4,Network Layer 4a-1
Chapter 4,Network Layer
Chapter goals:
? understand principles
behind network layer
services:
? routing (path selection)
? dealing with scale
? how a router works
? advanced topics,IPv6,
multicast
? instantiation and
implementation in the
Internet
Overview:
? network layer services
? hierarchical routing
? IP
? what’s inside a router?
? IPv6
4,Network Layer 4a-2
Network layer functions
? transport packet from
sending to receiving hosts
? network layer protocols in
every host,router
three important functions:
? path determination,route
taken by packets from source
to dest,Routing algorithms
? switching,move packets from
router’s input to appropriate
router output
? call setup,some network
architectures require router
call setup along path before
data flows
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
application
transport
network
data link
physical
application
transport
network
data link
physical
4,Network Layer 4a-3
Network service model
Q,What service model
for,channel”
transporting packets
from sender to
receiver?
? guaranteed bandwidth?
? preservation of inter-packet
timing (no jitter)?
? loss-free delivery?
? in-order delivery?
? congestion feedback to
sender?
virtual circuitor
datagram?
The most important
abstraction provided
by network layer:
4,Network Layer 4a-4
Virtual circuits
? call setup,teardown for each call before data can flow
? each packet carries VC identifier (not destination host OD)
? every router on source-dest path s maintain,state” for
each passing connection
? transport-layer connection only involved two end systems
? link,router resources (bandwidth,buffers) may be
allocated to VC
? to get circuit-like perf.
,source-to-dest path behaves much like telephone
circuit”
? performance-wise
? network actions along source-to-dest path
4,Network Layer 4a-5
Virtual circuits,signaling protocols
? used to setup,maintain teardown VC
? used in ATM,frame-relay,X.25
? not used in today’s Internet
application
transport
network
data link
physical
application
transport
network
data link
physical
1,Initiate call 2,incoming call
3,Accept call4,Call connected
5,Data flow begins 6,Receive data
4,Network Layer 4a-6
Datagram networks,the Internet model
? no call setup at network layer
? routers,no state about end-to-end connections
? no network-level concept of,connection”
? packets typically routed using destination host ID
? packets between same source-dest pair may take
different paths
application
transport
network
data link
physical
application
transport
network
data link
physical
1,Send data 2,Receive data
4,Network Layer 4a-7
Network layer service models:
Network
Architecture
Internet
ATM
ATM
ATM
ATM
Service
Model
best effort
CBR
VBR
ABR
UBR
Bandwidth
none
constant
rate
guaranteed
rate
guaranteed
minimum
none
Loss
no
yes
yes
no
no
Order
no
yes
yes
yes
yes
Timing
no
yes
yes
no
no
Congestion
feedback
no (inferred
via loss)
no
congestion
no
congestion
yes
no
Guarantees?
? Internet model being extented,Intserv,Diffserv
? Chapter 6
4,Network Layer 4a-8
Datagram or VC network,why?
Internet
? data exchange among
computers
?,elastic” service,no strict
timing req,
?,smart” end systems
(computers)
? can adapt,perform
control,error recovery
? simple inside network,
complexity at,edge”
? many link types
? different characteristics
? uniform service difficult
ATM
? evolved from telephony
? human conversation,
? strict timing,reliability
requirements
? need for guaranteed
service
?,dumb” end systems
? telephones
? complexity inside
network
4,Network Layer 4a-9
Routing
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) thru
network from source to dest.
Routing protocol
A
ED
CB
F
2
2
1 3
1
1
2
5
3
5
?, good” path:
? typically means minimum
cost path
? other def’s possible
4,Network Layer 4a-10
Routing Algorithm classification
Global or decentralized
information?
Global:
? all routers have complete
topology,link cost info
?,link state” algorithms
Decentralized:
? router knows physically-
connected neighbors,link
costs to neighbors
? iterative process of
computation,exchange of
info with neighbors
?,distance vector” algorithms
Static or dynamic?
Static:
? routes change slowly over
time
Dynamic:
? routes change more quickly
? periodic update
? in response to link cost
changes
4,Network Layer 4a-11
Hierarchical Routing
scale,with 50 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
Our routing study thus far - idealization
? all routers identical
? network,flat”
… not true in practice
4,Network Layer 4a-12
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
? special routers in AS
? run intra-AS routing
protocol with all other
routers in AS
? also responsible for
routing to destinations
outside AS
? run inter-AS routing
protocol with other
gateway routers
gateway routers
4,Network Layer 4a-13
Intra-AS and Inter-AS routing
Gateways:
?perform inter-AS
routing amongst
themselves
?perform intra-AS
routers with other
routers in their
AS
inter-AS,intra-AS
routing in
gateway A.c
network layer
link layer
physical layer
a
b
b
a
aC
A
B
d
A.a
A.c
C.b B.a
c
b
c
4,Network Layer 4a-14
Intra-AS and Inter-AS routing
Host
h2a
b
b
a
aC
A
B
d c
A.a
A.c
C.b B.a
c
b
Host
h1
Intra-AS routing
within AS A
Inter-AS
routing
between
A and B
Intra-AS routing
within AS B
? We’ll examine specific inter-AS and intra-AS
Internet routing protocols shortly
4,Network Layer 4a-15
The Internet Network layer
routing
table
Host,router network layer functions:
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
Network
layer
4,Network Layer 4a-16
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 may have multiple
interfaces
? IP addresses
associated with
interface,not host,
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
223.1.1.1 = 11011111 00000001 00000001 00000001
223 1 11
4,Network Layer 4a-17
IP Addressing
? IP address:
? network part (high
order bits)
? host part (low order
bits)
? What’s a network?
(from IP address
perspective)
? device interfaces with
same network part of
IP address
? can physically reach
each other without
intervening router
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
network consisting of 3 IP networks
(for IP addresses starting with 223,
first 24 bits are network address)
LAN
4,Network Layer 4a-18
IP Addressing
How to find the
networks?
? Detach each
interface from
router,host
? create,islands of
isolated networks
223.1.1.1
223.1.1.3
223.1.1.4
223.1.2.2223.1.2.1
223.1.2.6
223.1.3.2223.1.3.1
223.1.3.27
223.1.1.2
223.1.7.0
223.1.7.1
223.1.8.0223.1.8.1
223.1.9.1
223.1.9.2
Interconnected
system consisting
of six networks
4,Network Layer 4a-19
IP Addresses
0network host
10 network host
110 network host
1110 multicast address
A
B
C
D
class
1.0.0.0 to
127.255.255.255
128.0.0.0 to
191.255.255.255
192.0.0.0 to
223.255.255.255
224.0.0.0 to
239.255.255.255
32 bits
given notion of,network”,let’s re-examine IP addresses:
,class-full” addressing:
4,Network Layer 4a-20
IP addressing,CIDR
?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
?CIDR,Classless InterDomain Routing
? network portion of address of arbitrary length
? address format,a.b.c.d/x,where x is # bits in network
portion of address
11001000 00010111 00010000 00000000
network
part
host
part
200.23.16.0/23
4,Network Layer 4a-21
IP addresses,how to get one?
Hosts (host portion):
?hard-coded by system admin in a file
?DHCP,Dynamic Host Configuration Protocol,
dynamically get address:,plug-and-play”
?host broadcasts,DHCP discover” msg
?DHCP server responds with,DHCP offer” msg
?host requests IP address:,DHCP request” msg
?DHCP server sends address:,DHCP ack” msg
4,Network Layer 4a-22
IP addresses,how to get one?
Network (network portion):
?get allocated portion of ISP’s address 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
4,Network Layer 4a-23
Hierarchical addressing,route aggregation
,Send me anything
with addresses
beginning
200.23.16.0/20”
200.23.16.0/23
200.23.18.0/23
200.23.30.0/23
Fly-By-Night-ISP
Organization 0
Organization 7
Internet
Organization 1
ISPs-R-Us, Send me anythingwith addresses
beginning
199.31.0.0/16”
200.23.20.0/23
Organization 2
..
.
..
.
Hierarchical addressing allows efficient advertisement of routing
information:
4,Network Layer 4a-24
Hierarchical addressing,more specific
routes
ISPs-R-Us has a more specific route to Organization 1
,Send me anything
with addresses
beginning
200.23.16.0/20”
200.23.16.0/23
200.23.18.0/23
200.23.30.0/23
Fly-By-Night-ISP
Organization 0
Organization 7
Internet
Organization 1
ISPs-R-Us, Send me anythingwith addresses
beginning 199.31.0.0/16
or 200.23.18.0/23”
200.23.20.0/23
Organization 2
..
.
..
.
4,Network Layer 4a-25
IP addressing,the last word...
Q,How does an ISP get block of addresses?
A,ICANN,Internet Corporation for Assigned
Names and Numbers
? allocates addresses
? manages DNS
? assigns domain names,resolves disputes
4,Network Layer 4a-26
Getting a datagram from source to dest.
IP datagram:
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
A
B
E
misc
fields
source
IP addr
dest
IP addr data
? datagram remains
unchanged,as it travels
source to destination
? addr fields of interest
here
Dest,Net,next router Nhops
223.1.1 1
223.1.2 223.1.1.4 2
223.1.3 223.1.1.4 2
routing table in A
4,Network Layer 4a-27
Getting a datagram from source to dest.
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
Starting at A,given IP
datagram addressed to B:
? look up net,address of B
? find B is on same net,as A
? link layer will send datagram
directly to B inside link-layer
frame
? B and A are directly
connected
Dest,Net,next router Nhops
223.1.1 1
223.1.2 223.1.1.4 2
223.1.3 223.1.1.4 2
misc
fields 223.1.1.1 223.1.1.3 data
4,Network Layer 4a-28
Getting a datagram from source to dest.
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
Dest,Net,next router Nhops
223.1.1 1
223.1.2 223.1.1.4 2
223.1.3 223.1.1.4 2Starting at A,dest,E:
? look up network address of E
? E on different network
? A,E not directly attached
? routing table,next hop
router to E is 223.1.1.4
? link layer sends datagram to
router 223.1.1.4 inside link-
layer frame
? datagram arrives at 223.1.1.4
? continued…..
misc
fields 223.1.1.1 223.1.2.2 data
4,Network Layer 4a-29
Getting a datagram from source to dest.
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
Arriving at 223.1.4,
destined for 223.1.2.2
? look up network address of E
? E on same network as router’s
interface 223.1.2.9
? router,E directly attached
? link layer sends datagram to
223.1.2.2 inside link-layer
frame via interface 223.1.2.9
? datagram arrives at
223.1.2.2!!! (hooray!)
misc
fields 223.1.1.1 223.1.2.2 data network router Nhops interface223.1.1 - 1
223.1.1.4
223.1.2 - 1 223.1.2.9
223.1.3 - 1 223.1.3.27
Dest,next
Chapter 4,Network Layer
Chapter goals:
? understand principles
behind network layer
services:
? routing (path selection)
? dealing with scale
? how a router works
? advanced topics,IPv6,
multicast
? instantiation and
implementation in the
Internet
Overview:
? network layer services
? hierarchical routing
? IP
? what’s inside a router?
? IPv6
4,Network Layer 4a-2
Network layer functions
? transport packet from
sending to receiving hosts
? network layer protocols in
every host,router
three important functions:
? path determination,route
taken by packets from source
to dest,Routing algorithms
? switching,move packets from
router’s input to appropriate
router output
? call setup,some network
architectures require router
call setup along path before
data flows
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
application
transport
network
data link
physical
application
transport
network
data link
physical
4,Network Layer 4a-3
Network service model
Q,What service model
for,channel”
transporting packets
from sender to
receiver?
? guaranteed bandwidth?
? preservation of inter-packet
timing (no jitter)?
? loss-free delivery?
? in-order delivery?
? congestion feedback to
sender?
virtual circuitor
datagram?
The most important
abstraction provided
by network layer:
4,Network Layer 4a-4
Virtual circuits
? call setup,teardown for each call before data can flow
? each packet carries VC identifier (not destination host OD)
? every router on source-dest path s maintain,state” for
each passing connection
? transport-layer connection only involved two end systems
? link,router resources (bandwidth,buffers) may be
allocated to VC
? to get circuit-like perf.
,source-to-dest path behaves much like telephone
circuit”
? performance-wise
? network actions along source-to-dest path
4,Network Layer 4a-5
Virtual circuits,signaling protocols
? used to setup,maintain teardown VC
? used in ATM,frame-relay,X.25
? not used in today’s Internet
application
transport
network
data link
physical
application
transport
network
data link
physical
1,Initiate call 2,incoming call
3,Accept call4,Call connected
5,Data flow begins 6,Receive data
4,Network Layer 4a-6
Datagram networks,the Internet model
? no call setup at network layer
? routers,no state about end-to-end connections
? no network-level concept of,connection”
? packets typically routed using destination host ID
? packets between same source-dest pair may take
different paths
application
transport
network
data link
physical
application
transport
network
data link
physical
1,Send data 2,Receive data
4,Network Layer 4a-7
Network layer service models:
Network
Architecture
Internet
ATM
ATM
ATM
ATM
Service
Model
best effort
CBR
VBR
ABR
UBR
Bandwidth
none
constant
rate
guaranteed
rate
guaranteed
minimum
none
Loss
no
yes
yes
no
no
Order
no
yes
yes
yes
yes
Timing
no
yes
yes
no
no
Congestion
feedback
no (inferred
via loss)
no
congestion
no
congestion
yes
no
Guarantees?
? Internet model being extented,Intserv,Diffserv
? Chapter 6
4,Network Layer 4a-8
Datagram or VC network,why?
Internet
? data exchange among
computers
?,elastic” service,no strict
timing req,
?,smart” end systems
(computers)
? can adapt,perform
control,error recovery
? simple inside network,
complexity at,edge”
? many link types
? different characteristics
? uniform service difficult
ATM
? evolved from telephony
? human conversation,
? strict timing,reliability
requirements
? need for guaranteed
service
?,dumb” end systems
? telephones
? complexity inside
network
4,Network Layer 4a-9
Routing
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) thru
network from source to dest.
Routing protocol
A
ED
CB
F
2
2
1 3
1
1
2
5
3
5
?, good” path:
? typically means minimum
cost path
? other def’s possible
4,Network Layer 4a-10
Routing Algorithm classification
Global or decentralized
information?
Global:
? all routers have complete
topology,link cost info
?,link state” algorithms
Decentralized:
? router knows physically-
connected neighbors,link
costs to neighbors
? iterative process of
computation,exchange of
info with neighbors
?,distance vector” algorithms
Static or dynamic?
Static:
? routes change slowly over
time
Dynamic:
? routes change more quickly
? periodic update
? in response to link cost
changes
4,Network Layer 4a-11
Hierarchical Routing
scale,with 50 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
Our routing study thus far - idealization
? all routers identical
? network,flat”
… not true in practice
4,Network Layer 4a-12
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
? special routers in AS
? run intra-AS routing
protocol with all other
routers in AS
? also responsible for
routing to destinations
outside AS
? run inter-AS routing
protocol with other
gateway routers
gateway routers
4,Network Layer 4a-13
Intra-AS and Inter-AS routing
Gateways:
?perform inter-AS
routing amongst
themselves
?perform intra-AS
routers with other
routers in their
AS
inter-AS,intra-AS
routing in
gateway A.c
network layer
link layer
physical layer
a
b
b
a
aC
A
B
d
A.a
A.c
C.b B.a
c
b
c
4,Network Layer 4a-14
Intra-AS and Inter-AS routing
Host
h2a
b
b
a
aC
A
B
d c
A.a
A.c
C.b B.a
c
b
Host
h1
Intra-AS routing
within AS A
Inter-AS
routing
between
A and B
Intra-AS routing
within AS B
? We’ll examine specific inter-AS and intra-AS
Internet routing protocols shortly
4,Network Layer 4a-15
The Internet Network layer
routing
table
Host,router network layer functions:
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
Network
layer
4,Network Layer 4a-16
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 may have multiple
interfaces
? IP addresses
associated with
interface,not host,
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
223.1.1.1 = 11011111 00000001 00000001 00000001
223 1 11
4,Network Layer 4a-17
IP Addressing
? IP address:
? network part (high
order bits)
? host part (low order
bits)
? What’s a network?
(from IP address
perspective)
? device interfaces with
same network part of
IP address
? can physically reach
each other without
intervening router
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
network consisting of 3 IP networks
(for IP addresses starting with 223,
first 24 bits are network address)
LAN
4,Network Layer 4a-18
IP Addressing
How to find the
networks?
? Detach each
interface from
router,host
? create,islands of
isolated networks
223.1.1.1
223.1.1.3
223.1.1.4
223.1.2.2223.1.2.1
223.1.2.6
223.1.3.2223.1.3.1
223.1.3.27
223.1.1.2
223.1.7.0
223.1.7.1
223.1.8.0223.1.8.1
223.1.9.1
223.1.9.2
Interconnected
system consisting
of six networks
4,Network Layer 4a-19
IP Addresses
0network host
10 network host
110 network host
1110 multicast address
A
B
C
D
class
1.0.0.0 to
127.255.255.255
128.0.0.0 to
191.255.255.255
192.0.0.0 to
223.255.255.255
224.0.0.0 to
239.255.255.255
32 bits
given notion of,network”,let’s re-examine IP addresses:
,class-full” addressing:
4,Network Layer 4a-20
IP addressing,CIDR
?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
?CIDR,Classless InterDomain Routing
? network portion of address of arbitrary length
? address format,a.b.c.d/x,where x is # bits in network
portion of address
11001000 00010111 00010000 00000000
network
part
host
part
200.23.16.0/23
4,Network Layer 4a-21
IP addresses,how to get one?
Hosts (host portion):
?hard-coded by system admin in a file
?DHCP,Dynamic Host Configuration Protocol,
dynamically get address:,plug-and-play”
?host broadcasts,DHCP discover” msg
?DHCP server responds with,DHCP offer” msg
?host requests IP address:,DHCP request” msg
?DHCP server sends address:,DHCP ack” msg
4,Network Layer 4a-22
IP addresses,how to get one?
Network (network portion):
?get allocated portion of ISP’s address 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
4,Network Layer 4a-23
Hierarchical addressing,route aggregation
,Send me anything
with addresses
beginning
200.23.16.0/20”
200.23.16.0/23
200.23.18.0/23
200.23.30.0/23
Fly-By-Night-ISP
Organization 0
Organization 7
Internet
Organization 1
ISPs-R-Us, Send me anythingwith addresses
beginning
199.31.0.0/16”
200.23.20.0/23
Organization 2
..
.
..
.
Hierarchical addressing allows efficient advertisement of routing
information:
4,Network Layer 4a-24
Hierarchical addressing,more specific
routes
ISPs-R-Us has a more specific route to Organization 1
,Send me anything
with addresses
beginning
200.23.16.0/20”
200.23.16.0/23
200.23.18.0/23
200.23.30.0/23
Fly-By-Night-ISP
Organization 0
Organization 7
Internet
Organization 1
ISPs-R-Us, Send me anythingwith addresses
beginning 199.31.0.0/16
or 200.23.18.0/23”
200.23.20.0/23
Organization 2
..
.
..
.
4,Network Layer 4a-25
IP addressing,the last word...
Q,How does an ISP get block of addresses?
A,ICANN,Internet Corporation for Assigned
Names and Numbers
? allocates addresses
? manages DNS
? assigns domain names,resolves disputes
4,Network Layer 4a-26
Getting a datagram from source to dest.
IP datagram:
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
A
B
E
misc
fields
source
IP addr
dest
IP addr data
? datagram remains
unchanged,as it travels
source to destination
? addr fields of interest
here
Dest,Net,next router Nhops
223.1.1 1
223.1.2 223.1.1.4 2
223.1.3 223.1.1.4 2
routing table in A
4,Network Layer 4a-27
Getting a datagram from source to dest.
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
Starting at A,given IP
datagram addressed to B:
? look up net,address of B
? find B is on same net,as A
? link layer will send datagram
directly to B inside link-layer
frame
? B and A are directly
connected
Dest,Net,next router Nhops
223.1.1 1
223.1.2 223.1.1.4 2
223.1.3 223.1.1.4 2
misc
fields 223.1.1.1 223.1.1.3 data
4,Network Layer 4a-28
Getting a datagram from source to dest.
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
Dest,Net,next router Nhops
223.1.1 1
223.1.2 223.1.1.4 2
223.1.3 223.1.1.4 2Starting at A,dest,E:
? look up network address of E
? E on different network
? A,E not directly attached
? routing table,next hop
router to E is 223.1.1.4
? link layer sends datagram to
router 223.1.1.4 inside link-
layer frame
? datagram arrives at 223.1.1.4
? continued…..
misc
fields 223.1.1.1 223.1.2.2 data
4,Network Layer 4a-29
Getting a datagram from source to dest.
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
Arriving at 223.1.4,
destined for 223.1.2.2
? look up network address of E
? E on same network as router’s
interface 223.1.2.9
? router,E directly attached
? link layer sends datagram to
223.1.2.2 inside link-layer
frame via interface 223.1.2.9
? datagram arrives at
223.1.2.2!!! (hooray!)
misc
fields 223.1.1.1 223.1.2.2 data network router Nhops interface223.1.1 - 1
223.1.1.4
223.1.2 - 1 223.1.2.9
223.1.3 - 1 223.1.3.27
Dest,next
