[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-petithuguenin-avtext-multiple-clock-rates) 00 01 02 03 04 05 06 07 08 09 10 11 RFC 7160

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]


Html markup produced by rfcmarkup 1.129c, available from https://tools.ietf.org/tools/rfcmarkup/