TCP delayed acknowledgment
Encyclopedia
TCP delayed acknowledgment is a technique used by some implementations of the Transmission Control Protocol
in an effort to improve network performance
. However, in some circumstances it can in fact reduce performance.
The socket level tries to guess what the application level is doing, in order to manage communications efficiently. Often it guesses wrong.
If the socket layer immediately acked everything, this would result in a lot of wasted acks, as usually the application sends a response shortly after receiving something, so we would be issuing two acknowledgments, one from the socket layer, one from the application layer. So the socket layer waits one or two hundred milliseconds, and if there is still nothing to send, sends an ack.
Unfortunately, the socket layer at the other end, the TCP stack at the other end, may well be waiting for an ack (see Nagle's algorithm
).
If the socket layer at the other end has less than a full packet to send, well, it does not want to send unnecessarily small packets, so it waits a bit in the hope of having more data to send. It waits until it gets an ack for the data it has already sent. This can result in a brief delay.
Bob is sending stuff to Carol:
Bob's socket layer has less than a complete packet remaining to send. It will not be sent till he gets ack for the data already sent.
Carol's application layer will not send a response till it gets all the data. Carol's socket layer will not send an ack until it times out, or Carol makes a response.
If we are transmitting files in chunks of a hundred thousand bytes, over a gigabit network, all these two hundred millisecond delays can slow things down a great deal.
To prevent this delay, the application layer needs to emulate the socket layer, and keep on sending stuff without waiting for acknowledgment of the last thing it sent.
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...
in an effort to improve network performance
Network performance
Network performance refers to the service quality of a telecommunications product as seen by the customer. It should not be seen merely as an attempt to get "more through" the network....
. However, in some circumstances it can in fact reduce performance.
The socket level tries to guess what the application level is doing, in order to manage communications efficiently. Often it guesses wrong.
If the socket layer immediately acked everything, this would result in a lot of wasted acks, as usually the application sends a response shortly after receiving something, so we would be issuing two acknowledgments, one from the socket layer, one from the application layer. So the socket layer waits one or two hundred milliseconds, and if there is still nothing to send, sends an ack.
Unfortunately, the socket layer at the other end, the TCP stack at the other end, may well be waiting for an ack (see Nagle's algorithm
Nagle's algorithm
Nagle's algorithm, named after John Nagle, is a means of improving the efficiency of TCP/IP networks by reducing the number of packets that need to be sent over the network....
).
If the socket layer at the other end has less than a full packet to send, well, it does not want to send unnecessarily small packets, so it waits a bit in the hope of having more data to send. It waits until it gets an ack for the data it has already sent. This can result in a brief delay.
Bob is sending stuff to Carol:
Bob's socket layer has less than a complete packet remaining to send. It will not be sent till he gets ack for the data already sent.
Carol's application layer will not send a response till it gets all the data. Carol's socket layer will not send an ack until it times out, or Carol makes a response.
If we are transmitting files in chunks of a hundred thousand bytes, over a gigabit network, all these two hundred millisecond delays can slow things down a great deal.
To prevent this delay, the application layer needs to emulate the socket layer, and keep on sending stuff without waiting for acknowledgment of the last thing it sent.