One document matched: draft-lonvick-private-tax-03.xml
<?xml version="1.0"?>
<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
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<?rfc symrefs="yes"?>
<?rfc compact="yes"?>
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<rfc category="info" ipr="pre5378Trust200902" docName="draft-lonvick-private-tax-03.txt">
<front>
<title abbrev='Private Use Fields'>
A Taxonomy on Private Use Fields in Protocols
</title>
<author initials="C.M." surname="Lonvick"
fullname="Chris Lonvick">
<address>
<postal>
<street>213 El Rancho Grande</street>
<city>Kerrville</city><region>Texas</region>
<code>78028</code>
<country>US</country>
</postal>
<email>lonvick.ietf@gmail.com</email>
</address>
</author>
<date month="November" year="2011" />
<keyword>PEN</keyword>
<keyword>Private name space</keyword>
<keyword>Private use space</keyword>
<keyword>Private use</keyword>
<keyword>Private Enterprise Number</keyword>
<keyword>Private Enterprise Code</keyword>
<abstract>
<t>
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 is out there and offers a minimal amount of advice about how to design them.
</t>
</abstract>
</front>
<middle>
<section anchor="intro" title="Introduction">
<t>
Simply put, communications protocols are standardized ways for computing entities to
convey information. Within each communications protocol, there must be standardized
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. Fields reserved for private use
cannot provide interoperability unless their use is fully documented in openly
available documents.
</t><t>
The <xref target="RFC0868">Time Protocol</xref> is an example of a protocol that only
conveys standardized information. There is no way to add anything other than what is
specified in the document. On the other hand,
<xref target="RFC0761">DOD STANDARD TRANSMISSION CONTROL PROTOCOL</xref> does have
"options" but they must be registered through the IANA <xref target="IANAtcp" />,
which does not leave any room for optional information supplied by equipment vendors,
network operators, or experimenters. Finally,
<xref target="RFC3925">Vendor-Identifying Vendor Options for Dynamic Host Configuration Protocol version 4 (DHCPv4)</xref>
does allow for vendor specific options.
</t><t>
If a network operator wanted to add specific information to the
<xref target="RFC0868">Time Protocol</xref>, they could modify the code of the senders
and receivers and run this within their own domain without any problems. However, if
an equipment vendor wanted to include information specific to their equipment, they
would have to ensure that all senders and receivers within all network domains would
either accept the change in the protocol, or would not have problems with it. As a
final case, if several equipment vendors desired to add equipment-specific information
to this protocol, they would have to take great care that only their own receivers
would accept information from their own transmitters. An extension to that would be
that if one equipment vendor would like to transmit or receive the same information
that another vendor is using.
</t><t>
For the case of TCP <xref target="RFC0761" />, 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 <xref target="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 <xref target="RFC3925">DHCPv4</xref>, then vendors
and experimenters would each be able to use it for their own purposes as long as all
network participants could easily differentiate between the entities using the option.
</t><t>
This document explores the various ways that protocols have allowed optional
information to be included using fields designated as "private use". It uses examples
in some well known protocols. In well developed protocols, private use options may be
useful in avoiding allocation conflicts, and in dynamically extending a feature. As
with all good things, this will come 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,
a receiver will have to reserve buffers for an expected field in an inbound packet.
Since one way of implementing private use options is to only enable the field if it
is needed, then the allocation of buffers could be considered wasteful if it is
actually not used.
</t>
</section>
<section anchor="history" title="Origins of the Private Use Name Space">
<t>
<xref target="RFC2434">Guidelines for Writing an IANA Considerations Section in RFCs</xref>
describes that values of specific name spaces may either be registered with the IANA,
or not. In most cases, there are well defined values for name spaces. However, as
the document explains, not all name spaces require centralized administration.
</t><t>
In that document, it seems to be assumed that private use name spaces 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 name spaces refers to
<xref target="RFC2131">Dynamic Host Configuration Protocol</xref> and presumably
<xref target="RFC2132">DHCP Options and BOOTP Vendor Extensions</xref>. 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.
</t><t>
Later works identified a need to place a scope on private use name spaces. The second
example of private use name spaces in <xref target="RFC2434" /> is from
<xref target="RFC0822">STANDARD FOR THE FORMAT OF ARPA INTERNET TEXT MESSAGES</xref>
which describes X- headers. Again, there is no effort made to control the name space.
It appears however that the users of X- headers have self-organized; most consistently
use features that are universally useful and many have incorporated identifiers for
useful features that may overlap.
</t>
</section>
<section anchor="words" title="Nomenclature">
<t>
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.
<list style="symbols"><t>
Communications protocol - a formal description of digital message formats and the
rules for exchanging those messages in or between computing systems and in
telecommunications <xref target="wpProt" />
<list style="empty"><t>
Example: <xref target="RFC0959">The File Transfer Protocol</xref> is
an example of a communications protocol. It has well defined fields and standard
options. <xref target="RFC5424">The Syslog Protocol</xref> is another example of a
communications protocol. It has well defined fields, standard options, and it also
has standard and private use options. (See <xref target="syslog" />.)
</t></list>
</t><t>
Protocol frame - a defined container of fields used to convey information in a
communications protocol
<list style="empty"><t>
Example: An Internet Protocol packet <xref target="RFC0791" /> is considered
to be a protocol frame. In the case of
<xref target="RFC0959">The File Transfer Protocol</xref>, an FTP message from the
client to the server within the <xref target="RFC0791">Internet Protocol</xref>
containing an FTP command is a protocol frame. In the case of
<xref target="RFC5424">The Syslog Protocol</xref>, a message from the client to
the server within the <xref target="RFC0791">Internet Protocol</xref> containing
a syslog message is also a protocol frame.
</t></list>
</t><t>
Field - any defined container within a communications protocol frame
<list style="empty"><t>
Example: In the case of <xref target="RFC0959">The File Transfer Protocol</xref>,
a command will be contained within a field. In the case of
<xref target="RFC5424">The Syslog Protocol</xref>, the HOSTNAME is a field.
</t></list>
</t><t>
Standard option - a field in a protocol frame that may only use values that are
strictly defined within a specification
<list style="empty"><t>
Example: In the case of <xref target="RFC0959">The File Transfer Protocol</xref>,
an FTP command, such as CDUP or QUIT, is a standard option. The reason that a
command is a standard option is that only the values listed by the IANA in the
registry <xref target="IANAftp" /> may be used. The standard options are not
limited to the values defined in the original RFC, but also include any additions
to the registry. In the case of <xref target="RFC5424">The Syslog Protocol</xref>,
an SD-ID may be a standard option. The example given in Section 7.1.4 of
<xref target="RFC5424" /> of
<figure><artwork><![CDATA[
[timeQuality tzKnown="0" isSynced="0"]
]]></artwork></figure>
is a standard option because all of the fields are listed in the document and in
the IANA registry <xref target="IANAslg" />.
</t></list>
</t><t>
Private use option - a field in a protocol frame that is reserved for private or local
use only name spaces
<list style="empty"><t>
Example: In the case of The Syslog Protocol, an SD-ID may be a private
use option. Example 3 given in Section 6.5 contains a private use option.
<figure><artwork><![CDATA[
<165>1 2003-10-11T22:14:15.003Z mymachine.example.com
evntslog - ID47 [exampleSDID@32473 iut="3" eventSource=
"Application" eventID="1011"] BOMAn application
event log entry...
]]></artwork></figure>
Specifically, the SD-ID starting with "[exampleSDID@32473 ..." is not a
specifically defined option in the RFC, nor is it registered in the IANA registry
<xref target="IANAslg" />. It is a way for an equipment vendor to insert their
specific information without having to register anything. In this case if the
receiver knows the format of that SD-ID then it can immediately interpret its
meaning. However, if it does not know how to interpret that SD-ID, it can still
log the message and an Operator or Administrator can look up its meaning at a
later time.
</t></list>
</t><t>
Name space - the set of possible values a field may contain; its actual content may
be a name, a number or another kind of value
<list style="empty"><t>
Example: In the same Example 3 from Section 6.5 of
<xref target="RFC5424">The Syslog Protocol</xref>, "exampleSDID@32473" provides
the name space so the context of the rest of the SD-ID may be interpreted.
Specifically the Private Enterprise Number <xref target="IANApen" /> (PEN) is
used to associate the option with a private enterprise, and the text before the
"@" identifies the option defined within that private enterprise.
</t></list>
</t></list></t>
</section>
<section anchor="ways" title="Examples of Successful Private Use Options">
<t>
This section contains a review of RFCs that allow the use of private use options.
There seem to be three ways to address the name space: via a global origin, via a
truncated numerical origin, and via a name space based upon a domain name.
</t>
<section anchor="smi" title="SNMP">
<t>
Likely, the first private use option was defined in the
<xref target="RFC1155">Structure and Identification of Management Information for
TCP/IP-based Internets</xref> which was first used in
<xref target="RFC1067">A Simple Network Management Protocol</xref> (SNMP).
The structure of management information (SMI) has been updated and is currently
defined as the
<xref target="RFC2578">Structure of Management Information Version 2 (SMIv2)</xref>.
</t><t>
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
<xref target="IANAsmi" />.
</t><t>
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
<xref target="IANApen">Private Enterprise Number</xref> (PEN) and then the
objects defined by that enterprise.
</t><t>
Specific codes, known as error-indexes, are used to indicate when a request
cannot be processed because a device does not understand a request.
</t><t>
While this is very practical and practicable for SNMP, fully qualified OIDs do not
lend themselves well for other uses as a generic private use option.
</t>
</section>
<section anchor="pen" title="Private Enterprise Number">
<t>
Rather than using the entire SMI, protocol engineers started using just the
Private Enterprise Number <xref target="IANApen" />. This reduces the length of
the identifier but continues to provide an identifier through a globally unique
name space. This section provides examples of how the PEN has been used to
provide private use options.
</t>
<section anchor="radius" title="RADIUS">
<t>
<xref target="RFC2058">The Remote Authentication Dial In User Service (RADIUS)</xref>
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).
</t><t>
The updated RADIUS document, <xref target="RFC2865" />, gives guidance for
using the VSA.
<list style="symbols">
<t>
Servers not equipped to interpret the vendor-specific information
sent by a client MUST ignore it (although it may be reported).
</t><t>
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.
</t><t>
The Attribute-Specific field is dependent on the vendor's definition of
that attribute.
</t><t>
It SHOULD be encoded as a sequence of vendor type / vendor length / value
fields.
</t><t>
Multiple subattributes MAY be encoded within a single Vendor-Specific
Attribute, although they do not have to be.
</t>
</list>
</t><t>
There are many attributes defined in RADIUS <xref target="RFC2058" /> which
may be considered to be standard variables. Each of these attributes are
specified within a "type length value" (tlv) container. For this protocol,
the VSA "type" is a specific numerical value which separates it from other
attributes. Type 26 (decimal) denotes a VSA, and the PEN starts the "value"
which should then include a subsequent nested tlv so the vendor may enumerate
their own options within the field.
</t>
</section>
<section anchor="mobile" title="Mobile IP">
<t>
<xref target="RFC3115">Mobile IP Vendor Specific Extensions</xref>
defines two extensions that can be used for making organization
specific extensions by vendors/organizations for their own specific
purposes for Mobile IP <xref target="RFC2002" />. Mobile IP has been revised
several times and is currently specified in
<xref target="RFC5944">IP Mobility Support for IPv4, Revised</xref>.
</t><t>
In that specification, two tlv's have been defined to contain private use
options. These are called Vendor/Organization Specific Extensions (VSE).
<list style="symbols">
<t>
When the Critical Vendor/Organization Specific Extension (CVSE) is
encountered but not recognized, the message containing the extension MUST
be silently discarded.
</t><t>
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.
</t>
</list>
Having two VSEs of this nature for private use options is consistent with the
original Mobile IP specification <xref target="RFC2002" /> which states:
<list><t>
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.
</t></list>
</t>
</section>
<section anchor="dhcp" title="DHCP">
<t>
The introduction to <xref target="RFC3925">
Vendor-Identifying Vendor Options for
Dynamic Host Configuration Protocol version 4 (DHCPv4)</xref> states:
<list>
<t>
The DHCP protocol for IPv4, <xref target="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) <xref target="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.
</t>
</list>
</t><t>
This meant that
<xref target="RFC2131">Dynamic Host Configuration Protocol</xref>
specified that there was one instance of the vendor type, and the receiver
used that name space to set the scope for the fields in the vendor-specific
information option.
</t><t>
This situation was resolved with the publication of <xref target="RFC3925">
Vendor-Identifying Vendor Options for
Dynamic Host Configuration Protocol version 4 (DHCPv4)</xref> which states:
<list>
<t>
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.
</t>
</list>
</t><t>
That specification (<xref target="RFC3925" />) then used the
PEN <xref target="IANApen" /> to define a unique name space for private use
options in this protocol. Similar to other protocols of this era, tlv
containers were used.
</t>
</section>
<section anchor="syslog" title="Syslog">
<t>
<xref target="RFC5424">The Syslog Protocol</xref> also uses the PEN to
uniquely qualify the name space 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 name spaces 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.
</t><t>
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 really interested.
</t>
</section>
</section>
<section anchor="chars" title="Character strings">
<t>
An alternative to using numerical indicators is to use textual strings. Again,
the goal for using these strings is to disambiguate the identifiers and allow
freedom of expression by the vendors and experimenters using them.
</t>
<section anchor="ssh" title="Secure Shell">
<t>
<xref target="RFC4251">The Secure Shell (SSH) Protocol Architecture</xref>
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 name space field. For example, in
<xref target="RFC4254">The Secure Shell (SSH) Connection Protocol</xref> 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".
</t><t>
Obviously, these character strings are domain names <xref target="RFC1034" />
<xref target="RFC1035" />. This is specified in
<xref target="RFC4251">The Secure Shell (SSH) Protocol Architecture</xref>.
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.
</t>
</section>
<section anchor="yang" title="YANG and NETCONF">
<t>
<xref target="RFC6020">YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF)</xref>
and <xref target="RFC4741">NETCONF Configuration Protocol</xref> use URIs to
indicate private use name spaces.
The following is given as an example of a YANG and NETCONF configuration.
<figure><artwork><![CDATA[
module my-config {
namespace "http://example.com/schema/config";
prefix "co";
container system { ... }
container routing { ... }
}
That example could be encoded in NETCONF as the following.
<rpc message-id="101"
xmlns="urn:ietf:params:xml:ns:netconf:base:1.0"
xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0">
<edit-config>
<target>
<running/>
</target>
<config>This eternal association
<system xmlns="http://example.com/schema/config">
<!-- system data here -->
</system>
<routing xmlns="http://example.com/schema/config">
<!-- routing data here -->
</routing>
</config>
</edit-config>
</rpc>
]]></artwork></figure>
</t><t>
Section 8.3 of <xref target="RFC6020">YANG</xref> 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.
</t><t>
Similarly, <xref target="RFC4741">NETCONF</xref> contains much information
about processing requests that cannot be completed because elements or
values are not recognized.
</t>
</section>
</section>
</section>
<section anchor="means" title="Characteristics of Useful Private Use Options">
<t>
This section summarizes the observed characteristics
of private use options that are successful and deployed.
</t><t>
There seem to be three characteristics of successful private use options.
</t>
<section anchor="means1" title="Source of Authority">
<t>
A private use option requires a path to an origin that has the authority to
create and maintain the option. As shown above, this referent should be unique,
and not be dependent upon local interpretation.
</t><t>
The <xref target="pen">PEN</xref> is sourced by the Internet Assigned Numbers
Authority (IANA). These 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
required by the
<xref target="ICANN">Internet Corporation for Assigned Names and Numbers</xref>,
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 supplicant. This eternal association of the PEN,
versus the ephemeral association of domain names, has not been shown to present
any problems.
</t><t>
Domain names have similar problems as they can be more ephemeral than eternal.
The top level domains are maintained by the Internet Corporation for Assigned
Names and Numbers [ICANN] however the specific names are assigned much more
locally. 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 organizations. Again however, like the use of PENs there
have not been any problems reported from this.
</t><t>
It is vital to note that the usage of the option within the private space is the
full responsibility of the private entity. In the example of the PEN, each
entity registering a PEN must fully quantify the parameters of the use of the
option within their purview.
</t>
</section>
<section anchor="means2" title="Focus of the Name Space">
<t>
Once the source of authority is established, an actual option, or multiple
options, must be specified. This is usually an indicator of what value is
expected. Within the domain established by the source of authority, the focus
of each value must be unique. In a very simple example, a private use option
may consist of "PEN"@"focus"="value". The PEN will be unique and will specify
the source of authority. The focus will be unique as long as the source 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.
</t><t>
In some cases, multiple focuses and values need to be transmitted. When the PEN
has been used, this has most often been achieved by nesting tlv's within the
field. Each type is then a focus for the private use option. More recently URIs
have been used to point to a source of authority. This allows an organization to
organize an abundance of information about their names spaces.
</t>
</section>
<section anchor="means3" title="Value of the Option">
<t>
The value of each private use option must be extensible but bounded.
</t><t>
Generally speaking, values of private use options should follow the same guidance
given for standard options.
</t>
</section>
<section anchor="means4" title="Guidance on Incomplete Understanding">
<t>
Within the protocol, an understanding needs to be established between the
transmitter and receiver about what to do if the receiver does not understand a
name space. 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. While other protocols have no need for the receiver to
have any understanding of any private use options when it receives them. Each of
these behaviors is represented in the examples in this document.
</t><t>
Regardless of whether or not this understanding is established, the receiver
of any protocol must have a defined path of action to follow when receiving
anything that it may not understand.
</t>
</section>
</section>
<section anchor="examples" title="Examples of Characteristics of Useful Private Use Options">
<section anchor="snmp-chars" title="SNMP">
<t>
The globally unique origin in <xref target="smi">SNMP</xref> is the
<xref target="ISO">International Standards Organization</xref>
which is accredited by the United Nations to maintain this structure.
However, the name space resolves to the <xref target="pen">PEN</xref>.
</t><t>
After the vendor identifier (the PEN) in the management information base
(MIB), a vendor can create many different trees to identify objects. This may
result in a very large number of OBJECT IDENTIFIERs; each of which is an
identifier of the name space described in this document. Each of these are
uniquely identified by the vendor and do not require registration with any
coordinating authority.
</t><t>
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.
</t><t>
In this, each OBJECT IDENTIFIER contains a globally unique origin which is
ISO, a focus which is the OBJECT IDENTIFIER down to the last field, and a
value which is the last field in the SetRequest, and the last field in the
response to a GetRequest.
</t>
</section>
<section anchor="radius-chars" title="RADIUS">
<t>
As noted above, the globally unique origin for
<xref target="RFC2865">RADIUS</xref> is the PEN.
</t><t>
The remainder of the Attribute field after the PEN is deliberately undefined
in the specification. It is however suggested that the field contain embedded
tlv's. This is again very practical and practicable. 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"}. Since there is nothing to prevent vendors from
registering multiple PENs, each vendor may have a plethora of {type="1"}.
However, that is actually not needed since the focus may be extended by
enumerating multiple types. For example, the vendor attribute may contain
{PEN="M", type="1"(value), type="2"(value), type="3"(value)}.
</t><t>
The values for each type are bounded by the length of the attribute. Since
the entire vendor attribute is defined by the vendor, the values may be human
readable or binary. Since the protocol tends to be machine-to-machine, it is
likely that the values will be binary. In some cases, it is feasible that a
value has no length. In that case, the transmission of the type alone, would
be a signal of some sort to the receiver.
</t>
</section>
<section anchor="mobileip-chars" title="Mobile IP">
<t>
The structure of the origin, type, and value of the CVSEs and NVSEs for
<xref target="RFC3115">Mobile IP</xref> may 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 following that.
</t><t>
It should be noted that this does not have to be the case. Specifying CVSEs
and NVSEs in various packets can give a vendor another dimension in processing
these private use fields. If a vendor placed all CVSEs in a single packet,
and the receiver did not understand any one of them, the entire packet must
be discarded. However, if the vendor places individual CVSEs in separate
packets, only CVSEs that are not understood by the receiver will be discarded.
</t><t>
Similarly, a vendor may choose to not stack NVSEs so that a receiver won't
discard the entire cluster of NVSEs if a single one is not understood.
</t><t>
The values are constrained by the length of the types or subtypes.
</t><t>
It is not known by the author if this separation of CVSEs into multiple
packets, or if the separation of NVSEs by a single vendor is currently
practiced. Any clue from the clueful would be appreciated here.
</t>
</section>
<section anchor="dhcp-chars" title="DHCP">
<t>
The first version of the <xref target="RFC2131">DHCP protocol</xref> did not
allow for multiple origins; only a single origin was permitted and the types
were to be defined subsequent to that. The author of this document does not
know the history of this specification but it seems to be that this was
unworkable so the specification was changed to allow for multiple origins.
After that, it becomes very similar to CVSEs and NVSEs described in
<xref target="RFC3115">Mobile IP</xref>, however multiple uses of the PEN are
not permitted.
</t>
</section>
<section anchor="syslog-chars" title="Syslog">
<t>
The <xref target="RFC5424">Syslog protocol</xref> 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, multiple elements may
be used that have identifiers and values.
</t>
</section>
<section anchor="ssh-chars" title="Secure Shell">
<t>
In the <xref target="RFC4250">SSH protocol</xref>, 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 <xref target="RFC4250" />.
</t>
</section>
<section anchor="yang-chars" title="YANG and NETCONF">
<t>
Both <xref target="RFC6020">YANG</xref> and
<xref target="RFC4741">NETCONF</xref> use URIs to enumerate private use options
of a device. The use of this comes from
<xref target="W3C.REC-xpath-19991116">XPATH</xref>.
</t><t>
In both of these, the source 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.
</t><t>
The following is used to demonstrate this. First the YANG module is shown,
then a subset of the NETCONF is shown.
</t><t>
<figure><artwork><![CDATA[
YANG module:
// Contents of "acme-system.yang"
module acme-system {
namespace "http://acme.example.com/system";
prefix "acme";
organization "ACME Inc.";
contact "joe@acme.example.com";
description
"The module for entities implementing the ACME system.";
revision 2007-06-09 {
description "Initial revision.";
}
container system {
leaf host-name {
type string;
description "Hostname for this system";
}
leaf-list domain-search {
type string;
description "List of domain names to search";
}
container login {
leaf message {
type string;
description
"Message given at start of login session";
}
list user {
key "name";
leaf name {
type string;
}
leaf full-name {
type string;
}
leaf class {
type string;
}
}
}
}
}
NETCONF exchange:
<system>
<login>
<message>Good morning</message>
</login>
</system>
]]></artwork></figure>
</t><t>
In this example, YANG describes the source of authority and focus for the login
message, and the NETCONF exchange populates that specific value.
</t><t>
As noted above, both of these specifications have good descriptions of actions
to take if a name space is not recognized.
</t>
</section>
</section>
<section anchor="thinkaboutit" title="Issues to Consider">
<t>
This document is not an encouragement or recommendation to define private use fields
in IETF protocols. Rather, since private use options are useful to the community and
seem to be gaining popularity, this document is an attempt to document the ways in
which they have been successful so others may benefit.
</t><t>
Private use options are a way to allow vendors, network operators, and experimenters
to convey dynamic information without going through a rigorous process to register
each variable. There is no "one size fits all" mechanism. The use of a very specific
and fixed format works very well 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.
</t><t>
There seem to be four essential features to using a private use option. (The author
would be grateful if the community were to validate these and/or offer more or less.)
<list style="symbols">
<t>
One requirement 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 would be able to incorporate the
information available.
</t><t>
Instructions are needed on how to deal with private use options that are not
understood by a receiver. In some cases, a receiver may not need to understand
the options immediately upon receipt as in the case of Syslog. In other cases,
the options are immediately used and instructions must be clear on what to do if
the receiver cannot process them. It appears that Mobile IP has the best
thought-through instructions on this.
</t><t>
Private use options must 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 name space field. These are
becoming known as subattributes. This appears to be working in practice and it
may be assumed that this has become a de facto rule for RADIUS.
</t><t>
In most cases, a subattribute will only be named once within the context of an
exchange. RADIUS and DHCP either state or strongly imply this. However, while it
is not explicitly discussed, there is nothing to prevent this within Syslog.
Some guidance should be given about this in describing private use options in
protocols.
</t>
</list>
</t><t>
Clear documentation in full and open standards is needed to achieve uniformity and
interoperability in these features. Obviously implementers will need to adhere
closely to these standards for complete interoperability.
</t><t>
Finally, the usage of any private use values on the wire before any name space is
properly reserved with the IANA is entirely inadvisable.
</t>
</section>
<section anchor="notes" title="Authors Notes">
<t>
This section will be removed prior to publication.
</t><t>
This is version -03. I made several revisions based upon feedback from Dan Romascanu.
</t>
</section>
<section anchor="sec" title="Security Considerations">
<t>
This document reviews ways that options are being used in various protocols.
As such, there are no security considerations inherent in this document.
</t><t>
Readers and implementers should be aware of the context of implementing options
in their own protocols.
</t>
</section>
<section anchor="iana" title="IANA Considerations">
<t>
This document does not propose a standard and does not require the
IANA to do anything.
</t>
</section>
<section anchor="Acks" title="Acknowledgments">
<t>
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.
</t>
<t>
The following people have contributed to this document. Listing their names here does
not mean that they endorse the document, but that they have contributed to its
substance.
</t>
<t>
David Harrington, Dan Romascanu and Bert Wijnen.
</t>
</section>
</middle>
<back>
<references title="References">
<reference anchor="IANAtcp" target="http://www.iana.org/assignments/tcp-parameters/tcp-parameters.txt">
<front>
<title>IANA Transmission Control Protocol (TCP) Parameters, TCP Option Kind Numbers</title>
<author fullname="Internet Assigned Numbers Authority">
<organization abbrev="IANA">Internet Assigned Numbers Authority</organization>
</author>
<date year="2011" />
</front>
<format type="TXT" target="http://www.iana.org/assignments/tcp-parameters/tcp-parameters.txt" />
</reference>
<reference anchor="IANAftp" target="http://www.iana.org/assignments/ftp-commands-extensions/ftp-commands-extensions.txt">
<front>
<title>IANA FTP Commands and Extensions</title>
<author fullname="Internet Assigned Numbers Authority">
<organization abbrev="IANA">Internet Assigned Numbers Authority</organization>
</author>
<date year="2010" />
</front>
<format type="TXT" target="http://www.iana.org/assignments/ftp-commands-extensions/ftp-commands-extensions.txt" />
</reference>
<reference anchor="IANAslg" target="http://www.iana.org/assignments/syslog-parameters">
<front>
<title>IANA syslog Parameter</title>
<author fullname="Internet Assigned Numbers Authority">
<organization abbrev="IANA">Internet Assigned Numbers Authority</organization>
</author>
<date year="2010" />
</front>
<format type="TXT" target="http://www.iana.org/assignments/syslog-parameters" />
</reference>
<reference anchor="IANAsmi" target="http://www.iana.org/assignments/smi-numbers">
<front>
<title>Network Management Parameters</title>
<author fullname="Internet Assigned Numbers Authority">
<organization abbrev="IANA">Internet Assigned Numbers Authority</organization>
</author>
<date year="2011" />
</front>
<format type="TXT" target="http://www.iana.org/assignments/smi-numbers" />
</reference>
<reference anchor="IANApen" target="http://www.iana.org/assignments/enterprise-numbers">
<front>
<title>IANA PRIVATE ENTERPRISE NUMBERS</title>
<author fullname="Internet Assigned Numbers Authority">
<organization abbrev="IANA">Internet Assigned Numbers Authority</organization>
</author>
<date year="2011" />
</front>
<format type="TXT" target="http://www.iana.org/assignments/enterprise-numbers" />
</reference>
<reference anchor="wpProt" target="http://en.wikipedia.org/wiki/Communications_protocol">
<front>
<title>Wikipedia entry for communication protocol</title>
<author fullname="Wikipedia - the Free Dictionary">
<organization abbrev="Wikipedia">Wikipedia - the Free Dictionary</organization>
</author>
<date year="2011" />
</front>
<format type="TXT" target="http://en.wikipedia.org/wiki/Communications_protocol" />
</reference>
<reference anchor="ISO" target="http://www.iso.org">
<front>
<title>International Standards Organization</title>
<author fullname="ISO">
<organization abbrev="ISO">International Standards Organization</organization>
</author>
<date year="2011" />
</front>
<format type="TXT" target="http://www.iso.org" />
</reference>
<reference anchor="ICANN" target="http://www.icann.org">
<front>
<title>Internet Corporation for Assigned Names and Numbers</title>
<author fullname="ICANN">
<organization abbrev="ICANN">Internet Corporation for Assigned Names and Numbers</organization>
</author>
<date year="2011" />
</front>
<format type="TXT" target="http://www.icann.org" />
</reference>
<?rfc include="reference.RFC.0761" ?>
<?rfc include="reference.RFC.0791" ?>
<?rfc include="reference.RFC.0822" ?>
<?rfc include="reference.RFC.0868" ?>
<?rfc include="reference.RFC.0959" ?>
<?rfc include="reference.RFC.1034" ?>
<?rfc include="reference.RFC.1035" ?>
<?rfc include="reference.RFC.1067" ?>
<?rfc include="reference.RFC.1155" ?>
<?rfc include="reference.RFC.2002" ?>
<?rfc include="reference.RFC.2131" ?>
<?rfc include="reference.RFC.2132" ?>
<?rfc include="reference.RFC.2058" ?>
<?rfc include="reference.RFC.2434" ?>
<?rfc include="reference.RFC.2578" ?>
<?rfc include="reference.RFC.2865" ?>
<?rfc include="reference.RFC.3115" ?>
<?rfc include="reference.RFC.3925" ?>
<?rfc include="reference.RFC.4250" ?>
<?rfc include="reference.RFC.4251" ?>
<?rfc include="reference.RFC.4254" ?>
<?rfc include="reference.RFC.4741" ?>
<?rfc include="reference.RFC.5424" ?>
<?rfc include="reference.RFC.5944" ?>
<?rfc include="reference.RFC.6020" ?>
<?rfc include="reference.W3C.REC-xpath-19991116" ?>
</references>
</back>
</rfc>
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