Tail drop
Encyclopedia
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. With tail drop, when the queue is filled to its maximum capacity, the newly arriving packets are dropped until the queue has enough room to accept incoming traffic.
The name arises from the effect of the policy on incoming datagrams. Once a queue has been filled, the router begins discarding all additional datagrams, thus dropping the tail of the sequence of datagrams. The loss of datagrams causes the TCP
sender to enter slow-start
, which reduces throughput in that TCP session until the sender begins to receive acknowledgement
s again and increases its congestion window. A more severe problem occurs when datagrams from multiple TCP connections are dropped, causing global synchronization; i.e., all of the involved TCP senders enter slow-start. This happens because, instead of discarding many segments
from one connection, the router would tend to discard one segment from each connection.
Algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
used by Internet routers to decide when to drop packets. In contrast to the more complex algorithms like RED
Random early detection
Random early detection , also known as random early discard or random early drop is an active queue management algorithm. It is also a congestion avoidance algorithm....
and WRED
Weighted random early detection
Weighted random early detection is a queue management algorithm with congestion avoidance capabilities. It is an extension to random early detection where a single queue may have several different queue thresholds. Each queue threshold is associated to a particular traffic class.For example, a...
, in Tail Drop all the traffic is not differentiated. Each packet is treated identically. With tail drop, when the queue is filled to its maximum capacity, the newly arriving packets are dropped until the queue has enough room to accept incoming traffic.
The name arises from the effect of the policy on incoming datagrams. Once a queue has been filled, the router begins discarding all additional datagrams, thus dropping the tail of the sequence of datagrams. The loss of datagrams causes 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...
sender to enter 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...
, which reduces throughput in that TCP session until the sender begins to receive acknowledgement
Acknowledgement (data networks)
In data networking, an acknowledgment is a signal passed between communicating processes or computers to signify acknowledgment, or receipt of response, as part of a communications protocol...
s again and increases its congestion window. A more severe problem occurs when datagrams from multiple TCP connections are dropped, causing global synchronization; i.e., all of the involved TCP senders enter slow-start. This happens because, instead of discarding many segments
Protocol data unit
In telecommunications, the term protocol data unit has the following meanings:#Information that is delivered as a unit among peer entities of a network and that may contain control information, address information, or data....
from one connection, the router would tend to discard one segment from each connection.