1. Routing: Cores, Peers and Algorithms
Chapter 14

2. The Origin of Routing Tables
Routers form the base of an internet and handle all traffic except for direct delivery from one host to another
We have two questions:
What should be in routing tables?
How do routers get the information for the tables?
Initial routes may be determined:
from secondary storage into main memory at startup
from a set of addresses of connecting networks

3. The Origin of Routing Tables
In small internets, the routing tables may be established and modified by hand
In large internets, automated approaches are necessary

4. Routing with Partial Information
Hosts usually have two routes in their routing table
A route for the local network
A default route for nonlocal datagrams
Example of road signs with partial information, page 255
Extreme Architectural Approaches
Star shaped topology with one road to each town, one central point
Arbitrary roads with signs for all towns at each intersection

5. Routing with Partial Information
The two extreme architectural approaches fail
Star - one machine would have to work as switch between all
Arbitrary - to keep all routing information at all sites is cumbersome and difficult to change
A third approach
Half of the cities lie in the east and half in the west
A bridge spans a river that separates the east and west
Road signs in the east list all destinations in the east, and only points to the west, and so on

6. Original Internet Architecture and Cores
When TCP/IP was developed, research sites were connected to the ARPANET (Internet backbone)
Routing tables were initialized and modified by hand
To begin automation for routing,:
a small central set of routers kept complete information about all possible destinations
a larger set of outlying routers kept partial information
allows local administrators to make local changes
default routes at the outlying routers could indicate a central intersection

7. Core Routers Those early routers were either:
Core routers controlled by Internet Network Operations Center
or Noncore routers controlled by inividual groups
Core routers communicated among themselves
information was consistent and the system was reliable
Sites assigned an Internet network address agreed to advertise that address to the core system

8. Core Routers
The early Internet core routing system consisted of routers connecting local area networks to the ARPANET
See Figure 14.1
Hosts on the local networks passed nonlocal traffic to the core router
Routing information was exchanged between core routers
Default routes were not used; would be inefficient

9. Core Routers This approach became impractical because:
the Internet outgrew a single, centrally managed long-haul backbone
protocols needed to maintain consistency among core routers became nontrivial
maintaining correct routing information became difficult

10. Peer Backbones
NSFNET attached to ARPANET through a single router in Pittsburgh
The core had explicit routes to all destinations in NSFNET
Routers inside NSFNET knew local destinations and used a default route (the Pittsburgh router) to send all non-NSFNET traffic to the core
Multiple connections were added between NSFNET and ARPANET as shown in Figure 14.4 and the two became peer backbone networks, or peers

11. Peer Backbones
With such an architecture, how do we route with peer backbones?
Routing loops are possible as in Figure 14.5
Core systems work best for internets with a single, centrally managed backbone
Expanding the topology to multiple backbones makes routing complex

12. Automatic Route Propagation
The original Internet core system propagated complete routing information to all core routers
Today, many routers continue to communicate routing information
We need ways to automatically determine routes, and to continually update the information needed to determine the routes

13. Distance Vector Routing (aka Bellman-Ford)
A router keeps a list of all known routes in a table
When it boots, the routing table is initialized with an entry for each directly connected network
Each entry identifies a network and the distance to it, usually in hops - See Figure 14.6
Periodically, all routers send a copy of their routing table to any other router it can reach directly
If a shorter path is found in one of these, the current path is replaced with the shorter one

14. Distance Vector Routing
See Figure 14.7 which shows an existing table for router K, and an update from router J
Is this a good choice?
Easy to implement
If routes change rapidly, routes may not stabilize
Some routers may have incorrect information
Message size is proportional to the number of networks in an internet

15. Gateway to Gateway Protocol
GGP was used by original core routers to exchange routing information - no longer used
Designed to travel in IP datagrams like TCP and UDP
Messages had a fixed format header identifying message type

16. Distance Factoring
Distance values were small, and the same values tended to be repeated often
To reduce the message size, avoid sending copies of the same distance number
The list of pairs (Network, Distance) is sorted by distance
each distance is represented once and all networks at that distance follow

17. Reliability Most routing protocols use connectionless transport
Routing protocols that use UDP or IP must compensate for failures
by using checksums
sequence numbers for out of order delivery

18. Link-State Routing Primary alternative to distance vector routing
Tests the status of all neighbor routers
Short message asking if neighbor is alive
If the neighbor responds, the link is up, else down
Routers periodically broadcast a message with the status of each of its links
Indicates whether communication is possible between pairs of routers
When link status information arrives at a router, it modifies its view of the internet

19. Shortest Path First
When link status changes, the router recomputes routes by applying Dijkstra’s Shortest Path algorithm
SPF computes the shortest paths to all destinations from a single source
Advantages
each router computes routes independently
messages are propagated unchanged
size does not depend on number of networks in internet

20. Summary Hosts and routers usually contain partial routing information
The Internet used a core routing architecture in which cores had complete network information
Routing information is exchanged periodically
using distance-vector or link state algorithms

21. For Next Time
Read Chapter 15