BGP-Prefix Segment in large-scale data centers
draft-filsfils-spring-segment-routing-msdc-00
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Authors | Clarence Filsfils , Stefano Previdi , Jon Mitchell , Benjamin Black , Dmitry Afanasiev , Saikat Ray , Keyur Patel | ||
Last updated | 2014-10-26 | ||
Replaced by | draft-ietf-spring-segment-routing-msdc, RFC 8670 | ||
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draft-filsfils-spring-segment-routing-msdc-00
Network Working Group C. Filsfils, Ed. Internet-Draft S. Previdi, Ed. Intended status: Informational Cisco Systems, Inc. Expires: April 29, 2015 J. Mitchell B. Black Microsoft Corporation D. Afanasiev Yandex S. Ray K. Patel Cisco Systems, Inc. October 26, 2014 BGP-Prefix Segment in large-scale data centers draft-filsfils-spring-segment-routing-msdc-00 Abstract This document describes a practical use case where BGP segment routing can be used in a large-scale data center. 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 RFC 2119 [RFC2119]. 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 http://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 April 29, 2015. Filsfils, et al. Expires April 29, 2015 [Page 1] Internet-Draft BGP-Prefix SID in large-scale DCs October 2014 Copyright Notice Copyright (c) 2014 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 (http://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 1.1. Reference Diagram . . . . . . . . . . . . . . . . . . . . 3 2. BGP Prefix Segment . . . . . . . . . . . . . . . . . . . . . 5 3. Segment Routing Design . . . . . . . . . . . . . . . . . . . 5 3.1. Control Plane . . . . . . . . . . . . . . . . . . . . . . 6 3.2. Data Plane . . . . . . . . . . . . . . . . . . . . . . . 7 3.3. Network Design Variation . . . . . . . . . . . . . . . . 8 4. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.1. MPLS Dataplane with operational simplicity . . . . . . . 9 4.2. Minimizing the FIB table . . . . . . . . . . . . . . . . 9 4.3. Egress Peer Engineering . . . . . . . . . . . . . . . . . 10 4.4. Capacity Optimization . . . . . . . . . . . . . . . . . . 10 4.5. Incremental Deployments . . . . . . . . . . . . . . . . . 11 4.6. Anycast . . . . . . . . . . . . . . . . . . . . . . . . . 12 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 6. Manageability Considerations . . . . . . . . . . . . . . . . 12 7. Security Considerations . . . . . . . . . . . . . . . . . . . 12 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 9.1. Normative References . . . . . . . . . . . . . . . . . . 13 9.2. Informative References . . . . . . . . . . . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14 1. Introduction Segment Routing (SR), as described in [I-D.filsfils-spring-segment-routing] leverages the source routing paradigm. A node steers a packet through an ordered list of instructions, called segments. A segment can represent any instruction, topological or service-based. A segment can have a local semantic to an SR node or global within an SR domain. SR Filsfils, et al. Expires April 29, 2015 [Page 2] Internet-Draft BGP-Prefix SID in large-scale DCs October 2014 allows to enforce a flow through any topological path and service chain while maintaining per-flow state only at the ingress node to the SR domain. Segment Routing can be applied to the MPLS and IPv6 dataplanes. The use-case described in this document focuses on SR applied to the MPLS dataplane. In this context, a segment is encoded as an MPLS label. An ordered list of segments is encoded as a stack of labels. The segment to process is on the top of the stack. Upon completion of a segment, the related label is popped from the stack. No forwarding change is required to the MPLS dataplane. The use-case described in this document should be considered in the context of the BGP-based large-scale data-center (DC) design described in [I-D.ietf-rtgwg-bgp-routing-large-dc] where eBGP3107 described in [RFC3107] is used instead of eBGP. 1.1. Reference Diagram We reuse the 5-stage topology diagram from [I-D.ietf-rtgwg-bgp-routing-large-dc] while adapting the device naming to simplify the text. Tier-3 +-----+ |NODE | +->| 5 |--+ | +-----+ | Tier-2 | | Tier-2 +-----+ | +-----+ | +-----+ +------------>|NODE |--+->|NODE |--+--|NODE |-------------+ | +-----| 3 |--+ | 6 | +--| 9 |-----+ | | | +-----+ +-----+ +-----+ | | | | | | | | +-----+ +-----+ +-----+ | | | +-----+---->|NODE |--+ |NODE | +--|NODE |-----+-----+ | | | | +---| 4 |--+->| 7 |--+--| 10 |---+ | | | | | | | +-----+ | +-----+ | +-----+ | | | | | | | | | | | | | | +-----+ +-----+ | +-----+ | +-----+ +-----+ |NODE | |NODE | Tier-1 +->|NODE |--+ Tier-1 |NODE | |NODE | | 1 | | 2 | | 8 | | 11 | | 12 | +-----+ +-----+ +-----+ +-----+ +-----+ | | | | | | | | A O B O <- Servers -> Z O O O Figure 1: 5-stage Clos topology Filsfils, et al. Expires April 29, 2015 [Page 3] Internet-Draft BGP-Prefix SID in large-scale DCs October 2014 Briefly, we remind the salient points of the eBGP-3107 large-scale DC design ([I-D.ietf-rtgwg-bgp-routing-large-dc]) o Each node is its own AS: For simple and efficient route propagation filtering, Nodes 5, 6, 7 and 8 share the same AS, Nodes 3 and 4 share the same AS, nodes 9 and 10 share the same AS. For efficient usage of the scarce 2-byte private AS pool, different tier-1 nodes might share the same AS. Without loss of generality, we will simplify these details in this document and assume that each node has its own AS. o Each node peers with its neighbors via eBGP3107 session. o Each node originates its loopback into BGP and announces it to its neighbors. o The forwarding plane at Tier-2 and Tier-1 is MPLS. o The forwarding plane at Tier-3 is either IP2MPLS (if the host sends IP traffic) or MPLS2MPLS (if the host sends MPLS- encapsulated traffic). For illustration purpose, we assume that: o The AS of Node X is AS X. o The loopback of Node X is 1.1.1.x/32. In this document, we also refer to the Tier-3, Tier-2 and Tier-1 switches respectively as Spine, Leaf and ToR (top of rack) switches. When a ToR switch acts as a gateway to the "outside world", we call it a border switch. Filsfils, et al. Expires April 29, 2015 [Page 4] Internet-Draft BGP-Prefix SID in large-scale DCs October 2014 +-----+ +-----+ +-----+ +---------->|NODE | |NODE | |NODE | | | 4 |--+->| 7 |--+--| 10 |---+ | +-----+ +-----+ +-----+ | | | +-----+ +-----+ |NODE | |NODE | | 1 | | 11 | +-----+ +-----+ | | A <- Servers -> Z Figure 2: Path from A to Z via nodes 1, 4, 7, 10 and 11 2. BGP Prefix Segment A BGP-Prefix Segment is a segment associated with a BGP prefix. A BGP-Prefix Segment is a network-wide instruction to forward the packet along the ECMP-aware best path to the related prefix [I-D.keyupate-idr-bgp-prefix-sid]. In this document, we make the network design decision to assume that all the nodes are allocated the same SRGB, e.g. [16000, 23999]. This is important to fulfill the requirement for operational simplification as explained in [I-D.filsfils-spring-segment-routing] and [I-D.filsfils-spring-segment-routing-use-cases]. Note well that the use of a common SRGB in all nodes is not a requirement, one could use a different SRGB at every node. However, this would make the operation of the DC fabric more complex as the label allocated to the loopback of a remote switch is then different at every node. For illustration purpose, we assume that the segment index allocated to prefix 1.1.1.x/32 is X. As a result, a local label 1600x is allocated for prefix 1.1.1.x/32 by each node throughout the DC fabric. 3. Segment Routing Design Referring to Figure 1 and Figure 2 and assuming the IP address, AS and index allocation previously described, this section details the control plane operation and the data plane states for the prefix 1.1.1.11/32 (loopback of node 11). Filsfils, et al. Expires April 29, 2015 [Page 5] Internet-Draft BGP-Prefix SID in large-scale DCs October 2014 3.1. Control Plane Node 11 originates 1.1.1.11/32 in BGP and allocates to it the BGP- Prefix Segment attribute (index11). Node 11 sends the following eBGP3107 update to Node 10: . NLRI: 1.1.1.11/32 . Label: Implicit-Null . Next-hop: Node11's interface address on the link to Node10 . AS Path: {11} . BGP-Prefix Attribute: Index 11 Node 10 receives the above update. As it is SR capable, Node10 is able to interpret the BGP-Prefix Attribute and hence allocates the label 16011 to the NLRI (instead of asking a "random/local" label from its label manager). The implicit-null label in the update signals to Node 10 that it is the penultimate hop and MUST pop the top label on the stack before forwarding traffic for this prefix to Node 11. Then, Node 10 sends the following eBGP3107 update to Node 7: . NLRI: 1.1.1.11/32 . Label: 16011 . Next-hop: Node10's interface address on the link to Node7 . AS Path: {10, 11} . BGP-Prefix Attribute: Index 11 Node 7 receives the above update. As it is SR capable, Node 7 is able to interpret the BGP-Prefix Attribute and hence allocates the label 16011 to the NLRI (instead of asking a "random/local" label from its label manager). Node 7 sends the following eBGP3107 update to Node 4: . NLRI: 1.1.1.11/32 . Label: 16011 . Next-hop: Node7's interface address on the link to Node4 . AS Path: {7, 10, 11} . BGP-Prefix Attribute: Index 11 Node 4 receives the above update. As it is SR capable, Node 4 is able to interpret the BGP-Prefix Attribute and hence allocates the label 16011 to the NLRI (instead of asking a "random/local" label from its label manager). Node 4 sends the following eBGP3107 update to Node 1: Filsfils, et al. Expires April 29, 2015 [Page 6] Internet-Draft BGP-Prefix SID in large-scale DCs October 2014 . NLRI: 1.1.1.11/32 . Label: 16011 . Next-hop: Node4's interface address on the link to Node1 . AS Path: {4, 7, 10, 11} . BGP-Prefix Attribute: Index 11 Node 1 receives the above update. As it is SR capable, Node 1 is able to interpret the BGP-Prefix Attribute and hence allocates the label 16011 to the NLRI (instead of asking a "random/local" label from its label manager). 3.2. Data Plane Referring to figure 1, and assuming all nodes apply the same advertisement rules described above, here are the IP/MPLS forwarding tables for prefix 1.1.1.11/32 at nodes 1, 4, 7 and 10. ----------------------------------------------- Incoming label | outgoing label | Outgoing or IP destination | | Interface ------------------+----------------+----------- 16011 | 16011 | ECMP{3, 4} 1.1.1.11/32 | 16011 | ECMP{3, 4} ------------------+----------------+----------- Figure 3: Node 1 Forwarding Table ----------------------------------------------- Incoming label | outgoing label | Outgoing or IP destination | | Interface ------------------+----------------+----------- 16011 | 16011 | ECMP{7, 8} 1.1.1.11/32 | 16011 | ECMP{7, 8} ------------------+----------------+----------- Figure 4: Node-4 Forwarding Table ----------------------------------------------- Incoming label | outgoing label | Outgoing or IP destination | | Interface ------------------+----------------+----------- 16011 | 16011 | 10 1.1.1.11/32 | 16011 | 10 ------------------+----------------+----------- Figure 5: Node-7 Forwarding Table Filsfils, et al. Expires April 29, 2015 [Page 7] Internet-Draft BGP-Prefix SID in large-scale DCs October 2014 ----------------------------------------------- Incoming label | outgoing label | Outgoing or IP destination | | Interface ------------------+----------------+----------- 16011 | POP | 11 1.1.1.11/32 | N/A | 11 ------------------+----------------+----------- Node-10 Forwarding Table 3.3. Network Design Variation A network design choice could consist of switching all the traffic through tier-2 and tier-3 as MPLS traffic. In this case, one could filter away the IP entries at nodes 4, 7 and 10. This might be beneficial in order to optimize the forwarding table size. A network design choice could consist in allowing the hosts to send MPLS-encapsulated traffic (based on EPE use-case, [I-D.filsfils-spring-segment-routing-central-epe]). For example, Node 1 would receive Node11-destined MPLS-encapsulated traffic from its attached host A and would switch this traffic on the basis of the MPLS entry for 16011 (instead of classically receiving IP traffic from A and performing an IPtoMPLS switching operation). 4. Benefits The network design illustrated in this document retains all the benefits explained in [I-D.ietf-rtgwg-bgp-routing-large-dc], namely: o Bandwidth and traffic patterns o Capex minimization o Opex minimization o Traffic Engineering o Fast routing convergence o Anycast for extra availability and load-balancing Furthermore, it introduces the following benefits: o MPLS dataplane with operational simplicity o Minimization of the FIB table size Filsfils, et al. Expires April 29, 2015 [Page 8] Internet-Draft BGP-Prefix SID in large-scale DCs October 2014 o Egress Peer Engineering o Capacity Optimization o Incremental Deployment In the following sections, we detail the anycast benefit and the five "additional" benefits introduced by the BGP-Prefix Segment ([I-D.keyupate-idr-bgp-prefix-sid]). 4.1. MPLS Dataplane with operational simplicity As required by [I-D.ietf-rtgwg-bgp-routing-large-dc], no new signaling protocol is introduced. The Prefix Segment is a lightweight extension to BGP3107 [RFC3107]. LDP and RSVP-TE are not used. Thanks to the BGP-Prefix Segment extension ([I-D.keyupate-idr-bgp-prefix-sid]) and the design decision to use the same SRGB at each node in the DC fabric, the troubleshooting of the network is drastically simplified. At every node in the fabric, the same label is associated to each remote prefix/switch. When a controller (e.g. EPE controller in [I-D.filsfils-spring-segment-routing-central-epe]) programs a host A to send its traffic to host Z via the normal BGP multipath, the controller uses label 16011 associated with the ToR switch connected to the server Z. Specifically, the controller does not need to pick the label based on the source ToR that the source host is connected to. In a classic BGP3107 design applied to the DC fabric illustrated in Figure 1, the ToR switch 1 connected to server A would most likely allocate a different label for 1.1.1.11/32 than the one allocated by ToR switch 2. As a consequence, the controller would need to adapt the SR policy to each host, based on the ToR switch that they are connected to. This adds state maintenance and synchronization problems. All this unnecessary complexity is eliminated thanks to the BGP-Prefix Segment extension. Again, both the BGP-Prefix Segment and the design decision to use a common SRGB on all nodes have made this possible. 4.2. Minimizing the FIB table The designer may decide to switch all the traffic at tier2 and tier3's based on MPLS, hence drastically decreasing the IP table size at these nodes. Filsfils, et al. Expires April 29, 2015 [Page 9] Internet-Draft BGP-Prefix SID in large-scale DCs October 2014 This is easily accomplished by encapsulating the traffic directly at the host, or at the source ToR switch by pushing the BGP-Prefix Segment of the destination ToR for intra-DC traffic or border switch for inter-DC or DC-to-outside-world traffic. 4.3. Egress Peer Engineering It is straightforward to combine the design illustrated in this document with the EPE use-case [I-D.filsfils-spring-segment-routing-central-epe]. In such case, the operator is able to engineer its outbound traffic on a per host-flow basis, without incurring any additional state at intermediate points in the DC fabric. For example, the controller only needs to inject a per-flow state on the host A to force it to send its traffic destined to a specific internet destination D via a selected border switch (say 12 in Figure 1instead of another border switch 13) and a specific egress peer of border switch 12 (say peer AS 9999 of local PeerNode segment 9999 at border switch 12 instead of any other peer which provides a path to the destination D). Any packet matching this state at host A would be encapsulated with SR segment list (i.e.: label stack) {16012, 9999}. 16012 would steer the flow through the DC fabric, leveraging any ECMP, along the best path to border switch 12. Once the flow gets to border switch 12, the active segment is 9999. This EPE PeerNode segment forces border switch 12 to forward the packet to peer AS 9999, without any IP lookup at the border switch. There is no per-flow state for this engineered flow in the DC fabric. The per-flow state is only required at the source (source routing benefits). Note as well, that on top of allowing full engineering control, such a design also offer FIB table minimization benefits as the internet- scale IP lookup at border switch 12 might be avoided. 4.4. Capacity Optimization It is straightforward to combine the centralized capacity optimization process described in [I-D.filsfils-spring-segment-routing-use-cases] with the design introduced in this document. For example, in Figure 1, the controller may detect a hot spot on node 5. One way to alleviate the load is to deploy a set of per- destination flow states at a set of ToR switches such that they send they traffic via fabric paths that avoid Node 5. Filsfils, et al. Expires April 29, 2015 [Page 10] Internet-Draft BGP-Prefix SID in large-scale DCs October 2014 For example, host A could be forced to go to host Z via Node 4. This is conveniently programmed by the controller as a flow state for Z at host A which pushes the segment list {16004, 16011}. 16004 steers the traffic to node 4 via any ECMP path (e.g. multiple parallel links from Node 1 to Node 4). 16011 then steers the traffic from node 4 to node 11 load-balancing the traffic via nodes 7 and 8, and any ECMP along that path. This flow is thus avoiding Node 5 while still leveraging the maximum number of available ECMP paths. This is realized without any intermediate per-flow state. Another alternative state at A could be {16008, 16011}. In this case, this flow would use any ECMP path up to node 8 and then any ECMP path up to node 11. While traffic-engineering within a DC has been rarely used in the past, it is expected to eventually be required as Clos topologies get optimized for higher scale [DRAGONFLY]. 4.5. Incremental Deployments Referring to , let us assume that node 7 does not support the BGP- Prefix Segment attribute.Figure 2, let us assume that node 7 does not support the BGP-Prefix Segment attribute. From a signaling viewpoint, nothing would change as even if Node6 does not understand the BGP-Prefix Segment attribute, it does propagate it unmodified to its neighbors. From a label allocation viewpoint, the only difference is that Node7 would allocate a dynamic label to the prefix 1.1.1.11/32 (e.g. 12345) and would advertise that label to its neighbor Node4. Let's highlight that Node4 does understand the BGP-Prefix Segment attribute and hence allocates the indexed label in the SRGB (16011) for 1.1.1.11/32. As a result, all the dataplane entries across the network would be unchanged except the entries at Node7 and its neighbor Node4 as shown in the figures below. ------------------------------------------ Incoming label | outgoing | Outgoing or IP destination | label | Interface -------------------+---------------------- 12345 | 16011 | 10 Figure 7: Node 7 Forwarding Table Filsfils, et al. Expires April 29, 2015 [Page 11] Internet-Draft BGP-Prefix SID in large-scale DCs October 2014 ------------------------------------------ Incoming label | outgoing | Outgoing or IP destination | label | Interface -------------------+---------------------- 16011 | 12345 | 7 Figure 8: Node 4 Forwarding Table The BGP-Prefix Segment functionality can thus be deployed incrementally one node at a time. Where it is deployed, the operator enjoys its benefits without any dependency on the deployment state at any other node. 4.6. Anycast The design presented in this document preserves the availability and load-balancing properties of the base design presented in [I-D.filsfils-spring-segment-routing]. For example, one could assign an anycast loopback 1.1.1.20/32 to the border switches 11 and 12 (on top of their node-specific loopbacks). Doing so, the EPE controller could express a default "go-to-the- internet via any border switch" policy as segment list {16020}. Indeed, from any host in the DC fabric, from any ToR switch, 16020 steers the packet towards the border switches 11 or 12 leveraging any ECMP along the best paths to these switches. 5. IANA Considerations TBD 6. Manageability Considerations TBD 7. Security Considerations TBD 8. Acknowledgements TBD Filsfils, et al. Expires April 29, 2015 [Page 12] Internet-Draft BGP-Prefix SID in large-scale DCs October 2014 9. References 9.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3107] Rekhter, Y. and E. Rosen, "Carrying Label Information in BGP-4", RFC 3107, May 2001. 9.2. Informative References [DRAGONFLY] Kim, J., Dally, W., Scott, S., and D. Abts, "Cost- Efficient Dragonfly Topology for Large-Scale Systems", 2009. [I-D.filsfils-spring-segment-routing] Filsfils, C., Previdi, S., Bashandy, A., Decraene, B., Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R., Ytti, S., Henderickx, W., Tantsura, J., and E. Crabbe, "Segment Routing Architecture", draft-filsfils-spring- segment-routing-04 (work in progress), July 2014. [I-D.filsfils-spring-segment-routing-central-epe] Filsfils, C., Previdi, S., Patel, K., Aries, E., shaw@fb.com, s., Ginsburg, D., and D. Afanasiev, "Segment Routing Centralized Egress Peer Engineering", draft- filsfils-spring-segment-routing-central-epe-02 (work in progress), July 2014. [I-D.filsfils-spring-segment-routing-use-cases] Filsfils, C., Francois, P., Previdi, S., Decraene, B., Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R., Ytti, S., Henderickx, W., Tantsura, J., Kini, S., and E. Crabbe, "Segment Routing Use Cases", draft-filsfils- spring-segment-routing-use-cases-01 (work in progress), October 2014. [I-D.ietf-rtgwg-bgp-routing-large-dc] Lapukhov, P., Premji, A., and J. Mitchell, "Use of BGP for routing in large-scale data centers", draft-ietf-rtgwg- bgp-routing-large-dc-00 (work in progress), August 2014. [I-D.keyupate-idr-bgp-prefix-sid] Patel, K., Ray, S., Previdi, S., and C. Filsfils, "Segment Routing Prefix SID extensions for BGP", 2014. Filsfils, et al. Expires April 29, 2015 [Page 13] Internet-Draft BGP-Prefix SID in large-scale DCs October 2014 Authors' Addresses Clarence Filsfils (editor) Cisco Systems, Inc. Brussels BE Email: cfilsfil@cisco.com Stefano Previdi (editor) Cisco Systems, Inc. Via Del Serafico, 200 Rome 00142 Italy Email: sprevidi@cisco.com Jon Mitchell Microsoft Corporation One Microsoft Way Redmond, WA 98052 United States Email: Jon.Mitchell@microsoft.com Benjamin Black Microsoft Corporation One Microsoft Way Redmond, WA 98052 United States Email: benblack@microsoft.com Dmitry Afanasiev Yandex RU Email: fl0w@yandex-team.ru Filsfils, et al. Expires April 29, 2015 [Page 14] Internet-Draft BGP-Prefix SID in large-scale DCs October 2014 Saikat Ray Cisco Systems, Inc. 170, West Tasman Drive San Jose, CA 95134 US Email: sairay@cisco.com Keyur Patel Cisco Systems, Inc. US Email: keyupate@cisco.com Filsfils, et al. Expires April 29, 2015 [Page 15]