Intentional slowing of data transmission
Bandwidth throttling consists in the limitation of the communication speed (bytes or kilobytes per second), of the ingoing (received) or outgoing (sent) data in a network node or in a network device such as computers and mobile phones. 
The data speed and rendering may be limited depending on various parameters and conditions.
Bandwidth throttling should be done along with rate limiting pattern to minimize the number of throttling errors. 
Overview.
Limiting the speed of data sent by a data originator (a client computer or a server computer) is much more efficient than limiting the speed in an intermediate network device between client and server because while in the first case usually no network packets are lost, in the second case network packets can be lost / discarded whenever ingoing data speed overcomes the bandwidth limit or the capacity of device and data packets cannot be temporarily stored in a buffer queue (because it is full or it does not exist); the usage of such a buffer queue is to absorb the peaks of incoming data for very short time lapse.
In the second case discarded data packets can be resent by transmitter and received again.
When a low-level network device discards incoming data packets usually can also notify that fact to data transmitter in order to slow down the transmission speed (see also network congestion).
NOTE: Bandwidth throttling should not be confused with rate limiting which operates on client requests at application server level and/or at network management level (i.e. by inspecting protocol data packets). Rate limiting can also help in keeping peaks of data speed under control.
These bandwidth limitations can be implemented:
The first one (client/server program) is usually perfectly legal because it is a choice of the client manager or the server manager (by server administrator) to limit or not to limit the speed of data received from remote program via network or the speed of data sent to target program (server or client).
The second one (ISP) instead is considered an offense in the USA under FCC regulations. While Internet service providers (ISPs) prey on the individual's inability to fight them, fines can range up to $25,000 USD for throttling. In the United States, net neutrality, the principle that ISPs treat all data on the Internet the same, and not discriminate, has been an issue of contention between network users and access providers since the 1990s. With net neutrality, ISPs may not intentionally block, slow down, or charge money for specific online content.
Defined as the intentional slowing or speeding of an internet service by an ISP. It is a reactive measure employed in communication networks to regulate network traffic and minimize bandwidth congestion. Bandwidth throttling can occur at different locations on the network. On a local area network (LAN), a system administrator ("sysadmin") may employ bandwidth throttling to help limit network congestion and server crashes. On a broader level, the ISP may use bandwidth throttling to help reduce a user's usage of bandwidth that is supplied to the local network. Bandwidth throttling is also used as a measurement of data rate on Internet speed test websites.
Throttling can be used to actively limit a user's upload and download rates on programs such as video streaming, BitTorrent protocols and other file sharing applications, as well as even out the usage of the total bandwidth supplied across all users on the network. Bandwidth throttling is also often used in Internet applications, in order to spread a load over a wider network to reduce local network congestion, or over a number of servers to avoid overloading individual ones, and so reduce their risk of the system crashing, and gain additional revenue by giving users an incentive to use more expensive tiered pricing schemes, where bandwidth is not throttled.
Operation.
A computer network typically consists of a number of servers, which host data and provide services to clients. The Internet is a good example, in which web servers are used to host websites, providing information to a potentially very large number of client computers. Clients will make requests to servers, which will respond by sending the required data, which may be a song file, a video, and so on, depending on what the client has requested. As there will typically be many clients per server, the data processing demand on a server will generally be considerably greater than on any individual client. And so servers are typically implemented using computers with high data capacity and processing power. The traffic on such a network will vary over time, and there will be periods when client requests will peak or sent responses will be huge, sometimes exceeding the capacity of parts of network and causing congestion, especially in parts of the network that form bottlenecks. This can cause data request failures, or in worst cases, server crashes.
In order to prevent such occurrences, a client / server / system administrator may enable (if available) bandwidth throttling:
 examples: assuming to have a server site with speed access to Internet of 100MB/s (around 1000Mbit/s), assuming that most clients have a 1 MB/s (around 10 Mbit/s) network speed access to Internet and assuming to be able to download huge files (i.e. 1 GB each):
* with bandwidth throttling, a server using a max. output speed of 100kB/s (around 1 Mbit/s) for each TCP connection, could allow at least 1000 active TCP connections (or even 10000 if output is limited to 10 kB/s) (active connections means that data content, such as a big file, is being downloaded from server to client);
* without bandwidth throttling, a server could efficiently serve only 100 active TCP connections (100 MB/s / 1 MB/s) before saturating network bandwidth; a saturated network (i.e. with a bottleneck through an Internet Access Point) could slow down a lot the attempts to establish other new connections or even to force them to fail because of timeouts, etc.; besides this new active connections could not get easily or fastly their proper share of bandwidth.
Application.
A bandwidth intensive device, such as a server, might limit (throttle) the speed at which it receives or sends data, in order to avoid overloading its processing capacity or to saturate network bandwidth. This can be done both at the local network servers or at the ISP servers. ISPs often employ deep packet inspection (DPI), which is widely available in routers or provided by special DPI equipment. Additionally, today's networking equipment allows ISPs to collect statistics on flow sizes at line speed, which can be used to mark large flows for traffic shaping. Two ISPs, Cox and Comcast, have stated that they engage in this practice, where they limit users' bandwidth by up to 99%. Today most if not all ISPs throttle their users' bandwidth, with or without the user ever even realizing it. In the specific case of Comcast, an equipment vendor called Sandvine developed the network management technology that throttled P2P file transfers.
Those that could have their bandwidth throttled are typically someone who is constantly downloading and uploading torrents, or someone who just watches a lot of online videos. If this is done by an ISP, many consider this practice as an unfair method of regulating the bandwidth because consumers are not getting the required bandwidth even after paying the prices set by the ISPs. By throttling the people who are using so much bandwidth, the ISPs claim to enable their regular users to have a better overall quality of service.
Network neutrality.
Net neutrality is the principle that all Internet traffic should be treated equally. It aims to guarantee a level playing field for all websites and Internet technologies. With net neutrality, the network's only job is to move data—not to choose which data to privilege with higher quality, that is faster, service. In the US, on February 26, 2015, the Federal Communications Commission adopted Open Internet rules. They are designed to protect free expression and innovation on the Internet and promote investment in the nation's broadband networks. The Open Internet rules are grounded in the strongest possible legal foundation by relying on multiple sources of authority, including: Title II of the Communications Act and Section 706 of the Telecommunications Act of 1996. The new rules apply to both fixed and mobile broadband services. However, these rules were rolled back on December 14, 2017. On October 19, 2023, the FCC voted 3-2 to approve a Notice of Proposed Rulemaking (NPRM) that seeks comments on a plan to restore net neutrality rules and regulation of ISPs.On April 25, 2024, the FCC voted 3-2 to reinstate net neutrality in the United States by reclassifying the Internet under Title II.
Bright line rules:
Throttling vs. capping.
Bandwidth throttling works by limiting (throttling) the speed at which a bandwidth intensive device (a server) receives data or the speed (i.e. bytes or kilobytes per second) of each data response. If these limits are not in place, the device can overload its processing capacity.
Contrary to throttling, in order to use bandwidth when available, but prevent excess, each node in a proactive system should set an outgoing bandwidth cap that appropriately limits the total number of bytes sent per unit time. There are two types of bandwidth capping. A standard cap limits the bitrate or speed of data transfer on a broadband Internet connection. Standard capping is used to prevent individuals from consuming the entire transmission capacity of the medium. A lowered cap reduces an individual user's bandwidth cap as a defensive measure and/or as a punishment for heavy use of the medium's bandwidth. Oftentimes this happens without notifying the user.
The difference is that bandwidth throttling regulates a bandwidth intensive device (such as a server) by limiting how much data that device can receive from each node / client or can output or can send for each response. Bandwidth capping on the other hand limits the total transfer capacity, upstream or downstream, of data over a medium.
Court cases.
"Comcast Corp. v. FCC".
In 2007, Free Press, Public Knowledge, and the Federal Communications Commission filed a complaint against Comcast's Internet service. Several subscribers claimed that the company was interfering with their use of peer-to-peer networking applications. The Commission stated that it had jurisdiction over Comcast's network management practices and that it could resolve the dispute through negotiation rather than through rulemaking. The Commission believed that Comcast had "significantly impeded consumers' ability to access the content and use the applications of their choice", and that because Comcast "ha[d] several available options it could use to manage network traffic without discriminating" against peer-to-peer communications, its method of bandwidth management "contravene[d] ... federal policy". At this time, "Comcast had already agreed to adopt a new system for managing bandwidth demand, the Commission simply ordered it to make a set of disclosures describing the details of its new approach and the company's progress toward implementing it". Comcast complied with this Order but petitioned for a review and presented several objections.
ISP bandwidth throttling.
Canada.
In 2008, the Canadian Radio-television and Telecommunications Commission (CRTC) decided to allow Bell Canada to single out peer-to-peer (P2P) traffic for bandwidth throttling between the hours of 4:30 p.m. to 2 a.m. In 2009, the CRTC released a guideline for bandwidth throttling rules.
In 2011, following a major complaint by the Canadian Gamers Organization against Rogers for breaking the 2009 rules already in place, the CRTC created an addendum to their ITMP policy, allowing them to send the complaint to their Enforcements Division. The Canadian Gamers Organization in their submissions alluded to filing a complaint against Bell Canada. On December 20, 2011, Bell Canada announced they would end throttling by March 31, 2012 for their customers, as well as their wholesale customers. On February 4, 2012, in an effort to get out of trouble with the CRTC (which had continued its own testing and had found additional non-compliance and demanded immediate compliance), Rogers announced 50% of their customers would be throttle-free by June 2012, and 100% of their customers would be throttle-free by the end of 2012. Unfortunately for Rogers, this did not mollify the CRTC Enforcements Division.
ISPs in Canada that throttle bandwidth:
Europe.
In April 2011, the European Union launched an investigation into ISPs' methods for managing traffic on their networks. Some ISPs, for instance, restrict access to services such as Skype or the BBC iPlayer at peak times so that their users all receive an equal service. The EU's commissioner for the digital agenda, Neelie Kroes, said: "I am absolutely determined that everyone in the