chapter4 5th april 2014 final2014
TRANSCRIPT
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Network Layer 4-1
Chapter 4
Network 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 obviously
represent a lotof work on our part. In return for use, we only ask the
following:
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, and
note 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 Approach5thedition.Jim Kurose, Keith RossAddison-Wesley, April2009.
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Network Layer 4-2
Chapter 4: Network Layer
Chapter goals: understand principles behind network layer
services:
network layer service models forwarding versus routing how a router works routing (path selection)
dealing with scale advanced topics: IPv6, mobility
instantiation, implementation in the Internet
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Network Layer 4-4
Network layer
transport segment from
sending to receiving host on sending side
encapsulates segmentsinto datagrams
on rcving side, deliversdatagrams to transportlayer
network layer protocols
in everyhost, router router examines header
fields in all IP datagramspassing through it
applicationtransport
networkdata linkphysical
applicationtransportnetwork
data linkphysical
networkdata linkphysical network
data linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
network
data linkphysical
networkdata link
physicalnetworkdata linkphysical
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Network Layer 4-5
Two Key Network-Layer Functions
forwarding:movepackets from routersinput to appropriate
router output
routing:determineroute taken by
packets from sourceto dest.
routing algorithms
analogy:
routing:process of
planning trip fromsource to dest
forwarding:process
of getting throughsingle interchange
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Network Layer 4-6
1
23
0111
value in arriving
packets header
routing algorithm
local forwarding table
header value output link
0100
0101
01111001
3
2
21
Interplay between routing and forwarding
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Network Layer 4-7
Connection setup
3rdimportant function in somenetwork 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
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Network Layer 4-8
Network service model
Q:What service modelfor channel transportingdatagrams from sender to receiver?
Example services for
individual datagrams: guaranteed delivery
guaranteed deliverywith less than 40 msec
delay
Example services for aflow of datagrams:
in-order datagramdelivery
guaranteed minimumbandwidth to flow
restrictions onchanges in inter-packet spacing
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Network Layer 4-9
Network layer service models:
Network
Architecture
Internet
ATM
ATM
ATM
ATM
Service
Model
best effort
CBR
VBR
ABR
UBR
Bandwidth
none
constantrate
guaranteed
rate
guaranteed
minimumnone
Loss
no
yes
yes
no
no
Order
no
yes
yes
yes
yes
Timing
no
yes
yes
no
no
Congestion
feedback
no (inferred
via loss)
nocongestion
no
congestion
yes
no
Guarantees ?
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Network Layer 4-10
Chapter 4: Network Layer
4. 1 Introduction
4.2 Virtual circuit anddatagram networks
4.3 Whats inside arouter
4.4 IP: InternetProtocol Datagram format
IPv4 addressing
ICMP
IPv6
4.5 Routing algorithms Link state
Distance Vector
Hierarchical routing
4.6 Routing in theInternet RIP
OSPF
BGP 4.7 Broadcast and
multicast routing
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Network Layer 4-11
Network layer connection andconnection-less service
datagram network provides network-layerconnectionless service
VC network provides network-layer
connection service analogous to the transport-layer services,
but: service: host-to-host
no choice: network provides one or the other
implementation: in network core
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Network Layer 4-12
Virtual circuits
call setup, teardown for each call beforedata can flow
each packet carries VC identifier (not destination hostaddress)
everyrouter on source-dest path maintains state foreach passing connection
link, router resources (bandwidth, buffers) may beallocated to VC (dedicated resources = predictable service)
source-to-dest path behaves much like telephonecircuit performance-wise
network actions along source-to-dest path
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Network Layer 4-14
Forwarding table
12 22 32
12
3
VC number
interfacenumber
Incoming interface Incoming VC # Outgoing interface Outgoing VC #
1 12 3 222 63 1 183 7 2 17
1 97 3 87
Forwarding table innorthwest router:
Routers maintain connection state information!
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Network Layer 4-15
Virtual circuits: signaling protocols
used to setup, maintain teardown VC
used in ATM, frame-relay, X.25
not used in todays Internet
applicationtransportnetworkdata linkphysical
applicationtransport
networkdata linkphysical
1. Initiate call 2. incoming call3. Accept call4. Call connected
5. Data flow begins 6. Receive data
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Network Layer 4-16
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 packets between same source-dest pair may take
different paths
applicationtransportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1. Send data 2. Receive data
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Network Layer 4-17
Forwarding table
Destination Address Range Link Interface
11001000 00010111 00010000 00000000through 0
11001000 00010111 00010111 11111111
11001000 00010111 00011000 00000000through 1
11001000 00010111 00011000 11111111
11001000 00010111 00011001 00000000through 2
11001000 00010111 00011111 11111111
otherwise 3
4 billionpossible entries
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Router?
Network Layer 4-18
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Network Layer 4-19
Longest prefix matching
Prefix Match Link Interface11001000 00010111 00010 0
11001000 00010111 00011000 111001000 00010111 00011 2
otherwise 3
DA: 11001000 00010111 00011000 10101010
Examples
DA: 11001000 00010111 00010110 10100001 Which interface?
Which interface?
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Network Layer 4-20
Datagram or VC network: why?
Internet (datagram) data exchange among
computers
elastic service, no stricttiming req.
smart end systems(computers)
can adapt, performcontrol, error recovery
simple inside network,complexity at edge
many link types
different characteristics
uniform service difficult
ATM (VC) evolved from telephony
human conversation:
strict timing, reliability
requirements need for guaranteed
service
dumb end systems
telephones
complexity insidenetwork
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Network Layer 4-21
Chapter 4: Network Layer
4. 1 Introduction
4.2 Virtual circuit anddatagram networks
4.3 Whats inside arouter
4.4 IP: InternetProtocol Datagram format
IPv4 addressing
ICMP
IPv6
4.5 Routing algorithms Link state
Distance Vector
Hierarchical routing
4.6 Routing in theInternet RIP
OSPF
BGP 4.7 Broadcast and
multicast routing
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Network Layer 4-22
Router Architecture Overview
Two key router functions: run routing algorithms/protocol (RIP, OSPF, BGP)
forwarding datagrams from incoming to outgoing link
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Network Layer 4-24
Three types of switching fabrics
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Network Layer 4-25
Switching Via Memory
First generation routers:
traditional computers with switching under directcontrol of CPU
packet copied to systems memory
speed limited by memory bandwidth (2 buscrossings per datagram)
Input
Port
Output
Port
Memory
System Bus
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Network Layer 4-27
Switching Via An InterconnectionNetwork
overcome bus bandwidth limitations
Banyan networks, other interconnection nets
initially developed to connect processors inmultiprocessor
advanced design: fragmenting datagram into fixedlength cells, switch cells through the fabric.
Cisco 12000: switches 60 Gbps through theinterconnection network
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Network Layer 4-28
Output Ports
Bufferingrequired when datagrams arrive from
fabric faster than the transmission rate Scheduling disciplinechooses among queued
datagrams for transmission
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Network Layer 4-29
Output port queueing
buffering when arrival rate via switch exceedsoutput line speed
queueing (delay) and loss due to output portbuffer overflow!
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Network Layer 4-30
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 buffer Recent recommendation: with Nflows,
buffering equal to RTT C.N
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Network Layer 4-31
Input Port Queuing
Fabric slower than input ports combined -> queueing
may occur at input queues Head-of-the-Line (HOL) blocking:queued datagram
at front of queue prevents others in queue frommoving forward
queueing delay and loss due to input buffer overflow!
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Network Layer 4-32
Chapter 4: Network Layer
4. 1 Introduction
4.2 Virtual circuit anddatagram networks
4.3 Whats inside arouter
4.4 IP: InternetProtocol Datagram format
IPv4 addressing
ICMP
IPv6
4.5 Routing algorithms Link state
Distance Vector
Hierarchical routing
4.6 Routing in theInternet RIP
OSPF
BGP 4.7 Broadcast and
multicast routing
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Network Layer 4-33
The Internet Network layer
forwardingtable
Host, router network layer functions:
Routing protocols
path selectionRIP, OSPF, BGP
IP protocoladdressing conventions
datagram formatpacket handling conventions
ICMP protocolerror reportingrouter signaling
Transport layer: TCP, UDP
Link layer
physical layer
Networklayer
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Network Layer 4-34
Chapter 4: Network Layer
4. 1 Introduction
4.2 Virtual circuit anddatagram networks
4.3 Whats inside arouter
4.4 IP: InternetProtocol Datagram format
IPv4 addressing
ICMP
IPv6
4.5 Routing algorithms Link state
Distance Vector
Hierarchical routing
4.6 Routing in theInternet RIP
OSPF
BGP 4.7 Broadcast and
multicast routing
IP d t f t
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Network Layer 4-35
IP datagram format
ver length
32 bits
data
(variable length,typically a TCP
or UDP segment)
16-bit identifier
headerchecksum
time tolive
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 dataflgs
fragmentoffset
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 TCP
20 bytes of IP
= 40 bytes + app
layer overhead
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Network Layer 4-36
IP Fragmentation & Reassembly network links have MTU
(max.transfer size) - largestpossible link-level frame.
different link types,different MTUs
large IP datagram divided
(fragmented) within net one datagram becomes
several datagrams
reassembled only at finaldestination
IP header bits used toidentify, order relatedfragments
fragmentation:in:one large datagramout:3 smaller datagrams
reassembly
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Network Layer 4-37
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 bytedatagram
MTU = 1500 bytes
1480 bytes indata field
offset =1480/8
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Network Layer 4-38
Chapter 4: Network Layer
4. 1 Introduction
4.2 Virtual circuit anddatagram networks
4.3 Whats inside arouter
4.4 IP: InternetProtocol Datagram format
IPv4 addressing
ICMP
IPv6
4.5 Routing algorithms Link state
Distance Vector
Hierarchical routing
4.6 Routing in theInternet RIP
OSPF
BGP 4.7 Broadcast and
multicast routing
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Network Layer 4-39
IP Addressing: introduction
IP address:32-bitidentifier for host,router interface
interface:connection
between host/routerand physical link routers typically have
multiple interfaces
host typically has one
interface IP addresses
associated with eachinterface
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
223.1.1.1 = 11011111 00000001 00000001 00000001
223 1 11
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Page 40
IP Address
Host addressNetwork addressIP address
IP address uniquely identifies a network device, it consistsof 32 binary digits.
IP address is often represented in a dotted decimalformat.
IP address is divided into two parts: Network address portion
Host address portion
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Page 41
Classification of IP Address
Class A
Class B
Class C
Class D
Class E
128.0.0.0~191.255.255.255
192.0.0.0~223.255.255.255
224.0.0.0~239.255.255.255
240.0.0.0~255.255.255.255
1 1 1 1 0 reserve
1.0.0.0~126.255.255.255
Host(24bit)0 Network(7bit)
Host(8bit)Network(21bit)1 1 0
1 1 1 0 Multicast address
Host(16bit)1 0 Network(14bit)
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Page 42
Special IP Address
Network part Host part Address type Usage
Any all 0Network
AddressRepresents a network Segment
Any all 1
Broadcast
Address
All nodes of a specifically
designated network segment
127 AnyLoopback
AddressLoop diagnostic functionality
all 0 All NetworksDesignates default routes
in Huawei Quidway routers
all 1Broadcast
Address
All nodes of a local network
segment
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Page 44
Subnet Mask Introduction
Subnet Masks manipulate the network and host address portion
used to determine the network portion of an IP address
The subnet mask format is same as the IP address format
Network and subnet of subnet mask identified in binary as aseries of 1 bits, the host bits are 0
For example Class Bs subnetmask is
255.255.0.0
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Network Layer 4-45
Subnets
IP address: subnet part (high
order bits)
host part (low orderbits)
Whats a subnet ? device interfaces with
same subnet part of IPaddress
can 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
223.1.3.2223.1.3.1
223.1.3.27
network consisting of 3 subnets
subnet
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Network Layer 4-46
Subnets 223.1.1.0/24 223.1.2.0/24
223.1.3.0/24
Recipe To determine the
subnets, detach eachinterface from its
host or router,creating islands ofisolated networks.Each isolated network
is called a subnet.Subnet mask: /24
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Network Layer 4-47
Subnets
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
N t k Add ss d S b t
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Page 48
Network Address and SubnetMask
IP Address
Subnet Mask
Network Address:
192.168.1
255.255.255
192.168.1
.100
.0
.0
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Page 49
Binary to DecimalConversion
Decimal Summation is 255
11111111
1248163264128
8bit
Binary
20
21
22
24
25
26
27
23
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Page 50
Example of Conversion
10010111
1*10*20*41*80*161*321*641*128
233
+ + + + + + +64 32 0 0 10128 8
Example :
System Conversion of IP
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Page 51
System Conversion of IPAddress
IP address192.168.1.11(decimal)
byte8bits byte8bits byte8bits byte8bits
2726252423222120272625242322212027262524232221202726252423222120
192
168
1
11
1 1 0 0 0 0 0 0 1 0 1 0 1 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 1 0 1 1
binary digit
11000000.10101000.00000001.00001011
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Page 52
A. Classful Addressing
Classful Addressing uses default subnet masks, and thus nosubnet..
Classful Routing Updates
Subnet masks are not sent in routing updates
e.g. Class B segment 172.16.0.0 with mask 255.255.0.0
172.16.30.1/16 172.16.28.1/16
Ethernet
172.16.30.10/16
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Page 53
Calculation of Host Number
Host Number2n
Valid Host Number: 2n- 2
Subnet Mask
N bit
Network Portion Host Portion
11 1 111 1 111 1 111 000 0 000000 000 01 0
Example of Host Number
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Page 54
Example of Host NumberCalculation
IP Address192.168.1.100/28
/28=255.255.255.240
The binary representation of subnet mask:
28bits
networkportion
4bitsHost portion
The total number of host: 24
The valid number of host: 24-2
11111111111111111111111111110000
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B. Classless IP Addressing
Classless Routing Protocol Characteristics of classless routing
protocols: Routing updates include the subnet mask
Supports VLSM
Supports Route Summarization
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VLSMVLSMthe process
of sub-netting asubnetto fit yourneedsExample:
Subnet 10.1.0.0/16,8 more bits areborrowed again, tocreate 256 subnetswith a /24mask.
Mask allows for 254host addresses persubnet
Subnets range from:10.1.0.0 / 24 to10.1.255.0 / 24
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Page 57
Variable Length Subnet Mask(VLSM)
ISP announce192.168.1.0
192.168.1.32/27
192.168.1.64/27
192.168.1.96/27
192.168.1.128/27
192.168.1.160/30
192.168.1.164/30
192.168.1.168/30
192.168.1.172/30
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Page 58
Subnet Mask Representation
255 . 255 . 255 . 24011111111 11111111 11111111 11110000
192 . 168 . 1 . 7
11000000 10101000 00000001 00000111
8 + 8 + 8+ 4 = 28
192 . 168 . 1 . 7 / 28
IP Address
Subnet mask
Bits of subnet mask
Subnet mask representation
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Page 59
Calculation of Network address
192.168.1.0/28
11000000 10101000 00000001 00000000
IP Address is: 192.168.1.7/28
192 . 168 . 1 . 7
11000000 10101000 00000001 00000111
255 . 255 . 255 . 240
11111111 11111111 11111111 11110000
IP Address
Subnet Mask
Network Address
(Binary)
Network Address
Classless Inter Domain Routing
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Page 60
Internet
Classless Inter-Domain Routing(CIDR) CIDR reduces the scale of the routing table and enhances
network extensibility.
198.168.1.0
198.168.2.0
198.168.3.0
ISP
Announce route198.168.0.0/16
Classless Inter Domain Routing
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Classless Inter-Domain Routing(CIDR) CIDR
Uses IP addresses more efficiently through useof VLSM
VLSM is the process of subnetting a subnet
Allows for route summarization Route summarization is representing multiple
contiguous routes with a single route
Classless Routing Updates Subnet masks are included in updates
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Network Layer 4-62
IP addresses: how to get one?
Q:How does a hostget 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
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Network Layer 4-63
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 use
Allows 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
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Network Layer 4-64
DHCP client-server scenario
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
A
B
E
DHCPserver
arriving DHCP
clientneeds
address in this
network
DHCP client-server scenario
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Network Layer 4-65
DHCP client-server scenarioDHCP server: 223.1.2.5 arriving
client
time
DHCP discover
src : 0.0.0.0, 68
dest.: 255.255.255.255,67yiaddr: 0.0.0.0
transaction ID: 654
DHCP offer
src: 223.1.2.5, 67
dest: 255.255.255.255, 68
yiaddrr: 223.1.2.4
transaction ID: 654Lifetime: 3600 secs
DHCP request
src: 0.0.0.0, 68
dest:: 255.255.255.255, 67
yiaddrr: 223.1.2.4
transaction ID: 655Lifetime: 3600 secs
DHCP ACK
src: 223.1.2.5, 67
dest: 255.255.255.255, 68
yiaddrr: 223.1.2.4
transaction ID: 655
Lifetime: 3600 secs
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DHCP: example
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Network Layer 4-67
DHCP: example
connecting laptop needs itsIP address, addr of first-hop router, addr of DNSserver: use DHCP
router(runs DHCP)
DHCP
UDP
IPEth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulatedin UDP, encapsulated in IP,encapsulated in 802.1Ethernet
Ethernet frame broadcast(dest: FFFFFFFFFFFF) on LAN,received at router runningDHCP server
Ethernet demuxed to IPdemuxed, UDP demuxed toDHCP
168.1.1.1
DHCP: example
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Network Layer 4-68
DCP server formulatesDHCP ACK containing
clients IP address, IPaddress of first-hoprouter for client, name &IP address of DNS server
router(runs DHCP)
DHCP
UDP
IPEth
Phy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation of DHCPserver, frame forwardedto client, demuxing up toDHCP at client
client now knows its IPaddress, name and IPaddress of DSN server, IPaddress of its first-hop
router
DHCP: example
DHCP i h k M t B t R l (2)
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Network Layer 4-69
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 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
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Network Layer 4-70
IP addresses: how to get one?
Q:How does networkget subnet part of IPaddr?
A:gets allocated portion of its provider ISPsaddress space
ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20
Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23
Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23
Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23
... .. . .
Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23
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Network Layer 4-71
Hierarchical addressing: route aggregation
Send me anythingwith addressesbeginning200.23.16.0/20
200.23.16.0/23
200.23.18.0/23
200.23.30.0/23
Fly-By-Night-ISP
Organization 0
Organization 7Internet
Organization 1
ISPs-R-UsSend me anythingwith addressesbeginning199.31.0.0/16
200.23.20.0/23Organization 2
...
...
Hierarchical addressing allows efficient advertisement of routinginformation:
Hi hi l dd i ifi
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Network Layer 4-72
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 anythingwith addressesbeginning 199.31.0.0/16or 200.23.18.0/23
200.23.20.0/23Organization 2
...
...
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Network Layer 4-73
IP addressing: the last word...
Q:How does an ISP get block of addresses?A:ICANN: Internet Corporation for Assigned
Names and Numbers
allocates addressesmanages DNS
assigns domain names, resolves disputes
N N k dd l
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Network Layer 4-74
NAT: Network Address Translation
10.0.0.1
10.0.0.2
10.0.0.3
10.0.0.4
138.76.29.7
local network(e.g., home network)
10.0.0/24
rest ofInternet
Datagrams with source or
destination in this networkhave 10.0.0/24 address forsource, destination (as usual)
Alldatagrams leavinglocal
network have samesingle sourceNAT IP address: 138.76.29.7,different source port numbers
N T N k dd T l i
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Network Layer 4-75
NAT: Network Address Translation
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 devices
can change addresses of devices in local networkwithout notifying outside world
can change ISP without changing addresses ofdevices in local network
devices inside local net not explicitly addressable,visible by outside world (a security plus).
NAT N k Add T l i
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Network Layer 4-76
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 (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
N T N k dd T l i
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Network Layer 4-77
NAT: Network Address Translation
10.0.0.1
10.0.0.2
10.0.0.3
S: 10.0.0.1, 3345D: 128.119.40.186, 80
1
10.0.0.4
138.76.29.7
1: host 10.0.0.1sends datagram to128.119.40.186, 80
NAT translation tableWAN side addr LAN side addr
138.76.29.7, 5001 10.0.0.1, 3345
S: 128.119.40.186, 80D: 10.0.0.1, 3345 4
S: 138.76.29.7, 5001D: 128.119.40.186, 802
2: NAT routerchanges datagramsource addr from10.0.0.1, 3345 to138.76.29.7, 5001,updates table
S: 128.119.40.186, 80D: 138.76.29.7, 5001 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
NAT N k Add T l i
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Network Layer 4-78
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, eg, P2P applications
address shortage should instead be solved byIPv6
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NAT t l bl
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Network Layer 4-80
NAT traversal problem
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
NAT t l bl
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Network Layer 4-81
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
138.76.29.7Client
10.0.0.1
NATrouter
1.connection torelay initiatedby NATted host
2.connection torelay initiatedby client
3.relayingestablished
Ch t 4 N t k L
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Network Layer 4-82
Chapter 4: Network Layer
4. 1 Introduction 4.2 Virtual circuit and
datagram networks
4.3 Whats inside a
router 4.4 IP: Internet
Protocol Datagram format
IPv4 addressing ICMP
IPv6
4.5 Routing algorithms Link state
Distance Vector
Hierarchical routing
4.6 Routing in theInternet RIP
OSPF
BGP 4.7 Broadcast and
multicast routing
ICMP I t t C t l M P t l
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Network Layer 4-83
ICMP: Internet Control Message Protocol
used by hosts & routers tocommunicate network-levelinformation
error reporting:unreachable host, network,port, protocol
echo request/reply (usedby ping)
network-layer above IP:
ICMP msgs carried in IPdatagrams
ICMP message:type, code plusfirst 8 bytes of IP datagramcausing error
Type Code description
0 0 echo reply (ping)
3 0 dest. network unreachable
3 1 dest host unreachable
3 2 dest protocol unreachable
3 3 dest port unreachable3 6 dest network unknown
3 7 dest host unknown
4 0 source quench (congestion
control - not used)
8 0 echo request (ping)
9 0 route advertisement10 0 router discovery
11 0 TTL expired
12 0 bad IP header
T t d ICMP
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Network Layer 4-84
Traceroute and ICMP
Source sends series ofUDP segments to dest First has TTL =1
Second has TTL=2, etc.
Unlikely port number
When nth datagram arrivesto nth router: Router discards datagram
And sends to source anICMP message (type 11,
code 0) Message includes name of
router& IP address
When ICMP messagearrives, source calculatesRTT
Traceroute does this 3times
Stopping criterion UDP segment eventually
arrives at destination host
Destination returns ICMPhost unreachable packet(type 3, code 3)
When source gets thisICMP, stops.
Ch pt 4: N t k L
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Network Layer 4-85
Chapter 4: Network Layer
4. 1 Introduction 4.2 Virtual circuit and
datagram networks
4.3 Whats inside a
router 4.4 IP: Internet
Protocol Datagram format
IPv4 addressing ICMP
IPv6
4.5 Routing algorithms Link state
Distance Vector
Hierarchical routing
4.6 Routing in theInternet RIP
OSPF
BGP 4.7 Broadcast and
multicast routing
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IPv6 Header (Cont)
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Network Layer 4-87
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
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Transition From IPv4 To IPv6
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Network Layer 4-89
Transition From IPv4 To IPv6
Not all routers can be upgraded simultaneous no flag days
How will the network operate with mixed IPv4 andIPv6 routers?
Tunneling:IPv6 carried as payload in IPv4datagram among IPv4 routers
Tunneling
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Network Layer 4-90
TunnelingA B E F
IPv6 IPv6 IPv6 IPv6
tunnelLogical view:
Physical view:A B E F
IPv6 IPv6 IPv6 IPv6IPv4 IPv4
Tunneling
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Network Layer 4-91
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
Flow: XSrc: ADest: F
data
Flow: XSrc: ADest: F
data
Src:BDest: E
Flow: XSrc: ADest: F
data
Src:BDest: E
A-to-B:IPv6
E-to-F:IPv6
B-to-C:IPv6 inside
IPv4
B-to-C:IPv6 inside
IPv4
Chapter 4: Network Layer
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Network Layer 4-92
Chapter 4: Network Layer
4. 1 Introduction 4.2 Virtual circuit and
datagram networks
4.3 Whats inside a
router 4.4 IP: Internet
Protocol Datagram format
IPv4 addressing ICMP
IPv6
4.5 Routing algorithms Link state
Distance Vector
Hierarchical routing
4.6 Routing in theInternet RIP
OSPF
BGP 4.7 Broadcast and
multicast routing
Interplay between routing, forwarding
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Network Layer 4-93
1
23
0111
value in arriving
packets header
routing algorithm
local forwarding table
header value output link
0100
0101
01111001
3
2
21
p y g, g
I li
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Page 94
In reality
[RTB]display ip routing-table
Routing Tables: Public
Destinations : 10 Routes : 10
Destination/Mask Proto Pre Cost NextHop Interface
10.1.1.0/30 Direct 0 0 10.1.1.2 Serial0/0/0
10.1.1.1/32 Direct 0 0 10.1.1.1 Serial0/0/0
127.0.0.1/32 Direct 0 0 127.0.0.1 InLoopBack0
172.16.1.1/32 OSPF 10 1562 10.1.2.2 Serial0/0/1
192.168.2.0/24 RIP 100 1 10.1.1.1 Serial0/0/0
10.1.1.0/8
RTB
10.1.2.0/24
.1
.1
OSPF
RIP
RTA
RTC
172.16.1.1/32
192.168.2.1/24
.2
.2
Graph abstraction
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Network Layer 4-95
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) }
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
Graph abstraction: costs
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Network Layer 4-96
Graph abstraction: costs
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, orinversely related to bandwidth,or inversely related tocongestion
Cost of path (x1, x2, x3,, xp) = c(x1,x2) + c(x2,x3) + + c(xp-1,xp)
Question: Whats the least-cost path between u and z ?
Routing algorithm: algorithm that finds least-cost path
Routing Algorithm classification
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Network Layer 4-97
Routing Algorithm classification
Global or decentralizedinformation?
Global:
all routers have completetopology, link cost info
link state algorithmsDecentralized:
router knows physically-connected neighbors, linkcosts to neighbors
iterative process ofcomputation, exchange ofinfo with neighbors
distance vector algorithms
Static or dynamic?Static: routes change slowly
over time
Dynamic: routes change more
quickly
periodic update
in response to linkcost changes
Chapter 4: Network Layer
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Network Layer 4-98
Chapter 4: Network Layer
4. 1 Introduction 4.2 Virtual circuit and
datagram networks
4.3 Whats inside a
router 4.4 IP: Internet
Protocol Datagram format
IPv4 addressing ICMP
IPv6
4.5 Routing algorithms Link state
Distance Vector
Hierarchical routing
4.6 Routing in theInternet RIP
OSPF
BGP 4.7 Broadcast and
multicast routing
A Link-State Routing Algorithm
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Network Layer 4-99
A Link-State Routing Algorithm
Dijkstras algorithm net topology, link costs
known to all nodes
accomplished via linkstate broadcast
all nodes have same info
computes least cost pathsfrom one node (source) toall other nodes
gives forwarding tablefor that node
iterative: after kiterations, know least costpath to k dest.s
Notation: c(x,y):link cost from node
x to y; = if not directneighbors
D(v):current value of costof path from source todest. v
p(v):predecessor nodealong path from source to v
N':set of nodes whoseleast cost path definitivelyknown
Dijsktras Algorithm
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Network Layer4-100
Dijsktra s Algorithm
1Init ial ization:
2 N' = {u}
3 for all nodes v
4 if v adjacent to u
5 then D(v) = c(u,v)
6 else D(v) =
78 Loop
9 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 un t i l al l nodes in N'
Dijkstras algorithm: example
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Network Layer 4-101
Dijkstra s algorithm: example
Step0
1
2
3
4
5
N'u
ux
uxy
uxyv
uxyvw
uxyvwz
D(v),p(v)2,u
2,u
2,u
D(w),p(w)5,u
4,x
3,y
3,y
D(x),p(x)1,u D(y),p(y)2,x
D(z),p(z)
4,y
4,y
4,y
u
yx
wv
z2
21
3
1
1
2
53
5
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Chapter 4: Network Layer
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Network Layer4-104
Chapter 4: Network Layer
4. 1 Introduction 4.2 Virtual circuit and
datagram networks
4.3 Whats inside a
router 4.4 IP: Internet
Protocol Datagram format
IPv4 addressing ICMP
IPv6
4.5 Routing algorithms Link state
Distance Vector
Hierarchical routing
4.6 Routing in theInternet RIP
OSPF
BGP
4.7 Broadcast andmulticast routing
Distance Vector Algorithm
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Network Layer4-105
Distance Vector Algorithm
Bellman-Ford Equation (dynamic programming)Define
dx(y) := cost of least-cost path from x to y
Then
dx(y) = min {c(x,v) + dv(y) }
where min is taken over all neighbors v of x
v
Bellman-Ford example
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Network Layer4-106
Bellman Ford example
u
yx
wv
z2
21
3
1
1
2
53
5 Clearly, dv(z) = 5, dx(z) = 3, dw(z) = 3
du(z) = min { c(u,v) + dv(z),c(u,x) + dx(z),c(u,w) + dw(z) }
= min {2 + 5,1 + 3,
5 + 3} = 4Node that achieves minimum is nexthop in shortest path forwarding table
B-F equation says:
Distance Vector Algorithm
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Network Layer4-107
Distance Vector Algorithm
Dx(y)= estimate of least cost from x to yNode x knows cost to each neighbor v:
c(x,v)
Node x maintains distance vector Dx=[Dx(y): y N ]
Node x also maintains its neighborsdistance vectors For each neighbor v, x maintains
Dv= [Dv(y): y N ]
Distance vector algorithm (4)
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Network Layer4-108
Distance vector algorithm (4)
Basic idea: From time-to-time, each node sends its own
distance vector estimate to neighbors Asynchronous
When a node x receives new DV estimate fromneighbor, 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 estimateDx(y) converge to the actual least costdx(y)
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node x table
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
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Network Layer 4-110
x y z
xyz
0 2 7
from
cost to
from
from
x y z
xyz
0
from
cost to
x y z
xyz
cost to
x y zx
yz
7 1 0
cost to
2 0 1
2 0 17 1 0
time
x z12
7
y
node x table
node y table
node z table
m n{ , 7 }
32
node x table
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
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Network Layer 4-111
x y z
xyz
0 2 7
from
cost to
from
from
x y z
xyz
0 2 3
from
cost tox y z
xyz
0 2 3
from
cost to
x y z
xyz
cost tox y z
xyz
0 2 7
from
cost to
x y zx
yz
0 2 3
from
cost to
x y zx
yz
0 2 3
from
cost to
x y zx
yz
0 2 7
from
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 0
2 0 1
3 1 0
time
x z12
7
y
node x table
node y table
node z table
{ , }
Distance Vector: link cost changes
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Network Layer 4-112
D stance Vector l nk cost changes
Link cost changes: node detects local link cost change
updates routing info, recalculatesdistance vector
if DV changes, notify neighbors
goodnewstravelsfast
x z14
50
y1
At time t0, ydetects the link-cost change, updates its DV,and informs its neighbors.
At time t1, zreceives the update from yand updates its table.It computes a new least cost to x and sends its neighbors its DVAt time t2, yreceives zs update and updates its distance table.ys least costs do not change and hence y does notsend anymessage to z.
Distance Vector: link cost changes
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Network Layer 4-113
D V gLink cost changes:
good news travels fast bad news travels slow -
count to infinityproblem! **
44 iterations before
algorithm stabilizes: seetext
Poisoned reverse: If Z routes through Y to get
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?
Read ebook page 387 and 388 tounderstand this
x z14
50
y60
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Chapter 4: Network Layer
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Network Layer 4-115
Chapter 4 Network Layer
4. 1 Introduction 4.2 Virtual circuit and
datagram networks
4.3 Whats inside a
router 4.4 IP: Internet
Protocol Datagram format
IPv4 addressing ICMP
IPv6
4.5 Routing algorithms Link state
Distance Vector
Hierarchical routing
4.6 Routing in theInternet RIP
OSPF
BGP
4.7 Broadcast andmulticast routing
Hierarchical Routing
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Network Layer 4-116
g
scale:with 200 milliondestinations:
cant store all dests inrouting tables!
routing table exchangewould swamp links!
administrative autonomy internet = network of
networks
each network admin may
want to control routing in itsown network
Our routing study thus far - idealization all routers identical
network flat
nottrue in practice
Hierarchical Routing
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Network Layer 4-117
g
aggregate routers intoregions,autonomoussystems (AS)
routers in same AS run
same routing protocol intra-AS routingprotocol
routers in different AScan run different intra-
AS routing protocol
Gateway router Direct link to router in
another AS
Interconnected ASes
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Network Layer4-118
3b
1d
3a
1c2aAS3
AS1
AS21a
2c2b
1b
Intra-AS
Routing
algorithm
Inter-AS
Routing
algorithm
Forwarding
table
3c
forwarding tableconfigured by bothintra- and inter-ASrouting algorithm intra-AS sets entries
for internal dests
inter-AS & intra-Assets entries forexternal dests
Interconnected ASes
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Page 119
Interconnected ASes
IGPsRIP OSPF ISIS
EGPsBGP
AS100 AS200
Inter-AS (Exterior Gateway Protocol) tasksAS1 must:
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Network Layer4-120
3b
1d
3a
1c2aAS3
AS1
AS21a
2c2b
1b
3c
suppose router in AS1receives datagram
destined outside ofAS1: router should
forward packet togateway router, butwhich one?
1. learn which dests arereachable through
AS2, which throughAS3
2. propagate thisreachability info to allrouters in AS1
Job of inter-AS routing!
Example: Setting forwarding table in router 1d
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Network Layer4-121
suppose AS1 learns (via inter-AS protocol) that subnetxreachable 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
1c2aAS3
AS1
AS21a
2c2b
1b
3c
x
Example: Choosing among multiple ASes
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Network Layer4-122
now suppose AS1 learns from inter-AS protocol that
subnet xis reachable from AS3 andfrom 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 2aAS3
AS1
AS21a
2c
2b
1b
3cx
Example: Choosing among multiple ASes
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Network Layer4-123
Learn from inter-AS
protocol that subnetx is reachable via
multiple gateways
Use routing info
from intra-AS
protocol to determine
costs of least-cost
paths to each
of the gateways
Hot potato routing:
Choose the gatewaythat has the
smallest least cost
Determine from
forwarding table the
interface I that leadsto least-cost gateway.
Enter (x,I) in
forwarding table
now suppose AS1 learns from inter-AS protocol that
subnet xis reachable from AS3 andfrom 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.
Chapter 4: Network Layer
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Network Layer4-124
p y
4. 1 Introduction 4.2 Virtual circuit and
datagram networks
4.3 Whats inside a
router 4.4 IP: Internet
Protocol Datagram format
IPv4 addressing ICMP
IPv6
4.5 Routing algorithms Link state
Distance Vector
Hierarchical routing
4.6 Routing in theInternet RIP
OSPF
BGP
4.7 Broadcast andmulticast routing
Intra-AS Routing
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Network Layer4-125
g
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 (Ciscoproprietary)
Chapter 4: Network Layer
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Network Layer4-126
p y
4. 1 Introduction 4.2 Virtual circuit and
datagram networks
4.3 Whats inside a
router 4.4 IP: Internet
Protocol Datagram format
IPv4 addressing ICMP
IPv6
4.5 Routing algorithms Link state
Distance Vector
Hierarchical routing
4.6 Routing in theInternet RIP
OSPF
BGP
4.7 Broadcast andmulticast routing
RIP ( Routing Information Protocol)
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Network Layer4-127
g
RIPv1 , RIPv2 distance vector algorithm
included in BSD-UNIX Distribution in 1982
distance metric: # of hops (max = 15 hops)
DC
BA
u v
w
x
yz
destination hopsu 1v 2w 2
x 3y 3z 2
From router A to subnets:
RIPv1 vs. RIPv2
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Page 128
RIPv1 is a classful routing protocol, does notsupport VLSM and CIDR.
Sends messages via broadcast.
Doesnt support authentication.
RIPv2 is a classless routing protocol, supportsVLSM, route aggregation and CIDR.
Supports messages sent via broadcast ormulticast address (240.0.0.9).
Supports plain text and MD5 authentication.
RIP advertisements
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Network Layer4-129
distance vectors:exchanged amongneighbors every 30 sec via ResponseMessage (also called advertisement)
each advertisement: list of up to 25destination subnets within AS
RIP: Example
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Network Layer4-130
p
Destination Network Next Router Num. of hops to dest.
w A 2y B 2z B 7x -- 1. . ....
w x yz
A
C
D B
Routing/Forwarding table in D
RIP: Example
D t N t h
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Network Layer 4-131
Destination Network Next Router Num. of hops to dest.
w A 2
y B 2z B A 7 5x -- 1. . ....
Routing/Forwarding table in D
w x y
z
A
C
D B
Dest Next hopsw - 1x - 1
z C 4. ...
Advertisementfrom A to D
RIP: Link Failure and Recovery
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Network Layer4-132
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 (iftables changed)
link failure info quickly (?) propagates to entire net
poison reverseused to prevent ping-pong loops
(infinite distance = 16 hops)
RIP link failure and
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Page 133
Recovery
E0
S0 S0
S1
S0
E0
11.2.0.
0
11.3.0.
0
A
B
C
11.1.0.0
11.4.0.
0If the hop count is16, itmeans this route is
unreachable
16S011.4.0.0
1S011.3.0.0
0S011.2.0.0
0E011.1.0.0
countinterfac
e
destination
Routing Table
1S011.1.0.0
16S111.4.0.
0
0S111.3.0.0
0S011.2.0.0
countinterface
interface
Routing Table
2S011.1.0.0
1S011.2.0.0
16E011.4.0.0
0S011.3.0.0
countinterfac
e
destinatio
n
Routing Table
RIP Table processing
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Network Layer4-134
RIP routing tables managed by application-levelprocess called route-d (daemon)
advertisements sent in UDP packets, periodicallyrepeated
physical
link
network forwarding(IP) table
Transprt(UDP)
routed
physical
link
network(IP)
Transprt(UDP)
routed
forwardingtable
Chapter 4: Network Layer
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Network Layer4-135
4. 1 Introduction 4.2 Virtual circuit and
datagram networks
4.3 Whats inside a
router 4.4 IP: Internet
Protocol Datagram format
IPv4 addressing ICMP
IPv6
4.5 Routing algorithms Link state
Distance Vector
Hierarchical routing
4.6 Routing in theInternet RIPv1 & RIPv2
OSPF
BGP
4.7 Broadcast andmulticast routing
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OSPF advanced features (not inRIPv1)
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Network Layer4-137
RIPv1)
security:all OSPF messages authenticated (toprevent malicious intrusion)
multiple same-cost paths allowed (only one path inRIP)
For each link, multiple cost metrics for differentTOS (e.g., satellite link cost set low for best effort;high for real time)
integrated uni- and multicastsupport:
Multicast OSPF (MOSPF) uses same topology database as OSPF
hierarchicalOSPF in large domains.
Hierarchical OSPF
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Network Layer4-138
Hierarchical OSPF
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Network Layer4-139
two-level hierarchy:local area, backbone. Link-state advertisements only in area
each 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.
Chapter 4: Network Layer
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Network Layer4-140
4. 1 Introduction 4.2 Virtual circuit and
datagram networks
4.3 Whats inside a
router 4.4 IP: Internet
Protocol Datagram format
IPv4 addressing ICMP
IPv6
4.5 Routing algorithms Link state
Distance Vector
Hierarchical routing
4.6 Routing in theInternet RIP
OSPF
BGP
4.7 Broadcast andmulticast routing
Internet inter-AS (EGP) routing:BGP
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Network Layer 4-141
BGP
BGP (Border Gateway Protocol):thedefacto standard BGP 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
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Distributing reachability infoi BGP i b t 3 d 1 AS3 d
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Network Layer4-143
using eBGP session between 3a and 1c, AS3 sendsprefix 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 entryfor prefix in its forwarding table.
3b
1d
3a
1c2aAS3
AS1
AS21a
2c
2b
1b
3ceBGP session
iBGP session
Path attributes & BGP routes
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Network Layer4-144
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 17NEXT-HOP:indicates specific internal-AS router
to next-hop AS. (may be multiple links fromcurrent AS to next-hop-AS)
when gateway router receives routeadvertisement, uses import policytoaccept/decline.
BGP route selection
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Network Layer4-145
router may learn about more than 1 routeto some prefix. Router must select route.
elimination rules:1. local preference value attribute: policy
decision
2. shortest AS-PATH
3. closest NEXT-HOP router: hot potato routing
4. additional criteria
BGP messages
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Network Layer4-146
BGP messages exchanged using TCP. BGP messages:
OPEN:opens TCP connection to peer andauthenticates sender
UPDATE:advertises new path (or withdraws old) KEEPALIVEkeeps connection alive in absence of
UPDATES; also ACKs OPEN request
NOTIFICATION:reports errors in previous msg;
also used to close connection
BGP routing policy
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Network Layer4-147
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
A
B
C
WX
Y
legend:
customernetwork:
provider
network
BGP routing policy (2)
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Network Layer4-148
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 CBAWsince neither W nor C are Bs customers
B wants to force C to route to w via A
B wants to route only to/from its customers!
A
B
C
WX
Y
legend:
customernetwork:
provider
network
Why different Intra- and Inter-AS routing ?
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Network Layer4-149
Policy: Inter-AS: admin wants control over how its traffic
routed, who routes through its net.
Intra-AS: single admin, so no policy decisions needed
Scale: hierarchical routing saves table size, reduced update
traffic
Performance:
Intra-AS: can focus on performance Inter-AS: policy may dominate over performance
Chapter 4: Network Layer
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Network Layer4-150
4. 1 Introduction 4.2 Virtual circuit and
datagram networks
4.3 Whats inside a
router 4.4 IP: Internet
Protocol Datagram format
IPv4 addressing ICMP
IPv6
4.5 Routing algorithms Link state
Distance Vector
Hierarchical routing
4.6 Routing in theInternet RIP
OSPF
BGP
4.7 Broadcast andmulticast routing
Broadcast Routing
deliver packets from source to all other nodes
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Network Layer 4-151
R1
R2
R3 R4
source
duplication
R1
R2
R3 R4
in-network
duplication
duplicate
creation/transmissionduplicate
duplicate
deliver packets from source to all other nodes
source duplication is inefficient:
source duplication: how does sourcedetermine recipient addresses?
In-network duplication
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Network Layer4-152
flooding: when node receives brdcst pckt,sends copy to all neighbors Problems: cycles & broadcast storm
controlled flooding: node only brdcsts pkt
if it hasnt brdcst same packet beforeNode keeps track of pckt ids already brdcstedOr reverse path forwarding (RPF): only forward
pckt if it arrived on shortest path between
node and source spanning tree
No redundant packets received by any node
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Chapter 4: summary
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4. 1 Introduction 4.2 Virtual circuit and
datagram networks
4.3 Whats inside a
router 4.4 IP: Internet
Protocol Datagram format
IPv4 addressing ICMP
4.5 Routing algorithms Link state
Distance Vector
Hierarchical routing
4.6 Routing in theInternet RIP
OSPF
BGP
4.7 Broadcast andl i i