2023年12月30日发(作者：)

初始上下文建立失败后的处理

初始上下文建立过程用于在eNB内建立E-RAB上下文、安全上下文、切换限制列表等。其过程存在成功和失败等可能性。TS 36.413中有比较清楚的描述两种场景。如下，

8.3.1.2 Successful Operation

eNB

INITIAL CONTEXT SETUP REQUEST

MME

INITIAL CONTEXT SETUP RESPONSE

Figure 8.3.1.2-1: Initial Context Setup procedure. Successful operation.

8.3.1.3 Unsuccessful Operation

eNB

INITIAL CONTEXT SETUP REQUEST

MME

INITIAL CONTEXT SETUP FAILURE

Figure 8.3.1.3-1: Initial Context Setup procedure. Unsuccessful operation.

那么失败之后eNB和MME该怎么办呢？

在TS 36.413中没有描述这个问题，但是这涉及到两个网元的交互，必然要有一定的设计原则才能保证两个网元的资源对齐。OK，翻查其他协议吧。

首先就会想起要看看TS36.300，这是著名的百科协议，翻查到上下文建立一段，摘录如下，

19.2.2.3 Initial Context Setup procedure

The Initial Context Setup procedure establishes the necessary overall initial UE context in the eNB

in case of an Idle-to Active transition. The Initial Context Setup procedure is initiated by the

MME.

The Initial Context Setup procedure comprises the following steps:

- The MME initiates the Initial Context Setup procedure by sending INITIAL CONTEXT SETUP

REQUEST to the eNB. This message may include general UE Context (e.g. security context,

roaming restrictions, UE capability information, UE S1 signalling connection ID, etc.), E-RAB

context (Serving GW TEID, QoS information), and may be piggy-backed with the

corresponding NAS messages. When there are multiple NAS messages in the INITIAL

CONTEXT SETUP REQUEST message, the MME shall ensure that the NAS messages in the

E-RAB to be Setup List are aligned in the order of reception from the NAS layer to ensure the

in-sequence delivery of the NAS messages.

- Upon receipt of INITIAL CONTEXT SETUP REQUEST, the eNB setup the context of the

associated UE, and perform the necessary RRC signalling towards the UE, e.g. Radio Bearer

Setup procedure. When there are multiple NAS messages to be sent in the RRC message, the

order of the NAS messages in the RRC message shall be kept the same as that in the INITIAL

CONTEXT SETUP REQUEST message.

- The eNB responds with INITIAL CONTEXT SETUP COMPLETE to inform a successful

operation, and with INITIAL CONTEXT SETUP FAILURE to inform an unsuccessful

operation.

Note: In case of failure, eNB and MME behaviours are not mandated. Both implicit release

(local release at each node) and explicit release (MME-initiated UE Context Release

procedure) may in principle be adopted. The eNB should ensure that no hanging

resources remain at the eNB.

运气不错，看到了最后的Note，比较清楚该怎么做了。这段话表明的原则是，

 不强制定义网元的行为

 两边都要释放

 eNB不能挂住资源

那么，对于MME就有两种做法，一种是自己释放不通知eNB，另一种是通知eNB释放上下文；对于eNB来说就是发出上下文建立失败消息之后启动一个释放定时器，若定时器超时之前收到MME发来的上下文释放命令则释放自己并回复响应给MME，定时器超时后释放自己资源，不需要再通知MME。即如下过程对eNB来说是不必的。

eNB MME

UE CONTEXT RELEASE REQUEST