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SPRING                                                       C. Filsfils
Internet-Draft                                         P. Camarillo, Ed.
Intended status: Informational                       Cisco Systems, Inc.
Expires: March 29, 2021                                            Z. Li
                                                     Huawei Technologies
                                                           S. Matsushima
                                                                SoftBank
                                                             B. Decraene
                                                                  Orange
                                                            D. Steinberg
                                           Lapishills Consulting Limited
                                                               D. Lebrun
                                                                  Google
                                                               R. Raszuk
                                                            Bloomberg LP
                                                                J. Leddy
                                                  Individual Contributor
                                                      September 25, 2020


               Illustrations for SRv6 Network Programming
           draft-filsfils-spring-srv6-net-pgm-illustration-03

Abstract

   This document illustrates how SRv6 Network Programming
   [I-D.ietf-spring-srv6-network-programming] can be used to create
   interoperable and protected overlays with underlay optimization and
   service programming.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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/.




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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 29, 2021.

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
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Illustration  . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Simplified SID allocation . . . . . . . . . . . . . . . .   3
     2.2.  Reference diagram . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Basic security  . . . . . . . . . . . . . . . . . . . . .   5
     2.4.  SR-L3VPN  . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.5.  SR-Ethernet-VPWS  . . . . . . . . . . . . . . . . . . . .   6
     2.6.  SR-EVPN-FXC . . . . . . . . . . . . . . . . . . . . . . .   7
     2.7.  SR-EVPN . . . . . . . . . . . . . . . . . . . . . . . . .   7
       2.7.1.  EVPN Bridging . . . . . . . . . . . . . . . . . . . .   7
       2.7.2.  EVPN Multi-homing with ESI filtering  . . . . . . . .   9
       2.7.3.  EVPN Layer-3  . . . . . . . . . . . . . . . . . . . .  11
       2.7.4.  EVPN Integrated Routing Bridging (IRB)  . . . . . . .  11
     2.8.  SR TE for Underlay SLA  . . . . . . . . . . . . . . . . .  12
       2.8.1.  SR policy from the Ingress PE . . . . . . . . . . . .  12
       2.8.2.  SR policy at a midpoint . . . . . . . . . . . . . . .  13
     2.9.  End-to-End policy with intermediate BSID  . . . . . . . .  14
     2.10. TI-LFA  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     2.11. SR TE for Service programming . . . . . . . . . . . . . .  16
   3.  Benefits  . . . . . . . . . . . . . . . . . . . . . . . . . .  18
     3.1.  Seamless deployment . . . . . . . . . . . . . . . . . . .  18
     3.2.  Integration . . . . . . . . . . . . . . . . . . . . . . .  19
     3.3.  Security  . . . . . . . . . . . . . . . . . . . . . . . .  19
   4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19



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   5.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  19
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   Segment Routing leverages the source routing paradigm.  An ingress
   node steers a packet through a ordered list of instructions, called
   segments.  Each one of these instructions represents a function to be
   called at a specific location in the network.  A function is locally
   defined on the node where it is executed and may range from simply
   moving forward in the segment list to any complex user-defined
   behavior.  The network programming consists in combining segment
   routing functions, both simple and complex, to achieve a networking
   objective that goes beyond mere packet routing.

   [I-D.ietf-spring-srv6-network-programming] defines the SRv6 Network
   Programming concept and the main segment routing behaviors.

   This document illustrates how these concepts can be used to enable
   the creation of interoperable overlays with underlay optimization and
   service programming.

   The terminology for this document is defined in
   [I-D.ietf-spring-srv6-network-programming].

2.  Illustration

   We introduce a simplified SID allocation technique to ease the
   reading of the text.  We document the reference diagram.  We then
   illustrate the network programming concept through different use-
   cases.  These use-cases have been thought to allow straightforward
   combination between each other.

2.1.  Simplified SID allocation

   To simplify the illustration, we assume:

      2001:db8::/32 is an IPv6 block allocated by a RIR to the operator

      2001:db8:0::/48 is dedicated to the internal address space

      2001:db8:cccc::/48 is dedicated to the internal SRv6 SID space

      We assume a location expressed in 64 bits and a function expressed
      in 16 bits





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      Node k has a classic IPv6 loopback address 2001:db8::k/128 which
      is advertised in the IGP

      Node k has 2001:db8:cccc:k::/64 for its local SID space.  Its SIDs
      will be explicitly assigned from that block

      Node k advertises 2001:db8:cccc:k::/64 in its IGP

      Function :1:: (function 1, for short) represents the End function
      with PSP support

      Function :C2:: (function C2, for short) represents the End.X
      function towards neighbor 2

   Each node k has:

      An explicit SID instantiation 2001:db8:cccc:k:1::/128 bound to an
      End function with additional support for PSP

      An explicit SID instantiation 2001:db8:cccc:k:Cj::/128 bound to an
      End.X function to neighbor J with additional support for PSP

2.2.  Reference diagram

   Let us assume the following topology where all the links have IGP
   metric 10 except the link 3-4 which is 100.

   Nodes A, B and 1 to 8 are considered within the network domain while
   nodes CE-A, CE-B and CE-C are outside the domain.

                        CE-B
                           \
                            3------4---5
                            |       \ /
                            |        6
                            |       /
                    A--1--- 2------7---8--B
                      /                 \
                   CE-A                 CE-C
                Tenant100            Tenant100 with
                                       IPv4 203.0.113.0/24

                       Figure 1: Reference topology








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2.3.  Basic security

   Any edge node such as 1 would be configured with an ACL on any of its
   external interface (e.g. from CE-A) which drops any traffic with SA
   or DA in 2001:db8:cccc::/16.  See SEC-1.

   Any core node such as 6 could be configured with an ACL with the
   SEC-2 behavior "IF (DA == LocalSID) && (SA is not in 2001:db8:0::/48
   or 2001:db8:cccc::/16) THEN drop".

   SEC-3 protection is a default property of SRv6.  A SID must be
   explicitly instantiated.  In our illustration, the only available
   SIDs are those explicitly instantiated.

2.4.  SR-L3VPN

   Let us illustrate the SR-L3VPN use-case applied to IPv4.

   Nodes 1 and 8 are configured with a tenant 100, each respectively
   connected to CE-A and CE-C.

   Node 8 is configured with a locally instantiated End.DT4 SID
   2001:db8:cccc:8:D100:: bound to tenant IPv4 table 100.

   Via BGP signaling or an SDN-based controller, Node 1's tenant-100
   IPv4 table is programmed with an IPv4 SR-VPN route 203.0.113.0/24 via
   SRv6 policy <2001:db8:cccc:8:D100::>.

   When 1 receives a packet P from CE-A destined to 203.0.113.20, 1
   looks up 203.0.113.20 in its tenant-100 IPv4 table and finds an SR-
   VPN entry 203.0.113.0/24 via SRv6 policy <2001:db8:cccc:8:D100::>.
   As a consequence, 1 pushes an outer IPv6 header with SA=2001:db8::1,
   DA=2001:db8:cccc:8:D100:: and NH=4. 1 then forwards the resulting
   packet on the shortest path to 2001:db8:cccc:8::/64.

   When 8 receives the packet, 8 matches the DA in its "My SID Table",
   finds the bound function End.DT4(100) and confirms NH=4.  As a
   result, 8 decaps the outer header, looks up the inner IPv4 DA in
   tenant-100 IPv4 table, and forward the (inner) IPv4 packet towards
   CE-C.

   The reader can easily infer all the other SR-IPVPN instantiations:









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  +---------------------------------+----------------------------------+
  | Route at ingress PE(1)          | SR-VPN Egress SID of egress PE(8)|
  +---------------------------------+----------------------------------+
  | IPv4 tenant route with egress   | End.DT4 function bound to        |
  | tenant table lookup             | IPv4-tenant-100 table            |
  +---------------------------------+----------------------------------+
  | IPv4 tenant route without egress| End.DX4 function bound to        |
  | tenant table lookup             | CE-C (IPv4)                      |
  +---------------------------------+----------------------------------+
  | IPv6 tenant route with egress   | End.DT6 function bound to        |
  | tenant table lookup             | IPv6-tenant-100 table            |
  +---------------------------------+----------------------------------+
  | IPv6 tenant route without egress| End.DX6 function bound to        |
  | tenant table lookup             | CE-C (IPv6)                      |
  +---------------------------------+----------------------------------+

2.5.  SR-Ethernet-VPWS

   Let us illustrate the SR-Ethernet-VPWS use-case.

   Node 8 is configured a locally instantiated End.DX2 SID
   2001:db8:cccc:8:DC2C:: bound to local attachment circuit {ethernet
   CE-C}.

   Via BGP signalling or an SDN controller, node 1 is programmed with an
   Ethernet VPWS service for its local attachment circuit {ethernet CE-
   A} with remote endpoint 2001:db8:cccc:8:DC2C::.

   When 1 receives a frame F from CE-A, node 1 pushes an outer IPv6
   header with SA=2001:db8::1, DA=2001:db8:cccc:8:DC2C:: and NH=59.
   Note that no additional header is pushed. 1 then forwards the
   resulting packet on the shortest path to 2001:db8:cccc:8::/64.

   When 8 receives the packet, 8 matches the DA in its "My SID Table"
   and finds the bound function End.DX2.  After confirming that next-
   header=59, 8 decaps the outer IPv6 header and forwards the inner
   Ethernet frame towards CE-C.

   The reader can easily infer the Ethernet VPWS use-case:

      +------------------------+-----------------------------------+
      | Route at ingress PE(1) | SR-VPN Egress SID of egress PE(8) |
      +------------------------+-----------------------------------+
      | Ethernet VPWS          | End.DX2 function bound to         |
      |                        | CE-C (Ethernet)                   |
      +------------------------+-----------------------------------+





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2.6.  SR-EVPN-FXC

   Let us illustrate the SR-EVPN-FXC use-case (Flexible cross-connect
   service).

   Node 8 is configured with a locally instantiated End.DX2V SID
   2001:db8:cccc:8:DC2C:: bound to the L2 table T1.  Node 8 is also
   configured with local attachment circuits {ethernet CE1-C VLAN:100}
   and {ethernet CE2-C VLAN:200} in table T1.

   Via an SDN controller or derived from a BGP-based sginalling, the
   node 1 is programmed with an EVPN-FXC service for its local
   attachment circuit {ethernet CE-A} with remote endpoint
   2001:db8:cccc:8:DC2C::. For this purpose, the EVPN Type-1 route is
   used.

   When node 1 receives a frame F from CE-A, it pushes an outer IPv6
   header with SA=2001:db8::1, DA=2001:db8:cccc:8:DC2C:: and NH=59.
   Note that no additional header is pushed.  Node 1 then forwards the
   resulting packet on the shortest path to 2001:db8:cccc:8::/64.

   When node 8 receives the packet, it matches the IP DA in its "My SID
   Table" and finds the bound function End.DX2V.  After confirming that
   next-header=59, node 8 decaps the outer IPv6 header, performs a VLAN
   loopkup in table T1 and forwards the inner Ethernet frame to matching
   interface e.g. for VLAN 100, packet is forwarded to CE1-C and for
   VLAN 200, frame is forwarded to CE2-C.

   The reader can easily infer the Ethernet FXC use-case:

+---------------------------------+------------------------------------+
| Route at ingress PE (1)         | SR-VPN Egress SID of egress PE (8) |
+---------------------------------+------------------------------------+
| EVPN-FXC                        | End.DX2V function bound to         |
|                                 | CE1-C / CE2-C (Ethernet)           |
+---------------------------------+------------------------------------+

2.7.  SR-EVPN

   The following section details some of the particular use-cases of SR-
   EVPN.  In particular bridging (unicast and multicast), multi-homing
   ESI filtering, L3 EVPN and EVPN-IRB.

2.7.1.  EVPN Bridging

   Let us illustrate the SR-EVPN unicast and multicast bridging.





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   Nodes 1, 3 and 8 are configured with a EVPN bridging service (E-LAN
   service).

   Node 1 is configured with a locally instantiated End.DT2U SID
   2001:db8:cccc:1:D2AA:: bound to a local L2 table T1 where EVPN is
   enabled.  This SID will be used to attract unicast traffic.
   Additionally, Node 1 is configured with a locally instantiated
   End.DT2M SID 2001:db8:cccc:1:D2AF:: bound to the same local L2 table
   T1.  This SID will be used to attract multicast traffic.  Node 1 is
   also configured with local attachment circuit {ethernet CE-A
   VLAN:100} associated to table T1.

   A similar instantiation is done at Node 4 and Node 8 resulting in:

   - Node 1 - My SID table:

     - End.DT2U SID: 2001:db8:cccc:1:D2AA:: table T1

     - End.DT2M SID: 2001:db8:cccc:1:D2AF:: table T1

   - Node 3 - My SID table:

     - End.DT2U SID: 2001:db8:cccc:3:D2BA:: table T3

     - End.DT2M SID: 2001:db8:cccc:3:D2BF:: table T3

   - Node 8 - My SID table:

     - End.DT2U SID: 2001:db8:cccc:8:D2CA:: table T8

     - End.DT2M SID: 2001:db8:cccc:8:D2CF:: table T8

   Nodes 1, 4 and 8 are going to exchange the End.DT2M SIDs via BGP-
   based EVPN Type-3 route.  Upon reception of the EVPN Type-3 routes,
   each node build its own replication list per L2 table that will be
   used for ingress BUM traffic replication.  The replication lists are
   the following:

   - Node 1 - replication list: {2001:db8:cccc:3:D2BF:: and
     2001:db8:cccc:8:D2CF::}

   - Node 3 - replication list: {2001:db8:cccc:1:D2AF:: and
     2001:db8:cccc:8:D2CF::}

   - Node 8 - replication list: {2001:db8:cccc:1:D2AF:: and
     2001:db8:cccc:3:D2CF::}





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   When node 1 receives a BUM frame F from CE-A, it replicates that
   frame to every node in the replication list.  For node 3, it pushes
   an outer IPv6 header with SA=2001:db8::1, DA=2001:db8:cccc:3:D2BF::
   and NH=59.  For node 8, it performs the same operation but
   DA=2001:db8:cccc:8:D2CF::. Note that no additional headers are
   pushed.  Node 1 then forwards the resulting packets on the shortest
   path for each destination.

   When node 3 receives the packet, it matches the DA in its "My SID
   Table" and finds the bound function End.DT2M with its related layer2
   table T3.  After confirming that next-header=59, node 3 decaps the
   outer IPv6 header and forwards the inner Ethernet frame to all
   layer-2 output interface found in table T3.  Similar processing is
   also performed by node 8 upon packet reception.  This example is the
   same for any BUM stream coming from CE-B or CE-C.


   Node 1,3 and 8 are also performing software MAC learning to exchange
   MAC reachability information (unicast traffic) via BGP among
   themselves.

   Each MAC being learnt is exchanged using BGP-based EVPN Type-2 route.

   When node 1 receives an unicast frame F from CE-A, it learns its MAC-
   SA=CEA in software.  Node 1 transmits that MAC and its associated SID
   2001:db8:cccc:1:D2AA:: using BGP-based EVPN route-type 2 to all
   remote nodes.

   When node 3 receives an unicast frame F from CE-B destinated to MAC-
   DA=CEA, it performs a L2 lookup on T3 to find the associated SID.  It
   pushes an outer IPv6 header with SA=2001:db8::3,
   DA=2001:db8:cccc:1:D2AA:: and NH=59.  Node 3 then forwards the
   resulting packet on the shortest path to 2001:db8:cccc:1::/64.
   Similar processing is also performed by node 8.

2.7.2.  EVPN Multi-homing with ESI filtering

   In L2 network, support for traffic loop avoidance is mandatory.  In
   EVPN all-active multi-homing scenario enforces that requirement using
   ESI filtering.  Let us illustrate how it works:

   Nodes 3 and 4 are peering partners of a redundancy group where the
   access CE-B, is connected in an all-active multi-homing way with
   these two nodes.  Hence, the topology is the following:







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                              CE-B
                             /    \
                            3------4---5
                            |       \ /
                            |        6
                            |       /
                    A--1--- 2------7---8--B
                      /                 \
                   CE-A                 CE-C
                Tenant100            Tenant100 with
                                       IPv4 203.0.113.0/24

                  EVPN ESI filtering - Reference topology

   Nodes 3 and 4 are configured with an EVPN bridging service (E-LAN
   service).

   Node 3 is configured with a locally instantiated End.DT2M SID
   2001:db8:cccc:3:D2BF:: bound to a local L2 table T1 where EVPN is
   enabled.  This SID is also configured with the optional argument
   Arg.FE2 that specifies the attachment circuit.  Particularly, node 3
   assigns identifier 0xC1 to {ethernet CE-B}.

   Node 4 is configured with a locally instantiated End.DT2M SID
   2001:db8:cccc:4:D2BF:: bound to a local L2 table T1 where EVPN is
   enabled.  This SID is also configured with the optional argument
   Arg.FE2 that specifies the attachment circuit.  Particularly, node 3
   assigns identifier 0xC2 to {ethernet CE-B}.

   Both End.DT2M SIDs are exchanged between nodes via BGP-based EVPN
   Type-3 routes.  Upon reception of EVPN Type-3 routes, each node build
   its own replication list per L2 table T1.

   On the other hand, the End.DT2M SID arguments (Arg.F2) are exchanged
   between nodes via SRv6 VPN SID attached to the BGP-based EVPN Type-1
   route.  The BGP ESI-filtering extended community label is set to
   implicit-null [I-D.ietf-bess-srv6-services].

   Upon reception of EVPN Type-1 route and Type-3 route, node 3 merges
   merges the End.DT2M SID (2001:db8:cccc:4:D2BF:) with the
   Arg.FE2(0:0:0:C2::) from node 4 (its peering partner).  This is done
   by a simple OR bitwise operation.  As a result, the replication list
   on node 3 for the PEs 3,4 and 8 is: {2001:db8:cccc:1:D2AF::;
   2001:db8:cccc:4:D2BF:C2::; 2001:db8:cccc:8:D2CF::}.

   In a similar manner, the replication list on node 4 for the PEs 1,3
   and 8 is: {2001:db8:cccc:1:D2AF::; 2001:db8:cccc:3:D2BF:C1::;
   2001:db8:cccc:8:D2CF::}. Note that in this case the SID for PE3



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   contains the OR bitwise operation of SIDs 2001:db8:cccc:3:D2BF:: and
   0:0:0:C1::.

   When node 3 receives a BUM frame F from CE-B, it replicates that
   frame to remote PEs.  For node 4, it pushes an outer IPv6 header with
   SA=2001:db8::1, DA=2001:db8:cccc:4:D2AF:C2:: and NH=59.  Note that no
   additional header is pushed.  Node 3 then forwards the resulting
   packet on the shortest path to node 4, and once the packet arrives to
   node 4, the End.DT2M function is executed forwarding to all L2 OIFs
   except the ones corresponding to identifier 0xC2.

2.7.3.  EVPN Layer-3

   EVPN layer-3 works exactly in the same way than L3VPN.  Please refer
   to section Section 2.4

2.7.4.  EVPN Integrated Routing Bridging (IRB)

   EVPN IRB brings Layer-2 and Layer-3 together.  It uses BGP-based EVPN
   Type-2 route to achieve Layer-2 intra-subnet and Layer-3 inter-subnet
   forwarding.  The EVPN Type-2 route-2 maintains the MAC/IP
   association.

   Node 8 is configured with a locally instantiated End.DT2U SID
   2001:db8:cccc:8:D2C:: used for unicast L2 traffic.  Node 8 is also
   configured with locally instantiated End.DT4 SID
   2001:db8:cccc:8:D100:: bound to IPv4 tenant table 100.

   Node 1 is going to be configured with the EVPN IRB service.

   Node 8 signals to other remote PEs (1, 3) each ARP/ND request learned
   via BGP-based EVPN Type-2 route.  For example, when node 8 receives
   an ARP/ND packet P from a host (203.0.113.20) on CE-C destined to
   192.0.2.10, it learns its MAC-SA=CEC in software.  It also learns the
   ARP/ND entry (IP SA=203.0.113.20) in its cache.  Node 8 transmits
   that MAC/IP and its associated L3 SID (2001:db8:cccc:8:D100::) and L2
   SID (2001:db8:cccc:8:D2C::).

   When node 1 receives a packet P from CE-A destined to 203.0.113.20
   from a host (192.0.2.10), node 1 looks up its tenant-100 IPv4 table
   and finds an SR-VPN entry for that prefix.  As a consequence, node 1
   pushes an outer IPv6 header with SA=2001:db8::1,
   DA=2001:db8:cccc:8:D100:: and NH=4.  Node 1 then forwards the
   resulting packet on the shortest path to 2001:db8:cccc:8::/64.  EVPN
   inter-subnet forwarding is then achieved.

   When node 1 receives a packet P from CE-A destined to 203.0.113.20
   from a host (192.0.2.11), P looks up its L2 table T1 MAC-DA lookup to



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   find the associated SID.  It pushes an outer IPv6 header with
   SA=2001:db8::1, DA=2001:db8:cccc:8:D2C:: and NH=59.  Note that no
   additional header is pushed.  Node 8 then forwards the resulting
   packet on the shortest path to 2001:db8:cccc:8::/64.  EVPN intra-
   subnet forwarding is then achieved.

2.8.  SR TE for Underlay SLA

2.8.1.  SR policy from the Ingress PE

   Let's assume that node 1's tenant-100 IPv4 route "203.0.113.0/24 via
   2001:db8:cccc:8:D100::" is programmed with a color/community that
   requires low-latency underlay optimization
   [I-D.ietf-spring-segment-routing-policy].

   In such case, node 1 either computes the low-latency path to the
   egress node itself or delegates the computation to a PCE.

   In either case, the location of the egress PE can easily be found by
   looking for who originates the locator comprising the SID
   2001:db8:cccc:8:D100::. This can be found in the IGP's LSDB for a
   single domain case, and in the BGP-LS LSDB for a multi-domain case.

   Let us assume that the TE metric encodes the per-link propagation
   latency.  Let us assume that all the links have a TE metric of 10,
   except link 27 which has TE metric 100.

   The low-latency path from 1 to 8 is thus 1234678.

   This path is encoded in a SID list as: first a hop through
   2001:db8:cccc:3:C4:: and then a hop to 8.

   As a consequence the SR-VPN entry 203.0.113.0/24 installed in the
   Node1's Tenant-100 IPv4 table is: H.Encaps with SRv6 Policy
   <2001:db8:cccc:3:C4::, 2001:db8:cccc:8:D100::>.

   When 1 receives a packet P from CE-A destined to 203.0.113.20, P
   looks up its tenant-100 IPv4 table and finds an SR-VPN entry
   203.0.113.0/24.  As a consequence, 1 pushes an outer header with
   SA=2001:db8::1, DA=2001:db8:cccc:3:C4::, NH=SRH followed by SRH
   (2001:db8:cccc:8:D100::, 2001:db8:cccc:3:C4::; SL=1; NH=4). 1 then
   forwards the resulting packet on the interface to 2.

   2 forwards to 3 along the path to 2001:db8:cccc:3::/64.

   When 3 receives the packet, 3 matches the DA in its "My SID Table"
   and finds the bound function End.X to neighbor 4. 3 notes the PSP
   capability of the SID 2001:db8:cccc:3:C4::. 3 sets the DA to the next



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   SID 2001:db8:cccc:8:D100::. As 3 is the penultimate segment hop, it
   performs PSP and pops the SRH. 3 forwards the resulting packet to 4.

   4, 6 and 7 forwards along the path to 2001:db8:cccc:8::/64.

   When 8 receives the packet, 8 matches the DA in its "My SID Table"
   and finds the bound function End.DT(100).  As a result, 8 decaps the
   outer header, looks up the inner IPv4 DA (203.0.113.20) in tenant-100
   IPv4 table, and forward the (inner) IPv4 packet towards CE-B.

2.8.2.  SR policy at a midpoint

   Let us analyze a policy applied at a midpoint on a packet without
   SRH.

   Packet P1 is (2001:db8::1, 2001:db8:cccc:8:D100::).

   Let us consider P1 when it is received by node 2 and let us assume
   that that node 2 is configured to steer 2001:db8:cccc:8::/64 in a
   T.Insert behavior associated with SR policy <2001:db8:cccc:3:C4::>.

   In such a case, node 2 would send the following modified packet P1 on
   the link to 3:

   (2001:db8::1, 2001:db8:cccc:3:C4::)(2001:db8:cccc:8:D100::,
   2001:db8:cccc:3:C4::; SL=1).

   The rest of the processing is similar to the previous section.


   Let us analyze a policy applied at a midpoint on a packet with an
   SRH.

   Packet P2 is (2001:db8::1,
   2001:db8:cccc:7:1::)(2001:db8:cccc:8:D100::, 2001:db8:cccc:7:1::;
   SL=1).

   Let us consider P2 when it is received by node 2 and let us assume
   that node 2 is configured to steer 2001:db8:cccc:7::/64 in a T.Insert
   behavior associated with SR policy <2001:db8:cccc:3:C4::,
   2001:db8:cccc:5:1::>.

   In such a case, node 2 would send the following modified packet P2 on
   the link to 4:

   (2001:db8::1, 2001:db8:cccc:3:C4::)(2001:db8:cccc:7:1::,
   2001:db8:cccc:5:1::, 2001:db8:cccc:3:C4::;
   SL=2)(2001:db8:cccc:8:D100::, 2001:db8:cccc:7:1::; SL=1)



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   Node 3 would send the following packet to 4: (2001:db8::1,
   2001:db8:cccc:5:1::)(2001:db8:cccc:6:1::, 2001:db8:cccc:5:1::,
   2001:db8:cccc:3:C4::; SL=1)(2001:db8:cccc:8:D100::,
   2001:db8:cccc:7:1::; SL=1)

   Node 4 would send the following packet to 5: (2001:db8::1,
   2001:db8:cccc:5:1::)(2001:db8:cccc:6:1::, 2001:db8:cccc:5:1::,
   2001:db8:cccc:3:C4::; SL=1)(2001:db8:cccc:8:D100::,
   2001:db8:cccc:7:1::; SL=1)

   Node 5 would send the following packet to 6: (2001:db8::1,
   2001:db8:cccc:7:1::)(2001:db8:cccc:8:D100::, 2001:db8:cccc:7:1::;
   SL=1)

   Node 6 would send the following packet to 7: (2001:db8::1,
   2001:db8:cccc:7:1::)(2001:db8:cccc:8:D100::, 2001:db8:cccc:7:1::;
   SL=1)

   Node 7 would send the following packet to 8: (2001:db8::1,
   2001:db8:cccc:8:D100::)

2.9.  End-to-End policy with intermediate BSID

   Let us now describe a case where the ingress VPN edge node steers the
   packet destined to 203.0.113.20 towards the egress edge node
   connected to the tenant100 site with 203.0.113.0/24, but via an
   intermediate SR Policy represented by a single routable Binding SID.
   Let us illustrate this case with an intermediate policy which both
   encodes underlay optimization for low-latency and the service
   programming via two SR-aware container-based apps.

   Let us assume that the End.B6.Insert SID 2001:db8:cccc:2:B1:: is
   configured at node 2 and is associated with midpoint SR policy
   <2001:db8:cccc:3:C4::, 2001:db8:cccc:9:A1::, 2001:db8:cccc:6:A2::>.

   2001:db8:cccc:3:C4:: realizes the low-latency path from the ingress
   PE to the egress PE.  This is the underlay optimization part of the
   intermediate policy.

   2001:db8:cccc:9:A1:: and 2001:db8:cccc:6:A2:: represent two SR-aware
   NFV applications residing in containers respectively connected to
   node 9 and 6.

   Let us assume the following ingress VPN policy for 203.0.113.0/24 in
   tenant 100 IPv4 table of node 1: H.Encaps with SRv6 Policy
   <2001:db8:cccc:2:B1::, 2001:db8:cccc:8:D100::>.





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   This ingress policy will steer the 203.0.113.0/24 tenant-100 traffic
   towards the correct egress PE and via the required intermediate
   policy that realizes the SLA and NFV requirements of this tenant
   customer.

   Node 1 sends the following packet to 2: (2001:db8::1,
   2001:db8:cccc:2:B1::) (2001:db8:cccc:8:D100::, 2001:db8:cccc:2:B1::;
   SL=1)

   Node 2 sends the following packet to 4: (2001:db8::1,
   2001:db8:cccc:3:C4::) (2001:db8:cccc:6:A2::, 2001:db8:cccc:9:A1::,
   2001:db8:cccc:3:C4::; SL=2)(2001:db8:cccc:8:D100::,
   2001:db8:cccc:2:B1::; SL=1)

   Node 4 sends the following packet to 5: (2001:db8::1,
   2001:db8:cccc:9:A1::) (2001:db8:cccc:6:A2::, 2001:db8:cccc:9:A1::,
   2001:db8:cccc:3:C4::; SL=1)(2001:db8:cccc:8:D100::,
   2001:db8:cccc:2:B1::; SL=1)

   Node 5 sends the following packet to 9: (2001:db8::1,
   2001:db8:cccc:9:A1::) (2001:db8:cccc:6:A2::, 2001:db8:cccc:9:A1::,
   2001:db8:cccc:3:C4::; SL=1)(2001:db8:cccc:8:D100::,
   2001:db8:cccc:2:B1::; SL=1)

   Node 9 sends the following packet to 6: (2001:db8::1,
   2001:db8:cccc:6:A2::) (2001:db8:cccc:8:D100::, 2001:db8:cccc:2:B1::;
   SL=1)

   Node 6 sends the following packet to 7: (2001:db8::1,
   2001:db8:cccc:8:D100::)

   Node 7 sends the following packet to 8: (2001:db8::1,
   2001:db8:cccc:8:D100::) which decaps and forwards to CE-B.

   The benefits of using an intermediate Binding SID are well-known and
   key to the Segment Routing architecture: the ingress edge node needs
   to push fewer SIDs, the ingress edge node does not need to change its
   SR policy upon change of the core topology or re-homing of the
   container-based apps on different servers.  Conversely, the core and
   service organizations do not need to share details on how they
   realize underlay SLA's or where they home their NFV apps.

2.10.  TI-LFA

   Let us assume two packets P1 and P2 received by node 2 exactly when
   the failure of link 27 is detected.

      P1: (2001:db8::1, 2001:db8:cccc:7:1::)



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      P2: (2001:db8::1, 2001:db8:cccc:7:1::)(2001:db8:cccc:8:D100::,
      2001:db8:cccc:7:1::; SL=1)

   Node 2's pre-computed TI-LFA backup path for the destination
   2001:db8:cccc:7::/64 is <2001:db8:cccc:3:C4::>.  It is installed as a
   T.Insert transit behavior.

   Node 2 protects the two packets P1 and P2 according to the pre-
   computed TI-LFA backup path and send the following modified packets
   on the link to 4:

      P1: (2001:db8::1, 2001:db8:cccc:3:C4::)(2001:db8:cccc:7:1::,
      2001:db8:cccc:3:C4::; SL=1)

      P2: (2001:db8::1, 2001:db8:cccc:3:C4::)(2001:db8:cccc:7:1::,
      2001:db8:cccc:3:C4::; SL=1) (2001:db8:cccc:8:D100::,
      2001:db8:cccc:7:1::; SL=1)

   Node 4 then sends the following modified packets to 5:

      P1: (2001:db8::1, 2001:db8:cccc:7:1::)

      P2: (2001:db8::1, 2001:db8:cccc:7:1::)(2001:db8:cccc:8:D100::,
      2001:db8:cccc:7:1::; SL=1)

   Then these packets follow the rest of their post-convergence path
   towards node 7 and then go to node 8 for the VPN decaps.

2.11.  SR TE for Service programming

   We have illustrated the service programming through SR-aware apps in
   a previous section.

   We illustrate the use of End.AS function
   [I-D.ietf-spring-sr-service-programming] to service chain an IP flow
   bound to the internet through two SR-unaware applications hosted in
   containers.

   Let us assume that servers 20 and 70 are respectively connected to
   nodes 2 and 7.  They are respectively configured with SID spaces
   2001:db8:cccc:20::/64 and 2001:db8:cccc:70::/64.  Their connected
   routers advertise the related prefixes in the IGP.  Two SR-unaware
   container-based applications App2 and App7 are respectively hosted on
   server 20 and 70.  Server 20 (70) is configured explicitly with an
   End.AS SID 2001:db8:cccc:20:2:: for App2 (2001:db8:cccc:70:7:: for
   App7).





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   Let us assume a broadband customer with a home gateway CE-A connected
   to edge router 1.  Router 1 is configured with an SR policy which
   encapsulates all the traffic received from CE-A into a H.Encaps
   policy <2001:db8:cccc:20:2::, 2001:db8:cccc:70:7::,
   2001:db8:cccc:8:D0::> where 2001:db8:cccc:8:D0:: is an End.DT4 SID
   instantiated at node 8.

   P1 is a packet sent by the broadband customer to 1: (X, Y) where X
   and Y are two IPv4 addresses.

   1 sends the following packet to 2: (A1::,
   2001:db8:cccc:20:2::)(2001:db8:cccc:8:D0::, 2001:db8:cccc:70:7::,
   2001:db8:cccc:20:2::; SL=2; NH=4)(X, Y).

   2 forwards the packet to server 20.

   20 receives the packet (A1::,
   2001:db8:cccc:20:2::)(2001:db8:cccc:8:D0::, 2001:db8:cccc:70:7::,
   2001:db8:cccc:20:2::; SL=2; NH=4)(X, Y) and forwards the inner IPv4
   packet (X,Y) to App2.  App2 works on the packet and forwards it back
   to 20. 20 pushes the outer IPv6 header with SRH (A1::,
   2001:db8:cccc:70:7::)(2001:db8:cccc:8:D0::, 2001:db8:cccc:70:7::,
   2001:db8:cccc:20:2::; SL=1; NH=4) and sends the (whole) IPv6 packet
   with the encapsulated IPv4 packet back to 2.

   2 and 7 forward to server 70.

   70 receives the packet (A1::,
   2001:db8:cccc:70:7::)(2001:db8:cccc:8:D0::, 2001:db8:cccc:70:7::,
   2001:db8:cccc:20:2::; SL=1; NH=4)(X, Y) and forwards the inner IPv4
   packet (X,Y) to App7.  App7 works on the packet and forwards it back
   to 70. 70 pushes the outer IPv6 header with SRH (A1::,
   2001:db8:cccc:8:D0::)(2001:db8:cccc:8:D0::, 2001:db8:cccc:70:7::,
   2001:db8:cccc:20:2::; SL=0; NH=4) and sends the (whole) IPv6 packet
   with the encapsulated IPv4 packet back to 7.

   7 forwards to 8.

   8 receives (A1::, 2001:db8:cccc:8:D0::)(2001:db8:cccc:8:D0::,
   2001:db8:cccc:70:7::, 2001:db8:cccc:20:2::; SL=0; NH=4)(X, Y) and
   performs the End.DT4 function and sends the IP packet (X, Y) towards
   its internet destination.









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3.  Benefits

3.1.  Seamless deployment

   The VPN use-case can be realized with SRv6 capability deployed solely
   at the ingress and egress PE's.

      All the nodes in between these PE's act as transit routers as per
      [RFC8200].  No software/hardware upgrade is required on all these
      nodes.  They just need to support IPv6 per [RFC8200].

   The SRTE/underlay-SLA use-case can be realized with SRv6 capability
   deployed at few strategic nodes.

      It is well-known from the experience deploying SR-MPLS that
      underlay SLA optimization requires few SIDs placed at strategic
      locations.  This was illustrated in our example with the low-
      latency optimization which required the operator to enable one
      single core node with SRv6 (node 4) where one single and End.X SID
      towards node 5 was instantiated.  This single SID is sufficient to
      force the end-to-end traffic via the low-latency path.

   The TI-LFA benefits are collected incrementally as SRv6 capabilities
   are deployed.

      It is well-know that TI-LFA is an incremental node-by-node
      deployment.  When a node N is enabled for TI-LFA, it computes TI-
      LFA backup paths for each primary path to each IGP destination.
      In more than 50% of the case, the post-convergence path is loop-
      free and does not depend on the presence of any remote SRv6 SID.
      In the vast majority of cases, a single segment is enough to
      encode the post-convergence path in a loop-free manner.  If the
      required segment is available (that node has been upgraded) then
      the related back-up path is installed in FIB, else the pre-
      existing situation (no backup) continues.  Hence, as the SRv6
      deployment progresses, the coverage incrementally increases.
      Eventually, when the core network is SRv6 capable, the TI-LFA
      coverage is complete.

   The service programming use-case can be realized with SRv6 capability
   deployed at few strategic nodes.

      The service-programming deployment is again incremental and does
      not require any pre-deployment of SRv6 in the network.  When an
      NFV app A1 needs to be enabled for inclusion in an SRv6 service
      chain, all what is required is to install that app in a container
      or VM on an SRv6-capable server (Linux 4.10 or FD.io 17.04




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      release).  The app can either be SR-aware or not, leveraging the
      proxy functions.

      By leveraging the various End functions it can also be used to
      support any current VNF/CNF implementations and their forwarding
      methods (e.g.  Layer 2).

      The ability to leverage SR TE policies and BSIDs also permits
      building scalable, hierarchical service-chains.

3.2.  Integration

   The SRv6 network programming concept allows integrating all the
   application and service requirements: multi-domain underlay SLA
   optimization with scale, overlay VPN/Tenant, sub-50msec automated
   FRR, security and service programming.

3.3.  Security

   The combination of well-known techniques (SEC-1, SEC-2) and carefully
   chosen architectural rules (SEC-3) ensure a secure deployment of SRv6
   inside a multi-domain network managed by a single organization.

   Inter-domain security will be described in a companion document.

4.  Acknowledgements

   The authors would like to acknowledge Stefano Previdi, Dave Barach,
   Mark Townsley, Peter Psenak, Thierry Couture, Kris Michielsen, Paul
   Wells, Robert Hanzl, Dan Ye, Gaurav Dawra, Faisal Iqbal, Jaganbabu
   Rajamanickam, David Toscano, Asif Islam, Jianda Liu, Yunpeng Zhang,
   Jiaoming Li, Narendra A.K, Mike Mc Gourty, Bhupendra Yadav, Sherif
   Toulan, Satish Damodaran, John Bettink, Kishore Nandyala Veera Venk,
   Jisu Bhattacharya and Saleem Hafeez.

5.  Contributors

   Daniel Bernier
   Bell Canada
   Canada

   Email: daniel.bernier@bell.ca

   Daniel Voyer
   Bell Canada
   Canada

   Email: daniel.voyer@bell.ca



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   Bart Peirens
   Proximus
   Belgium

   Email: bart.peirens@proximus.com

   Hani Elmalky
   Ericsson
   United States of America

   Email: hani.elmalky@gmail.com

   Prem Jonnalagadda
   Barefoot Networks
   United States of America

   Email: prem@barefootnetworks.com

   Milad Sharif
   Barefoot Networks
   United States of America

   Email: msharif@barefootnetworks.com

   Stefano Salsano
   Universita di Roma "Tor Vergata"
   Italy

   Email: stefano.salsano@uniroma2.it

   Ahmed AbdelSalam
   Gran Sasso Science Institute
   Italy

   Email: ahmed.abdelsalam@gssi.it

   Gaurav Naik
   Drexel University
   United States of America

   Email: gn@drexel.edu

   Arthi Ayyangar
   Arista
   United States of America

   Email: arthi@arista.com




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   Satish Mynam
   Innovium Inc.
   United States of America

   Email: smynam@innovium.com

   Wim Henderickx
   Nokia
   Belgium

   Email: wim.henderickx@nokia.com

   Shaowen Ma
   Juniper
   Singapore

   Email: mashao@juniper.net

   Ahmed Bashandy
   Individual
   United States of America

   Email: abashandy.ietf@gmail.com

   Francois Clad
   Cisco Systems, Inc.
   France

   Email: fclad@cisco.com

   Kamran Raza
   Cisco Systems, Inc.
   Canada

   Email: skraza@cisco.com

   Darren Dukes
   Cisco Systems, Inc.
   Canada

   Email: ddukes@cisco.com

   Patrice Brissete
   Cisco Systems, Inc.
   Canada

   Email: pbrisset@cisco.com




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   Zafar Ali
   Cisco Systems, Inc.
   United States of America

   Email: zali@cisco.com

6.  References

   [I-D.ietf-bess-srv6-services]
              Dawra, G., Filsfils, C., Raszuk, R., Decraene, B., Zhuang,
              S., and J. Rabadan, "SRv6 BGP based Overlay services",
              draft-ietf-bess-srv6-services-04 (work in progress), July
              2020.

   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", draft-
              ietf-spring-segment-routing-policy-08 (work in progress),
              July 2020.

   [I-D.ietf-spring-sr-service-programming]
              Clad, F., Xu, X., Filsfils, C., daniel.bernier@bell.ca,
              d., Li, C., Decraene, B., Ma, S., Yadlapalli, C.,
              Henderickx, W., and S. Salsano, "Service Programming with
              Segment Routing", draft-ietf-spring-sr-service-
              programming-03 (work in progress), September 2020.

   [I-D.ietf-spring-srv6-network-programming]
              Filsfils, C., Camarillo, P., Leddy, J., Voyer, D.,
              Matsushima, S., and Z. Li, "SRv6 Network Programming",
              draft-ietf-spring-srv6-network-programming-20 (work in
              progress), September 2020.

   [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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.





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   [RFC8754]  Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
              <https://www.rfc-editor.org/info/rfc8754>.

Authors' Addresses

   Clarence Filsfils
   Cisco Systems, Inc.
   Belgium

   Email: cf@cisco.com


   Pablo Camarillo Garvia (editor)
   Cisco Systems, Inc.
   Spain

   Email: pcamaril@cisco.com


   Zhenbin Li
   Huawei Technologies
   China

   Email: lizhenbin@huawei.com


   Satoru Matsushima
   SoftBank
   1-9-1,Higashi-Shimbashi,Minato-Ku
   Tokyo  105-7322
   Japan

   Email: satoru.matsushima@g.softbank.co.jp


   Bruno Decraene
   Orange
   France

   Email: bruno.decraene@orange.com









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   Dirk Steinberg
   Lapishills Consulting Limited
   Cyprus

   Email: dirk@lapishills.com


   David Lebrun
   Google
   Belgium

   Email: david.lebrun@uclouvain.be


   Robert Raszuk
   Bloomberg LP
   United States of America

   Email: robert@raszuk.net


   John Leddy
   Individual Contributor
   United States of America

   Email: john@leddy.net

























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