SIP J. Rosenberg
Internet-Draft Cisco Systems
Expires: April 20, 2007 October 17, 2006
Construction of the Route Header Field in the Session Initiation
Protocol (SIP)
draft-rosenberg-sip-route-construct-02
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
The Route header field in the Session Initiation Protocol (SIP) is
used to cause a request to visit a set of hops on its way towards the
final destination. Several specifications have defined rules for how
a user agent obtains and then uses a set of Route header fields in
the transmission of a request. These include the SIP specification
itself, the Service-Route header field specification, the SIP server
option in the Dynamic Host Configuration Protocol (DHCP), and others.
Unfortunately, these specifications are not consistent and the
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resulting behavior at clients and servers is not clear or complete.
This document resolves this problem by defining a consistent set of
logic, and in the process, serves as an update to the Service-Route
specification.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Existing Sources . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Default Outbound Proxies . . . . . . . . . . . . . . . . . 3
2.2. Service Route . . . . . . . . . . . . . . . . . . . . . . 4
2.3. Record-Routes . . . . . . . . . . . . . . . . . . . . . . 4
2.4. Loose Routes . . . . . . . . . . . . . . . . . . . . . . . 4
3. Problems with Current Specifications . . . . . . . . . . . . . 5
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Overview of Operation . . . . . . . . . . . . . . . . . . . . 7
6. Detailed Processing Rules . . . . . . . . . . . . . . . . . . 7
6.1. Registrar Behavior . . . . . . . . . . . . . . . . . . . . 8
6.2. Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . 10
6.3. UAC Behavior . . . . . . . . . . . . . . . . . . . . . . . 10
6.3.1. REGISTER Processing . . . . . . . . . . . . . . . . . 10
6.3.2. Request Origination . . . . . . . . . . . . . . . . . 11
7. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 12
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
9.1. SIP Option Tag . . . . . . . . . . . . . . . . . . . . . . 13
9.2. Header Field Parameter . . . . . . . . . . . . . . . . . . 13
10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
12.1. Normative References . . . . . . . . . . . . . . . . . . . 14
12.2. Informative References . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . . . . 17
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1. Introduction
The Route header field in the Session Initiation Protocol (SIP)
protocol is used to cause a request to visit a set of hops on its way
towards the final destination. RFC 3261 [2] discusses how a client
constructs the Route header field for requests. However, this logic
is restricted to mid-dialog requests, where the route set was learned
as a result of record-routing.
However, additional sources of routes can exist for a UA. These
include default outbound proxies, a service route learned from the
Service-Route header field [3], and a redirection utilizing loose
routing [7]. In total, there are four sources of potential route
headers. The way in which these various sources are reconciled is
unclear. Furthermore, the various specifications are unclear about
which requests these Route headers are applicable to. Do they apply
to REGISTER? Do they apply to mid-dialog requests? Finally, RFC
3608 is underspecified, which can result in interoperability
problems.
Section 2 reviews the existing sources of route sources. Section 3
discusses problems with the existing specifications. Section 5
overviews the proposed changes in behavior. Section 6 provides a
detailed description of element behavior, and Section 7 defines the
grammar for the new parameters specified here.
2. Existing Sources
This section examines the current set of route header field sources.
2.1. Default Outbound Proxies
RFC 3261 discusses default outbound proxies. In Section 8.1.1.1, it
makes reference to its interaction with Route header fields:
In some special circumstances, the presence of a pre-existing
route set can affect the Request-URI of the message. A pre-
existing route set is an ordered set of URIs that identify a chain
of servers, to which a UAC will send outgoing requests that are
outside of a dialog. Commonly, they are configured on the UA by a
user or service provider manually, or through some other non-SIP
mechanism. When a provider wishes to configure a UA with an
outbound proxy, it is RECOMMENDED that this be done by providing
it with a pre-existing route set with a single URI, that of the
outbound proxy.
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When a pre-existing route set is present, the procedures for
populating the Request-URI and Route header field detailed in
Section 12.2.1.1 MUST be followed (even though there is no
dialog), using the desired Request-URI as the remote target URI.
The default outbound proxy can be learned either through DHCP [8],
through configuration (such as the SIP configuration framework [9]),
or through other means. In the IP Multimedia Subsystem (IMS), the
default outbound proxy is the P-CSCF and is learned through GPRS
specific techniques.
RFC 3261 does not explicitly say the set of messages to which this
route set applies. However, the text above implies that it is for
all requests outside of a dialog.
2.2. Service Route
RFC 3608 specifies the Service-Route header field. This header field
is provided to the UA in a 2xx response to a REGISTER request. The
client uses this to populate its Route header fields for outgoing
requests. However, RFC 3608 explicitly says that the decision a UA
makes about how it combines the service route with other outbound
routes is a matter of local policy. Furthermore, RFC 3608 does not
clearly define to which requests the service route applies, and in
particular, whether or not it applies to a REGISTER request or a mid-
dialog request.
Furthermore, RFC 3608 specifies that a service-route is associated
with an Address-of-Record (AOR), and is shared by all contacts
associated with the same AOR. It also specifies that the Service-
Route URI can only be ones known to the registrar apriori, as opposed
to learned through the registration itself.
2.3. Record-Routes
RFC 3261 provides a detailed description of the record-routing
mechanism, and how the user agents in a dialog construct route sets
from the Record-Route header field values. RFC 3261 is also clear
that the resulting route set applies to mid-dialog requests. It
implies (though does not explicitly say) that the resulting route set
overrides any default outbound proxies (which represent a pre-loaded
route set).
2.4. Loose Routes
Loose routing, introduced in [7], defines mechanisms for using Route
header fields to address and invoke services in a user agent. It
also specifies a redirection mechanism whereby a server can redirect
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a client, and ask it to either modify the topmost Route header field
of its request, or add a new Route header field to the topmost
request. The specification indicates that it is applicable to both
mid-dialog and out-of-dialog requests. Since the client can be a
user agent, this forms another potential source of a Route header
field for user agents.
3. Problems with Current Specifications
Numerous problems have arisen as a consequence of the combination of
these specifications. These problems fit into two categories. The
first are interoperability problems, and the second are missing
capabilities.
An interoperability problem that has arisen is keeping an outbound
proxy on the path for outbound requests. Consider a proxy in a hotel
which a client discovers via DHCP and uses as its outbound proxy.
This proxy wishes to be used for incoming and outgoing requests, both
in and out of a dialog. If the home proxy provides a service route,
the hotel proxy will not be able to determine what it needs to do in
order to stay on the path. If the client implementation is such that
it appends the service route to its default outbound proxy, then the
hotel proxy need not do anything to stay on the path. If, however,
the client abandons its default proxy in favor of the service route,
the hotel proxy would fall off the path of subsequent requests unless
it inserted a Service-Route into the response of a REGISTER request.
Interestingly, the latter is illegal behavior according to RFC 3608,
but is the only mechanism available for ensuring that a proxy stay on
the request path. Since RFC 3608 does not provide any normative
behavior for combining service routes and outbound proxies, the hotel
proxy cannot know what to do, thus causing the interoperability
problem.
This points to the first major functional gap with RFC 3608. There
is no standards-based way for keeping an outbound proxy on the path
for outbound requests, when it is not the default outbound proxy.
Consider a proxy in a hotel, PH-1 which a client discovers via DHCP
and uses as its outbound proxy. When the client sends a REGISTER to
this proxy, it forwards it to a second proxy in the hotel, PH-2,
which then forwards it to the home proxy of the user, PA. PH-2 needs
to be on the outbound path for all requests leaving the hotel. PA
includes itself in a Service-Route header field in the response. The
client receives this Service-Route. For an initial INVITE request,
the client constructs a route set which includes its outbound proxy
PH-1 followed by the URI from the Service-Route, PA. This request
will traverse PH-1, which now follows the next Route header, sending
it to PA. This is not the desired behavior. The problem is that the
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Service-Route URI has provided a route that overrides the default
outbound route behavior at PH-1.
Similarly, there is no way in RFC 3608 to change the outbound proxy,
outside of an update in the client configuration. Such changes are
extremely useful for many operational reasons. One example is
movement of subscribers between geographically distributed sites in
cases where a site must be gracefully taken out of service, and the
subscribers using it need to be moved gracefully, over a period of an
hour or two, from one site to the other. Since, at best, the impact
of Service-Route on the outbound proxy is ambiguous, there is
generally no way to affect it excepting configuration change. Using
configuration updates as the only way to alter the outbound proxy is
problematic. In practice, systems providing automated updates to
client configuration (when they exist, as they often do not) are
decoupled from the operational systems that manage subscriber
capacity and software upgrades of sites, making the change hard to
affect through configuration. Furthermore, configuration updates are
typically passed to clients once they are made. Here, however, the
objective is to gracefully move subscribers. Using the randomized
nature of REGISTER timings helps provide that; such a function is
difficult to accomplish through configuration updates. Finally, many
deployments use mechanisms other than [9] for updating client
configurations. As a consequence, there is no common way across
deployments to provide this very basic operational feature.
Another problem that has come up with RFC 3608 is that it will not
work well with mid-dialog failover techniques identified for SIP
Outbound [10]. These techniques require that the outbound proxy
construct a URI for the Service-Route that is used by the UA for new
requests outside of a dialog.
Finally, RFC 3608 is defined such that the service route is identical
for all contacts registered to a specific AOR. This makes it
applicable only for selecting a set of configured, well-known servers
to use, and only ones within the domain of the owner of that AOR.
This is a fairly narrow scope of applicability, and introduces a
configuration burden on the registrar.
Architecturally, there is an inconsistency between record-routing and
service route. With record-route, each proxy on the path of the
request inserts a Record-Route header field, and this dictates the
path of subsequent messages within a dialog both to and from the UA.
With REGISTER, each proxy on the path of the request inserts a Path
[4] header field to receive requests directed towards the client.
However, the Service-Route is not the inverse of the Path, and is
instead created through proprietary means by the registrar.
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4. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
5. Overview of Operation
This specification updates the behaviors in RFC 3608. In particular,
a registrar, upon receipt of a REGISTER, uses the Path header field
values to construct the Service-Route in the response. In addition,
the registrar retains an instance of the Path (and resulting Service
Route) for each registered contact. The Path and Service-Route
remain valid for the duration of the registration, and are updated
for each registration refresh.
In order to retain backwards compatibility with systems based on RFC
3608, proxies compliant to this specification include a flag, "p2sr",
in their Path header field values. When the registrar receives the
REGISTER request, it examines the sequence of Path header field URI.
If it sees that one or more contiguous proxies at the end of the Path
sequence do not support this mechanism, the registrar omits those URI
from the Service-Route, and omits the Require header field parameter
indicating support for this specification in the response. This
causes the UA to revert to existing behavior, augmenting the route
set with the outbound proxy [[OPEN ISSUE: well, thats true for IMS at
least. UA behavior isn't defined at all in this area in RFC 3608.
Alternative is to have two option tags - one that says to augment,
and one that says don't.]] If, however, all of the Path URI include
the "p2sr" flag, an option tag is placed into the Require header
field is placed in the response, indicating that the Service-Route
overrides the outbound proxy.
The rules for constructing the Route for a request at the UA follow
in a straightforward manner from this. Mid-dialog requests always
use the set of URI learned from the Record-Route. A request outside
of the scope of a dialog, including a REGISTER refresh, uses the
Service-Route, and based on the Require flag, may or may not override
the outbound proxy. Finally, in all cases, if a request generates a
redirect response that contains a loose route, the Route set is
further modified or augmented based on the redirection.
6. Detailed Processing Rules
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6.1. Registrar Behavior
This specification updates the procedures from RFC 3608.
The procedures in this specification MUST NOT be followed unless the
REGISTER request contains a Supported header field with the "sr"
option tag.
Assuming the REGISTER request contains this option tag, the registrar
examines the set of Path header field values, starting from the top
(the proxy closes to, but not including the registrar itself) towards
the bottom (the proxy farthest away from the registrar). If the
registrar is planning on adding itself to the Service-Route, it adds
itself to the top of the list. Its own URI MUST include the "p2sr"
Path header field parameter.
If the resulting list is such that there are 0 or more contiguous
values starting at the top which contain the "p2sr" Path header field
parameter, followed by 0 or more contiguous values which do not
contain this parameter, the registrar SHOULD follow the remaining
procedures of this specification in the construction of the Service-
Route header field in the response. If not, the procedures defined
here MUST NOT be used. In addition, if none of the Path header field
values contain the "p2sr" Path header field parameters, the
procedures defined here MUST NOT be used.
Consider the example network of Figure 1. The UAC is separated from
the registrar by three proxies, P1, P2 and P3. The UAC supports the
mechanism in this specification and indicates this through the "sr"
option tag in the Supported header field of its REGISTER request.
+------+ +------+ +------+ +------+ +------+
| | | | | | | | | |
| UAC |---->| P1 |----->| P2 |----->| P3 |----->| Reg |
| | | | | | | | | |
+------+ +------+ +------+ +------+ +------+
Figure 1
When the UAC registers, each of the proxies inserts itself onto the
Path header field of the REGISTER. Each of the proxies either
supports this extension (and thus inserts a "ps2r" parameter into its
Path header field value) or it does not (in which case no parameter
is inserted). The following table shows, for various Path sequences,
whether or not the modified Service-Route procedures of this
specification would be used.
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Path Header Field | Use New SR | Notes
| Procedures? |
--------------------------------------------------------------------
P3;p2sr, | |
P2;p2sr, | Y | All proxies support it
P1;p2sr | |
--------------------------------------------------------------------
P3;p2sr, | | Proxies closest to registrar
P2;p2sr, | Y | support, followed by ones
P1 | | that don't
--------------------------------------------------------------------
P3;p2sr, | | Proxies closest to registrar
P2 | Y | support, followed by ones
P1 | | that don't
--------------------------------------------------------------------
P3, | |
P2, | N | No proxies support it
P1 | |
--------------------------------------------------------------------
reg;p2sr | |
P3, | |Registrar planning on inserting
P2, | Y |itself onto the Service-Route
P1 | |
--------------------------------------------------------------------
P3, | | Set of proxies that support
P2;p2sr, | N | it must be contiguous and
P1 | | closest to registrar
--------------------------------------------------------------------
P3;p2sr, | | Set of proxies that support
P2, | N | it must be contiguous and
P1;p2sr | | closest to registrar
--------------------------------------------------------------------
Figure 2
This constraint basically says that the Path has to be built
either by a proxy chain which all support this spec, or by a chain
whereby a bunch didn't support it, followed by a bunch that did.
This works well in IMS deployments where the visited network
doesn't support the mechanism, but the home network does.
If the mechanisms in this specification are to be used, the registrar
MUST construct the Service-Route in the registration response by
taking each URI from the list which contained the "p2sr" header field
parameter, and inverting the order. The registrar MUST add an option
tag to the Require header field in the response (adding the header
field if necessary) with the value "sr". The URI in the Service-
Route header field values SHOULD NOT contain the "p2sr" parameter;
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that parameter is a Path header field value and is not needed in the
Service-Route.
The resulting Service-Route MUST be recomputed for each registration
refresh, and for each new registration. The server MAY store the
values associated with it, though this is not necessary for proper
operation of this specification.
In addition, the registrar MUST only return in a 200 OK response to
the REGISTER request, the Contact header field associated with the
registration which was just performed. [[OPEN ISSUE: This is really
orthogonal, and it is probably controversial. Basically it proposes
to use this new service route mechanism as a vehicle for eliminating
query registers and third party registrations.]]. A UA compliant to
this specification will never generate a registration with anything
except for a single Contact.
If the mechanisms in this specification are not used, the registrar
MUST follow the procedures of RFC 3608 and construct the Service-
Route as it would otherwise. It MUST omit the "sr" option tag from a
Require header field in the response.
6.2. Proxy Behavior
This specification updates the proxy processing rules in RFC 3608.
A proxy compliant to this specification which inserts a Path header
field value into a REGISTER request MUST include a "p2sr" Path header
field parameter with its value. If the response to the REGISTER
request includes the Require header field that includes the "sr"
option tag, it means that the UA will be using that URI (which will
be echoed in the Service-Route) as a Route header field value for
requests outside of a dialog. In this case, the proxy MAY remove its
value from the Service-Route in the response, or MAY modify it.
When the UA initiates a request outside of a dialog, that request
will contain a route set which includes the URIs learned from the
Service-Route. Consequently, a proxy MUST be prepared to receive
such a request, in which case the topmost Route header will be the
URI the proxy passed to the UA in the 200 OK response to REGISTER.
6.3. UAC Behavior
6.3.1. REGISTER Processing
A UA compliant to this specification MUST include the "sr" option tag
in the Supported header field of its REGISTER request. Such a UA
MUST include only a single Contact in each REGISTER request, which
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points to the UA performing the registration. It MUST NOT generate a
"query REGISTER" which contains no contacts, MUST NOT include
multiple Contact header field values in its registration, and MUST
NOT register a Contact which does not directly point to the UA
itself.
When the REGISTER response arrives, and it is a 2xx response, the UA
looks for the presence of a Supported header field in the response
with the "sr" option tag. If present, the UA is operating in
"override" mode, as described below. If not present, the UA is
operating in "augment" mode, as described below. In either case, the
UA MUST cache the contents of the Service-Route header field for the
duration of its registration.
A single UA may still be performing multiple registrations, for
purposes of high availability, as a consequence of the procedures
defined in SIP outbound [6]. In this case, the UA will end up with
multiple sets of Service-Route, each of which is bound to the
particular flow that was registered (and its associated Contact).
6.3.2. Request Origination
It is RECOMMENDED that a UA compliant to this specification also be
compliant to UA loose routing [7]. This allows proxies to utilize a
redirection to further augment the way in which the route set for a
request is constructed.
The primary question addressed by this specification is how the UA
constructs the route set for a request.
Determination of the route set for a request depends on whether the
request is generated as a consequence of a redirection. If the UA
indicated support for loose routing in its request (as described in
[7], the Route set for the recursed request is generated from the
request which generated the recursion, as described there.
This specification assumes that a UA may have one or more configured
outbound proxies, each in the form of a SIP URI. Each of these will
either be a loose route (in which case the request would contain that
URI in the Route header field) or not (in which case the UA will just
send the request to that target without including its URI in the
topmost Route request).
For a request sent by a UAC that is not the result loose route
recursion, the following logic MUST be used to compute the route set:
o If the request is a mid-dialog request, the route set is computed
per the procedures in Section 12.2.1.1 of RFC 3261. The route set
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will not contain any routes learned from configuration, DHCP,
Service-Route or any other mechanism.
o If the request is not a mid-dialog request, the client checks to
see if it has learned a service route as a result of registration.
The UAC may have learned numerous service routes, one for each
unique AOR/Contact that it registered. In the case of
registrations using the mechanisms of [6], the Contact includes
the flow ID and instance ID, so that the client may have a
distinct service route for each unique AOR/Flow ID/Instance ID
combination. As such, when sending a request, the client selects
the service route corresponding to the contact which is sending
the request. [[OPEN ISSUE: Need to say more about this
selection.]]. If the UA is operating in "override" mode for this
route set, the URIs from this service route become the route set.
If the UA is operating in the "augment" mode for this route set,
the UA takes the outbound proxy URI it used for the REGISTER
request which created the route set, and appends that URI to the
top of the service route.
o If the request is not a mid-dialog request, and there are are no
service routes associated with the contact generating the request,
the UAC uses the route set learned through configuration. [[OPEN
ISSUE: Do we need to specify how to reconcile route sources
learned across disparate configuration sources? For example DHCP
and a config file? These can come from different providers.]]
If the topmost URI in the resulting route set is not a loose route
(which can happen when there is a configured outbound proxy that is
not a loose route), the UA MUST remove that URI from the Route set,
but still use it for purposes of sending the request.
7. Grammar
This specification defines an option tag and a Path header field
parameter. Their syntax is as follows:
option-tag \= "sr"
rr-param \= "p2sr"
8. Security Considerations
The route set used by a user agent for generating initial requests
into the network is very sensitive information. If this information
is manipulated by an attacker, it can cause calls to be directed
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towards intermediaries, which can then observe call patterns,
intercept communications, and so on. Consequently, a UA using this
specification SHOULD use sips when performing a registration. This
makes sure that only entities along the request path can modify the
route set used by the UA.
Even with sips, it is possible that a malicious home proxy could
modify the route set used by the UA, eliminating the outbound proxy
that would otherwise be used by it. This kind of attack is only
meaningful in environments where the outbound proxy is in a different
domain than the home proxy. However, presumably the outbound proxy
is present to authorize access to services, and removing it will only
result in denial of service to the user, which would appear to
provide no benefit.
9. IANA Considerations
This specification registers a new option tag and a new Path header
field parameter.
9.1. SIP Option Tag
This specification registers a new SIP option tag, as per the
guidelines in Section 27.1 of RFC 3261.
Name: sr
Description: This option tag is used to identify the usage of Path
reflected Service-Route, as defined by RFC XXXX [[NOTE TO IANA:
Please replace XXXX with the RFC number of this specification]]
9.2. Header Field Parameter
This specification defines the "p2sr" header field parameter, as per
the registry created by [5]. The required information is as follows:
Header field in which the parameter can appear: Path
Name of the Parameter: p2sr
RFC Reference: RFC XXXX [[NOTE TO IANA: Please replace XXXX with the
RFC number of this specification.]]
10. Examples
TODO.
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11. Acknowledgements
The author would like to thank Paul Kyzivat and Anders Kristensen for
their comments.
12. References
12.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] 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.
[3] Willis, D. and B. Hoeneisen, "Session Initiation Protocol (SIP)
Extension Header Field for Service Route Discovery During
Registration", RFC 3608, October 2003.
[4] Willis, D. and B. Hoeneisen, "Session Initiation Protocol (SIP)
Extension Header Field for Registering Non-Adjacent Contacts",
RFC 3327, December 2002.
[5] Camarillo, G., "The Internet Assigned Number Authority (IANA)
Header Field Parameter Registry for the Session Initiation
Protocol (SIP)", BCP 98, RFC 3968, December 2004.
[6] Jennings, C. and R. Mahy, "Managing Client Initiated Connections
in the Session Initiation Protocol (SIP)",
draft-ietf-sip-outbound-04 (work in progress), June 2006.
[7] Rosenberg, J., "User Agent Loose Routing in the Session
Initiation Protocol (SIP)",
draft-rosenberg-sip-ua-loose-route-00 (work in progress),
October 2006.
12.2. Informative References
[8] Schulzrinne, H., "Dynamic Host Configuration Protocol (DHCP-
for-IPv4) Option for Session Initiation Protocol (SIP)
Servers", RFC 3361, August 2002.
[9] Petrie, D., "A Framework for Session Initiation Protocol User
Agent Profile Delivery", draft-ietf-sipping-config-framework-09
(work in progress), October 2006.
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[10] Rosenberg, J., "Discovering Outbound Proxies and Providing High
Availability with Client Initiated Connections in the Session
Initiation Protocol (SIP)",
draft-rosenberg-sip-outbound-discovery-mid-dialog-00 (work in
progress), October 2006.
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Author's Address
Jonathan Rosenberg
Cisco Systems
600 Lanidex Plaza
Parsippany, NJ 07054
US
Phone: +1 973 952-5000
Email: jdrosen@cisco.com
URI: http://www.jdrosen.net
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