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Re: "The connection is sender limited x% of the time"


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  • From: Richard Carlson < >
  • To: Clarke Morledge < >
  • Cc:
  • Subject: Re: "The connection is sender limited x% of the time"
  • Date: Fri, 23 Oct 2009 09:07:08 -0500

Hi Clarke;

The web100 kernel patches contain a set of variables to track where the sending TCP (the web100 server in this case) is spending it's time. From the RFC-4898 paper

tcpEStatsPerfSndLimTimeRwin OBJECT-TYPE
SYNTAX ZeroBasedCounter32
UNITS "milliseconds"
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The cumulative time spent in the 'Receiver Limited' state.
See tcpEStatsPerfSndLimTransRwin."
::= { tcpEStatsPerfEntry 34 }

tcpEStatsPerfSndLimTimeCwnd OBJECT-TYPE
SYNTAX ZeroBasedCounter32
UNITS "milliseconds"
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The cumulative time spent in the 'Congestion Limited'
state. See tcpEStatsPerfSndLimTransCwnd. When there is a
retransmission timeout, it SHOULD be counted in
tcpEStatsPerfSndLimTimeCwnd (and not the cumulative time
for some other state.)"
::= { tcpEStatsPerfEntry 35 }

tcpEStatsPerfSndLimTimeSnd OBJECT-TYPE
SYNTAX ZeroBasedCounter32
UNITS "milliseconds"
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The cumulative time spent in the 'Sender Limited' state.
See tcpEStatsPerfSndLimTransSnd."
::= { tcpEStatsPerfEntry 36 }

The NDT server extracts these variables and calculates the % of time in each state (you can see the time, bytes transfered, and transition count in the data below).

The test results below indicate that your server may have memory or CPU constraints that will impact the test results. The web100 data shows that this server has data to send (in the sender ready state) but it can't push this data down to the NIC. Most likely the CPU is busy handling other processes/interrupts. You can enable the -- cputime flag and it will track/report the CPU utilization numbers so you don't need to sit at a console running top or something like that to check the CPU util numbers.

While it is true that they haven't impacted this test, it may become an issue if you (1) want to run at gigabit rates; or (2) run 100 Mbps tests to remote clients.

Regards;
Rich

On Oct 22, 2009, at 3:58 PM, Clarke Morledge wrote:

I need some help decoding the "This connection is sender limited x% of the time", particularly when the value of x is greater than 80%.

What does the statement actually mean, and what are some potential troubleshooting remedies?

In this case, I am using NDT v.5.5.4a.

I am including the statistics output of a test below, where this example is sender limited 88.14% of the time....

Clarke Morledge
College of William and Mary
Information Technology - Network Engineering
Jones Hall (Room 18)
Williamsburg VA 23187

-------------------------------------------------------

checking for firewalls . . . . . . . . . . . . . . . . . . . Done
running 10s outbound test (client-to-server [C2S]) . . . . . 93.43Mb/s
running 10s inbound test (server-to-client [S2C]) . . . . . . 93.04Mb/s
Client: localhost/127.0.0.1
CurMSS: 1448
X_Rcvbuf: 33554432
X_Sndbuf: 33554432
AckPktsIn: 45541
AckPktsOut: 0
BytesRetrans: 0
CongAvoid: 0
CongestionOverCount: 0
CongestionSignals: 0
CountRTT: 45542
CurCwnd: 99912
CurRTO: 205
CurRwinRcvd: 63712
CurRwinSent: 6144
CurSsthresh: 2147483647
DSACKDups: 0
DataBytesIn: 0
DataBytesOut: 120854968
DataPktsIn: 0
DataPktsOut: 83303
DupAcksIn: 0
ECNEnabled: 0
FastRetran: 0
MaxCwnd: 99912
MaxMSS: 1448
MaxRTO: 207
MaxRTT: 9
MaxRwinRcvd: 66608
MaxRwinSent: 6144
MaxSsthresh: 0
MinMSS: 1428
MinRTO: 201
MinRTT: 1
MinRwinRcvd: 55024
MinRwinSent: 5792
NagleEnabled: 1
OtherReductions: 0
PktsIn: 45541
PktsOut: 83303
PktsRetrans: 0
RcvWinScale: 10
SACKEnabled: 3
SACKsRcvd: 0
SendStall: 0
SlowStart: 66
SampleRTT: 4
SmoothedRTT: 5
SndWinScale: 1
SndLimTimeRwin: 1223997
SndLimTimeCwnd: 5595
SndLimTimeSender: 9140777
SndLimTransRwin: 3624
SndLimTransCwnd: 10
SndLimTransSender: 3634
SndLimBytesRwin: 86086656
SndLimBytesCwnd: 123528
SndLimBytesSender: 34644784
SubsequentTimeouts: 0
SumRTT: 214553
Timeouts: 0
TimestampsEnabled: 1
WinScaleRcvd: 1
WinScaleSent: 10
DupAcksOut: 0
StartTimeUsec: 411289
Duration: 10370730
c2sData: 5
c2sAck: 5
s2cData: 5
s2cAck: 5
half_duplex: 0
link: 100
congestion: 0
bad_cable: 0
mismatch: 0
spd: 93.23
bw: 2344.96
loss: 0.000001000
avgrtt: 4.71
waitsec: 0.00
timesec: 10.00
order: 0.0000
rwintime: 0.1180
sendtime: 0.8814
cwndtime: 0.0005
rwin: 0.5082
swin: 256.0000
cwin: 0.7623
rttsec: 0.004711
Sndbuf: 33554432
aspd: 0.00000
CWND-Limited: 22054.01
minCWNDpeak: -1
maxCWNDpeak: -1
CWNDpeaks: -1
The slowest link in the end-to-end path is a 100 Mbps Full duplex Fast Ethernet subnet

Web100 reports the Round trip time = 4.71 msec; the Packet size = 1448 Bytes; and
No packet loss was observed.
This connection is receiver limited 11.79% of the time.
This connection is sender limited 88.14% of the time.
Server '128.239.22.8' is not behind a firewall. [Connection to the ephemeral port was successful]
Client is probably behind a firewall. [Connection to the ephemeral port failed]
The theoretical network limit is 2344.96 Mbps
The NDT server has a 16384.0 KByte buffer which limits the throughput to 54340.90 Mbps
Your PC/Workstation has a 65.0 KByte buffer which limits the throughput to 107.87 Mbps
The network based flow control limits the throughput to 161.81 Mbps

Richard Carlson

1000 Oakbrook Dr
Ann Arbor, MI 48104

P: 734-352-7043
C: 630-251-4572




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