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Versions: (draft-psarkar-spring-mpls-anycast-segments) 00 01

SPRING Working Group                                      P. Sarkar, Ed.
Internet-Draft                                              Arrcus, Inc.
Intended status: Standards Track                              H. Gredler
Expires: October 13, 2017                                   RtBrick Inc.
                                                             C. Filsfils
                                                              S. Previdi
                                                     Cisco Systems, Inc.
                                                             B. Decraene
                                                                  Orange
                                                            M. Horneffer
                                                        Deutsche Telekom
                                                          April 11, 2017


             Anycast Segments in MPLS based Segment Routing
               draft-ietf-spring-mpls-anycast-segments-01

Abstract

   Instead of forwarding to a specific device or to all devices in a
   group, anycast addresses, let network devices forward a packet to (or
   steer it through) one or more topologically nearest devices in a
   specific group of network devices.  The use of anycast addresses has
   been extended to the Segment Routing (SR) network, wherein a group of
   SR-capable devices can represent a anycast address, by having the
   same Segment Routing Global Block (SRGB) provisioned on all the
   devices and each one of them advertising the same anycast prefix
   segment (or Anycast SID).

   This document describes a proposal for implementing anycast prefix
   segments in a MPLS-based SR network, without the need to have the
   same SRGB block (label ranges) provisioned across all the member
   devices in the group.  Each node can be provisioned with a separate
   SRGB from the label range supported by the specfic hardware platform.

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



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   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://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 October 13, 2017.

Copyright Notice

   Copyright (c) 2017 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
   (http://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.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Solution  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   7
       3.1.1.  Common Anycast SRGB (CA-SRGB) . . . . . . . . . . . .   7
       3.1.2.  Common Anycast Prefix Segment Label (CAPSL) . . . . .   8
       3.1.3.  Anycast Prefix Segment Label (APSL) . . . . . . . . .   8
     3.2.  Procedures  . . . . . . . . . . . . . . . . . . . . . . .   9
       3.2.1.  Label Stack Computation . . . . . . . . . . . . . . .   9
       3.2.2.  Virtual SID Label Lookup Table  . . . . . . . . . . .  10
       3.2.3.  Advertising Anycast Prefix Segments . . . . . . . . .  13
       3.2.4.  Programming Anycast Prefix Segments . . . . . . . . .  14
     3.3.  Packet Flow . . . . . . . . . . . . . . . . . . . . . . .  16
   4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  17
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18




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1.  Introduction

   Anycast is a network addressing scheme and routing methodology in
   which packets from a single source device are forwarded to the
   topologically nearest node in a group of potential receiving devices,
   all identified by the same anycast address.  There are various useful
   usecases of anycast addresses, and discussion of the same are outside
   the scope of this document.

   [I-D.ietf-spring-segment-routing] extended the use of anycast
   addresses to SR networks.  An operator may combine a group of SR-
   enabled nodes to form a anycast group, by picking a anycast address
   and a segment identifier (hereon referred to as SID) to represent the
   group, and then provisioning all the nodes with the same address and
   SID.  Once provisioned, each device in the group advertises the
   corresponding anycast address in its IGP link-state advertisements
   along with the SID provisioned.  Source devices on receiving such
   anycast prefix segment advertisements, finds out the topologically
   nearest device that originated the anycast segment and forwards
   packets destined to the same on the shortest-path to the nearest
   device.

   [I-D.ietf-spring-segment-routing] requires all devices in a given
   anycast group to implement the exact same SRGB block(s).  This
   requirement will always be met in SR network deployed over IPV6
   forwarding plane [I-D.previdi-6man-segment-routing-header].  For SR
   over MPLS dataplane [I-D.ietf-spring-segment-routing-mpls], while
   this is a simple (and hence more desirable) solution, the same may
   not be possible in a multi-vendor networks deploying devices with
   varying hardware capabilities.

   In MPLS-based SR deployments, the segments on a given source router
   are actually mapped to a MPLS labels allocated from the local label
   pool carved out by the device for accomodating the SRGB.  In multi-
   vendor deployments with various types of devices deployed in the same
   network topology, such a anycast group may contain a good combination
   of devices from different vendors and have different internal
   hardware capabilities.  In such environments it is not sufficient to
   assume that all the devices in a anycast group will be able to
   allocate exactly the same range of labels for implementing the SRGB.
   In reality, getting a common range of labels among all the various
   vendors may not be feasible.

   This documents provides mechanisms to implement anycast segments with
   any kind of device in a multi-vendor network deployment without
   requiring to provision the same exact range of labels for SRGB on all
   the devices.




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2.  Problem Statement

   To better illustrate the problem let us consider an example topology
   using anycast segments as shown in Figure 1 below.


                             +--------------+
                             |   Group A    |
                             | 192.1.1.1/32 |
                             |    SID:100   |
                             |              |
                      +-----------A1---A3----------+
                      |      |    | \ / |   |      |
           SID:10     |      |    |  /  |   |      |     SID:30
         1.1.1.1/32   |      |    | / \ |   |      |   1.1.1.3/32
             PE1------R1----------A2---A4---------R3------PE3
               \     /|      |              |      |\     /
                \   / |      +--------------+      | \   /
                 \ /  |                            |  \ /
                  /   |                            |   /
                 / \  |                            |  / \
                /   \ |      +--------------+      | /   \
               /     \|      |              |      |/     \
             PE2------R2----------B1---B3----+----R4------PE4
         1.1.1.2/32   |      |    | \ / |   |      |   1.1.1.4/32
           SID:20     |      |    |  /  |   |      |     SID:40
                      |      |    | / \ |   |      |
                      +-----+-----B2---B4----+-----+
                             |              |
                             |   Group B    |
                             | 192.1.1.2/32 |
                             |    SID:200   |
                             +--------------+


                           Figure 1: Topology 1

   In Figure 1 above, there are two groups of transit devices.  Group A
   consists of devices {A1, A2, A3 and A4}. They are all provisioned
   with the anycast address 192.1.1.1/32 and the anycast SID 100.
   Similarly, group B consists of devices {B1, B2, B3 and B4} and are
   all provisioned with the anycast address 192.1.1.2/32, anycast SID
   200.  In the above network topology, each PE device is connected to
   two routers in each of the groups A and B.

   Following are all the possible ECMP paths between the various pairs
   of PE devices.




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   o  P1: via {R1, A1, A3, R3}

   o  P2: via {R1, A1, A4, R3}

   o  P3: via {R1, A2, A3, R3}

   o  P4: via {R1, A2, A4, R3}

   o  P5: via {R2, B1, B3, R4}

   o  P6: via {R2, B1, B4, R4}

   o  P7: via {R2, B2, B3, R4}

   o  P8: via {R2, B2, B4, R4}

   As seen above, there is always eight ECMP paths between each of pair
   of PE devices.  The network operator may not wish to utilize all
   possible ECMP paths for all possible types of traffic flowing between
   a given pair of PE devices.  It may be more useful for use paths P1,
   P2, P3 and P4 for certain types of traffic and use paths P5, P6, P7
   and P8 for all other types of traffic between the same PE devices.
   If so desired, operators may use these anycast groups A and B and the
   corresponding anycast segment to impose a segment-list (refer to
   [I-D.ietf-spring-segment-routing]) to forward the respective traffic
   flows over the desired specific paths as shown below.  Figure 2 below
   depicts a expanded view of the paths via group A.  The range labels
   allocated for SRGB on each of the devices in group A are also
   mentioned in this diagram.






















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                         +-------------------------+
                         |       Group A           |
                         |     192.1.1.1/32        |
                         |        SID:100          |
                         |-------------------------|
                         |                         |
                         |   SRGB:         SRGB:   |
      SID:10             |(1000-2000)   (3000-4000)|             SID:30
        PE1---+       +-------A1-------------A3-------+       +---PE3
               \     /   |    | \           / |    |   \     /
                \   /    |    |  +-----+   /  |    |    \   /
         SRGB:   \ /     |    |         \ /   |    |     \ /   SRGB:
      (7000-8000) R1     |    |          \    |    |      R3 (6000-7000)
                 / \     |    |         / \   |    |     / \
                /   \    |    |  +-----+   \  |    |    /   \
               /     \   |    | /           \ |    |   /     \
        PE2---+       +-------A2-------------A4-------+       +---PE4
      SID:20             |   SRGB:         SRGB:   |             SID:40
                         |(2000-3000)   (4000-5000)|
                         |                         |
                         +-------------------------+


                Figure 2: Transit paths via anycast group A

   In the above topology, if device PE1 (or PE2) requires to send a
   packet to the device PE3 (or PE4) it needs to encapsulate the packet
   in a MPLS payload with the following stack of labels.

   o  Label allocated R1 for anycast SID 100 (outer label)

   o  Label allocated by the nearest router in group A for SID 30 (for
      destination PE3)

   While the first label is easy to compute, in this case since there
   are more than one topologically nearest devices (A1 and A2), unless
   A1 and A2 implement same exact SRGB, determining the second label is
   impossible.  In all likeness, devices A1 and A2 may be devices from
   different hardware vendors and it may not implement the same exact
   SRGB label ranges.  In such cases, separate labels are allocated by
   A1 and A2 (1030 and 2030 respectively, in the above example).  Hence,
   PE1 (or PE2) cannot compute an appropriate label stack to steer the
   packet exclusively through the group A devices.  Same holds true for
   devices PE3 and PE4 when trying to send a packet to PE1 or PE2.







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3.  Solution

3.1.  Definitions

3.1.1.  Common Anycast SRGB (CA-SRGB)

   This document introduces the term 'Common-Anycast SRGB' (hereafter
   referred to as the CA-SRGB) to define the SRGB implemented by the
   majority of the devices in the network, that are participating in one
   or more anycast segments.  Each device MUST implement provisions to
   let the operators assign the CA-SRGB on the device.  Each vendor
   implementation MUST implement provisions to configure the CA-SRGB at
   all configuration levels (per-routing-instance/per-protocol/per-
   topology etc) wherein provisions to configure the local SRGB label
   ranges has also been implemented.  Essentially, for each SRGB
   configured on the device, vendor implementations MUST allow
   configuring a corresponding CA-SRGB value.

   For each configuration level (per-routing-instance/per-protocol/per-
   topology etc)supported, the operator MUST set the same exact CA-SRGB
   on all devices across the entire IGP domain (including different IS-
   IS levels and OSPF areas).  This ensures the proposal specified in
   Section 3.2.1 works flawlessly across all devices in any multi-vendor
   network deployment.

   However assigning the CA-SRGB (for a given routing-instance/protocol/
   topology etc.) on the device, does not mean the label ranges
   allocated by the device for the corresponding SRGB has to belong to
   the CA-SRGB defined.  The device may dynamically allocate the
   corresponding SRGB label ranges, or allocate the range provisioned by
   the operator, through an appropriate separate configuration (please
   refer to [I-D.ietf-spring-sr-yang] for more details).

   For devices that has the local SRGB to be exact same as the 'CA-SRGB'
   applicable for the entire network, operators need not explictly set
   the corresponding CA-SRGB values.  In such case, the vendor
   implementations MUST assume the local SRGB values to be the
   corresponding CA-SRGB values defined on the specific device.

   If the CA-SRGB defined on a device does not absolutely match the
   corresponding SRGB label ranges allocated (or provisioned) on the
   same device (i.e. the CA-SRGB is not an exact copy of the
   corresponding SRGB label ranges), and the device is provisoned with
   one or more anycast prefix segments, the device MUST implement all
   the additional functionalities specified in Section 3.2.2,
   Section 3.2.3 and Section 3.2.4.  On devices, where the SRGB label
   ranges is an exact copy of the corresponding CA-SRGB defined, the




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   device need not implement these additional functionalities (
   Section 3.2.2, Section 3.2.3 and Section 3.2.4).

3.1.2.  Common Anycast Prefix Segment Label (CAPSL)

   For each anycast prefix segment, this document also defines a 'Common
   Anycast Prefix Segment Label' (hereafter referred to CAPSL).  The
   value of this label is derived by applying the SID index associated
   with the prefix segment as an offset to the CA-SRGB configured on the
   specific device.  Since the operator MUST configure the same CA-SRGB
   values on all devices in the IGP domain, all devices shall associate
   the same CAPSL label value for a given anycast prefix segment.
   Table 1 below shows the CAPSL labels allocated by any device for the
   various prefix segments found in Figure 2, with CA-SRGB set to
   3000-4000 on all devices.

                   +-----+---------------+-------------+
                   | SID | CA-SRGB Range | CAPSL Value |
                   +-----+---------------+-------------+
                   | 10  | 2000-3000     | 2010        |
                   | 20  | 2000-3000     | 2020        |
                   | 30  | 2000-3000     | 2030        |
                   | 40  | 2000-3000     | 2040        |
                   | 100 | 2000-3000     | 2100        |
                   +-----+---------------+-------------+

          Table 1: Common Anycast Prefix Segment Label Allocation

3.1.3.  Anycast Prefix Segment Label (APSL)

   This document also introduces the term 'Anycast Prefix Segment Label'
   (hereafter referred to as APSL) to define the label allocated by a
   device to advertise reachability for the specific anycast prefix
   segment.  The value of this label is derived by applying the SID
   index associated with the anycast prefix segment as an offset to the
   SRGB of the specific device.  Table 2 below shows the labels
   allocated by the various devices in Figure 2 for the anycast prefix
   segment with SID 100.













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             +-------------+--------+-----------+------------+
             | Anycast-SID | Device | SRGB      | APSL-Label |
             +-------------+--------+-----------+------------+
             | 100         | R1     | 7000-8000 | 7100       |
             | 100         | A1     | 1000-2000 | 1100       |
             | 100         | A2     | 2000-3000 | 2100       |
             | 100         | A3     | 3000-4000 | 3100       |
             | 100         | A4     | 4000-5000 | 4100       |
             | 100         | R3     | 6000-7000 | 6100       |
             +-------------+--------+-----------+------------+

             Table 2: Anycast Prefix Segment Label Allocation

3.2.  Procedures

3.2.1.  Label Stack Computation

   A MPLS device that tries to encapsulate any kind of traffic into a
   SR-based MPLS payload (hereafter referred to as the ingress device)
   and steer it through a series of SR adjacency and/or unicast/anycast
   prefix segments, needs to compute an appropriate stack of MPLS labels
   and put it in the outgoing packet.  Alternatively, in a SDN
   environment, the SDN controller may need to compute the label stack
   and install it on the ingress device.

   However in both cases, as illustrated in Section 2, for a given
   ingress device (e.g.  PE1 or PE2), there maybe multiple topologically
   nearest devices in a specific anycast group (e.g.  A1 and A2), even
   through there is only out-going link from the source device(e.g.
   PE1->R1 or PE2-R1).  In such case, when the ingress device (or the
   SDN controller) wants to steer a packet through the anycast group A,
   it can use the anycast segment label advertised by the downstream
   neighbor of the ingress device for the specific anycast prefix
   segment.  Since the packet may reach any one of the multiple devices
   in the group and each of them may have a separate SRGB label range,
   choosing the MPLS label for the next segment providing reachability
   to the final destination.  Also, since the packet steered through a
   anycast segment can reach of any of the member device in the anycast
   group, it is sufficient to assume that the ingress (or the
   controller) cannot place an adjacency segment immediately after a
   anycast segment in the outgoing packet.

   This document proposes the ingress device (or the SDN controller) to
   derive the label for a prefix segment that immediately follows a
   given anycast segment, to be the CAPSL label associated with the
   corresponding SID index (refer to Section 3.1.2).  Note the prefix
   segment immediately following the given anycast segment may itself be
   another anycast segment.



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   The ingress (or the SDN controller) MUST follow the algorithm below
   to compute the label-stack, that it must use to steer a packet
   through a list of SR segments.

   o  Set previous_segment ==> NONE.

   o  Set label_stack ==> {EMPTY}.

   o  For [all segment in Segment_List]

      *  If {segment.type == Adjacency_Segment}

         +  Set label ==> segment.Adjacency_Segment_Label.

      *  Else

         +  If {previous_segment.type == Anycast_Prefix_Segment}

            -  Set label ==> CAPSL_Label(segment.SID_index).

         +  Else

            -  Set label ==> segment.Prefix_Segment_Label.

      *  Add label to label_stack.

      *  Set previous_segment ==> segment.

3.2.2.  Virtual SID Label Lookup Table

   When a MPLS packet on the wire first hits a device, the forwarding
   hardware reads the topmost label in the MPSL header and looks up the
   default label lookup table associated with the interface on which the
   label has been received.  This table is generally called LFIB.  The
   range of labels found in the LFIB constitutes the default label
   space.

   This document introduces a separate virtual label lookup table
   (hereafter referred to as Virtual LFIB or V-LFIB), that represents a
   label space which is also separate from the actual label space
   represented by the default LFIB.  The label value may be present in
   both the default and Virtual LFIB.  However the forwarding semantics
   associated with the label under the default and Virtual LFIB may not
   be same.  Following are the fields of a typical entry of this table.

   o  CAPSL-Label: The CAPSL label value derived from the SID index
      associated with a given prefix segment originated by another




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      device in the same network.  Refer to Section 3.1.2 for more
      details.  This is also the key field for this table.

   o  Forwarding Semantics: This is once again one or more tuples of
      following items.

      *  Outgoing-Label: The label(s) allocated by the neighbor
         device(s) on the shortest-path to the topologically nearest
         originator(s) of the prefix segment.

      *  Outgoing-link: The link(s) connecting the device to the
         neighbor device(s) on the shortest path to the topologically
         nearest originator(s) of the prefix segment.

   This document proposes that, any device, when provisioned with one or
   more anycast prefix segment (address and SID), and the CA-SRGB
   defined by the operator is not an exact copy of the corresponding
   SRGB label ranges allocated by the device, it MUST create a Virtual
   LFIB table.

   Such a device MUST add an entry in the Virtual LFIB for each unicast
   and anycast prefix segments learnt from a remote device, if and only
   if the same prefix has not been provisioned on the device.  The
   device SHOULD NOT add an entry for any of the Anycast or Node prefix
   segments that it has advertised itself.  However if the device has
   learnt any anycast prefix segment from a remote device, and the same
   is not provisioned on this device, the device MUST include the same
   in the Virtual LFIB table.

   In cases where a prefix segment is reachable via multiple shortest
   paths on a given device, the corresponding entry for the prefix SID
   MUST have as many forwarding entries in the Virtual LFIB table as the
   number of shortest-paths found for the corresponding prefix on the
   device.

   Figure 3 below shows how the Virtual LFIB table on each of devices in
   group A should look like.  Please note that some of the prefix
   segments has multiple forwarding semantics associated with them.  For
   example, on device A1, the prefix SID 10 (originated by PE3) is
   reachable through its neighbors A3 and A4.  And as per the SRGB
   advertised by A3 and A4, the labels allocated by A3 and A4 are 3030
   and 4030 respectively.  Hence A1 has added two forwarding entries for
   the prefix SID 30 in its Virtual LFIB table.








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              CA-SRGB configured on all devices: {2000-3000}
         +========+=============+=======+========================+
         |        |             |     Forwarding Semantics       |
         | Device | CAPSL-Label |--------------------------------|
         |        |             | Outgoing-Label | Outgoing-Link |
         +========+=============+================+===============+
         | A1     | 2010        | 7010           | A1->R1        |
         |        +-------------+----------------+---------------+
         |        | 2020        | 7020           | A1->R1        |
         |        +-------------+----------------+---------------+
         |        | 2030        | 3030           | A1->A3        |
         |        |             | 4030           | A1->A4        |
         |        +-------------+----------------+---------------+
         |        | 2040        | 3040           | A1->A3        |
         |        |             | 4040           | A1->A4        |
         +========+=============+================+===============+
         | A2     |  No V-LFIB Table created since CA-SRGB is    |
         |        |     identical to SRGB allocated locally      |
         +========+=============+================+===============+
         | A3     | 2010        | 1010           | A3->A1        |
         |        |             | 2010           | A3->A2        |
         |        +-------------+----------------+---------------+
         |        | 2020        | 1020           | A3->A1        |
         |        |             | 2020           | A3->A2        |
         |        +-------------+----------------+---------------+
         |        | 2030        | 6030           | A3->R3        |
         |        +-------------+----------------+---------------+
         |        | 2040        | 6040           | A3->R3        |
         +========+=============+================+===============+
         | A4     | 2010        | 1010           | A4->A1        |
         |        |             | 2010           | A4->A2        |
         |        +-------------+----------------+---------------+
         |        | 2020        | 1020           | A4->A1        |
         |        |             | 2020           | A4->A2        |
         |        +-------------+----------------+---------------+
         |        | 2030        | 6030           | A4->R3        |
         |        +-------------+----------------+---------------+
         |        | 2040        | 6040           | A4->R3        |
         +========+=============+================+===============+

                    Figure 3: Virtual LFIB Table Setup

   Please note that node A2 has not created a Virtual LFIB table since
   the CA-SRGB (2000-3000) is identical to the SRGB provisioned on it.

   Also please note that none of the devices in the anycast group have
   included the anycast SID 100 in the Virtual LFIB table, since the
   same has already been provisioned on these devices.



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   When a device receives a MPLS packet with the anycast segment label
   associated with one of the anycast prefix segments provisioned on the
   same device, and the CA-SRGB defined by the operator is not an exact
   copy of the corresponding SRGB label ranges allocated by it, it MUST
   use the Virtual LFIB table to lookup the next label that follows the
   anycast segment label in the stack of labels found in the MPLS
   header.  Refer to Section 3.2.4 for more details.

   Following forwarding instructions MUST be installed in the MPLS data-
   plane for each entry in the Virtual LFIB entry.

   o  If the label at the top of the stack matches any of the prefix
      SIDs in the Virtual LFIB table,

      *  If there are multiple forwarding tuples associated with
         matching table entry,

         +  Select one forwarding tuple.  (Criteria to select one is
            outside the scope of this document.)

      *  Else,

         +  Select the single forwarding tuple available.

      *  Replace the next label (should be a CAPSL label) found at top
         of the MPLS label stack in the incoming packet, with the
         'Outgoing-label' from the selected forwarding tuple.

      *  Forward the modified packet onto the 'Outgoing-link' as
         specified in the selected forwarding tuple.

      *  If the prefix SID is another anycast segment,

         +  Ensure the next label lookup is launched again on the
            Virtual LFIB table.

      *  Else,

         +  Ensure the next label lookup is launched on the default LFIB
            table.

3.2.3.  Advertising Anycast Prefix Segments

   Like unicast prefix segments, anycast prefix segments SHOULD be
   advertised in IGP Link-state advertsements using IGP protocol
   extension for SR specified in
   [I-D.ietf-isis-segment-routing-extensions],
   [I-D.ietf-ospf-segment-routing-extensions] and



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   [I-D.ietf-ospf-ospfv3-segment-routing-extensions].  This document
   does not propose any protocol extension for advertising anycast
   prefix segments.

   However when advertising the anycast segments, and the CA-SRGB
   defined by the operator is not an exact copy of the corresponding
   SRGB label ranges allocated by the originating device, it MUST set
   the corresponding P-Flag(No-PHP) in ISIS Prefix-SID SubTLV and/or the
   NP-Flag (No-PHP) in OSPFv2 and OSPFv3 Prefix-SID SubTLV to 1 and the
   E-Flag in the same SubTLVs to 0.  Please refer to following for more
   details on usage of these flags.

   o  ISIS Prefix-SID SubTLV [I-D.ietf-isis-segment-routing-extensions]

   o  OSPFv2 Prefix-SID SubTLV
      [I-D.ietf-ospf-segment-routing-extensions]

   o  OSPFv3 Prefix-SID SubTLV
      [I-D.ietf-ospf-ospfv3-segment-routing-extensions]

   The proposal above, ensures that a MPLS packet sent to (or taking
   transit through) a given anycast group, when reaching at a
   topologically nearest device in the group where CA-SRGB does not
   match SRGB provisioned on it, always arrives with the APSL-label that
   is derived from the device's SRGB, and the SID associated with the
   corresponding anycast prefix segment.  Note in the above topology,
   assuming domain-wide CA-SRGB is set to (2000-3000) on all nodes,
   while nodes A1, A3 and A4 will advertise the SID 100 with P-Flag(No-
   PHP) set to 1, node A2 will advertise the same anycast prefix SID
   with P-Flag unset.  This is because on node A1 the domain-wide CA-
   SRGB is identical to the local SRGB provisioned on A2.

   In Figure 2, when PE1 or PE2 intends to steer a packet destined for
   PE3 or PE4, through the anycast group A (SID 100), it needs to
   forward the packet to R1 (SRGB:7000-8000), after putting the label
   7100 (derived from R1's SRGB), at top of the label stack in the MPLS
   header.  However when the same packet is forwarded to A1 (one of the
   topologicaly nearest devices in group A), R1 shall not POP (or
   remove) the label 7100.  Instead R1 shall replace it with the label
   1100 while forwarding to A1.  While forwarding to A2, since A2 would
   have advertised the anycast SID 100 with P-Flag (No-PHP) unset, R1
   shall POP the incoming label 7100 before forwarding it to R1.

3.2.4.  Programming Anycast Prefix Segments

   The proposal specified in Section 3.2.3, ensures that a MPLS packet
   destined to (or steered via) a anycast prefix segment always arrives
   at the nearest device in the anycast group with a label derived from



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   the device's SRGB and the SID associated with the corresponding
   anycast prefix segment, as the top-most label label stack in its MPLS
   header.  If this label is also the bottom-most label (S=1), it means
   packet has been destined to the anycast segment, and should be
   consumed by the local device.  If the label is not the bottom-most
   label (S=0), the packet must be forwarded to the next segment, for
   which the next label in the stack should be consulted.  However
   Section 3.2.1 specifies that the next label in such case, shall be a
   label belonging to the CA-SRGB defined by the operator, derived from
   the SID associated with the next segment.  Since the CAPSL label for
   the SID index associated with a prefix segment may directly collide
   with another label in the default LFIB table, Section 3.2.2 also
   proposed to have a Virtual LFIB table to provide a separate label-
   space for looking up the next label.

   This document specifies that a device provisioned with a given prefix
   segment index MUST implement following forwarding semantics for the
   anycast segment label (refer to Section 3.1.3) associated with the
   anycast prefix segment, if the CA-SRGB label ranges defined is not an
   exact copy of the corresponding SRGB label range(s) locally
   allocated/provisioned on the device.

   o  If the label at the top the stack is a anycast segment label, and
      the CA-SRGB defined is not an exact copy of the corresponding SRGB
      label range(s),

      *  Pop the label.

      *  If bottom-most label in the stack (S=1),

         +  Send it to host stack for local consumption, as usual.

      *  Else if not the bottom-most label in the stack (S=0),

         +  Set the Virtual LFIB table as the lookup table for the next
            label lookup.

         +  Launch a lookup for the next label in the stack (should be a
            CAPSL label).

   o  Else

      *  Lookup the label in the default LFIB table as usual.








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3.3.  Packet Flow

   Figure 4 below ilustrate how SR-based MPLS packets destined for PE3
   and sourced by PE1 are expected to flow through when PE1 encapsulates
   the packet with an appropriate label stack to steer it through group
   A devices only



                         +-------------------------+
                         |       Group A           |
                         |     192.1.1.1/32        |
  CA-SRGB: {2000-3000}   |        SID:100          |
                         |-------------------------|
                         |                         |
                         |                         |
       --->            --->        --->          --->      --->     --->
 +----+----+--+   +----+----+--+   +----+--+   +----+--+   +--+     +--+
 |7100|2030|..|   |1100|2030|..|   |3030|..|   |6030|..|   |..|     |..|
 +----+----+--+   +----+----+--+   +----+--+   +----+--+   +--+     +--+
                         |                         |
                         |   SRGB:         SRGB:   |
      SID:10             |(1000-2000)   (3000-4000)|             SID:30
     ---PE1---+       +-------A1-------------A3-------+       +---PE3---
               \     /   |    | \           / |    |   \     /
                \   /    |    |  +-----+   /  |    |    \   /
         SRGB:   \ /     |    |         \ /   |    |     \ /   SRGB:
      (7000-8000) R1     |    |          \    |    |      R3 (6000-7000)
                 / \     |    |         / \   |    |     / \
                /   \    |    |  +-----+   \  |    |    /   \
               /     \   |    | /           \ |    |   /     \
     ---PE2---+       +-------A2-------------A4-------+       +---PE4---
      SID:20             |   SRGB:         SRGB:   |             SID:40
                         |(2000-3000)   (4000-5000)|
                         |                         |
                       --->        --->          --->
                  +----+--+      +----+--+     +----+--+
                  |2030|..|      |4030|..|     |6030|..|
                  +----+--+      +----+--+     +----+--+
                         |                         |
                         |                         |
                         +-------------------------+


       Figure 4: Packet Flow through MPLS-based SR Anycast Segments






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4.  Acknowledgements

   Many many thanks to Shraddha Hegde, Eric Rosen, Chris Bowers and
   Stephane Litkowski for their valuable inputs.

5.  IANA Considerations

   N/A. - No protocol changes are proposed in this document.

6.  Security Considerations

   This document does not introduce any change in any of the protocol
   specifications.

7.  References

7.1.  Normative References

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

7.2.  Informative References

   [I-D.ietf-isis-segment-routing-extensions]
              Previdi, S., Filsfils, C., Bashandy, A., Gredler, H.,
              Litkowski, S., Decraene, B., and J. Tantsura, "IS-IS
              Extensions for Segment Routing", draft-ietf-isis-segment-
              routing-extensions-04 (work in progress), May 2015.

   [I-D.ietf-ospf-ospfv3-segment-routing-extensions]
              Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
              Shakir, R., Henderickx, W., and J. Tantsura, "OSPFv3
              Extensions for Segment Routing", draft-ietf-ospf-ospfv3-
              segment-routing-extensions-02 (work in progress), February
              2015.

   [I-D.ietf-ospf-segment-routing-extensions]
              Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
              Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
              Extensions for Segment Routing", draft-ietf-ospf-segment-
              routing-extensions-04 (work in progress), February 2015.

   [I-D.ietf-spring-segment-routing]
              Filsfils, C., Previdi, S., Decraene, B., Litkowski, S.,
              and R. Shakir, "Segment Routing Architecture", draft-ietf-
              spring-segment-routing-03 (work in progress), May 2015.



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   [I-D.ietf-spring-segment-routing-mpls]
              Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
              Litkowski, S., Horneffer, M., Shakir, R., Tantsura, J.,
              and E. Crabbe, "Segment Routing with MPLS data plane",
              draft-ietf-spring-segment-routing-mpls-01 (work in
              progress), May 2015.

   [I-D.ietf-spring-sr-yang]
              Litkowski, S., Qu, Y., Sarkar, P., and J. Tantsura, "YANG
              Data Model for Segment Routing", draft-ietf-spring-sr-
              yang-00 (work in progress), July 2015.

   [I-D.previdi-6man-segment-routing-header]
              Previdi, S., Filsfils, C., Field, B., and I. Leung, "IPv6
              Segment Routing Header (SRH)", draft-previdi-6man-segment-
              routing-header-06 (work in progress), May 2015.

Authors' Addresses

   Pushpasis Sarkar (editor)
   Arrcus, Inc.
   Bangalore, KA  560103
   India

   Email: pushpasis.ietf@gmail.com


   Hannes Gredler
   RtBrick Inc.

   Email: hannes@rtbrick.com


   Clarence Filsfils
   Cisco Systems, Inc.
   Brussels
   BE

   Email: cfilsfil@cisco.com


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

   Email: sprevidi@cisco.com



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   Bruno Decraene
   Orange

   Email: bruno.decraene@orange.com


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

   Email: Martin.Horneffer@telekom.de






































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