aboutsummaryrefslogtreecommitdiffstats
path: root/erts/doc/src/erl_dist_protocol.xml
blob: a5949ce15fefff0aa03fca70c40092ea85e091c0 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE chapter SYSTEM "chapter.dtd">

<chapter>
  <header>
    <copyright>
      <year>2007</year>
      <year>2015</year>
      <holder>Ericsson AB, All Rights Reserved</holder>
    </copyright>
    <legalnotice>
  Licensed under the Apache License, Version 2.0 (the "License");
  you may not use this file except in compliance with the License.
  You may obtain a copy of the License at
 
      http://www.apache.org/licenses/LICENSE-2.0

  Unless required by applicable law or agreed to in writing, software
  distributed under the License is distributed on an "AS IS" BASIS,
  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  See the License for the specific language governing permissions and
  limitations under the License.

  The Initial Developer of the Original Code is Ericsson AB.
    </legalnotice>

    <title>Distribution Protocol</title>
    <prepared></prepared>
    <docno></docno>
    <date>2007-09-21</date>
    <rev>PA1</rev>
    <file>erl_dist_protocol.xml</file>
  </header>

  <p>This description is far from complete. It will be updated if the
    protocol is updated. However, the protocols, both from Erlang
    nodes to the Erlang Port Mapper Daemon (EPMD) and between Erlang nodes
    are stable since many years.</p>

  <p>The distribution protocol can be divided into four parts:</p>

  <list type="bulleted">
    <item>
      <p>Low-level socket connection (1)</p>
    </item>
    <item>
      <p>Handshake, interchange node name, and authenticate (2)</p>
    </item>
    <item>
      <p>Authentication (done by <seealso marker="kernel:net_kernel">
        <c>kernel:net_kernel(3)</c></seealso>) (3)</p>
    </item>
    <item>
      <p>Connected (4)</p> 
    </item>
  </list>

  <p>A node fetches the port number of another node through the EPMD (at the
    other host) to initiate a connection request.</p>

  <p>For each host, where a distributed Erlang node is running, also an EPMD
    is to be running. The EPMD can be started explicitly or automatically
    as a result of the Erlang node startup.</p>

  <p>By default the EPMD listens on port 4369.</p>

  <p>(3) and (4) above are performed at the same level but the <c>net_kernel</c>
    disconnects the other node if it communicates using an invalid cookie (after
    1 second).</p>

  <p>The integers in all multibyte fields are in big-endian order.</p>

  <section>
    <title>EPMD Protocol</title>
    <p>The requests served by the EPMD are summarized in the following
      figure.</p>

    <image file="erl_ext_fig.gif">
      <icaption>Summary of EPMD Requests</icaption>
    </image>

    <p>Each request <c>*_REQ</c> is preceded by a 2 byte length field.
      Thus, the overall request format is as follows:</p>

    <table align="left">
      <row>
        <cell align="center">2</cell>
        <cell align="center">n</cell>
      </row>
      <row>
        <cell align="center"><c>Length</c></cell>
        <cell align="center"><c>Request</c></cell>
      </row>
      <tcaption>Request Format</tcaption>
    </table>

    <section>
      <title>Register a Node in EPMD</title>
      <p>When a distributed node is started it registers itself in the EPMD.
        The message <c>ALIVE2_REQ</c> described below is sent from the node to
        the EPMD. The response from the EPMD is <c>ALIVE2_RESP</c>.</p>

      <table align="left">
        <row>
          <cell align="center">1</cell>
          <cell align="center">2</cell>
          <cell align="center">1</cell>
          <cell align="center">1</cell>
          <cell align="center">2</cell>
          <cell align="center">2</cell>
          <cell align="center">2</cell>
          <cell align="center">Nlen</cell>
          <cell align="center">2</cell>
          <cell align="center">Elen</cell>
        </row>
        <row>
          <cell align="center"><c>120</c></cell>
          <cell align="center"><c>PortNo</c></cell>
          <cell align="center"><c>NodeType</c></cell>
          <cell align="center"><c>Protocol</c></cell>
          <cell align="center"><c>HighestVersion</c></cell>
          <cell align="center"><c>LowestVersion</c></cell>
          <cell align="center"><c>Nlen</c></cell>
          <cell align="center"><c>NodeName</c></cell>
          <cell align="center"><c>Elen</c></cell>
          <cell align="center"><c>Extra</c></cell>
        </row>
        <tcaption>ALIVE2_REQ (120)</tcaption>
      </table>

      <taglist>
        <tag><c>PortNo</c></tag>
        <item>
          <p>The port number on which the node accept connection requests.</p>
        </item>
        <tag><c>NodeType</c></tag>
        <item>
          <p>77 = normal Erlang node, 72 = hidden node (C-node), ...</p>
        </item>
        <tag><c>Protocol</c></tag>
        <item>
          <p>0 = TCP/IPv4, ...</p>
        </item>
        <tag><c>HighestVersion</c></tag>
        <item>
          <p>The highest distribution version that this node can handle.
            The value in Erlang/OTP R6B and later is 5.</p>
        </item>
        <tag><c>LowestVersion</c></tag>
        <item>
          <p>The lowest distribution version that this node can handle.
            The value in Erlang/OTP R6B and later is 5.</p>
        </item>
        <tag><c>Nlen</c></tag>
        <item>
          <p>The length (in bytes) of field <c>NodeName</c>.</p>
        </item>
        <tag><c>NodeName</c></tag>
        <item>
          <p>The node name as an UTF-8 encoded string of <c>Nlen</c> bytes.</p>
        </item>
        <tag><c>Elen</c></tag>
        <item>
          <p>The length of field <c>Extra</c>.</p>
        </item>
        <tag><c>Extra</c></tag>
        <item>
          <p>Extra field of <c>Elen</c> bytes.</p>
        </item>
      </taglist>

      <p>The connection created to the EPMD must be kept as long as the
        node is a distributed node. When the connection is closed,
        the node is automatically unregistered from the EPMD.</p>

      <p>The response message <c>ALIVE2_RESP</c> is as follows:</p>

      <table align="left">
        <row>
          <cell align="center">1</cell>
          <cell align="center">1</cell>
          <cell align="center">2</cell>
        </row>
        <row>
          <cell align="center"><c>121</c></cell>
          <cell align="center"><c>Result</c></cell>
          <cell align="center"><c>Creation</c></cell>
        </row>
        <tcaption>ALIVE2_RESP (121)</tcaption>
      </table>

      <p>Result = 0 -> ok, result &gt; 0 -> error.</p>
    </section>

    <section>
      <title>Unregister a Node from EPMD</title>
      <p>A node unregisters itself from the EPMD by closing the TCP
        connection to EPMD established when the node was registered.</p>
    </section>

    <section>
      <title>Get the Distribution Port of Another Node</title>
      <p>When one node wants to connect to another node it starts with
        a <c>PORT_PLEASE2_REQ</c> request to the EPMD on the host where the
        node resides to get the distribution port that the node listens to.</p>

      <table align="left">
        <row>
          <cell align="center">1</cell>
          <cell align="center">N</cell>
        </row>
        <row>
          <cell align="center"><c>122</c></cell>
          <cell align="center"><c>NodeName</c></cell>
        </row>
        <tcaption>PORT_PLEASE2_REQ (122)</tcaption>
      </table>

      <p>where N = <c>Length</c> - 1.</p>

      <table align="left">
        <row>
          <cell align="center">1</cell>
          <cell align="center">1</cell>
        </row>
        <row>
          <cell align="center"><c>119</c></cell>
          <cell align="center"><c>Result</c></cell>
        </row>
        <tcaption>PORT2_RESP (119) Response Indicating Error, Result &gt; 0
        </tcaption>
      </table>

      <p>or</p>

      <table align="left">
        <row>
          <cell align="center">1</cell>
          <cell align="center">1</cell>
          <cell align="center">2</cell>
          <cell align="center">1</cell>
          <cell align="center">1</cell>
          <cell align="center">2</cell>
          <cell align="center">2</cell>
          <cell align="center">2</cell>
          <cell align="center">Nlen</cell>
          <cell align="center">2</cell>
          <cell align="center">Elen</cell>
        </row>
        <row>
          <cell align="center"><c>119</c></cell>
          <cell align="center"><c>Result</c></cell>
          <cell align="center"><c>PortNo</c></cell>
          <cell align="center"><c>NodeType</c></cell>
          <cell align="center"><c>Protocol</c></cell>
          <cell align="center"><c>HighestVersion</c></cell>
          <cell align="center"><c>LowestVersion</c></cell>
          <cell align="center"><c>Nlen</c></cell>
          <cell align="center"><c>NodeName</c></cell>
          <cell align="center"><c>Elen</c></cell>
          <cell align="center">><c>Extra</c></cell>
        </row>
        <tcaption>PORT2_RESP, Result = 0</tcaption>
      </table>

      <p>If <c>Result</c> &gt; 0, the packet only consists of
        <c>[119, Result]</c>.</p>

      <p>The EPMD closes the socket when it has sent the information.</p>
    </section>

    <section>
      <title>Get All Registered Names from EPMD</title>
      <p>This request is used through the Erlang function
        <seealso marker="kernel:net_adm#names/1,2">
        <c>net_adm:names/1,2</c></seealso>. A TCP connection is opened
        to the  EPMD and this request is sent.</p>

      <table align="left">
        <row>
          <cell align="center">1</cell>
        </row>
        <row>
          <cell align="center"><c>110</c></cell>
        </row>
        <tcaption>NAMES_REQ (110)</tcaption>
      </table>	

      <p>The response for a <c>NAMES_REQ</c> is as follows:</p>

      <table align="left">
        <row>
          <cell align="center">4</cell>
          <cell align="center">&nbsp;</cell>
        </row>
        <row>
          <cell align="center"><c>EPMDPortNo</c></cell>
          <cell align="center"><c>NodeInfo*</c></cell>
        </row>
        <tcaption>NAMES_RESP</tcaption>
      </table>

      <p><c>NodeInfo</c> is a string written for each active node.
        When all <c>NodeInfo</c> has been written the connection is
        closed by the EPMD.</p>

      <p><c>NodeInfo</c> is, as expressed in Erlang:</p>

      <code>
io:format("name ~ts at port ~p~n", [NodeName, Port]).</code>
    </section>

    <section>
      <title>Dump All Data from EPMD</title>
      <p>This request is not really used, it is to be regarded as a debug
        feature.</p>

      <table align="left">
        <row>
          <cell align="center">1</cell>
        </row>
        <row>
          <cell align="center"><c>100</c></cell>
        </row>
        <tcaption>DUMP_REQ</tcaption>
      </table>

      <p>The response for a <c>DUMP_REQ</c> is as follows:</p>

      <table align="left">
        <row>
          <cell align="center">4</cell>
          <cell align="center">&nbsp;</cell>
        </row>
        <row>
          <cell align="center"><c>EPMDPortNo</c></cell>
          <cell align="center"><c>NodeInfo*</c></cell>
        </row>
        <tcaption>DUMP_RESP</tcaption>
      </table>

      <p><c>NodeInfo</c> is a string written for each node kept in the EPMD.
        When all <c>NodeInfo</c> has been written the connection is
        closed by the EPMD.</p>

      <p><c>NodeInfo</c> is, as expressed in Erlang:</p>

      <code>
io:format("active name     ~ts at port ~p, fd = ~p~n",
          [NodeName, Port, Fd]).</code>

      <p>or</p>

      <code>
io:format("old/unused name ~ts at port ~p, fd = ~p ~n",
          [NodeName, Port, Fd]).</code>
    </section>

    <section>
      <title>Kill EPMD</title>
      <p>This request kills the running EPMD. It is almost never used.</p>

      <table align="left">
        <row>
          <cell align="center">1</cell>
        </row>
        <row>
          <cell align="center"><c>107</c></cell>
        </row>
        <tcaption>KILL_REQ</tcaption>
      </table>

      <p>The response for a <c>KILL_REQ</c> is as follows:</p>

      <table align="left">
        <row>
          <cell align="center">2</cell>
        </row>
        <row>
          <cell align="center"><c>OKString</c></cell>
        </row>
        <tcaption>KILL_RESP</tcaption>
      </table>

      <p>where <c>OKString</c> is "OK".</p>
    </section>

    <section>
      <title>STOP_REQ  (Not Used)</title>
      <table align="left">
        <row>
          <cell align="center">1</cell>
          <cell align="center">n</cell>
        </row>
        <row>
          <cell align="center"><c>115</c></cell>
          <cell align="center"><c>NodeName</c></cell>
        </row>
        <tcaption>STOP_REQ</tcaption>
      </table>

      <p>where n = <c>Length</c> - 1.</p>

      <p>The current implementation of Erlang does not care if the connection
        to the EPMD is broken.</p>

      <p>The response for a <c>STOP_REQ</c> is as follows:</p>

      <table align="left">
        <row>
          <cell align="center">7</cell>
        </row>
        <row>
          <cell align="center"><c>OKString</c></cell>
        </row>
        <tcaption>STOP_RESP</tcaption>
      </table>

      <p>where <c>OKString</c> is "STOPPED".</p>

      <p>A negative response can look as follows:</p>

      <table align="left">
        <row>
          <cell align="center">7</cell>
        </row>
        <row>
          <cell align="center"><c>NOKString</c></cell>
        </row>
        <tcaption>STOP_NOTOK_RESP</tcaption>
      </table>

      <p>where <c>NOKString</c> is "NOEXIST".</p>
    </section>
<!--
    <section>
      <title>ALIVE_REQ (97)</title>
      <table align="left">
        <row>
          <cell align="center">1</cell>
          <cell align="center">2</cell>
          <cell align="center">n</cell>
        </row>
        <row>
          <cell align="center"><c>97</c></cell>
          <cell align="center"><c>PortNo</c></cell>
          <cell align="center"><c>NodeName</c></cell>
        </row>
        <tcaption></tcaption>
      </table>

      <p>where n = <c>Length</c> - 3.</p>

      <p>The connection created to the EPMD must be kept until the node is
        not a distributed node any longer.</p>
    </section>

    <section>
      <title>ALIVE_OK_RESP (89)</title>
      <table align="left">
        <row>
          <cell align="center">1</cell>
          <cell align="center">2</cell>
        </row>
        <row>
          <cell align="center"><c>89</c></cell>
          <cell align="center"><c>Creation</c></cell>
        </row>
        <tcaption></tcaption>
      </table>
    </section>

    <section>
      <title>ALIVE_NOTOK_RESP ()</title>
      <p>The EPMD closed the connection.</p>
    </section>

    <section>
      <title>PORT_PLEASE_REQ (112)</title>
      <table align="left">
        <row>
          <cell align="center">1</cell>
          <cell align="center">n</cell>
        </row>
        <row>
          <cell align="center"><c>112</c></cell>
          <cell align="center"><c>NodeName</c></cell>
        </row>
        <tcaption></tcaption>
      </table>

      <p>where n = <c>Length</c> - 1.</p>
    </section>

    <section>
      <title>PORT_OK_RESP ()</title>
      <table align="left">
        <row>
          <cell align="center">2</cell>
        </row>
        <row>
          <cell align="center"><c>PortNo</c></cell>
        </row>
        <tcaption></tcaption>
      </table>
    </section>

    <section>
      <title>PORT_NOTOK_RESP ()</title>
      <p>The EPMD closed the connection.</p>
    </section>

    <section>
      <title>PORT_NOTOK_RESP ()</title>
      <p>The EPMD closed the connection.</p>
    </section>
-->
  </section>

  <section>
    <marker id="distribution_handshake"/>
    <title>Distribution Handshake</title>
    <p>This section describes the distribution handshake protocol introduced
      in Erlang/OTP R6. This description was previously located in
      <c>$ERL_TOP/lib/kernel/internal_doc/distribution_handshake.txt</c> and
      has more or less been copied and "formatted" here. It has been almost
      unchanged since 1999, but the handshake has not changed much since then
      either.</p>

    <section>
      <title>General</title>
      <p>The TCP/IP distribution uses a handshake that expects a
        connection-based protocol, that is, the protocol does not include any
        authentication after the handshake procedure.</p>

      <p>This is not entirely safe, as it is vulnerable against takeover
        attacks, but it is a tradeoff between fair safety and performance.</p>

      <p>The cookies are never sent in cleartext and the handshake procedure
        expects the client (called <c>A</c>) to be the first one to prove that
        it can generate a sufficient digest. The digest is generated with the
        MD5 message digest algorithm and the challenges are expected to be
        random numbers.</p>
    </section>

    <section>
      <title>Definitions</title>
      <p>A challenge is a 32-bit integer in big-endian order. Below the function
        <c>gen_challenge()</c> returns a random 32-bit integer used as a
        challenge.</p>

      <p>A digest is a (16 bytes) MD5 hash of the challenge (as text)
        concatenated with the cookie (as text). Below, the function
        <c>gen_digest(Challenge, Cookie)</c> generates a digest as described
        above.</p>

      <p>An <c>out_cookie</c> is the cookie used in outgoing communication to a
        certain node, so that <c>A</c>'s <c>out_cookie</c> for <c>B</c> is to
        correspond with <c>B</c>'s <c>in_cookie</c> for <c>A</c> and conversely.
        <c>A</c>'s <c>out_cookie</c> for <c>B</c> and <c>A</c>'s
        <c>in_cookie</c> for <c>B</c> need <em>not</em> be the same. Below the
        function <c>out_cookie(Node)</c> returns the current node's
        <c>out_cookie</c> for <c>Node</c>.</p>

      <p>An <c>in_cookie</c> is the cookie expected to be used by another node
        when communicating with us, so that <c>A</c>'s <c>in_cookie</c> for
        <c>B</c> corresponds with <c>B</c>'s <c>out_cookie</c> for <c>A</c>.
        Below the function <c>in_cookie(Node)</c> returns the current node's
        <c>in_cookie</c> for <c>Node</c>.</p>

      <p>The cookies are text strings that can be viewed as passwords.</p>

      <p>Every message in the handshake starts with a 16-bit big-endian integer,
        which contains the message length (not counting the two initial bytes).
        In Erlang this corresponds to option <c>{packet, 2}</c> in
        <seealso marker="kernel:gen_tcp"><c>kernel:gen_tcp(3)</c></seealso>.
        Notice that after the handshake, the distribution switches to 4 byte
        packet headers.</p>
    </section>

    <section>
      <title>The Handshake in Detail</title>
      <p>Imagine two nodes, <c>A</c> that initiates the handshake and <c>B</c>
        that accepts the connection.</p>

      <taglist>
      <tag>1) connect/accept</tag>
        <item>
          <p><c>A</c> connects to <c>B</c> through TCP/IP and <c>B</c> accepts
            the connection.</p>
        </item>
        <tag>2) <c>send_name</c>/<c>receive_name</c></tag>
        <item>
          <p><c>A</c> sends an initial identification to <c>B</c>, which
            receives the message. The message looks as follows (every "square"
            is one byte and the packet header is removed):</p>
          <pre>
+---+--------+--------+-----+-----+-----+-----+-----+-----+-...-+-----+
|'n'|Version0|Version1|Flag0|Flag1|Flag2|Flag3|Name0|Name1| ... |NameN|
+---+--------+--------+-----+-----+-----+-----+-----+-----+-... +-----+</pre>
           <p>'n' is the message tag. 'Version0' and 'Version1' is the
             distribution version selected by <c>A</c>, based on information
             from the EPMD. (16-bit big-endian) 'Flag0' ... 'Flag3' are
             capability flags, the capabilities are defined in
             <c>$ERL_TOP/lib/kernel/include/dist.hrl</c>. (32-bit big-endian)
             'Name0' ... 'NameN' is the full node name of <c>A</c>, as a string
             of bytes (the packet length denotes how long it is).</p>
        </item>
        <tag>3) <c>recv_status</c>/<c>send_status</c></tag>
        <item>
          <p><c>B</c> sends a status message to <c>A</c>, which indicates if the
            connection is allowed. The following status codes are defined:</p>
          <taglist>
            <tag><c>ok</c></tag>
            <item>
              <p>The handshake will continue.</p>
            </item>
            <tag><c>ok_simultaneous</c></tag>
            <item>
              <p>The handshake will continue, but <c>A</c> is informed that
                <c>B</c> has another ongoing connection attempt that will be
                shut down (simultaneous connect where <c>A</c>'s name is
                greater than <c>B</c>'s name, compared literally).</p>
            </item>
            <tag><c>nok</c></tag>
            <item>
              <p>The handshake will not continue, as <c>B</c> already has an
                ongoing handshake, which it itself has initiated (simultaneous
                connect where <c>B</c>'s name is greater than <c>A</c>'s).</p>
            </item>
            <tag><c>not_allowed</c></tag>
            <item>
              <p>The connection is disallowed for some (unspecified) security
                reason.</p>
            </item>
            <tag><c>alive</c></tag>
            <item>
              <p>A connection to the node is already active, which either means
                that node <c>A</c> is confused or that the TCP connection
                breakdown of a previous node with this name has not yet reached
                node <c>B</c>. See step 3B below.</p>
            </item>
          </taglist>
          <p>The format of the status message is as follows:</p>
          <pre>
+---+-------+-------+-...-+-------+
|'s'|Status0|Status1| ... |StatusN|
+---+-------+-------+-...-+-------+</pre>
          <p>'s' is the message tag. 'Status0' ... 'StatusN' is the status as a
            string (not terminated).</p>
        </item>
        <tag>3B) <c>send_status</c>/<c>recv_status</c></tag>
        <item>
          <p>If status was <c>alive</c>, node <c>A</c> answers with another
            status message containing either <c>true</c>, which means that the
            connection is to continue (the old connection from this node is
            broken), or <c>false</c>, which means that the connection is to be
            closed (the connection attempt was a mistake.</p>
        </item>
        <tag>4) <c>recv_challenge</c>/<c>send_challenge</c></tag>
        <item>
          <p>If the status was <c>ok</c> or <c>ok_simultaneous</c>, the
            handshake continues with <c>B</c> sending <c>A</c> another message,
            the challenge. The challenge contains the same type of information
            as the "name" message initially sent from <c>A</c> to <c>B</c>, plus
            a 32-bit challenge:</p>
          <pre>
+---+--------+--------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-...-+-----+
|'n'|Version0|Version1|Flag0|Flag1|Flag2|Flag3|Chal0|Chal1|Chal2|Chal3|Name0|Name1| ... |NameN|
+---+--------+--------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-... +-----+</pre>
          <p>'Chal0' ... 'Chal3' is the challenge as a 32-bit big-endian integer
            and the other fields are <c>B</c>'s version, flags, and full node
            name.</p>
        </item>
        <tag>5) <c>send_challenge_reply</c>/<c>recv_challenge_reply</c></tag>
        <item>
          <p>Now <c>A</c> has generated a digest and its own challenge. Those
            are sent together in a package to <c>B</c>:</p>
          <pre>
+---+-----+-----+-----+-----+-----+-----+-----+-----+-...-+------+
|'r'|Chal0|Chal1|Chal2|Chal3|Dige0|Dige1|Dige2|Dige3| ... |Dige15|
+---+-----+-----+-----+-----+-----+-----+-----+-----+-...-+------+</pre>
          <p>'r' is the tag. 'Chal0' ... 'Chal3' is <c>A</c>'s challenge for
            <c>B</c> to handle. 'Dige0' ... 'Dige15' is the digest that <c>A</c>
            constructed from the challenge <c>B</c> sent in the previous
            step.</p>
        </item>
        <tag>6) <c>recv_challenge_ack</c>/<c>send_challenge_ack</c></tag>
        <item>
          <p><c>B</c> checks that the digest received from <c>A</c> is correct
            and generates a digest from the challenge received from <c>A</c>.
            The digest is then sent to <c>A</c>. The message is as follows:</p>
          <pre>
+---+-----+-----+-----+-----+-...-+------+
|'a'|Dige0|Dige1|Dige2|Dige3| ... |Dige15|
+---+-----+-----+-----+-----+-...-+------+</pre>
          <p>'a' is the tag. 'Dige0' ... 'Dige15' is the digest calculated by
            <c>B</c> for <c>A</c>'s challenge.</p>
        </item>
        <tag>7) check</tag>
        <item>
          <p><c>A</c> checks the digest from <c>B</c> and the connection is
            up.</p>
        </item>
      </taglist>
    </section>

    <section>
      <title>Semigraphic View</title>
      <pre>
A (initiator)                                      B (acceptor)

TCP connect ------------------------------------&gt;
                                                   TCP accept

send_name --------------------------------------&gt;
                                                   recv_name

  &lt;---------------------------------------------- send_status
recv_status
(if status was 'alive'
 send_status - - - - - - - - - - - - - - - - - -&gt;
                                                   recv_status)
                                                   ChB = gen_challenge()
                          (ChB)
  &lt;---------------------------------------------- send_challenge
recv_challenge
ChA = gen_challenge(),
OCA = out_cookie(B),
DiA = gen_digest(ChB, OCA)
                          (ChA, DiA)
send_challenge_reply ---------------------------&gt;
                                                   recv_challenge_reply
                                                   ICB = in_cookie(A),
                                                   check:
                                                   DiA == gen_digest (ChB, ICB)?
                                                   - if OK:
                                                    OCB = out_cookie(A),
                                                    DiB = gen_digest (ChA, OCB)
                          (DiB)
  &lt;----------------------------------------------- send_challenge_ack
recv_challenge_ack                                  DONE
ICA = in_cookie(B),                                - else:
check:                                              CLOSE
DiB == gen_digest(ChA, ICA)?
- if OK:
 DONE
- else:
 CLOSE</pre>
    </section>

    <section>
      <marker id="dflags"/>
      <title>Distribution Flags</title>
      <p>The following capability flags are defined:</p>
      <taglist>
        <tag><c>-define(DFLAG_PUBLISHED,16#1).</c></tag>
        <item>
          <p>The node is to be published and part of the global namespace.</p>
        </item>
        <tag><c>-define(DFLAG_ATOM_CACHE,16#2).</c></tag>
        <item>
          <p>The node implements an atom cache (obsolete).</p>
        </item>
        <tag><c>-define(DFLAG_EXTENDED_REFERENCES,16#4).</c></tag>
        <item>
          <p>The node implements extended (3 &times; 32 bits) references. This
            is required today. If not present, the  connection is refused.</p>
        </item>
        <tag><c>-define(DFLAG_DIST_MONITOR,16#8).</c></tag>
        <item>
          <p>The node implements distributed process monitoring.</p>
        </item>
        <tag><c>-define(DFLAG_FUN_TAGS,16#10).</c></tag>
        <item>
          <p>The node uses separate tag for funs (lambdas) in the distribution
            protocol.</p>
        </item>
        <tag><c>-define(DFLAG_DIST_MONITOR_NAME,16#20).</c></tag>
        <item>
          <p>The node implements distributed named process monitoring.</p>
        </item>
        <tag><c>-define(DFLAG_HIDDEN_ATOM_CACHE,16#40).</c></tag>
        <item>
          <p>The (hidden) node implements atom cache (obsolete).</p>
        </item>
        <tag><c>-define(DFLAG_NEW_FUN_TAGS,16#80).</c></tag>
        <item>
          <p>The node understand new fun tags.</p>
        </item>
        <tag><c>-define(DFLAG_EXTENDED_PIDS_PORTS,16#100).</c></tag>
        <item>
          <p>The node can handle extended pids and ports. This is required
            today. If not present, the connection is refused.</p>
        </item>
        <tag><c>-define(DFLAG_EXPORT_PTR_TAG,16#200).</c></tag>
        <item>
        </item>
        <tag><c>-define(DFLAG_BIT_BINARIES,16#400).</c></tag>
        <item>
        </item>
        <tag><c>-define(DFLAG_NEW_FLOATS,16#800).</c></tag>
        <item>
          <p>The node understands new float format.</p>
        </item>
        <tag><c>-define(DFLAG_UNICODE_IO,16#1000).</c></tag>
        <item>
        </item>
        <tag><c>-define(DFLAG_DIST_HDR_ATOM_CACHE,16#2000).</c></tag>
        <item>
          <p>The node implements atom cache in distribution header.</p>
        </item>
        <tag><c>-define(DFLAG_SMALL_ATOM_TAGS, 16#4000).</c></tag>
        <item>
          <p>The node understand the <c>SMALL_ATOM_EXT</c> tag.</p>
        </item>
        <tag><c>-define(DFLAG_UTF8_ATOMS, 16#10000).</c></tag>
        <item>
          <p>The node understand UTF-8 encoded atoms.</p>
        </item>
      </taglist>
    </section>
  </section>

  <section>
    <marker id="connected_nodes"/>
    <title>Protocol between Connected Nodes</title>
    <p>As from <c>ERTS</c> 5.7.2 the runtime system passes a distribution flag
      in the handshake stage that enables the use of a
      <seealso marker="erl_ext_dist#distribution_header">distribution header
      </seealso> on all messages passed. Messages passed between nodes have in
      this case the following format:</p>

    <table align="left">
      <row>
        <cell align="center">4</cell>
        <cell align="center">d</cell>
        <cell align="center">n</cell>
        <cell align="center">m</cell>
      </row>
      <row>
        <cell align="center"><c>Length</c></cell>
        <cell align="center"><c>DistributionHeader</c></cell>
        <cell align="center"><c>ControlMessage</c></cell>
        <cell align="center"><c>Message</c></cell>
      </row>
      <tcaption>Format of Messages Passed between Nodes (as from ERTS 5.7.2)
      </tcaption>
    </table>

    <taglist>
      <tag><c>Length</c></tag>
      <item>
        <p>Equal to d + n + m.</p>
      </item>
      <tag><c>ControlMessage</c></tag>
      <item>
        <p>A tuple passed using the external format of Erlang.</p>
      </item>
      <tag><c>Message</c></tag>
      <item>
        <p>The message sent to another node using the '!' (in external format).
          Notice that <c>Message</c> is only passed in combination with a
          <c>ControlMessage</c> encoding a send ('!').</p>
      </item>
    </taglist>

    <p>Notice that <seealso marker="erl_ext_dist#overall_format">the version
      number is omitted from the terms that follow a distribution header
      </seealso>.</p>

    <p>Nodes with an <c>ERTS</c> version earlier than 5.7.2 does not pass the
      distribution flag that enables the distribution header. Messages passed
      between nodes have in this case the following format:</p>

    <table align="left">
      <row>
        <cell align="center">4</cell>
        <cell align="center">1</cell>
        <cell align="center">n</cell>
        <cell align="center">m</cell>
      </row>
      <row>
        <cell align="center"><c>Length</c></cell>
        <cell align="center"><c>Type</c></cell>
        <cell align="center"><c>ControlMessage</c></cell>
        <cell align="center"><c>Message</c></cell>
      </row>
      <tcaption>Format of Messages Passed between Nodes (before ERTS 5.7.2)
      </tcaption>
    </table>

    <taglist>
      <tag><c>Length</c></tag>
      <item>
        <p>Equal to 1 + n + m.</p>
      </item>
      <tag><c>Type</c></tag>
      <item>
        <p>Equal to <c>112</c> (pass through).</p>
      </item>
      <tag><c>ControlMessage</c></tag>
      <item>
        <p>A tuple passed using the external format of Erlang.</p>
      </item>
      <tag><c>Message</c></tag>
      <item>
        <p>The message sent to another node using the '!' (in external format).
          Notice that <c>Message</c> is only passed in combination with a
          <c>ControlMessage</c> encoding a send ('!').</p>
      </item>
    </taglist>

    <p>The <c>ControlMessage</c> is a tuple, where the first element indicates
      which distributed operation it encodes:</p>

    <taglist>
      <tag><c>LINK</c></tag>
      <item>
        <p><c>{1, FromPid, ToPid}</c></p>
      </item>
      <tag><c>SEND</c></tag>
      <item>
        <p><c>{2, Unused, ToPid}</c></p>
        <p>Followed by <c>Message</c>.</p>
        <p><c>Unused</c> is kept for backward compatibility.</p>
      </item>
      <tag><c>EXIT</c></tag>
      <item>
        <p><c>{3, FromPid, ToPid, Reason}</c></p>
      </item>
      <tag><c>UNLINK</c></tag>
      <item>
        <p><c>{4, FromPid, ToPid}</c></p>
      </item>
      <tag><c>NODE_LINK</c></tag>
      <item>
        <p><c>{5}</c></p>
      </item>
      <tag><c>REG_SEND</c></tag>
      <item>
        <p><c>{6, FromPid, Unused, ToName}</c></p>
        <p>Followed by <c>Message</c>.</p>
        <p><c>Unused</c> is kept for backward compatibility.</p>
      </item>
      <tag><c>GROUP_LEADER</c></tag>
      <item>
        <p><c>{7, FromPid, ToPid}</c></p>
      </item>	
      <tag><c>EXIT2</c></tag>
      <item>
        <p><c>{8, FromPid, ToPid, Reason}</c></p>
      </item>
    </taglist>
  </section>

  <section>
    <title>New Ctrlmessages for distrvsn = 1 (Erlang/OTP R4)</title>
    <taglist>
      <tag><c>SEND_TT</c></tag>
      <item>
        <p><c>{12, Unused, ToPid, TraceToken}</c></p>
        <p>Followed by <c>Message</c>.</p>
        <p><c>Unused</c> is kept for backward compatibility.</p>
      </item>
      <tag><c>EXIT_TT</c></tag>
      <item>
        <p><c>{13, FromPid, ToPid, TraceToken, Reason}</c></p>
      </item>
      <tag><c>REG_SEND_TT</c></tag>
      <item>
        <p><c>{16, FromPid, Unused, ToName, TraceToken}</c></p>
        <p>Followed by <c>Message</c>.</p>
        <p><c>Unused</c> is kept for backward compatibility.</p>
      </item>      
      <tag><c>EXIT2_TT</c></tag>
      <item>
        <p><c>{18, FromPid, ToPid, TraceToken, Reason}</c></p>
      </item>
    </taglist>
  </section>

  <section>
    <title>New Ctrlmessages for distrvsn = 2</title>
    <p><c>distrvsn</c> 2 was never used.</p>
  </section>

  <section>
    <title>New Ctrlmessages for distrvsn = 3 (Erlang/OTP R5C)</title>
    <p>None, but the version number was increased anyway.</p>
  </section>

  <section>
    <title>New Ctrlmessages for distrvsn = 4 (Erlang/OTP R6)</title>
    <p>These are only recognized by Erlang nodes, not by hidden nodes.</p>

    <taglist>
      <tag><c>MONITOR_P</c></tag>
      <item>
        <p><c>{19, FromPid, ToProc, Ref}</c>, where
          <c>FromPid</c> = monitoring process and
          <c>ToProc</c> = monitored process pid or name (atom)</p>
      </item>
      <tag><c>DEMONITOR_P</c></tag>
      <item>
        <p><c>{20, FromPid, ToProc, Ref}</c>, where
          <c>FromPid</c> = monitoring process and
          <c>ToProc</c> = monitored process pid or name (atom)</p>
        <p>We include <c>FromPid</c> just in case we want to trace this.</p>
      </item>
      <tag><c>MONITOR_P_EXIT</c></tag>
      <item>
        <p><c>{21, FromProc, ToPid, Ref, Reason}</c>, where
          <c>FromProc</c> = monitored process pid or name (atom),
          <c>ToPid</c> = monitoring process, and
          <c>Reason</c> = exit reason for the monitored process</p>
      </item>
    </taglist>
  </section>
</chapter>