Chapter 12 Congestion in Data Network

William Stallings

Data and Computer

Communications

Chapter 12

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}

Effects of Congestion

 _{Packets arriving are stored at input buffers}  _{Routing decision made}

 _{Packet moves to output buffer}

 _{Packets queued for output transmitted as fast as}

possible

 _{Statistical time division multiplexing}

 _{If packets arrive to fast to be routed, or to be}

output, buffers will fill

 _{Can discard packets}  _{Can use flow control}

Ideal

Practical Performance

 _{Ideal assumes infinite buffers and no} overhead

 _{Buffers are finite}

Effects of

Mechanisms for

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)

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 message

 _{Sent for every discarded packet, or} anticipated

Implicit Congestion Signaling

 _{Transmission delay may increase with}

congestion

 _{Packet may be discarded}

 _{Source can detect these as implicit}

indications of congestion

 _{Useful on connectionless (datagram)}

networks

 _{e.g. IP based}

 _{(TCP includes congestion and flow}

control - see chapter 17)

Explicit Congestion Signaling

 _{Network alerts end systems of increasing} congestion

 _{End systems take steps to reduce offered} load

 _Backwards

 _{Congestion avoidance in opposite} direction to packet required

 _Forwards

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}

Traffic Management

 _Fairness

 _{Quality of service}

 _{May want different treatment for} different connections

 _Reservations  _{e.g. ATM}

Congestion Control in Packet

Switched Networks

 _{Send control packet to some or all source}

nodes

 _{Requires additional traffic during}

congestion

 _{Rely on routing information}  _{May react too quickly}

 _{End to end probe packets}  _{Adds to overhead}

 _{Add congestion info to packets as they cross}

nodes

ATM Traffic Management

 _{High speed, small cell size, limited overhead}

bits

 _{Still evolving}  _Requirements

 _{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}

Latency/Speed Effects

 _{ATM 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

Cell Delay Variation

 _{For ATM voice/video, data is a stream of} cells

 _{Delay across network must be short}  _{Rate of delivery must be constant}

 _{There will always be some variation in} transit

Network Contribution 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}

Cell Delay Variation

At The UNI

 _{Application produces data at fixed rate}

 _{Processing at three layers of ATM causes delay}

 _{Interleaving cells from different connections}

 _{Operation and maintenance cell interleaving}

 _{If using synchronous digital hierarchy}

frames, these are inserted at physical layer

Traffic and Congestion

Control Framework

 _{ATM layer traffic and congestion control} should support QoS classes for all

foreseeable network services

 _{Should not rely on AAL protocols that are} network specific, nor higher level

application specific protocols

Timings Considered

 _{Cell insertion time}

 _{Round trip propagation time}  _{Connection duration}

 _{Long term}

 _{Determine whether a given new} connection can be accommodated

Traffic Management and

Congestion Control Techniques

 _{Resource management using virtual paths}  _{Connection admission control}

 _{Usage parameter control}  _{Selective cell discard}

Resource Management Using

Virtual Paths

 _{Separate traffic flow according to service} characteristics

 _{User to user application}

 _{User to network application}

 _{Network to network application}

 _{Concern with:}  _{Cell loss ratio}

Allocating VCCs within VPC

 _{All VCCs within VPC should experience} similar network performance

 _{Options for allocation:}

Connection Admission Control

 _{First line of defence}

 _{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}

Usage Parameter Control

 _{Monitor connection to ensure traffic cinforms}

to contract

 _{Protection of network resources from}

overload by one connection

 _{Done on VCC and VPC}

 _{Peak cell rate and cell delay variation}

 _{Sustainable cell rate and burst tolerance}  _{Discard cells that do not conform to traffic}

contract

Traffic Shaping

 _{Smooth out traffic flow and reduce cell} clumping

ATM-ABR 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}

 _{Closed loop control}

 _{ABR connections share available capacity}

 _{Share varies between minimum cell rate (MCR)}

and peak cell rate (PCR)

 _{ARB flow limited to available capacity by feedback}

 _{Buffers absorb excess traffic during feedback delay}

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 bit}

 _{No increase bit}

Rate Control Feedback

 _{EFCI (Explicit forward congestion} indication) marking

Frame Relay

Congestion Control

 _{Minimize discards}

 _{Miantain agreed QoS}

 _{Minimize probability of one end user monoply}  _{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}

Techniques

 _{Discard strategy}

 _{Congestion avoidance}  _{Explicit signaling}

 _{Congestion recovery}

Traffic Rate Management

 _{Must discard frames to cope with congestion}  _{Arbitrarily, no regard for source}

 _{No reward for restraint so end systems}

transmit as fast as possible

 _{Committed information rate (CIR)}

 _{Data in excess of this liable to discard}  _{Not guaranteed}

 _{Aggregate CIR should not exceed}

physical data rate

Relationship

Among

Explicit Signaling

 _{Network alerts end systems of growing} congestion

 _{Backward explicit congestion notification}  _{Forward explicit congestion notification}  _{Frame handler monitors its queues}

 _{May notify some or all logical connections}  _{User response}

Required Reading