TCP Maintenance and Minor                                     M. Bashyam
Extensions Working Group                           Ocarina Networks, Inc
Internet-Draft                                           M. Jethanandani
Intended status: Informational                                A. Ramaiah
Expires: January 11, 2010                                  Cisco Systems
                                                           July 10, 2009

        Clarification of sender behaviour in persist condition.

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   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.

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

   According to RFC 1122 [RFC1122] Section A TCP MAY keep it's
   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 important to remember that ACK (acknowledgement) 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 ZWP mode is typically
   known as the persist condition.

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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 state 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 mode.  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.  The 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.

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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 all the response data in it's send queue.  If
   the client never 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 in
   orphaned state.  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 is putting a 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 either
   explicitly requested by the application to do so.  The 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

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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.

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7.  Programming Considerations

   To enable a server to clear connections in persist condition and
   reclaim resources, a socket interface needs to be defined.  Note,
   this condition is mutually exclusive from a persist condition where
   we are not getting zero windows acknowledgement for the probes.


   Format: setsockopt(fd, SOL_TCP, PERSIST_TIMEOUT,

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

   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 is 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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