path: root/doc/rfc/rfc7758.txt

Internet Engineering Task Force (IETF)                        T. Mizrahi
Request for Comments: 7758                                      Y. Moses
Category: Experimental                                          Technion
ISSN: 2070-1721                                            February 2016


                       Time Capability in NETCONF

Abstract

   This document defines a capability-based extension to the Network
   Configuration Protocol (NETCONF) that allows time-triggered
   configuration and management operations.  This extension allows
   NETCONF clients to invoke configuration updates according to
   scheduled times and allows NETCONF servers to attach timestamps to
   the data they send to NETCONF clients.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7758.

















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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.





































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Table of Contents

   1. Introduction ....................................................4
   2. Conventions Used in This Document ...............................4
      2.1. Key Words ..................................................4
      2.2. Abbreviations ..............................................5
      2.3. Terminology ................................................5
   3. Using Time in NETCONF ...........................................5
      3.1. The Time Capability in a Nutshell ..........................5
      3.2. Notifications and Cancellation Messages ....................7
      3.3. Synchronization Aspects ....................................9
      3.4. Scheduled Time Format .....................................10
      3.5. Scheduling Tolerance ......................................10
      3.6. Scheduling the Near vs. Far Future ........................11
      3.7. Time-Interval Format ......................................13
   4. Time Capability ................................................14
      4.1. Overview ..................................................14
      4.2. Dependencies ..............................................14
      4.3. Capability Identifier .....................................14
      4.4. New Operations ............................................14
      4.5. Modifications to Existing Operations ......................15
           4.5.1. Affected Operations ................................15
           4.5.2. Processing Scheduled Operations ....................16
      4.6. Interactions with Other Capabilities ......................16
   5. Examples .......................................................17
      5.1. <scheduled-time> Example ..................................17
      5.2. <get-time> Example ........................................18
      5.3. Error Example .............................................19
   6. Security Considerations ........................................19
      6.1. General Security Considerations ...........................19
      6.2. YANG Module Security Considerations .......................20
   7. IANA Considerations ............................................21
   8. References .....................................................22
      8.1. Normative References ......................................22
      8.2. Informative References ....................................22
   Appendix A. YANG Module for the Time Capability ...................24
   Acknowledgments ...................................................32
   Authors' Addresses ................................................32













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1.  Introduction

   The Network Configuration Protocol (NETCONF), defined in [RFC6241],
   provides mechanisms to install, manipulate, and delete the
   configuration of network devices.  NETCONF allows clients to
   configure and monitor NETCONF servers using remote procedure calls
   (RPCs).

   NETCONF is asynchronous; when a client invokes an RPC, it has no
   control over the time at which the RPC is executed, nor does it have
   any feedback from the server about the execution time.

   Time-based configuration ([OneClock] [Time4]) can be a useful tool
   that enables an entire class of coordinated and scheduled
   configuration procedures.  Time-triggered configuration allows
   coordinated network updates in multiple devices; a client can invoke
   a coordinated configuration change by sending RPCs to multiple
   servers with the same scheduled execution time.  A client can also
   invoke a time-based sequence of updates by sending n RPCs with n
   different update times, T1, T2, ..., Tn, determining the order in
   which the RPCs are executed.

   This memo defines the :time capability in NETCONF.  This extension
   allows clients to determine the scheduled execution time of RPCs they
   send.  It also allows a server that receives an RPC to report its
   actual execution time to the client.

   The NETCONF time capability is intended for scheduling RPCs that
   should be performed in the near future, allowing the coordination of
   simultaneous configuration changes or specification of an order of
   configuration updates.  Time-of-day-based policies and far-future
   scheduling, e.g., [Cond], are outside the scope of this memo.

   This memo is defined for experimental purposes and will allow the
   community to experiment with the NETCONF time capability.  Based on
   the lessons learned from this experience, it is expected that the
   NETCONF working group will be able to consider whether to adopt the
   time capability.

2.  Conventions Used in This Document

2.1.  Key Words

   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 [RFC2119].





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2.2.  Abbreviations

   NETCONF  Network Configuration Protocol

   RPC      Remote Procedure Call

2.3.  Terminology

   o  Capability [RFC6241]: A functionality that supplements the base
      NETCONF specification.

   o  Client [RFC6241]: Invokes protocol operations on a server.  In
      addition, a client can subscribe to receive notifications from a
      server.

   o  Execution time: The execution time of an RPC is defined as the
      time at which a server completes the execution of an RPC, before
      it sends the <rpc-reply> message.

   o  Scheduled RPC: an RPC that is scheduled to be performed at a
      predetermined time, which is included in the <rpc> message.

   o  Scheduled time: The scheduled time of an RPC is the time at which
      the RPC should be started, as determined by the client.  It is the
      server's role to enforce the execution of the scheduled time.

   o  Server [RFC6241]: Executes protocol operations invoked by a
      client.  In addition, a server can send notifications to a client.

3.  Using Time in NETCONF

3.1.  The Time Capability in a Nutshell

   The :time capability provides two main functions:

   o  Scheduling:

      When a client sends an RPC to a server, the <rpc> message MAY
      include the scheduled-time element, denoted by Ts in Figure 1.
      The server then executes the RPC at the scheduled time Ts; once
      completed, the server can respond with an RPC reply message.










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   o  Reporting:

      When a client sends an RPC to a server, the <rpc> message MAY
      include a get-time element (see Figure 2), requesting the server
      to return the execution time of the RPC.  In this case, after the
      server performs the RPC, it responds with an RPC reply that
      includes the execution time, Te.

                      RPC _________
                    executed       \
                                   \/
                                   Ts
            server  ---------------+-------------        ----> time
                              /\      \
                          rpc /        \ rpc-reply
                         (Ts)/          \
                            /           \/
            client  -----------------------------

            Figure 1: Scheduled RPC

                   RPC _________
                 executed       \
                                \/
                                Te
            server  ------------+----------------        ----> time
                              /\   \
                       rpc    /     \ rpc-reply
                   (get-time)/       \ (Te)
                            /        \/
            client  -----------------------------

            Figure 2: Reporting the Execution Time of an RPC

   Example 1.  A client needs to trigger a commit at n servers, so that
   the n servers perform the commit as close as possible to
   simultaneously.  Without the time capability, the client sends a
   sequence of n commit messages; thus, each server performs the commit
   at a different time.  By using the time capability, the client can
   send commit messages that are scheduled to take place at a chosen
   time Ts, for example, 5 seconds in the future, causing the servers to
   invoke the commit as close as possible to time Ts.

   Example 2.  In many applications, it is desirable to monitor events
   or collect statistics regarding a common time reference.  A client
   can send a set of get-config messages that is scheduled to be
   executed at multiple servers at the same time, providing a




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   simultaneous system-wide view of the state of the servers.  Moreover,
   a client can use the get-time element in its get-config messages,
   providing a time reference to the sampled element.

   The scenarios of Figures 1 and 2 imply that a third scenario can also
   be supported (Figure 3), where the client invokes an RPC that
   includes a scheduled time, Ts, as well as the get-time element.  This
   allows the client to receive feedback about the actual execution
   time, Te.  Ideally, Ts=Te.  However, the server may execute the RPC
   at a slightly different time than Ts, for example, if the server is
   tied up with other tasks at Ts.

                      RPC _________
                    executed       \
                                   \/
                                Ts Te
            server  -------------+-+-------------        ----> time
                            /\        \
                   rpc      /          \ rpc-reply
            (Ts + get-time)/            \ (Te)
                          /             \/
            client  -----------------------------

            Figure 3: Scheduling and Reporting

3.2.  Notifications and Cancellation Messages

   Notifications

      As illustrated in Figure 1, after a scheduled RPC is executed, the
      server sends an <rpc-reply>.  The <rpc-reply> may arrive a long
      period of time after the RPC was sent by the client, leaving the
      client without a clear indication of whether the RPC was received.

      This document defines a new notification, the netconf-scheduled-
      message notification, which provides an immediate acknowledgement
      of the scheduled RPC.

      The <netconf-scheduled-message> notification is sent to the client
      if it is subscribed to the NETCONF notifications [RFC6470]; as
      illustrated in Figure 4, when the server receives a scheduled RPC,
      it sends a notification to the client.









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      The <netconf-scheduled-message> notification includes a <schedule-
      id> element.  The <schedule-id> is a unique identifier that the
      server assigns to every scheduled RPC it receives.  Thus, a client
      can keep track of all the pending scheduled RPCs; a client can
      uniquely identify a scheduled RPC by the tuple {server, schedule-
      id}.

                      RPC ____________
                    executed          \
                                      \/
                                      Ts
            server  -------------------+---------        ----> time
                        /\  \            \
                    rpc /    \notifi-     \ rpc-reply
                   (Ts)/      \cation      \
                      /       \/           \/
            client  -----------------------------

            Figure 4: Scheduled RPC with Notification

   Cancellation Messages

      A client can cancel a scheduled RPC by sending a <cancel-schedule>
      RPC.  The <cancel-schedule> RPC includes the <schedule-id> of the
      scheduled RPC that needs to be cancelled.

      The <cancel-schedule> RPC, defined in this document, can be used
      to perform a coordinated all-or-none procedure, where either all
      the servers perform the operation on schedule or the operation is
      aborted.

      Example 3.  A client sends scheduled <rpc> messages to server 1
      and server 2, both scheduled to be performed at time Ts.  Server 1
      sends a notification indicating that it has successfully scheduled
      the RPC, while server 2 replies with an unknown-element error
      [RFC6241] that indicates that it does not support the time
      capability.  The client sends a <cancel-schedule> RPC to server 1
      and receives an <rpc-reply>.  The message exchange between the
      client and server 1 in this example is illustrated in Figure 5.












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                                RPC not __________
                                executed          \
                                                  \/
                                                   Ts
            server  --------------------------------+---      ----> time
                        /\ \            /\        \
                    rpc /   \notifi-    /cancel-   \ rpc-reply
                   (Ts)/     \cation   /schedule    \
                      /      \/       /             \/
            client  ------------------------------------

                  Figure 5: Cancellation Message

   A <cancel-schedule> RPC MUST NOT include the scheduled-time
   parameter.  A server that receives a <cancel-schedule> RPC should try
   to cancel the schedule as soon as possible.  If the server is unable
   to cancel the scheduled RPC, for example, because it has already been
   executed, it should respond with an <rpc-error> [RFC6241], in which
   the error-type is 'protocol', and the error-tag is 'operation-
   failed'.

3.3.  Synchronization Aspects

   The time capability defined in this document requires clients and
   servers to maintain clocks.  It is assumed that clocks are
   synchronized by a method that is outside the scope of this document,
   e.g., [RFC5905] or [IEEE1588].

   This document does not define any requirements pertaining to the
   degree of accuracy of performing scheduled RPCs.  Note that two
   factors affect how accurately the server can perform a scheduled RPC:
   one factor is the accuracy of the clock synchronization method used
   to synchronize the clients and servers and the second factor is the
   server's ability to execute real-time configuration changes, which
   greatly depends on how it is implemented.  Typical networking devices
   are implemented by a combination of hardware and software.  While the
   execution time of a hardware module can typically be predicted with a
   high level of accuracy, the execution time of a software module may
   be variable and hard to predict.  A configuration update would
   typically require the server's software to be involved, thus
   affecting how accurately the RPC can be scheduled.

   Another important aspect of synchronization is monitoring; a client
   should be able to check whether a server is synchronized to a
   reference time source.  Typical synchronization protocols, such as
   the Network Time Protocol [RFC5905], provide the means ([RFC5907],
   [RFC7317]) to verify that a clock is synchronized to a time reference
   by querying its Management Information Base (MIB).  The get-time



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   feature defined in this document (see Figure 2) allows a client to
   obtain a rough estimate of the time offset between the client's clock
   and the server's clock.

   Since servers do not perform configuration changes instantaneously,
   the processing time of an RPC should not be overlooked.  The
   scheduled time always refers to the start time of the RPC, and the
   execution time always refers to its completion time.

3.4.  Scheduled Time Format

   The scheduled time and execution time fields in <rpc> messages use a
   common time format field.

   The time format used in this document is the date-and-time format,
   defined in Section 5.6 of [RFC3339] and Section 3 of [RFC6991].

       leaf scheduled-time {
         type yang:date-and-time;
         description
         "The time at which the RPC is scheduled to be performed.";
       }

       leaf execution-time {
         type yang:date-and-time;
         description
         "The time at which the RPC was executed.";
       }

3.5.  Scheduling Tolerance

   When a client sends an RPC that is scheduled to Ts, the server MUST
   verify that the value Ts is not too far in the past or in the future.
   As illustrated in Figure 6, the server verifies that Ts is within the
   scheduling-tolerance range.
















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                  RPC _________
                received       \
                               \/
                                     Ts
            -----+--------------+-----+------------+-------> time

                  <------------> <---------------->
                  sched-max-past  sched-max-future

                  <------------------------------->
                       scheduling tolerance

               Figure 6: Scheduling Tolerance

   The scheduling tolerance is determined by two parameters: sched-max-
   future and sched-max-past.  These two parameters use the time-
   interval format (Section 3.7.), and their default value is 15
   seconds.

   If the scheduled time, Ts, is within the scheduling-tolerance range,
   the scheduled RPC is performed; if Ts occurs in the past and within
   the scheduling tolerance, the server performs the RPC as soon as
   possible; whereas if Ts is a future time, the server performs the RPC
   at Ts.

   If Ts is not within the scheduling-tolerance range, the scheduled RPC
   is discarded, and the server responds with an error message [RFC6241]
   including a bad-element error-tag.  An example is provided in Section
   5.3.

3.6.  Scheduling the Near vs. Far Future

   The scheduling bound defined by sched-max-future guarantees that
   every scheduled RPC is restricted to a scheduling time in the near
   future.

   The scheduling mechanism defined in this document is intended for
   near-future scheduling, on the order of seconds.  Far-future
   scheduling is outside the scope of this document.

   Example 1 is a typical example of using near-future scheduling; the
   goal in the example is to perform the RPC at multiple servers at the
   same time; therefore, it is best to schedule the RPC to be performed
   a few seconds in the future.







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   The Challenges of Far-Future Scheduling

      When an RPC is scheduled to be performed at a far-future time,
      during the long period between the time at which the RPC is sent
      and the time at which it is scheduled to be executed, the
      following erroneous events may occur:

      o  The server may restart.

      o  The client's authorization level may be changed.

      o  The client may restart and send a conflicting RPC.

      o  A different client may send a conflicting RPC.

      In these cases, if the server performs the scheduled operation, it
      may perform an action that is inconsistent with the current
      network policy or inconsistent with the currently active clients.

      Near-future scheduling guarantees that external events, such as
      the examples above, have a low probability of occurring during the
      sched-max-future period, and even when they do, the period of
      inconsistency is limited to sched-max-future, which is a short
      period of time.

   The Trade-off in Setting the sched-max-future Value

      The sched-max-future parameter should be configured to a value
      that is high enough to allow the client to:

      1. Send the scheduled RPC, potentially to multiple servers.

      2. Receive notifications or <rpc-error> messages from the
         server(s) or wait for a timeout and decide that if no response
         has arrived then something is wrong.

      3. If necessary, send a cancellation message, potentially to
         multiple servers.

      On the other hand, sched-max-future should be configured to a
      value that is low enough to allow a low probability of the
      erroneous events above, typically on the order of a few seconds.
      Note that, even if sched-max-future is configured to a low value,
      it is still possible (with a low probability) that an erroneous
      event will occur.  However, this short, potentially hazardous
      period is not significantly worse than in conventional
      (unscheduled) RPCs, as even a conventional RPC may in some cases
      be executed a few seconds after it was sent by the client.



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   The Default Value of sched-max-future

      The default value of sched-max-future is defined to be 15 seconds.
      This duration is long enough to allow the scheduled RPC to be sent
      by the client, potentially to multiple servers, and in some cases
      to send a cancellation message, as described in Section 3.2.  On
      the other hand, the 15-second duration yields a very low
      probability of a reboot or a permission change.

3.7.  Time-Interval Format

   The time-interval format is used for representing the length of a
   time interval and is based on the date-and-time format.  It is used
   for representing the scheduling tolerance parameters, as described in
   the previous section.

   While the date-and-time type uniquely represents a specific point in
   time, the time-interval type defined below can be used to represent
   the length of a time interval without specifying a specific date.

   The time-interval type is defined as follows:

      typedef time-interval {
        type string {
          pattern '\d{2}:\d{2}:\d{2}(\.\d+)?';
        }
        description
          "Defines a time interval, up to 24 hours.
           The format is specified as HH:mm:ss.f,
           consisting of two digits for hours,
           two digits for minutes, two digits
           for seconds, and zero or more digits
           representing second fractions.";
      }

   Example

   The sched-max-future parameter is defined (Appendix A) as a time-
   interval, as follows:

      leaf sched-max-future {
        type time-interval;
        default 00:00:15.0;
      }

   The default value specified for sched-max-future is 0 hours, 0
   minutes, and 15 seconds.




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4.  Time Capability

   The structure of this section is as defined in Appendix D of
   [RFC6241].

4.1.  Overview

   A server that supports the time capability can perform time-triggered
   operations as defined in this document.

   A server implementing the :time capability:

   o  MUST support the ability to receive <rpc> messages that include a
      time element and perform a time-triggered operation accordingly.

   o  MUST support the ability to include a time element in the <rpc-
      reply> messages that it transmits.

4.2.  Dependencies

   With-defaults Capability

      The time-capability YANG module (Appendix A) uses default values;
      thus, it is assumed that the with-defaults capability [RFC6243] is
      supported.

4.3.  Capability Identifier

   The :time capability is identified by the following capability
   string:

   urn:ietf:params:netconf:capability:time:1.0

4.4.  New Operations

   <cancel-schedule>

      The <cancel-schedule> RPC is used for cancelling an RPC that was
      previously scheduled.

      A <cancel-schedule> RPC MUST include the <cancelled-message-id>
      element, which specifies the message ID of the scheduled RPC that
      needs to be cancelled.

      A <cancel-schedule> RPC MAY include the <get-time> element.  In
      this case, the <rpc-reply> includes the <execution-time> element,
      specifying the time at which the scheduled RPC was cancelled.




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4.5.  Modifications to Existing Operations

4.5.1.  Affected Operations

   The :time capability defined in this memo can be applied to any of
   the following operations:

   o  get-config

   o  get

   o  copy-config

   o  edit-config

   o  delete-config

   o  lock

   o  unlock

   o  commit

   Three new elements are added to each of these operations:

   o  <scheduled-time> This element is added to the input of each
      operation, indicating the time at which the server is scheduled to
      invoke the operation.  Every <rpc> message MAY include the
      <scheduled-time> element.  A server that supports the :time
      capability and receives an <rpc> message with a <scheduled-time>
      element MUST perform the operation as close as possible to the
      scheduled time.

      The <scheduled-time> element uses the date-and-time format
      (Section 3.4.).

   o  <get-time> This element is added to the input of each operation.
      An <rpc> message MAY include a <get-time> element, indicating that
      the server MUST include an <execution-time> element in its
      corresponding <rpc-reply>.

   o  <execution-time> This element is added to the output of each
      operation, indicating the time at which the server completed the
      operation.  An <rpc-reply> MAY include the <execution-time>
      element.  A server that supports the :time capability and receives
      an operation with the <get-time> element MUST include the
      execution time in its response.




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      The <execution-time> element uses the date-and-time format
      (Section 3.4.).

4.5.2.  Processing Scheduled Operations

   A server that receives a scheduled RPC MUST start executing the RPC
   as close as possible to its scheduled execution time.

   If a session between a client and a server is terminated, the server
   MUST cancel all pending scheduled RPCs that were received in this
   session.

   Scheduled RPCs are processed serially, in an order that is defined by
   their scheduled times.  Thus, the server sends <rpc-reply> messages
   to scheduled RPCs according to the order of their corresponding
   schedules.  Note that this is a modification to the behavior defined
   in [RFC6241], which states that replies are sent in the order the
   requests were received.  Interoperability with [RFC6241] is
   guaranteed by the NETCONF capability exchange; a server that does not
   support the :time capability responds to RPCs in the order the
   requests were received.  A server that supports the :time capability
   replies to conventional (non-scheduled) RPCs in the order they were
   received and replies to scheduled RPCs in the order of their
   scheduled times.

   If a server receives two or more RPCs that are scheduled to be
   performed at the same time, the server executes the RPCs serially in
   an arbitrary order.

4.6.  Interactions with Other Capabilities

   Confirmed Commit Capability

      The confirmed commit capability is defined in Section 8.4 of
      [RFC6241].  According to that document, a confirmed <commit>
      operation MUST be reverted if a confirming commit is not issued
      within the timeout period (which is 600 seconds by default).

      When the time capability is supported, and a confirmed <commit>
      operation is used with the <scheduled-time> element, the
      confirmation timeout MUST be counted from the scheduled time,
      i.e., the client begins the timeout measurement starting at the
      scheduled time.








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5.  Examples

5.1.  <scheduled-time> Example

   The following example extends the example presented in Section 7.2 of
   [RFC6241] by adding the time capability.  In this example, the
   <scheduled-time> element is used to specify the scheduled execution
   time of the configuration update (as shown in Figure 1).

   <rpc message-id="101"
       xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <edit-config>
       <target>
         <running/>
       </target>
       <scheduled-time
          xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-time">
           2015-10-21T04:29:00.235Z
       </scheduled-time>
       <config>
         <top xmlns="http://example.com/schema/1.2/config">
           <interface>
             <name>Ethernet0/0</name>
             <mtu>1500</mtu>
           </interface>
         </top>
       </config>
     </edit-config>
   </rpc>

   <rpc-reply message-id="101"
        xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <ok/>
   </rpc-reply>

















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5.2.  <get-time> Example

   The following example is similar to the one presented in Section 5.1,
   except that, in this example, the client includes a <get-time>
   element in its RPC and the server consequently responds with an
   <execution-time> element (as shown in Figure 2).

   <rpc message-id="101"
       xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <edit-config>
       <target>
         <running/>
       </target>
       <get-time
        xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-time">
       </get-time>
       <config>
         <top xmlns="http://example.com/schema/1.2/config">
           <interface>
             <name>Ethernet0/0</name>
             <mtu>1500</mtu>
           </interface>
         </top>
       </config>
     </edit-config>
   </rpc>

   <rpc-reply message-id="101"
        xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <ok/>
     <execution-time>
         2015-10-21T04:29:00.235Z
     </execution-time>
   </rpc-reply>

















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5.3.  Error Example

   The following example presents a scenario in which the scheduled-time
   is not within the scheduling tolerance, i.e., it is too far in the
   past; therefore, an <rpc-error> is returned.

   <rpc message-id="101"
       xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <edit-config>
       <target>
         <running/>
       </target>
       <scheduled-time
          xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-time">
           2010-10-21T04:29:00.235Z
       </scheduled-time>
       <config>
         <top xmlns="http://example.com/schema/1.2/config">
           <interface>
             <name>Ethernet0/0</name>
             <mtu>1500</mtu>
           </interface>
         </top>
       </config>
     </edit-config>
   </rpc>

   <rpc-reply message-id="101"
        xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <rpc-error>
       <error-type>application</error-type>
       <error-tag>bad-element</error-tag>
       <error-severity>error</error-severity>
       <error-info>
         <bad-element>scheduled-time</bad-element>
       </error-info>
     </rpc-error>
   </rpc-reply>

6.  Security Considerations

6.1.  General Security Considerations

   The security considerations of the NETCONF protocol in general are
   discussed in [RFC6241].






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   The usage of the time capability defined in this document can assist
   an attacker in gathering information about the system, such as the
   exact time of future configuration changes.  Moreover, the time
   elements can potentially allow an attacker to learn information about
   the system's performance.  Furthermore, an attacker that sends
   malicious <rpc> messages can use the time capability to amplify her
   attack; for example, by sending multiple <rpc> messages with the same
   scheduled time.  It is important to note that the security measures
   described in [RFC6241] can prevent these vulnerabilities.

   The time capability relies on an underlying time synchronization
   protocol.  Thus, by attacking the time protocol, an attack can
   potentially compromise NETCONF when using the time capability.  A
   detailed discussion about the threats against time protocols and how
   to mitigate them is presented in [RFC7384].

   The time capability can allow an attacker to attack a NETCONF server
   by sending malicious RPCs that are scheduled to take place in the
   future.  For example, an attacker can send multiple scheduled RPCs
   that are scheduled to be performed at the same time.  Another
   possible attack is to send a large number of scheduled RPCs to a
   NETCONF server, potentially causing the server's buffers to overflow.
   These attacks can be mitigated by a carefully designed NETCONF
   server; when a server receives a scheduled RPC that exceeds its
   currently available resources, it should reply with an <rpc-error>
   and discard the scheduled RPC.

   Note that if an attacker has been detected and revoked, its future
   scheduled RPCs are not executed; as defined in Section 4.5.2, once
   the session with the attacker has been terminated, the corresponding
   scheduled RPCs are discarded.

6.2.  YANG Module Security Considerations

   This memo defines a new YANG module, as specified in Appendix A.

   The YANG module defined in this memo is designed to be accessed via
   the NETCONF protocol [RFC6241].  The lowest NETCONF layer is the
   secure transport layer and the mandatory-to-implement secure
   transport is Secure SHell (SSH) [RFC6242].  The NETCONF access
   control model [RFC6536] provides the means to restrict access for
   particular NETCONF users to a preconfigured subset of all available
   NETCONF protocol operations and content.

   This YANG module defines <sched-max-future> and <sched-max-past>,
   which are writable/creatable/deletable.  These data nodes may be
   considered sensitive or vulnerable in some network environments.  An
   attacker may attempt to maliciously configure these parameters to a



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   low value, thereby causing all scheduled RPCs to be discarded.  For
   instance, if a client expects <sched-max-future> to be 15 seconds,
   but in practice it is maliciously configured to 1 second, then a
   legitimate scheduled RPC that is scheduled to be performed 5 seconds
   in the future will be discarded by the server.

   This YANG module defines the <cancel-schedule> RPC.  This RPC may be
   considered sensitive or vulnerable in some network environments.
   Since the value of the <schedule-id> is known to all the clients that
   are subscribed to notifications from the server, the <cancel-
   schedule> RPC may be used maliciously to attack servers by cancelling
   their pending RPCs.  This attack is addressed in two layers: (i)
   security at the transport layer, limiting the attack only to clients
   that have successfully initiated a secure session with the server,
   and (ii) the authorization level required to cancel an RPC should be
   the same as the level required to schedule it, limiting the attack
   only to attackers with an authorization level that is equal to or
   higher than that of the client that initiated the scheduled RPC.

7.  IANA Considerations

   The following capability identifier URN has been registered in the
   "Network Configuration Protocol (NETCONF) Capability URNs" registry:

      urn:ietf:params:netconf:capability:time:1.0

   The following XML namespace URN has been registered in the "IETF XML
   Registry", following the format defined in [RFC3688]:

      URI: urn:ietf:params:xml:ns:yang:ietf-netconf-time

      Registrant Contact: The IESG.

      XML: N/A, the requested URI is an XML namespace.

   The following module name has been registered in the "YANG Module
   Names" registry, defined in [RFC6020].

      name: ietf-netconf-time

      prefix: nct

      namespace: urn:ietf:params:xml:ns:yang:ietf-netconf-time

      RFC: 7758






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8.  References

8.1.  Normative References

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119,
               DOI 10.17487/RFC2119, March 1997,
               <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3339]   Klyne, G. and C. Newman, "Date and Time on the Internet:
               Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
               <http://www.rfc-editor.org/info/rfc3339>.

   [RFC3688]   Mealling, M., "The IETF XML Registry", BCP 81,
               RFC 3688, DOI 10.17487/RFC3688, January 2004,
               <http://www.rfc-editor.org/info/rfc3688>.

   [RFC6241]   Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J.,
               Ed., and A. Bierman, Ed., "Network Configuration Protocol
               (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
               <http://www.rfc-editor.org/info/rfc6241>.

   [RFC6470]   Bierman, A., "Network Configuration Protocol (NETCONF)
               Base Notifications", RFC 6470, DOI 10.17487/RFC6470,
               February 2012,
               <http://www.rfc-editor.org/info/rfc6470>.

   [RFC6991]   Schoenwaelder, J., Ed., "Common YANG Data Types",
               RFC 6991, DOI 10.17487/RFC6991, July 2013,
               <http://www.rfc-editor.org/info/rfc6991>.

8.2.  Informative References

   [Cond]      Watsen, K., "Conditional Enablement of Configuration
               Nodes", draft-kwatsen-conditional-enablement-00, Work in
               Progress, February 2013.

   [IEEE1588]  IEEE, "IEEE Standard for a Precision Clock
               Synchronization Protocol for Networked Measurement and
               Control Systems Version 2", IEEE Standard 1588.

   [OneClock]  Mizrahi, T. and Y. Moses, "OneClock to Rule Them All:
               Using Time in Networked Applications", IEEE/IFIP Network
               Operations and Management Symposium (NOMS), 2016.







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   [RFC5905]   Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
               "Network Time Protocol Version 4: Protocol and Algorithms
               Specification", RFC 5905,
               DOI 10.17487/RFC5905, June 2010,
               <http://www.rfc-editor.org/info/rfc5905>.

   [RFC5907]   Gerstung, H., Elliott, C., and B. Haberman, Ed.,
               "Definitions of Managed Objects for Network Time Protocol
               Version 4 (NTPv4)", RFC 5907,
               DOI 10.17487/RFC5907, June 2010,
               <http://www.rfc-editor.org/info/rfc5907>.

   [RFC6020]   Bjorklund, M., Ed., "YANG - A Data Modeling Language for
               the Network Configuration Protocol (NETCONF)",
               RFC 6020, DOI 10.17487/RFC6020, October 2010,
               <http://www.rfc-editor.org/info/rfc6020>.

   [RFC6242]   Wasserman, M., "Using the NETCONF Protocol over Secure
               Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011,
               <http://www.rfc-editor.org/info/rfc6242>.

   [RFC6243]   Bierman, A. and B. Lengyel, "With-defaults Capability for
               NETCONF", RFC 6243, DOI 10.17487/RFC6243, June 2011,
               <http://www.rfc-editor.org/info/rfc6243>.

   [RFC6536]   Bierman, A. and M. Bjorklund, "Network Configuration
               Protocol (NETCONF) Access Control Model", RFC 6536, DOI
               10.17487/RFC6536, March 2012,
               <http://www.rfc-editor.org/info/rfc6536>.

   [RFC7317]   Bierman, A. and M. Bjorklund, "A YANG Data Model for
               System Management", RFC 7317, DOI 10.17487/RFC7317,
               August 2014, <http://www.rfc-editor.org/info/rfc7317>.

   [RFC7384]   Mizrahi, T., "Security Requirements of Time Protocols in
               Packet Switched Networks", RFC 7384,
               DOI 10.17487/RFC7384, October 2014,
               <http://www.rfc-editor.org/info/rfc7384>.

   [Time4]     Mizrahi, T. and Y. Moses, "Software Defined Networks:
               It's About Time", IEEE INFOCOM, 2016.










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Appendix A.  YANG Module for the Time Capability

   This section is normative.

<CODE BEGINS> file "ietf-netconf-time@2016-01-26.yang"

module ietf-netconf-time {

   namespace "urn:ietf:params:xml:ns:yang:ietf-netconf-time";

   prefix nct;
   import ietf-netconf { prefix nc; }

   import ietf-yang-types { prefix yang; }

   import ietf-netconf-monitoring { prefix ncm; }

   organization
     "IETF";

   contact
     "Editor: Tal Mizrahi
         <dew@tx.technion.ac.il>
      Editor: Yoram Moses
         <moses@ee.technion.ac.il>";

   description
     "This module defines a capability-based extension to the
      Network Configuration Protocol (NETCONF) that allows
      time-triggered configuration and management operations.
      This extension allows NETCONF clients to invoke configuration
      updates according to scheduled times and allows NETCONF
      servers to attach timestamps to the data they send to NETCONF
      clients.

      Copyright (c) 2016 IETF Trust and the persons identified as
      the authors of the code.  All rights reserved.

      Redistribution and use in source and binary forms, with or
      without modification, is permitted pursuant to, and subject
      to the license terms contained in, the Simplified BSD License
      set forth in Section 4.c of the IETF Trust's Legal Provisions
      Relating to IETF Documents
      (http://trustee.ietf.org/license-info).";

   revision 2016-01-26 {
     description
       "Initial version.";



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     reference
       "RFC 7758:
        Time Capability in NETCONF";
   }

   typedef time-interval {
     type string {
       pattern '\d{2}:\d{2}:\d{2}(\.\d+)?';
     }
     description
       "Defines a time interval, up to 24 hours.
        The format is specified as HH:mm:ss.f,
        consisting of two digits for hours,
        two digits for minutes, two digits
        for seconds, and zero or more digits
        representing second fractions.";
   }

   grouping scheduling-tolerance-parameters {
     leaf sched-max-future {
       type time-interval;
       default 00:00:15.0;
       description
         "When the scheduled time is in the future, i.e., greater
          than the present time, this leaf defines the maximal
          difference between the scheduled time
          and the present time that the server is willing to
          accept.  If the difference exceeds this number, the
          server responds with an error.";
     }

     leaf sched-max-past {
       type time-interval;
       default 00:00:15.0;
       description
         "When the scheduled time is in the past, i.e., less
          than the present time, this leaf defines the maximal
          difference between the present time
          and the scheduled time that the server is willing to
          accept.  If the difference exceeds this number, the
          server responds with an error.";
     }

     description
       "Contains the parameters of the scheduling tolerance.";
   }
   // extending the get-config operation
   augment /nc:get-config/nc:input {



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     leaf scheduled-time {
       type yang:date-and-time;
       description
         "The time at which the RPC is scheduled to be performed.";
     }

     leaf get-time {
       type empty;
       description
         "Indicates that the rpc-reply should include the
          execution-time.";
     }

     description
       "Adds the time element to <get-config>.";
   }

   augment /nc:get-config/nc:output {
     leaf execution-time {
       type yang:date-and-time;
       description
         "The time at which the RPC was executed.";
     }

     description
       "Adds the time element to <get-config>.";
   }

   augment /nc:get/nc:input {
     leaf scheduled-time {
       type yang:date-and-time;
       description
         "The time at which the RPC is scheduled to be performed.";
     }

     leaf get-time {
       type empty;
       description
         "Indicates that the rpc-reply should include the
          execution-time.";
     }

     description
       "Adds the time element to <get>.";
   }

   augment /nc:get/nc:output {
     leaf execution-time {



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       type yang:date-and-time;
       description
         "The time at which the RPC was executed.";
     }

     description
       "Adds the time element to <get>.";
   }

   augment /nc:copy-config/nc:input {
     leaf scheduled-time {
       type yang:date-and-time;
       description
         "The time at which the RPC is scheduled to be performed.";
     }

     leaf get-time {
       type empty;
       description
         "Indicates that the rpc-reply should include the
          execution-time.";
     }

     description
       "Adds the time element to <copy-config>.";
   }

   augment /nc:copy-config/nc:output {
     leaf execution-time {
       type yang:date-and-time;
       description
         "The time at which the RPC was executed.";
     }

     description
       "Adds the time element to <copy-config>.";
   }

   augment /nc:edit-config/nc:input {
     leaf scheduled-time {
       type yang:date-and-time;
       description
         "The time at which the RPC is scheduled to be performed.";
     }

     leaf get-time {
       type empty;
       description



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         "Indicates that the rpc-reply should include the
          execution-time.";
     }

     description
       "Adds the time element to <edit-config>.";
   }

   augment /nc:edit-config/nc:output {
     leaf execution-time {
       type yang:date-and-time;
       description
         "The time at which the RPC was executed.";
     }

     description
       "Adds the time element to <edit-config>.";
   }

   augment /nc:delete-config/nc:input {
     leaf scheduled-time {
       type yang:date-and-time;
       description
         "The time at which the RPC is scheduled to be performed.";
     }

     leaf get-time {
       type empty;
       description
         "Indicates that the rpc-reply should include the
          execution-time.";
     }

     description
      "Adds the time element to <delete-config>.";
   }

   augment /nc:delete-config/nc:output {
     leaf execution-time {
       type yang:date-and-time;
       description
         "The time at which the RPC was executed.";
     }
     description
       "Adds the time element to <delete-config>.";
   }

   augment /nc:lock/nc:input {



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     leaf scheduled-time {
       type yang:date-and-time;
       description
         "The time at which the RPC is scheduled to be performed.";
     }

     leaf get-time {
       type empty;
       description
         "Indicates that the rpc-reply should include the
          execution-time.";
     }

     description
       "Adds the time element to <lock>.";
   }
   augment /nc:lock/nc:output {
     leaf execution-time {
       type yang:date-and-time;
       description
         "The time at which the RPC was executed.";
     }

     description
       "Adds the time element to <lock>.";
   }

   augment /nc:unlock/nc:input {
     leaf scheduled-time {
       type yang:date-and-time;
       description
         "The time at which the RPC is scheduled to be performed.";
     }

     leaf get-time {
       type empty;
       description
         "Indicates that the rpc-reply should include the
          execution-time.";
     }

     description
       "Adds the time element to <unlock>.";
   }

   augment /nc:unlock/nc:output {
     leaf execution-time {
       type yang:date-and-time;



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       description
         "The time at which the RPC was executed.";
     }

     description
       "Adds the time element to <unlock>.";
   }
   augment /nc:commit/nc:input {
     leaf scheduled-time {
       type yang:date-and-time;
       description
         "The time at which the RPC is scheduled to be performed.";
     }

     leaf get-time {
       type empty;
       description
         "Indicates that the rpc-reply should include the
          execution-time.";
     }

     description
       "Adds the time element to <commit>.";
   }

   augment /nc:commit/nc:output {
     leaf execution-time {
       type yang:date-and-time;
       description
         "The time at which the RPC was executed.";
     }

     description
       "Adds the time element to <commit>.";
   }

   augment /ncm:netconf-state {
     container scheduling-tolerance {
       uses scheduling-tolerance-parameters;
       description
         "The scheduling tolerance when the time capability
          is enabled.";
     }
     description
       "The scheduling tolerance of the server.";
   }

   rpc cancel-schedule {



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     description
       "Cancels a scheduled message.";
     reference
       "RFC 7758:
        Time Capability in NETCONF";

     input {
       leaf cancelled-message-id {
         type string;
         description
           "The ID of the message to be cancelled.";
       }
       leaf get-time {
         type empty;
         description
           "Indicates that the rpc-reply should include
            the execution-time.";
       }
     }
     output {
       leaf execution-time {
         type yang:date-and-time;
         description
           "The time at which the RPC was executed.";
       }
     }
   }

   notification netconf-scheduled-message {
     leaf schedule-id {
       type string;
       description
         "The ID of the scheduled message.";
     }

     leaf scheduled-time {
       type yang:date-and-time;
       description
         "The time at which the RPC is scheduled to be performed.";
     }
     description
       "Indicates that a scheduled message was received.";
     reference
       "RFC 7758:
        Time Capability in NETCONF";
   }

}



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<CODE ENDS>

Acknowledgments

   The authors gratefully acknowledge Joe Marcus Clarke, Andy Bierman,
   Balazs Lengyel, Jonathan Hansford, John Heasley, Robert Sparks, Al
   Morton, Olafur Gudmundsson, Juergen Schoenwaelder, Joel Jaeggli, Alon
   Schneider, and Eylon Egozi for their insightful comments.

   This work was supported in part by Israel Science Foundation grant
   ISF 1520/11.

Authors' Addresses

   Tal Mizrahi
   Department of Electrical Engineering
   Technion - Israel Institute of Technology
   Technion City, Haifa, 32000
   Israel

   Email: dew@tx.technion.ac.il


   Yoram Moses
   Department of Electrical Engineering
   Technion - Israel Institute of Technology
   Technion City, Haifa, 32000
   Israel

   Email: moses@ee.technion.ac.il





















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