One document matched: draft-ietf-sipping-v6-transition-03.txt
Differences from draft-ietf-sipping-v6-transition-02.txt
SIPPING Working Group G. Camarillo
Internet-Draft Ericsson
Updates: 3264 (if approved) K. El Malki
Expires: December 14, 2006 Athonet
V. Gurbani, Ed.
Lucent Technologies, Inc./Bell
Laboratories
June 12, 2006
IPv6 Transition in the Session Initiation Protocol (SIP)
draft-ietf-sipping-v6-transition-03.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document describes how IPv4 Session Initiation Protocol (SIP)
user agents can communicate with IPv6 SIP user agents (and vice
versa) at the signaling layer as well as exchange media once the
session has been successfully set up. Both single- and dual-stack
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(i.e., an IPv4-only and an IPv4/IPv6) user agents are considered in
this document.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The Signaling Layer . . . . . . . . . . . . . . . . . . . . . 4
3.1 Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . 4
3.1.1 Relaying Requests Across Different Networks . . . . . 5
3.2 UA Behavior . . . . . . . . . . . . . . . . . . . . . . . 8
4. The Media Layer . . . . . . . . . . . . . . . . . . . . . . . 9
4.1 Updates to RFC3264 . . . . . . . . . . . . . . . . . . . . 10
4.2 Initial Offer . . . . . . . . . . . . . . . . . . . . . . 10
4.3 Connectivity Checks . . . . . . . . . . . . . . . . . . . 11
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1 Normative References . . . . . . . . . . . . . . . . . . . 12
8.2 Informational References . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13
A. Sample IPv4/IPv6 DNS File . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . 15
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1. Introduction
SIP [3] is a protocol to establish and manage multimedia sessions.
After the exchange of signaling messages, SIP endpoints generally
exchange session, or media traffic, which is not transported using
SIP but a different protocol. For example, audio streams are
typically carried using Real-Time Transport Protocol (RTP [16]).
Consequently, a complete solution for IPv6 transition needs to handle
both the signaling layer and the media layer. While unextended SIP
can handle heterogeneous IPv6/IPv4 networks at the signaling layer as
long as proxy servers and their Domain Name Service (DNS) entries are
properly configured, user agents using different networks and address
spaces must implement extensions in order to exchange media between
them.
This document addresses the systems-level issues to make SIP work
successfully between IPv4 and IPv6. Section 3 and Section 4 provide
discussions on the topics that are pertinent to the signaling layer
and media layer, respectively, to establish a successful session
between heterogeneous IPv4/IPv6 networks.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in BCP 14, RFC 2119 [1] and indicate requirement levels for
compliant implementations.
IPv4-only user agent: An IPv4-only user agent supports SIP signaling
and media only on the IPv4 network. It does not understand IPv6
addresses.
IPv4-only node: A host that implements only IPv4. An IPv4-only node
does not understand IPv6. The installed base of IPv4 hosts existing
before the transition begins are IPv4-only nodes.
IPv6-only user agent: An IPv6-only user agent supports SIP signaling
and media only on the IPv6 network. It does not understand IPv4
addresses.
IPv6-only node: A host that implements IPv6 and does not implement
IPv4.
IPv4/IPv6 node: A host that implements both IPv4 and IPv6; such hosts
are also known as "dual-stack" hosts.
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IPv4/IPv6 user agent: An user agent that supports SIP signaling and
media on both IPv4 and IPv6 networks.
IPv4/IPv6 proxy: A proxy that supports SIP signaling on both IPv4 and
IPv6 networks.
3. The Signaling Layer
An autonomous domain sends and receives SIP traffic to and from its
user agents as well as to and from other autonomous domains. This
section describes the issues related to such traffic exchanges at the
signaling layer; i.e., the flow of SIP messages between participants
in order to establish the session. We assume that the network
administrators appropriately configure their networks such that the
SIP servers within an autonomous domain can communicate between
themselves. While this section contains systems-level issues, it
does not address implementation-specific issues (i.e., parser torture
tests). Such topics are outlined in detail in a separate document
[20].
3.1 Proxy Behavior
User agents typically send SIP traffic to an outbound proxy, which
takes care of routing it forward. In order to support both IPv4-only
and IPv6-only user agents, it is RECOMMENDED that domains deploy
dual-stack outbound proxy servers or, alternatively, deploy both
IPv4-only and IPv6-only outbound proxies. Furthermore, there SHOULD
exist both IPv6 and IPv4 DNS entries for outbound proxy servers.
This allows the user agent to query DNS and obtain an IP address most
appropriate for its use (i.e., an IPv4-only user agent will query DNS
for A resource records (RR), an IPv6-only user agent will query DNS
for AAAA RRs, and a dual-stack user agent will query DNS for all RRs
and choose a specific network.)
Some domains provide automatic means for user agents to discover
their proxy servers. It is RECOMMENDED that domains implement
appropriate discovery mechanisms to provide user agents with the IPv4
and IPv6 addresses of their outbound proxy servers. For example, a
domain may support both the DHCPv4 [14] and the DHCPv6 [13] options
for SIP servers.
On the receiving side, user agents inside an autonomous domain
receive SIP traffic from sources external to their domain through an
inbound proxy (which is sometimes co-located with the registrar of
the domain). As was the case previously, it is RECOMMENDED that
domains deploy dual-stack inbound proxies or, alternatively, deploy
both IPv4-only and IPv6-only inbound proxy servers. This allows the
user agents external to the autonomous domain to query DNS and
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receive an IP address of the inbound proxy most appropriate for its
use (i.e., an IPv4-only user agent will query DNS for A RRs, an IPv6-
only user agent will query DNS for AAAA RRs, and a dual-stack user
agent will query DNS for all RRs and choose a specific network.)
SIP uses DNS procedures to allow a proxy to resolve a SIP URI into
the equivalent IP address, port, and transport protocol of the next
hop to contact lookups [8]. When an IPv4/IPv6 proxy queries the DNS,
it gets an ordered set of IP addresses, ports, and transports to use.
If the server it is seeking is also dual-stacked, and has a DNS A and
AAAA RR, then the proxy must be prepared to contact the downstream
server on all the transports over both of the networks (i.e., IPv4
and IPv6). More specifically, when an IPv4/IPv6 proxy is prepared to
send a request to an IPv4/IPv6 next hop server, it must determine
whether to first send it to the IPv4 address of the server, or to the
IPv6 address. A technique to aid in this decision is outlined in RFC
3484 [18]. If SRV RR's have been used to derive the addresses of the
server, the "Priority" field of the SRV RR can also aid in such a
decision; however, the technique in RFC 3484 has precedence. SRV
priority levels MUST be used in addition to, but MUST NOT be used as
a replacement for RFC 3484. Regardless of which network is tried
first, each search corresponds to a different branch, and as such,
mandates that the the normative behavior in RFC3261 to handle
requests sent on a branch be followed.
Appendix A contains a sample DNS zone file entry that has been
populated with both IPv4 and IPv6 addresses.
3.1.1 Relaying Requests Across Different Networks
A SIP proxy server that receives a request using IPv6 and relays it
to a user agent (or another downstream proxy) using IPv4, and vice
versa, needs to remain in the path traversed by subsequent requests.
Therefore, such a SIP proxy server MUST be configured to Record-Route
in that situation.
Note that while this is the recommended practice, some problems
may still arise if a RFC2543 [17] endpoint is involved in
signaling. Since the ABNF in RFC2543 did not include production
rules to parse IPv6 network identifiers, there is a good chance
that a RFC2543-only compliant endpoint is not able to parse or
regenerate IPv6 network identifiers in headers. Thus, despite a
dual-stack proxy inserting itself into the session establishment,
the endpoint itself may not succeed in the signaling establishment
phase.
This is generally not a problem with RFC3261 endpoints; even if
such an endpoint runs on an IPv4-only node, it still is able to
parse and regenerate IPv6 network identifiers.
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Relaying a request across different networks in this manner has other
ramifications. For one, the proxy doing the relaying must remain in
the signaling path for the duration of the session since without it,
the upstream client and the downstream server would not be able to
communicate directly. Second, to remain in the signaling path, the
proxy MUST insert one or two Record-Route headers. If the proxy is
inserting a URI that contains a fully qualified domain name of the
proxy, then inserting one Record-Route header suffices; but if the
proxy is inserting raw IP addresses in the Record-Route header, then
it must insert two such headers. The first Record-Route header will
contain the proxy's IP address that is compatible with the network
type of the downstream server and the second Record-Route header will
contain the proxy's IP address that is compatible with the upstream
client.
An example helps illustrate this behavior. In the example, we use
only those headers pertinent to the discussion. Other headers have
been omitted for brevity. In addition, only the INVITE request and
final response (200 OK) are shown; it is not the intent of the
example to provide a complete call flow that includes provisional
responses and other requests.
In this example, proxy P, responsible for the domain example.com,
receives a request from an IPv4-only upstream client. It proxies
this request to an IPv6-only downstream server.
Proxy P is running on a dual-stack host; on the IPv4 interface, it
has an address of 192.0.2.1 and on the IPv6 interface, it sports an
address of 2001:db8::1.
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UAC Proxy UAS
| P |
| | |
+-----F1------------>| |
| +------F2------------>|
| | |
| |<-----F3-------------+
|<----F4-------------+ |
... ... ...
| | |
V V V
F1: INVITE sip:alice@example.com SIP/2.0
...
F2: INVITE sip:alice@2001:db8::10 SIP/2.0
Record-Route: <sip:2001:db8::1;lr>
Record-Route: <sip:192.0.2.1;lr>
...
F3: SIP/2.0 200 OK
Record-Route: <sip:2001:db8::1;lr>
Record-Route: <sip:192.0.2.1;lr>
...
F4: SIP/2.0 200 OK
Record-Route: <sip:2001:db8::1;lr>
Record-Route: <sip:192.0.2.1;lr>
...
When the UAS gets an INVITE and it accepts the invitation, it will
send a 200 OK (F3) and form a route set. It's route set will
include, as the first destination, the proxy's IPv6 interface.
Similarly, when the 200 OK reaches the UAC, it creates a route set by
following the guidelines of RFC3261 and reversing the Record-Route
headers. Its route set will include, as the first destination, the
proxy's IPv4 interface. In this manner, both the UAC and the UAS
will have the correct address of the proxy to which they can target
subsequent requests.
Strictly speaking, the proxy could have inserted its FQDN in the
Record-Route URI and the result would have been the same. This is
because the proxy has both IPv4 and IPv6 addresses in the DNS; thus
the URI resolution would have yielded an IPv4 address to the UAC and
an IPv6 address to the UAS.
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3.2 UA Behavior
SIP uses DNS procedures to allow a client to resolve a SIP URI into
the equivalent IP address, port, and transport protocol of the next
hop to contact lookups [8]. When an IPv4/IPv6 user agent queries the
DNS, it gets an ordered set of IP addresses, ports, and transports to
use. If the server it is seeking is also dual-stacked and has a DNS
A and AAAA RR, then the client will need to be prepared to contact
the server on all the transports over both of the networks (i.e.,
IPv4 and IPv6). More specifically, when an IPv4/IPv6 user agent is
prepared to send a request to an IPv4/IPv6 next hop server, it must
determine whether to first send it to the IPv4 address of the server,
or to the IPv6 address. A technique to aid in this decision is
outlined in RFC 3484 [18]. If SRV RR's have been used to derive the
addresses of the server, the "Priority" field of the SRV RR can also
aid in such a decision; however, the technique in RFC 3484 has
precedence. SRV priority levels MUST be used in addition to, but
MUST NOT be used as a replacement for RFC 3484. Regardless of which
network is tried first, each search corresponds to a different
branch, and as such, mandates that the normative behavior in RFC3261
to handle requests sent on a branch be followed.
When a request is to be sent on a branch, certain identifiers are
computed. Specifically, a branch ID is computed and inserted in the
topmost Via header, and certain headers are used to generate
signatures for Identity [21] and Authenticated Identity Body (AIB
[19]). The headers that are used to generate signatures can contain
either raw IP addresses or FQDNs; consequently, an implementation may
choose to insert a raw IP address in such headers. This choice has
ramifications, as discussed next.
When a request is re-targeted to a new branch because the old one did
not elicit a response, and the request is now destined to a new
network (i.e., old request went to an IPv4 downstream server while
the new one is destined towards an IPv6 downstream server) care must
be taken in re-computing the identifiers. More specifically,
implementations MUST ensure that, at the very least, the branch id is
recomputed. Additionally, if raw IP addresses were used to generate
the signatures for the Identity header and AIB, these signatures MUST
be recomputed.
To avoid recomputing the signatures used in the Identity header or
the AIB, it is RECOMMENDED that SIP user agents use FQDNs in those
headers that are used as the basis for the Identity header and the
AIB.
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4. The Media Layer
SIP establishes media sessions using the offer/answer model [4]. One
endpoint, the offerer, sends a session description (the offer) to the
other endpoint, the answerer. The offer contains all the media
parameters needed to exchange media with the offerer: codecs,
transport addresses, protocols to transfer media, etc.
When the answerer receives an offer, it elaborates an answer and
sends it back to the offerer. The answer contains the media
parameters that the answerer is willing to use for that particular
session. Offer and answer are written using a session description
protocol. The most widespread protocol to describe sessions at
present is called, aptly enough, the Session Description Protocol
(SDP[2]).
A direct offer/answer exchange between an IPv4-only user agent and an
IPv6-only user agent does not result in the establishment of a
session. The IPv6-only user agent wishes to receive media on one or
more IPv6 addresses, but the IPv4-only user agent cannot send media
to these addresses and generally does not even understand their
format.
This IP version incompatibility problem would not exist if hosts
implementing IPv6 also implemented IPv4, and were configured with
both IPv4 and IPv6 addresses. In such a case, a UA would be able to
pick a compatible media transport address type to communicate with
each other.
Consequently, user agents need a means to obtain both IPv4 and IPv6
addresses to receive media and to place them in offers and answers.
Note that pragmatism dictates that IPv6 user agents undertake the
greater burden in the transition period. Since IPv6 user agents are
not widely deployed yet, it seems appropriate that IPv6 user agents
obtain IPv4 addresses instead of mandating an upgrade on the
installed IPv4 base. Furthermore, IPv6 user agents are expected to
be dual-stacked and thus also support IPv4, unlike the larger IPv4-
only user agent base that does not or cannot support IPv6.
However, there will be deployments where an IPv4/IPv6 node is unable
to use both interfaces natively at the same time, and instead, runs
as an IPv6-only node. Examples of such deployments include:
1. Networks where public IPv4 addresses are scarce and it is
preferable to make large deployments only on IPv6.
2. Networks utilizing Layer-2s that do not support contemporary IPv4
and IPv6 usage on the same link (e.g., the 3rd Generation
Partnership Project, 3GPP).
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In such cases, relays (i.e., TURN) should be used to allow IPv6-only
nodes to utilize IPv4 addresses to communicate with IPv4-only nodes.
The advantage of this strategy is that the installed base of IPv4
user agents continues to function unchanged, but an operator that
introduces IPv6 must provide additional servers to allow IPv6 user
agents to obtain IPv4 addresses. This strategy may come at an
additional cost to SIP operators deploying IPv6. However, since
IPv4-only SIP operators are also likely deploy TURN relays as a NAT-
traversal, the additional effort to deploy IPv6 in an IPv4 SIP
network should be limited from this aspect.
4.1 Updates to RFC3264
This section provides a normative update to RFC 3264 [4] in the
following manner:
1. In some cases, especially those dealing with third party call
control (see Section 4.2 of [15]), there arises a need to specify
the IPv6 equivalent of the IPv4 unspecified address (0.0.0.0) in
the SDP offer. Instead of using the IPv6 unspecified address
(i.e., ::), implementations are REQUIRED to use a domain name
within the .invalid DNS top level domain.
2. The SDP answer MUST use the same network type as in the offer,
i.e., if the "c=" line of the offer contained a network type with
the value "IP4", the "c=" line of the answer MUST contain the
same network type. Similarly, if the "c=" line of the offer
contained a network type with the value "IP6", the "c=" line of
the answer MUST contain the same network type as well.
4.2 Initial Offer
We now describe how user agents can gather addresses following the
ICE (Interactive Connectivity Establishment) [12] procedures and how
they can encode them in an SDP session description using the ANAT
semantics [7] for the SDP grouping framework [5].
When following the ICE procedures, in addition to local addresses,
user agents need to obtain addresses from relays. For example, an
IPv6 user agent would obtain an IPv4 address from a relay. The relay
would forward the traffic received on this IPv4 address to the user
agent using IPv6. Such user agents MAY use any mechanism to obtain
addresses in relays, but, following the recommendations in ICE, it is
RECOMMENDED that user agents support TURN [10] [11] for this purpose.
It is RECOMMENDED that user agents gather IPv4 and IPv6 addresses
using the ICE procedures to generate all their offers. This way,
both IPv4-only and IPv6-only answerers will be able to generate an
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answer that establishes a session. Note that, in case of forking,
the same offer carried in an INVITE request can arrive to an IPv4-
only user agent and to an IPv6-only user agent at roughly the same
time. Having placed both IPv4 and IPv6 addresses in the offer
reduces the session establishment time because both all types of
answerers find the offer valid.
User agents that use SDP SHOULD support the ANAT semantics for the
SDP grouping framework. ANAT allows user agents to include both IPv4
and IPv6 addresses in their SDP session descriptions. The SIP usage
of the ANAT semantics is discussed in [9].
4.3 Connectivity Checks
Once the answerer has generated an answer following the ICE
procedures, both user agents MAY perform the STUN-based [6]
connectivity checks specified by ICE. This checks help prevent some
types of flooding attacks and allow user agents to discover new
addresses that can be useful in the presence of NATs (Network Address
Translators).
5. IANA Considerations
This document does not contain any actions for the IANA.
6. Security Considerations
This document describes how IPv4 SIP user agents can communicate with
IPv6 user agents (and vice versa). To do this, it uses additional
protocols (TURN, STUN, SDP); the threat model of each such protocol
is included in its respective document. This document does not
introduce the possibility of any new security threats; however, it
may make hosts more amenable to existing threats. Consider, for
instance, a UAC that allocates an IPv4 and IPv6 addresses locally and
inserts these into the SDP. Malicious user agents that may intercept
the request can mount a denial of service attack targeted to the
different network interfaces of the UAC.
7. Acknowledgment
The authors would like to thank Mohamed Boucadair, Cullen Jennings,
Aki Niemi, Jonathan Rosenberg, and Robert Sparks for discussions on
the WG list that improved the quality of this document.
8. References
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8.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[2] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[3] 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.
[4] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
the Session Description Protocol (SDP)", RFC 3264, June 2002.
[5] Camarillo, G., Holler, J., and H. Schulzrinne, "Grouping of
Media Lines in the Session Description Protocol (SDP)",
RFC 3388, December 2002.
[6] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, "STUN
- Simple Traversal of User Datagram Protocol (UDP) Through
Network Address Translators (NATs)", RFC 3489, March 2003.
[7] Camarillo, G. and J. Rosenberg, "The Alternative Network
Address Types (ANAT) Semantics for the Session Description
Protocol (SDP) Grouping Framework", RFC 4091, June 2005.
[8] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
(SIP): Locating SIP Servers", RFC 3263, June 2002.
[9] Camarillo, G. and J. Rosenberg, "Usage of the Session
Description Protocol (SDP) Alternative Network Address Types
(ANAT) Semantics in the Session Initiation Protocol (SIP)",
RFC 4092, June 2005.
[10] Rosenberg, J., Huitema, C., and R. Mahy, "Traversal Using Relay
NAT (TURN)", draft-rosenberg-midcom-turn-08 (work in progress),
September 2005.
[11] Camarillo, G. and O. Novo, "Traversal Using Relay NAT (TURN)
Extension for IPv4/IPv6 Transition",
draft-ietf-behave-turn-ipv6-00 (work in progress),
February 2006.
[12] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
Methodology for Network Address Translator (NAT) Traversal for
Offer/Answer Protocols", draft-ietf-mmusic-ice-08 (work in
progress), March 2006.
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8.2 Informational References
[13] Schulzrinne, H., "Dynamic Host Configuration Protocol (DHCPv6)
Options for Session Initiation Protocol (SIP) Servers",
RFC 3319, July 2003.
[14] Schulzrinne, H., "Dynamic Host Configuration Protocol (DHCP-
for-IPv4) Option for Session Initiation Protocol (SIP)
Servers", RFC 3361, August 2002.
[15] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo,
"Best Current Practices for Third Party Call Control (3pcc) in
the Session Initiation Protocol (SIP)", RFC 3725.
[16] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications",
RFC 3550, July 2003.
[17] Handley, M., Schulzrinne, H., Schooler, E., and J. Rosenberg,
"SIP: Session Initiation Protocol", RFC 2543, March 1999.
[18] Draves, R., "Default Address Selection for Internet Protocol
Version 6 (IPv6)", RFC 3484, Feb 2003.
[19] Peterson, J., "Session Initiation Protocol (SIP) Authenticated
Identity Body (AIB) Format", RFC 3893, September 2004.
[20] Gurbani, V., Boulton, C., and R. Sparks, "Recommendations on
the use of IPv6 in the Session Initiation Protocol (SIP)",
draft-gurbani-sipping-ipv6-sip-03 (work in progress), May 2006.
[21] Peterson, J. and C. Jennings, "Enhancements for Authenticated
Identity Management in the Session Initiation Protocol (SIP)",
draft-ietf-sip-identity-06 (work in progress), October 2005.
Authors' Addresses
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: Gonzalo.Camarillo@ericsson.com
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Karim El Malki
Athonet
Email: karim@athonet.com
Vijay K. Gurbani (editor)
Lucent Technologies, Inc./Bell Laboratories
2701 Lucent Lane
Rm 9F-546
Lisle, IL 60532
USA
Phone: +1 630 224 0216
Email: vkg@lucent.com
Appendix A. Sample IPv4/IPv6 DNS File
A portion of a sample DNS zone file entry is reproduced below that
has both IPv4 and IPv6 addresses. This entry corresponds to a proxy
server for the domain "example.com". The proxy server supports the
Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)
transport for both IPv4 and IPv6 networks.
...
_sip._tcp SRV 20 0 5060 sip1.example.com
SRV 0 0 5060 sip2.example.com
_sip._udp SRV 20 0 5060 sip1.example.com
SRV 0 0 5060 sip2.example.com
sip1 IN A 192.0.2.1
sip1 IN AAAA 2001:db8::1
sip2 IN A 192.0.2.2
sip2 IN AAAA 2001:db8::2
...
Camarillo, et al. Expires December 14, 2006 [Page 14]
Internet-Draft IPv6 Transition in SIP June 2006
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Camarillo, et al. Expires December 14, 2006 [Page 15]
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