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20 • Linux Symposium 2004 • Volume One 7.4 Restricted state transitions tcpcp could be used to advance TCP connection state past boundaries imposed by internal or external control mechanisms. In particular, conspiring applications may create TCP connections without ever exchanging SYN packets, bypassing SYN-filtering firewalls. Since SYN-filtering firewalls can already be avoided by privileged applications, sites depending on SYN-filtering firewalls should therefore use the default setting of tcpcp, which makes its use also a privileged operation. 7.5 Attacks on remote hosts The ability to set TCP_ICI makes it easy to commit all kinds of of protocol violations. While tcpcp may simplify implementing such attacks, this type of abuses has always been possible for privileged users, and therefore, tcpcp poses no new security threat to systems properly resistant against network attacks. However, if a site allows systems where only trusted users may be able to communicate with otherwise shielded systems with known remote TCP vulnerabilities, tcpcp could be used for attacks. Such sites should use the default setting, which makes setting TCP_ICI a privileged operation. 7.6 Security summary To summarize, the author believes that the design of tcpcp does not open any new exploits if tcpcp is used in its default configuration. Obviously, some subtleties have probably been overlooked, and there may be bugs inadvertently leading to vulnerabilities. Therefore, tcpcp should receive public scrutiny before being considered fit for regular use. 8 Future work To allow faster connection passing among hosts that share the same, or a very similar path to the peer, tcpcp should try to avoid going to slow start. To do so, it will have to pass more congestion control information, and integrate it properly at the destination. Although not strictly part of tcpcp, the redirection apparatus for the network should be further extended, in particular to allow individual connections to be redirected at that point too, and to include some middleware that coordinates the redirecting with the changes at the hosts passing the connection. It would be very interesting if connection passing could also be used for checkpointing. The analysis in Section 6 suggests that at least limited checkpointing capabilities should be feasible without interfering with regular TCP operation. The inner workings of TCP are complex and easily disturbed. It is therefore important to subject tcpcp to thorough testing, in particular in transient states, such as during recovery from lost segments. The umlsim simulator [12] allows to generate such conditions in a deterministic way, and will be used for these tests. 9 Conclusion tcpcp is a proof of concept implementation that successfully demonstrates that an endpoint of a TCP connection can be passed from one host to another without involving the host at the opposite end of the TCP connection. tcpcp also shows that this can be accomplished with a relatively small amount of kernel changes. tcpcp in its present form is suitable for experimental use as a building block for load balancing and process migration solutions. Future
Linux Symposium 2004 • Volume One • 21 work will focus on improving the performance of tcpcp, on validating its correctness, and on exploring checkpointing capabilities. References [1] RFC768; Postel, Jon. User Datagram Protocol, IETF, August 1980. [2] RFC793; Postel, Jon. Transmission Control Protocol, IETF, September 1981. [3] Stevens, W. Richard. TCP/IP Illustrated, Volume 1 – The Protocols, Addison-Wesley, 1994. [4] RFC2616; Fielding, Roy T.; Gettys, James; Mogul, Jeffrey C.; Frystyk Nielsen, Henrik; Masinter, Larry; Leach, Paul J.; Berners-Lee, Tim. Hypertext Transfer Protocol – HTTP/1.1, IETF, June 1999. [5] Bar, Moshe. OpenMosix, Proceedings of the 10th International Linux System Technology Conference (Linux-Kongress 2003), pp. 94–102, October 2003. [9] RFC1918; Rekhter, Yakov; Moskowitz, Robert G.; Karrenberg, Daniel; de Groot, Geert Jan; Lear, Eliot. Address Allocation for Private Internets, IETF, February 1996. [10] RFC1323; Jacobson, Van; Braden, Bob; Borman, Dave. TCP Extensions for High Performance, IETF, May 1992. [11] RFC2018; Mathis, Matt; Mahdavi, Jamshid; Floyd, Sally; Romanow, Allyn. TCP Selective Acknowledgement Options, IETF, October 1996. [12] Almesberger, Werner. UML Simulator, Proceedings of the Ottawa Linux Symposium 2003, July 2003. http://archive.linuxsymposium. org/ols2003/Proceedings/ All-Reprints/ Reprint-Almesberger-OLS2003. pdf [6] Kuntz, Bryan; Rajan, Karthik. MIGSOCK – Migratable TCP Socket in Linux, CMU, M.Sc. Thesis, February 2002. http://www-2.cs.cmu.edu/ ~softagents/migsock/MIGSOCK. pdf [7] Leite, Fábio Olivé. Load-Balancing HA Clusters with No Single Point of Failure, Proceedings of the 9th International Linux System Technology Conference (Linux-Kongress 2002), pp. 122–131, September 2002. http://www. linux-kongress.org/2002/ papers/lk2002-leite.html [8] Linux Virtual Server Project, http:// www.linuxvirtualserver.org/
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Page 7 and 8: Conference Organizers Andrew J. Hut
Page 9 and 10: TCP Connection Passing Werner Almes
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Methods to Improve Bootup Time in L
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Linux on NUMA Systems Martin J. Bli
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Improving Kernel Performance by Unm
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Linux Virtualization on IBM POWER5
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The State of ACPI in the Linux Kern
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Scaling Linux® to the Extreme From
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Big Servers—2.6 compared to 2.4 W
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e100 Weight Reduction Program Writi
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NFSv4 and rpcsec_gss for linux J. B
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Comparing and Evaluating epoll, sel
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IA64-Linux perf tools for IO dorks
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Hotplug Memory and the Linux VM Dav
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