Background

Please refer to Nomos Blend Network: Introduction [WIP Design] for more information about the Nomos Mix design.

Introduction

The Persistent Transmission Protocol is a protocol that works directly on connections between nodes. The protocol enables establishing a connection through which messages are sent at a constant rate. That is, there is a globally agreed parameter called Global Transmission Rate (GTR) which indicates the rate at which messages must be generated per connection. In other words, a node must emit messages to each of its peers at the rate defined by GTR.

The protocol defines two types of messages, data and drop. Data messages contain non-zero information and are used by other mechanisms of the Nomos Mix protocol. Drop messages are added by the connection endpoint in order to uphold the GTR in absence of data message. Drop messages are discarded by the other end of the connection. Both messages must be of the same length, otherwise they could be distinguished.

Finally, each of the established connections is encrypted. Therefore, for an external observer both drop and data messages are indistinguishable.

The Persistent Transmission gives us the following properties from the perspective of a global observer.

First, an observer cannot distinguish between messages due to application of the encryption.
Second, an observer cannot learn any statistical dependencies between timing of messages as they are propagated at a constant rate.
Third, an observer cannot learn when a node is transmitting a meaningful message as it cannot distinguish between drop and data messages.

Design

The Persistent Transmission Protocol is part of the system that is responsible for exchanging messages between nodes. The Persistent Transmission is a per-connection mechanism that: