Transcript Chapter 23 Internet Control Message Protocol

Chapter 23
Internet Control Message
Protocol

used to report information and errors
 IP uses ICMP when it sends an error
message
 ICMP uses IP to transport messages

ICMP Error Messages
(fig 23.1)
Source Quench
– used by router when it has discarded datagram due to
unavailable buffer memory.
– sent to source computer requesting it to slow down rate
of data transmission.

Time Exceeded
– whenever the TIME TO LIVE field in a datagram is
decremented to 0, the router discards the datagram and
sends a time exceeded message
– eg. Traceroute sends datagrams to a destination host
with TIME TO LIVE field set to 1,2,3… so that routers
along the path will send time exceeded message back,
allowing source to know of path or routes taken by
datagram.
ICMP Error Messages (cont.)

Destination Unreachable
– sent by router to source host whenever router
determines that a datagram cannot be delivered to its
final destination. A “host unreachable” or “network
unreachable” message is sent.

Redirect message
– sent by router to source host whenever router
determines that a host has incorrectly sent a datagram
that should be sent to a different router.

Fragmentation required message
– sent by a router to a source host if the router needs to
fragment a datagram but the DO NOT FRAGMENT bit
had been set in the datagram header. Such a datagram
is discarded by the router.
ICMP Informational
Messages

Echo Request message
– sent to a computer by setting the PROTOCOL TYPE
field in a IP header to 1, corresponding to ICMP
– Used by Ping to test for reachability.

Echo Reply message
– Sent by ICMP software on destination computer in
response to echo request.

address mask request & reply message
– can be broadcasted by host during bootup, asking for
router to send an address mask reply that contains the
correct 32-bit subnet mask for the network.
Encapsulation

ICMP messages are encapsulated in a IP
datagram
 IP datagram in turn is encapsulated in a
frame.
Path MTU

the smallest MTU along a path from a
source to a destination.
 can be found by sending increasing larger
datagrams with DO NOT FRAGMENT bit
set in the FLAGS field in the IP datagram
header.
– Eventually a ICMP fragmentation required
message will be sent by a router to the source
computer.