1. CS 428 Computer Networking
Internet Routing
Read Chapters 7 & 8
Routing - Process of choosing a path over which to send packets
Router - A computer making such a choice
Goal: “… provide a virtual network that encompasses multiple physical networks and offers a connectionless datagram delivery service.” - Comer
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CS 428 Computer Networking

2. CS 428 Computer Networking
Routers
An internet composed of multiple physical networks interconnected by computers called routers
Router - direct connections to 2 or more physical networks
Host computer - a single, direct connection
© MMII JW Ryder
CS 428 Computer Networking

3. CS 428 Computer Networking
Routers
Multi-homed hosts - direct connections to 2 or more physical networks
Both hosts and routers route IP datagrams
Hosts??
Two or more routers on network? Which to send to? Host must make decision
Any computer with multiple network connections can act as a router
© MMII JW Ryder
CS 428 Computer Networking

4. CS 428 Computer Networking
Delivery
Direct - Local
Sender encapsulates datagram in a physical frame
Binds dest IP addr to a physical hardware addr
Sends frame to destination
Known local because netid matches host’s netid
Indirect - Between physical networks
© MMII JW Ryder
CS 428 Computer Networking

5. CS 428 Computer Networking
Indirect Delivery
Sender must identify a router to send datagram to
Router then forwards it toward its destination
When frame reaches the router
Software extract the encapsulated datagram
IP software selects next router along the path
Datagram again placed in a frame
Sent over physical network to next router
© MMII JW Ryder
CS 428 Computer Networking

6. CS 428 Computer Networking
Table Driven Routing
IP Routing Table on each machine
netstat -nr
Want to contain most minimal information at all times
Next hop routing done by using the routing tables
(N, R) pairs
N = netid part of IP addr
R = Next router along path to N
© MMII JW Ryder
CS 428 Computer Networking

7. CS 428 Computer Networking
Next Hop Routing
Important - all routers identified in machine M’s routing table are physically located on the same network as M
When a datagram ready to leave M
IP software locates dest IP addr
extracts the network portion to make routing decision
M will select a router which M can directly reach
Hosts have minimal information, rely on routers
© MMII JW Ryder
CS 428 Computer Networking

8. CS 428 Computer Networking
Routing Example Q
IP Addr Gateway
Direct Delivery
Direct Delivery R S
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CS 428 Computer Networking

9. CS 428 Computer Networking
Consequences
All traffic destined for given network use same path
Even when multiple paths exist, won’t be used
Only final router can determine if dest. host exists or is operational
Need way for routers to return delivery problem errors
Datagrams travelling from A to B may follow a different path from B to A
© MMII JW Ryder
CS 428 Computer Networking

10. CS 428 Computer Networking
Default Routes
Especially useful when local network has small set of hosts and 1 connection to internet
2 tests
One for local network
Default that points to default router
© MMII JW Ryder
CS 428 Computer Networking

11. CS 428 Computer Networking
A Routing Algorithm
RouteDatagram (Datagram, RoutingTable)
Extract destination IP addr, D, from the datagram and compute the network prefix, N;
If N matches any directly connected network address, deliver datagram to destination D over that network (This involves resolving D to a physical address, encapsulating the datagram, and sending the frame)
else if the table contains a host-specific route for D send the datagram to next hop specified in the table
else if the table contains a route for network N, send datagram to next hop specified in the table
else if the table contains a default route, send datagram to the default router specified in the table
else declare a routing error
© MMII JW Ryder
CS 428 Computer Networking