<p>Please excuse the unfinished state of this paper. It is being actively worked on. The content here is made available early because it contains useful information for developers.</p>
<p>For more technical information about LBRY, visit <ahref="https://lbry.tech">lbry.tech</a>.</p>
<p>LBRY is a protocol for accessing and publishing digital content in a global, decentralized marketplace. Clients can use LBRY to publish, host, find, download, and pay for content — books, movies, music, or anything else that can be represented as a stream of bits. Anyone can participate and no permission is required, nor can anyone be blocked from participating. The system is distributed, so no single entity has unilateral control, nor will the removal of any single entity prevent the system from functioning.</p>
<p>This document defines the LBRY protocol, its components, and how they fit together. LBRY consists of several discrete components that are used together in order to provide the end-to-end capabilities of the protocol. There are two distributed data stores (blockchain and DHT), a peer-to-peer protocol for exchanging data, and several specifications for data structure, encoding, and retrieval.</p>
<p>This document assumes that the reader is familiar with Bitcoin and blockchain technology. It does not attempt to document the Bitcoin protocol or explain how it works. The <ahref="https://bitcoin.org/en/developer-reference">Bitcoin developer reference</a> is recommended for anyone wishing to understand the technical details.</p>
<dd>The output of a cryptographic hash function is applied to a blob. Hashes are used to uniquely identify blobs and to verify that the contents of the blob are correct. Unless otherwise specified, LBRY uses SHA384 as the hash function.</dd>
<p>The LBRY blockchain is a fork of the <ahref="https://bitcoin.org/bitcoin.pdf">Bitcoin</a> blockchain, with substantial modifications. This document does not cover or specify any aspects of LBRY that are identical to Bitcoin, and instead focuses on the differences.</p>
<p>A <em>stake</em> is a a single entry in the blockchain that commits credits toward a name. The two types of stakes are <ahref="#claims"><em>claims</em></a> and <ahref="#supports"><em>supports</em></a>.</p>
<p>All stakes have these properties:</p>
<dl>
<dt>id</dt>
<dd>A 20-byte hash unique among all stakes. See <ahref="#stake-identifier-generation">Stake Identifier Generation</a>.</dd>
<dt>amount</dt>
<dd>A quantity of tokens used to back the stake. See <ahref="#controlling">Controlling</a>.</dd>
</dl>
<h4id="claims">Claims</h4>
<p>A <em>claim</em> is a stake that stores metadata. There are two types of claims:</p>
<figcaption>Note: the blockchain treats the <code>value</code> as an opaque byte string and does not impose any structure on it. Structure is applied and validated <ahref="#metadata-validation">higher in the stack</a>. The value is shown here for demonstration purposes only.
<p>Supports are created and abandoned just like claims (see <ahref="#claim-operations">Claim Operations</a>). They cannot be updated or themselves supported.</p>
<p>The claimtrie is implemented as a <ahref="https://en.wikipedia.org/wiki/Merkle_tree">Merkle tree</a> that maps names to claims. Claims are stored as leaf nodes in the tree. Names are stored as the <ahref="#normalization">normalized</a> path from the root node to the leaf node.</p>
<p>The <em>root hash</em> is the hash of the root node. It is stored in the header of each block in the blockchain. Nodes use the root hash to efficiently and securely validate the state of the claimtrie.</p>
<p>Multiple claims can exist for the same name. They are all stored in the leaf node for that name, sorted by <ahref="#active">effective amount</a> (descending), then by block height (ascending), then by transaction order in the block (ascending).</p>
<p>For more details on the specific claimtrie implementation, see <ahref="https://github.com/lbryio/lbrycrd/blob/master/src/claimtrie.cpp">the source code</a>.</p>
<p>An <em>accepted</em> stake is one that has been been entered into the blockchain. This happens when the transaction containing it is included in a block.</p>
<p>An <em>abandoned</em> stake is one that was withdrawn by its creator or current owner. Spending a transaction that contains a stake will cause that claim to become abandoned. Abandoned claim stakes are removed from the claimtrie.</p>
<p>While data related to abandoned stakes still resides in the blockchain, it should be considered invalid and should not be used to resolve URLs or fetch the associated content. Active claim stakes signed by abandoned identities are also considered invalid.</p>
<p>An <em>active</em> stake is an accepted and non-abandoned stake that has been in the blockchain for an algorithmically determined number of blocks. This length of time required is called the <em>activation delay</em>.</p>
<p>If the stake is an update to an active claim, is the first claim for a name, or does not cause a change in which claim is controlling the name, the activation delay is 0 (i.e. the stake becomes active in the same block it is accepted).</p>
<p>Otherwise, the activation delay is determined by a formula covered in <ahref="#claimtrie-transitions">Claimtrie Transitions</a>. The formula’s inputs are the height of the current block, the height at which the stake was accepted, and the height at which the controlling claim for that name last changed.</p>
<p>The sum of the amount of an active claim and all of its active supports is called its <em>effective amount</em>. The effective amount affects the sort order of claims in a leaf node, and which claim is controlling for that name. Claims that are not active have an effective amount of 0.</p>
<p>A <em>controlling</em> claim is the active claim that is first in the sort order of a leaf node. That is, it has the highest effective amount of all claims with the same name.</p>
<li><p>Sort the claims by effective amount in descending order. Claims tied for the same amount are ordered by block height (lowest first), then by transaction order within the block.</p></li>
<li><p>Otherwise, a takeover is occurring. Set the takeover height for this name to the current height, recalculate which stakes are now active, and redo steps 1 and 2.</p></li>
<p>The purpose of 4 is to handle the case when multiple competing claims are made on the same name in different blocks, and one of those claims becomes active but another still-inactive claim has the greatest effective amount. Step 4 will cause the greater claim to also activate and become the controlling claim.</p>
<p>In written form, the delay before a stake becomes active is equal to the height at which the stake was accepted minus height of the last takeover, divided by 32. This delay is capped at a maximum of 4032 blocks, which is 7 days of blocks at 2.5 minutes per block (our target block time). It takes approximately 224 days without a takeover to reach the max delay.</p>
<p>The purpose of this delay is to give long-standing claimants time to respond to changes, while still keeping takeover times reasonable and allowing recent or contentious claims to change state quickly.</p>
<p><strong>Block 1010:</strong> Support X for 14LBC for claim A is accepted. Since it is a support for the controlling claim, it activates immediately.
<p><strong>Block 1020:</strong> Claim C for 50LBC is accepted. The activation height is <code>1020 + min(4032, floor((1020-13) / 32)) = 1020 + 31 = 1051</code>.
<br>State: A(10+14) is controlling, B(20) is accepted, C(50) is accepted.</p>
<p><strong>Block 1031:</strong> Claim B activates. It has 20LBC, while claim A has 24LBC (10 original + 14 from support X). There is no takeover, and claim A remains controlling.
<p><strong>Block 1040:</strong> Claim D for 300LBC is accepted. The activation height is <code>1040 + min(4032, floor((1040-13) / 32)) = 1040 + 32 = 1072</code>.
<br>State: A(10+14) is controlling, B(20) is active, C(50) is accepted, D(300) is accepted.</p>
<p><strong>Block 1051:</strong> Claim C activates. It has 50LBC, while claim A has 24LBC, so a takeover is initiated. The takeover height for this name is set to 1051, and therefore the activation delay for all the claims becomes <code>min(4032, floor((1051-1051) / 32)) = 0</code>. All the claims become active. The totals for each claim are recalculated, and claim D becomes controlling because it has the highest total.
<p>Names in the claimtrie are normalized when performing any comparisons. This is necessary to avoid confusion due to Unicode equivalence or casing. When names are being compared, they are first converted using <ahref="http://unicode.org/reports/tr15/#Norm_Forms">Unicode Normalization Form D</a> (NFD), then lowercased using the en_US locale. This means names are effectively case-insensitive. Since claims competing for the same name are stored in the same node in the claimtrie, names are also normalized to determine the claimtrie path to the node.</p>
grin: no, but we should probably include an example for how to do the validation using the root hash. its not strictly necessary because its similar to how bitcoin does it. so maybe link to https://lbry.tech/resources/claimtrie (which needs an update) and add a validation example there?
<p>The ultimate purpose of much of the claim and blockchain design is to provide human-readable URLs that can be provably resolved by clients without a full copy of the blockchain (i.e. <ahref="https://lbry.tech/glossary#spv">Simplified Payment Verification</a> wallets).</p>
<p>A URL is a name with one or more modifiers. A bare name on its own resolves to the <ahref="#controlling">controlling claim</a> at the latest block height. Here are some common URL structures.</p>
<h5id="channel-claim-name-and-stream-claim-name">Channel Claim Name and Stream Claim Name</h5>
<p>A URL containing both a channel and a stream claim name. URLs containing both are resolved in two steps. First, the channel is resolved to it’s associated claim. Then the stream claim name is resolved to get the appropriate claim from among the claims in the channel.</p>
<p>The <em>n</em>th accepted claim for this name. <em>n</em> must be a positive number. This can be used to resolve claims in the order in which they were made, rather than by the amount of credits backing a claim.</p>
<p>The <em>n</em>th claim for this name, ordered by total amount (highest first). <em>n</em> must be a positive number. This is useful for resolving non-winning bids in bid order.</p>
<p>Get all claims for the claim name whose IDs start with the given <code>ClaimID</code>. Sort the claims in ascending order by block height and position within the block. Return the first claim.</p>
<p>Get all claims for the claim name. Sort the claims in ascending order by block height and position within the block. Return the Nth claim, where N is the given <code>ClaimSequence</code> value.</p>
<p>Get all claims for the claim name. Sort the claims in descending order by total effective amount. Return the Nth claim, where N is the given <code>BidSequence</code> value.</p>
<h5id="channelname-and-claimname">ChannelName and ClaimName</h5>
<p>If both a channel name and a claim name are present, resolution happens in two steps. First, remove the <code>/</code> and <code>StreamClaimNameAndModifier</code> from the path, and resolve the URL as if it only had a <code>ChannelClaimNameAndModifier</code>. Then get the list of all claims in that channel. Finally, resolve the <code>StreamClaimNameAndModifier</code> as if it was its own URL, but instead of considering all claims, only consider the set of claims in the channel.</p>
<p>If multiple claims for the same name exist inside the same channel, they are resolved via the same resolution rules applied entirely within the sub-scope of the channel.</p>
<p>The most contentious aspect of this design is the choice to resolve naked names (sometimes called <em>vanity names</em>) to the claim with the highest effective amount.</p>
<p>First, it is important to note the problems in existing name allocation designs. Most existing public name schemes are first-come, first-serve with a fixed price. This leads to several bad outcomes:</p>
<li><p>Speculation and extortion. Entrepreneurs are incentivized to register common names even if they don’t intend to use them, in hopes of selling them to the proper owner in the future for an exorbitant price. While speculation in general can have positive externalities (stable prices and price signals), in this case it is pure value extraction. Speculation also harms the user experience, who will see the vast majority of URLs sitting unused (c.f. Namecoin).</p></li>
<li><p>Bureaucracy and transaction costs. While a centralized system can allow for an authority to use a process to reassign names based on trademark or other common use reasons, this system is also imperfect. Most importantly, it is a censorship point and an avenue for complete exclusion. Additionally, such processes are often arbitrary, change over time, involve significant transaction costs, and <em>still</em> lead to names being used in ways that are contrary to user expectation (e.g. <ahref="http://nissan.com">nissan.com</a>).</p></li>
<li><p>Inefficiencies from price controls. Any system that does not allow a price to float freely creates inefficiencies. If the set price is too low, there is speculation and rent-seeking. If the price is too high, people are excluded from a good that it would otherwise be beneficial for them to purchase.</p></li>
<p>Instead, LBRY has an algorithmic design built into consensus that encourage URLs to flow to their highest valued use. Following <ahref="https://en.wikipedia.org/wiki/Coase_theorem">Coase</a>, this staking design allows for clearly defined rules, low transaction costs, and no information asymmetry, minimizing inefficiency in URL allocation.</p>
<p>Finally, it’s important to note that <em>only</em> vanity URLs have this property. Short, memorable URLs like <code>lbry://myclaimname#a</code> exist and are available for the minimal cost of issuing a transaction.</p>
<p>To enable <ahref="#claim-operations">claim operations</a>, three new opcodes were added to the scripting language: <code>OP_CLAIM_NAME</code>, <code>OP_UPDATE_CLAIM</code>, and <code>OP_SUPPORT_CLAIM</code>. In Bitcoin they are respectively <code>OP_NOP6</code>, <code>OP_NOP7</code>, and <code>OP_NOP8</code>. The opcodes are used in output scripts to interact with the claimtrie. Each opcode is followed by one or more parameters. Here’s how these opcodes are used:</p>
<p>The <code><name></code> parameter is the [[name]] that the claim is associated with. <code><value></code> is the protobuf-encoded claim metadata and optional channel signature (see <ahref="#metadata">Metadata</a> for more about this value). The <code><claimID></code> is the claim ID of a previous claim that is being updated or supported.</p>
<p>Each opcode will push a zero on to the execution stack. Those zeros, as well as any additional parameters after the opcodes, are all dropped by <code>OP_2DROP</code> and <code>OP_DROP</code>. <code><outputScript></code> can be any valid script, so a script using these opcodes is also a pay-to-pubkey script. This means that claim scripts can be spent just like regular Bitcoin output scripts.</p>
<p>Like any standard Bitcoin output script, a claim script is associated with a transaction hash and output index. This combination of transaction hash and index is called an <em>outpoint</em>. Each claim script has a unique outpoint. The outpoint is hashed using SHA-256 and RIPEMD-160 to generate the claim ID for a claim. For the example above, let’s say claim script is included in transaction <code>7560111513bea7ec38e2ce58a58c1880726b1515497515fd3f470d827669ed43</code> at the output index <code>1</code>. Then the claim ID would be <code>529357c3422c6046d3fec76be2358004ba22e323</code>. An implementation of this is available <ahref="https://github.com/lbryio/lbry.go/blob/master/lbrycrd/blockchain.go">here</a>.</p>
<p>New claims are created using <code>OP_CLAIM_NAME</code>. For example, a claim transaction setting the name <code>Fruit</code> to the value <code>Apple</code> looks like this:</p>
<p><code>OP_UPDATE_CLAIM</code> updates a claim by replacing its metadata. An update transaction has an added requirement that it must spend the output for the existing claim that it wishes to update. Otherwise, it is considered invalid and will not make it into the claimtrie. Thus it must have the following redeem script:</p>
<p>The syntax is identical to the standard way of redeeming a pay-to-pubkey script in Bitcoin, with the caveat that <code><pubKeyForPreviousAddress></code> must be the public key for the address of the output that contains the claim that is being updated.</p>
<p>Since a claim is updated or abandoned by spending the claim’s transaction, most claimtrie transactions spend to an address controlled by the transaction’s creator. However, a claimtrie transaction can spend to any address. This can be used to create a <em>tip</em>. A tip is a support that also sends credits to the claim’s creator. Tips in LBRY are just like tips in real life. They can be used to express gratitude or make an optional payment to a content creator.</p>
<p>The proof-of-work target is adjusted every block to better adapt to sudden changes in hash rate. The exact adjustment algorithm can be seen <ahref="https://github.com/lbryio/lbrycrd/blob/master/src/lbry.cpp">here</a>.</p>
<p>LBRY uses a combination of SHA-256, SHA-512, and RIPEMD-160. The exact hashing algorithm can be seen <ahref="https://github.com/lbryio/lbrycrd/blob/master/src/hash.cpp#L18">here</a>.</p>
<p>The block reward schedule was adjusted to provide an initial testing period, a quick ramp-up to max block rewards, then a logarithmic decay to 0. The source for the algorithm is <ahref="https://github.com/lbryio/lbrycrd/blob/master/src/main.cpp#L1594">here</a>.</p>
<p>The address version byte is set to <code>0x55</code> for standard (pay-to-public-key-hash) addresses and <code>0x7a</code> for multisig (pay-to-script-hash) addresses. P2PKH addresses start with the letter <code>b</code>, and P2SH addresses start with <code>r</code>.</p>
<p>All the chain parameters are defined <ahref="https://github.com/lbryio/lbrycrd/blob/master/src/chainparams.cpp">here</a>.</p>
<p>Metadata is structured information about a stream or channel separate from the content itself (e.g. the title, language, media type, etc.). It is stored in the blockchain as the <ahref="#claim-properties">value property</a> of a claim.</p>
<p>Metadata is stored in a serialized binary format using <ahref="https://developers.google.com/protocol-buffers/">Protocol Buffers</a>. This allows for metadata to be:</p>
<li><strong>Extensibile</strong>. Metadata can encompass thousands of fields for dozens of types of content. It must be efficient to both modify the structure and maintain backward compatibility.</li>
<li><strong>Compact</strong>. Blockchain space is expensive. Data must be stored as compactly as possible.</li>
<li><strong>Interoperabile</strong>. Metadata will be used by many projects written in different languages.</li>
<p>The metadata specification is designed to grow and change frequently. The full specification is not detailed here. The <ahref="https://github.com/lbryio/types">types</a> repository is considered the precise specification.</p>
<p>Information on how to pay for the content. It includes the address that will receive the payment (the <em>fee address</em>), the the amount to be paid, and the currency. Only LBC and USD currencies are supported, though others may be added in the future.</p>
<p>Channels are the unit of identity. A channel is a claim for a name beginning with <code>@</code> that contains a metadata structure for identity rather than content. Included in the metadata is the channel’s public key. Here’s an example:</p>
<p>The purpose of channels is to allow content to be clustered under a single pseudonym or identity. This allows publishers to easily list all their content, maintain attribution, and build their brand.</p>
<p>A claim is considered part of a channel when its metadata is signed by the channel’s private key. Here’s the structure of a signed metadata value:</p>
<p>No enforcement or validation on metadata happens at the blockchain level. Instead, metadata encoding, decoding, and validation is done by clients. This allows evolution of the metadata without changes to consensus rules. Clients are responsible for validating metadata, including data structure and signatures.</p>
<p>Files published using LBRY are stored in a distributed fashion by the clients participating in the network. Each file is split into multiple small pieces. Each piece is encrypted and announced to the network. The pieces may also be uploaded to other hosts on the network that specialize in rehosting content.</p>
<p>The purpose of this process is to enable file storage and access without relying on centralized infrastructure, and to create a marketplace for data that allows hosts to be paid for their services. The design is strongly influenced by the Bittorrent protocol.</p>
<p>The smallest unit of data is called a <em>blob</em>. A blob is an encrypted chunk of data up to 2MiB in size. Each blob is indexed by its <em>blob hash</em>, which is a SHA384 hash of the blob. Addressing blobs by their hash protects against naming collisions and ensures that data cannot be accidentally or maliciously modified.</p>
<p>Blobs are encrypted using AES-256 in CBC mode and PKCS7 padding. In order to keep each encrypted blob at 2MiB max, a blob can hold at most 2097151 bytes (2MiB minus 1 byte) of plaintext data. The source code for the exact algorithm is available <ahref="https://github.com/lbryio/lbry.go/blob/master/stream/blob.go">here</a>. The encryption key and IV for each blob is stored as described below.</p>
<p>Multiple blobs are combined into a <em>stream</em>. A stream may be a book, a movie, a CAD file, etc. All content on the network is shared as streams. Every stream begins with the <em>manifest blob</em>, followed by one or more <em>content blobs</em>. The content blobs hold the actual content of the stream. The manifest blob contains information necessary to find the content blobs and decode them into a file. This includes the hashes of the content blobs, their order in the stream, and cryptographic material for decrypting them.</p>
<p>A manifest blob’s contents are encoded using a canonical JSON encoding. The JSON encoding must be canonical to support consistent hashing and validation. The encoding is the same as standard JSON, but adds the following rules:</p>
<ul>
<li>Object keys must be quoted and lexicographically sorted.</li>
<li>All strings are hex-encoded. Hex letters must be lowercase.</li>
<li>Whitespace before, after, or between tokens is not permitted.</li>
<li>Floating point numbers, leading zeros, and “minus 0” for integers are not permitted.</li>
<li>Trailing commas after the last item in an array or object are not permitted.</li>
<p>The <code>key</code> field contains the key to decrypt the stream, and is optional. The key may be stored by a third party and made available to a client when presented with proof that the content was purchased. The <code>version</code> field is always 1. It is intended to signal structure changes in future versions of this protocol. The <code>length</code> field for each blob is the length of the encrypted blob, not the original file chunk.</p>
<p>A file must be encoded into a stream before it can be published. Encoding involves breaking the file into chunks, encrypting the chunks into content blobs, and creating the manifest blob. Here are the steps:</p>
<p>After a [[stream]] is encoded, it must be <em>announced</em> to the network. Announcing is the process of letting other nodes on the network know that a client has content available for download. LBRY tracks announced content using a distributed hash table.</p>
<p><em>Distributed hash tables</em> (or DHTs) have proven to be an effective way to build a decentralized content network. Our DHT implementation follows the <ahref="https://pdos.csail.mit.edu/~petar/papers/maymounkov-kademlia-lncs.pdf">Kademlia</a>
<p>A distributed hash table is a key-value store that is spread over multiple nodes in a network. Nodes may join or leave the network anytime, with no central coordination necessary. Nodes communicate with each other using a peer-to-peer protocol to advertise what data they have and what they are best positioned to store.</p>
<p>When a host connects to the DHT, it announces the hash for every [[blob]] it wishes to share. Downloading a blob from the network requires querying the DHT for a list of hosts that announced that blob’s hash (called <em>peers</em>), then requesting the blob from the peers directly.</p>
<h4id="announcing-to-the-dht">Announcing to the DHT</h4>
<p>A host announces a hash to the DHT in two steps. First, the host looks for nodes that are closest to the target hash. Then the host asks those nodes to store the fact that the host has the target hash available for download.</p>
<p>Finding the closest nodes is done via iterative <code>FindNode</code> DHT requests. The host starts with the closest nodes it knows about and sends a <code>FindNode(target_hash)</code> request to each of them. If any of the requests return nodes that are closer to the target hash, the host sends <code>FindNode</code> requests to those nodes to try to get even closer. When the <code>FindNode</code> requests no longer return nodes that are closer, the search ends.</p>
<p>Once the search is over, the host takes the 8 closest nodes it found and sends a <code>Store(target_hash)</code> request to them. The nodes receiving this request store the fact that the host is a peer for the target hash.</p>
<p>A client wishing to download a [[stream]] must first query the [[DHT]] to find [[peers]] hosting the [[blobs]] in that stream, then contact those peers to download the blobs directly.</p>
<p>Querying works almost the same way as [[announcing]]. A client looking for a target hash starts by sending iterative <code>FindValue(target_hash)</code> requests to the nodes it knows that are closest to the target hash. If a node receives a <code>FindValue</code> request and knows of any peers for the target hash, it responds with a list of those peers. Otherwise, it responds with the closest nodes to the target hash that it knows about. The client then queries those closer nodes using the same <code>FindValue</code> call. This way, each call either finds the client some peers, or brings it closer to finding those peers. If no peers are found and no closer nodes are being returned, the client determines that the target hash is not available and gives up.</p>
<p>Downloading a blob from a peer is governed by the <em>Blob Exchange Protocol</em>. It is used by hosts and clients to exchange blobs and check data pricing and blob availability. The protocol is an RPC protocol using Protocol Buffers and the gRPC framework. It has five types of requests.</p>
<p>DownloadCheck checks whether the server has certain blobs available for download. For each hash in the request, the server returns a true or false to indicate whether the blob is available.</p>
<p>Download requests the blob for a given hash. The response contains the blob, its hash, and the address where to send payment for the data transfer. If the blob is not available on the server, the response instead contains an error.</p>
<p>UploadCheck asks the server whether blobs can be uploaded to it. For each hash in the request, the server returns a true or false to indicate whether it would accept a given blob for upload. In addition, if any of the hashes in the request is a stream hash and the server has the manifest blob for that stream but is missing some content blobs, it may include the hashes of those content blobs in the response.</p>
<h5id="upload">Upload</h5>
<p>Upload sends a blob to the server. If uploading many blobs, the client should use the UploadCheck request to check which blobs the server actually needs. This avoids needlessly uploading blobs that the server already has. If a client tries to upload too many blobs that the server does not want, this may be consider a denial of service attack.</p>
<p>The protocol calls and message types are defined in detail <ahref="https://github.com/lbryio/lbry.go/blob/master/blobex/blobex.proto">here</a>.</p>
<p>In order for a client to download content, there must be hosts online that have the content the client wants, when the client wants it. To incentivize the continued hosting of data, the blob exchange protocol supports data upload and payment for data. <em>Reflectors</em> are hosts that accept data uploads. They rehost (reflect) the uploaded data and charge for downloads. Using a reflector is optional, but most publishers will probably choose to use them. Doing so obviates the need for the publisher’s server to be online and connectable, which can be especially useful for mobile clients or those behind a firewall.</p>
<p>The current version of the protocol does not support sophisticated price negotiation between clients and hosts. The host simply chooses the price it wants to charge. Clients check this price before downloading, and pay the price after the download is complete. Future protocol versions will include more options for price negotiation, as well as stronger proofs of payment.</p>
