One additional problem in the cloud environment
is that multiple VMs in a single physical
machine may belong to different clients and thus
need to be in different virtual LANs (VLANs).
As discussed earlier, each of these VLANs may
span several data centers interconnected via L3
networks, as shown in Fig. 3.
Again, there are a number of competing proposals
to solve this problem. VMware and several
partner companies have proposed virtual
extensible LANs (VXLANs) [4]. Network virtualization
using generic routing encapsulation
(NVGRE) [5] and the Stateless Transport Tunneling
(STT) protocol [6] are two other proposals
being considered in the Network
Virtualization over L3 (NVO3) working group
of the Internet Engineering Task Force (IETF).