I have a (really dumb, kinda meme-y) project involving a dead hakko soldering iron and "Internet of Soldering", a custom ASIC for the actual front-end would be the appropriate level of silly and possibly could fit in the project budget.
I detailed the roadmap for custom ASIC design a bit here: https://talesonthewire.com/projects/two_weeks_until_tapeout/... and joining the Tiny Tapeout discord community is a great place to reach out for help.
Although I should point out that you missed the opportunity tor making the world's slowest counter and avoiding needing any expensive memory. Just declock it to an extreme degree. Hard to debug a design that never outputs anything for literal eons though.
Just saying, it's good form to do network stuff in big endian. Although since it's already done I dunno how hard you should work to retcon that to 0xFECA...
This used the librelane classic flow, so it's a lot of verilator, yosys, openroad, opensta, magic and klayout. :)
I wonder if it wouldn't make sense for one of the ports to be designated the "uplink" port and if the switch receives a packet destined for something not in the table it forwards it out the uplink port so long as the packet did not arrive via that port. It could even save a bit of memory by not storing entries for the uplink port in the table.
I'm surprised you managed to fit something like this in a TT grid square.
Also how much you can fit really depends on the node the shuttle is targeting. So I was in luck that this was on Global Foundries 180nm as you can cram in a lot for flip-flops onto a single tile compared with Skywater 130nm. On the other hand the gates are much slower than on Skywater 130nm.
Hopefully both programs will be around for a while, so if you are still interested when you have more time you can join the shuttle then.
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.
As far as advanced features go, even adding and removing VLAN tags is a headache in a cut-through switch.
It might be another switch. Even if it's not; VM/container networking is often implemented by bridging the host's NIC to a virtual switch, exposing the MAC addresses of all VMs/containers to the network.
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.
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).
"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."
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.
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..
“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.”
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.