can specify an integer field that must be in the range 0 to 100. The length of a sequence of values (an array) can also be specified, either as a fixed length or a range of permitted lengths. Constraints can also be specified as logical combinations of sets of basic constraints. Values used as constraints can either be literals used in the PDU specification, or ASN.1 values specified elsewhere in the schema file. Some ASN.1 tools will make these ASN.1 values available to programmers in the generated source code. Used as constants for the protocol being defined, developers can use these in the protocol's logic implementation. Thus all the PDUs and protocol constants can be defined in the schema, and all implementations of the protocol in any supported language draw upon those values. This avoids the need for developers to hand code protocol constants in their implementation's source code. This significantly aids protocol development; the protocol's constants can be altered in the ASN.1 schema and all implementations are updated simply by recompiling, promoting a rapid and low risk development cycle. 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. It is complex to implement all aspects of ASN.1 constraints in an ASN.1 compiler. Not all tools support the full range of possible constraints expressions. XML schema and JSON schema both support similar constraints concepts. Tool support for constraints varies. Microsoft's xsd.exe compiler ignores them. 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). ABNF, on the other hand, defines the encoding ("syntax") at the same time it defines the data structure ("semantics"). ABNF tends to be used more frequently for defining textual, human-readable protocols, and generally is not used to define type–length–value encodings. Many programming languages define language-specific serialization formats. For instance, Python's "pickle" module and Ruby's "Marshal" module. These formats are generally language specific. They also don't require a schema, which makes them easier to use in ad hoc storage scenarios, but inappropriate for communications protocols. JSON