< draft-xiao-tcp-prec-02.txt		draft-xiao-tcp-prec-03.txt >
	Network Working Group X. Xiao		A new Request for Comments is now available in online RFC libraries.
	Internet Draft Frontier Globalcenter
	A. Hannan
	iVMG
	V. Paxson
	ACIRI/ICSI
	E. Crabbe
	Expiration Date: October 2000 April 2000
	TCP Processing of the IPv4 Precedence Field
	<draft-xiao-tcp-prec-02.txt>
	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-
	Drafts.
	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
	http://www.ietf.org/ietf/1id-abstracts.txt
	The list of Internet-Draft Shadow Directories can be accessed at
	http://www.ietf.org/shadow.html.
	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
	ID TCP and the IPv4 Precedence Field April 2000
	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.
	ID TCP and the IPv4 Precedence Field April 2000
	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
	ID TCP and the IPv4 Precedence Field April 2000
	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),
	... [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
	ID TCP and the IPv4 Precedence Field April 2000
	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].
	ID TCP and the IPv4 Precedence Field April 2000
	8. Acknowledgments
	Our thanks to Al Smith for his careful review and comments.
	9. References
	[RFC791]
	Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.
	[RFC793]		RFC 2873
	Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
	September 1981.
	[RFC1349]		Title: TCP Processing of the IPv4 Precedence Field
	Almquist, P., "Type of Service in the Internet Protocol Suite",		Author(s): X. Xiao, A. Hannan, V. Paxson, E. Crabbe
	RFC 1349, July 1992.		Status: Standards Track
			Date: June 2000
			Mailbox: xipeng@gblx.net, alan@ivmg.net,
			edc@explosive.net, vern@aciri.org
			Pages: 8
			Characters: 15565
			Updates/Obsoletes/SeeAlso: None
	[RFC2460]		I-D Tag: draft-xiao-tcp-prec-03.txt
	Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
	Specification", RFC 2460, December 1998.
	[RFC2474]		URL: ftp://ftp.isi.edu/in-notes/rfc2873.txt
	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]		This memo describes a conflict between TCP [RFC793] and DiffServ
	Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z. and W.		[RFC2475] on the use of the three leftmost bits in the TOS octet of
	Weiss, "An Architecture for Differentiated Services", RFC 2475,		an IPv4 header [RFC791]. In a network that contains DiffServ-capable
	December 1998.		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.
	[RFC2597]		Because the IPv6 [RFC2460] traffic class octet does not have any
	Heinanen, J., Baker, F., Weiss, W. and J. Wroclawski, "Assured		defined meaning except what is defined in RFC 2474, and in particular
	Forwarding PHB Group", RFC 2587, June 1999.		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.
	[RFC2598]		This is now a Proposed Standard Protocol.
	Jacobson, V., Nichols, K. and K. Poduri, "An Expedited
	Forwarding PHB", RFC 2598, June 1999.
	ID TCP and the IPv4 Precedence Field April 2000		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.
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