2. 1 Figure 3-5: IP Packet Total Length (16 bits) Identification (16 bits) Header Checksum (16 bits) Time to Live (8 bits) Flags Protocol (8 bits) 1=ICMP, 6=TCP, 17=TCP Bit 0Bit 31 IP Version 4 Packet Source IP Address (32 bits) Fragment Offset (13 bits) Diff-Serv (8 bits) Header Length (4 bits) Version (4 bits) Destination IP Address (32 bits) Options (if any)Padding Data Field 0100

3. 2 Figure 3-5: IP Packet Version  Has value of four (0100) Time to Live (TTL)  Prevents the endless circulation of mis-addressed packets  Value is set by sender  Decremented by one by each router along the way  If reaches zero, router throws packet away

4. 3 Figure 3-5: IP Packet Protocol Field  Identifies contents of data field  1 = ICMP  6 = TCP  17 =UDP IP Header Protocol=1 IP Data Field ICMP Message IP Header Protocol=6 IP Data Field TCP Segment IP Header Protocol=17 IP Data Field UDP Datagram

5. 4 Figure 3-5: IP Packet Header checksum to check for errors in the header only  Faster than checking the whole packet  Stops bad headers from causing problems  IP Version 6 drops eve this checking Address Fields  32 bits long, of course Options field(s) give optional parameters Data field contains the payload of the packet.

6. 5 Figure 3-9: Layer Cooperation Through Encapsulation on the Source Host Application Process HTTP Message Transport Process HTTP Message TCP Hdr Internet Process HTTP Message TCP Hdr IP Hdr Encapsulation of HTTP message in data field of a TCP segment Encapsulation of TCP segment in data field of an IP packet

7. 6 Figure 3-9: Layer Cooperation Through Encapsulation on the Source Host Internet Process Data Link Process Physical Process Encapsulation of IP packet in data field of a frame HTTP Message TCP Hdr IP Hdr HTTP Message TCP Hdr IP Hdr DL Hdr DL Trlr Converts Bits of Frame into Signals

8. 7 Figure 3-9: Layer Cooperation Through Encapsulation on the Source Host DL Trlr Note: The following is the final frame for supervisory TCP segments: TCP Hdr IP Hdr DL Hdr

9. 8 Figure 3-10: Layer Cooperation Through Decapsulation on the Destination Host Application Process HTTP Message Transport Process HTTP Message TCP Hdr Internet Process HTTP Message TCP Hdr IP Hdr Decapsulation of HTTP message from data field of a TCP segment Decapsulation of TCP segment from data field of an IP packet

10. 9 Figure 3-10: Layer Cooperation Through Decapsulation on the Destination Host Internet Process Data Link Process Data Link Process Decapsulation of IP packet from data field of a frame HTTP Message TCP Hdr IP Hdr HTTP Message TCP Hdr IP Hdr DL Hdr DL Hdr Converts Signals into the Bits of the Frame

11. 10 Figure 3-11: Vertical Communication on Router R1 Port 1 DL Port 2 DL Port 3 DL Port 4 DL PHY Internet Layer Process Packet Frame Router R1 Switch X2 A Decapsulation Notes: A.Router R1 receives frame from Switch X2 in Port 1. Port 1 DL process decapsulates packet. Port 1 DL process passes packet to internet process.

12. 11 Figure 3-11: Vertical Communication on Router R1 Port 1 DL Port 2 DL Port 3 DL Port 4 DL PHY Internet Layer Process Packet Frame Router R1 Router 2 B Encapsulation B.Internet process sends packet out on Port 4. DL Process on Port 4 encapsulates packet in a PPP frame. DL process passes frame to Port 4 PHY.

13. 12 Packet Figure 3-12: Site Connection to an ISP 2. Packet Carried in ISP Carrier Frame 4. Data Link Between Site and ISP (Difficult to Attack) Border Firewall 3. Packet Carried in Site Frame Packet 1. Frame for This Data Link Site Network 5. Normally, Only the Arriving Packet is Dangerous—Not the Frame Fields ISP Packet ISP Router Internet Backbone