Abstract Syntax Notation One

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Abstract Syntax Notation One (ASN.1) is a standard and notation that describes rules and structures for representing, encoding, transmitting, and decoding data in telecommunications and computer networking. The formal rules enable representation of objects that are independent of machine-specific encoding techniques. Formal notation makes it possible to automate the task of validating whether a specific instance of data representation abides by the specifications. In other words, software tools can be used for the validation.[1]

ASN.1 is a joint standard of the International Organization for Standardization (ISO), International Electrotechnical Commission (IEC), and International Telecommunication Union Telecommunication Standardization Sector ITU-T, originally defined in 1984 as part of CCITT X.409:1984. ASN.1 moved to its own standard, X.208, in 1988 due to wide applicability. The substantially revised 1995 version is covered by the X.680 series. The latest available version is dated 2008, and is backward compatible with the 1995 version.

ASN.1 in transfer[edit]

Data generated at various sources of observation may need to be transmitted to other locations before it can be processed.

For example, a radio telescope in outer space might produce voluminous signal data that it cannot process locally. The system recording the data and the system processing it later may be very different in nature, and may even be produced by different vendors. A consistent mechanism is needed to record, transmit, and read data across diverse systems.

ASN.1 defines the abstract syntax of information but does not restrict the way the information is encoded. Various ASN.1 encoding rules provide the transfer syntax (a concrete representation) of the data values whose abstract syntax is described in ASN.1.

The standard ASN.1 encoding rules include:

ASN.1 together with specific ASN.1 encoding rules facilitates the exchange of structured data especially between application programs over networks by describing data structures in a way that is independent of machine architecture and implementation language.

Application layer protocols such as X.400 electronic mail, X.500 and Lightweight Directory Access Protocol (LDAP) directory services, H.323 (VoIP), Kerberos, BACnet and simple network management protocol (SNMP) use ASN.1 to describe the protocol data units (PDU) they exchange. It is also extensively used in the access and non-access strata of the Universal Mobile Telecommunications System (UMTS). There are many other application domains of ASN.1.[2]

A particularly useful new application of ASN.1 is Fast Infoset. Fast Infoset is an international standard that specifies a binary encoding format for the XML Information Set (XML Infoset) as an alternative to the XML document format. It aims to provide more efficient serialization than the text-based XML format.

Example[edit]

Data structures of FooProtocol defined using the ASN.1 notation:

FooProtocol DEFINITIONS ::= BEGIN

    FooQuestion ::= SEQUENCE {
        trackingNumber INTEGER,
        question       IA5String
    }

    FooAnswer ::= SEQUENCE {
        questionNumber INTEGER,
        answer         BOOLEAN
    }

END

This could be a specification published by creators of Foo protocol. ASN.1 does not define conversation flows. This is up to the textual description of the protocol.

Assuming a message, which complies with Foo protocol and which will be sent to the receiving party. This particular message (protocol data unit (PDU)) is:

myQuestion FooQuestion ::= {
    trackingNumber     5,
    question           "Anybody there?"
}

To send the above message through the network one needs to encode it to a string of bits. ASN.1 defines various algorithms to accomplish that task, called Encoding rules. There are plenty of them; one of the simplest is Distinguished Encoding Rules (DER).

The Foo protocol specification should explicitly name one set of encoding rules to use, so that users of the Foo protocol know which one they should use.

Example encoded in DER[edit]

Below is the data structure shown above encoded in DER format (all numbers are in hexadecimal):

30 — type tag indicating SEQUENCE
13 — length in octets of value that follows
02 — type tag indicating INTEGER
01 — length in octets of value that follows
05 — value (5)
16 — type tag indicating IA5String 
     (IA5 means the full 7-bit ISO 646 set, including variants, 
      but is generally US-ASCII)
0e — length in octets of value that follows
41 6e 79 62 6f 64 79 20 74 68 65 72 65 3f — value ("Anybody there?")

(Note: DER uses a pattern of type-length-value triplets, and uses well known byte constants for encoding type tags)

So what one actually gets is the string of 21 octets:

30 13 02 01 05 16 0e 41 6e 79 62 6f 64 79 20 74 68 65 72 65 3f

The scope of ASN.1 and DER ends here. It is possible to transmit the encoded message to the party by any means (utilizing Transmission Control Protocol (TCP) or any other protocol). The party should be able to decode the octets back using DER.

Example encoded in XER[edit]

Alternatively, it is possible to encode the same ASN.1 data structure with XML Encoding Rules (XER) to achieve greater human readability "over the wire". It would then appear like the following 108 octets, which includes the spaces used for indentation:

<FooQuestion>
    <trackingNumber>5</trackingNumber>
    <question>Anybody there?</question>
</FooQuestion>

Example encoded in PER (unaligned)[edit]

Alternatively, if Packed Encoding Rules are employed, the following 122 bits[clarification needed] (less than 16 octets) will be produced:

01 05 0e 83 bb ce 2d f9 3c a0 e9 a3 2f 2c af c0

In this format, type tags for required elements are not encoded, so it cannot be parsed without knowing the expected schemas used to encode. Additionally, the bytes for the value of the IA5String are packed using 7-bit units instead of 8-bit units, because the encoder knows that IA5String byte values only require 7 bits to encode each of them. However the length bytes are still encoded here, even for the first integer tag 01 (but a PER packer could also drop it if it knows that the allowed value range fits on 8 bits, and it could even compact the single value byte 05 with less than 8 bits, if it knows that allowed values can only fit in a smaller range).

Note also that the last 6 bits in the encoded PER are padded with null bits in the 6 least significant bits of the last byte c0 : these extra bits may not be transmitted or used for encoding something else if this sequence is inserted as a part of a longer unaligned PER sequence.

This means that unaligned PER data is essentially an ordered stream of bits, and not an ordered stream of bytes like with aligned PER, and that it will be a bit more complex to decode by software on usual processors because it will require additional contextual bit-shifting and masking and not direct byte addressing (but the same remark would be true with modern processors and memory/storage units whose minimum addressable unit is larger than 1 octet). However modern processors and signal processors include hardware support for fast internal decoding of bit streams with automatic handling of computing units that are crossing the boundaries of addressable storage units (this is needed for efficient processing in data codecs for compression/decompression or with some encryption/decryption algorithms).

If alignment on octet boundaries was required, an aligned PER encoder would produce:

01 05 0e 41 6e 79 62 6f 64 79 20 74 68 65 72 65 3f

(in this case, each octet is padded individually with null bits on their unused most significant bits).

ASN.1 versus other data structure definition schemes[edit]

Since it is commonly used for defining messages for communication protocols, ASN.1, with its associated encoding rules, results in a binary encoding.

Other communication protocols, such as Internet protocols HTTP and SMTP, define messages using text tags and values, sometimes based on the Augmented Backus-Naur form (ABNF) notation. The definition also defines the encoding, which is in text.

There has been much debate over the two approaches, and both have their merits; the ASN.1 approach is believed to be more efficient,[citation needed] and with Packed Encoding Rules, certainly provides a more compact encoding. The textual approach is claimed to be easier to implement (through creation and parsing of text strings) and easier to debug with only a text editor. In the case of the Megaco protocol, consensus between the two points of view was not reached and so two encodings, one based on ASN.1 and one on ABNF, were defined.

The ASN.1 XML Encoding Rules (XER) attempts to bridge the gap by providing a textual encoding of data structures defined using ASN.1 notation. Generic String Encoding Rules were also defined for the sole purpose of presenting and inputting data to/from a user.

Encoding Control Notation (ECN)[edit]

The Encoding Control Notation (ECN) is a notation to specify specific encodings of ASN.1 types. ECN is useful to describe legacy protocols in ASN.1. It is possible to specify only the encoding of some types and then complete with a standard encoding rules (usually unaligned PER).

ASN.1 Information Object Class[edit]

Information Object Classes is a concept used in ASN.1 to address specification needs similar to the ones addressed by CORBA/IDL specifications.

Using ASN.1 in practice[edit]

One may use an ASN compiler which takes as input an ASN.1 specification and generates computer code (for example in the language C) for an equivalent representation of the data structures. This computer code, together with supplied run-time libraries, can then convert encoded data structures to and from the computer language representation. Alternatively, one can manually write encoding and decoding routines.

Standards[edit]

Standards describing the ASN.1 notation:

Standards describing the ASN.1 encoding rules:

See also[edit]

Notes[edit]

  1. ^ Using ASN.1 (Abstract Syntax Notation 1): A Data Description Language
  2. ^ ITU-T website - Uses of ASN.1

References[edit]

This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.

External links[edit]