How does QUIC address head-of-line blocking and multiplexing relative to TCP?

• A. QUIC multiplexes independent streams within a single connection, delivering data per stream, reducing head-of-line blocking versus TCP’s single in-order stream.
• B. QUIC uses a single shared stream like TCP to guarantee in-order delivery.
• C. TCP multiplexes multiple streams per connection automatically causing head-of-line blocking.
• D. QUIC uses multiple separate connections per stream.

Answer: (A)

Head-of-line blocking comes from delivering data in a single, in-order stream. In TCP, a connection carries one byte-stream, so if a packet gets lost or arrives out of order, all data after that point can’t be delivered to the application until the missing piece is recovered, blocking everything on that connection.

QUIC tackles this by carrying multiple independent streams inside one connection. Each stream has its own sequence and flow control, so data on one stream can be delivered and consumed without waiting for data on other streams. If a packet for one stream is lost, only that stream’s data is delayed; other streams keep progressing. This per-stream multiplexing greatly reduces head-of-line blocking compared to TCP’s single in-order stream.

Note that QUIC still uses a single connection-level congestion control, but the key improvement is the independence of streams within the connection, which lets multiple data flows advance in parallel rather than being tied to one continuous, in-order byte stream.