Network Layer 4-2

Chapter 4Network Layer

Computer Networking: A Top Down Approach 5th edition. Jim Kurose, Keith RossAddison-Wesley, April 2009.

A note on the use of these ppt slides:We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you see the animations; and can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: If you use these slides (e.g., in a class) that you mention their source

(after all, we’d like people to use our book!) If you post any slides on a www site, that you note that they are adapted

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Thanks and enjoy! JFK/KWR

All material copyright 1996-2011J.F Kurose and K.W. Ross, All Rights Reserved

Legends Back to previous foil

Page contains animation

End of animation

Network Layer 4-3

Note: The original of these foils were provided by the authors. There are additions/deletions made by me, Parviz Kermani.

Chapter 4: network layerchapter goals: understand principles behind network layer

services: network layer service models forwarding versus routing how a router works routing (path selection) broadcast, multicast

instantiation, implementation in the Internet

Network Layer 4-4

Chapter 4: outline4.1 introduction4.2 virtual circuit and

datagram networks4.3 what’s inside a router4.4 IP: Internet Protocol

datagram format IPv4 addressing ICMP IPv6

4.5 routing algorithms link state distance vector hierarchical routing

4.6 routing in the Internet RIP OSPF BGP

4.7 broadcast and multicast routing

Network Layer 4-5

Network layer

transport segment from sending to receiving host

on sending side encapsulates segments into datagrams

on receiving side, delivers segments to transport layer

network layer protocols in every host, router

router examines header fields in all IP datagrams passing through it

Network Layer 4-6

applicationtransportnetworkdata linkphysical


networkdata linkphysical network

data linkphysical

networkdata linkphysical







networkdata linkphysicalnetwork

data linkphysical

Two key network-layer functions forwarding: move

packets from router’s input to appropriate router output

routing: determine route taken by packets from source to dest.

routing algorithms

analogy:

routing: process of planning trip from source to dest

forwarding: process of getting through single interchange

Network Layer 4-7

Interplay between routing and forwarding

Network Layer 4-8

1

23

0111

value in arrivingpacket’s header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

routing algorithm determinesend-end-path through network

forwarding table determineslocal forwarding at this router

Connection setup 3rd important function in some network

architectures: ATM, frame relay, X.25

before datagrams flow, two end hosts andintervening routers establish virtual connection routers get involved

network vs transport layer connection service: network: between two hosts (may also involve

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

Network Layer 4-9

Network service model

example services for individual datagrams:

guaranteed delivery guaranteed delivery with

less than 40 msec delay

example services for a flow of datagrams:

in-order datagram delivery

guaranteed minimum bandwidth to flow

restrictions on changes in inter-packet spacing

Network Layer 4-10

Q: What service model for “channel” transporting datagrams from sender to receiver?

Network layer service models:

Network Layer 4-11

NetworkArchitecture

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 ?







Network Layer 4-12

Connection, connection-less service

datagram network provides network-layer connectionless service

virtual-circuit network provides network-layer connection service

analogous to TCP/UDP connecton-oriented / connectionless transport-layer services, but: service: host-to-host no choice: network provides one or the other implementation: in network core

Network Layer 4-13

Virtual circuits

call setup, teardown for each call before data can flow each packet carries VC identifier (not destination host

address) every router on source-dest path maintains “state” for

each passing connection link, router resources (bandwidth, buffers) may be

allocated to VC (dedicated resources = predictable service)

Network Layer 4-14

“source-to-dest path behaves much like telephone circuit” performance-wise network actions along source-to-dest path

VC implementationa 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 path

packet 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

Network Layer 4-15

VC forwarding table

Network Layer 4-16

12 22 32

1 23

VC numberinterfacenumber

Incoming interface Incoming VC # Outgoing interface Outgoing VC #

1 12 3 222 63 1 18 3 7 2 171 97 3 87… … … …

forwarding table innorthwest router:

VC routers maintain connection state information!

Virtual circuits: signaling protocols

used to setup, maintain teardown VC used in ATM, frame-relay, X.25 not used in today’s Internet

Network Layer 4-17


1. initiate call 2. incoming call3. accept call4. call connected

5. data flow begins 6. receive dataapplicationtransportnetworkdata linkphysical

Datagram networks

no call setup at network layer routers: no state about end-to-end connections no network-level concept of “connection”

packets forwarded using destination host address

Network Layer 4-18

1. send datagrams



2. receive datagrams

Datagram forwarding table

Network Layer 4-19

1

23

IP destination address in arriving packet’s header

routing algorithm

local forwarding tabledest address output link

address-range 1address-range 2address-range 3address-range 4

3221

4 billion IP addresses, so rather than list individual destination addresslist range of addresses(aggregate table entries)

Datagram forwarding table

Network Layer 4-20

Destination Address Range

11001000 00010111 00010000 00000000through11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Q: but what happens if ranges don’t divide up so nicely?

Longest prefix matching

Network Layer 4-21

Destination Address Range

11001000 00010111 00010*** *********

11001000 00010111 00011000 *********

11001000 00010111 00011*** *********

otherwise

DA: 11001000 00010111 00011000 10101010

examples:DA: 11001000 00010111 00010110 10100001 which interface?

which interface?

when looking for forwarding table entry for given destination address, use longest address prefix that matches destination address.

longest prefix matching

Link interface

0

1

2

3

Datagram or VC network: why?Internet (datagram) data exchange among

computers “elastic” service, no strict

timing req.

many link types different characteristics uniform service difficult

“smart” end systems (computers) can adapt, perform control,

error recovery simple inside network,

complexity at “edge”

ATM (VC) evolved from telephony human conversation:

strict timing, reliability requirements

need for guaranteed service “dumb” end systems

telephones complexity inside

network

Network Layer 4-22







Network Layer 4-23

Router architecture overview

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

Network Layer 4-24

switchingfabric

routing processor

router input ports router output ports

Input port functions

Network Layer 4-25

decentralized switching: given datagram dest., lookup output port

using forwarding table in input port memory

goal: complete input port processing at ‘line speed’

queuing: if datagrams arrive faster than forwarding rate into switch fabric

linetermination

link layer

protocol(receive)

lookup,forwarding

queueing

physical layer:bit-level reception

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

switchfabric

Switching fabrics

transfer packet from input buffer to appropriate output buffer

switching rate: rate at which packets can be transfer from inputs to outputs often measured as multiple of input/output line rate N inputs: switching rate N times line rate desirable

three types of switching fabrics

Network Layer 4-26

memory

memory

bus crossbar

Switching via memory

first generation routers: traditional computers with switching under direct control

of CPU packet copied to system’s memory speed limited by memory bandwidth (2 bus crossings per

datagram)

Network Layer 4-27

inputport

(e.g.,Ethernet)

memoryoutputport

(e.g.,Ethernet)

system bus

Switching via a bus

datagram from input port memoryto output port memory via a shared bus

bus contention: switching speed limited by bus bandwidth

32 Gbps bus, Cisco 5600: sufficient speed for access and enterprise routers

Network Layer 4-28

bus

Switching via interconnection network

overcome bus bandwidth limitations

banyan networks, crossbar, other interconnection nets initially developed to connect processors in multiprocessor

advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric.

Cisco 12000: switches 60 Gbpsthrough the interconnection network

Network Layer 4-29

crossbar

Output ports

buffering required when datagrams arrive from fabric faster than the transmission rate

scheduling discipline chooses among queued datagrams for transmission

Network Layer 4-30

linetermination

link layer

protocol(send)

switchfabric

datagrambuffer

queueing

Output port queueing

buffering when arrival rate via switch exceeds output line speed

queueing (delay) and loss due to output port buffer overflow!

Network Layer 4-31

at t, packets morefrom input to output

one packet time later

switchfabric

switchfabric

How much buffering? RFC 3439 rule of thumb: average buffering equal

to “typical” RTT (say 250 msec) times link capacity C e.g., C = 10 Gpbs link: 2.5 Gbit buffer

recent recommendation: with N flows, buffering equal to

Network Layer 4-32

RTT C.N

Input port queuing fabric slower than input ports combined -> queueing may

occur at input queues queueing delay and loss due to input buffer overflow!

Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward

Network Layer 4-33

output port contention:only one red datagram can be

transferred.lower red packet is blocked

switchfabric

one packet time later: green packet

experiences HOL blocking

switchfabric







Network Layer 4-34

The Internet network layer

Network Layer 4-35

forwardingtable

routing protocols• path selection• RIP, OSPF, BGP

IP protocol• addressing conventions• datagram format• packet handling conventions

ICMP protocol• error reporting• router “signaling”

transport layer: TCP, UDP

link layer

physical layer

networklayer

host, router network layer functions:

IP datagram format

Network Layer 4-36

ver length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifierheader

checksumtime to

live

32 bit source IP address

head.len

type ofservice

flgsfragment

offsetupperlayer

32 bit destination IP address

options (if any)

IP protocol versionnumber

header length(bytes)

upper layer protocolto deliver payload to

total datagramlength (bytes)

“type” of data forfragmentation/reassemblymax number

remaining hops(decremented at

each router)

e.g. timestamp,record routetaken, specifylist of routers to visit.

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

layer overhead

IP Datagram Fragmentation 3 bits of flags Don’t fragment bit

• 0= may fragment• 1= don’t fragment

More bit• 1= more fragments to come• 0= last fragment

Spare bit

Network Layer 4-37

16-bit identifier flgs fragmentoffset

IP fragmentation, reassembly network links have MTU

(max.transfer size) -largest possible link-level frame different link types,

different MTUs large IP datagram divided

(“fragmented”) within net one datagram becomes

several datagrams “reassembled” only at

final destination IP header bits used to

identify, order related fragments

Network Layer 4-38

fragmentation:in: one large datagramout: 3 smaller datagrams

reassembly

…

…

IP fragmentation, reassembly

Network Layer 4-39

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 byte datagram MTU = 1500 bytes

1480 bytes in data field

offset =1480/8

IP Datagram & Reassembly

Network Layer 4-40

Example

1/8

Successive fragmentations

Network Layer 4-41

Initial datagram: Don’t fragment=0 in all datagrams

IPHDR 1024 “Data” octets

More bit (M) = 0Offset (OS) =0


More bit (M) = 1Offset (OS) =0


More bit (M) = 0Offset (OS) =64 (=512/8)


M = 1OS =0


M = 1OS =(0+256)/8=32


M = 1OS =(512+0)/8=64


M = 0OS =(512+256)/8=96

First fragmentation

Second fragmentation

Fragmentation Process

Create n fragment datagrams so that length of each will meet network limitations.

Copy IP header to each Divide data equally, along 8-octet boundaries.

The last segment can have any length. Calculate and insert fragment offsets for each fragment other than the

first. The offset value is length/8

Set a more bit for each fragment, except the last. Transport fragments independently through Internet.

Note: If the “Don’t Fragment" bit is 1 and fragmentation is needed, then the datagram is discarded

Network Layer 4-42

Datagram Reassembly

Occurs only in destination host, after fragments have transited the Internet.

Reassembly process: When first fragment arrives, create buffer, start reassembly timer. Note: The total length is not known. The length field is only for then

current datagram Insert data from each fragment in proper buffer position, based on

offset (may be received out of order). Continue until entire datagram is reassembled. Discard entire datagram if

Reassembly timer expires. Fragment error is detected.

Network Layer 4-43

Fragmentation: Last Word Fragmentation/reassembly puts additional load on routers Limit the TCP & UDP segments to a relatively small size All data link protocols supported by IP supposed to have

MTU of at least 576 bytes Fragmentation eliminated by using an MSS of 536 bytes

20 bytes of TCP segment header 20 bytes of IP datagram header

Most TCP segments for bulk data transfer (e.g. HTTP) are 512-536 bytes long

Network Layer 4-44







Network Layer 4-45

IP addressing: introduction

IP address: 32-bit identifier for host, router interface

interface: connection between host/router and physical link router’s typically have

multiple interfaces host typically has one

interface IP addresses associated with

each interface

Network Layer 4-46

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

Subnets

IP address:subnet part - high order bitshost part - low order bits

what’s a subnet ?device interfaces with same subnet part of IP addresscan physically reach each other without intervening router

Network Layer 4-47

network consisting of 3 subnets

223.1.1.1

223.1.1.3

223.1.1.4 223.1.2.9

223.1.3.2223.1.3.1

subnet

223.1.1.2

223.1.3.27223.1.2.2

223.1.2.1

Subnets

Network Layer 4-48

recipe to determine the

subnets, detach each interface from its host or router, creating islands of isolated networks

each isolated network is called a subnet

subnet mask: /24

223.1.1.0/24223.1.2.0/24

223.1.3.0/24

223.1.1.1

223.1.1.3

223.1.1.4 223.1.2.9

223.1.3.2223.1.3.1

subnet

223.1.1.2

223.1.3.27223.1.2.2

223.1.2.1

Network Layer 4-49

how many? 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

223.1.7.1223.1.8.0223.1.8.1

223.1.9.1

223.1.9.2

Subnets

(Classical) IP Address Structure

An IP address is broken in two parts Network address Host address

The division between network and host is determined by the size of network

Network Layer 4-50

Network host

IP Addressesgiven notion of “network”, let’s re-examine IP addresses:

Network Layer 4-51

0 network host

10 network host

110 network host

1110 multicast address

A

B

C

D

class1.0.0.0 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

“classful” addressing:

IP Addresses IP Classful Addresses:

Class A addresses begin with 0xxx, or 1 to 126 Class B addresses begin with 10xx, or 128 to 191 Class C addresses begin with 110x, or 192 to 223 Class D addresses begin with 1110, or 224 to 239

• Multicast Class E addresses begin with 1111, or 240 to 254

• Experimental

Network Layer 4-52

Classful Addressing Number of elements in each class

Network Layer 4-53

Class Number of classes

Number of local addresses

A 0xxx 128 16,777,216B 10xx 16,384 65,534C 110x 2,097,152 254

Classful Addressing Classful addressing: inefficient use of address space, address space

exhaustion e.g., class B net allocated enough addresses for 65K

hosts, even if only 2K hosts in that network No longer formally part of IP addressing architecture

Network Layer 4-54

IP addressing: CIDR

CIDR: Classless InterDomain Routing Adopted by IETF in 1993 Network (subnet) portion of address of arbitrary length subnet portion of address of arbitrary length address format: a.b.c.d/x, where x is # bits in network

(subnet) portion of address To support 2000 hosts, a block of 2048 addresses of the form

a.b.c.d/21 assigned• 11 bits needed to store 2048 (211=2048)

In practice the 11 bit rightmost addressing could be further divided (subnetting, more on this later)

Network Layer 4-55

11001000 00010111 00010000 00000000

networkpart

hostpart

200.23.16.0/23

Subnet Mask For routing traffic, subnet mask is used to extract the

network (subnet) portion of an IP address A string of 32 bits Bits corresponding to network (and subnet) part set to

‘1’ (one) Bits corresponding to host part set to ‘0’ Ex:

• Addr = 9 . 2 . 225 . 65/24= 00001001.00000010.11100001.01000001

• Mask = 11111111.11111111.11111111.00000000= 255 . 255 . 255 . 0

Network Layer 4-56

Subnet Mask: How To Use?

Logical AND the mask and the IP address EX:

• Addr = 9 . 2 . 225 . 65/24= 00001001.00000010.11100001.01000001

• Mask = 11111111.11111111.11111111.00000000• N/ADR= 00001001.00000010.11100001.00000000

= 9 . 2 . 225 . 0

Network Layer 4-57

Subnetting To minimize waste in classful addressing, IP

subnetting is used The host bits of a classful address is further

divided into “subnet” bits and “host” bits

Note: Only the classful Host bits should be subnetted Classful Network bits should stay intact

Network Layer 4-58

Subnetting Example Class B 162.150.0.0/16 16 subnets 162.150.xxxx0000.0/20 Mask:

• 11111111.11111111.11110000.00000000• 255 . 255 . 240 . 0

Example network 162.150.128.0/20 162.150.144.0/20

Network Layer 4-59

An IP address is “universally” unique Need for private IP address At home (NAT discussion later) Experiments Private networks

Private IP addresses Class A: 10.0.0.0 - 10.255.255.255 Class B: 172.16.0.0 - 172.31.255.255 Class C: 192.168.0.0 - 192.168.255.255

Note: Private IP addresses are not publically routable.

Network Layer 4-60

Private IP Addresses

Network Layer 4-61

IP addresses: how to get one?

Q: How does a host get IP address?

hard-coded by system admin in a file Windows: control-panel->network->configuration-

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

DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server “plug-and-play”

DHCP: Dynamic Host Configuration Protocol

goal: allow host to dynamically obtain its IP address from network server when it joins network can renew its lease on address in use allows reuse of addresses (only hold address while

connected/“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

Network Layer 4-62

DHCP client-server scenario

Network Layer 4-63

223.1.1.0/24

223.1.2.0/24

223.1.3.0/24

223.1.1.1

223.1.1.3

223.1.1.4 223.1.2.9

223.1.3.2223.1.3.1

223.1.1.2

223.1.3.27223.1.2.2

223.1.2.1

DHCPserver

arriving DHCPclient needs address in thisnetwork

DHCP client-server scenario

Network Layer 4-64

DHCP server: 223.1.2.5 arrivingclient

DHCP discover

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

DHCP offersrc: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 654lifetime: 3600 secs

DHCP requestsrc: 0.0.0.0, 68 dest:: 255.255.255.255, 67yiaddrr: 223.1.2.4transaction ID: 655lifetime: 3600 secs

DHCP ACKsrc: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 655lifetime: 3600 secs

DHCP: more than IP addressesDHCP can return more than just allocated IP

address on subnet: address of first-hop router for client name and IP address of DNS sever network mask (indicating network versus host portion

of address)

Network Layer 4-65

DHCP: Broadcast or Unicast” DHCPDISCOVER is broadcast over the subnet

attached to the DHCP client (UDP broadcast to 255.255.255.255)

If DHCP server does not reside on that subnet, the gateway need to direct the discover to the remote server (UDP unicast) The gateway router should be configured to do so. Otherwise DHCPDISCOVER fails

Network Layer 4-66

DHCP: Broadcast or Unicast” DHCPDISCOVER is broadcast over the subnet

attached to the DHCP client (UDP broadcast to 255.255.255.255)

If DHCP server does not reside on that subnet, The gateway needs to direct the discover to the

remote server (UDP unicast) The gateway router should be configured to do so. Otherwise DHCPDISCOVER fails

Network Layer 4-67

DHCP: example

Network Layer 4-68

Connecting laptop needs its IP address, addr of first-hop router, addr of DNS server: use DHCP

router with DHCP server built into router

DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in 802.1 Ethernet

Ethernet frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running DHCP server

Ethernet demuxed to IP demuxed, UDP demuxed to DHCP

168.1.1.1

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCPDHCP

DHCP: example

Network Layer 4-69

DHCP server formulates DHCP ACK containing client’s IP address, IP address of first-hop router for client, name & IP address of DNS server

encapsulation of DHCP server, frame forwarded to client, demuxing up to DHCP at client

router with DHCP server built into router

DHCP

DHCP

DHCP

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCPUDP

IPEthPhy

DHCP

DHCP

DHCP

DHCP

client now knows its IP address, name and IP address of DSN server, IP address of its first-hop router

DHCP: Wireshark output (home LAN)

Network Layer 4-70

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 ACKOption: (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

IP addresses: how to get one?Q: how does network get subnet part of IP address?A: gets allocated portion of its provider ISP’s

address space

Network Layer 4-71

ISP's block 10101000 00010111 00010000 00000000 168.23.16.0/20

Organization 0 10101000 00010111 00010000 00000000 168.23.16.0/23 Organization 1 10101000 00010111 00010010 00000000 168.23.18.0/23 Organization 2 10101000 00010111 00010100 00000000 168.23.20.0/23

... ….. …. ….Organization 7 10101000 00010111 00011110 00000000 168.23.30.0/23

Hierarchical addressing: route aggregation

Network Layer 4-72

“Send me anythingwith addresses beginning 168.23.16.0/20”

168.23.16.0/23

168.23.18.0/23

168.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us “Send me anythingwith addresses beginning 128.31.0.0/16”

168.23.20.0/23Organization 2

...

...

hierarchical addressing allows efficient advertisement of routing information:

Hierarchical addressing: more specific routes

Network Layer 4-73

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

“Send me anythingwith addresses beginning 168.23.16.0/20”

168.23.16.0/23

168.23.18.0/23

168.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us “Send me anythingwith addresses beginning 128.31.0.0/16(255.255.0.0)or 168.23.18.0/23”(255.255.252.0)

168.23.20.0/23Organization 2

...

...

Longest prefix matchingdetermines the route

Hierarchical addressing: Network Addresses and Masks

Network Layer 4-74

168 . 23 . 16 . 0/2311111111.11111111.11111110.00000000

255 . 255 . 254 . 0

Net addressMask (Binary)Mask (Decimal)

168 . 23 . 16 . 0/2011111111.11111111.11110000.00000000

255 . 255 . 240 . 0


128 . 31 . 0 . 0/1611111111.11111111.00000000.00000000

255 . 255 . 0 . 0


168 . 23 . 18 . 0/2311111111.11111111.11111110.00000000

255 . 255 . 254 . 0


IP addressing: the last word...

Q: how does an ISP get block of addresses?A: ICANN: Internet Corporation for Assigned

Names and Numbers http://www.icann.org/ allocates addresses manages DNS assigns domain names, resolves disputes

Network Layer 4-75

NAT: network address translation

Network Layer 4-76

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 for source, destination (as usual)

all datagrams leaving localnetwork have same single

source NAT IP address: 138.76.29.7,different source

port numbers

NAT: network address translationmotivation: 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)

Network Layer 4-77

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 (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 dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table

Network Layer 4-78

NAT: network address translation

Network Layer 4-79

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 10.0.0.1 sends datagram to 128.119.40.186, 80

NAT translation tableWAN side addr LAN side addr138.76.29.7, 5001 10.0.0.1, 3345…… ……

S: 128.119.40.186, 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, 80 D: 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

NAT: network address translation 16-bit port-number field: 60,000 simultaneous connections with a single

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

• NAT possibility must be taken into account by app designers, e.g., P2P applications

address shortage should instead be solved by IPv6

Network Layer 4-80

NAT traversal problem

client wants to connect to server with address 10.0.0.1 server address 10.0.0.1 local to

LAN (client can’t use it as destination addr)

only one externally visible NATedaddress: 138.76.29.7

solution1: statically configure NAT to forward incoming connection requests at given port to server e.g., (123.76.29.7, port 2500)

always forwarded to 10.0.0.1 port 25000

Network Layer 4-81

10.0.0.1

10.0.0.4

NAT router

138.76.29.7

client

?

NAT traversal problem

solution 2: Universal Plug and Play (UPnP) Internet Gateway Device (IGD) Protocol. Allows NATed host to: learn public IP address (138.76.29.7) add/remove port mappings (with lease

times)

i.e., automate static NAT port map configuration

Network Layer 4-82

10.0.0.1

NAT router

IGD

NAT traversal problem solution 3: relaying (used in Skype) NATed client establishes connection to relay external client connects to relay relay bridges packets between to connections

Network Layer 4-83

138.76.29.7

client

1. connection torelay initiatedby NATed host

2. connection torelay initiatedby client

3. relaying established

NAT router

10.0.0.1







Network Layer 4-84

ICMP: Internet Control Message Protocol

In contrast to a single network, in an internet, no special hardware can assist in reporting and resolving problems. In an internet, software assists in reporting problems

When things go wrong, ICMP comes to help ICMP Allows routers to send error messages or control messages to

other routers/hosts ICMP provides communication between the IP software not

application software All hosts and routers must be able to generate ICMP messages and

process the ICMP messages they receive. An arbitrary machine can send an ICMP message to any other machine.

ICMP is an error reporting mechanism. It does not fully specify the action to be taken for each possible error. ICMP is also used to help identify network problems (e.g. ping)

Network Layer 4-85

ICMP (Cont.)

ICMP reports problems to the original source. It cannot be used to inform intermediate routers about problems.

Why?

Network Layer 4-86

A datagram only contains source and destination address, not the intermediate nodes on a path

ICMP Message Delivery

Network Layer 4-87

ICMP message requires two levels of encapsulation

ICMP utilizes IP, but is considered to be at same level in protocol stack

ICMP messages are carried in IP datagram with ordinary IP headers with the Protocol field set to 1

IP Header

Protocol=1

ICMP Message

Types and Format of Error Messages

ICMP messages originate from a router or a host depending on type of error condition

Network Layer 4-88

ICMP Error MessagesMessage Description

Destination Unreachable

A datagram cannot reach its destination host, utility, or application

Time Exceeded The time-to-Live has expired at a router, or the Fragment Reassembly Time has expired at a destination host.

Parameter Problem There is a bad parameter in the IP header

Source Quench A router or destination is congested. (It is recommended that systems should not send Quench messages)

Redirect A host has routed a datagram to the wrong local router

When Not To Send ICMP

ICMP is used to send error messages when a network is under stress

Care should be taken that the ICMP traffic does not flood the network (making the situation worse!)

ICMP must not report problems caused by Routing or delivering ICMP messages Broadcast or multicast datagrams Datagram fragments other than the first Messages whose source address do not identify a unique host

• E.g., source IP addresses such as 127.0.0.1 or 0.0.0.0

Network Layer 4-89

ICMP Message Format

TYPE: identifies the messageCODE: further information about the

message typeCHECKSUM: only covers the ICMP message

In addition, ICMP messages that report errors always include the header and first 64 data bit of the datagram causing the problem.

Network Layer 4-90

ICMP: internet control message protocol

used by hosts & routers to communicate network-level information error reporting:

unreachable host, network, port, protocol

echo request/reply (used by ping)

network-layer “above” IP: ICMP msgs carried in IP

datagrams ICMP message: type, code

plus first 8 bytes of IP datagram causing error

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

Network Layer 4-91

traceroute and ICMP source sends series of

UDP segments to dest first set has TTL =1 second set has TTL=2, etc. unlikely port number

when nth set of datagrams arrives to nth router: router discards datagrams and sends source ICMP

messages (type 11, code 0) ICMP messages includes

name of router & IP address

when ICMP messages arrives, source records RTTs

stopping criteria: UDP segment eventually

arrives at destination host destination returns ICMP

“port unreachable” message (type 3, code 3)

source stops

Network Layer 4-92

3 probes

3 probes

3 probes

ICMP Example: Echo Request/Reply

Used by processors to test whether a destination is alive and reachable Example: ping

TYPE=8 (Request) or 0 (Reply) IDENTIFER/SEQUENCE

NUMBER: used by sender to match replies to request

DATA: Optional further matching information (returned by the sender)

Network Layer 4-93

ICMP Example: Destination Unreachable

Network Layer 4-94

Sent by a router to the source when it cannot forward or deliver an IP datagram

Code

ICMP Example: Redirect (Change Route)

Used by router to notify host to change its routing table There are more than one

routers on the network The host sends a datagram to

a wrong router (resulting in a longer route)

The router however forwards the datagram to the correct router and notifies the host

The host should send the subsequent traffic to the shorter route.

Network Layer 4-95

ICMP Redirect can be used to reduce manual network administration Hosts always use a default

router The default router redirects

the requests to optimal routers

Hosts update their tables dynamically

ICMP Example: Redirect (Cont.)

Network Layer 4-96

Format

Code

ICMP Example: Destination Unreachable & PathMTU Discovery

To transfer bulk data (e.g. file transfer), it is always better to send large datagrams IP and TCP headers are at least 40 bytes

The MTU for each medium is different Use of a small, conservative datagram size wasteful (e.g.

576 bytes) A simple Path Discovery procedure to determine the

biggest datagram size, the Path MTU Size The Don’t Fragment flag in IP header is set to 1 The Path MTU size is set for the local interface If the datagram is too large for some routers, the router sends

back an ICMP Destination Unreachable with code=4 Sending host reduces the datagram size and tries again

Network Layer 4-97

Viewing ICMP Activities

Use command “netstat -s” to view network activities Example: on Windows NT

C>netstat -s

IP Statistics

Packets Received = 21111

Received Header Errors = 194

Received Address Errors = 1063

Datagrams Forwarded = 0

Unknown Protocols Received = 0

Received Packets Discarded = 0

Received Packets Delivered = 20208

Output Requests = 11559

Routing Discards = 0

Discarded Output Packets = 0

Output Packet No Route = 0

Reassembly Required = 0

Reassembly Successful = 0

Reassembly Failures = 0

Datagrams Successfully Fragmented = 0

Datagrams Failing Fragmentation = 0

Fragments Created = 0

Network Layer 4-98

Viewing ICMP Activities (Cont.)

Example: Cont.ICMP Statistics

Received Sent

Messages 23 23

Errors 0 0

Destination Unreachable 0 0

Time Exceeded 0 0

Parameter Problems 0 0

Source Quenchs 0 0

Redirects 0 0

Echos 0 23

Echo Replies 23 0

Timestamps 0 0

Timestamp Replies 0 0

Address Masks 0 0

Address Mask Replies 0 0

Network Layer 4-99







Network Layer 4-100

IPv6: motivation initial motivation: 32-bit address space soon to be

completely allocated. additional motivation: header format helps speed processing/forwarding header changes to facilitate QoS

IPv6 datagram format: fixed-length 40 byte header no fragmentation allowed

Network Layer 4-101

IPv6 datagram format

Network Layer 4-102

priority: identify priority among datagrams in flowflow Label: identify datagrams in same “flow.”

(concept of “flow” not well defined).next header: identify upper layer protocol for data

data

destination address(128 bits)

source address(128 bits)

payload len next hdr hop limitflow labelpriver

32 bits

Other changes from IPv4 checksum: removed entirely to reduce processing

time at each hop options: allowed, but outside of header, indicated

by “Next Header” field ICMPv6: new version of ICMP additional message types, e.g. “Packet Too Big” multicast group management functions

Network Layer 4-103

Transition from IPv4 to IPv6 not all routers can be upgraded simultaneously no “flag days” how will network operate with mixed IPv4 and

IPv6 routers? tunneling: IPv6 datagram carried as payload in IPv4

datagram among IPv4 routers

Network Layer 4-104

IPv4 source, dest addr IPv4 header fields

IPv4 datagramIPv6 datagram

IPv4 payload

UDP/TCP payloadIPv6 source dest addr

IPv6 header fields

Tunneling

Network Layer 4-105

physical view:IPv4 IPv4

A B

IPv6 IPv6

E

IPv6 IPv6

FC D

logical view:

IPv4 tunnel connecting IPv6 routers E

IPv6 IPv6

FA B

IPv6 IPv6

Tunneling

Network Layer 4-106

flow: Xsrc: Adest: F

data

A-to-B:IPv6

Flow: XSrc: ADest: F

data

src:Bdest: E

B-to-C:IPv6 inside

IPv4

E-to-F:IPv6

flow: Xsrc: Adest: F

data

B-to-C:IPv6 inside

IPv4

Flow: XSrc: ADest: F

data

src:Bdest: E

physical view:A B

IPv6 IPv6

E

IPv6 IPv6

FC D

logical view:

IPv4 tunnel connecting IPv6 routers E

IPv6 IPv6

FA B

IPv6 IPv6

IPv4 IPv4







Network Layer 4-107

Interplay between routing, forwarding

Network Layer 4-108

1

23

IP destination address in arriving packet’s header

routing algorithm

local forwarding tabledest address output link

address-range 1address-range 2address-range 3address-range 4

3221

routing algorithm determinesend-end-path through network

forwarding table determineslocal forwarding at this router

Host Routing TableIntranet (Direct) Routing: Source and destination on the

same (sub)network (LAN) From: 130.15.12.131/24 To: 130.15.12.22/24 Subnet mask: 255.255.255.0

The source and destination addresses are ANDed with the Mask to extract the networkand subnet portion:

130.15.12.0 Both on the same subnet

Network Layer 4-109

The datagram must be wrappedin a frame and transmitted directly to its destination on the same LAN

The ARP table is checked to provide the physical address for the destination IP address If not there, ARP protocol is

used to create one

Datagram Encapsulation: Ethernet

Ethernet frame for the previous example

Network Layer 4-110

LinkHdr

IPHdr

Dest IP=130.15.12.22

Dest Enet= Enet address of 130.15.12.22

Datagram Encapsulation: Ethernet

Network Layer 4-111

IEEE 802.2/802.3 (RFC 1042) and Ethernet (RFC 894) Encapsulations

Getting a datagram from source to destination

Network Layer 4-112

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

BE

miscfields

sourceIP addr

destIP addr data

datagram remains unchanged, as it travels source to destination

addr fields of interest here

Dest. Net. next router Nhops

223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2

forwarding table in AMask=255.255.255.0

Getting a datagram from source to dest.

Network Layer 4-113

Starting at A, send IP datagram addressed to B:look up net. address of B in forwarding tablefind B is on same net. as Alink layer will send datagram directly to B inside link-layer frame

B and A are directly connected


223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 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

BE



Network Layer 4-114

Starting at A, send IP datagram addressed to E:look up net. address of E in forwarding tablefind E is not on same net. as AThe datagram is forwarded to the next router 223.1.1.4


223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2


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

BE


Host Routing Table

Internet (non-direct) routing From: 130.15.12.131 To: 192.45.89.5 Subnet mask: 255.255.255.0

Destination is not on the same LAN (network) Consult the routing table

If a destination is not on the local network, the only way to leave the local net is via a (the) router

Each host contains a routing table to route datagrams to “foreign” hosts

Network Layer 4-115

The default entry always points to a router: Forward any non-local

datagrams to the default router Destination address 0.0.0.0 is

used to mean default in routing tables

Destination is not on the same LAN (network)

Host Routing Table

Network Layer 4-116

Example of two routers on a LAN Interface 128.121.54.2 leads

to a small LAN A look at the routing

table at host “tigger”

Host Routing Table

First destination 127.0.01 is the loopback address For clients/servers within the same node

The default used for any destination not explicitly listed Datagrams to any systems on subnet 128.121.54.0 should be

forwarded to router 128.121.50.2 The last entry declares any destination on subnet 128.121.50.0 is

routed via 128.121.50.145, the node itself Flags indicate whether the route is up (U) and whether the next hop

is a host (H) or a gateway (G)

Network Layer 4-117

Rules for Routing Table Lookups

The routing table entry can be An individual host A subnet A network A supernet Default

General rule: The entry chosen should be based on the most precisematch to the destination IP address First search for a complete IP address match If not, search for destination subnet match If not, search for destination network match If not, search for a routing prefix entry match If not, the default route is used

Network Layer 4-118

Routing Example: Non-direct

Network Layer 4-119

From: bsdi (140.252.13.35) To: ftp.uu.net (192.48.96.9) Subnet mask: 255.255.255.0


Network Layer 4-120


223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2Starting at A, dest. E:

look up network address of E in forwarding tableE on different network

A, E not directly attachedrouting table: next hop router to E is 223.1.1.4 link layer sends datagram to router 223.1.1.4 inside link-layer framedatagram 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

BE

forwarding table in A


Network Layer 4-121

Arriving at 223.1.4, destined for 223.1.2.2look up network address of E in router’s forwarding tableE on same network as router’s interface 223.1.2.9

router, E directly attachedlink layer sends datagram to 223.1.2.2 inside link-layer frame via interface 223.1.2.9datagram arrives at 223.1.2.2!!!(hooray!)

miscfields 223.1.1.1 223.1.2.2 data Dest. Net router Nhops interface

223.1.1 - 1 223.1.1.4223.1.2 - 1 223.1.2.9223.1.3 - 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

BE

forwarding table in router

Routing Algorithms

Network Layer 4-122

Graph abstraction for routing algorithms:

graph nodes are routers graph edges are physical

links link cost: delay, $ cost, or

congestion level

Goal: determine “good” path(sequence of routers) through network from source to dest.

Routing Algorithm

A

ED

CB

F2

21

3

1

1

2

53

5

“good” path:typically means minimum cost pathother def’s possible

Graph abstraction

Network Layer 4-123

u

yx

wv

z2

21

3

1

1

2

53

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) }

aside: graph abstraction is useful in other network contexts, e.g., P2P, where N is set of peers and E is set of TCP connections

Graph abstraction: costs

Network Layer 4-124

u

yx

wv

z2

21

3

1

1

2

53

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 to congestion

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

key question: what is the least-cost path between u and z ?routing algorithm: algorithm that finds that least cost path

Routing Algorithm and internet: Graph Construction

Need to have graph abstraction of an internet General rule: All nodes in a subnet are fully connected Only routers are important Internet only worries about routing between routers

(and not within subnets) Procedure: Ignore all non-router hosts Remove all connections (links) Fully connect all routers on the same subnet

Network Layer 4-125

Graph abstraction of an internet

Network Layer 4-126

Non-router host

Router


Network Layer 4-127

Non-router host

Router

Step 1: ignore non-router hosts


Network Layer 4-128

Non-router host

Router

Step 2: remove all connections


Network Layer 4-129

Non-router host

Router

Step 3: fully connect all routers within the same subnet


Network Layer 4-130

Non-router host

Router

Step 4: Assign “weight” to links of the graph

Note: All links in a (sub)net have the same weight

Routing algorithm classificationQ: global or decentralized

information?global: all routers have complete

topology, link cost info “link state” algorithmsdecentralized: router knows physically-

connected neighbors, link costs to neighbors

iterative process of computation, exchange of info with neighbors

“distance vector” algorithms

Q: static or dynamic?static: routes change slowly over

timedynamic: routes change more

quickly periodic update in response to link

cost changes

Network Layer 4-131

Chapter 4: outline

Link State & Distance Vector Algorithms

Link State (LS) Each node reports the state/cost of its neighboring

links to all nodes in the network

Distance Vector (DV) Each node reports its distance to all nodes in the

network to its neighboring nodes

Network Layer 4-132







Network Layer 4-133

A Link-State Routing Algorithm

Dijkstra’s algorithm net topology, link costs

known to all nodes accomplished via “link state

broadcast” all nodes have same info

computes least cost paths from one node (‘source”) to all other nodes gives forwarding table for

that node iterative: after k

iterations, know least cost path to k dest.’s

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

p(v): predecessor node along path from source to v

N': set of nodes whose least cost path definitively known

Network Layer 4-134

Dijsktra’s Algorithm

Network Layer 4-135

1 Initialization: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 Loop9 find w not in N' such that 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 until all nodes in N'

Dijkstra's Algorithm: Alternate Method

Label a node v by (D(v),p(v)) Initially, label of the source, A, is (0,-)

Network Layer 4-136

(0,-)

(2,A)(5,A)

(1,A)

( ,-)∞

( ,-)∞


Network Layer 4-137

(0,-)

(2,A) (5,A)

(1,A) ( ,-)∞

( ,-)∞

(2,D)

(4,D)

(0,-)

(2,A) (5,A)

(1,A) ( ,-)∞

( ,-)∞

(2,D)

(4,D)

(0,-)

(2,A) (5,A)

(1,A) ( ,-)∞

( ,-)∞

(2,D)

(4,D) (3,E)

(4,E)(0,-)

(2,A) (5,A)

(1,A) ( ,-)∞

( ,-)∞

(2,D)

(4,D) (3,E)

(4,E)


Network Layer 4-138

(0,-)

(2,A)

(1,A) (2,D)

(3,E)

(4,E)(0,-)

(2,A) (5,A)

(1,A) ( ,-)∞

( ,-)∞

(2,D)

(4,D) (3,E)

(4,E)

Dijsktra’s Algorithm

Network Layer 4-139

1 Initialization: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 Loop9 find w not in N' such that 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 until all nodes in N'

Network Layer 4-140

w3

4

v

x

u

5

37 4

y8

z2

7

9

Dijkstra’s algorithm: example

Step N'D(v)

p(v)012345

D(w)p(w)

D(x)p(x)

D(y)p(y)

D(z)p(z)

u ∞ ∞ 7,u 3,u 5,uuw ∞ 11,w6,w 5,u

14,x 11,w 6,wuwxuwxv 14,x 10,v

uwxvy 12,y

notes: construct shortest path tree by

tracing predecessor nodes ties can exist (can be broken

arbitrarily)

uwxvyz

Network Layer 4-141

Dijkstra’s algorithm: another example

Step012345

N'u

uxuxy

uxyvuxyvw

uxyvwz

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,y4,y

u

yx

wv

z2

21

3

1

1

2

53

5

Shortest Path Tree

Network Layer 4-142

Shortest path tree rooted at node A

Note: Shortest Path Tree is Dependent on the “root”

Shortest path tree rooted at node B

Shortest Path Tree & Routing Table

Network Layer 4-143

Shortest path tree rooted at node a F

ED

CB

A

Dest. Next NodeA -B BC DD DE DF D

Routing table at node A

Dijkstra’s algorithm: example (2)

Network Layer 4-144

u

yx

wv

z

resulting shortest-path tree from u:

vxywz

(u,v)(u,x)

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

destination link

resulting forwarding table in u:

Dijkstra’s algorithm, discussion

algorithm complexity: n nodes each iteration: need to check all nodes, w, not in N n(n+1)/2 comparisons: O(n2) more efficient implementations possible: O(nlogn)

oscillations possible: e.g., support link cost equals amount of carried traffic:

Network Layer 4-145

AD

C

B1 1+e

e0

e

1 1

0 0

initially

AD

C

B

given these costs,find new routing….

resulting in new costs

2+e 0

001+e 1

AD

C

B



0 2+e

1+e10 0

AD

C

B



2+e 0

001+e 1

Some cures to Dijkstra’s Algorithm

Mandate link cost not depend on traffic Not acceptable

Ensure all routers do not run the algorithm at the same time

Observation: Routers in network self synchronize their operation Enforce a randomization

Network Layer 4-146







Network Layer 4-147

Distance Vector Routing Principle

Network Layer 4-148

The shortest distance from a node to a destination via a given neighbor is the shortest distance from the neighbor to the destination plus the distance from the node to that neighbor

Distance vector algorithm

Network Layer 4-149

Bellman-Ford equation (dynamic programming)

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

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

v

cost to neighbor v

min taken over all neighbors v of x

cost from neighbor v to destination y

Bellman-Ford example

Network Layer 4-150

u

yx

wv

z2

21

3

1

1

2

53

5clearly, 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 achieving minimum is nexthop in shortest path, used in forwarding table

B-F equation says:

Distance vector algorithm Dx(y) = estimate of least cost from x to y x maintains distance vector Dx = [Dx(y): y є N ]

node x: knows cost to each neighbor v: c(x,v) maintains its neighbors’ distance vectors. For

each neighbor v, x maintains Dv = [Dv(y): y є N ]

Network Layer 4-151

Distance vector algorithm key idea: from time-to-time, each node sends its own

distance vector estimate to neighbors when x receives new DV estimate from neighbor,

it updates its own DV using B-F equation:

Dx(y) ← minv{c(x,v) + Dv(y)} for each node y ∊ N

under minor, natural conditions, the estimate Dx(y) converge to the actual least cost dx(y)

Network Layer 4-152

Distance vector algorithm iterative, asynchronous:

each local iteration caused by:

local link cost change DV update message from

neighbordistributed: each node notifies

neighbors only when its DV changes neighbors then notify their

neighbors if necessary

each node:

Network Layer 4-153

wait for (change in local link cost or msg from neighbor)

recompute estimates

if DV to any dest has changed, notify neighbors

Distance Table: example

Network Layer 4-154

A

E D

CB7

81

2

1

2 D ()

A

B

C

D

A

1

7

6

4

B

14

8

9

11

D

5

5

4

2

Ecost to destination via

D (C,D)Ec(E,D) + min {D (C,w)}D

w== 2+2 = 4

D (A,D)Ec(E,D) + min {D (A,w)}D

w== 2+3 = 5 loop!

D (A,B)Ec(E,B) + min {D (A,w)}B

w== 8+6 = 14

loop!

D (Y,Z)Xdistance from X toY, via Z as next hop

c(X,Z) + min {D (Y,w)}Zw

=

=

Network Layer 4-155

x y z

xyz

0 2 7

∞ ∞ ∞∞ ∞ ∞

from

cost to

from

from

x y z

xyz

0

x y z

xyz

∞ ∞

∞ ∞ ∞

cost to

x y z

xyz

∞ ∞ ∞7 1 0

cost to

∞2 0 1

∞ ∞ ∞

2 0 17 1 0

time

x z12

7

y

node xtable

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

Dx(z) = min{c(x,y) +Dy(z), c(x,z) + Dz(z)}

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

32

node ytable

node ztable

cost to

from

Network Layer 4-156

x y z

xyz

0 2 3

from

cost to

x y z

xyz

0 2 7

from

cost tox y z

xyz

0 2 3

from

cost to

x y z

xyz

0 2 3fro

mcost to

x y z

xyz

0 2 7

from

cost to

2 0 17 1 0

2 0 13 1 0

2 0 13 1 0

2 0 1

3 1 0

2 0 1

3 1 0

time

x y z

xyz

0 2 7

∞ ∞ ∞∞ ∞ ∞

from

cost to

from

from

x y z

xyz

0

x y z

xyz

∞ ∞

∞ ∞ ∞

cost to

x y z

xyz

∞ ∞ ∞7 1 0

cost to

∞2 0 1

∞ ∞ ∞

2 0 17 1 0

time

x z12

7

y

node xtable

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

Dx(z) = min{c(x,y) +Dy(z), c(x,z) + Dz(z)}

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

32

node ytable

node ztable

cost to

from

Distance vector: link cost changes

Network Layer 4-157

link cost changes: node detects local link cost change updates routing info, recalculates

distance vector if DV changes, notify neighbors

“goodnews travelsfast”

x z14

50

y1

t0 : y detects link-cost change, updates its DV, informs its neighbors.

t1 : z receives update from y, updates its table, computes new least cost to x , sends its neighbors its DV.

t2 : y receives z’s update, updates its distance table. y’s least costs do not change, so y does not send a message to z.


Network Layer 4-158

link cost changes: node detects local link cost change updates routing info, recalculates

distance vector if DV changes, notify neighbors

“goodnews travelsfast”

x z14

50

y1

algorithmterminates

Distance Vector: link cost changes

Network Layer 4-159

Link cost changes:good news travels fast bad news travels slow - “count to infinity” problem! X Z

14

50

Y60

algorithmcontinues

on!

Distance Vector: poisoned reverse

Network Layer 4-160

If Z routes through Y to get to X :Z tells Y its (Z’s) distance to X is infinite (so Y won’t route to X via Z)will this completely solve count to infinity problem?

X Z14

50

Y60

algorithmterminates


Network Layer 4-161

link cost changes: node detects local link cost change bad news travels slow - “count to

infinity” problem! 44 iterations before algorithm

stabilizes: see text

x z14

50

y60

poisoned reverse: If Z routes through Y to get to X :

Z tells Y its (Z’s) distance to X is infinite (so Y won’t route to X via Z)

will this completely solve count to infinity problem?

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

incorrect link cost each node computes only

its own tableDV:

DV node can advertise incorrect path cost

each node’s table used by others

• error propagate thru network

Network Layer 4-162







Network Layer 4-163

Hierarchical routing

scale: with 600 million destinations:

can’t store all dest’s in routing tables!

routing table exchange would swamp links!

administrative autonomy internet = network of

networks each network admin may

want to control routing in its own network

Network Layer 4-164

our routing study thus far - idealization all routers identical network “flat”… not true in practice

Hierarchical routing aggregate routers into

regions, “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: at “edge” of its own AS has link to router in

another AS

Network Layer 4-165

Interconnected ASes

Network Layer 4-166

forwarding table configured by both intra-and inter-AS routing algorithm intra-AS sets entries

for internal dests inter-AS & intra-AS

sets entries for external dests

3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Inter-AS tasks suppose router in AS1

receives datagram destined outside of AS1: router should forward

packet to gateway router, but which one?

AS1 must:1. learn which dests are

reachable through AS2, which through AS3

2. propagate this reachability info to all routers in AS1

job of inter-AS routing!

Network Layer 4-167

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2bothernetworks

othernetworks

Example: setting forwarding table in router 1d

suppose AS1 learns (via inter-AS protocol) that subnet xreachable via AS3 (gateway 1c), but not via 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 installs forwarding table entry (x,I)

Network Layer 4-168

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2bothernetworks

othernetworks

x

Example: choosing among multiple ASes

now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2.

to configure forwarding table, router 1d must determine which gateway it should forward packets towards for destx this is also job of inter-AS routing protocol!

Network Layer 4-169

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2bothernetworks

othernetworks

x

?

Example: choosing among multiple ASes

now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2.

to configure forwarding table, router 1d must determine towards which gateway it should forward packets for dest x this is also job of inter-AS routing protocol!

hot potato routing: send packet towards closest of two routers.

Network Layer 4-170

learn from inter-AS protocol that subnet x is reachable via multiple gateways

use routing infofrom intra-AS

protocol to determinecosts of least-cost

paths to eachof the gateways

hot potato routing:choose the gateway

that has the smallest least cost

determine fromforwarding table the interface I that leads

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

forwarding table







Network Layer 4-171

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)

Network Layer 4-172

RIP ( Routing Information Protocol) included in BSD-UNIX distribution in 1982 distance vector algorithm

distance metric: # hops (max = 15 hops), each link has cost 1 DVs exchanged with neighbors every 30 sec in response message

(aka advertisement) each advertisement: list of up to 25 destination subnets (in IP

addressing sense)

Network Layer 4-173

DC

BAu v

w

x

yz

subnet hopsu 1v 2w 2x 3y 3z 2

from router A to destination subnets:

RIP: example

Network Layer 4-174

destination subnet next router # hops to destw A 2y B 2z B 7x -- 1…. …. ....

routing table in router D

w x yz

A

C

D B

RIP: example

Network Layer 4-175

w x yz

A

C

D B

destination subnet next router # hops to destw A 2y B 2z B 7x -- 1…. …. ....

routing table in router D

A 5

dest next hopsw - 1x - 1z C 4…. … ...

A-to-D advertisement

RIP: link failure, recovery

if no advertisement heard after 180 sec --> neighbor/link declared dead routes via neighbor invalidated new advertisements sent to neighbors neighbors in turn send out new advertisements (if

tables changed) link failure info quickly (?) propagates to entire net poison reverse used to prevent ping-pong loops (infinite

distance = 16 hops)

Network Layer 4-176

RIP table processing

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

advertisements sent in UDP packets, periodically repeated

Network Layer 4-177

physicallink

network forwarding(IP) table

transport(UDP)

routed

physicallink

network(IP)

transprt(UDP)

routed

forwardingtable

OSPF (Open Shortest Path First)

“open”: publicly available uses link state algorithm LS packet dissemination topology map at each node route computation using Dijkstra’s algorithm

OSPF advertisement carries one entry per neighbor

advertisements flooded to entire AS carried in OSPF messages directly over IP (rather than

TCP or UDP IS-IS routing protocol: nearly identical to OSPF

Network Layer 4-178

OSPF “advanced” features (not in RIP)

security: all OSPF messages authenticated (to prevent malicious intrusion)

multiple same-cost paths allowed (only one path in RIP)

for each link, multiple cost metrics for different TOS (e.g., satellite link cost set “low” for best effort ToS; high for real time ToS)

integrated uni- and multicast support: Multicast OSPF (MOSPF) uses same topology

data base as OSPF hierarchical OSPF in large domains.

Network Layer 4-179

Hierarchical OSPF

Network Layer 4-180

boundary router

backbone router

area 1area 2

area 3

backboneareaborderrouters

internalrouters

Hierarchical OSPF two-level hierarchy: local area, backbone. link-state advertisements only in area each nodes has detailed area topology; only

know direction (shortest path) to nets in other areas.

area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers.

backbone routers: run OSPF routing limited to backbone.

boundary routers: connect to other AS’s.

Network Layer 4-181

Internet inter-AS routing: BGP

BGP (Border Gateway Protocol): the de facto inter-domain routing protocol “glue that holds the Internet together”

BGP provides each AS a means to: eBGP: obtain subnet reachability information from

neighboring ASs. iBGP: propagate reachability information to all AS-

internal routers. determine “good” routes to other networks based on

reachability information and policy. allows subnet to advertise its existence to rest of

Internet: “I am here”

Network Layer 4-182

BGP basics BGP session: two BGP routers (“peers”) exchange BGP

messages: advertising paths to different destination network prefixes (“path

vector” protocol) exchanged over semi-permanent TCP connections

when AS3 advertises a prefix to AS1: AS3 promises it will forward datagrams towards that prefix AS3 can aggregate prefixes in its advertisement

Network Layer 4-183

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2bothernetworks

othernetworks

BGP message

BGP basics: distributing path information

using eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1. 1c can then use iBGP do distribute new prefix info to all routers

in AS1 1b can then re-advertise new reachability info to AS2 over 1b-to-

2a eBGP session

when router learns of new prefix, it creates entry for prefix in its forwarding table.

Network Layer 4-184

AS3

AS2

3b3a

AS1

1c1a

1d1b

2a2c

2bothernetworks

othernetworks

eBGP session

iBGP session

Path attributes and BGP routes advertised prefix includes BGP attributes prefix + attributes = “route”

two important attributes: AS-PATH: contains ASs through which prefix

advertisement has passed: e.g., AS 67, AS 17 NEXT-HOP: indicates specific internal-AS router to

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

gateway router receiving route advertisement uses import policy to accept/decline e.g., never route through AS x policy-based routing

Network Layer 4-185

BGP route selection router may learn about more than 1 route to

destination AS, selects route based on:1. local preference value attribute: policy decision2. shortest AS-PATH 3. closest NEXT-HOP router: hot potato routing4. additional criteria

Network Layer 4-186

BGP messages BGP messages exchanged between peers over TCP

connection BGP messages: OPEN: opens TCP connection to peer and

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

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

used to close connection

Network Layer 4-187

BGP routing policy

A,B,C are provider networks X,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

Network Layer 4-188

A

B

C

WX

Y

legend:

customer network:

providernetwork

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C?

No way! B gets no “revenue” for routing CBAW since neither W nor C are B’s customers

B wants to force C to route to w via A B wants to route only to/from its customers!

Network Layer 4-189

A

B

C

WX

Y

legend:

customer network:

providernetwork

Why different Intra-, 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

neededscale: hierarchical routing saves table size, reduced

update trafficperformance: intra-AS: can focus on performance inter-AS: policy may dominate over performance

Network Layer 4-190







Network Layer 4-191

Broadcast routing deliver packets from source to all other nodes source duplication is inefficient:

Network Layer 4-192

R1

R2

R3 R4

sourceduplication

R1

R2

R3 R4

in-networkduplication

duplicatecreation/transmissionduplicate

duplicate

source duplication: how does source determine recipient addresses?

In-network duplication flooding: when node receives broadcast packet,

sends copy to all neighbors problems: cycles & broadcast storm

controlled flooding: node only broadcasts pkt if it hasn’t broadcast same packet before node keeps track of packet ids already broadacsted or reverse path forwarding (RPF): only forward packet

if it arrived on shortest path between node and source spanning tree: no redundant packets received by any node

Network Layer 4-193

Spanning tree first construct a spanning tree nodes then forward/make copies only along

spanning tree

Network Layer 4-194

A

B

G

DE

c

F

A

B

G

DE

c

F

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

Spanning tree: creation center node each node sends unicast join message to center

node message forwarded until it arrives at a node already

belonging to spanning tree

Network Layer 4-195

A

B

G

DE

c

F1

2

3

4

5

(a) stepwise construction of spanning tree (center: E)

A

B

G

DE

c

F

(b) constructed spanning tree

Multicast routing: problem statement

goal: find a tree (or trees) connecting routers having local mcast group members

tree: not all paths between routers used

shared-tree: same tree used by all group members

Network Layer 4-196

shared tree source-based trees

group membernot group member

routerwith agroup member

routerwithoutgroup member

legend

source-based: different tree from each sender to rcvrs

Approaches for building mcast trees

approaches: source-based tree: one tree per source shortest path trees reverse path forwarding

group-shared tree: group uses one tree minimal spanning (Steiner) center-based trees

Network Layer 4-197

…we first look at basic approaches, then specific protocols adopting these approaches

Shortest path tree mcast forwarding tree: tree of shortest path

routes from source to all receivers Dijkstra’s algorithm

Network Layer 4-198

i

router with attachedgroup member

router with no attachedgroup member

link used for forwarding,i indicates order linkadded by algorithm

LEGEND

R1

R2

R3

R4

R5

R6 R7

21

6

3 45

s: source

Reverse path forwarding rely on router’s knowledge of unicast shortest

path from it to sender each router has simple forwarding behavior:

Network Layer 4-199

if (mcast datagram received on incoming link on shortest path back to center)

then flood datagram onto all outgoing linkselse ignore datagram

Reverse path forwarding: example

Network Layer 4-200

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



datagram will be forwarded

LEGENDR1

R2

R3

R4

R5

R6 R7

s: source

datagram will not be forwarded

Reverse path forwarding: pruning forwarding tree contains subtrees with no mcast group

members no need to forward datagrams down subtree “prune” msgs sent upstream by router with no

downstream group members

Network Layer 4-201



prune message

LEGEND

links with multicastforwarding

P

R1

R2

R3

R4

R5

R6R7

s: source

P

P

Shared-tree: steiner tree

steiner tree: minimum cost tree connecting all routers with attached group members

problem is NP-complete excellent heuristics exists not used in practice: computational complexity information about entire network needed monolithic: rerun whenever a router needs to

join/leave

Network Layer 4-202

Center-based trees single delivery tree shared by all one router identified as “center” of tree to join: edge router sends unicast join-msg addressed to center

router join-msg “processed” by intermediate routers and

forwarded towards center join-msg either hits existing tree branch for this center,

or arrives at center path taken by join-msg becomes new branch of tree for

this router

Network Layer 4-203

Center-based trees: example

Network Layer 4-204

suppose R6 chosen as center:



path order in which join messages generated

LEGEND

21

3

1

R1

R2

R3

R4

R5

R6R7

Internet Multicasting Routing: DVMRP

DVMRP: distance vector multicast routing protocol, RFC1075

flood and prune: reverse path forwarding, source-based tree RPF tree based on DVMRP’s own routing tables

constructed by communicating DVMRP routers no assumptions about underlying unicast initial datagram to mcast group flooded everywhere

via RPF routers not wanting group: send upstream prune msgs

Network Layer 4-205

DVMRP: continued… soft state: DVMRP router periodically (1 min.)

“forgets” branches are pruned: mcast data again flows down unpruned branch downstream router: reprune or else continue to

receive data routers can quickly regraft to tree following IGMP join at leaf

odds and ends commonly implemented in commercial router

Network Layer 4-206

TunnelingQ: how to connect “islands” of multicast routers in

a “sea” of unicast routers?

Network Layer 4-207

mcast datagram encapsulated inside “normal” (non-multicast-addressed) datagram

normal IP datagram sent thru “tunnel” via regular IP unicast to receiving mcast router (recall IPv6 inside IPv4 tunneling)

receiving mcast router unencapsulates to get mcastdatagram

physical topology logical topology

PIM: Protocol Independent Multicast

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

two different multicast distribution scenarios :

Network Layer 4-208

dense: group members densely

packed, in “close” proximity.

bandwidth more plentiful

sparse: # networks with group

members small wrt # interconnected networks

group members “widely dispersed”

bandwidth not plentiful

Consequences of sparse-dense dichotomy:

dense group membership by

routers assumed until routers explicitly prune

data-driven construction on mcast tree (e.g., RPF)

bandwidth and non-group-router processing profligate

sparse: no membership until

routers explicitly join receiver- driven

construction of mcast tree (e.g., center-based)

bandwidth and non-group-router processing conservative

Network Layer 4-209

PIM- dense mode

flood-and-prune RPF: similar to DVMRP but… underlying unicast protocol provides RPF info for

incoming datagram less complicated (less efficient) downstream flood

than DVMRP reduces reliance on underlying routing algorithm

has protocol mechanism for router to detect it is a leaf-node router

Network Layer 4-210

PIM - sparse mode center-based approach router sends join msg to

rendezvous point (RP) intermediate routers

update state and forward join

after joining via RP, router can switch to source-specific tree increased

performance: less concentration, shorter paths

Network Layer 4-211

all data multicastfrom rendezvouspoint

rendezvouspoint

join

join

join

R1

R2

R3

R4

R5

R6R7

PIM - sparse modesender(s): unicast data to RP,

which distributes down RP-rooted tree

RP can extend mcasttree upstream to source

RP can send stop msgif no attached receivers “no one is listening!”

Network Layer 4-212

all data multicastfrom rendezvouspoint

rendezvouspoint

join

join

join

R1

R2

R3

R4

R5

R6R7

Chapter 4: done!4.1 introduction4.2 virtual circuit and


datagram format, IPv4 addressing, ICMP, IPv6

4.5 routing algorithms link state, distance vector,

hierarchical routing4.6 routing in the Internet

RIP, OSPF, BGP4.7 broadcast and multicast

routing

Network Layer 4-213

understand principles behind network layer services: network layer service models, forwarding versus routing

how a router works, routing (path selection), broadcast, multicast

instantiation, implementation in the Internet

chapter 4: network layer & routing -...

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