Scheduling and flow control in CMT-SCTP

Abstract

Stream Control Transmission Protocol (SCTP) is a reliable transport layer protocol that supports multihoming. An extension of the SCTP that supports simultaneous data transfer over multiple paths is called Concurrent Multipath Transfer (CMT-SCTP), which can achieve an aggregated transmission throughput greater than that of a single path. The performance of CMT-SCTP in multipath networks with dissimilar path performance characteristics (PPCs) is challenging, especially when operating under the constraint of a small buffer. A problem arises when the fair round robin scheduling allows the paths of lower PPCs (slow paths) to affect the overall transmission efficiency due to the out-of-sequence outstanding data at the receiver. Data along the slow and fast paths occupy the shared receiver buffer while waiting for the out-of-sequence data, which causes performance degradation. Therefore, an intelligent scheduling of data transmissions as well as a proper flow control are necessary for efficient data transfer. I propose a flow control technique (CMT-RTTA), which logically divides a buffer by destination and in accordance with the corresponding RTT (round trip time). A destination with a shorter RTT will be allowed to occupy more buffer space than those with a longer RTT. With buffer splitting, destinations with shorter RTTs are allowed to receive their data first. I also propose an algorithm for scheduling data transmissions based mainly on the outstanding bytes (CMT-OUT). Firstly, CMT-OUT transmits data packets to the destinations that are ranked by a proposed Destination Selection Value (DSV) and only if the congestion and flow control allows the transmission. Secondly, the algorithm updates the path quality (PQU) after a successful transmission, which is a measure of how preferable the current destination should be selected in the next round. The third technique (OUT-BD), I propose is intended for heterogeneous networks having a higher dissimilarity among the paths. For that, I use CMTOUT for scheduling of data at the sender. For the flow control, the technique first selects a good path based on RTT and bandwidth. For the assumed good path, the sender is allowed to send data as long as the buffer space is available. For the other paths, if the transmission is allowed, the buffer size is intentionally delimited by the one-way bandwidth-delay-product (OWBDP). This technique is designed for those scenarios where higher OWBDP is because of the longer propagation delay. I established a real Internet testbed setup, which covers a two-path and a four-path network for testing all of the proposed techniques. The experiments show that CMT-RTTA and CMT-OUT improved the throughput of CMT-SCTP on average by 14% and 21% respectively when the maximum bandwidth dissimilarity is applied. Under the same scenario, the throughput improvement on average by OUT-BD is 59%. The mechanism in CMT-SCTP to handle the delay dissimilarity on a simple two-path scenario is very efficient; in the experiments on a slightly more complex four-path scenario, CMT-RTTA, CMT-OUT and OUT-BD improved the throughput by 26%, 54% and 41%, respectively.