Chapter4 5th Aug 2009

8/11/2019 Chapter4 5th Aug 2009

1/149

Network Layer 4-1

Chapter 4Network Layer

A note on the use of these ppt slides:Were making these slides freely available to all (faculty, students, readers).Theyre in PowerPoint form so you can add, modify, and delete slides(including this one) and slide content to suit your needs. They obviouslyrepresent a lot of work on our part. In return for use, we only ask thefollowing:

If you use these slides (e.g., in a class) in substantially unaltered form,that you mention their source (after all, wed like people to use our book!) If you post any slides in substantially unaltered form on a www site, that

you note that they are adapted from (or perhaps identical to) our slides, andnote our copyright of this material.

Thanks and enjoy! JFK/KWR

All material copyright 1996-2009

J.F Kurose and K.W. Ross, All Rights Reserved

Computer Networking:A Top Down Approach5 th edition.Jim Kurose, Keith RossAddison-Wesley, April2009.

8/11/2019 Chapter4 5th Aug 2009

2/149

Network Layer 4-2

Chapter 4: Network Layer

Chapter goals: understand principles behind network layerservices:

network layer service modelsforwarding versus routinghow a router worksrouting (path selection)

dealing with scaleadvanced topics: IPv6, mobility

instantiation, implementation in the Internet

8/11/2019 Chapter4 5th Aug 2009

3/149

Network Layer 4-3


4. 1 Introduction4.2 Virtual circuit anddatagram networks

4.3 Whats inside arouter4.4 IP: InternetProtocol

Datagram formatIPv4 addressingICMPIPv6

4.5 Routing algorithmsLink stateDistance VectorHierarchical routing

4.6 Routing in theInternet

RIPOSPFBGP

4.7 Broadcast andmulticast routing

8/11/2019 Chapter4 5th Aug 2009

4/149

Network Layer 4-4

Network layertransport segment fromsending to receiving hoston sending sideencapsulates segmentsinto datagramson rcving side, deliverssegments to transportlayernetwork layer protocolsin every host, routerrouter examines headerfields in all IP datagramspassing through it

applicationtransport

network data linkphysical

applicationtransportnetwork

data linkphysical

networkdata linkphysical network

data linkphysical

networkdata linkphysical







networkdata link

physical networkdata linkphysical

8/11/2019 Chapter4 5th Aug 2009

5/149

Network Layer 4-5

Two Key Network-Layer Functions

forwarding: movepackets from routersinput to appropriate

router outputrouting: determineroute taken by

packets from sourceto dest.

routing algorithms

analogy:

routing: process of

planning trip fromsource to dest

forwarding: process

of getting throughsingle interchange

8/11/2019 Chapter4 5th Aug 2009

6/149

Network Layer 4-6

1

23

0111

value in arrivingpackets header

routing algorithm

local forwarding tableheader value output link

01000101

01111001

32

21

Interplay between routing and forwarding

8/11/2019 Chapter4 5th Aug 2009

7/149

Network Layer 4-7

Connection setup

3rd important function in some network architectures:ATM, frame relay, X.25

before datagrams flow, two end hosts and intervening

routers establish virtual connectionrouters get involvednetwork vs transport layer connection service:

network: between two hosts (may also involve

intervening routers in case of VCs)transport: between two processes

8/11/2019 Chapter4 5th Aug 2009

8/149

Network Layer 4-8

Network service modelQ: What service model for channel transportingdatagrams from sender to receiver?

Example services for

individual datagrams:guaranteed deliveryguaranteed deliverywith less than 40 msec

delay

Example services for aflow of datagrams:in-order datagramdeliveryguaranteed minimumbandwidth to flowrestrictions onchanges in inter-packet spacing

8/11/2019 Chapter4 5th Aug 2009

9/149

Network Layer 4-9

Network layer service models:

Network Architecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed

minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss )nocongestionnocongestionyes

no

Guarantees ?

8/11/2019 Chapter4 5th Aug 2009

10/149

Network Layer 4-10


4. 1 Introduction4.2 Virtual circuit anddatagram networks

4.3 Whats inside arouter4.4 IP: InternetProtocol




RIPOSPF

BGP4.7 Broadcast andmulticast routing

8/11/2019 Chapter4 5th Aug 2009

11/149

Network Layer 4-11

Network layer connection andconnection-less service

datagram network provides network-layerconnectionless serviceVC network provides network-layerconnection serviceanalogous to the transport-layer services,but:

service: host-to-hostno choice: network provides one or the otherimplementation: in network core

8/11/2019 Chapter4 5th Aug 2009

12/149

Network Layer 4-12

Virtual circuits

call setup, teardown for each call before data can floweach packet carries VC identifier (not destination hostaddress)

every router on source- dest path maintains state foreach passing connectionlink, router resources (bandwidth, buffers) may beallocated to VC (dedicated resources = predictable service)

source -to-dest path behaves much like telephonecircuit

performance-wisenetwork actions along source-to-dest path

8/11/2019 Chapter4 5th Aug 2009

13/149

Network Layer 4-13

VC implementation

a VC consists of:1. path from source to destination2. VC numbers, one number for each link along

path3. entries in forwarding tables in routers along

pathpacket belonging to VC carries VC number(rather than dest address)VC number can be changed on each link.

New VC number comes from forwarding table

8/11/2019 Chapter4 5th Aug 2009

14/149

Network Layer 4-14

Forwarding table12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC # Outgoing interface Outgoing VC #

1 12 3 222 63 1 183 7 2 171 97 3 87

Forwarding table innorthwest router:

Routers maintain connection state information!

8/11/2019 Chapter4 5th Aug 2009

15/149

Network Layer 4-15

Virtual circuits: signaling protocols

used to setup, maintain teardown VCused in ATM, frame-relay, X.25not used in todays Internet

applicationtransportnetwork data linkphysical

applicationtransport

network data linkphysical

1. Initiate call 2. incoming call 3. Accept call 4. Call connected

5. Data flow begins 6. Receive data

8/11/2019 Chapter4 5th Aug 2009

16/149

Network Layer 4-16

Datagram networksno call setup at network layerrouters: no state about end-to-end connections

no network- level concept of connection packets forwarded using destination host address

packets between same source-dest pair may take

different paths

applicationtransportnetwork data linkphysical

application

transportnetwork data linkphysical

1. Send data 2. Receive data

8/11/2019 Chapter4 5th Aug 2009

17/149

Network Layer 4-17

Forwarding table

Destination Address Range Link Interface

11001000 00010111 00010000 00000000 through 0

11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000 through 1

11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000 through 2

11001000 00010111 00011111 11111111

otherwise 3

4 billionpossible entries

8/11/2019 Chapter4 5th Aug 2009

18/149

8/11/2019 Chapter4 5th Aug 2009

19/149

Network Layer 4-19

Datagram or VC network: why?

Internet (datagram) data exchange amongcomputers

elastic service, no stricttiming req.

smart end systems(computers)

can adapt, performcontrol, error recovery

simple inside network,complexity at edge many link types

different characteristicsuniform service difficult

ATM (VC) evolved from telephonyhuman conversation:

strict timing, reliability

requirementsneed for guaranteedservice

dumb end systems telephonescomplexity insidenetwork

8/11/2019 Chapter4 5th Aug 2009

20/149

8/11/2019 Chapter4 5th Aug 2009

21/149

Network Layer 4-21

Router Architecture Overview

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

8/11/2019 Chapter4 5th Aug 2009

22/149

Network Layer 4-22

Input Port Functions

Decentralized switching : given datagram dest., lookup output portusing forwarding table in input port

memorygoal: 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

8/11/2019 Chapter4 5th Aug 2009

23/149

Network Layer 4-23

Three types of switching fabrics

8/11/2019 Chapter4 5th Aug 2009

24/149

Network Layer 4-24

Switching Via MemoryFirst generation routers:

traditional computers with switching under directcontrol of CPUpacket copied to systems memory speed limited by memory bandwidth (2 bus

crossings per datagram)InputPort

OutputPort

Memory

System Bus

8/11/2019 Chapter4 5th Aug 2009

25/149

Network Layer 4-25

Switching Via a Bus

datagram from input port memory

to output port memory via a sharedbusbus contention: switching speedlimited by bus bandwidth

32 Gbps bus, Cisco 5600: sufficientspeed for access and enterpriserouters

8/11/2019 Chapter4 5th Aug 2009

26/149

Network Layer 4-26

Switching Via An InterconnectionNetwork

overcome bus bandwidth limitationsBanyan networks, other interconnection nets

initially developed to connect processors inmultiprocessoradvanced design: fragmenting datagram into fixedlength cells, switch cells through the fabric.

Cisco 12000: switches 60 Gbps through theinterconnection network

8/11/2019 Chapter4 5th Aug 2009

27/149

Network Layer 4-27

Output Ports

Buffering required when datagrams arrive fromfabric faster than the transmission rateScheduling discipline chooses among queueddatagrams for transmission

8/11/2019 Chapter4 5th Aug 2009

28/149

Network Layer 4-28

Output port queueing

buffering when arrival rate via switch exceedsoutput line speedqueueing (delay) and loss due to output portbuffer overflow!

8/11/2019 Chapter4 5th Aug 2009

29/149

Network Layer 4-29

How much buffering?

RFC 3439 rule of thumb: average bufferingequal to typical RTT (say 250 msec) timeslink capacity C

e.g., C = 10 Gps link: 2.5 Gbit bufferRecent recommendation: with N flows,buffering equal to RTT C.

N

8/11/2019 Chapter4 5th Aug 2009

30/149

Network Layer 4-30

Input Port QueuingFabric slower than input ports combined -> queueingmay occur at input queuesHead-of-the-Line (HOL) blocking: queued datagramat front of queue prevents others in queue frommoving forwardqueueing delay and loss due to input buffer overflow!

8/11/2019 Chapter4 5th Aug 2009

31/149

Network Layer 4-31


4. 1 Introduction4.2 Virtual circuit anddatagram networks4.3 Whats inside arouter4.4 IP: InternetProtocol




RIPOSPF


8/11/2019 Chapter4 5th Aug 2009

32/149

Network Layer 4-32

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

8/11/2019 Chapter4 5th Aug 2009

33/149

Network Layer 4-33






RIPOSPF


d f

8/11/2019 Chapter4 5th Aug 2009

34/149

Network Layer 4-34

IP datagram format

ver length

32 bits

data(variable length,typically a TCP

or UDP segment)

16-bit identifier header

checksum time to

live

32 bit source IP address

IP protocol versionnumber

header length(bytes)

max numberremaining hops

(decremented ateach router)

forfragmentation/reassembly

total datagramlength (bytes)

upper layer protocolto deliver payload to

head.len

type ofservice

type of data flgs fragment

offset upperlayer

32 bit destination IP address

Options (if any) E.g. timestamp,record routetaken, specifylist of routersto visit.

how much overheadwith TCP?20 bytes of TCP20 bytes of IP= 40 bytes + app

layer overhead

8/11/2019 Chapter4 5th Aug 2009

35/149

Network Layer 4-35

IP Fragmentation & Reassemblynetwork links have MTU(max.transfer size) - largestpossible link-level frame.

different link types,different MTUs

large IP datagram divided

(fragmented) within net one datagram becomesseveral datagramsreassembled only at finaldestination

IP header bits used toidentify, order relatedfragments

fragmentation:in: one large datagramout: 3 smaller datagrams

reassembly

8/11/2019 Chapter4 5th Aug 2009

36/149

Network Layer 4-36

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 bytedatagramMTU = 1500 bytes

1480 bytes indata field

offset =1480/8

8/11/2019 Chapter4 5th Aug 2009

37/149

Network Layer 4-37






RIPOSPF


8/11/2019 Chapter4 5th Aug 2009

38/149

Network Layer 4-38

IP Addressing: introduction

IP address: 32-bitidentifier for host,router interface interface: connection

between host/routerand physical linkrouters typically havemultiple interfaceshost typically has one

interfaceIP addressesassociated 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

223.1.3.2 223.1.3.1

223.1.3.27

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 1 1

8/11/2019 Chapter4 5th Aug 2009

39/149

Network Layer 4-39

Subnets

IP address: subnet part (highorder bits)host part (low orderbits)

Whats a subnet ? device interfaces withsame subnet part of IPaddresscan physically reacheach 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

223.1.3.2 223.1.3.1

223.1.3.27

network consisting of 3 subnets

subnet

8/11/2019 Chapter4 5th Aug 2009

40/149

Network Layer 4-40

Subnets 223.1.1.0/24 223.1.2.0/24

223.1.3.0/24

RecipeTo determine thesubnets, detach eachinterface from itshost or router,creating islands ofisolated networks.Each isolated network

is called a subnet .Subnet mask: /24

8/11/2019 Chapter4 5th Aug 2009

41/149

Network Layer 4-41

Subnets

How many? 223.1.1.1

223.1.1.3

223.1.1.4

223.1.2.2 223.1.2.1

223.1.2.6

223.1.3.2 223.1.3.1

223.1.3.27

223.1.1.2

223.1.7.0

223.1.7.1 223.1.8.0 223.1.8.1

223.1.9.1

223.1.9.2

8/11/2019 Chapter4 5th Aug 2009

42/149

8/11/2019 Chapter4 5th Aug 2009

43/149

Network Layer 4-43

IP addresses: how to get one?

Q: How does a host get IP address?

hard-coded by system admin in a fileWindows: control-panel->network->configuration->tcp/ip->propertiesUNIX: /etc/rc.config

DHCP: D ynamic Host Configuration Protocol:dynamically get address from as server

plug-andplay

8/11/2019 Chapter4 5th Aug 2009

44/149

Network Layer 4-44

DHCP: Dynamic Host Configuration Protocol

Goal: allow host to dynamically obtain its IP address fromnetwork server when it joins networkCan renew its lease on address in useAllows reuse of addresses (only hold address while connected an

on) Support for mobile users who want to join network (more shortly)

DHCP overview:host broadcasts DHCP discover msg [optional]

DHCP server responds with DHCP offer msg[optional]host requests IP address: DHCP request msg DHCP server sends address: DHCP ack msg

8/11/2019 Chapter4 5th Aug 2009

45/149

Network Layer 4-45

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

223.1.3.2 223.1.3.1

223.1.3.27

A

BE

DHCPserver

arriving DHCPclient needsaddress in thisnetwork

8/11/2019 Chapter4 5th Aug 2009

46/149

Network Layer 4-46

DHCP client-server scenarioDHCP server: 223.1.2.5 arriving

client

time

DHCP discover

src : 0.0.0.0, 68dest.: 255.255.255.255,67yiaddr: 0.0.0.0transaction ID: 654

DHCP offer src: 223.1.2.5, 67dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 654Lifetime: 3600 secs

DHCP request src: 0.0.0.0, 68dest:: 255.255.255.255, 67yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs

DHCP ACK src: 223.1.2.5, 67dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs

8/11/2019 Chapter4 5th Aug 2009

47/149

Network Layer 4-47

DHCP: more than IP address

DHCP can return more than just allocated IPaddress on subnet:

address of first-hop router for client

name and IP address of DNS severnetwork mask (indicating network versus hostportion of address)

8/11/2019 Chapter4 5th Aug 2009

48/149

Network Layer 4-48

DHCP: example

connecting laptop needs its

IP address, addr of first-hop router, addr of DNSserver: use DHCP

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDPIP

EthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP request encapsulatedin UDP, encapsulated in IP,encapsulated in 802.1EthernetEthernet frame broadcast(dest: FFFFFFFFFFFF) on LAN,received at router runningDHCP server

Ethernet demuxed to IPdemuxed, UDP demuxed toDHCP

168.1.1.1

8/11/2019 Chapter4 5th Aug 2009

49/149

Network Layer 4-49

DCP server formulatesDHCP ACK containingclients IP address, IPaddress of first-hoprouter for client, name &IP address of DNS server

router(runs DHCP)

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCPUDPIP

EthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation of DHCP

server, frame forwardedto client, demuxing up toDHCP at clientclient now knows its IPaddress, name and IPaddress of DSN server, IPaddress of its first-hoprouter

DHCP: example

8/11/2019 Chapter4 5th Aug 2009

50/149

Network Layer 4-50

DHCP: wiresharkoutput (home LAN)

Message type: Boot Reply (2)Hardware type: EthernetHardware address length: 6Hops: 0Transaction ID: 0x6b3a11b7Seconds elapsed: 0Bootp flags: 0x0000 (Unicast)Client IP address: 192.168.1.101 (192.168.1.101)Your (client) IP address: 0.0.0.0 (0.0.0.0)Next server IP address: 192.168.1.1 (192.168.1.1)Relay agent IP address: 0.0.0.0 (0.0.0.0)Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a)Server host name not givenBoot file name not givenMagic cookie: (OK)Option: (t=53,l=1) DHCP Message Type = DHCP ACK

Option: (t=54,l=4) Server Identifier = 192.168.1.1Option: (t=1,l=4) Subnet Mask = 255.255.255.0Option: (t=3,l=4) Router = 192.168.1.1Option: (6) Domain Name Server

Length: 12; Value: 445747E2445749F244574092;IP Address: 68.87.71.226;IP Address: 68.87.73.242;IP Address: 68.87.64.146

Option: (t=15,l=20) Domain Name = "hsd1.ma.comcast.net."

reply

Message type: Boot Request (1)Hardware type: EthernetHardware address length: 6Hops: 0Transaction ID: 0x6b3a11b7Seconds elapsed: 0Bootp flags: 0x0000 (Unicast)Client IP address: 0.0.0.0 (0.0.0.0)Your (client) IP address: 0.0.0.0 (0.0.0.0)Next server IP address: 0.0.0.0 (0.0.0.0)

Relay agent IP address: 0.0.0.0 (0.0.0.0)Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a)Server host name not givenBoot file name not givenMagic cookie: (OK)Option: (t=53,l=1) DHCP Message Type = DHCP RequestOption: (61) Client identifier

Length: 7; Value: 010016D323688A;Hardware type: EthernetClient MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a)

Option: (t=50,l=4) Requested IP Address = 192.168.1.101Option: (t=12,l=5) Host Name = "nomad"Option: (55) Parameter Request List

Length: 11; Value: 010F03062C2E2F1F21F92B1 = Subnet Mask; 15 = Domain Name3 = Router; 6 = Domain Name Server44 = NetBIOS over TCP/IP Name Server

request

8/11/2019 Chapter4 5th Aug 2009

51/149

Network Layer 4-51

IP addresses: how to get one?

Q: How does network get subnet part of IPaddr?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 200.23.16.0/23Organization 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

8/11/2019 Chapter4 5th Aug 2009

52/149

Network Layer 4-52

Hierarchical addressing: route aggregation

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 anything with addressesbeginning199.31.0.0/16

200.23.20.0/23Organization 2

. . .

. . .

Hierarchical addressing allows efficient advertisement of routinginformation:

8/11/2019 Chapter4 5th Aug 2009

53/149

Network Layer 4-53

Hierarchical addressing: more specificroutes

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 anything with addressesbeginning 199.31.0.0/16or 200.23.18.0/23

200.23.20.0/23Organization 2

. . .

. . .

8/11/2019 Chapter4 5th Aug 2009

54/149

Network Layer 4-54

IP addressing: the last word...

Q: How does an ISP get block of addresses?A: ICANN: I nternet Corporation for Assigned

Names and Numbers

allocates addressesmanages DNSassigns domain names, resolves disputes

8/11/2019 Chapter4 5th Aug 2009

55/149

Network Layer 4-55

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)

All datagrams leaving localnetwork have same single source

NAT IP address: 138.76.29.7,different source port numbers

8/11/2019 Chapter4 5th Aug 2009

56/149

Network Layer 4-56


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

range of addresses not needed from ISP: just one IPaddress for all devicescan change addresses of devices in local networkwithout notifying outside worldcan change ISP without changing addresses ofdevices in local networkdevices inside local net not explicitly addressable,visible by outside world (a security plus).

8/11/2019 Chapter4 5th Aug 2009

57/149

Network Layer 4-57

NAT: Network Address TranslationImplementation: NAT router must:

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

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

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

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

8/11/2019 Chapter4 5th Aug 2009

58/149

Network Layer 4-58


10.0.0.1

10.0.0.2

10.0.0.3

S: 10.0.0.1, 3345D: 128.119.40.186, 80

110.0.0.4

138.76.29.7

1: host 10.0.0.1sends datagram to128.119.40.186, 80

NAT translation tableWAN side addr LAN side addr138.76.29.7, 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 33: Reply arrivesdest. address:138.76.29.7, 5001

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

8/11/2019 Chapter4 5th Aug 2009

59/149

Network Layer 4-59


16-bit port-number field:60,000 simultaneous connections with a singleLAN-side address!

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

NAT possibility must be taken into account by app

designers, eg, P2P applicationsaddress shortage should instead be solved byIPv6

8/11/2019 Chapter4 5th Aug 2009

60/149

Network Layer 4-60

NAT traversal problem

client wants to connect toserver with address 10.0.0.1server address 10.0.0.1 localto LAN (client cant use it asdestination addr)

only one externally visibleNATted address: 138.76.29.7solution 1: staticallyconfigure NAT to forwardincoming connection

requests at given port toservere.g., (123.76.29.7, port 2500)always forwarded to 10.0.0.1port 25000

10.0.0.1

10.0.0.4

NATrouter

138.76.29.7

Client ?

8/11/2019 Chapter4 5th Aug 2009

61/149

Network Layer 4-61


solution 2: Universal Plug andPlay (UPnP) Internet GatewayDevice (IGD) Protocol. AllowsNATted host to:

learn public IP address(138.76.29.7)add/remove port mappings(with lease times)

i.e., automate static NAT portmap configuration

10.0.0.1

10.0.0.4

NATrouter

138.76.29.7

IGD

8/11/2019 Chapter4 5th Aug 2009

62/149

Network Layer 4-62


solution 3: relaying (used in Skype)NATed client establishes connection to relayExternal client connects to relayrelay bridges packets between to connections

138.76.29.7Client

10.0.0.1

NATrouter

1. connection torelay initiatedby NATted host

2. connection torelay initiatedby client

3. relayingestablished

8/11/2019 Chapter4 5th Aug 2009

63/149

8/11/2019 Chapter4 5th Aug 2009

64/149

Network Layer 4-64

ICMP: Internet Control Message Protocol

used by hosts & routers tocommunicate network-levelinformation

error reporting:unreachable host, network,

port, protocolecho request/reply (usedby ping)

networklayer above IP: ICMP msgs carried in IPdatagrams

ICMP message: type, code plusfirst 8 bytes of IP datagramcausing error

Type Code description0 0 echo reply (ping)3 0 dest. network unreachable3 1 dest host unreachable3 2 dest protocol unreachable

3 3 dest port unreachable3 6 dest network unknown3 7 dest host unknown4 0 source quench (congestion

control - not used)8 0 echo request (ping)9 0 route advertisement10 0 router discovery11 0 TTL expired12 0 bad IP header

8/11/2019 Chapter4 5th Aug 2009

65/149

Network Layer 4-65

Traceroute and ICMP

Source sends series ofUDP segments to dest

First has TTL =1Second has TTL=2, etc.Unlikely port number

When nth datagram arrivesto nth router:

Router discards datagramAnd sends to source anICMP message (type 11,code 0)Message includes name ofrouter& IP address

When ICMP messagearrives, source calculatesRTTTraceroute does this 3times

Stopping criterionUDP segment eventuallyarrives at destination hostDestination returns ICMP

host unreachable packet(type 3, code 3)When source gets thisICMP, stops.

8/11/2019 Chapter4 5th Aug 2009

66/149

Network Layer 4-66






RIPOSPF


8/11/2019 Chapter4 5th Aug 2009

67/149

8/11/2019 Chapter4 5th Aug 2009

68/149

Network Layer 4-68

IPv6 Header (Cont)

Priority: identify priority among datagrams in flowFlow Label: identify datagrams in same flow.(concept offlow not well defined).

Next header: identify upper layer protocol for data

8/11/2019 Chapter4 5th Aug 2009

69/149

Network Layer 4-69

Other Changes from IPv4

Checksum : removed entirely to reduceprocessing time at each hopOptions: allowed, but outside of header,

indicated by Next Header field ICMPv6: new version of ICMPadditional message types, e.g. Packet Too Big multicast group management functions

8/11/2019 Chapter4 5th Aug 2009

70/149

Network Layer 4-70

Transition From IPv4 To IPv6

Not all routers can be upgraded simultaneousno flag days How will the network operate with mixed IPv4 andIPv6 routers?

Tunneling: IPv6 carried as payload in IPv4datagram among IPv4 routers

T li

8/11/2019 Chapter4 5th Aug 2009

71/149

Network Layer 4-71

TunnelingA B E F

IPv6 IPv6 IPv6 IPv6

tunnelLogical view:

Physical view:A B E F

IPv6 IPv6 IPv6 IPv6IPv4 IPv4

T li

8/11/2019 Chapter4 5th Aug 2009

72/149

Network Layer 4-72

TunnelingA B E F

IPv6 IPv6 IPv6 IPv6

tunnelLogical view:

Physical view:A B E F

IPv6 IPv6 IPv6 IPv6

C D

IPv4 IPv4

Flow: XSrc: ADest: F

data


data


data

Src:BDest: E


data

Src:BDest: E

A-to-B:IPv6

E-to-F:IPv6B-to-C:IPv6 inside

IPv4

B-to-C:IPv6 inside

IPv4

8/11/2019 Chapter4 5th Aug 2009

73/149

Network Layer 4-73






RIPOSPF


8/11/2019 Chapter4 5th Aug 2009

74/149

8/11/2019 Chapter4 5th Aug 2009

75/149

Network Layer 4-75

u

y x

w v

z2

2 1

3

1

1

2

5 3

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

8/11/2019 Chapter4 5th Aug 2009

76/149

Network Layer 4-76

Graph abstraction: costs

u

y x

w v

z2

2

1 3

1

1

2

5 3

5 c(x,x) = cost of link (x,x)

- e.g., c(w,z) = 5

cost could always be 1, or

inversely related to bandwidth,or inversely related tocongestion

Cost of path (x 1, 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

8/11/2019 Chapter4 5th Aug 2009

77/149

Network Layer 4-77

Routing Algorithm classification

Global or decentralizedinformation? Global:

all routers have completetopology, link cost infolink state algorithms

Decentralized: router knows physically-connected neighbors, linkcosts to neighborsiterative process ofcomputation, exchange ofinfo with neighborsdistance vector algorithms

Static or dynamic?Static: routes change slowlyover time

Dynamic: routes change morequickly

periodic updatein response to linkcost changes

h k

8/11/2019 Chapter4 5th Aug 2009

78/149

Network Layer 4-78






RIPOSPF


8/11/2019 Chapter4 5th Aug 2009

79/149

Network Layer 4-79

A Link-State Routing Algorithm

Dijkstras algorithm net topology, link costsknown to all nodes

accomplished via linkstate broadcastall nodes have same info

computes least cost pathsfrom one node (source) toall other nodes

gives forwarding table for that nodeiterative: after kiterations, know least costpath to k dest.s

Notation: c(x,y): link cost from nodex to y; = if not directneighbors

D(v): current value of costof path from source todest. vp(v): predecessor nodealong path from source to vN': set of nodes whoseleast cost path definitivelyknown

k l h

8/11/2019 Chapter4 5th Aug 2009

80/149

Network Layer 4-80

Dijsktras Algorithm

1 Ini t ial izat ion: 2 N' = {u}3 for all nodes v4 if v adjacent to u5 then D(v) = c(u,v)6 else D(v) = 78 L o o p 9 find w not in N' such that D(w) is a minimum10 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 known14 shortest path cost to w plus cost from w to v */15 un t i l al l nodes in N '

k l h l

8/11/2019 Chapter4 5th Aug 2009

81/149

Network Layer 4-81

Dijkstras algorithm: example

Step0123

45

N'u

uxuxy

uxyv

uxyvwuxyvwz

D(v),p(v)2,u2,u2,u

D(w),p(w)5,u4,x3,y3,y

D(x),p(x)1,u D(y),p(y) 2,x

D(z),p(z)

4,y4,y

4,y

u

y x

w v z

2 2

1 3

1

1 2

5 3

5

ijk l i h l (2)

8/11/2019 Chapter4 5th Aug 2009

82/149

Network Layer 4-82

Dijkstras algorithm: example (2)

u

y x

w v

z

Resulting shortest-path tree from u:

vx ywz

(u,v)(u,x)(u,x)(u,x)(u,x)

destination link

Resulting forwarding table in u:

Dijk l i h di i

8/11/2019 Chapter4 5th Aug 2009

83/149

Network Layer 4-83

Dijkstras algorithm, discussion Algorithm complexity: n nodes

each iteration: need to check all nodes, w, not in Nn(n+1)/2 comparisons: O(n 2)more efficient implementations possible: O(nlogn)

Oscillations possible: e.g., link cost = amount of carried traffic

A D

C B

1 1+e

e 0

e 1 1

0 0

A

D

C B

2+e 0

0 0 1+e 1

A

D

C B

0 2+e

1+e 1 0 0

A

D C

B 2+e 0

e 0 1+e 1

initially recompute routing recompute recompute

Ch 4 N k L

8/11/2019 Chapter4 5th Aug 2009

84/149

Network Layer 4-84



Datagram format

IPv4 addressingICMPIPv6



RIPOSPF


Di V Al i h

8/11/2019 Chapter4 5th Aug 2009

85/149

Network Layer 4-85

Distance Vector Algorithm

Bellman-Ford Equation (dynamic programming)Definedx(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

8/11/2019 Chapter4 5th Aug 2009

86/149

8/11/2019 Chapter4 5th Aug 2009

87/149

Di t t l ith (4)

8/11/2019 Chapter4 5th Aug 2009

88/149

Network Layer 4-88

Distance vector algorithm (4)

Basic idea: From time-to-time, each node sends its owndistance vector estimate to neighborsAsynchronous

When a node x receives new DV estimate fromneighbor, it updates its own DV using B-F equation:D x (y) min v {c(x,v) + D v (y)} for each node y N

Under minor, natural conditions, the estimateD x (y) converge to the actual least cost dx(y)

Di t V t Al ith (5)

8/11/2019 Chapter4 5th Aug 2009

89/149

Network Layer 4-89

Distance Vector Algorithm (5)

Iterative, asynchronous:each local iteration causedby:local link cost changeDV update message from

neighborDistributed: each node notifiesneighbors only when its DVchanges

neighbors then notifytheir neighbors ifnecessary

wait for (change in local linkcost or msg from neighbor)

recompute estimates

if DV to any dest haschanged, notify neighbors

Each node:

Dx(y) = min{c(x,y) + D y(y), c(x,z) + Dz(y)}= min{2+0 , 7+1} = 2

D x (z) = min{c(x,y) +D y (z), c(x,z) + D z (z) }

8/11/2019 Chapter4 5th Aug 2009

90/149

Network Layer 4-90

x y z

x yz

0 2 7

f r o m

cost to

f r o m

f r o m

x y z

x yz

0

f r o m

cost to

x y zx

yz

cost to

x y zx

yz

7 1 0

cost to

2 0 1

2 0 17 1 0

time

x z1 2

7

y

node x table

node y table

node z table

{ , } y= min{2+1 , 7+0} = 3

32

Dx(y) = min{c(x,y) + D y(y), c(x,z) + Dz(y)}= min{2+0 , 7+1} = 2

D x (z) = min{c(x,y) +D y (z), c(x,z) + D z (z) }

8/11/2019 Chapter4 5th Aug 2009

91/149

Network Layer 4-91

x y z

x yz

0 2 7

f r o m

cost to

f r o m

f r o m

x y zx

yz

0 2 3

f r o m

cost tox y z

x yz

0 2 3

f r o m

cost to

x y zx

yz

cost tox y z

x yz

0 2 7

f r o m

cost to

x y zx

yz

0 2 3

f r o m

cost to

x y zx

yz

0 2 3

f r o m

cost to

x y zx

yz

0 2 7

f r o m

cost to

x y zx

yz

7 1 0

cost to

2 0 1

2 0 17 1 0

2 0 17 1 0

2 0 13 1 0

2 0 13 1 0

2 0 1

3 1 02 0 1

3 1 0

time

x z1 2

7

y

node x table

node y table

node z table

{ , }= min{2+1 , 7+0} = 3

Di t V t li k t h g

8/11/2019 Chapter4 5th Aug 2009

92/149

Network Layer 4-92

Distance Vector: link cost changes

Link cost changes: node detects local link cost changeupdates routing info, recalculatesdistance vectorif DV changes, notify neighbors

good newstravelsfast

x z 1 4

50

y 1

At time t 0 , y detects the link-cost change, updates its DV,and informs its neighbors.

At time t 1 , z receives the update from y and updates its table.It computes a new least cost to x and sends its neighbors its DV

At time t 2 , y receives z s update and updates its distance table.y s least costs do not change and hence y does not send anymessage to z .


8/11/2019 Chapter4 5th Aug 2009

93/149

Network Layer 4-93


Link cost changes: good news travels fastbad news travels slow -count to infinity problem! 44 iterations before

algorithm stabilizes: seetextPoisoned reverse:

If Z routes through Y toget to X :

Z tells Y its (Zs) distanceto X is infinite (so Y wontroute to X via Z)

will this completely solvecount to infinity problem?

x z 1 4

50

y 60

8/11/2019 Chapter4 5th Aug 2009

94/149


8/11/2019 Chapter4 5th Aug 2009

95/149

Network Layer 4-95



Datagram format



4.6 Routing in theInternetRIPOSPF


Hierarchical Routing

8/11/2019 Chapter4 5th Aug 2009

96/149

Network Layer 4-96


scale: with 200 milliondestinations:cant store all dests inrouting tables!routing table exchangewould swamp links!

administrative autonomy internet = network ofnetworkseach network admin maywant to control routing in itsown network

Our routing study thus far - idealizationall routers identicalnetwork flat

not true in practice


8/11/2019 Chapter4 5th Aug 2009

97/149

Network Layer 4-97


aggregate routers intoregions, autonomoussystems (AS) routers in same AS run

same routing protocolintra -AS routing protocolrouters in different AScan run different intra-

AS routing protocol

Gateway routerDirect link to router inanother AS

Interconnected ASes

8/11/2019 Chapter4 5th Aug 2009

98/149

Network Layer 4-98

3b

1d

3a

1c2a AS3

AS1

AS21a

2c 2b

1b

Intra-ASRoutingalgorithm

Inter-ASRoutingalgorithm

Forwardingtable

3c

Interconnected ASes

forwarding tableconfigured by bothintra- and inter-ASrouting algorithm

intra-AS sets entriesfor internal destsinter-AS & intra-Assets entries forexternal dests

Inter-AS tasks AS1 must:

8/11/2019 Chapter4 5th Aug 2009

99/149

Network Layer 4-99

3b

1d

3a

1c2a AS3

AS1AS2

1a

2c 2b

1b

3c

suppose router in AS1receives datagramdestined outside ofAS1:

router shouldforward packet to

gateway router, butwhich one?

AS1 must:1. learn which dests are

reachable throughAS2, which throughAS3

2. propagate thisreachability info to all

routers in AS1Job of inter-AS routing!

Example: Setting forwarding table in router 1d

8/11/2019 Chapter4 5th Aug 2009

100/149

Network Layer 4-100

suppose AS1 learns (via inter-AS protocol) that subnetx reachable via AS3 (gateway 1c) but not via AS2.inter-AS protocol propagates reachability info to allinternal routers.router 1d determines from intra-AS routing info thatits interface I is on the least cost path to 1c.

installs forwarding table entry (x,I)

3b

1d

3a

1c2a AS3

AS1AS2

1a

2c 2b

1b

3c

x

Example: Choosing among multiple ASes

8/11/2019 Chapter4 5th Aug 2009

101/149

Network Layer 4-101

p g g p

now suppose AS1 learns from inter-AS protocol thatsubnet x is reachable from AS3 and from AS2.to configure forwarding table, router 1d mustdetermine towards which gateway it should forwardpackets for dest x.

this is also job of inter-AS routing protocol!

3b

1d

3a

1c 2a AS3

AS1AS2

1a

2c 2b

1b

3c x

Example: Choosing among multiple ASes

8/11/2019 Chapter4 5th Aug 2009

102/149

Network Layer 4-102

Learn from inter-ASprotocol that subnet

x is reachable viamultiple gateways

Use routing info

from intra-ASprotocol to determinecosts of least-cost

paths to eachof the gateways

Hot potato routing:Choose the gateway

that has thesmallest least cost

Determine fromforwarding table theinterface I that leads

to least-cost gateway.Enter (x,I) in

forwarding table

now suppose AS1 learns from inter-AS protocol thatsubnet x is reachable from AS3 and from AS2.to configure forwarding table, router 1d mustdetermine towards which gateway it should forwardpackets for dest x.

this is also job of inter-AS routing protocol!hot potato routing: send packet towards closest oftwo routers.

8/11/2019 Chapter4 5th Aug 2009

103/149

8/11/2019 Chapter4 5th Aug 2009

104/149


8/11/2019 Chapter4 5th Aug 2009

105/149

Network Layer 4-105



Datagram format



4.6 Routing in theInternetRIPOSPFBGP


RIP ( Routing Information Protocol)

8/11/2019 Chapter4 5th Aug 2009

106/149

Network Layer 4-106

RIP ( Routing Information Protocol)

distance vector algorithmincluded in BSD-UNIX Distribution in 1982distance metric: # of hops (max = 15 hops)

D C

B A

u vw

x

yz

destination hopsu 1v 2w 2x 3

y 3z 2

From router A to subnets:

RIP advertisements

8/11/2019 Chapter4 5th Aug 2009

107/149

Network Layer 4-107

RIP advertisements

distance vectors: exchanged amongneighbors every 30 sec via ResponseMessage (also called advertisement )

each advertisement: list of up to 25destination subnets within AS

RIP: Example

8/11/2019 Chapter4 5th Aug 2009

108/149

Network Layer 4-108

RIP: Example

Destination Network Next Router Num. of hops to dest.w A 2y B 2

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

w x y z

A

C

D B

Routing/Forwarding table in D

RIP: Example

8/11/2019 Chapter4 5th Aug 2009

109/149

Network Layer 4-109

Destination Network Next Router Num. of hops to dest.w A 2y B 2z B A 7 5x -- 1. . ....

Routing/Forwarding table in D

w x y

z

A

C

DB

Dest Next hopsw - 1x - 1z C 4. ...

Advertisementfrom A to D

RIP: Link Failure and Recovery

8/11/2019 Chapter4 5th Aug 2009

110/149

Network Layer 4-110

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

neighbor/link declared deadroutes via neighbor invalidatednew advertisements sent to neighbors

neighbors in turn send out new advertisements (iftables changed)link failure info quickly (?) propagates to entire net

poison reverse used to prevent ping-pong loops

(infinite distance = 16 hops)

RIP Table processing

8/11/2019 Chapter4 5th Aug 2009

111/149

Network Layer 4-111

RIP Table processing

RIP routing tables managed by application-level process called route-d (daemon)advertisements sent in UDP packets, periodicallyrepeated

physicallink

network forwarding(IP) table

Transprt(UDP)

routed

physicallink

network(IP)

Transprt(UDP)

routed

forwardingtable


8/11/2019 Chapter4 5th Aug 2009

112/149

Network Layer 4-112



Datagram format





8/11/2019 Chapter4 5th Aug 2009

113/149

OSPF advanced features (not in RIP)

8/11/2019 Chapter4 5th Aug 2009

114/149

Network Layer 4-114

OSPF advanced features (not in RIP)

security: all OSPF messages authenticated (toprevent malicious intrusion)multiple same-cost path s allowed (only one path inRIP)For each link, multiple cost metrics for differentTOS (e.g., satellite link cost set low for best effort;high for real time)integrated uni- and multicast support:

Multicast OSPF (MOSPF) uses same topology database as OSPF

hierarchical OSPF in large domains.

Hierarchical OSPF

8/11/2019 Chapter4 5th Aug 2009

115/149

Network Layer 4-115

Hierarchical OSPF

Hierarchical OSPF

8/11/2019 Chapter4 5th Aug 2009

116/149

Network Layer 4-116

Hierarchical OSPF

two-level hierarchy: local area, backbone.Link-state advertisements only in areaeach nodes has detailed area topology; only knowdirection (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.


8/11/2019 Chapter4 5th Aug 2009

117/149

Network Layer 4-117



Datagram format





Internet inter-AS routing: BGP

8/11/2019 Chapter4 5th Aug 2009

118/149

Network Layer 4-118

Internet inter AS routing: BGP

BGP (Border Gateway Protocol): the defacto standardBGP provides each AS a means to:1. Obtain subnet reachability information from

neighboring ASs.2. Propagate reachability information to all AS-

internal routers.3. Determine good routes to subnets based on

reachability information and policy.allows subnet to advertise its existence torest of Internet: I am here

BGP basics

8/11/2019 Chapter4 5th Aug 2009

119/149

Network Layer 4-119

pairs of routers (BGP peers) exchange routing infoover semi-permanent TCP connections: BGP sessions

BGP sessions need not correspond to physicallinks.when AS2 advertises a prefix to AS1:

AS2 promises it will forward datagrams towardsthat prefix.AS2 can aggregate prefixes in its advertisement

3b

1d

3a

1c2a AS3

AS1

AS21a

2c 2b

1b

3c eBGP session

iBGP session

8/11/2019 Chapter4 5th Aug 2009

120/149

Path attributes & BGP routes

8/11/2019 Chapter4 5th Aug 2009

121/149

Network Layer 4-121

Path attributes & BGP routes

advertised prefix includes BGP attributes.prefix + attributes = route

two important attributes:AS-PATH: contains ASs through which prefixadvertisement has passed: e.g, AS 67, AS 17NEXT-HOP: indicates specific internal-AS routerto next-hop AS. (may be multiple links fromcurrent AS to next-hop-AS)

when gateway router receives routeadvertisement, uses import policy toaccept/decline.

BGP route selection

8/11/2019 Chapter4 5th Aug 2009

122/149

Network Layer 4-122

BGP route selection

router may learn about more than 1 routeto some prefix. Router must select route.elimination rules:

1.local preference value attribute: policydecision

2. shortest AS-PATH3. closest NEXT-HOP router: hot potato routing

4. additional criteria

BGP messages

8/11/2019 Chapter4 5th Aug 2009

123/149

Network Layer 4-123

g

BGP messages exchanged using TCP.BGP messages:OPEN: opens TCP connection to peer andauthenticates sender

UPDATE: advertises new path (or withdraws old)KEEPALIVE keeps connection alive in absence ofUPDATES; also ACKs OPEN requestNOTIFICATION: reports errors in previous msg;also used to close connection

BGP routing policy

8/11/2019 Chapter4 5th Aug 2009

124/149

Network Layer 4-124

g p y

A,B,C are provider networksX,W,Y are customer (of provider networks)X is dual-homed: attached to two networks

X does not want to route from B via X to C.. so X will not advertise to B a route to C

AB

C

WX

Y

legend :

customernetwork:

providernetwork

BGP routing policy (2)

8/11/2019 Chapter4 5th Aug 2009

125/149

Network Layer 4-125

g p y ( )

A advertises path AW to BB advertises path BAW to XShould B advertise path BAW to C?

No way! B gets no revenue for routing CBAWsince neither W nor C are Bs customersB wants to force C to route to w via AB wants to route only to/from its customers!

AB

C

WX

Y

legend :

customernetwork:

providernetwork

Why different Intra- and Inter-AS routing ?

8/11/2019 Chapter4 5th Aug 2009

126/149

Network Layer 4-126

y g

Policy: Inter-AS: admin wants control over how its trafficrouted, who routes through its net.Intra-AS: single admin, so no policy decisions needed

Scale: hierarchical routing saves table size, reduced updatetraffic

Performance: Intra-AS: can focus on performanceInter-AS: policy may dominate over performance

8/11/2019 Chapter4 5th Aug 2009

127/149

8/11/2019 Chapter4 5th Aug 2009

128/149

In-network duplication

8/11/2019 Chapter4 5th Aug 2009

129/149

Network Layer 4-129

p

flooding: when node receives brdcst pckt,sends copy to all neighborsProblems: cycles & broadcast storm

controlled flooding: node only brdcsts pktif it hasnt brdcst same packet before

Node keeps track of pckt ids already brdcstedOr reverse path forwarding (RPF): only forwardpckt if it arrived on shortest path between

node and sourcespanning treeNo redundant packets received by any node

Spanning Tree

8/11/2019 Chapter4 5th Aug 2009

130/149

Network Layer 4-130

A

B

G

D E

c

F

A

B

G

D

E

c

F

(a) Broadcast initiated at A (b) Broadcast initiated at D

p g

First construct a spanning treeNodes forward copies only along spanningtree

Spanning Tree: Creation

8/11/2019 Chapter4 5th Aug 2009

131/149

Network Layer 4-131

A

B

G

D E

c

F 1

2

3

4

5

(a) Stepwise constructionof spanning tree

A

B

G

D E

c

F

(b) Constructed spanningtree

Center node

Each node sends unicast join message to centernode

Message forwarded until it arrives at a node alreadybelonging to spanning tree

Multicast Routing: Problem Statement

8/11/2019 Chapter4 5th Aug 2009

132/149

gGoal: find a tree (or trees) connectingrouters having local mcast group members

tree: not all paths between routers usedsource-based: different tree from each sender to rcvrsshared-tree: same tree used by all group members

Shared tree Source-based trees

Approaches for building mcast trees

8/11/2019 Chapter4 5th Aug 2009

133/149

Approaches for building mcast trees

Approaches:source-based tree: one tree per source

shortest path trees

reverse path forwardinggroup-shared tree: group uses one treeminimal spanning (Steiner)center-based trees

we first look at basic approaches, then specificprotocols adopting these approaches

Shortest Path Tree

8/11/2019 Chapter4 5th Aug 2009

134/149

mcast forwarding tree: tree of shortestpath routes from source to all receiversDijkstras algorithm

R1

R2

R3

R4

R5

R6 R7

21

63 4

5

i

router with attachedgroup member

router with no attachedgroup memberlink used for forwarding,i indicates order linkadded by algorithm

LEGENDS: source

Reverse Path Forwarding

8/11/2019 Chapter4 5th Aug 2009

135/149

g

if (mcast datagram received on incoming linkon shortest path back to center)then flood datagram onto all outgoing links

else ignore datagram

rely on routers knowledge of unicastshortest path from it to sendereach router has simple forwarding behavior:

Reverse Path Forwarding: example

8/11/2019 Chapter4 5th Aug 2009

136/149

result is a source-specific reverse SPT may be a bad choice with asymmetric links

R1

R2

R3

R4

R5

R6 R7

router with attachedgroup member

router with no attached

group memberdatagram will beforwarded

LEGENDS: source

datagram will not beforwarded

8/11/2019 Chapter4 5th Aug 2009

137/149

Shared-Tree: Steiner Tree

8/11/2019 Chapter4 5th Aug 2009

138/149

Steiner Tree: minimum cost treeconnecting all routers with attached groupmembersproblem is NP-completeexcellent heuristics existsnot used in practice:

computational complexityinformation about entire network neededmonolithic: rerun whenever a router needs to

join/leave

Center-based trees

8/11/2019 Chapter4 5th Aug 2009

139/149

single delivery tree shared by allone router identified as center of treeto join:

edge router sends unicast join-msg addressedto center router join-msg processed by intermediate routersand forwarded towards center

join-msg either hits existing tree branch forthis center, or arrives at centerpath taken by join-msg becomes new branch oftree for this router

Center-based trees: an example

8/11/2019 Chapter4 5th Aug 2009

140/149

Suppose R6 chosen as center:

R1

R2

R3

R4

R5

R6 R7

router with attachedgroup memberrouter with no attachedgroup memberpath order in which joinmessages generated

LEGEND

21

3

1

Internet Multicasting Routing: DVMRP

8/11/2019 Chapter4 5th Aug 2009

141/149

g g

DVMRP: distance vector multicast routingprotocol, RFC1075flood and prune: reverse path forwarding,source-based tree

RPF tree based on DVMRPs own routing tablesconstructed by communicating DVMRP routersno assumptions about underlying unicast

initial datagram to mcast group floodedeverywhere via RPFrouters not wanting group: send upstream prunemsgs

DVMRP: continued

8/11/2019 Chapter4 5th Aug 2009

142/149

soft state: DVMRP router periodically (1 min.)forgets branches are pruned:mcast data again flows down unpruned branchdownstream router: reprune or else continue to

receive datarouters can quickly regraft to tree

following IGMP join at leafodds and ends

commonly implemented in commercial routersMbone routing done using DVMRP

Tunneling

8/11/2019 Chapter4 5th Aug 2009

143/149

Q: How to connect islands of multicastrouters in a sea of unicast routers?

mcast datagram encapsulated inside normal (non -multicast-

addressed) datagramnormal IP datagram sent thru tunnel via regular IP unicast toreceiving mcast routerreceiving mcast router unencapsulates to get mcast datagram

physical topology logical topology

PIM: Protocol Independent Multicast

8/11/2019 Chapter4 5th Aug 2009

144/149

p

not dependent on any specific underlying unicastrouting algorithm (works with all)

two different multicast distribution scenarios :

Dense :group membersdensely packed, inclose proximity.

bandwidth moreplentiful

Sparse:# networks with groupmembers small wrt #interconnected networks

group members widelydispersed bandwidth not plentiful

Consequences of Sparse-Dense Dichotomy:

8/11/2019 Chapter4 5th Aug 2009

145/149

q p y

Dense group membership byrouters assumed untilrouters explicitly prune

data-driven constructionon mcast tree (e.g., RPF)bandwidth and non-group-router processing

profligate

Sparse :no membership untilrouters explicitly joinreceiver- driven

construction of mcasttree (e.g., center-based)bandwidth and non-group-router processing

conservative

PIM- Dense Mode

8/11/2019 Chapter4 5th Aug 2009

146/149

flood-and-prune RPF , similar to DVMRP butunderlying unicast protocol provides RPF infofor incoming datagramless complicated (less efficient) downstreamflood than DVMRP reduces reliance onunderlying routing algorithmhas protocol mechanism for router to detect itis a leaf-node router

PIM - Sparse Mode

8/11/2019 Chapter4 5th Aug 2009

147/149

center-based approachrouter sends join msgto rendezvous point(RP)

intermediate routersupdate state andforward join

after joining via RP,router can switch tosource-specific tree

increased performance:less concentration,shorter paths

R1

R2

R3

R4

R5

R6R7

join

join

join

all data multicastfrom rendezvouspoint

rendezvouspoint

PIM - Sparse Mode

8/11/2019 Chapter4 5th Aug 2009

148/149

sender(s):unicast data to RP,which distributes downRP-rooted tree

RP can extend mcasttree upstream tosourceRP can send stop msg

if no attachedreceiversno one is listening!

R1

R2

R3

R4

R5

R6R7

join

join

join

all data multicastfrom rendezvouspoint

rendezvouspoint

Chapter 4: summary

8/11/2019 Chapter4 5th Aug 2009

149/149

4. 1 Introduction4.2 Virtual circuit anddatagram networks4.3 Whats inside a

router4.4 IP: InternetProtocol

Datagram format

IPv4 addressingICMP



4.7 Broadcast andl i i

Date post:	02-Jun-2018
Category:	Documents
Upload:	stevan-adjie
View:	217 times
Download:	0 times

Chapter4 5th Aug 2009

Documents