





Internet Engineering Task Force (IETF)                            Q. Xie
Request for Comments: 6354                                   August 2011
Updates: 2198, 4102
Category: Standards Track
ISSN: 2070-1721


             Forward-Shifted RTP Redundancy Payload Support

Abstract

   This document defines a simple enhancement to support RTP sessions
   with forward-shifted redundant encodings, i.e., redundant data sent
   before the corresponding primary data.  Forward-shifted redundancy
   can be used to conceal losses of a large number of consecutive media
   frames (e.g., consecutive loss of seconds or even tens of seconds of
   media).

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6354.

Copyright Notice

   Copyright (c) 2011 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
   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.





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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1. Introduction ....................................................2
      1.1. Sending Redundant Data Inband vs. Out-of-Band ..............3
   2. Conventions .....................................................4
   3. Allowing Forward-Shifted Redundant Data .........................4
   4. Registration of Media Type "fwdred" .............................5
   5. Mapping Media Type Parameters into SDP ..........................7
   6. Usage in Offer/Answer ...........................................7
   7. IANA Considerations .............................................7
   8. Security Considerations .........................................8
   9. Normative References ............................................8
   Appendix A. Anti-Shadow Loss Concealment Using
               Forward-Shifted Redundancy .............................9
      A.1. Sender-Side Operations .....................................9
      A.2. Receiver-Side Operations ..................................11
         A.2.1. Normal-Mode Operation ................................11
         A.2.2. Anti-Shadow-Mode Operation ...........................12

1.  Introduction

   This document defines a simple enhancement to RFC 2198 [RFC2198] to
   support RTP sessions with forward-shifted redundant encodings, i.e.,
   redundant data sent before the corresponding primary data.

   Forward-shifted redundancy can be used to conceal losses of a large
   number of consecutive media frames (e.g., consecutive loss of seconds
   of media).  Such capability is highly desirable, especially in
   wireless mobile communication environments where the radio signal to
   a mobile wireless media receiver can be temporarily blocked by tall
   buildings, mountains, tunnels, etc.  In other words, the receiver
   enters into a shadow of the radio coverage.  No new data will be
   received when the receiver is in a shadow.






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   In some extreme cases, the receiver may have to spend seconds or even
   tens of seconds in a shadow.  The traditional backward-shifted
   redundant encoding scheme (i.e., redundant data is sent after the
   primary data), as currently supported by RFC 2198 [RFC2198], does not
   address such consecutive frame losses.

   In contrast, the forward-shifted redundancy scheme allows one to
   apply effective anti-shadow loss management at the receiver (as
   illustrated in Appendix A), thus preventing service interruptions
   when a mobile receiver runs into such a shadow.

   Anti-shadow loss concealment as described in this document can be
   readily applied to the streaming of pre-recorded media.  Because of
   the need of generating the forward-shifted anti-shadow redundant
   stream, to apply anti-shadow loss concealment to the streaming of
   live media will require the insertion of a delay equal to or greater
   than the amount of forward-shifting at the source of media.

1.1.  Sending Redundant Data Inband vs. Out-of-Band

   Regardless of the direction of time shift (e.g., forward-shifting, or
   backward-shifting as in RFC 2198) or the encoding scheme (e.g.,
   Forward Error Correction (FEC), or non-FEC), there is always the
   option of sending the redundant data and the primary data either in
   the same RTP session (i.e., inband) or in separate RTP sessions
   (i.e., out-of-band).  There are pros and cons for either approach, as
   outlined below.

   Inband Approach:

   o  Pro: A single RTP session is faster to set up and easier to
      manage.

   o  Pro: A single RTP session presents a simpler problem for NAT/
      firewall traversal.

   o  Pro: Less overall overhead -- one source of RTP/UDP/IP overhead.

   o  Con: Lack of flexibility -- difficult for middle boxes such as
      gateways to add/remove the redundant data.

   o  Con: Need more specification -- special payload formats need to be
      defined to carry the redundant data inband.








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   Out-of-Band Approach:

   o  Pro: Flexibility -- redundant data can be more easily added,
      removed, or replaced by a middle box such as a gateway.

   o  Pro: Little or no specification -- no new payload format is
      needed.

   o  Con: Multiple RTP sessions may take longer to set up and may be
      more complex to manage.

   o  Con: Multiple RTP sessions for NAT/firewall traversal are harder
      to address.

   o  Con: Bigger overall overhead -- more than one source of RTP/UDP/IP
      overhead.

   It is noteworthy that the specification of inband payload formats, as
   described in this document and in RFC 2198, does not preclude a
   deployment from using the out-of-band approach.  Rather, it gives the
   deployment the choice to use whichever approach is deemed most
   beneficial under a given circumstance.

2.  Conventions

   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 RFC 2119 [RFC2119].

3.  Allowing Forward-Shifted Redundant Data

   In RFC 2198, the timestamp offset in the additional header
   corresponding to a redundant block is defined as a 14-bit unsigned
   offset of the timestamp relative to the timestamp given in the RTP
   header.  As stated in RFC 2198:

      The use of an unsigned offset implies that redundant data must be
      sent after the primary data, and is hence a time to be subtracted
      from the current timestamp to determine the timestamp of the data
      for which this block is the redundancy.

   This effectively prevents RFC 2198 from being used to support
   forward-shifted redundant blocks.








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   In order to support the use of forward-shifted redundant blocks, the
   media type "fwdred", which allows a parameter called "forwardshift",
   is introduced to indicate the capability of and willingness to use
   forward-shifted redundancy and the base value of timestamp forward-
   shifting.  The base value of "forwardshift" is an integer equal to or
   greater than '0' in RTP timestamp units.

   In an RTP session that uses forward-shifted redundant encodings, the
   timestamp of a redundant block in a received RTP packet is determined
   as follows:

      timestamp of redundant block = timestamp in RTP header
                          - timestamp offset in additional header
                          + forward-shift base value

   Note that generally, in a forward-shifted session, the timestamp
   offset in the additional header is set to '0'.

   The sender MUST NOT change the contents of a packet that appears in a
   forward-shifted stream when it is time to send it in the main stream.

4.  Registration of Media Type "fwdred"

   The definition is based on media type "red" defined in RFC 2198
   [RFC2198] and RFC 4102 [RFC4102], with the addition of the
   "forwardshift" parameter.

   Type names: audio, text

   Subtype names: fwdred

   Required parameters:

      rate: as defined in [RFC4102].

      pt: as defined in [RFC4102].

      forwardshift: An unsigned integer can be specified as the value.

         If this parameter has a value greater than '0', it indicates
         that the sender of this parameter will use forward-shifting
         with a base value as specified when sending out redundant data.
         This value is in RTP timestamp units.

         If this parameter has a value of '0', it indicates that the
         sender of this parameter will not use forward-shifting when
         sending its redundant data; i.e., the sender will have the same
         behaviors as defined in RFC 2198.



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   Optional parameters:

      ptime: as defined in [RFC4102] [RFC4566].

      maxptime: as defined in [RFC4102] [RFC4867].

   Encoding considerations:

      This media type is framed binary data (see RFC 4288, Section 4.8)
      and is only defined for transfer of RTP redundant data frames
      specified in RFC 2198.

   Security considerations: See Section 6 of RFC 2198.

   Interoperability considerations: none.

   Published specification:

      RTP redundant data frame format is specified in RFC 2198.

   Applications that use this media type:

      It is expected that real-time audio/video, text streaming, and
      conferencing tools/applications that want protection against
      losses of a large number of consecutive frames will be interested
      in using this type.

   Additional information: none.

   Person & email address to contact for further information:

      Qiaobing Xie <Qiaobing.Xie@gmail.com>

   Intended usage: COMMON

   Restrictions on usage:

      This media type depends on RTP framing, and hence is only defined
      for transfer via RTP (RFC 3550 [RFC3550]).  Transfer within other
      framing protocols is not defined at this time.

   Author:

      Qiaobing Xie

   Change controller:

      IETF Audio/Video Transport working group delegated from the IESG.



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5.  Mapping Media Type Parameters into SDP

   The information carried in the media type specification has a
   specific mapping to fields in the Session Description Protocol (SDP)
   [RFC4566], which is commonly used to describe RTP sessions.  When SDP
   is used to specify sessions employing the forward-shifted redundant
   format, the mapping is as follows:

   o  The media type ("audio") goes in SDP "m=" as the media name.

   o  The media subtype ("fwdred") goes in SDP "a=rtpmap" as the
      encoding name.

   o  The required parameter "forwardshift" goes in the SDP "a=fmtp"
      attribute by copying it directly from the media type string as
      "forwardshift=value".

   The following is an example of usage that indicates forward-shifted
   (by 5.1 sec) redundancy:

      m=audio 12345 RTP/AVP 121 0 5
      a=rtpmap:121 fwdred/8000/1
      a=fmtp:121 0/5 forwardshift=40800

   The following is an example of usage that indicates sending
   redundancy without forward-shifting (equivalent to RFC 2198):

      m=audio 12345 RTP/AVP 121 0 5
      a=rtpmap:121 fwdred/8000/1
      a=fmtp:121 0/5 forwardshift=0

6.  Usage in Offer/Answer

   The "forwardshift" SDP parameter specified in this document is
   declarative, and all reasonable values are expected to be supported.

7.  IANA Considerations

   IANA made the assignments described below per this document.

   o  IANA added the following to the "Audio Media Types" registry:

      fwdred     [RFC6354]

   o  IANA added the following to the "Text Media Types" registry:

      fwdred     [RFC6354]




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8.  Security Considerations

   Security considerations discussed in Section 6 of [RFC2198],
   Section 4 of [RFC4856], and Sections 9 and 14 of [RFC3550] apply to
   this specification.  In addition, to prevent denial-of-service
   attacks, a receiver SHOULD be prepared to ignore a 'forwardshift'
   parameter declaration if it considers the offset value in the
   declaration excessive.  In such a case, the receiver SHOULD also
   ignore the redundant stream in the resultant RTP session.

9.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2198]  Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,
              Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse-
              Parisis, "RTP Payload for Redundant Audio Data", RFC 2198,
              September 1997.

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

   [RFC4102]  Jones, P., "Registration of the text/red MIME Sub-Type",
              RFC 4102, June 2005.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC4856]  Casner, S., "Media Type Registration of Payload Formats in
              the RTP Profile for Audio and Video Conferences",
              RFC 4856, February 2007.

   [RFC4867]  Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie,
              "RTP Payload Format and File Storage Format for the
              Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband
              (AMR-WB) Audio Codecs", RFC 4867, April 2007.













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Appendix A.  Anti-Shadow Loss Concealment Using Forward-Shifted
             Redundancy

   (This Appendix is included for Informational purposes.)

   It is not unusual in a wireless mobile communication environment that
   the radio signal to a mobile wireless media receiver can be
   temporarily blocked by tall buildings, mountains, tunnels, etc. for a
   period of time.  In other words, the receiver enters into a shadow of
   the radio coverage.  When the receiver is in such a shadow, no new
   data will be received.  In some extreme cases, the receiver may have
   to spend seconds or even tens of seconds in such a shadow.

   Without special design considerations to compensate for the loss of
   data due to shadowing, a mobile user may experience an unacceptable
   level of service interruptions.  Traditional redundant encoding
   schemes (including RFC 2198 and most FEC schemes) are known to be
   ineffective in dealing with such losses of consecutive frames.

   However, the employment of forward-shifted redundancy, in combination
   with the anti-shadow loss concealment at the receiver, as described
   here, can effectively prevent service interruptions due to the effect
   of shadowing.

A.1.  Sender-Side Operations

   For anti-shadow loss management, the RTP sender simply adds a
   forward-shifted redundant stream (called anti-shadow or AS stream)
   while transmitting the primary media stream.  The amount of forward-
   shifting, which should remain constant for the duration of the
   session, will determine the maximal length of shadows that can be
   completely concealed at the receiver, as explained below.

   Except for the fact that the redundant stream is forward-shifted
   relative to the primary stream (i.e., the redundant data is sent
   ahead of the corresponding primary data), the design decision and
   trade-offs on the quality, encoding, bandwidth overhead, etc. of the
   redundant stream are not different from the traditional RTP payload
   redundant scheme.

   The following diagram illustrates a segment of the transmission
   sequence of a forward-shifted redundant RTP session, in which the AS
   stream is forward-shifted by 155 frames.  If, for simplicity here, we
   assume that the value of the timestamp is incremented by 1 between
   two consecutive frames, this forward-shifted redundancy can then be
   indicated with:

      forwardshift=155



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   and the setting of the timestamp offset to 0 in all the additional
   headers.  This can mean 3.1 seconds of forward-shifting if each frame
   represents 20 ms of original media.

                           Primary stream    AS stream

               ...               |                |
                                 v                v
               Pkt k+8        [ 111 ]          [ 266 ]
                                 |                |
                                 v                v
               Pkt k+7        [ 110 ]          [ 265 ]
                                 |                |
                                 v                v
           ^   Pkt k+6        [ 109 ]          [ 264 ]
           |                     |                |
           |                     v                v
               Pkt k+5        [ 108 ]          [ 263 ]
           T                     |                |
           I                     v                v
           M   Pkt k+4        [ 107 ]          [ 262 ]
           E                     |                |
                                 v                v
               Pkt k+3        [ 106 ]          [ 261 ]
                                 |                |
                                 v                v
               Pkt k+2        [ 105 ]          [ 260 ]
                                 |                |
                                 v                v
               Pkt k+1        [ 104 ]          [ 259 ]
                                 |                |
                                 v                v
               Pkt k          [ 103 ]          [ 258 ]
                                 |                |
                                 v                v

                                (Transmitted first)

       Figure 1: An Example of Forward-Shifted Redundant RTP Packet
                               Transmission











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A.2.  Receiver-Side Operations

   The anti-shadow receiver is illustrated in the following diagram.

                                                 +---------+
                               normal mode   sw1 | media   |     media
    Primary stream ======================o___o==>| decoder |===> output
    AS stream     ----                           +---------+     device
                     |             AS mode o
                     |       +---------+   |
                     |       | anti-   |   |
                     ------->| shadow  |----
                             | buffer  |
                             +---------+
                                  |
                                  V
                             expired frames
                             discarded

                    Figure 2: Anti-Shadow RTP Receiver

   The anti-shadow receiver operates between two modes -- "normal mode"
   and "AS mode".  When the receiver is not in a shadow (i.e., when it
   still receives new data), it operates in the normal mode.  Otherwise,
   it operates in the AS mode.

A.2.1.  Normal-Mode Operation

   In the normal mode, after receiving a new RTP packet that contains
   the primary data and forward-shifted AS data, the receiver passes the
   primary data directly to the appropriate media decoder for play-out
   (a de-jittering buffer may be used before the play-out, but for
   simplicity we assume that none is used here), while the received AS
   data is stored in an anti-shadow buffer.

   Moreover, data stored in the anti-shadow buffer will be continuously
   checked to determine whether it has expired.  If any redundant data
   in the anti-shadow buffer is found to have a timestamp older (i.e.,
   smaller) than that of the last primary frame passed to the media
   decoder, it will be considered expired and be purged from the
   anti-shadow buffer.

   The following example illustrates the operation of the anti-shadow
   buffer in normal mode.  We use the same assumption as in Figure 1,
   and assume that the initial timestamp value is 103 when the session
   starts.





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             Timestamp     Timestamp
     Time      being      of media in
    (in ms)  played out    AS buffer         Note
   ------------------------------------------------------------------
     t < 0                 --             (buffer empty)
      ...
     t=0       103         258            (hold 1 AS frame)
     t=20      104         258-259        (hold 2 AS frames)
     t=40      105         258-260        (hold 3 AS frames)

      ...
     t=3080    257         258-412        (full, hold 155 AS frames)
     t=3100    258         259-413        (full, frame 258 purged)
     t=3120    259         260-414        (full, frame 259 purged)
      ...

     t=6240    415         416-570        (always holds 3.1 sec
                                           worth of redundant data)
      ...

     Figure 3: Example of Anti-Shadow Buffer Operation in Normal Mode

   Before the beginning of the session (t<0), the anti-shadow buffer
   will be empty.  When the first primary frame is received, the play-
   out will start immediately, and the first received AS frame is stored
   in the anti-shadow buffer.  With the arrival of more forward-shifted
   redundant frames, the anti-shadow buffer will gradually be filled up.

   For the example shown in Figure 3, after 3.08 seconds (the amount of
   the forward-shifting minus one frame) from the start of the session,
   the anti-shadow buffer will be full, holding exactly 3.1 seconds
   worth of redundant data, with the oldest frame corresponding to
   t=3.1 sec and the youngest frame corresponding to t=6.18 sec.

   It is not difficult to see that in normal mode, because of the
   continuous purge of expired frames and the addition of new frames,
   the anti-shadow buffer will always be full, holding the next
   'forwardshift' amount of redundant frames.

A.2.2.  Anti-Shadow-Mode Operation

   When the receiver enters a shadow (or any other conditions that
   prevent the receiver from getting new media data), the receiver
   switches to the anti-shadow mode, in which it simply continues the
   play-out from the forward-shifted redundant data stored in the
   anti-shadow buffer.





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   For the example in Figure 3, if the receiver enters a shadow at
   t=3120, it can continue the play-out by using the forward-shifted
   redundant frames (ts=260-414) from the anti-shadow buffer.  As long
   as the receiver can move out of the shadow by t=6240, there will be
   no service interruption.

   When the shadow condition ends (meaning new data starts to arrive
   again), the receiver immediately switches back to the normal mode of
   operation, playing out from newly arrived primary frames.  At the
   same time, the arrival of new AS frames will start to re-fill the
   anti-shadow buffer.

   However, if the duration of the shadow is longer than the amount of
   forward-shifting, the receiver will run out of media frames from its
   anti-shadow buffer.  At that point, service interruption will occur.

Author's Address

   Qiaobing Xie
   15901 Wetherburn Rd.
   Chesterfield, MO  63017
   US

   Phone: +1-847-893-0222
   EMail: Qiaobing.Xie@gmail.com


























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