Nagle's algorithm
From Wikipedia, the free encyclopedia
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 that 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 combining 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 that do two successive writes to a TCP connection, followed
by a read that 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.
If possible an application should avoid consecutive small writes in
the first place, so that Nagle's algorithm will not be triggered. The
application should keep from sending small single writes and buffer up
application writes then send (or with the help of writev() call).
"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.
Negative effect on non-small writes
The algorithm applies to data of any size. If the data in a single write spans 2n
packets, the last packet will be withheld, waiting for the ACK for the previous packet.[
2]
In any request-response application protocols where request data can be
larger than a packet, this can artificially impose a few hundred
milliseconds latency between the requester and the responder, even if
the requester has properly buffered the request data. Nagle's algorithm
must be disabled by the requester in this case. If the response data can
be larger than a packet, the responder must also disable Nagle's
algorithm so the requester can promptly receive the whole response.
In general, since Nagle's algorithm is only a defense against
careless applications, it will not benefit a carefully written
application that takes proper care of buffering; the algorithm has
either no effect, or negative effect on the application.
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
efficiency at the expense of latency
. For this reason applications with low-bandwidth time-sensitive transmissions typically use TCP_NODELAY
to bypass the Nagle delay.[
3]
References
-
^
Boosting Socket Performance on Linux - Slashdot
-
^
http://www.stuartcheshire.org/papers/NagleDelayedAck/
-
^
Bug 17868 – Some Java applications are slow on remote X connections
External links
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