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
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
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
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
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
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
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
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
a. If the server understands one or more versions in the
client's range, it reports back the highest mutually
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
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
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
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
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
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
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
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).
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
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.
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
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
a. Payload fragmentation (often due to the introduction of
extensions or additional usage scenarios);
b. Problems with congestion control, transport reliability, or
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
a. NAT traversal concerns that were more easily addressed with UDP
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
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
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
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
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,
6.2. Informative References
Scudder, J., Chen, E., Mohapatra, P., and K. Patel,
"Revised Error Handling for BGP UPDATE Messages", Work in
Progress, June 2012.
Thaler, D., "Issues in Identifier Comparison for Security
Purposes", Work in Progress, August 2012.
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,
[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.
Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
Preparation and Comparison of Internationalized Strings in
Application Protocols", Work in Progress, August 2012.
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
[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
[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,
[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,
[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
[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,
[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,
[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,
[RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
for Application Designers", BCP 145, RFC 5405, November
[RFC5421] Cam-Winget, N. and H. Zhou, "Basic Password Exchange
within the Flexible Authentication via Secure Tunneling
Extensible Authentication Protocol (EAP-FAST)", RFC 5421,
[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,
[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,
[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.
This document is heavily based on an earlier draft by Scott Bradner
and Thomas Narten, other parts of which were eventually published as
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
8. IAB Members at the Time of Approval
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
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
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
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
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"
o New versions
o New cipher suites
o Expanded handshake messages
o New record types
o New handshake messages
The protocol also defines how implementations should handle unknown
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
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
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
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
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.
Department of Computer Science
University of Auckland
Bernard Aboba (editor)
15600 NE 8th Street, Suite B1
Bellevue, WA 98008
1 Infinite Loop
Cupertino, CA 95014