IBM REDP-4285-00 ユーザーズマニュアル

Chapter 1. Understanding the Linux operating system 

Draft Document for Review May 4, 2007 11:35 am

As an option, high-speed networks may use a technique called 

 to increase 

the maximum transfer window size even more. We will analyze the effects of these 
implementations in more detail in “Tuning TCP options” on page 132.

In the connection establishment and termination and data transfer, many timeouts and data 
retransmissions may be caused by various reasons (faulty network interface, slow router, 
network congestion, buggy network implementation, and so on). TCP/IP handles this situation 
by queuing packets and trying to send packets several times.

You can change some behavior of the kernel by configuring parameters. You may want to 
increase the number of attempts for TCP SYN connection establishment packet on the 
network with high rate of packet loss. You can also change some of timeout threshold through 
files under /proc/sys/net. For more information, see “Tuning TCP behavior” on page 131.

If the network adapter on your system supports hardware offload functionality, the kernel can 
offload part of its task to the adapter and it can reduce CPU utilization. 

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Checksum offload

IP/TCP/UDP checksum is performed to make sure if the packet is correctly transferred by 
comparing the value of checksum field in protocol headers and the calculated values by 
the packet data.

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TCP segmentation offload (TSO)

When the data that is lager than supported maximum transmission unit (MTU) is sent to 
the network adapter, the data should be divided into MTU sized packets. The adapter 
takes care of that on behalf of the kernel.

For more advanced network features, refer to redbook Tuning IBM System x Servers for 
Performance, SG24-5287. section 10.3. Advanced network features.