SPRING and DMM P. Camarillo, Ed.
Internet-Draft C. Filsfils
Intended status: Standards Track Cisco Systems, Inc.
Expires: February 16, 2020 H. Elmalky, Ed.
Individual
S. Matsushima
SoftBank
D. Voyer
Bell Canada
A. Cui
AT&T
B. Peirens
Proximus
August 15, 2019
SRv6 Mobility Use-Cases
draft-camarilloelmalky-springdmm-srv6-mob-usecases-02
Abstract
This document describes the SRv6 use-cases in the mobile network in
association with different mobile generations (3G, 4G, and 5G). It
also highlights potential interworking with SR-MPLS in relevant use-
cases.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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
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Drafts is at https://datatracker.ietf.org/drafts/current/.
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 February 16, 2020.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Use-cases . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. SP Network Simplification use-cases . . . . . . . . . . . 5
3.1.1. Radio-core Handoff . . . . . . . . . . . . . . . . . 5
3.1.1.1. Radio-transport programmability . . . . . . . . . 5
3.1.1.2. User-plane state transfer, offload, and mutation 6
3.1.1.3. Rip-n-replace of GTP with SRv6 . . . . . . . . . 8
3.1.2. End-to-end network slicing [N3, N9, N6 and transport] 9
3.1.3. GiLAN Service Programming [N6 and N9] . . . . . . . . 9
3.1.3.1. Service Programming on Gi-LAN for 3G/4G [SGi] . . 10
3.1.3.2. Service Programming for 5G [N6 and N9] . . . . . 10
3.1.4. ID-Location Isolation at anchors . . . . . . . . . . 10
3.2. New mobility use-cases . . . . . . . . . . . . . . . . . 10
3.2.1. eMBB (Enhanced Mobile Broadband) . . . . . . . . . . 10
3.2.1.1. Fixed/Mobile Convergence (HA, FWA & WA) . . . . . 10
3.2.1.2. Mobile Enforced SD-WAN . . . . . . . . . . . . . 11
3.2.2. mMTC (massive Machine Type Communications) . . . . . 11
3.2.2.1. Stationary IoT Devices (industrial applications) 11
3.2.3. URLLC (Ultra Reliable Low Latency Communications) . . 12
4. Work in progress . . . . . . . . . . . . . . . . . . . . . . 12
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.1. Normative References . . . . . . . . . . . . . . . . . . 12
6.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
4G/LTE mobile networks are complex and the use cases that 5G has been
architected to address, introduce new requirements and additional
complexity to both the RAN and the mobile core. The current
architecture employs the GPRS tunneling protocol (GTP) as the primary
vehicle for user plane interconnect in the RAN and 5GC. GTP is
currently used in two contexts, from the RAN to the first anchor
point; the S-PGW/UPF (S1-U/N3 interface) and for inter S-PGW/UPF
connectivity (S5-S8/N9 interface). While the tunnels themselves do
not impose significant state beyond that needed, they do have a
significant control plane setup component and are a potential target
for network delayering.
Segment Routing [I-D.ietf-spring-segment-routing] is a network
architecture that simplifies networks by removing state from the
network infrastructure, creating a scalable SDN architecture for
overlays (VPNs), underlay (SLA, Traffic Engineering, FRR) and service
programming (GiLAN). The IPv6 instantiation -also known as SRv6
[I-D.ietf-spring-srv6-network-programming]- takes this even further
with the introduction of the Network Programming concept, allowing to
bind segments to any kind of VNF anywhere in the network -from
private DCs to public cloud services.
Segment routing embodies a number of potentially useful properties
for consideration in a 4G/5G mobile networking context:
1.- Direct manipulation of path routing by the head-end
SRv6 provides the ability to direct traffic through an arbitrary path
without the imposition of path state in the network or requiring a
separate signaling system. It does this without signaling by
encoding the path state in the packet header. This means the path
head-end can instantly fulfill changes to a path by simply changing
the header encoded information.
This capability has numerous applications as far as networking in
general (traffic engineering, policy routing etc.), but has
additional applicability to mobile networks:
o The ability of a path head-end to manipulate the intermediate hops
in a path can be exploited for end system mobility, the
penultimate hop simply becomes the "care of" address.
o The ability of path head-end to imply an asymmetric return path
for a specific forwarding equivalence class (FEC).
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o Densification in the radio topology embodied in concepts like
coordinated multipoint and multi-connectivity require the
instantaneous redirection of traffic from the coordinating radio
controller to any of several base stations. This is critical to
exploit ephemeral "rich paths" that 4G & 5G radio technologies
depend upon to achieve high rates of information transfer.
2.- Network programmability
The ability to bind segments to network functions provides an
increased level of abstraction in service delivery combined with a
practical realization. This would have applications in the GiLAN/N6,
combined with the ability to specify a path from the head-end as
applications in the GiLAN/N6.
3.- Overall simplification of the control plane
As noted previously SRv6 dispenses with a signaling system. This has
obvious benefits as a simplification to overall network operation,
but may have additional benefits in the "signaling rich" environment
of mobile networks.
This memo serves to critically explore the applicability of SRv6 to
4G/5G mobile networks. It does that via an exploration of how SRv6
can simplify current mobile network architecture to improve the
status quo of eMBB operation, and then delves into the new use cases
that 5G is targeted towards.
2. Terminology
This document focuses on the use-cases, and it's associated
terminologies. The full list of terminologies exists in
[I-D.ietf-spring-srv6-network-programming].
In this document we focus on the 5G systems architecture, as
specified in [TS.23501]. This document also refers to 3G and 4G
networks as specified in [TS.23002].
The uplink/upstream traffic is the traffic originated at the UE,
while the downlink/downstream traffic is traffic destined towards the
UE.
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3. Use-cases
Use-cases have been classified into multiple categories depending on
their fit into the mobile-network domain (Radio, Transport, Core) or
mobile network generation (3G/4G, or 5G).
3.1. SP Network Simplification use-cases
3.1.1. Radio-core Handoff
3.1.1.1. Radio-transport programmability
Advances in radio technology, the deployment of new spectrum for 5G
and the quest for ever increasing spectral efficiency results in
increasingly complex RAN and air interface topologies. The result is
that the RAN end of a GTP tunnel may appear as a single end point to
the core network, but the actual realization is substantially more
complex.
Modern radio scheduling is increasingly focused on using techniques
such as MIMO to multiply the instantaneous bandwidth available for
information transfer for a given unit of spectrum. A "rich path" can
be very ephemeral so any latency between path measurement and
initiating data transfer to a UE can be parasitic in the overall
system efficiency.
3.1.1.1.1. Multi-connectiveity and coordinated multi-point
There are multiple scenarios where a UE can be associated with more
than one antenna and the associated spectrum:
Coordinated multipoint (CoMP) involves a UE associated with multiple
geographically distributed antennas serving a common block of
spectrum, and the radio controller selecting the best antenna at any
given time. The other antennas being quiescent in the sector
occupied by the UE at the time of transmission to avoid overlap.
This can be in the context of an RRC/RLC split (F1-U interface) or a
Phy Hi/Phy Lo split (F2-U interface).
Multi-connectivity can see a UE associated with multiple antennas
each serving different spectral allocations. Applications include
offload from a macro cell to a small cell. The possibility of
simultaneous transfer from multiple antennas also exists. And again
this can be on the basis of an F1 or F2 split in the RAN.
In both cases the radio controller is required to be able to
instantly redirect traffic based on current radio measurements to any
of a constellation of antennas serving a given UE.
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3.1.1.1.2. Fronthaul
Modern radio systems have been deconstructed in order to drive
efficiency across a variety of metrics. In essence various stages of
waveform construction have been abstracted and exposed on interfaces
as part of the 5G RAN architecture. In the most simplest form it
allows putting functionality where it is easy to service, such as the
equipment at the bottom of the tower rather than the top. In a rich
radio connectivity context it permits co-location of radio scheduling
and waveform generation which drives spectral efficiency, but where
applied also results in significant multiples of bandwidth, and very
tight jitter and delay requirements. The current specification for
the F2-U or e(CPRI) packet interface has a maximum latency of 75 usec
and correspondingly tight jitter requirements.
3.1.1.2. User-plane state transfer, offload, and mutation
A proper session handoff between radio, transport, and mobile-core
requires storing/recalling user-plane session state on multiple
levels. The use of SRv6 reduces the number of states needed in the
network nodes by mapping the UE session state into IPv6 SID (Segment
IDs) in SRH. Furthermore, mutation of SID-lists shall enable SMF to
program data-paths (handling-state) and policies (serving-state) on
per-subscriber / per-application level.
That session state can be broken down into two categories:
1.- Handling state: Who is the session handler?
o A 1-to-1 mapping between GTP tunnel (TEID) and S-PGW/UPF
o Usually stored at load-balancers deployed ahead of S-PGW/UPF
instances or embedded inside the S-PGW/UPF system.
2.- Serving state: What is the serving-policy associated with this
session?
o A 1-to-1 mapping between the UE and a specific policy to be
enforced on the subscriber traffic.
o The policy may include (but not limited to) the authorization &
accounting profile, one or multiple QoS profiles, one or multiple
service-chaining/programming profiles.
o A 1-to-1 mapping between a UE session and it's stats-registers at
the S-PGW/UPF.
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o It is typical that S-PGW/UPF may break down the service-state into
sub-states reflecting groups of 5-tuple flows, or employ other
techniques (ex. DPI, deep packet inspection) to break down the
serving-state even further within the same 5-tuple flow.
The ability to transfer, offload, or mutate the user-plane state with
no/minimum disruption to end-users is one of the most significant
challenges facing the mobile network's scalability towards mMTC use-
cases (The current GTP-U mandates a per-session tunnel creation &
handling). Moreover, the direct 1-to-1 binding between UE session ID
and Location affects the optimal-path selection, which is one of the
most significant challenges facing URLLC use-cases in 5G.
The use of SRv6 shall simplify the state storage dramatically where a
single SID-list embedded in the UE session packet can store the
handling-state and the majority of the serving-state. SRv6
programmability and traffic-engineering shall allow an easy way to
transfer, offload, or mutate that state.
3.1.1.2.1. State-offload:
Upstream state-offload:
the use of SRv6 shall allow the S-PGW/UPF anchor(s) to offload the
load-balancing function from a dedicated load-balancer in mobile-core
to be a standard function in packet-forwarding in transport network
where any SR-aware node on the path between eNB and S-PGW/UPF can
forward the UE session to the proper S-PGW/UPF handling instant by
relying on the handling-state stored in the SID-list in each packet.
Downstream state-offload:
The L3 anchor (PGW/UPF) is the first node that handles the subscriber
traffic in the downstream direction, depending on the policy
associated with the subscriber traffic. The PGW/UPF may decide to
hairpin the traffic through multiple application (service chain)
before sending it towards the radio-network. This implies double
packet-processing on PGW/UPF instant (50% penalty on the VNF useful
throughput).
The use of SRv6 shall allow the PGW/UPF to impose a specific data-
path on a group of 5-tuple flows without the need for hairpins all
the traffic through PGW/UPF. Which means the PGW/UPF can offload the
first packet processing towards another none-SR- node earlier in the
downstream path (ex. Service-proxy, or packet inspector) as per
specific service-pipeline policy.
Moreover, that offload-service can be programmed once the S-PGW/UPF
terminate the subs-session on the upstream direction. Alternatively,
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the offload-service can be programmed on-demand after the first few
packets been hair-pinned through the PGW/UPF on the downstream path.
3.1.1.2.2. State-transfer:
Handling-state:
SRv6 shall enable the handling-state to be embedded in the data-flow
as metadata (in a form of SID-list). This means that all load-
balancing operations can be performed by any of the SR-aware
intermediate nodes in a stateless fashion with a zero-state transfer
at failure scenarios.
Serving-state:
Depending on the applied policy, a significant portion of the
serving-state can be embedded in the data-flow as metadata (in a form
of SID-list). This means that serving nodes (S-PGW/UPF) have a
smaller amount of data to store/recall to serve the UE session.
3.1.1.2.3. State-mutation:
SRv6 provides a more natural way to mutate the handling-state and
serving-state to follow the optimal data path or fulfill traffic-
engineering constrain(s).
In contrast to the current limitation of mutating the state only at
SGW (session L2-anchor point) or PGW (session L3-anchor point). SRv6
shall allow the state mutation on any authorized SR-aware node
between radio and mobile core.
3.1.1.3. Rip-n-replace of GTP with SRv6
A possible mechanism to do an early-deployment of SRv6 is to keep the
tunnel-nature of GTP but do a simple data-plane replacement of
IP/UDP/GTP-U with SRv6 for specific PDU sessions. In this case,
there is no session aggregation, and the SRv6 segment corresponding
to the overlay creation now carries the TEID, QFI and RQI as part of
the SID arguments.
In this use-case there is no subscriber-traffic integration with the
underlay or service programming. There could be some integration but
it is based on static policies and not configured via the currently
existing mobility management.
This is an interworking mechanism that shall used for an early stage
implementation with no changes to the N4 interface.
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3.1.2. End-to-end network slicing [N3, N9, N6 and transport]
One of operator's main challenges is providing end-to-end network
slicing, taking into consideration the RAN, the S-PGW/UPF and the
VNFs in the GiLAN; but more importantly taking also into
consideration the transport network.
SRv6 can help bridging the gap in between all of these since it
integrates the overlay, underlay and service programming into a
single protocol. End-to-end SR policies can be defined that span
across the RAN, S-PGW/UPF and transport network, without requiring
any stitching configuration at the domain boundaries. From an
overlay perspective, it is clear that SRv6 can provide -if desired-
isolation among different RAN or S-PGW/UPF nodes.
In the transport network, the SRv6 overlay can integrate with an
existing SRv6 or SR-MPLS transport network to provide traffic
engineering in the underlay network infrastructure. SR provides
operators with a stateless mechanism to build network slices with
different optimization objectives or constrains i.e. low-latency
(uRLLC), resource isolation (disjointness), etc...
Also, SR provides mechanisms for in-band performance monitoring.
This implies that the end-to-end network slice can react upon
topology changes -that for example might change the low-latency
path-.
3.1.3. GiLAN Service Programming [N6 and N9]
Service Programming, in coordination with SRv6 can be used for
optimal placement of VNFs in the Gi-LAN of mobile operators for
flawless VNF management and placement -DC resource utilization-.
SRv6 transparently integrates VNFs
[I-D.xuclad-spring-sr-service-programming], in the same SR policy
used for overlay creation and underlay control. The VNFs are cloud-
infrastructure agnostic -can be hosted on a private DC or public
cloud-, and there is no state per-flow or per-chain in the network
infrastructure. This implies a huge flexibility for mobile
operators. Note that VMs can be distributed in different tenants, or
can be migrated while there is live traffic without any major
manageability complexity, state to update in the network
infrastructure or packet loss. Note also that in the case of network
slicing, the VNFs can be shared across multiple slices or can be
restricted to only a particular slice. This can be chosen on a per-
VNF granularity.
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In addition, SRv6 offers mechanisms to do VNF load-balancing and to
convey additional flow information to stateless VNFs using the SRv6
SID arguments, by leveraging the network programming concept.
3.1.3.1. Service Programming on Gi-LAN for 3G/4G [SGi]
SRv6-based NFV provides an approach to optimally steer traffic
through Gi-LAN network functions in 3G/4G networks.
The PGW can steer uplink traffic into a specific SR policy that
contains as many segments as VNFs that the packet must traverse. The
packet follows the path specified in the SR policy, traversing the
set of VNFs before getting delivered to the external PDN -i.e.
internet-.
3.1.3.2. Service Programming for 5G [N6 and N9]
In 5G networks SRv6 can offer NFV control, as done in the Gi-LAN for
3G-4G networks (N6 interface), but can also integrate the VNFs within
the N9 interface. This means that we can have more flexibility
regarding the distribution and association of the functions/VNFs/
micro-services, and bring applications closer to the user, where they
might be better located for the operator and improve the overall
customer experience.
3.1.4. ID-Location Isolation at anchors
TBD
3.2. New mobility use-cases
3.2.1. eMBB (Enhanced Mobile Broadband)
3.2.1.1. Fixed/Mobile Convergence (HA, FWA & WA)
The end users of different access networks under control of the same
service provider would obtain significant benefit if there is a tight
integration for service delivery in between the mobile access network
and the fixed network.
This is the example of a residential user that is accessing content
from his mobile phone, and once he arrives home his phone
automatically connects to his home wireless network provided whose
connectivity is provided by the same operator. As per today, these
networks have different architectures, with different control-planes
and data-planes, and with different policy control and service
management.
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SRv6 helps uniting the gap in between different access networks by
optimizing the data path in between hierarchical networks and
directly adding an SR policy that spans from the mobile packet core
up to the broadband network BNG. Such capability will simplify the
delivery of fixed-services on top of wireless infrastructure. it will
also enable the simultaneous use of wireless and fixed connections
towards end-user.
3.2.1.2. Mobile Enforced SD-WAN
TBD
3.2.2. mMTC (massive Machine Type Communications)
3.2.2.1. Stationary IoT Devices (industrial applications)
There are many types of IoT devices, ranging from connected cars to
massive machine type devices like meter readers, which are
stationary. One of these examples is electricity meters. These
devices are static and might only attach to other gNBs due to
changing RF conditions.
Massive machine type devices is projected to grow to 10's of billions
in operator networks in the next few years. However, the traditional
3GPP GTP tunnel/bearer based connection-oriented architecture does
not scale for billions of IoT devices due to the amount of signaling
overhead associated with GTP tunnel setup/tear- down and the UE
context information maintained at various parts of the mobile
network.
Unlike smart devices, electric meters never move and each generates
low RPU for carriers. For this reason, to efficiently support the
massive machine type of stationary IoT devices, a simpler and more
scalable control and user plane architecture is needed that can
reduce the amount of signaling overhead and the UE context
information kept in the network. This new architecture will need to
work across all types of access technologies to improve adaptability
to future RAT networks.
SRv6 can help improve scalability in the RAN, transport, and packet
core networks significantly by removing GTP tunnels for each
individual stationary IoT device, and replacing by the aggregated
SRv6 route information for all the similar stationary IoT devices.
For instance, at the eNB/gNB, only the first electric meter device
for an electric company needs the SRv6 route set up procedure, which
has one SRv6 look up table entry associated with it. No subsequent
SRv6 route set up procedures and no additional SRv6 table entries for
the succeeding electric meters are needed at the same eNB/gNB. This
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effectively reduces the signaling overhead and UE context overhead by
(1-1/N)% (where N is the number of the electric company meter readers
in the same eNB/gNB). In the case of RAN virtualization with an
aggregated vBBU for many cell sites, the reduction of the signaling
and UE context overhead will be greater since N is a much bigger
number.
The significant reduction of the signalling overhead and UE context
overhead can be translated to the cost reduction of running
operators' wireless network. In addition, this new architecture
using SRv6 allows flexible service edge treatment, service chaining,
such as billing, TE or other capabilities.
3.2.3. URLLC (Ultra Reliable Low Latency Communications)
TBD
4. Work in progress
o Use of SRv6 in optimizing interface (reference N4 as defined by
3GPP xxx r16) between control-plane and user-plane.
o Security implications & benefits of SRv6 in mobile networks.
5. Acknowledgements
We would like to thank Francois Clad, Darren Dukes, Zafar Ali, Peter
Bosch, Simon Spraggs and Tom Anschutz for their help.
6. References
6.1. 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>.
[TS.23501]
3GPP, "System Architecture for the 5G System", 3GPP TS
23.501 15.2.0, June 2018.
6.2. Informative References
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[I-D.ietf-spring-segment-routing]
Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing
Architecture", draft-ietf-spring-segment-routing-15 (work
in progress), January 2018.
[I-D.ietf-spring-srv6-network-programming]
Filsfils, C., Camarillo, P., Leddy, J.,
daniel.voyer@bell.ca, d., Matsushima, S., and Z. Li, "SRv6
Network Programming", draft-ietf-spring-srv6-network-
programming-01 (work in progress), July 2019.
[I-D.xuclad-spring-sr-service-programming]
Clad, F., Xu, X., Filsfils, C., daniel.bernier@bell.ca,
d., Li, C., Decraene, B., Ma, S., Yadlapalli, C.,
Henderickx, W., and S. Salsano, "Service Programming with
Segment Routing", draft-xuclad-spring-sr-service-
programming-02 (work in progress), April 2019.
[TS.23002]
3GPP, "Network Architecture", 3GPP TS 23.23002 15.0.0,
March 2018.
Authors' Addresses
Pablo Camarillo Garvia (editor)
Cisco Systems, Inc.
Spain
Email: pcamaril@cisco.com
Clarence Filsfils
Cisco Systems, Inc.
Belgium
Email: cf@cisco.com
Hani Elmalky (editor)
Individual
United States of America
Email: hani.elmalky@gmail.com
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Satoru Matsushima
SoftBank
1-9-1,Higashi-Shimbashi,Minato-Ku
Tokyo 105-7322
Japan
Email: satoru.matsushima@g.softbank.co.jp
Daniel Voyer
Bell Canada
Canada
Email: daniel.voyer@bell.ca
Anna Cui
AT&T
United States of America
Email: zc1294@att.com
Bart Peirens
Proximus
Belgium
Email: bart.peirens@proximus.com
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