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Versions: (draft-previdi-spring-problem-statement) 00 01 02 03 04 05 06 07 08 RFC 7855

Network Working Group                                    S. Previdi, Ed.
Internet-Draft                                          C. Filsfils, Ed.
Intended status: Informational                       Cisco Systems, Inc.
Expires: October 8, 2016                                     B. Decraene
                                                            S. Litkowski
                                                                  Orange
                                                            M. Horneffer
                                                        Deutsche Telekom
                                                               R. Shakir
                                                     Jive Communications
                                                           April 6, 2016


               SPRING Problem Statement and Requirements
                 draft-ietf-spring-problem-statement-08

Abstract

   The ability for a node to specify a forwarding path, other than the
   normal shortest path, that a particular packet will traverse,
   benefits a number of network functions.  Source-based routing
   mechanisms have previously been specified for network protocols, but
   have not seen widespread adoption.  In this context, the term
   'source' means 'the point at which the explicit route is imposed' and
   therefore it is not limited to the originator of the packet (i.e.:
   the node imposing the explicit route may be the ingress node of an
   operator's network).

   This document outlines various use cases, with their requirements,
   that need to be taken into account by the Source Packet Routing in
   Networking (SPRING) architecture for unicast traffic.  Multicast use-
   cases and requirements are out of scope of this document.

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
   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 October 8, 2016.

Copyright Notice

   Copyright (c) 2016 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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Dataplanes  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  SPRING Use Cases  . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  IGP-based MPLS Tunneling  . . . . . . . . . . . . . . . .   4
       3.1.1.  Example of IGP-based MPLS Tunnels . . . . . . . . . .   4
     3.2.  Fast Reroute (FRR)  . . . . . . . . . . . . . . . . . . .   5
     3.3.  Traffic Engineering . . . . . . . . . . . . . . . . . . .   5
       3.3.1.  Examples of Traffic Engineering Use Cases . . . . . .   7
     3.4.  Interoperability with non-SPRING nodes  . . . . . . . . .  13
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   6.  Manageability Considerations  . . . . . . . . . . . . . . . .  15
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  15
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   The ability for a node to specify a unicast forwarding path, other
   than the normal shortest path, that a particular packet will
   traverse, benefits a number of network functions, for example:



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      Some types of network virtualization, including multi-topology
      networks and the partitioning of network resources for VPNs

      Network, link, path and node protection such as fast re-route

      Network programmability

      OAM techniques

      Simplification and reduction of network signaling components

      Load balancing and traffic engineering

   Source-based routing mechanisms have previously been specified for
   network protocols, but have not seen widespread adoption other than
   in MPLS traffic engineering.

   These network functions may require greater flexibility and per
   packet source imposed routing than can be achieved through the use of
   the previously defined methods.  In the context of this document, the
   term 'source' means 'the point at which the explicit route is
   imposed' and therefore it is not limited to the originator of the
   packet (i.e.: the node imposing the explicit route may be the ingress
   node of an operator's network).  Throughout this document we refer to
   this definition of 'source'.

   In this context, Source Packet Routing in Networking (SPRING)
   architecture is being defined in order to address the use cases and
   requirements described in this document.

   The SPRING architecture MUST allow incremental and selective
   deployment without any requirement of flag day or massive upgrade of
   all network elements.

   The SPRING architecture MUST allow to put policy state in the packet
   header and not in the intermediate nodes along the path.  Hence, the
   policy is instantiated in the packet header and does not requires any
   policy state in midpoints and tail-ends.

   The SPRING architecture objective is not to replace existing source
   routing and traffic engineering mechanisms but rather complement them
   and address use cases where removal of signaling and path state in
   the core is a requirement.

   Multicast use-cases and requirements are out of scope of this
   document.





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2.  Dataplanes

   The SPRING architecture SHOULD be general in order to ease its
   applicability to different dataplanes.

   The SPRING architecture SHOULD leverage the existing MPLS dataplane
   without any modification and leverage IPv6 dataplane with a new IPv6
   Routing Header Type (IPv6 Routing Header is defined in [RFC2460]) and
   a proposal for a new type of routing header is made by
   [I-D.ietf-6man-segment-routing-header].

   The SPRING architecture MUST allow interoperability between SPRING
   capable and non-capable nodes and this in both MPLS and IPv6
   dataplanes.

3.  SPRING Use Cases

3.1.  IGP-based MPLS Tunneling

   The source-based routing model, applied to the MPLS dataplane, offers
   the ability to tunnel services like VPN ([RFC4364]), VPLS ([RFC4761],
   [RFC4762]) and VPWS ([RFC6624]), from an ingress PE to an egress PE,
   with or without the expression of an explicit path and without
   requiring forwarding plane or control plane state in intermediate
   nodes.  Point-to-multipoint and multipoint-to-multipoint tunnels are
   outside of the scope of this document.

3.1.1.  Example of IGP-based MPLS Tunnels

   This section illustrates an example use-case.

                                  P1---P2
                                 /       \
                    A---CE1---PE1         PE2---CE2---Z
                                 \       /
                                  P3---P4

                    Figure 1: IGP-based MPLS Tunneling

   In Figure 1 above, the four nodes A, CE1, CE2 and Z are part of the
   same VPN.  CE2 advertises to PE2 a route to Z.  PE2 binds a local
   label LZ to that route and propagates the route and its label via
   MPBGP to PE1 with nhop 192.0.2.2 (i.e.: the local address of PE2).
   PE1 installs the VPN prefix Z in the appropriate VRF and resolves the
   next-hop onto the IGP-based MPLS tunnel to PE2.

   In order to cope with the reality of current deployments, the SPRING
   architecture MUST allow PE to PE forwarding according to the IGP



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   shortest path without the addition of any other signaling protocol.
   The packet each PE forwards across the network will contain the
   necessary information derived from the topology database in order to
   deliver the packet to the remote PE.

3.2.  Fast Reroute (FRR)

   FRR (Fast Reroute) technologies have been deployed by network
   operators in order to cope with link or node failures through pre-
   computation of backup paths.

   Illustration of the problem statement for FRR and microloop avoidance
   are to be found in [I-D.ietf-spring-resiliency-use-cases].

   The SPRING architecture MUST address the following requirements:

   o  support of Fast Reroute (FRR) on any topology

   o  pre-computation and setup of backup path without any additional
      signaling (other than the regular IGP/BGP protocols)

   o  support of shared risk constraints

   o  support of node and link protection

   o  support of microloop avoidance

3.3.  Traffic Engineering

   Traffic Engineering (TE) is the term used to refer to techniques that
   enable operators to control how specific traffic flows are treated
   within their networks.  Different contexts and modes have been
   defined (single vs. multiple domains, with or without bandwidth
   admission control, centralized vs. distributed path computation,
   etc).

   Some deployments have a limited use of TE such as addressing specific
   application or customer requirements or address specific bandwidth
   limitation in the network (tactical TE).  In this situation, there is
   need to reduce as much of possible the cost (such as the number of
   signaling protocols and the number of nodes requiring specific
   configurations/features.  Some other deployments have a very high
   scale use of TE, such as fine tuning flows at the application level.
   In this situation, there is a need for a very high scalability, in
   particular on mid-points.

   The source-based routing model allows traffic engineering to be
   implemented without the need of a signaling component.



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   The SPRING architecture MUST support the following traffic
   engineering requirements:

   o  loose or strict options

   o  bandwidth admission control

   o  distributed vs. centralized model (PCE
      [I-D.ietf-pce-stateful-pce], SDN Controller)

   o  disjointness in dual-plane networks

   o  egress peering traffic engineering

   o  load-balancing among non-parallel links (i.e.: links connected to
      different adjacent neighbors).

   o  Limiting (scalable, preferably zero) per-service state and
      signaling on midpoint and tail-end routers.

   o  ECMP-awareness

   o  node resiliency property (i.e.: the traffic-engineering policy is
      not anchored to a specific core node whose failure could impact
      the service.

   In most cases, Traffic Engineering makes use of the "loose" route
   option where most of the explicit paths can be expressed through a
   small number of hops.  However, there are use cases where the
   "strict" option may be used and, in such case, each individual hop in
   the explicit path is specified.  This may incur into a long list of
   hops that is instantiated into a MPLS label stack (in the MPLS
   dataplane) or list of IPv6 addresses (in the IPv6 dataplane).

   It is obvious that in case of long strict source routing paths, the
   deployment is possible if the head-end of the explicit path supports
   the instantiation of long explicit paths.

   Alternatively, a controller could decompose the end-to-end path into
   a set of sub-paths such as each of these sub-paths is supported by
   its respective head-end and advertised with a single identifier.
   Hence, the concatenation (or stitching) of the sub-paths identifiers
   gives a compression scheme allowing an end-to-end path to be
   expressed in a smaller number of hops.







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3.3.1.  Examples of Traffic Engineering Use Cases

   Here follows the description of two sets of use cases:

   o  Traffic Engineering without Admission Control

   o  Traffic Engineering with Admission Control

3.3.1.1.  Traffic Engineering without Bandwidth Admission Control

   In this section, we describe Traffic Engineering use-cases without
   bandwidth admission control.

3.3.1.1.1.  Disjointness in dual-plane networks

   Many networks are built according to the dual-plane design, as
   illustrated in Figure 2:

      Each aggregation region k is connected to the core by
      two C routers C1k and C2k where k refers to the region.

      C1k is part of plane 1 and aggregation region k

      C2k is part of plane 2 and aggregation region k

      C1k has a link to C2j iff k = j.

         The core nodes of a given region are directly connected.
         Inter-region links only connect core nodes of the same plane.

      {C1k has a link to C1j} iff {C2k has a link to C2j}.

         The distribution of these links depends on the topological
         properties of the core of the AS. The design rule presented
         above specifies that these links appear in both core planes.

   We assume a common design rule found in such deployments: the inter-
   plane link costs (Cik-Cjk where i != j) are set such that the route
   to an edge destination from a given plane stays within the plane
   unless the plane is partitioned.











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                             Edge Router A
                                 /  \
                                /    \
                               /      \  Agg Region A
                              /        \
                             /          \
                            C1A----------C2A
                            | \         | \
                            |  \        |  \
                            |   C1B----------C2B
                  Plane1    |    |      |    |     Plane2
                            |    |      |    |
                            C1C--|-----C2C   |
                              \  |        \  |
                               \ |         \ |
                               C1Z----------C2Z
                                  \        /
                                   \      /  Agg Region Z
                                    \    /
                                     \  /
                                 Edge Router Z

               Figure 2: Dual-Plane Network and Disjointness

   In this scenario, the operator requires the ability to deploy
   different strategies.  For example, Edge Router A should be able to
   use the three following options:

   o  the traffic is load-balanced across any ECMP path through the
      network

   o  the traffic is load-balanced across any ECMP path within the
      Plane1 of the network

   o  the traffic is load-balanced across any ECMP path within the
      Plane2 of the network

   Most of the data traffic from A to Z would use the first option, such
   as to exploit the capacity efficiently.  The operator would use the
   two other choices for specific premium traffic that has requested
   disjoint transport.

   The SPRING architecture MUST support this use case with the following
   requirements:

   o  Zero per-service state and signaling on midpoint and tail-end
      routers.




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   o  ECMP-awareness.

   o  Node resiliency property: the traffic-engineering policy is not
      anchored to a specific core node whose failure could impact the
      service.

3.3.1.1.2.  Egress Peering Traffic Engineering

                                     +------+
                                     |      |
                                 +---D      F
                    +---------+ /    | AS 2 |\ +------+
                    |         |/     +------+ \|   Z  |
                    A         C                |      |
                    |         |\     +------+ /| AS 4 |
                    B   AS1   | \    |      |/ +------+
                    |         |  +---E      G
                    +---------+      | AS 3 |
                                     +------+\

               Figure 3: Egress peering traffic engineering

   Let us assume, in the network depicted in Figure 3, that:

      C in AS1 learns about destination Z of AS 4 via two BGP paths
      (AS2, AS4) and (AS3, AS4).

      C may or may not be configured so to enforce next-hop-self
      behavior before propagating the paths within AS1.

      C may propagate all the paths to Z within AS1 (BGP add-paths,
      [I-D.ietf-idr-add-paths]).

      C may install in its FIB only the route via AS2, or only the route
      via AS3, or both.

   In that context, the SPRING architecture MUST allow the operator of
   AS1 to apply a traffic-engineering policy such as the following one,
   regardless the configured behavior of next-hop-self:

      Steer 60% of the Z-destined traffic received at A via AS2 and 40%
      via AS3.

      Steer 80% of the Z-destined traffic received at B via AS2 and 20%
      via AS3.

   The SPRING architecture MUST allow an ingress node (i.e., an explicit
   route source node) to select the exit point of a packet as any



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   combination of an egress node, an egress interface, a peering
   neighbor, and a peering AS.

   The use cases and requirements for Egress Peer Engineering are
   described in [I-D.ietf-spring-segment-routing-central-epe].

3.3.1.1.3.  Load-balancing among non-parallel links

   The SPRING architecture MUST allow a given node to load share traffic
   across multiple non parallel links (i.e.: links connected to
   different adjacent routers) even if these lead to different
   neighbors.  This may be useful to support traffic engineering
   policies.

                                 +---C---D---+
                                 |           |
                       PE1---A---B-----F-----E---PE2

               Figure 4: Multiple (non-parallel) Adjacencies

   In the above example, the operator requires PE1 to load-balance its
   PE2-destined traffic between the ABCDE and ABFE equal-cost paths in a
   controlled way where the operator MUST be allowed to distribute
   traffic unevenly between paths (Weighted Equal Cost Multiplath,
   WECMP).

3.3.1.2.  Traffic Engineering with Bandwidth Admission Control

   The implementation of bandwidth admission control within a network
   (and its possible routing consequence which consists in routing along
   explicit paths where the bandwidth is available) requires a capacity
   planning process.

   The spreading of load among ECMP paths is a key attribute of the
   capacity planning processes applied to packet-based networks.

3.3.1.2.1.  Capacity Planning Process

   Capacity Planning anticipates the routing of the traffic matrix onto
   the network topology, for a set of expected traffic and topology
   variations.  The heart of the process consists in simulating the
   placement of the traffic along ECMP-aware shortest-paths and
   accounting for the resulting bandwidth usage.

   The bandwidth accounting of a demand along its shortest-path is a
   basic capability of any planning tool or PCE server.





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   For example, in the network topology described below, and assuming a
   default IGP metric of 1 and IGP metric of 2 for link GF, a 1600Mbps
   A-to-Z flow is accounted as consuming 1600Mbps on links AB and FZ,
   800Mbps on links BC, BG and GF, and 400Mbps on links CD, DF, CE and
   EF.

                                   C-----D
                                 /  \     \
                            A---B    +--E--F--Z
                                 \        /
                                  G------+

             Figure 5: Capacity Planning an ECMP-based demand

   ECMP is extremely frequent in SP, Enterprise and DC architectures and
   it is not rare to see as much as 128 different ECMP paths between a
   source and a destination within a single network domain.  It is a key
   efficiency objective to spread the traffic among as many ECMP paths
   as possible.

   This is illustrated in the below network diagram which consists of a
   subset of a network where already 5 ECMP paths are observed from A to
   M.

                                   C
                                  / \
                                 B-D-L--
                                / \ /   \
                               A   E     \
                                \         M
                                 \   G   /
                                  \ / \ /
                                   F   K
                                    \ /
                                     I

                      Figure 6: ECMP Topology Example

   When the capacity planning process detects that a traffic growth
   scenario and topology variation would lead to congestion, a capacity
   increase is triggered and if it cannot be deployed in due time, a
   traffic engineering solution is activated within the network.

   A basic traffic engineering objective consists of finding the
   smallest set of demands that need to be routed off their shortest
   path to eliminate the congestion, then to compute an explicit path
   for each of them and instantiating these traffic-engineered policies
   in the network.



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   The SPRING architecture MUST offer a simple support for ECMP-based
   shortest path placement as well as for explicit path policy without
   incurring additional signaling in the domain.  This includes:

   o  the ability to steer a packet across a set of ECMP paths

   o  the ability to diverge from a set of ECMP shortest paths to one or
      more paths not in the set of shortest paths

3.3.1.2.2.  SDN Use Case

   The SDN use-case lies in the SDN controller, (e.g.: Stateful PCE as
   described in [I-D.ietf-pce-stateful-pce].

   The SDN controller is responsible to control the evolution of the
   traffic matrix and topology.  It accepts or denies the addition of
   new traffic into the network.  It decides how to route the accepted
   traffic.  It monitors the topology and upon topological change,
   determines the minimum traffic that should be rerouted on an
   alternate path to alleviate a bandwidth congestion issue.

   The algorithms supporting this behavior are a local matter of the SDN
   controller and are outside the scope of this document.

   The means of collecting traffic and topology information are the same
   as what would be used with other SDN-based traffic-engineering
   solutions.

   The means of instantiating policy information at a traffic-
   engineering head-end are the same as what would be used with other
   SDN-based traffic-engineering solutions.

   In the context of Centralized-Based Optimization and the SDN use-
   case, here are the functionalities that the SPRING architecture MUST
   deliver:

      Explicit routing capability with or without ECMP-awareness.

      No signaling hop-by-hop through the network.

      Policy state is only maintained at the policy head-end.  No policy
      state is maintained at mid-points and tail-ends.

      Automated guaranteed FRR for any topology.

      The policy state is in the packet header and not in the
      intermediate nodes along the path.  The policy is absent from
      midpoints and tail-ends.



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      Highly responsive to change: the SDN Controller only needs to
      apply a policy change at the head-end.  No delay is introduced due
      to programming the midpoints and tail-end along the path.

3.4.  Interoperability with non-SPRING nodes

   SPRING nodes MUST inter-operate with non-SPRING nodes and in both
   MPLS and IPv6 dataplanes in order to allow a gradual deployment of
   SPRING on existing MPLS and IPv6 networks.

4.  Security Considerations

   SPRING reuses the concept of source routing by encoding the path in
   the packet.  As with other similar source routing architecture, an
   attacker may manipulate traffic path by modifying the packet header.
   By manipulating traffic path, an attacker may be able to cause
   outages on any part of the network.

   SPRING adds some meta-data on the packet, with the list of forwarding
   path elements that the packet must traverse.  Depending on the data
   plane, this list may shrink as the packet traverse the network, by
   only keeping the next elements and forgetting the past ones.

   SPRING architecture MUST provide clear trust domain boundaries, so
   that source routing information is only usable within the trusted
   domain and never exposed to the outside world.

   From a network protection standpoint, there is an assumed trust model
   such that any node imposing an explicit route on a packet is assumed
   to be allowed to do so.  This is a significant change compared to
   plain IP offering shortest path routing but not fundamentally
   different compared to existing techniques providing explicit routing
   capability.  It is expected that, by default, the explicit routing
   information is not leaked through the boundaries of the administered
   domain.

   Therefore, the dataplane MUST NOT expose any source routing
   information when a packet leaves the trusted domain.  Special care
   will be required for the existing dataplanes like MPLS, especially
   for the inter-provider scenario where a third-party provider may push
   MPLS labels corresponding to a SPRING header anywhere in the stack.
   The architecture document MUST analyze the exact security
   considerations of such scenario.

   Filtering routing information is typically performed in the control
   plane, but an additional filtering in the forwarding plane is also
   required.  In SPRING, as there is no control plane (related to source
   routed paths) between the source and the mid points, filtering in the



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   control plane is not possible (or not required, depending on the
   point of view).  Filtering MUST be performed on the forwarding plane
   on the boundaries and MAY require looking at multiple labels/
   instruction.

   For the MPLS data plane, this not a new requirement as the existing
   MPLS architecture already allow such source routing by stacking
   multiple labels.  And for security protection, RFC4381 section 2.4
   and RFC 5920 section 8.2 already calls for the filtering of MPLS
   packets on trust boundaries.

   If all MPLS labels are filtered at domain boundaries, then SPRING
   does not introduce any change.  If only a subset of labels are
   filtered, then SPRING introduces a change since the border router is
   expected to determine which information (e.g.: labels) are filtered
   while the border router is not the originator of these label
   advertisements.

   As the SPRING architecture must be based on clear trust domain,
   mechanisms allowing the authentication and validation of the source
   routing information must be evaluated by the SPRING architecture in
   order to prevent any form of attack or unwanted source routing
   information manipulation.

   Dataplane security considerations MUST be addressed in each SPRING
   dataplane related document (i.e.: MPLS and IPv6).

   The IPv6 data plane proposes the use of a cryptographic signature of
   the source routed path which would ease this configuration.  This is
   indeed more needed for the IPv6 data plane which is end to end in
   nature, compared to the MPLS data plane which is typically restricted
   to a controlled and trusted zone.

   In the forwarding plane, data plane extension documents MUST address
   the security implications of the required change.

   In term of privacy, SPRING does not propose change in term of
   encryption.  Each dataplane, may or may not provide existing or
   future encryption capability.

   In order to build the source routing information in the packet, a
   node in SPRING architecture will require learning information from a
   control layer.  As this control layer will be in charge of
   programming forwarding instructions, an attacker taking over this
   component may also manipulate the traffic path.  Any control protocol
   used in the SPRING architecture SHOULD provide security mechanisms or
   design to protect against such control layer attacker.  Control plane




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   security considerations MUST be addressed in each SPRING control
   plane related document.

5.  IANA Considerations

   This document does not request any IANA allocations.

6.  Manageability Considerations

   The SPRING WG MUST define Operations and Management (OAM) procedures
   applicable to SPRING enabled networks.

   In SPRING networks, the path the packet takes is encoded in the
   header.  SPRING architecture MUST include the necessary OAM
   mechanisms in order for the network operator to validate the
   effectiveness of a path as well as to check and monitor its liveness
   and performance.  Moreover, in SPRING architecture, a path may be
   defined in the forwarding layer (in both MPLS and IPv6 dataplanes) or
   as a service path (formed by a set of service instances).  The
   network operator MUST be capable to monitor, control and manage paths
   (network and service based) using OAM procedures.

   OAM use cases and requirements are detailed in
   [I-D.ietf-spring-oam-usecase] and
   [I-D.ietf-spring-sr-oam-requirement].

7.  Contributors

   The following individuals substantially contributed to the content of
   this documents:

   Ruediger Geib
   Deutsche Telekom
   Heinrich Hertz Str. 3-7
   Darmstadt  64295
   DE
   Email: Ruediger.Geib@telekom.de

   Robert Raszuk
   Mirantis Inc.
   615 National Ave.
   94043 Mt View, CA
   US
   Email: robert@raszuk.net







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8.  Acknowledgements

   The authors would like to thank Yakov Rekhter for his contribution to
   this document.

9.  References

9.1.  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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <http://www.rfc-editor.org/info/rfc2460>.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <http://www.rfc-editor.org/info/rfc4364>.

   [RFC4761]  Kompella, K., Ed. and Y. Rekhter, Ed., "Virtual Private
              LAN Service (VPLS) Using BGP for Auto-Discovery and
              Signaling", RFC 4761, DOI 10.17487/RFC4761, January 2007,
              <http://www.rfc-editor.org/info/rfc4761>.

   [RFC4762]  Lasserre, M., Ed. and V. Kompella, Ed., "Virtual Private
              LAN Service (VPLS) Using Label Distribution Protocol (LDP)
              Signaling", RFC 4762, DOI 10.17487/RFC4762, January 2007,
              <http://www.rfc-editor.org/info/rfc4762>.

   [RFC6624]  Kompella, K., Kothari, B., and R. Cherukuri, "Layer 2
              Virtual Private Networks Using BGP for Auto-Discovery and
              Signaling", RFC 6624, DOI 10.17487/RFC6624, May 2012,
              <http://www.rfc-editor.org/info/rfc6624>.

9.2.  Informative References

   [I-D.ietf-6man-segment-routing-header]
              Previdi, S., Filsfils, C., Field, B., Leung, I., Linkova,
              J., Kosugi, T., Vyncke, E., and D. Lebrun, "IPv6 Segment
              Routing Header (SRH)", draft-ietf-6man-segment-routing-
              header-01 (work in progress), March 2016.







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   [I-D.ietf-idr-add-paths]
              Walton, D., Retana, A., Chen, E., and J. Scudder,
              "Advertisement of Multiple Paths in BGP", draft-ietf-idr-
              add-paths-13 (work in progress), December 2015.

   [I-D.ietf-pce-stateful-pce]
              Crabbe, E., Minei, I., Medved, J., and R. Varga, "PCEP
              Extensions for Stateful PCE", draft-ietf-pce-stateful-
              pce-14 (work in progress), March 2016.

   [I-D.ietf-spring-oam-usecase]
              Geib, R., Filsfils, C., Pignataro, C., and N. Kumar, "Use
              Case for a Scalable and Topology-Aware Segment Routing
              MPLS Data Plane Monitoring System", draft-ietf-spring-oam-
              usecase-01 (work in progress), October 2015.

   [I-D.ietf-spring-resiliency-use-cases]
              Francois, P., Filsfils, C., Decraene, B., and R. Shakir,
              "Use-cases for Resiliency in SPRING", draft-ietf-spring-
              resiliency-use-cases-03 (work in progress), April 2016.

   [I-D.ietf-spring-segment-routing-central-epe]
              Filsfils, C., Previdi, S., Ginsburg, D., and D. Afanasiev,
              "Segment Routing Centralized BGP Peer Engineering", draft-
              ietf-spring-segment-routing-central-epe-01 (work in
              progress), March 2016.

   [I-D.ietf-spring-sr-oam-requirement]
              Kumar, N., Pignataro, C., Akiya, N., Geib, R., Mirsky, G.,
              and S. Litkowski, "OAM Requirements for Segment Routing
              Network", draft-ietf-spring-sr-oam-requirement-01 (work in
              progress), December 2015.

Authors' Addresses

   Stefano Previdi (editor)
   Cisco Systems, Inc.
   Via Del Serafico, 200
   Rome  00142
   Italy

   Email: sprevidi@cisco.com









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

   Email: cfilsfil@cisco.com


   Bruno Decraene
   Orange
   FR

   Email: bruno.decraene@orange.com


   Stephane Litkowski
   Orange
   FR

   Email: stephane.litkowski@orange.com


   Martin Horneffer
   Deutsche Telekom
   Hammer Str. 216-226
   Muenster  48153
   DE

   Email: Martin.Horneffer@telekom.de


   Rob Shakir
   Jive Communications, Inc.
   1275 West 1600 North, Suite 100
   Orem, UT  84057

   Email: rjs@rob.sh














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