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RFC 6429 - TCP Sender Clarification for Persist Condition

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Internet Engineering Task Force (IETF)                        M. Bashyam
Request for Comments: 6429                        Ocarina Networks, Inc.
Category: Informational                                  M. Jethanandani
ISSN: 2070-1721                                               A. Ramaiah
                                                           December 2011

             TCP Sender Clarification for Persist Condition


   This document clarifies the Zero Window Probes (ZWPs) described in
   RFC 1122 ("Requirements for Internet Hosts -- Communication Layers").
   In particular, it clarifies the actions that can be taken on
   connections that are experiencing the ZWP condition.  Rather than
   making a change to the standard, this document clarifies what has
   been until now a misinterpretation of the standard as specified in
   RFC 1122.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2011 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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   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 ....................................................2
      1.1. Requirements Language ......................................3
   2. Discussion of RFC 1122 Requirement ..............................3
   3. Description of One Simple Attack ................................4
   4. Clarification Regarding RFC 1122 Requirements ...................5
   5. Security Considerations .........................................5
   6. Acknowledgments .................................................5
   7. References ......................................................6
      7.1. Normative References .......................................6
      7.2. Informative References .....................................6

1.  Introduction

   Section of "Requirements for Internet Hosts -- Communication
   Layers" [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 needs to be supported to prevent a
   connection from hanging forever if ACK segments that re-open the
   window are 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 that connections
   be aborted regardless of whether or not they are in the persist
   condition.  The TCP implementation needs to, of course, comply by
   aborting such connections.  If such resource management is not
   performed external to the protocol implementation, TCP
   implementations that misinterpret Section of [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.

   Rather than making a change to the standard, this document clarifies
   what has been until now a misinterpretation of the standard as
   specified in RFC 1122 [RFC1122].

   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

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in

2.  Discussion of RFC 1122 Requirement

   Per [RFC1122], as long as the ACKs are being received for window
   probes, a connection can continue to stay in the persist condition.
   This is an important feature, because applications typically would
   want the TCP connection to stay open unless an application explicitly
   closes the connection.

   For example, take the case of a user running a network print job
   during which the printer runs out of paper and is waiting for the
   user to reload the paper tray (user intervention).  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
   connection 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 Section 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,
   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 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 such as the Stream Control Transmission Protocol

   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 connections with an 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 and therefore
   do not 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 starvation of TCP buffers and connection blocks, 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
   connections with no controlling process; such connections are
   referred to as orphaned connections.  These orphaned connections
   might be left holding the data indefinitely in their sending queue.

   The US Computer Emergency Readiness Team (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 [RFC1122], a TCP implementation MUST NOT close a
   connection merely because it seems to be stuck in the ZWP or persist
   condition.  Though 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

   An interface that allows an application to inform TCP on what to do
   when the connection stays in the persist condition, or that allows an
   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 with TCP
   [RFC0793] and [RFC1122].

5.  Security Considerations

   This document discusses one system security consideration that is
   listed in "Guidelines for Writing RFC Text on Security
   Considerations" [RFC3552].  In particular, it describes an
   inappropriate use of a system that is acting as a server for many
   users.  That use and a possible DoS attack are discussed in
   Section 3.

   This 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.

6.  Acknowledgments

   This document was inspired by the recent discussions that took place
   regarding the TCP persist condition issue in the TCP Maintenance and
   Minor Extensions (TCPM) Working Group mailing list [TCPM].  The
   outcome of those discussions was to come up with a document that
   would clarify the intentions of the ZWP as discussed in RFC 1122.  We

   would like to thank Mark Allman, Ted Faber, and David Borman for
   clarifying the objective behind this document.  Thanks also go to
   Wesley Eddy for his extensive editorial comments and to Dan Wing,
   Mark Allman, and Fernando Gont for providing feedback on this

7.  References

7.1.  Normative References

   [RFC0793]   Postel, J., "Transmission Control Protocol", STD 7,
               RFC 793, September 1981.

   [RFC1122]   Braden, R., Ed., "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.

7.2.  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., Ed., Rescorla, E., Ed., and IAB, "Internet
               Denial-of-Service Considerations", RFC 4732,
               December 2006.

   [TCPM]      IETF, "TCP Maintenance and Minor Extensions (tcpm) -
               Charter", <http://datatracker.ietf.org/wg/tcpm/charter/>.

   [VU723308]  Manion, A. and D. Warren, "TCP may keep its offered
               receive window closed indefinitely (RFC 1122)",
               November 2009, <http://www.kb.cert.org/vuls/id/723308>.

Authors' Addresses

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

   Phone: +1 (408) 512-2966
   EMail: mbashyam@ocarinanetworks.com

   Mahesh Jethanandani
   170 Tasman Drive
   San Jose, CA  95134

   Phone: +1 (408) 527-8230
   EMail: mjethanandani@gmail.com

   Anantha Ramaiah
   170 Tasman Drive
   San Jose, CA  95134

   Phone: +1 (408) 525-6486
   EMail: ananth@cisco.com


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