Network Working Group                                            X. Xiao
Internet Draft                                     Frontier Globalcenter
                                                               A. Hannan
                                                               V. Paxson
                                                               E. Crabbe
Expiration Date: October 2000                                 April 2000

              TCP Processing of the IPv4 Precedence Field

1. Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

2. Abstract

   This draft 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 draft 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

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   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.

3. 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.

4. 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.

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   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.

5. 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

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        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-
        ...  [unrelated statements snipped] If an incoming segment has a
        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

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   value when arriving at the other TCP module, the connection will not
   be reset.

6. 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 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 two ends' precedence may be the same or
   may be different (because precedence is ignored during connection
   setup time). The precedence fields may be changed by the intermediate
   nodes too. They will be ignored by the other end. The TCP connection
   will not be reset.

   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.

7. 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].

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8. Acknowledgments

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

9. References

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

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

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

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

        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.

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

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

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

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10. Authors' Addresses

   Xipeng Xiao <>
   Frontier Globalcenter
   141 Caspian Court
   Sunnyvale, CA 94087
   Phone: +1 408-543-4801

   Alan Hannan <>
   iVMG, Inc.
   112 Falkirk Court
   Sunnyvale, CA 94087
   Phone: +1 408-749-7084

   Edward Crabbe <>
   2650 San Tomas Expressway
   Santa Clara, CA 95051
   Phone: +1 408-888-3794

   Vern Paxson <>
   1947 Center Street
   Suite 600
   Berkeley, CA 94704-1198
   Phone: +1 510-666-2882

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