Summary

  • An individual IETF proposal would shift traffic from a locally congested interface onto a dedicated Offloading Flex-Algo path, one destination prefix and one measured step at a time.
  • Its High Utilization Affinity blocks new offload onto a crowded link while existing offloaded traffic may stay. Congestion Affinity removes that link from the offload topology and sends the existing detour traffic elsewhere.

The nasty moment comes after the first rescue. A router sees its primary interface filling up and diverts a slice of traffic. The alternate path has spare room, until several routers notice the same attractive corridor. It is now nearly full. The draft's first response is a velvet rope: advertise High Utilization Affinity, so no additional traffic is offloaded through that link. Flows already sent there may continue. If utilization reaches the congestion threshold, a separate Congestion Affinity excludes the link from the dedicated Offloading Flex-Algo, or OFA, topology. Existing offloaded traffic then has to find another OFA path. A network can therefore be stable at the first gate and suddenly much busier elsewhere at the second.

The proposed control is local. Each router measures its own outgoing interfaces and offloads by destination prefix using unequal-cost multipath between its primary route and the OFA route. It increases the offload share progressively, with jitter between iterations. The draft's 5% step and its 50/60/70/80/90% utilization diagram are examples, not common settings prescribed to every operator. The amount must account for the lowest-capacity link on the alternate path. If there is no valid OFA path, or a candidate crosses a high-utilization link, the router cannot add new offload. RFC 9350 provides the Flexible Algorithm machinery on which that separate topology rests.

The scheme buys time by refusing fresh detours before ejecting old ones. It also makes path state a moving target. An elephant flow can dominate a measurement interval; multiple routers can shift load at once; a topology change terminates that prefix's offload and starts the process again. The draft calls for prefix-specific damping to restrain oscillation. It recommends leaving more time between iterations than congestion detection, IGP advertisement throttling, flooding and a stability margin together require.

It also needs every router along a particular OFA path to support the mechanism; a legacy router in that path can erase the detour.

The IETF tracker lists an individual Internet-Draft, not a deployment result. Comparing revision 00 with November's revision 01 shows a date and document-header refresh rather than a newly invented two-gate mechanism. The missing receipt is a multi-router trace showing when the high-utilization gate stopped new offload, when Congestion Affinity expelled existing traffic, and whether the next path stayed below its own limit.