Network Architecture

Version 12 (Pau Escrich, 10/04/2013 09:35 am)

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h1. NetworkArchitecture
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h2. Objectives to achieve
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* Scalability
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* Network segmentation
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* Layer 2 roaming inside certain areas
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* Smart gateway selection with redundancy and possibility of user-choice
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* Compatibility of most of the existing scenarios
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* A single firmware for all the network (for ground level, for backbone, for companies, for quick deployments, etc.)
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h2. The basics
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The network architecture designed for libre-mesh is based in two layers:
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* The cloud layer 2 which runs Batman-Adv
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* The network layer 3 which runs BMX6
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!network1.png!
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By default all nodes are runing BMX6 and BATADV at same time in the same WiFi devices but different VLANS. 
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The BMX6 VLAN is always the same, so all link-layer connected nodes will see each other.
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The BATADV VLAN depends on the cloud identifier (CID) which is calculed (by default) from the AP SSID.
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!network2.png!
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This configuration allows to isolate the layer 2 clouds. 
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For instance a neighborhood, a company complex or a street level hotsposts network can choose isolate their LAN from the rest of the network. 
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However at the same time, they will be able to reach the rest of the community network using the layer 3 routed network.
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In case an entire network community wants to have a single cloud, they can just use the same CID.
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h2. The details
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All WiFi Access Points from the same layer 2 cloud share the same SSID, a special anycast IPv4/IPv6 address and a special anycast MAC address.
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It will allow real roaming inside the network, so the clients does not need to renew their IP configuration when they change from one AP to the other.
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!network3.png!
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All mesh nodes have a DHCP/RA server to provide valid IPs from the cloud to the clients.
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The DHCP leasses file is shared among the cloud to prevent colisions (using A.L.F.R.E.D).
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As long as all nodes share the same anycast MAC/IP, from the client point of view it is fully transparent. Thus the gateway is always the same nonetheless the mesh node where they are attached is different.
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!network4.png!
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When a client wants to reach the Internet or any other network which is not the one used in the layer 2 cloud, it sends the packet to the anycast gateway address.
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An ebtables rule in the LAN/AP bridge prevents the layer 2 packets propagation in the cloud sent to the anycast address. So the mesh node where the client is associated gets the packet but not the others.
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!network5.png!
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The packet is routed throw the BMX6 layer 3 network to the best Internet gateway (according to the BMX6 metrics). It may be a node from the same cloud or any other from another far away cloud.
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In the way back the packet will arreive to some node in the cloud but it may not be the same, it will depend on the routing protocol metrics. 
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!network6.png!
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h2. IP addressing
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LibreMesh uses both IPv4 and IPv6. While the routed network layer is done only with IPv6, the IPv4 is tunneled using IPIP tunnels between the layer 2 clouds. Each cloud uses a /22 of IPv4 addresses (1024 ips), and /48 of IPv6 addresses (2^80 ips).
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!ip1.png!