Which HTTP header is required in SOAP API requests to indicate compression support?

The required HTTP header for indicating compression support in SOAP API requests is the Accept-Encoding header. This header informs the server about the types of content encoding that the client is capable of understanding. When a client sends a request, including the Accept-Encoding header allows the server to compress the response using one of the specified methods, such as gzip or deflate, thus optimizing the data transfer by reducing the size of the response payload.

By utilizing this header, it facilitates better performance in data transmission because compressed responses typically consume less bandwidth and can be processed faster by the client. The server, upon receiving the validation of compression support through this header, can then tailor the response accordingly.

While other headers listed play important roles in HTTP communication, they do not specifically denote compression capability. Content-Type specifies the nature of the content being sent or expected, Transfer-Encoding deals with the encoding used in the transmission of the message body, and User-Agent identifies the client application making the request. However, none of these headers directly communicate the client's capacity for handling compressed responses. Thus, Accept-Encoding is crucial in this context.