Network Working Group P Faltstrom
Internet-Draft Tele2
Expires: July 25, 2000 January 25, 2000
E.164 number and DNS
draft-faltstrom-e164-05.txt
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This document discusses the use of DNS for storage of E.164 numbers.
More specifically, how DNS can be used for identifying available
services connected to one E.164 number. Routing of the actual
connection using the service selected using these methods is not
discussed.
Discussion on this Internet-Draft is to be held on the mailing list
ietf-e164-dns@imc.org, which is hosted by the Internet Mail
Consortium. To subscribe, send an email to
ietf-e164-dns-request@imc.org, with the text "subscribe" as the only
word in the body of the mail. There is an archive of the mailing
list at <http://www.imc.org/ietf-e164-dns/>.
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1. Introduction
The NAPTR[1] records in DNS[3] can be used for looking up what
services are available for a specific domainname. This technology is
used for finding what services exists given an E.164 number.
1.1 Terminology
"Must" or "Shall" - Software that does not behave in the manner that
this document says it must is not conformant to this document.
"Should" - Software that does not follow the behavior that this
document says it should may still be conformant, but is probably
broken in some fundamental way.
"May" - Implementations may or may not provide the described
behavior, while still remaining conformant to this document.
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2. E.164 numbers and DNS
The domain "e164.int." is being populated in order to provide the
infrastructure in DNS for storage of E.164 numbers. In order to
facilitate distributed operations, this domain is divided into
subdomains. Holders of E.164 numbers which want to be listed in DNS
should contact the appropriate zone administrator in order to be
listed, by examining the SOA resource record associated with the
zone, just like in normal DNS operations.
To find the DNS names for a specific E.164 number, the following
procedure is to be followed:
1. See that the E.164 number is written in its full form, including
the countrycode IDDD. Example: +46-8-56264000.
2. Remove all characters part from the digits. Example: 46856264000
3. Put dots (".") between each digit. Example:
4.6.8.5.6.2.6.4.0.0.0
4. Change the order of the digits. Example: 0.0.0.4.6.2.6.5.8.6.4
5. Append the domain "e164.int" to the end. Example:
0.0.0.4.6.2.6.5.8.6.4.e164.int
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3. Identifying available services
For a record in DNS, the NAPTR record is used for identifying
available ways of contacting a specific node identified by that
name. Specifically it can be used for knowing what services exists
for a specific domainname, including phone numbers by the use of the
e164.int domain as described above.
The identification is using the NAPTR resource record defined for
use in the URN resolution process, but it can be generalized in a
way that suits the needs specified in this document.
3.1 The NAPTR record
The key fields in the NAPTR RR are order, preference, service,
flags, regexp, and replacement. For a detailed description, see:
o The order field specifies the order in which records MUST be
processed when multiple NAPTR records are returned in response to
a single query.
o The preference field specifies the order in which records SHOULD
be processed when multiple NAPTR records have the same value of
"order".
o The service field specifies the resolution protocol and
resolution service(s) that will be available if the rewrite
specified by the regexp or replacement fields is applied.
o The flags field contains modifiers that affect what happens in
the next DNS lookup, typically for optimizing the process.
o The regexp field is one of two fields used for the rewrite rules,
and is the core concept of the NAPTR record.
o The replacement field is the other field that may be used for the
rewrite rule.
Note that the client applies all the substitutions and performs all
lookups, they are not performed in the DNS servers. Note also that
it is the belief that regexps should rarely be used. The replacement
field seems adequate for the vast majority of situations.
3.1.1 Specific use of some fields in the NAPTR record
The flags can be "s" or "a" for the next step in the resolution
process described in this document. "s" flag means that the next
lookup should be for SRV records, and "a" that the result of the
rewrite is a URI. Other flags are the "a" and the "p" flags.
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The service supported for a call must be N2R.
3.1.2 Example of use
tele2.se.
;; ord pr fl service re replacement
IN NAPTR 100 10 "a" "sip+N2R" "" "sip:information@tele2.se"
IN NAPTR 102 10 "a" "smtp+N2R" "" "mailto:information@tele2.se"
This describes that the domain tele2.se is preferrable contacted via
the SIP protocol, secondly via SMTP (for VPIM voicemail over SMTP
for example).
In both cases, the next step in the resolution process is to use the
resolution mechanism for each of the protocols, (SIP and SMTP) to
know what node to contact for each.
3.1.3 When the virtual address is a phone number
When the target address is a phone number, it is first translated
into a RR name in the e164.int domain according to the method
described above.
Example:
2.8.0.4.6.2.6.5.8.6.4.e164.int.
IN NAPTR 10 10 "a" "sip+N2R" "" "sip:paf@swip.net".
IN NAPTR 102 10 "s" "potscall+N2R" "" _potscall._tcp.paf.swip.net.
IN NAPTR 102 10 "a" "smtp+N2R" "" "mailto:paf@swip.net".
Note that the prefered method is to use the SIP protocol, but the
result of the rewrite of the NAPTR record is a URI (the "a" flag in
the NAPTR record). In the case of the protocol SIP, the URI might be
a SIP URI, which is resolved as described in RFC 2543[4].
The rest of the resolution of the routing is done as described
above.
3.1.4 The potscall protocol name
The potscall protocol name is just a placeholder so one knows that
the protocol to use is plain old telephony. Because the protocol is
not run on top of IP, the address to use when addressing the endnode
has to be a phone number. This address is given back when looking up
the SRV record for the _potscall._tcp service in the given domain.
Example:
_potscall._tcp.paf.swip.net.
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IN SRV 10 10 2.8.0.4.6.2.6.5.8.6.4.e164.int.
IN SRV 20 10 0.0.0.4.6.2.6.5.8.6.4.e164.int.
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4. IANA Considerations
IANA is to allocate the protocol name "potscall" as a placeholder
for a protocol name in the SRV record type. No portnumber have to be
allocated for this protocol name.
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5. Security Considerations
As this system is built on top of DNS, one can not be sure that the
information one get back from DNS is more secure than any DNS query.
To solve that, the use of DNSSEC for securing and verifying zones is
to be recommended.
The caching in DNS can make the propagation time for a change take
the same amount of time as the time to live for the NAPTR and SRV
records in the zone that is changed. The TTL should because of that
be kept to a minimum. The use of this in an environment where
IP-addresses are for hire (i.e. DHCP) must therefore be done very
carefully.
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6. Acknowledgement
I thank the people at Ericsson, especially Bjorn Larsson, for
support and ideas, and especially the group which implemented this
scheme in their lab to see that it worked. I also thank the people
of ITU-T SG2, Working Party 1/2 (Numbering, Routing, Global Mobility
and Service Definition) for comments, and Leif Sunnegardh at Tele2
for information about how SS7 really works.
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References
[1] Mealling, M and R Daniel, "The Naming Authority Pointer (NAPTR)
DNS Resource Record", Internet-Draft
draft-ietf-urn-naptr-rr-03.txt, June 1998.
[2] Gulbrandsen, A and R Daniel, "A DNS RR for specifying the
location of services (DNS SRV)", Internet-Draft
draft-ietf-urn-naptr-rr-03.txt, June 1998.
[3] Mockapetris, P, "Domain names - Implementation and
Specification", RFC 1035, November 1987.
[4] Handley, M, Schulzrinne, H, Schooler, E and J Rosenberg, "SIP:
Session Initiation Protocol", RFC 2543, March 1999.
Author's Address
Patrik Faltstrom
Tele2
Borgarfjordsgatan 16
127 61 Kista
Sweden
EMail: paf@swip.net
URI: http://www.tele2.se
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Appendix A. Example SIP
Caller (A) uses a phone, connected to the PSTN network, on number
+46-8-7525252.
Callee (B) is buying a service by provider X, which is telephony
over the Internet via the use of SIP.
Callee want to get reached on the message number +46-76-11223344,
which is in this example supposed to be directed to the correct SIP
URI.
On the buissness card, the callee have printed the number
+46-76-11223344 (and probably the SIP URI
"sip:foobar@x.example.net".
Caller reads the buissness card, lifts the handle, and punches the
number +46-76-11223344.
The SCP looks up the NAPTR record in DNS for
4.4.3.3.2.2.1.1.6.7.6.4.e164.int. The DNS server for Number Inc. has
the following information in its DNS:
4.4.3.3.2.2.1.1.6.7.6.4.e164.int. IN SOA ....
IN NS ....
IN NAPTR 100 10 "a" "sip+N2R" ""sip:foobar@x.example.net".
This shows to the switch that the only way B can be contacted is via
the SIP protocol, using the URI "sip:foobar@x.example.net".
The resolution of the SIP URI, using SRV records etc, is described
in appendix D of RFC 2543.
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