Download 15. Interior Routing Protocols

Chapter 15
Interior Routing Protocols
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Chapter 15 Interior Routing Protocols
Introduction
Routing protocols essential to operation of
an internet
 Routers forward IP datagrams from one
router to another on path from source to
destination
 Router must have idea of topology of
internet
 Routing protocols provide this information

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Chapter 15 Interior Routing Protocols
Internet Routing Principles
Routers receive and forward datagrams
 Make routing decisions based on
knowledge of topology and conditions on
internet
 Decisions based on some least cost
criterion (chapter 14)

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Chapter 15 Interior Routing Protocols
Fixed Routing

Single permanent route configured for
each source-destination pair
–
–
–
–
Routes fixed
May change when topology changes
Link cost not based on dynamic data
Based on estimated traffic volumes or capacity
of link
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Chapter 15 Interior Routing Protocols
Example Configuration
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Chapter 15 Interior Routing Protocols
Discussion of Example
5 networks, 8 routers
 Link cost for output side of each router for
each network

– Next slide shows how fixed cost routing may
be implemented
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Each router has routing table
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Chapter 15 Interior Routing Protocols
Routing Table
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One required for each router
Entry for each network
– Not for each destination
– Routing only needs network portion
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Once datagram reaches router attached to
destination network, that router can deliver to
host
IP address typically has network and host portion
Each entry shows next node on route
– Not whole route
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Chapter 15 Interior Routing Protocols
Routing Tables in Hosts

May also exist in hosts
– If attached to single network with single router
then not needed

All traffic must go through that router (called the
gateway)
– If multiple routers attached to network, host
needs table saying which to use
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Chapter 15 Interior Routing Protocols
Example Routing Tables
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Chapter 15 Interior Routing Protocols
Adaptive Routing

As conditions on internet changes, routes
may change
– Failure

Can route round problems
– Congestion
Can route round congestion
 Avoid, or at least not add to further congestion

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Chapter 15 Interior Routing Protocols
Drawbacks of Adaptive Routing

More complex routing decisions
– Router processing increases

Depends on information collected in one place but used
in another
– More information exchanged improves routing decisions but
increases overhead


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May react two fast causing congestion through oscillation
May react to slow, being irrelevant
Can produce pathologies
– Fluttering
– Looping
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Chapter 15 Interior Routing Protocols
Fluttering
Rapid oscillation in routing
 Due to router attempting load balancing or
splitting

– Splitting traffic among a number of routes
– May result in successive packets bound for
same destination taking very different routes
(see next slide)
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Chapter 15 Interior Routing Protocols
Example of Fluttering
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Chapter 15 Interior Routing Protocols
Problems with Fluttering

If in one direction only, route characteristics may
differ in the two directions
– Including timing and error characteristics
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Confuses management and troubleshooting applications that
measure these
Difficulty estimating round trip times
TCP packets arrive out of order
– Spurious retransmission
– Duplicate acknowledgements
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Chapter 15 Interior Routing Protocols
Looping
Packet forwarded by router eventually
returns to that router
 Algorithms designed to prevent looping
 May occur when changes in connectivity
not propagated fast enough to all other
routers

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Chapter 15 Interior Routing Protocols
Adaptive Routing Advantages
Improve performance as seen by user
 Can aid congestion control
 Benefits depend on soundness of design
 Adaptive routing very complex

– Continual evolution of protocols
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Chapter 15 Interior Routing Protocols
Classification of Adaptive
Routing Strategies

Based on information sources
– Local
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E.g. route each datagram to network with shortest queue
Balance loads on networks
May not be heading in correct direction
– Include preferred direction

Rarely used
– Adjacent nodes
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Distance vector algorithms
– All nodes

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Link-state algorithms
Both need routing protocol to exchange information
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Chapter 15 Interior Routing Protocols
Autonomous Systems (AS)
Group of routers exchanging information
via common routing protocol
 Set of routers and networks managed by
single organization
 Connected

– Except in time of failure
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Chapter 15 Interior Routing Protocols
Interior Routing Protocol (IRP)

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Passes routing information between routers
within AS
Does not need to be implemented outside AS
– Allows IRP to be tailored
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May be different algorithms and routing
information in different connected AS
Need minimum information from other
connected AS
– At least one router in each AS must talk
– Use Exterior Routing Protocol (ERP)
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Chapter 15 Interior Routing Protocols
Exterior Routing Protocol (ERP)
Pass less information than IRP
 Router in first system determines route to
target AS
 Routers in target AS then co-operate to
deliver datagram
 ERP does not deal with details within
target AS

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Chapter 15 Interior Routing Protocols
IRP and ERP
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Chapter 15 Interior Routing Protocols
Routing Information Protocol
(RIP)
Simple
 Suitable for small internets
 Widely used
 Uses Distance vector routing

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Chapter 15 Interior Routing Protocols
Distance Vector Routing
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Each node exchange information with neighbors
– Directly connected by same network
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Each node maintains three vectors
– Link cost
– Distance vector
– Next hop vector
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Every 30 seconds, exchange distance vector with
neighbors
Use this to update distance and next hop vector
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Chapter 15 Interior Routing Protocols
Distance Vector Example
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Chapter 15 Interior Routing Protocols
Distributed Bellman-Ford
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RIP is a distributed version of Bellman-Ford
Original routing algorithm in ARPANET
Each simultaneous exchange of vectors between
routers is equivalent to one iteration of step 2
In fact, asynchronous exchange used
– At start-up, get vectors from neighbors

Gives initial routing
– By own timer, update every 30 seconds
– Changes are propagated across network
– Routing converges within finite time

Proportional to number of routers
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Chapter 15 Interior Routing Protocols
RIP Details –
Incremental Update
Updates do not arrive from neighbors
within small time window
 RIP packets use UDP
 Tables updated after receipt of individual
distance vector

– Add any new destination network
– Replace existing routes with small delay ones
– If update from router R, update all routes
using R as next hop
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Chapter 15 Interior Routing Protocols
RIP Details –
Topology Change

If no updates received from a router within
180 seconds, mark route invalid
– Assumes router crash or network connection
unstable
– Set distance value to infinity
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Actually 16
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Chapter 15 Interior Routing Protocols
Counting to Infinity Problem (1)
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Slow convergence may cause:
All link costs 1
B has distance to network 5 as 2, next hop D
A & C have distance 3 and next hop B
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Chapter 15 Interior Routing Protocols
Counting to Infinity Problem (2)
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Suppose router D fails:
– B determines network 5 no longer reachable via D
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Sets distance to 4 based on report from A or C
– At next update, B tells A and C this
– A and C receive this and increment their network 5 distance to 5
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4 from B plus 1 to reach B
– B receives distance count 5 and assumes network 5 is 6 away
– Repeat until reach infinity (16)
– Takes 8 to 16 minutes to resolve
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Chapter 15 Interior Routing Protocols
Counting to Infinity Diagram
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Chapter 15 Interior Routing Protocols
Split Horizon

Counting to infinity problem caused by
misunderstanding between B and A, and B and C
– Each thinks it can reach network 5 via the other
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Split Horizon rule says do not send information
about a route back in the direction it came from
– Router sending information is nearer destination than
you
– Erroneous route now eliminated within time out
period (180 seconds)
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Chapter 15 Interior Routing Protocols
Poisoned Reverse

Send updates with hop count of 16 to
neighbors for route learned from those
neighbors
– If two routers have routes pointing at each
other advertising reverse route with metric 16
breaks loop immediately
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Chapter 15 Interior Routing Protocols
RIP Packet Format
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Chapter 15 Interior Routing Protocols
RIP Packet Format Notes
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Command: 1=request 2=reply
– Updates are replies whether asked for or not
– Initializing node broadcasts request
– Requests are replied to immediately
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Version: 1 or 2
Address family: 2 for IP
IP address: non-zero network portion, zero host portion
– Identifies particular network
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Metric
– Path distance from this router to network
– Typically 1, so metric is hop count
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Chapter 15 Interior Routing Protocols
RIP Limitations
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Destinations with metric more than 15 are
unreachable
– If larger metric allowed, convergence becomes
lengthy
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Simple metric leads to sub-optimal routing tables
– Packets sent over slower links
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Accept RIP updates from any device
– Misconfigured device can disrupt entire configuration
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Chapter 15 Interior Routing Protocols
Open Shortest Path First
(OSPF)
RIP limited in large internets
 OSPF preferred interior routing protocol
for TCP/IP based internets
 Link state routing used
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Chapter 15 Interior Routing Protocols
Link State Routing

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When initialized, router determines link cost on
each interface
Router advertises these costs to all other routers
in topology
Router monitors its costs
– When changes occurs, costs are re-advertised
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Each router constructs topology and calculates
shortest path to each destination network
Not distributed version of routing algorithm
Can use any algorithm
– Dijkstra
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Chapter 15 Interior Routing Protocols
Flooding
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Packet sent by source router to every neighbor
Incoming packet resent to all outgoing links except
source link
Duplicate packets already transmitted are discarded
– Prevent incessant retransmission

All possible routes tried so packet will get through if
route exists
– Highly robust

At least one packet follows minimum delay route
– Reach all routers quickly
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All nodes connected to source are visited
– All routers get information to build routing table
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High traffic load
Chapter 15 Interior Routing Protocols
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Flooding Example
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Chapter 15 Interior Routing Protocols
OSPF Overview
Router maintains descriptions of state of
local links
 Transmits updated state information to all
routers it knows about
 Router receiving update must acknowledge

– Lots of traffic generated

Each router maintains database
– Directed graph
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Chapter 15 Interior Routing Protocols
Router Database Graph
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Vertices
– Router
– Network
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Transit
Stub
Edges
– Connecting two routers
– Connecting router to network

Built using link state information from other
routers
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Chapter 15 Interior Routing Protocols
Sample
Autonomous
System
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Resultant
Directed
Graph
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Chapter 15 Interior Routing Protocols
Link Costs

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Cost of each hop in each direction is called
routing metric
OSPF provides flexible metric scheme based on
type of service (TOS)
–
–
–
–
–
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Normal (TOS) 0
Minimize monetary cost (TOS 2)
Maximize reliability (TOS 4)
Maximize throughput (TOS 8)
Minimize delay (TOS 16)
Each router generates 5 spanning trees (and 5
routing tables)
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Chapter 15 Interior Routing Protocols
SPF Tree
for
Router 6
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Chapter 15 Interior Routing Protocols
Areas
Make large internets more manageable
 Configure as backbone and multiple areas
 Area – Collection of contiguous networks
and hosts plus routers connected to any
included network
 Backbone – contiguous collection of
networks not contained in any area, their
attached routers and routers belonging to
multiple areas

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Chapter 15 Interior Routing Protocols
Operation of Areas

Each are runs a separate copy of the link
state algorithm
– Topological database and graph of just that
area
– Link state information broadcast to other
routers in area
– Reduces traffic
– Intra-area routing relies solely on local link
state information
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Chapter 15 Interior Routing Protocols
Inter-Area Routing

Path consists of three legs
– Within source area
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Intra-area
– Through backbone
Has properties of an area
 Uses link state routing algorithm for inter-area
routing
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– Within destination area
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Intra-area
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Chapter 15 Interior Routing Protocols
OSPF Packet Format
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Chapter 15 Interior Routing Protocols
Packet Format Notes
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Version number: 2 is current
Type: one of 5, see next slide
Packet length: in octets including header
Router id: this packet’s source, 32 bit
Area id: Area to which source router belongs
Authentication type: null, simple password or
encryption
Authentication data: used by authentication
procedure
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Chapter 15 Interior Routing Protocols
OSPF Packet Types
Hello: used in neighbor discovery
 Database description: Defines set of link
state information present in each router’s
database
 Link state request
 Link state update
 Link state acknowledgement

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Chapter 15 Interior Routing Protocols