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12/WAKU2-FILTER

previous versions: 00

WakuFilter is a protocol that enables subscribing to messages that a peer receives. This is a more lightweight version of WakuRelay specifically designed for bandwidth restricted devices. This is due to the fact that light nodes subscribe to full-nodes and only receive the messages they desire.

Content filtering​

Protocol identifiers:

  • filter-subscribe: /vac/waku/filter-subscribe/2.0.0-beta1
  • filter-push: /vac/waku/filter-push/2.0.0-beta1

Content filtering is a way to do message-based filtering. Currently the only content filter being applied is on contentTopic. This corresponds to topics in Waku v1.

Rationale​

Unlike the store protocol for historical messages, this protocol allows for native lower latency scenarios such as instant messaging. It is thus complementary to it.

Strictly speaking, it is not just doing basic request response, but performs sender push based on receiver intent. While this can be seen as a form of light pub/sub, it is only used between two nodes in a direct fashion. Unlike the Gossip domain, this is meant for light nodes which put a premium on bandwidth. No gossiping takes place.

It is worth noting that a light node could get by with only using the store protocol to query for a recent time window, provided it is acceptable to do frequent polling.

Design Requirements​

The effectiveness and reliability of the content filtering service enabled by WakuFilter protocol rely on the high availability of the full nodes as the service providers. To this end, full nodes must feature high uptime (to persistently listen and capture the network messages) as well as high Bandwidth (to provide timely message delivery to the light nodes).

Security Consideration​

Note that while using WakuFilter allows light nodes to save bandwidth, it comes with a privacy cost in the sense that they need to disclose their liking topics to the full nodes to retrieve the relevant messages. Currently, anonymous subscription is not supported by the WakuFilter, however, potential solutions in this regard are sketched below in Future Work section.

Terminology​

The term Personally identifiable information (PII) refers to any piece of data that can be used to uniquely identify a user. For example, the signature verification key, and the hash of one's static IP address are unique for each user and hence count as PII.

Adversarial Model​

Any node running the WakuFilter protocol i.e., both the subscriber node and the queried node are considered as an adversary. Furthermore, we consider the adversary as a passive entity that attempts to collect information from other nodes to conduct an attack but it does so without violating protocol definitions and instructions. For example, under the passive adversarial model, no malicious node intentionally hides the messages matching to one's subscribed content filter as it is against the description of the WakuFilter protocol.

The following are not considered as part of the adversarial model:

  • An adversary with a global view of all the nodes and their connections.
  • An adversary that can eavesdrop on communication links between arbitrary pairs of nodes (unless the adversary is one end of the communication). In specific, the communication channels are assumed to be secure.

Protobuf​

syntax = "proto3";

// 12/WAKU2-FILTER rfc: https://rfc.vac.dev/spec/12/
package waku.filter.v2;

// Protocol identifier: /vac/waku/filter-subscribe/2.0.0-beta1
message FilterSubscribeRequest {
  enum FilterSubscribeType {
    SUBSCRIBER_PING = 0;
    SUBSCRIBE = 1;
    UNSUBSCRIBE = 2;
    UNSUBSCRIBE_ALL = 3;
  }

  string request_id = 1;
  FilterSubscribeType filter_subscribe_type = 2;

  // Filter criteria
  optional string pubsub_topic = 10;
  repeated string content_topics = 11;
}

message FilterSubscribeResponse {
  string request_id = 1;
  uint32 status_code = 10;
  optional string status_desc = 11;
}

// Protocol identifier: /vac/waku/filter-push/2.0.0-beta1
message MessagePush {
  WakuMessage waku_message = 1;
  optional string pubsub_topic = 2;
}

Filter-Subscribe​

A filter service node MUST support the filter-subscribe protocol to allow filter clients to subscribe, modify, refresh and unsubscribe a desired set of filter criteria. The combination of different filter criteria for a specific filter client node is termed a "subscription". A filter client is interested in receiving messages matching the filter criteria in its registered subscriptions.

Since a filter service node is consuming resources to provide this service, it MAY account for usage and adapt its service provision to certain clients. An incentive mechanism is currently planned but underspecified.

Filter Subscribe Request​

A client node MUST send all filter requests in a FilterSubscribeRequest message. This request MUST contain a request_id. The request_id MUST be a uniquely generated string. Each request MUST include a filter_subscribe_type, indicating the type of request.

Filter Subscribe Response​

In return to any FilterSubscribeRequest, a filter service node SHOULD respond with a FilterSubscribeResponse with a requestId matching that of the request. This response MUST contain a status_code indicating if the request was successful or not. Successful status codes are in the 2xx range. Client nodes SHOULD consider all other status codes as error codes and assume that the requested operation had failed. In addition, the filter service node MAY choose to provide a more detailed status description in the status_desc field.

Filter matching​

In the description of each request type below, the term "filter criteria" refers to the combination of pubsub_topic and a set of content_topics. The request MAY include filter criteria, conditional to the selected filter_subscribe_type. If the request contains filter criteria, it MUST contain a pubsub_topic and the content_topics set MUST NOT be empty. A WakuMessage matches filter criteria when its content_topic is in the content_topics set and it was published on a matching pubsub_topic.

Filter Subscribe Types​

The following filter subscribe types are defined:

SUBSCRIBER_PING​

A filter client that sends a FilterSubscribeRequest with filter_subscribe_type set to SUBSCRIBER_PING requests that the service node SHOULD indicate if it has any active subscriptions for this client. The filter client SHOULD exclude any filter criteria from the request. The filter service node SHOULD respond with a success code if it has any active subscriptions for this client or an error code if not. The filter service node SHOULD ignore any filter criteria in the request.

SUBSCRIBE​

A filter client that sends a FilterSubscribeRequest with filter_subscribe_type set to SUBSCRIBE requests that the service node SHOULD push messages matching this filter to the client. The filter client MUST include the desired filter criteria in the request. A client MAY use this request type to modify an existing subscription by providing additional filter criteria in a new request. A client MAY use this request type to refresh an existing subscription by providing the same filter criteria in a new request. The filter service node SHOULD respond with a success code if it successfully honored this request or an error code if not. The filter service node SHOULD respond with an error code and discard the request if the subscribe request does not contain valid filter criteria, i.e. both a pubsub_topic and a non-empty content_topics set.

UNSUBSCRIBE​

A filter client that sends a FilterSubscribeRequest with filter_subscribe_type set to UNSUBSCRIBE requests that the service node SHOULD stop pushing messages matching this filter to the client. The filter client MUST include the filter criteria it desires to unsubscribe from in the request. A client MAY use this request type to modify an existing subscription by providing a subset of the original filter criteria to unsubscribe from in a new request. The filter service node SHOULD respond with a success code if it successfully honored this request or an error code if not. The filter service node SHOULD respond with an error code and discard the request if the unsubscribe request does not contain valid filter criteria, i.e. both a pubsub_topic and a non-empty content_topics set.

UNSUBSCRIBE_ALL​

A filter client that sends a FilterSubscribeRequest with filter_subscribe_type set to UNSUBSCRIBE_ALL requests that the service node SHOULD stop pushing messages matching any filter to the client. The filter client SHOULD exclude any filter criteria from the request. The filter service node SHOULD remove any existing subscriptions for this client. It SHOULD respond with a success code if it successfully honored this request or an error code if not.

Filter-Push​

A filter client node MUST support the filter-push protocol to allow filter service nodes to push messages matching registered subscriptions to this client.

A filter service node SHOULD push all messages matching the filter criteria in a registered subscription to the subscribed filter client. These WakuMessages are likely to come from 11/WAKU2-RELAY, but there MAY be other sources or protocols where this comes from. This is up to the consumer of the protocol.

If a message push fails, the filter service node MAY consider the client node to be unreachable. If a specific filter client node is not reachable from the service node for a period of time, the filter service node MAY choose to stop pushing messages to the client and remove its subscription. This period is up to the service node implementation. We consider 1 minute to be a reasonable default.

Message Push​

Each message MUST be pushed in a MessagePush message. Each MessagePush MUST contain one (and only one) waku_message. If this message was received on a specific pubsub_topic, it SHOULD be included in the MessagePush. A filter client SHOULD NOT respond to a MessagePush. Since the filter protocol does not include caching or fault-tolerance, this is a best effort push service with no bundling or guaranteed retransmission of messages. A filter client SHOULD verify that each MessagePush it receives originated from a service node where the client has an active subscription and that it matches filter criteria belonging to that subscription.

Future Work​

Anonymous filter subscription: This feature guarantees that nodes can anonymously subscribe for a message filter (i.e., without revealing their exact content filter). As such, no adversary in the WakuFilter protocol would be able to link nodes to their subscribed content filers. The current version of the WakuFilter protocol does not provide anonymity as the subscribing node has a direct connection to the full node and explicitly submits its content filter to be notified about the matching messages. However, one can consider preserving anonymity through one of the following ways:

  • By hiding the source of the subscription i.e., anonymous communication. That is the subscribing node shall hide all its PII in its filter request e.g., its IP address. This can happen by the utilization of a proxy server or by using Tor. Note that the current structure of filter requests i.e., FilterRPC does not embody any piece of PII, otherwise, such data fields must be treated carefully to achieve anonymity.
  • By deploying secure 2-party computations in which the subscribing node obtains the messages matching a content filter whereas the full node learns nothing about the content filter as well as the messages pushed to the subscribing node. Examples of such 2PC protocols are Oblivious Transfers and one-way Private Set Intersections (PSI).

Changelog​

Next​

  • Added initial threat model and security analysis.

2.0.0-beta2​

Initial draft version. Released 2020-10-28

  • Fix: Ensure contentFilter is a repeated field, on implementation
  • Change: Add ability to unsubscribe from filters. Make subscribe an explicit boolean indication. Edit protobuf field order to be consistent with libp2p.

2.0.0-beta1​

Initial draft version. Released 2020-10-05

Copyright and related rights waived via CC0.

References​

  1. Message Filtering (Wikipedia)

  2. Libp2p PubSub spec - topic validation