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 . 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.
(Update) Thus, for B2B environment, it is recommended to use VPN. However, VPN solution is costly to maintain, i.e. you either need VPN server on client end or create a desktop VPN client. In addition, for a lucrative target, the DDoS paradigm shift from web services port to VPN server.