ssh tunnel exits unexptected

[Ming]

Hi,

I have setup a tunnel from server A to server B across WAN by running 
"ssh -l user_X -L port_A:server_B:port_B server_C -N". Server B is in a 
private network, so I have to establish the tunnel via server C, which 
is in the same LAN with B but has a public IP.
The problem is, every time I start the tunnel on server B, the tunnel 
process will die nearly two hours later. And it always gives me the 
similar debugging message before it exits, e.g.:

... ...
Read from remote host XXX: Connection reset by peer
debug1: Transferred: stdin 0, stdout 0, stderr 69 bytes in 6959.0 seconds
debug1: Bytes per second: stdin 0.0, stdout 0.0, stderr 0.0
debug1: Exit status -1

Could anyboday give me some clues? Are there any problems with server A 
or with server C? I doubt it's something about timeout or KeepAlive 
values. But as a regular user, I don't how to check and set them. Your 
suggestions will be greatly appreciated.

Best regards,
- Ming