chapter 15 interior routing protocols 1 chapter 15 interior routing protocols

Chapter 15 Interior Routing Protocols1

Chapter 15Chapter 15Interior Routing Protocols


IntroductionIntroduction

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


Internet Routing PrinciplesInternet Routing Principles

Routers receive and forward datagramsMake routing decisions based on

knowledge of topology and conditions on internet

Decisions based on some least cost criterion (chapter 14)


Fixed RoutingFixed 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


Example ConfigurationExample Configuration


Discussion of ExampleDiscussion of Example

5 networks, 8 routersLink cost for output side of each router for

each network– Next slide shows how fixed cost routing may

be implementedEach router has routing table


Routing TableRouting Table

One required for each router Entry for each network

– Not for each destination– Routing only needs network portion

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


Routing Tables in HostsRouting Tables in Hosts

May also exist in hosts– If attached to single network with single

router then not neededAll traffic must go through that router (called the

gateway)

– If multiple routers attached to network, host needs table saying which to use


Example Routing TablesExample Routing Tables


Adaptive RoutingAdaptive Routing

As conditions on internet changes, routes may change– Failure

Can route round problems

– CongestionCan route round congestionAvoid, or at least not add to further congestion


Drawbacks of Adaptive RoutingDrawbacks 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 May react two fast causing congestion through

oscillation May react to slow, being irrelevant Can produce pathologies

– Fluttering– Looping


FlutteringFluttering

Rapid oscillation in routingDue 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)


Example of FlutteringExample of Fluttering


Problems with FlutteringProblems with Fluttering

If in one direction only, route characteristics may differ in the two directions– Including timing and error characteristics

Confuses management and troubleshooting applications that measure these

Difficulty estimating round trip times TCP packets arrive out of order

– Spurious retransmission

– Duplicate acknowledgements


LoopingLooping

Packet forwarded by router eventually returns to that router

Algorithms designed to prevent loopingMay occur when changes in connectivity

not propagated fast enough to all other routers


Adaptive Routing AdvantagesAdaptive Routing Advantages

Improve performance as seen by userCan aid congestion controlBenefits depend on soundness of designAdaptive routing very complex

– Continual evolution of protocols


Classification of Adaptive Classification of Adaptive Routing StrategiesRouting Strategies Based on information sources

– Local 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 Distance vector algorithms

– All nodes Link-state algorithms Both need routing protocol to exchange information


Autonomous Systems (AS)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


Interior Routing Protocol (IRP)Interior Routing Protocol (IRP)

Passes routing information between routers within AS

Does not need to be implemented outside AS– Allows IRP to be tailored

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)


Exterior Routing Protocol Exterior Routing Protocol (ERP)(ERP)Pass less information than IRPRouter in first system determines route to

target ASRouters in target AS then co-operate to

deliver datagramERP does not deal with details within

target AS


IRP and ERPIRP and ERP


Routing Information ProtocolRouting Information Protocol(RIP)(RIP)SimpleSuitable for small internetsWidely usedUses Distance vector routing


Distance Vector RoutingDistance Vector Routing

Each node exchange information with neighbors– Directly connected by same network

Each node maintains three vectors– Link cost

– Distance vector

– Next hop vector Every 30 seconds, exchange distance vector with

neighbors Use this to update distance and next hop vector


Distance Vector ExampleDistance Vector Example


Distributed Bellman-FordDistributed Bellman-Ford

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


RIP Details – RIP Details – Incremental UpdateIncremental UpdateUpdates do not arrive from neighbors

within small time windowRIP packets use UDPTables 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


RIP Details –RIP Details –Topology ChangeTopology ChangeIf no updates received from a router within

180 seconds, mark route invalid– Assumes router crash or network connection

unstable– Set distance value to infinity

Actually 16


Counting to Infinity Problem (1)Counting to Infinity Problem (1)

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


Counting to Infinity Problem (2)Counting to Infinity Problem (2)

Suppose router D fails:– B determines network 5 no longer reachable via D

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 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


Counting to Infinity DiagramCounting to Infinity Diagram


Split HorizonSplit 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

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)


Poisoned ReversePoisoned 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


RIP Packet FormatRIP Packet Format


RIP Packet Format NotesRIP Packet Format Notes

Command: 1=request 2=reply– Updates are replies whether asked for or not– Initializing node broadcasts request– Requests are replied to immediately

Version: 1 or 2 Address family: 2 for IP IP address: non-zero network portion, zero host portion

– Identifies particular network Metric

– Path distance from this router to network– Typically 1, so metric is hop count


RIP LimitationsRIP Limitations

Destinations with metric more than 15 are unreachable– If larger metric allowed, convergence becomes

lengthy Simple metric leads to sub-optimal routing tables

– Packets sent over slower links Accept RIP updates from any device

– Misconfigured device can disrupt entire configuration


Open Shortest Path First Open Shortest Path First (OSPF)(OSPF)RIP limited in large internetsOSPF preferred interior routing protocol

for TCP/IP based internetsLink state routing used


Link State RoutingLink State Routing

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

Each router constructs topology and calculates shortest path to each destination network

Not distributed version of routing algorithm Can use any algorithm

– Dijkstra


FloodingFlooding 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

All nodes connected to source are visited– All routers get information to build routing table

High traffic load


Flooding ExampleFlooding Example


OSPF OverviewOSPF 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


Router Database GraphRouter Database Graph

Vertices– Router– Network

Transit Stub

Edges– Connecting two routers– Connecting router to network

Built using link state information from other routers


Sample Sample Autonomous Autonomous SystemSystem


Resultant Resultant Directed Directed GraphGraph


Link CostsLink Costs

Cost of each hop in each direction is called routing metric

OSPF provides flexible metric scheme based on type of service (TOS)– 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)


SPF Tree SPF Tree for for Router 6Router 6


AreasAreas

Make large internets more manageableConfigure as backbone and multiple areasArea – 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


Operation of AreasOperation 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


Inter-Area RoutingInter-Area Routing

Path consists of three legs– Within source area

Intra-area

– Through backboneHas properties of an areaUses link state routing algorithm for inter-area

routing

– Within destination area Intra-area


OSPF Packet FormatOSPF Packet Format


Packet Format NotesPacket Format Notes

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


OSPF Packet TypesOSPF Packet Types

Hello: used in neighbor discoveryDatabase description: Defines set of link

state information present in each router’s database

Link state requestLink state updateLink state acknowledgement

chapter 15 interior routing protocols 1 chapter 15 interior routing protocols

Documents

fixed cost routing

single router

implementedeach router

destination network

single network

cost criterion chapter

table chapter

topology changeslink