On the Implementation of the TCP Urgent Mechanism
RFC 6093

Internet Engineering Task Force (IETF)                           F. Gont
Request for Comments: 6093                                       UTN/FRH
Updates: 793, 1011, 1122                                  A. Yourtchenko
Category: Standards Track                                          Cisco
ISSN: 2070-1721                                             January 2011

           On the Implementation of the TCP Urgent Mechanism

Abstract

   This document analyzes how current TCP implementations process TCP
   urgent indications and how the behavior of some widely deployed
   middleboxes affects how end systems process urgent indications.  This
   document updates the relevant specifications such that they
   accommodate current practice in processing TCP urgent indications,
   raises awareness about the reliability of TCP urgent indications in
   the Internet, and recommends against the use of urgent indications
   (but provides advice to applications that do).

Status of This Memo

   This is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in 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/rfc6093.

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.

Gont & Yourtchenko           Standards Track                    [Page 1]
RFC 6093               On the TCP Urgent Mechanism          January 2011

Table of Contents

   1. Introduction ....................................................3
   2. Specification of the TCP Urgent Mechanism .......................3
      2.1. Semantics of Urgent Indications ............................3
      2.2. Semantics of the Urgent Pointer ............................4
      2.3. Allowed Length of "Urgent Data" ............................4
   3. Current Implementation Practice of the TCP Urgent Mechanism .....5
      3.1. Semantics of Urgent Indications ............................5
      3.2. Semantics of the Urgent Pointer ............................5
      3.3. Allowed Length of "Urgent Data" ............................6
      3.4. Interaction of Middleboxes with TCP Urgent Indications .....6
   4. Updating RFC 793, RFC 1011, and RFC 1122 ........................6
   5. Advice to New Applications Employing TCP ........................7
   6. Advice to Applications That Make Use of the Urgent Mechanism ....7
   7. Security Considerations .........................................7
   8. Acknowledgements ................................................8
   9. References ......................................................8
      9.1. Normative References .......................................8
      9.2. Informative References .....................................8
   Appendix A.  Survey of the Processing of TCP Urgent
                Indications by Some Popular TCP Implementations ......10
      A.1. FreeBSD ...................................................10
      A.2. Linux .....................................................10
      A.3. NetBSD ....................................................10
      A.4. OpenBSD ...................................................11
      A.5. Cisco IOS software ........................................11
      A.6. Microsoft Windows 2000, Service Pack 4 ....................11
      A.7. Microsoft Windows 2008 ....................................11
      A.8. Microsoft Windows 95 ......................................11

Gont & Yourtchenko           Standards Track                    [Page 2]
RFC 6093               On the TCP Urgent Mechanism          January 2011

1.  Introduction

   This document analyzes how some current TCP implementations process
   TCP urgent indications, and how the behavior of some widely deployed