Internet DRAFT - draft-amante-irs-topology-use-cases


Internet Engineering Task Force                                J. Medved
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Informational                                 T. Nadeau
Expires: February 17, 2014                              Juniper Networks
                                                               S. Amante

                                                         August 16, 2013

                         Topology API Use Cases


   This document describes use cases for gathering routing, forwarding
   and policy information, (hereafter referred to as topology
   information), about the network.  It describes several applications
   that need to view the topology of the underlying physical or logical
   network.  This document further demonstrates a need for a "Topology
   Manager" and related functions that collects topology data from
   network elements and other data sources, coalesces the collected data
   into a coherent view of the overall network topology, and normalizes
   the network topology view for use by clients -- namely, applications
   that consume topology information.

Status of This Memo

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Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   ( in effect on the date of
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Statistics Collection . . . . . . . . . . . . . . . . . .   4
     1.2.  Inventory Collection  . . . . . . . . . . . . . . . . . .   5
     1.3.  Requirements Language . . . . . . . . . . . . . . . . . .   5
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  The Orchestration, Collection & Presentation Framework  . . .   7
     3.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  The Topology Manager  . . . . . . . . . . . . . . . . . .   8
     3.3.  The Policy Manager  . . . . . . . . . . . . . . . . . . .  10
     3.4.  Orchestration Manager . . . . . . . . . . . . . . . . . .  10
   4.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.1.  Virtualized Views of the Network  . . . . . . . . . . . .  11
       4.1.1.  Capacity Planning and Traffic Engineering . . . . . .  11  Present Mode of Operation . . . . . . . . . . . .  12  Proposed Mode of Operation  . . . . . . . . . . .  12
       4.1.2.  Services Provisioning . . . . . . . . . . . . . . . .  14
       4.1.3.  Troubleshooting & Monitoring  . . . . . . . . . . . .  14
     4.2.  Path Computation Element (PCE)  . . . . . . . . . . . . .  15
     4.3.  ALTO Server . . . . . . . . . . . . . . . . . . . . . . .  16
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  18
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   In today's networks, a variety of applications, such as Traffic
   Engineering, Capacity Planning, Security Auditing or Services
   Provisioning (for example, Virtual Private Networks), have a common
   need to acquire and consume network topology information.
   Unfortunately, all of these applications are (typically) vertically
   integrated: each uses its own proprietary normalized view of the
   network and proprietary data collectors, interpreters and adapters,

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   which speak a variety of protocols, (SNMP, CLI, SQL, etc.) directly
   to network elements and to back-office systems.  While some of the
   topological information can be distributed using routing protocols,
   unfortunately it is not desirable for some of these applications to
   understand or participate in routing protocols.

   This approach is incredibly inefficient for several reasons.  First,
   developers must write duplicate 'network discovery' functions, which
   then become challenging to maintain over time, particularly as/when
   new equipment are first introduced to the network.  Second, since
   there is no common "vocabulary" to describe various components in the
   network, such as physical links, logical links, or IP prefixes, each
   application has its own data model.  To solve this, some solutions
   have distributed this information in the normalized form of routing
   distribution.  However, this information still does not contain
   "inactive" topological information, thus not containing information
   considered to be part of a network's inventory.

   These limitations lead to applications being unable to easily
   exchange information with each other.  For example, applications
   cannot share changes with each other that are (to be) applied to the
   physical and/or logical network, such as installation of new physical
   links, or deployment of security ACL's. Each application must
   frequently poll network elements and other data sources to ensure
   that it has a consistent representation of the network so that it can
   carry out its particular domain-specific tasks.  In other cases,
   applications that cannot speak routing protocols must use proprietary
   CLI or other management interfaces which represent the topological
   information in non-standard formats or worse, semantic models.

   Overall, the software architecture described above at best results in
   inefficient use of both software developer resources and network
   resources, and at worst, it results in some applications simply not
   having access to this information.

   Figure 1 is an illustration of how individual applications collect
   data from the underlying network.  Applications retrieve inventory,
   network topology, state and statistics information by communicating
   directly with underlying Network Elements as well as with
   intermediary proxies of the information.  In addition, applications
   transmit changes required of a Network Element's configuration and/or
   state directly to individual Network Elements, (most commonly using
   CLI or Netconf).  It is important to note that the "data models" or
   semantics of this information contained within Network Elements are
   largely proprietary with respect to most configuration and state
   information, hence why a proprietary CLI is often the only choice to
   reflect changes in a NE's configuration or state.  This remains the
   case even when standards-based mechanisms such as Netconf are used

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   which provide a standard syntax model, but still often lack due to
   the proprietary semantics associated with the internal representation
   of the information.

                           +----------------+ |
                           |  Applications  |-+
                                 ^  ^  ^
             SQL, RPC, ReST      #  |  *  SQL, RPC, ReST ...
        ##########################  |  **********************
           #                        |                       *
     +------------+                 |                 +------------+
     | Statistics |                 |                 | Inventory  |
     | Collection |                 |                 | Collection |
     +------------+                 |                 +------------+
           ^                        | NETCONF, I2RS, SNMP,  ^
           |                        | CLI, TL1, ...         |
           |                        |                       |
           |                        |                       |
   +---------------+        +---------------+       +---------------+
   |Network Element|        |Network Element|       |Network Element|
   | +-----------+ |        | +-----------+ |       | +-----------+ |
   | |Information| |<-LLDP->| |Information| |<-LMP->| |Information| |
   | |   Model   | |        | |   Model   | |       | |   Model   | |
   | +-----------+ |        | +-----------+ |       | +-----------+ |
   +---------------+        +---------------+       +---------------+

               Figure 1: Applications getting topology data

   Figure 1 shows how current management interfaces such as NETCONF,
   SNMP, CLI, etc. are used to transmit or receive information to/from
   various Network Elements.  The figure also shows that protocols such
   as LLDP and LMP participate in topology discovery, specifically to
   discover adjacent network elements.

   The following sections describe the "Statistics Collection" and
   "Inventory Collection" functions.

1.1.  Statistics Collection

   In Figure 1, "Statistics Collection" is a dedicated infrastructure
   that collects statistics from Network Elements.  It periodically (for
   example, every 5-minutes) polls Network Elements for octets
   transferred per interface, per LSP, etc.  Collected statistics are
   stored and collated within a statistics data warehouse.  Applications
   typically query the statistics data warehouse rather than poll

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   Network Elements directly to get the appropriate set of link
   utilization figures for their analysis.

1.2.  Inventory Collection

   "Inventory Collection" is a network function responsible for
   collecting component and state information directly from Network
   Elements, as well as for storing inventory information about physical
   network assets that are not retrievable from Network Elements.  The
   collected data is hereafter referred to as the 'Inventory Asset
   Database.  Examples of information collected from Network Elements
   are: interface up/down status, the type of SFP/XFP optics inserted
   into physical port, etc.

   The Inventory Collection function may use SNMP and CLI to acquire
   inventory information from Network Elements.  The information housed
   in the Inventory Manager is retrieved by applications via a variety
   of protocols: SQL, RPC, REST etc.  Inventory information, retrieved
   from Network Elements, is periodically updated in the Inventory
   Collection system to reflect changes in the physical and/or logical
   network assets.  The polling interval to retrieve updated information
   is varied depending on scaling constraints of the Inventory
   Collection systems and expected intervals at which changes to the
   physical and/or logical assets are expected to occur.

   Examples of changes in network inventory that need be learned by the
   Inventory Collection function are as follows:

   o  Discovery of new Network Elements.  These elements may or may not
      be actively used in the network (i.e.: provisioned but not yet

   o  Insertion or removal of line cards or other modules, such as
      optics modules, during service or equipment provisioning.

   o  Changes made to a specific Network Element through a management
      interface by a field technician.

   o  Indication of an NE's physical location and associated cable run
      list, at the time of installation.

   o  Insertion of removal of cables that result in dynamic discovery of
      a new or lost adjacent neighbor, etc.

1.3.  Requirements Language

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   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119]

2.  Terminology

   The following briefly defines some of the terminology used within
   this document.

   Inventory Manager  is a function that collects Network Element
      inventory and state information directly from Network Elements and
      from associated offline inventory databases.  Inventory
      information may only be visible at a specific network layer; for
      example, a physical link is visible within the IGP, but a Layer-2
      switch through which the physical link traverses is unknown to the
      Layer-3 IGP.

   Policy Manager  is a function that attaches metadata to network
      components/attributes.  Such metadata may include security,
      routing, L2 VLAN ID, IP numbering, etc. policies, which enable the
      Topology Manager to:

      *  Assemble a normalized view of the network for clients (or
         upper-layer applications

      *  Allow clients (or upper-layer applications) access to
         information collected from various network layers and/or
         network components, etc.

      The Policy Manager function may be a sub-component of the Topology
      Manager or it may be a standalone function.

   Topology Manager  is a function that collects topological information
      from a variety of sources in the network and provides a normalized
      view of the network topology to clients and/or higher-layer

   Orchestration Manager  is a function that stitches together
      resources, such as compute or storage, and/or services with the
      network or vice-versa.  To realize a complete service, the
      Orchestration Manager relies on capabilities provided by the other
      "Managers" listed above.

   Normalized Topology Information Model  is an open, standards-based
      information model of the network topology.

   Information Model Abstraction:  The notion that one is able to
      represent the same set of elements in an information model at

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      different levels of "focus" in order to limit the amount of
      information exchanged in order to convey this information.

   Multi-Layer Topology:  Topology is commonly referred to using the OSI
      protocol layering model.  For example, Layer 3 represents routed
      topologies that typically use IPv4 or IPv6 addresses.  It is
      envisioned that, eventually, multiple layers of the network may be
      represented in a single, normalized view of the network to certain
      applications, (i.e.: Capacity Planning, Traffic Engineering, etc.)

   Network Element (NE)  refers to a network device that typically is
      addressable (but not always), or a host.  It is sometimes referred
      to as a 'Node'.

   Links:  Every NE contains at least 1 link.  These are used to connect
      the NE to other NEs in the network.  Links may be in a variety of
      states, such as up, down, administratively down, internally
      testing, or dormant.  Links are often synonymous with network
      ports on NEs.

3.  The Orchestration, Collection & Presentation Framework

3.1.  Overview

   Section 1 demonstrates the need for a network function that would
   provide a common, standards-based topology view to applications.
   Such topology collection/management/presentation function would be a
   part of a wider framework that should also include policy management
   and orchestration.  The framework is shown in Figure 2.

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                            +----------------+ |
                            |  Applications  |-+
                                    ^   Websockets, ReST, XMPP...
           |                        |                         |
     +------------+     +------------------------+     +-------------+
     |   Policy   |<----|    Topology Manager    |---->|Orchestration|
     |   Manager  |     | +--------------------+ |     |   Manager   |
     +------------+     | |Topology Information| |     +-------------+
                        | |       Model        | |
                        | +--------------------+ |
                                  ^ ^ ^
       Websockets, ReST, XMPP     # | *  Websockets, ReST, XMPP
            ####################### | ************************
            #                       |                        *
     +------------+                 |                  +------------+
     | Statistics |                 |                  | Inventory  |
     | Collection |                 |                  | Collection |
     +------------+                 |                  +------------+
           ^                        | I2RS, NETCONF, SNMP,   ^
           |                        | TL1 ...                |
           |                        |                        |
   +---------------+        +---------------+        +---------------+
   |Network Element|        |Network Element|        |Network Element|
   | +-----------+ |        | +-----------+ |        | +-----------+ |
   | |Information| |<-LLDP->| |Information| |<-LMP-->| |Information| |
   | |   Model   | |        | |   Model   | |        | |   Model   | |
   | +-----------+ |        | +-----------+ |        | +-----------+ |
   +---------------+        +---------------+        +---------------+

                        Figure 2: Topology Manager

   The following sections describe in detail the Topology Manager,
   Policy Manager and Orchestration Manager functions.

3.2.  The Topology Manager

   The Topology Manager is a function that collects topological
   information from a variety of sources in the network and provides a
   cohesive, abstracted view of the network topology to clients and/or
   higher-layer applications.  The topology view is based on a
   standards-based, normalized topology information model.

   Topology information sources can be:

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   o  The "live" Layer 3 IGP or an equivalent mechanism that provides
      information about links that are components of the active
      topology.  Active topology links are present in the Link State
      Database (LSDB) and are eligible for forwarding.  Layer 3 IGP
      information can be obtained by listening to IGP updates flooded
      through an IGP domain, or from Network Elements.

   o  The Inventory Collection system that provides information for
      network components not visible within the Layer 3 IGP's LSDB,
      (i.e.: links or nodes, or properties of those links or nodes, at
      lower layers of the network).

   o  The Statistics Collection system that provides traffic
      information, such as traffic demands or link utilizations.

   The Topology Manager provides topology information to Clients or
   higher-layer applications via a northbound interface, such as ReST,
   Websockets, or XMPP.

   The Topology Manager will contain topology information for multiple
   layers of the network: Transport, Ethernet and IP/MPLS, as well as
   information for multiple Layer 3 IGP areas and multiple Autonomous
   Systems (ASes).  The topology information can be used by higher-level
   applications, such as Traffic Engineering, Capacity Planning and
   Provisioning.  Such applications are typically used to design,
   augment and optimize IP/MPLS networks, and require knowledge of
   underlying Shared Risk Link Groups (SRLG) within the Transport and/or
   Ethernet layers of the network.

   The Topology Manager must be able to discover Network Elements that
   are not visible in the "live" L3 IGP's Link State Database (LSDB).
   Such Network Elements can either be inactive, or active but invisible
   in the L3 LSDB (e.g.: L2 Ethernet switches, ROADM's, or Network
   Elements that are in an underlying transport network).

   In addition static inventory information collected from the Inventory
   Manager, the Topology Manager will also collect dynamic inventory
   information.  For example, Network Elements utilize various Link
   Layer Discovery Protocols (i.e.: LLDP, LMP, etc.) to automatically
   identify adjacent nodes and ports.  This information can be pushed to
   or pulled by the Topology Manager in order to create an accurate
   representation of the physical topology of the network

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3.3.  The Policy Manager

   The Policy Manager is the function used to enforce and program
   policies applicable to network component/attribute data.  Policy
   enforcement is a network-wide function that can be consumed by
   various Network Elements and services, including the Inventory
   Manager, the Topology Manager and other Network Elements.  Such
   policies are likely to encompass the following:

   o  Logical Identifier Numbering Policies

      *  Correlation of IP prefix to link based on link type, such as
         P-P, P-PE, or PE-CE.

      *  Correlation of IP Prefix to IGP Area

      *  Layer-2 VLAN ID assignments, etc.

   o  Routing Configuration Policies

      *  OSPF Area or IS-IS Net-ID to Node (Type) Correlation

      *  BGP routing policies, such as nodes designated for injection of
         aggregate routes, max-prefix policies, or AFI/SAFI to node

   o  Security Policies

      *  Access Control Lists

      *  Rate-limiting

   o  Network Component/Attribute Data Access Policies.  Client's
      (upper-layer application) access to Network Components/Attributes
      contained in the "Inventory Manager" as well as Policies contained
      within the "Policy Manager" itself.

   The Policy Manager function may be either a sub-component of the
   Topology or Orchestration Manager or a standalone component.

3.4.  Orchestration Manager

   The Orchestration Manager provides the ability to stitch together
   resources (such as compute or storage) and/or services with the
   network or vice-versa.  Examples of 'generic' services may include
   the following:

   o  Application-specific Load Balancing

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   o  Application-specific Network (Bandwidth) Optimization

   o  Application or End-User specific Class-of-Service

   o  Application or End-User specific Network Access Control

   The above services could then enable coupling of resources with the
   network to realize the following:

   o  Network Optimization: Creation and Migration of Virtual Machines
      (VM's) so they are adjacent to storage in the same DataCenter.

   o  Network Access Control: Coupling of available (generic) compute
      nodes within the appropriate point of the data-path to perform
      firewall, NAT, etc. functions on data traffic.

   The Orchestration Manager will exchange information models with the
   Topology Manager, the Policy Manager and the Inventory Manager.  In
   addition, the Orchestration Manager must support publish and
   subscribe capabilities to those functions, as well as to Clients.

   The Orchestration Manager may receive requests from Clients
   (applications) for immediate access to specific network resources.
   However, Clients may request to schedule future appointments to
   reserve appropriate network resources when, for example, a special
   event is scheduled to start and end.

   Finally, the Orchestration Manager should have the flexibility to
   determine what network layer(s) may be able to satisfy a given
   Client's request, based on constraints received from the Client as
   well as constraints learned from the Policy and Topology Managers.
   This could allow the Orchestration Manager to, for example, satisfy a
   given service request for a given Client using the optical network
   (via OTN service) if there is insufficient IP/MPLS capacity at the
   specific moment the Client's request is received.

   The operational model is shown in the following figure.


                     Figure 3: Overall Reference Model

4.  Use Cases

4.1.  Virtualized Views of the Network

4.1.1.  Capacity Planning and Traffic Engineering

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   When performing Traffic Engineering and/or Capacity Planning of an IP
   /MPLS network, it is important to account for SRLG's that exist
   within the underlying physical, optical and Ethernet networks.
   Currently, it's quite common to take "snapshots" at infrequent
   intervals that comprise the inventory data of the underlying physical
   and optical layer networks.  This inventory data is then normalized
   to conform to data import requirements of sometimes separate Traffic
   Engineering and/or Capacity Planning tools.  This process is error-
   prone and inefficient, particularly as the underlying network
   inventory information changes due to introduction of new network
   element makes or models, line cards, capabilities, etc..

   The present mode of operation is inefficient with respect to Software
   Development, Capacity Planning and Traffic Engineering resources.
   Due to this inefficiency, the underlying physical network inventory
   information (containing SRLG and corresponding critical network
   assets information) is not updated frequently, thus exposing the
   network to, at minimum, inefficient utilization and, at worst,
   critical impairments.  Proposed Mode of Operation

   First, the Inventory Manager will extract inventory information from
   network elements and associated inventory databases.  Information
   extracted from inventory databases will include physical cross-
   connects and other information that is not available directly from
   network elements.  Standards-based information models and associated
   vocabulary will be required to represent not only components inside
   or directly connected to network elements, but also to represent
   components of a physical layer path (i.e.: cross-connect panels,
   etc.)  The inventory data will comprise the complete set of inactive
   and active network components.

   Second, the Topology Manager will augment the inventory information
   with topology information obtained from Network Elements and other
   sources, and provide an IGP-based view of the active topology of the
   network.  The Topology Manager will also include non-topology dynamic
   information from IGPs, such as Available Bandwidth, Reserved
   Bandwidth, Traffic Engineering (TE) attributes associated with links,

   Finally, the Statistics Collector will collect utilization statistics
   from Network Elements, and archive and aggregate them in a statistics
   data warehouse.  Selected statistics and other dynamic data may be
   distributed through IGP routing protocols
   ([I-D.ietf-isis-te-metric-extensions] and

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   [I-D.ietf-ospf-te-metric-extensions]) and then collected at the
   Statistics Collection Function via BGP-LS
   ([I-D.ietf-idr-ls-distribution]).  Statistics summaries then will be
   exposed in normalized information models to the Topology Manager,
   which can use them to, for example, build trended utilization models
   to forecast expected changes to physical and logical network

   It is important to recognize that extracting topology information
   from the network solely from Network Elements and IGPs (IS-IS TE or
   OSPF TE), is inadequate for this use case.  First, IGPs only expose
   the active components (e.g. vertices of the SPF tree) of the IP
   network, and are not aware of "hidden" or inactive interfaces within
   IP/MPLS network elements, such as unused line cards or ports.  IGPs
   are also not aware of components that reside at a layer lower than IP
   /MPLS, such as Ethernet switches, or Optical transport systems.
   Second, IGP's only convey SRLG information that have been first
   applied within a router's configurations, either manually or
   programatically.  As mentioned previously, this SRLG information in
   the IP/MPLS network is subject to being infrequently updated and, as
   a result, may inadequately account for critical, underlying network
   fate sharing properties that are necessary to properly design
   resilient circuits and/or paths through the network.

   Once the Topology Manager has assembled a normalized view of the
   topology and metadata associated with each component of the topology,
   it can expose this information via its northbound API to the Capacity
   Planning and Traffic Engineering applications.  The applications only
   require generalized information about nodes and links that comprise
   the network, e.g.: links used to interconnect nodes, SRLG information
   (from the underlying network), utilization rates of each link over
   some period of time, etc.

   Note that any client/application that understands the Topology
   Manager's northbound API and its topology information model can
   communicate with the Topology Manager.  Note also that topology
   information may be provided by Network Elements from different
   vendors, which may use different information models.  If a Client
   wanted to retrieve topology information directly from Network
   Elements, it would have to translate and normalize these different

   A Traffic Engineering application may run a variety of CSPF
   algorithms that create a list of TE tunnels that globally optimize
   the packing efficiency of physical links throughout the network.  The
   TE tunnels are then programmed into the network either directly or
   through a controller.  Programming of TE tunnels into the network is
   outside the scope of this document.

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   A Capacity Planning application may run a variety of algorithms the
   result of which is a list of new inventory that is required for
   purchase or redeployment, as well as associated work orders for field
   technicians to augment the network for expected growth.

4.1.2.  Services Provisioning

   Beyond Capacity Planning and Traffic Engineering applications, having
   a normalized view of just the IP/MPLS layer of the network is still
   very important for other mission critical applications, such as
   Security Auditing and IP/MPLS Services Provisioning, (e.g.: L2VPN,
   L3VPN, etc.).  With respect to the latter, these types of
   applications should not need a detailed understanding of, for
   example, SRLG information, assuming that the underlying MPLS Tunnel
   LSP's are known to account for the resiliency requirements of all
   services that ride over them.  Nonetheless, for both types of
   applications it is critical to have a common and up-to-date
   normalized view of the IP/MPLS network to, for example, instantiate
   new services at optimal locations in the network, or to validate
   proper ACL configuration to protect associated routing, signaling and
   management protocols on the network.

   A VPN Service Provisioning application must perform the following
   resource selection operations:

   o  Identify Service PE's in all markets/cities where the customer has
      identified they want service

   o  Identify one or more existing Servies PE's in each city with
      connectivity to the access network(s), e.g.: SONET/TDM, used to
      deliver the PE-CE tail circuits to the Service's PE.

   o  Determine that the Services PE have available capacity on both the
      PE-CE access interface and its uplinks to terminate the tail

   The VPN Provisioning application would iteratively query the Topology
   Manager to narrow down the scope of resources to the set of Services
   PEs with the appropriate uplink bandwidth and access circuit
   capability plus capacity to realize the requested VPN service.  Once
   the VPN Provisioning application has a candidate list of resources it
   requests programming of the Service PE's and associated access
   circuits to set up a customer's VPN service into the network.
   Programming of Service PEs is outside the scope of this document.

4.1.3.  Troubleshooting & Monitoring

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   Once the Topology Manager has a normalized view of several layers of
   the network, it can expose a rich set of data to network operators
   who are performing diagnosis, troubleshooting and repairs on the
   network.  Specifically, there is a need to (rapidly) assemble a
   current, accurate and comprehensive network diagram of a L2VPN or
   L3VPN service for a particular customer when either: a) attempting to
   diagnose a service fault/error; or, b) attempting to augment the
   customer's existing service.  Information that may be assembled into
   a comprehensive picture could include physical and logical components
   related specifically to that customer's service, i.e.: VLAN's or
   channels used by the PE-CE access circuits, CoS policies, historical
   PE-CE circuit utilization, etc.  The Topology Manager would assemble
   this information, on behalf of each of the network elements and other
   data sources in and associated with the network, and would present
   this information in a vendor-independent data model to applications
   to be displayed allowing the operator (or, potentially, the customer
   through a SP's Web portal) to visualize the information.

4.2.  Path Computation Element (PCE)

   As described in [RFC4655] a PCE can be used to compute MPLS-TE paths
   within a "domain" (such as an IGP area) or across multiple domains
   (such as a multi-area AS, or multiple ASes).

   o  Within a single area, the PCE offers enhanced computational power
      that may not be available on individual routers, sophisticated
      policy control and algorithms, and coordination of computation
      across the whole area.

   o  If a router wants to compute a MPLS-TE path across IGP areas its
      own TED lacks visibility of the complete topology.  That means
      that the router cannot determine the end-to-end path, and cannot
      even select the right exit router (Area Border Router - ABR) for
      an optimal path.  This is an issue for large-scale networks that
      need to segment their core networks into distinct areas, but which
      still want to take advantage of MPLS-TE.

   The PCE presents a computation server that may have visibility into
   more than one IGP area or AS, or may cooperate with other PCEs to
   perform distributed path computation.  The PCE needs access to the
   topology and the Traffic Engineering Database (TED) for the area(s)
   it serves, but [RFC4655] does not describe how this is achieved.
   Many implementations make the PCE a passive participant in the IGP so
   that it can learn the latest state of the network, but this may be
   sub-optimal when the network is subject to a high degree of churn, or
   when the PCE is responsible for multiple areas.

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   The following figure shows how a PCE can get its TED information
   using a Topology Server.

             |  -----   | TED synchronization via Topology API
             | | TED |<-+----------------------------------+
             |  -----   |                                  |
             |    |     |                                  |
             |    |     |                                  |
             |    v     |                                  |
             |  -----   |                                  |
             | | PCE |  |                                  |
             |  -----   |                                  |
             +----------+                                  |
                  ^                                        |
                  | Request/                               |
                  | Response                               |
                  v                                        |
    Service  +----------+   Signaling  +----------+   +----------+
    Request  | Head-End |   Protocol   | Adjacent |   | Topology |
    -------->|  Node    |<------------>|   Node   |   | Manager  |
             +----------+              +----------+   +----------+

           Figure 4: Topology use case: Path Computation Element

4.3.  ALTO Server

   An ALTO Server [RFC5693] is an entity that generates an abstracted
   network topology and provides it to network-aware applications over a
   web service based API.  Example applications are p2p clients or
   trackers, or CDNs.  The abstracted network topology comes in the form
   of two maps: a Network Map that specifies allocation of prefixes to
   PIDs, and a Cost Map that specifies the cost between PIDs listed in
   the Network Map. For more details, see [I-D.ietf-alto-protocol].

   ALTO abstract network topologies can be auto-generated from the
   physical topology of the underlying network.  The generation would
   typically be based on policies and rules set by the operator.  Both
   prefix and TE data are required: prefix data is required to generate
   ALTO Network Maps, TE (topology) data is required to generate ALTO
   Cost Maps.  Prefix data is carried and originated in BGP, TE data is
   originated and carried in an IGP.  The mechanism defined in this
   document provides a single interface through which an ALTO Server can
   retrieve all the necessary prefix and network topology data from the
   underlying network.  Note an ALTO Server can use other mechanisms to
   get network data, for example, peering with multiple IGP and BGP

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   The following figure shows how an ALTO Server can get network
   topology information from the underlying network using the Topology

   | Client |<--+
   +--------+   |
                |    ALTO    +--------+                  +----------+
   +--------+   |  Protocol  |  ALTO  | Network Topology | Topology |
   | Client |<--+------------| Server |<-----------------| Manager  |
   +--------+   |            |        |                  |          |
                |            +--------+                  +----------+
   +--------+   |
   | Client |<--+

                 Figure 5: Topology use case: ALTO Server

5.  Acknowledgements

   The authors wish to thank Alia Atlas, Dave Ward, Hannes Gredler,
   Stafano Previdi for their valuable contributions and feedback to this

6.  IANA Considerations

   This memo includes no request to IANA.

7.  Security Considerations

   At the moment, the Use Cases covered in this document apply
   specifically to a single Service Provider or Enterprise network.
   Therefore, network administrations should take appropriate
   precautions to ensure appropriate access controls exist so that only
   internal applications and end-users have physical or logical access
   to the Topology Manager.  This should be similar to precautions that
   are already taken by Network Administrators to secure their existing
   Network Management, OSS and BSS systems.

   As this work evolves, it will be important to determine the
   appropriate granularity of access controls in terms of what
   individuals or groups may have read and/or write access to various
   types of information contained with the Topology Manager.  It would
   be ideal, if these access control mechanisms were centralized within
   the Topology Manager itself.

8.  References

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8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

8.2.  Informative References

              Alimi, R., Penno, R., and Y. Yang, "ALTO Protocol", draft-
              ietf-alto-protocol-17 (work in progress), July 2013.

              Gredler, H., Medved, J., Previdi, S., Farrel, A., and S.
              Ray, "North-Bound Distribution of Link-State and TE
              Information using BGP", draft-ietf-idr-ls-distribution-03
              (work in progress), May 2013.

              Previdi, S., Giacalone, S., Ward, D., Drake, J., Atlas,
              A., and C. Filsfils, "IS-IS Traffic Engineering (TE)
              Metric Extensions", draft-ietf-isis-te-metric-
              extensions-00 (work in progress), June 2013.

              Giacalone, S., Ward, D., Drake, J., Atlas, A., and S.
              Previdi, "OSPF Traffic Engineering (TE) Metric
              Extensions", draft-ietf-ospf-te-metric-extensions-04 (work
              in progress), June 2013.

              Crabbe, E., Medved, J., Minei, I., and R. Varga, "PCEP
              Extensions for Stateful PCE", draft-ietf-pce-stateful-
              pce-05 (work in progress), July 2013.

   [RFC4655]  Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
              Element (PCE)-Based Architecture", RFC 4655, August 2006.

   [RFC5693]  Seedorf, J. and E. Burger, "Application-Layer Traffic
              Optimization (ALTO) Problem Statement", RFC 5693, October

Authors' Addresses

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   Jan Medved
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134


   Thomas D. Nadeau
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089


   Shane Amante

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