ng for information about a node. The ICMPv6 Echo Request is used to ask if a destination node is alive and to measure the RTT it takes for a message to traverse to the destination and back. The ICMPv6 Echo Request can also be used for trace routing purposes by purposefully sending several ICMPv6 Echo Requests with increasing Hop Limit values, starting from one. This causes each visible router on the way to the destination to drop the packet that reaches zero Hop Limit at the particular router and generate an ICMPv6 Time Exceeded message. By collecting the received ICMPv6 error packets and mapping them to the sent packets and used Hop Limit values, the originating host can find out what routers are visible on the path to the destination. The path may include invisible hops, such as bridges, Neighbor Discovery Proxies or firewalls, which perform their tricks without decrementing the Hop Limit field - the originating host will not learn of the presence of such entities. In addition, some routers can be partially invisible. This can happen if a router does decrement the Hop Limit field, but is configured not to send any ICMPv6 errors, or ICMPv6 rate limiting is restricting sending of ICMPv6 messages, or the ICMPv6 message is just lost in either direction. The originating node can learn about the presence of partially invisible routers even without getting ICMPv6 errors, if the originator gets ICMPv6 errors from subsequent routers for messages sent with a higher Hop Limit value. The ICMPv6 Node Information query can be used to poll a node for information regarding its hostname or fully qualified domain name. This functionality is partially Introduction to IPv6 overlapping with reverse Domain Name System (DNS) queries (see Section 3.10.2), but has the benefit of not requiring any servers for the resolution. Additionally, the Node Information query can be used to ask for information about the destination's IPv6 and IPv4 addresses. 3.4 Neighbor Discovery Protocol The most important user of the ICMPv6 is the IPv6 Neighbor