Internet Engineering Task Force (IETF) C. Filsfils, Ed.

Internet Engineering Task Force (IETF) C. Filsfils, Ed.
Request for Comments: 8670 S. Previdi
Category: Informational Cisco Systems, Inc.
ISSN: 2070-1721 G. Dawra
LinkedIn
E. Aries
Arrcus, Inc.
P. Lapukhov
Facebook
December 2019

BGP Prefix Segment in Large-Scale Data Centers

Abstract

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
https://www.rfc-editor.org/info/rfc8670.

Copyright Notice

Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.

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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction
2. Large-Scale Data-Center Network Design Summary
2.1. Reference Design
3. Some Open Problems in Large Data-Center Networks
4. Applying Segment Routing in the DC with MPLS Data Plane
4.1. BGP Prefix Segment (BGP Prefix-SID)
4.2. EBGP Labeled Unicast (RFC 8277)
4.2.1. Control Plane
4.2.2. Data Plane
4.2.3. Network Design Variation
4.2.4. Global BGP Prefix Segment through the Fabric
4.2.5. Incremental Deployments
4.3. IBGP Labeled Unicast (RFC 8277)
5. Applying Segment Routing in the DC with IPv6 Data Plane
6. Communicating Path Information to the Host
7. Additional Benefits
7.1. MPLS Data Plane with Operational Simplicity
7.2. Minimizing the FIB Table
7.3. Egress Peer Engineering
7.4. Anycast
8. Preferred SRGB Allocation
9. IANA Considerations
10. Manageability Considerations
11. Security Considerations
12. References
12.1. Normative References
12.2. Informative References
Acknowledgements
Contributors
Authors' Addresses

1. Introduction

Segment Routing (SR), as described in [RFC8402], 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 [RFC7938]. This document extends it by applying SR both
with IPv6 and MPLS data planes.

2. Large-Scale Data-Center Network Design Summary

This section provides a brief summary of the Informational RFC
[RFC7938], 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 [RFC7938]). In a Clos topology, the minimum number
of parallel paths between two elements is determined by the
"width" of the "Tier-1" stage. See Figure 1 for an illustration
of the concept.

* Large-scale data centers commonly use a routing protocol, such as
BGP-4 [RFC4271], 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.

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, it is assumed:

* Each node is its own autonomous system (AS) (Node X has AS X).
4-byte AS numbers are recommended ([RFC6793]).

- 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".

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 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.

4. Applying Segment Routing in the DC with MPLS Data Plane

4.1. 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
[RFC8669], 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 Section 8, 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.

4.2. EBGP Labeled Unicast (RFC 8277)

Referring to Figure 1 and [RFC7938], the following design
modifications are introduced:

* Each node peers with its neighbors via an EBGP session with
extensions defined in [RFC8277] (named "EBGP8277" throughout this
document) and with the BGP Prefix-SID attribute extension as
defined in [RFC8669].

* 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).

Figure 2 zooms into a path from ServerA to ServerZ 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 Figures 1 and 2, 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).

4.2.1. Control Plane

Node11 originates 192.0.2.11/32 in BGP and allocates to it a BGP
Prefix-SID with label-index: index11 [RFC8669].

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).

4.2.2. Data Plane

Referring 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 Node1, Node4, Node7, and Node10.

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

Table 1: Node1 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} |
+----------------------------------+----------------+------------+

Table 2: Node4 Forwarding Table

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

Table 3: Node7 Forwarding Table

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

Table 4: 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 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 [SR-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 ([SR-CENTRAL-EPE]) 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-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).

4.2.5. 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.

+----------------------------------+----------------+-----------+
| Incoming Label or IP Destination | Outgoing Label | Outgoing |
| | | Interface |
+----------------------------------+----------------+-----------+
| 12345 | 16011 | 10 |
+----------------------------------+----------------+-----------+

Table 5: Node7 Forwarding Table

+----------------------------------+----------------+-----------+
| Incoming Label or IP Destination | Outgoing Label | Outgoing |
| | | Interface |
+----------------------------------+----------------+-----------+
| 16011 | 12345 | 7 |
+----------------------------------+----------------+-----------+

Table 6: Node4 Forwarding Table

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.

4.3. 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 [RFC8277] (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.

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 |
+-----+ +-----+ +-----+ +-----+

Figure 3: IBGP Sessions with Reflection and Next-Hop-Self

* For simple and efficient route propagation filtering and as
illustrated in Figure 3:

- 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.

5. Applying Segment Routing in the DC with IPv6 Data Plane

The design described in [RFC7938] 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 ([RFC8402]).

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, [IPv6-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.

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, 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.

7. Additional Benefits

7.1. MPLS Data Plane with Operational Simplicity

As required by [RFC7938], no new signaling protocol is introduced.
The BGP Prefix-SID is a lightweight extension to BGP Labeled Unicast
[RFC8277]. 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
[RFC7938].

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 Section 8.

7.2. 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.

7.3. Egress Peer Engineering

It is straightforward to combine the design illustrated in this
document with the Egress Peer Engineering (EPE) use case described in
[SR-CENTRAL-EPE].

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
Figure 1 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.

7.4. Anycast

The design presented in this document preserves the availability and
load-balancing properties of the base design presented in [RFC8402].

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.

8. 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 [RFC8669]: 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.

9. IANA Considerations

This document has no IANA actions.

10. Manageability Considerations

The design and deployment guidelines described in this document are
based on the network design described in [RFC7938].

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 [RFC8402], the same SRGB should be allocated in all
nodes in order to facilitate the design, deployment, and operations
of the domain.

When EPE ([SR-CENTRAL-EPE]) is used (as explained in Section 7.3),
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.

11. Security Considerations

This document proposes to apply SR to a well-known scalability
requirement expressed in [RFC7938] using the BGP Prefix-SID as
defined in [RFC8669].

It has to be noted, as described in Section 10, that the design
illustrated in [RFC7938] 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 [RFC7938]
and [RFC8669]. 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
Section 10.

12. References

12.1. Normative References

[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,
<https://www.rfc-editor.org/info/rfc4271>.

[RFC7938] Lapukhov, P., Premji, A., and J. Mitchell, Ed., "Use of
BGP for Routing in Large-Scale Data Centers", RFC 7938,
DOI 10.17487/RFC7938, August 2016,
<https://www.rfc-editor.org/info/rfc7938>.

[RFC8277] Rosen, E., "Using BGP to Bind MPLS Labels to Address
Prefixes", RFC 8277, DOI 10.17487/RFC8277, October 2017,
<https://www.rfc-editor.org/info/rfc8277>.

[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.

[RFC8669] Previdi, S., Filsfils, C., Lindem, A., Ed., Sreekantiah,
A., and H. Gredler, "Segment Routing Prefix Segment
Identifier Extensions for BGP", RFC 8669,
DOI 10.17487/RFC8669, December 2019,
<https://www.rfc-editor.org/info/rfc8669>.

12.2. Informative References

[IPv6-SRH] Filsfils, C., Dukes, D., Previdi, S., Leddy, J.,
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", Work in Progress, Internet-Draft, draft-ietf-6man-
segment-routing-header-26, 22 October 2019,
<https://tools.ietf.org/html/draft-ietf-6man-segment-
routing-header-26>.

[RFC6793] Vohra, Q. and E. Chen, "BGP Support for Four-Octet
Autonomous System (AS) Number Space", RFC 6793,
DOI 10.17487/RFC6793, December 2012,
<https://www.rfc-editor.org/info/rfc6793>.

[SR-CENTRAL-EPE]
Filsfils, C., Previdi, S., Dawra, G., Aries, E., and D.
Afanasiev, "Segment Routing Centralized BGP Egress Peer
Engineering", Work in Progress, Internet-Draft, draft-
ietf-spring-segment-routing-central-epe-10, 21 December
2017, <https://tools.ietf.org/html/draft-ietf-spring-
segment-routing-central-epe-10>.

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: [email protected]

Gaurav Dawra
Cisco Systems
United States of America

Email: [email protected]

Dmitry Afanasiev
Yandex
Russian Federation

Email: [email protected]

Tim Laberge
Cisco
United States of America

Email: [email protected]

Edet Nkposong
Salesforce.com Inc.
United States of America

Email: [email protected]

Mohan Nanduri
Microsoft
United States of America

Email: [email protected]

James Uttaro
ATT
United States of America

Email: [email protected]

Saikat Ray
Unaffiliated
United States of America

Email: [email protected]

Jon Mitchell
Unaffiliated
United States of America

Email: [email protected]

Authors' Addresses

Clarence Filsfils (editor)
Cisco Systems, Inc.
Brussels
Belgium

Email: [email protected]

Stefano Previdi
Cisco Systems, Inc.
Italy

Email: [email protected]

Gaurav Dawra
LinkedIn
United States of America

Email: [email protected]

Ebben Aries
Arrcus, Inc.
2077 Gateway Place, Suite #400
San Jose, CA 95119
United States of America

Email: [email protected]

Petr Lapukhov
Facebook
United States of America

Email: [email protected]