19-10-2012, 04:37 PM
Underlay Interaction
19overlay-underlay.ppt (Size: 281.5 KB / Downloads: 80)
What’s Wrong With Internet Routing?
Restrictive path-selection model
Destination-based packet forwarding
Single best BGP path per prefix
BGP routing constrained by policies
Ignoring congestion and delay
Ignoring application requirements
Unappealing protocol dynamics
Persistent oscillation (due to policy conflicts)
Slow convergence (due to path exploration)
Lost reachability (due to route-flap damping)
Putting More Power in End Hosts
Source routing (e.g., Nimrod)
End host selects the end-to-end path
Routers simply forward packets on the path
Requires the routers to agree to participate
Overlay networks (e.g., RON)
Conventional computers act as logical routers
Real routers deliver packets to intermediate hosts
No need for cooperation from the real routers
Hybrid schemes
Source routing at the AS level
Source routing in the overlay network
Advantage: Flexible Routing
Paths that violate BGP routing policy
E.g., A to C goes through AT&T and Sprint
… and C to B goes through UUNET
BGP would not allow AT&T-Sprint-UUNET path
Quick adaptation to network problems
Fast detection of congestion and outages
… by probing as aggressively as necessary
Selecting paths based on different metrics
E.g., overlay selects paths based on latency
… whereas the underlay might try to balance
Disadvantage: Limitations of Active Probes
Bandwidth overhead
Probe traffic between two nodes
Propagating probe results to other nodes
Limited accuracy of end-to-end probes
Available bandwidth of logical link?
Losses due to congestion vs. failure?
Problem on forward vs. reverse path?
Limited visibility
Logical links may share underlay routers/links
May be hard to detect the dependencies