From: David Lebrun <david.leb...@uclouvain.be> Date: Tue, 29 Nov 2016 18:15:18 +0100
> When multiple nexthops are available for a given route, the routing engine > chooses a nexthop by computing the flow hash through get_hash_from_flowi6 > and by taking that value modulo the number of nexthops. The resulting value > indexes the nexthop to select. This method causes issues when a new nexthop > is added or one is removed (e.g. link failure). In that case, the number > of nexthops changes and potentially all the flows get re-routed to another > nexthop. > > This patch implements a consistent hash method to select the nexthop in > case of ECMP. The idea is to generate K slices (or intervals) for each > route with multiple nexthops. The nexthops are randomly assigned to those > slices, in a uniform manner. The number K is configurable through a sysctl > net.ipv6.route.ecmp_slices and is always an exponent of 2. To select the > nexthop, the algorithm takes the flow hash and computes an index which is > the flow hash modulo K. As K = 2^x, the modulo can be computed using a > simple binary AND operation (idx = hash & (K - 1)). The resulting index > references the selected nexthop. The lookup time complexity is thus O(1). > > When a nexthop is added, it steals K/N slices from the other nexthops, > where N is the new number of nexthops. The slices are stolen randomly and > uniformly from the other nexthops. When a nexthop is removed, the orphan > slices are randomly reassigned to the other nexthops. > > The number of slices for a route also fixes the maximum number of nexthops > possible for that route. > > Signed-off-by: David Lebrun <david.leb...@uclouvain.be> Interesting approach, but like Hannes I worry about the memory consumption bounds. Limiting to 1<<16 is interesting, but if you can limit to 1<<8 (256 nexthops) maybe the state requirement can be compressed even further? We can always increase this if necessary in the future if someone reports a reasonable use case that really needs it. Let's start simple and small first.
