ASN.1

Page edited by Emmanuel L=C3=A9charny

Changes (2)

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h2. Implementations

Two Three codecs are currently available :

* [LdapCodec|LdapCodec] whic= h encodes and decodes LDAP messages

* [KerberosCodec|Kerberos] which encodes an decodes KERBEROS message= s

* [SpnegoCodec] which encode= s an decodes SPNEGO messages

Full Content

ASN.1 encoder/deco= der

ASN.1 encoder/decoder=

Components

Compiler
Processing

Implementations

Components

Communications between clients and server can be seen as a two ways / mu= lti layers system. The client submits a request to the server, whi= ch replies.

Layers are just used to facilitate the implementation of this c= ommunication. From the developer point of view, working on a specific level= , he just has to know the two layers above and under, but can be seen as a = communication at she same level between the client and the server. Here is = a view of these layers :

We have depicted three layers:

Request/Response: This is the more abstract layer. Exchanged message= s are 'human readable'. Each message is a Java Bean, containing all the inf= ormation about a Request or a Response.
PDU: As communication petween the Client and the Server is done thro= ugh a network, we need to transform the beans to something more 'network fr= iendly'. The data are stored in PDU, or Protocol Data Unit. Those PDU contain an encoded form of messages, specified in RFC 2251 and ASN.1

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ByteBuffers: To trans= fer PDU from/to Client to/from Server, we need to store bytes in a structur= e that will permit to deal with network latency. Thus we are using byte buf= fers, which allow us to send pieces of PDU until the whole PDU has been tra= nsmitted. (Note : ByteBuffer= is also a Java NIO class, but can be seen just as a byte container. It cou= ld have been something totally different from the NIO class).

This layering allows many different implementations.

One can also imagine inter-layers used to trace debug informations.

Inter layer communication rely on a pipe-line: each layer push some piec= e of information to the next layer (up or down), and so on.

Each layer may implement its own strategy to fulfill the reception and t= ransmission of the data it is responsible of :

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asynchronous push
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synchronous push
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established and dedicated channel
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multiplexed channel
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reception =09
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listener
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established and dedicated channel
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multiplexed channel
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POJOs

POJOs are Java classes that contain high level informations.

A client create an instance of a class to communicate with the = server, which create an other one to reply. They implement a kind of ap= plication layer between clients and server.

Ideally, they are generated by an ASN.1 compiler, but can be hand= crafted.

PDUs/TLVs

PDU stands for Protocol Data Unit. An ASN.1 encoded= element is stored in a PDU. This is what is transfered between a client an= d a server.

TLV stands for Type/Length/Value. A PDU is made of TLV s. = Each TLV represent either a primitive element, and it has a V= alue, or a constructed element, and the Value is itself one ore more= TLV (The V can contain more than one TLV). The PDU= structure is like a tree, where the PDU is the whole tree, and = where TLV are leaves (primitives) and branches (constructed)

Further information about TLVs can be found here :

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TLV Page Info: Informations about Tlvs

ByteBuffer

Buffering the incoming request or the ourgoing response is essential. As= a request or a response can be huge (for example, if we want to store imag= es), it is necessary to store bytes in buffers in order to be able to pipel= ine the processing. Flushing informations byte by byte is totally insane, f= rom the network point of view.

We are using the NIO B= yteBuffer structure to store chunks of information, before pushing them= on the network, and reversly, store incoming bytes into buffers before pro= cessing the request.

Compiler

TO BE DONE ...

Processing

There are two kind of processing: encoding and decoding. E= ncoding is quite easy, decoding is much more complicated.

Important
Important : de= coding an ASN.1 PDU is generally not possible if you have no knowledge of t= he grammar being decoded. To limit the size of PDUs, the encoding schemes u= sed (PER, DER, BER, ...) permits the elimination of some TL if the construc= ted TLV that encapsulate the previous one is unambiguiously known. One who = want to decode a PDU MUST know which grammar has been used.

Encoder

The encoding process is quite easy. As we know what has to be encoded, t= he structure of the PDU is somehow dependent on the structure of the POJO w= hich contains the data. The only tricky things is the Length part, w= hich has to be computed. As a TLV may have a V part which is = itself one or more TLV s, its L part will be the sum of each = included TLV s length. This is typically a recursive processing, but= we can also process the POJO in two passes :

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the first pass compute each length
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the second pass generate the PDU

The DIRxSBOX:Encoding Asn.1= page gives informations about the encoding process.

Decoder

The decoding process is a loop which reads PDUs and constructs objects o= n the fly. It can stop and restart without loosing information, as PDU may = be very long (it also means that we must store a current state for each dec= oding).

The DIRxSBOX:Decoding Asn.1= page gives informations about the encoding process.

Performance

TODO : performance against memory/scalability/failover
TODO : which kind of performance should we deliver? Maximum throughput =3D = bandwith/average PDU size. For instance, with a 1Gb network connection, ass= uming that we have an average PDU size of 100 bytes, the system must delive= r 1 M Pdu/s to saturate the network.

Actually, the new decoder eats 110 000 BindRequest PDU or 37 000 Sea= rchResultEntry PDU per second on my 2.8Ghz computer, but we have to take in= to account the works that must be done aside.

Implementations

Three codecs are currently available :

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LdapCodec which encodes and decodes LDAP messages
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KerberosCodec which encodes an decodes KERBEROS messages
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SpnegoCodec which encodes an decodes SPNEGO messages

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