Fast retransmit
Encyclopedia
Fast Retransmit is an enhancement to TCP
which reduces the time a sender waits before retransmitting a lost segment
.
A TCP sender uses a timer to recognize lost segments. If an acknowledgement is not received for a particular segment within a specified time (a function of the estimated Round-trip delay time
), the sender will assume the segment was lost in the network
, and will retransmit the segment.
Duplicate acknowledgement is the basis for fast retransmit mechanism which works as follows: after receiving a packet (i.e., sequence number 1), the receiver sends an acknowledgement adding 1 with the sequence number (i.e., sequence number 2) which means that the receiver receives the packet number 1 and it expects packet number 2 from the sender. Lets assume that three subsequent packets have been lost. In the meantime the receiver receives the packet number 5 and 6. After receiving packet number 5, the receiver sends an acknowledgement with the sequence number 2 and 6. When the receiver receives packet number 6, it sends acknowledgement with the sequence number 2 and 7. In this way, the sender receives more than one acknowledgement with the same sequence number 2 which is called duplicate acknowledgement.
The fast retransmit enhancement works as follows: if a TCP sender receives a specified number of acknowledgements which is usually set to three duplicate acknowledgements with the same acknowledge number (that is, a total of four acknowledgements with the same acknowledgement number), the sender can be reasonably confident that the segment with the next higher sequence number was dropped, and will not arrive out of order. The sender will then retransmit the packet that was presumed dropped before waiting for its timeout.
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...
which reduces the time a sender waits before retransmitting a lost segment
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....
.
A TCP sender uses a timer to recognize lost segments. If an acknowledgement is not received for a particular segment within a specified time (a function of the estimated Round-trip delay time
Round-trip delay time
In telecommunications, the round-trip delay time or round-trip time is the length of time it takes for a signal to be sent plus the length of time it takes for an acknowledgment of that signal to be received...
), the sender will assume the segment was lost in the network
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....
, and will retransmit the segment.
Duplicate acknowledgement is the basis for fast retransmit mechanism which works as follows: after receiving a packet (i.e., sequence number 1), the receiver sends an acknowledgement adding 1 with the sequence number (i.e., sequence number 2) which means that the receiver receives the packet number 1 and it expects packet number 2 from the sender. Lets assume that three subsequent packets have been lost. In the meantime the receiver receives the packet number 5 and 6. After receiving packet number 5, the receiver sends an acknowledgement with the sequence number 2 and 6. When the receiver receives packet number 6, it sends acknowledgement with the sequence number 2 and 7. In this way, the sender receives more than one acknowledgement with the same sequence number 2 which is called duplicate acknowledgement.
The fast retransmit enhancement works as follows: if a TCP sender receives a specified number of acknowledgements which is usually set to three duplicate acknowledgements with the same acknowledge number (that is, a total of four acknowledgements with the same acknowledgement number), the sender can be reasonably confident that the segment with the next higher sequence number was dropped, and will not arrive out of order. The sender will then retransmit the packet that was presumed dropped before waiting for its timeout.
External links
- TCP Congestion Handling and Congestion Avoidance Algorithms — The TCP/IP Guide