Download Internet Control Message Protocol (ICMP) Chapter 7

Internet Control Message
Protocol (ICMP)
Chapter 7
ICMP
• One of the fundamental protocols that is
part of the TCP/IP suite
• It is used to control and manage information
that is transmitted using TCP/IP
• It allows nodes to share status and error
information
• Helps to re-route messages when a route is
busy or has failed
ICMP – Cont.
• A very common use of ICMP is the passing of
error information between host and router
• ICMP helps to keep the network running when a
router goes off line
• All messages are carried as IP datagrams
• Dynamic routers use ICMP messages to announce
the contents of their routing tables
• Hosts listen for these messages to acquire routing
information
• A router solicitation may be sent by a host when it
starts up. This is a request for immediate ads
When are ICMP Messages
Generated?
• A packet cannot reach its destination-sent by
router that cannot send the IP datagram any further
• A packet’s Time to Live (TTL) expires-reaches its
limit of hops before getting to destination-router
discards it and sends ICMP messageto sending
machine
• An error is detected in IP header data
• A router cannot buffer an incoming packer-buffer
in router is full
ICMP Messages – Cont.
• A router cannot keep up with an incoming
stream of packets
• A router sends a message to a host advising
of a shorter route to a destination
• A host sends an echo packet to determine if
another host is alive (ping)
• A host needs to determine the local network
ICMP – Cont.
• ICMP doesn’t make IP reliable
• It provides no facilities to provide
retransmission of IP datagrams that
encounter errors.
ICMP Echo/Echo Reply
• One of the most heavily used ICMP features
is the ability to send a simple message to an
IP node and have the message echoed back
to the sender.
• This is handy for network troubleshooting
and debugging.
• Ping and Tracert both use Echo/Echo Reply
• See p. 165 for message format
Ping Utility
• Tests reachability, name resolution, and
routing
• -t :sends Echoes until interrupted
• -w timeout :waits a specified amount of
time, in milliseconds, for the Echo Reply.
Default is 1000
• See p. 184 to see how ping works
Tracert Utility
• Determines a path from a host to a
destination node
• The default action is to trace the route,
report the round-trip time, and the near-side
router IP address
• See p. 186 for operation of tracert
ICMP Redirect
• Host have minimal routing tables
• The routers keep all other routing
information in their tables
• If there are multiple routers on a network,
the possibility exists for non-optimal
routing
• See p. 176, Figure 7-8
• See p. 177, for details of redirect process