Algorithms for In-Place, Consistent Network Update

Kedar S. Namjoshi, Sougol Gheissi, Krishan K. Sabnani

Abstract: Network configurations are regularly updated in response to issues such as congestion, failures, network changes, and modifications to security policies. We present a simple distributed algorithm for network update that operates on the fly and in place, and guarantees strong route-consistency. Existing methods are either weakly consistent, or do not operate in place and require excessive memory.We prove that our method, called a causal update, appears to take effect instantaneously at a quiescent network state, even though it is actually carried out over time and interleaved with normal network operation. This property ensures per-packet consistency: i.e., a packet is processed either entirely within the old configuration or within the new one, never by a mix of the two. The price paid for these strong guarantees is that the algorithm may be forced to occasionally drop packets for consistency. We prove that forced drops cannot be avoided: any in-place and on-the-fly update method must drop packets or violate consistency. We show how to exploit network structure and update characteristics to reduce or entirely eliminate packet drops. Simulation experiments indicate that the (temporary) loss in throughput from forced packet drops falls within acceptable limits.