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    1/*  Part of SWI-Prolog
    2
    3    Author:        Jeffrey Rosenwald and Jan Wielemaker
    4    E-mail:        jeffrose@acm.org
    5    WWW:           http://www.swi-prolog.org
    6    Copyright (c)  2012-2013, Jeffrey Rosenwald
    7		   2018, CWI Amsterdam
    8    All rights reserved.
    9
   10    Redistribution and use in source and binary forms, with or without
   11    modification, are permitted provided that the following conditions
   12    are met:
   13
   14    1. Redistributions of source code must retain the above copyright
   15       notice, this list of conditions and the following disclaimer.
   16
   17    2. Redistributions in binary form must reproduce the above copyright
   18       notice, this list of conditions and the following disclaimer in
   19       the documentation and/or other materials provided with the
   20       distribution.
   21
   22    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   23    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   24    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
   25    FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
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   34*/
   35
   36:- module(udp_broadcast,
   37          [ udp_broadcast_initialize/2,         % +IPAddress, +Options
   38            udp_broadcast_close/1,		% +Scope
   39
   40            udp_peer_add/2,                     % +Scope, +IP
   41            udp_peer_del/2,                     % +Scope, ?IP
   42            udp_peer/2                          % +Scope, -IP
   43          ]).   44:- use_module(library(socket)).   45:- use_module(library(broadcast)).   46:- use_module(library(option)).   47:- use_module(library(apply)).   48:- use_module(library(debug)).   49:- use_module(library(error)).   50
   51% :- debug(udp(broadcast)).

A UDP broadcast proxy

SWI-Prolog's broadcast library provides a means that may be used to facilitate publish and subscribe communication regimes between anonymous members of a community of interest. The members of the community are however, necessarily limited to a single instance of Prolog. The UDP broadcast library removes that restriction. With this library loaded, any member on your local IP subnetwork that also has this library loaded may hear and respond to your broadcasts.

This library support three styles of networking as described below. Each of these networks have their own advantages and disadvantages. Please study the literature to understand the consequences.

broadcast
Broadcast messages are sent to the LAN subnet. The broadcast implementation uses two UDP ports: a public to address the whole group and a private one to address a specific node. Broadcasting is generally a good choice if the subnet is small and traffic is low.
unicast
Unicast sends copies of packages to known peers. Unicast networks can easily be routed. The unicast version uses a single UDP port per node. Unicast is generally a good choice for a small party, in particular if the peers are in different networks.
multicast
Multicast is like broadcast, but it can be configured to work accross networks and may work more efficiently on VLAN networks. Like the broadcast setup, two UDP ports are used. Multicasting can in general deliver the most efficient LAN and WAN networks, but requires properly configured routing between the peers.

After initialization and, in the case of a unicast network managing the set of peers, communication happens through broadcast/1, broadcast_request/1 and listen/1,2,3.

A broadcast/1 or broadcast_request/1 of the shape udp(Scope, Term) or udp(Scope, Term, TimeOut) is forwarded over the UDP network to all peers that joined the same Scope. To prevent the potential for feedback loops, only the plain Term is broadcasted locally. The timeout is optional. It specifies the amount to time to wait for replies to arrive in response to a broadcast_request/1. The default period is 0.250 seconds. The timeout is ignored for broadcasts.

An example of three separate processes cooperating in the same scope called peers:

Process A:

   ?- listen(number(X), between(1, 5, X)).
   true.

   ?-

Process B:

   ?- listen(number(X), between(7, 9, X)).
   true.

   ?-

Process C:

   ?- findall(X, broadcast_request(udp(peers, number(X))), Xs).
   Xs = [1, 2, 3, 4, 5, 7, 8, 9].

   ?-

It is also possible to carry on a private dialog with a single responder. To do this, you supply a compound of the form, Term:PortId, to a UDP scoped broadcast/1 or broadcast_request/1, where PortId is the ip-address and port-id of the intended listener. If you supply an unbound variable, PortId, to broadcast_request, it will be unified with the address of the listener that responds to Term. You may send a directed broadcast to a specific member by simply providing this address in a similarly structured compound to a UDP scoped broadcast/1. The message is sent via unicast to that member only by way of the member's broadcast listener. It is received by the listener just as any other broadcast would be. The listener does not know the difference.

For example, in order to discover who responded with a particular value:

Host B Process 1:

   ?- listen(number(X), between(1, 5, X)).
   true.

   ?-

Host A Process 1:


   ?- listen(number(X), between(7, 9, X)).
   true.

   ?-

Host A Process 2:

   ?- listen(number(X), between(1, 5, X)).
   true.

   ?- bagof(X, broadcast_request(udp(peers,number(X):From,1)), Xs).
   From = ip(192, 168, 1, 103):34855,
   Xs = [7, 8, 9] ;
   From = ip(192, 168, 1, 103):56331,
   Xs = [1, 2, 3, 4, 5] ;
   From = ip(192, 168, 1, 104):3217,
   Xs = [1, 2, 3, 4, 5].

All incomming trafic is handled by a single thread with the alias udp_inbound_proxy. This thread also performs the internal dispatching using broadcast/1 and broadcast_request/1. Future versions may provide for handling these requests in seperate threads.

Caveats

While the implementation is mostly transparent, there are some important and subtle differences that must be taken into consideration:

author
- Jeffrey Rosenwald (JeffRose@acm.org), Jan Wielemaker
See also
- tipc.pl */
license
- BSD-2
  267:- multifile
  268    udp_term_string_hook/3,                     % +Scope, ?Term, ?String
  269    udp_unicast_join_hook/3,                    % +Scope, +From, +Data
  270    black_list/1.                               % +Term
  271
  272:- meta_predicate
  273    safely(0),
  274    safely_det(0).  275
  276safely(Predicate) :-
  277    Err = error(_,_),
  278    catch(Predicate, Err,
  279          print_message_fail(Err)).
  280
  281safely_det(Predicate) :-
  282    Err = error(_,_),
  283    catch(Predicate, Err,
  284          print_message_fail(Err)),
  285    !.
  286safely_det(_).
  287
  288print_message_fail(Term) :-
  289    print_message(error, Term),
  290    fail.
  291
  292udp_broadcast_address(IPAddress, Subnet, BroadcastAddress) :-
  293    IPAddress = ip(A1, A2, A3, A4),
  294    Subnet = ip(S1, S2, S3, S4),
  295    BroadcastAddress = ip(B1, B2, B3, B4),
  296
  297    B1 is A1 \/ (S1 xor 255),
  298    B2 is A2 \/ (S2 xor 255),
  299    B3 is A3 \/ (S3 xor 255),
  300    B4 is A4 \/ (S4 xor 255).
 udp_broadcast_service(?Scope, ?Address) is nondet
provides the UDP broadcast address for a given Scope. At present, only one scope is supported, udp_subnet.
 udp_scope(?ScopeName, ?ScopeDef)
  309:- dynamic
  310    udp_scope/2,
  311    udp_scope_peer/2.  312:- volatile
  313    udp_scope/2,
  314    udp_scope_peer/2.  315%
  316%  Here's a UDP proxy to Prolog's broadcast library
  317%
  318%  A sender may extend a broadcast  to  a   subnet  of  a UDP network by
  319%  specifying a =|udp_subnet|= scoping qualifier   in his/her broadcast.
  320%  The qualifier has the effect of  selecting the appropriate multi-cast
  321%  address for the transmission. Thus,  the   sender  of the message has
  322%  control over the scope of his/her traffic on a per-message basis.
  323%
  324%  All in-scope listeners receive the   broadcast and simply rebroadcast
  325%  the message locally. All broadcast replies, if any, are sent directly
  326%  to the sender via the port-id that   was received with the broadcast.
  327%
  328%  Each listener exposes two UDP ports,  a   shared  public port that is
  329%  bound to a well-known port number and   a  private port that uniquely
  330%  indentifies the listener. Broadcasts are received  on the public port
  331%  and replies are  sent  on  the   private  port.  Directed  broadcasts
  332%  (unicasts) are received on the private port   and replies are sent on
  333%  the private port.
  334
  335%  Thread 1 listens for directed traffic on the private port.
  336%
  337
  338:- dynamic
  339    udp_private_socket/3,                       % Port, Socket, FileNo
  340    udp_public_socket/4,                        % Scope, Port, Socket, FileNo
  341    udp_closed/1.				% Scope
  342
  343udp_inbound_proxy :-
  344    thread_at_exit(inbound_proxy_died),
  345    udp_inbound_proxy_loop.
  346
  347udp_inbound_proxy_loop :-
  348    make_private_socket,
  349    forall(udp_scope(Scope, ScopeData),
  350           make_public_socket(ScopeData, Scope)),
  351    retractall(udp_closed(_)),
  352    findall(FileNo, udp_socket_file_no(FileNo), FileNos),
  353    catch(dispatch_inbound(FileNos),
  354          E, dispatch_exception(E)),
  355    udp_inbound_proxy_loop.
  356
  357dispatch_exception(E) :-
  358    E = error(_,_),
  359    !,
  360    print_message(warning, E).
  361dispatch_exception(_).
 make_private_socket is det
Create our private socket. This socket is used for messages that are directed to me. Note that we only need this for broadcast networks. If we use a unicast network we use our public port to contact this specific server.
  371make_private_socket :-
  372    udp_private_socket(_Port, S, _F),
  373    !,
  374    (   (   udp_scope(Scope, broadcast(_,_,_))
  375        ;   udp_scope(Scope, multicast(_,_))
  376        ),
  377        \+ udp_closed(Scope)
  378    ->  true
  379    ;   tcp_close_socket(S),
  380        retractall(udp_private_socket(_,_,_))
  381    ).
  382make_private_socket :-
  383    udp_scope(_, broadcast(_,_,_)),
  384    !,
  385    udp_socket(S),
  386    tcp_bind(S, Port),
  387    tcp_getopt(S, file_no(F)),
  388    tcp_setopt(S, broadcast),
  389    assertz(udp_private_socket(Port, S, F)).
  390make_private_socket :-
  391    udp_scope(_, multicast(_,_)),
  392    !,
  393    udp_socket(S),
  394    tcp_bind(S, Port),
  395    tcp_getopt(S, file_no(F)),
  396    assertz(udp_private_socket(Port, S, F)).
  397make_private_socket.
 make_public_socket(+ScopeData, +Scope)
Create the public port Scope.
  403make_public_socket(_, Scope) :-
  404    udp_public_socket(Scope, _Port, S, _),
  405    !,
  406    (   udp_closed(Scope)
  407    ->  tcp_close_socket(S),
  408        retractall(udp_public_socket(Scope, _, _, _))
  409    ;   true
  410    ).
  411make_public_socket(broadcast(_SubNet, _Broadcast, Port), Scope) :-
  412    udp_socket(S),
  413    tcp_setopt(S, reuseaddr),
  414    tcp_bind(S, Port),
  415    tcp_getopt(S, file_no(F)),
  416    assertz(udp_public_socket(Scope, Port, S, F)).
  417make_public_socket(multicast(Group, Port), Scope) :-
  418    udp_socket(S),
  419    tcp_setopt(S, reuseaddr),
  420    tcp_bind(S, Port),
  421    tcp_setopt(S, ip_add_membership(Group)),
  422    tcp_getopt(S, file_no(F)),
  423    assertz(udp_public_socket(Scope, Port, S, F)).
  424make_public_socket(unicast(Port), Scope) :-
  425    udp_socket(S),
  426    tcp_bind(S, Port),
  427    tcp_getopt(S, file_no(F)),
  428    assertz(udp_public_socket(Scope, Port, S, F)).
  429
  430udp_socket_file_no(FileNo) :-
  431    udp_private_socket(_,_,FileNo).
  432udp_socket_file_no(FileNo) :-
  433    udp_public_socket(_,_,_,FileNo).
 dispatch_inbound(+FileNos)
Dispatch inbound traffic. This loop uses wait_for_input/3 to wait for one or more UDP sockets and dispatches the requests using the internal broadcast service. For an incomming broadcast request we send the reply only to the requester and therefore we must use a socket that is not in broadcast mode.
  443dispatch_inbound(FileNos) :-
  444    debug(udp(broadcast), 'Waiting for ~p', [FileNos]),
  445    wait_for_input(FileNos, Ready, infinite),
  446    debug(udp(broadcast), 'Ready: ~p', [Ready]),
  447    maplist(dispatch_ready, Ready),
  448    dispatch_inbound(FileNos).
  449
  450dispatch_ready(FileNo) :-
  451    udp_private_socket(_Port, Private, FileNo),
  452    !,
  453    udp_receive(Private, Data, From, [max_message_size(65535)]),
  454    debug(udp(broadcast), 'Inbound on private port', []),
  455    (   in_scope(Scope, From),
  456        udp_term_string(Scope, Term, Data) % only accept valid data
  457    ->  ld_dispatch(Private, Term, From, Scope)
  458    ;   true
  459    ).
  460dispatch_ready(FileNo) :-
  461    udp_public_socket(Scope, _PublicPort, Public, FileNo),
  462    !,
  463    udp_receive(Public, Data, From, [max_message_size(65535)]),
  464    debug(udp(broadcast), 'Inbound on public port from ~p for scope ~p',
  465          [From, Scope]),
  466    (   in_scope(Scope, From),
  467        udp_term_string(Scope, Term, Data) % only accept valid data
  468    ->  (   udp_scope(Scope, unicast(_))
  469        ->  ld_dispatch(Public, Term, From, Scope)
  470        ;   udp_private_socket(_PrivatePort, Private, _FileNo),
  471            ld_dispatch(Private, Term, From, Scope)
  472        )
  473    ;   udp_scope(Scope, unicast(_)),
  474        udp_term_string(Scope, Term, Data),
  475        unicast_out_of_scope_request(Scope, From, Term)
  476    ->  true
  477    ;   true
  478    ).
  479
  480in_scope(Scope, Address) :-
  481    udp_scope(Scope, ScopeData),
  482    in_scope(ScopeData, Scope, Address),
  483    !.
  484in_scope(Scope, From) :-
  485    debug(udp(broadcast), 'Out-of-scope ~p datagram from ~p',
  486          [Scope, From]),
  487    fail.
  488
  489in_scope(broadcast(Subnet, Broadcast, _PublicPort), _Scope, IP:_FromPort) :-
  490    udp_broadcast_address(IP, Subnet, Broadcast).
  491in_scope(multicast(_Group, _Port), _Scope, _From).
  492in_scope(unicast(_PublicPort), Scope, IP:_) :-
  493    udp_peer(Scope, IP:_).
 ld_dispatch(+PrivateSocket, +Term, +From, +Scope)
Locally dispatch Term received from From. If it concerns a broadcast request, send the replies to PrivateSocket to From. The multifile hook black_list/1 can be used to ignore certain messages.
  502ld_dispatch(_S, Term, From, _Scope) :-
  503    debug(udp(broadcast), 'ld_dispatch(~p) from ~p', [Term, From]),
  504    fail.
  505ld_dispatch(_S, Term, _From, _Scope) :-
  506    blacklisted(Term), !.
  507ld_dispatch(S, request(Key, Term), From, Scope) :-
  508    !,
  509    forall(safely(broadcast_request(Term)),
  510           safely((udp_term_string(Scope, reply(Key,Term), Message),
  511                   udp_send(S, Message, From, [])))).
  512ld_dispatch(_S, send(Term), _From, _Scope) :-
  513    safely_det(broadcast(Term)).
  514ld_dispatch(_S, reply(Key, Term), From, _Scope) :-
  515    (   reply_queue(Key, Queue)
  516    ->  safely(thread_send_message(Queue, Term:From))
  517    ;   true
  518    ).
  519
  520blacklisted(send(Term))      :- black_list(Term).
  521blacklisted(request(_,Term)) :- black_list(Term).
  522blacklisted(reply(_,Term))   :- black_list(Term).
 reload_udp_proxy
Update the UDP relaying proxy service. The proxy consists of three forwarding mechanisms:
  538reload_udp_proxy :-
  539    reload_outbound_proxy,
  540    reload_inbound_proxy.
  541
  542reload_outbound_proxy :-
  543    listening(udp_broadcast, udp(_,_), _),
  544    !.
  545reload_outbound_proxy :-
  546    listen(udp_broadcast, udp(Scope,Message),
  547           udp_broadcast(Message, Scope, 0.25)),
  548    listen(udp_broadcast, udp(Scope,Message,Timeout),
  549           udp_broadcast(Message, Scope, Timeout)),
  550    listen(udp_broadcast, udp_subnet(Message),  % backward compatibility
  551           udp_broadcast(Message, subnet, 0.25)),
  552    listen(udp_broadcast, udp_subnet(Message,Timeout),
  553           udp_broadcast(Message, subnet, Timeout)).
  554
  555reload_inbound_proxy :-
  556    catch(thread_signal(udp_inbound_proxy, throw(udp_reload)),
  557          error(existence_error(thread, _),_),
  558          fail),
  559    !.
  560reload_inbound_proxy :-
  561    thread_create(udp_inbound_proxy, _,
  562                  [ alias(udp_inbound_proxy),
  563                    detached(true)
  564                  ]).
  565
  566inbound_proxy_died :-
  567    thread_self(Self),
  568    thread_property(Self, status(Status)),
  569    (   catch(recreate_proxy(Status), _, fail)
  570    ->  print_message(informational,
  571                      httpd_restarted_worker(Self))
  572    ;   done_status_message_level(Status, Level),
  573        print_message(Level,
  574                      httpd_stopped_worker(Self, Status))
  575    ).
  576
  577recreate_proxy(exception(Error)) :-
  578    recreate_on_error(Error),
  579    reload_inbound_proxy.
  580
  581recreate_on_error('$aborted').
  582recreate_on_error(time_limit_exceeded).
  583
  584done_status_message_level(true, silent) :- !.
  585done_status_message_level(exception('$aborted'), silent) :- !.
  586done_status_message_level(_, informational).
 udp_broadcast_close(+Scope)
Close a UDP broadcast scope.
  593udp_broadcast_close(Scope) :-
  594    udp_scope(Scope, _ScopeData),
  595    !,
  596    assert(udp_closed(Scope)),
  597    reload_udp_proxy.
  598udp_broadcast_close(_).
 udp_broadcast(+What, +Scope, +TimeOut)
Send a broadcast request to my UDP peers in Scope. What is either of the shape Term:Address to send Term to a specific address or query the address from which term is answered or it is a plain Term.

If Term is nonground, it is considered is a request (see broadcast_request/1) and the predicate succeeds for each answer received within TimeOut seconds. If Term is ground it is considered an asynchronous broadcast and udp_broadcast/3 is deterministic.

  612udp_broadcast(Term:To, Scope, _Timeout) :-
  613    ground(Term), ground(To),           % broadcast to single listener
  614    !,
  615    udp_basic_broadcast(send(Term), Scope, single(To)).
  616udp_broadcast(Term, Scope, _Timeout) :-
  617    ground(Term),                       % broadcast to all listeners
  618    !,
  619    udp_basic_broadcast(send(Term), Scope, broadcast).
  620udp_broadcast(Term:To, Scope, Timeout) :-
  621    ground(To),                         % request to single listener
  622    !,
  623    setup_call_cleanup(
  624        request_queue(Id, Queue),
  625        ( udp_basic_broadcast(request(Id, Term), Scope, single(To)),
  626          udp_br_collect_replies(Queue, Timeout, Term:To)
  627        ),
  628        destroy_request_queue(Queue)).
  629udp_broadcast(Term:From, Scope, Timeout) :-
  630    !,                                  % request to all listeners, collect sender
  631    setup_call_cleanup(
  632        request_queue(Id, Queue),
  633        ( udp_basic_broadcast(request(Id, Term), Scope, broadcast),
  634          udp_br_collect_replies(Queue, Timeout, Term:From)
  635        ),
  636        destroy_request_queue(Queue)).
  637udp_broadcast(Term, Scope, Timeout) :-  % request to all listeners
  638    udp_broadcast(Term:_, Scope, Timeout).
  639
  640:- dynamic
  641    reply_queue/2.  642
  643request_queue(Id, Queue) :-
  644    Id is random(1<<63),
  645    message_queue_create(Queue),
  646    asserta(reply_queue(Id, Queue)).
  647
  648destroy_request_queue(Queue) :-         % leave queue to GC
  649    retractall(reply_queue(_, Queue)).
 udp_basic_broadcast(+Term, +Dest) is multi
Create a UDP private socket and use it to send Term to Address. If Address is our broadcast address, set the socket in broadcast mode.

This predicate succeeds with a choice point. Committing the choice point closes S.

Arguments:
Dest- is one of single(Target) or broadcast.
  662udp_basic_broadcast(Term, Scope, Dest) :-
  663    debug(udp(broadcast), 'UDP proxy outbound ~p to ~p', [Term, Dest]),
  664    udp_term_string(Scope, Term, String),
  665    udp_send_message(Dest, String, Scope).
  666
  667udp_send_message(single(Address), String, Scope) :-
  668    (   udp_scope(Scope, unicast(_))
  669    ->  udp_public_socket(Scope, _Port, S, _)
  670    ;   udp_private_socket(_Port, S, _F)
  671    ),
  672    safely(udp_send(S, String, Address, [])).
  673udp_send_message(broadcast, String, Scope) :-
  674    (   udp_scope(Scope, unicast(_))
  675    ->  udp_public_socket(Scope, _Port, S, _),
  676        forall(udp_peer(Scope, Address),
  677               ( debug(udp(broadcast), 'Unicast to ~p', [Address]),
  678                 safely(udp_send(S, String, Address, []))))
  679    ;   udp_scope(Scope, broadcast(_SubNet, Broadcast, Port))
  680    ->  udp_private_socket(_PrivatePort, S, _F),
  681        udp_send(S, String, Broadcast:Port, [])
  682    ;   udp_scope(Scope, multicast(Group, Port))
  683    ->  udp_private_socket(_PrivatePort, S, _F),
  684        udp_send(S, String, Group:Port, [])
  685    ).
  686
  687% ! udp_br_collect_replies(+Queue, +TimeOut, -TermAndFrom) is nondet.
  688%
  689%   Collect replies on Socket for  TimeOut   seconds.  Succeed  for each
  690%   received message.
  691
  692udp_br_collect_replies(Queue, Timeout, Reply) :-
  693    get_time(Start),
  694    Deadline is Start+Timeout,
  695    repeat,
  696       (   thread_get_message(Queue, Reply,
  697                              [ deadline(Deadline)
  698                              ])
  699       ->  true
  700       ;   !,
  701           fail
  702       ).
 udp_broadcast_initialize(+IPAddress, +Options) is semidet
Initialized UDP broadcast bridge. IPAddress is the IP address on the network we want to broadcast on. IP addresses are terms ip(A,B,C,D) or an atom or string of the format A.B.C.D. Options processed:
scope(+ScopeName)
Name of the scope. Default is subnet.
subnet_mask(+SubNet)
Subnet to broadcast on. This uses the same syntax as IPAddress. Default classifies the network as class A, B or C depending on the the first octet and applies the default mask.
port(+Port)
Public port to use. Default is 20005.
method(+Method)
Method to send a message to multiple peers. One of
broadcast
Use UDP broadcast messages to the LAN. This is the default
multicast
Use UDP multicast messages. This can be used on WAN networks, provided the intermediate routers understand multicast.
unicast
Send the messages individually to all registered peers.

For compatibility reasons Options may be the subnet mask.

  731udp_broadcast_initialize(IP, Options) :-
  732    with_mutex(udp_broadcast,
  733               udp_broadcast_initialize_sync(IP, Options)).
  734
  735udp_broadcast_initialize_sync(IP, Options) :-
  736    nonvar(Options),
  737    Options = ip(_,_,_,_),
  738    !,
  739    udp_broadcast_initialize(IP, [subnet_mask(Options)]).
  740udp_broadcast_initialize_sync(IP, Options) :-
  741    to_ip4(IP, IPAddress),
  742    option(method(Method), Options, broadcast),
  743    must_be(oneof([broadcast, multicast, unicast]), Method),
  744    udp_broadcast_initialize_sync(Method, IPAddress, Options),
  745    reload_udp_proxy.
  746
  747udp_broadcast_initialize_sync(broadcast, IPAddress, Options) :-
  748    option(subnet_mask(Subnet), Options, _),
  749    mk_subnet(Subnet, IPAddress, Subnet4),
  750    option(port(Port), Options, 20005),
  751    option(scope(Scope), Options, subnet),
  752
  753    udp_broadcast_address(IPAddress, Subnet4, Broadcast),
  754    udp_broadcast_close(Scope),
  755    assertz(udp_scope(Scope, broadcast(Subnet4, Broadcast, Port))).
  756udp_broadcast_initialize_sync(unicast, _IPAddress, Options) :-
  757    option(port(Port), Options, 20005),
  758    option(scope(Scope), Options, subnet),
  759    udp_broadcast_close(Scope),
  760    assertz(udp_scope(Scope, unicast(Port))).
  761udp_broadcast_initialize_sync(multicast, IPAddress, Options) :-
  762    option(port(Port), Options, 20005),
  763    option(scope(Scope), Options, subnet),
  764    udp_broadcast_close(Scope),
  765    multicast_address(IPAddress),
  766    assertz(udp_scope(Scope, multicast(IPAddress, Port))).
  767
  768to_ip4(Atomic, ip(A,B,C,D)) :-
  769    atomic(Atomic),
  770    !,
  771    (   split_string(Atomic, ".", "", Strings),
  772        maplist(number_string, [A,B,C,D], Strings)
  773    ->  true
  774    ;   syntax_error(illegal_ip_address)
  775    ).
  776to_ip4(IP, IP).
  777
  778mk_subnet(Var, IP, Subnet) :-
  779    var(Var),
  780    !,
  781    (   default_subnet(IP, Subnet)
  782    ->  true
  783    ;   domain_error(ip_with_subnet, IP)
  784    ).
  785mk_subnet(Subnet, _, Subnet4) :-
  786    to_ip4(Subnet, Subnet4).
  787
  788default_subnet(ip(A,_,_,_), ip(A,0,0,0)) :-
  789    between(1,126, A), !.
  790default_subnet(ip(A,B,_,_), ip(A,B,0,0)) :-
  791    between(128,191, A), !.
  792default_subnet(ip(A,B,C,_), ip(A,B,C,0)) :-
  793    between(192,223, A), !.
  794
  795multicast_address(ip(A,_,_,_)) :-
  796    between(224, 239, A),
  797    !.
  798multicast_address(IP) :-
  799    domain_error(multicast_network, IP).
  800
  801
  802		 /*******************************
  803		 *          UNICAST PEERS	*
  804		 *******************************/
 udp_peer_add(+Scope, +Address) is det
 udp_peer_del(+Scope, ?Address) is det
 udp_peer(?Scope, ?Address) is nondet
Manage and query the set of known peers for a unicast network. Address is either a term IP:Port or a plain IP address. In the latter case the default port registered with the scope is used.
Arguments:
Address- has canonical form ip(A,B,C,D):Port.
  816udp_peer_add(Scope, Address) :-
  817    must_be(ground, Address),
  818    peer_address(Address, Scope, Canonical),
  819    (   udp_scope_peer(Scope, Canonical)
  820    ->  true
  821    ;   assertz(udp_scope_peer(Scope, Canonical))
  822    ).
  823
  824udp_peer_del(Scope, Address) :-
  825    peer_address(Address, Scope, Canonical),
  826    retractall(udp_scope_peer(Scope, Canonical)).
  827
  828udp_peer(Scope, IPAddress) :-
  829    udp_scope_peer(Scope, IPAddress).
  830
  831peer_address(IP:Port, _Scope, IPAddress:Port) :-
  832    !,
  833    to_ip4(IP, IPAddress).
  834peer_address(IP, Scope, IPAddress:Port) :-
  835    (   udp_scope(Scope, unicast(Port))
  836    ->  true
  837    ;   existence_error(udp_scope, Scope)
  838    ),
  839    to_ip4(IP, IPAddress).
  840
  841
  842
  843		 /*******************************
  844		 *             HOOKS		*
  845		 *******************************/
 udp_term_string_hook(+Scope, +Term, -String) is det
udp_term_string_hook(+Scope, -Term, +String) is semidet
Hook for serializing the message Term. The default writes %prolog\n, followed by the Prolog term in quoted notation while ignoring operators. This hook may use alternative serialization such as fast_term_serialized/2, use library(ssl) to realise encrypted messages, etc.
Arguments:
Scope- is the scope for which the message is broadcasted. This can be used to use different serialization for different scopes.
Term- encapsulates the term broadcasted by the application as follows:
send(ApplTerm)
Is sent by broadcast(udp(Scope, ApplTerm))
request(Id, ApplTerm)
Is sent by broadcast_request/1, where Id is a unique large (64 bit) integer.
reply(Id, ApplTerm)
Is sent to reply on a broadcast_request/1 request that has been received. Arguments are the same as above.
throws
- The hook may throw udp(invalid_message) to stop processing the message.
 udp_term_string(+Scope, +Term, -String) is det
udp_term_string(+Scope, -Term, +String) is semidet
Serialize an arbitrary Prolog term as a string. The string is prefixed by a magic key to ensure we only accept messages that are meant for us.

In mode (+,-), Term is written with the options ignore_ops(true) and quoted(true).

This predicate first calls udp_term_string_hook/3.

  885udp_term_string(Scope, Term, String) :-
  886    catch(udp_term_string_hook(Scope, Term, String), udp(Error), true),
  887    !,
  888    (   var(Error)
  889    ->  true
  890    ;   Error == invalid_message
  891    ->  fail
  892    ;   throw(udp(Error))
  893    ).
  894udp_term_string(_Scope, Term, String) :-
  895    (   var(String)
  896    ->  format(string(String), '%-prolog-\n~W',
  897               [ Term,
  898                 [ ignore_ops(true),
  899                   quoted(true)
  900                 ]
  901               ])
  902    ;   sub_string(String, 0, _, _, '%-prolog-\n'),
  903        term_string(Term, String,
  904                    [ syntax_errors(quiet)
  905                    ])
  906    ).
 unicast_out_of_scope_request(+Scope, +From, +Data) is semidet
 udp_unicast_join_hook(+Scope, +From, +Data) is semidet
This multifile hook is called if an UDP package is received on the port of the unicast network identified by Scope. From is the origin IP and port and Data is the message data that is deserialized as defined for the scope (see udp_term_string/3).

This hook is intended to initiate a new node joining the network of peers. We could in theory also omit the in-scope test and use a normal broadcast to join. Using a different channal however provides a basic level of security. A possibe implementation is below. The first fragment is a hook added to the server, the second is a predicate added to a client and the last initiates the request in the client. The excanged term (join(X)) can be used to exchange a welcome handshake.

:- multifile udp_broadcast:udp_unicast_join_hook/3.
udp_broadcast:udp_unicast_join_hook(Scope, From, join(welcome)) :-
    udp_peer_add(Scope, From),
join_request(Scope, Address, Reply) :-
    udp_peer_add(Scope, Address),
    broadcast_request(udp(Scope, join(X))).
?- join_request(myscope, "1.2.3.4":10001, Reply).
Reply = welcome.
  944unicast_out_of_scope_request(Scope, From, send(Term)) :-
  945    udp_unicast_join_hook(Scope, From, Term).
  946unicast_out_of_scope_request(Scope, From, request(Key, Term)) :-
  947    udp_unicast_join_hook(Scope, From, Term),
  948    udp_public_socket(Scope, _Port, Socket, _FileNo),
  949    safely((udp_term_string(Scope, reply(Key,Term), Message),
  950            udp_send(Socket, Message, From, [])))