rfc:rfc8670
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.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction
2. 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: 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
Clarence Filsfils (editor) Cisco Systems, Inc. Brussels Belgium
Email: cfilsfil@cisco.com
Stefano Previdi Cisco Systems, Inc. Italy
Email: stefano@previdi.net
Gaurav Dawra LinkedIn United States of America
Email: gdawra.ietf@gmail.com
Ebben Aries Arrcus, Inc. 2077 Gateway Place, Suite #400 San Jose, CA 95119 United States of America
Email: exa@arrcus.com
Petr Lapukhov Facebook United States of America
Email: petr@fb.com
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