GTP的N3/T3计时器及Echo探测机制

decembersn

有个问题，请教楼主。
在最新的TS29.060英文版协议中，对echo request的描述是这样的：
Echo Request messages may be sent for each path associated with at least one of the active PDP context, or MBMS UE context, or MBMS bearer context. When and how often an Echo Request message may be sent is implementation specific but an Echo Request shall not be sent more often than every 60 s on each path. Even if the path is not associated with any active PDP contexts, or MBMS UE contexts, or MBMS bearer contexts, a GSN or RNC shall be prepared to receive an Echo Request at any time and it shall reply with an Echo Response.  
每个路径上至少有一个激活的PDP上下文或MBMS UE上下文或MBMS承载上下文，才有可能发送echo请求消息。即使路径上没有任何激活PDP上下文，那么GSN也应该随时准备接收echo request。
这句话如何理解，感觉有些矛盾。而且跟上面描述的保活机制也有些冲突。发echo请求还必须要存在PDP上下文么？