path: root/lib/asn1/doc/src/asn1_spec.xmlsrc

<?xml version="1.0" encoding="iso-8859-1" ?>
<!DOCTYPE chapter SYSTEM "chapter.dtd">

<chapter>
  <header>
    <copyright>
      <year>2003</year><year>2011</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
      compliance with the License. You should have received a copy of the
      Erlang Public License along with this software. If not, it can be
      retrieved online at http://www.erlang.org/.
    
      Software distributed under the License is distributed on an "AS IS"
      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
      the License for the specific language governing rights and limitations
      under the License.
    
    </legalnotice>

    <title>Specialized Decodes</title>
    <prepared>EAB/UAB/UKH/KD Bertil Karlsson</prepared>
    <docno></docno>
    <date>2003-04-24</date>
    <rev>D</rev>
    <file>asn1_spec.xml</file>
  </header>
  <marker id="SpecializedDecodes"></marker>
  <p>When performance is of highest priority and one is interested in
    a limited part of the ASN.1 encoded message, before one decide what
    to do with the rest of it, one may want to decode only this small
    part. The situation may be a server that has to decide to which
    addressee it will send a message. The addressee may be interested in
    the entire message, but the server may be a bottleneck that one want
    to spare any unnecessary load. Instead of making two <em>complete decodes</em> (the normal case of decode), one in the server and one
    in the addressee, it is only necessary to make one <em>specialized decode</em>(in the server) and another complete decode(in the
    addressee). The following specialized decodes <em>exclusive decode</em> and <em>selected decode</em> support to solve this and
    similar problems.
    </p>
  <p>So far this functionality is only provided when using the
    optimized BER_BIN version, that is when compiling with the
    options <c>ber_bin</c> and <c>optimize</c>. It does also work
    using the <c>nif</c> option. We have no intent to make this
    available on the default BER version, but maybe in the PER_BIN
    version (<c>per_bin</c>).
    </p>

  <section>
    <title>Exclusive Decode</title>
    <p>The basic idea with exclusive
      decode is that you specify which parts of the message you want to
      exclude from being decoded. These parts remain encoded and are
      returned in the value structure as binaries. They may be decoded
      in turn by passing them to a certain <c>decode_part/2</c>
      function. The performance gain is high when the message is large
      and you can do an exclusive decode and later on one or several
      decodes of the parts or a second complete decode instead of two or
      more complete decodes.
      </p>

    <section>
      <title>How To Make It Work</title>
      <p>In order to make exclusive decode work you have to do the
        following:
        </p>
      <list type="bulleted">
        <item>First,decide the name of the function for the exclusive
         decode.</item>
        <item>Second, write instructions that must consist of the name
         of the exclusive decode function, the name of the ASN.1
         specification and a notation that tells which parts of the
         message structure will be excluded from decode. These
         instructions shall be included in a configuration
         file. </item>
        <item>Third, compile with the additional option
        <c>asn1config</c>. The compiler searches for a configuration
         file with the same name as the ASN.1 spec but with the
         extension .asn1config. This configuration file is not the same
         as used for compilation of a set of files. See section
        <seealso marker="#UndecodedPart">Writing an Exclusive Decode Instruction.</seealso></item>
      </list>
    </section>

    <section>
      <title>User Interface</title>
      <p>The run-time user interface for exclusive decode consists of
        two different functions. First, the function for an exclusive
        decode, whose name the user decides in the configuration
        file. Second, the compiler generates a <c>decode_part/2</c>
        function when exclusive decode is chosen. This function decodes
        the parts that were left undecoded during the exclusive
        decode. Both functions are described below.
        </p>
      <p>If the exclusive decode function has for example got the name
        <c>decode_exclusive</c> and an ASN.1 encoded message
        <c>Bin</c> shall be exclusive decoded, the call is:</p>
      <pre>
{ok,Excl_Message} = 'MyModule':decode_exclusive(Bin)      </pre>
      <marker id="UndecodedPart"></marker>
      <p>The result <c>Excl_Message</c> has the same structure as an
        complete decode would have, except for the parts of the top-type
        that were not decoded. The undecoded parts will be on their place
        in the structure on the format <c>{Type_Key,Undecoded_Value}</c>.
        </p>
      <p>Each undecoded part that shall be decoded must be fed into the <c>decode_part/2</c> function,like:</p>
      <pre>
{ok,Part_Message} = 'MyModule':decode_part(Type_Key,Undecoded_Value)      </pre>
    </section>

    <section>
      <marker id="Exclusive Instruction"></marker>
      <title>Writing an Exclusive Decode Instruction</title>
      <p>This instruction is written in the configuration file on the
        format:</p>
      <pre>

Exclusive_Decode_Instruction = {exclusive_decode,{Module_Name,Decode_Instructions}}.

Module_Name = atom()

Decode_Instructions = [Decode_Instruction]+

Decode_Instruction = {Exclusive_Decode_Function_Name,Type_List}

Exclusive_Decode_Function_Name = atom()

Type_List = [Top_Type,Element_List]

Element_List = [Element]+

Element = {Name,parts} |
          {Name,undecoded} |
          {Name,Element_List}

Top_Type = atom()

Name = atom()
      </pre>
      <p>Observe that the instruction must be a valid Erlang term ended
        by a dot.
        </p>
      <p>In the <c>Type_List</c> the "path" from the top type to each
        undecoded sub-components is described. The top type of the path is
        an atom, the name of it. The action on each component/type that
        follows will be described by one of <c>{Name,parts}, {Name,undecoded}, {Name,Element_List}</c></p>
      <p>The use and effect of the actions are:
        </p>
      <list type="bulleted">
        <item><c>{Name,undecoded}</c> Tells that the element will be
         left undecoded during the exclusive decode. The type of Name may
         be any ASN.1 type. The value of element Name will be returned as a
         tuple,as mentioned <seealso marker="#UndecodedPart">above</seealso>, in the value structure of the top type.</item>
        <item><c>{Name,parts}</c> The type of Name may be one of
         SEQUENCE OF or SET OF. The action implies that the different
         components of Name will be left undecoded. The value of Name
         will be returned as a tuple, as <seealso marker="#UndecodedPart">above </seealso>, where the second element is a list of
         binaries. That is because the representation of a SEQUENCE OF/
         SET OF in Erlang is a list of its internal type. Any of the
         elements of this list or the entire list can be decoded by the
        <c>decode_part</c> function.</item>
        <item><c>{Name,Element_List}</c>This action is used when one or
         more of the sub-types of Name will be exclusive decoded.</item>
      </list>
      <p>Name in the actions above may be a component name of a
        SEQUENCE or a SET or a name of an alternative in a CHOICE.
        </p>
    </section>

    <section>
      <title>Example</title>
      <p>In the examples below we use the definitions from the following ASN.1 spec:</p>
      <marker id="Asn1spec"></marker>
      <codeinclude file="Seq.asn" tag="" type="none"></codeinclude>
      <p>If <c>Button</c> is a top type and we want to exclude
        component <c>number</c> from decode the Type_List in the
        instruction in the configuration file will be
        <c>['Button',[{number,undecoded}]]</c>. If we call the decode
        function <c>decode_Button_exclusive</c> the Decode_Instruction
        will be
        <c>{decode_Button_exclusive,['Button',[{number,undecoded}]]}</c>.
        </p>
      <p>We also have another top type <c>Window</c> whose sub
        component actions in type <c>Status</c> and the parts of component
        <c>buttonList</c> shall be left undecoded. For this type we name
        the function <c>decode__Window_exclusive</c>. The whole
        Exclusive_Decode_Instruction configuration is as follows: </p>
      <codeinclude file="Seq.asn1config" tag="" type="none"></codeinclude>
      <p></p>
      <image file="exclusive_Win_But.gif">
        <icaption>Figure symbolizes the bytes of a Window:status message. The components buttonList and actions are excluded from decode. Only state and enabled are decoded when decode__Window_exclusive is called. </icaption>
      </image>
      <p></p>
      <p>Compiling GUI.asn including the configuration file is done like:</p>
      <pre>
unix> erlc -bber_bin +optimize +asn1config GUI.asn

erlang> asn1ct:compile('GUI',[ber_bin,optimize,asn1config]).      </pre>
      <p>The module can be used like:</p>
      <pre>

1> Button_Msg = {'Button',123,true}.
{'Button',123,true}
2> {ok,Button_Bytes} = 'GUI':encode('Button',Button_Msg).
{ok,[&lt;&lt;48&gt;&gt;,
     [6],
     [&lt;&lt;128&gt;&gt;,
      [1],
      123],
     [&lt;&lt;129&gt;&gt;,
      [1],
      255]]}
3> {ok,Exclusive_Msg_Button} = 'GUI':decode_Button_exclusive(list_to_binary(Button_Bytes)).
{ok,{'Button',{'Button_number',&lt;&lt;28,1,123&gt;&gt;},
         true}}
4> 'GUI':decode_part('Button_number',&lt;&lt;128,1,123&gt;&gt;).
{ok,123}
5> Window_Msg = 
{'Window',{status,{'Status',35,
              [{'Button',3,true},
               {'Button',4,false},
               {'Button',5,true},
               {'Button',6,true},
               {'Button',7,false},
               {'Button',8,true},
               {'Button',9,true},
               {'Button',10,false},
               {'Button',11,true},
               {'Button',12,true},
               {'Button',13,false},
               {'Button',14,true}],
              false,
              {possibleActions,[{'Action',16,{'Button',17,true}}]}}}}. 
{'Window',{status,{'Status',35,
              [{'Button',3,true},
               {'Button',4,false},
               {'Button',5,true},
               {'Button',6,true},
               {'Button',7,false},
               {'Button',8,true},
               {'Button',9,true},
               {'Button',10,false},
               {'Button',11,true},
               {'Button',12,true},
               {'Button',13,false},
               {'Button',14,true}],
              false,
              {possibleActions,[{'Action',16,{'Button',17,true}}]}}}}
6> {ok,Window_Bytes}='GUI':encode('Window',Window_Msg).
{ok,[&lt;&lt;161&gt;&gt;,
     [127],
     [&lt;&lt;128&gt;&gt;, ...


8> {ok,{status,{'Status',Int,{Type_Key_SeqOf,Val_SEQOF},
BoolOpt,{Type_Key_Choice,Val_Choice}}}}=
'GUI':decode_Window_status_exclusive(list_to_binary(Window_Bytes)).
{ok,{status,{'Status',35,
        {'Status_buttonList',[&lt;&lt;48,6,128,1,3,129,1,255&gt;&gt;,
                              &lt;&lt;48,6,128,1,4,129,1,0&gt;&gt;,
                              &lt;&lt;48,6,128,1,5,129,1,255&gt;&gt;,
                              &lt;&lt;48,6,128,1,6,129,1,255&gt;&gt;,
                              &lt;&lt;48,6,128,1,7,129,1,0&gt;&gt;,
                              &lt;&lt;48,6,128,1,8,129,1,255&gt;&gt;,
                              &lt;&lt;48,6,128,1,9,129,1,255&gt;&gt;,
                              &lt;&lt;48,6,128,1,10,129,1,0&gt;&gt;,
                              &lt;&lt;48,6,128,1,11,129,1,255&gt;&gt;,
                              &lt;&lt;48,6,128,1,12,129,1,255&gt;&gt;,
                              &lt;&lt;48,6,128,1,13,129,1,0&gt;&gt;,
                              &lt;&lt;48,6,128,1,14,129,1,255&gt;&gt;]},
        false,
        {'Status_actions',
&lt;&lt;163,21,160,19,48,17,2,1,16,160,12,172,10,171,8,48,6,128,1,...&gt;&gt;}}}}
10> 'GUI':decode_part(Type_Key_SeqOf,Val_SEQOF).
{ok,[{'Button',3,true},
     {'Button',4,false},
     {'Button',5,true},
     {'Button',6,true},
     {'Button',7,false},
     {'Button',8,true},
     {'Button',9,true},
     {'Button',10,false},
     {'Button',11,true},
     {'Button',12,true},
     {'Button',13,false},
     {'Button',14,true}]}
11> 'GUI':decode_part(Type_Key_SeqOf,hd(Val_SEQOF)).
{ok,{'Button',3,true}}
12> 'GUI':decode_part(Type_Key_Choice,Val_Choice).  
{ok,{possibleActions,[{'Action',16,{'Button',17,true}}]}}
      </pre>
    </section>
  </section>

  <section>
    <title>Selective Decode</title>
    <p>This specialized decode decodes one single subtype of a
      constructed value. It is the fastest method to extract one sub
      value. The typical use of this decode is when one want to
      inspect, for instance a version number,to be able to decide what
      to do with the entire value. The result is returned as
      <c>{ok,Value}</c> or <c>{error,Reason}</c>.
      </p>

    <section>
      <title>How To Make It Work</title>
      <p>The following steps are necessary:
        </p>
      <list type="bulleted">
        <item>Write instructions in the configuration
         file. Including the name of a user function, the name of the ASN.1
         specification and a notation that tells which part of the type
         will be decoded. </item>
        <item>Compile with the additional option
        <c>asn1config</c>. The compiler searches for a configuration file
         with the same name as the ASN.1 spec but with the extension
         .asn1config. In the same file you can provide configuration specs
         for exclusive decode as well. The generated Erlang module has the
         usual functionality for encode/decode preserved and the
         specialized decode functionality added. </item>
      </list>
    </section>

    <section>
      <title>User Interface</title>
      <p>The only new user interface function is the one provided by the
        user in the configuration file. You can invoke that function by
        the <c>ModuleName:FunctionName</c> notation.
        </p>
      <p>So, if you have the following spec
        <c>{selective_decode,{'ModuleName',[{selected_decode_Window,TypeList}]}}</c>
        in the con-fig file, you do the selective decode by
        <c>{ok,Result}='ModuleName':selected_decode_Window(EncodedBinary).</c></p>
    </section>

    <section>
      <marker id="Selective Instruction"></marker>
      <title>Writing a Selective Decode Instruction</title>
      <p>It is possible to describe one or many selective decode
        functions in a configuration file, you have to use the following
        notation:</p>
      <pre>
Selective_Decode_Instruction = {selective_decode,{Module_Name,Decode_Instructions}}.

Module_Name = atom()

Decode_Instructions = [Decode_Instruction]+

Decode_Instruction = {Selective_Decode_Function_Name,Type_List}

Selective_Decode_Function_Name = atom()

Type_List = [Top_Type|Element_List]

Element_List = Name|List_Selector

Name = atom()

List_Selector = [integer()]      </pre>
      <p>Observe that the instruction must be a valid Erlang term ended
        by a dot.
        </p>
      <p>The <c>Module_Name</c> is the same as the name of the ASN.1
        spec, but without the extension. A <c>Decode_Instruction</c> is
        a tuple with your chosen function name and the components from
        the top type that leads to the single type you want to
        decode. Notice that you have to choose a name of your function
        that will not be the same as any of the generated functions. The
        first element of the <c>Type_List</c> is the top type of the
        encoded message. In the <c>Element_List</c> it is followed by
        each of the component names that leads to selected type. Each of
        the names in the <c>Element_List</c> must be constructed types
        except the last name, which can be any type.
        </p>
      <p>The List_Selector makes it possible to choose one of the
        encoded components in a SEQUENCE OF/ SET OF. It is also possible
        to go further in that component and pick a sub type of that to
        decode. So in the <c>Type_List</c>: <c>['Window',status,buttonList,[1],number]</c> the
        component <c>buttonList</c> has to be a SEQUENCE OF or SET OF type. In
        this example component <c>number</c> of the first of the encoded
        elements in the SEQUENCE OF <c>buttonList</c> is selected. This apply on
        the ASN.1 spec <seealso marker="#Asn1spec">above</seealso>.
        </p>
    </section>

    <section>
      <title>Another Example</title>
      <p>In this example we use the same ASN.1 spec as <seealso marker="#Asn1spec">above</seealso>. A valid selective decode
        instruction is:</p>
      <pre>
{selective_decode,
    {'GUI',
        [{selected_decode_Window1,
            ['Window',status,buttonList, 
             [1],
             number]},
 {selected_decode_Action,
     ['Action',handle,number]},
 {selected_decode_Window2,
     ['Window',
      status,
      actions,
      possibleActions,
      [1],
      handle,number]}]}}.
      </pre>
      <p>The first <c>Decode_Instruction</c>,
        <c>{selected_decode_Window1,['Window',status,buttonList,[1],number]}</c>
        is commented in the previous section. The instruction
        <c>{selected_decode_Action,['Action',handle,number]}</c> picks
        the component <c>number</c> in the <c>handle</c> component of the type
        <c>Action</c>. If we have the value <c>ValAction = {'Action',17,{'Button',4711,false}}</c> the internal value 4711
        should be picked by <c>selected_decode_Action</c>. In an Erlang
        terminal it looks like:</p>
      <pre>
ValAction = {'Action',17,{'Button',4711,false}}.
{'Action',17,{'Button',4711,false}}
7> {ok,Bytes}='GUI':encode('Action',ValAction).
...
8> BinBytes = list_to_binary(Bytes).
&lt;&lt;48,18,2,1,17,160,13,172,11,171,9,48,7,128,2,18,103,129,1,0&gt;&gt;
9> 'GUI':selected_decode_Action(BinBytes).
{ok,4711}
10>       </pre>
      <p>The third instruction,
        <c>['Window',status,actions,possibleActions,[1],handle,number]</c>,
        which is a little more complicated,</p>
      <list type="bulleted">
        <item>starts with type <em>Window</em>. </item>
        <item>Picks component <em>status</em> of <c>Window</c> that is
         of type <c>Status</c>.</item>
        <item>Then takes component <em>actions</em> of type
        <c>Status</c>.</item>
        <item>Then <em>possibleActions</em> of the internal defined
         CHOICE type.</item>
        <item>Thereafter it goes into the first component of the
         SEQUENCE OF by <em>[1]</em>. That component is of type
        <c>Action</c>.</item>
        <item>The instruction next picks component
        <em>handle</em>.</item>
        <item>And finally component <em>number</em> of the type
        <c>Button</c>.</item>
      </list>
      <p>The following figures shows which components are in the
        TypeList
        <c>['Window',status,actions,possibleActions,[1],handle,number]</c>. And
        which part of a message that will be decoded by
        selected_decode_Window2.
        </p>
      <p></p>
      <image file="selective_TypeList.gif">
        <icaption>The elements specified in the config file for selective decode of a sub-value in a Window message</icaption>
      </image>
      <p></p>
      <image file="selective_Window2.gif">
        <icaption>Figure symbolizes the bytes of a Window:status message. Only the marked element is decoded when selected_decode_Window2 is called. </icaption>
      </image>
      <p>With the following example you can examine that both
        <c>selected_decode_Window2</c> and
        <c>selected_decode_Window1</c> decodes the intended sub-value
        of the value <c>Val</c></p>
      <pre>
1> Val = {'Window',{status,{'Status',12,
                    [{'Button',13,true},
                     {'Button',14,false},
                     {'Button',15,true},
                     {'Button',16,false}],
                    true,
                    {possibleActions,[{'Action',17,{'Button',18,false}},
                                      {'Action',19,{'Button',20,true}},
                                      {'Action',21,{'Button',22,false}}]}}}}
2> {ok,Bytes}='GUI':encode('Window',Val).
...
3> Bin = list_to_binary(Bytes).
&lt;&lt;161,101,128,1,12,161,32,48,6,128,1,13,129,1,255,48,6,128,1,14,129,1,0,48,6,128,1,15,129,...&gt;&gt;
4> 'GUI':selected_decode_Window1(Bin).
{ok,13}
5> 'GUI':selected_decode_Window2(Bin).
{ok,18}      </pre>
      <p>Observe that the value feed into the selective decode
        functions must be a binary.
        </p>
    </section>
  </section>

  <section>
    <title>Performance</title>
    <p>To give an indication on the possible performance gain using
      the specialized decodes, some measures have been performed. The
      relative figures in the outcome between selective, exclusive and
      complete decode (the normal case) depends on the structure of
      the type, the size of the message and on what level the
      selective and exclusive decodes are specified.
      </p>

    <section>
      <title>ASN.1 Specifications, Messages and Configuration</title>
      <p>The specs <seealso marker="#Asn1spec">GUI</seealso> and
        <url href="http://www.itu.int/ITU-T/asn1/database/itu-t/h/h248/2002/MEDIA-GATEWAY-CONTROL.html">MEDIA-GATEWAY-CONTROL</url>
        was used in the test.
        </p>
      <p>For the GUI spec the configuration looked like:</p>
      <pre>
{selective_decode,
  {'GUI',
    [{selected_decode_Window1,
         ['Window',
          status,buttonList,
          [1],
          number]},
     {selected_decode_Window2,
         ['Window',
          status,
          actions,
          possibleActions,
          [1],
          handle,number]}]}}.
     {exclusive_decode,
         {'GUI',
            [{decode_Window_status_exclusive,
                ['Window',
                 [{status,
                     [{buttonList,parts},
                      {actions,undecoded}]}]]}]}}.
      </pre>
      <p>The MEDIA-GATEWAY-CONTROL configuration was:</p>
      <pre>
{exclusive_decode,
  {'MEDIA-GATEWAY-CONTROL',
    [{decode_MegacoMessage_exclusive,
        ['MegacoMessage',
         [{authHeader,undecoded},
          {mess,
             [{mId,undecoded},
              {messageBody,undecoded}]}]]}]}}.
{selective_decode,
  {'MEDIA-GATEWAY-CONTROL',
    [{decode_MegacoMessage_selective,
         ['MegacoMessage',mess,version]}]}}.
      </pre>
      <p>The corresponding values were:</p>
      <pre>
{'Window',{status,{'Status',12,
              [{'Button',13,true},
               {'Button',14,false},
               {'Button',15,true},
               {'Button',16,false},
               {'Button',13,true},
               {'Button',14,false},
               {'Button',15,true},
               {'Button',16,false},
               {'Button',13,true},
               {'Button',14,false},
               {'Button',15,true},
               {'Button',16,false}],
              true,
              {possibleActions,
                 [{'Action',17,{'Button',18,false}},
                  {'Action',19,{'Button',20,true}},
                  {'Action',21,{'Button',22,false}},
                  {'Action',17,{'Button',18,false}},
                  {'Action',19,{'Button',20,true}},
                  {'Action',21,{'Button',22,false}},
                  {'Action',17,{'Button',18,false}},
                  {'Action',19,{'Button',20,true}},
                  {'Action',21,{'Button',22,false}},
                  {'Action',17,{'Button',18,false}},
                  {'Action',19,{'Button',20,true}},
                  {'Action',21,{'Button',22,false}},
                  {'Action',17,{'Button',18,false}},
                  {'Action',19,{'Button',20,true}},
                  {'Action',21,{'Button',22,false}},
                  {'Action',17,{'Button',18,false}},
                  {'Action',19,{'Button',20,true}},
                  {'Action',21,{'Button',22,false}}]}}}}


{'MegacoMessage',asn1_NOVALUE,
  {'Message',1,
    {ip4Address,
      {'IP4Address',[125,125,125,111],55555}},
  {transactions,
    [{transactionReply,
      {'TransactionReply',50007,asn1_NOVALUE,
       {actionReplies,
        [{'ActionReply',0,asn1_NOVALUE,asn1_NOVALUE,
          [{auditValueReply,{auditResult,{'AuditResult',
            {'TerminationID',[],[255,255,255]},
             [{mediaDescriptor,
               {'MediaDescriptor',asn1_NOVALUE,
                {multiStream,
                 [{'StreamDescriptor',1,
                   {'StreamParms',
                    {'LocalControlDescriptor',
                     sendRecv,
                     asn1_NOVALUE,
                     asn1_NOVALUE,
                     [{'PropertyParm',
                       [0,11,0,7],
                       [[52,48]],
                       asn1_NOVALUE}]},
                    {'LocalRemoteDescriptor',
                     [[{'PropertyParm',
                        [0,0,176,1],
                        [[48]],
                        asn1_NOVALUE},
                       {'PropertyParm',
                         [0,0,176,8],
                         [[73,78,32,73,80,52,32,49,50,53,46,49,
                           50,53,46,49,50,53,46,49,49,49]],
                         asn1_NOVALUE},
                       {'PropertyParm',
                         [0,0,176,15],
                         [[97,117,100,105,111,32,49,49,49,49,32,
                           82,84,80,47,65,86,80,32,32,52]],
                         asn1_NOVALUE},
                       {'PropertyParm',
                         [0,0,176,12],
                         [[112,116,105,109,101,58,51,48]],
                         asn1_NOVALUE}]]},
                    {'LocalRemoteDescriptor',
                     [[{'PropertyParm',
                         [0,0,176,1],
                         [[48]],
                         asn1_NOVALUE},
                       {'PropertyParm',
                         [0,0,176,8],
                         [[73,78,32,73,80,52,32,49,50,52,46,49,50,
                           52,46,49,50,52,46,50,50,50]],
                         asn1_NOVALUE},
                       {'PropertyParm',
                         [0,0,176,15],
                         [[97,117,100,105,111,32,50,50,50,50,32,82,
                           84,80,47,65,86,80,32,32,52]],
                         asn1_NOVALUE},
                       {'PropertyParm',
                         [0,0,176,12],
                         [[112,116,105,109,101,58,51,48]],
                         asn1_NOVALUE}]]}}}]}}},
              {packagesDescriptor,
               [{'PackagesItem',[0,11],1},
                {'PackagesItem',[0,11],1}]},
              {statisticsDescriptor,
               [{'StatisticsParameter',[0,12,0,4],[[49,50,48,48]]},
                {'StatisticsParameter',[0,11,0,2],[[54,50,51,48,48]]},
                {'StatisticsParameter',[0,12,0,5],[[55,48,48]]},
                {'StatisticsParameter',[0,11,0,3],[[52,53,49,48,48]]},
                {'StatisticsParameter',[0,12,0,6],[[48,46,50]]},
                {'StatisticsParameter',[0,12,0,7],[[50,48]]},
                {'StatisticsParameter',[0,12,0,8],[[52,48]]}]}]}}}]}]}}}]}}}      
      </pre>
      <p>The size of the encoded values was 458 bytes for GUI and 464
        bytes for MEDIA-GATEWAY-CONTROL.
        </p>
    </section>

    <section>
      <title>Results</title>
      <p>The ASN.1 specs in the test are compiled with the options
        <c>ber_bin, optimize, driver</c> and <c>asn1config</c>. If the
        <c>driver</c> option had been omitted there should have been
        higher values for <c>decode</c> and <c>decode_part</c>. These tests have
	not been re-run using nifs, but are expected to perform about 5% better
	than the linked-in driver.
        </p>
      <p>The test program runs 10000 decodes on the value, resulting
        in a printout with the elapsed time in microseconds for the
        total number of decodes.
        </p>
      <table>
        <row>
          <cell align="left" valign="top"><em>Function</em></cell>
          <cell align="left" valign="top"><em>Time</em>(microseconds)</cell>
          <cell align="left" valign="top"><em>Kind of Decode</em></cell>
          <cell align="left" valign="top"><em>ASN.1 spec</em></cell>
          <cell align="left" valign="top"><em>% of time vs. complete decode</em></cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>decode_MegacoMessage_selective/1</c></cell>
          <cell align="left" valign="middle"><c>374045</c></cell>
          <cell align="left" valign="middle"><c>selective</c></cell>
          <cell align="left" valign="middle"><c>MEDIA-GATEWAY-CONTROL</c></cell>
          <cell align="left" valign="middle"><em>8.3</em></cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>decode_MegacoMessage_exclusive/1</c></cell>
          <cell align="left" valign="middle"><c>621107</c></cell>
          <cell align="left" valign="middle"><c>exclusive</c></cell>
          <cell align="left" valign="middle"><c>MEDIA-GATEWAY-CONTROL</c></cell>
          <cell align="left" valign="middle"><em>13.8</em></cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>decode/2</c></cell>
          <cell align="left" valign="middle"><c>4507457</c></cell>
          <cell align="left" valign="middle"><c>complete</c></cell>
          <cell align="left" valign="middle"><c>MEDIA-GATEWAY-CONTROL</c></cell>
          <cell align="left" valign="middle"><em>100</em></cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>selected_decode_Window1/1</c></cell>
          <cell align="left" valign="middle"><c>449585</c></cell>
          <cell align="left" valign="middle"><c>selective</c></cell>
          <cell align="left" valign="middle"><c>GUI</c></cell>
          <cell align="left" valign="middle"><em>7.6</em></cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>selected_decode_Window2/1</c></cell>
          <cell align="left" valign="middle"><c>890666</c></cell>
          <cell align="left" valign="middle"><c>selective</c></cell>
          <cell align="left" valign="middle"><c>GUI</c></cell>
          <cell align="left" valign="middle"><em>15.1</em></cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>decode_Window_status_exclusive/1</c></cell>
          <cell align="left" valign="middle"><c>1251878</c></cell>
          <cell align="left" valign="middle"><c>exclusive</c></cell>
          <cell align="left" valign="middle"><c>GUI</c></cell>
          <cell align="left" valign="middle"><em>21.3</em></cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>decode/2</c></cell>
          <cell align="left" valign="middle"><c>5889197</c></cell>
          <cell align="left" valign="middle"><c>complete</c></cell>
          <cell align="left" valign="middle"><c>GUI</c></cell>
          <cell align="left" valign="middle"><em>100</em></cell>
        </row>
        <tcaption>Results of complete, exclusive and selective decode</tcaption>
      </table>
      <p>Another interesting question is what the relation is between
        a complete decode, an exclusive decode followed by
        <c>decode_part</c> of the excluded parts and a selective decode
        followed by a complete decode. Some situations may be compared to
        this simulation, e.g. inspect a sub-value and later on look at
        the entire value. The following table shows figures from this
        test. The number of loops and time unit is the same as in the
        previous test.
        </p>
      <table>
        <row>
          <cell align="left" valign="top"><em>Actions</em></cell>
          <cell align="left" valign="top"><em>Function</em>&nbsp;&nbsp;&nbsp;&nbsp;</cell>
          <cell align="left" valign="top"><em>Time</em>(microseconds)</cell>
          <cell align="left" valign="top"><em>ASN.1 spec</em></cell>
          <cell align="left" valign="top"><em>% of time vs. complete decode</em></cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>complete</c></cell>
          <cell align="left" valign="middle"><c>decode/2</c></cell>
          <cell align="left" valign="middle"><c>4507457</c></cell>
          <cell align="left" valign="middle"><c>MEDIA-GATEWAY-CONTROL</c></cell>
          <cell align="left" valign="middle"><em>100</em></cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>selective and complete</c></cell>
          <cell align="left" valign="middle"><c>decode_&shy;MegacoMessage_&shy;selective/1</c></cell>
          <cell align="left" valign="middle"><c>4881502</c></cell>
          <cell align="left" valign="middle"><c>MEDIA-GATEWAY-CONTROL</c></cell>
          <cell align="left" valign="middle"><em>108.3</em></cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>exclusive and decode_part</c></cell>
          <cell align="left" valign="middle"><c>decode_&shy;MegacoMessage_&shy;exclusive/1</c></cell>
          <cell align="left" valign="middle"><c>5481034</c></cell>
          <cell align="left" valign="middle"><c>MEDIA-GATEWAY-CONTROL</c></cell>
          <cell align="left" valign="middle"><em>112.3</em></cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>complete</c></cell>
          <cell align="left" valign="middle"><c>decode/2</c></cell>
          <cell align="left" valign="middle"><c>5889197</c></cell>
          <cell align="left" valign="middle"><c>GUI</c></cell>
          <cell align="left" valign="middle"><em>100</em></cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>selective and complete</c></cell>
          <cell align="left" valign="middle"><c>selected_&shy;decode_&shy;Window1/1</c></cell>
          <cell align="left" valign="middle"><c>6337636</c></cell>
          <cell align="left" valign="middle"><c>GUI</c></cell>
          <cell align="left" valign="middle"><em>107.6</em></cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>selective and complete</c></cell>
          <cell align="left" valign="middle"><c>selected_&shy;decode_&shy;Window2/1</c></cell>
          <cell align="left" valign="middle"><c>6795319</c></cell>
          <cell align="left" valign="middle"><c>GUI</c></cell>
          <cell align="left" valign="middle"><em>115.4</em></cell>
        </row>
        <row>
          <cell align="left" valign="middle"><c>exclusive and decode_part</c></cell>
          <cell align="left" valign="middle"><c>decode_&shy;Window_&shy;status_&shy;exclusive/1</c></cell>
          <cell align="left" valign="middle"><c>6249200</c></cell>
          <cell align="left" valign="middle"><c>GUI</c></cell>
          <cell align="left" valign="middle"><em>106.1</em></cell>
        </row>
        <tcaption>Results of complete, exclusive + decode_part and selective + complete decodes</tcaption>
      </table>
      <p>Other ASN.1 types and values can differ much from these
        figures. Therefore it is important that you, in every case where
        you intend to use either of these decodes, perform some tests
        that shows if you will benefit your purpose.
        </p>
    </section>

    <section>
      <title>Comments</title>
      <p>Generally speaking the gain of selective and exclusive decode
        in advance of complete decode is greater the bigger value and the
        less deep in the structure you have to decode. One should also
        prefer selective decode instead of exclusive decode if you are
        interested in just one single sub-value.</p>
      <p>Another observation is that the exclusive decode followed by
        decode_part decodes is very attractive if the parts will be sent
        to different servers for decoding or if one in some cases not is
        interested in all parts.</p>
      <p>The fastest selective decode are when the decoded type is a
        primitive type and not so deep in the structure of the top
        type. The <c>selected_decode_Window2</c> decodes a big constructed
        value, which explains why this operation is relatively slow.</p>
      <p>It may vary from case to case which combination of
        selective/complete decode or exclusive/part decode is the fastest.</p>
    </section>
  </section>
</chapter>