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Network Working Group                                            X. XiaoRequest for Comments: 2873                               Global CrossingCategory: Standards Track                                      A. Hannan                                                                    iVMG                                                               V. Paxson                                                              ACIRI/ICSI                                                               E. Crabbe                                                   Exodus Communications                                                               June 2000TCP Processing of the IPv4 Precedence FieldStatus of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2000).  All Rights Reserved.Abstract   This memo describes a conflict between TCP [RFC793] and DiffServ   [RFC2475] on the use of the three leftmost bits in the TOS octet of   an IPv4 header [RFC791]. In a network that contains DiffServ-capable   nodes, such a conflict can cause failures in establishing TCP   connections or can cause some established TCP connections to be reset   undesirably. This memo proposes a modification to TCP for resolving   the conflict.   Because the IPv6 [RFC2460] traffic class octet does not have any   defined meaning except what is defined inRFC 2474, and in particular   does not define precedence or security parameter bits, there is no   conflict between TCP and DiffServ on the use of any bits in the IPv6   traffic class octet.1. Introduction   In TCP, each connection has a set of states associated with it. Such   states are reflected by a set of variables stored in the TCP Control   Block (TCB) of both ends. Such variables may include the local and   remote socket number, precedence of the connection, security levelXiao, et al.                Standards Track                     [Page 1]

RFC 2873           TCP and the IPv4 Precedence Field           June 2000   and compartment, etc.  Both ends must agree on the setting of the   precedence and security parameters in order to establish a connection   and keep it open.   There is no field in the TCP header that indicates the precedence of   a segment. Instead, the precedence field in the header of the IP   packet is used as the indication.  The security level and compartment   are likewise carried in the IP header, but as IP options rather than   a fixed header field.  Because of this difference, the problem with   precedence discussed in this memo does not apply to them.   TCP requires that the precedence (and security parameters) of a   connection must remain unchanged during the lifetime of the   connection. Therefore, for an established TCP connection with   precedence, the receipt of a segment with different precedence   indicates an error. The connection must be reset [RFC793, pp. 36, 37,   40, 66, 67, 71].   With the advent of DiffServ, intermediate nodes may modify the   Differentiated Services Codepoint (DSCP) [RFC2474] of the IP header   to indicate the desired Per-hop Behavior (PHB) [RFC2475,RFC2597,RFC2598]. The DSCP includes the three bits formerly known as the   precedence field.  Because any modification to those three bits will   be considered illegal by endpoints that are precedence-aware, they   may cause failures in establishing connections, or may cause   established connections to be reset.2. Terminology   Segment: the unit of data that TCP sends to IP   Precedence Field: the three leftmost bits in the TOS octet of an IPv4   header. Note that in DiffServ, these three bits may or may not be   used to denote the precedence of the IP packet. There is no   precedence field in the traffic class octet in IPv6.   TOS Field: bits 3-6 in the TOS octet of IPv4 header [RFC 1349].   MBZ field: Must Be Zero   The structure of the TOS octet is depicted below:                   0     1     2     3     4     5     6     7                +-----+-----+-----+-----+-----+-----+-----+-----+                |   PRECEDENCE    |          TOS          | MBZ |                +-----+-----+-----+-----+-----+-----+-----+-----+Xiao, et al.                Standards Track                     [Page 2]

RFC 2873           TCP and the IPv4 Precedence Field           June 2000   DS Field: the TOS octet of an IPv4 header is renamed the   Differentiated Services (DS) Field by DiffServ.   The structure of the DS field is depicted below:                  0   1   2   3   4   5   6   7                +---+---+---+---+---+---+---+---+                |         DSCP          |  CU   |                +---+---+---+---+---+---+---+---+   DSCP: Differentiated Service Code Point, the leftmost 6 bits in the   DS field.   CU:   currently unused.   Per-hop Behavior (PHB): a description of the externally observable   forwarding treatment applied at a differentiated services-compliant   node to a behavior aggregate.3. Problem Description   The manipulation of the DSCP to achieve the desired PHB by DiffServ-   capable nodes may conflict with TCP's use of the precedence field.   This conflict can potentially cause problems for TCP implementations   that conform toRFC 793.  First, page 36 ofRFC 793 states:       If the connection is in any non-synchronized state (LISTEN, SYN-       SENT, SYN-RECEIVED), and the incoming segment acknowledges       something not yet sent (the segment carries an unacceptable ACK),       or if an incoming segment has a security level or compartment       which does not exactly match the level and compartment requested       for the connection, a reset is sent. If our SYN has not been       acknowledged and the precedence level of the incoming segment is       higher than the precedence level requested then either raise the       local precedence level (if allowed by the user and the system) or       send a reset; or if the precedence level of the incoming segment       is lower than the precedence level requested then continue as if       the precedence matched exactly (if the remote TCP cannot raise       the precedence level to match ours this will be detected in the       next segment it sends, and the connection will be terminated       then). If our SYN has been acknowledged (perhaps in this incoming       segment) the precedence level of the incoming segment must match       the local precedence level exactly, if it does not a reset must       be sent.   This leads to Problem #1:  For a precedence-aware TCP module, if   during TCP's synchronization process, the precedence fields of the   SYN and/or ACK packets are modified by the intermediate nodes,Xiao, et al.                Standards Track                     [Page 3]

RFC 2873           TCP and the IPv4 Precedence Field           June 2000   resulting in the received ACK packet having a different precedence   from the precedence picked by this TCP module, the TCP connection   cannot be established, even if both modules actually agree on an   identical precedence for the connection.   Then, on page 37,RFC 793 states:       If the connection is in a synchronized state (ESTABLISHED, FIN-       WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT),       security level, or compartment, or precedence which does not       exactly match the level, and compartment, and precedence       requested for the connection, a reset is sent and connection goes       to the CLOSED state.   This leads to Problem #2:  For a precedence-aware TCP module, if the   precedence field of a received segment from an established TCP   connection has been changed en route by the intermediate nodes so as   to be different from the precedence specified during the connection   setup, the TCP connection will be reset.   Each of problems #1 and #2 has a mirroring problem. They cause TCP   connections that must be reset according toRFC 793 not to be reset.   Problem #3:  A TCP connection may be established between two TCP   modules that pick different precedence, because the precedence fields   of the SYN and ACK packets are modified by intermediate nodes,   resulting in both modules thinking that they are in agreement for the   precedence of the connection.   Problem #4:  A TCP connection has been established normally by two   TCP modules that pick the same precedence. But in the middle of the   data transmission, one of the TCP modules changes the precedence of   its segments. According toRFC 793, the TCP connection must be reset.   In a DiffServ-capable environment, if the precedence of the segments   is altered by intermediate nodes such that it retains the expected   value when arriving at the other TCP module, the connection will not   be reset.4. Proposed Modification to TCP   The proposed modification to TCP is that TCP must ignore the   precedence of all received segments. More specifically:   (1) In TCP's synchronization process, the TCP modules at both ends   must ignore the precedence fields of the SYN and SYN ACK packets. The   TCP connection will be established if all the conditions specified byRFC 793 are satisfied except the precedence of the connection.Xiao, et al.                Standards Track                     [Page 4]

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 ofRFC 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.Xiao, et al.                Standards Track                     [Page 5]

RFC 2873           TCP and the IPv4 Precedence Field           June 20007. References   [RFC791]  Postel, J., "Internet Protocol", STD 5,RFC 791, September             1981.   [RFC793]  Postel, J., "Transmission Control Protocol", STD 7,RFC793, 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.Xiao, et al.                Standards Track                     [Page 6]

RFC 2873           TCP and the IPv4 Precedence Field           June 20008. Authors' Addresses   Xipeng Xiao   Global Crossing   141 Caspian Court   Sunnyvale, CA 94089   USA   Phone: +1 408-543-4801   EMail: xipeng@gblx.net   Alan Hannan   iVMG, Inc.   112 Falkirk Court   Sunnyvale, CA 94087   USA   Phone: +1 408-749-7084   EMail: alan@ivmg.net   Edward Crabbe   Exodus Communications   2650 San Tomas Expressway   Santa Clara, CA 95051   USA   Phone: +1 408-346-1544   EMail: edc@explosive.net   Vern Paxson   ACIRI/ICSI   1947 Center Street   Suite 600   Berkeley, CA 94704-1198   USA   Phone: +1 510-666-2882   EMail: vern@aciri.orgXiao, et al.                Standards Track                     [Page 7]

RFC 2873           TCP and the IPv4 Precedence Field           June 20009.  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.Xiao, et al.                Standards Track                     [Page 8]