Internet Protocol:
Error and Control Messages
(ICMP)
Chapter 8
We have

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Unreliable
Connectionless
datagram delivery…
Need a way to inform original source if:

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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
Internet Control Message
Protocol
Routers operate autonomously
If everything works correctly, all ok.
But:
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Communication lines fail
Processors fail
Destination machines get disconnected
from the network
TTL counters expire
Intermediate routers get congested
In an internet, no special hardware
mechanisms to inform hosts of problems


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Don’t know if failure is local or remote problem
Debugging is difficult
IP has nothing itself to help out
Added special-purpose message mechanism
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Internet Control Message Protocol (ICMP)
Considered a required part of IP
ICMP messages travel in data portion of
IP datagrams
ICMP:
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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
Error Reporting vs Correction
ICMP is technically a reporting mechanism
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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
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Some do not
ICMP only informs original source
Source may not be able to determine problem
Have to restrict communication to original
source:
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Datagrams don’t usually record complete route
Can’t 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
ICMP Message Delivery
ICMP uses two levels of encapsulation
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ICMP message in datagram data portion
Datagram in frame data portion
Datagrams carrying ICMP messages:
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Are forwarded the same way as others
May themselves get lost or discarded
May cause further congestion
Figure 8.1
Exception for error messages

ICMP messages not generated if error
results from datagram carrying ICMP
error message
ICMP not a higher-level protocol

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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
ICMP Message Format
Each ICMP message has own format
But, all begin with three fields:
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8-bit integer message TYPE
8-bit CODE field
16-bit CHECKSUM
Also includes first 64 data bits of
datagram causing the problem
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
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Verifies source, outbound path, destination, and
return path
Sophisticated versions send a series of requests
and provide statistics about datagram loss
C:\>ping google.com
Pinging google.com [64.233.187.99] with 32 bytes of data:
Reply from 64.233.187.99: bytes=32 time=31ms TTL=239
Reply from 64.233.187.99: bytes=32 time=32ms TTL=239
Reply from 64.233.187.99: bytes=32 time=31ms TTL=239
Reply from 64.233.187.99: bytes=32 time=30ms TTL=239
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:\>
Echo request and reply message format:
0
8
Type (8 or 0)
16
31
Code (0)
Checksum
Identifier
Sequence Number
Optional Data
...
Unreachable Destination (3)
Sent when router cannot forward or
deliver a datagram
0
8
Type (3)
16
Code (0-12)
31
Checksum
Unused (must be zero)
Internet header + first 64 bits of datagram
...
Discarding datagrams not taken lightly
Messages sent when router determines
destination unreachable
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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 doesn’t provide acknowledgements
Source Quench (4)
Congestion: routers overrun with traffic
Can happen for two reasons:
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High-speed computer generate traffic too
fast for network to handle
Many computers sending thru same router
Datagrams buffered if arrive too fast
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Helps with small burst
If continues, memory gets exhausted
Source quench used to report congestion
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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
0
8
Type (4)
16
Code (0)
31
Checksum
Unused (must be zero)
Internet header + first 64 bits of datagram
...
Redirect (Route Change)(5)
Routers are assumed to know correct
routes
Hosts begin with minimal forwarding info
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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
Limitation:
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Only have interactions between directly
connected host and router
Later routers cannot send ICMP redirect
Don’t know non-directly connected router’s address

Redirect not used to solve general problem of
propagating routing information
0
8
Type (5)
16
Code (0 to 3)
31
Checksum
Router Internet Address
Internet header + first 64 bits of datagram
...
Code 0: redirect for net
Code 1: redirect for host
Code 2: redirect for TOS and net
Code 3: redirect for TOS and host
Figure 8.7
Time Exceeded (11)
Can get routing cycles
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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:
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Sent when datagram discarded due to TTL=0
Or, when timed out waiting for datagram
fragments
0
8
Type (11)
16
Code (0 or 1)
31
Checksum
Unused (must be zero)
Internet header + first 64 bits of datagram
...
Code 0: TTL count exceeded
Code 1: Fragment reassembly time exceeded
Parameter Problem (12)
Problem exists with a datagram
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Severe enough to cause it to be discarded
Uses pointer to ID octet that caused the problem
0
8
Type (12)
Pointer
16
Code (0 or 1)
31
Checksum
Unused (must be zero)
Internet header + first 64 bits of datagram
...
Code 1: required option is missing (pointer field not used)
Timestamp Request & Reply (13 or 14)
Can have clock synchronization problem
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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
0
8
Type (13 or 14)
16
31
Code (0)
Identifier
Checksum
Sequence Number
Originate Timestamp
Receive Timestamp
Transmit Timestamp
From originate time:
- can compute total time
From receive & transmit times:
- can compute network transit time
Then estimate differences in clocks
Information Request and Reply (15 or 16)
*OBSOLETE*
Intended to allow hosts to discover internet
address at startup
Made obsolete by DHCP
Address Mask Request & Reply (17 or 18)
*OBSOLETE*
Chap 9 – motivation for subnets
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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
Router Solicitation & Advertisement
*OBSOLETE*
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)
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Routers advertise their information periodically
Host discards route when timer for it expires
Summary
Routers may need to communicate with
network software on a particular host
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Report abnormal condition
Send control information
Internet Control Message Protocol is
used
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Travels in data area of IP datagram
Three fixed-length fields at the beginning
Message type determines rest of format