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In: IS-auth-wait
Network Working Group                                         C. Lonvick
Internet-Draft                                          October 17, 2019
Intended status: Informational
Expires: April 19, 2020

             A Taxonomy on Private Use Fields in Protocols


   This document discusses how private use fields in IETF protocols are

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on April 19, 2020.

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   Contributions published or made publicly available before November

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   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Nomenclature  . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Note on References  . . . . . . . . . . . . . . . . . . .   3
   2.  Origins of the Private Use Namespace  . . . . . . . . . . . .   3
   3.  Observed Characteristics of Private Use Options . . . . . . .   4
     3.1.  Parts and Identification of the Authority . . . . . . . .   5
       3.1.1.  Authority . . . . . . . . . . . . . . . . . . . . . .   5
       3.1.2.  Path and Value  . . . . . . . . . . . . . . . . . . .   6
     3.2.  Incomplete Understanding  . . . . . . . . . . . . . . . .   7
     3.3.  Bounds and Extensibility  . . . . . . . . . . . . . . . .   7
     3.4.  Use and Reuse . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Examples of Private Use Options . . . . . . . . . . . . . . .   7
     4.1.  SNMP  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  RADIUS  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.3.  Mobile IP . . . . . . . . . . . . . . . . . . . . . . . .  10
     4.4.  DHCP  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.5.  Syslog  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.6.  Secure Shell  . . . . . . . . . . . . . . . . . . . . . .  12
     4.7.  YANG and NETCONF  . . . . . . . . . . . . . . . . . . . .  13
     4.8.  Extensible Provisioning Protocol  . . . . . . . . . . . .  13
   5.  Observations  . . . . . . . . . . . . . . . . . . . . . . . .  14
   6.  Authoritative Guidance  . . . . . . . . . . . . . . . . . . .  15
   7.  Authors Notes . . . . . . . . . . . . . . . . . . . . . . . .  15
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  16
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     11.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   Protocols having options reserved for testing and experimentation
   have been found to be beneficial to the community as discussed in
   "Assigning Experimental and Testing Numbers Considered Useful"

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   [RFC3692].  As with any protocol detail, the effectiveness of private
   use fields depends upon a shared understanding of their syntax and
   semantics by all participating implementations.  For open use in the
   Internet, this requires that such fields be fully specified in openly
   available documents.

   This taxonomy uses examples of some protocols to discuss how some
   private use options are used.

1.1.  Nomenclature

   In this document, the following terms are defined to prevent
   ambiguity.  Some of these words have not been used in the referenced
   works but their meanings can be ascertained and applied.

   o  Standard Option - a field in a protocol frame that may only use
      values that are strictly defined within a specification.

   o  Private Use Option - a field in a protocol frame that is reserved
      for private, experimental, testing, or local use only.

   o  Namespace - a fully qualified Standard Option or Private Use

1.2.  Note on References

   In many cases throughout this document, RFCs are referenced even
   though they are not the most current version of their respective
   protocol.  This is only done to reference the first occurrence of a
   private use option, or to point out a distinct feature in that
   specification.  When an RFC is referenced that is not the current
   version, the reference will be followed by the RFC number of the
   current version in curly braces.

2.  Origins of the Private Use Namespace

   Some standards permit private use options in different ways, while
   others do not.  The "Time Protocol" [RFC0868] is an example of a
   protocol that only conveys standardized information; there is no way
   to use private options and no way to add anything other than what is
   specified in the document.  In a more open way, "DOD STANDARD
   TRANSMISSION CONTROL PROTOCOL" [RFC0761] {[RFC0793]} {[RFC7805]} does
   have "options" but they must be registered through the IANA [IANAtcp]
   before use, which does not leave any room for optional information
   supplied by equipment vendors, network operators, or experimenters.
   An even more open way may be seen in "Vendor-Identifying Vendor
   Options for Dynamic Host Configuration Protocol version 4 (DHCPv4)"

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   [RFC3925], which allows for vendor specific options that do not need
   to be registered with anyone.

   For the case of TCP [RFC0761] {[RFC0793]} {[RFC7805]}, standard
   options are expected; senders may use them and receivers may be
   configured to act upon that information, or to ignore it.  If an
   experimenter wants to add an option, they will have to create a new
   IETF RFC with specific details, or obtain approval from the IESG to
   have the IANA add to the registry [IANAtcp].  Similarly, if equipment
   vendors Foo and Bar were to have a need for a similar option within
   TCP, they would each have to go through the process to add to the
   registry.  They may then need to negotiate how they would interpret
   each others options for some level of interoperability.  On the other
   hand, if a properly crafted multipurpose private use option were to
   be registered, such as in the case of multiple vendor instances
   within "DHCPv4" [RFC3925], then vendors and experimenters would each
   be able to use it for their own purpose as long as all network
   participants could easily differentiate between the entities using
   the option.

   "Guidelines for Writing an IANA Considerations Section in RFCs"
   [RFC2434] {[RFC8126]} describes that values of specific namespaces
   may either be registered with the IANA, or not.  In most cases, there
   are well defined values for their respective namespaces.  However, as
   the document explains, not all namespaces require centralized
   administration.  In that document, it seems to be assumed that
   private use namespaces will be domain specific and it will be up to
   the administrators of any domain to avoid conflicts.  The first
   example given about private use namespaces refers to "Dynamic Host
   Configuration Protocol" [RFC2131] and presumably "DHCP Options and
   BOOTP Vendor Extensions" [RFC2132].  In this the example states that
   "site-specific options in DHCP have significance only within a single
   site".  As noted below this became a problem that was rectified in a
   later revision of DHCP.

   Later works identified a need to place a scope on private use
   namespaces.  Another example of private use namespace in the IANA
   guidelines [RFC2434] {[RFC8126]} is from "STANDARD FOR THE FORMAT OF
   ARPA INTERNET TEXT MESSAGES" [RFC0822] {[RFC5322]} which describes X-
   headers.  There was no effort made to control their scope, and the
   use of the namespace was removed when the specification was updated
   in 2001 in "Internet Message Format" [RFC2822] {[RFC5322]}.

3.  Observed Characteristics of Private Use Options

   This section summarizes the observed characteristics of some private
   use options that have been developed and deployed.

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3.1.  Parts and Identification of the Authority

   There appear to be three identifiable parts of private use

   o  Authority

   o  Path

   o  Value

3.1.1.  Authority

   A private use option requires an Authority that can create and
   maintain the option.  Presumably, the goal for an Authority is to
   regulate, codify, and disambiguate each namespace.  Therefore, the
   referent most often seen has been globally unique, and not dependent
   upon local interpretation.  For example, several vendors have
   published their RADIUS VSAs on web pages, which are easy to find.
   From that, anyone creating or updating a RADIUS server would have
   access to, and be able to incorporate the information available.

   Likely, the first private use option with a globally unique source
   was defined in the "Structure and Identification of Management
   Information for TCP/IP-based Internets" [RFC1155] which was first
   used in "A Simple Network Management Protocol" [RFC1067] {[RFC1157]}
   (SNMP).  The globally unique Authority in SNMP is the International
   Standards Organization [ISO] which is accredited by the United
   Nations to maintain this structure.

   While SNMP used the entire OBJECT IDENTIFIER with the prefix, other
   protocols truncated this to only used the Private Enterprise Number
   [IANApen] (PEN) as an identification of an Authority.  This reduced
   the length of the identifier but continued to provide a unique
   Authority through a globally managed scope.

   The PEN is sourced by the Internet Assigned Numbers Authority (IANA).
   PENs may be viewed as being similar to domain names in that they are
   acquired by individuals, corporations, or other organizations.
   However, a notable difference is that when domain names fall into
   disuse they may be acquired and used by entirely different people or
   organizations - as per the conditions set forth by the Internet
   Corporation for Assigned Names and Numbers [ICANN].  The structure of
   the PEN registry does not place any limits on the time that a PEN
   will be active or associated with the requester.  This is no
   different from many other registries maintained by the IANA; they are
   just a snapshot at the time of the reservation based on the
   information required by the IANA and provided by the applicant.  This

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   eternal association of the PEN, versus the ephemeral association of
   domain names, has not been shown to present any problems to private
   use options.  This may, in fact, be a feature as this methodology
   ensures that these namespaces will stay unique for the foreseeable

   Some additional information on the PEN may be found in "Enterprise
   Number for Documentation Use" [RFC5612], and in "Private Enterprise
   Number (PEN) practices and Internet Assigned Numbers Authority (IANA)
   registration considerations" [I-D.liang-iana-pen].

   One observed alternative to using a numerical indicator such as the
   OBJECT IDENTIFIER or PEN, is to use textual strings such as names.

   In some cases, domain names have been used for this purpose.
   However, as noted above, domain names may be more ephemeral than
   eternal.  Unlike PENs that usually become unserviceable when their
   owning organization ceases operation, domain names that fall into
   disuse may be acquired and used by entirely different organizations.
   Similar to the use of PENs however, there have not been any problems
   reported from this in normal use.

   Uniform Resource Names (URNs) have also been used to convey options.
   They seem to provide flexibility for many different needs.  URNs were
   first defined in "Uniform Resource Names (URN) Namespace Definition
   Mechanisms" [RFC3406] {[RFC8141]}.  "An IETF URN Sub-namespace for
   Registered Protocol Parameters" [RFC3553] provides guidance for ways
   to use URNs as protocol parameters and how to define an Authority.

3.1.2.  Path and Value

   Once the Authority is established as a globally unique source, an
   actual option, or in some cases multiple options, may be specified.
   This has usually been observed to be an indicator of what Value is
   expected.  Within the scope established by the Authority, the Path to
   each Value has been seen to be unique.

   In a very simple example, the namespace of a private use option may
   consist of "Authority"+"Path"="Value".  Since the Authority is
   unique, each individual Path will be unique as long as the Authority
   maintains that uniqueness; e.g., it would be poor form for an
   Authority to define a namespace, then to redefine it in a conflicting
   way at a later time.

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3.2.  Incomplete Understanding

   Guidance has frequently been provided on how to deal with incomplete
   understanding when private use options are not understood by a
   receiver.  Within the example protocol specifications given in this
   discussion, some behavior has usually been established about what to
   do if the receiver does not understand a namespace.  Some protocols
   have defined that a receiver will silently discard packets that
   contain private use options they do not understand.  Other protocols
   have defined that they will only discard the private use option
   rather than the entire packet.  On the other hand some other
   protocols have no need for the receiver to have any understanding of
   any private use options when it receives any.

   In some cases, guidance has given describing appropriate error
   message responses for incomplete understanding or processing that
   cannot be performed.

3.3.  Bounds and Extensibility

   The Values of private use options have frequently followed the same
   guidance given for standard options in their respective
   specifications.  In most of the examples given, the Value of each
   private use option has been well defined and bounded.

   Private use options may be extensible if they are clearly designed to
   be so.

3.4.  Use and Reuse

   In some cases, a unique option may only be used once within the
   context of an exchange.  This may define a Value of an option once
   and will not change that Value during the remainder of the session.
   RADIUS and DHCP seem to either state this or strongly imply it.
   However, while it is not explicitly discussed, it appears that
   nothing prevents this within Syslog, and it seems to be acceptable
   behavior to resend unique options multiple times within EPP.

4.  Examples of Private Use Options

   This section contains a review of RFCs that allow the use of private
   use options.

4.1.  SNMP

   SNMP is syntactically complex but has features in the GetRequest PDU
   that are consistent with the observed characteristics of private use
   options.  The structure of management information (SMI) is currently

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   defined by the "Structure of Management Information Version 2
   (SMIv2)" [RFC2578].  SMI is a well described tree of OBJECT
   IDENTIFIERs (OIDs).  OIDs have an Authority and Path for defined
   object identifiers which this document describes as standard options.
   The specification also allows for experimental and vendor specific
   object identifiers, which are described as private use options in
   this document.  The IANA maintains a registry of these Network
   Management Parameters [IANAsmi].

   As was noted, the globally unique Authority in SNMP is the
   International Standards Organization [ISO].

   The Internet subtree of experimental OBJECT IDENTIFIERs starts with
   the prefix:

   The Internet subtree of private enterprise OBJECT IDENTIFIERs starts
   with the prefix: and is followed by a Private Enterprise
   Number [IANApen] (PEN) and then the objects defined by that
   enterprise.  After the vendor identifier (the PEN) in the management
   information base (MIB), a vendor may create many different trees to
   identify objects.  This may result in a very large number of OBJECT
   IDENTIFIERs, each of which is an identifier, or Value, of a Path.
   Each of these are uniquely identified by the vendor and do not
   require registration with any coordinating authority.

   The last part of each OBJECT IDENTIFIER is the Path and a placeholder
   for its Value; the varbind.  In a GetRequest the SNMP Manager (the
   client) fills the first part of the varbind with the object
   identifier.  The other portion is transmitted with an ASN.1 NULL
   value.  In a typical case, the SNMP Agent (the server) responds by
   replacing the NULL with the actual Value in the response.  Since this
   namespae is defined by the vendor, it may actually be a concatenation
   of Values.

   The SetRequest PDU is similar to the GetRequest PDU in that it has an
   OID and may use a PID to identify the objects, however, the varbind
   is populated differently than in a GetRequest PDU.  The other PDUs
   also use the OID and may use a PID, but behave differently than the
   GetRequest PID.

   The SNMP namespace is extensible.  A varbind may be considered to be
   a TLV wherein the Value may be another TLV.

   Specific codes, known as error-indexes, are used to indicate when a
   request cannot be processed because a device does not understand a

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   GetRequests and SetRequests may be sent repetitively, even with the
   same Path and with the same or different Values.  For GetRequests, a
   client may be monitoring a server to chronologically record
   parameters of interest.  In some cases, the analysis of the Values
   obtained by GetRequests may trigger an event that causes one or more
   SetRequests to be sent.

4.2.  RADIUS

   There are many attributes defined in "The Remote Authentication Dial
   In User Service (RADIUS)" [RFC2058] {[RFC2865]}, which may be
   considered to be standard options.  Each of these attributes is
   specified within a "type length value" (TLV) container.  For this
   protocol, the "type" attribute is a specific numerical value, which
   differentiates it other types.

   [RFC2058] documented how to use just the PEN (without the rest of the
   SMI path to the root) to allow "vendors" to articulate their own
   options.  In that document, these are called Vendor-Specific
   Attributes (VSA).

   One example of a RADIUS standard option is Type 26, which denotes the
   Vendor Specified Attribute.  It is "available to allow vendors to
   support their own extended Attributes not suitable for general
   usage".  The PEN of the "vendor" is the Authority that starts the
   namespace.  The remainder of the namespace after the PEN is
   deliberately undefined in the specification.  It is practically
   suggested that the field contain embedded TLVs.  This may be seen as
   the Path and Value.

   The values for each RADIUS type are bounded by the length of the
   attribute.  In some cases, it is feasible that a value has no length.
   In that case, the transmission of the type alone has been seen to be
   a signal of some sort to the receiver.

   The original specification of [RFC2058] {[RFC2865]} provided guidance
   that invalid packets were to be silently discarded.  That was
   augmented in [RFC2865], along with guidance about reusing the

   o  Servers not equipped to interpret the vendor-specific information
      sent by a client MUST ignore it (although it may be reported).

   o  Clients which do not receive desired vendor-specific information
      SHOULD make an attempt to operate without it, although they may do
      so (and report they are doing so) in a degraded mode.

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   o  The Attribute-Specific field is dependent on the vendor's
      definition of that attribute.

   o  It SHOULD be encoded as a sequence of vendor type / vendor length
      / value fields.

   o  Multiple subattributes MAY be encoded within a single Vendor-
      Specific Attribute, although they do not have to be.

4.3.  Mobile IP

   "Mobile IP Vendor Specific Extensions" [RFC3115] defines two
   extensions that can be used for making organization specific
   extensions by vendors/organizations for their own specific purposes
   for Mobile IP [RFC2002] {[RFC5944]}.  These are the Critical Vendor/
   Organization Specific Extension (CVSE) and the Normal Vendor/
   Organization Specific Extension (NVSE).  These are collectively
   called Vendor/Organization Specific Extensions (VSE).

   The structure of the namespace of the VSEs for "Mobile IP" [RFC3115]
   is similar to that of RADIUS.  The PEN is the Authority, and types
   and values (the Path and Value) may be stacked in TLVs.  The values
   are constrained by the respective lengths of the types or subtypes.

   Guidance is given for incomplete understanding in [RFC3115], which is
   consistent with the guidance given in the original Mobile IP
   specification [RFC2002] {[RFC5944]}.

   o  When the Critical Vendor/Organization Specific Extension (CVSE) is
      encountered but not recognized, the message containing the
      extension MUST be silently discarded.

   o  When a Normal Vendor/Organization Specific Extension (NVSE) is
      encountered but not recognized, the extension SHOULD be ignored,
      but the rest of the Extensions and message data MUST still be

   Error codes are provided in responses to registration requests that
   are denied because of incomplete understanding.

   Multiple TLV's with the types CVSE-TYPE-NUMBER and NVSE-TYPE-NUMBER
   can be included in a message.  RFC 3115 is silent on reusing the same
   VSE in subsequent messages.

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4.4.  DHCP

   "Dynamic Host Configuration Protocol" [RFC2131] specified that there
   was to be a single instance of the vendor type, and the receiver was
   to use that namespace to set and limit the scope for the fields in
   the vendor-specific information option.  This early version of DHCP
   did not allow for multiple Authorities; only a single Authority was
   permitted where the Path and Value were to be defined referring
   exclusively to that scope.  Evidently this was found to be unworkable
   when different vendors needed to expand private use options in the

   "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)" [RFC3315]
   {[RFC8415]} was created to provide DHCP for IPv6.  This used the PEN
   as the way to identify the Authority of each private use option.
   This methodology was subsequently adopted in "Vendor-Identifying
   Vendor Options for Dynamic Host Configuration Protocol version 4
   (DHCPv4)" [RFC3925], which provided for multiple vendors to identify
   and set their own private use options.  TLVs were used in this
   instance with its inherent bounds and extensibility.

   [RFC3925] provides guidance on actions to take if servers and clients
   do not comprehend a request or a response: servers must ignore
   options they are not equipped to comprehend and clients should make
   an attempt to get along without any desired vendor specific response
   they expect.

   [RFC2131] allowed options to be sent only once.  However, it
   acknowledged that multiple values for an option may be transmitted.
   This may be, for example, for a list of routers where the list is too
   long to fit within a single option.  Guidance is given that the
   client must concatenate the values into a single list.  This
   sentiment is echoed in [RFC3925], which states that behavior is
   undefined if a sequence of vendors options reuses the same PEN.

4.5.  Syslog

   "The Syslog Protocol" [RFC5424] also uses the PEN within structured
   data (SD) to uniquely qualify the namespace for private use options.
   The format for options, called SD-ELEMNENTs, consists of an SD-ID and
   SD-PARAMs.  For standard options the "@" character cannot be used in
   the SD-ID.  Private use options must have the PEN following the "@"
   character in the SD-ID.  This allows a vendor or experimenter to have
   disambiguated Paths and Values.

   Simply put, a standard option is an SD-ID that does not have the "@"
   character in it, while a private use option is an SD-ID that does
   contain the "@" character.

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   For example the standard option of the SD-ID timeQuality may only
   have PARAM-VALUEs of "0" and "1" for the tzKnown PARAM-NAME.  The SD-
   ELEMENT Authority for the standard option timeQuality is then the
   IANA.  However the SD-ID timeQuality@32473 is a private use option
   controlled by the Authority that controls enterprise number 32473.
   Therefore, the tzKnown SD-PARAM may have any PARAM-VALUE assigned to
   it by the owner of enterprise number 32473.

   Syslog transport receivers are supposed to accept all correctly
   formatted Syslog messages.  Unlike RADIUS, the receiving Syslog
   application does not have to have immediate knowledge of all variable
   options to continue operations.  If a private use option is not
   immediately known to the receiving application, it may still store
   the message and an Operator or Administrator may look it up at a
   later time.

   An SD-ID may not be reused within a Syslog message.

   Bounds are given in [RFC5424].

4.6.  Secure Shell

   "The Secure Shell (SSH) Protocol Architecture" [RFC4251] uses
   character strings rather than PENs to establish Authority.  Similar
   to Syslog, but actually predating it, standard options must not have
   the "@" character in them.  Private use options will have an
   Authority identifier preceding an "@" character followed by a Value
   field.  For example, in "The Secure Shell (SSH) Connection Protocol"
   [RFC4254] SSH channels may be opened by specifying a channel type
   when sending the SSH_MSG_CHANNEL_OPEN message.  Standard options for
   the channel type include "session" and "x11".  A private use option
   for a channel type could be "example_session@example.com".

   The character strings are domain names as defined in [RFC1034] and
   [RFC1035].  This is specified in "The Secure Shell (SSH) Protocol
   Architecture" [RFC4251].  The rational for choosing the manner of
   providing a format for private use options is given in Section 4.2 of

      We have chosen to identify algorithms, methods, formats, and
      extension protocols with textual names that are of a specific
      format.  DNS names are used to create local Paths and Values where
      experimental or classified extensions can be defined without fear
      of conflicts with other implementations.

   In the SSH protocol [RFC4250], the Authority is a domain name and the
   path and value of the option is dependent upon context.  For example,
   ourcipher-cbc@example.com can only be used when negotiating ciphers,

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   while example_session@example.com can only be used when negotiating
   channel types, per the examples in [RFC4250].

   Guidance is given throughout the SSH series of RFCs (4250 - 4254) for
   incomplete understanding.  The guidance differes based upon the
   context; in some cases, the guidance is to ignore a private use
   option when it cannot be understood, while in other cases, a negative
   response must be sent to indicate that a received private use option
   could not be understood.

   Similarly, reuse of a private use option is dependent upon the
   context.  The same is true for checking bounds of any private use

4.7.  YANG and NETCONF

   One example of a protocol utilizing URNs is "Network Configuration
   Protocol (NETCONF)" [RFC6241].  NETCONF may utilize "YANG - A Data
   Modeling Language for the Network Configuration Protocol (NETCONF)"
   [RFC6020] as a means to describe and convey options.

   "YANG - A Data Modeling Language for the Network Configuration
   Protocol (NETCONF)" [RFC6020] and "Network Configuration Protocol
   (NETCONF)" [RFC6241] use URIs to indicate private use Paths and

   Section 8.3 of YANG [RFC6020] describes the parsing of the YANG
   payload.  It contains a good deal of information about how to process
   elements or values that are not recognized.

   Similarly, NETCONF [RFC6241] contains much information about
   processing requests that cannot be completed because elements or
   values are not recognized.

   Both YANG [RFC6020] and NETCONF [RFC6241] use URIs to enumerate
   private use options of a device.  The use of this comes from XPATH
   [W3C.REC-xpath-19991116].  In both of these, the start of authority
   is the domain name in the URI and the Authority is the full URI path.
   Many private use options may be described within YANG.  From that,
   each private use option may be populated in NETCONF.

4.8.  Extensible Provisioning Protocol

   The "Extensible Provisioning Protocol (EPP)" [RFC5730] is another
   example of a protocol that utilizes URN Path and Values.  From the
   Protocol Description section 2:

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      EPP uses XML namespaces to provide an extensible object management
      framework and to identify schemas required for XML instance
      parsing and validation.  These namespaces and schema definitions
      are used to identify both the base protocol schema and the schemas
      for managed objects.

   The specification provides clear guidance and an example on how to
   extend the base protocol and how to map new objects through the use
   of separate documents.  However, commands and responses may be
   extended through the use of an <extension> element.  In this
   protocol, and the extensions, the start of authority is the domain
   name in the URI and the focus is the full URI path.

   Guidance has been provided about incomplete understanding.  First, a
   section is provided on responses for received messages that are not
   understandable, are beyond boundaries, or are not in compliance with
   policy.  Additionally, guidance is given about incomplete
   understanding of a response:

      Command success or failure MUST NOT be assumed if no response is
      returned or if a returned response is malformed.  Protocol
      idempotency ensures the safety of retrying a command in cases of
      response-delivery failure.

   The associated RFCs of "Extensible Provisioning Protocol (EPP) Domain
   Name Mapping" [RFC5731] and "Extensible Provisioning Protocol (EPP)
   Host Mapping" [RFC5732] provide a mechanism to temporally bind

5.  Observations

   Private use options are a way to allow vendors, network operators,
   and experimenters to convey dynamic information without going through
   any process to register each variable.  However, there is no one size
   fits all.  The use of a very specific and fixed format works for
   RADIUS which requires speed in processing.  On the other hand, the
   open nature of the private use options in Syslog are appropriate for
   that protocol where all event messages need not be fully parsed at
   the time of reception.

   As with all good things, the use of private use options comes with a
   cost.  Adding any extra fields to a protocol will require additional
   processing for both the sender and the receiver.  Also, larger
   packets will take up more bandwidth in transmission.  In another
   aspect, the code needed to deal with private use options may be
   considered wasteful if it is not used.

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   Clear documentation has been seen to achieve uniformity and
   interoperability in these features.  Obviously implementers will need
   to adhere closely to these standards for complete interoperability.

6.  Authoritative Guidance

   This document is not an encouragement or recommendation to define
   private use fields in IETF protocols.  Rather, since private use
   options are being used by the community, this document is an attempt
   to document the ways in which they have been used.

   However, "Design Considerations for Protocol Extensions" [RFC6709] is
   intended to provide guidance on designing protocol extensions.  It
   has several examples and pointers to other material that will aid in
   the development of protocol extensions.

   "Procedures for Protocol Extensions and Variations" [RFC4775] is a
   companion document to [RFC6709] and provides the procedures for
   review and standardization for extensions to be added to protocols.

   Finally, the usage of any private use values on the wire before any
   namespace is properly reserved with the IANA is entirely inadvisable.

7.  Authors Notes

   This section will be removed prior to publication.

   This is version -18.  I have received ISE feedback and am integrating
   that into this document.  Unfortunately, that's going to take a while
   as life and the day job keep getting in the way.

   I'm revising the flow of the document to be consistent and to
   accomodate the feedback that I've received.

8.  Security Considerations

   This document reviews ways that options are being used in various
   protocols.  As such, there are no security considerations inherent in
   this document.

   While it is not a problem that can be technically addressed,
   fraudulently purporting to be an owner of a domain name, a PEN, or
   other identifier may allow the misuse of private namespaces.

   Readers and implementers should be aware of the context of
   implementing options in protocols they are using or that are being

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9.  IANA Considerations

   This document does not propose a standard and does not require the
   IANA to do anything.

10.  Acknowledgments

   The idea for documenting this came from questions asked in the SIP-
   CLF Working Group and the author is grateful for the discussion
   around this topic.

   The following people have contributed to this document.  Listing
   their names here does not mean that they agree with or endorse the
   document, but that they have contributed to its substance:

   David Harrington, Dan Romascanu, Bert Wijnen, Ralph Droms, Juergen
   Schoenwalder, Nevil Brownlee, Klaas Wierenga, Brian Carpenter, Randy
   Presuhn, and Dave Crocker.

11.  References

11.1.  Normative References

   [RFC4775]  Bradner, S., Carpenter, B., Ed., and T. Narten,
              "Procedures for Protocol Extensions and Variations",
              BCP 125, RFC 4775, DOI 10.17487/RFC4775, December 2006,

   [RFC6709]  Carpenter, B., Aboba, B., Ed., and S. Cheshire, "Design
              Considerations for Protocol Extensions", RFC 6709,
              DOI 10.17487/RFC6709, September 2012,

11.2.  Informative References

              Liang, P., Melnikov, A., and D. Conrad, "Private
              Enterprise Number (PEN) practices and Internet Assigned
              Numbers Authority (IANA) registration considerations",
              draft-liang-iana-pen-06 (work in progress), July 2015.


   [IANAsmi]  Authority, I. A. N., "Network Management Parameters",
              2011, <http://www.iana.org/assignments/smi-numbers>.

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   [IANAtcp]  Authority, I. A. N., "IANA Transmission Control Protocol
              (TCP) Parameters, TCP Option Kind Numbers", 2011,

   [ICANN]    ICANN, "Internet Corporation for Assigned Names and
              Numbers", 2011, <http://www.icann.org>.

   [ISO]      ISO, "International Standards Organization", 2011,

   [RFC0761]  Postel, J., "DoD standard Transmission Control Protocol",
              RFC 761, DOI 10.17487/RFC0761, January 1980,

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,

              TEXT MESSAGES", STD 11, RFC 822, DOI 10.17487/RFC0822,
              August 1982, <https://www.rfc-editor.org/info/rfc822>.

   [RFC0868]  Postel, J. and K. Harrenstien, "Time Protocol", STD 26,
              RFC 868, DOI 10.17487/RFC0868, May 1983,

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC1067]  Case, J., Fedor, M., Schoffstall, M., and J. Davin,
              "Simple Network Management Protocol", RFC 1067,
              DOI 10.17487/RFC1067, August 1988,

   [RFC1155]  Rose, M. and K. McCloghrie, "Structure and identification
              of management information for TCP/IP-based internets",
              STD 16, RFC 1155, DOI 10.17487/RFC1155, May 1990,

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   [RFC1157]  Case, J., Fedor, M., Schoffstall, M., and J. Davin,
              "Simple Network Management Protocol (SNMP)", RFC 1157,
              DOI 10.17487/RFC1157, May 1990,

   [RFC2002]  Perkins, C., Ed., "IP Mobility Support", RFC 2002,
              DOI 10.17487/RFC2002, October 1996,

   [RFC2058]  Rigney, C., Rubens, A., Simpson, W., and S. Willens,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2058, DOI 10.17487/RFC2058, January 1997,

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 2434,
              DOI 10.17487/RFC2434, October 1998,

   [RFC2578]  McCloghrie, K., Ed., Perkins, D., Ed., and J.
              Schoenwaelder, Ed., "Structure of Management Information
              Version 2 (SMIv2)", STD 58, RFC 2578,
              DOI 10.17487/RFC2578, April 1999,

   [RFC2822]  Resnick, P., Ed., "Internet Message Format", RFC 2822,
              DOI 10.17487/RFC2822, April 2001,

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, DOI 10.17487/RFC2865, June 2000,

   [RFC3115]  Dommety, G. and K. Leung, "Mobile IP Vendor/Organization-
              Specific Extensions", RFC 3115, DOI 10.17487/RFC3115,
              April 2001, <https://www.rfc-editor.org/info/rfc3115>.

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   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <https://www.rfc-editor.org/info/rfc3315>.

   [RFC3406]  Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom,
              "Uniform Resource Names (URN) Namespace Definition
              Mechanisms", RFC 3406, DOI 10.17487/RFC3406, October 2002,

   [RFC3553]  Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An
              IETF URN Sub-namespace for Registered Protocol
              Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June
              2003, <https://www.rfc-editor.org/info/rfc3553>.

   [RFC3692]  Narten, T., "Assigning Experimental and Testing Numbers
              Considered Useful", BCP 82, RFC 3692,
              DOI 10.17487/RFC3692, January 2004,

   [RFC3925]  Littlefield, J., "Vendor-Identifying Vendor Options for
              Dynamic Host Configuration Protocol version 4 (DHCPv4)",
              RFC 3925, DOI 10.17487/RFC3925, October 2004,

   [RFC4250]  Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Protocol Assigned Numbers", RFC 4250,
              DOI 10.17487/RFC4250, January 2006,

   [RFC4251]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251,
              January 2006, <https://www.rfc-editor.org/info/rfc4251>.

   [RFC4254]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Connection Protocol", RFC 4254, DOI 10.17487/RFC4254,
              January 2006, <https://www.rfc-editor.org/info/rfc4254>.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,

   [RFC5424]  Gerhards, R., "The Syslog Protocol", RFC 5424,
              DOI 10.17487/RFC5424, March 2009,

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   [RFC5612]  Eronen, P. and D. Harrington, "Enterprise Number for
              Documentation Use", RFC 5612, DOI 10.17487/RFC5612, August
              2009, <https://www.rfc-editor.org/info/rfc5612>.

   [RFC5730]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)",
              STD 69, RFC 5730, DOI 10.17487/RFC5730, August 2009,

   [RFC5731]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
              Domain Name Mapping", STD 69, RFC 5731,
              DOI 10.17487/RFC5731, August 2009,

   [RFC5732]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
              Host Mapping", STD 69, RFC 5732, DOI 10.17487/RFC5732,
              August 2009, <https://www.rfc-editor.org/info/rfc5732>.

   [RFC5944]  Perkins, C., Ed., "IP Mobility Support for IPv4, Revised",
              RFC 5944, DOI 10.17487/RFC5944, November 2010,

   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,

   [RFC7805]  Zimmermann, A., Eddy, W., and L. Eggert, "Moving Outdated
              TCP Extensions and TCP-Related Documents to Historic or
              Informational Status", RFC 7805, DOI 10.17487/RFC7805,
              April 2016, <https://www.rfc-editor.org/info/rfc7805>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,

   [RFC8141]  Saint-Andre, P. and J. Klensin, "Uniform Resource Names
              (URNs)", RFC 8141, DOI 10.17487/RFC8141, April 2017,

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   [RFC8415]  Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
              Richardson, M., Jiang, S., Lemon, T., and T. Winters,
              "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
              RFC 8415, DOI 10.17487/RFC8415, November 2018,

              Clark, J. and S. DeRose, "XML Path Language (XPath)
              Version 1.0", World Wide Web Consortium Recommendation
              REC-xpath-19991116, November 1999,

Author's Address

   Chris Lonvick
   1307 Kent Oak Dr.
   Houston, Texas  77077

   Email: lonvick.ietf@gmail.com

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