4 Network Layer

8/14/2019 4 Network Layer

1/68

Network Layer 4-1

Chapter 4

Network Layer


2/68

Network Layer 4-2

Chapter 4 roadmap

4.1 Introduction and Network Service Models4.2 Routing Principles

4.3 Hierarchical Routing

4.4 The Internet (IP) Protocol4.5 Routing in the Internet

4.6 Whats Inside a Router


3/68

Network Layer 4-3

Network layer functions

transport packet fromsending to receiving hosts

network layer protocols ineveryhost, router

three important functions: path determination:route

taken by packets from sourceto dest. Routing algorithms

forwarding:move packets

from routers input toappropriate router output

call setup:some networkarchitectures require routercall setup along path before

data flows

networkdata linkphysical

networkdata link

physical





networkdata link

physical


applicationtransportnetworkdata linkphysical

application

transportnetworkdata linkphysical


4/68

Network Layer 4-4

Network service model

Q: What servicemodelfor channeltransporting

packets from senderto receiver?

guaranteed bandwidth? loss-free delivery? in-order delivery? congestion feedback to

sender?

???

virtual circuitor

datagram?

The most importantabstraction provided

by network layer:

servicea

bstraction


5/68

Network Layer 4-5

Virtual circuits

call setup, teardown for each call beforedata can flow each packet carries VC identifier (not destination host ID) everyrouter on source-dest path maintains state for

each passing connectionr transport-layer connection only involved two end systems

link, router resources (bandwidth, buffers) may beallocatedto VCr to get circuit-like perf.

source-to-dest path behaves much like telephonecircuitr performance-wiser network actions along source-to-dest path


6/68

Network Layer 4-6

Virtual circuits: signaling protocols

used to setup, maintain teardown VC not used in todays Internet


applicationtransport


1. Initiate call 2. incoming call

3. Accept call4. Call connected5. Data flow begins 6. Receive data


7/68

Network Layer 4-7

Datagram networks: the Internet model

no call setup at network layer routers: no state about end-to-end connections

r no network-level concept of connection

packets forwarded using destination host address

r packets between same source-dest pair may takedifferent paths


application

transportnetworkdata linkphysical

1. Send data 2. Receive data


8/68

Network Layer 4-8

Chapter 4 roadmap


r Link state routingr

Distance vector routing4.3 Hierarchical Routing

4.4 The Internet (IP) Protocol

4.5 Routing in the Internet



9/68

Network Layer 4-9

Routing

Graph abstraction forrouting algorithms:

graph nodes arerouters

graph edges arephysical linksr link cost: delay, or

congestion level

Goal: determine good path(sequence of routers) thru

network from source to dest.

Routing protocol

A

ED

CB

F

2

2

1 3

1

1

2

53

5

good path:r typically means minimum

cost pathr other defs possible


10/68

Network Layer 4-10

Routing Algorithm classification

Global or decentralizedinformation?

Global: all routers have complete

topology, link cost info link state algorithms

Decentralized: router knows physically-

connected neighbors, link

costs to neighbors iterative process of

computation, exchange ofinfo with neighbors

distance vector algorithms

Static or dynamic?Static: routes change slowly

over time

Dynamic: routes change more

quicklyr periodic updater in response to link

cost changes


11/68

Network Layer 4-11

Distance Vector Routing: overview

Iterative, asynchronous:each local iteration causedby:

local link cost change message from neighbor: its

least cost path changefrom neighbor

Distributed: each node notifies

neighbors onlywhen itsleast cost path to anydestination changesr neighbors then notify

their neighbors ifnecessary

waitfor (change in local linkcost of msg from neighbor)

recompute distance table

if least cost path to any desthas changed, notifyneighbors

Each node:


12/68

Network Layer 4-12

Chapter 4 roadmap






13/68

Network Layer 4-13

Hierarchical Routing

scale: with 200 milliondestinations:

cant store all dests in

routing tables! routing table exchange

would swamp links!

administrative autonomy internet = network of

networks

each network admin maywant to control routing in itsown network

Our routing study thus far - idealization all routers identical network flat

nottrue in practice


14/68

Network Layer 4-14

Hierarchical Routing

aggregate routers intoregions, autonomoussystems (AS)

routers in same AS run

same routing protocolr intra-AS routing

protocolr 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 alsoresponsible for

routing to destinationsoutside ASr run inter-AS routing

protocol with othergateway routers

gateway routers


15/68

Network Layer 4-15

Intra-AS and Inter-AS routing

Gateways:perform inter-ASrouting amongstthemselvesperform intra-AS

routers with otherrouters in theirAS

inter-AS, intra-ASrouting ingateway A.c

network layer

link layerphysical layer

a

b

b

aaC

A

B

d

A.a

A.c

C.b B.a

c

b

c


16/68

Network Layer 4-16

Intra-AS and Inter-AS routing

Hosth2

a

b

b

aa

C

A

Bd c

A.a

A.c

C.bB.a

c

bHosth1

Intra-AS routingwithin AS A

Inter-ASroutingbetweenA and B

Intra-AS routingwithin AS B

Well examine specific inter-AS and intra-ASInternet routing protocols shortly


17/68

Network Layer 4-17

Chapter 4 roadmap

4.1 Introduction and Network Service Models4.2 Routing Principles4.3 Hierarchical Routing4.4 The Internet (IP) Protocol

4.5 Routing in the Internet4.6 Whats Inside a Router


18/68

Network Layer 4-18

The Internet Network layer

forwardingtable

Host, router network layer functions:

Routing protocols

path selectionRIP, OSPF, BGP

IP protocoladdressing conventionsdatagram formatpacket handling conventions

ICMP protocolerror reportingrouter signaling

Transport layer: TCP, UDP

Link layer

physical layer

Networklayer

ICMP : Internet Control Message Protocol


19/68

Network Layer 4-19

IP Addressing: introduction

IP address: 32-bitidentifier for host,router interface

interface:connection

between host/routerand physical linkr routers typically have

multiple interfacesr host may have multiple

interfacesr IP addresses

associated with eachinterface

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


20/68

Network Layer 4-20

IP Addressing

IP address:r network part (high

order bits)r host part (low order

bits)

Whats a network ?(from IP addressperspective)r device interfaces with

same network part ofIP addressr can physically reach

each other withoutintervening 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 IP networks(for IP addresses starting with 223,first 24 bits are network address)

LAN


21/68

Network Layer 4-21

IP Addressing

How to find thenetworks? Detach each

interface from

router, host create islands of

isolated networks

223.1.1.1

223.1.1.3

223.1.1.4

223.1.2.2223.1.2.1

223.1.2.6

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.2

223.1.7.0

223.1.7.1

223.1.8.0223.1.8.1

223.1.9.1

223.1.9.2

Interconnectedsystem consisting

of six networks


22/68

Network Layer 4-22

IP Addresses

0network host

10 network host

110 network host

1110 multicast address

A

B

C

D

class

1.0.0.0 to127.255.255.255

128.0.0.0 to191.255.255.255

192.0.0.0 to223.255.255.255

224.0.0.0 to239.255.255.255

32 bits

given notion of network, lets re-examine IP addresses:class-full addressing:


23/68

Network Layer 4-23

IP addressing: CIDR

Classful addressing:r inefficient use of address space, address space exhaustionr e.g., class B net allocated enough addresses for 65K hosts,

even if only 2K hosts in that network

CIDR:Classless InterDomain Routingr network portion of address of arbitrary lengthr address format: a.b.c.d/x, where x is # bits in network

portion of address

11001000 00010111 00010000 00000000

networkpart

hostpart

200.23.16.0/23


24/68

Network Layer 4-24

IP addresses: how to get one?

Q: How does hostget IP address?

hard-coded by system admin in a filer Wintel: control-panel->network->configuration-

>tcp/ip->propertiesr UNIX: /etc/rc.config

DHCP:Dynamic Host Configuration Protocol:dynamically get address from as serverr plug-and-play

(more shortly)


25/68

Network Layer 4-25

IP addresses: how to get one?

Q: How does networkget network part of IPaddr?

A: gets allocated portion of its provider ISPsaddress 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


26/68

Network Layer 4-26

IP addressing: the last word...

Q: How does an ISP get block of addresses?

A: ICANN: Internet Corporation for AssignedNames and Numbers

r allocates addressesr manages DNSr assigns domain names, resolves disputes


27/68

Network Layer 4-27

Getting a datagram from source to dest.

IP datagram:

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

A

B

E

miscfields

sourceIP addr

destIP addr data

datagram remainsunchanged, as it travelssource to destination

addr fields of interesthere

Dest. Net. next router Nhops

223.1.1 1223.1.2 223.1.1.4 2

223.1.3 223.1.1.4 2

forwarding table in A


28/68

Network Layer 4-28


Starting at A, send IPdatagram addressed to B:

look up net. address of B inforwarding table

find B is on same net. as A link layer will send datagram

directly to B inside link-layerframer B and A are directly

connected


223.1.1 1223.1.2 223.1.1.4 2

223.1.3 223.1.1.4 2

miscfields 223.1.1.1 223.1.1.3 data

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

A

B

E



29/68

Network Layer 4-29



223.1.1 1223.1.2 223.1.1.4 2

223.1.3 223.1.1.4 2Starting at A, dest. E: look up network address of E

in forwarding table E on differentnetwork

r A, E not directly attached routing table: next hop

router to E is 223.1.1.4 link layer sends datagram to

router 223.1.1.4 inside link-layer frame

datagram arrives at 223.1.1.4

continued..


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

A

B

E



30/68

Network Layer 4-30


Arriving at 223.1.4,destined for 223.1.2.2

look up network address of Ein routers forwarding table

E on samenetwork as routers

interface 223.1.2.9r

router, E directly attached link layer sends datagram to

223.1.2.2 inside link-layer

frame via interface 223.1.2.9 datagram arrives at

223.1.2.2!!! (hooray!)


Dest. Net router Nhops interface

223.1.1 - 1 223.1.1.4223.1.2 - 1 223.1.2.9

223.1.3 - 1 223.1.3.27

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

A

B

E

forwarding table in router


31/68

Network Layer 4-31

ICMP: Internet Control Message Protocol

used by hosts, routers,gateways to communicationnetwork-level informationr error reporting:

unreachable host, network,

port, protocolr echo request/reply (used

by ping) network-layer above IP:

r ICMP msgs carried in IP

datagrams ICMP message: type, code plus

first 8 bytes of IP datagramcausing error

Type Code description

0 0 echo reply (ping)

3 0 dest. network unreachable

3 1 dest host unreachable

3 2 dest protocol unreachable

3 3 dest port unreachable3 6 dest network unknown

3 7 dest host unknown

4 0 source quench (congestion

control - not used)

8 0 echo request (ping)

9 0 route advertisement

10 0 router discovery

11 0 TTL expired

12 0 bad IP header


32/68

Network Layer 4-32

DHCP: Dynamic Host Configuration Protocol

Goal: allow host to dynamicallyobtain its IP addressfrom network server when it joins networkCan renew its lease on address in use

Allows reuse of addresses (only hold address while connectedan on

Support for mobile users who want to join network (moreshortly)

DHCP overview:

r host broadcasts DHCP discover msgr DHCP server responds with DHCP offer msgr host requests IP address: DHCP request msgr DHCP server sends address: DHCP ack msg


33/68

Network Layer 4-33

DHCP client-server scenario

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

A

B

E

DHCPserver

arriving DHCP

client needs

address in this

network


34/68

Network Layer 4-34

DHCP client-server scenarioDHCP server: 223.1.2.5 arriving

client

time

DHCP discover

src : 0.0.0.0, 68dest.: 255.255.255.255,67

yiaddr: 0.0.0.0

transaction ID: 654

DHCP offer

src: 223.1.2.5, 67

dest: 255.255.255.255, 68

yiaddrr: 223.1.2.4

transaction ID: 654

Lifetime: 3600 secs

DHCP request

src: 0.0.0.0, 68

dest:: 255.255.255.255, 67

yiaddrr: 223.1.2.4

transaction ID: 655

Lifetime: 3600 secs

DHCP ACK

src: 223.1.2.5, 67

dest: 255.255.255.255, 68

yiaddrr: 223.1.2.4

transaction ID: 655

Lifetime: 3600 secs


35/68

Network Layer 4-35

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 leavinglocal

network have same single sourceNAT IP address: 138.76.29.7,different source port numbers


36/68

Network Layer 4-36


Motivation: local network uses just one IP address asfar as outside word is concerned:r no need to be allocated range of addresses from ISP:

- just one IP address is used for all devicesr can change addresses of devices in local network

without notifying outside worldr can change ISP without changing addresses of

devices in local networkr devices inside local net not explicitly addressable,

visible by outside world (a security plus).


37/68

Network Layer 4-37

NAT: Network Address TranslationImplementation: NAT router must:

r outgoing datagrams:replace(source IP address, port#) of every outgoing datagram to (NAT IP address,new port #). . . remote clients/servers will respond using (NAT

IP address, new port #) as destination addr.

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

r incoming datagrams:replace(NAT IP address, newport #) in dest fields of every incoming datagramwith corresponding (source IP address, port #)stored in NAT table


38/68

Network Layer 4-38


10.0.0.1

10.0.0.2

10.0.0.3

S: 10.0.0.1, 3345

D: 128.119.40.186, 80

1

10.0.0.4

138.76.29.7

1: host 10.0.0.1sends datagram to128.119.40, 80

NAT translation tableWAN side addr LAN side addr

138.76.29.7, 5001 10.0.0.1, 3345

S: 128.119.40.186, 80D: 10.0.0.1, 3345 4

S: 138.76.29.7, 5001D: 128.119.40.186, 802

2: NAT routerchanges datagramsource addr from10.0.0.1, 3345 to138.76.29.7, 5001,

updates table

S: 128.119.40.186, 80D: 138.76.29.7, 5001 3

3: Reply arrivesdest. address:138.76.29.7, 5001

4: NAT routerchanges datagramdest addr from138.76.29.7, 5001 to 10.0.0.1, 3345


39/68

Network Layer 4-39


16-bit port-number field:r 60,000 simultaneous connections with a single

LAN-side address!

NAT is controversial:r routers should only process up to layer 3r violates end-to-end argument

NAT possibility must be taken into account by app

designers, eg, P2P applicationsr address shortage should instead be solved by

IPv6


40/68

Network Layer 4-40

Chapter 4 roadmap




r4.5.1 Intra-AS routing: RIP and OSPFr

4.5.2 Inter-AS routing: BGP4.6 Whats Inside a Router?


41/68

Network Layer 4-41

Routing in the Internet

The Global Internet consists of Autonomous Systems(AS) interconnected with each other:r Stub AS: small corporation: one connection to other ASsr Multihomed AS: large corporation (no transit): multiple

connections to other ASsr Transit AS: provider, hooking many ASs together

Two-level routing:r Intra-AS: administrator responsible for choice of routing

algorithm within networkr Inter-AS: unique standard for inter-AS routing: BGP


42/68

Network Layer 4-42

Internet AS Hierarchy

Intra-AS border (exterior gateway) routers

Inter-ASinterior (gateway) routers


43/68

Network Layer 4-43

Intra-AS Routing

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

r RIP: Routing Information Protocol

r OSPF: Open Shortest Path First

r IGRP: Interior Gateway Routing Protocol (Ciscoproprietary)


44/68

Network Layer 4-44

RIP ( Routing Information Protocol)

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

Distance vectors: exchanged among neighbors every30 sec via Response Message (also calledadvertisement)

Each advertisement: list of up to 25 destination netswithin AS


45/68

Network Layer 4-45

RIP: Example

Destination Network Next Router Num. of hops to dest.

w A 2y B 2

z B 7x -- 1. . ....

w x yz

A

C

D B

Routing table in D


46/68

Network Layer 4-46

RIP: Example

Destination Network Next Router Num. of hops to dest.

w A 2y B 2

z B A 7 5x -- 1. . ....

Routing table in D

w x y

z

A

C

D B

Dest Next hops w - -

x - -z C 4. ...

Advertisement

from A to D


47/68

Network Layer 4-47

RIP: Link Failure and Recovery

If no advertisement heard after 180 sec -->neighbor/link declared deadr routes via neighbor invalidatedr new advertisements sent to neighbors

r neighbors in turn send out new advertisements (iftables changed)

r link failure info quickly propagates to entire net


48/68

Network Layer 4-48

RIP Table processing

RIP routing tables managed by application-levelprocess called route-d (daemon)

advertisements sent in UDP packets, periodicallyrepeated

physical

link

network forwarding(IP) table

Transprt(UDP)

routed

physical

link

network(IP)

Transprt(UDP)

routed

forwardingtable


49/68

Network Layer 4-49

OSPF (Open Shortest Path First)

open: publicly available Uses Link State algorithm

r LS packet disseminationr Topology map at each node

r Route computation using Dijkstras algorithm

OSPF advertisement carries one entry per neighborrouter

Advertisements disseminated to entire AS (viaflooding)r Carried in OSPF messages directly over IP (rather than TCP

or UDP


50/68

Network Layer 4-50

OSPF advanced features (not in RIP)

Security: all OSPF messages authenticated (toprevent malicious intrusion)

Multiple same-cost paths allowed (only one path inRIP)

For each link, multiple cost metrics for differentTOS (e.g., satellite link cost set low for besteffort; high for real time)

Integrated uni- and multicast support:r Multicast OSPF (MOSPF) uses same topology data

base as OSPF Hierarchical OSPF in large domains.


51/68

Network Layer 4-51

Hierarchical OSPF


52/68

Network Layer 4-52

Hierarchical OSPF

Two-level hierarchy: local area, backbone.r Link-state advertisements only in arear each nodes has detailed area topology; only know

direction (shortest path) to nets in other areas.

Area border routers:summarize distances to netsin own area, advertise to other Area Border routers.

Backbone routers: run OSPF routing limited tobackbone.

Boundary routers: connect to other ASs.


53/68

Network Layer 4-53

Inter-AS routing in the Internet: BGP

Figure 4.5.2-new2: BGP use for inter-domain routing

AS2(OSPF

intra-AS

routing)

AS1(RIP intra-AS

routing) BGP

AS3(OSPF intra-AS

routing)

BGP

R1 R2

R3

R4

R5

BGB : Border Gateway Protocol


54/68

Network Layer 4-54

BGP: controlling who routes to you

Figure 4.5-BGPnew: a simple BGP scenario

A

B

C

WX

Y

legend:

customer

network:

provider

network

A,B,C are provider networks X,W,Y are customer (of provider networks)

X is dual-homed: attached to two networksr X does not want to route from B via X to Cr .. so X will not advertise to B a route to C


55/68

Network Layer 4-55

BGP: controlling who routes to you

Figure 4.5-BGPnew: a simple BGP scenario

A

B

C

WX

Y

legend:

customer

network:

provider

network

A advertises to B the path AW B advertises to X the path BAW

Should B advertise to C the path BAW?r No way! B gets no revenue for routing CBAW since neitherW nor C are Bs customers

r B wants to force C to route to w via Ar B wants to route onlyto/from its customers!


56/68

Network Layer 4-56

BGP operation

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

directly attached neighbor(s).

Route selection.r To route to destination X, which path (of

several advertised) will be taken? Sending route advertisements to neighbors.


57/68

Network Layer 4-57

BGP messages

BGP messages exchanged using TCP. BGP messages:

r OPEN: opens TCP connection to peer andauthenticates sender

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

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

also used to close connection


58/68

Network Layer 4-58

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


59/68

Network Layer 4-59

Chapter 4 roadmap




4.6 Whats Inside a Router?

R t A hit t O i


60/68

Network Layer 4-60

Router Architecture Overview

Two key router functions: run routing algorithms/protocol (RIP, OSPF, BGP) switchingdatagrams from incoming to outgoing link


61/68

Network Layer 4-61

Input Port Functions

Decentralized switching: given datagram dest., lookup output port

using routing table in input port memory

goal: complete input port processing atline speed

queuing: if datagrams arrive faster thanforwarding rate into switch fabric

Physical layer:bit-level reception

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

I t P t Q i


62/68

Network Layer 4-62

Input Port Queuing

Fabric slower that input ports combined -> queueing

may occur at input queues Head-of-the-Line (HOL) blocking: queued datagram

at front of queue prevents others in queue frommoving forward

queueing delay and loss due to input buffer overflow!

Th t f it hi f b i


63/68

Network Layer 4-63

Three types of switching fabrics

Switching Via Memory


64/68

Network Layer 4-64

Switching Via Memory

First generation routers:

packet copied by systems (single) CPU speed limited by memory bandwidth (2 buscrossings per datagram)

Input

Port

Output

Port

Memory

System Bus

Modern routers: input port processor performs lookup, copy intomemory

Cisco Catalyst 8500


65/68

Network Layer 4-65

Switching Via a Bus

datagram from input port memory

to output port memory via a sharedbus bus contention: switching speed

limited by bus bandwidth

1 Gbps bus, Cisco 1900: sufficientspeed for access and enterpriserouters (not regional or backbone)


66/68

Network Layer 4-66

Switching Via An InterconnectionNetwork

overcome bus bandwidth limitations Banyan networks, other interconnection nets

initially developed to connect processors inmultiprocessor Advanced design: fragmenting datagram into fixed

length cells, switch cells through the fabric.

Cisco 12000: switches Gbps through theinterconnection network


67/68

Network Layer 4-67

Output Ports

Bufferingrequired when datagrams arrive from

fabric faster than the transmission rate Scheduling disciplinechooses among queued

datagrams for transmission


68/68

Output port queueing

buffering when arrival rate via switch exceedsoutput line speed queueing (delay) and loss due to output port

buffer overflow!

Documents

4 Network Layer