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IETF RFC 6093
On the Implementation of the TCP Urgent Mechanism
Last modified on Tuesday, January 4th, 2011
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Internet Engineering Task Force (IETF) F. Gont
Request for Comments: 6093 UTN/FRH
Updates: 793, 1011, 1122 A. Yourtchenko
Category: Standards Track Cisco
ISSN: 2070-1721 January 2011
On the Implementation of the TCP Urgent Mechanism
 Abstract
This document analyzes how current TCP implementations process TCP
urgent indications and how the behavior of some widely deployed
middleboxes affects how end systems process urgent indications. This
document updates the relevant specifications such that they
accommodate current practice in processing TCP urgent indications,
raises awareness about the reliability of TCP urgent indications in
the Internet, and recommends against the use of urgent indications
(but provides advice to applications that do).
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 6093.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Gont & Yourtchenko Standards Track PAGE 1 
RFC 6093 On the TCP Urgent Mechanism January 2011
Table of Contents
1. Introduction ....................................................3
2. Specification of the TCP Urgent Mechanism .......................3
2.1. Semantics of Urgent Indications ............................3
2.2. Semantics of the Urgent Pointer ............................4
2.3. Allowed Length of "Urgent Data" ............................4
3. Current Implementation Practice of the TCP Urgent Mechanism .....5
3.1. Semantics of Urgent Indications ............................5
3.2. Semantics of the Urgent Pointer ............................5
3.3. Allowed Length of "Urgent Data" ............................6
3.4. Interaction of Middleboxes with TCP Urgent Indications .....6
4. Updating RFC 793, RFC 1011, and RFC 1122 ........................6
5. Advice to New Applications Employing TCP ........................7
6. Advice to Applications That Make Use of the Urgent Mechanism ....7
7. Security Considerations .........................................7
8. Acknowledgements ................................................8
9. References ......................................................8
9.1. Normative References .......................................8
9.2. Informative References .....................................8
Appendix A. Survey of the Processing of TCP Urgent
Indications by Some Popular TCP Implementations ......10
A.1. FreeBSD ...................................................10
A.2. Linux .....................................................10
A.3. NetBSD ....................................................10
A.4. OpenBSD ...................................................11
A.5. Cisco IOS software ........................................11
A.6. Microsoft Windows 2000, Service Pack 4 ....................11
A.7. Microsoft Windows 2008 ....................................11
A.8. Microsoft Windows 95 ......................................11
Gont & Yourtchenko Standards Track PAGE 2
RFC 6093 On the TCP Urgent Mechanism January 2011
1. Introduction
This document analyzes how some current TCP implementations process
TCP urgent indications, and how the behavior of some widely deployed
middleboxes affects the processing of urgent indications by hosts.
This document updates RFC 793 [RFC 793], RFC 1011 [RFC 1011], and RFC
1122 [RFC 1122] such that they accommodate current practice in
processing TCP urgent indications. It also provides advice to
applications using the urgent mechanism and raises awareness about
the reliability of TCP urgent indications in the current Internet.
Given the above issues and potential interoperability issues with
respect to the currently common default mode operation, it is
strongly recommended that applications do not employ urgent
indications. Nevertheless, urgent indications are still retained as
a mandatory part of the TCP protocol to support the few legacy
applications that employ them. However, it is expected that even
these applications will have difficulties in environments with
middleboxes.
Section 2 describes what the current IETF specifications state with
respect to TCP urgent indications. Section 3 describes how current
TCP implementations actually process TCP urgent indications. Section
4 updates RFC 793 [RFC 793], RFC 1011 [RFC 1011], and RFC 1122
[RFC 1122], such that they accommodate current practice in processing
TCP urgent indications. Section 5 provides advice to new
applications employing TCP, with respect to the TCP urgent mechanism.
Section 6 provides advice to existing applications that use or rely
on the TCP urgent mechanism.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC 2119].
2. Specification of the TCP Urgent Mechanism
2.1. Semantics of Urgent Indications
TCP implements an "urgent mechanism" that allows the sending user to
stimulate the receiving user to accept some "urgent data" and that
permits the receiving TCP to indicate to the receiving user when all
the currently known "urgent data" have been read.
The TCP urgent mechanism permits a point in the data stream to be
designated as the end of urgent information. Whenever this point is
in advance of the receive sequence number (RCV.NXT) at the receiving
TCP, that TCP must tell the user to go into "urgent mode"; when the
receive sequence number catches up to the urgent pointer, the TCP
Gont & Yourtchenko Standards Track PAGE 3
RFC 6093 On the TCP Urgent Mechanism January 2011
must tell user to go into "normal mode" [RFC 793]. This means, for
example, that data that was received as "normal data" might become
"urgent data" if an urgent indication is received in some successive
TCP segment before that data is consumed by the TCP user.
The URG control flag indicates that the "Urgent Pointer" field is
meaningful and must be added to the segment sequence number to yield
the urgent pointer. The absence of this flag indicates that there is
no "urgent data" outstanding [RFC 793].
The TCP urgent mechanism is NOT a mechanism for sending "out-of-band"
data: the so-called "urgent data" should be delivered "in-line" to
the TCP user.
2.2. Semantics of the Urgent Pointer
There is some ambiguity in RFC 793 [RFC 793] with respect to the
semantics of the Urgent Pointer. Section 3.1 (page 17) of RFC 793
[RFC 793] states that the Urgent Pointer "communicates the current
value of the urgent pointer as a positive offset from the sequence
number in this segment. The urgent pointer points to the sequence
number of the octet following the urgent data. This field is only be
interpreted in segments with the URG control bit set" (sic).
However, Section 3.9 (page 56) of RFC 793 [RFC 793] states, when
describing the processing of the SEND call in the ESTABLISHED and
CLOSE-WAIT states, that "If the urgent flag is set, then SND.UP <-
SND.NXT-1 and set the urgent pointer in the outgoing segments".
RFC 1011 [RFC 1011] clarified this ambiguity in RFC 793 stating that
"Page 17 is wrong. The urgent pointer points to the last octet of
urgent data (not to the first octet of non-urgent data)". RFC 1122
[RFC 1122] formally updated RFC 793 by stating, in Section 4.2.2.4
(page 84), that "the urgent pointer points to the sequence number of
the LAST octet (not LAST+1) in a sequence of urgent data".
2.3. Allowed Length of "Urgent Data"
RFC 793 [RFC 793] allows TCP peers to send "urgent data" of any
length, as the TCP urgent mechanism simply provides a pointer to an
interesting point in the data stream. In this respect, Section
4.2.2.4 (page 84) of RFC 1122 [RFC 1122] explicitly states that "A TCP
MUST support a sequence of urgent data of any length".
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RFC 6093 On the TCP Urgent Mechanism January 2011
3. Current Implementation Practice of the TCP Urgent Mechanism
3.1. Semantics of Urgent Indications
As discussed in Section 2, the TCP urgent mechanism simply permits a
point in the data stream to be designated as the end of urgent
information but does NOT provide a mechanism for sending "out-of-
band" data.
Unfortunately, virtually all TCP implementations process TCP urgent
indications differently. By default, the last byte of "urgent data"
is delivered "out of band" to the application. That is, it is not
delivered as part of the normal data stream [UNPv1]. For example,
the "out-of-band" byte is read by an application when a recv(2)
system call with the MSG_OOB flag set is issued.
Most implementations provide a socket option (SO_OOBINLINE) that
allows an application to override the (broken) default processing of
urgent indications, so that "urgent data" is delivered "in line" to
the application, thus providing the semantics intended by the IETF
specifications.
3.2. Semantics of the Urgent Pointer
All the popular implementations that the authors of this document
have been able to test interpret the semantics of the TCP Urgent
Pointer as specified in Section 3.1 of RFC 793. This means that even
when RFC 1122 formally updated RFC 793 to clarify the ambiguity in
the semantics of the Urgent Pointer, this clarification was never
reflected in actual implementations (i.e., virtually all
implementations default to the semantics of the urgent pointer
specified in Section 3.1 of RFC 793).
Some operating systems provide a system-wide toggle to override this
behavior and interpret the semantics of the Urgent Pointer as
clarified in RFC 1122. However, this system-wide toggle has been
found to be inconsistent. For example, Linux provides the sysctl
"tcp_stdurg" (i.e., net.ipv4.tcp_stdurg) that, when set, supposedly
changes the system behavior to interpret the semantics of the TCP
Urgent Pointer as specified in RFC 1122. However, this sysctl changes
the semantics of the Urgent Pointer only for incoming segments (i.e.,
not for outgoing segments). This means that if this sysctl is set,
an application might be unable to interoperate with itself if both
the TCP sender and the TCP receiver are running on the same host.
Gont & Yourtchenko Standards Track PAGE 5
RFC 6093 On the TCP Urgent Mechanism January 2011
3.3. Allowed Length of "Urgent Data"
While Section 4.2.2.4 (page 84) of RFC 1122 explicitly states that "A
TCP MUST support a sequence of urgent data of any length", in
practice, all those implementations that interpret TCP urgent
indications as a mechanism for sending "out-of-band" data keep a
buffer of a single byte for storing the "last byte of urgent data".
Thus, if successive indications of "urgent data" are received before
the application reads the pending "out-of-band" byte, that pending
byte will be discarded (i.e., overwritten by the new byte of "urgent
data").
In order to avoid "urgent data" from being discarded, some
implementations queue each of the received "urgent bytes", so that
even if another urgent indication is received before the pending
"urgent data" are consumed by the application, those bytes do not
need to be discarded. Some of these implementations have been known
to fail to enforce any limits on the amount of "urgent data" that
they queue; thus, they become vulnerable to trivial resource
exhaustion attacks [CPNI-TCP].
It should be reinforced that the aforementioned implementations are
broken. The TCP urgent mechanism is not a mechanism for delivering
"out-of-band" data.
3.4. Interaction of Middleboxes with TCP Urgent Indications
As a result of the publication of Network Intrusion Detection System
(NIDS) evasion techniques based on TCP urgent indications [phrack],
some middleboxes clear the urgent indications by clearing the URG
flag and setting the Urgent Pointer to zero. This causes the "urgent
data" to become "in line" (that is, accessible by the read(2) call or
the recv(2) call without the MSG_OOB flag) in the case of those TCP
implementations that interpret the TCP urgent mechanism as a facility
for delivering "out-of-band" data (as described in Section 3.1). An
example of such a middlebox is the Cisco PIX firewall [Cisco-PIX].
This should discourage applications from depending on urgent
indications for their correct operation, as urgent indications may
not be reliable in the current Internet.
4. Updating RFC 793, RFC 1011, and RFC 1122
Considering that as long as both the TCP sender and the TCP receiver
implement the same semantics for the Urgent Pointer there is no
functional difference in having the Urgent Pointer point to "the
sequence number of the octet following the urgent data" vs. "the
last octet of urgent data", and that all known implementations
interpret the semantics of the Urgent Pointer as pointing to "the
Gont & Yourtchenko Standards Track PAGE 6
RFC 6093 On the TCP Urgent Mechanism January 2011
sequence number of the octet following the urgent data", we hereby
update RFC 793 [RFC 793], RFC 1011 [RFC 1011], and RFC 1122 [RFC 1122]
such that "the urgent pointer points to the sequence number of the
octet following the urgent data" (in segments with the URG control
bit set), thus accommodating virtually all existing TCP
implementations.
5. Advice to New Applications Employing TCP
As a result of the issues discussed in Section 3.2 and Section 3.4,
new applications SHOULD NOT employ the TCP urgent mechanism.
However, TCP implementations MUST still include support for the
urgent mechanism such that existing applications can still use it.
6. Advice to Applications That Make Use of the Urgent Mechanism
Even though applications SHOULD NOT employ the urgent mechanism,
applications that still decide to employ it MUST set the SO_OOBINLINE
socket option, such that "urgent data" is delivered in line, as
intended by the IETF specifications.
Additionally, applications that still decide to use the urgent
mechanism need to be designed for correct operation even when the URG
flag is cleared by middleboxes.
7. Security Considerations
Multiple factors can affect the data flow that is actually delivered
to an application when the TCP urgent mechanism is employed: for
example, the two possible interpretations of the semantics of the
Urgent Pointer in current implementations (e.g., depending on the
value of the tcp_stdurg sysctl), the possible implementation of the
urgent mechanism as an "out-of-band" (OOB) facility (versus "in-band"
as intended by the IETF specifications), or middleboxes (such as
packet scrubbers) or the end-systems themselves that could cause the
"urgent data" to be processed "in line". This might make it
difficult for a Network Intrusion Detection System (NIDS) to track
the application-layer data transferred to the destination system and
thus lead to false negatives or false positives in the NIDS
[CPNI-TCP] [phrack].
Probably the best way to avoid the security implications of TCP
"urgent data" is to avoid having applications use the TCP urgent
mechanism altogether. Packet scrubbers could probably be configured
to clear the URG bit and set the Urgent Pointer to zero. This would
basically cause the "urgent data" to be put "in line". However, this
Gont & Yourtchenko Standards Track PAGE 7
RFC 6093 On the TCP Urgent Mechanism January 2011
might cause interoperability problems or undesired behavior in those
applications that rely on the TCP urgent mechanism, such as Telnet
[RFC 854] and FTP [RFC 959].
8. Acknowledgements
The authors of this document would like to thank (in alphabetical
order) Jari Arkko, Ron Bonica, David Borman, Dave Cridland, Ralph
Droms, Wesley Eddy, John Heffner, Alfred Hoenes, Alexey Melnikov,
Keith Moore, Carlos Pignataro, Tim Polk, Anantha Ramaiah, Joe Touch,
Michael Welzl, Dan Wing, and Alexander Zimmermann for providing
valuable feedback on earlier versions of this document.
Fernando would like to thank David Borman and Joe Touch for a
fruitful discussion about the TCP urgent mechanism at IETF 73
(Minneapolis).
Fernando Gont's attendance to IETF meetings was supported by ISOC's
"Fellowship to the IETF" program.
Finally, Fernando Gont wishes to express deep and heartfelt gratitude
to Jorge Oscar Gont and Nelida Garcia for their precious motivation
and guidance.
9. References
9.1. Normative References
[RFC 793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC 1011] Reynolds, J. and J. Postel, "Official Internet
protocols", RFC 1011, May 1987.
[RFC 1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References
[CPNI-TCP] Gont, F., "Security Assessment of the Transmission
Control Protocol (TCP)", "http://www.cpni.gov.uk/
Docs/tn-03-09-security-assessment-TCP.pdf", 2009.
[Cisco-PIX] Cisco PIX, "http://www.cisco.com/en/US/docs/security/
asa/asa70/command/reference/tz.html#wp1288756".
Gont & Yourtchenko Standards Track PAGE 8
RFC 6093 On the TCP Urgent Mechanism January 2011
[FreeBSD] The FreeBSD project, "http://www.freebsd.org".
[Linux] The Linux Project, "http://www.kernel.org".
[NetBSD] The NetBSD project, "http://www.netbsd.org".
[OpenBSD] The OpenBSD project, "http://www.openbsd.org".
[RFC 854] Postel, J. and J. Reynolds, "Telnet Protocol
Specification", STD 8, RFC 854, May 1983.
[RFC 959] Postel, J. and J. Reynolds, "File Transfer Protocol",
STD 9, RFC 959, October 1985.
[UNPv1] Stevens, W., "UNIX Network Programming, Volume 1.
Networking APIs: Sockets and XTI", Prentice Hall PTR,
1997.
[Windows2000] Microsoft Windows 2000, "http://technet.microsoft.com/
en-us/library/bb726981(printer).aspx".
[Windows95] Microsoft Windows 95, "ftp://ftp.demon.co.uk/pub/
mirrors/win95netfaq/faq-c.html".
[phrack] Ko, Y., Ko, S., and M. Ko, "NIDS Evasion Method named
"SeolMa"", Phrack Magazine, Volume 0x0b, Issue 0x39,
Phile #0x03 of 0x12 http://www.phrack.org/
issues.html?issue=57&id=3#article, 2001.
Gont & Yourtchenko Standards Track PAGE 9
RFC 6093 On the TCP Urgent Mechanism January 2011
Appendix A. Survey of the Processing of TCP Urgent Indications by Some
Popular TCP Implementations
A.1. FreeBSD
FreeBSD 8.0 [FreeBSD] interprets the semantics of the urgent pointer
as specified in Section 4 of this document. It does not provide any
sysctl to override this behavior.
FreeBSD provides the SO_OOBINLINE socket option that, when set,
causes TCP "urgent data" to remain "in line". That is, it will be
accessible by the read(2) call or the recv(2) call without the
MSG_OOB flag.
FreeBSD supports only one byte of "urgent data". That is, only the
byte preceding the Urgent Pointer is considered "urgent data".
A.2. Linux
Linux 2.6.15-53-386 [Linux] interprets the semantics of the urgent
pointer as specified in Section 4 of this document. It provides the
net.ipv4.tcp_stdurg sysctl to override this behavior to interpret the
Urgent Pointer as specified in RFC 1122 [RFC 1122]. However, this
sysctl only affects the processing of incoming segments (the Urgent
Pointer in outgoing segments will still be set as specified in
Section 4 of this document).
Linux provides the SO_OOBINLINE socket option that, when set, causes
TCP "urgent data" to remain "in line". That is, it will be
accessible by the read(2) call or the recv(2) call without the
MSG_OOB flag.
Linux supports only one byte of "urgent data". That is, only the
byte preceding the Urgent Pointer is considered "urgent data".
A.3. NetBSD
NetBSD 5.0.1 [NetBSD] interprets the semantics of the urgent pointer
as specified in Section 4 of this document. It does not provide any
sysctl to override this behavior.
NetBSD provides the SO_OOBINLINE socket option that, when set, causes
TCP "urgent data" to remain "in line". That is, it will be
accessible by the read(2) call or the recv(2) call without the
MSG_OOB flag.
NetBSD supports only one byte of "urgent data". That is, only the
byte preceding the Urgent Pointer is considered "urgent data".
Gont & Yourtchenko Standards Track PAGE 10
RFC 6093 On the TCP Urgent Mechanism January 2011
A.4. OpenBSD
OpenBSD 4.2 [OpenBSD] interprets the semantics of the urgent pointer
as specified in Section 4 of this document. It does not provide any
sysctl to override this behavior.
OpenBSD provides the SO_OOBINLINE socket option that, when set,
causes TCP "urgent data" to remain "in line". That is, it will be
accessible by the read(2) or recv(2) calls without the MSG_OOB flag.
OpenBSD supports only one byte of "urgent data". That is, only the
byte preceding the Urgent Pointer is considered "urgent data".
A.5. Cisco IOS software
Cisco IOS Software Releases 12.2(18)SXF7, 12.4(15)T7 interpret the
semantics of the urgent pointer as specified in Section 4 of this
document.
The behavior is consistent with having the SO_OOBINLINE socket option
turned on, i.e., the data is processed "in line".
A.6. Microsoft Windows 2000, Service Pack 4
Microsoft Windows 2000 [Windows2000] interprets the semantics of the
urgent pointer as specified in Section 4 of this document. It
provides the TcpUseRFC1122UrgentPointer system-wide variable to
override this behavior, interpreting the Urgent Pointer as specified
in RFC 1122 [RFC 1122].
Tests performed with a sample server application compiled using the
cygwin environment has shown that the default behavior is to return
the "urgent data" "in line".
A.7. Microsoft Windows 2008
Microsoft Windows 2008 interprets the semantics of the urgent pointer
as specified in Section 4 of this document.
A.8. Microsoft Windows 95
Microsoft Windows 95 interprets the semantics of the urgent pointer
as specified in Section 4 of this document. It provides the
BSDUrgent system-wide variable to override this behavior,
interpreting the Urgent Pointer as specified in RFC 1122 [RFC 1122].
Windows 95 supports only one byte of "urgent data". That is, only
the byte preceding the Urgent Pointer is considered "urgent data"
[Windows95].
Gont & Yourtchenko Standards Track PAGE 11
RFC 6093 On the TCP Urgent Mechanism January 2011
Authors' Addresses
Fernando Gont
Universidad Tecnologica Nacional / Facultad Regional Haedo
Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706
Argentina
Phone: +54 11 4650 8472
EMail: fernando@gont.com.ar
URI: http://www.gont.com.ar
Andrew Yourtchenko
Cisco
De Kleetlaan, 7
Diegem B-1831
Belgium
Phone: +32 2 704 5494
EMail: ayourtch@cisco.com
Gont & Yourtchenko Standards Track PAGE 12
On the Implementation of the TCP Urgent Mechanism
RFC TOTAL SIZE: 25921 bytes
PUBLICATION DATE: Tuesday, January 4th, 2011
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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