Assessing and improving TCP rate shaping over edge gateways

Abstract

Computers installed with commercial/open-source software have been widely employed as organizational edge gateways to provide policy-based network management. Such gateways include firewalls for access control, and bandwidth managers for managing the narrow Internet access links. When managing the TCP traffic, pass-through TCP flows can introduce large buffer requirements, large latency, frequent buffer overflows, and unfairness among flows competing for the same queue. So, how to allocate the bandwidth for a TCP flow without the above drawbacks becomes an important issue. This study assesses and improves TCP rate shaping algorithms to solve the above problems through self-developed implementations in Linux, testbed emulations, live Internet measurements, computer simulations, modeling, and analysis. The widely deployed TCP Rate control (TCR) approach is found to be more vulnerable to Internet packet losses and less compatible to some TCP sending operating systems. The proposed PostACK approach can preserve TCR's advantages while avoiding TCR's drawbacks. PostACK emulates per-flow queuing, but relocates the queuing of data to the queuing of ACKs in the reverse direction, hence minimizing the buffer requirement up to 96 percent. PostACK also has 10 percent goodput improvement against TCR under lossy WAN environments. A further scalable design of PostACK can scale up to 750Mbps while seamlessly cooperating with the link-sharing architecture. Experimental results can be reproduced through our open sources: 1) tcp-masq: a modified Linux kernel, 2) wan-emu: a testbed for conducting switched LAN-to-WAN or WAN-to-LAN experiments with RTT/loss/jitter emulations.