- [Altruistic and centralized operator risk](#altruistic-and-centralized-operator-risk)
- [Privacy concerns](#privacy-concerns)
- [Denial-of-service](#denial-of-service)
- [Changelog](#changelog)
- [Version 0.1](#version-01)
## Abstract
Being mostly offline is an intrinsic property of mobile clients. They need to save network transfer and battery consumption to avoid spending too much money or constant charging. Waku protocol, on the other hand, is an online protocol. Messages are available in the Waku network only for short period of time calculate in seconds.
Waku Mailserver is a specification that allows messages to be stored permanently and to allows the stored messages to be delivered to requesting client nodes, regardless if the messages are not available in the network due to the message TTL expiring.
## `Mailserver`
From the network perspective, a `Mailserver` is just like any other Waku node. The only difference is that a `Mailserver` has the capability of archiving messages and delivering them to its peers on-demand.
It is important to notice that a `Mailserver` will only handle requests from its direct peers and exchanged packets between a `Mailserver` and a peer are p2p messages.
### Archiving messages
A node which wants to provide `Mailserver` functionality MUST store envelopes from
incoming message packets (Waku packet-code `0x01`). The envelopes can be stored in any
format, however they MUST be serialized and deserialized to the Waku envelope format.
A `Mailserver` SHOULD store envelopes for all topics to be generally useful for any peer, however for specific use cases it MAY store envelopes for a subset of topics.
### Requesting messages
In order to request historic messages, a node MUST send a packet P2P Request (`0x7e`) to a peer providing `Mailserver` functionality. This packet requires one argument which MUST be a Waku envelope.
In the Waku envelope's payload section, there MUST be RLP-encoded information about the details of the request:
```
[ Lower, Upper, Bloom, Limit, Cursor ]
```
`Lower`: 4-byte wide unsigned integer (UNIX time in seconds; oldest requested envelope's creation time)
`Upper`: 4-byte wide unsigned integer (UNIX time in seconds; newest requested envelope's creation time)
`Bloom`: 64-byte wide array of Waku topics encoded in a bloom filter to filter envelopes
`Limit`: 4-byte wide unsigned integer limiting the number of returned envelopes
`Cursor`: an array of a cursor returned from the previous request (optional)
The `Cursor` field SHOULD be filled in if a number of envelopes between `Lower` and `Upper` is greater than `Limit` so that the requester can send another request using the obtained `Cursor` value. What exactly is in the `Cursor` is up to the implementation. The requester SHOULD NOT use a `Cursor` obtained from one `Mailserver` in a request to another `Mailserver` because the format or the result MAY be different.
The envelope MUST be encrypted with a symmetric key agreed between the requester and the `Mailserver`.
### Receiving historic messages
Historic messages MUST be sent to a peer as a packet with a P2P Message code (`0x7f`) followed by an array of Waku envelopes.
In order to receive historic messages from a `Mailserver`, a node MUST trust the selected `Mailserver`, that is allowed to send packets with the P2P Message code. By default, the node discards such packets.
Received envelopes MUST be passed through the Waku envelope pipelines so that they are picked up by registered filters and passed to subscribers.
For a requester, to know that all messages have been sent by a `Mailserver`, it SHOULD handle P2P Request Complete code (`0x7d`). This code is followed by the following parameters:
```
[ RequestID, LastEnvelopeHash, Cursor ]
```
-`RequestID`: 32-byte wide array with a Keccak-256 hash of the envelope containing the original request
-`LastEnvelopeHash`: 32-byte wide array with a Keccak-256 hash of the last sent envelope for the request
-`Cursor`: an array of a cursor returned from the previous request (optional)
If `Cursor` is not empty, it means that not all messages were sent due to the set `Limit` in the request. One or more consecutive requests MAY be sent with `Cursor` field filled in order to receive the rest of the messages.
suboptimal from a decentralization, continuance and risk point of view. Coming
up with a better system for this is ongoing research.
A Status client SHOULD allow the `Mailserver` selection to be customizable.
### Privacy concerns
In order to use a `Mailserver`, a given node needs to connect to it directly,
i.e. add the `Mailserver` as its peer and mark it as trusted. This means that the
`Mailserver` is able to send direct p2p messages to the node instead of
broadcasting them. Effectively, it will have access to the bloom filter of
topics that the user is interested in, when it is online as well as many
metadata like IP address.
### Denial-of-service
Since a `Mailserver` is delivering expired envelopes and has a direct TCP connection with the recipient, the recipient is vulnerable to DoS attacks from a malicious `Mailserver` node.
