structure ipdatagram = struct

exception ip_Exception of string;

type ipheader = {
	vershl 	: Word8.word,
	serv		: Word8.word,
	totlen	: Word8Array.array,
	ident		: Word8Array.array,
	frags		: Word8Array.array,
	ttl		: Word8.word,
	proto		: Word8.word,
	cksum		: Word8Array.array,
	source	: LargeWord.word,
	dest		: LargeWord.word
};

val empty_ipheader = 
{
	vershl	= Word8.fromInt(0),
	serv		= Word8.fromInt(0),
	totlen	= double.get(Word8.fromInt(0), Word8.fromInt(0)),
	ident		= double.get(Word8.fromInt(0), Word8.fromInt(0)),
	frags		= double.get(Word8.fromInt(0), Word8.fromInt(0)),
	ttl		= Word8.fromInt(0),
	proto		= Word8.fromInt(0),
	cksum		= double.get(Word8.fromInt(0), Word8.fromInt(0)),
	source	= LargeWord.fromInt(0),
	dest		= LargeWord.fromInt(0)
}:ipheader;

type payload = Word8Array.array;

fun getipheader(data:Word8Array.array) = 
if (Word8Array.length(data) < 20) then raise ip_Exception("The IP header is incomplete\n")

else
{ 
	vershl = Word8Array.sub(data, 0), 
	serv	 = Word8Array.sub(data,1),
	totlen = double.get(Word8Array.sub(data,2), Word8Array.sub(data,3)),
      ident  = double.get(Word8Array.sub(data,4), Word8Array.sub(data,5)),
	frags	 = double.get(Word8Array.sub(data,6), Word8Array.sub(data,7)),
	ttl	 = Word8Array.sub(data, 8),
	proto	 = Word8Array.sub(data, 9),
	cksum  = double.get(Word8Array.sub(data,10), Word8Array.sub(data,11)),
	source = Pack32Big.subArr(data, 3),
	dest	 = Pack32Big.subArr(data, 4)
}:ipheader
;

fun sum (tot: Word32.word, i, header:Word8Array.array, ip_header_length) =
	let
		val word16 = Pack16Big.subArr(header, i) 
	in
		if (i<ip_header_length) then
			sum (tot + word16, i+2, header, ip_header_length)
		else
			tot+word16
	end
;
 
(* ip_header_length is the length of the header in terms of octets *)
(*
fun chksum(ip_header_length, header:Word8Array.array) =
	let
		val total = sum(Word32.fromInt(0), 0, header, ip_header_length)
	in
		
	end
;
*)
end;