One document matched: draft-ietf-avtext-multiple-clock-rates-00.txt
Network Working Group M. Petit-Huguenin
Internet-Draft Stonyfish, Inc.
Updates: 3550 (if approved) June 1, 2011
Intended status: Standards Track
Expires: December 3, 2011
Support for multiple clock rates in an RTP session
draft-ietf-avtext-multiple-clock-rates-00
Abstract
This document clarifies the RTP specification when different clock
rates are used in an RTP session. It also provides guidance on how
to interoperate with legacy RTP implementations that use multiple
clock rates.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 3, 2011.
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.
Petit-Huguenin Expires December 3, 2011 [Page 1]
Internet-Draft Multiple Clock Rates June 2011
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Legacy RTP . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Different SSRC . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Same SSRC . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2.1. Monotonic timestamps . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. RTP Sender . . . . . . . . . . . . . . . . . . . . . . . . 6
4.2. RTP Receiver . . . . . . . . . . . . . . . . . . . . . . . 7
5. Interoperability Analysis . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . . 8
Appendix A. Using a fixed clock rate . . . . . . . . . . . . . . 9
Appendix B. Release notes . . . . . . . . . . . . . . . . . . . . 9
B.1. Modifications between
draft-ietf-avtext-multiple-clock-rates-00 and
draft-petithuguenin-avtext-multiple-clock-rates-01 . . . . 9
B.2. Modifications between
draft-petithuguenin-avtext-multiple-clock-rates-01 and
draft-petithuguenin-avtext-multiple-clock-rates-00 . . . . 9
B.3. Modifications between
draft-petithuguenin-avtext-multiple-clock-rates-00 and
draft-petithuguenin-avt-multiple-clock-rates-03 . . . . . 9
B.4. Modifications between
draft-petithuguenin-avt-multiple-clock-rates-03 and
draft-petithuguenin-avt-multiple-clock-rates-02 . . . . . 10
B.5. Modifications between
draft-petithuguenin-avt-multiple-clock-rates-02 and
draft-petithuguenin-avt-multiple-clock-rates-01 . . . . . 10
B.6. Modifications between
draft-petithuguenin-avt-multiple-clock-rates-01 and
draft-petithuguenin-avt-multiple-clock-rates-00 . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
Petit-Huguenin Expires December 3, 2011 [Page 2]
Internet-Draft Multiple Clock Rates June 2011
1. Introduction
The clock rate is a parameter of the payload format. It is often
defined as been the same as the sampling rate but it is not always
the case (see e.g. the G722 and MPA audio codecs in [RFC3551]).
An RTP sender can switch between different payloads during the
lifetime of an RTP session and because clock rates are defined by
payload types, it is possible that the clock rate also varies during
an RTP session. RTP [RFC3550] lists using multiple clock rates as
one of the reasons to not use different payloads on the same SSRC but
unfortunately this advice was not always followed and some RTP
implementations change the payload in the same SSRC even if the
different payloads use different clock rates.
This creates three problems:
o The method used to calculate the RTP timestamp field in an RTP
packet is underspecified.
o When the same SSRC is used for different clock rates, it is
difficult to know what clock rate was used for the RTP timestamp
field in an RTCP SR packet.
o When the same SSRC is used for different clock rates, it is
difficult to know what clock rate was used for the interarrival
jitter field in an RTCP RR packet.
Table 1 contains a non-exhaustive list of fields in RTCP packets that
uses a clock rate as unit:
+---------------------+------------------+-----------+
| Field name | RTCP packet type | Reference |
+---------------------+------------------+-----------+
| RTP timestamp | SR | [RFC3550] |
| Interarrival jitter | RR | [RFC3550] |
| min_jitter | XR Summary Block | [RFC3611] |
| max_jitter | XR Summary Block | [RFC3611] |
| mean_jitter | XR Summary Block | [RFC3611] |
| dev_jitter | XR Summary Block | [RFC3611] |
| Interarrival jitter | IJ | [RFC5450] |
| RTP timestamp | SMPTETC | [RFC5484] |
| Jitter | RSI Jitter Block | [RFC5760] |
| Median jitter | RSI Stats Block | [RFC5760] |
+---------------------+------------------+-----------+
Table 1
This document first tries to list in Section 2 and subsections all
the various algorithms used by existing RTP implementations. This
sections are not normative.
Petit-Huguenin Expires December 3, 2011 [Page 3]
Internet-Draft Multiple Clock Rates June 2011
Section 4 and subsections then recommend a unique algorithm that
modifies [RFC3550]. This sections are normative.
Section 5 and subsections then analyze what happen when the legacy
algorithms listed in Section 2 are used with the new algorithm listed
in Section 4. This sections are not normative.
2. Legacy RTP
The following sections describe the various ways legacy RTP
implementations behave when multiple clock rates are used. Legacy
RTP refers to RFC 3550 without the modifications introduced by this
document.
[[We need to list here all the methods used in the field. Please
send them to the author. NDA can be arranged if needed]]
2.1. Different SSRC
One way of managing multiple clock rates is to use a different SSRC
for each different clock rate, as in this case there is no ambiguity
on the clock rate used by fields in the RTCP packets. This method
also seems to be the original intent of RTP as can be deduced from
points 2 and 3 of section 5.2 of RFC 3550.
On the other hand changing the SSRC can be a problem for some
implementations designed to work only with unicast IP addresses,
where having multiple SSRCs is considered a corner case. Lip
synchronization can also be a problem in the interval between the
beginning of the new stream and the first RTCP SR packet. This is
not different than what happen at the beginning of the RTP session
but it can be more annoying for the end-user.
2.2. Same SSRC
The simplest way of managing multiple clock rates is to use the same
SSRC for all the payload types regardless of the clock rates.
Unfortunately there is no clear definition on how the RTP timestamp
should be calculated in this case. The following subsection presents
one algorithm used in the field.
2.2.1. Monotonic timestamps
The most common method of calculating the RTP timestamp ensures that
the value increases monotonically. The formula used by this method
is as follow:
Petit-Huguenin Expires December 3, 2011 [Page 4]
Internet-Draft Multiple Clock Rates June 2011
timestamp = previous_timestamp + (current_capture_time -
previous_capture_time) * current_clock_rate
The problem with this method is that the jitter calculation on the
receiving side gives invalid result during the transition between two
clock rates, as shown in Table 2. The capture and arrival time are
in seconds, starting at the beginning of the capture of the first
packet; clock rate is in Hz; the RTP timestamp does not include the
random offset; the transit, jitter, and average jitter use the clock
rate as unit.
+-------+-------+-----------+---------+---------+--------+----------+
| Capt. | Clock | RTP | Arrival | Transit | Jitter | Average |
| time | rate | timestamp | time | | | jitter |
+-------+-------+-----------+---------+---------+--------+----------+
| 0 | 8000 | 0 | 0.1 | 800 | | |
| 0.02 | 8000 | 160 | 0.12 | 800 | 0 | 0 |
| 0.04 | 8000 | 320 | 0.14 | 800 | 0 | 0 |
| 0.06 | 8000 | 480 | 0.16 | 800 | 0 | 0 |
| 0.08 | 16000 | 800 | 0.18 | 2080 | 480 | 30 |
| 0.1 | 16000 | 1120 | 0.2 | 2080 | 0 | 28 |
| 0.12 | 16000 | 1440 | 0.22 | 2080 | 0 | 26 |
| 0.14 | 8000 | 1600 | 0.24 | 320 | 720 | 70 |
| 0.16 | 8000 | 1760 | 0.26 | 320 | 0 | 65 |
+-------+-------+-----------+---------+---------+--------+----------+
Table 2
Calculating the correct transit time on the receiving side can be
done by using the following formulas:
(1) current_time_capture = current_timestamp - previous_timestamp) /
current_clock_rate + previous_time_capture
(2) transit = current_clock_rate * (time_arrival -
current_time_capture)
(3) previous_time_capture = current_time_capture
The main problem with this method, in addition to the fact that the
jitter calculation described in RFC 3550 cannot be used, is that is
it dependent on the previous RTP packets, packets that can be
reordered or lost in the network. But it seems that this is what
most implementations are using.
3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
Petit-Huguenin Expires December 3, 2011 [Page 5]
Internet-Draft Multiple Clock Rates June 2011
document are to be interpreted as described in [RFC2119].
Clock rate: The multiplier used to convert from a wallclock value in
seconds to an equivalent RTP timestamp value (without the fixed
random offset). Note that RFC 3550 uses various terms like "clock
frequency", "media clock rate", "timestamp unit", "timestamp
frequency", and "RTP timestamp clock rate" as synonymous to clock
rate.
RTP Sender: A logical network element that sends RTP packets, sends
RTCP SR packets, and receives RTCP RR packets.
RTP Receiver: A logical network element that receives RTP packets,
receives RTCP SR packets, and sends RTCP RR packets.
4. Recommendations
4.1. RTP Sender
An RTP Sender with RTCP turned off (i.e. by setting the RS and RR
bandwidth modifiers defined in [RFC3556] to 0) SHOULD use a different
SSRC for each different clock rate but MAY use different clock rates
on the same SSRC as long as the RTP timestamp without the random
offset is calculated as explained below:
[[This was designed to help VoIP implementations who anyway never
cared about RTCP. Do we want to keep this?]]
Each time the clock rate changes, the start_offset and capture_start
values are calculated with the following formulas:
start_offset = (capture_time - capture_start) * previous_clock_rate
capture_start = capture_time
For the first RTP packet, the values are initialized with the
following formulas:
start_offset = 0
capture_start = capture_time
After eventualy updating this values, the RTP timestamp is calculated
with the following formula:
timestamp = (capture_time - capture_start) * clock_rate +
start_offset
An RTP Sender with RTCP turned on MUST use a different SSRC for each
different clock rate. An RTCP BYE MUST be sent and a new SSRC MUST
be used if the clock rate switches back to a value already seen in
Petit-Huguenin Expires December 3, 2011 [Page 6]
Internet-Draft Multiple Clock Rates June 2011
the RTP stream.
To accelerate lip synchronization, the next compound RTCP packet sent
by the RTP sender MUST contain multiple SR packets, the first one
containing the mapping for the current clock rate and the next SR
packets containing the mapping for the other clock rates seen during
the last period.
[[Some legacy implementations may dislike receiving multiple SR
packets. What should we do?]]
The RTP extension defined in [RFC6051] MAY be used to accelerate the
synchronization.
4.2. RTP Receiver
An RTP Receiver MUST calculate the jitter using the following
formula:
D(i,j) = (arrival_time_j * clock_rate_i - timestamp_j) -
(arrival_time_i * clock_rate_i - timestamp_i)
An RTP Receiver MUST be able to handle a compound RTCP packet with
multiple SR packets.
For interoperability with legacy RTP implementations, an RTP receiver
MAY use the information in two consecutive SR packets to calculate
the clock rate used, i.e. if Ni is the NTP timestamp for the SR
packet i, Ri the RTP timestamp for the SR packet i and Nj and Rj the
NTP timestamp and RTP timestamp for the previous SR packet j, then
the clock rate can be guessed as the closest to (Ri - Rj) / (Ni -
Nj).
5. Interoperability Analysis
The next subsections analyze the various combinations between legacy
RTP implementations and RTP implementations that follow this document
specifications.
TBD
6. Security Considerations
TBD
Petit-Huguenin Expires December 3, 2011 [Page 7]
Internet-Draft Multiple Clock Rates June 2011
7. IANA Considerations
No IANA considerations.
8. Acknowledgements
Thanks to Colin Perkins, Ali C. Begen and Magnus Westerlund for their
comments, suggestions and questions that helped to improve this
document.
Thanks to Robert Sparks and the attendees of SIPit 26 for the survey
on multiple clock rates interoperability.
This document was written with the xml2rfc tool described in
[RFC2629].
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 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.
9.2. Informative References
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
June 1999.
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551,
July 2003.
[RFC3556] Casner, S., "Session Description Protocol (SDP) Bandwidth
Modifiers for RTP Control Protocol (RTCP) Bandwidth",
RFC 3556, July 2003.
[RFC3611] Friedman, T., Caceres, R., and A. Clark, "RTP Control
Protocol Extended Reports (RTCP XR)", RFC 3611,
November 2003.
[RFC5450] Singer, D. and H. Desineni, "Transmission Time Offsets in
RTP Streams", RFC 5450, March 2009.
Petit-Huguenin Expires December 3, 2011 [Page 8]
Internet-Draft Multiple Clock Rates June 2011
[RFC5484] Singer, D., "Associating Time-Codes with RTP Streams",
RFC 5484, March 2009.
[RFC5760] Ott, J., Chesterfield, J., and E. Schooler, "RTP Control
Protocol (RTCP) Extensions for Single-Source Multicast
Sessions with Unicast Feedback", RFC 5760, February 2010.
[RFC6051] Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP
Flows", RFC 6051, November 2010.
[uRTR] Wenger, S. and C. Perkins, "RTP Timestamp Frequency for
Variable Rate Audio Codecs",
draft-ietf-avt-variable-rate-audio-00 (work in progress),
October 2004.
Appendix A. Using a fixed clock rate
An alternate way of fixing the multiple clock rates issue was
proposed in [uRTR]. This document proposed to define a unified clock
rate, but the proposal was rejected at IETF 61.
Appendix B. Release notes
This section must be removed before publication as an RFC.
B.1. Modifications between draft-ietf-avtext-multiple-clock-rates-00
and draft-petithuguenin-avtext-multiple-clock-rates-01
o Changed capture_state to capture_start.
B.2. Modifications between
draft-petithuguenin-avtext-multiple-clock-rates-01 and
draft-petithuguenin-avtext-multiple-clock-rates-00
o Clarified the goals for this documents
o Removed the non-monotonic method (replaced by Magnus formula).
o Moved the "RTP Sender and RTP Receiver section inside a new
"Recommendations" section.
o Inserted the new Sender formula inside the Recommendation section.
o Inserted the new jitter formula in the RTP Receiver section.
o Emptied the Analysis sections.
B.3. Modifications between
draft-petithuguenin-avtext-multiple-clock-rates-00 and
draft-petithuguenin-avt-multiple-clock-rates-03
Petit-Huguenin Expires December 3, 2011 [Page 9]
Internet-Draft Multiple Clock Rates June 2011
o Initial release for avtext WG.
B.4. Modifications between
draft-petithuguenin-avt-multiple-clock-rates-03 and
draft-petithuguenin-avt-multiple-clock-rates-02
o Updated RFC reference.
B.5. Modifications between
draft-petithuguenin-avt-multiple-clock-rates-02 and
draft-petithuguenin-avt-multiple-clock-rates-01
o Having multiple SRs in a compound RTCP packet is OK.
o If RTCP is used, must send a BYE and not reuse the SSRC.
o Removed resolved notes.
o Acknowledged SIPit 26 survey.
o Fixed some nits.
B.6. Modifications between
draft-petithuguenin-avt-multiple-clock-rates-01 and
draft-petithuguenin-avt-multiple-clock-rates-00
o Complete rewrite as a Standard Track I-D modifying RFC 3550.
Author's Address
Marc Petit-Huguenin
Stonyfish, Inc.
Email: petithug@acm.org
Petit-Huguenin Expires December 3, 2011 [Page 10]
| PAFTECH AB 2003-2026 | 2026-04-23 09:13:36 |