Internet DRAFT - draft-xie-mboned-bier-entropy-staged-dc-clos
draft-xie-mboned-bier-entropy-staged-dc-clos
Network Working Group J. Xie
Internet-Draft Huawei Technologies
Intended status: Standards Track X. Xu
Expires: January 3, 2019 Alibaba Inc.
G. Yan
M. McBride
Huawei Technologies
July 2, 2018
Use of BIER Entropy for Data Center CLOS Networks
draft-xie-mboned-bier-entropy-staged-dc-clos-00
Abstract
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.
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 [RFC2119].
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 January 3, 2019.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Problem Statement and Considerations . . . . . . . . . . . . 3
3.1. Problem Statement . . . . . . . . . . . . . . . . . . . . 3
3.2. Considerations . . . . . . . . . . . . . . . . . . . . . 4
4. Use of BIER Entropy for DC CLOS Network . . . . . . . . . . . 5
4.1. Use of BIER Entropy for DC CLOS Network . . . . . . . . . 5
4.2. Steering for elephant flows . . . . . . . . . . . . . . . 6
4.3. Path Division for Tenant flows to different SIs . . . . . 6
4.4. Link Failure and Convergence . . . . . . . . . . . . . . 6
5. Data-Plane Processing . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
Bit Index Explicit Replication (BIER) [RFC8279] 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. [RFC8296] 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.
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2. Terminology
Readers of this document are assumed to be familiar with the
terminology and concepts of the documents listed as Normative
References.
3. Problem Statement and Considerations
3.1. Problem Statement
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
[RFC7938].
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.
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Tier 1
+-----+
Cluster |SUPER|
+----------------------------+ +--| S11 |--+
| | | +-----+ |
| Tier 2 | | | Tier 2
| +-----+ | | +-----+ | +-----+
| +-------------|SPINE|------+--|SUPER|--+--|SPINE|-------------+
| | +-----| S1 |------+ | S12 | +--| S3 |-----+ |
| | | +-----+ | +-----+ +-----+ | |
| | | | | |
| | | +-----+ | +-----+ +-----+ | |
| | +-----------|SPINE|------+ |SUPER| +--|SPINE|-----------+ |
| | | | +---| S2 |------+--| S21 |--+--| S4 |---+ | | |
| | | | | +-----+ | | +-----+ | +-----+ | | | |
| | | | | | | | | | | |
| +-----+ +-----+ | | +-----+ | +-----+ +-----+
| | LEAF| | LEAF| | +--|SUPER|--+ | LEAF| | LEAF|
| | L1 | | L2 | Tier 3 | | S22 | Tier 3 | L3 | | L4 |
| +-----+ +-----+ | +-----+ +-----+ +-----+
| | | | | | | | | |
| O O O O | X Y O O
| Servers | Servers
+----------------------------+
Figure 1: 5-Stage Clos Topology
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 [I-D.ietf-spring-segment-routing-msdc].
A deterministic path for a multicast path, with multiple staged equal
cost paths, is comparable to a traffic-engineering path defined in
[I-D.ietf-mpls-spring-entropy-label] for a unicast path with multiple
hop equal cost paths.
3.2. Considerations
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
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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.
4. Use of BIER Entropy for DC CLOS Network
4.1. Use of BIER Entropy for DC CLOS Network
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 Tier 3 and Tier 2 switch the the example have two
parameters, X and Y, for using part of entropy label to do path
selection, then in example 2:
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o Each of Tier 3 (Stage 1) switches has a pair of parameters (X1=1,
Y1=4)
o Each of Tier 2 (Stage 2) switches has a pair of parameters
(X2=X1*Y1=4, Y2=64)
o Each of Tier 3 (Stage 1) switches populates its BIFTs for ECMP,
for example, BIFT-0 to BIFT-3.
o 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:
o 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.
4.2. Steering for elephant flows
One can steer an "elephant" flow to an end-to-end deterministic path,
or some divided end-to-end deterministic paths across different SIs.
4.3. Path Division for Tenant flows to 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.
4.4. Link Failure and Convergence
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 will then
probably not being the 'best' path. For example, the link between S1
and L2 fail, then the prefered neighbor of BIFT-0 of LEAF L1, S1, is
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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' path, because the flow doesn't have to replicate
on L1, only one 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.
5. Data-Plane Processing
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.
6. Security Considerations
This document introduces no new security considerations beyond those
already specified in [RFC8279] and [RFC8296].
7. IANA Considerations
This document contains no actions for IANA.
8. Acknowledgements
TBD.
9. References
9.1. Normative References
[I-D.ietf-mpls-spring-entropy-label]
Kini, S., Kompella, K., Sivabalan, S., Litkowski, S.,
Shakir, R., and J. Tantsura, "Entropy label for SPRING
tunnels", draft-ietf-mpls-spring-entropy-label-11 (work in
progress), May 2018.
[I-D.ietf-spring-segment-routing-msdc]
Filsfils, C., Previdi, S., Dawra, G., Aries, E., and P.
Lapukhov, "BGP-Prefix Segment in large-scale data
centers", draft-ietf-spring-segment-routing-msdc-09 (work
in progress), May 2018.
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[RFC7938] Lapukhov, P., Premji, A., and J. Mitchell, Ed., "Use of
BGP for Routing in Large-Scale Data Centers", RFC 7938,
DOI 10.17487/RFC7938, August 2016,
<https://www.rfc-editor.org/info/rfc7938>.
[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>.
[RFC8365] Sajassi, A., Ed., Drake, J., Ed., Bitar, N., Shekhar, R.,
Uttaro, J., and W. Henderickx, "A Network Virtualization
Overlay Solution Using Ethernet VPN (EVPN)", RFC 8365,
DOI 10.17487/RFC8365, March 2018,
<https://www.rfc-editor.org/info/rfc8365>.
9.2. Informative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
Authors' Addresses
Jingrong Xie
Huawei Technologies
Email: xiejingrong@huawei.com
Xiaohu Xu
Alibaba Inc.
Email: xiaohu.xxh@alibaba-inc.com
Gang Yan
Huawei Technologies
Email: yangang@huawei.com
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Mike McBride
Huawei Technologies
Email: mmcbride7@gmail.com
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