Wireshark · Wireshark-users: Re: [Wireshark-users] Possible network latency

["Albert Jurado" <ajurado@xxxxxxxxxxxxxxxx>]

Thx.

 

One particular capture shows a count of 1534 suspected retransmissions (that number seems high).  When I look at the details I see [TCP Retransmission] Close Response.  The protocol in question is SMB.  The communication is between a workstation and a SQL server.  Not sure what the close response means.  Is this a valid retransmission?  Or is there a preference setting I’m missing?

 

Albert Jurado

Network Manager

First Commercial Insurance Company

2300 W 84 St.

Hialeah, FL 33016

Phone: (305) 820-4848 ex. 1206

Mobile: (305) 873-4400

Email:  ajurado@xxxxxxxxxxxxxxxx

 

From: wireshark-users-bounces@xxxxxxxxxxxxx [mailto:wireshark-users-bounces@xxxxxxxxxxxxx] On Behalf Of Visser, Martin
Sent: Thursday, July 03, 2008 12:18 AM
To: Community support list for Wireshark
Subject: Re: [Wireshark-users] Possible network latency

 

You won't find a "average re-transmission" statement because there really isn't one. On a well provisioned local area network you could expect it to be zero. On a heavily congested wireless or WAN network it might be 20% before it becomes unworkable.

 

If netstat -s shows retransmission by the client than either the workstation or the LAN should show retransmissions. (The client will retransmit either if it receives if receives duplicate ACKs for segments it has said or if the retransmission timeout (RTO) the TCP stack calculates has exceeded before it receives an ACK.

 

On Wireshark if you seclect Analyze:Expert Info Composite and Notes tab you will see a TCP retransmission count if you are getting any. You can also just apply the Display Filter "tcp.analysis.retransmission" to see all relevant packets.

 

If you are getting a lot of retransmissions where client and server are on a local switched LAN you may want to look at the physical error counts on your switches - and look for physical layer issues.

 

Regards, Martin

Martin Visser

Technology Consultant
Technology Solutions Group

Hewlett-Packard
410 Concord Road
Rhodes NSW  2138
Australia

Mobile: +61-411-254-513
Fax: +61-2-9022-1800    
E-mail: martin.visserAThp.com

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From: wireshark-users-bounces@xxxxxxxxxxxxx [mailto:wireshark-users-bounces@xxxxxxxxxxxxx] On Behalf Of Albert Jurado
Sent: Thursday, 3 July 2008 12:40 AM
To: wireshark-users@xxxxxxxxxxxxx
Subject: [Wireshark-users] Possible network latency

I’m in the mist of troubleshooting possible high network re-transmissions.  I’m basically attempting to capture enough data to prove that the network is not the bottle neck.  I have complaints from user that their systems are slow but it seems that the application they are using is the bottleneck.  We have several in-house developed applications that the end users uses that communicates with a SQL server.  They also browse the internet frequently.  I’ve been looking for articles that describe what the average re-transmission rate is for a standard TCP/IP networked workstation but I could not find any.  I’ve attempted a simple test like running the trouble application and then performing a simple copy & paste (of a 1gb file) from a file server to the workstation’s desktop while pinging the SQL server at the same time and I did not see the time change from <1ms.  The application ran slow.  Plus the file copied over without any issue.

 

A brief description of the network is as follows.  We have 5 floors with each floor having a wiring closet.  In each closet we have a Cisco 3750 cluster of switches.  Each floor has fiber running down to the core switch on the 2^nd floor.

 

The reason we suspect re-transmission is because some workstations show a high “segments retransmitted” when you run netstat –s.  If I run Wireshark on the suspect workstations what should I be looking for in the capture?  Will I capture re-transmission that corresponds to the netstat –s output?

 

Thx.

 

 

Albert