Internet DRAFT - draft-bryant-rtgwg-enhanced-vpn
draft-bryant-rtgwg-enhanced-vpn
Routing Area Working Group S. Bryant
Internet-Draft J. Dong
Intended status: Informational Huawei
Expires: September 6, 2018 Z. Li
China Mobile
T. Miyasaka
KDDI Corporation
March 05, 2018
Enhanced Virtual Private Networks (VPN+)
draft-bryant-rtgwg-enhanced-vpn-02
Abstract
This draft describes a number of enhancements that need to be made to
virtual private networks (VPNs) to support the needs of new
applications, particularly applications that are associated with 5G
services. A network enhanced with these properties may form the
underpin of network slicing, but will also be of use in its own
right.
Status of This Memo
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This Internet-Draft will expire on September 6, 2018.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
3. Overview of the Requirements . . . . . . . . . . . . . . . . 4
3.1. Isolation between Virtual Networks . . . . . . . . . . . 4
3.2. Diverse Performance Guarantees . . . . . . . . . . . . . 6
3.3. A Pragmatic Approach to Isolation . . . . . . . . . . . . 7
3.4. Integration . . . . . . . . . . . . . . . . . . . . . . . 8
3.5. Dynamic Configuration . . . . . . . . . . . . . . . . . . 8
3.6. Customized Control Plane . . . . . . . . . . . . . . . . 9
4. Architecture and Components of VPN+ . . . . . . . . . . . . . 9
4.1. Communications Layering . . . . . . . . . . . . . . . . . 9
4.2. Multi-Point to Multi-point . . . . . . . . . . . . . . . 10
4.3. Candidate Underlay Technologies . . . . . . . . . . . . . 10
4.3.1. FlexE . . . . . . . . . . . . . . . . . . . . . . . . 11
4.3.2. Dedicated Queues . . . . . . . . . . . . . . . . . . 12
4.3.3. Time Sensitive Networking . . . . . . . . . . . . . . 12
4.3.4. Deterministic Networking . . . . . . . . . . . . . . 12
4.3.5. MPLS Traffic Engineering (MPLS-TE) . . . . . . . . . 13
4.3.6. Segment Routing . . . . . . . . . . . . . . . . . . . 13
4.4. Control Plane Considerations . . . . . . . . . . . . . . 16
4.5. Application Specific Network Types . . . . . . . . . . . 17
4.6. Integration with Service Functions . . . . . . . . . . . 17
5. Scalability Considerations . . . . . . . . . . . . . . . . . 17
5.1. Maximum Stack Depth . . . . . . . . . . . . . . . . . . . 18
5.2. RSVP scalability . . . . . . . . . . . . . . . . . . . . 18
6. OAM and Instrumentation . . . . . . . . . . . . . . . . . . . 19
7. Enhanced Resiliency . . . . . . . . . . . . . . . . . . . . . 19
8. Security Considerations . . . . . . . . . . . . . . . . . . . 20
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
10.1. Normative References . . . . . . . . . . . . . . . . . . 21
10.2. Informative References . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
Virtual networks, often referred to as virtual private networks
(VPNs) have served the industry well as a means of providing
different groups of users with logically isolated access to a common
network. The common or base network that is used to provide the VPNs
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is often referred to as the underlay, and the VPN is often called an
overlay.
Driven largely by needs surfacing from 5G, the concept of network
slicing has gained traction. There is a need to create a VPN with
enhanced characteristics. Specifically there is a need for a
transport network supporting a set of virtual networks each of which
provides the client with dedicated (private) networking, computing
and storage resources drawn from a shared pool.
The tenant of such a network can require a degree of isolation and
performance that previously could only be satisfied by dedicated
networks. Additionally the tenant may ask for some level of control
of their virtual network e.g. to customize the service paths in the
network slice.
These properties cannot be met with pure overlay networks, as they
require tighter coordination and integration between the underlay and
the overlay network. This document introduces a new network service
called enhanced VPN (VPN+). VPN+ refers to a virtual network which
has dedicated network resources allocated from the underlay network.
Unlike traditional VPN, an enhanced VPN can achieve greater isolation
and guaranteed performance.
These new network layer properties, which have general applicability,
may also be of interest as part of a network slicing solution.
This document specifies a framework for using the existing, modified
and potential new networking technologies as components to provide an
enhanced VPN (VPN+) service. Specifically we are concerned with:
o The design of the enhanced VPN data-plane
o The necessary protocols in both, underlay and the overlay of
enhanced VPN, and
o The mechanisms to achieve integration between overlay and underlay
o The necessary method of monitoring an enhanced VPN
o The methods of instrumenting an enhanced VPN to ensure that the
required tenant Service Level Agreement (SLA) is maintained
The required layer structure necessary to achieve this is shown in
Section 4.1.
One use for enhanced VPNs is to create network slices with different
isolation requirements. Such slices may be used to provide different
tenants of vertical industrial markets with their own virtual network
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with the explicit characteristics required. These slices may be
"hard" slices providing a high degree of confidence that the VPN+
characteristics will be maintained over the slice life cycle, of they
may be "soft" slices in which case some degree of interaction may be
experienced.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
3. Overview of the Requirements
In this section we provide an overview of the requirements of an
enhanced VPN.
3.1. Isolation between Virtual Networks
The requirement is to provide both hard and soft isolation between
the tenants/applications using one enhanced VPN and the tenants/
applications using another enhanced VPN. Hard isolation is needed so
that applications with exacting requirements can function correctly
despite a flash demand being created on another VPN competing for the
underlying resources. An example might be a network supporting both
emergency services and public broadband multi-media services.
During a major incident the VPNs supporting these services would both
be expected to experience high data volumes, and it is important that
both make progress in the transmission of their data. In these
circumstances the VPNs would require an appropriate degree of
isolation to be able to continue to operate acceptably.
We introduce the terms hard (static) and soft (dynamic) isolation to
cover cases such as the above. A VPN has soft isolation if the
traffic of one VPN cannot be inspected by the traffic of another.
Both IP and MPLS VPNs are examples of soft isolated VPNs because the
network delivers the traffic only to the required VPN endpoints.
However the traffic from one or more VPNs and regular network traffic
may congest the network resulting in delays for other VPNs operating
normally. The ability for a VPN to be sheltered from this effect is
called hard isolation, and this property is required by some critical
applications. Although these isolation requirements are triggered by
the needs of 5G networks, they have general utility. In the
remainder of this section we explore how isolation may be achieved in
packet networks.
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It is of course possible to achieve high degrees of isolation in the
optical layer. However this is done at the cost of allocating
resources on a long term basis and end-to-end basis. Such an
arrangement means that the full cost of the resources must be borne
by the service that is allocated the resources. On the other hand,
isolation at the packet layer allows the resources to be shared
amongst many services and only dedicated to a service on a temporary
basis. This allows greater statistical multiplexing of network
resources and amortizes the cost over many services, leading to
better economy. However, the degree of isolation required by network
slicing cannot easily be met with MPLS-TE packet LSPs as they
guarantee long-term bandwidth, but not latency.
Thus some trade-off between the two approaches needs to be considered
to provide the required isolation between virtual networks while
still allows reasonable sharing inside each VPN.
The work of the IEEE project on Time Sensitive Networking is
introducing the concept of packet scheduling where a high priority
packet stream may be given a scheduled time slot thereby guaranteeing
that it experiences no queuing delay and hence a reduced latency.
However where no scheduled packet arrives its reserved time-slot is
handed over to best effort traffic, thereby improving the economics
of the network. Such a scheduling mechanism may be usable directly,
or with extension to achieve isolation between multiple VPNs.
One of the key areas in which isolation needs to be provided is at
the interfaces. If nothing is done the system falls back to the
router queuing system in which the ingress places it on a selected
output queue. Modern routers have quite sophisticated output queuing
systems, traditionally these have not provided the type of scheduling
system needed to support the levels of isolation needed for the
applications that are the target of VPN+ networks. However some of
the more modern approaches to queuing allow the construction of
logical virtual channelized sub-interfaces (VCSI). With VCSIs there
is only one physical interface, and routing sees a single adjacency,
but the queuing system is used to provide virtual interfaces at
various priorities. Sophisticated queuing systems of this type may
be used to provide end-to-end virtual isolation between tenant's
traffic in an otherwise homogeneous network.
[FLEXE] provides the ability to multiplex multiple channels over an
Ethernet link in a way that provides hard isolation. However it is a
only a link technology. When packets are received by the downstream
node they need to be processed in a way that preserves that
isolation. This in turn requires a queuing and forwarding
implementation that preserves the isolation, such as a sliced
hardware system, or an LVI system of the type described above.
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3.2. Diverse Performance Guarantees
There are several aspects to guaranteed performance, guaranteed
maximum packet loss, guaranteed maximum delay and guaranteed delay
variation.
Guaranteed maximum packet loss is a common parameter, and is usually
addressed by setting the packet priorities, queue size and discard
policy. However this becomes more difficult when the requirement is
combine with the latency requirement. The limiting case is zero
congestion loss, and than is the goal of the Deterministic Networking
work that the IETF and IEEE are pursuing. In modern optical networks
loss due to transmission errors is already asymptotic to zero due,
but there is always the possibility of failure of the interface and
the fiber itself. This can only be addressed by some form of packet
duplication and transmission over diverse paths.
Guaranteed maximum latency is required in a number of applications
particularly real-time control applications and some types of virtual
reality applications. The work of the IETF Deterministic Networking
(DetNet) Working Group is relevant, however the scope needs to be
extended to methods of enhancing the underlay to better support the
delay guarantee, and to integrate these enhancements with the overall
service provision.
Guaranteed maximum delay variation is a service that may also be
needed. Time transfer is one example of a service that needs this,
although the fungible nature of time means that it might be delivered
by the underlay as a shared service and not provided through
different virtual networks. Alternatively a dedicated virtual
network may be used to provide this as a shared service. The need
for guaranteed maximum delay variation as a general requirement is
for further study.
This leads to the concept that there is a spectrum of grades of
service guarantee that need to be considered when deploying and
enhanced VPN. As a guide to understanding the design requirements we
can consider four types:
o Guaranteed latency,
o Enhanced delivery
o Assured bandwidth,
o Best effort
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In Section 3.1 we considered the work of the IEEE Time Sensitive
Networking (TSN) project and the work of the IETF DetNet Working
group in the context of isolation. However this work is of greater
relevance in assuring end-to-end packet latency. It is also of
importance in considering enhanced delivery.
A service that is guaranteed latency has a latency upper bound
provided by the network. It is important to note that assuring the
upper bound is more important than achieving the minimum latency.
A service that is offered enhanced delivery is one in which the
network (at layer 3) attempts to deliver the packet through multiple
paths in the hope of avoiding transient congestion
[I-D.ietf-detnet-dp-sol].
A useful mechanism to provide these guarantees is to use Flex
Ethernet [FLEXE] as the underlay. This is a method of bonding
Ethernets together and of providing time-slot based channelization
over an Ethernet bearer. Such channels are fully isolated from other
channels running over the same Ethernet bearer. As noted elsewhere
this produces hard isolation but at the cost of making the
reclamation of unused bandwidth harder.
These approaches can usefully be used in tandem. It is possible to
use FlexE to provide tenant isolation, and then to use the TSN
approach over FlexE to provide service performance guarantee inside
the a slice/tenant VPN.
3.3. A Pragmatic Approach to Isolation
A key question to consider is whether whether it is possible to
achieve hard isolation in packet networks? Packet networks were
never designed to support hard isolation, just the opposite, they
were designed to provide a high degree of statistical multiplexing
and hence a significant economic advantage when compared to a
dedicated, or a Time Division Multiplexing (TDM) network. However
the key thing to bear in mind is that the concept of hard isolation
needs to be viewed from the perspective of the application, and there
is no need to provide any harder isolation than is required by the
application. From a historical perspective it is good to think about
pseudowires [RFC3985] which emulate services that in many would have
had hard isolation in their native form. However experience has
shown that in most cases an approximation to this requirement is
sufficient for most uses.
Thus, for example, using FlexE or channelized sub-interface,together
with packet scheduling as interface slicing, and optionally, also
together with the slicing of node resources (Network Processor Unit
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(NPU), etc.), it may be possible to provide a type of hard isolation
that is adequate for many applications. Other applications may be
satisfied with a classical VPN and reserved bandwidth, but yet others
may require dedicated point to point fiber. The requirement is thus
to qualify the needs of each application and provide an economic
solution that satisfies those needs without over-engineering.
3.4. Integration
A solution to the enhanced VPN problem will need to provide seamless
integration of both Overlay VPN and the underlay network resources.
This needs be done in a flexible and scalable way so that it can be
widely deployed in operator networks. Given the targeting of both
this technology and service function chaining at mobile networks and
in particular 5G the co-integration of service functions is a likely
requirement.
3.5. Dynamic Configuration
It is necessary that new enhanced VPNs can be introduced to the
network, modified, and removed from the network according to service
demand. In doing so due regard must be given to the impact of other
enhanced VPNs that are operational. An enhanced VPN that requires
hard isolation must not be disrupted by the installation or
modification of another enhanced VPN.
Whether modification of an enhanced VPN can be disruptive to that
VPN, and in particular the traffic in flight is to be determined, but
is likely to be a difficult problem to address.
The data-plane aspect of this are discussed further in Section 4.3.
The control-plane and management-plane aspects of this, particularly
the garbage collection are likely to be challenging and are for
further study.
As well as managing dynamic changes to the VPN in a seamless way,
dynamic changes to the underlay and its transport network need to be
managed in order to avoid disruption to sensitive services.
In addition to non-disruptively managing the network as a result of
gross change such as the inclusion of a new VPN endpoint or a change
to a link, consideration has to be given to the need to move VPN
traffic as a result of traffic volume changes.
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3.6. Customized Control Plane
In some cases it is desirable that an enhanced VPN has a custom
control-plane, so that the tenant of the enhanced VPN can have some
control to the resources and functions partitioned for this VPN.
Each enhanced VPN may have its own dedicated controller, it may be
provided with an interface to a control-plane that is shared with a
set of other tenants, or it may be provided with an interface to the
control-plane of the underlay provided by the underlay network
operator.
Further detail on this requirement will be provided in a future
version of the draft.
4. Architecture and Components of VPN+
Normally a number of enhanced VPN services will be provided by a
common network infrastructure. Each enhanced VPN consists of both
the overlay and a specific set of dedicated network resources and
functions allocated in the underlay to satisfy the needs of the VPN
tenant. The integration between overlay and underlay ensures the
isolation and between different enhanced VPNs, and facilitates the
guaranteed performance for different services.
An enhanced VPN needs to be designed with consideration given to:
o Isolation of enhanced VPN data plane.
o A scalable control plane to match the data plane isolation.
o The amount of state in the packet vs the amount of state in the
control plane.
o Mechanism for diverse performance guarantee within an enhanced VPN
o Support of the required integration between network functions and
service functions.
4.1. Communications Layering
The communications layering model use to build an enhanced VPN is
shown in Figure 1.
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Tenant Tenant connection Tenant
CE1 ----------------------------------------CE2
\ /
AC \ OP Provider VPN OP /AC
+- PE1------------------------------PE1 -+
Enhanced Path
==============================
Underlay
++++++++++++++++++++++++++++++
Figure 1: Communication Layering
The network operator is required to provide a tenant connection
between the tenant's Customer Equipment (CE) (CE1 and CE2). These
CEs attach to the Operator's Provider Edge Equipments (PE) (PE1 and
PE2 respectively). The attachment circuits (AC) are outside the
scope of this document other than to note that they obviously need to
provide a connection of sufficient quality in terms of isolation,
latency etc so as to satisfy the needs of the user. The subtlety to
be aware of is that the ACs are often provided by a network rather
than a fixed point to point connection and thus the considerations in
this document may apply to the network that provides the AC.
A provider VPN is constructed between PE1 and PE2 to carry tenant
traffic. This is a normal VPN, and provides one stage of isolation
between tenants.
An enhanced path is constructed to carry the provider VPN using
dedicated resources drawn from the underlay.
4.2. Multi-Point to Multi-point
At a VPN level connections are frequently multi-point-to-multi-point
(MP2MP). As far as such services are concerned the underlay is also
an abstract MP2MP medium. However when service guarantees are
provided, such as with an enhanced VPN, each point to point path
through the underlay needs to be specifically engineered to meet the
required performance guarantees.
4.3. Candidate Underlay Technologies
A VPN is a network created by applying a multiplexing technique to
the underlying network (the underlay) in order to distinguish the
traffic of one VPN from that of another. A VPN path that travels by
other than the shortest path through the underlay normally requires
state in the underlay to specify that path. State is normally
applied to the underlay through the use of the RSVP Signaling
protocol, or directly through the use of an SDN controller, although
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other techniques may emerge as this problem is studied. This state
gets harder to manage as the number of VPN paths increases.
Furthermore, as we increase the coupling between the underlay and the
overlay to support the VPN which requires enhanced VPN service, this
state will increase further.
In an enhanced VPN different subsets of the underlay resources are
dedicated to different VPNs. Any enhanced VPN solution thus needs
tighter coupling with underlay than is the case with classical VPNs.
We cannot for example share the tunnel between enhanced VPNs which
require hard isolation.
In the following sections we consider a number of candidate underlay
solutions for proving the required VPN separation.
o FlexE
o Time Sensitive Networking
o Deterministic Networking
o Dedicated Queues
We then consider the problem of slice differentiation and resource
representation. Candidate technologies are:
o MPLS
o MPLS-SR
o Segment Routing over IPv6 (SRv6)
4.3.1. FlexE
FlexE [FLEXE] is a method of creating a point-to-point Ethernet with
a specific fixed bandwidth. FlexE supports the bonding of multiple
links, which supports creating larger links out of multiple slower
links in a more efficient way that traditional link aggregation.
FlexE also supports the sub-rating of links, which allows an operator
to only use a portion of a link. FlexE also supports the
channelization of links, which allows one link to carry several
lower-speed or sub-rated links from different sources.
If different FlexE channels are used for different services, then no
sharing is possible between the services. This in turn means that it
is not possible to dynamically re-distribute unused bandwidth to
lower priority services increasing the cost of operation of the
network. FlexE can on the other hand be used to provide hard
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isolation between different tenants by providing hard isolation on an
interface. The tenant can then use other methods to manage the
relative priority of their own traffic.
Methods of dynamically re-sizing FlexE channels and the implication
for enhanced VPN are under study.
4.3.2. Dedicated Queues
In an enhanced VPN providing multiple isolated virtual networks the
conventional Diff-Serv based queuing system is insufficient for our
purposes due to the limited number of queues which cannot
differentiate between traffic of different VPNs and the range of
service classes that each need to provide their tenants. This
problem is particularly acute with an MPLS underlay due to the small
number of traffic class services available. In order to address this
problem and thus reduce the interference between VPNs, it is likely
to be necessary to steer traffic of VPNs to dedicated input and
output queues.
4.3.3. Time Sensitive Networking
Time Sensitive Networking (TSN) is an IEEE project that is designing
a method of carrying time sensitive information over Ethernet. As
Ethernet this can obviously be tunneled over a Layer 3 network in a
pseudowire. However the TSN payload would be opaque to the underlay
and thus not treated specifically as time sensitive data. The
preferred method of carrying TSN over a layer 3 network is through
the use of deterministic networking as explained in the following
section of this document.
The machanisms defined in TSN can be used to meet the requirements of
time sensitive services of an enhanced VPN.
4.3.4. Deterministic Networking
Deterministic Networking (DetNet) [I-D.ietf-detnet-architecture] is a
technique being developed in the IETF to enhance the ability of layer
3 networks to deliver packets more reliably and with greater control
over the delay. The design cannot use classical re-transmission
techniques such as TCP since can add delay that is above the maximum
tolerated by the applications. Even the delay improvements that are
achieved with SCTP-PR are outside the bounds set by application
demands. The approach is to pre-emptively send copies of the packet
over various paths in the expectation that this minimizes the chance
of all packets being lost, but to trim duplicate packets to prevent
excessive flooding of the network and to prevent multiple packets
being delivered to the destination. It also seeks to set an upper
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bound on latency. Note that it is not the goal to minimize latency,
and the optimum upper bound paths may not be the minimum latency
paths.
DetNet is based on flows. It currently makes no comment on the
underlay, and so at this stage must be assumed to use the base
topology. To be of use in this application DetNet there needs to be
a description of how to deal with the concept of flows within an
enhanced VPN.
How we use DetNet in a multi-tenant (VPN) network, and how to improve
the scalability of DetNet in a multi-tenant (VPN) network is for
further study.
4.3.5. MPLS Traffic Engineering (MPLS-TE)
Normal MPLS runs on the base topology and has the concepts of
reserving end to end bandwidth for an LSP, and of creating VPNs. VPN
traffic can be run over RSVP-TE tunnels to provide reserved bandwidth
for a specific VPN connection. This is rarely deployed in practice
due to scaling and management overhead concerns.
4.3.6. Segment Routing
Segment Routing [I-D.ietf-spring-segment-routing] is a method that
prepends instructions to packets at entry and sometimes at various
points as it passes though the network. These instructions allow
packets to be routed on paths other than the shortest path for
various traffic engineering reasons. These paths can be strict or
loose paths, depending on the compactness required of the instruction
list and the degree of autonomy granted to the network (for example
to support ECMP).
With SR, a path needs to be dynamically created through a set of
resources by simply specifying the Segment IDs (SIDs), i.e.
instructions rooted at a particular point in the network. Thus if a
path is to be provisioned from some ingress point A to some egress
point B in the underlay, A is provided with the A..B SID list and
instructions on how to identify the packets to which the SID list is
to be prepended.
By encoding the state in the packet, as is done in Segment Routing,
state is transitioned out of the network.
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A-------B-----E
| | |
| | |
C-------D-----+
Figure 2: An SR Network Fragment
Consider the network fragment shown in Figure 2. To send a packet
from A to E via B, D & E: Node A prepends the ordered list of SIDs:D,
E to the packet and pushes the packet to B. SID list {B, D, E} can
be used as a VPN path. Thus, to create a VPN, a set of SID Lists is
created and provided to each ingress node of the VPN together with
packet selection criteria. In this way it is possible to create a
VPN with no state in the core. However this is at the expense of
creating a larger packet with possible MTU and hardware restriction
limits that need to be overcome.
Note in the above if A and E support multiple VPN an additional VPN
identifier will need to be added to the packet, but this is omitted
from this text for simplicity.
A---P---B---S---E
| | |
| Q |
| | |
C---R---D-------+
Figure 3: Another SR Network Fragment
Consider a further network fragment shown in Figure 3, and further
consider VPN A+D+E.
A has lists: {P, B, Q, D}, {P, B, S, E}
D has lists: {Q, B, P, A}, {E}
E has lists: {S, B, P, A}, {D}
To create a new VPN C+D+B the following list are introduced:
C lists: {R, D}, {A, P, B}
D lists: {R, C}, {Q, B}
B lists: {Q, D}, {P, A, C}
Thus VPN C+D+B was created without touching the settings of the core
routers, indeed it is possible to add endpoints to the VPNs, and move
the paths around simply by providing new lists to the affected
endpoints.
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There are a number of limitations in SR as it is currently defined
that limit its applicability to enhanced VPNs:
o Segments are shared between different VPNs,
o There is no reservation of bandwidth,
o There is limited differentiation in the data plane.
Thus some extensions to SR are needed to provide isolation between
different enhanced VPNs. This can be achieved by including a finer
granularity of state in the core in anticipation of its future use by
authorized services. We therefore need to evaluate the balance
between this additional state and the performance delivered by the
network.
Both MPLS Segment Routing and SRv6 Segment Routing are candidate
technologies for enhanced VPN.
With current segment routing, the instructions are used to specify
the nodes and links to be traversed. However, in order to achieve
the required isolation between different services, new instructions
can be created which can be prepended to a packet to steer it through
specific dedicated network resources and functions, e.g. links,
queues, processors, services etc.
Clearly we can use traditional constructs to create a VPN, but there
are advantages to the use of other constructs such as Segment Routing
(SR) in the creation of virtual networks with enhanced properties.
Traditionally a traffic engineered path operates with a granularity
of a link with hints about priority provided through the use of the
traffic class field in the header. However to achieve the latency
and isolation characteristics that are sought by VPN+ users, steering
packets through specific queues resources will likely be required.
The extent to which these needs can be satisfied through existing QoS
mechanisms is to be determined. What is clear is that a fine control
of which services wait for which, with a fine granularity of queue
management policy is needed. Note that the concept of a queue is a
useful abstraction for many types of underlay mechanism that may be
used to provide enhanced latency support. From the perspective of
the control plane and from the perspective of the segment routing the
method of steering a packet to a queue that provides the required
properties is a universal construct. How the queue satisfies the
requirement is outside the scope of these aspect of the enhanced VPN
system. Thus for example a FlexE channel, or time sensitive
networking packet scheduling slot are abstracted to the same concept
and bound to the data plane in a common manner.
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We can introduce the specification of finer, deterministic,
granularity to path selection through extensions to traditional path
construction techniques such as RSVP-TE and MPLS-TP.
We can also introduce it by specifying the queue through an SR
instruction list. Thus new SR instructions may be created to specify
not only which resources are traversed, but in some cases how they
are traversed. For example, it may be possible to specify not only
the queue to be used but the policy to be applied when enqueuing and
dequeuing.
This concept can be further generalized, since as well as queuing to
the output port of a router, it is possible to queue to any resource,
for example:
o A network processor unit (NPU)
o A Central Processing Unit (CPU) Core
o A Look-up engine such as TCAMs
4.4. Control Plane Considerations
It is expected that VPN+ would be based on a hybrid control
mechanism, which takes advantage of the logically centralized
controller for on-demand provisioning and global optimization, whilst
still relies on distributed control plane to provide scalability,
high reliability, fast reaction, automatic failure recovery etc.
Extension and optimization to the distributed control plane is needed
to support the enhanced properties of VPN+.
Where SR is used as a the data-plane construct it needs to be noted
that it does not have the capability of reserving resources along the
path nor do its currently specified distributed control plane (the
link state routing protocols). An SDN controller can clearly do
this, from the controllers point of view, and no resource reservation
is done on the device. Thus if a distributed control plane is needed
either in place of an SDN controller or as an assistant to it, the
design of the control system needs to ensure that resources are
uniquely allocated to the correct service, and no allocated to
multiple services casing unintended resource conflict. This needs
further study.
On the other hand an advantage of using an SR approach is that it
provides a way of efficiently binding the network underlay and the
enhanced VPN overlay. With a technology such as RSVP-TE LSPs, each
virtual path in the VPN is bound to the underlay with a dedicated TE-
LSP.
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RSVP-TE could be enhanced to bind the VPN to specific resources
within the underlay, but as noted elsewhere in this document there
are concerns as to the scalability of this approach. With an SR-
based approach to resource reservation (per-slice reservation), it is
straightforward to create dedicated SR network slices, and the VPN
can be bound to a particular SR network slice.
4.5. Application Specific Network Types
Although a lot of the traffic that will be carried over the enhanced
VPN will likely be IPv4 or IPv6, the design has to be capable of
carrying other traffic types. In particular the design SHOULD be
capable of carrying Ethernet traffic. This is easily accomplished
through the various pseudowire (PW) techniques [RFC3985]. Where the
underlay is MPLS Ethernet can be carried over the enhanced VPN
encapsulated according to the method specified in [RFC4448]. Where
the underlay is IP Layer Two Tunneling Protocol - Version 3 (L2TPv3)
[RFC3931] can be used with Ethernet traffic carried according to
[RFC4719]. Encapsulations have been defined for most of the common
layer two type for both PW over MPLS and for L2TPv3.
4.6. Integration with Service Functions
There is a significant overlap between the problem of routing a
packet though a set of network resources and the problem of routing a
packet through a set of compute resources. Service Function Chain
technology is designed to forward a packet through a set of compute
resources.
A future version of this document will discuss this further.
5. Scalability Considerations
For a packet to transit a network, other than on a best effort,
shortest path basis, it is necessary to introduce additional state,
either in the packet, or in the network of some combination of both.
There are at least three ways of doing this:
o Introduce the complete state into the packet. That is how SR does
this, and this allows the controller to specify the precise series
of forwarding and processing instructions that will happen to the
packet as it transits the network. The cost of this is an
increase in the packet header size. The cost is also that systems
will have capabilities enabled in case they are called upon by a
service. This is a type of latent state, and increases as we more
precisely specify the path and resources that need to be
exclusively available to a VPN.
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o Introduce the state to the network. This is normally done by
creating a path using RSVP-TE, which can be extended to introduce
any element that needs to be specified along the path, for example
explicitly specifying queuing policy. It is of course possible to
use other methods to introduce path state, such as via a Software
Defined Network (SDN) controller, or possibly by modifying a
routing protocol. With this approach there is state per path per
path characteristic that needs to be maintained over its life-
cycle. This is more state than is needed using SR, but the packet
are shorter.
o Provide a hybrid approach based on using binding SIDs to create
path fragments, and bind them together with SR.
Dynamic creation of a VPN path using SR requires less state
maintenance in the network core at the expense of larger VPN headers
on the packet. The scaling properties will reduce roughly from a
function of (N/2)^2 to a function of N, where N is the VPN path
length in intervention points (hops plus network functions).
Reducing the state in the network is important to VPN+, as VPN+
requires the overlay to be more closely integrated with the underlay
than with traditional VPNs. This tighter coupling would normally
mean that significant state needed to be created and maintained in
the core. However, a segment routed approach allows much of this
state to be spread amongst the network ingress nodes, and transiently
carried in the packets as SIDs.
These approaches are for further study.
5.1. Maximum Stack Depth
One of the challenges with SR is the stack depth that nodes are able
to impose on packets. This leads to a difficult balance between
adding state to the network and minimizing stack depth, or minimizing
state and increasing the stack depth.
5.2. RSVP scalability
The traditional method of creating a resource allocated path through
an MPLS network is to use the RSVP protocol. However there have been
concerns that this requires significant continuous state maintenance
in the network. There are ongoing works to improve the scalability
of RSVP-TE LSPs in the control plane
[I-D.ietf-teas-rsvp-te-scaling-rec]. This will be considered further
in a future version of this document.
There is also concern at the scalability of the forwarder footprint
of RSVP as the number of paths through an LSR grows
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[I-D.sitaraman-mpls-rsvp-shared-labels] proposes to address this by
employing SR within a tunnel established by RSVP-TE. This work will
be considered in a future version of this document.
6. OAM and Instrumentation
A study of OAM in SR networks has been documented in
[I-D.ietf-spring-oam-usecase].
The enhanced VPN OAM design needs to consider the following
requirements:
o Instrumentation of the underlay so that the network operator can
be sure that the resources committed to a tenant are operating
correctly and delivering the required performance.
o Instrumentation of the overlay by the tenant. This is likely to
be transparent to the network operator and to use existing
methods. Particular consideration needs to be given to the need
to verify the isolation and the various committed performance
characteristics.
o Instrumentation of the overlay by the network provider to
proactively demonstrate that the committed performance is being
delivered. This needs to be done in a non-intrusive manner,
particularly when the tenant is deploying a performance sensitive
application
o Verification of the conformity of the path to the service
requirement. This may need to be done as part of a commissioning
test.
These issues will be discussed in a future version of this document.
7. Enhanced Resiliency
Each enhanced VPN, of necessity, has a life-cycle, and needs
modification during deployment as the needs of its user change.
Additionally as the network as a whole evolves there will need to be
garbage collection performed to consolidate resources into usable
quanta.
Systems in which the path is imposed such as SR, or some form of
explicit routing tend to do well in these applications because it is
possible to perform an atomic transition from one path to another.
However implementations and the monitoring protocols need to make
sure that the new path is up before traffic is transitioned to it.
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There are however two manifestations of the latency problem that are
for further study in any of these approaches:
o The problem of packets overtaking one and other if a path latency
reduces during a transition.
o The problem of the latency transient in either direction as a path
migrates.
There is also the matter of what happens during failure in the
underlay infrastructure. Fast reroute is one approach, but that
still produces a transient loss with a normal goal of rectifying this
within 50ms. An alternative is some form of N+1 delivery such as has
been used for many years to support protection from service
disruption. This may be taken to a different level using the
techniques proposed by the IETF deterministic network work with
multiple in-network replication and the culling of later packets.
In addition to the approach used to protect high priority packets,
consideration has to be given to the impact of best effort traffic on
the high priority packets during a transient. Specifically if a
conventional re-convergence process is used there will inevitably be
micro-loops and whilst some form of explicit routing will protect the
high priority traffic, lower priority traffic on best effort shortest
paths will micro-loop without the use of a loop prevention
technology. To provide the highest quality of service to high
priority traffic, either this traffic must be shielded from the
micro-loops, or micro-loops must be prevented.
8. Security Considerations
All types of virtual network require special consideration to be
given to the isolation between the tenants. However in an enhanced
virtual network service hard isolation needs to be considered. If a
service requires a specific latency then it can be damaged by simply
delaying the packet through the activities of another tenant. In a
network with virtual functions, depriving a function used by another
tenant of compute resources can be just as damaging as delaying
transmission of a packet in the network.
9. IANA Considerations
There are no requested IANA actions.
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10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
10.2. Informative References
[FLEXE] "Flex Ethernet Implementation Agreement", March 2016,
<http://www.oiforum.com/wp-content/uploads/
OIF-FLEXE-01.0.pdf>.
[I-D.ietf-detnet-architecture]
Finn, N., Thubert, P., Varga, B., and J. Farkas,
"Deterministic Networking Architecture", draft-ietf-
detnet-architecture-04 (work in progress), October 2017.
[I-D.ietf-detnet-dp-sol]
Korhonen, J., Andersson, L., Jiang, Y., Finn, N., Varga,
B., Farkas, J., Bernardos, C., Mizrahi, T., and L. Berger,
"DetNet Data Plane Encapsulation", draft-ietf-detnet-dp-
sol-01 (work in progress), January 2018.
[I-D.ietf-spring-oam-usecase]
Geib, R., Filsfils, C., Pignataro, C., and N. Kumar, "A
Scalable and Topology-Aware MPLS Dataplane Monitoring
System", draft-ietf-spring-oam-usecase-10 (work in
progress), December 2017.
[I-D.ietf-spring-segment-routing]
Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing
Architecture", draft-ietf-spring-segment-routing-15 (work
in progress), January 2018.
[I-D.ietf-teas-rsvp-te-scaling-rec]
Beeram, V., Minei, I., Shakir, R., Pacella, D., and T.
Saad, "Techniques to Improve the Scalability of RSVP
Traffic Engineering Deployments", draft-ietf-teas-rsvp-te-
scaling-rec-09 (work in progress), February 2018.
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[I-D.sitaraman-mpls-rsvp-shared-labels]
Sitaraman, H., Beeram, V., Parikh, T., and T. Saad,
"Signaling RSVP-TE tunnels on a shared MPLS forwarding
plane", draft-sitaraman-mpls-rsvp-shared-labels-03 (work
in progress), December 2017.
[NETCALC] "Applicability of Network Calculus to DetNet", November
2017, <https://datatracker.ietf.org/meeting/100/materials/
slides-100-detnet-applicability-of-network-calculus-to-
detnet>.
[RFC3931] Lau, J., Ed., Townsley, M., Ed., and I. Goyret, Ed.,
"Layer Two Tunneling Protocol - Version 3 (L2TPv3)",
RFC 3931, DOI 10.17487/RFC3931, March 2005,
<https://www.rfc-editor.org/info/rfc3931>.
[RFC3985] Bryant, S., Ed. and P. Pate, Ed., "Pseudo Wire Emulation
Edge-to-Edge (PWE3) Architecture", RFC 3985,
DOI 10.17487/RFC3985, March 2005,
<https://www.rfc-editor.org/info/rfc3985>.
[RFC4448] Martini, L., Ed., Rosen, E., El-Aawar, N., and G. Heron,
"Encapsulation Methods for Transport of Ethernet over MPLS
Networks", RFC 4448, DOI 10.17487/RFC4448, April 2006,
<https://www.rfc-editor.org/info/rfc4448>.
[RFC4719] Aggarwal, R., Ed., Townsley, M., Ed., and M. Dos Santos,
Ed., "Transport of Ethernet Frames over Layer 2 Tunneling
Protocol Version 3 (L2TPv3)", RFC 4719,
DOI 10.17487/RFC4719, November 2006,
<https://www.rfc-editor.org/info/rfc4719>.
Authors' Addresses
Stewart Bryant
Huawei
Email: stewart.bryant@gmail.com
Jie Dong
Huawei
Email: jie.dong@huawei.com
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Zhenqiang Li
China Mobile
Email: lizhenqiang@chinamobile.com
Takuya Miyasaka
KDDI Corporation
Email: ta-miyasaka@kddi.com
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