Carrier Grade Minimalist Multicast (CGM2) using Bit Index Explicit Replication (BIER) with Recursive BitString Structure (RBS) Addresses
draft-eckert-bier-cgm2-rbs-00
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| Author | Toerless Eckert | ||
| Last updated | 2021-10-25 | ||
| Replaced by | draft-eckert-bier-rbs | ||
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draft-eckert-bier-cgm2-rbs-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.
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This Internet-Draft will expire on 28 April 2022.
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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/
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Please review these documents carefully, as they describe your rights
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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
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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)
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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
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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.
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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.
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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.
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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
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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.
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+-+ +-+ +-+
| |-----| |------|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.
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.............. 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).
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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.
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+--+---------+---------+
|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
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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.
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+-------+------+-----+
|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
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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).
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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.
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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.
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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.
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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.
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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].
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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
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Email: tte@cs.fau.de
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