2-11
Chapter 2 Overview
Redline Communications Inc. Confidential and Proprietary
By default, four packets are sent, at approximately 1-second intervals. The timeout for
each request depends on the version of Windows. For Windows XP the timeout is 4
seconds.
The default packet size is 32 bytes. This is the size of the data field in the ICMP packet,
but it is not the size of the entire packet. When you add 8 bytes of ICMP overhead, 20 bytes
of IP overhead and 18 bytes of Ethernet overhead, we get an Ethernet frame size (for a
default ping) which is actually 78 bytes. If you want to test how ping works with the
maximum Ethernet frame size, use the following command:
ping 192.168.101.3 -l 1472
If the specified size is larger than 1472 bytes, your computer will send more than one IP
packet. It will send fragments that will be reassembled at the destination. If the transfer is
completed successfully, then a packet (or packets) of the same size will be sent back.
The maximum value that you can enter for the ping size is 65500. With 8 bytes of ICMP
header, this makes the 65508 bytes of IP payload. IP will fragment this into 45 packets. So
for the "maximum ping" to succeed, 45 Ethernet packets of maximum size (actually 44
packets will be 1518 Bytes, the last one will be 426 Bytes long) have to be successfully
transmitted, and then in response, an additional 45 packets have to be successfully
received.
Although ping sends packets at approximately 1-second intervals, fragmentation is under
control of the TCP/IP stack, and all of the 45 packets will be sent at the line rate of the
network card, which in some cases could cause ping to drop packets simply because
buffers across the network are full.
Using the Iperf Utility
Iperf is a free, open source utility that can be used for testing TCP and UDP performance.
It is available for download at http://dast.nlanr.net/Projects/Iperf/.
In order to perform an iperf test, you need two computer's, one at either end of the
RedMAX link. The first computer acts as the client, and the other acts as the server. The
determination of which computer operates in which mode, and how they are configured,
depends on the nature of the test to be performed.
Iperf is a CLI-based tool. Arguments are supplied during execution determine the nature
of the test to be performed. The following table lists the most commonly used arguments:
Where: -l indicates the buffer size.
In this example 1472 bytes of data are sent. 1472 bytes of ping "data" maps to
1518 bytes of an Ethernet packet.
