Currently, a VMD (VNET server) tunnels Ethernet traffic for a particular address to another VMD over a TCP connection, a relationship we refer to as a handler, as described in Section 4.2. In principle, multiple handlers can be multiplexed over a single TCP connection. Hence, the edges in the VMD graph are simply TCP connections. In such a configuration, the VMDs supporting a user form an overlay network with a star topology centered on the proxy machine on the user's network. All messages are routed through the proxy machine.
It is straightforward to see why it is possible to support arbitrary topologies. Each VMD can effectively behave like a switch or router, sending an packet that arrives on a TCP connection to another connection instead of injecting it onto the local network. Indeed, for Ethernet topologies, we can simply emulate the Ethernet switch protocols, as in VIOLIN . Each Ethernet packet contains a source and destination address. A modern Ethernet switch learns the location of Ethernet addresses on its ports based on the source addresses of traffic it sees. Switches also run a distributed algorithm that assures that they form a spanning tree topology.
Because VNET understands that it is supporting VMs, it can go beyond simply emulating an IP or Ethernet network, however. For example, hierarchical routing of Ethernet packets is possible because the Ethernet addresses of the user's VMs are not chosen by Ethernet card vendors, but are assigned by VNET.