Karlstad University Studies
ISSN 1403-8099 ISBN 91-7063-046-1
Faculty of Economy, Communication and IT Computer Science
DISSErTaTIoN Karlstad University Studies
2006:8
Karl-Johan Grinnemo
Transport Services for Soft real-Time applications
in IP Networks
Karl-Johan Grinnemo Transport Services for Soft Real-Time Applications in IP Networks
Transport Services for Soft real-Time applications
in IP Networks
Providing a transport service over IP that meets the timeliness and availability require- ments of soft real-time applications has turned out to be a complex task. although net- work solutions such as IntServ, DiffServ, MPLS, and VrrP have been suggested, these solutions many times fail to provide a transport service for soft real-time applications end to end. In light of this, this thesis considers transport protocols for soft real-time applications.
Part I of the thesis focuses on the design and analysis of transport protocols for soft real- time multimedia applications with lax deadlines such as image-intensive Web applica- tions. Specifically, Part I investigates the feasibility of designing retransmission-based, partially reliable transport protocols that are congestion aware and fair to competing traffic. Two transport protocols are presented in Part I, PrTP and PrTP-ECN, which are both extensions to TCP for partial reliability. Part I also presents a taxonomy for retransmission-based, partially reliable transport protocols.
Part II of the thesis considers the Stream Control Transmission Protocol (SCTP), which was developed by the IETF to transfer telephony signaling traffic over IP. The main focus of Part II is on evaluating the SCTP failover mechanism.
Karlstad University Studies 2006:8
Karl-Johan Grinnemo
Transport Services for
Soft Real-Time Applications
in IP Networks
Karl-Johan Grinnemo. Transport Services for Soft Real-Time Applications in IP Networks.
Dissertation
Karlstad University Studies 2006:8 ISSN 1403-8099
ISBN 91-7063-046-1
© The author
Distribution:
Karlstad University
Faculty of Economy, Communication and IT Computer Science
SE-651 88 KARLSTAD SWEDEN
+46 54-700 10 00 www.kau.se
Printed at: Universitetstryckeriet, Karlstad 2006
To my parents
Transport Services for
Soft Real-Time Applications in IP Networks
KARL-JOHAN GRINNEMO
Department of Computer Science, Karlstad University
Abstract
In recent years, Internet and IP technologies have made inroads into almost every commu- nication market ranging from best-effort services such as email and Web, to soft real-time applications such as VoIP, IPTV, and video. However, providing a transport service over IP that meets the timeliness and availability requirements of soft real-time applications has turned out to be a complex task. Although network solutions such as IntServ, DiffServ, MPLS, and VRRP have been suggested, these solutions many times fail to provide a trans- port service for soft real-time applications end to end. Additionally, they have so far only been modestly deployed. In light of this, this thesis considers transport protocols for soft real-time applications.
Part I of the thesis focuses on the design and analysis of transport protocols for soft real- time multimedia applications with lax deadlines such as image-intensive Web applications.
Many of these applications do not need a completely reliable transport service, and to this end Part I studies so-called partially reliable transport protocols, i.e., transport protocols that enable applications to explicitly trade reliability for improved timeliness. Specifically, Part I investigates the feasibility of designing retransmission-based, partially reliable transport protocols that are congestion aware and fair to competing traffic. Two transport protocols are presented in Part I, PRTP and PRTP-ECN, which are both extensions to TCP for partial reliability. Simulations and theoretical analysis suggest that these transport protocols could give a substantial improvement in throughput and jitter as compared to TCP. Additionally, the simulations indicate that PRTP-ECN is TCP friendly and fair against competing congestion- aware traffic such as TCP flows. Part I also presents a taxonomy for retransmission-based, partially reliable transport protocols.
Part II of the thesis considers the Stream Control Transmission Protocol (SCTP), which was developed by the IETF to transfer telephony signaling traffic over IP. The main focus of Part II is on evaluating the SCTP failover mechanism. Through extensive experiments, it is suggested that in order to meet the availability requirements of telephony signaling, SCTP has to be configured much more aggressively than is currently recommended by IETF. Fur- thermore, ways to improve the transport service provided by SCTP, especially with regards to the failover mechanism, are suggested. Part II also studies the effects of Head-of-Line Blocking (HoLB) on SCTP transmission delays. HoLB occurs when packets in one flow block packets in another, independent, flow. The study suggests that the short-term effects of HoLB could be substantial, but that the long-term effects are marginal.
Keywords: transport protocol, congestion control, partial reliability, soft real-time, SCTP, failover, SIGTRAN, head-of-line blocking
Acknowledgments
This thesis has benefited from the help of many people. First and foremost, I would like to acknowledge my supervisor, Prof. Anna Brunstrom (Department of Computer Science, Karlstad University, Sweden), for being an excellent research mentor, and for guiding me through my doctoral studies. Also, I would like to express my sincere gratitude to my em- ployer and main sponsor, TietoEnator. Without their economical and administrative support, I would not have been able to pursue any doctoral studies in the first place.
Second, I would like to thank my current co-supervisor, Dr. Reiner Ludwig (Senior Spe- cialist, Ericsson Research, Aachen, Germany) for his assistance in my work on SCTP, and my former co-supervisor, Dr. Jakob ¨Angeby (Mecel, ˙Amal, Sweden) for his support during my licentiate thesis. I would also like to acknowledge all co-authors of the papers included in this thesis and written during my doctoral studies: Prof. Anna Brunstrom, Stephan Baucke (Ericsson Research, Aachen, Germany), Dr. Reiner Ludwig, Assistant Prof. Johan Garcia (Department of Computer Science, Karlstad University, Sweden), Torbj¨orn Andersson (Ti- etoEnator, Karlstad, Sweden), Assistant Prof. Stefan Lindskog (Department of Computer Science, Karlstad University, Sweden), Annika Wennstr¨om (Department of Computer Sci- ence, Karlstad University, Sweden), Katarina Asplund (Department of Computer Science, Karlstad University, Sweden), Sean Schneyer (Ericsson Inc., Richardson, Texas, U.S.), and Prof. Adam Wolisz (Department of Electrical Engineering, Technical University of Berlin, Germany).
Third, I would like to acknowledge the past and present members of the Distributed Systems and Communications Research Group (DISCO) at the Department of Computer Science at Karlstad University including Prof. Anna Brunstrom, Assistant Prof. Johan Garcia, Johan Eklund, Stefan Alfredsson, Annika Wennstr¨om, Hannes Persson, Assistant Prof. Andreas Kassler, and Katarina Asplund; especially, I would like to thank Johan Gar- cia and Stefan Alfredsson for helping me several times with Unix and Linux questions. I would also like to acknowledge the past and present members of the PRTP/SCTP reference group: Gunnar Lorentzon (TietoEnator, Karlstad, Sweden), Magnus Larsson (TietoEnator, Karlstad, Sweden), Mikael Blom (TietoEnator, Karlstad, Sweden), S¨oren Torstensson (Ti- etoEnator, Karlstad, Sweden), Eivind Nordby (Department of Computer Science, Karlstad University, Sweden), and Prof. Erland Jonsson (Department of Computer Science and Engi- neering, Chalmers University of Technology, Sweden). In this context, I would also like to thank Ulf Melin (TietoEnator, Karlstad, Sweden) and Nils B¨ojeryd (TietoEnator, Karlstad, Sweden) for their assistance with telecom and SIGTRAN questions, and Rickard Persson (TietoEnator, Karlstad, Sweden) for reviewing and assisting me with Paper VI.
Last but not least, this thesis would not have been possible without the support of my parents, G¨oran and Ingrid Grinnemo, and my brother Karl-Henrik Grinnemo. I am also indebted to Rikard Ed-Svensson for being a good friend.
List of Appended Papers
This thesis consists of an introductory summary and reprints of the following ten papers:
Part I: Partially Reliable Transport Protocols for Multimedia Applications
Paper I: K-J Grinnemo, J. Garcia, and A. Brunstrom. Taxonomy and Survey of Retransmission-based Partially Reliable Transport Protocols. Computer Com- munications, Elsevier, Vol. 27, Issue 15, September 2004, pp. 1441–1452.
Paper II: K-J Grinnemo and A. Brunstrom. A Simulation Based Performance Eval- uation of PRTP. Karlstad University Studies 2002:35, Karlstad University, Swe- den, October 2002.
Paper III: K-J Grinnemo and A. Brunstrom. Evaluation of the QoS offered by PRTP- ECN – A TCP Compliant Partially Reliable Transport Protocol. In Proceedings of the 9th International Workshop on QoS (IWQoS), pp. 217–231, Karlsruhe, Germany, June 2001.
Paper IV: K-J Grinnemo and A. Brunstrom. A Simulation Based Performance Anal- ysis of a TCP Extension for Best Effort Multimedia Applications. In Proceedings of the 35th Annual Simulation Symposium (ANSS35), pp. 327–336, San Diego, California, USA, April 2002.
Paper V: K-J Grinnemo and A. Brunstrom. Enhancing TCP for Applications with Soft Real-Time Constraints. In Proceedings of the Convergence of Information Technologies and Communications (ITCom), Vol. 4518, pp. 18–31, Denver, Colorado, USA, August 2001.
Part II: Transport Service for Telephony Signaling
Paper VI: K-J Grinnemo and A. Brunstrom. Towards the Next Generation Network:
The Softswitch Solution. Karlstad University Studies 2006:6, Karlstad Univer- sity, Sweden, April 2006.
Paper VII: K-J Grinnemo, T. Andersson, and A. Brunstrom. Performance Benefits of Avoiding Head-of-Line Blocking in SCTP. In Proceedings of the Joint Inter- national Conference on Autonomic/Autonomous Systems (ICAS)/
International Conference on Networking and Services (ICNS), Tahiti, French Polynesia, October 2005.
Paper VIII: K-J Grinnemo and A. Brunstrom. Performance of SCTP-controlled Failovers in M3UA-based SIGTRAN Networks. In Proceedings of the Advanced Simulation Technologies Conference (ASTC), Applied Telecommunication Sym- posium (ATS), Arlington, Virginia, USA, April 2004.
Paper IX: K-J Grinnemo and A. Brunstrom. Impact of Traffic Load on SCTP Failovers in SIGTRAN. In Proceedings of the 4th International Conference on Networking (ICN), Reunion Island, April 2005.
Paper X: S. Baucke, K-J Grinnemo, R. Ludwig, A. Brunstrom, and A. Wolisz. Using Relaxed Timer Backoff to Reduce SCTP Failover Times. Under submission.
Minor editorial changes have been made to some of the papers.
Comments on My Participation
I am the principal contributor to all papers except Papers I, VII, and X. The taxonomy pre- sented in Paper I builds upon an earlier work conducted by Assistant Prof. Johan Garcia and Prof. Anna Brunstrom at the Dept. of Computer Science at Karlstad University. However, I have substantially re-worked their taxonomy, and I am the main author of Paper I. In Paper VII, Torbj¨orn Andersson, at that time assistant at the Dept. of Computer Science at Karlstad University, is responsible for the design and execution of the tests presented in the paper.
My participation in Paper VII includes the analysis of the test results and the writing of the paper. Paper X is a joint effort between Ericsson Eurolab in Aachen and Karlstad University.
My work on this paper includes participation in the discussions which led to the proposed retransmission timeout strategy; taking part in the design and analysis of the simulations and experiments; executing the experiments; and co-authoring the paper.
Other Papers
Apart from the papers included in this thesis, I have authored or co-authored the following papers:
[1] K. Asplund, A. Brunstrom, J. Garcia, K-J Grinnemo, and S. Schneyer. PRTP – A Par- tially Reliable Transport Protocol for Multimedia Applications: Background Informa- tion and Analysis. Karlstad University Studies 1999:5, Karlstad University, Sweden, June 1999.
[2] K-J Grinnemo, J. Garcia, and A. Brunstrom. A Taxonomy and Survey of Retrans- mission Based Partially Reliable Transport Protocols. Karlstad University Studies 2002:34, Karlstad University, Sweden, October 2002.
[3] K-J Grinnemo and A. Brunstrom. A Survey of TCP-Friendly Congestion Control Mechanisms for Multimedia Traffic. Karlstad University Studies 2003:1, January 2003.
[4] K-J Grinnemo and A. Brunstrom. Impact of SCTP-controlled Failovers for M3UA Users in a Dedicated SIGTRAN Network. In Proceedings of the Second Swedish National Computer Networking Workshop (SNCNW). Stockholm, Sweden, September 2003.
[5] K-J Grinnemo and A. Brunstrom. Some Observations on the Performance of SCTP- controlled Failovers in M3UA-based SIGTRAN Networks. In Proceedings of the Sec- ond Swedish National Computer Networking Workshop (SNCNW). Karlstad, Sweden, November 2004.
[6] S. Lindskog, K-J Grinnemo, and A. Brunstrom. Physical Separation for Data Protec- tion based on SCTP Multihoming. In Proceedings of the Second Swedish National Computer Networking Workshop (SNCNW). Karlstad, Sweden, November 2004.
vi
[7] S. Lindskog, K-J Grinnemo, and A. Brunstrom. Data Protection Based on Physical Separation: Concepts and Application Scenarios. In Proceedings of the International Conference on Computational Science and its Application (ICCSA). Singapore, May 2005.
[8] K-J Grinnemo, S. Baucke, A. Brunstrom, R. Ludwig, and A. Wolisz. An Easy Way to Reduce SCTP Failover Times. In Proceedings of the Third Swedish National Com- puter Networking Workshop (SNCNW). Halmstad, Sweden, November 2005.
Contents
Introductory Summary 1
1 Introduction 3
2 Research Objectives 4
3 Contributions 5
3.1 Part I: Partially Reliable Transport Protocols for Multimedia Applications . 5
3.2 Part II: Transport Service for Telephony Signaling . . . 6
4 Thesis Outline 7 4.1 Part I: Partially Reliable Transport Protocols for Multimedia Applications . 7 4.2 Part II: Transport Service for Telephony Signaling . . . 9
5 Conclusions and Future Research 10 Part I: Partially Reliable Transport Protocols for Multimedia Applications 13 Paper I: Taxonomy and Survey of Retransmission-based Partially Reliable Trans- port Protocols 15 1 Introduction 17 2 Preliminaries 19 3 The Taxonomy 20 3.1 Classification with Respect to Reliability Service . . . 20
3.2 Classification with Respect to Error Control Scheme . . . 23
4 A Classification and Survey of Existing Protocols 28 4.1 PECC . . . 28
4.2 POCv2 . . . 30
4.3 SRP . . . 31
4.4 HPF . . . 32
4.5 PR-SCTP . . . 32
4.6 PRTP-ECN . . . 33
5 Concluding Remarks 34
Paper II: A Simulation Based Performance Evaluation of PRTP 39
1 Introduction 41
2 Protocol Design 42
3 The PRTP Simulation Model 44
4 Validation of the PRTP Simulation Model 44
4.1 Methodology . . . 45
4.2 Results . . . 46
5 Stationary Analysis 51 5.1 Simulation Experiment . . . 52
5.2 Results . . . 53
6 Transient Analysis 58 6.1 Simulation Experiment . . . 58
6.2 Results . . . 63
7 Conclusions 66 Paper III: Evaluation of the QoS Offered by PRTP-ECN – A TCP Compliant Par- tially Reliable Transport Protocol 71 1 Introduction 73 2 Related Work 74 3 Overview of PRTP-ECN 75 4 Description of Simulation Experiment 76 4.1 Implementation . . . 77
4.2 Simulation Methodology . . . 77
4.3 Selection of PRTP-ECN Configurations . . . 78
4.4 Performance Metrics . . . 79
5 Results 80 6 Conclusions and Future Work 83 Paper IV: A Simulation Based Performance Analysis of a TCP Extension for Best Effort Multimedia Applications 87 1 Introduction 89 2 Overview of PRTP-ECN 91 3 Description of the Simulation Experiment 92 3.1 Statistical Design and Analysis . . . 92
3.2 Simulation Procedure . . . 94
x
4 Results of the Simulation Experiment 95
4.1 Average Interarrival Jitter . . . 96
4.2 Average Throughput and Average Goodput . . . 98
4.3 Average Link Utilization . . . 100
4.4 Average Fairness and TCP-Friendliness . . . 101
5 Conclusions 104 Paper V: Enhancing TCP for Applications with Soft Real-Time Constraints 107 1 Introduction 109 2 Related Work 110 3 The PRTP-ECN Retransmission Scheme 112 4 Packet-Loss Behavior of the PRTP-ECN Retransmission Scheme 114 4.1 The Startup Behavior . . . 115
4.2 The Steady-State Behavior . . . 116
4.3 The Maximum Allowable Packet-Loss Burst Length . . . 120
5 Simulation Experiment 123 5.1 Methodology . . . 123
5.2 Results . . . 124
6 Conclusion 127 Part II: Transport Service for Telephony Signaling 131 Paper VI: Towards the Next Generation Network: The Softswitch Solution 133 1 Introduction 136 2 Signaling in Today’s Telecommunication Networks 137 2.1 Taxonomy of Signaling . . . 137
2.2 SS7 Network Architecture . . . 139
2.3 The SS7 Protocol Stack . . . 141
2.4 SS7 in PSTN . . . 147
2.5 SS7 in PLMN . . . 149
2.6 Intelligent Networks . . . 154
3 The Softswitch Architecture 156 4 Applications and Services 162 4.1 Application Programming Languages . . . 163
4.2 API Frameworks . . . 169
5 Call Control Signaling 181
5.1 H.323 . . . 182
5.2 SIP . . . 188
6 Bearer Signaling 194 7 Interworking with Legacy Circuit-Switched Networks 199 7.1 SCTP . . . 199
7.2 Adaptation Component . . . 204
7.3 M2PA . . . 207
7.4 M2UA . . . 207
7.5 M3UA . . . 209
7.6 SUA . . . 212
8 Future Outlook 212
9 Summary 220
Paper VII: Performance Benefits of Avoiding Head-of-Line Blocking in SCTP 233
1 Introduction 235
2 SCTP and HoLB 237
3 Methodology 238
4 Results 240
5 Conclusions 246
Paper VIII: Performance of SCTP-controlled Failovers in M3UA-based SIGTRAN
Networks 249
1 Introduction 251
2 Methodology 253
3 Results 256
4 Conclusions 261
Paper IX: Impact of Traffic Load on SCTP Failovers in SIGTRAN 265
1 Introduction 267
2 Failovers in SCTP 269
3 Methodology 270
xii
4 Results 274
5 Conclusions 276
Paper X: Using Relaxed Timer Backoff to Reduce SCTP Failover Times 279
1 Introduction 282
2 SCTP and SCTP Failover 283
2.1 Overview . . . 283
2.2 SCTP Multi-Homing and Failure Detection . . . 283
3 Background and Related Work 286 4 Stability of Relaxed Exponential Backoff 287 5 Theoretical Estimation of Failover Times 288 6 Experimental Estimation of Failover Times 291 6.1 Simulation Setup . . . 291
6.2 Traffic Scenarios . . . 293
6.3 Simulation Parameters . . . 294
6.4 Simulation Results . . . 294
6.5 Validation of Simulations . . . 297
7 Conclusions 298
Introductory Summary
1. Introduction 3
1 Introduction
Over the course of the last decade, the phenomenal success of the Internet and the universal adoption of Internet technologies have driven profound changes in the data- and telecommu- nication industry. Maybe the most remarkable outcome of this evolution is the vision of the Internet as a ubiquitous service platform for basically every known communication service;
from being the platform of basic data services such as file transfer, email, and Web browsing, to also become a platform for applications such as video broadcasting, IPTV, and, not the least, wireline and wireless telecommunication. However, using Internet as a ubiquitous ser- vice platform greatly challenges the overall architectural philosophy of the Internet Protocol (IP) which prescribes an end-to-end architecture with “smart” hosts and a “dumb” switching network [19]. In particular, IP does not lend itself easily to soft real-time applications, such as video and telephony, with time deadlines that need to be met most of the time, and with fairly stringent availability requirements.
To address the timeliness and availability requirements of soft real-time applications, quality-of-service (QoS) architectures such as the Integrated Services (IntServ) [5] and Dif- ferentiated Services (DiffServ) [7] architectures have been proposed; traffic engineering so- lutions such as MultiProtocol Label Switching (MPLS) [18] have been developed; and avail- ability/redundancy solutions such as the Virtual Router Redundancy Protocol (VRRP) [11]
and IP-based Fast Rerouting [21] have been suggested. However, in spite of these network solutions, many times IP still has problems meeting soft real-time requirements. The reasons to this are many and include the fact that neither of the proposed network solutions have so far enjoyed widespread deployment. They are also fairly expensive and need to be supported by a complex management architecture. Furthermore, even from a theoretical viewpoint the proposed network solutions have difficulties to provide a real-time service from one end host to another [12]. Often the solutions fall back to the end-to-end architecture, and it becomes the transport protocols of the end hosts that try to provide an end-to-end real-time service to the best of their abilities. To this end, this thesis is concerned with transport protocols for soft real-time applications. The thesis considers both timeliness and availability issues, and focuses on two important categories of soft real-time applications in the Internet: multimedia and telephony signaling.
Part I of the thesis considers the design and analysis of transport protocols for multimedia applications; particularly, for multimedia applications with lax deadlines such as image- intensive Web applications. Many of these applications do not require a completely reliable transport service, and, in view of this, Part I studies so-called partially reliable transport protocols. These protocols enable an application to explicitly trade reliability for improved timeliness.
From an implementation perspective, we may differentiate between two major classes of partially reliable transport protocols: open- and closed-loop protocols (cf. Paper I). Open- loop protocols comprise those protocols which do not employ feedback from the network or end nodes when they perform error recovery, while closed-loop protocols do employ feed- back. Part I studies a subclass of closed-loop protocols, retransmission-based protocols, i.e., partially reliable transport protocols which recover from packet losses by retransmitting lost packets. Specifically, Part I investigates the feasibility of designing retransmission-based,
4 Introductory Summary
partially reliable transport protocols that are congestion aware and fair to contending flows.
Part II of the thesis considers the Stream Control Transmission Protocol (SCTP) [22].
The SCTP transport protocol was developed by the Internet Engineering Task Force (IETF) [1] for the transfer of Public Switched Telephony Network (PSTN) signaling traffic over IP. From a broad viewpoint, Part II studies how well SCTP meets the timeliness and availability requirements of PSTN signaling in the SIGnaling TRANsport (SIGTRAN) ar- chitecture [16], i.e., in the interworking architecture between traditional telecom networks and carrier-grade Voice over IP (VoIP) networks proposed by the IETF (cf. Paper VI).
The majority of Part II considers the performance of the network path recovery mecha- nism of SCTP, the so-called SCTP failover mechanism. SCTP supports redundant network paths between two end points: one primary path and one or several backup or alternative paths. Normally, all traffic goes on the primary path, however, if this path becomes un- available, traffic is rerouted to one of the alternative paths. The detection of an unavailable path, and the rerouting of traffic from the primary to the selected alternative path are done by the SCTP failover mechanism. To be able to interwork with the corresponding recovery mechanisms in the traditional telecom network, it is essential that SCTP exhibits the same failover performance as these mechanisms. To this end, Part II evaluates the performance of the SCTP failover mechanism, and, on the basis of this, suggests improvements to its current design.
Part II also studies the deteriorating effect of Head-of-Line Blocking (HoLB) on the end- to-end transmission delay. HoLB occurs when packets from one flow block packets from another separate, independent flow. Since mitigating the impact of HoLB was one of the main reasons SCTP was developed in the first place, Part II tries to quantify the effects of HoLB on PSTN signaling traffic under various network conditions.
2 Research Objectives
The overall objective of this thesis can be formulated as follows:
The objective of this thesis is to design and analyze IP-based transport protocols for soft real-time applications.
The thesis applies this objective to two important categories of soft real-time applica- tions in the Internet: multimedia and PSTN signaling. Part I of the thesis considers mul- timedia applications, and studies the design and analysis of retransmission-based, partially reliable transport protocols for multimedia applications with lax deadlines. Apart from mak- ing it possible for these applications to explicitly trade reliability for improved timeliness, the transport protocols should be congestion aware and compatible with existing Internet transport protocols. Furthermore, it is advantageous if the protocols exhibit a fair and TCP- friendly behavior. More formally, the objective of Part I can be formulated as follows:
The objective of Part I of this thesis is to design and analyze retransmission- based, partially reliable transport protocols for soft real-time multimedia appli- cations with lax deadlines. The transport protocols should be compatible with
3. Contributions 5
existing Internet transport protocols; be congestion-aware; and, ideally, react to congestion in a fair and TCP-friendly manner.
Part II of the thesis considers PSTN signaling over IP, and, in so doing, studies to what extent SCTP is able to meet important timeliness and availability requirements of PSTN signaling traffic in the IETF SIGTRAN architecture. Part II also studies ways of improving the transport service offered by SCTP. The objective of Part II can be formally stated in the following way:
The objective of Part II of this thesis is to evaluate the transport service offered by SCTP in terms of timeliness and availability, and investigate to what extent SCTP is able to meet PSTN signaling requirements in the IETF SIGTRAN ar- chitecture. The objective also includes studying ways of improving the SCTP transport service.
3 Contributions
The main contributions of this thesis are summarized in this section. The contributions of Part I are summarized in Subsection 3.1, and the contributions of Part II are summarized in Subsection 3.2.
3.1 Part I: Partially Reliable Transport Protocols for Multimedia Ap- plications
The main contribution of Part I of this thesis is the design and analysis of two retransmission- based, partially reliable transport protocols: Partially Reliable Transport Protocol (PRTP) and Partially Reliable Transport Protocol using Explicit Congestion Notification (PRTP- ECN). Both protocols are extensions to TCP, and both protocols build upon the same se- lective retransmission scheme. An application atop PRTP/PRTP-ECN explicitly specifies a minimum acceptable reliability level by setting some parameters of the retransmission scheme. Implicitly, the parameters govern the tradeoff between reliability, throughput, and jitter. By relaxing the reliability, the application can receive less jitter and better throughput.
An attractive feature of PRTP/PRTP-ECN is that neither one of them entail any elaborate changes to standard TCP. This in contrast to other suggested TCP extensions for partial reliability such as POC [3, 9] and TLTCP [15]. Further, unlike POC and TLTCP, both PRTP and PRTP-ECN are purely receiver based, i.e., only involve changes on the receiver side of a TCP connection.
Extensive simulations complemented with theoretical studies suggest that both PRTP and PRTP-ECN could give substantial improvements in average throughput compared to TCP for both long- and short-lived connections. They also suggest that both PRTP and PRTP-ECN could significantly reduce the average interarrival jitter for long-lived connections. Addi- tionally, the simulations indicate that PRTP-ECN is TCP friendly and fair against contending TCP flows.
6 Introductory Summary
Another contribution of our work on retransmission-based, partially reliable transport protocols is a taxonomy for this class of transport protocols. Apart from serving as a frame- work for classification, the taxonomy articulates the principal components of retransmission- based, partially reliable transport protocols; introduces a uniform terminology; and makes clear those aspects that need further research.
3.2 Part II: Transport Service for Telephony Signaling
The overall contribution of Part II of this thesis is that it provides guidelines for the configu- ration of SCTP in the SIGTRAN architecture, and suggests ways of improving the transport service offered by SCTP. In this way, Part II serves as a complement to the recommendations provided by IETF [8, 22]. However, the guidelines and improvements in Part II may also be applicable in other application domains where SCTP can be used and where timeliness and availability are important issues, e.g., multimedia applications.
The majority of Part II focuses on the SCTP failover mechanism and its ability to meet PSTN signaling availability requirements. Extensive experimental studies presented in Part II indicate that the SCTP failover mechanism must be configured much more aggressively than is recommended in the SCTP specification, RFC 2960 [22], in order to meet PSTN signaling availability requirements. In part, this finding is in agreement with work conducted by Jungmaier et al. [13]. The studies also suggest that bursty cross traffic in combination with large router queues could significantly deteriorate the performance of the SCTP failover mechanism. This implies that the routers in the SIGTRAN architecture should be configured with relatively small buffers, a finding which concurs with contemporary work [10, 17, 23].
On the basis of the studies made on the SCTP failover performance, Part II suggests con- figuration guidelines and improvements to the existing failover mechanism. Notably, Paper X recognizes that one of the main problems with the failover mechanism is that it relies on the SCTP retransmission strategy to detect a possible path failure. SCTP essentially counts retransmission timeouts, and when the number of consecutive timeouts reaches a predeter- mined retransmission threshold, the path is considered unavailable. However, since SCTP, like TCP, employs a binary exponential retransmission backoff scheme, this means that the failover time increases with a factor of two for each increase of the retransmission threshold.
To this end, Paper X studies the use of relaxed retransmission backoff schemes which employ backoff factors of less than two. Through simulations and experiments, it is shown that the proposed backoff schemes could significantly improve SCTP failover times. Further, since the SCTP retransmission and congestion control schemes are intertwined, Paper X argues, on the basis of existing research on Multiple Access Control (MAC) protocols, that such a scheme does not threaten network stability during congestion periods.
Part II also includes a fairly comprehensive experimental study on the impact of HoLB on ordered delivery in SCTP. The study suggests that HoLB could have substantial short-term effects on single messages or groups of messages in an SCTP flow, but that the long-term effects are marginal. Compared to previous studies on the effects of HoLB, our study more or less corroborates the work of Camarillo et al. [6], but to some extent refutes the work of De Marco et al. [14] and the prestudy on HoLB of Telcordia [20].
4. Thesis Outline 7
4 Thesis Outline
This thesis is arranged in two parts. Part I considers the design and analysis of retrans- mission-based, partially reliable transport protocols for soft real-time multimedia applica- tions. It comprises five papers: Paper I – Paper V. Paper I presents a taxonomy and survey of retransmission-based, partially reliable transport protocols. Furthermore, Paper I provides an introduction to the subject. Paper II introduces PRTP, and Papers III - V discuss PRTP-ECN.
Part II of the thesis concerns the transport service offered by SCTP in the SIGTRAN ar- chitecture. It consists of five papers: Paper VI – Paper X. Paper VI, provides a background to our research on SCTP. It gives a comprehensive introduction to the softswitch solution, the common way of implementing carrier-grade VoIP networks, and how softswitch networks interwork with traditional telecom networks in the SIGTRAN architecture. Paper VII con- siders HoLB and its short- and long-term impact on SCTP traffic, and Paper VIII – Paper X address SIGTRAN availability and the SCTP failover mechanism. In particular, Paper VIII and Paper IX evaluate the SCTP failover performance in unloaded and loaded SIGTRAN networks, and Paper X presents our study of using relaxed retransmission backoff schemes to improve the failover performance.
A more detailed description of the papers included in this thesis is provided in Subsec- tions 4.1 and 4.2, below.
4.1 Part I: Partially Reliable Transport Protocols for Multimedia Ap- plications
Paper I: Taxonomy and Survey of Retransmission-based Partially Reliable Transport Protocols
This paper presents a taxonomy for retransmission-based, partially reliable transport pro- tocols. The taxonomy consists of two classification schemes. The first scheme classifies retransmission-based, partially reliable transport protocols with respect to their offered re- liability service, and the second scheme classifies them with respect to their error control scheme. The paper also includes a survey of retransmission-based, partially reliable trans- port protocols. The survey primarily focuses on the services offered by the protocols; how they are realized; and how they map into the taxonomy. Taken together, the taxonomy and survey not only provides the foundation for retransmission-based, partially reliable transport protocols, they also serve as an introduction to this class of transport protocols, and to the remaining material in Part I of the thesis.
Paper II: A Simulation Based Performance Evaluation of PRTP
This paper introduces PRTP. The core principles and the design of the protocol are described.
However, the largest portion of the paper is devoted to an extensive simulation study of PRTP which comprises three simulation experiments. In the first simulation experiment, our simu- lation model of PRTP is validated against a prototype of PRTP in Linux that has previously been developed by Asplund et al. [4] in our research group. The second simulation exper- iment evaluates the stationary performance of PRTP compared to TCP. In particular, the
8 Introductory Summary
performance of PRTP for long-lived connections in terms of average interarrival jitter, av- erage throughput, and average fairness is considered. The second simulation experiment also studies whether PRTP is TCP friendly or not. Finally, the third simulation experiment evaluates the transient performance of PRTP compared to TCP. Specifically, the third sim- ulation experiment studies the throughput performance of PRTP in a typical Web browsing scenario. Since, the throughput obtained in Web browsing is very much dependent on the type of Internet connection, three types of connections are studied: fixed, modem, and GSM.
Paper III: Evaluation of the QoS Offered by PRTP-ECN – A TCP Compliant Partially Reliable Transport Protocol
The simulations presented in Paper II found PRTP to be TCP unfriendly and not altogether fair. To address this, PRTP-ECN was conceived. This paper considers PRTP-ECN: the principal ideas behind the protocol and its design. The stationary performance of PRTP- ECN is evaluated using the same simulation testbed as was used in the stationary analysis of PRTP (see Paper II). The paper gives a detailed description of the stationary analysis of PRTP-ECN. Specifically, it evaluates the stationary performance of PRTP-ECN compared to TCP in terms of average interarrival jitter, average throughput, average goodput, average fairness, and TCP friendliness.
Paper IV: A Simulation Based Performance Analysis of a TCP Extension for Best Effort Multimedia Applications
In the same way as Paper III, this paper considers the stationary performance analysis of PRTP-ECN. However, here the focus is on the statistical design and analysis of the simula- tion experiment. The simulation experiment is designed as a series of factorial experiments, one for each studied performance metric. The paper elaborates on the underlying effects model. Examples of issues discussed are model fitting, e.g., variance stabilizing transforms, and the statistical hypotheses tested. In addition to the performance metrics studied in Pa- per III, the paper considers the link utilization of PRTP-ECN as compared to TCP.
Paper V: Enhancing TCP for Applications with Soft Real-Time Constraints
This paper presents a theoretical analysis of the transient and stationary behavior of PRTP- ECN. The paper presents analytical expressions for the packet-loss tolerance of PRTP-ECN at startup; explicit formulae for the upper an lower bounds of the stationary packet-loss tolerance of PRTP; and, finally, an expression for the maximum packet-loss burst tolerated by PRTP-ECN in stationary state. Although the central theme of the paper is a theoretical evaluation of PRTP-ECN, it also provides a summary of the stationary performance analysis of PRTP-ECN as described in Paper III.
4. Thesis Outline 9
4.2 Part II: Transport Service for Telephony Signaling
Paper VI: Towards the Next Generation Network: The Softswitch Solution
This paper serves as an introduction to the application domain studied in Part II. It provides a comprehensive introduction to the softswitch solution, and how it interworks with traditional telecom networks in the SIGTRAN architecture. It also discusses the steps following the softswitch in the migration towards an all-IP telecom network. In particular, the paper briefly reviews the IP Multimedia Subsystem (IMS) [2] architecture and how it is envisioned to provide multimedia services in future IP-based wireless and wireline telecom core networks.
Paper VII: Performance Benefits of Avoiding Head-of-Line Blocking in SCTP
This paper studies the effect of HoLB on ordered delivery in SCTP. The paper considers both the short- and long-term effects of HoLB. Furthermore, the paper takes into account both packet-loss rate and distribution when studying the impact of HoLB. The major contribution of the paper is that it indicates that while HoLB could, indeed, substantially increase the transmission delay of a small fraction of messages in an SCTP session, it seems to have only a marginal impact on the average message transmission delay.
Paper VIII: Performance of SCTP-controlled Failovers in M3UA-based SIGTRAN Networks
This paper examines the performance of the SCTP failover mechanism in an unloaded SIG- TRAN network. The performance metrics considered in the paper are failover time and end-to-end transmission delay. The main contribution of the paper is that it shows that to have a failover performance on par with the failover performance in a traditional telecom network, SCTP has to be configured much more aggressively than what is suggested in the SCTP specification, RFC 2960 [22].
Paper IX: Impact of Traffic Load on SCTP Failovers in SIGTRAN
As a complement to Paper VIII, this paper examines the performance of the SCTP failover mechanism in a loaded SIGTRAN network. Apart from the impact of cross traffic, the impact of router queues on the SCTP failover performance is studied. Again, the performance metrics considered are failover time and end-to-end transmission delay. The tests presented in the paper cover a range of traffic loads with different degrees of burstiness. The major finding of the tests is that cross traffic, especially bursty cross traffic, could significantly deteriorate the SCTP failover performance. Moreover, the tests suggest that the sizes of the router queues in a SIGTRAN network is an important factor on the performance of SCTP failovers. In fact, the tests indicate that a combination of bursty cross traffic and large router queues could result in the SCTP failover performance failing to comply with PSTN signaling requirements.
10 Introductory Summary
Paper X: Using Relaxed Timer Backoff to Reduce SCTP Failover Times
The Papers VIII and IX suggest that SCTP may have difficulties accommodating the failover requirements of PSTN signaling, to this end this paper proposes an improvement to the ex- isting SCTP failover mechanism. Particularly, the paper studies a modified retransmission strategy which involves using a relaxed backoff factor of less than two. The paper shows through both simulations and experiments that such a modified retransmission strategy could substantially improve the SCTP failover performance. The paper also presents strong argu- ments from research on MAC protocols that SCTP, in spite of using a relaxed backoff factor, remains stable.
5 Conclusions and Future Research
This thesis considers IP-based transport protocols for soft real-time applications. The thesis is concerned with both timeliness and availability issues, and focuses on two particular cate- gories of applications: multimedia and PSTN signaling. Part I of the thesis considers the de- sign and analysis of a subclass of partially reliable transport protocols: retransmission-based, partially reliable transport protocols. The objective with this work was to study the feasibil- ity of designing retransmission-based, partially reliable transport protocols for soft real-time applications that are compatible with existing Internet transport protocols; are congestion aware; and are, if possible, fair and TCP friendly. Our work resulted in two extensions to TCP for partial reliability, PRTP and PRTP-ECN, and Part I shows through simulations and theoretical analysis that these protocols could give a substantial improvement in throughput and jitter compared to TCP. Furthermore, the simulations in Part I suggest that while PRTP is not altogether TCP friendly, PRTP-ECN is both TCP friendly and reasonably fair against competing TCP flows. Part I of the thesis also presents a taxonomy for retransmission- based, partially reliable transport protocols which, apart from serving as a classification framework, provides a uniform terminology for the subject. The work presented in Part I opens up a number of avenues for future research including effective image and/or video coding techniques for partially reliable transport protocols, and alternative partially reliable retransmission schemes.
Part II of the thesis evaluates the transport service provided by SCTP, and studies to what extent SCTP is able to meet PSTN signaling requirements in the IETF SIGTRAN architec- ture. The main focus of Part II is on the SCTP failover mechanism and its ability to meet the availability requirements of PSTN signaling. Through extensive experiments, it is suggested that in order to meet the availability requirements of PSTN signaling, SCTP has to be con- figured much more aggressively than is recommended in RFC 2960. Ways to improve the transport service provided by SCTP are also presented. In particular, a relaxed retransmis- sion scheme is proposed. Simulations and complementary experiments suggest that such a retransmission scheme could significantly improve the SCTP failover performance. Part II also studies the effects of HoLB on SCTP transmission delays. The study suggests that the short-term effects of HoLB could be substantial, but that the long-term effects are marginal.
This thesis does not by any means signify the end of our study of timeliness and availabil- ity issues in SCTP. Currently, SCTP uses more or less the same congestion control mecha-
REFERENCES 11
nism as TCP, a congestion control mechanism which is believed by many, such as Camarillo et al. [6], to be less than ideal for signaling traffic. In future research, we intend to study and evaluate alternative congestion control mechanisms that, in better ways, take into account the properties of signaling traffic, e.g., in terms of burstiness and duration. Also our research on the SCTP failover mechanism will be continued. Notably, we intend to further our study of relaxed retransmission timeout schemes and consider alternative solutions. Furthermore, we intend to more formally analyze the stability of congestion control schemes that utilize a relaxed retransmission strategy.
References
[1] Internet engineering task force (IETF).http://www.ietf.org.
[2] 3GPP. 3rd generation partnership project; technical specification group services and system aspects; IP multimedia subsystem (IMS); stage 2 (release 7). Technical Speci- fication TS 23.228 v.7.1.0, 3GPP, September 2005.
[3] P. D. Amer, C. Chassot, T. Connolly, M. Diaz, and P. T. Conrad. Partial order transport service for multimedia and other applications. ACM/IEEE Transactions on Networking, 2(5), October 1994.
[4] K. Asplund, J. Garcia, A. Brunstrom, and S. Schneyer. Decreasing transfer delay through partial reliability. In Protocols for Multimedia Systems (PROMS), Cracow, Poland, October 2000.
[5] R. Braden, D. Clark, and S. Shenker. Integrated services in the internet architecture.
RFC 1633, IETF, June 1994.
[6] G. Camarillo and H. Schulzrinne. Signalling transport protocols. Technical report, Dept. of Computer Science, Columbia University, February 2002.
[7] M. Carlson, W. Weiss, S. Blake, Z. Wang, D. Black, and E. Davies. An architecture for differentiated services. RFC 2475, IETF, December 1998.
[8] L. Coene and J. Pastor-Balbas. Telephony signalling transport over stream control transmission control (SCTP) applicability statement. RFC 4166, IETF, February 2006.
[9] M. Diaz, A. Lopez, C. Chassot, and P. D. Amer. Partial order connections: A new concept for high speed and multimedia services and protocols. Annals of Telecomuni- cations, 49(5–6):270–281, 1994.
[10] M. Enachescu, Y. Ganjali, A. Goel, N. McKeown, and T. Roughgarden. Part iii:
Routers with very small buffers. ACM Computer Communication Review, 35(3):83–89, July 2005.
[11] R. Hinden. Virtual router redundancy protocol (VRRP). RFC 3768, IETF, April 2004.
12 Introductory Summary
[12] G. Huston. Internet Performance Survival Guide – QoS Strategies for Multiservice Networks. John Wiley & Sons, Inc., 1st edition, 2000.
[13] A. Jungmaier, E P. Rathgeb, and M. Tuexen. On the use of SCTP in failover scenar- ios. In 6th World Multiconference on Systemics, Cybernetics and Informatics, Orlando, Florida, USA, July 2002.
[14] G. De Marco, D. De Vito, M. Longo, and S. Loreto. SCTP as a transport for SIP: a case study. In 7th World Multiconference on Systemics, Cybernetics and Informatics (SCI), Orlando, Florida, USA, July 2003.
[15] B. Mukherjee and T. Brecht. Time-lined TCP for the TCP-friendly delivery of stream- ing media. In IEEE International Conference on Network Protocols (ICNP), pages 165–176, Osaka, Japan, November 2000.
[16] L. Ong, I. Rytina, M. Garcia, H. Schwarzbauer, L. Coene, H. Lin, I. Juhasz, M. Hol- drege, and C. Sharp. Framework architecture for signalling transport. RFC 2719, IETF, October 1999.
[17] G. Raina, D. Towsley, and D. Wischik. Part ii: Control theory for buffer sizing. ACM Computer Communication Review, 35(3):79–82, July 2005.
[18] E. Rosen, A. Viswanathan, and R. Callon. Multiprotocol label switching architecture.
RFC 3031, IETF, January 2001.
[19] J. H. Saltzer, D. P. Reed, and D. D. Clark. End-to-end arguments in system design.
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[20] T. Seth, A. Broscius, C. Huitema, and H-A P. Lin. Performance requirements for signaling in internet telephony. Internet draft, IETF, November 1998. Work in Progress.
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[22] R. Stewart, Q. Xie, K. Morneault, C. Sharp, H. Schwarzbauer, T. Taylor, I. Rytina, M. Kalla, L. Zhang, and V. Paxson. Stream control transmission protocol. RFC 2960, IETF, October 2000.
[23] D. Wischik and N. McKeown. Part i: Buffer sizes for core routers. ACM Computer Communication Review, 35(3):75–78, July 2005.
Karlstad University Studies
Faculty of Economy, Communication and IT Computer Science
DISSErTaTIoN Karlstad University Studies
Karl-Johan Grinnemo
Transport Services for Soft real-Time applications
in IP Networks
Karl-Johan Grinnemo Transport Services for Soft Real-Time Applications in IP Networks
Transport Services for Soft real-Time applications
in IP Networks
Providing a transport service over IP that meets the timeliness and availability require- ments of soft real-time applications has turned out to be a complex task. although net- work solutions such as IntServ, DiffServ, MPLS, and VrrP have been suggested, these solutions many times fail to provide a transport service for soft real-time applications end to end. In light of this, this thesis considers transport protocols for soft real-time applications.
Part I of the thesis focuses on the design and analysis of transport protocols for soft real- time multimedia applications with lax deadlines such as image-intensive Web applica- tions. Specifically, Part I investigates the feasibility of designing retransmission-based, partially reliable transport protocols that are congestion aware and fair to competing traffic. Two transport protocols are presented in Part I, PrTP and PrTP-ECN, which are both extensions to TCP for partial reliability. Part I also presents a taxonomy for retransmission-based, partially reliable transport protocols.
Part II of the thesis considers the Stream Control Transmission Protocol (SCTP), which was developed by the IETF to transfer telephony signaling traffic over IP. The main focus of Part II is on evaluating the SCTP failover mechanism.
