Line Down Condition

When A Client-Server Connection is fully established and normal routing of packages is tacking place, how we can keep track that the connection is alive or not. If not it should be notified immediately both to Client & Server. The answer should be cost effective. Please take into consideration that repeated polling between Client/Server proves to be an expensive method as it uses net connection over and over again. Before any routing is done, the line live condition should be checked. The Checkshould maintain the integrity while in midst of delivering a packet. The following answer should be compatible with all Unix flavors.