HP StorageWorks XPath OS 7.4.X Command Reference Guide (AA-RVHCC-TE, September 2005)

XPath OS 7.4.x command reference guide 161

produced if there is no response. When not using the -f (flood) option, the first interrupt, usually

generated by Ctrl-C or Delete, causes ping to wait for its outstanding requests to return. It will wait no

longer than the longest round-trip time encountered by previous, successful pings. The second interrupt

stops ping immediately.

This command is intended for use in network testing, measurement, and management. Because of the load

it can impose on the network, it is unwise to use ping during normal operations or from automated

scripts.

ICMP packet details

An IP header without options is 20 bytes. An ICMP ECHO_REQUEST packet contains an additional 8

bytes worth of ICMP header, followed by an arbitrary amount of data. When packetsize is specified,

this indicates the size of this extra piece of data (the default is 56). Thus, the amount of data received

inside of an IP packet of type ICMP ECHO_REPLY is always 8 bytes more than the requested data space

(the ICMP header).

If the data space is at least 8 bytes large, ping uses the first 8 bytes of this space to include a time stamp

to compute round-trip times. If less than 8 bytes of pad are specified, no round-trip times are given.

Duplicate and damaged packets

ping reports duplicate and damaged packets. Duplicate packets should not occur; when they do, they

are often caused by inappropriate link-level retransmissions.

Damaged packets are cause for alarm and often indicate broken hardware somewhere in the ping

packet path (in the network or in the hosts).

Trying different data patterns

The (inter)network layer should never treat packets differently based on the data contained in the data

portion. Unfortunately, data-dependent problems have been known to occur in networks and remain

undetected for long periods of time. Patterns that have problems often do not have sufficient transitions,

such as a pattern of all ones or all zeroes, or of nearly all ones or zeroes. It is not necessarily enough to

specify a data pattern of all zeroes, for example, on the command line, because the pattern that is of

interest is at the data link level, and the relationship between what you type and what the controllers

transmit can be complicated.

This means that if you have a data-dependent problem, you will probably have to do a lot of testing to find

it. You might find a file that either cannot be sent across your network or that takes much longer to transfer

than other similar-length files. You can then examine this file for repeated patterns that you can test using

the -p option of ping.