MVPN/EVPN Tunnel Aggregation with Common Labels
draft-ietf-bess-mvpn-evpn-aggregation-label-11

BESS                                                            Z. Zhang
Internet-Draft                                          Juniper Networks
Updates: 7432, 6514, 7582 (if approved)                         E. Rosen
Intended status: Standards Track                              Individual
Expires: 23 February 2024                                         W. Lin
                                                        Juniper Networks
                                                                   Z. Li
                                                     Huawei Technologies
                                                             I. Wijnands
                                                              Individual
                                                          22 August 2023

            MVPN/EVPN Tunnel Aggregation with Common Labels
             draft-ietf-bess-mvpn-evpn-aggregation-label-11

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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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Terminologies . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Problem Description . . . . . . . . . . . . . . . . . . .   5
     2.2.  Proposed Solution . . . . . . . . . . . . . . . . . . . .   6
       2.2.1.  MP2MP Tunnels . . . . . . . . . . . . . . . . . . . .   7
       2.2.2.  Segmented Tunnels . . . . . . . . . . . . . . . . . .   8
       2.2.3.  Summary of Label Allocation Methods . . . . . . . . .  10
   3.  Specification . . . . . . . . . . . . . . . . . . . . . . . .  11
     3.1.  Context-Specific Label Space ID Extended Community  . . .  11
     3.2.  Procedures  . . . . . . . . . . . . . . . . . . . . . . .  11
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  14
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14

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     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Terminologies

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

   *  BUM [RFC7432]: Broadcast, Unknown Unicast, or Multicast (traffic).

   *  BD [RFC7432]: Broadcast Domain.

   *  PMSI [RFC6513]: Provider Multicast Service Interface - a pseudo
      overlay interface for PEs to send certain overlay/customer
      multicast traffic via underlay/provider tunnels.  It includes
      I/S-PMSI (often referred to as x-PMSI) for Inclusive/Selective
      PMSI.  A PMSI is instantiated by the underlay/provider tunnel.

   *  Inclusive PMSI: A PMSI that enables traffic to be sent to all PEs
      of a VPN/BD.  The underlay/provider tunnel that instantiates the
      Inclusive PMSI is referred to as an inclusive tunnel.

   *  Selective PMSI: A PMSI that enables traffic to be sent to a sub
      set of PEs of a VPN/BD.  The underlay/provider tunnel that
      instantiates the Selective PMSI is referred to as a selective
      tunnel.

   *  Aggregate Tunnel: a tunnel that instantiates x-PMSIs of multiple
      MVPNs or EVPN BDs.

   *  IMET [RFC7432]: Inclusive Multicast Ethernet Tag route.  An EVPN
      specific name for I-PMSI A-D route.

   *  PTA [RFC6514]: PMSI Tunnel Attribute.  A BGP attribute that may be
      attached to an BGP-MVPN/EVPN x-PMXI A-D routes.

   *  RBR: Regional Border Routers.  Border routers between segmentation
      regions that participate in segmentation procedures.

   *  (C-S,C-G): A Customer/overlay <S,G> multicast flow.

   *  (C-*,C-G): Customer/overlay <*,G> multicast flows.

   *  (C-*,C-*): All Customer/overlay multicast flows.

   *  ESI [RFC7432]: Ethernet Segment Identifier.

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   *  ESI Label[RFC7432]: A label that identifies an Ethernet Segment

   *  SRGB [RFC8402]: SR Global Block, the set of global segments in the
      SR domain.  In SR-MPLS, SRGB is a local property of a node and
      identifies the set of local labels reserved for global segments.

   *  DCB: Domain-wide Common Block, a common block of labels reserved
      on all nodes in a domain.

   *  Context-specific Label Space [RFC5331]: A router may maintain
      additional label spaces besides its default label space.  When the
      label at the top of the stack is not from the default label space,
      there must be some context in the packet that identifies which one
      of those additional label spaces is to be used to lookup the
      label, hence those label spaces are referred to as context-
      specific label spaces.

   *  Upstream-assigned [RFC5331]: When the label at the top of the
      label stack is not assigned by the router receiving the traffic
      from its default label space, the label is referred to as
      upstream-assigned.  Otherwise it is downstream-assigned.  An
      upstream-assigned label must be looked up in a context-specific
      label space specific for the assigner.

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
   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.

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   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,
   for each ingress PE, the egress PE has a context-specific label space
   [RFC5331] that matches the default label space from which the ingress
   PE assigns the 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-specific label space for the ingress PE of the packet.
   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-specific 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:

   *  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.

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

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   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-)assigned
   from the source PE's label space, so the receiving PE needs to
   maintain context-specific 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 entity in
   the provider network 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 default
   label space for assignment by a central entity.  We refer to this
   reserved portion as the "Domain-wide Common Block" (DCB) of labels.
   This is analogous to the identical "Segment Routing Global Block"
   (SRGB) on all nodes that is described in [RFC8402].  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 maps to VPN 0, 1001 to VPN 1, and so
   forth.  Now only 1000 label instead of 1,000,000 labels are needed
   for 1000 VPNs.

   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.

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   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 necessary 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 at the PEs.  Therefore, it is
   better to not include internal P routers in the "domain".  That way
   they do not have to set 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-specific label spaces independent of each PE.  For
   example, if it is too 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-specific label space can be dedicated to those
   10,000 labels.  Each separate context-specific 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-specific 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-specific label
   space for 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 a context-specific
   label space for an ingress PE.  We also augment the signaling so that
   it is possible to indicate that a particular label is from an
   identified context-specific label space that is not for an ingress
   PE.

   Notice that, the VPN/BD/ES-identifying labels from the DCB or from
   those few context-specific label spaces are very similar to VNIs in
   VXLAN.  Allocating a label from the DCB or from a context-specific
   label spaces and communicating them to all PEs is not different from
   allocating VNIs, and is feasible especially with controllers.

2.2.1.  MP2MP Tunnels

   MP2MP tunnels present the same problem (Section 2.1) that can be
   solved the same way (Section 2.2), with the following additional
   requirement.

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   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" (section 4 of
   [RFC6513].  These labels are 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" that instantiate S-PMSIs (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-
   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

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   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 entity 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-specific 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 ([RFC6514] [RFC7432]
   [I-D.ietf-bess-evpn-bum-procedure-updates]), 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
   (Regional Border Routers) 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 that
   requires coordination and a simpler way is for each ASBR/RBR to
   allocate a label from the label block allocated to itself (see
   Section 2.2.2.1).

   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-
   assigned label, so per-region PMSIs is 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-advertises 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.

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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 needs to 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 in the following
   ways:

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

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

   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.

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3.  Specification

3.1.  Context-Specific Label Space ID Extended Community

   Context-Specific 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                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   *  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.

   *  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:

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

   *  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 MAY be used
   when BIER, or P2MP/MP2MP tunnel aggregation is used for MVPN/EVPN, or
   BIER/P2MP/MP2MP tunnels are used with EVPN multi-homing.  When these
   procedures are used, all PE routers and segmentation points MUST
   support the procedures.  It is outside the scope of this document how
   that is ensured.

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   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-specific
   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 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-specific 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-specific label spaces, a Context-Specific Label Space ID
   Extended Community MUST be 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-specific label space.  When an ingress
   PE sends traffic, it imposes the DCB label that identifies the
   context-specific label space after it imposes the label (that is
   advertised in the Label field of the PTA in 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-Specific
   Label Space ID EC, it MUST place an entry in its default MPLS
   forwarding table to map the label in the EC to a corresponding
   context-specific label table.  That table is used for the next label
   lookup for incoming data traffic with the label signaled in the EC.

   Then, the receiving PE MUST place an entry for 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-specific label table (if
   the Context-Specific Label Space ID EC is present) according to the
   x-PMSI/IMET route.

   An x-PMSI/IMET route MUST NOT both carry the DCB-flag and the
   Context-Specific Label Space ID EC.  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-Specific Label Space ID EC is attached, the
   label in the PTA or ESI Label EC MUST be treated as the upstream-
   assigned from the label space of the source PE, and procedures in
   [RFC6514][RFC7432] MUST be followed.

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   If a PE originates two x-PMSI/IMET routes with the same tunnel, it
   MUST ensure one of the following so that the PE receiving the routes
   can correctly interpret the label that follows the tunnel
   encapsulation of data packets arriving on the tunnel.

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

   *  They MUST all carry the Context-Specific Label Space ID EC, or,

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

   *  None of them carry the Context-Specific Label Space ID EC.

   Otherwise, a receiving PE MUST ignore the routes.

4.  Security Considerations

   This document allows three methods (Section 2.2.3) of label
   allocation for MVPN [RFC6514] or EVPN [RFC7432] PEs and specifies
   corresponding signaling and procedures.  The first method is the
   equivalent of using common SRGBs [RFC8402] from the regular per
   platform label space.  The second one is the equivalent of using
   common SRGBs from a third party label space [RFC5531].  The third
   method is a variation of the second, in that the third party 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 label
   forwarding tables to forward incoming labeled traffic.

   None of the [RFC6514], [RFC7432], [RFC8402] and [RFC5331]
   specifications lists any security concerns related to label
   allocation methods, and this document does not introduce new security
   concerns either.

5.  IANA Considerations

   IANA has made the following assignments:

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

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

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

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

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>.

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   [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>.

   [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. J., Lin, W., Rabadan, J., Patel, K., and A.
              Sajassi, "Updates on EVPN BUM Procedures", Work in
              Progress, Internet-Draft, draft-ietf-bess-evpn-bum-
              procedure-updates-14, 18 November 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-bess-
              evpn-bum-procedure-updates-14>.

   [I-D.ietf-bier-evpn]
              Zhang, Z., Przygienda, A., Sajassi, A., and J. Rabadan,
              "EVPN BUM Using BIER", Work in Progress, Internet-Draft,
              draft-ietf-bier-evpn-09, 22 August 2023,
              <https://datatracker.ietf.org/api/v1/doc/document/draft-
              ietf-bier-evpn/>.

   [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>.

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   [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

   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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