1. William Stallings Data and Computer Communications
Chapter 12
Congestion in
Data Networks

2. What Is Congestion? Data network is a network of queues
If the arrival rate is larger than the transmission rate queues grow to infinity
If the arrival rate is smaller than the transmission rate
queues grow as the arrival rate approaches to the tranmission rate
Generally 80% utilization is critical
growth in queue = more delay
queue may overflow since it is of finite capacity
Congestion control aims to keep number of packets below level at which performance falls off dramatically

3. Queues at a Node

4. Effects of Congestion Packets arriving are stored at input buffers
Routing decision made
Packet moves to appropriate output buffer
Packets queued for output transmitted as fast as possible
If packets
arrive to fast to be routed or
arrive faster than can be trnamistted, buffers will fill
Precautions
discard packets
flow control over neighbors

5. Effects of Congestion
Flow control can propagate congestion through network

6. Ideal Network Performance
Performance under the assumptions of infinite buffers (queues) and zero overhead for congestion control

7. Practical Performance
Ideal assumes infinite buffers and no overhead
Buffers are finite
Overheads occur in exchanging congestion control messages

8. Practical Performance
Point A:
Packet discard starts at heavily congested nodes
re-routing and congestion control messages cause overhead
Point B: PANIC
Retransmission of discarded packets causes heavier traffic
More buffers overflow
delay increase
even transmitted packets are retransmitted due to timeout

9. Mechanisms for Congestion Control

10. Backpressure
If node becomes congested it can slow down or halt flow of packets from its neighbors
That may cause longer queues at neighbors and they do the same
Propagates back to source
Used in connection oriented networks that allow hop by hop congestion control (e.g. X.25)
Not used in ATM and frame relay

11. Choke Packet Control packet Generated at congested node
Sent to source node
e.g. ICMP source quench
From router or destination
Source cuts back until no more source quench messages
Sent for every discarded packet, or when congestion is anticipated

12. Implicit Congestion Signaling
Transmission delay may increase with congestion
Packet may be discarded
Source can detect these as implicit indications of congestion
Used in TCP and frame relay

13. Explicit Congestion Signaling
Network alerts end systems of increasing congestion
End systems take steps to reduce offered load
Backward
notifies the source about congestion
Forward
notifies the destination about congestion
Could be added to data packets
Mechanisms
binary notification
credit based
rate based

14. Traffic Management Fairness Quality of service Reservations
“discard the last received” is not fair
multiple queues for multiple connections
Quality of service
different priorities for different connection types (voice, video, , etc.)
Reservations
congestion avoidance mechanism
traffic contract between user and network
network makes necessary reservations to keep its promise
user tries not to overuse the reserved capacity