An attack defense processing method, comprising: receiving, by a protection device, a first packet; parsing, by the protection device, the first packet to obtain an Internet Protocol, IP, address of a source node, an IP address of a destination node, and a maximum transmission unit, MTU, value that are carried in the first packet, if the protection device determines that the first packet is an Internet Control Message Protocol version 6, ICMPv6, Packet Too Big packet; determining, by the protection device, a range of valid MTUs on a path between the source node and the destination node according to the IP address of the source node and the IP address of the destination node; and performing, by the protection device, attack defense processing for the first packet if the protection device determines that the MTU value does not belong to the range of the valid MTUs on the path between the source node and the destination node, characterized in that the determining, by the protection device, a range of valid MTUs on a path between the source node and the destination node according to the IP address of the source node and the IP address of the destination node comprises: obtaining, by the protection device according to the IP address of the source node and the IP address of the destination node, a value of an MTU on a path from the source node to the destination node and a value of an MTU on a path from the destination node to the source node that are saved on the protection device, determining that an upper limit of the range of the valid MTUs on the path between the source node and the destination node is the greater one of the value of the MTU on the path from the source node to the destination node and the value of the MTU on the path from the destination node to the source node, and determining that a lower limit of the range of the valid MTUs on the path between the source node and the destination node is the smaller one of the value of the MTU on the path from the source node to the destination node and the value of the MTU on the path from the destination node to the source node. receiving, by a protection device, a first packet; parsing, by the protection device, the first packet to obtain an Internet Protocol, IP, address of a source node, an IP address of a destination node, and a maximum transmission unit, MTU, value that are carried in the first packet, if the protection device determines that the first packet is an Internet Control Message Protocol version 6, ICMPv6, Packet Too Big packet; determining, by the protection device, a range of valid MTUs on a path between the source node and the destination node according to the IP address of the source node and the IP address of the destination node; and performing, by the protection device, attack defense processing for the first packet if the protection device determines that the MTU value does not belong to the range of the valid MTUs on the path between the source node and the destination node, obtaining, by the protection device according to the IP address of the source node and the IP address of the destination node, a value of an MTU on a path from the source node to the destination node and a value of an MTU on a path from the destination node to the source node that are saved on the protection device, determining that an upper limit of the range of the valid MTUs on the path between the source node and the destination node is the greater one of the value of the MTU on the path from the source node to the destination node and the value of the MTU on the path from the destination node to the source node, and determining that a lower limit of the range of the valid MTUs on the path between the source node and the destination node is the smaller one of the value of the MTU on the path from the source node to the destination node and the value of the MTU on the path from the destination node to the source node.