http://hadoop.apache.org/docs/r1.0.4/cn/hdfs_design.html
通訊協議
所有的HDFS通訊協議都是建立在TCP/IP協議之上。客戶端通過一個可配置的TCP端口連接到Namenode，通過ClientProtocol協議與Namenode交互。而Datanode使用DatanodeProtocol協議與Namenode交互。一個遠程過程調用(RPC)模型被抽象出來封裝ClientProtocol和Datanodeprotocol協議。在設計上，Namenode不會主動發起RPC，而是響應來自客戶端或 Datanode 的RPC請求。
http://hadoop.apache.org/docs/current/hadoop-project-dist/hadoop-hdfs/HdfsDesign.html
The Communication Protocols
All HDFS communication protocols are layered on top of the TCP/IP protocol. A client establishes a connection to a configurable TCP port on the NameNode machine. It talks the ClientProtocol with the NameNode. The DataNodes talk to the NameNode using the DataNode Protocol. A Remote Procedure Call (RPC) abstraction wraps both the Client Protocol and the DataNode Protocol. By design, the NameNode never initiates any RPCs. Instead, it only responds to RPC requests issued by DataNodes or clients.