MOSH Wiki

The Open Systems Interconnect (OSI) model is a framework that describes how two or more computer systems communicate over a network. Each layer of the framework provides a specific purpose in communications. The OSI framework is as follows:

• Layer 7 - Application
• Layer 6 - Presentation
• Layer 5 - Session
• Layer 4 - Transport
• Layer 3 - Network
• Layer 2 - Data Link
• Layer 1 - Physical

Depending on the type of transport being used, MOSH transmits at different layers of the OSI model. It is important to know that in all cases, MOSH implements its own framework of security.

Another key concept in network communications is that of encapsulation. Simply put, encapsulation is the process of wrapping existing data in new data. In OSI (and TCP/IP), data at higher layers is encapsulated by data at lower layers. Consequently, MOSH messages are encapsulated, or wrapped, by other lower-level protocols used by transports.

Depending on the transport being used, MOSH may rely on the functionality of these existing protocols for things like reliable message delivery. For example, TCP (used in IP networks) ensures messages sent by one node are received by the destination node. In this scenario, MOSH doesn't need to perform that function since it's included it the transport.

A MOSH transport is the radio (or serial connection) where MOSH transmits and receives MOSH messages. Transport options are largely determined by the developers choice in hardware. However, there are some scenarios where transports can be chosen in software (ie ESP-NOW vs Wifi on ESP radios). The MOSH transport library currently provides support for the following protocols:

• Espressif ESP-NOW(V1)
• Espressif ESP-NOW(V2)
• WiFi
• Serial

ESP-NOW is a protocol designed by Espressif to be a connectionless, low-overhead, peer-to-peer protocol. It does not natively ensure delivery of messages, or include any built-in security features. As far as MOSH transports go, ESP-NOW is a sort-of middle ground – offering maximum message lengths of 250 bytes in version 1, and up to 1470 bytes in version 2.

WiFi is a wireless protocol defined by the IEEE in the 802.11 protocol standard. And while MOSH can optionally use WiFi as a transport for MOSH messages, providing additional features, it is certainly not a requirement for a complete MOSH network.

Modern Microcontroller Units (MCU) commonly use TTL Serial/UART to communicate with a host computer when initially programmed. A MOSH node can use this same transport to communicate with another MOSH node. Most commonly, this would be done on a MOSH Controller in order to bridge transports.