Download Chapter 14

Routing: Cores, Peers and
Algorithms
Chapter 14
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
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
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
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
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
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
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
Core Routers
• This approach became impractical because:
– the Internet outgrew a single, centrally managed longhaul backbone
– protocols needed to maintain consistency among core
routers became nontrivial
– maintaining correct routing information became
difficult
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 nonNSFNET 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
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
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
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
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
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
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
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
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
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
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
For Next Time
• Read Chapter 15