Network Working Group G. Trewitt Request for Comments: 1076 Stanford University Obsoletes: RFC 1023 C. Partridge BBN/NNSC November 1988 HEMS Monitoring and Control Language TABLE OF CONTENTS 1. Status of This Memo 1 Introduction 2 2. Overview and Scope 2 3. Overview of Query Processor Operation 4 4. Encoding of Queries and Responses 5 4.1 Notation Used in This Proposal 5 5. Data Organization 6 5.1 Example Data Tree 7 5.2 Arrays 8 6. Components of a Query 9 7. Reply to a Query 10 8. Query Language 12 8.1 Moving Around in the Data Tree 14 8.2 Retrieving Data 15 8.3 Data Attributes 16 8.4 Examining Memory 18 8.5 Control Operations: Modifying the Data Tree 19 8.6 Associative Data Access: Filters 21 8.7 Terminating a Query 26 9. Extending the Set of Values 27 10. Authorization 27 11. Errors 28 I. ASN.1 Descriptions of Query Language Components 29 I.1 Operation Codes 30 I.2 Error Returns 31 I.3 Filters 33 I.4 Attributes 34 I.5 VendorSpecific 36 II. Implementation Hints 36 III. Obtaining a Copy of the ASN.1 Specification 42 1. STATUS OF THIS MEMO This RFC specifies a query language for monitoring and control of network entities. This RFC supercedes RFC-1023, extending the query language and providing more discussion of the underlying issues. Trewitt & Partridge [Page 1] RFC 1076 HEMS Monitoring and Control Language November 1988 This language is a component of the High-Level Entity Monitoring System (HEMS) described in RFC-1021 and RFC-1022. Readers may wish to consult these RFCs when reading this memo. RFC-1024 contains detailed assignments of numbers and structures used in this system. Portions of RFC-1024 that define query language structures are superceded by definitions in this memo. This memo assumes a knowledge of the ISO data encoding standard, ASN.1. Distribution of this memo is unlimited. INTRODUCTION This RFC specifies the design of a general-purpose, yet efficient, monitoring and control language for managing network entities. The data in the entity is modeled as a hierarchy and specific items are named by giving the path from the root of the tree. Most items are read-only, but some can be "set" in order to perform control operations. Both requests and responses are represented using the ISO ASN.1 data encoding rules. 2. OVERVIEW AND SCOPE The basic model of monitoring and control used in this memo is that a query is sent to a monitored entity and the entity sends back a response. The term query is used in the database sense -- it may request information, modify data, or both. We will use gateway- oriented examples, but it should be understood that this query- response mechanism is applicable to any IP entity. In particular, there is no notion of an interactive "conversation" as in SMTP [RFC-821] or FTP [RFC-959]. A query is a complete request that stands on its own and elicits a complete response. In order to design the query language, we had to define a model for the data to be retrieved by the queries, which required some understanding of and assumptions to be made about the data. We ended up with a fairly flexible data model, which places few limits on the type or size of the data. Wherever possible, we give motivations for the design decisions or assumptions that led to particular features or definitions. Some of the important global considerations and assumptions are: - The query processor should place as little computational burden on the monitored entity as possible. - It should not be necessary for a monitored entity to store the complete query. Nothing in the query language should Trewitt & Partridge [Page 2] RFC 1076 HEMS Monitoring and Control Language November 1988 preclude an implementation from being able to process the query on the fly, producing portions of the response while the query is still being read and parsed. There may be other constraints that require large amounts of data to be buffered, but the query language design must not be one. - It is assumed that there is some mechanism to transport a sequence of octets to a query processor within the monitored entity and that there is some mechanism to return a sequence of octets to the entity making the query. In HEMS, this is provided by HEMP and its underlying transport layer. The query language design is independent of these details, however, and could be grafted onto some other protocol. - The data model must provide organization for the data, so that it can be conveniently named. - Much of the data to be monitored will be contained in tables. Some tables may contain other tables. The query language should be able to deal with such tables. - We don't provide capabilities for data reduction in the query language. We will provide for data selection, for example, only retrieving certain table entries, but we will not provide general facilities for processing data, such as computing averages. - Because one monitoring center may be querying many (possibly hetrogenous) hosts, it must be possible to write generic queries that can be sent to all hosts, and have the query elicit as much information as is available from each host. i.e., queries must not be aborted just because they requested non-existent data. There were some assumptions that we specifically did not make: - It is up to the implementation to choose what degree of concurrency will be allowed when processing queries. By locking only portions of the database, it should be possible to achieve good concurrency while still preventing deadlock. - This specification makes no statement about the use of the "definite" and "indefinite" length forms in ASN.1. There is currently some debate about this usage in the ISO community; implementors should note the recommendations in the ASN.1 specification. Trewitt & Partridge [Page 3] RFC 1076 HEMS Monitoring and Control Language November 1988 Other RFCs associated with HEMS are: RFC-1021 Overview; RFC-1022 Transport protocol and message encapsulation; RFC-1024 Precise data definitions. The rest of this report is organized as follows: Section 3 Gives a brief overview of the data model and the operation of the query processor. Section 4 Describes the encoding used for queries and responses, and the notation used to represent them in this report. Section 5 Describes how the data is organized in the monitored entity, and the view provided of it by the query processor. Section 6 Describes the basic data types that may be given to the query processor as input. Section 7 Describes how a reply to a query is organized. Section 8 Describes the operations available in the query language. Section 9 Describes how the set of data in the tree may be extended. Section 10 Describes how authorization issues affect the execution of a query. Section 11 Describes how errors are reported, and their effect on the processing of the query. Appendix I Gives precise ASN.1 definitions of the data types used by the query processor. Appendix II Gives extensive implementation hints for the core of the query processor. 3. OVERVIEW OF QUERY PROCESSOR OPERATION In this section, we give an overview of the operation of the query processor, to provide a framework for the later sections. The query language models the manageable data as a tree, with each Trewitt & Partridge [Page 4] RFC 1076 HEMS Monitoring and Control Language November 1988 branch representing a different aspect of the entity, such as different layers of protocols. Subtrees are further divided to provide additional structure to the data. The leaves of the tree contain the actual data. Given this data representation, the task of the query processor is to traverse this tree and retrieve (or modify) data specified in a query. A query consists of instructions to move around in the tree and to retrieve (or modify) named data. The result of a query is an exact image of the parts of the tree that the query processor visited. The query processor is very simple -- it only understands eight commands, most of which share the same structure. It is helpful to think of the query processor as an automaton that walks around in the tree, directed by commands in the query. As it moves around, it copies the tree structure it traverses to the query result. Data that is requested by the query is copied into the result as well. Data that is changed by a query is copied into the result after the modification is made. 4. ENCODING OF QUERIES AND RESPONSES Both queries and responses are encoded using the representation defined in ISO Standard ASN.1 (Abstract Syntax Notation 1). ASN.1 represents data as sequences of triples that are encoded as a stream of octets. The data tuples may be recursively nested to represent structured data such as arrays or records. For a full description, see the ISO standards IS 8824 and IS 8825. See appendix for information about obtaining these documents. 4.1 Notation Used in This Proposal The notation used in this memo is similar to that used in ASN.1, but less formal, smaller, and (hopefully) easier to read. We will refer to a tuple as a "data object". In this RFC, we will not be concerned with the details of the object lengths. They exist in the actual ASN.1 encoding, but will be omitted in the examples here. Data objects that have no internal ASN.1 structure such as integer or octet string are referred to as "simple types" or "simple objects". Objects which are constructed out of other ASN.1 data objects will be referred to as "composite types" or "composite objects". Trewitt & Partridge [Page 5] RFC 1076 HEMS Monitoring and Control Language November 1988 The notation ID(value) represents a simple object whose tag is "ID" with the given value. A composite object is represented as ID{ ... contents ... } where contents is a sequence of data objects. The contents may include both simple and structured types, so the structure is fully recursive. The difference between simple and composite types is close to the meaning of the "constructor" bit in ASN.1. For the uses here, the distinction is made based upon the semantics of the data, not the representation. Therefore, even though an OctetString can be represented in ASN.1 using either constructed or non-constructed forms, it is conceptually a simple type, with no internal structure, and will always be written as ID("some arbitrary string") in this RFC. There are situations where it is necessary to specify a type but give no value, such as when referring to the name of the data. In this situation, the same notation is used, but with the value omitted: ID or ID() or ID{} Such objects have zero length and no contents. The latter two forms are used when a distinction is being made between simple and composite data, but the difference is just notation -- the representation is the same. ASN.1 distinguishes between four "classes" of tags: universal, application-specific, context-dependent, and reserved. HEMS and this query language use the first three. Universal tags are assigned in the ASN.1 standard and its addendums for common types, and are understood by any application using ASN.1. Application-specific tags are limited in scope to a particular application. These are used for "well-known" identifiers that must be recognizable in any context, such as derived data types. Finally, context-dependent tags are used for objects whose meaning is dependent upon where they are encountered. Most tags that identify data are context-dependent. 5. DATA ORGANIZATION Data in a monitored entity is modeled as a hierarchy. Implementations are not required to organize the data internally as a hierarchy, but they must provide this view of the data through the query language. A hierarchy offers useful structure for the following operations: Trewitt & Partridge [Page 6] RFC 1076 HEMS Monitoring and Control Language November 1988 Organization A hierarchy allows related data to be grouped together in a natural way. Naming The name of a piece of data is just the path from the root to the data of interest. Mapping onto ASN.1 ASN.1 can easily represent a hierarchy by using a "constructor" type as an envelope for an entire subtree. Efficient Representation Hierarchical structures are compact and can be traversed quickly. Safe Locking If it is necessary to lock part of the hierarchy (for example, when doing an update), locking an entire subtree can be done efficiently and safely, with no danger of deadlock. We will use the term "data tree" to refer to this entire structure. Note that this internal model is completely independent of the external ASN.1 representation -- any other suitable representation would do. For the sake of efficiency, we do make a one-to-one mapping between ASN.1 tags and the (internal) names of the nodes. The same could be done for any other external representation. Each node in the hierarchy must have names for its component parts. Although we would normally think of names as being ASCII strings such as "input errors", the actual name is just an ASN.1 tag. Such names are small integers (typically, less than 30) and so can easily be mapped by the monitored entity onto its internal representation. We use the term "dictionary" to mean an internal node in the hierarchy. Leaf nodes contain the actual data. A dictionary may contain both leaf nodes and other dictionaries. 5.1 Example Data Tree Here is a possible organization of the hierarchy in an entity that has several network interfaces and does IP routing. The exact organization of data in entities is specified in RFC-1024. This skeletal data tree will be used throughout this RFC in query examples. System { name -- host name clock-msec -- msec since boot Trewitt & Partridge [Page 7] RFC 1076 HEMS Monitoring and Control Language November 1988 interfaces -- # of interfaces memory } Interfaces { -- one per interface InterfaceData{ address, mtu, netMask, ARP{...}, ... } InterfaceData{ address, mtu, netMask, ARP{...}, ... } : } IPRouting { Entry{ ip-addr, interface, cost, ... } Entry{ ip-addr, interface, cost, ... } : } There are three top-level dictionaries in this hierarchy (System, Interfaces, and IPRouting) and three other dictionary types (InterfaceData, Entry, and ARP), each with multiple instances. The "name" of the clock in this entity would be: system{ clock-msec } and the name of a routing table entry's IP address would be: IPRouting{ Entry{ ip-addr } }. More than one piece of data can be named by a single ASN.1 object. The entire collection of system information is named by: system and the name of a routing table's IP address and cost would be: IPRouting{ Entry{ ip-addr, cost } }. 5.2 Arrays There is one sub-type of a dictionary that is used as the basis for tables of objects with identical types. We call these dictionaries arrays. In the example above, the dictionaries for interfaces, routing tables, and ARP tables are all arrays. In the examples above, the "ip-addr" and "cost" fields are named. In fact, these names refer to the field values for ALL of the routing table entries -- the name doesn't (and can't) specify which routing table entry is intended. This ambiguity is a problem wherever data is organized in tables. If there was a meaningful index for such tables (e.g., "routing table entry #1"), there would be no problem. Unfortunately, there usually isn't such an index. The solution to this problem requires that the data be accessed on the basis of some of its content. Filters, discussed in section 8.6, provide this mechanism. The primary difference between arrays and plain dictionaries is that Trewitt & Partridge [Page 8] RFC 1076 HEMS Monitoring and Control Language November 1988 arrays may contain only one type of item, while dictionaries, in general, will contain many different types of items. For example, the dictionary IPRouting (which is an array) will contain only items of type Entry. The fact that these objects are viewed externally as arrays or tables does not mean that they are represented in an implementation as linear lists of objects. Any collection of same-typed objects is viewed as an array, even though it might be stored internally in some other format, for example, as a hash table. 6. COMPONENTS OF A QUERY A HEMS query consists of a sequence of ASN.1 objects, interpreted by a simple stack-based interpreter. [Although we define the query language in terms of the operations of a stack machine, the language does not require an implementation to use a stack machine. This is a well-understood model, and is easy to implement.] One ASN.1 tag is reserved for operation codes; all other tags indicate data that will eventually be used by an operation. These objects are pushed onto the stack when received. Opcodes are immediately executed and may remove or add items to the stack. Because ASN.1 itself provides tags, very little needs to be done to the incoming ASN.1 objects to make them suitable for use by the query interpreter. Each ASN.1 object in a query will fit into one of the following categories: Opcode An opcode tells the query interpreter to perform an action. They are described in detail in section 8. Opcodes are represented by an application-specific type whose value determines the operation. Template These are objects that name one or more items in the data tree. Named items may be either simple items (leaf nodes) or entire dictionaries, in which case the entire subtree "underneath" the dictionary is understood. Templates are used to select specific data to be retrieved from the data tree. A template may be either simple or structured, depending upon what it is naming. A template only names the data -- there are no values contained in it. Therefore the leaf objects in a template will all have a length of zero. Examples of very simple templates are: name() or System{} Each of these is just one ASN.1 data object, with zero length. The first names a single data item in the "System" Trewitt & Partridge [Page 9] RFC 1076 HEMS Monitoring and Control Language November 1988 dictionary (and must appear in that context), and the second names the entire "System" dictionary. A more complex template such as: Interfaces{ InterfaceData{ address, netMask, ARP } } names two simple data items and a dictionary, iterated over all occurrences of "InterfaceData" within the Interfaces array. Path A path is a special case of a template that names only a single node in the tree. It specifies a path down into the dictionary tree and names exactly one node in the dictionary tree. Value These are used to give data values when needed in a query, for example, when changing a value in the data tree. A value can be thought of as either a filled-in template or as the ASN.1 representation some part of the data tree. Filter A boolean expression that can be executed in the context of a particular dictionary that is used to select or not select items in the dictionary. The expressions consist of the primitives "equal", "greater-or-equal", "less-or-equal", and "present" possibly joined by "and", "or", and "not". (See section 8.6.) Values, Paths, and Templates usually have names in the context- dependent class, except for a few special cases, which are in the application-specific class. 7. REPLY TO A QUERY The data returned to the monitoring entity is a sequence of ASN.1 data items. Conceptually, the reply is a subset of the data tree, where the query selects which portions are to be included. This is exactly true for data retrieval requests, and essentially true for data modification requests -- the reply contains the data after it has been modified. The key point is that the data in a reply represents the state of the data tree immediately after the query was executed. The sequence of the data is determined by the sequence of query language operations and the order of data items within Templates and Values given as input to these operations. If a query requests data from two of the top-level dictionaries in the data tree, by giving two templates such as: System{ name, interfaces } Interfaces{ Trewitt & Partridge [Page 10] RFC 1076 HEMS Monitoring and Control Language November 1988 InterfaceData { address, netMask, mtu } } then the response will consist of two ASN.1 data objects, as follows: System { name("system name"), interfaces(2) } Interfaces { InterfaceData { address(36.8.0.1), netMask(FFFF0000), mtu(1500) } InterfaceData { address(10.1.0.1), mtu(1008), netMask(FF000000) } } With few exceptions, each of the data items in the hierarchy is named in the context-specific ASN.1 type space. Because of this, the returned objects must be fully qualified. For example, the name of the entity must always be returned encapsulated inside an ASN.1 object for "System". If it were not, there would be no way to tell if the object that was returned was "name" inside the "System" dictionary or "address" inside the "interfaces" dictionary (assuming in this case that "name" and "address" were assigned the same integer as their ASN.1 tags). Having fully-qualified data simplifies decoding of the data at the receiving end and allows the tags to be locally chosen. Definitions for tags within routing tables won't conflict with definitions for tags within interfaces. Therefore, the people doing the name assignments are less constrained. In addition, most of the identifiers will be fairly small integers, which is an advantage because ASN.1 can fit tag numbers up to 30 in a one-octet tag field. Larger numbers require a second octet. If data is requested that doesn't exist, either because the tag is not defined, or because an implementation doesn't provide that data (such as when the data is optional), the response will contain an ASN.1 object that is empty. The tag will be the same as in the query, and the object will have a length of zero. The same response is given if the requested data does exist, but the invoker of the query does not have authorization to access it. See section 10 for more discussion of authorization mechanisms. Trewitt & Partridge [Page 11] RFC 1076 HEMS Monitoring and Control Language November 1988 This allows completely generic queries to be composed without regard to whether the data is defined or implemented at all of the entities that will receive the query. All of the available data will be returned, without generating errors that might otherwise terminate the processing of the query. 8. QUERY LANGUAGE The query language is designed to be expressive enough to write useful queries with, yet simple enough to be easy to implement. The query processor should be as simple and fast as possible, in order to avoid placing a burden on the monitored entity, which may be a critical node such as a gateway. Although queries are formed in a flexible way using what we term a "language", this is not a programming language. There are operations that operate on data, but most other features of programming languages are not present. In particular: - Programs are not stored in the query processor. - The only form of temporary storage is a stack, of limited depth. - There are no subroutines. - There are no explicit control structures defined in the language. The central element of the language is the stack. It may contain templates, (and therefore paths), values, and filters taken from the query. In addition, it can contain dictionaries (and therefore arrays) from the data tree. At the beginning of a query, it contains one item, the root dictionary. The overall operation consists of reading ASN.1 objects from the input stream. All objects that aren't opcodes are pushed onto the stack as soon as they are read. Each opcode is executed immediately and may remove items from the stack, may generate ASN.1 objects and send them to the output stream, and may leave items on the stack. Because each input object is dealt with immediately, portions of the response may be generated while the query is still being received. In the descriptions below, operator names are in capital letters, preceded by the arguments used from the stack and followed by results left on the stack. For example: Trewitt & Partridge [Page 12] RFC 1076 HEMS Monitoring and Control Language November 1988 OP a b OP a t means that the OP operator takes and off of the stack and leaves on the stack. Most of the operators in the query language leave the first operand ( in this example) on the stack for future use. If both and were received as part of the query (as opposed to being calculated by previous operations), then this part of the query would have consisted of the sequence: OP So, like other stack-based languages, the arguments and operators must be presented in postfix order, with an operator following its operands. Here is a summary of all of the operators defined in the query language. Most of the operators can take several different sets of operands and behave differently based upon the operand types. Details and examples are given later. BEGIN dict1 path BEGIN dict1 dict array path filter BEGIN array dict Move down in the data tree, establishing a context for future operations. END dict END -- Undo the most recent BEGIN. GET dict GET dict dict template GET dict array template filter GET array Retrieve data from the data tree. GET-ATTRIBUTES dict GET-ATTRIBUTES dict dict template GET-ATTRIBUTES dict array template filter GET-ATTRIBUTES array Retrieve attribute information about data in the data tree. GET-RANGE dict path start length GET-RANGE dict Retrieve a subrange of an OctetString. Used for reading memory. SET dict value SET dict array value filter SET array Change values in the data tree, possibly performing control functions. Trewitt & Partridge [Page 13] RFC 1076 HEMS Monitoring and Control Language November 1988 CREATE array value CREATE dict Create new table entries. DELETE array filter DELETE array Delete table entries. These operators are defined so that it is impossible to generate an invalid query response. Since a response is supposed to be a snapshot of a portion (or portions) of the data tree, it is important that only data that is actually in the tree be put in the response. Two features of the language help guarantee this: - Data is put in the response directly from the tree (by GET-*). Data does not go from the tree to the stack and then into the response. - Dictionaries on the stack are all derived from the initial, root dictionary. The operations that manipulate dictionaries (BEGIN and END) also update the response with the new location in the tree. 8.1 Moving Around in the Data Tree The initial point of reference in the data tree is the root. That is, operators name data starting at the root of the tree. It is useful to be able to move to some other dictionary in the tree and then name data from that point. The BEGIN operator moves down in the tree and END undoes the last unmatched BEGIN. BEGIN is used for two purposes: - By moving to a dictionary closer to the data of interest, the name of the data can be shorter than if the full name (from the root) were given. - It is used to establish a context for filtered operations to operate in. Filters are discussed in section 8.6. BEGIN dict1 path BEGIN dict1 dict Follow down the dictionary starting from . Push the final dictionary named by onto the stack. must name a dictionary (not a leaf node). At the same time, produce the beginning octets of an ASN.1 object corresponding to the new dictionary. It is up to the implementation to choose between using the "indefinite length" representation or the "definite length" form and going back and filling the length in later. Trewitt & Partridge [Page 14] RFC 1076 HEMS Monitoring and Control Language November 1988 END dict END -- Pop off of the stack and terminate the open ASN.1 object(s) started by the matching BEGIN. Must be paired with a BEGIN. If an END operation pops the root dictionary off of the stack, the query is terminated. must point to a regular dictionary. If any part of it refers to a non-existent node, if it points to a leaf node, or if it refers to a node inside an array-type dictionary, then it is in error, and the query is terminated immediately. An additional form of BEGIN, which takes a filter argument, is described later. 8.2 Retrieving Data The basic model that all of the data retrieval operations follow is that they take a template and fill in the leaf nodes of the template with the appropriate data values. GET dict template GET dict Emit an ASN.1 object with the same "shape" as the given template, except with values filled in for each node. The first ASN.1 tag of