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rfc:rfc3312

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.

Camarillo, et. al. Standards Track [Page 1] RFC 3312 Integration of Resource Management and SIP October 2002

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.

Camarillo, et. al. Standards Track [Page 2] RFC 3312 Integration of Resource Management and SIP October 2002

 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

Camarillo, et. al. Standards Track [Page 3] RFC 3312 Integration of Resource Management and SIP October 2002

 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.

Camarillo, et. al. Standards Track [Page 4] RFC 3312 Integration of Resource Management and SIP October 2002

    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.

Camarillo, et. al. Standards Track [Page 5] 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 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.

Camarillo, et. al. Standards Track [Page 6] RFC 3312 Integration of Resource Management and SIP October 2002

 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.

Camarillo, et. al. Standards Track [Page 7] RFC 3312 Integration of Resource Management and SIP October 2002

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:

Camarillo, et. al. Standards Track [Page 8] RFC 3312 Integration of Resource Management and SIP October 2002

                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.

Camarillo, et. al. Standards Track [Page 9] RFC 3312 Integration of Resource Management and SIP October 2002

 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".

Camarillo, et. al. Standards Track [Page 10] RFC 3312 Integration of Resource Management and SIP October 2002

 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.)

Camarillo, et. al. Standards Track [Page 11] RFC 3312 Integration of Resource Management and SIP October 2002

 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.

Camarillo, et. al. Standards Track [Page 12] RFC 3312 Integration of Resource Management and SIP October 2002

 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.

Camarillo, et. al. Standards Track [Page 13] RFC 3312 Integration of Resource Management and SIP October 2002

 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]

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