diff options
Diffstat (limited to 'lib')
| -rw-r--r-- | lib/asn1/doc/src/asn1_ug.xml | 119 |
1 files changed, 54 insertions, 65 deletions
diff --git a/lib/asn1/doc/src/asn1_ug.xml b/lib/asn1/doc/src/asn1_ug.xml index 2475eaa153..a16aac0e03 100644 --- a/lib/asn1/doc/src/asn1_ug.xml +++ b/lib/asn1/doc/src/asn1_ug.xml @@ -879,19 +879,31 @@ Pdu ::= SEQUENCE { <pre> MyPdu = #'Pdu'{a=22,b=77.99,c={0,1,2,3,4},d='NULL'}. </pre> <p>The decode functions will return a record as result when decoding - a <c>SEQUENCE</c> or a <c>SET</c>. - <marker id="DEFAULT"></marker> -</p> - <p>A <c>SEQUENCE</c> and a <c>SET</c> may contain a component with a - <c>DEFAULT</c> key word followed by the actual value that is the - default value. In case of BER encoding it is optional to encode the - value if it equals the default value. If the application uses the - atom asn1_DEFAULT as value or if the value is a primitive value - that equals the default value the encoding omits the bytes for - this value, which is more efficient and it results in fever - bytes to send to the receiving application.</p> - <p>For instance, if the following types exists in a file "File.asn":</p> + a <c>SEQUENCE</c> or a <c>SET</c>.</p> + + <p>A <c>SEQUENCE</c> and a <c>SET</c> may contain a component + with a <c>DEFAULT</c> key word followed by the actual value that + is the default value. The <c>DEFAULT</c> keyword means that the + application doing the encoding can omit encoding of the value, + thus resulting in fewer bytes to send to the receiving + application.</p> + + <p>An application can use the atom <c>asn1_DEFAULT</c> to indicate + that the encoding should be omitted for that position in + the SEQUENCE.</p> + + <p>Depending on the encoding rules, the encoder may also compare + the given value to the default value and automatically omit the + encoding if they are equal. How much effort the encoder makes to + to compare the values depends on the encoding rules. The DER + encoding rules forbids encoding a value equal to the default value, + so it has a more thorough and time-consuming comparison than the + encoders for the other encoding rules.</p> + + <p>In the following example we will use this ASN.1 specification:</p> <pre> +File DEFINITIONS AUTOMATIC TAGS ::= +BEGIN Seq1 ::= SEQUENCE { a INTEGER DEFAULT 1, b Seq2 DEFAULT {aa TRUE, bb 15} @@ -901,60 +913,37 @@ Seq2 ::= SEQUENCE { aa BOOLEAN, bb INTEGER } - </pre> - <p>Some values and the corresponding encoding in an Erlang terminal - is shown below:</p> + +Seq3 ::= SEQUENCE { + bs BIT STRING {a(0), b(1), c(2)} DEFAULT {a, c} +} +END </pre> + <p>Here is an example where the BER encoder is able to omit encoding + of the default values:</p> <pre> -1> <input>asn1ct:compile('File').</input> -Erlang ASN.1 version "1.3.2" compiling "File.asn1" -Compiler Options: [] ---{generated,"File.asn1db"}-- ---{generated,"File.hrl"}-- ---{generated,"File.erl"}-- +1> <input>asn1ct:compile('File', [ber]).</input> ok -2> <input>'File':encode('Seq1',{'Seq1',asn1_DEFAULT,asn1_DEFAULT}).</input> -{ok,["0",[0],[[],[]]]} -3> <input>lists:flatten(["0",[0],[[],[]]]).</input> -[48,0] -4> <input>'File':encode('Seq1',{'Seq1',1,{'Seq2',true,15}}).</input> -{ok,["0","\\b",[[],["\\241",[6],[[[128],[1],"\\377"],[[129],[1],[15]]]]]]} -5> <input>lists:flatten(["0","\\b",[[],["\\241",[6],[[[128],[1],"\\377"],[[129],[1],[15]]]]]]).</input> -[48,8,161,6,128,1,255,129,1,15] -6> </pre> - <p>The result after command line 3, in the example above,shows that the - encoder omits the encoding of default values when they are specific - by asn1_DEFAULT. Line 5 shows that even primitive values that equals - the default value are detected and not encoded. But the constructed - value of component <c>b</c> in <c>Seq1</c> is not recognized as the - default value. Checking of default values in <c>BER</c> is not done - in case of complex values, because it would be to expensive. - <marker id="DEFAULT DER"></marker> -</p> - <p>But, the DER encoding format has stronger requirements regarding - default values both for SET and SEQUENCE. A more elaborate and time - expensive check of default values will take place. The following is - an example with the same types and values as above but with der - encoding format.</p> - <pre> -1> <input>asn1ct:compile('File',[der]).</input> -Erlang ASN.1 version "1.3.2" compiling "File.asn1" -Compiler Options: [der] ---{generated,"File.asn1db"}-- ---{generated,"File.hrl"}-- ---{generated,"File.erl"}-- +2> <input>'File':encode('Seq1', {'Seq1',asn1_DEFAULT,asn1_DEFAULT}).</input> +{ok,<<48,0>>} +3> <input>'File':encode('Seq1', {'Seq1',1,{'Seq2',true,15}}).</input> +{ok,<<48,0>>} </pre> + + <p>And here is an example with a named BIT STRING where the BER + encoder will not omit the encoding:</p> + <pre> +4> <input>'File':encode('Seq3', {'Seq3',asn1_DEFAULT).</input> +{ok,<<48,0>>} +5> <input>'File':encode('Seq3', {'Seq3',<<16#101:3>>).</input> +{ok,<<48,4,128,2,5,160>>} </pre> + + <p>The DER encoder will omit the encoding for the same BIT STRING:</p> + <pre> +6> <input>asn1ct:compile('File', [ber,der]).</input> ok -2> <input>'File':encode('Seq1',{'Seq1',asn1_DEFAULT,asn1_DEFAULT}).</input> -{ok,["0",[0],[[],[]]]} -3> <input>lists:flatten(["0",[0],[[],[]]]).</input> -[48,0] -4> <input>'File':encode('Seq1',{'Seq1',1,{'Seq2',true,15}}).</input> -{ok,["0",[0],[[],[]]]} -5> <input>lists:flatten(["0",[0],[[],[]]]).</input> -[48,0] -6> - </pre> - <p>Line 5 shows that even values of constructed types is checked and if - it equals the default value it will not be encoded.</p> +7> <input>'File':encode('Seq3', {'Seq3',asn1_DEFAULT).</input> +{ok,<<48,0>>} +8> <input>'File':encode('Seq3', {'Seq3',<<16#101:3>>).</input> +{ok,<<48,0>>} </pre> </section> <section> @@ -1005,9 +994,9 @@ Bad ::= SET {i INTEGER, [<cite id="X.680"></cite>]. </p> <p>Encoding of a SET with components with DEFAULT values behaves - similar as a SEQUENCE, <seealso marker="#DEFAULT">see above</seealso>. The DER encoding format restrictions on DEFAULT + similar as a SEQUENCE. The DER encoding format restrictions on DEFAULT values is the same for SET as for SEQUENCE, and is supported by - the compiler, <seealso marker="#DEFAULT DER">see above</seealso>.</p> + the compiler.</p> <p>Moreover, in DER the elements of a SET will be sorted. If a component is an un-tagged choice the sorting have to take place in run-time. This fact emphasizes the following recommendation |