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 RFC2474, 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 [RFC1349].

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 RFC793. First, page 36 of RFC793 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, RFC793 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 RFC793 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 RFC793, 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
RFC793 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 RFC793 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, RFC791, 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", RFC1349, July 1992.

[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC2460, 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", RFC2474, December 1998.

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

[RFC2597] Heinanen, J., Baker, F., Weiss, W. and J. Wroclawski,
"Assured Forwarding PHB Group", RFC2587, June 1999.

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

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