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111 lines
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111 lines
5.0 KiB
Markdown
Tunnelling through the Serval Mesh network
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==========================================
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[Serval Project][], May 2014
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The [Serval Mesh network][], based on the [Mesh Datagram Protocol][MDP],
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provides better packet delivery rates than conventional [UDP][] over a
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multi-hop wireless network by employing per-hop retransmission (and, in future,
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[linear network coding][]) to reduce the per-hop packet loss rate. This
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significantly reduces the cumulative multi-hop packet loss rate. For this
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reason, it is useful to carry [Internet Protocol][IP] traffic over the Serval
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Mesh wireless network using a technique called [tunnelling][].
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At present, only one tunnelling service is available for the Serval Mesh:
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* tunnelling of [TCP][] byte streams over the [Mesh Stream Protocol][MSP]
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(funded by a [grant][] from the [New America Foundation][NAF]'s [Open
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Technology Institute][OTI])
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Tunnelling TCP over MSP
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-----------------------
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An [MSP][] tunnel can carry many [TCP][] streams simultaneously. The tunnel
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is set up by running a process on each of the two end-point nodes:
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* the “server” node's process acts as an MSP *listener*
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* the “client” node's process *connects* to the server's MSP listener once for
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each TCP stream that is to be tunnelled.
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While an MSP tunnel is running, the client can initiate new TCP connections to
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the server, but not vice versa. The tunnel creates one MSP stream per TCP
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stream. Each MSP stream carries data in both directions.
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When either process terminates, the tunnel is broken and all of its connections
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immediately break.
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### Server end - `msp listen`
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$ servald msp listen [--once] --forward=TCPPORT [--service=NAME] MDPPORT
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The `msp listen` command acts as a server process which connects to the [Serval
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DNA][] daemon, binds to the [MDP port][] number given by `MDPPORT`, creates an
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[MSP][] *listening socket* on that port, and waits for connections from remote
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MSP clients to that socket. Whenever a connection is received, it:
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* connects to the TCP port `TCPPORT` on the local host (loopback interface),
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then:
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* writes all inbound MSP messages to the TCP stream in byte order, and
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* reads bytes from the local TCP stream and sends them as MSP messages
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* until either the local TCP stream is broken or the MSP connection is closed,
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whereupon it:
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* closes the TCP stream and the MSP connection.
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If the `--once` option is given then the server process only accepts a single
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MSP connection and exits as soon as the connection is closed.
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If the `--service=NAME` option is given, then the server process also binds to
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the standard [Serval service discovery][] [MDP port][] and responds to any
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matching query with the response `NAME.mdp.port=MDPPORT`.
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If the `--forward` option is omitted, then instead of connecting to a TCP port,
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accepts only a single connection (the `--once` option is implied, and need not
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be given explicitly) and forwards the MSP data to and from standard output and
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input.
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### Client end - `msp connect`
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$ servald msp connect [--once] --forward=TCPPORT SID MDPPORT
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The `msp connect` command listens on TCP port `TCPPORT` of the localhost
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(loopback interface) for a TCP connection from a local process. Whenever a
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connection is received, it:
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* makes an MSP connection to the given [SID][] and [MDP port][] number, then:
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* reads bytes from the local TCP stream and sends them as MSP messages, and
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* writes all inbound MSP messages to the local TCP stream in byte order
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* until either the TCP stream is broken or the MSP connection is closed,
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whereupon it:
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* closes the TCP stream and the MSP connection.
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If the `--once` option is given then the server process only accepts a single
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TCP connection and exits as soon as the connection is closed.
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If the `--forward` option is omitted, then instead of listening on a TCP port,
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makes the MSP connection immediately and forwards the MSP data to and from
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standard output and input.
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-----
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**Copyright 2014 Serval Project Inc.**
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Available under the [Creative Commons Attribution 4.0 International licence][CC BY 4.0].
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[Serval Project]: http://www.servalproject.org/
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[CC BY 4.0]: ../LICENSE-DOCUMENTATION.md
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[grant]: http://developer.servalproject.org/dokuwiki/doku.php?id=content:activity:naf6
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[NAF]: http://www.newamerica.net/
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[OTI]: http://oti.newamerica.net/
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[Serval mesh network]: http://developer.servalproject.org/dokuwiki/doku.php?id=content:tech:mesh_network
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[Serval DNA]: http://developer.servalproject.org/dokuwiki/doku.php?id=content:servaldna:
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[Serval service discovery]: ./Service-Discovery.md
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[MDP]: ./Mesh-Datagram-Protocol.md
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[MSP]: ./Mesh-Stream-Protocol.md
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[UDP]: http://en.wikipedia.org/wiki/User_Datagram_Protocol
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[TCP]: http://en.wikipedia.org/wiki/Transmission_Control_Protocol
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[IP]: http://en.wikipedia.org/wiki/Internet_Protocol
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[SID]: http://developer.servalproject.org/dokuwiki/doku.php?id=content:tech:sid
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[MDP port]: http://developer.servalproject.org/dokuwiki/doku.php?id=content:tech:mdp_port_number
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[linear network coding]: http://en.wikipedia.org/wiki/Linear_network_coding
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[tunnelling]: http://en.wikipedia.org/wiki/Tunneling_protocol
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