This is a purely informative rendering of an RFC that includes verified errata. This rendering may not be used as a reference.
The following 'Verified' errata have been incorporated in this document:
Network Working Group A. Farrel
Request for Comments: 5513 Old Dog Consulting
Category: Informational 1 April 2009
IANA Considerations for Three Letter Acronyms
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Three Letter Acronyms (TLAs) are commonly used to identify components
of networks or protocols as designed or specified within the IETF. A
common concern is that one acronym may have multiple expansions.
While this may not have been an issue in the past, network
convergence means that protocols that did not previously operate
together are now found in close proximity. This results in
contention for acronyms, and confusion in interpretation. Such
confusion has the potential to degrade the performance of the
Internet as misunderstandings lead to misconfiguration or other
Given the growing use of TLAs and the relatively small number
available, this document specifies a Badly Construed Proposal (BCP)
for the management of a registry of TLAs within the IETF, and the
procedures for the allocation of new TLAs from the registry.
A Three-Letter Acronym (TLA) is a popular form of abbreviation
usually based on the initial letters of a three-word term. A formal
definition of a TLA is provided in Section 2.
TLAs are particularly popular within the Internet community where
they serve as abbreviations in the spoken and written word. As their
popularity has grown, the measure of the value of an RFC (q.v.) is
not only its successful implementation, interoperability, and
deployment, but also the number of TLAs included in the text.
For example, the Transmission Control Protocol (itself a TLA - TCP)
[RFC0793] is extremely successful. The specification contains no
fewer than 20 distinct TLAs (although it should be noted that some
are simple abbreviations rather than proper acronyms). On the other
hand, the Internet Stream Protocol Version 2 [RFC1819] is ambiguously
referred to using the TLA ST2, and also as STII which is clearly not
a TLA. Further, the STII specification contains only 12 distinct
TLAs, and it should be no surprise that STII has been far less
successful than TCP.
A common concern amongst diligent protocol implementers is that one
acronym may have multiple expansions. While this may not have been
an issue in the past, network convergence means that protocols that
did not previously operate together are now found in close proximity.
Not only does this result in contention for acronyms, and confusion
in interpretation of specification, it also leads to many wasted
hours trying to select appropriate and suitably-unique names for
variables within source code implementations. Such confusion has the
potential to degrade the performance of the Internet as
misunderstandings lead to coding errors, compilation failures,
misconfiguration, and other operating errors.
Furthermore, it should be noted that we are rapidly approaching World
Acronym Depletion (WAD). It has been estimated that, at the current
rate of TLA allocation, we will run out by the end of September this
year. This timescale could be worsened if there is the expected
growth in demand for mobile acronyms, IP-TLAs, and TLA-on-demand.
According to the definition provided in Section 2, there are 36**3 -
10**3 = 45656 TLAs in total. This number will so easily be depleted
that we must institute some policy for conservation.
The Internet Assigned Numbers Authority (IANA, helpfully, a four-
letter acronym - although note that a four-letter acronym is an FLA
and hence is, in its own way, a TLA) maintains registries of names
and numbers for use within the Internet in order to avoid duplicate
allocation of one of those names or numbers and the consequent
confusion and failed interoperability that would arise. It is,
therefore, wholly appropriate that the IANA should manage the
assignment and use of TLAs within the Internet.
This document specifies a Badly Construed Proposal for the management
of a registry of TLAs within the IETF, and the procedures for the
allocation of new TLAs from the registry.
1.1. RFC Editor Terminology List
It is worth observing that the RFC Editor currently maintains a list
of common terms, abbreviations, and acronyms. While this list is
highly useful for the construction of documents, it does not provide
unambiguous interpretation of acronyms.
2. Formal Definition of TLA
Acronym - a word made up of the initial letters of the words in a
For example, IETF is an acronym formed from the first letters of
the phrase International Essential Tremor Foundation [URL-IETF].
Three Letter Acronym (TLA) - an acronym comprising exactly three
For example, RFC is a TLA formed of the first letters of the
phrase Rugby Football Club [URL-CARDIFF].
For our usage, we also allow digits within a TLA. Thus, P2P is an
acronym meaning Purchase to Pay [URL-P2P]. The digits 2 and 4 are
specially used by clever people who have noticed that, when spoken,
they sound like the words 'to' and 'for'. Whether this is helpful
may be left as an exercise for the user considering the brief
A - Do you use the Internet Streams Protocol?
B - Yes. Do you use ST, too?
A - No, I use ST2.
B - That's interesting. C uses ST2, too.
A - I have a car horn application called Toot-toot.
B - Really? Do you use ST2 to Toot-toot, too?
Note, however, that an acronym made up entirely of digits might be
Lastly, we must consider case-sensitivity. Although acronyms often
include upper or lowercase letters, no assumptions should be made
about the interpretation of the acronym based on the case of its
letters, so that both QOS and QoS clearly refer to the Queen of the
South football club [URL-QOS] and [URL-QoS].
2.1. A Note on Vocalization
Acronyms are often articulated as words in spoken text. This can be
helpful in generating a cosy feel or a marketing buzz around a
concept that offers a less-favorable reality. For example, Claws and
Teeth (CAT) can be pronounced "cat" making it seem quite cuddly.
Other acronyms are always spelled out in order to avoid accidental
misinterpretation or litigation. For example, do not refer to your
neighbor's Daughter or Granddaughter as anything other than their
But care should be taken with vocalization, as well. It will be
noted that some letters have more syllables than the words they are
used to represent. In these cases, acronyms are to be avoided.
Thus, the world wide web must never be assigned the acronym WWW.
Finally, a word of caution about attempting to pronounce acronyms as
words. This can lead to serious injury for the inexperienced unless
they happen to be native speakers of Czech. Do not try to say XML in
front of your mother-in-law, and don't attempt to talk about Open
Office dot Org in polite company.
3. Backward and Forward Compatibility
It should be obvious to most RFC readers (MRRs) that TLAs are already
widely used in Internet specifications. This work is not intended to
unnecessarily invalidate existing RFCs, although where such
invalidation is necessary or desirable, this work can be used for
In order to support existing documents, IANA is required to search
all existing RFCs for every existing acronym usage (EAU), but may
filter that search to exclude non-TLAs.
It will be noted that, as a result of that search, many duplicate
meanings will be discovered. For example, "OAM" will be found in a
large number of RFCs, yet its meaning may be as diverse as "on a
mission", "order of Australia medal", and "orbital angular momentum".
This contention is best resolved by the judgement of Solomon -- each
acronym usage will be allocated its share of the letters currently in
use. If there are three uses of an acronym, they will get one letter
each; two existing uses would get one-and-a-half letters each; etc.
4. IANA Considerations
4.1. New Registry
The Internet TLA Registry (ITR) should track the following
- Unique interpretation
- Defining RFC
4.2. Reserved Values
Certain key values are reserved. That is, they are allocated in the
registry by this document and may not be used for any other purpose.
Acronym Expansion Reference
TLA Two Letter Acronym [RFC5513]
TBD Two Be Deleted [RFC5513]
RFC Ready for Compost [RFC5513]
PoS Not particularly good [RFC5513]
VPN Very possibly no use [RFC5513]
TCP Totally bad proposal [RFC5513]
USA Universal Source of Acronyms [RFC5513]
NBG This document [RFC5513]
BCP Badly construed proposal [RFC5513]
4.3. Allocation Policy
IANA shall apply the following allocation policies according to
All TLAs of the form XX* where * is any letter or digit.
First Come First Served
All TLAs of the form X**, Y**, or Z** where * is any letter or
digit. Excepted from this are the TLAs of the form XX* as above.
All other TLAs.
5. Security Considerations
Many security algorithms are identified by TLAs. It is a clear
requirement that someone implementing, for example, MD5 should be
understood to have encoded the well-known Maybe-Decrypted-
Deciphered-Decoded-Disambiguated-and-Degraded algorithm, and not any
other security algorithm with the same acronym.
I would like to thank the MPLS-TP design team for holding seemingly
endless meetings during which the need for this document became
Thanks to Daniel King for noticing that this document is a BCP.
7.1. Normative References
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26, RFC
5226, May 2008.
7.2. Informative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC1819] Delgrossi, L., Ed., and L. Berger, Ed., "Internet
Stream Protocol Version 2 (ST2) Protocol Specification
- Version ST2+", RFC 1819, August 1995.
[URL-IETF] International Essential Tremor Foundation,
[URL-CARDIFF] Cardiff Rugby Football Club, http://www.cardiffrfc.com/
[URL-QOS] Queen of the South Football Club, http://www.qosfc.com/
[URL-QoS] Queen of the South Football Club,
EID 1889 (Verified) is as follows:Section: 7.2.
This appears at the “[URL-QoS]” reference, the second reference to the Queen of the South Football Club.
According to the URI schemes registry maintained by IANA, “ahttp” is not a valid URI scheme. It should probably be “http” instead.
The URI in its current form is inaccessible, because its meaning is not well-defined for users.