1. Congestion in Data Networks
Data Communications
Congestion in
Data Networks

2. What Is Congestion?
Congestion occurs when the number of packets being transmitted through the network approaches the packet handling capacity of the network
Congestion control aims to keep number of packets below level at which performance falls off dramatically
Data network is a network of queues
Generally 80% utilization is critical
Finite queues mean data may be lost

3. Figure 23.5 Incoming packet

4. Interaction of Queues

5. Effects of Congestion
Packets arriving are stored at input buffers (not ATM)
Routing decision made
Packet moves to output buffer
Packets queued for output transmitted as fast as possible
Statistical time division multiplexing
If packets arrive too fast to be routed, or to be output, buffers will fill
Can discard packets
Can use flow control
Can propagate congestion through network

6. Ideal Performance
Top: As load increases, throughput directly increases
Middle: As load increases, delay increases
Bottom: Power is ratio of throughput to delay

7. Practical Performance
Ideal assumes infinite buffers and no overhead
Unfortunately, buffers are finite
Additional overhead occurs in exchanging congestion control messages

8. Effects of Congestion - No Control

9. Congestion Control Objectives
In general, we want to:
Minimize discards
Maintain agreed QoS (if applicable)
Minimize probability of one end user monopoly over other end users
Simple to implement
Little overhead on network or user
Create minimal additional traffic
Distribute resources fairly
Limit spread of congestion
Operate effectively regardless of traffic flow
Minimum impact on other systems
Minimize variance in QoS

10. Basic Mechanisms for Congestion Control
Open-Loop Congestion Control (rely on other layers for feedback
and control)
Retransmission policy - a good policy can reduce congestion
Window policy - sel-reject better than go-back-N; use a bigger
window size
Acknowledgment policy - don’t ack each packet individually
Discard policy - a good policy by routers may prevent
congestion and at the same time may not harm the integrity
of the transmission
Admission policy - QOS mechanism

11. Basic Mechanisms for Congestion Control
Closed-Loop Congestion Control
Backpressure - when a router is congested, it informs the
previous upstream router to reduce the rate of outgoing
packets
Choke packet of choke point - sent by router to source, similar
to ICMP’s source quench packet
Implicit signaling - look for delay in some other action
Explicit signaling - router sends an explicit signal
Backward signaling - bit is set in packet moving in the
direction opposite to the congestion
Forward signaling - bit is set in packet moving in the direction
of congestion. Receiver can use policy such as slowing
down acks to alleviate congestion

12. Basic Mechanisms for Congestion Control (visual examples)

13. Backpressure
If node becomes congested it can slow down or halt flow of packets from other nodes
May mean that other nodes have to apply control on incoming packet rates
Propagates back to source
Can restrict to logical connections generating most traffic
Used in connection oriented that allow hop by hop congestion control (e.g. X.25)
Not used in ATM or frame relay
Only recently developed for IPv6 (PRI field)

14. Choke Packet Control packet Rather crude mechanism
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 message
Sent for every discarded packet, or anticipated
Rather crude mechanism

15. Implicit Congestion Signaling
Transmission delay may increase with congestion
Packets may be discarded
Source can detect these as implicit indications of congestion (source is responsible, not network)
Useful on connectionless (datagram) networks
e.g. IP based (TCP includes congestion and flow control)
Used in frame relay LAPF

16. Explicit Congestion Signaling
Network alerts end systems of increasing congestion
Used on connection-oriented networks
End systems take steps to reduce offered load
Backwards
Congestion avoidance info sent in opposite direction of packet travel
Forwards
Congestion avoidance info sent in same direction as packet travel - when end system receives info, either sends it back to source or hands it to higher layer to take action

17. Categories of Explicit Signaling
Binary
A bit set in a packet indicates congestion
Credit based
Indicates how many packets source may send
Common for end to end flow control
Rate based
The source may transmit data at a rate up to the set limit
Any node along the path of the connection can reduce the data rate limit in a control message to the source
e.g. ATM

18. How Does TCP Handle / Avoid Congestion
How Does TCP Handle / Avoid Congestion? (details in TDC 365 and TDC 463)
To Handle: TCP has a sender window size. Sender window size is minimum of receiver window size or network congestion window size.
To Avoid: TCP can use Slow Start and Additive Increase - at beginning, TCP sets congestion window size to maximum segment size, then increases window size with each ack.
Can also use Multiplicative Decrease - after timeout, threshold set to 1/2 previous threshold, and congestion window size reset to 1 (then slow start)

19. Figure 23.8 Multiplicative decrease

20. How Does Frame Relay Handle Congestion?
Connection management, coupled with
Discard strategy
Explicit signaling
Implicit signaling
In more detail:

21. Connection Management
Before a frame relay network allows a user to transmit data, they agree on a connection
Some call this an SLA (service level agreement)
Frame relay calls it the CIR (committed information rate)
Committed burst size - max amount of data the network agrees to transfer, under normal conditions
Excess burst size - max amount of data in excess of committed burst size
Different frame relay companies have different agreements

22. Connection Management
What happens if you exceed your CIR and the network experiences congestion?
Frame relay may start discarding your frames (Discard Eligible bit = 1) (Discard Strategy)
Does frame relay tell you that your frames are being tossed?
No. Frame relay assumes a higher layer protocol (such as TCP) will monitor lost or missing frames
Frame relay could discard arbitrarily with no regard for source, but then no reward for restraint so end systems transmit as fast as possible
CIR not 100% guaranteed, but network tries hard
Aggregate CIR should not exceed physical data rate

23. Figure 23.1 Traffic descriptors

24. Relationship Among Congestion Parameters

25. Explicit Signaling Network alerts end systems of growing congestion
Backward explicit congestion notification
Notifies the user that CA procedures should be initiated for traffic in the opposite direction; simpler
Forward explicit congestion notification
Notification goes forward, so end user has to somehow get signal back to other end to tell them to slow down
Frame handler monitors its queues
May notify some or all logical connections
User response
Reduce rate

26. Figure BECN

27. Figure FECN

28. Figure 23.11 Four cases of congestion

29. Implicit Signaling Implicit Congestion Notification Telecom Definition
In frame relay, inference by user equipment that congestion has occurred in the network. The inference is triggered by realization of the receiving frame relay access device (FRAD) of transmission delays. Based on block, frame or packet sequence numbers, another protocol may recognize that one or more frames have been lost in transit. Control mechanisms at the upper protocol layers of the end devices then deal with frame loss by requesting retransmissions.
From: YourDictionary.com

30. What About ATM? High speed, small cell size, limited overhead bits
Requirements (difficult)
Majority of traffic not amenable to flow control
Feedback slow due to reduced transmission time compared with propagation delay
Wide range of application demands
Different traffic patterns
Different network services
High speed switching and transmission increases volatility

31. Latency/Speed Effects
Consider a typical ATM transmission speed of 150Mbps
~2.8x10-6 seconds to insert single cell
Time to traverse network depends on propagation delay, switching delay
Assume propagation at two-thirds speed of light
If source and destination on opposite sides of USA, propagation time ~ 48x10-3 seconds
Given implicit congestion control, by the time dropped cell notification has reached source, 7.2x106 bits have been transmitted
So, this is not a good strategy for ATM

32. Cell Delay Variation For ATM voice/video, data is a stream of cells
Delay across network must be short
AND Rate of delivery must be constant
There will always be some variation in transit
Delay cell delivery to application so that constant bit rate can be maintained to application

33. Time Re-assembly of CBR Cells D(i)=end to end delay of ith cell V(0)= estimate of amount of cell delay variation that an application can tolerate

34. Various Network Contributions to Cell Delay Variation
Packet switched networks
Queuing delays
Routing decision time
Frame relay
As above but to lesser extent
ATM
Less than frame relay
ATM protocol designed to minimize processing overheads at switches
ATM switches have very high throughput
Only noticeable delay is from congestion
Must not accept load that causes congestion

35. Cell Delay Variation At The UNI in ATM
Even if application produces data at fixed rate, processing at (potentially) three layers of ATM causes delay
Interleaving cells from different connections
Operation and maintenance signals need to be interleaved
If using synchronous digital hierarchy frames, potential delays here are inserted at physical layer
Can not predict these delays
(See figure next slide)

36. Origins of Cell Delay Variation

37. Traffic and Congestion Control Objectives for ATM
ATM layer traffic and congestion control should support QoS classes for all foreseeable network services
ATM layer traffic and congestion control should not rely on AAL protocols that are network specific, nor on higher level application specific protocols
Any traffic and congestion controls should minimize network and end to end system complexity

38. Traffic Management and Congestion Control Techniques
ITU-T and ATM Forum have defined a range of traffic management functions to maintain the QoS of ATM connections:
Resource management using virtual paths - separate traffic flow according to service characteristics (1)
Connection admission control (2)
Usage parameter control (3)
Traffic shaping (4)
Let’s examine these in more detail

39. Resource Management Using Virtual Paths (1)
ATM network can use the virtual path to group similar virtual channels

40. Connection Admission Control (2)
Good first line of defense
User specifies traffic characteristics for new connection (VCC or VPC) by selecting a QoS
Network accepts connection only if it can meet the demand
Traffic contract
Peak cell rate - upper bound, CBR and VBR
Cell delay variation - CBR and VBR
Sustainable cell rate - average rate, VBR
Burst tolerance - VBR

41. Usage Parameter Control (3)
Monitor established connection to ensure traffic conforms to contract
Protection of network resources from overload by one connection
Done on VCC and VPC
Control of peak cell rate and cell delay variation, or
Control of sustainable cell rate and burst tolerance
Discard cells that do not conform to traffic contract
Called traffic policing

42. Traffic Shaping (4) Smooth out traffic flow and reduce cell clumping
Token bucket and leaky bucket are examples of traffic shaping
Token bucket allows bursts, while leaky bucket maintains an even flow
(See figures next slides)

43. Figure Token bucket

44. Token Bucket

45. Figure Leaky bucket

46. Leaky bucket keeps an average flow moving. Queue overflows?
Figure Leaky bucket implementation
Leaky bucket keeps an average flow moving. Queue overflows?
Discard packets. Unlike token bucket, no credit for no transmissions.

47. ATM’s Real-time Traffic Management
QoS provided for CBR, and rt-VBR is based on a traffic contract (connection admission control) and UBC (usage parameter control)
There is no feedback in these systems. Cells are simply discarded. This is called open-loop control.
This is not used for ABR or UBR traffic.

48. Non-real-time Traffic Management
Some applications (Web, file transfer) do not have well defined traffic characteristics
Best efforts
Allow these applications to share unused capacity
If congestion builds, cells are dropped, eg UBR
Closed loop control
ABR connections share available capacity
Share varies between minimum cell rate (MCR) and peak cell rate (PCR)
ABR flow limited to available capacity by feedback
Buffers absorb excess traffic during feedback delay
Low cell loss

49. Feedback Mechanisms Transmission rate characteristics:
Allowed cell rate
Minimum cell rate
Peak cell rate
Initial cell rate
Start with ACR=ICR
Adjust ACR based on feedback from network
Resource management cells
Congestion indication (CI) bit
No increase (NI) bit
Explicit cell rate (ER) field

50. Variations in Allowed Cell Rate

51. Cell Flow (RM = resource management)

52. 23.7 Integrated Services Integrated Services (IntServ) is a model
used to provide QoS in the Internet
at the IP layer.
IntServ is a flow-based model, in that
a user needs to create a flow or virtual
circuit between source and destination.
But IP is connectionless. How do you
create a connection? Use RSVP.

53. Path messages are sent from sender (S1) to all receivers
Figure Path messages
Path messages are sent from sender (S1) to all receivers
(multiple if multi-cast). This establishes the path.

54. Once the path is set, receivers return Resv messages. Note
Figure Resv messages
Once the path is set, receivers return Resv messages. Note
how reservations are merged (next slide).

55. R3 takes the larger of the two reservations and sends that upstream.
Figure Reservation merging
R3 takes the larger of the two reservations and sends that
upstream.

56. Wild card filter - the router creates a single reservation for all
Figure Reservation styles
Wild card filter - the router creates a single reservation for all
the senders, based on the largest request.
Fixed filter - the router creates a distinct reservation for each
flow.
Shared explicit - the router creates a single reservation which
can be shared by a set of flows.

57. 23.8 Differentiated Services
An alternative to Integrated Services.
Produced by the IETF to create a class-based QoS model
for IP.
Beyond the scope of this class.

58. Meter checks to see if incoming flow matches negotiated traffic
Figure Traffic conditioner
Meter checks to see if incoming flow matches negotiated traffic
profile. Marker can re-mark a packet that is using best-effort
delivery or down-mark a packet based on info received from meter.
Shaper uses the info received from meter to reshape the traffic.
Dropper works like a shaper with no buffer, discarding packets if
the flow severely violates the negotiated profile.