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.
Nagle's document, Congestion Control in IP/TCP Internetworks (RFC 896) describes what he called the 'small packet problem', where an application repeatedly emits data in small chunks, frequently only 1 byte in size. Since TCP packets have a 40 byte header (20 bytes for TCP, 20 bytes for IPv4), this results in a 41 byte packet for 1 byte of useful information, a huge overhead. This situation often occurs in Telnet sessions, where most keypresses generate a single byte of data which is transmitted immediately. Worse, over slow links, many such packets can be in transit at the same time, potentially leading to congestion collapse.
Nagle's algorithm works by coalescing a number of small outgoing messages, and sending them all at once. Specifically, as long as there is a sent packet for which the sender has received no acknowledgment, the sender should keep buffering its output until it has a full packet's worth of output, so that output can be sent all at once.
Algorithm
if there is new data to send
if the window size >= MSS and available data is >= MSS
send complete MSS segment now
else
if there is unconfirmed data still in the pipe
enqueue data in the buffer until an acknowledge is received
else
send data immediately
end if
end if
end if
where MSS = maximum segment size.
This algorithm interacts badly with TCP delayed acknowledgments, a feature introduced into TCP at roughly the same time in the early 1980s, but by a different group. With both algorithms enabled, applications which do two successive writes to a TCP connection, followed by a read which will not be fulfilled until after the data from the second write has reached the destination, experience a constant delay of up to 500 milliseconds, the "ACK delay". For this reason, TCP implementations usually provide applications with an interface to disable the Nagle algorithm. This is typically called the TCP_NODELAY option. It is however not recommended to disable this. A solution, recommended by Nagle himself, is to keep from sending small single writes and buffer up application writes then send.
"The user-level solution is to avoid write-write-read sequences on sockets. write-read-write-read is fine. write-write-write is fine. But write-write-read is a killer. So, if you can, buffer up your little writes to TCP and send them all at once. Using the standard UNIX I/O package and flushing write before each read usually works."[1]
The tinygram problem and silly window syndrome are sometimes confused. The tinygram problem occurs when the window is almost empty. Silly window syndrome occurs when the window is almost full.
Interactions with real-time systems
Applications that expect real time responses can react poorly with Nagle's algorithm. Applications such as networked multiplayer video games expect that actions in the game are sent immediately, while the algorithm purposefully delays transmission, increasing bandwidth at the expense of latency. For this reason applications with low-bandwidth time-sensitive transmissions typically use TCP_NODELAY to bypass the Nagle delay.[2]
References
- Computer Networks: A Systems Approach (4 ed.). 2007. p. 402–403. ISBN 0123740134.
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