kim-spring-mobile-network-use-cases-03.txt

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draft-kim-spring-mobile-network-use-cases

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Network Working Group J.Y. Kim
Internet Draft ETRI
Intended status: Informational G.M. Lee
LJMU
Expires: April 2017
October 31, 2016

SPRING Use cases for Mobile Network
draft-kim-spring-mobile-network-use-cases-03

Abstract

The ability for a node to specify a forwarding path, other than the
normal shortest path, that a particular packet will traverse,
benefits a number of network functions. Source-based routing
mechanisms have previously been specified for network protocols, but
have not seen widespread adoption. In this context, the term
'source' means 'the point at which the explicit route is imposed'.

The objective of this document is to illustrate some use cases that
provide the traffic engineering and the load balancing for mobile
and transport network, applying segment routing.

Status of this Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

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The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

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The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html

This Internet-Draft will expire on April 31, 2016.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with
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warranty as described in the Simplified BSD License.

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Table of Contents

1. Introduction ................................................ 4
1.1. Terminology and abbreviations ........................... 5
2. Mobile network overview ...................................... 6
2.1. GTP tunneling 8
2.2. Quality of Service (QoS) 9
3. Use case ................................................... 10
4. Security Considerations ..................................... 11
5. IANA Considerations ........................................ 11
6. References ................................................. 11
6.1. Normative References 11
6.2. Informative References 12
7. Acknowledgments ............................................ 13

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1. Introduction

In a mobile network, GTP is the protocol developed for tunneling and
encapsulation of data units and control messages in GPRS. GTP for
the Evolved 3GPP system comes in two variants, control and user
plane. The control plane GTP-C handles the signaling, and it is
needed in addition to the protocol for pure transfer of user related
data, GTP-U. [3GPP TS 23.060]

GTP-U is used for carrying user data within the GPRS core network
and between the radio access network and the core network. The user
data transported can be packets in any of IPv4 or IPv6 formats. GTP-
C is out of scope in this document.

IP packets are forwarded through the GTP tunnel between the P-GW and
the eNB for transmission to the UE in a mobile network. These GTP
tunnels are established per EPS bearer when a user is attached to
the LTE network. EPS uses the concept of EPS bearers to route IP
traffic from a gateway in the PDN to the UE. A bearer is an IP
packet flow with a defined quality of service (QoS) between the
gateway and the UE.

On the other hand, IP transport networks may provide data
transmission and interaction between the gateways and the UE. The
mobile nodes like the gateways are connected to an IP transport
network in overlay. Therefore the mapping between transport nodes
and mobile nodes may be needed to consistently guarantee QoS in
terms of priority control, traffic engineering and load balancing.

For simplicity we only describe GTP tunneling in the context of LTE
(Long Term Evolution), which aims to provide seamless Internet
Protocol (IP) connectivity between user equipment (UE) and the
packet data network (PDN). Indeed GTP tunneling also applies to
earlier generations of mobile networks, such as purely UMTS-based
mobile networks.

On the other hand, segment routing mechanisms [draft-ietf-spring-
problem-statement] have been developed to provide the traffic
engineering and the load balancing for networks. These mechanisms
could be also applicable to an IP-based mobile and transport network.
Therefore this document addresses how to utilize segment routing
mechanisms in a mobile network.

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The objective of this document is to illustrate some use cases that
provide QoS in a mobile and transport network by applying segment
routing.

1.1. Terminology and abbreviations

Much of the terminology used in this document has been defined by
the 3rd Generation Partnership Project (3GPP), which defines
standards for mobile service provider networks. Although a few terms
are defined here for convenience, further terms can be found in
[RFC6459].

AS Access Switch

AR Aggregation Router

ARP Allocation and Retention Priority

CE Customer Edge Router

D-IP Destination IP address

eNB enhanced NodeB

ECMP Equal Cost Multi-path Protocol

EPC Evolved Packet Core

EPS Evolved Packet System

ER Edge Router

G-PDU GTP-U Protocol Data Unit

GTP GPRS (General Packet Radio Service) Tunneling Protocol

GTP-U GTP layer for the user plane

HSS Home Subscriber System (control plane element)

IMSI The International Mobile Subscriber Identity

LTE Long Term Evolution

MME Mobility Management Entity (control plane element)

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P-GW Packet Gateway

PCE Path Computation Element

PCRF Policy and Charging Rules Function

PDN Packet Data Network

PDU Protocol Data Unit

PE Provider Edge Router

QCI QoS Class Identifier

QoS Quality of Service

S-GW Serving Gateway, primary function is user plane mobility

S-IP Source IP address

T-PDU Transport Protocol Data Unit

TEID 3GPP standardized Policy and Charging Rules Function

UDP User Datagram Protocol

UE User Equipment

VoIP Voice over IP

2. Mobile network overview

The major functional components of a LTE network are shown in Figure
1 and include user equipment (UE) like smartphones or tablets, the
LTE radio unit named enhanced NodeB (eNB), the serving gateway (S-
GW) which together with the mobility management entity (MME) takes
care of mobility and the packet gateway (P-GW), which finally
terminates the actual mobile service. These elements are described
in detail in [TS.23.401]. Other important components are the home
subscriber system (HSS) and the policy and charging rule function
(PCRF), which are described in [TS.23.203]. The P-GW interface
towards the SGi-LAN is called the SGi-interface, which is described
in [TS.29.061].

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In LTE, a mobile network consists of some LTE components
(specifically EPS) and GTP connections between eNB and S-GW, and
between S-GW and P-GW are established, as shown in Figure 1. An IP
packet for a UE is encapsulated in a GTP-U packet and tunneled cross
multiple interfaces (the S5/S8 interface from the P-GW to the S-GW,
the S1 interface from the S-GW to the eNB), and the radio interface
from the eNB to the UE.

The LTE components would be connected in overlay to MPLS components
such as AS, AR and ER in an IP transport network. The interactions
between LTE components occur through MPLS components.

EPS provides the user with IP connectivity to a PDN for accessing
the Internet, as well as for running services such as Voice over IP
(VoIP). An EPS bearer is typically associated with a QoS. Multiple
bearers can be established for a user in order to provide different
QoS streams or connectivity to different PDNs.

+----------------------------------------+
| Mobile Network [HSS] | [OTT Appl. Platform]
| | | |
| +--------- [MME] [PCRF]-----+--------+ |
| | | | | | |
| + [S-GW] [P-GW] | | Internet
| | S1-U |S5/S8 | | | |
| [UE] -- [eNB]----------+------+ | | |
| | | | | |
+===========|===================|========+ +-----+-----+-------+
| | | | | |
| [AS] - [AR(PE)] == [ER(PE)] == +--+----[SGi-LAN] |
| | | | |
| | | | |
| | | [Appl. Platform] |
| IP Transport Network | | |
+----------------------------------------+ +-------------------+

Figure 1 Architecture for mobile and transport network

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2.1. GTP tunneling

GTP-U tunnels are used to carry encapsulated T-PDUs and signalling
messages between a given pair of GTP-U Tunnel Endpoints. The Tunnel
Endpoint ID (TEID) which is present in the GTP header shall indicate
which tunnel a particular T-PDU belongs to. In this manner, packets
are multiplexed and de-multiplexed by GTP-U between a given pair of
Tunnel Endpoints. G-PDU is user data packet (T-PDU) plus GTP-U
header, sent between GTP network nodes. T-PDU is a user data packet,
for example an IP datagram, sent between a UE and a network entity
in an external packet data network. A T-PDU is the payload that is
tunneled in the GTP-U tunnel [3GPP TS 29.281].

GTPv1-U tunnel endpoints do not need to perform any IP routing
functions in respect to inner IP packet since it shall be
encapsulated at the GTPv1-U sender with a GTP header, Outer UDP and
IP header.

Outer UDP/IP is the only path protocol defined to transfer GTP
messages in a mobile network. The UDP source port may be allocated
dynamically by the sending GTP-U entity. Dynamic allocation of the
UDP source port may help balancing the load in the network even
though the scheme does not allow to deterministically force a
specific path, using ECMP.

For illustration, Figure 2 shows how the GTP encapsulation process
works. The user data packet itself including the header and payload
is preserved and kept unchanged. The packet is just added a GTP
header used by the receiving end to identify which tunnel the packet
is associated to. This GTP encapsulated packet is transported
between the two tunnel endpoints using a transport layer header,
specifically an outer IP/UDP header.

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Figure 2 GTP encapsulation

2.2. Quality of Service (QoS)

In a typical case, multiple applications may be running in a UE at
any time, each one having different QoS requirements. For example, a
UE can be engaged in a VoIP call while at the same time browsing a
web page or downloading an FTP file. VoIP has more stringent
requirements for QoS in terms of delay and delay jitter than web
browsing and FTP, while the latter requires a much lower packet loss
rate. In order to support multiple QoS requirements, different
bearers are set up within the Evolved Packet System, each being
associated with a QoS.

In the access network, it is the responsibility of the eNB to ensure
the necessary QoS for a bearer over the radio interface. Each bearer
has an associated QoS Class Identifier (QCI), and an Allocation and
Retention Priority (ARP). The QCI specifies values for the priority
handling, acceptable delay budget and packet loss rate for each QCI
label. The QCI label for a bearer determines how it is handled in
the eNB.

IP packets mapped to the same EPS bearer receive the same bearer-
level packet forwarding treatment. In order to provide different
bearer-level QoS, a separate EPS bearer must therefore be
established for each QoS flow. User IP packets must then be filtered
into the appropriate EPS bearers [3GPP TS.23.203].

Meanwhile, considerations about traffic engineering schemes and
different QoS requirements would not explicitly mentioned until now
in terms of IP transport network.

In addition, a simple traffic engineering scheme using ECMP would
lead to no load balancing and result to inefficient use of transport
resources because mobile services have different type of application
and QoS requirements. Congestion on the shortest path link could be
difficult to avoid when network status is not normal, for example
the higher than usual traffic demand and dynamically changing
traffic patterns.

In this regard, segment routing is regarded as a good way to provide
traffic engineering and load balancing for IP transport network in
mobile environment.

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3. Use case

The term "source" means "the point at which the explicit route is
imposed". Therefore GTP-U Tunnel Endpoint (e.g., eNB, S-GW, P-GW)
could be a "source" by applying segment routing. In addition, an EPS
bearer is typically associated with a QoS. Therefore, in order to
provide different bearer-level QoS, GTP-U Tunnel Endpoints could
ensure the necessary QoS for a bearer. With this the transport
network can provide a same level of QoS as much as mobile network
does.

LTE nodes (e.g., eNB, S-GW and P-GW) in overlay are connected to the
routers (e.g., A, B, C, etc.) in a transport network and have
interactions with each other via them. LTE components are assumed to
have multiple interfaces to the routers and an interface for packet
forwarding would be selected, depending on PCE's decision. PCE (Path
Computation Element) makes path computation using traffic matrix
that describes the bandwidth requirements between sources and
destinations in a network.

In order to more clearly explain use cases for a mobile network, LTE
nodes with multi-paths through IP transport nodes are shown in
Figure 3. Each LTE node is connected to IP transport nodes that have
corresponding two transport paths alternatively. Two shortest paths
between LTE nodes, B-C and G-H, are assumed respectively. Two
alternative paths between LTE nodes, A-E-D and F-I, are assumed
respectively.

An UE is connected to Internet (actually, a certain server) via both
mobile and transport nodes. In the example when the higher than
usual traffic is occurring, the shortest path between eNB and S-GW
as well as S-GW and P-GW (segment list B-C and G-H, respectively)
has low delay but is highly utilized. As a result, no load balancing
will happen and QoS requirements of the different applications will
not be satisfied. In this regard, an alternative path should be
selected by considering network status (segment list A-E-D and F-I,
respectively). With explicit path allocation to dynamically changing
traffic patterns, inefficient use of transport resources and QoS
degradations could be avoided. Therefore in a mobile network,
segment routing would lead to more optimal load distribution and
forwarding for different types of applications.

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For example, a specific path would be chosen by analyzing QCI since
it could represent QoS characteristics in terms of packet delay and
packet loss. Higher delay and underutilized path could be used for
delivery of delay insensitive service as represented by QCI.

UE-----eNB=====S-GW=====P-GW----Internet
| \ / | |\ / |
A B-C D F G--H I
\ / \ |
E----+ +-----+

Figure 3 LTE nodes connected to multi-paths via IP transport nodes

4. Security Considerations

This document doesn't introduce new security considerations when
applied to the MPLS dataplane.

There are a number of security concerns with source routing at the
IPv6 dataplane [RFC5095]. The new IPv6-based segment routing header
defined in [I-D.ietf-6man-segment-routing-header] and its associated
security measures address these concerns.

5. IANA Considerations

This document does not require any action from IANA.

6. References

6.1. Normative References

None

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6.2. Informative References

[3GPP TS.23.060]

"General Packet Radio Service (GPRS); Service description",
3GPP TS 23.060 13.3.0, June 2015.

[3GPP TS.23.203]

"Policy and charging control architecture", 3GPP TS 23.203
13.4.0, June 2015.

[3GPP TS.23.401]

"General Packet Radio Service (GPRS) enhancements for
Evolved Universal Terrestrial Radio Access Network (E-
UTRAN) access", 3GPP TS 23.401 12.3.0, December 2013.

[3GPP TS. 29.061]

"Interworking between the Public Land Mobile Network
(PLMN) supporting packet based services and Packet Data
Networks (PDN)", 3GPP TS 29.061 12.4.0, December 2013.

[3GPP TS. 29.274]

"3GPP Evolved Packet System (EPS); Evolved General Packet
Radio Service (GPRS) Tunnelling Protocol for Control plane
(GTPv2-C); Stage 3", 3GPP TS 29.274 13.2.0, June 2015.

[3GPP TS.29.281]

"General Packet Radio System (GPRS) Tunnelling Protocol
User Plane (GTPv1-U)", 3GPP TS 29.281 12.1.0, December
2014.

[draft-ietf-6man-segment-routing-header]

Previdi, S., Filsfils, C., Field, B., Leung, I., Linkova,
J., Kosugi, T., Vyncke, E., and D. Lebrun, "IPv6 Segment
Routing Header (SRH)", draft-ietf-6man-segment-routing-
header-01 (work in progress), March 2016.

[draft-ietf-sfc-use-case-mobility]

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W. Haeffner, J. Napper, M. Stiemerling, D. Lopez and J.
Uttaro, "Service Function Chaining Use Cases in Mobile
Networks", draft-ietf-sfc-use-case-mobility-01, July 2014.

[RFC7855]

S. Previdi, C. Filsfils, B. Decraene, S. Litkowski, R.
Geib, R. Shakir and R. Raszuk, " Source Packet Routing in
Networking (SPRING) Problem Statement and Requirements",
RFC 7855, May 2016.

[draft-ietf-spring-ipv6-use-cases]

J. Brzozowski, J. Leddy, I. Leung, S. Previdi, M. Townsley,
C. Martin, C. Filsfils and R. Maglione, "IPv6 SPRING Use
Cases", draft-ietf-spring-ipv6-use-cases-05, September
2015.

[RFC5095]

Abley, J., Savola, P., and G. Neville-Neil, "Deprecation
of Type 0 Routing Headers in IPv6", RFC 5095, December
2007.

[wan-traffic-engineering]

"Wide Area Network Traffic Engineering", Ericsson White
Paper, December 2015.

7. Acknowledgments

TBD

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Authors' Addresses

Jeongyun Kim (editor)
ETRI
218 Gajeong-ro, Yuseong-gu, Daejeon, 305-700, KR

Email: jykim@etri.re.kr

Gyu Myoung Lee
Liverpool John Moores University
James Parsons Building, Byrom Street, Liverpool, L3 3AF, UK

Email: G.M.Lee@ljmu.ac.uk

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