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rfc:rfc8577

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.

Sitaraman, et al. Standards Track [Page 1] RFC 8577 RSVP-TE Shared Labels April 2019

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
 (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.

Sitaraman, et al. Standards Track [Page 2] RFC 8577 RSVP-TE Shared Labels April 2019

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

Sitaraman, et al. Standards Track [Page 3] RFC 8577 RSVP-TE Shared Labels April 2019

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

Sitaraman, et al. Standards Track [Page 4] RFC 8577 RSVP-TE Shared Labels April 2019

     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.

Sitaraman, et al. Standards Track [Page 5] RFC 8577 RSVP-TE Shared Labels April 2019

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

Sitaraman, et al. Standards Track [Page 6] RFC 8577 RSVP-TE Shared Labels April 2019

 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,

Sitaraman, et al. Standards Track [Page 7] RFC 8577 RSVP-TE Shared Labels April 2019

 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.

Sitaraman, et al. Standards Track [Page 8] RFC 8577 RSVP-TE Shared Labels April 2019

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.

Sitaraman, et al. Standards Track [Page 9] RFC 8577 RSVP-TE Shared Labels April 2019

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.

Sitaraman, et al. Standards Track [Page 10] RFC 8577 RSVP-TE Shared Labels April 2019

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

Sitaraman, et al. Standards Track [Page 11] RFC 8577 RSVP-TE Shared Labels April 2019

 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.

Sitaraman, et al. Standards Track [Page 12] RFC 8577 RSVP-TE Shared Labels April 2019

 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}.

Sitaraman, et al. Standards Track [Page 13] RFC 8577 RSVP-TE Shared Labels April 2019

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.

Sitaraman, et al. Standards Track [Page 14] RFC 8577 RSVP-TE Shared Labels April 2019

 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.

Sitaraman, et al. Standards Track [Page 15] RFC 8577 RSVP-TE Shared Labels April 2019

  • 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

Sitaraman, et al. Standards Track [Page 16] RFC 8577 RSVP-TE Shared Labels April 2019

 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)'.

Sitaraman, et al. Standards Track [Page 17] RFC 8577 RSVP-TE Shared Labels April 2019

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.

Sitaraman, et al. Standards Track [Page 18] RFC 8577 RSVP-TE Shared Labels April 2019

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

Sitaraman, et al. Standards Track [Page 19] RFC 8577 RSVP-TE Shared Labels April 2019

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.

Sitaraman, et al. Standards Track [Page 20] RFC 8577 RSVP-TE Shared Labels April 2019

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>.

Sitaraman, et al. Standards Track [Page 21] RFC 8577 RSVP-TE Shared Labels April 2019

 [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.

Sitaraman, et al. Standards Track [Page 22] RFC 8577 RSVP-TE Shared Labels April 2019

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

Sitaraman, et al. Standards Track [Page 23] RFC 8577 RSVP-TE Shared Labels April 2019

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

Sitaraman, et al. Standards Track [Page 24]

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