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content:tech:overlay_network [21/05/2013 11:05] (current)
Andrew Bettison created, moved from the [[MDP]] page and improved
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 +====== Overlay network ======
 +
 +The first implementation of [[MDP]] was over [[http://en.wikipedia.org/wiki/WiFi|Wi-Fi]], by encapsulating it inside the [[http://en.wikipedia.org/wiki/Transport-layer|OSI “Transport” Layer 4]] protocol [[http://en.wikipedia.org/wiki/User_Datagram_Protocol|UDP/IP]] and dynamically translating [[SID|MDP addresses]] to IP addresses.  This encapsulation is called **MDP Overlay**.
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 +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 [[http://en.wikipedia.org/wiki/Maximum_transmission_unit|MTU]], less all the fixed overheads (encapsulation costs) of the [[https://en.wikipedia.org/wiki/User_Datagram_Protocol#Packet_structure|UDP header]], [[https://en.wikipedia.org/wiki/IPv4_header#Header|IP header]] and [[https://en.wikipedia.org/wiki/Ethernet_frame|Ethernet frame]].  The initial implementation of MDP Overlay used a fixed UDP MTU of 1200 bytes, which was compatible with the standard [[https://en.wikipedia.org/wiki/Ethernet|Ethernet]] MTU of 1500 and coincides with the default [[https://en.wikipedia.org/wiki/IPv6|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 [[MDP]] packets:
 + 
 +{{https://raw.github.com/servalproject/serval-docs/master/linux.conf.au-2013/slides/png/Protocol%202%20-%20MDP%20overlay%20frame.png?800}}
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 +To further conserve space, all [[SID|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.)
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