协议配置选项PCO（Protocol Configuration Option）用于为UE提供连接网络的附加信息。这是一个可选协议字段，只有当UE请求PCO时，PCO才会出现在Attach Accept信令消息中。
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PCO Spec
According to 3GPP TS24.008 10.5.6.3, Protocol Configuration Option (PCO) is used to provide additional information about the network to which a UE is connected. This is an optional protocol field and PCO appears in the Attach Accept signaling message only when the UE requests the PCO option. PCO contains two subfields: Configuration Protocol options list, which is the identifier of the protocol and is related to a particular configuration protocol (PPP). The Additional Parameter list contains special parameters or requests that need to be sent to the EPC or NGC, independent of the specific configuration protocol (PPP). The main one is the option to get DNS information.
PCO List
The configuration protocol options list contains a variable number of lo
