Framework of Vertical Multi-homing in IPv6-based NGN

ITU-T Recommendation Y.ipv6-vmh Framework of Vertical Multi-homing in IPv6-based NGN Summary This Recommendation describes a framework of vertical multi-homing in IPv6-based NGN. This Recommendation identifies the definition, requirements, functional architecture, and applications of vertical multi-homing in IPv6-based NGN. Keywords IPv6, vertical multi-homing, NGN ALL RIGHTS RESERVED. NO PART OF THIS PUBLICATION MAY BE REPRODUCED, BY ANY MEANS WHATSOEVER, WITHOUT THE PRIOR WRITTEN PERMISSION OF ITU.

- 2 - Table of Contents 1 Scope...3 2 References...3 3 Definitions...4 3.1 Terms defined elsewhere...4 3.2 Terms defined in this Recommendation...4 4 Abbreviations and acronyms...4 5 Conventions...4 6 Definition of vertical multi-homing in IPv6-based NGN...5 7 Requirements of vertical multi-homing in IPv6-based NGN...8 7.1 Multi-homing features and requirement of each layer...8 7.2 Interaction across several layers...8 7.3 Resource management of vertical multi-homing...9 7.4 Other requirements for vertical multi-homing...9 8 Vertical multi-homing scenarios...10 8.1 Vertical multi-homing scenarios for establishing multiple network connections..10 8.2 Vertical multi-homing scenarios as an aspect of each layer s multi-homing capabilities...11 8.2.1 Only PHY/MAC layer has multi-homing feature...11 8.2.2 PHY/MAC layer and network layer has multi-homing features...11 8.2.3 PHY/MAC layer, network layer, and transport layer has multi-homing features...12 8.2.4 All layers has multi-homing features...13 9 Functional architecture of vertical multi-homing...13 9.1 Functional architecture of vertical multi-homing in host side...13 9.2 Functional architecture of vertical multi-homing in network side...14 10 Applications of vertical multi-homing...15 10.1 Use cases related to lower layer...15 10.2 Use cases related to upper layer...17 11 Security considerations...17

- 3 - ITU-T Recommendation Y.IPv6-VMH Framework of Vertical Multi-homing in IPv6-based NGN 1 Scope In Y.2052 (Framework of Multi-homing in IPv6-based NGN), multi-homing based on network layer (especially, IPv6) is described. As heterogeneous access technologies have been developed, network node has the ability to simultaneously connect multiple access networks to support mobility, reliability, load sharing, etc. These simultaneous multiple connections are established through multiple accesses to NGN networks using multiple radios, multiple network interfaces, multiple IPv6 addresses, and multiple transport sessions. To provide efficient simultaneous multiple connections, an IPv6 based solution is required to be considered to support multiple connections and its impact from/on multiple radios, multiple network interfaces, and multiple transport sessions as well. We assume that vertical multi-homing is a multi-homing based on simultaneous multiple connections characteristics of several layers and managed vertically across several layers. This Recommendation defines framework to provide efficient simultaneous multiple connections by using vertical multi-homing. The scope of this Recommendation which is subject to IPv6-based NGN includes: Definition of vertical multi-homing; Requirements of vertical multi-homing; Scenarios for vertical multi-homing Functional architecture of vertical multi-homing; Applications of vertical multi-homing; Editor Note: Vertical multi-homing is also related to the Q9/13 (Multi-connection), which may have more general view on multi-connection issue, including various multi-connection scenarios, such as application level, session control level and access level multi-connection. This proposed draft recommendation of vertical multi-homing in IPv6-based NGN focuses on the IPv6 based multihoming, which is a multi-connection solution for IP network layer. So, the work of this proposed draft recommendation will be coordinated with the work of multi-connection by Q9/13 2 References The following ITU-T Recommendations and other references contain provisions which, through reference in this text, constitute provisions of this Recommendation. At the time of publication, the editions indicated were valid. All Recommendations and other references are subject to revision; users of this Recommendation are therefore encouraged to investigate the possibility of applying the most recent edition of the Recommendations and other references listed below. A list of the currently valid ITU-T Recommendations is published regularly. The reference to a document within this Recommendation does not give it, as a standalone document, the status of a Recommendation. [ITU-T Y.2001] ITU-T Recommendation Y.2001 (2004), General Overview of NGN. [ITU-T Y.2011] ITU-T Recommendation Y.2011 (2004), General Principles and general reference model for Next Generation Networks. [ITU-T Y.2012] ITU-T Recommendation Y.2012 (2006), Functional requirements and architecture of the NGN. [ITU-T Y.2201] ITU-T Recommendation Y.2201 (2006), NGN Release 1 Requirements. [ITU-T Y.2701] ITU-T Recommendation Y.2701 (2006), Security Requirements for NGN release 1.

- 4 - [ITU-T Y.2051] [ITU-T Y.2052] NGN. ITU-T Recommendation Y.2051 (2008), General Overview of IPv6-based NGN. ITU-T Recommendation Y.2052 (2008), Framework of Multi-homing in IPv6-based 3 Definitions 3.1 Terms defined elsewhere This Recommendation uses the following terms defined elsewhere: TBD 3.2 Terms defined in this Recommendation This Recommendation defines the following terms: TBD 4 Abbreviations and acronyms This Recommendation uses the following abbreviations and acronyms: TBD 5 Conventions TBD

- 5-6 Definition of vertical multi-homing in IPv6-based NGN Using IPv6 multi-homing features, a network and/or a network node is able to have multiple network connections with multiple network interfaces and multiple IPv6 addresses. In IPv6 multihoming, although a network and/or a network node have multiple network connections, generally, it uses only one network connection at one time. Other network connections are remained to prepare secondary (backup) connection and used for special cases such as the failure of primary network connection, necessity of higher bandwidth, load balancing, and mobility. It is different between providing multiple network connections and providing simultaneous multiple network connections with an aspect of how many network connections are used at a moment. In IPv6-based NGN, as one of example of an IPv6-based NGN network architecture, they consists of an IPv6-based core network and heterogeneous access networks which support IPv6-based IP connectivity [ITU-T Y.2052] Figure 1 One example of IPv6-based NGN architecture In Figure 1, we assume that a user terminal has an ability to access each access network. It means that the user terminal can access IPv6-based core network through each access network. In this case, the user terminal can use one access network or multiple access networks at a moment. If the user terminal use one access network and establish one network connection, the communication paradigm is similar to general communication mechanism. The communication method using TCP/IP at a legacy application is based on single network connection. In Figure 2, host 1 and host 2 has single network connection to communicate to each other. If we analyze this single network connection, it is composed of single network card (network interface, PHY layer), single MAC protocol (MAC layer), single IPv6 address (network layer), single transport session (transport layer), and fixed QoS parameters (application layer). Figure 2 Communication based on single network connection

- 6 - In this environment of communication based on single network connection, single MAC, single IPv6 address, and single transport session are together joined and single network connection is linked to an application. Figure 3 shows this case in TCP/IP aspects. Figure 3 TCP/IP application in single network connection In Figure 1, if the user terminal uses more than two access networks and establishes more than two network connections, the communication paradigm is not same as Figure 2 and Figure 3. (Typically, we can consider the case that the user terminal uses one access network but establishes more than two network connections through one access network.) As heterogeneous access technologies are widely deployed, a network node can have the ability to connect various access networks simultaneously. Figure 4 shows a case where a network node communicates to another node with simultaneously connecting heterogeneous access networks. In this figure, host 1 makes a communication to host 2 through three different network connections. Multiple network connections are composed of multiple network interfaces (PHY layer), multiple MAC protocols (MAC layer), multiple IPv6 addresses (network layer), multiple transport sessions (transport layer), and variable QoS parameters (application layer). Figure 4 Communication based on multiple network connections Communication mechanisms and legacy applications based on single network connection are different to those of multiple network connections. In single network connection environment, oneto-one mapping among single MAC and single IPv6 address and single transport session is established. But, in multiple network connections environment, instead of one-to-one mapping, multi-to-multi mapping is established. Figure 5 shows a TCP/IP application in multiple network connections environment.

- 7 - Figure 5 TCP/IP application in multiple network connections As shown in Figure 4, 5, in multiple network connections due to heterogeneous access networks, there are many layers used to establish multiple network connections. In PHY/MAC layer s aspect, there may be multiple network interfaces/mac protocols which are directly related to each access network and these multiple network interfaces/mac protocols may be simultaneously used. So, in PHY/MAC layer, there are multiple link layer (layer 1 and layer 2) connections to communicate with other node. In network layer s aspect, there may be multiple IPv6 addresses which are simultaneously used to establish communication path. So, in network layer, there are multiple network layer (layer 3) connections to communicate with other node. In transport layer s aspect, there may be multiple transport layer (layer 4) sessions which are simultaneously used (in this case, if SCTP and DCCP is used as a transport layer protocol). When we consider how multiple network connections are established, we can see that each layer has a role for multiple network connections. Even though multiple network connections are made by the help of each layer, there is no interaction or relationship between each layer until now. But, to make efficient multiple network connections and mange these connections, we should consider their interfaces and relationship between each layer. To do this, we consider vertical multihoming. In vertical multi-homing, each layer has their specific multi-homing features and their multi-homing features of each layer should be harmonized and interacted with each other. Figure 6 show each layer multi-homing and vertical multi-homing. We defines that vertical multi-homing is a multi-homing based on simultaneous multiple connections characteristics of several layers and managed vertically across several layers. Figure 6 Concept of vertical multi-homing In vertical multi-homing, each layer has a multi-homing feature and there are many network resources used to establish multiple network connections. In PHY/MAC layer, multiple network

- 8 - access technologies, multiple network interfaces, multiple channels, and multiple radios are network resources. In network layer, multiple IPv6 addresses and multiple prefix information are network resources. In transport layer, multiple transport sessions are network resources. To efficiently establish multiple network connections and mange network resources, it is needed to manage with integrated and harmonized fashion. 7 Requirements of vertical multi-homing in IPv6-based NGN In Y.2052, to apply IPv6 multi-homing to IPv6-based NGN, general requirements of multi-homing in IPv6-based NGN, requirements of site multi-homing, and requirements of host multi-homing are described. For vertical multi-homing, including the requirement of IPv6-multi-homing, other layers such as PHY/MAC layer multi-homing and transport layer multi-homing is required to be considered to support IPv6 multi-homing. In this section, we identify each layer s multi-homing features and describe the requirements of each layer for vertical multi-homing. After describing the requirements of each layer, we describe the requirement of interaction across several layers. Because the vertical multi-homing handles several layers, there are many network resources to be considered. So, resource management is one of important requirements for vertical multi-homing. 7.1 Multi-homing features and requirement of each layer Typically, IPv6 multi-homing means that a feature of IPv6 hosts and/or IPv6 network that enables the host and/or network to be multi-homed to networks through multiple network interfaces and multiple IPv6 addresses. To provide multiple network connections, not only IPv6-multi-homing, but also other layers features play an important role. In PHY/MAC layer, there may be many PHY entities (physical interfaces, radios, channels, etc.,) and many MAC entities (MAC protocols, etc.,). If we assume that the connection on PHY/MAC layer is link layer connection, there may be many link layer connections due to heterogeneous access networks. As heterogeneous access technologies and access networks have been deployed in IPv6-based NGN, a host can use multi-homing features such as multiple radios, multiple channels, multiple interfaces, and multiple MAC protocols. We assume that using these multi-homing features makes it possible to establish multiple link layer connections. For vertical multi-homing, multiple link layer connections are required to be provided and these multiple link layer connections are required to be simultaneously activated and utilized. And these multiple link layer connections are required to be harmonized with network layer connections. In network layer, for multi-homing features, multiple IPv6 addresses and multiple prefix information are used. By using these multi-homing features, multiple network connections are established at network layer. For vertical multi-homing, these network layer connections are required to be provided and these network layer connections are required to be simultaneously activated and utilized. And these multiple network layer connections are required to be harmonized with link layer connections and transport layer connections. In transport layer, for multi-homing features, multiple transport streams, ports can be used. By using these multi-homing features, multiple transport connections are established at transport layer. For vertical multi-homing, these transport layer connections are required to be provided and these transport layer connections are required to be simultaneously activated and utilized. And these transport layer connections are required to be harmonized with link layer connections and network layer connections. 7.2 Interaction across several layers Generally, layer concept (e.g., TCP/IP layer) simplifies design, implementation, and testing by partitioning overall communications process into parts. Protocol in each layer can be designed

- 9 - separately from those in other layers. Layer concept provides flexibility for modifying and evolving protocols and services without having to change layers above/below. Typically, Internet architecture and services have been developed based on this layer concept and operated properly. In NGN environments, where a host has the capability to connect to heterogeneous access networks and simultaneously use multiple connections, there is strong need to modify current layer concept. For example, cross-layer mechanism is that redesign of layer concept by the interconnection between each layer. It can include creation of new interfaces, merging of adjacent layers, design coupling without new interfaces, and vertical calibration across layers. Similarly, for vertical multihoming, interaction across several layers is needed. In figure 6, multi-homing at each layer are depicted; physical layer multi-homing, MAC layer multi-homing, network layer multi-homing, transport layer multi-homing, and application layer multi-homing, If multi-homing of each layer are not managed with integrated fashion, it seems that the synergy effects of multi-homing of each layer is not good. For example, if there are multiple link layer connections at PHY/MAC layer and multiple network layer connections at network layer and there is no interaction between two adjacent layers, network layer do not know how many link layer connections are provided for them. Without knowing the below link layer connections characteristics, it is difficult to efficiently utilize network layer connections. As same manner, multiple transport connections are required to interact to below network layer connections and link layer connections to efficiently utilize. So, for the vertical multi-homing, the interaction across several layers is required. 7.3 Resource management of vertical multi-homing Vertical multi-homing is related to several layers; PHY/MAC layer, network layer, transport layer, and application layer. In each layer, there are specific network resources. In PHY/MAC layer, wire/wireless interfaces, radios, channels are included. In network layer, IPv6 address and prefix information are network resources. In transport layer, transport streams and port numbers are network resources. If we consider legacy layer concept, network resources at each layer do not affect other network resources at adjacent layer. But, in vertical multi-homing, due to the interaction across several layers, network resources one layer affects other adjacent layers. So, it is required to manage network resource with different ways. As described in section 7.2, in vertical multi-homing, interaction between adjacent layers is performed and network resources at adjacent layer affect other layer. Differently with legacy layer concept, in vertical multi-homing, it is required to manage network resource with integration fashion. And it is also required to consider the effect of using network resources in one layer to adjacent layer. 7.4 Other requirements for vertical multi-homing For vertical multi-homing, the following requirements are required to be satisfied to handle multiple interfaces efficiently over multiple network connections. - Routing optimization through multiple network connections over multiple interfaces: Routing optimization focuses on choosing the data path on multiple network connections for forwarding data efficiently in terms of diverse application requirements. - QoS based connection selection over multiple network connections: In heterogeneous environments, each interface technology has different transmission capability. Hence, the multi-homed host can forward data through different routing paths with different transmission capability over multiple network connections established with multiple interfaces. Also, dividing data streaming among different interfaces can increase transmission capabilities through simultaneous usage of multiple interfaces.

- 10 - - Bandwidth utilization over multiple network connections: Bandwidth utilization is to provide better transmission stability, reliability and performance. Hence, these issues included in bandwidth utilization are how to separate data stream and how to recombine data stream over multiple network connections. - Recovery scheme for interface failure: When original interface used for data transmission fails because of the problem of link disconnection in layer 2 or the routing problem in Layer 3, the multi-homed host must perform fast connection recovery by available any interface in the multi-homed host. Recovery Scheme or interface failure is to locally recover the disconnected route. - Interface (network) selection: Interface Selection defines which to use and when to change with interface selection algorithm and policy which depend on choice parameter such as signal strength, transmission rate, service stability and reliability. 8 Vertical multi-homing scenarios 8.1 Vertical multi-homing scenarios for establishing multiple network connections In a vertical multi-homing environment, there are two different scenarios for establishing multiple network connections with a network host according to the number of IPv6 source addresses as following: - Multiple network connections through unique IPv6 address per interface - Multiple network connections through shared IPv6 address per interface In the first scenario where the multi-homed host is assigned the unique IPv6 address per interface, the multi-homed host can send and receive packet simultaneously over the multiple network connections established with a network host through unique IPv6 address assigned to an interface. As shown in Figure 7 (a), this scenario addresses that the multi-homed host is able to create multiple connectivity via multiple network interfaces and uses multiple network interfaces simultaneously for sending and receiving packets without a particular IP stack support. Because a particular application which wants to establish multiple network connections contacts a network host by creating client-local TCP/UDP socket of transport layer with unique IPv6 address assigned to multiple network interfaces, that is, from the application s perspective, there exist multiple network connections between the multi-homed hosts. Figure 7 Multiple network connections through (a) unique IPv6 address per interface and (b) shared IPv6 address per interface In the second scenario, the multi-homed host is assigned the shared IPv6 address across multiple interfaces. Although this scenario makes only one IPv6 source address visible to the application, the

- 11 - multi-homed host is able to access different network through multiple network interfaces. Therefore, it uses multiple network resources which can be accessed by multiple network interfaces. However, in this scenario, though a particular application in the multi-homed host wants to establish multiple network connections with a network host, the application is not able to establish multiple network connections. Since the application does not know multiple network interfaces support. Thus, a particular function for the establishment of multiple network connections is needed in the IP stack. Here, the function may perform the flow assignment for an application flow to be serviced on a particular network interface. 8.2 Vertical multi-homing scenarios as an aspect of each layer s multi-homing capabilities If a host has vertical multi-homing features, it influences not only PHY/MAC layer but also other layer such as network layer, transport layer, and application layer (and a user). As the deployment of vertical multi-homing to a host, there may be various scenarios of the usage of a host. At the beginning time, even though a host adopts multi-homing features in PHY/MAC layer, other layers could not have capability to support the multi-homing features in PHY/MAC layer. Also, a user who uses a host with multi-homing feature in PHY/MAC layer could not acknowledge the existence of this multi-homing feature. As the deployment of multi-homing feature in PHY/MAC layer, other layer such as network layer and transport layer could have capability to support multi-homing features in PHY/MAC layer. But in some cases, a specific layer could not have capability to support multi-homing features in another layer in a host. 8.2.1 Only PHY/MAC layer has multi-homing feature Because of some reasons, a host may adopt multi-homing feature in PHY/MAC layer such as multiple interfaces, multiple channels, and multiple radios but other layers do not support the multihoming features in PHY/MAC layer (Other layers are not aware of the existence of multi-homing features in PHY/MAC layer). Because of the overhead and cost of modification of original network layer, transport layer, and application layer, it is difficult to modify other layer all at once to support multi-homing features in a host. For specific applications and/or specific OS, it could be difficult to modify them to support multi-homing features in PHY/MAC layer. In this scenario, the host must have a method to handle multi-homing features in PHY/MAC layer even though original network layer and transport layer do not support multi-homing features in PHY/MAC layer. Figure 8 - A host with multi-homing features in only PHY/MAC layer 8.2.2 PHY/MAC layer and network layer has multi-homing features In this scenario, a host adopts multi-homing features in PHY/MAC layer and network layer and other layers such as transport layer, application layer are not aware of the existence of multi-homing features in PHY/MAC layer and network layer. For example, original Mobile IPv6 assumes the usage of single IPv6 address and single network interface but the extension of Mobile IPv6 considers multiple multi-homing features such as multiple interfaces, multiple CoA (Care-of

- 12 - Address), multiple HoA (Home-of Address), and multiple HA (Home Agent). In Mobile IPv6 case, multiple interfaces can be multi-homing features in PHY/MAC layer and multiple CoA, multiple HoA, and multiple HA can be multi-homing features in network layer. In this scenario, a host must have a method to effectively combine multi-homing features in PHY/MAC layer and multi-homing features in network layer. Also, a host must have a method to handle multi-homing features in PHY/MAC layer and network layer even though original transport layer and application layer do not support multi-homing features in PHY/MAC layer and network layer. Figure 9 - A host with multi-homing features in PHY/MAC layer and network layer 8.2.3 PHY/MAC layer, network layer, and transport layer has multi-homing features In this scenario, a host may adopt multi-homing features in PHY/MAC layer, network layer, and transport layer and application layer (including user) is not aware of the existence of these multihoming features in other layer. It seems that existing TCP/UDP could not have the capability to support multi-homing features and it seems that SCTP/DCCP could have the capability to support multi-homing features but it needs to be more enhanced to simultaneously use multi-homing features. In this scenario, a host must have a method to effectively combine multi-homing features among PHY/MAC layer, network layer, and transport layer. Also, a host must have a method to handle multi-homing features in PHY/MAC layer, network layer, and transport layer even though original application layer do not support multi-homing features in other layers. Figure 10 - A host with multi-homing features in PHY/MAC layer, network layer, and transport layer Editor Note: In IETF, shim6(site Multihoming by IPv6 Intermediation, a layer 3 shim for providing locator agility below the transport protocols, so that multihoming can be provided for IPv6 with failover and load-sharing properties, without assuming that a multihomed site will have a provider-independent IPv6 address prefix announced in the global IPv6 routing table) was lately published. Multi-homing feature in network layer and transport layer has some relationship with

- 13 - shim6 protocol. Any contribution for describing the relationship between shim6 and multi-homing features in network/transport layer is welcome. 8.2.4 All layers has multi-homing features A host may adopt multi-homing features in every layer and it is the ultimate vertical multi-homing. So, there are multiple connection characteristics of each layer and these characteristics may be simultaneously utilized and managed vertically across several layers. In this scenario, a host must have a method to interact multi-homing features across several layers and resource management of each layer for vertical multi-homing. Figure 11 - A host with multi-homing features in all layers 9 Functional architecture of vertical multi-homing 9.1 Functional architecture of vertical multi-homing in host side The required functions for vertical multi-homing are as follows. - Identifying resource on each layer - Resource management - Recognize network status and adjust resource - Interaction across several layers To provide these functions for vertical multi-homing in IPv6-based NGN, these functions may be located on both a host side and network side. This section describes required functions for vertical multi-homing in a host side. As show in Figure 1, multi-homing features exist in each layer and vertical multi-homing functions are related to multi-homing features in each layer. Figure 12 - Relation between vertical multi-homing function and multi-homing features in each layer For vertical multi-homing, resource identifying function, resource management function, network status recognize & adjust function, and interaction across layer function are tightly coupled and interacted. The basic capability of each functions are as follows.

- 14 - Figure 13 - Functions for vertical multi-homing - Resource identifying function: For vertical multi-homing, there are many resources in each layer. It is important to identify what kind of resources are related to vertical multihoming and what kind of attribute of each resources has influence to vertical multihoming. Resource identifying function does the identifying resources on each layer for vertical multi-homing and the gathering some necessary information for resource management. - Resource management function: In vertical multi-homing, resource on one layer is closely related to resources on another layer. The role of resource management function for vertical multi-homing is controlling and managing resource on each layer with integrated fashion. - Network status recognize & adjust function: Because vertical multi-homing is interacted with each layer and related to resources on each layer, it is important to precisely recognize each layer status. To satisfy the requirement of specific service (such as guarantee end-to-end performance, guarantee minimum bandwidth for some application), vertical multi-homing has the capability to adjust resource on each layer. - Interaction across layer function: For vertical multi-homing, one specific layer must collaborate with other layer. Interaction across layer function manage the interaction between different layer for vertical multi-homing 9.2 Functional architecture of vertical multi-homing in network side The functions for vertical multi-homing in network side are also related to functions for vertical multi-homing in host side. Because the primary purpose of vertical multi-homing is to provide efficient multiple connections with consideration of multi-homing characteristics of each layer, the functions for vertical multi-homing in host side is basically charge of providing vertical multihoming and the functions for vertical multi-homing in network side assistance the functions in host side. In section 9.2 in Y.2052, functional architecture to support multi-homing is described. To provide functions for vertical multi-homing in network side, existing functional entities can be used to provide functions for vertical multi-homing. In contrast to functional architecture which is defined in Y.2052, RACF is required to be participated in providing vertical multi-homing. The following figure shows functional architecture related to vertical multi-homing in network side.

- 15 - Figure 14 Functional architecture of vertical multi-homing in network side (Editor note: In Figure 14, the violet colour box functions are already described in Y.2052. And the red colour box function is newly considered function for vertical multi-homing. Further contribution will be needed for clarification.) (Editor note: In IETF MPTCP (Multi-path TCP) working group and Trilogy project, to provide multipath transport session, it is studied that it is necessary to provide resource pooling method. Although the resource pooling concept to provide multipath transport session is not completed, we think that similar approach is necessary to provide vertical multi-homing in network side. It is more need to study resource pooling method in NGN environment.) 10 Applications of vertical multi-homing In IPv6-based NGN, NGN users will benefit from always-on connectivity, load sharing, traffic engineering, fault tolerance with redundancy, and session continuity with the help of IPv6 multihoming [ITU-T Y.2052]. With vertical multi-homing, these usage cases are also applied into IPv6- based NGN. Besides these usage cases, vertical multi-homing has additional use cases as follows. - Use cases related to lower layer (e.g., PHY layer, MAC layer) - Use cases related to upper layer (e.g., transport layer, application layer) 10.1 Use cases related to lower layer Generally, a legacy user terminal utilizes one network interface for communication at a time. Even though, it has multiple network interfaces, it doesn t utilize multiple network interfaces simultaneously. The reason why it uses single interface at a time may be the compliance of traditional TCP/IP layer concept and there is no need to use multiple interfaces simultaneously. In traditional TCP/IP layer concept, typically, one network interface is directly related to one IP address. Even though IPv6 is widely deployed, this situation is not changed. The big difference between IPv6 and IPv4 is that there are lots numbers of IPv6 address and we can use IPv6 address fluently. We can use multiple IPv6 addresses per single network interface and multiple network interfaces in a host. So, we can utilize multiple network connections with using abundant number of

- 16 - IPv6 addresses and multiple network interfaces. The benefit of IPv6 multi-homing, usage of multiple network connections with multiple IPv6 addresses and multiple network interfaces is not increased as expected due to the inefficient relation between one IPv6 address and single network interface. If a host has multiple interfaces and multiple IPv6 addresses, the relation between them may be exist as various ways and it may be dynamically changeable. Among multiple interfaces, some of them may be utilized (connected to access network) and some of them may not be utilized (is not connected to access network). Among multiple IPv6 addresses, some of them may be utilized (there is routing path to communicate) and some of them may not be utilized (there is no routing path to communicate). If a host can recognize the existence of multiple interfaces and multiple IPv6 addresses and the utilized conditions of each interface and each IPv6 address, the host may find the optimal association between network interface and IPv6 address. Figure 15 An example of relation in multiplicity of network interfaces and IPv6 addresses For example, in traditional TCP/IP layer concept, one specific interface and one specific IPv6 address is combined at a starting time of communication. The selection of one IPv6 address among multiple IPv6 addresses and the selection of one network interface among multiple interfaces are completely independent. Even though there are lots of advantages of this independent layer concept, this complete independent layer concept cannot utilize the benefit of existence of multiple interfaces and multiple IPv6 addresses. Because there is no specific rule for combining a network interface and an IPv6 address and one IPv6 address and one network interface is selected for communication, there is no guarantee to select optimal association between a network interface and an IPv6 address. After communication between nodes starts, the combination between specific interface and specific IPv6 address cannot be modified. In communication mechanisms based on traditional TCP/IP layer concept, the combination between a network interface and an IPv6 address should be used for the entire time of communication. During communication, if the relation combination between a network interface and an IPv6 address is modified, the communication is disrupted. Because there are multiple network interfaces and multiple IPv6 addresses for communication, a host can utilize multiple network interfaces and multiple IPv6 addresses simultaneously. But in traditional TCP/IP layer mechanism, a host cannot utilize these multiplicities of network interfaces and IPv6 addresses. If we use vertical multi-homing, we can utilize these multiplicities of network interfaces and IPv6 addresses. Vertical multi-homing can recognize the existence of multiple network interfaces and IPv6 addresses and coordinate among them. One-to-one mapping between a network interface and an IPv6 address in traditional TCP/IP layer concept is enhanced to multi-to-multi mapping between them. The host can find optimal mapping between network interfaces and IPv6 addresses and dynamically update the combination between them to adapt changeable exterior environment.

- 17-10.2 Use cases related to upper layer TBD 11 Security considerations TBD

- 18 - Bibliography [b-ietf RFC2260] IETF RFC 2260 (1998), Scalable Support for Multi-homed Multi-provider Connectivity. [b-ietf RFC2772] IETF RFC 2772 (2000), 6Bone Backbone Routing Guidelines. [b-ietf RFC2894] IETF RFC 2894 (2000), Router Renumbering for IPv6. [b-ietf RFC2960] IETF RFC 2960 (2000), Stream Control Transmission Protocol. [b-ietf RFC3582] IETF RFC 3582 (2003), Goals for IPv6 Site-Multihoming Architectures. [b-ietf RFC4116] IETF RFC 4116 (2005), IPv4 Multihoming Practices and Limitations. [b-ietf RFC4177] IETF RFC 4177 (2005), Architectural Approaches to Multi-Homing for IPv6. [b-ietf RFC4218] IETF RFC 4218 (2005), Threats relating to IPv6 Multihoming Solutions. [b-ietf RFC4219] IETF RFC 4219 (2005), Things Multihoming in IPv6 (MULTI6) Developers Should Think About. [b-ietf RFC4340] IETF RFC 4340 (2006), Datagram Congestion Control Protocol (DCCP).