Consistent Modeling of Operational State Data in YANG
draft-openconfig-netmod-opstate-00

Network Working Group                                          R. Shakir
Internet-Draft                                                        BT
Intended status: Informational                                 A. Shaikh
Expires: September 10, 2015                                     M. Hines
                                                                  Google
                                                           March 9, 2015

         Consistent Modeling of Operational State Data in YANG
                   draft-openconfig-netmod-opstate-00

Abstract

   This document proposes an approach for modeling configuration and
   operational state data in YANG that is geared toward network
   management systems that require capabilities beyond those typically
   envisioned in a NETCONF-based management system.  The document
   presents the requirements of such systems and proposes a modeling
   approach to meet these requirements, along with implications and
   design patterns for modeling operational state in YANG.

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   to this document.  Code Components extracted from this document must
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1.  Introduction

   Retrieving the operational state of a network element (NE) is a
   critical process for a network operator, both because it determines
   how the network is currently running (for example, how many errors
   are occurring on a certain link, what is the load of that link); but
   also because it determines whether the intended configuration applied
   by a network management system is currently operational.  Whilst
   changing of the configuration of NE may be a process which occurs
   relatively infrequently, the accessing of the state of the network is
   significantly more often - with knowledge of the real-time state of
   the network by external analysis and diagnostic systems being desired
   (implying reading of this data on the order of millisecond
   timescales).

   It is desirable to model configuration and operational state within a
   single schema.  This allows a network operator or management system
   to retrieve information as to the intended state of the network
   system (the configuration), and easily relate to this to the actual
   running state.  There are numerous reasons that the intended state
   may not be reflected by the running config:

   o  Protocol negotiations may be required for multiple NEs to agree on
      a certain running value - for example, the HOLDTIME of a BGP
      session is chosen by taking the minimum value of the locally
      configured and advertised value received from the remote speaker.
      The operational value of the HOLDTIME may therefore be lower than
      the configured value on the local system.

   o  Where the application of a change is asynchronous - due to system
      operations, or a pre-requisite for another event to occur before
      the configuration value is applied (e.g., a protocol session
      restart) - then the intended configuration value may not determine
      whether the configuration has been committed to the running
      configuration; or whether the pre-requisite event has occurred.

   Based on such differences between intended and running state, the
   operation of checking one state and then subsequently applying a
   change is very common.  For example, checking the current IGP metric
   of a certain link, and if it is not reflective of the desired value,
   subsequently applying a change.  Rather than viewing configuration
   and operational state separately, having both types of values in a
   common location within the same data schema is advantageous.  In this

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   way, no complex mapping between the path where the value is read, and
   the path by which it is configured is required.

   The majority of existing designs of the layout and presentation of a
   YANG [RFC6020] model considers only the semantics of the NETCONF
   protocol - however, it is advantageous that any data model (expressed
   in YANG) should be suitable for use with multiple protocols.  Such
   protocols may be alternatives to NETCONF - e.g., RESTCONF - but also
   may be specifically engineered for accessing particular operational
   data (e.g., streamed data from a network element, rather than 'SNMP-
   like' polled mechanisms).

   Based on the inherent link between the configuration and state data
   for a NE, and the importance of state for a network operator, YANG's
   focus solely on configuration data is suboptimal[RFC6244].  We
   therefore consider that there is a requirement to consider (and
   define common approaches for) the definition of state and operational
   data within a YANG model.  Such considerations should be cognisant of
   the protocols used to interact with the data schema.

2.  Operational requirements

   Our proposal is motivated by a number of operational requirements as
   described below.

2.1.  Intended configuration as part of operational state

   The definition of operational state in [RFC6244] includes read-only
   transient data that is the result of system operation or protocol
   interactions, and data that is typically thought of as counters or
   statistics.  In many operational use cases it is also important to
   distinguish between the intended value of a configuration variable
   and its actual configured state, analogous to candidate and running
   configuration, respectively, in NETCONF datastores.  In non-
   transactional or asynchronous environments, for example, these may be
   different and it is important to know when they are different or when
   they have converged (see requirement #2).  For this reason, we
   consider the intended configuration as an additional important
   element of the operational state.  This is not considered in
   [RFC6244].

2.2.  Support for both transactional, synchronous management systems as
      well as distributed, asynchronous management systems

   In a synchronous system, configuration changes are transactional and
   committed as an atomic unit.  This implies that the management system
   knows the success or failure of the configuration change based on the
   return value, and hence knows that the intended configuration matches

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   what is on the system.  In particular, the value of any configuration
   variable should always reflect the (intended) configured value.
   Synchronous operation is generally associated with a NETCONF-based
   system that provides transactional semantics for all changes.

   In an asynchronous system, configuration changes to the system may
   not be reflected immediately, even though the change operation
   returns success.  Rather, the change is verified by observing the
   state of the system, for example based on notifications, or
   continuously streamed values of the state.  In this case, the value
   of a configuration variable may not reflect the intended configured
   value at a given point in time.

   The asynchronous use case is important because synchronous operation
   may not always be possible.  For example, in a large scale
   environment, the management system may not need to wait for all
   changes to complete if it is acceptable to proceed while some
   configuration values are being updated.  In addition, not all devices
   may support transactional changes, making asynchronous operation a
   requirement.  Moreover, using observed state to infer the configured
   value allows the management system to learn the time taken to
   complete various configuration changes.

2.3.  Separation of configuration and operational state data; ability to
      retrieve them independently

   These requirements are also mentioned in [RFC3535]:

   o  It is necessary to make a clear distinction between configuration
      data, data that describes operational state and statistics.

   o  It is required to be able to fetch separately configuration data,
      operational state data, and statistics from devices, and to be
      able to compare these between devices.

2.4.  Ability to retrieve operational state corresponding only to
      derived values, statistics, etc.

   When the management system operates in synchronous mode, it should be
   able to retrieve only the operational state corresponding to the
   system determined values, such as negotiated values, protocol
   determined values, or statistics and counters.  Since in synchronous
   mode the intended and actual configuration values are identical,
   sending the intended configuration state is redundant.

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2.5.  Consistent schema locations for configuration and corresponding
      operational state data

   This requirement implies that a common convention is used throughout
   the schema to locate configuration and state data so that the
   management system can infer how to access one or the other without
   needing significant external context.  When considering intended
   configuration as part of operational state (as discussed in
   Section 2.1), it is similarly required that the intended value vs.
   actual value for a particular configuration variable should be
   possible to locate with minimal, if any, mapping information.

   This requirement becomes more evident when considering the
   composition of individual data models into a higher-level model for a
   complete device (e.g., /device[name=devXY]/protocols/routing/...) or
   even higher layer models maintained by network operators (e.g., /ope
   ratorX/global/continent[name=eur]/pop[name=paris]/device[name=devXY]
   /...).  If each model has it's own way to separate configuration and
   state data, then this information must be known at potentially every
   subtree of the composed model.

3.  Implications on modeling operational state

   The requirements in Section 2 give rise to a number of new
   considerations for modeling operational state.  Some of the key
   implications are summarized below.

3.1.  Inclusion of intended configuration as part of operational state

   This implies that a copy of the configurable (i.e., writable) values
   should be included as read-only variables in containers for
   operational state, in addition to the variables that are
   traditionally thought of as state variables (counters, negotiated
   values, etc.).

3.2.  Corresponding leaves for configuration and state

   Any configuration leaf should have a corresponding state leaf.  The
   opposite is clearly not true -- some parts of the model may only have
   derived state variables, for example the contents of a routing table
   that is populated by a dynamic routing protocols like BGP or IS-IS.

3.3.  Retrieval of only the derived, or NE-generated part of the
      operational state

   YANG and NETCONF do not currently differentiate between state that is
   derived by the NE, state representing statistics, and state
   representing intended configuration -- all state is simply marked as

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   'config false' or read-only.  To retrieve only the state that is not
   part of intended configuration, we require a new way to tag such
   data.  This is proposed in this document as a YANG extension.
   Alternatively, as described in [RFC6244], a new NETCONF datastore for
   operational state that is just for NE- generated state could also be
   used to allow <get> (or similar) operations to specify just that part
   of the state.

3.4.  Consistency and predictability in the paths where corresponding
      state and configuration data may be retrieved

   To avoid arbitrary placement of state and configuration data
   containers, the most consistent options would be at the root of the
   model (as done in [YANG-IF]) or at the leaves, i.e., at the start or
   end of the paths.  When operators compose models into a higher level
   model, the root of the model is no longer well-defined, and hence
   neither is the start of the path.  For these reasons, we propose
   placing configuration and state separation at leaves of the model.

3.5.  Reuse of existing NETCONF conventions where applicable

   Though not a specific requirement, models for operational state
   should take advantage of existing protocol mechanisms where possible,
   e.g., to retrieve configuration and state data.

4.  Proposed operational state structure

   Below we show an example model structure that meets the requirements
   described above for all four types of data we are considering:

   o  configuration (writable) data

   o  operational state data on the NE that is derived, negotiated, set
      by a protocol, etc.

   o  operational state data for counters or statistics

   o  operational state data representing intended configuration

4.1.  Example model structure

   The example below shows a partial model (in ascii tree format) for
   managing Ethernet aggregate interfaces (leveraging data definitions
   from [RFC7223]):

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   +--rw interfaces
       +--rw interface* [name]
          +--rw name       -> ../config/name
          +--rw config
          |    ...
          +--ro state
          |  | ...
          |  +--ro counters
          |     +--ro discontinuity-time    yang:date-and-time
          |     +--ro in-octets?            yang:counter64
          |     +--ro in-unicast-pkts?      yang:counter64
          |     +--ro in-broadcast-pkts?    yang:counter64
          |     +--ro in-multicast-pkts?    yang:counter64
          |     +--ro in-discards?          yang:counter64
          |     +--ro in-errors?            yang:counter64
          |     +--ro in-unknown-protos?    yang:counter64
          |     +--ro out-octets?           yang:counter64
          |     +--ro out-unicast-pkts?     yang:counter64
          |     +--ro out-broadcast-pkts?   yang:counter64
          |     +--ro out-multicast-pkts?   yang:counter64
          |     +--ro out-discards?         yang:counter64
          |     +--ro out-errors?           yang:counter64
          +--rw aggregation!
             +--rw config
             |  +--rw lag-type?    aggregation-type
             |  +--rw min-links?   uint16
             +--ro state
             |  +--ro lag-type?    aggregation-type
             |  +--ro min-links?   uint16
             |  +--ro members*     ocif:interface-ref
             +--rw lacp!
                +--rw config
                |  +--rw interval?   lacp-period-type
                +--rw members* [interface]
                |  +--rw interface    ocif:interface-ref
                |  +--ro state
                |     +--ro activity?          lacp-activity-type
                |     +--ro timeout?           lacp-timeout-type
                |     +--ro synchronization?   lacp-synch-type
                |     +--ro aggregatable?      boolean
                |     +--ro collecting?        boolean
                |     +--ro distributing?      boolean
                +--ro state
                   +--ro interval?   lacp-period-type

   In this model, the path to the configurable (rw) items at the
   aggregate interface level is:

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   /interfaces/interface[name=ifName]/aggregation/config/...

   The corresponding operational state is located at:

   /interfaces/interface[name=ifName]/aggregation/state/...

   This container holds a read-only copy of the intended configuration
   variables (lag-type and min-links), as well as a generated list of
   member interfaces (the members leaf-list) for the aggregate that is
   active when the lag-type indicates a statically configured aggregate.
   Note that although the paths to config and state containers are
   symmetric, the state container contains additional derived variables.

   The model has an additional hierarchy level for aggregate interfaces
   that are maintained using LACP.  For these, the configuration path
   is:

   /interfaces/interface[name=ifName]/aggregation/lacp/config/...

   with the corresponding state container (in this case with only the
   state corresponding to the intended configuration) at:

   /interfaces/interface[name=ifName]/aggregation/lacp/state/...

   There is an additional list of members for LACP-managed aggregates
   with only a state container:

   /interfaces/interface[name=ifName]/aggregation/lacp/
   members[name=ifName]/state/...

   Note that it is not required that both a state and a config container
   be present at every leaf.  It may be convenient to include an empty
   config container to make it more explicit to the management system
   that there are no configuration variables at this location in the
   data tree.

   Finally, we can see that the generic interface object also has config
   and state containers (these are abbreviated for clarity).  The state
   container has a subcontainer for operational state corresponding to
   counters and statistics that are valid for any interface type:

   /interfaces/interface[name=ifName]/state/counters/...

5.  Impact on model authoring

   One drawback of structuring operational and configuration data in
   this way is the added complexity in authoring the models, relative to
   the way some models are currently built with state and config split

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   at the root of the individual model (e.g., in [RFC7223], [RFC7317],
   and [IETF-RTG]).  Moving the config and state containers to each leaf
   adds a one-time modeling effort, which is somewhat dependent on the
   model structure itself (how many layers of container hierarchy,
   number of lists, etc.)  However, we feel this effort is justified by
   the resulting simplicity with which management systems can access and
   correlate state and configuration data.

5.1.  Modeling design patterns

   We propose some specific YANG modeling design patterns that may be
   useful for building models following these conventions.

5.1.1.  Basic structure

   Since leaves that are created under the 'config' container are
   duplicated under the 'state' container, it is recommended that the
   following conventions are used to ensure that the schema remain as
   simple as possible:

   o  A grouping for the 'config' data items is created - with a
      specific naming convention to indicate that such variables are
      configurable, such as a suffix like '-config' or '_config'.  For
      example, the OpenConfig BGP model [OC-BGP] adopts the convention
      of appending "_config" to the name of the container.

   o  A grouping for the 'state' data items is created, with a similar
      naming convention as above, i.e., with a suffix such as '-state'
      or '_state'.  The BGP model uses "_state".

   o  A structure grouping is created that instantiates both the
      'config' and 'state' containers.  The 'config' container should
      include the "-config" grouping, whilst the state container has
      both the "-config" and "-state" groupings, along with the 'config
      false' statement.

   A simple example in YANG is shown in Appendix B.

5.1.2.  Handling lists

   In YANG 1.0, lists have requirements that complicate the creation of
   the parallel configuration and state data structures.  First, keys
   must be children of the list; they cannot be further down the data
   hierarchy within a subsequent container.  For example, the
   'interface' list cannot be keyed by /interfaces/interface/config/
   name.  Second YANG requires that the list key is part of the
   configuration or state data in each list member.

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   We consider two possible approaches for lists:

   1.  list keys appear only at the top level of the list, i.e., not
       duplicated under the 'config' or 'state' containers within the
       list

   2.  the data represented by the list key appears in the config and
       state containers, and a key with type leafref is used in the top
       level of the list pointing to the corresponding data node in the
       config (or state) container.

   Option 1 has the advantage of not duplicating data, but treats the
   data item (or items) that are keys as special cases, i.e., not
   included in the config or state containers.  Option 2 is appealing in
   that configurable data always appears in the config container, but
   requires an arguably unnecessary key pointing to the data from the
   top level of the list.

   Appendix C shows a simple example of both options.

5.1.3.  Selective use of state data from common groupings

   In a number of cases, it is desirable that the same grouping be used
   within different places in a model - but state information is only
   relevant in one of these paths.  For example, considering BGP, peer
   configuration is relevant to both a "neighbor" (i.e., an individual
   BGP peer), and also to a peer-group (a set of peers).  Counters
   relating to the number of received prefixes, or queued messages, are
   relevant only within the 'state' container of the peer (rather than
   the peer-group).  In this case, use of the 'augment' statement to add
   specific leaves to only one area of the tree is recommended, since it
   allows a common container to be utilized otherwise.

5.1.4.  Non-corresponding configuration and state data

   There are some instances where only an operational state container is
   relevant without a corresponding configuration data container.  For
   example, the list of currently active member interfaces in a LACP-
   managed LAG is typically reported by the system as operational state
   that is governed by the LACP protocol.  Such data is not directly
   configured.  Similarly, counters and statistics do not have
   corresponding configuration.  In these cases, we can either omit the
   config container from such leaves, or provide an empty container as
   described earlier.  With both options, the management system is able
   to infer that such data is not configurable.

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6.  YANG language considerations

   In adopting the approach described in this document for modeling
   operational state data in YANG, we encounter several language
   limitations that are described below.  We disucss some initial
   thoughts on possible changes to the language to more easily enable
   the proposed model for operational state modeling.

6.1.  Distinguishing derived operational state data and intended
      configuration

   As mentioned in Section 2, we require a way to separately query
   operational state that is not part of intended configuration (e.g.,
   protocol-determined data, counters, etc.).  YANG and NETCONF do not
   distinguish types of operational state data, however.  To overcome
   this, we currently use a YANG language extension to mark such data as
   'operational: true'.  Ideally, this could be generalized beyond the
   current 'config: true / false' to something like "operational:
   false", "operational: intent", and "operational:true".

6.2.  YANG lists as maps

   YANG has two list constructs, the 'leaf-list' which is similar to a
   list of scalars in other programming languages, and the 'list' which
   allows a keyed list of complex structures, where the key is also part
   of the data values.  As described in Section [impact], the current
   requirements on YANG list keys require either duplication of data, or
   treating some data (i.e., those that comprise list keys) as a special
   case.  One solution is to generalize lists to be more like map data
   structures, where each list member has a key that is not required to
   part of the configuration or state data.  This allows list keys to be
   arbitrarily defined by the user if desired, or based on values of
   data nodes.  In the latter case, the specification of which data
   nodes are used in constructing the list key could be indicated in the
   meta-data associated with the key.

6.3.  Configuration and state data hierarchy

   YANG does not allow read-write configuration data to be child nodes
   of read-only operational state data.  This requires the definition of
   separate state and config containers as described above.  However, it
   may be desirable to simplify the schema by 'flattening', e.g., having
   the operational state as the root of the data tree, with only config
   containers needed to specify the variables that are writable (in
   general, the configuration data is much smaller than operational
   state data).  Naming the containers explicitly according the config /
   state convention makes the intent of the data clear, and should allow
   relaxing of the current YANG restrictions.  That is, a read-write

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   config container makes explicit the nature of the enclosed data even
   if the parent data nodes are read-only.  This of course requires that
   all data in a config container are in fact configurable -- this is
   one of the motivations of pushing such containers as far down in the
   schema hierarchy as possible.

7.  Security Considerations

   This document addresses the structure of configuration and
   operational state data, both of which should be considered sensitive
   from a security standpoint.  Any data models that follows the
   proposed structuring must be carefully carefully evaluated to
   determine its security risks.  In general, access to both
   configuration (write) and operational state (read) data must be
   carefully controlled through appropriate access control and
   authorization mechanisms.

8.  References

8.1.  Normative references

   [RFC6020]  Bjorklund, M., "YANG - A Data Modeling Language for the
              Network Configuration Protocol (NETCONF)", RFC 6020,
              October 2010.

   [RFC6244]  Shafer, P., "An Architecture for Network Management Using
              NETCONF and YANG", RFC 6244, June 2011.

   [RFC3535]  Schoenwaelder, J., "Overview of the 2002 IAB Network
              Management Workshop", RFC 3535, May 2003.

   [RFC7223]  Bjorklund, M., "A YANG Data Model for Interface
              Management", RFC 7223, May 2014.

   [RFC7317]  Bierman, A. and M. Bjorklund, "A YANG Data Model for
              System Management", RFC 7317, August 2014.

8.2.  Informative references

   [IETF-RTG]
              Lhotka, L., "A YANG Data Model for Routing Management",
              draft-ietf-netmod-routing-cfg-16 (work in progress),
              October 2014.

   [OC-BGP]   Shaikh, A., D'Souza, K., Bansal, D., and R. Shakir, "BGP
              Configuration Model for Service Provider Networks", draft-
              shaikh-idr-bgp-model-01 (work in progress), March 2015.

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Appendix A.  Acknowledgements

   The authors are grateful for valuable input to this document from:
   Martin Bjorklund, Paul Borman, Chris Chase, Feihong Chen, Josh
   George, Carl Moberg, Jason Sterne, and Jim Uttaro.

Appendix B.  Example YANG base structure

   Below we show an example of the basic YANG building block for
   organizing configuration and operational state data as described in
   Section 4

   grouping example-config {
       description "configuration data for example container";

       leaf conf-1 {
         type empty;
       }

       leaf conf-2 {
         type string;
       }
     }

     grouping example-state {
       description
         "operational state data (derived, counters, etc.) for example
         container";

       leaf state-1 {
         type boolean;
       }

       leaf state-2 {
         type string;
       }

       container counters {
         description
           "operational state counters for example container";

         leaf counter-1 {
           type uint32;
         }

         leaf counter-2 {
           type uint64;
         }

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       }
     }

     grouping example-structure {
       description
         "top level grouping for the example container -- this is used
         to put the config and state subtrees in the appropriate
         location";

       container example {
         description
           "top-level container for the example data";

         container config {

           uses example-config;

         }

         container state {

           config false;
           uses example-config;
           uses example-state;
         }
       }
     }

     uses example-structure;

   The corresponding YANG data tree is:

      +--rw example
         +--rw config
         |  +--rw conf-1?   empty
         |  +--rw conf-2?   string
         +--ro state
            +--ro conf-1?     empty
            +--ro conf-2?     string
            +--ro state-1?    boolean
            +--ro state-2?    string
            +--ro counters
               +--ro counter-1?   uint32
               +--ro counter-2?   uint64

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Appendix C.  Example YANG list structure

   As described in Section 5.1.2, there are two options we consider for
   building lists according to the proposed structure.  Both are shown
   in the example YANG snippet below.  The groupings defined above in
   Appendix B are reused here.

   grouping example-no-conf2-config {
       description
       "configuration data for example container but without the conf-2
       data leaf which is used as a list key";

       leaf conf-1 {
         type empty;
       }

     }

     grouping example-structure {
       description
         "top level grouping for the example container -- this is used
         to put the config and state subtrees in the appropriate
         location";

       list example {

         key conf-2;
         description
           "top-level list for the example data";

         leaf conf-2 {
           type leafref {
             path "../config/conf-2";
           }
         }

         container config {

           uses example-config;

         }

         container state {

           config false;
           uses example-config;
           uses example-state;
         }

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       }

       list example2 {

         key conf-2;
         description
           "top-level list for the example data";

         leaf conf-2 {
           type string;
         }

         container config {

           uses example-no-conf2-config;

         }

         container state {

           config false;
           uses example-no-conf2-config;
           uses example-state;
         }
       }
     }

     uses example-structure;

   The corresponding YANG data tree is shown below for both styles of
   lists.

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   +--rw example* [conf-2]
      |  +--rw conf-2    -> ../config/conf-2
      |  +--rw config
      |  |  +--rw conf-1?   empty
      |  |  +--rw conf-2?   string
      |  +--ro state
      |     +--ro conf-1?     empty
      |     +--ro conf-2?     string
      |     +--ro state-1?    boolean
      |     +--ro state-2?    string
      |     +--ro counters
      |        +--ro counter-1?   uint32
      |        +--ro counter-2?   uint64
      +--rw example2* [conf-2]
         +--rw conf-2    string
         +--rw config
         |  +--rw conf-1?   empty
         +--ro state
            +--ro conf-1?     empty
            +--ro state-1?    boolean
            +--ro state-2?    string
            +--ro counters
               +--ro counter-1?   uint32
               +--ro counter-2?   uint64

Authors' Addresses

   Rob Shakir
   BT
   pp. C3L, BT Centre
   81, Newgate Street
   London  EC1A 7AJ
   UK

   Email: rob.shakir@bt.com
   URI:   http://www.bt.com/

   Anees Shaikh
   Google
   1600 Amphitheatre Pkwy
   Mountain View, CA  94043
   US

   Email: aashaikh@google.com

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   Marcus Hines
   Google
   1600 Amphitheatre Pkwy
   Mountain View, CA  94043
   US

   Email: hines@google.com

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