path: root/doc/rfc/rfc3312.txt

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.





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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











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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.






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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







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   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).









































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











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   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).


















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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) 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:








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   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".






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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



















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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 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.





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   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.





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      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.



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   [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.





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   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






















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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]
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