Internet Protocol Version 6 (IPv6) is expected for widespread deployment worldwide. Such rapid development of IPv6 may lead to safety problems. The main threats in IPv6 networks are denial of service (DoS) attacks and distributed DoS (DDoS) attacks. In addition to the similar threats in Internet Protocol Version 4 (IPv4), IPv6 has introduced new potential vulnerabilities, which are DoS and DDoS attacks based on Internet Control Message Protocol version 6 (ICMPv6). We divide such new attacks into two categories: pure flooding attacks and source address spoofing attacks. We propose P4-NSAF, a scheme to defend against the above two IPv6 DoS and DDoS attacks in the programmable data plane. P4-NSAF uses Count-Min Sketch to defend against flooding attacks and records information about IPv6 agents into match tables to prevent source address spoofing attacks. We implement a prototype of P4-NSAF with P4 and evaluate it in the programmable data plane. The result suggests that P4-NSAF can effectively protect IPv6 networks from DoS and DDoS attacks based on ICMPv6.