Download Chapter 25 Internet Routing

Chapter 25 Internet Routing
Static Routing

manually configured routes that do not
change
 Used by hosts whose routing table contains
one static route describing the network to
with the host is attached
 a default route directs all other traffic to a
specific router (fig 25.1)
Dynamic Routing

routing tables can change automatically
over time via information learned via
routing messages from other routers.
 Each router learns about destinations other
routers can reach, and informs other routers
about destinations that it can reached (fig
25.2)
Routing in the Global Internet

the Internet uses a 2-level routing hierarchy
 Routers and networks are divided into groups
where each group is known as an autonomous
system (a contiguous set of networks and routers
all under control of one administrative authority).
 All routers within a group(autonomous system)
exchange routing information while one router in
each group summarizes the information before
passing it to other groups.
Internet Routing Protocols

Interior Gateway Protocol (IGP)
– eg. RIP, OSPF, IGRP: used by routers within
an autonomous system

Exterior Gateway Protocols(EGP)
– eg.BGP: used by a router in one autonomous
system to exchange routing information with a
router in another autonomous system (fig 25.3)
Routing Metrics

eg. hop count, administrative cost, throughput,
delay.
 Used by interior gateway protocols but not by
exterior gateway protocols due to existence of
different metrics.
 Within an autonomous system, IGP software uses
a routing metric to choose an optimal path to each
destination.
 EGP software finds a path to each destination, but
cannot find an optimal path because it cannot
compare routing metrics from multiple
autonomous systems.
Border Gateway Protocol
(BGP)

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routing messages among autonomous systems contain routes,
each of which is described as a path of autonomous systems
( eg. route to autonomous system 34 is achieve via
autonomous systems 17, 2, 56, and 12.
Provision for policies: manager can configure BGP to restrict
routes advertised to outsiders
Each autonomous system is classified as a transit system if it
agrees to pass traffic through to another autonomous system or
as a stub system if it does not
Uses TCP for reliable transport of routing messages
Used by all ISPs to exchange routing information with each
other and from an authoritative route server (which has a copy
distributed database of all possible destinations in the Internet
with information about the ISP that owns each destination)
Routing Information Protocol
(RIP)

implemented by a program called routed
 hop count metric with origin-one counting ( ie.
A directly connected network is 1 hop away,
not 0)
 uses unreliable transport (UDP)
 uses broadcast(RIP version 1) or multicast(RIP
version2) for message delivery
 support for default route propagation
Routing Information Protocol
(cont.)


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

uses distance-vector algorithm
RIP advertises the destinations it can reach along
with a distance to each destination
adjacent routers receive the information and
update their routing tables
allows hosts to be operate in passive listen-only
mode ( ie. no advertising)
RIP packet format (fig 25.5)
drawbacks of RIP include large routing messages,
slow propagation of route changes (one router at a
time), and limited scalability.
Open Shortest Path First
Protocol (OSPF)

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
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

scales well to large organizations
designed as an interior gateway protocol
full CIDR and subnet support
authenticated message exchange
can import routes from BGP
Uses hierarchical routing by dividing routers and
networks in an autonomous system into subnets
known as areas.
– Each router is within a given area exchange link-status
messages via broadcasts.
– Summarized routing information are exchanged by one
router in each area with routers in other areas.
Open Shortest Path First
Protocol (cont)

uses link-state routing algorithm.
– Each router must periodically probe adjacent
routers and then broadcast a link-status message
– routers that receive the message use Dijkstra’s
SPF algorithm to compute the shortest paths
using it local copy of network graph(fig 25.6) .
Multicasting

an application running on any computer can join a
multicast group at any time and begin receiving a
copy of all packets sent to the group.
 To join or leave a group, the computer informs a
nearby router via IGMP (Internet Group Multicast
Protocol).
 To leave a group, the computer informs the local
router that it is no longer participating in the
group.
Multicasting (cont)

An IP multicast group is anonymous
– sender and receiver do not know the identity or the number of
group members
– routers do not know which applications will send a datagram
to a group since any application on any computer can send a
datagram to any multicast group at any time.

Membership in a multicast group only defines a set of
receivers
– sender does not need to join a multicast group before sending
a message to the group.

Multicast packets are forwarded using techniques such
as flood-and-prune, configuration-and-tunneling, or
core-base discovery.
Multicast routing protocols

DVMRP (distance vector multicast routing
protocol)
– used by Unix program mrouted and Internet Multicast
backBONE (MBONE)

CBT (Core Base Trees)
 PIM-SM (protocol independent multicast–sparse
mode)
 PIM-DM (protocol independent multicast–dense
mode)
 MOSPF(multicast extensions to the open shortest
path first protocol)