RFC2873 - TCP Processing of the IPv4 Precedence Field

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.

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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