BESS WorkGroup S. Mohanty
Internet-Draft Cisco Systems
Intended status: Informational A. Vayner
Expires: September 16, 2021 Google
A. Gattani
A. Kini
Arista Networks
March 15, 2021
Cumulative DMZ Link Bandwidth and load-balancing
draft-mohanty-bess-ebgp-dmz-03
Abstract
The DMZ Link Bandwidth draft provides a way to load-balance traffic
to a destination (which is in a different AS than the source) which
is reachable via more than one path. Typically, the link bandwidth
(either configured on the link of the EBGP egress interface or set
via a policy) is encoded in an extended community and then sent to
the IBGP peer which employs multi-path. The link-bandwidth value is
then extracted from the path extended community and is used as a
weight in the FIB, which does the load-balancing. This draft extends
the usage of the DMZ link bandwidth to another setting where the
ingress BGP speaker requires knowledge of the cumulative bandwidth
while doing the load-balancing. The draft also proposes neighbor-
level knobs to enable the link bandwidth extended community to be
regenerated and then advertised to EBGP peers to override the default
behavior of not advertising optional non-transitive attributes to
EBGP peers.
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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material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 16, 2021.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Problem Description . . . . . . . . . . . . . . . . . . . . . 3
4. Large Scale Data Centers Use Case . . . . . . . . . . . . . . 6
5. Non-Conforming BGP Topologies . . . . . . . . . . . . . . . . 8
6. Protocol Considerations . . . . . . . . . . . . . . . . . . . 10
7. Operational Considerations . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
The Demilitarized Zone (DMZ) Link Bandwidth (LB) extended community
along with the multi-path feature can be used to provide unequal cost
load-balancing as per user control. In [I-D.ietf-idr-link-bandwidth]
the EBGP egress link bandwidth is encoded in the link bandwidth
extended community and sent along with the BGP update to the IBGP
peer. It is assumed that either a labeled path exists to each of the
EBGP links or alternatively the IGP cost to each link is the same.
When the same prefix/net is advertised into the receiving AS via
different egress-points or next-hops, the receiving IBGP peer that
employs multi-path will use the value of the DMZ LB to load-balance
traffic to the egress BGP speakers (ASBRs) in the proportion of the
link-bandwidths.
The link bandwidth extended community cannot be advertised over EBGP
peers as it is defined to be optional non-transitive. This draft
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discusses a new use-case where we need to advertise the link
bandwidth over EBGP peers. The new use-case mandates that the router
calculates the aggregated link-bandwidth, regenerate the DMZ link
bandwidth extended community, and advertise it to EBGP peers. The
new use case also negates the [I-D.ietf-idr-link-bandwidth]
restriction that the DMZ link bandwidth extended community not be
sent when the the advertising router sets the next-hop to itself.
In draft [I-D.ietf-idr-link-bandwidth], the DMZ link bandwidth
advertised by EBGP egress BGP speaker to the IBGP BGP speaker
represents the Link Bandwidth of the EBGP link. However, sometimes
there is a need to aggregate the link bandwidth of all the paths that
are advertising a given net and then send it to an upstream neighbor.
This is represented pictorially in Figure 1. The aggregated link
bandwidth is used by the upstream router to do load-balancing as it
may also receive several such paths for the same net which in turn
carry the accumulated bandwidth.
R1- -20 - - |
R3- -100 - -|
R2- -10 - - | |
|
R6- -40 - - | |- - R4
| |
R5- -100 - -|
R7- -30 - - |
EBGP Network with cumulative DMZ requirement
Figure 1
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Problem Description
Figure 1 above represents an all-EBGP network. Router R3 is peering
with two other EBGP downstream routers, R1 and R2, over the eBGP link
and another upstream EBGP router R4. There is another router, R5,
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which is peering with two downstream routers R6 and R7. R5 peers
with R4. A net, p/m, is learnt by R1, R2, R6, and R7 from their
downstream routers (not shown). From the perspective of R4, the
topology looks like a directed tree. The link bandwidths of the EBGP
links are shown alongside the links (The exact units are not really
important and for simplicity these can be assumed to be weights
proportional to the operational link bandwidths). It is assumed that
R3, R4 and R5 have multi-path configured and paths having different
value as-path attributes can still be considered as multi-path (knobs
exist in many implementations for this). When the ingress router,
R4, sends traffic to the destination p/m, the traffic needs to be
spread amongst the links in the ratio of their link bandwidths.
Today this is not possible as there is no way to signal the link
bandwidth extended community over the EBGP session from R3 to R4. In
absence of a mechanism to regenerate the link bandwidth over the EBGP
session from R3 to R4 and from R5 to R4, the assumed link bandwidth
for paths received over the R3 to R4 and R5 to R4 EBGP sessions would
be equal to the operational link bandwidth of the corresponding EBGP
links.
As per EBGP rules at the advertising router, the next-hop will be set
to the advertising router itself. Accordingly, R3 computes the best-
path from the advertisements received from R1 and R2 and R5 computes
the best-path from advertisements received from R6 and R7
respectively. R4 receives the update from R3 and R5 and in-turn
computes the best-path and may advertises it upstream (not shown).
The expected behavior is that when R4 sends traffic for p/m towards
R3 and R5, and then on to to R1, R2, R6, and R7, the traffic should
be load-balanced based on the calculated weights at the routers which
employ multi-path. R4 should send 30% of the traffic to R3 and the
remaining 70% to R5. R3 in turn should send 67% of the traffic that
it received from R4 to R1 and 33% to R2. Similarly, R5 should send
57% of the traffic received from R4 to R6 and the remaining 43% to
R7. Instead what is happening is that R4 sends 50% of the traffic
towards both R3 and R5. R3 in turn sends more traffic than is
desired towards R1 and R2. R4 in turn sends less traffic than is
desired towards R6 and R7. Effectively the load balancing is getting
skewed towards R1 and R2 even as R1 and R2's egress link bandwidth
relative to R6 and R7 is less.
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R1- -20 - - |
R3- -30 (100) - -|
R2- -10 - - | |
|
R6- -40 - - | |- - R4
| |
R5- -70 (100) - -|
R7- -30 - - |
EBGP Network showing advertisement of cumulative link bandwidth
Figure 2
With the existing rules for the DMZ link bandwidth, this is not
possible. First the LB extended community is not sent over EBGP.
Secondly the DMZ does not have a notion of conveying the cumulative
link bandwidth (of the directed tree rooted at a node) to an upstream
router. To enable the use case described above, the cumulative link
bandwidth of R1 and R2 has to be advertised by R3 to R4, and,
similarly, the cumulative bandwidth of R6 and R7 has to be advertised
by R5 to R4. This will enable R4 to load-balance based on the
proportion of the cumulative link bandwidth that it receives from its
downstream routers R3 and R5. Figure 2 shows the cumulative link
bandwidth advertised by R3 towards R4 and R5 towards R4 with the
original link bandwidth values in '()' for comparison.
To address cases like the above example, rather than introducing a
new attribute for aggregate link bandwidth, we will reuse the link
bandwidth extended community attribute and relax a few assumptions.
With neighbor-specific knobs or policy configuration applied to the
neighbor outbound or inbound as may be the case, we can regenerate
and advertise and/or accept the link bandwidth extended community
over the EBGP link. In addition, we can define neighbor specific
knobs that will aggregate the link bandwidth values from the LB
extended communities learnt from the downstream routers (either
received as link bandwidth extended community in the path update or
assigned at ingress using a neighbor inbound policy configuration or
derived from the operational link-speed of the peer link) and then
regenerate and advertise (via neighbor outbound policy knob) this
aggregate link bandwidth value in the form of the LB extended
community to the upstream EBGP router. Since the advertisement is
being made to EBGP neighbors, the next-hop is going to be reset at
the advertising router.
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Speaking of overall traffic profile, if we assume that on ingress at
R4 traffic flow for net p/m is received at a data rate of 'x', then
in absence of link bandwidth regeneration at R3 and R5 the resultant
traffic profile is below:
link ratio percent approximation(~)
R4-R3 1/2x 50%
R4-R5 1/2x 50%
R3-R1 1/3x (1/2 * 2/3) 33%
R3-R2 1/6x (1/2 * 1/3) 17%
R5-R6 2/7x (1/2 * 4/7) 29%
R5-R7 3/14x (1/2 * 3/7) 21%
For comparison the resultant traffic profile in presence of
cumulative link bandwidth regeneration at R3 and R5 is as below:
link ratio percent approximation(~)
R4-R3 3/10x 30%
R4-R5 7/10x 70%
R3-R1 1/5x (3/10 * 2/3) 20%
R3-R2 1/10x (3/10 * 1/3) 10%
R5-R6 2/5x (7/10 * 4/7) 40%
R5-R7 3/10x (7/10 * 3/7) 30%
As is evident, the second table is closer to the desired traffic
profile that shoud be received by the leaf nodes (R1, R2, R6, R7)
compared to the first one.
4. Large Scale Data Centers Use Case
The "Use of BGP for Routing in Large-Scale Data Centers" [RFC7938]
describes a way to design large scale data centers using EBGP across
the different routing layers. [RFC7938] section 6.3 ("Weighted
ECMP") describes a use case in which a service (most likely
represented using an anycast virtual IP) has an unequal set of
resources serving across the data center regions. Figure 3 shows a
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typical data center topology as described in section 3.1 of [RFC7938]
where an unequal number of servers are deployed advertising a certain
BGP prefix. As can be seen in the figure, the left side of the data
center hosts only 3 servers while the right side hosts 10 servers.
+------+ +------+
| | | |
| AS1 | | AS1 | Tier 1
| | | |
+------+ +------+
| | | |
+---------+ | | +----------+
| +-------+--+------+--+-------+ |
| | | | | | | |
+----+ +----+ +----+ +----+
| | | | | | | |
|AS2 | |AS2 | |AS3 | |AS3 | Tier 2
| | | | | | | |
+----+ +----+ +----+ +----+
| | | |
| | | |
| +-----+ | | +-----+ |
+-| AS4 |-+ +-| AS5 |-+ Tier 3
+-----+ +-----+
| | | | | |
<- 3 Servers -> <- 10 Servers ->
Typical Data Center Topology (RFC7938)
Figure 3
In a regular ECMP environment, the tier 1 layer would see an ECMP
path equally load-sharing across all 4 tier 2 paths. This would
cause the servers on the left part of the data center to be
potentially overloaded, while the servers on the right to be
underutilized. Using link bandwidth advertisements the servers could
add a link bandwidth extended community to the advertised service
prefix. Another option is to add the extended community on the tier
3 network devices as the routes are received from the servers or
generated locally on the network devices. If the link bandwidth
value advertised for the service represents the server capacity for
that service, each data center tier would aggregate the values up
when sending the update to the higher tier. The result would be a
set of weighted load-sharing metrics at each tier allowing the
network to distribute the flow load among the different servers in
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the most optimal way. If a server is added or removed to the service
prefix, it would add or remove its link bandwidth value and the
network would adjust accordingly.
Figure 4 shows a more popular Spine Leaf architecture similar to
[RFC7938] section 3.2. Tor1, Tor2 and Tor3 are in the same tier,
i.e. the leaf tier (The representation shown in Figure 3 here is the
unfolded Clos). Using the same example above, it is clear that the
LB extended community value received by each of Spine1 and Spine2
from Tor1 and Tor2 is in the ratio 3 to 10 respectively. The Spines
will then aggregate the bandwidth, regenerate and advertise the LB
extended-community to Tor3. Tor3 will do equal cost sharing to both
the spines which in turn will do the traffic-splitting in the ratio 3
to 10 when forwarding the traffic to the Tor1 and Tor2 respectively.
+------+
| Tor3 | Tier 1
+------+
|
+- - - - -+- - - - +
| |
+----+ +----+
| | | |
|Spine1 |Spine2
| | | |
+----+--+ +-+----+
| \ / |
- + - -
| / \ |
+-----+- + -+-----+
|Tor1 | |Tor2 | Tier 1
+-----+ +-----+
| | | | | |
<- 3 Servers -> <- 10 Servers ->
Two-tier Clos Data Center Topology
Figure 4
5. Non-Conforming BGP Topologies
This use-case will not readily apply to all topologies. Figure 5
shows a all EBGP topology: R1, R2, R3, R4, R5 and R6 are in AS1, AS2,
AS3, AS4, AS5 and AS6 respectively. A net p/m, is being advertised
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from a server S1 with LB extended-community value 10 to R1 and R5.
R1 advertises p/m to R2 and R3 and also regenerates the LB extended-
community with value 10. R4 receives the advertisements from R2, R3
and R5 and computes the aggregate bandwidth to be 30. R4 advertises
p/m to R6 with LB extended-community value 30. The link bandwidths
are as shown in the figure.
In the example as can be seen, R4 will do the cumulative bandwidth of
the LB that it receives from R2, R3 and R5 which is 30. When R4
receives the traffic from R6, it will load-balance it across R2, R3
and R5. As a result R1 will receive twice the volume of traffic that
R5 does. This is not desirable because the bandwidth from R1 to S1
and the bandwidth from S1 to R5 is the same i.e. 10. The discrepancy
arose because when R4 aggregated the link bandwidth values from the
received advertisements, the contribution from R1 was actually
factored in twice.
|- - R2 - 10 --|
| |
| |
S1- - 10- R1 R4- - - --30 - -R6
| | |
| | |
10 |- - -R3- 10 - -|
| |
|- - - R5 - - -- - -- - - -|
A non-conforming topology for the Cumulative DMZ
Figure 5
One way to make the topology in the figure above conforming would be
to regenerate a normalized value of the aggregate link bandwidth when
the aggregate link bandwidth is being advertised over more than one
eBGP peer link. Such normalization can be achieved through outbound
policy application on top of the aggregate link bandwidth value. A
couple of options in this context are:
1. divide the aggregate link bandwidth across the eBGP peers equally
2. divide the aggregate link bandwidth across the eBGP peers as per
the ratio of the operational link capacity of the eBGP peer links
These and similar options for regeneration of link-bandwidth to cater
to load-balancing requirements in such topologies are outside the
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scope of this document and can be implementated as additional
outbound policy enhancements on top of a computed aggregate link
bandwidth.
6. Protocol Considerations
[I-D.ietf-idr-link-bandwidth] needs to be refreshed. No Protocol
Changes are necessary if the knobs are implemented as recommended.
The other way to achieve the same purpose would be to use some
complicated policy frameworks. But that is only a conjecture.
7. Operational Considerations
A note may be made that these solutions also are applicable to many
address families such as L3VPN [RFC2547] , IPv4 with labeled unicast
[RFC8277] and EVPN [RFC7432].
In topologies and implementation where there is an option to
advertise all multipath (equal cost) eligible paths to eBGP peers
(i.e. 'ecmp' form of additional-path advertisement is enabled),
aggregate link bandwidth advertisement may not be required or may be
redundant since the receiving BGP speaker receives the link bandwidth
extended community values with all eligible paths, so the aggregate
link bandwidth is effectively received by the downstream eBGP speaker
and can be used in the local computation to affect the forwarding
behaviour. This assumes the additional paths are advertised with
next-hop self.
8. Security Considerations
This document raises no new security issues.
9. Acknowledgements
Viral Patel did substantial work on an implementation along with the
first author. The authors would like to thank Acee Lindem and Jakob
Heitz for their help in reviewing the draft and valuable suggestions.
The authors would like to thank Shyam Sethuram, Sameer Gulrajani,
Nitin Kumar, Keyur Patel and Juan Alcaide for discussions related to
the draft.
10. References
10.1. Normative References
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[I-D.ietf-idr-link-bandwidth]
Mohapatra, P. and R. Fernando, "BGP Link Bandwidth
Extended Community", draft-ietf-idr-link-bandwidth-06
(work in progress), January 2013.
[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>.
[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>.
10.2. Informative References
[RFC2547] Rosen, E. and Y. Rekhter, "BGP/MPLS VPNs", RFC 2547,
DOI 10.17487/RFC2547, March 1999,
<https://www.rfc-editor.org/info/rfc2547>.
[RFC7432] Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
2015, <https://www.rfc-editor.org/info/rfc7432>.
[RFC8277] Rosen, E., "Using BGP to Bind MPLS Labels to Address
Prefixes", RFC 8277, DOI 10.17487/RFC8277, October 2017,
<https://www.rfc-editor.org/info/rfc8277>.
Authors' Addresses
Satya Ranjan Mohanty
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
USA
Email: satyamoh@cisco.com
Arie Vayner
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
USA
Email: avayner@google.com
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Akshay Gattani
Arista Networks
5453 Great America Parkway
Santa Clara, CA 95054
USA
Email: akshay@arista.com
Ajay Kini
Arista Networks
5453 Great America Parkway
Santa Clara, CA 95054
USA
Email: ajkini@arista.com
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