Network Working Group                                            X. Xiao
Request for Comments: 2873                               Global Crossing
Category: Standards Track                                      A. Hannan
                                                                    iVMG
                                                               V. Paxson
                                                              ACIRI/ICSI
                                                               E. Crabbe
                                                   Exodus Communications
                                                               June 2000


              TCP Processing of the IPv4 Precedence Field

Status 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 in RFC 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 level

   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 |
                +-----+-----+-----+-----+-----+-----+-----+-----+

   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 to RFC 793.  First, page 36 of RFC 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,

   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 to RFC 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 to RFC 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 by
   RFC 793 are satisfied except the precedence of the connection.

   (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.

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.

8. 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.org

9.  Full Copyright Statement

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

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Acknowledgement

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