1. Tree Structured Internet(c. 1990)
Stanford
NSFNET backbone
ISU
BARRNET
MidNet …
regional W
estnet
regional
regional
Berkeley P
ARC
UNL
UNM KU
NCAR UA
Autonomous Systems
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Internetworking

2. The Global Internet We know How to internetwork(heterogeneous)
IP scalability
Routers don’t need to know about every host
Need to know about all the networks
Ten of thousands of networks
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3. Scaling Two related scaling issues Scalability of routing
Minimize # of network numbers in routing protocols
Minimize # of entries in the routing tables
Address utilization
Minimize the usage rate of IP addresses
Classless routing
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4. Making Routing Scale Need efficient Hierarchical Addressing
Reduce the # of network numbers assigned
Inefficient use of Hierarchical Address Space
class C with 2 hosts (2/254 = 0.78% efficient)
class B with 256 hosts (256/65534 = 0.39% efficient) 7 24
(a)
Network
Host 14 16
Any network with
> 255 hosts
(b) 1
Network
Host 21 8
(c) 1 1
Network
Host
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5. Subnetting Split a single IP network number into several subnets
Add another level to address/routing hierarchy: subnet
Subnet masks enable physical networks to share the same subnet number
Subnets visible only within site, world sees single network
Network number
Host number
Class B address
Subnet mask ( )
Subnetted address
Host ID
Subnet ID
Configure each node with a
subnet mask
Hosts now configured with
IP address and subnet address
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Internetworking

6. Subnet Example Forwarding table at router R1
Subnet mask:
Subnet number: H1 R1
Subnet number: R2 H2
Subnet mask:
Subnet number: H3
Forwarding table at router R1
Subnet Number Subnet Mask Next Hop
interface 0
interface 1 R2
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Internetworking

7. Datagram Forwarding Algorithm
D = destination IP address
for each entry (SubnetNum, SubnetMask, NextHop)
D1 = SubnetMask & D
if D1 = SubnetNum
if NextHop is an interface
deliver datagram directly to D
else
deliver datagram to NextHop
Use a default router if nothing matches
Not necessary for all 1s in subnet mask to be contiguous
Can put multiple subnets on one physical network
Subnets not visible from the rest of the Internet
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Internetworking

8. Chapter 4 “There is more than one network”
Point-to-point links
Shared media
Switches
Goal – connect these things together
Heterogeneity
Scale
Routing(connection of nodes)
Addressing
Introduce the Internet Protocol(IP)
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9. Internet, internet or internetwork
Concatenation of Networks
Protocol Stack R2 R1 H4 H5 H3 H2 H1
Network 2 (Ethernet)
Network 1 (Ethernet) H6
Network 3 (FDDI)
Network 4
(point-to-point) H7 R3 H8
Routers/Gateways R1
ETH
FDDI IP
TCP R2
PPP R3 H1 H8
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10. Service Model the services that are to be provided from host-to-host
IP provides Datagram data delivery service
Connectionless (datagram-based) / full dest
Best-effort delivery (unreliable service)
packets are lost
packets are delivered out of order
duplicate copies of a packet are delivered
packets can be delayed for a long time
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11. Datagram Datagram format Version-IP e.g. IPv4, IPv6 Hlen-Header length
TOS-Type Of Service
TTL-Time To Live
Protocol-UDP/TCP V
ersion
HLen
TOS
Length
Ident
Flags
Offset
TTL
Protocol
Checksum
SourceAddr
DestinationAddr
Options (variable)
Pad
(variable) 4 8 16 19 31
Data
Assigned at
32-bit
Boundary
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Internetworking

12. Fragmentation and Reassembly
Each network has some
Maximum Transmission Unit(MTU) – largest IP datagram that it can carry in a frame.
Strategy
fragment when necessary (MTU < Datagram)
try to avoid fragmentation at source host
re-fragmentation is possible when travel to a different network
fragments are self-contained datagrams
delay reassembly until destination host
do not recover from lost fragments
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Internetworking

13. Global Addresses
All hosts attached to the same network have the same network field
The host field unique
Routers have two interfaces R2 R1 H4 H5 H3 H2 H1
Network 2 (Ethernet)
Network 1 (Ethernet) H6
Network 3 (FDDI)
Network 4
(point-to-point) H7 R3 H8
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14. Datagram Forwarding How do IP routers forward datagrams?
Strategy (main points)
every datagram contains destination’s address
if directly connected to destination network, then forward to host(i.e. same network number)
if not directly connected to destination network, then forward to some router(next hop router-host selected)
Hosts = 1 interface, routers >= 2 interfaces
forwarding table maps network number into next hop
each host has a default router
each router maintains a forwarding table
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Internetworking

15. Datagram Forwarding Example (R2) Network Number Next Hop 1 1 R3
Network 2 (Ethernet)
Network 1 (Ethernet) H6
Network 3 (FDDI)
Network 4
(point-to-point) H7 R3 H8
Example (R2) Network Number Next Hop
1 R3
2 R1
3 interface 1
interface 0 1
Routers/Gateways
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16. Bridges, Switches, and Routers
Confusion?
Bridges
Link-level to forward frames from one link to another to create extended LANs
Switches
Network-level to forward packets from one link to another to create packet-switched networks
Routers
Internet-level to forward datagrams from network to another
Bridges/Switches (above the physical below the internet)
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17. IP Mapping IP datagrams contain IP addresses
Physical interface only understands the addressing specific to its link-level(i.e. Ethernet, Token-Ring, etc)
Problem – how to put the two together?
Solution – have each host maintain a table of address pairs.
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Internetworking

18. ARP - Address Translation
Map IP addresses into physical addresses
destination host
next hop router
ARP cache or ARP table
ARP (Address Resolution Protocol)
table of IP to physical address bindings dynamically created
broadcast request if IP address not in table
target machine responds with its link-level and physical address
table entries are discarded if not refreshed
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19. Routing For VC’s (one-off setup)
For datagrams, every packet must be routed
Regardless, switches and routers must
Look at destination port
Determine the output port best suited transmission
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20. Routing & Forwarding
Routing tables are constructed as precursors to forwarding table construction
Routing algorithms
Map Ntwk #s to NHs
Optimized for changes in topology
Forwarding tables
Ntwk #s, Output interfaces, MAC address of NH
Optimized to look up Ntwk #s
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21. Routing & Forwarding Routing Table Network NH (NextHop)
Forwarding table
Network Int MAC Address
10 if0 8:0:2 B:E 4:B:1:2
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22. Routing Domain What is a routing domain?
it is a matter of graph theory
hosts, switches, routers, or networks
network links w/associated costs
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23. Routing Protocol How to store the shortest paths?
calculate and store in non-volatile memory?
Nope!
Node/link failures not handled
Node/link additions not handled
Edge costs do not change
Run routing protocols among nodes instead
Distance vector and link state
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Internetworking

24. Distance Vector(Bellman-Ford)
Each node maintains a set of triples
(Destination, Cost, NextHop)
Exchange updates directly connected neighbors
periodically (on the order of several seconds)
whenever table changes (called triggered update)
Each update is a list of pairs:
(Destination, Cost)
Update local table if receive a “better” route
smaller cost
came from next-hop
Refresh existing routes; delete if they time out
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Internetworking

25. Distance Vector Destination Cost NextHop Destination Cost NextHop
B B
C C D
E E
F F G
Destination Cost NextHop
B B
C C
D C
E E
F F
G F
With no topological changes
Only a few exchanges for completion
Realization of routing tables at nodes is called convergence
The beauty - no one node is responsible for being the central authority
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Internetworking

26. Routing Updates Two circumstances under which a node sends an update
Periodic updates (I’m alive!)
Triggered updates (recipient)
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27. Routing Protocols Interior Gateway Protocols(IGP) Distance Vector
Updates based upon directly connected neighbors
Destination, cost, and NH (NextHop)
Count-to-Infinity
RIP
Link State
Updates communicated through network
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28. Routing Information Protocol(RIP)
Probably the most used routing protocol today
Example distance vector protocol
RIP send advertisements every 30 seconds
Routers also send updates when it receives an update from another router
All link costs = 1
Minimum hop route
Valid distance = 1 16()
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29. Link State Routing Strategy Link State Packet (LSP)
send to all nodes (not just neighbors) information about directly connected links (but not entire routing table)
Reliable flooding of Link State Packet (LSP)
Each node then has complete topology of network
Apply any shortest path algorithm (e.g. Dijkstra) to find the shortest route
Link State Packet (LSP)
id of the node that created the LSP
cost of link to each directly connected neighbor
sequence number (SEQNO)
time-to-live (TTL) for this packet
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30. OSPF Open Shortest Path First (OSPF) Most commonly used type of LSP
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31. Distance Vector vs. Link State
Each node talks to
only its directly connected neighbors
all other nodes via LSP(Flooding)
It relays
everything it knows – distance to all other nodes
only the costs of its directly connected links
CS565
Internetworking