1. Introduction to Communication Networks
Lecture 5-6
LAN continuation + Network Layer
Introduction to Communication Networks 2/2006

2. Extending Local Area Networks
Motivation
Distance limitation
Performance degradation as number of users increases
Hub
Repeater
Bridge
Switch
Introduction to Communication Networks 2/2006

3. Hub Hardware device Creates one LAN segment
Each Ethernet frame received on one port will be forwarded to all other ports
Logically operates on signals and propagates each incoming signal to all connections
Creates one collision domain
Introduction to Communication Networks 2/2006

4. Repeaters R Hardware device Connects two LAN segments Repeater
Ethernet segment
Ethernet segment R
Direct connection
Hardware device
Connects two LAN segments
Copies signal from one segment to the other
Amplifies signals from one segment and sends to the other
Propagates noise and collisions
Operates in two directions simultaneously
Repeater
Introduction to Communication Networks 2/2006

5. Bridges Hardware, Link Level device Connects two LAN segments
Forwards frames
stores and forwards Ethernet frames
examines frame header and selectively forwards frame based on MAC destination address
when frame is to be forwarded on segment, uses CSMA/CD to access segment
Does not forward noise or collisions
Learns addresses and filters
Allows independent transmission
transparent
hosts are unaware of presence of bridges
plug-and-play, self-learning
bridges do not need to be configured
Introduction to Communication Networks 2/2006

6. Bridges: traffic isolation
Bridge installation breaks LAN into LAN segments
Bridges filter packets:
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
bridge
collision
domain
= hub
= host
LAN segment
Introduction to Communication Networks 2/2006

7. Bridge Learning Algorithm
Listen in promiscuous mode (all frames are copied & analyzed)
Watch source address in incoming frames
Make list of computers on each segment
Only forward if necessary
Always forward broadcast/multicast
Introduction to Communication Networks 2/2006

8. Bridge Learning cont. A bridge has a bridge table
entry in bridge table:
(Node LAN Address, Bridge Interface, Time Stamp)
stale entries in table dropped (TTL can be 60 min)
bridges learn which hosts can be reached through which interfaces
when frame received, bridge “learns” location of sender: incoming LAN segment
records sender/location pair in bridge table
Introduction to Communication Networks 2/2006

9. Filtering/Forwarding
When bridge receives a frame:
index bridge table using MAC dest address
if entry found for destination then{
if dest on segment from which frame arrived then drop the frame
else forward the frame on interface indicated }
else floodforward on all but the interface on which the frame arrived
Introduction to Communication Networks 2/2006

10. Bridge example
Suppose C sends frame to D and D replies back with frame to C.
Bridge receives frame from C
notes in bridge table that C is on interface 1
because D is not in table, bridge sends frame into interfaces 2 and 3
frame received by D
Introduction to Communication Networks 2/2006

11. Bridge example D generates frame for C, sends bridge receives frame
notes in bridge table that D is on interface 2
bridge knows C is on interface 1, so selectively forwards frame to interface 1
Introduction to Communication Networks 2/2006

12. Distributed Spanning Tree Algorithm
Introduction to Communication Networks 2/2006

13. Spanning Tree Algorithm
The network of bridges is a graph.
Usually built in hierarchical way
If bridges at top hierarchy fails -> LAN can become disconnected
Therefore LANs are usually connected in more than one point (redundancy)
Can cause same information to be copied many times over the LAN  can even crash the network.
The Spanning Tree Protocol finds a a subgraph that spans all the vertices without loops.
Spanning => all LAN segments are included.
Tree => the topology has no loops.
The distributed protocol runs:
To determine which bridge is the root of the tree, and
Each bridge turns off ports that are not part of the tree.
Introduction to Communication Networks 2/2006

14. Example Spanning Tree B8 B3 B5 Protocol operation: B7 B2 B1 B6 B4
Picks a root
For each LAN, picks a designated bridge that is closest to the root.
All bridges on a LAN send packets towards the root via the designated bridge. B8
Introduction to Communication Networks 2/2006

15. Example Spanning Tree Spanning Tree: Root B8 B3 B5 B7 B2 B1 B6 B4
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16. Switch High performance multiple interface bridge!
Physically similar to a hub
Logically similar to a bridge
Operates on packets
Understands addresses
Only forwards when necessary
More fancy forwarding methods
Permits separate pairs of computers to communicate at the same time (full duplex)
Introduction to Communication Networks 2/2006

17. Typical Configuration
Introduction to Communication Networks 2/2006

18. Network Layer Protocol Stack App Transport Network IP + Routing
Data
Hdr
IP Datagram
Link
Introduction to Communication Networks 2/2006

19. Network Layer Role of the Network Layer Main Functions:
Move packets from sending host to receiving host.
Main Functions:
Host-to-Host delivery:
Internetworking
Addressing
Routing
Path determination  Routing Algorithms
Forwarding  select the correct outgoing port
Call Setup:
Some network layer architectures (ATM, RSVP) requires that routers along a chosen path will agree on the path.
Packetizing
Fragmenting
Introduction to Communication Networks 2/2006

20. The Routing Problem “A” “B” R1 R2 R3 R4
How does R1 choose a route to host B?
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21. Switching
Introduction to Communication Networks 2/2006

22. Circuit Switching It’s the method used by the telephone network.A B
Source
Destination
It’s the method used by the telephone network.
A call has three phases:
Establish circuit from end-to-end (“dialing”),
Communicate,
Close circuit (“tear down”).
Originally, a circuit was an end-to-end physical wire.
Nowadays, a circuit is like a virtual private wire: each call has its own private, guaranteed data rate from end-to-end.
Introduction to Communication Networks 2/2006

23. Circuit Switching Telephone Network
Source
“Caller”
Central
Office
“C.O.”
Destination
“Callee”
Trunk
Exchange
Each phone call is allocated 64kb/s. So, a 2.5Gb/s trunk line can carry about 39,000 calls.
Introduction to Communication Networks 2/2006

24. Packet Switching
Virtual circuit approach – relationship between all packets belonging to a message is preserved – a single route is chosen, and all packets take that route
Datagram approach – each packet is treated independently of all others – thus, packets in the same message can take different routes, and possibly arrive out of order
Introduction to Communication Networks 2/2006

25. Datagram approach It’s the method used by the Internet.
Each packet is individually routed packet-by-packet, using the router’s local routing table.
The routers maintain no per-flow state.
Different packets may take different paths.
Several packets may arrive for the same output link at the same time, therefore a packet switch has buffers.
Introduction to Communication Networks 2/2006

26. Store-and-Forward Basic paradigm used in packet switched network
Sent from source computer
Travels router/switch-to-router/switch
Delivered to destination
Router
‘‘Stores’’ packet in memory
Examines packet’s destination address
‘‘Forwards’’ packet toward destination
Introduction to Communication Networks 2/2006

27. Routing Table Configuration
Manual
Table created by hand
Useful in small networks
Useful if routes never change
Automatic routing
Software creates/ updates table
Needed in large networks
Changes routes when failures occur
Introduction to Communication Networks 2/2006

28. Routing & Graph Theory Graph Node models switch Edge models connection
Introduction to Communication Networks 2/2006

29. An algorithm ensuring all nodes get a copy of an update packet
Reliable Flooding
An algorithm ensuring all nodes get a copy of an update packet R1
Un update packet:
Node sequence update
id number data
Algorithm:
A source node sends out the packet on all ports
Each node receiving the packet checks the sequence no. and if more
recent, stores it and sends it out on all ports but the ingress port
Each node receiving the packet acks the directly node which sent it
A sending node, not receiving an ack within a T/O, retransmits
Introduction to Communication Networks 2/2006

30. Reliable Flooding
Advantages:
Reliable transmission.
Works also when network topology is unknown.
If the network is connected, every node gets a copy
of the packet
Disadvantages:
Some nodes receive multiple copies of the packet
Packets can go round in loops forever.
Waste of network resources
To prevent looping for ever, a TTL is added to packet
Useful only for important updates – not for regular routing
Node sequence TTL update
id number data
Introduction to Communication Networks 2/2006

31. Routing Approaches Source Routing Virtual Circuit Destination Routing
complete path in packet
Used in practice in ad hoc networks
Virtual Circuit
Path is setup in advance
Used in practice in ATM/MPLS/RSVP
Destination Routing
Forwarding according to destination address
Used in practice in IP networks
Introduction to Communication Networks 2/2006

32. Routing Algorithm classification
Global or decentralized information?
Global:
all routers have complete topology, link cost info
“link state” algorithms
Decentralized:
router knows physically-connected neighbors, link costs to neighbors
iterative process of computation, exchange of info with neighbors
“distance vector” algorithms
Static or dynamic?
Static:
routes change slowly over time
Dynamic:
routes change more quickly
periodic update
in response to link cost changes
Introduction to Communication Networks 2/2006

33. Shortest Path Algorithms
Algorithms from graph theory
Dijkstra
Bellman-Ford
May be distributed (No central authority)
A Router
Must learn route to each destination
Only communicates with directly attached neighbors
Introduction to Communication Networks 2/2006

34. Shortest Path Label on edge represents ‘‘distance’’
Possible distance metric
Geographic distance
Economic cost
Inverse of capacity
Introduction to Communication Networks 2/2006

35. Algorithms For Computing Shortest Paths
Distance Vector (DV)
Routers exchange information in their routing tables  Local
Link-state
Routers exchange link status information  Global
Algorithm has complete information about the network.
Both used in practice
Introduction to Communication Networks 2/2006

36. Distance Vector Periodic, two-way exchange between neighbors
Iterative, asynchronous and distributed
During exchange, switch sends
List of pairs
Each pair gives (destination, distance)
Receiver
Compares each item in list to local routes
Changes routes if better path exists
Introduction to Communication Networks 2/2006

37. Distance Vector Intuition
Let
N be neighbor that sent the routing message
V be destination in a pair
D be distance in a pair
C = D + WN (WN the cost to reach the sender)
If no local route to V or local route has cost greater than C,
install a route with next hop N and cost C
else ignore pair
Introduction to Communication Networks 2/2006

38. Example Of Distance Vector Routing
Consider transmission of one DV message
Node 2 sends to 3, 5, and 6
Node 6 installs cost 8 route to 2
Later 3 sends update to 6
6 changes route to make 3 the next hop for destination 2
Introduction to Communication Networks 2/2006

39. Bellman-Ford Algorithm
Objective: Finds the minimum cost path from every node to
every other node
Example: Determine the route from (R1, …, R7) to R8
that minimizes the cost. R5 R3 R7 R6 R4 R2 R1 1 4 2 3 R8
Examples of link cost:
Distance, data rate, price, congestion/delay, …
Introduction to Communication Networks 2/2006

40. B-F is a Distance Vector Algorithm1 1 4 R1 R2 R4 R6 3 2 2 2 R7 R5 2 3 4 R3 R8
The solution is a spanning tree with R8 as the root
of the tree (link costs are cost in the direction to R8).
The Bellman-Ford Algorithm finds all spanning trees.
Introduction to Communication Networks 2/2006

41. The Distributed Bellman-Ford Algorithm
Introduction to Communication Networks 2/2006

42. Bellman-Ford Algorithm - Example1 1 4 R2 R4 R6 2 3 2 2 R7 3 R5 2 R3 4 R8 R1
Inf R2 R3
4, R8 R4 R5
2, R8 R6 R7
3, R8 R2 R5 R7 R4 R6 R8 R3 R1 1 4 2 3
Introduction to Communication Networks 2/2006

43. Bellman-Ford Algorithm - Example1 4 2 3 R1
6, R3 R2
4, R5 R3
4, R8 R4
6, R7 R5
2, R8 R6 R7
3, R8 R8 R6 R4 R2 R1 R3 R5 1 4 2 3 R7
Solution
R1Z
5, R2 R2
4, R5 R3
4, R8 R4 R5
2, R8 R6 R7
3, R8
Introduction to Communication Networks 2/2006

44. Bellman-Ford Algorithm
Questions:
How long can the algorithm take to run?
How do we know that the algorithm always converges?
What happens when link costs change, or when routers/links fail?
Introduction to Communication Networks 2/2006

45. Link-State Routing Overcomes instabilities in DV
Pair of routers periodically
Test link between them
Broadcast link status message
Router
Receives status messages
Computes new routes
Uses Dijkstra’s algorithm
Introduction to Communication Networks 2/2006

46. Dijkstra’s Shortest Path First (SPF) Alg
Objective: Finds the minimum cost path from every node to
every other node
Executed by every node based on the following complete
network topology and link costs
When completed at node, say R8, C(n) will contain the
minimum cost path from R8 to node n
Introduction to Communication Networks 2/2006

47. Dijkstra’s Shortest Path First (SPF) Alg
Each router calculates lowest cost path to all its neighbors, starting from itself;
At each step of the algorithm, a router
adds a node to M to which it can reach with its current minimum cost
For every node outside M, updates the minimum path cost to it
Introduction to Communication Networks 2/2006

48. Getting Network Topology for SPF
Done by the Reliable Flooding algorithm that sends the following Link State Packet (LSP):
LSP are sent out whenever the state of an attached link changes (up/down), or when an Interval Timer expires
Node sequence TTL attached links
id number and their costs
Introduction to Communication Networks 2/2006

49. Dijkstra’s Shortest Path First Algorithm- Example1 4 2 3 R8
Introduction to Communication Networks 2/2006

50. Dijkstra’s Shortest Path First Algorithm- Example1 1 4 R2 R4 R6 2 3 2 2 R7 3 R5 2 R3 4 R8
is the minimum
Introduction to Communication Networks 2/2006

51. Dijkstra’s Shortest Path First Algorithm- Example
is the minimum R1 1 1 4 R2 R4 R6 2 3 2 2 R7 3 R5 2 R3 4 R8
Introduction to Communication Networks 2/2006

52. Dijkstra’s Shortest Path First Algorithm- Example
is the minimum R1 1 1 4 R2 R4 R6 2 3 2 2 R7 3 R5 2 R3 4 R8
Introduction to Communication Networks 2/2006

53. Dijkstra’s Shortest Path First Algorithm- Example1 1 4 R2 R4 R6 2 3 2 2 R7 3 R5 2 R3 4 R8
is the minimum
Introduction to Communication Networks 2/2006

54. Dijkstra’s Shortest Path First Algorithm- Example1 1 4 R2 R4 R6 2 3 2 2 R7 3 R5 2 R3 4 R8
is the minimum
Introduction to Communication Networks 2/2006

55. Dijkstra’s Shortest Path First Algorithm- Example1 1 4 R2 R4 R6 2 3 2 2 R7 3 R5 2 R3 4 R8
is the minimum
Introduction to Communication Networks 2/2006

56. Dijkstra’s Shortest Path First Algorithm- Example1 1 4 R2 R4 R6 2 3 2 2 R7 3 R5 2 R3 4 R8
is the minimum
Introduction to Communication Networks 2/2006

57. Comparison of LS and DV algorithms
Message complexity
LS: with n nodes, E links, O(nE) msgs sent
DV: exchange between neighbors only
larger msgs
convergence time varies
Speed of Convergence
LS: requires O(nE) msgs
may have oscillations
DV: convergence time varies
may be routing loops
count-to-infinity problem
Robustness: what happens if router malfunctions?
LS:
node can advertise incorrect link cost
each node computes only its own table
DV:
DV node can advertise incorrect path cost
each node’s table used by others
error propagate thru network
Introduction to Communication Networks 2/2006

58. Network Services Paradigms
Connection Oriented
Paradigm: phone calls
3 phase protocol:
connection establishment
data transmission
clearing
Example: ATM (Asynchronous Transfer Mode)
Connectionless
Paradigm: snap mail
1 Phase protocol
Example: IP (Internet Protocol)
Introduction to Communication Networks 2/2006

59. Connection-Oriented Networks (CON)
Sender
Requests ‘‘connection’’ to receiver
Waits for network to form connection
Leaves connection in place while sending data
Terminates connection when no longer needed
Network
Receives connection request
Forms path to specified destination and informs sender
Transfers data across connection
Removes connection when sender requests
Introduction to Communication Networks 2/2006

60. Connectionless Networks (CLN)
Sender
Forms packet to be sent
Places address of intended recipient in packet
Transfers packet to network for delivery
Network
Uses destination address to forward packet
Each packet handled independently
One phase protocol
No setup required before transmitting data
No cleanup required after sending data
Introduction to Communication Networks 2/2006

61. Network layer service models:
Architecture
Internet
ATM
Service
Model
best effort
CBR
VBR
ABR
UBR
Bandwidth
none
constant
rate
guaranteed
avg. rate
minimum
Loss no
yes
Order
Timing
Congestion
feedback
no (inferred
via loss)
congestion
Guarantees ?
Internet model being extended: Intserv, Diffserv
More in the QoS lecture
Introduction to Communication Networks 2/2006

62. CLN vs CON CON - Connection Oriented Networks CLN - Connectionless
More intelligence in network
Can reserve bandwidth
Connection setup overhead
Well-suited to real-time applications
CLN - Connectionless
Less overhead
Permits asynchronous use
Allows broadcast/ multicast
Introduction to Communication Networks 2/2006

63. Routing in the Internet
The Internet uses hierarchical routing
The Internet is split into Autonomous Systems (AS’s)
Examples of AS’s: Stanford (32), HP (71), MCI Worldcom (17373)
Try (unix): whois –h whois.arin.net ASN “MCI Worldcom”
Within an AS, the administrator chooses an Interior Gateway Protocol (IGP)
Examples of IGPs: RIP (rfc 1058), OSPF (rfc 1247).
Between AS’s, the Internet uses an Exterior Gateway Protocol
AS’s today use the Border Gateway Protocol, BGP-4 (rfc 1771)
Introduction to Communication Networks 2/2006

64. Interior Routing Protocols
RIP (Routing Information Protocol)
Uses distributed Bellman-Ford algorithm (DV).
Updates (in a RIP packet) are sent every 30 seconds.
No authentication in RIP ver. 1; password in ver. 2
Originally in BSD UNIX.
OSPF (Open Shortest Path First)
Link-state updates (LSU) sent (using reliable flooding) as and when required.
Every router runs Dijkstra’s algorithm.
Authenticated updates.
Autonomous system may be partitioned into “areas”.
Allows multiple paths with the same cost to a destination
Introduction to Communication Networks 2/2006

65. Interior Routing Protocols
Authentication of LSU packets
To prevent, e.g., forged advertisement of zero cost
Early OSPF uses 8-byte password
Strong cryptography in newer versions
“Areas” as a further hierarchy layer
An AS can be partitioned into “areas”
Adds scalability to the OSPF alg
A router knows only how to reach the “areas” – not to all networks
Introduction to Communication Networks 2/2006