[noncoml]

“ Cut-through vs Store-and-Forward”

I had a Google interview collapse because the last interviewer was a Principle Engineer who just couldn’t accept that a switch will ever forward a frame before reading in full and checking the FCS.

I was too much of an idiot to go along with it and instead ended up having an argument with him.

[inigyou]

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.

[userbinator]

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.

[crote]

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.

[userbinator]

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.

[bcrl]

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>

[foobiekr]

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.

[bcrl]

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.

[sophacles]

I'm struggling to think of any real-world networking scenario where cut-through switching for ethernet can provide any sort of benefit.

Most data-center networks are involve aggregation/spine/core switches at higher speed than TOR/leaf switches - so the packet has to be buffered.

Most every workload is going to involve port collisions (2 packets at the same time needing the same output port) - again a full buffering required (or drops and throughput reduction).

Bigger networks start using vlans, mpls, ip routing at switches, and vxlan - requiring more lookups per-packet, larger "minimum chunks of packet seen before port is determined" sizes, etc reducing the value of cut-through.

And so on.

But i also struggle to find a scenario where a principle engineer at google is refusing to accept that cut-through switches exist, but rather, was refusing to accept that the scenario they're talking through with you would ever involve an effectively deployed cut-through switch (since that's basically never).

[BornToLag]

Ethernet frame preemption has been standardized for about 10 years for ultra low latency applications. See 802.1Qbu:

"Defines a class of service for time-critical frames that requests the transmitter in a bridged Local Area Network to suspend the transmission of a non-time-critical frame, and allow for one or more time-critical frames to be transmitted. When the time-critical frames have been transmitted, the transmission of the preempted frame is resumed. A non-time-critical frame could be preempted multiple times."

[toast0]

> I'm struggling to think of any real-world networking scenario where cut-through switching for ethernet can provide any sort of benefit.

When it works, it reduces latency and reduces buffer memory usage a smidge. This ASIC switch can't afford to buffer any packets, so cut through is the only option... AFAIK, most ethernet switches have at least some buffer memory...

> Most data-center networks are involve aggregation/spine/core switches at higher speed than TOR/leaf switches - so the packet has to be buffered.

If you have traffic flowing within a rack, you can cut through for that where the machines attach to the TOR switch at the same speed.

If you have traffic between racks, and it goes through a spine, it likely needs to buffer at the TORs, but it could cut through at the spine.

> Most every workload is going to involve port collisions (2 packets at the same time needing the same output port) - again a full buffering required (or drops and throughput reduction).

Depends on the workload and redundancy requirements. If you want to have dual link aggregation and no loss of throughput with a single lost link, you'll upgrade line speed when link utilization is at 50%, so maybe 50% of packets could be cut through (depending on burstiness). If latency is a key driver, you may have line speed much higher than throughput and have very little utilization.

[noncoml]

> but rather, was refusing to accept that the scenario they're talking through with you would ever involve an effectively deployed cut-through switch (since that's basically never).

Nope, the argument was exactly if a cut-through switch exists or ever existed

Edit: Actually the conversation came back to me:

He asked me about the frame format. Once I drew that on the board, he asked why is the DST before the SRC. My answer was in order for the switch to start doing the lookups ASAP and that's when I mentioned that a cut-through switch can switch the packet before it even finishes receiving it.

After that we spend the rest of the interview time arguing if cut-through switches actually exist or not. Both he and I having too big of an ego to let it go..

[icedchai]

So he didn't want to google it?

[inigyou]

If you Google it, you find many copies of theory, but not a single reference to a cut-through switch ever existing in real life.

[userbinator]

What "real life" are you living in? The references in https://en.wikipedia.org/wiki/Cut-through_switching already link to several. Here's a few more from all the major vendors.

Cisco:

https://www.cisco.com/c/en/us/td/docs/switches/datacenter/ne...

HPE Aruba:

https://arubanetworking.hpe.com/techdocs/AOS-CX/AOSCX-CLI-Ba...

Fortinet:

https://docs.fortinet.com/document/fortiswitch/7.2.7/adminis...

Arista:

https://arista.my.site.com/AristaCommunity/s/question/0D55w0...

Nvidia/Mellanox:

https://enterprise-support.nvidia.com/s/article/switch-forwa...

Extreme Networks:

https://documentation.extremenetworks.com/Switch%20Engine%20...

[noncoml]

From Arista documentation:

“Arista data planes support these packet forwarding modes: Store and forward: the switch accumulates entire packets before forwarding them. Cut through: the switch begins forwarding frames before their reception is complete.”

[icedchai]

You sure? There is one described here: https://en.wikipedia.org/wiki/Cut-through_switching (Kalpana switch from the early 90's)

[stinkbeetle]

I see you've never had a run-in with a blowhard in a position of power. Lucky.

[icedchai]

I've had it happen in an interview, many years ago. I told him we'd have to agree to disagree. After I was hired, he admitted he looked it up and I was correct (some weird SQL question he asked.)

[userbinator]

After I was hired, he admitted he looked it up and I was correct

The best possible outcome.

[noncoml]

“So he didn't want to google it?”

No, he didn’t want interrupt the interview to start checking out phones.

“I told him we'd have to agree to disagree.”

That’s where I went wrong. But I like to think learned my lesson.

[tuetuopay]

The two scenarios I can think of that definitely make use of cut-through are timekeeping systems, and HFT. The latter goes even further, where the switches at the exchange do cut-through, but also ensure the packet is broadcast to multiple ports at the exact same time. This is done so that no specific HFT firm get an advantage from being on port 10 rather than port 20. They even measure out the outgoing fibers to be the same length.

[empty_set35204]

The amount of effort an exchange goes through to ensure fairness varies substantially, unfortunately. Many do have measurable and consistent /semi-consistent differences between gateways that can be attributed to network asymmetries.

[brcmthrowaway]

What do HFT switches do?

[random__duck]

Sounds like they could have used you on that team. Bad luck, but it at least you can walk away proud knowing it was not an issue with your technical skills. :)

[Aurornis]

I remember studying cut-through forwarding in a class and thinking it was obviously going to be used everywhere.

Then I was disappointed to enter the real world of networking and learn that it wasn’t actually used as often as I had been led to believe due to conflicting with higher priority features.

So the interview may have been a conflict between textbook learning and the Google engineer’s practical experience. It’s tough to encounter that conflict in interviews because you get two people talking past each other.

[gjvc]

"principal"