Network Working Group                                    S. Previdi, Ed.
Internet-Draft                                          C. Filsfils, Ed.
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: September 29, October 6, 2014                                     B. Decraene
                                                            S. Litkowski
                                                            M. Horneffer
                                                                 R. Geib
                                                        Deutsche Telekom
                                                               R. Shakir
                                                         British Telecom
                                                               R. Raszuk
                                                          March 28,
                                                           April 4, 2014

               SPRING Problem Statement and Requirements


   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'.

   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. 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",
   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.

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   This Internet-Draft will expire on September 29, October 6, 2014.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Dataplanes . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  IGP-based MPLS Tunneling . . . . . . . . . . . . . . . . . . .  5
     3.1.  Example of IGP-based MPLS Tunnels  . . . . . . . . . . . .  5
   4.  Fast Reroute . . . . . . . . . . . . . . . . . . . . . . . . .  5
   5.  Traffic Engineering  . . . . . . . . . . . . . . . . . . . . .  6
     5.1.  Examples of Traffic Engineering Use Cases  . . . . . . . .  7
       5.1.1.  Traffic Engineering without Bandwidth Admission
               Control  . . . . . . . . . . . . . . . . . . . . . . .  7
       5.1.2.  Traffic Engineering with Bandwidth Admission
               Control  . . . . . . . . . . . . . . . . . . . . . . . 11
   6.  Interoperability with non-SPRING nodes . . . . . . . . . . . . 14
   7.  OAM  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   8.  Security . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   10. Manageability Considerations . . . . . . . . . . . . . . . . . 15
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 16
     13.2. Informative References . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18

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:

      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

   The term 'source' means 'the point at which the explicit route is

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

   SPRING architecture should allow incremental and selective deployment
   without any requirement of flag day or massive upgrade of all network

   SPRING architecture should allow optimal virtualization: put policy
   state in the packet header and not in the intermediate nodes along
   the path.  Hence, the policy is completely virtualized away from
   midpoints and tail-ends.

2.  Dataplanes

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

   MPLS dataplane doesn't require any modification in order to apply a
   source-based routed model (e.g.:

   IPv6 specification [RFC2460] [RFC2460], amended by [RFC6564] and [RFC7045],
   defines the Routing Extension Header which provides IPv6 source-based
   routing capabilities.

   The SPRING architecture should leverage existing MPLS dataplane
   without any modification and leverage IPv6 dataplane with minor

3.  IGP-based MPLS Tunneling

   The source-based routing model, applied to the MPLS dataplane, offers
   the ability to tunnel services (VPN, VPLS, VPWS) from an ingress PE
   to an egress PE, without any other protocol than IGPs (ISIS with or OSPF).
   LDP without the expression of an explicit path
   and RSVP-TE signaling protocols are not required. without requiring forwarding plane or control plane state in
   intermediate nodes.

3.1.  Example of IGP-based MPLS Tunnels

   This section illustrates an example use-case taken from

                                 /       \
                    A---CE1---PE1         PE2---CE2---Z
                                 \       /

                    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  PE1 installs the VPN prefix Z in
   the appropriate VRF and resolves the next-hop onto the node segment
   associated with PE2.  Upon receiving a packet from A destined to Z,
   PE1 pushes two labels onto the packet: the top label is the Prefix
   SID attached to, the bottom label is the VPN label LZ
   attached to the VPN route Z.

   In order to cope with the reality of current deployments, the SPRING
   architecture should allow PE to PE forwarding according to the IGP
   shortest path without the addition of any other signaling protocol.
   The packet each PE forwards across the network will contain (within
   their label stack) the necessary information derived from the
   topology database in order to deliver the packet to the remote PE.

4.  Fast Reroute

   FRR technologies have been deployed by network operators in order to
   cope with link or node failures through pre-computation of backup

   The SPRING architecture should address following requirements:

   o  support of 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

   Further illustrations of the problem statement for FRR are to be
   found in [I-D.francois-spring-resiliency-use-case].

5.  Traffic Engineering

   Traffic Engineering has been addressed using IGP protocol extensions
   (for resources information propagation) and RSVP-TE for signaling
   explicit paths.  Different contexts and modes have been defined
   (single vs. multiple domains, with or without bandwidth admission
   control, centralized vs. distributed path computation, etc).

   In all cases, one of the major components of the TE architecture is
   the soft state based signaling protocol (RSVP-TE) which is used in
   order to signal and establish the explicit path.  Each path, once
   computed, need to be signaled and state for each path must be present
   in each node traversed by the path.  This incurs a scalability
   problem especially in the context of SDN where traffic
   differentiation may be done at a finer granularity (e.g.: application
   specific).  Also the amount of state needed to be maintained and
   periodically refreshed in all involved nodes contributes
   significantly to complexity and the number of failures cases, and
   thus increases operational effort while decreasing overall network

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

   The SPRING architecture should support traffic engineering,

   o  loose or strict options

   o  bandwidth admission control

   o  distributed vs. centralized model (PCE, SDN Controller)
   o  disjointness in dual-plane networks

   o  egress peering traffic engineering

   o  load-balancing among non-parallel links

   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.

5.1.  Examples of Traffic Engineering Use Cases

   As documented in [I-D.filsfils-rtgwg-segment-routing-use-cases] [I-D.filsfils-spring-segment-routing-use-cases] here
   follows the description of two sets of use cases:

   o  Traffic Engineering without Admission Control

   o  Traffic Engineering with Admission Control

5.1.1.  Traffic Engineering without Bandwidth Admission Control

   In this section, we describe Traffic Engineering use-cases without
   bandwidth admission control.  Disjointness in dual-plane networks

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

      Each access region k is connected to the core by two C routers
      (C(1,k) and C(2,k)).

      C(1,k) is part of plane 1 and aggregation region K

      C(2,k) is part of plane 2 and aggregation region K

      C(1,k) has a link to C(2, j) iff k = j.

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

      {C(1,k) has a link to C(1, j)} iff {C(2,k) has a link to C(2, j)}.

         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.
                             Edge Router A
                                 /  \
                                /    \
                               /      \  Agg Region A
                              /        \
                             /          \
                            | \         | \
                            |  \        |  \
                            |   C1B----------C2B
                  Plane1    |    |      |    |     Plane2
                            |    |      |    |
                            C1C--|-----C2C   |
                              \  |        \  |
                               \ |         \ |
                                  \        /
                                   \      /  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, A should be able to use the three
   following options:

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

   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 should support this use case with the
   following requirements:

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

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

      C propagates all the paths to Z within AS1 (add-path).

      C only installs the path via AS2 in its RIB.

   In that context, SPRING should allow the operator of AS1 cannot to apply the
   following traffic-engineering policy: policy, 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.

   While egress routers are known in the routing domain (generally
   through their loopback address), the SPRING architecture should
   enable following:

   o  identify the egress interfaces of an egress node

   o  identify the peering neighbors of an egress node

   o  identify the peering ASes of an egress node

   With these identifiers known in the domain, the SPRING architecture
   should allow an ingress node to select the exit point of a packet as
   any combination of an egress node, an egress interface, a peering
   neighbor, and a peering AS.  Load-balancing among non-parallel links

   The SPRING architecture should allow a given node should be able to
   load share traffic across multiple non parallel links even if these
   ones lead to different neighbors.  This may be useful to support
   traffic engineering policies.

                                 |           |

               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 paths.

5.1.2.  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.  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.

   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
                                 /  \     \
                            A---B    +--E--F--Z
                                 \        /

             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

                                   / \
                                 / \ /   \
                                A   E     \
                                 \         M
                                  \   G   /
                                   \ / \ /
                                    F   K
                                     \ /

                      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.

   SPRING architecture should 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  SDN/SR 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 (e.g.  [RFC7011] and [I-D.ietf-idr-ls-distribution].

   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 (e.g.:
   [I-D.ietf-i2rs-architecture], [I-D.crabbe-pce-pce-initiated-lsp] and

   In the context of Centralized-Based Optimization and the SDN use-
   case, here are the benefits that the SPRING architecture should

      Explicit routing capability with or without ECMP-awareness.

      No signaling hop-by-hop through the network.

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

      Automated guaranteed FRR for any topology.

      Optimum virtualization: the policy state is in the packet header
      and not in the intermediate nodes along the path.  The policy is
      completely virtualized away from midpoints and tail-ends.

      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.  SDN Example

   The data-set consists in a full-mesh of 12000 explicitly-routed
   tunnels observed on a real network.  These tunnels resulted from
   distributed headend-based CSPF computation.

   We measured that only 65% of the traffic is forwarded over its
   shortest path.

   Three well-known defects are illustrated in this data set:

      The lack of ECMP support in explicitly routed tunnels: ATM-alike
      traffic-steering mechanisms steer the traffic along a non-ECMP

      The increase of the number of explicitly-routed non-ECMP tunnels
      to enumerate all the ECMP options.

      The inefficiency of distributed optimization: too much traffic is
      forwarded off its shortest path.

   We applied the SDN use-case to this dataset implying a source route
   model where the path of the packet is encoded within the packet
   itself.  This means that:

      The distributed CSPF computation is replaced by centralized
      optimization and BW admission control, supported by the SDN

         As part of the optimization, we also optimized the IGP-metrics
         such as to get a maximum of traffic load-spread among ECMP
         paths by default.

      The traffic-engineering policies are supported by a source route
      model (e.g.: [I-D.filsfils-rtgwg-segment-routing]).

   As a result, we measured that 98% of the traffic would be kept on its
   normal policy (over the shortest-path) and only 2% of the traffic
   requires a path away from the shortest-path.

   Let us highlight a few benefits:

      98% of the traffic-engineering head-end policies are eliminated.

         Indeed, by default, an ingress edge node capable of injecting
         source routed packets steers the traffic to the egress edge
         node.  No configuration or policy needs to be maintained at the
         ingress edge node to realize this.

      100% of the states at mid/tail nodes are eliminated.

6.  Interoperability with non-SPRING nodes

   SPRING must inter-operate with non-SPRING nodes.

   An illustration of interoperability between SPRING and other MPLS
   Signalling Protocols (LDP) is described here in

   Interoperability with IPv6 non-SPRING nodes will be described in a
   future document.

7.  OAM

   The SPRING WG should provide OAM and the management needed to manage
   SPRING enabled networks.  The SPRING procedures may also be used as a
   tool for OAM in SPRING enabled networks.

   OAM use cases and requirements are described in
   [I-D.geib-spring-oam-usecase] and

8.  Security

   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.  In
   such context trust boundaries should strip explicit routes from a

   For each data plane technology that SPRING specifies, a security
   analysis must be provided showing how protection is provided against
   an attacker disrupting the network by for example, maliciously
   injecting SPRING packets.

9.  IANA Considerations


10.  Manageability Considerations


11.  Security Considerations


12.  Acknowledgements

   The authors would like to thank Robert Raszuk and Yakov Rekhter for
   their his contribution to
   this document.

13.  References
13.1.  Normative References

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

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC6564]  Krishnan, S., Woodyatt, J., Kline, E., Hoagland, J., and
              M. Bhatia, "A Uniform Format for IPv6 Extension Headers",
              RFC 6564, April 2012.

   [RFC7011]  Claise, B., Trammell, B., and P. Aitken, "Specification of
              the IP Flow Information Export (IPFIX) Protocol for the
              Exchange of Flow Information", STD 77, RFC 7011,
              September 2013.

   [RFC7045]  Carpenter, B. and S. Jiang, "Transmission and Processing
              of IPv6 Extension Headers", RFC 7045, December 2013.

13.2.  Informative References

              Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "PCEP
              Extensions for PCE-initiated LSP Setup in a Stateful PCE
              Model", draft-crabbe-pce-pce-initiated-lsp-03 (work in
              progress), October 2013.

              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.


              Filsfils, C., Francois, P., Previdi, S., Bashandy, A., 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 interoperability with LDP",
              draft-filsfils-spring-segment-routing-ldp-interop-00 (work
              in progress), October 2013.


              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 interoperability with LDP",
              draft-filsfils-spring-segment-routing-ldp-interop-00 MPLS data plane",
              draft-filsfils-spring-segment-routing-mpls-00 (work in
              progress), October 2013.


              Filsfils, C., Francois, P., Previdi, S., Bashandy, A., Decraene, B.,
              Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,
              Ytti, S., Henderickx, W., Tantsura, J., Kini, S., and E.
              Crabbe, "Segment Routing with MPLS data plane",
              draft-filsfils-spring-segment-routing-mpls-00 Use Cases",
              draft-filsfils-spring-segment-routing-use-cases-00 (work
              in progress), October 2013. March 2014.

              Francois, P., Filsfils, C., Decraene, B., and R. Shakir,
              "Use-cases for Resiliency in Segment Routing",
              draft-francois-spring-resiliency-use-case-00 SPRING",
              draft-francois-spring-resiliency-use-case-01 (work in
              progress), January April 2014.

              Geib, R. and C. Filsfils, "Use case for a scalable and
              topology aware MPLS data plane monitoring system",
              draft-geib-spring-oam-usecase-01 (work in progress),
              February 2014.

              Atlas, A., Halpern, J., Hares, S., Ward, D., and T.
              Nadeau, "An Architecture for the Interface to the Routing
              System", draft-ietf-i2rs-architecture-02 (work in
              progress), February 2014.

              Gredler, H., Medved, J., Previdi, S., Farrel, A., and S.
              Ray, "North-Bound Distribution of Link-State and TE
              Information using BGP", draft-ietf-idr-ls-distribution-04
              (work in progress), November 2013.

              Crabbe, E., Medved, J., Minei, I., and R. Varga, "PCEP
              Extensions for Stateful PCE",
              draft-ietf-pce-stateful-pce-08 (work in progress),
              February 2014.

              Kumar, N., Pignataro, C., Akiya, N., Geib, R., and G.
              Mirsky, "OAM Requirements for Segment Routing Network",
              draft-kumar-spring-sr-oam-requirement-00 (work in
              progress), February 2014.

              Sivabalan, S., Medved, J., Filsfils, C., Crabbe, E., and
              R. Raszuk, "PCEP Extensions for Segment Routing",
              draft-sivabalan-pce-segment-routing-02 (work in progress),
              October 2013.

Authors' Addresses

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


   Clarence Filsfils (editor)
   Cisco Systems, Inc.


   Bruno Decraene


   Stephane Litkowski


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

   Ruediger Geib
   Deutsche Telekom
   Heinrich Hertz Str. 3-7
   Darmstadt  64295


   Rob Shakir
   British Telecom


   Robert Raszuk