4. Considerations for the Base Protocol Good extension design depends on a well-designed base protocol. To promote interoperability, designers should: 1. Ensure a well-written base protocol specification. Does the base protocol specification make clear what an implementer needs to support, and does it define the impact that individual operations (e.g., a message sent to a peer) will have when invoked? 2. Design for backward compatibility. Does the base protocol specification describe how to determine the capabilities of a peer and negotiate the use of extensions? Does it indicate how implementations handle extensions that they do not understand? Is it possible for an extended implementation to negotiate with an unextended (or differently-extended) peer to find a common subset of useful functions? 3. Respect underlying architectural or security assumptions. Is there a document describing the underlying architectural assumptions, as well as considerations that have arisen in operational experience? Or are there undocumented considerations that have arisen as the result of operational experience, after the original protocol was published? For example, will backward-compatibility issues arise if extensions reverse the flow of data, allow formerly static parameters to be changed on the fly, or change assumptions relating to the frequency of reads/writes?
4. Minimize impact on critical infrastructure. For a protocol that represents a critical element of Internet infrastructure, it is important to explain when it is appropriate to isolate new uses of the protocol from existing ones. For example, is it explained when a proposed extension (or usage) has the potential for negatively impacting critical infrastructure to the point where explicit steps would be appropriate to isolate existing uses from new ones? 5. Provide guidance on data model extensions. Is there a document that explains when a protocol extension is routine and when it represents a major change? For example, is it clear when a data model extension represents a major versus a routine change? Are there guidelines describing when an extension (such as a new data type) is likely to require a code change within existing implementations? 4.1. Version Numbers Any mechanism for extension by versioning must include provisions to ensure interoperability, or at least clean failure modes. Imagine someone creating a protocol and using a "version" field and populating it with a value (1, let's say), but giving no information about what would happen when a new version number appears in it. This would be a bad protocol design and description; it should be clear what the expectation is and how it can be tested. For example, stating that 1.X must be compatible with any version 1 code, but version 2 or greater is not expected to be compatible, has different implications than stating that version 1 must be a proper subset of version 2. An example of an under-specified versioning mechanism is provided by the MIME-Version header, originally defined in "MIME (Multipurpose Internet Mail Extensions)" [RFC1341]. As noted in Section 1 of the MIME specification [RFC1341]: A MIME-Version header field ... uses a version number to declare a message to be conformant with this specification and allows mail processing agents to distinguish between such messages and those generated by older or non-conformant software, which is presumed to lack such a field.
Beyond this, the 1992 MIME specification [RFC1341] provided little guidance on versioning behavior, or even the format of the MIME- Version header, which was specified to contain "text". The 1993 update [RFC1521] better defined the format of the version field but still did not clarify the versioning behavior: Thus, future format specifiers, which might replace or extend "1.0", are constrained to be two integer fields, separated by a period. If a message is received with a MIME-version value other than "1.0", it cannot be assumed to conform with this specification.... It is not possible to fully specify how a mail reader that conforms with MIME as defined in this document should treat a message that might arrive in the future with some value of MIME- Version other than "1.0". However, conformant software is encouraged to check the version number and at least warn the user if an unrecognized MIME-version is encountered. Thus, even though the 1993 update [RFC1521] defined a MIME-Version header with a syntax suggestive of a "Major/Minor" versioning scheme, in practice the MIME-Version header was little more than a decoration. An example of a protocol with a better versioning scheme is ROHC (Robust Header Compression). ROHCv1 [RFC3095] supports a certain set of profiles for compression algorithms. But experience had shown that these profiles had limitations, so the ROHC WG developed ROHCv2 [RFC5225]. A ROHCv1 implementation does not contain code for the ROHCv2 profiles. As the ROHC WG charter said during the development of ROHCv2: It should be noted that the v2 profiles will thus not be compatible with the original (ROHCv1) profiles, which means less complex ROHC implementations can be realized by not providing support for ROHCv1 (over links not yet supporting ROHC, or by shifting out support for ROHCv1 in the long run). Profile support is agreed through the ROHC channel negotiation, which is part of the ROHC framework and thus not changed by ROHCv2. Thus, in this case, both backward-compatible and backward- incompatible deployments are possible. The important point is to have a clearly thought out approach to the question of operational compatibility.
In the past, protocols have utilized a variety of strategies for versioning, each with its own benefits and drawbacks in terms of capability and complexity of implementation: 1. No versioning support. This approach is exemplified by the Extensible Authentication Protocol (EAP) [RFC3748] as well as the Remote Authentication Dial In User Service (RADIUS) protocol [RFC2865], both of which provide no support for versioning. While lack of versioning support protects against the proliferation of incompatible dialects, the need for extensibility is likely to assert itself in other ways, so that ignoring versioning entirely may not be the most forward thinking approach. 2. Highest mutually supported version (HMSV). In this approach, implementations exchange the version numbers of the highest version each supports, with the negotiation agreeing on the highest mutually supported protocol version. This approach implicitly assumes that later versions provide improved functionality and that advertisement of a particular version number implies support for all lower version numbers. Where these assumptions are invalid, this approach breaks down, potentially resulting in interoperability problems. An example of this issue occurs in the Protected Extensible Authentication Protocol [PEAP] where implementations of higher versions may not necessarily provide support for lower versions. 3. Assumed backward compatibility. In this approach, implementations may send packets with higher version numbers to legacy implementations supporting lower versions, but with the assumption that the legacy implementations will interpret packets with higher version numbers using the semantics and syntax defined for lower versions. This is the approach taken by "Port- Based Network Access Control" [IEEE-802.1X]. For this approach to work, legacy implementations need to be able to accept packets of known types with higher protocol versions without discarding them; protocol enhancements need to permit silent discard of unsupported extensions; and implementations supporting higher versions need to refrain from mandating new features when encountering legacy implementations. 4. Major/minor versioning. In this approach, implementations with the same major version but a different minor version are assumed to be backward compatible, but implementations are required to negotiate a mutually supported major version number. This approach assumes that implementations with a lower minor version number but the same major version can safely ignore unsupported protocol messages.
5. Min/max versioning. This approach is similar to HMSV, but without the implied obligation for clients and servers to support all versions back to version 1, in perpetuity. It allows clients and servers to cleanly drop support for early versions when those versions become so old that they are no longer relevant and no longer required. In this approach, the client initiating the connection reports the highest and lowest protocol versions it understands. The server reports back the chosen protocol version: a. If the server understands one or more versions in the client's range, it reports back the highest mutually understood version. b. If there is no mutual version, then the server reports back some version that it does understand (selected as described below). The connection is then typically dropped by client or server, but reporting this version number first helps facilitate useful error messages at the client end: * If there is no mutual version, and the server speaks any version higher than client max, it reports the lowest version it speaks that is greater than the client max. The client can then report to the user, "You need to upgrade to at least version <xx>". * Else, the server reports the highest version it speaks. The client can then report to the user, "You need to request the server operator to upgrade to at least version <min>". Protocols generally do not need any version-negotiation mechanism more complicated than the mechanisms described here. The nature of protocol version-negotiation mechanisms is that, by definition, they don't get widespread real-world testing until *after* the base protocol has been deployed for a while, and its deficiencies have become evident. This means that, to be useful, a protocol version- negotiation mechanism should be simple enough that it can reasonably be assumed that all the implementers of the first protocol version at least managed to implement the version-negotiation mechanism correctly. 4.2. Reserved Fields Protocols commonly include one or more "reserved" fields, clearly intended for future extensions. It is good practice to specify the value to be inserted in such a field by the sender (typically zero) and the action to be taken by the receiver when seeing some other
value (typically no action). In packet format diagrams, such fields are typically labeled "MBZ", to be read as, "Must Be Zero on transmission, Must Be Ignored on reception". A common mistake of inexperienced protocol implementers is to think that "MBZ" means that it's their software's job to verify that the value of the field is zero on reception and reject the packet if not. This is a mistake, and such software will fail when it encounters future versions of the protocol where these previously reserved fields are given new defined meanings. Similarly, protocols should carefully specify how receivers should react to unknown extensions (headers, TLVs, etc.), such that failures occur only when that is truly the intended outcome. 4.3. Encoding Formats Using widely supported encoding formats leads to better interoperability and easier extensibility. As described in "IAB Thoughts on Encodings for Internationalized Domain Names" [RFC6055], the number of encodings should be minimized, and complex encodings are generally a bad idea. As soon as one moves outside the ASCII repertoire, issues arise relating to collation, valid code points, encoding, normalization, and comparison, which extensions must handle with care [ID-COMPARISON][PRECIS-STATEMENT][PRECIS-FRAMEWORK]. An example is the Simple Network Management Protocol (SNMP) Structure of Managed Information (SMI). Guidelines exist for defining the Management Information Base (MIB) objects that SNMP carries [RFC4181]. Also, multiple textual conventions have been published, so that MIB designers do not have to "reinvent the wheel" when they need a commonly encountered construct. For example, "Textual Conventions for Internet Network Addresses" [RFC4001] can be used by any MIB designer needing to define objects containing IP addresses, thus ensuring consistency as the body of MIBs is extended. 4.4. Parameter Space Design In some protocols, the parameter space either has no specified limit (e.g., Header field names) or is sufficiently large that it is unlikely to be exhausted. In other protocols, the parameter space is limited and, in some cases, has proven inadequate to accommodate demand. Common mistakes include: a. A version field that is too small (e.g., two bits or less). When designing a version field, existing as well as potential versions of a protocol need to be taken into account. For example, if a
protocol is being standardized for which there are existing implementations with known interoperability issues, more than one version for "pre-standard" implementations may be required. If two "pre-standard" versions are required in addition to a version for an IETF Standard, then a two-bit version field would only leave one additional version code point for a future update, which could be insufficient. This problem was encountered during the development of the PEAPv2 protocol [PEAP]. b. A small parameter space (e.g., 8 bits or less) along with a First Come, First Served (FCFS) allocation policy [RFC5226]. In general, an FCFS allocation policy is only appropriate in situations where parameter exhaustion is highly unlikely. In situations where substantial demand is anticipated within a parameter space, the space should either be designed to be sufficient to handle that demand, or vendor extensibility should be provided to enable vendors to self-allocate. The combination of a small parameter space, an FCFS allocation policy, and no support for vendor extensibility is particularly likely to prove ill-advised. An example of such a combination was the design of the original 8-bit EAP Type space [RFC2284]. Once the potential for parameter exhaustion becomes apparent, it is important that it be addressed as quickly as possible. Protocol changes can take years to appear in implementations and by then the exhaustion problem could become acute. Options for addressing a protocol parameter exhaustion problem include: Rethinking the allocation regime Where it becomes apparent that the size of a parameter space is insufficient to meet demand, it may be necessary to rethink the allocation mechanism, in order to prevent or delay parameter space exhaustion. In revising parameter allocation mechanisms, it is important to consider both supply and demand aspects so as to avoid unintended consequences such as self-allocation or the development of black markets for the resale of protocol parameters. For example, a few years after publication of PPP EAP [RFC2284] in 1998, it became clear that the combination of an FCFS allocation policy [RFC5226] and lack of support for vendor-extensions had created the potential for exhaustion of the EAP Method Type space within a few years. To address the issue, Section 6.2 of the 2004 update [RFC3748] changed the allocation policy for EAP Method Types from FCFS to Expert Review, with Specification Required. Since this allocation policy revision did not change the demand
for EAP Method Types, it would have been likely to result in self- allocation within the standards space had mechanisms not been provided to expand the Method Type space (including support for vendor-specific method types). Support for vendor-specific parameters If the demand that cannot be accommodated is being generated by vendors, merely making allocation harder could make things worse if this encourages vendors to self-allocate, creating interoperability problems. In such a situation, support for vendor-specific parameters should be considered, allowing each vendor to self-allocate within their own vendor-specific space based on a vendor's Private Enterprise Code (PEC). For example, in the case of the EAP Method Type space, Section 6.2 of the 2004 EAP specification [RFC3748] also provided for an Expanded Type space for "functions specific only to one vendor's implementation". Extensions to the parameter space If the goal is to stave off exhaustion in the face of high demand, a larger parameter space may be helpful; this may require a new version of the protocol (such as was required for IPv6). Where vendor-specific parameter support is available, this may be achieved by allocating a PEC for IETF use. Otherwise, it may be necessary to try to extend the size of the parameter fields, which could require a new protocol version or other substantial protocol changes. Parameter reclamation In order to gain time, it may be necessary to reclaim unused parameters. However, it may not be easy to determine whether a parameter that has been allocated is in use or not, particularly if the entity that obtained the allocation no longer exists or has been acquired (possibly multiple times). Parameter transfer When all the above mechanisms have proved infeasible and parameter exhaustion looms in the near future, enabling the transfer of ownership of protocol parameters can be considered as a means for improving allocation efficiency. However, enabling transfer of parameter ownership can be far from simple if the parameter allocation process was not originally designed to enable title searches and ownership transfers. A parameter allocation process designed to uniquely allocate code points is fundamentally different from one designed to enable title search and transfer. If the only goal is to ensure that a parameter is not allocated more than once, the parameter registry
will only need to record the initial allocation. On the other hand, if the goal is to enable transfer of ownership of a protocol parameter, then it is important not only to record the initial allocation, but also to track subsequent ownership changes, so as to make it possible to determine and transfer the title. Given the difficulty of converting from a unique allocation regime to one requiring support for title search and ownership transfer, it is best for the desired capabilities to be carefully thought through at the time of registry establishment. 4.5. Cryptographic Agility Extensibility with respect to cryptographic algorithms is desirable in order to provide resilience against the compromise of any particular algorithm. Section 3 of "Guidance for Authentication, Authorization, and Accounting (AAA) Key Management" BCP 132 [RFC4962] provides some basic advice: The ability to negotiate the use of a particular cryptographic algorithm provides resilience against compromise of a particular cryptographic algorithm.... This is usually accomplished by including an algorithm identifier and parameters in the protocol, and by specifying the algorithm requirements in the protocol specification. While highly desirable, the ability to negotiate key derivation functions (KDFs) is not required. For interoperability, at least one suite of mandatory-to-implement algorithms MUST be selected.... This requirement does not mean that a protocol must support both public-key and symmetric-key cryptographic algorithms. It means that the protocol needs to be structured in such a way that multiple public-key algorithms can be used whenever a public-key algorithm is employed. Likewise, it means that the protocol needs to be structured in such a way that multiple symmetric-key algorithms can be used whenever a symmetric-key algorithm is employed. In practice, the most difficult challenge in providing cryptographic agility is providing for a smooth transition in the event that a mandatory-to-implement algorithm is compromised. Since it may take significant time to provide for widespread implementation of a previously undeployed alternative, it is often advisable to recommend implementation of alternative algorithms of distinct lineage in addition to those made mandatory-to-implement, so that an alternative algorithm is readily available. If such a recommended alternative is not in place, then it would be wise to issue such a recommendation as soon as indications of a potential weakness surface. This is particularly important in the case of potential weakness in
algorithms used to authenticate and integrity-protect the cryptographic negotiation itself, such as KDFs or message integrity checks (MICs). Without secure alternatives to compromised KDF or MIC algorithms, it may not be possible to secure the cryptographic negotiation while retaining backward compatibility. 4.6. Transport In the past, IETF protocols have been specified to operate over multiple transports. Often the protocol was originally specified to utilize a single transport, but limitations were discovered in subsequent deployment, so that additional transports were subsequently specified. In a number of cases, the protocol was originally specified to operate over UDP, but subsequent operation disclosed one or more of the following issues, leading to the specification of alternative transports: a. Payload fragmentation (often due to the introduction of extensions or additional usage scenarios); b. Problems with congestion control, transport reliability, or efficiency; and c. Lack of deployment in multicast scenarios, which had been a motivator for UDP transport. On the other hand, there are also protocols that were originally specified to operate over reliable transport that have subsequently defined transport over UDP, due to one or more of the following issues: a. NAT traversal concerns that were more easily addressed with UDP transport; b. Scalability problems, which could be improved by UDP transport. Since specification of a single transport offers the highest potential for interoperability, protocol designers should carefully consider not only initial but potential future requirements in the selection of a transport protocol. Where UDP transport is selected, the guidance provided in "Unicast UDP Usage Guidelines for Application Designers" [RFC5405] should be taken into account.
After significant deployment has occurred, there are few satisfactory options for addressing problems with the originally selected transport protocol. While specification of additional transport protocols is possible, removal of a widely used transport protocol is likely to result in interoperability problems and should be avoided. Mandating support for the initially selected transport protocol while designating additional transport protocols as optional may have limitations. Since optional transport protocols are typically introduced due to the advantages they afford in certain scenarios, in those situations, implementations not supporting optional transport protocols may exhibit degraded performance or may even fail. While mandating support for multiple transport protocols may appear attractive, designers need to realistically evaluate the likelihood that implementers will conform to the requirements. For example, where resources are limited (such as in embedded systems), implementers may choose to only support a subset of the mandated transport protocols, resulting in non-interoperable protocol variants. 4.7. Handling of Unknown Extensions IETF protocols have utilized several techniques for the handling of unknown extensions. One technique (often used for vendor-specific extensions) is to specify that unknown extensions be "silently discarded". While this approach can deliver a high level of interoperability, there are situations in which it is problematic. For example, where security functionality is involved, "silent discard" may not be satisfactory, particularly if the recipient does not provide feedback as to whether or not it supports the extension. This can lead to operational security issues that are difficult to detect and correct, as noted in Appendix A.2 and in Section 2.5 of "Common Remote Authentication Dial In User Service (RADIUS) Implementation Issues and Suggested Fixes" [RFC5080]. In order to ensure that a recipient supports an extension, a recipient encountering an unknown extension may be required to explicitly reject it and to return an error, rather than ignoring the unknown extension and proceeding with the remainder of the message. This can be accomplished via a "Mandatory" bit in a TLV-based protocol such as the Layer 2 Tunneling Protocol (L2TP) [RFC2661], or a "Require" or "Proxy-Require" header in a text-based protocol such as SIP [RFC3261] or HTTP [RFC2616].
Since a mandatory extension can result in an interoperability failure when communicating with a party that does not support the extension, this designation may not be permitted for vendor-specific extensions and may only be allowed for Standards Track extensions. To enable fallback operation with degraded functionality, it is good practice for the recipient to indicate the reason for the failure, including a list of unsupported extensions. The initiator can then retry without the offending extensions. Typically, only the recipient will find itself in the position of rejecting a mandatory extension, since the initiator can explicitly indicate which extensions are supported, with the recipient choosing from among the supported extensions. This can be accomplished via an exchange of TLVs, such as in the Internet Key Exchange Protocol Version 2 (IKEv2) [RFC5996] or Diameter [RFC3588], or via use of "Accept", "Accept-Encoding", "Accept-Language", "Allow", and "Supported" headers in a text-based protocol such as SIP [RFC3261] or HTTP [RFC2616]. 5. Security Considerations An extension must not introduce new security risks without also providing adequate countermeasures; in particular, it must not inadvertently defeat security measures in the unextended protocol. Thus, the security analysis for an extension needs to be as thorough as for the original protocol -- effectively, it needs to be a regression analysis to check that the extension doesn't inadvertently invalidate the original security model. This analysis may be simple (e.g., adding an extra opaque data element is unlikely to create a new risk) or quite complex (e.g., adding a handshake to a previously stateless protocol may create a completely new opportunity for an attacker). When the extensibility of a design includes allowing for new and presumably more powerful cryptographic algorithms to be added, particular care is needed to ensure that the result is, in fact, increased security. For example, it may be undesirable from a security viewpoint to allow negotiation down to an older, less secure algorithm.
6. References 6.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4775] Bradner, S., Carpenter, B., Ed., and T. Narten, "Procedures for Protocol Extensions and Variations", BCP 125, RFC 4775, December 2006. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. 6.2. Informative References [ERROR-HANDLING] Scudder, J., Chen, E., Mohapatra, P., and K. Patel, "Revised Error Handling for BGP UPDATE Messages", Work in Progress, June 2012. [ID-COMPARISON] Thaler, D., "Issues in Identifier Comparison for Security Purposes", Work in Progress, August 2012. [IEEE-802.1X] Institute of Electrical and Electronics Engineers, "Local and Metropolitan Area Networks: Port-Based Network Access Control", IEEE Standard 802.1X-2004, December 2004. [LISP] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "Locator/ID Separation Protocol (LISP)", Work in Progress, May 2012. [PEAP] Palekar, A., Simon, D., Salowey, J., Zhou, H., Zorn, G., and S. Josefsson, "Protected EAP Protocol (PEAP) Version 2", Work in Progress, October 2004. [PRECIS-FRAMEWORK] Saint-Andre, P. and M. Blanchet, "PRECIS Framework: Preparation and Comparison of Internationalized Strings in Application Protocols", Work in Progress, August 2012. [PRECIS-STATEMENT] Blanchet, M. and A. Sullivan, "Stringprep Revision and PRECIS Problem Statement", Work in Progress, August 2012.
[RFC822] Crocker, D., "STANDARD FOR THE FORMAT OF ARPA INTERNET TEXT MESSAGES", STD 11, RFC 822, August 1982. [RFC1263] O'Malley, S. and L. Peterson, "TCP Extensions Considered Harmful", RFC 1263, October 1991. [RFC1341] Borenstein, N. and N. Freed, "MIME (Multipurpose Internet Mail Extensions): Mechanisms for Specifying and Describing the Format of Internet Message Bodies", RFC 1341, June 1992. [RFC1521] Borenstein, N. and N. Freed, "MIME (Multipurpose Internet Mail Extensions) Part One: Mechanisms for Specifying and Describing the Format of Internet Message Bodies", RFC 1521, September 1993. [RFC2058] Rigney, C., Rubens, A., Simpson, W., and S. Willens, "Remote Authentication Dial In User Service (RADIUS)", RFC 2058, January 1997. [RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor Extensions", RFC 2132, March 1997. [RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999. [RFC2284] Blunk, L. and J. Vollbrecht, "PPP Extensible Authentication Protocol (EAP)", RFC 2284, March 1998. [RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, December 1998. [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. [RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"", RFC 2661, August 1999. [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671, August 1999. [RFC2822] Resnick, P., Ed., "Internet Message Format", RFC 2822, April 2001.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson, "Remote Authentication Dial In User Service (RADIUS)", RFC 2865, June 2000. [RFC2882] Mitton, D., "Network Access Servers Requirements: Extended RADIUS Practices", RFC 2882, July 2000. [RFC3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H., Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le, K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K., Wiebke, T., Yoshimura, T., and H. Zheng, "RObust Header Compression (ROHC): Framework and four profiles: RTP, UDP, ESP, and uncompressed", RFC 3095, July 2001. [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. [RFC3427] Mankin, A., Bradner, S., Mahy, R., Willis, D., Ott, J., and B. Rosen, "Change Process for the Session Initiation Protocol (SIP)", RFC 3427, December 2002. [RFC3575] Aboba, B., "IANA Considerations for RADIUS (Remote Authentication Dial In User Service)", RFC 3575, July 2003. [RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, "Diameter Base Protocol", RFC 3588, September 2003. [RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record (RR) Types", RFC 3597, September 2003. [RFC3692] Narten, T., "Assigning Experimental and Testing Numbers Considered Useful", BCP 82, RFC 3692, January 2004. [RFC3735] Hollenbeck, S., "Guidelines for Extending the Extensible Provisioning Protocol (EPP)", RFC 3735, March 2004. [RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, Ed., "Extensible Authentication Protocol (EAP)", RFC 3748, June 2004. [RFC3935] Alvestrand, H., "A Mission Statement for the IETF", BCP 95, RFC 3935, October 2004.
[RFC4001] Daniele, M., Haberman, B., Routhier, S., and J. Schoenwaelder, "Textual Conventions for Internet Network Addresses", RFC 4001, February 2005. [RFC4181] Heard, C., Ed., "Guidelines for Authors and Reviewers of MIB Documents", BCP 111, RFC 4181, September 2005. [RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and T. Wright, "Transport Layer Security (TLS) Extensions", RFC 4366, April 2006. [RFC4485] Rosenberg, J. and H. Schulzrinne, "Guidelines for Authors of Extensions to the Session Initiation Protocol (SIP)", RFC 4485, May 2006. [RFC4521] Zeilenga, K., "Considerations for Lightweight Directory Access Protocol (LDAP) Extensions", BCP 118, RFC 4521, June 2006. [RFC4727] Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4, ICMPv6, UDP, and TCP Headers", RFC 4727, November 2006. [RFC4929] Andersson, L., Ed., and A. Farrel, Ed., "Change Process for Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS) Protocols and Procedures", BCP 129, RFC 4929, June 2007. [RFC4962] Housley, R. and B. Aboba, "Guidance for Authentication, Authorization, and Accounting (AAA) Key Management", BCP 132, RFC 4962, July 2007. [RFC5080] Nelson, D. and A. DeKok, "Common Remote Authentication Dial In User Service (RADIUS) Implementation Issues and Suggested Fixes", RFC 5080, December 2007. [RFC5201] Moskowitz, R., Nikander, P., Jokela, P., Ed., and T. Henderson, "Host Identity Protocol", RFC 5201, April 2008. [RFC5218] Thaler, D. and B. Aboba, "What Makes For a Successful Protocol?", RFC 5218, July 2008. [RFC5225] Pelletier, G. and K. Sandlund, "RObust Header Compression Version 2 (ROHCv2): Profiles for RTP, UDP, IP, ESP and UDP-Lite", RFC 5225, April 2008. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, October 2008. [RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines for Application Designers", BCP 145, RFC 5405, November 2008. [RFC5421] Cam-Winget, N. and H. Zhou, "Basic Password Exchange within the Flexible Authentication via Secure Tunneling Extensible Authentication Protocol (EAP-FAST)", RFC 5421, March 2009. [RFC5422] Cam-Winget, N., McGrew, D., Salowey, J., and H. Zhou, "Dynamic Provisioning Using Flexible Authentication via Secure Tunneling Extensible Authentication Protocol (EAP- FAST)", RFC 5422, March 2009. [RFC5704] Bryant, S., Ed., Morrow, M., Ed., and IAB, "Uncoordinated Protocol Development Considered Harmful", RFC 5704, November 2009. [RFC5727] Peterson, J., Jennings, C., and R. Sparks, "Change Process for the Session Initiation Protocol (SIP) and the Real- time Applications and Infrastructure Area", BCP 67, RFC 5727, March 2010. [RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen, "Internet Key Exchange Protocol Version 2 (IKEv2)", RFC 5996, September 2010. [RFC6055] Thaler, D., Klensin, J., and S. Cheshire, "IAB Thoughts on Encodings for Internationalized Domain Names", RFC 6055, February 2011. [RFC6158] DeKok, A., Ed., and G. Weber, "RADIUS Design Guidelines", BCP 158, RFC 6158, March 2011. [RFC6648] Saint-Andre, P., Crocker, D., and M. Nottingham, "Deprecating the "X-" Prefix and Similar Constructs in Application Protocols", BCP 178, RFC 6648, June 2012.
7. Acknowledgments This document is heavily based on an earlier draft by Scott Bradner and Thomas Narten, other parts of which were eventually published as RFC 4775. That draft stated: "The initial version of this document was put together by the IESG in 2002. Since then, it has been reworked in response to feedback from John Loughney, Henrik Levkowetz, Mark Townsley, Randy Bush and others." Valuable comments and suggestions on the current form of the document were made by Loa Andersson, Ran Atkinson, Stewart Bryant, Leslie Daigle, Alan DeKok, Roy Fielding, Phillip Hallam-Baker, Ted Hardie, Alfred Hoenes, John Klensin, Barry Leiba, Eric Rescorla, Adam Roach, and Pekka Savola. The text on TLS experience was contributed by Yngve Pettersen. 8. IAB Members at the Time of Approval Bernard Aboba Jari Arkko Marc Blanchet Ross Callon Alissa Cooper Spencer Dawkins Joel Halpern Russ Housley David Kessens Danny McPherson Jon Peterson Dave Thaler Hannes Tschofenig
Appendix A. Examples This section discusses some specific examples as case studies. A.1. Already-Documented Cases There are certain documents that specify a change process or describe extension considerations for specific IETF protocols: The SIP change process [RFC3427], [RFC4485], [RFC5727] The (G)MPLS change process (mainly procedural) [RFC4929] LDAP extensions [RFC4521] EPP extensions [RFC3735] DNS extensions [RFC2671][RFC3597] SMTP extensions [RFC5321] It is relatively common for MIBs, which are all in effect extensions of the SMI data model, to be defined or extended outside the IETF. BCP 111 [RFC4181] offers detailed guidance for authors and reviewers. A.2. RADIUS Extensions The RADIUS [RFC2865] protocol was designed to be extensible via addition of Attributes. This extensibility model assumed that Attributes would conform to a limited set of data types and that vendor extensions would be limited to use by vendors in situations in which interoperability was not required. Subsequent developments have stretched those assumptions. From the beginning, uses of the RADIUS protocol extended beyond the scope of the original protocol definition (and beyond the scope of the RADIUS Working Group charter). In addition to rampant self- allocation within the limited RADIUS standard attribute space, vendors defined their own RADIUS commands. This led to the rapid proliferation of vendor-specific protocol variants. To this day, many common implementation practices have not been documented. For example, authentication server implementations are often typically based on a Data Dictionary, enabling addition of Attributes without requiring code changes. Yet, the concept of a Data Dictionary is not mentioned in the RADIUS specification [RFC2865]. As noted in "Extended RADIUS Practices" [RFC2882], Section 1: The RADIUS Working Group was formed in 1995 to document the protocol of the same name, and was chartered to stay within a set of bounds for dial-in terminal servers. Unfortunately the real world of Network Access Servers (NASes) hasn't stayed that small and simple, and continues to evolve at an amazing rate.
This document shows some of the current implementations on the market have already outstripped the capabilities of the RADIUS protocol. A quite a few features have been developed completely outside the protocol. These features use the RADIUS protocol structure and format, but employ operations and semantics well beyond the RFC documents. The limited set of data types defined in the RADIUS specification [RFC2865] led to subsequent documents defining new data types. Since new data types are typically defined implicitly as part of defining a new attribute and because RADIUS client and server implementations differ in their support of these additional specifications, there is no definitive registry of RADIUS data types, and data type support has been inconsistent. To catalog commonly implemented data types as well as to provide guidance for implementers and attribute designers, Section 2.1 of "RADIUS Design Guidelines" [RFC6158] includes advice on basic and complex data types. Unfortunately, these guidelines [RFC6158] were published in 2011, 14 years after the RADIUS protocol was first documented [RFC2058] in 1997. Section 6.2 of the RADIUS specification [RFC2865] defines a mechanism for Vendor-Specific extensions (Attribute 26) and states that use of Vendor-Specific extensions: should be encouraged instead of allocation of global attribute types, for functions specific only to one vendor's implementation of RADIUS, where no interoperability is deemed useful. However, in practice, usage of Vendor-Specific Attributes (VSAs) has been considerably broader than this. In particular, VSAs have been used by Standards Development Organizations (SDOs) to define their own extensions to the RADIUS protocol. This has caused a number of problems. One issue concerns the data model for VSAs. Since it was not envisaged that multi-vendor VSA implementations would need to interoperate, the RADIUS specification [RFC2865] does not define the data model for VSAs and allows multiple sub-attributes to be included within a single Attribute of type 26. Since this enables VSAs to be defined that would not be supportable by current implementations if placed within the standard RADIUS attribute space, this has caused problems in standardizing widely deployed VSAs, as discussed in Section 2.4 of "RADIUS Design Guidelines" BCP 158 [RFC6158]: RADIUS attributes can often be developed within the vendor space without loss (and possibly even with gain) in functionality. As a result, translation of RADIUS attributes developed within the vendor space into the standard space may provide only modest
benefits, while accelerating the exhaustion of the standard space. We do not expect that all RADIUS attribute specifications requiring interoperability will be developed within the IETF, and allocated from the standard space. A more scalable approach is to recognize the flexibility of the vendor space, while working toward improvements in the quality and availability of RADIUS attribute specifications, regardless of where they are developed. It is therefore NOT RECOMMENDED that specifications intended solely for use by a vendor or SDO be translated into the standard space. Another issue is how implementations should handle unknown VSAs. Section 5.26 of the RADIUS specification [RFC2865] states: Servers not equipped to interpret the vendor-specific information sent by a client MUST ignore it (although it may be reported). 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. However, since VSAs do not contain a "mandatory" bit, RADIUS clients and servers may not know whether it is safe to ignore unknown VSAs. For example, in the case where VSAs pertain to security (e.g., Filters), it may not be safe to ignore them. As a result, Section 2.5 of "Common Remote Authentication Dial In User Service (RADIUS) Implementation Issues and Suggested Fixes" [RFC5080] includes the following caution: To avoid misinterpretation of service requests encoded within VSAs, RADIUS servers SHOULD NOT send VSAs containing service requests to RADIUS clients that are not known to understand them. For example, a RADIUS server should not send a VSA encoding a filter without knowledge that the RADIUS client supports the VSA. In addition to extending RADIUS by use of VSAs, SDOs have also defined new values of the Service-Type attribute in order to create new RADIUS commands. Since the RADIUS specification [RFC2865] defined Service-Type values as being allocated First Come, First Served (FCFS) [RFC5226], this permitted new RADIUS commands to be allocated without IETF review. This oversight has since been fixed in "IANA Considerations for RADIUS" [RFC3575].
A.3. TLS Extensions The Secure Sockets Layer (SSL) Version 2 Protocol was developed by Netscape to be used to secure online transactions on the Internet. It was later replaced by SSLv3, also developed by Netscape. SSLv3 was then further developed by the IETF as the Transport Layer Security (TLS) 1.0 [RFC2246]. The SSLv3 protocol was not explicitly specified to be extended. Even TLS 1.0 did not define an extension mechanism explicitly. However, extension "loopholes" were available. Extension mechanisms were finally defined in "Transport Layer Security (TLS) Extensions" [RFC4366]: o New versions o New cipher suites o Compression o Expanded handshake messages o New record types o New handshake messages The protocol also defines how implementations should handle unknown extensions. Of the above extension methods, new versions and expanded handshake messages have caused the most interoperability problems. Implementations are supposed to ignore unknown record types but to reject unknown handshake messages. The new version support in SSL/TLS includes a capability to define new versions of the protocol, while allowing newer implementations to communicate with older implementations. As part of this functionality, some Key Exchange methods include functionality to prevent version rollback attacks. The experience with this upgrade functionality in SSL and TLS is decidedly mixed: o SSLv2 and SSLv3/TLS are not compatible. It is possible to use SSLv2 protocol messages to initiate an SSLv3/TLS connection, but it is not possible to communicate with an SSLv2 implementation using SSLv3/TLS protocol messages. o There are implementations that refuse to accept handshakes using newer versions of the protocol than they support. o There are other implementations that accept newer versions but have implemented the version rollback protection clumsily.
The SSLv2 problem has forced SSLv3 and TLS clients to continue to use SSLv2 Client Hellos for their initial handshake with almost all servers until 2006, much longer than would have been desirable, in order to interoperate with old servers. The problem with incorrect handling of newer versions has also forced many clients to actually disable the newer protocol versions, either by default or by automatically disabling the functionality, to be able to connect to such servers. Effectively, this means that the version rollback protection in SSL and TLS is non-existent if talking to a fatally compromised older version. SSLv3 and TLS also permitted extension of the Client Hello and Server Hello handshake messages. This functionality was fully defined by the introduction of TLS extensions, which make it possible to add new functionality to the handshake, such as the name of the server the client is connecting to, request certificate status information, and indicate Certificate Authority support, maximum record length, etc. Several of these extensions also introduce new handshake messages. It has turned out that many SSLv3 and TLS implementations that do not support TLS extensions did not ignore the unknown extensions, as required by the protocol specifications, but instead failed to establish connections. Since several of the implementations behaving in this manner are used by high-profile Internet sites, such as online banking sites, this has caused a significant delay in the deployment of clients supporting TLS extensions, and several of the clients that have enabled support are using heuristics that allow them to disable the functionality when they detect a problem. Looking forward, the protocol version problem, in particular, can cause future security problems for the TLS protocol. The strength of the digest algorithms (MD5 and SHA-1) used by SSL and TLS is weakening. If MD5 and SHA-1 weaken to the point where it is feasible to mount successful attacks against older SSL and TLS versions, the current error recovery used by clients would become a security vulnerability (among many other serious problems for the Internet). To address this issue, TLS 1.2 [RFC5246] makes use of a newer cryptographic hash algorithm (SHA-256) during the TLS handshake by default. Legacy ciphersuites can still be used to protect application data, but new ciphersuites are specified for data protection as well as for authentication within the TLS handshake. The hashing method can also be negotiated via a Hello extension. Implementations are encouraged to implement new ciphersuites and to enable the negotiation of the ciphersuite used during a TLS session to be governed by policy, thus enabling a more rapid transition away from weakened ciphersuites.
The lesson to be drawn from this experience is that it isn't sufficient to design extensibility carefully; it must also be implemented carefully by every implementer, without exception. Test suites and certification programs can help provide incentives for implementers to pay attention to implementing extensibility mechanisms correctly. A.4. L2TP Extensions The Layer Two Tunneling Protocol (L2TP) [RFC2661] carries Attribute- Value Pairs (AVPs), with most AVPs having no semantics to the L2TP protocol itself. However, it should be noted that L2TP message types are identified by a Message Type AVP (Attribute Type 0) with specific AVP values indicating the actual message type. Thus, extensions relating to Message Type AVPs would likely be considered major extensions. L2TP also provides for vendor-specific AVPs. Because everything in L2TP is encoded using AVPs, it would be easy to define vendor- specific AVPs that would be considered major extensions. L2TP also provides for a "mandatory" bit in AVPs. Recipients of L2TP messages containing AVPs that they do not understand but that have the mandatory bit set, are expected to reject the message and terminate the tunnel or session the message refers to. This leads to interesting interoperability issues, because a sender can include a vendor-specific AVP with the M-bit set, which then causes the recipient to not interoperate with the sender. This sort of behavior is counter to the IETF ideals, as implementations of the IETF standard should interoperate successfully with other implementations and not require the implementation of non-IETF extensions in order to interoperate successfully. Section 4.2 of the L2TP specification [RFC2661] includes specific wording on this point, though there was significant debate at the time as to whether such language was by itself sufficient. Fortunately, it does not appear that the potential problems described above have yet become a problem in practice. At the time of this writing, the authors are not aware of the existence of any vendor- specific AVPs that also set the M-bit.
Authors' Addresses Brian Carpenter Department of Computer Science University of Auckland PB 92019 Auckland, 1142 New Zealand EMail: email@example.com Bernard Aboba (editor) PMB 606 15600 NE 8th Street, Suite B1 Bellevue, WA 98008 USA EMail: firstname.lastname@example.org Stuart Cheshire Apple Inc. 1 Infinite Loop Cupertino, CA 95014 USA EMail: email@example.com