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From: directory-cvs@incubator.apache.org
To: directory-cvs@incubator.apache.org
Subject: 
 =?iso-8859-1?q?=5BApache_Directory_Project_Wiki=5D_Updated=3A__TLVPageInf?=
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Date: Fri, 18 Feb 2005 06:21:23 -0000
Message-ID: <20050218062123.78019.96806@minotaur.apache.org>

   Date: 2005-02-17T22:21:22
   Editor: AlexKarasulu
   Wiki: Apache Directory Project Wiki
   Page: TLVPageInfo
   URL: http://wiki.apache.org/directory/TLVPageInfo

   no comment

Change Log:

---------------------------------------------------------------------------=
---
@@ -19,11 +19,13 @@
 =

 Labels are numbers used to identify elements in a '''SET''' (see Asn.1 gra=
mmar), for instance. Generally, we don't have to deal with label above 30, =
which can be encoded in 5 bits (so this kind of '''Tag''' will be 1 byte lo=
ng), and never above 1024. In the LDAP ASN.1 grammar, no label exceed 19 (i=
n ''LdapMessage'', the ''ExtendedResponse'' label is 19), so we can focus o=
n 1 byte tags. Whatever, it could be interesting to accept longer labels to=
 be able to support any LDAP evolution (or other protocols, as this '''Tag'=
'' decoder is not specifically written for LDAP)
 =

-''In the current setup we use a Java primitive int to store a maximum of 4=
 Tag field bytes.  The Tags within a protocol can be pre-fabricated.  An en=
umeration constant can be reused.  This way all we need is a 4 byte referen=
ce to the constant to represent the tag value.  This works well especially =
for using the enumeration type's value for switch statments.  Today here's =
how we pack Tag bytes into a Java primitive int:''
+''In the current setup we use a Java primitive int to store a maximum of 4=
 Tag field bytes.  The Tags within a protocol can be pre-fabricated.  An en=
umeration constant can be reused.  This way all we need is a 4 byte referen=
ce to the constant to represent the tag value.  This works well especially =
for using the enumeration type's value for switch statments.  Today here's =
how we pack Tag bytes into a Java primitive int.''
 =

 [http://incubator.apache.org/directory/subprojects/asn1/images/tag-integer=
-encoding.png]
 =

-''Looking at the automaton below it occurred to me that we can create two =
different Tag decoders.  A stateless one for protocols that only requires a=
 single byte Tag.  You get whole bytes delivered in buffers so no need to k=
eep state right?  The stateless Tag really simplifies the automaton ;).  Fo=
r multibyte protocols we do collect bytes in a TagOctetCollector which is j=
ust a wrapper around a Java int.  Do you think this needs to be revamped?''
+''Looking at the automaton below it occurred to me that we can create two =
different Tag decoders.  A stateless one for protocols that only requires a=
 single byte Tag.  You get whole bytes delivered in buffers so no need to k=
eep state right?  The stateless Tag really simplifies the automaton ;).  Fo=
r multibyte protocols we do collect bytes in a TagOctetCollector which is j=
ust a wrapper around a Java int.  Do you think this needs to be revamped?  =
In the case of a protocol where the number of Tag octets can be greater tha=
n 1 we can use a less efficient stateful Tag decoder which needs the full l=
ogic of the automaton.  I know this is just some extra if-then-else logic b=
ut that adds up for every byte.  So we can swap out the Tag decoder used ba=
sed on the protocol.  Most hopefully will take the simple tag decoder and n=
ot pay too much of a penalty.''
+
+''BTW note that if we have a protocol with more that 30 tags we only need =
one multibyte tag decoder per stream.  This is a per connection setup cost =
paid once and not for every Tag that comes through.''
 =

 Decoding a Tag has to follow the finite state automaton showed on this pic=
ture :
=20