Module 5.3: Internet Control Message Protocol

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Module 5.3 Internet Control Message Protocol

• Why need ICMP?
• IP and ICMP
• ICMP Message Format
• ICMP Error Reporting messages
• ICMP Query messages
• ICMPv6

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ICMP

• IP provides an unreliable, connectionless
  datagram delivery service.
• Datagram travels from gateway to gateway until it
  reaches the destination hosts network, where it
  would be sent directly.
• Question is What happens if a gateway cannot
  route or deliver a datagram or if the gateway
  detects an unusual condition like network
  congestion?
• What mechanisms are there in TCP/IP suite for a
  gateway to inform the sender about an error in
  the IP Datagram deliver?
• How do gateways use the mechanism to report
  delivery problems and how do hosts use it to test
  whether or not destinations are reachable?
• In a connectionless system each gateway operates
  autonomously, routing or delivering datagrams
  without any coordination from the sender.
• Failures to communication lines and processors,
  or the expiry of time-to-live counter, or the
  congestion of intermediate gateways could disrupt
  the whole operation.
• The IP itself contains nothing to help sender
  test connectivity or learn about such failures.
  So we use ICMP.

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Encapsulation of ICMP message
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Format of ICMP message

• 16-bit CHECKSUM field (additive checksum covering
  the ICMP message (header options (if any)
  data) of the message ones complement sum of all
  16-bit words, with checksum originally being
  zero)

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Message Types

• 0 Echo Reply
• 3 Destination Unreachable
• 4 Source Quench
• 5 Redirect (change a route)
• 8 Echo Request
• 9 Router Advertisement
• 10 Router solicitation
• 11 Time Exceeded for a Datagram
• 12 Parameter Problem on a Datagram
• 13 Timestamp Request
• 14 Timestamp Reply
• 15 Information Request (obsolete)
• 16 Information Reply (obsolete)
• 17 Address Mask Request
• 18 Address Mask Reply

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(No Transcript)
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Error Reporting Messages
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Data field for error messages
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Error Reporting vs. Error Correction

• ICMP only reports error conditions to the
  original source the source must relate errors to
  individual application programs and take action
  to correct the problem.
• ICMP reports problems to the original source even
  if they are due to misbehaving intermediate
  gateways. This is due to the format of the IP
  header that has information about the source and
  destination and not the record of path.

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Destination Unreachable

• Code 0 15
• 0 network unreachable, 1host unreachable, 2
  protocol unreachable, 3 port unreachable, 4
  fragmentation needed and DF set, 5 source route
  failed, 6 destination network unknown, 7
  destination host unknown, 8 source host isolated,
  9 communication with destination network
  administratively prohibited, 10 communication
  with destination host administratively
  prohibited.

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Source-Quench

• There is no flow-control mechanism in IP
• A source-quench message informs the source that
  a datagram has been discarded due to congestion
  in a router or the destination host.
• The source must slow down the sending of
  datagrams until the congestion is relieved.
• One source-quench message should be sent for each
  datagram that is discarded due to congestion.

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Time-Exceeded

• Code 0 Time to live, Code 1 Fragmentation
• Whenever a router receives a datagram whose
  time-to-live field has the value of zero, it
  discards the datagram and sends a time-exceeded
  message to the original source.
• When the final destination does not receive all
  of the fragments in a set time, it discards the
  received fragments and sends a time-exceeded
  message to the original source.
• Traceroute program. How it works?
• MTU Path Discovery? How it works?

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Parameter-Problem

• Code 0 Main header problem,
• Code 1 Problem in the option field
• Bad header field or option field.

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Redirection

• If R2 was newly added, R1 forwards B message to
  R2 and tells host A to redirect future B messages
  to R2.
• Code field specifies redirect packet for host,
  for network, for host and TOS, and for network
  and TOS.

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Query Messages
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Echo-Request and Echo-Reply

• Ping program uses these to check if a host is
  alive.
• Note that Identifier and Sequence Number fields
  are used to match Request and replies.

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Timestamp-Request and Timestamp-Reply

• Timestamp is 32 bit field in ms. Original
  timestamp is filled by sender. Receive timestamp
  is filled by Receiver. Transmit timestamp is
  filled by receiver just before sending the Reply.
• Sending time value of receive timestamp
  value of original timestamp
• Receiving time time the packet returned
  value of transmit timestamp
• Round-trip time sending time receiving time

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Mask Request and Mask Reply

• This is used to obtain the network subnet mask.
• The Request can be sent directly to a router, or
  broadcast to on the network.
• This is used by diskless hosts and alternative to
  BOOTP and DHCP protocol

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Router Solicitation

• ICMP router discovery consists of Router
  Solicitation and Router Discovery
• Discovery is initiated by the host and only
  pertaining to discovering local routers on
  directly connected network.
• Router Solicitation has a multicast address of
  224.0.0.2
• Router Advertisement is sent in reply to
  Solicitation message, and also is sent
  periodically (as a multicast) every 10 minutes.
• Router Advertisement message can be sent by all
  routers or by one designated router.

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Router Advertisement

• List of IP addresses of routers that are
  available with a preference indicating for the
  host the priority of being used as a default
  router.
• Lifetime filed indicates the age of such
  information.

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ICMPv6
Type of Message V4 V6 Destination unreachable Yes
Yes Source quench Yes No Packet too
big No Yes Time exceeded Yes Yes Parameter
problem Yes Yes Redirection Yes Yes
Type of Message V4 V6 Echo request and
reply Yes Yes Timestamp request and
reply Yes No Address mask request and
reply Yes No Router solicitation
Yes Yes Neighbor solicitation ARP
Yes Group membership IGMP Yes