Internet Routing - CS

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29 Οκτ 2013 (πριν από 3 χρόνια και 9 μήνες)

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Internet Routing

Chapter 27

Static Versus Dynamic Routing


Does not change (except in the event of an error)


Assumes a basic static routing table

Route Propagation Software

Goal: optimal routes

Static Routing in Hosts and a Default

Typical Situation

A host with one connection to a network with one

Use static routing! Do you need to know how to get to

One Host, One Router

Dynamic Routing and Routers

Usually, a router should
not use static routing

In this figure the
administrator of R1 does
not want to have to
manually change routes
on Net 3

Routing in the Global Internet

A two
level routing hierarchy

Routers and networks are broken up into groups

One or more routers in each group summarize
information and pass it on to other groups

A group is called an Autonomous System

Each such system chooses its own routing protocol

If Calvin adds a subnet for CS, we do NOT tell Hope

The Two Types of Routing Protocols

Interior Gateway Protocols


Several are available

Each autonomous system makes its own choice

Exterior Gateway Protocols


Summarize the data

Implement policy constraints

When EGPs and IGPs Are Used

Optimal Routes, Routing Metrics, and

Finding one optimal path is not realistic

What does “optimal” mean?

Interactive login

Least delay

Downloading a larger graphics file

Maximum throughput

Audio webcast

Least jitter

Typical internet routing combines hop count and
administrative cost


An administrative fence

Two routes from A to B

The administration does not want internal traffic
to go over the first route

Add administrative cost

Since each autonomous system chooses its own

EGPs cannot compare apples and oranges so they
only care about the existence of a path

Routes and Data Traffic

The Border Gateway Protocol


Version 4



Routing among autonomous systems

no use of routing metrics

Provision for Policies

Classifies each AS as transit or stub

Reliable transport using TCP

The Routing Information Protocol


Routing within an AS

Hop Count Metric

Unreliable transport using UDP

Broadcast (v1) or multicast (v2)

Support fro default route propagation

vector algorithm

Passive version for hosts

RIP Packet Format

The Open Shortest Path First Protocol


Much more scalable than RIP

Routes within an AS

Full CIDR and subnet support

Authenticated message exchange

Authentication refers to a guarantee of the source of a

Imported routes

From another protocol, say BGP

State algorithm

Support for metrics

Designates a single router to broadcast on the

An Example OSPF Graph

OSPF Areas

A manager divides an AS into areas

Routers only communicate within their own

Except for designated routers which exchange
information with other areas

This allows OSPF to scale well

Multicast Routing

Up to now we have only considered



Multicast is a major complication

Multicast uses designated IP addresses

Characterized by

dynamic membership

anonymous senders

Internet Group Membership Protocol


Only used between a host and a router

Group members are hosts, not applications

Multiple copies are made for each application

Last one to leave turns out the lights, ie, host
computer informs the router that it is no longer a
member of the group

Forwarding and Discovery Techniques

Routers, not hosts, have responsibility for the
propagation of multicast routing information

A large group may span the globe

Routing protocols must adapt quickly to
changes in membership

Three different approaches have emerged


Ideal for small groups in which members are attached to
contiguous LANs

In the flooding stage routers forward each datagram to all
networks to which they are attached

Reverse Path Broadcasting (RPB) is used to avoid loops

A router receives a datagram from A on interface 3, say

It checks its routing tables to see if it would send to A on
interface 3

If not, it drops the datagram

Eventually, a router learns that no hosts on a given
network are members of the group and prunes that


Ideal for groups spread out over long distances

A router at each site is configured to learn about
other sites

When a multicast datagram arrives, the router at
a site transmits on all attached LANs and
consults its configuration table to see which
other sites should receive a copy

Uses IP
IP tunneling (why?) to send a copy
to other sites

Based Discovery

Allows multicast to scale gracefully from a
small group in one area to a large group with
members at arbitrary locations

Each multicast group has a core unicast address

When a router needs to reach a group, it sends a
datagram to the core address

A router that participates in the group

forwards a multicast datagram to the group

responds to a request to join the group by adding the
information to its routes and forwards a copy of each
subsequent multicast datagram to the new member

Multicast Protocols (Proposed)

Distance Vector Multicast Routing Protocol


local multicast

IP to other sites

Core Based Trees


protocol software builds a delivery tree from a central point

Protocol Independent Multicast

Sparse Mode (PIM

tree approach

no particular unicast protocol

Dense Mode (PIM

within an organization


Multicast extensions (MOSPF)