TCP Maintenance and Minor M. Bashyam
Extensions Working Group Ocarina Networks, Inc
Internet-Draft M. Jethanandani
Intended status: Informational A. Ramaiah
Expires: August 18, 2011 Cisco
February 14, 2011
Clarification of sender behavior in persist condition.
draft-ietf-tcpm-persist-03.txt
Abstract
This document clarifies the Zero Window Probes (ZWP) described in
Requirements for Internet Hosts [RFC1122]. In particular, it
clarifies the actions that can be taken on connections which are
experiencing the ZWP condition.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 18, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Discussion on RFC 1122 Requirement . . . . . . . . . . . . . . 5
4. Description of one Simple Attack . . . . . . . . . . . . . . . 6
5. Clarification Regarding RFC 1122 Requirements . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
9. Appendix A, Programming Considerations . . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
10.1. Normative References . . . . . . . . . . . . . . . . . . 13
10.2. Informative References . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
Section 4.2.2.17 of Requirements for Internet Hosts [RFC1122] says:
"A TCP MAY keep its offered receive window closed indefinitely.
As long as the receiving TCP continues to send acknowledgments in
response to the probe segments, the sending TCP MUST allow the
connection to stay open."
DISCUSSION:
It is extremely important to remember that ACK (acknowledgment)
segments that contain no data are not reliably transmitted by
TCP.
Therefore zero window probing SHOULD be supported to prevent a
connection from hanging forever if ACK segments that re-opens the
window is lost. The condition where the sender goes into the Zero-
Window Probe (ZWP) mode is typically known as the 'persist
condition'.
This guidance is not intended to preclude resource management by the
operating system or application, which may request connections to be
aborted regardless of them being in the persist condition, and the
TCP implementation should, of course, comply by aborting such
connections. TCP implementations strictly adhering to Section
4.2.2.17 of Requirements for Internet Hosts [RFC1122] have the
potential to make systems vulnerable to Denial of Service (DoS)
scenarios where attackers tie up resources by keeping connections in
the persist condition, if such resource management is not performed
external to the protocol implementation.
Section 2 of this document describes why implementations must not
close connections merely because they are in the persist condition,
yet must still allow such connections to be closed on command.
Section 3 outlines a simple attack on systems that do not
sufficiently manage connections in this state. Section 4 concludes
with a requirements-language clarification to the RFC 1122
requirement.
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2. Requirements
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.
When used in lowercase, these words convey their typical use in
common language, and they are not to be interpreted as described in
Key words for use in RFCs [RFC2119].
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3. Discussion on RFC 1122 Requirement
Per Requirements for Internet Hosts [RFC1122] as long as the ACK's
are being received for window probes, a connection can continue to
stay in the persist condition. This is an important feature because
typically applications would want the TCP connection to stay open
unless an application explicitly closes the connection.
For example take the case of user running a network print job during
which the printer runs out of paper and is waiting for the user
intervention to reload the paper tray. The printer may not be
reading data from the printing application during this time.
Although this may result in a prolonged ZWP state, it would be
premature for TCP to take action on its own and close the printer
connecting merely due to its lack of progress. Once the printer's
paper tray is reloaded (which may be minutes, hours, or days later),
the print job should be able to continue uninterrupted over the same
TCP connection.
Systems that adhere too strictly to the above verbiage of
Requirements for Internet Hosts [RFC1122] may fall victim to DoS
attacks, by not supporting sufficient mechanisms to allow release of
system resources tied up by connections in the persist condition
during times of resource exhaustion. For example, if we take the
case of a busy server where multiple (attacker) clients can advertise
a zero window forever (by reliably acknowledging the ZWPs). This
could eventually lead to the resource exhaustion in the server
system. In such cases the application or operating system would need
to take appropriate action on the TCP connection to reclaim their
resources and continue to persist legitimate connections.
The problem is applicable to TCP and TCP derived flow-controlled
transport protocols like SCTP.
Clearly, a system should be robust to such attacks and allow
connections in the persist condition to be aborted in the same way as
any other connection. Section 4 of this document provides the
requisite clarification, in standards language, to permit such
resource management
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4. Description of one Simple Attack
To illustrate a potential DoS scenario, consider the case where many
client applications open TCP connection with a HTTP [RFC2616] server,
and each sends a GET request for a large page and stops reading the
response partway through. This causes the client's TCP
implementation to advertise a zero window to the server. For every
large HTTP response, the server is left holding on to the response
data in its sending queue. The amount of response data held will
depend on the size of the send buffer and the advertised window. If
the clients never read the data in their receive queues in order to
clear the persist condition, the server will continue to hold that
data indefinitely. Since there may be a limit to the operating
system kernel memory available for TCP buffers, this may result in
DoS to legitimate connections by locking up the necessary resources.
If the above scenario persists for an extended period of time, it
will lead to TCP buffers and connection blocks starvation causing
legitimate existing connections and new connection attempts to fail.
A clever application might detect such attacks with connections that
are not making progress, and could close these connections. However,
some applications might have transferred all the data to the TCP
socket and subsequently closed the socket leaving the connection with
no controlling process, hereby referred to as orphaned connections.
Such orphaned connections might be left holding the data indefinitely
in their sending queue.
CERT has released an advisory in this regard[VU723308] and is making
vendors aware of this DoS scenario.
Appendix A of this document provides a simple mitigation to such
attacks. More sophisticated attacks are possible which can build on
this vulnerability and may remain effective even when mitigated with
the mechanism prescribed in Appendix A of this document.
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5. Clarification Regarding RFC 1122 Requirements
As stated in Requirements for Internet Hosts [RFC1122], a TCP
implementation MUST NOT close a connection merely because it seems to
be stuck in the ZWP or persist condition. Unstated in RFC 1122, but
implicit for system robustness, a TCP implementation MUST allow
connections in the ZWP or persist condition to be closed or aborted
by their applications or other resource management routines in the
operating system.
In order to provide some level of robustness to DoS attacks, a TCP
implementation MAY provide a feedback regarding the persist condition
to the application if requested to do so or an application or other
resource manager can query the health of the TCP connection allowing
it to take the desired action. All such techniques are in complete
compliance of TCP [RFC0793] and Requirements for Internet Hosts
[RFC1122].
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6. IANA Considerations
This document has no actions for IANA.
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7. Security Considerations
This document discusses one system security consideration as
described in Security Considerations Guidelines [RFC3552]. In
particular it describes a inappropriate use of a system that is
acting as a server for many users. That and a possible DoS attack is
discussed in Section 3.
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8. Acknowledgments
This document was inspired by the recent discussions that took place
regarding the TCP persist condition issue in the TCPM WG mailing list
[TCPM]. The outcome of those discussions was to come up with a draft
that would clarify the intentions of the ZWP referred by RFC 1122.
We would like to thank Mark Allman, Ted Faber and David Borman for
clarifying the objective behind this draft. To Wesley Eddy for his
extensive editorial comments and to Dan Wing, Mark Allman and
Fernando Gont on providing feedback on the document.
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9. Appendix A, Programming Considerations
As a potential implementation guideline, the authors are documenting
some of the programming considerations. This should not be in any
way construed as the only way that the mitigation against the DoS
condition can be achieved. Applications can choose their own
implementations on how to deal with this DoS scenario, and should be
aware that this mitigation is only effective at combating the simple
attack scenario described in this document, and does not handle even
slightly more sophisticated attacks based on the same or similar
concepts.
Note, this persist condition is mutually exclusive from a persist
condition where we are not getting zero windows acknowledgement for
the probes.
The technique described here allows an application to specify to the
operating system that it consents to aborting such connections.
Implementers can choose to in addition provide an asynchronous
notification interface to inform the application of the connection in
the persist condition, if they want the application to abort the
connection. In the case where the application has terminated or
orphaned the connection, the TCP or kernel code will go ahead and
clear the connection and reclaim its resources.
The key consideration in putting a solution together is to be able to
detect a connection that is in persist condition. The application
through the socket interface will be able to inform TCP
implementation or kernel of how long they are willing to have
connections wait in the persist condition.
PERSIST_TIMEOUT
Format:
int setsockopt (sockfd, SOL_TCP, SO_PERSISTTIMEO,
persist_timeout_value, length)
int getsockopt (sockfd, SOL_TCP, SO_PERSISTTIMEO,
persist_timeout_value, length)
where persist_timeout_value recorded in seconds is of type int, the
length is set to four.
The above interface allows applications to inform TCP what to do when
the local connection stays in the persist condition. Note that the
default value of persist_timeout_value is -1 which implies it is
infinite.
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TCP sender will save the current time in the connection block when it
receives a zero window ACK. This time is referred to as the persist
entry time. Thereafter every time the probe timer expires and before
it sends another probe or an ACK carrying zero window is received a
check will be done to see how long the connection has been in persist
condition by comparing the current time to the persist entry time.
If the timeout has been exceeded, the connection will be aborted.
Any time a ACK is received that advertises a non-zero window, the
persist entry time is cleared to take the connection out of the
persist condition.
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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
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
July 2003.
[TCPM] TCPM, "IETF TCPM Working Group and mailing list
http://www.ietf.org/html.charters/tcpm.charter.html".
[VU723308]
Manion, "Vulnerability in Web Servers
http://www.kb.cert.org/vuls/id/723308", July 2009.
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Authors' Addresses
Murali Bashyam
Ocarina Networks, Inc
42 Airport Parkway
San Jose, CA 95110
USA
Phone: +1 (408) 512-2966
Email: mbashyam@ocarinanetworks.com
Mahesh Jethanandani
Cisco
170 Tasman Drive
San Jose, CA 95134
USA
Phone: +1 (408) 527-8230
Email: mahesh@cisco.com
Anantha Ramaiah
Cisco
170 Tasman Drive
San Jose, CA 95134
USA
Phone: +1 (408) 525-6486
Email: ananth@cisco.com
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