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.


   This document clarifies the Zero Window Probes (ZWP) described in
   [RFC1122].  In particular, it clarifies the actions that can be taken
   on connections which are experiencing the ZWP condition.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on August 18, 2011.

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   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.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Discussion on RFC 1122 Requirement . . . . . . . . . . . . . .  4
   3.  Description of one Simple Attack . . . . . . . . . . . . . . .  5
   4.  Clarification Regarding RFC 1122 Requirements  . . . . . . . .  6
   5.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  7
   6.  Appendix A, Programming Considerations . . . . . . . . . . . .  8
   7.  Informative References . . . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11

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1.  Introduction

   Section of [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."


         It is extremely important to remember that ACK (acknowledgment)
         segments that contain no data are not reliably transmitted by

   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

   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 of [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

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2.  Discussion on RFC 1122 Requirement

   Per [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 [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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3.  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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4.  Clarification Regarding RFC 1122 Requirements

   As stated in [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 [RFC0793] and [RFC1122].

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5.  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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6.  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

   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.



   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

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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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7.  Informative 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.

   [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.

   [TCPM]     TCPM, "IETF TCPM Working Group and mailing list

              Manion, "Vulnerability in Web Servers
    ", July 2009.

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Authors' Addresses

   Murali Bashyam
   Ocarina Networks, Inc
   42 Airport Parkway
   San Jose, CA  95110

   Phone: +1 (408) 512-2966

   Mahesh Jethanandani
   170 Tasman Drive
   San Jose, CA  95134

   Phone: +1 (408) 527-8230

   Anantha Ramaiah
   170 Tasman Drive
   San Jose, CA  95134

   Phone: +1 (408) 525-6486

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