Network Layer.ppt

7/29/2019 Network Layer.ppt

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Network Layer

Lecture # 2

MAHS


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4: Network Layer 4b-2

Hierarchical Routing

scale: with 200 milliondestinations:

cant store all dests inrouting tables!

routing table exchange

would swamp links!

administrative autonomy internet = network of

networks each network admin may

want to control routing (costmetrics, etc.) in its ownnetwork

Our routing study thus far - idealization all routers identical

network flat

nottrue in practice

Why?


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Hierarchical Routing

Organization:aggregate routers intoregions,calledautonomous systems

(AS) routers in same AS run

same routing protocol intra-AS routing (i.e.,

within an AS) protocol routers in different AS

can run different intra-AS routing protocol

special routersin (onthe edgeof) an AS

run intra-AS routingprotocol with all otherrouters in AS

alsoresponsible forrouting to destinationsoutside AS run inter-AS routing

(i.e., between AS)protocol with othergateway routers

gateway routers


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Intra-AS and Inter-AS routing

Gateway routers:perform inter-ASrouting amongstthemselvesperform intra-AS

routing with otherrouters in theirAS

inter-AS, in

tra-ASrouting ingateway A.c

network layer

data link layerphysical layer

a

b

b

aaC

A

B

d

A.a

A.c

C.b

B.a

c

b

c


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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 (section 4.5)


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IP datagram format

ver length

32 bits

data(variable length,

typically a TCP segment,a UDP segment,

or an ICMP message)

16-bit identifier

Headerchecksum

time tolive

32 bit source IP address

IP protocol versionnumber

header length(4-byte multiples)

max numberremaining hops

(decremented ateach router)

forfragmentation/reassembly

total datagram

length (bytes)

upper layer protocolto deliver payload to

(RFC 1700, 3232)

head.len

type ofservice

DS codepoint, ECNflgs

fragment

offsetupper

layer

32 bit destination IP address

Options (if any) E.g. timestamp,record route

taken, specifylist of routersto visit.

how much overheadwith TCP?

20 bytes of TCP

20 bytes of IP

= 40 bytes + app

layer overhead


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IP Fragmentation & Reassembly

network links have MTU (Max.Transfer Unit) size - largestpossible link-level frame.

different link types,different MTUs

large IP datagram is divided

(fragmented) within network one datagram becomes

several datagrams

reassembled only at thefinal destination

IP header bits are used toidentify and order relatedfragments

fragmentation:in: one large datagramout: 3 smaller datagrams

reassembly


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IP Fragmentation and ReassemblyID

=x

offset

=0

More bit

=0

bytes*

=3980

ID=x offset=0More bit=1bytes*=1480

ID=x

offset=1480

More bit=1

bytes*=1480

ID=x offset=2960More bit=0bytes*=1020

One large datagram becomesseveral smaller datagrams

Note: Offset is actuallyspecified as number of8-byte (64-bit) units.

Example

4000 byte

datagram MTU = 1500 bytes

* This is the number ofdata bytes in the IPdatagram. The IP lengthfield would show this +20. Why?


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DHCP: Dynamic Host Configuration Protocol

Goal: allow host to dynamicallyobtain its IP addressfrom network server when it joins a networkCan renew its lease on address in useAllows reuse of addresses (only hold address while connected

an onSupport for mobile users who want to join network (more

shortly)

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


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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


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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: 223.1.2.4transaction ID: 654

Lifetime: 3600 secsDHCP request

src: 0.0.0.0, 68

dest:: 255.255.255.255, 67

yiaddrr: 223.1.2.4

transaction ID: 655Lifetime: 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


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

Alldatagrams leavinglocalnetwork have same single source

NAT IP address: 138.76.29.7,different source port numbers


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Motivation: local network uses just one IP address asfar as outside word is concerned:

no need to be allocated range of addresses from ISP:- just one IP address is used for all devices

can change addresses of devices in local networkwithout notifying outside world

can change ISP without changing addresses ofdevices in local network

devices inside local net not explicitly addressable,visible by outside world (a security plus).


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NAT: Network Address TranslationImplementation: NAT router must:

outgoing datagrams:replace(source IP address, port #) ofevery outgoing 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 #) indest fields of every incoming datagram with corresponding(source IP address, port #) stored in NAT table


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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, 80

D: 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


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

LAN-side address!

Reserved address space (rfc 1918)NAT is controversial: routers should only process up to layer 3 violates end-to-end argument

NAT possibility must be taken into account by appdesigners, eg, P2P applications

address shortage should instead be solved byIPv6
ftp://ftp.rfc-editor.org/in-notes/rfc1918.txthttp://www.icann.org/registrars/accredited-list.htmlhttp://www.icann.org/registrars/accredited-list.htmlftp://ftp.rfc-editor.org/in-notes/rfc1918.txtftp://ftp.rfc-editor.org/in-notes/rfc1918.txt


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Intra-AS Routing

Also known as Interior Gateway Protocols (IGP)

Most common IGPs:

RIP: Routing Information Protocol (legacy,

RIPv2 still in use)

OSPF: Open Shortest Path First (common)

EIGRP: Enhanced Interior Gateway Routing

Protocol (proprietary Cisco Systems)


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RIP ( Routing Information Protocol)

Distance vector algorithm

Included in BSD-UNIX Distribution in 1982 RFC 1058 (version 1), RFC 2453 (version 2)

Distance metric: # of hops (max = 15 hops) Can you guess why?

Distance vectors: exchanged every 30 seconds viaResponse Message (also called advertisement)

Each advertisement: routing info for maximum of 25destination nets within the AS

Uses UDP transport, port 520


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Problems/limitations with RIP

Good for small systems, but doesnt scalewell

Count-to-infinity problem poisonedreverse only

Comparatively slow convergence

1979 RIP version 1

1988 IETF initiates work on replacement 1990 OSPF became new standard 1990s RIP version 2


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OSPF (Open Shortest Path First)

open: publicly available Uses Link State algorithm

LS packet dissemination

Topology map at each node

Route computation using Dijkstras algorithm

However.

OSPF advertisement carries only one entry per

neighbor router Advertisements disseminated to entire AS (via

flooding)

Sent as payload in IP datagram


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EIGRP (Enhanced Interior GatewayRouting Protocol)

CISCO proprietary; successor of RIP (mid 80s)

uses Distance Vector, like RIP

several cost metrics (delay, bandwidth, reliability,

load etc) uses TCP (!) to exchange routing updates

Loop-free routing via a distributed update routingalgorithm (called DUAL) based on diffused

computation


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Inter-AS routing

Network Layer.ppt

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