9.3]) uses a
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simple initial block download (IBD) method we'll call *blocks-first*.
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The goal is to download the blocks from the best block chain in sequence.
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The first time a node is started, it only has a single block in its
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local best block chain---the hardcoded genesis block (block 0). This
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node chooses a remote peer, called the sync node, and sends it the
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`getblocks` message illustrated below.
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In the header hashes field of the `getblocks` message, this new node
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sends the header hash of the only block it has, the genesis block
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(6fe2...0000 in internal byte order). It also sets the stop hash field
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to all zeroes to request a maximum-size response.
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Upon receipt of the `getblocks` message, the sync node takes the first
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(and only) header hash and searches its local best block chain for a
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block with that header hash. It finds that block 0 matches, so it
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replies with 500 block inventories (the maximum response to a
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`getblocks` message) starting from block 1. It sends these inventories
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in the `inv` message illustrated below.
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Inventories are unique identifiers for information on the network. Each
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inventory contains a type field and the unique identifier for an
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instance of the object. For blocks, the unique identifier is a hash of
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the block's header.
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The block inventories appear in the `inv` message in the same order they
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appear in the block chain, so this first `inv` message contains
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inventories for blocks 1 through 501. (For example, the hash of block 1
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is 4860...0000 as seen in the illustration above.)
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The IBD node uses the received inventories to request 128 blocks from
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the sync node in the `getdata` message illustrated below.
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It's important to blocks-first nodes that the blocks be requested and
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sent in order because each block header references the header hash of
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the preceding block. That means the IBD node can't fully validate a
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block until its parent block has been received. Blocks that can't be
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validated because their parents haven't been received are called orphan
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blocks; a subsection below describes them in more detail.
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Upon receipt of the `getdata` message, the sync node replies with each
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of the blocks requested. Each block is put into serialized block format
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and sent in a separate `block` message. The first `block` message sent
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(for block 1) is illustrated below.
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The IBD node downloads each block, validates it, and then requests the
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next block it hasn't requested yet, maintaining a queue of up to 128
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blocks to download. When it has requested every block for which it has
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an inventory, it sends another `getblocks` message to the sync node
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requesting the inventories of up to 500 more blocks. This second
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`getblocks` message contains multiple header hashes as illustrated
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below:
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Upon receipt of the second `getblocks` message, the sync node searches
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its local best block chain for a block that matches one of the header
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hashes in the message, trying each hash in the order they were received.
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If it finds a matching hash, it replies with 500 block inventories
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starting with the next block from that point. But if there is no
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matching hash (besides the stopping hash), it assumes the only block the
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two nodes have in common is block 0 and so it sends an `inv` starting with
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block 1 (the same `inv` message seen several illustrations above).
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This repeated search allows the sync node to send useful inventories even if
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the IBD node's local block chain forked from the sync node's local block
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chain. This fork detection becomes increasingly useful the closer the
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IBD node gets to the tip of the block chain.
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When the IBD node receives the second `inv` message, it will request
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those blocks using `getdata` messages. The sync node will respond with
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`block` messages. Then the IBD node will request more inventories with
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another `getblocks` message---and the cycle will repeat until the IBD
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node is synced to the tip of the block chain. At that point, the node
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will accept blocks sent through the regular block broadcasting described
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in a later subsection.
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{% endautocrossref %}
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##### Blocks-First Advantages & Disadvantages
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{:.no_toc}
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{% include helpers/subhead-links.md %}
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{% autocrossref %}
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The primary advantage of blocks-first IBD is its simplicity. The primary
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disadvantage is that the IBD node relies on a single sync node for all
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of its downloading. This has several implications:
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* **Speed Limits:** All requests are made to the sync node, so if the
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sync node has limited upload bandwidth, the IBD node will have slow
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download speeds. Note: if the sync node goes offline, Bitcoin Core
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will continue downloading from another node---but it will still only
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download from a single sync node at a time.
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* **Download Restarts:** The sync node can send a non-best (but
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otherwise valid) block chain to the IBD node. The IBD node won't be
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able to identify it as non-best until the initial block download nears
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completion, forcing the IBD node to restart its block chain download
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over again from a different node. Bitcoin Core ships with several
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block chain checkpoints at various block heights selected by
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developers to help an IBD node detect that it is being fed an
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alternative block chain history---allowing the IBD node to restart
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its download earlier in the process.
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* **Disk Fill Attacks:** Closely related to the download restarts, if
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the sync node sends a non-best (but otherwise valid) block chain, the
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chain will be stored on disk, wasting space and possibly filling up
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the disk drive with useless data.
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* **High Memory Use:** Whether maliciously or by accident, the sync node
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can send blocks out of order, creating orphan blocks which can't be
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validated until their parents have been received and validated.
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Orphan blocks are stored in memory while they await validation,
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which may lead to high memory use.
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All of these problems are addressed in part or in full by the
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headers-first IBD method used in Bitcoin Core 0.10.0.
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**Resources:** The table below summarizes the messages mentioned
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throughout this subsection. The links in the message field will take you
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to the reference page for that message.
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| **Message** | [`getblocks`][getblocks message] | [`inv`][inv message] | [`getdata`][getdata message] | [`block`][block message]
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| **From→To** | IBD→Sync | Sync→IBD | IBD→Sync | Sync→IBD
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| **Payload** | One or more header hashes | Up to 500 block inventories (unique identifiers) | One or more block inventories | One serialized block
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{% endautocrossref %}
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### Block Broadcasting
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{% include helpers/subhead-links.md %}
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{% autocrossref %}
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At the start of a connection with a peer, both nodes send `getblocks` messages containing the hash of the latest known block. If a peer believes they have newer blocks or a longer chain, that peer will send an `inv` message which includes a list of up to 500 hashes of newer blocks, stating that it has the longer chain. The receiving node would then request these blocks using the command `getdata`, and the remote peer would reply via `block`<!--noref--> messages. After all 500 blocks have been processed, the node can request another set with `getblocks`, until the node is caught up with the network. Blocks are only accepted when validated by the receiving node.
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New blocks are also discovered as miners publish their found blocks, and these messages are propagated in a similar manner. Through previously established connections, an `inv` message is sent with the new block hashed, and the receiving node requests the block via the `getdata` message.
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{% endautocrossref %}
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#### Orphan Blocks
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{% include helpers/subhead-links.md %}
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{% autocrossref %}
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Blocks-first nodes may download orphan blocks---blocks whose previous
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block header hash field refers to a block header this node
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hasn't seen yet. In other words, orphan blocks have no known parent
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(unlike stale blocks, which have known parents but which aren't part of
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the best block chain).
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When a blocks-first node downloads an orphan block, it will not validate
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it. Instead, it will send a `getblocks` message to the node which sent
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the orphan block; the broadcasting node will respond with an `inv` message
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containing inventories of any blocks the downloading node is missing (up
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to 500); the downloading node will request those blocks with a `getdata`
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message; and the broadcasting node will send those blocks with a `block`
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message. The downloading node will validate those blocks, and once the
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parent of the former orphan block has been validated, it will validate
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the former orphan block.
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{% endautocrossref %}
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### Transaction Broadcasting
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{% include helpers/subhead-links.md %}
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{% autocrossref %}
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In order to send a transaction to a peer, an `inv` message is sent. If a `getdata` response message is received, the transaction is sent using `tx`. The peer receiving this transaction also forwards the transaction in the same manner, given that it is a valid transaction.
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{% endautocrossref %}
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#### Memory Pool
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{% include helpers/subhead-links.md %}
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{% autocrossref %}
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Full peers may keep track of unconfirmed transactions which are eligible to
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be included in the next block. This is essential for miners who will
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actually mine some or all of those transactions, but it's also useful
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for any peer who wants to keep track of unconfirmed transactions, such
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as peers serving unconfirmed transaction information to SPV clients.
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Because unconfirmed transactions have no permanent status in Bitcoin,
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Bitcoin Core stores them in non-persistent memory, calling them a memory
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pool or mempool. When a peer shuts down, its memory pool is lost except
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for any transactions stored by its wallet. This means that never-mined
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unconfirmed transactions tend to slowly disappear from the network as
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peers restart or as they purge some transactions to make room in memory
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for others.
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Transactions which are mined into blocks that later become stale blocks may be
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added back into the memory pool. These re-added transactions may be
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re-removed from the pool almost immediately if the replacement blocks
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include them. This is the case in Bitcoin Core, which removes stale
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blocks from the chain one by one, starting with the tip (highest block).
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As each block is removed, its transactions are added back to the memory
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pool. After all of the stale blocks are removed, the replacement
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blocks are added to the chain one by one, ending with the new tip. As
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each block is added, any transactions it confirms are removed from the
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memory pool.
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SPV clients don't have a memory pool for the same reason they don't
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relay transactions. They can't independently verify that a transaction
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hasn't yet been included in a block and that it only spends UTXOs, so
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they can't know which transactions are eligible to be included in the
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next block.
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{% endautocrossref %}
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### Misbehaving Nodes
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{% include helpers/subhead-links.md %}
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{% autocrossref %}
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Take note that for both types of broadcasting, mechanisms are in place to punish misbehaving peers who take up bandwidth and computing resources by sending false information. If a peer gets a banscore above the `-banscore=<n>` threshold, he will be banned for the number of seconds defined by `-bantime=<n>`, which is 86,400 by default (24 hours).
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{% endautocrossref %}
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### Alerts
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{% include helpers/subhead-links.md %}
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{% autocrossref %}
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In case of a bug or attack,
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the Bitcoin Core developers provide a
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[Bitcoin alert service](https://bitcoin.org/en/alerts) with an RSS feed
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and users of Bitcoin Core can check the error field of the `getinfo` RPC
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results to get currently active alerts for their specific version of
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Bitcoin Core.
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These messages are aggressively broadcast using the `alert` message, being sent to each peer upon connect for the duration of the alert.
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These messages are signed by a specific ECDSA private key that only a small number of developers control.
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**Resource:** More details about the structure of messages and a complete list of message types can be found in
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the [P2P reference section][section P2P reference].
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{% endautocrossref %}
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