Head-of-line blocking (HOL blocking) in

computer networking A computer network is a collection of communicating computers and other devices, such as printers and smart phones. In order to communicate, the computers and devices must be connected by wired media like copper cables, optical fibers, or b ...

is a performance-limiting phenomenon that occurs when a queue of packets is held up by the first packet in the queue. This occurs, for example, in input-buffered

network switch A network switch (also called switching hub, bridging hub, Ethernet switch, and, by the IEEE, MAC bridge) is networking hardware that connects devices on a computer network by using packet switching to receive and forward data to the destinat ...

es, out-of-order delivery and multiple requests in HTTP pipelining.

Network switches

A switch may be composed of buffered input ports, a switch fabric, and buffered output ports. If first-in first-out (FIFO) input buffers are used, only the oldest packet is available for forwarding. If the oldest packet cannot be transmitted due to its target output being busy, then more recent arrivals cannot be forwarded. The output may be busy if there is output contention. Without HOL blocking, the new arrivals could potentially be forwarded around the stuck oldest packet to their respective destinations. HOL blocking can produce performance-degrading effects in input-buffered systems. This phenomenon limits the throughput of switches. For FIFO input buffers, a simple model of fixed-sized cells to uniformly distributed destinations, causes the throughput to be limited to 58.6% of the total as the number of links becomes large. One way to overcome this limitation is by using virtual output queues. Only switches with input buffering can suffer HOL blocking. With sufficient internal bandwidth, input buffering is unnecessary; all buffering is handled at outputs and HOL blocking is avoided. This no-input-buffering architecture is common in small to medium-sized

ethernet switch A network switch (also called switching hub, bridging hub, Ethernet switch, and, by the IEEE, MAC bridge) is networking hardware that connects devices on a computer network by using packet switching to receive and forward data to the destinat ...

es.

Out-of-order delivery

Out-of-order delivery occurs when sequenced packets arrive out of order. This may happen due to different paths taken by the packets or from packets being dropped and resent. HOL blocking can significantly increase packet reordering. Reliably broadcasting messages across a lossy network among a large number of peers is a difficult problem. While atomic broadcast algorithms solve the

single point of failure A single point of failure (SPOF) is a part of a system that would Cascading failure, stop the entire system from working if it were to fail. The term single point of failure implies that there is not a backup or redundant option that would enab ...

problem of centralized servers, those algorithms introduce a head-of-line blocking problem. The Bimodal Multicast algorithm, a

randomized algorithm A randomized algorithm is an algorithm that employs a degree of randomness as part of its logic or procedure. The algorithm typically uses uniformly random bits as an auxiliary input to guide its behavior, in the hope of achieving good performan ...

that uses a

gossip protocol A gossip protocol or epidemic protocol is a procedure or process of computer peer-to-peer communication that is based on the way epidemics spread. Some distributed systems use peer-to-peer gossip to ensure that data is disseminated to all members ...

, avoids head-of-line blocking by allowing some messages to be received out-of-order.

In HTTP

One form of HOL blocking in HTTP/1.1 is when the number of allowed parallel requests in the browser is used up, and subsequent requests need to wait for the former ones to complete.

HTTP/2 HTTP/2 (originally named HTTP/2.0) is a major revision of the HTTP network protocol used by the World Wide Web. It was derived from the earlier experimental SPDY protocol, originally developed by Google. HTTP/2 was developed by the HTTP Working ...

addresses this issue through request multiplexing, which eliminates HOL blocking at the application layer, but HOL still exists at the transport (TCP) layer.

In reliable byte streams

Head-of-line blocking can occur in

reliable byte stream A reliable byte stream is a common service paradigm in computer networking; it refers to a byte stream in which the bytes which emerge from the communication channel at the recipient are exactly the same, and in exactly the same order, as they we ...

s: if packets are reordered or lost and need to be retransmitted (and thus arrive out-of-order), data from sequentially later parts of the stream may be received before sequentially earlier parts of the stream; however, the later data cannot typically be used until the earlier data has been received, incurring

network latency Network delay is a design and performance characteristic of a telecommunications network. It specifies the latency for a bit of data to travel across the network from one communication endpoint to another. It is typically measured in multiples ...

. If multiple independent higher-level messages are encapsulated and multiplexed onto a single reliable byte stream, then head-of-line blocking can cause processing of a fully-received message that was sent later to wait for delivery of a message that was sent earlier. This affects, for example,

, which frames multiple request–response pairs onto a single stream; HTTP/3, which has an

application-layer framing Application-layer framing or application-level framing (ALF) is a method of allowing an application to use its semantics for the design of its network protocols. This procedure was first proposed by D. D. Clark and David L. Tennenhouse.Clark, ...

design and uses

datagram A datagram is a basic transfer unit associated with a packet-switched network. Datagrams are typically structured in header and payload sections. Datagrams provide a connectionless communication service across a packet-switched network. The de ...

rather than stream transport, avoids this problem. The latency degradation from head-of-line blocking depends on the underlying packet loss rate and round-trip time, with higher losses producing worse latency. Without changing the stream abstraction, reducing packet loss can reduce the harm from head-of-line blocking; an alternative is to implement the reliable byte stream using forward error correction to send redundant data so that a certain amount of loss can be tolerated without incurring retransmissions.

References

Bibliography