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SPRING                                                          S. Hegde
Internet-Draft                                                 C. Bowers
Intended status: Standards Track                   Juniper Networks Inc.
Expires: January 13, 2021                                          X. Xu
                                                            Alibaba Inc.
                                                                A. Gulko
                                                               Refinitiv
                                                           July 12, 2020


                        Seamless Segment Routing
                 draft-hegde-spring-mpls-seamless-sr-00

Abstract

   In order to operate networks with large numbers of devices, network
   operators organize networks into multiple smaller network domains.
   Each network domain typically runs an IGP which has complete
   visibility within its own domain, but limited visibility outside of
   its domain.  Seamless Segment Routing (Seamless SR) provides
   flexible, scalable and reliable end-to-end connectivity for services
   across independent network domains.  Seamless SR accomodates domains
   using SR, LDP, and RSVP for MPLS label distribution as well as
   domains running IP without MPLS (IP-Fabric).

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 https://datatracker.ietf.org/drafts/current/.

   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 January 13, 2021.




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Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
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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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Service provider network  . . . . . . . . . . . . . . . .   5
     3.2.  Large scale WAN networks  . . . . . . . . . . . . . . . .   6
     3.3.  Data Center Interconnect (DCI) Networks . . . . . . . . .   7
     3.4.  Multicast Usecases  . . . . . . . . . . . . . . . . . . .   7
   4.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  MPLS Transport  . . . . . . . . . . . . . . . . . . . . .   8
     4.2.  SLA Guarantee . . . . . . . . . . . . . . . . . . . . . .   9
     4.3.  Scalability . . . . . . . . . . . . . . . . . . . . . . .   9
     4.4.  Availability  . . . . . . . . . . . . . . . . . . . . . .   9
     4.5.  Operations  . . . . . . . . . . . . . . . . . . . . . . .   9
     4.6.  Service Mapping . . . . . . . . . . . . . . . . . . . . .  10
   5.  Seamless Segment Routing architecture . . . . . . . . . . . .  10
     5.1.  Solution Concepts . . . . . . . . . . . . . . . . . . . .  10
     5.2.  BGP Classful Transport  . . . . . . . . . . . . . . . . .  11
     5.3.  SLA Guarantee . . . . . . . . . . . . . . . . . . . . . .  15
       5.3.1.  Low latency . . . . . . . . . . . . . . . . . . . . .  15
       5.3.2.  Traffic Engineering (TE) constraints  . . . . . . . .  16
       5.3.3.  Bandwidth constraints . . . . . . . . . . . . . . . .  16
     5.4.  Scalability . . . . . . . . . . . . . . . . . . . . . . .  16
       5.4.1.  Access node scalability . . . . . . . . . . . . . . .  16
       5.4.2.  Label stack depth . . . . . . . . . . . . . . . . . .  17
       5.4.3.  Label Resources . . . . . . . . . . . . . . . . . . .  17
     5.5.  Reliability . . . . . . . . . . . . . . . . . . . . . . .  20
       5.5.1.  Intra domain link and node protection . . . . . . . .  20
       5.5.2.  Egress Link and node protection . . . . . . . . . . .  20
       5.5.3.  Border Node protection  . . . . . . . . . . . . . . .  20
     5.6.  Operations  . . . . . . . . . . . . . . . . . . . . . . .  20
       5.6.1.  MPLS ping and Traceroute  . . . . . . . . . . . . . .  20



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       5.6.2.  Counters and Statistics . . . . . . . . . . . . . . .  21
     5.7.  Service Mapping . . . . . . . . . . . . . . . . . . . . .  21
     5.8.  Migrations  . . . . . . . . . . . . . . . . . . . . . . .  22
     5.9.  Interworking with v6 transport technologies . . . . . . .  22
     5.10. BGP based Multicast . . . . . . . . . . . . . . . . . . .  22
   6.  Backward Compatibility  . . . . . . . . . . . . . . . . . . .  22
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  22
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  22
   10. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  22
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  23
     11.2.  Informative References . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

1.  Introduction

   The Seamless SR architecture builds upon the Seamless MPLS
   architecture, which has been widely deployed to provide end-to-end
   transport for service in 3G/4G networks.
   [I-D.ietf-mpls-seamless-mpls], contains a good description of the
   Seamless MPLS architecture.  Although [I-D.ietf-mpls-seamless-mpls]
   has not been published as an RFC, it serves as a useful description
   of the Seamless MPLS architecture.  [I-D.ietf-mpls-seamless-mpls]
   describes the Seamless MPLS architecture, which uses LDP and/or RSVP
   for intra-domain label distribution, and BGP-LU [RFC3107] for end-to-
   end label distribution.  The Seamless SR architecture builds on the
   the Seamless MPLS architecture.  Seamless SR focuses on using segment
   routing for intra-domain label distribution.

   By using segment routing for intra-domain label distribution,
   Seamless SR is able to easily support both SR-MPLS on IPv4 and IPv6
   networks.  This overcomes a limitation of the classic Seamless MPLS
   architecture, which was limited to run MPLS on IPv4 networks in
   practice.  Seamless SR (like Seamless MPLS) can use BGP-LU (RFC 3107)
   to stitch different domains.  However, Seamless SR can also take
   advantage of BGP Prefix-SID [RFC8669] to provide predictable and
   deterministic labels for the inter-domain connectivity.

   5G technology is expected to place new requirements on the packet
   transport networks that support it.  To enable 5G technology, packet
   transport networks will need to be capable of handling much greater
   bandwidth than today's 3G/4G networks. 5G networks are expected to
   require up to 250Gbps in the fronthaul and up to 400Gbps in the
   backhaul.  The number of transport network devices is also expected
   to grow significantly to cater to 5G needs.  Overall service
   availabilty requirements for 5G will place significant requirements
   on the resiliency of packet transport networks.



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   There is a desire to allow many 5G network functions to be
   virtualized and cloud native.  In order to support latency-sensitive
   cloud-native 5G network functions, packet transport networks should
   be capable of providing low-latency paths end-to-end.  Some services
   will require low-latency paths while others may require different QoS
   properties.  The network should be able to differentiate the services
   and provide corresponding SLA transport paths.

   The basic functionality of the Seamless SR architecture does not
   require any enhancements to existing protocols.  However, in order to
   support end-to-end service requirements across multiple domains,
   protocol extensions may be needed.  This draft discusses usecases,
   requirements, and potential protocol enhancements.

2.  Terminology

 This document uses the following terminology

   o  Access Node (AN): An access node is a node which processes
      customers frames or packets at Layer 2 or above.  This includes
      but is not limited to DSLAMs and Cell Site Routers in 5G networks.
      Access nodes have only limited MPLS functionalities
      in order to reduce complexity in the access network.

   o  Pre-Aggregation Node (P-AGG): A pre-aggregation node (P-AGG) is a node
      which aggregates several access nodes (ANs).

   o  Aggregation Node (AGG): A aggregation node (AGG) is a node which
      aggregates several pre-aggregation nodes (P-AGG).

   o  Area Border Router (ABR): Router between aggregation and core
      domain.

   o  Label Switch Router (LSR): Label Switch router are pure transit nodes.
      ideally have no customer or service state and are therefore decoupled
      from service creation.


   o  Use Case: Describes a typical network including service creation
      points and distribution of remote node loopback prefixes.

                           Figure 1: Terminology

3.  Use Cases







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3.1.  Service provider network

   Service provider transport networks use multiple domains to support
   scalability.  For this analysis, we consider a representative network
   design with four level of hierarchy: access domains, pre-aggregation
   domains, aggregation domains and a core.  (See Figure 2).  The 5G
   transport networks in particular are expected to scale to very large
   number of access nodes due to the shorter range of the 5G radio
   technology.  The networks are expected to scale up to one million
   nodes.


                 +-------+   +-------+   +------+   +------+
                 |       |   |       |   |      |   |      |
              +--+ P-AGG1+---+ AGG1  +---+ ABR1 +---+ LSR1 +--> to ABR
             /   |       |  /|       |   |      |   |      |
      +----+/    +-------+\/ +-------+   +------+  /+------+
      | AN |              /\                     \/
      +----+\    +-------+  \+-------+   +------+/\ +------+
             \   |       |   |       |   |      |  \|      |
              +--+ P-AGG2+---+ AGG2  +---+ ABR2 +---+ LSR2 +--> to ABR
                 |       |   |       |   |      |   |      |
                 +-------+   +-------+   +------+   +------+

      ISIS L1       ISIS L2                   ISIS L2

      |-Access-|--Aggregation Domain--|---------Core-----------------|


                           Figure 2: 5G network

   Many network functions in a 5G network will be virtualized and
   distributed across multiple data centers.  Virtualized network
   functions are instantiated dynamically across different compute
   resources.  This requires that the underlying transport network
   supports the stringent SLA on end-to-end paths.

   5G networks support variety of service use cases that require end-to-
   end slicing.  In certain cases the end-to-end connectivity requires
   differentiated forwarding capabilities.  Seamless SR architecture
   should provide ability to establish end-to-end paths that satisfy the
   required SLAs.  For Example, End user requirement could be to
   establish low latency path end-to-end.  The System Architecture for
   the 5G System [TS.23.501-3GPP] currently defines four standardized
   Slice/Service Types: Enhanced Mobile Broadband (eMBB), Ultra-Reliable
   Low Latency Communication (URLLC), massive Internet of Things (mIoT),
   Vehicle to everything (V2X).  The Seamless SR should support end-to-




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   end QoS mechansisms to allow the creation of network slices with
   these four Slice/Service Types.

   Many deployments consist of ring topologies in the access and
   aggregation networks.  In the ring topologies, there are atmost two
   forwarding paths for the traffic, where as the core networks consist
   of nodes with more denser connectivity compared to ring topologies.
   Thus core networks may have larger number of TE paths while access
   networks will have smaller number of TE paths.  The Seamless SR
   architecture should support ability to have more TE paths in one
   domain and lesser number of TE paths in another domain and provide
   ability to effectively connect the domain end-to-end satisfying end-
   to-end constraints.

3.2.  Large scale WAN networks

   As WAN networks grow beyond several thousand nodes, it is often
   useful to divide the network into multiple IGP domains.  The
   different IGP domains provide better fault isolation.  Smaller IGP
   domains can also reduce FIB scale.


                 +-------+     +-------+     +-------+
                 |       |     |       |     |       |
                 |      ABR1  ABR2    ABR3   ABR4    |
                 |       |     |       |     |       |
              PE1+DOMAIN1+-----+DOMAIN2+-----+DOMAIN3+PE2
                 |       |     |       |     |       |
                 |      ABR11  ABR22  ABR33  ABR44   |
                 |       |     |       |     |       |
                 +-------+     +-------+     +-------+


                |-ISIS1-|      |-ISIS2-|     |-ISIS3-|


                           Figure 3: WAN Network

   Large WAN networks often cross national boundaries.  In order to meet
   data sovereignity requirements, operators need to maintain strict
   control over end-to-end traffic-engineered(TE) paths.  Segment
   Routing provides two main solutions to implement highly constrained
   TE paths.  Flex-algo (defined in [I-D.ietf-lsr-flex-algo]) uses
   prefix-SIDs computed by all nodes in the IGP domain using the same
   pruned topology.  Highly constrained TE paths for the data
   soveriegnty use case can also be implemented using SR-TE policies
   ([I-D.ietf-spring-segment-routing-policy]) built using unprotected
   adjacency SIDs.



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   Both of these approaches work well for intra-domain TE paths.
   However, they both have limitations when one tries to extend them to
   the creation of highly constrained inter-domain TE paths.  A goal of
   seamless SR is to be able to create highly constrained inter-domain
   TE paths in a scalable manner.

3.3.  Data Center Interconnect (DCI) Networks

   Data centers are playing an increasingly important role in providing
   access to information and applications.  Geographically diverse data
   centers usually connect via a high speed, reliable and secure core
   network.


                 +-------+     +-------+     +-------+
                 |       ASBR1 ASBR2 ASBR3   ASBR4   |
                 |       |     |       |     |       |
              PE1+  DC1  +-----+  CORE +-----+  DC2  +PE2
                 |    ASBR11  ASBR22 ASBR33 ASBR44   |
                 |       |     |       |     |       |
                 +-------+     +-------+     +-------+


                 |-ISIS1-|      |-ISIS2-|    |-ISIS3-|


                           Figure 4: DCI Network

   In many Data Center deployments, applications require end-to-end path
   diversity and/or end-to-end low latency paths.  It is desirable to
   have a uniform technology deployed in the core as well as in the Data
   Centers to create these SLA paths.  Such uniformity simplifies the
   network to a great extent.  It is desirable for a solution to only
   require service-related configurations on the access end-points where
   services are attached, avoiding service-related configurations on the
   ABR/ASBR nodes.

3.4.  Multicast Usecases

   Multicast services such as IPTV and multicast also need to be support
   across a multi-domain service provider network.  Multicast services
   such as IPTV, multicast VPN etc need to be supported in a service
   provider network.








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                 +---------+---------+---------+
                 |         |         |         |
                 S1       ABR1      ABR2       R1
                 | Metro1  |  Core   |  Metro2 |
                 |         |         |         |
                 S2       ABR11     ABR22      R2
                 |         |         |         |
                 +---------+---------+---------+


                 |-ISIS1-|  |-ISIS2-|  |-ISIS3-|


                       Figure 5: Multicast usecases

   Figure 5 shows a simplified multi-domain network supporting
   multicast.  Multicast sources S1 and S2 lie in a different domain
   from the receivers R1 and R2.  Using multiple IGP domains presents a
   problem for the establishment of multicast replication trees.
   Typically, a multicast receiver does a reverse path forwarding (RPF)
   lookup for a multicast source.  One solution is to leak the routes
   for multicast sources across the IGP domains.  However, this can
   compromise the scaling properties of the multi-domain architecture.
   SR-P2MP [I-D.voyer-pim-sr-p2mp-policy] offers a solution for both
   intra-domain and inter-domain multicast.  However, it does accomodate
   deployments using existing intra-domain multicast technology, such as
   mLDP [RFC6388] in some of the domains.  A solution should accomodate
   a mixture of existing and newer technologies to better facilitate
   coexistence and migration.

4.  Requirements

   This section provides a summary of requirements derived from the use
   cases described in previous sections.

4.1.  MPLS Transport

      The architecture should provide MPLS transport between two service
      endpoints regardless of whether the two end-points are in the same
      IGP domain, different IGP domains, or in different autonomous
      systems.

      The MPLS transport should be supported on IPv4, IPv6, and dual-
      stack networks.







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4.2.  SLA Guarantee

      The architecture should allow the creation of paths that support
      end-to-end SLAs.  The paths should for example obey constraints
      related to latency, diversity, and availability.

      The architecture should support end-to-end network slicing as
      described by 5G transport requirements [TS.23.501-3GPP].

4.3.  Scalability

      The architecture should be able to support up to 1 million nodes.

      The architecture should facilitate the use of access nodes with
      low RIB/FIB and low CPU capabilities.

      The architecture should facilitate the use of access nodes with
      low label stacking capability.

      The architecture should allow for a scalable response to network
      events.  An individual node should only need to respond to a
      limited subset of network events.

      Service routes on the border nodes should be minimized.

4.4.  Availability

      Traffic should be Fast Reroute (FRR) protected against link, node,
      and SRLG failures within a domain.

      Traffic should be Fast Reroute (FRR) protected against border node
      failures.

      Traffic should be Fast Reroute (FRR) protected against egress node
      and egress link failures.

4.5.  Operations

      Each domain should be independent and should not depend on the
      transport technology in another domain.  This allows for more
      flexible evolution of the network.

      Basic MPLS OAM mechanisms described in [RFC8029] should be
      supported.

      End-to-end mpls ping and traceroute procedures should be
      supported.




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      Ability to validate the path inside each domain should be
      supported.

      Statistics for inter-domain paths on the ingress and egress PE
      nodes as well as border nodes should be supported.

4.6.  Service Mapping

      The architecture should support the automated steering of traffic
      on to transport paths based on communities carried in the service
      prefix advertisements.

      The architecture should support the steering of traffic on to
      transport paths based the DSCP value carried in IPv4/IPv6 packets.

      Traffic steering based on EXP bits in the mpls header should be
      supported.

      Traffic steering based on 5-tuple packet filter should be
      supported.  Source address, destination address, source port,
      destination port and protocol fields should be allowed.

      All traffic steering mechanims should be supported for all kinds
      of service traffic including VPN traffic as well as global
      internet traffic.

      The core domain is expected to have more traffic enginnering
      constraints as compared to metros.  The ability to map the
      services to appropriate transport tunnels at service attachment
      points should be supported.

5.  Seamless Segment Routing architecture

5.1.  Solution Concepts

















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The solution described below makes use of the following concepts.

   o  Transport Class (TC): A Transport Class is defined as a collection of
      end-to-end MPLS paths that satisfy a set of constraints or
      Service Level Aggreements.

   o  BGP-Classful Transport (BGP-CT): A new BGP family used to
      establish Transport Class paths across different domains.

   o  Route Distinguisher (RD):  The Route Distinguisher is
      defined in RFC4364.  In BGP-CT, the RD is used in BGP advertisements
      to differentiate multiple paths to the same loopback address.
      It may be useful to automatically generate RDs in order to
      simplify configuration.

   o  Route Target (RT): The Route Target extended community is
      carried in BGP-CT advertisements. The RT represents the Transport Class
      of an advertised path.

   o  Mapping Community (MC): The Mapping Community is the standard BGP community
      as defined in RFC1997. In the Seamless SR architecture,
      an MC is carried by a service route.  The MC is used to identify
      the specific local policy used to map traffic for a service route
      to different Transport Class paths. The local policy can include additional
      traffic steering properties for placing traffic on different
      Transport Class paths.  The values of the MCs and the corresponding local
      policies for service mapping are defined by the network operator.

                        Figure 6: Solution Concepts

5.2.  BGP Classful Transport




















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                 ----IBGP------EBGP----IBGP------EBGP-----IBGP---
                |            |     |           |     |           |

                 +-----------+     +-----------+     +-----------+
                 |           |     |           |     |           |
                 |        ASBR1+--+ASBR2    ASBR3+--+ASBR4       |
              PE1+     D1    |  X  |     D2    |  X  |     D3    +PE2
                 |        ASBR5+--+ASBR6    ASBR7+--+ASBR8       |
                 |           |     |           |     |           |
                 +-----+-----+     +-----------+     +-----------+
                      PE3

                 |---ISIS1---|      |---ISIS2---|      |---ISIS3---|




                           Figure 7: WAN Network

   The above diagram shows a WAN network divided into 3 different
   domains.  Within each domain, BGP sessions are established between
   the PE nodes and the border nodes as well as between border nodes.
   BGP sessions are also established between border nodes across
   domains.  The goal is for PE1 to have MPLS connectivity to PE2,
   satisfying specific characteristics.  Multiple MPLS paths from PE1 to
   PE2 are required in order to satisfy diffrent SLAs.
   [I-D.kaliraj-idr-bgp-classful-transport-planes] defines a new BGP
   family called BGP-Classful Transport.  The NLRI for this new family
   consists of a prefix and a Route Distinguisher.  The prefix
   corresponds to the loopback of the destination PE, and RD is used to
   distinguish multiple paths to the same PE loopback.  The BGP-CT
   advertisement also carries a Route Target.  The RT specifies the
   Transport Class to which the BGP-CT advertisement belongs.


















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                 BGP-CT advertisements for red Transport Class

            Prefix:PE2    Prefix:PE2  Prefix:PE2   Prefix:PE2   Prefix:PE2
            RD:RD1        RD:RD1      RD:RD1       RD:RD1       RD:RD1
            RT:Red        RT:Red      RT:Red       RT:Red       RT:Red
            nh:ASBR1      nh:ASBR2    nh:ASBR3     nh:ASBR4     nh:PE2
            Label:L1      Label:L2    Label:L3     Label:L4     Label:L5


        PE1-------ASBR1------ASBR2---------ASBR3-------ASBR4--------PE2


            +------+              +------+                   +------+
            | IL71 |              | IL72 |                   | IL73 |
            +------+   +------+   +------+      +------+     +------+
            | L1   |   | L2   |   |  L3  |      | L4   |     |  L5  |
            +------+   +------+   +------+      +------+     +------+
            | S1   |   | S1   |   |  S1  |      | S1   |     |  S1  |
            +------+   +------+   +------+      +------+     +------+

                      Label stacks along end-to-end path
                      S1 is the end-to-end service label.
            IL71, IL72, and IL73 are intra-domain labels corresponding to
                            red intra-domain paths.

             Figure 8: BGP-CT Advertisements and Label Stacks

























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                  BGP-CT advertisements for blue Transport Class

            Prefix:PE2    Prefix:PE2  Prefix:PE2   Prefix:PE2   Prefix:PE2
            RD:RD2        RD:RD2      RD:RD2       RD:RD2       RD:RD2
            RT:Blue       RT:Blue     RT:Blue      RT:Blue      RT:Blue
            nh:ASBR1      nh:ASBR2    nh:ASBR3     nh:ASBR4     nh:PE2
            Label:L11     Label:L12   Label:L13    Label:L14    Label:L15


        PE1-------ASBR1----ASBR2----------ASBR3-------ASBR4--------PE2


            +------+              +------+                   +------+
            | IL81 |              | IL82 |                   | IL83 |
            +------+   +------+   +------+      +------+     +------+
            | L11  |   | L12  |   |  L13 |      | L14  |     |  L15 |
            +------+   +------+   +------+      +------+     +------+
            | S2   |   | S2   |   |  S2  |      | S2   |     |  S2  |
            +------+   +------+   +------+      +------+     +------+

                      Label stacks along end-to-end path
                      S2 is the end-to-end service label.
            IL81, IL82, and IL83 are intra-domain labels corresponding to
                            blue intra-domain paths.


             Figure 9: BGP-CT Advertisements and Label Stacks

   For example, consider the diagram in Figure 8 and Figure 9 .  The
   diagram shows the BGP-CT advertisements corresponding to two
   different end-to-end paths between PE1 and PE2.  The two different
   paths belong to two different Transport Classes, red and blue.  In
   order to create unique NLRIs for the two advertisements, PE2 uses two
   different RDs.  In the example above, the red BGP-CT advertisement
   has an RD of RD1 and the blue BGP-CT advertisement has an RD of RD2.
   The advertisements will have RTs corresponding to the red and blue
   Transport Classes respectively.  The RT MAY be directly mapped from
   the color extended community defined in [I-D.ietf-idr-tunnel-encaps].
   In addition to the red and blue BGP-CT advertisments, the diagram
   shows the label stacks at different points along the end-to-end paths
   for the forwarding entries which are established by the two
   advertisements.  Labels L1-L4 are red BGP-CT labels advertised by
   border nodes ASBR1,2,3,and 4, while label L5 is advertised by PE2 for
   the red Transport Class.  Labels L11-L14 are blue BGP-CT labels
   advertised by border nodes ASBR1,2,3,and 4, while label L15 is
   advertised by PE2 for the blue Transport Class.





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   IL71, IL72, and IL73 represent tunnels internal to the domains 1, 2,
   and 3 which correspond to the red Transport Class.  IL81, IL82, and
   IL83 represent tunnels internal to the domains 1, 2, and 3 which
   correspond to the blue Transport Class.  In this example, we assume
   that the intra-domain tunnels correspond to SRTE policies having red
   SRTE-policy-color and blue SRTE-policy-color.  Service labels are
   represented by S1 and S2.  In this example, we assume that the
   service advertisement corresponding to S1 carries the red extended-
   color community, while the service advertisement corresponding to S2
   carries the blue extended-color community.  By default, the Transport
   Class carried in the BGP-CT route target maps to the extend-color
   community as well as the SRTE-policy-color.  Therefore, based on the
   simple BGP-CT advertisment originated by PE2, PE1 is able to
   automatically steer traffic for service S1 over an end-to-end path
   made up of red SRTE policies in each domain.

   Note that this example focuses on how signalling originated by PE2
   results in forwarding state used by PE1 to reach PE2 on a specific
   Transport Class path.  The solution supports the establishment of
   forwarding state for an arbitrary number of PEs to reach PE2.  For
   example, PE3 in Figure 8 can reach PE2 on a red Transport Class path
   established using the same BGP-CT signalling.  The signalling and
   forwarding state from ASBR1 all the way to PE2 is common to the paths
   used by both PE1 and PE3.  This merging of signalling and forwarding
   state is essentially to the good scaling properties of the Seamless
   SR architecture.  Millions of end-to-end Transport Class paths can be
   established in a scalable manner.

5.3.  SLA Guarantee

5.3.1.  Low latency

   In a 5G network, many network functions are virtualized and
   distributed.  Certain functions are time and latency sensitive.
   Latency is one of the main SLA parameter for 5G networks.  In inter-
   domain networks, End-to-End latency measurement is required.  Inside
   a domain, latency measurement mechanisms such as TWAMP [RFC5357] are
   used and link latency is advertised in IGP using extensions described
   in [RFC8570]and [RFC7471] .

   [I-D.ietf-idr-performance-routing] extends the BGP AIGP attribute
   [RFC7311] by adding a sub TLV to carry an accumulated latency metric.
   The BGP best path selection algorithm used for a Transport Class
   requiring low latency will consider the accumulated latency metric to
   choose lowest latency path.






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5.3.2.  Traffic Engineering (TE) constraints

   TE constraints generally include the ability to send traffic via
   certain nodes or links or avoid using certain nodes or links.  In the
   Seamless SR architecture, the intra-domain transport technology is
   responsible for ensuring the TE constraints inside the domain, BGP-CT
   ensures that the end-to-end path is construct from intra-domain paths
   and inter-AS links that individually satisfy the TE constraints.

   For example, in order to construct a pair of diverse paths, we can
   define a red and a blue Transport Class.  Within each domain, the red
   and blue Transport Class path are realized using intra-domain path
   diversity mechansisms.  For example, in a domain using flex-algo, red
   and blue Transport Classes are realized using red and blue flex-algo
   which don't share any links.  To maintain path diversity on inter-AS
   links, BGP policies are used to associate two inter-AS peers with the
   red Transport Class and another two inter-AS peers with the blue
   Transport Class.

5.3.3.  Bandwidth constraints

   The Seamless SR architecture does not natively support end-to-end
   bandwidth reservations.  In this architecture, the bandwidth
   utilization characteristics of each domain are managed independently.
   The intra-domain bandwidth management can make use of a variety of
   tools.

   Link bandwidth extended community as defined in
   [I-D.ietf-idr-link-bandwidth] allows for efficient weighted load-
   balancing of traffic on multiple BGP-CT paths that belong to the same
   Transport Class.  For optimized path placement, a seperate tool may
   be deployed and BGP policies/communites used for path placement.

5.4.  Scalability

5.4.1.  Access node scalability

   The Seamless SR architecture needs to be able to accommodate very
   large numbers of access devices.  These access devices are expected
   to be low-end devices with limited FIB capacity.  The Seamless MPLS
   architecture, as described in [I-D.ietf-mpls-seamless-mpls],
   recommends the use of LDP DOD mode to limit the size of both the RIB
   and the FIB needed on the access devices.  In the Seamless SR
   architecture, networks use IGP based label distribution and do not
   have this selective label request mechanism.  However, RIB
   scalability of access nodes has not been a problem for real seamless
   MPLS deployments.  In cases where access devices are low on CPU and
   memory and unable to support large a RIB, BGP filtering policies can



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   be applied at the ABR/ASBR routers to restrict the number of BGP-CT
   advertisements towards the access devices.  The access devices will
   receive only the PE loopbacks that it needs to connect to.

5.4.2.  Label stack depth

   The ability for a device to push multiple MPLS labels on a packet
   depends on hardware capabilities.  Access devices are expected to
   have limited label stack push capabilities.  The Seamless SR
   architecture can provide cross-domain MPLS connectivity with a single
   label.  The access devices push one service label, one BGP-CT label,
   and one intra-domain transport label.  Assuming shortest path SR-MPLS
   in the access domain, the access domain transport will use a single
   label.  Light weight traffic-engineering and slicing could also be
   achieved with a single label as described in
   [I-D.ietf-lsr-flex-algo].  The access nodes will need to be able to
   push a minimum of 3 labels.

5.4.3.  Label Resources
































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               -----IBGP----- -----IBGP----- -----IBGP------
              |              |              |              |

                                                         BGP-CT Prefix:PE2
                                                         RD:2.2.2.2
                                                         RT: 128
                          Label:100       Label:100      Label:101
                          Next hop:ABR3   Next hop:ABR3  Next hop: PE2
        ----------------------------------------------------------------

                                          BGP-CT Prefix: ABR3
                                          RD:30.30.30.30
                                          RT:128
                        Label:200         Label:201
                        Nexthop:ABR1      Nexthop:ABR3

               +-----------+   +------------+  +-----------+
              /             \ /              \/             \
              |             ABR1            ABR3            |
              |              |               |              |
           PE1+    Metro1    +     Core      +    Metro2    +PE2
              |              |               |              |
              |             ABR2            ABR4            |
              \              /\             /\              /
               +------------+  +-----------+  +------------+


                 |-ISIS1-|      |-ISIS2-|       |-ISIS3-|

        +------+        +------+          +------+
        | 2000 |        |  201 |          | 101  |
        +------+        +------+          +------+
        | 200  |        | 100  |          | VPN  |
        +------+        +------+          + -----+
        | 100  |        | VPN  |
        +------+        +------+
        |vpn   |
        +------+


                   Figure 10: Recursive Route Resolution

   The label resources are an important consideration in MPLS networks.
   On access devices, labels are consumed by services as well as for
   transport loopbacks inside IGP domain where the access device
   resides.  For example, in the above diagram PE1 would have to
   allocate label resources equal to the number of customers connecting
   (i.e. the number of L2/L3 VPNs).  Based on the size of the IGP domain



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   that PE1 resides in, it will also have to allocate labels for IGP
   loopbacks.  This number is at most a few thousands.  So overall a
   typical access device should have adequate label resources in
   Seamless SR architecture.  The P routers need to allocate labels for
   IGP loopbacks.  This number again is small.  At most it will be a few
   thousand based on number of nodes in the largest IGP domains.  The
   metro networks connect to the core network through ABRs.  It is
   possible that a given ABR may end up having to maintain forwarding
   entries for a large subset of the transport loopback routes.  There
   may be a large number of metro networks connecting to a given ABR,
   and in this case, the ABR will need forwarding entries for every
   access node in the directly connected metros.  So, this ABR may have
   to maintain on the order of 100k routes.  With BGP-CT each Transport
   Class will have to be separately allocated a label.  So, in the above
   example, the ABR1 would have to use 300k labels if there were 3
   Transport Classes.  MPLS labels are 20 bit long and the label range
   of 16-1 million is available for general applications.This label
   space is shared between transport protocols and services.  However,
   in a well-designed network, ABRs are not expected to host service
   routes.  This leaves with 1 million labels completely available for
   transport infrastructure.  This is sufficient in most cases.

   In certain cases, it is desirable to reduce the forwarding state on
   the ABRs.  This reduction can be achieved with label stacking as a
   result of recursive route resolution.  In the Figure 10, PE2
   advertises a BGP-CT prefix with nexthop being PE2 and 101 label.
   ABR3 advertises a label 100 for this BGP-CT prefix and changes the
   nexthop to self.  When ABR1 receives this BGP-CT advertisement for
   PE2, it does not change the nexthop and advertises same label
   advertised by ABR3.  When PE1 receives the BGP-CT advetisement for
   PE2 with a nexthop of ABR3, it resolves on another BGP-CT prefix for
   ABR3.  As shown in the diagram, ABR3 advertises BGP-CT prefix with
   201 and ABR1 advertises label 200 and sets nexthop to self.  On PE1,
   the data packet consists of a VPN label at the bottom followed by 2
   BGP-CT labels 100 and 200.  The top most label 2000 is the transport
   label for the metro1 domain.  There is 1 additional BGP-CT label on
   the datapacket.

   Recursive route resolution provides significant forwarding state
   reduction on the ABRs.  ABRs have to allocate label resources for the
   PE loopback that they directly connect to.  This number is
   significantly lower as compared to the total number of PEs in the
   network.








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5.5.  Reliability

   Transport layer redundancy is very important in 5G networks.  Any
   link or node failure must be repaired with 50ms conevergence time. 50
   ms convergence time can be achieved with Fast ReRoute (FRR)
   mechanisms.  Seamless SR architecture supports Intra-domain link/node
   failures, Border node failures and the egress node and link failures
   for 50 ms convergence.  Details of the FRR techniques are described
   in below sections.

5.5.1.  Intra domain link and node protection

   In the seamless SR architecture, protection against node and link
   failure is achieved with the relevant FRR techniques for the
   corresponding transport mechanism used inside the domain.  In the
   case of an IP fabric, ECMP FRR or LFA can be used.  In SR networks,
   TI-LFA [I-D.ietf-rtgwg-segment-routing-ti-lfa] provides link and node
   protection.  For SR-TE [I-D.ietf-spring-segment-routing-policy]
   transport, link and node protection can be achieved using TI-LFA,
   combined with mechanisms described in
   [I-D.hegde-spring-node-protection-for-sr-te-paths].

5.5.2.  Egress Link and node protection

   [RFC8679] describes the mechanisms for providing protection for
   border nodes and PE devices where services are hosted.  The mechanism
   can be further simplified operationally with anycast SIDs and anycast
   service labels, as described in
   [I-D.hegde-rtgwg-egress-protection-sr-networks].

5.5.3.  Border Node protection

   Border node protection is very important in a network consisting of
   multiple domains.  Seamless SR architecture proposes to achieve 50ms
   FRR protection in the event of node failure with anycast address for
   the ABR/ASBRs and allocates same label for the BGP-CT Prefix.The
   detailed mechanism is described in
   [I-D.hegde-rtgwg-egress-protection-sr-networks].

5.6.  Operations

5.6.1.  MPLS ping and Traceroute

   Seamless SR Architecture is based on hierarchical network modeling.
   The End-to-end BGP-CT connectivity can be verified.  A new FEC is
   defined for BGP-CT as defined in draft
   [I-D.kaliraj-idr-bgp-classful-transport-planes] that describes End-
   to-End connectivity verification as well as fault isolation.  The



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   BGP-CT verification happens only on the BGP nodes.  The intra-domain
   connectivity verification and fault isolation will be based on the
   technology deployed in that domain as defined in [RFC8029] and
   [RFC8287].

5.6.2.  Counters and Statistics

   Traffic accounting and ability to build demand matrix for PE to PE
   traffic is very important.  With BGP-CT, per-label transit counters
   should be supported on every transit router. per-label transit
   counters provide details of total traffic towards a remote PE
   measured at every BGP transit router. per-label egress counter should
   be supported on ingress PE router. per-label egress counter provides
   total traffic from ingress PE to the specific remote PE.

5.7.  Service Mapping

   Service mapping is an imprtant aspect of any architecture.  It
   provides means to translate end users SLA requirements into
   operator's network configurations.  Seamless SR architecture supports
   automatic steering with extended color community.  The Transport
   Class and the route target carried by the BGP-CT advertisement
   directly map to the extended color community.  Services that require
   specific SLA carry the extended color community which maps to the
   Transport Class to which the BGP-CT advertisement belongs.

   Other types of traffic steering such as DSCP based forwarding is
   expressed with mapping-community.  Mapping community is a standard
   BGP community and is completely generic and user defined.  The
   mapping community will have a specific service mapping feature
   associated with it along with required fallback behaviour when the
   primary transport goes down.  The below list provides a general
   guideline into the different service mapping features and fallback
   options an implementation should provide.

      DSCP based mapping with each DSCP mapping to a Transport Class.

      DSCP based mapping with default mapping to a best-effort transport

      DSCP based mapping with fallback to best-effort when primary
      transport tunnel goes down.

      Extended color community based mapping with fallback to best
      effort

      Fallback options with specific protocol during migrations

      Falback options to a different Transport Class.



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      No Fallback permitted.

5.8.  Migrations

   Networks that migrate from Seamless MPLS architecture to Seamless SR
   architecture, require that all the border nodes and PE devices be
   upgraded and enable new family on the BGP session.  In cases where
   legacy nodes that cannot be upgraded exporting from BGP-LU into BGP-
   CT and vice versa SHOULD be supported.

5.9.  Interworking with v6 transport technologies

   A later version of this document will address interworking with other
   v6 technologies, including SRv6, SRm6, and MPLS over GRE6.

5.10.  BGP based Multicast

   BGP based multicast as described in draft
   [I-D.zzhang-bess-bgp-multicast] serves two main purposes.  It can
   replace PIM/ mLDP inside a domain to natively do a BGP based
   multicast.  It can also serve as an overlay stitching protocol to
   stitch multiple P2MP LSPs across the domain.  This gives the ability
   to easily transition each domain independently from one technology to
   the other.  BGP based multicast defines a new SAFI for carrying the
   MULTICAST TREE SAFI.  Different route types are defined to support
   the various usecases.

6.  Backward Compatibility

7.  Security Considerations

   TBD

8.  IANA Considerations

9.  Acknowledgements

   Many thanks to Kireeti Kompella, Ron Bonica, Krzysztof Szarcowitz,
   Srihari Salngi,Julian Lucek for discussions and inputs.

10.  Contributors

   1.Kaliraj Vairavakkalai

   Juniper Networks

   kaliraj@juniper.net




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   2.  Jeffrey Zhang

   Juniper Networks

   zzhang@juniper.net

11.  References

11.1.  Normative References

   [I-D.hegde-rtgwg-egress-protection-sr-networks]
              Hegde, S. and W. Lin, "Egress Protection for Segment
              Routing (SR) networks", draft-hegde-rtgwg-egress-
              protection-sr-networks-00 (work in progress), March 2020.

   [I-D.ietf-idr-performance-routing]
              Xu, X., Hegde, S., Talaulikar, K., Boucadair, M., and C.
              Jacquenet, "Performance-based BGP Routing Mechanism",
              draft-ietf-idr-performance-routing-02 (work in progress),
              October 2019.

   [I-D.kaliraj-idr-bgp-classful-transport-planes]
              Vairavakkalai, K., Venkataraman, N., and B. Rajagopalan,
              "BGP Classful Transport Planes", draft-kaliraj-idr-bgp-
              classful-transport-planes-00 (work in progress), May 2020.

   [I-D.zzhang-bess-bgp-multicast]
              Zhang, Z., Giuliano, L., Patel, K., Wijnands, I., mishra,
              m., and A. Gulko, "BGP Based Multicast", draft-zzhang-
              bess-bgp-multicast-03 (work in progress), October 2019.

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

   [RFC3107]  Rekhter, Y. and E. Rosen, "Carrying Label Information in
              BGP-4", RFC 3107, DOI 10.17487/RFC3107, May 2001,
              <https://www.rfc-editor.org/info/rfc3107>.

   [RFC8669]  Previdi, S., Filsfils, C., Lindem, A., Ed., Sreekantiah,
              A., and H. Gredler, "Segment Routing Prefix Segment
              Identifier Extensions for BGP", RFC 8669,
              DOI 10.17487/RFC8669, December 2019,
              <https://www.rfc-editor.org/info/rfc8669>.






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11.2.  Informative References

   [I-D.hegde-spring-node-protection-for-sr-te-paths]
              Hegde, S., Bowers, C., Litkowski, S., Xu, X., and F. Xu,
              "Node Protection for SR-TE Paths", draft-hegde-spring-
              node-protection-for-sr-te-paths-05 (work in progress),
              July 2019.

   [I-D.ietf-idr-link-bandwidth]
              Mohapatra, P. and R. Fernando, "BGP Link Bandwidth
              Extended Community", draft-ietf-idr-link-bandwidth-07
              (work in progress), March 2018.

   [I-D.ietf-idr-tunnel-encaps]
              Patel, K., Velde, G., and S. Ramachandra, "The BGP Tunnel
              Encapsulation Attribute", draft-ietf-idr-tunnel-encaps-15
              (work in progress), December 2019.

   [I-D.ietf-lsr-flex-algo]
              Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
              A. Gulko, "IGP Flexible Algorithm", draft-ietf-lsr-flex-
              algo-08 (work in progress), July 2020.

   [I-D.ietf-mpls-seamless-mpls]
              Leymann, N., Decraene, B., Filsfils, C., Konstantynowicz,
              M., and D. Steinberg, "Seamless MPLS Architecture", draft-
              ietf-mpls-seamless-mpls-07 (work in progress), June 2014.

   [I-D.ietf-rtgwg-segment-routing-ti-lfa]
              Litkowski, S., Bashandy, A., Filsfils, C., Decraene, B.,
              Francois, P., Voyer, D., Clad, F., and P. Camarillo,
              "Topology Independent Fast Reroute using Segment Routing",
              draft-ietf-rtgwg-segment-routing-ti-lfa-03 (work in
              progress), March 2020.

   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Sivabalan, S., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", draft-
              ietf-spring-segment-routing-policy-07 (work in progress),
              May 2020.

   [I-D.voyer-pim-sr-p2mp-policy]
              Voyer, D., Filsfils, C., Parekh, R., Bidgoli, H., and Z.
              Zhang, "Segment Routing Point-to-Multipoint Policy",
              draft-voyer-pim-sr-p2mp-policy-02 (work in progress), July
              2020.





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   [RFC1997]  Chandra, R., Traina, P., and T. Li, "BGP Communities
              Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996,
              <https://www.rfc-editor.org/info/rfc1997>.

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

   [RFC5357]  Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
              Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
              RFC 5357, DOI 10.17487/RFC5357, October 2008,
              <https://www.rfc-editor.org/info/rfc5357>.

   [RFC6388]  Wijnands, IJ., Ed., Minei, I., Ed., Kompella, K., and B.
              Thomas, "Label Distribution Protocol Extensions for Point-
              to-Multipoint and Multipoint-to-Multipoint Label Switched
              Paths", RFC 6388, DOI 10.17487/RFC6388, November 2011,
              <https://www.rfc-editor.org/info/rfc6388>.

   [RFC7311]  Mohapatra, P., Fernando, R., Rosen, E., and J. Uttaro,
              "The Accumulated IGP Metric Attribute for BGP", RFC 7311,
              DOI 10.17487/RFC7311, August 2014,
              <https://www.rfc-editor.org/info/rfc7311>.

   [RFC7471]  Giacalone, S., Ward, D., Drake, J., Atlas, A., and S.
              Previdi, "OSPF Traffic Engineering (TE) Metric
              Extensions", RFC 7471, DOI 10.17487/RFC7471, March 2015,
              <https://www.rfc-editor.org/info/rfc7471>.

   [RFC8029]  Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N.,
              Aldrin, S., and M. Chen, "Detecting Multiprotocol Label
              Switched (MPLS) Data-Plane Failures", RFC 8029,
              DOI 10.17487/RFC8029, March 2017,
              <https://www.rfc-editor.org/info/rfc8029>.

   [RFC8287]  Kumar, N., Ed., Pignataro, C., Ed., Swallow, G., Akiya,
              N., Kini, S., and M. Chen, "Label Switched Path (LSP)
              Ping/Traceroute for Segment Routing (SR) IGP-Prefix and
              IGP-Adjacency Segment Identifiers (SIDs) with MPLS Data
              Planes", RFC 8287, DOI 10.17487/RFC8287, December 2017,
              <https://www.rfc-editor.org/info/rfc8287>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.





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   [RFC8570]  Ginsberg, L., Ed., Previdi, S., Ed., Giacalone, S., Ward,
              D., Drake, J., and Q. Wu, "IS-IS Traffic Engineering (TE)
              Metric Extensions", RFC 8570, DOI 10.17487/RFC8570, March
              2019, <https://www.rfc-editor.org/info/rfc8570>.

   [RFC8679]  Shen, Y., Jeganathan, M., Decraene, B., Gredler, H.,
              Michel, C., and H. Chen, "MPLS Egress Protection
              Framework", RFC 8679, DOI 10.17487/RFC8679, December 2019,
              <https://www.rfc-editor.org/info/rfc8679>.

   [TS.23.501-3GPP]
              3rd Generation Partnership Project (3GPP), "System
              Architecture for 5G System; Stage 2, 3GPP TS 23.501
              v16.4.0", March 2020.

Authors' Addresses

   Shraddha Hegde
   Juniper Networks Inc.
   Exora Business Park
   Bangalore, KA  560103
   India

   Email: shraddha@juniper.net


   Chris Bowers
   Juniper Networks Inc.

   Email: cbowers@juniper.net


   Xiaohu Xu
   Alibaba Inc.
   Beijing
   China

   Email: xiaohu.xxh@alibaba-inc.com


   Arkadiy Gulko
   Refinitiv

   Email: arkadiy.gulko@refinitiv.com







Hegde, et al.           Expires January 13, 2021               [Page 26]


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