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rfc:rfc8379

Internet Engineering Task Force (IETF) S. Hegde Request for Comments: 8379 Juniper Networks, Inc. Category: Standards Track P. Sarkar ISSN: 2070-1721 Arrcus, Inc.

                                                            H. Gredler
                                                          RtBrick Inc.
                                                            M. Nanduri
                                                      ebay Corporation
                                                              L. Jalil
                                                               Verizon
                                                              May 2018
                    OSPF Graceful Link Shutdown

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 value on one side of the link is not sufficient
 to divert the traffic flowing in the other direction.
 It is useful for the routers in an OSPFv2 or OSPFv3 routing domain to
 be able to advertise a link as being in a graceful-shutdown state to
 indicate impending maintenance activity on the link.  This
 information can be used by the network devices to reroute the traffic
 effectively.
 This document describes the protocol extensions to disseminate
 graceful-link-shutdown information in OSPFv2 and OSPFv3.

Status of This Memo

 This is an Internet Standards Track document.
 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).  Further information on
 Internet Standards is available in 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/rfc8379.

Hegde, et al. Standards Track [Page 1] RFC 8379 OSPF Graceful Link Shutdown May 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
   1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
 2.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   3
 3.  Flooding Scope  . . . . . . . . . . . . . . . . . . . . . . .   4
 4.  Protocol Extensions . . . . . . . . . . . . . . . . . . . . .   4
   4.1.  OSPFv2 Graceful-Link-Shutdown Sub-TLV . . . . . . . . . .   4
   4.2.  Remote IPv4 Address Sub-TLV . . . . . . . . . . . . . . .   4
   4.3.  Local/Remote Interface ID Sub-TLV . . . . . . . . . . . .   5
   4.4.  OSPFv3 Graceful-Link-Shutdown Sub-TLV . . . . . . . . . .   6
   4.5.  BGP-LS Graceful-Link-Shutdown 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  . . . . . . . . . . . . . . . . . .   9
   5.3.  Point-to-Multipoint Links . . . . . . . . . . . . . . . .  10
   5.4.  Unnumbered Interfaces . . . . . . . . . . . . . . . . . .  10
   5.5.  Hybrid Broadcast and P2MP Interfaces  . . . . . . . . . .  10
 6.  Backward Compatibility  . . . . . . . . . . . . . . . . . . .  10
 7.  Applications  . . . . . . . . . . . . . . . . . . . . . . . .  11
   7.1.  Overlay Network . . . . . . . . . . . . . . . . . . . . .  11
   7.2.  Controller-Based Deployments  . . . . . . . . . . . . . .  12
   7.3.  L3VPN Services and Sham Links . . . . . . . . . . . . . .  13
   7.4.  Hub and Spoke Deployment  . . . . . . . . . . . . . . . .  13
 8.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
 9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
 10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
   10.1.  Normative References . . . . . . . . . . . . . . . . . .  14
   10.2.  Informative References . . . . . . . . . . . . . . . . .  16
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  16
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

Hegde, et al. Standards Track [Page 2] RFC 8379 OSPF Graceful Link Shutdown May 2018

1. Introduction

 This document describes a mechanism for gracefully taking a link out
 of service while allowing it to be used if no other path is
 available.  It also provides a mechanism to divert the traffic from
 both directions of the link.
 Many OSPFv2 or OSPFv3 deployments run on overlay networks provisioned
 by means of pseudowires 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 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.  In a service provider's network, there may be many
 CE-to-CE connections that run over a single PE.  It is cumbersome to
 change the metric on every CE-to-CE connection in both directions.
 This document provides a mechanism to change the metric of the link
 on the remote side and also use the link as a last-resort link if no
 alternate paths are available.  An application specific to this use
 case is described in detail in Section 7.1.
 This document provides mechanisms to advertise graceful-link-shutdown
 state in the flexible encodings provided by "OSPFv2 Prefix/Link
 Attribute Advertisement" [RFC7684] and the E-Router-LSA [RFC8362] for
 OSPFv3.  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.

1.1. Requirements Language

 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. 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.

Hegde, et al. Standards Track [Page 3] RFC 8379 OSPF Graceful Link Shutdown May 2018

 3.  Advertise the maintenance activity to other nodes in the network
     so that Label Switched Path (LSP) ingress routers/controllers can
     learn about the impending maintenance activity and apply specific
     policies to reroute the LSPs for deployments based on Traffic
     Engineering (TE).
 4.  Allow the link to be used as a last-resort link to prevent
     traffic disruption when alternate paths are not available.

3. Flooding Scope

 The graceful-link-shutdown information is flooded in an area-scoped
 Extended Link Opaque LSA [RFC7684] for OSPFv2 and in an E-Router-LSA
 for OSPFv3 [RFC8362].  The Graceful-Link-Shutdown sub-TLV MAY be
 processed by the head-end nodes or the controller as described in the
 Section 7.  The procedures for processing the Graceful-Link-Shutdown
 sub-TLV are described in Section 5.

4. Protocol Extensions

4.1. OSPFv2 Graceful-Link-Shutdown Sub-TLV

 The Graceful-Link-Shutdown sub-TLV identifies the link as being
 gracefully shutdown.  It is advertised in the 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: Graceful-Link-Shutdown Sub-TLV for OSPFv2
 Type: 7
 Length: 0

4.2. Remote IPv4 Address Sub-TLV

 This sub-TLV specifies the IPv4 address of the 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 an
 area-scoped Extended Link Opaque LSA to identify the link when there
 are multiple parallel links between two nodes.

Hegde, et al. Standards Track [Page 4] RFC 8379 OSPF Graceful Link Shutdown May 2018

      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: 8
 Length: 4
 Value: Remote IPv4 address.  The remote IPv4 address is used to
 identify a particular link on the remote side 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 IDs.  It is
 advertised in the Extended Link TLV as defined in [RFC7684].  This
 sub-TLV is optional and MAY be advertised in an 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 the
 Remote Interface ID is described in [RFC4203].
      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: 9
 Length: 8
 Value: 4 octets of the Local Interface ID followed by 4 octets of the
 Remote Interface ID.

Hegde, et al. Standards Track [Page 5] RFC 8379 OSPF Graceful Link Shutdown May 2018

4.4. OSPFv3 Graceful-Link-Shutdown Sub-TLV

 The Graceful-Link-Shutdown sub-TLV is carried in the Router-Link TLV
 as defined in [RFC8362] for OSPFv3.  The Router-Link TLV contains the
 Neighbor 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: Graceful-Link-Shutdown Sub-TLV for OSPFv3
 Type: 8
 Length: 0

4.5. BGP-LS Graceful-Link-Shutdown TLV

 BGP-LS as defined in [RFC7752] is a mechanism that distributes
 network information to the external entities using the BGP routing
 protocol.  Graceful link shutdown is important link information that
 the external entities can use for various use cases as defined in
 Section 7.  BGP Link Network Layer Reachability Information (NLRI) is
 used to carry the link information.  A new TLV called "Graceful-Link-
 Shutdown" is defined to describe the link attribute corresponding to
 graceful-link-shutdown state.  The TLV format is as described in
 Section 3.1 of [RFC7752].  There is no Value field, and the Length
 field is set to zero for this TLV.
      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 5: Graceful-Link-Shutdown TLV for BGP-LS
 Type: 1121
 Length: 0

Hegde, et al. Standards Track [Page 6] RFC 8379 OSPF Graceful Link Shutdown May 2018

4.6. Distinguishing Parallel Links

                  ++++++++++I.w            I.y+++++++++++
                  |Router A|------------------|Router B |
                  |        |------------------|         |
                  ++++++++++I.x            I.z+++++++++++
                       Figure 6: Parallel Links
 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
 defined in [RFC7684] describes links using Link Type, Link ID, and
 Link Data.  For example, a link with the address I.w is described as
 below on Router A.
    Link Type = Point-to-point
    Link ID = Router ID of 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 on Router A based on the link information
 described above.  The interface with address I.y or I.z could be
 chosen due to this ambiguity.  In such cases, a Remote IPv4 Address
 sub-TLV should be originated and added to the Extended Link TLV.  The
 use cases as described in Section 7 require controller or head-end
 nodes to interpret the graceful-link-shutdown information and hence
 the need for the Remote IPv4 Address sub-TLV.  I.y is carried in the
 Extended Link TLV, which unambiguously identifies the interface on
 the remote side.  The OSPFv3 Router-Link TLV as described in
 [RFC8362] contains an Interface ID and a neighbor's Interface ID,
 which can uniquely identify connecting the interface on the remote
 side; hence, OSPFv3 does not require a separate remote IPv6 address
 to be advertised along with the OSPFv3 Graceful-Link-Shutdown
 sub-TLV.

Hegde, et al. Standards Track [Page 7] RFC 8379 OSPF Graceful Link Shutdown May 2018

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 MUST advertise the Graceful-Link-Shutdown sub-TLV in the
 Extended Link TLV of the Extended Link Opaque LSA for OSPFv2, as
 defined in [RFC7684], and in the Router-Link TLV of E-Router-LSA for
 OSPFv3.  The Graceful-Link-Shutdown sub-TLV indicates that the link
 identified by the sub-TLV is subjected to maintenance.
 For the purposes of changing the metric OSPFv2 and OSPFv3 Router-LSAs
 need to be reoriginated.  To change the Traffic Engineering metric,
 TE Opaque LSAs in OSPFv2 [RFC3630] and Intra-area-TE-LSAs in OSPFv3
 [RFC5329] need to be reoriginated.
 The graceful-link-shutdown information is advertised as a property of
 the link and is flooded through 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.
 When a link is ready to carry traffic, the Graceful-Link-Shutdown
 sub-TLV MUST be removed from the Extended Link TLV/Router-Link TLV,
 and the corresponding LSAs MUST be readvertised.  Similarly, the
 metric MUST be set to original values, and the corresponding LSAs
 MUST be readvertised.
 The procedures described in this document may be used to divert the
 traffic away from the link in scenarios other than link-shutdown or
 link-replacement activity.
 The precise action taken by the remote node at the other end of the
 link identified for graceful-shutdown 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 the
 metric of the link to MaxLinkMetric (0xffff) and reoriginate its
 Router-LSA.  The Traffic Engineering metric of the link SHOULD be set
 to (0xffffffff), and the node SHOULD reoriginate the corresponding TE
 Link Opaque LSAs.  When a Graceful-Link-Shutdown sub-TLV is received
 for a point-to-point link, the remote node MUST identify the local
 link that corresponds to the graceful-shutdown link and set its
 metric to MaxLinkMetric (0xffff), and the remote node MUST
 reoriginate its Router-LSA with the changed metric.  When TE is
 enabled, the Traffic Engineering metric of the link SHOULD be set to
 (0xffffffff) and follow the procedures in [RFC5817].  Similarly, the

Hegde, et al. Standards Track [Page 8] RFC 8379 OSPF Graceful Link Shutdown May 2018

 remote node SHOULD set the Traffic Engineering metric of the link to
 0xffffffff and SHOULD reoriginate the TE Link Opaque LSA for the link
 with the 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 Graceful-Link-Shutdown sub-TLV advertised in an
 Extended Link Opaque LSA corresponds to all the topologies that
 include the link.  The receiver node SHOULD change the metric in the
 reverse direction for all the topologies that include the remote link
 and reoriginate the Router-LSA as defined in [RFC4915].
 When the originator of the Graceful-Link-Shutdown sub-TLV purges the
 Extended Link Opaque LSA or reoriginates it without the
 Graceful-Link-Shutdown sub-TLV, the remote node must reoriginate the
 appropriate LSAs with the metric and TE metric values set to their
 original values.

5.2. Broadcast/NBMA Links

 Broadcast or Non-Broadcast Multi-Access (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 their adjacency to the DR.  Routers that
 do not act as DRs 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 all paths that traverse
 any of the neighbors connected on that broadcast link.
 The node that has the link to be taken out of service MUST set the
 metric of the link to MaxLinkMetric (0xffff) and reoriginate the
 Router-LSA.  The Traffic Engineering metric of the link SHOULD be set
 to (0xffffffff), and the node SHOULD reoriginate the corresponding TE
 Link Opaque LSAs.  For a broadcast link, the two-part metric as
 described in [RFC8042] is used.  The node originating the
 Graceful-Link-Shutdown sub-TLV MUST set the metric in the
 Network-to-Router Metric sub-TLV to MaxLinkMetric (0xffff) for OSPFv2
 and OSPFv3 and reoriginate 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.

Hegde, et al. Standards Track [Page 9] RFC 8379 OSPF Graceful Link Shutdown May 2018

5.3. Point-to-Multipoint Links

 Operation for the point-to-multipoint (P2MP) links is similar to the
 point-to-point links.  When a Graceful-Link-Shutdown sub-TLV is
 received for a point-to-multipoint link, the remote node MUST
 identify the neighbor that corresponds to the graceful-shutdown link
 and set its metric to MaxLinkMetric (0xffff).  The remote node MUST
 reoriginate the Router-LSA with the changed metric for the
 corresponding neighbor.

5.4. Unnumbered Interfaces

 Unnumbered interfaces 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
 Graceful-Link-Shutdown 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 is 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
 operation for the P2MP interfaces.  When a Graceful-Link-Shutdown
 sub-TLV is received for a hybrid link, the remote node MUST identify
 the neighbor that corresponds to the graceful-shutdown link and set
 its metric to MaxLinkMetric (0xffff).  All the remote nodes connected
 to the originator MUST reoriginate 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
 Graceful-Link-Shutdown sub-TLV as well as the node at the remote end
 of the graceful-shutdown link support the extensions described herein
 for the traffic to be diverted from the graceful-shutdown link.  If
 the remote node doesn't support the capability, it will still use the
 graceful-shutdown 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.

Hegde, et al. Standards Track [Page 10] RFC 8379 OSPF Graceful Link Shutdown May 2018

7. Applications

7.1. Overlay Network

 Many service providers offer L2 services to a customer connecting
 different locations.  The customer's IGP protocol creates a seamless
 private network (overlay network) across the locations for the
 customer.  Service providers want to offer graceful-shutdown
 functionality when the PE device is taken out for maintenance.  There
 can be large number of customers attached to a PE node, and the
 remote endpoints for these L2 attachment circuits are spread across
 the service provider's network.  Changing the metric for all
 corresponding L2 circuits in both directions is a tedious and error-
 prone process.  The graceful-link-shutdown feature simplifies the
 process by increasing the metric on the CE-CE overlay link so that
 traffic in both directions is diverted away from the PE undergoing
 maintenance.  The graceful-link-shutdown feature allows the link to
 be used as a last-resort link so that traffic is not disrupted when
 alternate paths are not available.
  1. —–PE3—————PE4——CE3

/ \

               /                               \
            CE1---------PE1----------PE2---------CE2
                                     \
                                      \
                                       ------CE4
 CE: Customer Edge
 PE: Provider Edge
                       Figure 7: Overlay Network
 In the example shown in Figure 7, when the PE1 node is going out of
 service for maintenance, a service provider sets the PE1 to stub-
 router state and communicates the pending maintenance action to the
 overlay customer networks.  The mechanisms used to communicate
 between PE1 and CE1 is outside the scope of this document.  CE1 sets
 the graceful-link-shutdown state on its links connecting CE3, CE2,
 and CE4, changes the metric to MaxLinkMetric, and reoriginates 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 PE1.

Hegde, et al. Standards Track [Page 11] RFC 8379 OSPF Graceful Link Shutdown May 2018

7.2. Controller-Based Deployments

 In controller-based deployments where the controller participates in
 the IGP protocol, the controller can also receive the
 graceful-link-shutdown information as a warning that link maintenance
 is imminent.  Using this information, the controller can find
 alternate paths for traffic that uses the affected link.  The
 controller can apply various policies and reroute the LSPs away from
 the link undergoing maintenance.  If there are no alternate paths
 satisfying the 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
 synchronization.  An explicit indication from the router using the
 Graceful-Link-Shutdown sub-TLV is needed to inform the controller or
 head-end routers.
                            _____________
                           |             |
             --------------| Controller  |--------------
             |             |____________ |             |
             |                                         |
             |--------- Primary Path ------------------|
             PE1---------P1----------------P2---------PE2
                         |                  |
                         |                  |
                         |________P3________|
                            Alternate Path
            Figure 8: Controller-Based Traffic Engineering
 In the above example, the 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 10 Gbps.  When the P1->P2 link
 is being prepared for maintenance, the controller receives the
 graceful-link-shutdown information, as there is no alternate path
 available that satisfies the constraints, and 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.

Hegde, et al. Standards Track [Page 12] RFC 8379 OSPF Graceful Link Shutdown May 2018

7.3. L3VPN Services and Sham Links

 Many service providers offer Layer 3 Virtual Private Network (L3VPN)
 services to customers, and CE-PE links run OSPF [RFC4577].  When the
 PE is taken out of service for maintenance, all the links on the PE
 can be set to graceful-link-shutdown state, which will guarantee that
 the traffic to/from dual-homed CEs gets diverted.  The interaction
 between OSPF and BGP is outside the scope of this document.  A
 mechanism based on [RFC6987] with summaries and externals that are
 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 use case is when ISPs provide sham-link services to
 customers [RFC4577].  When the PE goes out of service for
 maintenance, all sham links on the PE can be set to graceful-link-
 shutdown state, and traffic can be diverted from both ends without
 having to touch the configurations on the remote end of the sham
 links.

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 mechanism defined in [RFC6987] 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
 graceful-link-shutdown state, and traffic gets diverted from the
 Spoke sites as well without having to make configuration changes on
 the Spokes.

8. Security Considerations

 This document utilizes the OSPF packets and LSAs described in
 [RFC2328] , [RFC3630], [RFC5329], and [RFC5340].  The authentication
 procedures described in [RFC2328] for OSPFv2 and [RFC4552] for OSPFv3
 are applicable to this document as well.  This document does not
 introduce any further security issues other than those discussed in
 [RFC2328] and [RFC5340].

Hegde, et al. Standards Track [Page 13] RFC 8379 OSPF Graceful Link Shutdown May 2018

9. IANA Considerations

 IANA has registered the following in the "OSPFv2 Extended Link TLV
 Sub-TLVs" registry:
    7 - Graceful-Link-Shutdown Sub-TLV
    8 - Remote IPv4 Address Sub-TLV
    9 - Local/Remote Interface ID Sub-TLV
 IANA has registered the following value in the "OSPFv3 Extended-LSA
 Sub-TLVs" registry:
    8 - Graceful-Link-Shutdown sub-TLV
 IANA has registered the following value in the "BGP-LS Node
 Descriptor, Link Descriptor, Prefix Descriptor, and Attribute TLVs"
 registry [RFC7752]":
    1121 - Graceful-Link-Shutdown TLV

10. References

10.1. 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>.
 [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
            DOI 10.17487/RFC2328, April 1998,
            <https://www.rfc-editor.org/info/rfc2328>.
 [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
            (TE) Extensions to OSPF Version 2", RFC 3630,
            DOI 10.17487/RFC3630, September 2003,
            <https://www.rfc-editor.org/info/rfc3630>.
 [RFC5329]  Ishiguro, K., Manral, V., Davey, A., and A. Lindem, Ed.,
            "Traffic Engineering Extensions to OSPF Version 3",
            RFC 5329, DOI 10.17487/RFC5329, September 2008,
            <https://www.rfc-editor.org/info/rfc5329>.
 [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>.

Hegde, et al. Standards Track [Page 14] RFC 8379 OSPF Graceful Link Shutdown May 2018

 [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>.
 [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>.
 [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>.
 [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>.
 [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>.
 [RFC8362]  Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and
            F. Baker, "OSPFv3 Link State Advertisement (LSA)
            Extensibility", RFC 8362, DOI 10.17487/RFC8362, April
            2018, <https://www.rfc-editor.org/info/rfc8362>.

Hegde, et al. Standards Track [Page 15] RFC 8379 OSPF Graceful Link Shutdown May 2018

10.2. Informative References

 [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>.
 [RFC4552]  Gupta, M. and N. Melam, "Authentication/Confidentiality
            for OSPFv3", RFC 4552, DOI 10.17487/RFC4552, June 2006,
            <https://www.rfc-editor.org/info/rfc4552>.
 [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>.

Acknowledgements

 Thanks to Chris Bowers for valuable input and edits to the document.
 Thanks to Jeffrey Zhang, Acee Lindem, and Ketan Talaulikar for their
 input.  Thanks to Karsten Thomann for careful review and input on the
 applications where graceful link shutdown is useful.
 Thanks to Alia Atlas, Deborah Brungard, Alvaro Retana, Andrew G.
 Malis, and Tim Chown for their valuable input.

Hegde, et al. Standards Track [Page 16] RFC 8379 OSPF Graceful Link Shutdown May 2018

Authors' Addresses

 Shraddha Hegde
 Juniper Networks, Inc.
 Embassy Business Park
 Bangalore, KA  560093
 India
 Email: shraddha@juniper.net
 Pushpasis Sarkar
 Arrcus, Inc.
 Email: pushpasis.ietf@gmail.com
 Hannes Gredler
 RtBrick Inc.
 Email: hannes@rtbrick.com
 Mohan Nanduri
 ebay Corporation
 2025 Hamilton Avenue
 San Jose, CA  98052
 United States of America
 Email: mnanduri@ebay.com
 Luay Jalil
 Verizon
 Email: luay.jalil@verizon.com

Hegde, et al. Standards Track [Page 17]

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