Designing an Ethernet Switch ASIC

> So for everyone else also running multiple instances of experimental hardware on their local network I have added an additional 2 Byte magic number using the all so original 0xCAFE (little endian) allowing packet parsers to distinguish between this ASIC’s experimental traffic and your other experimental traffic.

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...

You can check out the tiles for this project and their exact location on the TT chip[0].

[0] https://tinytapeout.com/chips/ttgf26b/tt_um_coffeepot

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."

> 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.

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.

> 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..

What do HFT switches do?