When a network is connected to more than one Internet service provider (ISP) – who may be a connectivity provider, transit provider, or upstream provider -the technique is referred to as multihoming. The chief objective is to increase the quality and robustness of the Internet connection for the IP network. It is also possible to extend this concept to devices, especially when each of them has more than one interface, and each of the interfaces is attached to different networks. IPv6
Multihoming techniques are under serious consideration as the transition to the new IPv6 protocol is underway, specifically with the objective of imparting the desired level of resilience against malfunction of the links, hardware, and protocols within the system. The following additional advantages may also be derived: redundancy, load sharing, traffic engineering, policy constraints, transport-layer survivability, scalability, DNS compatibility, packet filtering capability and legacy compatibility.
Requirements for Multihoming
While the basic premise of using multihoming is to provide a solution to eliminate scope for the single point of failure (SPOF) in network connectivity, certain important factors have to be satisfied ensure flawless performance:
Upstream links and connectivity: Network operations centers must have multiple upstream links to individual service providers. Each of these upstream links should be located at a suitable distance away from one another, to obviate the possibility of simultaneous breakdown of all the connections happening even by accident.
Routers: The positioning of routers and switches must be so organized that all network access to a given host should under no circumstances be controlled by a single point of hardware control. Sometimes, multiple Internet uplinks are configured to converge on a single edge router. When this is done, any malfunction of that single router leads to disconnection of the Internet uplink, even where multiple ISPs are connected.
Host connectivity: A given host must be connected to the network over multiple network interfaces, each of which is connected to a separate router or switch. Again, the function of the specific host should be duplicated across multiple computers, each of which is connected to a different router or switch, to ensure maximum reliability.
Host referencing: A host must not only be accessible, but it should also be “referenced” with a functional name resolution to the particular server. This is important for ensuring high reliability.
Suggested approaches for IPv6 Multihoming
Five generic forms of architectural approaches towards smooth transition to IPv6 multi-homing have been identified:
Routing: The IPv4 multi-homing approach may be extended to IPv6 as well, with transit ISPs specifying the local site’s address prefix as a distinct routing entry. Provider Independent (PI) Address Space is offered in IPv6. However some people feel that the resultant increased routing table size is likely to be too high for current router hardware to handle efficiently. One possibility is that new hardware with higher memory can be produced at less cost and will be able to handle this.
Mobility: An IPv6-specific mobility approach to be devised New Protocol Element: A new element to be inserted in the protocol stack that manages a determined identity for the session. Modifying a Protocol Element: The transport or IP protocol stack element in the host may be suitably modified, to cope with dynamic changes to the forwarding locator. Modified Site-Exit Router: The site-exit router and local forwarding system can be suitably modified to allow various behaviors including source-based forwarding, site-exit hand-offs, and address rewriting by site-exit routers. (Source: RFC 4177 ftp://ftp.rfc-editor.org/in-notes/rfc4177.txt).
Suggested IPv6 Multihoming Solutions
- GSE/8+8: Global, Site, and End-System Address Elements
- GMultihoming with Route Aggregation
- GMultihoming Using Router Renumbering
- GMultihoming Support at Site Exit Routers
- GHost-Centric IPv6 Multihoming
- GGAPI: Geographically Aggregatable PI Addresses
- GMHAP: Multihoming Aliasing Protocol
- GProvider-Internal Aggregation Based on Geography to Support Multihoming in IPv6
- GAn IPv6 Provider-Independent Global Unicast Address Format
The two major methods for ID/Loc separation are Locator Identifier Separation Protocol (LISP) and SIX/One, and their variations were presented. LISP makes multihoming possible by packet capsuling and de-capsuling between tunnel routers with no impact on the hosts. The addresses given to tunnel routers for use for packet capsuling serve as Locator, while the in-site address is used as Identifier.
The SIX/One method is based on a host-based multihoming method called the shim6 protocol, and provides for change of the packet address field at the site of the intermediate routers. Presumably, shim6 will likely be adopted for multi-homing of residences and very small organizations.
As multihoming in the IPv6 protocol is still in its infancy, the various approaches are still under consideration, and it will be some time to come for a completely standardized solution once all the issues are resolved.