%%% @doc ICMPv4 (RFC 792) and ICMPv6 (RFC 4443) error-message
%%% builders used by CONNECT-IP proxies to synthesise errors for
%%% packets they cannot deliver.
%%%
%%% Each builder returns a complete IP packet (IP header + ICMP
%%% message) ready to hand back to the client via
%%% `masque:send_ip_packet/2' or a handler's
%%% `{send_ip_packet, _}' / `{icmp_error, _}' action.
%%%
%%% Invoking-packet truncation (RFC 4443 §3.1 and RFC 1812 §4.3.2.3):
%%% <ul>
%%%  <li>ICMPv6: "as much of invoking packet as possible without the
%%%      ICMPv6 packet exceeding the minimum IPv6 MTU" (1280 B).
%%%      Budget: 1280 − 40 (IPv6 hdr) − 8 (ICMPv6 hdr) = 1232 B.</li>
%%%  <li>ICMPv4: at least the IPv4 header plus 8 B of the original
%%%      datagram's data; we cap at the IPv4 minimum MTU 576, i.e.
%%%      576 − 20 − 8 = 548 B of the invoking packet.</li>
%%% </ul>
-module(masque_icmp).

-export([dest_unreachable/3,
         packet_too_big/2,
         time_exceeded/2]).

-export([apply_action/3]).

%% Limits.
-define(V4_INVOKING_CAP, 548).
-define(V6_INVOKING_CAP, 1232).

%% Default source address for proxy-synthesised ICMP errors.
-define(DEFAULT_V4_SRC, {0,0,0,0}).
-define(DEFAULT_V6_SRC, {0,0,0,0,0,0,0,0}).

%%====================================================================
%% API
%%====================================================================

%% @doc Build a Destination Unreachable ICMP packet.
%% Code maps to the RFC type/code tables.
%% For IPv4 (type 3): 0 = net unreachable, 1 = host unreachable,
%% 3 = port unreachable, 4 = frag needed (RFC 792 / RFC 1812).
%% For IPv6 (type 1): 0 = no route, 1 = admin prohibited, 3 = addr
%% unreachable, 4 = port unreachable, 5 = src addr failed
%% ingress/egress policy (RFC 4443 section 3.1).
-spec dest_unreachable(v4 | v6, non_neg_integer(), binary()) -> binary().
dest_unreachable(v4, Code, Invoking) ->
    build_v4(3, Code, <<0:32>>, Invoking);
dest_unreachable(v6, Code, Invoking) ->
    build_v6(1, Code, <<0:32>>, Invoking).

%% @doc Build an IPv6 Packet Too Big (type 2, RFC 4443 §3.2). `Mtu'
%% is the next-hop MTU that caused the drop.
-spec packet_too_big(non_neg_integer(), binary()) -> binary().
packet_too_big(Mtu, Invoking) ->
    build_v6(2, 0, <<Mtu:32>>, Invoking).

%% @doc Build a Time Exceeded ICMP packet. `Code' is 0 (TTL/HL
%% exceeded in transit) or 1 (fragment reassembly timeout).
-spec time_exceeded(v4 | v6, non_neg_integer(), binary()) -> binary().
time_exceeded(v4, Code, Invoking) ->
    build_v4(11, Code, <<0:32>>, Invoking);
time_exceeded(v6, Code, Invoking) ->
    build_v6(3, Code, <<0:32>>, Invoking).

time_exceeded(v4, Invoking) -> time_exceeded(v4, 0, Invoking);
time_exceeded(v6, Invoking) -> time_exceeded(v6, 0, Invoking).

%% @doc Translate a session-level `{icmp_error, Spec}' action into
%% the IP packet it represents. Used by the IP server session's
%% action interpreter. `Spec' accepts:
%% <ul>
%%  <li>`{dest_unreachable, v4|v6, Code}'</li>
%%  <li>`{packet_too_big, Mtu}'  (IPv6 only)</li>
%%  <li>`{time_exceeded,   v4|v6}'</li>
%% </ul>
-spec apply_action(atom(), term(), binary()) -> binary().
apply_action(dest_unreachable, {V, Code}, Invoking) ->
    dest_unreachable(V, Code, Invoking);
apply_action(packet_too_big, Mtu, Invoking) ->
    packet_too_big(Mtu, Invoking);
apply_action(time_exceeded, {V, Code}, Invoking) ->
    time_exceeded(V, Code, Invoking);
apply_action(time_exceeded, V, Invoking) when V =:= v4; V =:= v6 ->
    time_exceeded(V, Invoking).

%%====================================================================
%% Internal — IPv4 builder (type | code | csum | rest-of-header | body)
%%====================================================================

build_v4(Type, Code, RestHeader4, Invoking) ->
    Body = clamp(Invoking, ?V4_INVOKING_CAP),
    %% Preserve the invoking packet's destination as our source so
    %% the client sees the packet "from" the original target.
    {Src, Dst} = v4_endpoints(Invoking),
    Msg0 = <<Type:8, Code:8, 0:16, RestHeader4/binary, Body/binary>>,
    Csum = inet_checksum(Msg0),
    Msg  = <<Type:8, Code:8, Csum:16, RestHeader4/binary, Body/binary>>,
    TotalLen = 20 + byte_size(Msg),
    Header0 = <<16#45:8, 0:8, TotalLen:16,
                0:16, 2#010:3, 0:13,
                64:8, 1:8, 0:16,
                (ip4_bin(Src))/binary,
                (ip4_bin(Dst))/binary>>,
    HdrCsum = inet_checksum(Header0),
    Header = <<16#45:8, 0:8, TotalLen:16,
               0:16, 2#010:3, 0:13,
               64:8, 1:8, HdrCsum:16,
               (ip4_bin(Src))/binary,
               (ip4_bin(Dst))/binary>>,
    <<Header/binary, Msg/binary>>.

v4_endpoints(<<4:4, _IHL:4, _TOS:8, _:16, _:16, _:16, _:8, _:8, _:16,
               SA:8, SB:8, SC:8, SD:8,
               DA:8, DB:8, DC:8, DD:8, _/binary>>) ->
    {{DA,DB,DC,DD}, {SA,SB,SC,SD}};
v4_endpoints(_) ->
    {?DEFAULT_V4_SRC, ?DEFAULT_V4_SRC}.

ip4_bin({A,B,C,D}) -> <<A:8, B:8, C:8, D:8>>.

%%====================================================================
%% Internal — IPv6 builder (pseudo-header checksum, RFC 2460 §8.1)
%%====================================================================

build_v6(Type, Code, RestHeader4, Invoking) ->
    Body = clamp(Invoking, ?V6_INVOKING_CAP),
    {Src, Dst} = v6_endpoints(Invoking),
    Msg0 = <<Type:8, Code:8, 0:16, RestHeader4/binary, Body/binary>>,
    PayloadLen = byte_size(Msg0),
    %% ICMPv6 checksum includes the IPv6 pseudo-header.
    Pseudo = <<(ip6_bin(Src))/binary,
               (ip6_bin(Dst))/binary,
               PayloadLen:32, 0:24, 58:8>>,
    Csum = inet_checksum(<<Pseudo/binary, Msg0/binary>>),
    Msg = <<Type:8, Code:8, Csum:16, RestHeader4/binary, Body/binary>>,
    Header = <<6:4, 0:8, 0:20, PayloadLen:16, 58:8, 64:8,
               (ip6_bin(Src))/binary,
               (ip6_bin(Dst))/binary>>,
    <<Header/binary, Msg/binary>>.

v6_endpoints(<<6:4, _:28, _:16, _:8, _:8,
               SA:16, SB:16, SC:16, SD:16,
               SE:16, SF:16, SG:16, SH:16,
               DA:16, DB:16, DC:16, DD:16,
               DE:16, DF:16, DG:16, DH:16, _/binary>>) ->
    {{DA,DB,DC,DD,DE,DF,DG,DH}, {SA,SB,SC,SD,SE,SF,SG,SH}};
v6_endpoints(_) ->
    {?DEFAULT_V6_SRC, ?DEFAULT_V6_SRC}.

ip6_bin({A,B,C,D,E,F,G,H}) ->
    <<A:16, B:16, C:16, D:16, E:16, F:16, G:16, H:16>>.

%%====================================================================
%% Internal — shared helpers
%%====================================================================

clamp(Bin, Max) when byte_size(Bin) =< Max -> Bin;
clamp(Bin, Max) -> binary:part(Bin, 0, Max).

%% Standard 16-bit one's-complement Internet checksum.
inet_checksum(Bin) ->
    finish_csum(sum_words(Bin, 0)).

sum_words(<<A:16, Rest/binary>>, Acc) -> sum_words(Rest, Acc + A);
sum_words(<<A:8>>, Acc)                -> Acc + (A bsl 8);
sum_words(<<>>, Acc)                   -> Acc.

finish_csum(Sum) ->
    S = (Sum band 16#FFFF) + (Sum bsr 16),
    S2 = (S band 16#FFFF) + (S bsr 16),
    (bnot S2) band 16#FFFF.