NDT-DEV email list created

Revision: 607
Author:   

Date:     Thu Sep  8 07:15:52 2011
Log:      Fixes in the duplex mismatch detection section, renamed speed  
estimate to theoretical maximum bandwidth
http://code.google.com/p/ndt/source/detail?r=607

Modified:
 /wiki/NDTTestMethodology.wiki

=======================================
--- /wiki/NDTTestMethodology.wiki       Thu Sep  8 03:27:34 2011
+++ /wiki/NDTTestMethodology.wiki       Thu Sep  8 07:15:52 2011
@@ -173,15 +173,15 @@

 === Duplex Mismatch Detection ===

-Duplex mismatch is a condition whereby the host Network Interface Card  
(NIC) and building switch port fail to agree on this basic network  
operating parameter. While this failure will have a large impact on  
application performance, basic network connectivity still exists. This  
means that normal testing procedures (e.g., ping, traceroute) will report  
that no problem exists while real applications will run extremely slowly.
+Duplex mismatch is a condition whereby the host Network Interface Card  
(NIC) and building switch port fail to agree on whether to operate  
at 'half-duplex' or 'full-duplex'. While this failure will have a large  
impact on application performance, basic network connectivity still exists.  
This means that normal testing procedures (e.g., ping, traceroute) may  
report that no problem exists while real applications will run extremely  
slowly.

 The NDT contains two heuristics for the duplex mismatch detection.

-A duplex mismatch is detected by the Old Duplex-Mismatch algorithm when  
the connection is congestion limited 90% of the time, the !WiFi is not  
detected, the [NDTTestMethodology#Speed_estimate speed estimate] is greater  
than 2Mibps, there is a lot of packets retransmissions and the S2C  
throughput speed is much smaller than expected.
+A duplex mismatch is detected by the Old Duplex-Mismatch algorithm when  
the connection is congestion limited 90% of the time, the !WiFi is not  
detected, the [NDTTestMethodology#Theoretical_maximum_bandwidth theoretical  
maximum bandwidth] is greater than 2Mibps, there is a lot of packets  
retransmissions and the S2C throughput speed is much smaller than expected.

 This means that all of the following conditions should be met:
  * The  
[NDTTestMethodology#'Congestion_Limited'_state_time_share 'Congestion  
Limited' state time share] *is greater than 90%*
- * The [NDTTestMethodology#Speed_estimate speed estimate] *is greater than  
2Mibps*
+ * The [NDTTestMethodology#Theoretical_maximum_bandwidth theoretical  
maximum bandwidth] *is greater than 2Mibps*
  * The number of segments transmitted containing at least some  
retransmitted data *is greater than 2 per second*
  * The maximum slow start threshold, excluding the initial value, *is  
greater than 0*
  * The cumulative time of the expired retransmit timeouts RTO *is greater  
than 1% of the total test time*
@@ -209,27 +209,27 @@

 === Link Type Detection Heuristics ===

-The following link type detection heuristics are run only when there is no  
duplex mismatch condition detected and the  
[NDTTestMethodology#Total_send_throughput total send throughput] is the  
same or smaller than the [NDTTestMethodology#Speed_estimate speed estimate]  
(which is a correct situation).
+The following link type detection heuristics are run only when there is no  
duplex mismatch condition detected and the  
[NDTTestMethodology#Total_send_throughput total send throughput] is the  
same or smaller than the [NDTTestMethodology#Theoretical_maximum_bandwidth  
theoretical maximum bandwidth] (which is a correct situation).

 ==== DSL/Cable modem ====

-The link is treated as a DSL/Cable modem when the sender isn't a  
bottleneck and the [NDTTestMethodology#Total_send_throughput total send  
throughput] is less than 2 Mbps and less than the  
[NDTTestMethodology#Speed_estimate speed estimate].
+The link is treated as a DSL/Cable modem when the sender isn't a  
bottleneck and the [NDTTestMethodology#Total_send_throughput total send  
throughput] is less than 2 Mbps and less than the  
[NDTTestMethodology#Theoretical_maximum_bandwidth theoretical maximum  
bandwidth].

 This means that all of the following conditions should be met:
  * The cumulative time spent in the 'Sender Limited' state *is less than  
0.6 ms*
  * The number of transitions into the 'Sender Limited' state *is 0*
  * The [NDTTestMethodology#Total_send_throughput total send throughput]  
*is less than 2 Mbps*
- * The [NDTTestMethodology#Total_send_throughput total send throughput]  
*is less than* [NDTTestMethodology#Speed_estimate speed estimate]
+ * The [NDTTestMethodology#Total_send_throughput total send throughput]  
*is less than* [NDTTestMethodology#Theoretical_maximum_bandwidth  
theoretical maximum bandwidth]

 ==== IEEE 802.11 (!WiFi) ====

-The link is treated as a wireless one when the  
[NDTTestMethodology#DSL/Cable_modem DSL/Cable modem] is not detected, the  
receiver is a bottleneck and the [NDTTestMethodology#Total_send_throughput  
total send throughput] is less than 5 Mbps but the  
[NDTTestMethodology#Speed_estimate speed estimate] is greater than 50 Mibps.
+The link is treated as a wireless one when the  
[NDTTestMethodology#DSL/Cable_modem DSL/Cable modem] is not detected, the  
receiver is a bottleneck and the [NDTTestMethodology#Total_send_throughput  
total send throughput] is less than 5 Mbps but the  
[NDTTestMethodology#Theoretical_maximum_bandwidth theoretical maximum  
bandwidth] is greater than 50 Mibps.

 This means that all of the following conditions should be met:
  * The heuristic for DSL/Cable modem link *gives negative results*
  * The cumulative time spent in the 'Sender Limited' state *is 0 ms*
  * The [NDTTestMethodology#Total_send_throughput total send throughput]  
*is less than 5 Mbps*
- * The [NDTTestMethodology#Speed_estimate speed estimate] *is greater than  
50 Mibps*
+ * The [NDTTestMethodology#Theoretical_maximum_bandwidth theoretical  
maximum bandwidth] *is greater than 50 Mibps*
  * The number of transitions into the 'Receiver Limited' state *is the  
same* as the number of transitions into the 'Congestion Limited' state
  * The [NDTTestMethodology#'Receiver_Limited'_state_time_share 'Receiver  
Limited' state time share] *is greater than 90%*

@@ -398,9 +398,9 @@
 where:
  * *AvgRTT* - [NDTTestMethodology#Average_round_trip_time_(Latency/Jitter)  
Average round trip time (Latency/Jitter)]

-=== Speed estimate ===
-
-The speed estimate is computed using the following formula:
+=== Theoretical maximum bandwidth ===
+
+The theoretical maximum bandwidth is computed using the following formula:

 {{{
 (CurrentMSS / (AvgRTTSec * sqrt(PktsLoss))) * 8 / 1024 / 1024
@@ -411,9 +411,9 @@
  * *AvgRTTSec* - [NDTTestMethodology#Average_RTT_in_seconds Average RTT in  
seconds]
  * *!PktsLoss* - [NDTTestMethodology#Packet_loss Packet loss]

-The speed estimate is kept in Mibps.
-
-The above speed estimate comes from the matthis equation  
([http://www.psc.edu/networking/papers/model_ccr97.ps]):
+The theoretical maximum bandwidth is kept in Mibps.
+
+The above theoretical maximum bandwidth comes from the matthis equation  
([http://www.psc.edu/networking/papers/model_ccr97.ps]):

 {{{
 Rate < (MSS/RTT)*(1 / sqrt(p))
@@ -421,9 +421,9 @@

 where p is the loss probability.

-==== Known issues (Speed estimate) ====
-
-The speed estimate should be computed to receive Mbps instead of Mibps.  
This is the only variable in the NDT that is kept in Mibps, so it might  
lead to the inconsistent results when comparing it with the other values.
+==== Known issues (Theoretical maximum bandwidth) ====
+
+The theoretical maximum bandwidth should be computed to receive Mbps  
instead of Mibps. This is the only variable in the NDT that is kept in  
Mibps, so it might lead to the inconsistent results when comparing it with  
the other values.

 === 'Congestion Limited' state time share ===

@@ -470,4 +470,4 @@
  *  
[NDTTestMethodology#Known_issues/limitations_(Duplex_Mismatch_Detection)  
Duples Mismatch Detection]
  * [NDTTestMethodology#Known_issues_(Link_Type_Detection_Heuristics) Link  
Type Detection Heuristics]
  * [NDTTestMethodology#Known_issues_(Faulty_Hardware_Link_Detection)  
Faulty Hardware Link Detection]
- * [NDTTestMethodology#Known_issues_(Speed_estimate) Speed estimate]
+ * [NDTTestMethodology#Known_issues_(Theoretical_maximum_bandwidth)  
Theoretical maximum bandwidth]