Download Introduction - Ceng Anadolu

Network Layer:
Host-to-Host Communication
1
Network Layer: Motivation
•
Can we built a global network such as Internet
by extending LAN segments using bridges?
–
•
No! Bridged networks do not scale
4 problems
1. We can only bridge certain link-layer technologies
together
•
Link layers to be bridged must have similar MAC address
structure
2. Bridge table explosion
•
Bridges use MAC addresses for forwarding and MAC
addresses are flat, i.e., not hierarchical
–
the bridge table needs to have an entry per host in the
network bridge table explosion!!!
2
Network Layer: Motivation
3. Robustness
•
Change of network topology requires a new spanning tree
computation
4. Link-layer broadcast storms
–
–
–
–
–
Notice that a bridged network is still a single LAN!
A link-layer broadcast packet must still be delivered to ALL
hosts in the network.
Can you imagine receiving a link-layer broadcast packet from
a host 5000 km away at your host?
Bottom Line: Bridged/Switched LANs don’t
scale!
What’s the solution? --- Next
3
How to achieve scalable growth?
• Divide the network into separate LANs that are NOT part of the
same “LL broadcast” domain
• Connect the LANs using “routers”
– Notice that we CANNOT use bridges to connect separate LANs as
bridged LANs form a single LL broadcast domain, which is what we are
trying to avoid to achieve scalability
Network Core
C
A
Bridge
R1
B
D
E
R4
R2
R3
Router
Separate LANs
Each LAN is a separate
LL Broadcast
Domain
Switch
G
Hub
F
A collision
domain
within a LAN
H
Hub
Hub
I
M
Hub
N
O
K
L
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Communication Issue
• How do two hosts on separate LANs, e.g., A and E, communicate?
• Recall that using the Link Layer (LL), only hosts that are neighbors,
that is, hosts that are within the same LAN can communicate.
• Solution: Design a new layer, called the network layer, that would
provide host-to-host packet delivery for hosts that are in separate
LANs
Network Core
C
A
Bridge
R1
B
D
R4
R2
R3
Router
F
Separate LANs
Each LAN is a separate
LL Broadcast
Domain
Switch
G
Hub
E
A collision
domain
within a LAN
H
K
Hub
Hub
I
M
Hub
N
L
O
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Network Layer: Responsibility
• Responsibility: Deliver a packet from a sending host,
e.g., A, to one (unicast) or more (multicast) receiving
host(s)
C
Network
link
physical
Network Core
link
physical
Bridge
Network
link
physical
R1
A
R2
Network
link
physical
R4
Network
link
physical
Router
Network
link
physical
B
Network
link
physical
R3
D
Network
link
physical
Hub
E
F
link
physical
Switch
G
Network
link
physical
K
Hub
Hub
I
M
H
Network
link
physical
L
Hub
Network
link
physical
N
O
Network
link
physical6
Network Layer: Issues
• Addressing: Each host/router interface must have a
GLOBALLY unique network address
– Recall that within a LAN, this was achieved by globally unique
MAC addresses at the LL
– We need a similar GLOBALLY unique address at the network
layer – e.g., IP address
• Packet Forwarding: How does a packet sent from host A
to host E make it to host E?
– Recall that within a LAN, this was achieved transparently by
bridges/switches, with each bridge building a forwarding table
with respect to MAC addresses on the fly
– We need a similar “path determination” algorithm with respect
to the destination network address.
• Called path determination or the routing problem.
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Network Layer Service Model
• Every Network Layer must export a service
model (interface) to the layers on top of it
–
–
–
–
–
–
guaranteed bandwidth?
preservation of inter-packet timing (no jitter)?
loss-free delivery: reliable vs. unreliable?
in-order delivery: ordered vs. unordered?
unicast vs. multicast delivery?
congestion feedback to sender?
8
Network Layer Abstraction
The question that a network designer must
answer to satisfy the chosen service model: Will
the network be based on
virtual circuits
or
datagrams?
? ??
–
That is, should we establish an end-to-end path
through the network for the packets to flow?
•
•
Yes: Virtual-Circuit Networks (X.25, Frame-Relay, ATM)
No: Datagram Networks (the Internet)
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Virtual Circuits Networks: Signaling
A
Network
link
physical
R2
Network
R3link
physical
Network
link
physical
R6
Network
link
physical
R1
B
R5
Network
link
physical
Network
link
physical
R4
R7
C
R10
Network
link
physical
R9
Network
link
physical
Network
link
physical
D
Network
link
physical
R8
• Virtual Circuit Networks (e.g., X.25, Frame Relay, ATM)
– Establish a path along which the packets will flow between the
source and the destination. How?
• Use a signaling (virtual circuit establishment) protocol
• Ex: B tells its router (R1) that it wants to talk to C
• The call establishment message is forwarded by the routers in the
network until it reaches C. Then a reply comes back from C to B.
– Path established at call setup time remains fixed during packet
exchange
– Routers maintain state information for ongoing connections 10
Virtual Circuits Networks: Forwarding
A
45
1
12
R5
R2
R3
3
2 53
2 22
1
R4
R1
43
2
9
69
66
R9
R7
B
D
77
R8
VC table at R1:
–
–
–
3
R10
R6
C
VC table at R2:
Incoming
Interface
Incoming
VC #
Outgoing
interface
Outgoing
VC #
Incoming
Interface
Incoming
VC #
Outgoing
interface
Outgoing
VC #
1
12
2
22
1
45
3
53
2
38
1
19
3
8
1
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each packet carries tag (virtual circuit ID), which determines next hop
Path established at call setup time remains fixed during packet exchange
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Routers maintain state information for ongoing connections
Datagram Networks: Idea
A
Network
link
physical
Network
link
physical
D
R3
D
C
B
R2
Network
link
physical
R1
C
D
C
D
R5
R4
Network
link
physical
C
R10
C
R6
D
C
R7
D
Network
link
physical
D
C
C
D
C
C
R9
Network
link
physical
D
D
Network
link
physical
R8
• Datagram networks (e.g. the Internet):
• No call establishment before data exchange
• Simply put the destination address on top of the
packet and submit it to the network for delivery
• Similar to postal service
12
Datagram Networks: Forwarding
A
R2
1
D
2
1
C
3
D
R3
R1
B
C
D
2 C
R4
R5
C
R6
D
C
R10
C
C
C
C
D
R9
D
R7
R8
Forwarding table at R1:
Forwarding table at R2:
Destination
Address
Outgoing
interface
Next
Hop
Destination
Address
Outgoing
interface
Next
Hop
B
1
B
A
1
A
C
2
R3
C
3
R3
D
2
R3
D
3
R3
– Destination address is written on top of a packet and it is simply
submitted to the network for delivery (like postal service)
– Routers look at destination address in packet to determine the next hop
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– No connection-state information needed in the routers
– Routes may change during session
VC vs. Datagram network: why?
X.25, Frame Relay, ATM
(Virtual Circuit)
• evolved from telephony
• human conversation:
– strict timing, reliability
requirements
– need for guaranteed
service
• “dumb” end systems
– telephones
– complexity inside
network
Internet (Datagram)
• 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
14
Network layer service models
Network
Architecture
Internet
Service
Model
Guarantees ?
Congestion
Bandwidth Loss Order Timing feedback
best effort none
ATM
CBR
ATM
VBR
ATM
ABR
ATM
UBR
constant
rate
guaranteed
rate
guaranteed
minimum
none
no
no
no
yes
yes
yes
yes
yes
yes
no
yes
no
no (inferred
via loss)
no
congestion
no
congestion
yes
no
yes
no
no
• Internet model being extended: Intserv, Diffserv
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