MVPN/EVPN Tunnel Aggregation with Common Labels
draft-ietf-bess-mvpn-evpn-aggregation-label-08

BESS                                                            Z. Zhang
Internet-Draft                                          Juniper Networks
Updates: 7432, 6514, 7582 (if approved)                         E. Rosen
Intended status: Standards Track                              Individual
Expires: July 24, 2022                                            W. Lin
                                                        Juniper Networks
                                                                   Z. Li
                                                     Huawei Technologies
                                                             I. Wijnands
                                                              Individual
                                                        January 20, 2022

            MVPN/EVPN Tunnel Aggregation with Common Labels
             draft-ietf-bess-mvpn-evpn-aggregation-label-08

Abstract

   The MVPN specifications allow a single Point-to-Multipoint (P2MP)
   tunnel to carry traffic of multiple VPNs.  The EVPN specifications
   allow a single P2MP tunnel to carry traffic of multiple Broadcast
   Domains (BDs).  These features require the ingress router of the P2MP
   tunnel to allocate an upstream-assigned MPLS label for each VPN or
   for each BD.  A packet sent on a P2MP tunnel then carries the label
   that is mapped to its VPN or BD (in some cases, a distinct upstream-
   assigned label is needed for each flow.)  Since each ingress router
   allocates labels independently, with no coordination among the
   ingress routers, the egress routers may need to keep track of a large
   number of labels.  The number of labels may need to be as large (or
   larger) than the product of the number of ingress routers times the
   number of VPNs or BDs.  However, the number of labels can be greatly
   reduced if the association between a label and a VPN or BD is made by
   provisioning, so that all ingress routers assign the same label to a
   particular VPN or BD.  New procedures are needed in order to take
   advantage of such provisioned labels.  These new procedures also
   apply to Multipoint-to-Multipoint (MP2MP) tunnels.  This document
   updates RFCs 6514, 7432 and 7582 by specifying the necessary
   procedures.

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.

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Status of This Memo

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Table of Contents

   1.  Terminologies . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Problem Description . . . . . . . . . . . . . . . . . . .   4
     2.2.  Proposed Solution . . . . . . . . . . . . . . . . . . . .   5
       2.2.1.  MP2MP Tunnels . . . . . . . . . . . . . . . . . . . .   7
       2.2.2.  Segmented Tunnels . . . . . . . . . . . . . . . . . .   7
       2.2.3.  Summary of Label Allocation Methods . . . . . . . . .   9
   3.  Specification . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.1.  Context Label Space ID Extended Community . . . . . . . .  10
     3.2.  Procedures  . . . . . . . . . . . . . . . . . . . . . . .  10
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  13

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   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Terminologies

   Familiarity with MVPN/EVPN protocols and procedures is assumed.  Some
   terminologies are listed below for convenience.

   o  BUM: Broadcast, Unknown Unicast, or Multicast (traffic).

   o  BD: Broadcast Domain.

   o  PMSI: Provider Multicast Service Interface - a pseudo interface
      for a PE to send overlay/customer multicast traffic via underlay/
      provider tunnels.  Includes I/S-PMSI (often referred to as x-PMSI)
      for Inclusive/Selective-PMSI.

   o  IMET: Inclusive Multicast Ethernet Tag route.  An EVPN specific
      name for I-PMSI A-D route.

   o  PTA: PMSI Tunnel Attribute.  A BGP attribute that may be attached
      to an BGP-MVPN/EVPN A-D routes.

   o  ESI: Ethernet Segment Identifier.

2.  Introduction

   MVPN can use P2MP tunnels (set up by RSVP-TE, mLDP, or PIM) to
   transport customer multicast traffic across a service provider's
   backbone network.  Often, a given P2MP tunnel carries the traffic of
   only a single VPN.  There are however procedures defined that allow a
   single P2MP tunnel to carry traffic of multiple VPNs.  In this case,
   the P2MP tunnel is called an "aggregate tunnel".  The PE router that
   is the ingress node of an aggregate P2MP tunnel allocates an
   "upstream-assigned MPLS label" [RFC5331] for each VPN, and each
   packet sent on the P2MP tunnel carries the upstream-assigned MPLS
   label that the ingress PE has bound to the packet's VPN.

   Similarly, EVPN can use P2MP tunnels (set up by RSVP-TE, mLDP, or
   PIM) to transport BUM traffic (Broadcast traffic, Unicast traffic
   with an Unknown address, or Multicast traffic), across the provider
   network.  Often a P2MP tunnel carries the traffic of only a single
   BD.  However, there are procedures defined that allow a single P2MP
   tunnel to be an "aggregate tunnel" that carries traffic of multiple
   BDs.  The procedures are analogous to the MVPN procedures -- the PE
   router that is the ingress node of an aggregate P2MP tunnel allocates

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   an upstream-assigned MPLS label for each BD, and each packet sent on
   the P2MP tunnel carries the upstream-assigned MPLS label that the
   ingress PE has bound to the packet's BD.

   MVPN and EVPN can also use BIER [RFC8279] to transmit multicast
   traffic or BUM traffic [RFC8556] [I-D.ietf-bier-evpn].  Although BIER
   does not explicitly set up P2MP tunnels, from the perspective of
   MVPN/EVPN, the use of BIER transport is very similar to the use of
   aggregate P2MP tunnels.  When BIER is used, the PE transmitting a
   packet (the "BFIR" [RFC8279]) must allocate an upstream-assigned MPLS
   label for each VPN or BD, and the packets transmitted using BIER
   transport always carry the label that identifies their VPN or BD.
   (See [RFC8556] and [I-D.ietf-bier-evpn] for the details.)  In the
   remainder of this document, we will use the term "aggregate tunnels"
   to include both P2MP tunnels and BIER transport.

   When an egress PE receives a packet from an aggregate tunnel, it must
   look at the upstream-assigned label carried by the packet, and must
   interpret that label in the context of the ingress PE.  Essentially,
   each ingress PE has its own "context label space" [RFC5331] from
   which it allocates its upstream-assigned labels.  When an egress PE
   looks up the upstream-assigned label carried by a given packet, it
   looks it up in the context label space owned by the packet's ingress
   PE.  How an egress PE identifies the ingress PE of a given packet
   depends on the tunnel type.

2.1.  Problem Description

   Note that these procedures may require a very large number of labels.
   Suppose an MVPN or EVPN deployment has 1001 PEs, each hosting 1000
   VPN/BDs.  Each ingress PE has to assign 1000 labels, and each egress
   PE has to be prepared to interpret 1000 labels from each of the
   ingress PEs.  Since each ingress PE allocates labels from its own
   label space and does not coordinate label assignments with others,
   each egress PE must be prepared to interpret 1,000,000 upstream-
   assigned labels (across 1000 context label spaces - one for each
   ingress PE).  This is an evident scaling problem.

   At the present time, few if any MVPN/EVPN deployments use aggregate
   tunnels, so this problem has not surfaced.  However, the use of
   aggregate tunnels is likely to increase due to the following two
   factors:

   o  In EVPN, a single customer ("tenant") may have a large number of
      BDs, and the use of aggregate RSVP-TE or mLDP P2MP tunnels may
      become important, since each tunnel creates state at the
      intermediate nodes.

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   o  The use of BIER as transport for MVPN/EVPN is becoming more and
      more attractive and feasible.

   Note there are pros and cons with traditional P2MP tunnel aggregation
   (vs.  BIER), which are already discussed in Section 2.1.1 of
   [RFC6513].  This document simply specifies a way to increase label
   scaling when tunnel aggregation is used.

   A similar problem also exists with EVPN ESI labels used for multi-
   homing.  A PE attached to a multi-homed Ethernet Segment (ES)
   advertises an ESI label in its Ethernet A-D per Ethernet Segment
   Route.  The PE imposes the label when it sends frames received from
   the ES to other PEs via a P2MP/BIER tunnel.  A receiving PE that is
   attached to the source ES will know from the ESI label that the
   packet originated on the source ES, and thus will not transmit the
   packet on its local attachment circuit to that ES.  From the
   receiving PE's point of view, the ESI label is (upstream-)allocated
   from the source PE's label space, so the receiving PE needs to
   maintain context label tables, one for each source PE, just like the
   VRF/BD label case above.  If there are 1,001 PEs, each attached to
   1,000 ESes, this can require each PE to understand 1,000,000 ESI
   labels.  Notice that the issue exists even when no P2MP tunnel
   aggregation (i.e. one tunnel used for multiple BDs) is used.

2.2.  Proposed Solution

   The number of labels could be greatly reduced if a central authority
   assigned a label to each VPN, BD, or ES, and if all PEs used that
   same label to represent a given VPN , BD, or ES.  Then the number of
   total number of labels needed would just be the sum of the number of
   VPNs, BD, and/or ESes.

   One method of achieving this is to reserve a portion of the label
   space for assignment by a central authority.  We refer to this
   reserved portion as the "Domain-wide Common Block" (DCB) of labels.
   This is analogous to the "Segment Routing Global Block" (SRGB) that
   is described in [RFC8402].  The DCB is taken from the same label
   space that is used for downstream-assigned labels, but each PE would
   know not to allocate labels from that block for other purposes (e.g.,
   as a local label or for SRGB).  A PE that is attached (via L3VPN VRF
   interfaces or EVPN Access Circuits) would know by provisioning which
   label from the DCB corresponds to which of its locally attached VPNs,
   BDs, or ESes.

   For example, all PEs could reserve a DCB [1000, 2000] and they are
   all provisioned that label 1000 is for VPN 0, 1001 for VPN 1, and so
   on so forth.  Now only 1000 label instead of 1,000,000 labels are
   needed for 1000 VPNs.

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   The definition of "domain" is loose - it simply includes all the
   routers that share the same DCB.  In this document, it only needs to
   include all PEs of an MVPN/EVPN network.  (Though if tunnel
   segmentation [RFC6514] is used, each segmentation region could have
   its own DCB.  This will be explained in more detail later.)

   The "domain" could also include all routers in the provider network,
   making it not much different from a common SRGB across all the
   routers.  However, that is not necessarily as the labels used by PEs
   for the purposes defined in this document will only rise to the top
   of the label stack when traffic arrives the PEs.  Therefore, it is
   better to not include internal P routers in the "domain".  That way
   they do not have to aside the same DCB used for the purposes in this
   document.

   In some deployments, it may be impractical to allocate a DCB that is
   large enough to contain labels for all the VPNs/BDs/ESes.  In this
   case, it may be necessary to allocate those labels from one or a few
   separate context label spaces independent of each PE's default label
   space (that the DCB belongs to).  For example, if it is difficult to
   have a DCB of 10,000 labels across all PEs for all the VPNs/BDs/ESes
   that need to be supported, a separate context label space can be
   dedicated to those 10,000 labels.  Each separate context label space
   is identified in the forwarding plane by a label from the DCB (which
   does not need to be large).  Each PE is provisioned with the label-
   space-identifying DCB label and the common VPN/BD/ES labels allocated
   from that context label space.  When sending traffic, an ingress PE
   imposes all necessary service labels (for the VPN/BD/ES) first, then
   imposes the label-space-identifying DCB label.  From the label-space-
   identifying DCB label an egress PE can determine the label space
   where the inner VPN/BD/ES label is looked up.

   The MVPN/EVPN signaling defined in [RFC6514] and [RFC7432] assumes
   that certain MPLS labels are allocated from a context label space
   owned by a particular ingress PE.  In this document, we augment the
   signaling procedures so that it is possible to signal that a
   particular label is from the DCB, rather than from an ingress PE's
   context label space.  We also augment the signaling so that it is
   possible to indicate that a particular label is from an identified
   context label space that is different than the ingress PE's own
   context label space.

   Notice that, the VPN/BD/ES-identifying labels from the DCB or from
   those few context label spaces are very similar to VNIs in VXLAN.
   Allocating a label from the DCB or from those a few context label
   spaces and communicating them to all PEs should not be different from
   allocating VNIs, and should be feasible in today's networks since
   controllers are used more and more widely.

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2.2.1.  MP2MP Tunnels

   MP2MP tunnels present the same problem that can be solved the same
   way.

   Per RFC 7582 ("MVPN: Using Bidirectional P-tunnels"), when MP2MP
   tunnels are used for MVPN, the root of the MP2MP tunnel may need to
   allocate and advertise "PE Distinguisher Labels".  RFC 7582 states
   that these labels are upstream-assigned, from the label space used by
   the root node for its upstream-assigned labels.

   It is REQUIRED by this document that the PE Distinguisher labels
   allocated by a particular node come from the same label space that
   the node uses to allocate its VPN-identifying labels.

2.2.2.  Segmented Tunnels

   There are some additional issues to be considered when MVPN or EVPN
   is using "tunnel segmentation" (see [RFC6514], [RFC7524], and
   [I-D.ietf-bess-evpn-bum-procedure-updates] Sections 5 and 6).

2.2.2.1.  Selective Tunnels

   For "selective tunnels" (see [RFC6513] Sections 2.1.1 and 3.2.1, and
   [I-D.ietf-bess-evpn-bum-procedure-updates] Section 4), the procedures
   outlined above work only if tunnel segmentation is not used.

   A selective tunnel carries one or more particular sets of flows to a
   particular subset of the PEs that attach to a given VPN or BD.  Each
   set of flows is identified by a Selective PMSI A-D route [RFC6514].
   The PTA of the S-PMSI route identifies the tunnel used to carry the
   corresponding set of flows.  Multiple S-PMSI routes can identify the
   same tunnel.

   When tunnel segmentation is applied to a S-PMSI, certain nodes are
   "segmentation points".  A segmentation point is a node at the
   boundary between two "segmentation regions".  Let's call these
   "region A" and "region B".  A segmentation point is an egress node
   for one or more selective tunnels in region A, and an ingress node
   for one or more selective tunnels in region B.  A given segmentation
   point must be able to receive traffic on a selective tunnel from
   region A, and label switch the traffic to the proper selective tunnel
   in region B.

   Suppose one selective tunnel (call it T1) in region A is carrying two
   flows, Flow-1 and Flow-2, identified by S-PMSI route Route-1 and
   Route-2 respectively.  However, it is possible that, in region B,
   Flow-1 is not carried by the same selective tunnel that carries Flow-

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   2.  Let's suppose that in region B, Flow-1 is carried by tunnel T2
   and Flow-2 by tunnel T3.  Then when the segmentation point receives
   traffic from T1, it must be able to label switch Flow-1 from T1 to
   T2, while also label switching Flow-2 from T1 to T3.  This implies
   that Route-1 and Route-2 must signal different labels in the PTA.
   For comparison, when segmentation is not used, they can all use the
   common per-VPN/BD DCB label.

   In this case, it is not practical to have a central authority assign
   domain-wide unique labels to individual S-PMSI routes.  To address
   this problem, all PEs can be assigned disjoint label blocks in those
   few context label spaces, and each will allocate labels for segmented
   S-PMSI independently from its assigned label block that is different
   from any other PE's.  For example, PE1 allocates from label block
   [101~200], PE2 allocates from label block [201~300], and so on.

   Allocating from disjoint label blocks can be used for VPN/BD/ES
   labels as well, though it does not address the original scaling
   issue, because there would be one million labels allocated from those
   a few context label spaces in the original example, instead of just
   one thousand common labels.

2.2.2.2.  Per-PE/Region Tunnels

   Similarly, for segmented per-PE (MVPN (C-*,C-*) S-PMSI or EVPN IMET)
   or per-AS/region (MVPN Inter-AS I-PMSI or EVPN per-Region I-PMSI)
   tunnels, labels need to be allocated per PMSI route.  In case of per-
   PE PMSI route, the labels should be allocated from the label block
   allocated to the advertising PE.  In case of per-AS/region PMSI
   route, different ASBR/RBRs attached to the same source AS/region will
   advertise the same PMSI route.  The same label could be used when the
   same route is advertised by different ASBRs/RBRs, though a simpler
   way is for each ASBR/RBR to allocate its own label from the label
   block allocated to itself.

   In the rest of the document, we call the label allocated for a
   particular PMSI a (per-)PMSI label, just like we have (per-)VPN/BD/ES
   labels.  Notice that using per-PMSI label in case of per-PE PMSI
   still has the original scaling issue associated with the upstream
   allocated label, so per-region PMSIs should be preferred.  Within
   each AS/region, per-PE PMSIs are still used though they do not go
   across border and per-VPN/BD labels can still be used.

   Note that, when a segmentation point re-advertise a PMSI route to the
   next segment, it does not need to re-advertise a new label unless the
   upstream or downstream segment uses Ingress Replication.  [note -
   future revision may extend the applicability of this document to
   Ingress Replication as well]

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2.2.2.3.  Alternative to the per-PMSI Label Allocation

   The per-PMSI label allocation in case of segmentation, whether for
   S-PMSI or for per-PE/Region I-PMSI, is for the segmentation points to
   be able to label switch traffic w/o having to do IP or MAC lookup in
   VRFs (the segmentation points typically do not have those VRFs at
   all).  If the label scaling becomes a concern, alternatively the
   segmentation points could use (C-S,C-G) lookup in VRFs for flows
   identified by the S-PMSIs.  This allows the S-PMSIs for the same VPN/
   BD to share the a VPN/BD-identifying label that leads to lookup in
   the VRFs.  That label should be different from the label used in the
   per-PE/region I-PMSIs though, so that the segmentation points can
   label switch other traffic (not identified by those S-PMSIs).
   However, this moves the scaling problem from the number of labels to
   the number of (C-S/*,C-G) routes in VRFs on the segmentation points.

2.2.3.  Summary of Label Allocation Methods

   In summary, labels can be allocated and advertised the following
   ways:

   1.  A central authority allocates per-VPN/BD/ES labels from the DCB.
       PEs advertise the labels with an indication that they are from
       the DCB.

   2.  A central authority allocates per-VPN/BD/ES labels from a few
       common context label spaces, and allocate labels from the DCB to
       identify those context label spaces.  PEs advertise the VPN/BD
       labels along with the context-identifying labels.

   3.  A central authority assigns disjoint label blocks from those a
       few context label spaces to each PE, and allocate labels from the
       DCB to identify the context label spaces.  Each PE allocates
       labels from its assigned label block independently for its
       segmented S-PMSI, along with the context-identifying labels.

   Option 1 is simplest, but it requires that all the PEs set aside a
   common label block for the DCB that is large enough for all the
   VPNs/BDs/ESes combined.  Option 3 is needed only for segmented
   selective tunnels that are set up dynamically.  Multiple options
   could be used in any combination depending on the deployment
   situation.

3.  Specification

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3.1.  Context Label Space ID Extended Community

   Context Label Space ID Extended Community is a new Transitive Opaque
   EC with the following structure (Sub-Type value to be assigned by
   IANA):

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | 0x03 or 0x43  |   Sub-Type    |      ID-Type                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         ID-Value                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o  ID-Type: A 2-octet field that specifies the type of Label Space
      ID.  In this document, the ID-Type is 0, indicating that the ID-
      Value field is a label.

   o  ID-Value: A 4-octet field that specifies the value of Label Space
      ID.  When it is a label (with ID-Value 0), the most significant
      20-bit is set to the label value.

   This document introduces a DCB flag (to be assigned by IANA) in the
   "Additional PMSI Tunnel Attribute Flags" BGP Extended Community
   [RFC7902].

   In the remainder of the document, when we say a BGP-MVPN/EVPN A-D
   route "carries DCB-flag" or "has DCB-flag attached" we mean the
   following:

   o  The route carries a PMSI Tunnel Attribute (PTA) and its Flags
      field has the Extension bit set

   o  The route carries an "Additional PMSI Tunnel Attribute Flags" EC
      and its DCB flag is set

3.2.  Procedures

   The protocol and procedures specified in this section need not be
   applied unless when BIER, or P2MP/MP2MP tunnel aggregation is used
   for MVPN/EVPN, or BIER/P2MP/MP2MP tunnels are used with EVPN multi-
   homing.

   By means outside the scope of this document, each VPN/BD/ES is
   assigned a label from the DCB or one of those few context label
   spaces, and every PE that is part of the VPN/BD/ES is aware of the
   assignment.  The ES label and the BD label MUST be assigned from the

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   same label space.  If PE Distinguisher labels are used [RFC7582],
   they MUST be allocated from the same label space as well.

   In case of tunnel segmentation, each PE is also assigned a disjoint
   label block from one of those few context label spaces and it
   allocates labels for its segmented PMSI routes from its assigned
   label block.

   When a PE originates/re-advertises an x-PMSI/IMET route, the route
   MUST carry a DCB-flag if and only if the label in its PTA is assigned
   from the DCB.

   If the VPN/BD/ES/PMSI label is assigned from one of those few context
   label spaces, a Context Label Space ID Extended Community is attached
   to the route.  The ID-Type in the EC is set to 0 and the ID-Value is
   set to a label allocated from the DCB and identifies the context
   label space.  When an ingress PE sends traffic, it imposes the DCB
   label that identifies the context label space after it imposes the
   label (that is advertised in the PTA's Label field of the x-PMSI/IMET
   route) for the VPN/BD and/or the label (that is advertised in the ESI
   Label EC) for the ESI, and then imposes the encapsulation for the
   transport tunnel.

   When a PE receives an x-PMSI/IMET route with the Context Label Space
   ID EC, it MUST program its default MPLS forwarding table to map the
   label in the EC that identifies the context label space to a
   corresponding context label table in which the next label lookup is
   done for traffic that this PE receives.

   Then, the receiving PE MUST program the label in the PTA or ESI Label
   EC into either the default mpls forwarding table (if the route
   carries the DCB-flag) or the context label table (if the Context
   Label Space ID EC is present) according to the x-PMSI/IMET route.

   A PE MUST NOT both carry the DCB-flag in an x-PMSI/IMET route and
   attach the Context Label Space ID EC in the route.  A PE MUST ignore
   a received route with both the DCB-flag and the Context Label Space
   ID EC attached, treating as if it was not received.  If neither the
   DCB-flag nor the Context Label Space ID EC is attached, the label in
   the PTA or ESI Label EC MUST be treated as the upstream allocated
   from the source PE's label space, and procedures in
   [RFC6514][RFC7432] MUST be followed.

   In case of MPLS P2MP tunnels, if two x-PMSI/IMET routes specify the
   same tunnel, one of the following conditions MUST be met, so that a
   receiving PE can correctly interpret the label that follows the
   tunnel label in the right context.

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   o  They MUST all have the DCB-flag, or,

   o  They MUST all carry the Context Label Space ID EC, or,

   o  None of them has the DCB-flag, or,

   o  None of them carry the Context Label Space ID EC.

4.  Security Considerations

   This document allows three methods (Section 2.2.3) of label
   allocation for MVPN/EVPN PEs and specifies corresponding signaling
   and procedures.  The first method is the equivalent of using common
   SRGBs from the regular per platform label space.  The second one is
   the equivalent of using common SRGBs from a third party's label
   space, which is also considered as an upstream-assigned label
   allocation [RFC5331].  The third method is a variation of the second,
   in that the third party's label space is divided into disjoint blocks
   for use by different PEs, who will use labels from their respective
   block to send traffic.  In all cases, a receiving PE is able to
   identify one of a few LFIBs to forward incoming labeled traffic.

   Therefore, this document does not introduce new security issues
   besides what have been discussed in existing specifications [RFC5331]
   [RFC6514] [RFC7432] [RFC8402].

5.  IANA Considerations

   IANA has made the following assignments:

   o  Bit 47 (DCB) from the "Additional PMSI Tunnel Attribute Flags"
      registry

        Bit Flag    Name                             Reference
        --------    ----------------------           -------------
        47          DCB (temporary)                  This document

   o  Sub-type 0x08 for "Context Label Space ID Extended Community" from
      the "Transitive Opaque Extended Community Sub-Types" registry

        Sub-Type Value    Name                             Reference
        --------------    ----------------------           -------------
        0x08              Context Label Space ID           This document
                              Extended Community

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6.  Acknowledgements

   The authors thank Stephane Litkowski, Ali Sajassi and Jingrong Xie
   for their review of, comments on and suggestions for this document.

7.  Contributors

   The following also contributed to this document.

   Selvakumar Sivaraj
   Juniper Networks

   Email: ssivaraj@juniper.net

8.  References

8.1.  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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC6513]  Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/
              BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February
              2012, <https://www.rfc-editor.org/info/rfc6513>.

   [RFC6514]  Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
              Encodings and Procedures for Multicast in MPLS/BGP IP
              VPNs", RFC 6514, DOI 10.17487/RFC6514, February 2012,
              <https://www.rfc-editor.org/info/rfc6514>.

   [RFC7432]  Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
              Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
              Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
              2015, <https://www.rfc-editor.org/info/rfc7432>.

   [RFC7524]  Rekhter, Y., Rosen, E., Aggarwal, R., Morin, T.,
              Grosclaude, I., Leymann, N., and S. Saad, "Inter-Area
              Point-to-Multipoint (P2MP) Segmented Label Switched Paths
              (LSPs)", RFC 7524, DOI 10.17487/RFC7524, May 2015,
              <https://www.rfc-editor.org/info/rfc7524>.

   [RFC7582]  Rosen, E., Wijnands, IJ., Cai, Y., and A. Boers,
              "Multicast Virtual Private Network (MVPN): Using
              Bidirectional P-Tunnels", RFC 7582, DOI 10.17487/RFC7582,
              July 2015, <https://www.rfc-editor.org/info/rfc7582>.

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   [RFC7902]  Rosen, E. and T. Morin, "Registry and Extensions for
              P-Multicast Service Interface Tunnel Attribute Flags",
              RFC 7902, DOI 10.17487/RFC7902, June 2016,
              <https://www.rfc-editor.org/info/rfc7902>.

   [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>.

8.2.  Informative References

   [I-D.ietf-bess-evpn-bum-procedure-updates]
              Zhang, Z., Lin, W., Rabadan, J., Patel, K., and A.
              Sajassi, "Updates on EVPN BUM Procedures", draft-ietf-
              bess-evpn-bum-procedure-updates-14 (work in progress),
              November 2021.

   [I-D.ietf-bier-evpn]
              Zhang, Z., Przygienda, A., Sajassi, A., and J. Rabadan,
              "EVPN BUM Using BIER", draft-ietf-bier-evpn-05 (work in
              progress), December 2021.

   [RFC5331]  Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
              Label Assignment and Context-Specific Label Space",
              RFC 5331, DOI 10.17487/RFC5331, August 2008,
              <https://www.rfc-editor.org/info/rfc5331>.

   [RFC8279]  Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
              Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
              Explicit Replication (BIER)", RFC 8279,
              DOI 10.17487/RFC8279, November 2017,
              <https://www.rfc-editor.org/info/rfc8279>.

   [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>.

   [RFC8556]  Rosen, E., Ed., Sivakumar, M., Przygienda, T., Aldrin, S.,
              and A. Dolganow, "Multicast VPN Using Bit Index Explicit
              Replication (BIER)", RFC 8556, DOI 10.17487/RFC8556, April
              2019, <https://www.rfc-editor.org/info/rfc8556>.

Authors' Addresses

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   Zhaohui Zhang
   Juniper Networks

   EMail: zzhang@juniper.net

   Eric Rosen
   Individual

   EMail: erosen52@gmail.com

   Wen Lin
   Juniper Networks

   EMail: wlin@juniper.net

   Zhenbin Li
   Huawei Technologies

   EMail: lizhenbin@huawei.com

   IJsbrand Wijnands
   Individual

   EMail: ice@braindump.be

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