jlivingood
commented
1 year ago [SM] The Introduction says "Furthermore, unlike with bandwidth priority on a highly/fully utilized link, low latency using these new approaches is not a zero sum game - everyone can potentially have lower latency at no one else's expense."
[JL] I'm not focusing on the current OS implementation to which you refer - I am addressing how it is implemented in an ISP network, such as in a CMTS. Feel free to debate OS implementation with the devs of those operating systems or perhaps the authors of the main L4S and NQB documents. ;-) I will ponder a re-wording that does not say zero-sum game.
[SM] "L4S does not provide low latency in the same way as previous technologies like DiffServ (QoS). That prior QoS approach used packet prioritization, where it was possible to assign a higher relative priority to certain application traffic, such as Voice over IP (VoIP) telephony." Again, given the right conditions, L4S will do exactly what is claimed here it would do not, e.g. when a short RTT L4S flow shares a link with a long RTT conventional-TCP flow.
[JL] I'll consider a re-wording but I am trying to contrast this with layer-3 network prioritization, where IMO this clearly differs. I know you likely disagree and I am not sure either of us will ever convince the other, but appreciate your view nonetheless.
[SM] Nitpick: yield instead of yeild
[JL] Good catch - thx
...it seems to contain imprecise descriptions that I would prefer not to find in any RFC independent of the document track:
https://urldefense.com/v3/__https://datatracker.ietf.org/doc/html/draft-livingood-low-latency-deployment-01*NewThinking__;Iw!!CQl3mcHX2A!AmULDT9h4izDsq8Uv5SzAjBiTa3_f8SR8jQk-CGMmG8BQerRUvygrpILGdxwk0uDMgf5VInlAeUNKIzbR2wU7Xwo$ "The Introduction says "Furthermore, unlike with bandwidth priority on a highly/fully utilized link, low latency using these new approaches is not a zero sum game - everyone can potentially have lower latency at no one else's expense." But this bears a bit more discussion to understand more fully."
"Zero-sum game is a mathematical representation in game theory and economic theory of a situation which involves two sides, where the result is an advantage for one side and an equivalent loss for the other.[1] In other words, player one's gain is equivalent to player two's loss, therefore the net improvement in benefit of the game is zero."
https://urldefense.com/v3/__https://datatracker.ietf.org/doc/html/draft-livingood-low-latency-deployment-01*NewThinking__;Iw!!CQl3mcHX2A!AmULDT9h4izDsq8Uv5SzAjBiTa3_f8SR8jQk-CGMmG8BQerRUvygrpILGdxwk0uDMgf5VInlAeUNKIzbR2wU7Xwo$ "4S does not provide low latency in the same way as previous technologies like DiffServ (QoS). That prior QoS approach used packet prioritization, where it was possible to assign a higher relative priority to certain application traffic, such as Voice over IP (VoIP) telephony.
https://urldefense.com/v3/__https://datatracker.ietf.org/doc/html/draft-livingood-low-latency-deployment-01*NewThinking__;Iw!!CQl3mcHX2A!AmULDT9h4izDsq8Uv5SzAjBiTa3_f8SR8jQk-CGMmG8BQerRUvygrpILGdxwk0uDMgf5VInlAeUNKIzbR2wU7Xwo$ "This approach could provide consistent and relatively low latency by assigning high priority to a partition of the capacity of a link, and then policing the rate of packets using that partition. For example, on a 10 Mbps link, a high QoS priority could be assigned to VoIP with a dedicated capacity of 1 Mbps of the 10 Mbps link capacity. The other 9 Mbps would be available to lower QoS priority, such as best effort general Internet traffic that was not VoIP.¶
https://urldefense.com/v3/__https://datatracker.ietf.org/doc/html/draft-livingood-low-latency-deployment-01*NewThinking__;Iw!!CQl3mcHX2A!AmULDT9h4izDsq8Uv5SzAjBiTa3_f8SR8jQk-CGMmG8BQerRUvygrpILGdxwk0uDMgf5VInlAeUNKIzbR2wU7Xwo$ "But even when QoS was used in this manner, the latency may have been relatively good but it was not ultra low latency of the sort that low latency networking (NQB and L4S) can deliver. As well, that QoS approach is to some extent predicated on an idea that network capacity is very limited and that links are often highly utilized. But in today's Internet, it is increasingly the case that there is an abundance of capacity to end users, which makes QoS approaches ineffective in delivering ever-lower latency.¶"
https://urldefense.com/v3/__https://datatracker.ietf.org/doc/html/draft-livingood-low-latency-deployment-01*NewThinking__;Iw!!CQl3mcHX2A!AmULDT9h4izDsq8Uv5SzAjBiTa3_f8SR8jQk-CGMmG8BQerRUvygrpILGdxwk0uDMgf5VInlAeUNKIzbR2wU7Xwo$ "The result, as noted in the prior section, has been the role of dual queue networking. With these approaches, the new low latency packet processing queue is introduced on one or more links on the end-to-end path. The internal L4S queuing may still use a sort of internal prioritization, but this is not QoS in the typical sense because this is happening on an extremely short timescale - sub-round trip time (so microseconds or a few milliseconds)."
https://urldefense.com/v3/__https://datatracker.ietf.org/doc/html/draft-livingood-low-latency-deployment-01*NewThinking__;Iw!!CQl3mcHX2A!AmULDT9h4izDsq8Uv5SzAjBiTa3_f8SR8jQk-CGMmG8BQerRUvygrpILGdxwk0uDMgf5VInlAeUNKIzbR2wU7Xwo$ "A more important and impactful force at play is the rapid congestion signals that are exchanged that will cause a sender to dynamically yeild to other traffic (as if the other traffic had no QoS priority, which it does not) - which can be thought of as back pressure to signal the sender to backoff prior to packetloss occuring.¶"
[SM] Nitpick: yield instead of yeild
Again this is targeted as Informational and intended to document Comcast's recommendations, which might well be built on what the text describes, yet it would be helpful to make sure that technology is described in objective language that is free from bias.
P.S.: The L4S RFC's clearly state that L4S is built upon conditional priority scheduling between its two queues, so it seems rather surprising to claim that it does not. It is not hard to describe its mechanism in this draft in a way, that is correct and still shows how this design has the potential of higher utility than a strawman of a fixed prioritity assignment, but it will require an example where L4S actually deliver over the traditional hierarchical prioritization method.