Abstract Syntax Notation One (ASN.1) is a standard interface description language (IDL) for defining data structures that can be serialized and deserialized in a cross-platform way.
[1] Protocol developers define data structures in ASN.1 modules, which are generally a section of a broader standards document written in the ASN.1 language.
ASN.1 is a joint standard of the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) in ITU-T Study Group 17 and International Organization for Standardization/International Electrotechnical Commission (ISO/IEC), originally defined in 1984 as part of CCITT X.409:1984.
ASN.1 is closely associated with a set of encoding rules that specify how to represent a data structure as a series of bytes.
Conversation flows, transaction interchanges, and states are not defined in ASN.1, but are left to other notations and textual description of the protocol.
Below is the data structure shown above as myQuestion encoded in DER format (all numbers are in hexadecimal):
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).
ASN.1 is similar in purpose and use to Google Protocol Buffers and Apache Thrift, which are also interface description languages for cross-platform data serialization.
It also includes a wider variety of basic data types, some of which are obsolete, and has more options for extensibility.
If the ASN.1 tools properly implement constraints checking in the generated source code, this acts to automatically validate protocol data during program operation.
Generally ASN.1 tools will include constraints checking into the generated serialization / deserialization routines, raising errors or exceptions if out-of-bounds data is encountered.
ASN.1 is visually similar to Augmented Backus-Naur form (ABNF), which is used to define many Internet protocols like HTTP and SMTP.
However, in practice they are quite different: ASN.1 defines a data structure, which can be encoded in various ways (e.g. JSON, XML, binary).
They also don't require a schema, which makes them easier to use in ad hoc storage scenarios, but inappropriate for communications protocols.
Thus it is possible (though perhaps ill-advised) to have in a project an XSD schema being compiled by ASN.1 tools producing source code that serializes objects to/from JSON wireformat.