| Transport Area Working Group | G. White, Ed. |
| Internet-Draft | CableLabs |
| Intended status: Informational | July 30, 2020 |
| Expires: January 31, 2021 |
Operational Guidance for Deployment of L4S in the Internet
draft-white-tsvwg-l4sops-00
This is an early, work-in-progress draft - a start at getting some of the ideas from the mailing list and email exchanges on paper.
This draft is intended to provide guidance to operators of end-systems, operators of networks, and researchers in order to ensure reasonable fairness between L4S and Classic flows sharing a single-queue RFC3168 bottleneck link. This draft identifies opportunites to prevent and/or detect and resolve fairness problems in such networks.
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In the majority of network paths, including paths where the bottleneck link utilizes packet drops (either due to buffer overrun or active queue management) in response to congestion, as well as paths that implement a 'flow-queuing' scheduler such as fq_codel or Cobalt, and those that implement dual-Q-coupled AQM, L4S traffic coexists well with classic congestion controlled traffic.
On network paths where the bottleneck link implements a shared-queue (FIFO) with an Active Queue Management algorithm that provides Explicit Congestion Notification signaling according to RFC3168, it has been demonstrated that when a set of long-running flows comprising both "Classic" congestion controlled flows and L4S-compliant congestion controlled flows compete for bandwidth, the classic congestion controlled flows may achieve lower throughput when compared to the L4S congestion controlled flows. This 'unfairness' between the two classes appears to be more pronounced on longer RTT paths (e.g. 50ms and above) and/or at higher link rates (e.g. 50 Mbps and above).
The root cause of this unfairness is that RFC3168 does not differentiate between packets marked ECT0 (used by classic senders) and those marked ECT1 (used by L4S senders), and provides an identical congestion signal (CE marks) to both classes, whereas the two classes respond differently to that congestion signal. The classic senders expect that CE marks are sent very rarely (e.g. approximately 1 CE mark every 200 round trips on a 50 Mbps x 50ms path) while the L4S senders expect very frequent CE marking (e.g. approximately 2 CE marks per round trip). The result is that the classic senders respond to the CE marks provided by the bottleneck by yielding capacity to the L4S flows. While this has not been demonstrated to cause starvation of the classic flows, the resulting rate imbalance can be a cause of concern.
There are a number of factors that influence the relative rates achieved by a set of congestion controlled flows sharing a queue in a bottleneck link.
TODO: discuss startup & convergence times, short flows, RTT-unfairness, differences in deployed CC algorithms, etc.
TODO: also mention that flow sharding is commonplace, so per-flow fairness does not imply per-application fairness
Support for L4S involves both endpoints: ECT1 marking & L4S-compatible congestion control on the sender, and ECN feedback on the receiver. Between these two entities, it is incumbent upon the sender to evaluate the potential for unfairness and make decisions whether or not to use L4S congestion control. The receiver is not expected to perform any testing or monitoring for unfairness, and is also not expected to invoke any active response in the case that unfairness occurs.
Some hosts (such as CDN leaf nodes and servers internal to an ISP) are deployed in environments in which they serve content to a constrained set of networks or clients. The operator of such hosts may be able to determine whether there is the possibility of RFC3168 FIFO bottlenecks being present, and utilize this information to make decisions on selectively deploying L4S.
While it is, of course, preferred for networks to deploy L4S-capable high fidelity congestion signaling, a network operator who has deployed equipment in a likely bottleneck link location (i.e. a link that is expected to be fully saturated) that is configured with an RFC3168 FIFO AQM can take certain steps in order to improve rate fairness between classic traffic and L4S traffic.
If equipment is configurable in such as way as to only supply CE marks to ECT0 packets, and treat ECT1 packets identically to NotECT, or is upgradable to support this capability, doing so will eliminate the risk of unfairness.
Equipment supporting RFC3168 may be configurable to enable two parallel queues for the same traffic class, with classification done based on the ECN field.
Option 1:
Option 2:
This configuration is similar to Option 2 in the previous section, but uses a single queue with WRED functionality.
Using an RFC6040 compatibility mode tunnel, tunnel ECT1 traffic through the RFC3168 bottleneck with the outer header indicating Not-ECT.
Two variants
While not a recommended alternative, disabling RFC3168 ECN marking eliminates the fairness issue. Clearly a downside to this approach is that classic senders will no longer get the benefits of Explict Congestion Notification.
While not a recommended alternative, remarking ECT1 packets as NotECT ensures that they are treated identically to classic NotECT senders. However, this also eliminates the possibility of downstream L4S bottlenecks providing high fidelity congestion signals.
TODO: Describe active testing methods, in-band or out-of-band, that can distinguish FIFO from FQ.
TBD
Thanks to Bob Briscoe, Jake Holland, Koen De Schepper, Olivier Tilmans, Tom Henderson, Asad Ahmed, and members of the TSVWG mailing list for their contributions to this document.
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