4 ms·

I'm curious if you know what, at a switch level, would actually cause drops and buffering for a 1:N (near-) saturated multicast flow. If all the packets are com

I'm curious if you know what, at a switch level, would actually cause drops and buffering for a 1:N (near-) saturated multicast flow. If all the packets are coming from the same source machine at (perhaps) 9.9Gbps and flowing into the switch, I would expect the switch to robustly redirect all that data with near-zero latency or packet drops to all its output ports. I don't think 10G Ethernet has "backpressure" in a way that would allow some output ports to get slowed down. If there are other data flows also going through the switch, that could obviously change things, and the sending computer could drop packets if there's jitter in how quickly the application produces them, but it seems impossible for the sending computer to burst packets into the switch any faster than it can handle because all the incoming packets are coming over the same 10G link. Not an expert here, legitimately curious.

hinkley 2y ago
Periodic background traffic like DHCP and background noise causing packet loss. You can’t run a queue at 100% and have any expectations of latency. In fact the rule of thumb from queueing theory is 50% to avoid latency spikes.
- kristjansson 2y ago
  I mean it's not that hard to eliminate all other traffic on a closed network like that, at least where there's millions of dollars at stake. Must be nice to open Wireshark and see _nothing_.
  - hinkley 2y ago
    That would be highly uninteresting to the rest of us don’t you think?
bitcharmer 2y ago
The nature of this kind of traffic is that it's pretty bursty. Think 100x-200x the normal packet rate in the span of a millisecond. Perfect opportunity for drops. Ultra low latency switches have tiny buffers.
SkipperCat 2y ago
A modern cut thru switch typically has one ASIC for a group of ports, and that ASIC handles all the traffic. If the traffic for all of those ports is greater than what the ASIC can handle, you'll have buffering and/or drops. That being said, the ASIC can typically handle line rate on all the ports. You could have 10G input and fan it out to 10G output on all the ports with no drops, but if there is other cross port traffic, something could get dropped.

hinkley 2y ago

Periodic background traffic like DHCP and background noise causing packet loss. You can’t run a queue at 100% and have any expectations of latency. In fact the rule of thumb from queueing theory is 50% to avoid latency spikes.

kristjansson 2y ago
I mean it's not that hard to eliminate all other traffic on a closed network like that, at least where there's millions of dollars at stake. Must be nice to open Wireshark and see _nothing_.
- hinkley 2y ago
  That would be highly uninteresting to the rest of us don’t you think?

bitcharmer 2y ago

The nature of this kind of traffic is that it's pretty bursty. Think 100x-200x the normal packet rate in the span of a millisecond. Perfect opportunity for drops. Ultra low latency switches have tiny buffers.

SkipperCat 2y ago

A modern cut thru switch typically has one ASIC for a group of ports, and that ASIC handles all the traffic. If the traffic for all of those ports is greater than what the ASIC can handle, you'll have buffering and/or drops. That being said, the ASIC can typically handle line rate on all the ports. You could have 10G input and fan it out to 10G output on all the ports with no drops, but if there is other cross port traffic, something could get dropped.