path: root/erts/doc/src/erl_dist_protocol.xml

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<!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>
The description here is far from complete and will therefore be further
refined in upcoming releases.

The protocols both from Erlang nodes towards
EPMD (Erlang Port Mapper Daemon) and between Erlang nodes, however, are
stable since many years.
</p>

<p>The distribution protocol can be divided into four (4) parts:</p>
<list>
  <item>
    <p>
      1. Low level socket connection.
    </p>
  </item>
  <item>
    2. Handshake, interchange node name and authenticate.
  </item>
  <item>
    3. Authentication (done by net_kernel).
  </item>
  <item>
    4. Connected. 
  </item>
</list>
<p>
  A node fetches the Port number of another node through the EPMD (at the
  other host) in order to initiate a connection request.
</p>
<p>
For each host where a distributed Erlang node is running there should also
be an EPMD running. The EPMD can be started explicitly or automatically
as a result of the Erlang node startup.
</p>
<p>
By default EPMD listens on port 4369.
</p>
<p>
  3 and 4 are performed at the same level but the net_kernel disconnects the
  other node if it communicates using an invalid cookie (after one (1) second).
</p>

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

  <section>
    <title>EPMD Protocol</title>
    <p>
      The requests served by the EPMD (Erlang Port Mapper Daemon) are 
      summarized in the figure below.
    </p>

      <image file="erl_ext_fig.gif">
	<icaption>
	  Summary of EPMD requests. 
	</icaption>
      </image>
      <p>
	Each request <c>*_REQ</c> is preceded by a two-byte length field. 
	Thus, the overall request format is:
      </p>

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

      <section>
	<title>Register a node in the EPMD</title>
	<p>
	  When a distributed node is started it registers itself in EPMD.
	  The message ALIVE2_REQ described below is sent from the node towards
	  EPMD. The response from EPMD is ALIVE2_RESP.
	</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">120</cell>
	    <cell align="center">PortNo</cell>
	    <cell align="center">NodeType</cell>
	    <cell align="center">Protocol</cell>
	    <cell align="center">HighestVersion</cell>
	    <cell align="center">LowestVersion</cell>
	    <cell align="center">Nlen</cell>
	    <cell align="center">NodeName</cell>
	    <cell align="center">Elen</cell>
	    <cell align="center">Extra</cell>
	  </row>
	<tcaption>ALIVE2_REQ (120)</tcaption></table>
	  <taglist>
	    <tag><c>PortNo</c></tag>
	    <item>
	      The port number on which the node accept connection requests.
	    </item>
	    <tag><c>NodeType</c></tag>
	    <item>
	      77 = normal Erlang node, 72 = hidden node (C-node),...
	    </item>
	    <tag><c>Protocol</c></tag>
	    <item>
	      0 = tcp/ip-v4, ...
	    </item>
	    <tag><c>HighestVersion</c></tag>
	    <item>
	      The highest distribution version that this node can handle.
	      The value in R6B and later is 5.
	    </item>
	    <tag><c>LowestVersion</c></tag>
	    <item>
	      The lowest distribution version that this node can handle.
	      The value in R6B and later is 5.
	    </item>
	    <tag><c>Nlen</c></tag>
	    <item>
	      The length (in bytes) of the <c>NodeName</c> field.
	    </item>
	    <tag><c>NodeName</c></tag>
	    <item>
	      The NodeName as an UTF-8 encoded string of
	      <c>Nlen</c> bytes.
	    </item>
	    <tag><c>Elen</c></tag>
	    <item>
	      The length of the <c>Extra</c> field.
	    </item>
	    <tag><c>Extra</c></tag>
	    <item>
	      Extra field of <c>Elen</c> bytes.
	    </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 ALIVE2_RESP is described below.
	</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">121</cell>
	    <cell align="center">Result</cell>
	    <cell align="center">Creation</cell>
	  </row>
	<tcaption>ALIVE2_RESP (121)</tcaption></table>
	<p>
	  Result = 0 -> ok, Result > 0 -> error
	</p>
      </section>

      <section>
	<title>Unregister a node from the EPMD</title>
	<p>
	  A node unregisters itself from the EPMD by simply closing the
	  TCP connection towards 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 PORT_PLEASE2_REQ request towards EPMD on the host where the 
	  node resides in order 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">122</cell>
	  <cell align="center">NodeName</cell>
	</row>
      <tcaption>PORT_PLEASE2_REQ (122)</tcaption></table>
      <p>
	where N = Length - 1
      </p>

      <p>
      </p>
      <table align="left">
	<row>
	  <cell align="center">1</cell>
	  <cell align="center">1</cell>
	</row>
	<row>
	  <cell align="center">119</cell>
	  <cell align="center">Result</cell>
	</row>
	<tcaption>
	  PORT2_RESP (119) response indicating error, Result > 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">119</cell>
	  <cell align="center">Result</cell>
	  <cell align="center">PortNo</cell>
	  <cell align="center">NodeType</cell>
	  <cell align="center">Protocol</cell>
	  <cell align="center">HighestVersion</cell>
	  <cell align="center">LowestVersion</cell>
	  <cell align="center">Nlen</cell>
	  <cell align="center">NodeName</cell>
	  <cell align="center">Elen</cell>
	  <cell align="center">Extra</cell>
	</row>
      <tcaption>PORT2_RESP when Result = 0.</tcaption></table>
<p>
If Result > 0, the packet only consists of [119, Result].
</p>

        <p>EPMD will close the socket as soon as it has sent the information.</p>
      </section>

      <section>
	<title>Get all registered names from EPMD</title>
	<p>
	  This request is used via the Erlang function 
	  <c>net_adm:names/1,2</c>. A TCP connection is opened
	  towards EPMD and this request is sent.
	</p>
	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	  </row>
	  <row>
	    <cell align="center">110</cell>
	  </row>
	<tcaption>NAMES_REQ (110)</tcaption></table>
	

	<p>The response for a <c>NAMES_REQ</c> looks like this:</p>
	<table align="left">
	  <row>
	    <cell align="center">4</cell>
	    <cell align="center">&nbsp;</cell>
	  </row>
	  <row>
	    <cell align="center">EPMDPortNo</cell>
	    <cell align="center">NodeInfo*</cell>
	  </row>
	<tcaption>NAMES_RESP</tcaption></table>
	<p>
	  NodeInfo is a string written for each active node.
	  When all NodeInfo has been written the connection is
	  closed by EPMD.
	</p>
	<p>
	  NodeInfo 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 should be regarded as a debug
	  feature.
	</p>
	<table align="left">
	  <row>
	  <cell align="center">1</cell>
	  </row>
	  <row>
	    <cell align="center">100</cell>
	  </row>
	<tcaption>DUMP_REQ</tcaption></table>

	<p>The response for a <c>DUMP_REQ</c> looks like this:</p>
	<table align="left">
	  <row>
	    <cell align="center">4</cell>
	    <cell align="center">&nbsp;</cell>
	  </row>
	  <row>
	    <cell align="center">EPMDPortNo</cell>
	    <cell align="center">NodeInfo*</cell>
	  </row>
	<tcaption>DUMP_RESP</tcaption></table>
	<p>
	  NodeInfo is a string written for each node kept in EPMD.
	  When all NodeInfo has been written the connection is
	  closed by EPMD.
	</p>
	<p>
	  NodeInfo 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 the EPMD</title>
	<p>
	  This request will kill the running EPMD. It is almost never used.
	</p>
	<table align="left">
	  <row>
	  <cell align="center">1</cell>
	  </row>
	  <row>
	    <cell align="center">107</cell>
	  </row>
	<tcaption>KILL_REQ</tcaption></table>

	<p>The response fo a <c>KILL_REQ</c> looks like this:</p>
	<table align="left">
	  <row>
	    <cell align="center">2</cell>
	  </row>
	  <row>
	    <cell align="center">OKString</cell>
	  </row>
	<tcaption>KILL_RESP</tcaption></table>
	<p>
	  where <c>OKString</c> is "OK".
	</p>
      </section>

      <section>
	<title>STOP_REQ  (Not Used)</title>
	<p></p>
	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	    <cell align="center">n</cell>
	  </row>
	  <row>
	    <cell align="center">115</cell>
	    <cell align="center">NodeName</cell>
	  </row>
	<tcaption>STOP_REQ</tcaption></table>
	<p>
	  where n = Length - 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> looks like this.</p>
	<table align="left">
	  <row>
	    <cell align="center">7</cell>
	  </row>
	  <row>
	    <cell align="center">OKString</cell>
	  </row>
	<tcaption>STOP_RESP</tcaption></table>
	<p>
	  where OKString is "STOPPED".
	</p>
	<p>A negative response can look like this.</p>
	<table align="left">
	  <row>
	    <cell align="center">7</cell>
	  </row>
	  <row>
	    <cell align="center">NOKString</cell>
	  </row>
	<tcaption>STOP_NOTOK_RESP</tcaption></table>
	<p>
	  where NOKString is "NOEXIST".
	</p>
      </section>
<!--
      <section>
	<title>ALIVE_REQ (97)</title>
	<p></p>

	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	    <cell align="center">2</cell>
	    <cell align="center">n</cell>
	  </row>
	  <row>
	    <cell align="center">97</cell>
	    <cell align="center">PortNo</cell>
	    <cell align="center">NodeName</cell>
	  </row>
	<tcaption></tcaption></table>

	<p>
	  where n = Length - 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>
	<p></p>
	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	    <cell align="center">2</cell>
	  </row>
	  <row>
	    <cell align="center">89</cell>
	    <cell align="center">Creation</cell>
	  </row>
	<tcaption></tcaption></table>
      </section>


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

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

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

      </section>

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


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

    </section>
      <section>
	<title>Distribution Handshake</title>
	  <p>
            <marker id="distribution_handshake"/>
	    This section describes the distribution handshake protocol
	    introduced in the OTP-R6 release of Erlang/OTP. 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
	    more or less unchanged since the year 1999, but the handshake
	    should not have changed much since then either.
	  </p>
	<section>
	  <title>General</title>
	  <p>
	    The TCP/IP distribution uses a handshake which expects a
	    connection based protocol, i.e. 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 A) 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 very
	    random numbers.
	  </p>
	</section>
	<section>
	  <title>Definitions</title>
	  <p>
	    A challenge is a 32 bit integer number 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 out_cookie is the cookie used in outgoing communication to a certain node,
	    so that A's out_cookie for B should correspond with B's in_cookie for A and 
	    the other way around. A's out_cookie for B and A's in_cookie for B need <em>NOT</em>
	    be the same. Below the function <c>out_cookie(Node)</c> returns the current
	    node's out_cookie for <c>Node</c>.
	  </p>
	  <p>
	    An in_cookie is the cookie expected to be used by another node when 
	    communicating with us, so that A's in_cookie for B corresponds with B's 
	    out_cookie for A. 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 length of the message (not counting the two initial bytes).
	    In erlang this corresponds to the <c>gen_tcp</c> option <c>{packet, 2}</c>. Note 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, node A, which initiates the handshake and node B, which
	    accepts the connection.
	  </p>
	  <taglist>
	    <tag>1) connect/accept</tag>
	    <item><p>A connects to B via TCP/IP and B accepts the connection.</p></item>
	    <tag>2) send_name/receive_name</tag>
	    <item><p>A sends an initial identification to B. B receives the message.
	    The message looks like this (every "square" being one byte and the packet
	    header removed):
	  </p>
<pre>
+---+--------+--------+-----+-----+-----+-----+-----+-----+-...-+-----+
|'n'|Version0|Version1|Flag0|Flag1|Flag2|Flag3|Name0|Name1| ... |NameN|
+---+--------+--------+-----+-----+-----+-----+-----+-----+-... +-----+
</pre>
	  <p>
	    The 'n' is just a message tag.
	    Version0 and Version1 is the distribution version selected by node A,
            based on information from EPMD. (16 bit big endian)
	    Flag0 ... Flag3 are capability flags, the capabilities defined in
	    <c>$ERL_TOP/lib/kernel/include/dist.hrl</c>.
            (32 bit big endian)
	    Name0 ... NameN is the full nodename of A, as a string of bytes (the
	    packet length denotes how long it is).
	  </p></item>
	  <tag>3) recv_status/send_status</tag>
	  <item><p>B sends a status message to A, which indicates
	    if the connection is allowed. The following status codes are defined:</p>
	    <taglist>
	      <tag><c>ok</c></tag>
	      <item>The handshake will continue.</item>
	      <tag><c>ok_simultaneous</c></tag>
	      <item>The handshake will continue, but A is informed that B
              has another ongoing connection attempt that will be
	      shut down (simultaneous connect where A's name is 
	      greater than B's name, compared literally).</item>
	      <tag><c>nok</c></tag>
	      <item>The handshake will not continue, as B already has an ongoing handshake
	      which it itself has initiated. (simultaneous connect where B's name is 
	      greater than A's).</item>
	      <tag><c>not_allowed</c></tag>
	      <item>The connection is disallowed for some (unspecified) security 
              reason.</item>
	      <tag><c>alive</c></tag>
	      <item>A connection to the node is already active, which either means
	      that node A is confused or that the TCP connection breakdown
	      of a previous node with this name has not yet reached node B.
	      See 3B below.</item>
	    </taglist>
	    <p>This is the format of the status message:</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) send_status/recv_status</tag>
	  <item><p>If status was 'alive', node A will answer with
	  another status message containing either 'true' which means that the
	  connection should continue (The old connection from this node is broken), or
	  <c>'false'</c>, which simply means that the connection should be closed, the 
	  connection attempt was a mistake.</p></item>
	  <tag>4) recv_challenge/send_challenge</tag>
	  <item><p>If the status was <c>ok</c> or <c>ok_simultaneous</c>,
	  The handshake continues with B sending A another message, the challenge.
	  The challenge contains the same type of information as the "name" message 
	  initially sent from A to B, with the addition of a 32 bit challenge:</p>
<pre>
+---+--------+--------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-...-+-----+
|'n'|Version0|Version1|Flag0|Flag1|Flag2|Flag3|Chal0|Chal1|Chal2|Chal3|Name0|Name1| ... |NameN|
+---+--------+--------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-... +-----+
</pre>
          <p>
	    Where Chal0 ... Chal3 is the challenge as a 32 bit big endian integer
	    and the other fields are B's version, flags and full nodename.
	  </p></item>
	  <tag>5) send_challenge_reply/recv_challenge_reply</tag>
	  <item><p>Now A has generated a digest and its own challenge. Those are
	  sent together in a package to B:</p>
<pre>
+---+-----+-----+-----+-----+-----+-----+-----+-----+-...-+------+
|'r'|Chal0|Chal1|Chal2|Chal3|Dige0|Dige1|Dige2|Dige3| ... |Dige15|
+---+-----+-----+-----+-----+-----+-----+-----+-----+-...-+------+
</pre>
          <p>
	    Where 'r' is the tag, Chal0 ... Chal3 is A's challenge for B to handle and
	    Dige0 ... Dige15 is the digest that A constructed from the challenge B sent
	    in the previous step.
	  </p></item>
	  <tag>6) recv_challenge_ack/send_challenge_ack</tag>
	  <item><p>B checks that the digest received from A is correct and generates a
	  digest from the challenge received from  A. The digest is then sent to A. The
	  message looks like this:</p>
<pre>
+---+-----+-----+-----+-----+-...-+------+
|'a'|Dige0|Dige1|Dige2|Dige3| ... |Dige15|
+---+-----+-----+-----+-----+-...-+------+
</pre>
          <p>
	    Where 'a' is the tag and Dige0 ... Dige15 is the digest calculated by B
	    for A's challenge.</p></item>
	  <tag>7)</tag>
	  <item><p>A checks the digest from B 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
			(DiB)					(ChA, OCB)
	    &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>
	<marker id="dflags"/>
	<section>
	  <title>The Currently Defined Distribution Flags</title>
	  <p>
	    Currently (OTP-R16) the following capability flags are defined:
	  </p>
<pre>
%% The node should be published and part of the global namespace
-define(DFLAG_PUBLISHED,1).

%% The node implements an atom cache (obsolete)
-define(DFLAG_ATOM_CACHE,2).

%% The node implements extended (3 * 32 bits) references. This is
%% required today. If not present connection will be refused.
-define(DFLAG_EXTENDED_REFERENCES,4).

%% The node implements distributed process monitoring.
-define(DFLAG_DIST_MONITOR,8).

%% The node uses separate tag for fun's (lambdas) in the distribution protocol.
-define(DFLAG_FUN_TAGS,16#10).

%% The node implements distributed named process monitoring.
-define(DFLAG_DIST_MONITOR_NAME,16#20).

%% The (hidden) node implements atom cache (obsolete)
-define(DFLAG_HIDDEN_ATOM_CACHE,16#40).

%% The node understand new fun-tags
-define(DFLAG_NEW_FUN_TAGS,16#80).

%% The node is capable of handling extended pids and ports. This is
%% required today. If not present connection will be refused.
-define(DFLAG_EXTENDED_PIDS_PORTS,16#100).

%%
-define(DFLAG_EXPORT_PTR_TAG,16#200).

%%
-define(DFLAG_BIT_BINARIES,16#400).

%% The node understands new float format
-define(DFLAG_NEW_FLOATS,16#800).

%%
-define(DFLAG_UNICODE_IO,16#1000).

%% The node implements atom cache in distribution header.
-define(DFLAG_DIST_HDR_ATOM_CACHE,16#2000).

%% The node understand the SMALL_ATOM_EXT tag
-define(DFLAG_SMALL_ATOM_TAGS, 16#4000).

%% The node understand UTF-8 encoded atoms
-define(DFLAG_UTF8_ATOMS, 16#10000).

</pre>
	</section>
      </section>

      <section>
        <marker id="connected_nodes"/>
	<title>Protocol between connected nodes</title>
	<p>
	  As of erts version 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 are in this case on 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></tcaption></table>
	<p>
	  where:
	</p>
	<p>
	  <c>Length</c> is equal to d + n + m
	</p>
	<p>
	  <c>ControlMessage</c> is a tuple passed using the external format of
	  Erlang.
	</p>
	<p>
	  <c>Message</c> is the message sent to another node using the '!'
	  (in external format). Note that <c>Message</c> is only passed in
	  combination with a <c>ControlMessage</c> encoding a send ('!').
	</p>
	<p>
	  Also note 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 less than 5.7.2 does not pass the
	  distribution flag that enables the distribution header. Messages
	  passed between nodes are in this case on 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></tcaption></table>
	<p>
	  where:
	</p>
	<p>
	  <c>Length</c> is equal to 1 + n + m
  	</p>
	<p>
	  Type is: 112 (pass through)
	</p>
	<p>
	  <c>ControlMessage</c> is a tuple passed using the external format of
	  Erlang.
	</p>
	<p>
	  <c>Message</c> is the message sent to another node using the '!'
	  (in external format).  Note that <c>Message</c> is only passed in
	  combination with a <c>ControlMessage</c> encoding a send ('!').
	</p>
	<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, Cookie, ToPid}</c>
	    </p>
	    <p>
	    <em>Note</em> followed by <c>Message</c>
	    </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, Cookie, ToName}</c>
	    </p>
	    <p>
	    <em>Note</em> followed by <c>Message</c>
	    </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 (OTP R4)</title>
      <taglist>
	<tag><c>SEND_TT</c></tag>
	<item>
	<p>
	  <c>{12, Cookie, ToPid, TraceToken}</c>
	</p>
	<p>
	<em>Note</em> followed by <c>Message</c>
	</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, Cookie, ToName, TraceToken}</c>
	</p>
	<p>
	<em>Note</em> followed by <c>Message</c>
	</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>
	distrvsn 2 was never used.
      </p>
    </section>
    
    <section>
      <title>New Ctrlmessages for distrvsn = 3 (OTP R5C)</title>
      <p>
	None, but the version number was increased anyway.
      </p>
    </section>

    <section>
      <title>New Ctrlmessages for distrvsn = 4 (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>
	  
	  <c>FromPid</c> = monitoring process
	  <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>
	  We include the FromPid just in case we want to trace this.
	  
	  <c>FromPid</c> = monitoring process
	  <c>ToProc</c> = monitored process pid or name (atom)
	</p>
	</item>

	<tag><c>MONITOR_P_EXIT</c></tag>
	<item>
	<p>
	  <c>{21, FromProc, ToPid, Ref, Reason}</c>
	  
	  <c>FromProc</c> = monitored process pid or name (atom)
	  <c>ToPid</c> = monitoring process
	  <c>Reason</c> = exit reason for the monitored process
	</p>
	</item>
      </taglist>
    </section>
  </chapter>