HOL blocking happens in a Router when a Datagram in a queue of an Input Port must wait to be transferred through the Switching Fabric, even though its Output Port is free, because it is blocked by another packet in the head of line, that is itself waiting for its own different output port to be free.
Application Layer Mitigations
HTTP/1.1, which uses a single persistent connection, is vulnerable to this problem since an object of a large size can make subsequent objects queue. HTTP/1.1 mitigates this problem by opening multiple parallel TCP connections.
HTTP/2 mitigates this problem by implementing Binary Framing.
Examples
