The Secure Shell (SSH) Session Channel Break Extension
RFC 4335

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Network Working Group                                       J. Galbraith
Request for Comments: 4335                              VanDyke Software
Category: Standards Track                                     P. Remaker
                                                      Cisco Systems, Inc
                                                            January 2006

         The Secure Shell (SSH) Session Channel Break Extension

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   The Session Channel Break Extension provides a means to send a BREAK
   signal over a Secure Shell (SSH) terminal session.

Table of Contents

   1. Introduction ....................................................2
   2. Conventions Used in This Document ...............................2
   3. The Break Request ...............................................3
   4. Security Considerations .........................................4
   5. IANA Considerations .............................................4
   6. References ......................................................4
      6.1. Normative References .......................................4
      6.2. Informative References .....................................5

Galbraith & Remaker         Standards Track                     [Page 1]
RFC 4335                  SSH Break Extension               January 2006

1.  Introduction

   The Secure Shell (SSH) [5] session channel provides a mechanism for
   the client-user to interactively enter commands and receive output
   from a remote host while taking advantage of the SSH transport's
   privacy and integrity features.  SSH is increasingly being used to
   replace Telnet for terminal access applications.

   A common application of the Telnet protocol is the "Console Server"
   [7] whereby a Telnet Network Virtual Terminal (NVT) can be connected
   to a physical RS-232/V.24 asynchronous port, making the Telnet NVT
   appear as a locally attached terminal to that port, and making that
   physical port appear as a network-addressable device.  A number of
   major computer equipment vendors provide high-level administrative
   functions through an asynchronous serial port and generally expect
   the attached terminal to be capable of sending a BREAK signal.

   A BREAK signal is defined as the TxD signal being held in a SPACE
   ("0") state for a time greater than a whole character time.  In
   practice, a BREAK signal is typically 250 to 500 ms in length.

   The Telnet protocol furnishes a means to send a "BREAK" signal, which
   RFC 854 [1] defines as "a signal outside the USASCII set which is
   currently given local meaning within many systems".  Console Server
   vendors interpret the TELNET BREAK signal as a physical BREAK signal,
   which can then allow access to the full range of administrative
   functions available on an asynchronous serial console port.

   The lack of a similar facility in the SSH session channel has forced
   users to continue the use of Telnet for the "Console Server"
   function.

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [2].

   The "byte", "boolean", "uint32", and "string" data types are defined
   in [3].

Galbraith & Remaker         Standards Track                     [Page 2]
RFC 4335                  SSH Break Extension               January 2006

3.  The Break Request

   The following channel-specific request can be sent over a session
   channel (as described in [4]) to request that the remote host perform
   a BREAK operation.

        byte      SSH_MSG_CHANNEL_REQUEST
        uint32    recipient channel
        string    "break"
        boolean   want_reply
        uint32    break-length in milliseconds

   If the BREAK length cannot be controlled by the application receiving
   this request, the BREAK length parameter SHOULD be ignored and the
   default BREAK signal length of the chipset or underlying chipset
   driver SHOULD be sent.

   If the application receiving this request can control the BREAK
   length, the following suggestions are made regarding BREAK duration.
   If a BREAK duration request of greater than 3000 ms is received, it
   SHOULD be interpreted as a request for a 3000 ms BREAK.  This
   safeguard prevents an unreasonably long BREAK request from causing a
   port to become unavailable for as long as 49.7 days while executing
   the BREAK.  Applications that require a longer BREAK may choose to