Distributed Mobility Management (DMM) M. Liebsch
Internet-Draft NEC
Intended status: Informational J. Mutikainen
Expires: 4 September 2025 NTT Docomo
Z. Zhang
Juniper Networks
T. Jiang
China Mobile
3 March 2025
Mobile Traffic Steering
draft-liebsch-dmm-mts-02.txt
Abstract
The evolution of cellular mobile communication systems is aligned
with an increasing demand for customized deployments, energy
efficiency, dynamic re-configurability and the integration and use of
other network technologies, such as non-cellular radio access
technologies and non-terrestrial networks. In order to achieve and
maintain the expected service quality and continuity, such systems
should be designed and controllable end-to-end, taking all involved
network domains and segments into account. This document discusses
an end-to-end system from an advanced use cases perspective and
substantiates the demand for solutions to share information and
enable control interfaces between all connected network domains,
including the mobile communication system and the transport network
that stretches up to the data networks that host service instances.
Two architectural principles are described and discussed in the view
of existing or new IETF technology that enables end-to-end mobile
traffic treatment and steering in such complex and dynamically
changing networks.
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This Internet-Draft will expire on 4 September 2025.
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Reference Architecture in the view of advanced end-to-end
operations . . . . . . . . . . . . . . . . . . . . . . . 5
4. System Evolution and Use Cases . . . . . . . . . . . . . . . 6
4.1. General directions and impact . . . . . . . . . . . . . . 6
4.2. MCS proactive UPA relocation . . . . . . . . . . . . . . 8
4.3. MCS reactive UPA relocation . . . . . . . . . . . . . . . 9
4.4. Node ephemerality . . . . . . . . . . . . . . . . . . . . 9
4.5. Inter-UPA communication . . . . . . . . . . . . . . . . . 9
5. Framework and Deployment Options . . . . . . . . . . . . . . 10
5.1. Mobile User Plane and Data Plane aspects . . . . . . . . 11
5.2. Dedicated Control Plane . . . . . . . . . . . . . . . . . 11
5.3. Decentralized Control Plane . . . . . . . . . . . . . . . 12
5.4. Control Interfaces . . . . . . . . . . . . . . . . . . . 13
6. Operational Aspects . . . . . . . . . . . . . . . . . . . . . 14
6.1. Mode I Operation - Dedicated Control Plane . . . . . . . 15
6.2. Mode II Operation - Decentralized Control Plane . . . . . 16
6.3. Mode III Operation - Hybrid . . . . . . . . . . . . . . . 18
7. Exemplary Application of MTS to a 5G System . . . . . . . . . 19
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
9. Security Considerations . . . . . . . . . . . . . . . . . . . 21
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
11.1. Normative References . . . . . . . . . . . . . . . . . . 21
11.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. Information Models . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Terminology
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.
2. Introduction
The evolution of cellular mobile communication systems resulted in a
clear separation of control plane and user plane functions. The
control plane comprises functions for security, mobile subscriber
management, handover, and mobility management. The user plane
functions represent anchor points for a user's traffic and enforce
policies to user plane traffic, such as for traffic engineering or
chargeable event monitoring and reporting. Compared to early
standards, today's mobile communication systems support the demand of
mobile subscribers as well as mobile network operators better in
terms of deployment options and route optimization. This includes
the decentralized deployment and selection of user plane anchors,
which can, e.g., be topologically located on the path between a
mobile user and a user's currently connected and used application
service.
Such flexible deployment of user plane anchors is aligned with a
rising interest in distributing compute resources. Edge Computing
enables the provisioning of compute resources, which are
topologically close to mobile users. This helps on the one hand to
reduce end-to-end latency between a mobile user and a service or a
compute resource, that performs a certain computation task for the
user. On the other hand, it keeps data for certain use cases local,
which can be leveraged for certain analytics tasks, where data is
only of local interest, or for meeting some privacy and security
objectives.
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Figure 1 depicts the end-to-end view of a mobile user, which connects
to an application service function (ASF) through a mobile
communication system (MCS). The ASF serves a user's request at the
data plane layer, while the service may have a dedicated application
control function (ACF). The user, as an example, may connect to the
ACF to configure the service and gets served by the associated ASF.
The service components are deployed in a data network, which may be
represented by a central cloud, or a distributed data center with
cloud computing resources. In alignment with mobile communication
system standards, the MCS is separated into a radio access network
(RAN) part and a mobile core network, which comprises the MCN control
plane and the user plane anchors. The RAN offers cellular and non-
cellular radio access technologies, such as WiFi, to a mobile user to
connect to the MCS for mobile access to a target service.
While mobile communication standards focus on the components of the
MCS and their operation, details on the network in between the MCS
and a data network are often treated out-of-scope of the mobile
communication standard and deployment specific. Such network may
indeed be deployment specific and differs in the type of network
nodes and protocols that route or switch traffic in between the MCS
and the data network. Figure 1 depicts this network as a set of data
plane nodes (DPN), which can, for example, be routers with redundant
paths and implementing MPLS or SRv6 in their transport plane.
This document addresses relevant system components in an end-to-end
reference architecture to discuss advanced use cases and associated
deployment options. In particular use cases are addressed, where
components or network functions on the end-to-end path change, move
or discontinue operation while the mobile user is still connected to
the service. The objective of this document is to analyze these use
cases and elaborate how existing or new IETF technology can serve as
enabler to accomplish service continuity for a mobile user in such
agile and dynamic system by means of controlled treatment of a mobile
user's end-to-end traffic for steering, with an option to extend for
advanced treatment policies, such as traffic engineering. The
treatment for steering of traffic in between a mobile communication
system and a data network should be according to computed and agreed
policies taking mobile communication system-, transport network and
data network aspects into account. Traffic treatment goes beyond
path computation and can include quality-of-service aspects, e.g. in
the view of a mobile subscriber's service level agreements.
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Each case is analyzed and discussed in the view of technical
challenges and operational aspects in each of the two described
solution options. Advanced use cases are in-line with the view in
industry, research community as well as pre-standards effort. The
following deployment- and operational aspects of session and service
continuity are included:
0 Mid-session relocation of a mobile user's UPA, e.g. due to user
mobility, UPA failover or load balancing.
o Deployments with moving or ephemeral system-relevant nodes on
the end-to-end path. These nodes include system components, such
as radio access network components and a MCS's UPA that are on-
board of a moving resource, such as a low earth orbit (LEO)
satellite, or an energy constrained node whose schedule enforces a
power save- or inactive mode.
o Mid-session relocation of a mobile user's serving data network,
e.g. due to the service resource's mobility, service failover,
energy/costs or quality of service reasons.
This document includes the description of two solution options, that
complement a MCS without interference by means of well inter-
connected and collaborative control- and data planes. Operational
aspects of the two solution options are described and semantics as
well as information models, that apply to relevant reference points,
are specified.
3. Reference Architecture in the view of advanced end-to-end operations
Figure 1 depicts a reference architecture for end-to-end operations,
which includes communication between a mobile user and an Application
Service Function (ASF) as well as user-to-user communications. The
ACF can be a service control instance hosted in a central or a
distributed cloud resources, or a workload placed by the mobile user
to an assigned compute resource in either a central or a
decentralized cloud. While the MCS ensures mobile connectivity and
data services between a mobile user and its UPA, a Transport Network
comprises a network of data plane nodes (DPN) and ensures routing of
a mobile user's traffic between it's UPA and one or multiple data
networks.
The Transport Network may implement redundant paths and select the
most suitable route based on the topological location and associated
IP address of the ASF and mobile user, respectively. The mobile
user's IP address may be topologically correct and fit the UPA's
network, or it does not match the network where the mobile user's
assigned UPA is located. The latter case applies, for example, to
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mobile user subscriptions, which have static IP addresses assigned
even in a deployment of distributed UPAs. Routing of topologically
incorrect mobile device IP addresses can be tackled by host routes,
either on the PE routers of network-centric overlays, such as via
encapsulation or label switching, or on all relevant on-path DPNs in
the transport network in between the mobile user's UPA and its
connected data networks.
With reference to Figure 1, the DPNs that are depicted close to the
data network and to the MCS can represent the associated domains' PE
router, or be independent routers in the domain of the transport
network provider that connect to the PE routers of the MCS domain and
the data network domain respectively.
The reference architecture's domains, the MCS, the transport network
and the data network, may share the same AS or be located in
different ASs. The relevance and scope of solutions for mobile
traffic steering in advanced and anticipated use cases, that this
document describes in Section 4, depend on the involved ASs.
+----------MCS----------+ Transport Network +--Data Network--+
| | | |
| +-------------------+ | +---+ | +---+ |
| | MCS Control Plane | | +---+DPN+---+ | |ACF| |
| +--+-------------+--+ | | +---+ | | +-+-+ |
| | | | | | | | |
Mobile | +--+--+ +--+--+ | +-+-+ +-+-+ | +-+-+ |
User-----+ RAN +===/===+ UPA +----+DPN+-------+DPN+--+-----+ASF| |
| +-----+ +-----+ | +---+ +---+ | +---+ |
| | | |
+-----------------------+ +----------------+
Figure 1: Reference End-to-End Architecture.
4. System Evolution and Use Cases
4.1. General directions and impact
While recent standards for MCSs are still being evolved and extended,
the research community and standards organizations started to
elaborate on a new MCS generation and advanced use cases. Key
objectives include tighter integration of a variety of radio access
technologies, including non-terrestrial networks, e.g. satellites,
support of energy-efficiency schemes, and runtime changes of service
instances and network functions. Customized deployments place a set
of UPAs topologically close to or even inside distributed data
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networks, which may host compute resources for service instances or
temporary workload that has been offloaded from a mobile user.
In case of user mobility, the MCS may assign a new UPA to the mobile
device, which can lead to sub-optimal routes between a data network
and the mobile user. While the relocation of the UPA is handled by
the MCS, awareness of such change is required in the transport
network and the data network to update traffic treatment. For
efficiency reasons, even the use of temporarily available and
ephemeral resources is considered, which includes, for example,
energy constrained or solar powered resources, mobile resources, such
as vehicles, drones or satellites. Industry develops satellites with
radio- and optical units to enable inter-satellite links and to
connect to ground stations as well as mobile users on earth. Recent
directions consider regenerative payload on satellites, which goes
beyond the use of a satellite as a communication relay but offers on-
board compute, storage and networking resources. This enables the
deployment of functions from mobile communication system, such as a
UPA, and Internet technology, such as a routing function, on
satellites to enable mobile access to such satellite resources. Even
the placement of suitable compute resources on-board of satellites is
considered, which makes them pre-destined to host application service
instances and other types of workload.
These directions enable highly efficient deployments in terms of
customization, resources usage, energy saving, communication latency,
connectivity and Internet coverage, as well as workload placement and
distribution. On the other hand, the dynamics of system components'
availability as well as their geographic and topological location
requires sharing of information between the involved system
components across domains, i.e. MCS, transport network and data
network. Exposing such information and offering control interfaces
between these domains allows timely configuration of alternative
paths to continue a service and steer mobile data plane traffic
between a mobile user and its service or between two mobile users.
In the discussed end-to-end system, a solution should also enable
traffic engineering in the traversed network segments and data
networks.
UPA relocation and other dynamic changes in the network may result in
reordering of the data plane packets. Solutions for mobile traffic
treatment and steering should provide suitable enablers to mitigate
the impact of such changes to packet re-ordering.
[I-D.zzhang-dmm-5gdn-end-marker] describes the use of End Markers to
address this problem.
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The following sub-sections describe selected use cases, their impact
to the end-to-end system and the need for mobile traffic steering
solutions. Section 5 introduces two architectural principles and
options to tackle mobile traffic steering, which are subsequently
discussed in the view of enabling technology, functional operation
and required semantics on relevant control plane interfaces.
4.2. MCS proactive UPA relocation
Figure 2 depicts a use case, where the MCS relocates the mobile
user's current UPA, UPA1, to a new UPA, UPA2. The resulting route
between service instance ASF1 in the data network and UPA2 may differ
from the previous route to UPA1. In case the IP address of the
mobile user's device needs to survive in order to not break the
current session with the used service at ASF1, the transport network
needs to enforce rules for traffic steering, e.g. by applying host
routes on the path's DPNs, by applying source routing or an overlay
route, i.e. IP tunnel. In case the user's IP address needs to
continue only as long as the data session with ASF1 takes, the MCS
may assign a new, topologically correct IP address from the network
of UPA2 to the mobile user device. After the mobile user terminates
its data session with ASF1, new data sessions will use the new IP
address and transient host routes in the transport network can be
removed. Figure 2 depicts also the change of the serving ASF, which
may happen also as a result of user mobility. As example, in an
automotive use case, vehicles represent mobile users and connect to a
most suitable, nearby ASF that is hosted by one of many available
distributed cloud networks. Due to mobility, the MCS may relocate
the mobile user's UPA from UPA1 to UPA2. In order to keep routing
paths short and latency low, the ASF may be relocated from ASF1 to
ASF2, as ASF2 is deployed in a more suitable data network.
+----------MCS----------+ Transport Network +--Data Network--+
| | | |
| +-------------------+ | +---+ | +---+ |
| | MCS Control Plane | | +---+DPN+---+ | |ACF| |
| +--+-------------+--+ | | +---+ | | +-+-+ |
| | | | | | | | |
Mobile | +--+--+ +---+--+ | +-+-+ +-+-+ | +--+-+ |
User-----+ RAN +===/==+ UPA1 +----+DPN+-------+DPN+--+----+ASF1| |
| +--+--+ +------+ | +---+ +-+-+ | +----+ |
| | | | +----------------+
| | | | +----------------+
| | +------+ | +---+ +-+-+ | +----+ |
| +---------+ UPA2 +----+DPN+-------|DPN|-------+ASF2| |
| +------+ | +---+ +---+ | +----+ |
+-----------------------+ +----------------+
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Figure 2: Mid-session UPA relocation, e.g. due to user mobility.
4.3. MCS reactive UPA relocation
In this case the source and rationale behind a UPA relocation in the
MCS is different and results from a decision to change the serving
ASF from ASF1 to ASF2 (Figure 2). This may be because of an
anticipation that the targeted geographic region of the mobile user
gets better service from a data network that hosts ASF2. As further
example, the transport network applies changes in its routing
strategy and as a result, a route between the MCS and a new ASF
instance will result in better service quality and lower latency.
The new situation in the service data network and the transport
network can be exposed to the MCS, which may relocate the UPA from
UPA1 to UPA2 to optimize the end-to-end path.
4.4. Node ephemerality
Components from the transport network or the MCS may be of ephemeral
nature and discontinue service at a predictable or unpredictable
point in time. Predictable discontinuity may be due to a scheduled
power saving plan or mobility of the component. The latter case
applies, as example, to LEO satellites in space, which are non-
stationary. Advanced use cases consider the components of a radio
base station and a UPA instance from the MCS as well as a DPN
instance on-board of each satellite. In such case, the mobile user
on earth or a High Altitude Platform Station (HAPS) is associated
with the satellite's radio base station, the UPA and a DPN for
further routing of the mobile traffic. At some point in time, the
mobile user is not covered by the satellite's radio base station
anymore and a different satellite, including its on-board radio base
station, the UPA and DPN, takes over service the mobile user. The
end-to-end system needs to adapt to the changed triple of a radio
base station, UPA and DPN in terms of traffic steering.
Taking an example from a 3GPP-based MCS, a joint deployment of a
radio base station, UPA and virtual routing function can be realized
per the description in [I-D.zzhang-dmm-mup-evolution].
4.5. Inter-UPA communication
A MCS provide may deploy distributed UPAs in order to shorten the
route and resulting latency for the communication between two mobile
users. Figure 3 depicts such case where mobile user1 has UPA1
assigned, while mobile user2 has UPA3 assigned. Mobile traffic
between the two mobile users traverses UPA1 and UPA3 as well as the
transport network. In case the MCS relocates the mobile user1's UPA1
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to UPA2, a different route may apply. In case of demand for IP
address continuity or enforcement of particular traffic engineering
rules in the DPNs, the change in the UPAs must be notified to the
transport network to apply adjusted policies in the relevant DPNs.
+----------MCS----------+ Transport Network
| |
| +-------------------+ |
| | MCS Control Plane | |
| +--+-------------+--+ |
| | | |
Mobile | +--+--+ +---+--+ | +-+-+
User1-----+ RAN +===/==+ UPA1 +----+DPN+-----+
| +--+--+ +------+ | +---+ |
| |I | |
| || | |
| || +------+ | +---+ +-+-+
| +=========+ UPA2 +----+DPN+---|DPN|
| +------+ | +---+ +-+-+
Mobile | +-----+ +------+ | +---+ |
User 2----+ RAN +======+ UPA3 +----+DPN+-----+
| +-----+ +------+ | +---+
+-----------------------+
Figure 3: Mid-session UPA relocation, e.g. due to one user's
mobility.
5. Framework and Deployment Options
This section discusses two distinct architecture principles and
deployment options that enable end-to-end awareness of changes in the
network and its configuration that have impact on a mobile user's
access to a service and the expected service quality. One option
leverages a transport network controller (TN Controller) that shares
relevant states and information with the MCS through a control plane
interface. The same interface is used to receive notification from
the MCS and to apply control commands in the MCS. The second option
is based on decentralized control and requires tighter coupling of
the transport network's routing functions with the MCS.
Based on the two distinct architecture principles, Section 6
describes operational aspects in the view of three modes, that (i)
rely solely on the interface between the two dedicated control planes
of the MCS and the TN, (ii) solely on the decentralized control
plane, and (iii) a hybrid mode, that utilizes both, a dedicated
control plane on the TN as well as the decentralized control plane.
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5.1. Mobile User Plane and Data Plane aspects
A mobile user plane applies to the scope of the MCS and is managed by
the MCS control plane to enable bi-directional data traffic between a
mobile user's device and its assigned UPA. The network in between
the UPA and a data network includes DPNs of the transport network and
PE routers. This document focuses on IETF technology that applies to
the control and data plane in the transport network and data network.
The transport network's data plane transits into the MCS domain at a
PE router or the UPA.
5.2. Dedicated Control Plane
Figure 4 illustrates a deployment with a dedicated control function
in the transport network, which is denoted as Mobile Traffic Steering
(MTS) Control and may build on top of a TN Controller, leveraging its
northbound API. The MTS Controller leverages an interface to the MCS
control plane to receive notifications, such as during the change of
a mobile user's UPA, and to apply changes in the configuration of a
mobile user's states in the MCS, such as enforcement of a UPA change
in alignment with a change in the transport network or data network.
The data network may also use an interface either to the MTS
controller or to the MCS control plane, which is used to enforce re-
configurations in the transport network and/or the MCS in alignment
with changes in the data network or a mobile user's service, e.g. ASF
relocation or QoS settings.
The transport network's DPNs may make use of a dedicated ingress
router, such as a PE router, to reach the MCS's UPAs. In this
deployment option, the control of the UPAs is clearly separated from
the DPN control, though the two control planes, the MCS control plane
and the MTS Controller, are connected through a control plane
interface. The MCS control plane may enforce rules in a UPA for
uplink traffic treatment, e.g. to forward a mobile user's traffic to
a particular DPN.
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+----------MCS----------+ Transport Network +--Data Network--+
| | | |
| +-------------------+ | +---------------+ | +---+ |
| | MCS Control Plane +----+ MTS Control +--------+ACF| |
| +--+-------------+--+ | |+------+------+| | +---+ |
| | | | ||TN Controller|| | | |
| | | | +++-----------+++ | | |
| | | | | | | | |
Mobile | +--+--+ +--+---+ | +-+-+ +-+-+ | +--+-+ |
User----+ RAN +===/==+ UPA1 +----+DPN+-------+DPN+-------+ASF1| |
| +-----+ +------+ | +---+ +-+-+ | +----+ |
| || | +----------------+
| || | : : +----------------+
| || +------+ | +-+-+ +-+-+ | +----+ |
| +=========+ UPA2 +----+DPN+-------|DPN|-------+ASF2| |
| +------+ | +---+ +---+ | +----+ |
+-----------------------+ +----------------+
Figure 4: End-to-end architecture with a dedicated transport
network controller and loose UPA-DPN coupling
5.3. Decentralized Control Plane
Figure 5 illustrates a decentralized deployment without a dedicated
TN controller. DPNs expose routes through a routing protocol. To
cover cases where the MCS relocates a mobile user's UPA, the DPN must
either tightly couple with the UPAs and apply local APIs to learn
about the mobile user and it's changes configuration in the MCS, or
the UPA applies a protocol to share relevant states and information
with the DPN. This document does not consider a dedicated control
interface between the MCS control plane and the DPN. Relevant
semantic to enable the DPN to propagate updated routes towards the
transport network and the data network must be available at the UPA.
This may require an extension to the MCS control plane to configure
the UPA with information that is relevant for mobile traffic
treatment and steering in the transport network and the data network.
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+----------MCS----------+ Transport Network +--Data Network--+
| | | |
| +-------------------+ | | +---+ |
| | MCS Control Plane | | | +ACF| |
| +--+-------------+--+ | | +---+ |
| | | | | | |
Mobile | +--+--+ +---+--+ | +---+ +-+-+ | +--+-+ |
User-----+ RAN +===/==+ UPA1 +---+DPN+-------+DPN+---+----+ASF1| |
| +-----+ +------+ | +---+ +-+-+ | +----+ |
| || | | +----------------+
| || | | +----------------+
| || +------+ | +---+ +-+-+ | +----+ |
| +=========+ UPA2 +---+DPN+-------|DPN|--------+ASF2| |
| +------+ | +---+ +---+ | +----+ |
+-----------------------+ +----------------+
Figure 5: End-to-end architecture with a routing protocol-based
propagation of traffic steering policies and tight UPA-DPN
coupling
5.4. Control Interfaces
This document revolves around the specification of interface
semantics that applies to control endpoints that operate in the MCS
and the TN respectively. This control interface for mobile traffic
steering is denoted C_mts and can apply in between (i) the MCS
control plane and the MTS Control in the TN, as well as between (ii)
a UPA in the MCS and a DPN in the TN.
Figure 6 depicts the application of the C_mts interface and semantics
per this document in between endpoints (c1) and (c2) of the MCS
control plane and the MTS controller respectively. Figure 7 depicts
the application of the C_mts interface and semantics per this
document in between (c1) and (c2) of a UPA in the MCS and a DPN in
the TN. MTS policies and resulting forwarding rules (fwd) apply in
either case to the data plane endpoints (d1) and (d2) of a UPA in the
MCS and a first hop DPN in the TN. For end-to-end mobile traffic
steering, MTS policies may have impact on further DPNs in the TN
between the MCS UPFs and data networks.
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Note: (d2) per Figure 6 and Figure 7 applies to the first hop DPN
from a UPA perspective towards the TN. A UPA may connect to multiple
DPNs with a (d2) endpoint, and a DPN may connect to multiple UPAs
with a (d1) endpoint. MTS policies are further propagated to the
relevant DPNs in between a UPA and a data network by either
mechanism, through a dedicated control- / data-plane protocol in
between the MTS/TN Controller and DPNs, or a routing protocol that
applies to the DPNs' control plane.
+-------------------+ +-------------+
| MCS Control Plane +(c1)------//------(c2)+ MTS Control |
+-------------------+ C_mts +-------------+
: :
+-----+ +-----+
| | | |
| UPA | | DPN |
| +(d1)--------------(d2)+ +--- - - -//
+-----+ fwd +-----+ fwd
Figure 6: C_mts applies to reference point between MCS and MTS
Control Plane
+-------------------+
| MCS Control Plane |
+-------------------+
:
+-----+ C_mts +-----+
| +(c1)------//------(c2)+ |
| UPA | | DPN |
| +(d1)--------------(d2)+ +--- - - -//
+-----+ fwd +-----+ fwd
Figure 7: C_mts applies to reference point between an MCS's UPA
and an associated DPN
6. Operational Aspects
This section describes operational aspects in the view of three
modes, that (i) utilize solely a dedicated control plane for MTS,
(ii) a decentralized control plane, or (iii) a hybrid mode,
leveraging both, a dedicated MTS controller to make use of all
control features enabled by the inter-action with the MCS control
plane as well as a de-centralized routing protocol to propagate MTS
route updates into the TN.
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6.1. Mode I Operation - Dedicated Control Plane
This operational mode leverages the full feature set of the C_mts
interface semantics to interact between an MTS Controller and the MCS
control plane (Figure 8). MTS policies and their updates can be
initiated by both planes, the MCS control plane or the MTS control
plane. This mode assumes the existence of dedicated protocol planes
between the MCS control plane and the UPAs (C/U_mcs), as well as
between the MTS control plane and the TNs' DPNs (C/D_mts). Hence,
routing for a mobile device's traffic that applies to the forwarding
plane (fwd) between a UPA endpoint (d1) and a relevant first hop DPN
endpoint (d2) is configured through the associated control interfaces
of the MCS control plane towards the UPAs, and the MTS control plane
towards the DPNs. Details about a choice and implementation of the
C/U_mcs and C/D_mts are out of scope of this specification.
While this document focuses on the C_mts semantics and an associated
information model, REST is a suitable and recommended enabler to
implement operations in between the MCS control plane and a MTC
controller. Other protocols may apply as an alternative.
+-------------------+ +-----------------+
| MCS Control Plane +(c1)<<<==//==>>>(c2)+ MTS Control |
+-------------------+ C_mts +-----------------+
| | |
C/U_mcs | | C/D_mts |
V V V
V V V
+-----+ +-----+ +-----+
| | | | | |
| UPA | | DPN | | DPN |
| +(d1)-----------(d2)+ +-------+ +-- - -//
+-----+ fwd +-----+ fwd +-----+ fwd
Figure 8: Use of dedicated MTS Control that interacts with the
MCS control plane for mobile traffic steering
Key operations in this mode include the following:
O The MTS Control indicates to the MCS control plane that traffic
associated with a single or a group of users will be assigned a
new DPN and associated (d2) endpoint. In case further on-path
DPNs will change on the traffic's end-to-end path up to the data
network, the MTC Control will enforce updates routing policies to
the TN's DPN. These path changes are kept transparent to the MCS
control plane and the UPA. The MTS Control will provide single
DPN or a group of DPNs to the MCS control plane. In any case the
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MCS control plane may decide to keep the current UPA or change the
UPA in alignment with the single of group of potential DPNs. In
case the MTS control plane provides a group of potential DPNs with
its indication, the MCS control plane can choose for the group of
candidate DPNs. The MCS control plane provides details one the
selected UPA and DPN back to the MTS Control.
O The MCS control plane indicates to the MTS control that traffic
associated with a single or a group of users will be assigned a
new UPA, e.g. due to user mobility, load balancing or failover
reasons. In case the MCS control plane identifies a particular
UPA with the indication, the MTS Control can either keep the
current DPN or assign a new DPN to the associated traffic. Based
on the identified UPA, the MTS Control may interact with a backend
system, resulting in a decision to change the data network
associated with the single or group of users' traffic, e.g. due to
route- and traffic latency, costs or load balancing reasons. The
MTS Control may compute a new path based on the newly assigned UPA
and data network and provide information about the first hop DPN,
to be used by the UPA, to the MCS control plane. Based on the
identified DPN, the MCS control plane may re-select a more
suitable UPA and lets the MTS Control know about this decision for
final configuration of forwarding policies.
6.2. Mode II Operation - Decentralized Control Plane
This operational mode leverages the C_mts interfaces and semantics in
between a UPA and DPN to make the DPN propagating MTS routing
policies and updates into the TN (Figure 9). While the UPA receives
MTS policies from the MCS control plane through the protocol applying
to the C/U_mcs reference point, the DPN relies entirely on the C_mts
interfaces with a UPA to receive or retrieve relevant information to
propagate routes into the TN.
While this document focuses on the C_mts semantics and an associated
information model, different protocols can apply in between the (c1)
and (c2) endpoints of a UPA and a DPN. REST is a suitable enabler to
implement operations in between a UPA and a DPN, while a particular
routing protocol, such as RIP or BGP, can also be considered as
suitable feed of C_mts semantics from a UPA to a DPN, leveraging a
UPA's routing capabilities and possibly available support of basic
routing protocols.
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As mentioned before, in case of re-using existing protocols that
apply in between a UPA and a DPN, additional information and
semantics per this specification is desired for the routes announced
by the UPAs to the DPNs. Any existing routing protocol with an
extension to pass the additional information can be used, e.g., RIP/
BGP. APIs provided by the DPN can also be used for the UPA to
program the routes with the additional information into the DPN.
Note that the UPA running a routing protocol does not mean it needs
to be a full-function router, and BGP is a reasonable choice to carry
both the routes and the additional information as attributes. While
BGP can support many advanced features with many of its extensions,
only basic BGP functionality is needed here and that has become a
commodity.
The distributed UPAs may want the TN to rate-limit or shape the DL
traffic so that they are not overwhelmed by the traffic. This is
much like that in the centralized UPA case the UPA applies traffic
control. Therefore, traffic characteristics information may be
distributed along with the routes as route attributes, as described
in the "QoS Handling" section of [I-D.zzhang-dmm-mup-evolution].
When a mobile user's UPA changes (which is more often with the
distributed UPAs), packet re-ordering may happen and TN-triggered
End-Marker can help mitigate that, as described in
[I-D.zzhang-dmm-5gdn-end-marker]. In this case, the session
information (specifically the session ID and UPA address) can help
with the process and they can be attached to the routes advertised by
UPAs to DPNs.
+-------------------+
| MCS Control Plane +
+-------------------+
|
C/U_mcs |
V
V
+-----+ C_mts +-----+ C_rtg +-----+
| +(c1)===//==>>>(c2)+ +========>>>+ |
| UPA | | DPN | | DPN |
| +(d1)----------(d2)+ +-----------+ +- - -//
+-----+ fwd +-----+ fwd +-----+ fwd
Figure 9: Use of routing protocols to propagate mobile traffic
steering policies in the transport network
Key operations in this mode include the following:
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O The MCS control plane assigns to the traffic of a single or a
group of users a new UPF, e.g. due to user mobility, failover or
load balancing reasons. The current UPA indicates to the DPN that
is currently assigned for the treatment of this traffic that the
UPA will change. When the new UPA is identified in the
indication, the DPN may prepare, using the C_mts interface to the
new UPA, the retrieval of updated MTS policies for the user's
traffic. After the new UPA became active, the new UPA and the DPN
enforce updates forwarding rules per the updated MTS policy. For
end-to-end mobile traffic steering, The DPN may propagate the
updated MTS policies into the TN towards the data network
O A DPN may receive an indication to offload traffic associated
with a single or a group of users to an alternative DPN, e.g. due
to load balancing or failover reasons. The DPN uses the C_mts
reference point to indicate to the UPA, that treats the user's
traffic, about a change in the first hop DPN. The indication may
include a single or group of alternative DPNs. The UPA may select
one DPN out of the group of DPNs or use the single DPN from the
indication and applies the MTS policies to the new DPN through the
C_mts reference point. After the new MTS policies are enforced
and the new forwarding rules apply, the UPA may indicate to the
previous DPN to make the old rules obsolete.
6.3. Mode III Operation - Hybrid
This operational mode leverages the C_mts interfaces and semantics on
both (c1) and (c2) endpoints, in between the MCS control plane and
the MTS controller, as well as in between an applying UPA and DPN
(Figure 10). While the dedicated control plane between MCS and a MTS
controller is used to leverage the rich features enabled per Mode I
operations, this mode does not assume the enforcement of MTS policies
and their updates directly from the MTS controller, but leverages
Mode II operations to receive or retrieve MTS policies and their
updates through the C_mts interface that applies on the (c1) and (c2)
endpoints between a UPA and a DPN and to propagate updated routes
towards the TN by means of a routing protocol.
The high-level framework for a hybrid mode operation does not differ
from Figure 4 but integrating both the dedicated TN controller (i.e.,
for centralized framework) and the exposure of the routes via routing
protocol(s) between UPA & DPN (i.e., for distributed deployment).
Similar to that in Section 5.2, the TN or MTS controller in transport
network interfaces directly with the MCS control plane in Mobile
network for configurations and provisioning as the dedicated control
mode. While on the other aspect similar to Section 5.3, the (edge)
DPNs in transport network couple closely with UPAs in mobile network
to exchange routes via routing protocols (e.g., RIP, BGP, etc.) as
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the distributed framework. Here the traffic steering between a UPA
in MN and a DPN in TN may be influenced by the combined policies of
both the dedicated channel (i.e., MCS and MTS control) and the
distributed logic (i.e., tight coupling of UPA & DPN).
+-------------------+ +---------------+
| MCS Control Plane +(c1)<<<===//===>>>(c2)+ MTS Control |
+-------------------+ C_mts +---------------+
|
C/U_mcs |
V
V
+-----+ C_mts +-----+ C_rtg +-----+
| +(c1)===//==>>>(c2)+ +========>>>+ |
| UPA | | DPN | | DPN |
| +(d1)----------(d2)+ +-----------+ +- - -//
+-----+ fwd +-----+ fwd +-----+ fwd
Figure 10: Use of MTS Control to interact with the MCS control
plane and of a routing protocols to propagate MTS routes in the
transport network
7. Exemplary Application of MTS to a 5G System
The three architecture principles and deployment options as laid out
in Section 5, i.e., the dedicated, the decentralized, and the hybrid
framework, can perfectly fit into the 3GPP 5GS.
The Figure 11 shows the brief interaction between a 5GS (i.e., so-
named MCS in the draft) and TN & DN. The 5GS control-plane or '5GS
CP' interacts with the external control function, i.e., the TN
controller in the TN or transport network. Application servers or
ASFs are located in the (remote) DN or data network that is connected
to the 5GS user-plane, e.g., UPF off the N6 interface, via the
(intermediate) TN. Here, UPF acts as UPA. The N6 reference point is
between the UPA (UPF) and the external IP network (or TN).
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5GS CP: also the MCS controller
C-/L- PSA: Central/Local PSA UPF (i.e., UPA)
I-UPF: Intermediate UPF (also, UPA)
TN: Transport Network DN: Data Network
ASF: App Function in Data Domain (DN)
.......................................
: :
: [-- 5GS CP --]-----------------/----------\
: / | | N4 \ : / MTS w/TN \
: N1 N2 | \ : | Controller |
: / | | \ : | |
: / | +-------+ +-----+ : | + |
+----+ +-----+ | I-UPF | | UPF | : | : |
| UE |--| gNB |--| |--|C-PSA+-(N6)-+ : |
+----+ +-----+ +-------+ +-----+ : | : |
: | : | + |
: | : | |
: +-----+ : | DN w/ |
: | UPF | : \ ACF/ASF /
: |L-PSA|(N6)-------------\----------/
: +-----+ :
.......................................
Figure 11: 5GS Architecture w/ DN & TN
The 3GPP document TS 29.561 [TS.29.561] specifies a UPF (i.e., UPA)
can be seen as a normal IP router from the external IP network's
point of view. Therefore, the control & steering of mobile traffic
between UPA(s) in MCS and ASF(s) in DN via TN can be categorized
based on the three options as in Section 5:
* Dedicated CP: The 5GS CP (i.e., MCS controller), the TN/MTS
controller and the ACF together determine intelligently the
traffic steering from a UPA to an ASF. The 5G AF-provided session
provisioning or control can be bucketed into the type. For
example, the 5G Rel-19 eEdge has an enhancement to leverage the N6
path delay for (DN) ASF selection, through the interactions
between the 5GS-CP (MCS) and the external controller (MTS).
* Decentralized CP: Running native routing protocols. That is, as
per [TS.29.561], a UPA acts as an IP router to engage & peer with
the DPN PE(s) in the TN directly. The mobile traffic is steered
natively over the transport network.
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* Hybrid CP: It is a combination of both the dedicated CP and the
decentralized CP modes. One example is the mobile traffic
steering based on the N6 point-to-point tunnel (e.g., PMIPv6/GRE,
L2TP, etc.) between a UPA (i.e., UPF) and an ASF. The UPA acts as
the forwarding node that might acquire its routing intelligence
from multiple sources. One source could be from the interactions
between the 5GS-CP (or the MCS controller) and the (external MTS)
TN controller -- the dedicated mode. The second source could be
based on the routing protocols operating natively between the UPA
and the TN PEs -- the decentralized mode. Further, the third
source could be the overlay transfer of compute information from
the remote (DN) ACF (to the UPA). -- the dedicated mode.
8. IANA Considerations
This document has no request to IANA.
9. Security Considerations
TBD.
10. Acknowledgments
This document includes results from discussion with various IETF and
non-IETF delegates. Many thanks to David Lake for his immediate
interest in this topic and feedback at various opportunities and
through different channels. Many thanks also to Joel Halpern for his
clear and apt feedback during the public side meeting@IETF118 and to
Sri Gundavelli and Satoru Matsushima for their support and advice on
this matter.
11. References
11.1. 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>.
[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>.
11.2. Informative References
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[I-D.zzhang-dmm-mup-evolution]
Zhang, Z. J., Patel, K., Contreras, L. M., Islam, K.,
Mutikainen, J., Jiang, T., Jalil, L., Sejati, O. P., and
S. Zadok, "Mobile User Plane Evolution", Work in Progress,
Internet-Draft, draft-zzhang-dmm-mup-evolution-09, 2 July
2024, <https://datatracker.ietf.org/doc/html/draft-zzhang-
dmm-mup-evolution-09>.
[I-D.zzhang-dmm-5gdn-end-marker]
Zhang, Z. J., Liebsch, M., and T. Jiang, "End Marker in 5G
Data Network", Work in Progress, Internet-Draft, draft-
zzhang-dmm-5gdn-end-marker-01, 6 February 2024,
<https://datatracker.ietf.org/doc/html/draft-zzhang-dmm-
5gdn-end-marker-01>.
[TS.29.561]
"3GPP TS 29.561: Interworking between 5G Network and
external Data Networks; Stage 3", 3GPP TS 29.561,
December 2024.
Appendix A. Information Models
In a deployment with a dedicated control plane, the information model
applies to the reference points between the MCS Control Plane and the
MTS Control function, which builds on top of a Network Controller.
This reference point is extracted from Figure 4 and depicted with
suitable labels in Figure 6
In a decentralized deployment, the information model applies to the
reference points between a MCS's UPA and an associated DPN. This
reference points is extracted from Figure 5 and depicted in Figure 7.
Besides mobile device identifiers, MCS-specific session identifiers
and traffic classifiers, the following Information Elements (IE) are
considered relevant for mobile traffic steering:
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+--------------------------------------------------------------------+
| IE | Description | Note about Use/Format |
+====================================================================+
| UPA_ID | Identifier of the UPA | String or numerical ID |
+---------------+-------------------------+--------------------------+
| UPA_CP_URI | Control Plane API for | MCS control plane access |
| | the identified UPA | API or UPA control API |
+---------------+-------------------------+--------------------------+
| UPA_SUPPL_URI | API to retrieve supple- | Source of suppl. info, |
| | mentary information | e.g. map service, |
| | about UPA | mobility pattern, etc. |
+---------------+-------------------------+--------------------------+
| UPA_UP_IP | IP address of a UPA | UPA Data Plane IP to |
| | | be used by DPNs |
+---------------+-------------------------+--------------------------+
| UPA_UP_MAC | MAC address of a UPA | UPA Data Plane MAC to |
| | | be used b< DPNs |
+---------------+-------------------------+--------------------------+
| UPA_UP_ID | Lower layer ID for user | Label or segment ID, |
| | plane traffic Forwarding| type/format |
+---------------+-------------------------+--------------------------+
| UPA_GEO_LOC | UPA geographic locator | |
+---------------+-------------------------+--------------------------+
| UPA_TOPO_LOC | UPA topological locator | |
+---------------+-------------------------+--------------------------+
| UPA_TOPO_MAP | Reference to topology | Can be map of physical |
| | map | or virtual topology |
+---------------+-------------------------+--------------------------+
| UPA_ROLE | Role={current, target, | |
| | candidate} | |
+---------------+-------------------------+--------------------------+
| UPA_STAT_INF | Status info incl. load, | |
| | expected changes | |
+---------------+-------------------------+--------------------------+
Figure 12: UPA Information Elements
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+--------------------------------------------------------------------+
| IE | Description | Note about Use/Format |
+====================================================================+
| DPN_ID | Identifier of the DPN | String or numerical ID |
+---------------+-------------------------+--------------------------+
| DPN_CP_URI | Control Plane API for | MCS control plane access |
| | the identified DPN | API or DPN control API |
+---------------+-------------------------+--------------------------+
| DPN_SUPPL_URI | API to retrieve supple- | Source of suppl. info, |
| | mentary information | e.g. map service, |
| | about DPN | mobility pattern, etc. |
+---------------+-------------------------+--------------------------+
| DPN_FWD_IP | IP address of a DPN | DPN next hop IP to |
| | | be used by UPAs and DPNs |
+---------------+-------------------------+--------------------------+
| DPN_FWD_MAC | MAC address of a DPN | DPN next hop MAC to |
| | | be used by UPAs and DPNs |
+---------------+-------------------------+--------------------------+
| DPN_FWD_ID | Lower layer ID for | Label or segment ID, |
| | Forwarding | type/format |
+---------------+-------------------------+--------------------------+
| DPN_GEO_LOC | DPN geographic locator | |
+---------------+-------------------------+--------------------------+
| DPN_TOPO_LOC | DPN topological locator | |
+---------------+-------------------------+--------------------------+
| DPN_TOPO_MAP | Reference to topology | Can be map of physical |
| | map | or virtual topology |
+---------------+-------------------------+--------------------------+
| DPN_ADJ_TYPE | Adjacency={UPA adj.,UPA | Candidate router for |
| | attached, DN adj. } | UPA, DN/PE next hop |
+---------------+-------------------------+--------------------------+
| DPN_STAT_INF | Status info incl. load, | |
| | expected changes | |
+---------------+-------------------------+--------------------------+
Figure 13: DPN Information Elements
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+--------------------------------------------------------------------+
| IE | Description | Note about Use/Format |
+====================================================================+
| DN_ID | Identifier of the DN | String or numerical ID |
+---------------+-------------------------+--------------------------+
| DN_DPN_ID | Access to DN, e.g. PE | Reference to AP of DN, |
| | | type DN_ID |
+---------------+-------------------------+--------------------------+
| DN_GEO_LOC | DN geographic locator | |
+---------------+-------------------------+--------------------------+
| DN_TOPO_LOC | DN topological locator | |
+---------------+-------------------------+--------------------------+
| DN_TOPO_MAP | Reference to topology | Can be map of physical |
| | map | or virtual topology |
+---------------+-------------------------+--------------------------+
| DN_CAND_LIST | Candidate list of | List of DN_IDs |
| | alternative DNs | |
+---------------+-------------------------+--------------------------+
| DN_TARGET | Target DN | Single DN_ID |
| | | |
+---------------+-------------------------+--------------------------+
Figure 14: DN Information Elements
Authors' Addresses
Marco Liebsch
NEC Laboratories Europe GmbH
Kurfuersten-Anlage 36
D-69115 Heidelberg
Germany
Email: marco.liebsch@neclab.eu
Jari Mutikainen
NTT Docomo
Email: mutikainen@docomolab-euro.com
Zhaohui Zhang
Juniper Networks
Email: zzhang@juniper.net
Tianji Jiang
China Mobile
Email: tianjijiang@chinamobile.com
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