Re: AI12-0218: What is the portable representation clause for processing IETF packets on little-endian machines?

[AdaMagica <christ-usch.grein@t-online.de>]

Am Donnerstag, 10. Mai 2018 19:45:43 UTC+2 schrieb Dan'l Miller:
> The Challenge:
> Without the proposed solution in AI12-0218, I challenge anyone to write an Ada representation clause that is •portable• to both little-endian and big-endian processors for either of the following IETF packet formats that must always be in big-endian network byte order:
> https://en.wikipedia.org/wiki/IPv4#Packet_structure
> https://en.wikipedia.org/wiki/IPv6_packet

I'm not sure this is what you are looking for:

  for IPv4 use record
    Version                at  0 range  0 ..  3;
    IHL                    at  0 range  4 ..  7;
    DSCP                   at  0 range  8 .. 13;
    ECN                    at  0 range 14 .. 15;
    Total_Length           at  0 range 16 .. 31;
    Identification         at  4 range  0 .. 15;
    Flags                  at  4 range 16 .. 18;
    Fragment_Offset        at  4 range 19 .. 31;
    Time_to_Live           at  8 range  0 ..  7;
    Protocol               at  8 range  8 .. 15;
    Header_Checksum        at  8 range 16 .. 31;
    Source_IP_Address      at 12 range  0 .. 31;
    Destination_IP_Address at 16 range  0 .. 31;
  end record;
  for IPv4'Bit_Order use System.High_Order_First;

Of course you have to byte-swap after a transfer.

GNAT reports this upne compilation:
reverse bit order in machine scalar of length 32
little-endian range for component "Version" is 28 .. 31
reverse bit order in machine scalar of length 32
little-endian range for component "IHL" is 24 .. 27
reverse bit order in machine scalar of length 32
little-endian range for component "DSCP" is 18 .. 23
reverse bit order in machine scalar of length 32
little-endian range for component "ECN" is 16 .. 17
reverse bit order in machine scalar of length 32
little-endian range for component "Total_Length" is 0 .. 15
reverse bit order in machine scalar of length 32
little-endian range for component "Identification" is 16 .. 31
reverse bit order in machine scalar of length 32
little-endian range for component "Flags" is 13 .. 15
reverse bit order in machine scalar of length 32
little-endian range for component "Fragment_Offset" is 0 .. 12
reverse bit order in machine scalar of length 32
little-endian range for component "Time_to_Live" is 24 .. 31
reverse bit order in machine scalar of length 32
little-endian range for component "Protocol" is 16 .. 23
reverse bit order in machine scalar of length 32
little-endian range for component "Header_Checksum" is 0 .. 15
reverse bit order in machine scalar of length 32
little-endian range for component "Source_IP_Address" is 0 .. 31
reverse bit order in machine scalar of length 32
little-endian range for component "Destination_IP_Address" is 0 .. 31

You might want to look at:
https://en.wikibooks.org/wiki/Ada_Programming/Attributes/%27Bit_Order