Comonad Dev

pierre@comonad.dev

binary This specification describes a simple, decentrally extensible and efficient format for data serialization.

This specification aims at finding an original trade-off between transparency, syntax complexity, generality, extensibility, decentralization, discoverability, compactness, streamability, safety, processing speed and processing footprint for a data format (see definitions). It is our opinion that every widely used existing format occupy a different position than this one in the solution space for formats, that none is better on all axes, and that this one is the current best on several axes, hence this new design. It is also our opinion that some of those existing formats constitute an optimal solution for their specific use case, either in a absolute sense, or at least at the time of their design. But the ever-changing field of IT now faces new challenges that call for a new approach. In particular, whereas the previous trend for Internet and Web standards and programming tools has been to create human-readable syntaxes for data and protocols, the advent of technologies like protocol buffers, CBOR, Thrift, the various binary serializations for JSON like Avro or Smile, or the binary HTTP/2 seem to indicate that the time is ripe for a generalized use of binary, reserved until now for the low-level protocols. The lessons about flexibility learnt in the previous switch from binary to plain text can now be applied to efficient binary syntaxes.

By transparency, we mean the property of a format that can be parsed even by an application that doesn't understand the semantics of every part of the processed data. By syntax complexity, we mean the number of different syntactic structures of the format. High transparency and low syntax complexity mostly have value in the face of extension, as a fixed format doesn't need either, but even in that case, they might make the overall format and its implementation simpler, which can have a lot of value by reducing the risk of incompatible implementations and the likelihood that complex or opaque elements open up security vulnerabilities. Almost all extensible formats have a relatively high transparency and low syntax complexity for their extensible part. The goal was thus to achieve a low syntax complexity for the whole format, while still having high generality (i.e. that extending the format without a new syntactic structure is as easy and practical as possible). A good counter-example is found in most programming languages. Adding a new branching construct cannot be done in a terse way without modifying the underlying implementation. Such a construct either cannot be defined by user code (because of evaluation rules) or can in a terribly verbose and inconvenient way (with lots of boilerplate code). Notable exceptions to this limitation of programming languages are Lisp languages (e.g. Common Lisp, Scheme or Clojure), languages with lazy evaluation (e.g. Haskell, Purescript or Agda) and stack (or concatenative) languages (e.g. Forth, Postscript or Factor). On the other hand, stack languages are the canonical examples of non-transparent formats. Each operator takes a number of operands from the stack. Not knowing the arity of an operator makes it impossible to continue parsing, even when its evaluation was optional to the final processing. In the design space, stack languages completely sacrifice transparency to achieve one of the highest combination of extensibility, compactness and speed of processing. By generality, we mean the ability of a format to describe any type of data with a reasonable (or better yet, high) level of compactness and simplicity. By analogy with data structures, while both arrays and linked lists are both able to store any kind of data, they actually do at the cost of complexity and transparency for arrays (they need the embedding of data structure in the data or in the processing logic) and size for linked lists (in-memory linked lists can waste as much as half or two third of the space for the overhead of the data structure). By extensibility, we mean the ability of a format to encode types and values that were not anticipated when the syntax was designed. By decentralization, we mean the ability to encode new types and values while avoiding name collisions, but without the need of coordination. Note that the DNS, as we use it (e.g. in domain names in module names in some programming languages, or in URIs in XML Namespaces), is not decentralized in this sense, but distributed, as it cannot work without its root servers and prior knowledge of their location. By discoverability, we mean the ability for a processing application to automatically discover new extensions when it encounters their use in data, with no prior knowledge of them beforehand. By compactness, we mean the ability of a format to encode as many diverse types of data as possible with an overall size as small as possible. By streamability, we mean two levels. The first, being streamed, is the ability of a format to be transported in pieces that can be processed before the next piece is available. The second, being broadcasted, is, when a stream of data is cut in two at an arbitrary place, the ability of the second halt to be processed successfully. By safety, we mean the ability for a format to have a parser implementing the full specification while having a default behaviour that doesn't expose the system where it runs to attacks triggered by malicious input. By processing speed, we mean the property of a format that lends itself to benefit from current computing architectures to be processed at high speed overall (e.g. processing can be fast in itself, some shortcuts can be taken, or parallelization is possible). By processing footprint, we mean the ability of a format to be processed while using a low, possibly constrained quantity of memory.

Transparency, generality and extensibility are usually highly-valued traits in formats design. Programming languages obviously feature them foremost, although their generality usually stops at what they are supposed to express: procedures. Most of them are ill-suited to represent arbitrary data, but notable exceptions include Lisp (where "code is data") and Javascript, from which a subset has been extracted to exchange data, JSON, which has seen a tremendous success for this purpose. JSON may have some caveats with regards to generality and a relatively low compactness, but its design makes its parsing really straightforward and fast. All of them, though, lack decentralization and discoverability. Some of them make it possible to extend them in a distributed way if some discipline is followed (for example, by naming modules after domain names), but the discipline is not mandatory (and even with domain names, a change of ownership makes it possible for name collisions). The SGML/XML family of formats also feature good transparency, syntax complexity, generality and extensibility and actually fare much better than programming languages on those axes. XML namespaces also make XML naming distributed and there have been attempts at making it compact (e.g. EXI from W3C, Fast Infoset from ISO/ITU or EBML). All the previously cited formats clearly lack compactness, although just applying standard compression techniques would sacrifice only very little processing time to gain huge size reductions on most of their intended use cases, but compression may not address their ineffectiveness at storing arbitrary bytes (and compression of the base64 encoding of arbitrary bytes can be less efficient than compression of the arbitrary bytes). Neither JSON nor XML are suitable for streaming and the whole document must usually be parsed entirely before its content can be processed, which impacts processing speed and footprint. So-called binary formats pretty much exhibit the opposite trade-offs. Most of them have high syntax complexity, low generality and low extensibility to achieve better compactness. Some are specifically designed for a great generality, but many lack extensibility. When they are extensible, it's never in a decentralized way nor are they discoverable, both for reasons that have to do with compactness. They are usually extremely fast to parse, and while some are designed to be streamed, few can be broadcasted. Actually, many binary formats are not so much formats as they are formats frameworks, and exclude extensibility by design. For each use case, an IDL compiler creates a brand new format that is essentially incompatible with all other formats created by the same compiler (EBML specifically cites this property among its own disadvantages). If the IDL compiler and framework are well designed, such a format can represent an optimum in compactness and speed of processing, as the compiler can also automatically generate an ad-hoc optimized parser. Where extensibility has been planned in existing binary formats, it often doesn't get used that much or at all because of the complications around it. Many binary formats include reserved values meant to extend them to future uses, like the CM field in the ZIP format. A case like this one faces an chicken-and-egg problem: if you don't write and get a specification officially adopted, implementations might not want to include your extension, but if your extension is purely theoretical and hasn't been tested in the wild, you may face resistance to get it officially adopted. This is probably why even though most compression or compressed archive formats include the ability to later encode other compression methods, each new compression method usually comes with its own new format. When extensions are managed with any form of registry, another issue is that you usually need to reserve a large set of values for free experimentation, and once an extension gains any traction while in experimentation, its authors face the difficulty to switch all existing implementations to the definitive values they'll get. And how experimenters choose their temporary values makes them vulnerable to conflicts with others. Furthermore, the process of switching between the experimental and registered versions of the format or protocol might be error-prone and add a significant editorial workload ( details some of the pitfalls and suggests a process to deal with this transition).

Here are some cases where the use of BULK formats and protocols would make software engineers' and users' lives easier.

Without BULK	With BULK
If a user has a huge collection of pictures but many of them contain extended metadata that her image software doesn't support yet, there is no safe and easy way for her to access it.	The user's BULK image software likely is able to show the existence of different metadata in every image file and, with safely auto-discovered data, can visualize the extended metadata, at least in a raw form but with human-readable labels or, better yet, mapped into a form it knows.
If a user has a collection of pictures with some in a file format her image software doesn't support yet, she cannot access the kind of common metadata that she can expect most images to contain (like author or copyright information).	Generic BULK tools can let the user make queries about her whole image collection, and even her broader file collection, making use of either common metadata formats between different image or file formats, or the mapping between different metadata formats into a single one. Queries could be for all objects, whole files or entries inside files, that have been flagged "confidential" or belong to some entity, or all images or videos where some person has been tagged as visible, for example.
If a new image or video compression has been designed, its designers usually need to create a whole custom container format, with custom metadata format. A lot of work is needed for the many image or video software to be able to accommodate reading or writing this new container, new metadata and new codec. Because this new work involves parsing binary data, it often is a source of security vulnerabilities.	If a new image or video compression has been designed, its designers don't need to create anything more to embed it in BULK. If it has unusual features, it is still pretty easy to make what is backward-compatible with existing data models fit into the existing container structure. Even new kinds of structures leverage existing parsing code, minimizing attack surface.
If a new compression, signing or encryption algorithm has been designed, and its designers hope to see it used in existing formats, they have to work separately for each format where it may be used and then for each software project implementing them and in each case, they will face a chicken-and-egg problem where software projects may be reluctant to put effort into something users may not ultimately want or benefit from. The designers may also need to interact with one or several registries to get their algorithm registered, which may be a necessary upfront work. Users need to wait for each software to be updated to include the new algorithm and sometimes suffer from incompatible implementations at the format level (e.g. early adopters producing files with obsolete numbers allocated to the algorithm).	If a new compression, signing or encryption algorithm has been designed, and its designers hope to see it used in existing BULK formats, their BULK vocabulary is guaranteed to uniquely describe the data produced by their algorithm and can be readily embedded in any existing BULK format. The only work needed may be to implement their algorithm as a pure function that can be plugged into the BULK evaluation mechanism, for the relevant programming languages, so that software projects just have to register the link between BULK vocabulary and software plugin (or may not even need to intervene, if a safe mechanism is implemented to auto-discover software plugins providing pure functions).
If a user encounters a file of an unknown type, their system might give them useful media type identification or not, and based on that or file extension, an advanced user might search in her system's software registry or on the Internet and find some software that can visualize some or all of the file's content.	If a user encounters a BULK file of a yet unknown type, generic BULK tools can safely auto-discover human-readable information about the structure of the file, and parts of the file that the user can readily use can be easily found and viewed or extracted.
When a communication protocol is designed, its designers have to choose a trade-off between latency caused by message size, ease of debugging and complexity of the parser. It's deceptively easy to make mistakes in the syntax that make it harder to use or implement, but using an existing syntax ties the protocol with this syntax' limitations.	When a BULK-based communication protocol is designed, its designers can make it both very compact and easy to debug with very little effort. Generic tools can present data on the wire with the support of safely auto-discovered data about the protocol.

In a few of those cases, without BULK, a registry of executable software patches or plugins that can be automatically discovered could have been a solution, with the caveat that almost all of our current software architectures make that possibility more dangerous than it's worth (because of the broad ambient authority given to most code). With BULK, there is immediately a strong incentive to provide discoverability, at first for just data, not executable code, with safeties already in place on that process. The data model of BULK then makes it more likely that code plugins are provided that are designed to work with extremely limited privileges, to act as transformers of BULK expressions.

A BULK stream is a stream of 8-bit bytes, in big-endian order. Parsing a BULK stream yields a sequence of expressions, which can be either atoms or forms, which are sequences of expressions. Forms have a simple syntax: a starting byte marker, a sequence of expressions and an ending byte marker. Atoms each have a special syntax, for compactness purposes: they start with a marker byte, followed by a static or dynamic number of bytes, depending on the type. But there are only 5 kinds: the nil atom, generic arrays, small arrays, small unsigned integers and references. Even booleans and floating-point numbers use this existing syntax without the need for special cases. References consist of a namespace marker (in almost all cases, a single byte) followed by an identifier within this namespace (a single byte). All in all, a very little sacrifice is made in compactness for the benefit of a very simple and resilient syntax: apart from nil and small integers, nothing is smaller than 2 bytes, and as most forms involve a reference followed by some content, a form is usually 4 bytes + its content. A namespace marker in a BULK stream is associated to a namespace identified by some identifier guaranteed to be unique without coordination (like a UUID or cryptographical hash), thus ensuring decentralized extensibility. The stream can be processed even if the application doesn't recognize the namespace. Parsing remains possible thanks to the transparent syntax. Combination of BULK namespaces, BULK streams and even other formats doesn't need any content transformation to work. Here are some examples: The content of a BULK stream, enclosed in form starting and ending byte markers, constitute a valid BULK expression. Thus BULK streams can be aggregated or annotated within a BULK stream without modification. A BULK format could specify in its syntax the place for a metadata expression. Whether the specification provides its own metadata forms or not, an application could use a BULK serialization for MARC, TEI Header, XML or RDF for this metadata expression. The vocabulary selected would be univocally expressed by the namespace and every vocabulary would be parsed by the same mechanisms. Whenever a content must be stored as-is instead of serialized, or a highly-optimized ad hoc serialization exists for some data, anything can always be stored within an array. They can contain arbitrary bytes and there is no limit to their size. Furthermore, BULK expressions can be evaluated. Many expressions evaluate to themselves, but others evaluate to the result of executing a pure function, making it possible to serialize data in an even more compact form, by eliminating boilerplate data and repeated patterns.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in . Literal numerical values are provided in decimal or hexadecimal as appropriate. Hexadecimal literals are prefixed with 0x to distinguish them from decimal literals. The text notation of the BULK stream uses mnemonics for some bytes sequences. Mnemonics are sequences of characters, excluding all capital letters and white space, like this-is-one-mnemonic or what-the-%§!?#-is-that?. They are always separated by white space. Outside the use of mnemonics, a sequence of bytes (of one or more bytes) can be represented by its hexadecimal value as an unsigned integer prefixed by 0x (e.g. 0x3F or 0x3A0B770F). Such a sequence of bytes can include dashes to make it more readable (e.g. 0xDDA37D36-85E6-4E6D-9B51-959E1CCE366C). Some types in this specification define a special syntax for their representation in the text notation. In the grammar, a shape is a pattern of bytes, following the rules of the text notation for a BULK stream. Apart from mnemonics and fixed sequences of bytes, a shape can contain: an arbitrary sequence of a fixed number of bytes, represented by its size, i.e. a number of bytes in decimal immediately followed by a B uppercase letter (e.g. 4B) a typed sequence of bytes, represented by the name of its type, a capitalized word (e.g. Foo); this means a sequence of bytes whose specific yield (see ) has this type a named sequence of bytes (of zero or more bytes), represented by a sequence of any character excluding '{}' between '{' and '}' (e.g. {quux}); a named sequence can be typed or sized, in which case it is immediately followed by ':' and a type or size (e.g. {quux}:Bar or {quux}:12B) The shape that describes the byte sequence of an atom is called its parsing shape. When a shape is given for a form, it merely describes the semantics of evaluating forms of that shape. A reference used in such a shape can be used in different shapes, with unrelated semantics. For example, this specification defines a way do encode a string with explicit encoding with forms of the shape ( string {enc}:Expr {string}:Expr ). But the shapes ( string {arg1}:Int {arg2}:Int ) or ( {arg1}:Int string {arg2}:Int ) are syntactically valid. They just evaluate to themselves as lists of three expressions, as far as this specification is concerned.

A BULK stream is a sequence of 8-bit bytes. Bits and bytes are in big-endian order. The result of parsing a BULK stream is a list of abstract data, called the abstract yield. BULK parsing is injective: a BULK stream has only one abstract yield, but different BULK streams can have the same abstract yield (if they associate namespaces to different markers, see ). A processing application is not expected to actually produce the abstract yield, but an adaptation of the abstract yield to its own implementation, called the concrete yield. Also, some expressions in a BULK stream may have the semantics of a transformation of the abstract yield. A processing application MAY thus not produce or retain the concrete yield but the result of its transformation. This specification deals mainly with the byte sequence and the abstract yield and occasionally provides guidelines about the concrete yield. Of course, a processing application MAY not produce any concrete yield at all but produce various data structures and side effects from parsing the BULK stream (for example, an event sourced application may read its event log from a BULK stream and build its application state by applying the events, discarding each of them as soon as it has been applied). The abstract yield is a list of expressions. Expressions can be atoms or forms. Forms are lists of expressions. If a byte sequence is parsed as an expression, this byte sequence is said to encode this expression. When a sequence of bytes is named in a shape, its name can be used in this specification to designate either the byte sequence, the expression or sequence of expressions it encodes, or the result of evaluating those expressions. When there could be ambiguity, this specification specifies which is designated.

The parser operates with a context, which is a list of expressions. Each time an expression is parsed, it is appended at the end of the context. The initial context is the abstract yield. At the beginning of a BULK stream and after having consumed the byte sequence encoding a complete expression, the parser is at the dispatch stage. At this stage, the next byte is a marker byte, which tells the parser what kind of expression comes next (the marker byte is the first byte of the sequence that encodes an expression). The expression appended to the context after reading a byte sequence is called the specific yield of the byte sequence. The 0x01 and 0x02 marker bytes are special cases. When the parser reads 0x01, it immediately appends an empty list to the current context. This list becomes the new context. This new context has the previous context as parent. Then the parser returns to its dispatch stage. When the parser reads 0x02, it appends nothing to the context, but instead the parent of the current context becomes the new context and the parser returns to the dispatch stage. Thus it is a parsing error to read 0x02 when the context is the abstract yield. It is also a parsing error when the stream ends and the parser is not at the dispatch stage in the abstract yield. When a form contains at least one expression, the first expression is called the operator of that form. When a form contains more than one expression, all but the first expression are called the operands of that form. Some forms have side-effects in their semantics. Those side-effects MUST NOT affect the parsing of any expression. They can affect evaluation, in which case they MUST only affect the evaluation of expressions in the scope of the form. The scope of an expression is the part of its context that follows the expression. This makes BULK lexically scoped. Whenever a parsing error is encountered, parsing of the BULK stream MUST stop. The version of the BULK stream can affect parsing, see .

marker	shape
00	nil
01	(
02	)
03	# Nat {content}
04–0F		reserved
10–7F	Ref
80–BF	w6[value]
C0–FF	#[size] {content}

A processing application MAY implement evaluation of BULK expressions and streams. When evaluating a BULK stream, when the parser gets to the dispatch stage and the context is the abstract yield (this is called a dispatch point), the last expression in the context is replaced by what it evaluates to. (of course, this description is supposed to provide the semantics of BULK evaluation, but a processing application MAY implement evaluation with a different algorithm as long as it provides the same semantics) Evaluation cannot affect parsing, which means that parsing and evaluation can be done in parallel. The default evaluation rule is that an expression evaluates to itself. A name within a namespace can have a value, which is what a reference associated to this name evaluates to. A reference whose marker value is associated to no namespace or whose name has no value evaluates to itself. How self-evaluating BULK expressions are represented in the concrete yield is application-dependent, but future specifications MAY define a standard API to access it, similar to the Document Object Model for XML. The evaluation of a form obeys a special rule, though: if the operator of the form has type Function, that function is called with an argument list and the form evaluates to the return value if it's an atom or the evaluation of the return value if it is a form. If the function has type LazyFunction, the argument list is the operands of the form. If the function has type EagerFunction, the argument list is the result of evaluating the operands of the form, from left to right. Any expression that has type LazyFunction or EagerFunction also has type Function. In the abstract yield, if the first expression produced as a value by evaluation has type EagerFunction, then after the whole abstract yield has been evaluated, the resulting sequence of expressions MAY be evaluated as a form. This is called whole stream evaluation, and MAY be a configuration option. In particular, a processing application MAY choose to disable whole stream evaluation during streaming. When this specification describes the evaluation of a form starting with a LazyFunction, by default, named shapes designate the unevaluated expressions. When this specification describes the evaluation of a form starting with an EagerFunction, by default, named shapes designate the result of evaluating expressions. A form whose operand doesn't have the type Function evaluates to a form containing the result of evaluating each expression of the form, from left to right. When an application evaluates a BULK expression, it MUST verify that evaluation terminates in a finite number of evaluation steps. An application MAY verify finite termination statically or dynamically. For example, an application MAY stop evaluation in error after a predetermined number of steps. Whenever this specification describes the semantics of an expression, it describes the result of evaluating that expression, either in terms of the value returned or the side-effects executed by evaluation of the expression. When an evaluation error is encountered, a processing application MAY not stop processing with an error. If the processing application produces a partially evaluated concrete yield, it MUST convey enough information for the using agent to know the order of all expressions in the original BULK stream, which expressions were successfully evaluated, and which were not because of evaluation errors. A processing application MAY stop evaluation at the first evaluation error and produce the concrete yield in two separated sections, the successfully evaluated part, followed by the unevaluated one. A processing application MAY continue evaluation and produce the concrete yield as a sequence of expressions, each tagged with the fact that it was successfully evaluated or not. The version of the BULK stream can affect evaluation, see .

0x01
mnemonic: ( 0x02
mnemonic: )

There is a difference between a byte sequence encoding several expressions among the current context and a byte sequence encoding a form (i.e. a single expression that is a list of expressions). As an example, let's examine several forms of the shape ( foo {bar} ). In the form ( foo nil nil nil ), {bar} encodes 3 expressions, and they are three atoms in the yield. In the form ( foo nil ), {bar} is a single expression in the yield, and that expression is an atom. In the form ( foo ( nil nil nil ) ), {bar} is also a single expression in the yield, and that expression is a form, a list in the yield. In a shape, when a byte sequence must yield a single expression, it has the type Expr. So the last two examples fit the shape ( foo {bar}:Expr ) but not the first.

0x00
mnemonic: nil nil Apart from being a possible short marker value, the fact that the 0x00 byte represents a valid atom means that a sequence of null bytes is a valid part of a BULK stream, thus making the format less fragile. In a network communication, nil atoms can be sent to keep the channel open. They can also be used as padding at the end of a form or between forms.

Arrays can be used to store arbitrary bytes. An array can be interpreted either as a bits sequence or as an unsigned integer in binary notation. The choice depends on the context and the application. Actually, many processing applications may not need make any choice, as most programming language implementations actually also confuse unsigned integers and bits sequences to some extent. Expressions that are unsigned integers (that is, natural numbers) have type Nat (whether they are encoded as an array or not). Big arrays typically store the content of a file or a binary message of another format. They can also be used to store a vector or matrix of fixed-size elements. In any case, the semantics of the content must be inferred by the processing application; where ambiguity can appear, an application SHOULD enclose the array in a form that makes the semantics explicit (e.g. string, blob, or unsigned-int). Because BULK arrays have no end markers, the payload of a BULK array can constitute the end of the stream. The start and end of an array are known without reading its content, which means that its content can be skipped in constant time and mapped in memory (or read lazily by any other means). Because BULK can use integers with arbitrary size to store the size of an array, BULK arrays have no limit in size. Any array also has the type String if its contents can be decoded as a string in the current encoding. When this specification mentions "the bytes contained in the expression {foo}", it never means the bytes encoding that BULK expression, but the bytes that are the payload of that expression, which could be an array or some other byte container defined in a BULK vocabulary.

0x03
mnemonic: # # Nat {content} After consuming the marker byte, the parser returns to the dispatch stage. It is a parsing error if the parsed expression is not of type Nat or if its value cannot be recognized. This integer is not added to any context, but the parser consumes as many bytes as this integer and they constitute the content of this array. In the text notation, a quoted string is the notation for a generic array containing the encoding of that string in the current encoding, except if the size of the encoding is below 64 bytes, cf. small arrays. In the text notation, some text notation enclosed between balanced ([ and ]) is the notation for a generic array containing the encoding of that text notation, except if the size of the encoding is below 64 bytes, cf. small arrays. Types: Bytes, Nat

0xC0–0xFF
mnemonic: #[size] #[size] {content} The 6 least significant bits of the marker byte are treated as an unsigned integer. This integer is not added to any context, but the parser consumes as many bytes as this integer and they constitute the content of this array. In the text notation, the notation of the marker byte of a small array of size X is #[X]. For example, #[2] 0x1234 is a notation for the bytes 0xC2-1234. In the text notation, a quoted string is the notation for a small array containing the encoding of that string in the current encoding if the size of the encoding is below 64 bytes. For example, "abc" and #[3] 0x616263 are the notation for the same byte sequence if the current encoding is UTF-8. In the text notation, some text notation enclosed between balanced ([ and ]) is the notation for a small array containing the encoding of that text notation if the size of the encoding is below 64 bytes. For example, ([ nil 0 1 256 ]) and #[6] nil w6[0] w6[1] #[2] 0x0100 are the notation for the same byte sequence. Types: Bytes, Nat

0x80–0xBF
mnemonic: w6[value] w6[value] The 6 least significant bits of the marker byte are the value encoded by this byte (as bits or as an unsigned integer in binary notation). In the text notation, the notation of the marker byte of a small unsigned integer of value X is w6[X]. For example, w6[11] is a notation for the byte 0x8B (as is 11, cf. ). Types: Bytes, Nat

When the syntax of a BULK form mandates that an expression can only be a Nat, an application SHOULD encode it as the smallest possible array using one of the following sizes: 6, 8, 16, 32, or any multiple of 64 bits.

Marker bytes 0x04−0x0F are reserved for future major versions of BULK. It is a parsing error if a BULK stream with version 1.0 contains such a marker byte (see ).

0x10−0x7F {ns}:1B {name}:1B 0x7F {ns'} {name}:1B The {ns} byte is a value associated with a namespace, called the namespace marker. Values 0x10−0x13 are reserved for standard namespaces defined by BULK specifications. Greater values can be associated with namespaces identified by a unique identifier. The {name} byte is the name index within the namespace. Vocabularies with more than 256 names thus need to be spread across several namespaces. The specification of a namespace SHOULD include a mnemonic for the namespace and for each defined name. When descriptions use several namespaces, the mnemonic of a reference SHOULD be the concatenation of the namespace mnemonic, ":" and the name mnemonic if there can be an ambiguity. For example, the fft name in namespace math becomes math:fft. Type: Ref

References have a second parsing rule. In case a BULK stream needs an important number of namespaces, if the marker byte is 0x7F, the parser continues to read bytes until it finds a byte different than 0xFF. The sum of each of those bytes taken as unsigned integers is the namespace marker. For example, the reference encoded by the bytes 0x7F 0xFF 0x8C 0x1A is the name 26 in the namespace associated with 522.

Standard namespaces have a fixed marker value and are not identified by a unique identifier. Standard namespaces are immutable. It is an evaluation error when the reference in a name definition is in a standard namespace. Extension namespaces are defined with a unique identifier, to be associated to a marker value. By its decentralized nature, as far as a processing application is concerned, while standard namespaces are a special case, there is no difference between an extension namespace defined as part of the official BULK suite and any other one.

0x10
namespace mnemonic: bulk

name	mnemonic	type
00	version	LazyFunction
01	import (namespace, package)	LazyFunction
02	namespace
03	package
04	define (namespace, package, name)	LazyFunction
05	mnemonic	LazyFunction
06	explain	LazyFunction
07	string	EagerFunction
08	iana-charset	EagerFunction
09	bulk	EagerFunction
0A	blob	EagerFunction
0B	concat	EagerFunction
0C	indexed-bulk	EagerFunction
0D	indexed-array	EagerFunction
0E	true	Boolean
0F	false	Boolean
10	subst	LazyFunction
11	arg
12	rest
13	values	LazyFunction
14	unsigned-int	EagerFunction
15	signed-int	EagerFunction
16	fraction	EagerFunction
17	binary-float	EagerFunction
18	decimal-float	EagerFunction
19	prefix	LazyFunction
1A	postfix	LazyFunction
1B	arity	LazyFunction

( version {major}:Nat {minor}:Nat ) When parsing a BULK stream, a processing application MUST determine explicitly the major and minor version of the BULK specification that the stream obeys. This information MAY be exchanged out-of-band, if BULK is used to exchange a number a very small messages, where repeated headers of 6 bytes might become too big an overhead. A processing application MUST NOT assume a default version. If the version is expressed within a BULK stream, this form MUST be the first in the stream, where its semantics is the side-effect of declaring the version. In any other place, this form evaluates to itself. This specification defines BULK 1.0. When writing a BULK stream's version, an application MUST encode {major} and {minor} by the smallest byte sequence as described in . An application writing a BULK stream to long-term storage (e.g. in a file or a database record) SHOULD include a version form. Two BULK versions with the same major version MUST share the same parsing rules and the same definitions of marker bytes used by both. Changing the syntax or semantics of existing marker bytes warrants a new major version. Changing the syntax or semantics of existing standard names (meaning names in standard namespaces) also warrants a new major version. It is a parsing error when a processing application encounters a major version that it doesn't explicitly support. Using marker bytes in the reserved interval, adding standard names, or adding new syntactic uses of existing standard names that don't overlap with existing uses warrants a new minor version. If version A and version B of BULK have a different default profile, and the change of profile would only affect evaluation of BULK streams that use new marker bytes, new standard names or new syntactic uses of existing standard names, then the profile difference warrants a difference of minor version between A and B. Otherwise, the profile change warrants a difference of major version between A and B. Overall, the goal is that if an application writes a BULK stream of version 1.2 that only contains syntactic and semantic elements from BULK 1.1, a processing application that only supports BULK 1.1 will be able to parse and evaluate that stream. The writing application doesn't need to know the features that are specific to BULK 1.0 and BULK 1.1, because as soon as the processing application encounters a reserved marker byte, an unknown standard name or an unknown syntactic use of a known standard name, it can detect that it cannot correctly evaluate the stream. For that reason, it is an evaluation error for a processing application when there is an unknown syntactic use of a standard name, in a BULK stream with a major version supported by the processing application but a minor version not supported. As far as BULK 1.0 is concerned, using import, define, mnemonic, explain, and arity, when none of their operands is either a standard name or a form with a standard name as operator, or using version somewhere else than as first expression, don't constitute an unknown syntactic use. This makes it possible to overload those names in ways that don't interfere with forward-compatibility.

( import {marker}:Nat ( namespace {id}:Expr ) ) The semantics of this form is the side-effect of associating the namespace identified by {id} to the namespace marker {marker}, within the scope of this expression. {id} can be a form using one or several names whose namespace marker has the value of {marker}. This is called importing a bootstrapping namespace, see . It is not an evaluation error if the namespace is unknown to the processing application. A processing application MAY produce warnings when it encounters this case.

( import {base}:Nat ( package {id}:Expr ) {count}:Nat ) The semantics of this form is the side-effect of associating the first {count} namespaces in the package identified by {id} with a continuous range of marker bytes starting at {base}, within the scope of this expression. It is not an evaluation error if the package is unknown to the processing application, or if it is known but not all namespaces packaged inside are. A processing application MAY produce warnings when it encounters those cases. This forms needs an explicit number of namespaces to import to preserve transparency: if the number was implicit, there could be issues when a processing application that doesn't known how many namespaces are in the package wanted to modify the BULK stream. This makes BULK overall simpler and more predictable. Example: ( import 21 ( package {foo} ) 3 ) associates the first 3 namespaces of the package identified by {foo} to the markers 21, 22 and 23. ( import {base}:Nat ( package {id}:Expr ) {count}:Nat {increment}:Nat ) The semantics of this form is the side-effect of associating the first {count} namespaces in the package identified by {id} with a range of marker bytes starting at {base} at {increment} increments, within the scope of this expression. Example: ( import 21 ( package {foo} ) 3 2 ) associates the first 3 namespaces of the package identified by {foo} to the markers 21, 23 and 25.

A processing application MAY use any BULK expression as a namespace or package identifier, including atoms like numbers or arrays, but it is RECOMMENDED to use one kind of expression called a canonical identifier. Canonical identifiers have the shape ( {type}:Ref {content}:Bytes ). The role of {type} is to describe how to interpret {content}: is it a URI, a UUID, a SHA-3 checksum? Canonical identifiers can be used to efficiently encode IPLD's Content IDentifiers in BULK, but they have a broader use. Where IPLD's CIDs can only encode content-adressing, canonical identifiers can encode arbitrary identifiers, including some that can be created independently of the content, like UUIDs.

( define ( namespace {id}:Expr ) {def}:Bytes ) The semantics of this form is the side-effect of defining a new namespace. The bytes contained in {def} MUST be a BULK stream with a version form, and have the shape ( version {major}:Nat {minor}:Nat ) {config}:Expr {definitions}. {config} MUST be a form and its first element MUST have the shape ( namespace {marker}:Nat ). The state of the namespace associated to {marker} after evaluating all expressions in {definitions} (starting with a namespace with no definitions, no mnemonics, and no documentations) is made the definition of the namespace identified by {id}, within the scope of this expression. This also associates that namespace to the namespace marker {marker}, within the scope of this expression.

When a processing application recognizes that {id} designates a digest that matches the bytes contained in {def}, this creates a verifiable namespace. If more data than {id} is needed to verify {id} against the bytes contained in {def} (like the salt of a hash function, or the namespace of a UUID), this data MUST be provided in {config}. It is an evaluation error if the processing application recognizes that {id} designates a digest but it doesn't match the bytes contained in {def}. Verifiable namespaces are meant to be immutable, but that would be circumvented if they were built upon namespaces that aren't. A verifiable namespace that only uses names from immutable namespaces is an immutable namespace (see ). A processing application SHOULD only consider digest algorithms that are currently known to be cryptographically secure for the determination of namespace and package immutability. For example, a processing application could check that a namespace with an MD5 identifier is verifiable, but it SHOULD NOT determine it to be immutable. The BULK stream in {def} has a version form to prevent the possibility that a verified namespace definition could be evaluated to different results by applications using different BULK versions. The definitions are in a nested BULK stream because if the definition could use immutable namespaces imported outside of the definition, the same verifiable definition could be used in different contexts, also defeating the immutability.

When using canonical identifiers (see ), a verifiable namespace will use a form to express its digest. For immutable namespaces to exist, this means that at least one namespace needs to express its own digest with a name from within itself. This is called a bootstrapping namespace. A bootstrapping namespace MUST use a canonical identifier. When importing a bootstrapping namespace, the processing application looks up in its known namespaces if there is a namespace identified by a form whose operator is a name in itself with the same name index as the operator in the identifier in the import. If that name is associated with a digest algorithm and the logic of the digest algorithm determines that the digest form in the import matches the digest in the namespace definition (along with additional configuration data provided there), then the bootstrapping is successful (meaning that the definition has been found and can be imported). This process doesn't rely just on the digest in the import being identical to the digest in the known definition, because some digests algorithms (e.g. extended-output functions) have been designed to retain some collision resistance when using a prefix of the digest, so a definition could contain a 256 or 512 bits wide digest, but some small BULK streams could import it with the first 64 or 128 bits, when it is worth the trade-off between stream size and collision resistance. It is not an evaluation error if bootstrapping fails. The only consequence is that the bootstrapping namespace is not known, and any other namespace using this namespace for its identifier will not be known either. A processing application MAY produce warnings when it encounters this case. The same logic can be used to import a bootstrapping package, which is a package identified with a form from one of its own namespaces. When using immutable namespaces, bootstrapping packages are likely to be norm, as using almost any immutable namespace requires importing the bootstrapping namespace used to identify the target namespace, then that namespace. Every definition of an immutable namespace can be accompanied by the definition of a bootstrapping package packaging that namespace, its bootstrapping namespace and any other namespaces that are likely to be used with it.

( define ( package {id}:Expr ) {def}:Bytes ) The semantics of this form is the side-effect of creating a package identified by {id}. The bytes contained in {def} MUST be a BULK stream with a version form and have the shape ( version {major}:Nat {minor}:Nat ) {config}:Expr {preamble} {namespaces}:Expr. {config} MUST be a form. {namespaces} MUST be a form containing a sequence of expressions each identifying a BULK namespace. When a processing application recognizes that {id} designates a digest that matches the bytes contained in {def}, this creates a verifiable package. If more data than {id} is needed to verify {id} against the bytes contained in {def} (like the salt of a hash function, or the namespace of a UUID), this data MUST be provided in {config}. It is an evaluation error if the processing application recognizes that {id} designates a digest but it doesn't match the bytes contained in {def}. Packages are meant to be immutable, but that would be circumvented if they were built upon namespaces that aren't. A verifiable package that only contains immutable namespaces is an immutable package.

To define a reference is to make a value the semantics of any reference with the same associated namespace and the same name, in the scope of that definition. This change of semantics operates on the namespace as identified by its unique identifier, not the marker value. This means that if a namespace Foo is associated to markers 21 and 22 and a reference with namespace marker 21 and name 0 is defined to true, then in the scope of that definition, a reference with namespace marker 22 ans name 0 will be evaluated as true. When a BULK stream containing definitions for a namespace comes from a trusted source (i.e. in configuration files of the application, or in the communication with an agent that has been granted the relevant authority), an application MAY give those definitions long-lasting semantics (i.e. keep the values of the names at the end of parsing). This is the RECOMMENDED mechanism for bulk namespace definition when the semantics of the defined expressions can be expressed completely by BULK expressions (see ). ( define {ref}:Ref {value}:Expr ) The semantics of this form is the side-effect of defining the reference {ref} to the value of evaluating {value}.

( mnemonic ( namespace {marker}:Nat ) {mnemonic}:Expr ) This shape declares the value of evaluating {mnemonic} to be the mnemonic for the namespace associated with the marker {marker}. ( mnemonic Ref {mnemonic}:Expr ) This shape declares the value of evaluating {mnemonic} to be the mnemonic for the name designated by the reference.

( explain ( namespace {marker}:Nat ) {doc}:Expr ) This shape declares the value of evaluating {doc} to be the documentation for the namespace associated with the marker {marker}. ( explain Ref {doc}:Expr ) This shape declares the value of evaluating {doc} to be the documentation for the name designated by the reference. Documentation expressions can be strings with plain text or use any BULK vocabulary to use a richer documentation format (including BULK forms to make a format of plain text explicit).

( define string {encoding}:Expr ) The semantics of this form is the side-effect that, in the scope of this expression, the default encoding for expressions that are understood by the application as character strings is the encoding designated by {encoding}. As the abstract yield doesn't contain strings but expressions that will be used as strings by the application, it is not a parsing error if the application doesn't recognize {encoding}. In this situation, it is only a parsing error when the application actually needs to decode a byte sequence as a string with that encoding. It is not a parsing error when a processing application only transmits a byte sequence encoding a string, if it can accurately convey the encoding to the receiving application.

( string {string}:Expr ) This form indicates that the bytes contained in the expression {string} are meant to be interpreted as a string encoded with the current default string encoding.

( string {encoding}:Expr {string}:Epxr ) This form indicates that the bytes contained in the expression {string} are meant to be interpreted as a string encoded with the encoding designated by {encoding}.

( iana-charset {id}:Expr ) This designates the string encoding registered among the IANA Character Sets whose MIBenum is {id}. Type: Encoding.

( bulk {bulk}:Expr ) This form indicates that the bytes contained in the expression {bulk} are meant to be interpreted as a BULK stream. If the stream doesn't start with a version form, the stream explicitly has the same version as the parent stream. The semantics of this form is the same as the evaluation of the BULK stream in {bulk} taken as a form. For example, these two forms have the same evaluation: ( 4 5 ) ( bulk true #[2] 4 5 ) This form can be useful to let the application reading a BULK stream skip parsing a large section. In that case, it MUST be enclosed in a values form to prevent the security issue described in .

( blob {blob}:Expr ) This form indicates that the bytes contained in {blob} are meant be interpreted as just a raw sequence of bytes, not to be decoded.

( concat {arrays} ) The value of this form is an array that contains the bytes contained in the first expression in {arrays} followed by the bytes contained in the second expression, and so on for each expression. It is an evaluation error if any expression in {arrays} doesn't evaluate to a container of bytes. ( concat ) evaluates to an empty array like #[0].

When writing a stream containing a big number of expressions where an application may want to access one of those expression without parsing all expressions before, one could imagine as a solution to use pointer-like references that each use the offset of some expression in the stream. This solution creates a security risk, because if reading according to the pointers doesn't produce the same result as parsing the stream without using them, an attacker might use this inconsistency to their advantage, when they can expect one application to use pointers and another application to use normal parsing, especially when the stream is big enough that verifying the consistency of the pointers might be costly enough that it might not be done or not in time to prevent the attack. Because of that risk, whenever a stream includes indexed BULK expressions, that is, expressions that are meant to be accessed by their byte position, indexed reading SHOULD be the only way used to access them. To that end, indexed data SHOULD be stored in arrays. When the goal of indexed data is to selectively parse only part of the BULK stream, a values form MUST be used to prevent the security issue described in .

( indexed-bulk {container}:Expr {start}:Expr ) The semantics of this form is the value of evaluating the BULK expression starting at offset {start} in the bytes contained in expression {container}. Beware that, although evaluations of all other BULK definitions follow lexical scoping, any definition used inside an indexed expression that isn't defined inside that same indexed expression follows dynamic scoping with respect to any places where it's used. Any definition made inside an indexed expression still follows lexical scoping.

( indexed-array {container}:Expr {start}:Expr {size}:Expr ) The semantics of this form is the value of an array whose content are {size} bytes, starting at offset {start} in the bytes contained in the expression {container}. ( indexed-array {container}:Expr {start}:Expr ) The semantics of this form is the value of an array whose content are the bytes starting at offset {start} in the array {container} until its end. Compared to indexed-bulk, which can reference an array expression, indexed-array is useful when several different but overlapping sections of the same byte sequence are needed as arrays, or when reversing packing through evaluation (to avoid packing the marker bytes).

true false Type: Boolean.

( subst {code} ) The semantics of this form is an expression of type LazyFunction, called the substitution function. The semantics of the substitution function are the semantics of {code}, but where the names arg and rest are substituted as such: ( arg {n}:Nat ) is replaced by the element number {n} (starting at zero) of the substitution function's arguments list. ( rest {n}:Nat ) is replaced by the substitution function's arguments list without its first {n} elements. It is an evaluation error if the substitution function is called with too few arguments with respect to the arg and rest forms in {code}.

Here is a definition of the inverse followed by the numbers 1/2, 1/3 and 1/4:

( define inverse ( subst ( fraction 1 ( arg 0 ) ) ) ) ( inverse 2 ) ( inverse 3 ) ( inverse 4 )

Substitution will splice multiple expressions in place: The evaluation of:

( define foo ( subst 20 ( rest 0 ) 50 ) ) ( 10 ( foo 30 40 ) 60 )

must produce: ( 10 20 30 40 50 60 )

( values {exprs} ) This form's semantics is the values produced by the evaluation of {exprs}. This means that the side-effects in {exprs} can affect its own evaluation, but not the scope of the values form. It makes it possible to isolate evaluation side-effects. This MUST be used for selectively parseable data, to prevent the security issue described in . Because the semantics of this form is only the values produced by the evaluation of its operands, this evaluation can wait until those values are actually needed. This property means that if a processing application uses lazy evaluation of BULK expressions, every expression in a values form is selectively evaluated only when needed, which in turns means that any nested BULK stream within a values form is selectively parsed only when needed.

A processing application must recognize the type of all expressions defined in this specification that have the type Nat, but an application MAY consider a number as having an unknown value if it can't decode its value or has no adequate data type to store it. It is only a parsing error if the number is needed by the parsing algorithm. It is only an evaluation error if the number is needed by the evaluation algorithm. In the text notation of a BULK stream, a decimal integer is the notation for the smallest byte sequence that yields this integer as described in . For example, ( 31 256 ) is a notation for the bytes 0x01 0x9F 0xC2-0100 0x02.

( unsigned-int {bits}:Expr ) The semantics of this form is the value of the unsigned integer represented in binary notation in the bits contained in {bits}. This form exists in case disambiguation of the semantics of an array (or another bit container) is necessary. Type: Number, Real, Int, Nat.

( signed-int {bits}:Expr ) The semantics of this form is the value of the signed integer represented in two's-complement notation in the bits contained in {bits}. Type: Number, Real, Int.

( fraction {num}:Expr {div}:Expr ) The semantics of this form is the fraction with denominator {num} and divisor {div}. Type: Number.

fraction makes it possible to express fixed-point numbers in BULK: ( fraction 15 4 ) has value 11.11₂ (3.75₁₀) ( fraction 123 100 ) has value 1.23 A more extensive arithmetic vocabulary could define forms to express fixed-point numbers according to a given base, as well as predefined fixed points (e.g. the use of fixed-point numbers with two decimals is pretty common with financial data).

( binary-float {bits}:Expr ) The semantics of this form is the floating-point number expressed in IEEE 754-2008 binary interchange format by the bits contained in {bits}. {bits} can be of size 16, 32, 64, 128 or any bigger multiple of 32 bits, as per IEEE 754-2008 rules. Types: Number, Real, Float.

( decimal-float {bits}:Expr ) The semantics of this form is the floating-point number expressed in IEEE 754-2008 decimal interchange format by the bits contained in {bits}. {bits} can be of size 32, 64, 128 or any bigger multiple of 32 bits, as per IEEE 754-2008 rules. Types: Number, Real, Float.

This specification and other official BULK specifications use forms with a reference operator as their basic building blocks. Basically, these are a binary representation of an abstract syntax tree. As noted previously, this means that most representations weigh 4 bytes plus their actual content, which will in turn have some overhead because of one or several marker bytes. But when there is a special need for compactness, BULK makes it possible to design protocols and formats with different trade-offs, while retaining its property of being parseable by processing applications not knowing the protocol in its entirety. On one end of the spectrum, a format might choose to use an array to encapsulate an ad hoc binary format. An extreme use of this scheme would be to use BULK just to make explicit the binary format used and for nothing else. With a known profile (for example with a file extension and/or media type for such explicitly typed BLOBs), such a BULK stream can consist solely of the version form, a reference that describes the binary format and an array, which would amount to an overhead between 11 bytes and 20 bytes depending on the size of the content (11, 13, 14, 16 and 20 bytes for contents of no more than 63B, 255B, 65kB, 4GB and 18EB respectively). Without a profile, with the namespaces associations in a package, the minimum overhead is only between 32 and 41 bytes (the difference is a single import form, assuming a digest of 64 bits). Still, even this extreme in the design space retains the ability to insert expressions in the BULK stream, whatever their type. Thus metadata can be added about data that is represented in a format that doesn't allow for metadata or that allows only for limited metadata. gives a few examples of what encapsulating existing media types in BULK could bring. In-between these two extremes, several options are available to produce a format that leverages the BULK parser a lot more while being more compact than a basic BULK format. The following forms provide a standard way to create such formats, called BULK bytecodes. A BULK bytecode is a flat sequence of expressions. The evaluation of a bytecode form transforms that sequence to an abstract syntax tree of its contents (and then the resulting expression can be evaluated with the normal BULK evaluation rules). The expressions of the bytecode are divided among bytecode operators and bytecode operands. Operators are references that will end up as form operators in the abstract syntax tree. Operands are all other expressions. Prefix bytecodes are those where operators come before their operands, postfix bytecodes are those where operators come after their operands. In the following forms, operators MUST be references. When evaluating a bytecode, it is an evaluation error when the processing application encounters a reference for which it cannot determine if it is an operator or its arity (the number of operands it will have). An expression that is not a reference is always an operand.

( prefix {bytecode} ) This is a prefix bytecode form. The bytecode to be transformed is the sequence of expressions in {bytecode}. To transform a prefix bytecode, a processing application creates an alternate context. If the first expression of the bytecode is an operand, it is removed from the beginning of the bytecode and appended at the end of the alternate context. If the first expression of the bytecode is an operator, it is removed from the beginning of the bytecode and a list is created with the operator as the first expression, then as many next expressions as its arity are removed from the beginning of the bytecode and appended at the end of this list. Then that resulting list is appended at the end of the alternate context. The transformation continues until the bytecode is empty, in which case the transformation is complete and the alternate context is the value of evaluating the bytecode form. The resulting form can then be evaluated in turn. Example: the evaluation of

( define ( arity prefix ) ( nil game ) ( 2 black ) ) ( prefix game black 1 2 black 3 4 black 5 6 )

is

( game ( black 1 2 ) ( black 3 4 ) ( black 5 6 ) )

It is an evaluation error when there are less expressions remaining in the bytecode than the arity of the current operator. In the error information, a processing application MAY provide the alternate context and the remaining bytecode. The alternate context after a failed transformation MUST NOT appear in the abstract yield as if evaluation had successfully transformed the bytecode.

( postfix {bytecode} ) This is a postfix bytecode form. The bytecode to be transformed is the sequence of expressions in {bytecode}. To transform a postfix bytecode, a processing application creates a data stack. If the first expression of the bytecode is an operand, it is removed from the beginning of the bytecode and pushed on top of the stack. If the first expression of the bytecode is an operator, it is removed from the beginning of the bytecode and a list is created with the operator as the first expression, then as many next expressions as its arity are popped from the stack and appended at the end of this list (with the top of the stack as the last element). Then that resulting list is pushed on top of the stack. The transformation continues until the bytecode is empty, in which case the transformation is complete and the list of expressions on the stack (with the top of the stack as the last element) is the value of evaluating the bytecode form. The resulting form can then be evaluated in turn. Example: the evaluation of

( define ( arity postfix ) ( nil game ) ( 2 black white comment alternative ) ) ( postfix game 1 2 black "white tried an unorthodox opening" 3 4 white comment "a more classical opening would be" 8 9 white comment alternative 2 3 black 4 5 white )

is

( game ( black 1 2 ) ( alternative ( comment "white tried an unorthodox opening" ( white 3 4 ) ) ( comment "a more classical opening would be" ( white 8 9 ) ) ) ( black 2 3 ) ( white 4 5 ) )

The obvious advantage of postfix bytecode is that it makes it possible to compact nested forms when they have a known arity. When a reference in a vocabulary can be used in a form containing a variable number of expressions, if some arity is used frequently enough, an application can define a specific form for it. The trade-offs for this are explained in It is an evaluation error when there are less expressions remaining on the data stack than the arity of the current operator. In the error information, a processing application MAY provide the data stack and the remaining bytecode. The data stack after a failed transformation MUST NOT appear in the abstract yield as if evaluation had successfully transformed the bytecode.

( define ( arity {contexts} ) {arities} ) This form defines the arity of references in the context of bytecodes. {contexts} can contain prefix, postfix, or any other reference, to specify in the context of which kind of bytecode the arities are modified. If {contexts} is empty, the arities are modified in the context of all kinds of bytecodes. {arities} is a sequence of expressions that each can be shaped as such: nil: meaning all known arities should be forgotten ( {kind}:Expr {target} ): if {kind} is nil, it sets all references designated by {target} as operands if {kind} is typed Nat, it sets all references designated by {target} as operators of arity {kind} if {target} is nil, it designates all references with unknown arity if {target} is a sequence of references, it designates each of those

If the overhead of several marker bytes in some operands is too much, more compactness can be achieved by packing together small operands. For example, instead of an operator with two integers as its operands, one could specify an operator to take a single array as operand and extract the integers from it. When the processing application does this extraction, the format retains the ability to operate on many sizes of integers, because the processing application can still deduce the size of the integers by dividing the size of the array by two. This can be used outside or inside of bytecodes (to stack the compacting effects of both). For example, a BULK format representing player moves with a pair of coordinates on a large board might represent a single move with the following shapes: ( move/2 #[1] 0x41 #[1] 0x5A ) ( move/1 #[2] 0x41 0x5A ) move/2 #[1] 0x41 #[1] 0x5A move/1 #[2] 0x41 0x5A Packing can also be done without adding its burden on the logic of the processing application, by using evaluation to transform packed forms into simpler forms (but they need to be created separately for each size of operands). For example, the following:

( define move/1-16 ( subst ( move/2 ( indexed-array ( arg 0 ) 0 1 ) ( indexed-array ( arg 0 ) 1 1 ) ) ) ) ( move/1-16 #[2] 0x415A )

Would be evaluated into:

( move/2 #[1] 0x41 #[1] 0x5A )

More complex packing can be encoded as well. For example, this would be a form that packs two 16-bits unsigned integers, one 32-bits signed integer, one reference, and one variable-length string:

( define pack-224-ref-str ( subst ( bar ( indexed-array ( arg 0 ) 0 2 ) ( indexed-array ( arg 0 ) 2 2 ) ( signed-int ( indexed-array ( arg 0 ) 4 4 ) ) ( indexed-bulk ( arg 0 ) 8 ) ( indexed-array ( arg 0 ) 10 ) ) )

In essence, packing makes it possible to embed very simple ad hoc binary formats described within the BULK framework.

The transformation defined for the bytecode forms makes it possible to mix literal expressions and operations represented by a sequence of operators and operands. A typical example would be that instead of explicitly encoding which player is playing in turn, to only encode the move and let the player information be implicit, when the order of players is dictated by the game logic. In the previous go example, for instance, one might represent each alternating move by the two players as two integers, lowering the weight of each normal move to 2 bytes as coordinates are below 64:

( define ( arity postfix ) ( nil game ) ( 2 black white comment alternative ) ) ( postfix game 1 2 "white tried an unorthodox opening" 3 4 white comment "a more classical opening would be" 8 9 white comment alternative 2 3 4 5 )

The difference between all these schemes and an array containing fixed-size elements is that you keep the ability to insert other forms, like here to represent comments on the game or variants.

The most visible cost of the bytecode format is that if it contains operators whose arity is unknown to a processing application, the whole list after the first occurrence of them is unreadable to that processing application, whereas in the basic format, the processing application can still process all the forms it understands, and that requires no anticipation by the application creating the BULK stream. The only case where operators could have an unknown arity is when the application writing the stream didn't include the arities of every operator used in the stream to avoid the redundancy with their previous definition (typically in the definition of their respective namespaces). That redundancy would be offset by the space reduction of postfix bytecode for streams containing a few dozens forms. At that point, with all arities explicit, with packing and with literals for the most used forms, postfix bytecode gets on the Pareto front for size and generality, retaining the full generality of BULK while saving a lot of space. It is RECOMMENDED that whenever explicit arities would be a small fraction of the total stream size, they all be given. A processing application MAY choose to never include explicit arities for the names of the main namespace of the format, if that namespace's definition includes arities, when processing the stream without knowledge of that definition wouldn't make sense. But another cost of the bytecode format is the loss of resilience. If a BULK stream ends up with errors in the content, whether during creation or transit, but those errors don't affect the syntactic structure of the stream, then those errors will only prevent processing the form they are in. If errors occur within a metadata form, the data is still readable. If errors occur within one entry in an archive, other entries are still readable. But a far wider class or errors will make a bytecode impossible to evaluate, and the blast radius is everything after the error. When a protocol needs the exchange of messages where every byte spared counts and there is no sense in trying to recover a partial message after corruption, then using a packed bytecode is probably an excellent solution. When writing large quantities of small data elements to long-term storage, when overhead adds up significantly, it is RECOMMENDED to use a mechanism to limit the blast radius of possible data corruption. There are several possible solutions to limit where bytecode errors propagate: one is chunking the bytecode into several bytecode forms, another is inserting at regular intervals a beacon expression that can never be an operand, and that can "reset" the bytecode transformation process that had been corrupted before. The former is simpler but the latter might be better suited when data is streamed (see ).

A profile is a byte sequence parsed by a processing application just after the version form or before the first expression if there is no version form. Thus a parser SHOULD look ahead at the beginning of a stream to see if the first three bytes are ( bulk:version. With respect to the BULK stream, the profile is an out-of-band information, usually implicit. A processing application doesn't need to actually parse the profile or include the profile's yield in the concrete yield, as long as the semantics of the abstract yield are maintained. The same BULK stream might be processed with different profiles. A processing application MUST NOT deduce the profile from the content of a BULK stream.

A processing application SHOULD only rely on the use of a profile when it is a safe assumption that the profile is known, for example within a communication where the protocol dictates the profile. In particular, long-term storage of a BULK stream SHOULD preserve profile information, for example with a media type that dictates the profile. Otherwise, an application writing a BULK stream in a long-term storage SHOULD include the profile after the version form. For this reason, the expressions in a profile SHOULD have idempotent semantics.

This specification defines the default profile that a processing application MUST use when it is not using a specific profile: ( define string ( iana-charset 106 ) ) This means that the default string encoding in a BULK stream is UTF-8.

Fixed BULK is a mode of processing for a format or protocol where evaluation has been deemed detrimental (it could be that it's too expensive computationally, or that it adds too much complexity in the processing application's code or the protocol). In Fixed BULK mode, a processing application uses a profile that contains one or several namespace associations and possibly definitions. Because evaluation is disabled, no namespace association or definition will be executed during processing and namespaces are "fixed" to their markers as per the profile. Fixed BULK mode lets a format or protocol use BULK's syntax while operating more like binary format frameworks, like ASN.1, Protocol Buffers, or CBOR. An interesting difference is that the concatenation of the profile and the Fixed BULK stream is a normal BULK stream.

When a processing application encounters an unknown namespace or package identifier, it MAY ask several sources to provide the definition. This is called discovery. The possible sources include the agent that made use of the unknown identifier, known BULK registries (registries of definitions for BULK namespaces or packages), or protocols based on content-addressing. When discovery is done without user intervention while processing a BULK stream, in order to evaluate it fully, it is called immediate discovery. When missing identifiers are collected to be retrieved later, under human supervision, it is called deferred discovery. The fundamental security risk in this mechanism is when an attacker manages to be the first to use some identifier and poison the processing application by giving it a malicious definition, or managed to poison one or several BULK registries. To avoid that risk, a processing application MUST only accept definitions for immutable namespaces and immutable packages during immediate discovery. It is also RECOMMENDED that a BULK registry only store definitions for immutable namespaces and immutable packages. During deferred discovery, a processing application MUST NOT accept namespace or package definitions from an untrusted source when they are not immutable (including when the digest doesn't match the data, or when the identifier form is not known to be a digest by the processing application). A BULK registry MUST NOT store namespace or package definitions from an untrusted source when they are not immutable. While discovery of bootstrapping namespaces and packages can be done, the digest algorithm used in the identifier of a bootstrapping namespace or package MUST have been known before discovery (this can only be checked after discovery has retrieved and evaluated the definitions). This is possible when a bootstrapping package uses a digest from a namespace that was known by the processing application before discovery, or when the digest name comes from a namespace where it is aliased to a digest name that was known by the processing application before discovery. With immutable namespaces and immutable packages, though, automatic discovery can be a safe mechanism if some risks are mitigated: Asking untrusted registries might expose when other agents communicate with the processing application. Making the request asynchronously, with noise in the timing, can mitigate that risk. Application operators need to consider the trade-off between latency and privacy, or give the option to agents to make that determination. Making the request in ways that hide the processing application's identity, like using The Onion Router, is another option. Asking untrusted registries might expose what kind of data is sent by other agents to the processing application. Making the request in ways that hide the processing application's identity, like using The Onion Router, is an option. Asking the agent or not might expose the fact that a namespace was already known or not. Some processing applications SHOULD provide configurable policies so that operators can choose one that is relevant to the sensitivity of the services they operate. Examples could include: always asking for namespaces that haven't been explicitly marked safe, always or never asking to some classes of agents.

The parsing and evaluation algorithms allow for reading a BULK stream while it has not been completely received by the processing application. Once the parser reaches a dispatch point (see ), the fully parsed expression can be evaluated if evaluation is enabled. This makes BULK usable for streaming data, including in the case of a communication protocol with a long-lived connection where requests and responses must be processed immediately, including a full-duplex communication (see ).

It is possible to stream BULK data in a broadcast setting, meaning that a processing application could start receiving data mid-stream and never see the data sent before it started listening to the broadcasted stream. A broadcasted BULK stream MUST NOT contain expressions whose evaluation have side-effects when their scope is the abstract yield. If it contained such expressions, the same processing application that started listening at different points in the stream could produce different results for a common portion of the concrete yield. This means that a protocol that employs BULK broadcasting and uses any extension namespace MUST provide a profile, either in the protocol specification, or during connection establishment. For the latter, when a BULK stream is broadcasted by HTTP, the server can use the bulk-profile link relation in headers (see ). When broadcasting BULK, one issue is stream capture, to discover an offset in the stream that is a dispatch point. This specification describes four ways: server clipping, beacon expressions, Ogg encapsulation and Magrat encapsulation, but others are possible.

Conceptually, the simplest solution to stream capture is just for the server to always start sending data from a dispatch point. If the server streaming data can be aware of the internal structure of the BULK stream, it can make new clients wait until the next dispatch point to start sending them data.

To signal some of the dispatch points, the abstract yield of the BULK stream contains an expression whose byte pattern is unique in the stream, repeated frequently enough to minimize the length of bytes that a processing application must go through before achieving stream capture. After reading that byte pattern, the processing application can start parsing BULK expressions. The problem with the idea of a beacon expression is that because BULK arrays can contain arbitrary bytes, no beacon expression exists that cannot appear in a BULK stream. There are two solutions. First, in a variety of situations, an application could know or preclude a byte pattern to appear in the stream, and chose that expression as beacon. Second, an application could choose a byte pattern that can only appear in a BULK array and, whenever a BULK array contains that byte pattern, represent it in the broadcasted BULK stream as the concatenation of its split across the beacon pattern. For example, if the beacon expression is 0xC3AABBCC, the expression #[8] 0x0000-C3AABBCC-0000 would become ( concat #[4] 0x0000-C3AA #[4] 0xBBCC-0000 ).

The Ogg format already provides an efficient mechanism for stream capture with a relatively low overhead. It can stream and multiplex data from multiple media types. The BULK stream MUST begin with a version form, and the beginning of that form constitutes the codec identifier: ( version 1 (see ). Ogg packets provided to the Ogg encoder MUST start and end at dispatch points. This ensures than any complete Ogg packet provided to the processing application by the Ogg decoder is a valid BULK stream and can be parsed into zero or more expressions. Granule position SHOULD be the number of expressions parsed in the abstract yield after parsing the Ogg packet.

The Magrat encapsulation is inspired by the Ogg format and has similar properties, except that it is less concerned with audio and video, is less powerful, and takes less space (hence the name). A Magrat broadcast stream is a sequence of Magrat forms. The BULK stream that is encapsulated inside a Magrat broadcast stream is called the Magrat embedded stream. This specification documents basic Magrat encapsulation. In this version, a Magrat form has the following shape: ( values ( bulk {chunk}:Bytes ) {checksum}:Expr ) This means that a processing application can look for the sequence of six bytes 0x01-1013-01-1009 to mark the beginning of a Magrat form. In case those bytes were in a BULK array, there are several features that work to verify that they actually started a Magrat form: first, they must be followed by an array {chunk}, a form end, a single expression {checksum} and another form end, second, {checksum} MUST be a digest that matches the bytes contained in {chunk}. Those bytes are called the Magrat chunk and they MUST be a valid BULK stream. A protocol using Magrat encapsulation MAY specify which kind of checksum can be used. In the basic Magrat encapsulation, the concatenation of chunks is the embedded stream. An extension of basic Magrat encapsulation MAY add metadata forms inside the chunk that are removed before the chunks are concatenated as the embedded stream.

At the BULK level, the simplest way to do full-duplex communication is when the underlying protocol can create separate channels for each direction. Each agent streams its own BULK stream that is fully independent from the other agent's stream. There is no restriction on the semantics used in the streams and an agent can use namespace associations and definitions to build a complex data model. Each agent is faced with the usual security considerations of BULK processing (see ). An agent faced with BULK data that crosses a safety threshold SHOULD stop processing the other agent's stream. For that reason, a protocol using separate channels for BULK full-duplex communication SHOULD provide a way for an agent to end a channel. This SHOULD include the reason for ending the channel, and if the agent offers the option to restart the channel from scratch. It MAY also include the option to restart the channel up to a previous safe dispatch point.

When two agents want to communicate over a bidirectional channel and reference data sent by each other, one naive way to do it would be to consider a virtual BULK stream acting as a kind of shared whiteboard. Each agent sending a BULK expression would add that expression to the white board. When the agents have a mechanism to ensure some transactional safety, meaning that one agent cannot write without having properly read what the other agent had written, this is a valid option. This can even work for more than two agents. For when this safety is not available, this specification defines BULK's basic full-duplex protocol. In the basic protocol, the fact that it is used is an out-of-band information, which could be part of the underlying protocol statically or conveyed during connection establishment. Another BULK full-duplex protocol could instead define a namespace to convey its use and some configuration parameters. In the basic protocol, the agent that initiates the communication is called the initiating agent and the other agent is called the responding agent. The initiating agent and the responding agents each have a set of namespace markers designated for their use. The initiating agent's designated markers are even numbers. The responding agent's designated markers are odd numbers. Agents MUST only associate immutable namespaces and MUST only associate them to their designated markers and MUST NOT associate a namespace to a marker that already is associated to a namespace. Agents MUST only define names that don't already have a definition, and only to references whose namespace marker is in their designated markers. It is a protocol error when any of those rules is broken by either agent. There is a separate virtual BULK stream associated with each agent. Each agent has a pull position, which is an offset in the other agent's virtual stream. At the beginning of the exchange, each agent's pull position is 0, the beginning of the stream. Let Alice and Bob be two agents in a full-duplex communication. When Alice streams a BULK expression A1 that only contains references with standard namespaces or Alice's designated markers, this expression A1 is appended to Alice's virtual stream. But when Alice streams a BULK expression A2 that contains one or several references with Bob's designated markers that got a definition in Bob's virtual stream, through namespace association or by definition, after Alice's pull position, those definitions are pulled, i.e. the content of Bob's virtual stream between Alice's pull position and the first dispatch point where all those references have a definition gets appended to Alice's virtual stream, Alice's pull position becomes that dispatch point, then A2 is appended to Alice's virtual stream. When Alice streams a BULK expression A3 that contains one or several references with Bob's designated markers, but all those references got a definition before the pull position, only A3 is appended to Alice's virtual stream. If both agents have associated a single namespace to correct designated markers for each one, and both agents have independently defined a name that wasn't defined in the namespaces's immutable definition, it is a protocol error when either agent pulls the other agent's definition of that name. Whenever an agent sees a protocol error, it MUST end the connection. Before ending the connection, the agent MAY stream an expression shaped ( explain false {reason}:String ), in which case {reason} MUST contain a human-readable description of the issue that triggered the disconnect.

Parsing a BULK stream is designed to be free of side-effects for the processing application, apart from storing the parsed results. Arrays in BULK carry their size, to avoid the need for escaping their content. A malicious software, however, may announce an array with a size chosen to get an application to exhaust its available memory. When a BULK stream has been completely received, an array bigger than the remaining data is a parsing error. When a BULK stream's size is not known in advance, the application SHOULD use a growable data structure. Evaluation opens up some known attacks that appear whenever a format provides a way to express abstraction, like the billion laughs attack. As it is explained in , an implementation MAY stop evaluation after a predefined number of evaluation steps. As this has been demonstrated not to be sufficient to prevent attacks based on expansion, an implementation SHOULD also put predefined limits on the space that the concrete yield can take on disk or in memory. A processing application SHOULD use lazy immutable data structures to represent array concatenation and array indexing, as a defence against evaluaton attacks. For example, in the billion laughs attack, the resulting concatenation would produce 9 lists of 10 pointers and one actual array of 3 characters, instead of an array of 3 billion characters. Applications MAY use out-of-band information to select size limits (like HTTP attributes), or a BULK namespace MAY provide hints.

When a processing application forwards all or part of the data in a BULK stream to another application, care must be taken if part of the forwarded data was not entirely recognized, as it could be used by an attacker to benefit from the authority the forwarding application has on the recipient of the data. If a protocol deems it necessary for applications to be able to forward data they don't fully understand, a known protection from that threat is the use of capability security, where the agent that provides the data to be forwarded must also provide the explicit authority that will be used after forwarding. If the authority of the forwarding application is not used, it cannot be abused.

The architecture of a processing application SHOULD ensure that a malicious agent cannot abuse authority given to it to define a namespace in order to modify associations in other namespaces. Depending on the use of data structures storing BULK expressions, this could amount to giving an attacker a way to manipulate the application's state. See for an example of architecture that is resistant to that kind of attack.

It could be a security risk if a single BULK stream could be parsed into two different abstract yields by two conformant applications, so the evaluation of the whole stream cannot change whether some part that is designed to be selectively parseable is decoded or not. For that reason, any side-effects in the selectively parseable expressions that affect how BULK expressions are evaluated (like namespace associations or definitions) MUST be isolated. For that security reason, there isn't a ( bulk-with-size Nat Expr ) form to make the expression skippable, because it would open up that risk when the size given is not the actual size of the enclosed expression, accidentally or maliciously. Whenever BULK data is selectively parseable, it MUST be enclosed in a values form.

This specification doesn't address in too much detail the security considerations that a BULK format or protocol would need to include, because of the wide diversity of use cases for BULK. But gives some hints and resources on the subject.

This specification defines two new media types, application/bulk and text/bulk. Here are the informations for its registration to IANA :

application bulk N/A N/A none, content is self-describing cf. N/A this document the BARK manifest prototype this specification defines no semantics for addressing the data with a fragment identifier; a future specification MAY define fragment identifier syntaxes to address the content by byte offset or the parsed results by their position in the abstract yield
the constraint to start any BULK file with a version form has the side-effect that classes of BULK streams can be identified by a sequence of bytes acting as "magic number", at offset 0: any BULK stream a BULK stream of major version 1 a BULK stream of version 1.0 .bulk
this specification defines a suffix +bulk for naming media types that use BULK as their core syntax

text bulk N/A N/A content MUST be encoded in UTF-8 cf. N/A this document, the BARK manifest prototype this specification defines no semantics for addressing the data with a fragment identifier; a future specification MAY define fragment identifier syntaxes to address the content by byte offset or the parsed results by their position in the abstract yield
The text notation allows for arbitrary number and kinds of whitespaces around lexical elements, so there are no "magic numbers" as such but, in most cases, BULK streams in text notation will not have leading whitespace and use a single space within the first version form, so the first characters can identify classes of BULK streams: '( version ' or '( bulk:version ': any BULK stream '( version 1 ' or '( bulk:version 1 ': a BULK stream of major version 1 '( version 1 0 )' or '( bulk:version 1 0 )': a BULK stream of version 1.0 .bulktext

This specification defines a new link relation type, bulk-profile. Here are the informations for its registration to IANA : bulk-profile This link target is the BULK stream that a processing application SHOULD use as the profile of the link's context. this document

This specification defines a new Ogg logical bitstream type, for the media type application/bulk: char[4]: '\x01\x10\x00\x81' bulk1 For more details, see . A BULK-aware Ogg decoder could anticipate future BULK versions and recognize any version form conformant with as codec identifier with the encompassing codecs parameter "bulk".

The original author of this specification read Erik Naggum's famous rant about XML several years before, and while forgotten as such for a time, it definitively was the seed that slowly bloomed into the design of BULK. This format is dedicated to Erik. Unknowingly, work on BULK started just as CBOR was in Request for Last Call at IETF. The early design goals of BULK and the design goals of CBOR had both significant differences and a large common ground. It felt like an implicitly obvious choice to make the parser a simple state machine that could be implemented with a jump table but CBOR's inspiration was to make fast processing speed and low processing footprint explicit requirements. The idea to store together marking bits and a small argument in a marker byte was a direct inspiration from both CBOR and MessagePack and it made BULK's syntax and implementation both drastically simpler.

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BULK's text notation can be used as a full-fledged format alongside BULK's binary syntax. The text format has a different trade-off. On one hand, it is readily human-readable and it is easy to author in the absence of any tooling, because it is plain text. On the other hand, it is less robust in several ways: it is more dependent on the processing application knowing the definitions of namespaces and packages, it is rigid with respect to encoding (it MUST be encoded in UTF-8), and it is limited and inefficient in its representation of arbitrary bytes and strings. While parsing BULK text notation is a bit more involved than parsing binary BULK, the syntax of the text notation is still deliberately simple so as to limit even that part's processing footprint. Conceptually, parsing text notation involves translating the notation into binary BULK and then processing that. Each lexical element of BULK's text notation is separated from other elements by whitespace. Apart from ([ and ]), delimiting an array containing a BULK stream, every lexeme can be immediately translated into its binary representation. Translating reference mnemonics involves knowing the mnemonics of previously imported namespaces, which means that as complete expressions are produced by the parser, they need to be evaluated. Reference mnemonics can appear without a namespace mnemonic if there aren't two imported namespaces that both use that mnemonic for a name. When a reference mnemonic appears with a known namespace mnemonic but an unknown name mnemonic, a processing application MUST associate that mnemonic with the first name in that namespace that doesn't already have a mnemonic. This makes it easy to author a namespace definition without having to manually number names.

This constitutes a suggestion of architecture for a BULK processing application. It has the advantage that an agent cannot modify the values of names to which it has not specifically been given authority. This architecture doesn't ensure this property by checking the validity of definitions but by adhering to the Principle Of Least Authority, thus ensuring no false positives or TOCTOU race conditions. For each new context (including the abstract yield when parsing starts), the parser creates a new copy of each known namespace. These copies are available in this context to retrieve and define values. It implements the lexical scoping of definitions on top of providing the robustness properties discussed here. By default, all namespaces created in a context are discarded at the end of this context. Of course, an implementation of the architecture presented here can be optimized compared to the abstract algorithm, for example by using copy-on-demand. Any namespace that is not a copy for its context but the object retained by the application afterwards, gives authority to make long-lasting definitions. A namespace that is stored by the processing application after evaluating a BULK stream is called a lasting namespace. Note that there are two ways to define a namespace in a BULK stream: using only the definition in the ( define ( namespace {…} ) form, or using this (possibly empty) definition as modified by other definitions (in the same BULK stream or not). The former is called the initial definition, the latter the amended definition.

When the amended definitions of all namespaces constitute lasting namespaces, it means that the evaluation of the BULK stream can modify any existing namespace. This level of authority might be useful for a handful of privileged BULK streams acting as configuration of the application (e.g. to achieve reverse aliasing, see ).

A number of lasting namespaces are included for the abstract yield. Their unique identifiers are agreed out-of-band. The disadvantage of this solution is that it needs prior agreement on the definable namespaces. This may be a safer way than complete authority to achieve reverse aliasing (see ).

Any namespace definition for a unique identifier unknown to the processing application triggers the creation of a lasting namespace. The disadvantage of this solution is that it opens a denial of service vulnerability. If Bob is a processing application and Carol and Dave are agents communicating with Bob with an open authority, Dave can prevent Carol from defining a namespace if it manages to know the unique identifier and to start a communication with Bob before Carol. If an agent uses a secure way to create unique identifiers, this solution is both flexible and safe (the burden is not on the BULK processing application). This specification thus encourages the use of open authority restricted to verifiable namespaces (in which case several agents can present the same definition to a processing application without conflict). A processing application could have a configuration setting to select what will generate a lasting namespace: any time a namespace is defined with an identifier that was previously unknown, either its initial or amended definition constitutes a lasting namespace; this is the most lax open authority, most flexible but also most open to misuse and issues any time a namespace is defined with an identifier that was previously unknown and the identifier relies explicitly on an algorithm that the processing application deems giving a high enough guarantee that identifiers are unique, either the initial or amended definition of that namespace constitutes a lasting namespace any time an immutable namespace is defined, its initial definition constitutes a lasting namespace It is RECOMMENDED to use immutable open authority by default, as several agents can safely present the same definition to a processing application without conflict.

BULK makes it possible to create new versions of vocabularies that encompass previous versions, in a way that minimizes implementation complexity. The first tool is aliasing: reuse names and values from existing namespaces, even in bootstrapping namespaces:

( define ( namespace ( newhash:shake128 {newhashid} ) 20 ) ([ ( version 1 0 ) ( ( namespace 20 ) ) ( mnemonic ( namespace 20 ) "newhash" ) ( explain ( namespace 20 ) "The new, shiny hash namespace!" ) ( mnemonic newhash:shake128 "shake128" ) ( import 21 ( namespace ( oldhash:shake128 {oldhashid} ) ) ) ( define newhash:shake128 oldhash:shake128 ) ]) )

With this, new namespaces can be created and applications don't need to change the existing code. One possible downside with aliasing is that if the number of aliasing namespaces grow, you might end up with the implementation of an important namespace scattered across a bunch of aliased legacy namespaces. Also, the definition of the new namespace is tied to the old one, which means that you need to keep the old definition around for the lifetime of the new one. To prevent those issues, a second tool is to reverse the direction of aliasing: all the implementation lives in the current namespace, cohesively, and its definition can be used on its own, and the old namespace is aliased to the new:

( import 20 ( namespace ( oldhash:shake128 {oldhashid} ) ) ) ( import 21 ( namespace ( newhash:shake128 {newhashid} ) ) ) ( define oldhash:shake128 newhash:shake128 )

Following the Principle of Least Authority, it should not be possible by default for the evaluation of any BULK stream to make lasting modifications to existing namespaces. One obvious design would be for the application to have a privileged storage for reverse aliasing namespace definitions, with those namespaces given complete authority or, better yet, each being given selective authority for a specific existing namespace (see ). Where this could still not be deemed safe enough, reverse aliasing of namespaces could be defined in the application's code.

Sometimes a vocabulary will include forms that can contain an arbitrary number of expressions. When such a form is used in postfix bytecode, the simplest solution is just to use a nested postfix form:

( define ( arity ) ( 2 black white comment ) ) ( postfix game 1 2 black ( postfix alternative "white tried an unorthodox opening" 3 4 white comment "a more classical opening would be" 8 9 white comment ) 2 3 black ( postfix alternative "white played a bad move" 4 5 white comment "white could have played a decent move" 5 6 white comment "white could have played a great move" 5 7 white comment ) )

The nested postfix form costs 4 bytes, compared to an equivalent postfix bytecode. If those 4 bytes add up to too much space through repetition, an application could define a form for the sole purpose of assigning it an arity, while the evaluation of the arity-carrying form would just replace it with the original one. For example, after evaluating the postfix bytecode transformation and the resulting form of the last expression of

( define alt/2 alternative ) ( define alt/3 alternative ) ( define ( arity ) ( 2 black white comment alt/2 ) ( 3 alt/3 ) ) ( postfix game 1 2 black "white tried an unorthodox opening" 3 4 white comment "a more classical opening would be" 8 9 white comment alt/2 2 3 black "white played a bad move" 4 5 white comment "white could have played a decent move" 5 6 white comment "white could have played a great move" 5 7 white comment alt/3 )

it would be transformed into

( game ( black 1 2 ) ( alternative ( comment "white tried an unorthodox opening" ( white 3 4 ) ) ( comment "a more classical opening would be" ( white 8 9 ) ) ) ( black 2 3 ) ( alternative ( comment "white played a bad move" ( white 4 5 ) ) ( comment "white could have played a decent move" ( white 5 6 ) ) ( comment "white could have played a great move" ( white 5 7 ) ) ) ( white 4 5 ) )

Such an arity-carrying form costs 10 or 13 bytes to be usable when it is added to an existing form defining arities. Which means that compared to the nested postfix form, it pays for itself if it is used only 3 or 4 times.

BULK aims at being a useful framework for a wide variety of formats, including low-level communication protocols, higher-level RPC or REST APIs, media files, rich documents, archives and efficient serialization of existing data models (like XML, JSON or RDF). This had several implications on its design.

As such, BULK imposes no constraints on what kind of data can be represented. One is free to design a BULK format that mandates the use of EBCDIC. In keeping with , BULK chooses UTF-8 as the default string encoding and its core namespace only permits designating encodings from the IANA charset registry. But a BULK vocabulary would be free to define a new windows-codepage form to use Windows Codepages instead. Any protocol designed to transport human-readable text should be aware of , but BULK could be used to transport text emitted in a terminal, making full use of control characters, or even to create a file with examples of ill-formed UTF-8 strings containing surrogates. As such, BULK doesn't limit what code points are allowed in UTF-8 strings, or any other Unicode encoding. BULK doesn't include a grammar to define BULK formats, but a BULK grammar vocabulary, that could encode ABNF or CDDL in BULK, would do well to add the ability to express "Unicode Scalars", "XML Characters" and "Unicode Assignables" from , as well as classes and profiles from PRECIS.

For the same reason, BULK parsing and evaluation needed to be secure by default and feature a security model that would be safe enough that it can be a secure foundation almost everywhere. This is why BULK syntax and the BULK core namespace can't directly express notions like the inclusion of an outside BULK stream, or referencing a file or URI to access. This is also why this specification limits the discoverability of bootstrapping namespaces and strongly limits discoverabilty of non immutable namespaces and packages. A BULK format that can express such a dangerous combination of actions as reading files and making network connections SHOULD carefully consider the attack surface they present and the threat models for the format's use cases, and explain those in detail in the format's documentation. Dhall's Safety Guarantees are a prime example. The BULK core namespace can't directly express Turing complete functions, and not even functions that can take different execution paths depending on their arguments. The functions that can be expressed can only use their arguments in a static transformation, by design. While this drastically limit BULK's expressivity, it also drastically limit the attack surface on what arbitrary input can make the BULK parser or evaluator do, while still offering a decent power of abstraction (e.g. one could write a substitution function to unpack an array containing an IPv6 header into its fields, but not a substitution function to unpack an IPv6 extension header according to its type). The goal is that a BULK protocol or format designer should be able to trust that if they use BULK in accordance with the safety recommendations of this specification, they don't need to carefully weigh the benefits of evaluation vs. its dangers, like it has been the case with a couple of previous formats.

One possible use of BULK is the ability to add metadata around a file. The most basic metadata is the media type and its parameters. Although many media types can be expressed with a file extension, this usually doesn't encode parameters like the charset used for a plain text file format. This is why most spreadsheet software present the user with a preview of a few rows while asking them for the encoding, when importing CSV data. A media type vocabulary would make it possible to encode parameters and provide names for some known parameter values. While a .md file extension only encodes the media type text/markdown, a short BULK header could encode the full media type text/markdown; charset=ISO-8859-15; variant=GFM: ( version 1 0 ) ( import 20 ( package ( shake128 #[8] 0xDABBED01 ) 2 ) ) ( markdown ( iana-charset 111 ) "GFM" ) While a .csv file extension only encodes the media type text/csv, a short BULK header could encode the full media type text/csv; charset=UTF-8; header=absent ( version 1 0 ) ( import 20 ( package ( shake128 #[8] 0xDABBED01 ) 2 ) ) ( csv ( iana-charset 106 ) csv-header-absent ) Media type metadata could be mixed with other metadata: ( version 1 0 ) ( import 20 ( package ( shake128 #[8] 0xDABBED02 ) 4 ) ) ( description ( media-type ( csv ( iana-charset 106 ) csv-header-present ) ) ( licence cc0 ) )