# What is a stream?
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In AWS IoT, a *stream* is a publicly addressable resource that is an abstraction for a list of files that can be transferred to an IoT device. A typical stream contains the following information:
+ **An Amazon Resource Name (ARN)** that uniquely identifies a stream at a given time. This ARN has the pattern `arn:{{partition}}:iot:{{region}}:{{account-ID}}:stream/{{stream ID}}`. 
+ **A stream ID** that identifies your stream and is used (and usually required) in AWS Command Line Interface (AWS CLI) or SDK commands.
+ **A stream description** that provides a description of the stream resource.
+ **A stream version** that identifies a particular version of the stream. Because stream data can be modified immediately before devices start the data transfer, the stream version can be used by the devices to enforce a consistency check.
+ **A list of files** that can be transferred to devices. For each file in the list, the stream records a file ID, the file size, and the address information of the file, which consists of, for example, the Amazon S3 bucket name, object key, and object version.
+ **An AWS Identity and Access Management (IAM) role** that grants AWS IoT MQTT-based file delivery the permission to read stream files stored in data storage.

AWS IoT MQTT-based file delivery provides the following functionality so that devices can transfer data from the AWS Cloud: 
+ Data transfer using the MQTT protocol.
+ Support for JSON or CBOR formats.
+ The ability to describe a stream ([https://docs.aws.amazon.com/iot/latest/developerguide/mqtt-based-file-delivery-in-devices.html#mqtt-based-file-delivery-describe-stream](https://docs.aws.amazon.com/iot/latest/developerguide/mqtt-based-file-delivery-in-devices.html#mqtt-based-file-delivery-describe-stream) API) to get a stream file list, stream version, and related information.
+ The ability to send data in small blocks ([https://docs.aws.amazon.com/iot/latest/developerguide/mqtt-based-file-delivery-in-devices.html#mqtt-based-file-delivery-get-getstream](https://docs.aws.amazon.com/iot/latest/developerguide/mqtt-based-file-delivery-in-devices.html#mqtt-based-file-delivery-get-getstream) API) so that devices with hardware constraints can receive the blocks.
+ Support for a dynamic block size per request, to support devices that have different memory capacities.
+ Optimization for concurrent streaming requests when multiple devices request data blocks from the same stream file.
+ Amazon S3 as data storage for stream files.
+ Support for data transfer log publishing from AWS IoT MQTT-based file delivery to CloudWatch.

For MQTT-based file delivery quotas, see [AWS IoT Core Service Quotas](https://docs.aws.amazon.com//general/latest/gr/iot-core.html#limits_iot) in the *AWS General Reference*.