ref: 186363604d64fa1d2445c60caec3f268b40f3e21
dir: /doc/ietf/draft-valin-celt-rtp-profile.xml/
<?xml version='1.0'?>
<!DOCTYPE rfc SYSTEM 'rfc2629.dtd'>
<?rfc symrefs="yes" toc="yes" ?>
<rfc ipr="full3978" docName="RTP Payload Format for the CELT Codec">
<front>
<title>draft-valin-celt-rtp-profile-01</title>
<author initials="J-M" surname="Valin" fullname="Jean-Marc Valin">
<organization>Octasic Semiconductor</organization>
<address>
<postal>
<street>4101, Molson Street, suite 300</street>
<city>Montreal</city>
<region>Quebec</region>
<code>H1Y 3L1</code>
<country>Canada</country>
</postal>
<email>[email protected]</email>
</address>
</author>
<author initials="G" surname="Maxwell" fullname="Gregory Maxwell">
<organization>Juniper Networks</organization>
<address>
<postal>
<street>2251 Corporate Park Drive, Suite 100</street>
<city>Herndon</city>
<region>VA</region>
<code>20171-1817</code>
<country>USA</country>
</postal>
<email>[email protected]</email>
</address>
</author>
<date day="27" month="February" year="2009" />
<area>General</area>
<workgroup>AVT Working Group</workgroup>
<keyword>I-D</keyword>
<keyword>Internet-Draft</keyword>
<keyword>CELT</keyword>
<keyword>RTP</keyword>
<abstract>
<t>
CELT is an open-source voice codec suitable for use in very low delay
audio communication applications, including Voice over IP (VoIP).
This document describes the payload format for CELT generated bit
streams within an RTP packet. Also included here are the necessary
details for the use of CELT with the Session Description Protocol
(SDP). At the time of this writing, the CELT bit-stream has NOT
been finalized yet, and compatibility is usually broken with
every new release of the codec.
</t>
</abstract>
</front>
<middle>
<section anchor="Conventions used in this document" title="Conventions used in this document">
<t>
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 <xref target="rfc2119"></xref>.
</t>
</section>
<section anchor="Overview of the CELT Codec" title="Overview of the CELT Codec">
<t>
CELT stands for "Constrained Energy Lapped Transform". It applies some of the CELP principles, but does everything in the frequency domain, which removes some of the limitations of CELP. CELT is suitable for both speech and music and currently features:
</t>
<t>
<list style="symbols">
<t>Ultra-low algorithmic delay (typically from 3 to 9 ms)</t>
<t>Full audio bandwidth (better than 20kHz bandpass)</t>
<t>Support for both voice and music</t>
<t>Stereo support</t>
<t>Packet loss concealment</t>
<t>Constant bitrates from 32 kbps to 128 kbps and above</t>
<t>Free software/open-source</t>
</list>
</t>
</section>
<section anchor="RTP payload format for CELT" title="RTP payload format for CELT">
<t>
For RTP based transportation of CELT encoded audio the standard
RTP header <xref target="rfc3550"></xref> is followed by one or more payload data blocks.
An optional padding terminator may also be used.
</t>
<t>
<figure>
<artwork><![CDATA[
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP Header |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| one or more frames of CELT .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
</t>
<section anchor="RTP Header" title="RTP Header">
<t>
<figure>
<artwork><![CDATA[
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
</t>
<t>
The RTP header begins with an octet of fields (V, P, X, and CC) to
support specialized RTP uses (see <xref target="rfc3550"></xref> and <xref target="rfc3551"></xref> for details). For
CELT the following values are used.
</t>
<t>Version (V): 2 bits</t><t>
This field identifies the version of RTP. The version
used by this specification is two <xref target="rfc3550"></xref>.
</t>
<t>Padding (P): 1 bit</t><t>
If the padding bit is set, the packet contains one or more
additional padding octets at the end which are not part of the
payload. The last octet of the padding contains a count of how
many padding octets should be ignored, including itself. Padding
may be needed by some encryption algorithms with fixed block sizes
or for carrying several RTP packets in a lower-layer protocol data
unit.
</t>
<t>Extension (X): 1 bit</t><t>
If the extension, X, bit is set, the fixed header MUST be
followed by exactly one header extension, with a format defined
in Section 5.3.1. of <xref target="rfc3550"></xref>.
</t>
<t>CSRC count (CC): 4 bits</t><t>
The CSRC count contains the number of CSRC identifiers.
</t>
<t>Marker (M): 1 bit</t><t>
The M bit MUST be set to zero in all packets. The receiver MUST
ignore the M bit.
</t>
<t>Payload Type (PT): 7 bits</t><t>
An RTP profile for a class of applications is expected to assign
a payload type for this format, or a dynamically allocated
payload type SHOULD be chosen which designates the payload as
CELT.
</t>
<t>Sequence number: 16 bits</t><t>
The sequence number increments by one for each RTP data packet
sent, and may be used by the receiver to detect packet loss and
to restore packet sequence. This field is detailed further in
<xref target="rfc3550"></xref>.
</t>
<t>Timestamp: 32 bits</t><t>
A timestamp representing the sampling time of the first sample of
the first CELT frame in the RTP payload. The clock frequency
MUST be set to the sample rate of the encoded audio data and is
conveyed out-of-band (e.g., as an SDP parameter).
</t>
<t>SSRC/CSRC identifiers:</t><t>
These two fields, 32 bits each with one SSRC field and a maximum
of 16 CSRC fields, are as defined in <xref target="rfc3550"></xref>.
</t>
</section>
<section anchor="CELT payload" title="CELT payload">
<t>
For the purposes of packetizing the bit stream in RTP, it is only
necessary to consider the sequence of bits as output by the CELT
encoder <xref target="celt-website"></xref>, and present the same
sequence to the decoder. The payload format described here maintains
this sequence.
</t>
<t>
A typical CELT frame, encoded at a high bitrate, is approx.
128 octets and the total number of CELT frames SHOULD be kept
less than the path MTU to prevent fragmentation. CELT frames MUST
NOT be split across multiple RTP packets,
</t>
<t>
An RTP packet MAY contain CELT frames of the same bit rate or of
varying bit rates, since the bitrate for the frames is explicitly
conveyed in band with the signal.
</t>
<t>
The encoding and decoding algorithm can change the bit rate at any
frame boundary, with the bit rate change notification provided
in-band. No out-of-band notification is required for the decoder
to process changes in the bit rate sent by the encoder.
</t>
<t>
It is RECOMMENDED that sampling rates 32000, 44100, or 48000 Hz be used
for most applications, unless a specific reason exists -- such as
requirements for a very specific packetization time. For example,
51200 Hz sampling may be useful to obtain a 5 ms packetization time
with 256-sample frames. For compatibility reasons, the sender and
receiver MUST support 48000 Hz sampling rate.
</t>
<t>
The CELT codec always produces an integer number of bytes and can
produce any integer number of bytes, so no padding is ever required.
Bitrate adjustment SHOULD be used instead of padding.
</t>
</section>
<section anchor="Multiple CELT frames in a RTP packet" title="Multiple CELT frames in a RTP packet">
<t>
The bitrate used by CELT is implicitly determined by the size of the
compressed data. When more than one frame is encoded in the same packet,
it is not possible to determine the size of each encoded frame, so the
information MUST be explicitly encoded. If N frames are present in a
packet, N compressed frame sizes need to be encoded at the
beginning of the packet. Each size that is less than 255 bytes is encoded
in one byte (unsigned 8-bit integer). For sizes greater or equal to 255, a 0xff byte is encoded,
followed by the size-255. Multiple 0xff bytes are allowed if there are
more than 510 bytes transmitted. An payload of zero bytes MUST be interpreted as length zero
for all frames contained in that packet.
</t>
<t>
Below is an example of two CELT frames contained within one RTP
packet.
</t>
<t><figure>
<artwork><![CDATA[
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length frame 1| length frame 2| CELT frame 1... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (frame 1) | CELT frame 2... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (frame 2) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure></t>
</section>
<section anchor="Multiple channels" title="Multiple channels">
<t>CELT supports both mono streams and stereo streams. If more than two channels are desired, it is possible to use transmit multiple streams in the same packet. In this case, the number of streams S and the pairing must be agreed with out-of-band negotiation such as SDP. Each stream can be either mono or stereo, depending on whether the channels are assumed to be correlated. For example, a 5.1 surround could have the front-left and front-right channels in a stereo stream, the rear-left and rear-right channels in a separate stereo stream, while the center and low-frequency channels would be in separate mono streams. In that example, the RTP packet would be:</t>
<t><figure>
<artwork><![CDATA[
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Front length | rear length | center length | LFE length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Front stereo |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... | Rear stereo data... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Center mono data... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | LFE mono data... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure></t>
<t>In the case where streams for multiple channels are used with multiple frames of the same streams per packet, then all streams for a certain timestamp are encoded before all streams for the following timestamp. In the case of the 5.1 example above with two frames per packet, the number of compressed length fields would be S*N = 8.</t>
</section>
</section>
<section anchor="MIME registration of CELT" title="MIME registration of CELT">
<t>
Full definition of the MIME <xref target="rfc2045"></xref> type for CELT will be part of the Ogg
Vorbis MIME type definition application <xref target="rfc3534"></xref>.
</t>
<t>MIME media type name: audio</t>
<t>MIME subtype: celt</t>
<t>Optional parameters:</t>
<t>Required parameters: to be included in the Ogg MIME specification.</t>
<t>Encoding considerations:</t>
<t>Security Considerations:</t>
<t>See Section 6 of RFC 3047.</t>
<t>Interoperability considerations: none</t>
<t>Published specification: </t>
<t>Applications which use this media type:</t>
<t>Additional information: none</t>
<t>Person & email address to contact for further information:<vspace blankLines="1" />
<list style="empty">
<t>Jean-Marc Valin <[email protected]></t>
</list>
</t>
<t>Intended usage: COMMON</t>
<t>Author/Change controller:</t>
<t>
<list style="empty">
<t>Author: Jean-Marc Valin <[email protected]></t>
<t>Change controller: Jean-Marc Valin <[email protected]></t>
<t>Change controller: IETF AVT Working Group</t>
</list>
</t>
<t>
This transport type signifies that the content is to be interpreted
according to this document if the contents are transmitted over RTP.
Should this transport type appear over a lossless streaming protocol
such as TCP, the content encapsulation should be interpreted as an
Ogg Stream in accordance with <xref target="rfc3534"></xref>, with the exception that the
content of the Ogg Stream may be assumed to be CELT audio and
CELT audio only.
</t>
</section>
<section anchor="SDP usage of CELT" title="SDP usage of CELT">
<t>
When conveying information by SDP <xref target="rfc2327"></xref>, the encoding name MUST be
set to "CELT". The sampling frequency is typically between 32000 and 48000 Hz.
Implementations SHOULD support both 44100 Hz and 48000 Hz. The maximum bandwidth permitted for
the CELT audio is encoded using the "b=AS:" header, as explained in SDP <xref target="rfc2327"></xref>.
</t>
<t>
The SDP parameters have the following interpretation with respect to CELT:
</t>
<t>
<list style="empty">
<t>b=AS: The maximum bandwidth (in kbit/s) allowed for CELT, excluding the header overhead. The default is 64 kbit/s.</t>
<t>ptime: The desired packetization time. The sender SHOULD choose a number of frames per packet that corresponds to the smallest packetization time greater or equal to the specified ptime for the selected frame size. The default is 20 ms as specified in <xref target="rfc3551"></xref></t>
<t>maxptime: The maximum packetization time desired. If the maximum is lower than the smallest packetization time determined from the chosen frame size (as described above), then that packtization time SHOULD be used despite the maxptime value. The default is "no maximum".</t>
</list>
</t>
<t>
Several CELT-specific parameters can be given via the "a=fmtp:" directive. The
following parameters are defined for use in this way:
</t>
<t>
<list style="empty">
<t>frame-size: The frame size is the duration of each frame in samples. If more than one frame size is supported, a comma-separated list can be used. The default is 480.</t>
<t>mapping: Optional string describing the multi-channel mapping. </t>
</list>
</t>
<t>Several CELT specific parameters can be given in a single
a=fmtp line provided that they are separated by a semi-colon.</t>
<t>
The selected frame-size values MUST be even. They SHOULD be divisible by 8
and have a prime factorization which consists only of 2, 3, or 5 factors.
For example, powers-of-two and values such as 160, 320, 240, and 480 are
recommended. Implementations MUST support receiving and sending the default
value of 480, and if the size 480 is supported it MUST be offered. Implementations
SHOULD also support frame sizes of 256 and 512 since these are the ones that lead
to the lowest complexity.
</t>
<t>
Care must be taken when setting the value of ptime: and b=AS: so that the
RTP packet size does not exceed the path MTU.
</t>
<t>An example of the media representation in SDP for offering a single
channel of CELT at 48000 samples per second might be:
</t>
<t>
<vspace blankLines="1" />
<list style="empty">
<t>m=audio 8088 RTP/AVP 97</t>
<t>a=rtpmap:97 CELT/48000</t>
</list>
</t>
<t>
Note that the RTP payload type code of 97 is defined in this media
definition to be 'mapped' to the CELT codec at a 48kHz sampling
frequency using the 'a=rtpmap' line. Any number from 96 to 127
could have been chosen (the allowed range for dynamic types). If there
is more than one channel being encoded the rtpmap MUST specify the channel
count.
</t>
<t>
The following example illustrates the case where the offerer cannot receive more than 64 kbit/s.
</t>
<t>
<vspace blankLines="1" />
<list style="empty">
<t>m=audio 8088 RTP/AVP 97</t>
<t>b=AS:64</t>
<t>a=rtmap:97 CELT/48000</t>
</list>
</t>
<t>
In this case, if the remote party agrees, it should configure its
CELT encoder so that it does not use modes that produce more than
64 kbit/s. Note that the "b=" constraint also applies on all
payload types that may be proposed in the media line ("m=").
</t>
<t>The following example demonstrates the use of the a=fmtp: parameters:</t>
<t>
<vspace blankLines="1" />
<list style="empty">
<t>m=audio 8008 RTP/AVP 97</t>
<t>a=ptime: 21</t>
<t>a=rtpmap:97 CELT/44100</t>
<t>a=fmtp:97 frame-size=512;</t>
</list>
</t>
<t>
This examples illustrate an offerer that wishes to receive
a CELT stream at 44100 Hz, by packing two 512-sample frames in each packet.
</t>
</section>
<section anchor="Multichannel Mapping" title="Multichannel Mapping">
<t>
When more than two channels are used, a mapping parameter MUST be provided.
The mapping parameter is defined as comma separated list of integers which specify the
number of channels contained in each CELT stream, OPTIONALLY followed by a '/' and a
comma separated list of channel identifiers, then OPTIONALLY another '/' and a string
which provides an application specific elaboration on any speaker-feed definitions.
The channels per stream entries MUST be either 1 or 2. The total number of
channels is indicated by the sum of the channels per stream entries. The sum
of the channel counts MUST be equal to the total number of channels.
</t>
<t>
Channel identifiers are short alphanumeric strings.
Each identifier MUST begin with a letter indicating the type of channel. 'A' MUST be
used to indicate an ambisonic channel, 'S' to indicate a speaker-feed channel, or 'O'
indicating other usage.
</t>
<t>
A channel identifier MAY be repeated, but the meaning of such repetition is application specific.
Applications SHOULD attempt to utilize channel identifiers such that mixing all identical identifiers
would produce a reasonable result.
</t>
<t>
Non-surround usage such as individual performer tracks, effect send, "order wire", or
other administrative channels may be given application specific identifiers
which MUST not conflict with the identifiers defined in this draft. These
identifiers SHOULD begin with S if it would be sensible to include them in a
mono-downmix, or O if it would be most sensible to exclude them from a
mono-downmix.
An example usage might be
mapping=2,1,2,1,1/SLguitar,SRguitar,OheadsetG,SLkeyboard,SRkeyboard,OheadsetK,SMbass,OheadsetB"
</t>
<t>
Ambisonic channels MUST follow the Furse-Malham naming and weighing
conventions for up to third order spherical<xref target="Ambisonic"></xref>.
Higher order ambisonic support is application defined but MUST NOT reuse any of WXYZRSTUVKLMNOPQ
for higher order components. For example, second order spherical ambisonics SHOULD use the
mapping "mapping=1,1,1,1,1,1,1,1,1/AW,AX,AY,AZ,AR,AS,AT,AU,AV". Any set of Ambisonic channels
MUST contain at least one "AW" channel.
</t>
<t>
Speaker-feed identifiers are named based on the intended speaker locations. "L", "R" for the left and
right speakers, respectively, in conventional stereo or the front left and right in 4, 5,
5.1, or 7.1 channel surround. "LR", "RR" for the left and right rear speakers in 4,5 or 5.1 channel surround. C" is used for a center channel, "MLFE" for a low frequency extension channel. "LS", "RS" for the side
channels in 7.1 channel surround. Additional speaker-feeds are application specific but should not
reuse the prior identifiers.
For 5.1 surround in non-ambisonic form the mapping SHOULD be "mapping=2,2,1,1/L,R,LR,RR,C,MLFE/ITU-RBS.775-1".
When only one or two channels are used, the mapping parameter MAY be omitted, in which case the
default mapping is used. For one channel, the default is "mapping=1/C", while for two channels,
the default is "mapping=2/L,R".
</t>
<t>
For example a stereo configuration might signal:
<vspace blankLines="1" />
<list style="empty">
<t>m=audio 8008 RTP/AVP 97</t>
<t>a=ptime: 5</t>
<t>a=rtpmap:97 CELT/44100/2</t>
<t>a=fmtp:97 frame-size=256;</t>
</list>
Which specifies a single two-channel CELT stream according to the default
mapping.
</t>
</section>
<section anchor="Issues that need to be addressed" title="Issues that need to be addressed">
<t>
How do we minimize the possibility of encoder-decoder mode mismatch
</t>
<t>
Support for redundant data (how)?
</t>
</section>
<section anchor="RTP Payload Types" title="RTP Payload Types">
<t>
Dynamic payload type codes MUST be negotiated 'out-of-band'
for the assignment of a dynamic payload type from the
range of 96-127.
</t>
</section>
<section anchor="Congestion Control" title="Congestion Control">
<t>
CELT allows for bitrate adjustment in one byte per frame
increments without any signaling requirement or overhead.
Applications SHOULD utilize congestion control to regulate the transmitted bitrate.
</t>
</section>
<section anchor="Security Considerations" title="Security Considerations">
<t>
RTP packets using the payload format defined in this specification
are subject to the security considerations discussed in the RTP
specification <xref target="rfc3550"></xref>, and in any applicable RTP profile. The
main security considerations for the RTP packet carrying the RTP
payload format defined within this memo are confidentiality,
integrity and source authenticity. Confidentiality is achieved by
encryption of the RTP payload. Integrity of the RTP packets through
suitable cryptographic integrity protection mechanism. Cryptographic
system may also allow the authentication of the source of the
payload. A suitable security mechanism for this RTP payload format
should provide confidentiality, integrity protection and at least
source authentication capable of determining if an RTP packet is from
a member of the RTP session or not.
</t>
<t>
Note that the appropriate mechanism to provide security to RTP and
payloads following this memo may vary. It is dependent on the
application, the transport, and the signalling protocol employed.
Therefore a single mechanism is not sufficient, although if suitable
the usage of SRTP <xref target="rfc3711"></xref> is recommended. Other mechanism that may
be used are IPsec <xref target="rfc4301"></xref> and TLS <xref target="rfc5246"></xref> (RTP over TCP), but
also other alternatives may exist.
</t>
<t>
This RTP payload format and its media decoder do not exhibit any
significant non-uniformity in the receiver-side computational
complexity for packet processing, and thus are unlikely to pose a
denial-of-service threat due to the receipt of pathological data.
Nor does the RTP payload format contain any active content.
</t>
<t>
Because this format supports VBR operation small amounts of information
about the transmitted audio may be leaked by a length preserving
cryptographic transport. Accordingly, when CELT is used inside a secure
transport the sender SHOULD restrict the use of VBR to congestion control purposes.
</t>
<t>
CELT implementations will typically exhibit tiny content-sensitive encoding
time variances. Since transmission is usually triggered by an accurate
hardware clock and the encoded data is typically transmitted as soon as
encoding is complete this variance may result in a small amount of
additional frame to frame jitter which could be measured by a third-party.
Encrypted implementations SHOULD transmit packets at fixed intervals to
avoid the possible information leak.
</t>
</section>
<section anchor="Acknowledgments" title="Acknowledgments">
<t>
The authors would also like to thank the following members of the
CELT and AVT communities for their input:
</t>
</section>
</middle>
<back>
<references title="Normative References">
<reference anchor="rfc2119">
<front>
<title>Key words for use in RFCs to Indicate Requirement Levels </title>
<author initials="S." surname="Bradner" fullname="Scott Bradner"><organization/></author>
</front>
<seriesInfo name="RFC" value="2119" />
</reference>
<reference anchor="rfc3550">
<front>
<title>RTP: A Transport Protocol for real-time applications</title>
<author initials="H." surname="Schulzrinne" fullname=""><organization/></author>
<author initials="S." surname="Casner" fullname=""><organization/></author>
<author initials="R." surname="Frederick" fullname=""><organization/></author>
<author initials="V." surname="Jacobson" fullname=""><organization/></author>
</front>
<seriesInfo name="RFC" value="3550" />
</reference>
<reference anchor="rfc2045">
<front>
<title>Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
<author initials="" surname="" fullname=""><organization/></author>
<date month="November" year="1998" />
</front>
<seriesInfo name="RFC" value="2045" />
</reference>
<reference anchor="rfc2327">
<front>
<title>SDP: Session Description Protocol</title>
<author initials="V." surname="Jacobson" fullname=""><organization/></author>
<author initials="M." surname="Handley" fullname=""><organization/></author>
<date month="April" year="1998" />
</front>
<seriesInfo name="RFC" value="2327" />
</reference>
<reference anchor="rfc3551">
<front>
<title>RTP Profile for Audio and Video Conferences with Minimal Control.</title>
<author initials="H." surname="Schulzrinne" fullname=""><organization/></author>
<author initials="S." surname="Casner" fullname=""><organization/></author>
<date month="July" year="2003" />
</front>
<seriesInfo name="RFC" value="3551" />
</reference>
<reference anchor="rfc3534">
<front>
<title>The application/ogg Media Type</title>
<author initials="L." surname="Walleij" fullname=""><organization/></author>
<date month="May" year="2003" />
</front>
<seriesInfo name="RFC" value="3534" />
</reference>
<reference anchor="rfc3711">
<front>
<title>The Secure Real-time Transport Protocol (SRTP)</title>
<author initials="M." surname="Baugher" fullname=""><organization/></author>
<author initials="D." surname="McGrew" fullname=""><organization/></author>
<author initials="M." surname="Naslund" fullname=""><organization/></author>
<author initials="E." surname="Carrara" fullname=""><organization/></author>
<author initials="K." surname="Norrman" fullname=""><organization/></author>
<date month="March" year="2004" />
</front>
<seriesInfo name="RFC" value="3711" />
</reference>
<reference anchor="rfc4301">
<front>
<title>Security Architecture for the Internet Protocol</title>
<author initials="S." surname="Kent" fullname=""><organization/></author>
<author initials="K." surname="Seo" fullname=""><organization/></author>
<date month="December" year="2005" />
</front>
<seriesInfo name="RFC" value="4301" />
</reference>
<reference anchor="rfc5246">
<front>
<title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
<author initials="T." surname="Dierks" fullname=""><organization/></author>
<author initials="E." surname="Rescorla" fullname=""><organization/></author>
<date month="August" year="2008" />
</front>
<seriesInfo name="RFC" value="5246" />
</reference>
</references>
<references title="Informative References">
<reference anchor="celt-website">
<front>
<title>The CELT ultra-low delay audio codec</title>
<author initials="" surname="Xiph.Org Foundation" fullname="Xiph.Org Foundation"><organization/></author>
</front>
<seriesInfo name="CELT website" value="http://www.celt-codec.org/" />
</reference>
<reference anchor="Ambisonic">
<front>
<title>Higher order Ambisonic systems</title>
<author initials="D." surname="Malham" fullname="Dave
Malham"><organization/></author>
<date month="December" year="2003" />
</front>
<seriesInfo name="Paper"
value="http://www.york.ac.uk/inst/mustech/3d_audio/higher_order_ambisonics.pdf" />
</reference>
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