Delivery and Forwarding Chapter 18 COMP 3270 Computer Networks Computing Science Thompson Rivers University

Learning Objectives List the three next-hop methods for packet forwarding. List the four most basic fields in routing entries. Explain step-by-step how a router forwards an IP packet to the next router (or host). You need to use, in the explanation, the terms: destination address, routing table, interface, address of next hop, ARP. 2

1. DELIVERY The network layer supervises the handling of the packets by the underlying physical networks. We define this handling as the delivery of a packet. 3

Direct and indirect delivery 4

Internetwork: a network of different physical networks Internetworking: communications over an internetwork ☺ ☺ The Internet? ☺ ☺ How to deliver packets from A to D and from D to A? Which route? ☺ ☺ Can we use the data link layer? LAN f3f3 5

☺ ☺ How to decide the next hop for the traffic from A to D? Data link layer; Physical layer Host-to-host delivery Each data link handles its own network interface only; There is no interaction between them. -> We can not use the data link layer for internetworking. LAN f3 6

It decides the next hop (the IP address of S3 and the interface f3) f1f3f1f2 S1 is connected to S3 through f3. LAN f3 7

Datagram approach There is no specific route established from the source to the destination. They could arrive in a different order; Reordering does not occur in the network layer. ☺ ☺ Then what layer? 8

Communication at the network layer in the Internet is unreliable connectionless.  Packets could be lost; Packets could arrive in a different order. ☺ ☺ Does IP include an error checking routine? ☺ ☺ Yes; then? 9

2. IP PACKET FORWARDING IP packet forwarding means to place the packet in its route to its destination. Forwarding requires a host or a router to have a routing table. When a host has a packet to send or when a router has received a packet to be forwarded, it looks at this table to find the route to the final destination, i.e., ???. Forwarding Techniques Forwarding Process Routing Table Topics discussed in this section: 10

Route method versus next-hop method Do we have to keep this? Forwarding Techniques 11

Next-hop methods and their priorities: 1.Source routing methods 2.Host specific method 3.Network specific method 4.Default method Related to classless addressing and netmask Packet header options Next-hop method 12

Source routing methods A routing path from the source to the destination is included in an option field in the IP packet header - SSR (Strict Source Routing): all routers on a path … - LSR (Loose Source Routing): some routers on a path … X Y Z SSR LSR ☺ ☺ Any danger? Most routers in the public realm blocks SSR and LSR. 13

Host-specific versus network-specific method However, it is not still easy to keep all the routing information for the whole Internet, due to the routing traffic overhead. An ordinary router in an AS (Autonomous System) keeps the routing information for only the AS. ☺ ☺ What if a packet is supposed to go to a destination in another AS? ☺ ☺ How to distinguish host-specific entries and network specific entries in a routing table? Netmask: 32bits for host specific entries, or less bits for … Note. A host address is a special case of network address. 14

Default method Default router Default router: Border gateway: It knows the whole topology of the Internet, using BGP (Border Gateway Protocol). Now transferred to the destination AS through other ASes. In the destination AS, host specific or network specific methods are used. AS Just zero, i.e., /0 -> if all other entries do not match with the given destination address, then the entry of 0..0/0 will be matched. Or also called, default gateway 15 AS will be discussed more when routing is discussed.

Next-hop methods and their priorities: 1.Source routing methods 2.Host specific method 3.Network specific method 4.Default method ☺ ☺ How do routers support the above priorities? Using the length of netmasks Related to netmask Packet header options 16

Simplified forwarding module in classless address Forwarding Process 17

In classless addressing, we need at least four columns in a routing table. They are Routing Table 18

IP address; Not MAC address Longest prefix match first Destination address in the packet & mask, and then compare the result to the destination address  Time consuming process  Need to use good algorithm for searching  Any good idea? ☺ ☺ When the source routing option is not included, what are the next hops for the following destinations? It will be matched for the last because of the “longest prefix match first” policy. 19

☺ ☺ Can you make routing tables for routers R1 and R2? (One routing entry for each network to which the router is connected, and one default routing entry, and possibly more.) m3 R2 eth1 eth / /24 ☺ ☺ Where is /25? ☺ ☺ Aren’t they strange? Example 20

Routing table for router R1 ☺ 128: ☺ Are they okay? 192: /

(Very important) Show the forwarding process if a packet arrives at R1 with the destination addresses , , , ??? 128: : /

Address aggregation (or also called supernetting) 23

point-to-point -> addresses are not necessary.

From the same address block /24, But different locations

Example: As an example of hierarchical routing: A regional ISP is granted 16,384 addresses starting from /18. The regional ISP has decided to divide this block into four subblocks, each with 4096 addresses. 1.The first local ISP has divided its assigned subblock into 8 smaller blocks and assigned each to a small ISP. Each small ISP provides services to 128 households, each using four addresses. 2.The second local ISP has divided its block into 4 blocks and has assigned the addresses to four large organizations. 3.The third local ISP has divided its block into 16 blocks and assigned each block to a small organization. Each small organization has 256 addresses, and the mask is /24. There is a sense of hierarchy in this configuration. All routers in the Internet send a packet with destination address to to the regional ISP. 26

regional local

Related utilities: netstat route ifconfig 28