[Search] [txt|html|xml|pdf|bibtex] [Tracker] [Email] [Nits]

Versions: 00                                                            
BIER                                                           T. Eckert
Internet-Draft                                Futurewei Technologies USA
Intended status: Experimental                            25 October 2021
Expires: 28 April 2022


   Carrier Grade Minimalist Multicast (CGM2) using Bit Index Explicit
 Replication (BIER) with Recursive BitString Structure (RBS) Addresses
                     draft-eckert-bier-cgm2-rbs-00

Abstract

   This memo introduces the architecture of a multicast architecture
   derived from BIER-TE, which this memo calls Carrier Grade Minimalist
   Multicast (CGM2).  It reduces limitations and complexities of BIER-TE
   by replacing the representation of the in-packet-header delivery tree
   of packets through a "flat" BitString of adjacencies with a
   hierarchical structure of BFR-local BitStrings called the Recursive
   BitString Structure (RBS) Address.

   Benefits of CGM2 with RBS addresses include smaller/fewer BIFT in
   BFR, less complexity for the network architect and in the CGM2
   controller (compared to a BIER-TE controller) and fewer packet copies
   to reach a larger set of BFER.

   The additional cost of forwarding with RBS addresses is a slightly
   more complex processing of the RBS address in BFR compared to a flat
   BitString and the novel per-hop rewrite of the RBS address as opposed
   to bit-reset rewrite in BIER/BIER-TE.

   CGM2 can support the traditional deployment model of BIER/BIER-TE
   with the BIER/BIER-TE domain terminating at service provider PE
   routers as BFIR/BFER, but it is also the intention of this document
   to expand CGM2 domains all the way into hosts, and therefore
   eliminating the need for an IP Multicast flow overlay, further
   reducing the complexity of Multicast services using CGM2.  Note that
   this is not fully detailed in this version of the document.

   This document does not specify an encapsulation for CGM2/RBS
   addresses.  It could use existing encapsulations such as [RFC8296],
   but also other encapsulations such as IPv6 extension headers.

Status of This Memo

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





Eckert                    Expires 28 April 2022                 [Page 1]


Internet-Draft                bier-cgm2-rbs                 October 2021


   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://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 28 April 2022.

Copyright Notice

   Copyright (c) 2021 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 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.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Introduction  . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Encapsulation Considerations  . . . . . . . . . . . . . .   4
   2.  CGM2/RBS Architecture . . . . . . . . . . . . . . . . . . . .   5
   3.  CGM2/RBS forwarding plane . . . . . . . . . . . . . . . . . .   6
     3.1.  RBS BIFT  . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  Reference encoding of RBS addresses . . . . . . . . . . .   8
     3.3.  RBS Address . . . . . . . . . . . . . . . . . . . . . . .   8
       3.3.1.  RecursiveUnit . . . . . . . . . . . . . . . . . . . .   8
       3.3.2.  AddressingField . . . . . . . . . . . . . . . . . . .   9
   4.  BIER-RBS Example  . . . . . . . . . . . . . . . . . . . . . .   9
     4.1.  BFR B . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     4.2.  BFR R . . . . . . . . . . . . . . . . . . . . . . . . . .  12
     4.3.  BFR S . . . . . . . . . . . . . . . . . . . . . . . . . .  13
     4.4.  BFR C . . . . . . . . . . . . . . . . . . . . . . . . . .  14
     4.5.  BFR D . . . . . . . . . . . . . . . . . . . . . . . . . .  14
     4.6.  BFR E . . . . . . . . . . . . . . . . . . . . . . . . . .  15
   5.  RBS forwarding Pseudocode . . . . . . . . . . . . . . . . . .  16
   6.  Operational and design considerations (informational) . . . .  18
     6.1.  Comparison with BIER-TE / BIER  . . . . . . . . . . . . .  18



Eckert                    Expires 28 April 2022                 [Page 2]


Internet-Draft                bier-cgm2-rbs                 October 2021


       6.1.1.  Eliminating the need for large BIFT . . . . . . . . .  18
       6.1.2.  Reducing number of duplicate packet copies across
               BFR . . . . . . . . . . . . . . . . . . . . . . . . .  19
       6.1.3.  BIER-TE forwarding plane complexities . . . . . . . .  20
       6.1.4.  BIER-TE controller complexities . . . . . . . . . . .  20
       6.1.5.  BIER-TE specification complexities  . . . . . . . . .  20
       6.1.6.  Forwarding plane complexity . . . . . . . . . . . . .  21
     6.2.  CGM2 / RBS controller considerations  . . . . . . . . . .  21
     6.3.  Analysis of performance gain with CGM2  . . . . . . . . .  21
     6.4.  Example use case scenarios  . . . . . . . . . . . . . . .  21
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  21
   8.  Security considerations . . . . . . . . . . . . . . . . . . .  21
   9.  Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  21
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     10.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Overview

1.1.  Introduction

   Carrier Grade Minimalist Multicast (CGM2) is an architecture derived
   from the BIER-TE architecture [I-D.ietf-bier-te-arch] with the
   following changes/improvements.

   CGM2 forwarding is based on the principles of BIER-TE forwarding: It
   is based on an explicit, in-packet, "source routed" tree indicated
   through bits for each adjacency that the packet has to traverse.
   Like in BIER-TE, adjacencies can be L2 to a subnet local neighbor in
   support of "native" deployment of CGM2 and/or L3, so-called "routed"
   adjacencies to support incremental or partial deployment of CGM2 as
   needed.

   The address used to replicate packets in the network is not a flat
   network wide BitString as in BIER-TE, but a hierarchical structure of
   BitStrings called a Recursive BitString Structure (RBS) Address.  The
   significance of the BitPositions (BP) in each BitString is only local
   to the BIFT of the router/BFR that is processing this specific
   BitString.

   RBS addressing allows for a more compact representation of a large
   set of adjacencies especially in the common case of sparse set of
   receivers in large Service Provider Networks (SP).

   CGM2 thereby eliminates the challenges in BIER [RFC8279] and BIER-TE
   having to send multiple copies of the same packet in large SP
   networks and the complexities especially for BIER-TE (but also BIER)



Eckert                    Expires 28 April 2022                 [Page 3]


Internet-Draft                bier-cgm2-rbs                 October 2021


   to engineer multiple set identifier (SI) and/or sub-domains (SD)
   BIER-TE topologies for limited size BitStrings (e.g.: 265) to cover
   large network topologies.

   Like BIER-TE, CGM2 is intended to leverage a Controller to minimize
   the control plane complexity in the network to only a simple unicast
   routing underlay required only for routed adjacencies.

   The controller centric architecture provides most easily any type of
   required traffic optimization for its multicast traffic due to their
   need to perform often NP-complete calculations across the whole
   topology: reservation of bandwidth to support CIR/PIR traffic buffer/
   latency to support Deterministic Network (DetNet) traffic, cost
   optimized Steiner trees, failure point disjoint trees for higher
   resilience including DetNet deterministic services.

   CGM2 can be deployed as BIER/BIER-TE are specified today, by
   encapsulating IP Multicast traffic at Provider Edge (PE) routers, but
   it is also considered to be highly desirable to extend CGM2 all the
   way into Multicast Sender/Receivers to eliminate the overhead of an
   Overlay Control plane for that (legacy) IP Multicast layer and the
   need to deal with yet another IP multicast group addressing space.
   In this deployment option Controller signaling extends directly (or
   indirectly via BFIR) into senders.

1.2.  Encapsulation Considerations

   This document does not define a specific BIER-RBS encapsulation nor
   does it preclude that multiple different encapsulations may be
   beneficial to better support different use-cases or operator/user
   technology preferences.  Instead, it discusses considerations for
   specific choices.

   BIER-RBS can easily re-use [RFC8296] encapsulation.  The RBS address
   is inserted into the [RFC8296] BitString field.  The BFR forwarding
   plane needs to be configured (from Controller or control plane) that
   the BIFT-id(s) used with RBS addresses are mapped to BIFT and
   forwarding rules with RBS semantic.

   SI/SD fields of [RFC8296] may be used as in BIER-TE, but given that
   CGM2 is designed (as described in the Overview section) to simplify
   multicast services, a likely and desirable configuration would be to
   only use a single BIFT in each BFR for RBS addresses, and mapping
   these to a single SD and SI 0.

   IP Multicast [RFC1112] was defined as an extension of IP [RFC791],
   reusing the same network header, and IPv6 multicast inherits the same
   approach.  In comparison, [RFC8296] defines BIER encapsulation as a



Eckert                    Expires 28 April 2022                 [Page 4]


Internet-Draft                bier-cgm2-rbs                 October 2021


   completely separate (from IP) layer 3 protocol, and duplicates both
   IP and MPLS header elements into the [RFC8296] header.  This not only
   results in always unused, duplicate header parameters (such as TC vs.
   DSCP), but it also foregoes the option to use any non-considered IPv6
   extension headers with BIER and would require the introduction of a
   whole new BIER specific socket API into host operating systems if it
   was to be supported natively in hosts.

   Therefore an encapsulation of RBS addresses using an IP and/or IPv6
   extension header may be more desirable in otherwise IP and/or IPv6
   only deployments, for example when CGM2 is extended into hosts,
   because it would allow to support CGM2 via existing IP/IPv6 socket
   APIs as long as they support extension headers, which the most
   important host stacks do today.

2.  CGM2/RBS Architecture

   This section describes the basic CGM2 architecture via Figure 1
   through its key differences over the BIER-TE architecture.

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

          CGM2      [CGM2  Controller]
   control plane   .  ^      ^     ^
                  .  /       |      \     BIFT configuration
        ..........  |        |       |    per-flow RBS setup
       .            |        |       |
      .             v        v       v
   Src (-> ... ) -> BFIR-----BFR-----BFER -> (... ->) Rcvr

                   |<----------------->|
             CGM2 with RBS-address forwarding plane

    |<.............. <- CGM domain ---> ...............|

                 |<--------------------->|
                 Routing underlay (optional)

                      Figure 1: CGM2/RBS Architecture

   In the "traditional" option, when deployed with a domain spanning
   from BFIR to BFER, the CGM2 architecture is very much like the BIER-
   TE architecture, in which the BIER-TE forwarding rules for
   (BitString,SI,SD) addresses are replaced by the RBS address
   forwarding rules.




Eckert                    Expires 28 April 2022                 [Page 5]


Internet-Draft                bier-cgm2-rbs                 October 2021


   The CGM2 Controller replaces the BIER-TE controller, populating
   during network configuration the BIFT, which are very much like BIER-
   TE BIFT, except that they do not cover a network-wide BP address
   space, but instead each BFR BIFT only needs as many BP in its BIFT as
   it has link-local adjacencies, and in partial deployments also
   additional L3 adjacencies to tunnel across non-CGM capable routers.

   Per-flow operations in this "traditional" option is very much as in
   BIER/BIER-TE, with the CGM2 controller determining the RBS address
   (instead of the BIER-TE (BitString,SI,SD)) to be imposed as part of
   the RBS address header (compared to the BIER encapsulation [RFC8296])
   on the BFIR.

   To eliminate the need for an IP Multicast flow overlays, a CGM2
   domain may extend all the way into Sender/Receiver hosts.  This is
   called "end-to-end" deployment model.  In that case, the sender host
   and CGM2 controller collaborate to determine the desired receivers
   for a packet as well as desired path policy/requirements, the
   controller indicates to the sender of the packet the necessary RBS
   address and address of the BFIR, and the Sender imposes an
   appropriate RBS address header together with a unicast encapsulation
   towards the BFIR.

   CGM2 is also intended so especially simplify controller operations
   that also instantiate QoS policies for multicast traffic flows, such
   as bandwidth and latency reservations (e.g.: DetNet).  As in BIER-TE,
   this is orthogonal to the operations of the CGM2/RBS address
   forwarding operations and will be covered in separate documents.

3.  CGM2/RBS forwarding plane

   Instead of a (flat) BitString as in BIER-TE that use a network wide
   shared BP address space for adjacencies across multiple BFR, CGM2
   uses a structured address built from so-called RecursiveUnits (RU)
   that contain BitStrings, each of which is to be parsed by exactly one
   BFR along the delivery tree of the packet.

   The equivalent to a BIER/BIER-TE BitString is therefore called the
   RecursiveUnit BitString Structure (RBS) Address.  Forwarding for
   CGMP2 is therefore also called RBS forwarding.











Eckert                    Expires 28 April 2022                 [Page 6]


Internet-Draft                bier-cgm2-rbs                 October 2021


3.1.  RBS BIFT

   RBS BIFT as shown in Figure 2 are, like BIER-TE BIFT, tables that are
   indexed by BP, containing for each BP an adjacency.  The core
   difference over BIER-TE BIFT is that the BP of the BIFT are all local
   to the BFR, whereas in BIER-TE, the BP are shared across a BIER-TE
   domain, each BFR can only use a subset the BP for its own
   adjacencies, and only in some cases can BP be shared for adjacencies
   across two (or more) BFR.  Because of this difference, most of the
   complexities of BIER-TE BIFT are not required with BIER-RBS BIFT, see
   Section 6.1.3.

   +--+---------+-------------+
   |BP|Recursive|    Adjacency|
   +--+---------+-------------+
   | 1|        1|adjacenct BFR|
   +--+---------+-------------+
   | 2|        0|    punt/host|
   +--+---------+-------------+
   |     .....    ...         |
   +--+---------+-------------+
   | N|      ...|         ... |
   +--+---------+-------------+

                             Figure 2: RBS BIFT

   An RBS BIFT has a configured number of N addressable BP entries.
   When a BFR receives a packet with an RBS address, it expects that the
   BitString inside the RBS address that needs to be parsed by the BFR
   (see Section 3.3 has a length that matches N according to the
   encapsulation used for the RBS address.  Therefore, N MUST support
   configuration in increments of the supported size of the BitString in
   the encapsulation of the RBS Address.  In the reference encoding (see
   Section 3.3), the increment for N is 1 (bit).  If an encapsulation
   would call for a byte accurate encoding of the BitString, N would
   have to be configurable in increments of 8.

   BFR MUST support a value of N larger than the maximum number of
   adjacencies through which RBS forwarding/replication of a single
   packet is required, such as the number of physical interfaces on BFR
   that are intended to be deployed as a Provider Core (P) routers.

   RBS BIFT introduce a new "Recursive" flag for each BP.  These are
   used for adjacencies to other BFR to indicate that the BFR processing
   the packet RBS address BitString also has to expect for every BP with
   the recursive flag set another RU inside the RBS address.





Eckert                    Expires 28 April 2022                 [Page 7]


Internet-Draft                bier-cgm2-rbs                 October 2021


3.2.  Reference encoding of RBS addresses

   Structure elements of the RBS Address and its components are
   parameterized according to a specific encapsulation for RBS
   addresses, such as the total size of the TotalLen field and the unit
   in which it is counted (see Section 3.3).  These parameters are
   outside the scope of this document.  Instead, this document defines
   example parameters that together form the so called "Reference
   encoding of RBS addresses".  This encoding may or may not be adopted
   for any particular encapsulation of RBS addresses.

3.3.  RBS Address

   An RBS address is structured as shown in Figure 3.

   +----------+-----+---------------+---------+
   | TotalLen | Rsv | RecursiveUnit | Padding |
   +----------+-----+---------------+---------+
              .                     .
               .... TotalLen .......

                           Figure 3: RBS Address

   TotalLen counts in some unit, such as bits, nibbles or bytes the
   length of the RBS Address excluding itself and Padding.  For the
   reference encoding, TotalLen is an 8-bit field that counts the size
   of the RBS address in bits, permitting for up to 256 bit long RBS
   addresses.

   In case additional, non-recursive flags/fields are determined to be
   required in the RBS Address, they should be encoded in a field
   between TotalLen and RecursiveUnit, which is called Rsv. In the
   reference encoding, this field has a length of 0.

   Padding is used to align the RBS address as required by the
   encapsulation.  In the reference encoding, this alignment is to 8
   bits (byte boundaries).  Therefore, Padding (bits) = (8 - TotalLen %
   8).

3.3.1.  RecursiveUnit

   The RecursiveUnit field is structured as shown in Figure 4.

   +-+-+-+-+-+  -+-+-+-+-+-+-+-+-+  -+-+-+-+-+-+-+-+     -+
   | BitString...| AddressingField...| RecursiveUnit 1...M|
   +-+-+-+-+-+  -+-+-+-+-+-+-+-+-+  -+-+-+-+-+-+-+-+-    -+

                        Figure 4: RBS RecursiveUnit



Eckert                    Expires 28 April 2022                 [Page 8]


Internet-Draft                bier-cgm2-rbs                 October 2021


   The BitString field indicates the bit positions (BPs) to which the
   packet is to be replicated using the BIFT of the BFR that is
   processing the Recursive unit.

   For each of M BP set in the BitString of the RecursiveUnit for which
   the Recursive flag is set in the BIFT of the BFR, the RecursiveUnit
   contains a RecursiveUnit i, i=1...M, in order of increasing BP index.

   If adjacencies between BFR are not configured as recursive in the
   BIFT, this recursive extraction does not happen for an adjacency, no
   RecursiveUnit i has to be encoded for the BP, and BFRs across such
   adjacencies would have to share the BP of a common BIFT as in BIER-
   TE.  This option is not further discussed in this version of the
   document.

3.3.2.  AddressingField

   The AddressingField of an RBS address is structured as shown in
   Figure 5.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   +-+-+-+-+-+-+-+-+
   |      L1       |     L2        |...|      L(M-1)   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   +-+-+-+-+-+-+-+-+

                       Figure 5: RBS AddressingField

   The AddressingField consists of one or more fields Li, i=1...(M-1).
   Li is the length of RecursiveUnit i for the i'th recursive bit set in
   the BitString preceding it.

   In the reference encoding, the lengths are 8-bit fields indicating
   the length of RecursiveUnits in bits.

   The length of the M'th RecursiveUnit is not explicitly encoded but
   has to be calculated from TotalLen.

4.  BIER-RBS Example

   Figure 6 shows an example for RBS forwarding.












Eckert                    Expires 28 April 2022                 [Page 9]


Internet-Draft                bier-cgm2-rbs                 October 2021


                  +-+     +-+      +-+
                  | |-----| |------|C|-=> Client2
                  +-+     +-+      +-+
                 /   \      \   /=>/ \
                /     \      \ /     |
               +-+    +-+    +-+    +-+
    Client1 =>-|B|-=>-|R|-=>-|S|-=>-|D|-=> Client3
               +-+    +-+    +-+    +-+
                         \         /
                          \     +-+
                           \-=>-|E|-=> Client4
                                +-+

                     Figure 6: Example Network Topology

   A packet from Client1 connected to BFIR B is intended to be
   replicated to Client2,3,4.  The example initially assumes the
   traditional option of the architecture, in which the imposition of
   the header for the RBS address happens on BFIR B, for example based
   on functions of an IP multicast flow overlay.

   A controller determines that the packet should be forwarded hop-by-
   hop across the network as shown in Figure 7.

   Client 1 ->B(impose BIER-RBS)
               =>R(
                  => E (dispose BIER-RBS)
                       => Client4
                  => S(
                      =>C (dispose BIER-RBS)
                          => Client2
                      =>D (dispose BIER-RBS)
                          => Client3
                       )
                  )

                 Figure 7: Desired example forwarding tree

4.1.  BFR B

   The 34 bit long (without padding) RBS address shown in Figure 8 is
   constructed to represent the desired tree from Figure 7 and is
   imposed at B onto the packet through an appropriate header supporting
   the reference encoding of RBS addresses.







Eckert                    Expires 28 April 2022                [Page 10]


Internet-Draft                bier-cgm2-rbs                 October 2021


            .............. RecursiveUnit .................
           .                                              .
   +-------+----+-----+-----+-----+----+-----+------+-----+-----+
   |Tlen:34|B:01|R:011|L1:10|S:011|L1:3|C:001|D:0001|E:001|Pad:6|
   +-------+----+-----+-----+-----+----+-----+------+-----+-----+
     8bit   2bit  3bit 8bit  3bit  8bit 3bit  4bit   3bit  6bit

                  Figure 8: RBS Address imposed at BFIR-B

   In Figure 8 and further the illustrations of RBS addresses,
   BitStrings are preceded by the name of the BFR for whom they are
   destined and their values are shown as binary with the lowest BP 1
   starting on the left.  TotalLength (Tlen:), AddressingField (L1:) and
   Padding (Pad:) fields are shown with decimal values.

   RBS forwarding on B examines this address based on its RBS BIFT with
   N=2 BP entries, which is shown in Figure 9.

   +--+---------+---------+
   |BP|Recursive|Adjacency|
   +--+---------+---------+
   | 1|        0| client1 |
   +--+---------+---------+
   | 2|        1|       R |
   +--+---------+---------+

                        Figure 9: BIER-RBS BIFT on B

   This results in the parsing of the RBS address as shown in Figure 10,
   which shows that B does not need (nor can) parse all structural
   elements, but only those relevant to its own RBS forwarding
   procedure.

            ......... RecursiveUnit ...............
           .                                       .
           .     ......,.. RecursiveUnit 1 .........
           .    .                                  .
   +-------+----+----------------------------------+-----+
   |Tlen:34|B:01|R:01100001010011000000110010001001|Pad:6|
   +-------+----+----------------------------------+-----+
     8bit   2bit  32bit                             6bit

               Figure 10: RBS Address as processed by BFIR-B

   There is only one BP towards BFR R set in the BitString B:01, so the
   RecursiveUnit 1 follows directly after the end of the BitString B:01
   and it covers the whole Tlen - length of BitString (34 - 2 = 32 bit).




Eckert                    Expires 28 April 2022                [Page 11]


Internet-Draft                bier-cgm2-rbs                 October 2021


   B rewrites the RBS address by replacing the RecursiveUnit with
   RecursiveUnit 1 and adjusts the Padding to zero bits.  The resulting
   RBS address is shown in Figure 11.  It then sends the packet copy
   with that rewritten RBS address to BFR R.

4.2.  BFR R

   BFR R receives from BFR B the packet with that RBS address shown in
   Figure 11.

            .............. RecursiveUnit ............
           .                                         .
   +-------+-----+-----+-----+----+-----+------+-----+
   |Tlen:32|R:011|L1:18|S:011|L1:3|C:001|D:0001|E:001|
   +-------+-----+-----+-----+----+-----+------+-----+
     8bit    3bit  8bit  3bit 8bit 3bit  4bit   3bit
                       .                       .     .
                        . RecursiveUnit 1...... .....
                                                  .
                               RecursiveUnit 2 ...

                 Figure 11: RBS Address processed by BFR-R

   BFR R parses the RBS Address as shown in Figure 12 using its RBS BIFT
   of N=3 BP entries shown in Figure 13.

            .............. RecursiveUnit ............
           .                                         .
   +-------+-----+-----+--------------------+-----+
   |Tlen:32|R:011|L1:18|S:011000000110010001|E:001|
   +-------+-----+-----+--------------------+-----+
     8bit    3bit  8bit  18bit               3bit
                       .                    .     .
                        . RecursiveUnit 1... .....
                                               .
                            RecursiveUnit 2 ...

                 Figure 12: RBS Address processed by BFR-R

   Because there are two recursive BP set in the BitString for R, one
   for BFR S and one for BFR E, one Length field L1 is required in the
   AddressingField, indicating the length of the RecursiveUnit 1 for BFR
   S, followed by the remainder of the RBS address being the
   RecursiveUnit 2 for BFR E.







Eckert                    Expires 28 April 2022                [Page 12]


Internet-Draft                bier-cgm2-rbs                 October 2021


   +--+---------+---------+
   |BP|Recursive|Adjacency|
   +--+---------+---------+
   | 1|        1|       B |
   +--+---------+---------+
   | 2|        1|       S |
   +--+---------+---------+
   | 3|        1|       E |
   +--+---------+---------+

                        Figure 13: RBS BIFT on BFR R

   BFR R accordingly creates one copy for BFR S using RecursiveUnit 1,
   and only copy for BFR E using RecursiveUnit 2, updating Padding
   accordingly for each copy.

4.3.  BFR S

   BFR S receives from BFR B the packet and parses the RBS address as
   shown in Figure 14 using its RBS BIFT of N=3 BP shown in Figure 15.

            .... RecursiveUnit ....
           .                       .
   +-------+-----+----+-----+------+-----+
   |Tlen:18|S:011|L1:3|C:001|D:0001|Pad:6|
   +-------+-----+----+-----+------+-----+
     8bit    3bit 8bit  3bit   4bit  3bit
                      .    . .      .
                       ....   ......
        RecursiveUnit 1 .      .
                               .
        RecursiveUnit 2 .......

                 Figure 14: RBS Address processed by BFR-S

   +--+---------+---------+
   |BP|Recursive|Adjacency|
   +--+---------+---------+
   | 1|        1|       R |
   +--+---------+---------+
   | 2|        1|       C |
   +--+---------+---------+
   | 3|        1|       D |
   +--+---------+---------+

                        Figure 15: RBS BIFT on BFR-S





Eckert                    Expires 28 April 2022                [Page 13]


Internet-Draft                bier-cgm2-rbs                 October 2021


   BFR S accordingly sends one packet copy with RecursiveUnit 1 in the
   RBS address to BFR C and a second packet copy with RecursiveUnit 2 to
   BFR D.

4.4.  BFR C

   BFR C receives from BFR S the packet and parses the RBS address
   according to its N=3 BP entries BIFT (shown in Figure 17) as shown in
   Figure 16.

   +-------+-----+-----+
   |Tlen:3 |C:001|Pad:5|
   +-------+-----+-----+
     8bit    3bit 5bi

                 Figure 16: RBS Address processed by BFR-C

   +--+---------+-------------+
   |BP|Recursive|    Adjacency|
   +--+---------+-------------+
   | 1|        1|           S |
   +--+---------+-------------+
   | 2|        1|           D |
   +--+---------+-------------+
   | 3|        0|  local_decap|
   +--+---------+-------------+

                        Figure 17: RBS BIFT on BFR-C

   BFR S accordingly creates one packet copy for BP 3 where the RBS
   address encapsulation is disposed of, and the packet is ultimately
   forwarded to Client 2, for example because of an IP multicast payload
   for which the multicast flow overlay identifies Client 2 as an
   interested receiver, as in BIER/BIER-TE.

   To avoid having to use an IP flow overlay, the BIFT could instead
   have one BP allocated for every non-RBS destination, in this example
   BP 3 would then explicitly be allocated for Client 2, and instead of
   disposing of the RBS address encapsulation, BFR C would impose or
   rewrite a unicast encapsulation to make the packet become a unicast
   packet directed to Client 2.  This option is not further detailed in
   this version of the document.

4.5.  BFR D

   The procedures for processing of the packet on BFR D are very much
   the same as on BFR C.  Figure 18 shows the RBS address at BFR D,
   Figure 19 shows the N=4 bit RBS BIFT of BFR D.



Eckert                    Expires 28 April 2022                [Page 14]


Internet-Draft                bier-cgm2-rbs                 October 2021


   +-------+------+-----+
   |Tlen:4 |D:0001|Pad:4|
   +-------+------+-----+
     8bit    4bit   4bit

                 Figure 18: RBS Address processed by BFR-D

   +--+---------+-------------+
   |BP|Recursive|    Adjacency|
   +--+---------+-------------+
   | 1|        1|           S |
   +--+---------+-------------+
   | 2|        1|           C |
   +--+---------+-------------+
   | 3|        1|           E |
   +--+---------+-------------+
   | 4|        0|  local_decap|
   +--+---------+-------------+

                        Figure 19: RBS BIFT on BFR-D

4.6.  BFR E

   The procedures for processing of the packet on BFR E are very much
   the same as on BFR C and D.  Figure 20 shows the RBS address at BFR
   D, Figure 21 shows the N=E bit RBS BIFT of BFR E.

   +-------+-----+-----+
   |Tlen:3 |E:001|Pad:5|
   +-------+-----+-----+
     8bit    3bit   5bit

                 Figure 20: RBS Address processed by BFR-E

   +--+---------+-------------+
   |BP|Recursive|    Adjacency|
   +--+---------+-------------+
   | 1|        1|           R |
   +--+---------+-------------+
   | 2|        1|           D |
   +--+---------+-------------+
   | 3|        0|  local_decap|
   +--+---------+-------------+

                        Figure 21: RBS BIFT on BFR-E






Eckert                    Expires 28 April 2022                [Page 15]


Internet-Draft                bier-cgm2-rbs                 October 2021


5.  RBS forwarding Pseudocode

   The following example RBS forwarding Pseudocode assumes the reference
   encoding of bit-accurate length of BitStrings and RecursiveUnits as
   well as 8-bit long TotalLen and AddressingField Lengths.  All packet
   field addressing and address/offset calculations is therefore bit-
   accurate instead of byte accurate (which is what most CPU memory
   access today is).











































Eckert                    Expires 28 April 2022                [Page 16]


Internet-Draft                bier-cgm2-rbs                 October 2021


   void ForwardRBSPacket (Packet)
   {
     RBS = GetPacketMulticastAddr(Packet);
     Total_len = RBS;
     Rsv = Total_len + length(Total_Len);
     BitStringA = Rsv + length(Rsv);
     AddressingField =  BitStringA + BIFT.entries;

     // [1] calculate number of recursive bits set in BitString
     CopyBitString(*BitStringA, *RecursiveBits, BIFT.entries);
     And(*RecursiveBits,*BIFTRecursiveBits, BIFT.entries);
     N = CountBits(*RecursiveBits, BIFT.entries);

     // Start of first RecursiveUnit in RBS address
     // After AddressingField array with 8-bit length fields
     RecursiveUnit = AddressingField + (N - 1) * 8;

     RemainLength = *Total_len - length(Rsv)
                    - BIFT.entries;

     Index = GetFirstBitPosition(*BitStringA);
     while (Index) {
       PacketCopy = Copy(Packet);

       if (BIFT.BP[Index].recursive) {
         if(N == 1) {
           RecursiveUnitLength = RemainLength;
         } else {
           RecursiveUnitLength = *AddressingField;
           N--;
           AddressingField += 8;
           RemainLength -= RecursiveUnitLength;
           RemainLength -= 8; // 8 bit of AddressingField
         }
         RewriteRBS(PacketCopy, RecursiveUnit, RecursiveUnitLength);
         SendTo(PacketCopy, BIFT.BP[Index].adjacency);

         RecursiveUnit += RecursiveUnitLength;
       } else {
         DisposeRBSheader(PacketCopy);
         SendTo(PacketCopy, BIFT.BP[Index].adjacency);
       }
       Index = GetNextBitPosition(*BitStringA, Index);
     }

                Figure 22: RBS address forwarding Pseudocode

   Explanations for Figure 22.



Eckert                    Expires 28 April 2022                [Page 17]


Internet-Draft                bier-cgm2-rbs                 October 2021


   RBS is the (bit accurate) address of the RBS address in packet header
   memory.  BitStringA is the address of the RBS address BitString in
   memory.  length(Total_Len) and length(Rsv) are the bit length of the
   two RBS address fields, e.g.: 8 bit and 0 bit for the reference
   encoding.

   The BFR local BIFT has a total number of BIFT.entries addressable BP
   1...BIFTentries.  The BitString therefore has BIFT.entries bits.

   BIFT.RecursiveBits is a BitString pre-filled by the control plane
   with all the BP with the recursive flag set.  This is constructed
   from the Recursive flag setting of the BP of the BIFT.  The code
   starting at [1] therefore counts the number of recursive BP in the
   packets BitString.

   Because the AddressingField does not have an entry for the last (or
   only) RecursiveUnit, its length has to be calculated by taking
   TotalLen into account.

   RewriteRBS needs to replace RBS address with the RecursiveUnit
   address, keeping only Rsv, recalculating TotalLen and adding
   appropriate Padding.

   For non-recursive BP, the Pseudocode assumes disposition of the
   RBSheader.  This is not strictly necessary but non-disposing cases
   are outside of scope of this version of the document.

6.  Operational and design considerations (informational)

6.1.  Comparison with BIER-TE / BIER

   This section discusses informationally, how and where CGM2 can avoid
   different complexities of BIER/BIER-TE, and where it introduces new
   complexities.

6.1.1.  Eliminating the need for large BIFT

   In a BIER domain with M BFER, every BFR requires M BIFT entries.  If
   the supported BSL is N and M > 2 ^ N, then S = (M / 2 ^ N) set
   indices (SI) are required, and S copies of the packet have to be sent
   by the BFIR to reach all targeted BFER.

   In CGM2, the number of BIFT entries does not need to scale with the
   number of BFER or paths through the network, but can be limited to
   only the number of L2 adjacencies of the BFR.  Therefore CGM2
   requires minimum state maintenance on each BFR, and multiple SI are
   not required.




Eckert                    Expires 28 April 2022                [Page 18]


Internet-Draft                bier-cgm2-rbs                 October 2021


6.1.2.  Reducing number of duplicate packet copies across BFR

   If the total size of an RBS encoded delivery tree is larger than a
   supported maximum RBS header size, then the CGM2 controller simply
   needs to divide the tree into multiple subtrees, each only addressing
   a part of the BFER (leaves) of the target tree and pruning any
   unnecessary branches.

                B1
               /  \
         B2    B3
           /   \  /  \
          /     \/    \
        B4      B5     B6
      /..|     /  \    |..\
   B7..B99  B100..B200 B201...B300

                     Figure 23: Simple Topology Example

   Consider the simple topology in Figure 23 and a multicast packet that
   needs to reach all BFER B7...B300.  Assume that the desired maximum
   RBM header size is such that a RBS address size of <= 256 bits is
   desired.  The CGM2 controller could create an RBS address
   B1=>B2=>B4=>(B7..B99), for a first packet, an RBS address
   B1=>B3=>B5=>(B100..B200) for a second packet and a third RBS address
   B1=>B3=>B6=>B201...B300.

   The elimination of larger BIFT state in BFR through multiple SI in
   BIER/BIER-TE does come at the expense of replicating initial hops of
   a tree in RBS addresses, such as in the example the encoding of
   B1=>B3 in the example.

   Consider that the assignment of BFIR-ids with BIER in the above
   example is not carefully engineered.  It is then easily possible that
   the BFR-ids for B7..B99 are not sequentially, but split over a larger
   BFIR-id space.  If the same is true for all BFER, then it is possible
   that each of the three BFR B4,B5 and B6 has attached BFER from three
   different SI and one may need to send for example three multiple
   packets to B7 to address all BFER B7..B99 or to B5 to address all
   B100..B200 or B6 to address all B201...B300.  These unnecessary
   duplicate packets across B4, B5 or B6 are because of the addressing
   principle in BIER and are not necessary in CGM2, as long as the total
   length of an RBS address does not require it.

   For more analysis, see Section 6.3.






Eckert                    Expires 28 April 2022                [Page 19]


Internet-Draft                bier-cgm2-rbs                 October 2021


6.1.3.  BIER-TE forwarding plane complexities

   BIER-TE introduces forwarding plane complexities to allow reducing
   the BSL required.  While all of these could be supported /
   implemented with CGM2, this document contends that they are not
   necessary, therefore providing significant overall simplifications.

   *  BIER-TE supports multiple adjacencies in a single BIFT Index to
      allow compressing multiple adjacencies into a single Index for
      traffic that is known to always require replications to all those
      adjacencies (such as when flooding TV traffic).

   *  BIER-TE support ECMP adjacencies which have to calculate which out
      of 2 or more possible adjacencies a packet should be forwarded to.

   *  BIER-TE supports special Do-Not-Clear (DNC) behavior of
      adjacencies to permit reuse of such a bit for adjacencies on
      multiple consecutive BFR.  This behavior specifically also raises
      the risk of looping packets.

6.1.4.  BIER-TE controller complexities

   BIER-TE introduces BIER-TE controller plane mechanisms that allow to
   reuse bits of the flat BIER-TE BitStrings across multiple BFR solely
   to reduce the number of BP required but without introducing
   additional complexities for the BIER-TE forwarding plane.

   *  Shared BP for all Leaf BFR.

   *  Shared BP for both Interfaces of p2p links.

   *  Shared bits for multi-access subnets (LANs).

   These bit-sharing mechanisms are unnecessary and inapplicable to CGM2
   because there is no need to share BP across the BIFT of multiple BFR.

6.1.5.  BIER-TE specification complexities

   The BIER-TE specification distinguishes between forward (link scope)
   and routed (underlay routed) adjacencies to highlight, explain and
   emphasize on the ability of BIER-TE to be deployed in an overlay
   fashion especially also to reduce the necessary BSL, even when all
   routers in the domain could or do support BIER-TE.

   In CGM2, routed adjacencies are considered to be only required in
   partial deployments to forward across non-CGM2 enabled routers.  This
   specification does therefore not highlight link scope vs. routed
   adjacencies as core distinct features.



Eckert                    Expires 28 April 2022                [Page 20]


Internet-Draft                bier-cgm2-rbs                 October 2021


6.1.6.  Forwarding plane complexity

   CGM2 introduces some more processing calculation steps to extract the
   BitString that needs to be examined by a BFR from the RBS address.
   These additional steps are considered to be non-problematic for
   todays programmable forwarding planes such as P4.

   Whereas BIER-TE clears bit on each hops processing, CGM2 rewrites the
   address on every hop by extracting the recursive unit for the next
   hop and make it become the packet copies address.  This rewrite
   shortens the RBS address.  This hopefully has only the same
   complexity as (tunnel) encapsulations/decapsulations in existing
   forwarding planes.

6.2.  CGM2 / RBS controller considerations

   TBD.  Any aspects not covered in Section 6.1.

6.3.  Analysis of performance gain with CGM2

   TBD: Comparison of number of packets/header sizes required in large
   real-world operator topology between BIER/BIER-TE and CGM2.

6.4.  Example use case scenarios

   TBD.

7.  Acknowledgements

   This work is based on the design published by Sheng Jiang, Xu Bing,
   Yan Shen, Meng Rui, Wan Junjie and Wang Chuang {jiangsheng|bing.xu|ya
   nshen|mengrui|wanjunjie2|wangchuang}@huawei.com, see [CGM2Design].

8.  Security considerations

   TBD.

9.  Changelog

   [RFC-Editor: please remove this section].

   This document is written in https://github.com/cabo/kramdown-rfc2629
   markup language.  This documents source is maintained at
   https://github.com/toerless/bier-cgm2-rbs, please provide feedback to
   the appropriate IETF mailing list and submit an Issue to the GitHub.

   00 - Initial version from [CGM2Design].




Eckert                    Expires 28 April 2022                [Page 21]


Internet-Draft                bier-cgm2-rbs                 October 2021


10.  References

10.1.  Normative References

   [I-D.ietf-bier-te-arch]
              Eckert, T., Cauchie, G., and M. Menth, "Tree Engineering
              for Bit Index Explicit Replication (BIER-TE)", Work in
              Progress, Internet-Draft, draft-ietf-bier-te-arch-10, 9
              July 2021, <https://www.ietf.org/archive/id/draft-ietf-
              bier-te-arch-10.txt>.

   [RFC1112]  Deering, S., "Host extensions for IP multicasting", STD 5,
              RFC 1112, DOI 10.17487/RFC1112, August 1989,
              <https://www.rfc-editor.org/info/rfc1112>.

   [RFC791]   Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,
              <https://www.rfc-editor.org/info/rfc791>.

   [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>.

   [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>.

10.2.  Informative References

   [CGM2Design]
              Jiang, S., Xu, B.(., Shen, Y., Rui, M., Junjie, W., and W.
              Chuang, "Novel Multicast Protocol Proposal Introduction",
              10 October 2021,
              <https://github.com/BingXu1112/CGMM/blob/main/Novel%20Mult
              icast%20Protocol%20Proposal%20Introduction.pptx>.

Author's Address

   Toerless Eckert
   Futurewei Technologies USA
   2220 Central Expressway
   Santa Clara,  CA 95050
   United States of America




Eckert                    Expires 28 April 2022                [Page 22]


Internet-Draft                bier-cgm2-rbs                 October 2021


   Email: tte@cs.fau.de


















































Eckert                    Expires 28 April 2022                [Page 23]