NETCONF                                                      J. Lindblad
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                             8 June 2022
Expires: 10 December 2022


                  Transaction ID Mechanism for NETCONF
                draft-lindblad-netconf-transaction-id-02

Abstract

   NETCONF clients and servers often need to have a synchronized view of
   the server's configuration data stores.  The volume of configuration
   data in a server may be very large, while data store changes
   typically are small when observed at typical client resynchronization
   intervals.

   Rereading the entire data store and analyzing the response for
   changes is an inefficient mechanism for synchronization.  This
   document specifies an extension to NETCONF that allows clients and
   servers to keep synchronized with a much smaller data exchange and
   without any need for servers to store information about the clients.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Source for this draft and an issue tracker can be found at
   https://github.com/netconf-wg/netconf-etag.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 10 December 2022.





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Copyright Notice

   Copyright (c) 2022 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 (https://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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   4
   3.  NETCONF Txid Extension  . . . . . . . . . . . . . . . . . . .   4
     3.1.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  General Txid Principles . . . . . . . . . . . . . . . . .   5
     3.3.  Initial Configuration Retrieval . . . . . . . . . . . . .   6
     3.4.  Subsequent Configuration Retrieval  . . . . . . . . . . .   7
     3.5.  Conditional Transactions  . . . . . . . . . . . . . . . .  10
       3.5.1.  Transactions toward the Candidate Datastore . . . . .  12
     3.6.  Dependencies within Transactions  . . . . . . . . . . . .  13
     3.7.  Other NETCONF Operations  . . . . . . . . . . . . . . . .  16
     3.8.  YANG-Push Subscriptions . . . . . . . . . . . . . . . . .  17
   4.  Txid Mechanisms . . . . . . . . . . . . . . . . . . . . . . .  17
     4.1.  The etag attribute txid mechanism . . . . . . . . . . . .  17
     4.2.  The last-modified attribute txid mechanism  . . . . . . .  18
     4.3.  Common features to both etag and last-modified txid
           mechanisms  . . . . . . . . . . . . . . . . . . . . . . .  19
   5.  Txid Mechanism Examples . . . . . . . . . . . . . . . . . . .  21
     5.1.  Initial Configuration Response  . . . . . . . . . . . . .  21
       5.1.1.  With etag . . . . . . . . . . . . . . . . . . . . . .  21
       5.1.2.  With last-modified  . . . . . . . . . . . . . . . . .  25
     5.2.  Configuration Response Pruning  . . . . . . . . . . . . .  27
     5.3.  Configuration Change  . . . . . . . . . . . . . . . . . .  31
     5.4.  Conditional Configuration Change  . . . . . . . . . . . .  35
     5.5.  Using etags with Other NETCONF Operations . . . . . . . .  37
     5.6.  YANG-Push . . . . . . . . . . . . . . . . . . . . . . . .  38
   6.  YANG Modules  . . . . . . . . . . . . . . . . . . . . . . . .  40
     6.1.  Base module for txid in NETCONF . . . . . . . . . . . . .  40
     6.2.  Additional support for txid in YANG-Push  . . . . . . . .  43
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  45
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  45
   9.  Changes . . . . . . . . . . . . . . . . . . . . . . . . . . .  46



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     9.1.  Major changes in -02 since -01  . . . . . . . . . . . . .  46
     9.2.  Major changes in -01 since -00  . . . . . . . . . . . . .  47
   10. Normative References  . . . . . . . . . . . . . . . . . . . .  48
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  48
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  48

1.  Introduction

   When a NETCONF client connects with a NETCONF server, a frequently
   occurring use case is for the client to find out if the configuration
   has changed since it was last connected.  Such changes could occur
   for example if another NETCONF client has made changes, or another
   system or operator made changes through other means than NETCONF.

   One way of detecting a change for a client would be to retrieve the
   entire configuration from the server, then compare the result with a
   previously stored copy at the client side.  This approach is not
   popular with most NETCONF users, however, since it would often be
   very expensive in terms of communications and computation cost.

   Furthermore, even if the configuration is reported to be unchanged,
   that will not guarantee that the configuration remains unchanged when
   a client sends a subsequent change request, a few moments later.

   In order to simplify the task of tracking changes, a NETCONF server
   could implement a meta level checksum over the configuration over a
   datastore or YANG subtree, and offer clients a way to read and
   compare this checksum.  If the checksum is unchanged, clients can
   avoid performing expensive operations.  Such checksums are often
   referred to as a configuration id or transaction id (txid).

   Evidence of a transaction id feature being demanded by clients is
   that several server implementors have built proprietary and mutually
   incompatible mechanisms for obtaining a transaction id from a NETCONF
   server.

   RESTCONF, RFC 8040 (https://tools.ietf.org/html/rfc8040), defines a
   mechanism for detecting changes in configuration subtrees based on
   Entity-Tags (ETags) and Last-Modified txid values.

   In conjunction with this, RESTCONF provides a way to make
   configuration changes conditional on the server confiuguration being
   untouched by others.  This mechanism leverages RFC 7232
   (https://tools.ietf.org/html/rfc7232) "Hypertext Transfer Protocol
   (HTTP/1.1): Conditional Requests".






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   This document defines similar functionality for NETCONF, RFC 6241
   (https://tools.ietf.org/html/rfc6241), and ties this in with YANG-
   Push, RFC 8641 (https://tools.ietf.org/html/rfc8641).

2.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This document uses the terminology defined in RFC6241
   (https://tools.ietf.org/html/rfc6241), RFC7950
   (https://tools.ietf.org/html/rfc7950), RFC8040
   (https://tools.ietf.org/html/rfc8040), and RFC8641
   (https://tools.ietf.org/html/rfc8641).

   In addition, this document defines the following terms:

   Versioned node  A node in the instantiated YANG data tree for which
      the server maintains a transaction id (txid) value.

3.  NETCONF Txid Extension

   This document describes a NETCONF extension which modifies the
   behavior of get-config, get-data, edit-config, edit-data, discard-
   changes, copy-config, delete-config and commit such that clients are
   able to conditionally retrieve and update the configuration in a
   NETCONF server.

   For servers implementing YANG-Push, an extension for conveying txid
   updates as part of subscription updates is also defined.

   Several low level mechanisms could be defined to fulfill the
   requirements for efficient client-server txid synchronization.  This
   document defines two such mechanisms, the etag txid mechanism and the
   last-modified txid mechanism.  Additional mechanisms could be added
   in future.

3.1.  Use Cases

   The common use cases for such mecahnisms are briefly discussed here.

   Initial configuration retrieval  When the client initially connects






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      to a server, it may be interested to acquire a current view of
      (parts of) the server's configuration.  In order to be able to
      efficiently detect changes later, it may also be interested to
      store meta level txid information for subtrees of the
      configuration.

   Subsequent configuration retrieval  When a client needs to reread
      (parts of) the server's configuration, it may be interested to
      leverage the txid meta data it has stored by requesting the server
      to prune the response so that it does not repeat configuration
      data that the client is already aware of.

   Configuration update with txid return  When a client issues a
      transaction towards a server, it may be interested to also learn
      the new txid meta data the server has stored for the updated parts
      of the configuration.

   Configuration update with txid specification  When a client issues a
      transaction towards a server, it may be interested to also specify
      the new txid meta data that the server stores for the updated
      parts of the configuration.

   Conditional configuration change  When a client issues a transaction
      towards a server, it may specify txid meta data for the
      transaction in order to allow the server to verify that the client
      is up to date with any changes in the parts of the configuration
      that it is concerned with.  If the txid meta data in the server is
      different than the client expected, the server rejects the
      transaction with a specific error message.

   Subscribe to configuration changes with txid return  When a client
      subscribes to configuration change updates through YANG-Push, it
      may be interested to also learn the the updated txid meta data for
      the changed data trees.

3.2.  General Txid Principles

   All servers implementing a txid mechanism MUST maintain a txid meta
   data value for each configuration datastore supported by the server.
   Txid mechanism implementations MAY also maintain txid meta data
   values for nodes deeper in the YANG data tree.  The nodes for which
   the server maintains txids are collectively referred to as the
   "versioned nodes".








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   The server returning txid values for the versioned nodes MUST ensure
   the txid values are changed every time there has been a configuration
   change at or below the node associated with the txid value.  This
   means any update of a config true node will result in a new txid
   value for all ancestor versioned node, up to and including the
   datastore root itself.

   This also means a server MUST update the txid value for any nodes
   that change as a result of a configuration change, regardless of
   source, even if the changed nodes are not explicitly part of the
   change payload.  An example of this is dependent data under YANG RFC
   7950 (https://tools.ietf.org/html/rfc7950) when- or choice-
   statements.

   The server MUST NOT change the txid value of a versioned node unless
   the node itself or a child node of that node has been changed.  The
   server MUST NOT change any txid values due to changes in config false
   data.

3.3.  Initial Configuration Retrieval

   When a NETCONF server receives a get-config or get-data request
   containing requests for txid values, it MUST return txid values for
   all versioned nodes below the point requested by the client in the
   reply.

   The exact encoding varies by mechanism, but all txid mechanisms would
   have a special "txid-request" txid value (e.g. "?") which is
   guaranteed to never be used as a normal txid value.  Clients MAY use
   this special txid value associated with one or more nodes in the data
   tree to indicate to the server that they are interested in txid
   values below that point of the data tree.



















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        Client                                            Server
          |                                                 |
          |   ------------------------------------------>   |
          |   get-config                                    |
          |     acls (txid: ?)                              |
          |                                                 |
          |   <------------------------------------------   |
          |   data                                          |
          |     acls (txid: 5152)                           |
          |       acl A1 (txid: 4711)                       |
          |         aces (txid: 4711)                       |
          |           ace R1 (txid: 4711)                   |
          |             matches ipv4 protocol udp           |
          |       acl A2 (txid: 5152)                       |
          |         aces (txid: 5152)                       |
          |           ace R7 (txid: 4711)                   |
          |             matches ipv4 dscp AF11              |
          |           ace R8 (txid: 5152)                   |
          |             matches udp source-port port 22     |
          |           ace R9 (txid: 5152)                   |
          |             matches tcp source-port port 22     |
          v                                                 v

       Figure 1: Initial Configuration Retrieval.  The server returns
       the requested configuration, annotated with txid values.  The
      most recent change seems to have been an update to the R8 and R9
                                source-port.

   NOTE: In the call flow examples we are using a 4-digit, monotonously
   increasing integer as txid.  This is convenient and enhances
   readability of the examples, but does not reflect a typical
   implementation.  In general, the only operation defined on a pair of
   txid values is testing them for equality.

3.4.  Subsequent Configuration Retrieval

   Clients MAY request the server to return txid values in the response
   by adding one or more txid values received previously in get-config
   or get-data requests.

   When a NETCONF server receives a get-config or get-data request
   containing a node with a client specified txid value, there are
   several different cases:








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   *  The node is not a versioned node, i.e. the server does not
      maintain a txid value for this node.  In this case, the server
      MUST look up the closest ancestor that is a versioned node, and
      use the txid value of that node as the txid value of this node in
      the further handling below.  The datastore root is always a
      versioned node.

   *  The client specified txid value is different than the server's
      txid value for this node.  In this case the server MUST return the
      contents as it would otherwise have done, adding the txid values
      of all child versioned nodes to the response.  In case the client
      has specified txid values for some child nodes, then these cases
      MUST be re-evaluated for those child nodes.

   *  The node is a versioned node, and the client specified txid value
      matches the server's txid value.  In this case the server MUST
      return the node decorated with a special "txid-match" txid value
      (e.g. "=") to the matching node, pruning any value and child
      nodes.  A server MUST NOT ever use the txid-match value (e.g. "=")
      as an actual txid value.

   For list elements, pruning child nodes means that top-level key nodes
   MUST be included in the response, and other child nodes MUST NOT be
   included.  For containers, child nodes MUST NOT be included.

        Client                                            Server
          |                                                 |
          |   ------------------------------------------>   |
          |   get-config                                    |
          |     acls (txid: 5152)                           |
          |       acl A1 (txid: 4711)                       |
          |         aces (txid: 4711)                       |
          |       acl A2 (txid: 5152)                       |
          |         aces (txid: 5152)                       |
          |                                                 |
          |   <------------------------------------------   |
          |   data                                          |
          |     acls (txid: =)                              |
          v                                                 v

     Figure 2: Response Pruning.  Client sends get-config request with
       known txid values.  Server prunes response where txid matches
                               expectations.








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        Client                                            Server
          |                                                 |
          |   ------------------------------------------>   |
          |   get-config                                    |
          |     acls (txid: 5152)                           |
          |       acl A1 (txid: 4711)                       |
          |       acl A2 (txid: 5152)                       |
          |                                                 |
          |   <------------------------------------------   |
          |   data                                          |
          |     acls (txid: 6614)                           |
          |       acl A1 (txid: =)                          |
          |       acl A2 (txid: 6614)                       |
          |         aces (txid: 6614)                       |
          |           ace R7 (txid: 4711)                   |
          |             matches ipv4 dscp AF11              |
          |           ace R8 (txid: 5152)                   |
          |             matches udp source-port port 22     |
          |           ace R9 (txid: 6614)                   |
          |             matches tcp source-port port 830    |
          v                                                 v

      Figure 3: Out of band change detected.  Client sends get-config
       request with known txid values.  Server provides update where
                           changes have happened.

        Client                                            Server
          |                                                 |
          |   ------------------------------------------>   |
          |   get-config                                    |
          |     acls                                        |
          |       acls A2                                   |
          |         aces                                    |
          |           ace R7                                |
          |             matches                             |
          |               ipv4                              |
          |                 dscp (txid: 4711)               |
          |                                                 |
          |   <------------------------------------------   |
          |   data                                          |
          |     acls                                        |
          |       acl A2                                    |
          |         aces                                    |
          |           ace R7                                |
          |             matches                             |
          |               ipv4                              |
          |                 dscp (txid: =)                  |
          v                                                 v



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        Figure 4: Versioned nodes.  Server lookup of dscp txid gives
       4711, as closest ancestor is ace R7 with txid 4711.  Since the
      server's and client's txid match, the etag value is '=', and the
                           leaf value is pruned.

3.5.  Conditional Transactions

   Conditional transactions are useful when a client is interested to
   make a configuration change, being sure that relevant parts of the
   server configuration have not changed since the client last inspected
   it.

   By supplying the latest txid values known to the client in its change
   requests (edit-config etc.), it can request the server to reject the
   transaction in case any relevant changes have occurred at the server
   that the client is not yet aware of.

   This allows a client to reliably compute and send confiuguration
   changes to a server without either acquiring a global datastore lock
   for a potentially extended period of time, or risk that a change from
   another client disrupts the intent in the time window between a read
   (get-config etc.) and write (edit-config etc.) operation.

   Clients that are also interested to know the txid assigned to the
   modified versioned nodes in the model immediately in the response
   could set a flag in the rpc message to request the server to return
   the new txid with the ok message.

        Client                                            Server
          |                                                 |
          |   ------------------------------------------>   |
          |   edit-config (request new txid in response)    |
          |     config (txid: 5152)                         |
          |       acls (txid: 5152)                         |
          |         acl A1 (txid: 4711)                     |
          |           aces (txid: 4711)                     |
          |             ace R1 (txid: 4711)                 |
          |               matches ipv4 protocol tcp         |
          |                                                 |
          |   <------------------------------------------   |
          |   ok (txid: 7688)                               |
          v                                                 v

      Figure 5: Conditional transaction towards the Running datastore
      successfully executed.  As all the txid values specified by the
          client matched those on the server, the transaction was
                           successfully executed.




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        Client                                            Server
          |                                                 |
          |   ------------------------------------------>   |
          |   get-config                                    |
          |     acls (txid: ?)                              |
          |                                                 |
          |   <------------------------------------------   |
          |   data (txid: 7688)                             |
          |     acls (txid: 7688)                           |
          |       acl A1 (txid: 7688)                       |
          |         aces (txid: 7688)                       |
          |           ace R1 (txid: 7688)                   |
          |             matches ipv4 protocol tcp           |
          |       acl A2 (txid: 6614)                       |
          |         aces (txid: 6614)                       |
          |           ace R7 (txid: 4711)                   |
          |             matches ipv4 dscp AF11              |
          |           ace R8 (txid: 5152)                   |
          |             matches udp source-port port 22     |
          |           ace R9 (txid: 6614)                   |
          |             matches tcp source-port port 830    |
          v                                                 v

      Figure 6: For all leaf objects that were changed, and all their
      ancestors, the txids are updated to the value returned in the ok
                                  message.

   If the server rejects the transaction because the configuration txid
   value differs from the client's expectation, the server MUST return
   an rpc-error with the following values:

      error-tag:      operation-failed
      error-type:     protocol
      error-severity: error

   Additionally, the error-info tag SHOULD contain an sx:structure
   containing relevant details about the mismatching txids.














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        Client                                            Server
          |                                                 |
          |   ------------------------------------------>   |
          |   edit-config                                   |
          |     config                                      |
          |       acls                                      |
          |         acl A1 (txid: 4711)                     |
          |           aces (txid: 4711)                     |
          |             ace R1 (txid: 4711)                 |
          |               ipv4 dscp AF22                    |
          |                                                 |
          |   <------------------------------------------   |
          |   rpc-error                                     |
          |     error-tag       operation-failed            |
          |     error-type      protocol                    |
          |     error-severity  error                       |
          |     error-info                                  |
          |       mismatch-path /acls/acl[A1]               |
          |       mismatch-etag-value 6912                  |
          v                                                 v

      Figure 7: Conditional transaction that fails a txid check.  The
          client wishes to ensure there has been no changes to the
       particular acl entry it edits, and therefore sends the txid it
       knows for this part of the configuration.  Since the txid has
        changed (out of band), the server rejects the configuration
      change request and reports an error with details about where the
                           mismatch was detected.

3.5.1.  Transactions toward the Candidate Datastore

   When working with the Candidate datastore, the txid validation
   happens at commit time, rather than at individual edit-config or
   edit-data operations.  Clients add their txid attributes to the
   configuration payload the same way.  In case a client specifies
   different txid values for the same element in successive edit-config
   or edit-data operations, the txid value specified last MUST be used
   by the server at commit time.













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        Client                                            Server
          |                                                 |
          |   ------------------------------------------>   |
          |   edit-config                                   |
          |     config (txid: 5152)                         |
          |       acls (txid: 5152)                         |
          |         acl A1 (txid: 4711)                     |
          |           type ipv4                             |
          |                                                 |
          |   <------------------------------------------   |
          |   ok                                            |
          |                                                 |
          |   ------------------------------------------>   |
          |   edit-config                                   |
          |     config                                      |
          |       acls                                      |
          |         acl A1                                  |
          |           aces (txid: 4711)                     |
          |             ace R1 (txid: 4711)                 |
          |               matches ipv4 protocol tcp         |
          |                                                 |
          |   <------------------------------------------   |
          |   ok                                            |
          |                                                 |
          |   ------------------------------------------>   |
          |   commit (request new txid in response)         |
          |                                                 |
          |   <------------------------------------------   |
          |   ok (txid: 7688)                               |
          v                                                 v

     Figure 8: Conditional transaction towards the Candidate datastore
      successfully executed.  As all the txid values specified by the
          client matched those on the server, the transaction was
                           successfully executed.

3.6.  Dependencies within Transactions

   YANG modules that contain when-statements referencing remote parts of
   the model will cause the txid to change even in parts of the data
   tree that were not modified directly.

   Let's say there is an energy-example.yang module that defines a
   mechanism for clients to request the server to measure the amount of
   energy that is consumed by a given access control rule.  The energy-
   example module augments the access control module as follows:





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     augment /acl:acls/acl:acl {
       when /energy-example:energy/energy-example:metering-enabled;
       leaf energy-tracing {
         type boolean;
         default false;
       }
       leaf energy-consumption {
         config false;
         type uint64;
         units J;
       }
     }

   This means there is a system wide switch leaf metering-enabled in
   energy-example which disables all energy measurements in the system
   when set to false, and that there is a boolean leaf energy-tracing
   that controls whether energy measurement is happening for each acl
   rule individually.

   In this example, we have an initial configuration like this:

        Client                                            Server
          |                                                 |
          |   ------------------------------------------>   |
          |   get-config                                    |
          |     energy (txid: ?)                            |
          |     acls (txid: ?)                              |
          |                                                 |
          |   <------------------------------------------   |
          |   data (txid: 7688)                             |
          |     energy metering-enabled true (txid: 4711)   |
          |     acls (txid: 7688)                           |
          |       acl A1 (txid: 7688)                       |
          |         energy-tracing false                    |
          |         aces (txid: 7688)                       |
          |           ace R1 (txid: 7688)                   |
          |             matches ipv4 protocol tcp           |
          |       acl A2 (txid: 6614)                       |
          |         energy-tracing true                     |
          |         aces (txid: 6614)                       |
          |           ace R7 (txid: 4711)                   |
          |             matches ipv4 dscp AF11              |
          |           ace R8 (txid: 5152)                   |
          |             matches udp source-port port 22     |
          |           ace R9 (txid: 6614)                   |
          |             matches tcp source-port port 830    |
          v                                                 v




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     Figure 9: Initial configuration for the energy example.  Note the
      energy metering-enabled leaf at the top and energy-tracing leafs
                              under each acl.

   At this point, a client updates metering-enabled to false.  This
   causes the when-expression on energy-tracing to turn false, removing
   the leaf entirely.  This counts as a configuration change, and the
   txid MUST be updated appropriately.

        Client                                            Server
          |                                                 |
          |   ------------------------------------------>   |
          |   edit-config (request new txid in response)    |
          |     config                                      |
          |       energy metering-enabled false             |
          |                                                 |
          |   <------------------------------------------   |
          |   ok (txid: 9118)                               |
          v                                                 v

      Figure 10: Transaction changing a single leaf.  This leaf is the
        target of a when-statement, however, which means other leafs
         elsewhere may be indirectly modified by this change.  Such
             indirect changes will also result in txid changes.

   After the transaction above, the new configuration state has the
   energy-tracing leafs removed.
























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        Client                                            Server
          |                                                 |
          |   ------------------------------------------>   |
          |   get-config                                    |
          |     energy (txid: ?)                            |
          |     acls (txid: ?)                              |
          |                                                 |
          |   <------------------------------------------   |
          |   data (txid: 9118)                             |
          |     energy metering-enabled false (txid: 9118)  |
          |     acls (txid: 9118)                           |
          |       acl A1 (txid: 9118)                       |
          |         aces (txid: 7688)                       |
          |           ace R1 (txid: 7688)                   |
          |             matches ipv4 protocol tcp           |
          |       acl A2 (txid: 9118)                       |
          |         aces (txid: 6614)                       |
          |           ace R7 (txid: 4711)                   |
          |             matches ipv4 dscp AF11              |
          |           ace R8 (txid: 5152)                   |
          |             matches udp source-port port 22     |
          |           ace R9 (txid: 6614)                   |
          |             matches tcp source-port port 830    |
          v                                                 v

     Figure 11: The txid for the energy subtree has changed since that
       was the target of the edit-config.  The txids of the ACLs have
       also changed since the energy-tracing leafs are now removed by
                      the now false when- expression.

3.7.  Other NETCONF Operations

   discard-changes  The discard-changes operation resets the candidate
      datastore to the contents of the running datastore.  The server
      MUST ensure the txid values in the candidate datastore get the
      same txid values as in the running datastore when this operation
      runs.

   copy-config  The copy-config operation can be used to copy contents
      between datastores.  The server MUST ensure the txid values retain
      the same txid values as in the soruce datastore.

      If copy-config is used to copy from a file, URL or other source
      that is not a datastore, the server MUST ensure the txid values
      are changed for the versioned nodes that are changed or have child
      nodes changed by the operation.

   delete-config  The server MUST ensure the datastore txid value is



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      changed, unless it was already empty.

   commit  At commit, with regards to the txid values, the server MUST
      treat the contents of the candidate datastore as if any txid value
      provided by the client when updating the candidate was provided in
      a single edit-config towards the running datastore.  If the
      transaction is rejected due to txid value mismatch, an rpc-error
      as described in section Conditional Transactions (Section 3.5)
      MUST be sent.

3.8.  YANG-Push Subscriptions

   A client issuing a YANG-Push establish-subscription or modify-
   subscription request towards a server that supports both YANG-Push
   RFC 8641 (https://tools.ietf.org/html/rfc8641) and a txid mechanism
   MAY request that the server provides updated txid values in YANG-Push
   subscription updates.

4.  Txid Mechanisms

   This document defines two txid mechanisms:

   *  The etag attribute txid mechanism

   *  The last-modified attribute txid mechanism

   Servers implementing this specification MUST support the etag
   attribute txid mechanism and MAY support the last-modified attribute
   txid mechanism.

   Section NETCONF Txid Extension (Section 3) describes the logic that
   governs all txid mechanisms.  This section describes the mapping from
   the generic logic to specific mechanism and encoding.

   If a client uses more than one txid mechanism, such as both etag and
   last-modified in a particular message to a server, or patricular
   commit, the result is undefined.

4.1.  The etag attribute txid mechanism

   The etag txid mechanism described in this section is centered around
   a meta data XML attribute called "etag".  The etag attribute is
   defined in the namespace "urn:ietf:params:xml:ns:netconf:txid:1.0".
   The etag attribute is added to XML elements in the NETCONF payload in
   order to indicate the txid value for the YANG node represented by the
   element.





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   NETCONF servers that support this extension MUST announce the
   capability "urn:ietf:params:netconf:capability:txid:etag:1.0".

   The etag attribute values are opaque UTF-8 strings chosen freely,
   except that the etag string must not contain space, backslash or
   double quotes.  The point of this restriction is to make it easy to
   reuse implementations that adhere to section 2.3.1 in RFC 7232
   (https://tools.ietf.org/html/rfc7232).  The probability SHOULD be
   made very low that an etag value that has been used historically by a
   server is used again by that server if the configuration is
   different.

   It is RECOMMENDED that the same etag txid values are used across all
   management interfaces (i.e.  NETCONF, RESTCONF and any other the
   server might implement), if it implements more than one.

   The detailed rules for when to update the etag value are described in
   section General Txid Principles (Section 3.2).  These rules are
   chosen to be consistent with the ETag mechanism in RESTCONF, RFC 8040
   (https://tools.ietf.org/html/rfc8040), specifically sections 3.4.1.2,
   3.4.1.3 and 3.5.2.

4.2.  The last-modified attribute txid mechanism

   The last-modified txid mechanism described in this section is
   centered around a meta data XML attribute called "last-modified".
   The last-modified attribute is defined in the namespace
   "urn:ietf:params:xml:ns:netconf:txid:1.0".  The last-modified
   attribute is added to XML elements in the NETCONF payload in order to
   indicate the txid value for the YANG node represented by the element.

   NETCONF servers that support this extension MUST announce the
   capability "urn:ietf:params:netconf:capability:txid:last-
   modified:1.0".

   The last-modified attribute values are yang:date-and-time values as
   defined in ietf-yang-types.yang, RFC 6991
   (https://datatracker.ietf.org/doc/html/rfc6991).

   "2022-04-01T12:34:56.123456Z" is an example of what this time stamp
   format looks like.  It is RECOMMENDED that the time stamps provided
   by the server to closely match the real world clock.  Servers MUST
   ensure the timestamps provided are monotonously increasing for as
   long as the server's operation is maintained.







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   It is RECOMMENDED that server implementors choose the number of
   digits of precision used for the fractional second timestamps high
   enough so that there is no risk that multiple transactions on the
   server would get the same timestamp.

   It is RECOMMENDED that the same last-modified txid values are used
   across all management interfaces (i.e.  NETCONF and any other the
   server might implement), except RESTCONF.

   RESTCONF, as defined in RFC 8040 (https://tools.ietf.org/html/
   rfc8040), is using a different format for the time stamps which is
   limited to one second resolution.  Server implementors that support
   the Last-Modified txid mechanism over both RESTCONF and other
   management protocols are RECOMMENDED to use Last-Modified timestamps
   that match the point in time referenced over RESTCONF, with the
   fractional seconds part added.

   The detailed rules for when to update the last-modified value are
   described in section General Txid Principles (Section 3.2).  These
   rules are chosen to be consistent with the Last-Modified mechanism in
   RESTCONF, RFC 8040 (https://tools.ietf.org/html/rfc8040),
   specifically sections 3.4.1.1, 3.4.1.3 and 3.5.1.

4.3.  Common features to both etag and last-modified txid mechanisms

   Clients MAY add etag or last-modified attributes to zero or more
   individual elements in the get-config or get-data filter, in which
   case they pertain to the subtree(s) rooted at the element(s) with the
   attributes.

   Clients MAY also add such attributes directly to the get-config or
   get-data tags (e.g. if there is no filter), in which case it pertains
   to the txid value of the datastore root.

   Clients might wish to send a txid value that is guaranteed to never
   match a server constructed txid.  With both the etag and last-
   modified txid mechanisms, such a txid-request value is "?".

   Clients MAY add etag or last-modified attributes to the payload of
   edit-config or edit-data requests, in which case they indicate the
   client's txid value of that element.

   Clients MAY request servers that also implement YANG-Push to return
   configuration change subsription updates with etag or last-modified
   txid attributes.  The client requests this service by adding a with-
   etag or with-last-modified flag with the value 'true' to the
   subscription request or yang-push configuration.  The server MUST
   then return such txids on the YANG Patch edit tag and to the child



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   elements of the value tag.  The txid attribute on the edit tag
   reflects the txid associated with the changes encoded in this edit
   section, as well as parent nodes.  Later edit sections in the same
   push-update or push-change-update may still supercede the txid value
   for some or all of the nodes in the current edit section.

   Servers returning txid values in get-config, edit-config, get-data,
   edit-data and commit operations MUST do so by adding etag and/or
   last-modified txid attributes to the data and ok tags.  When servers
   prune output due to a matching txid value, the server MUST add a
   txid-match attribute to the pruned element, and MUST set the
   attribute value to "=", and MUST NOT send any element value.

   Servers returning a txid mismatch error MUST return an rpc-error as
   defined in section Conditional Transactions (Section 3.5) with an
   error-info tag containing a txid-value-mismatch-error-info structure.

   The txid attributes are valid on the following NETCONF tags, where
   xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0",
   xmlns:ncds="urn:ietf:params:xml:ns:yang:ietf-netconf-nmda",
   xmlns:sn="urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications",
   xmlns:yp="urn:ietf:params:xml:ns:yang:ietf-yang-patch" and
   xmlns:ypatch="urn:ietf:params:xml:ns:yang:ietf-yang-patch":

   In client messages sent to a server:

   *  /nc:rpc/nc:get-config

   *  /nc:rpc/nc:get-config/nc:filter//*

   *  /nc:rpc/ncds:get-data

   *  /nc:rpc/ncds:get-data/ncds:subtree-filter//*

   *  /nc:rpc/ncds:get-data/ncds:xpath-filter//*

   *  /nc:rpc/nc:edit-config/nc:config

   *  /nc:rpc/nc:edit-config/nc:config//*

   *  /nc:rpc/ncds:edit-data/ncds:config

   *  /nc:rpc/ncds:edit-data/ncds:config//*

   In server messages sent to a client:

   *  /nc:rpc-reply/nc:data




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   *  /nc:rpc-reply/nc:data//*

   *  /nc:rpc-reply/ncds:data

   *  /nc:rpc-reply/ncds:data//*

   *  /nc:rpc-reply/nc:ok

   *  /yp:push-update/yp:datastore-contents/ypatch:yang-patch/
      ypatch:edit

   *  /yp:push-update/yp:datastore-contents/ypatch:yang-patch/
      ypatch:edit/ypatch:value//*

   *  /yp:push-change-update/yp:datastore-contents/ypatch:yang-patch/
      ypatch:edit

   *  /yp:push-change-update/yp:datastore-contents/ypatch:yang-patch/
      ypatch:edit/ypatch:value//*

5.  Txid Mechanism Examples

5.1.  Initial Configuration Response

5.1.1.  With etag

   NOTE: In the etag examples below, we have chosen to use a txid value
   consisting of "nc" followed by a monotonously increasing integer.
   This is convenient for the reader trying to make sense of the
   examples, but is not an implementation requirement.  An etag would
   often be implemented as a "random" string of characters, with no
   comes-before/after relation defined.

   To retrieve etag attributes across the entire NETCONF server
   configuration, a client might send:

   <rpc xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" message-id="1"
        xmlns:txid="urn:ietf:params:xml:ns:netconf:txid:1.0">
     <get-config txid:etag="?"/>
   </rpc>

   The server's reply might then be:









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   <rpc-reply message-id="1"
              xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
              xmlns:txid="urn:ietf:params:xml:ns:netconf:txid:1.0">
     <data txid:etag="nc5152">
       <acls xmlns=
               "urn:ietf:params:xml:ns:yang:ietf-access-control-list"
             txid:etag="nc5152">
         <acl txid:etag="nc4711">
           <name>A1</name>
           <aces txid:etag="nc4711">
             <ace txid:etag="nc4711">
               <name>R1</name>
               <matches>
                 <ipv4>
                   <protocol>udp</protocol>
                 </ipv4>
               </matches>
             </ace>
           </aces>
         </acl>
         <acl txid:etag="nc5152">
           <name>A2</name>
           <aces txid:etag="nc5152">
             <ace txid:etag="nc4711">
               <name>R7</name>
               <matches>
                 <ipv4>
                   <dscp>AF11</dscp>
                 </ipv4>
               </matches>
             </ace>
             <ace txid:etag="nc5152">
               <name>R8</name>
               <matches>
                 <udp>
                   <source-port>
                     <port>22</port>
                   </source-port>
                 </udp>
               </matches>
             </ace>
             <ace txid:etag="nc5152">
               <name>R9</name>
               <matches>
                 <tcp>
                   <source-port>
                     <port>22</port>
                   </source-port>



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                 </tcp>
               </matches>
             </ace>
           </aces>
         </acl>
       </acls>
       <nacm xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-acm"
             txid:etag="nc3072">
         <groups txid:etag="nc3072">
           <group txid:etag="nc3072">
             <name>admin</name>
             <user-name>sakura</user-name>
             <user-name>joe</user-name>
           </group>
         </groups>
       </nacm>
     </data>
   </rpc>

   To retrieve etag attributes for a specific ACL using an xpath filter,
   a client might send:

   <rpc xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" message-id="2"
        xmlns:txid="urn:ietf:params:xml:ns:netconf:txid:1.0">
     <get-config>
       <source>
         <running/>
       </source>
       <filter type="xpath"
         xmlns:acl=
           "urn:ietf:params:xml:ns:yang:ietf-access-control-list"
         select="/acl:acls/acl:acl[acl:name='A1']"
         txid:etag="?"/>
     </get-config>
   </rpc>

   To retrieve etag attributes for "acls", but not for "nacm", a client
   might send:













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   <rpc xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" message-id="3"
        xmlns:txid="urn:ietf:params:xml:ns:netconf:txid:1.0">
     <get-config>
       <source>
         <running/>
       </source>
       <filter>
         <acls
           xmlns="urn:ietf:params:xml:ns:yang:ietf-access-control-list"
           txid:etag="?"/>
         <nacm xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-acm"/>
       </filter>
     </get-config>
   </rpc>

   If the server considers "acls", "acl", "aces" and "acl" to be
   versioned nodes, the server's response to the request above might
   look like:

   <rpc-reply message-id="3"
              xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
              xmlns:txid="urn:ietf:params:xml:ns:netconf:txid:1.0">
     <data>
       <acls xmlns=
               "urn:ietf:params:xml:ns:yang:ietf-access-control-list"
             txid:etag="nc5152">
         <acl txid:etag="nc4711">
           <name>A1</name>
           <aces txid:etag="nc4711">
             <ace txid:etag="nc4711">
               <name>R1</name>
               <matches>
                 <ipv4>
                   <protocol>udp</protocol>
                 </ipv4>
               </matches>
             </ace>
           </aces>
         </acl>
         <acl txid:etag="nc5152">
           <name>A2</name>
           <aces txid:etag="nc5152">
             <ace txid:etag="nc4711">
               <name>R7</name>
               <matches>
                 <ipv4>
                   <dscp>AF11</dscp>
                 </ipv4>



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               </matches>
             </ace>
             <ace txid:etag="nc5152">
               <name>R8</name>
               <matches>
                 <udp>
                   <source-port>
                     <port>22</port>
                   </source-port>
                 </udp>
               </matches>
             </ace>
             <ace txid:etag="nc5152">
               <name>R9</name>
               <matches>
                 <tcp>
                   <source-port>
                     <port>22</port>
                   </source-port>
                 </tcp>
               </matches>
             </ace>
           </aces>
         </acl>
       </acls>
       <nacm xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-acm"/>
         <groups>
           <group>
             <name>admin</name>
             <user-name>sakura</user-name>
             <user-name>joe</user-name>
           </group>
         </groups>
       </nacm>
     </data>
   </rpc>

5.1.2.  With last-modified

   To retrieve last-modified attributes for "acls", but not for "nacm",
   a client might send:










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   <rpc xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" message-id="4"
        xmlns:txid="urn:ietf:params:xml:ns:netconf:txid:1.0">
     <get-config>
       <source>
         <running/>
       </source>
       <filter>
         <acls
           xmlns="urn:ietf:params:xml:ns:yang:ietf-access-control-list"
           txid:last-modified="?"/>
         <nacm xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-acm"/>
       </filter>
     </get-config>
   </rpc>

   If the server considers "acls", "acl", "aces" and "acl" to be
   versioned nodes, the server's response to the request above might
   look like:

   <rpc-reply message-id="4"
              xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
              xmlns:txid="urn:ietf:params:xml:ns:netconf:txid:1.0">
     <data>
       <acls
         xmlns="urn:ietf:params:xml:ns:yang:ietf-access-control-list"
         txid:last-modified="2022-04-01T12:34:56.789012Z">
         <acl txid:last-modified="2022-03-20T16:20:11.333444Z">
           <name>A1</name>
           <ace txid:last-modified="2022-03-20T16:20:11.333444Z">
             <name>R1</name>
             <matches>
               <ipv4>
                 <protocol>udp</protocol>
               </ipv4>
             </matches>
           </ace>
         </acl>
         <acl txid:last-modified="2022-04-01T12:34:56.789012Z">
           <name>A2</name>
           <aces txid:last-modified="2022-04-01T12:34:56.789012Z">
             <ace txid:last-modified="2022-03-20T16:20:11.333444Z">
               <name>R7</name>
               <matches>
                 <ipv4>
                   <dscp>AF11</dscp>
                 </ipv4>
               </matches>
             </ace>



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             <ace txid:last-modified="2022-04-01T12:34:56.789012Z">
               <name>R8</name>
               <matches>
                 <udp>
                   <source-port>
                     <port>22</port>
                   </source-port>
                 </udp>
               </matches>
             </ace>
             <ace txid:last-modified="2022-04-01T12:34:56.789012Z">
               <name>R9</name>
               <matches>
                 <tcp>
                   <source-port>
                     <port>22</port>
                   </source-port>
                 </tcp>
               </matches>
             </ace>
           </aces>
         </acl>
       </acls>
       <nacm xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-acm"/>
         <groups>
           <group>
             <name>admin</name>
             <user-name>sakura</user-name>
             <user-name>joe</user-name>
           </group>
         </groups>
       </nacm>
     </data>
   </rpc>

5.2.  Configuration Response Pruning

   A NETCONF client that already knows some txid values MAY request that
   the configuration retrieval request is pruned with respect to the
   client's prior knowledge.

   To retrieve only changes for "acls" that do not have the last known
   etag txid value, a client might send:








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   <rpc xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" message-id="6"
        xmlns:txid="urn:ietf:params:xml:ns:netconf:txid:1.0">
     <get-config>
       <source>
         <running/>
       </source>
       <filter>
         <acls
           xmlns="urn:ietf:params:xml:ns:yang:ietf-access-control-list"
           txid:etag="nc5152">
           <acl txid:etag="nc4711">
             <name>A1</name>
             <aces txid:etag="nc4711"/>
           </acl>
           <acl txid:etag="nc5152">
             <name>A2</name>
             <aces txid:etag="nc5152"/>
           </acl>
       </filter>
     </get-config>
   </rpc>

   Assuming the NETCONF server configuration is the same as in the
   previous rpc-reply example, the server's response to request above
   might look like:

   <rpc-reply message-id="6"
              xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
              xmlns:txid="urn:ietf:params:xml:ns:netconf:txid:1.0">
     <data>
       <acls
         xmlns="urn:ietf:params:xml:ns:yang:ietf-access-control-list"
         txid:etag="="/>
     </data>
   </rpc>

   Or, if a configuration change has taken place under /acls since the
   client was last updated, the server's response may look like:













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   <rpc-reply message-id="6"
              xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
              xmlns:txid="urn:ietf:params:xml:ns:netconf:txid:1.0">
     <data>
       <acls
         xmlns="urn:ietf:params:xml:ns:yang:ietf-access-control-list"
         txid:etag="nc6614">
         <acl txid:etag="=">
           <name>A1</name>
         </acl>
         <acl txid:etag="nc6614">
           <name>A2</name>
           <aces txid:etag="nc6614">
             <ace txid:etag="nc4711">
               <name>R7</name>
               <matches>
                 <ipv4>
                   <dscp>AF11</dscp>
                 </ipv4>
               </matches>
             </ace>
             <ace txid:etag="nc5152">
               <name>R8</name>
               <matches>
                 <ipv4>
                   <source-port>
                     <port>22</port>
                   </source-port>
                 </ipv4>
               </matches>
             </ace>
             <ace txid:etag="nc6614">
               <name>R9</name>
               <matches>
                 <ipv4>
                   <source-port>
                     <port>830</port>
                   </source-port>
                 </ipv4>
               </matches>
             </ace>
           </aces>
         </acl>
       </acls>
     </data>
   </rpc>





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   In case the client provides a txid value for a non-versioned node,
   the server needs to treat the node as having the same txid value as
   the closest ancestor that does have a txid value.

   <rpc xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" message-id="7"
        xmlns:txid="urn:ietf:params:xml:ns:netconf:txid:1.0">
     <get-config>
       <source>
         <running/>
       </source>
       <filter>
         <acls
           xmlns="urn:ietf:params:xml:ns:yang:ietf-access-control-list">
           <acl>
             <name>A2</name>
             <aces>
               <ace>
                 <name>R7</name>
                 <matches>
                   <ipv4>
                     <dscp txid:etag="nc4711"/>
                   </ipv4>
                 </matches>
               </ace>
             </aces>
           </acl>
         </acls>
       </filter>
     </get-config>
   </rpc>

   If a txid value is specified for a leaf, and the txid value matches,
   the leaf value is pruned.


















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   <rpc-reply message-id="7"
              xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
              xmlns:txid="urn:ietf:params:xml:ns:netconf:txid:1.0">
     <data>
       <acls
         xmlns="urn:ietf:params:xml:ns:yang:ietf-access-control-list">
         <acl>
           <name>A2</name>
           <aces>
             <ace>
               <name>R7</name>
               <matches>
                 <ipv4>
                   <dscp txid:etag="="/>
                 </ipv4>
               </matches>
             </ace>
           </aces>
         </acl>
       </acls>
     </data>
   </rpc-reply>

5.3.  Configuration Change

   A client that wishes to update the ace R1 protocol to tcp might send:

























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   <rpc xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" message-id="8">
     <edit-config xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
                  xmlns:ietf-netconf-txid=
                   "urn:ietf:params:xml:ns:yang:ietf-netconf-txid">
       <target>
         <running/>
       </target>
       <test-option>test-then-set</test-option>
       <ietf-netconf-txid:with-etag>true<ietf-netconf-txid:with-etag>
       <config>
         <acls
           xmlns="urn:ietf:params:xml:ns:yang:ietf-access-control-list"
           txid:etag="nc5152">
           <acl txid:etag="nc4711">
             <name>A1</name>
             <aces txid:etag="nc4711">
               <ace txid:etag="nc4711">
                 <matches>
                   <ipv4>
                     <protocol>tcp</protocol>
                   </ipv4>
                 </matches>
               </ace>
             </aces>
           </acl>
         </acls>
       </config>
     </edit-config>
   </rpc>

   The server would update the protocol leaf in the running datastore,
   and return an rpc-reply as follows:

   <rpc-reply message-id="8"
              xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
              xmlns:txid="urn:ietf:params:xml:ns:netconf:txid:1.0">
     <ok txid:etag="nc7688"/>
   </rpc-reply>

   A subsequent get-config request for "acls", with txid:etag="?" might
   then return:

   <rpc-reply message-id="9"
              xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
              xmlns:txid="urn:ietf:params:xml:ns:netconf:txid:1.0">
     <data>
       <acls
         xmlns="urn:ietf:params:xml:ns:yang:ietf-access-control-list"



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         txid:etag="nc7688">
         <acl txid:etag="nc7688">
           <name>A1</name>
           <aces txid:etag="nc7688">
             <ace txid:etag="nc7688">
               <name>R1</name>
               <matches>
                 <ipv4>
                   <protocol>tcp</protocol>
                 </ipv4>
               </matches>
             </ace>
           </aces>
         </acl>
         <acl txid:etag="nc6614">
           <name>A2</name>
           <aces txid:etag="nc6614">
             <ace txid:etag="nc4711">
               <name>R7</name>
               <matches>
                 <ipv4>
                   <dscp>AF11</dscp>
                 </ipv4>
               </matches>
             </ace>
             <ace txid:etag="nc5152">
               <name>R8</name>
               <matches>
                 <udp>
                   <source-port>
                     <port>22</port>
                   </source-port>
                 </udp>
               </matches>
             </ace>
             <ace txid:etag="nc6614">
               <name>R9</name>
               <matches>
                 <tcp>
                   <source-port>
                     <port>830</port>
                   </source-port>
                 </tcp>
               </matches>
             </ace>
           </aces>
         </acl>
       </acls>



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     </data>
   </rpc>

   In case the server at this point received a configuration change from
   another source, such as a CLI operator, removing ace R8 and R9 in acl
   A2, a subsequent get-config request for acls, with txid:etag="?"
   might then return:

   <rpc-reply message-id="9"
              xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
              xmlns:txid="urn:ietf:params:xml:ns:netconf:txid:1.0">
     <data>
       <acls
         xmlns="urn:ietf:params:xml:ns:yang:ietf-access-control-list"
         txid:etag="cli2222">
         <acl txid:etag="nc7688">
           <name>A1</name>
           <aces txid:etag="nc7688">
             <ace txid:etag="nc7688">
               <name>R1</name>
               <matches>
                 <ipv4>
                   <protocol>tcp</protocol>
                 </ipv4>
               </matches>
             </ace>
           </aces>
         </acl>
         <acl txid:etag="cli2222">
           <name>A2</name>
           <aces txid:etag="cli2222">
             <ace txid:etag="nc4711">
               <name>R7</name>
               <matches>
                 <ipv4>
                   <dscp>AF11</dscp>
                 </ipv4>
               </matches>
             </ace>
           </aces>
         </acl>
       </acls>
     </data>
   </rpc>







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5.4.  Conditional Configuration Change

   If a client wishes to delete acl A1 if and only if its configuration
   has not been altered since this client last synchronized its
   configuration with the server, at which point it received the etag
   "nc7688" for acl A1, regardless of any possible changes to other
   acls, it might send:

   <rpc xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" message-id="10"
        xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0"
        xmlns:txid="urn:ietf:params:xml:ns:netconf:txid:1.0"
        xmlns:ietf-netconf-txid=
          "urn:ietf:params:xml:ns:yang:ietf-netconf-txid">
     <edit-config>
       <target>
         <runnign/>
       </target>
       <test-option>test-then-set</test-option>
       <ietf-netconf-txid:with-etag>true<ietf-netconf-txid:with-etag>
       <config>
         <acls xmlns=
             "urn:ietf:params:xml:ns:yang:ietf-access-control-list">
           <acl nc:operation="delete"
                txid:etag="nc7688">
             <name>A1</name>
           </acl>
         </acls>
       </config>
     </edit-config>
   </rpc>

   If acl A1 now has the etag txid value "nc7688", as expected by the
   client, the transaction goes through, and the server responds
   something like:

   <rpc-reply message-id="10"
              xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
              xmlns:txid="urn:ietf:params:xml:ns:netconf:txid:1.0">
     <ok txid:etag="nc8008"/>
   </rpc-reply>

   A subsequent get-config request for acls, with txid:etag="?" might
   then return:








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   <rpc-reply message-id="11"
              xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
              xmlns:txid="urn:ietf:params:xml:ns:netconf:txid:1.0">
     <data>
       <acls
         xmlns="urn:ietf:params:xml:ns:yang:ietf-access-control-list"
         txid:etag="nc8008">
         <acl txid:etag="cli2222">
           <name>A2</name>
           <aces txid:etag="cli2222">
             <ace txid:etag="nc4711">
               <name>R7</name>
               <matches>
                 <ipv4>
                   <dscp>AF11</dscp>
                 </ipv4>
               </matches>
             </ace>
           </aces>
         </acl>
       </acls>
     </data>
   </rpc>

   In case acl A1 did not have the expected etag txid value "nc7688",
   when the server processed this request, it rejects the transaction,
   and might send:
























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   <rpc-reply xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
              xmlns:acl=
               "urn:ietf:params:xml:ns:yang:ietf-access-control-list"
              xmlns:ietf-netconf-txid=
                "urn:ietf:params:xml:ns:yang:ietf-netconf-txid"
              message-id="11">
     <rpc-error>
       <error-type>protocol</error-type>
       <error-tag>operation-failed</error-tag>
       <error-severity>error</error-severity>
       <error-info>
         <ietf-netconf-txid:txid-value-mismatch-error-info>
           <ietf-netconf-txid:mismatch-path>
             /acl:acls/acl:acl[acl:name="A1"]
           </ietf-netconf-txid:mismatch-path>
           <ietf-netconf-txid:mismatch-etag-value>
             cli6912
           </ietf-netconf-txid:mismatch-etag-value>
         </ietf-netconf-txid:txid-value-mismatch-error-info>
       </error-info>
     </rpc-error>
   </rpc-reply>

5.5.  Using etags with Other NETCONF Operations

   The client MAY request that the new etag txid value is returned as an
   attribute on the ok response for a successful commit.  The client
   requests this by adding with-etag to the commit operation.

   For example, a client might send:

   <rpc message-id="12"
       xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
       xmlns:ietf-netconf-txid=
         "urn:ietf:params:xml:ns:yang:ietf-netconf-txid"
     <commit>
       <ietf-netconf-txid:with-etag>true<ietf-netconf-txid:with-etag>
     </commit>
   </rpc>

   Assuming the server accepted the transaction, it might respond:

   <rpc-reply message-id="12"
       xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
       xmlns:txid="urn:ietf:params:xml:ns:netconf:txid:1.0">
     <ok txid:etag="nc8008"/>
   </rpc-reply>




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5.6.  YANG-Push

   A client MAY request that the updates for one or more YANG Push
   subscriptions are annotated with the txid values.  The request might
   look like this:

   <netconf:rpc message-id="13"
                xmlns:netconf="urn:ietf:params:xml:ns:netconf:base:1.0">
     <establish-subscription
         xmlns=
           "urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications"
         xmlns:yp="urn:ietf:params:xml:ns:yang:ietf-yang-push"
         xmlns:ietf-netconf-txid-yp=
           "urn:ietf:params:xml:ns:yang:ietf-txid-yang-push">
       <yp:datastore
           xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores">
         ds:running
       </yp:datastore>
       <yp:datastore-xpath-filter
           xmlns:acl=
             "urn:ietf:params:xml:ns:yang:ietf-access-control-list">
         /acl:acls
       </yp:datastore-xpath-filter>
       <yp:periodic>
         <yp:period>500</yp:period>
       </yp:periodic>
       <ietf-netconf-txid-yp:with-etag>
         true
       </ietf-netconf-txid-yp:with-etag>
     </establish-subscription>
   </netconf:rpc>

   In case a client wishes to modify a previous subscription request in
   order to no longer receive YANG Push subscription updates, the
   request might look like this:
















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   <rpc message-id="14"
       xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <modify-subscription
         xmlns=
           "urn:ietf:params:xml:ns:yang:ietf-subscribed-notifications"
         xmlns:yp="urn:ietf:params:xml:ns:yang:ietf-yang-push"
         xmlns:ietf-netconf-txid-yp=
           "urn:ietf:params:xml:ns:yang:ietf-txid-yang-push">
       <id>1011</id>
       <yp:datastore
           xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores">
         ds:running
       </yp:datastore>
       <ietf-netconf-txid-yp:with-etag>
         false
       </ietf-netconf-txid-yp:with-etag>
     </modify-subscription>
   </rpc>

   A server might send a subscription update like this:

   <notification
     xmlns="urn:ietf:params:xml:ns:netconf:notification:1.0">
     <eventTime>2022-04-04T06:00:24.16Z</eventTime>
     <push-change-update
         xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-push">
       <id>89</id>
       <datastore-changes>
         <yang-patch>
           <patch-id>0</patch-id>
           <edit txid:etag="nc8008">
             <edit-id>edit1</edit-id>
             <operation>delete</operation>
             <target xmlns:acl=
               "urn:ietf:params:xml:ns:yang:ietf-access-control-list">
               /acl:acls
             </target>
             <value>
               <acl xmlns=
                 "urn:ietf:params:xml:ns:yang:ietf-access-control-list">
                 <name>A1</name>
               </acl>
             </value>
           </edit>
         </yang-patch>
       </datastore-changes>
     </push-change-update>
   </notification>



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6.  YANG Modules

6.1.  Base module for txid in NETCONF

   module ietf-netconf-txid {
     yang-version 1.1;
     namespace
       'urn:ietf:params:xml:ns:yang:ietf-netconf-txid';
     prefix ietf-netconf-txid;

     import ietf-netconf {
       prefix nc;
     }

     import ietf-netconf-nmda {
       prefix ncds;
     }

     import ietf-yang-structure-ext {
       prefix sx;
     }

     import ietf-yang-types {
       prefix yang;
     }

     organization
       "IETF NETCONF (Network Configuration) Working Group";

     contact
       "WG Web:   <http://tools.ietf.org/wg/netconf/>
        WG List:  <netconf@ietf.org>

        Author:   Jan Lindblad
                  <mailto:jlindbla@cisco.com>";

     description
       "NETCONF Transaction ID aware operations for NMDA.

        Copyright (c) 2022 IETF Trust and the persons identified as
        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
        (https://trustee.ietf.org/license-info).



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        This version of this YANG module is part of RFC XXXX
        (https://www.rfc-editor.org/info/rfcXXXX); see the RFC itself
        for full legal notices.

        The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL
        NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'NOT RECOMMENDED',
        'MAY', and 'OPTIONAL' in this document are to be interpreted as
        described in BCP 14 (RFC 2119) (RFC 8174) when, and only when,
        they appear in all capitals, as shown here.
        ";

     revision 2022-04-01 {
       description
         "Initial revision";
       reference
         "RFC XXXX: Xxxxxxxxx";
     }

     typedef etag-t {
       type string {
         pattern ".* .*" {
           modifier invert-match;
         }
         pattern '.*".*' {
           modifier invert-match;
         }
         pattern ".*\\.*" {
           modifier invert-match;
         }
       }
       description
         "Unique Entity-tag txid value representing a specific
         transaction.  Could be any string that does not contain
         spaces, double quotes or backslash.  The txid values '?'
         and '=' have special meaning.";
     }

     typedef last-modified-t {
       type union {
         type yang:date-and-time;
         type enumeration {
           enum ? {
             description "Txid value used by clients that is
               guaranteed not to match any txid on the server.";
           }
           enum = {
             description "Txid value used by servers to indicate
               that contents has been pruned due to txid match



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               between client and server.";
           }
         }
       }
       description
         "Last-modified txid value representing a specific transaction.
          The txid values '?' and '=' have special meaning.";
     }

     grouping txid-grouping {
       leaf with-etag {
         type boolean;
         description
           "Indicates whether the client requests the server to include
            a txid:etag txid attribute when the configuration has
            changed.";
       }
       leaf with-last-modified {
         type boolean;
         description
           "Indicates whether the client requests the server to include
            a txid:last-modified attribute when the configuration has
            changed.";
       }
       description
         "Grouping for txid mechanisms, to be augmented into
          rpcs that modify configuration data stores.";
     }

     augment /nc:edit-config/nc:input {
       uses txid-grouping;
       description
         "Injects the txid mechanisms into the
         edit-config operation";
     }

     augment /nc:commit/nc:input {
       uses txid-grouping;
       description
         "Injects the txid mechanisms into the
         commit operation";
     }

     augment /ncds:edit-data/ncds:input {
       uses txid-grouping;
       description
         "Injects the txid mechanisms into the
         edit-data operation";



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     }

     sx:structure txid-value-mismatch-error-info {
       container txid-value-mismatch-error-info {
         description
            "This error is returned by a NETCONF server when a client
             sends a configuration change request, with the additonal
             condition that the server aborts the transaction if the
             server's configuration has changed from what the client
             expects, and the configuration is found not to actually
             not match the client's expectation.";
         leaf mismatch-path {
           type instance-identifier;
           description
             "Indicates the YANG path to the element with a mismatching
              etag txid value.";
         }
         leaf mismatch-etag-value {
           type etag-t;
           description
             "Indicates server's txid value of the etag
             attribute for one mismatching element.";
         }
         leaf mismatch-last-modified-value {
           type last-modified-t;
           description
             "Indicates server's txid value of the last-modified
             attribute for one mismatching element.";
         }
       }
     }
   }

6.2.  Additional support for txid in YANG-Push

   module ietf-netconf-txid-yang-push {
     yang-version 1.1;
     namespace
       'urn:ietf:params:xml:ns:yang:ietf-netconf-txid-yang-push';
     prefix ietf-netconf-txid-yp;

     import ietf-subscribed-notifications {
       prefix sn;
       reference
         "RFC 8639: Subscription to YANG Notifications";
     }

     import ietf-netconf-txid {



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       prefix ietf-netconf-txid;
       reference
         "RFC XXXX: Xxxxxxxxx";
     }

     organization
       "IETF NETCONF (Network Configuration) Working Group";

     contact
       "WG Web:   <http://tools.ietf.org/wg/netconf/>
        WG List:  <netconf@ietf.org>

        Author:   Jan Lindblad
                  <mailto:jlindbla@cisco.com>";

     description
       "NETCONF Transaction ID aware operations for YANG Push.

        Copyright (c) 2022 IETF Trust and the persons identified as
        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
        (https://trustee.ietf.org/license-info).

        This version of this YANG module is part of RFC XXXX
        (https://www.rfc-editor.org/info/rfcXXXX); see the RFC itself
        for full legal notices.

        The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL
        NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'NOT RECOMMENDED',
        'MAY', and 'OPTIONAL' in this document are to be interpreted as
        described in BCP 14 (RFC 2119) (RFC 8174) when, and only when,
        they appear in all capitals, as shown here.
        ";

     revision 2022-04-01 {
       description
         "Initial revision";
       reference
         "RFC XXXX: Xxxxxxxxx";
     }

     augment "/sn:establish-subscription/sn:input" {
       description



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         "This augmentation adds additional subscription parameters
           that apply specifically to datastore updates to RPC input.";
       uses ietf-netconf-txid:txid-grouping;
     }
     augment "/sn:modify-subscription/sn:input" {
       description
         "This augmentation adds additional subscription parameters
           specific to datastore updates.";
       uses ietf-netconf-txid:txid-grouping;
     }
     augment "/sn:subscriptions/sn:subscription" {
       description
         "This augmentation adds additional subscription parameters
           specific to datastore updates.";
       uses ietf-netconf-txid:txid-grouping;
     }
   }

7.  Security Considerations

   TODO Security

8.  IANA Considerations

   This document registers the following capability identifier URN in
   the 'Network Configuration Protocol (NETCONF) Capability URNs'
   registry:

     urn:ietf:params:netconf:capability:txid:1.0

   This document registers three XML namespace URNs in the 'IETF XML
   registry', following the format defined in RFC 3688
   (https://tools.ietf.org/html/rfc3688).

     URI: urn:ietf:params:xml:ns:netconf:txid:1.0

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

     URI: urn:ietf:params:xml:ns:yang:ietf-netconf-txid-yang-push

     Registrant Contact: The NETCONF WG of the IETF.

     XML: N/A, the requested URIs are XML namespaces.

   This document registers two module names in the 'YANG Module Names'
   registry, defined in RFC 6020 (https://tools.ietf.org/html/rfc6020).





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     name: ietf-netconf-txid

     prefix: ietf-netconf-txid

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

     RFC: XXXX

   and

     name: ietf-netconf-txid-yp

     prefix: ietf-netconf-txid-yp

     namespace: urn:ietf:params:xml:ns:yang:ietf-netconf-txid-yang-push

     RFC: XXXX

9.  Changes

9.1.  Major changes in -02 since -01

   *  A last-modified txid mechanism has been added (back).  This
      mechanism aligns well with the Last-Modified mechanism defined in
      RESTCONF RFC 8040 (https://tools.ietf.org/html/rfc8040), but is
      not a carbon copy.

   *  YANG Push functionality has been added.  This allows YANG Push
      users to receive txid updates as part of the configuration
      updates.  This functionality comes in a separate YANG module, to
      allow implementors to cleanly keep all this functionality out.

   *  Changed name of "versioned elements".  They are now called
      "versioned nodes".

   *  Clarified txid behavior for transactions toward the Candidate
      datastore, and some not so common situations, such as when a
      client specifies a txid for a non-versioned node, and when there
      are when-statement dependencies across subtrees.

   *  Examples provided for the abstract mechanism level with simple
      message flow diagrams.

   *  More examples on protocol level, and with ietf-interfaces as
      example target module replaced with ietf-access-control to reduce
      confusion.





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   *  Explicit list of XPaths to clearly state where etag or last-
      modified attributes may be added by clients and servers.

   *  Document introduction restructured to remove duplication between
      sections and to allow multiple (etag and last-modified) txid
      mechanisms.

   *  Moved the actual YANG module code into proper module files that
      are included in the source document.  These modules can be
      compiled as proper modules without any extraction tools.

9.2.  Major changes in -01 since -00

   *  Updated the text on numerous points in order to answer questions
      that appeared on the mailing list.

   *  Changed the document structure into a general transaction id part
      and one etag specific part.

   *  Renamed entag attribute to etag, prefix to txid, namespace to
      urn:ietf:params:xml:ns:yang:ietf-netconf-txid.

   *  Set capability string to
      urn:ietf:params:netconf:capability:txid:1.0

   *  Changed YANG module name, namespace and prefix to match names
      above.

   *  Harmonized/slightly adjusted etag value space with RFC 7232 and
      RFC 8040.

   *  Removed all text discussing etag values provided by the client
      (although this is still an interesting idea, if you ask the
      author)

   *  Clarified the etag attribute mechanism, especially when it comes
      to matching against non-versioned elements, its cascading upwards
      in the tree and secondary effects from when- and choice-
      statements.

   *  Added a mechanism for returning the server assigned etag value in
      get-config and get-data.

   *  Added section describing how the NETCONF discard-changes, copy-
      config, delete-config and commit operations work with respect to
      etags.

   *  Added IANA Considerations section.



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   *  Removed all comments about open questions.

10.  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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

Acknowledgments

   The author wishes to thank Benoit Claise for making this work happen,
   and the following individuals, who all provided helpful comments: Per
   Andersson, Kent Watsen, Andy Bierman, Robert Wilton, Qiufang Ma.

Author's Address

   Jan Lindblad
   Cisco Systems
   Email: jlindbla@cisco.com



























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