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SIPPING                                                       B. Stucker
Internet-Draft                                                    Nortel
Intended status: Informational                             July 23, 2006
Expires: January 24, 2007


    Coping with Early Media in the Session Initiation Protocol (SIP)
              draft-stucker-sipping-early-media-coping-00

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Copyright Notice

   Copyright (C) The Internet Society (2006).














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Abstract

   It is widely known that early media is an area within SIP in which
   the existing mechanisms within the protocol are not fully capable of
   handling to make rendering of these media flows a reliable process.
   Unfortunately, the need for media to be presented to an originator
   prior to the terminator answering is persistent.  As such, a path
   forward to fixing this limitation is needed.  This document seeks to
   document common ways that SIP networks handle early media, what side-
   effects those mechanisms have, and what the desired behaviors are.
   From this, it goes on to try to address as many early media issues as
   possible using as simple of a generic mechanism as possible.







































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Types of Early Media . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Pre-routing early media  . . . . . . . . . . . . . . . . .  6
     3.2.  Pre-presentation early media . . . . . . . . . . . . . . .  6
     3.3.  Post-presentation early media  . . . . . . . . . . . . . .  7
     3.4.  Non-SDP early media  . . . . . . . . . . . . . . . . . . .  7
   4.  Current common coping mechanisms for early media . . . . . . .  9
     4.1.  Problems with current coping mechanisms  . . . . . . . . . 10
       4.1.1.  Proxy-side coping mechanisms . . . . . . . . . . . . . 10
         4.1.1.1.  Proxy SDP stripping  . . . . . . . . . . . . . . . 10
         4.1.1.2.  Proxy SDP weighting  . . . . . . . . . . . . . . . 10
       4.1.2.  Client-side coping mechanisms  . . . . . . . . . . . . 10
         4.1.2.1.  Client detection of forking  . . . . . . . . . . . 10
         4.1.2.2.  Client slow-start INVITE . . . . . . . . . . . . . 11
   5.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 12
     5.1.  Deprecation of forking . . . . . . . . . . . . . . . . . . 12
     5.2.  Deprecation of early media . . . . . . . . . . . . . . . . 13
     5.3.  Originating UA's to render early media . . . . . . . . . . 13
     5.4.  Downstream signaling of acceptance . . . . . . . . . . . . 13
     5.5.  Upstream signaling of importance . . . . . . . . . . . . . 14
     5.6.  Universal backward-compatibility . . . . . . . . . . . . . 14
     5.7.  Recursive forking  . . . . . . . . . . . . . . . . . . . . 14
   6.  Recommendations  . . . . . . . . . . . . . . . . . . . . . . . 15
     6.1.  Early Media Classification and Prioritization  . . . . . . 15
       6.1.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . 15
         6.1.1.1.  The Early-Media-Class header . . . . . . . . . . . 15
           6.1.1.1.1.  Early-Media-Class types  . . . . . . . . . . . 16
           6.1.1.1.2.  Early-Media-Q-Value  . . . . . . . . . . . . . 16
     6.2.  Early Media Flow Negotiation . . . . . . . . . . . . . . . 16
       6.2.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . 16
       6.2.2.  SDP parameters . . . . . . . . . . . . . . . . . . . . 17
       6.2.3.  Usage of emflow with offer/answer  . . . . . . . . . . 17
         6.2.3.1.  Meaning of a=emflow:none . . . . . . . . . . . . . 17
         6.2.3.2.  Meaning of a=emflow:send . . . . . . . . . . . . . 18
         6.2.3.3.  Meaning of a=emflow:recv . . . . . . . . . . . . . 18
         6.2.3.4.  Meaning of a=emflow:sendrecv . . . . . . . . . . . 18
       6.2.4.  Option tag for emflow  . . . . . . . . . . . . . . . . 19
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 23
   Intellectual Property and Copyright Statements . . . . . . . . . . 24






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1.  Introduction

   One of the mechanisms within SIP [2] that has caused much
   consternation (and interesting service scenarios) is forking,
   especially forking of INVITE requests.  This is where a SIP INVITE
   request sent to a SIP proxy is resolved into two or more destinations
   which are signaled in parallel.  When this occurs, multiple
   downstream parties will receive similar INVITE requests to initiate a
   SIP session from a given originating SIP user agent (UA).  This
   creates the possibility of race conditions where the ordering of the
   provisional and final responses to this request, as observed by the
   originating SIP UA, may potentially arrive in any order, or not at
   all.

   Another mechanism in SIP [2] that looks simple, but causes difficult
   interactions, was introduced to handle SIP to PSTN interworking.
   Because the PSTN has a specific set of behaviors which require that
   only one endpoint in the PSTN network (typically the last PSTN switch
   reached) may generate media back to the originator of a PSTN call,
   generation of early media (media produced prior to the intended
   terminator of a call answering the call) is relatively straight-
   forward.  In SIP, this PSTN interaction with early media was handled
   by allowing any endpoint that has received an SDP offer as part of
   setting up a session to be able to immediately generate media back to
   the to SDP offerer.  Further, the SDP offerer was obligated to be
   prepared to render any media received at the location specified in
   the SDP offer at any time as long as the session was in a setup or
   stable state.

   Each of these mechanisms, taken separately, can create complex
   signaling flows and difficult service interactions to resolve.
   Together, however, they compound the effects of one another to create
   an area of study that has been open within the SIP design community
   for some time.  Several extensions to SIP [2] have been proposed to
   handle some of the various effects that early media suffers from.
   However, none have fully attacked a few key areas of interest:

   o    Controlling the order and timing of early media stream rendering
        at the originating SIP UAC.

   o    Knowing under what general conditions early media flows are
        potentially being sent to the originating SIP UAC.

   This document seeks to capture the salient requirements for these
   areas, and propose a mechanism for handling these early media
   interactions in a more predictable manner.





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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].














































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3.  Types of Early Media

   Not all early media is created equal, some types are more problematic
   than others.  There are four generic types of early media within SIP:

   1.  Pre-routing early media - This is early media that is, conveyed
       via SDP, and is presented to the originator by a proxy before
       routing on the URI is started.

   2.  Pre-presentation early media - This is early media that is
       presented to the originator, conveyed via SDP, by a proxy after
       the URI has been routed upon, but before any forwarding of the
       INVITE request has occurred.

   3.  Post-presentation early media - This is early media that is
       presented to the originator, conveyed via SDP, by either a
       forking proxy or any subsequent hop after the INVITE request has
       been forwarded from the proxy.

   4.  Non-SDP early media - This is early media that may be presented
       to the originator at any time through means other than SDP, such
       as the Alert-Info header as defined in [2]

3.1.  Pre-routing early media

   Pre-routing early media is typically generated and characterized by a
   proxy that has an associated media resource.  An example of this type
   of early media would be a brief 'branding' message that is played to
   the originator thanking them for using the service provider
   associated with the originator's local outbound proxy.  When the
   message ends, the media resource signals this to the proxy and
   routing of the request continues per [2]

   This type of early media typically does not pose the originator's
   local outbound proxy any issues unless the client is using one of the
   mechanisms defined in Section 4.1.2 or something similar.  This is
   because the proxy is in complete control over the pace at which the
   terminator will be routed to relative to the media stream being
   presented.  If the proxy attempting to present pre-routing early
   media to the originator is a subsequent proxy from the originator's
   local outbound proxy, then the service may not work due to upstream
   proxies employing one of the mechanisms described in Section 4.1.1

3.2.  Pre-presentation early media

   Pre-presentation early media is similar to pre-routing early media
   except that it may take into account the routes that the proxy is
   about to route the INVITE request to in its decision of what to play.



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   This may allow the proxy to employ one of the proxy-side early media
   coping mechanisms defined in Section 4.1.1.  Likewise, the proxy may
   inject its own SDP answer into the signaling to the originator to
   kick off services like colorful ringback tone (CRBT) where the
   originator is hearing a recording (typically music) selected by the
   terminator while the network attempts to reach the terminating party.

   Pre-presentation early media also differs from non-SDP early media in
   that the proxy or proxies are manipulating the SDP offer/answer
   rather than SIP headers such as Alert-Info (as defined in [2]) to
   signify what media the originator should be rendering.  There are
   several potential reasons why the Alert-Info header is not used in
   this case: the service may be interactive, requiring two-way media in
   order to work (such as digit collection for a credit card number), or
   may not want to rely on the originator's ability to render the
   information in the Alert-Info header to the end user (such as a call
   originating from the PSTN through a SIP gateway).

3.3.  Post-presentation early media

   Post-presentation early media is most typically characterized by the
   ugly interactions that arise between it and forking.  Since this is
   early media that has come about after the proxy has potentially
   caused multiple endpoints to be contacted, and therefore the
   possibility that multiple early media streams may have been
   triggered, it is commonly considered to be the worst-case scenario
   with early media.  Even worse, because downstream proxies and
   endpoints may not be aware that the INVITE has forked, they may be
   generating what is ultimately going to be post-presentation early
   media while thinking that they are generating pre-routing, pre-
   presentation, or non-SDP early media erroneously.

3.4.  Non-SDP early media

   Non-SDP early media is typically characterized by the presence of an
   Alert-Info header [2].  The Alert-Info header specifies a URI that
   the originator may go to in order to receive a file or stream that
   contains information (such as a wave recording) about the ringback
   tone the terminator wishes the originator to hear.  It is somewhat
   simpler in that it is not part of the offer/answer model, and that it
   is not trying to create a two-way media stream.  The problem is that
   the interaction between this mechanism, inband ringback, client
   generated ringback, and other forms of early media is not well
   specified in SIP [2], the offer/answer model [4] or other early media
   related RFCs such as [5].

   At the moment, non-SDP early media is for future study, although it
   is envisioned that this document would clarify the behavioral



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   interactions between of the type of early media and other types of
   early media.

















































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4.  Current common coping mechanisms for early media

   A number of mechanisms exist for coping with early media.  They all
   rely, generally, on 'fixing' the early media problem by 'breaking'
   the behaviors specified in other RFCs (or at least bending the spirit
   of them to some extent):

   1.  Proxy SDP stripping - If a proxy detects that it is about to fork
       an INVITE, it keeps track of this fact in its processing state
       for the INVITE transaction.  Any SDP answers in provisional
       responses are stripped before being forwarded upstream.  The SDP
       answer may be added into a 200 response upstream from last
       provisional SDP answer received if SDP is not already present in
       the message to ensure that the offer/answer exchange is
       completed.  This effectively turns off early media.

   2.  Proxy SDP weighting - If a proxy detects that it had previously
       forked the INVITE to which it is now receiving a provisional
       response it may allow the first provisional response to retain
       the SDP answer in the message body and strip other SDP answers in
       provisional responses per the proxy SDP stripping methodology.
       This mechanism is used to favor SDP that the proxy may have some
       control over.  For instance, if the proxy knows that one forked
       leg is to a media server streaming CRBT media to the originator,
       it may allow that SDP answer to flow back, but block all other
       SDP answers on other legs in the meantime.

   3.  Client detection of forking - Clients may play audible ringback
       to the originator until the first SDP answer is received at which
       time they may switch to playing the media for that SDP answer.
       Upon detecting that the INVITE forked (reception of two or more
       distinct SDP answers or [2] 'TO' header tags) the client may
       irrevocably return to playing audible ringback until the call is
       answered for the duration of that call's setup period, or until
       an error condition arises.

   4.  Client slow-start - Clients may wish to simply not include any
       SDP in their initial INVITE message in order to accumulate a set
       of SDP offers from their prospective terminating endpoints.  Such
       INVITEs are known as 'slow-start' INVITEs, because the SDP offer/
       answer exchange gets off to a 'slow start'.  These may also be
       used in protocol interworkings (notably H.323 to SIP) with no
       intent as to managing early media.  The client can either use
       PRACK or UPDATE to respond to offers received in provisional
       responses at the point in time the originating client wishes to
       stage early media streams.





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4.1.  Problems with current coping mechanisms

4.1.1.  Proxy-side coping mechanisms

4.1.1.1.  Proxy SDP stripping

   This is a very common mechanism, perhaps second only to the two
   client mechansims mentioned above.  When a proxy employs this
   mechanism, it remembers when forking has occurred and removes any SDP
   in provisional responses as a result.  This means that if the
   originator supports reliable provisional responses (100rel) as
   defined in [6], that this option tag must be removed by the proxy
   before forwarding the INVITE to each forked leg.  Otherwise it may be
   forced to potentially handle SDP in negotiation within a PRACK
   transaction for the originating client with little or no information
   about the originating client's capabilities.  In the case that the
   originator requires support for PRACK the proxy may have to fail the
   call setup, handle very complex negotiation signaling in the case
   that the call forks, or simply not fork the call.

   Additionally, this mechanism also completely breaks any early media
   services or announcements, some of which may be critical to proper
   completion of the call or as to the billing disposition of the call
   upon answer.  For instance, the call may fork to a PSTN gateway that
   is trying to tell the originator that it is about to bill them $500
   to complete the current call.  With proxy SDP stripping this
   announcement would not be heard by the originator.

4.1.1.2.  Proxy SDP weighting

   Proxy weighting of SDP can be useful in situations where the proxy
   knows what is going on with the call routing for each leg.  However,
   lack of information as to why downstream elements are sending SDP in
   provisional responses can cause proxies to weight the SDP
   incorrectly.  Further, if multiple proxies are traversed, the SDP
   that is accepted for delivery to the originating UA may not be the
   SDP selected at any given proxy.  There is no indication to
   downstream network clients as to what has happened with their SDP as
   it traverses proxys back upstream towards the originator.  Likewise,
   the $500 warning announcement presented in the previous section may
   or may not be heard.

4.1.2.  Client-side coping mechanisms

4.1.2.1.  Client detection of forking

   This mechanism is where a client may play audible ringback while
   waiting for an initial provisonal or final response to an INVITE



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   message it originated.  When the first provisional response with SDP
   is received, it may switch from playing audible ringback to rendering
   the media stream defined in the SDP.  If a subsequent provisional
   response is received from a different endpoint (identifiable by a
   different to-tag in the 'TO' header as defined in [2]) it stops
   rendering any early media media packets it is receiving and typically
   returns to audible ringback.  Upon receiving a non-3xx final
   response, the UA switches media appropriately to the response.  For
   3xx responses, the client continues to play audible ringback if that
   was what is currently being rendered, or switches (typically) to
   ringback again if it was rendering media packets.  This mechanism is
   used by client devices for a number of reasons:

   o  What gets presented to the end user is predictable.

   o  Does not rely on the set of proxies handling any given INVITE
      request to do anything special.

   o  Is easy to implement.

   The problem with this approach is that it often causes early media to
   break altogether.  This means that if a leg that the call was forked
   to is awaiting media from the originating client (such as prompting
   for digit collection, like a credit card number or extension) this
   may fail depending on what is received at the client by other
   endpoints in provisional responses.  Network services that utilize
   early media are likely to fail when a client behaves in this manner.
   What's worse, is that there's no indication as to what the client is
   doing to the downstream network elements.  Clipping may occur if
   there is a significant delay between the answer to the INVITE being
   received and the first media packet being received at the originating
   client.

4.1.2.2.  Client slow-start INVITE

   Slow-start INVITEs circumvent the problem of having to immediately
   render media packets from an unknown set of terminating endpoints by
   not giving those endpoints anywhere to send the media to.  However,
   this mechanism has some serious drawbacks, most notably guaranteed
   clipping (potentially severe if the SDP offer is not received from
   the other end until a 200 response is received) and the potential for
   an increased number of messaging round-trips to setup a call.

   Due to some service designs and protocol interworking slow-start
   INVITEs will continue to be seen, but due to the clipping problems
   associated with slow-start INVITEs this coping mechanism is
   considered to be incomplete.




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5.  Requirements

   The following requirements are considered to be the starting point in
   more formally discussing improvements to SIP for early media
   interactions:

   R1:  Deprecation of forking within the SIP protocol [2] is considered
        to be out-of-scope of the possible solutions (sorry Dean).

   R2:  Deprecation of early media from within the SIP protocol [2] is
        considered to be out-of-scope of the possible solutions (sorry
        again, Dean).

   R3:  SIP UAs that are attempting to create a new SIP dialog using the
        INVITE method should no longer be obligated to blindly render
        media packets that are sent to them as part of a SIP offer sent
        in the INVITE.

   R4:  A mechanism should exist by which an originating SIP UA can
        signal to a downstream SIP endpoint that it is now willing to
        accept media packets.

   R5:  A mechanism should exist by which a terminating SIP UA can
        signal to an upstream SIP endpoint what type of early media (if
        known) it wishes to present to the originating UA, if it
        requires one-way, or two-way media flows, and the relative
        importance of the early media.

   R6:  Universal backwards-compatability is a secondary goal.  Where
        possible, backwards-compatability with clients that do not
        implement recommendations in this draft should be preserved.

   R7:  The mechanism must be able to deal with recursive forking
        scenarios.  This is where an INVITE passes two or more proxies
        that both choose to fork the request to two or more endpoints at
        each proxy in parallel.

5.1.  Deprecation of forking

   Deprecation of forking from SIP [2] is considered to be out of scope.
   This is due to the heavy deployment of forking in existing
   implementations for key routing services.  Changes of this nature are
   considered by the author (and others) to be of too large a scope
   relative to the problem at hand and are subsequently excluded from
   this draft in favor of searching for less radical solutions.






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5.2.  Deprecation of early media

   Deprecation of early media from SIP [2] is considered to be out of
   scope.  Early media is required in order to handle certain PSTN
   interactions as defined in RFC-3398 [3] and elsewhere.  In addition,
   the desire to provide announcements and other media prior to the
   terminating party answering the call is considered desirable and must
   therefore use some form of "pre-answer" media (currently known as
   early media).

5.3.  Originating UA's to render early media

   Currently, section 5.1 of the offer/answer model [4] states that the
   offerer in an SDP offer/answer exchange must be prepared to receive
   media from media streams described in the offer as being 'recvonly'
   or 'sendrecv'.  Further, in section 6.1 of [4] states that the
   answerer in an SDP offer/answer exchange may immediately send media
   to media streams that are described in the answer as being 'sendrecv'
   (note: [4] does not explicitly state as much, but it is assumed that
   media streams that are 'sendonly' in the answer can also have media
   immediately sent to them by the answering UA).

   These statements, taken together, create an obligation upon the
   originating UA to render any early media sent to them by anyone to
   whom their SDP was sent to unless the media stream was defined to be
   'sendonly' or 'inactive' state when offered.  This is useful in
   resolving the PSTN interactions in [3], especially as noted in the
   example call flows and ACM message processing in section 7 of that
   document.  This obligation on the part of the originating UA has
   subsequently been used in the absence of actual PSTN interworking to
   provide services that mimic the PSTN network (such as providing far-
   end announcements), or provide other services such as colorful
   ringback tones (CRBT) in which media is streamed to the originator
   while the terminator is being located/alerted.  An argument can, and
   has, been made that simply because a service exists in the PSTN
   world, that it does not mean that it must exist within SIP.  However,
   given the prevalence of services that utilize early media, and the
   number of RFCs that talk about dealing with various aspects of early
   media, this particular train appears to have long ago left the
   station.  It is not the intent of this document to pass judgement
   upon these services, but to find a way to cope with them in a more
   robust manner than currently is available.

5.4.  Downstream signaling of acceptance

   An INVITE with SDP should serve two simple purposes: establish the
   path by which all signaling shall follow to/from the originating and
   set of terminating clients, let the terminating party know what sort



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   of communications the originator can and will engage in.  Currently,
   SDP offers also imply tacit acceptance of any and all media that
   might be generated in the reverse path upstream towards the
   originator.  This should not necessarily always be the case, and a
   mechanism whereby the originator may assert that it is further ready
   to receive media packets is needed.

5.5.  Upstream signaling of importance

   A provisional response from a terminating party currently implies
   that the terminating party is listening to the SIP signaling it is
   receiving, and (if an SDP answer is present) the type of
   communications that the terminator wishes to engage in (if any).
   What is missing is a way for the terminating party to tell upstream
   entities what sort of demands it has upon the originator for
   rendering of its early media, and the relative importance associated
   with the media that it generates towards the originator.  This helps
   the originator decide what is important and what is not when choosing
   which media stream it should render (if it wishes to, see
   Section 4.1.2.1).

5.6.  Universal backward-compatibility

   There are scenarios in which there is no way to cope appropriately
   with early media streams.  An example would be a call that forks to
   an ISUP PSTN gateway as defined in [3] that is ignorant of the
   content with which it is generating media packets.  There is no
   reliable indication in ISUP as to what the other end might be doing
   for early media in the CPG or ACM messages.  It is possible that a
   cause code is present in the CPG in some ISUP to legacy platform
   interworking scenarios, but these are not present generally in ISUP
   signaling flows, and therefore cannot be relied upon.  Mechansims to
   deal with these types of devices is currently for future study and
   not explored further here at this time.

5.7.  Recursive forking

   The mechanism should be able to deal with recursive forking
   scenarios.  This would be where two or more independent proxies fork
   a given INVITE request from an originating client.  In this case, the
   proxies are normally not coordinated in their operations.  As a
   result, the mechanism proposed should be robust enough to allow for
   both end-to-middle and end-to-end negotiation of early media.








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6.  Recommendations

   The following sections include recommendations that create a
   framework that is capable of both identifying/prioritizing the type
   of early media being presented to the originator, and giving the
   originating client a means by which it can control the order in which
   early media flows are presented to it.

6.1.  Early Media Classification and Prioritization

6.1.1.  Overview

   Regardless of the mechanism that is used to control the presentation
   of early media, if at any point more than one endpoint is attempting
   to stream early media to the originator a two problems arise:

   o  Nobody upstream of the device attempting to stream early media to
      the originator is aware of what exactly it is that the early media
      generator is generating.  Is it advertising?  Is it an important
      message?  Who knows.  This is important not only for the
      originating client (see Section 4.1.2.1), but proxies as well as
      they may be employing a weighting mechanism as described in
      Section 4.1.1.2.

   o  The device generating the early media may have no idea how many
      other devices peer to it or downstream from it are also trying to
      generate early media.  Again, this is important if the client is
      using the client-side detection of forking mechanism defined in
      Section 4.1.2.1.

   In order to rectify this situation a new SIP header 'Early-Media-
   Class' is defined, which includes information such as the category
   and priority of the SDP being offered.  If there is SDP with the
   Early-Session disposition, as defined in [5], the header applies to
   the section of SDP that the Early-Session disposition refers to.
   Upon answer of the INVITE, all processing of media streams and SDP
   shall revert to [2]RFC-3261 rules.  It is presented as a SIP header
   instead of in the SDP so that it can be read by endpoints that may
   not have access to the SDP, and so it can be changed without having
   to touch the message body.

6.1.1.1.  The Early-Media-Class header

   The Early-Media-Class header is inserted by an endpoint generating
   media in an INVITE response to identify the type and priority of any
   SDP associated with an early-session disposition, or absent that, and
   SDP answer received from a given terminating endpoint.  It is defined
   as follows:



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      Early-Media-Class = "Early-Media-Class" HCOLON Early-Media-Type
      ";q=" Early-Media-Q-Value

      Early-Media-Type = "Advertisement" / "Warning" / "Critical" /
      "Two-way" / "RFC-3264" / "Unknown"

      Early-Media-Q-Value = "q" EQUAL qvalue

      qvalue = ( "0" [ "." 0*3DIGIT ] ) / ( "1" [ "." 0*3("0") ] )

6.1.1.1.1.  Early-Media-Class types

   The early-media-class type gives the type of early media that is
   being presented to the originator:

   1.  Advertisement - A non-critical advertisement.

   2.  Warning - A non-critical announcement.

   3.  Critical - A critical announcement, such as: "We're about to bill
       you for $10k".

   4.  Two-way - The endpoint presenting early media wishes to establish
       a two-way early media session before completing the call.

   5.  RFC-3264 - The default behaivor defined in RFC-3264 is requested.

   6.  Unknown - The nature of the early media being presented to the
       originator is unknown (such as from a PSTN gateway receiving a
       generic announcement.)

6.1.1.1.2.  Early-Media-Q-Value

   This is simply a q-value such as that defined in section 20.10 of [2]
   that is used for ordering of early media types.  Ties should be
   resolved in favor of the more important type of early media.

6.2.  Early Media Flow Negotiation

   The following sections take the requirements from the earlier section
   and try to create a mechanism that can satisfy the requirements in
   Section 5.  This mechanism is built along similar lines as the SIP
   preconditions framework [7].

6.2.1.  Overview

   A simple mechanism is introduced that tells terminators what the
   originator expects to have happen with respect to early media.  This



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   information may also be of use to intermediate nodes that also wish
   to generate early media.  The mechanism differs from the SDP [8]
   'a=recvonly', 'a=sendonly', 'a=sendrecv', and 'a=inactive' attributes
   in that the final media flow mode can be negotiated and ready upon
   answer without further messaging, and from the preconditions [5] SDP
   attributes in that QoS can be negotiated separately as well.

6.2.2.  SDP parameters

   The following media-level parameters are defined:

      early-media-flow-status = "a=emflow:" direction-tag

      direction-tag = ("none" | "send" | "recv" | "sendrecv")

   The early-media-flow-status 'emflow' denotes the current state of the
   early media from the perspective of the originator.  It is expected
   that the directionality indicators such as 'sendrecv', 'sendonly',
   'recvonly', and 'inactive' are otherwise unaffected.  These values
   may be changed in subsequent offer/answer exchanges to allow the
   originator to properly stage multiple early media streams according
   to the Early-Media-Class header values.  For example, an originator
   may specify 'a=emflow:none' initially to suppress all early media
   flows, and then send an UPDATE with a new SDP offer to an endpoint
   the originator received an early media indication from with
   'a=emflow:recv' to denote that the originator is now willing to
   receive early media.

   Regardless of the value of this parameter, upon answer, both
   endpoints may immediately begin exchanging media packets upon answer
   according to [2], [4] and [8].Intermediate proxies should honor this
   indication, and adjust their behavior accordingly, potentally causing
   them to divert from their normal early media coping mechanisms.

6.2.3.  Usage of emflow with offer/answer

6.2.3.1.  Meaning of a=emflow:none

   If the emflow value of 'none' is set in an the SDP offer, it
   indicates that the endpoint generating the offer will not accept
   early media and that anyone accepting this SDP offer MUST NOT send
   early media.  If the emflow value in the SDP offer was 'none', then
   the emflow value in the SDP answer MUST be set to 'none' as well.

   If the emflow value of 'none' is set in an SDP answer, it indicates
   that the endpoint generating the answer will not generate early
   media.  Any early media sent to it per [4] MAY be discarded.  The SDP
   offeror can take this indication to mean that they should not expect



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   early media packets from this endpoint, and that any received prior
   to answer from this source may safely be discarded.

6.2.3.2.  Meaning of a=emflow:send

   If the emflow value of 'send' is set in an the SDP offer, it
   indicates that the endpoint generating the offer may send early media
   packets, but will not accept early media.  Anyone accepting this SDP
   offer MUST NOT send early media, but SHOULD process received early
   media packets if it is appropriate to the device receiving packets to
   do so.  If the emflow value in the SDP offer was 'send', then the
   emflow value in the SDP answer MUST be set to 'none' or 'recv'
   depending on whether the application intends to process the early
   media packets that the offeror may send to it.

   If the emflow value of 'send' is set in an SDP answer, it indicates
   that the endpoint generating the answer may generate early media but
   will not process any sent to it.  Any early media sent to it per [4]
   MAY be discarded.  The SDP offeror can take this indication to mean
   that they should expect early media packets from this endpoint and
   behave appropriately.

6.2.3.3.  Meaning of a=emflow:recv

   If the emflow value of 'recv' is set in an the SDP offer, it
   indicates that the endpoint generating the offer may be sent early
   media packets, but will not generate early media.  Anyone accepting
   this SDP offer MAY send early media, but SHOULD NOT expect to receive
   early media from the SDP offeror, and that any media packets received
   prior to answer from this the offeror may safely be discarded.  If
   the emflow value in the SDP offer was 'recv', then the emflow value
   in the SDP answer MUST be set to 'none' or 'send' depending on
   whether the application intends to send the early media packets to
   the offeror or not.

   If the emflow value of 'recv' is set in an SDP answer, it indicates
   that the endpoint generating the answer will accept early media but
   will not generate any.The SDP offeror can take this indication to
   mean that they should not expect early media packets from this
   endpoint and may safely discard any received prior to answer.

6.2.3.4.  Meaning of a=emflow:sendrecv

   If the emflow value of 'sendrecv' is set in an the SDP offer, it
   indicates that the endpoint generating the offer may send and receive
   early media packets.  Anyone accepting this SDP offer MAY send early
   media, and SHOULD process received early media packets if it is
   appropriate to the device receiving packets to do so.  If the emflow



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   value in the SDP offer was 'sendrecv', then the emflow value in the
   SDP answer MAY be set to any value.  The value set in the SDP answer
   depends on if the endpoint answering the SDP offer intends to send
   and/or receive early media packets.

   If the emflow value of 'sendrecv' is set in an SDP answer, it
   indicates that the endpoint generating the answer may generate and
   receive early media and behave appropriately.

6.2.4.  Option tag for emflow

   The option tag "emflow" is defined for use in the Require and
   Supported header fields [2].  An offerer MAY include this tag in a
   Require header if they wish to ensure that any endpoint reached
   supports this extension (typically when 'a=emflow:' is not set to
   'sendrecv').  The if the party generating an SDP offer or answer
   supports this extension it MUST include this tag in a Supported
   header if it is not already in a Require header of any message
   containing SDP.  This allows the other party or parties involved in
   the signaling flow to know that the other end is processing their
   emflow values.






























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7.  Security Considerations

   TBD.
















































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8.  IANA Considerations

   TBD.
















































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9.  References

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

   [2]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
        Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
        Session Initiation Protocol", RFC 3261, June 2002.

   [3]  Camarillo, G., Roach, A., Peterson, J., and L. Ong, "Integrated
        Services Digital Network (ISDN) User Part (ISUP) to Session
        Initiation Protocol (SIP) Mapping", RFC 3398, December 2002.

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

   [5]  Camarillo, G., "The Early Session Disposition Type for the
        Session Initiation Protocol (SIP)", RFC 3959, December 2004.

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

   [7]  Camarillo, G., Marshall, W., and J. Rosenberg, "Integration of
        Resource Management and Session Initiation Protocol (SIP)",
        RFC 3312, October 2002.

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






















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Author's Address

   Brian Stucker
   Nortel
   2201 Lakeside
   Richardson, TX  75082
   US

   Phone: +1 972 685 7724
   Email: bstucker@nortel.com
   URI:   http://www.nortel.com/








































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Full Copyright Statement

   Copyright (C) The Internet Society (2006).

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