BESS WorkGroup S. Mohanty Internet-Draft A. Millisor Intended status: Informational Cisco Systems Expires: September 4, 2018 A. Vayner Google March 3, 2018 Cumulative DMZ Link Bandwidth and load-balancing draft-mohanty-bess-ebgp-dmz-00 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. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on September 4, 2018. Mohanty, et al. Expires September 4, 2018 [Page 1] Internet-Draft Cumulative DMZ Link Bandwidth March 2018 Copyright Notice Copyright (c) 2018 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/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as 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 . . . . . . . . . . . . . . 5 5. Non-Conforming BGP Topologies . . . . . . . . . . . . . . . . 7 6. Protocol Considerations . . . . . . . . . . . . . . . . . . . 8 7. Operational Considerations . . . . . . . . . . . . . . . . . 8 8. Security Considerations . . . . . . . . . . . . . . . . . . . 8 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 10.1. Normative References . . . . . . . . . . . . . . . . . . 9 10.2. Informative References . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 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 Mohanty, et al. Expires September 4, 2018 [Page 2] Internet-Draft Cumulative DMZ Link Bandwidth March 2018 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, 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 Mohanty, et al. Expires September 4, 2018 [Page 3] Internet-Draft Cumulative DMZ Link Bandwidth March 2018 links are shown alongside the links (The exact units are not really important). 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. 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 to R6 and the remaining 43% to R7. 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. To address cases like the above example, rather than inventing something new from scratch, we will relax a few assumptions of the link bandwidth extended community. With neighbor-specific knobs outbound/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 (received via the neighbor inbound policy knobs) from the downstream routers and then regenerate and advertise (via neighbor outbound policy knob) this aggregate link bandwidth value stored in 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. Mohanty, et al. Expires September 4, 2018 [Page 4] Internet-Draft Cumulative DMZ Link Bandwidth March 2018 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 2 shows a 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 2 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 Mohanty, et al. Expires September 4, 2018 [Page 5] Internet-Draft Cumulative DMZ Link Bandwidth March 2018 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 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 3 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. Mohanty, et al. Expires September 4, 2018 [Page 6] Internet-Draft Cumulative DMZ Link Bandwidth March 2018 +------+ | Tor3 | Tier 1 +------+ | +- - - - -+- - - - + | | +----+ +----+ | | | | |Spine1 |Spine2 | | | | +----+--+ +-+----+ | \ / | - + - - | / \ | +-----+- + -+-----+ |Tor1 | |Tor2 | Tier 1 +-----+ +-----+ | | | | | | <- 3 Servers -> <- 10 Servers -> Two-tier Clos Data Center Topology Figure 3 5. Non-Conforming BGP Topologies This use-case will not readily apply to all topologies. Figure 4 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 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. Mohanty, et al. Expires September 4, 2018 [Page 7] Internet-Draft Cumulative DMZ Link Bandwidth March 2018 |- - R2 - 10 --| | | | | S1- - 10- R1 R4- - - --30 - -R6 | | | | | | 10 |- - -R3- 10 - -| | | |- - - R5 - - -- - -- - - -| A non-conforming topology for the Cumulative DMZ Figure 4 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]. 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 Mohanty, et al. Expires September 4, 2018 [Page 8] Internet-Draft Cumulative DMZ Link Bandwidth March 2018 10.1. Normative References [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, . [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 USA Email: satyamoh@cisco.com Mohanty, et al. Expires September 4, 2018 [Page 9] Internet-Draft Cumulative DMZ Link Bandwidth March 2018 Aaron Millisor Cisco Systems 170 W. Tasman Drive San Jose, CA 95134 USA Email: amilliso@cisco.com Arie Vayner Google 1600 Amphitheatre Pkwy Mountain View, CA 94043 USA Email: avayner@google.com Mohanty, et al. 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