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


   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

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on September 16, 2021.

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Copyright Notice

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   described in the Simplified BSD License.

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

   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",
   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

   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

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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              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,

   [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,

10.2.  Informative References

   [RFC2547]  Rosen, E. and Y. Rekhter, "BGP/MPLS VPNs", RFC 2547,
              DOI 10.17487/RFC2547, March 1999,

   [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, <>.

   [RFC8277]  Rosen, E., "Using BGP to Bind MPLS Labels to Address
              Prefixes", RFC 8277, DOI 10.17487/RFC8277, October 2017,

Authors' Addresses

   Satya Ranjan Mohanty
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134


   Arie Vayner
   1600 Amphitheatre Parkway
   Mountain View, CA  94043


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   Akshay Gattani
   Arista Networks
   5453 Great America Parkway
   Santa Clara, CA  95054


   Ajay Kini
   Arista Networks
   5453 Great America Parkway
   Santa Clara, CA  95054


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