What DCD feature does in the Cisco ASA firewall?

DCD (Dead Connection Detection)  detects a dead connection and allows it to expire, without expiring connections that can still handle traffic. You can configure DCD when you want idle, but valid connections to persist. After a TCP connection times out, the ASA sends Dead Connection Detection probes to the end hosts to find out the validity of the connection. If one of the end hosts failed to respond after the maximum retries are exhausted, the ASA frees the connection. If both end hosts respond that the connection is valid, the ASA updates the activity timeout to the current time and reschedules the idle timeout accordingly

Routing:TCP/IP Volume I is a really good book for anyone who is going for their CCIE R&S preparation or interested in Studying the Interior Routing Protocols. The best feature of book is that it contains Configuration as well as troubleshooting exercises which is a must for CCIE level.