school of computing science simon fraser university

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School of Computing Science

Simon Fraser University

CMPT 771/471: Internet Architecture & CMPT 771/471: Internet Architecture & Protocols Protocols

TCP-Friendly Transport ProtocolsTCP-Friendly Transport Protocols

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Motivations

Congestion Control Prevents congestive network collapse

Improve Quality of Service (QoS) for UDP Control transmission rate

Fair bandwidth utilization Prevents starvation of TCP traffic ~95% of Internet traffic are TCP

In short, a TCP-Friendly protocol based on UDP

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Approaches

Resources Reservation Impossible to know exact bandwidth May leads to over-allocate

Priority Mechanisms Require supports by path routers

Adaptive Sending Rate Easy to implement, application level Can adapt to changes in bandwidth availability Improve QoS through loss reduction

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Adaptive Sending Rate

Congestion Control Achieved by varying sending rate

12+ algorithms for calculating transmission rate

Main idea is to use TCP throughput model All claims to be the most effective

Loss-Delay Based Adjustment Algorithm (LDA) TCP like approach Increase sending rate during network under-load Uses feedback to accurately measure RTT Intended for video and audio streaming Network and Operating System Support for Digital Audio

and Video (NOSSDAV ‘98)

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Loss-Delay Based Adjustment Algorithm

Loss-Delay Based Adjustment Algorithm (LDA)

Start with a small value, 10 kb/s Additive increase rate (AIR) If packet drop, back off to initial value (10 kb/s) Increase until similar rate as TCP

Relies on feedback Uses Real Time Protocol (RTP) on top of UDP Feedback contains losses and round-trip time (RTT)

How much to increase? Bf = bandwidth factor AIR = initial value (10 kb/s)

BfAIRAIRi *

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Loss-Delay Based Adjustment Algorithm

How do we calculate ?

r = current transmission rate b = bottleneck bandwidth

Calculating bottleneck bandwidth b = probe packet size / gap between 2 probe

packets

Two sequential packets with small gap means less delay

b

rBf 1

Bf

Understanding TCP

Detecting Congestion Recall TCP average throughput Fairness mean we must not exceed

Recall TCP average throughput simplistic model

TCP throughput is inversely proportional to RTT and square root of packet loss probability p

pRTTpX

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31)(

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Loss-Delay Based Adjustment Algorithm

Calculating RTT No ACK in UDP Use feedback report

Where t= arrival time, = time elapsed since last report, and = last received sender report

LSRDLSR ttt

DLSRtLSRt

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Performance of LDA

LDA and TCP

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Performance of LDA

LDA scalability

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In Theory, It Works

Not In Practice Additional network overhead (RTP) Additional application level complexity Rogue UDP process could starve TCP-Friendly

protocols

Performance Driven UDP is intended to be light weight and fast TCP-Friendly protocols would have to yield to UDP Developers are lazy, they want the fastest

connection with minimal amount of work

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Summary

TCP-Friendly Protocols UDP based protocols with congestion control Able to sense the network and adjust send rate

accordingly Promote fair bandwidth sharing

Prevents Network Collapse Work together with TCP to balance bandwidth

Fill The Gap TCP-Friendly protocols can fill the gap between TCP

and UDP

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References

TCP-Friendly, Advanced Networking: Pittsburgh Supercomputing Center, http://www.psc.edu/networking/projects/tcpfriendly/

D. Sisalem, H. Schulzrinne, “The Loss-Delay Adjustment Algorithm: A TCP-friendly Adaptation Scheme”, Network and Operating System Support for Digital Audio and Video (NOSSDAV ‘98), Cambridge, UK, July 8-10, 1998.

J. Mahdavi, S. Floyd, TCP-Friendly Unicast Rate-Based Flow Control http://www.psc.edu/networking/papers/tcp_friendly.html

Lorenzo Vicisano, Luigi Rizzo (Pisa) and Jon Crowcroft, TCP-like Congestion Control for Layered Multicast Data Transfer (INFOCOM ‘98).