1. Guide to TCP/IP, Third Edition
Chapter 4:
Internet Control Message Protocol

2. Objectives Understand the Internet Control Message Protocol
Test and troubleshoot sequences for Internet Control Message Protocol
Work with Internet Control Message Protocol packet fields and functions
Internet Control Message Protocol

3. Understanding The Internet Control Message Protocol
ICMP
Provides information about network connectivity and routing behavior
Provides a way to return information to senders
Messages are nothing more than specially formatted IP datagrams
Internet Control Message Protocol

4. Overview of RFC 792 RFC 792 According to RFC 792, ICMP
Provides basic specification for all ICMP messages
According to RFC 792, ICMP
Provides mechanism for gateways (routers) or destination hosts to communicate with source hosts
Takes the form of specially formatted IP datagrams
Required in some implementations of TCP/IP
Reports errors about processing of non-ICMP IP datagrams
Internet Control Message Protocol

5. ICMP’s Vital Role on IP Networks
ICMP’s job is to provide information about
IP routing behavior
Reachability
Routes between specific pairs of IP hosts
Delivery errors
Internet Control Message Protocol

6. Internet Control Message Protocol

7. Internet Control Message Protocol

8. Testing And Troubleshooting Sequences For ICMP: Connectivity Testing with Ping
PING and TRACEROUTE
Rely on ICMP to perform connectivity tests and path discovery
PING
Actually a form of ICMP Echo communication
ICMP Echo Request
Connectionless process with no guarantee of delivery
Internet Control Message Protocol

9. Internet Control Message Protocol

10. Connectivity Testing with PING (continued)
Most PING utilities
Send series of several Echo Requests to the target in order to obtain average response time
PING utility
Sends series of four ICMP Echo Requests with a one-second ICMP Echo Reply Timeout value
Supports IP addresses and names
Uses traditional name resolution processes
Internet Control Message Protocol

11. Internet Control Message Protocol

12. Connectivity Testing with PING (cont’d)
Parameters available with the PING utility
-l size -f
-i TTL
-v TOS,
-w timeout
Internet Control Message Protocol

13. Path Discovery with TRACEROUTE
TRACEROUTE utility
Uses route tracing to identify a path from sender to target host
Available parameters -d -h -w
Internet Control Message Protocol

14. Internet Control Message Protocol

15. Path Discovery with PATHPING
PATHPING utility
Command-line utility
Uses ICMP Echo packets to test router and link latency, as well as packet loss
PMTU Discovery
Enables source to learn the currently supported MTU across an entire path
Internet Control Message Protocol

16. Path MTU Discovery with ICMP
PMTU process
Host A sends a 4,096-byte packet to Host B
Router 1 discards packet and sends Host A a “Fragmentation Needed and Don’t Fragment Flag was Set” ICMP packet
Host A re-sends packet using maximum MTU size of 1,500
Router 1 strips off token ring header and applies Ethernet header before forwarding packet
Internet Control Message Protocol

17. Internet Control Message Protocol

18. Internet Control Message Protocol

19. Routing Sequences for ICMP
Can provide some routing information to hosts
Used by routers to provide a default gateway setting to a host
Routers
Can send ICMP messages
Internet Control Message Protocol

20. Router Discovery IP hosts By default
Typically learn about routes through manual configuration of
Default gateway parameter and redirection messages
Send ICMP Router Solicitations and routers reply with ICMP Router Advertisements
By default
ICMP Router Solicitation packet is sent to the all-routers IP multicast address
Internet Control Message Protocol

21. Internet Control Message Protocol

22. Router Advertising ICMP Router Advertisements
Allow hosts to passively learn about available routes
Default Lifetime value for route entries
30 minutes
Default advertising rate
Between seven and ten minutes
Internet Control Message Protocol

23. Internet Control Message Protocol

24. Security Issues For ICMP
Can be used as an information-gathering tool
IP address scanning process
One method of obtaining a list of the active hosts
IP host probe
Performed by sending a PING packet to each host within a range and noting the responses
Internet Control Message Protocol

25. ICMP Redirect Attack ICMP Attacker can
Used to manipulate traffic flow between hosts
Attacker can
Redirect traffic to his machine and perform any number of man-in-the-middle style attacks
Internet Control Message Protocol

26. ICMP Router Discovery
Susceptible to attack on the local network segment
During discovery process
Router solicitation message finds its way to attacker’s machine
Timing is critical
Internet Control Message Protocol

27. Inverse Mapping One method of determining live targets on a network
Firewalking
Describes the concept of walking a firewall ACL or ruleset to determine what it filters and how
A two-phase attack method
Internet Control Message Protocol

28. ICMP Packet Fields and Functions
Value 1 in IP header Protocol field
Denotes that an ICMP header follows the IP header
ICMP header portions
Constant portion
Variable portion
Internet Control Message Protocol

29. Internet Control Message Protocol

30. Constant ICMP Fields
ICMP packets contain three required fields after the IP header
Type
Code
Checksum
Internet Control Message Protocol

31. The Variable ICMP Structures and Functions
ICMP Type 0
Used for Echo Reply packets
ICMP Type 8
Used for Echo Request packets
RFC 792
Identifier and Sequence fields are used to aid in matching Echo messages with Echo Replies
Internet Control Message Protocol

32. Internet Control Message Protocol

33. Internet Control Message Protocol

34. Type 3: Destination Unreachable Packets
Network troubleshooters
Often closely track ICMP Destination Unreachable packets
Host that sends Destination Unreachable packet
Must return IP header and eight bytes of original datagram that triggered this response
Total of 16 (0 through 15) possible codes
Currently assigned to ICMP Destination Unreachable type number
Internet Control Message Protocol

35. Internet Control Message Protocol

36. Internet Control Message Protocol

37. Type 4: Source Quench Router or host By default
May use Source Quench to indicate that it is becoming congested or overloaded
By default
Most current routers do not issue Source Quench messages
Internet Control Message Protocol

38. Internet Control Message Protocol

39. Type 5: Redirect Routers ICMP Redirect packet Ideally
Send ICMP Redirect messages to hosts to indicate that a preferable route exists
ICMP Redirect packet
Four-byte field for the preferred gateway’s address
Ideally
Clients should update routing tables to indicate optimal path
Internet Control Message Protocol

40. Types 9 and 10: Router Advertisement and Router Solicitation
ICMP Router Advertisement packets include the following fields
# of Addresses
Address Size
Lifetime
Router Address 1
Precedence Level 1
Router Address 2 and Precedence Level 2
Internet Control Message Protocol

41. Type 11: Time Exceeded Routers or hosts Codes that can be used
Can send these ICMP packets
Codes that can be used
Code 0 and Code 1
Internet Control Message Protocol

42. Type 12: Parameter Problem
Errors indicate problems not covered by other ICMP error messages
Codes used in ICMP Parameter Problem messages
Code 0: Pointer Indicates the Error
Code 1: Missing a Required Option
Code 2: Bad Length
Internet Control Message Protocol

43. Types 13 and 14: Timestamp and Timestamp Reply
Defined as a method for one IP host to obtain the current time
Value returned
The number in milliseconds since midnight, Universal Time (UT)
ICMP Timestamp and Timestamp Reply packets
Use the same structure
Internet Control Message Protocol

44. Types 15 and 16: Information Request and Information Reply
Provides a way for a host to find out what network it is on
ICMP Information Request and Information Reply packets
Use the same structure
Internet Control Message Protocol

45. Types 17 and 18: Address Mask Request and Address Mask Reply
Intended to provide diskless hosts with a method to determine their network mask information
ICMP Address Mask Request and Address Mask Reply packets
Use the same structure
Internet Control Message Protocol

46. Type 30: TRACEROUTE Documented in RFC 1393 but not currently in use
Requires some added functionality in the IP routers it traverses
Adding functionality to routers
Costly and requires numerous resources to build, implement, and test new code
Internet Control Message Protocol

47. Internet Control Message Protocol

48. Summary
ICMP
Provides vital feedback about IP routing and delivery problems
Really part of IP itself
Support is required in any standards-compliant IP implementation
Used by PING and TRACEROUTE to measure round-trip times
Supports PMTU Discovery between a sender and a receiver
Internet Control Message Protocol

49. Summary (continued) Route and routing error information from ICMP ICMP
Derives from numerous types of ICMP messages
ICMP
Supports route optimization through its ICMP Redirect message type
Security issues are important
Message structures and functions can vary
Internet Control Message Protocol