```
```

```
```xiejingrong@huawei.com

```
```

```
```
Alibaba Inc.
xiaohu.xxh@alibaba-inc.com
Huawei Technologies
yangang@huawei.com
Futurewei
```
```

```
```mmcbride7@gmail.com

```
```

```
```
Bit Index Explicit Replication (BIER) introduces a new
multicast-specific BIER Header. BIER can be applied to the Multi
Protocol Label Switching (MPLS) data plane or Non-MPLS data plane.
Entropy is a technique used in BIER to support load-balancing. This
document examines and describes how BIER Entropy is to be applied to
Data Center Clos networks for path selection.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP 14
when, and only when,
they appear in all capitals, as shown here.

```
```
Bit Index Explicit Replication (BIER) is an
architecture that provides optimal multicast forwarding without
requiring intermediate routers to maintain any per-flow state by using a
multicast-specific BIER header. defines two
types of BIER encapsulation formats: one is MPLS encapsulation, the
other is non-MPLS encapsulation. Entropy is a technique used in BIER to
support load-balancing. This document examines and describes how BIER
Entropy is to be applied to Data Center Clos networks for path
selection.
Readers of this document are assumed to be familiar with the
terminology and concepts of the documents listed as Normative
References.
A common choice for a horizontally scalable topology used in Data
Center is a Clos topology. This topology features an odd number of
stages, for example, a 5-Stage Clos Topology as a example in .
ECMP is the fundamental load-sharing mechanism used by a Clos
topology. Effectively, every lower-tier device will use all of its
directly attached upper-tier devices to load-share traffic destined to
the same IP prefix. The number of ECMP paths between any two Tier 3
devices in Clos topology is equal to the number of the devices in the
middle stage (Tier 1). For example, Figure 1 illustrates a topology
where Tier 3 device L1 has four paths to reach servers X and Y, via
Tier 2 devices S1 and S2 and then Tier 1 devices S11, S12, S21 and S22
respectively.
When BIER is deployed in a multi-tenant data center network
environment for efficient delivery of Broadcast, Unknown-unicast and
Multicast (BUM) traffic, a network operator may want a deterministic
path for every packet. For example, when L1 needs to send a BUM packet
to L3 and L4, which are in different SIs, L1 has to send the packet
twice, and expects the packet along two deterministic paths of
L1->S1->S11-->L3 and L1->S2->S21-->L4 seperately.
Another example of using a deterministic path in a DC is for per-flow
steering of "elephant" flows defined in .
A deterministic path for a multicast packet, with multiple staged
equal cost paths, is comparable to a traffic-engineering path defined
in for a unicast
path with multiple hop equal cost paths.
The idea behind entropy is that the ingress router computes a hash
based on several fields from a given packet and places the result in
an additional label, named "entropy label". Then this entropy label
can be used as part of the hash keys used by an transit router. When
entropy label is used, the keys used in the hashing functions are
still a local configuration matter. A router may soley use the entropy
label or use a combination of multiple fields from the incoming
packet. The hashing function is to randomly load balance the mass of
flows between the small number of equal cost paths.
If one wants, however, to get a deterministic path from the equal
cost paths, one can use part of the 20-bit entropy field. For example,
bit 0 to bit 2 of entropy label can represent a value of 0 to 7, and
thus can be used to select a deterministic path from 8 equal cost
paths. And thus, a 20-bit entropy label can be used by routers in
different tiers to select a deterministic path independently by using
different parts of the 20-bit entropy label, and form an end-to-end
deterministic path.
This is simple and applicable especially for DC Clos networks,
because data delivery in DC Clos networks for tenants is always
multi-staged, with the upstream direction stages having equal cost
paths.
Take the 5-stage Clos network in figure 1 as an example.
Tier 2 in every cluster has N nodes, and the Tier 1 has M nodes. M
is equal to N multiplied by P.
Tier 3 switches, in upstream direction, act as stage 1 of data
delivery and have N equal cost paths to every BFERs in other clusters.
Tier 2 switches, in upstream direction, act as stage 2 of data
delivery and have P equal cost paths to every BFERs in other
clusters.
Example 1: One can configure, on each Tier 3 switch, the use of bit
0 for path selection when N is equal to 2, and configure, on each Tier
2 switch, to use bit 1 for path selection when P is equal to 2.
Example 2: One can configure, on each Tier 3 switch, the use of bit
0 to bit 1 for path selection when N is equal to 4, and configure on
each Tier 2 switches the use of bit 2 to bit 7 for path selection when
P is equal to 48.
Assume that, each of the Tier 3 and Tier 2 switchs in the example
has two parameters, X and Y, configured locally for using part of
entropy label to do path selection, then in example 2:
Each of Tier 3 (Stage 1) switches has a pair of parameters
(X1=1, Y1=4)
Each of Tier 2 (Stage 2) switches has a pair of parameters
(X2=X1*Y1=4, Y2=64)
Each of Tier 3 (Stage 1) switches populates its BIFTs for ECMP,
for example, BIFT-0 to BIFT-3.
Each of Tier 2 (Stage 2) switches populates its BIFTs for ECMP,
for example, BIFT-0 to BIFT-47.

For each of Tier 3 (Stage 1) switches, each of the BIFT will have a
prefered neighboring BFR. For example, LEAF L1 will have a prefered
neighbor S1/S2 for BIFT-0/1 seperately, and when forming the BIFT-0
table through the underlay routing to every BFER, the prefered
neighboring BFR will has a highest priority among all the locally
available ECMP path.
Then an end-to-end deterministic path for a BIER packet can be had
by calculating an entropy label value like this:
Entropy = (P1-1)*X1 + (P2-1)*X2

Where P1 represents one of the Stage 1 equal cost paths with a
value between 1 and N, and P2 represents one of the Stage 2 equal cost
paths with a value between 1 and P.
One can steer an "elephant" flow to an end-to-end deterministic
path, or some divided end-to-end deterministic paths across different
SIs.
When the VNEs for a tenant span multiple SIs, then it is useful to
divide the BUM packets paths across different SIs.
One can configure a policy to use different paths for BIER SIs when
using BIER as the BUM tunnel, on each VNE for each VNI.
As stated above, each of the BIFT on a BFR will have a prefered
neighboring BFR. But when the link to the prefered neighbor of some
BIFT (say BIFT-X) fail, BIFT-X will converge normally, and the path of
this BIFT-X will then probably not being the 'best optimized' path.
For example, the link between S1 and L2 fail, then the prefered
neighbor of BIFT-0 of LEAF L1, S1, is no longer the neighboring BFR
for LEAF L2, and the flow using a Entropy using LEAF L1's BIFT-0 will
have to replicate on L1, one packet to S1 for BFER L3 and L4, and one
packet to S2 for BFER L2. If the flow changes to use a Entropy using
LEAF L1's BIFT-1, it will then be the 'best optimized' path, because
the flow doesn't have to replicate on L1, and it need to forward only
one copy to S1 for BFER L2 and L3 and L4. Such a change to a flow's
entropy is the Ingress switch's responsibility, possibly with the
assisstance of a controller.
The use of BIER entropy label to select a path between some equal
cost paths is a local configuration matter. This draft defines a method
to use part of the 20-bit entropy label in each router, and this needs a
data-plane to do some bit operation function. It is expected to be
easier than hashing function.
This document introduces no new security considerations beyond those
already specified in [RFC8279] and [RFC8296].
This document contains no actions for IANA.
The authors wish to thank Tony Przygienda, Greg Shepherd, Alia Atlas,
Jeffery Zhang, Andrew Dolganow, and Toerless Eckert for their reviews,
comments and suggestions.

```
```