source-specific routing
{{distinguish|Source routing}}
Source-specific routing,{{cite conference|author=Matthieu Boutier|author2=Juliusz Chroboczek|title=Source-specific routing|work=Proc. IFIP Networking 2015|date=2015|arxiv=1403.0445|bibcode=2014arXiv1403.0445B}} also called source-address dependent routing (SADR),{{cite web | url=https://tools.ietf.org/html/draft-troan-homenet-sadr-01 | title=Draft-troan-homenet-sadr-01 }} 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.
The problem
File:Multihoming-incorrect-source.svg
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.{{IETF RFC|2827}}
Multihoming with source-specific routing
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.
Desirable host changes
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,{{IETF RFC|6724}} above the network layer (see Shim6), or by using multipath techniques at the higher layers (see Multipath TCP and Multipath Mosh{{cite arXiv|author=Matthieu Boutier|author2=Juliusz Chroboczek|title=User-space multipath UDP in Mosh|year=2015|eprint=1502.02402|class=cs.NI}}).
Support in routing protocols
On a network with a single edge router, it is possible to implement source-specific routing by manual manipulation of routing tables.
http://www.lartc.org/, Section 4.2 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;{{IETF RFC|9079}} this is implemented for IPv6 in babeld and in BIRD (earlier versions of babeld supported source-specific routing for IPv4{{cite web | url=https://alioth-lists.debian.net/pipermail/babel-users/2021-April/003818.html | title=[Babel-users] ANNOUNCE: Babeld-1.10 }});
- There exists an implementation of IS-IS with support for source-specific routing for IPv6 only.{{cite web | url=https://tools.ietf.org/html/draft-baker-ipv6-isis-dst-src-routing-07 | title=Draft-baker-ipv6-isis-DST-SRC-routing-07 }}
The IETF Homenet protocol suite requires support for source-specific routing in its routing protocol.{{IETF RFC|7368}}, Section 3.2.4