1. Datagram Forwarding

2. Routers and the IP Addressing Principle
How to assign IP to a router, which interconnects multiple networks?

3. Routers and the IP Addressing Principle
Each router is assigned two or more IP addresses, one address for each network 3 3

4. Forwarding an IP Datagram
To make the selection of a next hop for an IP datagram, a router uses a forwarding table. 4 4

5. Network Prefix Extraction
The mask field in a forwarding table entry is used to extract the network portion (prefix) of an IP address during lookup
if ( (Mask[i] & IP Address) == Destination[i] ) forward to NextHop[i]
Examples
A datagram to
The first entry fails since & ≠
The second entry: & ≠
The third entry: & ≠
The last entry: & ==  Forward to

6. Exercise 1
What is the next hop for IP datagram to ?

7. Longest Matching Prefix
Prefix in the table might overlap!
AT&T Pennsylvania  AT&T Pittsburgh
Longest matching prefix rule:
For each packet, examine entries in the forwarding table with the longest prefix first

8. Exercise 2 Given the following forwarding table:
Determine the next hop for each of the following destination address:
Network
Next Hop /8 A
/12 B
/16 C
/24 D

9. IP Encapsulation: Virtual Packets
An IP datagram is encapsulated in a frame by network hardware.
Datagram: Layer-3 packet
Encapsulation requires the sender to supply the MAC address of the next computer
Router or host

10. Example: A Tiny Internetwork
NIC-1-1
NIC-1-2 A D R2
NIC-2-1
NIC-2-2 E B
NIC-L
NIC-R F A
/16
/16 R1
/16 R2 F

11. Exercise 3 Complete R2’s forwarding table: F A R2 R1 166.200.0.0/16
/16 R1
/16 R2 F
Complete R2’s forwarding table:
Network
Mask
Next Hop

12. Steps for A to send “I like you” to F
P-A will compose the packet
I like you
NIC-A’s MAC
NIC-1-1’s MAC
P-A forwards the packet to NIC-A
NIC-A calculates the CRC, adds SOH, EOT
I like you
NIC-A’s MAC
NIC-1-1’s MAC
SOH
EOT
CRC
NIC-A modulates the packet  send out using CSMA/CD

13. ARP (Address Resolution Protocol)
Internet uses ARP to map the next hop’s IP address to its Link layer (MAC) address
always restricted to a single network at a time.
ARP uses broadcast to reach all nodes
Broadcast MAC address: FF:FF:FF:FF:FF:FF
“I'm looking for the MAC address of a computer that has IP address ”
“I'm the computer with IP address , and my MAC address is 11:22:33:44:55:66”

14. Steps for A to send “I like you” to F
After a while, NIC-1-1 will receive the m-carrier
NIC-1-1 demodulates the m-carrier and put it into the cache
NIC-1-1 does address filtering
NIC-1-1 does length and CRC checkings
If the packet passes all checkings, NIC-1-1 will strip off SOH, EOT, CRC, and forward the frame to P-R1
P-R1 will strip off the Layer-2 (MAC) header:
I like you
NIC-A’s MAC
NIC-1-1’s MAC
I like you

15. Steps for A to send “I like you” to F
P-R1 will do routing based on the routing table.
Destination address:
P-R1 adds the Layer-2 (MAC) header and forward it to NIC-1-2
Network
Mask
Next Hop
Deliver direct
(NIC-2-1)
I like you
NIC-1-2’s MAC
NIC-2-1’s MAC

16. Steps for A to send “I like you” to F
NIC-1-2 calculates the CRC, adds SOH, EOT; modulates the packet  send out using CSMA/CD…
NIC-2-1 does the same as NIC-1-1…
P-R2 does the same as P-R1 except that it will use a different routing table:
Destination address:
Network
Mask
Next Hop
(NIC-1-2)
Deliver direct

17. Steps for A to send “I like you” to F
P-R2 adds the new Layer-2 (MAC) header and forward it to NIC-2-2
NIC-2-2 does the same as NIC-1-2 to send out the frame…
The bridge will forward the frame to computer F based on the MAC address as we discussed before (week10-2.pptx)
Note: the bridge does not understand IP addresses!
After a while, NIC-F will receive the m-carrier…
I like you
NIC-2-2’s MAC
NIC-F’s MAC

18. Exercise 4
Q1: During the whole process, how many processors (incl. computers and routers) have seen the Internet layer header?
Q2: During the whole process, how many times is the Internet layer header changed? How about the Link layer header?