HTTP Head of line blocking: Why responses must come back in order

Question

Head of Line blocking (in HTTP/1.1 terms) is often referring to the fact that each client has a limited number of TCP connections to a server (usually 6 connections per hostname) and doing a new request over one of those connections has to wait for the previous request on the same connection to complete before the client can make a new request.

HTTP/1.1 introduced a feature called "Pipelining" which allowed a client to send several HTTP requests over the same TCP connection. However, HTTP/1.1 still required the responses to arrive in order so it didn't really solve the HOL issue and as of today it is not widely adopted.

My question is: in HTTP pipelining, why responses must come back in order?

Answer 1

Because there is no identifier in the response indicating to which request it belongs.

HTTP/2 solves this with stream identifiers.