ress probing depends on the timing. If, for example, A starts sending probing packets to B first, when the packets leave the internal network, NAT A will replace the inter- nal transport address of A with an external one. If the NAT implements an endpoint-independent mapping behavior, it will assign A the same external transport address registered with the rendezvous server S. In addition, since the packet is targeted to the external transport address of B, if the NAT implements an address-dependent filtering behavior, future packets coming from host B will not be dropped. If NAT B implements an address-dependent filtering behavior, the probing packets from A will not reach B. However, on the other side, B in turn sends probing packets to A, thus "activating" NAT B to the forwarding of packets going to B from A. Obviously, if both NATs (or at least one of them) implement an endpoint-independent filtering behavior, the communication establishment will be successful as well. The last scenario involving multiple layers of NAT, as depicted in Figure 12.6, is quite common on the network of many Internet service providers (ISPs) where NAT C is a large ISP NAT, while NAT A and NAT B are small resi- dential NATs. Such a deployment is required because the addressing space of the ISP is private, as well as the addressing space of the customers' networks. It is not unlikely that the customers' address spaces overlap with the address- ing space of the ISP and with each other, i.e., external transport addresses assigned by NAT A may collide with the internal ones of NAT B and vice versa. So the most conservative choice would be to have host A and host B communicating using the external transport addresses assigned by NAT C. This requires S to be in the external network of NAT C and NAT C to sup- port hairpinning. Indeed, it is more frequent to have the rendezvous server S placed in the external network of NAT C rather than in the intenal one. If this is the case, host A and host B cannot communicate using the exter- nal trans