AS PATH Inference is one of the prominent techniques used for creating Internet maps. This technique relies on various BGP collectors that collect information such as routing updates and tables and provide this information publicly. Each BGP entry contains a Path Vector attribute called the AS Path. This path represents an autonomous system forwarding path from a given origin for a given set of prefixes. These paths can be used to infer AS-level connectivity and in turn be used to build AS topology graphs. However, these paths do not necessarily reflect how data is actually forwardeA, Ddjacencies between AS nodes only represent a policy relationship between them. A single AS link can in reality be several router links. It is also much harder to infer peering between two AS nodes, as these peering relationships are only propagated to an ISP's customer networks. Nevertheless, support for this type of mapping is increasing as more and more ISP's offer to peer with public route collectors such as Route-Views and RIPE. New toolsets are emerging such as Cyclops and NetViews that take advantage of a new experimental BGP collector BGPMon. NetViews can not only build topology maps in seconds but visualize topology changes moments after occurring at the actual router. Hence, routing dynamics can be visualized in real time. Answer: B is incorrect. There is no such Internet mapping technique.
Answer: D is incorrect. Firewalking is a technique for gathering information about a remote network protected by a firewall. This technique can be used effectively to perform information gathering attacks. In this technique, an attacker sends a crafted packet with a TTL value that is set to expire one hop past the firewall. If the firewall allows this crafted packet through, it forwards the packet to the next hop. On the next hop, the packet expires and elicits an ICMP "TTL expired in transit" message to the attacker. If the firewall does not allow the traffic, there should be no response, or an ICMP "administratively prohibited" message should be returned to the attacker. A malicious attacker can use firewalking to determine the types of ports/protocols that can bypass the firewall. To use firewalking, the attacker needs the IP address of the last known gateway before the firewall and the IP address of a host located behind the firewall. The main drawback of this technique is that if an administrator blocks ICMP packets from leaving the network, it is ineffective. Answer: A is incorrect. Path MTU discovery (PMTUD) is a technique in computer networking for determining the maximum transmission unit (MTU) size on the network path between two Internet Protocol (IP) hosts, usually with the goal of avoiding IP fragmentation. Path MTU discovery works by setting the DF (Don't Fragment) option bit in the IP headers of outgoing packets. Then, any device along the path whose MTU is smaller than the packet will drop it, and send back an ICMP "Fragmentation Needed" (Type 3, Code 4) message containing its MTU, allowing the source host to reduce its path MTU appropriately. The process repeats until the MTU is small enough to traverse the entire path without fragmentation. If the path MTU changes after the connection is set up and is lower than the previously determined path MTU, the first large packet will cause an ICMP error and the new, lower path MTU will be found. Conversely, if PMTUD finds that the path allows a larger MTU than what is possible on the lower link, the OS will periodically reprobe to see if the path has changed and now allows larger packets. On Linux this timer is set by default to ten minutes.
