Just add extra facts to complement the answer.

Intrusion Detection Systems(IDS) is just a firewall build with extra programs that act upon incoming traffics, cache the counter and state of each incoming traffic to decide what to do next. 

[SYN-flood attack mitigation should be done by ISP ][1]. However, there is little a IDS can do to prevent degradation-of-service attacks.  Since most web services run on a specific TCP/IP port, IDS has no way to evade such slow fix port "flood" attacks.  


  [1]: https://security.stackexchange.com/questions/34298/stopping-ddos-tcp-syn-and-udp-flood-attacks