Head-of-line blocking

In computer networking, head-of-line blocking (HOL blocking) refers to a performance bottleneck that occurs when a queue of packets is held up by the first packet in the queue, even though other packets in the queue could be processed.

In HTTP/1.1, requests on a single TCP connection are usually sent sequentially — a new request can’t be made on the connection while waiting for a response to the previous request. This can lead to HOL blocking problems even if there are several TCP connections between the client and server.

HTTP/1.1 defines an optional feature called HTTP pipelining that unsuccessfully attempted to work around HOL blocking, by allowing requests to be sent without waiting for earlier responses. Unfortunately the design of HTTP/1.1 means that responses must be returned in the same order as the requests were received, so HOL blocking can still occur if a request takes a long time to complete. Network conditions such as congestion, packet loss (and the resulting TCP retransmissions), or TCP slow start can also delay transmission and cause later responses to be blocked by earlier ones.