Transient BGP Loops Do they matter, and what can be done about them? Nate Kushman MIT/Akamai Srikanth Kandula, Dina Katabi and John Wroclawski

What causes: “Transient BGP Loops” MIT Bob JoeAT&TSprint Maintenance Withdraw MIT

What causes: “Transient BGP Loops” MIT Bob JoeAT&TSprint Maintenance

What causes: “Transient BGP Loops” MIT Bob JoeAT&TSprint Maintenance Withdraw MIT

What causes: “Transient BGP Loops” MIT Bob JoeAT&TSprint Maintenance Routing Loop

What causes: “Transient BGP Loops” MIT Bob JoeAT&TSprint Maintenance Withdraw MIT

What causes: “Transient BGP Loops” MIT Bob JoeAT&TSprint Maintenance

What causes: “Transient BGP Loops” MIT Bob JoeAT&TSprint Maintenance

How common are: “Transient Inter-domain Routing Loops” Sprint Study (IMC 2003, IMW 2002): –Looked at packet traces from the Sprint backbone –Up to 90% of the observed packet-loss was caused by routing loops –60-100% of the loops attributable to BGP

Routing Loop Damage Our Study: –20 vantage points with BGP feeds –2 Months –70,000 unique prefixes –Pinged once every 2 minutes –Trace-routed once every 30 minutes –TTL Exceeded responses to detect loops –Additional pings and traceroutes when loops detected

Routing Loop Damage 10-15% of updates cause routing loops

Collateral Damage AS A AS F AS E AS D AS C AS B

Collateral Damage AS A AS F AS E AS D AS C AS B Collateral Damage X

Collateral Damage Prefixes sharing a loopy link see 19% loss

What should be done? We should prevent forwarding loops

A loop occurs because: One AS pushes a route update to the data plane, but other AS's, unaware yet of the move, try to send packets on the old route

How can we avoid Routing Loops? MIT Bob JoeAT&TSprint Maintenance Withdraw MIT

How can we avoid Routing Loops? MIT Bob JoeAT&TSprint Maintenance Withdraw MIT AT&T still thinks Joe is routing through Bob

How can we avoid Routing Loops? MIT Bob JoeAT&TSprint Maintenance What if: AT&T knew about Joe’s change before making its own?

Suspension Continue to route traffic Tell control system not to propagate the route

How can we avoid Routing Loops? MIT Bob JoeAT&TSprint Maintenance Withdraw MIT

How can we avoid Routing Loops? MIT Bob JoeAT&TSprint Maintenance Withdraw MIT What if: Joe sends it’s update before changing it’s forwarding table?

How can we avoid Routing Loops? MIT Bob JoeAT&TSprint Maintenance

How can we avoid Routing Loops? MIT Bob JoeAT&TSprint Maintenance And also waits for an Ack from AT&T before updating it’s forwarding table?

How can we avoid Routing Loops? MIT Bob JoeAT&TSprint Maintenance Then we can be sure that AT&T knows about the path change before it happens and will not use the path

How can we avoid Routing Loops? MIT Bob JoeAT&TSprint Maintenance Instead, AT&T will move immediately to the Sprint path and the loop is avoided.

More Generally We have proven: –Loops are prevented in the general case –Convergence properties similar to normal BGP All sorts of good proofs and stuff: –

Your feedback Clearly: –Planned Maintenance events 20% of update events caused by planned maintenance –Link up events What about? –Unplanned Link down events –Trade-off between loss on current path and collateral damage

In Short Routing loops cause significant performance problems Even prefixes with no BGP updates are significantly affected by loops A simple change to BGP can avoid all routing loops

Questions?