Crc in the child packet

[ErfanBabakhani]

Hello , suppose that I have a structure like this :

Pdu DEFINITIONS AUTOMATIC TAGS ::= BEGIN

AddressSize ::= INTEGER (0..7)

HeaderSize ::= INTEGER (0..31)

Packet ::= SEQUENCE {
--Common fields
field1 INTEGER(0..255) ,
--Common fields
child Child
}

SrcAndDesAdd ::= OCTET STRING (SIZE (1..7))

Child ::= CHOICE {
first First,
second Second
}

FirstPacket ::= SEQUENCE { --in this PDU HeaderSize is always const excluding SrcAndDesAdd <We don't consider SrcAndDesAdd>
--Because nobodies size depends on sizeHeader we declare it in ASN
--Size of Address Field 3 bits: must declare in ACN
sizeHeader HeaderSize,
srcAndDesAddress SrcAndDesAdd,
crcHeaderr CrcHeader
}

CrcHeader ::= OCTET STRING (SIZE (2))

SegmentCrc ::= OCTET STRING (SIZE (4))

SecondPacket ::= SEQUENCE { --In this case size of header is var + consult so we declare it in ACN and by using the mapping function
add an offset for it
--Size of Address Field 3 bits: must declare in ACN
--Size of Header Field 5 bits: must declare in ACN
srcAndDesAddress SrcAndDesAdd,
var Var,
crcHeaderr CrcHeader
}

Var ::= OCTET STRING (SIZE (0..16))

END

Is it possible to calculate crcHeaderr in the child packets <FirstPacket , SecondPacket> ?
This scenario happen when I want have access to common fields that are exist in Packet and also what are exist for Child Packet

As I see for solving this problem I should change my structure to body and header for all childs and after header add crc in the Packet type instead of adding it in all children. Am I right?

[ErfanBabakhani]

Packet ::= SEQUENCE { --add post encoding property in ACN
--Common fields
field1 INTEGER(0..255) ,
--Common fields
header Header,
--crc in the ACN
body Body

}

SrcAndDesAdd ::= OCTET STRING (SIZE (1..7))

Header ::= CHOICE {
firstHeader FirstHeader,
secondHeader SecondHeader
}
Body ::= CHOICE {
firstBody FirstBody,
secondBody SecondBody
}

Something like that I mean can solve the problem
Is there any alternative to achieve goal without changing structure?

[ErfanBabakhani]

If I have a variable size type in body and it's size declare by the external field in header then I can not pass size to it

[ErfanBabakhani]

May be if we have just a property to returning position of a specefic field we would be more powefull

// Suggestion//

Packet ::= SEQUENCE {
field1 INTEGER(0..255) ,
child Child
}

CrcHeader ::= OCTET STRING (SIZE (2))

Child ::= CHOICE {
first FirstPacket,
second SecondPacket
}

FirstPacket ::= SEQUENCE {
field1 INTEGER(0..255),
crcHeaderr CrcHeader
}

-- Imaginary ACN --
Packet [] {
field1 [],
child []
}

FirstPacket ::= SEQUENCE {
field1 [],
crcHeaderr [return-position] --for example by such property we will return position of this field
}

When the user encode/decode it with Packet then we will return crcHeaderr position + offset
offset is what we encode/decode until entering child <in this example just 1 byte + 1 bit >

And calculating the offset is not complicated because you will just add what ever you encode before calling FirstPacket_ACN_encode in function Packet_ACN_encode

Maybe passing an extra argument as a int* to Packet_ACN_encode (.... , int * returned_positions) is enough for that
Packet_ACN_encode (.... , int * returned_positions){
//Encode what are in Packet
offset = current byte*8 + current bit
if child type is equal to FirstPacket
FirstPacket_ACN_encode (.... , int * returned_positions)
for int i=0 ; i < size returned_positions ; i++
returned_positions [i] +=offset
}

I think this is better approach to handling post encoding situations
We can set returned_positions size to max size of elements that have return-position property
returned_positions elements order can be as the order of fields order that has return-position property