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OSPF Link Overload
draft-ietf-ospf-link-overload-11

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8379.
Authors Shraddha Hegde , Pushpasis Sarkar , Hannes Gredler , Mohan Nanduri , Luay Jalil
Last updated 2018-01-16 (Latest revision 2018-01-01)
Replaces draft-hegde-ospf-link-overload
RFC stream Internet Engineering Task Force (IETF)
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Stream WG state Submitted to IESG for Publication
Document shepherd Acee Lindem
Shepherd write-up Show Last changed 2017-12-06
IESG IESG state Became RFC 8379 (Proposed Standard)
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Telechat date (None)
Responsible AD Alia Atlas
Send notices to Acee Lindem <acee@cisco.com>
IANA IANA review state IANA OK - Actions Needed
draft-ietf-ospf-link-overload-11
Open Shortest Path First IGP                                    S. Hegde
Internet-Draft                                    Juniper Networks, Inc.
Intended status: Standards Track                               P. Sarkar
Expires: July 5, 2018                                         H. Gredler
                                                              Individual
                                                              M. Nanduri
                                                        ebay Corporation
                                                                L. Jalil
                                                                 Verizon
                                                         January 1, 2018

                           OSPF Link Overload
                    draft-ietf-ospf-link-overload-11

Abstract

   When a link is being prepared to be taken out of service, the traffic
   needs to be diverted from both ends of the link.  Increasing the
   metric to the highest metric on one side of the link is not
   sufficient to divert the traffic flowing in the other direction.

   It is useful for routers in an OSPFv2 or OSPFv3 routing domain to be
   able to advertise a link as being in an overload state to indicate
   impending maintenance activity on the link.  This information can be
   used by the network devices to re-route the traffic effectively.

   This document describes the protocol extensions to disseminate link-
   overload information in OSPFv2 and OSPFv3.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of This Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any

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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 5, 2018.

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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Flooding Scope  . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Link-Overload sub-TLV . . . . . . . . . . . . . . . . . . . .   4
     4.1.  OSPFv2 Link-overload sub-TLV  . . . . . . . . . . . . . .   4
     4.2.  Remote IPv4 Address Sub-TLV . . . . . . . . . . . . . . .   4
     4.3.  Local/Remote Interface ID Sub-TLV . . . . . . . . . . . .   5
     4.4.  OSPFv3 Link-Overload sub-TLV  . . . . . . . . . . . . . .   6
     4.5.  BGP-LS Link-overload TLV  . . . . . . . . . . . . . . . .   6
     4.6.  Distinguishing parallel links . . . . . . . . . . . . . .   7
   5.  Elements of procedure . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Point-to-point links  . . . . . . . . . . . . . . . . . .   8
     5.2.  Broadcast/NBMA links  . . . . . . . . . . . . . . . . . .   8
     5.3.  Point-to-multipoint links . . . . . . . . . . . . . . . .   9
     5.4.  Unnumbered interfaces . . . . . . . . . . . . . . . . . .   9
     5.5.  Hybrid Broadcast and P2MP interfaces  . . . . . . . . . .   9
   6.  Backward compatibility  . . . . . . . . . . . . . . . . . . .  10
   7.  Applications  . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Pseudowire Services . . . . . . . . . . . . . . . . . . .  10
     7.2.  Controller based Traffic Engineering Deployments  . . . .  11
     7.3.  L3VPN Services and sham-links . . . . . . . . . . . . . .  12
     7.4.  Hub and spoke deployment  . . . . . . . . . . . . . . . .  13
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14

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     11.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     11.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   When a node is being prepared for a planned maintenance or upgrade,
   [RFC6987] provides mechanisms to advertise the node being in an
   overload state by setting all outgoing link costs to MaxLinkMetric
   (0xffff).  These procedures are specific to the maintenance activity
   on a node and cannot be used when a single link on the node requires
   maintenance.

   In traffic-engineering deployments, LSPs need to be diverted from the
   link without disrupting the services.  [RFC5817] describes
   requirements and procedures for graceful shutdown of MPLS links.  It
   is useful to be able to advertise the impending maintenance activity
   on the link and to have LSP re-routing policies at the ingress to
   route the LSPs away from the link.

   Many OSPFv2 or OSPFv3 deployments run on overlay networks provisioned
   by means of pseudo-wires or L2-circuits.  Prior to devices in the
   underlying network going offline for maintenance, it is useful to
   divert the traffic away from the node before the maintenance is
   actually performed.  Since the nodes in the underlying network are
   not visible to OSPF, the existing stub router mechanism described in
   [RFC6987] cannot be used.  An application specific to this use case
   is described in Section 7.1.

   This document provides mechanisms to advertise link-overload state in
   the flexible encodings provided by OSPFv2 Prefix/Link Attribute
   Advertisement [RFC7684].  Throughout this document, OSPF is used when
   the text applies to both OSPFv2 and OSPFv3.  OSPFv2 or OSPFv3 is used
   when the text is specific to one version of the OSPF protocol.

2.  Motivation

   The motivation of this document is to reduce manual intervention
   during maintenance activities.  The following objectives help to
   accomplish this in a range of deployment scenarios.

   1.  Advertise impending maintenance activity so that traffic from
       both directions can be diverted away from the link.

   2.  Allow the solution to be backward compatible so that nodes that
       do not understand the new advertisement do not cause routing
       loops.

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   3.  Advertise the maintenance activity to other nodes in the network
       so that LSP ingress routers/controllers can learn of the
       impending maintenance activity and apply specific policies to re-
       route the LSPs for traffic-engineering based deployments.

   4.  Allow the link to be used as last resort link to prevent traffic
       disruption when alternate paths are not available.

3.  Flooding Scope

   The link-overload information is flooded in area-scoped Extended Link
   Opaque LSA [RFC7684].  The Link-Overload sub-TLV MAY be processed by
   the head-end nodes or the controller as described in the Section 7.
   The procedures for processing the Link-Overload sub-TLV are described
   in Section 5.

4.  Link-Overload sub-TLV

4.1.  OSPFv2 Link-overload sub-TLV

   The Link-Overload sub-TLV identifies the link as being in overload
   state.It is advertised in extended Link TLV of the Extended Link
   Opaque LSA as defined in [RFC7684].

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Type             |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 1: Link-Overload sub-TLV for OSPFv2

   Type : TBA (suggested value 7)

   Length: 0

4.2.  Remote IPv4 Address Sub-TLV

   This sub-TLV specifies the IPv4 address of remote endpoint on the
   link.  It is advertised in the Extended Link TLV as defined in
   [RFC7684].  This sub-TLV is optional and MAY be advertised in area-

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   scoped Extended Link Opaque LSA to identify the link when there are
   multiple parallel links between two nodes.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Type             |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Remote IPv4 address                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 2: Remote IPv4 Address Sub-TLV

   Type : TBA (suggested value 8)

   Length: 4

   Value: Remote IPv4 address.  The remote IP4 address is used to
   identify the particular link when there are multiple parallel links
   between two nodes.

4.3.  Local/Remote Interface ID Sub-TLV

   This sub-TLV specifies local and remote interface identifiers.  It is
   advertised in the Extended Link TLV as defined in [RFC7684].  This
   sub-TLV is optional and MAY be advertised in area-scoped Extended
   Link Opaque LSA to identify the link when there are multiple parallel
   unnumbered links between two nodes.  The local interface-id is
   generally readily available.  One of the mechanisms to obtain remote
   interface-id is described in [RFC4203].

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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Type             |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Local Interface ID                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Remote Interface ID                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 3: Local/Remote Interface ID Sub-TLV

   Type : TBA (suggested value 9)

   Length: 8

   Value: 4 octets of Local Interface ID followed by 4 octets of Remote
   interface ID.

4.4.  OSPFv3 Link-Overload sub-TLV

   The Link Overload sub-TLV is carried in the Router-Link TLV as
   defined in the [I-D.ietf-ospf-ospfv3-lsa-extend] for OSPFv3.  The
   Router-Link TLV contains the neighbour interface-id and can uniquely
   identify the link on the remote node.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Type             |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 4: Link-Overload sub-TLV for OSPFv3

   Type : TBA (Suggested value 7)

   Length: 0

4.5.  BGP-LS Link-overload TLV

   BGP-LS as defined in [RFC7752] is a mechanism to distribute network
   information to external entities using BGP routing protocol. link-
   overload is an imporatant link information that the external entities
   can use for various usecases as defined in Section 7.  BGP Link NLRI

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   is used to carry the link information.  a new TLV called Link-
   Overload is defined to describe the link attribute corresponding to
   link-overload state.

4.6.  Distinguishing parallel links

       ++++++++++I.w            I.y +++++++++
       |Router A|------------------|Router B |
       |        |------------------|         |
       ++++++++++I.x             I.z++++++++++

                         Figure 5: Parallel Linkls

   Consider two routers A and B connected with two parallel point-to-
   point interfaces.  I.w and I.x represent the Interface address on
   Router A's side and I.y and I.z represent Interface addresses on
   Router B's side.  The extended link opaque LSA as described in
   [RFC7684] describes links using link-type, Link-ID and Link-data.
   For ex.  Link with address I.w is described as below on Router A.

      Link-type = Point-to-point

      Link-ID: Router-ID B

      Link-Data = I.w

   A third node (controller or head-end) in the network cannot
   distinguish the Interface on router B which is connected to this
   particular Interface with the above information.  Interface with
   address I.y or I.z could be chosen due to this ambiguity.  In such
   cases Remote-IPv4 Address sub-TLV should be originated and added to
   the extended link-TLV.  The usecases as described in Section 7
   require controller or head-end nodes to interpret the link-overload
   information and hence the need for the RemoteIPv4 address sub-TLV.
   I.y is carried in the extended-link-TLV which unambiguously
   identifies the interface on the remote side.  OSPFv3 Router-link-TLV
   as described in [I-D.ietf-ospf-ospfv3-lsa-extend] contains Interface
   ID and neighbor's Interface-ID which can uniquely identify connecting
   interface on the remote side and hence OSPFv3 does not require
   seperate Remote-IPv6 address to be advertised along with OSPFv2-link-
   overload-sub-TLV.

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5.  Elements of procedure

   As defined in [RFC7684] every link on the node will have a separate
   Extended Link Opaque LSA.  The node that has the link to be taken out
   of service SHOULD advertise the Link-Overload sub-TLV in the Extended
   Link TLV of the Extended Link Opaque LSA as defined in [RFC7684] for
   OSPFv2.  The Link-Overload sub-TLV indicates that the link identified
   by the sub-TLV is overloaded.  The Link-Overload information is
   advertised as a property of the link and is flooded across the area.
   This information can be used by ingress routers or controllers to
   take special actions.  An application specific to this use case is
   described in Section 7.2.

   The precise action taken by the remote node at the other end of the
   link identified as overloaded depends on the link type.

5.1.  Point-to-point links

   The node that has the link to be taken out of service MUST set metric
   of the link to MaxLinkMetric (0xffff) and re-originate its router-
   LSA.  The TE metric SHOULD be set to MAX-TE-METRIC (0xfffffffe) and
   the node SHOULD re-originate the corresponding TE Link Opaque LSAs.
   When a Link-Overload sub-TLV is received for a point-to-point link,
   the remote node MUST identify the local link which corresponds to the
   overloaded link and set the metric to MaxLinkMetric (0xffff)and the
   remote node MUST re-originate its router-LSA with the changed metric.
   The TE metric SHOULD be set to MAX-TE-METRIC (0xfffffffe) and the TE
   opaque LSA for the link SHOULD be re-originated with new value.

   The Extended link opaque LSAs and the Extended link TLV are not
   scoped for multi-topology [RFC4915].  In multi-topology deployments
   [RFC4915], the Link-Overload sub-TLV advertised in an Extended Link
   opaque LSA corresponds to all the topologies which include the link.
   The receiver node SHOULD change the metric in the reverse direction
   for all the topologies which include the remote link and re-originate
   the router-LSA as defined in [RFC4915].

   When the originator of the Link-Overload sub-TLV purges the Extended
   Link Opaque LSA or re-originates it without the Link-Overload sub-
   TLV, the remote node must re-originate the appropriate LSAs with the
   metric and TE metric values set to their original values.

5.2.  Broadcast/NBMA links

   Broadcast or NBMA networks in OSPF are represented by a star topology
   where the Designated Router (DR) is the central point to which all
   other routers on the broadcast or NBMA network logically connect.  As
   a result, routers on the broadcast or NBMA network advertise only

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   their adjacency to the DR.  Routers that do not act as DR do not form
   or advertise adjacencies with each other.  For the Broadcast links,
   the MaxLinkMetric on the remote link cannot be changed since all the
   neighbors are on same link.  Setting the link cost to MaxLinkMetric
   would impact paths going via all neighbors.

   The node that has the link to be taken out of service MUST set metric
   of the link to MaxLinkMetric (0xffff) and re-originate the Router-
   LSA.  The TE metric SHOULD be set to MAX-TE-METRIC( 0xfffffffe) and
   the node SHOULD re-originate the corresponding TE Link Opaque LSAs.
   For a broadcast link, the two part metric as described in [RFC8042]
   is used.  The node originating the Link-Overload sub-TLV MUST set the
   metric in the Network-to-Router Metric sub-TLV to MaxLinkMetric
   (0xffff) for OSPFv2 and OSPFv3 and re-originate the corresponding
   LSAs.  The nodes that receive the two-part metric should follow the
   procedures described in [RFC8042].  The backward compatibility
   procedures described in [RFC8042] should be followed to ensure loop
   free routing.

5.3.  Point-to-multipoint links

   Operation for the point-to-multipoint links is similar to the point-
   to-point links.  When a Link-Overload sub-TLV is received for a
   point-to-multipoint link the remote node MUST identify the neighbour
   which corresponds to the overloaded link and set the metric to
   MaxLinkMetric (0xffff).  The remote node MUST re-originate the
   router-LSA with the changed metric for the correponding neighbor.

5.4.  Unnumbered interfaces

   Unnumbered interface do not have a unique IP address and borrow their
   address from other interfaces.  [RFC2328] describes procedures to
   handle unnumbered interfaces in the context of the router-LSA.  We
   apply a similar procedure to the Extended Link TLV advertising the
   Link-Overload sub-TLV in order to handle unnumbered interfaces.  The
   link-data field in the Extended Link TLV includes the Local
   interface-id instead of the IP address.  The Local/Remote Interface
   ID sub-TLV MUST be advertised when there are multiple parallel
   unnumbered interfaces between two nodes.  One of the mechanisms to
   obtain the interface-id of the remote side are defined in [RFC4203].

5.5.  Hybrid Broadcast and P2MP interfaces

   Hybrid Broadcast and P2MP interfaces represent a broadcast network
   modeled as P2MP interfaces.  [RFC6845] describes procedures to handle
   these interfaces.  Operation for the Hybrid interfaces is similar to
   the P2MP interfaces.  When a Link-Overload sub-TLV is received for a
   hybrid link, the remote node MUST identify the neighbor which

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   corresponds to the overloaded link and set the metric to
   MaxLinkMetric (0xffff).  All the remote nodes connected to originator
   MUST re-originate the router-LSA with the changed metric for the
   neighbor.

6.  Backward compatibility

   The mechanisms described in the document are fully backward
   compatible.  It is required that the node adverting the Link-Overload
   sub-TLV as well as the node at the remote end of the overloaded link
   support the extensions described herein for the traffic to diverted
   from the overloaded link.  If the remote node doesn't support the
   capability, it will still use the overloaded link but there are no
   other adverse effects.  In the case of broadcast links using two-part
   metrics, the backward compatibility procedures as described in
   [RFC8042] are applicable.

7.  Applications

7.1.  Pseudowire Services

   Many service providers offer pseudo-wire services to customers using
   L2 circuits.  The IGP protocol that runs in the customer network
   would also run over the pseudo-wire to create a seamless private
   network for the customer.  Service providers want to offer overload
   functionality when the PE device is taken-out for maintenance.  The
   provider should guarantee that the PE is taken out for maintenance
   only after the service is successfully diverted on an alternate path.
   There can be large number of customers attached to a PE node and the
   remote end-points for these pseudo-wires are spread across the
   service provider's network.  It is a tedious and error-prone process
   to change the metric for all pseudo-wires in both directions.  The
   link-overload feature simplifies the process by increasing the metric
   on the link in the reverse direction as well so that traffic in both
   directions is diverted away from the PE undergoing maintenance.  The
   Link-Overload feature allows the link to be used as a last resort
   link so that traffic is not disrupted when alternative paths are not
   available.

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                   Private VLAN
           =======================================
          |                                       |
          |                                       |
          |     ------PE3---------------PE4------CE3
          |   /                             \
          | /                                 \
        CE1---------PE1----------PE2---------CE2
          |                       \
          |                        \
          |                         ------CE4
          |                                 |
          |                                 |
          |                                 |
           =================================
                   Private VLAN

                       Figure 6: Pseudowire Services

   In the example shown in Figure 6, when the PE1 node is going out of
   service for maintenance, service providers set the PE1 to overload
   state.  The PE1 going in to overload state triggers all the CEs
   connected to the PE (CE1 in this case) to set their pseudowire links
   passing via PE1 to link-overload state.  The mechanisms used to
   communicate between PE1 and CE1 is outside the scope of this
   document.  CE1 sets the link-overload state on its private VLAN
   connecting CE3, CE2 and CE4 and changes the metric to MAX_METRIC and
   re-originates the corresponding LSA.  The remote end of the link at
   CE3, CE2, and CE4 also set the metric on the link to MaxLinkMetric
   and the traffic from both directions gets diverted away from the
   pseudowires.

7.2.  Controller based Traffic Engineering Deployments

   In controller-based deployments where the controller participates in
   the IGP protocol, the controller can also receive the link-overload
   information as a warning that link maintenance is imminent.  Using
   this information, the controller can find alternate paths for traffic
   which uses the affected link.  The controller can apply various
   policies and re-route the LSPs away from the link undergoing
   maintenance.  If there are no alternate paths satisfying the traffic
   engineering constraints, the controller might temporarily relax those
   constraints and put the service on a different path.  Increasing the
   link metric alone does not specify the maintenance activity as the
   metric could increase in events such as LDP-IGP synchronisation.  An
   explicit indication from the router using the link-overload sub-TLV
   is needed to inform the Controller or head-end routers.

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                         _____________
                        |             |
           -------------| Controller  |--------------
          |             |____________ |             |
          |                                         |
          |--------- Primary Path ------------------|
          PE1---------P1----------------P2---------PE2
                      |                  |
                      |                  |
                      |________P3________|

                         Alternate Path

              Figure 7: Controller based Traffic Engineering

   In the above example, PE1->PE2 LSP is set-up to satisfy a constraint
   of 10 Gbps bandwidth on each link.  The links P1->P3 and P3->P2 have
   only 1 Gbps capacity and there is no alternate path satisfying the
   bandwidth constraint of 10Gbps.  When P1->P2 link is being prepared
   for maintenance, the controller receives the link-overload
   information, as there is no alternate path available which satisfies
   the constraints, the controller chooses a path that is less optimal
   and temporarily sets up an alternate path via P1->P3->P2.  Once the
   traffic is diverted, the P1->P2 link can be taken out of service for
   maintenance/upgrade.

7.3.  L3VPN Services and sham-links

   Many service providers offer L3VPN services to customers and CE-PE
   links run OSPF [RFC4577].  When PE is taken out of service for
   maintenance, all the links on the PE can be set to link-overload
   state which will gurantee that the traffic to/from dual-homed CEs
   gets diverted.  The interaction between OSPF and BGP is outside the
   scope of this document.  [RFC6987] based mechanism with summaries and
   externals advertised with high metrics could also be used to achieve
   the same functionality when implementations support high metrics
   advertisement for summaries and externals.

   Another useful usecase is when ISPs provide sham-link services to
   customers [RFC4577].  When PE goes out of service for maintenance,
   all sham-links on the PE can be set to link-overload state and
   traffic can be divered from both ends without having to touch the
   configurations on the remote end of the sham-links.

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7.4.  Hub and spoke deployment

   OSPF is largely deployed in Hub and Spoke deployments with a large
   number of spokes connecting to the Hub. It is a general practice to
   deploy multiple Hubs with all spokes connecting to these Hubs to
   achieve redundancy.  The [RFC6987]  mechanism can be used to divert
   the spoke-to-spoke traffic from the overloaded hub router.  The
   traffic that flows from spokes via the hub into an external network
   may not be diverted in certain scenarios.When a Hub node goes down
   for maintenance, all links on the Hub can be set to link-overload
   state and traffic gets divered from the spoke sites as well without
   having to make configuration changes on the spokes.

8.  Security Considerations

   This document does not introduce any further security issues other
   than those discussed in [RFC2328] and [RFC5340].

9.  IANA Considerations

   This specification updates one OSPF registry:

   OSPFv2 Extended Link TLV Sub-TLVs

   i) Link-Overload Sub-TLV - Suggested value 7

   ii) Remote IPv4 Address Sub-TLV - Suggested value 8

   iii) Local/Remote Interface ID Sub-TLV - Suggested Value 9

   OSPFv3 Extended-LSA sub-TLV Registry

   i) Link-Overload sub-TLV - suggested value 7

   BGP-LS Link NLRI Registry [RFC7752]

   i)Link-Overload TLV - Suggested 1101

10.  Acknowledgements

   Thanks to Chris Bowers for valuable inputs and edits to the document.
   Thanks to Jeffrey Zhang, Acee Lindem and Ketan Talaulikar for inputs.
   Thanks to Karsten Thomann for careful review and inputs on the
   applications where link-overload is useful.

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11.  References

11.1.  Normative References

   [I-D.ietf-ospf-ospfv3-lsa-extend]
              Lindem, A., Mirtorabi, S., Roy, A., and F. Baker, "OSPFv3
              LSA Extendibility", draft-ietf-ospf-ospfv3-lsa-extend-10
              (work in progress), May 2016.

   [RFC6845]  Sheth, N., Wang, L., and J. Zhang, "OSPF Hybrid Broadcast
              and Point-to-Multipoint Interface Type", RFC 6845,
              DOI 10.17487/RFC6845, January 2013,
              <https://www.rfc-editor.org/info/rfc6845>.

   [RFC7684]  Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
              Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
              Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
              2015, <https://www.rfc-editor.org/info/rfc7684>.

   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,
              <https://www.rfc-editor.org/info/rfc7752>.

   [RFC8042]  Zhang, Z., Wang, L., and A. Lindem, "OSPF Two-Part
              Metric", RFC 8042, DOI 10.17487/RFC8042, December 2016,
              <https://www.rfc-editor.org/info/rfc8042>.

11.2.  Informative 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>.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <https://www.rfc-editor.org/info/rfc2328>.

   [RFC4203]  Kompella, K., Ed. and Y. Rekhter, Ed., "OSPF Extensions in
              Support of Generalized Multi-Protocol Label Switching
              (GMPLS)", RFC 4203, DOI 10.17487/RFC4203, October 2005,
              <https://www.rfc-editor.org/info/rfc4203>.

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   [RFC4577]  Rosen, E., Psenak, P., and P. Pillay-Esnault, "OSPF as the
              Provider/Customer Edge Protocol for BGP/MPLS IP Virtual
              Private Networks (VPNs)", RFC 4577, DOI 10.17487/RFC4577,
              June 2006, <https://www.rfc-editor.org/info/rfc4577>.

   [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>.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
              <https://www.rfc-editor.org/info/rfc5340>.

   [RFC5817]  Ali, Z., Vasseur, JP., Zamfir, A., and J. Newton,
              "Graceful Shutdown in MPLS and Generalized MPLS Traffic
              Engineering Networks", RFC 5817, DOI 10.17487/RFC5817,
              April 2010, <https://www.rfc-editor.org/info/rfc5817>.

   [RFC6987]  Retana, A., Nguyen, L., Zinin, A., White, R., and D.
              McPherson, "OSPF Stub Router Advertisement", RFC 6987,
              DOI 10.17487/RFC6987, September 2013,
              <https://www.rfc-editor.org/info/rfc6987>.

Authors' Addresses

   Shraddha Hegde
   Juniper Networks, Inc.
   Embassy Business Park
   Bangalore, KA  560093
   India

   Email: shraddha@juniper.net

   Pushpasis Sarkar
   Individual

   Email: pushpasis.ietf@gmail.com

   Hannes Gredler
   Individual

   Email: hannes@gredler.at

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   Mohan Nanduri
   ebay Corporation
   2025 Hamilton Avenue
   San Jose, CA  98052
   US

   Email: mnanduri@ebay.com

   Luay Jalil
   Verizon

   Email: luay.jalil@verizon.com

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