On shaping TCP traffic at edge gateways

Many security and QoS functions have been deployed at edge gateways to provide policy-based network management. For QoS functions, the bandwidth management system can manage the narrow WAN access links. When managing the TCP traffic, pass-through TCP flows can introduce large buffer requirement, latency, buffer overflows, and unfairness among flows competing for the same queue. This study evaluates possible TCP-aware approaches through self-developed implementations in Linux, testbed emulation, and live WAN measurement. The widely deployed TCP rate control (TCR) approach is found to be more vulnerable to WAN packet losses and less compatible to several 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 minimizes buffer requirement up to 96%. PostACK also has 10% goodput improvement against TCR under lossy WAN. 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.