Internet Engineering Task Force (IETF) Q. Xie
Request for Comments: 6354 August 2011
Updates: 2198, 4102
Category: Standards Track
Forward-Shifted RTP Redundancy Payload Support
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
Status of This Memo
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Table of Contents
1. Introduction ....................................................21.1. Sending Redundant Data Inband vs. Out-of-Band ..............32. Conventions .....................................................43. Allowing Forward-Shifted Redundant Data .........................44. Registration of Media Type "fwdred" .............................55. Mapping Media Type Parameters into SDP ..........................76. Usage in Offer/Answer ...........................................77. IANA Considerations .............................................78. Security Considerations .........................................89. Normative References ............................................8Appendix A. Anti-Shadow Loss Concealment Using
Forward-Shifted Redundancy .............................9A.1. Sender-Side Operations .....................................9A.2. Receiver-Side Operations ..................................11A.2.1. Normal-Mode Operation ................................11A.2.2. Anti-Shadow-Mode Operation ...........................121. 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.
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
o Pro: A single RTP session is faster to set up and easier to
o Pro: A single RTP session presents a simpler problem for NAT/
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.
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
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
o Con: Bigger overall overhead -- more than one source of RTP/UDP/IP
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.
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.
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
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
Type names: audio, text
Subtype names: fwdred
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.
ptime: as defined in [RFC4102] [RFC4566].
maxptime: as defined in [RFC4102] [RFC4867].
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.
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.
IETF Audio/Video Transport working group delegated from the IESG.
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
o The required parameter "forwardshift" goes in the SDP "a=fmtp"
attribute by copying it directly from the media type string as
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=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=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:
o IANA added the following to the "Text Media Types" registry:
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,
[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.
Appendix A. Anti-Shadow Loss Concealment Using Forward-Shifted
(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
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
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
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
... | |
Pkt k+8 [ 111 ] [ 266 ]
Pkt k+7 [ 110 ] [ 265 ]
^ Pkt k+6 [ 109 ] [ 264 ]
| | |
| v v
Pkt k+5 [ 108 ] [ 263 ]
T | |
I v v
M Pkt k+4 [ 107 ] [ 262 ]
E | |
Pkt k+3 [ 106 ] [ 261 ]
Pkt k+2 [ 105 ] [ 260 ]
Pkt k+1 [ 104 ] [ 259 ]
Pkt k [ 103 ] [ 258 ]
Figure 1: An Example of Forward-Shifted Redundant RTP Packet
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 |
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
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
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
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
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.
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