1. Chapter 10. Packet Switching
Packet Switching Principles
Routing
Congestion Control
X.25

2. Packet-Switching Principles
Advantages over circuit switching
Greater line efficiency
Can perform data-rate conversion
When traffic becomes heavy, a circuit-switch network blocks calls, while on a packet-switch network, packets are still accepted, but delivery delay increases
Priorities can be used

3. Packet-Switching Principles

4. Packet Switching Technique
Datagram
Each packet is treated independently
Virtual Circuit
a preplanned route is established before any packets are sent
Call-Request packet
Call-Accept packet
Routing decision is made only once for all packets using that virtual circuit
Better for two stations wish to exchange data over an extended period of time

5. Packet Switching Technique (cont)
Advantage of datagram
Call setup phase is avoided, better for a station wishes to send only one or a few packets
More primitive, more flexible
e.g. incoming datagrams can be routed away from the congestion
More reliable
if a node fails, all virtual circuits that pass through that node are lost
Is common in internetworks

6. Packet Size v.s. Transmission Time

7. Circuit Switching v.s. Packet Switching

8. Three Types of Delay Propagation delay Transmission time Node delay
The time it takes a signal to propagate from one node to the next, generally negligible
The speed of electromagnetic signals through a wired medium is typically 2x108 m/s
Transmission time
The time it takes for a transmitter to send out a block of data
Node delay
The time it takes for a node to perform the necessary processing

9. Comparison Datagram packet switching Virtual-circuit packet switching
Circuit switching
Dedicated
transmission path
No dedicated path
No dedicated path
Continuous
transmission of data
Transmission of
packets
Transmission of
packets
Fast enough for
interactive
Fast enough for
interactive
Fast enough for
interactive
Messages are not
stored
Packets may be
stored until delivered
Packets stored
until delivered
Path is established
for entire conversation
Route is established
for each packet
Route is established
for entire conversation

10. Comparison (cont) Datagram packet switching Virtual-circuit
Circuit switching
Call setup delay;
negligible tx delay
Packet transmission
delay
Call setup delay;
Packet tx delay
Busy signal if called
party busy
Sender may be notified
if packet not delivered
Sender notified of
connection denial
Overload may block
call setup
Overload increases
packet delay
Overload may block
call setup
User responsible for
msg loss protection
Network may be
responsible for pkt loss
Network may be
responsible for pkt loss
Fixed bandwidth
transmission
Dynamic use of
bandwidth
Dynamic use of
bandwidth

11. External and Internal Operation
Interface between a station and a network node
Connection-oriented (external virtual circuit)
Connectionless (external datagram service)
E.g. X.25  external virtual circuit
External virtual circuit, internal virtual circuit
External virtual circuit, internal datagram
External datagram, internal datagram
External datagram, internal virtual circuit

12. External Virtual Circuit

13. External Datagram
Out of order!

14. Internal Virtual Circuit

15. Internal Datagram

16. Discussion
External datagram service, coupled with internal datagram operation
allows for efficient use of network
no call setup, not need to hold up packets while a packet in error is retransmitted
Desirable in some real-time applications
External virtual-circuit service
Provide end-to-end sequencing and error control
Attractive for supporting connection-oriented applications, such as file transfer

17. Routing

18. Routing Strategies Fixed Routing
A route is selected for each source-destination pair of nodes in the network
No difference between routing for datagrams and virtual circuits
Simplicity, but lack of flexibility
Refinement: supply the nodes with an alternate next node for each destination

19. Routing Strategies (cont)
Flooding
A packet is sent by a source node to every one of its neighbors
At each node, an incoming packet is retx on all outgoing link except for the link on which it arrived
Hop count field deals with duplicate copies of a pkt
Properties
All possible routes btw source and destination are tried
At least one copy of the packet to arrive at the destination will have a minimum-hop route
All nodes connected to the source node are visited

20. Routing Strategies (cont)
Random Routing
Selects only one outgoing path for retx of an incoming packet
Assign a probability to each outgoing link and to select the link based on that probability
Adaptive Routing
Routing decisions that are made change as conditions on the network change
Failure
Congestion

21. Routing Strategies (cont)
Adaptive Routing
State of the network must be exchanged among the nodes
Routing decision is more complex
Introduces traffic of state information to the network
Reacting too quickly will cause congestion-producing oscillation
If it reacts too slowly, the strategy will be irrelevant

22. Shortest Path Algorithm
Dijkstra’s Algorithm
Bellman-Ford Algorithm 8 5 3 3 5 2 6 2 6 8 3 3 2 1 4 1 2 3 1 1 2 1 7 4 5 1

23. Reduced graph 5 3 3 5 2 2 6 1 1 2 3 2 1 4 5 1

24. Dijkstra’s Algorithm 2 3 2 3 1 1 6 6 4 5 4 5 M = { 1 } M = { 1, 4 } 2

25. Dijkstra’s Algorithm (cont)
Node D M 2 3 4 5 6 1 2 5 1
1, 4 2 4 1 2
1, 4, 2 2 4 1 2
1, 4, 2, 5 2 3 1 2 4
1, 4, 2, 5, 3 2 3 1 2 4
1, 4, 2, 5, 3, 6 2 3 1 2 4

26. Dijkstra’s Algorithm (cont)
w(i, j) = link cost, L(n) = path cost from node s to n
1. [Initialization]
T = {s}
L(n) = w (s, n) for n ≠ s
2. [Get next node]
Find x Ï T such that L(x) = min L(j)
Add x to T
3. [Update Least-Cost Paths]
L(n) = min [ L(n), L(x)+w(x, n) ] for all n Ï T
Go to step 2
jÏT

27. Bellman-Ford Algorithm2 3 2 3
D(1)2 = 2
D(1)3 = 5
D(2)2 = 2 1 1 6 6 4 5 4 5
D(2)6 = 10
D(1)4 = 1
D(2)4 = 1
D(2)5 = 2
h = 1
h = 2
D(3)3 = 3 2 3
D(3)2 = 2 1 6 4 5
D(3)6 = 4
D(3)4 = 1
D(3)5 = 2
h = 3

28. Bellman-Ford Algorithm (cont)
Node D h 2 3 4 5 6
Source = 1 1 2 5 1 2 2 4 1 2 10 3 2 3 1 2 4 4 2 3 1 2 4

29. Bellman-Ford Algorithm (cont)
Lh(n) = path cost from s to n w/ no more than h links
1. [Initialization]
L0(n) = ∞, for all n ≠ s
Lh(s) = 0, for all h
2. [Update]
For each successive h ≥ 0
For each n ≠ s, compute
Lh+1(n) = min [ Lh(j) + w(j, n) ] s j n
<= h links
link j

30. Comparisons … …… L(n) = min [ L(n), L(x)+w(x, n) ] Lh(x, D) S D S D
Dijkstra’s
(Link State)
Bellman-Ford
(Distance Vector)

31. Routing in ARPANET First generation(RIP), 1969
Adaptive Routing is adopted
Use Bellman-Ford algorithm
Estimated link delay is simply the queue length for that link
Every 128ms, each node exchanges its delay vector(routing table) with all its neighbors
Information about a change in network condition would gradually ripple through the network

32. Routing in ARPANET (cont)
Each node i maintains
di j = current estimate of min delay from i to j
si j = next node in the current min-delay route from i to j
Node k updates its vectors as follows
dk j = Min [ lk i + di j ] i Î A
sk j = i using i that minimizes the expression above where A = set of neighbor nodes for k lk i = current estimate of delay from k to i k i j

33. Routing in ARPANET (cont)
Major shortcomings of RIP
It did not consider line speed, merely queue length. Higher capacity links were not given the favored status
Queue length is an artificial measure of delay
The algorithm was not very accurate. It responded slowly to congestion and delay increases.

34. Routing in ARPANET (cont)
Second generation, 1979
OSPF: Open Shortest Path First protocol
Link-state routing protocol
The delay is measured directly
Every 10 seconds, the node computes the average delay on each outgoing link
Information of changes in delay is sent to all others nodes using flooding
Using Dijkstra’s algorithm

35. Routing in ARPANET (cont)

36. Routing in ARPANET (cont)
Third generation, 1987
Problem
The correlation between the reported values (delay) and those actually experienced after rerouting
Conclusion
Under heavy load, the goal of routing should be to give the average route a good path instead of attempting to give all routes the best path
Solution
Also consider the average utilization of links
Revised cost function: delay-based metric under light loads, capacity-based metric under heavy loads

37. Calculate Link Costs Measure the avg. delay over the last 10 sec
Using the single-server queuing model, the measured delay is transformed into an estimate of link utilization
Average the link utilization with the previous estimate of utilization
The link cost is set as a function of average utilization

38. ARPANET Delay Metrics (3rd)
Theoretical queueing delay 5 4
Delay (hops) 3
Metric for satellite link 2
Metric for terrestrial link 1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Estimated utilization

39. Congestion Control Throughput (packet delivered) 1.0 0.8 1.0
ideal
overhead
Controlled
Uncontrolled
0.8
1.0
Offered load (packets sent)

40. Congestion Control (cont)
Uncontrolled
Average Packet Delay
ideal
Controlled
0.8
1.0
Offered load (packets sent)

41. Congestion Control (cont)
Examples of control mechanism
Send the choke packet from a congested node to some or all source node
Rely on routing information
Link delay information to influence the packet rate
But the delay may vary too rapidly to be used effectively
Make use of an end-to-end probe packet
Measure the delay between two particular stations
Allow packet-switching nodes to add congestion information to packets as they go by
Two possible directions

42. X.25 Interface, since 1976 User process To remote user process Packet
Multi-channel logical
interface
Link
access
Link
access
Lap-B Link-level
logical interface
Physical
Physical
X.21 physical interface
DTE
DCE

43. User Data and X.25 Protocol

44. X.25 Protocol (e.g.)

46. X.25 Packet Format
24-bit (3-bit seq.) , 32-bit (7-bit seq.), 56-bit (15-bit seq.) header
Q bit: not defined in the standard
D, M: for complete pkt sequence

47. X.25 Packet Format (cont)

48. X.25 Packet Types

49. X.25 Flow and Error Control
Virtually identical in format and procedure to flow control used for HDLC
Sliding-window protocol is used
Each packet carries
P(S): send seq. # and P(R): receive seq. #
Go-back-N ARQ error control
D bit and M bit
Complete packet sequence
End-to-end acknowledgement (D=1)

50. X.25 Acknowledgement P(R) in the data, RR, RNR Packet Switch Net.
End-to-end (D=1)
Local (D=0, the usual case)
Local (D=0, the usual case)
X.25
X.25
X.25
X.25
LAPB
LAPB
LAPB
LAPB
Packet Switch Net.
X.21
X.21
X.21
X.21
DTE
DCE
DCE
DTE

51. X.25 Packet Sequence A packet: (M=1, D=0), B packet: not an A packet
Complete packet sequence = A…AB (zero or more A packets followed by a B packet)
Original seq.
Combined seq.
Pkt type M D
Pkt type M D A 1 A A 1 1 A 1 A 1 A 1 A 1 B 1 B 1
Segmented seq. B A 1 B

52. X.25 Packet Sequence (cont)
E.g. Packet seq. with intermediate E-E Ack
Pkt type M D A 1 A 1
Group of pkt that can be combined
more bit A 1 B 1 1
Require an end-to-end acknowledgement A 1 A 1 B 1 1 A 1 A 1 A 1 B 1
end of sequence