Which of these countermeasures might be helpful in preventing against a SYN flood attack?

a. End systems perform egress screening of IP packets to ensure they are generated from valid sources.

b. Routers perform ingress screening of IP packets to make sure that they are coming from valid sources.

c. Prevent all further connections after the limit of connection requests is reached.

d. Use an intrusion detection system (IDS) to monitor for rates of SYN requests greater than normal, and try to limit these.