1. Internet Control Message Protocol
Week-9
Internet Control Message Protocol
Objectives
Upon completion you will be able to:
Be familiar with the ICMP message format
Know the types of error reporting messages
Know the types of query messages
Be able to calculate the ICMP checksum
Know how to use the ping and traceroute commands
Understand the modules and interactions of an ICMP package
TCP/IP Protocol Suite

2. Figure 9.1 Position of ICMP in the network layer
TCP/IP Protocol Suite

3. Figure 9.2 ICMP encapsulation
Note that ICMP messages are encapsulated within
IP datagrams.
TCP/IP Protocol Suite

4. Table 9.1 ICMP messages
ICMP messages are divided into error-reporting messages and query messages. The error-reporting messages report problems that a router or a host (destination) may encounter. The query messages get specific information from a router or another host.
TCP/IP Protocol Suite

5. Figure 9.4 General format of ICMP messages
All the ICMP messages are different. The only thing that is
common amongst all is the first 4 bytes.
Type field tells what type of message (as we saw on the
previous slide); Code field gives further information on the
type of message.
TCP/IP Protocol Suite

6. 9.3 ERROR REPORTING
IP, as an unreliable protocol, is not concerned with error checking and error control. ICMP was designed, in part, to compensate for this shortcoming.
ICMP does not correct errors, it simply reports them.
ICMP always reports the message back to the source.
The topics discussed in this section include:
Destination Unreachable
Source Quench
Time Exceeded
Parameter Problem
Redirection
TCP/IP Protocol Suite

7. Figure 9.5 Error-reporting messages
TCP/IP Protocol Suite

8. The following are important points about ICMP error messages:
Note:
The following are important points about ICMP error messages:
❏ No ICMP error message will be generated in response to a datagram carrying an ICMP error message. ❏ No ICMP error message will be generated for a fragmented datagram that is not the first fragment. ❏ No ICMP error message will be generated for a datagram having a multicast address. ❏ No ICMP error message will be generated for a datagram having a special address such as or
TCP/IP Protocol Suite

9. Figure 9.6 Contents of data field for the error messages
Destination Unreachable
When a router cannot route a datagram or a host cannot
deliver a datagram, the datagram is discarded and the router
or host sends a destination-unreachable ICMP message.
There are 15 different codes:
network unreachable(0); host unreachable(1); protocol
unreachable(2); port unreachable(3); fragmentation required
but DF bit set(4); source routing cannot be accomplished (5);
destination network unknown(6); destination host unknown(7);
source host isolated(8); communication with destination network
is prohibited(9); and six more
TCP/IP Protocol Suite

10. Figure 9.7 Destination-unreachable format
TCP/IP Protocol Suite

11. Other destination-unreachable messages can be created only by routers.
Note:
Destination-unreachable messages with codes 2 or 3 can be created only by the destination host.
Other destination-unreachable messages can be created only by routers.
TCP/IP Protocol Suite

12. Figure 9.8 Source-quench format
There is no flow control mechanism in IP, so ICMP
allows a router to tell a source to slow down (we
just discarded one of your datagrams because our
buffers are full). This is source quench.
One source quench message is sent for each datagram
discarded.
TCP/IP Protocol Suite

13. Figure 9.9 Time-exceeded message format
When a router decrements a Time-to-Live counter to zero,
the datagram is tossed and a time-exceeded message is
returned to the source.
Likewise if a destination does not receive all the fragments
in a set amount of time.
TCP/IP Protocol Suite

14. Note:
In a time-exceeded message, code 0 is used only by routers to show that the value of the time-to-live field is zero. Code 1 is used only by the destination host to show that not all of the fragments have arrived within a set time.
TCP/IP Protocol Suite

15. Figure 9.10 Parameter-problem message format
A parameter problem message can be created by a router
or the destination host.
Code 0 - there is an error or ambiguity in one of the header
fields.
Code 1 - the required part of an option is missing.
TCP/IP Protocol Suite

16. Figure 9.11 Redirection concept
Routers are constantly updating themselves with new
routing information.
Not so with hosts. A host has to know where the next
router is too, but its table is usually static (for efficiency).
Host routing tables start off small, usually with only one
entry - the default router.
When the host sends a datagram to the wrong router
(because its routing table is incomplete), the router that
receives the datagram sends the datagram to the
correct router AND sends a redirection message to the
host.
The host uses this info to update its routing table.
TCP/IP Protocol Suite

17. Figure 9.11 Redirection concept
TCP/IP Protocol Suite

18. Figure 9.12 Redirection message format
TCP/IP Protocol Suite

19. 9.4 QUERY
ICMP can also diagnose some network problems through the query messages, a group of four different pairs of messages. In this type of ICMP message, a node sends a message that is answered in a specific format by the destination node.
The four different types of request and reply are:
Echo Request and Reply
Timestamp Request and Reply
Address-Mask Request and Reply
Router Solicitation and Advertisement
TCP/IP Protocol Suite

20. Note:
An echo-request message can be sent by a host or router. An echo-reply message is sent by the host or router which receives an echo-request message.
TCP/IP Protocol Suite

21. If a router returns a reply, then it and IP are working.
Note:
Echo-request and echo-reply messages can be used by network managers to check the operation of the IP protocol.
If a router returns a reply, then it and IP are working.
TCP/IP Protocol Suite

22. Note:
Echo-request and echo-reply messages can test the reachability of a host. This is usually done by invoking the ping command.
TCP/IP Protocol Suite

23. Figure 9.14 Echo-request and echo-reply messages
Identifier field and Sequence number field are not used
and can be arbitrarily set by the sender.
Identifier field is often the same as the process ID.
TCP/IP Protocol Suite

24. Note:
Timestamp-request and timestamp- reply messages can be used to calculate the round-trip time between a source and a destination machine even if their clocks are not synchronized.
TCP/IP Protocol Suite

25. Note:
The timestamp-request and timestamp- reply messages can be used to synchronize two clocks in two machines if the exact one-way time duration is known.
TCP/IP Protocol Suite

26. Figure 9.15 Timestamp-request and timestamp-reply message format
Sending time = receive timestamp - original timestamp
Receiving time = returned time - transmit time
Round-trip time = sending time + receiving time
TCP/IP Protocol Suite

27. Figure 9.16 Address-mask-request and address-mask-reply message format
If a thin-client host does not know its IP mask, it may
issue an address-mask-request. Can also use BOOTP
or DHCP to obtain this information.
Address mask is 0 in request message.
Identifier and Sequence number fields are not used.
TCP/IP Protocol Suite

28. Figure 9.17 Router-solicitation message format
How does a host find out what routers are connected to its
own network? Or if those routers are functioning? It can
send out a router-solicitation message.
This can be broadcast on current network.
Identifier and Sequence number fields are not used.
TCP/IP Protocol Suite

29. Figure 9.18 Router-advertisement message format
This is the reply that comes back from the previous request.
Lifetime field shows the number of seconds that the entries
are considered to be valid.
If address preference = 0, then this should be the default router.
TCP/IP Protocol Suite

30. Figure 9.18 Ping and Traceroute
Ping and Traceroute (in Unix) and Ping and Tracert in Microsoft
both use ICMP and its various options to perform.
For the Ping command, the source host sends ICMP echo request
messages (type 8, code 0); the destination,if alive, responds
with ICMP echo reply messages.
TCP/IP Protocol Suite

31. 9.7 ICMP PACKAGE
To give an idea of how ICMP can handle the sending and receiving of ICMP messages, we present our version of an ICMP package made of two modules: an input module and an output module.
The topics discussed in this section include:
Input Module
Output Module
TCP/IP Protocol Suite

32. Figure ICMP package
TCP/IP Protocol Suite