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Versions: (draft-xia-avtext-splicing-for-rtp) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 6828

AVTEXT Working Group                                              J. Xia
Internet-Draft                                                    Huawei
Intended status: Informational                         February 20, 2012
Expires: August 23, 2012


                   Content Splicing for RTP Sessions
                 draft-ietf-avtext-splicing-for-rtp-07

Abstract

   This memo outlines RTP splicing.  Splicing is a process that replaces
   the content of the main multimedia stream with other multimedia
   content, and delivers the substitutive multimedia content to receiver
   for a period of time.  This memo provides some RTP splicing use
   cases, then we enumerate a set of requirements and analyze whether an
   existing RTP level middlebox can meet these requirements, at last we
   provide concrete guidelines for how the chosen middlebox works to
   handle RTP splicing.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at http://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on August 23, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must



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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  RTP Splicing Discussion and Requirements . . . . . . . . . . .  4
   4.  Recommended Solution for RTP Splicing  . . . . . . . . . . . .  7
     4.1.  RTP Processing in RTP Mixer  . . . . . . . . . . . . . . .  7
     4.2.  RTCP Processing in RTP Mixer . . . . . . . . . . . . . . .  9
     4.3.  Media Clipping Considerations  . . . . . . . . . . . . . . 10
     4.4.  Congestion Control Considerations  . . . . . . . . . . . . 11
     4.5.  Processing Splicing in User Invisibility Case  . . . . . . 13
   5.  Implementation Considerations  . . . . . . . . . . . . . . . . 13
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 14
   9.  Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     9.1.  draft-xia-avtext-splicing-for-rtp-01 . . . . . . . . . . . 14
     9.2.  draft-xia-avtext-splicing-for-rtp-00 . . . . . . . . . . . 14
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 15
     10.2. Informative References . . . . . . . . . . . . . . . . . . 16
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
























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1.  Introduction

   This document outlines how splicing can be used for RTP sessions.
   Splicing is a process that replaces the content of the main RTP
   stream with other multimedia content, and delivers the substitutive
   content to receiver for a period of time.  The substitutive content
   can be provided for example via another RTP stream or local media
   file storage.

   One representative use case for splicing is advertisements insertion,
   which allows operators to replace the national advertising content
   with its own regional advertising content prior to delivering the
   regional advertising content to receiver.

   Besides the advertisement insertion use case, there are other use
   cases to which RTP splicing technology can apply.  For example,
   splicing a recorded video into a video conferencing session, and
   implementing a playlist server that stitches pieces of video together
   and so forth.

   So far [SCTE30] and [SCTE35] have standardized MPEG2-TS splicing
   running over cable.  The introduction of multimedia splicing into
   internet requires changes to transport layer, but to date there is no
   guideline for how to handle content splicing for RTP sessions
   [RFC3550].

   In this document, we first describe a set of requirements of RTP
   splicing.  Then we provide a method about how an intermediary node
   can be used to process RTP splicing to meet these requirements from
   the aspects of feasibility, implementation complexity and backward
   compatibility.


2.  Terminology

   This document uses the following terminologies.

   Current RTP Stream

      The RTP stream that the RTP receiver is currently receiving.  The
      content of current RTP stream can be either main content or
      substitutive content.

   Main Content

      The multimedia content that are conveyed in main RTP stream.  Main
      content will be replaced by the substitutive content during
      splicing.



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   Main RTP Stream

      The RTP stream that the Splicer is receiving.  The content of main
      RTP stream can be replaced by substitutive content for a period of
      time.

   Substitutive Content

      The multimedia content that replaces the main content during
      splicing.  The substitutive content can for example be contained
      in an RTP stream from a media sender or fetched from local media
      file storage.

   Substitutive RTP Stream

      A RTP stream that may provide substitutive content.  Substitutive
      RTP stream and main RTP stream are two separate streams.  If the
      substitutive content is provided via substitutive RTP stream, the
      substitutive RTP Stream must pass through Splicer before the
      substitutive content is delivered to receiver.

   Splicing In Point

      A virtual point in the RTP stream, suitable for substitutive
      content entry, that exists in the boundary of two independently
      decodable frames.

   Splicing Out Point

      A virtual point in the RTP stream, suitable for substitutive
      content exit, that exists in the boundary of two independently
      decodable frames.

   Splicer

      An intermediary node that inserts substitutive content into main
      RTP stream.  Splicer sends substitutive content to RTP receiver
      instead of main content during splicing.  It is also responsible
      for processing RTCP traffic between media source and RTP receiver.


3.  RTP Splicing Discussion and Requirements

   In this document, we assume an intermediary network element, which is
   referred to as Splicer, to play the key role to handle RTP splicing.
   A simplified RTP splicing diagram is depicted in Figure 1, in which
   only one main content flow and one substitutive content flow are
   given.



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      +---------------+
      |               | Main Content +-----------+
      |Main RTP Sender|------------->|           | Current Content
      |               |              |  Splicer  |---------->
      +---------------+   ---------->|           |
                         |           +-----------+
                         |
                         | Substitutive Content
                         |
                         |
               +-----------------------+
               |Substitutive RTP Sender|
               |          or           |
               |   Local File Storage  |
               +-----------------------+


               Figure 1: RTP Splicing Architecture


   When RTP splicing begins, Splicer stops delivering the main content,
   instead delivering the substitutive content to RTP receiver for a
   period of time, and then resumes the main content when splicing ends.
   The methods how Splicer learns when to start and end the splicing is
   out of scope for this document.  The RTP splicing may happen more
   than once in case that substitutive content will be dispersedly
   inserted in multiple time slots during the lifetime of the main RTP
   stream.

   When realizing splicing technology on RTP layer, there are a set of
   requirements that must be satisfied to at least some degree on
   Splicer:

   REQ-1:

      Splicer must operate in either unicast or multicast session
      environment.

   REQ-2:

      Splicer should not cause perceptible media clipping at the
      splicing point and adverse impact on the quality of user
      experience.








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   REQ-3:

      Splicer must be backward compatible with RTP/RTCP protocols, and
      its associated profiles and extensions to those protocols.  For
      example, Splicer must be robust to packet loss, network congestion
      etc.

   REQ-4:

      Splicer must be trusted by media source and receiver, and has the
      valid security context with media source and RTP receiver
      respectively.

   REQ-5:

      Splicer should allow the media source to learn the performance of
      the downstream receiver when its content is being passed to RTP
      receiver.


   In a number of deployment scenarios, especially advertisement
   insertion, there may be one specific requirement.  Given that it is
   unacceptable for advertisers that their advertising content is not
   delivered to user, this may require RTP splicing to be operated
   within the following constraint:

   REQ-6:

      If Splicer intends to prevent RTP receiver from identifying and
      filtering the substitutive content, it should eliminate the
      visibility of splicing process on RTP level from RTP receiver
      point of view.

      However, substitutive content and main content are encoded by
      different encoders and have different parameter sets.  In such
      case, a full media transcoding must be done on Splicer to ensure
      the completely invisible impact on RTP receiver, but this may be
      prohibitively expensive and complex.  As a trade-off, it is
      recommended to minimize the splicing visibility on RTP receiver,
      i.e., maintaining RTP header parameters consistent but leaving the
      RTP payload untranscoded.  If one wants to realize complete
      invisibility, the cost of transcoding must be taken into account.

      Henceforth, we refer to the minimum and complete invisibility
      requirement as User Invisibility Requirement.


   To improve the versatility of existing implementations and better



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   interoperability, it is recommended to use existing tools in RTP/RTCP
   protocol family to realize RTP splicing without any protocol
   extension unless the existing tools are incompetent for splicing.


4.  Recommended Solution for RTP Splicing

   Given that Splicer is an intermediary node exists between the main
   media source and the RTP receiver and splicing is not a very
   complicated processing, there are some chance that any existing RTP-
   level middlebox may has the incidental capability to meet the
   requirements described in previous section.

   Since Splicer needs to select substitutive content or main content as
   the input content at one point of time, an RTP mixer seems to have
   such capability to do this under its own SSRC.  Moreover, mixer may
   include the CSRC list in outgoing packets to indicate the source(s)
   of content in some use cases like conferencing, this facilitates the
   system debugging and loop detection.  From this point of view, an RTP
   mixer may have some chance to be Splicer.  In next four subsections
   (from subsection 4.1 to subsection 4.4), we start analyzing how an
   RTP mixer handles RTP splicing and how it satisfies the general
   requirements listed in section 3.

   In subsection 4.5, we specially consider the special requirement 6
   (i.e., User Invisibility Requirement) since it needs to mask any RTP
   splicing clue on receiver (e.g, CSRC list must not be included in
   outgoing packets to prevent receiver from identifying the difference
   between main RTP stream and substitutive RTP stream) when mixer is
   used.

4.1.  RTP Processing in RTP Mixer

   Once mixer has learnt when to do splicing, it must get ready for the
   coming splicing in advance, e.g., fetches the substitutive content
   either from local media file storage or via substitutive RTP stream
   earlier than splicing in point.  If the substitutive content comes
   from local media file storage, mixer should leave the CSRC list blank
   in the output stream.

   Even if splicing does not begin, mixer still needs to receive the
   main RTP stream, and generate a media stream as defined in RFC3550.
   Using main content, mixer generates the current media stream with its
   own SSRC, sequence number space and timing model.  Moreover, mixer
   may insert the SSRC of main RTP stream into CSRC list in the current
   media stream.

   When splicing begins, mixer chooses the substitutive RTP stream as



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   input stream at splicing in point, and extracts the payload data
   (i.e., substitutive content).  After that, mixer encapsulates
   substitutive content instead of main content as the payload of the
   current media stream, and then outputs the current media stream to
   receiver.  Moreover, mixer may insert the SSRC of substitutive RTP
   stream into CSRC list in the current media stream.

   When splicing ends, mixer retrieves the main RTP stream as input
   stream at splicing out point, and extracts the payload data (i.e.,
   main content).  After that, mixer encapsulates main content instead
   of substitutive content as the payload of the current media stream,
   and then outputs the current media stream to receiver.  Moreover,
   mixer may insert the SSRC of main RTP stream into CSRC list in the
   current media stream.

   The whole RTP splicing procedure is perhaps best explained by a
   pseudo code example:

   if (splicing begins) {
      the substitutive RTP stream is terminated on mixer and
      substitutive content is encapsulated by mixer with its own SSRC
      identifier;

      the sequence numbers of the current RTP packets which contain
      substitutive content are allocated by mixer and maintain
      consistent with the sequence numbers of previous current RTP
      packets, until the splicing end;

      the timestamp of the current RTP packet increments linearly;

      the CSRC list of the current RTP packet may include SSRC of
      substitutive RTP stream;
   }

   else {
      the main RTP stream is terminated on mixer and main content is
      encapsulated by mixer with its own SSRC identifier;

      the sequence numbers of the current RTP packets which contain main
      content are allocated by mixer and maintain consistent with the
      sequence numbers of previous current RTP packets, until the
      splicing begins;

      the timestamp of the current RTP packets increments linearly;

      the CSRC list of the current RTP may include SSRC of main RTP
      stream;
   }



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   Splicing may occur more than one time during the lifetime of main RTP
   stream, this means mixer needs to output main content and
   substitutive content in turn with its own SSRC identifier.  From
   receiver point of view, the only source of the current stream is
   mixer wherever the content comes from.

   Note that, the substitutive content should be outputted in the range
   of splicing duration.  Any gap or overlap between main RTP stream and
   substitutive RTP stream may induce media clipping at splicing point.
   More details about preventing media clipping are introduced in
   section 4.3.

4.2.  RTCP Processing in RTP Mixer

   By monitoring available bandwidth and buffer levels and by computing
   network metrics such as packet loss, network jitter, and delay, RTP
   receiver can learn the situation on it and can communicate this
   information to media source via RTCP reception reports.

   According to the description in section 7.3 of [RFC3550], mixer
   divides RTCP flow between media source and receiver into two separate
   RTCP loops, media source probably has no idea about the situation on
   receiver.  Hence, mixer can use some mechanisms, allowing media
   source to at least some degree to have some knowledge of the
   situation on receiver when its content is being passed to receiver.

   Because splicing is a processing that mixer selects one media stream
   from multiple streams but neither mixing nor transcoding them, upon
   receiving an RTCP receiver report from downstream receiver, mixer can
   forward it to original media source with its SSRC identifier intact
   (i.e., the SSRC of downstream receiver).  Given that the number of
   output RTP packets containing substitutive content is equal to the
   number of input substitutive RTP packets (from substitutive RTP
   stream) during splicing.  In the same manner, the number of output
   RTP packets containing main content is equal to the number of input
   main RTP packets (from main RTP stream) during non-splicing, so mixer
   does not need to modify loss packet fields in Receiver Report Blocks
   unless the reporting intervals spans the splicing point.  But mixer
   needs to change the SSRC field in report block to the SSRC identifier
   of original media source and rewrite the extended highest sequence
   number field to the corresponding original extended highest sequence
   number before forwarding the RTCP receiver report to original media
   source.

   When a RTCP receiver report spans the splicing point, it reflects the
   characteristics of the combination of main RTP packets and
   substitutive RTP packets, in which case, mixer needs to divide the
   receiver report into two separated receiver reports and send them to



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   their original media sources respectively.  For each separated
   receiver report, mixer also needs to make the corresponding changes
   to the packet loss fields in report block besides the SSRC field and
   the extended highest sequence number field.

   The mixer can also inform the media source of quality with which the
   content reaches the mixer.  This is done by the mixer generating RTCP
   reports for the RTP stream, which it sends upstream towards the media
   source.  These RTCP reports use the SSRC of the mixer.

   Based on above RTCP operating mechanism, the media source whose
   content is being passed to receiver, will see the reception quality
   of its stream received on mixer, and the reception quality of spliced
   stream received on receiver.  The media source whose content is not
   being passed to receiver, will only see the reception quality of its
   stream received on mixer.

   If the substitutive content comes from local media file storage (
   i.e., mixer can be regarded as the substitutive media source), the
   reception reports received from downstream relate to the substitutive
   content should be terminated on mixer without any further processing.

4.3.  Media Clipping Considerations

   This section provides informative guideline about how media clipping
   may shape and how mixer deal with the media clipping.

   If the time slot for substitutive RTP stream mismatches (shorter or
   longer than) the duration of the reserved main RTP stream for
   replacing, the media clipping may occur at the splicing point which
   usually is the joint between two independently decodable frames.

   At the splicing in point, mixer can fill up receiver's buffer with
   substitutive content several seconds earlier than the presentation
   time of substitutive content so that smooth playback can be achieved
   without pauses or stuttering on RTP receiver.

   Compared to buffering method used at splicing in point, things become
   somewhat complex at splicing out point.  The case that insertion
   duration is shorter than the reserved gap time may cause a little
   playback latency of main RTP stream on RTP receiver, but not
   adversely impact the quality of user experience.  One alternative
   approach is that mixer may pad some blank content (e.g., all black
   sequence) to fill up the gap.  Another alternative approach is that
   main media source may send filler content (e.g., static channel
   identifier) during splicing, the mixer can switch back to early when
   it runs out of substitutive content.




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   However, in case that insertion duration is longer than the reserved
   gap duration, there exists an overlap of the substitutive RTP stream
   and the main RTP stream at splicing out point.  One straightforward
   approach is that mixer takes a ungracefule action, terminating the
   splicing and switching back to main RTP stream even if this may cause
   media stuttering on receiver.  There is an alternative approach which
   may be mild but somewhat complex, mixer buffers main content for a
   while until substitutive content is finished, and then transmits
   buffered main content to receiver at an acceleated bitrate (as
   compared to the nominal bitrate of main RTP stream) until its buffer
   level returns to normal.  At this point in time, mixer transmits main
   content to receiver at an nominal bitrate of main RTP stream.  Note
   that mixer should take into account a variety of parameters, such as
   available bandwidth between mixer and receiver, mixer buffer level
   and receiver buffer level, to count the accelerated bitrate value.

   Another reason to cause media clipping is synchronization delay at
   splicing point if RTP receiver needs to synchronize multiple current
   streams for playback.  How to address this issue is discussed in
   detail in [RFC6051], which provides three feasible approaches to
   reduce synchronization delay.

4.4.  Congestion Control Considerations

   Provided that the substitutive content has somewhat different
   characteristics to the main content it replaces (e.g., the more
   dynamic content, the higher bandwidth occupation), or substitutive
   content may be encoded with different codec and has different
   encoding bitrate, some challenge raise to network capacity and
   receiver buffer size.  A more dynamic content or a higher encoding
   bitrate stream might overload the network and possibly exceed the
   receiver's media consumption rate, which might flood receiver's
   buffer and eventually result in a buffer overflow.  Either network
   overload or buffer overflow would induce network congestion and
   congestion-caused packet loss.

   To be robust to network congestion and packet loss, mixer must
   continuously monitor the network situation by means of a variety of
   manners:

   1.  RTCP receiver reports indicate packet loss [RFC3550].

   2.  RTCP NACKs for lost packet recovery [RFC4585].

   3.  RTCP ECN Feedback information [I-D.ietf-avtcore-ecn-for-rtp].

   Upon detection of above three types of RTCP reports during splicing,
   mixer will treat them with three different manners as following:



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   1.  If mixer receives the RTCP receiver reports with packet loss
       indication, it will process them as the description given in
       section 7.3 of [RFC3550].

   2.  If mixer receives the RTCP NACK packets defined in [RFC4585] from
       RTP receiver for packet loss recovery, it first identifies the
       content category of lost packets to which the NACK corresponds.
       Then, mixer will generate new RTCP NACK for the lost packets with
       its own SSRC, and make corresponding changes to their sequence
       numbers to match original, pre-spliced, packets.  If the lost
       substitutive content comes from local media file storage, mixer
       acting as substitutive media source will directly fetch the lost
       substitutive content and retransmit it to RTP receiver.

       It is somewhat complex that the lost packets requested in a
       single RTCP NACK message not only contain the main content but
       also the substitutive content.  To address this, mixer must
       divide the RTCP NACK packet into two separate RTCP NACK packets:
       one requests for the lost main content, and another requests for
       the lost substitutive content.

   3.  In [I-D.ietf-avtcore-ecn-for-rtp], two RTCP extensions are
       defined for ECN feedback: RTP/AVPF transport layer ECN feedback
       packet for urgent ECN information, and RTCP XR ECN summary report
       block for regular reporting of the ECN marking information.

       If an ECN-aware mixer receives any RTCP ECN feedback (i.e., RTCP
       ECN feedback packets or RTCP XR summary reports) from RTP
       receiver, it must operates as description given in section 8.4 of
       [I-D.ietf-avtcore-ecn-for-rtp], terminating the RTCP ECN feedback
       packets from downstream receivers, and driving congestion control
       loop and bitrate adaptation between itself and downstream
       receiver as if it were the media source.  In addition, an ECN-
       aware RTP mixer must generate RTCP ECN feedback relating to the
       input RTP streams it terminates, and driving congestion control
       loop and bitrate adaptation between itself and upstream sender as
       if it were the RTP sender.

   Once mixer learns that congestion is being experienced on its
   downstream link by means of above three detection mechanisms, it
   should adapt the bitrate of output stream in response to network
   congestion.  The bitrate adaptation may be determined by a TCP-
   friendly bitrate adaptation algorithm specified in [RFC5348], or by a
   DCCP congestion control algorithms defined in [RFC5762].

   In practice, during splicing, the real reason to cause congestion
   usually is the different characteristic of substitutive RTP stream
   (more dynamic content or higher encoding bitrate) with main RTP



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   stream, and that stream transcoding or thinning on mixer is very
   inefficient and difficult operation.  Therefore, a means that enables
   substitutive media source to limit the media bitrate it is currently
   generating even in the absence of congestion on the path between
   itself and mixer is desirable.  The TMMBR message defined in
   [RFC5104] provides an effective method.  When mixer detects
   congestion on its downstream link during splicing, it uses TMMBR to
   request substitutive media source to reduce the media bitrate to a
   value that is in compliance with congestion control principles for
   the slowest link.  Upon reception of TMMBR, substitutive media source
   applies its congestion control algorithm and responds Temporary
   Maximum Media Stream Bit Rate Notification (TMMBN) to mixer.

   If the substitutive content comes from local media file storage,
   mixer must directly reduce the substitutive media bitrate as the
   substitutive media source when it detects any congestion on its
   downstream link during splicing.

   From above analysis, to reduce the risk of congestion and remain the
   bandwidth consumption stable over time, the substitutive RTP stream
   is recommended to be encoded at an appropriate bitrate to match that
   of main RTP stream.  If the substitutive RTP stream comes from
   substitutive media source, the source had better has some knowledge
   about the media encoding bitrate of main content in advance.  How it
   knows that is out of scope in this draft.

4.5.  Processing Splicing in User Invisibility Case

   Mixer will not includes CRSC list in outgoing RTP packets to prevent
   user from detecting the splicing occurred on RTP level.  Due to the
   absence of CRSC list in current RTP stream, RTP receiver only
   initiates SDES, BYE and APP packets to mixer without any knowledge of
   main media source and substitutive media source.  This creates a
   danger that loops involving those sources could not be detected.


5.  Implementation Considerations

   When mixer is used to handle RTP splicing, RTP receiver does not need
   any RTP/RTCP extension for splicing.  As a trade-off, additional
   overhead could be induced on mixer which uses its own sequence number
   space and timing model.  So mixer will rewrite RTP sequence number
   and timestamp whatever splicing is active or not, and generate RTCP
   flows for both sides.  In case mixer serves multiple main RTP streams
   simultaneously, this may lead to more overhead on mixer.

   In addition, there is a potential issue with loop detection, which
   would be problematic if User Invisibility Requirement is required.



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6.  Security Considerations

   If any payload internal security mechanisms (e.g., ISMACryp
   [ISMACryp]) are used, only media source and RTP receiver can learn
   the security keying material generated by such internal security
   mechanism, any middlebox (e.g., mixer) between media source and RTP
   receiver can't get such keying material.  Only when regular transport
   security mechanisms (e.g., SRTP, IPSec, etc) are used, mixer will
   process the packets passing through it.

   The security considerations of the RTP specification [RFC3550], the
   Extended RTP profile for RTCP-Based Feedback [RFC4585], and the
   Secure Real-time Transport Protocol [RFC3711] apply.  Mixer must be
   trusted by main media source and insertion media source, and must be
   included in the security context.


7.  IANA Considerations

   No IANA actions are required.


8.  Acknowledgments

   The following individuals have reviewed the earlier versions of this
   specification and provided very valuable comments: Colin Perkins,
   Magnus Westerlund, Roni Even, Tom Van Caenegem, Joerg Ott, David R
   Oran, Cullen Jennings, Ali C Begen, Charles Eckel and Ning Zong.


9.  Change Log

9.1.  draft-xia-avtext-splicing-for-rtp-01

   The following are the major changes compared to previous version 00:

   o  Use mixer to handle both user visible and invisible splicing.

   o  Add one subsection to describe media clipping considerations.

   o  Add one subsection to describe congestion control considerations.

9.2.  draft-xia-avtext-splicing-for-rtp-00

   The following are the major changes compared to previous AVT I-D
   version 00:





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   o  Change primary RTP stream to main RTP stream, add current RTP
      stream as the streaming received by RTP receiver.

   o  Eliminate the ambiguity of inserted content with substitutive
      content which replaces the main content rather than pause it.

   o  Clarify the signaling requirements.

   o  Delete the description on Mixer and MCU in section 4, mainly focus
      on the direction whether a Translator can act as a Splicer.

   o  Add section 5 to describe the exact guidance on how an RTP
      Translator is used to handle splicing.

   o  Modify the security considerations section and add acknowledges
      section.


10.  References

10.1.  Normative References

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
              July 2006.

   [RFC5104]  Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
              "Codec Control Messages in the RTP Audio-Visual Profile
              with Feedback (AVPF)", RFC 5104, February 2008.

   [RFC6051]  Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP
              Flows", RFC 6051, November 2010.

   [I-D.ietf-avtcore-ecn-for-rtp]
              Westerlund, M., "Explicit Congestion Notification (ECN)
              for RTP over UDP", draft-ietf-avtcore-ecn-for-rtp-06 (work
              in progress), February 2012.





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10.2.  Informative References

   [RFC5348]  Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
              Friendly Rate Control (TFRC): Protocol Specification",
              RFC 5348, September 2008.

   [RFC5762]  Perkins, C., "RTP and the Datagram Congestion Control
              Protocol (DCCP)", RFC 5762, April 2010.

   [SCTE30]   Society of Cable Telecommunications Engineers (SCTE),
              "Digital Program Insertion Splicing API", 2001.

   [SCTE35]   Society of Cable Telecommunications Engineers (SCTE),
              "Digital Program Insertion Cueing Message for Cable",
              2004.

   [ISMACryp]
              Internet Streaming Media Alliance (ISMA), "ISMA Encryption
              and Authentication Specification 2.0", November 2007.


Author's Address

   Jinwei Xia
   Huawei
   Software No.101
   Nanjing, Yuhuatai District  210012
   China

   Phone: +86-025-86622310
   Email: xiajinwei@huawei.com




















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