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In: MissingRef
Network Working Group                                            S. Kini
Internet-Draft
Intended status: Informational                               K. Kompella
Expires: October 26, 2017                                        Juniper
                                                            S. Sivabalan
                                                                   Cisco
                                                            S. Litkowski
                                                                  Orange
                                                               R. Shakir
                                                                  Google
                                                             J. Tantsura
                                                          April 24, 2017


                    Entropy label for SPRING tunnels
                draft-ietf-mpls-spring-entropy-label-05

Abstract

   Source routed tunnels with label stacking is a technique that can be
   leveraged to provide a method to steer a packet through a controlled
   set of segments.  This can be applied to the Multi Protocol Label
   Switching (MPLS) data plane.  Entropy label (EL) is a technique used
   in MPLS to improve load balancing.  This document examines and
   describes how ELs are to be applied to source routed tunnels with
   label stacks.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 26, 2017.








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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
   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
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Abbreviations and Terminology . . . . . . . . . . . . . . . .   4
   3.  Use-case requiring multipath load balancing . . . . . . . . .   4
   4.  Entropy Readable Label Depth  . . . . . . . . . . . . . . . .   5
   5.  Maximum SID Depth . . . . . . . . . . . . . . . . . . . . . .   7
   6.  LSP stitching using the binding SID . . . . . . . . . . . . .   8
   7.  Insertion of entropy labels for SPRING path . . . . . . . . .  10
     7.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  10
       7.1.1.  Example 1 . . . . . . . . . . . . . . . . . . . . . .  11
       7.1.2.  Example 2 . . . . . . . . . . . . . . . . . . . . . .  12
     7.2.  Considerations for the placement of entropy labels  . . .  12
       7.2.1.  ERLD value  . . . . . . . . . . . . . . . . . . . . .  13
       7.2.2.  Segment type  . . . . . . . . . . . . . . . . . . . .  14
         7.2.2.1.  Node-SID  . . . . . . . . . . . . . . . . . . . .  14
         7.2.2.2.  Adjacency-SID representing an ECMP bundle . . . .  14
         7.2.2.3.  Adjacency-SID representing a single IP link . . .  15
         7.2.2.4.  Adjacency-SID representing a single link within a
                   L2 bundle . . . . . . . . . . . . . . . . . . . .  15
         7.2.2.5.  Adjacency-SID representing a L2 bundle  . . . . .  15
       7.2.3.  Maximizing number of LSRs that will loadbalance . . .  15
       7.2.4.  Preference for a part of the path . . . . . . . . . .  16
       7.2.5.  Combining criteria  . . . . . . . . . . . . . . . . .  16
   8.  A simple algorithm example  . . . . . . . . . . . . . . . . .  16
   9.  Deployment Considerations . . . . . . . . . . . . . . . . . .  17
   10. Options considered  . . . . . . . . . . . . . . . . . . . . .  18
     10.1.  Single EL at the bottom of the stack of tunnels  . . . .  18
     10.2.  An EL per tunnel in the stack  . . . . . . . . . . . . .  18
     10.3.  A re-usable EL for a stack of tunnels  . . . . . . . . .  19
     10.4.  EL at top of stack . . . . . . . . . . . . . . . . . . .  20
     10.5.  ELs at readable label stack depths . . . . . . . . . . .  20



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   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  20
   12. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  21
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  21
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     15.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   The source routed tunnels with label stacking paradigm is leveraged
   by techniques such as Segment Routing (SR)
   [I-D.ietf-spring-segment-routing] to steer a packet through a set of
   segments.  This can be directly applied to the MPLS data plane, but
   it has implications on the label stack depth.

   Clarifying statements on label stack depth have been provided in
   [RFC7325] but the RFC does not address the case of source routed
   stacked MPLS tunnels as described in
   [I-D.ietf-spring-segment-routing] where deeper label stacks are more
   prevalent.

   Entropy label (EL) [RFC6790] is a technique used in the MPLS data
   plane to provide entropy for load balancing.  When using LSP
   hierarchies there are implications on how [RFC6790] should be
   applied.  The current document addresses the case where the hierarchy
   is created at a single LSR as required by source routed tunnels with
   label stacks.

   A use-case requiring load balancing with source routed tunnels with
   label stacks is given in Section 3.  A recommended solution is
   described in Section 7 keeping in consideration the limitations of
   implementations when applying [RFC6790] to deeper label stacks.
   Options that were considered to arrive at the recommended solution
   are documented for historical purposes in Section 10.

1.1.  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].

   Although this document is not a protocol specification, the use of
   this language clarifies the instructions to protocol designers
   producing solutions that satisfy the requirements set out in this
   document.




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2.  Abbreviations and Terminology

      EL - Entropy Label

      ELI - Entropy Label Identifier

      ELC - Entropy Label Capability

      ERLD - Entropy Readable Label Depth

      SR - Segment Routing

      ECMP - Equal Cost Multi Paths

      LSR - Label Switch Router

      MPLS - Multiprotocol Label Switching

      MSD - Maximum SID Depth

      SID - Segment Identifier

      RLD - Readable Label Depth

      OAM - Operation, Administration and Maintenance

3.  Use-case requiring multipath load balancing

                            +------+
                            |      |
                    +-------|  P3  |-----+
                    | +-----|      |---+ |
                  L3| |L4   +------+ L1| |L2     +----+
                    | |                | |    +--| P4 |--+
      +-----+     +-----+            +-----+  |  +----+  |  +-----+
      |  S  |-----| P1  |------------| P2  |--+          +--|  D  |
      |     |     |     |            |     |--+          +--|     |
      +-----+     +-----+            +-----+  |  +----+  |  +-----+
                                              +--| P5 |--+
                                                 +----+
          S=Source LSR, D=Destination LSR, P1,P2,P3,P4,P5=Transit LSRs,
                                L1,L2,L3,L4=Links


                  Figure 1: Traffic engineering use-case

   Traffic-engineering (TE) is one of the applications of MPLS and is
   also a requirement for source routed tunnels with label stacks



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   [RFC7855].  Consider the topology shown in Figure 1.  The LSR S
   requires data to be sent to LSR D along a traffic- engineered path
   that goes over the link L1.  Good load balancing is also required
   across equal cost paths (including parallel links).  To engineer
   traffic along a path that takes link L1, the label stack that LSR S
   creates consists of a label to the node SID of LSR P3, stacked over
   the label for the adjacency SID of link L1 and that in turn is
   stacked over the label to the node SID of LSR D.  For simplicity lets
   assume that all LSRs use the same label space (SRGB) for source
   routed label stacks.  Let L_N-Px denote the label to be used to reach
   the node SID of LSR Px.  Let L_A-Ln denote the label used for the
   adjacency SID for link Ln.  The LSR S must use the label stack <L_N-
   P3, L_A-L1, L_N-D> for traffic-engineering.  However to achieve good
   load balancing over the equal cost paths P2-P4-D, P2-P5-D and the
   parallel links L3, L4, a mechanism such as Entropy labels [RFC6790]
   should be adapted for source routed label stacks.  Indeed, the SPRING
   architecture with the MPLS dataplane uses nested MPLS LSPs composing
   the source routed label stacks.  As each MPLS node may have
   limitations in the number of labels it can push when it is ingress or
   inspect when doing loadbalancing, entropy labels insertion strategy
   becomes important to keep benefit of the loadbalancing.  Multiple
   ways to apply entropy labels were considered and are documented in
   Section 10 along with their tradeoffs.  A recommended solution is
   described in Section 7.

4.  Entropy Readable Label Depth

   The Entropy Readable Label Depth (ERLD) is defined as the number of
   labels a router can:

   a.  read in an MPLS packet received on its incoming interface
       (starting from the top of the stack) and

   b.  use in its loadbalancing function.

   The ERLD means that the router will perform load-balancing using the
   EL label if the EL is placed within the ERLD first labels.

   A router capable of reading N labels but not using an EL located
   within those N labels MUST consider its ERLD to be 0.  In a
   distributed switching architecture, each linecard may have a
   different capability in term of ERLD.  For simplicity reason, an
   implementation MAY use the minimum ERLD between each linecard as the
   ERLD value for the system.

   Examples:





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                                                         | Payload  |
                                                         +----------+
                                           | Payload  |  |    EL    | P7
                                           +----------+  +----------+
                             | Payload  |  |    EL    |  |    ELI   |
                             +----------+  +----------+  +----------+
               | Payload  |  |   EL     |  |    ELI   |  | Label 50 |
               +----------+  +----------+  +----------+  +----------+
 |  Payload |  |     EL   |  |   ELI    |  | Label 40 |  | Label 40 |
 +----------+  +----------+  +----------+  +----------+  +----------+
 |     EL   |  |    ELI   |  | Label 30 |  | Label 30 |  | Label 30 |
 +----------+  +----------+  +----------+  +----------+  +----------+
 |    ELI   |  | Label 20 |  | Label 20 |  | Label 20 |  | Label 20 |
 +----------+  +----------+  +----------+  +----------+  +----------+
 | Label 16 |  | Label 16 |  | Label 16 |  | Label 16 |  | Label 16 | P1
 +----------+  +----------+  +----------+  +----------+  +----------+
   Packet 1      Packet 2      Packet 3      Packet 4      Packet 5

                    Figure 2: Label stacks with ELI/EL

   In the figure below, we consider the displayed packets received on a
   router interface.  We consider also a single ERLD value for the
   router.

   o  If the router has an ERLD of 3, it will be able to loadbalance the
      Packet 1 displayed in the Figure 2 using the EL as part of the
      loadbalancing keys.  The ERLD value of 3 means that the router can
      read and take into account the entropy label for loadbalancing if
      it is placed between position 1 (top) and position 3.

   o  If the router has an ERLD of 5, it will be able to loadbalance the
      Packet 1 to 3 in the Figure 2 using the EL as part of the
      loadbalancing keys.  The Packet 4 and 5 have the EL placed at a
      position greater than 5, so the router is not able to read it and
      take it into account during the hashing.

   o  If the router has an ERLD of 10, it will be able to loadbalance
      all the packets displayed in the Figure 2 using the EL as part of
      the loadbalancing keys.

   To allow an efficient loadbalancing based on entropy labels, a router
   running SPRING SHOULD advertise its ERLD (or ERLDs), so all the other
   SPRING routers in the network are aware of its capability.  How this
   advertisement is done is out of scope of this document.

   To advertise an ERLD value, a SPRING router:





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   o  MUST be entropy label capable and as a consequence MUST apply all
      the procedures defined in [RFC6790].

   o  MUST be able to read an ELI/EL which is located within its ERLD
      value

   o  MUST take into account this EL in its load balancing function

5.  Maximum SID Depth

   The Maximum SID Depth defines the maximum number of labels that a
   particular node can impose on a packet.  This includes any kind of
   labels (service, entropy, transport...).  In an MPLS network, the MSD
   is a limit of the Ingress LSR (I-LSR) or any stitching node that
   would perform an imposition of additional labels on an existing label
   stack.

   Depending of the number of MPLS operations (POP, SWAP...) to be
   performed before the PUSH, the MSD may vary due to the hardware or
   software limitations.  As for the ERLD, there may also be different
   MSD limits based on the linecard type used in a distributed switching
   system.

   When an external controller is used to program a label stack on a
   particular node, this node MAY advertise its MSD value or a subset of
   its MSD value to the controller.  How this advertisement is done is
   out of scope of this document.  As the controller does not have the
   knowledge of the entire label stack to be pushed by the node, the
   node may advertise an MSD value which is lower than its real limit.
   This gives the ability for the controller to program a label stack up
   to the advertised MSD value while leaving room for the local node to
   add more labels (e.g. service, entropy, transport...) without
   reaching the hardware/software limit.

                 P7 ---- P8 ---- P9
               /                   \
       PE1 --- P1 --- P2 --- P3 --- P4 --- P5 --- P6 --- PE2
                                           |  \            |
   ---->                                  P10  \           |
   IP Pkt                                  |    \          |
                                          P11 --- P12 --- P13
                                              100    10000

                                 Figure 3

   In the Figure 3, an IP packet comes in the MPLS network at PE1.  All
   metrics are considered equal to 1 except P12-P13 which is 10000 and
   P11-P12 which is 100.  PE1 wants to steer the traffic using a SPRING



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   path to PE2 along
   PE1->P1->P7->P8->P9->P4->P5->P10->P11->P12->P13->PE2.  By using
   Adjacency SIDs only, PE1 will be required to push (as an I-LSR) 10
   labels on the IP packet received, it so requires an MSD of 10.  If
   the IP packet should be carried over an MPLS service like a regular
   layer 3 VPN, an additional service label will be imposed, requiring
   an MSD of 11 for PE1.  In addition, if PE1 wants to insert an ELI/EL
   for loadbalancing purpose, PE1 will need to push 13 labels on the IP
   packet requiring an MSD of 13.

   In the SPRING architecture, Node SIDs or Binding SIDs can be used to
   reduce the label stack size.  As an example, to steer the traffic on
   the same path as before, PE1 may be able to use the following label
   stack: <Node_P9, Node_P5, Binding_P5, Node_PE2>.  In this example we
   consider a combination of Node SIDs and a Binding SID advertised by
   P5 that will stitch the traffic along the path P10->P11->P12->P13.
   The instruction associated with the binding SID at P5 is thus to swap
   Binding_P5 to Adj_P12-P13 and then push <Adj_P11-P12, Node_P11>.  P5
   acts as a stitching node that pushes additional labels on an existing
   label stack, P5 MSD needs also to be taken into account and may limit
   the number of labels that could be imposed.

6.  LSP stitching using the binding SID

   The binding SID allows to bind a segment identifier to an existing
   LSP.  As examples, the binding SID can represent an RSVP-TE tunnel,
   an LDP path (through the mapping server advertisement), a SPRING
   path...  Each LSP associated with a binding SID has its own entropy
   label capability.

   In the figure 3, if we consider that:

   o  P6, PE2, P10, P11, P12 are pure LDP routers.

   o  PE1, P1, P2, P3, P4, P7, P8, P9 are pure SPRING routers.

   o  P5 is running SPRING and LDP.

   o  P5 acts as a mapping server (MS) and advertises Prefix SIDs for
      the LDP FECs: an index value of 20 is used for PE2.

   o  All SPRING routers use an SRGB of [1000, 1999].

   o  P6 advertises label 20 for the PE2 FEC.

   o  Traffic from PE1 to PE2 uses the shortest path.





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           PE1 ----- P1 -- P2 -- P3 -- P4 ---- P5 --- P6 --- PE2

       -->    +----+                   +----+   +----+  +----+
     IP Pkt   | IP |                   | IP |   | IP |  | IP |
              +----+                   +----+   +----+  +----+
              |1020|                   |1020|   | 20 |
              +----+                   +----+   +----+
                                       SPRING    LDP

   In term of packet forwarding, by learning the MS advertisement from
   PE5, PE1 imposes a label 1020 to an IP packet destinated to PE2.
   SPRING routers along the shortest path to PE2 will switch the traffic
   until it reaches P5 which will perform the LSP stitching.  P5 will
   swap the SPRING label 1020 to the LDP label 20 advertised by the
   nexthop P6.  P6 will then forward the packet using the LDP label
   towards PE2.

   PE1 cannot push an ELI/EL for the binding SID without knowing that
   the tail-end of the LSP associated with the binding (PE2) is entropy
   label capable.

   To accomodate the mix of signalling protocols involved during the
   stitching, the entropy label capability SHOULD be propagated between
   the signalling protocols.  Each binding SID SHOULD have its own
   entropy label capability that MUST be inherited from the entropy
   label capability of the associated LSP.  If the router advertising
   the binding SID does not know the ELC state of the target FEC, it
   MUST NOT set the ELC for the binding SID.  An ingress node MUST NOT
   push an ELI/EL associated to a binding SID unless this binding SID
   has the entropy label capability.  How the entropy label capability
   is advertised for a binding SID is out of scope of this document.

   In our example, if PE2 is LDP entropy label capable, it will add the
   entropy label capability in its LDP advertisement.  When P5 receives
   the FEC/label binding for PE2, it learns about the ELC and can set
   the ELC in the mapping server advertisement.  Thus PE1 learns about
   the ELC of PE2 and may push an ELI/EL associated with the binding
   SID.

   The proposed solution works only if the SPRING router advertising the
   binding SID is also performing the dataplane LSP stitching.  In our
   example, if the mapping server function is hosted on P8 instead of
   P5, P8 does not know about the ELC state of PE2 LDP FEC.  As a
   consequence, it does not set the ELC on the associated binding SID.







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7.  Insertion of entropy labels for SPRING path

7.1.  Overview

   The solution described in this section follows [RFC6790].  Within a
   SPRING path, a node may be ingress, egress, transit (regarding the
   entropy label processing described in [RFC6790] or it can be any
   combination of those.  For example:

   o  The ingress node of a SPRING domain may be an ingress node from an
      entropy label perspective.

   o  Any LSR terminating a segment of the SPRING path is an egress node
      (because it terminates the segment) but may also be a transit node
      if the SPRING path is not ended here because there is a subsequent
      SPRING MPLS label in the stack.

   o  Any LSR processing a binding SID may be a transit node and an
      ingress node (because it may push additional labels when
      processing the binding SID).

   As described earlier, an LSR may have a limitation on the depth of
   the label stack that it can read and process in order to do multipath
   load balancing based on entropy labels: we called it the ERLD.

   If an EL does not occur within the ERLD of an LSR in the label stack
   of the MPLS packet that it receives, then it would lead to poor load
   balancing at that LSR.  Hence an ELI/EL pair MUST be within the ERLD
   of the LSR in order for the LSR to use the EL during load balancing.

   Adding a single ELI/EL pair for the entire SPRING path may lead also
   to poor loadbalancing as well because the EL/ELI may not occur within
   the ERLD of some LSR on the path (if too deep) or may not be present
   anymore in the stack for some LSRs if too shallow.

   In order for the EL to occur within the ERLD of LSRs along the path
   corresponding to a SPRING label stack, multiple <ELI, EL> pairs MAY
   be inserted in this label stack.

   The insertion of the ELI/EL SHOULD occur only with a SPRING label
   advertised by an LSR that advertised an ERLD (the LSR is entropy
   label capable) or with a SPRING label associated with a binding SID
   that has the ELC set.

   The ELs among multiple <ELI, EL> pairs inserted in the stack MAY be
   same or different.  The LSR that inserts <ELI, EL> pairs MAY have
   limitations on the number of such pairs that it can insert and also
   the depth at which it can insert them.  If due to any limitation, the



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   inserted ELs are at positions such that an LSR along the path
   receives an MPLS packet without an EL in the label stack within that
   LSR's ERLD, then the load balancing performed by that LSR would be
   poor.  An implementation MAY consider multiple criterias when
   inserting <ELI, EL> pairs.

7.1.1.  Example 1


                        ECMP          LAG           LAG
      PE1 --- P1 --- P2 --- P3 --- P4 --- P5 --- P6 --- PE2


                                 Figure 4

   In the Figure 4, PE1 wants to forward some MPLS VPN traffic over an
   explicit path to PE2 resulting in the following label stack to be
   pushed onto the received IP header: {VPN_label, Adj_P6PE2, Adj_P5P6,
   Adj_P4P5, Adj_P3P4, Adj_Bundle_P2P3, Adj_P1P2}.  PE1 is limited to
   push a maximum of 11 labels (MSD=11).  P2, P3 and P6 have an ERLD of
   3 while others have an ERLD of 10.

   PE1 can only add two ELI/EL pairs in the label stack due to its MSD
   limitation.  It should place them in a smart way to benefit of load
   balancing along the longest part of the path.

   PE1 may take into account multiple parameters when placing the ELs,
   as examples:

   o  the ERLD value advertised by transit nodes.

   o  the requirement of load balancing for a particular label value.

   o  any service provider preference: favor beginning of the path or
      end of the path.

   In the Figure 4, a good strategy may be to use the following stack
   {VPN_label, ELI2,EL2, Adj_P6PE2, Adj_P5P6, Adj_P4P5, Adj_P3P4, ELI1,
   EL1, Adj_Bundle_P2P3, Adj_P1P2}.  The original stack requests P2 to
   forward based on a bundle Adjacency segment that will require load
   balancing.  Therefore it is important to ensure that P2 can
   loadbalance correctly.  As P2 has a limited ERLD of 3, ELI/EL must be
   inserted just next to the label P2 will use to forward.  On the path
   to PE2, P3 has also a limited ERLD, but P3 will forward based on a
   basic adjacency segment that may require no load balancing.
   Therefore it does not seem important to ensure that P3 can do load
   balancing despite of its limited ERLD.  The next nodes along the
   forwarding path have a high ERLD that does not cause any issue,



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   except P6, moreover P6 is using some LAGs to PE2 and is so expected
   to loadbalance.  It becomes important to insert a new ELI/EL just
   next to P6 forwarding label.

   In the case above, the ingress node had enough label push capacity to
   ensure end to end load balancing taking into the path attributes.
   There might be some cases, where the ingress node may not have the
   necessary label push capacity.

7.1.2.  Example 2


                      ECMP          LAG           ECMP         ECMP
    PE1 --- P1 --- P2 --- P3 --- P4 --- P5 --- P6 --- P7 --- P8 --- PE2


                                 Figure 5

   In the Figure 5, PE1 wants to forward MPLS VPN traffic over an
   explicit path to PE2 resulting in the following label stack to be
   pushed onto the IP header : {VPN_label, Adj_Bundle_P8PE2, Adj_P7P8,
   Adj_Bundle_P6P7, Adj_P5P6, Adj_P4P5, Adj_P3P4, Adj_Bundle_P2P3,
   Adj_P1P2}.  PE1 is limited to push a maximum of 11 labels, P2, P3 and
   P6 have a ERLD of 3 while others have a ERLD of 15.

   Using a similar strategy as the previous case may lead to a dilemma,
   as PE1 can only push a single ELI/EL while we may need a minimum of
   three to loadbalance the end to end path.  An optimized stack that
   would enable end-to-end load balancing may be: {VPN_label, ELI3, EL3,
   Adj_Bundle_P8PE2, Adj_P7P8, ELI2, EL2, Adj_Bundle_P6P7, Adj_P5P6,
   Adj_P4P5, Adj_P3P4, ELI1, EL1, Adj_Bundle_P2P3, Adj_P1P2}.

   A decision needs to be taken to favor some part of the path for load
   balancing considering that load balancing may not work on the other
   part.  A service provider may decide to place the ELI/EL after P6
   forwarding label as it will allow P4 and P6 to loadbalance.  Placing
   the ELI/EL at bottom of the stack is also a possibility enabling load
   balancing for P4 and P8.

7.2.  Considerations for the placement of entropy labels

   The sample cases described in the previous section shown that placing
   the ELI/EL when the maximum number of labels to be pushed is limited
   is not an easy decision and multiple criteria may be taken into
   account.

   This section describes some considerations that could be taken into
   account when placing ELI/ELs.  This list of criteria is not



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   considered as exhaustive and an implementation MAY take into account
   additional criteria or tie breakers that are not documented here.

   An implementation SHOULD try to maximize the load-balancing where
   multiple ECMP paths are available and minimize the number of EL/ELIs
   that need to be inserted.  In case of trade-off, an implementation
   MAY provide flexibility to operator to select the criteria to be
   taken into account when placing EL/ELIs or the sub-objective to be
   optimized for.

      PE1 -- P1 -- P2 -- P3 -- P4 -- P5 -- ... -- P8 -- P9 -- PE2
                        |             |
                        P3'--- P4'--- P5'

                                 Figure 6

   The figure above will be used as reference in the following sub
   sections.

7.2.1.  ERLD value

   As mentioned in Section 7.1, the ERLD value is an important parameter
   to take into account when inserting ELI/EL as if an ELI/EL does not
   fall within the ERLD of a node on the path, the node will not be able
   to loadbalance the traffic in an efficient way.

   The ERLD value can be advertised via protocols and those extensions
   are described in separate documents [I-D.ietf-isis-mpls-elc] and
   [I-D.ietf-ospf-mpls-elc].

   Let's consider a path from PE1 to PE2 using the following stack
   pushed by PE1: {Service_label, Adj_PE2P9, Node_P9, Adj_P1P2}.

   Using the ERLD as an input parameter may help to minimize the number
   of required ELI/EL pairs to be inserted.  An ERLD value must be
   retrieved for each SPRING label in the label stack.

   For a label bound to an adjacency segment, the ERLD is the ERLD of
   the node that advertised the adjacency segment.  In the example
   above, the ERLD associated with Adj_P1P2 would be the ERLD of router
   P1 as P1 will perform the forwarding based on Adj_P1P2 label.

   For a label bound to a node segment, multiple strategies MAY be
   implemented.  An implementation may try to evaluate the minimum ERLD
   value along the node segment path.  If the implementation cannot find
   the minimum ERLD along the path of the segment, it can use the ERLD
   of the starting node instead.  In the example above, if the
   implementation supports computation of minimum ERLD along the path,



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   the ERLD associated to label Node_P9 would be the minimum ERLD
   between nodes {P2,P3,P4 ..., P8}.  If the implementation does not
   support the computation of minimum ERLD, it should consider the ERLD
   of P2 (starting node that will forward based on Node_P9 label).

   For a label bound to a binding segment, if the binding segment
   describes a path, an implementation may also try to evaluate the
   minimum ERLD along this path.  If the implementation cannot find the
   minimum ERLD along the path of the segment, it can use the ERLD of
   the starting node instead.

7.2.2.  Segment type

   Depending of the type of segment a particular label is bound to, an
   implementation may deduce that this particular label will be subject
   to load balancing on the path.

7.2.2.1.  Node-SID

   An MPLS label bound to a Node-SID represents a path that may cross
   multiple hops.  Load balancing may be needed on the node starting
   this path but also on any node along the path.

   Let's consider a path from PE1 to PE2 using the following stack
   pushed by PE1: {Service_label, Adj_PE2P9, Node_P9, Adj_P1P2}.

   If, for example, PE1 is limited to 6 labels to be pushed, it can add
   a single ELI/EL within the label stack.  An operator may want to
   favor a placement that would allow load balancing along the node-SID
   path.  In the figure above, P3 which is along the node-SID path
   requires load balancing on two equal cost paths.

   An implementation may try to evaluate if load balancing is really
   required within a node segment path: this could be done by running
   additional SPT computation and by analysis the node segment path.  So
   a node segment that does not really require load balancing may not be
   preferred when placing EL/ELIs.  Such inspection may be time
   consuming for implementations without 100% guarantee, as a node
   segment path may use LAG that could be invisible from the IP
   topology.  A simpler approach would be to consider that a label bound
   to a Node-SID will be subject to load balancing and so requires an
   EL/ELI.

7.2.2.2.  Adjacency-SID representing an ECMP bundle

   When an adjacency segment representing an ECMP bundle is used within
   a label stack, an implementation can deduce that load balancing is
   expected at the node that advertised this adjacency segment.  An



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   implementation could then favor this particular label value when
   placing ELI/ELs.

7.2.2.3.  Adjacency-SID representing a single IP link

   When an adjacency segment representing a single IP link is used
   within a label stack, an implementation can deduce that load
   balancing may not be expected at the node that advertised this
   adjacency segment.

   The implementation could then decide to place ELI/ELs to favor other
   LSRs than the one advertising this adjacency segment.

   Readers should note that an adjacency segment representing a single
   IP link may require load balancing.  This is the case when a LAG (L2
   bundle) is implemented between two IP nodes and L2 bundle SR
   extensions [I-D.ietf-isis-l2bundles] are not implemented.  In such
   case, it may be interesting to keep the possibility to insert an EL/
   ELI in a readable position for the LSR advertising the label
   associated to the adjacency segment.

7.2.2.4.  Adjacency-SID representing a single link within a L2 bundle

   When L2 bundle SR extensions [I-D.ietf-isis-l2bundles] are used,
   adjacency segments may be advertised for each member of the bundle.
   In this case, an implementation can deduce that load balancing is not
   expected on the LSR advertising this segment and could then decide to
   place ELI/ELs to favor other LSRs than the one advertising this
   adjacency segment.

7.2.2.5.  Adjacency-SID representing a L2 bundle

   When L2 bundle SR extensions [I-D.ietf-isis-l2bundles] are used, an
   adjacency segment may be advertised to represent the bundle.  In this
   case, an implementation can deduce that load balancing is expected on
   the LSR advertising this segment and could then decide to place ELI/
   ELs to favor this LSR.

7.2.3.  Maximizing number of LSRs that will loadbalance

   When placing ELI/ELs, an implementation may try to maximize the
   number of LSRs that LSRs that both needs to load balance (i.e. have
   ECMP paths) and that will be able to perform load balancing (i.e.  EL
   label is within their ERLD).

   Let's consider a path from PE1 to PE2 using the following stack
   pushed by PE1: {Service_label, Adj_PE2P9, Node_P9, Adj_P1P2}.  All




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   routers have ERLD of 10, expect P1 and P2 which have a ERLD of 4.
   PE1 is able to push 6 labels, so only a single ELI/EL can be added.

   In the example above, adding ELI/EL next to Adj_P1P2 will only allow
   load balancing at P1 while inserting it next to Adj_PE2P9, will allow
   load balancing at P2,P3 ... P9 so maximizing the number of LSRs that
   could perform load balancing.

7.2.4.  Preference for a part of the path

   An implementation may propose to favor a part of the end-to-end path
   when the number of EL/ELI that can be pushed is not enough to cover
   the entire path.  As example, a service provider may want to favor
   load balancing at the beginning of the path or at the end of path, so
   the implementation should prefer putting the ELI/ELs near the top or
   near of the bottom of the stack.

7.2.5.  Combining criteria

   An implementation can combine multiple criteria to determine the best
   EL/ELIs placement.  But combining too much criteria may lead to
   implementation complexity and high control plane resource
   consumption.  Each time, the network topology will change, a new
   evaluation of the EL/ELI placement will be necessary.

8.  A simple algorithm example

   A simple implementation can only take into account ERLD when placing
   ELI/EL while keep minimizing the number of EL/ELIs inserted and
   maximizing the number of LSRs that can loadbalance.

   The algorithm example is based on the following considerations:

   o  An LSR that is limited in the number of <ELI, EL> pairs that it
      can insert SHOULD insert such pairs deeper in the stack.

   o  An LSR should try to insert <ELI, EL> pairs at positions so that
      for the maximum number of transit LSRs, the EL occurs within the
      ERLD of the incoming packet to that LSR.

   o  An LSR should try to insert the minimum number of such pairs while
      trying to satisfy the above criteria.

   The pseudocode of the example is shown below.







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      Initialize the current EL insertion point to the
        bottommost label in the stack that is EL-capable
      while (local-node can push more <ELI,EL> pairs OR
                insertion point is not above label stack) {
          insert an <ELI,EL> pair below current insertion point
          move new insertion point up from current insertion point until
              ((last inserted EL is below the ERLD) AND (ERLD > 2)
                                AND
               (new insertion point is EL-capable))
          set current insertion point to new insertion point
      }

     Figure 7: Example algorithm to insert <ELI, EL> pairs in a label
                                   stack

   When this algorithm is applied to the example described in Section 3
   it will result in ELs being inserted in two positions, one below the
   label L_N-D and another below L_N-P3.  Thus the resulting label stack
   would be {L_N-P3, ELI, EL, L_A-L1, L_N-D, ELI, EL}

9.  Deployment Considerations

   As long as LSR node dataplane capabilities with be limited (number of
   labels that can be pushed, or number of labels that can be
   inspected), hop-by-hop load balancing of SPRING encapsulated flows
   will require trade-offs.

   Entropy label is still a good and usable solution as it allows load
   balancing without having to perform a deep packet inspection on each
   LSR: it does not sound reasonable to have a LSR inspecting UDP port
   within a GRE tunnel carried over a 15 labels SPRING tunnel.

   Due to the limited capacity of reading a deep stack of MPLS labels,
   multiple EL/ELIs may be required within the stack which directly
   impacts the capacity of the head-end to push a deep stack: each EL/
   ELI inserted requiring two additional labels to be pushed.

   Placement strategies of EL/ELIs are so required to find the best
   trade-off.  Multiple criteria may be taken into account and some
   level of customization (by user) may be required to accommodate the
   different deployments.  Analyzing the path of each destination to
   determine the best EL/ELI placement may be time consuming for the
   control plane, we encourage implementations to find the best trade-
   off between simplicity, resource consumption and load balancing
   efficiency.






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   In future, hardware and software capacity may increase dataplane
   capabilities and may be remove some of those limits: this may
   increase the capacity of load balancing using entropy labels.

10.  Options considered

   Different options that were considered to arrive at the recommended
   solution are documented in this section.

10.1.  Single EL at the bottom of the stack of tunnels

   In this option a single EL is used for the entire label stack.  The
   source LSR S encodes the entropy label (EL) at the bottom of the
   label stack.  In the example described in Section 3, it will result
   in the label stack at LSR S to look like {L_N-P3, L_A-L1, L_N-D, ELI,
   EL} {remaining packet header}.  Note that the notation in [RFC6790]
   is used to describe the label stack.  An issue with this approach is
   that as the label stack grows due an increase in the number of SIDs,
   the EL goes correspondingly deeper in the label stack.  Hence transit
   LSRs have to access a larger number of bytes in the packet header
   when making forwarding decisions.  In the example described in
   Section 3, the LSR P1 would poorly load-balance traffic on the
   parallel links L3, L4 since the EL is below the RLD of the packet
   received by P1.  A load balanced network design using this approach
   must ensure that all intermediate LSRs have the capability to
   traverse the maximum label stack depth as required for that
   application that uses source routed stacking.

   In the case where the hardware is capable of pushing a single <ELI,
   EL> pair at any depth, this option is the same as the recommended
   solution in Section 7.

   This option was rejected since there exist a number of hardware
   implementations which have a low maximum readable label depth.
   Choosing this option can lead to a loss of load-balancing using EL in
   a significant part of the network but that is a critical requirement
   in a service provider network.

10.2.  An EL per tunnel in the stack

   In this option each tunnel in the stack can be given its own EL.  The
   source LSR pushes an <ELI, EL> before pushing a tunnel label when
   load balancing is required to direct traffic on that tunnel.  In the
   example described in Section 3, the source LSR S encoded label stack
   would be {L_N-P3, ELI, EL, L_A-L1, L_N-D, ELI, EL} where all the ELs
   can be the same.  Accessing the EL at an intermediate LSR is
   independent of the depth of the label stack and hence independent of
   the specific application that uses source routed tunnels with label



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   stacking in that network.  A drawback is that the depth of the label
   stack grows significantly, almost 3 times as the number of labels in
   the label stack.  The network design should ensure that source LSRs
   should have the capability to push such a deep label stack.  Also,
   the bandwidth overhead and potential MTU issues of deep label stacks
   should be accounted for in the network design.

   In the case where the RLD is the minimum value (3) for all LSRs, all
   LSRs are EL capable and the LSR that is inserting <ELI, EL> pairs has
   no limit on how many it can insert then this option is the same as
   the recommended solution in Section 7.

   This option was rejected due to the existence of hardware
   implementations that can push a limited number of labels on the label
   stack.  Choosing this option would result in a hardware requirement
   to push two additional labels per tunnel label.  Hence it would
   restrict the number of tunnels that can be stacked in a LSP and hence
   constrain the types of LSPs that can be created.  This was considered
   unacceptable.

10.3.  A re-usable EL for a stack of tunnels

   In this option an LSR that terminates a tunnel re-uses the EL of the
   terminated tunnel for the next inner tunnel.  It does this by storing
   the EL from the outer tunnel when that tunnel is terminated and re-
   inserting it below the next inner tunnel label during the label swap
   operation.  The LSR that stacks tunnels should insert an EL below the
   outermost tunnel.  It should not insert ELs for any inner tunnels.
   Also, the penultimate hop LSR of a segment must not pop the ELI and
   EL even though they are exposed as the top labels since the
   terminating LSR of that segment would re-use the EL for the next
   segment.

   In Section 3 above, the source LSR S encoded label stack would be
   {L_N-P3, ELI, EL, L_A-L1, L_N-D}.  At P1 the outgoing label stack
   would be {L_N-P3, ELI, EL, L_A-L1, L_N-D} after it has load balanced
   to one of the links L3 or L4.  At P3 the outgoing label stack would
   be {L_N-D, ELI, EL}.  At P2 the outgoing label stack would be {L_N-D,
   ELI, EL} and it would load balance to one of the nexthop LSRs P4 or
   P5.  Accessing the EL at an intermediate LSR (e.g.  P1) is
   independent of the depth of the label stack and hence independent of
   the specific use-case to which the label stack is applied.

   This option was rejected due to the significant change in label swap
   operations that would be required for existing hardware.






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10.4.  EL at top of stack

   A slight variant of the re-usable EL option is to keep the EL at the
   top of the stack rather than below the tunnel label.  In this case
   each LSR that is not terminating a segment should continue to keep
   the received EL at the top of the stack when forwarding the packet
   along the segment.  An LSR that terminates a segment should use the
   EL from the terminated segment at the top of the stack when
   forwarding onto the next segment.

   This option was rejected due to the significant change in label swap
   operations that would be required for existing hardware.

10.5.  ELs at readable label stack depths

   In this option the source LSR inserts ELs for tunnels in the label
   stack at depths such that each LSR along the path that must load
   balance is able to access at least one EL.  Note that the source LSR
   may have to insert multiple ELs in the label stack at different
   depths for this to work since intermediate LSRs may have differing
   capabilities in accessing the depth of a label stack.  The label
   stack depth access value of intermediate LSRs must be known to create
   such a label stack.  How this value is determined is outside the
   scope of this document.  This value can be advertised using a
   protocol such as an IGP.

   Applying this method to the example in Section 3 above, if LSR P1
   needs to have the EL within a depth of 4, then the source LSR S
   encoded label stack would be {L_N-P3, ELI, EL, L_A-L1, L_N-D, ELI,
   EL} where all the ELs would typically have the same value.

   In the case where the RLD has different values along the path and the
   LSR that is inserting <ELI, EL> pairs has no limit on how many pairs
   it can insert, and it knows the appropriate positions in the stack
   where they should be inserted, then this option is the same as the
   recommended solution in Section 7.

   Note that a refinement of this solution which balances the number of
   pushed labels against the desired entropy is the solution described
   in Section 7.

11.  Acknowledgements

   The authors would like to thank John Drake, Loa Andersson, Curtis
   Villamizar, Greg Mirsky, Markus Jork, Kamran Raza, Carlos Pignataro,
   Bruno Decraene and Nobo Akiya for their review comments and
   suggestions.




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12.  Contributors

      Xiaohu Xu
      Huawei

      Email: xuxiaohu@huawei.com


      Wim Hendrickx
      Nokia

      Email: wim.henderickx@nokia.com


      Gunter Van De Velde
      Nokia

      Email: gunter.van_de_velde@nokia.com


      Acee Lindem
      Cisco

      Email: acee@cisco.com


13.  IANA Considerations

   This memo includes no request to IANA.  Note to RFC Editor: Remove
   this section before publication.

14.  Security Considerations

   This document does not introduce any new security considerations
   beyond those already listed in [RFC6790].

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,
              <http://www.rfc-editor.org/info/rfc2119>.







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   [RFC6790]  Kompella, K., Drake, J., Amante, S., Henderickx, W., and
              L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
              RFC 6790, DOI 10.17487/RFC6790, November 2012,
              <http://www.rfc-editor.org/info/rfc6790>.

   [RFC7855]  Previdi, S., Ed., Filsfils, C., Ed., Decraene, B.,
              Litkowski, S., Horneffer, M., and R. Shakir, "Source
              Packet Routing in Networking (SPRING) Problem Statement
              and Requirements", RFC 7855, DOI 10.17487/RFC7855, May
              2016, <http://www.rfc-editor.org/info/rfc7855>.

15.2.  Informative References

   [RFC4206]  Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)
              Hierarchy with Generalized Multi-Protocol Label Switching
              (GMPLS) Traffic Engineering (TE)", RFC 4206,
              DOI 10.17487/RFC4206, October 2005,
              <http://www.rfc-editor.org/info/rfc4206>.

   [RFC7325]  Villamizar, C., Ed., Kompella, K., Amante, S., Malis, A.,
              and C. Pignataro, "MPLS Forwarding Compliance and
              Performance Requirements", RFC 7325, DOI 10.17487/RFC7325,
              August 2014, <http://www.rfc-editor.org/info/rfc7325>.

   [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-11 (work in progress), February
              2017.

   [I-D.ietf-isis-mpls-elc]
              Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
              Litkowski, "Signaling Entropy Label Capability Using IS-
              IS", draft-ietf-isis-mpls-elc-02 (work in progress),
              October 2016.

   [I-D.ietf-ospf-mpls-elc]
              Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
              Litkowski, "Signaling Entropy Label Capability Using
              OSPF", draft-ietf-ospf-mpls-elc-04 (work in progress),
              November 2016.

   [I-D.ietf-isis-l2bundles]
              Ginsberg, L., Bashandy, A., Filsfils, C., Nanduri, M., and
              E. Aries, "Advertising L2 Bundle Member Link Attributes in
              IS-IS", draft-ietf-isis-l2bundles-04 (work in progress),
              April 2017.




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Authors' Addresses

   Sriganesh Kini

   EMail: sriganeshkini@gmail.com


   Kireeti Kompella
   Juniper

   EMail: kireeti@juniper.net


   Siva Sivabalan
   Cisco

   EMail: msiva@cisco.com


   Stephane Litkowski
   Orange

   EMail: stephane.litkowski@orange.com


   Rob Shakir
   Google

   EMail: rjs@rob.sh


   Jeff Tantsura

   EMail: jefftant@gmail.com

















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