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Network Working Group example This document describes the motivation for, and benefits of, applying Segment Routing (SR) in BGP-based large-scale data centers. It describes the design to deploy SR in those data centers for both the MPLS and IPv6 data planes.

Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are candidates for any level of Internet Standard; see Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at .

Copyright Notice Copyright (c) 2019 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 () 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

• .  Introduction
• .  Large-Scale Data-Center Network Design Summary
  • .  Reference Design
• .  Some Open Problems in Large Data-Center Networks
• .  Applying Segment Routing in the DC with MPLS Data Plane
  • .  BGP Prefix Segment (BGP Prefix-SID)
  • .  EBGP Labeled Unicast (RFC 8277)
    • .  Control Plane
    • .  Data Plane
    • .  Network Design Variation
    • .  Global BGP Prefix Segment through the Fabric
    • .  Incremental Deployments
  • .  IBGP Labeled Unicast (RFC 8277)
• .  Applying Segment Routing in the DC with IPv6 Data Plane
• .  Communicating Path Information to the Host
• .  Additional Benefits
  • .  MPLS Data Plane with Operational Simplicity
  • .  Minimizing the FIB Table
  • .  Egress Peer Engineering
  • .  Anycast
• .  Preferred SRGB Allocation
• .  IANA Considerations
• . Manageability Considerations
• . Security Considerations
• . References
  • .  Normative References
  • .  Informative References
• Acknowledgements
• Contributors
• Authors' Addresses

Introduction Segment Routing (SR), as described in , 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 a global semantic within an SR domain. SR allows the enforcement of a flow through any topological path while maintaining per-flow state only from the ingress node to the SR domain. SR can be applied to the MPLS and IPv6 data planes. The use cases described in this document should be considered in the context of the BGP-based large-scale data-center (DC) design described in . This document extends it by applying SR both with IPv6 and MPLS data planes.

Large-Scale Data-Center Network Design Summary This section provides a brief summary of the Informational RFC , which outlines a practical network design suitable for data centers of various scales:

• 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 (as described in ). In a Clos topology, the minimum number of parallel paths between two elements is determined by the "width" of the "Tier-1" stage. See for an illustration of the concept.
• Large-scale data centers commonly use a routing protocol, such as BGP-4 , in order to provide endpoint connectivity. Therefore, recovery after a network failure is driven either by local knowledge of directly available backup paths or by distributed signaling between the network devices.
• Within data-center networks, traffic is load shared using the Equal Cost Multipath (ECMP) mechanism. With ECMP, every network device implements a pseudorandom decision, mapping packets to one of the parallel paths by means of a hash function calculated over 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 "Tier-1" stage. Notice that the number of paths between Node1 and Node12 equals four; the paths have to cross all of the 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, only the topologies that have a single path from Tier-1 to Tier-3 are considered below. The rest could be treated similarly, with a few modifications to the logic.

Reference Design

5-Stage Clos Topology 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

In the reference topology illustrated in , it is assumed:

• Each node is its own autonomous system (AS) (Node X has AS X). 4-byte AS numbers are recommended ().
  • For simple and efficient route propagation filtering, Node5, Node6, Node7, and Node8 use the same AS; Node3 and Node4 use the same AS; and Node9 and Node10 use the same AS.
  • In the case in which 2-byte autonomous system numbers are used for efficient usage of the scarce 2-byte Private Use AS pool, different Tier-3 nodes might use the same AS.
  • Without loss of generality, these details will be simplified in this document. It is to be assumed that each node has its own AS.
• Each node peers with its neighbors with a BGP session. If not specified, external BGP (EBGP) is assumed. In a specific use case, internal BGP (IBGP) will be used, but this will be called out explicitly in that case.
• Each node originates the IPv4 address of its loopback interface into BGP and announces it to its neighbors.
  • The loopback of Node X is 192.0.2.x/32.

In this document, the Tier-1, Tier-2, and Tier-3 nodes are referred to as "Spine", "Leaf", and "ToR" (top of rack) nodes, respectively. When a ToR node acts as a gateway to the "outside world", it is referred to as a "border node".

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 a design:

• 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 may reduce efficiency of per-flow load sharing. In other words, per-flow ECMP does not perform efficiently when flow-lifetime 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.
• Shortest-path routing with ECMP implements an oblivious routing model that 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 the 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.
• Isolating faults in the network with multiple parallel paths and ECMP-based routing is nontrivial 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.

Applying Segment Routing in the DC with MPLS Data Plane

BGP Prefix Segment (BGP Prefix-SID) 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. The BGP Prefix Segment is defined as the BGP Prefix-SID Attribute in , which contains an index. Throughout this document, the BGP Prefix Segment Attribute is referred to as the "BGP Prefix-SID" and the encoded index as the label index. In this document, the network design decision has been made to assume that all the nodes are allocated the same SRGB (Segment Routing Global Block), e.g., [16000, 23999]. This provides operational simplification as explained in , but this is not a requirement. For illustration purposes, when considering an MPLS data plane, it is assumed that the label index allocated to prefix 192.0.2.x/32 is X. As a result, a local label (16000+x) is allocated for prefix 192.0.2.x/32 by each node throughout the DC fabric. When the IPv6 data plane is considered, it is assumed that Node X is allocated IPv6 address (segment) 2001:DB8::X.

EBGP Labeled Unicast (RFC 8277) Referring to and , the following design modifications are introduced:

• Each node peers with its neighbors via an EBGP session with extensions defined in (named "EBGP8277" throughout this document) and with the BGP Prefix-SID attribute extension as defined in .
• The forwarding plane at Tier-2 and Tier-1 is MPLS.
• The forwarding plane at Tier-3 is either IP2MPLS (if the host sends IP traffic) or MPLS2MPLS (if the host sends MPLS-encapsulated traffic).

zooms into a path from ServerA to ServerZ within the topology of .

Path from A to Z via Nodes 1, 4, 7, 10, and 11 +-----+ +-----+ +-----+ +---------->|NODE | |NODE | |NODE | | | 4 |--+->| 7 |--+--| 10 |---+ | +-----+ +-----+ +-----+ | | | +-----+ +-----+ |NODE | |NODE | | 1 | | 11 | +-----+ +-----+ | | A <- Servers -> Z

Referring to Figures and , and assuming the IP address with the AS and label-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).

Control Plane Node11 originates 192.0.2.11/32 in BGP and allocates to it a BGP Prefix-SID with label-index: index11 . Node11 sends the following EBGP8277 update to Node10:

• IP Prefix:

  192.0.2.11/32

  Label:

  Implicit NULL

  Next hop:

  Node11's interface address on the link to Node10

  AS Path:

  {11}

  BGP Prefix-SID:

  Label-Index 11

Node10 receives the above update. As it is SR capable, Node10 is able to interpret the BGP Prefix-SID; therefore, it understands that it should allocate the label from its own SRGB block, offset by the label index received in the BGP Prefix-SID (16000+11, hence, 16011) to the Network Layer Reachability Information (NLRI) instead of allocating a 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 that it must pop the top label on the stack before forwarding traffic for this prefix to Node11. Then, Node10 sends the following EBGP8277 update to Node7:

• IP Prefix:

  192.0.2.11/32

  Label:

  16011

  Next hop:

  Node10's interface address on the link to Node7

  AS Path:

  {10, 11}

  BGP Prefix-SID:

  Label-Index 11

Node7 receives the above update. As it is SR capable, Node7 is able to interpret the BGP Prefix-SID; therefore, it 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 EBGP8277 NLRI as the outgoing label (the index is only used to derive the local/incoming label). Node7 sends the following EBGP8277 update to Node4:

• IP Prefix:

  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-SID:

  Label-Index 11

Node4 receives the above update. As it is SR capable, Node4 is able to interpret the BGP Prefix-SID; therefore, it 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 EBGP8277 NLRI as an outgoing label (the index is only used to derive the local/incoming label). Node4 sends the following EBGP8277 update to Node1:

• IP Prefix:

  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-SID:

  Label-Index 11

Node1 receives the above update. As it is SR capable, Node1 is able to interpret the BGP Prefix-SID; therefore, it 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 EBGP8277 NLRI as an outgoing label (the index is only used to derive the local/incoming label).

Data Plane Referring to , 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 Node1, Node4, Node7, and Node10. Node1 Forwarding Table

Incoming Label or IP Destination	Outgoing Label	Outgoing Interface
16011	16011	ECMP{3, 4}
192.0.2.11/32	16011	ECMP{3, 4}

Node4 Forwarding Table

Incoming Label or IP Destination	Outgoing Label	Outgoing Interface
16011	16011	ECMP{7, 8}
192.0.2.11/32	16011	ECMP{7, 8}

Node7 Forwarding Table

Incoming Label or IP Destination	Outgoing Label	Outgoing Interface
16011	16011	10
192.0.2.11/32	16011	10

Node10 Forwarding Table

Incoming Label or IP Destination	Outgoing Label	Outgoing Interface
16011	POP	11
192.0.2.11/32	N/A	11

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 Node4, Node7, and Node10. This might be beneficial in order to optimize the forwarding table size. A network design choice could consist of allowing the hosts to send MPLS-encapsulated traffic based on the Egress Peer Engineering (EPE) use case as defined in . 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.

Global BGP Prefix Segment through the Fabric When the previous design is deployed, the operator enjoys global BGP Prefix-SID and label allocation throughout the DC fabric. A few examples follow:

• Normal forwarding to Node11: A packet with top label 16011 received by any node in the fabric will be forwarded along the ECMP-aware BGP best path towards Node11, and the label 16011 is penultimate popped at Node10 (or at Node 9).
• Traffic-engineered path to Node11: An application on a host behind Node1 might want to restrict its traffic to paths via the Spine node Node5. The application achieves this by sending its packets with a label stack of {16005, 16011}. BGP Prefix-SID 16005 directs the packet up to Node5 along the path (Node1, Node3, Node5). BGP Prefix-SID 16011 then directs the packet down to Node11 along the path (Node5, Node9, Node11).

Incremental Deployments The design previously described can be deployed incrementally. Let us assume that Node7 does not support the BGP Prefix-SID, and let us show how the fabric connectivity is preserved. From a signaling viewpoint, nothing would change; even though Node7 does not support the BGP Prefix-SID, 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-SID. The neighbors of Node7 adapt automatically as they always use the label in the BGP8277 NLRI as an outgoing label. Node4 does understand the BGP Prefix-SID; therefore, it 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 Label Switched Path (LSP) is preserved because the outgoing label is always derived from the received label within the BGP8277 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. Node7 Forwarding Table

Incoming Label or IP Destination	Outgoing Label	Outgoing Interface
12345	16011	10

Node4 Forwarding Table

Incoming Label or IP Destination	Outgoing Label	Outgoing Interface
16011	12345	7

The BGP Prefix-SID can thus be deployed incrementally, i.e., one node at a time. When deployed together with a homogeneous SRGB (the same SRGB across the fabric), the operator incrementally enjoys the global prefix segment benefits as the deployment progresses through the fabric.

IBGP Labeled Unicast (RFC 8277) The same exact design as EBGP8277 is used with the following modifications:

• All nodes use the same AS number.
• Each node peers with its neighbors via an internal BGP session (IBGP) with extensions defined in (named "IBGP8277" throughout this document).
• Each node acts as a route reflector for each of its neighbors and with the next-hop-self option. Next-hop-self is a well-known operational feature that consists of rewriting the next hop of a BGP update prior to sending it to the neighbor. Usually, it's a common practice to apply next-hop-self behavior towards IBGP peers for EBGP-learned routes. In the case outlined in this section, it is proposed to use the next-hop-self mechanism also to IBGP-learned routes.

IBGP Sessions with Reflection and Next-Hop-Self Cluster-1 +-----------+ | Tier-1 | | +-----+ | | |NODE | | | | 5 | | Cluster-2 | +-----+ | Cluster-3 +---------+ | | +---------+ | Tier-2 | | | | Tier-2 | | +-----+ | | +-----+ | | +-----+ | | |NODE | | | |NODE | | | |NODE | | | | 3 | | | | 6 | | | | 9 | | | +-----+ | | +-----+ | | +-----+ | | | | | | | | | | | | | | +-----+ | | +-----+ | | +-----+ | | |NODE | | | |NODE | | | |NODE | | | | 4 | | | | 7 | | | | 10 | | | +-----+ | | +-----+ | | +-----+ | +---------+ | | +---------+ | | | +-----+ | | |NODE | | Tier-3 | | 8 | | Tier-3 +-----+ +-----+ | +-----+ | +-----+ +-----+ |NODE | |NODE | +-----------+ |NODE | |NODE | | 1 | | 2 | | 11 | | 12 | +-----+ +-----+ +-----+ +-----+

• For simple and efficient route propagation filtering and as illustrated in :
  • Node5, Node6, Node7, and Node8 use the same Cluster ID (Cluster-1).
  • Node3 and Node4 use the same Cluster ID (Cluster-2).
  • Node9 and Node10 use the same Cluster ID (Cluster-3).
• The control-plane behavior is mostly the same as described in the previous section; the only difference is that the EBGP8277 path propagation is simply replaced by an IBGP8277 path reflection with next hop changed to self.
• The data-plane tables are exactly the same.

Applying Segment Routing in the DC with IPv6 Data Plane The design described in is reused with one single modification. It is highlighted using the example of the reachability to Node11 via Spine node Node5. Node5 originates 2001:DB8::5/128 with the attached BGP Prefix-SID for IPv6 packets destined to segment 2001:DB8::5 (). Node11 originates 2001:DB8::11/128 with the attached BGP Prefix-SID advertising the support of the Segment Routing Header (SRH) 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 that needs to send traffic to HostZ via only Node5 (Spine node) can do so by sending IPv6 packets with a Segment Routing Header (SRH, ). 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.

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, it is noted 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 requests for a path to a 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. Note also 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 to both 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.

Additional Benefits

MPLS Data Plane with Operational Simplicity As required by , no new signaling protocol is introduced. The BGP Prefix-SID is a lightweight extension to BGP Labeled Unicast . 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 . Provided the same SRGB is configured on all nodes, all nodes use the same MPLS label for a given IP prefix. This is simpler from an operation standpoint, as discussed in .

Minimizing the FIB Table The designer may decide to switch all the traffic at Tier-1 and Tier-2 based on MPLS, thereby 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 node. The encapsulation is done by pushing the BGP Prefix-SID of the destination ToR for intra-DC traffic, or by pushing the BGP Prefix-SID for the border node for inter-DC or DC-to-outside-world traffic.

Egress Peer Engineering It is straightforward to combine the design illustrated in this document with the Egress Peer Engineering (EPE) use case described in . In such a 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 node (say Node12 in instead of another border node, Node11) and a specific egress peer of Node12 (say peer AS 9999 of local PeerNode segment 9999 at Node12 instead of any other peer that provides a path to the destination D). Any packet matching this state at HostA 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 node Node12. Once the flow gets to border node Node12, the active segment is 9999 (because of Penultimate Hop Popping (PHP) on the upstream neighbor of Node12). This EPE PeerNode segment forces border node Node12 to forward the packet to peer AS 9999 without any IP lookup at the border node. There is no per-flow state for this engineered flow in the DC fabric. A benefit of SR is that 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 node Node12 is not required if all FIB lookups are avoided there by using EPE.

Anycast The design presented in this document preserves the availability and load-balancing properties of the base design presented in . For example, one could assign an anycast loopback 192.0.2.20/32 and associate segment index 20 to it on the border nodes Node11 and Node12 (in addition to their node-specific loopbacks). Doing so, the EPE controller could express a default "go-to-the-Internet via any border node" policy as segment list {16020}. Indeed, from any host in the DC fabric or from any ToR node, 16020 steers the packet towards the border nodes Node11 or Node12 leveraging ECMP where available along the best paths to these nodes.

Preferred SRGB Allocation In the MPLS case, it is recommended to use the same SRGBs at each node. Different SRGBs in each node likely increase the complexity of the solution both from an operational viewpoint and from a controller viewpoint. From an operational viewpoint, it is much simpler to have the same global label at every node for the same destination (the MPLS troubleshooting is then similar to the IPv6 troubleshooting where this global property is a given). From a controller viewpoint, this allows us to construct simple policies applicable across the fabric. Let us consider two applications, A and B, respectively connected to Node1 and Node2 (ToR nodes). Application 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 Node5 and Node8. Assuming a consistent unique SRGB across the fabric as described in this 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 and where the SRGB of each node is advertised using the Originator SRGB TLV of the BGP Prefix-SID as defined in : SRGB of Node K starts at value K*1000, and the SRGB length is 1000 (e.g., Node1's SRGB is [1000, 1999], Node2's SRGB is [2000, 2999], ...). In this case, the controller would need to collect and store all of these different SRGBs (e.g., through the Originator SRGB TLV of the BGP Prefix-SID); furthermore, it would also 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 needs to carefully update the label of each segment. This is obviously more error prone, more complex, and more difficult to troubleshoot.

IANA Considerations This document has no IANA actions.

Manageability Considerations The design and deployment guidelines described in this document are based on the network design described in . The deployment model assumed in this document is based on a single domain where the interconnected DCs are part of the same administrative domain (which, of course, is split into different autonomous systems). The operator has full control of the whole domain, and the usual operational and management mechanisms and procedures are used in order to prevent any information related to internal prefixes and topology to be leaked outside the domain. As recommended in , the same SRGB should be allocated in all nodes in order to facilitate the design, deployment, and operations of the domain. When EPE () is used (as explained in ), the same operational model is assumed. EPE information is originated and propagated throughout the domain towards an internal server, and unless explicitly configured by the operator, no EPE information is leaked outside the domain boundaries.

Security Considerations This document proposes to apply SR to a well-known scalability requirement expressed in using the BGP Prefix-SID as defined in . It has to be noted, as described in , that the design illustrated in and in this document refer to a deployment model where all nodes are under the same administration. In this context, it is assumed that the operator doesn't want to leak outside of the domain any information related to internal prefixes and topology. The internal information includes Prefix-SID and EPE information. In order to prevent such leaking, the standard BGP mechanisms (filters) are applied on the boundary of the domain. Therefore, the solution proposed in this document does not introduce any additional security concerns from what is expressed in and . It is assumed that the security and confidentiality of the prefix and topology information is preserved by outbound filters at each peering point of the domain as described in .

References Normative References This document discusses the Border Gateway Protocol (BGP), which is an inter-Autonomous System routing protocol. The primary function of a BGP speaking system is to exchange network reachability information with other BGP systems. This network reachability information includes information on the list of Autonomous Systems (ASes) that reachability information traverses. This information is sufficient for constructing a graph of AS connectivity for this reachability from which routing loops may be pruned, and, at the AS level, some policy decisions may be enforced. BGP-4 provides a set of mechanisms for supporting Classless Inter-Domain Routing (CIDR). These mechanisms include support for advertising a set of destinations as an IP prefix, and eliminating the concept of network "class" within BGP. BGP-4 also introduces mechanisms that allow aggregation of routes, including aggregation of AS paths. This document obsoletes RFC 1771. [STANDARDS-TRACK] Some network operators build and operate data centers that support over one hundred thousand servers. In this document, such data centers are referred to as "large-scale" to differentiate them from smaller infrastructures. Environments of this scale have a unique set of network requirements with an emphasis on operational simplicity and network stability. This document summarizes operational experience in designing and operating large-scale data centers using BGP as the only routing protocol. The intent is to report on a proven and stable routing design that could be leveraged by others in the industry. This document specifies a set of procedures for using BGP to advertise that a specified router has bound a specified MPLS label (or a specified sequence of MPLS labels organized as a contiguous part of a label stack) to a specified address prefix. This can be done by sending a BGP UPDATE message whose Network Layer Reachability Information field contains both the prefix and the MPLS label(s) and whose Next Hop field identifies the node at which said prefix is bound to said label(s). This document obsoletes RFC 3107. Segment Routing (SR) 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 semantic local to an SR node or global within an SR domain. SR provides a mechanism that allows a flow to be restricted to a specific topological path, while maintaining per-flow state only at the ingress node(s) to the SR domain. SR can be directly applied to the MPLS architecture with no change to the forwarding plane. 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. SR can be applied to the IPv6 architecture, with a new type of routing header. A segment is encoded as an IPv6 address. An ordered list of segments is encoded as an ordered list of IPv6 addresses in the routing header. The active segment is indicated by the Destination Address (DA) of the packet. The next active segment is indicated by a pointer in the new routing header. Informative References Segment Routing can be applied to the IPv6 data plane using a new type of Routing Extension Header called the Segment Routing Header. This document describes the Segment Routing Header and how it is used by Segment Routing capable nodes. Work in Progress The Autonomous System number is encoded as a two-octet entity in the base BGP specification. This document describes extensions to BGP to carry the Autonomous System numbers as four-octet entities. This document obsoletes RFC 4893 and updates RFC 4271. [STANDARDS-TRACK] Segment Routing (SR) leverages source routing. A node steers a packet through a controlled set of instructions, called segments, by prepending the packet with an SR header. A segment can represent any instruction topological or service-based. SR allows to enforce a flow through any topological path while maintaining per-flow state only at the ingress node of the SR domain. The Segment Routing architecture can be directly applied to the MPLS dataplane with no change on the forwarding plane. It requires a minor extension to the existing link-state routing protocols. This document illustrates the application of Segment Routing to solve the BGP Egress Peer Engineering (BGP-EPE) requirement. The SR-based BGP-EPE solution allows a centralized (Software Defined Network, SDN) controller to program any egress peer policy at ingress border routers or at hosts within the domain. Work in Progress

Acknowledgements The authors would like to thank Benjamin Black, Arjun Sreekantiah, Keyur Patel, Acee Lindem, and Anoop Ghanwani for their comments and review of this document.

Contributors Gaya Nagarajan Facebook United States of America Email: gaya@fb.com Gaurav Dawra Cisco Systems United States of America Email: gdawra.ietf@gmail.com Dmitry Afanasiev Yandex Russian Federation Email: fl0w@yandex-team.ru Tim Laberge Cisco United States of America Email: tlaberge@cisco.com Edet Nkposong Salesforce.com Inc. United States of America Email: enkposong@salesforce.com Mohan Nanduri Microsoft United States of America Email: mohan.nanduri@oracle.com James Uttaro ATT United States of America Email: ju1738@att.com Saikat Ray Unaffiliated United States of America Email: raysaikat@gmail.com Jon Mitchell Unaffiliated United States of America Email: jrmitche@puck.nether.net

Authors' Addresses Cisco Systems, Inc.

Brussels Belgium cfilsfil@cisco.com

Cisco Systems, Inc.

Italy stefano@previdi.net

LinkedIn

United States of America gdawra.ietf@gmail.com

Arrcus, Inc.

2077 Gateway Place, Suite #400 San Jose CA 95119 United States of America exa@arrcus.com

Facebook

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