matt9j:Matt Johnson

Rethinking network congestion (Especially in community networks)

·7 mins

What is congestion in a network anyway? #

A network link suffers from congestion when demand for the link’s resources exceeds the amount of service it can supply within some reasonable duration of time, resulting in data being dropped on the floor or delayed. Both the “resources” and “reasonable duration of time” are highly situational and vary by application and type of network.[^1] Ultimately this becomes “ugh, why is the WiFi so slow today!”

In the presence of resource congestion, everyone on the network cannot get everything that they want.

I argue that networking practitioners, myself included, need to rethink how we reason about network behavior in the presence of congestion. We need to consider how humans on their computers and phones experience congestion and how those humans expect to resolve these fundamentally resource allocation problems. The answers are not apparent.

Hidden Mechanisms with Hidden Assumptions #

Most users don’t get much visibility into, or control over, the congestion in their networks that causes slow service at their device. The most congested link, known as the bottleneck, could be very distant from the user, deep in the heart of the network, or directly connected to the user (“at the edge”). To make matters worse, congestion can sometimes cause even more network congestion: when a bottleneck link drops packets it cannot forward in time, those packets must be re-transmitted, resulting in even more traffic, which leads to even more congestion… and eventually total network failure (This tragic fate, congestion collapse, is as terrible as it sounds. See Congestion Avoidance and Control, Jacobson 1988)!

The Internet relies on congestion control protocols (like TCP, BBR, or Timely) to help traffic sources know when the network is congested and scale back the amount of data they transmit accordingly. Congestion control protocols have evolved extensively over the lifetime of the Internet, using a variety of different congestion signals and strategies for managing the congestion, but all protocols seek to accomplish this same basic task. Ultimately, congestion management is a resource allocation problem — there is more demand for network resources (i.e. bandwidth) than can be supplied, creating scarcity. The available resources must somehow be allocated between the competing flows in the network. In this context, though, modern internet networks fall woefully short of expectations.

For many years academics have relied on the notion of flow rate fairness to conceptualize how resources are divided in the Internet. The idea is that all flows should altruistically follow compatible rules, and thus will evenly share the network resources naturally in a distributed fashion. For anyone who has worked with real systems, however, there are two immediately apparent problems with this approach:

  • Relying on altruistic behavior from all sources is unrealistic (whether by malicious intent, negligence, or honest mistakes in applications).
  • It’s unclear how flows (which are concretely visible on the network) map to actual users (real-world entities outside the network).

Researchers and engineers have articulated and argued these (and other) drawbacks for decades (see Flow Rate Fairness: Dismantling a Religion, Briscoe 2007 for a well-written example, Charging and Rate Control for Elastic Traffic, Kelly 1997 for an early theoretical analysis, and On the Future of Congestion Control for the Public Internet, Brown et al. 2020, which addresses congestion in the core of the modern Internet). Whole fields of study have developed around solving these challenges (Check out the terms “Weighted Proportional Fairness” and “Quality of Service” or “QoS”). Yet the public and hobbyist discourse around network traffic management is negligible, limiting the exposure community network operators have to the possible tradeoffs and options available.

Challenges to Transparency and Control #

Experienced system administrators have access to some sophisticated tools for designing and deploying network rules via professional network management suites (with both software and network hardware components). Yet real-world local, community, and/or mesh networking tools (in our experience) lack the capability for enforcing meaningful resource allocation outside flow-rate fairness. A range of factors combine to make meaningful congestion management incredibly hard, including but not limited to:

  • The large number of abstractions between traffic “on the wire” and the actual applications creating that traffic.
  • Poor choices for network equipment defaults which tend toward not enforcing any policy.
  • The inherent complexity of network performance, especially with wireless links serving multiple devices.
  • No reliable way to map network traffic to a real-world entity without extra outside information.
  • The proliferation of transport layer encryption (which is a good thing!), but which limits visibility and makes it hard to understand how the network is actually used.
  • A lack of support of meaningful QoS tagging, even in sophisticated applications, depending on the device operating system and network medium.
  • Non-linear and time-varying real economic costs such as monthly data caps or wireless promotions that make it hard to decide on an optimal usage strategy.

Computer networks are engineered to account for flows of traffic, streams of data sent from one application to another through the network’s layers and many links. Network engineers reason about flows because they are easy to measure on the network itself, but in reality flows have limited meaning. Flows on their own don’t reveal how to allocate network resources to real people and their activities on the net.

Understanding Resources in Community Networks #

Community networks in particular pose unique resource allocation challenges. In community networks, a group of people organize and run their own network for their own benefit. The goal is to maximize the utility of the network’s physical capabilities for all members. This is in contrast to commercial networks that seek to maximize profit, providing the least costly amount of acceptable service while maximizing the return from end-user subscriptions. Commercial operators often mask the fundamental capabilities of the underlying network infrastructure for competitive advantage, and their immediate profit motive overshadows messy concerns around resource allocation and fairness.

A community can be defined in as many ways as you can imagine, but importantly, a community network is bigger than a single person or household. Community networks care about achieving equitable division of underlying resources, and rely on human governance structures to make decisions about fairness. The unit entity of resource consumption in community networks can be surprisingly complex: should resources be allocated between families, households, devices, individuals, or other possibly overlapping identity groups (elders, students, government workers, etc.)? Current implementations lack the flexibility to handle all cases.

Beyond just allocating bandwidth proportionally between entities, community networks may wish to incorporate other values into network management. Examples could be prioritizing certain classes of applications over others (like relatively lightweight real-time audio calls over bulk media consumption), or encouraging using the network during off-peak hours to improve utilization.[^2] Additionally the metric of interest may not be bandwidth at all, but could be latency (as real-time digital communication has grown in importance with the ongoing COVID pandemic), or subjective application performance. While these ideas are easy to express abstractly, they are extremely difficult to encode in concrete and enforceable network policies with existing systems.

Reconceptualizing congestion management as resource allocation opens a frontier of new possibilities for system designs and correspondingly new requirements on the network to support resource allocations in an efficient manner. While there is much theory to draw from, both in computer science and economics, the challenge now is to move meaningfully beyond the status quo and actually realize the potential of congestion management to improve the predictability, understandability, and performance of networks for everyday people in the real world. This will involve both the creation of new systems as well as improving the availability and accessibility of existing techniques, lowering the threshold to effective use.

[^1] A video call requires ~3mbps of bandwidth with a maximum latency of a few hundred ms. Browsing the web can tolerate longer latencies and smaller amounts of bandwidth.

[^2] The implementation details of traffic management could easily have negative implications for Net Neutrality, the principle that all traffic on the network should be treated equally. I want to emphasize the difference between prioritizing a few specific popular applications, versus prioritizing an entire class of applications, regardless of which specific application each user should choose to use. Unfortunately, current affordances make the former easier to implement than the latter, which can lead to reinforcement of incumbent market positions and lead to long-term harm to the Internet ecosystem. I hope to enable communities to implement the latter via more general and even-handed forms of class-based prioritization should they desire to do so.

(See original post on medium)