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As datacenter network speeds rise, an increasing fraction of server CPU cycles is consumed by TCP packet processing, in particular for remote procedure calls (RPCs). To free server CPUs from this burden, various existing approaches have attempted to mitigate these overheads, by bypassing the OS kernel, customizing the TCP stack for an application, or by offloading packet processing to dedicated hardware. In doing so, these approaches trade security, agility, or generality for efficiency. Neither trade-off is fully desirable in the fast-evolving commodity cloud. We present TAS, TCP acceleration as a service. TAS splits the common case of TCP processing for RPCs in the datacenter from the OS kernel and executes it as a fastpath OS service on dedicated CPUs. Doing so allows us to streamline the common case, while still supporting all of the features of a stock TCP stack, including security, agility, and generality. In particular, we remove code and data of less common cases from the fastpath, improving performance on the wide, deeply pipelined CPU architecture common in today's servers. To be workload proportional, TAS dynamically allocates the appropriate amount of CPUs to accommodate the fastpath, depending on the traffic load. TAS provides up to 90% higher throughput and 57% lower tail latency than the IX kernel bypass OS for common cloud applications, such as a key-value store and a realtime analytics framework. TAS also scales to more TCP connections, providing 2.2x higher throughput than IX with 64K connections.