A Taxonomy on Private Use Fields in Protocols
draft-lonvick-private-tax-12
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| Author | Chris M. Lonvick | ||
| Last updated | 2017-06-17 | ||
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draft-lonvick-private-tax-12
Network Working Group C. Lonvick
Internet-Draft June 17, 2017
Intended status: Informational
Expires: December 19, 2017
A Taxonomy on Private Use Fields in Protocols
draft-lonvick-private-tax-12.txt
Abstract
This document attempts to provide some clarification for the way that
private use fields have been used in protocols developed in the IETF.
It is strictly a taxonomy of what has been published and offers no
advice about how to design or use private use options.
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 December 19, 2017.
Copyright Notice
Copyright (c) 2017 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
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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.
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This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
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material may not have granted the IETF Trust the right to allow
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Without obtaining an adequate license from the person(s) controlling
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than English.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Nomenclature . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Note on References . . . . . . . . . . . . . . . . . . . . 4
2. Origins of the Private Use Namespace . . . . . . . . . . . . . 4
3. Observed Characteristics of Private Use Options . . . . . . . 5
3.1. Start of Authority . . . . . . . . . . . . . . . . . . . . 5
3.2. Focus of the Namespace . . . . . . . . . . . . . . . . . . 7
4. Examples of Private Use Options . . . . . . . . . . . . . . . 7
4.1. SNMP . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. RADIUS . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.3. Mobile IP . . . . . . . . . . . . . . . . . . . . . . . . 9
4.4. DHCP . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.5. Syslog . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.6. Secure Shell . . . . . . . . . . . . . . . . . . . . . . . 12
4.7. YANG and NETCONF . . . . . . . . . . . . . . . . . . . . . 12
4.8. Extensible Provisioning Protocol . . . . . . . . . . . . . 13
5. Observations . . . . . . . . . . . . . . . . . . . . . . . . . 14
6. Authoritative Guidance . . . . . . . . . . . . . . . . . . . . 15
7. Authors Notes . . . . . . . . . . . . . . . . . . . . . . . . 15
8. Security Considerations . . . . . . . . . . . . . . . . . . . 16
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
11.1. Normative References . . . . . . . . . . . . . . . . . . . 16
11.2. Informative References . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 20
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1. Introduction
Simply put, communications protocols are standardized ways for
computing entities to convey information. Within each communications
protocol, there must be quantified pieces of information that will be
communicated, and there may be non-standardized pieces that can be
communicated. Since one of the goals of standards is to provide
interoperability, all parties participating in any communications
protocol must be aware of how to deal with all fields in the
protocol.
Some protocols have reserved fields for private and experimental use.
Indeed, having options reserved for testing and experimentation has
been found to be beneficial to the community as has been outlined in
"Assigning Experimental and Testing Numbers Considered Useful"
[RFC3692].
Fields reserved for private use cannot provide interoperability
unless their use is fully documented in openly available documents.
This specification uses examples of some protocols to exemplify how
some private use options are used.
1.1. 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 "Key words for use in
RFCs to Indicate Requirement Levels" [RFC2119].
1.2. Nomenclature
In this document, the following words 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 namespaces.
o Namespace - the set of possible values a field may contain; its
actual content may be a name, a number, or another kind of value.
Additionally, the terms "Start of Authority" and "Focus of the
Namespace" are defined within their respective contexts and further
discussed below.
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The name "Start of Authority" comes from the domain name system
configuration file which describes a "SoA" in "DOMAIN NAMES -
CONCEPTS AND FACILITIES" [RFC1034] and "DOMAIN NAMES - IMPLEMENTATION
AND SPECIFICATION" [RFC1035]. This includes the person or entity who
has ultimate control and decision making powers over the scope of the
domain. Some liberties have been taken with using this name in this
specification, but the intent is to identify an authoritative source
for the namespace.
1.3. 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 usually 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)"
[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. 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 purposes as
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long as all network participants could easily differentiate between
the entities using the option.
"Guidelines for Writing an IANA Considerations Section in RFCs"
[RFC2434] {[RFC5226]} describes that values of specific namespaces
may either be registered with the IANA, or not. In most cases, there
are well defined values for 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. The second example of private use namespaces in the IANA
guidelines [RFC2434] {[RFC5226]} is from "STANDARD FOR THE FORMAT OF
ARPA INTERNET TEXT MESSAGES" [RFC0822] {[RFC5322]} which describes X-
headers. There was no effort made to control the namespace 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. Subsequent
sections will explain how these characteristics may have been applied
to specific protocols that are used in the Internet.
There appear to be three quantifiable characteristics of private use
options:
o Start of Authority
o Focus of the Namespace
o Value of the Option
3.1. Start of Authority
A private use option requires a path to an origin that has the
authority to create and maintain the option. The goal for a globally
unique origin is to disambiguate each focus of a namespace to allow
freedom of expression by the vendors and experimenters using them.
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Therefore the referent has most often been seen to be globally
unique, and not dependent upon local interpretation.
Likely, the first private use option 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 origin 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 only used the Private Enterprise Number [IANApen] (PEN) as
a truncated identification of an origin. This reduced the length of
the identifier but continued to provide a unique identifier through a
globally managed namespace.
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. 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 source of the
domain names. 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 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 stay
unique for the foreseeable future.
Some additional information on the PEN may be found in "Enterprise
Number for Documentation Use" [RFC5612].
An alternative to using numerical indicators is to use textual
strings such as names.
Domain names have similar problems as they may be more ephemeral than
eternal. Unlike PENs that become unserviceable when their owning
organization goes out of business, domain names that fall into disuse
may be acquired and used by entirely different organization. Similar
to the use of PENs there have not been any problems reported from
this.
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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 a start of
authority.
3.2. Focus of the Namespace
Once the start of authority is established as a globally unique
source, an actual option, or in some cases multiple options, may be
specified. This has been seen to usually be an indicator of what
value is expected. Within the domain established by the start of
authority, the focus of each value has been seen to be unique.
In a very simple example, a private use option may consist of
"SoA"+"focus"="value". The SoA will be unique and will specify the
start of authority. The focus will be unique as long as the start of
authority maintains that uniqueness; e.g., it would be poor form for
a private enterprise to define a focus, then to redefine it at a
later time.
4. Examples of Private Use Options
This section contains a review of RFCs that allow the use of private
use options.
4.1. SNMP
As was noted, the globally unique origin in SNMP is the International
Standards Organization [ISO] which is accredited by the United
Nations to maintain this structure. The Internet subtree of
experimental OBJECT IDENTIFIERs starts with the prefix: 1.3.6.1.3.,
and the Internet subtree of private enterprise OBJECT IDENTIFIERs
starts with the prefix: 1.3.6.1.4.1. This is followed by a Private
Enterprise Number [IANApen] (PEN) and then the objects defined by
that enterprise.
The structure of management information (SMI) is currently defined as
the "Structure of Management Information Version 2 (SMIv2)"
[RFC2578]. SMI is a well described tree of OBJECT IDENTIFIERs
(OIDs). OIDs have an origin and a path for defined object
identifiers which this document describes as standard options. It
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
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[IANAsmi].
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 focus, of a
namespace. 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 value corresponding to
the focus, which is known as the varbind. In a GetRequest the server
fills this field with a "0" and the client responds by replacing the
"0" with the actual value. Since this field is defined by the
vendor, it may actually be a concatenation of values. In a
SetRequest transmitted to the receiver, this is the last field.
Each OBJECT IDENTIFIER contains a globally unique origin which is
ISO, a focus which is the OBJECT IDENTIFIER, and a value which is the
last field in the SetRequest. This is also the last field in the
response to a GetRequest.
Specific codes, known as error-indexes, are used to indicate when a
request cannot be processed because a device does not understand a
request.
4.2. RADIUS
"The Remote Authentication Dial In User Service (RADIUS)" [RFC2058]
{[RFC2865]} specification 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).
The updated RADIUS document, [RFC2865], gives guidance for using the
VSA.
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.
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.
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o Multiple subattributes MAY be encoded within a single Vendor-
Specific Attribute, although they do not have to be.
There are many attributes defined in RADIUS [RFC2058] {[RFC2865]}
which may be considered to be standard options. Each of these
attributes is specified within a "type length value" (TLC) container.
For this protocol, the attribute "type" is a specific numerical value
which differentiates it from other attributes.
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" starts the "value" which should then
include a subsequent nested TLV so the vendor may define and
enumerate their own options within that field.
As noted above, the globally unique origin for "RADIUS" [RFC2865] is
the PEN. The remainder of the Attribute field after the PEN is
deliberately undefined in the specification. It is however suggested
that the field contain embedded TLVs. This is again very practical.
Each vendor may then have conflicting "types" (e.g. "1") which would
be disambiguated by the origin. For example [PEN="N", type="1"] is
different from [PEN="M", type="1"].
The values for each type are bounded by the length of the attribute.
The protocols exemplified here tend to be machine-to-machine readable
therefore it is likely that the values will not be human readable.
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] to say that RADIUS clients and servers may
ignore Attributes with an unknown type.
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]}.
In that specification, two TLVs have been defined to contain private
use options. These are collectively called Vendor/Organization
Specific Extensions (VSE).
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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
processed.
Having two VSEs of this nature for private use options is consistent
with the original Mobile IP specification [RFC2002] {[RFC5944]} which
states:
When an Extension numbered in either of these sets within the
range 0 through 127 is encountered but not recognized, the message
containing that Extension MUST be silently discarded. When an
Extension numbered in the range 128 through 255 is encountered
which is not recognized, that particular Extension is ignored, but
the rest of the Extensions and message data MUST still be
processed.
The structure of the origin, type, and value of the CVSEs and NVSEs
for "Mobile IP" [RFC3115] has been seen to be used in a manner very
similar to that of RADIUS. The PEN is the origin and types and
values may be stacked within the field. The values are constrained
by the length of the types or subtypes.
4.4. DHCP
The introduction to "Vendor-Identifying Vendor Options for Dynamic
Host Configuration Protocol version 4 (DHCPv4)" [RFC3925] states:
The DHCP protocol for IPv4, [RFC2131], defines options that allow
a client to indicate its vendor type (option 60), and the DHCP
client and server to exchange vendor-specific information (option
43) [RFC2132]. Although there is no prohibition against passing
multiple copies of these options in a single packet, doing so
would introduce ambiguity of interpretation, particularly if
conveying vendor-specific information for multiple vendors.
This appears to indicate that "Dynamic Host Configuration Protocol"
[RFC2131] specified that there was one instance of the vendor type,
and the receiver used that namespace to set the scope for the fields
in the vendor-specific information option. This version of DHCP did
not allow for multiple origins; only a single origin was permitted
and the types were to be defined subsequent to that. Evidently this
was found to be unworkable when different vendors needed to expand
private use options in the protocol.
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This situation was resolved with the publication of "Vendor-
Identifying Vendor Options for Dynamic Host Configuration Protocol
version 4 (DHCPv4)" [RFC3925] which cautions:
The Dynamic Host Configuration Protocol (DHCP) options for Vendor
Class and Vendor-Specific Information can be limiting or ambiguous
when a DHCP client represents multiple vendors.
That specification ([RFC3925]) then used the PEN [IANApen] to define
a unique namespace for private use options in this protocol. Similar
to other protocols of this era, TLV containers were used.
When this protocol was updated to conform to the requirements of
IPv6, the PEN was again used as the way to identify the origin of the
private use option.
[RFC3925] provides guidance on actions to take if servers and clients
do not comprehend a request or a response.
Servers not equipped to interpret the vendor-specific information
sent by a client MUST ignore it. Clients that do not receive
desired vendor-specific information SHOULD make an attempt to
operate without it.
4.5. Syslog
"The Syslog Protocol" [RFC5424] also uses the PEN to uniquely qualify
the namespace for a private use option. Standard options do not
contain the "@" character. Private use options must have the PEN
following the "@" character. This allows a vendor or experimenter to
have overlapping namespaces which the PEN will then uniquely
identify. For example the standard option of tzKnown may only have
associated values of "0" and "1". However tzKnown@32473 may have any
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 if they are interested.
The Syslog protocol [RFC5424] uses the PEN as the origin and allows
for the focus of the private use option to be fully defined by the
vendor within the structured data. Specifically, a vendor may define
a "type" of private use option by concatenating it with the PEN by
using the @ character. Within the bounds of the structured data,
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multiple elements may be used that have identifiers and values.
4.6. Secure Shell
"The Secure Shell (SSH) Protocol Architecture" [RFC4251] uses
character strings rather than PENs. Similar to Syslog, but actually
predating it, standard options must not have the "@" character in
them. Private use options will have an origin identifier preceding
an "@" character followed by a namespace 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 rational for choosing the manner of providing a format for
private use options is given in Section 4.2 of [RFC4251].
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 namespaces where
experimental or classified extensions can be defined without fear
of conflicts with other implementations.
The character strings are domain names as defined in [RFC1034] and
[RFC1035]. This is specified in "The Secure Shell (SSH) Protocol
Architecture" [RFC4251].
Generally, the guidance given is that if a private use option of this
nature is not understood it is to convey an error code to its peer.
In the SSH protocol [RFC4250], the origin is a domain name and the
focus of the option is dependent upon context. For example,
ourcipher-cbc@example.com can only be used when negotiating ciphers,
while example_session@example.com can only be used when negotiating
channel types, per the examples in [RFC4250].
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 namespaces.
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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 origin 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 namespaces. From the
Protocol Description section 2:
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
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Name Mapping" [RFC5731] and "Extensible Provisioning Protocol (EPP)
Host Mapping" [RFC5732] provide a mechanism to temporally bind
options.
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 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.
There appear to be five quantifiable features that have been
documented in using a private use option.
o One feature is to have a definable way for the community to
ascertain the nature of all private use options. For example,
several vendors have published their RADIUS VSAs on web pages,
which are easy to find. From that, anyone creating a new RADIUS
server would have access to, and be able to incorporate the
information available.
o Instructions have frequently been provided on how to deal with
incomplete understanding; where private use options are not
understood by a receiver. Within the example protocol
specifications given in this document, 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.
o The values of private use options have frequently followed the
same guidance given for standard options in their respective
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specifications. In most of the examples given, the value of each
private use option has been well defined and bounded. Most have
been defined to be extensible so as to accomodate future
requirements.
o Private use options may be extensible in a clearly designed way.
RADIUS suggests that the string containing the option be another
TLV. This allows a vendor to define multiple private use options
within their own namespace field. These are known as
subattributes.
o 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.
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.
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This is version -12. I finally got some time to work on this.
A recommendation has been made to update RFC5226 with ID
draft-leiba-cotton-iana-5225bis. If that becomes an RFC while this
document is pending, I'll do that.
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.
Readers and implementers should be aware of the context of
implementing options in protocols they are using or that are being
developed.
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, and Brian Carpenter.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4775] Bradner, S., Carpenter, B., Ed., and T. Narten,
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"Procedures for Protocol Extensions and Variations",
BCP 125, RFC 4775, DOI 10.17487/RFC4775, December 2006,
<http://www.rfc-editor.org/info/rfc4775>.
[RFC6709] Carpenter, B., Aboba, B., Ed., and S. Cheshire, "Design
Considerations for Protocol Extensions", RFC 6709,
DOI 10.17487/RFC6709, September 2012,
<http://www.rfc-editor.org/info/rfc6709>.
11.2. Informative References
[IANAtcp] Internet Assigned Numbers Authority, "IANA Transmission
Control Protocol (TCP) Parameters, TCP Option Kind
Numbers", 2011, <http://www.iana.org/assignments/
tcp-parameters/tcp-parameters.txt>.
[IANAsmi] Internet Assigned Numbers Authority, "Network Management
Parameters", 2011,
<http://www.iana.org/assignments/smi-numbers>.
[IANApen] Internet Assigned Numbers Authority, "IANA PRIVATE
ENTERPRISE NUMBERS", 2011,
<http://www.iana.org/assignments/enterprise-numbers>.
[ISO] International Standards Organization, "International
Standards Organization", 2011, <http://www.iso.org>.
[ICANN] Internet Corporation for Assigned Names and Numbers,
"Internet Corporation for Assigned Names and Numbers",
2011, <http://www.icann.org>.
[RFC0761] Postel, J., "DoD standard Transmission Control Protocol",
RFC 761, January 1980.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<http://www.rfc-editor.org/info/rfc793>.
[RFC0822] Crocker, D., "Standard for the format of ARPA Internet
text messages", STD 11, RFC 822, August 1982.
[RFC0868] Postel, J. and K. Harrenstien, "Time Protocol", STD 26,
RFC 868, May 1983.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
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specification", STD 13, RFC 1035, November 1987.
[RFC1067] Case, J., Fedor, M., Schoffstall, M., and J. Davin,
"Simple Network Management Protocol", RFC 1067,
August 1988.
[RFC1155] Rose, M. and K. McCloghrie, "Structure and identification
of management information for TCP/IP-based internets",
STD 16, RFC 1155, May 1990.
[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., "IP Mobility Support", RFC 2002,
October 1996.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, March 1997.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
[RFC2058] Rigney, C., Rubens, A., Simpson, W., and S. Willens,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2058, January 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC2578] McCloghrie, K., Ed., Perkins, D., Ed., and J.
Schoenwaelder, Ed., "Structure of Management Information
Version 2 (SMIv2)", STD 58, RFC 2578, April 1999.
[RFC2822] Resnick, P., Ed., "Internet Message Format", RFC 2822,
DOI 10.17487/RFC2822, April 2001,
<http://www.rfc-editor.org/info/rfc2822>.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.
[RFC3115] Dommety, G. and K. Leung, "Mobile IP Vendor/
Organization-Specific Extensions", RFC 3115, April 2001.
[RFC3406] Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom,
"Uniform Resource Names (URN) Namespace Definition
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Mechanisms", BCP 66, RFC 3406, 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, <http://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,
<http://www.rfc-editor.org/info/rfc3692>.
[RFC3925] Littlefield, J., "Vendor-Identifying Vendor Options for
Dynamic Host Configuration Protocol version 4 (DHCPv4)",
RFC 3925, October 2004.
[RFC4250] Lehtinen, S. and C. Lonvick, "The Secure Shell (SSH)
Protocol Assigned Numbers", RFC 4250, January 2006.
[RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
[RFC4254] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Connection Protocol", RFC 4254, January 2006.
[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>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<http://www.rfc-editor.org/info/rfc5322>.
[RFC5424] Gerhards, R., "The Syslog Protocol", RFC 5424, March 2009.
[RFC5612] Eronen, P. and D. Harrington, "Enterprise Number for
Documentation Use", RFC 5612, August 2009.
[RFC5730] Hollenbeck, S., "Extensible Provisioning Protocol (EPP)",
STD 69, RFC 5730, August 2009.
[RFC5731] Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
Domain Name Mapping", STD 69, RFC 5731, DOI 10.17487/
RFC5731, August 2009,
<http://www.rfc-editor.org/info/rfc5731>.
[RFC5732] Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
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Host Mapping", STD 69, RFC 5732, August 2009.
[RFC5944] Perkins, C., "IP Mobility Support for IPv4, Revised",
RFC 5944, November 2010.
[RFC6020] Bjorklund, M., "YANG - A Data Modeling Language for the
Network Configuration Protocol (NETCONF)", RFC 6020,
October 2010.
[RFC6241] Enns, R., Bjorklund, M., Schoenwaelder, J., and A.
Bierman, "Network Configuration Protocol (NETCONF)",
RFC 6241, 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, <http://www.rfc-editor.org/info/rfc7805>.
[RFC8141] Saint-Andre, P. and J. Klensin, "Uniform Resource Names
(URNs)", RFC 8141, DOI 10.17487/RFC8141, April 2017,
<http://www.rfc-editor.org/info/rfc8141>.
[W3C.REC-xpath-19991116]
Clark, J. and S. DeRose, "XML Path Language (XPath)
Version 1.0", World Wide Web Consortium
Recommendation REC-xpath-19991116, November 1999,
<http://www.w3.org/TR/1999/REC-xpath-19991116>.
Author's Address
Chris Lonvick
1307 Kent Oak Dr.
Houston, Texas 77077
US
Email: lonvick.ietf@gmail.com
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