SPRING Use cases for Mobile Network
draft-kim-spring-mobile-network-use-cases-00
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| Authors | Jeong Yun Kim , Gyu Myoung Lee | ||
| Last updated | 2015-10-19 | ||
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draft-kim-spring-mobile-network-use-cases-00
Network Working Group J.Y. Kim
Internet Draft ETRI
Intended status: Informational G.M. Lee
Expires: April 2016 LJMU
October 19, 2015
SPRING Use cases for Mobile Network
draft-kim-spring-mobile-network-use-cases-00
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.
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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 7
2.2. QoS 8
3. Use case .................................................... 9
4. Security Considerations ..................................... 10
5. IANA Considerations ........................................ 10
6. References ................................................. 10
6.1. Normative References 10
6.2. Informative References 11
7. Acknowledgments ............................................ 12
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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 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.
Segment routing mechanisms [draft-ietf-spring-problem-statement]
have been developed to provide the traffic engineering and the load
balancing for networks. These mechanisms are 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
CE Customer Edge Router
EPC Evolved Packet Core
EPS Evolved Packet System
ER Edge Router
GTP GPRS (General Packet Radio Service) Tunneling Protocol
GTP-U GTP user data tunneling
UE User equipment like tablets or smartphones
eNB enhanced NodeB, radio access part of the LTE system
S-GW Serving Gateway, primary function is user plane mobility
P-GW Packet Gateway, actual service creation point, terminates 3GPP
mobile network, interface to Packet Data Networks (PDN)
PE Provider Edge Router
HSS Home Subscriber System (control plane element)
MME Mobility Management Entity (control plane element)
S-IP Source IP address
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D-IP Destination IP address
IMSI The International Mobile Subscriber Identity that identifies a
mobile subscriber
PCRF 3GPP standardized Policy and Charging Rules Function
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].
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.
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+----------------------------------------+
| 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
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.
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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.
+----------+-----------+--------+----------+-----------+
| Outer IP | Outer UDP | GTP-U | User Data Packet |
| Header | Header | Header | (IP datagram) |
+----------+-----------+--------+----------+-----------+
|<-------------------->|<----------------------------->|
|Transport layer header| GTP-U message |
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].
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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.
3. Use case
LTE nodes (e.g., eNB, S-GW and P-GW) in overlay are connected to the
routers 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 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 P-GW
(segment list eNB=B=C=S-GW=G=H=P-GW) 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 using a
segment list (eNB=A=E=D=S-GW=F=I=P-GW) instead. With explicit path
allocation to dynamically changing traffic patterns, inefficient use
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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.
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
TBD
5. IANA Considerations
TBD
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-sfc-use-case-mobility]
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.
[draft-ietf-spring-problem-statement]
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S. Previdi, C. Filsfils, B. Decraene, S. Litkowski, R.
Geib, R. Shakir and R. Raszuk, "SPRING Problem Statement
and Requirements", draft-ietf-spring-problem-statement-04,
April 2015.
[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.
[wan-traffic-engineering]
"Wide Area Network Traffic Engineering", Ericsson White
Paper, December 2015.
7. Acknowledgments
TBD
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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