Loco
07-24-2002, 08:45 PM
Taken from the original FAQ.
2.17 - What exactly does the Nagle algorithm do?
It groups together as much data as it can between ACK's from the other end of the connection. I found this really confusing until Andrew Gierth (andrew@erlenstar.demon.co.uk) drew the following diagram, and explained:
This diagram is not intended to be complete, just to illustrate the point better...
Case 1: client writes 1 byte per write() call. The program on host B is tcpserver.c from the FAQ examples.
CLIENT SERVER
APP TCP TCP APP
[connection setup omitted]
"h" ---------> [1 byte]
------------------>
-----------> "h"
[ack delayed]
"e" ---------> [Nagle alg. .
now in effect] .
"l" ---------> [ditto] .
"l" ---------> [ditto] .
"o" ---------> [ditto] .
"\n"---------> [ditto] .
.
.
[ack 1 byte]
<------------------
[send queued
data]
[5 bytes]
------------------>
------------> "ello\n"
<------------ "HELLO\n"
[6 bytes, ack 5 bytes]
<------------------
"HELLO\n" <----
[ack delayed]
.
.
. [ack 6 bytes]
------------------>
Total segments: 5. (If TCP_NODELAY was set, could have been up to 10.) Time for response: 2*RTT, plus ack delay.
Case 2: client writes all data with one write() call.
CLIENT SERVER
APP TCP TCP APP
[connection setup omitted]
"hello\n" ---> [6 bytes]
------------------>
------------> "hello\n"
<------------ "HELLO\n"
[6 bytes, ack 6 bytes]
<------------------
"HELLO\n" <----
[ack delayed]
.
.
. [ack 6 bytes]
------------------>
Total segments: 3.
Time for response = RTT (therefore minimum possible).
Hope this makes things a bit clearer...
Note that in case 2, you don't want the implementation to gratuitously delay sending the data, since that would add straight onto the response time.
2.17 - What exactly does the Nagle algorithm do?
It groups together as much data as it can between ACK's from the other end of the connection. I found this really confusing until Andrew Gierth (andrew@erlenstar.demon.co.uk) drew the following diagram, and explained:
This diagram is not intended to be complete, just to illustrate the point better...
Case 1: client writes 1 byte per write() call. The program on host B is tcpserver.c from the FAQ examples.
CLIENT SERVER
APP TCP TCP APP
[connection setup omitted]
"h" ---------> [1 byte]
------------------>
-----------> "h"
[ack delayed]
"e" ---------> [Nagle alg. .
now in effect] .
"l" ---------> [ditto] .
"l" ---------> [ditto] .
"o" ---------> [ditto] .
"\n"---------> [ditto] .
.
.
[ack 1 byte]
<------------------
[send queued
data]
[5 bytes]
------------------>
------------> "ello\n"
<------------ "HELLO\n"
[6 bytes, ack 5 bytes]
<------------------
"HELLO\n" <----
[ack delayed]
.
.
. [ack 6 bytes]
------------------>
Total segments: 5. (If TCP_NODELAY was set, could have been up to 10.) Time for response: 2*RTT, plus ack delay.
Case 2: client writes all data with one write() call.
CLIENT SERVER
APP TCP TCP APP
[connection setup omitted]
"hello\n" ---> [6 bytes]
------------------>
------------> "hello\n"
<------------ "HELLO\n"
[6 bytes, ack 6 bytes]
<------------------
"HELLO\n" <----
[ack delayed]
.
.
. [ack 6 bytes]
------------------>
Total segments: 3.
Time for response = RTT (therefore minimum possible).
Hope this makes things a bit clearer...
Note that in case 2, you don't want the implementation to gratuitously delay sending the data, since that would add straight onto the response time.