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I'm curious, what makes the natting/routing performance much better on a router? Is it that the router is an ASIC designed only to process packets vs a cpu that

I'm curious, what makes the natting/routing performance much better on a router? Is it that the router is an ASIC designed only to process packets vs a cpu that can execute any code?

dsr_ 6y ago
In particular, a modern high-end router has a CPU (which might be an x86 of some kind) which handles administrative and auxiliary tasks (NTP, ssh, DHCP reflection, whatever you want) while making one or more dedicated ASICs handle packets. When you manage the router, you're actually talking to the supervisory CPU.
dahfizz 6y ago
> Is it that the router is an ASIC designed only to process packets vs a cpu that can execute any code? Yes. Most routers + switches are "line rate" meaning the packets go through the switch at the speed of electricity, as if the switch wasn't there are all.
rayiner 6y ago
That's not really the whole explanation. There are a couple of things in play. First, while Linux can perform routing, it's mainly intended to be an end host. So for example software like VPP that is designed with the assumption that it can use polling and burn up entire CPU cores to optimize packet forwarding can perform 10x faster on the same hardware. Second, router ASICs can be designed to sped transistors on a predictable packet flow path, instead of intelligence like out-of-order execution to make general purpose code fast. For example, routers have big expensive content-addressable memories called TCAMs that are used to store things like routing tables and ACLs. The ASIC, moreover, can implement a highly tuned pipeline designed around the latencies in the underlying memories. E.g. you get a packet, grab the destination address, look up the next hop in the routing table, etc. Each step takes a predictable amount of time that you can account for and optimize. Third, parallelism is much cheaper in hardware than in general purpose CPUs. It takes a lot more transistors to be able to execute a second general-purpose instruction stream than to have a single-purpose circuit that does some work in parallel with something else.

I'm curious, what makes the natting/routing performance much better on a router? Is it that the router is an ASIC designed only to process packets vs a cpu that can execute any code?

dsr_ 6y ago

In particular, a modern high-end router has a CPU (which might be an x86 of some kind) which handles administrative and auxiliary tasks (NTP, ssh, DHCP reflection, whatever you want) while making one or more dedicated ASICs handle packets. When you manage the router, you're actually talking to the supervisory CPU.

dahfizz 6y ago

> Is it that the router is an ASIC designed only to process packets vs a cpu that can execute any code? Yes. Most routers + switches are "line rate" meaning the packets go through the switch at the speed of electricity, as if the switch wasn't there are all.

rayiner 6y ago

That's not really the whole explanation. There are a couple of things in play. First, while Linux can perform routing, it's mainly intended to be an end host. So for example software like VPP that is designed with the assumption that it can use polling and burn up entire CPU cores to optimize packet forwarding can perform 10x faster on the same hardware. Second, router ASICs can be designed to sped transistors on a predictable packet flow path, instead of intelligence like out-of-order execution to make general purpose code fast. For example, routers have big expensive content-addressable memories called TCAMs that are used to store things like routing tables and ACLs. The ASIC, moreover, can implement a highly tuned pipeline designed around the latencies in the underlying memories. E.g. you get a packet, grab the destination address, look up the next hop in the routing table, etc. Each step takes a predictable amount of time that you can account for and optimize. Third, parallelism is much cheaper in hardware than in general purpose CPUs. It takes a lot more transistors to be able to execute a second general-purpose instruction stream than to have a single-purpose circuit that does some work in parallel with something else.