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Routing Architecture and Protocols

NETS3303/3603

Week 6

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Outline• Intro concepts

– Issues– Architecture

• Routing protocols:– Distance-vector– Link-state

• Autonomous System and Exterior Gateway Protocol– BGP

• Interior Gateway Protocol– RIP1 and RIP2– OSPF

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Review - Internet Routing

• IP implements datagram forwarding

• Both hosts and routers– Have an IP module– Forward datagrams

• IP forwarding is table-driven– Table known as routing table

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Elements of Routing

• routing protocols that allow info to be gathered and distributed – routers communicate with these protocols

• routing algorithms – compute good routes based on gathered data (like Bellman-Ford and Dijkstra)

• routing table – database of routes

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Its Classic problem!

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How / When Are IP Routing Tables Built?

• Depends on size / complexity of internet• Static routing

– Fixes routes at boot time

– Useful only for simplest cases

• Dynamic routing– Table initialized at boot time

– Values inserted / updated by protocols that propagate route information

– Necessary in large internets

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Routing Tables

• Two sources of information– Initialization (e.g., from disk)– Update (e.g., from protocols)

• Hosts tend to freeze the routing table after initialization

• But, routers use protocols to learn new information and update their routing table dynamically

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Original Arpanet Routing Architecture

• Small set of ‘‘core’’ routers with complete information about all destinations

• Other routers know local destinations and use the core as central router (default route)

• Disadvantages of original core:– Central bottleneck for all traffic

– No shortcut routes possible

– Does not scale

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General Idea – Better!

• Have a set of core routers know routes to all locations

• Devise a mechanism that allows other routers to contact the core to learn routes (spread necessary routing information automatically)

• Continually update routing information

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Automatic Route Propagation

• Two basic algorithms used by routing update protocols– Distance-vector– Link-state

• Many variations in implementation details

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Distance-Vector Algorithm

• Initialize routing table with one entry for each directly connected network

• Periodically run a distance-vector update to exchange information with routers that are reachable over directly connected networks

• Each router sends list of its routes to another

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DV algorithm

• examples: RIP, BGP• Its algorithm elements:

– send: every N seconds out all connected interfaces broadcast 2-tuples : (to network X, hop count Y) ...

– recv: if new tuple, add to routing table; if better tuple, change existing; if “dead” tuple, remove

– timeout: if no refresh, timeout entry in N * Y seconds

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Example Of DV Update

• Router K received an update from router J

• (a) is existing routing table at K

• (b) incoming update (marked items cause change)

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slow convergence/count to infinity

• DV’s big problem!• changes can be sent when they occur, but must

recompute a bit so convergence takes time (made worse by possible loops)

• count to infinity problem can occur too - routing loop until hopcount reaches impossible value

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Count to infinity

• C crashes, B knows C crashed but hasn’t told A, but unfortunately A talks to B first B is told by A: – I can get to C in two hops (and note it doesn’t mention to

B that the path is thru B)

• B says AHA!, that means I can get to C in three hops and reports that to A

• A says AHA!, it’s now four hops to B and tells B etc...

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split-horizon and poison reverse fixup

• Split-horizon:– A does not tell B that it can reach C (white lie)– Because its through B

• Poison reverse:– When B loses connection to C, its distance to C is

changed to infinity– An immediate update is triggered, without wait for

regular update• when link goes away, B will know that there is no

path to C, and tell A• Still doesn’t work in all cases…

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Link-State Algorithm

• Alternative to distance-vector• Distributed computation

– Broadcast information– Allow each router to compute shortest paths

• Avoids problem where one router can damage the entire internet by passing incorrect information

• Also called Shortest Path First (SPF)

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Link-State Update

• Participating routers learn internet topology• Think of routers as nodes in a graph, and networks

connecting them as edges or links• Pairs of directly-connected routers periodically

– Test link between them– Propagate (broadcast) status of link

• All routers– Receive link status messages– Recompute routes from their local copy of information

A

ED

CB

F

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1 - Determine link-state

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2 - Send LS-update

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3 - Compute shortest path

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link-state: pros/cons

• pros– converges faster, no count to infinity problem +

router can forward LSP immediately– more functionality; e.g., each router has map

of net, can make network debugging easier

• cons– more compute than DV (does this matter?)

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ROUTING: EXTERIOR GATEWAY

PROTOCOLS AND AUTONOMOUSSYSTEMS (BGP)

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General Principle for Internet Routing

• Although it is desirable for routers to exchange routing information, it is impractical for all routers in an arbitrarily large internet to participate in a single routing update protocol

• Consequence: routers must be divided into groups

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A Practical Limit On Group Size

• Up to a dozen routers to participate in a single routing area across a WAN

• approximately five times as many can safely participate across a set of LANs

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Router Outside A Group

• Does not participate directly in group’s routing information propagation algorithm

• Problems:– Will not choose optimal routes if it uses a

member of the group for general delivery– May not know all networks from other groups

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The Extra HopProblem

• Non-participating router picks one participating router to use (e.g., R2)• Non-participating router routes all packets to R2 across backbone• Router R2 routes some packets back across backbone to R1

• So, a mechanism is needed that allows nonparticipating routers to learn routes from participating routers– so they can choose optimal routes.

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The Hidden Networks Problem

• Group must learn routes from nonparticipating routers

• Example: owner of networks 1 and 3 must tell group that there is a route to network 4

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Autonomous System Concept (AS)

• Group of networks under one administrative authority

• Free to choose internal routing update mechanism

• Connects to one or more other autonomous systems

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EGPs: Exterior GatewayProtocols

• A protocol for communicating routes between two autonomous systems

• Solves two problems– Allows router outside a group to advertise

networks hidden in another autonomous system– Allows router outside a group to learn

destinations in the group

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Border Gateway Protocol

• The most popular (virtually the only) EGP in use in the Internet

• Current version is BGP-4• Supports CIDR (mask accompanies each

route)• Each AS designates a border router to

speak on its behalf• Two border routers become BGP peers

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Key Characteristics Of BGP

• Provides inter-autonomous system communication• Propagates reachability information• Follows next-hop paradigm• Provides support for policies• Sends path information• Permits incremental updates• Allows route aggregation• Allows authentication

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Additional BGP Facts

• Uses reliable transport (i.e., TCP)– Unusual: most routing update protocols use

connectionless transport (e.g., UDP)

• Sends keepalive messages so other end knows connection is valid (even if no new routing information is needed)

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Four BGP Message Types

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BGP Message Header

• Each BGP message starts with this header

• Marker is used by peers to indicate message boundary => synchronisation

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BGP Open Message

• Used to start a connection

• HOLD TIME specifies max time that can elapse between BGP messages

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BGP Update Message

• Sender can advertise new routes or withdraw old routes

• Each route entry consists of address and mask• Entry can be compressed to eliminate zero bytes

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BGP Must Consider Receiver’s Perspective

• Two issues are considered: policies and optimal routes

• Advertise not just destinations but report reachability too

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Path Metric Interpretation

• Each AS use own IGP may be with different metric (hop count, delay, policy-based values)

• So, an exterior gateway protocol does not communicate or interpret metrics, even if metrics are available!

• BGP only propagates reachability information– a receiver can implement policy constraints

– BUT cannot choose a least cost route.

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ROUTING: INSIDE ANAUTONOMOUS SYSTEM

(RIP, OSPF)• Static routes

– Initialized at startup– Never change– Typical for host– Sometimes used for router

• Dynamic routes– Initialized at startup– Updated by route propagation protocols– Typical for router– Sometimes used in host

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Exchanging Routing Information Within An Autonomous System

• Mechanisms called interior gateway protocols, IGPs

• Choice of IGP is made by autonomous system

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Example Of Two Autonomous Systems

And the Routing Protocols Used

RIP OSPF

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Routing Information Protocol (RIP)

• Implemented by UNIX program routed• Uses hop count metric• Distance-vector protocol• Relies on broadcast• Assumes low-delay LAN• Uses split horizon and poison reverse techniques

to solve inconsistencies• Current standard is RIP2

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Two Forms Of RIP

• Active– Form used by routers– Broadcasts routing updates periodically– Uses incoming messages to update routes

• Passive– Form used by hosts– Uses incoming messages to update routes– Does not send updates

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RIP Operation

• Each router sends update every 30 seconds

• Update contains pairs of

(destination address, distance)

• Distance of 16 is infinity (i.e., no route)– This limits span of its internet to 16 hops– Any 2 nodes have at most 15 routers between

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Illustration Of HostsUsing Passive RIP

• Host listens for RIP broadcast and uses data to update table

• Eliminates ICMP redirects

• Host routing table initialized to:

Destination Route

128.10.0.0

default

Direct

128.10.0.200

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Changes To RIP In Version 2

• RIP1 does not include subnet mask– Only suitable for classful or fixed-len subnets

• Update includes subnet mask

• Authentication supported

• Explicit next-hop information

• Messages can be multicast (optional)– IP multicast address is 224.0.0.9

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RIP2 Update Format

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Open Shortest Path First (OSPF)

• Developed by IETF in response to vendors’ proprietary protocols

• Uses link-state algorithm

• More powerful than most predecessors

• Permits hierarchical topology

• More complex to install and manage

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OSPF Features

• Type of service routing – the first to offer• Load balancing across multiple paths• Networks partitioned into subsets called areas

– Designated router per area

• Message authentication• Virtual network topology abstracts away details• Can import external routing information

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OSPF Message Header

• Each message starts with same header• OSPF Message Types

– 1 Hello (used to test reachability)– 2 Database description (topology)– 3 Link status request– 4 Link status update– 5 Link status acknowledgement

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OSPF HELLO Message Format

• Used to establish and test reachability

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OSPF Database Description Message Format

• Initialises network topology database• One serve as a Master; other slave• Can be large => separate into different msgs

– I is 1 for first message, M is 1 for more messages

RouterNetwork

Summary of netsAS boundary

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Summary

• Internet is too large for all routers to participate in one routing update protocol

• Group of networks and routers under one administrative authority is called Autonomous System (AS)

• EGP is used to communicate routing information between two autonomous systems

• Each AS chooses its own interior routing update protocol• Popular IGPs include

– RIP (distance vector algorithm)– OSPF (link-state algorithm)