1. Connecting Networks © 2004 Cisco Systems, Inc. All rights reserved. Defining the IP Packet Delivery Process INTRO v2.0—4-1

2. © 2004 Cisco Systems, Inc. All rights reserved. INTRO v2.0—4-2 Outline Overview Overview of IP Packet Delivery IP Packet Structure Protocol Field Internet Control Message Protocol Using Internet Control Message Protocol Testing Summary

3. © 2004 Cisco Systems, Inc. All rights reserved. INTRO v2.0—4-3 IP Packet Delivery 1. Host sends packet to default gateway 2. Packet placed in frame 3. Router receives frame 4. Router finds destination network in route table 5. Router chooses next hop toward destination 6. MAC address of next hop determined 7. Packet placed in frame 8. Repeats steps 2 through 7 as necessary 9. Router receives frame 10. Router finds network directly connected 11. MAC address of end host determined 12. Packet placed in frame to final destination

4. © 2004 Cisco Systems, Inc. All rights reserved. INTRO v2.0—4-4 IP Datagram Header

5. © 2004 Cisco Systems, Inc. All rights reserved. INTRO v2.0—4-5 Protocol Field

6. © 2004 Cisco Systems, Inc. All rights reserved. INTRO v2.0—4-6 ICMP Announces network problems –Destination unreachable Announces network congestion –Source Quench Assists in troubleshooting –Echo request and Echo reply Announces timeouts –Time to Live exceeded

7. © 2004 Cisco Systems, Inc. All rights reserved. INTRO v2.0—4-7 ICMP Testing Process D:\>ping 192.110.1.140 Pinging 192.110.1.140 with 32 bytes of data: Request timed out D:\>ping 192.110.1.40 Pinging 192.110.1.40 with 32 bytes of data: Reply from 192.110.1.40: bytes=32 time<10ms TTL=64 D:\>ping 10.140.244.217 Pinging 192.110.1.40 with 32 bytes of data: Destination unreachable

8. © 2004 Cisco Systems, Inc. All rights reserved. INTRO v2.0—4-8 Summary The steps involved in transmitting an IP packet over a routed network are similar to that of sending a letter through a mail delivery service and include finding the destination address in a routing table, putting the data into a frame and sending it to a router at the destination location, unpacking the frame, and sending the data to its destination. The structure of an IP packet (datagram) includes the necessary information to deliver the packet. This information includes items such as version, header length, priority and type of service, total length, identification, and so on.

9. © 2004 Cisco Systems, Inc. All rights reserved. INTRO v2.0—4-9 Summary (Cont.) IP uses a protocol number in the datagram header to identify which protocol to use for a particular datagram. ICMP defines a small number of messages used for diagnostic and management purposes in the IP transmission process. These messages include announcing network errors, congestion and timeouts and assisting with troubleshooting. The ping command is used to test the connectivity. The ping command uses the echo function in ICMP.