以MBS为例理解ASN.1编解码

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Notation解读

参考3GPP规范38.413 Version17.0.0，在Ch.9.3.5.7可以看到MBS Distrubution Setup Request Transfer的描述

image.png

实际定义在Ch.9.4.5--F(P463):

MBS-DistributionSetupRequestTransfer ::= SEQUENCE {
	mBS-SessionID                       MBS-SessionID,
	mBS-AreaSessionID                   MBS-AreaSessionID            OPTIONAL,
	sharedNG-U-Unicast-TNL-Information  UPTransportLayerInformation  OPTIONAL,
	iE-Extensions                       ProtocolExtensionContainer { { MBS-DistributionSetupRequestTransfer-ExtIEs} }  OPTIONAL,
	...
}

SEQUENCE关键字定义于X.680 Ch.25，可类比于C的struct声明，以上定义了MBS-DistributionSetupRequestTransfer是一个Type，其中的字段有mBS-SessionID，mBS-AreaSessionID，...都是有序的

类似的，MBS-SessionID，MBS-AreaSessionID，...都是Type，可查找对应的定义

略复杂一些的是iE-Extensions的定义，可以看出与上面的字段有所不同：

iE-Extensions  ProtocolExtensionContainer { { MBS-DistributionSetupRequestTransfer-ExtIEs} }  OPTIONAL,

这个是Referencing parameterized definitions（实例化参数化类型）的语法，定义于X.683 Ch.9

这里的ProtocolExtensionContainer是一个ParameterizedType（参数化类型）（X.683 Ch.9）

这里的 {MBS-DistributionSetupRequestTransfer-ExtIEs} 是一个ObjectSet（X.681 Ch.12.3）

先来看一下ProtocolExtensionContainer的ParameterizedTypeAssignment（参数化类型定义）（X.683 Ch.8）:

ProtocolExtensionContainer {NGAP-PROTOCOL-EXTENSION : ExtensionSetParam} ::=
    SEQUENCE (SIZE (1..maxProtocolExtensions)) OF
    ProtocolExtensionField {{ExtensionSetParam}}

其中ExtensionSetParam是一个无意义的名称，用于指代实例化参数化类型中的实际参数（也即上面例子中的 {MBS-DistributionSetupRequestTransfer-ExtIEs} ）

其中NGAP-PROTOCOL-EXTENSION是一个DefinedObjectClass（X.683 Ch.8.3），其定义为：

NGAP-PROTOCOL-EXTENSION ::= CLASS {
    &id             ProtocolExtensionID         UNIQUE,
    &criticality    Criticality,
    &Extension,
    &presence       Presence
}
WITH SYNTAX {
    ID              &id
    CRITICALITY     &criticality
    EXTENSION       &Extension
    PRESENCE        &presence
}

InformationObjectClass定义语法出自X.681 Ch.9，可以参考资料 Understanding Information Objects

可以将InformationObjectClass理解为一个类型模板，可以指定其中的field而将其特化为InformationObject，但在38.413中更多地可以看到将InformationObjectClass特化为一组InformationObjectSet的例子，比如：

UserLocationInformationEUTRA-ExtIEs NGAP-PROTOCOL-EXTENSION ::= {
    { ID id-PSCellInformation  CRITICALITY ignore  EXTENSION NGRAN-CGI  PRESENCE optional},
    ...
}

虽然这个例子只特化了一个值，但确实使用的是声明ObjectSet的语法

其实前面例子中的 {MBS-DistributionSetupRequestTransfer-ExtIEs} 就是一个InformationObjectSet，只是在此版本的规范中没有定义任何特化值：

MBS-DistributionSetupRequestTransfer-ExtIEs NGAP-PROTOCOL-EXTENSION ::= {
    ...
}

个人理解，InformationObjectClass和InformationObject的关系可以对标Type和Value的关系，只是有几个区别：

  1. Value是由用户特化的，而InformationObjectSet是一组预定义的枚举值
  2. Value是用户数据的载体，而InformationObjectSet应该是用于约束Type的特化的，并不用于承载用户数据

那么InformationObjectSet是如何约束Type特化的？我们回到前面ProtocolExtensionContainer的定义：

ProtocolExtensionContainer {NGAP-PROTOCOL-EXTENSION : ExtensionSetParam} ::=
    SEQUENCE (SIZE (1..maxProtocolExtensions)) OF
    ProtocolExtensionField {{ExtensionSetParam}}

现在我们应该能看出这段的大概意思了，语义比较接近于：

typedef ProtocolExtensionContainer List<ProtocolExtensionField<Set<NGAP-PROTOCOL-EXTENSION>>>

这次我们看ProtocolExtensionField的定义：

ProtocolExtensionField {NGAP-PROTOCOL-EXTENSION : ExtensionSetParam} ::= SEQUENCE {
    id             NGAP-PROTOCOL-EXTENSION.&id             ({ExtensionSetParam}),
    criticality    NGAP-PROTOCOL-EXTENSION.&criticality    ({ExtensionSetParam}{@id}),
    extensionValue NGAP-PROTOCOL-EXTENSION.&Extension      ({ExtensionSetParam}{@id})
}

从这里我们可以看出InformationObjectSet的“约束”作用是什么意思（X.682 Ch.10），这段语义接近于：

public class ProtocolExtensionField<T extends Set<NGAP-PROTOCOL-EXTENSION>> {
    public ProtocolExtensionField(ProtocolExtensionID id, Criticality criticality, Object extensionValue) {
        assert(T.contains(id));
        assert(T[id].CRITICALITY == criticality);
        assert(extensionValue instanceof T[id].EXTENSION);
    }
}

说到这里我们再回到最初的IE定义：

MBS-DistributionSetupRequestTransfer ::= SEQUENCE {
	mBS-SessionID                       MBS-SessionID,
	mBS-AreaSessionID                   MBS-AreaSessionID            OPTIONAL,
	sharedNG-U-Unicast-TNL-Information  UPTransportLayerInformation  OPTIONAL,
	iE-Extensions                       ProtocolExtensionContainer { { MBS-DistributionSetupRequestTransfer-ExtIEs} }  OPTIONAL,
	...
}

其实就说iE-Extensions是一个数组，其元素必须符合MBS-DistributionSetupRequestTransfer-ExtIEs中定义过的枚举值。而由于MBS-DistributionSetupRequestTransfer-ExtIEs目前是空集合，应该是无法为iE-Extensions定义合法元素的

我们再来看一个类型特化的例子，MBS-DistributionSetupResponseTransfer也即响应的类型

MBS-DistributionSetupResponseTransfer ::= SEQUENCE {
    protocolIEs     ProtocolIE-Container        { {MBS-DistributionSetupResponseTransferIEs} },
    ...
}

这个形式我们很熟悉了，来看一下ProtocolIE-Container的定义：

ProtocolIE-Container {NGAP-PROTOCOL-IES : IEsSetParam} ::=
    SEQUENCE (SIZE (0..maxProtocolIEs)) OF
    ProtocolIE-Field {{IEsSetParam}}
    
NGAP-PROTOCOL-IES ::= CLASS {
    &id             ProtocolIE-ID                   UNIQUE,
    &criticality    Criticality,
    &Value,
    &presence       Presence
}
WITH SYNTAX {
    ID              &id
    CRITICALITY     &criticality
    TYPE            &Value
    PRESENCE        &presence
}
    
ProtocolIE-Field {NGAP-PROTOCOL-IES : IEsSetParam} ::= SEQUENCE {
    id              NGAP-PROTOCOL-IES.&id               ({IEsSetParam}),
    criticality     NGAP-PROTOCOL-IES.&criticality      ({IEsSetParam}{@id}),
    value           NGAP-PROTOCOL-IES.&Value            ({IEsSetParam}{@id})
}

特化使用的枚举集合定义为：

MBS-DistributionSetupResponseTransferIEs NGAP-PROTOCOL-IES ::= {
    { ID id-MBS-SessionID                                      CRITICALITY reject  TYPE MBS-SessionID                           PRESENCE mandatory }|
    { ID id-MBS-AreaSessionID                                  CRITICALITY reject  TYPE MBS-AreaSessionID                       PRESENCE optional  }|
    { ID id-SharedNG-U-Multicast-TNL-Information               CRITICALITY reject  TYPE SharedNG-U-Multicast-TNL-Information    PRESENCE optional  }|
    { ID id-Alternative-SharedNG-U-Multicast-TNL-Information   CRITICALITY ignore  TYPE SharedNG-U-Multicast-TNL-Information    PRESENCE optional  }|
    { ID id-MBS-QoSFlows-ToBeSetupList                         CRITICALITY reject  TYPE MBS-QoSFlows-ToBeSetupList              PRESENCE mandatory }|
    { ID id-MBSSessionStatus                                   CRITICALITY reject  TYPE MBSSessionStatus                        PRESENCE mandatory }|
    { ID id-MBS-ServiceArea                                    CRITICALITY reject  TYPE MBS-ServiceArea                         PRESENCE optional  }, 
    ...
}

有疑问的一点是Presence在这里似乎没有办法检验？

APER编解码

于38.413 Ch.9.5指出NGAP协议使用Aligned Basic PER作为编解码规范，此规范定义于X.691

MBS-DistributionSetupRequestTransfer编码

我们尝试从MBS-DistributionSetupRequestTransfer的编码开始，MBS-DistributionSetupRequestTransfer的定义为：

MBS-DistributionSetupRequestTransfer ::= SEQUENCE {
	mBS-SessionID                       MBS-SessionID,
	mBS-AreaSessionID                   MBS-AreaSessionID            OPTIONAL,
	sharedNG-U-Unicast-TNL-Information  UPTransportLayerInformation  OPTIONAL,
	iE-Extensions                       ProtocolExtensionContainer { { MBS-DistributionSetupRequestTransfer-ExtIEs} }  OPTIONAL,
	...
}

是一个SEQUENCE类型，参考X.691 Ch.19 Encoding the sequence type

根据X.691 Ch.19.1，如果SEQUENCE有extension marker（即...），则第1个bit（称作extension bit）用于指定数据是否携带extension additions

由于MBS-DistributionSetupRequestTransfer并未定义任何extension additions，所以此bit总应置0

根据X.691 Ch.19.2，如果SEQUENCE中有n个带有OPTIONAL或DEFAULT标记的字段，就应有n个bit用于记录对应字段是否存在

对MBS-DistributionSetupRequestTransfer来说，就应是3个bit位

根据X.691 Ch.19.4，从第5个bit开始应逐个编码对应字段

mBS-SessionID编码

看第一个字段类型MBS-SessionID及相关定义：

MBS-SessionID ::= SEQUENCE {
    tMGI            TMGI,
    nID             NID                                                         OPTIONAL,
    iE-Extensions   ProtocolExtensionContainer { {MBS-SessionID-ExtIEs} }       OPTIONAL,
    ...
}

TMGI ::=  OCTET STRING (SIZE(6))

NID ::= BIT STRING (SIZE(44))

MBS-SessionID-ExtIEs NGAP-PROTOCOL-EXTENSION ::= {
    ...
}

MBS-SessionID是SEQUENCE类型，所以同样地，第5个bit是extension bit，第6，7个bit代表nID和iE-Extensions是否出现

由于MBS-SessionID-ExtIEs这个约束集合是空的，目前没有合法的iE-Extensions，所以第7个bit总应是0

tMGI编码

从第8个bit开始编码tMGI，参考X.691 Ch.17 Encoding the octetstring type

根据X.691 Ch.17.7，2<长度<64K的定长OCTET STRING要字节对齐，无length determinant（长度指示符）

所以tMGI从第2个字节开始编码，即第2,3,4,5,6,7个字节，第1字节的第8bit填充0

nID编码

第8个字节开始编码nID（如果存在），参考X.691 Ch.16 Encoding the bitstring type

根据X.691 Ch.16.10，16bit<长度<64K bit的BIT STRING要字节对齐，无长度指示符

所以第8，9，10，11，12字节，以及第13字节的前4个bit用于编码nID

根据X.691 Ch.16.3，如果nID之后没有字段需要编码了，并且第13个字节的前4个bit都是0，则第13个字节可以省略，整体消息编码结束

mBS-AreaSessionID编码

再看下一个例子，MBS-AreaSessionID的定义为：

MBS-AreaSessionID ::= INTEGER (0..65535, ...)

假设mBS-SessionID到第13字节第4bit为止，则mBS-AreaSessionID的编码从第13字节第5bit开始

INTEGER的编码参考X.691 Ch.13 Encoding the integer type

根据X.691 Ch.13.1，INTERGER的约束中有extension marker时，应有1个bit表明此INTEGER实际值是否超出约束范围

在此例子中即第13字节第5bit：

  1. 若置0代表mBS-SessionID实际值在约束0..65535之间
  2. 若置1则需要按无约束的INTEGER规则进行编码，参考X.691 13.2.4~13.2.6

按约束编码

0..65535的范围参考X.691 Ch.13.2.2，跳转到X.691 Ch.11.5 Encoding of a constrained whole number，适用于X.691 Ch.11.5.7.3

根据X.691 Ch.11.5.7.3，实际值在0..65535之间的mBS-SessionID将按照2个字节的非负整数进行编码（字节对齐），也即第14，15个字节用于编码mBS-SessionID，第13字节的第6，7，8bit置0

无约束编码

但是若mBS-SessionID实际值范围不在0..65535之间，即第13字节第5bit为1时，mBS-SessionID将作为无约束整数进行编码

根据X.691 Ch.13.2.4，无约束整数参考X.691 Ch.11.8 Encoding of an unconstrained whole number规则进行编码

根据X.691 Ch.11.8，无约束整数以2的补码方式编码，并且有长度指示符

长度指示符的编码规则在X.691 Ch.11.9 General rules for encoding a length determinant指出

从X.691 Ch.11.9.3.6开始对无约束整数编码长度指示符的描述，实际上这种编码很类似UTF-8的原理：

值长度长度指示符
<=127单个字节，bit8置0，其余7个bit作非负整数编码
127<l<16K2个字节，bit16置1，bit15置0，剩余14个bit作非负整数编码
>16K单个字节，bit8，7置1，剩余6个比特可选值为1，2，3，4，代表后面跟着最多16K*4的值，然后再次插入长度指示符，直到分片结束

>16K的情况有个图可以很好地说明：

image.png

sharedNG-U-Unicast-TNL-Information编码

UPTransportLayerInformation类型及相关定义：

UPTransportLayerInformation ::= CHOICE {
    gTPTunnel               GTPTunnel,
    choice-Extensions       ProtocolIE-SingleContainer { {UPTransportLayerInformation-ExtIEs} }
}

CHOICE类型编码参考X.691 Ch.23 Encoding the choice type

根据X.691 Ch.23.6，首先将选中的项序号作为有约束整数编码，然后再跟上对应项的编码

由于一共只有2项，所以可选项的序号为0..1，根据X.691 Ch.15.7.5.1使用1个bit编码

gTPTunnel编码

GTPTunnel ::= SEQUENCE {
    transportLayerAddress       TransportLayerAddress,
    gTP-TEID                    GTP-TEID,
    iE-Extensions               ProtocolExtensionContainer { {GTPTunnel-ExtIEs} } OPTIONAL,
    ...
}

TransportLayerAddress ::= BIT STRING (SIZE(1..160, ...))

GTP-TEID ::= OCTET STRING (SIZE(4))

GTPTunnel-ExtIEs NGAP-PROTOCOL-EXTENSION ::= {
    ...
}

这段定义没多少特殊的，只说一下TransportLayerAddress这个带extension marker的BIT STRING怎么编码

根据X.691 Ch.16.6，带有extension marker的BIT STRING第一个bit是extension bit，若置1代表实际值不在约束范围中

若extension bit置0，代表transportLayerAddress实际长度在约束1..160中，根据X.691 Ch.16.11应先将长度指示符作为有约束整数编码，参考X.691 Ch.11.9.3.3，1.。160的长度应使用8个bit进行编码，之后应以字节对齐的方式接上transportLayerAddress的实际值

若extension bit置1，首先应将长度指示符作为半约束整数编码，参考X.691 Ch.11.9.3.6~，以字节对齐方式插入长度指示符，再以字节对齐方式接上transportLayerAddress的实际值

choice-Extensions编码

由于UPTransportLayerInformation-ExtIEs目前是空集合，不存在合法的choice-Extensions实例，所以不考虑编码

ProtocolIE-SingleContainer {NGAP-PROTOCOL-IES : IEsSetParam} ::=
    ProtocolIE-Field {{IEsSetParam}}
    
ProtocolIE-Field {NGAP-PROTOCOL-IES : IEsSetParam} ::= SEQUENCE {
    id              NGAP-PROTOCOL-IES.&id               ({IEsSetParam}),
    criticality     NGAP-PROTOCOL-IES.&criticality      ({IEsSetParam}{@id}),
    value           NGAP-PROTOCOL-IES.&Value            ({IEsSetParam}{@id})
}
    
UPTransportLayerInformation-ExtIEs NGAP-PROTOCOL-IES ::= {
    ...
}

MBS-DistributionSetupResponseTransfer

再来看一个复杂些的例子，响应类型MBS-DistributionSetupResponseTransfer的定义为：

MBS-DistributionSetupResponseTransfer ::= SEQUENCE {
    protocolIEs     ProtocolIE-Container        { {MBS-DistributionSetupResponseTransferIEs} },
    ...
}

ProtocolIE-Container {NGAP-PROTOCOL-IES : IEsSetParam} ::=
    SEQUENCE (SIZE (0..maxProtocolIEs)) OF
    ProtocolIE-Field {{IEsSetParam}}
    
NGAP-PROTOCOL-IES ::= CLASS {
    &id             ProtocolIE-ID                   UNIQUE,
    &criticality    Criticality,
    &Value,
    &presence       Presence
}
WITH SYNTAX {
    ID              &id
    CRITICALITY     &criticality
    TYPE            &Value
    PRESENCE        &presence
}
    
ProtocolIE-Field {NGAP-PROTOCOL-IES : IEsSetParam} ::= SEQUENCE {
    id              NGAP-PROTOCOL-IES.&id               ({IEsSetParam}),
    criticality     NGAP-PROTOCOL-IES.&criticality      ({IEsSetParam}{@id}),
    value           NGAP-PROTOCOL-IES.&Value            ({IEsSetParam}{@id})
}

MBS-DistributionSetupResponseTransferIEs NGAP-PROTOCOL-IES ::= {
    { ID id-MBS-SessionID                                      CRITICALITY reject  TYPE MBS-SessionID                           PRESENCE mandatory }|
    { ID id-MBS-AreaSessionID                                  CRITICALITY reject  TYPE MBS-AreaSessionID                       PRESENCE optional  }|
    { ID id-SharedNG-U-Multicast-TNL-Information               CRITICALITY reject  TYPE SharedNG-U-Multicast-TNL-Information    PRESENCE optional  }|
    { ID id-Alternative-SharedNG-U-Multicast-TNL-Information   CRITICALITY ignore  TYPE SharedNG-U-Multicast-TNL-Information    PRESENCE optional  }|
    { ID id-MBS-QoSFlows-ToBeSetupList                         CRITICALITY reject  TYPE MBS-QoSFlows-ToBeSetupList              PRESENCE mandatory }|
    { ID id-MBSSessionStatus                                   CRITICALITY reject  TYPE MBSSessionStatus                        PRESENCE mandatory }|
    { ID id-MBS-ServiceArea                                    CRITICALITY reject  TYPE MBS-ServiceArea                         PRESENCE optional  }, 
    ...
}

同样的，首先编码MBS-DistributionSetupResponseTransfer的extension bit

然后是protocolIEs是一个SEQUENCE OF类型，参考X.691 Ch.20 Encoding the sequence-of type

根据X.691 Ch.20.6，编码双字节的长度指示符，然后跟上数组元素

数组元素是ProtocolIE-Field类型，即SEQUENCE类型，无extension marker和optional，即无preamble

值得一提的是Criticality是ENUMERATED类型，根据X.691 Ch.14 Encoding the enumerated type，其中的枚举项应该以升序排列，并按有约束整数编码其序号（？）

还有一点需要注意，ProtocolIE-Field中的value是按照OpenType进行编码的，参考X.691 Ch.11.2 Open type fields要先编码无约束长度指示符再加上实际值

实践

使用online ASN.1的工具来验证编码是否正确： asn1.io/asn1playgro…

MBS-DistributionSetupRequestTransfer编码

输入schema并编译：

--<ASN1.HugeInteger World-Schema.Rocket.range>--
World-Schema DEFINITIONS AUTOMATIC TAGS ::= 
BEGIN
MBS-DistributionSetupRequestTransfer ::= SEQUENCE {
  mBS-SessionID                       MBS-SessionID,
  mBS-AreaSessionID                   MBS-AreaSessionID            OPTIONAL,
  sharedNG-U-Unicast-TNL-Information  UPTransportLayerInformation  OPTIONAL,
  iE-Extensions                       ProtocolExtensionContainer { { MBS-DistributionSetupRequestTransfer-ExtIEs} }  OPTIONAL,
  ...
}                     

MBS-SessionID ::= SEQUENCE {
    tMGI            TMGI,
    nID             NID                                                         OPTIONAL,
    iE-Extensions   ProtocolExtensionContainer { {MBS-SessionID-ExtIEs} }       OPTIONAL,
    ...
}

TMGI ::=  OCTET STRING (SIZE(6))

NID ::= BIT STRING (SIZE(44))

ProtocolExtensionContainer {NGAP-PROTOCOL-EXTENSION : ExtensionSetParam} ::=
    SEQUENCE (SIZE (1..maxProtocolExtensions)) OF
    ProtocolExtensionField {{ExtensionSetParam}}

maxProtocolExtensions					INTEGER ::= 65535

NGAP-PROTOCOL-EXTENSION ::= CLASS {
    &id             ProtocolExtensionID         UNIQUE,
    &criticality    Criticality,
    &Extension,
    &presence       Presence
}
WITH SYNTAX {
    ID              &id
    CRITICALITY     &criticality
    EXTENSION       &Extension
    PRESENCE        &presence
}

ProtocolExtensionID	::= INTEGER (0..65535)

Criticality		::= ENUMERATED { reject, ignore, notify }

Presence		::= ENUMERATED { optional, conditional, mandatory }

ProtocolExtensionField {NGAP-PROTOCOL-EXTENSION : ExtensionSetParam} ::= SEQUENCE {
    id             NGAP-PROTOCOL-EXTENSION.&id             ({ExtensionSetParam}),
    criticality    NGAP-PROTOCOL-EXTENSION.&criticality    ({ExtensionSetParam}{@id}),
    extensionValue NGAP-PROTOCOL-EXTENSION.&Extension      ({ExtensionSetParam}{@id})
}

MBS-SessionID-ExtIEs NGAP-PROTOCOL-EXTENSION ::= {
    ...
}

MBS-AreaSessionID ::= INTEGER (0..65535, ...)

UPTransportLayerInformation ::= CHOICE {
    gTPTunnel               GTPTunnel,
    choice-Extensions       ProtocolIE-SingleContainer { {UPTransportLayerInformation-ExtIEs} }
}

GTPTunnel ::= SEQUENCE {
    transportLayerAddress       TransportLayerAddress,
    gTP-TEID                    GTP-TEID,
    iE-Extensions               ProtocolExtensionContainer { {GTPTunnel-ExtIEs} } OPTIONAL,
    ...
}

TransportLayerAddress ::= BIT STRING (SIZE(1..160, ...))

GTP-TEID ::= OCTET STRING (SIZE(4))

GTPTunnel-ExtIEs NGAP-PROTOCOL-EXTENSION ::= {
    ...
}

ProtocolIE-SingleContainer {NGAP-PROTOCOL-IES : IEsSetParam} ::=
    ProtocolIE-Field {{IEsSetParam}}

NGAP-PROTOCOL-IES ::= CLASS {
    &id             ProtocolIE-ID                   UNIQUE,
    &criticality    Criticality,
    &Value,
    &presence       Presence
}
WITH SYNTAX {
    ID              &id
    CRITICALITY     &criticality
    TYPE            &Value
    PRESENCE        &presence
}
    
ProtocolIE-Field {NGAP-PROTOCOL-IES : IEsSetParam} ::= SEQUENCE {
    id              NGAP-PROTOCOL-IES.&id               ({IEsSetParam}),
    criticality     NGAP-PROTOCOL-IES.&criticality      ({IEsSetParam}{@id}),
    value           NGAP-PROTOCOL-IES.&Value            ({IEsSetParam}{@id})
}

ProtocolIE-ID		::= INTEGER (0..65535)
    
UPTransportLayerInformation-ExtIEs NGAP-PROTOCOL-IES ::= {
    ...
}

MBS-DistributionSetupRequestTransfer-ExtIEs NGAP-PROTOCOL-EXTENSION ::= {
    ...
}
END

输入值（自选示例值）并encode：

value MBS-DistributionSetupRequestTransfer ::= {
  mBS-SessionID {
    tMGI '112233445566'H
  },
  mBS-AreaSessionID 922,
  sharedNG-U-Unicast-TNL-Information gTPTunnel : {
    transportLayerAddress '010010011'B,
    gTP-TEID 'AABBCCDD'H
  }
}

APER方式得到的编码为：6011 2233 4455 6600 039a 0080 4980 aabb ccdd，我们来看一下是否符合上述理解：

bit位BinaryHex解释
1-4011060: MBS-DistributionSetupRequestTransfer层无extension addition
1: OPTIONAL mBS-AreaSessionID存在
1: OPTIONAL sharedNG-U-Unicast-TNL-Information存在
0: OPTIONAL iE-Extensions不存在
5-8000000: MBS-SessionID层无extension addition
0: OPTIONAL nID不存在
0: OPTIONAL iE-Extensions不存在
0: 由于后面tMGI的编码要求字节对齐而产生的填充位
9-56112233445566tMGI的值
57-6400000000000: mBS-AreaSessionID的值在约束中
0000000: 由于mBS-AreaSessionID双字节编码要求字节对齐而产生的填充位
65-80039amBS-AreaSessionID的值
81-8800000000000: 选择UPTransportLayerInformation第0项choice
0: GTPTunnel层无extension addition
0: GTPTunnel的OPTIONAL iE-Extensions不存在
0: TransportLayerAddress的值在约束中
0000: 由于编码TransportLayerAddress的长度指示符需要字节对齐而产生的填充位
89-961000000080这里我不理解为什么是80，最大range 160，应当使用8bit编码长度，实际长度为9，所以应编码9-1=8，编码应为0x08
97-11201001001100000004980010010011: transportLayerAddress的值
0000000: 由于后面GTP-TEID要求字节对齐而产生的填充位
113-144aabbccddgTP-TEID的值

MBS-DistributionSetupResponseTransfer编码

输入schema并编译：

--<ASN1.HugeInteger World-Schema.Rocket.range>--
World-Schema DEFINITIONS AUTOMATIC TAGS ::= 
BEGIN                   
MBS-DistributionSetupResponseTransfer ::= SEQUENCE {
    protocolIEs     ProtocolIE-Container        { {MBS-DistributionSetupResponseTransferIEs} },
    ...
}

ProtocolIE-Container {NGAP-PROTOCOL-IES : IEsSetParam} ::=
    SEQUENCE (SIZE (0..maxProtocolIEs)) OF
    ProtocolIE-Field {{IEsSetParam}}

maxProtocolIEs							INTEGER ::= 65535

NGAP-PROTOCOL-IES ::= CLASS {
    &id             ProtocolIE-ID                   UNIQUE,
    &criticality    Criticality,
    &Value,
    &presence       Presence
}
WITH SYNTAX {
    ID              &id
    CRITICALITY     &criticality
    TYPE            &Value
    PRESENCE        &presence
}

ProtocolIE-ID		::= INTEGER (0..65535)

ProtocolIE-Field {NGAP-PROTOCOL-IES : IEsSetParam} ::= SEQUENCE {
    id              NGAP-PROTOCOL-IES.&id               ({IEsSetParam}),
    criticality     NGAP-PROTOCOL-IES.&criticality      ({IEsSetParam}{@id}),
    value           NGAP-PROTOCOL-IES.&Value            ({IEsSetParam}{@id})
}

MBS-DistributionSetupResponseTransferIEs NGAP-PROTOCOL-IES ::= {
    { ID id-MBS-SessionID                                      CRITICALITY reject  TYPE MBS-SessionID                           PRESENCE mandatory }|
    { ID id-MBS-AreaSessionID                                  CRITICALITY reject  TYPE MBS-AreaSessionID                       PRESENCE optional  }|
    { ID id-SharedNG-U-Multicast-TNL-Information               CRITICALITY reject  TYPE SharedNG-U-Multicast-TNL-Information    PRESENCE optional  }|
    { ID id-Alternative-SharedNG-U-Multicast-TNL-Information   CRITICALITY ignore  TYPE SharedNG-U-Multicast-TNL-Information    PRESENCE optional  }|
    { ID id-MBS-QoSFlows-ToBeSetupList                         CRITICALITY reject  TYPE MBS-QoSFlows-ToBeSetupList              PRESENCE mandatory }|
    { ID id-MBSSessionStatus                                   CRITICALITY reject  TYPE MBSSessionStatus                        PRESENCE mandatory }|
    { ID id-MBS-ServiceArea                                    CRITICALITY reject  TYPE MBS-ServiceArea                         PRESENCE optional  }, 
    ...
}

id-MBS-SessionID 										ProtocolIE-ID ::= 299
id-MBS-AreaSessionID 									ProtocolIE-ID ::= 295
id-SharedNG-U-Multicast-TNL-Information					ProtocolIE-ID ::= 321
id-Alternative-SharedNG-U-Multicast-TNL-Information		ProtocolIE-ID ::= 308
id-MBS-QoSFlows-ToBeSetupList							ProtocolIE-ID ::= 296
id-MBSSessionStatus							 			ProtocolIE-ID ::= 320
id-MBS-ServiceArea 										ProtocolIE-ID ::= 298

MBS-SessionID ::= SEQUENCE {
    tMGI            TMGI,
    nID             NID                                                         OPTIONAL,
    iE-Extensions   ProtocolExtensionContainer { {MBS-SessionID-ExtIEs} }       OPTIONAL,
    ...
}

TMGI ::=  OCTET STRING (SIZE(6))

NID ::= BIT STRING (SIZE(44))

ProtocolExtensionContainer {NGAP-PROTOCOL-EXTENSION : ExtensionSetParam} ::=
    SEQUENCE (SIZE (1..maxProtocolExtensions)) OF
    ProtocolExtensionField {{ExtensionSetParam}}

maxProtocolExtensions					INTEGER ::= 65535

NGAP-PROTOCOL-EXTENSION ::= CLASS {
    &id             ProtocolExtensionID         UNIQUE,
    &criticality    Criticality,
    &Extension,
    &presence       Presence
}
WITH SYNTAX {
    ID              &id
    CRITICALITY     &criticality
    EXTENSION       &Extension
    PRESENCE        &presence
}

ProtocolExtensionID	::= INTEGER (0..65535)

Criticality		::= ENUMERATED { reject, ignore, notify }

Presence		::= ENUMERATED { optional, conditional, mandatory }

ProtocolExtensionField {NGAP-PROTOCOL-EXTENSION : ExtensionSetParam} ::= SEQUENCE {
    id             NGAP-PROTOCOL-EXTENSION.&id             ({ExtensionSetParam}),
    criticality    NGAP-PROTOCOL-EXTENSION.&criticality    ({ExtensionSetParam}{@id}),
    extensionValue NGAP-PROTOCOL-EXTENSION.&Extension      ({ExtensionSetParam}{@id})
}

MBS-SessionID-ExtIEs NGAP-PROTOCOL-EXTENSION ::= {
    ...
}

MBS-AreaSessionID ::= INTEGER (0..65535, ...)

SharedNG-U-Multicast-TNL-Information ::= SEQUENCE {
	iP-MulticastAddress 		TransportLayerAddress,
	iP-SourceAddress			TransportLayerAddress,
	gTP-TEID						GTP-TEID,
	iE-Extensions				ProtocolExtensionContainer { {SharedNG-U-Multicast-TNL-Information-ExtIEs} } 	OPTIONAL,
	...
}

TransportLayerAddress ::= BIT STRING (SIZE(1..160, ...))

GTP-TEID ::= OCTET STRING (SIZE(4))

SharedNG-U-Multicast-TNL-Information-ExtIEs NGAP-PROTOCOL-EXTENSION ::= {
	...
}


MBS-QoSFlows-ToBeSetupList ::= SEQUENCE (SIZE(1.. maxnoofMBSQoSFlows)) OF MBS-QoSFlows-ToBeSetupItem

maxnoofMBSQoSFlows					INTEGER ::= 64

MBS-QoSFlows-ToBeSetupItem ::= SEQUENCE {
	mBSqosFlowIdentifier				QosFlowIdentifier,
	mBSqosFlowLevelQosParameters		QosFlowLevelQosParameters,
	iE-Extensions						ProtocolExtensionContainer { {MBS-QoSFlows-ToBeSetupItem-ExtIEs} }	OPTIONAL,
	...
}

QosFlowIdentifier ::= INTEGER (0..63, ...)

QosFlowLevelQosParameters ::= SEQUENCE {
	qosCharacteristics					QosCharacteristics,
	allocationAndRetentionPriority		AllocationAndRetentionPriority,
	gBR-QosInformation					GBR-QosInformation									OPTIONAL,
	reflectiveQosAttribute				ReflectiveQosAttribute								OPTIONAL,
	additionalQosFlowInformation		AdditionalQosFlowInformation						OPTIONAL,
	iE-Extensions		ProtocolExtensionContainer { {QosFlowLevelQosParameters-ExtIEs} }	OPTIONAL,
	...
}

QosCharacteristics ::= CHOICE {
	nonDynamic5QI		NonDynamic5QIDescriptor,
	dynamic5QI			Dynamic5QIDescriptor,
	choice-Extensions		ProtocolIE-SingleContainer { {QosCharacteristics-ExtIEs} }
}

NonDynamic5QIDescriptor ::= SEQUENCE {
	fiveQI						FiveQI,
	priorityLevelQos			PriorityLevelQos										OPTIONAL,
	averagingWindow				AveragingWindow											OPTIONAL,
	maximumDataBurstVolume		MaximumDataBurstVolume									OPTIONAL,
	iE-Extensions		ProtocolExtensionContainer { {NonDynamic5QIDescriptor-ExtIEs} }	OPTIONAL,
	...
}

FiveQI ::= INTEGER (0..255, ...)

PriorityLevelQos ::= INTEGER (1..127, ...)

AveragingWindow ::= INTEGER (0..4095, ...)

MaximumDataBurstVolume ::= INTEGER (0..4095, ..., 4096.. 2000000)

NonDynamic5QIDescriptor-ExtIEs NGAP-PROTOCOL-EXTENSION ::= {
	{ ID id-CNPacketDelayBudgetDL	CRITICALITY ignore	EXTENSION ExtendedPacketDelayBudget		PRESENCE optional		}|
	{ ID id-CNPacketDelayBudgetUL	CRITICALITY ignore	EXTENSION ExtendedPacketDelayBudget		PRESENCE optional		},
	...
}

id-CNPacketDelayBudgetDL								ProtocolIE-ID ::= 187
id-CNPacketDelayBudgetUL								ProtocolIE-ID ::= 188

ExtendedPacketDelayBudget ::= INTEGER (1..65535, ...)

Dynamic5QIDescriptor ::= SEQUENCE {
	priorityLevelQos			PriorityLevelQos,
	packetDelayBudget			PacketDelayBudget,
	packetErrorRate				PacketErrorRate,
	fiveQI						FiveQI													OPTIONAL,
	delayCritical				DelayCritical											OPTIONAL,
-- The above IE shall be present in case of GBR QoS flow
	averagingWindow				AveragingWindow											OPTIONAL,
-- The above IE shall be present in case of GBR QoS flow
	maximumDataBurstVolume		MaximumDataBurstVolume									OPTIONAL,
	iE-Extensions		ProtocolExtensionContainer { {Dynamic5QIDescriptor-ExtIEs} }	OPTIONAL,
	...
}

PacketDelayBudget ::= INTEGER (0..1023, ...)

PacketErrorRate ::= SEQUENCE {
	pERScalar		INTEGER (0..9, ...),
	pERExponent		INTEGER (0..9, ...),
	iE-Extensions		ProtocolExtensionContainer { {PacketErrorRate-ExtIEs} }	OPTIONAL,
	...
}

PacketErrorRate-ExtIEs NGAP-PROTOCOL-EXTENSION ::= {
	...
}

DelayCritical ::= ENUMERATED {
	delay-critical,
	non-delay-critical,
	...
}

Dynamic5QIDescriptor-ExtIEs NGAP-PROTOCOL-EXTENSION ::= {
	{ ID id-ExtendedPacketDelayBudget	CRITICALITY ignore	EXTENSION ExtendedPacketDelayBudget		PRESENCE optional		}|
	{ ID id-CNPacketDelayBudgetDL		CRITICALITY ignore	EXTENSION ExtendedPacketDelayBudget		PRESENCE optional		}|
	{ ID id-CNPacketDelayBudgetUL		CRITICALITY ignore	EXTENSION ExtendedPacketDelayBudget		PRESENCE optional		},
	...
}

id-ExtendedPacketDelayBudget							ProtocolIE-ID ::= 189

ProtocolIE-SingleContainer {NGAP-PROTOCOL-IES : IEsSetParam} ::= 
	ProtocolIE-Field {{IEsSetParam}}

QosCharacteristics-ExtIEs NGAP-PROTOCOL-IES ::= {
	...
}

AllocationAndRetentionPriority ::= SEQUENCE {
	priorityLevelARP				PriorityLevelARP,
	pre-emptionCapability			Pre-emptionCapability,
	pre-emptionVulnerability		Pre-emptionVulnerability,
	iE-Extensions		ProtocolExtensionContainer { {AllocationAndRetentionPriority-ExtIEs} } OPTIONAL,
	...
}

PriorityLevelARP ::= INTEGER (1..15)

Pre-emptionCapability ::= ENUMERATED {
	shall-not-trigger-pre-emption,
	may-trigger-pre-emption,
	...
}

Pre-emptionVulnerability ::= ENUMERATED {
	not-pre-emptable,
	pre-emptable,
	...
}

AllocationAndRetentionPriority-ExtIEs NGAP-PROTOCOL-EXTENSION ::= {
	...
}

GBR-QosInformation ::= SEQUENCE {
	maximumFlowBitRateDL		BitRate,
	maximumFlowBitRateUL		BitRate,
	guaranteedFlowBitRateDL		BitRate,
	guaranteedFlowBitRateUL		BitRate,
	notificationControl			NotificationControl									OPTIONAL,
	maximumPacketLossRateDL		PacketLossRate										OPTIONAL,
	maximumPacketLossRateUL		PacketLossRate										OPTIONAL,
	iE-Extensions		ProtocolExtensionContainer { {GBR-QosInformation-ExtIEs} }	OPTIONAL,
	...
}

BitRate	::= INTEGER (0..4000000000000, ...)

NotificationControl ::= ENUMERATED {
	notification-requested,
	...
}

PacketLossRate ::= INTEGER (0..1000, ...)

GBR-QosInformation-ExtIEs NGAP-PROTOCOL-EXTENSION ::= {
	{ ID id-AlternativeQoSParaSetList	CRITICALITY ignore	EXTENSION AlternativeQoSParaSetList	PRESENCE optional	},
	...
}

id-AlternativeQoSParaSetList							ProtocolIE-ID ::= 220

AlternativeQoSParaSetList ::= SEQUENCE (SIZE(1..maxnoofQosParaSets)) OF AlternativeQoSParaSetItem

maxnoofQosParaSets					INTEGER ::= 8

AlternativeQoSParaSetItem ::= SEQUENCE {
	alternativeQoSParaSetIndex			AlternativeQoSParaSetIndex,
	guaranteedFlowBitRateDL				BitRate												OPTIONAL,
	guaranteedFlowBitRateUL				BitRate												OPTIONAL,
	packetDelayBudget					PacketDelayBudget									OPTIONAL,
	packetErrorRate						PacketErrorRate										OPTIONAL,
	iE-Extensions		ProtocolExtensionContainer { {AlternativeQoSParaSetItem-ExtIEs} }	OPTIONAL,
	...
}

AlternativeQoSParaSetIndex ::= INTEGER (1..8, ...)

AlternativeQoSParaSetItem-ExtIEs NGAP-PROTOCOL-EXTENSION ::= {
	...
}

ReflectiveQosAttribute ::= ENUMERATED {
	subject-to,
	...
}

AdditionalQosFlowInformation ::= ENUMERATED {
	more-likely,
	...
}

QosFlowLevelQosParameters-ExtIEs NGAP-PROTOCOL-EXTENSION ::= {
	{ID id-QosMonitoringRequest	CRITICALITY ignore	EXTENSION QosMonitoringRequest	PRESENCE optional}|
	{ID id-QosMonitoringReportingFrequency	CRITICALITY ignore	EXTENSION QosMonitoringReportingFrequency	PRESENCE optional},
	...
}

id-QosMonitoringRequest									ProtocolIE-ID ::= 181
id-QosMonitoringReportingFrequency						ProtocolIE-ID ::= 276

QosMonitoringRequest ::= ENUMERATED {ul, dl, both, ..., stop}

QosMonitoringReportingFrequency ::= INTEGER (1..1800, ...)

MBS-QoSFlows-ToBeSetupItem-ExtIEs NGAP-PROTOCOL-EXTENSION ::= {
	...
}

MBSSessionStatus ::= ENUMERATED {
	activated,
	deactivated,
	...
}

MBS-ServiceArea ::= CHOICE {
	locationindependent		MBS-ServiceAreaInformation,
	locationdependent		MBS-ServiceAreaInformationList,
	choice-Extensions		ProtocolIE-SingleContainer { {MBS-ServiceArea-ExtIEs} }
}

MBS-ServiceAreaInformation ::= SEQUENCE {
	mBS-ServiceAreaCellList		MBS-ServiceAreaCellList																OPTIONAL,
	mBS-ServiceAreaTAIList		MBS-ServiceAreaTAIList																OPTIONAL,
	iE-Extensions					ProtocolExtensionContainer { {MBS-ServiceAreaInformation-ExtIEs} }	OPTIONAL,
	...
}

MBS-ServiceAreaCellList ::= SEQUENCE (SIZE(1.. maxnoofCellsforMBS)) OF NR-CGI

maxnoofCellsforMBS					INTEGER ::= 8192

NR-CGI ::= SEQUENCE {
	pLMNIdentity		PLMNIdentity,
	nRCellIdentity		NRCellIdentity,
	iE-Extensions		ProtocolExtensionContainer { {NR-CGI-ExtIEs} } OPTIONAL,
	...
}

PLMNIdentity ::= OCTET STRING (SIZE(3))

NRCellIdentity ::= BIT STRING (SIZE(36))

NR-CGI-ExtIEs NGAP-PROTOCOL-EXTENSION ::= {
	...
}

MBS-ServiceAreaTAIList ::= SEQUENCE (SIZE(1.. maxnoofTAIforMBS)) OF TAI

maxnoofTAIforMBS					INTEGER ::= 1024

TAI ::= SEQUENCE {
	pLMNIdentity		PLMNIdentity,
	tAC					TAC,
	iE-Extensions		ProtocolExtensionContainer { {TAI-ExtIEs} } OPTIONAL,
	...
}

TAC ::= OCTET STRING (SIZE(3))

TAI-ExtIEs NGAP-PROTOCOL-EXTENSION ::= {
	...
}

MBS-ServiceAreaInformation-ExtIEs NGAP-PROTOCOL-EXTENSION ::= {
	...
}

MBS-ServiceAreaInformationList ::= SEQUENCE (SIZE(1..maxnoofMBSServiceAreaInformation)) OF MBS-ServiceAreaInformation

maxnoofMBSServiceAreaInformation		INTEGER ::= 256 -- FFS

MBS-ServiceArea-ExtIEs NGAP-PROTOCOL-IES ::= {
	...
}
END

输入值（自选示例值）并encode：

value MBS-DistributionSetupResponseTransfer ::= {
    protocolIEs {
        {
            id id-MBS-SessionID,
            criticality reject,
            value MBS-SessionID : {
                tMGI '112233445566'H
            }
        },
        {
            id id-MBS-AreaSessionID,
            criticality reject,
            value MBS-AreaSessionID : 922
        },
        {
            id id-MBS-QoSFlows-ToBeSetupList,
            criticality reject,
            value MBS-QoSFlows-ToBeSetupList : {
                {
                    mBSqosFlowIdentifier 7,
                    mBSqosFlowLevelQosParameters {
                        qosCharacteristics nonDynamic5QI : {
                            fiveQI 126
                        },
                        allocationAndRetentionPriority {
                            priorityLevelARP 13,
                            pre-emptionCapability shall-not-trigger-pre-emption,
                            pre-emptionVulnerability not-pre-emptable
                        }
                    }
                }
            }
        },
        {
            id id-MBSSessionStatus,
            criticality reject,
            value MBSSessionStatus : deactivated
        }
    }
}

APER方式得到的编码为：0000 0401 2b00 0700 1122 3344 5566 0127 0003 0003 9a01 2800 0700 0e00 007e 3000 0140 0001 40，我们来看一下是否符合上述理解：

bit位BinaryHex解释
1-800000000000: MBS-DistributionSetupResponseTransfer层无extension addition
0000000: 由于protocolIEs的长度指示符为双字节编码要求字节对齐而产生的填充位
9-240004双字节编码长度指示符: 4
25-40012b双字节编码id: id-MBS-SessionID(299)
41-48000000000000: criticality枚举值需要2个bit编码，00是第一项reject
000000: 由于OpenType无约束长度指示符要求字节对齐而产生的填充位
49-5607OpenType无约束长度指示符，代表MBS-SessionID长度为7个字节
57-6400000000000: MBS-SessionID层无extension addition
0: OPTIONAL nID不存在
0: OPTIONAL iE-Extensions不存在
00000: 由于后面tMGI的编码要求字节对齐而产生的填充位
65-112112233445566tMGI的值
113-1280127双字节编码id: id-MBS-AreaSessionID(295)
129-136000000000000: criticality reject
000000: 由于OpenType无约束长度指示符要求字节对齐而产生的填充位
137-14403OpenType无约束长度指示符，代表mBS-AreaSessionID长度为3个字节
145-15200000000000: mBS-AreaSessionID的值在约束中
0000000: 由于mBS-AreaSessionID双字节编码要求字节对齐而产生的填充位
153-168039amBS-AreaSessionID的值
169-1840128双字节编码id: id-MBS-QoSFlows-ToBeSetupList(296)
185-192000000000000: criticality reject
000000: 由于OpenType无约束长度指示符要求字节对齐而产生的填充位
193-2000000011107OpenType无约束长度指示符，代表MBS-QoSFlows-ToBeSetupList长度为7个字节
201-23200000000000011100000000000000000000e0000000000: MBS-QoSFlows-ToBeSetupList最大长度是64，所以长度指示符为6bit，实际长度为1，所以编码1-1=0
0: MBS-QoSFlows-ToBeSetupItem无extention addition
0: OPTIONAL iE-Extensions不存在
0: QosFlowIdentifier值在约束中
000111: QosFlowIdentifier最大长度为64，用6bit编码(7)
0: QosFlowLevelQosParameters无extension addition
0000: 4个OPTIONAL均不存在
00: QosCharacteristics choice用2bit编码，选中00(nonDynamic5QI)
0: NonDynamic5QIDescriptor无extension addition
0000: 4个OPTIONAL均不存在
0: FiveQI值在约束中
000: FiveQI单字节编码要求字节对齐而产生的填充位
233-2407e单字节编码FiveQI(126)
241-256001100000000000030000: AllocationAndRetentionPriority无extension addition
0: OPTIONAL不存在
1100: 4bit编码PriorityLevelARP(13-1=12)
0: Pre-emptionCapability无extension addition
0: 1bit编码Pre-emptionCapability(shall-not-trigger-pre-emption)
0: Pre-emptionVulnerability无extension addition
0: 1bit编码Pre-emptionVulnerability(not-pre-emptable)
000000: 由于双字节编码id-MBSSessionStatus而产生的填充位
257-2720140双字节编码id: id-MBSSessionStatus(320)
273-280000000000000: criticality reject
000000: 由于OpenType无约束长度指示符要求字节对齐而产生的填充位
281-28801OpenType无约束长度指示符，代表MBSSessionStatus长度为1个字节
289-29601000000400: MBSSessionStatus无extension additon
1: 1bit编码MBSSessionStatus(deactivated)
000000: 填充满1个字节

参考资料

Specification #38.413 NGAP协议

X.680: Specification of basic notation

X.681: Information object specification

X.682: Constraint specification

X.683: Parameterization of ASN.1 specification

Understanding Information Objects

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