Network Service Header
draft-ietf-sfc-nsh-05
The information below is for an old version of the document.
Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8300.
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Authors | Paul Quinn , Uri Elzur | ||
Last updated | 2016-05-26 | ||
Replaces | draft-quinn-sfc-nsh | ||
RFC stream | Internet Engineering Task Force (IETF) | ||
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IESG | IESG state | Became RFC 8300 (Proposed Standard) | |
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draft-ietf-sfc-nsh-05
IEN 148 J. Postel RFC 764 ISI June 1980 TELNET PROTOCOL SPECIFICATION INTRODUCTION The purpose of the TELNET Protocol is to provide a fairly general, bi-directional, eight-bit byte oriented communications facility. Its primary goal is to allow a standard method of interfacing terminal devices and terminal-oriented processes to each other. It is envisioned that the protocol may also be used for terminal-terminal communication ("linking") and process-process communication (distributed computation). GENERAL CONSIDERATIONS A TELNET connection is a Transmission Control Protocol (TCP) connection used to transmit data with interspersed TELNET control information. TCP and the connection establishment procedure are documentented in the ARPA Internet Protocol Handbook. The TELNET Protocol is built upon three main ideas: first, the concept of a "Network Virtual Terminal"; second, the principle of negotiated options; and third, a symmetric view of terminals and processes. 1. When a TELNET connection is first established, each end is assumed to originate and terminate at a "Network Virtual Terminal", or NVT. An NVT is an imaginary device which provides a standard, network-wide, intermediate representation of a canonical terminal. This eliminates the need for "server" and "user" Hosts* to keep information about the characteristics of each other's terminals and terminal handling conventions. All Hosts, both user and server, map their local device characteristics and conventions so as to appear to be dealing with an NVT over the network, and each can assume a similar mapping by the other party. The NVT is intended to strike a balance between being overly restricted (not providing Hosts a rich enough vocabulary for mapping into their local character sets), and being overly inclusive (penalizing users with modest terminals). *NOTE: The "user" Host is the Host to which the physical terminal is normally attached, and the "server" host is the Host which is normally providing some service. As an alternate point of view, applicable even in terminal-to-terminal or process-to-process communications, the "user" Host is the Host which initiated the communication. Postel [Page 1] June 1980 RFC 764, IEN 148 Telnet Protocol Specification 2. The principle of negotiated options takes cognizance of the fact that many sites will wish to provide additional services over and above those available within an NVT, and many users will have sophisticated terminals and would like to have elegant, rather than minimal, services. Independent of, but structured within, the TELNET Protocol various "options" will be sanctioned which can be used with the "DO, DON'T, WILL, WON'T" structure (discussed below) to allow a user and server to agree to use a more elaborate (or perhaps just different) set of conventions for their TELNET connection. Such options could include changing the character set, the echo mode, the line width, the page length, etc. The basic strategy for setting up the use of options is to have either party (or both) initiate a request that some option take effect. The other party may then either accept or reject the request. If the request is accepted the option immediately takes effect; if it is rejected the associated aspect of the connection remains as specified for an NVT. Clearly, a party may always refuse a request to enable, and must never refuse a request to disable, some option since all parties must be prepared to support the NVT. The syntax of option negotiation has been set up so that if both parties request an option simultaneously, each will see the other's request as the positive acknowledgment of its own. 3. The symmetry of the negotiation syntax can potentially lead to nonterminating acknowledgment loops -- each party seeing the incoming commands not as acknowledgments but as new requests which must be acknowledged. To prevent such loops, the following rules prevail: a. Parties may only request a change in option status; i.e., a party may not send out a "request" merely to announce what mode it is in. b. If a party receives what appears to be a request to enter some mode it is already in, the request should not be acknowledged. c. Whenever one party sends an option command to a second party, whether as a request or an acknowledgment, and use of the option will have any effect on the processing of the data being sent from the first party to the second, then the command must be inserted in the data stream at the point where it is desired that it take effect. (It should be noted that some time will elapse between the transmission of a request and the receipt of an acknowledgment, which may be negative. Thus, a site may wish to buffer data, after requesting an [Page 2] Postel RFC 764, IEN 148 June 1980 Telnet Protocol Specification option, until it learns whether the request is accepted or rejected, in order to hide the "uncertainty period" from the user.) Option requests are likely to flurry back and forth when a TELNET connection is first established, as each party attempts to get the best possible service from the other party. Beyond that, however, options can be used to dynamically modify the characteristics of the connection to suit changing local conditions. For example, the NVT, as will be explained later, uses a transmission discipline well suited to the many "line at a time" applications such as BASIC, but poorly suited to the many "character at a time" applications such as NLS. A server might elect to devote the extra processor overhead required for a "character at a time" discipline when it was suitable for the local process and would negotiate an appropriate option. However, rather than then being permanently burdened with the extra processing overhead, it could switch (i.e., negotiate) back to NVT when the more "taut" control was no longer necessary. It is possible for requests initiated by processes to stimulate a nonterminating request loop if the process responds to a rejection by merely re-requesting the option. To prevent such loops from occurring, rejected requests should not be repeated until something changes. Operationally, this can mean the process is running a different program, or the user has given another command, or whatever makes sense in the context of the given process and the given option. A good rule of thumb is that a re-request should only occur as a result of subsequent information from the other end of the connection or when demanded by local human intervention. Option designers should not feel constrained by the somewhat limited syntax available for option negotiation. The intent of the simple syntax is to make it easy to have options--since it is correspondingly easy to profess ignorance about them. If some particular option requires a richer negotiation structure than possible within "DO, DON'T, WILL, WON'T", the proper tack is to use "DO, DON'T, WILL, WON'T" to establish that both parties understand the option, and once this is accomplished a more exotic syntax can be used freely. For example, a party might send a request to alter (establish) line length. If it is accepted, then a different syntax can be used for actually negotiating the line length--such a "sub-negotiation" perhaps including fields for minimum allowable, maximum allowable and desired line lengths. The important concept is that such expanded negotiations should never begin until some prior (standard) negotiation has established that both parties are capable of parsing the expanded syntax. Postel [Page 3] June 1980 RFC 764, IEN 148 Telnet Protocol Specification In summary, WILL XXX is sent, by either party, to indicate that party's desire (offer) to begin performing option XXX, DO XXX and DON'T XXX being its positive and negative acknowledgments; similarly, DO XXX is sent to indicate a desire (request) that the other party (i.e., the recipient of the DO) begin performing option XXX, WILL XXX and WON'T XXX being the positive and negative acknowledgments. Since the NVT is what is left when no options are enabled, the DON'T and WON>. [RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery for IP version 6", RFC 1981, DOI 10.17487/RFC1981, August 1996, <http://www.rfc-editor.org/info/rfc1981>. [RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina, "Generic Routing Encapsulation (GRE)", RFC 2784, DOI 10.17487/RFC2784, March 2000, <http://www.rfc-editor.org/info/rfc2784>. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 2008, <http://www.rfc-editor.org/info/rfc5226>. [RFC6071] Frankel, S. and S. Krishnan, "IP Security (IPsec) and Internet Key Exchange (IKE) Document Roadmap", RFC 6071, DOI 10.17487/RFC6071, February 2011, <http://www.rfc-editor.org/info/rfc6071>. [RFC6830] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The Locator/ID Separation Protocol (LISP)", RFC 6830, DOI 10.17487/RFC6830, January 2013, <http://www.rfc-editor.org/info/rfc6830>. Quinn & Elzur Expires November 27, 2016 [Page 36] Internet-Draft Network Service Header May 2016 [VXLAN-gpe] Quinn, P., Manur, R., Agarwal, P., Kreeger, L., Lewis, D., Maino, F., Smith, M., Yong, L., Xu, X., Elzur, U., Garg, P., and D. Melman, "Generic Protocol Extension for VXLAN", <https://datatracker.ietf.org/doc/ draft-ietf-nvo3-vxlan-gpe/>. [broadalloc] Napper, J., Kumar, S., Muley, P., and W. Hendericks, "NSH Context Header Allocation -- Mobility", 2016, <https:// datatracker.ietf.org/doc/ draft-napper-sfc-nsh-broadband-allocation/>. [dcalloc] Guichard, J., Smith, M., and et al., "Network Service Header (NSH) Context Header Allocation (Data Center)", 2016, <https://datatracker.ietf.org/doc/ draft-guichard-sfc-nsh-dc-allocation/>. [nsh-sec] Reddy, T., Migault, D., Pignataro, C., Quinn, P., and C. Inacio, "NSH Security and Privacy requirements", 2016, <ht tps://datatracker.ietf.org/doc/ draft-reddy-sfc-nsh-security-req/>. Quinn & Elzur Expires November 27, 2016 [Page 37] Internet-Draft Network Service Header May 2016 Authors' Addresses Paul Quinn (editor) Cisco Systems, Inc. Email: paulq@cisco.com Uri Elzur (editor) Intel Email: uri.elzur@intel.com Quinn & Elzur Expires November 27, 2016 [Page 38]