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Versions: 00 01 02 03 04 05 06 RFC 3312

Internet Engineering Task Force                                   SIP WG
Internet Draft                                     G. Camarillo (Editor)
                                                                Ericsson
                                                    W. Marshall (Editor)
                                                                    AT&T
                                                      Jonathan Rosenberg
                                                             dynamicsoft
draft-ietf-sip-manyfolks-resource-06.txt
March 25, 2002
Expires: September, 2002


               Integration of Resource Management and SIP

STATUS OF THIS MEMO

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress".

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   To view the list Internet-Draft Shadow Directories, see
   http://www.ietf.org/shadow.html.


Abstract

   This document defines a generic framework for preconditions which is
   extensible through IANA registration. This document also discusses
   how network quality of service can be made a precondition to
   establishment of sessions initiated by the Session Initiation
   Protocol (SIP). These preconditions require that the participant
   reserve network resources before continuing with the session. We do
   not define new quality of service reservation mechanisms; these
   preconditions simply require a participant to use existing resource
   reservation mechanisms before beginning the session.




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                           Table of Contents



   1          Introduction ........................................    3
   2          Terminology .........................................    3
   3          Overview ............................................    3
   4          SDP parameters ......................................    4
   5          Usage of preconditions with offer/answer ............    7
   5.1        Generating an offer .................................    8
   5.1.1      SDP encoding ........................................    9
   5.2        Generating an Answer ................................   10
   6          Suspending and Resuming Session Establishment .......   12
   7          Status Confirmation .................................   13
   8          Refusing an offer ...................................   14
   8.1        Rejecting a Media Stream ............................   15
   9          Unknown Precondition Type ...........................   15
   10         Option Tag for Preconditions ........................   15
   11         Indicating Capabilities .............................   16
   12         Examples ............................................   16
   12.1       End-to-end Status Type ..............................   16
   12.2       Segmented Status Type ...............................   21
   12.3       Offer in a SIP response .............................   22
   13         Security Considerations .............................   26
   14         IANA considerations .................................   26
   15         Contributors ........................................   26
   16         Acknowledgments .....................................   27
   17         Authors' Addresses ..................................   27
   18         Bibliography ........................................   28



















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1 Introduction

   Some architectures require that at session establishment time, once
   the callee has been alerted, the chances of a session establishment
   failure are minimum. One source of failure is the inability to
   reserve network resources for a session. In order to minimize "ghost
   rings", it is necessary to reserve network resources for the session
   before the callee is alerted. However, the reservation of network
   resources frequently requires learning the IP address, port, and
   session parameters from the callee. This information is obtained as a
   result of the initial offer/answer exchange carried in SIP. This
   exchange normally causes the "phone to ring", thus introducing a
   chicken-and-egg problem: resources cannot be reserved without
   performing an initial offer/answer exchange, and the initial
   offer/answer exchange can't be done without performing resource
   reservation.

   The solution is to introduce the concept of a precondition. A
   precondition is a set of constraints about the session which are
   introduced in the offer. The recipient of the offer generates an
   answer, but does not alert the user or otherwise proceed with session
   establishment. That only occurs when the preconditions are met. This
   can be known through a local event (such as a confirmation of a
   resource reservation), or through a new offer sent by the caller.

   This document deals with sessions that use SIP [1] as signalling
   protocol and SDP [2] to describe the parameters of the session.

   We have chosen to include the quality of service preconditions in the
   SDP description rather than in the SIP header because preconditions
   are stream specific.

2 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 [3].

3 Overview

   In order to ensure that session establishment does not take place
   until certain preconditions are met we distinguish between two
   different state variables that affect a particular media stream:
   current status and desired status. This document defines quality of
   service status.

   The desired status consists of a threshold for the current status.
   Session establishment stops until the current status reaches or



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   surpasses this threshold. Once this threshold is reached or
   surpassed, session establishment resumes.

   For example, the following values for current and desired status
   would not allow session establishment to resume:

     current status = resources reserved in the send direction
     desired status = resources reserved in both (sendrecv) directions


   On the other hand, the values of the example below would make session
   establishment resume:

     current status = resources reserved in both (sendrecv) directions
     desired status = resources reserved in the send direction


   These two state variables define a certain piece of state of a media
   stream the same way as the direction attribute or the codecs in use,
   define other pieces of state. Consequently, we treat these two new
   variables in the same way as other SDP media attributes are treated
   in the offer/answer model used by SIP [4]: they are exchanged between
   two user agents using an offer and an answer in order to have a
   shared view of the status of the session.


   Figure 1 shows a typical message exchange between two SIP user agents
   using preconditions. A includes quality of service preconditions in
   the SDP of the initial INVITE. A does not want B to be alerted until
   there is network resources reserved in both directions (sendrecv)
   end-to-end. B agrees to reserve network resources for this session
   before alerting the callee. B will handle resource reservation in the
   B->A direction, but needs A to handle the A->B direction. To indicate
   so, B returns a 183 response to A asking A to start resource
   reservation and to confirm to B as soon as the A->B direction is
   ready for the session. A and B both start resource reservation. B
   finishes reserving resources in the B->A direction, but does not
   alert the user yet, because network resources in both directions are
   needed. When A finishes reserving resources in the A->B direction, it
   sends an UPDATE [5] to B. B returns a 200 (OK) response for the
   UPDATE indicating that all the preconditions for the session have
   been met. At this point of time, B starts alerting the user, and
   session establishment completes normally.

4 SDP parameters

   We define the following media level SDP attributes:




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        current-status     =  "a=curr:" precondition-type
                              SP status-type SP direction-tag
        desired-status     =  "a=des:" precondition-type
                              SP strength-tag SP status-type
                              SP direction-tag
        confirm-status     =  "a=conf:" precondition-type
                              SP status-type SP direction-tag
        precondition-type  =  "qos" | token
        strength-tag       =  ("mandatory" | "optional" | "none"
                           =  | "failure" | "unknown")
        status-type        =  ("e2e" | "local" | "remote")
        direction-tag      =  ("none" | "send" | "recv" | "sendrecv")


        Current status: The current status attribute carries the current
             status of network resources for a particular media stream.

        Desired status: The desired status attribute carries the
             preconditions for a particular media stream. When the
             direction-tag of the current status attribute with a given
             precondition-type/status-type for a particular stream is
             equal to (or better than) the direction-tag of the desired
             status attribute with the same precondition-type/status-
             type for that stream, then the preconditions are considered
             to be met for that stream.

        Confirmation status: The confirmation status attribute carries
             threshold conditions for a media stream. When the status of
             network resources reach these conditions, the peer user
             agent will send an update of the session description
             containing an updated current status attribute for this
             particular media stream.

        Precondition type: This document defines quality of service
             preconditions. Extensions may define other types of
             preconditions.

        Strength tag: The strength-tag indicates whether or not the
             callee can be alerted in case the network fails to meet the
             preconditions.

        Status type: We define two types of status: end-to-end and
             segmented. The end-to-end status reflects the status of the
             end-to-end reservation of resources. The segmented status
             reflects the status of the access network reservations of
             both user agents. The end-to-end status corresponds to the
             tag "e2e" defined above and the segmented status to the
             tags "local" and "remote". End-to-end status is useful when



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          A                                            B

          |                                            |
          |-------------(1) INVITE SDP1--------------->|
          |                                            |
          |<------(2) 183 Session Progress SDP 2-------|
          |  ***                                 ***   |
          |--*R*-----------(3) PRACK-------------*R*-->|
          |  *E*                                 *E*   |
          |<-*S*-------(4) 200 OK (PRACK)--------*S*---|
          |  *E*                                 *E*   |
          |  *R*                                 *R*   |
          |  *V*                                 *V*   |
          |  *A*                                 *A*   |
          |  *T*                                 *T*   |
          |  *I*                                 *I*   |
          |  *O*                                 *O*   |
          |  *N*                                 *N*   |
          |  ***                                 ***   |
          |  ***                                       |
          |  ***                                       |
          |-------------(5) UPDATE SDP3--------------->|
          |                                            |
          |<--------(6) 200 OK (UPDATE) SDP4-----------|
          |                                            |
          |<-------------(7) 180 Ringing---------------|
          |                                            |
          |-----------------(8) PRACK----------------->|
          |                                            |
          |<------------(9) 200 OK (PRACK)-------------|
          |                                            |
          |                                            |
          |                                            |
          |<-----------(10) 200 OK (INVITE)------------|
          |                                            |
          |------------------(11) ACK----------------->|
          |                                            |
          |                                            |





   Figure 1: Basic session establishment using preconditions


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             end-to-end resource reservation mechanisms are available.
             The segmented status is useful when one or both UAs perform
             resource reservations on their respective access networks.
             Note that the use of the segmented status type does not
             prevent bottlenecks in the backbone, only in the access
             networks.

        Direction tag: The direction-tag indicates the direction a
             particular attribute (current, desired or confirmation
             status) is applicable to.

   The values of the tags "send", "recv", "local" and "remote" represent
   the point of view of the entity generating the SDP description. In an
   offer, "send" is the direction offerer->answerer and "local" is the
   offerer's access network. In an answer, "send" is the direction
   answerer->offerer and "local" is the answerer's access network.

   The following example shows these new SDP attributes in two media
   lines of a session description:

          m=audio 20000 RTP/AVP 0
          a=curr:qos e2e send
          a=des:qos optional e2e send
          a=des:qos mandatory e2e recv
          m=audio 20002 RTP/AVP 0
          a=curr:qos local sendrecv
          a=curr:qos remote none
          a=des:qos optional local sendrecv
          a=des:qos mandatory remote sendrecv



5 Usage of preconditions with offer/answer

   Parameter negotiation in SIP is carried out using the offer/answer
   model described in [4]. The idea behind this model is to provide a
   shared view of the session parameters for both user agents once the
   answer has been received by the offerer. This section describes which
   values our new SDP attributes can take in an answer depending on
   their value in the offer.

   To achieve a shared view of the status of a media stream, we define a
   model that consists of three tables: both user agents implement a
   local status table, and each offer/answer exchange has a transaction
   status table associated to it. The offerer generates a transaction
   status table identical to its local status table and sends it to the
   answerer in the offer. The anwerer uses the information of this
   transaction status table to update its local status table. The



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   answerer also updates the transaction status table fields that were
   out of date and returns this table to the offerer in the answer. The
   offerer can then update its local status table with the information
   received in the answer. After this offer/answer exchange, the local
   status tables of both user agents are synchronised. They now have a
   common view of the status of the media stream. Sessions that involve
   several media streams implement these tables per media stream. Note,
   however, that this is a model of user agent behavior, not of
   software. An implementation is free to take any approach that
   replicates the external behavior this model defines.

5.1 Generating an offer

   Both user agents MUST maintain local precondition status, which is
   referred to as a "local status table". Tables 1 and 2 show the format
   of these tables for both the end-to-end and the segmented status
   types. For the end-to-end status type, the table contains two rows;
   one for each direction (i.e., send and recv). A value of "yes" in the
   "Current" field indicates that resource has been successfully
   reserved in the corresponding direction. "No" indicates that
   resources have not been reserved yet. The "Desired Strength" field
   indicates the strength of the preconditions in the corresponding
   direction. The table for the segmented status type contains four
   rows: both directions in the local access network and in the peer's
   access network. The meaning of the fields is the same as in the end-
   to-end case.

   Before generating an offer, the offerer MUST build a transaction
   status table with the current and the desired status for each media
   stream. The different values of the strength-tag for the desired
   status attribute have the following semantics:

        o None: no resource reservation is needed.

        o Optional: the user agents SHOULD try to provide resource
          reservation, but the session can continue regardless of
          whether this provision is possible or not.

        o Mandatory: the user agents MUST provide resource reservation.
          Otherwise, session establishment MUST NOT continue.

   The offerer then decides whether it is going to use the end-to-end
   status type or the segmented status type. If the status type of the
   media line will be end-to-end, the user agent generates records with
   the desired status and the current status for each direction (send
   and recv) independently, as shown in table 1:





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                   Direction  Current  Desired Strength
                   ____________________________________
                     send       no        mandatory
                     recv       no        mandatory


   Table 1: Table for the end-to-end status type

   If the status type of the media line will be segmented, the user
   agent generates records with the desired status and the current
   status for each direction (send and recv) and each segment (local and
   remote) independently, as shown in table 2:


                   Direction   Current  Desired Strength
                  ______________________________________
                  local send     no           none
                  local recv     no           none
                  remote send    no         optional
                  remote recv    no           none


   Table 2: Table for the segmented status type



   At the time of sending the offer, the offerer's local status table
   and the transaction status table contain the same values.

   With the transaction status table, the user agent MUST generate the
   current-status and the desired status lines following the syntax of
   Section 4 and the rules described below in Section 5.1.1.

5.1.1 SDP encoding

   For the end-to-end status type, the user agent MUST generate one
   current status line with the tag "e2e" for the media stream. If the
   strength-tags for both directions are equal (e.g., both "mandatory")
   in the transaction status table, the user agent MUST add one desired
   status line with the tag "sendrecv". If both tags are different, the
   user agent MUST include two desired status lines, one with the tag
   "send" and the other with the tag "recv".

        The semantics of two lines with the same strength-tag, one
        with a "send" tag and the other with a "recv" tag, is the
        same as one "sendrecv" line. However, in order to achieve a
        more compact encoding, we have chosen to make mandatory the
        latter format.




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   For the segmented status type, the user agent MUST generate two
   current status lines: one with the tag "local" and the other with the
   tag "remote". The user agent MUST add one or two desired status lines
   per segment (i.e., local and remote). If for a particular segment
   (local or remote) the tags for both directions in the transaction
   status table are equal (e.g., both "mandatory"), the user agent MUST
   add one desired status line with the tag "sendrecv". If both tags are
   different, the user agent MUST include two desired status lines, one
   with the tag "send" and the other with the tag "recv".

   Note that the rules above apply to the desired strength-tag "none" as
   well. This way, a user agent that supports quality of service but
   does not intend to use them, adds desired status lines with the
   strength-tag "none". Since this tag can be upgraded in the answer, as
   described in Section 5.2, the answerer can request quality of service
   reservation without a need of another offer/answer exchange.

   The example below shows the SDP corresponding to tables 1 and 2.

          m=audio 20000 RTP/AVP 0
          a=curr:qos e2e none
          a=des:qos mandatory e2e sendrecv
          m=audio 20002 RTP/AVP 0
          a=curr:qos local none
          a=curr:qos remote none
          a=des:qos optional remote send
          a=des:qos optional local none



5.2 Generating an Answer

   When the answerer receives the offer, it recreates the transaction
   status table using the SDP attributes contained in the offer. The
   answerer updates both its local status and the transaction status
   table following the rules below:

        Desired Strength: We define an absolute ordering for the
             strength-tags: "none", "optional" and "mandatory".
             "Mandatory" is the tag with highest grade and "none" the
             tag with lowest grade. An answerer MAY upgrade the desired
             strength in any entry of the transaction status table, but
             it MUST NOT downgrade it. Therefore, it is OK to upgrade a
             row from "none" to "optional", from "none" to "mandatory"
             or from "optional" to "mandatory", but not the other way
             around.

        Current Status: For every row, the value of the "Current" field



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             in the transaction status table and in the local status
             table of the answerer have to be compared. Table 3 shows
             the four possible combinations. If both fields have the
             same value (two first rows of table 3, nothing needs to be
             updated. If the "Current" field of the transaction status
             table is "Yes" and the field of the local status table is
             "No" (third row of table 3), the latter MUST be set to
             "Yes". If the "Current" field of the transaction status
             table is "No" and the field of the local status table is
             "Yes" (forth row of table 3), the answerer needs to check
             if it has local information (e.g., a confirmation of a
             resource reservation has been received) about that
             particular current status. If it does, the "Current" field
             of the transaction status table is set to "Yes". If the
             answerer does not have local information about that current
             status, the "Current" field of the local status table MUST
             be set to "No".


   Transac. status table  Local status table  New values transac./local
   ____________________________________________________________________
            no                    no                    no/no
            yes                  yes                   yes/yes
            yes                   no                   yes/yes
            no                   yes            depends on local info


   Table 3: Possible values for the "Current" fields


   Once both tables have been updated, an answer MUST be generated
   following the rules described in Section 5.1.1 and taking into
   account that "send", "recv", "local" and "remote" tags have to be
   inverted in the answer, as shown in table 4.


                              Offer   Answer
                              ______________
                               send    recv
                               recv    send
                              local   remote
                              remote  local


   Table 4: Values of tags in offer and answers


   At the time the answer is sent, the transaction status table and the
   answerer's local status table contain the same values. Therefore,




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   this answer contains the shared view of the status of the media line
   in the current-status attribute and the negotiated strength and
   direction-tags in the desired-status attribute.

   If the resource reservation mechanism used requires participation of
   both user agents, the answerer SHOULD start resource reservation
   after having sent the answer and the offerer SHOULD start resource
   reservation as soon as the answer is received. If participation of
   the peer user agent is not needed (e.g., segmented status type), the
   offerer MAY start resource reservation before sending the offer and
   the answerer MAY start it before sending the answer.

   The status of the resource reservation of a media line can change
   between two consecutive offer/answer exchanges. Therefore, both user
   agents MUST keep their local status tables up to date using local
   information through the duration of the session.

6 Suspending and Resuming Session Establishment

   A user agent server that receives an offer with preconditions SHOULD
   NOT alert the user until all the mandatory preconditions are met;
   session establishment is suspended until that moment (e.g., a PSTN
   gateway reserves resources without sending signalling to the PSTN.)

   A user agent server may receive an INVITE request with no offer in
   it. In this case, following normal procedures defined in [1] and in
   [5], the user agent server will provide an offer in a reliable 1xx
   response. The user agent client will send the answer in another SIP
   request (i.e., the PRACK for the 1xx). If the offer and the answer
   contain preconditions, the user agent server SHOULD NOT alert the
   user until all the mandatory preconditions in the answer are met.

        Note that in this case, a user agent server providing a
        initial offer with preconditions, a 180 (Ringing) response
        will never be sent, since the user agent server cannot
        alert the user until all the preconditions are met.

   A UAS that is not capable of unilaterally meeting all of the
   mandatory preconditions MUST include a confirm-status attribute in
   the SDP (offer or answer) that it sends (see Section 7). Further, the
   SDP (offer or answer) that contains this confirm-status attribute
   MUST be sent as soon as allowed by the SIP offer/answer rules.

   While session establishment is suspended, user agents SHOULD not send
   any data over any media stream. In the case of RTP [6], neither RTP
   nor RTCP packets are sent.

   A user agent server knows that all the preconditions are met for a



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   media line when its local status table has a value of "yes" in all
   the rows whose strength-tag is "mandatory". When the preconditions of
   all the media lines of the session are met, session establishment
   SHOULD resume.

   For an initial INVITE suspending and resuming session establishment
   is very intuitive. The callee will not be alerted until all the
   mandatory preconditions are met. However, offers containing
   preconditions sent in the middle of an ongoing session need further
   explanation. Both user agents SHOULD continue using the old session
   parameters until all the mandatory preconditions are met. At that
   moment, the user agents can begin using the new session parameters.
   Section 12 contains an example of this situation.

7 Status Confirmation

   The confirm-status attribute MAY be used in both offers and answers.
   This attribute represents a threshold for the resource reservation.
   When this threshold is reached or surpassed, the user agent MUST send
   an offer to the peer user agent reflecting the new current status of
   the media line as soon as allowed by the SIP offer/answer rules. If
   this threshold is crossed again (e.g., the network stops providing
   resources for the media stream), the user agent MUST send a new offer
   as well as soon as allowed by the SIP offer/answer rules.

   If a peer has requested confirmation on a particular stream, an agent
   MUST mark that stream with a flag in its local status table. When all
   the rows with this flag have a value of "yes", the user agent MUST
   send a new offer to the peer. This offer will contain the current
   status of resource reservation in the current-status attributes. If
   later any of the rows with this flag transition to "No", a new offer
   MUST be sent as well.

   Confirmation attributes are not negotiated. The answerer uses the
   value of the confirm-status attribute in the offer and the offerer
   uses the value of this attribute in the answer.

   For example, if a user agent receives an SDP description with the
   following attributes:

          m=audio 20002 RTP/AVP 0
          a=curr:qos local none
          a=curr:qos remote none
          a=des:qos mandatory local sendrecv
          a=des:qos mandatory remote sendrecv
          a=conf:qos remote sendrecv





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   It will send an offer as soon as it reserves resources in its access
   network ("remote" tag in the received message) for both directions
   (sendrecv).

8 Refusing an offer

   We define a new SIP status code:

          Server-Error =  "580"  ;Precondition Failure


   When a UAS acting as an answerer cannot or is not willing to meet the
   preconditions in the offer it SHOULD reject the offer by returning a
   580 (Precondition-Failure) response. This response SHOULD contain an
   SDP description indicating which desired status triggered the
   failure. The corresponding desired status line MUST use the "failure"
   strength-tag, as shown in the example below:

          m=audio 20000 RTP/AVP 0
          a=des:qos failure e2e send


   SDP description indicating this type of failure MUST follow the
   format for describing media capabilities defined in the SIP
   offer/answer model [4].

   Using the 580 (Precondition Failure) status code to refuse an offer
   is useful when the offer came in an INVITE or in an UPDATE request.
   However, SIP does not provide a means to refuse offers that contained
   in a response (1xx or 2xx) to an INVITE.

   If a UAC generates an initial INVITE without an offer and receives an
   offer in a 1xx or 2xx response which is not acceptable, it SHOULD
   respond to this offer with a correctly formed answer and immediately
   after that send a CANCEL or a BYE.

   If the offer comes in a 1xx or 2xx response to a re-INVITE, A would
   not have a way to reject it without terminating the session at the
   same time. The same recommendation given in Section 14.2 of [1]
   applies here:

        "The UAS MUST ensure that the session description overlaps
        with its previous session description in media formats,
        transports, other parameters that require support from the
        peer. This is to avoid the need for the peer to reject the
        session description. If, however, it is unacceptable to A,
        A SHOULD generate an answer with a valid session
        description, and then send a BYE to terminate the session."



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8.1 Rejecting a Media Stream

   In the offer/answer model when an answerer wishes to reject a media
   stream it sets its port to zero. The presence of preconditions does
   not change this behaviour; streams are still rejected by setting
   their port to zero.

   Both the offerer and the answerer MUST ignore all the preconditions
   that affect a stream with its port set to zero. They are not taken
   into consideration to decide whether or not session establishment can
   resume.

9 Unknown Precondition Type

   This document defines the "qos" tag for quality of service
   preconditions. New precondition-types defined in the future will have
   new associated tags. A UA that receives an unknown precondition-type
   with a "mandatory" strength-tag in an offer MUST refuse the offer
   unless the only unknown mandatory preconditions have the "local" tag.
   In this case, the UA does not need to be involved in order to meet
   the preconditions. The UA will ask for confirmation of the
   preconditions and, when the confirmation arrives, it will resume
   session establishment.

   A UA refusing an offer follows the rules described in section 8, but
   instead of the tag "failure", it uses the tag "unknown", as shown in
   the example below:


          m=audio 20000 RTP/AVP 0
          a=des:foo unknown e2e send



10 Option Tag for Preconditions

   We define the option tag "precondition" for use in the Require and
   Supported header fields. An offerer MUST include this tag in the
   Require header field if the offer contains one or more "mandatory"
   strength-tags. If all the strength-tags in the description are
   "optional" or "none" the offerer MUST include this tag either in a
   Supported header field or in a Require header field. It is, however,
   RECOMMENDED, that the Supported header field is used in this case.
   The lack of preconditions in the answer would indicate that the
   answerer did not support this extension.

   The mapping of offers and answers to SIP requests and responses is
   performed following the rules given in [5]. Therefore, a user agent



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   including preconditions in the SDP MUST support the PRACK method, and
   consequently, MUST include the "100rel" [7] tag in the Require header
   field.

   A user agent including preconditions in the SDP SHOULD support the
   UPDATE method. If it is supported, an "update" [5] tag MUST be
   included in the Require header field.

        User agents that use preconditions but do not support
        UPDATE can only be used in a limited set of scenarios, such
        as the one described in figure 4.

11 Indicating Capabilities

   The offer/answer model [4] describes the format of a session
   description to indicate capabilities. This format is used in
   responses to OPTIONS requests. A UA that supports preconditions
   SHOULD add desired status lines indicating the precondition-types
   supported for each media stream. These lines MUST have the "none"
   strength-tag, as shown in the example below:


          m=audio 0 RTP/AVP 0
          a=rtpmap:0 PCMU/8000
          a=des:foo none e2e sendrecv
          a=des:qos none local sendrecv



   Note that when this document was published, the precondition-type
   "foo" has not been registered. It is used here in the session
   description above to provide an example with multiple precondition-
   types.

   A UA that supports this framework SHOULD add a "precondition" tag to
   the Supported header field of its responses to OPTIONS requests.

12 Examples

   The following examples cover both status types; end-to-end and
   segmented.

12.1 End-to-end Status Type

   The call flow of figure 2 shows a basic session establishment using
   the end-to-end status type. The SDP descriptions of this example are
   shown below:




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   SDP1: A includes end-to-end quality of service preconditions in the
   initial offer.

          m=audio 20000 RTP/AVP 0
          c=IN IP4 192.0.2.1
          a=curr:qos e2e none
          a=des:qos mandatory e2e sendrecv


   SDP2: Since B uses RSVP, it can know when resources in its "send"
   direction are available, because it will receive RESV messages from
   the network. However, it does not know the status of the reservations
   in the other direction. B requests confirmation for resource
   reservations in its "recv" direction to the peer user agent A in its
   answer.

          m=audio 30000 RTP/AVP 0
          c=IN IP4 192.0.2.4
          a=curr:qos e2e none
          a=des:qos mandatory e2e sendrecv
          a=conf:qos e2e recv


   After having sent the answer B starts reserving network resources for
   the media stream. When A receives this answer (2) it starts
   performing resource reservation as well. Both UAs use RSVP, so A
   sends PATH messages towards B and B sends PATH messages towards A.

   As time passes by, B receives RESV messages confirming the
   reservation. However, B waits until resources in the other direction
   are reserved as well since it did not receive any confirmation and
   the preconditions still have not been met.

   SDP3: When A receives RESV messages it sends an updated offer (5) to
   B:

          m=audio 20000 RTP/AVP 0
          c=IN IP4 192.0.2.1
          a=curr:qos e2e send
          a=des:qos mandatory e2e sendrecv


   SDP4: B responds with an answer (6) which contains the current status
   of the resource reservation (i.e., sendrecv):

          m=audio 30000 RTP/AVP 0
          c=IN IP4 192.0.2.4



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          a=curr:qos e2e sendrecv
          a=des:qos mandatory e2e sendrecv


   At this point of time, session establishment resumes and B returns a
   180 (Ringing) response (7).

   Note that now the media stream has been already established, and A
   has received a 180 (Ringing) response. Since the direction of the
   stream is "sendrecv", A will not generate local ringback, since it
   assumes that it will receive early media over this stream.

   However, if B wants A to generate local ringback, it can put the
   media stream on hold in SDP4. In this case, B would put the media
   stream off hold by sending an offer in an UPDATE request which would
   be sent at the same time as the 200 OK for the INVITE (10). The
   contents of the messages for this alternative flow are shown below:

   SDP4 (on hold):

          m=audio 30000 RTP/AVP 0
          c=IN IP4 192.0.2.4
          a=recvonly
          a=curr:qos e2e sendrecv
          a=des:qos mandatory e2e sendrecv


   SDP5 in an UPDATE:

          m=audio 30000 RTP/AVP 0
          c=IN IP4 192.0.2.4
          a=sendrecv
          a=curr:qos e2e sendrecv
          a=des:qos mandatory e2e sendrecv


   SDP6 in the 200 OK for the UPDATE:

          m=audio 20000 RTP/AVP 0
          c=IN IP4 192.0.2.1
          a=sendrecv
          a=curr:qos e2e sendrecv
          a=des:qos mandatory e2e sendrecv


   Let's assume that in the middle of the session A wishes to change the
   IP address where it is receiving media. Figure 3 shows this scenario.




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          A                                            B

          |                                            |
          |-------------(1) INVITE SDP1--------------->|
          |                                            |
          |<------(2) 183 Session Progress SDP 2-------|
          |  ***                                 ***   |
          |--*R*-----------(3) PRACK-------------*R*-->|
          |  *E*                                 *E*   |
          |<-*S*-------(4) 200 OK (PRACK)--------*S*---|
          |  *E*                                 *E*   |
          |  *R*                                 *R*   |
          |  *V*                                 *V*   |
          |  *A*                                 *A*   |
          |  *T*                                 *T*   |
          |  *I*                                 *I*   |
          |  *O*                                 *O*   |
          |  *N*                                 *N*   |
          |  ***                                 ***   |
          |  ***                                       |
          |  ***                                       |
          |-------------(5) UPDATE SDP3--------------->|
          |                                            |
          |<--------(6) 200 OK (UPDATE) SDP4-----------|
          |                                            |
          |<-------------(7) 180 Ringing---------------|
          |                                            |
          |-----------------(8) PRACK----------------->|
          |                                            |
          |<------------(9) 200 OK (PRACK)-------------|
          |                                            |
          |                                            |
          |                                            |
          |<-----------(10) 200 OK (INVITE)------------|
          |                                            |
          |------------------(11) ACK----------------->|
          |                                            |
          |                                            |





   Figure 2: Example using the end-to-end status type


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          A                                            B

          |                                            |
          |-------------(1) INVITE SDP1--------------->|
          |                                            |
          |<------(2) 183 Session Progress SDP 2-------|
          |  ***                                 ***   |
          |--*R*-----------(3) PRACK-------------*R*-->|
          |  *E*                                 *E*   |
          |<-*S*-------(4) 200 OK (PRACK)--------*S*---|
          |  *E*                                 *E*   |
          |  *R*                                 *R*   |
          |  *V*                                 *V*   |
          |  *A*                                 *A*   |
          |  *T*                                 *T*   |
          |  *I*                                 *I*   |
          |  *O*                                 *O*   |
          |  *N*                                 *N*   |
          |  ***                                 ***   |
          |  ***                                       |
          |  ***                                       |
          |-------------(5) UPDATE SDP3--------------->|
          |                                            |
          |<--------(6) 200 OK (UPDATE) SDP4-----------|
          |                                            |
          |<-----------(7) 200 OK (INVITE)-------------|
          |                                            |
          |------------------(8) ACK------------------>|
          |                                            |
          |                                            |



   Figure 3: Session modification with preconditions



   SDP1: A includes an offer in a re-INVITE (1). A continues to receive
   media on the old IP address (192.0.2.1), but it is ready to receive
   media on the new one as well (192.0.2.2):

          m=audio 20000 RTP/AVP 0
          c=IN IP4 192.0.2.2
          a=curr:qos e2e none
          a=des:qos mandatory e2e sendrecv




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   SDP2: B includes a "conf" attribute in its answer. B continues
   sending media to the old remote IP address (192.0.2.1)

          m=audio 30000 RTP/AVP 0
          c=IN IP4 192.0.2.4
          a=curr:qos e2e none
          a=des:qos mandatory e2e sendrecv
          a=conf:qos e2e recv


   SDP3: When A receives RESV messages it sends an updated offer (5) to
   B:

          m=audio 20000 RTP/AVP 0
          c=IN IP4 192.0.2.2
          a=curr:qos e2e send
          a=des:qos mandatory e2e sendrecv


   SDP4: B responds with an answer (6) indicating that the preconditions
   have been met (current status "sendrecv). It is now when B begins
   sending media to the new remote IP address (192.0.2.2).

          m=audio 30000 RTP/AVP 0
          c=IN IP4 192.0.2.4
          a=curr:qos e2e sendrecv
          a=des:qos mandatory e2e sendrecv



12.2 Segmented Status Type

   The call flow of figure 4 shows a basic session establishment using
   the segmented status type. The SDP descriptions of this example are
   shown below:


   SDP1: A includes local and remote QoS preconditions in the initial
   offer. Before sending the initial offer, A reserves resources in its
   access network. This is indicated in the local current status of the
   SDP below:

          m=audio 20000 RTP/AVP 0 8
          c=IN IP4 192.0.2.1
          a=curr:qos local sendrecv
          a=curr:qos remote none
          a=des:qos mandatory local sendrecv
          a=des:qos mandatory remote sendrecv



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   SDP2: B reserves resources in its access network and, since all the
   preconditions are met, returns an answer in a 180 (Ringing) response
   (3).

          m=audio 30000 RTP/AVP 0 8
          c=IN IP4 192.0.2.4
          a=curr:qos local sendrecv
          a=curr:qos remote sendrecv
          a=des:qos mandatory local sendrecv
          a=des:qos mandatory remote sendrecv


   Let's assume that after receiving this response A decides that it
   wants to use only PCM u-law (payload 0), as opposed to both PCM u-law
   and A-law (payload 8). It would send an UPDATE to B possibly before
   receiving the 200 OK for the INVITE (5). The SDP would look like:

          m=audio 20000 RTP/AVP 0
          c=IN IP4 192.0.2.1
          a=curr:qos local sendrecv
          a=curr:qos remote sendrecv
          a=des:qos mandatory local sendrecv
          a=des:qos mandatory remote sendrecv


   B would generate an answer for this offer and place it in the 200 OK
   for the UPDATE.

   Note that this last offer/answer to reduce the number of supported
   codecs may arrive to the user agent server after the 200 OK response
   has been generated. This would mean that the session is established
   before A has reduced the number of supported codecs. To avoid this
   situation, the user agent client could wait for the first answer from
   the user agent before setting its local current status to "sendrecv".

12.3 Offer in a SIP response

   The call flow of figure 5 shows a basic session establishment where
   the initial offer appears in a reliable 1xx response. This example
   uses the end-to-end status type. The SDP descriptions of this example
   are shown below:


   The first INVITE) (1) does not contain a session description.
   Therefore, the initial offer is sent by B in a reliable 183 response.

   SDP1: B includes end-to-end quality of service preconditions in the
   initial offer. Since B uses RSVP, it can know when resources in its



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          A                                            B

          | ***                                        |
          | *R*                                        |
          | *E*                                        |
          | *S*                                        |
          | *E*                                        |
          | *R*                                        |
          | *V*                                        |
          | *A*                                        |
          | *T*                                        |
          | *I*                                        |
          | *O*                                        |
          | *N*                                        |
          | ***                                        |
          |-------------(1) INVITE SDP1--------------->|
          |                                     ***    |
          |                                     *R*    |
          |                                     *E*    |
          |                                     *S*    |
          |                                     *E*    |
          |                                     *R*    |
          |                                     *V*    |
          |                                     *A*    |
          |                                     *T*    |
          |                                     *I*    |
          |                                     *O*    |
          |                                     *N*    |
          |                                     ***    |
          |<----------(2) 180 Ringing SDP2-------------|
          |                                            |
          |----------------(3) PRACK------------------>|
          |                                            |
          |<-----------(4) 200 OK (PRACK)--------------|
          |                                            |
          |                                            |
          |<-----------(5) 200 OK (INVITE)-------------|
          |                                            |
          |------------------(6) ACK------------------>|
          |                                            |
          |                                            |








   Figure 4: Example using the segmented status type

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          A                                            B

          |                                            |
          |----------------(1) INVITE----------------->|
          |                                            |
          |<------(2) 183 Session Progress SDP 1-------|
          |                                            |
          |---------------(3) PRACK SDP 2------------->|
          |  ***                                 ***   |
          |<-*R*--------(4) 200 OK (PRACK)-------*R*---|
          |  *E*                                 *E*   |
          |  *S*                                 *S*   |
          |  *E*                                 *E*   |
          |  *R*                                 *R*   |
          |  *V*                                 *V*   |
          |  *A*                                 *A*   |
          |  *T*                                 *T*   |
          |  *I*                                 *I*   |
          |  *O*                                 *O*   |
          |  *N*                                 *N*   |
          |  ***                                 ***   |
          |-------------(5) UPDATE SDP3----------***-->|
          |                                      ***   |
          |<--------(6) 200 OK (UPDATE) SDP4-----***---|
          |                                      ***   |
          |                                      ***   |
          |                                      ***   |
          |<-------------(7) 180 Ringing---------------|
          |                                            |
          |-----------------(8) PRACK----------------->|
          |                                            |
          |<------------(9) 200 OK (PRACK)-------------|
          |                                            |
          |                                            |
          |                                            |
          |<-----------(10) 200 OK (INVITE)------------|
          |                                            |
          |------------------(11) ACK----------------->|
          |                                            |





   Figure 5: Example of an initial offer in a 1xx response



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   "send" direction are available, because it will receive RESV messages
   from the network. However, it does not know the status of the
   reservations in the other direction. B requests confirmation for
   resource reservations in its "recv" direction to the peer user agent
   A in its answer.

          m=audio 30000 RTP/AVP 0
          c=IN IP4 192.0.2.4
          a=curr:qos e2e none
          a=des:qos mandatory e2e sendrecv
          a=conf:qos e2e recv



   SDP2: A includes its answer if the PRACK for the 183 response.

          m=audio 20000 RTP/AVP 0
          c=IN IP4 192.0.2.1
          a=curr:qos e2e none
          a=des:qos mandatory e2e sendrecv



   After having sent the answer A starts reserving network resources for
   the media stream. When B receives this answer (3) it starts
   performing resource reservation as well. Both UAs use RSVP, so A
   sends PATH messages towards B and B sends PATH messages towards A.

   SDP3: When A receives RESV messages it sends an updated offer (5) to
   B:

          m=audio 20000 RTP/AVP 0
          c=IN IP4 192.0.2.1
          a=curr:qos e2e send
          a=des:qos mandatory e2e sendrecv


   SDP4: B responds with an answer (6) which contains the current status
   of the resource reservation (i.e., recv):

          m=audio 30000 RTP/AVP 0
          c=IN IP4 192.0.2.4
          a=curr:qos e2e recv
          a=des:qos mandatory e2e sendrecv



   As time passes by, B receives RESV messages confirming the



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   reservation. At this point of time, session establishment resumes and
   B returns a 180 (Ringing) response (7).

13 Security Considerations

   An entity in the middle of two user agents establishing a session may
   add desired-status attributes making session establishment
   impossible. It could also modify the content of the current-status
   parameters so that the session is established without meeting the
   preconditions. Integrity protection can be used to avoid these
   attacks.

   An entity performing resource reservations upon reception of
   unathenticated requests carrying preconditions can be an easy target
   for a denial of service attack. Requests with preconditions SHOULD be
   authenticated.

14 IANA considerations

   This document defines three media level SDP attributes: desired-
   status, current-status and conf-status. Their format is defined in
   Section 4.

   Section 4 also defines one standard precondition-type related to the
   attributes above: "qos". If in the future it was needed to
   standardize further precondition-types, they would need to be defined
   in a standards track document. Future precondition-types MUST define
   the semantics with respect to the offer/answer model, as this
   document defined these semantics for quality of service preconditions
   in Section 5.

   This document also defines a new SIP status code (580). Its default
   reason phrase (Precondition Failure) is defined in section 8.

   This document defines a SIP option tag (precondition) in section 10.

15 Contributors

   The following persons contributed and were co-authors on earlier
   versions of this spec:

        K. K. Ramakrishnan (TeraOptic Networks), Ed Miller
        (Terayon), Glenn Russell (CableLabs), Burcak Beser (Pacific
        Broadband Communications), Mike Mannette (3Com), Kurt
        Steinbrenner (3Com), Dave Oran (Cisco), Flemming Andreasen
        (Cisco), Michael Ramalho (Cisco), John Pickens (Com21),
        Poornima Lalwaney (Nokia), Jon Fellows (Copper Mountain
        Networks), Doc Evans (D. R. Evans Consulting), Keith Kelly



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        (NetSpeak), Adam Roach (dynamicsoft), Dean Willis
        (dynamicsoft), Steve Donovan (dynamicsoft), Henning
        Schulzrinne (Columbia University).

   This "manyfolks" draft is the culmination of over two years of work
   by many individuals, most are listed here and in the following
   acknowledgements section. A special note is due to Flemming
   Andreasen, Burcak Beser, Dave Boardman, Bill Guckel, Chuck Kalmanek,
   Keith Kelly, Poornima Lalwaney, John Lawser, Bill Marshall, Mike
   Mannette, Dave Oran, K.K. Ramakrishnan, Michael Ramalho, Adam Roach,
   Jonathan Rosenberg, and Henning Schulzrinne for spearheading the
   initial "single INVITE" quality of service preconditions work from
   previous, non-SIP compatible, "two-stage Invite" proposals. These
   "two-stage INVITE" proposals had their origins from Distributed Call
   Signaling work in PacketCable, which, in turn, had architectural
   elements from AT&T's Distributed Open Systems Architecture (DOSA)
   work [8].

16 Acknowledgments

   The Distributed Call Signaling work in the PacketCable project is the
   work of a large number of people, representing many different
   companies. The authors would like to recognize and thank the
   following for their assistance: John Wheeler, Motorola; David
   Boardman, Daniel Paul, Arris Interactive; Bill Blum, Jay Strater,
   Jeff Ollis, Clive Holborow, General Instruments; Doug Newlin, Guido
   Schuster, Ikhlaq Sidhu, 3Com; Jiri Matousek, Bay Networks; Farzi
   Khazai, Nortel; John Chapman, Bill Guckel, Cisco; Chuck Kalmanek,
   Doug Nortz, John Lawser, James Cheng, Tung-Hai Hsiao, Partho Mishra,
   AT&T; Telcordia Technologies; and Lucent Cable Communications.

   Miguel Angel Garcia, Rohan May and Mark Watson provided helpful
   comments and suggestions.

17 Authors' Addresses

   Gonzalo Camarillo
   Ericsson
   Advanced Signalling Research Lab.
   FIN-02420 Jorvas
   Finland
   electronic mail: Gonzalo.Camarillo@ericsson.com

   Bill Marshall
   AT&T
   Florham Park, NJ 07932
   USA
   electronic mail: wtm@research.att.com



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   Jonathan Rosenberg
   dynamicsoft
   West Orange, NJ 07052
   USA
   electronic mail: jdrosen@dynamicsoft.com

18 Bibliography

   [1] J. Rosenberg, H. Schulzrinne, et al.  , "SIP: Session initiation
   protocol," Internet Draft, Internet Engineering Task Force, Feb.
   2002.  Work in progress.

   [2] M. Handley and V. Jacobson, "SDP: session description protocol,"
   Request for Comments 2327, Internet Engineering Task Force, Apr.
   1998.

   [3] S. Bradner, "Key words for use in RFCs to indicate requirement
   levels," Request for Comments 2119, Internet Engineering Task Force,
   Mar. 1997.

   [4] J. Rosenberg and H. Schulzrinne, "An offer/answer model with
   SDP," Internet Draft, Internet Engineering Task Force, Feb. 2002.
   Work in progress.

   [5] J. Rosenberg, "The SIP UPDATE method," Internet Draft, Internet
   Engineering Task Force, Mar. 2002.  Work in progress.

   [6] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP: a
   transport protocol for real-time applications," Request for Comments
   1889, Internet Engineering Task Force, Jan. 1996.

   [7] J. Rosenberg and H. Schulzrinne, "Reliability of provisional
   responses in SIP," Internet Draft, Internet Engineering Task Force,
   Feb. 2002.  Work in progress.

   [8] C. Kalmanek, W. Marshall, P. Mishra, D. Nortz, and K. K.
   Ramakrishnan, "DOSA: an architecture for providing robust IP
   telephony service," in Proceedings of the Conference on Computer
   Communications (IEEE Infocom) , (Tel Aviv, Israel), Mar. 2000.


   Full Copyright Statement

   Copyright (c) The Internet Society (2002). All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published



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   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works. However, this
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