What is NAT Acceleration?
NAT Acceleration is a set of software rules and hardware features used to speed up fast internet connections through a NAT router, typically used with internet bandwidth over 100 Mbits/s. It improves throughput and reduces CPU/hardware utilization on the router by bypassing some of the features of the TCP/IP stack. NAT acceleration (a.k.a. hardware acceleration) is usually a NAT router admin panel setting, with some router models allowing you to also set the level, as follows:
No NAT Acceleration
When NAT Acceleration is disabled, routers use the full TCP/IP stack with flow/congestion control, QoS, etc. causing a bit higher CPU/resource utilization. In this mode, NAT routers store the entire frame before sending it out to its destination. It is better with faster routers, multiple simultaneous local clients, when you need to use QoS, and with speeds that do not overwhelm the router's CPU/RAM (typically under 150Mbits/s).
Level 1: CTF (Cut Through Forwarding)
Software optimization technique to accelerate NAT traffic, usually used with speeds over 100Mbits/s, may be required for peak speeds over 200Mbits/s depending on your router hardware.
When CTF/Level 1 acceleration is enabled, the router sends out frames as soon as it knows their destination, without waiting for any acknowledgements, or flow control. This method relies on the destination device to provide information whether the data is corrupt and needs to be resent, however, it only supports adaptive QoS and may cause issues with some applications. It is recommended for fast connections (over 150Mbps) and few local clients, as it improves throughput while reducing latency and router CPU/memory utilization.
Level 2: CTF+FA (CTF/Level 1 + Flow Acceleration)
Hardware NAT acceleration mechanism designed for accelerating wired internet connections. This is typically only needed with very fast broadband connections at speeds over ~500 Mbps, as the traditional full TCP/IP stack and congestion/flow control becomes very taxing on the router's CPU and limited hardware resources.
CTF+FA (Level 2 NAT Acceleration) does not support QoS, and you may have to disable PPPoE and STP (Spanning Tree Protocol).
Pros: NAT Acceleration improves throughput while reducing latency and router CPU utilization. It may be needed to achieve higher WAN throghput.
Cons: NAT Acceleration supports only adaptive QoS (Level 1 CTF), no QoS (Level 2 CTF+FA), may not support port forwarding (hosting game servers, etc.), parental controls, PPPoE, STP. The increased retransmissions caused by NAT Acceleration may also cause shuttering on some streaming devices (Apple TV, Chromecast, VoIP).
Notes: "Auto" NAT Acceleration may be an option on some routers.
If the router admin interface provides CPU utilization statistics, that information can be used as an indicator of whether you need to turn on NAT Acceleration to reduce resource utilization under load for your particular internet speed.