1. Fred Kuhns Applied Research Laboratory
ICMP and IP Options
Fred Kuhns
Applied Research Laboratory

2. ICMP – RFC 792 Purposes of ICMP (Protocol == 1) ICMP Message
IP Hdr
ICMP
Hdr
Data
20B
4B+
Variable
Type
Code
Checksum
Optional Data
ICMP Message
Purposes of ICMP (Protocol == 1)
Error reporting from routers or destination host to source host.
ICMP data includes header and first 64 bytes of data from the IP packet that caused the error
Only fragment 0 of fragmented messages generate ICMP error messages
Control messages between routers/hosts.

3. ICMP Message Types
Type Field
Code
Message
Echo Reply 3 -
Destination Unreachable (Error)
Network Unreachable 1
Host Unreachable 2
Protocol Unreachable
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 11
Network unreachable for type of service 12
Host unreachable for type of service
Source Quench Report congestion to original host
Redirect – request host use different route
Redirect for network (obsolete)
Redirect for host
Redirect for type-of-service and network
Redirect for type-of-service and host
Type Field
Code
Message 8
Echo Request 9
Router Advertisement 10
Router Solicitation 11 -
Time Exceeded for a Datagram
Time-to-live equals 0 during transit (traceroute) 1
Time-to-live equals 0 during reassembly Timeout occurred while waiting for fragments 12
Parameter Problem – any other error condition (incorrect option
IP Header bad
Required option missing 13
Timestamp Request 14
Timestamp Reply 15
Information Request (obsolete) 16
Information Reply (obsolete) 17
Address Mask Request 18
Address Mask Reply
From Comer, “Internetworking with TCP/IP”, volume 1, 4th edition, 2000.

4. ICMP Echo Request type = 8 Reply type = 0 ICMP Message Type = 0/8
Code = 0
Checksum
Identifier
Sequence Number
Optional Data
ICMP Message
Request type = 8
Reply type = 0

5. Typical ICMP Error Message
Type
Code
Checksum
Unused, must be zero
Internet Header plus first 64 bits (8 Bytes) of datagram
ICMP Message
Destination unreachable
blah, blah, blah

6. Internet Header plus first 64 bits of datagram
ICMP - Redirect
Type
Code
Checksum
Router 32 bit IP address
Internet Header plus first 64 bits of datagram
tells a host to use a different route, however the original packet is still forwarded.

7. ICMP - Parameter Problem
Type = 12
Code = 0/1
Checksum
Pointer
Unused, must be zero
Internet Header plus first 64 bits of datagram
Pointer identifies the problem octet

8. ICMP - Timestamps Times in milliseconds since midnight, universal time
Type = 13/14
Code = 0
Checksum
Identifier
Sequence Number
Originate Timestamp
Receive Timestamp
Transmit Timestamp
Times in milliseconds since midnight, universal time
Originate Timestamp – filled in by sender
Receive timestamp – filled in by receiver immediately on receiving
Transmit timestamp - filled in by receiver immediately before sending

9. ICMP - Subnet Mask ICMP request for subnet mask – host to router
Type = 17/18
Code = 0
Checksum
Identifier
Sequence Number
Address Mask
ICMP request for subnet mask – host to router

10. ICMP - Router Discovery
Type = 9
Code = 0
Checksum
# Addresses
Address Size
Lifetime
Router Address 1
Preference Level 1
Additional Router Addres/Preferences
Lifetime default is 30 minutes
Periodic retransmission default time is 10 minutes
Multicast to (all-systems address) or broadcast if multicast is not supported
Router advertises

11. ICMP - Router solicitation
Type = 10
Code = 0
Checksum
Reserved
Request router to reply with a router advertisement
If multicast supported sent to (all-routers address)
Host to routers

12. IP Options IP Header (20 Bytes) Code Variable (0 – 43) ...
Code Byte Expanded
Copy
(bit)
Option Class
(2 bits)
Option Number
(5 bits)
IP header field HLEN (4bits) = total header length in 4-Byte words. Max is Byte words or 60 Bytes. IP header is 20 bytes, leaving a max of 40 Bytes for IP options.
Two valid formats:
1) single byte option (code) or
2) Code byte, length byte (total bytes in option) followed by option data.
Code Byte:
Copy bit – indicates if option should be copied into all fragments or just first.
Option class specify the general
0 – Datagram or network control; 1 – reserved; 2 – debugging and measurement; 3 – reserved.
Option number – specific class of option. See next slide

13. IP Options With the exception of LFS, none seem particularly useful.
7/8/2019
IP Options
Class
Number
Length
Description 1
End of option list – used if options do not end at end of header
No operation – used to align octets in a list of options 2 11
DoD Basic security (rfc 1108) 3
var
Loose source routing, used to request route that include specified routers 5
DoD extended security (rfc 1108) 7
Record route, used to trace a route 8 4
Stream identifier, used to carry a SATNET stream identifier (obsolete) 9
Strict source routing, use to specify an exact path through network
MTU probe, used for Path MTU discovery (new method uses ICMP) 12
MTU reply, used for path MTU discovery 20
Router alert, Router should examine this datagram (rfc 2113) 21
Selective directed broadcast (rfc 1770) 22
LFS – Light Weight Flow setup protocol
Internet timestamp, used to record timestamp along a route 18
Traceroute, record route in option (rfc 1393)
With the exception of LFS, none seem particularly useful.
IP Header plus options must end on a 4 byte boundary