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RoutingUnicast routing protocols

Jens A AnderssonElectrical and Information

Technology

Choosing an Optimal Path

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Router

• A router is a type of internetworking device that passes data packets between networks, based on Layer 3 or Network Layer addresses.

• A router has the ability to make intelligent decisions regarding the best path for delivery of data on the network.

Figure 21.1 Unicasting

Packet Switched Routing

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VPN, VPC

• Virtual Private Network• Virtual Path Connection

• Build a virtual and private network overlaying a public network

• Often by tunnelling (encapsulation of packets inside packets)

• Often encrypted

Figure 8.1 Circuit-switched network

Circuit-switched routing

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Local Routing

routerNetid: 192.168.1.0/24

.1

.105 .10

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ARP (1)

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ARP (2)

Reminder

• Routing performed on net ids• One net id per link/subnet! (at least)• (What if links/subnets without net ids?)

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Technology of Distributed Routing

• Topology information is flooded within the routing domain

• Best end-to-end paths are computed locally at each router.

• Best end-to-end paths determine next-hops.

• Based on minimizing some notion of distance

• Works only if policy is shared and uniform

• Examples: OSPF, IS-IS

• Each router knows little about network topology

• Only best next-hops are chosen by each router for each destination network.

• Best end-to-end paths result from composition of all next-hop choices

• Does not require any notion of distance

• Does not require uniform policies at all routers

• Examples: RIP, BGP

Link StateDistance Vector

Routing Tables and Forwarding Table

Forwarding Table

OSPFDomain

RIPDomain

BGP

OS kernel

OSPF Process

OSPF Routing tables

RIP Process

RIP Routing tables

BGP Process

BGP Routing tables

Forwarding Table Manager

Static routing table

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The Link Metric

• Possible metrics– hop count– inverse of the link bandwidth– delay– dynamically calculated – administratively assigned– combination

• Traffic should be monitored and metrics adjusted

Figure 21.2 Popular routing protocols

Routing Protocols

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Figure 21-18The Concept of Distance

Vector Routing

Table 21.1 A distance vector routing table

Destination Hop Count

Next Router Other information

163.5.0.0 7 172.6.23.4

197.5.13.0 5 176.3.6.17

189.45.0.0 4 200.5.1.6

115.0.0.0 6 131.4.7.19

Distance Vector Routing Table

Network id Host id of interfaceMetric

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RIP ( Routing Information Protocol)

• Distance vector algorithm• Included in BSD-UNIX Distribution in 1982• Distance metric: # of hops (max = 15 hops)

– Can you guess why there is a max?

• Distance vectors: exchanged among neighbours every 30 sec via Response Message (also called advertisement)

• Each advertisement: list of up to 25 destination nets within AS

RIP Updating Algorithm(1) if (advertised destination not in table) then

update table(2) else(2.a) if (advertised next-hop = next-hop in table) then

replace entry(2.b) else(2.b.i) if (advertised hop count < hop count in table) then

replace entry(2.b.ii) else

do nothing

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Figure 21-23

Example 21.1

RIP: Link Failure and RecoveryIf no advertisement heard after 180 sec -->

neighbour/link declared dead– routes via neighbour invalidated– new advertisements sent to neighbours– neighbours in turn send out new advertisements (if

tables changed)– link failure info quickly propagates to entire net– split horizon and poison reverse used to prevent

ping-pong loops (infinite distance = 16 hops)

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The Count to Infinity Problem

I can reach A at cost 1A I can reach

A at cost 2I can reach A at cost 3

I can reach A at cost 3

I don’t know A, so I will not tellA

I can reach A at cost 5A I can reach

A at cost 3

I can reach A at cost 2

I can reach A at cost 4

The Count to Infinity Problem (cont)

I can reach A at cost 6

I can reach A at cost 7A I can reach

A at cost 5

I can reach A at cost 8

I can reach A at cost 9A I can reach

A at cost 7

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Split Horizon

A I can’t reach A

1 2 3

I can reach A at cost 2 but I won’t tell 1

A

1 2 3

I can’t reach A

I can’t reach A

A I can reach A at cost 2 but I won’t tell 1

1 2 3

I can reach A at cost 1

Split Horizon withPoison Reverse

A

I can reach A at cost 2 but I tell 1 I can’t. Tells

1 that info about A came from 1.

1 2 3

I can reach A at cost 1

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Unsolved routing-loop

C B A

I tell D I can reach

C at cost 2,But I’ll tell

B that I can’t!

I can reach C at cost 1

DI tell A I can

reachC at cost 2,But I’ll tell

B that I can’t!

Figure 21-24

Concept of Link State Routing

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Figure 21-28

Link State Database

The Dijkstra Algorithm

1. Identify the root (the node itself)2. Attach all neighbour nodes temporarily3. Make arc and node with least cumulative

cost permanent4. Choose this node5. Repeat 2 and 3 until all nodes are

permanent

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Table 21.2 Link state routing table for router A

Network Cost Next Router Other Information

N1 5 C

N2 7 D

N3 10 B

N4 11 D

N5 15 C

Link State Routing Table

Network id Host id of interfaceMetric

Figure 21.7 Areas in an autonomous system

OSPF hierarchy

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Figure 21.8 Types of links

Types of Links

Figure 21.9 Point-to-point link

Point-to-Point Link

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Figure 21.10 Transient link

Transient Link

Figure 21.11 Stub link

Stub Link

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Figure 21.14 Types of LSAs

Types of LSAs

Figure 21.15 Router link

Router Link

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Figure 21.16 Network link

Network Links

Figure 21.17 Summary link to network

Summary Link to Network

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Figure 21.18 Summary link to AS boundary router

Summary link to AS boundary router

Figure 21.19 External link

External Link

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Comparison of LS and DV algorithms

Message complexity• LS: with n nodes, E links, O(nE)

msgs for full knowledge, changessent to all nodes

• DV: exchange - periodically or triggered - between neighbours only

Speed of Convergence• LS: O(n2) algorithm requires

O(nE) msgs– may have oscillations

• DV: convergence time varies– may be routing loops– count-to-infinity problem

Robustness: what happens if router malfunctions?

LS:– node can advertise incorrect

link cost for its “own” links– node can break broadcast path

or change broadcasted info– each node computes only its

own tableDV:

– DV node can advertise incorrect path cost to any path

– each node’s table used by others

• error propagate thru network

Flooding

• Routing without routing information• Forward each packet to all ports except

incoming port• Detect and remove looping packets• Very robust• This is not broadcast

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Source Address routing

• Route according to source address?!• Example:

– Customers share subnet (different ISPs; unlikely)

– Customers have different traffic policies

Internet AS HierarchyIntra-AS border (exterior gateway) routers

Inter-AS interior (gateway) routers

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Hierarchical Routing

• aggregate routers into regions, “autonomous systems” (AS)

• routers in same AS run same routing protocol– “intra-AS” routing

protocol– routers in different AS

can run different intra-AS routing protocol

• special routers in AS• run intra-AS routing

protocol with all other routers in AS

• also responsible for routing to destinations outside AS– run inter-AS routing

protocol with other gateway routers

gateway routers

Why different Intra- and Inter-AS routing ?

Policy:• Inter-AS: admin wants control over how its traffic

routed, who routes through its net. • Intra-AS: single admin, so no policy decisions neededScale:• hierarchical routing saves table size, reduced update

trafficPerformance:• Intra-AS: can focus on performance• Inter-AS: policy may dominate over performance

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Internet inter-AS routing: BGP

• BGP (Border Gateway Protocol): the de facto standard• Path Vector protocol:

– similar to Distance Vector protocol– each Border Gateway broadcast to neighbors (peers)

entire path (i.e., sequence of AS’s) to destination– BGP routes to networks (ASs), not individual hosts– E.g., Gateway X may send its path to dest. Z:

Path (X,Z) = X,Y1,Y2,Y3,…,Z

Figure 21.21 Path vector messages

Path Vector Messages

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Table 21.3 Path vector routing table

Network Next Router Path

N01 R01 AS14, AS23, AS67

N02 R05 AS22, AS67, AS05, AS89

N03 R06 AS67, AS89, AS09, AS34

N04 R12 AS62, AS02, AS09

Path Vector Routing Table

AS = Autonomous System = Organisation

BGP operation

Q: What does a BGP router do?• Receiving and filtering route advertisements from

directly attached neighbor(s). • Route selection.

– To route to destination X, which path (of several advertised) will be taken?

• Sending route advertisements to neighbors.

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BGP Operations (Simplified)

Establish session onTCP port 179

Exchange allactive routes

Exchange incrementalupdates

AS1

AS2

While connection is ALIVE exchangeroute UPDATE messages

BGP session

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BGP Route Processing

Best RouteSelection

Apply ImportPolicies

Best Route Table

Apply ExportPolicies

Install forwardingEntries for bestRoutes.

ReceiveBGPUpdates

BestRoutes

TransmitBGP Updates

Apply Policy =filter routes & tweak attributes

Based onAttributeValues

IP Forwarding Table

Apply Policy =filter routes & tweak attributes

Open ended programming.Constrained only by vendor configuration language

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Tweak Tweak Tweak

• For inbound traffic– Filter outbound routes– Tweak attributes on

outbound routes in the hope of influencing your neighbor’s best route selection

• For outbound traffic– Filter inbound routes– Tweak attributes on

inbound routes to influence best route selection

outboundroutes

inboundroutes

inboundtraffic

outboundtraffic

In general, an AS has morecontrol over outbound traffic

BGP messages• BGP messages exchanged using TCP.• BGP messages:

– OPEN: opens TCP connection to peer and authenticates sender

– UPDATE: advertises new path (or withdraws old)– KEEPALIVE keeps connection alive in absence of

UPDATES; also ACKs OPEN request– NOTIFICATION: reports errors in previous msg;

also used to close connection

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BGP AttributesValue Code Reference----- --------------------------------- ---------

1 ORIGIN [RFC1771]2 AS_PATH [RFC1771]3 NEXT_HOP [RFC1771]4 MULTI_EXIT_DISC [RFC1771]5 LOCAL_PREF [RFC1771]6 ATOMIC_AGGREGATE [RFC1771]7 AGGREGATOR [RFC1771]8 COMMUNITY [RFC1997]9 ORIGINATOR_ID [RFC2796]

10 CLUSTER_LIST [RFC2796]11 DPA [Chen]12 ADVERTISER [RFC1863]13 RCID_PATH / CLUSTER_ID [RFC1863]14 MP_REACH_NLRI [RFC2283] 15 MP_UNREACH_NLRI [RFC2283] 16 EXTENDED COMMUNITIES [Rosen]

...255 reserved for development

From IANA: http://www.iana.org/assignments/bgp-parametersNot all attributesneed to be present inevery announcement

Route Selection Summary

Highest Local Preference

Shortest ASPATH

Lowest MED

i-BGP < e-BGP

Lowest IGP cost to BGP egress

Lowest router ID

traffic engineering

Enforce relationships

Throw up hands andbreak ties

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Hot Potato Routing: Go for the Closest Egress Point

192.44.78.0/24

15 56 IGP distances

egress 1 egress 2

This Router has two BGP routes to 192.44.78.0/24.

Hot potato: get traffic off of your network as Soon as possible. Go for egress 1!

Hot Potato Routing

• Also:

Routing without queuing

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Customer-Provider Hierarchy

IP trafficprovider customer

Address aggregation!•Local•Nation•Global

Geographical

Deaggregation Due to Multihoming

AS 1

customerAS 2

provider

12.0.0.0/8

AS 3provider

12.2.0.0/1612.2.0.0/16

12.2.0.0/16

If AS 1 doesnot announce themore specific prefix,then most traffic to AS 2 will go through AS 3 because it is a longer match

AS 2 is “punching a hole” inThe CIDR block of AS 1

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Scarry?

• BGP is not guaranteed to converge on a stable routing. Policy interactions could lead to “livelock” protocol oscillations. See “Persistent Route Oscillations in Inter-domain Routing” by K. Varadhan, R. Govindan, and D. Estrin. ISI report, 1996

• Corollary: BGP is not guaranteed to recover from network failures.

Is There A Problem?packet switching

routing

interdomain routing

Border Gateway Protocol v4

Single Point of failure!

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Figure 19.25 NAT

19.64

Figure 19.12 NAT address translation

Alternative: External Source Address 200.24.5.8 goes here

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19.65

Table 19.4 Five-column translation table

Alternative: External source address 200.24.5.8 goes here

Alternative: External source port goes here