draft-ietf-tcpm-persist-04.txt   draft-ietf-tcpm-persist-05.txt 
TCP Maintenance and Minor Extensions M. Bashyam TCP Maintenance and Minor Extensions M. Bashyam
Working Group Ocarina Networks, Inc Working Group Ocarina Networks, Inc
Internet-Draft M. Jethanandani Internet-Draft M. Jethanandani
Intended status: Informational A. Ramaiah Intended status: Informational A. Ramaiah
Expires: September 30, 2011 Cisco Expires: February 23, 2012 Cisco
March 29, 2011 August 22, 2011
Clarification of sender behavior in persist condition. Clarification of sender behavior in persist condition.
draft-ietf-tcpm-persist-04.txt draft-ietf-tcpm-persist-05.txt
Abstract Abstract
This document clarifies the Zero Window Probes (ZWP) described in This document clarifies the Zero Window Probes (ZWP) described in
Requirements for Internet Hosts [RFC1122]. In particular, it Requirements for Internet Hosts [RFC1122]. In particular, it
clarifies the actions that can be taken on connections which are clarifies the actions that can be taken on connections which are
experiencing the ZWP condition. experiencing the ZWP condition.
Status of this Memo Status of this Memo
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 30, 2011. This Internet-Draft will expire on February 23, 2012.
Copyright Notice Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
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publication of this document. Please review these documents publication of this document. Please review these documents
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to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Discussion on RFC 1122 Requirement . . . . . . . . . . . . . . 4
3. Discussion on RFC 1122 Requirement . . . . . . . . . . . . . . 5 3. Description of one Simple Attack . . . . . . . . . . . . . . . 5
4. Description of one Simple Attack . . . . . . . . . . . . . . . 6 4. Clarification Regarding RFC 1122 Requirements . . . . . . . . 6
5. Clarification Regarding RFC 1122 Requirements . . . . . . . . 7 5. Scope of Changes . . . . . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9 7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . . 11 9.1. Normative References . . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . . 11 9.2. Informative References . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction 1. Introduction
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As long as the receiving TCP continues to send acknowledgments in As long as the receiving TCP continues to send acknowledgments in
response to the probe segments, the sending TCP MUST allow the response to the probe segments, the sending TCP MUST allow the
connection to stay open." connection to stay open."
DISCUSSION: DISCUSSION:
It is extremely important to remember that ACK (acknowledgment) It is extremely important to remember that ACK (acknowledgment)
segments that contain no data are not reliably transmitted by segments that contain no data are not reliably transmitted by
TCP. TCP.
Therefore zero window probing SHOULD be supported to prevent a Therefore zero window probing should be supported to prevent a
connection from hanging forever if ACK segments that re-opens the connection from hanging forever if ACK segments that re-opens the
window is lost. The condition where the sender goes into the Zero- window is lost. The condition where the sender goes into the Zero
Window Probe (ZWP) mode is typically known as the 'persist Window Probe (ZWP) mode is typically known as the 'persist
condition'. condition'.
This guidance is not intended to preclude resource management by the This guidance is not intended to preclude resource management by the
operating system or application, which may request connections to be operating system or application, which may request connections to be
aborted regardless of them being in the persist condition, and the aborted regardless of them being in the persist condition, and the
TCP implementation should, of course, comply by aborting such TCP implementation should, of course, comply by aborting such
connections. TCP implementations strictly adhering to Section connections. TCP implementations that misinterpret Section 4.2.2.17
4.2.2.17 of Requirements for Internet Hosts [RFC1122] have the of Requirements for Internet Hosts [RFC1122] have the potential to
potential to make systems vulnerable to Denial of Service (DoS) make systems vulnerable to Denial of Service (DoS) [RFC4732]
scenarios where attackers tie up resources by keeping connections in scenarios where attackers tie up resources by keeping connections in
the persist condition, if such resource management is not performed the persist condition, if such resource management is not performed
external to the protocol implementation. external to the protocol implementation.
Section 3 of this document describes why implementations must not Section 2 of this document describes why implementations must not
close connections merely because they are in the persist condition, close connections merely because they are in the persist condition,
yet must still allow such connections to be closed on command. yet must still allow such connections to be closed on command.
Section 4 outlines a simple attack on systems that do not Section 3 outlines a simple attack on systems that do not
sufficiently manage connections in this state. Section 5 concludes sufficiently manage connections in this state. Section 4 concludes
with a requirements-language clarification to the RFC 1122 with a requirements-language clarification to the RFC 1122
requirement. requirement.
2. Requirements 2. Discussion on RFC 1122 Requirement
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].
3. Discussion on RFC 1122 Requirement
Per Requirements for Internet Hosts [RFC1122] as long as the ACK's Per Requirements for Internet Hosts [RFC1122] as long as the ACK's
are being received for window probes, a connection can continue to are being received for window probes, a connection can continue to
stay in the persist condition. This is an important feature because stay in the persist condition. This is an important feature because
typically applications would want the TCP connection to stay open typically applications would want the TCP connection to stay open
unless an application explicitly closes the connection. unless an application explicitly closes the connection.
For example take the case of user running a network print job during 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 which the printer runs out of paper and is waiting for the user
intervention to reload the paper tray. The printer may not be intervention to reload the paper tray. The printer may not be
reading data from the printing application during this time. reading data from the printing application during this time.
Although this may result in a prolonged ZWP state, it would be 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 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 connecting merely due to its lack of progress. Once the printer's
paper tray is reloaded (which may be minutes, hours, or days later), paper tray is reloaded (which may be minutes, hours, or days later),
the print job should be able to continue uninterrupted over the same the print job should be able to continue uninterrupted over the same
TCP connection. TCP connection.
Systems that adhere too strictly to the above verbiage of Systems that misinterpret the above section of Requirements for
Requirements for Internet Hosts [RFC1122] may fall victim to DoS Internet Hosts [RFC1122] may fall victim to DoS attacks, by not
attacks, by not supporting sufficient mechanisms to allow release of supporting sufficient mechanisms to allow release of system resources
system resources tied up by connections in the persist condition tied up by connections in the persist condition during times of
during times of resource exhaustion. For example, if we take the resource exhaustion. For example, if we take the case of a busy
case of a busy server where multiple (attacker) clients can advertise server where multiple (attacker) clients can advertise a zero window
a zero window forever (by reliably acknowledging the ZWPs). This forever (by reliably acknowledging the ZWPs). This could eventually
could eventually lead to the resource exhaustion in the server lead to the resource exhaustion in the server system. In such cases
system. In such cases the application or operating system would need the application or operating system would need to take appropriate
to take appropriate action on the TCP connection to reclaim their action on the TCP connection to reclaim their resources and continue
resources and continue to persist legitimate connections. to maintain legitimate connections.
The problem is applicable to TCP and TCP derived flow-controlled The problem is applicable to TCP and TCP derived flow-controlled
transport protocols like SCTP. transport protocols like SCTP.
Clearly, a system should be robust to such attacks and allow Clearly, a system should be robust to such attacks and allow
connections in the persist condition to be aborted in the same way as connections in the persist condition to be aborted in the same way as
any other connection. Section 5 of this document provides the any other connection. Section 4 of this document provides the
requisite clarification, in standards language, to permit such requisite clarification, in standards language, to permit such
resource management resource management
4. Description of one Simple Attack 3. Description of one Simple Attack
To illustrate a potential DoS scenario, consider the case where many To illustrate a potential DoS scenario, consider the case where many
client applications open TCP connection with a HTTP [RFC2616] server, client applications open TCP connection with a HTTP [RFC2616] server,
and each sends a GET request for a large page and stops reading the and each sends a GET request for a large page and stops reading the
response partway through. This causes the client's TCP response partway through. This causes the client's TCP
implementation to advertise a zero window to the server. For every implementation to advertise a zero window to the server. For every
large HTTP response, the server is left holding on to the response large HTTP response, the server is left holding on to the response
data in its sending queue. The amount of response data held will 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 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 the clients never read the data in their receive queues in order to
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are not making progress, and could close these connections. However, are not making progress, and could close these connections. However,
some applications might have transferred all the data to the TCP some applications might have transferred all the data to the TCP
socket and subsequently closed the socket leaving the connection with socket and subsequently closed the socket leaving the connection with
no controlling process, hereby referred to as orphaned connections. no controlling process, hereby referred to as orphaned connections.
Such orphaned connections might be left holding the data indefinitely Such orphaned connections might be left holding the data indefinitely
in their sending queue. in their sending queue.
CERT has released an advisory in this regard[VU723308] and is making CERT has released an advisory in this regard[VU723308] and is making
vendors aware of this DoS scenario. vendors aware of this DoS scenario.
5. Clarification Regarding RFC 1122 Requirements 4. Clarification Regarding RFC 1122 Requirements
As stated in Requirements for Internet Hosts [RFC1122], a TCP As stated in Requirements for Internet Hosts [RFC1122], a TCP
implementation MUST NOT close a connection merely because it seems to implementation MUST NOT close a connection merely because it seems to
be stuck in the ZWP or persist condition. Unstated in RFC 1122, but be stuck in the ZWP or persist condition. Unstated in RFC 1122, but
implicit for system robustness, a TCP implementation MUST allow implicit for system robustness, a TCP implementation must allow
connections in the ZWP or persist condition to be closed or aborted connections in the ZWP or persist condition to be closed or aborted
by their applications or other resource management routines in the by their applications or other resource management routines in the
operating system. operating system.
An interface that allows an application to inform TCP on what to do An interface that allows an application to inform TCP on what to do
when the connection stays in persist condition, or for application or when the connection stays in persist condition, or for application or
other resource manager to query the health of the TCP connection is other resource manager to query the health of the TCP connection is
considered outside the scope of this document. All such techniques considered outside the scope of this document. All such techniques
however are in complete compliance of TCP [RFC0793] and Requirements however are in complete compliance of TCP [RFC0793] and Requirements
for Internet Hosts [RFC1122]. for Internet Hosts [RFC1122].
5. Scope of Changes
There was a question within the IETF TCP Maintenance and Minor
Extensions (TCPM) working group about the scope of this document.
After a lot of discussion it came down to whether this draft was
suggesting a change in the standard. The workgroup consensus was
that the draft clarifies what has been till now a misinterpretation
of the standard as specified in RFC 1122 [RFC1122], rather than a
change in standard. Therefore it felt that the document should be
published as a Information RFC rather than a standards document.
6. IANA Considerations 6. IANA Considerations
This document has no actions for IANA. This document has no actions for IANA.
7. Security Considerations 7. Security Considerations
This document discusses one system security consideration as This document discusses one system security consideration as
described in Security Considerations Guidelines [RFC3552]. In described in Security Considerations Guidelines [RFC3552]. In
particular it describes a inappropriate use of a system that is 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 acting as a server for many users. That and a possible DoS attack is
discussed in Section 3. discussed in Section 3.
The document limits itself to clarifying RFC 1122. It does not
discuss what should happen with orphaned connections and other
possible mitigation techniques, as these are considered outside the
scope of this document.
8. Acknowledgments 8. Acknowledgments
This document was inspired by the recent discussions that took place This document was inspired by the recent discussions that took place
regarding the TCP persist condition issue in the TCPM WG mailing list 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 [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. 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 We would like to thank Mark Allman, Ted Faber and David Borman for
clarifying the objective behind this draft. To Wesley Eddy for his clarifying the objective behind this draft. To Wesley Eddy for his
extensive editorial comments and to Dan Wing, Mark Allman and extensive editorial comments and to Dan Wing, Mark Allman and
Fernando Gont on providing feedback on the document. Fernando Gont on providing feedback on the document.
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9.2. Informative References 9.2. Informative References
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC [RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552, Text on Security Considerations", BCP 72, RFC 3552,
July 2003. July 2003.
[RFC4732] Handley, M., Rescorla, E., and IAB, "Internet Denial-of-
Service Considerations", RFC 4732, December 2006.
[TCPM] TCPM, "IETF TCPM Working Group and mailing list [TCPM] TCPM, "IETF TCPM Working Group and mailing list
http://www.ietf.org/html.charters/tcpm.charter.html". http://www.ietf.org/html.charters/tcpm.charter.html".
[VU723308] [VU723308]
Manion, "Vulnerability in Web Servers Manion, "Vulnerability in Web Servers
http://www.kb.cert.org/vuls/id/723308", July 2009. http://www.kb.cert.org/vuls/id/723308", July 2009.
Authors' Addresses Authors' Addresses
Murali Bashyam Murali Bashyam
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Phone: +1 (408) 512-2966 Phone: +1 (408) 512-2966
Email: mbashyam@ocarinanetworks.com Email: mbashyam@ocarinanetworks.com
Mahesh Jethanandani Mahesh Jethanandani
Cisco Cisco
170 Tasman Drive 170 Tasman Drive
San Jose, CA 95134 San Jose, CA 95134
USA USA
Phone: +1 (408) 527-8230 Phone: +1 (408) 527-8230
Email: mahesh@cisco.com Email: mjethanandani@gmail.com
Anantha Ramaiah Anantha Ramaiah
Cisco Cisco
170 Tasman Drive 170 Tasman Drive
San Jose, CA 95134 San Jose, CA 95134
USA USA
Phone: +1 (408) 525-6486 Phone: +1 (408) 525-6486
Email: ananth@cisco.com Email: ananth@cisco.com
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