27 KiB
%%% Title = "LBRY: A Decentralized Digital Content Marketplace" area = "Internet"
[seriesInfo] name = "Internet-Draft" value = "draft-grintsvayg-00" stream = "IETF" status = "informational"
date = 2018-08-21T00:00:00Z
author initials="A." surname="Grintsvayg" fullname="Alex Grintsvayg" %%%
LBRY: A Decentralized Digital Content Marketplace
A> 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.
A> For more technical information about LBRY, visit lbry.tech.
Introduction
(introduction)
Table of Contents
Overview
(overview)
The LBRY protocol consists of n discrete parts (sub-protocols?) designed to be used in conjunction in order to provide the end-to-end capabilities covered in the Introduction:
Conventions and Terminology
(Rather than this section, maybe we can use a syntax like brackets around keywords to inline key definitions?)
- file
- stream
- blob
- metadata
- hash
- name
- claim
- channel
- url
Blockchain
The LBRY blockchain is a public, proof-of-work blockchain. It serves three key purposes:
- An index of the content available on the network
- A payment and proof system for priced content
- Trustful publisher identities (fixme: should this even be listed here?)
The LBRY blockchain is a fork of the Bitcoin blockchain, with substantial modifications. This document will not cover or specify any aspects of LBRY that are identical to Bitcoin, and will instead focus on the differences.
Claims
A single metadata entry in the blockchain is called a claim. It records an item that was published to the network or a publisher's identity.
Every claim has a globally-unique claimID, an amount (how many credits were set aside to back the claim), and a value. The value may contain metadata about a piece of content, a publisher's public key, or other information. See the Metadata for more information about what may be stored in the value.
Every claim is associated with a name, which is a bytestring of 0-255 bytes. Every name must be a valid UTF8 string.
Here is an example claim:
{
"claimId": "fa3d002b67c4ff439463fcc0d4c80758e38a0aed",
"name": "lbry",
"amount": 100000000,
"value": "{\"ver\": \"0.0.3\", \"description\": \"What is LBRY? An introduction with Alex Tabarrok\",
\"license\": \"LBRY inc\", \"title\": \"What is LBRY?\", \"author\": \"Samuel Bryan\",
\"language\": \"en\", \"sources\": {\"lbry_sd_hash\":
\"e1e324bce7437540fac6707fa142cca44d76fc4e8e65060139a88ff7cdb218b4540cb9cff8bb3d5e06157ae6b08e5cb5\"},
\"content_type\": \"video/mp4\", \"nsfw\": false, \"thumbnail\":
\"https://s3.amazonaws.com/files.lbry.io/logo.png\"}",
"txid": "53ed05d9dfd728a94bedf952d67783bbe9da5d2ab436a84338bb53f0b85301b5",
"n": 0,
"height": 146117
}
Claim Operations
There are four claim operations: create, support, update, and abandon.
A create operation makes a new claim for a name, or submits a competing claim on an existing name.
A support is a claim that adds to the credit total of an existing claim. A support does not have it’s own claim ID or data. Instead, it has the claim ID of the claim to which its amount will be added.
An update changes the data or the amount stored in an existing claim or support. Updates do not change the claim ID, so an updated claim retains any supports attached to it.
An abandon withdraws a claim or support, freeing the associated credits to be used for other purposes.
Claimtrie
The claimtrie is the data structure that LBRY uses to store claims and prove the correctness of name resolution. It is a Merkle Tree that maps names to claims. Claims are stored as leaf nodes in the tree. Names are stored as the path from the root node to the leaf node.
The hash of the root node (the root hash) is stored in the header of each block in the blockchain. Nodes in the LBRY network use the root hash to efficiently and securely validate the state of the claimtrie.
Multiple claims can exist for the same name. They are all stored in the leaf node for that name, sorted in decreasing order by the total amount of credits backing each claim.
For more details on the specific claimtrie impelementation, see the source code.
Claim States
A claim can have the following states at a given block:
Accepted
An accepted claim or support is simply one that has been entered into the blockchain. This happens when the transaction containing the claim is included in a block.
Abandoned
An abandoned claim or support is one that was withdrawn by its creator. It is no longer in contention to control a name. Spending the transaction that contains the claim will also cause the claim to become abandoned.
While data related to abandoned claims technically still resides in the blockchain, it is considered inappropriate (and potentially illegal? #fixme), to use this data to fetch the associated content.
Active
A claim is active when it is in contention for controlling a name (or a support for such a claim). An active claim must be accepted and not abandoned. The time it takes an accepted claim to become active is called the activation delay, and it depends on the claim type, the height of the current block, and the height at which the last takeover occurred for the claim’s name.
If the claim is an update or support to the current controlling claim, or if it is the first claim for a name (T = 0), the claim becomes active as soon as it is accepted. Otherwise it becomes active at height A, where A = C + D, and D = min(4032, floor((H-T) / 32)).
- A = activation height
- D = activation delay
- C = claim height (height when the claim was accepted)
- H = current height
- T = takeover height (the most recent height at which the controlling claim for the name changed)
In plain English, the delay before a claim becomes active is equal to the claim’s height minus height of the last takeover, divided by 32. The delay is capped at 4032 blocks, which is 7 days of blocks at 2.5 minutes per block (our target block time). The max delay is reached 224 (7x32) days after the last takeover. The goal of this delay function is to give long-standing claimants time to respond to takeover attempts, while still keeping takeover times reasonable and allowing recent or contentious claims to be taken over quickly.
Controlling
The controlling claim is the claim that has the highest total effective amount, which is the sum of its own amount and the amounts of all of its supports. It must be active and cannot itself be a support.
Only one claim can be controlling for a given name at a given block. To determine which claim is controlling for a given name at a given block, the following algorithm is used:
-
For each active claim for the name, add up the amount of the claim and the amount of all the active supports for that claim.
-
Determine if a takeover is happening
-
If the claim with the greatest total is the controlling claim from the previous block, then nothing changes. That claim is still controlling at this block.
-
Otherwise, a takeover is occurring. Set the takeover height for this name to the current height, recalculate which claims and supports are now active, and then perform step 1 again.
-
At this point, the claim with the greatest total is the controlling claim at this block.
The purpose of 2b 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 amount. Step 2b will cause this claim to also activate and become the controlling claim.
Here is a step-by-step example to illustrate the different scenarios. All claims are for the same name.
Block 13: Claim A for 10LBC is accepted. It is the first claim, so it immediately becomes active and controlling.
State: A(10) is controlling
Block 1001: Claim B for 20LBC is accepted. It’s activation height is 1001 + min(4032, floor((1001-13) / 32)) = 1001 + 30 = 1031.
State: A(10) is controlling, B(20) is accepted.
Block 1010: Support X for 14LBC for claim A is accepted. Since it is a support for the controlling claim, it activates immediately.
State: A(10+14) is controlling, B(20) is accepted.
Block 1020: Claim C for 50LBC is accepted. The activation height is 1020 + min(4032, floor((1020-13) / 32)) = 1020 + 31 = 1051.
State: A(10+14) is controlling, B(20) is accepted, C(50) is accepted.
Block 1031: 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.
State: A(10+14) is controlling, B(20) is active, C(50) is accepted.
Block 1040: Claim D for 300LBC is accepted. The activation height is 1040 + min(4032, floor((1040-13) / 32)) = 1040 + 32 = 1072.
State: A(10+14) is controlling, B(20) is active, C(50) is accepted, D(300) is accepted.
Block 1051: 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 min(4032, floor((1051-1051) / 32)) = 0. All the claims become active. The totals for each claim are recalculated, and claim D becomes controlling because it has the highest total.
State: A(10+14) is active, B(20) is active, C(50) is active, D(300) is controlling.
Normalization
(Talk about how claim names are normalized.)
URLs
LBRY has URLs that can be resolved to return claim metadata or directly fetched to retrieve the content referenced by the metadata.
The ultimate purpose of much of the claim design, including controlling claims and the claimtrie structure, is to provide human readable URLs that can be trustfully resolved by Simple Payment Verification wallets.
Components
A URL is generally a name with one or more modifiers. A bare name on its own will resolve to the controlling claim at the latest block height, for reasons covered in Design Notes. The available modifiers are:
Name: a basic claim for a name
lbry://meet-LBRY
Channel: a claim for a channel
lbry://@lbry
Claim in Channel: a claim for this name that has been signed with a key connected to the controlling claim for this channel
lbry://@lbry/meet-LBRY
Claim ID: a claim for this name with this claim ID (does not have to be the controlling claim). Partial prefix matches are allowed.
lbry://meet-LBRY#7a0aa95c5023c21c098
lbry://meet-LBRY#7a
Claim Sequence: the Nth claim for this name, in the order the claims entered the blockchain. N must be a positive number. This can be used to determine which claim came first, rather than which claim has the most support.
lbry://meet-LBRY:1
Bid Position: the Nth claim for this name, in order of most support to least support. N must be a positive number. This is useful for resolving non-winning bids in bid order if you, for example, want to list the top three winning claims in a voting contest or want to ignore the activation delay.
lbry://meet-LBRY$2
lbry://meet-LBRY$3
Query Params: extra parameters (reserved for future use)
lbry://meet-LBRY?arg=value+arg2=value2
The full URL grammar is defined below using Xquery EBNF notation:
URL ::= Protocol (ChannelAndhModifier '/')? ClaimNameAndModifier Query?
Protocol ::= 'lbry://'
ClaimNameAndModifier ::= ClaimName Modifier?
ChannelAndModifier ::= Channel Modifier?
ClaimName ::= Allowed+
Channel ::= '@' ClaimName
Modifier ::= ClaimID | ClaimSequence | BidPosition
ClaimID ::= '#' Hex+
ClaimSequence ::= ':' Number
BidPosition ::= '$' Number
Path ::= '/' Allowed+
Query ::= '?' QueryParameterList
QueryParameterList ::= QueryParameter ( '&' QueryParameterList )*
QueryParameter ::= QueryParameterName ( '=' QueryParameterValue )?
QueryParameterName ::= Allowed+
QueryParameterValue ::= Allowed+
PosDigit ::= [123456789]
Digit ::= '0' | PosDigit
Number ::= PosDigit Digit*
HexAlpha ::= [abcdef]
Hex ::= (Digit | HexAlpha)+
Reserved ::= [=&#:$@?/]
Allowed ::= [^=&#:$@?/]
Design Notes
Most existing public name schemes are first-come, first-serve. This leads to several bad outcomes. When the system is young, users 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. In a centralized system, the authority may allow for appeals to reassign names based on trademark or other common use reasons. There may also be a process to "verify" that a name belongs to the entity you think it does (e.g. Twitter's verified accounts). Such processes are often arbitrary, change over time, involve significant transaction costs, and may still lead to names being used in ways that are contrary to user expectation (e.g. nissan.com is not what you’d expect).
In a decentralized system, such approaches are not possible, so name squatting is especially dangerous (see Namecoin). Instead, LBRY creates an efficient allocation of names via a market. Following Coase, we believe that if the rules for name ownership and exchange are clearly defined, transaction costs are low, and there is no information asymmetry, then control of URLs will flow to their highest-valued use.
Note that only vanity URLs (i.e. URLs without a ClaimID or or ClaimSequence modifier) have this property. Permanent URLs like lbry://myclaimname#abc exist and are available for the small cost of issuing a create claim transactions.
Transactions
To support claims, the LBRY blockchain adds or modifies behavior related to transactions.
Operations and Opcodes
To enable claim operations, 3 new opcodes were added to the blockchain scripting language: OP_CLAIM_NAME, OP_SUPPORT_CLAIM, and OP_UPDATE_CLAIM (in Bitcoin they are respectively OP_NOP6, OP_NOP7, and OP_NOP8). Each op code will push a zero on to the execution stack, and will trigger the claimtrie to perform calculations necessary for each bid type. Below are the three supported transactions scripts using these opcodes.
OP_CLAIM_NAME <name> <value> OP_2DROP OP_DROP <pubKey>
OP_UPDATE_CLAIM <name> <claimId> <value> OP_2DROP OP_2DROP <pubKey>
OP_SUPPORT_CLAIM <name> <claimId> OP_2DROP OP_DROP <pubKey>
<pubKey> can be any valid Bitcoin payout script, so a claimtrie script is also a pay-to-pubkey script to a user-controlled address. Note that the zeros pushed onto the stack by the claimtrie opcodes and vectors are all dropped by OP_2DROP and OP_DROP. This means that claimtrie transactions exist as prefixes to Bitcoin payout scripts and can be spent just like standard transactions.
For example, a claim transaction setting the name “Fruit” to “Apple” and using a pay-to-pubkey script will have the following payout script:
OP_CLAIM_NAME Fruit Apple OP_2DROP OP_DROP OP_DUP OP_HASH160 <addressOne>
OP_EQUALVERIFY OP_CHECKSIG
Like any standard Bitcoin transaction output script, it will be associated with a transaction hash and output index. The transaction hash and output index are concatenated and hashed to create the claimID for this claim. For the example above, let's say the above transaction hash is 7560111513bea7ec38e2ce58a58c1880726b1515497515fd3f470d827669ed43 and the output index is 1. Then the claimID would be 529357c3422c6046d3fec76be2358004ba22e323.
A support for this bid will have the following payout script:
OP_SUPPORT_CLAIM Fruit 529357c3422c6046d3fec76be2358004ba22e323 OP_2DROP OP_DROP
OP_DUP OP_HASH160 <addressTwo> OP_EQUALVERIFY OP_CHECKSIG
And now let's say we want to update the original claim to change the value to “Banana”. An update transaction has a special requirement that it must spend the existing claim that it wishes to update in its redeem script. Otherwise, it will be considered invalid and will not make it into the claimtrie. Thus it will have the following redeem script:
<signature> <pubKeyForAddressOne>
This is identical to the standard way of redeeming a pay-to-pubkey script in Bitcoin.
The payout script for the update transaction is:
OP_UPDATE_CLAIM Fruit 529357c3422c6046d3fec76be2358004ba22e323 Banana OP_2DROP
OP_2DROP OP_DUP OP_HASH160 <addressThree> OP_EQUALVERIFY OP_CHECKSIG
Addresses
The address version byte is set to 0x55 for standard (pay-to-public-key-hash) addresses and 0x7a for multisig (pay-to-script-hash) addresses. P2PKH addresses start with the letter b, and P2SH addresses start with r.
Proof of Payment
Explain how transactions serve as proof that a client has made a valid payment for a piece of content.
Consensus
LBRY makes some small changes to consensus timing and methodology.
Block Timing
The target block time was lowered from 10 to 2.5 minutes to facilitate faster transaction confirmation.
Difficulty Adjustment
The proof-of-work target is adjusted every block to better adapt to sudden changes in hashrate. The exact adjustment algorithm can be seen here.
Hash Algorithm
LBRY uses a combination of SHA256, SHA512, and RIPEMD160. The exact hashing algorithm can be seen here.
URLs
The purpose of including a name value in claims is to create human readable URLs like lbry://whatever.
Metadata
Claim metadata is stored in a structured, stored, and validated in a serialized format via [Protocol Buffers]. This facilitate interoperability across varied systems and languages as well as making it easy to add properties in backwards-compatible way.
No enforcement or validation on metadata happens at the blockchain level. Instead, metadata encoding, decoding, and validation happens at the client level. This allows evolution of the metadata without soft or hard forks.
Metadata Specification
A useful index of LBRY’s content must be succinct yet meaningful. It should be machine-readable, comprehensive, and should ideally point you toward the content you’re looking for. LBRY achieves this by defining a set of common properties for streams.
The metadata contains structured information describing the content, such as the title, author, language, and so on.
Here’s an example:
"metadata": {
"author": "",
"description": "All proceeds go to holly for buying toys, i will post the video with those toys on Xmas day",
"language": "en",
"license": "All rights reserved.",
"licenseUrl": "",
"nsfw": false,
"preview": "",
"thumbnail": "http://www.thetoydiscounter.com/happy.jpg",
"title": "Holly singing The Happy Working Song",
"source": {
"contentType": "video/mp4",
"source": "92b8aae7a901c56901fd5602c1f1acc0e63fb5492ef2a3cd5b9c631d92cab2e060e2a908baa922c24dee6c5229a98136",
"sourceType": "lbry_sd_hash",
"version": "_0_0_1"
},
"version": "_0_1_0"
}
Because the metadata structure can and does change frequently, a complete specification is omitted from this document. Instead, https://github.com/lbryio/types should be consulted for the precise definition of current metadata structure.
Key Metadata Fields
Despite not covering the full metadata structure, a few specific metadata fields are extremely unlikely to ever change.
Streams and Stream Hashes
(The metadata property sd_hash contains a unique identifier to locate and find the content in the data network. Reference Data.)
Fees and Fee Structure
- LBC
- Currencies?
More?
Identities
Channels are the unit of identity in the LBRY system. A channel is simply a claim that start with @ and contains a metadata structure for identities rather than content. Once a channel claim is accepted on the blockchain, content claims that are signed with the channel’s private key will appear in lists under that channel (fixme: how does the protocol enforce/provide this?).
The purpose of channels is to allow content to be clustered under a single pseudonym or identity, sort of like a username.
Here’s the value of an example channel claim:
"certificate": {
"keyType": "SECP256k1",
"publicKey": "3056301006072a8648ce3d020106052b8104000a0342
0004180488ffcb3d1825af538b0b952f0eba6933faa6
d8229609ac0aeadfdbcf49C59363aa5d77ff2b7ff06c
ddc07116b335a4a0849b1b524a4a69d908d69f1bcebb",
"version": "_0_0_1"
}
When a claim published into a channel, the claim data is signed and the following is added to the claim:
"publisherSignature": {
"channelClaimID": "2996b9a087c18456402b57cba6085b2a8fcc136d",
"signature": "bf82d53143155bb0cac1fd3d917c000322244b5aD17
e7865124db2ed33812ea66c9b0c3f390a65a9E2d452
e315e91ae695642847d88e90348ef3c1fa283a36a8",
"signatureType": "SECP256k1",
"version": "_0_0_1"
}
Metadata Validation
(expand)
- Validation 101
- Channel / identity validation
Data
(This portion covers how content is actually encoded and decoded, fetched, and announced. Expand/fix.)
Encoding and Decoding
Blobs
The unit of content in our network is called a blob. A blob is an encrypted chunk of data up to 2MB in size. Each blob is indexed by its blob hash, which is a SHA384 hash of the blob contents. Addressing blobs by their hashes simultaneously protects against naming collisions and ensures that the content you get is what you expect.
Streams
Multiple blobs may be combined into a stream. 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 stream descriptor blob, which contains a JSON list of the hashes and keys of the content blobs. The content blobs hold the actual content of the stream. Every stream ends with an empty content blob, to signify that the stream has finished (this is similar to a null-terminated string, and is necessary to support streaming content).
Download
Data can be downloaded via one of two methods: the distriuted data network and from centralized blob providers.
Distributed Hash Table
Distributed hash tables have proven to be an effective way to build a decentralized content network. Our DHT implementation follows the Kademlia spec fairly closely, with some modifications.
A distributed hash table is a key-value store that is spread over multiple host 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 able to store.
When a host connects to the DHT, it advertises the blob hash for every blob it wishes to share. Downloading a blob from the network requires querying DHT for a list of hosts that advertised that blob’s hash (called peers), then requesting the blob from the peers directly.
Blob Mirrors
(fill me in)
Announcing
(how stuff gets created / published)
Stream Descriptor Blob Contruction (better title)
(talk about this)
Blob Mirrors
(Blob mirrors can also help you announce your content.)
Data Markets
(Price negotiation.)
Conclusion
TODO