ENUM L. Conroy
Internet-Draft RMRL
Expires: December 28, 2005 K. Fujiwara
JPRS
June 26, 2005
ENUM Implementation Issues and Experiences
<draft-ietf-enum-experiences-02.txt>
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Abstract
This document captures experience in implementing systems based on
the ENUM protocol, and experience of ENUM data that have been created
by others. As such, it is informational only, and produced as a help
to others in reporting what is "out there" and the potential pitfalls
in interpreting the set of documents that specify the protocol.
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Character Sets and ENUM . . . . . . . . . . . . . . . . . . . 6
3.1 Character Sets - Non-ASCII considered harmful . . . . . . 6
3.2 Case Sensitivity . . . . . . . . . . . . . . . . . . . . . 9
3.3 Regexp Field Delimiter . . . . . . . . . . . . . . . . . . 9
3.4 Regexp Meta-character Issue . . . . . . . . . . . . . . . 10
4. ORDER/PRIORITY Processing . . . . . . . . . . . . . . . . . . 11
4.1 Order/Priority values - general processing . . . . . . . . 11
4.2 NAPTRs with identical ORDER/PRIORITY values . . . . . . . 13
4.3 Processing Order value across Zones . . . . . . . . . . . 14
5. Non-final NAPTR Processing . . . . . . . . . . . . . . . . . . 16
5.1 Non-Final NAPTRs - necessity . . . . . . . . . . . . . . . 16
5.2 Non-Final NAPTRs - future implementation . . . . . . . . . 17
5.2.1 Non-Final NAPTRs - general . . . . . . . . . . . . . . 17
5.2.2 Non-Final NAPTRs - loop detection and response . . . . 17
6. DNS record size and DNS software issue . . . . . . . . . . . . 19
7. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 21
7.1 Service field syntax . . . . . . . . . . . . . . . . . . . 21
8. Security Considerations . . . . . . . . . . . . . . . . . . . 23
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 25
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
11.1 Normative References . . . . . . . . . . . . . . . . . . . 26
11.2 Informative References . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 28
Intellectual Property and Copyright Statements . . . . . . . . 29
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1. Terminology
This document is Informational, and does not specify a standard of
any kind. Note that recommendations here contain the words "MUST",
"REQUIRE", "SHOULD", and "MAY". In this particular document, these
do not form a standard, and so do not hold their normative
definitions. The proposals include these terms from observation of
behaviour and for internal consistency, where Client and Server
recommendations have to match.
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2. Introduction
The ENUM protocol (RFC3761 [1]) and the Dynamic Delegation Discovery
System (DDDS, [2] [3] [4] [5] [6]) are defined elsewhere, and those
documents alone form the normative definition of the ENUM system.
Unfortunately, this document cannot provide an overview of the
specifications, so the reader is assumed to have read and understood
the complete set of ENUM normative documents.
From experience of creating ENUM data and of developing client
systems to process that data it is apparent that there are some
subtleties in the specifications that have led to different
interpretations; in addition there are common syntactic mistakes in
data currently "out there" on the Internet.
This document is intended to help others avoid the potential pitfalls
in interpreting the set of documents that specify the protocol. It
also reports the kind of data they will "find" and so how to process
the intent of the publisher of that ENUM data, regardless of the
syntax used. As such, it is in keeping with the principle evinced in
RFC791 that "In general, an implementation must be conservative in
its sending behavior, and liberal in its receiving behavior".
Note that the DDDS system is intricate and so in some places there
are several potential interpretations of the specifications. This
document proposes a suggested interpretation for some of these
points, but they are just that; suggestions.
This draft covers 10 issues in five areas, making 37 recommendations
and giving 2 clarifications.
Any ENUM implementation issue has two sides:
o the "Server" side covering the expected behaviour of the ENUM zone
provisioning system and expectations Registrants may make, and
o the "Client" side covering behaviour that has been observed and
that can be expected of the Client, together with the expectations
that an end user who requests an ENUM lookup may make.
For each of the issues, we have split the recommendations into
"Client" and "Server" proposals. In three cases, we have indicated
proposals that relate to ENUMservice specifications, rather than
implementations; these are labelled as "Spec".
There are undoubtedly other issues, and developers are asked to raise
any others they find on the IETF ENUM Working group's mailing list
and/or by mail to the authors (see later for contact information).
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Finally, note that the authors are not aware of any IPR issues that
are involved in the suggestions made in this document.
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3. Character Sets and ENUM
3.1 Character Sets - Non-ASCII considered harmful
RFC3761 [1] and RFC3403 [2] specify that ENUM (and NAPTRs) support
Unicode using the UTF-8 encoding specified in RFC3629 [7]. This
raises an issue where implementations use "single byte" string
processing routines. If there are multi-byte characters within an
ENUM NAPTR, incorrect processing may well result from these non
"UTF-8 aware" systems.
The UTF-8 encoding has a "US-ASCII equivalent range", so that all
characters in US-ASCII [20] from 0x00 to 0x7F hexadecimal have an
identity map to the UTF-8 encoding; the encodings are the same. In
UTF-8, characters with Unicode code points above this range will be
encoded using more than one byte, all of which will be in the range
0x80 to 0xFF hexadecimal. Thus it is important to consider the
different fields of a NAPTR and whether or not multi-byte characters
can or should appear in them.
In addition, characters in the "non-printable" portion of US-ASCII
(0x00 to 0x1F hexadecimal, plus 0x7F hexadecimal) are "difficult".
Although NAPTRs are processed by machine, they may sometimes need to
be written in a "human readable" form. Similarly, if NAPTR content
is shown to an end user so that they may choose, it is important that
the content is "human readable". Thus it is unwise to use non-
printable characters within the US-ASCII range; the Client may have
good reason to reject NAPTRs that include these characters as they
cannot be shown.
There are two numeric fields in a NAPTR; the ORDER and PREFERENCE
fields. As these contain binary values, no risk is involved as
string processing should not be applied to them. The "string based"
fields are the flags, services, and RegExp fields. The Replacement
field holds a domain name encoded according to the standard DNS
mechanism [8][9]. With the introduction of Internationalized Domain
Name (IDN) support, this domain name MUST be further encoded using
Punycode [10]. As this holds a domain name that is not subject to
replacement or modification (other than Punycode processing), it is
not of concern here.
Taking the "string" fields in turn, the flags field contains
characters that indicate the disposition of the NAPTR. This may be
empty, in which case the NAPTR is "non-final", or it may include a
flag character as specified in RFC3761. These characters all fall
into the US-ASCII equivalent range, so multi-byte characters cannot
occur.
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The services field includes the DDDS Application identifier ("E2U")
used for ENUM, the '+' character used to separate tokens, and a set
of ENUMservice identifiers, any of which may include the ':'
separator character. In section 2.4.2 of RFC3761 these identifiers
are specified as 1*32 ALPHA/DIGIT, so there is no possibility of non-
ASCII characters in the services field.
The RegExp field is more complex. It forms a SED-like substitution
expression, defined in [2], and consists of two sub-fields:
o the POSIX Extended Regular Expression (ERE) sub-field [11]
o a replacement (repl) sub-field [2].
Additionally, RFC3403 specifies that a flag character may be
appended, but the only flag currently defined there (the 'i' case
insensitivity flag) is not appropriate for ENUM - see later in this
document.
The ERE sub-field matches against the "Application Unique String";
for ENUM, this is defined in RFC3761 to consist of digit characters,
with an initial '+' character. It is similar to a global-number-
digits production of a tel: URI, as specified in [12], but with
visual-separators removed. All of these characters fall into the US-
ASCII equivalent range of UTF-8 encoding, as do the characters
significant to the ERE processing. Thus, for ENUM, there will be no
multi-byte characters within this sub-field.
The repl sub-field can include a mixture of explicit text used to
construct a URI and characters significant to the substitution
expression, as defined in RFC3403. Whilst the latter set all fall
into the US-ASCII equivalent range of UTF-8 encoding, this might not
be the case for all conceivable text used to construct a URI.
Presence of multi-byte characters could complicate URI generation and
processing routines.
URI generic syntax is defined in [13] as a sequence of characters
chosen from a limited subset of the repertoire of US-ASCII
characters. The current URIs use the standard URI character
"escaping" rules specified in the URI generic syntax, and so any
multi-byte characters will be pre-processed; they will not occur in
the explicit text used to construct a URI within the repl sub-field.
However, the Internationalized Resource Identifier (IRI) is defined
in [14] as extending the syntax of URIs, and specifies a mapping from
an IRI to a URI. IRI syntax allows characters with multi-byte UTF-8
encoding.
Given that this is the only place within an ENUM NAPTR where such
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multi-byte encodings might reasonably be found, a simple solution is
to use the mapping method specified in section 3.1 of [14] to convert
any IRI into its equivalent URI.
This process consists of two elements; the domain part of a IRI MUST
be processed using Punycode if it has a non-ASCII domain name, and
the remainder MUST be processed using the extended "escaping" rules
specified in the IRI document if it contains characters ouside the
normal URI repertoire. Using this process, there will be no non-
ASCII characters in any part of any URI, even if it has been
converted from an IRI which contains such characters.
Taking into account the existing client base, it is RECOMMENDED that:
Spec ENUMservice registrations SHOULD REQUIRE that any static
text in the repl sub-field is encoded using only characters
in the US-ASCII equivalent range that are "printable". If
any of the static text characters do fall outside this
range then they MUST be pre-processed using an IRI/
URI-specific "escape" mechanism to re-encode them only
using US-ASCII equivalent printable characters (those in
the range 0x20 to 0x7E).
At the least, it is RECOMMENDED that:
Spec Any ENUMservice registration that allows characters
requiring multi-byte UTF-8 encoding to be present in the
repl sub-field MUST have a clear indication that there may
be characters outside of the US-ASCII equivalent range.
Such an ENUMservice registration is strongly discouraged,
as the mechanisms specified in section 3.1 of [14] will
suffice.
Finally, the majority of ENUM Clients in use today do not support
multi-byte encodings of UCS. This is a reasonable choice,
particularly for "small footprint" implementations, and they may not
be able to support NAPTR content that is non-printable as they need
to present the content to an end user for selection. Thus, it is
RECOMMENDED that:
Client Clients MAY discard NAPTRs in which they detect characters
not in the US-ASCII "printable" range (0x20 to 0x7E
hexadecimal).
ENUM provisioning systems should consider this. It is RECOMMENDED
that:
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Server ENUM zone content population systems SHOULD NOT use non-
ASCII characters in the NAPTRs they generate unless they
are sure that all Clients they intend to support will be
able correctly to process them.
3.2 Case Sensitivity
The only place where NAPTR field content is case sensitive is in any
static text in the repl sub-field of the RegExp field. Everywhere
else, case insensitive processing can be used.
The case insensitivity flag ('i') may be added at the end of the
RegExp field. However, in ENUM, the ERE sub-field operates on a
string defined as the '+' character, followed by a sequence of digit
characters. Thus this flag is redundant for E2U NAPTRs, as it does
not act on the repl sub-field contents.
To avoid the confusion that this generates, It is RECOMMENDED that:
Server When populating ENUM zones with NAPTRs, population systems
SHOULD NOT use the 'i' flag, as it has no effect and some
Clients don't expect it.
Client Clients SHOULD NOT assume that the field delimiter is the
last character.
3.3 Regexp Field Delimiter
It is not possible to select a delimiter character that cannot appear
in one of the sub-fields. Some old clients are "hardwired" to expect
the character '!' as a delimiter.
This is used in an example in RFC3403.
It is RECOMMENDED that:
Server ENUM zone population systems SHOULD use '!' (U+0021) as
their RegExp delimiter character.
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Client Clients MAY discard NAPTRs that do not use '!' as a RegExp
delimiter.
This cannot appear in the ERE sub-field. It may appear in the
content of some URIs, as it is a valid character (e.g.in http URLs).
Thus, it is further RECOMMENDED that:
Server ENUM zone population systems MUST ensure that, if the
RegExp delimiter is a character in the static text of the
repl sub-field, it MUST be "escaped" using the escaped-
delimiter production of the BNF specification shown in
section 3.2 of RFC3402 (i.e. "\!", U+005C U+0021).
Client Clients SHOULD discard NAPTRs that have more or less than 3
"unescaped" instances of the delimiter character within the
RegExp field.
3.4 Regexp Meta-character Issue
In ENUM, the ERE sub-field may include a literal character '+', as
the Application Unique String on which it operates includes this.
However, if it is present, then '+' must be "escaped" using a
backslash character as '+' is a meta-character in POSIX Extended
Regular Expression syntax.
The following NAPTR example is incorrect:
* IN NAPTR 100 10 "u" "E2U+sip" "!^+46555(.*)$!sip:\1@sipcsp.se!" .
This example MUST be written as:
* IN NAPTR 100 10 "u" "E2U+sip" "!^\+46555(.*)$!sip:\1@sipcsp.se!" .
Thus, it is RECOMMENDED that:
Server If present in the ERE sub-field of an ENUM NAPTR, '+' MUST
be written as "\+" (i.e. U+005C U+002B).
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4. ORDER/PRIORITY Processing
4.1 Order/Priority values - general processing
RFC3761 and RFC3403 state that the client MUST sort the NAPTRs using
the ORDER field value ("lowest value is first") and SHOULD order the
NAPTRs using the PRIORITY field value as the minor sort term (again,
lowest value first). The NAPTRs in the sorted list must be processed
in order. Subsequent NAPTRs with less preferred ORDER values must
only be dealt with once the current ones with a "winning" ORDER value
have been processed.
However, this expected behaviour is a simplification; clients may not
behave this way in practice, and so there is a conflict between the
specification and practice. For example, ENUM clients will be
incapable of using most NAPTRs as they don't support the ENUMservice
(and the URI generated by those NAPTRs). As such, they will discard
the "unusable" NAPTRs and continue with processing the "next best"
NAPTR in the list.
The end user may have pre-specified their own preference for services
to be used. Thus, an end user may specify that they would prefer to
use contacts with a "sip" ENUMservice, and then those with "email:
mailto" service, and are not interested in any other options. Thus
the sorted list as proposed by the Registrant (and published via
ENUM) may be reordered. For example, a NAPTR with a "sip"
ENUMservice may have a "losing" ORDER field value, and yet is chosen
before a NAPTR with an "h323" ENUMservice and a "winning" ORDER
value. This may occur even if the node the end user controls is
capable of handling other ENUMservices.
ENUM Clients may also include the end user "in the decision loop",
offering the end user the choice from a list of possible NAPTRs.
Given that the ORDER field value is the major sort term, one would
expect a conforming ENUM Client to present only those NAPTRs with a
"winning" ORDER field value as choices. However, if all the options
presented had been rejected, then the ENUM Client might offer those
with the "next best" ORDER field value, and so on. As this may be
inconvenient for the end user, some clients simply offer all of the
available NAPTRs as options to the end user for their selection "in
one go".
In summary, some clients will take into account the service field
value along with the ORDER and PRIORITY field values, and may
consider the preferences of the end user.
The Registrant and the ENUM zone population system they use MUST be
aware of this and SHOULD NOT rely on Clients taking account of the
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value of the ORDER and the PRIORITY fields.
Specifically, it is unsafe to assume that a Client WILL NOT consider
another NAPTR until they have discarded one with a "winning" ORDER
value. The instruction (in RFC3403 section 4.1 and section 8) may or
may not be followed strictly by different ENUM clients for perfectly
justifiable reasons.
To avoid the risk of variable behaviour, it is RECOMMENDED that:
Server An ENUM zone population system SHOULD NOT use different
ORDER values for NAPTRs within a zone.
In our experience, incorrect ORDER values in ENUM zones is a major
source of problems. Although it is by no means required, it is
further RECOMMENDED that:
Server An ENUM zone population system SHOULD use a value of 100 as
the default ORDER value to be used with all NAPTRs.
As such, when populating a zone with NAPTRS, it is RECOMMENDED that:
Server A Registrant SHOULD NOT expect the client to ignore NAPTRs
with higher ORDER field values - the "winning" ones may
have been discarded.
Server Server - A Registrant SHOULD NOT expect the client to
conform to the ORDER and PRIORITY sort order they have
specified for their NAPTRs; the end user may have their own
preferences for ENUMservices.
Client Client - Clients MAY reorder the NAPTRs only to match an
explicit preference pre-specified by their end user.
Client Client - Clients that offer a list of contacts to the end
user for their choice MAY present all NAPTRs, not just the
ones with the highest currently unprocessed ORDER field
value.
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Server A Registrant SHOULD NOT assume which NAPTR choices will be
presented "at once".
The impact of this is that a Registrant should place into their zone
only contacts that they are willing to support; even those with the
"least preferred" ORDER/PRIORITY values may be selected by an end
user.
Finally, we have noticed a number of ENUM zones with NAPTRs that have
identical PRIORITY field values and different ORDER values. This may
be the result of a zone population system "bug" or a misunderstanding
over the uses of the two fields.
To clarify, the ORDER field value is the major sort term, and the
PRIORITY field value is the minor sort term. Thus one should expect
to have a set of NAPTRs in a zone with identical ORDER field values
and different PRIORITY field values.
4.2 NAPTRs with identical ORDER/PRIORITY values
From experience, there are zones that hold discrete NAPTRs with
identical ORDER and PRIORITY field values. This will lead to
indeterminate client behaviour and so should not occur. However, in
the spirit of being liberal in what is allowed:
It is RECOMMENDED that:
Client Clients should accept all NAPTRs with identical ORDER and
identical PRIORITY field values, and process them in the
order in which they appear in the DNS response.
(There is no benefit in further randomizing the order in
which these are processed, as intervening DNS servers may
do this already).
Conversely, populating the records with these identical values is a
mistake, and so it is RECOMMENDED that:
Server When populating ENUM zones with NAPTRs, ENUM zone
population systems SHOULD NOT have more than one with the
same ORDER and the same PRIORITY field values, as Clients
MAY reject the response.
There is a special case in which one could derive a set of NAPTRs
with identical ORDER/PRIORITY fields. With RFC3761, it is possible
to have more than one ENUMservice associated with a single NAPTR. Of
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course, the different ENUMservices share the same RegExp field and so
generate the same URI. Such a "compound ENUMservice" NAPTR could
well be used to indicate, for example, a mobile phone that supports
both voice:tel and sms:tel ENUMservices.
This compound NAPTR may be reconstructed into a set of NAPTRs each
holding a single ENUMservice. However, in this case the members of
this set all hold the same ORDER and PRIORITY field values.
In this case, it is RECOMMENDED that:
Client Clients receiving "compound" NAPTRs (i.e ones with more
than one ENUMservice) should process these ENUMservices
using a "left-to-right" sort ordering, so that the first
ENUMservice to be processed will be the leftmost one, and
the last will be the rightmost one.
Server An ENUM zone population system SHOULD assume that, if it
generates compound NAPTRs, the ENUMservices will be
processed in "left to right" order within such NAPTRs.
As a final point on client processing of "compound" NAPTRs, it is
quite possible that the client is incapable of processing one of the
ENUMservices indicated.
To clarify, it is RECOMMENDED that:
Client When a client encounters a "compound" NAPTR and cannot
process one of the ENUMservices within it, the client
should continue with the "next" ENUMservice within this
NAPTR's service field, discarding the NAPTR only if it
cannot handle any of the ENUMservices contained.
4.3 Processing Order value across Zones
Using a different ORDER field value in different zones is unimportant
for most queries. However, DDDS includes a mechanism for continuing
a search for NAPTRs in another zone by including a reference to that
other zone in a "non-final" NAPTR. The treatment of non-final NAPTRs
is covered in the next section, but if these are supported then it
does have a bearing on the way that ORDER and Priority field values
are processed.
Two main questions remain from the specifications in DDDS and
RFC3671:
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o if there's a different (lower) order field value in a zone
referred to by a non-final NAPTR, then does this mean that the
ENUM client discards the other NAPTRs in the referring zone?
o Conversely, if the zone referred to by a non-final NAPTR contains
entries that have a higher ORDER field value, then does the ENUM
client ignore those?
Whilst one interpretation of section 1.3 of RFC3761 is that the
answer to both questions is "yes", this is not the way that those
examples of non-final NAPTRs that do exist (and those ENUM Clients
that support them) seem to be designed.
Thus, to reflect the interpretation that is made by those systems
that have implemented non-final NAPTRs, it is RECOMMENDED that:
Server Registrants SHOULD assume that, once a non-final NAPTR has
been selected for processing, the ORDER field value in a
zone referred to by that non-final NAPTR will be considered
only within the context of that referenced zone (i.e. the
ORDER value will be used only to sort within the referenced
zone, and will not be used in the processing of NAPTRs in
the referring zone).
Client ENUM Clients SHOULD consider the ORDER field value only
when sorting NAPTRs within a single zone. The ORDER field
value SHOULD NOT be taken into account when processing
NAPTRs across a multi-zone ENUM query created by a chain of
non-final NAPTR references.
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5. Non-final NAPTR Processing
5.1 Non-Final NAPTRs - necessity
Consider an ENUM domainname that contains a non-final NAPTR record.
This non-final NAPTR "points to" another domain that has a set of
NAPTRs. In effect, this is similar to the non-final NAPTR being
replaced by the NAPTRs contained in the domain to which it points.
It is possible to have a non-final NAPTR in a domain that is, itself,
pointed to by another non-final NAPTR. Thus a set of domains forms a
"chain", and the list of NAPTRs considered is the set of all NAPTRs
contained in all of the domains in that chain.
For an ENUM management system to support non-final NAPTRs, it is
necessary for it to be able to analyse, validate and (where needed)
correct not only the NAPTRs in its current ENUM domain but also those
"pointed to" by non-final NAPTRs in its domain. If the domains
pointed to have non-final NAPTRs of their own, the management system
will have to check each of the referenced domains in turn, as their
contents forms part of the result of a query on the "main" ENUM
domain. The domain content in the referenced domains may well not be
under the control of the ENUM management system, and so it may not be
possible to correct any errors in those zones. This is both complex
and prone to error in the management system design, and any reported
errors in validation may well be non-intuitive for users.
For an ENUM Client to support non-final NAPTRs can also be difficult.
Processing non-final NAPTRs causes a set of sequential DNS queries
that can take an indeterminate time, and requires extra resources and
complexity to handle fault conditions like non-final loops. The
indeterminacy of response time makes ENUM supported Telephony
Applications difficult (such as in an "ENUM-aware" PBX), whilst the
added complexity and resources needed makes support problematic in
embedded devices like "ENUM-aware" mobile phones.
Given that, in principle, a non-final NAPTR can be replaced by the
NAPTRs in the domain to which it points, support of non-final NAPTR
is not needed and non-final NAPTRs may not be useful.
Furthermore, most existing ENUM clients do not support non-final
NAPTRs and ignore them if received. To avoid interoperability
problems, some kind of acceptable requirement is needed on non-final
NAPTRs. Given the lack of current support and the issues raised, we
propose that one SHOULD NOT use non-final NAPTRs in ENUM.
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Thus, it is RECOMMENDED that:
Server ENUM zone population systems SHOULD NOT generate non-final
NAPTRs (i.e. NAPTRs with an empty flags field).
Client ENUM clients MAY discard non-final NAPTRs (i.e. they MAY
only support ENUM NAPTRs with a flags field of "u").
5.2 Non-Final NAPTRs - future implementation
The following specific issues need to be considered if non-final
NAPTRs are to be supported in the future. These issues are gleaned
from experience, and indicate the kinds of conditions that should be
considered before support for non-final NAPTRs is contemplated. Note
that these issues are in addition to the point just mentioned on ENUM
provisioning or management system complexity and the potential for
that management system to have no control over the zone contents to
which non-final NAPTRs in "its" zones refer.
5.2.1 Non-Final NAPTRs - general
To clarify, if all NAPTRs in a domain traversed as a result of a
reference in a non-final NAPTR have been discarded, then the Client
should continue its processing with the next NAPTR in the zone
including the non-final NAPTR that caused the traversal.
A non-final NAPTR in one zone refers to the NAPTRs contained in
another zone. The NAPTRs in the zone referred to by the non-final
NAPTR may have a different ORDER value from that in the referring
non-final NAPTR. See section 4.3 for details.
5.2.2 Non-Final NAPTRs - loop detection and response
Where a "chain" of non-final NAPTRs refers back to a domain already
traversed in the current query, this implies a "non-final loop". To
ensure consistent behaviour, it is RECOMMENDED that:
Client Clients SHOULD consider processing more than 5 "non-final"
NAPTRs in a single ENUM query to indicate that a loop may
have been detected, and act accordingly.
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Server When populating a set of domains with NAPTRs, one should
not configure non-final NAPTRs so that more than 5 such
NAPTRs will be processed in an ENUM query.
Client Where a domain is about to be entered as the result of a
reference in a non-final NAPTR, and the client has detected
a potential "non-final loop", then the Client should
discard the non-final NAPTR from its processing and
continue with the next NAPTR in its list.
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6. DNS record size and DNS software issue
An ENUM RRset may be large. The size of an ENUM DNS response may
easily exceed the 512 octet limit for "basic" UDP-based DNS messages.
In this case, a basic DNS response will be truncated (indicated by
the "TC" flag being set to '1' in the response).
The full DNS content can be carried only in TCP, or UDP packets where
the Client has indicated a suitably large packet size with EDNS0
[15].
Limiting ENUM RRSet size is not easily possible in ENUM, as the
Registrant has control of the RRSet held in "their" zone, not the
Authoritative Name Server or the ENUM Client.
If the Registrant chooses to hold a large set of NAPTRs in their
zone, then all entities involved in ENUM, (ENUM clients, their ISP's
recursive resolver/cache servers, and the authoritative servers that
hold ENUM RRSets) MUST support TCP DNS queries and responses (or
EDNS0 with a suitably large packet size specification) or they cannot
resolve ENUM requests.
The DNS protocol is defined in [8][9] and clarified in [16], whilst
Requirements for Internet Hosts are specified in [17].
Supporting UDP queries is mandatory, but support for TCP queries is
recommended also, and is (in effect) required as RFC1123 requires
that a DNS client discard a truncated response and retry using
another protocol. In effect, Authoritative name servers which do not
answer TCP queries after returning truncated responses are
misconfigured.
Given that an ENUM query is likely to return a large RRset that might
cause a truncated response using the standard DNS transport, lack of
TCP and/or EDNS0 support for the entities involved in an ENUM query
may trigger some problems that can be difficult to determine and
resolve. For more details of the issues involved, see [21].
As the normal process for a DNS Client is to make a TCP query only
once a truncated response has been received, and there may be issues
with the overall performance of the DNS when TCP is used for queries,
it may be worthwhile for Clients (both the ENUM Client and any
Recursive Resolver they use) to support EDNS0 and indicate their
ability to receive larger response packets from DNS servers. Note
that this is not necessarily within the control of the ENUM Client,
as they may well not have control over the Recursive Resolver (e.g.
the ISP's Cache Server) they use and its capabilities.
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Thus, it is RECOMMENDED that (in keeping with the standards):
Server ENUM zone population systems SHOULD be careful of the DNS
response size required to support the ENUM RRSet when
writing NAPTRs. It may be apropriate to warn the
Registrant that by adding more NAPTRs they will slow down
resolution of ENUM client queries.
Server Authoritative servers SHOULD support TCP queries and EDNS0.
Client Recursive Resolvers (e.g. ISP's DNS Cache Servers) SHOULD
support TCP queries and EDNS0.
Client ENUM clients SHOULD support TCP queries and EDNS0 queries.
As a final comment, from our experiences a Name Server may support
TCP queries, but there may well be an intervening packet filter that
does not allow TCP traffic to pass correctly. It is unfortunately
common for people managing a firewall to block traffic to or from the
DNS TCP port without considering the impact. Thus if TCP queries do
not seem to work, it is worthwhile considering this possibility; the
Name Server may be operating correctly, but the TCP SYN or SYN-ACK
packets may be blocked, effectively disabling the server from contact
with the World outside the firewall. This can be very hard to debug.
Thus, although it should be obvious, we RECOMMEND that:
MidBox Name Servers SHOULD support TCP queries. Thus intermediate
systems such as firewalls SHOULD NOT be configured to
filter traffic to or from a Name Server. This is
particularly important for Servers hosting ENUM data, as
these may well need to support TCP queries to operate
correctly
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7. Backwards Compatibility
7.1 Service field syntax
RFC3761 is the current standard for the syntax for NAPTRs supporting
the ENUM DDDS application. This obsoletes the original specification
that was given in RFC2916. There has been a change to the syntax of
the services field of the NAPTR that reflects a refinement of the
concept of ENUM processing.
As defined in RFC3403, there is now a single identifier that
indicates the DDDS Application. In the obsolete specification
(RFC2915), there were zero or more "Resolution Service" identifiers
(the equivalent of the DDDS Application). The same identifier string
is defined in both RFC3761 and in the old RFC2916 specifications for
the DDDS identifier or the Resolution Service; "E2U".
Also, RFC3761 defines at least one but potentially several
ENUMservice sub-fields; in the obsolete specification, only one
"protocol" sub-field was allowed.
In many ways, the most important change for implementations is that
the order of the sub-fields has been reversed. RFC3761 specifies
that the DDDS Application identifier is the leftmost sub-field,
followed by one or more ENUMservice sub-fields, each separated by the
'+' character delimiter. RFC2916 specified that the protocol sub-
field was the leftmost, followed by the '+' delimiter, in turn
followed by the "E2U" resolution service tag.
RFC2915 and RFC2916 have been obsoleted by RFC3401-RFC3404 and by
RFC3761. Thus it is RECOMMENDED that:
Server ENUM zone population systems MUST NOT generate NAPTRs
according to the syntax defined in RFC2916. All zones MUST
hold ENUM NAPTRs according to RFC3761 (and ENUMservice
specifications according to the framework specified there).
However, RFC3824 [18] suggests that clients SHOULD be prepared to
accept NAPTRs with the obsolete syntax. Thus, a Client
implementation may have to deal with both forms.
It is RECOMMENDED that:
Client Clients MUST support ENUM NAPTRs according to RFC3761
syntax. Clients and SHOULD also support ENUM NAPTRs
according to the obsolete syntax of RFC2916; there are
still zones that hold "old" syntax NAPTRs.
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This need not be difficult. For example, an implementation could
process the services field into a set of tokens, and expect exactly
one of these tokens to be "E2U". In this way, the client might be
designed to handle both the old and the current forms without added
complexity.
There is one subtle implication of this scheme. It is RECOMMENDED
that:
Spec Registrations for an ENUMservice with the type string of
"E2U" and an empty sub-type string MUST NOT be accepted.
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8. Security Considerations
This document does not specify any standard. It does however make
some recommendations, and so the implications of following those
suggestions have to be considered.
In addition to these issues, those in the basic use of ENUM (and
specified in the normative documents for this protocol) should be
considered as well; this document does not negate those in any way.
The clarifications throughout this document are intended only as
that; clarifications of text in the normative documents. They do not
appear to have any security implications above those mentioned in the
normative documents.
The suggestions in sections 3, 4, and 7 do not appear to have any
security considerations (either positive or negative).
The suggestions in section 5.2.2 are a valid approach to a known
security threat. It does not open an advantage to an attacker in
client excess processing or memory usage. It does, however, mean
that a client will traverse a "tight loop" of non-final NAPTRs in two
domains 5 times before the client detects this as a loop; this does
introduce slightly higher processing load that would be provided
using other methods, but avoids the risks they incur.
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9. IANA Considerations
This document is informational only, and does not include any IANA
considerations other than the suggestion at the end of section 7.1
that no-one should specify an enumservice with the identifying tag
"E2U".
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10. Acknowledgements
We would like to thank the various development teams who implemented
ENUM (both creation systems and clients) and who read the normative
documents differently - without these differences it would have been
harder for us all to develop robust clients and suitably conservative
management systems. We would also thank those who allowed us to
check their implementations to explore behaviour; their trust and
help were much appreciated.
In particular, thanks to Richard Stastny for his hard work on a
similar task TS 102 172 [22] under the aegis of ETSI, and for
supporting some of the ENUM implementations that exist today.
Finally, thanks for the dedication of Michael Mealling in giving us
such detailed DDDS specifications, without which the ENUM development
effort would have had a less rigorous framework on which to build.
This document reflects how complex a system it is - without the
intricacy of RFC3401-RFC3404 and the work that went into them, it
could have been quite different.
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11. References
11.1 Normative References
[1] Faltstrom, P. and M. Mealling, "The E.164 to Uniform Resource
Identifiers (URI) Dynamic Delegation Discovery System (DDDS)
Application (ENUM)", RFC 3761, April 2004.
[2] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Three: The Domain Name System (DNS) Database", RFC 3403,
October 2002.
[3] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
One: The Comprehensive DDDS", RFC 3401, October 2002.
[4] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Two: The Algorithm", RFC 3402, October 2002.
[5] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Four: The Uniform Resource Identifiers (URI)", RFC 3404,
October 2002.
[6] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Five: URI.ARPA Assignment Procedures", RFC 3405, October 2002.
[7] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
STD 63, RFC 3629, November 2003.
[8] Mockapetris, P., "DOMAIN NAMES - CONCEPTS AND FACILITIES",
RFC 1034, November 1987.
[9] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[10] Costello, A., "Punycode: A Bootstring encoding of Unicode for
Internationalized Domain Names in Applications (IDNA)",
RFC 3492, March 2003.
[11] Institute of Electrical and Electronics Engineers, "Information
Technology - Portable Operating System Interface (POSIX) - Part
2: Shell and Utilities (Vol. 1)", IEEE Standard 1003.2,
January 1993.
[12] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC 3966,
December 2004.
[13] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", RFC 3986,
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January 2005.
[14] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
[15] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671,
August 1999.
[16] Elz, R. and R. Bush, "Clarifications to the DNS Specification",
RFC 2181, July 1997.
[17] Braden, R., "Requirements for Internet Hosts -- Application and
Support", RFC 1123, October 1989.
[18] Peterson, J., Liu, H., Yu, J., and B. Campbell, "Using E.164
numbers with the Session Initiation Protocol (SIP)", RFC 3824,
June 2004.
[19] ITU-T, "The International Public Telecommunication Number
Plan", Recommendation E.164, May 1997.
11.2 Informative References
[20] American National Standards Institute, "Coded Character Set --
7-bit American Standard Code for Information Interchange",
ANSI X3.4, 1986.
[21] Fujiwara, K., Toyama, K., Ishibashi, K., and C. Yoshimura, "DNS
authoritative server misconfiguration and a countermeasure in
resolver", draft-fujiwara-dnsop-bad-dns-auth-02.txt (work in
progress), February 2005.
[22] ETSI, "Minimum Requirements for Interoperability of European
ENUM Implementations", ETSI TS 102 172, October 2004.
[23] Bradner, S., "The Internet Standards Process -- Revision 3",
RFC 2026, BCP 9, October 1996.
[24] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, BCP 14, March 1997.
[25] Bradner, S., "IETF Rights in Contributions", BCP 78, RFC 3978,
March 2005.
[26] Bradner, S., "Intellectual Property Rights in IETF Technology",
BCP 79, RFC 3979, March 2005.
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Authors' Addresses
Lawrence Conroy
Roke Manor Research
Roke Manor
Old Salisbury Lane
Romsey
United Kingdom
Phone: +44-1794-833666
Email: lwc@roke.co.uk
URI: http://www.sienum.co.uk
Kazunori Fujiwara
Japan Registry Service Co., Ltd.
Chiyoda First Bldg. East 13F
3-8-1 Nishi-Kanda Chiyoda-ku
Tokyo 101-0165
JAPAN
Email: fujiwara@jprs.co.jp
URI: http://jprs.jp/en/
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