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Network Working Group                                       S. Kini, Ed.
Internet-Draft                                                  Ericsson
Intended status: Informational                               K. Kompella
Expires: March 02, 2014                                          Juniper
                                                            S. Sivabalan
                                                                   Cisco
                                                         August 29, 2013


            Entropy labels for source routed stacked tunnels
          draft-kini-mpls-entropy-label-src-stacked-tunnels-01

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

   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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on March 02, 2014.

Copyright Notice

   Copyright (c) 2013 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



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
   2.  Abbreviations and Terminology . . . . . . . . . . . . . . . .   2
   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 . . . . . . . . . .   4
     3.4.  ELs at readable label stack depths  . . . . . . . . . . .   5
   4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

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.  Entropy labels (EL) [RFC6790] is a
   technique used by the MPLS data plane to do load balancing.  Applying
   ELs to stacked tunnels brings up some issues and these are documented
   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



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      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.



                        +-----+               +-----+
                        | 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



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                    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
   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.
   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



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   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.

3.4.  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
   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 TBD for their
   comments.

5.  IANA Considerations

   This memo includes no request to IANA.

6.  Security Considerations

7.  References

7.1.  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., and E. Crabbe,
              "Segment Routing Use Cases", draft-filsfils-rtgwg-segment-
              routing-use-cases-01 (work in progress), July 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-00 (work in progress), June 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.

7.2.  Informative References

   [I-D.previdi-isis-segment-routing-extensions]
              Previdi, S., Filsfils, C., Bashandy, A., Gredler, H., and
              S. Litkowski, "IS-IS Extensions for Segment Routing",
              draft-previdi-isis-segment-routing-extensions-02 (work in
              progress), July 2013.

   [I-D.psenak-ospf-segment-routing-extensions]
              Psenak, P., Previdi, S., Filsfils, C., Gredler, H., and R.
              Shakir, "OSPF Extensions for Segment Routing", draft-
              psenak-ospf-segment-routing-extensions-02 (work in
              progress), July 2013.

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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