| Network Working Group | C. Lonvick |
| Internet-Draft | February 25, 2019 |
| Intended status: Informational | |
| Expires: August 29, 2019 |
A Taxonomy on Private Use Fields in Protocols
draft-lonvick-private-tax-16
This document provides clarification about private use fields in IETF protocols.
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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 general use in the open Internet, this requires that such fields be fully specified in openly available documents. This discussion provides examples of some protocols to explore the role of private use options.
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".
Fields reserved for private use cannot provide interoperability unless their use is fully documented in openly available documents. This discussion uses examples of some protocols to exemplify how some private use options are used.
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.
Additionally, the terms "Start of Authority" and "Focus of the Namespace" are defined within their respective contexts and further discussed below.
The name "Start of Authority" comes from the domain name system configuration file which describes a "SoA" in "DOMAIN NAMES - CONCEPTS AND FACILITIES" and "DOMAIN NAMES - IMPLEMENTATION AND SPECIFICATION". 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 discussion, but the intent is to identify an authoritative source for the namespace.
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.
Some standards permit private use options in different ways, while others do not. The "Time Protocol" 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" {[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)", 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", 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.
"Guidelines for Writing an IANA Considerations Section in RFCs" {[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 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" and presumably "DHCP Options and BOOTP Vendor Extensions". 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 {[RFC8126]} is from "STANDARD FOR THE FORMAT OF ARPA INTERNET TEXT MESSAGES" {[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" {[RFC5322]}.
This section summarizes the observed characteristics of some private use options that have been developed and deployed.
There appear to be three quantifiable characteristics of private use options:
A private use option requires a path to an origin that has the authority to create and maintain the option. If the goal for an origin is to disambiguate each focus of a namespace, then the referent most often seen has been globally unique, and not dependent upon local interpretation.
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" which was first used in "A Simple Network Management Protocol" {[RFC1157]} (SNMP). The globally unique origin in SNMP is the International Standards Organization 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 (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, 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", and in "Private Enterprise Number (PEN) practices and Internet Assigned Numbers Authority (IANA) registration considerations".
An alternative to using a numerical indicator such as the OBJECT IDENTIFIER or PEN, is to use textual strings such as names.
Domain names may be more ephemeral than eternal. Unlike PENs that usually 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. Similar to the use of PENs, there have not been any problems reported from this from 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" {[RFC8141]}. "An IETF URN Sub-namespace for Registered Protocol Parameters" provides guidance for ways to use URNs as protocol parameters and how to define a start of authority.
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 usually been seen to 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.
This section contains a review of RFCs that allow the use of private use options.
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 defined by the "Structure of Management Information Version 2 (SMIv2)". 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 [IANAsmi].
As was noted, the globally unique origin in SNMP is the International Standards Organization 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.
The Internet subtree of private enterprise OBJECT IDENTIFIERs starts with the prefix: 1.3.6.1.4.1. and is followed by a Private Enterprise Number (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 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 focus 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 field 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.
Specific codes, known as error-indexes, are used to indicate when a request cannot be processed because a device does not understand a request.
"The Remote Authentication Dial In User Service (RADIUS)" {[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).
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" (TLV) 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" is the PEN. The remainder of the Attribute field after the PEN is deliberately undefined in the specification. It is practically suggested that the field contain embedded TLVs. 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. 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] as follows:
"Mobile IP Vendor Specific Extensions" 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). [RFC2002] {[RFC5944]} which states:
Having two VSEs of this nature for private use options is consistent with the original Mobile IP specification
The structure of the origin, type, and value of the CVSEs and NVSEs for "Mobile IP" 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 respective lengths of the types or subtypes.
The introduction to "Vendor-Identifying Vendor Options for Dynamic Host Configuration Protocol version 4 (DHCPv4)" states:
This appears to indicate that "Dynamic Host Configuration Protocol" 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.
This situation was resolved with the publication of "Vendor-Identifying Vendor Options for Dynamic Host Configuration Protocol version 4 (DHCPv4)" which cautions:
The DHCP specification 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.
DHCP provides guidance on actions to take if servers and clients do not comprehend a request or a response.
"The Syslog Protocol" 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 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.
"The Secure Shell (SSH) Protocol Architecture" 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" 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". The rational for choosing the manner of providing a format for private use options is given in Section 4.2 of [RFC4251].
In the SSH protocol, 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].
Generally, the guidance given is that if a private use option is not understood the receiver is to convey an error code to its peer.
One example of a protocol utilizing URNs is "Network Configuration Protocol (NETCONF)". NETCONF may utilize "YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF)" as a means to describe and convey options.
"YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF)" and "Network Configuration Protocol (NETCONF)" use URIs to indicate private use namespaces.
Section 8.3 of YANG 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 contains much information about processing requests that cannot be completed because elements or values are not recognized.
Both YANG and NETCONF use URIs to enumerate private use options of a device. The use of this comes from XPATH. 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.
The "Extensible Provisioning Protocol (EPP)" is another example of a protocol that utilizes URN namespaces. From the Protocol Description section 2:
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:
The associated RFCs of "Extensible Provisioning Protocol (EPP) Domain Name Mapping" and "Extensible Provisioning Protocol (EPP) Host Mapping" provide a mechanism to temporally bind options.
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.
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.
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" 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" 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.
This section will be removed prior to publication.
This is version -16. 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.
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 developed.
This document does not propose a standard and does not require the IANA to do anything.
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.
| [RFC4775] | Bradner, S., Carpenter, B. and T. Narten, "Procedures for Protocol Extensions and Variations", BCP 125, RFC 4775, DOI 10.17487/RFC4775, December 2006. |
| [RFC6709] | Carpenter, B., Aboba, B. and S. Cheshire, "Design Considerations for Protocol Extensions", RFC 6709, DOI 10.17487/RFC6709, September 2012. |