Note on protocol synch sequences
RFC 529
Document | Type | RFC - Unknown (June 1973) | |
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Authors | |||
Last updated | 2013-03-02 | ||
RFC stream | Legacy stream | ||
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IESG | Responsible AD | (None) | |
Send notices to | (None) |
RFC 529
Global Routing Operations J. Mauch Internet-Draft J. Snijders Intended status: Standards Track NTT Expires: May 4, 2017 G. Hankins Nokia October 31, 2016 Default IPv4 and IPv6 Unicast EBGP Route Propagation Behavior Without Policies draft-ietf-grow-bgp-reject-02 Abstract This document defines the default behavior of a BGP speaker when there is no import or export policy associated with a BGP session for the IPv4 or IPv6 Unicast Address Family. Requirements Language 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 RFC 2119 [RFC2119]. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on May 4, 2017. Copyright Notice Copyright (c) 2016 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 Mauch, et al. Expires May 4, 2017 [Page 1] Internet-Draft BGP Default Reject October 2016 (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" signal. For the TELNET protocol the "out of band" signal is the INS of Host- Host Protocol and the "in band" marker is the TELNET Data Mark character (DM). Ignoring for the moment the use of TELNET as a basis for higher level protocols (such as FTP), the class of characters "interesting" to a TELNET interpreter is the set of TELNET commands (including the commands for option negotiation and sub-negotiation [2]). One might reasonably argue that this class could be enlarged by a server Host to include the set of signals of interest to the terminal support software of that particular Host. For example, in case of TENEX such a set would include the "terminal interrupt" characters enabled by the process reading from the TELNET connection (e.g., ^C, ^T, etc.). Other hosts, such as Multics, might look only for the TELNET commands, such as Interrupt Process (IP), Abort Output (AO), etc. Whether or not one chooses to consider additional signals as interesting during the processing of a TELNET SYNCH sequence should cause the implementer no problem: He must treat all TELNET commands as interesting by interpreting them. He may choose either to ignore such additional signals or to pass them on to the process; in either case there is no vagueness since the implementer knows which characters his terminal support software considers interesting. The difficulty noted in RFC 513 concerning the vagueness of "interesting things" occurs when a higher level protocol makes use of the TELNET SYNCH sequence to force commands of interest to it through to its interpreter. A higher level protocol designed in such a way represents a violation of the protocol layering discipline: The TELNET SYNCH mechanism is being misused by attempting to give it meaning at two different levels of protocol. The problem stems from the fact that, in general, a (increasing) number of different higher level protocols can be designed with TELNET as a base. A TELNET interpreter has no way of knowing the higher level protocol interpreter (if any) to which it is passing characters, and therefore, can not tell which things are "interesting" to the higher level protocol interpreter. That is, just as an NCP should not have to know whether the data it handles is McKenzie, et. al. [Page 2] RFC 529 A Note on Protocol Synch Sequences 29 June 1973 for a TELNET connection, an FTP data connection, etc., a TELNET interpreter should not be required to know the kind of process for which it is handling characters. This should, in fact, result in a simplification of the design and implementation of TELNET protocol interpreters. This difficulty can be resolved by proper design of protocols that make use of TELNET as a base. In particular, if in such a higher level protocol it is important to be able to force commands through to the protocol interpreter, the higher level protocol should include its own synch sequence: i.e., an "out of band" signal used with an "in band" data mark. The TELNET protocol provides the Interrupt Process character (IP) for use as an "out of band" signal. A synch sequence for a protocol built upon TELNET would be: 1. Insert the TELNET IP control character into the data stream; 2. Insert the higher level protocol data mark character (HDM) into the data stream following whatever higher level protocol commands are important at the time. Receipt of the IP TELNET command causes the higher level protocol interpreter to be interrupted, enabling it to scan the data stream (up to and including the HDM) for commands it considers important. As an example, consider the case of the File Transfer Protocol (RFC 454) and the problem of aborting a file transfer in progress. To accomplish such an abort the FTP user (process) should: 1. Send the TELNET IP character; 2. Send the TELNET SYNC sequence, that is: a. Send the TELNET Data Mark (DM); b. Send the Host-Host Protocol INS; 3. Send the FTP ABOR command; and 4. Send the FTP data mark character [3]. The user (or process acting on his behalf) must transmit the TELNET SYNCH sequence of step 2 above to ensure that the TELNET IP gets through to the server's TELNET interpreter. McKenzie, et. al. [Page 3] RFC 529 A Note on Protocol Synch Sequences 29 June 1973 Endnotes [1] I.e., any TELNET commands appearing before the Data Mark are to be interpreted; the Data Mark is to be used to terminate the scan initiated by the INS; characters that are not TELNET commands may be discarded or passed to the user process as the implementer sees fit. [2] We support Hathaway's proposal to fully parenthesize sub- negotiations. Further, we believe that the "closing parenthesis" should be a new command rather than a second SB command; this will aid the receiver in recovering from errors, either in parsing at the receiver or in generation at the transmitter. We disagree with his proposal that sub-negotiations be discarded when encountered during processing of a TELNET SYNCH. [3] For FTP such a data mark character has not yet been defined and, in fact, may not be necessary under the constraint that the FTP command interpreter should look for exactly one command after being interrupted; this is consistent with the general command-reply orientation of FTP. [This RFC was put into machine readable form for entry] [into the online RFC archives by Helene Morin, Via Genie 12/1999] McKenzie, et. al. [Page 4]