OMS Cannot Communicate with OMA on another Host

GC 10.2.0.4 on Windows Server 2003 R2
Agent 10.2.0.4 on RHEL 4

Problem:
Inside Grid Control, click on host and it takes a long time to see home page of that host.
Click on Host Performance page, error show up, something like "oracle.sysman.emSDK.emd.comm.CommException: No route to host: connect".
Check emagent.log, it seems all right. Check emoms.log, the same error is reported.

Solution:
From the error, it's something like no communication between oms and agent.
Restart agent several times, it succeed every time. Agent uploaded file successfully!
The communication from agent to oms seems all right. The problem might be in the communication from oms to agent. Use telnet to contact with agent using "telnet host port", it got refused! Some firewall or proxy might be between oms and agent. Contacted with SA, updated the firewall configuration on that port, it worked. The Host Performance page show up and reponsed quickly.