Internet DRAFT - draft-ali-spring-network-slicing-building-blocks


SPRING Working Group                                              Z. Ali
Internet-Draft                                               C. Filsfils
Intended status: Standards Track                            P. Camarillo
Expires: May 6, 2020                                       Cisco Systems
                                                                D. Voyer 
                                                             Bell Canada
                                                        November 3, 2019

          Building blocks for Slicing in Segment Routing Network 

   This document describes how to build network slice using the 
   Segment Routing based technology. It explains how the existing  
   building blocks specified for the Segment Routing can be used for 
   this purpose.  
   Status of This Memo 
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   Internet-Draft      Network Slicing Using SR 

   Table of Contents 

      1  Introduction...................................................2 
      2  Segment Routing Policy.........................................3 
         2.1  Flex-Algorithm Based SR Policies .........................4 
         2.2  On-demand SR policy ......................................5 
         2.3  Automatic Steering .......................................6 
         2.4  Inter-domain Considerations ..............................6 
      3  TI-LFA and Microloop Avoidance.................................7 
      4  SR VPN.........................................................7 
      5  Stateless Service Programming..................................7 
      6  Operations, Administration, and Maintenance (OAM)..............8 
      7  QoS............................................................8 
      8  Orchestration at the Controller................................9 
      9  Illustration...................................................9 
      10   Security Considerations .....................................9 
      11   IANA Considerations ........................................10 
      12   References .................................................10 
         12.1   Normative References ..................................10 
      13   Acknowledgments ............................................10 
      14   Contributors ...............................................10 

   1  Introduction 
   As more and more Service Providers and Enterprises operate a 
   single network infrastructure to support an ever-increasing 
   number of services, the ability to custom fit transport to 
   application needs is critically important. This includes 
   creating network slices with different characteristics can 
   coexist on top of the shared network infrastructure.  
   Network Slicing is meant to create (end-to-end) partitioned 
   network infrastructure that can be used to provide 
   differentiated connectivity behaviors to fulfill the 
   requirements of a diverse set of services. Services belonging to 
   different Network slices can be wholly disjoint or can share 
   different parts of the network infrastructure. Network Slicing 
   is one of the requirements in 5G [3GPP 23501].  
   Segment Routing enables Service Providers to support Network 
   Slicing without any additional protocol, other than the SR IGP 
   extensions. The network as a whole, in a distributed and 
   entirely automated manner, can share a single infrastructure 
   resource along multiple virtual services (slices). For example, 
   one slice is optimized continuously for low-cost transport; a 
   second Slice is optimized continuously for low-latency 
   transport; a third Slice is orchestrated to support disjoint 

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   services, etc. The optimization objective of each of these 
   slices is programmable by the operator.  
   The Segment Routing specification already contains the various 
   building blocks required to create network slices. This includes 
   the following.  
     . SR Policy with or without Flexible Algorithm.  
     . TI-LFA with O(50 msec) protection in the slice underlay.  
     . SR VPN.  
     . SR Service Programming (NFV, SFC).  
     . Operation, Administration and Management (OAM) and 
        Performance Management (PM).  
     . QoS using DiffServ.  
     . Orchestration at the Controller.
   Each of these building blocks works independently of each other. 
   Their functionality can be combined to satisfy service 
   provider's requirement for the Network Slicing. An external 
   controller plays an important role to orchestrate these building 
   blocks into a Slicing service.  
   This document elaborates on the attributes of each of these 
   building blocks for Network Slicing. The document also highlights 
   how services in each Slice can benefit from traffic engineering, 
   network function virtualization/ service chaining (service 
   programming), OAM, performance management, SDN readiness, O (50 msec) 
   TI-LFA protection, etc. features of SR while respecting resource 
   partitioning employed over the common networking infrastructure.  
   The document equally applicable to the MPLS and SRv6 
   instantiations of segment routing.  
   The following subsection elaborates on each of these build 

   2  Segment Routing Policy  
   Segment Routing (SR) allows a headend node to steer a packet 
   flow along any path without creating intermediate per-flow 
   states [I-D.ietf-spring-segment-routing-policy]. The headend 
   node steers a flow into a Segment Routing Policy (SR Policy).  
   I.e., the SR Policy can be used to steer traffic along any 
   arbitrary path in the network. This allows operators to enforce 
   low-latency and / or disjoint paths, regardless of the normal 
   forwarding paths.  

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   The SR policy is able to support various optimization objectives 
   [I-D.draft-filsfils-spring-sr-policy-considerations]. The 
   optimization objectives can be instantiated for the IGP metric 
   ([RFC1195] [RFC2328] [RFC5340]) xor the TE metric ([RFC5305], 
   [RFC3630]) xor the latency extended TE metric ([RFC7810] 
   [RFC7471]). In addition, an SR policy is able to various 
   constraints, including inclusion and/or exclusion of TE 
   affinity, inclusion and/or exclusion of IP address, inclusion 
   and/or exclusion of SRLG, inclusion and/or exclusion of admin-
   tag, maximum accumulated metric (IGP, TE, and latency), maximum 
   number of SIDs in the solution SID-List, maximum number of 
   weighted SID-Lists in the solution set, diversity to another 
   service instance (e.g., link, node, or SRLG disjoint paths 
   originating from different head-ends), etc. [I-D.draft-filsfils-
   spring-sr-policy-considerations]. The supports for various 
   optimization objectives and constraints enables SR policy to 
   create Slices in the network. 
   SR policy can be instantiated with or without IGP Flexible 
   Algorithm feature. The following subsection describes the SR 
   Flexible Algorithm feature and how SR policy can utilize this 
   2.1 Flex-Algorithm Based SR Policies 
   Flexible Algorithm enriches the SR Policy solution by adding 
   additional segments having different properties than the IGP 
   Prefix segments. Flex Algo adds flexible, user-defined segments 
   to the SRTE toolbox. Specifically, it allows for association of 
   the "intent" to Prefix SIDs. [I-D.ietf-lsr-flex-algo] defines 
   the IGP based Flex-Algorithm 
   solution which allows IGPs themselves to compute paths 
   constraint by the "intent" represented by the Flex-Algorithm.  
   The Flex-Algorithm has the following attributes:  
     . Algorithm associate to the SID a specific TE intent 
        expressed as an optimization objective (an algorithm) [I-
     . Flexibility includes the ability of network operators to 
        define the intent of each algorithm they implement.  
     . By design the mapping between the Flex-Algorithm and its 
        meaning is flexible and is defined by the user.  
     . Flexibility also includes ability for operators to make the 
        decision to exclude some specific links from the shortest 
        path computation, e.g.,  

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          o operator 1 may define Algo 128 to compute the shortest 
      path for TE metric and exclude red affinity links. 
          o operator 2 may define Algo 128 to compute the shortest 
      path for latency metric and exclude blue affinity 
   A Network Slice can be created by associating a Flexible-
   Algorithm value with the Slice via provisioning.  
   Flex Alg leverages SR on-demand next hop (ODN) and Automated 
   Steering for intent-based instantiation of traffic engineered 
   paths described in the following sub-sections. Specifically, as 
   specified in [I-D.ietf-spring-segment-routing] the IGP Flex Algo 
   Prefix SIDs can also be used as segments within SR Policies 
   thereby leveraging the underlying IGP Flex Algo solution. 
   2.2 On-demand SR policy  
   Segment Routing On-Demand Next-hop (ODN) functionality enables 
   on-demand creation of SR Policies for service traffic. Using a 
   Path Computation Element (PCE), end-to-end SR Policy paths can 
   be computed to provide end-to-end Segment Routing connectivity, 
   even in multi-domain networks running with or without IGP 
   Flexible-Algorithm [I-D.draft-ietf-spring-segment-routing-
   The On-Demand Next-hop functionality provides optimized service 
   paths to meet customer and application SLAs (such as latency, 
   disjointness) without any pre-configured TE tunnel and with the 
   automatic steering of the service traffic on the SR Policy 
   without a static route, autoroute-announce, or policy-based 
   With this functionality, a Network Service Orchestrator can 
   deploy the service based on their requirements. The service 
   head-end router requests the PCE to compute the path for the 
   service and then instantiates an SR Policy with the computed 
   path and steers the service traffic into that SR Policy. If the 
   topology changes, the stateful PCE updates the SR Policy path. 
   This happens seamlessly, while TI-LFA protects the traffic in 
   case the topology change happened due to a failure. 

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   2.3 Automatic Steering 
   Automatically steering traffic into a Network Slice is one of 
   the fundamental requirement for Slicing. That is made possible 
   by the "Automated Steering" functionality of SR. Specifically, 
   SR policy can be used for traffic engineer paths 
   within a slice, "automatically steer" traffic to the right slice 
   and connect IGP Flex-Algorithm domains sharing the same 
   A headend can steer a packet flow into a valid SR Policy within 
   a slice in various ways [I-D.draft-ietf-spring-segment-routing-
     . Incoming packets have an active SID matching a local 
        Binding SID (BSID) at the headend. 
     . Per-destination Steering: incoming packets match a 
        BGP/Service route which recurses on an SR policy. 
     . Per-flow Steering: incoming packets match or recurse on a 
        forwarding array of where some of the entries are SR 
     . Policy-based Steering: incoming packets match a routing 
        policy which directs them on an SR policy. 
   2.4 Inter-domain Considerations 
   The network slicing needs to be extended across multiple domains 
   such that each domain can satisfy the intent consistently. SR 
   has native inter-domain mechanisms, e.g., SR policies are 
   designed to span multiple domains using a PCE based solution [I-
   D.ietf-spring-segment-routing], [I-D.ietf-spring-segment-
   routing-central-epe]. An edge router upon service configuration 
   automatically requests to the Segment Routing PCE an inter-
   domain path to the remote service endpoint. The path can either 
   be for simple best-effort inter-domain reachability or for 
   reachability with an SLA contract and can be restricted to a
   Network Slice.   
   The SR native mechanisms for inter-domain are easily extendable 
   to include the case when different IGP Flex-Algorithm values are 
   used to represent the same intent. E.g., in domain1 Service 
   Provider 1 (SP1) may use flex-algo 128 to indicate low latency 
   Slice and in domain2 Service Provider 2 (SP2) may use flex-algo 
   129 to indicate low latency Slice. When an automation system at 
   a PE1 in SP1 network configures a service with next hop (PE2) in 
   SP2 network, SP1 contacts a Path Computation Element (PCE) to 
   find a route to PE2. In the request, the PE1 also indicates the 
   intent (i.e., the Flex-Algo 128) in the PCEP message. As the PCE 
   has a complete understanding of both Domains, it can understand 
   the path computation in Domain1 needs to be performed for 
   Algorithm 128 and path computation in Domain2 needs to be 

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   performed for Algorithm 129 (i.e., in the Low Latency Network 
   Slice in both domains).  

   3  TI-LFA and Microloop Avoidance 
   The Segment Routing-based fast-reroute solution, TI-LFA, can 
   provide per-destination sub-50msec protection upon any single 
   link, node or SRLG failure regardless of the topology. The 
   traffic is rerouted straight to the post-convergence path, hence 
   avoiding any intermediate flap via an intermediate path. The 
   primary and backup path computation is completely automatic by 
   the IGP.  
   [I-D.draft-bashandy-rtgwg-segment-routing-ti-lfa] proposes a 
   Topology Independent Loop-free Alternate Fast Re-route (TI-LFA), 
   aimed at protecting node and adjacency segments within O(50 
   msec) in the Segment Routing networks. Furthermore, [I-D. draft-
   bashandy-rtgwg-segment-routing-uloop] provides a mechanism 
   leveraging Segment Routing to ensure loop-freeness during the 
   IGP reconvergence process following a link-state change event.  
   As mentioned earlier, Network Slicing in Segment Routing works 
   seamlessly with all the other components of the Segment Routing. 
   This, of course, includes TI-LFA and microloop avoidance within 
   a Slice, with the added benefit that backup path only uses 
   resources available to the Slice. For example, when Flexible 
   Algorithm is used, the TI-LFA backup path computation is 
   performed such that it is optimized per Flexible-Algorithm. The 
   backup path shares the same properties are the primary path. The 
   backup path does not use a resource outside the Slice of the 
   primary path it is protecting.  

   4  SR VPN 
   Virtual Private Networks (VPNs) provides a mean for creating a 
   logically separated network to a different set of users access 
   to a common network. Segment Routing is equipped with the rich 
   multi-service virtual private network (VPN) capabilities, 
   including Layer 3 VPN (L3VPN), Virtual Private Wire Service 
   (VPWS), Virtual Private LAN Service (VPLS), and Ethernet VPN 
   (EVPN). The ability of Segment Routing to support different VPN 
   technologies is one of the fundamental building blocks for  
   creating slicing an SR network.  

   5  Stateless Service Programming 

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   An important part of the Network Slicing is the orchestration of 
   virtualized service containers. [I-D.draft-xuclad-spring-sr-
   service-chaining] describes how to implement service segments 
   and achieve stateless service programming in SR-MPLS and SRv6 
   networks. It introduces the notion of service segments. The 
   ability of encoding the service segments along with the 
   topological segment enables service providers to forward packets 
   along a specific network path, but also steer them through VNFs 
   or physical service appliances available in the network. 
   In an SR network, each of the service, running either on a 
   physical appliance or in a virtual environment, is associated 
   with a segment identifier (SID) for the service.  These service 
   SIDs are then leveraged as part of a SID-list to steer packets 
   through the corresponding services.  Service SIDs may be 
   combined with topological SIDs to achieve service programming 
   while steering the traffic through a specific topological path 
   in the network. In this fashion, SR provides a fully integrated 
   solution for overlay, underlay and service programming building 
   blocks needed to satisfy network slicing requirements. 

   6  Operations, Administration, and Maintenance (OAM) 
   There are various OAM elements that are critical to satisfy 
   Network Slicing requirements. These includes but not limited to 
   the following:    
     . Measuring per-link TE Matric.  
     . Flooding per-link TE Matric.  
     . Taking TE Matric into account during path calculation.  
     . Taking TE Matric bound into account during path calculation.  
     . SLA Monitoring: Service Provider can monitor each SR Policy 
        in a Slice to Monitor SLA offered by the Policy using 
        technique described in [I-D.draft-gandhi-spring-udp-pm]. 
        This includes monitoring end-to-end delays on all ECMP 
        paths of the Policy as well as monitoring traffic loss on a 
        Policy. Remedial mechanisms can be used to ensure that the 
        SR policy conforms to the SLA contract.  

   7  QoS 
   Segment Routing relies on MPLS and IP Differentiated Services. 
   Differentiated services enhancements are intended to enable 
   scalable service discrimination in the Internet without the need 
   for per-flow state and signaling at every hop. [RFC2475] defines 

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   an architecture for implementing scalable service 
   differentiation in the Internet. This architecture is composed 
   of many functional elements implemented in network nodes, 
   including a small set of per-hop forwarding behaviors, packet 
   classification functions, and traffic conditioning functions 
   including metering, marking, shaping, and policing.   
   The DiffServ architecture achieves scalability by implementing 
   complex classification and conditioning functions only at 
   network boundary nodes, and by applying per-hop behaviors to 
   aggregates of traffic depending on the traffic marker. 
   Specifically, the node at the ingress of the DiffServ domain 
   conditions, classifies and marks the traffic into a limited 
   number of traffic classes. The function is used to ensure that 
   the slice's traffic conforms to the contract associated with the 
   Per-hop behaviors are defined to permit a reasonably granular 
   means of allocating buffer and bandwidth resources at each node 
   among competing traffic streams. Specifically, per class 
   scheduling and queuing control mechanisms are applied at each IP 
   hop to the traffic classes depending on packet's marking. 
   Techniques such as queue management and a variety of scheduling 
   mechanisms are used to get the required packet behavior to meet 
   the slice's SLA.  

   8  Orchestration at the Controller  
   A controller plays a vital role in orchestrating the SR building 
   blocks discussed above to create Network Slices. The controller also 
   performs admission control and traffic placement for slice 
   management at the transport layer. The SDN friendliness of the 
   SR technology becomes handy to realize the orchestration. The 
   controller may use PCEP or Netconf to interact with the routers. 
   The router implements Yang model for SR-based network slicing.  
   Specification of the controller technology for orchestrating 
   Network Slices, services and admission control for the services 
   is outside the scope of this draft.  

   9  Illustration  
   To be added in a later revision.  

   10 Security Considerations 
   This document does not impose any additional security 

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   11 IANA Considerations 
   This document does not define any new protocol or any extension 
   to an existing protocol.   

   12 References 
   12.1 Normative References 
   7.2. Informative References 
         Filsfils, C., Sivabalan, et al, "Segment Routing Policy  
         For Traffic Engineering", draft-filsfils-spring-segment- 
         routing-policy-05 (work in progress), February 2018. 
   [I-D.ietf-lsr-flex-algo] P. Psenak, et al,  
         draft-ietf-lsr-flex-algo, work in progress.  
         Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B., 
         Litkowski, S., and R. Shakir, "Segment Routing 
         Architecture", draft-ietf-spring-segment-routing-15 
         (work in progress), January 2018. 
         Filsfils, C., et al. draft-filsfils-spring-sr-policy- 
         considerations (work in progress) 

   13 Acknowledgments 

   14 Contributors 
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   Authors' Addresses 
      Zafar Ali 
      Cisco Systems, Inc. 
      Clarence Filsfils 
      Cisco Systems, Inc. 

      Pablo Camarillo Garvia 
      Cisco Systems, Inc. 
      Daniel Voyer 
      Bell Canada