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Versions: 00 01 02 03 04 05 06 draft-ietf-bier-te-arch

Network Working Group                                          T. Eckert
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Standards Track                              G. Cauchie
Expires: January 6, 2016                                Bouygues Telecom
                                                            July 5, 2015


     Traffic Enginering for Bit Index Explicit Replication BIER-TE
                      draft-eckert-bier-te-arch-01

Abstract

   This document proposes an architecture for BIER-TE: Traffic
   Engineering for Bit Index Explicit Replication (BIER).

   BIER-TE shares part of its architecture with BIER as described in
   [I-D.wijnands-bier-architecture].  It also proposes to share the
   packet format with BIER.

   BIER-TE forwards and replicates packets like BIER based on a
   BitString in the packet header but it does not require an IGP.  It
   does support traffic engineering by explicit hop-by-hop forwarding
   and loose hop forwarding of packets.  It does support Fast ReRoute
   (FRR) for link and node protection and incremental deployment.
   Because BIER-TE like BIER operates without explicit in-network tree-
   building but also supports traffic engineering, it is more similar to
   SR than RSVP-TE.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at http://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on January 6, 2016.







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Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Layering  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  The Multicast Flow Overlay  . . . . . . . . . . . . . . .   4
     2.2.  The BIER-TE Controller Host . . . . . . . . . . . . . . .   5
       2.2.1.  Assignment of BitPositions to adjacencies of the
               network topology  . . . . . . . . . . . . . . . . . .   5
       2.2.2.  Changes in the network topology . . . . . . . . . . .   5
       2.2.3.  Set up per-multicast flow BIER-TE state . . . . . . .   6
       2.2.4.  Link/Node Failures and Recovery . . . . . . . . . . .   6
     2.3.  The BIER-TE Forwarding Layer  . . . . . . . . . . . . . .   6
     2.4.  The Routing Underlay  . . . . . . . . . . . . . . . . . .   6
   3.  BIER-TE Forwarding  . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  The Bit Index Forwarding Table (BIFT) . . . . . . . . . .   7
     3.2.  Adjacency Types . . . . . . . . . . . . . . . . . . . . .   8
       3.2.1.  Forward Connected . . . . . . . . . . . . . . . . . .   8
       3.2.2.  Forward Routed  . . . . . . . . . . . . . . . . . . .   8
       3.2.3.  ECMP  . . . . . . . . . . . . . . . . . . . . . . . .   8
       3.2.4.  Local Decap . . . . . . . . . . . . . . . . . . . . .   9
     3.3.  Encapsulation considerations  . . . . . . . . . . . . . .   9
     3.4.  Basic BIER-TE Forwarding Example  . . . . . . . . . . . .   9
   4.  BIER-TE Controller Host BitPosition Assignments . . . . . . .  11
     4.1.  P2P Links . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.2.  BFER  . . . . . . . . . . . . . . . . . . . . . . . . . .  12
     4.3.  Leaf BFIRs  . . . . . . . . . . . . . . . . . . . . . . .  12
     4.4.  LANs  . . . . . . . . . . . . . . . . . . . . . . . . . .  12
     4.5.  Hub and Spoke . . . . . . . . . . . . . . . . . . . . . .  13
     4.6.  Rings . . . . . . . . . . . . . . . . . . . . . . . . . .  13
     4.7.  Equal Cost MultiPath (ECMP) . . . . . . . . . . . . . . .  13
     4.8.  Routed adjacencies  . . . . . . . . . . . . . . . . . . .  16



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       4.8.1.  Reducing BitPositions . . . . . . . . . . . . . . . .  16
       4.8.2.  Supporting nodes without BIER-TE  . . . . . . . . . .  16
     4.9.  Using multiple BIFTs  . . . . . . . . . . . . . . . . . .  16
   5.  Avoiding loops and duplicates . . . . . . . . . . . . . . . .  16
     5.1.  Loops . . . . . . . . . . . . . . . . . . . . . . . . . .  17
     5.2.  Duplicates  . . . . . . . . . . . . . . . . . . . . . . .  17
   6.  BIER-TE FRR . . . . . . . . . . . . . . . . . . . . . . . . .  17
     6.1.  The BIER-TE Adjacency FRR Table (BTAFT) . . . . . . . . .  18
     6.2.  FRR in BIER-TE forwarding . . . . . . . . . . . . . . . .  18
     6.3.  FRR in the BIER-TE Controller Host  . . . . . . . . . . .  18
     6.4.  BIER-TE FRR Benefits  . . . . . . . . . . . . . . . . . .  19
   7.  BIER-TE Forwarding Pseudocode . . . . . . . . . . . . . . . .  19
   8.  Further considerations  . . . . . . . . . . . . . . . . . . .  22
     8.1.  BIER-TE and existing FRR  . . . . . . . . . . . . . . . .  22
     8.2.  BIER-TE and Segment Routing . . . . . . . . . . . . . . .  22
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  22
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  23
   12. Change log [RFC Editor: Please remove]  . . . . . . . . . . .  23
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

1.1.  Overview

   This document specifies the architecture for BIER-TE: traffic
   engineering for Bit Index Explicit Replication BIER.

   BIER-TE shares architecture and packet formats with BIER as described
   in [I-D.wijnands-bier-architecture].

   BIER-TE forwards and replicates packets like BIER based on a
   BitString in the packet header but it does not require an IGP.  It
   does support traffic engineering by explicit hop-by-hop forwarding
   and loose hop forwarding of packets.  It does support Fast ReRoute
   (FRR) for link and node protection and incremental deployment.
   Because BIER-TE like BIER operates without explicit in-network tree-
   building but also supports traffic engineering, it is more similar to
   SR than RSVP-TE.

   The key differences over BIER are:

   o  BIER-TE replaces in-network autonomous path calculation by
      explicit paths calculated offpath by the BIER-TE controller host.

   o  In BIER-TE every BitPosition of the BitString of a BIER-TE packet
      indicates one or more adjacencies - instead of a BFER as in BIER.



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   o  BIER-TE in each BFR has no routing table but only a BIER-TE
      Forwarding Table (BIFT) indexed by BitPosition and populated with
      only those adjacencies to which the BFR should replicate packets
      to.

   Currently, BIER-TE does not support BIER-sub-domains and it does not
   not use BFR-id.  BIER-TE headers use the same format as BIER headers
   (BFR-id is set to 0).

1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Layering

   End to end BIER-TE operations consists of four components: The
   "Multicast Flow Overlay", the "BIER-TE Controller Host", the "Routing
   Underlay" and the "BIER-TE forwarding layer".

      Picture 2: Layers of BIER-TE

                   <------BGP/PIM----->
      |<-IGMP/PIM->  multicast flow   <-PIM/IGMP->|
                        overlay

                   [Bier-TE Controller Host]
                      ^      ^     ^
                     /       |      \   BIER-TE control protocol
                    |        |       |  eg.: Netconf/Restconf/Yang
                    v        v       v
    Src -> Rtr1 -> BFIR-----BFR-----BFER -> Rtr2 -> Rcvr

                   |--------------------->|
                   BIER-TE forwarding layer

                   |<- BIER-TE domain-->|

                  |<--------------------->|
                      Routing underlay

2.1.  The Multicast Flow Overlay

   The Multicast Flow Overlay operates as in BIER.  See
   [I-D.wijnands-bier-architecture].  Instead of interacting with the
   BIER layer, it interacts with the BIER-TE Controller Host




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2.2.  The BIER-TE Controller Host

   The BIER-TE controller host is representing the control plane of
   BIER-TE.  It communicates two sets of informations with BFRs:

   During bring-up or modifications of the network topology, the
   controller discovers the network topology, assigns BitPositions to
   adjacencies and signals the resulting mapping of BitPositions to
   adjacencies to each BFR connecting to the adjacency.

   During day-to-day operations of the network, the controller signals
   to BFIRs what multicast flows are mapped to what BitStrings.

   Communications between the BIER-TE controller host to BFRs is ideally
   via standardized protocols and data-models such as Netconf/Retconf/
   Yang.  This is currently outside the scope of this document.  Vendor-
   specific CLI on the BFRs is also a posible stopgap option (as in many
   other SDN solutions lacking definition of standardized data model).

   For simplicity, the procedures of the BIER-TE controller host are
   described in this document as if it is a single, centralized
   automated entity, such as an SDN controller.  It could equally be an
   operator setting up CLI on the BFRs.  Distribution of the functions
   of the BIER-TE controller host is currently outside the scope of this
   document.

2.2.1.  Assignment of BitPositions to adjacencies of the network
        topology

   The BIER-TE controller host tracks the BFR topology of the BIER-TE
   domain.  It determines what adjacencies require BitPositions so that
   BIER-TE explicit paths can be built through them as desired by
   operator policy.

   The controller then pushes the BitPositions/adjacencies to the BIFT
   of the BFRs, populating only those BitPositions to the BIFT of each
   BFR to which that BFR should be able to send packets to - adjacencies
   connecting to this BFR.

2.2.2.  Changes in the network topology

   If the network topology changes (not failure based) so that
   adjacencies that are assigned to BitPositions are no longer needed,
   the controller can re-use those BitPositions for new adjacencies.
   First, these BitPositions need to be removed from any BFIR flow state
   and BFR BIFT state (and BTAFT if FRR is supported, see below), then
   they can be repopulated, first into BIFT (and if FRR is supported
   BTAFT), then into BFIR.



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2.2.3.  Set up per-multicast flow BIER-TE state

   The BIER-TE controller host tracks the multicast flow overlay to
   determine what multicast flow needs to be sent by a BFIR to which set
   of BFER.  It calculates the desired distribution tree across the
   BIER-TE domain based on algorithms outside the scope of this document
   (eg.: CSFP, Steiner Tree,...).  It then pushes the calculated
   BitString into the BFIR.

2.2.4.  Link/Node Failures and Recovery

   When link or nodes fail or recover in the topology, BIER-TE can
   quickly respond with the optional FRR procedures described below.  It
   can also more slowly react by recalculating the BitStrings of
   affected multicast flows.  This reaction is slower than the FR
   procedure because the controller needs to receive link/node up/down
   indications, recalculate the desired BitStrings and push them down
   into the BFIRs. with FRR, this is all performed locally on a BFR
   receiving the adjacency up/down notification.

2.3.  The BIER-TE Forwarding Layer

   When the BIER-TE Forwarding Layer receives a packet, it simply looks
   up the BitPositions that are set in the BitString of the packet in
   the Bit Index Forwarding Table (BIFT) that was populated by the BIER-
   TE controller host.  For every BP that is set in the BitString, and
   that has one or more adjacencies in the BIFT, a copy is made
   according to the type of adjacencies for that BP in the BIFT.  Before
   sending any copy, the BFR resets all BitPositions in the BitString of
   the packet to which it can create a copy.  This is done to inhibit
   that packets can loop.

   If the BFR support BIER-TE FRR operations, then the BIER-TE
   forwarding layer will receive fast adjacency up/down notification
   uses the BIER-TE FRR Adjacency Table to modify the BitString of the
   packet before it performs BIER-TE forwarding.  This is detailed in
   the FRR section.

2.4.  The Routing Underlay

   BIER-TE is sending BIER packets to directly connected BIER-TE
   neighbors as L2 (unicasted) BIER packets without requiring a routing
   underlay.  BIER-TE forwarding uses the Routing underlay for
   forward_routed adjacencies which copy BIER-TE packets to not-
   directly-connected BFRs (see below for adjacency definitions).

   If the BFR intends to support FRR for BIER-TE, then the BIER-TE
   forwarding plane needs to receive fast adjacency up/down



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   notifications: Link up/down or neighbor up/down, eg.: from BFD.
   Providing these notifications is considered to be part of the routing
   underlay in this document.

3.  BIER-TE Forwarding

3.1.  The Bit Index Forwarding Table (BIFT)

   The Bit Index Forwarding Table (BIFT) exists in every BFR.  It is a
   table indexed by BitPosition and is populated by the BIER-TE control
   plane.  Each index can be empty or contain a list of one or more
   adjacencies.

   If the network is so large that the number of BitPositions in a
   single BIFT does not suffice to identify the necessary adjacencies,
   multiple BIFT need to be used, each identified via a separate SI (Set
   Identifier) value.

     ------------------------------------------------------------------
     | Index           |  Adjacencies                                 |
     ==================================================================
     | 1               |  forward_connected(interface,neighbor,DNR)   |
     ------------------------------------------------------------------
     | 2               |  forward_connected(interface,neighbor,DNR)   |
     |                 |  forward_connected(interface,neighbor,DNR)   |
     ------------------------------------------------------------------
     | 3               |  local_decap([VRF])                          |
     ------------------------------------------------------------------
     | 4               |  forward_routed([VRF,]l3-neighbor)           |
     ------------------------------------------------------------------
     | 5               |  <empty>                                     |
     ------------------------------------------------------------------
     | 6               |  ECMP({adjacency1,...adjacencyN}, seed)      |
     ------------------------------------------------------------------
     ...
     | BitStringLength |  ...                                         |
     ------------------------------------------------------------------
                      Bit Index Forwarding Table


   The BIFT is programmed into the data plane of BFRs by the BIER-TE
   controller host and used to forward packets, according to the rules
   specified in the BIER-TE Forwarding Procedures.

   Adjacencies for the same BP when populated in more than one BFR by
   the controller do not have to have the same adjacencies.  This is up
   to the controller.  BPs for p2p links are one case (see below).




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3.2.  Adjacency Types

3.2.1.  Forward Connected

   A "forward_connected" adjacency is towards a directly connected BFR
   neighbor using an interface address of that BFR on the connecting
   interface.  A forward_connected adjacency does not route packets but
   only L2 forwards them to the neighbor.

   Packets sent to an adjacency with "DoNotReset" (DNR) set in the BIFT
   will not have the BitPosition for that adjacency reset when the BFR
   creates a copy for it.  The BitPosition will still be reset for
   copies of the packet made towards other adjacencies.  The can be used
   for example in ring topologies as explained below.

3.2.2.  Forward Routed

   A "forward_routed" adjacency is an adjacency towards a BFR that is
   not a forward_connected adjacency: towards a loopback address of a
   BFR or towards an interface address that is non-directly connected.
   Forward_routed packets are forwarded via the Routing Underlay.

   If the Routing Underlay has multiple paths for a forward_routed
   adjacency, it will perform ECMP independent of BIER-TE for packets
   forwarded across a forward_routed adjacency.

   If the Routing Underlay has FRR, it will perform FRR independent of
   BIER-TE for packets forwarded across a forward_routed adjacency.

3.2.3.  ECMP

   The ECMP mechanisms in BIER are tied to the BIER BIFT and are are
   therefore not directly useable with BIER-TE.  The following
   procedures describe ECMP for BIER-TE that we consider to be
   lightweight but also well manageable.  It leverages the existing
   entropy parameter in the BIER header to keep packets of the flows on
   the same path anbd it introduces a "seed" parameter to allow
   engineering traffic to be polarized or randomized across multiple
   hops.

   An "Equal Cost Multipath" (ECMP) adjacency has a list of two or more
   adjacencies included in it.  It copies the BIER-TE to one of those
   adjacencies based on the ECMP hash calculation.  The BIER-TE ECMP
   hash algorithm must select the same adjacency from that list for all
   packets with the same "entropy" value in the BIER-TE header if the
   same number of adjacencies and same seed are given as parameters.
   Further use of the seed parameter is explained below.




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3.2.4.  Local Decap

   A "local_decap" adjacency passes a copy of the payload of the BIER-TE
   packet to the packets NextProto within the BFR (IPv4/IPv6,
   Ethernet,...).  A local_decap adjacency turns the BFR into a BFER for
   matching packets.  Local_decap adjacencies require the BFER to
   support routing or switching for NextProto to determine how to
   further process the packet.

3.3.  Encapsulation considerations

   Specifications for BIER-TE encapsulation are outside the scope of
   this document.  This section gives explanations and guidelines.

   Because a BFR needs to interpret the BitString of a BIER-TE packet
   differently from a BIER packet, it is necessary to distinguish BIER
   from BIER-TE packets.  BIER MPLS encapsulation for example assigns
   one label by which BFRs recognize BIER packets.  BIER-TE packets
   should be recognized via a second equally assigned label.  If an
   encapsulation does not permit such differentiation, then
   modifications in the BIER header may be necessary to support
   simultaneous BIER and BIER-TE forwarding.

   "forward_routed" requires an encapsulation permitting to unicast
   BIER-TE packets to a specific interface address on a target BFR.
   With MPLS encapsulation, this can simply be done via a label stack
   with that addresses label as the top label - followed by the label
   identifying BIER-TE packets.  With a non-MPLS encapsulation, some
   form of IP tunneling (IP in IP, LISP, GRE) would be required.

   The encapsulation used for "forward_routed" adjacencies can equally
   support existing advanced adjacency information such as "loose source
   routes" via eg: MPLS label stacks or appropriate header extensions
   (eg: for IPv6).

3.4.  Basic BIER-TE Forwarding Example

   Step by step example of basic BIER-TE forwarding.  This does not use
   ECMP or forward_routed adjacencies nor does it try to minimize the
   number of required BitPositions for the topology.











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     Picture 1: Forwarding Example

               [Bier-Te Controller Host]
                       /   | \
                      v    v  v

           | p13   p1 |
           +- BFIR2 --+          |
           |          | p2   p6  |           LAN2
           |          +-- BFR3 --+           |
           |          |          |  p7  p11  |
      Src -+                     +-- BFER1 --+
           |          | p3   p8  |           |
           |          +-- BFR4 --+           +-- Rcv1
           |          |          |           |
           |          |
           | p14  p4  |
           +- BFIR1 --+          |
           |          +-- BFR5 --+ p10  p12  |
         LAN1         | p5   p9  +-- BFER2 --+
                                 |           +-- Rcv2
                                             |
                                             LAN3

          IP  |..... BIER-TE network......| IP

   pXX indicate the BitPositions number assigned by the BIER-TE
   controller host to adjacencies in the BIER-TE topology.  For example,
   p9 is the adjacency towards BFR9 on the LAN connecting to BFER2.

      BIFT BFIR2:
        p13: local_decap()
         p2: forward_connected(BFR3)

      BIFT BFR3:
         p1: forward_connected(BFIR2)
         p7: forward_connected(BFER1)
         p8: forward_connected(BFR4)

      BIFT BFER1:
        p11: local_decap()
         p6: forward_connected(BFR3)
         p8: forward_connected(BFR4)

   ...and so on.

   Traffic needs to flow from BFIR2 towards Rcv1, Rcv2.  The controller
   determines it wants it to pass across the following paths:



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                 -> BFER1 ---------------> Rcv1
    BFIR2 -> BFR3
                 -> BFR4 -> BFR5 -> BFER2 -> Rcv2

   These paths equal to the following BitString: p2, p5, p7, p8, p10,
   p11, p12

   This BitString is set up in BFIR2.  Multicast packets arriving at
   BFIR2 from Src are assigned this BitString.

   BFIR2 forwards based on that BitString.  It has p2 and p13 populated.
   Only p13 is in BitString which has an adjacency towards BFR3.  BFIR2
   resets p2 in BitString and sends a copy towards BFR2.

   BFR3 sees a BitString of p5,p7,p8,p10,p11,p12.  It is only interested
   in p1,p7,p8.  It creates a copy of the packet to BFER1 (due to p7)
   and one to BFR4 (due to p8).  It resets p7, p8 before sending.

   BFER1 sees a BitString of p5,p10,p11,p12.  It is only interested in
   p6,p7,p8,p11 and therefore considers only p11. p11 is a "local_decap"
   adjacency installed by the BIER-TE controller host because BFER1
   should pass packets to IP multicast.  The local_decap adjacency
   instructs BFER1 to create a copy, decapsulate it from the BIER header
   and pass it on to the NextProtocol, in this example IP multicast.  IP
   multicast will then forward the packet out to LAN2 because it did
   receive PIM or IGMP joins on LAN2 for the traffic.

   Further processing of the packet in BFR4, BFR5 and BFER2 accordingly.

4.  BIER-TE Controller Host BitPosition Assignments

   This section describes how the BIER-TE controller host can use the
   different BIER-TE adjacency types to define the BitPositions of a
   BIER-TE domain.

   Because the size of the BitString is limiting the size of the BIER-TE
   domain, many of the options described exist to support larger
   topologies with fewer BitPositions (4.1, 4.3, 4.4, 4.5, 4.6, 4.7,
   4.8).

4.1.  P2P Links

   Each P2p link in the BIER-TE domain is assigned one unique
   BitPosition with a forward_connected adjacency pointing to the
   neighbor on the p2p link.






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4.2.  BFER

   Every BFER is given a unique BitPosition with a local_decap
   adjacency.

4.3.  Leaf BFIRs

   Leaf BFIRs are BFIRs where incoming BIER-TE packets never need to be
   forwarded to another BFR but are only sent to the BFIR to exit the
   BIER-TE domain.  For example, in networks where PEs are spokes
   connected to P routers, those PEs are Leaf BFIRs unless there is a
   U-turn between two PEs.

   All leaf-BFIR in a BIER-TE domain can share a single BitPosition.
   This is possible because the BitPosition for the adjacency to reach
   the BFIR can be used to distinguish whether or not packets should
   reach the BFIR.

   This optimization will not work if an upstream interface of the BFIR
   is using a BitPosition optimized as described in the following two
   sections (LAN, Hub and Spoke).

4.4.  LANs

   In a LAN, the adjacency to each neighboring BFR on the LAN is given a
   unique BitPosition.  The adjacency of this BitPosition is a
   forward_connected adjacency towards the BFR and this BitPosition is
   populated into the BIFT of all the other BFRs on that LAN.

            BFR1
             |p1
      LAN1-+-+---+-----+
          p3|  p4|   p2|
          BFR3 BFR4  BFR7

   If Bandwidth on the LAN is not an issue and most BIER-TE traffic
   should be copied to all neighbors on a LAN, then BitPositions can be
   saved by assigning just a single BitPosition to the LAN and
   populating the BitPosition of the BIFTs of each BFRs on the LAN with
   a list of forward_connected adjacencies to all other neighbors on the
   LAN.

   This optimization does not work in the face of BFRs redundantly
   connected to more than one LANs with this optimization because these
   BFRs would receive duplicates and forward those duplicates into the
   opposite LANs.  Adjacencies of such BFRs into their LANs still need a
   separate BitPosition.




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4.5.  Hub and Spoke

   In a setup with a hub and multiple spokes connected via separate p2p
   links to the hub, all p2p links can share the same BitPosition.  The
   BitPosition on the hubs BIFT is set up with a list of
   forward_connected adjacencies, one for each Spoke.

   This option is similar to the BitPosition optimization in LANs:
   Redundantly connected spokes need their own BitPositions.

4.6.  Rings

   In L3 rings, instead of assigning a single BitPosition for every p2p
   link in the ring, it is possible to save BitPositions by setting the
   "Do Not Reset" (DNR) flag on forward_connected adjacencies.

   For the rings shown in the following picture, a single BitPosition
   will suffice to forward traffic entering the ring at BFRa or BFRb all
   the way up to BFR1:

   On BFRa, BFRb, BFR30,... BFR3, the BitPosition is populated with a
   forward_connected adjacency pointing to the clockwise neighbor on the
   ring and with DNR set.  On BFR2, the adjacency also points to the
   clockwise neighbor BFR1, but without DNR set.  Handling DNR this way
   ensures that copies forwarded from any BFR in the ring to a BFR
   outside the ring will not have this BitPosition, therefore minimizing
   the chance to create loops.

                  v        v
                  |        |
           L1     |   L2   |   L3
       /-------- BFRa ---- BFRb --------------------\
       |                                            |
       \- BFR1 - BFR2 - BFR3 - ... - BFR29 - BFR30 -/
           |      |    L4               |      |
        p33|                         p15|
           BFRd                       BFRc

4.7.  Equal Cost MultiPath (ECMP)

   The ECMP adjacency allows to use just one BP per link bundle between
   two BFRs instead of one BP for each p2p member link of that link
   bundle.  In the following picture, one BP is used across L1,L2,L3 and
   BFR1/BFR2 have for the BP







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                --L1-----
           BFR1 --L2----- BFR2
                --L3-----

     BIFT entry in BFR1:
     ------------------------------------------------------------------
     | Index |  Adjacencies                                           |
     ==================================================================
     | 6     |  ECMP({L1-to-BFR2,L2-to-BFR2,L3-to-BFR2}, seed)        |
     ------------------------------------------------------------------

     BIFT entry in BFR2:
     ------------------------------------------------------------------
     | Index |  Adjacencies                                           |
     ==================================================================
     | 6     |  ECMP({L1-to-BFR1,L2-to-BFR1,L3-to-BFR1}, seed)        |
     ------------------------------------------------------------------

   In the following example, all traffic from BFR1 towards BFR10 is
   intended to be ECMP load split equally across the topology.  This
   example is not mean as a likely setup, but to illustrate that ECMP
   can be used to share BPs not only across link bundles, and it
   explains the use of the seed parameter.




























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                    BFR1
                  /     \
                 /L11    \L12
             BFR2         BFR3
            /    \       /    \
           /L21   \L22  /L31   \L32
          BFR4  BFR5   BFR6  BFR7
           \      /     \      /
            \    /       \    /
             BFR8         BFR9
                 \       /
                  \     /
                   BFR10

     BIFT entry in BFR1:
     ------------------------------------------------------------------
     | 6     |  ECMP({L11-to-BFR2,L12-to-BFR3}, seed)                 |
     ------------------------------------------------------------------

     BIFT entry in BFR2:
     ------------------------------------------------------------------
     | 6     |  ECMP({L21-to-BFR4,L22-to-BFR5}, seed)                 |
     ------------------------------------------------------------------

     BIFT entry in BFR3:
     ------------------------------------------------------------------
     | 6     |  ECMP({L31-to-BFR6,L32-to-BFR7}, seed)                 |
     ------------------------------------------------------------------

   With the setup of ECMP in above topology, traffic would not be
   equally load-split.  Instead, links L22 and L31 would see no traffic
   at all: BFR2 will only see traffic from BFR1 for which the ECMP hash
   in BFR1 selected the first adjacency in a list of 2 adjacencies: link
   L11-to-BFR2.  When forwarding in BFR2 performs again an ECMP with two
   adjacencies on that subset of traffic, then it will again select the
   first of its two adjacencies to it: L21-to-BFR4.  And therefore L22
   and BFR5 sees no traffic.

   To resolve this issue, the ECMP adjaceny on BFR1 simply needs to be
   set up with a different seed than the ECMP adjacncies on BFR2/BFR3

   This issue is called polarization.  It depends on the ECMP hash.  It
   is possible to build ECMP that does not have polarization, for
   example by taking entropy from the actual adjacency members into
   account, but that can make it harder to achieve evenly balanced load-
   splitting on all BFR without making the ECMP hash algorithm
   potentially too complex for fast forwarding in the BFRs.




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4.8.  Routed adjacencies

4.8.1.  Reducing BitPositions

   Routed adjacencies can reduce the number of BitPositions required
   when the traffic engineering requirement is not hop-by-hop explicit
   path selection, but loose-hop selection.

              ...............             ...............
       BFR1--... Redundant ...--L1-- BFR2... Redundant ...---
          \--... Network   ...--L2--/    ... Network   ...---
       BFR4--... Segment 1 ...--L3-- BFR3... Segment 2 ...---
              ...............             ...............

   Assume the requirement in above network is to explicitly engineer
   paths such that specific traffic flows are passed from segment 1 to
   segment 2 via link L1 (or via L2 or via L3).

   To achieve this, BFR1 and BFR4 are set up with a forward_routed
   adjacency BitPosition towards an address of BFR2 on link L1 (or link
   L2 BFR3 via L3).

   For paths to be engineered through a specific node BFR2 (or BFR3),
   BFR1 and BFR4 are set up up with a forward_routed adjacency
   BitPosition towards a loopback address of BFR2 (or BFR3).

4.8.2.  Supporting nodes without BIER-TE

   Routed adjacencies also enable incremental deployment of BIER-TE.
   Only the nodes through which BIER-TE traffic needs to be steered -
   with or without replication - need to support BIER-TE.  Where they
   are not directly connected to each other, forward_routed adjacencies
   are used to pass over non BIER-TE enabled nodes.

4.9.  Using multiple BIFTs

   In a large network, multiple BIFT may be necessary, each one
   identified by a different SI value in the BIER header.  Transit
   adjacencies may need to be given BitPositions in multiple BIFTs to
   achieve the desired path engineering for packets replicated with
   different SIs/BIFTs.

5.  Avoiding loops and duplicates








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5.1.  Loops

   Whenever BIER-TE creates a copy of a packet, the BitString of that
   copy will have all BitPositions cleared that are associated with
   adjacencies in the BFR.  This inhibits looping of packets.  The only
   exception are adjacencies with DNR set.

   With DNR set, looping can happen.  Consider in the ring picture that
   link L4 from BFR3 is plugged into the L1 interface of BFRa.  This
   creates a loop where the rings clockwise BitPosition is never reset
   for copies of the packets traveling clockwise around the ring.

   To inhibit looping in the face of such physical misconfiguration,
   only forward_connected adjacencies are permitted to have DNR set, and
   the link layer destination address of the adjacency (eg.: MAC
   address) protects against closing the loop.  Link layers without port
   unique link layer addresses should not used with the DNR flag set.

5.2.  Duplicates

   Duplicates happen when the topology of the BitString is not a tree
   but redundantly connecting BFRs with each other.  The controller must
   therefore ensure to only create BitStrings that are trees in the
   topology.

   When links are incorrectly physically re-connected before the
   controller updates BitStrings in BFIRs, duplicates can happen.  Like
   loops, these can be inhibited by link layer addressing in
   forward_connected adjacencies.

   If interface or loopback addresses used in forward_routed adjacencies
   are moved from one BFR to another, duplicates can equally happen.
   Such re-addressing operations must be coordinated with the
   controller.

6.  BIER-TE FRR

   FRR is an optional procedure.  To leverage it, the BIER-TE controller
   host and BFRs need to support it.  It does not have to be supported
   on all BFRs, but only those that are attached to a link/adjacency for
   which FRR support is required.

   If BIER-TE FRR is supported by the BIER-TE controller host, then it
   needs to calculate the desired backup paths for link and/or node
   failures in the BIER-TE domain and download this information into the
   BIER-TE Adjacency FRR Table (BTAFT) of the BFRs.  The BTAFT then
   drives FRR operations in the BIER-TE forwarding plane of that BFR.




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6.1.  The BIER-TE Adjacency FRR Table (BTAFT)

   The BIER-TE IF FRR Table exists in every BFR that is supporting BIER-
   TE FRR procedures.  It is indexed by FRR Adjacency Index.  Associated
   with each FRR Adjacency Index is a ResetBitmask, AddBitmask and
   BitPosition.

     -----------------------------------------------------------
     | FRR Adjacency | BitPosition | ResetBitmask | AddBitmask |
     | Index         |             |              |            |
     ===========================================================
     | 1             |   5         |  ..0010000   | ..11000000 |
     -----------------------------------------------------------
     ...

   An FRR Adjacency is an adjacency that is used in the BIFT of the BFR.
   The BFR has to be able to determine whether the adjacency is up or
   down in less than 50msec.  An FRR adjacency can be a
   forward_connected adjacency with fast L2 link state Up/Down state
   notifications or a forward_connected or forward_routed adjacency with
   a fast aliveness mechanism such as BFD.  Details of those mechanism
   are outside the scope of this architecture.

   The FRR Adjacency Index is the index that would be indicated on the
   fast Up/Down notifications to the BIER-TE forwarding plane

   The BitPosition is the BP in the BIFT in which the FRR Adjacency is
   used

6.2.  FRR in BIER-TE forwarding

   The BIER-TE forwarding plane receives fast Up/Down notifications with
   the FRR Adjacency Index.  From the BitPosition in the BTAFT entry, it
   remembers which BPs are currently affected (have a down adjacency).

   When a packet is received, BIER-TE forwarding checks if it has
   affected BPs to which it would forward.  If it does, it will remove
   the ResetBitmask bits from the packets BitString and add the
   AddBitmask bits to the packets BitString.

   Afterwards, normal BIER-TE forwarding occurs, taking the modified
   BitString into account.

6.3.  FRR in the BIER-TE Controller Host

   The basic rules how the BIER-TE controller host would calculate
   ResetBitMask and AddBitmask are as follows:




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   1.  The BIER-TE controller host has to determine whether a failure of
       the adjacency should be taken to indicate link or node failure.
       This is a policy decision.

   2.  The ResetBitmask has the BitPosition of the failed adjacency.

   3.  In the case of link protection, the AddBitmask are the segments
       forming a path from the BFR over to the BFR on the other end of
       the failed link.

   4.  In the case of node protection, the AddBitmask are the segments
       forming a tree from the BFR over to all necessary BFR downstream
       of the (assumed to be failed) BFR across the failed adjacency.

   5.  The ResetBitmask is extended with those segments that could lead
       to duplicate packets if the AddBitmask is added to possible
       BitStrings of packets using the failing BitPosition.

6.4.  BIER-TE FRR Benefits

   Compared to other FRR solutions, such as RSVP-TE/P2MP FRR, BIER-TE
   FRR has two key distinctions

   o  It maintains the goal of BIER-TE not to establish in-network per
      multicast traffic flow state.  For that reason, the backup path/
      trees are only tied to the topology but not to individual
      distribution trees.

   o  For the case of node failure, it allows to build a path engineered
      backup tree (4.) as opposed to only a set of p2p backup tunnels.

7.  BIER-TE Forwarding Pseudocode

   The following sections of Pseudocode are meant to illustrate the
   BIER-TE forwarding plane.  This code is not meant to be normative but
   to serve both as a potentially easier to read and more precise
   representation of the forwarding functionality and to illustrate how
   simple BIER-TE forwarding is and that it can be efficiently be
   implemented.

   The following procedure is executed on a BFR whenever the BIFT is
   changed by the BIER-TE controller host:









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      global MyBitsOfInterest

      void BIFTChanged()
      {

          for (Index = 0; Index++ ; Index <= BitStringLength)
              if(BIFT[Index] != <empty>)
                  MyBitsOfInterest != 2<<(Index-1)
      }

   The following procedure is executed whenever an adjacency used for
   BIER-TE FRR changes state:

      global ResetBitMaskByBT[BitStringLength]
      global AddtBitMaskByBT[BitStringLength]
      global FRRaffectedBP

      void FrrUpDown(FrrAdjacencyIndex, UpDown)
      {
          global FRRAdjacenciesDown
          local Idx = FrrAdjacencyIndex

          if (UpDown == Up)
              FRRAdjacenciesDown &= ~ 2<<(FrrAdjacencyIndex-1)
          else
              FRRAdjacenciesDown |=   2<<(FrrAdjacencyIndex-1)

          for (Index = GetFirstBitPosition(FRRAdjacenciesDown); Index ;
              Index = GetNextBitPosition(FRRAdjacenciesDown, Index))

              local BP = BTAFT[Index].BitPosition
              FRRaffectedBP |= 2<<(Index)
              ResetBitMaskByBT[BP] |= BTAFT[Index].ResetBitMask
              AddBitMaskByBT[BP]   |= BTAFT[Index].AddBitMask
      }

   The following procedure is executed whenever a BIER-TE packet is to
   be forwarded:













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      void ForwardBierTePacket (Packet)
      {
          // We calculate in BitMask the subset of BPs of the BitString
          // for which we have adjacencies. This is purely an
          // optimization to avoid to replicate for every BP
          // set in BitString only to discover that for most of them,
          // the BIFT has no adjacency.

          local BitMask = Packet->BitString
          Packet->BitString &= ~MyBitsOfInterest
          BitMask &= MyBitsOfInterest

          // FRR Operations
          // Note: this algorithm is not optimal yet for ECMP cases
          // it performs FRR replacement for all candidate ECMP paths

          local MyFRRBP = BitMask & FRRaffectedBP
          for (BP = GetFirstBitPosition(MyFRRNP); BP ;
               BP = GetNextBitPosition(MyFRRNP, BP))
              BitMask &= ~ResetBitMaskByBT[BP]
              BitMask |=  ResetBitMaskByBT[BT]

          // Replication
          for (Index = GetFirstBitPosition(BitMask); Index ;
               Index = GetNextBitPosition(BitMask, Index))
              foreach adjacency BIFT[Index]

                  if(adjacency == ECMP(ListOfAdjacencies, seed) )
                      I = ECMP_hash(sizeof(ListOfAdjacencies),
                                    Packet->Entropy, seed)
                      adjacency = ListOfAdjacencies[I]

                  PacketCopy = Copy(Packet)

                  switch(adjacency)
                      case forward_connected(interface,neighbor,DNR):
                          if(DNR)
                              PacketCopy->BitString |= 2<<(Index-1)
                          SendToL2Unicast(PacketCopy,interface,neighbor)

                      case forward_routed([VRF],neighbor):
                          SendToL3(PacketCopy,[VRF,]l3-neighbor)

                      case local_decap([VRF],neighbor):
                          DecapBierHeader(PacketCopy)
                          PassTo(PacketCopy,[VRF,]Packet->NextProto)
      }




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8.  Further considerations

8.1.  BIER-TE and existing FRR

   BIER-TE as described above is an advanced method for mode-protection
   where the replication in a failed node is on the fly replaced by
   another replication tree through bit operations on the BitString.

   If BIER-TE is not feasible or necessary, it is also possible for
   BIER-TE to leverage any existing form of "link" protection.  For
   example: instead of dorectly setting up a forward_connected adjacency
   to a next-hop neighbor, this can be a "protected" adjacency that is
   maintained by RSVP-TE (or another FRR mechanism) and passes via a
   backup path if the link fails.

8.2.  BIER-TE and Segment Routing

   Segment Routing aims to achieve lightweight path engineering via
   loose source routing.  Compared for example to RSVP-TE, it does not
   require per-path signaling to each of these hops.

   BIER-TE is supports the same design philosophy for multicast.  Like
   in SR, it relies on source-routing - via the definition of a
   BitString.  Like SR, it only requires to consider the "hops" on which
   either replication has to happen, or across which the traffic should
   be steered (even without replication).  Any other hops can be skipped
   via the use of routed adjacencies.

   Instead of defining BitPositions for non-replicating hops, it is
   equally possible to use segment routing encapsulations (eg: MPLS
   label stacks) for "forward_routed" adjacencies.

9.  Security Considerations

   The security considerations are the same as for BIER with the
   following differences:

   BFR-ids and BFR-prefixes are not used in BIER-TE, nor are procedures
   for their distribution, so these are not attack vectors against BIER-
   TE.

10.  IANA Considerations

   This document requests no action by IANA.







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11.  Acknowledgements

   The authors would like to thank Greg Shepherd, Ijsbrand Wijnands and
   Neale Ranns for their extensive review and suggestions.

12.  Change log [RFC Editor: Please remove]

      01: Added explanation of SI, difference to BIER ECMP,
      consideration for Segment Routing, unicast FRR, considerations for
      encapsulation, explanations of BIER-TE controller host and CLI.

      00: Initial version.

13.  References

   [I-D.wijnands-bier-architecture]
              Wijnands, I., Rosen, E., Dolganow, A., Przygienda, T., and
              S. Aldrin, "Multicast using Bit Index Explicit
              Replication", draft-wijnands-bier-architecture-05 (work in
              progress), March 2015.

   [I-D.wijnands-mpls-bier-encapsulation]
              Wijnands, I., Rosen, E., Dolganow, A., Tantsura, J., and
              S. Aldrin, "Encapsulation for Bit Index Explicit
              Replication in MPLS Networks", draft-wijnands-mpls-bier-
              encapsulation-02 (work in progress), December 2014.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

Authors' Addresses

   Toerless Eckert
   Cisco Systems, Inc.

   Email: eckert@cisco.com


   Gregory Cauchie
   Bouygues Telecom

   Email: GCAUCHIE@bouyguestelecom.fr









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