Week9 Internet Control Message Protocol Objectives Upon completion

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 1

Figure 9. 1 TCP/IP Protocol Suite Position of ICMP in the network layer 2

Figure 9. 2 ICMP encapsulation Note that ICMP messages are encapsulated within IP datagrams. TCP/IP Protocol Suite 3

Table 9. 1 ICMP messages TCP/IP Protocol Suite ICMP messages are divided into error-reporting messages and query messages. The errorreporting messages report problems that a router or a host (destination) may encounter. The query messages get specific information from a router or another host. 4

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 5

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 6

Figure 9. 5 TCP/IP Protocol Suite Error-reporting messages 7

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 127. 0. 0. 0 or 0. 0. TCP/IP Protocol Suite 8

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 9

Figure 9. 7 TCP/IP Protocol Suite Destination-unreachable format 10

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 11

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 12

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 13

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 14

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 15

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 16

Figure 9. 11 TCP/IP Protocol Suite Redirection concept 17

Figure 9. 12 TCP/IP Protocol Suite Redirection message format 18

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 19

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 20

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 21

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 22

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 23

Note: Timestamp-request and timestampreply 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 24

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

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 26

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 27

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 28

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 29

Figure 9. 18 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 30

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 31

Figure 9. 21 TCP/IP Protocol Suite ICMP package 32