SCTP and TCP Variants: Congestion Control Under Multiple Losses

We characterize an inefficiency in the current specification of SCTP’s congestion control, which degrades performance (more than necessary to be “TCP-friendly”) when there are multiple packet losses in a single window. We present an SCTP variant, called New-Reno SCTP, which introduces three modifications. First, a Fast Recovery mechanism, similar to that of New-Reno TCP, is included to avoid multiple congestion window (cwnd) reductions in a single round-trip time. Second, we introduce a new policy which restricts the cwnd from being increased during Fast Recovery, thus ensuring that the newly introduced Fast Recovery mechanism maintains conservative behavior. Third, we modify SCTP’s HTNA (Highest TSN Newly Acked) algorithm to ensure that Fast Retransmits are not unnecessarily delayed. We show that New-Reno SCTP performs better, and still conforms to AIMD principles. Also, we compare these two variants of SCTP with New-Reno TCP and SACK TCP under five different loss scenarios. Our results show that New-Reno SCTP performs significantly better than New-Reno TCP, maintains conservative behavior similar to SACK TCP, and is as robust as SACK TCP to multiple losses in a window. Prepared through collaborative participation in the Communications and Networks Consortium sponsored by the U.S. Army Research Laboratory under the Collaborative Technology Alliance Program, Cooperative Agreement DAAD19-01-2-0011. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation thereon.