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Comment by inigyou | Hacker News Reader

Cut-through is applicable in a minority of scenarios which are sometimes common. The majority of switches, especially the ones that support any advanced features at all, can't use it. Even when you can use it, you still can't use it all the time because of port conflicts.

As far as advanced features go, even adding and removing VLAN tags is a headache in a cut-through switch.

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As far as advanced features go, even adding and removing VLAN tags is a headache in a cut-through switch.

Not really; it just needs to inject the tag right after the source address, or not pass it through at all; and of course recompute throughout and replace the FCS at the end. Other more advanced packet editing can be done in a streaming fashion too, with the latency only limited by the length of any "forward references" needed.

But "just recompute the FCS" isn't quite the right approach, is it?

With a traditional cut-through switch the entire packet is passed as-is, so you're not modifying a corrupt package. You don't drop it, but the corruption is trivially detectable on the other side. If you naively recompute the FCS on transmit you essentially mark a corrupted package as valid, so you have to compute the FCS on transmit and receive, and intentionally send out the wrong FCS on transmit if you determine the receive FCS is invalid.

It's easy enough to write a bit of Verilog to perform partial updates on the CRC to only compensate for what has changed. I did so for an FPGA project I implemented a number of years ago when processing VLAN tags and doing IP forwarding.

That said, there are lazy hardware designers that just slap on a block to recompute the CRC and replace the old one with the new. An experience I had years ago at Red Hat taught me that the hard way as a shiny new Cisco ethernet switch IT deployed ended up costing us weeks of frustration and delayed a release when it caused bit flips in NFS packets during stress testing. The switch was so awesomely advanced that it recomputed the CRC on the bit flipped packets causing kernel builds to fail. <sigh>

Wow that rings a bell.

The lack of end to end ECC protection in some packet paths on very high end ASICs is something I just can’t get over.

ECC tends to be too expensive to be used widely in hardware data paths. Most of the time hardware will use a parity bit as checking it is far less costly (both in terms of gates and power). Parity bits are usually sufficient, as the purpose is really limited to identifying hardware failure. The CRC should prevent delivery of a corrupted packet provided the hardware doesn't cheat and blindly recompute it.

ECC is _not_ too expensive to use in hardware data paths. The existence proof of this is almost every medium to high end NPU has end to end ECC.

so you have to compute the FCS on transmit and receive, and intentionally send out the wrong FCS on transmit if you determine the receive FCS is invalid.

Yes, exactly; but that isn't particularly difficult either, compared to all the other packet-editing capabilities that switch ASICs have.

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