TCP Maintenance and Minor Extensions                          M. Bashyam
Working Group                                      Ocarina Networks, Inc
Internet-Draft                                           M. Jethanandani
Intended status: Informational                                A. Ramaiah
Expires: March 12, 18, 2012                                            Cisco
                                                      September 9, 15, 2011

         Clarification of

            TCP sender behavior in persist condition.
                     draft-ietf-tcpm-persist-06.txt clarification for Persist Condition.


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

   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 March 12, 18, 2012.

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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.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  9
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11

1.  Introduction

   Section of Requirements for Internet Hosts [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." open.


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

   Therefore zero window probing needs to 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 needs to, of course, comply by aborting such
   connections.  TCP implementations that misinterpret Section
   of Requirements for Internet Hosts [RFC1122] have the potential to
   make systems vulnerable to Denial of Service (DoS) [RFC4732]
   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.

   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,
   yet need to 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

2.  Discussion on RFC 1122 Requirement

   Per Requirements for Internet Hosts [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 needs to be able to continue uninterrupted over the
   same TCP connection.


   However, systems that misinterpret the above section of Requirements
   for Internet Hosts [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 maintain legitimate connections.

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

   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
   any other connection.  Section 4 of this document provides the
   requisite clarification, to permit such resource management

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 needs to 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.

4.  Clarification Regarding RFC 1122 Requirements

   As stated in Requirements for Internet Hosts [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 needs to 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.

   An interface that allows an application to inform TCP on what to do
   when the connection stays in persist condition, or for application or
   other resource manager to query the health of the TCP connection is
   considered outside the scope of this document.  All such techniques
   however are in complete compliance of TCP [RFC0793] and Requirements
   for Internet Hosts [RFC1122].

5.  IANA Considerations

   This document has no actions for IANA.

6.  Security Considerations

   This document discusses one system security consideration as
   described in Security Considerations Guidelines [RFC3552].  In
   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
   discussed in Section 3.

   The document limits itself to clarifying RFC 1122.  It does not
   discuss what can happen with orphaned connections and other possible
   mitigation techniques, as these are considered outside the scope of
   this document.

7.  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.

8.  References

8.1.  Normative 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.

8.2.  Informative References

   [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.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              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

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

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