Multihoming is the process of configuring multiple network interfaces or IP addresses on a single computer. In means having as many internet roots assigned to an IP network as possible. This strategy is increasingly becoming popular in many companies as a technique to increase reliability of network applications. Although maximum connectivity is achieved, this application is not guaranteed to improve performance1. Multihoming is first done by installing a network adaptor (second hardware) on a computer that normally has one, then configuring both adaptors to utilise one local IP address. The second technique requires connecting these adaptors to different internet access points although increment of total bandwidth to be distributed across multiple applications is required2.
Multihoming uses Single Link and Multiple IP addresses features. For example assigning multiple IP addresses to the same adaptor in one physical upstream such as (2001:db8:: 1 and 2001:db8: :2 and so on). Multihoming is increasingly becoming popular in many offices across the country. For example, Mitchell states that ‘IPv6 offers extensive network protocol support for multihoming compared to the tradition IPv4. It helps solve the problem of migrating between different types of networks while travelling’1. He adds that ‘many network hosts have only a single network interface with multiple interfaces that connect the host to a network’1. A single interface is sufficient for multiple users since they only use one host as their main client which use services provided by servers. It may also have simultaneous connected network interfaces, with different properties. For example, a node may have an Ethernet network interface and a wireless interface. Router may serve as an example of a node having multiple interfaces. Also, a host or a node may use a mechanism where it has one internet interface assigned multiple addresses. The use of routing protocol, which announces the address to the upstream links helps in picking up network when one link fails in one of the traffics3.
Nodes (determining the addressing plan) belong to a site, in this case Cisco networking equipment. In this regard, Cisco networking equipment will operate on a network using IP and a routing policy for the network. Hinden states that ‘a transit provider will operate the site which directly provides connectivity to the internet to one or more external sites’4. Cisco networking equipment multihoming will have more than one transit provider5.
Since multihoming uses special links such as Link Load Balancer or WAN Load Balancer, features applicable between firewall and link routers, no special configurations required in the ISP routers. In this case an organisation will only require increasing its bandwidth. Another variant would be performing control routing between separate addresses used by each interface-that is if Cisco Networking Equipment will be using PC server based firewalls.
While multihoming is generally used to eliminate network failure, it also helps to save data transfer costs as the site can use several Internet Service Providers (ISP) and route data traffic to a certain ISP. Some data may also require special kind of service level requirements and can be directed to the link which can provide the requested service level of all links do not have the adequate characteristics i.e. sufficient bandwidth or lowest transfer delay. Special features such as Voip or video conferencing are examples of programmes that require either low latencies or bulk data transfers e.g. (FTP) and might require high bandwidth but not low latencies. Multihoming increases fault tolerance and redundancy because when one hosts links goes down, it picks up from another link and continues the communication.
When there’s a major hardware problem in a certain ISP, multihoming will enable a site to use another ISP is connected to continue communication. Multihomed networks also provides away for load sharing. For example; outgoing traffic to the hosts belonging to network M is routed via ISP B and all other traffic via ISP C. This is because network M is routed via ISP B. Also, in cases where traffic is routed by default via slow ISP B and traffic generated busy hosts requiring more bandwidth is routed via faster and more expensive ISP C-which increases network connectivity and cost saving. Having multiple addresses eases renumbering transition since Nodes and Hosts can be configured to use new address prefixes and routes while the transition is in progress and remove the old ones from the nodes/hosts transition is finished. Miltihomed networks also enables use of multiple addresses at the same time when Node is changing location and received new address while still maintaining the old address. The IPv6 for example is designed to that a node might have multiple unicast addresses which are used for different reachability scopes.
Administrative and Technical issues of multihoming
Making service arrangements with several ISPs and setting up routing tables may not be enough for setting multihoming as most problems are linked to the internetworking and transport. Multihoming increases size of routing tables in the routers which decreases routing performance6. Other hosts on the operating system are also slowed down because the backbone routers are overloaded. Selecting addresses for sources and the destination causes problems with multihoming. Also, the selection process can be affected by; administrative problems, quality of service, costs of using certain interface, application requirements, use/operator7. Decision based on costs on how to select the most suitable interface for a given destination address requires careful consideration8. In this case, Cisco Networking Equipment should be well equipped to detect changes when interface changes between states, i.e. when interface goes down or up and characteristics change. When a problem of selecting a source address is experienced, chances are that the data may not reach the intended destination because the selected source address may have insufficient scope2.
Abley, James. 2010. “Goals for IPv6 Site-Multihoming Architectures- IETF.” Available at: http://www.ietf.org/internet-drafts/ draft-ietf-multi6-multihoming-requirements-05.txt
Deering, Stevens. 1999. “Overview of ip(v6) multihoming issues.” Presented at IPng Working Group Meeting, Tokyo.
Draves, Richard. 2003. “Default address selection for internet protocol version 6 (ipv6) Technical report.” Internet Engineering Task Force.
Hinden, Richard. 2010. “RFC 3513: Internet Protocol Version 6 (IPv6) Addressing Architecture. IETF”. Available at: http://www.ietf.org/rfc/rfc3513.txt. .
Kousa, Mika. 2006. “Supporting Multiple IPsec Security Association in Host Identity Protocol Mobility and Multihoming.”Telecommunication Software and Multimedia Labaratory Espoo : 1-86.
Mitchell, Bradley. 2010. “Multihoming.” Available at: http://compnetworking.about.com/od/tcpip/g/multihoming.htm. Accessed September 10, 2010
Stevens, Williams. 1997. UNIX Network programming. New York: Prentice-Hall.
Mitchell, Bradley. 2010. “Multiholming”. Web.
Kousa, Mika. 2006. “Supporting Multiple IPsec Security Association in Host Identity Protocol Mobility and Multihoming”. Telecommunication Software and Multimedia Labaratory Espoo : 1.
Mitchell 2010,1. Abley, James. 2010. “Goals for IPv6 Site-Multihoming Architectures-IETF”. Web.
Hinden, Richard. 2010. “RFC 3513: Internet Protocol Version 6 (IPv6) Addressing Architecture- IETF”. Web.
Hinden, Richard. 2010. “RFC 3513: Internet Protocol Version 6 (IPv6). Addressing Architecture- IETF”. Web.
Stevens, Williams. 1997. UNIX Network programming. New York: Prentice-Hall.
Deering, Stevens. 1999. “Overview of ip(v6) multihoming issues”. Presented at IPng Working Group Meeting, Tokyo.
. Kousa 2006, 18. Draves, Richard. 2003 “Default address selection for internet protocol version 6 (ipv6) Technical report”. Internet Engineering Task Force.