Analysis and scenarios of multiple interfaces in a host

A host with multiple interfaces may simultaneously use its interfaces for communication with a correspondent node. If a host simultaneously uses multiple interfaces, it seems that the following two cases can be considered. - All interfaces activates and simultaneously sends/receives packets at a time. (all of them has layer 3 connections) - All interfaces activates but just some of them sends/receives packets at a time. (some of them just have layer 2 connections) If a host has single interface, the relation between network interface and destination address is static and is one-to-one mapping. But, if a host has multiple interfaces, this relation is different. Let's consider the first case (All interfaces activates and simultaneously sends/receives packets at a time) and identify the relation between network interface and destination address. In the following, we assume a host with two network interfaces IF1 and IF2 of different access technologies. Each interface is configured with an IP address, respectively IP1 and IP2, and the address of a correspondent node is IP3. Because the host simultaneously use two interfaces and two interfaces are involved in sending/receiving packets, the routing information (for relation between network interface and destination address) could be configured as shown in the following table. In this table, to send packets to IP3, there are two available network interface; IF1 and IF2. The relation between network interface and destination address which is one-to-one mapping is changed to one(destination address)-to-many(network interface) mapping. So other relevant routing tables and routing caches should be support this change.

_________ | CN | |_________| | IP3 / \ +------------------+ +------------------+ | Network 1 (N1) | | Network 2 (N2) | | | | | +------------------+ +------------------| Interface 1(IF1) \ / \ / Interface 2 (IF2) IP1 | | IP2 |_________| | host | |_________| Location (Used I/F) (Source Address, Destination Address) Relation btw a Destination Addr. and a network I/F N1 (IF1) (IP1, IP3) IP3 -> IF1 N2 (IF2) (IP2, IP3) IP3 -> IF2 Let's consider the second case (All interfaces activates but just some of them sends/receives packets at a time) and identify the relation between network interface and destination address. Let's assume that in the beginning time, IF1 is used and as time goes by, IF 2 is used. At the beginning time of communication, when the host uses (IF1, IP1) and sends packets to a correspondent node, the source address and destination address of packets is (IP1, IP3) and the used network interface is IF1. As time goes by and the host selects IF2 to send/receive packets, and then the host uses (IF2, IP2) and the used network interface is IF2. Before this change, the destination address (IP3) of the correspondent node is mapped to a network interface IF1. However after this change, when an application tries to send packets to the correspondent node, it continually tries to use a network interface IF1 if the relation between destination address IP3 and network interface IF1 is not properly updated. So, if all interfaces activates but just some of them sends/receives packets at a time, the proper update between network interface and destination address is needed.

In current network, a host can be equipped one or more network interfaces. Each network interface can be assigned one or multiple IP address. With this configuration, a host can connect to different networks and use different services at the same time. Since a host usually has a node-scoped routing table, in some cases it cannot differentiate exactly which interface that a packet will be forwarded. We consider the following scenario: A host H1 has two network interfaces IF1 and IF2. Each network interface connects to a specific sub-network with a corresponding IP address as showed in the table.

_________ | CN | |_________| | IP3 \ +------------------+ \ +------------------+ | Network 1 (N1) | \| Network 2 (N2) | | Gateway (G1) | | Gateway (G2) | +------------------+ +------------------| \ / Interface 1(IF1) \ / Interface 2 (IF2) IP1 | | IP2 |_________| | host(H1)| |_________| Interface Gateway IP address Network IF1 G1 IP1 N1 IF2 G2 IP2 N2 The node has a routing table as follow. Destination Gateway Interface * G1 IF1 N3 G2 IF2 N4 G2 IF2 Now we assume that there is a correspond node CN in the network N5. CN has only one connection with the network N2 via the gateway G2. Let CN send a TCP packet to H1 via the gateway G2. When host H1 receives this packet, it will send an ACK packet back to CN. First H1 will check its routing table to find the gateway for sending ACK packet. Because the routing table does not have any dedicated routing entry for the network N3, H1 will send ACK packet to the default gateway G1. However CN does not have any connection with this network, the ACK packet cannot reach CN. Thus to maintain a connection, the response of a packet should be routed via the same interface that it is received.

If a host has only single network interface (or although it has multiple interfaces but only one interface is activated at one time) and it is connected to single network, the activated interface is established to one layer 2 entity (e.g., WLAN Access Point) and it is also established to one layer 3 entity (e.g., router). It means that single layer 2 connection (where the single network interface is activated via a layer 2 entity) is only mapped to single layer 3 connection. Meanwhile, if a host has multiple interfaces (or it has single interface and it is connected to multiple networks) and it is connected to multiple networks, the host may have multiple activated interfaces. And each activated interface may be established to different layer 2 entity. Because each layer 2 entity may be connected to different networks, some activated interfaces may be connected to global networks and some activated interfaces may be connected to non-global networks (e.g., private networks). In this case, the one-to-one mapping between single layer 2 connection and single layer 3 connection is not made. There may be various usage scenarios by the relation between layer 2 networks and layer 3 networks.

This scenario illustrates a host with single interface that is connected to single layer 2 network and single layer 3 network. This scenario is a general network configuration now a days. In this case, there is no issues as an aspects of MIF.

+--------------------+ +--------------------+ ( ) ( ) ( Public ) ( Private ) ( Networks ) ( Networks ) ( ) ( ) +-------\\-----------+ +--------------------+ \\ +-----\\---------+ +----------------+ ( +----------+ ) ( +----------+ ) ( |L3 Router1| ) ( |L3 Router2| ) ( +----------+ ) ( +----------+ ) +--------\\------+ +----------------+ \\Prefix1 +---------\\---+ +--------------+ ( +----------+ ) ( +----------+ ) ( |L2 Entity1| ) ( |L2 entity2| ) ( +----------+ ) ( +----------+ ) +-------------\+ +--------------+ \ \ +----+ |I/F | +-----+ 1 +-----+ | +----+ | | Host | +----------------+ The following table shows status of a host on this scenario. L2 counter part L3 counter part (L3 information), (# of IP address, # of default router) Path to Pub. Net. L2 Entity1 L3 Router1 (Prefix1), (1, 1) via Router1

This scenario illustrates a host with single interface that is connected to multiple layer 2 networks and single layer 3 network. In layer 3 operation's aspect, same link and same subnet is used. This scenario seems that it is not possible when we use typical layer 2 technologies such as WLAN. But it is acknowledged that some particular access technology such as CDMA makes it possible. But, in real field, it is not easy to see this configuration. Although the single interface is connected to multiple layer 2 entity, the host is connected to single layer 3 network, because the two layer 2 entities are connected to same router and receive same prefix information. In this case, the host must properly decide which layer 2 entity should be used. But, there is no issue as an aspect of MIF.

+--------------------+ +--------------------+ ( ) ( ) ( Public ) ( Private ) ( Networks ) ( Networks ) ( ) ( ) +-------\\-----------+ +--------------------+ \\ +-----\\---------+ +----------------+ ( +----------+ ) ( +----------+ ) ( |L3 Router1| === ( |L3 Router2| ) ( +----------+ ) \\ ( +----------+ ) +--------\\------+ \\ +----------------+ Prefix1\\ \\Prefix1 +---------\\---+ \\ +--------------+ ( +----------+ ) == +----------+ ) ( |L2 Entity1| ) ( |L2 entity2| ) ( +----------+ ) ( +----------+ ) +-------------\+ +/-------------+ \ / \ / +----+ |I/F | +-----+ 1 +-----+ | +----+ | | Host | +----------------+ L2 counter part L3 counter part (L3 information), (# of IP address, # of default router) Path to Pub. Net. L2 Entity1, L2 Entity2 L3 Router1 (Prefix1), (1, 1) via Router1

This scenario illustrates a host with single interface that is connected to multiple layer 2 networks and single layer 3 network. In layer 3 operation's aspect, same link but different subnet is used. This scenario seems that it is not possible when we use typical layer 2 technologies such as WLAN. But it is acknowledged that some particular access technology such as CDMA makes it possible. But, in real field, it is not easy to see this configuration. Because the host receives two different prefix information from same Router, MIF issues exists in this case. The host can have two IP addresses and two default routers. The host must decide which IP address and which default router should be used to make proper connection.

+--------------------+ +--------------------+ ( ) ( ) ( Public ) ( Private ) ( Networks ) ( Networks ) ( ) ( ) +-------\\-----------+ +--------------------+ \\ +-----\\---------+ +----------------+ ( +----------+ ) ( +----------+ ) ( |L3 Router1| === ( |L3 Router2| ) ( +----------+ ) \\ ( +----------+ ) +--------\\------+ \\ +----------------+ Prefix1\\ \\Prefix2 +---------\\---+ \\ +--------------+ ( +----------+ ) == +----------+ ) ( |L2 Entity1| ) ( |L2 entity2| ) ( +----------+ ) ( +----------+ ) +-------------\+ +/-------------+ \ / \ / +----+ |I/F | +-----+ 1 +-----+ | +----+ | | Host | +----------------+ L2 counter part L3 counter part (L3 information), (# of IP address, # of default router) Path to Pub. Net. L2 Entity1, L2 Entity2 L3 Router1 (Prefix1, Prefix2), (2, 2) via Router1

This scenario illustrates a host with single interface that is connected to single layer 2 networks and multiple layer 3 networks. Because the host is connected to different router, it receives different prefix information. So, MIF issues exist in this case. The host can have two IP addresses and two default routers and the host must decide which IP address and which default router should be used. In this case, only Router 1 is connected to public network, the host must decide which default router should be used to connect to public network.

+--------------------+ +--------------------+ ( ) ( ) ( Public ) ( Private ) ( Networks ) ( Networks ) ( ) ( ) +-------\\-----------+ +-------//-----------+ \\ // +-----\\---------+ +-----//---------+ ( +----------+ ) ( +----------+ ) ( |L3 Router1| ) === |L3 Router2| ) ( +----------+ ) // ( +----------+ ) +--------\\------+ // +----------------+ Prefix1\\ //Prefix2 +---------\\---+ // +--------------+ ( +----------+ === ( +----------+ ) ( |L2 Entity1| ) ( |L2 entity2| ) ( +----------+ ) ( +----------+ ) +-------------\+ +--------------+ \ \ +----+ |I/F | +-----+ 1 +-----+ | +----+ | | Host | +----------------+ L2 counter part L3 counter part (L3 information), (# of IP address, # of default router) Path to Pub. Net. L2 Entity1 L3 Router1, L3 Router2 (Prefix1, Prefix2), (2, 2) via Router1

This scenario illustrates a host with single interface that is connected to single layer 2 networks and multiple layer 3 networks. Because the host is connected to different router, it receives different prefix information. So, MIF issues exist in this case. The host can have two IP addresses and two default routers and the host must decide which IP address and which default router should be used. In this case, both Router 1 and Router 2 are connected to public network, the host can use any default router to connect to public network.

+--------------------+ +--------------------+ ( ) ( ) ( Public ) ( Private ) ( Networks ===== ( Networks ) ( ) \\ ( ) +-------\\-----------+ \\ +--------------------+ \\ \\ +-----\\---------+ \\----------------+ ( +----------+ ) ( +----------+ ) ( |L3 Router1| ) === |L3 Router2| ) ( +----------+ ) // ( +----------+ ) +--------\\------+ // +----------------+ Prefix1\\ //Prefix2 +---------\\---+ // +--------------+ ( +----------+ === ( +----------+ ) ( |L2 Entity1| ) ( |L2 entity2| ) ( +----------+ ) ( +----------+ ) +-------------\+ +--------------+ \ \ +----+ |I/F | +-----+ 1 +-----+ | +----+ | | Host | +----------------+ L2 counter part L3 counter part (L3 information), (# of IP address, # of default router) Path to Pub. Net. L2 Entity1 L3 Router1, L3 Router2 (Prefix1, prefix2), (2, 2) via Router1, Router2

This scenario illustrates a host with single interface that is connected to multiple layer 2 networks and multiple layer 3 networks. Because the host is connected to different layer 2 entity and different router, it receives different prefix information. So, MIF issues exist in this case. The host can have two IP addresses and two default routers and the host must decide which IP address and which default router should be used. In this case, only Router 1 is connected to public network, the host must decide which default router should be used to connect to public network.

+--------------------+ +--------------------+ ( ) ( ) ( Public ) ( Private ) ( Networks ) ( Networks ) ( ) ( ) +-------\\-----------+ +-----------//-------+ \\ // +-----\\---------+ +---------//-----+ ( +----------+ ) ( +----------+ ) ( |L3 Router1| ) ( |L3 Router2| ) ( +----------+ ) ( +----------+ ) +--------\\------+ +-----//---------+ Prefix1\\ //Prefix2 +---------\\---+ +---//---------+ ( +----------+ ) ( +----------+ ) ( |L2 Entity1| ) ( |L2 entity2| ) ( +----------+ ) ( +----------+ ) +-------------\+ +/-------------+ \ / \ / +----+ |I/F | +-----+ 1 +-----+ | +----+ | | Host | +----------------+ L2 counter part L3 counter part (L3 information), (# of IP address, # of default router) Path to Pub. Net. L2 Entity1, L2 Entity2 L3 Router1, L3 Router2 (Prefix1, Prefix2), (2, 2) via Router1

This scenario illustrates a host with single interface that is connected to multiple layer 2 networks and multiple layer 3 networks. Because the host is connected to different layer 2 entity and different router, it receives different prefix information. So, MIF issues exist in this case. The host can have two IP addresses and two default routers and the host must decide which IP address and which default router should be used. In this case, both Router 1 and Router 2 are connected to public network, the host can use any default router to connect to public network.

+--------------------+ +--------------------+ ( ) ( ) ( Public ) ( Private ) ( Networks ===== ( Networks ) ( ) \\ ( ) +-------\\-----------+ \\ +--------------------+ \\ \\ +-----\\---------+ \\----------------+ ( +----------+ ) ( +----------+ ) ( |L3 Router1| ) ( |L3 Router2| ) ( +----------+ ) ( +----------+ ) +--------\\------+ +-----//---------+ Prefix1\\ //Prefix2 +---------\\---+ +---//---------+ ( +----------+ ) ( +----------+ ) ( |L2 Entity1| ) ( |L2 entity2| ) ( +----------+ ) ( +----------+ ) +-------------\+ +/-------------+ \ / \ / +----+ |I/F | +-----+ 1 +-----+ | +----+ | | Host | +----------------+ L2 counter part L3 counter part (L3 information), (# of IP address, # of default router) Path to Pub. Net. L2 Entity1, L2 Entity2 L3 Router1, L3 Router2 (Prefix1, Prefix2), (2, 2) via Router1, Router2

This scenario illustrates a host with multiple interfaces that are connected to single layer 2 network and single layer 3 network. We assume that in this scenario, although the host has multiple interfaces but only single interface is activated and connected to layer 2 and layer 3 network at a time. It is same case of scenario 1. This scenario is a general network configuration now a days. In this case, there is no issues as an aspects of MIF.

+--------------------+ +--------------------+ ( ) ( ) ( Public ) ( Private ) ( Networks ) ( Networks ) ( ) ( ) +-------\\-----------+ +--------------------+ \\ +-----\\---------+ +----------------+ ( +----------+ ) ( +----------+ ) ( |L3 Router1| ) ( |L3 Router2| ) ( +----------+ ) ( +----------+ ) +--------\\------+ +----------------+ \\Prefix1 +---------\\---+ +--------------+ ( +----------+ ) ( +----------+ ) ( |L2 Entity1| ) ( |L2 entity2| ) ( +----------+ ) ( +----------+ ) +-------------\+ +--------------+ \ \ +---+ +---+ |I/F| |I/F| +--+ 1 +--+ 2 +--+ | +---+ +---+ | | Host | +----------------+ L2 counter part L3 counter part (L3 information), (# of IP address, # of default router) Path to Pub. Net. L2 Entity1 L3 Router1 (Prefix1), (1, 1) via Router1

This scenario illustrates a host with multiple interfaces that are connected to multiple layer 2 networks and single layer 3 network. Although, multiple layer 2 entity is used, in layer 3 operation's aspect, same link and same subnet is used. In this case, the host has only one prefix information and other MIF issues do not exist except for the choosing proper network interface.

+--------------------+ +--------------------+ ( ) ( ) ( Public ) ( Private ) ( Networks ) ( Networks ) ( ) ( ) +-------\\-----------+ +--------------------+ \\ +-----\\---------+ +----------------+ ( +----------+ ) ( +----------+ ) ( |L3 Router1| === ( |L3 Router2| ) ( +----------+ ) \\ ( +----------+ ) +--------\\------+ \\ +----------------+ Prefix1\\ \\Prefix1 +---------\\---+ \\ +--------------+ ( +----------+ ) == +----------+ ) ( |L2 Entity1| ) ( |L2 entity2| ) ( +----------+ ) ( +----------+ ) +-------------\+ +/-------------+ \ / \ / +---+ +---+ |I/F| |I/F| +--+ 1 +--+ 2 +--+ | +---+ +---+ | | Host | +----------------+ L2 counter part L3 counter part (L3 information), (# of IP address, # of default router) Path to Pub. Net. L2 Entity1, L2 Entity2 L3 Router1 (Prefix1), (1, 1) via Router1

This scenario illustrates a host with multiple interfaces that are connected to multiple layer 2 networks and single layer 3 network. In layer 3 operation's aspect, same link but different subnet is used. Because the host receives two different prefix information from same router, MIF issues exist in this case. The host can have two IP addresses and two default routers. The host must decide which IP address and which default router should be used to make proper connection.

+--------------------+ +--------------------+ ( ) ( ) ( Public ) ( Private ) ( Networks ) ( Networks ) ( ) ( ) +-------\\-----------+ +--------------------+ \\ +-----\\---------+ +----------------+ ( +----------+ ) ( +----------+ ) ( |L3 Router1| === ( |L3 Router2| ) ( +----------+ ) \\ ( +----------+ ) +--------\\------+ \\ +----------------+ Prefix1\\ \\Prefix2 +---------\\---+ \\ +--------------+ ( +----------+ ) == +----------+ ) ( |L2 Entity1| ) ( |L2 entity2| ) ( +----------+ ) ( +----------+ ) +-------------\+ +/-------------+ \ / \ / +---+ +---+ |I/F| |I/F| +--+ 1 +--+ 2 +--+ | +---+ +---+ | | Host | +----------------+ L2 counter part L3 counter part (L3 information), (# of IP address, # of default router) Path to Pub. Net. L2 Entity1, L2 Entity2 L3 Router1 (Prefix1, Prefix2), (2, 2) via Router1

This scenario illustrates a host with multiple interfaces that are connected to single layer 2 networks and multiple layer 3 networks. Because the host is connected to different router, it receives different prefix information. So, MIF issues exist in this case. The host can have two IP addresses and two default routers and the host must decide which IP address and which default router should be used. In this case, only Router 1 is connected to public network, the host must decide which default router should be used to connect to public network.

+--------------------+ +--------------------+ ( ) ( ) ( Public ) ( Private ) ( Networks ) ( Networks ) ( ) ( ) +-------\\-----------+ +-------//-----------+ \\ // +-----\\---------+ +-----//---------+ ( +----------+ ) ( +----------+ ) ( |L3 Router1| ) === |L3 Router2| ) ( +----------+ ) // ( +----------+ ) +--------\\------+ // +----------------+ Prefix1\\ //Prefix2 +---------\\---+ // +--------------+ ( +----------+ === ( +----------+ ) ( |L2 Entity1| ) ( |L2 entity2| ) ( +----------+ ) ( +----------+ ) +-------------\\_____ +--------------+ \ \ \ \ +---+ +---+ |I/F| |I/F| +--+ 1 +--+ 2 +--+ | +---+ +---+ | | Host | +----------------+ L2 counter part L3 counter part (L3 information), (# of IP address, # of default router) Path to Pub. Net. L2 Entity1 L3 Router1, L3 Router2 (Prefix1, Prefix2), (2, 2) via Router1

This scenario illustrates a host with multiple interfaces that are connected to single layer 2 networks and multiple layer 3 networks. Because the host is connected to different router, it receives different prefix information. So, MIF issues exist in this case. The host can have two IP addresses and two default routers and the host must decide which IP address and which default router should be used. In this case, both Router 1 and Router 2 are connected to public network, the host can use any default router to connect to public network.

+--------------------+ +--------------------+ ( ) ( ) ( Public ) ( Private ) ( Networks ===== ( Networks ) ( ) \\ ( ) +-------\\-----------+ \\ +--------------------+ \\ \\ +-----\\---------+ \\----------------+ ( +----------+ ) ( +----------+ ) ( |L3 Router1| ) === |L3 Router2| ) ( +----------+ ) // ( +----------+ ) +--------\\------+ // +----------------+ Prefix1\\ //Prefix2 +---------\\---+ // +--------------+ ( +----------+ === ( +----------+ ) ( |L2 Entity1| ) ( |L2 entity2| ) ( +----------+ ) ( +----------+ ) +-------------\\_____ +--------------+ \ \ \ \ +---+ +---+ |I/F| |I/F| +--+ 1 +--+ 2 +--+ | +---+ +---+ | | Host | +----------------+ L2 counter part L3 counter part (L3 information), (# of IP address, # of default router) Path to Pub. Net. L2 Entity1 L3 Router1, L3 Router2 (Prefix1, Prefix2), (2, 2) via Router1, Router2

This scenario illustrates a host with multiple interfaces that are connected to multiple layer 2 networks and multiple layer 3 networks. Because the host is connected to different layer 2 entity and different router, it receives different prefix information. So, MIF issues exist in this case. The host can have two IP addresses and two default routers and the host must decide which IP address and which default router should be used. In this case, only Router 1 is connected to public network, the host must decide which default router should be used to connect to public network.

+--------------------+ +--------------------+ ( ) ( ) ( Public ) ( Private ) ( Networks ) ( Networks ) ( ) ( ) +-------\\-----------+ +-----------//-------+ \\ // +-----\\---------+ +---------//-----+ ( +----------+ ) ( +----------+ ) ( |L3 Router1| ) ( |L3 Router2| ) ( +----------+ ) ( +----------+ ) +--------\\------+ +-----//---------+ Prefix1\\ //Prefix2 +---------\\---+ +---//---------+ ( +----------+ ) ( +----------+ ) ( |L2 Entity1| ) ( |L2 entity2| ) ( +----------+ ) ( +----------+ ) +-------------\+ +/-------------+ \ / \ / +---+ +---+ |I/F| |I/F| +--+ 1 +--+ 2 +--+ | +---+ +---+ | | Host | +----------------+ L2 counter part L3 counter part (L3 information), (# of IP address, # of default router) Path to Pub. Net. L2 Entity1, L2 Entity2 L3 Router1, L3 Router2 (Prefix1, Prefix2), (2, 2) via Router1

This scenario illustrates a host with multiple interfaces that are connected to multiple layer 2 networks and multiple layer 3 networks. Because the host is connected to different layer 2 entity and different router, it receives different prefix information. So, MIF issues exist in this case. The host can have two IP addresses and two default routers and the host must decide which IP address and which default router should be used. In this case, both Router 1 and Router 2 are connected to public network, the host can use any default router to connect to public network.

+--------------------+ +--------------------+ ( ) ( ) ( Public ) ( Private ) ( Networks ===== ( Networks ) ( ) \\ ( ) +-------\\-----------+ \\ +--------------------+ \\ \\ +-----\\---------+ \\----------------+ ( +----------+ ) ( +----------+ ) ( |L3 Router1| ) ( |L3 Router2| ) ( +----------+ ) ( +----------+ ) +--------\\------+ +-----//---------+ Prefix1\\ //Prefix2 +---------\\---+ +---//---------+ ( +----------+ ) ( +----------+ ) ( |L2 Entity1| ) ( |L2 entity2| ) ( +----------+ ) ( +----------+ ) +-------------\+ +/-------------+ \ / \ / +---+ +---+ |I/F| |I/F| +--+ 1 +--+ 2 +--+ | +---+ +---+ | | Host | +----------------+ L2 counter part L3 counter part (L3 information), (# of IP address, # of default router) Path to Pub. Net. L2 Entity1, L2 Entity2 L3 Router1, L3 Router2 (Prefix1, Prefix2), (2, 2) via Router1, Router2

It is understood that MIF working group tries to describe the issues of attaching to multiple networks on hosts. In some point, it seems that MIF issues are not directly related to the configuration of layer 2 network and layer 3 network, because we are focusing on host itself in MIF. But, after we describe the usage scenarios of multiple interfaces, we found that there are many possible usage scenarios and MIF issues exist in some scenarios and do not exist in other scenarios. To make effective works in MIF, we must find which scenario we should focus on. The optimal MIF's goal should be worked with irrelevant of the network configuration of layer 2 network and layer 3 network. In describing scenarios, a host with single interface looks like having similar MIF issues with a host with multiple interfaces if we assume some particular access technology such as CDMA. It is needed to discuss whether MIF working group includes this special cases or not.