An examination of IP/ATM cut-through forwarding in dynamically routed networks

Multiprotocol over ATM (MPOA), IP switching and multiprotocol label switching (MPLS) have distinctly different mechanisms for cut-through packet forwarding. MPOA and IP switching use flow-based cut-through (FBC) forwarding while MPLS uses routing table linked cut-through forwarding (TLC). This paper examines the sensitivity of each these cut-through forwarding mechanisms to changes in underlying routing tables. We examine a scenario where a congestion-sensitive dynamic routing protocol, such as OSPF optimised multipath, leads to frequently changing routing tables. We show that FBC forwarding reacts significantly worse than flow length distributions predict, taking up to 1200 seconds to react to route changes and forward at least 50% of packets on the new route. Flow characteristics are examined to determine ways to improve FBC sensitivity. We show that implementing a maximum flow length of 200 seconds improves the response to route changes significantly with a minimal decrease in the number of switched packets (1.5%).