| Network Working Group | B. Carpenter |
| Internet-Draft | Univ. of Auckland |
| Intended status: Informational | L. Ciavaglia |
| Expires: April 28, 2018 | Nokia |
| S. Jiang | |
| Huawei Technologies Co., Ltd | |
| P. Peloso | |
| Nokia | |
| October 25, 2017 |
Guidelines for Autonomic Service Agents
draft-carpenter-anima-asa-guidelines-03
This document proposes guidelines for the design of Autonomic Service Agents for autonomic networks. It is based on the Autonomic Network Infrastructure outlined in the ANIMA reference model, making use of the Autonomic Control Plane and the Generic Autonomic Signaling Protocol.
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This document proposes guidelines for the design of Autonomic Service Agents (ASAs) in the context of an Autonomic Network (AN) based on the Autonomic Network Infrastructure (ANI) outlined in the ANIMA reference model [I-D.ietf-anima-reference-model]. This infrastructure makes use of the Autonomic Control Plane (ACP) [I-D.ietf-anima-autonomic-control-plane] and the Generic Autonomic Signaling Protocol (GRASP) [I-D.ietf-anima-grasp].
There is a considerable literature about autonomic agents with a variety of proposals about how they should be characterized. Some examples are [DeMola06], [Huebscher08], [Movahedi12] and [GANA13]. However, for the present document, the basic definitions and goals for autonomic networking given in [RFC7575] apply . According to RFC 7575, an Autonomic Service Agent is "An agent implemented on an autonomic node that implements an autonomic function, either in part (in the case of a distributed function) or whole."
The reference model [I-D.ietf-anima-reference-model] expands this by adding that an ASA is "a process that makes use of the features provided by the ANI to achieve its own goals, usually including interaction with other ASAs via the GRASP protocol [I-D.ietf-anima-grasp] or otherwise. Of course it also interacts with the specific targets of its function, using any suitable mechanism. Unless its function is very simple, the ASA will need to be multi-threaded so that it can handle overlapping asynchronous operations. It may therefore be a quite complex piece of software in its own right, forming part of the application layer above the ANI."
A basic property of an ASA is that it is a relatively complex software component that will in many cases control and monitor simpler entities in the same host or elsewhere. For example, a device controller that manages tens or hundreds of simple devices might contain a single ASA.
The remainder of this document offers guidance on the design of ASAs.
As mentioned above, all but the simplest ASAs will be multi-threaded programs.
A typical ASA will have a main thread that performs various initial housekeeping actions such as:
The logic of the main loop will depend on the details of the autonomic function concerned. Whenever asynchronous operations are required, extra threads will be launched. Examples of such threads include:
These threads should all either exit after their job is done, or enter a wait state for new work, to avoid blocking other threads unnecessarily.
Note: If the programming environment does not support multi-threading, an 'event loop' style of implementation could be adopted, in which case each of the above threads would be implemented as an event handler called in turn by the main loop. In this case, the GRASP API (Section 3.3) must provide non-blocking calls. If necessary, the GRASP session identifier will be used to distinguish simultaneous negotiations.
According to the degree of parallelism needed by the application, some of these threads might be launched in multiple instances. In particular, if negotiation sessions with other ASAs are expected to be long or to involve wait states, the ASA designer might allow for multiple simultaneous negotiating threads, with appropriate use of queues and locks to maintain consistency.
The main loop itself could act as the initiator of synchronization requests or negotiation requests, when the ASA needs data or resources from other ASAs. In particular, the main loop should watch for changes in policy Intent that affect its operation. It should also do whatever is required to avoid unnecessary resource consumption, such as including an arbitrary wait time in each cycle of the main loop.
The self-monitoring thread is of considerable importance. Autonomic service agents must never fail. To a large extent this depends on careful coding and testing, with no unhandled error returns or exceptions, but if there is nevertheless some sort of failure, the self-monitoring thread should detect it, fix it if possible, and in the worst case restart the entire ASA.
An ASA by definition runs in an autonomic node. Before any normal ASAs are started, such nodes must be bootstrapped into the autonomic network's secure key infrastructure in accordance with [I-D.ietf-anima-bootstrapping-keyinfra]. This key infrastructure will be used to secure the ACP (next section) and may be used by ASAs to set up additional secure interactions with their peers, if needed.
Note that the secure bootstrap process itself may include special-purpose ASAs that run in a constrained insecure mode.
In a normal autonomic network, ASAs will run as clients of the ACP. It will provide a fully secured network environment for all communication with other ASAs, in most cases mediated by GRASP (next section).
Note that the ACP formation process itself may include special-purpose ASAs that run in a constrained insecure mode.
GRASP [I-D.ietf-anima-grasp] is expected to run as a separate process with its API [I-D.liu-anima-grasp-api] available in user space. Thus ASAs may operate without special privilege, unless they need it for other reasons. The ASA's view of GRASP is built around GRASP objectives (Section 4), defined as data structures containing administrative information such as the objective's unique name, and its current value. The format and size of the value is not restricted by the protocol, except that it must be possible to serialise it for transmission in CBOR [RFC7049], which is no restriction at all in practice.
The GRASP API offers the following features:
For further details and some additional housekeeping functions, see [I-D.liu-anima-grasp-api].
This API is intended to support the various interactions expected between most ASAs, such as the interactions outlined in Section 2. However, if ASAs require additional communication between themselves, they can do so using any desired protocol. One option is to use GRASP discovery and synchronization as a rendez-vous mechanism between two ASAs, passing communication parameters such as a TCP port number as the value of a GRASP objective. As noted above, either the ACP or in special cases the autonomic key infrastructure will be used to secure such communications.
At the time of writing, the Intent mechanism for the ANI is undefined. It is expected to operate by an information distribution mechanism that can reach all autonomic nodes, and therefore every ASA. However, each ASA must be capable of operating "out of the box" in the absence of locally defined Intent, so every ASA implementation must include carefully chosen default values and settings for all parameters and choices that might depend on Intent.
The general rules for the format of GRASP Objective options, their names, and IANA registration are given in [I-D.ietf-anima-grasp]. Additionally that document discusses various general considerations for the design of objectives, which are not repeated here. However, we emphasize that the GRASP protocol does not provide transactional integrity. In other words, if an ASA is capable of overlapping several negotiations for a given objective, then the ASA itself must use suitable locking techniques to avoid interference between these negotiations. For example, if an ASA is allocating part of a shared resource to other ASAs, it needs to ensure that the same part of the resource is not allocated twice. This might impact the design of the objective as well as the logic flow of the ASA.
In particular, if 'dry run' mode is defined for the objective, its specification, and every implementation, must consider what state needs to be saved following a dry run negotiation, such that a subsequent live negotiation can be expected to succeed. It must be clear how long this state is kept, and what happens if the live negotiation occurs after this state is deleted. An ASA that requests a dry run negotiation must take account of the possibility that a successful dry run is followed by a failed live negotiation. Because of these complexities, the dry run mechanism should only be supported by objectives and ASAs where there is a significant benefit from it.
The actual value field of an objective is limited by the GRASP protocol definition to any data structure that can be expressed in Concise Binary Object Representation (CBOR) [RFC7049]. For some objectives, a single data item will suffice; for example an integer, a floating point number or a UTF-8 string. For more complex cases, a simple tuple structure such as [item1, item2, item3] could be used. Nothing prevents using other formats such as JSON, but this requires the ASA to be capable of parsing and generating JSON. The formats acceptable by the GRASP API will limit the options in practice. A fallback solution is for the API to accept and deliver the value field in raw CBOR, with the ASA itself encoding and decoding it via a CBOR library.
Autonomic functions could be permanent, in the sense that ASAs are shipped as part of a product and persist throughout the product's life. However, a more likely situation is that ASAs need to be installed or updated dynamically, because of new requirements or bugs. Because continuity of service is fundamental to autonomic networking, the process of seamlessly replacing a running instance of an ASA with a new version needs to be part of the ASA's design.
The implication of service continuity on the design of ASAs can be illustrated along the three main phases of the ASA life-cycle, namely Installation, Instantiation and Operation.
+--------------+
Undeployed ------>| |------> Undeployed
| Installed |
+-->| |---+
Mandate | +--------------+ | Receives a
is revoked | +--------------+ | Mandate
+---| |<--+
| Instantiated |
+-->| |---+
set | +--------------+ | set
down | +--------------+ | up
+---| |<--+
| Operational |
| |
+--------------+
Figure 1: Life cycle of an Autonomic Service Agent
Before being able to instantiate and run ASAs, the operator must first provision the infrastructure with the sets of ASA software corresponding to its needs and objectives. The provisioning of the infrastructure is realized in the installation phase and consists in installing (or checking the availability of) the pieces of software of the different ASA classes in a set of Installation Hosts.
There are 3 properties applicable to the installation of ASAs:
These three properties are very important in the context of the installation phase as their variations condition how the ASA class could be installed on the infrastructure.
Inputs are:
The main output of the installation phase is an up-to-date directory of installed ASAs which corresponds to [list of ASA classes] installed on [list of installation Hosts]. This output is also useful for the coordination function and corresponds to the static interaction map (see next section).
The condition to validate in order to pass to next phase is to ensure that [list of ASA classes] are well installed on [list of installation Hosts]. The state of the ASA at the end of the installation phase is: installed. (not instantiated). The following commands or messages are foreseen: install(list of ASA classes, Installation_target_Infrastructure, ASA class placement function), and un-install (list of ASA classes).
Once the ASAs are installed on the appropriate hosts in the network, these ASA may start to operate. From the operator viewpoint, an operating ASA means the ASA manages the network resources as per the objectives given. At the ASA local level, operating means executing their control loop/algorithm.
But right before that, there are two things to take into consideration. First, there is a difference between 1. having a piece of code available to run on a host and 2. having an agent based on this piece of code running inside the host. Second, in a coupled case, determining which resources are controlled by an ASA is straightforward (the determination is embedded), in a decoupled mode determining this is a bit more complex (hence a starting agent will have to either discover or be taught it).
The instantiation phase of an ASA covers both these aspects: starting the agent piece of code (when this does not start automatically) and determining which resources have to be controlled (when this is not obvious).
Through this phase, the operator wants to control its autonomic network in two things:
Additionally in this phase, the operator may want to set objectives to autonomic functions, by configuring the ASAs technical objectives.
The operator's goal can be summarized in an instruction to the ANIMA ecosystem matching the following pattern:
Inputs are:
Outputs are:
The instructions described in section 4.2 could be either:
This set of instructions can be materialized through a message that is named an Instance Mandate (description TBD).
The conclusion of this instantiation phase is a ready to operate ASA (or interacting set of ASAs), then this (or those) ASA(s) can describe themselves by depicting which are the resources they manage and what this means in terms of metrics being monitored and in terms of actions that can be executed (like modifying the parameters values). A message conveying such a self description is named an Instance Manifest (description TBD).
Though the operator may well use such a self-description "per se", the final goal of such a description is to be shared with other ANIMA entities like:
Note: This section is to be further developed in future revisions of the document, especially the implications on the design of ASAs.
During the Operation phase, the operator can:
During the Operation phase, running ASAs can interact the one with the other:
During the Operation phase, running ASAs are expected to apply coordination schemes
The ASA life-cycle is discussed in more detail in "A Day in the Life of an Autonomic Function" [I-D.peloso-anima-autonomic-function].
Some autonomic functions will be completely independent of each other. However, others are at risk of interfering with each other - for example, two different optimization functions might both attempt to modify the same underlying parameter in different ways. In a complete system, a method is needed of identifying ASAs that might interfere with each other and coordinating their actions when necessary. This issue is considered in "Autonomic Functions Coordination" [I-D.ciavaglia-anima-coordination].
It is of great importance that all components of an autonomic system are highly robust. In principle they must never fail. This section lists various aspects of robustness that ASA designers should consider.
ASAs are intended to run in an environment that is protected by the Autonomic Control Plane [I-D.ietf-anima-autonomic-control-plane], admission to which depends on an initial secure bootstrap process [I-D.ietf-anima-bootstrapping-keyinfra]. However, this does not relieve ASAs of responsibility for security. In particular, when ASAs configure or manage network elements outside the ACP, they must use secure techniques and carefully validate any incoming information. As appropriate to their specific functions, ASAs should take account of relevant privacy considerations [RFC6973].
Authorization of ASAs is a subject for future study. At present, ASAs are trusted by virtue of being installed on a node that has successfully joined the ACP.
This document makes no request of the IANA.
TBD.
| [I-D.ietf-anima-autonomic-control-plane] | Behringer, M., Eckert, T. and S. Bjarnason, "An Autonomic Control Plane (ACP)", Internet-Draft draft-ietf-anima-autonomic-control-plane-12, October 2017. |
| [I-D.ietf-anima-bootstrapping-keyinfra] | Pritikin, M., Richardson, M., Behringer, M., Bjarnason, S. and K. Watsen, "Bootstrapping Remote Secure Key Infrastructures (BRSKI)", Internet-Draft draft-ietf-anima-bootstrapping-keyinfra-08, October 2017. |
| [I-D.ietf-anima-grasp] | Bormann, C., Carpenter, B. and B. Liu, "A Generic Autonomic Signaling Protocol (GRASP)", Internet-Draft draft-ietf-anima-grasp-15, July 2017. |
| [RFC7049] | Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049, October 2013. |
draft-carpenter-anima-asa-guidelines-03, 2017-10-25:
Added details on life cycle.
Added details on robustness.
Added co-authors.
draft-carpenter-anima-asa-guidelines-02, 2017-07-01:
Expanded description of event-loop case.
Added note about 'dry run' mode.
draft-carpenter-anima-asa-guidelines-01, 2017-01-06:
More sections filled in
draft-carpenter-anima-asa-guidelines-00, 2016-09-30:
Initial version