CS 457 Lecture 23 Congestion Fall 2011 Defining Fairness: MaxMin - - PowerPoint PPT Presentation

CS 457 – Lecture 23 Congestion

Fall 2011

Defining Fairness: MaxMin

• Given a resource utotal and several requests pi
• Assign allocations of ui to node i.
• Allocation is fair if

– No one receives more than they requested: ui <= pi for all i – No other allocation has higher minimum allocation (for allocations meeting requirement above) – Recursive true after removal of minimum node: let j = user with minimum allocation let utotal = utotal - uj remove user j above conditions should still hold

A Simple (Incorrect) Approach

• Allocate buffer space for each flow

– Only drop packet if individual flow buffer overflows – Seems fair in terms of buffer allocation….

• Round-Robin Rule for Sending Packets

– Transmit packet from queue 1, then queue 2, etc.

• Is this approach fair?

– What should a sender do to optimize its share of bandwidth? – Answer: Send VERY big packets!

A Simple (Impractical) Approach

• Allocate buffer space for each flow

– Only drop packet if individual flow buffer overflows – Seems fair in terms of buffer allocation….

• Round-Robin Rule for Sending Bits

– Transmit one bit from queue 1, then queue 2, etc.

• Is this approach fair?
• Some notation:

R(t) = the number of rounds up to time t Nac(t) = the number of active flows at time t S(i,a) = the time packet i from flow a arrives F(i,a) = the time packet i from flow a left

Notations

• Some notation:

R(t) = the number of rounds up to time t Nac(t) = the number of active flows at time t S(i,a) = the round packet i from flow starts F(i,a) = the round packet i from flow finishes

• In which round is a packet transmitted?

S(i,a) = Max(R(arrival_time), F(i-1,a)) F(i,a) = S(i,a) + P

• Can order packets by their finishing time F(i,a)

A Fair Queuing Algorithm

• Important factor is the finishing time F(i,a)

– Packet arrived bit by bit (impractical approach) – But it does not matter how bits arrived – Packet is not available until F(i,a) anyway

• Fair Queuing Approach

– Send packet with lowest F(i,a)

• Pre-emption

– What if packet arrives with lower F(i,a) then current packet? – Could choose to pre-empt (easier to analyze) or wait (easier to implement)

Simple Congestion Detection

• Packet loss

– Packet gets dropped along the way

• Packet delay

– Packet experiences high delay

• How does TCP sender learn these?

– Loss

• Timeout
• Triple-duplicate acknowledgment

– Delay

• Round-trip time estimate

Simple Congestion Detection

• Packet loss

– Packet gets dropped along the way

• Packet delay

– Packet experiences high delay

• How does TCP sender learn these?

– Loss

• Timeout
• Triple-duplicate acknowledgment

– Delay

• Round-trip time estimate

TCP Congestion Control Basics

• Each source determines available capacity

– … and how many packets is allowed to have in transit

• Congestion window

– Maximum # of unack’ed bytes allowed to be in transit (the congestion-control equivalent of receiver window) – MaxWindow = min{congestion window, receiver window} - send at the rate of the slowest component

• How to adapt the congestion window?

– Decrease upon losing a packet: back-off – Increase upon success: explore new capacity

Additive Increase, Multiplicative Decrease

• How much to increase and decrease?

– Increase linearly, decrease multiplicatively – A necessary condition for stability of TCP – Consequences of oversized window are much worse than having an under-sized window

• Oversized window: packets dropped, retransmitted, pain

for all

• Undersized window: lower throughput for one flow
• Multiplicative decrease

– On loss of packet, divide congestion window in half

• Additive increase

– On success for last window of data, increase linearly, adding one MSS per RTT

TCP “Sawtooth” Behavior

t Window halved Loss

Practical Details

• Congestion window (cwnd)

– Represented in bytes, not in packets (Why?) – Packets typically one MSS (Maximum Segment Size)

• Increasing the congestion window

– Increase by MSS on success for last window of data – In practice, increase a fraction of MSS per received ACK

• # packets per window: CWND / MSS
• Increment per ACK: MSS * (MSS / CWND)
• Decreasing the congestion window

– Cut in half, but never below 1 MSS

Getting Started

t Window

But, could take a long time to get started!

Need to start with a small CWND to avoid overloading the network.

“Slow Start” Phase

• Start with a small congestion window

– Initially, CWND is 1 MSS – So, initial sending rate is MSS/RTT

• That could be pretty wasteful

– Might be much less than the available bandwidth – Linear increase takes a long time to accelerate

• Slow-start phase (but in reality it’s “fast start”)

– Sender starts at a slow rate (hence the name) – … but increases the rate exponentially – … until the first loss event

Slow Start in Action

Double CWND per round-trip time

D A D D A A D D A A D A

Src Dest

D A

1 2 4 8

What’s Next

• Read Chapter 1, 2, 3, 4.1-4.3, and 5.1-5.2
• Next Lecture Topics from Chapter 6.4 and 6.5

– Congestion Control

• Homework

– Due Thursday in lecture

• Project 3

– Report Any Changes in Your Project Group