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Network Working Group                                          T. Eckert
Internet-Draft                                                    Huawei
Intended status: Informational                               Mar 5, 2018
Expires: September 6, 2018

  Framework for Traffic Engineering with BIER-TE forwarding (Bit Index
             Explicit Replication with Traffic Engineering)


   BIER-TE is an application-state free, (loose) source routed multicast
   forwarding method where every hop and destination is identified via
   bits in a bitstring of the data packets.  It is described in
   [I-D.ietf-bier-te-arch].  BIER-TE is a variant of [RFC8279] in
   support of such explicit path engineering.

   This document described the traffic engineering control framework for
   use with the BIER-TE forwarding plane: How to enable the ability to
   calculate paths and integrate this forwarding plane into an overall
   TE solution.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   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 September 6, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of

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   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 and Overview . . . . . . . . . . . . . . . . . .   2
   2.  BIER-TE Topology management . . . . . . . . . . . . . . . . .   6
     2.1.  Operational model . . . . . . . . . . . . . . . . . . . .   6
     2.2.  BIER-TE topology model  . . . . . . . . . . . . . . . . .   7
     2.3.  Consistency checking  . . . . . . . . . . . . . . . . . .  10
     2.4.  Auto-configuration  . . . . . . . . . . . . . . . . . . .  11
   3.  Flow Management . . . . . . . . . . . . . . . . . . . . . . .  12
     3.1.  Operational / Architectural Models  . . . . . . . . . . .  12
       3.1.1.  Overprovisioning  . . . . . . . . . . . . . . . . . .  13
       3.1.2.  PCEC  . . . . . . . . . . . . . . . . . . . . . . . .  13  per-flow QoS - policer/shaper/EF  . . . . . . . .  14  DiffServ QoS  . . . . . . . . . . . . . . . . . .  15
     3.2.  BIER-TE flow model  . . . . . . . . . . . . . . . . . . .  15
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   7.  Change log [RFC Editor: Please remove]  . . . . . . . . . . .  17
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction and Overview

   This document proposes a framework and abstract data model for the
   control plane of BIER-TE as defined in [I-D.ietf-bier-te-arch] (BIER-
   TE-ARCH).  That document primarily defines the forwarding plane and
   provides some example scenarios how to use it.

   BIER-TE is a forwarding plane derived from BIER ([RFC8279]) in which
   the destinations of packets are bits in a bitstring.  Every bit
   indicates a destination (BFER - BIER Forwarding Exit Router) and an
   IGP is used to flood those "bit addresses" so hops along the path
   from sender (BFIR - BIER Forwarding Ingres Router) through
   intermediate nodes (BFR) can calculate the shortest path for each
   destination (bit) and simply copy the received packet to every
   interface to one or more bits set in the packet.

   In BIER-TE, shortest path calculation is replaced by bits of the
   bitstring indicating intermediate hops and pre-populated forwarding
   tables (BIFT - Bit Index Forwarding Tables) on every BFR indicating

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   those bits.  In the simplest case, every interface on a BFR has a
   unique bit assigned to it, and the BIFT of only that BFR will have in
   its BIFT for this bit an adjacency entry indicating that interface.
   This ultimately allows to indicate any sub-graph of the network
   topology as a bitstring and hop-by-hop perform the necessary
   forwarding/replication for a packet with such a bitstring.  More
   complex semantics of bits are used to help saving bits.  A typical
   bitstring size supportable is 256 bits, the original BIER
   specification allows up to 1024 bits.  BIER-TE may be specifically
   interesting for typically smaller topologies such as often
   encountered in DetNet scenarios, or else through intelligent
   allocating and saving of bits for larger topologies, some of which is
   exemplified in BIER-TE-ARCH.

   One can compare BIER-TE in function to Segment Routing in so far that
   it attempts to be as much as possible a per-packet "source-routed"
   (for lack of better term) forwarding paradigm without per-
   application/flow state in the network.  Whereas SR primarily supports
   simple sequential paths indicated as a sequence of SIDs, in BIER-TE,
   the bitstring indicate a directed and acyclic graphs (DAG) - with
   replications.  BIER-TE can also be combined with SR and then bits in
   the bitstring are only required for the nodes (BFR) where replication
   is desired, and the paths between any two such replication nodes
   could be SIDs or stack of SIDs that are selected by assigning bits to
   them (routed adjacencies in the BIER-TE terminology).

   In BIER-TE-ARCH, the control plane is not considered.  In its place,
   a theoretical BIER-TE Controller Host uses unspecified signaling to
   control the setup of the BIER-TE forwarding-plane end to end (all
   bits/adjacencies in all BFR BFITs) and during the lifecycle of
   network device install through the determination of paths for
   specific traffic and changes to the topology.  This document expands
   and refines this simplistic model and intends to serve as the
   framework for follow-up protocol and data model specification work.

   The core forwarding documents relevant to this document are as

   o  [RFC8279] (BIER-ARCH): as summarized above.

   o  [RFC8296] (BIER-ENCAP): The encapsulation for BIER packets using
      MPLS or non-MPLS networks underneath.

   o  [I-D.ietf-bier-te-arch] (BIER-TE-ARCH): as summarized above.

   o  [I-D.thubert-bier-replication-elimination] (BIER-EF-OAM): Extends
      the BIER-TE forwarding from BIER-TE-ARCH to support the
      Elimination Function (EF) and an OAM function.  The Elimination

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      Function is a term from DetNets resilience architecture: Multiple
      copies of traffic flows are carried across disjoint path, merged
      in a BFR running the EF and duplicates are eliminated on that BFR
      based on recognizing duplicate sequence numbers.  Engineered
      multiple transmission paths are a key reason to leverage BIER-TE.

   o  [I-D.huang-bier-te-encapsulation] (BIER-TE-ENCAP): Proposed
      encapsulation based on an extension of BIER-ENCAP.  Identifies
      whether the packet expects to use a BIER or BIER-TE BIFT.  Also
      adds a control-word in support of (optional) elimination function
      (EF) and interprets the pre-existing BFIR-ID and entropy fields as
      a flow-id.

   o  [I-D.eckert-bier-te-frr] (BIER-TE-FRR): This document describes
      protections methods applicable to BIER-TE. 1:1 / end-to-end path
      protection is referenced in this document in the context of DetNet
      style PREF path protection.  The options not discucced yet (TBD)
      in this document are link protection tunnels (such as used in
      RSVP-TE as well) and the novel BIER-TE specific protection method,
      in which nodes modify the bitstring upon local discovery of a

   The relevant routing underlay documents are as follows:

   o  [I-D.ietf-bier-isis-extensions] (BIER-ISIS),
      [I-D.ietf-bier-ospf-bier-extensions] (BIER-OSPF): The BIER-ISIS
      and BIER-OSPF documents describe extensions to those two IGPs in
      support of BIER.  Effictively, every BFR announces the <SD,SI-
      range> BIFTs it is configured for, the MT-ID (IGP Multitopology-
      ID) they are using, and the BFR-ID it has in each SD (none if it
      does not need to operate as a BFER).  For MPLS encapsulation, the
      base label for every SD is announced as well as the SI-range (one
      label per <SD,SI> is used).

   o  There is currently no document describing IGP extensions for BIER-
      TE, but the goal is to define those based, using the proposals
      made in this framework, and as feasible re-using and/or amending
      those existing BIER IGP extensions.

   o  [I-D.ietf-bier-bier-yang] (BIER-YANG): This document describes the
      YANG data model to provision on every BFR BIER.  It also provides
      OAM functions.  There is currently no model expanding this to
      support BIER-TE.  This framework document defines elements that
      should be included in a BIER-TE YANG model.

   o  TBD: incomplete list ?.

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                 |<--- BIER-TE domain-->|

                 [Bier-TE Controller Host]
      {PCE controller}, [Provisioning], [Monitoring]

                      ^      ^     ^
                     /       |      \   BIER-TE control protocol
                    |        |       |  Yang(netconf/restconf), PCEP
                    v        v       v

         {per-flow QoS}    ......   {EF,OAM}  Optional per-flow BFIR/BFER
                                           functions (for per-flow TE)

                     BIER-TE forwarding

                   |<------------------>|  IGP extensions for BIER-TE

                   |<------------------>|  Existing IGP
                      Routing underlay    {Existing IGP TE extensions}

                 Unicast forwarding underlay  - IPv4/v6/SR

                 Figure 1: BIER-TE signaling architecture

   The above picture is a modified version of Picture 2 from BIER-TE-
   ARCH reduced by the elements not considered in this document, and
   refined with those that are intended to be described by this

   In comparison with BIER-TE-ARCH, Picture 2, this picture and this
   document do not include considerations for specific multicast flow
   overlay elements.  Instead, it adds description of optional BFIR/BFER
   elements for per-flow QoS/EF (Elimination Function) and OAM, which
   are optional parts of an overall BIER-TE traffic engineering
   architecture.  See BIER-EF-OAM for more background.

   The routing underlay is refined in this document to consider a
   unicast forwarding underlay of IPv4/IPv6 and/or unicast SR (Segment
   Routing) for BIER-TE "forward_routed" adjacencies.  It also assumes
   an existing IGP, such as ISIS or OSPF as the routing underlay.  This
   may include (TBD) extensions already supporting TE aspects (like
   those IGP extensions done for RSVP-TE).

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   This framework intends to support a wide range of options to
   instantiate it:

   In one extreme (PCEC only), there is no IGP in the network that BIER-
   TE depends on, but all BIER-TE operations is managed in an SDN-style
   fashion from centralized components called "BIER-TE Controller Host"
   in BIER-TE-ARCH.  This central packend can be further subdivided into
   a Configuration/Provisioning component to install the BIER-TE
   topology into the network and a PCEC (Pat Computation Engine
   Controller) and (TBD) monitoring components.  After BIER-TE is
   operational, the PCEC calculates BIER-TE bitstrings for BFIR when
   they need to send traffic flow to

   In the other extreme (IGP only), there is no need for a PCEC or NMS.
   The initial setup of the BIER-TE topology can be performed manually,
   using configuration options to support automatic consistency checking
   and partial auto-configuration to simplify this work.  BIER-TE
   extensions of the IGP are used for consistency checking and
   autoconfiguration and finally to provide the whole BIER-TE topology
   to BFIR that can then autonomously calculate BIER-TE bitstrings
   without the help of a PCEC.

2.  BIER-TE Topology management

2.1.  Operational model

   When a network is installed, BIER-TE is added as a service or later
   when it is meant to change, BFR need to be (re)provisioned.  This
   involves a planning phase which physical adjacencies (links) should
   be used in the BIER-TE topology, and which virtual adjacencies
   (routed adjacencies) should be created and assigned bits.  Ultimately
   this means the definition of the BIER-TE topology.

   When the physical topology if the network is smaller than the
   possible bitstring size (e.g.: 256 bits), then this can be a simple,
   fully automated process.  Likewise, if multiple disjoined services
   for BIER-TE each require active subsets of the network topology
   smaller than the network topology, it likewise can be simple to
   create a different SD (subdomain) BIER-TE topologies for each such

   When the required network topology for a BIER-TE service exceeds the
   supportable bitstring size, bit-saving mechanisms can be employed as
   described in BIER-ARCH.  Some of them such as p2p link bits or lan-
   bits are easily automatically calculated.  Creation of virtual
   adjacencies (routed adjacencies) may likely best be done with
   operator defined policies applied to a system a system calculating
   the bits for the BIER-TE topology.

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   Ultimately, if the set of required destinations plus transit hops
   exceeds the size of available bitstrings after optimization, multiple
   BIFT == bitstrings need to be allocated to support this case.  These
   multiple BIFT will likely need to be engineered to minimize duplicate
   traffic load on the network and minimize bit use.  One example shown
   in BIER-TE-ARCH is to allocate different <SD,SI> BIFT to different
   areas of a network, therefore having to create one BIER-TE packet
   copy per required destination region, but in result having only one
   packet copy in each of those regions.

   Provisioning / initial setup can be done manually in simpler networks
   or through a provisioning system.  A PCEP may equally perform this
   function.  If a PCEP is not used to perform this function, but a PCEP
   is used later for Flow Management, then the PCEP does of course need
   to also learn the BIER-TE topologies created by the provisioning

   Unless a PCEC is used for provisioning/initial setup, YANG is likely
   the prefered model to install the BIER-TE topology information into
   the BFR.  If a PCEC is used, YANG or PCEC seem to be valid choices.

   When the network topology expands, bit assignements for the new parts
   of the topology need to be made.  If expansion was not factored into
   the initial bit assignment plans, this can lead to the need to
   reassign bits for existing parts of the topology.  Support for such
   processes could be simplified through additional topology
   information, for example to enable seamless switching of traffic
   flows from bits in one SD over to bits in another SD.  This is
   currently not considered in this document.

2.2.  BIER-TE topology model

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   <BFR> BIFT information:
     Instance: "configured", "operational",
               "learned-configured", "learned-operational" (pce, igp)
       BIFT-ID: <SD subdomain,BSL bitstring length,SI Set Identifier>
       BIFT-Name: string (optional)
       BFR-ID: 16 bit (BIER-TE ID of the <bfr> in this BIFT
                       or undefined if not BFER in this BIFT)
       Ingres-groups: (list of) string (1..16 bytes)
                      (that <bfr> is a member of)
       EF:  <TBD> (optional, parameters for EF Function on this BIFT)
       OAM: <TBD> (optional, parameter for OAM Function on this BIFT)
       Bits: (#BSL - BitStringLength)
         BitIndex: 1...BSL
         BitType(/Tag): "unassigned",
                        (if unassigned, must have no adjacencies)
                        "unique", "p2p", "lan", "leaf", "node", "flood",
                        (more BitTypes defined in text below)
         Names: (list of 0 or more) string (1..16 bytes)
                (for BitTypes that require it)
         List of 0 or more adjacencies:
           (The following is the list of possible types of adjacencies,
            as defined in BIER-TE-ARCH with parameters)
             VRFcontext: string (TBD)
             destination-id: ip-addr (4/16 bytes, router-id/link-local)
             link-id: ifIndex Value (connecting to destination)
             boolean: DNR (Do Not Reset)
             destination-id: 20 bit (SID), 4 or 16 bytes (router-id)
             TBD: path/encap information (e.g: SR SID stack)
             list of 2 or more forward_connect and/or
             forward_routed adjacencies

                  Figure 2: BIER-TE topology information

   The above picture shows informally the data model for BIER-TE
   topology information.  <BFR> is a domain-wide unique identifier of a
   BFR, for example the router-id of the IGP (if an IGP is used).  Every
   <BFR> has a "configured" instance of the BIFT information for every
   BIFT configured on it.  This configuration could be created from
   legacy models, a YANG model, PCEP, or other means.

   Every <BFR> also has an "operational" instance of the BIFT
   information.  If the BFR has nor "learned-configured" / "learned-
   operational" information, then the "operational" instance is just a

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   copy of the "configuration" instance, but would take additional local
   information into account.  For example, if resource limits do not
   allow to activate configured BIFT.  Or when bits in the BIFT point to
   interfaces/adjacencies that are down, this could potentially also be
   reflected in the operational instance.  While the "configuration"
   instance is read/write, the operational instance is read-only (from
   NMS or PCEC).

   To calculate paths/bitstrings through the topology without the help
   of a PCEC, a BIFT would need to know the network wide BIER-TE
   topology.  This topology consists of the "operational" BIFT
   informations of the BFR itself plus the "learned-operational" BIFT
   information from all other BIER-TE nodes in the network plus the
   underlay routing topology information, for example from an IGP.  When
   an IGP is used, the "learned-operational" information of another BFR
   is simply learned because the BFRs are flooding this information as
   IGP information.

   In the absence of any IGP, or the desire not to use it to distribute
   BIER-TE topology information, an NMS or PCEC could collect the
   "operational" BIER-TE topology information from BFRs and distribute
   it to BFIR to enable them to calculate BIER-TE bitstrings

   The operational instance of the topology information can depend on
   the presence of an IGP.  If the adjacency of a bit in the BIFT is
   configured to use a nexthop identifier that has to be learned from an
   IGP, such as a Segment Routing SID or a router-ID, then the
   operational instance (as well as distributed learned-operational
   ones) would indicate that such an adjacency is non-operational if the
   BFR could not resolve this nexthop information.  Forward_connected
   adjacencies do not require a routing underlay, but just link-local

   Some information elements in the BIER-TE topology information is
   metadata to support automatic consistency checking of learned
   topology information which permit to prohibit use of adjacencies that
   would not lead to working paths or worst case could create loops.
   The same information can also be used to auto-configure some
   adjacencies, specifically routed adjacencies, allowing to minimize
   operator work in case BIFT topology information is not auto-created
   from an NMS/PCEP but through manual mechanisms, but also to
   automatically discover mis-wirings and avoid them to be used.

   The semantic of BitType and Names are described in conjunction with
   consistency checking and autoconfiguration in the following sections.

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2.3.  Consistency checking

   The BitType and associated Name or Names for the bit are intended to
   support automated consistency checking and different reactions. an
   NMS can for example discover misconfiguration or miscablings and
   alert the operator.  BFIR can likewise discover misconfiguration when
   the "configured" and "operational" instances of BFR are distributed
   via the IGP and are therefore available as "learned-configured" and
   "learned-operational" on the BFIR.  The BFIR can then fr example stop
   using those misconfigured bits in any bitstrings it calculates and
   further escalate (e.g.: overlay signaling) unreachability of any BFER
   (or inability to calculate paths supporting required TE features).

   "Unique" bits doe not require a name, but the <SD,SI> bit in question
   must only have an adjacency on one BFR.  If it shows up with
   adjacencies on more than one BFR, this is an inconsistency.

   "p2p" bits need to be the same bit on both BFR connected to each
   other via a subnet, and must be pointing to each other via
   "forward_connected" adjacencies.  A "p2p" bit needs to have one Name
   parameter unique in the domain - for example constructed from
   concatenating the IfIndex of both sides.  Note that the actual subnet
   does not need to be p2p, a BFR can have multiple bits across a
   multiaccess subnet, one for each neighbor.

   Not listed in the above picture, but a "remote-p2p" could be a
   BitType when a bidirectional adjacency between two remode BFR using
   forward_routed adjacencies.

   A "leaf" bit is the one shared bit in a <SD,SI> bitstring assigned to
   the "local_decap" adjacency on all leaf BFER.  Leaf BFER do not need
   a separate bit.  See BIER-TE-ARCH.  If more then one "lead" bits are
   used in an <SD,SI> across the domain that is an inconsistency - waste
   of bits.

   A "node" bit is associated with a Name that follows a standardized
   form to identify a node - e.g.: its router-id.  On a non-leaf BFER,
   this bit can only have one local_decap adjacency on the node
   indicated itself.  On a leaf BFER, the "node" bit must be assigned to
   adjacencies on one or BFR that connect to the indicated BFER.  Other
   configurations (or wirings) are a misconfiguration.

   A "lan" bit indicates a bit for a LAN, as discussed in BIER-TE-ARCH.
   It must have one domain wide unique name.  It must only be used by
   BFR connecting to the same subnet with a set of forward_connected
   adjacencies pointing to the other BFR on that subnet.  Disabling the
   use of a "lan" bit either on a BFIR when sending packets, or even
   more son on the actual BFR connecting to a subnet and recognizing

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   inconsistent BIER-TE topolocy configuraiton for it - is the most
   important automatic function to avoid mis-routing of BIER-TE packets.
   The looping will be also stopped because bits are reset when packets
   traverse the paths, or ultimately by TTL, but neither mechanism can
   provide as specifica OAM information about what went wrong than
   recognizing inconsistencies via the IGP.

   TBD: flood bit, DNR (like lan bit, but more complex.

   Consistency checking may happen directly during configuration as well
   as later during rewiring/remot changes of topology.

   In general, the operational instance of the BIER-TE topology are
   relevant to topology consistency checking (as hey are for path
   calculations).  For example, future extensions may actually introduce
   some form of node/BFR redundancy where different BFR are configured
   for the same bits, but only one at a time is actively using a bit,
   and therefore announcing it in the operational instance of the BIER-
   TE topology.

2.4.  Auto-configuration

   For subnets, the actual adjacency to the neighbor on a link may not
   actually be configured explicitly, but only the interface.  Discovery
   of the neighbor via the IGP would result in a complete working
   adjacency for a bit, and that adjacency would show then in the
   operational instance - while the configured instance would only show
   an incomplete adjacency and the bit that was configured for the
   adjacency.  The Name parameter can be used in configuration to lock
   in the BFR that is expected to be on the other side of a subnet
   interface.  If that node is not the one actually connected, the
   adjacency in the operational instance would not be completed.

   When a "p2p" BitType is used, but the bit is configured
   inconsistently on both sides of a p2p link, an autoconfiguration
   mechanism may be specified to select which of the two bits should be
   used (e.g.: bit number configured on the higher router-id peer).
   This could help to auto-correct a configuration mistake, but it does
   of course not recover the inconsistently configured bit directly, it
   just ignores it.

   When a "lan" or "flood" BitType is configured, likewise auto-
   configuration can be done to overcome misconfigurations.  TBD: more

   Most importantly, configuration of routed adjacencies can create most
   need for network-wide consistent configuration.  This can be
   automated with the proposed "group" bittype.

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            (Source)    (midpoint1)  (midpoint2)     (receivers)

            GrpA1          GrpB1          GrpC1          GrpD1

               GrpA2          GrpB2          GrpC2          GrpD2
               ...                                          ...
            GrpA10         GrpB3          GrpC3          GrpD200

                        Figure 3: Group BitType use

   The typical set of forward_routed adjacency is to allow steering of
   BIER-TE packets through a sequence of one or more members of a hop-
   group, load-balancing across them for TE reasons.  In the above
   picture, those paths would start from a BFIR in GrpA and go via one
   (or more) nodes in GrpB, then GrpC and then BFER (GrpD).

   To half-automate the setup of such loose hops, each member of GrpC
   would for example be configured with one unique bit of BitType
   "group" and the Name parameter would be set to "GrpB".  Each
   midpoint1 BFR would "GrpB" in the list of strings for the BIFT
   Ingres-Group parameter.  When such a BFR discovers (e.g.: via the
   IGP) a BFR "learned-operational" bit of BitType group with a name
   "GrpB" (and no adjacency!), then that midpoint1 BFR would create an
   adjacency in its "operational" instance, pointing to the announcing
   BFR with a "forward_routed" adjacency.

   The saving through such group BitTypes is therefore that the bit had
   only to be configured on one node (the receiver side of the
   forward_routed adjacency), but would be configured on any number of
   ingres BFR for the adjacency.  In the above picture, the benefit
   would be biggest if forward_routed adjacencies where used from Source
   to midpoint1, because the number of Sources is potentially largest
   (e.g: as shown in the picture 10 BFIR in Source group).

3.  Flow Management

3.1.  Operational / Architectural Models

   Once a BIER-topology is active in a network, it can be used to pass
   BIER-TE packets.  Typically this also requires the provisioning of
   some routing overlay because today, all applications defined for BIER
   today are classical SP PE-PE application where some customer traffic
   is mapped to SP traffic via PE-PE "overlay" signaling.

   Applications in future environments such as industrial control or IoT
   may result in different overlay signaling.  Even native end-to-end
   BIER-TE from application stacks is possible, but has so far not been

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   Overlay signaling is currently out of scope of this document.

3.1.1.  Overprovisioning

   In the "overprovisioning flow management" model, the network operator
   is responsible to engineer the available network resources, BIER-TE
   Topology and applications generating BIER-TE flows such that the
   required resources can be guaranteed without contention - and
   potentially without the help of either PCEP or IGP, but simply using
   provisioning to configure BFIR and overlay signaling to determine
   active destinations.

   Overprovisioning is the most control/signaling lightweight approach
   and currently the standard approach in most enterprises and service
   provider for IP multicast traffic.

   For example: An ISP with a ++40Gbps network and a comparable small
   amount of high-value muticast traffic requiring in aggregate less
   than 5 Gbps can easily carry all of that multicast traffic across any
   available path.  This is especially easy when the mayority of traffic
   is best effort traffic (such as Internet traffic).  In that case, the
   multicast traffic would be carried in a traffic class that is
   overprovisioned, for example with 6 Gbps guaranteed on every link.
   Calculated BIER-TE bitstrings would for example be used to reduce
   cost of multicast distribution (e.g.: steiner tree calculation), use
   disjoint paths (in conjunction with EF), or simply load-balance
   across all available non-ECMP paths.  Overprovisioning flow
   management is traditional in most SP networks (core/edge/access) for
   IP multicast traffic and requires no additional signaling.

   The overprovisioning flow management model is one that likely would
   request for (only) a YANG model to provision the BIER-TE topology.

3.1.2.  PCEC

   In the PCEC based flow management model, a PCEP determines
   (calculates) the (flow-id,<SD,SI>,bitstring) for a traffic flows and
   signals this to the BFIR sourcing the flow (its BFR-ID is part of the
   flow-id).  If the flow was not statically defined, then this step
   would be preceeded with the BFIR requesting the resources for the,
   indicating the requested resources as well as the set of
   destinations.  The destinations could be indicated as BFR-ID or
   (likely easier for the BFIR) by their unique identifiers in unicast
   routing (e.g.: router-ID).  The bitstring returned by the PCEP would
   include not only engineered paths to all these destinations, but
   those paths could also be disjoint paths, carrying the traffic twice
   towards each destination and merging them via the EF function.  The
   BFIR could be fully agnostic to these PCEP choices.

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   One of the core benefits of using BIER-TE forwarding is the ability
   to change the bitstring on a per-packet basis to re-route traffic by
   setting different transit bits, or to quickly add/delete
   destinations.  When the BFIR should be empowered to perform any of
   these functions without the need for help by the PCEP, then the PCEP
   needs to provide additiona information back to the BFIR.

   If a BFIR has for example an OAM capability to determine without the
   help of a controller that a path has failed (too much packet loss on
   destination, signalled back to BFIR), and dual-transmission is not
   desired (due to double resource usage), then the PCP and BFIR could
   co-operate on a path-protection scheme in which the PCEP provides for
   flows not one, but two bitstrings, one being the backup path which is
   used by the BFIR when it discovers via OAM loss on the currently used
   path.  This approach can extremely reduce the need to rely on
   controller help during failures.

   When the destinations for a particular flow can potentially change
   over time, this can often be faster and more efficiently signalled
   directly via the overlay signaling to the BFIR instead of going
   through the PCEP.  To support this mode of operations, the BFIR could
   request from the PCEP not simply the current set of destinations for
   a flow, but instead the maximum superset of receivers and request
   per-destination information.  The PCEP would then return not just one
   bitstring, but one bitstring per destination (BFER).  The BFIR would
   simply OR the bitstrings for all required destinations for each
   packet to create the final bitstring for that packet.  Note that this
   description of of course on a per-<SD,SI> (aka: per BIFT) basis.
   Destinations using different BIFTs require always different BIER-TE
   packets to be sent by the BFIR.  per-flow QoS - policer/shaper/EF

   In the PCEP based resource management model, it is up to the PCEP to
   determine how explicit resource reservations should be managed, e.g.:
   whether or how it tracks resource consumption.  The BIER-TE
   forwarding plane itself does not support per-flow state with the
   exception of EF, which would usually be a function enabled on BFER.

   Likewise, per-flow policer and/or shaper state may be a useful
   optional feature that the PCEP should be able to request to be
   enabled on a BFIR to ensure that the traffic passed by the BFIR into
   the BIER-TE domain does not overrun resources available.  In the
   simplest case, such a shaper/policer could simply reflect the
   resources indicated by the BFIR in its request to the PCEP.

   Per-flow policer/shaper or EF may need to be explicitly instantiated
   by BFIR/BFER.  Instantiation of the Policer/Shaper on the BFIR can

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   happen as a function of the PCEP signaling to the BFIR, but
   instantiation of the EF would also require signaling of the PCEP to
   the BFER(s) for flows.  Note that EF could also be instantiated on
   any midpoint BFR, so the PCEP would need to know the BIER-TE topology
   including where EF is considered and manage it through appropriate

   Note that it is unclear yet, if EF implemenations could or should be
   implemented with or wihthout the need for explicit instantiation, the
   BIER-TE-EF-OAM document allows both options.  Even in the absence of
   explicit signaling, per-flow Policer/Shaper and EF are limited
   resources and PCEP should keep track of how much of these resources
   are allocated and available for future flows.  Like other path
   resources, exhaustion may require PCEP failure to allocate responses
   or other mitigating options.  DiffServ QoS

   The only resource management that could be expected to exist in the
   BIER-TE domain hop-by-hop would be DiffServ QoS.  As outlined in the
   above overprovisioning resource management model, it can serve as an
   easy method for lightweight resource management, and as soon as the
   network intends to use more than one such DiffServ codepoint across
   different BIER-TE flows, the PCEP should likely be able to understand
   and manage the DiffServ assignments of BIER-TE flows and signal the
   selected codepoint back to the BFIR.

3.2.  BIER-TE flow model

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        BIER-TE traffic flow (change) request (from BFIR):
          Flow-control-ID: <identifier>
          Ingres BFIR of flow: (IGP router-id ?!)
          Destination-ID: set of BFER identifiers (IGP router-id ?!)
          extended-reply-required (boolean)
            TSPEC (bandwidth, burst size,...)
            resilience: dual-transmission with EF
            shared-group: name

        BIER-TE traffic flow reply/command (to BFIR):
          Flow-control-ID: <identifier>
          Ingres Policer/Shaper parameters (applies to each BIFT)
          Set of 1 or more BIFT:
            <SD, SI, BSL>
            BFIR-ID, entropy  (form together flow-ID)
            QoS, TTL,

        BIER-TE traffic flow extended reply/command (to BFIR):
          Flow-control-ID: <identifier>
          Ingres Policer/Shaper parameters (applies to each BIFT)
          Set of 1 or more BIFT:
            <SD, SI, BSL>
            BFIR-ID, entropy  (form together flow-ID)
            QoS, TTL
            List of 1 or more destinations
              Destination-ID, Bitstring

        BIER-TE traffic flow command (to BFER):
          Flow-control-ID: <identifier>
          Ingres BFIR of flow: BFIR-ID (in BIER-TE packet header)
          Set of 1 or more BIFT:
            <SD, SI, BSL>
            BFIR-ID, entropy  (form together flow-ID)
            EF parameter (window size etc..)

                   Figure 4: Flow request/reply/commands

   The above picture shows an initial abstract representation of the
   data models for the different type of request/replies discussed in
   the previous section between PCEC and BFIR (and in one case BFER).

   The Flow-conrol-ID identifies the managed object itself: a flow to be
   sent from one BFIR to a set of BFER with some TE requirements, which
   ultimately may require BIER-TE packets for one or more BIFT.

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   BFIR and BFER need to be identified in the request in a form not
   specific to the bits of BIFT, so the PCEP can select the appropriate
   BIFT(s) to use.  The above picture assumes the router-id of BFIR and
   BFER are appropriate.

   The request includes TE requirements, including (something like a)
   TSPEC for bandwidth, burst-size or the like, whether or not dual-
   transmsision via PREF is required, and if the resource used are to be
   shared across multiple flows, then the name of a shared group.  One
   example of sharing would for example be a video-conference where the
   speaker transmits video, every speaker requests/allocates a BIER-TE
   flow from the PCEP, but the resources for those flows are of course
   shared (only one flow active at a time).

   The reply from the PCEP lists the BIFTS/packets that must be sent by
   the BFIR to reach the desired destinations as well as any other BIER-
   TE packet header fields relevant <SD,SI,BSL>, BFIR-ID, entropy, QoS,
   TTL.  Beside the BIER-TE packet header, the parameters for the
   policer and/or shaper to be used by the BFIR are signalled back.

   The extended reply does not provide simply the bitstring to use for
   each BIFT, but instead lists the bitstrings required for each
   destination so that (as described above), the BFIR can simply add/
   delete destinations on a packet-by-packet basis OR'ing those

   Finally, a command to BFER is required to instruct the creation of EF
   state in case this can not be done automatically.

4.  Security Considerations


5.  IANA Considerations

   This document requests no action by IANA.

6.  Acknowledgements


7.  Change log [RFC Editor: Please remove]

      00: Initial version.

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8.  References

              Eckert, T., Cauchie, G., Braun, W., and M. Menth,
              "Protection Methods for BIER-TE", draft-eckert-bier-te-
              frr-02 (work in progress), June 2017.

              Huang, R., Eckert, T., Wei, N., and P. Thubert,
              "Encapsulation for BIER-TE", draft-huang-bier-te-
              encapsulation-00 (work in progress), March 2018.

              Chen, R., hu, f., Zhang, Z., dai.xianxian@zte.com.cn, d.,
              and M. Sivakumar, "YANG Data Model for BIER Protocol",
              draft-ietf-bier-bier-yang-03 (work in progress), February

              Ginsberg, L., Przygienda, T., Aldrin, S., and Z. Zhang,
              "BIER support via ISIS", draft-ietf-bier-isis-
              extensions-09 (work in progress), February 2018.

              Psenak, P., Kumar, N., Wijnands, I., Dolganow, A.,
              Przygienda, T., Zhang, Z., and S. Aldrin, "OSPF Extensions
              for BIER", draft-ietf-bier-ospf-bier-extensions-15 (work
              in progress), February 2018.

              Eckert, T., Cauchie, G., Braun, W., and M. Menth, "Traffic
              Engineering for Bit Index Explicit Replication (BIER-TE)",
              draft-ietf-bier-te-arch-00 (work in progress), January

              Thubert, P., Eckert, T., Brodard, Z., and H. Jiang, "BIER-
              TE extensions for Packet Replication and Elimination
              Function (PREF) and OAM", draft-thubert-bier-replication-
              elimination-03 (work in progress), March 2018.

   [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,

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   [RFC8296]  Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
              Tantsura, J., Aldrin, S., and I. Meilik, "Encapsulation
              for Bit Index Explicit Replication (BIER) in MPLS and Non-
              MPLS Networks", RFC 8296, DOI 10.17487/RFC8296, January
              2018, <https://www.rfc-editor.org/info/rfc8296>.

Author's Address

   Toerless Eckert
   Futurewei Technologies Inc.
   2330 Central Expy
   Santa Clara  95050

   Email: tte+ietf@cs.fau.de

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