Network Working Group C. Filsfils, Ed. Internet-Draft S. Previdi, Ed. Intended status: Informational Cisco Systems, Inc. Expires: October 15, 2016 J. Mitchell Unaffiliated E. Aries P. Lapukhov Facebook April 13, 2016 BGP-Prefix Segment in large-scale data centers draft-ietf-spring-segment-routing-msdc-01 Abstract This document describes the motivation and benefits for applying segment routing in the data-center. It describes the design to deploy segment routing in the data-center, for both the MPLS and IPv6 dataplanes. 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 October 15, 2016. Filsfils, et al. Expires October 15, 2016 [Page 1] Internet-Draft BGP-Prefix SID in large-scale DCs April 2016 Copyright Notice Copyright (c) 2016 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 . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Large Scale Data Center Network Design Summary . . . . . . . 3 2.1. Reference design . . . . . . . . . . . . . . . . . . . . 4 3. Some open problems in large data-center networks . . . . . . 5 4. Applying Segment Routing in the DC with MPLS dataplane . . . 6 4.1. BGP Prefix Segment . . . . . . . . . . . . . . . . . . . 6 4.2. eBGP Labeled Unicast (RFC3107) . . . . . . . . . . . . . 7 4.2.1. Control Plane . . . . . . . . . . . . . . . . . . . . 7 4.2.2. Data Plane . . . . . . . . . . . . . . . . . . . . . 9 4.2.3. Network Design Variation . . . . . . . . . . . . . . 10 4.2.4. Global BGP Prefix Segment through the fabric . . . . 10 4.2.5. Incremental Deployments . . . . . . . . . . . . . . . 11 4.3. iBGP Labeled Unicast (RFC3107) . . . . . . . . . . . . . 12 5. Applying Segment Routing in the DC with IPv6 dataplane . . . 12 6. Communicating path information to the host . . . . . . . . . 13 7. Addressing the open problems . . . . . . . . . . . . . . . . 14 7.1. Per-packet and flowlet switching . . . . . . . . . . . . 14 7.2. Performance-aware routing . . . . . . . . . . . . . . . . 15 7.3. Non-oblivious routing . . . . . . . . . . . . . . . . . . 16 7.4. Deterministic network probing . . . . . . . . . . . . . . 16 8. Additional Benefits . . . . . . . . . . . . . . . . . . . . . 16 8.1. MPLS Dataplane with operational simplicity . . . . . . . 16 8.2. Minimizing the FIB table . . . . . . . . . . . . . . . . 17 8.3. Egress Peer Engineering . . . . . . . . . . . . . . . . . 17 8.4. Incremental Deployments . . . . . . . . . . . . . . . . . 18 8.5. Anycast . . . . . . . . . . . . . . . . . . . . . . . . . 18 9. Preferred SRGB Allocation . . . . . . . . . . . . . . . . . . 18 10. Alternative Options . . . . . . . . . . . . . . . . . . . . . 19 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 12. Manageability Considerations . . . . . . . . . . . . . . . . 20 13. Security Considerations . . . . . . . . . . . . . . . . . . . 20 Filsfils, et al. Expires October 15, 2016 [Page 2] Internet-Draft BGP-Prefix SID in large-scale DCs April 2016 14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20 15. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 20 16. References . . . . . . . . . . . . . . . . . . . . . . . . . 21 16.1. Normative References . . . . . . . . . . . . . . . . . . 21 16.2. Informative References . . . . . . . . . . . . . . . . . 21 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22 1. Introduction Segment Routing (SR), as described in [I-D.ietf-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 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 data-planes. The use-case use-cases 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]We extend it by applying SR both with IPv6 and MPLS dataplane. 2. Large Scale Data Center Network Design Summary This section provides a brief summary of the informational document [I-D.ietf-rtgwg-bgp-routing-large-dc] that outlines a practical network design suitable for data-centers of various scales: o Data-center networks have highly symmetric topologies with multiple parallel paths between two server attachment points. The well-known Clos topology is most popular among the operators. In a Clos topology, the minimum number of parallel paths between two elements is determined by the "width" of the middle stage. See Figure 1 below for an illustration of the concept. o Large-scale data-centers commonly use a routing protocol, such as BGP4 [RFC4271] in order to provide endpoint connectivity. Recovery after a network failure is therefore driven either by local knowledge of directly available backup paths or by distributed signaling between the network devices. o Within data-center networks, traffic is load-shared using the Equal Cost Multipath (ECMP) mechanism. With ECMP, every network device implements a pseudo-random decision, mapping packets to one of the parallel paths by means of a hash function calculated over Filsfils, et al. Expires October 15, 2016 [Page 3] Internet-Draft BGP-Prefix SID in large-scale DCs April 2016 certain parts of the packet, typically a combination of various packet header fields. The following is a schematic of a five-stage Clos topology, with four devices in the middle stage. Notice that number of paths between Node1 and Node12 equals to four: the paths have to cross all of Tier-1 devices. At the same time, the number of paths between Node1 and Node2 equals two, and the paths only cross Tier-2 devices. Other topologies are possible, but for simplicity we'll only look into the topologies that have a single path from Tier-1 to Tier-3. The rest could be treated similarly, with a few modifications to the logic. 2.1. Reference design Tier-1 +-----+ |NODE | +->| 5 |--+ | +-----+ | Tier-2 | | Tier-2 +-----+ | +-----+ | +-----+ +------------>|NODE |--+->|NODE |--+--|NODE |-------------+ | +-----| 3 |--+ | 6 | +--| 9 |-----+ | | | +-----+ +-----+ +-----+ | | | | | | | | +-----+ +-----+ +-----+ | | | +-----+---->|NODE |--+ |NODE | +--|NODE |-----+-----+ | | | | +---| 4 |--+->| 7 |--+--| 10 |--+ | | | | | | | +-----+ | +-----+ | +-----+ | | | | | | | | | | | | | | +-----+ +-----+ | +-----+ | +-----+ +-----+ |NODE | |NODE | Tier-3 +->|NODE |--+ Tier-3 |NODE | |NODE | | 1 | | 2 | | 8 | | 11 | | 12 | +-----+ +-----+ +-----+ +-----+ +-----+ | | | | | | | | A O B O <- Servers -> Z O O O Figure 1: 5-stage Clos topology In the reference topology illustrated in Figure 1, we assume: o Each node is its own AS (Node X has AS X) * 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. Filsfils, et al. Expires October 15, 2016 [Page 4] Internet-Draft BGP-Prefix SID in large-scale DCs April 2016 * For efficient usage of the scarce 2-byte Private Use AS pool, different Tier-3 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 BGP session * If not specified, eBGP is assumed. In a specific use-case, iBGP will be used but this will be called out explicitly in that case. o Each node originates the IPv4 address of it's loopback interface into BGP and announces it to its neighbors. * The loopback of Node X is 192.0.2.x/32. In this document, we also refer to the Tier-1, Tier-2 and Tier-3 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. 3. Some open problems in large data-center networks The data-center network design summarized above provides means for moving traffic between hosts with reasonable efficiency. There are few open performance and reliability problems that arise in such design: o ECMP routing is most commonly realized per-flow. This means that large, long-lived "elephant" flows may affect performance of smaller, short-lived "mouse" flows and reduce efficiency of per- flow load-sharing. In other words, per-flow ECMP that does not perform efficiently when flow life-time distribution is heavy- tailed. Furthermore, due to hash-function inefficiencies it is possible to have frequent flow collisions, where more flows get placed on one path over the others o Shortest-path routing with ECMP implements oblivious routing model, which is not aware of the network imbalances. If the network symmetry is broken, for example due to link failures, utilization hotspots may appear. For example, if a link fails between Tier-1 and Tier-2 devices (e.g. "Node5" and "Node9"), Tier-3 devices "Node1" and "Node2" will not be aware of that, since there are other paths available from perspective of "Node3". They will continue sending roughly equal traffic to Node3 and Node4 as if the failure didn't exist which may cause a traffic hotspot. Filsfils, et al. Expires October 15, 2016 [Page 5] Internet-Draft BGP-Prefix SID in large-scale DCs April 2016 o Absence of path visibility leaves transport protocols, such as TCP, with a "blackbox" view of the network. Some TCP metrics, such as SRTT, MSS, CWND and few others could be inferred and cached based on past history, but those apply to destinations, regardless of the path that has been chosen to get there. Thus, for instance, TCP is not capable of remembering "bad" paths, such as those that exhibited poor performance in the past. This means that every new connection will be established obliviously (memory- less) with regards to the paths chosen before, or chosen by other nodes. o Isolating faults in the network with multiple parallel paths and ECMP-based routing is non-trivial due to lack of determinism. Specifically, the connections from HostA to HostB may take a different path every time a new connection is formed, thus making consistent reproduction of a failure much more difficult. This complexity scales linearly with the number of parallel paths in the network, and stems from the random nature of path selection by the network devices. Further in this document, we are going to demonstrate how these problems could be addressed within the framework of Segment Routing. First, we will explain how to apply SR in the DC, for MPLS and IPv6 data-planes. 4. Applying Segment Routing in the DC with MPLS dataplane 4.1. 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.ietf-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 recommendation for operational simplification as explained in [I-D.ietf-spring-segment-routing]. 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. This also may increase the complexity of the centralized controller. Filsfils, et al. Expires October 15, 2016 [Page 6] Internet-Draft BGP-Prefix SID in large-scale DCs April 2016 For illustration purpose, when considering an MPLS data-plane, we assume that the segment index allocated to prefix 192.0.2.x/32 is X. As a result, a local label 1600x is allocated for prefix 192.0.2.x/32 by each node throughout the DC fabric. When IPv6 data-plane is considered, we assume that Node X is allocated IPv6 address (segment) 2001:DB8::X. 4.2. eBGP Labeled Unicast (RFC3107) Referring to Figure 1 and [[I-D.ietf-rtgwg-bgp-routing-large-dc], the following design modifications are introduced: o Each node peers with its neighbors via eBGP3107 session 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). Figure 2 zooms on a path from server A to server Z within the topology of Figure 1. +-----+ +-----+ +-----+ +---------->|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 Referring to Figure 1 and Figure 2 and assuming the IP address, AS and index allocation previously described, the following sections detail the control plane operation and the data plane states for the prefix 192.0.2.11/32 (loopback of Node11) 4.2.1. Control Plane Node11 originates 192.0.2.11/32 in BGP and allocates to it the BGP- Prefix Segment attribute (index11). Filsfils, et al. Expires October 15, 2016 [Page 7] Internet-Draft BGP-Prefix SID in large-scale DCs April 2016 Node11 sends the following eBGP3107 update to Node10: . NLRI: 192.0.2.11/32 . Label: Implicit-Null . Next-hop: Node11's interface address on the link to Node10 . AS Path: {11} . BGP-Prefix Attribute: Index 11 Node10 receives the above update. As it is SR capable, Node10 is able to interpret the BGP-Prefix Attribute and hence understands that it should allocate the label LOCAL-SRGB (16000) + "index" 11 (hence 16011) to the NLRI instead of allocating an nondeterministic label out of a dynamically allocated portion of the local label space. The implicit-null label in the NLRI tells Node10 that it is the penultimate hop and MUST pop the top label on the stack before forwarding traffic for this prefix to Node11. Then, Node10 sends the following eBGP3107 update to Node7: . NLRI: 192.0.2.11/32 . Label: 16011 . Next-hop: Node10's interface address on the link to Node7 . AS Path: {10, 11} . BGP-Prefix Attribute: Index 11 Node7 receives the above update. As it is SR capable, Node7 is able to interpret the BGP-Prefix Attribute and hence allocates the local (incoming) label 16011 (16000 + 11) to the NLRI (instead of allocating a "dynamic" local label from its label manager). Node7 uses the label in the received eBGP3107 NLRI as the outgoing label (the index is only used to derive the local/incoming label). Node7 sends the following eBGP3107 update to Node4: . NLRI: 192.0.2.11/32 . Label: 16011 . Next-hop: Node7's interface address on the link to Node4 . AS Path: {7, 10, 11} . BGP-Prefix Attribute: Index 11 Node4 receives the above update. As it is SR capable, Node4 is able to interpret the BGP-Prefix Attribute and hence allocates the local (incoming) label 16011 to the NLRI (instead of allocating a "dynamic" local label from its label manager). Node4 uses the label in the received eBGP3107 NLRI as outgoing label (the index is only used to derive the local/incoming label). Node4 sends the following eBGP3107 update to Node1: Filsfils, et al. Expires October 15, 2016 [Page 8] Internet-Draft BGP-Prefix SID in large-scale DCs April 2016 . NLRI: 192.0.2.11/32 . Label: 16011 . Next-hop: Node4's interface address on the link to Node1 . AS Path: {4, 7, 10, 11} . BGP-Prefix Attribute: Index 11 Node1 receives the above update. As it is SR capable, Node1 is able to interpret the BGP-Prefix Attribute and hence allocates the local (incoming) label 16011 to the NLRI (instead of allocating a "dynamic" local label from its label manager). Node1 uses the label in the received eBGP3107 NLRI as outgoing label (the index is only used to derive the local/incoming label). 4.2.2. Data Plane Referring to Figure 1Referring to Figure 1, and assuming all nodes apply the same advertisement rules described above and all nodes have the same SRGB (16000-23999), here are the IP/MPLS forwarding tables for prefix 192.0.2.11/32 at Nodes 1, 4, 7 and 10. ----------------------------------------------- Incoming label | outgoing label | Outgoing or IP destination | | Interface ------------------+----------------+----------- 16011 | 16011 | ECMP{3, 4} 192.0.2.11/32 | 16011 | ECMP{3, 4} ------------------+----------------+----------- Figure 3: Node1 Forwarding Table ----------------------------------------------- Incoming label | outgoing label | Outgoing or IP destination | | Interface ------------------+----------------+----------- 16011 | 16011 | ECMP{7, 8} 192.0.2.11/32 | 16011 | ECMP{7, 8} ------------------+----------------+----------- Figure 4: Node4 Forwarding Table Filsfils, et al. Expires October 15, 2016 [Page 9] Internet-Draft BGP-Prefix SID in large-scale DCs April 2016 ----------------------------------------------- Incoming label | outgoing label | Outgoing or IP destination | | Interface ------------------+----------------+----------- 16011 | 16011 | 10 192.0.2.11/32 | 16011 | 10 ------------------+----------------+----------- Figure 5: Node7 Forwarding Table ----------------------------------------------- Incoming label | outgoing label | Outgoing or IP destination | | Interface ------------------+----------------+----------- 16011 | POP | 11 192.0.2.11/32 | N/A | 11 ------------------+----------------+----------- Node10 Forwarding Table 4.2.3. Network Design Variation A network design choice could consist of switching all the traffic through Tier-1 and Tier-2 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.ietf-spring-segment-routing-central-epe]). For example, applications at HostA would send their Z-destined traffic to Node1 with an MPLS label stack where the top label is 16011 and the next label is an EPE peer segment at Node11 directing the traffic to Z. 4.2.4. Global BGP Prefix Segment through the fabric When the previous design is deployed, the operator enjoys global BGP prefix segment (label) allocation throughout the DC fabric. A few examples follow: o Normal forwarding to Node11: a packet with top label 16011 received by any switch in the fabric will be forwarded along the ECMP-aware BGP best-path towards Node11 and the label 16011 is penultimate-popped at Node10. o Traffic-engineered path to Node11: an application on a host behind Node1 might want to restrict its traffic to paths via the Spine Filsfils, et al. Expires October 15, 2016 [Page 10] Internet-Draft BGP-Prefix SID in large-scale DCs April 2016 switch Node5. The application achieves this by sending its packets with a label stack of {16005, 16011}. BGP Prefix segment 16005 directs the packet up to Node5 along the path (Node1, Node3, Node5). BGP Prefix Segment 16011 then directs the packet down to Node11 along the path (Node5, Node9, Node11). 4.2.5. Incremental Deployments The design previously described can be deployed incrementally. Let us assume that Node7 does not support the BGP-Prefix Segment attribute and let us show how the fabric connectivity is preserved. From a signaling viewpoint, nothing would change: if Node7 does not understand the BGP-Prefix Segment attribute, it does propagate the attribute unmodified to its neighbors. From a label allocation viewpoint, the only difference is that Node7 would allocate a dynamic (random) label to the prefix 192.0.2.11/32 (e.g. 123456) instead of the "hinted" label as instructed by the BGP Prefix Segment attribute. The neighbors of Node7 adapt automatically as they always use the label in the BGP3107 NLRI as outgoing label. Node4 does understand the BGP-Prefix Segment attribute and hence allocates the indexed label in the SRGB (16011) for 192.0.2.11/32. As a result, all the data-plane entries across the network would be unchanged except the entries at Node7 and its neighbor Node4 as shown in the figures below. The key point is that the end-to-end LSP is preserved because the outgoing label is always derived from the received label within the BGP3107 NLRI. The index in the BGP Prefix SID is only used as a hint on how to allocate the local label (the incoming label) but never for the outgoing label. ------------------------------------------ Incoming label | outgoing | Outgoing or IP destination | label | Interface -------------------+---------------------- 12345 | 16011 | 10 Figure 7: Node7 Forwarding Table Filsfils, et al. Expires October 15, 2016 [Page 11] Internet-Draft BGP-Prefix SID in large-scale DCs April 2016 ------------------------------------------ Incoming label | outgoing | Outgoing or IP destination | label | Interface -------------------+---------------------- 16011 | 12345 | 7 Figure 8: Node4 Forwarding Table The BGP-Prefix Segment functionality can thus be deployed incrementally one node at a time. When deployed together with a homogeneous SRGB (same SRGB across the fabric), the operator incrementally enjoys the global prefix segment benefits as the deployment progresses through the fabric. 4.3. iBGP Labeled Unicast (RFC3107) The same exact design as eBGP3107 is used with the following modifications: All switches share the same AS iBGP3107 reflection with nhop-self is used instead of eBGP3107 For simple and efficient route propagation filtering, Nodes 5, 6, 7 and 8 share the same Cluster ID, Nodes 3 and 4 share the same Cluster ID, Nodes 9 and 10 share the same Cluster ID. AIGP metric ([RFC7311]) is likely applied to the BGP prefix segments as part of a large-scale multi-domain design such as Seamless MPLS [I-D.ietf-mpls-seamless-mpls]. The control-plane behavior is mostly the same as described in the previous section: the only difference is that the eBGP3107 path propagation is simply replaced by an iBGP3107 path reflection with next-hop changed to self. The data-plane tables are exactly the same. 5. Applying Segment Routing in the DC with IPv6 dataplane The design described in I-D.ietf-rtgwg-bgp-routing-large-dc [I-D.ietf-rtgwg-bgp-routing-large-dc] is reused with one single modification. We highlight it using the example of the reachability to Node11 via spine switch Node5. Spine5 originates 2001:DB8::5/128 with the attached BGP Prefix Attribute adverting the support of the Segment Routing extension Filsfils, et al. Expires October 15, 2016 [Page 12] Internet-Draft BGP-Prefix SID in large-scale DCs April 2016 header (SRH, [I-D.ietf-6man-segment-routing-header]) for IPv6 packets destined to segment 2001:DB8::5. Tor11 originates 2001:DB8::11/128 with the attached BGP Prefix Attribute adverting the support of the Segment Routing extension header (SRH, [I-D.ietf-6man-segment-routing-header]) for IPv6 packets destined to segment 2001:DB8::11. The control-plane and data-plane processing of all the other nodes in the fabric is unchanged. Specifically, the routes to 2001:DB8::5 and 2001:DB8::11 are installed in the FIB along the eBGP best-path to Node5 (spine node) and Node11 (ToR node) respectively. An application on HostA which needs to send traffic to HostZ via only Node5 (spine node) can do so by sending IPv6 packets with a SR extension header. The destination address and active segment is set to 2001:DB8::5. The next and last segment is set to 2001:DB8::11. The application must only use IPv6 addresses that have been advertised as capable for SRv6 segment processing (e.g. for which the BGP prefix segment capability has been advertised). How applications learn this (e.g.: centralized controller and orchestration) is outside the scope of this document. 6. Communicating path information to the host There are two general methods for communicating path information to the end-hosts: "proactive" and "reactive", aka "push" and "pull" models. There are multiple ways to implement either of these methods. Here, we note that one way could be using a centralized controller: the controller either tells the hosts of the prefix-to- path mappings beforehand and updates them as needed (network event driven push), or responds to the hosts making request for a path to specific destination (host event driven pull). It is also possible to use a hybrid model, i.e., pushing some state from the controller in response to particular network events, while the host pulls other state on demand. We note, that when disseminating network-related data to the end- hosts a trade-off is made to balance the amount of information vs the level of visibility in the network state. This applies both to push and pull models. In the extreme case, the host would request path information on every flow, and keep no local state at all. On the other end of the spectrum, information for every prefix in the network along with available paths could be pushed and continuously updated on all hosts. Filsfils, et al. Expires October 15, 2016 [Page 13] Internet-Draft BGP-Prefix SID in large-scale DCs April 2016 7. Addressing the open problems This section demonstrates how the problems describe above could be solved using the segment routing concept. It is worth noting that segment routing signaling and data-plane are only parts of the solution. Additional enhancements, e.g. such as the centralized controller mentioned previously, and host networking stack support are required to implement the proposed solutions. 7.1. Per-packet and flowlet switching With the ability to choose paths on the host, one may go from per- flow load-sharing in the network to per-packet or per-flowlet (see [KANDULA04] for information on flowlets). The host may select different segment routing instructions either per packet, or per flowlet, and route them over different paths. This allows for solving the "elephant flow" problem in the data-center and avoiding link imbalances. Note that traditional ECMP routing could be easily simulated with on- host path selection, using method proposed in VL2 (see [GREENBERG09]). The hosts would randomly pick a Tier-2 or Tier-1 device to "bounce" the packet off of, depending on whether the destination is under the same Tier-2 switches, or has to be reached across Tier-1. The host would use a hash function that operates on per-flow invariants, to simulate per-flow load-sharing in the network. Using Figure 1 as reference, let's illustrate this assuming that HostA has an elephant flow to Z called Flow-f. Normally, a flow is hashed on to a single path. Let's assume HostA sends its packets associated with Flow-f with top label 16011 (the label for the remote ToR, Node11, where HostZ is connected) and Node1 would hash all the packets of Flow-F via the same nhop (e.g. Node3). Similarly, let's assume that leaf Node3 would hash all the packets of Flow-F via the same next-hop (e.g.: spine switch Node1). This normal operation would restrict the elephant flow on a small subset of the ECMP paths to HostZ and potentially create imbalance and congestion in the fabric. Leveraging the flowlet proposal, assuming A is made aware of 4 disjoint paths via intermediate segment 16005, 16006, 16007 and 16008 (the BGP prefix SID's of the 4 spine switches) and also made aware of the prefix segment of the remote ToR connected to the destination (16011), then the application can break the elephant flow F into flowlets F1, F2, F3, F4 and associate each flowlet with one of the following 4 label stacks: {16005, 16011}, {16006, 16011}, {16007, Filsfils, et al. Expires October 15, 2016 [Page 14] Internet-Draft BGP-Prefix SID in large-scale DCs April 2016 16011} and {16008, 16011}. This would spread the load of the elephant flow through all the ECMP paths available in the fabric and rebalance the load. 7.2. Performance-aware routing Knowing the path associated with flows/packets, the end host may deduce certain characteristics of the path on its own, and additionally use the information supplied with path information pushed from the controller or received via pull request. The host may further share its path observations with the centralized agent, so that the latter may keep up-to-date network health map to assist other hosts with this information. For example, an application A.1 at HostA may pin a TCP flow destined to HostZ via Spine switch Node5 using label stack {16005, 16011}. The application A.1 may collect information on packet loss, deduced from TCP retransmissions and other signals (e.g. RTT increases). A.1 may additionally publish this information to a centralized agent, e.g. after a flow completes, or periodically for longer lived flows. Next, using both local and/or global performance data, application A.1 as well as other applications sharing the same resources in the DC fabric may pick up the best path for the new flow, or update an existing path (e.g.: when informed of congestion on an existing path). One particularly interesting instance of performance-aware routing is dynamic fault-avoidance. If some links or devices in the network start discarding packets due to a fault, the end-hosts could detect the path(s) being affected and steer their flows away from the problem spot. Similar logic applies to failure cases where packets get completely black-holed, e.g. when a link goes down. For example, an application A.1 informed about 5 paths to Z {16005, 16011}, {16006, 16011}, {16007, 16011}, {16008, 16011} and {16011} might use the latter one by default (for simplicity). When performance is degrading, A.1 might then start to pin TCP flows to each of the 4 other paths (each via a distinct spine) and monitor the performance. It would then detect the faulty path and assign a negative preference to the faulty path to avoid further flows using it. Gradually, over time, it may re-assign flows on the faulty path to eventually detect the resolution of the trouble and start reusing the path. Filsfils, et al. Expires October 15, 2016 [Page 15] Internet-Draft BGP-Prefix SID in large-scale DCs April 2016 7.3. Non-oblivious routing By leveraging Segment Routing, one avoids issues associated with oblivious ECMP hashing. For example, if in the topology depicted on Figure 1 a link between spine switch Node5 and leaf node Node9 fails, HostA may exclude the segment corresponding to Node5 from the prefix matching the servers under Tier-2 devices Node9. In the push path discovery model, the affected path mappings may be explicitly pushed to all the servers for the duration of the failure. The new mapping would instruct them to avoid the particular Tier-1 switch until the link has recovered. Alternatively, in pull path, the centralized controller may start steering new flows immediately after it discovers the issue. Until then, the existing flows may recover using local detection of the path issues, as described in Section 7.2. 7.4. Deterministic network probing Active probing is a well-known technique for monitoring network elements health, constituting of sending continuous packet streams simulating network traffic to the hosts in the data-center. Segment routing makes possible to prescribe the exact paths that each probe or series of probes would be taking toward their destination. This allows for fast correlation and detection of failed paths, by processing information from multiple actively probing agents. This complements the data collected from the hosts routing stacks as described inSection 7.2. For example, imagine a probe agent sending packets to all machines in the data-center. For every host, it may send packets over each of the possible paths, knowing exactly which links and devices these packets will be crossing. Correlating results for multiple destinations with the topological data, it may automatically isolate possible problem to a link or device in the network. 8. Additional Benefits 8.1. 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 BGP Labelled Unicast (RFC3107 [RFC3107]). It applies either to eBGP or iBGP based designs. Specifically, LDP and RSVP-TE are not used. These protocols would drastically impact the operational complexity of the Data Center and would not scale. This is in line with the requirements expressed in [I-D.ietf-rtgwg-bgp-routing-large-dc] Filsfils, et al. Expires October 15, 2016 [Page 16] Internet-Draft BGP-Prefix SID in large-scale DCs April 2016 A key element of the operational simplicity is the deployment of the design with a single and consistent SRGB across the DC fabric. At every node in the fabric, the same label is associated to a given BGP prefix segment and hence a notion of global prefix segment arises. When a controller programs HostA to send traffic to HostZ via the normally available BGP ECMP paths, the controller uses label 16011 associated with the ToR switch connected to the HostZ. The controller does not need to pick the label based on the ToR that the source host is connected to. In a classic BGP Labelled Unicast design applied to the DC fabric illustrated in Figure 1, the ToR Node1 connected to HostA would most likely allocate a different label for 192.0.2.11/32 than the one allocated by ToR Node2. 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 of this unnecessary complexity is eliminated if a single consistent SRGB is utilized across the fabric. 8.2. Minimizing the FIB table The designer may decide to switch all the traffic at Tier-1 and Tier- 2's based on MPLS, hence drastically decreasing the IP table size at these nodes. This is easily accomplished by encapsulating the traffic either 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. 8.3. Egress Peer Engineering It is straightforward to combine the design illustrated in this document with the Egress Peer Engineering (EPE) use-case described in [I-D.ietf-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 HostA to force it to send its traffic destined to a specific Internet destination D via a selected border switch (say Node12 in Figure 1 instead of another border switch Node11) and a specific egress peer of Node12 (say peer AS 9999 of local PeerNode segment Filsfils, et al. Expires October 15, 2016 [Page 17] Internet-Draft BGP-Prefix SID in large-scale DCs April 2016 9999 at Node12 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 (label stack) {16012, 9999}. 16012 would steer the flow through the DC fabric, leveraging any ECMP, along the best path to border switch Node12. Once the flow gets to border switch Node12, the active segment is 9999 (thanks to PHP on the upstream neighbor of Node12). This EPE PeerNode segment forces border switch Node12 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. A benefit of segment routing is the per-flow state is only required at the source. As well as allowing full traffic engineering control such a design also offers FIB table minimization benefits as the Internet- scale FIB at border switch Node12 is not required if all FIB lookups are avoided there by using EPE. 8.4. Incremental Deployments As explained in Section 4.2.5, this design can be deployed incrementally. 8.5. Anycast The design presented in this document preserves the availability and load-balancing properties of the base design presented in [I-D.ietf-spring-segment-routing]. For example, one could assign an anycast loopback 192.0.2.20/32 and associate segment index 20 to it on the border switches 11 and 12 (in addition to 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 or from any ToR switch, 16020 steers the packet towards the border switches 11 or 12 leveraging ECMP where available along the best paths to these switches. 9. Preferred SRGB Allocation In the MPLS case, we do not recommend to use different SRGBs at each node. Different SRGBs in each node likely increase the complexity of the solution both from an operation viewpoint and from a controller viewpoint. From an operation viewpoint, it is much simpler to have the same global label at every node for the same destination (the MPLS Filsfils, et al. Expires October 15, 2016 [Page 18] Internet-Draft BGP-Prefix SID in large-scale DCs April 2016 troubleshooting is then similar to the IPv6 troubleshooting where this global property is a given). From a controller viewpoint, this allows to construct simple policies applicable across the fabric. Let us consider two applications A and B respectively connected to ToR1 and ToR2. A has two flows FA1 and FA2 destined to Z. B has two flows FB1 and FB2 destined to Z. The controller wants FA1 and FB1 to be load-shared across the fabric while FA2 and FB2 must be respectively steered via Spine5 and spine 8. Assuming a consistent unique SRGB across the fabric as described in the document, the controller can simply do it by instructing A and B to use {16011} respectively for FA1 and FB1 and by instructing A and B to use {16005 16011} and {16008 16011} respectively for FA2 and FB2. Let us assume a design where the SRGB is different at every node: SRGB of Node K starts at value K*1000 and the SRGB length is 1000 (e.g. ToR1's SRGB is [1000, 1999], ToR2's SRGB is [2000, 2999]...). In this case, not only the controller would need to collect and store all of these different SRGB's, furthermore it would need to adapt the policy for each host. Indeed, the controller would instruct A to use {1011} for FA1 while it would have to instruct B to use {2011} for FB1 (while with the same SRGB, both policies are the same {16011}). Even worse, the controller would instruct A to use {1005, 5011} for FA1 while it would instruct B to use {2011, 8011} for FB1 (while with the same SRGB, the second segment is the same across both policies: 16011). When combining segments to create a policy, one need to carefully update the label of each segment. This is obviously more error-prone, more complex and more difficult to troubleshoot. 10. Alternative Options In order to support all the requirements and get consensus, the BGP Prefix SID attribute has been extended to allow this design. Specifically, the ORIGINATOR_SRGB TLV in the BGP Prefix SID signals the SRGB of the switch that originated the BGP Prefix Segment. This allows to determine the local label allocated by any switch for any BGP Prefix Segment, despite the lack of a consistent unique SRGB in the domain. Filsfils, et al. Expires October 15, 2016 [Page 19] Internet-Draft BGP-Prefix SID in large-scale DCs April 2016 11. IANA Considerations TBD 12. Manageability Considerations TBD 13. Security Considerations TBD 14. Acknowledgements The authors would like to thank Benjamin Black, Arjun Sreekantiah, Keyur Patel and Acee Lindem for their comments and review of this document. 15. Contributors Gaya Nagarajan Facebook US Email: gaya@fb.com Dmitry Afanasiev Yandex RU Email: fl0w@yandex-team.ru Tim Laberge Cisco US Email: tlaberge@cisco.com Edet Nkposong Microsoft US Email: edetn@microsoft.com Filsfils, et al. Expires October 15, 2016 [Page 20] Internet-Draft BGP-Prefix SID in large-scale DCs April 2016 Mohan Nanduri Microsoft US Email: mnanduri@microsoft.com James Uttaro ATT US Email: ju1738@att.com Saikat Ray Unaffiliated US Email: raysaikat@gmail.com 16. References 16.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC3107] Rekhter, Y. and E. Rosen, "Carrying Label Information in BGP-4", RFC 3107, DOI 10.17487/RFC3107, May 2001, . [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, DOI 10.17487/RFC4271, January 2006, . [RFC7311] Mohapatra, P., Fernando, R., Rosen, E., and J. Uttaro, "The Accumulated IGP Metric Attribute for BGP", RFC 7311, DOI 10.17487/RFC7311, August 2014, . 16.2. Informative References [GREENBERG09] Greenberg, A., Hamilton, J., Jain, N., Kadula, S., Kim, C., Lahiri, P., Maltz, D., Patel, P., and S. Sengupta, "VL2: A Scalable and Flexible Data Center Network", 2009. Filsfils, et al. Expires October 15, 2016 [Page 21] Internet-Draft BGP-Prefix SID in large-scale DCs April 2016 [I-D.ietf-6man-segment-routing-header] Previdi, S., Filsfils, C., Field, B., Leung, I., Linkova, J., Kosugi, T., Vyncke, E., and D. Lebrun, "IPv6 Segment Routing Header (SRH)", draft-ietf-6man-segment-routing- header-01 (work in progress), March 2016. [I-D.ietf-idr-bgp-prefix-sid] Previdi, S., Filsfils, C., Lindem, A., Patel, K., Sreekantiah, A., Ray, S., and H. Gredler, "Segment Routing Prefix SID extensions for BGP", draft-ietf-idr-bgp-prefix- sid-02 (work in progress), December 2015. [I-D.ietf-mpls-seamless-mpls] Leymann, N., Decraene, B., Filsfils, C., Konstantynowicz, M., and D. Steinberg, "Seamless MPLS Architecture", draft- ietf-mpls-seamless-mpls-07 (work in progress), June 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-09 (work in progress), March 2016. [I-D.ietf-spring-segment-routing] Filsfils, C., Previdi, S., Decraene, B., Litkowski, S., and R. Shakir, "Segment Routing Architecture", draft-ietf- spring-segment-routing-07 (work in progress), December 2015. [I-D.ietf-spring-segment-routing-central-epe] Filsfils, C., Previdi, S., Ginsburg, D., and D. Afanasiev, "Segment Routing Centralized BGP Peer Engineering", draft- ietf-spring-segment-routing-central-epe-01 (work in progress), March 2016. [KANDULA04] Sinha, S., Kandula, S., and D. Katabi, "Harnessing TCP's Burstiness with Flowlet Switching", 2004. Authors' Addresses Clarence Filsfils (editor) Cisco Systems, Inc. Brussels BE Email: cfilsfil@cisco.com Filsfils, et al. Expires October 15, 2016 [Page 22] Internet-Draft BGP-Prefix SID in large-scale DCs April 2016 Stefano Previdi (editor) Cisco Systems, Inc. Via Del Serafico, 200 Rome 00142 Italy Email: sprevidi@cisco.com Jon Mitchell Unaffiliated Email: jrmitche@puck.nether.net Ebben Aries Facebook US Email: exa@fb.com Petr Lapukhov Facebook US Email: petr@fb.com Filsfils, et al. Expires October 15, 2016 [Page 23]