1 Michele Pagano – HET-NET 2005 - Ilkely, July - 2005

Modeling the Behavior of a DVB- RCS Satellite Network: an Empirical

Validation

Davide Adami, Stefano Giordano, Michele Pagano, Raffaello Secchi

Dipartimento di Ingegneria dell’InformazioneUniversita’ di Pisa

2 Michele Pagano – HET-NET 2005 - Ilkely, July - 2005

Outline

• Motivation

• Introduction to DVB-RCS architecture

– satellite network elements

– bandwidth allocation strategies

• The modeling methodology

• Modeling Validation through actual traffic

measurements

– impact on UDP Constant Rate Traffic

– behavior of TCP Short Lived Flows

3 Michele Pagano – HET-NET 2005 - Ilkely, July - 2005

Motivations

• Satellite Networks provide access to vast regions at a low cost:– The satellite link bandwidth is a scarce resource and its

use should optimized – The DVB-RCS is designed to support user interactivity

from satellite link and integrate satellite networks into the global Internet infrastructure

• An analytical framework is required:– To evaluate the behavior of BoD algorithms at IP layer– To evaluate the impact of satellite MAC on TCP/IP traffic– To the project of new satellite access scheme

4 Michele Pagano – HET-NET 2005 - Ilkely, July - 2005

DVB-RCS Architecture

• The Regional Network Control Center (RNCC) provides control and management to a group of terminals

• The MAC allocation is based on Multi-Frequency Time Division Multiple Access (MF-TDMA) scheme

bandwidth

request

bandwidthallocation

end-to-end connection

5 Michele Pagano – HET-NET 2005 - Ilkely, July - 2005

DVB-RCS Allocation Strategies

• Constant Rate Assignment (CRA)– Bandwidth is negotiated between the traffic terminal and

RNCC at the beginning of connection

• Rate Based Dynamic Capacity (RBDC)– Traffic terminals submit to RNCC bandwidth request

messages based on rate measurement of local incoming traffic

• Volume Based Dynamic Capacity (VBDC)– Each terminals request the amount of bandwidth per frame

needed to empty its buffer

• Free Capacity Assignment (FCA)– No explicit requests comes from terminals. The RNCC assign

bandwidth using some fairness criteria

6 Michele Pagano – HET-NET 2005 - Ilkely, July - 2005

RBDC Allocation Strategy

11 krkxkr

The requested bandwidth is the smoothing of amount of traffic seen during k-th resource allocation

BoD controller

Traffic Terminal

BoD Processing

system response time (L frames)

safe frame period

k-th resource allocation period

r(k) a(k)

The BoD Controller assign bandwidth as long as is lower than the ceiling threshold RBDCmax and at least the Committed Information Rate (CIR) max,,maxmin RBDCCIRLkrka

1 2 3 4 5 6 7 8 9 10 11 12 13

14 15 1617 19 20 21 22

7 Michele Pagano – HET-NET 2005 - Ilkely, July - 2005

Continuous Time Approximation (1)

tx

ta

tLtxta Fs

Let define … Instantaneous input rate measured at traffic station

Rate assigned from RNCC to TT

If less input rate is less than RBDCmax and enough bandwidth is available, the bandwidth reserved for a traffic station is given by

The F is a positive noisy term that takes the discrete nature of time-slot allocation into account

trRequested bandwidth from TT to RNCC

The requested bandwidth is a smoothed version of input rate. Assuming high time constant r(t) = x(t)

8 Michele Pagano – HET-NET 2005 - Ilkely, July - 2005

Continuous Time Approximation (2)

tatxtatxtq tq 01

By applying heavy load approximation to Lyndley’s recursion, we have

Rta

RtqTtRTT

0

Thus, assuming packet buffering only at traffic terminal

t t

Fs ddLxxtq

The queue size evolution is obtained by substituting previous expression and integrating

9 Michele Pagano – HET-NET 2005 - Ilkely, July - 2005

Satellite Measurement Test-Bed

Total Capacity (Ct) 2122 Kbps

Frame Period 273.3 ms

slot bandwidth (Cs) 44 kbps

Resource Allocation Period 400 ms

TT TT

SAT

SKYPLEX data

terminal

Ethernet LAN

A B

DBV-RCS DBV-S

Characteristics of satellite link

Laptop PC

Laptop PC

Since 18 TT were active, the available bandwidth was

KbpsCCRBDC st 136417max

10 Michele Pagano – HET-NET 2005 - Ilkely, July - 2005

Measurement Sessions

• UDP Traffic Measurements– We use constant rate UDP traffic to evaluate the

characteristics of satellite link and validate our RTT model

• TCP Traffic Measurements – We schedule a new TCP connection carrying 600KB every

60 seconds and we evaluate the mean behavior of TCP cwnd and RTT

– Since the TCP connection does not meet losses, TCP never from slow-start phase and the ssthresh remains unset

– A simple rate profile is a rate pulse with exponential increasing

Dt

Rt

R

w

R

0max

1

1,2

min

11 Michele Pagano – HET-NET 2005 - Ilkely, July - 2005

UDP Traffic Measurements

Packet Size (bytes)

direction mean RTT (ms) std. dev. (ms)

1024A-B-A 607.71 13.28

B-A-B 596.50 17.69

512A-B-A 601.35 12.40

B-A-B 590.07 15.19

• The path is symmetrical and introduces low jitter (2%)

• The behavior weakly dependent from packet size

• We observe very low drop rate (<10-4)

12 Michele Pagano – HET-NET 2005 - Ilkely, July - 2005

UDP Flow Measurements: Throughput

The output presents an evident overshot after the transition time.

This phenomenon should be attributed to presence of non-linear term F.

Comparison between experimental and theoretical throughput

13 Michele Pagano – HET-NET 2005 - Ilkely, July - 2005

UDP Flow Measurements: RTT

After the transition time the RTT undergoes a drastical increase. The extent of RTT increasing is inversely proportional to rate

Comparison between experimental and theoretical RTT

14 Michele Pagano – HET-NET 2005 - Ilkely, July - 2005

Average TCP Congestion Window

Wmax = 32KB

Slow-start exponential increasing advertised-window

saturated

three-way

handshake

TCP is unable to saturate channel capacity !

15 Michele Pagano – HET-NET 2005 - Ilkely, July - 2005

TCP Round Trip Time Dynamics

• The theoretical RTT shows a good match with the real RTT dynamic

• TCP packets may experiment nearly two times the RTT observed during steady state period

We fed the RTT model with actual traffic data and compare the results with experimental RTT

16 Michele Pagano – HET-NET 2005 - Ilkely, July - 2005

Conclusions

In this work, we evaluate the impact of BoD mechanisms on TCP/IP traffic by means of an analytical approach

Our analysis highlights some issues:– large delay variations determine long delays and

performance degradations– Short-lived TCP connections may achieve low

throughput due to RTT increasing during connections start-up

– The advertised-window allow 64KB at most, but the satellite link bandwidth-delay product is higher than that