TCP Sender Clarification for Persist Condition
RFC 6429

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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
                                                                   Cisco
                                                           December 2011

             TCP Sender Clarification for Persist Condition

Abstract

   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
   http://www.rfc-editor.org/info/rfc6429.

Bashyam, et al.               Informational                     [Page 1]
RFC 6429                  TCP Persist Condition            December 2011

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 4.2.2.17 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.

      DISCUSSION:

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

Bashyam, et al.               Informational                     [Page 2]
RFC 6429                  TCP Persist Condition            December 2011

   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
   condition'.

   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 4.2.2.17 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