tomcat session复制 UDP多播

在5.5文档中有说明
In this release of session replication, Tomcat performs an all-to-all replication of session state. This is an algorithm that is only efficient when the clusters are small. For large clusters, the next release will support a primary-secondary session replication where the session will only be stored at one or maybe two backup servers.
在5.5中只支持session 在所有节点复制，这个应该用的是多播

在6.0文档中
In this release of session replication, Tomcat can perform an all-to-all replication of session state using the DeltaManager or perform backup replication to only one node using the BackupManager.
这个只在一个节点备份应该是用TCP实现的

如果某个节点失效了，怎么装到备份节点，这些逻辑有知道的吗，对java不熟，看源码不知道怎么着手