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<chapter>
  <header>
    <copyright>
      <year>2007</year>
      <year>2016</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>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>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 ERTS 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 ERTS 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>
