132
Chapter 3
Internet Protocols
Buffer Full If a router's memory buffer for receiving incoming data-
grams is full, it will use ICMP to send out this message.
Hops Each IP datagram is allotted a certain number of routers, called
hops, that it may go through. If it reaches its limit of hops before arriving
at its destination, the last router to receive that datagram deletes it. The
executioner router then uses ICMP to send an obituary message, inform-
ing the sending machine of the demise of its datagram.
Ping Ping (Packet Internet Groper) uses ICMP echo messages to check
the physical connectivity of machines on an internetwork.
Traceroute Using ICMP timeouts, Traceroute is used to discover the
path a packet takes as it traverses an internetwork.
Both Ping and Traceroute (also just called Trace; Microsoft Windows uses
tracert) allow you to verify address configurations in your internetwork.
The following data is from a network analyzer catching an ICMP echo
request. Take a look at it--notice anything unusual? Do you catch the fact
that even though ICMP works at the Internet layer, it still uses IP to do the
Ping request? The Type field in the IP header is 0x01, which specifies the
ICMP protocol.
Flags: 0x00
Status: 0x00
Packet Length: 78
Timestamp: 14:04:25.967000 05/06/2002
Ethernet Header
Destination: 00:a0:24:6e:0f:a8
Source: 00:80:c7:a8:f0:3d
Ether-Type: 08-00 IP
IP Header - Internet Protocol Datagram
Version: 4
Header Length: 5
Precedence: 0
Type of Service: %000
Unused: %00
Copyright ©2002 SYBEX, Inc., Alameda, CA
www.sybex.com