BGP Usage for SDWAN Overlay Networks
draft-ietf-bess-bgp-sdwan-usage-04
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draft-ietf-bess-bgp-sdwan-usage-04
Network Working Group L. Dunbar
Internet Draft J. Guichard
Intended status: Informational Futurewei
Expires: April 18, 2022 Ali Sajassi
Cisco
J. Drake
Juniper
B. Najem
Bell Canada
Ayan Barnerjee
D. Carrel
IPsec Research
October 18, 2021
BGP Usage for SDWAN Overlay Networks
draft-ietf-bess-bgp-sdwan-usage-04
Abstract
The document discusses the usage and applicability of BGP as the
control plane for multiple SDWAN scenarios. The document aims to
demonstrate how the BGP-based control plane is used for large-scale
SDWAN overlay networks with little manual intervention.
SDWAN edge nodes are commonly interconnected by multiple types of
underlay networks owned and managed by different network providers.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. This document may not be modified,
and derivative works of it may not be created, except to publish it
as an RFC and to translate it into languages other than English.
Internet-Drafts are working documents of the Internet Engineering
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other groups may also distribute working documents as Internet-
Drafts.
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Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction...................................................3
2. Conventions used in this document..............................4
3. Use Case Scenario Description and Requirements.................5
3.1. Requirements..............................................6
3.1.1. Supporting SDWAN Segmentation........................6
3.1.2. Client Service Requirement...........................6
3.1.3. SDWAN Traffic Segmentation...........................6
3.1.4. Zero Touch Provisioning..............................7
3.1.5. Constrained Propagation of SDWAN Edge Properties.....8
3.2. Scenario #1: Homogeneous WAN..............................9
3.3. Scenario #2: Hybrid WAN Underlay.........................10
3.4. Scenario #3: Private VPN PE based SDWAN..................12
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4. Provisioning Model............................................13
4.1. Client Service Provisioning Model........................13
4.2. Policy Configuration.....................................14
4.3. IPsec related parameters Provisioning....................14
5. BGP Controlled SDWAN..........................................14
5.1. BGP Walk Through for Homogeneous SDWAN...................14
5.2. BGP Walk Through for Hybrid WAN Underlay.................16
5.3. BGP Walk Through for Application Flow Based Segmentation.17
5.4. Benefit of Using Recursive Next Hop Resolution...........19
5.5. Why BGP as Control Plane for SDWAN?......................19
6. SDWAN Forwarding Model........................................20
6.1. Forwarding Model for Homogeneous SDWAN...................20
6.1.1. Network and Service Startup Procedures..............20
6.1.2. Packet Walk-Through.................................21
6.2. Forwarding Model for Hybrid Underlay SDWAN...............22
6.2.1. Network and Service Startup Procedures..............22
6.2.2. Packet Walk-Through.................................22
6.3. Forwarding Model for PE based SDWAN......................23
6.3.1. Network and Service Startup Procedures..............23
6.3.2. Packet Walk-Through.................................23
7. Manageability Considerations..................................24
8. Security Considerations.......................................25
9. IANA Considerations...........................................25
10. References...................................................25
10.1. Normative References....................................25
10.2. Informative References..................................26
11. Acknowledgments..............................................27
1. Introduction
SDWAN optimizes transport of IP Packets over multiple underlay
connectivity services. Here are some of the main characteristics of
"SDWAN" networks:
- Augment of transport, which refers to utilizing paths over
different underlay networks. Very often, there are multiple
parallel overlay paths between any two SDWAN edges; some of them
are private networks over which traffic can traverse with or
without encryption; others require encryption, e.g., over
untrusted public networks.
- Direct Internet breakout from remote branch offices is allowed
instead of all traffic hauled to Corporate HQ for centralized
policy control.
- Some traffic can be forwarded based on their application
identifiers instead of based on destination IP addresses by the
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edge nodes placing the traffic onto specific overlay paths based
on the application-specific policies.
- The traffic forwarding can also be based on specific performance
criteria (e.g., packets delay, packet loss, jitter) to provide
better application performance by choosing the underlay that
meets or exceeds the specified policies.
[Net2Cloud-Problem] describes the network-related problems to
connect enterprises' branch offices to dynamic workloads in
different Cloud Data Centers (DC). SDWAN has been positioned as a
flexible way to reach dynamic workloads in third-party Cloud DCs.
However, scaling becomes a significant issue when hundreds or
thousands of nodes need to be interconnected by SDWAN overlay
networks.
This document describes using BGP as the control plane to scale
large SDWAN overlay networks.
2. Conventions used in this document
Cloud DC: Third party data centers that host applications and
workloads owned by different organizations or tenants.
Controller: Used interchangeably with SDWAN controller to manage
SDWAN overlay path creation/deletion and monitor the
path conditions between sites.
CPE: Customer Premise Equipment
CPE-Based VPN: Virtual Private Secure network formed among CPEs.
This is to differentiate from more commonly used PE-
based VPNs [RFC 4364].
Homogeneous SDWAN: A SDWAN network in which all traffic to/from the
SDWAN edge nodes are carried by IPsec tunnels regardless
of underlay networks. I.e., the client traffic is
carried by IPsec tunnel even over MPLS private networks.
ISP: Internet Service Provider
NSP: Network Service Provider. NSP usually provides more
advanced network services, such as MPLS VPN, private
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leased lines, or managed Secure WAN connections, many
times within a private trusted domain, whereas an ISP
usually provides plain Internet services over public
untrusted domains.
PE: Provider Edge
SDWAN Edge Node: an edge node, which can be physical or virtual,
maps the attached clients' traffic to the wide area
network (WAN) overlay tunnels.
SDWAN: Software Defined Wide Area Network. A connectivity
service, offered by a Service Provider, that optimizes
transport of IP Packets over multiple underlay
connectivity services by recognizing applications at
Ingress and determining forwarding behavior by applying
policies to them.
SDWAN IPsec SA: IPsec Security Association between two SDWAN ports
or nodes.
SDWAN over Hybrid Networks: SDWAN over Hybrid Networks typically
have edge nodes utilizing bandwidth resources from
different types of underlay networks, some being private
networks and others being public Internet.
WAN Port: A Port or Interface facing an ISP or Network Service
Provider (NSP), with address allocated by the ISP or the
NSP.
C-PE: SDWAN Edge node, which can be CPE for customer managed
SDWAN, or PE for provider managed SDWAN services.
ZTP: Zero Touch Provisioning
3. Use Case Scenario Description and Requirements
This section describes some essential requirements for SDWAN
networks and several SDWAN scenarios used by the subsequent sections
to explain how the BGP control plane is applied.
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3.1. Requirements
3.1.1. Supporting SDWAN Segmentation
"SDWAN Segmentation" is a frequently used term in SDWAN deployment,
referring to policy-driven network partitioning. An SDWAN segment is
a virtual private network (SDWAN VPN) consisting of a set of edge
nodes interconnected by the tunnels, such as IPsec tunnels and MPLS
VPN tunnels.
This document assumes that an SDWAN VPN configuration on a PE
follows the same way as MPLS VPN, i.e., via VRFs. One SDWAN VPN can
be mapped to one or multiple SD-WAN virtual topologies, governed by
the SDWAN controller's policies.
When using BGP for SDWAN, the Client Route UPDATE is the same as
MPLS VPN. Route Target in the BGP Extended Community can be used to
differentiate routes belonging to different SDWAN VPNs.
As SDWAN is an overlay network arching over multiple types of
networks, MPLS L2VPN/L3VPN or pure L2 underlay can continue using
the VPN ID, VN-ID, or VLAN in the data plane to differentiate
packets belonging to different SDWAN VPNs. For packets carried by an
IPsec tunnel, the IPsec's inner encapsulation header can have the
SDWAN VPN Identifier to distinguish the packets belonging to
different SDWAN VPNs.
3.1.2. Client Service Requirement
The Client interface of SDWAN nodes can be IP or Ethernet-based.
For Ethernet-based client interfaces, SDWAN edge should support
VLAN-based service interfaces (EVI100), VLAN bundle service
interfaces (EVI200), or VLAN-Aware bundling service interfaces. EVPN
service requirements apply to the Client traffic, as described in
Section 3.1 of RFC8388.
For IP-based client interfaces, L3VPN service requirements are
applicable.
3.1.3. SDWAN Traffic Segmentation
SD-WAN Traffic Segmentation enables the separation of the traffic
based on the business and the security needs of different users'
groups and/or application requirements. Each user group and/or
application may need different isolated topologies and/or policies
to fulfill the business requirements. The SD-WAN Traffic
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Segmentation is enabled on a single SD-WAN Service to a single
subscriber.
For example, a retail business requires the point-of-sales (PoS)
application to be on a different topology from other applications.
The PoS application is routed only to the payment processing entity
at a hub site; other applications can be routed to all other sites.
The traffic from the PoS application follows a Tree topology in the
figure below, whereas other traffic can be multipoint-to-multipoint
topology.
+--------+
Payment traffic |Payment |
+------+----+-+gateway +------+----+-----+
/ / | +----+---+ | \ \
/ / | | | \ \
+-+--+ +-+--+ +-+--+ | +-+--+ +-+--+ +-+--+
|Site| |Site| |Site| | |Site| |Site| |Site|
| 1 | | 2 | | 3 | | |4 | | 5 | | 6 |
+--+-+ +--+-+ +--|-+ | +--|-+ +--|-+ +--|-+
| | | | | | |
==+=======+=======+====+======+=======+=======+===
Multi-point connection for Other traffic
Another example is an enterprise that wants to isolate the traffic
from different departments, with each department having its unique
topology and policy. The HR department may need to access specific
applications that are NOT accessible by the engineering department.
Also, the contractors may have limited access to the enterprise
resources.
3.1.4. Zero Touch Provisioning
SDWAN zero-tour provisioning (ZTP) allows devices to be configured
and provisioned centrally. When an SDWAN edge is installed at a
remote location, ZTP automates follow-up steps, including updates to
the OS, software version, and configuration before client traffic
being forwarded. The ZTP can bootstrap a remote SDWAN node and
establish a secure connection to the local SDWAN Controller, making
it convenient to add or delete an SDWAN edge node (virtual or
physical). From the network control perspective, ZTP includes the
following:
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- Upon power-up, an SDWAN node can establish the transport layer
secure connection (such as TLS, SSL, etc.) to its controller,
whose address can be burned or preconfigured on the device.
- The SDWAN Controller can designate a local network controller
in the proximity of the SDWAN node. Like the Route-Reflector (RR)
for BGP controlled SDWAN, the local network controller manages and
monitors the communication policies for traffic to/from the edge
node.
3.1.5. Constrained Propagation of SDWAN Edge Properties
One SDWAN edge node may only be authorized to communicate with a
small number of other SDWAN edge nodes. Under this circumstance, the
property of the SDWAN edge node cannot be propagated to other nodes
that are not authorized to communicate. But a remote SDWAN edge
node, upon powering up, might not have the right policies to know
which peers are authorized to communicate. Therefore, SDWAN
deployment needs to have a central point to distribute the
properties of an SDWAN edge node to its authorized peers.
BGP is well suited for this purpose. RFC4684 has specified the
procedure to constrain the distribution of BGP UPDATE to only a
subset of nodes. Each edge node informs the Route-Reflector (RR)
[RFC4456] on its interested SDWAN VPNs. The RR only propagates the
BGP UPDATE for the relevant SDWAN VPNs to the edge.
The connection between an SDWAN edge node and its RR can be over an
insecure network. Therefore, an SDWAN node needs to establish a
secure transport layer connection (TLS, SSL, etc.) to its designated
RR upon power-up. The BGP UPDATE messages need to be sent over the
secure channel (TLS, SSL, etc.) to the RR.
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+---+
Peer Group 1 |RR | Peer Group 2
+======+====+=+ +======+====+=====+
/ / | +---+ | \ \
/ / | | \ \
+-+--+ +-+--+ +-+--+ +-+--+ +-+--+ +-+--+
|C-PE| |C-PE| |C-PE| |C-PE| |C-PE| |C-PE|
| 1 | | 2 | | 3 | |4 | | 5 | | 6 |
+----+ +----+ +----+ +----+ +----+ +----+
Tenant 1 Tenant 2
Figure 1: Peer Groups managed by RR
Tenant separation is achieved by the SDWAN VPN identifiers
represented in the control plane and data plane, respectively.
3.2. Scenario #1: Homogeneous WAN
Homogeneous WAN refers to a type of SDWAN network with edge nodes
encrypting all traffic over WAN to other edge nodes, regardless of
whether the underlay is private or public. For lack of better
terminology, we call this Homogeneous SDWAN throughout this
document.
Some typical scenarios for the use of a Homogeneous SDWAN network
are as follows:
- A small branch office to connect to its HQ offices via the
Internet. All sensitive traffic to/from this small branch office
must be encrypted, usually achieved by IPsec Tunnels.
- A store in a shopping mall may need to securely connect to its
applications in one or more Cloud DCs via the Internet. A common way
of achieving this is to establish IPsec SAs to the Cloud DC gateway
to carry the sensitive data to/from the store.
As described in [SECURE-EVPN], the granularity of the IPsec SAs for
Homogeneous SDWAN can be per site, per subnet, per tenant, or
address. Once the IPsec SA is established for a specific
subnet/tenant/site, all traffic to/from the subnets/tenants/site is
encrypted.
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+---+
+--------------|RR |------------+
/ Untrusted +-+-+ \
/ \
/ \
+----+ +---------+ +------+ +----+
| CN3|--| P1-----+ -------------+------ P1 |--| CN3|
+----+ | C-PE1 P2-----+ | | C-PE2| +----+
+----+ | P3-----+ Wide +------ P2 | +----+
| CN2|--| | | area +------ P3 |--| CN1|
+-+--+ +---------+ | network | +------+ +-+--+
\ | | /
\ +---------+ | all packets | +------+ /
+--| P1-----+ encrypted +------ P1 |-+
| C-PE3 P2-----+ by | | C-PE4|
+----+ | P3-----+ IPsec SAs +------ P2 | +----+
| CN1|--| P4-----+--------------+ | |--| CN2|
+----+ +---------+ +------+ +----+
CN: Client Networks, which is same as Tenant Networks used by NVo3
Figure 2: Homogeneous SDWAN
One of the properties of a homogeneous SDWAN is that the SDWAN Local
Network Controller (RR)might be connected to C-PEs via an untrusted
public network, therefore, requiring a secure connection between RR
and C-PEs (TLS, DTLS, etc.).
Homogeneous SDWAN has some similarity to commonly deployed IPsec
VPN, albeit the IPsec VPN is usually point-to-point among a small
number of nodes and with heavy manual configuration for IPsec
between nodes. In contrast, an SDWAN network can have many edge
nodes and has a central controller to manage the configurations on
the edge nodes.
Existing Private VPNs (e.g., MPLS based) can use homogeneous SDWAN
to extend over the public network to remote sites to which the VPN
operator does not own or lease infrastructural connectivity, as
described in [SECURE-EVPN] and [SECURE-L3VPN]
3.3. Scenario #2: Hybrid WAN Underlay
Since IPsec requires additional processing power and the encrypted
traffic over the Internet does not have the premium SLA commonly
offered by Private VPNs, especially over a long distance, it is more
desired for traffic over private VPN to be forwarded without
encryption. The Hybrid WAN Underlay scenario refers to an SDWAN
network in which traffic over IP VPN is forwarded natively without
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IPsec protection. IPsec tunnels protect only the traffic sent over
the public Internet.
One C-PE might have the Internet-facing WAN ports managed by
different ISPs/NSPs with the WAN ports' addresses assigned by the
corresponding ISPs/NSPs. Clients might have policies to specify:
1) Some flows can only be forwarded over private VPNs.
2) Some flows can be forwarded over either private VPNs or public
Inter. The packets over the public Internet are encrypted.
3) Some flows, especially Internet-bound browsing ones, can be handed
off to the Internet without any encryption.
Suppose a flow, traversing multiple segments such as A<->B<->C<->D,
has the Policy 2) above. The flow can cross different underlays in
different segments, such as over Private underlay between A<->B
without encryption or over the public Internet between B<->C
protected by an IPsec SA.
As shown in the figure below, C-PE-1 has two different types of
interfaces (A1 to Internet and A2 & A3 to VPN). The C-PE's loopback
address and the C-PE attached client addresses may or may not be
visible to the ISPs/NSPs. The WAN ports' addresses can be allocated
by the service providers or dynamically assigned (e.g., by DHCP).
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+---+
+--------------|RR |----------+
/ Untrusted +-+-+ \
/ \
/ \
+----+ +---------+ packets encrypted over +------+ +----+
| CN3|--| A1-----+ Untrusted +------ B1 |--| CN1|
+----+ | C-PE1 A2-\ | C-PE2| +----+
+----+ | A3--+--+ +---+---B2 | +----+
| CN2|--| | |PE+--------------+PE |---B3 |--| CN3|
+----+ +---------+ +--+ trusted +---+ +------+ +----+
| WAN |
+----+ +---------+ +--+ packets +---+ +------+ +----+
| CN1|--| C1--|PE| go natively |PE |-- D1 |--| CN1|
+----+ | C-PE3 C2--+--+ without encry+---+ | C-PE4| +----+
| | +--------------+ | |
| | | |
+----+ | | without encrypt over | | +----+
| CN2|--| C3--+---- Untrusted --+------D2 |--| CN2|
+----+ +---------+ +------+ +----+
CN: Client Network
Figure 3: Hybrid SDWAN
Also, the connection between C-PEs and their Controller (RR) might
be via the untrusted public network. It is necessary to encrypt
the communication between RR and C-PEs, by TLS, DTLS, etc.
There could be multiple SDWAN edges (C-PEs) sharing common
property, such as a geographic location. Some applications over
SDWAN may need to traverse specific geographic areas for various
reasons, such as to comply with regulatory rules, to utilize
specific value-added services, or others.
Services may not be congruent, i.e., the packets from A-> B may
traverse one underlay network, and the packets from B -> A may go
over a different underlay.
3.4. Scenario #3: Private VPN PE based SDWAN
This scenario refers to the existing VPN (e.g., EVPN or IPVPN) being
expanded by adding extra ports facing the untrusted Internet for PEs
to offload low-priority traffic when the VPN paths are congested.
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Throughout this document, this scenario is also called Internet
Offload for Private VPN, or PE-based SDWAN.
Here are some differences from the Hybrid Underlay scenario (Section
3.3):
- For MPLS-based VPN, PEs would have MPLS as payload
encapsulated within the IPsec tunnel egressing the Internet
WAN ports, MPLS-in-IP/GRE-in-IPsec.
- The BGP RR is connected to PEs in the same way as VPN, i.e.,
via the trusted network.
The PE-based SDWAN can be used by VPN service providers to
temporarily increase bandwidth between sites when not sure if the
demand will sustain for an extended period or as a temporary
solution before the permanent infrastructure is built or leased.
+---+
+======>|PE2|
// +---+
// ^
// || VPN
// VPN v
++--+ ++-+ +---+
|PE1| <====> |RR| <===> |PE3|
+-+-+ +--+ +-+-+
| |
+--- Public Internet -- +
Offload
Figure 4: Additional Internet paths added to the VPN
4. Provisioning Model
4.1. Client Service Provisioning Model
Client service provisioning can follow the same approach as MPLS
VRFs. A client VPN can establish the communication policy by
specifying the Route Targets to be imported and exported.
Alternatively, traditional Match and Action ACLs can identify the
specific routes allowed or denied to or from the client VPN.
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For an SDWAN edge node dedicated to one subscriber with one virtual
network, provisioning can be automated. All the prefixes attached to
the client port(s) of the edge node can be considered in one VRF,
and the RR can manage the policies for import/export of the VRF.
4.2. Policy Configuration
One of the characteristics of an SDWAN service is that packets can
be forwarded over multiple types of underlays. Policies are needed
to govern which underlay paths can carry an application flow, as
described by Section 8 of MEF70.1. An Application Flow consists of
packets that match specific criteria. For example, client-prefix-x
can only be mapped to MPLS topology.
4.3. IPsec related parameters Provisioning
For the IPsec tunnel to be established, the SDWAN edge nodes need to
support the common IPsec encryption algorithms (DES, 3DES, or AES),
the hash algorithm (SHA or MD5), and the DH groups. Each SDWAN edge
node can have the default supported values for those attributes or
get the attributes from its controller to minimize the
configuration. For BGP-controlled SDWAN, BGP UPDATE messages can
propagate each node's IPsec related attributes values for peers to
choose the common values supported, which is traditionally done by
IPsec IKEv2.
5. BGP Controlled SDWAN
5.1. BGP Walk Through for Homogeneous SDWAN
For the BGP-controlled homogeneous SDWAN, a C-PE can advertise its
attached client routes and the properties of the IPsec tunnel for
carrying the traffic towards the client routes in one BGP UPDATE
message.
In the Figure below, the BGP UPDATE message from C-PE2 to RR can
have the client routes encoded in the MP-NLRI Path Attribute and the
IPsec Tunnel associated information encoded in the Tunnel-Encap
[RFC9012] Path Attributes as described in the [SECURE-EVPN].
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+---+
+---------|RR |----------+
/ Untrusted+---+ \
/ \
C-PE1/ \
+---------+ +------+
--+---+---------------------------------> |-10.1.x.x/16
| / | |C-PE2 |- VLAN = 15
| / | +-----> |
--|/1.1.1.1 | | | |-12.1.1.x/24
+---------+ | +------+
| 2.2.2.2
|
C-PE3 |
+---------+ |
--|---+---------------------------+
| / |
| / |
--|/3.3.3.3 |
+---------+
Figure 5: Homogeneous SDWAN
Alternatively, the C-PE2 can use two separate BGP UPDATE messages to
reduce the size of the BGP UPDATE messages, as described by Section
4 and 8 of [RFC9012]. UPDATE U1 has its Nexthop to the node loopback
address and is reclusively resolved to the IPsec tunnel detailed
attributes advertised by the UPDATE U2 for the Node Loopback
address:
Here are the details of the UPDATE messages:
- Suppose that a given packet P destined towards the client
addresses attached to C-PE2 (e.g., prefix 10.1.x.x/16) can be
carried by any IPsec tunnels terminated at C-PE2;
- The path along which P is to be forwarded is determined by BGP
UPDATE U1;
- UPDATE U1 does not have a Tunnel Encapsulation attribute;
- UPDATE U1 can include the Encapsulation Extended Community with
the option to have the Color Extended Community;
- The address of the next-hop of UPDATE U1 is router C-PE2;
- The best route to router C-PE2 is a BGP route advertised in
UPDATE U2;
- UPDATE U2 has a Tunnel Encapsulation attribute to describe the
IPsec detailed attributes.
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UPDATE U1:
- MP-NLRI Path Attribute:
10.1.x.x/16
12.1.1.x/24
- Nexthop: 2.2.2.2 (C-PE2)
- Encapsulation Extended Community: Type = IPsec
UPDATE U2:
- MP-NLRI Path Attribute:
2.2.2.2 (C-PE2)
- Tunnel Encapsulation Path Attributes (as described in the
[SECURE-EVPN])
If different client routes attached to C-PE2 need to be reached by
separate IPsec tunnels, the Color-Extended-Community [RFC9012] is
used to associate routes with the tunnels. See Section 8 of
[RFC9012].
Suppose C-PE2 does not have the policy on the authorized peers for
the specific client routes. In that case, RR needs to check the
client routes policies to constrain the BGP UPDATE messages
propagation only to the remote authorized edge nodes.
5.2. BGP Walk Through for Hybrid WAN Underlay
In this scenario, some client routes can be forwarded by any tunnels
terminated at the edge node, and some client routes can be sent over
some specific tunnels (such as only MPLS VPN).
An edge node can use the Color Extended Community (Section 4 & 8 of
[RFC9012]) in its BGP UPDATE message to associate the client routes
with the specific tunnels.
For example, in Figure 5 above, suppose that Route 10.1.x.x/16 can
be carried by either MPLS or IPsec and Route 12.1.1.x/24 can only be
carried by MPLS; C-PE2 can use the following UPDATE messages:
UPDATE #1a for Route Route 10.1.x.x/16:
- MP-NLRI Path Attribute:
10.1.x.x/16
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Nexthop: 2.2.2.2 (C-PE2)
- Encapsulation Extended Community: Type = SDWAN-Hybrid
- Color Extended Community: RED
UPDATE #1b for Route Route 12.1.1.x/24:
- MP-NLRI Path Attribute:
12.1.1.x/24
Nexthop: 2.2.2.2 (C-PE2)
- Encapsulation Extended Community: Type= SDWAN-Hybrid
- Color Extended Community: YELLOW
UPDATE #2a: for IPsec tunnels terminated at the node:
- MP-NLRI Path Attribute:
2.2.2.2 (C-PE2)
- Tunnel Encapsulation Path Attributes: TYPE=SDWAN-Hybrid
- Color Extended Community: RED
UPDATE #2b: for MPLS-in-GRE terminated at the node:
- MP-NLRI Path Attribute:
2.2.2.2 (C-PE2)
- Tunnel Encapsulation Path Attributes: TYPE=SDWAN-Hybrid
- Color Extended Community: YELLOW
SDWAN-Hybrid Tunnel Type is specified by [SDWAN-EDGE-Discovery].
5.3. BGP Walk Through for Application Flow Based Segmentation
Suppose the application flow can be identified by the source or
destination IP addresses. In that case, constraining the BGP UPDATE
messages for the application only to the nodes that meet the
criteria of the application flow can achieve the Application Flow
based Segmentation described in Section 3.1.2. In the Figure below,
the following BGP Updates can be advertised to ensure that the
Payment Application only communicates with the Payment Gateway:
BGP UPDATE #1a from C-PE2 to RR for the P2P topology that is only
propagated to Payment GW node:
UPDATE #1a (only to the Payment GW node):
- MP-NLRI Path Attribute:
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- 30.1.1.x/24
- Nexthop: 2.2.2.2
- Encapsulation extended community: Tunneltype=IPSEC
- Color Extended Community: BLUE
BGP UPDATE #1b from C-PE2 to RR for the routes to be reached by C-
PE1 and C-PE2:
- MP-NLRI Path Attribute:
- 10.1.x.x
- 12.4.x.x
- Nexthop:2.2.2.2
- Encapsulation extended community: Tunnel-type=IPSEC
- Color Extended Community: RED
BGP UPDATE #2 describes the IPsec detailed attributes for IPsec
tunnels terminated at C-PE2 2.2.2.2.
UPDATE #2a: for all IPsec SAs terminated at the node:
- MP-NLRI Path Attribute:
2.2.2.2 (C-PE2)
- Tunnel Encapsulation Path Attributes: TYPE=IPsec (for all IPsec
SAs)
- Color Extended Community: RED
UPDATE #2b: for the IPsec SA to the Payment Gateway:
- MP-NLRI Path Attribute:
2.2.2.2 (C-PE2)
- Tunnel Encapsulation Path Attributes: TYPE=IPsec (for the IPsec
SA to Payment GW).
- Color Extended Community: Blue
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+-------+
|Payment|
+-------->| GW |<-------+
/Hub-spoke +-------+ \
/for Payment App \
C-PE1 / \ C-PE2
+------/--+ +----\-+
--|-----/ | | -|- 30.1.1.x/24
+ ---------------------------------------> |-10.1.x.x/16
| | | |-
| | +------------> |- 12.1.1.x/24
--|---------------------------+ | |
+---------+ +------> |- VLAN=25;
/ +------+ 22.1.1.x/24
+---------+ /
--| -----------------------------+
| C-PE3 | /
| | /
--| --------------------------+
+---------+
Figure 6: Application Based SDWAN Segmentation
5.4. Benefit of Using Recursive Next Hop Resolution
Using the Recursive Next Hop Resolution described in Section 8 of
[RFC9012], the clients' routes UPDATE messages become very compact,
and any changes of the underlay network tunnels attributes can be
advertised without client route update. This method is handy when
the underlay tunnels are IPsec based, which requires periodic
message exchange for the pairwise re-keying process.
5.5. Why BGP as Control Plane for SDWAN?
For an SDWAN network with a small number of nodes, the traditional
hub & spoke model utilizing NHRP or DSVPN/DMVPN protocol had worked
reasonably well. DSVPN/DMVPN has a hub node (or controller) managing
the edge nodes, including local & public addresses and tunnel
identifiers mapping. However, for a sizeable SDWAN network, say more
than 100 nodes with different underlays, the traditional approach
becomes very messy, complex, and error prone.
Here are some of the compelling reasons for using BGP:
- With a secure management channel already established between an
edge node and RR, RR can perform the peer authentication on behalf
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of the edge node. Not only RR has policies on peer communication,
but RR also has the built-in capability to constrain the propagation
of the UPDATE messages to the authorized edge nodes [RFC4684].
- When multiple IPsec tunnels are established between two pairwise
edge nodes, BGP Tunnel Attribute Update can associate multiple IPsec
tunnels with the client routes. In traditional IPsec VPN, separate
routing protocols must run in parallel in each IPsec Tunnel if the
client routes can be load shared among the IPsec tunnels.
- The IPsec tunnel's traffic selector or admission control can be
inherently realized by specifying importing/exporting the Route
Targets representing the SDWAN VPNs.
6. SDWAN Forwarding Model
This section describes how client traffic is forwarded in BGP
Controlled SDWAN for the use cases described in Section 3.
The procedures described in Section 6 of RFC8388 are applicable for
the SDWAN client traffic. Like the BGP-based VPN/EVPN client routes
UPDATE message, Route Target can distinguish routes from different
clients.
6.1. Forwarding Model for Homogeneous SDWAN
6.1.1. Network and Service Startup Procedures
A single IPsec security association (SA) protects data in one
direction. Under the homogeneous SDWAN Scenario, two SAs must be
present to secure traffic in both directions between two C-PE nodes,
two client ports, or two prefixes. Using Figure 2 of Section 3.2 as
an example, for client CN2 attached to C-PE1, C-PE3, and C-PE4 to
have full-mesh connection, six one-directional IPsec SAs must be
established: C-PE1 <-> C-PE3; C-PE1 <-> C-PE4; C-PE3 <-> C-PE4.
SDWAN services to clients can be IP-based or Ethernet-based. An
SDWAN node can learn client routes from the client-facing ports via
OSPF, RIP, BGP or Statically configuration for its IP-based
services. For Layer-2 SDWAN services, the relevant EVPN parameters,
such as the ESI (Ethernet Segment Identifier), EVI, CE-VID to EVI
mapping, can be configured in the same way as EVPN described in
RFC8388.
Instead of running IGP within each IPsec tunnel as done by the
traditional IPsec VPN, BGP UPDATE messages propagate the client
routes attached to SDWAN edge nodes.
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In addition, BGP-RR (SDWAN Controller) facilitates the IPsec SA
establishment and rekey management described in [SECURE-EVPN]. The
Controller manages how client's routes are associated with
individual IPsec SA. Therefore, it is no longer necessary to
manually configure the IPsec tunnel's endpoint addresses on each
SDWAN edge node and set up policies for the allowed client prefixes.
6.1.2. Packet Walk-Through
For an IPsec SA terminated at a C-PE node, multiple client routes
can be multiplexed in the IPsec SA (or tunnel). Traffic to the
client prefixes is encapsulated in an inner tunnel, such as GRE or
VxLAN, carried by the IPsec SA ESP tunnel. Different client traffic
can be differentiated by a unique value in the inner encapsulation
key or ID field.
For unicast packets forwarding:
the C-PE node address (or loopback address) acts as the Next Hop
addresses for the prefixes attached to the C-PE nodes.
C-PE Node-based IPsec tunnel is inherently protected when the C-PE
has multiple WAN ports to different underlay paths. As shown in
Figure 2, when one of the underlay paths fails, the IPsec traffic
can be forwarded to or received from a different physical port.
When a C-PE receives a packet from its client port, the packet is
encapsulated inside the IPsec SA, whose destination address
matches the Next Hop address of the packet's destination and
forwarded to the target C-PE.
When a C-PE receives an IPsec encrypted packet from its WAN ports,
it decrypts the packet and forwards the inner packet to the client
port based on the inner packet's destination address.
For multicast packets forwarding:
IPsec was created to be a security protocol between two and only
two devices, so multicast service using IPsec is problematic. An
IPsec peer encrypts a packet so that only one other IPsec peer can
successfully perform the de-encryption. A straight way to forward
a multicast packet for the homogeneous SDWAN is to encapsulate the
multicast packet in separate unicast IPsec SA tunnels. More
optimized forwarding multicast packets for the homogeneous SDWAN
is out of the scope of this document.
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6.2. Forwarding Model for Hybrid Underlay SDWAN
In this scenario as shown in Figure 3 of Section 3.3, traffic
forwarded over the trusted VPN paths can be native (i.e.,
unencrypted). The traffic forwarded over untrusted networks need to
be protected by IPsec SA.
6.2.1. Network and Service Startup Procedures
Infrastructure setup: The proper MPLS infrastructure must be set up
among the edge nodes, i.e., the C-PE1/C-PE2/C-PE3/C-PE4 of Figure 3.
The IPsec SA between WAN ports or nodes must be set up as well.
IPsec SA related attributes on edge nodes can be distributed by BGP
UPDATE messages as described in Section 5.
There could be policies governing how flows can be forwarded, as
specified by MEF70.1. For example, "Private-only" indicates that
the flows can only traverse the MPLS VPN underlay paths.
6.2.2. Packet Walk-Through
For unicast packets forwarding:
Upon receiving a packet from a client port, if the packet belongs
to a flow that can only be forwarded over the MPLS VPN, the
forwarding processing is the same as the MPLS VPN. Otherwise, the
C-PE node can make the local decision in choosing the least cost
path, including the prior established MPLS paths and IPsec
Tunnels, to forward the packet. Packets forwarded over the trusted
MPLS VPN can be native without any additional encryption, while
the packets sent over the untrusted networks need to be encrypted
by IPsec SA.
For a C-PE with multiple WAN ports provided by different ISPs,
separate IPsec SAs can be established for the different WAN ports.
In this case, the C-PE have multiple IPsec tunnels in addition to
the MPLS path to choose from to forward the packets from the
client ports.
If the IPsec SA is chosen, the packet is encapsulated by the IPsec
inner packet header and encrypted by the IPsec SA before
forwarding to the WAN.
For packets received from a MPLS path, processing is the same as
MPLS VPN.
For IPsec SA encrypted packets received from the WAN ports, the
packets are decrypted, and the inner payload is decapsulated and
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forward per the forwarding table of the C-PE. For all packets from
the Internet-facing WAN ports, the additional anti-DDoS mechanism
has to be enabled to prevent potential attacks from the Internet-
facing ports. Control Plane should not learn routes from the
Internet-facing WAN ports.
+---+
+--------------|RR |----------+
/ +-+-+ \
/ \
/ \
+----+ +---------+ packets encrypted over +--------+ +----+
| CN3|--| A1-----+ Untrusted +------ B1 |--| CN1|
+----+ | C-PE-a A2-----+ +-------B2 C-PE-b| +----+
|10.1.1.1 | |10.1.2.1|
+----+ | | +--+ +---+ | | +----+
| CN2|--| A3 |PE+--------------+PE |---B3 |--| CN3|
+----+ +---------+ +--+ trusted +---+ +--------+ +----+
| VPN |
+--------------+
Figure 8: SDWAN with Hybrid Underlays
For multicast packets forwarding:
For multicast traffic, MPLS multicast [RFC6513, RFC6514, or
RFC7988] can be used to forward multicast traffic.
If IPsec tunnels are chosen for a multicast packet, the packet is
encapsulated and encrypted by multiple separate IPsec tunnels to
the desired destinations.
6.3. Forwarding Model for PE based SDWAN
6.3.1. Network and Service Startup Procedures
In this scenario, all PEs have secure interfaces facing the clients
and facing the MPLS backbone, with some PEs having extra connections
by untrusted public Internet. The public Internet paths are for
offloading low priority traffic when the MPLS paths get congested.
The PEs are already connected to their RRs, and the configurations
for the clients and policies are already established.
6.3.2. Packet Walk-Through
For PEs to offload some MPLS packets to the Internet path, each MPLS
packet is wrapped by an outer IP header as MPLS-in-IP or MPLS-in-GRE
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[RFC4023]. The outer IP address can be an interface address or the
PE's loopback address.
When IPsec Tunnel mode is used to protect an MPLS-in-IP packet, the
entire MPLS-in-IP packet is placed after the IPsec tunnel header.
When IPsec transport mode is used to protect the MPLS packet, the
MPLS-in-IP packet's IP header becomes the outer IP header of the
IPsec packet, followed by an IPsec header, and then followed by the
MPLS label stack. The IPsec header must set the payload type to MPLS
by using the IP protocol number specified in section 3 of RFC4023.
If IPsec transport mode is applied to an MPLS-in-GRE packet, the GRE
header follows the IPsec header.
The IPsec SA's endpoints should not be the client-facing interface
addresses unless the traffic to/from those clients always goes
through the IPsec SA even when the MPLS backbone has enough capacity
to transport the traffic.
When the PEs' Internet-facing ports are behind the NAT [RFC3715], an
outer UDP field can be added outside the encrypted payload
[RFC3948]. Three UDP ports must be open on the PEs: UDP port 4500
(used for NAT traversal), UDP port 500 (used for IKE), and IP
protocol 50 (ESP). IPsec IKE (Internet Key Exchange) between the two
PEs would be over the NAT [RFC3947] as well.
Upon receiving a packet from a client port, the forwarding
processing is the same as the MPLS VPN. If the MPLS backbone path to
the destination is deemed congested, the IPsec tunnel towards the
target PEs is used to encrypt the MPLS-in-IP packet.
Upon receiving a packet from the Internet-facing WAN port, the
packet is decrypted, and the inner MPLS payload is extracted to be
sent to the MPLS VPN engine.
Same as Scenario #2, the additional anti-DDoS mechanism must be
enabled to prevent potential attacks from the Internet-facing port.
Control Plane should not learn routes from the Internet-facing WAN
ports.
7. Manageability Considerations
BGP-controlled SDWAN utilizes the BGP RR to facilitate the routes
and underlay properties distribution among the authorized edge
nodes. With RR having the preconfigured policies about the
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authorized peers, the peer-wise authentications of the IPsec IKE
(Internet Key Exchange) are significantly simplified.
8. Security Considerations
Adding an Internet-facing WAN port to a C-PE can introduce the
following security risks:
1) Potential DDoS attacks from the Internet-facing ports.
2) Potential risk of provider VPN network being injected with
illegal traffic from the Internet-facing WAN ports.
Therefore, the additional anti-DDoS mechanism must be enabled on all
Internet-facing ports to prevent potential attacks from those ports.
Control Plane should not learn any routes from the Internet-facing
WAN ports.
9. IANA Considerations
No Action is needed.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4364] E. rosen, Y. Rekhter, "BGP/MPLS IP Virtual Private
networks (VPNs)", Feb 2006.
[RFC7296] C. Kaufman, et al, "Internet Key Exchange Protocol Version
2 (IKEv2)", Oct 2014.
[RFC7432] A. Sajassi, et al, "BGP MPLS-Based Ethernet VPN", Feb
2015.
[RFC8365] A. Sajassi, et al, "A network Virtualization Overlay
Solution Using Ethernet VPN (EVPN)", March 2018.
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[RFC9012] K.Patel, et al "The BGP Tunnel Encapsulation Attribute",
RFC9012, April 2021.
10.2. Informative References
[RFC8192] S. Hares, et al, "Interface to Network Security Functions
(I2NSF) Problem Statement and Use Cases", July 2017
[RFC5521] P. Mohapatra, E. Rosen, "The BGP Encapsulation Subsequent
Address Family Identifier (SAFI) and the BGP Tunnel
Encapsulation Attribute", April 2009.
[RFC8388] J. Rabadan, et al, "Usage and Applicability of BGP MPLS-
Based Ethernet VPN", May 2018.
[Net2Cloud-Gap] L. Dunbar, A. Malis, C. Jacquenet, "Gap Analysis of
Interconnecting Underlay with Cloud Overlay", draft-dm-
net2cloud-gap-analysis-02, work in progress, Oct. 2018.
[SDWAN-EDGE-Discovery] L. Dunbar, S. Hares, R. Raszuk, K. Majumdar,
"BGP UPDATE for SDWAN Edge Discovery", draft-dunbar-idr-
sdwan-edge-discovery-01, work-in-progress, Nov 2020.
[VPN-over-Internet] E. Rosen, "Provide Secure Layer L3VPNs over
Public Infrastructure", draft-rosen-bess-secure-l3vpn-00,
work-in-progress, July 2018
[DMVPN] Dynamic Multi-point VPN:
https://www.cisco.com/c/en/us/products/security/dynamic-
multipoint-vpn-dmvpn/index.html
[DSVPN] Dynamic Smart VPN:
http://forum.huawei.com/enterprise/en/thread-390771-1-
1.html
[SECURE-EVPN] A. Sajassi, et al, "Secure EVPN", draft-sajassi-bess-
secure-evpn-01, Work-in-progress, March 2019.
[SECURE-L3VPN] E. Rosen, R. Bonica, "Secure Layer L3VPN over Public
Infrastructure", draft-rosen-bess-secure-l3vpn-00, Work-
in-progress, June 2018.
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[ITU-T-X1036] ITU-T Recommendation X.1036, "Framework for creation,
storage, distribution and enforcement of policies for
network security", Nov 2007.
[Net2Cloud-Problem] L. Dunbar and A. Malis, "Seamless Interconnect
Underlay to Cloud Overlay Problem Statement", draft-dm-
net2cloud-problem-statement-02, June 2018
[Net2Cloud-gap] L. Dunbar, A. Malis, and C. Jacquenet, "Gap Analysis
of Interconnecting Underlay with Cloud Overlay", draft-dm-
net2cloud-gap-analysis-02, work-in-progress, Aug 2018.
11. Acknowledgments
Acknowledgements to Adrian Farrel, Joel Halpern, John Scudder,
Darren Dukes, Andy Malis and Donald Eastlake for their review and
contributions.
This document was prepared using 2-Word-v2.0.template.dot.
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Authors' Addresses
Linda Dunbar
Futurewei
Email: ldunbar@futurewei.com
James Guichard
Futurewei
Email: james.n.guichard@futurewei.com
Ali Sajassi
Cisco
Email: sajassi@cisco.com
John Drake
Juniper
Email: jdrake@juniper.net
Basil Najem
Bell Canada
Email: basil.najem@bell.ca
David Carrel
IPsec Research
Email: carrel@ipsec.org
Ayan Banerjee
Cisco
Email: ayabaner@cisco.com
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