Overlay network

The first implementation of Mesh Datagram Protocol (MDP) was over Wi-Fi, by encapsulating it inside the OSI “Transport” Layer 4 protocol UDP/IP and dynamically translating MDP addresses to IP addresses. This encapsulation is called MDP Overlay.

In order to maximise bandwith utilisation, the MDP Overlay layer stuffs as many MDP packets into a single UDP packet as it can, up to the Overlay network's MTU. The standard implementation of UDP/IP over Wi-Fi will accept a maximum datagram size equal to the Wi-Fi interface's MTU, less all the fixed overheads (encapsulation costs) of the UDP header, IP header and Ethernet frame. The initial implementation of MDP Overlay used a fixed UDP MTU of 1200 bytes, which was compatible with the standard Ethernet MTU of 1500 and coincides with the default IPv6 MTU of 1200.

The Overlay layer fills each UDP frame with a single MDP Overlay Frame which consists of a frame header followed by one or more stuffed Mesh Datagram Protocol (MDP) packets:

To further conserve space, all addresses (SIDs) within the stuffed MDP packets are represented using a “ditto notation” which avoids repetition and can significantly shorten some or even all of the MDP packets. (This is in addition to the SID abbreviation scheme used within MDP frames independently of the MDP Overlay layer.)