Audio Video Transport WG Q. Xie
Internet-Draft TRG
Updates: 2198, 4102 June 24, 2010
(if approved)
Intended status: Standards Track
Expires: December 26, 2010
Forward-shifted RTP Redundancy Payload Support
draft-ietf-avt-forward-shifted-red-05.txt
Abstract
This document defines a simple enhancement to RFC 2198 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).
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Table of Contents
1. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Sending Redundant Data Inband vs. Out-of-band . . . . . . 3
3. Allowing Forward-shifted Redundant Data . . . . . . . . . . . 4
4. Registration of Media Type "fwdred" . . . . . . . . . . . . . 5
5. Mapping Media Type Parameters into SDP . . . . . . . . . . . . 6
6. Usage in Offer/Answer . . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. Normative References . . . . . . . . . . . . . . . . . . . . . 7
Appendix A. Anti-shadow Loss Concealment Using
Forward-shifted Redundancy . . . . . . . . . . . . . 8
A.1. Sender Side Operations . . . . . . . . . . . . . . . . . . 8
A.2. Receiver Side Operations . . . . . . . . . . . . . . . . . 10
A.2.1. Normal Mode Operation . . . . . . . . . . . . . . . . 11
A.2.2. Anti-shadow Mode Operation . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. 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].
2. Introduction
This document defines a simple enhancement to RFC 2198 [RFC2198] to
support RTP sessions with forward-shifted redundant encodings, i.e.,
redundant data is 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], is known
to be ineffective in dealing with such consecutive frame losses.
In contrast, the forward-shifted redundancy, when used in combination
with the anti-shadow loss management at the receiver (as described in
Appendix A), can effectively prevent service interruptions when a
mobile receiver runs into such a shadow.
Anti-shadow loss concealment 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.
2.1. 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., 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.
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Inband Approach:
o Pro: A single RTP session is faster to setup and easier to manage.
o Pro: A single RTP session presents a simpler problem for NAT/
firewall traversal.
o Pro: Less overall overhead - one 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.
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 none specification - no new payload format is
needed.
o Con: Multiple RTP sessions may take longer to setup and more
complexity to manage.
o Con: Multiple RTP sessions NAT/firewall traverse are harder to
address.
o Con: Bigger overall overhead - more than one RTP/UDP/IP overhead.
It is noteworthy that the specification of inband payload formats
such as this and 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 deemed most beneficial under a given
circumstance.
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 bits unsigned
offset of timestamp relative to 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
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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 an optional parameter,
"forwardshift", is introduced for indicating the capability and
willingness of using forward-shifted redundancy and the base value of
timestamp forward-shifting. The base value of "forwardshift" is an
integer equal or greater than '0' in RTP timestamp units.
In an RTP session which 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, generally in a forward-shifted session, the timestamp offset in
the additional header SHOULD be set to '0'.
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 optional
"forwardshift" parameter.)
Type names: audio, text
Subtype names: fwdred
Required parameters: none
Optional parameters:
forwardshift: An unsigned integer can be specified as value.
If this parameter is present with 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 is absent or present with 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
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sender will have the same behaviors as defined in RFC 2198.
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 "Security Considerations" 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 and 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.
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:
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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 optional parameter "forwardshift" goes in the SDP "a=fmtp"
attribute by copying it directly from the media type string as
"forwardshift=value".
Example of usage of indicating 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
Example of usage of indicating 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 optional "forwardshift" SDP parameter specified in this document
is declarative, and all reasonable values are expected to be
supported.
7. IANA Considerations
The registration of the new subtype "fwdred", as described in
Section 4, is required.
8. Security Considerations
See Section 6 "Security Considerations" of RFC 2198 [RFC2198]. In
addition, since an excessive forwardshift value can be signalled, as
a denial of service, a receive SHOULD be prepared to ignore
outrageous values.
9. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
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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", 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.
Appendix A. Anti-shadow Loss Concealment Using Forward-shifted
Redundancy
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 the loss of data
due to shadowing, a mobile user may experience an unacceptable level
of service interruptions. And 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.
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Except for the fast that it 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 is 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 the value of timestamp is incremented by 1 between two
consecutive frames, this forward-shifted redundancy can then be
indicated with:
forwardshift=155
and the setting of timestamp offset to 0 in all the additional
headers. This can mean a 3.1 second of forward shifting if each
frame represents 20 ms of original media,
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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
Transmit first
Figure 1. An example of forward-shifted redundant RTP packet
transmission.
A.2. Receiver Side Operations
The anti-shadow receiver is illustrated in the following diagram.
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+---------+
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 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 a 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-
shadowing 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 154 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 3.08 sec
worth of redundant data)
...
Figure 3. Example of anti-shadow buffer operation in normal mode.
At 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. And with the arriving of more forward-shifted
redundant frames, the anti-shadow buffer will gradually be filled up.
For the example shown in Figure 1, 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 3.08 seconds
worth of redundant data, with the oldest frame corresponding to t=3.1
sec and youngest frame corresponding to t=6.18 sec.
And 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-shadowing buffer will always be full holding the next
forward-shift 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 far 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 normal mode of
operation, playing out from newly arrived primary frames. And 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
The Resource Group
1700 Pennsylvania Ave. NW
Washington, DC 20006
US
Phone: +1-314-420-8248
Email: Qiaobing.Xie@gmail.com
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