Multihoming is the practice of connecting a host or a computer network to more than one network. This can be done in order to increase reliability or performance.
A typical host or end-user network is connected to just one network. Connecting to multiple networks can increase reliability because if one connection fails, packets can still be routed through the remaining connection. Connecting to multiple networks can also improve performance because data can be transmitted and received through the multiple connections simultaneously multiplying throughput and, depending on the destination, it may be more efficient to route through one network or the other.
There are several different ways to perform multihoming.
A single host may be connected to multiple networks. For example, a mobile phone might be simultaneously connected to a WiFi network and a 3G network, and a desktop computer might be connected to both a home network and a VPN. A multihomed host usually is assigned multiple addresses, one per connected network.
In network multihoming, [1] [2] a network is connected to multiple providers, and uses its own range of addresses (typically from a Provider Independent (PI) range). The network's edge routers communicate with the providers using a dynamic routing protocol, typically BGP, which announces the network's address range to all providers. If one of the links fails, the dynamic routing protocol recognizes the failure and reconfigures its routing tables to use the remaining links, transparently to the hosts.
Network multihoming is costly since it requires the use of address space that is accepted by all providers, a public Autonomous System (AS) number, and a dynamic routing protocol. Since multihomed address space cannot be aggregated, it causes growth of the global routing table. [3]
In this approach, the network is connected to multiple providers, and assigned multiple address ranges, one for each provider. Hosts are assigned multiple addresses, one for each provider. [4]
Multihoming with multiple addresses is cheaper than network multihoming, and can be used without any cooperation from the providers (e.g. in a home network) but requires additional technology in order to perform routing: [5]
When multihoming is used to improve reliability, care must be taken to eliminate any single point of failure (SPOF):
By increasing the number of interfaces and links being used and making routing less deterministic, multihoming complicates network administration[ citation needed ].
Network multihoming is the dominant technique for IPv4. This requires that a network have its own public IP address range and a public AS number.
While multihoming with multiple addresses has been implemented for IPv4, [6] it is not generally used, as host implementations do not deal well with multiple addresses per interface which requires the use of "virtual interfaces". [7] It is also possible to implement multihoming for IPv4 using multiple NAT gateways. [8]
Both network multihoming and multihoming with multiple addresses may be used in IPv6.
Provider Independent Address Space (PI) is available in IPv6. [9] This technique has the advantage of working like IPv4, supporting traffic balancing across multiple providers, and maintaining existing TCP and UDP sessions through cut-overs. Critics say that the increased size of routing tables needed to handle multi-homing in this way will overwhelm current router hardware. Proponents say that new hardware will be able to handle the increase due to cheaper memory, which drops in price according to Moore's law. Proponents also say this is the only viable solution right now, and the worse is better philosophy supports the idea that it is better to deploy an imperfect solution now than a perfect solution after it is too late.
Because many ISPs filter out route announcements with small prefixes, this will generally require a large "ISP-sized" IP allocation, such as a /32, to ensure global reachability. Using such large prefixes is an inefficient use of IPv6's address space; there are only about 4 billion /32 prefixes. However, from a pragmatic perspective, allocating a /32 is equivalent in global address space cost to allocating a single IPv4 address, and this may be acceptable if, as seems to be likely for the foreseeable future, the number of multihomed sites can be numbered only in the millions, as opposed to the many billions of non-multihomed endpoints which are anticipated to comprise the vast majority of IPv6 endpoints.[ citation needed ] Some regional Internet registries (RIR) such as RIPE have started to allocate /48 from a specific prefix for this purpose. RIPE allocates IPv6 provider-independent address spaces /48 or shorter from 2001:0678::/29.
Multihoming with multiple addresses has been implemented for IPv6. [6] [10] For outgoing traffic, this requires support on the host, either protocol agnostic (Multipath TCP, SCTP, QUIC, etc.) or specific to IPv6 (e.g. SHIM6).
An Internet Protocol address is a numerical label such as 192.0.2.1 that is assigned to a device connected to a computer network that uses the Internet Protocol for communication. IP addresses serve two main functions: network interface identification, and location addressing.
Internet Protocol version 4 (IPv4) is the fourth version of the Internet Protocol (IP). It is one of the core protocols of standards-based internetworking methods in the Internet and other packet-switched networks. IPv4 was the first version deployed for production on SATNET in 1982 and on the ARPANET in January 1983. It is still used to route most Internet traffic today, even with the ongoing deployment of Internet Protocol version 6 (IPv6), its successor.
Internet Protocol version 6 (IPv6) is the most recent version of the Internet Protocol (IP), the communications protocol that provides an identification and location system for computers on networks and routes traffic across the Internet. IPv6 was developed by the Internet Engineering Task Force (IETF) to deal with the long-anticipated problem of IPv4 address exhaustion, and was intended to replace IPv4. In December 1998, IPv6 became a Draft Standard for the IETF, which subsequently ratified it as an Internet Standard on 14 July 2017.
The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet.
In computer networking, the User Datagram Protocol (UDP) is one of the core communication protocols of the Internet protocol suite used to send messages to other hosts on an Internet Protocol (IP) network. Within an IP network, UDP does not require prior communication to set up communication channels or data paths.
Classless Inter-Domain Routing is a method for allocating IP addresses for IP routing. The Internet Engineering Task Force introduced CIDR in 1993 to replace the previous classful network addressing architecture on the Internet. Its goal was to slow the growth of routing tables on routers across the Internet, and to help slow the rapid exhaustion of IPv4 addresses.
Border Gateway Protocol (BGP) is a standardized exterior gateway protocol designed to exchange routing and reachability information among autonomous systems (AS) on the Internet. BGP is classified as a path-vector routing protocol, and it makes routing decisions based on paths, network policies, or rule-sets configured by a network administrator.
A multicast address is a logical identifier for a group of hosts in a computer network that are available to process datagrams or frames intended to be multicast for a designated network service. Multicast addressing can be used in the link layer, such as Ethernet multicast, and at the internet layer for Internet Protocol Version 4 (IPv4) or Version 6 (IPv6) multicast.
Open Shortest Path First (OSPF) is a routing protocol for Internet Protocol (IP) networks. It uses a link state routing (LSR) algorithm and falls into the group of interior gateway protocols (IGPs), operating within a single autonomous system (AS).
Network address translation (NAT) is a method of mapping an IP address space into another by modifying network address information in the IP header of packets while they are in transit across a traffic routing device. The technique was originally used to bypass the need to assign a new address to every host when a network was moved, or when the upstream Internet service provider was replaced, but could not route the network's address space. It has become a popular and essential tool in conserving global address space in the face of IPv4 address exhaustion. One Internet-routable IP address of a NAT gateway can be used for an entire private network.
A virtual private network (VPN) is a mechanism for creating a secure connection between a computing device and a computer network, or between two networks, using an insecure communication medium such as the public Internet.
A subnetwork or subnet is a logical subdivision of an IP network. The practice of dividing a network into two or more networks is called subnetting.
Anycast is a network addressing and routing methodology in which a single IP address is shared by devices in multiple locations. Routers direct packets addressed to this destination to the location nearest the sender, using their normal decision-making algorithms, typically the lowest number of BGP network hops. Anycast routing is widely used by content delivery networks such as web and name servers, to bring their content closer to end users.
In Internet networking, a private network is a computer network that uses a private address space of IP addresses. These addresses are commonly used for local area networks (LANs) in residential, office, and enterprise environments. Both the IPv4 and the IPv6 specifications define private IP address ranges.
6to4 is an Internet transition mechanism for migrating from Internet Protocol version 4 (IPv4) to version 6 (IPv6) and a system that allows IPv6 packets to be transmitted over an IPv4 network without the need to configure explicit tunnels. Special relay servers are also in place that allow 6to4 networks to communicate with native IPv6 networks.
An IPv6 transition mechanism is a technology that facilitates the transitioning of the Internet from the Internet Protocol version 4 (IPv4) infrastructure in use since 1983 to the successor addressing and routing system of Internet Protocol Version 6 (IPv6). As IPv4 and IPv6 networks are not directly interoperable, transition technologies are designed to permit hosts on either network type to communicate with any other host.
Locator/ID Separation Protocol (LISP) is a "map-and-encapsulate" protocol which is developed by the Internet Engineering Task Force LISP Working Group. The basic idea behind the separation is that the Internet architecture combines two functions, routing locators and identifiers in one number space: the IP address. LISP supports the separation of the IPv4 and IPv6 address space following a network-based map-and-encapsulate scheme. In LISP, both identifiers and locators can be IP addresses or arbitrary elements like a set of GPS coordinates or a MAC address.
An Internet Protocol Version 6 address is a numeric label that is used to identify and locate a network interface of a computer or a network node participating in a computer network using IPv6. IP addresses are included in the packet header to indicate the source and the destination of each packet. The IP address of the destination is used to make decisions about routing IP packets to other networks.
An IPv6 packet is the smallest message entity exchanged using Internet Protocol version 6 (IPv6). Packets consist of control information for addressing and routing and a payload of user data. The control information in IPv6 packets is subdivided into a mandatory fixed header and optional extension headers. The payload of an IPv6 packet is typically a datagram or segment of the higher-level transport layer protocol, but may be data for an internet layer or link layer instead.
Source-specific routing, also called source-address dependent routing (SADR), is a routing technique in which a routing decision is made by looking at the source address of a packet in addition to its destination address. The main application of source-specific routing is to allow a cheap form of multihoming without the need for provider-independent addresses or any cooperation from upstream ISPs.
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