NOSSDAV '97

Understanding TCP Dynamics in an Integrated Services Internet

Wu-chang Feng, Dilip Kandlur, Debanjan Saha, and Kang Shin

NOSSDAV '97

Motivation

• Many TCP-based applications can take advantage of guarantees in the network

• Majority of these applications don’t require strict delay bound guarantees

• Examples– Non-interactive audio and video– Data streaming applications– Elastic applications (ftp, http)

NOSSDAV '97

Controlled-load Service

• IETF defined service which provides more flexible guarantees to applications than Guaranteed Service

• Application provides TSpec• Compliant traffic receives service similar to

that in an “unloaded” network• Non-compliant traffic is treated as best-

effort

NOSSDAV '97

Question

Can TCP-based applications take advantage of a network which provides controlled-load service?

NOSSDAV '97

System Model

• Source– Compliance check done at the source using a

token bucket filter derived from TSpec– Compliant packets sent marked– Non-compliant packets sent unmarked

• Network– Enhanced Random Early Detection (ERED)

NOSSDAV '97

Source Model

Sending source

TCP Send

Compliance Check

Network

NOSSDAV '97

Network support

• RED queues– Random early packet dropping for congestion

avoidance– Keep queue lengths small– Avoid synchronization– Remove biases against bursty traffic

NOSSDAV '97

Enhanced RED queues (ERED)

• Same as RED, but marked packets have a much lower drop probability than unmarked packets– Single queue implementation– Retains FIFO ordering– Does not require per-flow information in the

data forwarding path

NOSSDAV '97

Example scenario

• Reserved connections should get reserved rate and a share of the excess bandwidth

• 3 sources with 1Mbs, 2Mbs and 4Mbs policed with token buckets of depth 50ms

• 3 best-effort sources• 80KB ERED queues at each router• Simulated using ns-1.1

S D

NOSSDAV '97

TCP with reservations

00.5

11.5

22.5

33.5

44.5

Bandwidth (Mbs)

1 Mbs 2 Mbs 4 Mbs BEReservation Level

Total Bandwidth

IdealObserved

NOSSDAV '97

Problem

• TCP uses acknowledgement based triggers to send data

• Well-known problem of ACK compression which can cause gaps in ACK stream

• Transmission credits build up in token bucket as TCP waits for an ACK

• Credits overflow and are lost

NOSSDAV '97

TCP losing tokens

DS

NOSSDAV '97

TCP timer modification

After every acknowledgementif (room under cwnd and awnd)

if (tokens available > packet size)send packet marked

else send packet unmarkedAfter every timer expiry

reset timerif (room under awnd)

if (tokens available > packet size)send packet marked

NOSSDAV '97

TCP timer modification

00.5

11.5

22.5

33.5

44.5

Bandwidth (Mbs)

1Mbs 2Mbs 4Mbs BEReservation Level

Total Bandwidth

IdealObserved

NOSSDAV '97

TCP timer modification

00.10.20.30.40.50.60.70.8

Bandwidth (Mbs)

1 Mbs 2 Mbs 4 Mbs BEReservation Level

Share of Residual Bandwidth

+TIdeal

NOSSDAV '97

Rate-adaptive windowingNormal Windowing

Rate Adaptive Windowing

WindowSize

Time

Time

WindowSize

NOSSDAV '97

TCP windowing modification

After every new acknowledgementif (cwnd < ssthresh)

cwnd = cwnd + (cwnd-rwnd)/cwndelse cwnd = cwnd + 1/cwnd

Upon detection of loss from DUPACKscwnd = rwnd + (cwnd-rwnd)/2 + ndupssthresh = rwnd + (cwnd-rwnd)/2

Upon RTOcwnd = rwnd + 1ssthresh = rwnd + (cwnd-rwnd)/2

NOSSDAV '97

TCP w/ timer and window mods

00.10.20.30.40.50.60.70.8

Bandwidth (Mbs)

1 Mbs 2 Mbs 4 Mbs BEReservation Level

Share of Residual Bandwidth

+T+T+WIdeal

NOSSDAV '97

Additional Experiments

• Performance when a subset or when no network routers support service differentiation

• Integration into a more elaborate packet scheduling and/or link scheduling experiments

• Influence on pricing• Reservations vs. adaptation

NOSSDAV '97

Summary

• TCP’s ack-clocking and windowing algorithm limit its performance in an integrated services environment

• Fine-grained timer and rate-adaptive windowing can solve this problem

• Extended version and simulation results at http://www.eecs.umich.edu/~wuchang/ered/

• TCP Brooklyn?

(We don’t play chess all day)