TCP Maintenance and Minor F. Gont
Extensions (tcpm) UTN/FRH
Internet-Draft A. Yourtchenko
Intended status: Standards Track Cisco
Expires: May 14, 2010 November 10, 2009
On the implementation of the TCP urgent mechanism
draft-ietf-tcpm-urgent-data-01.txt
Abstract
This document analyzes how current TCP implementations process TCP
urgent indications, and how the behavior of some widely-deployed
middle-boxes affect how urgent indications are processed by end
systems. This document updates the relevant specifications such that
they accommodate current practice in processing TCP urgent
indications, provides advice to applications that make use of the
urgent mechanism, and raises awareness about the reliability of TCP
urgent indications in the current Internet.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Specification of the TCP urgent mechanism . . . . . . . . . . 3
2.1. Semantics of urgent inications . . . . . . . . . . . . . . 3
2.2. Semantics of the Urgent Pointer . . . . . . . . . . . . . 4
2.3. Allowed length of urgent data . . . . . . . . . . . . . . 4
3. Current implementation practice of TCP urgent data . . . . . . 4
3.1. Semantics of urgent indications . . . . . . . . . . . . . 4
3.2. Semantics of the Urgent Pointer . . . . . . . . . . . . . 5
3.3. Allowed length of urgent data . . . . . . . . . . . . . . 5
3.4. Interaction of middle-boxes with TCP urgent indications . 6
4. Updating 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. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . . 8
Appendix A. Survey of the processing of TCP urgent
indications by some popular TCP implementations . . . 9
A.1. FreeBSD . . . . . . . . . . . . . . . . . . . . . . . . . 9
A.2. Linux . . . . . . . . . . . . . . . . . . . . . . . . . . 9
A.3. NetBSD . . . . . . . . . . . . . . . . . . . . . . . . . . 10
A.4. OpenBSD . . . . . . . . . . . . . . . . . . . . . . . . . 10
A.5. Cisco IOS, versions 12.2(18)SXF7, 12.4(15)T7 . . . . . . . 10
A.6. Microsoft Windows 2000, Service Pack 4 . . . . . . . . . . 10
A.7. Microsoft Windows 2008 . . . . . . . . . . . . . . . . . . 11
A.8. Microsoft Windows 95 . . . . . . . . . . . . . . . . . . . 11
Appendix B. Changes from previous versions of the draft (to
be removed by the RFC Editor before publishing
this document as an RFC) . . . . . . . . . . . . . . 11
B.1. Changes from draft-ietf-tcpm-urgent-data-00 . . . . . . . 11
B.2. Changes from draft-gont-tcpm-urgent-data-01 . . . . . . . 11
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
TCP incorporates an "urgent mechanism" that allows the sending user
to stimulate the receiving user to accept some "urgent data" and to
permit the receiving TCP to indicate to the receiving user when all
the currently known urgent data has been received by the user. This
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 must tell user to go
into "normal mode" [RFC0793].
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].
This document analyzes how current TCP implementations process TCP
urgent indications, and how the behavior of some widely-deployed
middle-boxes affect the processing of urgent indications by hosts.
This document updates RFC 1122 [RFC1122] such that IT accommodates
current practice in processing TCP urgent indications, provides
advice to applications using urgent the urgent mechanism, and raises
awareness about the reliability of TCP urgent indications in the
current Internet.
Section 2 describes what the current IETF secifications state with
respect to TCP urgent indications. Section 3 describes how current
TCP implementations actually process TCP urgent indications.
Section 4 updates RFC 1122 [RFC1122] such that it accommodates
current practice in processing TCP urgent indications. Section 5
provides advice to 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 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 inications
As discussed in Section 1, 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
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(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 must tell user to go into "normal mode".
This means, for example, that data that were received as "normal
data" might become "urgent data" if an urgent indication is received
in some successive TCP segment before those data are consumed by the
TCP user.
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". 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 961 [RFC0961] 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 explicitly states that "A TCP MUST
support a sequence of urgent data of any length".
3. Current implementation practice of TCP urgent data
3.1. Semantics of urgent indications
As discussed in Section 1, the TCP urgent mechanism simply permits a
point in the data stream to be designated as the end of urgent
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information, but does NOT provide a mechanism for sending out of band
data.
Unfortunately, virtually all TCP implementations process TCP urgent
data 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. 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 data, so that they are delivered "in band" 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 officially updated RFC 793 to clarify the ambiguity in
the semantics of the Urgent Pointer, this clarification never
reflected into 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.ivp4.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, but 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.
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
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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 resulting 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 middle-boxes with TCP urgent indications
As a result of the publication of Network Intrusion Detection (NIDs)
evasion techniques based on TCP urgent indications [phrack], some
middle-boxes 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 implement the urgent mechanism as out-of-band
data (as described in Section 3.1). Examples of such middle-boxes
are Cisco PIX firewall [Cisco-PIX]. This should discourage
applications to depend on urgent indications for their correct
operation, as urgent indications may not be not reliable in the
current Internet.
4. Updating 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 since all known implementations interpret
the semantics of the Urgent Pointer as pointing to "the sequence
number of the octet following the urgent data", hereby we update 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.
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5. Advice to new applications employing TCP
As a result of the issues discussed in 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
Applications that employ the Sockets API MUST set the SO_OOBINLINE
socket option, such that "urgent data" are delivered inline, as
intended by the IETF specifications.
7. Security Considerations
Given that there are two different interpretations of the semantics
of the Urgent Pointer in current implementations (e.g., depnding on
the value of the tcp_stdurg sysctl), and that middle-boxes (such as
packet scrubbers) or the end-systems themselves could cause the
urgent data to be processed "in band", there exists ambiguity in how
"urgent data" sent by a TCP will be processed by the intended
recipient. 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].
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 band". However, this
might cause interoperability problems or undesired behavior in the
applications running on top of TCP.
8. IANA Considerations
This document has no actions for IANA.
9. Acknowledgements
The authors of this document would like to thank (in alphabetical
order) David Borman, Alfred Hoenes, Carlos Pignataro, Anantha
Ramaiah, Joe Touch, and Dan Wing for providing valuable feedback on
earlier versions of this document.
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Additionally, Fernando would like to thank David Borman and Joe Touch
for a fruitful discussion about TCP urgent mode at IETF 73
(Minneapolis).
10. References
10.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC1122] Braden, R., "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.
10.2. Informative References
[CPNI-TCP]
CPNI, "Security Assessment of the Transmission Control
Protocol (TCP)", (to be published) .
[Cisco-PIX]
Cisco PIX, "http://www.cisco.com/en/US/docs/security/asa/
asa70/command/reference/tz.html#wp1288756".
[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".
[RFC0961] Reynolds, J. and J. Postel, "Official ARPA-Internet
protocols", RFC 961, December 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]
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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.
Appendix A. Survey of the processing of TCP urgent indications by some
popular TCP implementations
A.1. FreeBSD
FreeBSD [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 band". 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 as "urgent data".
A.2. Linux
Linux [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 band". 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 as "urgent data".
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A.3. NetBSD
NetBSD [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 band". That is, they 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 as "urgent data".
A.4. OpenBSD
OpenBSD [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 band". That is, they 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 as "urgent data".
A.5. Cisco IOS, versions 12.2(18)SXF7, 12.4(15)T7
Cisco IOS, versions 12.2(18)SXF7, 12.4(15)T7 interpret the semantics
of the urgent pointer as specified in Section 4 of this document.
Tests performed with an HTTP server running on Cisco IOS version
12.2(18)SXF7 and 12.4(15)T7 suggest that urgent data is processed "in
band". That is, they are accessible together with "normal" data.
The TCP debugs on the Cisco IOS device do explicitly mention the
presence of urgent data, and thus we infer that the behavior is
different depending on the application.
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
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cygwin environment, has shown that the default behavior is to return
the urgent data "in band".
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 as "urgent data".
[Windows95]
Appendix B. Changes from previous versions of the draft (to be removed
by the RFC Editor before publishing this document as an
RFC)
B.1. Changes from draft-ietf-tcpm-urgent-data-00
o Minor editorial changes.
o Incorporated the specific changes/advice stated in
http://www.ietf.org/mail-archive/web/tcpm/current/msg04548.html in
different sections (Section 4, Section 5, Section 6).
B.2. Changes from draft-gont-tcpm-urgent-data-01
o Draft resubmitted as draft-ietf, as a result of wg consensus on
adopting the document as a tcpm wg item.
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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
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