PCO(Protocol Configuration Options)协议配置选项是NSA消息中重要部分之一，由以下三部分内容构成：
PDN连接请求：PDN Connectivity Request
激活的EPS默认承载上下文请求：ActivateDefaultEPSBearerContextRequest
激活的EPS默认承载上下文接受：ActivateDefaultEPSBearerContextAccept
*我个人理解PCO就是各节点（如DNS等IP）地址的请求和分配。
1.PCO数据结构：PCO数据结构灵活，主要由以下信元组成
2.消息ID内容定义
Container ID | Description (Network to MS Direction) |
0001H | P-CSCF IPv6 Address Request |
0002H | IM CN Subsystem Signaling Flag |
0003H | DNS Server IPv6 Address Request |
0004H | Policy Control rejection code |
0005H | Selected Bearer Control Mode; |
0006H | Reserved |
0007H | DSMIPv6 Home Agent Address |
0008H | DSMIPv6 Home Network Prefix |
0009H | DSMIPv6 IPv4 Home Agent Address |
000AH | reserved |
000BH | reserved |
000CH | P-CSCF IPv4 Address |
000DH | DNS Server IPv4 Address |
000EH | MSISDN Request |
C021H | LCP (refer to RFC 3232) |
C023H | PAP (refer to RFC 3232) |
C223H | CHAP (refer to RFC 3232) |
8021H | IPCP (refer to RFC 3232) |
FF00H | FFFFH reserved for operator specific use. |
3.PCO数据配置
PCO数据的配置可由网络侧进行，UE也可以配置请求，具体如下：
i) UE --> NW : PCO in PDN Connectivity Request (UE request the information it want to get)
ii) UE <-- NW : PCO in Activate Default EPS Bearer Request (NW informs UE of what it want)
4.PCO示例
Protocol Configuration Options - in PDN Connectivity Request
Element ID: 0x27
Length: 26
[Link direction: MS to network (0)]
1... .... = Extension: True
Configuration Protocol: PPP for use with IP PDP type or IP PDN type (0)
Protocol or Container ID: Internet Protocol Control Protocol (0x8021)
Length: 0x10 (16)
PPP IP Control Protocol
Code: Configuration Request (1)
Identifier: 0 (0x00)
Length: 16
Options: (12 bytes), Primary DNS Server IP Address, Secondary DNS Server IP Address
Primary DNS Server IP Address: 0.0.0.0
Type: Primary DNS Server IP Address (129)
Length: 6
Primary DNS Address: 0.0.0.0 (0.0.0.0)
Secondary DNS Server IP Address: 0.0.0.0
Type: Secondary DNS Server IP Address (131)
Length: 6
Secondary DNS Address: 0.0.0.0 (0.0.0.0)
Protocol or Container ID: DNS Server IPv4 Address Request (0x000d)
Length: 0x00 (0)
Protocol or Container ID: IP address allocation via NAS signalling (0x000a)
Length: 0x00 (0)
Protocol Configuration Options - in Activate Default EPS Bearer Request
Element ID: 0x27
Length: 27
[Link direction: Network to MS (1)]
1... .... = Extension: True
Configuration Protocol: PPP for use with IP PDP type or IP PDN type (0)
Protocol or Container ID: Internet Protocol Control Protocol (0x8021)
Length: 0x10 (16)
PPP IP Control Protocol
Code: Configuration Nak (3)
Identifier: 0 (0x00)
Length: 16
Options: (12 bytes), Primary DNS Server IP Address, Secondary DNS Server IP Address
Primary DNS Server IP Address: 192.168.1.2
Type: Primary DNS Server IP Address (129)
Length: 6
Primary DNS Address: 192.168.1.2 (192.168.1.2)
Secondary DNS Server IP Address: 192.168.1.2
Type: Secondary DNS Server IP Address (131)
Length: 6
Secondary DNS Address: 192.168.1.2 (192.168.1.2)
Protocol or Container ID: DNS Server IPv4 Address (0x000d)
Length: 0x04 (4)
IPv4: 192.168.1.2
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整理：kangguoying20201009
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