Signaling RSVP-TE tunnels on a shared MPLS forwarding plane
draft-sitaraman-mpls-rsvp-shared-labels-02

Versions: 00 01 02                                      IPR declarations
MPLS Working Group                                          H. Sitaraman
Internet-Draft                                                 V. Beeram
Intended status: Standards Track                        Juniper Networks
Expires: March 10, 2018                                        T. Parikh
                                                                 Verizon
                                                                 T. Saad
                                                           Cisco Systems
                                                       September 6, 2017


      Signaling RSVP-TE tunnels on a shared MPLS forwarding plane
             draft-sitaraman-mpls-rsvp-shared-labels-02.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.  That is, it allows many more LSPs to be
   supported than there are forwarding plane labels available.

   This work introduces the notion of pre-installed 'pop labels' that
   are applied per Traffic Engineering link and that can be shared by
   MPLS RSVP-TE LSPs that traverse these 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.

   This document also introduces the ability to mix different types of
   label operations along the path of an LSP, thereby allowing the
   ingress router or an external controller to influence how to
   optimally place a LSP in the network.








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Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

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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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on March 10, 2018.

Copyright Notice

   Copyright (c) 2017 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
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   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Allocation of Pop Labels  . . . . . . . . . . . . . . . . . .   5
   4.  RSVP-TE Pop and Forward 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



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     5.2.  Explicit Delegation . . . . . . . . . . . . . . . . . . .  10
     5.3.  Automatic Delegation  . . . . . . . . . . . . . . . . . .  10
       5.3.1.  Effective Transport Label-Stack Depth (ETLD)  . . . .  10
   6.  Mixing Pop and Swap Labels in an RSVP-TE Tunnel . . . . . . .  11
   7.  Construction of Label Stacks  . . . . . . . . . . . . . . . .  12
   8.  Facility Backup Protection  . . . . . . . . . . . . . . . . .  13
     8.1.  Link Protection . . . . . . . . . . . . . . . . . . . . .  13
     8.2.  Node Protection . . . . . . . . . . . . . . . . . . . . .  14
   9.  Quantifying Pop Labels  . . . . . . . . . . . . . . . . . . .  14
   10. Protocol Extensions . . . . . . . . . . . . . . . . . . . . .  14
     10.1.  Requirements . . . . . . . . . . . . . . . . . . . . . .  14
     10.2.  Attribute Flags TLV: Pop Label . . . . . . . . . . . . .  15
     10.3.  RRO Label Subobject Flag: Pop Label  . . . . . . . . . .  15
     10.4.  Attribute Flags TLV: LSI-D . . . . . . . . . . . . . . .  15
     10.5.  RRO Label Subobject Flag: Delegation Label . . . . . . .  16
     10.6.  Attributes Flags TLV: LSI-D-S2E  . . . . . . . . . . . .  16
     10.7.  Attributes TLV: ETLD . . . . . . . . . . . . . . . . . .  16
   11. OAM Considerations  . . . . . . . . . . . . . . . . . . . . .  17
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   13. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  17
   14. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
     14.1.  Attribute Flags: Pop Label, LSI-D, LSI-D-S2E . . . . . .  18
     14.2.  Attribute TLV: ETLD  . . . . . . . . . . . . . . . . . .  18
     14.3.  Record Route Label Sub-object Flags: Pop Label,
            Delegation Label . . . . . . . . . . . . . . . . . . . .  18
   15. Security Considerations . . . . . . . . . . . . . . . . . . .  19
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     16.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   The scaling of RSVP-TE [RFC3209] control plane implementations can be
   improved by adopting the guidelines and mechanisms described in
   [RFC2961] and [I-D.ietf-teas-rsvp-te-scaling-rec].  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 'pop labels' that
   are allocated by an LSR for each of its Traffic Engineering (TE)



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   links.  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.  These 'pop labels' can be shared by MPLS RSVP-
   TE LSPs that traverse the same TE link.

   This pop and forward data plane behavior is similar to that used by
   Segment Routing (SR) [I-D.ietf-spring-segment-routing] using a MPLS
   forwarding plane and a series of adjacency segments.  This couples
   the feature benefits of the RSVP-TE control plane with the simplicity
   of the Segment Routing MPLS forwarding plane.  The RSVP-TE pop and
   forward tunnels can co-exist with MPLS-SR LSPs
   [I-D.ietf-spring-segment-routing-mpls] as described in
   [I-D.ietf-teas-sr-rsvp-coexistence-rec].

   RSVP-TE using a pop and forward data plane offers the following
   benefits:

   1.  Shared forwarding plane: 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 'pop' and 'swap' 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
       'swap' labels in Resv messages.

   No additional extensions are required to routing protocols (IGP-TE)
   in order to support this pop and forward data plane.  Functionalities
   such as bandwidth admission control, LSP priorities, preemption,



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   auto-bandwidth and Fast Reroute continue to work with this forwarding
   plane.

2.  Terminology

   The following terms are defined for use in this document:

   Pop 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.

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

   RSVP-TE pop and forward tunnel:   An MPLS RSVP-TE tunnel that uses a
      pop and forward labels on every hop of the LSP.

   Pop and forward data plane:   A forwarding plane where every LSR uses
      pop labels on every LSP.

3.  Allocation of Pop Labels

   An LSR SHOULD allocate a unique pop label for each TE link.  When an
   LSR encounters a pop 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 labels MAY be allocated for the TE link to accommodate
   tunnels requesting no protection, link-protection and node-protection
   over the specific TE link.

4.  RSVP-TE Pop and Forward Tunnel Setup

   This section provides an example of how the RSVP-TE signaling
   procedure works to set up a tunnel utilizing a pop and forward data
   plane.  The sample topology below is used to explain the example.
   Labels shown at each node are pop 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: Pop and Forward Label Topology

   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 pop label
   attributes flag defined in Section 10.2).  When LSR D receives the
   Resv message from the egress LER E, it checks the next-hop TE link
   (D-E) and provides the pop 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 pop label flag as
   defined in Section 10.3.

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

   For tunnel T2, the transit LSRs provide the same pop 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 pop 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 pop and forward data 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 an RSVP-
   TE pop and forward tunnel.  Prior to signaling the LSP, the ingress
   LER may decide that it would be unable to push a label stack
   containing one label for each hop along the path.  In this case the
   LER can choose either to not signal an RSVP-TE pop and forward tunnel
   (using normal LSP signaling instead), or can adopt the techniques
   described in Section 5 or Section 6.

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 - pop 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 10.6.






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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 10.4.  The extension required for the delegation hop to
   indicate that the recorded label is a delegation label is defined in
   Section 10.5.

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.  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 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 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 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



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   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

   Signaling extension for the ingress LER to request automatic
   delegation is defined in Section 10.4.  The extension for signaling
   the ETLD is defined in Section 10.7.  The extension required for the
   delegation hop to indicate that the recorded label is a delegation
   label is defined in Section 10.5.

6.  Mixing Pop and Swap Labels in an RSVP-TE Tunnel

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









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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  +---+

            Notation : (#) denotes swap labels
                       Other labels are pop labels


                Figure 6: Mixed Pop and Swap Label Topology

   If the transit LSR allocates a swap 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 pop
   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.

   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:

      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
      pop label, then any label from the next downstream hop MUST also
      be pushed on the constructed label stack.  If the label type is a
      swap 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 stacking procedure stops at that




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      delegation hop.  Approaches in Section 5.1 SHOULD be used to
      determine how the delegation labels are pushed in the label stack.

8.  Facility Backup Protection

   The following section describe how link and node protection works
   with facility backup protection [RFC4090] for the RSVP-TE pop and
   forward tunnels.

8.1.  Link Protection

   To provide link protection at a Point of Local Repair (PLR) with a
   pop and forward data plane, the LSR SHOULD allocate a separate pop
   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
   pop and forward data plane.

   At each LSR, link protected pop 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 pop 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 pop 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 pop labels to offer protection on a TE link


                    Figure 7: Link Protection Topology



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8.2.  Node Protection

   The solutions for the PLR to provide node-protection for the pop and
   forward RSVP-TE tunnel will be explained in a future version of this
   document.

9.  Quantifying Pop Labels

   This section quantifies the number of labels required in the
   forwarding plane to provide sharing of labels across RSVP-TE pop and
   forward tunnels.  An MPLS RSVP-TE tunnel offers either no protection,
   link protection, or node protection and only one of these labels is
   required per tunnel during signaling.  The scale of the number of pop
   labels required per LSR can be deduced as follows:


   o  For an LSR having X neighbors reachable across Y interfaces, the
      number of unprotected pop labels is X.

   o  For a PLR having X neighbors reachable across Y interfaces, the
      number of link protected pop labels is X.

   o  For a PLR having X neighbors, each having Nx neighbors (i.e. next-
      nexthops for the PLR), number of node protected pop labels is
      SUM_OF_ALL(Nx).

   The total number of pop labels is given by:
   Unprotected pop labels +
   link protected pop labels +
   node protected pop labels = 2X + SUM_OF_ALL(Nx)

10.  Protocol Extensions

10.1.  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 pop labels at all hops along the path of
      the LSP.

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

      *  the node recording the pop label SHOULD have the ability to
         indicate if the recorded label is a pop label.





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      *  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.

10.2.  Attribute Flags TLV: Pop Label

   Bit Number (TBD1): Pop 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 pop 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' and an error value of 'pop label usage failure'.


10.3.  RRO Label Subobject Flag: Pop Label

   Bit Number (TBD2): Pop Label

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

10.4.  Attribute Flags TLV: LSI-D

   Bit Number (TBD3): 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 pop labels is requested/mandated for this LSP.




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   Explicit Delegation: The presence of this flag in the HOP_ATTRIBUTES
   subobject [RFC7570] of an ERO object 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 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 pop 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' and an error value of
   'label stack imposition failure'.

10.5.  RRO Label Subobject Flag: Delegation Label

   Bit Number (TBD4): 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 pop labels and delegation are requested/mandated for
   the LSP.

10.6.  Attributes Flags TLV: LSI-D-S2E

   Bit Number (TBD5): 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 pop labels and
   delegation are requested/mandated for this LSP.

10.7.  Attributes TLV: ETLD

   The format of the ETLD Attributes TLV is shown in Figure 8.  The
   Attribute TLV Type is TBD6.












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       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 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 pop labels is requested/mandated and automatic
   delegation is requested for the LSP.  The ETLD field specifies the
   maximum number of transport labels that this hop can potentially send
   to its downstream hop.

11.  OAM Considerations

   MPLS LSP ping and traceroute [RFC8029] are applicable for RSVP-TE pop
   and forward 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.

12.  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.

13.  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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14.  IANA Considerations

14.1.  Attribute Flags: Pop 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
      TBD1 Pop Label         Yes       No        No  No  This document
                                                         (Section 11.2)
      TBD3 LSI-D             Yes       No        No  Yes This document
                                                         (Section 11.4)
      TBD5 LSI-D-S2E         Yes       No        No  No  This document
                                                         (Section 11.6)

14.2.  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


      TBD6   ETLD    No          No           Yes         This document
                                                          (Section 11.7)

14.3.  Record Route Label Sub-object Flags: Pop 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.

      Flag  Name                    Reference

      0x1   Global Label            RFC 3209
      TBD2  Pop Label               This document (Section 11.3)
      TBD4  Delegation Label        This document (Section 11.5)




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15.  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.

16.  References

16.1.  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>.

   [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>.




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16.2.  Informative References

   [I-D.ietf-spring-segment-routing]
              Filsfils, C., Previdi, S., Decraene, B., Litkowski, S.,
              and R. Shakir, "Segment Routing Architecture", draft-ietf-
              spring-segment-routing-12 (work in progress), June 2017.

   [I-D.ietf-spring-segment-routing-mpls]
              Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing with MPLS
              data plane", draft-ietf-spring-segment-routing-mpls-10
              (work in progress), June 2017.

   [I-D.ietf-teas-rsvp-te-scaling-rec]
              Beeram, V., Minei, I., Shakir, R., Pacella, D., and T.
              Saad, "Implementation Recommendations to Improve the
              Scalability of RSVP-TE Deployments", draft-ietf-teas-rsvp-
              te-scaling-rec-06 (work in progress), August 2017.

   [I-D.ietf-teas-sr-rsvp-coexistence-rec]
              Sitaraman, H., Beeram, V., Minei, I., and S. Sivabalan,
              "Recommendations for RSVP-TE and Segment Routing LSP co-
              existence", draft-ietf-teas-sr-rsvp-coexistence-rec-01
              (work in progress), June 2017.

   [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>.

Authors' Addresses

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

   Email: hsitaraman@juniper.net








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   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


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

   Email: tsaad@cisco.com


























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