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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/rfc6093.

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 [RFC0793], RFC 1011 [RFC1011], and RFC
 1122 [RFC1122] 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 [RFC0793], RFC 1011 [RFC1011], and RFC 1122
 [RFC1122], 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 [RFC2119].

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" [RFC0793].  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 [RFC0793].
 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 [RFC0793] with respect to the
 semantics of the Urgent Pointer.  Section 3.1 (page 17) of RFC 793
 [RFC0793] 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 [RFC0793] 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 [RFC1011] 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
 [RFC1122] 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 [RFC0793] 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 [RFC1122] explicitly states that "A TCP
 MUST support a sequence of urgent data of any length".

Gont & Yourtchenko Standards Track [Page 4] 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 [RFC0793], RFC 1011 [RFC1011], and RFC 1122 [RFC1122]
 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
 [RFC0854] and FTP [RFC0959].

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

 [RFC0793]     Postel, J., "Transmission Control Protocol", STD 7, RFC
               793, September 1981.
 [RFC1011]     Reynolds, J. and J. Postel, "Official Internet
               protocols", RFC 1011, May 1987.
 [RFC1122]     Braden, R., Ed., "Requirements for Internet Hosts -
               Communication Layers", STD 3, RFC 1122, October 1989.
 [RFC2119]     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".
 [RFC0854]     Postel, J. and J. Reynolds, "Telnet Protocol
               Specification", STD 8, RFC 854, May 1983.
 [RFC0959]     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 [RFC1122].  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 [RFC1122].
 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 [RFC1122].
 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]

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