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Versions: (draft-kini-mpls-spring-entropy-label) 00 01 02 03 04 05 06 07 08 09 10 11 12

Network Working Group                                            S. Kini
Internet-Draft
Intended status: Standards Track                             K. Kompella
Expires: November 24, 2018                                       Juniper
                                                            S. Sivabalan
                                                                   Cisco
                                                            S. Litkowski
                                                                  Orange
                                                               R. Shakir
                                                                  Google
                                                             J. Tantsura
                                                            May 23, 2018


                    Entropy label for SPRING tunnels
                draft-ietf-mpls-spring-entropy-label-11

Abstract

   Segment Routing (SR) leverages the source routing paradigm.  A node
   steers a packet through an ordered list of instructions, called
   segments.  Segment Routing 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 Segment Routing MPLS.

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 https://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 November 24, 2018.

Copyright Notice

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




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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Abbreviations and Terminology . . . . . . . . . . . . . . . .   4
   3.  Use-case requiring multipath load-balancing . . . . . . . . .   5
   4.  Entropy Readable Label Depth  . . . . . . . . . . . . . . . .   6
   5.  Maximum SID Depth . . . . . . . . . . . . . . . . . . . . . .   7
   6.  LSP stitching using the binding SID . . . . . . . . . . . . .   9
   7.  Insertion of entropy labels for SPRING path . . . . . . . . .  10
     7.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  10
       7.1.1.  Example 1 where the ingress node has a sufficient MSD  11
       7.1.2.  Example 2 where the ingress node has not a sufficient
               MSD . . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.2.  Considerations for the placement of entropy labels  . . .  13
       7.2.1.  ERLD value  . . . . . . . . . . . . . . . . . . . . .  14
       7.2.2.  Segment type  . . . . . . . . . . . . . . . . . . . .  14
         7.2.2.1.  Node-SID  . . . . . . . . . . . . . . . . . . . .  15
         7.2.2.2.  Adjacency-set SID . . . . . . . . . . . . . . . .  15
         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 . . . . . . . . . . . . . . . . . . . .  16
         7.2.2.5.  Adjacency-SID representing a L2 bundle  . . . . .  16
       7.2.3.  Maximizing number of LSRs that will load-balance  . .  16
       7.2.4.  Preference for a part of the path . . . . . . . . . .  16
       7.2.5.  Combining criteria  . . . . . . . . . . . . . . . . .  17
   8.  A simple example algorithm  . . . . . . . . . . . . . . . . .  17
   9.  Deployment Considerations . . . . . . . . . . . . . . . . . .  18
   10. Options considered  . . . . . . . . . . . . . . . . . . . . .  18
     10.1.  Single EL at the bottom of the stack . . . . . . . . . .  18
     10.2.  An EL per segment in the stack . . . . . . . . . . . . .  19
     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
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  21
   12. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  21
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  22



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   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     15.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   Segment Routing [I-D.ietf-spring-segment-routing] is based on source
   routed tunnels to steer a packet along a particular path.  This path
   is encoded as an ordered list of segments.  When applied to the MPLS
   dataplane [I-D.ietf-spring-segment-routing-mpls], each segment is an
   LSP with an associated MPLS label value.  Hence, label stacking is
   used to represent the ordered list of segments and the label stack
   associated with an SR tunnel can be seen as nested LSPs (LSP
   hierarchy) in the MPLS architecture.

   Using label stacking to encode the list of segment has implications
   on the label stack depth.

   Traffic load-balancing over ECMP (Equal Cost MultiPath) or LAGs (Link
   Aggregation Groups) is usually based on a hashing function.  The
   local node who performs the load-balancing is required to read some
   header fields in the incoming packets and then computes a hash based
   on these fields.  The result of the hash is finally mapped to a list
   of outgoing nexthops.  The hashing technique is required to perfom a
   per-flow load-balancing and thus prevent packet disordering.  For IP
   traffic, the usual fields that are looked up are the source address,
   the destination address, the protocol type, and, if the upper layer
   is TCP or UDP, the source port and destination port can be added as
   well in the hash.

   The MPLS architecture brings some challenges on the load-balancing as
   an LSR (Label Switch Router) should be able to look at header fields
   that are beyond the MPLS label stack.  An LSR must perform a deeper
   inspection compared to an ingress router which could be challenging
   for some hardware.  Entropy label (EL) [RFC6790] is a technique used
   in the MPLS data plane to provide entropy for load-balancing.  The
   idea behind entropy label is that the ingress router computes a hash
   based on several fields from a given packet and place the result in
   an additional label, named "entropy label".  Then, this entropy label
   can be used as part of the hash keys used by an LSR.  Using the
   entropy label in the hash keys reduces the need of a deep packet
   inspection in the LSR while keeping a good level of entropy in the
   load balancing.  When entropy label is used, the keys used in the
   hashing functions are still a local configuration matter and an LSR
   may use solely the entropy label or a combination of multiple fields
   from the incoming packet.




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   When using LSP hierarchies, there are implications on how [RFC6790]
   should be applied.  The current document addresses the case where a
   hierarchy is created at a single LSR as required by Segment Routing.

   A use-case requiring load-balancing with SR 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", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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 Path

      LSR - Label Switch Router

      MPLS - Multiprotocol Label Switching

      MSD - Maximum SID Depth

      SID - Segment Identifier

      RLD - Readable Label Depth

      OAM - Operation, Administration and Maintenance







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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 is one of the applications of MPLS and is also a
   requirement for source routed tunnels with label stacks [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 ([I-D.ietf-spring-segment-routing-mpls]) uses nested
   MPLS LSPs composing the source routed label stacks.

   An MPLS node may have limitations in the number of labels it can
   push.  It may also have a limitation in the number of labels it can
   inspect when looking for hash keys during load-balancing.  While
   entropy label is normally inserted at the bottom of the transport
   tunnel, this may prevent an LSR to take into account the EL in its
   load-balancing function if the EL is too deep in the stack.  In a
   segment routing environment, it is important to define the



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   considerations that needs to be taken into account when inserting EL.
   Multiple ways to apply entropy labels were considered and are
   documented in Section 10 along with their trade-offs.  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 both:

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

   b.  Use in its load-balancing 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 terms of ERLD.  For simplicity, an
   implementation MAY use the minimum ERLD between each linecard as the
   ERLD value for the system.

   Examples:

                                                         | 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





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   In the figure 2, 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 load-balance
      Packet 1 displayed in Figure 2 using the EL as part of the load-
      balancing keys.  The ERLD value of 3 means that the router can
      read and take into account the entropy label for load-balancing 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 load-balance
      Packets 1 to 3 in Figure 2 using the EL as part of the load-
      balancing keys.  Packets 4 and 5 have the EL placed at a position
      greater than 5, so the router is not able to read it and use as
      part of the load-balancing keys.

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

   To allow an efficient load-balancing 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 outside the scope of this document.

   To advertise an ERLD value, a SPRING router:

   o  MUST be entropy label capable and, as a consequence, MUST apply
      the dataplane 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




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   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
   outside the 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 actual 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
   path to PE2 along
   PE1->P1->P7->P8->P9->P4->P5->P10->P11->P12->P13->PE2.  By using
   adjacency SIDs only, PE1 (acting as an I-LSR) will be required to
   push 10 labels on the IP packet received and thus 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 should be imposed,
   requiring an MSD of 11 for PE1.  In addition, if PE1 wants to insert
   an ELI/EL for load-balancing 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




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   label stack, P5's 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 binding 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), or a SPRING
   path.  Each LSP associated with a binding SID has its own entropy
   label capability.

   In the figure 3, we consider that:

   o  P6, PE2, P10, P11, P12, P13 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 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.

           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 mapping-server
   advertisement from P5, 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.



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   To accomodate the mix of signaling protocols involved during the
   stitching, the entropy label capability SHOULD be propagated between
   the signaling domains.  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 with a binding SID unless this binding SID has
   the entropy label capability.  How the entropy label capability is
   advertised for a binding SID is outside the 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 only works 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's LDP FEC.  As a
   consequence, it does not set the ELC for the associated binding SID.

7.  Insertion of entropy labels for SPRING path

7.1.  Overview

   The solution described in this section follows the dataplane
   processing defined in [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 terminated 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).






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

   If an EL does not occur within the ERLD of an LSR in the label stack
   of an 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 load-balancing 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 in the stack when it reaches some LSRs if it is 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
   the 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 limitations, the
   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 criteria when
   inserting <ELI, EL> pairs.

7.1.1.  Example 1 where the ingress node has a sufficient MSD


                        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: <Adj_P1P2, Adj_set_P2P3,
   Adj_P3P4, Adj_P4P5, Adj_P5P6, Adj_P6PE2, VPN_label>.  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.



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

   PE1 may take into account multiple parameters when inserting 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
   <Adj_P1P2, Adj_set_P2P3, ELI1, EL1, Adj_P3P4, Adj_P4P5, Adj_P5P6,
   Adj_P6PE2, VPN_label>.  The original stack requests P2 to forward
   based on a L3 adjacency set that will require load-balancing.
   Therefore it is important to ensure that P2 can load-balance
   correctly.  As P2 has a limited ERLD of 3, ELI/EL must be inserted
   just next to the label that 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, except P6,
   moreover P6 is using some LAGs to PE2 and so is expected to load-
   balance.  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 imposition capacity.

7.1.2.  Example 2 where the ingress node has not a sufficient MSD


                      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: <Adj_P1P2, Adj_set_P2P3, Adj_P3P4,
   Adj_P4P5, Adj_P5P6, Adj_set_P6P7, Adj_P7P8; Adj_set_P8PE2,



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   VPN_label>.  PE1 is limited to push a maximum of 11 labels, P2, P3
   and P6 have an ERLD of 3 while others have an 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 load-balance the end-to-end path.  An optimized stack that
   would enable end-to-end load-balancing may be: <Adj_P1P2,
   Adj_set_P2P3, ELI1, EL1, Adj_P3P4, Adj_P4P5, Adj_P5P6, Adj_set_P6P7,
   ELI2, EL2, Adj_P7P8; Adj_set_P8PE2, ELI3, EL3, VPN_label>.

   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 the P6
   forwarding label as it will allow P4 and P6 to load-balance.  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 showed 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
   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 a trade-off, an implementation
   MAY provide flexibility to the operator to select the criteria to be
   considered when placing EL/ELIs or the sub-objective for which to
   optimize.

                            2      2
      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
   subsections.  All metrics are equal to 1, except P3-P4 and P4-P5
   which have a metric 2.





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7.2.1.  ERLD value

   As mentioned in Section 7.1, the ERLD value is an important parameter
   to consider when inserting ELI/EL.  If an ELI/EL does not fall within
   the ERLD of a node on the path, the node will not be able to load-
   balance the traffic efficiently.

   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: <Adj_P1P2, Node_P9, Adj_P9PE2, Service_label>.

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






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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.

   In the figure 6, let's consider a path from PE1 to PE2 using the
   following stack pushed by PE1: <Adj_P1P2, Node_P9, Adj_P9PE2,
   Service_label>.

   If, for example, PE1 is limited to push 6 labels, 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
   an additional SPT computation and analysis of the node segment path
   to prevent a node segment that does not really require load-balancing
   from being preferred when placing EL/ELIs.  Such inspection may be
   time consuming for implementations and without a 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 requires an EL/
   ELI.

7.2.2.2.  Adjacency-set SID

   An adjacency-set is an adjacency SID that refers to a set of
   adjacencies.  When an adjacency-set segment is used within a label
   stack, an implementation can deduce that load-balancing is expected
   at the node that advertised this adjacency segment.  An
   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



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   bundle) is implemented between two IP nodes and the L2 bundle SR
   extensions [I-D.ietf-isis-l2bundles] are not implemented.  In such a
   case, it may be useful to insert an EL/ELI in a readable position for
   the LSR advertising the label associated with 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 load-balance

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

   Let's consider a path from PE1 to PE2 using the following stack
   pushed by PE1: <Adj_P1P2, Node_P9, Adj_P9PE2, Service_label>.  All
   routers have an ERLD of 10, expect P1 and P2 which have an 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 and 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.




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7.2.5.  Combining criteria

   An implementation can combine multiple criteria to determine the best
   EL/ELIs placement.  However, combining too many criteria may lead to
   implementation complexity and high resource consumption.  Each time
   the network topology changes, a new evaluation of the EL/ELI
   placement will be necessary for each impacted LSPs.

8.  A simple example algorithm

   A simple implementation might take into account ERLD when placing
   ELI/EL while trying to minimize the number of EL/ELIs inserted and
   trying to maximize the number of LSRs that can load-balance.

   The example algorithm 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 those LSRs.

   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 algorithm is shown below.

      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>




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9.  Deployment Considerations

   As long as LSR node dataplane capabilities are 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 seem reasonable to have an LSR inspecting UDP ports
   within a GRE tunnel carried over a 15 label 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 requires two additional labels to be pushed.

   Placement strategies of EL/ELIs are required to find the best trade-
   off.  Multiple criteria may be taken into account and some level of
   customization (by the 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.

   In future, hardware and software capacity may increase dataplane
   capabilities and may be remove some of these limitations, increasing
   load-balancing capability using entropy labels.

10.  Options considered

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

   These options are detailed here only for historical purposes.

10.1.  Single EL at the bottom of the stack

   In this option, a single EL is used for the entire label stack.  The
   source LSR S encodes the entropy label 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



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   header when making forwarding decisions.  In the example described in
   Section 3, if we consider that the LSR P1 has an ERLD of 3, P1 would
   load-balance traffic poorly on the parallel links L3 and L4 since the
   EL is below the ERLD of P1.  A load-balanced network design using
   this approach must ensure that all intermediate LSRs have the
   capability to read the maximum label stack depth as required for the
   application that uses source routed stacking.

   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 when that is a critical requirement
   in a service-provider network.

10.2.  An EL per segment in the stack

   In this option, each segment/label in the stack can be given its own
   EL.  When load-balancing is required to direct traffic on a segment,
   the source LSR pushes an <ELI, EL> before pushing the label
   associated to this segment .  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 stacking.  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 have the capability to push such a deep label
   stack.  Also, the bandwidth overhead and potential MTU issues of deep
   label stacks should be considered in the network design.

   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



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   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.

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.




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   In the case where the ERLD 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, 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, Chris Bowers and Nobo Akiya for their review comments
   and suggestions.

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.




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

   [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,
              <https://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, <https://www.rfc-editor.org/info/rfc7855>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [I-D.ietf-spring-segment-routing]
              Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing
              Architecture", draft-ietf-spring-segment-routing-15 (work
              in progress), January 2018.

   [I-D.ietf-spring-segment-routing-mpls]
              Bashandy, A., Filsfils, C., Previdi, S., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing with MPLS
              data plane", draft-ietf-spring-segment-routing-mpls-13
              (work in progress), April 2018.

15.2.  Informative References

   [I-D.ietf-isis-mpls-elc]
              Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
              Litkowski, "Signaling Entropy Label Capability and
              Readable Label-stack Depth Using IS-IS", draft-ietf-isis-
              mpls-elc-03 (work in progress), January 2018.



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   [I-D.ietf-ospf-mpls-elc]
              Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
              Litkowski, "Signaling Entropy Label Capability and
              Readable Label-stack Depth Using OSPF", draft-ietf-ospf-
              mpls-elc-05 (work in progress), January 2018.

   [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-07 (work in progress),
              May 2017.

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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