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Versions: (draft-sitaraman-mpls-rsvp-shared-labels) 00 01 02 03 04 05

MPLS Working Group                                          H. Sitaraman
Internet-Draft                                                 V. Beeram
Intended status: Standards Track                        Juniper Networks
Expires: April 18, 2019                                        T. Parikh
                                                                 Verizon
                                                                 T. Saad
                                                           Cisco Systems
                                                        October 15, 2018


      Signaling RSVP-TE tunnels on a shared MPLS forwarding plane
               draft-ietf-mpls-rsvp-shared-labels-05.txt

Abstract

   As the scale of MPLS RSVP-TE networks has grown, so 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 control plane state.

   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 pre-installed 'per
   Traffic Engineering (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 MPLS
   forwarding plane.

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.








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Status of This Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on April 18, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Allocation of TE Link Labels  . . . . . . . . . . . . . . . .   5
   4.  Segment Routed RSVP-TE Tunnel Setup . . . . . . . . . . . . .   5
   5.  Delegating Label Stack Imposition . . . . . . . . . . . . . .   7
     5.1.  Stacking at the Ingress . . . . . . . . . . . . . . . . .   8
       5.1.1.  Stack to Reach Delegation Hop . . . . . . . . . . . .   8
       5.1.2.  Stack to Reach Egress . . . . . . . . . . . . . . . .   9
     5.2.  Explicit Delegation . . . . . . . . . . . . . . . . . . .  10
     5.3.  Automatic Delegation  . . . . . . . . . . . . . . . . . .  10
       5.3.1.  Effective Transport Label-Stack Depth (ETLD)  . . . .  10
   6.  Mixing TE Link Labels and Regular Labels in an RSVP-TE Tunnel  12
   7.  Construction of Label Stacks  . . . . . . . . . . . . . . . .  12
   8.  Facility Backup Protection  . . . . . . . . . . . . . . . . .  13



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     8.1.  Link Protection . . . . . . . . . . . . . . . . . . . . .  13
   9.  Protocol Extensions . . . . . . . . . . . . . . . . . . . . .  14
     9.1.  Requirements  . . . . . . . . . . . . . . . . . . . . . .  14
     9.2.  Attribute Flags TLV: TE Link Label  . . . . . . . . . . .  15
     9.3.  RRO Label Subobject Flag: TE Link Label . . . . . . . . .  15
     9.4.  Attribute Flags TLV: LSI-D  . . . . . . . . . . . . . . .  15
     9.5.  RRO Label Subobject Flag: Delegation Label  . . . . . . .  16
     9.6.  Attributes Flags TLV: LSI-D-S2E . . . . . . . . . . . . .  16
     9.7.  Attributes TLV: ETLD  . . . . . . . . . . . . . . . . . .  16
   10. OAM Considerations  . . . . . . . . . . . . . . . . . . . . .  17
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   12. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  17
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
     13.1.  Attribute Flags: TE Link Label, LSI-D, LSI-D-S2E . . . .  18
     13.2.  Attribute TLV: ETLD  . . . . . . . . . . . . . . . . . .  18
     13.3.  Record Route Label Sub-object Flags: TE Link Label,
            Delegation Label . . . . . . . . . . . . . . . . . . . .  18
     13.4.  Error Codes and Error Values . . . . . . . . . . . . . .  19
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  19
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     15.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

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 make any difference
   to 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 pre-installed 'per Traffic
   Engineering (TE) link labels' that are allocated by an LSR.  Each
   such label is installed in the MPLS forwarding plane with a 'pop'
   operation and the instruction 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.  To make use of this feature, the ingress Label Edge Router
   (LER) pushes a stack of labels [RFC3031] as received in the RRO.



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   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 same as that exhibited by Segment Routing (SR)
   [RFC8402] when using an MPLS forwarding plane and a series of
   adjacency segments [I-D.ietf-spring-segment-routing-mpls].  This work
   couples the feature benefits of the RSVP-TE control plane with the
   simplicity of the Segment Routing 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 independent 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 time
       for provisioning and deprovisioning LSPs.

   3.  Hitless re-routing: 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 and
       so makes 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.






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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 could 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.

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.



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    +---+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 and also
   in the Label subobject carried in the RRO and setting 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 subobjects in the RRO are of interest to the
   ingress LERs in order to construct the stack of labels to impose on
   the packets.




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

   There MAY be 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 would be 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




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5.1.  Stacking at the Ingress

   When delegation labels come into play, there are two stacking
   approaches that the ingress can choose from.  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.  And at LSR I,
   the delegation label 1500 will get popped and the remaining set of
   labels {550, 600} will get pushed.








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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.  And
   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.






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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 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.

   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 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 act as if it does not support TE link
   labels.  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 limitations at
   that hop.  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



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   delegation hop is selected or the egress is reached.  The net result
   is that 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 3 transport labels while the remaining nodes can push
   up to 5 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.






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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 could be used by the node constructing the label
   stack:





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      Each RRO label sub-object SHOULD 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 SHOULD 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.








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         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 protection 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 SHOULD 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 SHOULD have the ability to
         indicate if the recorded label is a TE link label.

      *  the ingress SHOULD 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 SHOULD have the ability to indicate the chosen
            stacking approach (and)

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



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9.2.  Attribute Flags TLV: TE Link Label

   Bit Number 16 (Early allocation by IANA): TE Link Label

   The presence of this in the LSP_ATTRIBUTES/LSP_REQUIRED_ATTRIBUTES
   object 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 does not cater to the mandate
   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 (TBD3)'.  A transit hop that caters to this request/mandate
   MUST also check for the presence of other Attribute Flags introduced
   in this document (Section 9.4 and Section 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 Subobject Flag: TE Link Label

   Bit Number (TBD1): 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 is requested/mandated for the LSP.

9.4.  Attribute Flags TLV: LSI-D

   Bit Number 17 (Early allocation by IANA): 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 is requested/mandated for this LSP.
   If the transit hop does not support this flag, it MUST act as if it
   does not support TE link labels.  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 (TBD3)'.

   Explicit Delegation: The presence of this flag in the HOP_ATTRIBUTES
   subobject [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 subobject has been picked as an
   explicit delegation hop.  The HOP_ATTRIBUTES subobject carrying this



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   flag MUST have the R (Required) bit set.  This flag MUST be set in
   the HOP_ATTRIBUTES subobject of an ERO object in the Path message
   only if the use and recording of TE link labels is requested/mandated
   for this LSP.  If the hop is not able to comply with this mandate, it
   MUST send a PathErr message with an error code of 'Routing Problem
   (24)' and an error value of 'Label stack imposition failurei (TBD4)'.

9.5.  RRO Label Subobject Flag: Delegation Label

   Bit Number (TBD2): 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 (Early allocation by IANA): 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 (TBD4)'.

9.7.  Attributes TLV: ETLD

   The format of the ETLD Attributes TLV is shown in Figure 8.  The
   Attribute TLV Type is 6 (Early allocation by IANA).


       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




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   The presence of this TLV in the HOP_ATTRIBUTES subobject of an RRO
   object in the Path message indicates that the hop identified by the
   preceding IPv4 or IPv6 or Unnumbered Interface ID subobject supports
   automatic delegation.  This attribute MUST be used only if the use
   and recording of TE link labels is 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.  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 text suggestion for clarity and
   readability.

12.  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






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13.  IANA Considerations

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

   IANA manages the 'Attribute 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.  This document introduces three new Attribute Flags.

      Bit  Name              Attribute Attribute RRO ERO Reference
      No.                    FlagsPath FlagsResv
      16   TE Link Label     Yes       No        No  No  [This.ID]
                                                         (Section 9.2)
      17   LSI-D             Yes       No        No  Yes [This.ID]
                                                         (Section 9.4)
      18   LSI-D-S2E         Yes       No        No  No  [This.ID]
                                                         (Section 9.6)

   Note: The code points specified for TE Link Label, LSI-D and LSI-
   D-S2E are early allocations by IANA.

13.2.  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         LSP REQUIRED LSP Hop
                     ATTRIBUTES  ATTRIBUTES   Attributes


        6    ETLD    No          No           Yes         [This.ID]
                                                          (Section 9.7)

   Note: The code point specified for ETLD is an early allocation by
   IANA.

13.3.  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.  This registry currently does not include Label
   Sub-object Flags.  This document requests the addition of a new sub-
   registry for Label Sub-object Flags as shown below.



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      Flag  Name                    Reference

      0x1   Global Label            RFC 3209
      TBD1  TE Link Label           [This.ID] (Section 9.3)
      TBD2  Delegation Label        [This.ID] (Section 9.5)


   All assignments in this sub-registry are to be performed via
   Standards Action.

13.4.  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
   there is a definition of the "Routing Problem" error code with error
   code value 24.  The definition lists a number of error values that
   may be used with this error code.  IANA is requested to allocate
   further error values for use with this error code as described in
   this document.  The resulting entry in the registry should look as
   follows.

      24  Routing Problem                             [RFC3209]

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

           TBD3 = TE link label usage failure        [This.ID]
           TBD4 = Label stack imposition failure     [This.ID]

14.  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.

15.  References

15.1.  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>.






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   [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>.

   [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>.

15.2.  Informative References

   [I-D.ietf-spring-segment-routing-mpls]
              Bashandy, A., Filsfils, C., Previdi, S., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing with MPLS
              data plane", draft-ietf-spring-segment-routing-mpls-14
              (work in progress), June 2018.

   [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>.



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   [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>.

Authors' Addresses

   Harish Sitaraman
   Juniper Networks
   1133 Innovation Way
   Sunnyvale, CA  94089
   US

   Email: hsitaraman@juniper.net


   Vishnu Pavan Beeram
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886
   US

   Email: vbeeram@juniper.net


   Tejal Parikh
   Verizon
   400 International Parkway
   Richardson, TX  75081
   US

   Email: tejal.parikh@verizon.com










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

   Email: tsaad@cisco.com












































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