请问Bearer Context中的to be removal or to be created ?

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  只能说下个人理解，不一定对。

我的问题是在TAU或者切换的过程中，如果eNodeB不接受切换后的某些承载（比如承载 A），是在随后MME发送的Create Session Request中,由MME将 承载 A 标记为to be removal 吗？而MME将eNodeB可以接受的承载标记为to be created，发送给SGW，如果经过SGW判断后，又不接受一些承载（比如承载 B），则在Create Session Response中的承载B的Bearer Context的cause中指明未接受（not accepted），而如果随后发送Modify Bearer Request，则在Bearer Context中将承载 B指明为to be removal，告诉SGW删除该承载。请问是这个意思吗？
就是说在MME发送的Create Session中的to be removal是指eNodeB未能接受的承载，而如果是SGW未能接受的承载，则在Create Session Response中指明为未接受，而在Modify中指明为to be removal？
  应该是的。查了下规范，在create session response消息里携带的IE有两个：
“Bearer Contexts created M EPS bearers corresponding to Bearer Contexts sent in request message. Several IEs with the same type and instance value may be included as necessary to represent a list of Bearers.
One bearer shall be included for "eUTRAN Initial Attach", "PDP Context Activation" or "UE Requested PDN Connectivity ".
One or more created bearers shall be included for a Handover/TAU/RAU with an SGW change.
Bearer Contexts marked for removal C EPS bearers corresponding to Bearer Contexts to be removed that were sent in the Create Session Request message.
For each of those bearers an IE with the same type and instance value shall be included.”
   在modify bearer response消息里有两个IE：
“Bearer Contexts modified C EPS bearers corresponding to Bearer Contexts to be modified that were sent in Modify Bearer Request message. Several IEs with the same type and instance value may be included as necessary to represent a list of the Bearers which are modified.
Bearer Contexts marked for removal C EPS bearers corresponding to Bearer Contexts to be removed sent in the Modify Bearer Request message. Shall be included if request message contained Bearer Contexts to be removed.
For each of those bearers an IE with the same type and instance value shall be included.”
  从IE的名字可以看出，是marked for removal，因此并不是真正删除，只是打上一个标记。然后，create session response消息里携带的“Bearer Contexts created“用的是过去时，代表的是已创建的bearer，并且有提到是对request的响应。
  

还有一个问题是，不论是Create Session Request，还是Modify Bearer Request中指明为to be removal的承载，最后如果确定要删除，是不是都要经过Delete Bearer Request/Response这个消息才能删除，就是说to be removal只是一个标识，指明某些承载需要去除，而真正的删除还是要通过delete才可以？
   同上个问题的理解。个人认为是。

还有一个问题是，如果是切换过程中的Create Session Request，是不是不需要建立默认承载，默认承载可以从eNodeB接收过来的？就是Bearer Context中EBI=5，Instance是to be created的那个承载？
切换过程中发送create session request消息，对应的场景是SGW发生重选的情况，所以默认承载是不需要重新创建的。

最后一个问题是，是不是每一个Create Session Request/Response都能对应一个Delete Session Request/Response？
  是的。即便是default bearer对应的session在detach的时候也有delete消息去删除。

仅供参考哈。