Network Working Group                                       S. Kini, Ed.
Internet-Draft                                                  Ericsson
Intended status: Informational                               K. Kompella
Expires: August 18, 2014                                         Juniper
                                                            S. Sivabalan
                                                                   Cisco
                                                       February 14, 2014


            Entropy labels for source routed stacked tunnels
                draft-kini-mpls-spring-entropy-label-00

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 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 August 18, 2014.

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   document authors.  All rights reserved.

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   carefully, as they describe your rights and restrictions with respect



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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Abbreviations and Terminology . . . . . . . . . . . . . . . .   3
   3.  Entropy Labels for source routed stacked tunnels  . . . . . .   3
     3.1.  Single EL at the bottom of the stack of tunnels . . . . .   4
     3.2.  An EL per tunnel in the stack . . . . . . . . . . . . . .   4
     3.3.  A re-usable EL for a stack of tunnels . . . . . . . . . .   5
       3.3.1.  EL at top of stack  . . . . . . . . . . . . . . . . .   5
     3.4.  ELs at readable label stack depths  . . . . . . . . . . .   5
   4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Change Log  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   7
     9.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   The source routed stacked tunnels paradigm is leveraged by techniques
   such as Segment Routing (SR) [I-D.filsfils-rtgwg-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
   [I-D.ietf-mpls-forwarding] but they do not address the case of source
   routed stacked tunnels as described in [I-D.gredler-spring-mpls] or
   [I-D.filsfils-rtgwg-segment-routing] where deeper label stacks are
   more prevalent.

   Entropy label (EL) [RFC6790] is a technique used by the MPLS data
   plane to do 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 at different routers.
   This draft addresses the case where the hierarchy is created at a




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   single LSR as required by source stacked tunnels.  The different
   options and their issues are discussed in Section 3.

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].

2.  Abbreviations and Terminology

      EL - Entropy Label

      ELI - Entropy Label Identifier

      SR - Segment Routing

      ECMP - Equal Cost Multi Paths

      MPLS - Multi Protocol Label Switching

      SID - Segment Identifier

3.  Entropy Labels for source routed stacked tunnels

   Stacked tunnels have several use-cases, one of which is service
   chaining [I-D.filsfils-rtgwg-segment-routing-use-cases].  Consider a
   service-chaining network in Figure 1.  The source LSR S wants to send
   traffic to destination LSR D. This traffic is required to go through
   service nodes S1 and S2 to produce the service chain S-S1-S2-D.
   Segment Routing can be used to achieve this.  Load balancing is
   required across the parallel links between P1 and S1.  Load balancing
   is also required between the ECMP paths from S1 to S2, S1-P1-P2-P3-S2
   and S1-P1-P2-P4-S2.  The source LSR wants the intermediate LSRs P1
   and P2 to take local load balancing decisions and does not specify
   the Segment Identifiers (SIDs) of specific interfaces.  Entropy
   labels should be used to achieve the desired load balancing.  Two
   possible ways to use the entropy labels and their associated
   tradeoffs are discussed below.  We denote SN to be the node segment
   identifier (SID) of LSR N and SN{L1,L2,...} to denote the SID of the
   adjacency set for links {L1,L2,...} of LSR N and S-N to denote the
   SID for a service at service node N. The label stack that the source
   LSR S uses for the service chain can be <SS1, S-S1, SS2, S-S2, SD> or
   <SP1, SP1{L1,L2}, S-S1, SS2, S-S2, SD>.  The issues discussed in this
   document are equally applicable to both of these options.






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


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

                    Figure 1: Service chaining use-case

3.1.  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, SP1{L1,L2}, SS1, S-S1, SS2, S-S2, SD, 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
   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 to perform load balancing decisions have to access
   a larger number of bytes in the packet header when making forwarding
   decisions.  A network design using this approach, should 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.

3.2.  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
   <SP1, SP1{L1,L2}, ELI, EL1, SS1, S-S1, SS2, ELI, EL2, SD> where all



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   the ELs would typically 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.

3.3.  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 <SP1, ELI, EL, SP1{L1,L2}, SS1, S-S1, SS2, SD>.  At P1 the
   outgoing label stack would be <SS1, ELI, EL, S-S1, SS2, SD> after it
   has load balanced to one of the links L1 or L2.  At S1 the outgoing
   label stack would be <SS2, ELI, EL, SD>.  At P2 the outgoing label
   stack would be <SS2, ELI, EL, SD> and it would load balance to one of
   the nexthop LSRs P3 or P4.  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.

3.3.1.  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 is under discussion.

3.4.  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.  Details of this will follow in subsequent
   versions if this option is found to be worth pursuing.  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 <SP1, SP1{L1,L2},
   ELI, EL1, SS1, S-S1, SS2, ELI, EL2, SD> where all the ELs would
   typically have the same value.

4.  Acknowledgements

   The authors would like to thank Rob Shakir and John Drake for their
   comments.

5.  Contributors

   Stephane Litkowski

   Email: stephane.litkowski@orange.com

6.  Change Log

   This draft is a renamed update of
   [I-D.kini-mpls-entropy-label-src-stacked-tunnels] that was presented
   at the IETF88 meeting in Vancouver [1].

7.  IANA Considerations

   This memo includes no request to IANA.

8.  Security Considerations

9.  References

9.1.  Normative References

   [I-D.filsfils-rtgwg-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-rtgwg-
              segment-routing-use-cases-02 (work in progress), October
              2013.




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   [I-D.filsfils-rtgwg-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-rtgwg-
              segment-routing-01 (work in progress), October 2013.

   [I-D.gredler-spring-mpls]
              Gredler, H., Rekhter, Y., Jalil, L., and S. Kini,
              "Supporting Source/Explicitly Routed Tunnels via Stacked
              LSPs", draft-gredler-spring-mpls-02 (work in progress),
              October 2013.

   [I-D.ietf-mpls-forwarding]
              Villamizar, C., Kompella, K., Amante, S., Malis, A., and
              C. Pignataro, "MPLS Forwarding Compliance and Performance
              Requirements", draft-ietf-mpls-forwarding-07 (work in
              progress), February 2014.

   [I-D.kini-mpls-entropy-label-src-stacked-tunnels]
              Kini, S., Kompella, K., and S. Sivabalan, "Entropy labels
              for source routed stacked tunnels", draft-kini-mpls-
              entropy-label-src-stacked-tunnels-01 (work in progress),
              August 2013.

   [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-01 (work in progress), October 2013.

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

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

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.







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   [I-D.psenak-ospf-segment-routing-extensions]
              Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
              Shakir, R., and W. Henderickx, "OSPF Extensions for
              Segment Routing", draft-psenak-ospf-segment-routing-
              extensions-04 (work in progress), February 2014.

9.3.  URIs

   [1] http://www.ietf.org/proceedings/88/slides/slides-88-spring-0.pdf

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























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