Datagram Forwarding

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

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

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

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 192.4.10.3 The first entry fails since 255.0.0.0 & 192.4.10.3 ≠ 30.0.0.0 The second entry: 255.0.0.0 & 192.4.10.3 ≠ 40.0.0.0 The third entry: 255.255.0.0 & 192.4.10.3 ≠ 128.1.0.0 The last entry: 255.255.255.0 & 192.4.10.3 == 192.4.10.0  Forward to 128.1.0.9

Exercise 1 What is the next hop for IP datagram to 30.0.1.2?

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

Exercise 2 Given the following forwarding table: Determine the next hop for each of the following destination address: 112.32.54.188 112.33.45.12 112.34.8.60 Network Next Hop 60.0.0.0/8 A 112.32.0.0/12 B 112.32.0.0/16 C 112.33.45.0/24 D

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

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 166.200.0.0/16 167.234.0.0/16 R1 168.16.0.0/16 R2 F 166.200.1.5 166.200.0.7 167.234.0.7 167.234.0.8 168.16.0.8 168.16.2.4

Exercise 3 Complete R2’s forwarding table: F A R2 R1 166.200.0.0/16 167.234.0.0/16 R1 168.16.0.0/16 R2 F 166.200.1.5 166.200.0.7 167.234.0.7 167.234.0.8 168.16.0.8 168.16.2.4 Complete R2’s forwarding table: Network Mask Next Hop 166.200.0.0 167.234.0.0 168.16.0.0

Steps for A to send “I like you” to F 166.200.1.5 168.16.2.4 P-A will compose the packet I like you 166.200.1.5 168.16.2.4 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 166.200.1.5 168.16.2.4 NIC-A’s MAC NIC-1-1’s MAC SOH EOT CRC NIC-A modulates the packet  send out using CSMA/CD

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 1.2.3.4” “I'm the computer with IP address 1.2.3.4, and my MAC address is 11:22:33:44:55:66”

Steps for A to send “I like you” to F 166.200.1.5 168.16.2.4 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 166.200.1.5 168.16.2.4 NIC-A’s MAC NIC-1-1’s MAC I like you 166.200.1.5 168.16.2.4

Steps for A to send “I like you” to F P-R1 will do routing based on the routing table. Destination address: 168.16.2.4 P-R1 adds the Layer-2 (MAC) header and forward it to NIC-1-2 Network Mask Next Hop 166.200.0.0 255.255.0.0 Deliver direct 167.234.0.0 168.16.0.0 167.234.0.8 (NIC-2-1) I like you 166.200.1.5 168.16.2.4 NIC-1-2’s MAC NIC-2-1’s MAC

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: 168.16.2.4 Network Mask Next Hop 166.200.0.0 255.255.0.0 167.234.0.7 (NIC-1-2) 167.234.0.0 Deliver direct 168.16.0.0

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 166.200.1.5 168.16.2.4 NIC-2-2’s MAC NIC-F’s MAC

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?