ME LECTURE NOTES-- Effects of Congestion

ME LECTURE NOTES Effects of Congestion

  ‘Congestion-Control Mechanisms

l  Backpressure

–      Request from destination to source to reduce rate

–      Useful only on a logical connection basis

–      Requires hop-by-hop flow control mechanism

l  Policing

–      Measuring and restricting packets as they enter the network

l  Choke packet

–      Specific message back to source

–      E.g., ICMP Source Quench

l  Implicit congestion signaling

–      Source detects congestion from transmission delays and lost packets and reduces flow

Explicit congestion signaling

 Frame Relay reduces network overhead by implementing simple congestion-notification mechanisms rather than explicit, per-virtual-circuit flow control. Frame Relay typically is implemented on reliable network media, so data integrity is not sacrificed because flow control can be left to higher-layer protocols. Frame Relay implements two congestion-notification mechanisms:

• Forward-explicit congestion notification (FECN)

• Backward-explicit congestion notification (BECN)

FECN and BECN each is controlled by a single bit contained in the Frame Relay frame header. The Frame Relay frame header also contains a Discard Eligibility (DE) bit, which is used to identify less important traffic that can be dropped during periods of congestion.

The FECN bit is part of the Address field in the Frame Relay frame header. The FECN mechanism is initiated when a DTE device sends Frame Relay frames into the network. If the network is congested, DCE devices (switches) set the value of the frames' FECN bit to 1. When the frames reach the destination DTE device, the Address field (with the FECN bit set) indicates that the frame experienced congestion in the path from source to destination. The DTE device can relay this information to a higher-layer protocol for processing. Depending on the implementation, flow control may be initiated, or the indication may be ignored.

The BECN bit is part of the Address field in the Frame Relay frame header. DCE devices set the value of the BECN bit to 1 in frames traveling in the opposite direction of frames with their FECN bit set. This informs the receiving DTE device that a particular path through the network is congested. The DTE device then can relay this information to a higher-layer protocol for processing. Depending on the implementation, flow-control may be initiated, or the indication may be ignored.

Frame Relay Discard Eligibility

The Discard Eligibility (DE) bit is used to indicate that a frame has lower importance than other frames. The DE bit is part of the Address field in the Frame Relay frame header.

DTE devices can set the value of the DE bit of a frame to 1 to indicate that the frame has lower importance than other frames. When the network becomes congested, DCE devices will discard frames with the DE bit set before discarding those that do not. This reduces the likelihood of critical data being dropped by Frame Relay DCE devices during periods of congestion.

Frame Relay Error Checking

Frame Relay uses a common error-checking mechanism known as the cyclic redundancy check (CRC). The CRC compares two calculated values to determine whether errors occurred during the transmission from source to destination. Frame Relay reduces network overhead by implementing error checking rather than error correction. Frame Relay typically is implemented on reliable network media, so data integrity is not sacrificed because error correction can be left to higher-layer protocols running on top of Frame Relay.

Traffic Management in Congested Network – Some Considerations

l  Fairness

–      Various flows should “suffer” equally

–      Last-in-first-discarded may not be fair

l  Quality of Service (QoS)

–      Flows treated differently, based on need

–      Voice, video: delay sensitive, loss insensitive

–      File transfer, mail: delay insensitive, loss sensitive

–      Interactive computing: delay and loss sensitive

l  Reservations

–      Policing: excess traffic discarded or handled on best-effort basis

–       

Frame Relay Congestion Control

l  Minimize frame discard

l  Maintain QoS (per-connection bandwidth)

l  Minimize monopolization of network

l  Simple to implement, little overhead

l  Minimal additional network traffic

l  Resources distributed fairly

l  Limit spread of congestion

l  Operate effectively regardless of flow

l  Have minimum impact other systems in network

l  Minimize variance in QoS

Congestion Avoidance with Explicit Signaling

Two general strategies considered:

l  Hypothesis 1: Congestion always occurs slowly, almost always at egress nodes

–      forward explicit congestion avoidance

l  Hypothesis 2: Congestion grows very quickly in internal nodes and requires quick action

–      backward explicit congestion avoidance

Explicit Signaling Response

l  Network Response

–      each frame handler monitors its queuing behavior and takes action

–      use FECN/BECN bits

–      some/all connections notified of congestion

l  User (end-system) Response

–      receipt of BECN/FECN bits in frame

–      BECN at sender: reduce transmission rate

–      FECN at receiver: notify peer (via LAPF or higher layer) to restrict flow

Frame Relay Traffic Rate Management Parameters

l  Committed Information Rate (CIR)

–      Average data rate in bits/second that the network agrees to support for a connection

l  Data Rate of User Access Channel (Access Rate)

–      Fixed rate link between user and network (for network access)

l  Committed Burst Size (Bc)

–      Maximum data over an interval agreed to by network

l  Excess Burst Size (Be)

–      Maximum data, above Bc, over an interval that network will attempt to transfer

Relationship of Congestion Parameters