Chapter 1. Understanding the Linux operating system 33
Draft Document for Review May 4, 2007 11:35 am 4285ch01.fm
Figure 1-29 Sliding window and delayed ack
As an option, high-speed networks may use a technique called window scaling 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.
Retransmission
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.
1.5.3 Offload
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.
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.
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.
Sender Receiver
Sender Receiver
Delayed Ack
Sliding
window
