RFC 2760
Ongoing TCP Research Related to Satellites
Pages: 46
Informational
Informational
Part 3 of 3 – Pages 29 to 46
3.8.1 Mitigation Description
Persistent TCP state information can be used to overcome limitations in the configuration of the initial state, and to automatically tune TCP to environments using satellite links and to coordinate multiple TCP connections sharing a satellite link. TCP includes a variety of parameters, many of which are set to initial values which can severely affect the performance of TCP connections traversing satellite links, even though most TCP parameters are adjusted later after the connection is established. These parameters include initial size of cwnd and initial MSS size. Various suggestions have been made to change these initial conditions, to more effectively support satellite links. However, it is difficult to select any single set of parameters which is effective for all environments. An alternative to attempting to select these parameters a-priori is sharing state across TCP connections and using this state when initializing a new connection. For example, if all connections to a subnet result in extended congestion windows of 1 megabyte, it is probably more efficient to start new connections with this value, than to rediscover it by requiring the cwnd to increase using slow start over a period of dozens of round-trip times.3.8.2 Research
Sharing state among connections brings up a number of questions such as what information to share, with whom to share, how to share it, and how to age shared information. First, what information is to be shared must be determined. Some information may be appropriate to share among TCP connections, while some information sharing may be inappropriate or not useful. Next, we need to determine with whom to share information. Sharing may be appropriate for TCP connections sharing a common path to a given host. Information may be shared among connections within a host, or even among connections between different hosts, such as hosts on the same LAN. However, sharing information between connections not traversing the same network may not be appropriate. Given the state to share and the parties that share it, a mechanism for the sharing is required. Simple state, like MSS and RTT, is easy to share, but congestion window information can be shared a variety of ways. The sharing mechanism determines priorities among the sharing connections, and a variety of fairness criteria need to be considered. Also, the mechanisms by which information is aged require further study. See RFC 2140 for a discussion of the security issues in both sharing state within a single host and sharing state among hosts on a subnet. Finally, the security concerns associated with sharing a piece of information need
to be carefully considered before introducing such a mechanism. Many of these open research questions must be answered before state sharing can be widely deployed. The opportunity for such sharing, both among a sequence of connections, as well as among concurrent connections, is described in more detail in [Tou97]. The state management itself is largely an implementation issue, however what information should be shared and the specific ways in which the information should be shared is an open question. Sharing parts of the TCB state was originally documented in T/TCP [Bra92], and is used there to aggregate RTT values across connection instances, to provide meaningful average RTTs, even though most connections are expected to persist for only one RTT. T/TCP also shares a connection identifier, a sequence number separate from the window number and address/port pairs by which TCP connections are typically distinguished. As a result of this shared state, T/TCP allows a receiver to pass data in the SYN segment to the receiving application, prior to the completion of the three-way handshake, without compromising the integrity of the connection. In effect, this shared state caches a partial handshake from the previous connection, which is a variant of the more general issue of TCB sharing. Sharing state among connections (including transfers using non-TCP protocols) is further investigated in [BRS99].3.8.3 Implementation Issues
Sharing TCP state across connections requires changes to the sender's TCP stack, and possibly the receiver's TCP stack (as in the case of T/TCP, for example). Sharing TCP state may make a particular TCP connection more aggressive. However, the aggregate traffic should be more conservative than a group of independent TCP connections. Therefore, sharing TCP state should be safe for use in shared networks. Note that state sharing does not present any new security problems within multiuser hosts. In such a situation, users can steal network resources from one another with or without state sharing.3.8.4 Topology Considerations
It is expected that sharing state across TCP connections may be useful in all network environments presented in section 2.
3.8.5 Possible Interaction and Relationships with Other Research
The state sharing outlined above is very similar to the Congestion Manager proposal [BRS99] that attempts to share congestion control information among both TCP and UDP flows between a pair of hosts.3.9 ACK Congestion Control
In highly asymmetric networks, a low-speed return link can restrict the performance of the data flow on a high-speed forward link by limiting the flow of acknowledgments returned to the data sender. For example, if the data sender uses 1500 byte segments, and the receiver generates 40 byte acknowledgments (IPv4, TCP without options), the reverse link will congest with ACKs for asymmetries of more than 75:1 if delayed ACKs are used, and 37:1 if every segment is acknowledged. For a 1.5 Mb/second data link, ACK congestion will occur for reverse link speeds below 20 kilobits/sec. These levels of asymmetry will readily occur if the reverse link is shared among multiple satellite receivers, as is common in many VSAT satellite networks. If a terrestrial modem link is used as a reverse link, ACK congestion is also likely, especially as the speed of the forward link is increased. Current congestion control mechanisms are aimed at controlling the flow of data segments, but do not affect the flow of ACKs. In [KVR98] the authors point out that the flow of acknowledgments can be restricted on the low-speed link not only by the bandwidth of the link, but also by the queue length of the router. The router may limit its queue length by counting packets, not bytes, and therefore begin discarding ACKs even if there is enough bandwidth to forward them.3.9.1 Mitigation Description
ACK Congestion Control extends the concept of flow control for data segments to acknowledgment segments. In the method described in [BPK97], any intermediate router can mark an acknowledgment with an Explicit Congestion Notification (ECN) bit once the queue occupancy in the router exceeds a given threshold. The data sender (which receives the acknowledgment) must "echo" the ECN bit back to the data receiver (see section 3.3.3 for a more detailed discussion of ECN). The proposed algorithm for marking ACK segments with an ECN bit is Random Early Detection (RED) [FJ93]. In response to the receipt of ECN marked data segments, the receiver will dynamically reduce the rate of acknowledgments using a multiplicative backoff. Once segments without ECN are received, the data receiver speeds up acknowledgments using a linear increase, up to a rate of either 1 (no
delayed ACKs) or 2 (normal delayed ACKs) data segments per ACK. The authors suggest that an ACK be generated at least once per window, and ideally a few times per window. As in the RED congestion control mechanism for data flow, the bottleneck gateway can randomly discard acknowledgments, rather than marking them with an ECN bit, once the queue fills beyond a given threshold.3.9.2 Research
[BPK97] analyze the effect of ACK Congestion Control (ACC) on the performance of an asymmetric network. They note that the use of ACC, and indeed the use of any scheme which reduces the frequency of acknowledgments, has potential unwanted side effects. Since each ACK will acknowledge more than the usual one or two data segments, the likelihood of segment bursts from the data sender is increased. In addition, congestion window growth may be impeded if the receiver grows the window by counting received ACKs, as mandated by [Ste97,APS99]. The authors therefore combine ACC with a series of modifications to the data sender, referred to as TCP Sender Adaptation (SA). SA combines a limit on the number of segments sent in a burst, regardless of window size. In addition, byte counting (as opposed to ACK counting) is employed for window growth. Note that byte counting has been studied elsewhere and can introduce side-effects, as well [All98]. The results presented in [BPK97] indicate that using ACC and SA will reduce the bursts produced by ACK losses in unmodified (Reno) TCP. In cases where these bursts would lead to data loss at an intermediate router, the ACC and SA modification significantly improve the throughput for a single data transfer. The results further suggest that the use of ACC and SA significantly improve fairness between two simultaneous transfers. ACC is further reported to prevent the increase in round trip time (RTT) that occurs when an unmodified TCP fills the reverse router queue with acknowledgments. In networks where the forward direction is expected to suffer losses in one of the gateways, due to queue limitations, the authors report at best a very slight improvement in performance for ACC and SA, compared to unmodified Reno TCP.
3.9.3 Implementation Issues
Both ACC and SA require modification of the sending and receiving hosts, as well as the bottleneck gateway. The current research suggests that implementing ACC without the SA modifications results in a data sender which generates potentially disruptive segment bursts. It should be noted that ACC does require host modifications if it is implemented in the way proposed in [BPK97]. The authors note that ACC can be implemented by discarding ACKs (which requires only a gateway modification, but no changes in the hosts), as opposed to marking them with ECN. Such an implementation may, however, produce bursty data senders if it is not combined with a burst mitigation technique. ACC requires changes to the standard ACKing behavior of a receiving TCP and therefore is not recommended for use in shared networks.3.9.4 Topology Considerations
Neither ACC nor SA require the storage of state in the gateway. These schemes should therefore be applicable for all topologies, provided that the hosts using the satellite or hybrid network can be modified. However, these changes are expected to be especially beneficial to networks containing asymmetric satellite links.3.9.5 Possible Interaction and Relationships with Other Research
Note that ECN is a pre-condition for using ACK congestion control. Additionally, the ACK Filtering algorithm discussed in the next section attempts to solve the same problem as ACC. Choosing between the two algorithms (or another mechanism) is currently an open research question.3.10 ACK Filtering
ACK Filtering (AF) is designed to address the same ACK congestion effects described in 3.9. Contrary to ACC, however, AF is designed to operate without host modifications.3.10.1 Mitigation Description
AF takes advantage of the cumulative acknowledgment structure of TCP. The bottleneck router in the reverse direction (the low speed link) must be modified to implement AF. Upon receipt of a segment which represents a TCP acknowledgment, the router scans the queue for redundant ACKs for the same connection, i.e. ACKs which acknowledge portions of the window which are included in the most recent ACK. All of these "earlier" ACKs are removed from the queue and discarded.
The router does not store state information, but does need to implement the additional processing required to find and remove segments from the queue upon receipt of an ACK.3.10.2 Research
[BPK97] analyzes the effects of AF. As is the case in ACC, the use of ACK filtering alone would produce significant sender bursts, since the ACKs will be acknowledging more previously-unacknowledged data. The SA modifications described in 3.9.2 could be used to prevent those bursts, at the cost of requiring host modifications. To prevent the need for modifications in the TCP stack, AF is more likely to be paired with the ACK Reconstruction (AR) technique, which can be implemented at the router where segments exit the slow reverse link. AR inspects ACKs exiting the link, and if it detects large "gaps" in the ACK sequence, it generates additional ACKs to reconstruct an acknowledgment flow which more closely resembles what the data sender would have seen had ACK Filtering not been introduced. AR requires two parameters; one parameter is the desired ACK frequency, while the second controls the spacing, in time, between the release of consecutive reconstructed ACKs. In [BPK97], the authors show the combination of AF and AR to increase throughput, in the networks studied, over both unmodified TCP and the ACC/SA modifications. Their results also strongly suggest that the use of AF alone, in networks where congestion losses are expected, decreases performance (even below the level of unmodified TCP Reno) due to sender bursting. AF delays acknowledgments from arriving at the receiver by dropping earlier ACKs in favor of later ACKs. This process can cause a slight hiccup in the transmission of new data by the TCP sender.3.10.3 Implementation Issues
Both ACK Filtering and ACK Reconstruction require only router modification. However, the implementation of AR requires some storage of state information in the exit router. While AF does not require storage of state information, its use without AR (or SA) could produce undesired side effects. Furthermore, more research is required regarding appropriate ranges for the parameters needed in AR.
3.10.4 Topology Considerations
AF and AR appear applicable to all topologies, assuming that the storage of state information in AR does not prove to be prohibitive for routers which handle large numbers of flows. The fact that TCP stack modifications are not required for AF/AR makes this approach attractive for hybrid networks and networks with diverse types of hosts. These modifications, however, are expected to be most beneficial in asymmetric network paths. On the other hand, the implementation of AF/AR requires the routers to examine the TCP header, which prohibits their use in secure networks where IPSEC is deployed. In such networks, AF/AR can be effective only inside the security perimeter of a private, or virtual private network, or in private networks where the satellite link is protected only by link-layer encryption (as opposed to IPSEC). ACK Filtering is safe to use in shared networks (from a congestion control point-of-view), as the number of ACKs can only be reduced, which makes TCP less aggressive. However, note that while TCP is less aggressive, the delays that AF induces (outlined above) can lead to larger bursts than would otherwise occur.3.10.5 Possible Interaction and Relationships with Other Research
ACK Filtering attempts to solve the same problem as ACK Congestion Control (as outlined in section 3.9). Which of the two algorithms is more appropriate is currently an open research question.4 Conclusions
This document outlines TCP items that may be able to mitigate the performance problems associated with using TCP in networks containing satellite links. These mitigations are not IETF standards track mechanisms and require more study before being recommended by the IETF. The research community is encouraged to examine the above mitigations in an effort to determine which are safe for use in shared networks such as the Internet.5 Security Considerations
Several of the above sections noted specific security concerns which a given mitigation aggravates. Additionally, any form of wireless communication link is more susceptible to eavesdropping security attacks than standard wire- based links due to the relative ease with which an attacker can watch the network and the difficultly in finding attackers monitoring the network.
6 Acknowledgments
Our thanks to Aaron Falk and Sally Floyd, who provided very helpful comments on drafts of this document.7 References
[AFP98] Allman, M., Floyd, S. and C. Partridge, "Increasing TCP's Initial Window", RFC 2414, September 1998. [AGS99] Allman, M., Glover, D. and L. Sanchez, "Enhancing TCP Over Satellite Channels using Standard Mechanisms", BCP 28, RFC 2488, January 1999. [AHKO97] Mark Allman, Chris Hayes, Hans Kruse, Shawn Ostermann. TCP Performance Over Satellite Links. In Proceedings of the 5th International Conference on Telecommunication Systems, March 1997. [AHO98] Mark Allman, Chris Hayes, Shawn Ostermann. An Evaluation of TCP with Larger Initial Windows. Computer Communication Review, 28(3), July 1998. [AKO96] Mark Allman, Hans Kruse, Shawn Ostermann. An Application- Level Solution to TCP's Satellite Inefficiencies. In Proceedings of the First International Workshop on Satellite-based Information Services (WOSBIS), November 1996. [All97a] Mark Allman. Improving TCP Performance Over Satellite Channels. Master's thesis, Ohio University, June 1997. [All97b] Mark Allman. Fixing Two BSD TCP Bugs. Technical Report CR-204151, NASA Lewis Research Center, October 1997. [All98] Mark Allman. On the Generation and Use of TCP Acknowledgments. ACM Computer Communication Review, 28(5), October 1998. [AOK95] Mark Allman, Shawn Ostermann, Hans Kruse. Data Transfer Efficiency Over Satellite Circuits Using a Multi-Socket Extension to the File Transfer Protocol (FTP). In Proceedings of the ACTS Results Conference, NASA Lewis Research Center, September 1995. [AP99] Mark Allman, Vern Paxson. On Estimating End-to-End Network Path Properties. ACM SIGCOMM, September 1999.
[APS99] Allman, M., Paxson, V. and W. Richard Stevens, "TCP Congestion Control", RFC 2581, April 1999. [BCC+98] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering, S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G., Partridge, C., Peterson, L., Ramakrishnan, K., Shenker, S., Wroclawski, J. and L. Zhang, "Recommendations on Queue Management and Congestion Avoidance in the Internet", RFC 2309, April 1998. [BKVP97] B. Bakshi and P. Krishna and N. Vaidya and D. Pradham, "Improving Performance of TCP over Wireless Networks", 17th International Conference on Distributed Computing Systems (ICDCS), May 1997. [BPK97] Hari Balakrishnan, Venkata N. Padmanabhan, and Randy H. Katz. The Effects of Asymmetry on TCP Performance. In Proceedings of the ACM/IEEE Mobicom, Budapest, Hungary, ACM. September, 1997. [BPK98] Hari Balakrishnan, Venkata Padmanabhan, Randy H. Katz. The Effects of Asymmetry on TCP Performance. ACM Mobile Networks and Applications (MONET), 1998 (to appear). [BPSK96] H. Balakrishnan and V. Padmanabhan and S. Sechan and R. Katz, "A Comparison of Mechanisms for Improving TCP Performance over Wireless Links", ACM SIGCOMM, August 1996. [Bra89] Braden, R., "Requirements for Internet Hosts -- Communication Layers", STD 3, RFC 1122, October 1989. [Bra92] Braden, R., "Transaction TCP -- Concepts", RFC 1379, September 1992. [Bra94] Braden, R., "T/TCP -- TCP Extensions for Transactions: Functional Specification", RFC 1644, July 1994. [BRS99] Hari Balakrishnan, Hariharan Rahul, and Srinivasan Seshan. An Integrated Congestion Management Architecture for Internet Hosts. ACM SIGCOMM, September 1999. [ddKI99] M. deVivo, G.O. deVivo, R. Koeneke, G. Isern. Internet Vulnerabilities Related to TCP/IP and T/TCP. Computer Communication Review, 29(1), January 1999.
[DENP97] Mikael Degermark, Mathias Engan, Bjorn Nordgren, Stephen Pink. Low-Loss TCP/IP Header Compression for Wireless Networks. ACM/Baltzer Journal on Wireless Networks, vol.3, no.5, p. 375-87. [DMT96] R. C. Durst and G. J. Miller and E. J. Travis, "TCP Extensions for Space Communications", Mobicom 96, ACM, USA, 1996. [DNP99] Degermark, M., Nordgren, B. and S. Pink, "IP Header Compression", RFC 2507, February 1999. [FF96] Kevin Fall, Sally Floyd. Simulation-based Comparisons of Tahoe, Reno, and SACK TCP. Computer Communication Review, V. 26 N. 3, July 1996, pp. 5-21. [FF99] Sally Floyd, Kevin Fall. Promoting the Use of End-to-End Congestion Control in the Internet, IEEE/ACM Transactions on Networking, August 1999. [FH99] Floyd, S. and T. Henderson, "The NewReno Modification to TCP's Fast Recovery Algorithm", RFC 2582, April 1999. [FJ93] Sally Floyd and Van Jacobson. Random Early Detection Gateways for Congestion Avoidance, IEEE/ACM Transactions on Networking, V. 1 N. 4, August 1993. [Flo91] Sally Floyd. Connections with Multiple Congested Gateways in Packet-Switched Networks, Part 1: One-way Traffic. ACM Computer Communications Review, V. 21, N. 5, October 1991. [Flo94] Sally Floyd. TCP and Explicit Congestion Notification, ACM Computer Communication Review, V. 24 N. 5, October 1994. [Flo99] Sally Floyd. "Re: TCP and out-of-order delivery", email to end2end-interest mailing list, February, 1999. [Hah94] Jonathan Hahn. MFTP: Recent Enhancements and Performance Measurements. Technical Report RND-94-006, NASA Ames Research Center, June 1994. [Hay97] Chris Hayes. Analyzing the Performance of New TCP Extensions Over Satellite Links. Master's Thesis, Ohio University, August 1997. [HK98] Tom Henderson, Randy Katz. On Improving the Fairness of TCP Congestion Avoidance. Proceedings of IEEE Globecom `98 Conference, 1998.
[HK99] Tim Henderson, Randy Katz. Transport Protocols for Internet-Compatible Satellite Networks, IEEE Journal on Selected Areas of Communications, February, 1999. [Hoe95] J. Hoe, Startup Dynamics of TCP's Congestion Control and Avoidance Schemes. Master's Thesis, MIT, 1995. [Hoe96] Janey Hoe. Improving the Startup Behavior of a Congestion Control Scheme for TCP. In ACM SIGCOMM, August 1996. [IL92] David Iannucci and John Lakashman. MFTP: Virtual TCP Window Scaling Using Multiple Connections. Technical Report RND-92-002, NASA Ames Research Center, January 1992. [Jac88] Van Jacobson. Congestion Avoidance and Control. In Proceedings of the SIGCOMM '88, ACM. August, 1988. [Jac90] Jacobson, V., "Compressing TCP/IP Headers", RFC 1144, February 1990. [JBB92] Jacobson, V., Braden, R. and D. Borman, "TCP Extensions for High Performance", RFC 1323, May 1992. [JK92] Van Jacobson and Mike Karels. Congestion Avoidance and Control. Originally appearing in the proceedings of SIGCOMM '88 by Jacobson only, this revised version includes an additional appendix. The revised version is available at ftp://ftp.ee.lbl.gov/papers/congavoid.ps.Z. 1992. [Joh95] Stacy Johnson. Increasing TCP Throughput by Using an Extended Acknowledgment Interval. Master's Thesis, Ohio University, June 1995. [KAGT98] Hans Kruse, Mark Allman, Jim Griner, Diepchi Tran. HTTP Page Transfer Rates Over Geo-Stationary Satellite Links. March 1998. Proceedings of the Sixth International Conference on Telecommunication Systems. [Kes91] Srinivasan Keshav. A Control Theoretic Approach to Flow Control. In ACM SIGCOMM, September 1991. [KM97] S. Keshav, S. Morgan. SMART Retransmission: Performance with Overload and Random Losses. Proceeding of Infocom. 1997.
[KVR98] Lampros Kalampoukas, Anujan Varma, and K. K.Ramakrishnan. Improving TCP Throughput Over Two-Way Asymmetric Links: Analysis and Solutions. Measurement and Modeling of Computer Systems, 1998, Pages 78-89. [MM96a] M. Mathis, J. Mahdavi, "Forward Acknowledgment: Refining TCP Congestion Control," Proceedings of SIGCOMM'96, August, 1996, Stanford, CA. Available from http://www.psc.edu/networking/papers/papers.html [MM96b] M. Mathis, J. Mahdavi, "TCP Rate-Halving with Bounding Parameters" Available from http://www.psc.edu/networking/papers/FACKnotes/current. [MMFR96] Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP Selective Acknowledgment Options", RFC 2018, October 1996. [MSMO97] M. Mathis, J. Semke, J. Mahdavi, T. Ott, "The Macroscopic Behavior of the TCP Congestion Avoidance Algorithm",Computer Communication Review, volume 27, number3, July 1997. Available from http://www.psc.edu/networking/papers/papers.html [MV98] Miten N. Mehta and Nitin H. Vaidya. Delayed Duplicate- Acknowledgments: A Proposal to Improve Performance of TCP on Wireless Links. Technical Report 98-006, Department of Computer Science, Texas A&M University, February 1998. [Nic97] Kathleen Nichols. Improving Network Simulation with Feedback. Com21, Inc. Technical Report. Available from http://www.com21.com/pages/papers/068.pdf. [PADHV99] Paxson, V., Allman, M., Dawson, S., Heavens, I. and B. Volz, "Known TCP Implementation Problems", RFC 2525, March 1999. [Pax97] Vern Paxson. Automated Packet Trace Analysis of TCP Implementations. In Proceedings of ACM SIGCOMM, September 1997. [PN98] Poduri, K. and K. Nichols, "Simulation Studies of Increased Initial TCP Window Size", RFC 2415, September 1998. [Pos81] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981.
[RF99] Ramakrishnan, K. and S. Floyd, "A Proposal to add Explicit Congestion Notification (ECN) to IP", RFC 2481, January 1999. [SF98] Nihal K. G. Samaraweera and Godred Fairhurst, "Reinforcement of TCP error Recovery for Wireless Communication", Computer Communication Review, volume 28, number 2, April 1998. [SP98] Shepard, T. and C. Partridge, "When TCP Starts Up With Four Packets Into Only Three Buffers", RFC 2416, September 1998. [Ste97] Stevens, W., "TCP Slow Start, Congestion Avoidance, Fast Retransmit, and Fast Recovery Algorithms", RFC 2001, January 1997. [Sut98] B. Suter, T. Lakshman, D. Stiliadis, and A. Choudhury. Design Considerations for Supporting TCP with Per-flow Queueing. Proceedings of IEEE Infocom `98 Conference, 1998. [Tou97] Touch, J., "TCP Control Block Interdependence", RFC 2140, April 1997. [VH97a] Vikram Visweswaraiah and John Heidemann. Improving Restart of Idle TCP Connections. Technical Report 97-661, University of Southern California, 1997. [VH97b] Vikram Visweswaraiah and John Heidemann. Rate-based pacing Source Code Distribution, Web page: http://www.isi.edu/lsam/publications/rate_based_pacing/README.html November, 1997. [VH98] Vikram Visweswaraiah and John Heidemann. Improving Restart of Idle TCP Connections (revised). Submitted for publication.
8 Authors' Addresses
Mark Allman NASA Glenn Research Center/BBN Technologies Lewis Field 21000 Brookpark Rd. MS 54-2 Cleveland, OH 44135 EMail: mallman@grc.nasa.gov http://roland.grc.nasa.gov/~mallman Spencer Dawkins Nortel P.O.Box 833805 Richardson, TX 75083-3805 EMail: Spencer.Dawkins.sdawkins@nt.com Dan Glover NASA Glenn Research Center Lewis Field 21000 Brookpark Rd. MS 3-6 Cleveland, OH 44135 EMail: Daniel.R.Glover@grc.nasa.gov http://roland.grc.nasa.gov/~dglover Jim Griner NASA Glenn Research Center Lewis Field 21000 Brookpark Rd. MS 54-2 Cleveland, OH 44135 EMail: jgriner@grc.nasa.gov http://roland.grc.nasa.gov/~jgriner Diepchi Tran NASA Glenn Research Center Lewis Field 21000 Brookpark Rd. MS 54-2 Cleveland, OH 44135 EMail: dtran@grc.nasa.gov
Tom Henderson University of California at Berkeley Phone: +1 (510) 642-8919 EMail: tomh@cs.berkeley.edu URL: http://www.cs.berkeley.edu/~tomh/ John Heidemann University of Southern California/Information Sciences Institute 4676 Admiralty Way Marina del Rey, CA 90292-6695 EMail: johnh@isi.edu Joe Touch University of Southern California/Information Sciences Institute 4676 Admiralty Way Marina del Rey, CA 90292-6601 USA Phone: +1 310-448-9151 Fax: +1 310-823-6714 URL: http://www.isi.edu/touch EMail: touch@isi.edu Hans Kruse J. Warren McClure School of Communication Systems Management Ohio University 9 S. College Street Athens, OH 45701 Phone: 740-593-4891 Fax: 740-593-4889 EMail: hkruse1@ohiou.edu http://www.csm.ohiou.edu/kruse Shawn Ostermann School of Electrical Engineering and Computer Science Ohio University 416 Morton Hall Athens, OH 45701 Phone: (740) 593-1234 EMail: ostermann@cs.ohiou.edu
Keith Scott The MITRE Corporation M/S W650 1820 Dolley Madison Blvd. McLean VA 22102-3481 EMail: kscott@mitre.org Jeffrey Semke Pittsburgh Supercomputing Center 4400 Fifth Ave. Pittsburgh, PA 15213 EMail: semke@psc.edu http://www.psc.edu/~semke
9 Full Copyright Statement
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