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Extending Networks
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Three Levels of Extension
• Physical Layer– Repeaters
• Link Layer– Bridges– Switches
• Network– Routers: “Connecting networks”
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(Physical Layer) 5-4-3 Rule
• For IEEE 802.3 LANs, there is a limit on the length of a segment, how to extend the limit?– Adding repeaters (Hubs) while respecting the
5-4-3 rule:• Any path should traverse at most 5 segments• Any path should traverse at most 4 repeaters (hubs)• Any path should traverse at most 3 populated
segments
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(Link Layer) Bridges
• Filter network component– Back learning
• Internetworking component (between LANs of different standards)
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DIX and 802.3 Frames
Preamble Dest. Source. Type Type CRC
DIX
Preamble Dest. Source. Length Type CRC
IEEE 802.3
8 6 6 2 Up to 1500 4
8 6 6 2 Up to 1500 4
How to make the difference?
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Connecting Networks
• LANs are connected with point-to-point links
• Packets are “routed” using another level of addresses other than MAC addresses
• Paths may be multihop
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Network Layer
Read Tanenbaum (Chapter 5)
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Where in the OSI Reference Model ?
Transport Layer
Network Layer
Link Layer
Physical Layer
Session Layer
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Fundamental Functions of a Network Layer
• Addressing
• Routing
• Congestion control– Note that not all network protocols provide
congestion control
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Network Layer Protocols : two families
• Connectionless– Each piece of information is sent as an
independent entity. No state information is kept in hosts or routers
• Connection oriented– There exists a virtual circuit over which all
pieces of information will transit.
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Connectionless vs Connection-orientedTanenbaum Figure 5.4
• Circuit setup
• Addressing
• State information
• Routing
• Effect of routing failures
• Congestion control
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Routing
• Routing policy: – updates the routing table
• Routing mechanism:– decides how to route a packet
depending on some policy (i.e, chooses the output line)
Routing structure
modifies
reads
Quite elementary
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Routing Policy: Updating the Routing Tables
• Manual
• Using routing Daemons such as:– RIP– OSPF– EGP– BGP
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Routing Mechanism
• Takes the decision how to route
• For example, IP protocol implements in each Internet host a mechanism to route
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Desirable Properties of Routing
• Correct
• Simple
• Fair
• Robust
• STABLE
• Optimal
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Routing Mechanisms
• Flooding
• Hot-potatoe
• Shortest path
• Fixed routing
• Dynamic routing
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Overview
• Three big classes of routing policies– Distance vector routing (DVR)– Link state routing (LSR)– Hierarchical Routing (HR)
• Application to Internet– RIP is a DVR– OSPF is an LSR– EGP, BGP (DVR)
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Distance Vector
• A node – tells its neighbors only– its distance to EVERY NODE in the network
• Example:– Initial A(0,1,4,inf),– B(1,0,1,1)– C(4,1,0,2)– D(inf,1,2,0)
A
B
D
C
1 1
1
4 4
A receives update from B ….
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Distance Vector (Problems!!!)
• When ? – Links go down
• What kind of problem ?– Count-to-infinity– Example
• Solution– path vector
A B C
1 1
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Link-State Routing
• A node – tells its EVERY node– its distance to its NEIGHBORS
• How ?– Send link-state packets (LSP)– using controlled flooding– Use Dijkstra’s algorithm
A
B
D
C
1 1
1
4 4
A receives update from B ….
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LANs
Internal and External Protocols
Internet Backbone
Autonomous Systems
Autonomous Systems
Autonomous Systems
Autonomous Systems
Exterior GatewayProtocol
Interior GatewayProtocol
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RIP (RFC 1058)(Routing Information Protocol)
• Interior gateway protocol
• Distance vector protocol
• Uses split horizon to avoid count-to-infinity (Does not advertise a cost to a neighbor if it is a next hop for that destination)
• Exchange each 30s
• Time-out after 180 s.
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OSPF (RFC 1247)(Open Shortest Path First)
• Interior gateway protocol
• Link state protocol
• Uses directly IP (while RIP uses UDP)
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BGP (RFC 1267)(Exterior Gateway Protocol)
• Exterior gateway protocol
• (Exception: BGP uses TCP !!!!)
• Path Vector protocol (+ policy attributes)
• Topology may be loop-free (BGP guarantees loop-freeness)
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The Internet Protocol (IP) RFC 791
Read 5.5
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A Connectionless Network Layer: the Internet Protocol (IP)
• The Internet Protocol is found on every:– host that is connected to Internet– router on a LAN connected to the Internet– router on the backbone
Application
Transport
Network IP
Link Layer
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Fundamental Idea of IP
• Routes between Networks, not between hosts
• This allows shorter routing tables
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IP Header (Figure 3.1, p. 34)
310 15 16V HL TOS Total Length
16-bit identification Flgs13-bit frag. offset
TTL Protocol 16-bit Hdr Checksum
32-bit source IP address
32-bit destination IP address
Options (Variable 0 ---> ??)
Data (TCP segment, or UDP Dtg,or ICMP ….)
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Internet Addresses
• Class A
• Class B
• Class C
• Class D
• Class E
0 netid hostid7bits 24 bits
1 netid hostid14 bits 16 bits
0
1 netid hostid21 bits 8 bits
1 0
1 Multicast group ID28 bits
1 1 0
1 Future use27 bits
1 1 1 0
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Special Internet Addresses
• This host
• Local host
• Local broadcast
• Remote
• Loopback 127.X.X.X
000000000000…..00000000000000000032 bits
00000….0000000 hostid
1111111111111…..11111111111111111
11111111111…..111111111NetID
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Internet Addresses (Cont’d)
• Unicast addresses (Classes A,B, and C)
• Multicast addresses (Class D)
• Dotted notation : the 32-address is divided in 4 groups of 8 bits (byte, octet). Each byte is expressed in base 10 separated by dots
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Subnetting
• Why to do subnetting ?
• How is it done ?
• Notion of subnet mask
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Subnetting (RFC 950)
1 netid hostid14 bits 16 bits
0Class B
netid hostidn bits m bits
Example : netid hostid5 bits 11 bits
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Subnetmask
Idea : IP1 IP2& (Bitwise AND) &
Subnetmask Subnetmask
= R1 = R2
R1 = R2 if IP1 and IP2 are on the same subnet
R1 # R2 if IP1 and IP2 are on different subnets
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IP Routing
• Based on a routing table with entries having– Destination IP addr (host ID or net ID)– Next hop router IP address– flags (hostid or netid, next hop or connected
interface…)– network interface
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IP Routing Cont’d
• 1) Try to find a complete IP address match in the routing table
• 2) If 1) fails, then try to find a match with network id
• if 1) and 2) fail search for default router
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Some IP Helpers
• ARP (Address Resolution Protocol) : RFC 826
• RARP : Reverse Address Protocol (RFC 903)
• BOOTP (RFC 951, 1048, 1084) replaced by
• DHCP (RFC 1541)
• ICMP (RFC 792)
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Extensions to IP
• IPv6
• Mobile IP
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Conclusion
• You must know:– The functions of the network layer– the difference between connectionless and
connection oriented network protocols (what they can do and cannot do)
– IP protocol and helpers (ICMP, DHCP, ARP)– the general features introduced by IPv6– globally what is Mobile IP.