17. Appendix G - Applicability Statement As stated in the introduction, PGM has been designed with a specific class of applications in mind in recognition of the fact that a general solution for reliable multicast has proven elusive. The applicability of PGM is narrowed further, and perhaps more significantly, by the prototypical nature of at least four of the transport elements the protocol incorporates. These are congestion control, router assist, local retransmission, and a programmatic API for reliable multicast protocols of this class. At the same time as standardization efforts address each of these elements individually, this publication is intended to foster experimentation with these elements in general, and to inform that standardization process with results from practise. This section briefly describes some of the experimental aspects of PGM and makes non-normative references to some examples of current practise based upon them. At least 3 different approaches to congestion control can be explored with PGM: a receiver-feedback based approach, a router-assist based approach, and layer-coding based approach. The first is supported by the negative acknowledgement mechanism in PGM augmented by an application-layer acknowledgement mechanism. The second is supported by the router exception processing mechanism in PGM. The third is supported by the FEC mechanisms in PGM. An example of a receiver- feedback based approach is provided in , and a proposal for a router-assist based approach was proposed in . Open issues for the researchers include how do each of these approaches behave in the presence of multiple competing sessions of the same discipline or of different disciplines, TCP most notably; how do each of them behave over a particular range of topologies, and over a particular range of loads; and how do each of them scale as a function of the size of the receiver population. Router assist has applications not just to implosion control and retransmit constraint as described in this specification, but also to congestion control as described above, and more generally to any feature which may be enhanced by access to per-network-element state and processing. The full range of these features is as yet
unexplored, but a general mechanism for providing router assist in a transport-protocol independent way (GRA) is a topic of active research . That effort has been primarily informed by the router assist component of PGM, and implementation and deployment experience with PGM will continue to be fed back into the specification and eventual standardization of GRA. Open questions facing the researchers (, , ) include how router-based state scales relative to the feature benefit obtained, how system-wide factors (such as throughput and retransmit latency) vary relative to the scale and topology of deployed router assistance, and how incremental deployment considerations affect the tractability of router-assist based features. Router assist may have additional implications in the area of congestion control to the extent that it may be applied in multi-group layered coding schemes to increase the granularity and reduce the latency of receiver based congestion control. GRA itself explicitly incorporates elements of active networking, and to the extent that the router assist component of PGM is reflected in GRA, experimentation with the narrowly defined network-element functionality of PGM will provide some of the first real world experience with this promising if controversial technology. Local retransmission is not a new idea in general in reliable multicast, but the specific approach taken in PGM of locating re- transmitters on the distribution tree for the session, diverting repair requests from network elements to the re-transmitters, and then propagating repairs downward from the repair point on the distribution tree raises interesting questions concerning where to locate re-transmitters in a given topology, and how network elements locate those re-transmitters and evaluate their efficiency relative to other available sources of retransmissions, most notably the source itself. This particular aspect of PGM, while fully specified, has only been implemented on the network element side, and awaits a host-side implementation before questions like these can be addressed. PGM presents the opportunity to develop a programming API for reliable multicast applications that reflects both those applications' service requirements as well as the services provided by PGM in support of those applications that may usefully be made visible above the transport interface. At least a couple of host- side implementations of PGM and a concomitant API have been developed for research purposes (, ), and are available as open source explicitly for the kind of experimentation described in this section. Perhaps the broadest experiment that PGM can enable in a community of researchers using a reasonable scale experimental transport protocol is simply in the definition, implementation, and deployment of IP
multicast applications for which the reliability provided by PGM is a significant enabler. Experience with such applications will not just illuminate the value of reliable multicast, but will also provoke practical examination of and responses to the attendant policy issues (such as peering, billing, access control, firewalls, NATs, etc.), and, if successful, will ultimately encourage more wide spread deployment of IP multicast itself. 18. Abbreviations ACK Acknowledgment AFI Address Family Indicator ALF Application Level Framing APDU Application Protocol Data Unit ARQ Automatic Repeat reQuest DLR Designated Local Repairer GSI Globally Unique Source Identifier FEC Forward Error Correction MD5 Message-Digest Algorithm MTU Maximum Transmission Unit NAK Negative Acknowledgment NCF NAK Confirmation NLA Network Layer Address NNAK Null Negative Acknowledgment ODATA Original Data POLL Poll Request POLR Poll Response RDATA Repair Data RSN Receive State Notification SPM Source Path Message SPMR SPM Request TG Transmission Group TGSIZE Transmission Group Size TPDU Transport Protocol Data Unit TSDU Transport Service Data Unit TSI Transport Session Identifier TSN Transmit State Notification
19. Acknowledgements The design and specification of PGM has been substantially influenced by reviews and revisions provided by several people who took the time to read and critique this document. These include, in alphabetical order: Bob Albrightson Joel Bion Mark Bowles Steve Deering Tugrul Firatli Dan Harkins Dima Khoury Gerard Newman Dave Oran Denny Page Ken Pillay Chetan Rai Yakov Rekhter Dave Rossetti Paul Stirpe Brian Whetten Kyle York 20. References  B. Whetten, T. Montgomery, S. Kaplan, "A High Performance Totally Ordered Multicast Protocol", in "Theory and Practice in Distributed Systems", Springer Verlag LCNS938, 1994.  S. Floyd, V. Jacobson, C. Liu, S. McCanne, L. Zhang, "A Reliable Multicast Framework for Light-weight Sessions and Application Level Framing", ACM Transactions on Networking, November 1996.  J. C. Lin, S. Paul, "RMTP: A Reliable Multicast Transport Protocol", ACM SIGCOMM August 1996.  Miller, K., Robertson, K., Tweedly, A. and M. White, "Multicast File Transfer Protocol (MFTP) Specification", Work In Progress.  Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC 1112, August 1989.  Katz, D., "IP Router Alert Option", RFC 2113, February 1997.  C. Partridge, "Gigabit Networking", Addison Wesley 1994.
 H. W. Holbrook, S. K. Singhal, D. R. Cheriton, "Log-Based Receiver-Reliable Multicast for Distributed Interactive Simulation", ACM SIGCOMM 1995.  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April 1992.  Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1700, October 1994.  J. Nonnenmacher, E. Biersack, D. Towsley, "Parity-Based Loss Recovery for Reliable Multicast Transmission", ACM SIGCOMM September 1997.  L. Rizzo, "Effective Erasure Codes for Reliable Computer Communication Protocols", Computer Communication Review, April 1997.  V. Jacobson, "Congestion Avoidance and Control", ACM SIGCOMM August 1988.  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP, 14, RFC 2119, March 1997.  J. Bolot, T. Turletti, I. Wakeman, "Scalable Feedback Control for Multicast Video Distribution in the Internet", Proc. ACM/Sigcomm 94, pp. 58-67.  L. Rizzo, "pgmcc: A TCP-friendly Single-Rate Multicast Congestion Control Scheme", Proc. of ACM SIGCOMM August 2000.  M. Luby, L. Vicisano, T. Speakman. "Heterogeneous multicast congestion control based on router packet filtering", RMT working group, June 1999, Pisa, Italy.  Cain, B., Speakman, T. and D. Towsley, "Generic Router Assist (GRA) Building Block, Motivation and Architecture", Work In Progress.  C. Papadopoulos, and E. Laliotis,"Incremental Deployment of a Router-assisted Reliable Multicast Scheme,", Proc. of Networked Group Communications (NGC2000), Stanford University, Palo Alto, CA. November 2000.
 C. Papadopoulos, "RAIMS: an Architecture for Router-Assisted Internet Multicast Services." Presented at ETH, Zurich, Switzerland, October 23 2000.  J. Chesterfield, A. Diana, A. Greenhalgh, M. Lad, and M. Lim, "A BSD Router Implementation of PGM", http://www.cs.ucl.ac.uk/external/m.lad/rpgm/  L. Rizzo, "A PGM Host Implementation for FreeBSD", http://www.iet.unipi.it/~luigi/pgm.html  M. Psaltaki, R. Araujo, G. Aldabbagh, P. Kouniakis, and A. Giannopoulos, "Pragmatic General Multicast (PGM) host implementation for FreeBSD.", http://www.cs.ucl.ac.uk/research/darpa/pgm/PGM_FINAL.html 21. Authors' Addresses Tony Speakman EMail: email@example.com Dino Farinacci Procket Networks 3850 North First Street San Jose, CA 95134 USA EMail: firstname.lastname@example.org Steven Lin Juniper Networks 1194 N. Mathilda Ave. Sunnyvale, CA 94086 USA EMail: email@example.com Alex Tweedly EMail: firstname.lastname@example.org Nidhi Bhaskar EMail: email@example.com Richard Edmonstone EMail: firstname.lastname@example.org Rajitha Sumanasekera EMail: email@example.com
Lorenzo Vicisano Cisco Systems, Inc. 170 West Tasman Drive, San Jose, CA 95134 USA EMail: firstname.lastname@example.org Jon Crowcroft Department of Computer Science University College London Gower Street London WC1E 6BT UK EMail: email@example.com Jim Gemmell Microsoft Bay Area Research Center 301 Howard Street, #830 San Francisco, CA 94105 USA EMail: firstname.lastname@example.org Dan Leshchiner Tibco Software 3165 Porter Dr. Palo Alto, CA 94304 USA EMail: email@example.com Michael Luby Digital Fountain, Inc. 39141 Civic Center Drive Fremont CA 94538 USA EMail: firstname.lastname@example.org Todd L. Montgomery Talarian Corporation 124 Sherman Ave. Morgantown, WV 26501 USA EMail: email@example.com
Luigi Rizzo Dip. di Ing. dell'Informazione Universita` di Pisa via Diotisalvi 2 56126 Pisa Italy EMail: firstname.lastname@example.org
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