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

                    SPRING Use cases for Mobile Network


   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

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Copyright Notice

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   This document is subject to BCP 78 and the IETF Trust's Legal
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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

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

   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.

   | Outer IP | Outer UDP | GTP-U  |   User Data Packet   |
   |  Header  |  Header   | Header |    (IP datagram)     |
   |Transport layer header|        GTP-U message          |

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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

   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

   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.

             | \   /  | |\    / |
             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


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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


            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.


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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.


            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.


           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


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


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

7. Acknowledgments


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Authors' Addresses

   Jeongyun Kim (editor)
   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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