path: root/doc/rfc/rfc8430.txt

Internet Engineering Task Force (IETF)                   N. Bahadur, Ed.
Request for Comments: 8430                                          Uber
Category: Informational                                     S. Kini, Ed.
ISSN: 2070-1721
                                                               J. Medved
                                                                   Cisco
                                                          September 2018


                         RIB Information Model

Abstract

   Routing and routing functions in enterprise and carrier networks are
   typically performed by network devices (routers and switches) using a
   Routing Information Base (RIB).  Protocols and configurations push
   data into the RIB, and the RIB manager installs state into the
   hardware for packet forwarding.  This document specifies an
   information model for the RIB to enable defining a standardized data
   model.  The IETF's I2RS WG used this document to design the I2RS RIB
   data model.  This document is being published to record the higher-
   level information model decisions for RIBs so that other developers
   of RIBs may benefit from the design concepts.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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 candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8430.












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Copyright Notice

   Copyright (c) 2018 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 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
     1.1.  Conventions Used in This Document . . . . . . . . . . . .   6
   2.  RIB Data  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.1.  RIB Definition  . . . . . . . . . . . . . . . . . . . . .   7
     2.2.  Routing Instance  . . . . . . . . . . . . . . . . . . . .   7
     2.3.  Route . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     2.4.  Nexthop . . . . . . . . . . . . . . . . . . . . . . . . .  10
       2.4.1.  Base Nexthops . . . . . . . . . . . . . . . . . . . .  12
       2.4.2.  Derived Nexthops  . . . . . . . . . . . . . . . . . .  14
       2.4.3.  Nexthop Indirection . . . . . . . . . . . . . . . . .  15
   3.  Reading from the RIB  . . . . . . . . . . . . . . . . . . . .  16
   4.  Writing to the RIB  . . . . . . . . . . . . . . . . . . . . .  16
   5.  Notifications . . . . . . . . . . . . . . . . . . . . . . . .  17
   6.  RIB Grammar . . . . . . . . . . . . . . . . . . . . . . . . .  17
     6.1.  Nexthop Grammar Explained . . . . . . . . . . . . . . . .  20
   7.  Using the RIB Grammar . . . . . . . . . . . . . . . . . . . .  20
     7.1.  Using Route Preference  . . . . . . . . . . . . . . . . .  20
     7.2.  Using Different Nexthop Types . . . . . . . . . . . . . .  20
       7.2.1.  Tunnel Nexthops . . . . . . . . . . . . . . . . . . .  21
       7.2.2.  Replication Lists . . . . . . . . . . . . . . . . . .  21
       7.2.3.  Weighted Lists  . . . . . . . . . . . . . . . . . . .  21
       7.2.4.  Protection  . . . . . . . . . . . . . . . . . . . . .  22
       7.2.5.  Nexthop Chains  . . . . . . . . . . . . . . . . . . .  22
       7.2.6.  Lists of Lists  . . . . . . . . . . . . . . . . . . .  23
     7.3.  Performing Multicast  . . . . . . . . . . . . . . . . . .  24
   8.  RIB Operations at Scale . . . . . . . . . . . . . . . . . . .  25
     8.1.  RIB Reads . . . . . . . . . . . . . . . . . . . . . . . .  25
     8.2.  RIB Writes  . . . . . . . . . . . . . . . . . . . . . . .  25
     8.3.  RIB Events and Notifications  . . . . . . . . . . . . . .  25
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  25
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  26
     11.2.  Informative References . . . . . . . . . . . . . . . . .  27
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28













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1.  Introduction

   Routing and routing functions in enterprise and carrier networks are
   traditionally performed in network devices.  Customarily, routers run
   routing protocols, and the routing protocols (along with static
   configuration information) populate the Routing Information Base
   (RIB) of the router.  The RIB is managed by the RIB manager, and the
   RIB manager provides a northbound interface to its clients (i.e., the
   routing protocols) to insert routes into the RIB.  The RIB manager
   consults the RIB and decides how to program the Forwarding
   Information Base (FIB) of the hardware by interfacing with the FIB
   manager.  The relationship between these entities is shown in
   Figure 1.

         +-------------+        +-------------+
         |RIB Client 1 | ...... |RIB Client N |
         +-------------+        +-------------+
                ^                      ^
                |                      |
                +----------------------+
                           |
                           V
                +---------------------+
                |    RIB Manager      |
                |                     |
                |     +--------+      |
                |     | RIB(s) |      |
                |     +--------+      |
                +---------------------+
                           ^
                           |
          +---------------------------------+
          |                                 |
          V                                 V
   +----------------+               +----------------+
   | FIB Manager 1  |               | FIB Manager M  |
   |   +--------+   |  ..........   |   +--------+   |
   |   | FIB(s) |   |               |   | FIB(s) |   |
   |   +--------+   |               |   +--------+   |
   +----------------+               +----------------+

           Figure 1: RIB Manager, RIB Clients, and FIB Managers

   Routing protocols are inherently distributed in nature, and each
   router makes an independent decision based on the routing data
   received from its peers.  With the advent of newer deployment
   paradigms and the need for specialized applications, there is an
   emerging need to guide the router's routing function [RFC7920].  The



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   traditional network-device RIB population that is protocol based
   suffices for most use cases where distributed network control is
   used.  However, there are use cases that the network operators
   currently address by configuring static routes, policies, and RIB
   import/export rules on the routers.  There is also a growing list of
   use cases in which a network operator might want to program the RIB
   based on data unrelated to just routing (within that network's
   domain).  Programming the RIB could be based on other information
   (such as routing data in the adjacent domain or the load on storage
   and compute) in the given domain.  Or, it could simply be a
   programmatic way of creating on-demand dynamic overlays (e.g., GRE
   tunnels) between compute hosts (without requiring the hosts to run
   traditional routing protocols).  If there was a standardized,
   publicly documented programmatic interface to a RIB, it would enable
   further networking applications that address a variety of use cases
   [RFC7920].

   A programmatic interface to the RIB involves two types of operations:
   reading from the RIB and writing (adding/modifying/deleting) to the
   RIB.

   In order to understand what is in a router's RIB, methods like per-
   protocol SNMP MIBs and screen scraping are used.  These methods are
   not scalable since they are client pull mechanisms and not proactive
   push (from the router) mechanisms.  Screen scraping is error prone
   (since the output format can change) and is vendor dependent.
   Building a RIB from per-protocol MIBs is error prone since the MIB
   data represents protocol data and not the exact information that went
   into the RIB.  Thus, just getting read-only RIB information from a
   router is a hard task.

   Adding content to the RIB from a RIB client can be done today using
   static configuration mechanisms provided by router vendors.  However,
   the mix of what can be modified in the RIB varies from vendor to
   vendor, and the method of configuring it is also vendor dependent.
   This makes it hard for a RIB client to program a multi-vendor network
   in a consistent and vendor-independent way.

   The purpose of this document is to specify an information model for
   the RIB.  Using the information model, one can build a detailed data
   model for the RIB.  That data model could then be used by a RIB
   client to program a network device.  One data model that has been
   based on this document is the I2RS RIB data model [RFC8431].

   The rest of this document is organized as follows.  Section 2 goes
   into the details of what constitutes and can be programmed in a RIB.
   Guidelines for reading and writing the RIB are provided in Sections 3
   and 4, respectively.  Section 5 provides a high-level view of the



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   events and notifications going from a network device to a RIB client
   to update the RIB client on asynchronous events.  The RIB grammar is
   specified in Section 6.  Examples of using the RIB grammar are shown
   in Section 7.  Section 8 covers considerations for performing RIB
   operations at scale.

1.1.  Conventions Used in This Document

   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.

2.  RIB Data

   This section describes the details of a RIB.  It makes forward
   references to objects in the RIB grammar (see Section 6).  A high-
   level description of the RIB contents is as shown in Figure 2.
   Please note that for ease of representation in ASCII art, this
   drawing shows a single routing instance, a single RIB, and a single
   route.  Subsections of this section describe the logical data nodes
   that should be contained within a RIB.  Sections 3 and 4 describe the
   high-level read and write operations.

                          network-device
                                |
                                | 0..N
                                |
                         routing instance(s)
                          |             |
                          |             |
                    0..N  |             | 0..N
                          |             |
                     interface(s)     RIB(s)
                                        |
                                        |
                                        | 0..N
                                        |
                                      route(s)

                      Figure 2: RIB Information Model









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2.1.  RIB Definition

   A RIB, in the context of the RIB information model, is an entity that
   contains routes.  It is identified by its name and is contained
   within a routing instance (see Section 2.2).  A network device MAY
   contain routing instances, and each routing instance MAY contain
   RIBs.  The name MUST be unique within a routing instance.  All routes
   in a given RIB MUST be of the same address family (e.g., IPv4).  Each
   RIB MUST belong to a routing instance.

   A routing instance may contain two or more RIBs of the same address
   family (e.g., IPv6).  A typical case where this can be used is for
   multi-topology routing [RFC4915] [RFC5120].

   Each RIB MAY be associated with an ENABLE_IP_RPF_CHECK attribute that
   enables Reverse Path Forwarding (RPF) checks on all IP routes in that
   RIB.  The RPF check is used to prevent spoofing and limit malicious
   traffic.  For IP packets, the IP source address is looked up and the
   RPF interface(s) associated with the route for that IP source address
   is found.  If the incoming IP packet's interface matches one of the
   RPF interfaces, then the IP packet is forwarded based on its IP
   destination address; otherwise, the IP packet is discarded.

2.2.  Routing Instance

   A routing instance, in the context of the RIB information model, is a
   collection of RIBs, interfaces, and routing parameters.  A routing
   instance creates a logical slice of the router.  It allows different
   logical slices across a set of routers to communicate with each
   other.  Layer 3 VPNs, Layer 2 VPNs (L2VPNs), and Virtual Private LAN
   Service (VPLS) can be modeled as routing instances.  Note that
   modeling an L2VPN using a routing instance only models the Layer 3
   (RIB) aspect and does not model any Layer 2 information (like ARP)
   that might be associated with the L2VPN.

   The set of interfaces indicates which interfaces are associated with
   this routing instance.  The RIBs specify how incoming traffic is to
   be forwarded, and the routing parameters control the information in
   the RIBs.  The intersection set of interfaces of two routing
   instances MUST be the null set.  In other words, an interface MUST
   NOT be present in two routing instances.  Thus, a routing instance
   describes the routing information and parameters across a set of
   interfaces.








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   A routing instance MUST contain the following mandatory fields:

   o  INSTANCE_NAME: A routing instance is identified by its name,
      INSTANCE_NAME.  This MUST be unique across all routing instances
      in a given network device.

   o  rib-list: This is the list of RIBs associated with this routing
      instance.  Each routing instance can have multiple RIBs to
      represent routes of different types.  For example, one would put
      IPv4 routes in one RIB and MPLS routes in another RIB.  The list
      of RIBs can be an empty list.

   A routing instance MAY contain the following fields:

   o  interface-list: This represents the list of interfaces associated
      with this routing instance.  The interface list helps constrain
      the boundaries of packet forwarding.  Packets coming in on these
      interfaces are directly associated with the given routing
      instance.  The interface list contains a list of identifiers, with
      each identifier uniquely identifying an interface.

   o  ROUTER_ID: This field identifies the network device in control
      plane interactions with other network devices.  This field is to
      be used if one wants to virtualize a physical router into multiple
      virtual routers.  Each virtual router MUST have a unique
      ROUTER_ID.  A ROUTER_ID MUST be unique across all network devices
      in a given domain.

   A routing instance may be created purely for the purposes of packet
   processing and may not have any interfaces associated with it.  For
   example, an incoming packet in routing instance A might have a
   nexthop of routing instance B, and after packet processing in B, the
   nexthop might be routing instance C.  Thus, routing instance B is not
   associated with any interface.  And, given that this routing instance
   does not do any control-plane interaction with other network devices,
   a ROUTER_ID is also not needed.

2.3.  Route

   A route is essentially a match condition and an action following the
   match.  The match condition specifies the kind of route (IPv4, MPLS,
   etc.) and the set of fields to match on.  Figure 3 represents the
   overall contents of a route.  Please note that for ease of depiction
   in ASCII art, only a single instance of the route-attribute, match
   flags, and nexthop is depicted.






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                                 route
                                 | | |
                       +---------+ | +----------+
                       |           |            |
                  0..N |           |            |

         route-attribute         match         nexthop
                                   |
                                   |
                   +-------+-------+-------+--------+
                   |       |       |       |        |
                   |       |       |       |        |

                  IPv4    IPv6    MPLS    MAC    Interface

                           Figure 3: Route Model

   This document specifies the following match types:

   o  IPv4: Match on destination and/or source IP address in the IPv4
      header

   o  IPv6: Match on destination and/or source IP address in the IPv6
      header

   o  MPLS: Match on an MPLS label at the top of the MPLS label stack

   o  MAC: Match on Media Access Control (MAC) destination addresses in
      the Ethernet header

   o  Interface: Match on the incoming interface of the packet

   A route MAY be matched on one or more of these match types by policy
   as either an "AND" (to restrict the number of routes) or an "OR" (to
   combine two filters).

   Each route MUST have the following mandatory route-attributes
   associated with it:

   o  ROUTE_PREFERENCE: This is a numerical value that allows for
      comparing routes from different protocols.  Static configuration
      is also considered a protocol for the purpose of this field.  It
      is also known as "administrative distance".  The lower the value,
      the higher the preference.  For example, there can be an OSPF
      route for 192.0.2.1/32 (or IPv6 2001:DB8::1/128) with a preference
      of 5.  If a controller programs a route for 192.0.2.1/32 (or IPv6
      2001:DB8::1/128) with a preference of 2, then the controller's
      route will be preferred by the RIB manager.  Preference should be



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      used to dictate behavior.  For more examples of preference, see
      Section 7.1.

   Each route can have one or more optional route-attributes associated
   with it.

   o  route-vendor-attributes: Vendors can specify vendor-specific
      attributes using this.  The details of this attribute are outside
      the scope of this document.

   Each route has a nexthop associated with it.  Nexthops are described
   in Section 2.4.

   Additional features to match multicast packets were considered (e.g.,
   TTL of the packet to limit the range of a multicast group), but these
   were not added to this information model.  Future RIB information
   models should investigate these multicast features.

2.4.  Nexthop

   A nexthop represents an object resulting from a route lookup.  For
   example, if a route lookup results in sending the packet out of a
   given interface, then the nexthop represents that interface.

   Nexthops can be either fully resolved or unresolved.  A resolved
   nexthop has adequate information to send the outgoing packet to the
   destination by forwarding it on an interface to a directly connected
   neighbor.  For example, a nexthop to a point-to-point interface or a
   nexthop to an IP address on an Ethernet interface has the nexthop
   resolved.  An unresolved nexthop is something that requires the RIB
   manager to determine the final resolved nexthop.  For example, a
   nexthop could be an IP address.  The RIB manager would resolve how to
   reach that IP address; for example, is the IP address reachable by
   regular IP forwarding, by an MPLS tunnel, or by both?  If the RIB
   manager cannot resolve the nexthop, then the nexthop remains in an
   unresolved state and is NOT a candidate for installation in the FIB.
   Future RIB events can cause an unresolved nexthop to get resolved
   (e.g., an IP address being advertised by an IGP neighbor).
   Conversely, resolved nexthops can also become unresolved (e.g., in
   the case of a tunnel going down); hence, they would no longer be
   candidates to be installed in the FIB.

   When at least one of a route's nexthops is resolved, then the route
   can be used to forward packets.  Such a route is considered eligible
   to be installed in the FIB and is henceforth referred to as a FIB-
   eligible route.  Conversely, when all the nexthops of a route are
   unresolved, that route can no longer be used to forward packets.
   Such a route is considered ineligible to be installed in the FIB and



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   is henceforth referred to as a FIB-ineligible route.  The RIB
   information model allows a RIB client to program routes whose
   nexthops may be unresolved initially.  Whenever an unresolved nexthop
   gets resolved, the RIB manager will send a notification of the same
   (see Section 5).

   The overall structure and usage of a nexthop is as shown in the
   figure below.  For ease of description using ASCII art, only a single
   instance of any component of the nexthop is shown in Figure 4.

                               route
                                 |
                                 | 0..N
                                 |
                               nexthop <-------------------------------+
                                 |                                     |
          +-------+----------------------------+-------------+         |
          |       |              |             |             |         |
          |       |              |             |             |         |
       base   load-balance   protection      replicate     chain       |
          |       |              |             |             |         |
          |       |2..N          |2..N         |2..N         |1..N     |
          |       |              |             |             |         |
          |       |              V             |             |         |
          |       +------------->+<------------+-------------+         |
          |                      |                                     |
          |                      +-------------------------------------+
          |
          +-------------------+
                              |
                              |
                              |
                              |
     +---------------+--------+--------+--------------+----------+
     |               |                 |              |          |
     |               |                 |              |          |
  nexthop-id  egress-interface  ip-address     logical-tunnel    |
                                                                 |
                                                                 |
                          +--------------------------------------+
                          |
       +----------------------+------------------+-------------+
       |                      |                  |             |
       |                      |                  |             |
 tunnel-encapsulation   tunnel-decapsulation  rib-name   special-nexthop

                          Figure 4: Nexthop Model




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   This document specifies a very generic, extensible, and recursive
   grammar for nexthops.  A nexthop can be a base nexthop or a derived
   nexthop.  Section 2.4.1 details base nexthops, and Section 2.4.2
   explains various kinds of derived nexthops.  There are certain
   special nexthops, and those are described in Section 2.4.1.1.
   Lastly, Section 2.4.3 delves into nexthop indirection and its use.
   Examples of when and how to use tunnel nexthops and derived nexthops
   are shown in Section 7.2.

2.4.1.  Base Nexthops

   At the lowest level, a nexthop can be one of the following:

   o  Identifier: This is an identifier returned by the network device
      representing a nexthop.  This can be used as a way of reusing a
      nexthop when programming derived nexthops.

   o  Interface nexthops: These are nexthops that are pointing to an
      interface.  Various attributes associated with these nexthops are:

      *  Egress-interface: This represents a physical, logical, or
         virtual interface on the network device.  Address resolution
         must not be required on this interface.  This interface may
         belong to any routing instance.

      *  IP address: A route lookup on this IP address is done to
         determine the egress-interface.  Address resolution may be
         required depending on the interface.

         +  An optional rib-name can also be specified to indicate the
            RIB in which the IP address is to be looked up.  One can use
            the rib-name field to direct the packet from one domain into
            another domain.  By default the RIB will be the same as the
            one that route belongs to.

      These attributes can be used in combination as follows:

      *  Egress-interface and IP address: This can be used in cases
         where, e.g., the IP address is a link-local address.

      *  Egress-interface and MAC address: The egress-interface must be
         an Ethernet interface.  Address resolution is not required for
         this nexthop.








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   o  Tunnel nexthops: These are nexthops that are pointing to a tunnel.
      The types of tunnel nexthops are:

      *  tunnel-encapsulation: This can be an encapsulation representing
         an IP tunnel, MPLS tunnel, or others as defined in this
         document.  An optional egress-interface can be chained to the
         tunnel-encapsulation to indicate which interface to send the
         packet out on.  The egress-interface is useful when the network
         device contains Ethernet interfaces and one needs to perform
         address resolution for the IP packet.

      *  tunnel-decapsulation: This is to specify decapsulating a tunnel
         header.  After decapsulation, further lookup on the packet can
         be done via chaining it with another nexthop.  The packet can
         also be sent out via an egress-interface directly.

      *  logical-tunnel: This can be an MPLS Label Switched Path (LSP)
         or a GRE tunnel (or others as defined in this document) that is
         represented by a unique identifier (e.g., name).

   o  rib-name: A nexthop pointing to a RIB.  This indicates that the
      route lookup needs to continue in the specified RIB.  This is a
      way to perform chained lookups.

   Tunnel nexthops allow a RIB client to program static tunnel headers.
   There can be cases where the remote tunnel endpoint does not support
   dynamic signaling (e.g., no LDP support on a host); in those cases,
   the RIB client might want to program the tunnel header on both ends
   of the tunnel.  The tunnel nexthop is kept generic with
   specifications provided for some commonly used tunnels.  It is
   expected that the data model will model these tunnel types with
   complete accuracy.

2.4.1.1.  Special Nexthops

   Special nexthops are for performing specific well-defined functions
   (e.g., DISCARD).  The purpose of each of them is explained below:

   o  DISCARD: This indicates that the network device should drop the
      packet and increment a drop counter.

   o  DISCARD_WITH_ERROR: This indicates that the network device should
      drop the packet, increment a drop counter, and send back an
      appropriate error message (like ICMP error).







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   o  RECEIVE: This indicates that the traffic is destined for the
      network device, for example, protocol packets or Operations,
      Administration, and Maintenance (OAM) packets.  All locally
      destined traffic SHOULD be throttled to avoid a denial-of-service
      attack on the router's control plane.  An optional rate limiter
      can be specified to indicate how to throttle traffic destined for
      the control plane.  The description of the rate limiter is outside
      the scope of this document.

2.4.2.  Derived Nexthops

   Derived nexthops can be:

   o  weighted lists, which are used for load-balancing;

   o  preference lists, which are used for protection using primary and
      backup;

   o  replication lists, which are lists of nexthops to which to
      replicate a packet;

   o  nexthop chains, which are for chaining multiple operations or
      attaching multiple headers; or

   o  lists of lists, which are a recursive application of the above.

   Nexthop chains (see Section 7.2.5 for usage) are a way to perform
   multiple operations on a packet by logically combining them.  For
   example, one can chain together "decapsulate MPLS header" and "send
   it out a specific egress-interface".  Chains can be used to specify
   multiple headers over a packet before a packet is forwarded.  One
   simple example is that of MPLS over GRE, wherein the packet has an
   inner MPLS header followed by a GRE header followed by an IP header.
   The outermost IP header is decided by the network device, whereas the
   MPLS header or GRE header is specified by the controller.  Not every
   network device will be able to support all kinds of nexthop chains
   and an arbitrary number of headers chained together.  The RIB data
   model SHOULD provide a way to expose a nexthop chaining capability
   supported by a given network device.

   It is expected that all network devices will have a limit on how many
   levels of lookup can be performed, and not all hardware will be able
   to support all kinds of nexthops.  RIB capability negotiation becomes
   very important for this reason, and a RIB data model MUST specify a
   way for a RIB client to learn about the network device's
   capabilities.





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2.4.2.1.  Nexthop List Attributes

   For nexthops that are of the form of a list(s), attributes can be
   associated with each member of the list to indicate the role of an
   individual member of the list.  Two attributes are specified:

   o  NEXTHOP_PREFERENCE: This is used for protection schemes.  It is an
      integer value between 1 and 99.  A lower value indicates higher
      preference.  To download a primary/standby pair to the FIB, the
      nexthops that are resolved and have the two highest preferences
      are selected.  Each <NEXTHOP_PREFERENCE> should have a unique
      value within a <nexthop-protection> (see Section 6).

   o  NEXTHOP_LB_WEIGHT: This is used for load-balancing.  Each list
      member MUST be assigned a weight between 1 and 99.  The weight
      determines the proportion of traffic to be sent over a nexthop
      used for forwarding as a ratio of the weight of this nexthop
      divided by the weights of all the nexthops of this route that are
      used for forwarding.  To perform equal load-balancing, one MAY
      specify a weight of "0" for all the member nexthops.  The value
      "0" is reserved for equal load-balancing and, if applied, MUST be
      applied to all member nexthops.  Note that a weight of 0 is
      special because of historical reasons.

2.4.3.  Nexthop Indirection

   Nexthops can be identified by an identifier to create a level of
   indirection.  The identifier is set by the RIB manager and returned
   to the RIB client on request.

   One example of usage of indirection is a nexthop that points to
   another network device (e.g., a BGP peer).  The returned nexthop
   identifier can then be used for programming routes to point to the
   this nexthop.  Given that the RIB manager has created an indirection
   using the nexthop identifier, if the transport path to the network
   device (BGP peer) changes, that change in path will be seamless to
   the RIB client and all routes that point to that network device will
   automatically start going over the new transport path.  Nexthop
   indirection using identifiers could be applied to not only unicast
   nexthops but also nexthops that contain chains and nested nexthops.
   See Section 2.4.2 for examples.










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3.  Reading from the RIB

   A RIB data model MUST allow a RIB client to read entries for RIBs
   created by that entity.  The network device administrator MAY allow
   reading of other RIBs by a RIB client through access lists on the
   network device.  The details of access lists are outside the scope of
   this document.

   The data model MUST support a full read of the RIB and subsequent
   incremental reads of changes to the RIB.  When sending data to a RIB
   client, the RIB manager SHOULD try to send all dependencies of an
   object prior to sending that object.

4.  Writing to the RIB

   A RIB data model MUST allow a RIB client to write entries for RIBs
   created by that entity.  The network device administrator MAY allow
   writes to other RIBs by a RIB client through access lists on the
   network device.  The details of access lists are outside the scope of
   this document.

   When writing an object to a RIB, the RIB client SHOULD try to write
   all dependencies of the object prior to sending that object.  The
   data model SHOULD support requesting identifiers for nexthops and
   collecting the identifiers back in the response.

   Route programming in the RIB MUST result in a return code that
   contains the following attributes:

   o  Installed: Yes/No (indicates whether the route got installed in
      the FIB)

   o  Active: Yes/No (indicates whether a route is fully resolved and is
      a candidate for selection)

   o  Reason: E.g., "Not authorized"

   The data model MUST specify which objects can be modified.  An object
   that can be modified is one whose contents can be changed without
   having to change objects that depend on it and without affecting any
   data forwarding.  To change a non-modifiable object, one will need to
   create a new object and delete the old one.  For example, routes that
   use a nexthop that is identified by a nexthop identifier should be
   unaffected when the contents of that nexthop changes.







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5.  Notifications

   Asynchronous notifications are sent by the network device's RIB
   manager to a RIB client when some event occurs on the network device.
   A RIB data model MUST support sending asynchronous notifications.  A
   brief list of suggested notifications is as below:

   o  Route change notification (with a return code as specified in
      Section 4)

   o  Nexthop resolution status (resolved/unresolved) notification

6.  RIB Grammar

   This section specifies the RIB information model in Routing Backus-
   Naur Form (rBNF) [RFC5511].  This grammar is intended to help the
   reader better understand Section 2 in order to derive a data model.

 <routing-instance> ::= <INSTANCE_NAME>
                        [<interface-list>] <rib-list>
                        [<ROUTER_ID>]


 <interface-list> ::= (<INTERFACE_IDENTIFIER> ...)


 <rib-list> ::= (<rib> ...)
 <rib> ::= <rib-name> <address-family>
                     [<route> ... ]
                     [ENABLE_IP_RPF_CHECK]
 <address-family> ::= <IPV4_ADDRESS_FAMILY> | <IPV6_ADDRESS_FAMILY> |
                      <MPLS_ADDRESS_FAMILY> | <IEEE_MAC_ADDRESS_FAMILY>


 <route> ::= <match> <nexthop>
             [<route-attributes>]
             [<route-vendor-attributes>]


 <match> ::= <IPV4> <ipv4-route> | <IPV6> <ipv6-route> |
             <MPLS> <MPLS_LABEL> | <IEEE_MAC> <MAC_ADDRESS> |
             <INTERFACE> <INTERFACE_IDENTIFIER>
 <route-type> ::= <IPV4> | <IPV6> | <MPLS> | <IEEE_MAC> | <INTERFACE>








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 <ipv4-route> ::= <ip-route-type>
                  (<destination-ipv4-address> | <source-ipv4-address> |
                   (<destination-ipv4-address> <source-ipv4-address>))
 <destination-ipv4-address> ::= <ipv4-prefix>
 <source-ipv4-address> ::= <ipv4-prefix>
 <ipv4-prefix> ::= <IPV4_ADDRESS> <IPV4_PREFIX_LENGTH>


 <ipv6-route> ::= <ip-route-type>
                  (<destination-ipv6-address> | <source-ipv6-address> |
                   (<destination-ipv6-address> <source-ipv6-address>))
 <destination-ipv6-address> ::= <ipv6-prefix>
 <source-ipv6-address> ::= <ipv6-prefix>
 <ipv6-prefix> ::= <IPV6_ADDRESS> <IPV6_PREFIX_LENGTH>
 <ip-route-type> ::= <SRC> | <DEST> | <DEST_SRC>

 <route-attributes> ::= <ROUTE_PREFERENCE> [<LOCAL_ONLY>]
                        [<address-family-route-attributes>]

 <address-family-route-attributes> ::= <ip-route-attributes> |
                                       <mpls-route-attributes> |
                                       <ethernet-route-attributes>
 <ip-route-attributes> ::= <>
 <mpls-route-attributes> ::= <>
 <ethernet-route-attributes> ::= <>
 <route-vendor-attributes> ::= <>


 <nexthop> ::= <nexthop-base> |
               (<NEXTHOP_LOAD_BALANCE> <nexthop-lb>) |
               (<NEXTHOP_PROTECTION> <nexthop-protection>) |
               (<NEXTHOP_REPLICATE> <nexthop-replicate>) |
               <nexthop-chain>

 <nexthop-base> ::= <NEXTHOP_ID> |
                    <nexthop-special> |
                    <egress-interface> |
                    <ipv4-address> | <ipv6-address> |
                    (<egress-interface>
                        (<ipv4-address> | <ipv6-address>)) |
                    (<egress-interface> <IEEE_MAC_ADDRESS>) |
                    <tunnel-encapsulation> | <tunnel-decapsulation> |
                    <logical-tunnel> |
                    <rib-name>

 <egress-interface> ::= <INTERFACE_IDENTIFIER>





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 <nexthop-special> ::= <DISCARD> | <DISCARD_WITH_ERROR> |
                       (<RECEIVE> [<COS_VALUE>])


 <nexthop-lb> ::= <NEXTHOP_LB_WEIGHT> <nexthop>
                  (<NEXTHOP_LB_WEIGHT> <nexthop) ...


 <nexthop-protection> = <NEXTHOP_PREFERENCE> <nexthop>
                       (<NEXTHOP_PREFERENCE> <nexthop>)...

 <nexthop-replicate> ::= <nexthop> <nexthop> ...


 <nexthop-chain> ::= <nexthop> ...


 <logical-tunnel> ::= <tunnel-type> <TUNNEL_NAME>
 <tunnel-type> ::= <IPV4> | <IPV6> | <MPLS> | <GRE> | <VxLAN> | <NVGRE>

 <tunnel-encapsulation> ::= (<IPV4> <ipv4-header>) |
                            (<IPV6> <ipv6-header>) |
                            (<MPLS> <mpls-header>) |
                            (<GRE> <gre-header>) |
                            (<VXLAN> <vxlan-header>) |
                            (<NVGRE> <nvgre-header>)

 <ipv4-header> ::= <SOURCE_IPv4_ADDRESS> <DESTINATION_IPv4_ADDRESS>
                   <PROTOCOL> [<TTL>] [<DSCP>]

 <ipv6-header> ::= <SOURCE_IPV6_ADDRESS> <DESTINATION_IPV6_ADDRESS>
                   <NEXT_HEADER> [<TRAFFIC_CLASS>]
                   [<FLOW_LABEL>] [<HOP_LIMIT>]

 <mpls-header> ::= (<mpls-label-operation> ...)
 <mpls-label-operation> ::= (<MPLS_PUSH> <MPLS_LABEL> [<S_BIT>]
                                         [<TOS_VALUE>] [<TTL_VALUE>]) |
                            (<MPLS_SWAP> <IN_LABEL> <OUT_LABEL>
                                        [<TTL_ACTION>])

 <gre-header> ::= <GRE_IP_DESTINATION> <GRE_PROTOCOL_TYPE> [<GRE_KEY>]
 <vxlan-header> ::= (<ipv4-header> | <ipv6-header>)
                    [<VXLAN_IDENTIFIER>]
 <nvgre-header> ::= (<ipv4-header> | <ipv6-header>)
                    <VIRTUAL_SUBNET_ID>
                    [<FLOW_ID>]





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 <tunnel-decapsulation> ::= ((<IPV4> <IPV4_DECAP> [<TTL_ACTION>]) |
                            (<IPV6> <IPV6_DECAP> [<HOP_LIMIT_ACTION>]) |
                            (<MPLS> <MPLS_POP> [<TTL_ACTION>]))

                        Figure 5: RIB rBNF Grammar

6.1.  Nexthop Grammar Explained

   A nexthop is used to specify the next network element to forward the
   traffic to.  It is also used to specify how the traffic should be
   load-balanced, protected using preference, or multicast using
   replication.  This is explicitly specified in the grammar.  The
   nexthop has recursion built in to address complex use cases like the
   one defined in Section 7.2.6.

7.  Using the RIB Grammar

   The RIB grammar is very generic and covers a variety of features.
   This section provides examples on using objects in the RIB grammar
   and examples to program certain use cases.

7.1.  Using Route Preference

   Using route preference, a client can preinstall alternate paths in
   the network.  For example, if OSPF has a route preference of 10, then
   another client can install a route with a route preference of 20 to
   the same destination.  The OSPF route will get precedence and will
   get installed in the FIB.  When the OSPF route is withdrawn, the
   alternate path will get installed in the FIB.

   Route preference can also be used to prevent denial-of-service
   attacks by installing routes with the best preference, which either
   drops the offending traffic or routes it to some monitoring/analysis
   station.  Since the routes are installed with the best preference,
   they will supersede any route installed by any other protocol.

7.2.  Using Different Nexthop Types

   The RIB grammar allows one to create a variety of nexthops.  This
   section describes uses for certain types of nexthops.











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7.2.1.  Tunnel Nexthops

   A tunnel nexthop points to a tunnel of some kind.  Traffic that goes
   over the tunnel gets encapsulated with the tunnel-encapsulation.
   Tunnel nexthops are useful for abstracting out details of the network
   by having the traffic seamlessly route between network edges.  At the
   end of a tunnel, the tunnel will get decapsulated.  Thus, the grammar
   supports two kinds of operations: one for encapsulation and another
   for decapsulation.

7.2.2.  Replication Lists

   One can create a replication list for replicating traffic to multiple
   destinations.  The destinations, in turn, could be derived nexthops
   in themselves (at a level supported by the network device); point to
   multipoint and broadcast are examples that involve replication.

   A replication list (at the simplest level) can be represented as:

   <nexthop> ::= <NEXTHOP_REPLICATE> <nexthop> [ <nexthop> ... ]

   The above can be derived from the grammar as follows:

   <nexthop> ::= <nexthop-replicate>
   <nexthop> ::= <NEXTHOP_REPLICATE> <nexthop> <nexthop> ...

7.2.3.  Weighted Lists

   A weighted list is used to load-balance traffic among a set of
   nexthops.  From a modeling perspective, a weighted list is very
   similar to a replication list, with the difference that each member
   nexthop MUST have a NEXTHOP_LB_WEIGHT associated with it.

   A weighted list (at the simplest level) can be represented as:

   <nexthop> ::= <NEXTHOP_LOAD_BALANCE> (<nexthop> <NEXTHOP_LB_WEIGHT>)
                      [(<nexthop> <NEXTHOP_LB_WEIGHT>)... ]

   The above can be derived from the grammar as follows:

   <nexthop> ::= <nexthop-lb>
   <nexthop> ::= <NEXTHOP_LOAD_BALANCE>
                   <NEXTHOP_LB_WEIGHT> <nexthop>
                   (<NEXTHOP_LB_WEIGHT> <nexthop>) ...
   <nexthop> ::= <NEXTHOP_LOAD_BALANCE> (<NEXTHOP_LB_WEIGHT> <nexthop>)
                   (<NEXTHOP_LB_WEIGHT> <nexthop>) ...





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7.2.4.  Protection

   A primary/backup protection can be represented as:

   <nexthop> ::= <NEXTHOP_PROTECTION> <1> <interface-primary>
                                      <2> <interface-backup>)

   The above can be derived from the grammar as follows:

<nexthop> ::= <nexthop-protection>
<nexthop> ::= <NEXTHOP_PROTECTION> (<NEXTHOP_PREFERENCE> <nexthop>
                      (<NEXTHOP_PREFERENCE> <nexthop>)...)
<nexthop> ::= <NEXTHOP_PROTECTION> (<NEXTHOP_PREFERENCE> <nexthop>
                      (<NEXTHOP_PREFERENCE> <nexthop>))
<nexthop> ::= <NEXTHOP_PROTECTION> ((<NEXTHOP_PREFERENCE> <nexthop-base>
                      (<NEXTHOP_PREFERENCE> <nexthop-base>))
<nexthop> ::= <NEXTHOP_PROTECTION> (<1> <interface-primary>
                      (<2> <interface-backup>))

   Traffic can be load-balanced among multiple primary nexthops and a
   single backup.  In such a case, the nexthop will look like:

   <nexthop> ::= <NEXTHOP_PROTECTION> (<1>
                 (<NEXTHOP_LOAD_BALANCE>
                  (<NEXTHOP_LB_WEIGHT> <nexthop-base>
                  (<NEXTHOP_LB_WEIGHT> <nexthop-base>) ...))
                   <2> <nexthop-base>)

   A backup can also have another backup.  In such a case, the list will
   look like:

   <nexthop> ::= <NEXTHOP_PROTECTION> (<1> <nexthop>
                 <2> <NEXTHOP_PROTECTION>(<1> <nexthop> <2> <nexthop>))

7.2.5.  Nexthop Chains

   A nexthop chain is a way to perform multiple operations on a packet
   by logically combining them.  For example, when a VPN packet comes on
   the WAN interface and has to be forwarded to the correct VPN
   interface, one needs to pop the VPN label before sending the packet
   out.  Using a nexthop chain, one can chain together "pop MPLS header"
   and "send it out a specific egress-interface".









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   The above example can be derived from the grammar as follows:

   <nexthop-chain> ::= <nexthop> <nexthop>
   <nexthop-chain> ::= <nexthop-base> <nexthop-base>
   <nexthop-chain> ::= <tunnel-decapsulation> <egress-interface>
   <nexthop-chain> ::= (<MPLS> <MPLS_POP>) <interface-outgoing>

   Elements in a nexthop chain are evaluated left to right.

   A nexthop chain can also be used to put one or more headers on an
   outgoing packet.  One example is a pseudowire, which is MPLS over
   some transport (MPLS or GRE, for instance).  Another example is
   Virtual eXtensible Local Area Network (VXLAN) over IP.  A nexthop
   chain thus allows a RIB client to break up the programming of the
   nexthop into independent pieces (one per encapsulation).

   A simple example of MPLS over GRE can be represented as follows:

   <nexthop-chain> ::= (<MPLS> <mpls-header>) (<GRE> <gre-header>)
                       <interface-outgoing>

   The above can be derived from the grammar as follows:

   <nexthop-chain> ::= <nexthop> <nexthop> <nexthop>
   <nexthop-chain> ::= <nexthop-base> <nexthop-base> <nexthop-base>
   <nexthop-chain> ::= <tunnel-encapsulation> <tunnel-encapsulation>
                       <egress-interface>
   <nexthop-chain> ::= (<MPLS> <mpls-header>) (<GRE> <gre-header>)
                       <interface-outgoing>

7.2.6.  Lists of Lists

   Lists of lists is a derived construct.  One example of usage of such
   a construct is to replicate traffic to multiple destinations with
   load-balancing.  In other words, for each branch of the replication
   tree, there are multiple interfaces on which traffic needs to be
   load-balanced.  So, the outer list is a replication list for
   multicast and the inner lists are weighted lists for load-balancing.
   Let's take an example of a network element that has to replicate
   traffic to two other network elements.  Traffic to the first network
   element should be load-balanced equally over two interfaces:
   outgoing-1-1 and outgoing-1-2.  Traffic to the second network element
   should be load-balanced over three interfaces: outgoing-2-1,
   outgoing-2-2, and outgoing-2-3 (in the ratio 20:20:60).







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   This can be derived from the grammar as follows:

<nexthop> ::= <nexthop-replicate>
<nexthop> ::= <NEXTHOP_REPLICATE> (<nexthop> <nexthop>...)
<nexthop> ::= <NEXTHOP_REPLICATE> (<nexthop> <nexthop>)
<nexthop> ::= <NEXTHOP_REPLICATE> ((<NEXTHOP_LOAD_BALANCE> <nexthop-lb>)
              (<NEXTHOP_LOAD_BALANCE> <nexthop-lb>))
<nexthop> ::= <NEXTHOP_REPLICATE> ((<NEXTHOP_LOAD_BALANCE>
              (<NEXTHOP_LB_WEIGHT> <nexthop>
              (<NEXTHOP_LB_WEIGHT> <nexthop>) ...))
               ((<NEXTHOP_LOAD_BALANCE>
                (<NEXTHOP_LB_WEIGHT> <nexthop>
                (<NEXTHOP_LB_WEIGHT> <nexthop>) ...))
<nexthop> ::= <NEXTHOP_REPLICATE> ((<NEXTHOP_LOAD_BALANCE>
              (<NEXTHOP_LB_WEIGHT> <nexthop>
               (<NEXTHOP_LB_WEIGHT> <nexthop>)))
                ((<NEXTHOP_LOAD_BALANCE>
                (<NEXTHOP_LB_WEIGHT> <nexthop>
                (<NEXTHOP_LB_WEIGHT> <nexthop>)
                (<NEXTHOP_LB_WEIGHT> <nexthop>)))
<nexthop> ::= <NEXTHOP_REPLICATE> ((<NEXTHOP_LOAD_BALANCE>
               (<NEXTHOP_LB_WEIGHT> <nexthop>)
               (<NEXTHOP_LB_WEIGHT> <nexthop>)))
               ((<NEXTHOP_LOAD_BALANCE>
               (<NEXTHOP_LB_WEIGHT> <nexthop>)
               (<NEXTHOP_LB_WEIGHT> <nexthop>)
               (<NEXTHOP_LB_WEIGHT> <nexthop>)))
<nexthop> ::= <NEXTHOP_REPLICATE>
               ((<NEXTHOP_LOAD_BALANCE>
                 (50 <outgoing-1-1>)
                 (50 <outgoing-1-2>)))
                ((<NEXTHOP_LOAD_BALANCE>
                  (20 <outgoing-2-1>)
                  (20 <outgoing-2-2>)
                  (60 <outgoing-2-3>)))

7.3.  Performing Multicast

   IP multicast involves matching a packet on (S,G) or (*,G), where both
   S (Source) and G (Group) are IP prefixes.  Following the match, the
   packet is replicated to one or more recipients.  How the recipients
   subscribe to the multicast group is outside the scope of this
   document.

   In PIM-based multicast, the packets are IP forwarded on an IP
   multicast tree.  The downstream nodes on each point in the multicast
   tree are one or more IP addresses.  These can be represented as a
   replication list (see Section 7.2.2).



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   In MPLS-based multicast, the packets are forwarded on a Point-to-
   Multipoint (P2MP) LSP.  The nexthop for a P2MP LSP can be represented
   in the nexthop grammar as a <logical-tunnel> (P2MP LSP identifier) or
   a replication list (see Section 7.2.2) of <tunnel-encapsulation>,
   with each tunnel-encapsulation representing a single MPLS downstream
   nexthop.

8.  RIB Operations at Scale

   This section discusses the scale requirements for a RIB data model.
   The RIB data model should be able to handle a large scale of
   operations to enable deployment of RIB applications in large
   networks.

8.1.  RIB Reads

   Bulking (grouping of multiple objects in a single message) MUST be
   supported when a network device sends RIB data to a RIB client.
   Similarly, the data model MUST enable a RIB client to request data in
   bulk from a network device.

8.2.  RIB Writes

   Bulking (grouping of multiple write operations in a single message)
   MUST be supported when a RIB client wants to write to the RIB.  The
   response from the network device MUST include a return-code for each
   write operation in the bulk message.

8.3.  RIB Events and Notifications

   There can be cases where a single network event results in multiple
   events and/or notifications from the network device to a RIB client.
   On the other hand, due to timing of multiple things happening at the
   same time, a network device might have to send multiple events and/or
   notifications to a RIB client.  The network-device-originated event/
   notification message MUST support the bulking of multiple events and
   notifications in a single message.

9.  Security Considerations

   The information model specified in this document defines a schema for
   data models that are designed to be accessed via network management
   protocols such as NETCONF [RFC6241] or RESTCONF [RFC8040].  The
   lowest NETCONF layer is the secure transport layer, and the
   mandatory-to-implement secure transport is Secure Shell (SSH)
   [RFC6242].  The lowest RESTCONF layer is HTTPS, and the mandatory-to-
   implement secure transport is TLS [RFC8446].




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   The NETCONF access control model [RFC8341] provides the means to
   restrict access for particular NETCONF or RESTCONF users to a
   preconfigured subset of all available NETCONF or RESTCONF protocol
   operations and content.

   The RIB information model specifies read and write operations to
   network devices.  These network devices might be considered sensitive
   or vulnerable in some network environments.  Write operations to
   these network devices without proper protection can have a negative
   effect on network operations.  Due to this factor, it is recommended
   that data models also consider the following in their design:

   o  Require utilization of the authentication and authorization
      features of the NETCONF or RESTCONF suite of protocols.

   o  Augment the limits on how much data can be written or updated by a
      remote entity built to include enough protection for a RIB data
      model.

   o  Expose the specific RIB data model implemented via NETCONF/
      RESTCONF data models.

10.  IANA Considerations

   This document has no IANA actions.

11.  References

11.1.  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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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,
              <https://www.rfc-editor.org/info/rfc6241>.

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

   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
              <https://www.rfc-editor.org/info/rfc8040>.




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   [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>.

   [RFC8341]  Bierman, A. and M. Bjorklund, "Network Configuration
              Access Control Model", STD 91, RFC 8341,
              DOI 10.17487/RFC8341, March 2018,
              <https://www.rfc-editor.org/info/rfc8341>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

11.2.  Informative References

   [RFC4915]  Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
              Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
              RFC 4915, DOI 10.17487/RFC4915, June 2007,
              <https://www.rfc-editor.org/info/rfc4915>.

   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", RFC 5120,
              DOI 10.17487/RFC5120, February 2008,
              <https://www.rfc-editor.org/info/rfc5120>.

   [RFC5511]  Farrel, A., "Routing Backus-Naur Form (RBNF): A Syntax
              Used to Form Encoding Rules in Various Routing Protocol
              Specifications", RFC 5511, DOI 10.17487/RFC5511, April
              2009, <https://www.rfc-editor.org/info/rfc5511>.

   [RFC7920]  Atlas, A., Ed., Nadeau, T., Ed., and D. Ward, "Problem
              Statement for the Interface to the Routing System",
              RFC 7920, DOI 10.17487/RFC7920, June 2016,
              <https://www.rfc-editor.org/info/rfc7920>.

   [RFC8431]  Wang, L., Chen, M., Dass, A., Ananthakrishnan, H., Kini,
              S., and N. Bahadur, "A YANG Data Model for the Routing
              Information Base (RIB)", RFC 8431, DOI 10.17487/RFC8431,
              September 2018, <http://www.rfc-editor.org/info/rfc8431>.











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Acknowledgements

   The authors would like to thank Ron Folkes, Jeffrey Zhang, the WG
   co-Chairs, and reviewers for their comments and suggestions on this
   document.  The following people contributed to the design of the RIB
   information model as part of the I2RS Interim meeting in April 2013:

   Wes George, Chris Liljenstolpe, Jeff Tantsura, Susan Hares, and
   Fabian Schneider.

Authors' Addresses

   Nitin Bahadur (editor)
   Uber
   900 Arastradero Rd
   Palo Alto, CA  94304
   United States of America

   Email: nitin_bahadur@yahoo.com


   Sriganesh Kini (editor)

   Email: sriganeshkini@gmail.com


   Jan Medved
   Cisco

   Email: jmedved@cisco.com





















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