CSE 30264 Computer Networks

CSE 30264

Computer Networks

Prof. Aaron StriegelDepartment of Computer Science & Engineering

University of Notre Dame

Lecture 12 – February 18, 2010

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Today’s Lecture

• Routing– Distance Vector– Link State

• Global Routing

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Physical

Data

Network

Transport

Application

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Routing (Revisited)

OutlineDistance VectorLink State


Overview• Forwarding vs Routing

– Forwarding: to select an output port based on destination address and routing table

– Routing: process by which routing table is built• Network as a Graph

• Problem: Find lowest cost path between two nodes– How much do you know?

• Distance vector: Just my neighbors and who they can reach• Link state: Everything on my network


Distance Vector• Each node maintains a set of triples

– (Destination, Cost, NextHop)

Destination Cost Next HopA 0 Local

B 2 E

C 7 B

D 2 E

.. .. ..

Exchange the vector (the table) of my current routing information

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Routing Revisited

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F

EB

C D

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Time t=0, Start Time

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F

EB

C D

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Perspective of A

A’s Routing Table

A C=0 NH: *

Assume know our neighbors and link cost

B C=3 NH: BE C=1 NH: EF C=6 NH: F

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Symmetry

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B

3 A

B

3

3

Cost in either direction is the same

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A sends to its neighbors

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A C=0 NH: Me


Routing Table

Vector of reachability

B,3

E,1

F,1

Send this to our neighbors

A

F

EB

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E broadcasts to its neighbors

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F

EB

C D

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4A,1

B,1

D,1

F,2

Distance Vector

Vector = Here is my best cost to get to these locations if you come through me

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Gather round….

• Send our distance vector– Cost to reach nodes through us– Two ways

• Cost in the vector• Let external node put in cost

• When we receive a DV– Are any routes better than our own?– Could we take the cost of using

that link and navigate via that node?

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What happens at A?

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F

EB

C D

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Perspective of A

A’s Routing Table

A C=0 NH: *


A,1

B,1

D,1

F,2

DV from EThat’s us, duh

Compare new vs. current

3 vs. 1+1 via EInf vs. 1+1 via E6 vs. 1+2 via E

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What happens at A?

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F

EB

C D

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Perspective of A

A’s Routing Table

A C=0 NH: *


A C=0 NH: *

B C=2 NH: EE C=1 NH: EF C=3 NH: ED C=E NH: E

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Convergence

• Convergence– Stop when no more changes– Non-convergence bad

• We’ll see that in a moment• How?

– Only send on change of local table– Stop seeing bcast, stop

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Routing Loops• Example 1

– F detects that link to G has failed– F sets distance to G to infinity and sends update t o A– A sets distance to G to infinity since it uses F to reach G– A receives periodic update from C with 2-hop path to G– A sets distance to G to 3 and sends update to F– F decides it can reach G in 4 hops via A

• Example 2– link from A to E fails– A advertises distance of infinity to E– B and C advertise a distance of 2 to E– B decides it can reach E in 3 hops; advertises this to A– A decides it can read E in 4 hops; advertises this to C– C decides that it can reach E in 5 hops…

Count to Infinity Problem

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Loop-Breaking Heuristics

• Set infinity to 16• Split horizon• Split horizon with poison reverse

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Is DV used?

• Yes– RIP – Routing Information Protocol– BGP – Border Gateway Protocol*

• Sort of, BGP is kind of a hybrid– Various mobile routing protocols

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Routing Information Protocol (RIP)

• Distributed along with BSD Unix• Straightforward implementation of DV• Updates sent every 30 seconds• Link costs constant at 1 (16 = infinity)

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Link State• Strategy

– Two-fold• Exchange HELLO msgs

with neighbors• Broadcast changes to all

of neighbor state

• Link State Packet (LSP)– ID of the node that created the LSP– Cost of link– Sequence number – TTL

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Link State (cont)

• Changes Reliably flood– store most recent LSP from each node– forward LSP to all nodes but one that sent it– generate new LSP periodically

• increment SEQNO– start SEQNO at 0 when reboot– decrement TTL of each LSP

• discard when TTL=0

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Flooding

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Route Calculation• Dijkstra’s shortest-path algorithm• Let

– N denotes set of nodes in the graph– l (i, j) denotes non-negative cost (weight) for edge (i, j)– s denotes this node– M denotes the set of nodes incorporated so far– C(n) denotes cost of the path from s to node n

M = {s}for each n in N - {s}

C(n) = l(s, n)while (N != M)

M = M union {w} such that C(w) is the minimum for all w in (N - M)

for each n in (N - M)C(n) = MIN(C(n), C (w) + l(w, n ))

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Link State Example

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F

EB

C D

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Assume that A gets updates from everyone else via reliably flooded updates, i.e. A knows the topology of the graph

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Link State Example

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F,*

E,*B,*

C,*D,*

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Step 0: Set everyone’s weight to infinity Add ourselves as the lowest cost node

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Link State Example

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F,*

E,*B,*

C,*D,*

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Lowest cost total reachable cost EWho has the lowest total reachable cost from our covered net?

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E wins, add it to our covered graph

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A

F,*

E,1B,*

C,*D,*

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Lowest cost total reachable cost B or D tied at 2Who has the lowest total reachable cost from our covered net?

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B wins, add it to our covered graph

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A

F,*

E,1B,2

C,*D,*

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Lowest cost total reachable cost D at 2Who has the lowest total reachable cost from our covered net?

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D wins, add it to our covered graph

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F,*

E,1B,2

C,*D,2

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Lowest cost total reachable cost F with a cost of 3Who has the lowest total reachable cost from our covered net?

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F wins, add it to our covered graph

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F,3

E,1B,2

C,*D,2

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Lowest cost total reachable cost C with a cost of 6Who has the lowest total reachable cost from our covered net?

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C wins, all done

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F,3

E,1B,2

C,6D,2

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What does it mean?

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F,3

E,1B,2

C,6D,2

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4 Target Next HopB E

C E

D EE E

F E

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OSPF

• Open Shortest Path First Protocol• Authentication• Additional hierarchy• Load balancing

Quintessential link state protocol

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Metrics • Original ARPANET metric

– measures number of packets queued on each link– took neither latency or bandwidth into consideration

• New ARPANET metric– stamp each incoming packet with its arrival time (AT)– record departure time (DT)– when link-level ACK arrives, compute

Delay = (DT - AT) + Transmit + Latency– if timeout, reset DT to departure time for retransmission – link cost = average delay over some time period

• Revised ARPANET metric– compressed dynamic range– replaced Delay with link utilization

• Practice– static metrics (e.g., 1/bandwidth)

Why might variable weights be bad?

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Global Internet

OutlineSubnettingSupernetting

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How to Make Routing Scale

• Flat versus Hierarchical Addresses• Inefficient use of Hierarchical Address Space

– Class C with 2 hosts (2/254 = 0.78% efficient)– Class B with 256 hosts (256/65534 = 0.39% efficient)

• Too many networks– Routing tables do not scale– Route propagation protocols do not scale

Current routers – IPv4 – core 160k entries

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Internet StructureRecent Past

NSFNET backboneStanford

BARRNETregional

Berkeley PARCNCAR

UA

UNM

Westnetregional

UNL KU

ISU

MidNetregional■ ■ ■

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Internet StructureToday

Backbone service providerPeeringpoint Peering

point

Large corporation

Large corporationSmall

corporation

“Consumer” ISP

“Consumer” ISP

“Consumer” ISP

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Abilene – aka Internet 2

Spring 2010

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Subnetting• Add another level to address/routing hierarchy: subnet• Subnet masks define variable partition of host part• Subnets visible only within site

Network number Host number

Class B address

Subnet mask (255.255.255.0)

Subnetted address

11111111111111111111111100000000

Network number Host IDSubnet ID

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Forwarding AlgorithmD = destination IP addressfor each entry (SubnetNum, SubnetMask, NextHop) D1 = SubnetMask & D if D1 = SubnetNum if NextHop is an interface deliver datagram directly to D else deliver datagram to NextHop

• Use a default router if nothing matches• Not necessary for all 1s in subnet mask to be contiguous

– More likely than not, it will be• Can put multiple subnets on one physical network• Subnets not visible from the rest of the Internet

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Supernetting

• Assign block of contiguous network numbers to nearby networks

• Called CIDR: Classless Inter-Domain Routing• Represent blocks with a single pair (first_network_address, count)

• Restrict block sizes to powers of 2• Use a bit mask (CIDR mask) to identify block size• All routers must understand CIDR addressing

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CIDR

Border gateway(advertises path to

11000000000001) Regional network

Corporation X

(11000000000001000001)

Corporation Y

(11000000000001000000)

Instead of advertising two networks upstream, just advertise one

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Route Propagation• Know a smarter router

– hosts know local router– local routers know site routers– site routers know core router– core routers know everything

• Autonomous System (AS)– corresponds to an administrative domain– examples: University, company, backbone network– assign each AS a 16-bit number

• Two-level route propagation hierarchy– interior gateway protocol (each AS selects its own)– exterior gateway protocol (Internet-wide standard)

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Inter-Domain/Intra-Domain

R1

Autonomous system 1R2

R3

Autonomous system 2R4

R5 R6

Edge router

Core router

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Popular Interior Gateway Protocols

• RIP: Route Information Protocol– developed for XNS– distributed with Unix– distance-vector algorithm– based on hop-count

• OSPF: Open Shortest Path First– recent Internet standard– uses link-state algorithm– supports load balancing – supports authentication

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EGP: Exterior Gateway Protocol• Overview

– designed for tree-structured Internet– concerned with reachability, not optimal routes

• Protocol messages– Neighbor acquisition: one router requests that another

be its peer; peers exchange reachability information– Neighbor reachability: one router periodically tests if

the another is still reachable; exchange HELLO/ACK messages; uses a k-out-of-n rule

– Routing updates: peers periodically exchange their routing tables (distance-vector)

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BGP-4: Border Gateway Protocol• AS Types

– Stub AS: has a single connection to one other AS• carries local traffic only

– Multihomed AS: has connections to more than one AS• refuses to carry transit traffic

– Transit AS: has connections to more than one AS• carries both transit and local traffic

• Each AS has:– one or more border routers– one BGP speaker that advertises:

• local networks• other reachable networks (transit AS only)• gives path information

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Multi-Backbone Internet

Backbone service providerPeeringpoint Peering

point

Large corporation

Large corporationSmall

corporation

“Consumer” ISP

“Consumer” ISP

“Consumer” ISP

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BGP Example• Speaker for AS2 advertises reachability to P and Q

– network 128.96, 192.4.153, 192.4.32, and 192.4.3, can be reached directly from AS2

• Speaker for backbone advertises– networks 128.96, 192.4.153, 192.4.32, and 192.4.3 can be reached

along the path (AS1, AS2).• Speaker can cancel previously advertised paths

Regional provider A(AS 2)

Regional provider B(AS 3)

Customer P(AS 4)

Customer Q(AS 5)

Customer R(AS 6)

Customer S(AS 7)

128.96192.4.153

192.4.32192.4.3

192.12.69

192.4.54192.4.23

Backbone network(AS 1)

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IP Version 6• Features

– 128-bit addresses (classless)– multicast– real-time service– authentication and security – autoconfiguration – end-to-end fragmentation– protocol extensions

• Header– 40-byte “base” header– extension headers (fixed order, mostly fixed length)

• fragmentation• source routing• authentication and security• other options

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Addresses125- m- n- o- pponm3

SubscriberIDProviderIDRegistryID010 InterfaceIDSubnetID

Version TrafficClass FlowLabel

PayloadLen NextHeader HopLimit

SourceAddress

DestinationAddress

0 4 12 16 24 31

Next header/data

NextHeader Reserved Offset RES M

Ident

0 8 16 29 31

CSE 30264 Computer Networks

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