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Versions: (draft-filsfils-spring-segment-routing-mpls) 00 01 02 03 04 05 06 07 08 09 10

Network Working Group                                   C. Filsfils, Ed.
Internet-Draft                                           S. Previdi, Ed.
Intended status: Standards Track                             A. Bashandy
Expires: December 24, 2017                           Cisco Systems, Inc.
                                                             B. Decraene
                                                            S. Litkowski
                                                                  Orange
                                                               R. Shakir
                                                                  Google
                                                           June 22, 2017


                  Segment Routing with MPLS data plane
               draft-ietf-spring-segment-routing-mpls-10

Abstract

   Segment Routing (SR) leverages the source routing paradigm.  A node
   steers a packet through a controlled set of instructions, called
   segments, by prepending the packet with an SR header.  In the MPLS
   dataplane, the SR header is instantiated through a label stack.  A
   segment can represent any instruction, topological or service-based.
   Additional segments can be defined in the future.  SR allows to
   enforce a flow through any topological path and/or service chain
   while maintaining per-flow state only at the ingress node to the SR
   domain.

   Segment Routing can be directly applied to the MPLS architecture with
   no change in the forwarding plane.  This drafts describes how Segment
   Routing operates on top of the MPLS data plane.

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].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.





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   Internet-Drafts are draft documents valid for a maximum of six months
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  MPLS Instantiation of Segment Routing . . . . . . . . . . . .   3
   3.  IGP Segments Examples . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Example 1 . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Example 2 . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.3.  Example 3 . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.4.  Example 4 . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.5.  Example 5 . . . . . . . . . . . . . . . . . . . . . . . .   8
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   5.  Manageability Considerations  . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   8
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   The Segment Routing architecture [I-D.ietf-spring-segment-routing]
   can be directly applied to the MPLS architecture with no change in
   the MPLS forwarding plane.  This drafts describes how Segment Routing
   operates on top of the MPLS data plane.



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   The Segment Routing problem statement is described in [RFC7855].

   Link State protocol extensions for Segment Routing are described in
   [I-D.ietf-isis-segment-routing-extensions],
   [I-D.ietf-ospf-segment-routing-extensions] and
   [I-D.ietf-ospf-ospfv3-segment-routing-extensions].

   Segment Routing, applied to the MPLS data plane, offers the ability
   to tunnel services (VPN, VPLS, VPWS) from an ingress PE to an egress
   PE, without any other protocol than ISIS or OSPF
   ([I-D.ietf-isis-segment-routing-extensions] and
   [I-D.ietf-ospf-segment-routing-extensions]).  LDP and RSVP-TE
   signaling protocols are not required.

   Note that [I-D.ietf-spring-segment-routing-ldp-interop] documents SR
   co-existence and interworking with other MPLS signaling protocols, if
   present in the network during a migration, or in case of non-
   homogeneous deployments.

2.  MPLS Instantiation of Segment Routing

   MPLS instantiation of Segment Routing fits in the MPLS architecture
   as defined in [RFC3031] both from a control plane and forwarding
   plane perspective:

   o  From a control plane perspective, [RFC3031] does not mandate a
      single signaling protocol.  Segment Routing makes use of Link
      State IGPs since their flooding mechanism fits very well with
      label stacking on ingress.

   o  From a forwarding plane perspective, Segment Routing does not
      require any change to the forwarding plane.

   When applied to MPLS, a Segment is a LSP and the 20 right-most bits
   of the SID are encoded as a label.  This implies that, in the MPLS
   instantiation, the SID values are allocated within a reduced 20-bit
   space out of the 32-bit SID space.

   The notion of indexed global segment, defined in
   [I-D.ietf-spring-segment-routing], fits the MPLS architecture
   [RFC3031] as the absolute value allocated to any segment (global or
   local) can be managed by a local allocation process (similarly to
   other MPLS signaling protocols).

   Local segments MAY be allocated from the SR Local Block (SRLB) which
   consists of the range of local labels reserved by the node for local
   segments.  In a controller-driven network, some controllers or
   applications MAY use the control plane to discover the available set



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   of local SIDs on a particular router.  In such cases, the SRLB is
   advertised in the control plane (e.g., using
   [I-D.ietf-isis-segment-routing-extensions]).

   Contrary to RSVP-based explicit routes where tunnel midpoints
   maintain states, SR-based explicit routes only require per-flow
   states at the ingress edge router where the traffic engineer policy
   is applied.

   Contrary to RSVP-based explicit routes which consist in non-ECMP
   circuits (similar to ATM/FR), SR-based explicit routes can be built
   as list of ECMP-aware node segments and hence ECMP-aware traffic
   engineering is natively supported by SR.

   When Segment Routing is instantiated over the MPLS data plane the
   following applies:

   o  A list of segments is represented as a stack of labels.

   o  The active segment is the top label.

   o  The CONTINUE operation (defined in
      [I-D.ietf-spring-segment-routing]) is implemented as an MPLS swap
      operation.  The outgoing label value is computed as follows:

      *  When the same Segment Routing Global Block (SRGB, defined in
         [I-D.ietf-spring-segment-routing] is used throughout the SR
         domain, the outgoing label value is equal to the incoming label
         value.

      *  When different SRGBs are used, the outgoing label value is set
         as: [SRGB(next_hop)+index].  If the index can't be applied to
         the SRGB (i.e.: if the index points outside the SRGB of the
         next-hop or the next-hop has not advertised a valid SRGB), then
         no outgoing label value can be computed and the next-hop MUST
         be considered as not supporting the MPLS operations for that
         particular SID.

      *  The index and the SRGB may be learned through different means.
         Obviously, the SRGB MUST be the one the index is related to.

   o  The NEXT operation (defined in [I-D.ietf-spring-segment-routing])
      is implemented as an MPLS pop operation.  The NEXT operation does
      not require any mapping to an outgoing label hence the SRGB is
      irrelevant for this operation.

   o  The PUSH operation (defined in [I-D.ietf-spring-segment-routing])
      is implemented as an MPLS push of a label stack.



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   o  The Segment Routing Global Block (SRGB) values MUST be greater
      than 15 in order to preserve values 0-15 as defined in [RFC3032].

   o  As described in [I-D.ietf-spring-segment-routing], using the same
      SRGB on all nodes within the SR domain eases operations and
      troubleshooting and is expected to be a deployment guideline.

   In conclusion, there are no changes in the operations of the data-
   plane currently used in MPLS networks.

   Note that the kind of deployment of Segment Routing may affect the
   depth of the MPLS label stack.  As every segment in the list is
   represented by an additional MPLS label, the length of the segment
   list directly correlates to the depth of the label stack.
   Implementing a long path with many explicit hops as a segment list
   may thus yield a deep label stack that would need to be pushed at the
   head of the SR tunnel.

   However, many use cases would need very few segments in the list.
   This is especially true when taking good advantage of the ECMP aware
   routing within each segment.  In fact, most use cases need just one
   additional segment and thus lead to a similar label stack depth as
   e.g.  RSVP-based routing.

   Moreover, the use of the binding segment as specified in
   [I-D.ietf-spring-segment-routing], also allows to substantially
   reduce the length of the segment list and hence the depth of the
   label stack.

   Nodes will often have limits with respect to the label depth
   supported for a PUSH operation.  Two ways can be seen to deal with
   this limitation:

      When Segment Routing tunnels are computed by a centralized
      controller, the controller can consider the Maximum SID depth
      capability of a node as it may be signaled through routing
      protocols extensions.

      When Segment Routing tunnels are not computed by a centralized
      controller but derived from an operator designed policy, the
      operator needs to be aware of the limits of the used nodes and
      take this into account in the design.

3.  IGP Segments Examples

   Assuming the network diagram of Figure 1 and the IP address and IGP
   Segment allocation of Figure 2, the following examples can be
   constructed.



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                                +--------+
                               /          \
                       R1-----R2----------R3-----R8
                              | \        / |
                              |  +--R4--+  |
                              |            |
                              +-----R5-----+

                   Figure 1: IGP Segments - Illustration

       +-----------------------------------------------------------+
       | IP address allocated by the operator:                     |
       |                      192.0.2.1/32 as a loopback of R1     |
       |                      192.0.2.2/32 as a loopback of R2     |
       |                      192.0.2.3/32 as a loopback of R3     |
       |                      192.0.2.4/32 as a loopback of R4     |
       |                      192.0.2.5/32 as a loopback of R5     |
       |                      192.0.2.8/32 as a loopback of R8     |
       |              198.51.100.9/32 as an anycast loopback of R4 |
       |              198.51.100.9/32 as an anycast loopback of R5 |
       |                                                           |
       | SRGB defined by the operator as 1000-5000                 |
       |                                                           |
       | Global IGP SID allocated by the operator:                 |
       |                      1001 allocated to 192.0.2.1/32       |
       |                      1002 allocated to 192.0.2.2/32       |
       |                      1003 allocated to 192.0.2.3/32       |
       |                      1004 allocated to 192.0.2.4/32       |
       |                      1008 allocated to 192.0.2.8/32       |
       |                      2009 allocated to 198.51.100.9/32    |
       |                                                           |
       | Local IGP SID allocated dynamically by R2                 |
       |                     for its "north" adjacency to R3: 9001 |
       |                     for its "north" adjacency to R3: 9003 |
       |                     for its "south" adjacency to R3: 9002 |
       |                     for its "south" adjacency to R3: 9003 |
       +-----------------------------------------------------------+

        Figure 2: IGP Address and Segment Allocation - Illustration

3.1.  Example 1

   R1 may send a packet P1 to R8 simply by pushing an SR header with
   segment list {1008}.

   1008 is a global IGP segment attached to the IP prefix 192.0.2.8/32.
   Its semantic is global within the IGP domain: any router forwards a




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   packet received with active segment 1008 to the next-hop along the
   ECMP-aware shortest-path to the related prefix.

   In conclusion, the path followed by P1 is R1-R2--R3-R8.  The ECMP-
   awareness ensures that the traffic be load-shared between any ECMP
   path, in this case the two north and south links between R2 and R3.

3.2.  Example 2

   R1 may send a packet P2 to R8 by pushing an SR header with segment
   list {1002, 9001, 1008}.

   1002 is a global IGP segment attached to the IP prefix 192.0.2.2/32.
   Its semantic is global within the IGP domain: any router forwards a
   packet received with active segment 1002 to the next-hop along the
   shortest-path to the related prefix.

   9001 is a local IGP segment attached by node R2 to its north link to
   R3.  Its semantic is local to node R2: R2 switches a packet received
   with active segment 9001 towards the north link to R3.

   In conclusion, the path followed by P2 is R1-R2-north-link-R3-R8.

3.3.  Example 3

   R1 may send a packet P3 along the same exact path as P1 using a
   different segment list {1002, 9003, 1008}.

   9003 is a local IGP segment attached by node R2 to both its north and
   south links to R3.  Its semantic is local to node R2: R2 switches a
   packet received with active segment 9003 towards either the north or
   south links to R3 (e.g. per-flow loadbalancing decision).

   In conclusion, the path followed by P3 is R1-R2-any-link-R3-R8.

3.4.  Example 4

   R1 may send a packet P4 to R8 while avoiding the links between R2 and
   R3 by pushing an SR header with segment list {1004, 1008}.

   1004 is a global IGP segment attached to the IP prefix 192.0.2.4/32.
   Its semantic is global within the IGP domain: any router forwards a
   packet received with active segment 1004 to the next-hop along the
   shortest-path to the related prefix.

   In conclusion, the path followed by P4 is R1-R2-R4-R3-R8.





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3.5.  Example 5

   R1 may send a packet P5 to R8 while avoiding the links between R2 and
   R3 while still beneficing from all the remaining shortest paths (via
   R4 and R5) by pushing an SR header with segment list {2009, 1008}.

   2009 is a global IGP segment attached to the anycast IP prefix
   198.51.100.9/32.  Its semantic is global within the IGP domain: any
   router forwards a packet received with active segment 2009 to the
   next-hop along the shortest-path to the related prefix.

   In conclusion, the path followed by P5 is either R1-R2-R4-R3-R8 or
   R1-R2-R5-R3-R8 .

4.  IANA Considerations

   This document does not make any request to IANA.

5.  Manageability Considerations

   This document describes the applicability of Segment Routing over the
   MPLS data plane.  Segment Routing does not introduce any change in
   the MPLS data plane.  Manageability considerations described in
   [I-D.ietf-spring-segment-routing] applies to the MPLS data plane when
   used with Segment Routing.

6.  Security Considerations

   This document does not introduce additional security requirements and
   mechanisms other than the ones described in
   [I-D.ietf-spring-segment-routing].

7.  Contributors

   The following contributors have substantially helped the definition
   and editing of the content of this document:

   Martin Horneffer
   Deutsche Telekom
   Email: Martin.Horneffer@telekom.de

   Wim Henderickx
   Nokia
   Email: wim.henderickx@nokia.com

   Jeff Tantsura
   Email: jefftant@gmail.com




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   Edward Crabbe
   Email: edward.crabbe@gmail.com

   Igor Milojevic
   Email: milojevicigor@gmail.com

   Saku Ytti
   Email: saku@ytti.fi

8.  Acknowledgements

   The authors would like to thank Les Ginsberg and Shah Himanshu for
   their comments on this document.

9.  References

9.1.  Normative References

   [I-D.ietf-spring-segment-routing]
              Filsfils, C., Previdi, S., Decraene, B., Litkowski, S.,
              and R. Shakir, "Segment Routing Architecture", draft-ietf-
              spring-segment-routing-12 (work in progress), June 2017.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,
              <http://www.rfc-editor.org/info/rfc3031>.

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
              <http://www.rfc-editor.org/info/rfc3032>.

9.2.  Informative References

   [I-D.ietf-isis-segment-routing-extensions]
              Previdi, S., Filsfils, C., Bashandy, A., Gredler, H.,
              Litkowski, S., Decraene, B., and j. jefftant@gmail.com,
              "IS-IS Extensions for Segment Routing", draft-ietf-isis-
              segment-routing-extensions-13 (work in progress), June
              2017.





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   [I-D.ietf-ospf-ospfv3-segment-routing-extensions]
              Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
              Shakir, R., Henderickx, W., and J. Tantsura, "OSPFv3
              Extensions for Segment Routing", draft-ietf-ospf-ospfv3-
              segment-routing-extensions-09 (work in progress), March
              2017.

   [I-D.ietf-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-ietf-ospf-segment-
              routing-extensions-16 (work in progress), May 2017.

   [I-D.ietf-spring-segment-routing-ldp-interop]
              Filsfils, C., Previdi, S., Bashandy, A., Decraene, B., and
              S. Litkowski, "Segment Routing interworking with LDP",
              draft-ietf-spring-segment-routing-ldp-interop-08 (work in
              progress), June 2017.

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

Authors' Addresses

   Clarence Filsfils (editor)
   Cisco Systems, Inc.
   Brussels
   BE

   Email: cfilsfil@cisco.com


   Stefano Previdi (editor)
   Cisco Systems, Inc.
   Italy

   Email: stefano@previdi.net











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   Ahmed Bashandy
   Cisco Systems, Inc.
   170, West Tasman Drive
   San Jose, CA  95134
   US

   Email: bashandy@cisco.com


   Bruno Decraene
   Orange
   FR

   Email: bruno.decraene@orange.com


   Stephane Litkowski
   Orange
   FR

   Email: stephane.litkowski@orange.com


   Rob Shakir
   Google

   Email: robjs@google.com
























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