rfc2873.txt

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RFC 2873           TCP and the IPv4 Precedence Field           June 2000


   (2) After a connection is established, each end sends segments with
   its desired precedence. The precedence picked by one end of the TCP
   connection may be the same or may be different from the precedence
   picked by the other end (because precedence is ignored during
   connection setup time). The precedence fields may be changed by the
   intermediate nodes too. In either case, the precedence of the
   received packets will be ignored by the other end. The TCP connection
   will not be reset in either case.

   Problems #1 and #2 are solved by this proposed modification. Problems
   #3 and #4 become non-issues because TCP must ignore the precedence.
   In a DiffServ-capable environment, the two cases described in
   problems #3 and #4 should be allowed.

5. Security Considerations

   A TCP implementation that terminates a connection upon receipt of any
   segment with an incorrect precedence field, regardless of the
   correctness of the sequence numbers in the segment's header, poses a
   serious denial-of-service threat, as all an attacker must do to
   terminate a connection is guess the port numbers and then send two
   segments with different precedence values; one of them is certain to
   terminate the connection.  Accordingly, the change to TCP processing
   proposed in this memo would yield a significant gain in terms of that
   TCP implementation's resilience.

   On the other hand, the stricter processing rules of RFC 793 in
   principle make TCP spoofing attacks more difficult, as the attacker
   must not only guess the victim TCP's initial sequence number, but
   also its precedence setting.

   Finally, the security issues of each PHB group are addressed in the
   PHB group's specification [RFC2597, RFC2598].

6. Acknowledgments

   Our thanks to Al Smith for his careful review and comments.














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RFC 2873           TCP and the IPv4 Precedence Field           June 2000


7. References

   [RFC791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
             1981.

   [RFC793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
             793, September 1981.

   [RFC1349] Almquist, P., "Type of Service in the Internet Protocol
             Suite", RFC 1349, July 1992.

   [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
             (IPv6) Specification", RFC 2460, December 1998.

   [RFC2474] Nichols, K., Blake, S., Baker, F. and D. Black, "Definition
             of the Differentiated Services Field (DS Field) in the IPv4
             and IPv6 Headers", RFC 2474, December 1998.

   [RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z. and
             W.  Weiss, "An Architecture for Differentiated Services",
             RFC 2475, December 1998.

   [RFC2597] Heinanen, J., Baker, F., Weiss, W. and J. Wroclawski,
             "Assured Forwarding PHB Group", RFC 2587, June 1999.

   [RFC2598] Jacobson, V., Nichols, K. and K. Poduri, "An Expedited
             Forwarding PHB", RFC 2598, June 1999.
























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RFC 2873           TCP and the IPv4 Precedence Field           June 2000


8. Authors' Addresses

   Xipeng Xiao
   Global Crossing
   141 Caspian Court
   Sunnyvale, CA 94089
   USA

   Phone: [phone]
   EMail: [e-mail]


   Alan Hannan
   iVMG, Inc.
   112 Falkirk Court
   Sunnyvale, CA 94087
   USA

   Phone: [phone]
   EMail: [e-mail]


   Edward Crabbe
   Exodus Communications
   2650 San Tomas Expressway
   Santa Clara, CA 95051
   USA

   Phone: [phone]
   EMail: [e-mail]


   Vern Paxson
   ACIRI/ICSI
   1947 Center Street
   Suite 600
   Berkeley, CA 94704-1198
   USA

   Phone: [phone]
   EMail: [e-mail]










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RFC 2873           TCP and the IPv4 Precedence Field           June 2000


9.  Full Copyright Statement

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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