Signaling RSVP-TE Tunnels on a Shared MPLS Forwarding Plane

Internet Engineering Task Force (IETF)                      H. Sitaraman
Request for Comments: 8577                                     V. Beeram
Category: Standards Track                               Juniper Networks
ISSN: 2070-1721                                                T. Parikh
                                                                 Verizon
                                                                 T. Saad
                                                           Cisco Systems
                                                              April 2019


      Signaling RSVP-TE Tunnels on a Shared MPLS Forwarding Plane

Abstract

   As the scale of MPLS RSVP-TE networks has grown, the number of Label
   Switched Paths (LSPs) supported by individual network elements has
   increased.  Various implementation recommendations have been proposed
   to manage the resulting increase in the amount of control-plane state
   information.

   However, those changes have had no effect on the number of labels
   that a transit Label Switching Router (LSR) has to support in the
   forwarding plane.  That number is governed by the number of LSPs
   transiting or terminated at the LSR and is directly related to the
   total LSP state in the control plane.

   This document defines a mechanism to prevent the maximum size of the
   label space limit on an LSR from being a constraint to control-plane
   scaling on that node.  It introduces the notion of preinstalled
   'per-TE link labels' that can be shared by MPLS RSVP-TE LSPs that
   traverse these TE links.  This approach significantly reduces the
   forwarding-plane state required to support a large number of LSPs.
   This couples the feature benefits of the RSVP-TE control plane with
   the simplicity of the Segment Routing (SR) MPLS forwarding plane.

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/rfc8577.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   6
   3.  Allocation of TE Link Labels  . . . . . . . . . . . . . . . .   6
   4.  Segment Routed RSVP-TE Tunnel Setup . . . . . . . . . . . . .   6
   5.  Delegating Label Stack Imposition . . . . . . . . . . . . . .   8
     5.1.  Stacking at the Ingress . . . . . . . . . . . . . . . . .   8
       5.1.1.  Stack to Reach Delegation Hop . . . . . . . . . . . .   9
       5.1.2.  Stack to Reach Egress . . . . . . . . . . . . . . . .  10
     5.2.  Explicit Delegation . . . . . . . . . . . . . . . . . . .  11
     5.3.  Automatic Delegation  . . . . . . . . . . . . . . . . . .  11
       5.3.1.  Effective Transport Label-Stack Depth (ETLD)  . . . .  11
   6.  Mixing TE Link Labels and Regular Labels in an RSVP-TE Tunnel  13
   7.  Construction of Label Stacks  . . . . . . . . . . . . . . . .  14
   8.  Facility Backup Protection  . . . . . . . . . . . . . . . . .  14
     8.1.  Link Protection . . . . . . . . . . . . . . . . . . . . .  14
   9.  Protocol Extensions . . . . . . . . . . . . . . . . . . . . .  15
     9.1.  Requirements  . . . . . . . . . . . . . . . . . . . . . .  15
     9.2.  Attribute Flags TLV: TE Link Label  . . . . . . . . . . .  16
     9.3.  RRO Label Sub-object Flag: TE Link Label  . . . . . . . .  16
     9.4.  Attribute Flags TLV: LSI-D  . . . . . . . . . . . . . . .  16
     9.5.  RRO Label Sub-object Flag: Delegation Label . . . . . . .  17
     9.6.  Attributes Flags TLV: LSI-D-S2E . . . . . . . . . . . . .  17
     9.7.  Attributes TLV: ETLD  . . . . . . . . . . . . . . . . . .  18
   10. OAM Considerations  . . . . . . . . . . . . . . . . . . . . .  18
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
     11.1.  Attribute Flags: TE Link Label, LSI-D, LSI-D-S2E . . . .  19
     11.2.  Attribute TLV: ETLD  . . . . . . . . . . . . . . . . . .  19
     11.3.  Record Route Label Sub-object Flags: TE Link Label,
            Delegation Label . . . . . . . . . . . . . . . . . . . .  20
     11.4.  Error Codes and Error Values . . . . . . . . . . . . . .  20
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  20
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     13.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  23
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

1.  Introduction

   The scaling of RSVP-TE [RFC3209] control-plane implementations can be
   improved by adopting the guidelines and mechanisms described in
   [RFC2961] and [RFC8370].  These documents do not affect the
   forwarding-plane state required to handle the control-plane state.
   The forwarding-plane state remains unchanged and is directly
   proportional to the total number of Label Switching Paths (LSPs)
   supported by the control plane.

   This document describes a mechanism that prevents the size of the
   platform-specific label space on a Label Switching Router (LSR) from
   being a constraint to pushing the limits of control-plane scaling on
   that node.

   This work introduces the notion of preinstalled 'per-TE link labels'
   that are allocated by an LSR.  Each such label is installed in the
   MPLS forwarding plane with a 'pop' operation and instructions to
   forward the received packet over the TE link.  An LSR advertises this
   label in the Label object of a Resv message as LSPs are set up, and
   they are recorded hop by hop in the Record Route Object (RRO) of the
   Resv message as it traverses the network.  The ingress Label Edge
   Router (LER) constructs and pushes a stack of labels [RFC3031] using
   the labels received in the RRO.  These 'TE link labels' can be shared
   by MPLS RSVP-TE LSPs that traverse the same TE link.

   This forwarding-plane behavior fits in the MPLS architecture
   [RFC3031] and is the same as that exhibited by Segment Routing (SR)
   [RFC8402] when using an MPLS forwarding plane and a series of
   adjacency segments [SEG-ROUTING].  This work couples the feature
   benefits of the RSVP-TE control plane with the simplicity of the SR
   MPLS forwarding plane.

   RSVP-TE using a shared MPLS forwarding plane offers the following
   benefits:

   1.  Shared labels: The transit label on a TE link is shared among
       RSVP-TE tunnels traversing the link and is used independently of
       the ingress and egress of the LSPs.

   2.  Faster LSP setup time: No forwarding-plane state needs to be
       programmed during LSP setup and teardown, resulting in faster
       provisioning and deprovisioning of LSPs.

   3.  Hitless rerouting: New transit labels are not required during
       make-before-break (MBB) in scenarios where the new LSP instance
       traverses the exact same path as the old LSP instance.  This
       saves the ingress LER and the services that use the tunnel from
       needing to update the forwarding plane with new tunnel labels,
       thereby making MBB events faster.  Periodic MBB events are
       relatively common in networks that deploy the 'auto-bandwidth'
       feature on RSVP-TE LSPs to monitor bandwidth utilization and
       periodically adjust LSP bandwidth.

   4.  Mix-and-match labels: Both 'TE link labels' and regular labels
       can be used on transit hops for a single RSVP-TE tunnel (see
       Section 6).  This allows backward compatibility with transit LSRs
       that provide regular labels in Resv messages.

   No additional extensions to routing protocols are required in order
   to support key functionalities such as bandwidth admission control,
   LSP priorities, preemption, and auto-bandwidth on this shared MPLS
   forwarding plane.  This document also discusses how Fast Reroute
   [RFC4090] via facility backup link protection using regular bypass
   tunnels can be supported on this forwarding plane.

   The signaling procedures and extensions discussed in this document do
   not apply to Point to Multipoint (P2MP) RSVP-TE tunnels.

2.  Terminology

   The following terms are used in this document:

   TE link label:   An incoming label at an LSR that will be popped by
      the LSR with the packet being forwarded over a specific outgoing
      TE link to a neighbor.

   Shared MPLS forwarding plane:   An MPLS forwarding plane where every
      participating LSR uses TE link labels on every LSP.

   Segment Routed RSVP-TE tunnel:   An MPLS RSVP-TE tunnel that requests
      the use of a shared MPLS forwarding plane at every hop of the LSP.
      The corresponding LSPs are referred to as "Segment Routed RSVP-TE
      LSPs".

   Delegation hop:   A transit hop of a Segment Routed RSVP-TE LSP that
      is selected to assist in the imposition of the label stack in
      scenarios where the ingress LER cannot impose the full label
      stack.  There can be multiple delegation hops along the path of a
      Segment Routed RSVP-TE LSP.

   Delegation label:   A label assigned at the delegation hop to
      represent a set of labels that will be pushed at this hop.

2.1.  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.

3.  Allocation of TE Link Labels

   An LSR that participates in a shared MPLS forwarding plane MUST
   allocate a unique TE link label for each TE link.  When an LSR
   encounters a TE link label at the top of the label stack, it MUST pop
   the label and forward the packet over the TE link to the downstream
   neighbor on the RSVP-TE tunnel.

   Multiple TE link labels MAY be allocated for the TE link to
   accommodate tunnels requesting protection.

   Implementations that maintain per-label bandwidth accounting at each
   hop must aggregate the reservations made for all the LSPs using the
   shared TE link label.

4.  Segment Routed RSVP-TE Tunnel Setup

   This section provides an example of how the RSVP-TE signaling
   procedure works to set up a tunnel utilizing a shared MPLS forwarding
   plane.  The sample topology below is used to explain the example.
   Labels shown at each node are TE link labels that, when present at
   the top of the label stack, indicate that they should be popped and
   that the packet should be forwarded on the TE link to the neighbor.

    +---+100  +---+150  +---+200  +---+250  +---+
    | A |-----| B |-----| C |-----| D |-----| E |
    +---+     +---+     +---+     +---+     +---+
      |110      |450      |550      |650      |850
      |         |         |         |         |
      |         |400      |500      |600      |800
      |       +---+     +---+     +---+     +---+
      +-------| F |-----|G  |-----|H  |-----|I  |
              +---+300  +---+350  +---+700  +---+

                Figure 1: Sample Topology -- TE Link Labels

   Consider two tunnels:

      RSVP-TE tunnel T1: From A to E on path A-B-C-D-E

      RSVP-TE tunnel T2: From F to E on path F-B-C-D-E

   Both tunnels share the TE links B-C, C-D, and D-E.

   RSVP-TE is used to signal the setup of tunnel T1 (using the TE link
   label attributes flag defined in Section 9.2).  When LSR D receives
   the Resv message from the egress LER E, it checks the next-hop TE
   link (D-E) and provides the TE link label (250) in the Resv message
   for the tunnel placing the label value in the Label object.  It also
   provides the TE link label (250) in the Label sub-object carried in
   the RRO and sets the TE link label flag as defined in Section 9.3.

   Similarly, LSR C provides the TE link label (200) for the TE link
   C-D, and LSR B provides the TE link label (150) for the TE link B-C.

   For tunnel T2, the transit LSRs provide the same TE link labels as
   described for tunnel T1 as the links B-C, C-D, and D-E are common
   between the two LSPs.

   The ingress LERs (A and F) will push the same stack of labels (from
   top of stack to bottom of stack) {150, 200, 250} for tunnels T1 and
   T2, respectively.

   It should be noted that a transit LSR does not swap the top TE link
   label on an incoming packet (the label that it advertised in the Resv
   message it sent); all it has to do is pop the top label and forward
   the packet.

   The values in the Label sub-objects in the RRO are of interest to the
   ingress LERs when constructing the stack of labels to impose on the
   packets.

   If, in this example, there were another RSVP-TE tunnel T3 from F to I
   on path F-B-C-D-E-I, then this tunnel would also share the TE links
   B-C, C-D, and D-E and traverse link E-I.  The label stack used by F
   would be {150, 200, 250, 850}.  Hence, regardless of where the LSPs
   start and end, they will share LSR labels at shared hops in the
   shared MPLS forwarding plane.

   There MAY be a local operator policy at the ingress LER that
   influences the maximum depth of the label stack that can be pushed
   for a Segment Routed RSVP-TE tunnel.  Prior to signaling the LSP, the
   ingress LER may determine that it is unable to push a label stack
   containing one label for each hop along the path.  In some scenarios,
   the ingress LER may not have sufficient information to make that
   determination.  In these cases, the LER SHOULD adopt the techniques
   described in Section 5.

5.  Delegating Label Stack Imposition

   One or more transit LSRs can assist the ingress LER by imposing part
   of the label stack required for the path.  Consider the example in
   Figure 2 with an RSVP-TE tunnel from A to L on path
   A-B-C-D-E-F-G-H-I-J-K-L.  In this case, the LSP is too long for LER A
   to impose the full label stack, so it uses the assistance of
   delegation hops LSR D and LSR I to impose parts of the label stack.

   Each delegation hop allocates a delegation label to represent a set
   of labels that will be pushed at this hop.  When a packet arrives at
   a delegation hop LSR with a delegation label, the LSR pops the label
   and pushes a set of labels before forwarding the packet.


                                   1250d
    +---+100p  +---+150p  +---+200p  +---+250p  +---+300p  +---+
    | A |------| B |------| C |------| D |------| E |------| F |
    +---+      +---+      +---+      +---+      +---+      +---+
                                                             |350p
                                                             |
                                   1500d                     |
    +---+  600p+---+  550p+---+  500p+---+  450p+---+  400p+---+
    | L |------| K |------| J |------| I |------| H |------+ G +
    +---+      +---+      +---+      +---+      +---+      +---+

           Notation: <Label>p - TE link label
                      <Label>d - Delegation label

                Figure 2: Delegating Label Stack Imposition

5.1.  Stacking at the Ingress

   When delegation labels come into play, there are two stacking
   approaches from which the ingress can choose.  Section 7 explains how
   the label stack can be constructed.

5.1.1.  Stack to Reach Delegation Hop

   In this approach, the stack pushed by the ingress carries a set of
   labels that will take the packet to the first delegation hop.  When
   this approach is employed, the set of labels represented by a
   delegation label at a given delegation hop will include the
   corresponding delegation label from the next delegation hop.  As a
   result, this delegation label can only be shared among LSPs that are
   destined to the same egress and traverse the same downstream path.

   This approach is shown in Figure 3.  The delegation label 1250
   represents the stack {300, 350, 400, 450, 1500}, and the delegation
   label 1500 represents the label stack {550, 600}.


    +---+               +---+               +---+
    | A |-----.....-----| D |-----.....-----| I |-----.....
    +---+               +---+               +---+

                   Pop 1250 &           Pop 1500 &
     Push                Push                Push
    ......              ......              ......
    : 150:        1250->: 300:        1500->: 550:
    : 200:              : 350:              : 600:
    :1250:              : 400:              ......
    ......              : 450:
                        :1500:
                        ......

                  Figure 3: Stack to Reach Delegation Hop

   With this approach, the ingress LER A will push {150, 200, 1250} for
   the tunnel in Figure 2.  At LSR D, the delegation label 1250 will get
   popped, and {300, 350, 400, 450, 1500} will get pushed.  At LSR I,
   the delegation label 1500 will get popped, and the remaining set of
   labels {550, 600} will get pushed.

5.1.2.  Stack to Reach Egress

   In this approach, the stack pushed by the ingress carries a set of
   labels that will take the packet all the way to the egress so that
   all the delegation labels are part of the stack.  When this approach
   is employed, the set of labels represented by a delegation label at a
   given delegation hop will not include the corresponding delegation
   label from the next delegation hop.  As a result, this delegation
   label can be shared among all LSPs traversing the segment between the
   two delegation hops.

   The downside of this approach is that the number of hops that the LSP
   can traverse is dictated by the label stack push limit of the
   ingress.

   This approach is shown in Figure 4.  The delegation label 1250
   represents the stack {300, 350, 400, 450}, and the delegation label
   1500 represents the label stack {550, 600}.

    +---+               +---+               +---+
    | A |-----.....-----| D |-----.....-----| I |-----.....
    +---+               +---+               +---+

                   Pop 1250 &           Pop 1500 &
     Push                Push                Push
    ......              ......              ......
    : 150:        1250->: 300:        1500->: 550:
    : 200:              : 350:              : 600:
    :1250:              : 400:              ......
    :1500:              : 450:
    ......              ......
                        |1500|
                        ......

                      Figure 4: Stack to Reach Egress

   With this approach, the ingress LER A will push {150, 200, 1250,
   1500} for the tunnel in Figure 2.  At LSR D, the delegation label
   1250 will get popped, and {300, 350, 400, 450} will get pushed.  At
   LSR I, the delegation label 1500 will get popped, and the remaining
   set of labels {550, 600} will get pushed.  The signaling extension
   required for the ingress to indicate the chosen stacking approach is
   defined in Section 9.6.

5.2.  Explicit Delegation

   In this delegation option, the ingress LER can explicitly delegate
   one or more specific transit LSRs to handle pushing labels for a
   certain number of their downstream hops.  In order to accurately pick
   the delegation hops, the ingress needs to be aware of the label stack
   depth push limit (total number of MPLS labels that can be imposed,
   including all service/transport/special labels) of each of the
   transit LSRs prior to initiating the signaling sequence.  The
   mechanism by which the ingress or controller (hosting the path
   computation element) learns this information is outside the scope of
   this document.  Base MPLS Imposition MSD (BMI-MSD) advertisement,
   specified in [RFC8491], is an example of such a mechanism.

   The signaling extension required for the ingress LER to explicitly
   delegate one or more specific transit hops is defined in Section 9.4.
   The extension required for the delegation hop to indicate that the
   recorded label is a delegation label is defined in Section 9.5.

5.3.  Automatic Delegation

   In this approach, the ingress LER lets the downstream LSRs
   automatically pick suitable delegation hops during the initial
   signaling sequence.  The ingress does not need to be aware up front
   of the label stack depth push limit of each of the transit LSRs.
   This approach SHOULD be used if there are loose hops [RFC3209] in the
   explicit route.  The delegation hops are picked based on a per-hop
   signaled attribute called the Effective Transport Label-Stack Depth
   (ETLD), as described in the next section.

5.3.1.  Effective Transport Label-Stack Depth (ETLD)

   The ETLD is signaled as a per-hop recorded attribute in the Path
   message [RFC7570].  When automatic delegation is requested, the
   ingress MUST populate the ETLD with the maximum number of transport
   labels that it can potentially send to its downstream hop.  This
   value is then decremented at each successive hop.  If a node is
   reached and it is determined that this hop cannot support automatic
   delegation, then it MUST NOT use TE link labels and use regular
   labels instead.  If a node is reached where the ETLD set from the
   previous hop is 1, then that node MUST select itself as the
   delegation hop.  If a node is reached and it is determined that this
   hop cannot receive more than one transport label, then that node MUST
   select itself as the delegation hop.  If there is a node or a
   sequence of nodes along the path of the LSP that do not support ETLD,
   then the immediate hop that supports ETLD MUST select itself as the
   delegation hop.  The ETLD MUST be decremented at each non-delegation
   transit hop by either 1 or some appropriate number based on the local
   policy.  For example, consider a transit node with a local policy
   that mandates it to take the label stack read limit into account when
   decrementing the ETLD.  With this policy, the ETLD is decremented in
   such a way that the transit hop does not receive more labels in the
   stack than it can read.  At each delegation hop, the ETLD MUST be
   reset to the maximum number of transport labels that the hop can
   send, and the ETLD decrements start again at each successive hop
   until either a new delegation hop is selected or the egress is
   reached.  As a result, by the time the Path message reaches the
   egress, all delegation hops are selected.  During the Resv
   processing, at each delegation hop, a suitable delegation label is
   selected (either an existing label is reused or a new label is
   allocated) and recorded in the Resv message.

   Consider the example shown in Figure 5.  Let's assume ingress LER A
   can push up to three transport labels while the remaining nodes can
   push up to five transport labels.  The ingress LER A signals the
   initial Path message with ETLD set to 3.  The ETLD value is adjusted
   at each successive hop and signaled downstream as shown.  By the time
   the Path message reaches the egress LER L, LSRs D and I are
   automatically selected as delegation hops.

          ETLD:3    ETLD:2    ETLD:1    ETLD:5    ETLD:4
          ----->    ----->    ----->    ----->    ----->
                                    1250d
      +---+100p +---+150p +---+200p +---+250p +---+300p +---+
      | A |-----| B |-----| C |-----| D |-----| E |-----| F |  ETLD:3
      +---+     +---+     +---+     +---+     +---+     +---+    |
                                                          |350p  |
                                                          |      |
                                    1500d                 |      |
      +---+ 600p+---+ 550p+---+ 500p+---+ 450p+---+ 400p+---+    v
      | L |-----| K |-----| J |-----| I |-----| H |-----+ G +
      +---+     +---+     +---+     +---+     +---+     +---+

          ETLD:3    ETLD:4    ETLD:5    ETLD:1    ETLD:2
          <-----    <-----    <-----    <-----    <-----

                              Figure 5: ETLD

   When an LSP that requests automatic delegation also requests facility
   backup protection [RFC4090], the ingress or the delegation hop MUST
   account for the bypass tunnel's label(s) when populating the ETLD.
   Hence, when a regular bypass tunnel is used to protect the facility,
   the ETLD that gets populated on these nodes is one less than what
   gets populated for a corresponding unprotected LSP.

   Signaling extension for the ingress LER to request automatic
   delegation is defined in Section 9.4.  The extension for signaling
   the ETLD is defined in Section 9.7.  The extension required for the
   delegation hop to indicate that the recorded label is a delegation
   label is defined in Section 9.5.

6.  Mixing TE Link Labels and Regular Labels in an RSVP-TE Tunnel

   Labels can be mixed across transit hops in a single MPLS RSVP-TE LSP.
   Certain LSRs can use TE link labels and others can use regular
   labels.  The ingress can construct a label stack appropriately based
   on what type of label is recorded from every transit LSR.

                             (#)       (#)
    +---+100  +---+150  +---+200  +---+250  +---+
    | A |-----| B |-----| C |-----| D |-----| E |
    +---+     +---+     +---+     +---+     +---+
      |110      |450      |550      |650      |850
      |         |         |         |         |
      |         |400      |500      |600      |800
      |       +---+     +---+     +---+     +---+
      +-------| F |-----|G  |-----|H  |-----|I  |
              +---+300  +---+350  +---+700  +---+

            Notation: (#) denotes regular labels
                       Other labels are TE link labels

      Figure 6: Sample Topology -- TE Link Labels and Regular Labels

   If the transit LSR allocates a regular label to be sent upstream in
   the Resv, then the label operation at the LSR is a swap to the label
   received from the downstream LSR.  If the transit LSR is using a TE
   link label to be sent upstream in the Resv, then the label operation
   at the LSR is a pop and forward regardless of any label received from
   the downstream LSR.  There is no change in the behavior of a
   penultimate hop popping (PHP) LSR [RFC3031].

   Section 7 explains how the label stack can be constructed.  For
   example, the LSP from A to I using path A-B-C-D-E-I will use a label
   stack of {150, 200}.

7.  Construction of Label Stacks

   The ingress LER or delegation hop MUST check the type of label
   received from each transit hop as recorded in the RRO in the Resv
   message and generate the appropriate label stack to reach the next
   delegation hop or the egress.

   The following logic is used by the node constructing the label stack:

      Each RRO label sub-object MUST be processed starting with the
      label sub-object from the first downstream hop.  Any label
      provided by the first downstream hop MUST always be pushed on the
      label stack regardless of the label type.  If the label type is a
      TE link label, then any label from the next downstream hop MUST
      also be pushed on the constructed label stack.  If the label type
      is a regular label, then any label from the next downstream hop
      MUST NOT be pushed on the constructed label stack.  If the label
      type is a delegation label, then the type of stacking approach
      chosen by the ingress for this LSP (Section 5.1) MUST be used to
      determine how the delegation labels are pushed in the label stack.

8.  Facility Backup Protection

   The following section describes how link protection works with
   facility backup protection [RFC4090] using regular bypass tunnels for
   the Segment Routed RSVP-TE tunnels.  The procedures for supporting
   node protection are not discussed in this document.  The use of
   Segment Routed bypass tunnels for providing facility protection is
   left for further study.

8.1.  Link Protection

   To provide link protection at a Point of Local Repair (PLR) with a
   shared MPLS forwarding plane, the LSR MUST allocate a separate TE
   link label for the TE link that will be used for RSVP-TE tunnels that
   request link protection from the ingress.  No signaling extensions
   are required to support link protection for RSVP-TE tunnels over the
   shared MPLS forwarding plane.

   At each LSR, link-protected TE link labels can be allocated for each
   TE link, and a link-protecting facility backup LSP can be created to
   protect the TE link.  The link-protected TE link label can be sent by
   the LSR for LSPs requesting link protection over the specific TE
   link.  Since the facility backup terminates at the next hop (merge
   point), the incoming label on the packet will be what the merge point
   expects.

   Consider the network shown in Figure 7.  LSR B can install a facility
   backup LSP for the link-protected TE link label 151.  When the TE
   link B-C is up, LSR B will pop 151 and send the packet to C.  If the
   TE link B-C is down, the LSR can pop 151 and send the packet via the
   facility backup to C.

         101(*)     151(*)     201(*)     251(*)
    +---+100   +---+150   +---+200   +---+250   +---+
    | A |------| B |------| C |------| D |------| E |
    +---+      +---+      +---+      +---+      +---+
      |110       |450       |550       |650       |850
      |          |          |          |          |
      |          |400       |500       |600       |800
      |        +---+      +---+      +---+      +---+
      +--------| F |------|G  |------|H  |------|I  |
               +---+300   +---+350   +---+700   +---+

     Notation: (*) denotes link-protected TE link labels

                    Figure 7: Link Protection Topology

9.  Protocol Extensions

9.1.  Requirements

   The functionality discussed in this document imposes the following
   requirements on the signaling protocol.

   o  The ingress of the LSP needs to have the ability to mandate/
      request the use and recording of TE link labels at all hops along
      the path of the LSP.

   o  When the use of TE link labels is mandated/requested for the path:

      *  the node recording the TE link label needs to have the ability
         to indicate whether the recorded label is a TE link label.

      *  the ingress needs to have the ability to delegate label stack
         imposition by:

         +  explicitly mandating specific hops to be delegation hops, or

         +  requesting automatic delegation.

      *  When explicit delegation is mandated or automatic delegation is
         requested:

         +  the ingress needs to have the ability to indicate the chosen
            stacking approach, and

         +  the delegation hop needs to have the ability to indicate
            that the recorded label is a delegation label.

9.2.  Attribute Flags TLV: TE Link Label

   Bit Number 16: TE Link Label

   The presence of this flag in the LSP_ATTRIBUTES/
   LSP_REQUIRED_ATTRIBUTES object [RFC5420] of a Path message indicates
   that the ingress has requested/mandated the use and recording of TE
   link labels at all hops along the path of this LSP.  When a node that
   recognizes this flag but does not cater to the mandate because of
   local policy receives a Path message carrying the
   LSP_REQUIRED_ATTRIBUTES object with this flag set, it MUST send a
   PathErr message with an error code of 'Routing Problem (24)' and an
   error value of 'TE link label usage failure (70)'.  A transit hop
   that caters to this request/mandate MUST also check for the presence
   of other Attribute Flags introduced in this document (Sections 9.4
   and 9.6) and process them as specified.  An ingress LER that sets
   this bit MUST also set the "label recording desired" flag [RFC3209]
   in the SESSION_ATTRIBUTE object.

9.3.  RRO Label Sub-object Flag: TE Link Label

   Flag (0x02): TE Link Label

   The presence of this flag indicates that the recorded label is a TE
   link label.  This flag MUST be used by a node only if the use and
   recording of TE link labels are requested/mandated for the LSP.

9.4.  Attribute Flags TLV: LSI-D

   Bit Number 17: Label Stack Imposition - Delegation (LSI-D)

   Automatic Delegation: The presence of this flag in the LSP_ATTRIBUTES
   object of a Path message indicates that the ingress has requested
   automatic delegation of label stack imposition.  This flag MUST be
   set in the LSP_ATTRIBUTES object of a Path message only if the use
   and recording of TE link labels are requested/mandated for this LSP.
   If the transit hop does not support this flag, it MUST NOT use TE
   link labels and use regular labels instead.  If the use of TE link
   labels was mandated in the LSP_REQUIRED_ATTRIBUTES object, it MUST
   send a PathErr message with an error code of 'Routing Problem (24)'
   and an error value of 'TE link label usage failure (70)'.

   Explicit Delegation: The presence of this flag in the HOP_ATTRIBUTES
   sub-object [RFC7570] of an Explicit Route Object (ERO) in the Path
   message indicates that the hop identified by the preceding IPv4 or
   IPv6 or Unnumbered Interface ID sub-object has been picked as an
   explicit delegation hop.  The HOP_ATTRIBUTES sub-object carrying this
   flag MUST have the R (Required) bit set.  This flag MUST be set in
   the HOP_ATTRIBUTES sub-object of an ERO object in the Path message
   only if the use and recording of TE link labels are requested/
   mandated for this LSP.  If the hop recognizes this flag but is not
   able to comply with this mandate because of local policy, it MUST
   send a PathErr message with an error code of 'Routing Problem (24)'
   and an error value of 'Label stack imposition failure (71)'.

9.5.  RRO Label Sub-object Flag: Delegation Label

   Flag (0x04): Delegation Label

   The presence of this flag indicates that the recorded label is a
   delegation label.  This flag MUST be used by a node only if the use
   and recording of TE link labels and delegation are requested/mandated
   for the LSP.

9.6.  Attributes Flags TLV: LSI-D-S2E

   Bit Number 18: Label Stack Imposition - Delegation - Stack to Reach
   Egress (LSI-D-S2E)

   The presence of this flag in the LSP_ATTRIBUTES object of a Path
   message indicates that the ingress has chosen to use the "Stack to
   reach egress" approach for stacking.  The absence of this flag in the
   LSP_ATTRIBUTES object of a Path message indicates that the ingress
   has chosen to use the "Stack to reach delegation hop" approach for
   stacking.  This flag MUST be set in the LSP_ATTRIBUTES object of a
   Path message only if the use and recording of TE link labels and
   delegation are requested/mandated for this LSP.  If the transit hop
   is not able to support the "Stack to reach egress" approach, it MUST
   send a PathErr message with an error code of 'Routing Problem (24)'
   and an error value of 'Label stack imposition failure (71)'.

9.7.  Attributes TLV: ETLD

   The format of the ETLD Attributes TLV is shown in Figure 8.  The
   Attribute TLV Type is 6.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Reserved                              |     ETLD      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 8: The ETLD Attributes TLV

   The presence of this TLV in the HOP_ATTRIBUTES sub-object of an RRO
   object in the Path message indicates that the hop identified by the
   preceding IPv4 or IPv6 or Unnumbered Interface ID sub-object supports
   automatic delegation.  This attribute MUST be used only if the use
   and recording of TE link labels are requested/mandated and automatic
   delegation is requested for the LSP.

   The ETLD field specifies the effective number of transport labels
   that this hop (in relation to its position in the path) can
   potentially send to its downstream hop.  It MUST be set to a non-zero
   value.

   The Reserved field is for future specification.  It SHOULD be set to
   zero on transmission and MUST be ignored on receipt to ensure future
   compatibility.

10.  OAM Considerations

   MPLS LSP ping and traceroute [RFC8029] are applicable for Segment
   Routed RSVP-TE tunnels.  The existing procedures allow for the label
   stack imposed at a delegation hop to be reported back in the Label
   Stack Sub-TLV in the MPLS echo reply for traceroute.

11.  IANA Considerations

11.1.  Attribute Flags: TE Link Label, LSI-D, LSI-D-S2E

   IANA manages the 'Attribute Flags' subregistry as part of the
   'Resource Reservation Protocol-Traffic Engineering (RSVP-TE)
   Parameters' registry located at <http://www.iana.org/assignments/
   rsvp-te-parameters>.  This document introduces three new Attribute
   Flags:

   Bit  Name              Attribute   Attribute  RRO ERO Reference
   No                     Flags Path  Flags Resv

   16   TE Link Label     Yes         No         No  No  [RFC8577],
                                                         Section 9.2

   17   LSI-D             Yes         No         No  Yes [RFC8577],
                                                         Section 9.4

   18   LSI-D-S2E         Yes         No         No  No  [RFC8577],
                                                         Section 9.6

11.2.  Attribute TLV: ETLD

   IANA manages the "Attribute TLV Space" registry as part of the
   'Resource Reservation Protocol-Traffic Engineering (RSVP-TE)
   Parameters' registry located at <http://www.iana.org/assignments/
   rsvp-te-parameters>.  This document introduces a new Attribute TLV.

   Type  Name  Allowed on     Allowed on    Allowed on  Reference
               LSP_ATTRIBUTES LSP_REQUIRED  LSP Hop
                              _ATTRIBUTES   Attributes

   6     ETLD      No               No         Yes       [RFC8577],
                                                         Section 9.7

11.3.  Record Route Label Sub-object Flags: TE Link Label, Delegation
       Label

   IANA manages the "Record Route Object Sub-object Flags" registry as
   part of the "Resource Reservation Protocol-Traffic Engineering (RSVP-
   TE) Parameters" registry located at <http://www.iana.org/assignments/
   rsvp-te-parameters>.  Prior to this document, this registry did not
   include Label Sub-object Flags.  This document creates the addition
   of a new subregistry for Label Sub-object Flags as shown below.

      Flag  Name                    Reference

      0x1   Global Label            [RFC3209]
      0x02  TE Link Label           [RFC8577], Section 9.3
      0x04  Delegation Label        [RFC8577], Section 9.5


11.4.  Error Codes and Error Values

   IANA maintains a registry called "Resource Reservation Protocol
   (RSVP) Parameters" with a subregistry called "Error Codes and
   Globally-Defined Error Value Sub-Codes".  Within this subregistry is
   a definition of the "Routing Problem" Error Code (24).  The
   definition lists a number of error values that may be used with this
   error code.  IANA has allocated further error values for use with
   this Error Code as described in this document.  The resulting entry
   in the registry is as follows.

      24  Routing Problem                             [RFC3209]

          This Error Code has the following globally defined Error
          Value sub-codes:

           70 = TE link label usage failure        [RFC8577]
           71 = Label stack imposition failure     [RFC8577]

12.  Security Considerations

   This document does not introduce new security issues.  The security
   considerations pertaining to the original RSVP protocol [RFC2205] and
   RSVP-TE [RFC3209] and those that are described in [RFC5920] remain
   relevant.

13.  References

13.1.  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>.

   [RFC2205]  Braden, R., Ed., Zhang, L., Berson, S., Herzog, S., and
              S. Jamin, "Resource ReSerVation Protocol (RSVP) -- Version
              1 Functional Specification", RFC 2205,
              DOI 10.17487/RFC2205, September 1997,
              <https://www.rfc-editor.org/info/rfc2205>.

   [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>.

   [RFC4090]  Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast
              Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
              DOI 10.17487/RFC4090, May 2005,
              <https://www.rfc-editor.org/info/rfc4090>.

   [RFC5420]  Farrel, A., Ed., Papadimitriou, D., Vasseur, JP., and
              A. Ayyangarps, "Encoding of Attributes for MPLS LSP
              Establishment Using Resource Reservation Protocol Traffic
              Engineering (RSVP-TE)", RFC 5420, DOI 10.17487/RFC5420,
              February 2009, <https://www.rfc-editor.org/info/rfc5420>.

   [RFC7570]  Margaria, C., Ed., Martinelli, G., Balls, S., and
              B. Wright, "Label Switched Path (LSP) Attribute in the
              Explicit Route Object (ERO)", RFC 7570,
              DOI 10.17487/RFC7570, July 2015,
              <https://www.rfc-editor.org/info/rfc7570>.

   [RFC8029]  Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N.,
              Aldrin, S., and M. Chen, "Detecting Multiprotocol Label
              Switched (MPLS) Data-Plane Failures", RFC 8029,
              DOI 10.17487/RFC8029, March 2017,
              <https://www.rfc-editor.org/info/rfc8029>.

   [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>.

13.2.  Informative References

   [RFC2961]  Berger, L., Gan, D., Swallow, G., Pan, P., Tommasi, F.,
              and S. Molendini, "RSVP Refresh Overhead Reduction
              Extensions", RFC 2961, DOI 10.17487/RFC2961, April 2001,
              <https://www.rfc-editor.org/info/rfc2961>.

   [RFC5920]  Fang, L., Ed., "Security Framework for MPLS and GMPLS
              Networks", RFC 5920, DOI 10.17487/RFC5920, July 2010,
              <https://www.rfc-editor.org/info/rfc5920>.

   [RFC8370]  Beeram, V., Ed., Minei, I., Shakir, R., Pacella, D., and
              T. Saad, "Techniques to Improve the Scalability of RSVP-TE
              Deployments", RFC 8370, DOI 10.17487/RFC8370, May 2018,
              <https://www.rfc-editor.org/info/rfc8370>.

   [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>.

   [RFC8491]  Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
              "Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491,
              DOI 10.17487/RFC8491, November 2018,
              <https://www.rfc-editor.org/info/rfc8491>.

   [SEG-ROUTING]
              Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing with MPLS data plane", Work in Progress,
              draft-ietf-spring-segment-routing-mpls-18, December 2018.

Acknowledgements

   The authors would like to thank Adrian Farrel, Kireeti Kompella,
   Markus Jork, and Ross Callon for their input from discussions.
   Adrian Farrel provided a review and a text suggestion for clarity and
   readability.

Contributors

   The following individuals contributed to this document:

   Raveendra Torvi
   Juniper Networks
   Email: rtorvi@juniper.net

   Chandra Ramachandran
   Juniper Networks
   Email: csekar@juniper.net

   George Swallow
   Email: swallow.ietf@gmail.com

Authors' Addresses

   Harish Sitaraman
   Juniper Networks
   1133 Innovation Way
   Sunnyvale, CA  94089
   United States of America

   Email: harish.ietf@gmail.com


   Vishnu Pavan Beeram
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886
   United States of America

   Email: vbeeram@juniper.net


   Tejal Parikh
   Verizon
   400 International Parkway
   Richardson, TX  75081
   United States of America

   Email: tejal.parikh@verizon.com


   Tarek Saad
   Cisco Systems
   2000 Innovation Drive
   Kanata, Ontario  K2K 3E8
   Canada

   Email: tsaad.net@gmail.com