Packet loss
Encyclopedia
Packet loss occurs when one or more packets of data travelling across a computer network
fail to reach their destination. Packet loss is distinguished as one of the three main error types encountered in digital communications; the other two being bit error and spurious packets caused due to noise.
, corrupted packets rejected in-transit, faulty networking hardware, faulty network drivers or normal routing routines (such as DSR
in ad-hoc networks ).
In addition to this, packet loss probability is also affected by signal-to-noise ratio
and distance between the transmitter and receiver.
with streaming technologies
, voice over IP
, online gaming and videoconferencing
, and will affect all other network applications to a degree. However, it is important to note that packet loss does not always indicate a problem. If the latency and the packet loss at the destination hop are acceptable then the hops prior to that one don't matter.
provide for reliable delivery of packets. In the event of packet loss, the receiver asks for retransmission or the sender automatically resends any segments that have not been acknowledged. Although TCP can recover from packet loss, retransmitting missing packets causes the throughput
of the connection to decrease. This drop in throughput is due to the sliding window protocols used for acknowledgment of received packets. In certain variants of TCP, if a transmitted packet is lost, it will be re-sent along with every packet that had been sent after it. This retransmission causes the overall throughput of the connection to drop.
Protocols such as UDP
provide no recovery for lost packets. Applications that use UDP are expected to define their own mechanisms for handling packet loss.
traffic, the only effect seen due to the occasional dropped packet is jitter, and therefore “[m]issing one or two packets every now and then will not affect the quality of the conversation. Losses between 5% and 10% of the total packet stream will affect the quality significantly.” On the other hand, when transmitting a text document or web page, a single dropped packet could result in losing part of the file, which is where the aforementioned packet retransmission schemes are used.
When given a situation where the amount of content due to be pushed through a connection is growing at a rate greater than it is possible to push through that connection, also known as a bottleneck
, then there is no other solution than to drop packets. The TCP
protocol is designed with a slow-start
connection strategy so that excessive packet loss will cause the sender to throttle back and stop flooding the bottleneck point with data (using perceived packet loss as feedback to discover congestion). The data packets will be transmitted over a longer duration.
There are many methods used for determining which packets to drop. Most basic networking equipment will use FIFO
queuing for packets waiting to go through the bottleneck and they will drop the packet if the queue is full at the time the packet is received. This type of packet dropping is called tail drop
. However, dropping packets when the queue is full is a poor solution for any connection that requires real-time throughput. For these types of connections, quality of service
and other methods are applied.
In some connections, packets may be intentionally dropped in order to slow down specific services for no other reason than to dissuade users from using those services. For this reason, packet loss is not necessarily an indication of poor connection reliability or a bottleneck.
Packet loss is closely associated with quality of service
considerations, and is related to the erlang unit
of measure.
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
fail to reach their destination. Packet loss is distinguished as one of the three main error types encountered in digital communications; the other two being bit error and spurious packets caused due to noise.
Causes
Packet loss can be caused by a number of factors including signal degradation over the network medium due to multi-path fading, packet drop because of channel congestionNetwork congestion
In data networking and queueing theory, network congestion occurs when a link or node is carrying so much data that its quality of service deteriorates. Typical effects include queueing delay, packet loss or the blocking of new connections...
, corrupted packets rejected in-transit, faulty networking hardware, faulty network drivers or normal routing routines (such as DSR
Dynamic Source Routing
'Dynamic Source Routing' is a routing protocol for wireless mesh networks. It is similar to AODV in that it forms a route on-demand when a transmitting computer requests one...
in ad-hoc networks ).
In addition to this, packet loss probability is also affected by signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
and distance between the transmitter and receiver.
Effects
When caused by network problems, lost or dropped packets can result in highly noticeable performance issues or jitterJitter
Jitter is the undesired deviation from true periodicity of an assumed periodic signal in electronics and telecommunications, often in relation to a reference clock source. Jitter may be observed in characteristics such as the frequency of successive pulses, the signal amplitude, or phase of...
with streaming technologies
Streaming media
Streaming media is multimedia that is constantly received by and presented to an end-user while being delivered by a streaming provider.The term "presented" is used in this article in a general sense that includes audio or video playback. The name refers to the delivery method of the medium rather...
, voice over IP
Voice over IP
Voice over Internet Protocol is a family of technologies, methodologies, communication protocols, and transmission techniques for the delivery of voice communications and multimedia sessions over Internet Protocol networks, such as the Internet...
, online gaming and videoconferencing
Videoconferencing
Videoconferencing is the conduct of a videoconference by a set of telecommunication technologies which allow two or more locations to interact via two-way video and audio transmissions simultaneously...
, and will affect all other network applications to a degree. However, it is important to note that packet loss does not always indicate a problem. If the latency and the packet loss at the destination hop are acceptable then the hops prior to that one don't matter.
Packet recovery
Some network transport protocols such as TCPTransmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
provide for reliable delivery of packets. In the event of packet loss, the receiver asks for retransmission or the sender automatically resends any segments that have not been acknowledged. Although TCP can recover from packet loss, retransmitting missing packets causes the throughput
Throughput
In communication networks, such as Ethernet or packet radio, throughput or network throughput is the average rate of successful message delivery over a communication channel. This data may be delivered over a physical or logical link, or pass through a certain network node...
of the connection to decrease. This drop in throughput is due to the sliding window protocols used for acknowledgment of received packets. In certain variants of TCP, if a transmitted packet is lost, it will be re-sent along with every packet that had been sent after it. This retransmission causes the overall throughput of the connection to drop.
Protocols such as UDP
User Datagram Protocol
The User Datagram Protocol is one of the core members of the Internet Protocol Suite, the set of network protocols used for the Internet. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol network without requiring...
provide no recovery for lost packets. Applications that use UDP are expected to define their own mechanisms for handling packet loss.
Acceptable packet loss
“The fraction of lost packets increases as the traffic intensity increases. Therefore, performance at a node is often measured not only in terms of delay, but also in terms of the probability of packet loss…a lost packet may be retransmitted on an end-to-end basis in order to ensure that all data are[sic] eventually transferred from source to destination.” The amount of packet loss that is acceptable depends on the type of data being sent. For example, for Voice over IPVoice over IP
Voice over Internet Protocol is a family of technologies, methodologies, communication protocols, and transmission techniques for the delivery of voice communications and multimedia sessions over Internet Protocol networks, such as the Internet...
traffic, the only effect seen due to the occasional dropped packet is jitter, and therefore “[m]issing one or two packets every now and then will not affect the quality of the conversation. Losses between 5% and 10% of the total packet stream will affect the quality significantly.” On the other hand, when transmitting a text document or web page, a single dropped packet could result in losing part of the file, which is where the aforementioned packet retransmission schemes are used.
When given a situation where the amount of content due to be pushed through a connection is growing at a rate greater than it is possible to push through that connection, also known as a bottleneck
Bottleneck
A bottleneck is a phenomenon where the performance or capacity of an entire system is limited by a single or limited number of components or resources. The term bottleneck is taken from the 'assets are water' metaphor. As water is poured out of a bottle, the rate of outflow is limited by the width...
, then there is no other solution than to drop packets. The TCP
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
protocol is designed with a slow-start
Slow-start
Slow-start is part of the congestion control strategy used by TCP, the data transmission protocol used by many Internet applications. Slow-start is used in conjunction with other algorithms to avoid sending more data than the network is capable of transmitting, that is, to avoid causing network...
connection strategy so that excessive packet loss will cause the sender to throttle back and stop flooding the bottleneck point with data (using perceived packet loss as feedback to discover congestion). The data packets will be transmitted over a longer duration.
There are many methods used for determining which packets to drop. Most basic networking equipment will use FIFO
FIFO
FIFO is an acronym for First In, First Out, an abstraction related to ways of organizing and manipulation of data relative to time and prioritization...
queuing for packets waiting to go through the bottleneck and they will drop the packet if the queue is full at the time the packet is received. This type of packet dropping is called tail drop
Tail drop
Tail Drop, or Drop Tail, is a simple queue management algorithm used by Internet routers to decide when to drop packets. In contrast to the more complex algorithms like RED and WRED, in Tail Drop all the traffic is not differentiated. Each packet is treated identically...
. However, dropping packets when the queue is full is a poor solution for any connection that requires real-time throughput. For these types of connections, quality of service
Quality of service
The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...
and other methods are applied.
In some connections, packets may be intentionally dropped in order to slow down specific services for no other reason than to dissuade users from using those services. For this reason, packet loss is not necessarily an indication of poor connection reliability or a bottleneck.
Packet loss is closely associated with quality of service
Quality of service
The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...
considerations, and is related to the erlang unit
Erlang unit
The erlang is a dimensionless unit that is used in telephony as a statistical measure of offered load or carried load on service-providing elements such as telephone circuits or telephone switching equipment. It is named after the Danish telephone engineer A. K...
of measure.