NDT-DEV email list created

Revision: 629
Author:   

Date:     Fri Sep 16 07:27:38 2011
Log:      Note that NDT requires Reno/interrupt coalescing disabled
Explain the use of tcpdump
Note a limitation of the average rtt calculation


http://code.google.com/p/ndt/source/detail?r=629

Modified:
 /wiki/NDTTestMethodology.wiki

=======================================
--- /wiki/NDTTestMethodology.wiki       Fri Sep 16 06:31:11 2011
+++ /wiki/NDTTestMethodology.wiki       Fri Sep 16 07:27:38 2011
@@ -96,8 +96,8 @@

 Next, the client connects to the server's newly bound port. When the  
connection is successfully established, the server initializes the  
following routines:
  * libpcap routines to perform packet trace used by the  
[NDTTestMethodology#Bottleneck_Link_Detection Bottleneck Link Detection]  
algorithm.
- * [NDTDataFormat#tcpdump_trace tcpdump trace] to dump all packets sent  
during the [NDTTestMethodology#C2S_Throughput_Test C2S throughput test] on  
the newly created connection. This tcpdump trace dump is only started when  
the `-t, --tcpdump` options are set.
- * [NDTDataFormat#web100_snaplog_trace web100 snaplog trace] to dump  
web100 kernel MIB variables' values written in a fixed time (default is 5  
msec) increments during the [NDTTestMethodology#C2S_Throughput_Test C2S  
throughput test] for the newly created connection. This snaplog trace dump  
is only started when the `--snaplog` option is set.
+ * [NDTDataFormat#tcpdump_trace tcpdump trace] to save to a standard  
tcpdump file all packets sent during the  
[NDTTestMethodology#C2S_Throughput_Test C2S throughput test] on the newly  
created connection. This tcpdump trace dump is only started when the `-t,  
--tcpdump` options are set. The use of tcpdump duplicates work being done  
by the libpcap trace above. However, this approach simplifies the NDT  
codebase.
+ * [NDTDataFormat#web100_snaplog_trace web100 snaplog trace] to dump  
web100 kernel MIB variables' values written in a fixed time interval  
(default is 5 msec) during the [NDTTestMethodology#C2S_Throughput_Test C2S  
throughput test] for the newly created connection. This snaplog trace dump  
is only started when the `--snaplog` option is set.

 In the next step the client starts a 10 seconds throughput test using the  
newly created connection. The NDT client sends packets as fast as possible  
(i.e. without any delays) during the test. These packets are written using  
the 8192 Byte buffer containing a pre-generated pseudo random data  
(including only US-ASCII printable characters).

@@ -418,6 +418,9 @@

 The average round trip time is kept in milliseconds.

+==== Known Limitations (Average round trip time) ====
+The average round trip time is calculated during the S2C throughput test.  
Because NDT is attempting to fill the link to discover what throughput it  
can obtain, the RTT calculations will be skewed by NDT. In this way, NDT's  
calculation of the RTT is conservative since the actual RTT should be no  
worse than the RTT when NDT is running the throughput test.
+
 === Theoretical maximum throughput ===

 The theoretical maximum throughput is computed using the following formula:
@@ -489,7 +492,9 @@

 == Known issues/limitations ==

-Some issues/limitations have been found in the NDT regarding the following  
areas:
+Two overall known limitations are that NDT requires that the TCP  
congestion algorithms be Reno, and that it requires packet coalescing to be  
disabled. If these are not the case, some of NDT's heuristics may not be  
accurate. These limitations, however, will negatively impact the throughput  
tests. NDT's results are, thus, conservative, showing the worst performance  
a client might see.
+
+Some specific issues/limitations have been found in the NDT regarding the  
following areas:
  * [NDTTestMethodology#Known_issues_(Middlebox_Test) Middlebox Test]
  * [NDTTestMethodology#Known_limitations_(Bottleneck_Link_Detection)  
Bottleneck Link Detection]
  *  
[NDTTestMethodology#Known_issues/limitations_(Duplex_Mismatch_Detection)  
Duples Mismatch Detection]