| NETCONF Data Modeling Language Working Group (netmod) | E. Voit |
| Internet-Draft | A. Clemm |
| Intended status: Informational | Cisco Systems |
| Expires: March 17, 2016 | S. Mertens |
| Prismtech | |
| September 14, 2015 |
Requirements for mounting of local and remote YANG subtrees
draft-voit-netmod-peer-mount-requirements-03
Network integrated applications want simple ways to reference and access YANG objects and subtrees. These simplifications might include aliasing of local YANG information. These simplifications might include remote referencing of YANG information distributed across network.
For such applications, development complexity must be minimized. Specific aspects of complexity developers want to ignore include:
The solution requirements described in this document detail what is needed to support application access to authoritative network YANG objects locally (via aliasing), or remotely from controllers or peering network devices in such a way to meet these goals.
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This Internet-Draft will expire on March 17, 2016.
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Instrumenting Physical and Virtual Network Elements purely along device boundaries and via a fully normalized object representation is insufficient for today's requirements. Instead, users, applications, and operators are asking for the ability to interact with local and remote information exposed as simply as possible from a familiar local datastore.
Achieving an easy, local abstract representation of any remote information can be difficult since a running network is comprised of a distributed mesh of object ownership. Solutions require the transparent assembly of local and remote objects in order to provide context specific, time synchronized, and consistent views required for a simple local abstraction.
Ultimately network application programming must be simplified. To do this:
These steps will eliminate much unnecessary overhead currently required of today’s network programmer.
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 RFC 2119 [RFC2119].
Alias Mount – A type of YANG Mount performed against a subtree located in a local datastore.
Authoritative Datastore – A datastore containing the authoritative copy of an object, i.e. the source and the “owner” of the object.
Client Datastore – a datastore containing an object whose source and “owner” is a remote datastore.
Data Node - An instance of management information in a YANG datastore.
Datastore - A conceptual store of instantiated information, with individual data items represented by data nodes which are arranged in hierarchical manner.
Data Subtree - An instantiated data node and the data nodes that are hierarchically contained within it.
Mount Client - The system at which the mount point resides, into which one or more subtrees may be mounted.
Mount Binding - An instance of YANG mount from a specific Mount Point to a datastore. Types include:
Mount Point - Point in the local data store which may reference a single remote subtree
Mount Server - The server with which the Mount Client communicates and which provides the Mount Client with access to the mounted information. Can be used synonymously with Mount Target.
Peer Mount - A method of YANG Mount which enables the representation of remote objects within a local datastore.
Target Data Node - Data Node on Mount Server against which a Mount Binding is established
YANG Mount - A method of including YANG data node from another location as part of a specific YANG model and on a specific path in the data tree via an explicit reference to a subtree to be included. Two subtypes are Alias Mount and Peer Mount.
YANG modeling has emerged as a preferred way to offer network abstractions. The requirements in this document can be enabled by expanding of the syntax of YANG capabilities embodied within RFC 6020 [RFC6020] and YANG 1.1 [rfc6020bis]. A companion draft to this one which details a potential set of YANG technology extensions which can support key requirements within this document are contained in . [draft-clemm-mount].
To date systems built upon YANG models have been missing two capabilities:
First this document will dive deeper into YANG Datastore Mount (a.k.a., "YANG Mount"). Two subtypes of YANG Mount are “Alias Mount” and “Peer Mount”.
Alias Mount allows access to the same YANG data node along different paths within the same YANG datastore, allowing a given subtree to subtend from different YANG models within the same system. This provides a means to:
Considering there are YANG models incorporating intersected and replicated information today, adding an Alias Mount capability should reduce YANG model development and model mapping requirements.
For Peer Mount, we need the capability to refer to managed resources that reside on different systems. This allows applications on the same system as the YANG datastore server, as well as remote clients that access the datastore through a management protocol such as NETCONF, to access all data from a familiar local YANG model.
In this way, an application developer is projected an image of one virtual consolidated datastore. Peer Mount builds on Alias Mount by allowing to incorporate redirection to remote systems into the structure.
The value in YANG Mount comes from its under-the-covers federation. The datastore transparently exposes information about objects that can be reached along multiple paths, allowing to make the same data nodes part of multiple concurrent hierarchies. The user does not need to be aware of the precise distribution and ownership of data themselves, nor is there a need for the application to discover those data sources, maintain separate associations with them, and partition its operations to fit along remote system boundaries. The effect is that a network device can broaden and customize the information available for local access. Life for the application is easier.
At the same time, the authoritative ownership of a data node is never in question. The original hierarchy and path that was defined when the data node was first defined in a YANG module remain in effect, and any validation involved in creating, modifying, or deleting the data node always occurs in the same context in which it was originally introduced. All that YANG Mount allows to do is to define alternative, additional paths and hierarchies to which the object could also be accessed.
Any Object or subtree type can be exposed via such a reference. This can include configuration data that is either persistent or ephemeral, and which is valid within only a single device or across a domain of devices. This can include operational data that represents state across a single device or across a multiple devices.
Another useful aspect of YANG Mount is its ability to embed information from existing into newly defined models without requiring additional normalization effort. Normalization is a good thing, but the massive human efforts invested in uber-data-models have never gained industry traction due to the resulting models' brittle nature and complexity. By mounting subtrees/objects into local datastores it is possible to expose objects under a locally optimized hierarchy without having to transpose remote objects into a separate local model.
It should be noted that YANG Mount does not require knowledge of the entire subtree being mounted. For example, there might be augmentations of that subtree, or even mounted information in the subtree itself. Likewise, mounted objects might dynamically change, or even come into being. These dynamic changes can be reflected as needed under the "attachment points" within the namespace hierarchy where the data subtrees from remote systems have been mounted. In this case, the precise details of what these subtrees exactly contain does not need to be understood by the system implementing the attachment point, it simply acts as a single point of entry and "proxy" for the attached data. .
The CAP theorem states that it is impossible for a distributed computer system to simultaneously provide Consistency, Availability, and Partition tolerance. (I.e., distributed network state management is hard.) Mostly for this reason YANG implementations have shied away from distributed datastore implementations where ACID transactional guarantees cannot be given. This of course limits the universe of applicability for YANG technology.
Leveraging YANG concepts, syntax, and models for objects which might be happening to undergo network convergence is valuable. Such reuse greatly expands the universe of information visible to networking applications. The good news is that there is nothing in YANG syntax that prohibits its reapplication for distributed datastores. Extensions are needed however.
Requirements described within this document can be used to define technology extensions to YANG 1.1 for remote datastore mounting. Because of the CAP theorem, it must be recognized that systems built upon these extensions MAY choose to support eventual consistency rather than ACID guarantees. Some applications do not demand ACID guarantees (examples are contained in this document’s Use Case section). Therefore for certain classes of applications, eventual consistency should be viewed as a cornerstone feature capability rather than a bug.
Other industries have been able to identify and realize the value in such model. The Object Management Group Data-Distribution Service for Real-Time Systems has even standardized these capabilities for non-YANG deployments [OMG-DDS]. Commercial deployments exist.
Example Use Cases for Alias Mount can easily be seen from the description within Section 3.1. Therefore these are not detailed within this document. In general, those use cases involve imposing an alternative structure over YANG data models. YANG allows to extend and augment data models, allowing to add new data nodes as child nodes or as siblings to existing data nodes. However, YANG does not allow to superimpose a new data node on top of an existing one, or move an existing node under a newly defined node. Peer Mount closes that gap and allows to define models with alternative hierarchies and insert existing data nodes into that hierarchy.
For Peer Mount, many types of applications can benefit from the simple and quick availability of objects from peer network devices. Because network management and orchestration systems have been fulfilling a subset of the requirements for decades, it is important to focus on what has changed. Changes include:
Such changes can enable a new class of applications. These applications are built upon fast-feedback-loops which dynamically tune the network based on iterative interactions upon a distributed datastore.
A Cloud Policer enables a single aggregated data rate to tenants/users of a data center cloud that applies across their VMs; a rate independent of where specific VMs are physically hosted. This works by having edge router based traffic counters available to a centralized application, which can then maintain an aggregate across those counters. Based on the sum of the counters across the set of edge routers, new values for each device based Policer can be recalculated and installed. Effectively policing rates are continuously rebalanced based on the most recent traffic offered to the aggregate set of edge devices.
The cloud policer provides a very simple cloud QoS model. Many other QoS models could also be implemented. Example extensions include:
It is possible to implement such a cloud policer application with maximum application developer simplicity using peer mount. To do this the application accesses a local datastore which in turn does a peer mount from edge routers the objects which house current traffic counter statistics. These counters are accessed as if they were part of the local datastore structures, without concern for the fact that the actual authoritative copies reside on remote systems.
Beyond this centralized counter collection peer mount, it is also possible to have distributed edge routers mount information in the reverse direction. In this case local edge routers can peer mount centrally calculated policer rates for the device, and access these objects as if they were locally configured.
For both directions of mounting, the authoritative copy resides in a single system and is mounted by peers. Therefore issues with regards to inconsistent configuration of the same redundant data across the network are avoided. Also as can be seen in this use case, the same system can act as a mount client of some objects while acting as server for other objects.
Another extension of the “Cloud Policer” application is the creation of additional action thresholds at bandwidth rates far greater than might be expected. If these higher thresholds are hit, it is possible to connect in DDoS scrubbers to ingress traffic. This can be done in seconds after a bandwidth spike. This can also be done if non-bandwidth counters are available. For example, if TCP flag counts are available it is possible to look for changes in SYN/ACK ratios which might signal a different type of attack. In all cases, when network counters indicate a return to normal traffic profiles the DDoS Scrubbers can be automatically disconnected.
Benefits of only connecting a DDoS scrubber in the rare event an attack might be underway include:
Service Chains will dynamically change ingress classification filters, allocate paths from many ingress devices across shared resources. This information needs to be updated in real time as available capacity is allocated or failures are discovered. It is possible to simplify service chain configuration and dynamic topology maintenance by transparently updating remote cached topologies when an authoritative object is changed within a central repository. For example if the CPU in one VM spikes, you might want to recalculate and adjust many chained paths to relieve the pressure. Or perhaps after the recalculation you want to spin up a new VM, and then adjust chains when that capacity is on-line.
A key value here is central calculation and transparent auto-distribution. In other words, a change only need be updated by an application in a single location, and the infrastructure will automatically synchronize changes across any number of subscribing devices without application involvement. In fact, the application need not even know many devices are monitoring the object which has been changed.
Beyond 1:n policy distribution, applications can step back from aspects of failure recovery. What happens if a device is rebooting or simply misses a distribution of new information? With peer mount there is no doubt as to where the authoritative information resides if things get out of synch.
While this ability is certainly useful for dynamic service chain filtering classification and next hop mapping, this use case has more general applicability. With a distributed datastore, diverse applications and hosts can locally access a single device’s current VM CPU and Bandwidth values. They can do it without needing to explicitly query that remote machine. Updates from a device would come from a periodic push of stats to a transparent cache to subscribed, or via an unsolicited update which is only sent when these value exceed established norms.
To achieve the objectives described above, the network needs to support a number of requirements
A major obstacle to network programmability are any requirements which force applications to use abstractions more complicated than the developer cares to touch. To simplify applications development and reduce unnecessary code, the following needs must be met.
Applications MUST be able to access a local datastore which includes objects whose authoritative source perhaps is located in a elsewhere in some datastore.
Local datastores MUST be able to provide a hierarchical view of objects assembled from objects whose authoritative source may potentially originate from different and overlapping namespaces.
Applications MUST be able to access all objects of a datastore without concern where the actual object is located, i.e. whether the authoritative copy of the object is hosted on the same system as the local datastore or whether it is hosted in a remote datastore.
A datastore's application facing interfaces MUST make no differentiation whether individual objects exposed are authoritatively owned by the datastore or mounted from elsewhere
When a change is made to an object, that change will be reflected in any datastore in which the object is included.
A datastore supporting YANG Mount MUST allow the same object to be mounted from multiple places.
Applications SHOULD be able to extract a time synchronized set of operational data from the datastore. (In other words, the application asks for a subset of network state at time-stamp or time-range “X”. The datastore would then deliver time synchronized snapshots of the network state per the request. The datastore may work with NTP and operational counter to optimize the synchronization results of such a query. It is understood that some types of data might be undergoing convergence conditions.)
Authoritative datastore retain full ownership of “their” objects. This means that while remote datastores may access the data, any modifications to objects that are initiated at those remote datastores need to be authorized by the authoritative owner of the data. Likewise, the authoritative owner of the data may make changes to objects, including modifications, additions, and deletions, without needing to first ask for permission from remote clients.
Applications MUST be designed to deal with incomplete data if remote objects are not accessible, e.g. due to temporal connectivity issues preventing access to the authoritative source. (This will be true for many protocols and programming languages. Mount is unlikely to add anything new here unless applications have extra error handling routines to deal with when there is no response from a remote system.).
Remote objects in a datastore can be accessed “on demand”, when the application interacting with the datastore demands it. In that case, a request made to the local datastore is forwarded to the remote system. The response from the remote system, e.g. the retrieved data, is subsequently merged and collated with the other data to return a consolidated response to the invoking application.
A downside of a datastore which is distributed across devices can be the latency induced when remote object acquisition is necessary. There are plenty of applications which have requirements which simply cannot be served when latency is introduced. The good news is that the concept of caching lends itself well to distributed datastores. It is possible to transparently store some types of objects locally even when the authoritative copy is remote. Instead of fetching data on demand when an application demands it, the application is simply provided with the local copy. It is then up to the datastore infrastructure to keep selected replicated info in synch, e.g. by prefetching information, or by having the remote system publish updates which are then locally stored. At this point, it is expected that a preferred method of subscribing to and publishing updates will be accomplished via [i2rs-pub-sub-reqts] and [draft-clemm-datastore-push]. Other methods could work equally well .
This is not a new idea. Caching and Content Delivery Networks (CDN) have sped read access for objects within the Internet for years. This has enabled greater performance and scale for certain content. Just as important, these technologies have been employed without end user applications being explicitly aware of their involvement. Such concepts are applicable for scaling the performance of a distributed datastore.
Where caching occurs, it MUST be possible for the Mount Client to store object copies of a remote data node or subtree in such a way that applications are unaware that any caching is occurring. However, the interface to a datastore MAY provide applications with a special mode/flag to allow them to force a read-through.
Where caching occurs, system administration facilities SHOULD allow facilities to flush either the entire cache, or information associated with select Mount Points.
When caching occurs, data can go stale. [draft-clemm-datastore-push] provides a mechanism where changes in an authoritative data node or subtree can be monitored. If changes occur, these changes can be delivered to any subscribing datastores. In this way remote caches can be kept up-to-date. In this way, directly monitoring remote applications can quickly receive notifications without continuous polling.
A Mount Server SHOULD support [draft-clemm-datastore-push] Periodic and/or On-Change pub/sub capabilities in which one or more remote clients subscribe to updates of a target data node / subtree, which are then automatically published by the Mount Server.
It MUST be possible for Applications to bind to subscribed Data Node / Subtrees so that upon Mount Client receipt of subscribed information, it is immediately passed to the application.
It MUST be possible for a Target Data Node to support 1:n Mount Bindings to many subscribed Mount Points.
Mount can drive a dynamic and richly interconnected mesh of peer-to-peer of object relationships. Each of these Mounts will be independently established by a Mount Client.
It MUST be possible to bootstrap the Mount Client by providing the YANG paths to resources on the Mount Server.
There SHOULD be the ability to add Mount Client bindings during run-time.
A Mount Client MUST be able to be able to create, delete, and timeout Mount Bindings.
Any Subscription MUST be able to inform the Mount Client of an intentional/graceful disconnect.
A Mount Client MUST be able to verify the status of Subscriptions, and drive re-establishment if it has disappeared.
Application visibility into an ever-changing set of network objects is not trivial. While some applications can be easily configured to know the Devices and available Mount Points of interest, other applications will have to balance many aspects of dynamic device availability, capabilities, and interconnectedness. Maintenance of these dynamic elements can be done on the YANG objects themselves without anything needed new for YANG Mount.
Mount Clients MUST NOT create recursive Mount bindings (i.e., the Mount Client should not load any object or subtree which it has already delivered to another in the role of a Mount Server.) Note: Objects mounted from a controller as part of orchestration are *not* considered the same objects as those which might be mounted back from a network device showing the actual running config.
The Mount Server default MUST be to deliver the same Data Node / Subtree that would have been delivered via direct YANG access.
It SHOULD be possible for a Mount Client to request something less than the full subtree or a target node as defined in [i2rs-pub-sub-reqts].
The interest that a Mount Client expresses in a particular subtree SHOULD include the non-functional data delivery requirements (QoS) on the data that is being mounted. Additionally, Mount Servers SHOULD advertise their data delivery capabilities. With this information the Mount Client can decide whether the quality of the delivered data is sufficient to serve applications residing above the Mount Client.
An example here is reliability. A reliable protocol might be overkill for a state that is republished with high frequency. Therefore a Mount Server may sometimes choose to not provide a reliable method of communication for certain objects. It is up to the Mount Client to determine whether what is offered is sufficiently reliable for its application. Only when the Mount Server is offering data delivery QoS better or equal to what is requested, shall a mount binding be established.
Another example is where subscribed objects must be pushed from the Mount Server within a certain interval from when an object change is identified. In such a scenario the interval period of the Mount Server must be equal or smaller than what is requested by a Mount Client. If this “deadline” is not met by the Mount Server the infrastructure MAY take action to notify clients.
It is conceivable to differentiate between different datastores on the remote server, that is, to designate the name of the actual datastore to mount, e.g. "running" or "startup". If on the target node there are multiple datastores available, but there has no specific datastore identified by the Mount Client, then the running or "effective" datastore is the assumed target.
It is conceivable to use such Datastore Qualification in conjunction with ephemeral datastores, to address requirements being worked in the I2RS WG [draft-i2rs-ephemeral].
It is possible for the mounted subtree to in turn contain a mountpoint. However, circular mount relationships MUST NOT be introduced. For this reason, a mounted subtree MUST NOT contain a mountpoint that refers back to a mount target that directly or indirectly contains the originating mountpoint. As part of a mount operation, the mount points of the mounted system need to be checked accordingly.
Many secured transports are viable assuming transport, data security, scale, and performance objectives are met. Netconf and/or Restconf should be considered as starting points. Other transports may be proposed over time.
It MUST be possible to support Netconf or Restconf Transport of subscribed Nodes and Subtrees.
Many security mechanisms exist to protect data access for CLI and API on network devices. To the degree possible these mechanisms should transparently protect data when performing a Peer Mount.
The same mechanisms used to determine whether a remote host has access to a particular YANG Data Node or Subtree MUST be invoked to determine whether a Mount Client has access to that information.
The same traditional transport level security mechanism security used for YANG over a particular transport MUST be used for the delivery of objects from a Mount Server to a Mount Client.
A Mount Server implementation MUST NOT change any credentials passed by the Mount Client system for any Mount Binding request.
The Mount Server MUST deliver no more objects from a Data Node or Subtree than allowable based on the security credentials provided by the Mount Client.
To ensure the ensuring maximum scale limits, it MUST be possible to for a Mount Server to limit the number of bindings and transactional limits
It SHOULD be possible to prioritize which Mount Binding instances should be serviced first if there is CPU, bandwidth, or other capacity constraints.
A key intent for YANG Mount is to allow access to an authoritative copy of an object for a particular domain. Of course system and software failures or scheduled upgrades might mean that the primary copy is not consistently accessible from a single device. In addition, system failovers might mean that the authoritative copy might be housed on a different device than the one where the binding was originally established. YANG Mount architectures must be built to enable Mount Clients to transparently provide access to objects where the authoritative copy moves due to dynamic network reconfigurations .
A YANG Mount architecture MUST guarantee that mount bindings between a Mount Server and Mount Clients drive system behavior which is at least eventually consistent. The infrastructure providing this level of consistency MUST be able to operate in scenarios where a system is (temporarily) not fully connected. Furthermore, Mount Clients MAY have various requirements on the boundaries under which eventual consistency is allowed to take place. This subject can be decomposed in the following items:
A scenario that deserves attention in particular is when a subset of Mount Clients receive and cache a pushed subscription update. If a Mount Server loses connectivity, cross network element consistency can be lost. In such a scenario Mount Clients MAY elect a new designated Mount Server from the set of Mount Clients which have received the latest state.
When a mount binding is established a Mount Server SHOULD provide the Mount Client with the latest state of the requested data. In order to increase availability and fault tolerance an infrastructure MAY support the capability to have multiple alignment sources. In (temporary) absence of a Mount Server, Mount Clients MAY elect a temporary Mount Server to service late joining Mount Clients.
Upon losing liveliness and being unable to refresh cached data provided from a Mount Server, a Mount Client MAY decide to purge the mount bindings of that server. Purging mount bindings under such conditions however makes a system vulnerable to losing network-wide consistency. A Mount Client can take proactive action based on the assumption that the Mount Server is no longer available. When connectivity is only temporarily lost, this assumption could be false for other datastores. This can introduce a potential for decision-making based on semantical disagreement. To properly handle these scenarios, application behavior MUST be designed accordingly and timeouts with regards to liveliness detection MUST be carefully determined.
A traditional problem with merging replicated datasets during the failover and recovery of Mount Servers is handling the corresponding target data node lifecycle management. When two replicas of a dataset experienced a prolonged loss of connectivity a merge between the two is required upon re-establishing connectivity. A replica might have been modifying contents of the set, including deletion of objects. A naive merge of the two replicas would discard these deletes by aligning the now stale, deleted objects to the replica that deleted them.
Authoritative ownership is an elegant solution to this problem since modifications of content can only take place at the owner. Therefore a Mount Client SHOULD, upon reestablishing connectivity with a newly authoritative Mount Server, replace any existing cache contents from a mount binding with the latest version.
For selected objects, Mount Bindings SHOULD be allowed to Anycast addresses so that a Distributed Mount Server implementation can transparently provide (a) availability during failure events to Mount Clients, and (b) load balancing on behalf of Mount Clients.
At the Mount Client, it MUST be possible for all Mount bindings to configure the following such that the application needs no knowledge. This will include a diverse list of elements such as the YANG URI path to the remote subtree.
API usage for YANG should be tracked via existing mechanisms. There is no intent to require additional transaction tracking than would have been provided normally. However there are additional requirements which should allow the state of existing and historical bindings to be provided.
A Mount Client MUST be able to poll a Mount Server for the state of Subsciptions maintained between the two devices.
A Mount Server MUST be able to publish the set of Subscriptions which are currently established on or below any identified data node.
This document makes no request of IANA.
We wish to acknowledge the helpful contributions, comments, and suggestions that were received from Ambika Prasad Tripathy. Shashi Kumar Bansal, Prabhakara Yellai, Dinkar Kunjikrishnan, Harish Gumaste, Rohit M., Shruthi V. , Sudarshan Ganapathi, and Swaroop Shastri.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC6020] | Bjorklund, M., "YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF)", RFC 6020, DOI 10.17487/RFC6020, October 2010. |
| [draft-clemm-datastore-push] | Clemm, A., "Subscribing to datastore push updates", July 2015. |
| [draft-clemm-mount] | Clemm, Alexander., Mounting YANG-Defined Information from Remote Datastores", April 2015. |
| [draft-i2rs-ephemeral] | Haas, J., "I2RS Ephemeral State Requirements", June 2015. |
| [i2rs-pub-sub-reqts] | Voit, Eric., Clemm, Alexander. and Alberto. Gonzalez Prieto, "Requirements for Subscription to YANG Datastores", March 2015. |
| [OMG-DDS] | Data Distribution Service for Real-time Systems, version 1.2", January 2007. | , "
| [rfc6020bis] | Bjorklund, Martin., "YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF)", May 2015. |