    Network Working Group                                  G. Camarillo, Ed.
    Request for Comments: 3312                                      Ericsson
    Category: Standards Track                               W. Marshall, Ed.
                                                                        AT&T
                                                                J. Rosenberg
                                                                 dynamicsoft
                                                                October 2002

                      Integration of Resource Management
                     and Session Initiation Protocol (SIP)

    Status of this Memo

       This document specifies an Internet standards track protocol for the
       Internet community, and requests discussion and suggestions for
       improvements.  Please refer to the current edition of the "Internet
       Official Protocol Standards" (STD 1) for the standardization state
       and status of this protocol.  Distribution of this memo is unlimited.

    Copyright Notice

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

    Abstract

       This document defines a generic framework for preconditions, which
       are extensible through IANA registration.  This document also
       discusses how network quality of service can be made a precondition
       for 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 ...................................................  2
       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 .................. 11
       7 Status Confirmation ............................................ 12
       8 Refusing an offer .............................................. 13
       8.1 Rejecting a Media Stream ..................................... 14
       9 Unknown Precondition Type ...................................... 15
       10 Multiple Preconditions per Media Stream ....................... 15
       11 Option Tag for Preconditions .................................. 16
       12 Indicating Capabilities ....................................... 16
       13 Examples ...................................................... 16
       13.1 End-to-end Status Type ...................................... 17
       13.2 Segmented Status Type ....................................... 21
       13.3 Offer in a SIP response ..................................... 23
       14 Security Considerations ....................................... 26
       15 IANA Considerations ........................................... 26
       16 Notice Regarding Intellectual Property Rights ................. 27
       17 References .................................................... 27
       18 Contributors .................................................. 28
       19 Acknowledgments ............................................... 28
       20 Authors' Addresses ............................................ 29
       21 Full Copyright Statement ...................................... 30

    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.

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       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 a 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 BCP 14, 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 the quality
       of service status.

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

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       These two state variables define a certain piece of state of a media
       stream the same way 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 are 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 (Session Progress) 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 in time, B starts alerting
       the user, and session establishment completes normally.

    4 SDP parameters

       We define the following media level SDP attributes:

          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.

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          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
                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.

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                   A                                            B

                   |                                            |
                   |-------------(1) INVITE SDP1--------------->|
                   |                                            |
                   |<------(2) 183 Session Progress SDP2--------|
                   |  ***                                 ***   |
                   |--*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

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

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       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 answerer uses the information of this
       transaction status table to update its local status table.  The
       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.

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    5.1 Generating an offer

       Both user agents MUST maintain a 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 the successful reservation of that
       resource 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
             or not this provision is possible.

          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:

                      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:

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                      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 the latter format mandatory.

       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.

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       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 none remote recv
          a=des:qos none local sendrecv

    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 the highest grade and "none" the
                tag with the 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 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".

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       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, 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 offers 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,
       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 throughout 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.)

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       A user agent server may receive an INVITE request with no offer in
       it.  In this case, following normal procedures defined in [1] and
       [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 an
             initial offer with preconditions, a 180 (Ringing) response with
             preconditions 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
       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 13 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.

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       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 "Current" 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.  Later, if 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

       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.

       Using the 580 (Precondition Failure) status code to refuse an offer
       is useful when the offer comes in an INVITE or in an UPDATE request.
       However, SIP does not provide a means to refuse offers that arrive 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 send a CANCEL or a
       BYE.

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       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 15.2 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."

       580 (Precondition Failure) responses and BYE and CANCEL requests,
       indicating failure to meet certain preconditions, SHOULD contain an
       SDP description, indicating which desired status triggered the
       failure.  Note that this SDP description is not an offer or an
       answer, since it does not lead to the establishment of a session.
       The format of such a description is based on the last SDP (an offer
       or an answer) received from the remote UA.

       For each "m=" line in the last SDP description received, there MUST
       be a corresponding "m=" line in the SDP description indicating
       failure.  This SDP description MUST contain exactly the same number
       of "m=" lines as the last SDP description received.  The port number
       of every "m=" line MUST be set to zero, but the connection address is
       arbitrary.

       The desired status line corresponding to the precondition that
       triggered the failure MUST use the "failure" strength-tag, as shown
       in the example below:

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

    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.

    Camarillo, et. al.          Standards Track                    [Page 14]

    RFC 3312       Integration of Resource Management and SIP   October 2002

    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 Multiple Preconditions per Media Stream

       A media stream MAY contain multiple preconditions. Different
       preconditions MAY have the same precondition-type and different
       status-types (e.g., end to end and segmented quality of service
       preconditions) or different precondition-types (this document only
       defines the "qos" precondition type, but extensions may define more
       precondition-types in the future).

       All the preconditions for a media stream MUST be met in order to
       resume session establishment. The following example shows a session
       description that uses both end-to-end and segmented status-types for
       a media stream.

             m=audio 20000 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=curr:qos e2e none
             a=des:qos optional e2e sendrecv

    Camarillo, et. al.          Standards Track                    [Page 15]

    RFC 3312       Integration of Resource Management and SIP   October 2002

    11 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 in either a
       Supported header field or in a Require header field.  It is, however,
       RECOMMENDED that the Supported header field be 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
       including preconditions in the SDP MUST support the PRACK and UPDATE
       methods. Consequently, it MUST include the "100rel" [7] tag in the
       Supported header field and SHOULD include an Allow header field with
       the "UPDATE" tag [5].

    12 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.

    13 Examples

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

    Camarillo, et. al.          Standards Track                    [Page 16]

    RFC 3312       Integration of Resource Management and SIP   October 2002

    13.1 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:

       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, 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

    Camarillo, et. al.          Standards Track                    [Page 17]

    RFC 3312       Integration of Resource Management and SIP   October 2002

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

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

    Camarillo, et. al.          Standards Track                    [Page 18]

    RFC 3312       Integration of Resource Management and SIP   October 2002

                   A                                            B

                   |                                            |
                   |-------------(1) INVITE SDP1--------------->|
                   |                                            |
                   |<------(2) 183 Session Progress SDP2--------|
                   |  ***                                 ***   |
                   |--*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

    Camarillo, et. al.          Standards Track                    [Page 19]

    RFC 3312       Integration of Resource Management and SIP   October 2002

       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.

       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 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

       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
       that B begins sending media to the new remote IP address (192.0.2.2).

    Camarillo, et. al.          Standards Track                    [Page 20]

    RFC 3312       Integration of Resource Management and SIP   October 2002

                   A                                            B

                   |                                            |
                   |-------------(1) INVITE SDP1--------------->|
                   |                                            |
                   |<------(2) 183 Session Progress SDP2--------|
                   |  ***                                 ***   |
                   |--*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

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

    13.2 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:

    Camarillo, et. al.          Standards Track                    [Page 21]

    RFC 3312       Integration of Resource Management and SIP   October 2002

       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

       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".

    Camarillo, et. al.          Standards Track                    [Page 22]

    RFC 3312       Integration of Resource Management and SIP   October 2002

    13.3 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 (Session
       Progress) 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
       "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 in the PRACK for the 183 (Session
       Progress) 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

    Camarillo, et. al.          Standards Track                    [Page 23]

    RFC 3312       Integration of Resource Management and SIP   October 2002

                   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

    Camarillo, et. al.          Standards Track                    [Page 24]

    RFC 3312       Integration of Resource Management and SIP   October 2002

                   A                                            B

                   |                                            |
                   |----------------(1) INVITE----------------->|
                   |                                            |
                   |<------(2) 183 Session Progress SDP1--------|
                   |                                            |
                   |---------------(3) PRACK SDP2-------------->|
                   |  ***                                 ***   |
                   |<-*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

       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.

    Camarillo, et. al.          Standards Track                    [Page 25]

    RFC 3312       Integration of Resource Management and SIP   October 2002

       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, B receives RESV messages confirming the reservation.
       At this point in time, session establishment resumes and B returns a
       180 (Ringing) response (7).

    14 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
       unauthenticated requests carrying preconditions can be an easy target
       for a denial of service attack.  Requests with preconditions SHOULD
       be authenticated.

    15 IANA Considerations

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

       This document defines a framework for using preconditions with SIP.
       Precondition-types to be used with this framework are registered by
       the IANA when they are published in standards track RFCs.  The IANA
       Considerations section of the RFC MUST include the following
       information, which appears in the IANA registry along with the RFC
       number of the publication.

    Camarillo, et. al.          Standards Track                    [Page 26]

    RFC 3312       Integration of Resource Management and SIP   October 2002

          o  Name of the precondition-type. The name MAY be of any length,
             but SHOULD be no more than ten characters long.

          o  Descriptive text that describes the extension.

       The only entry in the registry for the time being is:

       Pecondition-Type    Reference   Description
       ----------------    ---------   -----------
       qos                 RFC 3312    Quality of Service preconditions

       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 11.

    16 Notice Regarding Intellectual Property Rights

       The IETF has been notified of intellectual property rights claimed in
       regard to some or all of the specification contained in this
       document.  For more information consult the online list of claimed
       rights.

    17 References

       [1] 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.

       [2] Handley, M. and V. Jacobson, "SDP: Session Description Protocol",
           RFC 2327, April 1998.

       [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
           Levels", BCP 14, RFC 2119, March 1997.

       [4] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
           Session Description Protocol (SDP)", RFC 3264, June 2002.

       [5] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
           Method," RFC 3311, September 2002.

       [6] Schulzrinne, S., Casner, S., Frederick, R. and V. Jacobson, "RTP:
           A Transport Protocol for Real-Time Applications", RFC 1889,
           January 1996.

       [7] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional
           Responses in Session Initiation Protocol (SIP)", RFC 3262, June
           2002.

    Camarillo, et. al.          Standards Track                    [Page 27]

    RFC 3312       Integration of Resource Management and SIP   October 2002

       [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.

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

       This "manyfolks" document 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].

    19 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.

    Camarillo, et. al.          Standards Track                    [Page 28]

    RFC 3312       Integration of Resource Management and SIP   October 2002

       Miguel Angel Garcia-Martin, Rohan Mahy and Mark Watson provided
       helpful comments and suggestions.

    20 Authors' Addresses

       Gonzalo Camarillo
       Ericsson
       Advanced Signalling Research Lab.
       FIN-02420 Jorvas
       Finland

       EMail: Gonzalo.Camarillo@ericsson.com

       Bill Marshall
       AT&T
       Florham Park, NJ 07932
       USA

       EMail: wtm@research.att.com

       Jonathan Rosenberg
       dynamicsoft
       72 Eagle Rock Ave
       East Hanover, NJ 07936
       USA

       EMail: jdrosen@dynamicsoft.com

    Camarillo, et. al.          Standards Track                    [Page 29]

    RFC 3312       Integration of Resource Management and SIP   October 2002

    21 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
       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
       document itself may not be modified in any way, such as by removing
       the copyright notice or references to the Internet Society or other
       Internet organizations, except as needed for the purpose of
       developing Internet standards in which case the procedures for
       copyrights defined in the Internet Standards process must be
       followed, or as required to translate it into languages other than
       English.

       The limited permissions granted above are perpetual and will not be
       revoked by the Internet Society or its successors or assigns.

       This document and the information contained herein is provided on an
       "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
       TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
       BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
       HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
       MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

    Acknowledgement

       Funding for the RFC Editor function is currently provided by the
       Internet Society.

    Camarillo, et. al.          Standards Track                    [Page 30]