Internet Protocol: Error and Control Messages ICMP

1
Internet Protocol Error and Control Messages
(ICMP)

• Chapter 8

2

• We have
• Unreliable
• Connectionless
• datagram delivery
• Need a way to inform original source if
• Router cannot forward or deliver a datagram
• Router detects unusual condition affecting the
  ability to forward a datagram
• Original source will need to avoid or correct the
  problem

3
Internet Control Message Protocol

• Routers operate autonomously
• If everything works correctly, all ok. But
• Communication lines fail
• Processors fail
• Destination machines get disconnected from the
  network
• TTL counters expire
• Intermediate routers get congested

4

• In an internet, no special hardware mechanisms to
  inform hosts of problems
• Dont know if failure is local or remote problem
• Debugging is difficult
• IP has nothing itself to help out
• Added special-purpose message mechanism
• Internet Control Message Protocol (ICMP)
• Considered a required part of IP

5

• ICMP messages travel in data portion of IP
  datagrams
• ICMP
• Allows sending of error or control messages to
  other routers or hosts
• Provides communication between IP software on one
  machine and IP software on another
• ICMP module handles ICMP messages
• Any machine can send an ICMP message

1
6
Error Reporting vs Correction
2

• ICMP is technically a reporting mechanism
• ICMP only reports error condition to source
• Source must relate error to a specific
  application program or take other action
• Most errors stem from original source
• Some do not
• ICMP only informs original source
• Source may not be able to determine problem

7

• Have to restrict communication to original
  source
• Datagrams dont usually record complete route
• Cant know path taken to a given router
• So cannot inform intermediate routers
• Inform original source
• Hope host administrators and network
  administrators will cooperate to solve problem

3
8
ICMP Message Delivery

• ICMP uses two levels of encapsulation
• ICMP message in datagram data portion
• Datagram in frame data portion
• Datagrams carrying ICMP messages
• Are forwarded the same way as others
• May themselves get lost or discarded
• May cause further congestion

9
Figure 8.1
10

• Exception for error messages
• ICMP messages not generated if error results from
  datagram carrying ICMP error message
• ICMP not a higher-level protocol
• It is a required part of IP
• IP used to carry because messages may have to
  travel across several networks
• Cannot be delivered by physical transport alone

11
ICMP Message Format

• Each ICMP message has own format
• But, all begin with three fields
• 8-bit integer message TYPE
• 8-bit CODE field
• 16-bit CHECKSUM
• Also includes first 64 data bits of datagram
  causing the problem

12
Echo Reply and Request (0 8)

• Command on many systems is ping
• Sends echo request to a machine
• Receiver sends echo reply to sender
• Tests if destination is reachable responding
• Verifies source, outbound path, destination, and
  return path
• Sophisticated versions send a series of requests
  and provide statistics about datagram loss

13

• C\gtping google.com
• Pinging google.com 64.233.187.99 with 32 bytes
  of data
• Reply from 64.233.187.99 bytes32 time31ms
  TTL239
• Reply from 64.233.187.99 bytes32 time32ms
  TTL239
• Reply from 64.233.187.99 bytes32 time31ms
  TTL239
• Reply from 64.233.187.99 bytes32 time30ms
  TTL239
• Ping statistics for 64.233.187.99
• Packets Sent 4, Received 4, Lost 0 (0
  loss),
• Approximate round trip times in milli-seconds
• Minimum 30ms, Maximum 32ms, Average
  31ms
• C\gt

14

• Echo request and reply message format

15
Unreachable Destination (3)

• Sent when router cannot forward or deliver a
  datagram

16

• Discarding datagrams not taken lightly
• Messages sent when router determines destination
  unreachable
• Network unreachable usually forwarding failure
• Host unreachable usually delivery failure
• 13 types possible specified in CODE field
• However, routers cannot know all delivery
  failures
• Destination on Ethernet
• Network HW doesnt provide acknowledgements

17
Source Quench (4)

• Congestion routers overrun with traffic
• Can happen for two reasons
• High-speed computer generate traffic too fast for
  network to handle
• Many computers sending thru same router
• Datagrams buffered if arrive too fast
• Helps with small burst
• If continues, memory gets exhausted

18

• Source quench used to report congestion
• Requests source to reduce rate of transmission
• Usually one message per discarded datagram
• May be more sophisticated
• Quench source with highest transmission rate
• Avoid congestion by sending before overflow

19
Redirect (Route Change)(5)

• Routers are assumed to know correct routes
• Hosts begin with minimal forwarding info
• Usually know the address of single router
• Learn new routes from routers
• One way is the ICMP redirect message
• Router detects host using a non-optimal route
• Redirect requests the host change its route
• Host routing table stays small but optimal

20

• Limitation
• Only have interactions between directly connected
  host and router
• Later routers cannot send ICMP redirect
• Dont know non-directly connected routers
  address
• Redirect not used to solve general problem of
  propagating routing information
• Code 0 redirect for net Code 2 redirect
  for TOS and net
• Code 1 redirect for host Code 3
  redirect for TOS and host

21
Figure 8.7
22
Time Exceeded (11)

• Can get routing cycles
• R1 sends datagrams for D to R2
• R2 sends datagrams for D to R1
• TTL (hop count) timer used to stop circle
• ICMP time exceeded message
• Sent when datagram discarded due to TTL0
• Or, when timed out waiting for datagram fragments

23

• Code 0 TTL count exceeded
• Code 1 Fragment reassembly time exceeded

24
Parameter Problem (12)

• Problem exists with a datagram
• Severe enough to cause it to be discarded
• Uses pointer to ID octet that caused the problem
• Code 1 required option is missing (pointer
  field not used)

25
Timestamp Request Reply (13 or 14)

• Can have clock synchronization problem
• Widely different clocks can confuse operations
• Several protocols exist to synchronize clocks
• One of simplest
• Machine sends ICMP timestamp request message
• Receiver returns a timestamp reply

26

• From originate time
• - can compute total time
• From receive transmit times
• - can compute network transit time
• Then estimate differences in clocks

27
Information Request and Reply (15 or
16)OBSOLETE

• Intended to allow hosts to discover internet
  address at startup
• Made obsolete by DHCP

28
Address Mask Request Reply (17 or 18)OBSOLETE

• Chap 9 motivation for subnets
• Understand for now
• Some bits in hostid identify a physical network
• Host needs to know
• Which bits correspond to the physical network
• Which bits correspond to the host identifiers
• Subnet mask
• 32-bit quantity allowing address interpretation
• Intended to allow host to obtain the address mask
  used on the local network
• Made obsolete by DHCP

29
Router Solicitation AdvertisementOBSOLETE

• Intended to allow a host to discover routers
  available on the local network
• Made obsolete by DHCP
• Two differences from DHCP
• Host got info directly from router
• Prevented hosts from retaining routes after a
  router crashes (soft state technique)
• Routers advertise their information periodically
• Host discards route when timer for it expires

30
Summary

• Routers may need to communicate with network
  software on a particular host
• Report abnormal condition
• Send control information
• Internet Control Message Protocol is used
• Travels in data area of IP datagram
• Three fixed-length fields at the beginning
• Message type determines rest of format