Scalability Considerations for Enhanced VPN (VPN+)
draft-dong-teas-enhanced-vpn-vtn-scalability-03
TEAS Working Group J. Dong
Internet-Draft Z. Li
Intended status: Informational Huawei Technologies
Expires: January 12, 2022 L. Gong
China Mobile
G. Yang
China Telecom
J. Guichard
Futurewei Technologies
G. Mishra
Verizon Inc.
F. Qin
China Mobile
July 11, 2021
Scalability Considerations for Enhanced VPN (VPN+)
draft-dong-teas-enhanced-vpn-vtn-scalability-03
Abstract
Enhanced VPN (VPN+) aims to provide enhancements to existing VPN
services to support the needs of new applications, particularly
including the applications that are associated with 5G services.
VPN+ could be used to provide network slicing, and may also be of use
in more generic scenarios, such as enterprise services which have
demanding requirement. With the requirement for VPN+ services
increase, scalability would become an important factor for the
deployment of VPN+. This document describes the scalability
considerations in the control plane and data plane to enable VPN+
services, some optimization mechanisms are also described.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 12, 2022.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. VPN+ Scalability Requirements . . . . . . . . . . . . . . . . 3
3. VPN+ Scalability Considerations . . . . . . . . . . . . . . . 5
3.1. Control Plane Scalability . . . . . . . . . . . . . . . . 5
3.1.1. Distributed Control Plane . . . . . . . . . . . . . . 5
3.1.2. Centralized Control Plane . . . . . . . . . . . . . . 6
3.2. Data Plane Scalability . . . . . . . . . . . . . . . . . 6
3.3. Gap Analysis of Existing Mechanisms . . . . . . . . . . . 7
4. Possible Scalability Optimizations . . . . . . . . . . . . . 8
4.1. Control Plane Optimizations . . . . . . . . . . . . . . . 8
4.2. Data Plane Optimizations . . . . . . . . . . . . . . . . 10
5. Solution Evolution for Improved Scalability . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 12
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
Virtual Private Networks (VPNs) have served the industry well as a
means of providing different customers with logically isolated
connectivity over a common network infrastructure. The common or
base network that is used to provide the VPNs is often referred to as
the underlay, and the VPN is often called an overlay. The underlay
is responsible for establishing the network connectivity and managing
network resources to meet the service requirement. The overlay is
used to distribute the membership and reachability information of the
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customer, and provide logical separation of service delivery between
different customers.
Enhanced VPN service (VPN+) [I-D.ietf-teas-enhanced-vpn] is targeted
at new applications which require better isolation between customers
and/or services, and have more stringent performance requirements
than can be provided with existing VPNs. To meet the requirement of
VPN+ services, a number of Virtual Transport Networks (VTNs) need to
be created, each has a subset of the underlay network topology and a
set of network resources allocated from the physical network to meet
the requirements of one or a group of VPN+ services. The overlay
VPNs together with the corresponding underlay VTN provide the VPN+
service.
Section 6 of [I-D.ietf-teas-enhanced-vpn] provides some general
analysis of the scalability of VPN+. This document gives detailed
analysis of the scalability considerations when a large number of
VPN+ services are provided. Since the scalability of the overlay is
not the major bottleneck, this document mainly focuses on the
scalability of the underlay VTN.
2. VPN+ Scalability Requirements
As described in [I-D.ietf-teas-enhanced-vpn], VPN+ services may
require additional state to be introduced into the network to take
advantage of the enhanced functionality. This introduces some
scalability considerations to the network. This section gives some
analysis of the number of VPN+ services that might be needed in a
network.
There are several use cases where VPN+ may be needed, and these
determine how many VPN+ will be required in a network. One typical
use case of VPN+ is to deliver IETF network slice
[I-D.ietf-teas-ietf-network-slices] for applications or services in
5G and other scenarios, thus the number of IETF network slices needed
could reflect the number of VPN+ services. With the development and
evolution of 5G, it is expected that an increasing number of network
slices will be deployed. The number of network slices required
depends on how IETF network slices will be used, and the progress of
5G for the vertical industrial services. The potential number of
network slices is analyzed by classifying the network slicing
deployment into three typical scenarios:
1. Network slices can be used by a network operator internally for
different types of services. For example, in a converged multi-
service network, different network slices can be created to carry
mobile transport service, fixed broadband service and enterprise
services respectively, each type of service could be managed by a
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separate department or management team. Some service types, such
as multicast service may also be deployed in a dedicated network
slice. It is also possible that an infrastructure network
operator provides network slices to other network operators as a
wholesale service. In this scenario, the number of network
slices in a network would be relatively small, such as on the
order of 10 or so. This could be the typical case in the
beginning of the network slice deployment.
2. Network slices can be used to provide isolated and customized
virtual networks for customers in different vertical industries.
At the early stage of the vertical industrial service deployment,
a few top customers in some industries will begin to use network
slices to ensure the performance of their business, such as smart
grid, manufacturing, public safety, on-line gaming, etc.
Considering the number of the vertical industries, and the number
of top customers in each industry, the number of network slices
may increase to the order of 100.
3. With the evolution of 5G, network slices could be widely used by
both vertical industrial customers and enterprise customers which
require guaranteed or predictable service performance. The total
amount of network slices may increase to the order of 1000 or
more. However, it is expected that the number of network slices
would still be less than the number of traditional VPN services
in the network.
In 3GPP [TS23501], a 5G network slice is identified using Single
Network Slice Selection Assistance Information (S-NSSAI), which is a
32-bit identifier comprised of 8-bit Slice/Service Type (SST) and
24-bit Slice Differentiator (SD). This allows the mobile networks
(RAN and CN) to provide a large number of network slices. Although
it is possible that multiple 5G network slices in RAN and CN are
mapped to the same IETF network slice, the number of IETF network
slices may still be comparable with the number of 5G network slices.
Thus the scalability of IETF network slices needs to be taken into
consideration.
8-bit 24-bit
+------------+-------------------------+
| SST | Slice Differentiator |
+------------+-------------------------+
Figure 1. Format of S-NSSAI in 3GPP
VPN+ needs to meet the scalability requirement of network slicing in
different scenarios. The increased number of VPN+ services will
introduce additional complexity and overhead to both the control
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plane and data plane, especially in the aspects related to the
underlying VTNs. Although multiple VPN+ services can be mapped to
the same VTN as the underlay, there still can be scalability
challenges with the increased number of VTNs.
3. VPN+ Scalability Considerations
In this section, the scalability of VTN in the control plane and data
plane is analyzed to understand the possible gaps in meeting the
scalability requirement of VPN+.
3.1. Control Plane Scalability
As described in [I-D.ietf-teas-enhanced-vpn], the control plane of
VPN+ could be based on the hybrid of a centralized controller and the
distributed control plane.
3.1.1. Distributed Control Plane
At part of the construction of VPN+ services, it is necessary to
create multiple VTNs which provide customized topology and resource
attributes. The attributes and state information of each VTN needs
to be exchanged in the control plane. The scalability of the
distributed control plane for the establishment and maintenance of
VTNs needs to be considered in the following aspects:
o The number of control protocol instances maintained on each node
o The number of protocol sessions maintained on each link
o The number of routes advertised by each node
o The amount of attributes associated with each route
o The number of route computation (i.e. SPF computation) executed
on each node
As the number of VTNs increases, it is expected that in some of the
above aspects, the overhead in the control plane may increase
dramatically. For example, the overhead of maintaining separated
control protocol instances (e.g. IGP instances) for different VTNs
is considered higher than maintaining the information of separated
VTNs in the same control protocol instance with appropriate
separation, and the overhead of maintaining separate protocol
sessions for different VTNs is considered higher than using a shared
protocol session for the information exchange of multiple VTNs. To
meet the requirement of the increasing number of VTNs, It is
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suggested to choose the control plane mechanisms which could improve
the scalability while still provide the required functionality.
3.1.2. Centralized Control Plane
Although the SDN approach can reduce the amount of control plane
overhead in the distributed control plane, it may transfer some of
the scalability concerns from network nodes to the centralized
controller, thus the scalability of the controller also needs to be
considered.
To provide global optimization for the Traffic Engineered (TE) paths
in different VTNs, the controller needs to keep the topology and
resource information of all the VTNs up to date. To achieve this,
the controller may need to maintain a communication channel with each
network node in the network. When there is significant change in the
network, or multiple VTNs requires global optimization concurrently,
there may be a heavy processing burden at the controller, and a heavy
load in the network surrounding the controller for the distribution
of the updated network state and the TE paths.
3.2. Data Plane Scalability
To provide different VPN+ services with the required isolation and
performance characteristics, it is necessary to allocate different
sets of network resources to different VTNs. As the number of VPN+
increases, the number of VTNs will increase accordingly. This
requires the underlying network to provide fine-granular network
resource partitioning, which means the amount of state about the
reserved network resources to be maintained on network nodes will
also increase.
In data plane, traffic of different VPN+ services need to be
processed separately according to the topology and resource
constraints of the associated VTN , thus the information used for VTN
identification needs to be carried either directly or implicitly in
the data packet. Different approaches of encapsulating the VTN
information in data packet can have different scalability
implications.
One approach is to reuse some existing fields in the data packet to
additionally identify the VTN the packet belongs to. This avoids the
cost of introducing new fields in the data packet, while since it
introduces additional semantics to an existing field, it requires to
change the processing of the existing field in packet forwarding.
And when the identifiers which were used to identify a node or link
are reused to further identify a VTN, the number of the identifiers
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may be increased in proportion to the number of the VTNs, which may
cause scalability problem in some networks.
Another alternative approach is to introduce a dedicated field in the
packet for VTN identification. This could avoid the impact to the
existing fields in the packet. And if this new field carries a
global-significant VTN identifier, it could be used together with the
existing fields to determine the VTN-specific packet forwarding. The
potential issue with this approach is the difficulty in introducing a
new field in some types of the data plane.
In addition, the introduction of per VTN packet forwarding has impact
on the scalability of the forwarding entries on network nodes, as a
network node may need to maintain separate forwarding entries for
each VTN it participates in.
3.3. Gap Analysis of Existing Mechanisms
One candidate approach to build VTN is to use VTN specific Segment
Routing (either SR-MPLS or SRv6) Identifiers in the data plane
[I-D.ietf-spring-sr-for-enhanced-vpn], and define and distribute the
associated topology and resource attribute of each VTN based on
Multi-topology [RFC4915] [RFC5120] [I-D.ietf-lsr-isis-sr-vtn-mt],
Flex-Algo [I-D.ietf-lsr-flex-algo] [I-D.zhu-lsr-isis-sr-vtn-flexalgo]
or the combination of these mechanisms in the control plane. This
mechanism is suitable for networks with a limited number of VTNs. As
the number of VTNs increases, there may be several scalability
challenges with this approach:
1. The number of SR SIDs needed will increase in proportion to the
number of VTNs in the network, which will bring challenges both
to the distribution of SIDs and the related information in the
control plane, and to the installation of forwarding entries for
VTN-specific SIDs in the data plane.
2. The number of route computation (e.g. SPF computation) will
increase in proportion to the number of VTNs in the network,
which may introduce significant overhead to the control plane of
network nodes.
3. The maximum number of logical topologies supported by OSPF is
128, and the maximum number of Flex-Algo is 128, which may not
meet the required number of VTNs in some network scenarios.
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4. Possible Scalability Optimizations
4.1. Control Plane Optimizations
For the distributed control plane, several optimizations can be
considered to reduce the control plane overhead and improve the
scalability.
The first optimization mechanism is to reduce the amount of control
plane sessions used for the establishment and maintenance of the
VTNs. For multiple VTNs which have the same peering relationship
between two adjacent network nodes, it is proposed that one single
control protocol session is used for the establishment of multiple
VTNs. The information of different VTNs can be exchanged over the
same session, with necessary identification information to
distinguish the VTNs in the control messages. This could reduce the
overhead of maintaining a large number of control protocol sessions
for different VTNs, and could also reduce the amount of control plane
messages flooded in the network.
The second optimization mechanism is to decompose the attributes of a
VTN into different groups, so that different types of VTN attribute
can be advertised and processed separately in control plane. There
are two basic types of attributes associated with a VTN: the topology
attribute and the network resource attribute. In a network, it is
possible that multiple VTNs share the same topology, and multiple
VTNs may share the same set of network resources on particular
network segments. Then it is more efficient if only one copy of the
topology attribute is advertised, and multiple VTNs sharing the same
topology could refer to this topology information. More importantly,
with this approach the result of topology-based route computation
could be shared by multiple VTNs, so that the overhead of per-VTN
route computation could be reduced . Similarly, information of a
subset of network resources reserved on a particular network segment
could be advertised once and be referred to by multiple VTNs which
share the same set of resources. This methodology could also apply
to other attributes of VTN which may be introduced later and can be
processed independently.
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O#####O#####O O*****O*****O
# # # * * *
# # # * * *
O#####O#####O O*****O*****O
VTN-1 VTN-2
O-----O-----O
| | |
| | |
O-----O-----O
Shared Network Topology
Legend
O Virtual node
### Virtual links with a set of reserved resources
*** Virtual links with another set of reserved resources
Figure 2. Topology Sharing between VTNs
FIG-2
Figure 2 gives an example of two VTNs which share the same logical
topology attribute. As shown in the figure, VTN-1 and VTN-2 have the
same topology, while the link resource attributes of each VTN are
different. In this case, only one copy of the network topology
information needs to be advertised, and the topology-based route
computation result can be shared by the two VTNs to generate the
corresponding routing and forwarding tables.
O#####O#####O O----O#####O
# # # \/ # #
# # # /\ # #
O#####O#####O O----O#####O
VTN-1 VTN-2
Legend
O Virtual node
### Virtual links with a set of reserved resource
--- Virtual links with another set of reserved resource
Figure 3. Resource Sharing between VTNs
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Figure 3 gives another example of two VTNs which share the same set
of network resources on some links. In this case, information about
the reserved resource on each link only needs to be advertised once,
then both VTN-1 and VTN-2 could refer to the link resource for
constraint based path computation.
For the optimization of the centralized control plane, it is
suggested that the centralized controller is used as a complementary
mechanism to the distributed control plane rather than a replacement,
so that the VTN specific path computation burden in control plane
could be shared by both the centralized controller and the network
nodes, thus the scalability of both systems could be improved.
4.2. Data Plane Optimizations
To support more VPN+ services while keeping the amount of data plane
state at a reasonable scale, one possible approach is to classify a
set of VPN+ services which have similar service characteristics and
performance requirements into a group, and such group of VPN+
services is mapped to one VTN, which is allocated with an aggregated
set of network topology and resources to meet the service requirement
of the whole group of VPN+. Different groups of VPN+ services need to
be mapped to different VTNs with different set of network resources
allocated. With appropriate grouping of VPN+ services, a reasonable
number of VTNs with network resources reservation and aggregation
could still meet the service requirements.
Another optimization in the data plane is to decouple the identifier
used for topology-based forwarding and the identifier used for the
resource-specific processing introduced by VTN. One possible
mechanism is to introduce a dedicated VTN-ID in the packet header to
uniquely identify the set of local network resources allocated to a
VTN on each network node for the processing and forwarding of the
received packet. Then the existing identifier in the packet header
used for topology based forwarding is kept unchanged. The benefit is
the amount of topology-specific identifiers is in proportion to the
number of topologies rather than the number of VTNs, so that its
scalability will not be impacted by the increased number of VTN.
Since this new VTN-ID field will be used together with the existing
fields to determine the VTN-specific packet forwarding, this MAY
require network nodes to support a hierarchical forwarding table in
the data plane. Figure 4 shows the concept of using different data
plane identifiers for topology-based and VTN resource-based packet
processing respectively.
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+--------------------------+
| Packet Header |
| |
| +----------------------+ |
| | Topology-specific ID | |
| +----------------------+ |
| |
| +----------------------+ |
| | VTN Resource ID | |
| +----------------------+ |
+--------------------------+
Figure 4. Decoupled Data Plane Identifiers
In an IPv6 [RFC8200] based network, this could be achieved by
introducing a dedicated field in either the IPv6 fixed header or the
extension headers to carry the VTN identifier for the resource-
specific forwarding, while keeping the destination IP address field
used for routing towards the destination prefix in the corresponding
topology. Note that the VTN-ID needs to be parsed by every node
along the path which is capable of VTN-specific forwarding. In an
MPLS [RFC3032] based network, this may be achieved by introducing a
dedicated MPLS label to identify the VTN, while the existing MPLS
labels could be used for topology-based packet forwarding towards the
associated destination prefix. This requires that both labels be
parsed by each node along the forwarding path of the packet, and the
forwarding behavoir depends on the position of the VTN label in the
label stack. Another option with the MPLS data plane is to introduce
a new MPLS extension header which follows the MPLS label stack to
carry the VTN-ID and the associated information. The detailed
extensions in IPv6 and MPLS data plane encapsulation are out of the
scope of this document.
5. Solution Evolution for Improved Scalability
Based on the analysis in this document, the control plane and data
plane for VPN+ needs to evolve to support the increasing number of
VPN+ services in the network.
At the first step, by introducing resource-awareness to segment
routing SIDs [I-D.ietf-spring-resource-aware-segments], and using
Multi-Topology or Flex-Algo as the control plane, it could provide a
solution for building a limited number of VTNs in the network to meet
the requirement of a relatively small number of VPN+ services in the
network. This mechanism is considered as the basic SR VTN.
As the number of required VPN+ services increases, more VTNs may be
needed, then the control plane scalability could be improved by
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decoupling the topology attribute from other attributes (e.g.
resource attribute) of VTN, so that multiple VTNs could share the
same topology or resource attribute. This mechanism is considered as
the scalable SR VTN. Both the basic and the scalable SR VTN
mechanisms are described in [I-D.ietf-spring-sr-for-enhanced-vpn].
If the data plane scalability becomes a concern, dedicated data plane
VTN-ID can be introduced to decouple the topology-specific
identifiers from the VTN-specific resource identifiers in the data
plane, this could help to reduce the number of SR SIDs needed to
support a large number of VTNs. This mechanism is considered as the
Resource-Independent (RI) VTN.
6. Security Considerations
TBD
7. IANA Considerations
This document makes no request of IANA.
8. Contributors
Zhibo Hu
Email: huzhibo@huawei.com
Hongjie Yang
Email: hongjie.yang@huawei.com
9. Acknowledgments
The authors would like to thank Adrian Farrel for the review and
discussion of this document.
10. References
10.1. Normative References
[I-D.ietf-teas-enhanced-vpn]
Dong, J., Bryant, S., Li, Z., Miyasaka, T., and Y. Lee, "A
Framework for Enhanced Virtual Private Network (VPN+)
Services", draft-ietf-teas-enhanced-vpn-07 (work in
progress), February 2021.
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10.2. Informative References
[I-D.ietf-lsr-flex-algo]
Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
A. Gulko, "IGP Flexible Algorithm", draft-ietf-lsr-flex-
algo-15 (work in progress), April 2021.
[I-D.ietf-lsr-isis-sr-vtn-mt]
Xie, C., Ma, C., Dong, J., and Z. Li, "Using IS-IS Multi-
Topology (MT) for Segment Routing based Virtual Transport
Network", draft-ietf-lsr-isis-sr-vtn-mt-00 (work in
progress), March 2021.
[I-D.ietf-spring-resource-aware-segments]
Dong, J., Bryant, S., Miyasaka, T., Zhu, Y., Qin, F., Li,
Z., and F. Clad, "Introducing Resource Awareness to SR
Segments", draft-ietf-spring-resource-aware-segments-02
(work in progress), February 2021.
[I-D.ietf-spring-sr-for-enhanced-vpn]
Dong, J., Bryant, S., Miyasaka, T., Zhu, Y., Qin, F., Li,
Z., and F. Clad, "Segment Routing based Virtual Transport
Network (VTN) for Enhanced VPN", draft-ietf-spring-sr-for-
enhanced-vpn-00 (work in progress), February 2021.
[I-D.ietf-teas-ietf-network-slices]
Farrel, A., Gray, E., Drake, J., Rokui, R., Homma, S.,
Makhijani, K., Contreras, L. M., and J. Tantsura,
"Framework for IETF Network Slices", draft-ietf-teas-ietf-
network-slices-00 (work in progress), April 2021.
[I-D.zhu-lsr-isis-sr-vtn-flexalgo]
Zhu, Y., Dong, J., and Z. Hu, "Using Flex-Algo for Segment
Routing based VTN", draft-zhu-lsr-isis-sr-vtn-flexalgo-02
(work in progress), February 2021.
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
<https://www.rfc-editor.org/info/rfc3032>.
[RFC4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
RFC 4915, DOI 10.17487/RFC4915, June 2007,
<https://www.rfc-editor.org/info/rfc4915>.
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[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120,
DOI 10.17487/RFC5120, February 2008,
<https://www.rfc-editor.org/info/rfc5120>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[TS23501] "3GPP TS23.501", 2016,
<https://portal.3gpp.org/desktopmodules/Specifications/
SpecificationDetails.aspx?specificationId=3144>.
Authors' Addresses
Jie Dong
Huawei Technologies
Huawei Campus, No. 156 Beiqing Road
Beijing 100095
China
Email: jie.dong@huawei.com
Zhenbin Li
Huawei Technologies
Huawei Campus, No. 156 Beiqing Road
Beijing 100095
China
Email: lizhenbin@huawei.com
Liyan Gong
China Mobile
No. 32 Xuanwumenxi Ave., Xicheng District
Beijing
China
Email: gongliyan@chinamobile.com
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Guangming Yang
China Telecom
No.109 West Zhongshan Ave., Tianhe District
Guangzhou
China
Email: yangguangm@chinatelecom.cn
James N Guichard
Futurewei Technologies
2330 Central Express Way
Santa Clara
USA
Email: james.n.guichard@futurewei.com
Gyan Mishra
Verizon Inc.
Email: gyan.s.mishra@verizon.com
Fengwei Qin
China Mobile
No. 32 Xuanwumenxi Ave., Xicheng District
Beijing
China
Email: qinfengwei@chinamobile.com
Dong, et al. Expires January 12, 2022 [Page 15]