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From: Paul Rubin <no.email@nospam.invalid>
Newsgroups: comp.lang.ada
Subject: Re: AI12-0218:  What is the portable representation clause for
 processing IETF packets on little-endian machines?
Date: Thu, 10 May 2018 13:32:09 -0700
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Date: 2018-05-10T13:32:09-07:00
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"Dan'l Miller" <optikos@verizon.net> writes:
> And just in case anyone is thinking, “Oh, every single one of the new
> designs of ISAs have been big-endian for decades

No it's for real, you do have to consider enddianness in any network
program, though like Dmitry I'm unconvinced that representation clauses
are the way to do it in Ada.  In C programs you usually just call some
macros to convert machine integers to and from network byte order.  In
the case where the machine int's bytes are already in network order, the
macro does nothing.

Example 4 of this page

  http://erlang.org/doc/programming_examples/bit_syntax.html

shows how you describe an IP datagram in Erlang:

    -define(IP_VERSION, 4).
    -define(IP_MIN_HDR_LEN, 5).

    DgramSize = byte_size(Dgram),
    case Dgram of 
        <<?IP_VERSION:4, HLen:4, SrvcType:8, TotLen:16, 
          ID:16, Flgs:3, FragOff:13,
          TTL:8, Proto:8, HdrChkSum:16,
          SrcIP:32,
          DestIP:32, RestDgram/binary>> when HLen>=5, 4*HLen=<DgramSize ->
            OptsLen = 4*(HLen - ?IP_MIN_HDR_LEN),
            <<Opts:OptsLen/binary,Data/binary>> = RestDgram,
        ...
    end.

It isn't bad, if you are used to Erlang syntax.  Those fields like
FragOff:13 are bit fields of the specified lengths.