Download How Networks Differ

ELEC3030 (EL336) Computer Networks
S Chen
How Networks Differ
• Differences that can occur at network layer, which makes internetworking difficult:
• It is impossible to resolve all differences, and the solution is to take a simple
approach (as in the Internet)
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ELEC3030 (EL336) Computer Networks
S Chen
Internetworking
• Problem: Networks with different protocol stacks → how to let them talk to each other?
• Nonsolution: Why not enforce all networks to run same protocol stack? → ask for troubles ! and
this is effectively saying no progress is allowed
• Solution: Construct some gateways that connect different kinds of networks
– Repeaters or hubs at physical layer:
Mainframe
operate on bits, do not understand
protocols, just regenerate signals
SNA network
ATM network
– Bridges or switches at data link
layer: operate on frames, examine
MAC addresses, do minor protocol
Router
Switch
translation, e.g. Ethernet to 802.11
FDDI
– Multiprotocol router at network
Notebook
ring
computer
layer: operate on packets, translate Connection
Ethernet
802.11
between different packet formats, to Internet
possibly split up packets
– Transport gateways at transport layer: coupling byte streams in different networks, interface
between different transport connections
– Application gateways at application layer: translate message semantics between OSI and TCP/IP
networks
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ELEC3030 (EL336) Computer Networks
S Chen
Internetworking at Network Layer
• With a router, packet is extracted from frame and address in packet is used to
decide where to send it → so router has to understand network protocols
Legend
Header
Packet
Trailer
Switch
Router
S
D
LAN 1
LAN 2
(a)
S
D
LAN 2
LAN 1
(b)
• Switch or bridge do not understand network protocols. In a switch (or bridge)
entire frame is transported on basis of its MAC address
• Two basic internetworking approaches: connection-oriented concatenation of
virtual-circuit subnets, and connectionless datagram internet style
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ELEC3030 (EL336) Computer Networks
S Chen
Concatenated Virtual Circuits
• A sequence of virtual circuits is set up from source through gateways (multiprotocol
routers) to destination
SNA
Multiprotocol
router
1
X. 25
M
M
ATM
Router
OSI
M
M
2
Host
End-to-end concatenated
virtual circuits
Each gateway maintain tables telling which virtual circuits pass through it, where
they are to be routed and what new virtual circuit number is
• If one of constituent subnets does not support virtual circuits, simple concatenation
becomes hard
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ELEC3030 (EL336) Computer Networks
S Chen
Using Datagram
• This connectionless internetworking approach is similar to datagram service in
subnets
1
M
M
Packets travel individually
and can take different routes
Router
M
M
2
Multiprotocol
router
Host
• Main problem: Addressing – different networks use completely different address
systems. How do you address a host in a IP subnet when you are in a SNA subnet?
• Solution: Don’t solve it, instead consider possibly some universal network protocol,
e.g. IP, which can be carried through many networks
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ELEC3030 (EL336) Computer Networks
S Chen
Tunneling
• Solving general internetworking problem is extremely hard, but thing becomes easier
if source and destination are on same type of subnet – tunnelling can be used
Acts like a serial line
Ethernet in Paris
Multiprotocol
router
Tunnel
Ethernet in London
WAN
1
2
Header
IP
Ethernet frame
IP
IP packet inside payload
field of the WAN packet
IP
Ethernet frame
• Multiprotocol router simply places source host’s packet inside payload of WAN
packet and sends through
• Essentially, what between two hosts’ subnets, WAN, can be seen as a “tunnel”
extending from one multiprotocol router to the other
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ELEC3030 (EL336) Computer Networks
S Chen
Internetworking Routing
A
• Routing through an internetwork is
similar to routing within a subnet
but with added complications
• Two-level routing algorithm needed
2
A
1
C
B
B
3
(Multiprotocol)
router
D
C
D
E
Network
4
– Interior gateway protocol used
(a)
within each network
– Exterior gateway protocol used between networks
F
5
E
F
(b)
• Thus, one has intranetwork routing and internetwork routing
• Consider an internet packet starts out from its LAN addressed to the local multiprotocol routine (in
the MAC layer header)
– After the packet has reached the local router, the network layer decides which mutliprotocol
router to forward the packet to using its own routing table
– If that router can be reached using the packet’s native network protocol, the packet is directly
forward there
– Otherwise, it is tunnelled there (see previous slide), encapsulated in the protocol required by the
intervening network
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ELEC3030 (EL336) Computer Networks
S Chen
Fragmentation
• Different networks have different maximum packet sizes, and solution is to allow gateway to split a
packet into smaller ones → fragmentation
Network 1
Network 2
Packet
G1
G2
G1 fragments
a large packet
G3
G2
reassembles
the fragments
G4
G3 fragments
again
G4
reassembles
again
(a)
Packet
G1
G1 fragments
a large packet
G2
G3
G4
The fragments are not reassembled
until the final destination (a host) is reached
(b)
• Transparent fragmentation: “small-packet” network splits packet and recombines fragments back.
Others do not need to know
• Nontransparent fragmentation: destination host recombines fragments back
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ELEC3030 (EL336) Computer Networks
S Chen
Fragmentation → Reassembly
• A fragment may be fragmented again by successive intermediate networks → when a fragment
arrives at destination, we have to know exactly where it fits into original packet
• Solution: requires two sequence fields in header, original packet number and fragment number
Number of the first elementary fragment in this packet
Packet
number
27
End of
packet bit
0
1
A
B
1 byte
C
D
E
F
G
H
I
27
8
J
Header
(a)
27
0
0
A
B
C
D
E
F
G
H
Header
1
I
J
Header
(b)
27
0
Header
0
A
B
C
D
E
27
5
Header
0
F
G
H
27
8
1
I
J
Header
(c)
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ELEC3030 (EL336) Computer Networks
S Chen
Firewalls
• Sometimes it is better not all your computers can be easily reached by others, or vice versa →
firewall
Application
gateway
Backbone
Packet
filtering
router
Corporate
network
Packet
filtering
router
Connections
to outside
networks
Security
perimeter
Inside
LAN
Outside
LAN
Firewall
• Packet filter: standard router with extra functionality to inspect packets → Those packets meeting
some criterion are forwarded normally, and those failed are dropped
• Application gateway: operates at application level
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ELEC3030 (EL336) Computer Networks
S Chen
Summary
• Internetworking in general
• Internetworking at network layer
Concatenated virtual circuits
Connectionless internetworking
Tunnelling
Internetwork routing
• Transparent and nontransparent fragmentation, reassembly
• Firewalls
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