Testing the Impairments of Dynamic Optical Switching on TCP Traffic through the European FEDERICA Testbed Infrastructure Marco Ruffini, Donal O’Mahony, Linda Doyle CTVR Telecommunications Research Centre, University of Dublin, Trinity College, Ireland Tel: +353 (0)1 896 8441, e-mail: ruffinm@tcd.ie ABSTRACT In this article we examine the impairments that dynamic optical circuit switching can cause on the TCP transport protocol. Although the TCP Reno implementation solves some of the issues that occurred in legacy TCP during path switch, we found out that if optical path switching is used to bypass congested routers, impairments on TCP flows can still occur. We show the results of experiments carried out both in our laboratory and on the newly deployed European FEDERICA testbed infrastructure. Keywords: TCP impairments, dynamic optical circuit switching, FEDERICA testbed infrastructure. 1. INTRODUCTION Many in the research community believe that dynamic Optical Circuit Switching (OCS) will be the near-future solution to the bottleneck currently experienced at the routing layer. Many commercial optical circuit switched networks have been already deployed, where optical paths are created over relatively long time scales (days to weeks). Current research activity aims at bringing the dynamics of path provisioning down to seconds (for example with Optical IP Switching [1]) and sub-second scales (for example with Optical Flow Switching [2]). As optical paths are provisioned and deleted at progressively higher rates, the impairments that each path modification might cause on the Internet transport protocols (e.g., TCP and UDP) needs to be evaluated. Such studies currently exist mostly for optical burst-switched networks, where packet out-of-order arrival can often occur at the receiver. The main reference for TCP impairments in Optical Circuit Switching is the work in [3], where the authors show the improvement that the TCP RENO algorithm brings over legacy TCP. In this paper we extend such results with tests carried out on our Optical IP Switching (OIS) testbed. OIS is a novel optical circuit switching architecture that automatically creates optical paths that allow long-lived IP flows to bypass the IP layer. The reader can refer to [1] for further details. We examine the effect of optical switching first on high rate flows in our laboratory testbed. We then extend our tests, by using the FEDERICA [5] network infrastructure, to a more complex and large-scale topology, spanning multiple European countries. 2. TRANSPORT LEVEL ISSUES CAUSED BY PATH SWITCHING Dynamic creation, modification or cancellation of optical paths can degrade the quality of applications transported over TCP and UDP protocols, because it can increase jitter and cause loss and out of order arrival of packets at their destination. For live stream applications transported through UDP, packet loss and jitter deteriorate the perceived quality of the signal in a similar way. The issue is different for the TCP protocol. Although the effect of packet jitter is usually negligible, as long as it is not above the TCP timeout for the acknowledgment (usually of the order of a few seconds), out of order arrival and packet loss can negatively affect the protocol performance. Packet loss causes both the retransmission of packets and the shrinking of the acknowledgment window, significantly slowing down the data transfer process. In optical circuit switching, where packet losses are caused by the switching time of the optical devices, the legacy TCP protocol erroneously attributes such anomalies to network congestion, reducing the size of the TCP window. The out of order arrival can instead produce duplicate acknowledgments. Although such problems have been solved with the wide deployment of improved congestion algorithms in network stacks (e.g., TCP Reno), our experiments shows that there are some cases where switching optical paths can still cause disruption to the TCP protocol. 2.1 Effects of switching from a default to a dedicated link Issues on packet delivery arise in OCS networks when a packet flow is switched from a default into a transparent optical path that bypasses some of the IP routing nodes. As optical switches have negligible transit time, packets travelling on the new optical path can overtake those on the IP-routed path, causing out of order arrival at the destination node. This situation occurs when the sum of the transit times of the routers bypassed by the optical path is higher than the time gap ( ) between two consecutive packets belonging to the same flow. If indicates the transit time of the i-th router, and we express the packet inter-arrival time as a function of the application rate (R [MB/s]) and packet length (B [Bytes]) (with the simplification that packets are sent at uniform interval time, all of the same length), we could state that out of order arrival occurs if: . (1)