Experimental Versions of IP
Overlays are ideal for deploying experimental versions of IP that you hope
will eventually take over the world. For example, IPmulticast (Section 4.2)
started off as an extension to IP and even today is not enabled in many
Internet routers. The MBone (multicast backbone) was an overlay network
that implemented IP multicast on top of the unicast routing provided
by the Internet. A number of multimedia conference tools were
developed for and deployed on the Mbone. For example, IETF meetings—
which are a week long and attract thousands of participants—were for
many years broadcast over the MBone.
Like VPNs, the MBone used both IP tunnels and IP addresses, but
unlike VPNs, theMBone implemented a different forwarding algorithm—
forwarding packets to all downstream neighbors in the shortest path
multicast tree. As an overlay, multicast-aware routers tunnel through legacy routers, with the hope that one day there will be no more legacy
routers.
The 6-BONE was a similar overlay that was used to incrementally
deploy IPv6. Like the MBone, the 6-BONE used tunnels to forward packets
through IPv4 routers. Unlike the MBone, however, 6-BONE nodes
did not simply provide a new interpretation of IPv4’s 32-bit addresses.
Instead, they forwarded packets based on IPv6’s 128-bit address space.
The 6-BONE also supported IPv6 multicast.