Congestion Control in Networks ATM and TCP Balazs Attila-Mihaly

Myths about congestion

Congestion control possibilities

ATM – Vocabulary

TCP

TCP – Congestion Control

TCP – CC evaluation

Bibliography

Queuing delay

Jitter

Low troughput

Traffic access control

Packet scheduling (prioritization)

Buffer management

Flow control (speed matching)

QoS routing

Traffic access control

Packet scheduling (prioritization)

Buffer management

Flow control (speed matching)

avoidance

recovery

Min + Equal share

Maximum of the previous too

Allocation proportional with Min

Implementability

Fixed sized cells (53 bytes – 48 byte of data)

PCR – Peak Cell Rate

SCR – Sustained Cell Rate

MBR – Maximum Burst Size

MCR – Minimum Cell Rate

CDV – Cell Delay Variation (peak-to-peak / instant.)

rt-VBR – Real-Time Variable Bit Rate

nrt-VBR – Non-Real-Time Variable Bit Rate

UBR – Unspecified Bit Rate

ABR – Available Bit Rate

I – Increment

L - Limit Cell arrival at t t <= TAT TAT = t t+L < TAT Conforming cell TAT = TAT + I Non-conforming cell

Closed loop, Rate based, Binary & Explicit feedback and Queue growth rate based CC

BN – Backward Notification

CI – Congestion Indication

NI – No Increase

Considered out of band cells

Can be sent by the intermediate switches

Tagged packets (CLP) can be dropped at intermediate switches if a treshold is hit

Each switch can control the flow

Needs ”smart-network”

Three-way handshake

Flow multiplexing with source/destination ports

Positive acknowledgement

Windowing

Not part of the original spec

As a result in 1986 NSFnet capacity dropped more than 800 (!!!) times

Only reactive – needs to respect the ”smart endpoints – dumb network” principle

There are alternatives (SACK, RVSP, etc)

Slow start

Fast retransmit and recovery

It's reactive

It actually relies on congestion happening to detect the link speed (for slow start)

It assumes that all packed loss is due to congestion

Jagannathan Sarangapani – Wireless, Ad Hoc and Senso Networks – Protocol, Performance and Control

Steve Sjoquist, Andrew Tucker – Comparison of ATM and TCP Congestion Control

http://www.cis.ohio-state.edu/~jain/cis788-95/atm_cong/index.html

Thank you