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SIP                                                         J. Rosenberg
Internet-Draft                                             Cisco Systems
Expires: September 7, 2006                                 March 6, 2006


    Construction of the Route Header Field in the Session Initiation
                             Protocol (SIP)
                 draft-rosenberg-sip-route-construct-01

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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.

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  305 Use Proxy  . . . . . . . . . . . . . . . . . . . . . .   4
   3.   Problems with Current Specifications . . . . . . . . . . . .   5
   4.   Overview of Operation  . . . . . . . . . . . . . . . . . . .   7
   5.   Detailed Processing Rules  . . . . . . . . . . . . . . . . .   9
     5.1  Registrar Behavior . . . . . . . . . . . . . . . . . . . .   9
     5.2  Proxy Behavior . . . . . . . . . . . . . . . . . . . . . .  10
     5.3  UAC Behavior . . . . . . . . . . . . . . . . . . . . . . .  11
     5.4  Client Behavior  . . . . . . . . . . . . . . . . . . . . .  13
     5.5  Server Behavior  . . . . . . . . . . . . . . . . . . . . .  14
   6.   Grammar  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   7.   Security Considerations  . . . . . . . . . . . . . . . . . .  15
   8.   IANA Considerations  . . . . . . . . . . . . . . . . . . . .  15
   9.   Examples . . . . . . . . . . . . . . . . . . . . . . . . . .  15
   10.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  15
   11.  References . . . . . . . . . . . . . . . . . . . . . . . . .  15
     11.1   Normative References . . . . . . . . . . . . . . . . . .  15
     11.2   Informative References . . . . . . . . . . . . . . . . .  16
        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 coming from a 305
   response.  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, the
   existing specifications are incomplete and inconsistent.

   Section 2 reviews the existing sources of route sources.  Section 3
   discusses problems with the existing specifications.  Section 4
   overviews the proposed changes in behavior.  Section 5 provides a
   detailed description of element behavior, and Section 6 defines the
   grammar for the new headers 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.

      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



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      dialog), using the desired Request-URI as the remote target URI.

   The default outbound proxy can be learned either through DHCP [6],
   through configuration (such as the SIP configuration framework [8]),
   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  305 Use Proxy

   RFC 3261 defines the 305 "Use Proxy" response code, but says
   extremely little about exactly how it is used.  It has this to say:

      The requested resource MUST be accessed through the proxy given by
      the Contact field.  The Contact field gives the URI of the proxy.
      The recipient is expected to repeat this single request via the



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      proxy. 305 (Use Proxy) responses MUST only be generated by UASs.

   It is unclear how the Contact in the redirect is used.  Does it
   populate the request URI of the resulting request?  Or, does it get
   used to populate the Route header field?  The restriction to UASs is
   also not explained.

   Historically, the reason for the restriction to UAs was to avoid
   routing loops.  Consider an outbound proxy that generates a 305,
   instead of proxying the request.  The concern was that the client
   would then recurse on the response, populate the Contact into a new
   request URI, and send the request to its default outbound proxy,
   which redirects once more.  To avoid this, the RFC says that only a
   UAS can redirect with a 305, not a proxy.

   However, this design decision on 305 handling was made prior to the
   conception of loose routing, although both ended up in RFC 3261.  The
   design of the 305 mechanism, unfortunately, was not revisited after
   loose routing was specified.  As such, the draft is not clear about
   whether or not the contact gets utilized as a Route header field
   value or whether it replaces the Request URI.  The assumption,
   unstated, is that it populates the Request-URI, since redirection
   works that way in general.

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



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   problem.

   This points to the first major functional gap with the existing
   specifications.  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 Service-Route URI has
   provided a route that overrides the default outbound route behavior
   at PH-1.

   Similarly, there is no way in the current specifications 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 [8] for updating
   client configurations.  As a consequence, there is no common way
   across deployments to provide this very basic operational feature.

   Finally, it is important to note that there is an architectural
   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



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   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.

4.  Overview of Operation

   Firstly, new behavior for generation and processing of the 305 Use
   Proxy is specified.  Any element in the network, proxy or UAS, can
   generate a 305, not just a UAS as specified in RFC3261.  The 305 is
   directed towards a specific upstream element, whether it is a proxy
   or UAC, through the inclusion of the Redirect-Target header field in
   the response.  This header field contains a counter that is
   decremented as the response is forwarded upstream.  When it hits
   zero, that element recurses on it.

   This only works if the server that sends the 305 can be sure that all
   of the upstream elements between it and the target of the redirect
   support this mechanism.  To make this determination, the Target-Range
   header field is used.  This header field contains a pair of integers,
   start-range and end-range.  These integers correspond to the values
   of the Max-Forwards header field across a set of compliant elements.
   When the first element in a compliant chain (for example, a UAC
   supporting this mechanism) emits a request, it sets both start-range
   and end-range to the value of Max-Forwards in the request it emits.
   A compliant element decrements both Max-Forwards and end-range before
   forwarding the request if its policy says that downstream elements
   are permitted to redirect elements upstream from it.  If this is not
   permitted, the element sets both start-range and end-range to Max-
   Forwards in the outbound request, effectively starting the chain
   afresh.  However, a non-compliant element will only decrement the
   value of Max-Forwards.  As such, a compliant server can determine
   whether the previous hop was compliant by comparing the value of Max-
   Forwards in the received request with the value of end-range.  If
   they are identical, it means the previous hop supports the mechanism.
   Therefore, this proxy is is extended an existing chain of compliant
   elements, and it decrements both end-range and Max-Forwards before
   sending the request.  However, if the values of Max-Forwards and end-
   range in the received request were not identical, it means that the
   previous upstream element (and possibly ones upstream from that) were
   not compliant.  Therefore, this proxy is the first in a chain of
   compliant elements.  It therefore resets start-range and end-range to
   the value of Max-Forwards in the request it emits.

   Now, when a server receives a request, if Max-Forwards equals end-
   range, the server knows some number of upstream elements support this
   mechanism.  Indeed, the exact number of such elements will be start-
   range minus end-range plus one.  Thus, the server can insert the



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   Redirect-Target header field into the response with a value between 0
   (directing the immediate upstream element to recurse) and start-range
   minus end-range.



             MF:70        MF:69        MF:68        MF:67 +
      +-----+SR:70 .......SR:70 +-----+SR:68 +-----+SR:68 |-----+
      |     |ER:70 .     .ER:70 |     |ER:68 |     |ER:67 |     |
      |     |----->.     .----->|     |----->|     |----->|     |
      | UAC |      . P-1 .      | P-2 |      | P-3 |      | P-4 |
      |     |      .     .      |     |<-----|     |<-----|     |
      |     |      .     .      |     |RT:0  |     |RT:1  +     |
      +-----+      .......      +-----+      +-----+       -----+
                                   |
                                   |
                                   V



                                 Figure 1

   This procedure is shown pictorially in Figure 1.  The figure shows a
   UAC and four proxies, P-1 through P-4.  The UAC and proxies P-2
   through P-4 support the mechanism, but P-1 does not.  The UAC emits
   an INVITE request.  It populates Max-Forwards (shown as MF in the
   figure) with the initial value of 70.  It also adds a Target-Range
   header field to the request, with a start-range value (SR in the
   figure) of 70, and an end-range value (ER in the figure) of 70.  This
   request is received by P-1.  Since P-1 does not understand Target-
   Range, it only decrements Max-Forwards.  The request is now received
   by P-2.  P-2 sees that the value of Max-Forwards in the received
   request (69) does not match the value of end-range (70).  Thus, it
   knows that its immediately upstream neighbor didn't support the
   mechanism.  As such, when it emits its request, it sets the value of
   both start-range and end-range to the value of Max-Forwards, 68.
   This request arrives at P-3.  P-3 sees that the value of Max-Forwards
   matches end-range.  So, it decrements both in the request it emits.
   This request arrives at P-4.  Again, the value of Max-Forwards equals
   end-range.  It subtracts end-range from start-range (68-67) and adds
   1, which results in 2.  This means that the 2 upstream elements
   support the redirect targeting mechanism, and P-4 generates a 305
   response with a Redirect-Target value of 1.  This is received by P-3,
   which decrements Redirect-Target to zero.  This is received by P-2,
   which sees that the value is zero.  This means that it is the
   intended target of the redirect.  It therefore recurses on the
   redirect and emits a new request.




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   [[OPEN ISSUE: This backwards compatibility mechanism could actually
   be used for all redirects; providing a mechanism to know and control
   where and when recursion is done.  Indeed, the mechanism could
   provide a general framework for allowing downstream servers to
   determine whether a sequence of upstream servers supports some
   extension.  If one uses a sequence of ranges instead of a single
   range, full proxy support information can be delivered to the UAS.
   Is there a need for such a thing?]]

   In addition to specifying new rules for generation and processing of
   the 305, 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.
   The values from the Path are copied into the Service-Route, and the
   registrar can then add some additional ones if they are within the
   domain of the provider.  By allowing the registrar to add values, the
   mechanism defined here is made a superset of the behaviors defined in
   RFC 3608.  There, the registrar can only add URI in its own domain.
   Here, the registrar can add such URI, but also reflects Path headers
   from the request, which may have come from other domains.  In
   addition, RFC 3608 defined the service route to be associated with an
   AOR, rather than a registered contact.  This specification modifies
   that behavior.  Instead, the service route will be associated with
   each registered contact.  Note that the registrar never needs to
   store the service-route; it is computed as a function of the Path
   header in the REGISTER request.

5.  Detailed Processing Rules

5.1  Registrar Behavior

   This specification updates the procedures from RFC 3608.

   The registrar MUST construct the Service-Route in the registration
   response by taking each URI from the Path header field in the
   REGISTER request, and inverting the order.  After inversion, the
   registrar MAY add additional URIs at the end of the list (that is,
   the right hand side of the list, corresponding to proxy elements that
   will be the farthest away from the UA).

   Furthermore, the registrar MAY replace or remove any URIs that are
   within a domain under the control of the registrar.  When replacing a
   URI, one or more new ones can take its place.  If the registrar is in
   example.com, this would include any URIs whose domain part is
   example.com.  It would also include any URIs whose domain is a
   subdomain of example.com, as long as that subdomain is under the
   control of example.com.  It could also include URIs whose domain part
   is unrelated to example.com, as long as those are within the control



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   of example.com.  It is difficult to provide a concise definition of
   "under the control", but generally it means that the administrative
   policies for the subservient domain are completely defined by the
   controlling domain.

   This behavior ensures that proxies outside of the domain of the
   registrar have a way to appear on the service route, but provides a
   way, within a domain, to provide service routes that are not coupled
   to the Path.

   [[OPEN ISSUE: Its unclear whether this mechanism is backwards
   compatible with current IMS procedures.  The P-CSCF will insert Path,
   but not expect for its URI to be in the outbound Route set.  The
   procedures here, for a UA compliant to this specification, will
   result in an outgoing INVITE being delivered to the P-CSCF as a
   consequence of it being the default outbound proxy, but the request
   will arrive with the topmost route equal to the outbound proxy URI,
   and the next route will be the Path URI.  The route after that will
   be the S-CSCF (or I-CSCF in the home domain if added to the service
   route).  Not clear that this will work.  If it doesn't, it is easily
   remedied by including a flag in the Path header which indicates
   whether it needs to be reflected into Service-Route.]]

5.2  Proxy Behavior

   When a proxy receives a request that contains the Target-Range header
   field, it examines the value of end-range.  If end-range is not equal
   to the value of Max-Forwards in the received request, the proxy MUST
   set both end-range and start-range equal to the value of the Max-
   Forwards header field in the request it emits.  If they are equal,
   the proxy MAY extend the chain of compliant elements, or MAY reset
   it, starting with itself.  The decision depends on whether the proxy
   wishes downstream elements to be able to generate redirects towards
   upstream elements, and is a matter of local policy.  If the proxy
   decides to reset it, it sets both end-range and start-range equal to
   the value of the Max-Forwards header field in the request it emits.
   If it decides to extend it, it sets end-range equal to the value of
   the Max-Forwards header field in the request it emits and retains the
   value of start-range.

   When a proxy receives a 305 response, it MUST check the value of the
   Redirect-Target header field.  If this value is non-zero, the proxy
   MUST decrement it by one before forwarding the 305 upstream, and MUST
   NOT recurse on the 305.  If the value is zero, the proxy follows the
   procedures in Section 5.4.

   This specification updates the proxy processing rules in RFC 3608.
   In particular, if a proxy inserts a Path header field in a REGISTER



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   request, it means that a compliant registrar will echo the Path
   header field back into the REGISTER response as a Service-Route
   header field value.  The proxy MAY remove its value from the Service-
   Route in the response, or MAY modify it.  If the REGISTER response
   does not contain a Service-Route value that includes the Path URI
   inserted by the proxy, it means that the registrar is not compliant
   to this specification.  [[OPEN ISSUE: and then it does what??]

   [[OPEN ISSUE: not sure if the following belongs here or not; its not
   elaborated on anywhere else and is just a placeholder]]

   If the proxy receives a request destined for the AOR of a subscriber,
   and the proxy is the responsible proxy for that domain, it looks up
   the AOR in the location database, and retrieves the Path URI and the
   registered Contact.  However, instead of rewriting the request-URI to
   be equal to the registered contact, if that contact contains the URI
   loose-route parameter, the proxy retains the request URI, and instead
   uses that registered contact URI as the last Route header field
   value.  In this way, the UAS will receive new requests with the AOR
   retained in the request URI, and a topmost Route header field
   present, with a URI containing the registered contact.

5.3  UAC Behavior

   A UAC which supports the 305 recursion mechanism, including the
   Response-Target and Target-Range header fields, MUST include the
   Target-Range header field in all requests it emits, excepting CANCEL
   and ACK.  This header field MUST have the start range and end range
   values equal to the value of Max-Forwards in the request emitted by
   the UAC.

   Determination of the route set for a request is done in two steps.
   The first is the determination of a baseline route set.  This set is
   the route set determined strictly by protocol mechanisms, and has not
   yet been subject to UA policies which might require alteration of the
   route set.  Those policies are then applied, and the result is the
   final route set for the request.

   For a request sent by a UAC that is not the result of recursion on a
   305, the following logic MUST be used to compute the baseline 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 baseline
      route set will not contain any routes learned from configuration,
      DHCP, Service-Route or any other mechanism.





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   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 [5], 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.]].  Once a service route has been selected, the URIs
      from this service route become the baseline route set.  Here too,
      the baseline route set will not contain any routes learned from
      configuration, DHCP, or other service routes.

   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 request is being generated as a consequence of a 305, the
   baseline route set is computed as described in Section 5.4.

   With the baseline route set computed, the UAC applies policy to
   determine whether this route set needs to be modified.  The primary
   factor involved is whether or not the client needs to send this
   request through its outbound proxy or not.  The following logic is
   RECOMMENDED.  If the topmost URI in the baseline route set equals the
   configured default outbound proxy for the UAC, then the baseline
   route set is used unmodified, and used as the final route set.  If,
   however, it does not, the UAC checks a white list of URI that it
   maintains.  If the topmost URI appears on that white list, the
   baseline route set is used as the final route set.  If it is not
   present, the default outbound proxy URI is appended to the top of the
   route set.

   The white list represents destinations that the UA has confidence are
   ones permitted to be used.  Here, this implies that the provider of
   the default outbound proxy allows that URI to be used in its place.
   This white list can be provided by configuration, but this is
   cumbersome and NOT RECOMMENDED.  Instead, the following algorithm is
   RECOMMENDED.  Initially, the white list is empty when a UA starts up.
   If a UA receives a 305 to a REGISTER request it generates, the URI in
   the Contact header field of the redirect are added to the white list.
   This will cause the REGISTER to be sent with one of these URI as the
   topmost URI in the route set.  If that registration succeeds, the URI



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   are retained in the white list for the duration of the registration.
   The client maintains a separate white list for each registered
   contact.

   This algorithm works because of two factors.  Firstly, registrations
   are always targeted towards the domain of the subscriber, and are
   never delivered to user agents.  As such, the mechanism relies on
   trust between the provider of the outbound proxy and provider of the
   AOR for the subscriber to follow the 305 mechanism described here
   correctly.  However, if that trust doesn't exist, basic call
   processing is not possible in any case.  The second factor is the 305
   mechanism itself.  If the outbound proxy doesn't support this
   specification, or the provider of the outbound proxy doesn't wish the
   provider of the AOR to bypass the outbound proxy, the 305 mechanism
   allows this to be known to the provider of the AOR.  Thus, the
   provider of the AOR can only bypass the outbound proxy if permitted
   by the provider of the outbound proxy.  Typically, this would only be
   allowed when they are the same provider.  The 305 mechanism allows
   the outbound proxy to bypass itself, since the outbound proxy can
   generate a 305 as long as the client supports the mechanism in this
   specification.

5.4  Client Behavior

   The following logic defined here applies to all clients, both UAC and
   proxies, and applies to the processing of a 305 response.

   When a client receives a 305 response, it MUST check for the presence
   of the Response-Target header field.  If this header field is absent,
   the client MAY recurse on the request.  However, in this case, the
   recursion MUST be accomplished by replacing the request URI of the
   request it generates with the value of the Contact header field in
   the 305 response.  This provides backwards compatibility with the
   existing usage of 305, since all redirection defined in RFC 3261
   updates the Request-URI.  If the Response-Target header field is
   present, but has a non-zero value, the client MUST NOT recurse on the
   redirect.  If the Response-Target header field is present and has a
   value of zero, the client MUST recurse on the redirect.

   To compute the request that is sent as a result of the recursion, the
   client MUST take the route set used for the request that generated
   the 305 response.  If that request had a Route header field, the
   first value MUST be replaced with the value of the Contact header
   field in the 305 with the highest q-value.  If there are multiple
   such Contacts with the same q-value, one is chosen at random.  The
   result is used as the route set for the new request.  If the client
   is a UAC, it follows the procedures defined in Section 5.3.  If the
   client is a proxy, it follows the procedures definde in Section 5.2.



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   [[ISSUE: There are three choices about how to process the contact in
   the 305.  The URI there can replace the route set at the client
   completely, they can be appended to the route set, or they can
   replace the topmost route.  This draft employs the latter technique.
   Further consideration is needed to determine whether or not this is
   the right thing.  Since the contact can contain multiple URI, we
   could actually have it contain the entire route set that should
   replace the one from the request.]]

   If a 305 response had multiple Contact header field values, and the
   recursed request generated a 503 response, and the client had
   exhausted all alternative servers learned from DNS [7] for the
   previous Contact header field value, the client SHOULD choose the
   Contact from the 305 with the next highest q-value, and construct
   another recursed request using the procedures defined above.  In the
   event the 305 had multiple Contact header field values with
   equivalent q-values, the next highest one might have a q-value equal
   to the one that was just tried.

5.5  Server Behavior

   Any server, either a UAS or a proxy, MAY generate a 305 in response
   to a request.  However, it MUST NOT do so unless the request contains
   a Target-Range header field, and the value of end-range in that
   header field equals the value of Max-Forwards in the received
   request.  If the server generates a 305, it MUST direct that redirect
   to a specific upstream element.  To do so, it includes a Redirect-
   Target header field in the response.  That header field identifies a
   specific element that is the target of the redirect.  A value of 0
   indicates that the element immediately upstream is the target, 1
   indicates that the target is the second upstream element, and so on.
   The value of Redirect-Target MUST be between 0 and start range minus
   end range.

6.  Grammar

   This specification defines two new header fields - the Target-Range
   and Redirect-Target header fields.  Their syntax is as follows:



   Target-Range = "Target-Range" HCOLON start-range SWS "-" SWS end-range
   Redirect-Target = "Redirect-Target" HCOLON target-val
   start-range = 1*DIGIT
   end-range = 1*DIGIT
   target-val = 1*DIGIT





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7.  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
   towards intermediaries, which can then observe call patterns,
   intercept communications, and so on.  As a consequence, the
   mechanisms in this specification to take care that this route set can
   only be updated on very specific conditions.  Furthermore, the 305
   mechanism defined here gives service providers policy hooks that
   allow them to control whether such redirection can be employed by
   exteranl service providers.

   [[ISSUE: needs more verbiage here]]

8.  IANA Considerations

9.  Examples

10.  Acknowledgements

   The author would like to thank Paul Kyzivat and Anders Kristensen for
   their comments.

11.  References

11.1  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]  Jennings, C. and R. Mahy, "Managing Client Initiated Connections
        in the Session Initiation Protocol  (SIP)",
        draft-ietf-sip-outbound-02 (work in progress), March 2006.





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11.2  Informative References

   [6]  Schulzrinne, H., "Dynamic Host Configuration Protocol (DHCP-for-
        IPv4) Option for Session Initiation Protocol (SIP) Servers",
        RFC 3361, August 2002.

   [7]  Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
        (SIP): Locating SIP Servers", RFC 3263, June 2002.

   [8]  Petrie, D., "A Framework for Session Initiation Protocol User
        Agent Profile Delivery", draft-ietf-sipping-config-framework-07
        (work in progress), July 2005.


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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