TEAS Working Group T. Saad
Internet-Draft V. Beeram
Intended status: Standards Track Juniper Networks
Expires: January 12, 2022 B. Wen
Comcast
D. Ceccarelli
J. Halpern
Ericsson
S. Peng
R. Chen
ZTE Corporation
X. Liu
Volta Networks
L. Contreras
Telefonica
R. Rokui
Nokia
July 11, 2021
Realizing Network Slices in IP/MPLS Networks
draft-bestbar-teas-ns-packet-03
Abstract
Network slicing provides the ability to partition a physical network
into multiple logical networks of varying sizes, structures, and
functions so that each slice can be dedicated to specific services or
customers. Network slices need to operate in parallel while
providing slice elasticity in terms of network resource allocation.
The Differentiated Service (Diffserv) model allows for carrying
multiple services on top of a single physical network by relying on
compliant nodes to apply specific forwarding treatment (scheduling
and drop policy) on to packets that carry the respective Diffserv
code point. This document proposes a solution based on the Diffserv
model to realize network slicing in IP/MPLS networks.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 12, 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Acronyms and Abbreviations . . . . . . . . . . . . . . . 6
2. Network Resource Slicing Membership . . . . . . . . . . . . . 6
2.1. Dedicated Network Resources . . . . . . . . . . . . . . . 6
2.2. Shared Network Resources . . . . . . . . . . . . . . . . 7
3. Path Selection . . . . . . . . . . . . . . . . . . . . . . . 7
4. Slice Policy Modes . . . . . . . . . . . . . . . . . . . . . 8
4.1. Data plane Slice Policy Mode . . . . . . . . . . . . . . 8
4.2. Control Plane Slice Policy Mode . . . . . . . . . . . . . 9
4.3. Data and Control Plane Slice Policy Mode . . . . . . . . 11
5. Slice Policy Instantiation . . . . . . . . . . . . . . . . . 12
5.1. Slice Policy Definition . . . . . . . . . . . . . . . . . 13
5.1.1. Slice Policy Data Plane Selector . . . . . . . . . . 14
5.1.2. Slice Policy Resource Reservation . . . . . . . . . . 17
5.1.3. Slice Policy Per Hop Behavior . . . . . . . . . . . . 18
5.1.4. Slice Policy Topology . . . . . . . . . . . . . . . . 19
5.2. Slice Policy Boundary . . . . . . . . . . . . . . . . . . 19
5.2.1. Slice Policy Edge Nodes . . . . . . . . . . . . . . . 19
5.2.2. Slice Policy Interior Nodes . . . . . . . . . . . . . 20
5.2.3. Slice Policy Incapable Nodes . . . . . . . . . . . . 20
5.2.4. Combining Slice Policy Modes . . . . . . . . . . . . 21
5.3. Mapping Traffic on Slice Aggregates . . . . . . . . . . . 22
6. Control Plane Extensions . . . . . . . . . . . . . . . . . . 22
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7. Applicability to Path Control Technologies . . . . . . . . . 23
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
9. Security Considerations . . . . . . . . . . . . . . . . . . . 23
10. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 24
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 24
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
12.1. Normative References . . . . . . . . . . . . . . . . . . 24
12.2. Informative References . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
1. Introduction
Network slicing allows a Service Provider to create independent and
logical networks on top of a common or shared physical network
infrastructure. Such network slices can be offered to customers or
used internally by the Service Provider to facilitate or enhance
their service offerings. A Service Provider can also use network
slicing to structure and organize the elements of its infrastructure.
This document provides a path control technology agnostic solution
that a Service Provider can deploy to realize network slicing in IP/
MPLS networks.
The definition of network slice for use within the IETF and the
characteristics of IETF network slice are specified in
[I-D.ietf-teas-ietf-network-slice-definition]. A framework for
reusing IETF VPN and traffic-engineering technologies to realize IETF
network slices is discussed in [I-D.nsdt-teas-ns-framework]. These
documents also discuss the function of an IETF Network Slice
Controller and the requirements on its northbound and southbound
interfaces.
This document introduces the notion of a slice aggregate which
comprises of one of more IETF network slice traffic streams. It
describes how a slice policy can be used to realize a slice aggregate
by instantiating specific control and data plane behaviors on select
topological elements in IP/MPLS networks. The onus is on the IETF
Network Slice Controller to maintain the mapping between one or more
IETF network slices and a slice aggregate. The mechanisms used by
the controller to determine the mapping are outside the scope of this
document. The focus of this document is on the mechanisms required
at the device level to address the requirements of network slicing in
packet networks.
In a Differentiated Service (Diffserv) domain [RFC2475], packets
requiring the same forwarding treatment (scheduling and drop policy)
are classified and marked with a Class Selector (CS) at domain
ingress nodes. At transit nodes, the CS field inside the packet is
inspected to determine the specific forwarding treatment to be
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applied before the packet is forwarded further. Similar principles
are adopted by this document to realize network slicing.
When logical networks representing slice aggregates are realized on
top of a shared physical network infrastructure, it is important to
steer traffic on the specific network resources allocated for the
slice aggregate. In packet networks, the packets that traverse a
specific slice aggregate MAY be identified by one or more specific
fields carried within the packet. A slice policy ingress boundary
node populates the respective field(s) in packets that enter a slice
aggregate to allow interior slice policy nodes to identity those
packets and apply the specific Per Hop Behavior (PHB) that is
associated with the slice aggregate. The PHB defines the scheduling
treatment and, in some cases, the packet drop probability.
The slice aggregate traffic may further carry a Diffserv CS to allow
differentiation of forwarding treatments for packets within a slice
aggregate. For example, when using MPLS as a dataplane, it is
possible to identify packets belonging to the same slice aggregate by
carrying a global MPLS label in the label stack that identifies the
slice aggregate in each packet. Additional Diffserv classification
may be indicated in the Traffic Class (TC) bits of the global MPLS
label to allow further differentiation of forwarding treatments for
traffic traversing the same slice aggregate network resources.
This document covers different modes of slice policy and discusses
how each slice policy mode can ensure proper placement of slice
aggregate paths and respective treatment of slice aggregate traffic.
1.1. Terminology
The reader is expected to be familiar with the terminology specified
in [I-D.ietf-teas-ietf-network-slice-definition] and
[I-D.nsdt-teas-ns-framework].
The following terminology is used in the document:
IETF network slice:
a well-defined composite of a set of endpoints, the connectivity
requirements between subsets of these endpoints, and associated
requirements; the term 'network slice' in this document refers to
'IETF network slice' as defined in
[I-D.ietf-teas-ietf-network-slice-definition].
IETF Network Slice Controller (NSC):
controller that is used to realize an IETF network slice
[I-D.ietf-teas-ietf-network-slice-definition].
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Slice policy:
a policy construct that enables instantiation of mechanisms in
support of IETF network slice specific control and data plane
behaviors on select topological elements; the enforcement of a
slice policy results in the creation of a slice aggregate.
Slice aggregate:
a collection of packets that match a slice policy selection
criteria and are given the same forwarding treatment; a slice
aggregate comprises of one or more IETF network slice traffic
streams; the mapping of one or more IETF network slices to a slice
aggregate is maintained by the IETF Network Slice Controller.
Slice policy capable node:
a node that supports one of the slice policy modes described in
this document.
Slice policy incapable node:
a node that does not support any of the slice policy modes
described in this document.
Slice aggregate traffic:
traffic that is forwarded over network resources associated with a
specific slice aggregate.
Slice aggregate path:
a path that is setup over network resources associated with a
specific slice aggregate.
Slice aggregate packet:
a packet that traverses network resources associated with a
specific slice aggregate.
Slice policy topology:
a set of topological elements associated with a slice policy.
Slice aggregate aware TE:
a mechanism for TE path selection that takes into account the
available network resources associated with a specific slice
aggregate.
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
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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1.2. Acronyms and Abbreviations
BA: Behavior Aggregate
CS: Class Selector
SS: Slice Selector
S-PHB: Slice policy Per Hop Behavior as described in Section 5.1.3
SSL: Slice Selector Label as described in Section 5.1.1
SSLI: Slice Selector Label Indicator
SLA: Service Level Agreement
SLO: Service Level Objective
Diffserv: Differentiated Services
MPLS: Multiprotocol Label Switching
LSP: Label Switched Path
RSVP: Resource Reservation Protocol
TE: Traffic Engineering
SR: Segment Routing
VRF: VPN Routing and Forwarding
2. Network Resource Slicing Membership
A slice aggregate can be instantiated over parts of an IP/MPLS
network (e.g., all or specific network resources in the access,
aggregation, or core network), and can stretch across multiple
domains administered by a provider. A slice policy topology may
include all or a sub-set of the physical nodes and links of an IP/
MPLS network; it may be comprised of dedicated and/or shared network
resources (e.g., in terms of processing power, storage, and
bandwidth).
2.1. Dedicated Network Resources
Physical network resources may be fully dedicated to a specific slice
aggregate. For example, traffic belonging to a slice aggregate can
traverse dedicated network resources without being subjected to
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contention from traffic of other slice aggregates. Dedicated network
resource slicing allows for simple partitioning of the physical
network resources amongst slice aggregates without the need to
distinguish packets traversing the dedicated network resources since
only one slice aggregate traffic stream can traverse the dedicated
resource at any time.
2.2. Shared Network Resources
To optimize network utilization, sharing of the physical network
resources may be desirable. In such case, the same physical network
resource capacity is divided among multiple slice aggregates. Shared
network resources can be partitioned in the data plane (for example
by applying hardware policers and shapers) and/or partitioned in the
control plane by providing a logical representation of the physical
link that has a subset of the network resources available to it.
3. Path Selection
Path selection in a network can be network state dependent, or
network state independent as described in Section 5.1 of
[I-D.ietf-teas-rfc3272bis]. The latter is the choice commonly used
by IGPs when selecting a best path to a destination prefix, while the
former is used by ingress TE routers, or Path Computation Engines
(PCEs) when optimizing the placement of a flow based on the current
network resource utilization.
For example, when steering traffic on a delay optimized path, the IGP
can use its link state database's view of the network topology to
compute a path optimizing for the delay metric of each link in the
network resulting in a cumulative lowest delay path.
When path selection is network state dependent, the path computation
can leverage Traffic Engineering mechanisms (e.g., as defined in
[RFC2702]) to compute feasible paths taking into account the incoming
traffic demand rate and current state of network. This allows
avoiding overly utilized links, and reduces the chance of congestion
on traversed links.
To enable TE path placement, the link state is advertised with
current reservations, thereby reflecting the available bandwidth on
each link. Such link reservations may be maintained centrally on a
network wide network resource manager, or distributed on devices (as
usually done with RSVP). TE extensions exist today to allow IGPs
(e.g., [RFC3630] and [RFC5305]), and BGP-LS [RFC7752] to advertise
such link state reservations.
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When network resource reservations are also slice aggregate aware,
the link state can carry per slice aggregate state (e.g., reservable
bandwidth). This allows path computation to take into account the
specific network resources available for a slice aggregate when
determining the path for a specific flow. In this case, we refer to
the process of path placement and path provisioning as slice
aggregate aware TE.
4. Slice Policy Modes
A slice policy can be used to dictate if the partitioning of the
shared network resources amongst multiple slice aggregates can be
achieved by realizing slice aggregates in:
a) data plane only, or
b) control plane only, or
c) both control and data planes.
4.1. Data plane Slice Policy Mode
The physical network resources can be partitioned on network devices
by applying a Per Hop forwarding Behavior (PHB) onto packets that
traverse the network devices. In the Diffserv model, a Class
Selector (CS) is carried in the packet and is used by transit nodes
to apply the PHB that determines the scheduling treatment and drop
probability for packets.
When data plane slice policy mode is applied, packets need to be
forwarded on the specific slice aggregate network resources and need
to be applied a specific forwarding treatment that is dictated in the
slice policy (refer to Section 5.1 below). A Slice Selector (SS)
MUST be carried in each packet to identify the slice aggregate that
it belongs to.
The ingress node of a slice policy domain, in addition to marking
packets with a Diffserv CS, MAY also add an SS to each slice
aggregate packet. The transit nodes within a slice policy domain MAY
use the SS to associate packets with a slice aggregate and to
determine the Slice policy Per Hop Behavior (S-PHB) that is applied
to the packet (refer to Section 5.1.3 for further details). The CS
MAY be used to apply a Diffserv PHB on to the packet to allow
differentiation of traffic treatment within the same slice aggregate.
When data plane only slice policy mode is used, routers may rely on a
network state independent view of the topology to determine the best
paths to reach destinations. In this case, the best path selection
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dictates the forwarding path of packets to the destination. The SS
field carried in each packet determines the specific S-PHB treatment
along the selected path.
For example, the Segment-Routing Flexible Algorithm
[I-D.ietf-lsr-flex-algo] may be deployed in a network to steer
packets on the IGP computed lowest cumulative delay path. A slice
policy may be used to allow links along the least latency path to
share its data plane resources amongst multiple slice aggregates. In
this case, the packets that are steered on a specific slice policy
carry the SS field that enables routers (along with the Diffserv CS)
to determine the S-PHB and enforce slice aggregate traffic streams.
4.2. Control Plane Slice Policy Mode
The physical network resources in the network can be logically
partitioned by having a representation of network resources appear in
a virtual topology. The virtual topology can contain all or a subset
of the physical network resources by applying specific topology
filters on the native topology. The logical network resources that
appear in the virtual topology can reflect a part, whole, or in-
excess of the physical network resource capacity (when
oversubscription is desirable). For example, a physical link
bandwidth can be divided into fractions, each dedicated to a slice
aggregate. Each fraction of the physical link bandwidth MAY be
represented as a logical link in a virtual topology that is used when
determining paths associated with a specific slice aggregate. The
virtual topology associated with the slice policy can be used by
routing protocols, or by the ingress/PCE when computing slice
aggregate aware TE paths.
To perform network state dependent path computation in this mode
(slice aggregate aware TE), the resource reservation on each link
needs to be slice aggregate aware. Details of required IGP
extensions to support SA-TE are described in
[I-D.bestbar-lsr-slice-aware-te].
The same physical link may be member of multiple slice policies that
instantiate different slice aggregates. The slice aggregate network
resource availability on such a link is updated (and may be
advertised) whenever new paths are placed in the network. The slice
aggregate resource reservation, in this case, MAY be maintained on
each device or off the device on a resource reservation manager that
holds reservation states for those links in the network.
Multiple slice aggregates can form a group and share the available
network resources allocated to each slice aggregate. In this case, a
node can update the reservable bandwidth for each slice aggregate to
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take into consideration the available bandwidth from other slice
aggregates in the same group.
For illustration purposes, the diagram below represents bandwidth
isolation or sharing amongst a group of slice aggregates. In
Figure 1a, the slice aggregates: S_AGG1, S_AGG2, S_AGG3 and S_AGG4
are not sharing any bandwidths between each other. In Figure 1b, the
slice aggregates: S_AGG1 and S_AGG2 can share the available bandwidth
portion allocated to each amongst them. Similarly, S_AGG3 and S_AGG4
can share amongst themselves any available bandwidth allocated to
them, but they cannot share available bandwidth allocated to S_AGG1
or S_AGG2. In both cases, the Max Reservable Bandwidth may exceed
the actual physical link resource capacity to allow for over
subscription.
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I-----------------------------I I-----------------------------I
<--S_AGG1-> I I-----------------I I
I---------I I I <-S_AGG1-> I I
I I I I I-------I I I
I---------I I I I I I I
I I I I-------I I I
<-----S_AGG2------> I I I I
I-----------------I I I <-S_AGG2-> I I
I I I I I---------I I I
I-----------------I I I I I I I
I I I I---------I I I
<---S_AGG3----> I I I I
I-------------I I I S_AGG1 + S_AGG2 I I
I I I I-----------------I I
I-------------I I I I
I I I I
<---S_AGG4----> I I-----------------I I
I-------------I I I <-S_AGG3-> I I
I I I I I-------I I I
I-------------I I I I I I I
I I I I-------I I I
I S_AGG1+S_AGG2+S_AGG3+S_AGG4 I I I I
I I I <-S_AGG4-> I I
I-----------------------------I I I---------I I I
<--Max Reservable Bandwidth--> I I I I I
I I---------I I I
I I I
I S_AGG3 + S_AGG4 I I
I-----------------I I
I S_AGG1+S_AGG2+S_AGG3+S_AGG4 I
I I
I-----------------------------I
<--Max Reservable Bandwidth-->
(a) No bandwidth sharing (b) Sharing bandwidth between
between slice aggregates. slice aggregates of the
same group
Figure 1: Bandwidth Isolation/Sharing.
4.3. Data and Control Plane Slice Policy Mode
In order to support strict guarantees for slice aggregates, the
network resources can be partitioned in both the control plane and
data plane.
The control plane partitioning allows the creation of customized
topologies per slice aggregate that routers or a Path Computation
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Engine (PCE) can use to determine optimal path placement for specific
demand flows (Slice aggregate aware TE).
The data plane partitioning protects slice aggregate traffic from
network resource contention that could occur due to bursts in traffic
from other slice aggregates traversing the same shared network
resource.
5. Slice Policy Instantiation
A network slice can span multiple technologies and multiple
administrative domains. Depending on the network slice consumer's
requirements, a network slice can be differentiated from other
network slices in terms of data, control or management planes.
The consumer of a network slice expresses their intent by specifying
requirements rather than mechanisms to realize the slice. The
requirements for a network slice can vary and can be expressed in
terms of connectivity needs between end-points (point-to-point,
point-to-multipoint or multipoint-to-multipoint) with customizable
network capabilities that may include data speed, quality, latency,
reliability, security, and services (refer to
[I-D.ietf-teas-ietf-network-slice-definition] for more details).
These capabilities are always provided based on a Service Level
Agreement (SLA) between the network slice consumer and the provider.
The onus is on the network slice controller to consume the service
layer slice intent and realize it with an appropriate slice policy.
Multiple IETF network slices can be mapped to the same slice policy
resulting in a slice aggregate. The network wide consistent slice
policy definition is distributed to the devices in the network as
shown in Figure 2. The specification of the network slice intent on
the northbound interface of the controller and the mechanism used to
map the network slice to a slice policy are outside the scope of this
document.
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|
| IETF Network Slice
| (service)
+--------------------+
| IETF Network |
| Slice Controller |
+--------------------+
|
| Slice Policy
/|\
/ | \
slice policy capable
nodes/controllers
/ / | \ \
v v v v v
xxxxxxxxxxxxxxxxxxxx
xxxx xxxx
xxxx Slice xxxx
xxxx Aggregate xxxx
xxxx xxxx
xxxxxxxxxxxxxxxxxxxx
<------ Path Control ------>
RSVP-TE/SR-Policy/SR-FlexAlgo
Figure 2: Slice Policy Instantiation.
5.1. Slice Policy Definition
The slice policy is network-wide construct that is consumed by
network devices, and may include rules that control the following:
o Data plane specific policies: This includes the SS, any firewall
rules or flow-spec filters, and QoS profiles associated with the
slice policy and any classes within it.
o Control plane specific policies: This includes guaranteed
bandwidth, any network resource sharing amongst slice policies,
and reservation preference to prioritize any reservations of a
specific slice policy over others.
o Topology membership policies: This defines topology filter
policies that dictate node/link/function network resource topology
association for a specific slice policy.
There is a desire for flexibility in realizing network slices to
support the services across networks consisting of products from
multiple vendors. These networks may also be grouped into disparate
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domains and deploy various path control technologies and tunnel
techniques to carry traffic across the network. It is expected that
a standardized data model for slice policy will facilitate the
instantiation and management of slice aggregates on slice policy
capable nodes. A YANG data model for the slice policy instantiation
on network devices is described in
[I-D.bestbar-teas-yang-slice-policy].
It is also possible to distribute the slice policy to network devices
using several mechanisms, including protocols such as NETCONF or
RESTCONF, or exchanging it using a suitable routing protocol that
network devices participate in (such as IGP(s) or BGP). The
extensions to enable specific protocols to carry a slice policy
definition will be described in separate documents.
5.1.1. Slice Policy Data Plane Selector
A router MUST be able to identify a packet belonging to a slice
aggregate before it can apply the associated forwarding treatment or
S-PHB. One or more fields within the packet MAY be used as an SS to
do this.
Forwarding Address Based Slice Selector:
It is possible to assign a different forwarding address (or MPLS
forwarding label in case of MPLS network) for each slice aggregate
on a specific node in the network. [RFC3031] states in
Section 2.1 that: 'Some routers analyze a packet's network layer
header not merely to choose the packet's next hop, but also to
determine a packet's "precedence" or "class of service"'.
Assigning a unique forwarding address (or MPLS forwarding label)
to each slice aggregate allows slice aggregate packets destined to
a node to be distinguished by the destination address (or MPLS
forwarding label) that is carried in the packet.
This approach requires maintaining per slice aggregate state for
each destination in the network in both the control and data plane
and on each router in the network. For example, consider a
network slicing provider with a network composed of 'N' nodes,
each with 'K' adjacencies to its neighbors. Assuming a node can
be reached over 'M' different slice aggregates, the node assigns
and advertises reachability to 'N' unique forwarding addresses, or
MPLS forwarding labels. Similarly, each node assigns a unique
forwarding address (or MPLS forwarding label) for each of its 'K'
adjacencies to enable strict steering over the adjacency for each
slice. The total number of control and data plane states that
need to be stored and programmed in a router's forwarding is
(N+K)*M states. Hence, as 'N', 'K', and 'M' parameters increase,
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this approach suffers from scalability challenges in both the
control and data planes.
Global Identifier Based Slice Selector:
A slice policy MAY include a Global Identifier Slice Selector
(GISS) field as defined in [I-D.kompella-mpls-mspl4fa] that is
carried in each packet in order to associate it to a specific
slice aggregate, independent of the forwarding address or MPLS
forwarding label that is bound to the destination. Routers within
the slice policy domain can use the forwarding address (or MPLS
forwarding label) to determine the forwarding next-hop(s), and use
the GISS field in the packet to infer the specific forwarding
treatment that needs to be applied on the packet.
The GISS can be carried in one of multiple fields within the
packet, depending on the dataplane used. For example, in MPLS
networks, the GISS can be encoded within an MPLS label that is
carried in the packet's MPLS label stack. All packets that belong
to the same slice aggregate MAY carry the same GISS in the MPLS
label stack. It is also possible to have multiple GISS's map to
the same slice aggregate.
The GISS can be encoded in an MPLS label and may appear in several
positions in the MPLS label stack. For example, the VPN service
label may act as a GISS to allow VPN packets to be associated with
a specific slice aggregate. In this case, a single VPN service
label acting as a GISS MAY be allocated by all Egress PEs of a
VPN. Alternatively, multiple VPN service labels MAY act as GISS's
that map a single VPN to the same slice aggregate to allow for
multiple Egress PEs to allocate different VPN service labels for a
VPN. In other cases, a range of VPN service labels acting as
multiple GISS's MAY map multiple VPN traffic to a single slice
aggregate. An example of such deployment is shown in Figure 3.
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SR Adj-SID: GISS (VPN service label) on PE2: 1001
9012: P1-P2
9023: P2-PE2
/-----\ /-----\ /-----\ /-----\
| PE1 | ----- | P1 | ------ | P2 |------ | PE2 |
\-----/ \-----/ \-----/ \-----/
In
packet:
+------+ +------+ +------+ +------+
| IP | | 9012 | | 9023 | | 1001 |
+------+ +------+ +------+ +------+
| Pay- | | 9023 | | 1001 | | IP |
| Load | +------+ +------+ +------+
+----- + | 1001 | | IP | | Pay- |
+------+ +------+ | Load |
| IP | | Pay- | +------+
+------+ | Load |
| Pay- | +------+
| Load |
+------+
Figure 3: GISS or VPN label at bottom of label stack.
In some cases, the position of the GISS may not be at a fixed
position in the MPLS label header. In this case, the GISS label
can show up in any position in the MPLS label stack. To enable a
transit router to identify the position of the GISS label, a
special purpose label (ideally a base special purpose label
(bSPL)) can be used as a GISS label indicator.
[I-D.kompella-mpls-mspl4fa] proposes a new bSPL called Forwarding
Actions Identifier (FAI) that is assigned to alert of the presence
of multiple actions and action data (including the presence of the
GISS) that are carried within the MPLS label stack. The slice
policy ingress boundary node, in this case, imposes two labels:
the FAI label and a forwarding actions label that includes the
GISS to identify the slice aggregate that packets belong to as
shown in Figure 4.
[I-D.decraene-mpls-slid-encoded-entropy-label-id] also proposes to
repurpose the ELI/EL [RFC6790] to carry the Slice Identifier in
order to minimize the size of the MPLS stack and ease incremental
deployment.
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SR Adj-SID: GISS: 1001
9012: P1-P2
9023: P2-PE2
/-----\ /-----\ /-----\ /-----\
| PE1 | ----- | P1 | ------ | P2 |------ | PE2 |
\-----/ \-----/ \-----/ \-----/
In
packet:
+------+ +------+ +------+ +------+
| IP | | 9012 | | 9023 | | FAI |
+------+ +------+ +------+ +------+
| Pay- | | 9023 | | FAI | | 1001 |
| Load | +------+ +------+ +------+
+------+ | FAI | | 1001 | | IP |
+------+ +------+ +------+
| 1001 | | IP | | Pay- |
+------+ +------+ | Load |
| IP | | Pay- | +------+
+------+ | Load |
| Pay- | +------+
| Load |
+------+
Figure 4: FAI and GISS label in the label stack.
When the slice is realized over an IP dataplane, the GISS can be
encoded in the IP header. For example, the SSL can be encoded in
portion of the IPv6 Flow Label field as described in
[I-D.filsfils-spring-srv6-stateless-slice-id].
5.1.2. Slice Policy Resource Reservation
Bandwidth and network resource allocation strategies for slice
policies are essential to achieve optimal placement of paths within
the network while still meeting the target SLOs.
Resource reservation allows for the managing of available bandwidth
and for prioritization of existing allocations to enable preference-
based preemption when contention on a specific network resource
arises. Sharing of a network resource's available bandwidth amongst
a group of slice policies may also be desirable. For example, a
slice aggregate may not always be using all of its reservable
bandwidth; this allows other slice policies in the same group to use
the available bandwidth resources.
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Congestion on shared network resources may result from sub-optimal
placement of paths in different slice policies. When this occurs,
preemption of some slice aggregate specific paths may be desirable to
alleviate congestion. A preference based allocation scheme enables
prioritization of slice aggregate paths that can be preempted.
Since network characteristics and its state can change over time, the
slice policy topology and its state also need to be propagated in the
network to enable ingress TE routers or Path Computation Engine
(PCEs) to perform accurate path placement based on the current state
of the slice policy network resources.
5.1.3. Slice Policy Per Hop Behavior
In Diffserv terminology, the forwarding behavior that is assigned to
a specific class is called a Per Hop Behavior (PHB). The PHB defines
the forwarding precedence that a marked packet with a specific CS
receives in relation to other traffic on the Diffserv-aware network.
A Slice policy Per Hop Behavior (S-PHB) is the externally observable
forwarding behavior applied to a specific packet belonging to a slice
aggregate. The goal of an S-PHB is to provide a specified amount of
network resources for traffic belonging to a specific slice
aggregate. A single slice policy may also support multiple
forwarding treatments or services that can be carried over the same
logical network.
The slice aggregate traffic may be identified at slice policy ingress
boundary nodes by carrying a SS to allow routers to apply a specific
forwarding treatment that guarantee the SLA(s).
With Differentiated Services (Diffserv) it is possible to carry
multiple services over a single converged network. Packets requiring
the same forwarding treatment are marked with a Class Selector (CS)
at domain ingress nodes. Up to eight classes or Behavior Aggregates
(BAs) may be supported for a given Forwarding Equivalence Class (FEC)
[RFC2475]. To support multiple forwarding treatments over the same
slice aggregate, a slice aggregate packet MAY also carry a Diffserv
CS to identify the specific Diffserv forwarding treatment to be
applied on the traffic belonging to the same slice policy.
At transit nodes, the CS field carried inside the packets are used to
determine the specific PHB that determines the forwarding and
scheduling treatment before packets are forwarded, and in some cases,
drop probability for each packet.
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5.1.4. Slice Policy Topology
A key element of the slice policy is a customized topology that may
include the full or subset of the physical network topology. The
slice policy topology could also span multiple administrative domains
and/or multiple dataplane technologies.
A slice policy topology can overlap or share a subset of links with
another slice policy topology. A number of topology filtering
policies can be defined as part of the slice policy to limit the
specific topology elements that belong to a slice policy. For
example, a topology filtering policy can leverage Resource Affinities
as defined in [RFC2702] to include or exclude certain links for a
specific slice aggregate. The slice policy may also include a
reference to a predefined topology (e.g., derived from a Flexible
Algorithm Definition (FAD) as defined in [I-D.ietf-lsr-flex-algo], or
Multi-Topology ID as defined [RFC4915].
5.2. Slice Policy Boundary
A network slice originates at the edge nodes of a network slice
provider. Traffic that is steered over the corresponding slice
aggregate may traverse slice policy capable interior nodes as well as
slice policy incapable interior nodes.
The network slice may encompass one or more domains administered by a
provider. For example, an organization's intranet or an ISP. The
network provider is responsible for ensuring that adequate network
resources are provisioned and/or reserved to support the SLAs offered
by the network end-to-end.
5.2.1. Slice Policy Edge Nodes
Slice policy edge nodes sit at the boundary of a network slice
provider network and receive traffic that requires steering over
network resources specific to a slice aggregate. These edge nodes
are responsible for identifying slice aggregate specific traffic
flows by possibly inspecting multiple fields from inbound packets
(e.g., implementations may inspect IP traffic's network 5-tuple in
the IP and transport protocol headers) to decide on which slice
policy it can be steered.
Network slice ingress nodes may condition the inbound traffic at
network boundaries in accordance with the requirements or rules of
each service's SLAs. The requirements and rules for network slice
services are set using mechanisms which are outside the scope of this
document.
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When data plane slice policy is applied, the slice policy ingress
boundary nodes are responsible for adding a suitable SS onto packets
that belong to specific slice aggregate. In addition, edge nodes MAY
mark the corresponding Diffserv CS to differentiate between different
types of traffic carried over the same slice aggregate.
5.2.2. Slice Policy Interior Nodes
A slice policy interior node receives slice traffic and MAY be able
to identify the packets belonging to a specific slice aggregate by
inspecting the SS field carried inside each packet, or by inspecting
other fields within the packet that may identify the traffic streams
that belong to a specific slice aggregate. For example, when data
plane slice policy is applied, interior nodes can use the SS carried
within the packet to apply the corresponding S-PHB forwarding
behavior. Nodes within the network slice provider network may also
inspect the Diffserv CS within each packet to apply a per Diffserv
class PHB within the slice policy, and allow differentiation of
forwarding treatments for packets forwarded over the same slice
aggregate network resources.
5.2.3. Slice Policy Incapable Nodes
Packets that belong to a slice aggregate may need to traverse nodes
that are slice policy incapable. In this case, several options are
possible to allow the slice traffic to continue to be forwarded over
such devices and be able to resume the slice policy forwarding
treatment once the traffic reaches devices that are slice policy
capable.
When data plane slice policy is applied, packets carry a SS to allow
slice interior nodes to identify them. To enable end-to-end network
slicing, the SS MUST be maintained in the packets as they traverse
devices within the network - including slice policy incapable
devices.
For example, when the SS is an MPLS label at the bottom of the MPLS
label stack, packets can traverse over devices that are slice policy
incapable without any further considerations. On the other hand,
when the SSL is at the top of the MPLS label stack, packets can be
bypassed (or tunneled) over the slice policy incapable devices
towards the next device that supports slice policy as shown in
Figure 5.
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SR Node-SID: SSL: 1001 @@@: slice policy enforced
1601: P1 ...: slice policy not enforced
1602: P2
1603: P3
1604: P4
1605: P5
@@@@@@@@@@@@@@ ........................
.
/-----\ /-----\ /-----\ .
| P1 | ----- | P2 | ----- | P3 | .
\-----/ \-----/ \-----/ .
| @@@@@@@@@@
|
/-----\ /-----\
| P4 | ------ | P5 |
\-----/ \-----/
+------+ +------+ +------+
| 1001 | | 1604 | | 1001 |
+------+ +------+ +------+
| 1605 | | 1001 | | IP |
+------+ +------+ +------+
| IP | | 1605 | | Pay- |
+------+ +------+ | Load |
| Pay- | | IP | +------+
| Load | +------+
+----- + | Pay- |
| Load |
+------+
Figure 5: Extending network slice over slice policy incapable
device(s).
5.2.4. Combining Slice Policy Modes
It is possible to employ a combination of the slice policy modes that
were discussed in Section 4 to realize a network slice. For example,
data and control plane slice policy mode can be employed in parts of
a network, while control plane slice policy mode can be employed in
the other parts of the network. The path selection, in such case,
can take into account the slice aggregate specific available network
resources. The SS carried within packets allow transit nodes to
enforce the corresponding S-PHB on the parts of the network that
apply the data plane slice policy mode. The SS can be maintained
while traffic traverses nodes that do not enforce data plane slice
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policy mode, and so slice PHB enforcement can resume once traffic
traverses capable nodes.
5.3. Mapping Traffic on Slice Aggregates
The usual techniques to steer traffic onto paths can be applicable
when steering traffic over paths established for a specific slice
aggregate.
For example, one or more (layer-2 or layer-3) VPN services can be
directly mapped to paths established for a slice aggregate. In this
case, the per Virtual Routing and Forwarding (VRF) instance traffic
that arrives on the Provider Edge (PE) router over external
interfaces can be directly mapped to a specific slice aggregate path.
External interfaces can be further partitioned (e.g., using VLANs) to
allow mapping one or more VLANs to specific slice aggregate paths.
Another option is steer traffic to specific destinations directly
over multiple slice policies. This allows traffic arriving on any
external interface and targeted to such destinations to be directly
steered over the slice paths.
A third option that can also be used is to utilize a data plane
firewall filter or classifier to enable matching of several fields in
the incoming packets to decide whether the packet is steered on a
specific slice aggregate. This option allows for applying a rich set
of rules to identify specific packets to be mapped to a slice
aggregate. However, it requires data plane network resources to be
able to perform the additional checks in hardware.
6. Control Plane Extensions
Routing protocols may need to be extended to carry additional per
slice aggregate link state. For example, [RFC5305], [RFC3630], and
[RFC7752] are ISIS, OSPF, and BGP protocol extensions to exchange
network link state information to allow ingress TE routers and PCE(s)
to do proper path placement in the network. The extensions required
to support network slicing may be defined in other documents, and are
outside the scope of this document.
The instantiation of a slice policy may need to be automated.
Multiple options are possible to facilitate automation of
distribution of a slice policy to capable devices.
For example, a YANG data model for the slice policy may be supported
on network devices and controllers. A suitable transport (e.g.,
NETCONF [RFC6241], RESTCONF [RFC8040], or gRPC) may be used to enable
configuration and retrieval of state information for slice policies
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on network devices. The slice policy YANG data model is outside the
scope of this document, and is defined in
[I-D.bestbar-teas-yang-slice-policy].
7. Applicability to Path Control Technologies
The slice policy modes described in this document are agnostic to the
technology used to setup paths that carry slice aggregate traffic.
One or more paths connecting the endpoints of the mapped IETF network
slices may be selected to steer the corresponding traffic streams
over the resources allocated for the slice aggregate.
For example, once the feasible paths within a slice policy topology
are selected, it is possible to use RSVP-TE protocol [RFC3209] to
setup or signal the LSPs that would be used to carry the slice
aggregate traffic. Specific extensions to RSVP-TE protocol to enable
signaling of slice aggregate aware RSVP LSPs are outside the scope of
this document.
Alternatively, Segment Routing (SR) [RFC8402] may be used and the
feasible paths can be realized by steering over specific segments or
segment-lists using an SR policy. Further details on how the slice
policy modes presented in this document can be realized over an SR
network is discussed in [I-D.bestbar-spring-scalable-ns], and
[I-D.bestbar-lsr-spring-sa].
8. IANA Considerations
This document has no IANA actions.
9. Security Considerations
The main goal of network slicing is to allow for varying treatment of
traffic from multiple different network slices that are utilizing a
common network infrastructure and to allow for different levels of
services to be provided for traffic traversing a given network
resource.
A variety of techniques may be used to achieve this, but the end
result will be that some packets may be mapped to specific resources
and may receive different (e.g., better) service treatment than
others. The mapping of network traffic to a specific slice policy is
indicated primarily by the SS, and hence an adversary may be able to
utilize resources allocated to a specific slice policy by injecting
packets carrying the same SS field in their packets.
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Such theft-of-service may become a denial-of-service attack when the
modified or injected traffic depletes the resources available to
forward legitimate traffic belonging to a specific slice policy.
The defense against this type of theft and denial-of-service attacks
consists of a combination of traffic conditioning at slice policy
domain boundaries with security and integrity of the network
infrastructure within a slice policy domain.
10. Acknowledgement
The authors would like to thank Krzysztof Szarkowicz, Swamy SRK,
Navaneetha Krishnan, Prabhu Raj Villadathu Karunakaran and Jie Dong
for their review of this document, and for providing valuable
feedback on it.
11. Contributors
The following individuals contributed to this document:
Colby Barth
Juniper Networks
Email: cbarth@juniper.net
Srihari R. Sangli
Juniper Networks
Email: ssangli@juniper.net
Chandra Ramachandran
Juniper Networks
Email: csekar@juniper.net
12. References
12.1. Normative References
[I-D.bestbar-lsr-slice-aware-te]
Britto, W., Shetty, R., Barth, C., Wen, B., Peng, S., and
R. Chen, "IGP Extensions for Support of Slice Aggregate
Aware Traffic Engineering", draft-bestbar-lsr-slice-aware-
te-00 (work in progress), February 2021.
[I-D.bestbar-lsr-spring-sa]
Saad, T., Beeram, V. P., Chen, R., Peng, S., Wen, B., and
D. Ceccarelli, "IGP Extensions for SR Slice Aggregate
SIDs", draft-bestbar-lsr-spring-sa-00 (work in progress),
February 2021.
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[I-D.bestbar-spring-scalable-ns]
Saad, T., Beeram, V. P., Chen, R., Peng, S., Wen, B., and
D. Ceccarelli, "Scalable Network Slicing over SR
Networks", draft-bestbar-spring-scalable-ns-01 (work in
progress), February 2021.
[I-D.bestbar-teas-yang-slice-policy]
Saad, T., Beeram, V. P., Wen, B., Ceccarelli, D., Peng,
S., Chen, R., Contreras, L. M., and X. Liu, "YANG Data
Model for Slice Policy", draft-bestbar-teas-yang-slice-
policy-00 (work in progress), February 2021.
[I-D.decraene-mpls-slid-encoded-entropy-label-id]
Decraene, B., Filsfils, C., Henderickx, W., Saad, T.,
Beeram, V. P., and L. Jalil, "Using Entropy Label for
Network Slice Identification in MPLS networks.", draft-
decraene-mpls-slid-encoded-entropy-label-id-01 (work in
progress), February 2021.
[I-D.filsfils-spring-srv6-stateless-slice-id]
Filsfils, C., Clad, F., Camarillo, P., and K. Raza,
"Stateless and Scalable Network Slice Identification for
SRv6", draft-filsfils-spring-srv6-stateless-slice-id-02
(work in progress), January 2021.
[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.kompella-mpls-mspl4fa]
Kompella, K., Beeram, V. P., Saad, T., and I. Meilik,
"Multi-purpose Special Purpose Label for Forwarding
Actions", draft-kompella-mpls-mspl4fa-00 (work in
progress), February 2021.
[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>.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031,
DOI 10.17487/RFC3031, January 2001,
<https://www.rfc-editor.org/info/rfc3031>.
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[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
<https://www.rfc-editor.org/info/rfc3209>.
[RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630,
DOI 10.17487/RFC3630, September 2003,
<https://www.rfc-editor.org/info/rfc3630>.
[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>.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, DOI 10.17487/RFC5305, October
2008, <https://www.rfc-editor.org/info/rfc5305>.
[RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and
L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
RFC 6790, DOI 10.17487/RFC6790, November 2012,
<https://www.rfc-editor.org/info/rfc6790>.
[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752,
DOI 10.17487/RFC7752, March 2016,
<https://www.rfc-editor.org/info/rfc7752>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
12.2. Informative References
[I-D.ietf-teas-ietf-network-slice-definition]
Rokui, R., Homma, S., Makhijani, K., Contreras, L. M., and
J. Tantsura, "Definition of IETF Network Slices", draft-
ietf-teas-ietf-network-slice-definition-01 (work in
progress), February 2021.
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[I-D.ietf-teas-rfc3272bis]
Farrel, A., "Overview and Principles of Internet Traffic
Engineering", draft-ietf-teas-rfc3272bis-11 (work in
progress), April 2021.
[I-D.nsdt-teas-ns-framework]
Gray, E. and J. Drake, "Framework for IETF Network
Slices", draft-nsdt-teas-ns-framework-05 (work in
progress), February 2021.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
<https://www.rfc-editor.org/info/rfc2475>.
[RFC2702] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and J.
McManus, "Requirements for Traffic Engineering Over MPLS",
RFC 2702, DOI 10.17487/RFC2702, September 1999,
<https://www.rfc-editor.org/info/rfc2702>.
[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
<https://www.rfc-editor.org/info/rfc6241>.
[RFC8040] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
<https://www.rfc-editor.org/info/rfc8040>.
Authors' Addresses
Tarek Saad
Juniper Networks
Email: tsaad@juniper.net
Vishnu Pavan Beeram
Juniper Networks
Email: vbeeram@juniper.net
Bin Wen
Comcast
Email: Bin_Wen@cable.comcast.com
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Daniele Ceccarelli
Ericsson
Email: daniele.ceccarelli@ericsson.com
Joel Halpern
Ericsson
Email: joel.halpern@ericsson.com
Shaofu Peng
ZTE Corporation
Email: peng.shaofu@zte.com.cn
Ran Chen
ZTE Corporation
Email: chen.ran@zte.com.cn
Xufeng Liu
Volta Networks
Email: xufeng.liu.ietf@gmail.com
Luis M. Contreras
Telefonica
Email: luismiguel.contrerasmurillo@telefonica.com
Reza Rokui
Nokia
Email: reza.rokui@nokia.com
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