Overview Network Layer

8/12/2019 Overview Network Layer

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Wireless Networking & Mobile ComputingNetwork Layer Overview

ECE 256

Romit Roy Choudhury

Dept. of ECE and CS


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Recall Layering

transport segment fromsending to receiving host

on sending side encapsulatessegments into datagrams

on rcving side, delivers

segments to transport layer network layer protocols in

everyhost, router

Router examines header fieldsin all IP datagrams passing

through it

networkdata linkphysical

networkdata link

physical





networkdata link

physical


applicationtransportnetworkdata linkphysical

application

transportnetworkdata linkphysical


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Routing - Why Difficult ?

Several algorithmic problems: Many many paths - which is the best?

Each path has changing characteristics

Queuing time varies, losses happen, router down

How do you broadcast (find where someone is)

How do you multicast (webTV, conference call)

How do routers perform routing at GBbps scale

Several management problems:

How do you detect/diagnose faults

How do you do pricing, accounting


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Chapter 4: Network Layer

4. 1 Introduction 4.2 Virtual circuit and

datagram networks

4.3 Whats inside a router

4.4 IP: Internet Protocol Datagram format

IPv4 addressing

ICMP

IPv6

4.5 Routing algorithms Link state

Distance Vector

Hierarchical routing

4.6 Routing in theInternet

RIP

OSPF

BGP

4.7 Broadcast and

multicast routing


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Key Network-Layer Functions

forwarding:movepackets from routersinput to appropriaterouter output

routing:determineroute taken by packetsfrom source to dest.

Routing algorithms

analogy:

routing:process of

planning trip fromsource to dest

forwarding:process of

getting through actual

traffic intersections


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1

23

0111

value in arriving

packets header

routing algorithm

local forwarding table

header value output link

0100

0101

01111001

3

2

21

Interplay between routing and forwarding


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Two types of Network Architecture

Connection-OrientedandConnection-Less

Virtual Circuit Switching

Example:ATM, X.25

Analogy: Telephone

Datagram forwarding

Example: IP networks

Analogy: Postal service


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Virtual circuits: signaling protocols

used to setup, maintain teardown VC used in ATM, frame-relay, X.25

not used in todays Internet


applicationtransport


1. Initiate call

2. incoming call

3. Accept call

4. Call connected5. Data flow begins 6. Receive data


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No call setup at network layer

@ routers: no state about end-to-end connections

no concept of connection

packets forwarded using destination host address May take different path for same source-dest pair

Datagram networks


application

transportnetworkdata linkphysical

1. Send data 2. Receive data


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Design Decisions

Thoughts on why VC isnt great?

Thoughts on why dataram may not be great?

Think of an application thats better with VC


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Datagram or VC network: why?

Internet data traffic

elastic service, no strict

timing req.

smart end computers

simple network

complexity at edge

many link types

different characteristics

uniform service difficult

ATM

evolved from telephony

Call admission control

human conversation: strict timing, reliability

requirements

need for guaranteed

service

dumb end systems

telephones

complexity inside

network


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IP Addressing


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IP Addressing: introduction

IP address:32-bitidentifier for host,

router interface

interface:connection

between host/routerand physical link

routers typically have

multiple interfaces

host typically has one

interface IP addresses associated

with each interface

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11


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Subnets

IP address: subnet part (high order

bits)

host part (low order bits)

Whats a subnet ?

device interfaces with

same subnet part of IP

address

can physically reacheach other without

intervening router

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

network consisting of 3 subnets

subnet


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IP addressing: CIDR

CIDR:Classless InterDomain Routing subnet portion of address of arbitrary length

address format: a.b.c.d/x, where x is # bits in subnet

portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

200.23.16.0/23


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IP addresses: how to get one?

Q:How does networkget subnet part of IP addr?A:gets allocated portion of its provider ISPs

address space

ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20

Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23

Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23

Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23... .. . .

Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23


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Scalability Problem

Internet growing very fast Many million devices

Each device needs an address for communication

Question is

How do you address each of them

IP addresing can give you 232

May not be enough


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NAT: Network Address Translation

10.0.0.1

10.0.0.2

10.0.0.3

10.0.0.4

138.76.29.7

local network(e.g., home network)

10.0.0/24

rest ofInternet

Datagrams with source or

destination in this networkhave 10.0.0/24 address forsource, destination (as usual)

Alldatagrams leavinglocalnetwork have samesingle source

NAT IP address: 138.76.29.7,different source port numbers


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NAT makes Globally non-routable hosts

Non-routable Means you cannot ping 192.168.0.3 (your home

machines) from Duke Lab

But, Skype, GotoMyPC, etc. can access / callyour home machine

How ?


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An Alternate Approach: IPv6

Initial motivation:Make space for 64 bit addressspace

How can this be made compatible to IPv4 routers?

IPv6 not flying

NAT coping fine with todays needs


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Routing Algorithms


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u

y

x

wv

z2

2

1

3

1

1

2

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5

Graph: G = (N,E)

N = set of routers = { u, v, w, x, y, z }

E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) }

Graph abstraction

Remark: Graph abstraction is useful in other network contexts

Example: P2P, where N is set of peers and E is set of TCP connections


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Graph abstraction: costs

u

y

x

wv

z2

2

1

3

1

1

2

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5 What factors influence this cost ?

Cost of path (x1, x2, x3,, xp) = c(x1,x2) + c(x2,x3) + + c(xp-1,xp)

Question: Whats the least-cost path between u and z ?

Routing algorithm: algorithm that finds least-cost path

Should costs be only on links ?


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Routing Algorithm classification

2 main classes:

Centralized

all routers have complete topology, link cost info

link state algorithms

Distributed:

Each router knows link costs to neighbor routers only

distance vector algorithms


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A Link-State Routing Algorithm

Dijkstras algorithm

Link costs known to all nodes

computes least cost paths from one node (source) to allother nodes

gives forwarding tablefor that node

iterative: after k iterations, know least cost path to kdest.s


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Dijkstras Algorithm

1 Init ial ization:2 N' = {u}

3 for all nodes v

4 if v adjacent to u

5 then D(v) = c(u,v)

6 else D(v) = 7

8 Loop

9 find w not in N' s.t. D(w) is a minimum

10 add w to N'

11 update D(v) for all v adjacent to w and not in N' :

12 D(v) = min( D(v), D(w) + c(w,v) )

13 /* new cost to v is either old cost to v or known

14 shortest path cost to w plus cost from w to v */

15 unt i l al l nodes in N'

Notation:

c(x,y):link cost from node x to y;= if not direct neighbors

D(v):current value of cost of path

from source to dest. v

u

y

x

wv

z2 2

13

1

1

2

5

3

5


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Dijkstras algorithm: example (2)

u

y

x

wv

z

Resulting shortest-path tree from u:

vx

y

w

z

(u,v)(u,x)

(u,x)

(u,x)

(u,x)

destination link

Resulting forwarding table in u:


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Distributed: Distance Vector

To find D, node S asks each neighbor X How far X is from D

X asks its neighbors comes back and says C(X,D)

Node S deduces C(S,D) = C(S,X) + C(X,D)

S chooses neighbor Xithat provides min C(S,D)

Later, Xjmay find better route to D

Xjadvertizes C(Xj,D)

All nodes update their cost to D if new min found


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Distance Vector Algorithm

Bellman-Ford Equation (dynamic programming)

Define

dx(y) := cost of least-cost path from x to y

Then

dx(y) = min {c(x,v) + dv(y) }

where min is taken over all neighbors v of x

v

x y

v2

v1


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Bellman-Ford example

u

y

x

wv

z2

2

1

3

1

1

2

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5 Clearly, dv(z) = 5, dx(z) = 3, dw(z) = 3

du(z) = min { c(u,v) + dv(z),c(u,x) + dx(z),c(u,w) + dw(z) }

= min {2 + 5,1 + 3,5 + 3} = 4

Node that achieves minimum is nexthop in shortest path forwarding table

B-F equation says:


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Distance Vector: link cost changes

Link cost changes: if DV changes, notify neighbors

x z

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y1

At time t0, ydetects the link-cost change, updates its DV,and informs its neighbors.

At time t1, zreceives the update from yand updates its table.It computes a new least cost to x and sends its neighbors its DV.

At time t2, yreceives zs update and updates its distance table.ys least costs do not change and hence y does notsend anymessage to z.

When can it get complicated ?


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Distance Vector: link cost changes

Link cost changes: Y thinks Zs best cost is 5

Thus C(y,x) = 5 + 1 = 6

Announces this cost

Z thinks C(z,x) = 6 + 1

Poissoned reverse:

If Z routes through Y to get

to X :

Z tells Y its (Zs) distance to

X is infinite (so Y wont route

to X via Z)

will this completely solve

count to infinity problem?

x z1

4

50

y

60

Food for thought

Will this converge ?

If so, after how many rounds ?How can this be solved?

Should Y announce change from 4 to 60?


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Routing in Internet

Similar to international FedEx routing FedEx figures out best route within country

Uses google maps say This is link state -- All info available

USA FedEx does not have international map,

also no permission to operate outside USA

Gets price quote from Germany FedEx, Japan FedEx

etc. to route to India Chooses minimum price and handles package to say

Germany (Distance Vector)

Germany has country map (link state)

Germany asks for cost from Egypt, South Africa


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Internet Routing

Think of each country FedEx as ISPs Routing on internet very similar to prior example

The link state and DV routing protocols used in

internet routing RIP (routing information protocol)

OSPF (Open shortest path first)

BGP (Border gateway protocol)

They utilize the concepts of Link state

Distance vector routing


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How is this different in wireless?


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Routing in wireless Mobile Networks

Imagine hundreds of hosts moving Routing algorithm needs to cope up with varying

wireless channel and node mobility

Wheres

RED guy


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Questions ?


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Backup Slides


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Comparison of LS and DV algorithms

Message complexity

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

msgs sent

DV: exchange between neighbors

only

convergence time varies

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 incorrectlinkcost

each node computes only its

owntableDV:

DV node can advertiseincorrectpathcost

each nodes table used byothers

error propagate thru network


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Hierarchical Routing

aggregate routers intoregions,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

Gateway router Direct link to router in

another AS


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3b

1d

3a

1c2a

AS3

AS1

AS21a

2c

2b

1b

Intra-AS

Routing

algorithm

Inter-AS

Routing

algorithm

Forwarding

table

3c

Interconnected ASes

Forwarding table isconfigured by both

intra- and inter-AS

routing algorithm

Intra-AS sets entries forinternal dests

Inter-AS & Intra-As sets

entries for external dests


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3b

1d

3a

1c2a

AS3

AS1

AS21a

2c

2b

1b

3c

Inter-AS tasks

Suppose router in AS1

receives datagram forwhich dest is outside of

AS1

Router should forward

packet towards one of

the gateway routers, butwhich one?

AS1 needs:

1. to learn which dests

are reachable throughAS2 and which through

AS3

2. to propagate this

reachability info to allrouters in AS1

Job of inter-AS routing!


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3b

1d

3a

1c2a

AS3

AS1

AS21a

2c

2b

1b

3c

Inter-AS tasks

Suppose router in AS1

receives datagram forwhich dest is outside of

AS1

Router should forward

packet towards one of

the gateway routers, butwhich one?

AS1 needs:

1. to learn which dests

are reachable throughAS2 and which through

AS3

2. to propagate this

reachability info to allrouters in AS1

Job of inter-AS routing!


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Example: Setting forwarding table in router 1d

Suppose AS1 learns from the inter-AS protocolthat subnetxis reachable from AS3 (gateway

1c) but not from AS2.

Inter-AS protocol propagates reachability info to

all internal routers.

Router 1d determines from intra-AS routing info

that its interface I is on the least cost path to 1c.

Puts in forwarding table entry (x,I).


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Learn from inter-ASprotocol that subnet

x is reachable via

multiple gateways

Use routing info

from intra-ASprotocol to determine

costs of least-cost

paths to each

of the gateways

Hot potato routing:Choose the gateway

that has the

smallest least cost

Determine from

forwarding table theinterface I that leads

to least-cost gateway.

Enter (x,I) in

forwarding table

Example: Choosing among multiple ASes

Now suppose AS1 learns from the inter-AS protocolthat subnetxis reachable from AS3 andfrom AS2.

To configure forwarding table, router 1d mustdetermine towards which gateway it should forwardpackets for dest x.

This is also the job on inter-AS routing protocol!

Hot potato routing:send packet towards closest of tworouters.


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4. 1 Introduction 4.2 Virtual circuit and

datagram networks



IPv4 addressing

ICMP

IPv6

4.5 Routing algorithms Link state

Distance Vector


4.6 Routing in the

Internet

RIP

OSPF

BGP

4.7 Broadcast andmulticast routing


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Intra-AS Routing

Also known as Interior Gateway Protocols (IGP) Most common Intra-AS routing protocols:

RIP: Routing Information Protocol

OSPF: Open Shortest Path First

IGRP: Interior Gateway Routing Protocol (Cisco

proprietary)


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4. 1 Introduction

4.2 Virtual circuit and

datagram networks



IPv4 addressing

ICMP

IPv6

4.5 Routing algorithms

Link state

Distance Vector


4.6 Routing in the

Internet

RIP

OSPF

BGP



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Internet inter-AS routing: BGP

BGP (Border Gateway Protocol):thede factostandard

BGP provides each AS a means to:1. Obtain subnet reachability information from

neighboring ASs.

2. Propagate the reachability information to all routersinternal to the AS.

3. Determine good routes to subnets based onreachability information and policy.

Allows a subnet to advertise its existence torest of the Internet: I am here

BGP b i


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BGP basics

Pairs of routers (BGP peers) exchange routing info over semi-permanentTCP conctns: BGP sessions

Note that BGP sessions do not correspond to physical links. When AS2 advertises a prefix to AS1, AS2 ispromisingit will forward

any datagrams destined to that prefix towards the prefix.

AS2 can aggregate prefixes in its advertisement

3b

1d

3a

1c

2aAS3

AS1

AS21a

2c

2b

1b

3c

eBGP session

iBGP session

Di t ib ti h bilit i f


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Distributing reachability info

With eBGP session between 3a and 1c, AS3 sends prefixreachability info to AS1.

1c can then use iBGP do distribute this new prefix reach info toall routers in AS1

1b can then re-advertise the new reach info to AS2 over the 1b-to-2a eBGP session

When router learns about a new prefix, it creates an entry for theprefix in its forwarding table.

3b

1d

3a

1c

2a

AS3

AS1

AS21a

2c

2b

1b

3c

eBGP session

iBGP session


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Path attributes & BGP routes

When advertising a prefix, advert includes BGP attributes.

prefix + attributes = route

Two important attributes:

AS-PATH:contains the ASs through which the advert for the prefix

passed: AS 67 AS 17

NEXT-HOP:Indicates the specific internal-AS router to next-hop AS.

(There may be multiple links from current AS to next-hop-AS.)

When gateway router receives route advert, uses import policyto

accept/decline.


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BGP route selection

Router may learn about more than 1 route tosome prefix. Router must select route.

Elimination rules:

1. Local preference value attribute: policy decision

2. Shortest AS-PATH

3. Closest NEXT-HOP router: hot potato routing

4. Additional criteria


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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)

KEEPALIVEkeeps 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 routing policy (2)

Figure 4.5-BGPnew: a simple BGP scenario

A

B

C

W

X

Y

legend:

customer

network:

provider

network

Aadvertises to B the path AW

B advertises to X the path BAW

Should B advertise to C the path BAW?

No way! B gets no revenue for routing CBAW since neither W

nor C are Bs customers

B wants to force C to route to w via A

B wants to route only to/from its customers!


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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 needed

Scale:

hierarchical routing saves table size, reduced update traffic

Performance:

Intra-AS: can focus on performance

Inter-AS: policy may dominate over performance


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Questions ?


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Motivation:local network uses just one IP address as far as

outside world is concerned:

range of addresses not needed from ISP: just one IP

address for all devices

can change addresses of devices in local network

without notifying outside world

can change ISP without changing addresses of devices

in local network

devices inside local net not explicitly addressable,

visible by outside world (a security plus).


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Implementation:NAT router must:

outgoing datagrams:replace(source IP address, port #) of everyoutgoing datagram to (NAT IP address, new port #)

. . . remote clients/servers will respond using (NAT IP address,new port #) as destination addr.

remember (in NAT translation table) every (source IP address, port#) to (NAT IP address, new port #) translation pair

incoming datagrams:replace(NAT IP address, new port #) in destfields of every incoming datagram with corresponding (source IPaddress, port #) stored in NAT table


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Distance Vector Algorithm

Dx(y)= estimate of least cost from x to yDistance vector: Dx= [Dx(y): y N ]

Node x knows cost to each neighbor v: c(x,v)

Node x maintains Dx

= [Dx

(y): y N ]

Node x also maintains its neighbors distance

vectors

For each neighbor v, x maintains

Dv= [Dv(y): y N ]


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4. 1 Introduction

4.2 Virtual circuit and

datagram networks



IPv4 addressing

ICMP

IPv6

4.5 Routing algorithms

Link state

Distance Vector


4.6 Routing in the

Internet RIP

OSPF

BGP


Ro ter Architect re O er ie


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Router Architecture Overview

Two key router functions:

run routing algorithms/protocol (RIP, OSPF, BGP) forwarding datagrams from incoming to outgoing link


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Input Port Functions

Decentralized switching:

given datagram dest., lookup output port using

forwarding table

goal: complete input port processing at line

speed

queuing: if datagrams arrive faster than

forwarding rate into switch fabric

Physical layer:bit-level reception

Data link layer:e.g., Ethernetsee chapter 5

Th t f it hi f b i


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Three types of switching fabrics


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The Internet Network layer

forwardingtable

Host, router network layer functions:

Routing protocolspath selection

RIP, OSPF, BGP

IP protocoladdressing conventions

datagram formatpacket handling conventions

ICMP protocolerror reportingrouter signaling

Transport layer: TCP, UDP

Link layer

physical layer

Networklayer


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Hierarchical addressing: route aggregation

Send me anythingwith addressesbeginning200.23.16.0/20

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-UsSend me anythingwith addressesbeginning199.31.0.0/16

200.23.20.0/23Organization 2

.

.

.

.

.

.

Hierarchical addressing allows efficient advertisement of routinginformation:


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Hierarchical addressing: more specific routes

ISPs-R-Us has a more specific route to Organization 1

Send me anything

with addressesbeginning200.23.16.0/20

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us Send me anythingwith addressesbeginning 199.31.0.0/16or 200.23.18.0/23

200.23.20.0/23Organization 2

.

.

.

.

.

.


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IP addressing: the last word...

Q:How does an ISP get block of addresses?A:ICANN: Internet Corporation forAssigned

Names and Numbers

allocates addresses

manages DNS

assigns domain names, resolves disputes

Network layer connection and connection-less


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Network layer connection and connection less

service

Datagram network provides network-layerconnectionless service

VC network provides network-layer connection

service

Analogous to the transport-layer services, but: Service: host-to-host

No choice: network provides one or the other

Implementation: in the core


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Virtual circuits

Call setup, teardown for each call beforedata can flow

Each packet carries VC identifier (not destination host

address)

Everyrouter on source-dest path maintains state for

each passing connection

Link, router resources (bandwidth, buffers) may beallocated to VC

VC i l t ti


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VC implementation

A VC consists of:1. Path from source to destination

2. VC numbers, one number for each link along path

3. Entries in forwarding tables in routers along path

Packet belonging to VC carries a VC number.

VC number must be changed on each link. New VC number comes from forwarding table


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D t F di T bl


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Datagram Forwarding Table

Destination Address Range Link Interface

11001000 00010111 00010000 00000000through 0

11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through 1

11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through 211001000 00010111 00011111 11111111

otherwise 3

4 billionpossible entries

Longest prefi matching


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Longest prefix matching

Prefix Match Link Interface11001000 00010111 00010 0

11001000 00010111 00011000 111001000 00010111 00011 2

otherwise 3

DA: 11001000 00010111 00011000 10101010

Examples

DA: 11001000 00010111 00010110 10100001 Which interface?

Which interface?

Documents

Overview Network Layer