extending networks. three levels of extension physical layer –repeaters link layer –bridges...
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Extending Networks
Three Levels of Extension
• Physical Layer– Repeaters
• Link Layer– Bridges– Switches
• Network– Routers: “Connecting networks”
(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
(Link Layer) Bridges
• Filter network component– Back learning
• Internetworking component (between LANs of different standards)
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?
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
Network Layer
Read Tanenbaum (Chapter 5)
Where in the OSI Reference Model ?
Transport Layer
Network Layer
Link Layer
Physical Layer
Session Layer
Fundamental Functions of a Network Layer
• Addressing
• Routing
• Congestion control– Note that not all network protocols provide
congestion control
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.
Connectionless vs Connection-orientedTanenbaum Figure 5.4
• Circuit setup
• Addressing
• State information
• Routing
• Effect of routing failures
• Congestion control
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
Routing Policy: Updating the Routing Tables
• Manual
• Using routing Daemons such as:– RIP– OSPF– EGP– BGP
Routing Mechanism
• Takes the decision how to route
• For example, IP protocol implements in each Internet host a mechanism to route
Desirable Properties of Routing
• Correct
• Simple
• Fair
• Robust
• STABLE
• Optimal
Routing Mechanisms
• Flooding
• Hot-potatoe
• Shortest path
• Fixed routing
• Dynamic routing
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)
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 ….
Distance Vector (Problems!!!)
• When ? – Links go down
• What kind of problem ?– Count-to-infinity– Example
• Solution– path vector
A B C
1 1
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 ….
LANs
Internal and External Protocols
Internet Backbone
Autonomous Systems
Autonomous Systems
Autonomous Systems
Autonomous Systems
Exterior GatewayProtocol
Interior GatewayProtocol
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.
OSPF (RFC 1247)(Open Shortest Path First)
• Interior gateway protocol
• Link state protocol
• Uses directly IP (while RIP uses UDP)
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)
The Internet Protocol (IP) RFC 791
Read 5.5
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
Fundamental Idea of IP
• Routes between Networks, not between hosts
• This allows shorter routing tables
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 ….)
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
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
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
Subnetting
• Why to do subnetting ?
• How is it done ?
• Notion of subnet mask
Subnetting (RFC 950)
1 netid hostid14 bits 16 bits
0Class B
netid hostidn bits m bits
Example : netid hostid5 bits 11 bits
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
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
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
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)
Extensions to IP
• IPv6
• Mobile IP
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.