TCP Congestion Control

TCP Congestion Control
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Motivation

History:
- In the late 1980s congestion became a serious problem (leading to many dropped packets)
Kinds of Responses:
- Pre-allocate resources to avoid congestion
- Detect congestion and control it when it occurs

Where to Manage Resources

Congestion and Window Size

TCP Tahoe

History:
- Van Jacobson (1988)
Components:
- Additive Increase, Multiplicative Decrease (AIMD)
- Slow Start
- Fast Retransmit

TCP Tahoe (cont.)

AIMD:
- Increase the size of the congestion window by a fixed increment (e.g., one packet) for each successful round trip
- Divide the size of the congestion window by 2 each time there is a timeout (or four ACKS for a given segment -- an implicit loss event)
Slow Start:
- Start with a congestion window size of 1
- Use additive increase until there's a timeout or a threshold is exceeded
Fast Retransmit:
- Send an ACK for each packet
- Send a duplicate ACK when a packet is received out of order
- Use duplicate ACKs to force retransmission

TCP Reno (and Derivatives)

Startup:
- Don't use slow start; use the last successful congestion window instead.
Consecutive Packet Drops:
- Use slow start (to try and determine the available capacity)
In Case of Fast Retransmit:
- Decrease size of congestion window by 2

Retransmission Timers in Tahoe/Reno

TCP Vegas

The Concept:
- Predict the onset of congestion and react accordingly
Possible Congestion Indicators:
- Increasing round trip time
- Flat sending rate

Aside: Congestion Avoidance in Routers

DECbit:
- Router uses the queue length to predict congestion
- Use a congestion bit in the packet header to inform hosts
Random Early Detection (RED):
- Drop packets (probabilistically) before the queue becomes full (which will result in a TCP timeout)
RED with In and Out (RIO):
- Categorize packets and drop the low priority (a.k.a., out of profile) packets before the high priority (a.k.a., in profile) packets