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

1. Modeling the Behavior of a DVB- RCS Satellite Network: an Empirical Validation Davide Adami, Stefano Giordano, Michele Pagano, Raffaello Secchi Dipartimento di Ingegneria dell’Informazione Universita’ di Pisa

2. 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. 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. DVB-RCS Architecture bandwidth request end-to-end connection bandwidth allocation 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

5. 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. RBDC Allocation Strategy BoD controller BoD Processing k-th resource allocation period safe frame period Traffic Terminal 1 2 3 4 6 8 9 10 11 14 15 16 13 20 5 7 12 17 19 21 22 r(k) a(k) system response time (L frames) The requested bandwidth is the smoothing of amount of traffic seen during k-th resource allocation The BoD Controller assign bandwidth as long as is lower than the ceiling threshold RBDCmax and at least the Committed Information Rate (CIR)

7. Continuous Time Approximation (1) Let define … Instantaneous input rate measured at traffic station Requested bandwidth from TT to RNCC Rate assigned from RNCC to TT The requested bandwidth is a smoothed version of input rate. Assuming high time constant r(t) = x(t) If less input rate is less than RBDCmax and enough bandwidth is available, the bandwidth reserved for a traffic station is given by The Fis a positive noisy term that takes the discrete nature of time-slot allocation intoaccount

8. Continuous Time Approximation (2) By applying heavy load approximation to Lyndley’s recursion, we have The queue size evolution is obtained by substituting previous expression and integrating Thus, assuming packet buffering only at traffic terminal

9. Satellite Measurement Test-Bed SAT DBV-RCS DBV-S Laptop PC Ethernet LAN B A TT TT SKYPLEX data terminal Laptop PC Characteristics of satellite link Since 18 TT were active, the available bandwidth was

10. 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

11. UDP Traffic Measurements 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. UDP Flow Measurements: Throughput Comparison between experimental and theoretical throughput The output presents an evident overshot after the transition time. This phenomenon should be attributed to presence of non-linear term F.

13. UDP Flow Measurements: RTT Comparison between experimental and theoretical RTT After the transition time the RTT undergoes a drastical increase. The extent of RTT increasing is inversely proportional to rate

14. Average TCP Congestion Window Slow-start exponential increasing advertised-window saturated Wmax = 32KB TCP is unable to saturate channel capacity ! three-way handshake

15. TCP Round Trip Time Dynamics We fed the RTT model with actual traffic data and compare the results with experimental RTT 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

16. 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