Our Unixadmin, John from xyz company,  recently upgraded one of his server’s network interface card  from the old one with 100MB speed to a new one with 10Gig speed , and the reason is to improve the speed for the backup of the server. After the NIC Upgrade  john found that the server network performance was improved a little, but still the server is not able to utilize all the available network bandwidth. 

After a bit of investigation john realised that his server TCP window size has to be tuned  to utilize maximum network bandwidth available to the server.  In this post we will be discussing the procedure about “how john tuned the TCP window size  to increase his server network performance?”.

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Virtual LANs or VLANs are virtual separations within a switch that provide distinct logical LANs that each behave if there were configured on a separate physical switch. Some Important points about VLANS.

•  All the servers connected with in a VLAN will have same broadcasting domain, and can connect to each other directly
•  Servers belongs to different VLANs has to communicate with each other using external Routing device.  
•  To Expand the VLAN across multiple physical switches, physical switches needs to be interconnected using links called Trunks

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Network connectivity checks For the server without OS  ( just Racked hardware and powered up)

For X86 Hardware

We have to make LED checks – like when we disconnect the cable at the server side / switch side the Link / Act LEDs should lit up on powered on machines.

For Sparc based Servers:

We can watch individual interfaces to see if they have a connection and can ‘see’ the network. At the OBP prompt on the client, use the “watch-net-all” command to test and see the network devices.

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This post explains you the information on the TCP retransmission measurement using netstat -s for Solaris. A system with a high retransmission rate indicates network congestion and can cause network oriented applications (like web servers) to operate slowly.

Procedure to Calculate TCP retransmission rate:

TCP expects acknowledgments from the destination system when it successfully receives segments from the sender. If it does not receive the acknowledgment within a certain time, it will retransmit the segment.

To implement this scheme of retransmission, TCP starts a timer  for each packet transmitted.  Unless the acknowledgment is received before this timer expires, TCP will assume that the packet is lost and will retransmit the packet.  The rate at which packets are retransmitted is called the ”retransmission rate”, and is an indicator of network health.

To calculate the retransmission rate, use the output of the ‘netstat -s -P tcp‘ command. We need the values for the counters

• tcpOutDataBytes
• tcpRetransBytes
• tcpOutDataSegs
• tcpRetransSegs. Full Story

If you want to make a TCP application bind to a different port instead of standard port then here is what you do.  This will enable to telnet to a machine only if you know the port number.

1. vi /etc/services file and edit the line for telnet

telnet          23/tcp

change this line to

telnet          12121/tcp

where 12121 is port number that it is going to listen for telnet requests.Make sure that you use port number bigger than 1k (1024 bytes) as the ports less than 1K are reserved ports.

2.  Kill inetd daemon and restart it or do HUP on it.

% ps -ef | grep inetd

% kill -HUP

3. Telnet to the client by specifying port number to connect to this machine.

% telnet hostname 12121

Follow these steps to:

• ensure dns client configuration is in place and enabled;
• verify mail exchanger lookups are successful (if using DNS for mail exchanger lookups).
• To resolve error conditions, consult your DNS or Sendmail administrator for proper configuration parameters for your environment.

Is system configured as a DNS client:

Solaris 8, 9 and 10:

/etc/nsswitch.conf  “hosts:” line contains “dns” keyword
/etc/resolv.conf exists with a minimum configuration of at least 2 (3 preferred) nameserver IP addresses

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