1. Chapter 5 Network Layer

2. Network Layer Functions: connecting different
application
transport
network
data link
physical
Functions:
Routing issues
determine “good” path
(sequence of routers) thru
network from source to dest.
Congestion (Not Contention!)
More packets enter an
area than can be processed
Internetworking
connecting different
network technologies together
Network layer protocols in every host, router
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
application
transport
network
data link
physical

3. Network Layer Design Issues
Store-and-forward packet switching
Services provided to transport layer
Implementation of connectionless service
Implementation of connection-oriented service
Comparison of virtual-circuit and datagram networks

4. Network layer design issues
Store-and-Forward packet switching
After each reception checksum is done in router and a new packet is transmitted to the next router. 1

5. Network layer design issues
Services provided to transport layer
Independent of subnet technology
Transport layer shielded from number, type, and topology of subnets
Uniform network address numbering
Even across LANs and WANs

6. Network layer design issues
Two basic kind of services
Connectionless (routers only send and receive packets)
Packets also called datagram
This kind of network is also called datagram subnet
Connection oriented (error control and flow control is done end to end)
Virtual circuit should set up before sending packets
This kind of network is also called virtual circuit subnet

7. Connectionless: Datagram
No call setup at network layer
Packets forwarded using destination host address
packets between same source-dest. pair may take different paths
Use in Internet
application
transport
network
data link
physical
2. Receive Data
application
transport
network
data link
physical
1. Send Data

8. Connectionless: Datagram
Routing within a diagram subnet:
Store-and-Forward packet
Subnet
The table of router A is changed because of some reasons!
Management and update this tables for routing = Routing algorithm
Routing tables

9. Connection Oriented-Virtual Circuits
Call setup, do for each call before data can flow
Each packet carries VC identifier
Used in ATM, frame-relay, X.25
application
transport
network
data link
physical
6. Receive data
3. Accept call
2. Incoming call
application
transport
network
data link
physical
5. Data flow begins
4. Call connected
1. Initiate call

10. Virtual Circuits
Routing within a virtual-circuit subnet:

11. Comparison of Virtual-Circuit and Datagram Subnets

12. Routing Algorithms
The network layer is responsible for routing packets from the source to destination.
The routing algorithm is the piece of software that decides where a packet goes next (e.g., which output line, or which node on a broadcast channel).
For connectionless networks, the routing decision is made for each datagram. For connection-oriented networks, the decision is made once, at circuit setup time.
The routing algorithm must deal with the following issues:
Correctness, simplicity, stability, fairness and optimality
Minimizing mean packet delay or maximizing total network throughput
Routing is different from Forwarding!:
Forwarding: Select the output path using routing table
Routing: Management and updating the routing tables

13. Desirable properties of the routing algorithms:
Correctness:
Simplicity: for efficiency
Robustness: must be able to sustain the changes in the networks (cannot just rely on reboot)
Stability: should converge to equilibrium
Fairness: every one gets to send
Optimality: (Selection of Best Route)as efficient as possible

14. Conflict between fairness and optimality.
Routing Algorithms
Conflict between fairness and optimality.
If traffic flows between A and A’, B and B’, and C and C’, the utilization of the network will be good, but flow between X and X’ will be severely restricted. Fairness must often be imposed at the expense of overall utilization.

15. Optimality Principle Optimality principle: This means:
If router J is on the optimal path from router I to router K, then the optimal path from J to K also falls along the same route.
This means:
Given the final destination, routers only need to know the optimal route to the next router.
Construct a sink tree with the destination to be root.
The goal of all routing algorithms is to discover and use the sink tree for all routers.
Since it is a tree, there is no loops.
There would be different sink trees for each final destination.

16. The Optimality Principle
(a) A subnet. (b) A sink tree for router B.

17. Routing There are two types:
Static (Non-Adaptive)
routes never update or update slowly over time
Examples: Dijkstra, Flooding algorithm
Dynamic (Adaptive)
routes update more quickly use dynamic information of current topology such as load, delay, …
Examples: Distance Vector, Link State Routing
From another view:
Global: all routers have complete topology, link cost info
Decentralized: router knows physically-connected neighbors

18. Routing Algorithms Shortest Path Routing (Dijkstra’s) Flooding
Distance Vector Routing
Link State routing
Hierarchical routing
Broadcast routing
Multicast routing
Routing for mobile hosts
Routing in Ad Hoc Networks

19. Dijkstra Algorithm Net topology, link costs known to all nodes
Non-Adaptive Algorithm
Net topology, link costs known to all nodes
Global algorithm
Cost of a link is a function of :
Number of Hops, Distance, Average traffic, Delay, …
Computes least cost paths (Minimum path) from one node (‘source”) to all other nodes
gives routing table for that node
Iterative: after k iterations, know least cost path to k dest.’s

20. The shortest path from A to D is: ABEFHD
Next steps?
Figure. The first five steps used in computing the shortest path from A to D. The arrows indicate the working node.
The shortest path from A to D is:
ABEFHD

22. Example
Find the Shortest path from s to y using dijsktra algorithm for the following graph.
Solution

23. Dijkstra Animation

24. Flooding Algorithm P flooding P P P
Transmit a copy of each packet
it receives on every one of its
transmission links
flooding P P P
How to reduce looping even further:
1. hop count
2. time stamp
Using Hop Counter:
Hop count is number of nodes that a packet may have to pass through on the way to its destination.
Each router decrements a hop count contained in the packet header.
Whenever the hop count decrements to zero, the router discards the packet

25. Flooding How to Keep track of which packet have been flood
Add a sequence number to each packet's header.
Each router maintains a private sequence number. When it sends a new packet, it copies the sequence number into the packet, and increments its private sequence number.
Keeps track of the highest sequence number seen from S.
Whenever it receives a packet from S containing a sequence number lower than the one stored in its table, it discards the packet. Otherwise, it updates the entry for S and forwards the packet on

26. Selective Flooding
Another variation of flooding is Selective Flooding:
Don’t send incoming packets to ALL output lines
Just forward on that lines which are going approximately in right direction
Flooding is not practical in most applications, but it does have some uses
Uses:
In military applications, the network must remain robust in the face of (extreme) hostility
Sending routing updates, because updates can't rely on the correctness of a router's routing table.
Theoretical-chooses all possible paths, so it chooses the shortest one

27. Distance Vector Routing
Adaptive Algorithm
Distance Vector Routing
Each router
maintains a table (vector) giving the best known distance to a destination and the line to use for sending there.
Tables are updated by exchanging information with neighbors.
Each router knows the distance (cost) of reaching its neighbors (e.g. send echo requests).
Routers periodically exchange routing tables with each of their neighbors.
The distance vector routing algorithm is sometimes called by other names, most commonly Bellman-Ford routing algorithm and the Ford-Fulkerson algorithm.
This algorithm was used in the original ARPANET

28. Distance Vector Routing
Distance table for J
DJ (G,?)=
c(J,H)+minw{DH(G,w)}=
6+12=18
A subnet.
Input from A, I, H, K, and the new routing table for J.
Suppose that J has measured its delay to its neighbors, A, I, H, and K as 8, 10,12,6 msec respectively.

29. Count –to-infinity Problem
Convergence is slow!
Good news travels quickly, bad news travels slowly (count-to-infinity) problem
Example: Propagation of good news
Initially A is down and all other routers know this
When A comes up, the other routers learn about it via the vector exchanges
There is no path to A
Table for dest.=A
In a subnet with longest subnet path=N, after N exchanges everyone will know

30. Count-to-Infinity Problem
Example: Propagation of bad news
The count-to-infinity problem
A goes down after initially
After this A goes down
B thinks that there is a path to A thru C but C itself go to A via B!
Counting will continuous to infinity

31. Link State Routing Each router must do the following:
 Discover its neighbors, learn their network address.
Sends ‘hello' message on booting.
 Measure the delay or cost to each of its neighbors.
Delay= (Send ‘Echo’ message + Receive its reply)/2
 Construct a packet telling all it has just learned.
Construct Link State (LS) packet, it contains:
Source Add., Seq. No., Age No., List of neighbors + their delay
 Send this packet to all other routers.
Forwards link state packets to all other routers using Flooding algorithm.
 Compute the shortest path to every other router.
Each router uses an Dijkstra algorithm to calculate shortest paths based on the current values in its database.
Adaptive Algorithm

32. Link State Routing
Link state routing is based on the assumption that, although the global knowledge about the topology is not clear, each node has partial knowledge: it knows the state (type, condition, and cost) of its links
In other words, the whole topology can be compiled from the partial knowledge of each node

33. Creation of Link State Packet (LSP)
A link state packet (LSP) can carry a large amount of information such as the node identity, the list of links, a sequence number, and age
Node identity and the list of links are needed to make the topology
Sequence number facilitates flooding and distinguishes new LSPs from old ones
Age prevents old LSPs from remaining in the domain for a long time

34. Link State Routing
The link state packets for this subnet.

35. Hierarchical Routing
As a network becomes larger, the amount of information that must be propagated increases, and the routing calculation becomes increasingly expensive.
(Increase the memory amount and calculation)
Hierarchical routing:
Divide the network into regions, with a router only knowing the details of how to route to other routers in its region.
Hides information from far-away nodes, reducing the amount of information a given router needs to perform routing
Router don’t know about the internal topology of other regions.
Gateway is a router that knows about other regions

36. Hierarchical Routing Advantage: Disadvantage:
Scaling. Each router needs less information
In Ex. Distance table reduce from 17 entries to 7
Disadvantage:
Sub optimal routes. The average path length increases
Optimal path for 1A to 5C is thru region 2 while in hierarchical is thru region 3

37. Broadcast routing Send message to all other hosts: Poor methods:
Update distributed database
Distribute weather reports
Distribute live radio/TV programs
Poor methods:
Send a distinct packet to each destination
List of addresses needed
High usage of bandwidth
Flooding
Too many packets
Multi-destination routing
Each packet contains a list of destination
On each line a single packet

38. Broadcast routing Best method: use sink tree
Broadcast source = root of sink tree
Forwarding on sink tree lines
Excellent use of bandwidth
Source tree must be known at each intermediate node
Best method: use sink tree
Which sink tree?
How many sink trees?

39. Broadcast routing Sink tree approximation: reverse path forwarding
if a packet arrives on line used for traffic to source of broadcast
then forward packet on all lines, except the one it arrived on
else discard packet

40. Routing Algorithms Properties Shortest Path Routing Flooding
Distance Vector Routing
Link State routing
Hierarchical routing
Broadcast routing
Multicast routing
Routing for mobile hosts
Routing in Ad Hoc Networks

41. Multicast routing Send a message to a well-defined group
Large in size
Small compared to network as a whole
Group management
Create and destroy groups
Process can leave and join a group
algorithm
How will routers know about groups?

42. Multicast routing Algorithm Pruning: Source computes spanning tree
Remove lines that do not lead to hosts of group ( = pruning)
Pruning:
Link state routing
Each router knows full topology
Distance vector routing
Reverse path forwarding +
PRUNE messages to remove arcs

43. Multicast routing Pruning: source Link state routing
Each router knows full topology
Distance vector routing
Reverse path forwarding +
PRUNE messages to remove arcs

44. Routing Algorithms Shortest Path Routing Flooding
Distance Vector Routing
Link State routing
Hierarchical routing
Broadcast routing
Multicast routing
Routing for mobile hosts
Routing in Ad Hoc Networks

45. Routing for Mobile Hosts
4. Network Layer - routing
February 2006
Routing for Mobile Hosts
Model of world: WAN + LANs, wireless cells
Migratory users
Roaming users
Move from time to time
Use network when connected
Compute on the run
Maintain connections as they move around
Computer Networks

46. Routing for Mobile Hosts
Foreign agent: keeps track of users:
who are currently visiting the area
Home agent: keeps track of users
whose home is in the area
who are currently visiting another area
Permanent home location
Permanent home address

47. Routing for Mobile Hosts
How does it work?
Registration procedure with foreign agents
Sending packets
Leaving an area
Registration procedure with foreign agent
Announcing existence of foreign agent
Broadcast by foreign agent
Broadcast query by arriving mobile user
Mobile user gives to foreign agent
Home address
Current data link address
Security information
Foreign agent contacts home agent of user

48. Routing for Mobile Hosts
Registration procedure with foreign agent (cont.)
Announcing existence of foreign agent
Mobile user gives to foreign agent
Foreign agent contacts home agent of user
Identity of user
Security info
Network address of foreign agent
Home agent
Checks security info
Sends ack to foreign agent
Foreign agent
Stores state
Informs mobile user

49. Routing for Mobile Hosts
Sending a packet to a mobile user
Home address is used  packet routed to home LAN
Packet intercepted by home agent
Packet forwarded to the foreign agent
Encapsulation – tunneling
Foreign agent forwards packet to mobile user
Sender is given address of foreign agent
Encapsulation – tunneling used

50. Routing for Mobile Hosts
4. Network Layer - routing
February 2006
Routing for Mobile Hosts
Home agent
Foreign agent
Computer Networks

51. Routing for Mobile Hosts
Leaving an area
Announced by user  deregistration
Automatic detection by foreign agent
Various different schemes:
Protocol carried out by routers or hosts
Routers along the way intercept and redirect traffic
Modify original packet instead of encapsulation

52. Routing Algorithms Properties Shortest Path Routing Flooding
Distance Vector Routing
Link State routing
Hierarchical routing
Broadcast routing
Multicast routing
Routing for mobile hosts
Routing in Ad Hoc Networks

53. Routing in Ad Hoc Networks
Networks of nodes that just happen to be near each other are called ad hoc networks or MANETs (Mobile Ad hoc NETworks).
Ad Hoc Network = routers are mobile
No fixed topologies
No fixed or known neighbors
Valid paths can disappear at any time
Node = router + host
Routing quite different from routing in wired networks
Examples
Military vehicles on a battlefield
Fleet of ships at see
People with notebooks (lacking )
AODV = Ad hoc On-demand distance vector routing
On-demand: route computed when needed
Distance vector for mobile world
Taking into account limited bandwidth + low battery life

54. Routing in Ad Hoc Networks
Graph presentation
Node
Arc = nodes connected can communicate directly by radio
Routing table:
Line for known routes
Can be valid or invalid
A wants to send a packet
To H: forward to D
To I: start route discovery broadcasting a route request packet

55. Routing in Ad Hoc Networks
(a) Range of A's broadcast.
(b) After B and D have received A's broadcast.
(c) After C, F, and G have received A's broadcast.
(d) After E, H, and I have received A's broadcast.
Shaded nodes are new recipients. Arrows show possible reverse routes.

56. Routing in Ad Hoc Networks
Route request packet:
Source & destination address: e.g. IP address
Request ID:
local counter incremented for each new route request packet broadcasted
allows to discard duplicate requests at other nodes
Source sequence #
counter to distinguish old routes to destination from new ones
Destination sequence #
most recent sequence counter of destination seen by source

57. Routing in Ad Hoc Networks
Processing a route request packet
duplicate request?
Yes: discard
Fresh route to destination known?
Fresh = local destination sequence # >= destination sequence # in request
Yes: send route reply packet
No fresh route to destination is known!
Store info from route request in reverse route table; to enable the forwarding of route reply packets
Increment hop count
Rebroadcast route request

58. Routing in Ad Hoc Networks
Route reply packet
Returned by
Intermediate node knowing a fresh route
Destination node
Destination sequence#
Number known by sender of reply packet
Hop count
Set to length of path known by sender of reply packet
For destination = 0

59. Routing in Ad Hoc Networks
Route maintenance
Detect that neighbors are not reachable anymore
Broadcast Hello packet periodically
Failure to send packet
Cleanup routing table
Additional field in routing table: active neighbor
Nodes that have offered path to destination in recent past

60. Routing in Ad Hoc Networks
Route maintenance
Routing table of D G
Node G goes down!

61. Routing in Ad Hoc Networks
Route maintenance
New routing table of D:
Delete routes with G as next hop
Delete G as active neigbor