CSE679: Buffer Management

Drop Tail RED Advanced RED

Network Congestion

Congestion results in buffer backlog Eventually, packets have to be dropped Packet drops indicate congestion to senders Senders eventually backoff to reduce

congestion Usual mode of dropping packets at router -

DropTail

Problems with DropTail

No indication of congestion until late May drop several packets at once - leads to

global synchronization of flows Can lead to oscillations of high-low link

utilization Aggressive flows can kill all other flows

Early Congestion Indication

Notify congestion earlier Individual flows reduce rates at different times

-- reducing synchronization problems Various techniques for early congestion

indication/notification Random Early Drop -- drop packets randomly

when queues are starting to buildup

Random Early Detection

Two thresholds, min, max of queue lengths In between , mark/drop packets randomly Inform flows early on, avoid global

synchronization Reduces average queue lengths, delays Flows with higher rates have more packets

dropped - fairer distribution of BW

RED Work Flow

For each packet arrival, calculate the average queue size avg if min avg < max calculate probability pa, mark arriving

packet with probability pa else if max avg, mark the arriving packet

RED Parameters

Pa = maxp*(avg - min)/(max -min) avg = (1-w)*avg + w * q count measures the number of packet arrivals

since the last marked packet if queue length is between min and max

RED

Queue length

Drop

p

min max

RED Properties

Maintains queue lengths low Allows high utilization of links without

oscillations/global synchronization Higher rate flows get dropped more often -

leads to fairer sharing than DropTail Aggressive flows can still shut down ‘nice’

applications Small RTT bias of TCP still present

RED Properties (more)

FIFO scheduling - doesn’t provide flow isolation - one aggressive flow can hog all the buffers

Favors ‘robust’ applications (ftp) over ‘fragile’ applications (telnet)

FRED - Per Flow RED employs RED and per-flow queuing to solve some of these problems

How to Enhance RED?

Per-flow Queuing based RED QFRED by Harvard LQD-RED by Bell Lab

FRED

Applies RED per flow Aggressive flows may have packets dropped

while others have no drops Per-flow RED isolates flows Solves the problem of aggressive flows Can allocate BW fairly

Issues in FRED

Difficult to maintain queues for each flow In backbone routers, may have thousands or

millions of flows Difficult to sort packets based on “flows” and

to figure out scheduling order Scalability is an issue How to provide flow isolation that can scale?

More New RED

BRED (Balanced RED) by Maryland at College Park RED + recoding buffer occupancy of each flows

SRED (Stabilized RED) by Bell core RED + estimating the buffer occupancy of each flows

Choke by Stanford RED + random picking a packet from queue and

comparing

Etc.

Conclusion

RED provides fairer sharing than DropTail RED simple to implement RED cannot contain aggressive flows Enhanced RED