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Diffstat (limited to 'lib/asn1')
| -rw-r--r-- | lib/asn1/doc/src/asn1_ug.xml | 57 |
1 files changed, 0 insertions, 57 deletions
diff --git a/lib/asn1/doc/src/asn1_ug.xml b/lib/asn1/doc/src/asn1_ug.xml index ce8da39e6d..2522333d77 100644 --- a/lib/asn1/doc/src/asn1_ug.xml +++ b/lib/asn1/doc/src/asn1_ug.xml @@ -1415,62 +1415,5 @@ T2 ::= General{BIT STRING} encode/decode functions will only be generated for T1 and T2. </p> </section> - - <section> - <title>Encoding Rules</title> - <p>When the first recommendation on ASN.1 was released 1988 it was - accompanied with the Basic Encoding Rules, BER, as the only - alternative for encoding. - BER is a somewhat verbose protocol. It adopts a so-called TLV (type, - length, value) approach to encoding in which every element of the - encoding carries some type information, some length information and - then the value of that element. Where the element is itself - structured, then the Value part of the element is itself a series of - embedded TLV components, to whatever depth is necessary. In summary, - BER is not a compact encoding but is relatively fast and easy to - produce.</p> - <p>The DER (Distinguished Encoding Rule) encoding format was included in - the standard in 1994. It is a specialized form of BER, which gives - the encoder the option to encode some entities differently. For - instance, is the value for TRUE any octet with any bit set to one. But, - DER does not leave any such choices. The value for TRUE in the DER - case is encoded as the octet <c>11111111</c>. So, the same value - encoded by two different DER encoders must result in the same bit - stream.</p> - <p>A more compact encoding is achieved with the Packed Encoding - Rules PER which was introduced together with the revised - recommendation in 1994. PER takes a rather different approach from - that taken by BER. The first difference is that the tag part in - the TLV is omitted from the encodings, and any tags in the - notation are not encoded. The potential ambiguities are resolved - as follows:</p> - <list type="bulleted"> - <item> - <p>A CHOICE is encoded by first encoding a choice index which - identifies the chosen - alternative by its position in the notation.</p> - </item> - <item> - <p>The elements of a SEQUENCE are transmitted in textual - order. OPTIONAL or DEFAULT elements are preceded by a bit map - to identify which elements are present. After sorting the - elements of a SET in the "canonical tag order" as defined in - X.680 8.6 they are treated as a SEQUENCE regarding OPTIONAL - and DEFAULT elements. A SET is transferred in the sorted - order.</p> - </item> - </list> - <p>A second difference is that PER takes full account of the sub-typing - information in that the encoded bytes are affected by the constraints. - The BER encoded bytes are unaffected by the constraints. - PER uses the sub-typing information to for example omit length fields - whenever possible. </p> - <p>The run-time functions, sometimes take the constraints into account - both for BER and PER. For instance are SIZE constrained strings checked.</p> - <p>There are two variants of PER, <em>aligned</em> and <em>unaligned</em>. - In summary, PER results in compact encodings which require much more - computation to produce than BER. - </p> - </section> </chapter> |