ICMP Messages

Explain how ICMP is used to test network connectivity.

ICMPv4 and ICMPv6 Messages

• Internet Control Message Protocol (ICMP) provides feedback about issues related to the processing of IP packets under certain conditions.
• ICMPv4 is the messaging protocol for IPv4.
• ICMPv6 is the messaging protocol for IPv6 and includes additional functionality.
• The ICMP messages common to both ICMPv4 and ICMPv6 include:
• • Host reachability
  • Destination or Service Unreachable
  • Time exceeded

Note: ICMPv4 messages are not required and are often not allowed within a network for security reasons.

Host Reachability

• ICMP Echo Message can be used to test the reachability of a host on an IP network.

• In the example:
  • The local host sends an ICMP Echo Request to a host.
  • If the host is available, the destination host responds with an Echo Reply.

Destination or Service Unreachable

• An ICMP Destination Unreachable message can be used to notify the source that a destination or service is unreachable.
• The ICMP message will include a code indicating why the packet could not be delivered.
• A few Destination Unreachable codes for ICMPv4 are as follows:
  • 0 – Net unreachable
  • 1 – Host unreachable
  • 2 – Protocol unreachable
  • 3 – Port unreachable
• A few Destination Unreachable codes for ICMPv6 are as follows:
  • 0 – No route to destination
  • 1 – Communication with the destination is administratively prohibited (e.g., firewall)
  • 2 – Beyond scope of the source address
  • 3 – Address unreachable
  • 4 – Port unreachable

Note: ICMPv6 has similar but slightly different codes for Destination Unreachable messages.

Time Exceeded

• When the Time to Live (TTL) field in a packet is decremented to 0, an ICMPv4 Time Exceeded message will be sent to the source host.
• ICMPv6 also sends a Time Exceeded message.
• Instead of the IPv4 TTL field, ICMPv6 uses the IPv6 Hop Limit field to determine if the packet has expired.

Note: Time Exceeded messages are used by the traceroute tool.

ICMPv6 Messages

• ICMPv6 has new features and improved functionality not found in ICMPv4, including four new protocols as part of the Neighbor Discovery Protocol (ND or NDP).
• Messaging between an IPv6 router and an IPv6 device, including dynamic address allocation are as follows:
• • Router Solicitation (RS) message
  • Router Advertisement (RA) message
• Messaging between IPv6 devices, including duplicate address detection and address resolution are as follows:
• • Neighbor Solicitation (NS) message
  • Neighbor Advertisement (NA) message

Note: ICMPv6 ND also includes the redirect message, which has a similar function to the redirect message used in ICMPv4.

• RA messages are sent by IPv6-enabled routers every 200 seconds to provide addressing information to IPv6-enabled hosts.
• RA message can include addressing information for the host such as the prefix, prefix length, DNS address, and domain name.  
• A host using Stateless Address Autoconfiguration (SLAAC) will set its default gateway to the link-local address of the router that sent the RA.
• An IPv6-enabled router will also send out an RA message in response to an RS message.
• In the figure, PC1 sends a RS message to determine how to receive its IPv6 address information dynamically.
  • R1 replies to the RS with an RA message.
  • PC1 sends an RS message, “Hi, I just booted up. Is there an IPv6 router on the network? I need to know how to get my IPv6 address information dynamically.”
  • R1 replies with an RA message. “Hi all IPv6-enabled devices. I’m R1 and you can use SLAAC to create an IPv6 global unicast address. The prefix is 2001:db8:acad:1::/64. By the way, use my link-local address fe80::1 as your default gateway.”

• A device assigned a global IPv6 unicast or link-local unicast address, may perform duplicate address detection (DAD) to ensure that the IPv6 address is unique.
• To check the uniqueness of an address, the device will send an NS message with its own IPv6 address as the targeted IPv6 address.
• If another device on the network has this address, it will respond with an NA message notifying to the sending device that the address is in use.

Note: DAD is not required, but RFC 4861 recommends that DAD is performed on unicast addresses.

• To determine the MAC address for the destination, the device will send an NS message to the solicited node address.
• The message will include the known (targeted) IPv6 address.
• The device that has the targeted IPv6 address will respond with an NA message containing its Ethernet MAC address.
• In the figure, R1 sends a NS message to 2001:db8:acad:1::10 asking for its MAC address.

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