computer science

Rate limiting

Rate limiting is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Rate limiting rather than just read about it. In short: In computer networks, rate limiting is used to control the rate of requests sent or received by a network interface controller. It can be used to prevent DoS attacks and limit web scraping.

Key takeaways

Rate limiting belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Rate limiting to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Rate limiting from memory before moving on to harder problems.

Reference excerpt

In computer networks, rate limiting is used to control the rate of requests sent or received by a network interface controller. It can be used to prevent DoS attacks and limit web scraping. Research indicates flooding rates for one zombie machine are in excess of 20 HTTP GET requests per second, legitimate rates much less. Rate limiting should be used along with throttling pattern to minimize the number of throttling errors.

Hardware appliances Hardware appliances can limit the rate of requests on layer 4 or 5 of the OSI model. Rate limiting can be induced by the network protocol stack of the sender due to a received ECN-marked packet and also by the network scheduler of any router along the way. While a hardware appliance can limit the rate for a given range of IP-addresses on layer 4, it risks blocking a network with many users which are masked by NAT with a single IP address of an ISP. Deep packet inspection can be used to filter on the session layer but will effectively disarm encryption protocols like TLS and SSL between the appliance and the protocol server (i.e. web server).

Protocol servers Protocol servers using a request / response model, such as FTP servers or typically web servers may use a central in-memory key-value database, like Redis or Aerospike, for session management. A rate limiting algorithm is used to check if the user session (or IP address) has to be limited based on the information in the session cache. In case a client made too many requests within a given time frame, HTTP servers can respond with status code 429: Too Many Requests. However, in some cases (i.e. web servers) the session management and rate limiting algorithm should be built into the application (used for dynamic content) running on the web server, rather than the web server itself. When a protocol server or a network device notice that the configured request limit is reached, then it will offload new requests and not respond to them. Sometimes they may be added to a queue to be processed once the input rate reaches an acceptable level, but at peak times the request rate can even exceed the capacities of such queues and requests have to be thrown away.

Data centers Data centers widely use rate limiting to control the share of resources given to different tenants and applications according to their service level agreement. A variety of rate limiting techniques are applied in data centers using software and hardware. Virtualized data centers may also apply rate limiting at the hypervisor layer. Two important performance metrics of rate limiters in data centers are resource footprint (memory and CPU usage) which determines scalability, and precision. There usually exists a trade-off, that is, higher precision can be achieved by dedicating more resources to the rate limiters. A considerable body of research with focus on improving performance of rate limiting in data centers.

See also Bandwidth management Bandwidth throttling Project Shield Algorithms Token bucket Leaky bucket Fixed window counter Sliding window log Sliding window counter Libraries ASP.NET Web API rate limiter ASP.NET Core rate limiting middleware Rate limiting for .NET (PCL Library) Rate limiting for Node.JS

References

Worked examples

Example 1 — a first encounter with Rate limiting

Start with the simplest possible case. Write down what Rate limiting claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Rate limiting before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Rate limiting ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Rate limiting

In research: Rate limiting appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Rate limiting in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Rate limiting is common in secondary-school and first-year university syllabi. It links to neighbouring topics Network performance, so understanding it makes those chapters shorter.
In everyday life: Look for Rate limiting outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Rate limiting in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Rate limiting means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Rate limiting out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Rate limiting in simple terms?

In computer networks, rate limiting is used to control the rate of requests sent or received by a network interface controller. It can be used to prevent DoS attacks and limit web scraping.

Why does Rate limiting matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Rate limiting?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Rate limiting.