authors: Adam Babik <adam@status.im>, Andrea Maria Piana <andreap@status.im>, Dean Eigenmann <dean@status.im>, Kim De Mey <kimdemey@status.im>, Oskar Thorén <oskar@status.im>
redirect_from:
- /waku/waku.html
---
## Table of Contents
- [Abstract](#abstract)
- [Motivation](#motivation)
- [Definitions](#definitions)
- [Underlying Transports and Prerequisites](#underlying-transports-and-prerequisites)
This specification describes the format of Waku packets within the ÐΞVp2p Wire Protocol. This spec substitutes [EIP-627](https://eips.ethereum.org/EIPS/eip-627). Waku is a fork of the original Whisper protocol that enables better usability for resource restricted devices, such as mostly-offline bandwidth-constrained smartphones. It does this through (a) light node support, (b) historic envelopes (with a mailserver) (c) expressing topic interest for better bandwidth usage and (d) basic rate limiting.
Waku was created to incrementally improve in areas that Whisper is lacking in, with special attention to resource restricted devices. We specify the standard for Waku packets in order to ensure forward compatibility of different Waku clients, backwards compatibility with Whisper clients, as well as to allow multiple implementations of Waku and its capabilities. We also modify the language to be more unambiguous, concise and consistent.
For nodes to communicate, they MUST implement devp2p and run RLPx. They MUST have some way of connecting to other nodes. Node discovery is largely out of scope for this spec, but see the appendix for some suggestions on how to do this.
This protocol needs to advertise the `waku/1` [capability](https://ethereum.gitbooks.io/frontier-guide/devp2p.html).
In Whisper, envelopes are gossiped between peers. Whisper is a form of rumor-mongering protocol that works by flooding to its connected peers based on some factors. Envelopes are eligible for retransmission until their TTL expires. A node SHOULD relay envelopes to all connected nodes if an envelope matches their PoW and bloom filter settings. If a node works in light mode, it MAY choose not to forward envelopes. A node MUST NOT send expired envelopes, unless the envelopes are sent as a [mailserver](./mailserver.md) response. A node SHOULD NOT send an envelope to a peer that it has already sent before.
All Waku packets are sent as devp2p RLPx transport protocol, version 5[^1] packets. These packets MUST be RLP-encoded arrays of data containing two objects: packet code followed by another object (whose type depends on the packet code). See [informal RLP spec](https://github.com/ethereum/wiki/wiki/RLP) and the [Ethereum Yellow Paper, appendix B](https://ethereum.github.io/yellowpaper/paper.pdf) for more details on RLP.
Waku is a RLPx subprotocol called `waku` with version `0`. The version number corresponds to the major version in the header spec. Minor versions should not break compatibility of `waku`, this would result in a new major. (Some exceptions to this apply in the Draft stage of where client implementation is rapidly change).
When a node is receiving other Waku packets from a peer before a Status
packet is received, the node MUST ignore these packets and SHOULD disconnect from that peer. Status packets received after the handshake is completed MUST also be ignored.
The Status packet MUST contain an association list containing various options. All options within this association list are OPTIONAL, ordering of the key-value pairs is not guaranteed and therefore MUST NOT be relied on. Unknown keys in the association list SHOULD be ignored.
PoW is defined as average number of iterations, required to find the current BestBit (the number of leading zero bits in the hash), divided by envelope size and TTL:
The bloom filter is used to identify a number of topics to a peer without compromising (too much) privacy over precisely what topics are of interest. Precise control over the information content (and thus efficiency of the filter) may be maintained through the addition of bits.
Blooms are formed by the bitwise OR operation on a number of bloomed topics. The bloom function takes the topic and projects them onto a 512-bit slice. At most, three bits are marked for each bloomed topic.
The projection function is defined as a mapping from a 4-byte slice S to a 512-bit slice D; for ease of explanation, S will dereference to bytes, whereas D will dereference to bits.
LET D[*] = 0
FOREACH i IN { 0, 1, 2 } DO
LET n = S[i]
IF S[3] & (2 ** i) THEN n += 256
D[n] = 1
END FOR
A full bloom filter (all the bits set to 1) means that the node is to be considered a `Full Node` and it will accept any topic.
If only bloom filter is specified, the current topic interest MUST be discarded and only the updated bloom filter MUST be used when forwarding or posting envelopes.
Topic interest is used to share a node's interest in envelopes with specific topics. It does this in a more bandwidth considerate way, at the expense of some metadata protection. Peers MUST only send envelopes with specified topics.
It is currently bounded to a maximum of 10000 topics. If you are interested in more topics than that, this is currently underspecified and likely requires updating it. The constant is subject to change.
If only topic interest is specified, the current bloom filter MUST be discarded and only the updated topic interest MUST be used when forwarding or posting envelopes.
In order to provide basic Denial-of-Service attack protection, each node SHOULD define its own rate limits. The rate limits SHOULD be applied on IPs, peer IDs, and envelope topics.
Each node MAY decide to whitelist, i.e. do not rate limit, selected IPs or peer IDs.
If a peer exceeds node's rate limits, the connection between them MAY be dropped.
Each node SHOULD broadcast its rate limits to its peers using the `status-update` packet. The rate limits MAY also be sent as an optional parameter in the handshake.
Each node SHOULD respect rate limits advertised by its peers. The number of packets SHOULD be throttled in order not to exceed peer's rate limits. If the limit gets exceeded, the connection MAY be dropped by the peer.
When the node's `light-node` field is set to true, the node SHOULD NOT forward Envelopes from its peers.
A node connected to a peer with the `light-node` field set to true MUST NOT depend on the peer for forwarding Envelopes.
##### Confirmations Enabled Field
When the node's `confirmations-enabled` field is set to true, the node SHOULD send [message confirmations](#batch-ack-and-message-response) to its peers.
Message confirmations tell a node that a envelope originating from it has been received by its peers, allowing a node to know whether an envelope has or has not been received.
A message confirmation is sent using Batch Ack packet (`0x0B`) or Message Response packet (`0x0C`). The message confirmation is specified in the ABNF specification below.
The drawback of sending message confirmations is that it increases the noise in the network because for each sent envelope, a corresponding confirmation is broadcast by one or more peers.
This packet is used for sending Dapp-level peer-to-peer requests, e.g. Waku Mail Client requesting historic (expired) envelopes from the [Waku Mail Server](./mailserver.md).
This packet is used for sending the peer-to-peer envelopes, which are not supposed to be forwarded any further. E.g. it might be used by the Waku Mail Server for delivery of historic (expired) envelopes, which is otherwise not allowed.
This packet is used to indicate that all envelopes, requested earlier with a P2P Request packet (`0x7E`), have been sent via one or more P2P Message packets (`0x7F`).
Asymmetric encryption uses the standard Elliptic Curve Integrated Encryption Scheme with SECP-256k1 public key.
Symmetric encryption uses AES GCM algorithm with random 96-bit nonce.
### Packet code Rationale
Packet codes `0x00` and `0x01` are already used in all Waku / Whisper versions. Packet code `0x02` and `0x03` were previously used in Whisper but are deprecated as of Waku v0.4
Packet code `0x22` is used to dynamically change the settings of a node.
Packet codes `0x7E` and `0x7F` may be used to implement Waku Mail Server and Client. Without the P2P Message packet it would be impossible to deliver the historic envelopes, since they will be recognized as expired, and the peer will be disconnected for violating the Waku protocol. They might be useful for other purposes when it is not possible to spend time on PoW, e.g. if a stock exchange will want to provide live feed about the latest trades.
Waku supports multiple capabilities. These include light node, rate limiting and bridging of traffic. Here we list these capabilities, how they are identified, what properties they have and what invariants they must maintain.
Additionally there is the capability of a mailserver which is documented in its on [specification](mailserver.md).
### Light node
The rationale for light nodes is to allow for interaction with waku on resource restricted devices as bandwidth can often be an issue.
Light nodes MUST NOT forward any incoming envelopes, they MUST only send their own envelopes. When light nodes happen to connect to each other, they SHOULD disconnect. As this would result in envelopes being dropped between the two.
Nodes MAY implement accounting, keeping track of resource usage. It is heavily inspired by Swarm's [SWAP protocol](https://www.bokconsulting.com.au/wp-content/uploads/2016/09/tron-fischer-sw3.pdf), and works by doing pairwise accounting for resources.
Each node keeps track of resource usage with all other nodes. Whenever an envelope is received from a node that is expected (fits bloom filter or topic interest, is legal, etc) this is tracked.
Every epoch (say, every minute or every time an event happens) statistics SHOULD be aggregated and saved by the client:
The currently advertised capability is `waku/1`. This needs to be advertised in the `hello``ÐΞVp2p` [packet](https://ethereum.gitbooks.io/frontier-guide/devp2p.html).
If a node supports multiple versions of `waku`, those needs to be explicitly advertised. For example if both `waku/0` and `waku/1` are supported, both `waku/0` and `waku/1` MUST be advertised.
These are policies that guide how we make decisions when it comes to upgradability, compatibility, and extensibility:
1. Waku aims to be compatible with previous and future versions.
2. In cases where we want to break this compatibility, we do so gracefully and as a single decision point.
3. To achieve this, we employ the following two general strategies:
- a) Accretion (including protocol negotiation) over changing data
- b) When we want to change things, we give it a new name (for example, a version number).
Examples:
- We enable bridging between `shh/6` and `waku/1` until such a time as when we are ready to gracefully drop support for `shh/6` (1, 2, 3).
- When we add parameter fields, we (currently) do so by accreting them in a list, so old clients can ignore new fields (dynamic list) and new clients can use new capabilities (1, 3).
- To better support (2) and (3) in the future, we will likely release a new version that gives better support for open, growable maps (association lists or native map type) (3)
- When we we want to provide a new set of packets that have different requirements, we do so under a new protocol version and employ protocol versioning. This is a form of accretion at a level above - it ensures a client can support both protocols at once and drop support for legacy versions gracefully. (1,2,3)
Waku is a different subprotocol from Whisper so it isn't directly compatible. However, the data format is the same, so compatibility can be achieved by the use of a bridging mode as described below. Any client which does not implement certain packet codes should gracefully ignore the packets with those codes. This will ensure the forward compatibility.
### Waku-Whisper bridging
`waku/1` and `shh/6` are different DevP2P subprotocols, however they share the same data format making their envelopes compatible. This means we can bridge the protocols naively, this works as follows.
**Roles:**
- Waku client A, only Waku capability
- Whisper client B, only Whisper capability
- WakuWhisper bridge C, both Waku and Whisper capability
**Note**: This flow means if another bridge C1 is active, we might get duplicate relaying for a envelope between C1 and C2. I.e. Whisper(<>Waku<>Whisper)<>Waku, A-C1-C2-B. Theoretically this bridging chain can get as long as TTL permits.
It is desirable to have a strategy for maintaining forward compatibility between `waku/1` and future version of waku. Here we outline some concerns and strategy for this.
- **Connecting to nodes with multiple versions:** The way this SHOULD be accomplished is by negotiating the versions of subprotocols, within the `hello` packet nodes transmit their capabilities along with a version. The highest common version should then be used.
- **Adding new packet codes:** New packet codes can be added easily due to the available packet codes. Unknown packet codes SHOULD be ignored. Upgrades that add new packet codes SHOULD implement some fallback mechanism if no response was received for nodes that do not yet understand this packet.
- **Adding new options in `status-options`:** New options can be added to the `status-options` association list in the `status` and `status-update` packet as options are OPTIONAL and unknown option keys SHOULD be ignored. A node SHOULD NOT disconnect from a peer when receiving `status-options` with unknown option keys.
There are several security considerations to take into account when running Waku. Chief among them are: scalability, DDoS-resistance and privacy. These also vary depending on what capabilities are used. The security considerations for extra capabilities such as [mailservers](./mailserver.md#security-considerations) can be found in their respective specifications.
In version 0 of Waku, bandwidth usage is likely to be an issue. For more investigation into this, see the theoretical scaling model described [here](https://github.com/vacp2p/research/tree/dcc71f4779be832d3b5ece9c4e11f1f7ec24aac2/whisper_scalability).
Use of gossip-based routing doesn't necessarily scale. It means each node can see an envelope multiple times, and having too many light nodes can cause propagation probability that is too low. See [Whisper vs PSS](https://our.status.im/whisper-pss-comparison/) for more and a possible Kademlia based alternative.
The main privacy concern with light nodes is that directly connected peers will know that an envelope originates from them (as it are the only ones it sends). This means nodes can make assumptions about what envelopes (topics) their peers are interested in.
By having a bloom filter where only the topics you are interested in are set, you reveal which envelopes you are interested in. This is a fundamental tradeoff between bandwidth usage and privacy, though the tradeoff space is likely suboptimal in terms of the [Anonymity](https://eprint.iacr.org/2017/954.pdf) [trilemma](https://petsymposium.org/2019/files/hotpets/slides/coordination-helps-anonymity-slides.pdf).
Privacy for Whisper / Waku haven't been studied rigorously for various threat models like global passive adversary, local active attacker, etc. This is unlike e.g. Tor and mixnets.
Proof of work is a poor spam prevention mechanism. A mobile device can only have a very low PoW in order not to use too much CPU / burn up its phone battery. This means someone can spin up a powerful node and overwhelm the network.
### Censorship resistance
**Devp2p TCP port blockable:**
By default Devp2p runs on port `30303`, which is not commonly used for any other service. This means it is easy to censor, e.g. airport WiFi. This can be mitigated somewhat by running on e.g. port `80` or `443`, but there are still outstanding issues. See libp2p and Tor's Pluggable Transport for how this can be improved.
To avoid duplicate envelopes, only connect to one Waku node. Benign duplicate envelopes is an intrinsic property of Whisper which often leads to a N factor increase in traffic, where N is the number of peers you are connected to.
2. Topic specific recommendations
Consider partition topics based on some usage, to avoid too much traffic on a single topic.
### Node discovery
Resource restricted devices SHOULD use [EIP-1459](https://eips.ethereum.org/EIPS/eip-1459) to discover nodes.
Released [February 21, 2020](https://github.com/vacp2p/specs/commit/17bd066e317bbe33af07146b721d73f24de47e88).
- Simplify implementation matrix with latest state
- Introduces a new required packet code Status Code (`0x22`) for communicating option changes
- Deprecates the following packet codes: PoW Requirement (`0x02`), Bloom Filter (`0x03`), Rate limits (`0x20`), Topic interest (`0x21`) - all superseded by the new Status Code (`0x22`)
- Increased `topic-interest` capacity from 1000 to 10000
### Version 0.3
Released [February 13, 2020](https://github.com/vacp2p/specs/commit/73138d6ba954ab4c315e1b8d210ac7631b6d1428).
- Recommend DNS based node discovery over other Discovery methods.
- Mark spec as Draft mode in terms of its lifecycle.
