Network Working Group Q. Xie, Ed.
Request for Comments: 3557 Motorola, Inc.
Category: Standards Track July 2003
RTP Payload Format for
European Telecommunications Standards Institute (ETSI) European Standard
ES 201 108 Distributed Speech Recognition Encoding
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document specifies an RTP payload format for encapsulating
European Telecommunications Standards Institute (ETSI) European
Standard (ES) 201 108 front-end signal processing feature streams for
distributed speech recognition (DSR) systems.
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RFC 3557 RTP Payload Format for DSR ES 201 108 July 2003
Table of Contents
1. Conventions and Acronyms . . . . . . . . . . . . . . . . . . . 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1. ETSI ES 201 108 DSR Front-end Codec. . . . . . . . . . . 3
2.2. Typical Scenarios for Using DSR Payload Format . . . . . 4
3. ES 201 108 DSR RTP Payload Format. . . . . . . . . . . . . . . 5
3.1. Consideration on Number of FPs in Each RTP Packet. . . . 6
3.2. Support for Discontinuous Transmission . . . . . . . . . 6
4. Frame Pair Formats . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Format of Speech and Non-speech FPs. . . . . . . . . . . 7
4.2. Format of Null FP. . . . . . . . . . . . . . . . . . . . 8
4.3. RTP header usage . . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 9
5.1. Mapping MIME Parameters into SDP . . . . . . . . . . . . 10
6. Security Considerations. . . . . . . . . . . . . . . . . . . . 11
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . 12
10. IPR Notices. . . . . . . . . . . . . . . . . . . . . . . . . . 12
11. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13
12. Editor's Address . . . . . . . . . . . . . . . . . . . . . . . 14
13. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 15
1. Conventions and Acronyms
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 [RFC2119].
The following acronyms are used in this document:
DSR - Distributed Speech Recognition
ETSI - the European Telecommunications Standards Institute
FP - Frame Pair
DTX - Discontinuous Transmission
2. Introduction
Motivated by technology advances in the field of speech recognition,
voice interfaces to services (such as airline information systems,
unified messaging) are becoming more prevalent. In parallel, the
popularity of mobile devices has also increased dramatically.
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RFC 3557 RTP Payload Format for DSR ES 201 108 July 2003
However, the voice codecs typically employed in mobile devices were
designed to optimize audible voice quality and not speech recognition
accuracy, and using these codecs with speech recognizers can result
in poor recognition performance. For systems that can be accessed
from heterogeneous networks using multiple speech codecs, recognition
system designers are further challenged to accommodate the
characteristics of these differences in a robust manner. Channel
errors and lost data packets in these networks result in further
degradation of the speech signal.
In traditional systems as described above, the entire speech
recognizer lies on the server. It is forced to use incoming speech
in whatever condition it arrives after the network decodes the
vocoded speech. To address this problem, we use a distributed speech
recognition (DSR) architecture. In such a system, the remote device
acts as a thin client, also known as the front-end, in communication
with a speech recognition server, also called a speech engine. The
remote device processes the speech, compresses the data, and adds
error protection to the bitstream in a manner optimal for speech
recognition. The speech engine then uses this representation
directly, minimizing the signal processing necessary and benefiting
from enhanced error concealment.
To achieve interoperability with different client devices and speech
engines, a common format is needed. Within the "Aurora" DSR working
group of the European Telecommunications Standards Institute (ETSI),
a payload has been defined and was published as a standard [ES201108]
in February 2000.
For voice dialogues between a caller and a voice service, low latency
is a high priority along with accurate speech recognition. While
jitter in the speech recognizer input is not particularly important,
many issues related to speech interaction over an IP-based connection
are still relevant. Therefore, it is desirable to use the DSR
payload in an RTP-based session.
2.1 ETSI ES 201 108 DSR Front-end Codec
The ETSI Standard ES 201 108 for DSR [ES201108] defines a signal
processing front-end and compression scheme for speech input to a
speech recognition system. Some relevant characteristics of this
ETSI DSR front-end codec are summarized below.
The coding algorithm, a standard mel-cepstral technique common to
many speech recognition systems, supports three raw sampling rates: 8
kHz, 11 kHz, and 16 kHz. The mel-cepstral calculation is a frame-
based scheme that produces an output vector every 10 ms.
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RFC 3557 RTP Payload Format for DSR ES 201 108 July 2003
After calculation of the mel-cepstral representation, the
representation is first quantized via split-vector quantization to
reduce the data rate of the encoded stream. Then, the quantized
vectors from two consecutive frames are put into an FP, as described
in more detail in Section 4.1.
2.2 Typical Scenarios for Using DSR Payload Format
The diagrams in Figure 1 show some typical use scenarios of the ES
201 108 DSR RTP payload format.
+--------+ +----------+
|IP USER | IP/UDP/RTP/DSR |IP SPEECH |
|TERMINAL|-------------------->| ENGINE |
| | | |
+--------+ +----------+
a) IP user terminal to IP speech engine
+--------+ DSR over +-------+ +----------+
| Non-IP | Circuit link | | IP/UDP/RTP/DSR |IP SPEECH |
| USER |:::::::::::::::>|GATEWAY|--------------->| ENGINE |
|TERMINAL| ETSI payload | | | |
+--------+ format +-------+ +----------+
b) non-IP user terminal to IP speech engine via a gateway
+--------+ +-------+ DSR over +----------+
|IP USER | IP/UDP/RTP/DSR | | circuit link | Non-IP |
|TERMINAL|----------------->|GATEWAY|::::::::::::::::>| SPEECH |
| | | | ETSI payload | ENGINE |
+--------+ +-------+ format +----------+
c) IP user terminal to non-IP speech engine via a gateway
Figure 1: Typical Scenarios for Using DSR Payload Format.
For the different scenarios in Figure 1, the speech recognizer always
resides in the speech engine. A DSR front-end encoder inside the
User Terminal performs front-end speech processing and sends the
resultant data to the speech engine in the form of "frame pairs"
(FPs). Each FP contains two sets of encoded speech vectors
representing 20ms of original speech.
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RFC 3557 RTP Payload Format for DSR ES 201 108 July 2003
3. ES 201 108 DSR RTP Payload Format
An ES 201 108 DSR RTP payload datagram consists of a standard RTP
header [RFC3550] followed by a DSR payload. The DSR payload itself
is formed by concatenating a series of ES 201 108 DSR FPs (defined in
Section 4).
FPs are always packed bit-contiguously into the payload octets
beginning with the most significant bit. For ES 201 108 front-end,
the size of each FP is 96 bits or 12 octets (see Sections 4.1 and
4.2). This ensures that a DSR payload will always end on an octet
boundary.
The following example shows a DSR RTP datagram carrying a DSR payload
containing three 96-bit-long FPs (bit 0 is the MSB):
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 in [RFC3550] /
\ \
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| |
+ +
| FP #1 (96 bits) |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| FP #2 (96 bits) |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| FP #3 (96 bits) |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2. An example of an ES 201 108 DSR RTP payload.
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RFC 3557 RTP Payload Format for DSR ES 201 108 July 2003
3.1 Consideration on Number of FPs in Each RTP Packet
The number of FPs per payload packet should be determined by the
latency and bandwidth requirements of the DSR application using this
payload format. In particular, using a smaller number of FPs per
payload packet in a session will result in lowered bandwidth
efficiency due to the RTP/UDP/IP header overhead, while using a
larger number of FPs per packet will cause longer end-to-end delay
and hence increased recognition latency. Furthermore, carrying a
larger number of FPs per packet will increase the possibility of
catastrophic packet loss; the loss of a large number of consecutive
FPs is a situation most speech recognizers have difficulty dealing
with.
It is therefore RECOMMENDED that the number of FPs per DSR payload
packet be minimized, subject to meeting the application's
requirements on network bandwidth efficiency. RTP header compression
techniques, such as those defined in [RFC2508] and [RFC3095], should
be considered to improve network bandwidth efficiency.
3.2 Support for Discontinuous Transmission
The DSR RTP payloads may be used to support discontinuous
transmission (DTX) of speech, which allows that DSR FPs are sent only
when speech has been detected at the terminal equipment.
In DTX a set of DSR frames coding an unbroken speech segment
transmitted from the terminal to the server is called a transmission
segment. A DSR frame inside such a transmission segment can be
either a speech frame or a non-speech frame, depending on the nature
of the section of the speech signal it represents.
The end of a transmission segment is determined at the sending end
equipment when the number of consecutive non-speech frames exceeds a
pre-set threshold, called the hangover time. A typical value used
for the hangover time is 1.5 seconds.
After all FPs in a transmission segment are sent, the front-end
SHOULD indicate the end of the current transmission segment by
sending one or more Null FPs (defined in Section 4.2).
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RFC 3557 RTP Payload Format for DSR ES 201 108 July 2003
4. Frame Pair Formats
4.1 Format of Speech and Non-speech FPs
The following mel-cepstral frame MUST be used, as defined in
[ES201108]:
As defined in [ES201108], pairs of the quantized 10ms mel-cepstral
frames MUST be grouped together and protected with a 4-bit CRC,
forming a 92-bit long FP:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Frame #1 (44 bits) |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Frame #2 (44 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
| | CRC |0|0|0|0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The length of each frame is 44 bits representing 10ms of voice. The
following mel-cepstral frame formats MUST be used when forming an FP:
Frame #1 in FP:
===============
(MSB) (LSB)
0 1 2 3 4 5 6 7
+-----+-----+-----+-----+-----+-----+-----+-----+
: idx(2,3) | idx(0,1) | Octet 1
+-----+-----+-----+-----+-----+-----+-----+-----+
: idx(4,5) | idx(2,3) (cont) : Octet 2
+-----+-----+-----+-----+-----+-----+-----+-----+
| idx(6,7) |idx(4,5)(cont) Octet 3
+-----+-----+-----+-----+-----+-----+-----+-----+
idx(10,11) | idx(8,9) | Octet 4
+-----+-----+-----+-----+-----+-----+-----+-----+
: idx(12,13) | idx(10,11) (cont) : Octet 5
+-----+-----+-----+-----+-----+-----+-----+-----+
| idx(12,13) (cont) : Octet 6/1
+-----+-----+-----+-----+
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RFC 3557 RTP Payload Format for DSR ES 201 108 July 2003
Frame #2 in FP:
===============
(MSB) (LSB)
0 1 2 3 4 5 6 7
+-----+-----+-----+-----+
: idx(0,1) | Octet 6/2
+-----+-----+-----+-----+-----+-----+-----+-----+
| idx(2,3) |idx(0,1)(cont) Octet 7
+-----+-----+-----+-----+-----+-----+-----+-----+
: idx(6,7) | idx(4,5) | Octet 8
+-----+-----+-----+-----+-----+-----+-----+-----+
: idx(8,9) | idx(6,7) (cont) : Octet 9
+-----+-----+-----+-----+-----+-----+-----+-----+
| idx(10,11) |idx(8,9)(cont) Octet 10
+-----+-----+-----+-----+-----+-----+-----+-----+
| idx(12,13) | Octet 11
+-----+-----+-----+-----+-----+-----+-----+-----+
Therefore, each FP represents 20ms of original speech. Note, as
shown above, each FP MUST be padded with 4 zeros to the end in order
to make it aligned to the 32-bit word boundary. This makes the size
of an FP 96 bits, or 12 octets. Note, this padding is separate from
padding indicated by the P bit in the RTP header.
The 4-bit CRC MUST be calculated using the formula defined in 6.2.4
in [ES201108]. The definition of the indices and the determination of
their value are also described in [ES201108].
4.2 Format of Null FP
A Null FP for the ES 201 108 front-end codec is defined by setting
the content of the first and second frame in the FP to null (i.e.,
filling the first 88 bits of the FP with 0's). The 4-bit CRC MUST be
calculated the same way as described in 6.2.4 in [ES201108], and 4
zeros MUST be padded to the end of the Null FP to make it 32-bit word
aligned.
4.3 RTP header usage
The format of the RTP header is specified in [RFC3550]. This payload
format uses the fields of the header in a manner consistent with that
specification.
The RTP timestamp corresponds to the sampling instant of the first
sample encoded for the first FP in the packet. The timestamp clock
frequency is the same as the sampling frequency, so the timestamp
unit is in samples.
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RFC 3557 RTP Payload Format for DSR ES 201 108 July 2003
As defined by ES 201 108 front-end codec, the duration of one FP is
20 ms, corresponding to 160, 220, or 320 encoded samples with
sampling rate of 8, 11, or 16 kHz being used at the front-end,
respectively. Thus, the timestamp is increased by 160, 220, or 320
for each consecutive FP, respectively.
The DSR payload for ES 201 108 front-end codes is always an integral
number of octets. If additional padding is required for some other
purpose, then the P bit in the RTP in the header may be set and
padding appended as specified in [RFC3550].
The RTP header marker bit (M) should be set following the general
rules defined in [RFC3551].
The assignment of an RTP payload type for this new packet format is
outside the scope of this document, and will not be specified here.
It is expected that the RTP profile under which this payload format
is being used will assign a payload type for this encoding or specify
that the payload type is to be bound dynamically.
5. IANA Considerations
One new MIME subtype registration is required for this payload type,
as defined below.
This section also defines the optional parameters that may be used to
describe a DSR session. The parameters are defined here as part of
the MIME subtype registration. A mapping of the parameters into the
Session Description Protocol (SDP) [RFC2327] is also provided in 5.1
for those applications that use SDP.
Media Type name: audio
Media subtype name: dsr-es201108
Required parameters: none
Optional parameters:
rate: Indicates the sample rate of the speech. Valid values include:
8000, 11000, and 16000. If this parameter is not present, 8000
sample rate is assumed.
maxptime: The maximum amount of media which can be encapsulated in
each packet, expressed as time in milliseconds. The time shall be
calculated as the sum of the time the media present in the packet
represents. The time SHOULD be a multiple of the frame pair size
(i.e., one FP <-> 20ms).
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RFC 3557 RTP Payload Format for DSR ES 201 108 July 2003
If this parameter is not present, maxptime is assumed to be 80ms.
Note, since the performance of most speech recognizers are
extremely sensitive to consecutive FP losses, if the user of the
payload format expects a high packet loss ratio for the session,
it MAY consider to explicitly choose a maxptime value for the
session that is shorter than the default value.
ptime: see RFC2327 [RFC2327].
Encoding considerations : This type is defined for transfer via RTP
[RFC3550] as described in Sections 3 and 4 of RFC 3557.
Security considerations : See Section 6 of RFC 3557.
Person & email address to contact for further information:
[email protected]
Intended usage: COMMON. It is expected that many VoIP applications
(as well as mobile applications) will use this type.
Author/Change controller:
[email protected]
IETF Audio/Video transport working group
5.1 Mapping MIME Parameters into SDP
The information carried in the MIME media type specification has a
specific mapping to fields in the Session Description Protocol (SDP)
[RFC2327], which is commonly used to describe RTP sessions. When SDP
is used to specify sessions employing ES 201 018 DSR codec, the
mapping is as follows:
o The MIME type ("audio") goes in SDP "m=" as the media name.
o The MIME subtype ("dsr-es201108") goes in SDP "a=rtpmap" as the
encoding name.
o The optional parameter "rate" also goes in "a=rtpmap" as clock
rate.
o The optional parameters "ptime" and "maxptime" go in the SDP
"a=ptime" and "a=maxptime" attributes, respectively.
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RFC 3557 RTP Payload Format for DSR ES 201 108 July 2003
Example of usage of ES 201 108 DSR:
m=audio 49120 RTP/AVP 101
a=rtpmap:101 dsr-es201108/8000
a=maxptime:40
6. Security Considerations
Implementations using the payload defined in this specification are
subject to the security considerations discussed in the RTP
specification [RFC3550] and the RTP profile [RFC3551]. This payload
does not specify any different security services.
7. Contributors
The following individuals contributed to the design of this payload
format and the writing of this document: Q. Xie (Motorola), D. Pearce
(Motorola), S. Balasuriya (Motorola), Y. Kim (VerbalTek), S. H. Maes
(IBM), and, Hari Garudadri (Qualcomm).
8. Acknowledgments
The design presented here benefits greatly from an earlier work on
DSR RTP payload design by Jeff Meunier and Priscilla Walther. The
authors also wish to thank Brian Eberman, John Lazzaro, Magnus
Westerlund, Rainu Pierce, Priscilla Walther, and others for their
review and valuable comments on this document.
9. References
9.1 Normative References
[ES201108] European Telecommunications Standards Institute (ETSI)
Standard ES 201 108, "Speech Processing, Transmission
and Quality Aspects (STQ); Distributed Speech
Recognition; Front-end Feature Extraction Algorithm;
Compression Algorithms," Ver. 1.1.2, April 11, 2000.
[RFC3550] Schulzrinne, H., Casner, S., Jacobson, V. and R.
Frederick, "RTP: A Transport Protocol for Real-Time
Applications", RFC 3550, July 2003.
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC2327] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
9.2 Informative References
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio
and Video Conferences with Minimal Control", RFC 3551,
July 2003.
[RFC2508] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP
Headers for Low-Speed Serial Links", RFC 2508, February
1999.
[RFC3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima,
H., Hannu, H., Jonsson, L-E, Hakenberg, R., Koren, T.,
Le, K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro,
K., Wiebke, T., Yoshimura, T. and H. Zheng, "RObust
Header Compression (ROHC): Framework and four profiles",
RFC 3095, July 2001.
10. IPR Notices
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made be made available, or the result of an attempt
made to obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
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RFC 3557 RTP Payload Format for DSR ES 201 108 July 2003
11. Authors' Addresses
David Pearce
Motorola Labs
UK Research Laboratory
Jays Close
Viables Industrial Estate
Basingstoke, HANTS, RG22 4PD
Phone: +44 (0)1256 484 436
EMail:
[email protected]
Senaka Balasuriya
Motorola, Inc.
600 U.S Highway 45
Libertyville, IL 60048, USA
Phone: +1-847-523-0440
EMail:
[email protected]
Yoon Kim
VerbalTek, Inc.
2921 Copper Rd.
Santa Clara, CA 95051
Phone: +1-408-768-4974
EMail:
[email protected]
Stephane H. Maes, PhD,
Oracle
500 Oracle Parkway, M/S 4op634
Redwood City, CA 94065 USA
Phone: +1-650-607-6296.
EMail:
[email protected]
Hari Garudadri
Qualcomm Inc.
5775, Morehouse Dr.
San Diego, CA 92121-1714, USA
Phone: +1-858-651-6383
EMail:
[email protected]
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12. Editor's Address
Qiaobing Xie
Motorola, Inc.
1501 W. Shure Drive, 2-F9
Arlington Heights, IL 60004, USA
Phone: +1-847-632-3028
EMail:
[email protected]
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13. Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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