Source-specific routing

Source-specific routing,^[1] also called source-address dependent routing (SADR),^[2] 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.

In traditional next-hop routing, a packet is routed according to its destination only, towards the closest router that announces a route that matches that destination. Consider a multihomed end-user network connected to two ISPs, BT&T and PacketCast; such a network will typically have two edge routers, each of which is connected to one ISP.

Both edge routers announce a default route, meaning that they are willing to accept packets destined for the Internet. If a packet with a source in BT&T's network is routed through PacketCast's edge router, PacketCast will assume it is a spoofed packet, and drop it in accordance to BCP 38.^[3]

With source-specific routing, each edge router announces a source-specific default route: a route that applies to packets destined to the Internet but only if their source is in a given prefix. The effect is that each edge router only attracts packets that have a source address in that provider's prefix.

With source-specific routing, each host interface has multiple addresses, one per provider-dependent prefix. For outgoing traffic, host software must choose the right source address. Various techniques for doing that have been suggested, at the network layer,^[4] above the network layer (see Shim6), or by using multipath techniques at the higher layers (see Multipath TCP and Multipath Mosh^[5]).

On a network with a single edge router, it is possible to implement source-specific routing by manual manipulation of routing tables.^[6] With multiple routers, explicit support for source-specific routing is required in the routing protocol.

As of early 2016, there are two routing protocols that implement support for source-specific routing:

• The Babel routing protocol has support for source-specific routing for both IPv4 and IPv6;^[7] this is implemented for IPv6 in babeld and in BIRD (earlier versions of babeld supported source-specific routing for IPv4^[8]);
• There exists an implementation of IS-IS with support for source-specific routing for IPv6 only.^[9]

The IETF Homenet protocol suite requires support for source-specific routing in its routing protocol.^[10]