Entropy labels for source routed stacked tunnels
draft-kini-mpls-spring-entropy-label-02

Network Working Group                                       S. Kini, Ed.
Internet-Draft                                                  Ericsson
Intended status: Informational                               K. Kompella
Expires: April 28, 2015                                          Juniper
                                                            S. Sivabalan
                                                                   Cisco
                                                            S. Litkowski
                                                                  Orange
                                                               R. Shakir
                                                                    B.T.
                                                                   X. Xu
                                                                  Huawei
                                                            W. Hendrickx
                                                          Alcatel-Lucent
                                                             J. Tantsura
                                                                Ericsson
                                                        October 25, 2014

            Entropy labels for source routed stacked tunnels
                draft-kini-mpls-spring-entropy-label-02

Abstract

   Source routed tunnel 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 stacked tunnels.

Status of This Memo

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   This Internet-Draft will expire on April 28, 2015.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Abbreviations and Terminology . . . . . . . . . . . . . . . .   3
   3.  Use-case for multipath load balancing in source stacked
       tunnels . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Recommended EL solution for SPRING  . . . . . . . . . . . . .   5
   5.  Options considered  . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Single EL at the bottom of the stack of tunnels . . . . .   6
     5.2.  An EL per tunnel in the stack . . . . . . . . . . . . . .   7
     5.3.  A re-usable EL for a stack of tunnels . . . . . . . . . .   8
       5.3.1.  EL at top of stack  . . . . . . . . . . . . . . . . .   8
     5.4.  ELs at readable label stack depths  . . . . . . . . . . .   8
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   The source routed stacked tunnels paradigm is leveraged by techniques
   such as Segment Routing (SR) [I-D.filsfils-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 label
   stack depth.

   Clarifying statements on label stack depth have been provided in
   [RFC7325] but they do not address the case of source routed stacked
   MPLS tunnels as described in [I-D.gredler-spring-mpls] or

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   [I-D.filsfils-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.  One such issue is addressed by
   [I-D.ravisingh-mpls-el-for-seamless-mpls] but that is when different
   levels of the hierarchy are created at different LSRs.  The current
   document addresses the case where the hierarchy is created at a
   single LSR as required by source stacked tunnels.

   A use-case requiring load balancing with source stacked tunnels is
   given in Section 3.  A recommended solution is described in Section 4
   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 5.

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 RFC 2119 [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.

2.  Abbreviations and Terminology

      EL - Entropy Label

      ELI - Entropy Label Identifier

      ELC - Entropy Label Capability

      SR - Segment Routing

      ECMP - Equal Cost Multi Paths

      MPLS - Multiprotocol Label Switching

      SID - Segment Identifier

      RLD - Readable Label Depth

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      OAM - Operation, Administration and Maintenance

3.  Use-case for multipath load balancing in source stacked tunnels

   Source stacked tunnels have several use-cases, one of which is
   service chaining [I-D.filsfils-spring-segment-routing-use-cases].
   Consider the service-chaining network in Figure 1 that has MPLS as
   the data plane.  The requirement of the use-case is to create a LSP
   from source LSR S, apply the services S1, S2 and finally terminate
   the LSP at destination LSR D.  Local load balancing is required
   across the parallel links between P1 and S1.  Local load balancing is
   also required between the ECMP paths from S1 to S2 i.e., between the
   paths S1-P1-P2-P3-S2 and S1-P1-P2-P4-S2.  Segment routing can be used
   to achieve this.  A segment to S1 is stacked above the segment to S2
   which in turn is stacked above the segment to D.  Labels for service
   instructions are also inserted in the stack at appropriate depths so
   that services S1 and S2 are executed.  To achieve local load
   balancing the SIDs of specific interfaces is not specified.  Since
   entropy label is a standardized [RFC6790] mechanism defined for MPLS
   it can be adapted to the case of source stacked tunnels.  Multiple
   ways to apply entropy labels exist and a recommended solution is
   described in Section 4 and all the options considered are listed in
   Section 5 along with their tradeoffs.  We denote SN to be the node
   SID of LSR N and SN{L1,L2,...} to denote the SID of the adjacency for
   the set for links {L1,L2,...} of LSR N and S-SvcN to denote the SID
   for a service at service LSR N.  The label stack that the source LSR
   S uses for the LSP can be <SS1, S-SvcS1, SS2, S-SvcS2, SD> or <SP1,
   SP1{L1,L2}, S-SvcS1, SS2, S-SvcS2, SD>.

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               +-----+               +-----+
               | S1  |        +------| P3  |------+
               +-----+        |      +-----+      |
               L1| |L2        |                   |
   +-----+     +-----+     +-----+             +-----+
   |  S  |-----| P1  |-----| P2  |             | S2  |
   +-----+     +-----+     +-----+             +-----+
                              |                   |
                              |      +-----+      |
                              +------| P4  |------+
                                     +-----+
                                        |
                                     +-----+
                                     |  D  |
                                     +-----+

     S=Source LSR, D=Destination LSR, S1,S2=service-LSRs, L1,L2=links,
                         P1,P2,P3,P4=Transit LSRs

                    Figure 1: Service chaining use-case

4.  Recommended EL solution for SPRING

   The solution described in this section follows [RFC6790].

   An LSR may have a limitation in its ability to read and process the
   label stack in order to do multipath load balancing.  This limitation
   expressed in terms of the number of label stack entries that the LSR
   can read is henceforth referred to as the Readable Label Depth (RLD)
   capability of that LSR.  If an EL does not occur within the RLD 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.  The RLD of an LSR is
   a characteristic of the forwarding plane of that LSR's implementation
   and determining it is outside the scope of this document.

   In order for the EL to occur within the RLD of LSRs along the path
   corresponding to a label stack, multiple <ELI, EL> pairs MAY be
   inserted in the label stack as long as the labels below which they
   are inserted are entropy label capable.  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 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 RLD, then the load balancing performed by
   that LSR would be poor.  Special attention should be paid when a
   forwarding adjacency LSP (FA-LSP) [RFC4206] is used as a link along
   the path of a source stacked LSP, since the labels of the FA-LSP

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   would additionally count towards the depth of the label stack when
   calculating the appropriate positions to insert the ELs.  The
   recommendations for inserting <ELI, EL> pairs are:

   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 an <ELI, EL> pair within the RLD of
      the maximum number of LSRs along the path as it can.

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

   A sample algorithm to insert ELs is shown below.  Implementations can
   choose any algorithm as long as it follows the above recommendations.

     Initialize the current EL insertion point to the
       bottommost label in the stack that is EL-capable
     while local-node can push more labels OR
       top of stack has been reached {
         insert an ELI+EL at current insertion point
         move insertion point up until current EL is out of RLD
                                              AND
                                     insertion point is EL-capable
         set current insertion point to new insertion point
     }

      Figure 2: Algorithm to insert <ELI, EL> pairs in a label stack

   The RLD can be advertised via protocols and those extensions would be
   described in a separate document.

   The recommendations above are not expected to bring any additional
   OAM considerations beyond those described in section 6 of [RFC6790].
   However, the OAM requirements and solutions for source stacked
   tunnels are still under discussion and future revisions of this
   document will address those if needed.

5.  Options considered

5.1.  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) below the labels of all
   the stacked tunnels.  In Figure 1 label stack at LSR S would look
   like <SP1, SS1, S-SvcS1, SS2, S-SvcS2, SD, ELI, EL> <remaining packet
   header>.  Note that the notation in [RFC6790] is used to describe the

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   label stack.  An issue with this approach is that as the label stack
   grows due an increase in the number of SIDs, the EL correspondingly
   goes deeper in the label stack.  As a result, intermediate LSRs (such
   as P1) that have to walk the label stack at least until the EL (if
   found) to perform load balancing decisions have to access a larger
   number of bytes in the packet header when making forwarding
   decisions.  A load balanced network design using this approach must
   ensure that all intermediate LSRs have the capability to traverse the
   maximum label stack depth in order to do effective load balancing.
   The use-case for which the tunnel stacking is applied would determine
   the maximum label stack depth.

   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 4.

   This option was discounted 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.

5.2.  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.  For the
   same Figure 1 above, the source LSR S encoded label stack would be
   <SS1, ELI, EL1, S-SvcS1, SS2, ELI, EL2, S-SvcS2, SD> where all the
   ELs can have the same value.  Accessing the EL at an intermediate LSR
   is independent of the depth of the label stack and hence independent
   of the specific use-case to which the stacked tunnels are applied.  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 4.

   This option was discounted 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

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   restrict the number of tunnels that can form a LSP and constrain the
   types of LSPs that can be created.  This was considered unacceptable.

5.3.  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.

   For the same Figure 1 above, the source LSR S encoded label stack
   would be <SS11, ELI, EL, S-SvcS1, SS2, S-SvcS2, SD>.  At P1 the
   outgoing label stack would be <SS1, ELI, EL, S-SvcS1, SS2, S-SvcS2,
   SD> after it has load balanced to one of the links L1 or L2.  At S1
   the outgoing label stack would be <SS2, S-SvS2, ELI, EL, SD>.  At P2
   the outgoing label stack would be <SS2, ELI, EL, S-SvcS2, SD> and it
   would load balance to one of the nexthop LSRs P3 or P4.  Accessing
   the EL at an intermediate LSR (e.g.  P3) is independent of the depth
   of the label stack and hence independent of the specific use-case to
   which the stacked tunnels are applied.

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

5.3.1.  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 discounted due to the significant change in label
   swap operations that would be required for existing hardware.

5.4.  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

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   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.  For the same Figure 1 above, if LSR P1
   needs to have the EL within a depth of 4, then the source LSR S
   encoded label stack would be <SS1, S-SvcS1, ELI, EL2, SS2, SD> 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 4.

   A variant of this solution was selected which balances the number of
   labels that need to be pushed against the requirement for entropy.

6.  Acknowledgements

   The authors would like to thank John Drake and Loa Andersson for
   their comments.

7.  IANA Considerations

   This memo includes no request to IANA.

8.  Security Considerations

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

9.  References

9.1.  Normative References

   [I-D.filsfils-spring-segment-routing]
              Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
              Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,
              Ytti, S., Henderickx, W., Tantsura, J., and E. Crabbe,
              "Segment Routing Architecture", draft-filsfils-spring-
              segment-routing-04 (work in progress), July 2014.

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   [I-D.filsfils-spring-segment-routing-use-cases]
              Filsfils, C., Francois, P., Previdi, S., Decraene, B.,
              Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,
              Ytti, S., Henderickx, W., Tantsura, J., Kini, S., and E.
              Crabbe, "Segment Routing Use Cases", draft-filsfils-
              spring-segment-routing-use-cases-01 (work in progress),
              October 2014.

   [I-D.gredler-spring-mpls]
              Gredler, H., Rekhter, Y., Jalil, L., Kini, S., and X. Xu,
              "Supporting Source/Explicitly Routed Tunnels via Stacked
              LSPs", draft-gredler-spring-mpls-06 (work in progress),
              May 2014.

   [I-D.ravisingh-mpls-el-for-seamless-mpls]
              Singh, R., Shen, Y., and J. Drake, "Entropy label for
              seamless MPLS", draft-ravisingh-mpls-el-for-seamless-
              mpls-02 (work in progress), July 2014.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4206]  Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)
              Hierarchy with Generalized Multi-Protocol Label Switching
              (GMPLS) Traffic Engineering (TE)", RFC 4206, October 2005.

   [RFC6790]  Kompella, K., Drake, J., Amante, S., Henderickx, W., and
              L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
              RFC 6790, November 2012.

   [RFC7325]  Villamizar, C., Kompella, K., Amante, S., Malis, A., and
              C. Pignataro, "MPLS Forwarding Compliance and Performance
              Requirements", RFC 7325, August 2014.

9.2.  Informative References

   [I-D.previdi-isis-segment-routing-extensions]
              Previdi, S., Filsfils, C., Bashandy, A., Gredler, H.,
              Litkowski, S., and J. Tantsura, "IS-IS Extensions for
              Segment Routing", draft-previdi-isis-segment-routing-
              extensions-05 (work in progress), February 2014.

   [I-D.psenak-ospf-segment-routing-extensions]
              Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
              Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
              Extensions for Segment Routing", draft-psenak-ospf-
              segment-routing-extensions-05 (work in progress), June
              2014.

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Authors' Addresses

   Sriganesh Kini (editor)
   Ericsson

   Email: sriganesh.kini@ericsson.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
   B.T.

   Email: rob.shakir@bt.com

   Xiaohu Xu
   Huawei

   Email: xuxiaohu@huawei.com

   Wim Hendrickx
   Alcatel-Lucent

   Email: wim.henderickx@alcatel-lucent.com

   Jeff Tantsura
   Ericsson

   Email: jeff.tantsura@ericsson.com

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