1. COMPUTER NETWORKS
CS610
Lecture-33
Hammad Khalid Khan

2. Review Lecture 32 Fragmentation in IPV6 Use of Multiple Headers
IPV6 Addressing
ICMP
Best Effort Semantics and Error Detection
Types of ICMP Messages

3. ICMP Message Transport
ICMP encapsulated in IP

4. ICMP Message Transport
How it Works?
ICMP messages sent in response to incoming datagrams with problems
ICMP message not sent for ICMP message

5. Using ICMP to Test Reachability
An internet host, A, is reachable from another host, B, if datagrams can be delivered from A to B
Ping program tests reachability - sends datagram from B to A that A echoes back to B
Uses ICMP echo request and echo reply messages
Internet layer includes code to reply to incoming ICMP echo request messages

6. Using ICMP to Trace a Route
List of all routers on path from A to B is called the route from A to B
Intermediate routers send ICMP time exceeded message to the source
destination sends an ICMP destination unreachable message to the source

7. Using ICMP to Trace a Route
tracert (Windows version) Sends ICMP echo messages with increasing TTL
Router that decrements TTL to 0 sends ICMP time exceeded message, with router's address as source address
First, with TTL 1, gets to first router, which discards and sends time exceeded message
Next, with TTL 2, gets through first router to second router
Continue until an ICMP echo reply message from destination received

8. The Last Address printed by Traceroute
There are two possibilities used to detect the destination:
Send an ICMP echo request, destination host will generate an ICMP echo reply
Send a datagram to a non-existent application, destination host will generate an ICMP destination unreachable message

9. Using ICMP for Path MTU Discovery
Fragmentation should be avoided
How can source configure outgoing datagrams to avoid fragmentation?
Source determines path MTU - smallest network MTU on path from source to destination
Source probes path using IP datagrams with don't fragment flag
Router responds with ICMP fragmentation required message
Source sends smaller probes until destination reached

10. Summary ICMP Message Transport Using ICMP to Test Reachability
Using ICMP to Trace a Route
The Address Printed by Trace-route
Using ICMP for Path MTU Discovery