One document matched: draft-ietf-enum-softswitch-req-02.txt
Differences from draft-ietf-enum-softswitch-req-01.txt
ENUM J. Lim
Internet-Draft W. Kim
Intended status: Informational C. Park
Expires: October 30, 2008 NIDA
L. Conroy
RMRL
April 28, 2008
ENUM-based Softswitch Requirement
<draft-ietf-enum-softswitch-req-02.txt>
Status of this Memo
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Abstract
This document describes experiences of operational requirements and
several considerations for ENUM-based softswitches concerning call
routing between two Korean VoIP carriers, gained during the ENUM pre-
commercial trial hosted by National Internet Development Agency of
Korea (NIDA) in 2006.
These experiences show that an interim solution can maintain the
stability of on-going commercial softswitch system operations during
the initial stage of ENUM service, where the DNS does not have
sufficient data for the majority of calls.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Call Routing on Softswitch . . . . . . . . . . . . . . . . . . 4
3. Infrastructure ENUM trial in Korea . . . . . . . . . . . . . . 5
4. Requirement for ENUM-based Softswitches . . . . . . . . . . . 6
4.1. Call routing cases for DNS response codes . . . . . . . . 6
4.2. Call routing cases for domainparts . . . . . . . . . . . . 7
5. Trial Results . . . . . . . . . . . . . . . . . . . . . . . . 9
6. 'e164.arpa' considerations . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
10.1. Normative References . . . . . . . . . . . . . . . . . . . 14
10.2. Informative References . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . . . . 17
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1. Introduction
ENUM[RFC3761] is a mapping system based on DNS[RFC1034] [RFC1035]
that converts from an E.164[E164] number to a domain name using the
Naming Authority Pointer (NAPTR) [RFC3403] resource record type.
ENUM is able to store different service types (such as fax, email,
homepage, SIP, H.323 and so on), for every E.164 number. It
originally focused on how end-users could gain access to other end-
user's communications contact information through the Internet.
Recently, discussion on the need to update RFC 3761 has begun, to
ensure that the standard also works in the "Infrastructure ENUM"
scenario, where ENUM provides routing information between carriers.
This has resulted in two documents, the updated ENUM specification
[draft-rfc3761bis] and an Enumservice guide
[draft-enumservice-guide].
When providing VoIP service, a VoIP carrier that wants to integrate
various protocols typically uses a softswitch. However, such a
system is still inefficient for interconnection, number portability,
and sharing protocol support information among carriers, because each
softswitch does not have complete end-to-end routing information for
all carriers. This information can be stored in DNS, using ENUM.
Therefore, carriers can expect to gain many advantages if they use
ENUM within the call routing functions of their softswitches.
To confirm these benefits and to verify the performance of ENUM-
enabled softswitches, NIDA cooperated with two Korean VoIP service
providers for an Infrastructure ENUM trial in 2006. NIDA is a non-
profit organization with a mandate to manage 2.8.e164.arpa.
(representing the +82 country code of Korea). NIDA promotes and
facilitates technical cooperation on a national scale between
partners, and this includes ENUM. During the trial, NIDA provided a
centralized ENUM DNS to each VoIP service provider for call routing.
The data used in this Infrastructure trial was also accessible to the
public (i.e. it was an Internet-based system, rather than a closed
scheme).
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2. Call Routing on Softswitch
In the PSTN (Public Switched Telephone Network), hardware-based
switches predominate in the network. A softswitch provides similar
functionality, but is implemented on a computer system by software.
It typically has to support various signalling protocols (such as
SIP[RFC3261], H.323[H323], MGCP[RFC3435], and others), to make call
connections for VoIP service, often on the boundary point between the
circuit and packet network.
To make a call, first of all a softswitch must discover routing
information. It has to process the E.164 number that comes from a
caller through its own routing table. The goal is to determine how
the call can be routed towards the callee, so that either the call
can be processed through the softswitch or the caller can connect to
the callee directly.
Today, call routing is often based on a prefix of the dialled number.
This is used very widely not only for legacy PSTN switches, but also
for softswitches. So, if a softswitch exclusively uses ENUM DNS for
call routing, then in the beginning most of the calls queried to ENUM
DNS would fail if there are only a small group of carriers
provisioning data into ENUM. However a softswitch will have a higher
success rate with ENUM DNS as the number of carriers grows, and so
the overall percentage of numbers provisioned in ENUM increases. In
short, ENUM as a long term solution has obvious benefits, but the
problem lies in migrating from today's prefix based routing towards
that long term ENUM-based solution.
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3. Infrastructure ENUM trial in Korea
As described in Section 1, NIDA and two VoIP Service Providers built
ENUM-processing modules into their softswitches, interconnected these
via the IP network, and provisioned their trial users' numbers into a
centralized ENUM DNS system operated by NIDA. The carriers
provisioned their E.164 numbers using EPP [RFC4114] to a centralized
Registration Server (also operated by NIDA). Changes to the ENUM
data were implemented and updated to the ENUM DNS instantly, using
DNS Dynamic Update [RFC2136].
In the trial, the EPP-based provisioning sub-system was developed and
operated separately from the carriers' main customer provisioning
systems and protocols. It was not integrated as the carriers already
operated their own customer provisioning systems that were totally
different from the EPP-based model, and (as mission-critical
components) those were not open to modification.
Call routing
+---------------------------------------------+
| |
| |
+-----+------+ +-----------------+ +------+-----+
|Softswitch A|------| ENUM DNS(+82) |------|Softswitch B|
+-----+------+ | (Tier1,2) | +------+-----+
| +--------+--------+ |
+-----+------+ | +------+-----+
| IP Phone A | |Dynamic update | IP Phone B |
+------------+ |(RFC 2136) +------------+
|
+------------+ +--------+--------+ +------------+
| EPP Client | | Registration | | EPP Client |
| |------| server |------| |
+------------+ +-----------------+ +------------+
Provisioning E.164 Numbers(RFC 4114)
Carrier A NIDA Carrier B
Figure 1: Infrastructure ENUM Trial System Topology
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4. Requirement for ENUM-based Softswitches
4.1. Call routing cases for DNS response codes
To use ENUM DNS, each softswitch needs to have an ENUM module that
converts from an E.164 number to the ENUM domain name (as defined in
RFC 3761) and processes a query to ENUM DNS. This ENUM module uses
the algorithm specified in RFC 3761.
However, in the initial stage of ENUM DNS roll-out, ENUM shares call
routing information from a limited number of carriers. There is the
problem that a softswitch can't find all of the call routing
information it needs just using ENUM. To solve this problem, ENUM-
based softswitches have to follow a consistent set of rules. In the
Korean trial, the rules were:
1. The ENUM module of the softswitch converts an E.164 number coming
from the VoIP subscriber to an ENUM domain name (as defined in
RFC 3761).
2. The ENUM module, acting as a DNS stub resolver, sends a query to
a recursive name server.
3. if the ENUM module receives a DNS answer, the call routing
process may branch off in several ways, depending on the Rcode
value in the DNS response message, as shown below.
A. Rcode=0 (No error condition)
There is, potentially, an answer to the corresponding query.
The normal call routing process needs to differentiate
between the following conditions:
+ The response includes no URI that can be used to initiate
a call (such as SIP or H.323) -
The call fails immediately.
Note: A domain that DOES exist but DOES NOT include any
supported NAPTRs was used to indicate that the telephone
number could ONLY be reached by the Internet. A
softswitch receiving this response meant that the call
could be dropped immediately - it would fail if passed to
the PSTN.
+ There is at least one usable URI (such as SIP and/or H.323
URIs) -
The softswitch picks one based on the preference and order
values in the NAPTR Resource Record Set, and routes the
call using the method described in Section 4.2.
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B. Rcode=3 (Name error), 1 (Format Error), 2 (Server Failure), 4
(Not Implemented) or 5 (Refused)
There is no valid answer for the query.
The softswitch has no choice but to route the call using the
E.164 number with its vendor-specific method (such as a
prefix-based method). In this case, it means that the call
has to be delivered through the PSTN for onward call routing.
It is also important to stress that the ENUM DNS servers MUST
respond to all queries they receive from the softswitches.
If the ENUM module in a softswitch does not receive a
response, it will eventually time out, and the ENUM module
will treat this as a DNS error. However, the delay involved
is long in terms of the normal call setup time, and should be
avoided.
4.2. Call routing cases for domainparts
If the DNS response has a valid URI such as SIP or H.323, the
softswitch can resolve the domain name part of that URI to route a
call by searching two different sources. One is a recursive
nameserver, and the other is a fixed routing table held in the
softswitch, mapping from the domain name to the corresponding
gateway's host name and IP address.
If there are many points of interconnection, using a recursive
nameserver is useful for resolving a domain name, but if there are
just a few known carriers and they do not change this interconnection
information frequently, a fixed (internal) routing table mapping from
domain name to the corresponding gateway hostname and IP address is
more efficient (rather than querying the recursive nameserver every
time). In addition, carriers would like to charge an interconnection
fee for all received calls, so they tend to make interconnection only
with trusted carriers based on some sort of bilateral agreement
between these carriers. They may agree on a specific gateway for
this purpose, so the interconnection information is often private to
the parties of this particular agreement.
These two types of domain routing are also affected by the Rcode=0
case described in Section 4.1.
There are two choices for routing. These are described and compared
here:
1. Case when using a fixed routing table:
A. If the domain name part of the URI is found in the internal
fixed routing table, the softswitch can use it.
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B. If the domain name part of URI does not exist in the fixed
routing table, the call is forwarded to the PSTN.
2. Case when using a recursive nameserver:
A. If the domain name part of the URI can be resolved via the
recursive nameserver, the softswitch can use it.
B. If the domain name part of the URI cannot be resolved on the
recursive nameserver for any reason (such as a response with
no usable resource records according to [RFC3263] and
[RFC3261], or with Rcode=1, 2, 3, 4, or 5), the call must be
forwarded to the PSTN.
Case (1) seems inefficient because the administrator maintains two
management points for numbers; the ENUM DNS and the softswitch
itself. However this configuration can minimize the call routing
failure ratio during the transition period of ENUM (when there are
relatively few provisioned ENUM entries and so few IP-based Points Of
Interconnection). Thus case (1) could reasonably be implemented on
the softswitches during the trial phase, and hereafter as ENUM
entries are populated, case (2) would be a reasonable choice.
With these choices, the two carriers could use ENUM DNS for call
routing without any impact on their on-going commercial VoIP service.
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5. Trial Results
To provide a stable commercial service, an ENUM-based softswitch must
have a defined performance, in the same way as must any non-ENUM-
based softswitch. The only difference between these two types of
softswitch is the searching mechanism for call routing information,
which can be stored in the softswitch itself or in the DNS.
Therefore a similar delay time for call routing is important to
guarantee quality of service. During the trial, each carrier
measured this delay time when using the SIP protocol. This was based
on the "Answer Delay time", defined as the elapsed time between
requesting a call ('INVITE' message) and receiving a response ('200
OK' message) [RFC3261].
+------------------------+------+----------+
| Call Type | ENUM | Non-ENUM |
+------------------------+------+----------+
| Carrier A->A | 2.33 | 2.28 |
| | | |
| Carrier A->B | 2.23 | 2.25 |
| | | |
| Carrier A->other(PSTN) | 4.11 | 3.79 |
| | | |
| Carrier B->B | 2.18 | 2.05 |
| | | |
| Carrier B->A | 2.19 | 2.19 |
| | | |
| Carrier B->other(PSTN) | 3.95 | 3.41 |
+------------------------+------+----------+
Table 1: Average Answer Delay time (sec)
As shown in Table 1, there is little difference in time (under a
second) between the ENUM and non-ENUM cases. Therefore it is
difficult for a caller with either carrier to detect the choice (ENUM
or non-ENUM) as an aspect of quality when a call initiates. This
means that ENUM definitely works well with softswitches on a
commercial basis.
To make the trial more realistic, the resolver that was used by these
ENUM-based softswitches was a recursive nameserver that could be
accessed publicly. This was done as it was felt that a tough
condition would be better to verify the fact that an ENUM-based
softswitch works as well as non-ENUM-based softswitch in providing a
commercial VoIP service.
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6. 'e164.arpa' considerations
During the trial, the Infrastructure ENUM deployed in the
2.8.e164.arpa zone could be accessed via the (public) Internet. In
this situation, each carrier questioned whether the centralized
number management under the ENUM DNS was realistic or not.
Another issue concerned responsibility for routing errors. All
carriers can use the shared ENUM data to route their calls. However,
if there are routing errors (due to the data being provisioned
incorrectly), it is not always clear who has responsibility for these
errors and who can correct the data. The errors occur in the
networks of the carriers placing the calls. Unless the identity of
the carrier responsible for delivering service to this telephone
number is known, it is not obvious (to the carrier handling the
error) who should be informed of these problems. This is a
particular issue when number portability is introduced.
In addition, the carriers also question whether or not Infrastructure
ENUM needs to be accessible publicly. To prevent disclosure of
telephone numbers, they would prefer to access the ENUM DNS
privately. Therefore any ENUM module embedded in a softswitch needs
to be flexible, to adopt these considerations during the interim
period of ENUM, before common policies and agreements have been
forged.
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7. Security Considerations
This document inherits the security considerations described in RFC
3761 and 'draft-ietf-enum-infrastructure-05.txt'
[draft-enum-infrastructure], as the ENUM DNS used with softswitches
in this trial could be accessed publicly.
In addition, if the recursive resolvers handling ENUM queries coming
from a softswitch were to be compromised by an attacker, that
attacker would be able to force calls to fail or cause delay to
calls. Therefore, the DNS resolvers used should allow access from
the local network to which the softswitch is connected, whilst
restricting access from outside, using a proper access-list policy.
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8. IANA Considerations
This document is only advisory, and does not have any IANA
considerations.
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9. Acknowledgements
Thanks to Richard Shockey, Jason Livingood, Karsten Fleischhauer, Jim
Reid and Otmar Lendl who helped guide the direction of this document.
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10. References
10.1. Normative References
[E164] ITU-T, "The International Public Telecommunication Number
Plan", Recommendation E.164, February 2005.
[RFC1034] Mockapetris, P., "DOMAIN NAMES - CONCEPTS AND FACILITIES",
RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC3403] Mealling, M., "Dynamic Delegation Discovery System (DDDS)
Part Three: The Domain Name System (DNS) Database",
RFC 3403, October 2002.
[RFC3761] Faltstrom, P. and M. Mealling, "The E.164 to Uniform
Resource Identifiers (URI) Dynamic Delegation Discovery
System (DDDS) Application (ENUM)", RFC 3761, April 2004.
10.2. Informative References
[H323] ITU-T, "Packet-based multimedia communications systems",
Recommendation H.323, 2003.
[RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, April 1997.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3263] Rosenberg, J., "Session Initiation Protocol (SIP):
Locating SIP Servers", RFC 3263, June 2002.
[RFC3435] Andreasen, F. and B. Foster, "Media Gateway Control
Protocol (MGCP) Version 1.0", RFC 3435, January 2003.
[RFC4114] Hollenbeck, S., "E.164 Number Mapping for the Extensible
Provisioning Protocol (EPP)", RFC 4114, June 2005.
[draft-enum-infrastructure]
Livingood, J., Pfautz, P., and R. Stastny, "The E.164 to
Uniform Resource Identifiers (URI) Dynamic Delegation
Discovery System (DDDS) Application for Infrastructure
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ENUM", draft ietf-enum-infrastructure-05.txt (work in
progress), January 2007.
[draft-enumservice-guide]
Hoeneisen, B., Mayrhofer, A., and J. Livingood, "Guide and
Template for IANA Registrations of Enumservices",
draft-ietf-enum-enumservices-guide-06.txt (work in
progress), November 2007.
[draft-rfc3761bis]
Bradner, S., Conroy, L., and K. Fujiwara, "The E.164 to
Uniform Resource Identifiers (URI) Dynamic Delegation
Discovery System (DDDS) Application (ENUM)",
draft-ietf-enum-rfc3761bis-03.txt (work in progress),
February 2008.
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Authors' Addresses
JoonHyung Lim
National Internet Development Agency of Korea(NIDA)
3F. KTF B/D 1321-11, Seocho-dong, Seocho-gu
Seoul
Korea
Phone: +82-2-2186-4548
Email: jhlim@nida.or.kr
URI: http://www.nida.or.kr
Weon Kim
National Internet Development Agency of Korea(NIDA)
3F. KTF B/D 1321-11, Seocho-dong, Seocho-gu
Seoul
Korea
Phone: +82-2-2186-4502
Email: wkim@nida.or.kr
URI: http://www.nida.or.kr
ChanKi Park
National Internet Development Agency of Korea(NIDA)
3F. KTF B/D 1321-11, Seocho-dong, Seocho-gu
Seoul
Korea
Phone: +82-2-2186-4504
Email: ckp@nida.or.kr
URI: http://www.nida.or.kr
Lawrence Conroy
Roke Manor Research
Roke Manor
Old Salisbury Lane
Romsey
United Kingdom
Phone: +44-1794-833666
Email: lconroy@insensate.co.uk
URI: http://www.sienum.co.uk
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