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Troubleshooting IP
- Internet Control Message Protocol (ICMP) is a TCP/IP protocol that was designed specifically
to help manage and control the operation of TCP/IP networks. It provides a wide variety of
information about the operational status of a network.
- ICMP is considered as a part of the network layer. ICMP messages are encapsulated within IP
packets and sent using the basic IP header only, with no transport layer header at all – it is really
just an extension of the network layer.
- Below lists some ICMP message types and their usages:
Message Usage
Echo Request (Type 8)
and Echo Reply (Type 0)
Used by the ping command when testing network connectivity.
Destination Unreachable Used to notify the source host that there is a problem when
(Type 3)
delivering a packet.
Redirect (Type 5) Sent by a router to notify the sender to use a better route for
subsequent connection attempts.
Time Exceeded
(Type 11)
Used to notify a host when a packet sent by it has been discarded
due the time for a packet to exist in a network (Time-to-Live,
TTL) when being delivered to the destination has expired.
- ICMP Destination Unreachable messages are sent to notify the sender when a message cannot
be delivered to the destination host. Packet delivery can fail with many reasons. Below lists the 5
common ICMP Unreachable codes that can be sent in ICMP Destination Unreachable messages:
Unreachable Code When it is being sent? Typically sent by
Network Unreachable Unable to match a packet’s destination network Routers
(Code 0)
in the routing table and therefore unable to
forward the packet.
Host Unreachable
(Code 1)
Can’t Fragment
(Code 4)
Protocol Unreachable
(Code 2)
Port Unreachable
(Code 3)
A packet can be routed to the router attached to
the destination network, but the destination host
is not responding to ARP Request sent by the
router.
Note: Cisco routers do not generate
Host Unreachable ICMP messages.
A packet has the Don’t Fragment (DF) bit set,
but the router must fragment the packet in order
to forward it.
A packet is delivered to the destination host,
but the transport layer protocol is not available
or running on that host. This is very unlikely to
happen as most operating systems that support
TCP/IP should have provided IP, TCP and UDP
services.
A packet is delivered to the destination host,
but there is no application listening to the
destination port number.
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Routers
Routers
Endpoint hosts
Endpoint hosts
Copyright © 2008 Yap Chin Hoong
yapchinhoong@hotmail.com