Internet Engineering Task Force (IETF)                  A. Bashandy, Ed.
Request for Comments: 8661                                    Individual
Category: Standards Track                               C. Filsfils, Ed.
ISSN: 2070-1721                                               S. Previdi                                      Cisco Systems, Inc.
                                                              S. Previdi
                                                     Huawei Technologies
                                                             B. Decraene
                                                            S. Litkowski
                                                                  Orange
                                                          September
                                                            October 2019

               Segment Routing MPLS Interworking with LDP

Abstract

   A Segment Routing (SR) node steers a packet through a controlled set
   of instructions, called segments, by prepending the packet with an SR
   header.  A segment can represent any instruction, topological or
   service based.  SR allows enforcing a flow through any topological
   path while maintaining per-flow state only at the ingress node to the
   SR domain.

   The Segment Routing architecture can be directly applied to the MPLS
   data plane with no change in the forwarding plane.  This document
   describes how Segment Routing MPLS operates in a network where LDP is
   deployed and in the case where SR-capable and non-SR-capable nodes
   coexist.

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/rfc8661.

Copyright Notice

   Copyright (c) 2019 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
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   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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  SR/LDP  SR-LDP Ships-in-the-Night Coexistence . . . . . . . . . . . .   3
     2.1.  MPLS2MPLS, MPLS2IP, and IP2MPLS Coexistence . . . . . . .   5
   3.  SR and LDP Interworking . . . . . . . . . . . . . . . . . . .   6
     3.1.  LDP to SR . . . . . . . . . . . . . . . . . . . . . . . .   6
       3.1.1.  LDP to SR Behavior  . . . . . . . . . . . . . . . . .   7
     3.2.  SR to LDP . . . . . . . . . . . . . . . . . . . . . . . .   7
       3.2.1.  Segment Routing Mapping Server (SRMS) . . . . . . . .   9
       3.2.2.  SR to LDP Behavior  . . . . . . . . . . . . . . . . .  10
       3.2.3.  Interoperability of Multiple SRMSes and Prefix
               SID Advertisements  . . . . . . . . . . . . . . . . . . .  10
   4.  SR/LDP  SR-LDP Interworking Use Cases . . . . . . . . . . . . . . . .  12
     4.1.  SR Protection of LDP-Based Traffic  . . . . . . . . . . .  12
     4.2.  Eliminating Targeted LDP Sessions . . . . . . . . . . . .  14
     4.3.  Guaranteed FRR Coverage . . . . . . . . . . . . . . . . .  15
     4.4.  Inter-AS Option C, Carrier's Carrier  . . . . . . . . . .  16
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   6.  Manageability Considerations  . . . . . . . . . . . . . . . .  17
     6.1.  SR and LDP Coexistence  . . . . . . . . . . . . . . . . .  17
     6.2.  Data-Plane Verification . . . . . . . . . . . . . . . . .  17
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  18
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  19
   Appendix A.  Migration from LDP to SR . . . . . . . . . . . . . .  20
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  21
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   Segment Routing, as described in [RFC8402], can be used on top of the
   MPLS data plane without any modification as described in [RFC8660].

   Segment Routing control plane can coexist with current label
   distribution protocols such as LDP [RFC5036].

   This document outlines the mechanisms through which SR interworks
   with LDP in cases where a mix of SR-capable and non-SR-capable
   routers coexist within the same network and more precisely in the
   same routing domain.

   Section 2 describes the coexistence of SR with other MPLS control-
   plane protocols.  Section 3 documents the interworking between SR and
   LDP in the case of nonhomogeneous deployment.  Section 4 describes
   how a partial SR deployment can be used to provide SR benefits to
   LDP-based traffic including a possible application of SR in the
   context of interdomain MPLS use cases.  Appendix A documents a method
   to migrate from LDP to SR-based MPLS tunneling.

   Typically, an implementation will allow an operator to select
   (through configuration) which of the described modes of SR and LDP
   coexistence to use.

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.  SR/LDP  SR-LDP Ships-in-the-Night Coexistence

   "MPLS Control-Plane Client (MCC)" refers to any control-plane
   protocol installing forwarding entries in the MPLS data plane.  SR,
   LDP [RFC5036], RSVP-TE [RFC3209], BGP [RFC8277], etc., are examples
   of MCCs.

   An MCC, operating at node N, must ensure that the incoming label it
   installs in the MPLS data plane of node N has been uniquely allocated
   to itself.

   Segment Routing makes use of the Segment Routing Global Block (SRGB,
   as defined in [RFC8402]) for the label allocation.  The use of the
   SRGB allows SR to coexist with any other MCC.

   This is clearly the case for the adjacency segment: it is a local
   label allocated by the label manager, as is the case for for any MCC.

   This is clearly the case for the prefix segment: the label manager
   allocates the SRGB set of labels to the SR MCC client, and the
   operator ensures the unique allocation of each global prefix segment
   or label within the allocated SRGB set.

   Note that this static label-allocation capability of the label
   manager has existed for many years across several vendors and is
   therefore not new.  Furthermore, note that the label manager's
   ability to statically allocate a range of labels to a specific
   application is not new either.  This is required for MPLS-TP
   operation.  In this case, the range is reserved by the label manager,
   and it is the MPLS-TP [RFC5960] Network Management System (acting as
   an MCC) that ensures the unique allocation of any label within the
   allocated range and the creation of the related MPLS forwarding
   entry.

   Let us illustrate an example of ship-in-the-night (SIN) coexistence.

                              PE2          PE4
                                \          /
                          PE1----A----B---C---PE3

                         Figure 1: SIN Coexistence

   The EVEN VPN service is supported by PE2 and PE4 while the ODD VPN
   service is supported by PE1 and PE3.  The operator wants to tunnel
   the ODD service via LDP and the EVEN service via SR.

   This can be achieved in the following manner:

   *  The operator configures PE1, PE2, PE3, and PE4 with respective
      loopbacks 192.0.2.201/32, 192.0.2.202/32, 192.0.2.203/32, and
      192.0.2.204/32.  These PEs advertised their VPN routes with next-
      hop set on their respective loopback address.

   *  The operator configures A, B, C with respective loopbacks
      192.0.2.1/32, 192.0.2.2/32, 192.0.2.3/32.

   *  The operator configures PE2, A, B, C, and PE4 with SRGB [100,
      300].

   *  The operator attaches the respective Node Segment Identifiers
      (Node-SIDs, as defined in [RFC8402]), 202, 101, 102, 103, and 204,
      to the loopbacks of nodes PE2, A, B, C, and PE4.  The Node-SIDs
      are configured to request Penultimate Hop Popping.

   *  PE1, A, B, C, and PE3 are LDP capable.

   *  PE1 and PE3 are not SR capable.

   PE3 sends an ODD VPN route to PE1 with next-hop 192.0.2.203 and VPN
   label 10001.

   From an LDP viewpoint, PE1 received an LDP label binding (1037) for a
   forwarding equivalence class
   Forwarding Equivalence Class (FEC) 192.0.2.203/32 from its next-hop
   A; A received an LDP label binding (2048) for that FEC from its next-
   hop B; B received an LDP label binding (3059) for that FEC from its
   next-hop C; and C received implicit-null LDP binding from its next-
   hop PE3.

   As a result, PE1 sends its traffic to the ODD service route
   advertised by PE3 to next-hop A with two labels: the top label is
   1037 and the bottom label is 10001.  Node A swaps 1037 with 2048 and
   forwards to B; B swaps 2048 with 3059 and forwards to C; C pops 3059
   and forwards to PE3.

   PE4 sends an EVEN VPN route to PE2 with next-hop 192.0.2.204 and VPN
   label 10002.

   From an SR viewpoint, PE2 maps the IGP route 192.0.2.204/32 onto
   Node-SID 204; node A swaps 204 with 204 and forwards to B; B swaps
   204 with 204 and forwards to C; and C pops 204 and forwards to PE4.

   As a result, PE2 sends its traffic to the VPN service route
   advertised by PE4 to next-hop A with two labels: the top label is 204
   and the bottom label is 10002.  Node A swaps 204 with 204 and
   forwards to B.  B swaps 204 with 204 and forwards to C.  C pops 204
   and forwards to PE4.

   The two modes of MPLS tunneling coexist.

   *  The ODD service is tunneled from PE1 to PE3 through a continuous
      LDP LSP traversing A, B, and C.

   *  The EVEN service is tunneled from PE2 to PE4 through a continuous
      SR node segment traversing A, B, and C.

2.1.  MPLS2MPLS, MPLS2IP, and IP2MPLS Coexistence

   MPLS2MPLS refers to the forwarding behavior where a router receives a
   labeled packet and switches it out as a labeled packet.  Several
   MPLS2MPLS entries may be installed in the data plane for the same
   prefix.

   Let us examine A's MPLS forwarding table as an example:

      Incoming label: 1037

      - outgoing  Outgoing label: 2048

      - outgoing  Outgoing next-hop: B

      Note: this This entry is programmed by LDP for 192.0.2.203/32 192.0.2.203/32.

      Incoming label: 203

      - outgoing  Outgoing label: 203

      - outgoing  Outgoing next-hop: B

      Note: this This entry is programmed by SR for 192.0.2.203/32 192.0.2.203/32.

   These two entries can coexist because their incoming label is unique.
   The uniqueness is guaranteed by the label manager allocation rules.

   The same applies for the MPLS2IP forwarding entries.  MPLS2IP is the
   forwarding behavior where a router receives a labeled IPv4/IPv6
   packet with one label only, pops the label, and switches the packet
   out as IPv4/IPv6.  For IP2MPLS coexistence, refer to Section 6.1.

3.  SR and LDP Interworking

   This section analyzes the case where SR is available in one part of
   the network and LDP is available in another part.  It describes how a
   continuous MPLS tunnel can be built throughout the network.

                             PE2            PE4
                               \            /
                        PE1----P5--P6--P7--P8---PE3

                        <-----SR---->
                                  <------LDP------>

                     Figure 2: SR and LDP Interworking

   Let us analyze the following example:

   *  P6, P7, P8, PE4, and PE3 are LDP capable.

   *  PE1, PE2, P5, and P6 are SR capable.  PE1, PE2, P5, and P6 are
      configured with SRGB (100, 200) and respectively with node segments 101, 102,
      105, and 106. 106, respectively.

   *  A service flow must be tunneled from PE1 to PE3 over a continuous
      MPLS tunnel encapsulation and encapsulation; therefore, SR and LDP need to
      interwork.

3.1.  LDP to SR

   In this section, a right-to-left traffic flow is analyzed.

   PE3 has learned a service route whose next-hop is PE1.  PE3 has an
   LDP label binding from the next-hop P8 for the FEC "PE1".  Therefore,
   PE3 sends its service packet to P8 as per classic LDP behavior.

   P8 has an LDP label binding from its next-hop P7 for the FEC "PE1"
   and therefore, P8 forwards to P7 as per classic LDP behavior.

   P7 has an LDP label binding from its next-hop P6 for the FEC "PE1"
   and therefore, P7 forwards to P6 as per classic LDP behavior.

   P6 does not have an LDP binding from its next-hop P5 for the FEC
   "PE1".  However, P6 has an SR node segment to the IGP route "PE1".
   Hence, P6 forwards the packet to P5 and swaps its local LDP label for
   FEC "PE1" by the equivalent node segment (i.e., 101).

   P5 pops 101 (assuming PE1 advertised its node segment 101 with the
   penultimate-pop flag set) and forwards to PE1.

   PE1 receives the tunneled packet and processes the service label.

   The end-to-end MPLS tunnel is built from by stitching an LDP LSP from PE3
   to P6 and the related node segment from P6 to PE1.

3.1.1.  LDP to SR Behavior

   It has to be noted that no additional signaling or state is required
   in order to provide interworking in the direction LDP to SR.

   An SR node having LDP neighbors MUST create LDP bindings for each
   Prefix SID learned in the SR domain by treating SR-learned labels as
   if they were learned through an LDP neighbor.  In addition, for each
   FEC, the SR node stitches the incoming LDP label to the outgoing SR
   label.  This has to be done in both LDP-independent and ordered label
   distribution control modes as defined in [RFC5036].

3.2.  SR to LDP

   In this section, the left-to-right traffic flow is analyzed.

   This section defines the Segment Routing Mapping Server (SRMS).  The
   SRMS is an IGP node advertising mapping between Segment Identifiers
   (SID) and prefixes advertised by other IGP nodes.  The SRMS uses a
   dedicated IGP extension (IS-IS, OSPFv2, and OSPFv3), which is
   protocol specific and defined in [RFC8665], [RFC8666], and [RFC8667].

   The SRMS function of an SR-capable router allows distribution of
   mappings for prefixes not locally attached to the advertising router
   and therefore allows advertisement of mappings on behalf of non-SR-
   capable routers.

   The SRMS is a control-plane-only function that may be located
   anywhere in the IGP flooding scope.  At least one SRMS server MUST
   exist in a routing domain to advertise Prefix SIDs on behalf of
   non-SR nodes, thereby allowing non-LDP routers to send and receive
   labeled traffic from LDP-only routers.  Multiple SRMSes may be
   present in the same network (for redundancy).  This implies that
   there are multiple ways a prefix-to-SID mapping can be advertised.
   Conflicts resulting from inconsistent advertisements are addressed by
   [RFC8660].

   The example depicted in Figure 2 assumes that the operator configures
   P5 to act as a Segment Routing Mapping Server (SRMS) and advertises
   the following mappings: (P7, 107), (P8, 108), (PE3, 103), and (PE4,
   104).

   The mappings advertised by one or more SRMSes result from local
   policy information configured by the operator.

   If PE3 had been SR capable, the operator would have configured PE3
   with node segment 103.  Instead, as PE3 is not SR capable, the
   operator configures that policy at the SRMS and it is the latter that
   advertises the mapping.

   The mapping server advertisements are only understood by SR-capable
   routers.  The SR-capable routers install the related node segments in
   the MPLS data plane exactly like the node segments had been
   advertised by the nodes themselves.

   For example, PE1 installs the node segment 103 with next-hop P5
   exactly as if PE3 had advertised node segment 103.

   PE1 has a service route whose next-hop is PE3.  PE1 has a node
   segment for that IGP route: 103 with next-hop P5.  Hence, PE1 sends
   its service packet to P5 with two labels: the bottom label is the
   service label and the top label is 103.

   P5 swaps 103 for 103 and forwards to P6.

   P6's next-hop for the IGP route "PE3" is not SR capable (P7 does not
   advertise the SR capability).  However, P6 has an LDP label binding
   from that next-hop for the same FEC (e.g., LDP label 1037).  Hence,
   P6 swaps 103 for 1037 and forwards to P7.

   P7 swaps this label with the LDP label received from P8 and forwards
   to P8.

   P8 pops the LDP label and forwards to PE3.

   PE3 receives the tunneled packet and processes the service label.

   The end-to-end MPLS tunnel is built from by stitching an SR node segment
   from PE1 to P6 and an LDP LSP from P6 to PE3.

   SR-mapping advertisement for a given prefix provides no information
   about Penultimate Hop Popping.  Other mechanisms, such as IGP-
   specific mechanisms ([RFC8665], [RFC8666] [RFC8666], and [RFC8667]), MAY be
   used to determine the Penultimate Hop Popping in such case.

      |  Note: In the previous example, Penultimate Hop Popping is not
      |  performed at the SR/LDP SR-LDP border for segment 103 (PE3), because
      |  none of the routers in the SR domain are Penultimate Hop for
      |  segment 103.  In this case, P6 requires the presence of the
      |  segment 103 such as to map it to the LDP label 1037.

3.2.1.  Segment Routing Mapping Server (SRMS)

   This section specifies the concept and externally visible
   functionality of a segment routing mapping server (SRMS).

   The purpose of SRMS functionality is to support the advertisement of
   Prefix SIDs to a prefix without the need to explicitly advertise such
   assignment within a prefix reachability advertisement.  Examples of
   explicit Prefix SID advertisement are the prefix-SID Prefix SID sub-TLVs defined
   in [RFC8665], [RFC8666], and [RFC8667].

   The SRMS functionality allows assigning of Prefix SIDs to prefixes
   owned by non-SR-capable routers as well as to prefixes owned by SR-
   capable nodes.  It is the former capability that is essential to the
   SR-LDP interworking described later in this section.

   The SRMS functionality consists of two functional blocks: the Mapping
   Server (MS) and Mapping Client (MC).

   An MS is a node that advertises an SR mappings.  Advertisements sent
   by an MS define the assignment of a Prefix SID to a prefix
   independent of the advertisement of reachability to the prefix
   itself.  An MS MAY advertise SR mappings for any prefix whether or
   not it advertises reachability for the prefix and irrespective of
   whether that prefix is advertised by or even reachable through any
   router in the network.

   An MC is a node that receives and uses the MS mapping advertisements.
   Note that a node may be both an MS and an MC.  An MC interprets the
   SR-mapping advertisement as an assignment of a Prefix SID to a
   prefix.  For a given prefix, if an MC receives an SR-mapping
   advertisement from a mapping server and also has received a Prefix
   SID advertisement for that same prefix in a prefix reachability
   advertisement, then the MC MUST prefer the SID advertised in the
   prefix reachability advertisement over the mapping server
   advertisement, i.e., the mapping server advertisement MUST be ignored
   for that prefix.  Hence, assigning a Prefix SID to a prefix using the
   SRMS functionality does not preclude assigning the same or different
   Prefix SID(s) to the same prefix using explicit Prefix SID
   advertisement such as the aforementioned prefix-SID Prefix SID sub-TLVs.

   For example, consider an IPv4 prefix advertisement received by an
   IS-IS router in the Extended IP reachability TLV (TLV 135).  Suppose
   TLV 135 contained the prefix-SID Prefix SID sub-TLV.  If the router that
   receives TLV 135 with the prefix-SID Prefix SID sub-TLV also received an SR-
   mapping advertisement for the same prefix through the SID/Label
   binding
   Binding TLV, then the receiving router must prefer the prefix-SID Prefix SID
   sub-TLV over the SID/Label binding Binding TLV for that prefix.  Refer to
   [RFC8667] for details about the prefix-SID Prefix SID sub-TLV and SID/Label
   binding
   Binding TLV.

3.2.2.  SR to LDP Behavior

   SR to LDP interworking requires an SRMS as defined above.

   Each SR-capable router installs in the MPLS data-plane Node-SIDs
   learned from the SRMS exactly like as if these SIDs had been advertised by
   the nodes themselves.

   An SR node having LDP LDP-only neighbors MUST stitch the incoming SR
   label (whose SID is advertised by the SRMS) to the outgoing LDP
   label.

   It has to be noted that the SR to LDP behavior does not propagate the
   status of the LDP FEC that was signaled if by LDP was when configured to use
   the in
   ordered mode.

   It has to be noted that in the case of SR to LDP, the label binding
   is equivalent to the independent LDP Label Distribution Control Mode
   [RFC5036] where a label in is bound to an a FEC independently from the
   received binding for the same FEC.

3.2.3.  Interoperability of Multiple SRMSes and Prefix SID
        Advertisements

   In the case of SR/LDP SR-LDP interoperability through the use of an SRMS,
   mappings are advertised by one or more SRMSes.

   SRMS functionality is implemented in the link-state protocol (such as
   IS-IS and OSPF).  Link-state protocols allow propagation of updates
   across area boundaries and, therefore, SRMS advertisements are
   propagated through the usual inter-area advertisement procedures in
   link-state protocols.

   Multiple SRMSes can be provisioned in a network for redundancy.
   Moreover, a preference mechanism may also be used among SRMSes to
   deploy a primary/secondary SRMS scheme allowing controlled
   modification or migration of SIDs.

   The content of SRMS advertisement (i.e., mappings) are a matter of
   local policy determined by the operator.  When multiple SRMSes are
   active, it is necessary that the information (mappings) advertised by
   the different SRMSes is aligned and consistent.  The following
   mechanism is applied to determine the preference of SRMS
   advertisements:

   If a node acts as an SRMS, it MAY advertise a preference to be
   associated with all SRMS SID advertisements sent by that node.  The
   means of advertising the preference is defined in the protocol-
   specific documents, e.g., [RFC8665], [RFC8666], and [RFC8667].  The
   preference value is an unsigned 8-bit integer with the following
   properties:

           +-------+------------------------------------------+
           |   0 -   | Reserved value indicating advertisements |
           |       | from that node MUST NOT be used.

      1 - 255 used          |
           +-------+------------------------------------------+
           | 1-255 | Preference value (255 is most preferred) |
           +-------+------------------------------------------+

                                 Table 1

   Advertisement of a preference value is optional.  Nodes that do not
   advertise a preference value are assigned a preference value of 128.

   An MCC on a node receiving one or more SRMS mapping advertisements
   applies them as follows:

   -

   *  For any prefix for which it did not receive a Prefix SID
      advertisement, the MCC applies the SRMS mapping advertisements
      with the highest preference.  The mechanism by which a Prefix SID
      is advertised for a given prefix is defined in the protocol
      specifications [RFC8665], [RFC8666], and [RFC8667].

   -

   *  If there is an incoming label collision as specified in [RFC8660],
      apply the steps specified in [RFC8660] to resolve the collision.

   When the SRMS advertises mappings, an implementation should provide a
   mechanism through which the operator determines which of the IP2MPLS
   mappings are preferred among the one advertised by the SRMS and the
   ones advertised by LDP.

4.  SR/LDP  SR-LDP Interworking Use Cases

   SR can be deployed, for example, to enhance LDP transport.  The SR
   deployment can be limited to the network region where the SR benefits
   are most desired.

4.1.  SR Protection of LDP-Based Traffic

   In Figure 3, let us assume:

   *  All link costs are 10 except FG, which is 30.

   *  All routers are LDP capable.

   *  X, Y, and Z are PEs participating in an important service S.

   *  The operator requires 50 msec link-based Fast Reroute (FRR) for
      service S.

   *  A, B, C, D, E, F, and G are SR capable.

   *  X, Y, and Z are not SR capable, e.g., as part of a staged
      migration from LDP to SR, the operator deploys SR first in a
      subpart of the network and then everywhere.

                                     X
                                     |
                              Y--A---B---E--Z
                                 |   |    \
                                 D---C--F--G
                                         30

                   Figure 3: SR/LDP SR-LDP Interworking Example

   The operator would like to resolve the following issues:

   *  To protect the link BA along the shortest-path of the important
      flow XY, B requires a Remote Loop-Free Alternate (RLFA; see
      [RFC7490]) repair tunnel to D and, therefore, a targeted LDP
      session from B to D.  Typically, network operators prefer avoiding
      these dynamically established multi-hop LDP sessions in order to
      reduce the number of protocols running in the network and,
      therefore, simplify network operations.

   *  There is no LFA/RLFA solution to protect the link BE along the
      shortest path of the important flow XZ.  The operator wants a
      guaranteed link-based FRR solution.

   The operator can meet these objectives by deploying SR only on A, B,
   C, D, E, F, and G:

   *  The operator configures A, B, C, D, E, F, and G with SRGB [100,
      200] and respective with node segments 101, 102, 103, 104, 105, 106, and 107. 107,
      respectively.

   *  The operator configures D as an SR Mapping Server with the
      following policy mapping: (X, 201), (Y, 202), and (Z, 203).

   *  Each SR node automatically advertises a local adjacency segment
      for its IGP adjacencies.  Specifically, F advertises adjacency
      segment 9001 for its adjacency FG.

   A, B, C, D, E, F F, and G keep their LDP capability and hence capability.  Therefore, the
   flows XY and XZ are transported over end-to-end LDP LSPs.

   For example, LDP at B installs the following MPLS data-plane entries:

      Incoming label: local LDP label bound by B for FEC Y

      -  Outgoing label: LDP label bound by A for FEC Y

      -  Outgoing next-hop: A

      Incoming label: local LDP label bound by B for FEC Z

      -  Outgoing label: LDP label bound by E for FEC Z

      -  Outgoing next-hop: E

   The novelty comes from how the backup chains are computed for these
   LDP-based entries.  While LDP labels are used for the primary next-
   hop and outgoing labels, SR information is used for the FRR
   construction.  In steady state, the traffic is transported over LDP
   LSP.  In transient FRR state, the traffic is backup thanks to the SR-
   enhanced capabilities.

   The RLFA paths are dynamically precomputed as defined in [RFC7490].
   Typically, implementations allow to enable an RLFA mechanism through
   a simple configuration command that triggers both the precomputation
   and installation of the repair path.  The details on how RLFA
   mechanisms are implemented and configured is outside the scope of
   this document and not relevant to the aspects of SR/LDP SR-LDP interwork
   explained in this document.

   This helps meet the requirements of the operator:

   *  Eliminate targeted LDP sessions.

      Guarantee

   *  Provide guaranteed FRR coverage.

   *  Keep the traffic over LDP LSP in steady state.

   *  Partially deploy SR only where needed.

4.2.  Eliminating Targeted LDP Sessions

   B's MPLS entry to Y becomes:

   -

      Incoming label: local LDP label bound by B for FEC Y

      -  Outgoing label: LDP label bound by A for FEC Y

      -  Backup outgoing label: SR node segment for Y {202}

      -  Outgoing next-hop: A

      -  Backup next-hop: repair tunnel: node segment to D {104} with
         outgoing next-hop: C

   It has to be noted that D is selected as a Remote Loop-Free Alternate
   (RLFA) as defined in [RFC7490].

   In steady state, X sends its Y-destined traffic to B with a top
   label, which is the LDP label bound by B for FEC Y.  B swaps that top
   label for the LDP label bound by A for FEC Y and forwards to A.  A
   pops the LDP label and forwards to Y.

   Upon failure of the link BA, B swaps the incoming top label with the
   node segment for Y (202) and sends the packet onto a repair tunnel to
   D (node segment 104).  Thus, B sends the packet to C with the label
   stack {104, 202}.  C pops the node segment 104 and forwards to D.  D
   swaps 202 for 202 and forwards to A.  A's next-hop to Y is not SR
   capable, and therefore, node A swaps the incoming node segment 202 to
   the LDP label announced by its next-hop (in this case, implicit
   null).

   After IGP convergence, B's MPLS entry to Y will become:

   -

      Incoming label: local LDP label bound by B for FEC Y

      -  Outgoing label: LDP label bound by C for FEC Y

      -  Outgoing next-hop: C

   And the traffic XY travels again over the LDP LSP.

   Conclusion: the operator has eliminated the need for targeted LDP
   sessions (no longer required) and the steady-state traffic is still
   transported over LDP.  The SR deployment is confined to the area
   where these benefits are required.

   Despite that, in general, an implementation would not require a
   manual configuration of targeted LDP sessions.  However, it is always
   a gain if the operator is able to reduce the set of protocol sessions
   running on the network infrastructure.

4.3.  Guaranteed FRR Coverage

   As mentioned in Section 4.1, in the example topology described in
   Figure 3, there is no RLFA-based solution for protecting the traffic
   flow YZ against the failure of link BE because there is no
   intersection between the extended P-space and Q-space (see [RFC7490]
   for details).  However:

   -

   *  G belongs to the Q space of Z.

   -

   *  G can be reached from B via a "repair SR path" {106, 9001} that is
      not affected by failure of link BE.  (The method by which G and
      the repair tunnel to it from B are identified are outside the
      scope of this document.)

   B's MPLS entry to Z becomes:

   -

      Incoming label: local LDP label bound by B for FEC Z

      -  Outgoing label: LDP label bound by E for FEC Z

      -  Backup outgoing label: SR node segment for Z {203}

      -  Outgoing next-hop: E

      -  Backup next-hop: repair tunnel to G: {106, 9001}

      G is reachable from B via the combination of a node segment to F
      {106} and an adjacency segment FG {9001} {9001}.

      Note that {106, 107} would have equally work. worked.  Indeed, in many
      cases, P's shortest path to Q is over the link PQ.  The adjacency
      segment from P to Q is required only in very rare topologies where
      the shortest-path from P to Q is not via the link PQ.

   In steady state, X sends its Z-destined traffic to B with a top
   label, which is the LDP label bound by B for FEC Z.  B swaps that top
   label for the LDP label bound by E for FEC Z and forwards to E.  E
   pops the LDP label and forwards to Z.

   Upon failure of the link BE, B swaps the incoming top label with the
   node segment for Z (203) and sends the packet onto a repair tunnel to
   G (node segment 106 followed by adjacency segment 9001).  Thus, B
   sends the packet to C with the label stack {106, 9001, 203}.  C pops
   the node segment 106 and forwards to F.  F pops the adjacency segment
   9001 and forwards to G.  G swaps 203 for 203 and forwards to E.  E's
   next-hop to Z is not SR capable, and thus, E swaps the incoming node
   segment 203 for the LDP label announced by its next-hop (in this
   case, implicit null).

   After IGP convergence, B's MPLS entry to Z will become:

   -

      Incoming label: local LDP label bound by B for FEC Z

      -  Outgoing label: LDP label bound by C for FEC Z

      -  Outgoing next-hop: C

   And the traffic XZ travels again over the LDP LSP.

   Conclusions:

   -

   *  the operator has eliminated its second problem: guaranteed FRR
      coverage is provided.  The steady-state traffic is still
      transported over LDP.  The SR deployment is confined to the area
      where these benefits are required.

   -

   *  FRR coverage has been achieved without any signaling for setting
      up the repair LSP and without setting up a targeted LDP session
      between B and G.

4.4.  Inter-AS Option C, Carrier's Carrier

   In inter-AS Option C [RFC4364], two interconnected ASes sets up
   inter-AS MPLS connectivity.  SR may be independently deployed in each
   AS.

                       PE1---R1---B1---B2---R2---PE2
                       <----------->   <----------->
                            AS1            AS2

                        Figure 4: Inter-AS Option C

   In Inter-AS Option C, B2 advertises to B1 a labeled BGP route
   [RFC8277] for PE2, and B1 reflects it to its internal peers, e.g.,
   PE1.  PE1 learns from a service route reflector a service route whose
   next-hop is PE2.  PE1 resolves that service route on the labeled BGP
   route to PE2.  That labeled BGP route to PE2 is itself resolved on
   the AS1 IGP route to B1.

   If AS1 operates SR, then the tunnel from PE1 to B1 is provided by the
   node segment from PE1 to B1.

   PE1 sends a service packet with three labels: the top one is the node
   segment to B1, the next one is the label in the labeled BGP route
   provided by B1 for the route "PE2", and the bottom one is the service
   label allocated by PE2.

5.  IANA Considerations

   This document has no IANA actions.

6.  Manageability Considerations

6.1.  SR and LDP Coexistence

   When both SR and LDP coexist, the following applies:

   -

   *  If both SR and LDP propose an IP2MPLS entry for the same IP
      prefix, then by default the LDP route SHOULD be selected.  This is
      because it is expected that SR is introduced into networks that
      contain routers that do not support SR.  Hence, by having a
      behavior that prefers LDP over SR, traffic flow is unlikely to be
      disrupted

   -
      disrupted.

   *  A local policy on a router MUST allow to prefer the SR-provided
      IP2MPLS entry.

   -

   *  Note that this policy MAY be locally defined.  There is no
      requirement that all routers use the same policy.

6.2.  Data-Plane Verification

   When Label switch paths (LSPs) are defined by stitching LDP LSPs with
   SR LSPs, it is necessary to have mechanisms allowing the verification
   of the LSP connectivity as well as validation of the path.  These
   mechanisms are described in [RFC8287].

7.  Security Considerations

   This document does not introduce any change to the MPLS data plane
   [RFC3031] and therefore no additional security of the MPLS data plane
   is required.

   This document introduces another form of label binding
   advertisements.  The security associated with these advertisements is
   part of the security applied to routing protocols such as IS-IS
   [RFC5304] and OSPF [RFC5709], which both optionally make use of
   cryptographic authentication mechanisms.  This form of advertisement
   is more centralized, on behalf of the node advertising the IP
   reachability, which presents a different risk profile.  This document
   also specifies a mechanism by which the ill effects of advertising
   conflicting label bindings can be mitigated.  In particular,
   advertisements from the node advertising the IP reachability is more
   preferred than the centralized one.  Because this  This document recognizes that reachability, which presents a different risk profile.  This
   document miscofiguration
   errors in configuration and/or programming may result in false or
   conflicting also specifies a mechanism by which the ill effects of
   advertising label binding advertisements, thereby advertisements compromising traffic conflicting label bindings can be mitigated.  In particular,
   forwarding, the
   forwarding.  Therefore, this document recommends the operator
   implement strict configuration/
   advertisements from configuration/programmability control, the node advertising
   monitoring of the advertised SIDs, the preference of an IP
   reachability is more
   programmability control as well as montoring the SID advertised and
   preferred than the advertisement over a centralized one. log/error messages by SID advertisement,
   and the
   operator logging of any error message in order to avoid avoid, or at least
   significantly minimize minimize, the possibility of such risk.

8.  References

8.1.  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>.

   [RFC5036]  Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
              "LDP Specification", RFC 5036, DOI 10.17487/RFC5036,
              October 2007, <https://www.rfc-editor.org/info/rfc5036>.

   [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>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8660]  Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing with MPLS Data Plane", RFC 8660,
              DOI 10.17487/RFC8660, September October 2019,
              <https://www.rfc-editor.org/info/rfc8660>.

8.2.  Informative References

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,
              <https://www.rfc-editor.org/info/rfc3031>.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
              <https://www.rfc-editor.org/info/rfc3209>.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <https://www.rfc-editor.org/info/rfc4364>.

   [RFC5304]  Li, T. and R. Atkinson, "IS-IS Cryptographic
              Authentication", RFC 5304, DOI 10.17487/RFC5304, October
              2008, <https://www.rfc-editor.org/info/rfc5304>.

   [RFC5709]  Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M.,
              Li, T., and R. Atkinson, "OSPFv2 HMAC-SHA Cryptographic
              Authentication", RFC 5709, DOI 10.17487/RFC5709, October
              2009, <https://www.rfc-editor.org/info/rfc5709>.

   [RFC5960]  Frost, D., Ed., Bryant, S., Ed., and M. Bocci, Ed., "MPLS
              Transport Profile Data Plane Architecture", RFC 5960,
              DOI 10.17487/RFC5960, August 2010,
              <https://www.rfc-editor.org/info/rfc5960>.

   [RFC7490]  Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
              So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
              RFC 7490, DOI 10.17487/RFC7490, April 2015,
              <https://www.rfc-editor.org/info/rfc7490>.

   [RFC8277]  Rosen, E., "Using BGP to Bind MPLS Labels to Address
              Prefixes", RFC 8277, DOI 10.17487/RFC8277, October 2017,
              <https://www.rfc-editor.org/info/rfc8277>.

   [RFC8287]  Kumar, N., Ed., Pignataro, C., Ed., Swallow, G., Akiya,
              N., Kini, S., and M. Chen, "Label Switched Path (LSP)
              Ping/Traceroute for Segment Routing (SR) IGP-Prefix and
              IGP-Adjacency Segment Identifiers (SIDs) with MPLS Data
              Planes", RFC 8287, DOI 10.17487/RFC8287, December 2017,
              <https://www.rfc-editor.org/info/rfc8287>.

   [RFC8355]  Filsfils, C., Ed., Previdi, S., Ed., Decraene, B., and R.
              Shakir, "Resiliency Use Cases in Source Packet Routing in
              Networking (SPRING) Networks", RFC 8355,
              DOI 10.17487/RFC8355, March 2018,
              <https://www.rfc-editor.org/info/rfc8355>.

   [RFC8665]  Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
              H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
              Extensions for Segment Routing", RFC 8665,
              DOI 10.17487/RFC8665, September 2019. October 2019,
              <https://doi.org/10.17487/RFC8665>.

   [RFC8666]  Psenak, P., Ed. and S. Previdi, "OSPFv3 Extensions for
              Segment Routing", RFC 8666, DOI 10.17487/RFC8666,
              September 2019. October
              2019, <https://doi.org/10.17487/RFC8666>.

   [RFC8667]  Previdi, S., Ginsberg, L., Filsfils, C., Bashandy, A.,
              Gredler, H., and B. Decraene, "IS-IS Extensions for
              Segment Routing", RFC 8667, DOI 10.17487/RFC8667,
              September 2019. October
              2019, <https://doi.org/10.17487/RFC8667>.

Appendix A.  Migration from LDP to SR

                               PE2        PE4
                                 \        /
                          PE1----P5--P6--P7---PE3

                            Figure 5: Migration

   Several migration techniques are possible.  The technique described
   here is inspired by the commonly used method to migrate from one IGP
   to another.

   At time T0, all the routers run LDP.  Any service is tunneled from an
   ingress PE to an egress PE over a continuous LDP LSP.

   At time T1, all the routers are upgraded to SR.  They are configured
   with the SRGB range [100, 300].  PE1, PE2, PE3, PE4, P5, P6, and P7
   are respectively configured with the node segments 101, 102, 103,
   104, 105, 106, and 107 (attached to their service-recursing
   loopback).

      |  Note: At this time, the service traffic is still tunneled over
      |  LDP LSP. LSPs.  For example, PE1 has an SR node segment to PE3 and
      |  an LDP LSP to
      PE3 but by default, as PE3.  As seen earlier, however, the LDP IP2MPLS
      |  encapsulation is preferred. preferred by default.  However, it has to be
      |  noted that the SR infrastructure is usable, e.g., for Fast
      |  Reroute (FRR) or IGP Loop-Free Convergence to protect existing
      |  IP and LDP traffic.  FRR mechanisms are described in [RFC8355].

   At time T2, the operator enables the local policy at PE1 to prefer SR
   IP2MPLS encapsulation over LDP IP2MPLS.

      The service from PE1 to any other PE is now riding over SR.  All
      other service traffic is still transported over LDP LSPs.

   At time T3, gradually, the operator enables the preference for SR
   IP2MPLS encapsulation across all the edge routers.

      All the service traffic is now transported over SR.  LDP is still
      operational and services could be reverted to LDP.

   At time T4, LDP is unconfigured from all routers.

Acknowledgements

   The authors would like to thank Pierre Francois, Ruediger Geib, and
   Alexander Vainshtein for their contributions to the content of this
   document.

Contributors

   Edward Crabbe
   Individual
   Email: edward.crabbe@gmail.com

   Igor Milojevic
   Email: milojevicigor@gmail.com

   Saku Ytti
   TDC
   Email: saku@ytti.fi

   Rob Shakir
   Google
   Email: robjs@google.com

   Martin Horneffer
   Deutsche Telekom
   Email: Martin.Horneffer@telekom.de

   Wim Henderickx
   Nokia
   Email: wim.henderickx@nokia.com

   Jeff Tantsura
   Apstra, Inc.
   Email: jefftant.ietf@gmail.com

   Les Ginsberg
   Cisco Systems
   Email: ginsberg@cisco.com

Authors' Addresses

   Ahmed Bashandy (editor)
   Individual
   United States of America

   Email: abashandy.ietf@gmail.com

   Clarence Filsfils (editor)
   Cisco Systems, Inc.
   Brussels
   Belgium

   Email: cfilsfil@cisco.com

   Stefano Previdi
   Cisco Systems, Inc.
   Huawei Technologies
   Italy

   Email: stefano@previdi.net

   Bruno Decraene
   Orange
   France

   Email: bruno.decraene@orange.com

   Stephane Litkowski
   Orange
   France

   Email: stephane.litkowski@orange.com slitkows.ietf@gmail.com