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PubSub interface for libp2p

Generalized publish/subscribe interface for libp2p.

Lifecycle Stage Maturity Status Latest Revision
3A Recommendation Active r3, 2020-09-25

Authors: @whyrusleeping, @protolambda, @raulk, @vyzo.

Interest Group: @yusefnapora, @raulk, @vyzo, @Stebalien, @jamesray1, @vasco-santos

See the lifecycle document for context about maturity level and spec status.

Table of Contents

Overview

This is the specification for generalized pubsub over libp2p. Pubsub in libp2p is currently still experimental and this specification is subject to change. This document does not go over specific implementation of pubsub routing algorithms, it merely describes the common wire format that implementations will use.

libp2p pubsub currently uses reliable ordered streams between peers. It assumes that each peer is certain of the identity of each peer it is communicating with. It does not assume that messages between peers are encrypted, however encryption defaults to being enabled on libp2p streams.

You can find information about the PubSub research and notes in the following repos:

Implementations

The RPC

All communication between peers happens in the form of exchanging protobuf RPC messages between participating peers.

The RPC protobuf is as follows:

message RPC {
	repeated SubOpts subscriptions = 1;
	repeated Message publish = 2;

	message SubOpts {
		optional bool subscribe = 1;
		optional string topicid = 2;
	}
}

This is a relatively simple message containing zero or more subscription messages, and zero or more content messages. The subscription messages contain a topicid string that specifies the topic, and a boolean signifying whether to subscribe or unsubscribe to the given topic. True signifies 'subscribe' and false signifies 'unsubscribe'.

The Message

The RPC message can contain zero or more messages of type 'Message'. The Message protobuf looks like this:

message Message {
	optional string from = 1;
	optional bytes data = 2;
	optional bytes seqno = 3;
        required string topic = 4;
	optional bytes signature = 5;
	optional bytes key = 6;
}

The optional fields may be omitted, depending on the signature policy and message ID function.

The from field (optional) denotes the author of the message. This is the peer who initially authored the message, and NOT the peer who propagated it. Thus, as the message is routed through a swarm of pubsubbing peers, the original authorship is preserved.

The seqno field (optional) is a 64-bit big-endian uint that is a linearly increasing number that is unique among messages originating from each given peer. No two messages on a pubsub topic from the same peer should have the same seqno value, however messages from different peers may have the same sequence number. In other words, this number is not globally unique. It is used in conjunction with from to derive a unique message_id (in the default configuration).

Henceforth, we define the term origin-stamped messaging to refer to messages whose from and seqno fields are populated.

The data (optional) field is an opaque blob of data representing the payload. It can contain any data that the publisher wants it to.

The topic field specifies a topic that this message is being published to.

The signature and key fields (optional) are used for message signing, if such feature is enabled, as explained below.

The size of the Message should be limited, say to 1 MiB, but could also be configurable, for more information see issue 118, while messages should be rejected if they are over this size. Note that for applications where state such as messages is stored, such as blockchains, it is suggested to have some kind of storage economics (see e.g. here, here and here).

Message Identification

Pubsub requires to uniquely identify messages via a message ID. This enables a wide range of processes like de-duplication, tracking, scoring, circuit-breaking, and others.

The message_id is calculated from the Message struct.

By default, origin-stamping is in force. This strategy relies on the string concatenation of the from and seqno fields, to uniquely identify a message based on the author.

Alternatively, a user-defined message_id_fn may be supplied, where message_id_fn(Message) => message_id. Such a function could compute the hash of the data field within the Message, and thus one could reify content-addressed messaging.

If fabricated collisions are not a concern, or difficult enough within the window the message is relevant in, a message_id based on a short digest of inputs may benefit performance.

[[ Margin note ]]: There's a potential caveat with using hashes instead of seqnos: the peer won't be able to send identical messages (e.g. keepalives) within the timecache interval, as they will get treated as duplicates. This consequence may or may not be relevant to the application at hand. Reference: #116.

Note that the availability of these fields on the Message object will depend on the signature policy configured for the topic.

Whichever the choice, it is crucial that all peers participating in a topic implement identical message ID calculation logic, or the topic will malfunction.

[[ Implementation note ]]: At the time of writing this section, go-libp2p-pubsub (reference implementation of this spec) only allows configuring a single top-level message_id_fn. This function may, however, vary its behaviour based on the topic (contained inside its Message) argument. Thus, it's feasible to implement a per-topic policy using branch selection control flow logic. In the near future, go-libp2p-pubsub plans to push down the configuration of the message_id_fn to the topic level. Other implementations are encouraged to do the same.

Message Signing

Signature behavior is configured in two axes: signature creation, and signature verification.

Signature creation. There are two configurations possible:

  • Sign: when publishing a message, perform origin-stamping and produce a signature.
  • NoSign: when publishing a message, do not perform origin-stamping and do not produce a signature.

For signing purposes, the signature and key fields are used:

  • The signature field contains the signature.
  • The key field contains the signing key when it cannot be inlined in the source peer ID (from). When present, it must match the peer ID.

The signature is computed over the marshalled message protobuf excluding the signature field itself.

This includes any fields that are not recognized, but still included in the marshalled data.

The protobuf blob is prefixed by the string libp2p-pubsub: before signing.

[[ Margin note: ]] Protobuf serialization is non-deterministic/canonical, and the same data structure may result in different, valid serialised bytes across implementations, as well as other issues. In the near future, the signature creation and verification algorithm will be made deterministic.

Signature verification. There are two configurations possible:

  • Strict: either expect or not expect a signature.
  • Lax (legacy, insecure, underterministic, to be deprecated): accept a signed message if the signature verification passes, or if it's unsigned.

When signature validation fails for a signed message, the implementation must drop the message and omit propagation. Locally, it may treat this event in whichever manner it wishes (e.g. logging, penalization, etc.).

Signature Policy Options

The usage of the signature, key, from, and seqno fields in Message is configurable per topic.

[[ Implementation note ]]: At the time of writing this section, go-libp2p-pubsub (reference implementation of this spec) allows for configuring the signature policy at the global pubsub instance level. This needs to be pushed down to topic-level configuration. Other implementations should support topic-level configuration, as this spec mandates.

The intersection of signing behaviours across the two axes (signature creation and signature verification) gives way to four signature policy options:

  • StrictSign, StrictNoSign. Deterministic, usage encouraged.
  • LaxSign, LaxNoSign. Non-deterministic, legacy, usage discouraged. Mostly for backwards compatibility. Will be deprecated. If the implementation decides to support these, their use should be discouraged through deprecation warnings.

StrictSign option

On the producing side:

  • Build messages with the signature, key (from may be enough for certain inlineable public key types), from and seqno fields.

On the consuming side:

  • Enforce the fields to be present, reject otherwise.
  • Propagate only if the fields are valid and signature can be verified, reject otherwise.

StrictNoSign option

On the producing side:

  • Build messages without the signature, key, from and seqno fields.
  • The corresponding protobuf key-value pairs are absent from the marshalled message, not just empty.

On the consuming side:

  • Enforce the fields to be absent, reject otherwise.
  • Propagate only if the fields are absent, reject otherwise.
  • A message_id function will not be able to use the above fields, and should instead rely on the data field. A commonplace strategy is to calculate a hash.

LaxSign legacy option

Not required for backwards-compatibility. Considered insecure, nevertheless defined for completeness.

Always sign, and verify incoming signatures, and but accept unsigned messages.

On the producing side:

  • Build messages with the signature, key (from may be enough), from and seqno fields.

On the consuming side:

  • signature may be absent, and not verified.
  • Verify signature, iff the signature is present, then reject if signature is invalid.

LaxNoSign option

Previous default for 'no signature verification' mode.

Do not sign nor origin-stamp, but verify incoming signatures, and accept unsigned messages.

On the producing side:

  • Build messages without the signature, key, from and seqno fields.

On the consuming side:

  • Accept and propagate messages with above fields.
  • Verify signature, iff the signature is present, then reject if signature is invalid.

[[ Margin note: ]] For content-addressed messaging, StrictNoSign is the most appropriate policy option, coupled with a user-defined message_id_fn, and a validator function to verify protocol-defined signatures.

When publisher anonymity is being sought, StrictNoSign is also the most appropriate policy, as it refrains from outputting the from and seqno fields.

The Topic Descriptor

The topic descriptor message is used to define various options and parameters of a topic. It currently specifies the topic's human readable name, its authentication options, and its encryption options. The AuthOpts and EncOpts of the topic descriptor message are not used in current implementations, but may be used in future. For clarity, this is added as a comment in the file, and may be removed once used.

The TopicDescriptor protobuf is as follows:

message TopicDescriptor {
	optional string name = 1;
	// AuthOpts and EncOpts are unused as of Oct 2018, but
	// are planned to be used in future.
	optional AuthOpts auth = 2;
	optional EncOpts enc = 3;

	message AuthOpts {
		optional AuthMode mode = 1;
		repeated bytes keys = 2;

		enum AuthMode {
			NONE = 0;
			KEY = 1;
			WOT = 2;
		}
	}

	message EncOpts {
		optional EncMode mode = 1;
		repeated bytes keyHashes = 2;

		enum EncMode {
			NONE = 0;
			SHAREDKEY = 1;
			WOT = 2;
		}
	}
}

The name field is a string used to identify or mark the topic. It can be descriptive or random or anything that the creator chooses.

Note that instead of using TopicDescriptor.name, for privacy reasons the TopicDescriptor struct may be hashed, and used as the topic ID. Another option is to use a CID as a topic ID. While a consensus has not been reached, for forwards and backwards compatibility, using an enum TopicID that allows custom types in variants (i.e. Name, hashedTopicDescriptor, CID) may be the most suitable option if it is available within an implementation's language (otherwise it would be implementation defined).

The auth field specifies how authentication will work for this topic. Only authenticated peers may publish to a given topic. See 'AuthOpts' below for details.

The enc field specifies how messages published to this topic will be encrypted. See 'EncOpts' below for details.

AuthOpts

The AuthOpts message describes an authentication scheme. The mode field specifies which scheme to use, and the keys field is an array of keys. The meaning of the keys field is defined by the selected AuthMode.

There are currently three options defined for the AuthMode enum:

AuthMode 'NONE'

No authentication, anyone may publish to this topic.

AuthMode 'KEY'

Only peers whose peerIDs are listed in the keys array may publish to this topic, messages from any other peer should be dropped.

AuthMode 'WOT'

Web Of Trust: any trusted peer may publish to the topic. A trusted peer is one whose peerID is listed in the keys array, or any peer who is 'trusted' by another trusted peer. The mechanism of signifying trust in another peer is yet to be defined.

EncOpts

The EncOpts message describes an encryption scheme for messages in a given topic. The mode field denotes which encryption scheme will be used, and the keyHashes field specifies a set of hashes of keys whose purpose may be defined by the selected mode.

There are currently three options defined for the EncMode enum:

EncMode 'NONE'

Messages are not encrypted, anyone can read them.

EncMode 'SHAREDKEY'

Messages are encrypted with a preshared key. The salted hash of the key used is denoted in the keyHashes field of the EncOpts message. The mechanism for sharing the keys and salts is undefined.

EncMode 'WOT'

Web Of Trust publishing. Messages are encrypted with some certificate or certificate chain shared amongst trusted peers. (Spec writer's note: this is the least clearly defined option and my description here may be wildly incorrect, needs checking).

Topic Validation

Implementations MUST support attaching validators to topics.

Validators have access to the Message and can apply any logic to determine its validity. When propagating a message for a topic, implementations will invoke all validators attached to that topic, and will only continue propagation if, and only if all, validations pass.

In its simplest form, a validator is a function with signature (peer.ID, *Message) => bool, where the return value is true if validation passes, and false otherwise.

Local handling of failed validation is left up to the implementation (e.g. logging).

Implementations MAY allow dynamically adding and removing validators at runtime.