IPFS Architecture Overview(英文版)

来源:牛链IPFS  作者:牛链IPFS  时间:2018-07-01

Reviewers:

This spec document defines the IPFS protocol stack, the subsystems, the interfaces, and how it all fits together. It delegates non-interface details to other specs as much as possible. This is meant as a top-level view of the protocol and how the system fits together.

Note, this document is not meant to be an introduction of the concepts in IPFS and is not recommended as a first pass to understanding how IPFS works. For that, please refer to theIPFS paper.

Table of Contents IPFS and the Merkle DAG

Nodes and Network Model

The Stack

Applications and Datastructures -- on top of IPFS

Lifetime of fetching an object

IPFS User Interfaces

1. IPFS and the Merkle DAG At the heart of IPFS is the MerkleDAG, a directed acyclic graph whose links are hashes. This gives all objects in IPFS useful properties:

authenticated: content can be hashed and verified against the link

permanent: once fetched, objects can be cached forever

universal: any datastructure can be represented as a merkledag

decentralized: objects can be created by anyone, without centralized writers

In turn, these yield properties for the system as a whole:

links are content addressed

objects can be served by untrusted agents

objects can be cached permanently

objects can be created and used offline

networks can be partitioned and merged

any datastructure can be modelled and distributed

(todo: list more)

IPFS is a stack of network protocols that organize agent networks to create, publish, distribute, serve, and download merkledags. It is the authenticated, decentralized, permanent web.

2. Nodes and Network Model The IPFS network uses PKI based identity. An "ipfs node" is a program that can find, publish, and replicate merkledag objects. Its identity is defined by a private key. Specifically:

privateKey, publicKey := keygen() nodeID := multihash(publicKey) TODO: constraints on keygen.

2.1 multihash and upgradeable hashing All hashes in ipfs are encoded withmultihash, a self-describing hash format. The actual hash function used depends on security requirements. The cryptosystem of IPFS is upgradeable, meaning that as hash functions are broken, networks can shift to stronger hashes. There is no free lunch, as objects may need to be rehashed, or links duplicated. But ensuring that tools built do not assume a pre-defined length of hash digest means tools that work with today's hash functions will also work with tomorrows longer hash functions too.

As of this writing, IPFS nodesmustsupport:

sha2-256 sha2-512 sha3 3. The Stack IPFS has a stack of modular protocols. Each layer may have multiple implementations, all in different modules. This spec will only address the interfaces between the layers, and briefly mention possible implementations. Details are left to the other specs.

IPFS has five layers:

naming- a self-certifying PKI namespace (IPNS)

merkledag- datastructure format (thin waist)

exchange- block transport and replication

routing- locating peers and objects

network- establishing connections between peers

These are briefly described bottom-up.

3.1 Network Thenetworkprovides point-to-point transports (reliable and unreliable) between any two IPFS nodes in the network. It handles:

NAT traversal - hole punching, port mapping, and relay

supports multiple transports - TCP, SCTP, UTP, ...

supports encryption, signing, or clear communications

multi-multiplexes -multiplexes connections, streams, protocols, peers, ...

See more in thelibp2p specs.

3.2 Routing -- finding peers and data The IPFSRoutinglayer serves two important purposes:

peer routing-- to find other nodes

content routing-- to find data published to ipfs

The Routing Sytem is an interface that is satisfied by various kinds of implementations. For example:

DHTs:perhaps the most common, DHTs can be used to create a semi-persistent routing record distributed cache in the network.

mdns:used to find services advertised locally.mdns(ordnssd) is a local discovery service. We will be using it.

snr:supernode routing is a delegated routing system: it delegates to one of a set of supernodes. This is roughly like federated routing.

dns:ipfs routing could even happen over dns.

See more in thelibp2p specs.

3.3 Block Exchange -- transfering content-addressed data The IPFSBlock Exchangetakes care of negotiating bulk data transfers. Once nodes know each other -- and are connected -- the exchange protocols govern how the transfer of content-addressed blocks occurs.

The Block Exchange is an interface that is satisfied by various kinds of implementations. For example:

Bitswap:our main protocol for exchanging data. It is a generalization of BitTorrent to work with arbitrary (and not known apriori) DAGs.

HTTP:a simple exchange can be implemented with HTTP clients and servers.

3.4. Merkledag -- making sense of data As discussed above, the IPFSmerkledagis the datastructure at the heart of IPFS. It is anacyclic directed graphwhose edges are hashes. Another name for it is the merkleweb.

The merkledag data structure is:

message MDagLink { bytes Hash = 1; // multihash of the target object string Name = 2; // utf string name. should be unique per object uint64 Tsize = 3; // cumulative size of target object}message MDagNode { MDagLink Links = 2; // refs to other objects bytes Data = 1; // opaque user data} The merkledag is the "thin waist" of authenticated datastructures. It is a minimal set of information needed to represent transfer arbitrary authenticated datastructures. More complex datastructures are implemented on top of the merkledag, such as:

gitand other version control systems

bitcoinand other blockchains

unixfs, a content-addressed unix filesystem

See more in the merkledag spec (TODO).

3.4.1 Merkledag Paths The merkledag is enough to resolve paths:

/ipfs/QmdpMvUptHuGysVn6mj69K53EhitFd2LzeHCmHrHasHjVX/test/foo (a) Would first fetch resolveQmdpMvUptHuGysVn6mj69K53EhitFd2LzeHCmHrHasHjVX

(b) Then look into the links of (a), find the hash fortest, and resolve it

(c) Then look into the links of (b), find the hash forfoo, and resolve it

See more in the path resolution spec (TODO).

3.5 Naming -- PKI namespace and mutable pointers IPFS is mostly concerned with content-addressed data, which by nature is immutable: changing an object would change its hash -- and thus its address, making it adifferentobject altogether. (Think of it as a copy-on-write filesystem).

The IPFSnaminglayer -- or IPNS -- handles the creation of:

mutable pointers to objects

human-readable names

IPNS is based onSFS. It is a PKI namespace -- a name is simply the hash of a public key. Whoever controls the private key controls the name. Records are signed by the private key and distributed anywhere (in IPFS, via the routing system). This is an egalitarian way to assign mutable names in the internet at large, without any centralization whatsoever, or certificate authorities.

See more in the namin spec (TODO).

4. Applications and Datastructures -- on top of IPFS The stack described so far is enough to represent arbitrary datastructures and replicate them across the internet. It is also enough to build and deploy decentralized websites.

Applications and datastructures on top of IPFS are represented as merkledags. Users can create arbitrary datastructures that extend the merkledag and deploy them to the rest of the world using any of the tools that understand IPFS.

See more in the datastructures and applications specs (TODO).

4.1 unixfs -- representing traditional files The unix filesystem abstractions -- files and directories -- are the main way people conceive of files in the internet. In IPFS,unixfsis a datastructure that represents unix files on top of IPFS. We need a separate datastructure to carry over information like:

whether the object represents a file or directory.

total sizes, minus indexing overhead

See more in the unixfs spec (TODO).

5 Lifetime of fetching an object. Suppose we ask an IPFS node to retrieve

/ipfs/QmdpMvUptHuGysVn6mj69K53EhitFd2LzeHCmHrHasHjVX/test/foo The IPFS node first splits the path into components (discarding theipfsprefix):

[ "QmdpMvUptHuGysVn6mj69K53EhitFd2LzeHCmHrHasHjVX", "test", "foo" ] Then, the IPFS node resolves the components. The first component in an/ipfs/...path is always a multihash. The rest are names of links, to be resolved into multihashes.

6 IPFS User Interfaces IPFS is not just a protocol. It is also a toolset. IPFS implementations include various tools for working with the merkledag, how to publish something, how to name something, etc. These interfaces may be critical to the survival of an implementation, or the project as a whole. These interfaces govern how people use IPFS, thus careful attention must be given to their design and implementation. Examples:

TheIPFS api- an HTTP service

TheIPFS cli- a unix cli

TheIPFS libs- implementations in various languages

The IPFS gateways - nodes in the internet that serve HTTP over IPFS

WIP Stack Dump: How the layers fit together

How they call on each other

Mention all the ports

Mention all the interfaces with the user

Mention gateways

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