Scalable TCP Session Monitoring with Symmetric Receive-side Scaling

Receive-side scaling (RSS) is a technique that stores the arriving IP packets in the same flow into the same hardware queue of a modern network interface card (NIC). It allows scalable processing of the received packets by allowing exclusive access to the NIC queues by each CPU core. This removes the lock contention when accessing the NIC queue, and it allows concurrent access to different queues by multiple CPU cores. One problem with the existing RSS mechanism, however, is that it maps the IP packets in the same TCP connection to different NIC queues depending on the direction of the packets. In this paper, we present symmetric RSS, which allows mapping the packets in the same TCP connection to the same NIC queue regardless of whether they are upstream or downstream. The basic idea is to manipulate the RSS seeds such that the RSS hashes of the IP packets in the reverse direction to take on the same values with those in the forward direction. Since RSS hash calculation is done in the NIC hardware, it does not consume extra CPU cycles and promises high performance.