1. Internet Control Message Protocol (ICMP)
IP provides unreliable and connectionless delivery
Provides unreliable delivery
Make efficient use of network resources
No error reporting, or correcting mechanism
No management of queries
Network manager might need information about a host/router
What happens if
Router discards a datagram ?
TTL expires ?
Host didn’t receive all datagram’s fragments ?

2. Position of ICMP in the network layer
And Encapsulation
Special purpose message mechanism added to the TCP/IP protocols
Destination of an ICMP message is the ICMP software module
ICMP is a network layer protocol, but its messages are first encapsulated into IP datagrams.

3. Error reporting vs. Error Correction
ICMP can only report an error to the original source
Up to the source to deal with it.
ICMP cannot be used to inform intermediate routers.
Source has no responsibility of routers problems.
Why restrict ICMP messages to original source?
Except for record route option, datagrams only contains source +
No global knowledge of routes (i.e. routers establish and change their own routing tables)

4. ICMP messages

5. General format of ICMP messages
Data section in
Error Messages carries information to find the original packet that had the error
Rest of Header unused (all 0s), except for Redirection message format
Query Messages carries extra information based on type of the query.
Rest of Header = Identifier (8 bits) + Sequence Number (8 bits)

6. Error-reporting messages
Important points about ICMP error messages: 1. No ICMP error message for a datagram carrying an ICMP error message No ICMP error message for a fragmented datagram that is not the first fragment No ICMP error message for a datagram having a multicast address No ICMP error message for a datagram with a special address such as or

7. Contents of data field for error messages
In ICMP error messages
The first 8 bytes of the Transport layer header is included
Provides Information about the port numbers (TCP or UDP) and sequence number (TCP)

8. Destination-unreachable (Type 3)
ICMP destination unreachable message for codes = 2 and 3 only created by a host
All others are created by a routers
!!! Routers cannot detect all Problems that prevent
the delivery of a packet.

9. Source-quench (type =4, code =0)
IP do not provide a flow-control mechanism
Source never knows if routers of destination is congested
A source-quench message informs the source that a datagram has been discarded due to congestion in a router or the destination host
Two purposes: (1) informs the source of dropped packet (2) inform of congestion along the path
Source must slow down (quench) the sending of datagrams until the congestion is relieved.
One source-quench message should be sent to each datagram that has been discarded due to congestion
One-to-one congestion
Many-to-one congestion (congested router has no idea which source is sending datagrams faster)

10. Time-exceeded message (Type 11)
If router receives a datagram with TTL = 0
Discard the datagram
Inform the source using a Time-exceeded message (code = 0)
If a host does not receive all fragments of a datagram during within a certain time of receiving the first fragment
Discard all fragments
Inform the source using a Time-exceeded message (code = 1)
Code 0: used only by routers
Code 1: used only by Hosts

11. Parameter-problem message
A parameter problem message is created by a router or destination host
If there is an error or ambiguity in the header field (code = 0), pointer points to the byte with problem
An option is missing or incorrect (code = 1) pointer not used
Router discards the datagram and sends a Parameter-problem message

12. Redirection message format
Updates to routers’ routing tables are dynamic
Updates to hosts’ routing tables are static
Starts with the small routing tables that gets updated one of the tools is redirection message format
Redirection always sent from a router to a host in the same network
Code 0: Network specific Code 1: Host specific Code 2: Network specific (specified service)
Code 3: Host specific (specified service)

13. Query messages Diagnose some network problems.
Information request/reply is now obsolete (replaced by RARP/BOOTP)

14. Timestamp-request and timestamp-reply message format
Can be used to determine RTT needed for an IP datagram to travel between two machines.
Identifier and Sequence fields allows machine to associate request with replies

15. Timestamp-request and timestamp-reply message
Can be used to synchronize two machines clocks.
Example:
Orig = , recv = ; trans = ;
With RTT = 2 ms, |diff| = 6 ms; implies that recv is 7 ms late.
Orig = , recv = ; trans = ;
With RTT = 2 ms; |diff|=336 ms Timestamp req/reply is useless
In general:
SendingT = recv – orig
ReceivingT = packet arrived – trans
RTT = (sendingT + ReceivingT)
If: One_way_time = RTT/2
outOfSyncT = recv – (orig + RTT/2)

16. Router solicitation message format
An important issue is how to accommodate routers in the same netwok:
Designers provided routers advertisement,
Default value 10 mn
Compromise between rapid failure detection and low overhead.
From a Host point of view this delay is very expensive
Example: when a host boots cannot wait 10 mn
Designers provide Router solicitation message to request immediate advertisement.
Host multicasts/broadcasts a router solicitation message
Identifier + Sequence number not used.

17. Router Advertisement
Static routing works well for a network that has only one router. (no need to discover routes or change routes)
For a network with many routers, and if a router crashes, then host uses router advertisement message.
ICMP route discovery helps in:
Host can get the router address from the router itself, instead via a bootstrap protocol (i.e. static configuration)
Use of Timers to update routes (Soft state technique)

18. Router advertisement message format
Number of addresses (that follows usually 1)
Address size (IPv4 = 1)
Lifetime: time to use the specified address (a default 30 mn)
Address Preference: Shows the preference of the router’s address.

19. Mask-request and mask-reply message format
In general to request a subnet mask from a router
Diskless machine can use Mask-request to get its subnet mask.

20. Echo-request and echo-reply message format
Designed for diagnosis purposes
Host or router can send a echo-request
Receivers echoes back the message with an echo-reply
Echo-request/reply used by network admin to test the reachability of a specific host
Identifier & Sequence Number are not formally used by the protocol,
Can be set to anything by the sender
Example : Ping program : a statistical tool
Does not use Transport protocols (TCP or UDP)

21. Ping Program
Ping programs uses Echo-request/reply to test reachability of a host
Identifiers : Process ID
If many ping programs are running
Sequence Number : increment for each echo-request
RTT = received_reply_time – requested_time(stored in ICMP data packet)
Other Options:
Use ICMP request message encapsulated into an IP packet with record route option.
Example: Ping –R machine
Limited number of IP addresses
IP header lenght = 4 bits
Allows 15* 4 bytes (60 – 20 IP header– 3 bytes for option information ) Only 9 IP addresses
Use ICMP request message encapsulated into IP packet with timestamp option
More severe limitation

22. ICMP package

23. Input Module (handles all types of received ICMP messages)
Receive an ICMP packet from the IP layer
1- if (type = request type)
1- Create a reply
2- Send the reply
2- if (type = router solicitation)
1- if (station is a router)
Create router advertisement
Send the advertisement
3- if (type = one of the 3 reply messages or router advertisement)
1- Extract information in the data section of the packet
2- Deliver extracted information to the process that requested it
4- if (type defines a redirection)
Modify the routing table
5- if (type = error messages other that redirection)
Inform the appropriate source protocol
6- return.

24. Output Module (responsible for: creating requests, solicitation, error messages requested by higher level protocols or IP)
Receive : a demand
1- if (demand = error messages)
If (demand is from IP)
If (demand is forbidden i.e. 4 cases where ICMP does not error message)
Return
If (type defines a redirection message)
If (station is not a router)
Create the error message using type, code, and original IP packet
2- If (demand = request or solicitation)
Create a request or solicitation message
3- send the message
4- return

25. Ping Program (example)

26. Traceroute Program (ICMP “Time Exceeded” revisited)
If a router crashes or connection between two routers is lost
Can take time to re-route
In this case TTL function is to avoid routing loops.
If TTL =0/1 must not forward the datagram.
Router sends ICMP “Exceeded Time” to sender.
TRACEROUTE
Create UDP datagram(“with unreachable port#”) to send to destination
UDP datagram has 12bytes of data, (sequence#, TTL,time).
TTL = 1;
Send ‘n’ datagrams to destination
while (!receive (ICMP “port_unreachable”)
(TTLth Router
TTL--; Send ICMP “TimeExceeded”)
Sender knows IP address of TTLth router in ICMP packet
Print (IP address, 1st RTT, 2nd RTT, .., nth RTT)
TTL++; }

27. Traceroute Program ( IP Source route revisited)
Traceroute can specify a Strict Source routing
If failed, ICMP error source route unfeasible (type =3, code =5)
Can also specify loose source routing
Traceroute [-LooseRoute Destination] Source
Traceroute Round Trips may be different
How many source route IP addresses?

28. Traceroute or Ping RTT?
Traceroute stores original time in UDP data. (12 bytes).
Ping stores original time in ICMP packet, echoed by receiver.