draft-ietf-tcpm-persist-05.txt   draft-ietf-tcpm-persist-06.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: February 23, 2012 Cisco Expires: March 12, 2012 Cisco
August 22, 2011 September 9, 2011
Clarification of sender behavior in persist condition. Clarification of sender behavior in persist condition.
draft-ietf-tcpm-persist-05.txt draft-ietf-tcpm-persist-06.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. This draft clarifies what has been
till now a misinterpretation of the standard as specified in RFC 1122
[RFC1122] rather than making a change to the standard.
Status of this Memo Status of this Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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 February 23, 2012. This Internet-Draft will expire on March 12, 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
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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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. Discussion on RFC 1122 Requirement . . . . . . . . . . . . . . 4 2. Discussion on RFC 1122 Requirement . . . . . . . . . . . . . . 4
3. Description of one Simple Attack . . . . . . . . . . . . . . . 5 3. Description of one Simple Attack . . . . . . . . . . . . . . . 5
4. Clarification Regarding RFC 1122 Requirements . . . . . . . . 6 4. Clarification Regarding RFC 1122 Requirements . . . . . . . . 6
5. Scope of Changes . . . . . . . . . . . . . . . . . . . . . . . 7 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 8.1. Normative References . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . . 11 8.2. Informative References . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction 1. Introduction
Section 4.2.2.17 of Requirements for Internet Hosts [RFC1122] says: Section 4.2.2.17 of Requirements for Internet Hosts [RFC1122] says:
"A TCP MAY keep its offered receive window closed indefinitely. "A TCP MAY keep its offered receive window closed indefinitely.
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 needs to 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 needs to, of course, comply by aborting such
connections. TCP implementations that misinterpret Section 4.2.2.17 connections. TCP implementations that misinterpret Section 4.2.2.17
of Requirements for Internet Hosts [RFC1122] have the potential to of Requirements for Internet Hosts [RFC1122] have the potential to
make systems vulnerable to Denial of Service (DoS) [RFC4732] 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 2 of this document describes why implementations must not This draft clarifies what has been till now a misinterpretation of
the standard as specified in RFC 1122 [RFC1122], rather than making a
change to the standard.
Section 2 of this document describes why implementations might 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 need to still allow such connections to be closed on command.
Section 3 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 4 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. Discussion on RFC 1122 Requirement 2. 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
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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 needs to be able to continue uninterrupted over the
TCP connection. same TCP connection.
Systems that misinterpret the above section of Requirements for Systems that misinterpret the above section of Requirements for
Internet Hosts [RFC1122] may fall victim to DoS attacks, by not Internet Hosts [RFC1122] may fall victim to DoS attacks, by not
supporting sufficient mechanisms to allow release of system resources supporting sufficient mechanisms to allow release of system resources
tied up by connections in the persist condition during times of tied up by connections in the persist condition during times of
resource exhaustion. For example, if we take the case of a busy resource exhaustion. For example, if we take the case of a busy
server where multiple (attacker) clients can advertise a zero window server where multiple (attacker) clients can advertise a zero window
forever (by reliably acknowledging the ZWPs). This could eventually forever (by reliably acknowledging the ZWPs). This could eventually
lead to the resource exhaustion in the server system. In such cases lead to the resource exhaustion in the server system. In such cases
the application or operating system would need to take appropriate the application or operating system would need to take appropriate
action on the TCP connection to reclaim their resources and continue action on the TCP connection to reclaim their resources and continue
to maintain 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 needs to 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 4 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, to permit such resource management
resource management
3. 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
clear the persist condition, the server will continue to hold that clear the persist condition, the server will continue to hold that
data indefinitely. Since there may be a limit to the operating data indefinitely. Since there may be a limit to the operating
system kernel memory available for TCP buffers, this may result in system kernel memory available for TCP buffers, this may result in
DoS to legitimate connections by locking up the necessary resources. DoS to legitimate connections by locking up the necessary resources.
If the above scenario persists for an extended period of time, it If the above scenario persists for an extended period of time, it
will lead to TCP buffers and connection blocks starvation causing will lead to TCP buffers and connection blocks starvation causing
legitimate existing connections and new connection attempts to fail. legitimate existing connections and new connection attempts to fail.
A clever application might detect such attacks with connections that A clever application needs to detect such attacks with connections
are not making progress, and could close these connections. However, that are not making progress, and could close these connections.
some applications might have transferred all the data to the TCP However, some applications might have transferred all the data to the
socket and subsequently closed the socket leaving the connection with TCP socket and subsequently closed the socket leaving the connection
no controlling process, hereby referred to as orphaned connections. with no controlling process, hereby referred to as orphaned
Such orphaned connections might be left holding the data indefinitely connections. Such orphaned connections might be left holding the
in their sending queue. data indefinitely 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.
4. 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 needs to 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 5. IANA Considerations
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
This document has no actions for IANA. This document has no actions for IANA.
7. Security Considerations 6. 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 The document limits itself to clarifying RFC 1122. It does not
discuss what should happen with orphaned connections and other discuss what can happen with orphaned connections and other possible
possible mitigation techniques, as these are considered outside the mitigation techniques, as these are considered outside the scope of
scope of this document. this document.
8. Acknowledgments 7. 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.
9. References 8. References
9.1. Normative References 8.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, [RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981. RFC 793, September 1981.
[RFC1122] Braden, R., "Requirements for Internet Hosts - [RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989. Communication Layers", STD 3, RFC 1122, October 1989.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References 8.2. 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- [RFC4732] Handley, M., Rescorla, E., and IAB, "Internet Denial-of-
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