TCP Maintenance and Minor                                     M. Bashyam
Extensions Working Group                           Ocarina Networks, Inc
Internet-Draft                                           M. Jethanandani
Intended status: Informational                                A. Ramaiah
Expires: May 14, 2011                                      Cisco Systems
                                                       November 10, 2010

        Clarification of sender behaviour in persist condition.


   This document attempts to clarify the notion of the Zero Window
   Probes (ZWP) described in RFC 1122 [RFC1122].  In particular, it
   clarifies the actions that can be taken on connections which are
   experiencing the ZWP condition.  The motivation for this document
   stems from the belief that TCP implementations strictly adhering to
   the current RFC language have the potential to become vulnerable to
   Denial of Service (DoS) scenarios.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on May 14, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Discussion on RFC 1122 Requirement . . . . . . . . . . . . . .  4
   3.  Description of Attack  . . . . . . . . . . . . . . . . . . . .  5
   4.  Clarification Regarding RFC 1122 Requirements  . . . . . . . .  6
   5.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  8
   7.  Programming Considerations . . . . . . . . . . . . . . . . . .  9
   8.  Informative References . . . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11

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1.  Introduction

   TCP implementations strictly adhering to Section of
   [RFC1122] have the potential to become vulnerable to Denial of
   Service (DoS) scenarios.  That 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
   condition'.  It is under this condition that the sending TCP can
   become vulnerable to DoS.

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2.  Discussion on RFC 1122 Requirement

   It needs to be emphasised that TCP MUST NOT take any action of its
   own when a particular connection is in persist condition for a long
   time.  As per RFC 1122 as long as the ACK's are being received for
   window probes, it can continue to stay in persist condition.  This is
   important because typically applications would want the TCP
   connection to stay open unless it explicitly closes the connection.
   For example take the case of user running a print job and the printer
   ran out of paper waiting for the user intervention.  It would be
   premature for TCP to take action on its own.  Hence TCP cannot act as
   a resource manager and it is the system or application's
   responsibility to take appropriate action.

   At the same time, many existing TCP implementations that adhere
   strictly to the above verbiage of RFC 1122 may fall victim to DOS
   attacks, if appropriate measures are not followed.  For example, if
   we take the case of a busy server where multiple clients can
   advertise a zero forever (by reliably acknowledging the ZWP's), it
   could eventually lead to the resource exhaustion in the system.  In
   such cases the system would need to take appropriate action on the
   TCP connection to reclaim the resources.

   This document is not intended to provide any advice on any particular
   resource management scheme that can be implemented to circumvent DOS
   issues arising due to the connections stuck in the persist state.

   The problem is applicable to TCP and TCP derived transport protocols
   like SCTP.

   In summary, TCP MUST NOT take any action on its own to abort a
   connection in persist condition.  Applications however can request
   that a connection in persist condition be aborted.  The resource
   manager in the operating system when faced with depleted resources
   can also ask TCP to abort a connection.

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3.  Description of Attack

   If TCP implementations strictly follow RFC 1122 and there is no
   instruction on what to do in persist condition, connections will
   encounter an indefinite wait.  To illustrate this, consider the case
   where the client application opens a TCP connection with a HTTP
   [RFC2616] server, sends a GET request for a large page and stops
   reading the response.  This would cause the client TCP 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 send queue.  The
   amount of response data held will depend on the size of the send
   buffer and the advertised window.  If the client never reads the data
   in its receive queue or clears the persist condition, the server will
   continue to hold that data indefinitely.  Multiple such TCP
   connections stuck in the same scenario on the server would cause
   resource depletion resulting in a DoS situation on the server.

   Applications on the sender can transfer all the data to the TCP
   socket and subsequently close the socket leaving the connection with
   no controlling process, hereby referred to as orphaned connection.
   If the application on the receiver refuses to read the data, the
   orphaned connection will be left holding the data indefinitely in its
   send queue.

   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.

   CERT has released an advisory in this regard[VU723308] and is making
   vendors aware of this DoS scenario.

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4.  Clarification Regarding RFC 1122 Requirements

   A consequence of adhering to the above requirement mandated by RFC
   1122 is that multiple TCP receivers advertising a zero window to a
   server could exhaust the connection and buffer resources of the
   sender.  In such cases, and specially when the receiver is reliably
   acknowledging zero window probe, to achieve robustness, the system
   should be able to take appropriate action on those TCP connections
   and reclaim resources.  A possible action could be to terminate the
   connection and such an action is in the spirit of RFC 1122.

   In order to accomplish this action, TCP MAY provide a feedback
   regarding the persist condition to the application if requested to do
   so or the application or the resource manager can query the health of
   the TCP connection which would allow it to take the desired action.
   All such actions are in complete compliance of RFC 793 and RFC 1122.

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5.  Conclusion

   The document addresses the fact that terminating TCP connections
   stuck in the persist condition does not violate RFC 1122 or RFC 793.
   It also suggests that TCP must not abort any connection until
   explicitly requested by the application or the operating system to do
   so.  The potential implementation guidelines of the request and the
   action are documented in Section 7, and the details of mitigating the
   DoS attack are left to the implementer.

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6.  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 and David Borman for clarifying
   the objective behind this draft.  To Dan Wing, Mark Allman and
   Fernando Gont on providing feedback on the document.

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7.  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 sceanrio.

   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 can inform TCP or kernel of how long
   they are willing to wait in persist condition.  When the connection
   reaches that particular timeout value a EPERSISTTIMEOUT notification
   will be sent to the application.  The application on receiving the
   notification can turn around and issue a close.  In the case, the
   application has terminated, TCP or kernel will go ahead and clear the
   connection and reclaim the resoruces.  Note, this persist condition
   is mutually exclusive from a persist condition where we are not
   getting zero windows acknowledgement for the probes.



   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 and
   the length is four.

   The above interface allows applications to inform TCP that when the
   local connection stays in persist condition it can be aborted after a
   set time.  Note that the default value of this option is infinite.

   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 persist

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8.  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
   Cisco Systems
   170 Tasman Drive
   San Jose, CA  95134

   Phone: +1 (408) 527-8230

   Anantha Ramaiah
   Cisco Systems
   170 Tasman Drive
   San Jose, CA  95134

   Phone: +1 (408) 525-6486

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