This draft from the IETF based upon work from Cisco, Juniper, and Google is titled Enterprise Multihoming using Provider-Assigned Addresses without Network Prefix Translation: Requirements and Solution.
Quoting from the lead in section:
Connecting an enterprise site to multiple ISPs using provider- assigned addresses is difficult without the use of some form of Network Address Translation (NAT). Much has been written on this topic over the last 10 to 15 years, but it still remains a problem without a clearly defined or widely implemented solution. Any multihoming solution without NAT requires hosts at the site to have addresses from each ISP and to select the egress ISP by selecting a source address for outgoing packets. It also requires routers at the site to take into account those source addresses when forwarding packets out towards the ISPs.
This document attempts to define a complete solution to this problem. It covers the behavior of routers to forward traffic taking into account source address, and it covers the behavior of host to select appropriate source addresses. It also covers any possible role that routers might play in providing information to hosts to help them select appropriate source addresses. In the process of exploring potential solutions, this documents also makes explicit requirements for how the solution would be expected to behave from the perspective of an enterprise site network administrator.
Some interesting quotes:
"It is not reasonable to expect an enterprise network operator to change the routing topology of the site in order to deploy IPv6."
"Mechanisms have been proposed to allow hosts to choose the source address for packets in a fine grained manner. We will discuss these proposals in Section 4. However, interacting with host operating systems in some manner to ensure a particular source address is chosen for a particular destination prefix is not what an enterprise network administrator would expect to have to do to implement this routing policy."
"With IPv4, this problem is commonly solved by using [RFC1918] private address space within the multi-homed site and Network Address Translation (NAT) or Network Address/Port Translation (NAPT) on the uplinks to the ISPs. However, one of the goals of IPv6 is to eliminate the need for and the use of NAT or NAPT. Therefore, requiring the use of NAT or NAPT for an enterprise site to multihome with provider-assigned addresses is not an attractive solution."