When the router or destination host informs something is corrupted in the IP datagram, the protocol used is the Internet Control Message Protocol (ICMP). The characteristics of ICMP include:
- ICMP uses IP
- ICMP reports crash
- ICMP cannot report crashes using ICMP messages, to avoid retrying.
- For fragmented data, ICMP messages only send corruption messages on the first fragmentation.
- ICMP messages do not respond by sending data via broadcast or multicast.
- ICMP will not respond to IP datagrams that do not have a sender IP header.
- ICMP messages can create corruption processes in IP datagrams.
The ICMP specification can be seen in RFC 792 with the update RFC 950.
1. ICMP Messages
ICMP messages are sent in IP Datagram. In the IP header, the protocol will contain the number 1 (ICMP). And the type of service (TOS) is 0 (routine). The ICMP format can be seen in Figure 5.1.
Figure 5.1 ICMP Message Format
Information:
Type : message type:
- 0 : Echo reply
- 3 : Destination Unreachable
- 4 : Source quench
- 5 : Redirect
- 8 : Echo
- 9 : Router Advertisement
- 10 : Router Solicitation
- 11 : Time exceeded
- 12 : Parameter problem
- 13 : Timestamp request
- 14 : Timestamp reply
- 15 : Information request (expired)
- 16 : Information reply (expired)
- 17 : Address mask request
- 18 : Address mask reply
- 30 : Traceroute
- 31 : Datagram conversion error
- 32 : Mobile host redirect
- 33 : IPv6 Where-are-you
- 34 : IPv6 I-Am-Here
- 35 : Mobile registration request
- 36 : Mobile registration reply
- 37 : Domain name request
- 38 : Domain name reply
- 39 : SKIP
40 : Photuris.
- Code: contains the response to the damage report from the ICMP message sent.
- Checksum: used to check for damage to ICMP messages sent.
- Data: contains the ICMP message sent. Explanation of the ICMP message type.
1.1. Echo (8) and Echo reply (0)
Echo is used to check the activity of a host. Where if the host is active it will be replied with an Echo Reply message.
1.2. Unreachable Destinations (3)
This message comes from a router informing that the destination host is unreachable.
1.3. Source Quench (4)
This message comes from a router where the router does not have buffer space to forward the datagram.
1.4. Redirect (5)
This message originates from the router, where the host must send the next datagram to the router on the network that the ICMP message is addressed to.
1.5. Router Advertisement (9) and Router Solicitation (10)
This message is used by routers that have the discover protocol. Where the router will provide an IP address to its network and the host that receives the IP address will reply with a Router Solicitation message.
1.6. Time Exceeded (11)
This message comes from a router, where the message sent has timed out according to the TTL limit.
1.7. Problem Parameters (12)
This message is caused by an error in the process of preparing to send an ICMP message.
1.8. Timestamp Request (13) and Timestamp Reply (14)
This message is used for debugging process.
1.9. Information Request (15) and Information Reply (16)
The message used to obtain an IP address, this message has been replaced by ARP and RARP.
1.10. Address Mask Request (17) and Address Mask Reply (18)
This message is used to get the subnetmask of a network.
Understanding ICMP Applications
Examples of applications that use the ICMP protocol include: PING and TRACEROUTE.
1. PING
Ping is the simplest program of TCP/IP application. Ping sends IP datagram to a host and measures the round trip time and receives the response. Ping is short for Packet Internet Groper.
Ping uses ICMP echo and echo reply messages.
Ping can also be used to ensure the installation of IP addresses on a host. The steps that can be taken are:
- Ping loopback: test TCP/IP software
- Ping my IP address : test network devices on that host
- Ping the IP address of another host: test whether the path is correct
- Ping the name of a host: test whether the DNS system is running.
2. TRACEROUTE
The traceroute application traces all paths taken to reach a destination host.
How traceroute works by sending a message with TTL = 1. Where if it has reached a target the TTL number will be 0, and this will give a message to the sender with a time exceeded message, so the host will send another ICMP message with an enlarged TTL value. This process is carried out continuously until it reaches the target host.
Understanding Internet Group Management Protocol (IGMP)
IGMP is used to check whether a host can join IP Multicast. The IGMP protocol provides facilities for routers to check hosts that are interested in using the multicast system.
The IGMP specification can be seen in RFC 1112 with updates in RFC 2236.
Unicast, Broadcast, Multicast & Anycast Delivery Methods
Data delivery on IP address is generally 1 delivery package, this is called Unicast. Unicast connection is a connection with a one-to-one relationship between 1 sender address and 1 recipient address.
For recipients with more than 1 number, there are several delivery methods, namely broadcast, multicast and anycast. Can be seen in
Figure 4.5 Data transmission mode
1. Broadcast
Sending data to all addresses in 1 network, this data sending mode is called Broadcast. Applications that use this method will send to the broadcast address. Example 192.168.0.255, if sending data to this address then all hosts in the network will receive the data.
2. Multicast
Sending data to a group address in 1 network, this data delivery mode is called Multicast. This address uses class D, so some hosts will be registered using this class D address. If there is a sender who sends data to this class D address, it will be forwarded to the hosts that have been registered in this class D IP.
3. Anycast
If a service uses several different IP addresses, then if a sender sends data to the service, it will be forwarded to one of the IP addresses, this delivery mode is called Anycast. Example: If there are 5 servers with the same FTP application, then if a user accesses the FTP service, it will be directed to one of the 5 servers.