TCP global synchronization
Encyclopedia
TCP global synchronization in Computer networks can happen to
TCP
/IP
flows during periods
of congestion because each sender will reduce their transmission rate at the same
time when packet loss occurs.
Routers on the Internet normally have packet queues, to allow them to hold packets when the network is busy, rather than discarding them.
Because routers have limited resources, the size of these queues is also limited. The simplest technique to limit queue size is known as tail drop
. The queue is allowed to fill to its maximum size, and then any new packets are simply discarded, until there is space in the queue again.
This causes problems when used on TCP/IP routers handling multiple TCP
streams, especially when bursty traffic is present. While the network is stable, the queue is constantly full, and there are no problems except that the full queue results in high latency. However, the introduction of a sudden burst of traffic may cause large numbers of established, steady streams to lose packets simultaneously.
TCP
has automatic recovery from dropped packets, which it interprets as congestion on the network (which is usually correct). The sender reduces its sending rate for a certain amount of time, and then tries to find out if the network is no longer congested by increasing the rate again subject to a ramp-up. This is known as the slow-start
algorithm.
Almost all the senders will use the same time delay before increasing their rates. When these delays expire, at the same time, all the senders will send additional packets, the router queue will again overflow, more packets will be dropped, the senders will all back off for a fixed delay... ad infinitum.
This pattern of each sender decreasing and increasing transmission rates at the same time as other senders is referred to as "global synchronization" and leads to inefficient use of bandwidth, due to the large numbers of dropped packets, which must be retransmitted.
This problem has been the subject of much research. The consensus appears to be that the tail drop algorithm is the leading cause of the problem, and other queue size management algorithms
such as Random Early Detection (RED)
and Weighted RED
will reduce the likelihood of global synchronization, as well as keeping queue sizes down in the face of heavy load and bursty traffic.
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...
/IP
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
flows during periods
of congestion because each sender will reduce their transmission rate at the same
time when packet loss occurs.
Routers on the Internet normally have packet queues, to allow them to hold packets when the network is busy, rather than discarding them.
Because routers have limited resources, the size of these queues is also limited. The simplest technique to limit queue size is known as 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...
. The queue is allowed to fill to its maximum size, and then any new packets are simply discarded, until there is space in the queue again.
This causes problems when used on TCP/IP routers handling multiple 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...
streams, especially when bursty traffic is present. While the network is stable, the queue is constantly full, and there are no problems except that the full queue results in high latency. However, the introduction of a sudden burst of traffic may cause large numbers of established, steady streams to lose packets simultaneously.
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...
has automatic recovery from dropped packets, which it interprets as congestion on the network (which is usually correct). The sender reduces its sending rate for a certain amount of time, and then tries to find out if the network is no longer congested by increasing the rate again subject to a ramp-up. This is known as the 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...
algorithm.
Almost all the senders will use the same time delay before increasing their rates. When these delays expire, at the same time, all the senders will send additional packets, the router queue will again overflow, more packets will be dropped, the senders will all back off for a fixed delay... ad infinitum.
This pattern of each sender decreasing and increasing transmission rates at the same time as other senders is referred to as "global synchronization" and leads to inefficient use of bandwidth, due to the large numbers of dropped packets, which must be retransmitted.
This problem has been the subject of much research. The consensus appears to be that the tail drop algorithm is the leading cause of the problem, and other queue size management algorithms
Active Queue Management
In Internet routers, active queue management is a technique that consists in dropping or ECN-marking packets before a router's queue is full.-Queue management:...
such as Random Early Detection (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 Weighted RED
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...
will reduce the likelihood of global synchronization, as well as keeping queue sizes down in the face of heavy load and bursty traffic.
See also
- Transmission Control ProtocolTransmission Control ProtocolThe 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...
(TCP) - TCP congestion avoidance algorithms
- Network congestion avoidance